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A hot interaction between immune cells and adipose tissue
van den Berg, S.M.
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Download date: 22 May 2020
A hot interaction between immune
cells and adipose tissue
Susanna Maria van den Berg
A hot interaction between immune cells and adipose tissue
PhD thesis, University of Amsterdam, the Netherlands
ISBN: 978‐94‐6299‐591‐8
Cover design: Emily van ‘t Wout and Susan van den Berg
Lay‐out: Susan van den Berg
Printing: Ridderprint BV
Copyright 2017 © S.M. van den Berg
A hot interaction between immune
cells and adipose tissue
ACADEMISCH PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Universiteit van Amsterdam
op gezag van de Rector Magnificus
prof. dr. ir. K.I.J. Maex
ten overstaan van een door het College voor Promoties ingestelde commissie,
in het openbaar te verdedigen in de Agnietenkapel
op vrijdag 2 juni 2017, te 14:00 uur
door Susanna Maria van den Berg
geboren te Leiden
Promotiecommissie:
Promotores: Prof. Dr. E. Lutgens AMC‐Universiteit van Amsterdam
Prof. Dr. M.P.J. de Winther AMC‐Universiteit van Amsterdam
Copromotor: Prof. Dr. P.C.N. Rensen Universiteit Leiden
Overige leden: Prof. Dr. N. Zelcer AMC‐Universiteit van Amsterdam
Prof. Dr. M. Nieuwdorp AMC‐Universiteit van Amsterdam
Prof. Dr. R.P.J. Oude Elferink AMC‐Universiteit van Amsterdam
Prof. Dr. R. Shiri‐Sverdlov Universiteit van Maastricht
Dr. B.G.A. Guigas Universiteit Leiden
Dr. R.H.L. Houtkooper AMC‐Universiteit van Amsterdam
Faculteit der Geneeskunde
Financial support by the Dutch Heart Foundation for the publication of this thesis is gratefully acknowledged. Further financial support for printing this thesis was kindly provided by Special Diets Services.
Table of contents
Chapter 1 General introduction………………………………………………………………………………………… 9
Chapter 2 Immune modulation of brown(ing) adipose tissue in obesity……………………………. 15
Chapter 3 Diet‐induced obesity induces rapid inflammatory changes in brown
adipose tissue in mice…………………………………………………………….……………..…………. 49
Chapter 4 Type 2 inflammatory response by helminth‐derived antigens induces
beiging of white adipose tissue in mice…………………………………………………………….. 65
Chapter 5 Diet‐induced obesity in mice diminishes hematopoietic stem and progenitor
cells in the bone marrow………………………………………………………………………………….. 81
Chapter 6 Blocking CD40‐TRAF6 interactions by small‐molecule inhibitor 6860766
ameliorates the complications of diet‐induced obesity in mice………………………… 101
Chapter 7 General discussion……………………………………………………………………………………………. 121
Appendix I. Summary…………………………………………………….………………………………………………… 133
II. Samenvatting……………………………………………………………………………………………….. 137
III. PhD Portfolio………………………………………………………………………………………….……. 141
Dankwoord …………………………………………………………………………………………………………………………. 147
1
BattlingobesityGeneralintroduction
Chapter 1
10
General introduction
11
Battling obesity
Cookies, candy and chocolate are found at every counter, hamburgers and cola are the cheapest
items on the menu and exercising takes too much time and effort; it is so easy to become fat. It has
come to a point where obesity rates worldwide are ridiculously high; 39% of the world’s adult
population is overweight, of which 13% is obese [World Health Organization]. We love to eat,
especially when it is cheap, greasy, sweet and fast.
Willingness to eat is of course essential to survive but this is becoming a problem when food is
everywhere. Why is being overweight or obese a bad thing? Obesity is linked with high risks of type 2
diabetes (T2D), fatty liver disease, cardiovascular disease (CVD), and cancer [1]. Chronic obesity may
shorten healthy lifespan by 5‐20 years, which results in a tremendous socio‐economic burden [2].
When energy intake exceeds energy expenditure, white adipose tissue stores excessive lipids. With
the accumulation of lipids, adipocytes enlarge and the tissue becomes hypoxic. Dysfunctional
adipocytes secrete adipokines, cytokines and chemokines that recruit inflammatory cells, which
results in a state of chronic low‐grade inflammation [3].
In the current era of scientific research, we are stepping in and manipulate our survival mechanisms
by finding strategies that limit the damage of our unhealthy eating life style. This thesis describes
three approaches that contribute to the battle against obesity‐associated diseases. First, we
hypothesize that obesity induces immunological changes in brown adipose tissue (BAT), which
affects brown adipocyte activity. Our second hypothesis is that the chronic low‐grade inflammatory
state in obesity and the continuous recruitment of immune cells affects hematopoietic stem cells in
the bone marrow. Lastly, we hypothesize that inhibiting the interaction between the co‐stimulatory
molecule CD40 and its adaptor protein TRAF6 using a small‐molecule inhibitor will improve white
adipose tissue inflammation and the associated metabolic dysfunction in a model of diet‐induced
obesity.
Brown adipose tissue burns fat
One intriguing target to reduce excessive energy storage in obesity is BAT. BAT is involved in adaptive
thermogenesis. Cold exposure activates the production of heat by brown adipocytes using glucose
and fat as fuel [4]. Chapter 2 of this thesis elaborates on how BAT is regulated, describes that brown
adipocytes can occur within white adipose tissue, describes how different components of the
immune system are altered in obese adipose tissue, and extensively discusses recent data on how
immune cells contribute to the regulation of brown and beige adipocyte activity.
The underlying key component of adverse effects in obesity is inflammation. Although this has been
extensively studied in white adipose tissue, effects of obesity on the inflammatory status in BAT are
still largely unknown. In chapter 3 we explored what inflammatory changes occur in BAT in the
course of obesity. In chapter 4 we apply a model of helminth antigens, which induces a type 2
immune response, to study whether skewing macrophages to a Th2 and M2 anti‐inflammatory
phenotype affects brown(ing) adipose tissue in a setting of high‐fat diet.
Chapter 1
12
Does diet‐induced chronic inflammation affect hematopoietic stem cells?
Hematopoietic stem cells and progenitor cells (HSPCs) are the most primitive precursors of immune
cells and are mostly present in the bone marrow. Their most important feature is that they have a
self‐renewal capacity. HSPCs generally remain quiescent but can generate an appropriate immune
response when needed [5]. Homeostasis of the system is disturbed after chronic immune stressors,
creating a dysbalance in different immune cell lineages. Activation of HSPCs leads to proliferation,
differentiation and mobilization which upon prolonged stimuli can even lead to exhaustion of the
stem cell pool [6]. Dysregulation of the earliest hematopoietic stem cells can have major effects on
later lineages.
Obesity is characterized by an ongoing inflammatory response [3]. Due to the chronic nature of
obesity, immune cells are continuously recruited from the bone marrow. In chapter 5 we study
whether the immune system suffers from chronic activation by looking at HSPCs in the bone marrow
after different durations of HFD.
Targeting the co‐stimulatory molecule CD40 to improve metabolism in
obesity
Co‐stimulatory molecules have a central role in inflammation in which they help with the activation
of immune cells. Antigen presenting cells (APCs) activate T cells by presenting an antigen to the T cell
receptor, secondly, the interaction between co‐stimulatory molecules then provides an additional
stimuli to activate the APC and the T cell [7]. Co‐stimulatory molecules can also directly activate
other cell types including monocytes and granulocytes but also adipocytes or endothelial cells. An
important co‐stimulatory dyad is CD40‐CD40L, which is involved in the activation of APCs and T cells,
the induction of cytokine production, but also B cell isotype switching as well as the activation of
endothelial cells and the migration of monocytes [8, 9].
CD40 is involved in mediating a wide variety of immune responses. However, it cannot signal on its
own and upon binding of CD40L, it recruits adaptor proteins, TNFR‐associated factors (TRAFs) for
signal transduction. CD40 has different binding sites for TRAF 2/3/5 and for TRAF6. This allows CD40
to activate different signalling pathways depending on which adaptor protein binds, which cell type is
involved and other local conditions [9].
The CD40‐CD40L dyad has an important role in obesity. Interactions between CD40‐CD40L on
adipocytes and immune cells promote adipose tissue inflammation [10, 11]. In diet‐induced obesity,
CD40‐/‐ mice have increased insulin resistance, more adipose tissue inflammation and enhanced
hepatosteatosis compared to wild‐type mice. CD40‐TRAF2/3/5 deficient mice exhibit a similar
phenotype, however, deficient CD40‐TRAF6 signalling does not result in insulin resistance but
reduces adipose tissue inflammation and hepatosteatosis in diet‐induced obesity. In chapter 6 we
approach the diet‐induced inflammatory response in adipose tissue by targeting CD40‐TRAF6
interactions using a small molecule inhibitor, previously designed in our lab.
General introduction
13
References
[1] Berrington de Gonzalez A, Hartge P, Cerhan JR, Flint AJ, Hannan L, MacInnis RJ et al. 2010. Body‐Mass Index and Mortality among 1.46 Million White Adults. New England Journal of Medicine 363(23):2211‐2219.
[2] Kanneganti T‐D, Dixit VD. 2012. Immunological complications of obesity. Nat Immunol 13(8):707‐712. [3] Lumeng CN, Saltiel AR. 2011. Inflammatory links between obesity and metabolic disease. Journal of Clinical
Investigation 121(6):2111‐2117. [4] Kajimura S, Spiegelman Bruce M, Seale P. 2015. Brown and Beige Fat: Physiological Roles beyond Heat
Generation. Cell Metabolism 22(4):546‐559. [5] Mendelson A, Frenette PS. 2014. Hematopoietic stem cell niche maintenance during homeostasis and
regeneration. Nat Med 20(8):833‐846. [6] King KY, Goodell MA. 2011. Inflammatory modulation of HSCs: viewing the HSC as a foundation for the immune
response. Nat Rev Immunol 11(10):685‐692. [7] Zirlik A, Lutgens E. 2015. An inflammatory link in atherosclerosis and obesity. Co‐stimulatory molecules.
Hämostaseologie 35(3):272‐278. [8] Seijkens T, Kusters P, Chatzigeorgiou A, Chavakis T, Lutgens E. 2014. Immune Cell Crosstalk in Obesity: A Key Role
for Costimulation? Diabetes 63(12):3982. [9] Engel D, Seijkens T, Poggi M, Sanati M, Thevissen L, Beckers L et al. 2009. The immunobiology of CD154–CD40–
TRAF interactions in atherosclerosis. Semin Immunol 21(5):308‐312. [10] Poggi M, Jager J, Paulmyer‐Lacroix O, Peiretti F, Gremeaux T, Verdier M et al. 2009. The inflammatory receptor
CD40 is expressed on human adipocytes: contribution to crosstalk between lymphocytes and adipocytes. Diabetologia 52(6):1152‐1163.
[11] Poggi M, Engel D, Christ A, Beckers L, Wijnands E, Boon L et al. 2011. CD40L Deficiency Ameliorates Adipose Tissue Inflammation and Metabolic Manifestations of Obesity in Mice. Arterioscler Thromb Vasc Biol 31(10):2251‐2260.
Chapter 1
14
2
Immunemodulationofbrown(ing)adiposetissueinobesity
Susan M. van den Berg1, Andrea D. van Dam2,3, Patrick C.N. Rensen2,3, Menno P.J. de Winther1,4,*,
Esther Lutgens1,4,*
1Department of Medical Biochemistry, Subdivision of Experimental Vascular Biology, Academic Medical Centre, University
of Amsterdam, The Netherlands. 2Department of Medicine, Division of Endocrinology, Leiden University Medical Center, Leiden, The Netherlands. 3Einthoven Laboratory for Experimental Vascular Medicine, Leiden University Medical Center, Leiden, The Netherlands. 4Institute for Cardiovascular Prevention (IPEK), Ludwig Maximilians University of Munich, Munich, Germany.
*these authors contributed equally to this work.
Endocrine Reviews 2017 38(1): 46‐68
Chapter 2
16
Abstract
Obesity is associated with a variety of medical conditions such as type 2 diabetes and cardiovascular
diseases and is therefore responsible for high morbidity and mortality rates. Increasing energy
expenditure by brown adipose tissue (BAT) is a current novel strategy to reduce the excessive energy
stores in obesity. Brown adipocytes burn energy to generate heat and are mainly activated upon cold
exposure. As prolonged cold exposure is not a realistic therapy, researchers worldwide are searching
for novel ways to activate BAT and/or induce beiging of WAT. Recently the contribution of immune
cells in the regulation of brown adipocyte activity and beiging of WAT has gained increased attention,
with a prominent role for eosinophils and alternatively activated macrophages. This review will
discuss the re‐discovery of BAT, present an overview of modes of activation and differentiation of
beige and brown adipocytes and describe the recently discovered immunological pathways that are
key in mediating brown/beige adipocyte development and function. Interventions in immunological
pathways harbour the potential to provide novel strategies to increase beige and brown adipose
tissue activity as therapeutic target for obesity.
Immune regulation in adipose tissue
17
Introduction
According to the latest statistics of the world health organization, the worldwide prevalence of
obesity, defined as a BMI>30, has nearly doubled since 1980 and at least 2.8 million people die each
year as a result of obesity. This number is expected to further increase over the next decade. Obesity
leads to adverse effects on blood pressure, plasma lipid levels and insulin resistance. Health
problems related to these adverse effects include coronary artery disease, atherosclerosis, fatty liver
disease, type 2 diabetes, cancer and degenerative diseases [1]. The immune system plays a key role
in the pathogenesis of obesity, and extensive research is ongoing to identify critical players involved
to find novel therapeutic targets to combat obesity. Currently, adequate treatment options are
limited. Interestingly, recent studies show that the activation of beige or brown adipocytes and the
subsequent increase in energy expenditure can lower body fat mass and potentially lower adipose
tissue inflammation [2]. Moreover, there is increasing evidence that brown and beige adipose tissue
function is subject to immune regulation [3‐6]. Brown and beige adipose tissue are therefore of great
interest as a novel therapeutic target for obesity.
Brown adipose tissue
BAT regulates adaptive thermogenesis by mitochondrial uncoupling Whereas white adipose tissue (WAT) is specialized in the storage of energy, brown adipose tissue
(BAT) plays a central role in energy expenditure. Brown adipocytes convert energy from glucose and
fatty acids into heat via non‐shivering thermogenesis, which contributes to the maintenance of body
temperature [7]. The regulation of body temperature is crucial to ensure that cellular functions and
physiological processes continue in cold environments [8]. This regulation is particularly important in
small organisms with a relatively large surface area. Even new‐borns of large organisms have distinct
depots of BAT that regress with increasing age. BAT in adult humans is most commonly present in the
supraclavicular and neck region, but also along the vertebrae and aorta and near the kidneys (Figure
1) [9]. In rodents, the major BAT depot is found in the interscapular region, whereas smaller depots
include axillary BAT, cervical BAT and perirenal and periaortic BAT (Figure 1) [10, 11].
BAT is highly innervated by the sympathetic nervous system and a well‐structured vascularization
enables the supply of oxygen and transport of heat. Brown adipocytes have numerous small lipid
droplets for fast energy supply and a large number of mitochondria that can produce heat via
mitochondrial uncoupling. The wealth of mitochondria and the extensive vascularization pattern
account for their dark colour and hence its name: brown adipose tissue.
Cold activates the hypothalamus, which induces sympathetic outflow towards BAT and results in
release of noradrenaline by efferent sympathetic nerve endings. Noradrenaline binds to β‐adrenergic
receptors present on brown, but also on white adipocytes [12, 13]. Cold‐induced adrenergic‐receptor
stimulation has both acute and chronic effects on BAT [14]. Acute thermogenesis results in lipolysis,
degradation of fatty acids, glucose uptake and activation of uncoupling protein‐1 (UCP1). Chronic
activation leads to increased gene transcription of UCP1 and mitochondrial biogenesis [14]. BAT
expresses substantial amounts of UCP1, an inner‐membrane mitochondrial protein that uncouples
oxidative phosphorylation from ATP synthesis, resulting in dissipation of energy into heat (Figure 2)
[15].
Chapter 2
18
Figure 1. BAT locations Human new‐borns have a distinct interscapular BAT depot which regresses with age. Human adults have supraclavicular BAT and BAT depots in the neck region. Smaller depots are found along the aorta, vertebrae and kidneys. In mice, cervical BAT is located underneath the muscles running from the back of the head to the interscapular area. Ventrolateral of the scapulae is an axillary BAT depot. The major BAT depot is found interscapular, but also around the aorta and the hilum of the kidneys.
β‐adrenergic receptors signal via cAMP, leading to activation of protein kinase A (PKA) and p38
mitogen‐activated protein kinase (MAPK) [16, 17]. Eventually, this results in phosphorylation of
relevant transcription factors, including cAMP‐response element binding protein (CREB), which
control the expression of genes involved in BAT activation, such as peroxisome proliferator‐activated
receptor γ co‐activator 1α (PGC‐1α). PGC‐1α is a transcriptional cofactor that increases mitochondrial
biogenesis and induces expression of UCP1 [15, 18, 19].
White, beige and brown adipocytes
Currently, two distinct types of brown adipocytes, each of a different origin, have been described.
The classical brown adipocyte, which is found at distinct anatomical sites in mice, including the
interscapular, perirenal and axillary BAT depots, and the so‐called ‘brite’ or ‘beige’ adipocytes which
are found within WAT [20]. Inducing these beige adipocytes in WAT is referred to as ‘browning’ or
‘beiging’. Because humans lack a large classical BAT depot, it is attractive to identify approaches to
stimulate formation of beige cells within WAT that share functional characteristics with classical
brown adipocytes. An important difference between the two cell types is that classical brown
adipocytes constitutively express UCP1, while beige adipocytes only do so upon appropriate stimuli,
such as cold and β‐adrenergic receptor stimulation [21‐23]. Increased biogenesis of beige adipocytes
may contribute to increased energy expenditure, improved metabolic parameters and improved
tolerance to cold.
Cervical
InterscapularPeriaorticPerirenal
Axillary
Interscapular
Supraclavicular
Periaortic
Perirenal
Paravertebral
Neck
Immune regulation in adipose tissue
19
Figure 2. Activation of brown adipocytes by cold. Cold activates the hypothalamus in the brain, which activates the SNS and results in the release of noradrenaline that binds to β‐adrenergic receptors on brown adipocytes. The acute result is intracellular lipolysis, degradation of fatty acids via beta‐oxidation and activation of UCP1. Other effects of activated β‐adrenergic receptors occur via cAMP, PKA, CREB and p38 mitogen‐activated protein kinase. Oxidative phosphorylation in mitochondria drives protons from the mitochondrial matrix into the intermembrane space, generating an electrochemical gradient by which protons flow back into the mitochondrial matrix via ATP synthase, activating ATP synthesis in e.g. heart and skeletal muscle. In brown adipocytes, UCP1 is found on the inner‐mitochondrial membrane where it causes mitochondrial uncoupling. UCP1 increases the permeability of the inner mitochondrial membrane and thus causes a reflux of protons into the mitochondrial matrix, bypassing ATP synthase. This proton leakage leads to dissipation of energy into heat. ADP; adenosine diphosphate, ATP; adenosine triphosphate, cAMP; cyclic adenosine monophosphate, CREB; cAMP‐response element binding protein, FFA; free fatty acids, P38; p38 mitogen‐activated protein kinase, PKA; protein kinase A, SNS; sympathetic nervous system, UCP1; uncoupling protein 1.
Chapter 2
20
Microarray analysis as well as in vivo lineage tracing studies showed a common developmental
ancestry between the classical brown adipocyte and skeletal muscle cells, in which the common
progenitor is myogenic factor 5 (Myf5)‐expressing muscle precursor cell [20, 24]. Whereas myogenin
stimulates myocyte development, transcriptional regulator PR domain zinc finger protein 16
(Prdm16) regulates the developmental switch to brown adipocytes. PRDM16 forms a transcriptional
complex with activated transcription factor CCAAT/enhancer‐binding protein β (C/EBPβ), which
enables the switch from a myogenic precursor to a brown adipocyte [25]. This in turn leads to the
expression of Peroxisome proliferator‐activated receptor γ (Pparγ) and Pgc‐1α, which are key
regulators of brown adipocyte differentiation, and BAT associated genes such as Ucp1, Cidea,
Cox7α1, Cox8β, Elovl3 and Cpt1b. PPARγ itself can stimulate brown adipocyte differentiation as well.
Indeed, continuous treatment with a PPARγ agonist, rosiglitazone, enhances brown adipocyte
differentiation in vitro [26]. Although the Myf5 expressing precursors were assumed to be precursors
of brown adipocytes and muscle cells only, recent lineage studies contradict this. It was found that
Myf5+ precursor cells can also give rise to unilocular white adipocytes in subcutaneous and
retroperitoneal WAT [21, 27‐29]. In classic myogenic transcription, Myf5 is thought to function
downstream or simultaneously with paired box 3 (Pax3) and upstream of myogenic differentiation 1
(MyoD1). Indeed, expression of Myf5 and Pax3, not MyoD1, overlaps in adipocyte progenitors. This
holds true for most adipose tissue depots, but not in perigonadal WAT, indicating depot and gender
differences [28]. Although all brown adipocytes in interscapular BAT descend from Myf5+ precursors,
only a subset of classical brown adipocytes in cervical and no brown adipocytes in perirenal or
periaortic BAT could be traced back to a Myf5+ precursor, suggesting a large heterogeneity within
and between adipose tissue depots [28]. Other markers for committed brown adipocyte precursors
that differentiate into mature brown adipocytes are early B‐cell factor 2 (EBF2) and platelet‐derived
growth factor receptor‐α (PDGFRα) (Figure 3) [30].
Beige cells are present within WAT and, when present in sufficient quantity, significantly increase
energy expenditure and thereby are thought to contribute to a reduction in body fat [31‐34]. In mice,
cold exposure and β‐adrenergic receptor agonist treatment increases Ucp1 gene expression and
mitochondrial biogenesis in WAT, which increases the presence of beige cells within WAT [35]. Beige
cells are mostly present in subcutaneous WAT. Adipocyte‐specific PRDM16 transgenic mice are able
to induce the development of beige cells in subcutaneous WAT and have an increased energy
expenditure, are resistant to weight gain and show improved glucose tolerance on a HFD [36]. Other
stimuli that can induce beige adipogenesis include noradrenaline [5, 37], lactate [38], irisin [39],
fibroblast growth factor (FGF) 21 [40], BMP (bone morphogenetic protein) 4 [41] and BMP7 [42].
Beige adipocytes arise from white (i.e. unilocular and Ucp1 negative) adipocytes, via
transdifferentiation. The transdifferentiation hypothesis is based on experiments in rodents in which
beige adipogenesis was induced and examined by microscopy [33, 43, 44], analysis of DNA content
and labelling with bromodeoxyuridine [32, 45, 46] and UCP1‐Cre reporter mice, which allows
inducible permanent labelling of Ucp1+ cells and tracing of white, beige and brown adipocytes [47].
However, other studies show that beige adipocytes can arise de novo from specific precursor cells.
Using AdipoChaser mice, which have an inducible adipocyte‐tagging system with an adiponectin
promoter‐driven tetracycline‐on (Tet) transcription factor, a Tet responsive Cre (activated by
doxycycline) and a Rosa26 promoter‐driven loxP‐stop‐loxP‐β‐galactosidase. Upon doxycycline
treatment, all adipocytes become positive for β‐galactosidase (β‐gal). Upon induction of beige
Immune regulation in adipose tissue
21
Myf5-Pax3-
Precursor
Myf5+Pax3+
Precursor
Whitening
Myf5+Pax3+
Precursor
Transdifferentiation
Dedifferentiation
Beige adipocyte
Brown
adipocyteWhite
adipocyte
Myocyte
Myogenin
De novo adipogenesis
Inguinal subcutaneous white adipose tissue Interscapular brown adipose tissue
EBF2+PDGFRα+
Preadipocyte
PRDM16
PPARγ
EBF2+PDGFRα+
Preadipocyte
Figure 3. Beige and brown adipogenesis. Model showing beige and brown adipocyte development in inguinal subcutaneous adipose tissue and interscapular brown adipose tissue. White adipocytes can derive from both Myf5+Pax3+ as well as Myf5‐Pax3‐ precursors. Beige adipocytes can either transdifferentiate from mature white adipocytes or directly differentiate from EBF2+PDGFRα+ preadipocytes, called de novo adipogenesis. EBF2 is a selective marker for brown and beige preadipocytes.In interscapular BAT, brown adipocytes are derived from a multipotent Myf5
+Pax3+ expressing precursor population. When these precursors are exposed to myogenin they will develop into myocytes. PRDM16 and PPARy promote brown adipocyte differentiation. Brown adipocytes in BAT can undergo whitening upon exposure to thermoneutrality, obesity, ageing or sympathetic denervation. EBF2; early B‐cell factor 2, Myf5; myogenic factor 5, MyoD1; myogenic differentiation 1, Pax3; paired box 3, PDGFRα; platelet‐derived growth factor receptor‐α, PPARγ; peroxisome proliferator‐activated receptor γ, PRDM16; transcriptional regulator PR domain zinc finger protein 16.
Chapter 2
22
adipogenesis by cold exposure or β3 adrenergic receptor agonist treatment, β‐gal negative beige
adipocytes appear in subcutaneous WAT [48, 49]. As in interscapular BAT, Ebf2 expression marks
beige adipocytes, regardless of their developmental origin [30] (Figure 3). These discrepant insights
might result from technical restraints, or the fact that transdifferentiation and de novo adipogenesis
occur simultaneously, maybe depending on whether adipocytes have experienced beige stimuli
before [47]. Formerly beige, dedifferentiated white adipocytes might be capable of
transdifferentiation into beige adipocytes whereas a first encounter with beige stimuli induces de
novo development. Alternatively, pre‐encoded beige adipocytes may exist disguised as white
adipocytes and only develop their beige phenotype upon the adequate stimuli.
The vasculature may be another important mediator of beiging, as BAT is highly vascularized to
enable a fast supply of oxygen and nutrients and transport of generated heat [50]. In WAT, exercise,
as well as treatment with a β3‐adrenergic receptor agonist or a PPARγ ligand such as rosiglitazone,
not only induces beiging but also increases angiogenesis. Overexpression of Vegf increases WAT
vascularization and induces increased gene expression of Ucp1 and Pgc‐1a in retroperitoneal WAT
[51]. The importance of VEGF as an essential downstream target in the process of beiging was further
confirmed by administration of an anti‐VEGF antibody, which reduced both angiogenesis and beiging
in WAT [51].
Contrary to beiging of WAT, whitening of BAT also occurs. Sympathetic denervation,
thermoneutrality, ageing and excessive energy supplies in obesity causes BAT to accumulate lipid
droplets, resulting in whitening of the tissue [52, 53]. This is accompanied by decreased
vascularization, hypoxia and mitochondrial dysfunction. A whitened phenotype of BAT in obesity is
therefore predictive of decreased BAT activity and is associated with complications such as insulin
resistance [50]. Although the mechanisms of BAT whitening are largely unknown, VEGF‐mediated
vascularization seems to play an important role; obesity reduces Vegfa expression and deletion of
Vegfa in adipose tissue results in whitened BAT [50].
Immune regulation in adipose tissue
23
BAT activity and obesity: what we have learned from mouse models
Much of our current knowledge on the role of BAT in obesity is derived from experimental animal
studies, often including genetically modified mice that were subjected to different models of obesity.
In these studies, decreased BAT activity was associated with aggravated metabolic dysfunction and
an increase in obesity, whereas increased BAT activity was associated with improved metabolism.
Over the years, several experimental animal studies have been published in which the relation
between obesity and BAT was studied (Table 1). BAT activity is dependent on environmental
temperature and therefore, housing temperature has major metabolic consequences. Mice housed
at room temperature (18‐22°C) are thermally stressed, leading to increased metabolism and
activated BAT to maintain their body temperature. Elimination of thermal stress is accomplished by
thermoneutral housing conditions of 30°C. It is important to realize that in some experimental set‐
ups, it is thus appropriate to house mice under thermoneutral conditions, for example when
comparing studies to thermoneutral conditions in humans (i.e. 25°C) or to distinguish central
mediated effects from direct activation of BAT.
Genetic models of decreased BAT activity
Energy expenditure and heat production in BAT occurs via UCP1 and mice lacking UCP1 are indeed
unable to maintain their body temperature when exposed to acute cold. However, at room
temperature, the decreased ability to dissipate energy into heat does not result in increased obesity
neither on chow nor upon high‐fat diet (HFD) feeding [54], however, UCP1 knockout mice are more
susceptible to develop obesity with age [55]. Strikingly, when UCP1 knockout mice are housed under
thermoneutral conditions (30°C), they do develop more severe obesity on a HFD and also gain slightly
more weight when on chow diet [56].
Other studies in mice have revealed that besides UCP1 deficiency, additional models with decreased
BAT activity also suffer from cold intolerance and display an obesity‐sensitive phenotype, as listed in
Table 1. Genetic ablation of BAT function via expression of a diphtheria toxin A‐chain driven by
regulatory elements of the Ucp1 gene (UCP‐DTA) decreased the UCP1 content in BAT, which resulted
in a reduction in non‐shivering thermogenesis and obesity. Consequently, these mice did not show a
thermogenic response upon treatment with a β3‐adrenergic receptor agonist [57]. β‐adrenergic
receptor knockout mice lacking all three known β‐adrenergic receptors, as well as dopamine β‐
hydroxylase deficient mice, which fail to convert dopamine into noradrenaline, both present inactive
BAT and are sensitive to cold [58, 59]. Furthermore, β‐adrenergic receptor knockout mice are slightly
obese on a low fat diet and develop massive obesity on a HFD [59]. Leptin‐deficient ob/ob mice also
have reduced Ucp1 expression in BAT and, similar to UCP1 knockout mice, are cold sensitive, but will
survive when they are gradually exposed to extreme cold (4°C). However, mice lacking both UCP1
and leptin do not survive temperatures below 12°C. These models show that alternative mechanisms
for maintaining body temperatures do exist, but that these are only effective within certain limits
[60].
Increasing BAT activity improves metabolism in mice
The disruption of BAT activation results in an obesity‐prone phenotype in mice. Conversely,
increasing the amount and/or function of BAT can induce a healthy metabolic phenotype. Cold
exposure and β3‐adrenergic receptor agonist treatment increase Ucp1 expression, mitochondrial
Chapter 2
24
Table 1. Experimental animal studies showing the metabolic effects of increasing or decreasing BAT
activity.
Model Effect Reference
Decreased BAT activity
UCP1‐/‐
Room temperature (18‐22°C)
Chow diet
Cold sensitive Enerbäck 1997 [54]
Room temperature (18‐22°C)
HFD Increased susceptibility to obesity with age
Kontani 2005 [55]
Thermoneutral housing (30°C)
Chow diet
Obese Feldmann 2009 [56]
UCP‐DTA Reduction in NST Obese phenotype
Lowell 1993 [57]
β‐adrenergic receptor‐/‐
Chow diet
Slightly obese Bachman 2002 [59]
HFD Severe obesity
Dopamine β‐hydroxylase‐/‐ Inactive BAT Cold sensitive Slightly obese
Thomas 1997 [58]
ob/ob Obese Cold sensitive
Ukropec 2006 [60]
UCP1‐/‐ ob/ob Obese Extremely cold sensitive
Ukropec 2006 [60]
FABP4/5‐/‐ Cold sensitive Syamsunarno 2014 [61]
Adipocyte specific p62 Decreased BAT activity Obese
Muller 2013 [62]
Increased BAT activity
Cold exposure Increases UCP1 expression Increases thermogenic capacity Accelerates atherosclerosis
Ravussin 2014 [63] Bartelt 2011 [64] Dong 2013 [65]
β3‐ adrenergic receptor agonist treatment
Increases UCP1 expression Increases thermogenic capacity Reduces body weight Reduces atherosclerosis
Himms‐Hagen 1994 [66] Barbatelli 2010 [43] Berbée 2015 [35]
Constitutive UCP1 expression in adipocytes
Prevents obesity Kopecky 1995 [67]
Increased expression of UCP1 by FOXC2 overexpression on HFD
Less adiposity Improved glucose tolerance
Cederberg 2001 [68] Kim 2005 [69]
Increased BAT activity in Cidea‐/‐ Prevents obesity Zhou 2003 [70]
BAT transplantation Reversal obese phenotype Gunawarda 2012 [71] Stanford 2013 [72] Zhu 2014 [73]
P53 Prevents obesity Increases energy expenditure
Al‐Massadi 2016 [74]
USF1 Prevents obesity Laurila 2016 [75]
Immune regulation in adipose tissue
25
Table 1. continued
Model Effect Reference
Immune cell models
16 weeks HFD Increases inflammatory markers in BAT
Roberts‐Toler 2015 [76]
Cancer‐associated cachexia Increases M2 macrophage markers in beiging WAT
Petruzelli 2014 [77]
Macrophage depletion Cold sensitive Nguyen 2011 [3]
Cold exposure Increases M2 macrophages in WAT and BAT
Nguyen 2011 [3]
Il‐4/IL‐13‐/‐
STAT6‐/‐
IL‐4R‐/‐
4get‐ΔdblGATA‐/‐ (lacking eosinophils) CCR2‐/‐
Impairs cold induced beiging Qiu 2014 [5]
IL‐4 injections Reduces body weight Improves insulin sensitivity Increases beiging
Qiu 2014 [5]
IL‐33‐/‐ Obesity‐prone on a HFD Less beige adipocytes in WAT
Brestoff 2015 [6]
IL‐33 injections Increases energy expenditure Increases beiging
Lee 2015 [4]
Treg depletion Increased inflammation in BAT Cold sensitive
Medrikova 2015 [78]
Adiponectin‐/‐ Impairs cold induced beiging Hui 2015 [79]
Germ‐free mice Antibiotic‐depleted gut microbiota
Increases Eosinophils, type 2 cytokines and M2 macrophages Improves insulin sensitivity Increases beiging
Suarez‐Zamorano 2015 [80] Chevalier 2015 [81]
Macrophage depletion Decreases inflammation in WAT Lowers body weight Decreases glucose levels
Sakamoto 2016 [82]
IL‐6 injections Increases Ucp1 in WAT Knudsen 2014 [83]
Anti‐IL‐6 antibody Decreases Ucp1 in WAT Petruzelli 2014 [77]
IL‐6‐/‐ Reduces energy expenditure Cold sensitive
Knudsen 2014 [83]
Adipocyte specific CXCR4‐/‐ Severe obesity upon HFD Increased inflammation in WAT Cold sensitive
Yao 2014 [84]
BAT: brown adipose tissue, HFD: high fat diet, IL: interleukin, NST: non‐shivering thermogenesis, ob/ob: leptin deficient mice, Treg: regulatory T cells, UCP1: uncoupling protein 1, UCP‐DTA: expression of a diphtheria toxin A‐chain on regulatory elements of the Ucp1 gene, USF1: upstream stimulatory factor 1, WAT: white adipose tissue.
Chapter 2
26
biogenesis and differentiation of brown adipocytes. This results in BAT hyperplasia and an increased thermogenic capacity, ultimately leading to a reduction in body weight [34, 43, 63, 66, 85]. BAT activation also increases the influx of nutrients into brown adipocytes, including glucose and fatty acids. Glucose is taken up by GLUT1/4 receptors [86, 87] and (triglyceride‐derived) fatty acids by CD36 and LPL [64], which is regulated by Angptl4 [88]. Whether BAT takes up TRL‐derived fatty acids by holoparticle uptake of TRL or after lipolysis of TRLs is under debate [89], although current evidence points to uptake of fatty acids derived from triglycerides after lipolysis [90]. By taking up substrates, BAT activation leads to lowering of plasma glucose and triglyceride levels and can even attenuate hypercholesterolemia by indirectly enhancing hepatic clearance of TRL remnants [35, 89].
Whereas UCP1 knockout mice are more prone to develop obesity, constitutive overexpression of
UCP1 in adipocytes prevents obesity in mice [67] (Table 1). Transgenic mice with adipocyte‐specific
overexpression of winged helix/forkhead transcription factor (Foxc2), which plays a regulatory role in
adipocyte metabolism, have an enhanced sensitivity of the β‐adrenergic/cAMP/PKA pathway and an
increased expression of Ucp1. These mice develop adiposity and display improved glucose tolerance
following a HFD compared to wild‐type mice [68, 69]. BAT mitochondria express high levels of cell‐
death activator CIDE‐A (Cidea), that also plays a role in energy homeostasis. Mice deficient for CIDE‐A
have an increased amount of BAT and are protected from diet‐induced obesity (DIO) [70]. Loss of
Cidea stabilizes the AMP‐activated protein kinase (AMPK) complex, which enhances basal AMPK
activity leading to elevated fatty acid oxidation and energy expenditure [91]. In line with this,
treatment of mice with the AMPK activator metformin enhances BAT activity [92]. Fatty acid binding
proteins (s) are also largely involved in energy metabolism. Mice with mutations in two related
adipocyte FABPs, aP2 and mal1, are protected from DIO and exhibit increased energy expenditure
[93]. Moreover, FABP4/5 knockout mice have a severely impaired thermogenesis during fasting due
to the depletion of energy storage and reduced energy supply in BAT and skeletal muscle [61].
Three research groups have been able to increase the amount of BAT by successfully transplanting
BAT in diabetic or obese mice and observed a reversal of their obese phenotype [71‐73]. These
studies thus suggest that increasing BAT activity has a great potential as treatment to reverse
obesity.
BAT activation and atherosclerosis
An interesting therapeutic effect of activating BAT includes the reduction of dyslipidemia‐ associated
atherosclerosis. In APOE*3‐Leiden.CETP mice, which have a human‐like lipoprotein metabolism
through expression of a mutated variant of the human APOE*3 gene combined with human
cholesteryl ester transfer protein (CETP), BAT activation by β3‐adrenergic receptor stimulation was
shown to be protective against diet‐induced atherosclerosis [35]. β3‐adrenergic receptor agonist
treatment decreased plasma triglyceride and cholesterol levels [35]. Notably, using ApoE knockout or
LDLR knockout mice, β3‐adrenergic receptor agonist treatment similarly reduced plasma triglycerides
but did not affect plasma cholesterol levels or atherosclerosis [35]. In fact, cold exposure of ApoE
knockout or LDLR knockout mice increased plasma levels of small low‐density lipoprotein (LDL)
remnants, leading to unfavourable plasma lipid levels and even accelerated development of
atherosclerotic lesions [65]. These findings show that in the presence of an intact ApoE‐LDLR
clearance pathway, as is present in APOE*3‐Leiden.CETP mice, BAT‐mediated local lipolysis of
triglyceride‐rich lipoproteins stimulates the hepatic clearance of lipoprotein remnants via ApoE and
Immune regulation in adipose tissue
27
the LDLR. As a result, BAT activation reduces atherosclerotic lesion size and severity in APOE*3‐
Leiden.CETP mice but not in mice lacking ApoE or the LDL receptor [35].
BAT and obesity in humans
Presence of active BAT in human adults
In humans, BAT was initially considered to be only present in new‐borns, but was recently detected
to be still present in adults as well [9, 94, 95]. Simultaneous examinations of positron‐emission
tomographic (PET) and X‐ray computed tomography (CT) revealed sites in human adult adipose
tissue with increased 18F‐fluorodeoxy‐glucose (FDG) uptake, indicating metabolically active tissue.
Tissue biopsies confirmed that the FDG‐intense tissue was indeed composed of brown adipocytes,
and thus could be classified as BAT [95]. In humans, cold exposure increases glucose uptake rate in
BAT by 12‐15‐fold compared to thermoneutral conditions [95, 96]. This was reflected by an increased
FDG uptake, as well as by an increased fatty acid uptake and a higher activation of oxidative
metabolism, which was demonstrated by subsequent studies in which humans were subjected to
cold [95, 97]. The β‐adrenergic blocker propranolol reduces uptake of FDG in BAT depots in humans
[98] and treatment with mirabegron, a β3‐adrenergic receptor agonist approved to treat the
overactive bladder, increases the metabolic activity of BAT evidenced by increased FDG uptake in
healthy male subjects [99].
Studies in humans comparing gene expression profiles of white, beige and brown adipocytes claim
that BAT in adult humans is mainly composed of beige adipocytes [21‐23, 100, 101] while others
have found markers for classical BAT in humans [102]. Controversy exists on brown and beige
markers and further studies will be needed to unequivocally characterize BAT in humans. Other
discrepancies can be caused by sampling bias (i.e. variations in location where the biopsies were
taken) and differences in age or body mass index of the studied population. It is likely that beige
adipocytes remain present in the adult state when hypothermia is a less frequent threat than in new‐
borns or rodents. The capability of beige cells to switch between a state of energy storage and
energy dissipation is intriguing and further studies on how this switch works are needed.
BAT activity and obesity
Along with the discovery of BAT in adult humans by PET/CT studies, it also became clear that the
presence of BAT, visualized through FDG uptake, showed a negative correlation with BMI and body
fat percentage [9, 94]. A study of 2000 randomly selected PET/CT scans showed a higher frequency of
functionally active BAT in females than in males and a negative correlation with age, BMI, but also
beta‐blocker use, outdoor temperature at the time of the scan and season [103, 104]. Interestingly,
recent data show that cold‐induced fatty acid uptake by BAT is similar in individuals with type 2
diabetes, age‐matched controls and in healthy young controls [105], suggesting that previous
conclusions based on FDG uptake may also reflect altered insulin sensitivity of BAT rather than BAT
activity.
Since stimulating BAT results in enhanced energy expenditure, it is an intriguing target for the control
of whole‐body energy balance, adiposity and obesity. The first indication that BAT activity could be
increased in humans was shown in a PET/CT study, where ten morbidly obese patients with an
average BMI of 42 were scanned before and 1 year after bariatric surgery. Before surgery, FDG
Chapter 2
28
uptake by BAT was seen in only two patients, whereas 1 year after surgery, when the average BMI
was 30, active BAT was present in five patients [106]. Interestingly, also repeated cold exposure can
increase BAT activity in humans. Successful cold acclimation protocols include 6 hours of cold
exposure for 10 consecutive days or daily exposure (17°C) for 2 hours during 6 weeks [2, 107]. Using
these protocols, it was possible to increase BAT volume by 37% or increase FDG uptake by 60% and
decrease fat mass in humans [2].
Although still scarce, more and more studies on human brown or beige adipocyte development,
function and how they are modulated are published. Min et al. reported that pro‐angiogenic factors
drive the proliferation of human beige adipocyte progenitors and activate beige adipocytes which,
when transplanted in mice, improves systemic glucose turnover [108]. Accordingly, human
individuals with a higher abundance of BAT have lower blood glucose levels and glucose uptake in
BAT is associated with improvements in systemic glucose homeostasis and insulin sensitivity [109,
110]. Increased supraclavicular BAT activity in humans is inversely associated with arterial
inflammation and reduced risk of cardiovascular events [111]. Interestingly, South‐Asians have a
higher prevalence of hyperglycemia, dyslipidemia and cardiovascular disease and a lower amount of
BAT [112]. Glucose uptake by BAT in humans is associated with heat production following a circadian
rhythm in which BAT may buffer glucose fluctuations and maintain whole‐body glucose homeostasis
over time [113]. In another study, 48‐60 hrs of fasting induced insulin resistance resulted in a
considerable decrease in BAT glucose uptake and non‐shivering thermogenesis during cold
stimulation [114]. The effects of anti‐obesity treatments on human BAT have not been studied in
detail but mouse studies suggest a mechanism mediated by BAT for some treatments. Compounds
used in humans to improve dyslipidemia, hyperglycemia and/or lower plasma triglyceride levels such
as metformin, rimonabant, salsalate and activation of the glucagon‐like peptide 1 receptor all
activate BAT in mice [92, 115‐117].
Chronic low‐grade inflammation in obesity
Dysfunctional adipocytes in WAT attract immune cells
In obesity, excessive energy intake is accompanied by an increased storage of lipids in adipose tissues
leading to hypertrophy of adipocytes, hypoxia, and cell death, causing WAT dysfunction and fibrosis.
Dysfunctional adipocytes change the local microenvironment with leakage of fatty acids and other
products resulting from adipocyte cell death. This causes a release of adipokines, chemokines and
cytokines by WAT and subsequent recruitment of inflammatory cells [118, 119]. The chronic nature
of obesity affects steady‐state homeostasis and leads to continuous activation of the immune system
[118]. Due to the extensive communication between adipocytes and immune cells, chronic
inflammation disturbs the homeostatic regulation of insulin signalling and adipogenesis in white
adipocytes, leading to reduced insulin sensitivity and development of type 2 diabetes [118, 120].
Thus, by increasing circulating cytokines and attracting immune cells, WAT contributes to metabolic
dysfunction.
Immune regulation in adipose tissue
29
Inflammation in obese white adipose tissue
Myeloid cell recruitment into WAT
The first proof that inflammation is important in the pathogenesis of obesity and the resulting
metabolic dysfunction was provided by Hotamisligil et al [121]. The pro‐inflammatory cytokine tumor
necrosis factor‐α (TNF) was found to be present in WAT and correlated with insulin resistance in
humans and mice [121]. It became evident that the infiltration of pro‐inflammatory macrophages in
WAT plays a central role in the inflammatory response as a dominant source of TNF [122]. In WAT of
lean mice, 10‐15% of the cells are macrophages, whereas obese WAT contains 45‐60% macrophages
[122]. Resident macrophages in lean WAT have a predominant anti‐inflammatory phenotype,
whereas in obesity, inflammatory Ly6ChighCCR2+ monocytes are recruited to WAT, where they
differentiate, acquire a pro‐inflammatory or M1 phenotype and form the majority of macrophages
[120, 122‐127]. Anti‐inflammatory or M2 macrophages depend on the cytokines IL‐4 and IL‐13 and
require STAT6 to maintain their alternative activation state [6]. Other myeloid cells that play a role in
WAT include neutrophils and eosinophils. Neutrophils are very short‐lived cells that are already
present in WAT within 3 days of HFD [128, 129]. In contrast, the amount of eosinophils is inversely
correlated with adiposity, and exhaustion of eosinophils in mice results in increased body weight,
glucose intolerance and insulin resistance [130]. Both type 2 innate lymphoid cells (ILC2s) and
eosinophils have only recently been shown to be an important cell population in WAT and are a
predominant source of IL‐4 and IL‐13, the cytokines required for the induction of M2 macrophage
polarization [130, 131].
Lymphoid cell infiltration in WAT
T cells are also a component of the repertoire of immune cells found in WAT. Ten percent of the
stromal vascular fraction of lean WAT consists of T cells. A large part of these are CD4+ T‐helper cells,
of which approximately 50% are regulatory T cells (Tregs). In humans, the number of T cells in WAT
correlates with BMI [132]. In mice, the amount of T cells in WAT increases within 2 weeks of HFD.
There are only a few CD8+ cytotoxic T cells and CD4+ effector T cells in lean WAT, but both
populations increase drastically in an obese state, whereas CD4+ Tregs decrease [133]. Similarly, as
the ratio between M1 and M2 macrophages increases in obese WAT, the Th1 and Th2 T cell ratio
does as well. This results in a decrease in Th2 derived cytokines such as IL‐4 and IL‐13, thereby
reducing M2 macrophage polarization. An increase in Th1 T cells and cytotoxic T cells results in
excessive secretion of TNF and IFNγ, which polarizes macrophages to a pro‐inflammatory state,
resulting in increased inflammation in obese WAT [130, 134‐136].
The chronic low‐grade inflammation in obese WAT also includes the recruitment of B cells, natural
killer (NK) cells and mast cells [137, 138]. NK cells are activated by recognition of lipid antigens and
mast cells contain granules that can release a variety of mediators, including histamine, serotonin
and cytokines, which also promote recruitment of inflammatory cell types [139].
Overall, a variety of immune cells infiltrate WAT in DIO, inducing a switch from a homeostatic anti‐
inflammatory environment to a state of chronic low‐grade inflammation.
Chapter 2
30
The immune system in brown and beige adipose tissue
In contrast to the established role of the different immune cells in WAT, the contribution of the
immune system to the development, function and activity of BAT is still largely unknown. However,
we do know that obese individuals have a decreased amount of active BAT, based on FDG uptake,
which is related to their low‐grade inflammatory state. Moreover, an inactive brown adipocyte
accumulates lipids, similar to a white adipocyte and ablation of noradrenergic input by selective
sympathetic denervation of BAT indeed results in a ‘whitened’ appearance of brown adipocytes with
large intracellular vacuoles [53]. Since the recruitment of macrophages into WAT is correlated with
lipolysis of stored triglycerides [120], it is likely that release of fatty acids also induces recruitment of
immune cells in BAT. However, whether this is indeed the case is still unknown.
In diet‐induced obesity (DIO), thermoneutral housing leads to an additive increase in inflammation in
white adipose tissue and in the vasculature compared to normal housing conditions. Although not
causing increased insulin resistance, the increase in vascular inflammation does cause enhanced
progression of atherosclerosis [140], indicating that BAT protects against obesity‐induced
atherosclerosis. In another study, a similar phenomenon was observed. Immune compromised
C57Bl/6 nude mice experience cold stress when housed at 23°C which modulates energy and body
weight homeostasis and are therefore protected from DIO. However, at thermoneutrality (33°C),
they do develop DIO with increased adiposity, hepatic triglyceride accumulation, increased
inflammatory markers and glucose intolerance [141], showing that BAT activity protects against
metabolic disarray and adipose tissue inflammation.
Besides environmental temperature, other incentives such as the biological clock [53, 142],
hormones [143, 144] and food intake not only modulate energy expenditure via the hypothalamus
but also affect inflammation. For instance, time‐restricted feeding of a HFD for 8 hours per day
increases BAT activity and reduces adipocyte hypertrophy and inflammation in WAT compared to ad
libitum HFD‐fed mice [145]. Gut hormones such as GLP‐1 mediate effects on food intake, energy
expenditure and inflammation. GLP‐1 receptor signalling activates BAT and promotes beiging of WAT
[117, 146] while GLP‐1 also reduces macrophage infiltration and inflammatory signalling in white
adipocytes and macrophages [147, 148]. Other hormonal changes such as menopause also affect
energy metabolism. Estradiol inhibits AMPK in the hypothalamus, which activates thermogenesis in
BAT [143]. Indeed, ovariectomised mice with reduced estradiol levels gain more weight than sham
operated mice and have reduced energy expenditure and increased WAT inflammation [144].
Currently, many other factors that affect both BAT activity and inflammation, such as p53 [74] and
USF1 [75] are being investigated (Table 1). As described below, reports have also provided evidence
that immune cells are directly involved in regulating the activity of brown as well as beige adipocytes.
Inflammatory mediators in BAT
Remarkably, Roberts‐Toler et al. [76] discovered that feeding mice a HFD for 16 weeks not only
reduced insulin signalling, but also increased mRNA levels of markers of inflammation in both WAT
and BAT, including Tnf and the macrophage marker F4/80. Microarrays of interscapular BAT showed
an upregulation of immune response gene networks after 24 weeks of HFD, including genes that
indicate infiltration of leukocytes, monocytes and macrophages (CD44, CD52, CD68 and CD84).
Furthermore, after 2, 4 and 8 weeks of HFD, immune gene networks were upregulated, including
Immune regulation in adipose tissue
31
genes responsive to IFNγ; immunity‐related GTPase family M member 2 (Irgm2), guanylate binding
protein 4 (Gbp4) and interferon gamma induced GTPase (Igtp) [149]. These gene expression profiles
hint towards the presence of immune cell trafficking, leukocyte activation and lymphocyte activation
in BAT.
In another study, gene expression analysis in BAT of obese mice showed an upregulation of genes
encoding the inflammatory cytokines TNF, IL‐6, CCL2 and CCL5 [150]. Furthermore, activation of the
pattern recognition receptors (PRRs), nucleotide‐oligomerization domain‐containing protein (NOD) 1,
Toll‐like receptor (TLR) 2 and TLR4 in brown adipocytes induces a pro‐inflammatory response
through NF‐κB and MAPK signalling pathways. PRRs are receptors responsible for the sensing of
invading pathogens and can activate specific signalling pathways leading to distinct inflammatory
responses. Enhanced PRRs expression decreased UCP1 expression as well as mitochondrial
respiration in brown adipocytes [150].
Another factor affecting BAT function and BAT inflammation is p62, a protein involved in cell growth
and differentiation, energy metabolism and inflammation. Global ablation, as well as adipocyte
specific ablation of p62 results in obesity and insulin resistance. Moreover, these mice have
decreased non‐shivering thermogenesis and a lower metabolic rate, which is caused by impaired
mitochondrial function in brown adipocytes, due to decreased activation of p38α MAPK and its
downstream regulators of thermogenesis, including UCP1, PGC‐1α and CREB. Furthermore, gene
expression of obese BAT in adipocyte‐specific p62‐deficient mice reveals induction of pathways of
inflammation and increased macrophage infiltration in BAT [62] (Table 1).
BAT activation and beiging of WAT: Role of the immune system
Macrophages have been shown to play a role in adaptive thermogenesis. Nguyen et al. [3]
demonstrated that short term (6 h) cold exposure increases M2 macrophage markers in both WAT
and interscapular BAT in mice. Furthermore, absence of M2 macrophages blunts BAT activity and
induces cold‐intolerance in DIO mice. Acute cold exposure stimulates anti‐inflammatory M2
macrophages to produce tyrosine hydroxylase (TH), a catecholamine‐synthesizing enzyme, via IL‐4‐
STAT6 signalling. This results in the release of noradrenaline, which binds to the β3‐adrenergic
receptor on the brown adipocyte membrane. Consequently, BAT is activated, evidenced by the
induction of Ucp1 and other thermogenic gene expression, such as Pgc‐1α and acyl‐CoA synthase
long‐chain family member 1 (Acsl1). An acute cold‐induced increase of M2 macrophages in BAT and
WAT was not observed in Il‐4/Il‐13 and Stat6 knockout mice, indicating that the IL‐4/IL‐13‐STAT6
pathway is crucial to increase the amount of M2 macrophages in interscapular BAT after a cold
challenge [3]. However, other reports show that upon prolonged, 72h cold exposure, depletion of
macrophages by clodronate or deficiency of CCR2 does not affect adaptive thermogenesis in classical
BAT, but induces beiging of WAT. This suggests a less pronounced role for macrophages in adaptive
thermogenesis in BAT, but rather an involvement in beiging of WAT. Especially M2 macrophages
have been shown to induce beiging of subcutaneous WAT. Impaired monocyte recruitment in CCR2
knockout mice results in decreased biogenesis of beige adipocytes upon cold exposure, proving that
CCR2 is responsible for cold‐induced monocyte recruitment [5]. Moreover, beiging of WAT is also
observed when M1 to M2 polarization is enhanced through lowering of Receptor Interacting Protein
140 (RIP140) [151].
Chapter 2
32
Not only can macrophages produce TH, they are also capable of producing the catecholamine
noradrenaline [152, 153] and express β2‐adrenergic receptors. Binding of noradrenaline to β2‐
adrenergic receptors promotes M2 polarization, enabling a paracrine loop and suggesting auto‐
regulation of catecholamine levels [154]. Protein expression of the catecholamine producing enzyme
TH increases upon cold exposure in BAT as well as subcutaneous WAT and epididymal WAT [5].
Interestingly, myeloid specific TH deficient mice have impaired cold‐induced biogenesis of beige
adipocytes, but exhibit normal BAT activity. BAT is highly innervated by the sympathetic nervous
system, along with a constitutively high expression of catecholamine synthesizing enzymes. The
contribution of noradrenaline production by macrophages may therefore be less crucial in BAT then
in the far less innervated WAT, which has a low basal TH expression [5].
Cold‐induced remodelling of subcutaneous WAT into thermogenic beige fat by noradrenaline
producing macrophages was shown to be induced by eosinophils, via IL‐4, IL‐13 and STAT6 [5].
Mouse models with a genetic loss of Il‐4/Il‐13, Il‐4Rα, Stat6 or IL‐4‐producing eosinophils have
impaired cold‐induced biogenesis of beige adipocytes. In addition, administration of IL‐4 into DIO
mice at thermoneutrality reduces body weight, improves insulin sensitivity and increases protein
expression of UCP1 in subcutaneous WAT, but not in classical interscapular BAT [5]. Meteorin‐like
(Metrnl) hormone was found to be responsible for inducing IL‐4 secretion by eosinophils [37]. Metrnl
is a target of PGC‐1α4 and is released by skeletal muscle during exercise and by adipose tissue upon
cold exposure. It does not have a direct effect on brown adipocytes, but stimulates eosinophils to
produce IL‐4 which promotes activation of M2 macrophages and causes the increased expression of
thermogenic and anti‐inflammatory gene programs [37].
Besides macrophages, ILC2s are important mediators of beiging of WAT (Figure 4). This subtype of
innate lymphoid cells controls eosinophil and M2 macrophage responses by secreting IL‐5 and IL‐13,
initiates type 2 immune responses and is designed to protect against helminth infections but also
promote allergies [155]. In humans and mice suffering from obesity, decreased numbers of ILC2s
have been detected in subcutaneous WAT [6]. The cytokine IL‐33 is critical for the maintenance of
ILC2s and experimental mouse studies have shown that IL‐33 limits the development of spontaneous
obesity. IL‐33 deficient mice gain more weight on a HFD, have a reduced number of ILC2s and exhibit
decreased numbers of beige adipocytes in WAT. Administration of IL‐33 increases numbers of ILC2s
and eosinophils in subcutaneous WAT, leading to increased energy expenditure in mice by inducing
beiging of WAT [4, 6]. IL‐33 induced beiging is a result of proliferation and differentiation of PDGFRα+
adipocyte precursor cells. PDGFRα+ pre‐adipocytes do not express receptors required for signalling by
IL‐33 (IL1RL1) or IL‐5 (IL‐5Rα), but they do express IL‐4Rα, which is required for both IL‐4 and IL‐13
signalling [4]. Therefore, beiging of WAT is also dependent on IL‐4/IL‐13 signalling in adipocyte
precursor cells, and not necessarily solely on signalling of these cytokines in myeloid cells [4].
Furthermore, IL‐33 sensing ILC2s could be dysregulated in the setting of increased adiposity [156].
The presence of ILC2s and eosinophils in BAT and WAT in mice and the decrease in ILC2s and IL‐33
upon high‐fat feeding and leptin deficiency‐induced obesity was confirmed by Ding et al [157]. Again,
administration of IL‐33 increased the number of ILC2s, eosinophils and induced Ucp1 and TH in WAT,
but not BAT, of HFD‐fed mice, resulting in beiging of subcutaneous WAT. In addition, cold‐exposure
increased IL‐33 levels as well as the numbers of ILC2s and eosinophils in subcutaneous WAT.
Denervation of subcutaneous WAT by injection of 6‐hydroxydopamine (6‐OHDA) suppressed basal
and cold‐induced IL‐33 levels and lowered the ILC2 and eosinophil fraction. Sympathetic denervation
did not alter CD206+ M2 macrophages, showing the importance of catecholaminergic regulation in
the IL‐33‐ILC2‐eosinophil axis.
Immune regulation in adipose tissue
33
IL-5
IL-13
Eosinophil
IL-4
IL-33
Metrnl
CCR2+ monocyte
Macrophage
M2
NA
UCP1
ILC2MetEnk
Treg
Interestingly, Brestoff et al. [6] demonstrated that beige adipocytes can also develop via an
alternative mechanism, independently of IL‐4, IL‐13, eosinophils or macrophages. DblGata1 knockout
mice, which lack eosinophils, IL‐4Rα knockout mice, which do not have IL‐4 and IL‐13 signalling, or
ILC2‐sufficient Rag2 knockout mice, which lack mature lymphocytes but have reconstituted ILC2s, still
exhibit beiging of WAT, although all factors combined most likely contribute to optimal beiging. ILC2s
produce an opioid‐like peptide, methionine‐enkephalin (MetEnk) peptide, which upregulates UCP1 in
subcutaneous WAT and induces beiging of adipocytes without affecting IL‐4 or IL‐13 levels or the
number of eosinophils or macrophages in WAT [6]. This mechanism was only restricted to
subcutaneous WAT. MetEnk receptors MetEnk receptor δ1 opioid receptor (Oprd1) and MetEtnk
receptor Opioid growth factor receptor (Ogfr) were differentially expressed in WAT and BAT,
explaining these tissue‐specific effects [6].
These two different mechanisms as to how ILC2s control beiging of WAT; PDGFRα+ adipocyte
precursor cell proliferation and differentiation and MetEnk production, might synergize in the
generation of beige adipocytes. The previously unrecognized feature of ILC2s to produce MetEnk
would need to be confirmed in future studies, as well as the role for beige adipocyte precursors.
Further studies are needed to determine the relative contribution of IL‐4, IL‐13 and MetEnk to beige
Figure 4. Cold‐induced remodelling of subcutaneous white adipose tissue into thermogenic beige adipocytes via immune cells. ILC2s are important players in beiging of white adipocytes. ILC2s are maintained by IL‐33 and produce MetEnk peptide, which upregulates UCP1 in white adipose tissue. IL‐33 also increases the percentage of regulatory T cells in WAT, the T cell fraction contributing to glucose homeostasis. ILC2s secrete IL‐5 and IL‐13, which stimulate eosinophils to produce IL‐4. IL‐4 can directly cause beiging or act via M2‐macrophages that induce beiging by the production of noradrenaline. The increased number of macrophages upon cold exposure is dependent on monocyte recruitment via CCR2. Eosinophil‐dependent IL‐4 secretion can also occur via Metrnl hormone, released by the adipose tissue upon cold exposure. IL; interleukin, ILC2s; group 2 innate lymphoid cells, MetEnk; methionine‐enkephalin, Metrnl; meteorin‐like, NA; noradrenaline Treg; regulatory T cells, UCP1; uncoupling protein 1.
Chapter 2
34
adipogenesis. It would also be interesting to determine the source of IL‐33 production, the cytokine
that puts the ILC2 pathway in motion.
Improved metabolic health by caloric restriction in mice also indicates that type 2 cytokine signalling
is important in beiging of WAT. Caloric restriction increases the amount of eosinophils, type 2
cytokines and M2 macrophages in WAT, but does not affect the number of ILC2s, and consequently
promotes beiging. The development of beige adipocytes upon caloric restriction is ablated in Stat6
knockout and IL4R knockout mice, suggesting that type 2 cytokines play an important role in beiging
of WAT [158].
Toll‐like receptors (TLRs) respond to various pathogen‐associated molecules, including saturated
fatty acids, by inducing signal transduction and transcription of various chemokines and cytokines,
and are predominantly found on cells of the innate immune system. However, adipocytes also
express several TLRs (TLR3 and TLR4) and contain relevant downstream signalling elements.
Members of the interferon regulatory factor (IRF) family regulate both immune cell activation as well
as adipocyte differentiation, via TLR3 and TLR4. HFD‐fed IRF3 deficient mice have improved insulin
sensitivity, reduced adipose‐ and systemic inflammation and enhanced beiging of subcutaneous WAT
[159]. TLR4 activation in obesity or by lipopolysaccharide indeed suppresses beiging of subcutaneous
WAT and causes defective BAT. In human primary white adipocytes, TLR4 activation by LPS
suppresses UCP1 induction [160]. Irf4flox/flox;UCP1‐Cre mice, lacking IRF4 specifically in UCP1 positive
cells, are more obese on a HFD. IRF4 is induced by cold, promotes energy expenditure and increases
thermogenic gene expression, including PGC1α and PRDM16 [161]. IRF4 was also shown to be a key
transcription factor in controlling M2 polarization [162]. These data indicate that TLRs in both
immune cells and adipocytes play a role in adaptive thermogenesis.
The circuit in which metabolic improvements are associated with beige adipogenesis and mediated
by eosinophils, type 2 cytokines and M2 macrophages is also linked with the status of the gut
microbiome [80]. Germ‐free mice, or mice with antibiotic‐depleted microbiota have more beige
adipocytes in both lean and obese WAT, leading to improved glucose tolerance and insulin sensitivity
mediated by eosinophils, type 2 cytokines and M2 macrophages [80]. Cold exposure alters the gut
microbiota by remodelling of the intestine and adipose tissue, increasing the absorptive surface of
the gut. Transfer of cold‐adapted microbiota improves insulin sensitivity and enhances beiging of
WAT [81]. With the current high prevalence of human obesity and the frequent use of antibiotics, the
relation between beiging and microbiome is an interesting topic of future research.
Another player related to beiging of WAT by M2 macrophages is adiponectin, an adipokine regulating
glucose levels and fatty acid degradation. Adiponectin can promote the polarization of macrophages
towards an M2 phenotype, correcting obesity‐induced macrophage infiltration and inflammation.
Hui et al. show that cold exposure induces elevated gene expression and protein levels of
adiponectin in subcutaneous WAT, and, using adiponectin knockout mice, show that adiponectin is
essential for beiging of the subcutaneous adipose tissue via M2 macrophage proliferation [79].
Adiponectin deficiency has no effect on ILC2s or type 2 cytokines, suggesting that these are not the
downstream effectors of adiponectin during cold‐induced beiging [79].
Immune regulation in adipose tissue
35
Increased numbers of M1 macrophages in obese WAT suppress cold‐induced beiging of
subcutaneous WAT in mice. Depletion of macrophages, using clodronate liposomes, eliminates the
suppressive effects of M1 macrophages on UCP1 induction and reduces the level of TNF in obese
WAT. Cold exposure in clodronate treated obese mice has metabolic benefits, such as lower body
weight and decreased plasma glucose levels. Mitochondrial biogenesis is not the underlying
mechanism by which M1 macrophages suppress UCP1, since this was not affected by clodronate nor
intraperitoneal TNF injections [82].
Interestingly, IL‐33 also increases the percentage of Tregs in WAT, the T cell fraction contributing to
glucose homeostasis [6]. Medrikova et al. [78] performed a microarray on sorted Tregs from BAT and
identified a BAT‐specific subset of Tregs, which contribute to metabolic control of BAT. Upon
depletion of Tregs, the tissue exerted an increased inflammatory state with an increase in
macrophages, resulting in mice that were more sensitive to cold [78]. However, nude mice on a
C57Bl/6 background, which lack mature T cells, experience cold stress at 23°C and are protected
from DIO by increased thermogenesis and energy expenditure. This suggests that T cells are not
required for adaptive thermogenesis in C57Bl/6 nude mice. At thermoneutrality, C57Bl/6 nude mice
are susceptible to DIO and associated increased inflammatory markers in adipose tissues [141].
Cytokines
Besides the effects of IL‐4 and IL‐33, which induce beiging as described above, other cytokines have
also been shown to play a role in thermogenesis. Adipose tissue becomes hypoxic in the
development of obesity, which contributes to the inflammatory state and the induction of TNF. This
cytokine further affects adipocyte homeostasis by reducing expression of Pgc‐1α and Pparγ in both
white and brown adipocytes [163]. The induction of beige adipocytes in vitro by a β‐adrenergic
receptor agonist is blunted upon stimulation with TNF or pro‐inflammatory cytokines derived from
LPS‐activated‐macrophages. TNF is also involved in apoptosis of brown adipocytes as shown in TNF
receptor deficient ob/ob mice [164]. Furthermore, TNF impairs mitochondrial biogenesis in WAT, BAT
and muscle of ob/ob and DIO mice, and leptin receptor‐deficient Zucker fa/fa rats [165]. Extracellular
signal‐related kinase (ERK), which mediates the induction of inflammatory cytokines, has been shown
to be an important factor in the suppression of UCP1 transcriptional activation in white adipocytes
and activated macrophages [166]. Together, these data indicate that cytokines can also directly
influence BAT activity.
Moisan et al. performed a small‐molecule screen to find pathways that induce a brown‐like
thermogenic program in human adipocytes in vitro [167]. They identified tofacitinib and R406 which
both target the JAK‐STAT1/3 pathway in adipocytes, with R406 having additional targets including
AKT and ERK1/2. TNF can signal through JAK‐STAT and ERK pathways and although TNF represses
Ucp1 expression, simultaneous stimulation with TNF and the compounds (partially) restored Ucp1
levels [167]. Furthermore, IFNγ treatment reduces Ucp1 expression in human brown adipocytes.
Interferons (α/β/γ) bind to JAKs and activate STAT1/2/3 and repression of IFN signalling by JAK
inhibition contributes to the upregulation of Ucp1 and promotes the metabolic beiging of adipocytes
[167].
Another major cytokine in regulating inflammatory responses in obesity is IL‐6, although it is unclear
yet whether IL‐6 serves a harmful or a protective role. IL‐6 has a homeostatic role in limiting
Chapter 2
36
inflammation but can also have pro‐inflammatory effects [168]. This leads to controversial results in
studies on IL‐6 in obesity. Plasma IL‐6 levels increase in obesity and correlate with C‐reactive protein
levels [169] and infusion of IL‐6 immediately impairs insulin sensitivity in mice [170]. However, IL‐6
deficiency results in more obese mice [171], but not in all models [172]. Myeloid specific IL‐6R
inactivation leads to a greater propensity to develop obesity‐induced inflammation and glucose
intolerance. IL‐6 induces the IL‐4 receptor and is therefore an important determinant in M2
macrophage polarization [168]. With respect to BAT, IL‐6 deficient mice have reduced energy
expenditure and fatty acid oxidation at room temperature. Housed at 4°C, IL‐6 deficient mice have a
lower core body temperature than WT mice [173]. Knudsen et al. [83] gave daily i.p. injections of IL‐6
for 7 days which increased Ucp1 mRNA but not UCP1 protein levels in inguinal WAT (iWAT). Cold
exposure increases iWAT Ucp1 mRNA content similarly in IL‐6 deficient as WT mice, while exercise
training (which induces the release of IL‐6 from contracting muscle) increases iWAT Ucp1 mRNA in
WT but not in IL‐6 deficient mice. Taken together, the appropriate cytokine stimulation can induce
beiging of WAT, which could be a potential mechanism to increase energy expenditure in obesity.
Beiging of WAT is also found in cancer‐associated cachexia (wasting syndrome), where β‐adrenergic
activation and/or inflammatory cytokine‐induced lipolysis cause WAT atrophy. These beige
adipocytes are present within WAT and cause increased lipid mobilization and energy expenditure
[77]. The cytokine IL‐6 increases Ucp1 expression in WAT, whereas silencing IL‐6, an anti‐IL‐6
antibody, the nonsteroidal anti‐inflammatory drug sulindac or β3‐adrenergic blockade in mice
reduces WAT beiging and ameliorates the severity of cachexia. This is also observed in patients [77].
The involvement of the sympathetic nervous system in BAT also contributes to a homeostatic
cytokine environment. Surgical denervation of BAT or β‐adrenergic antagonist propranolol treatment
upregulates gene expression of Tnf, ll‐6 and Ccl2, without a change in F4/80 expression, indicating
that the SNS and β‐adrenergic signalling are necessary to maintain an anti‐inflammatory state in BAT
[174].
Chemokines
Chemokines and chemokine receptors regulate leukocyte influx into obese WAT. Chemokine
receptors on immune cells in WAT and serum chemokine levels are increased in obese versus lean
individuals [175]. Chemokines secreted by the stromal vascular fraction (SVF) and/or white
adipocytes include CCL5, CCL20, CXCL10 and CXCL12, which promote the accumulation of leukocytes
into WAT through binding to their receptors CCR5, CCR6, CXCR3 and CXCR4, respectively [119, 176,
177].
The regulation of BAT homeostasis and leukocyte influx by chemokines is scarcely studied. DIO
adipocyte‐specific CXCR4 knockout mice develop severe obesity upon HFD feeding, have an
increased pro‐inflammatory leukocyte content in WAT and exhibit reduced thermogenic capacity
when exposed to cold. This reveals that CXCR4 prevents inflammatory leukocyte influx in WAT and
that adipocyte Cxcr4 expression is required for the thermogenic capacity of BAT, thereby increasing
overall energy expenditure and decreasing susceptibility to DIO [84].
Lipokines
Besides metabolic effects, lipids have inflammatory signalling properties. While saturated fatty acids
promote adipose tissue inflammation and insulin resistance through TLR signalling [178], n‐3 fatty
Immune regulation in adipose tissue
37
acids [179] and palmitic acid esters of hydroxyl‐stearic acids (PAHSAs) [180] have anti‐inflammatory
and anti‐diabetic effects. The lipokine palmitoleate supresses cytokine expression in adipocytes and
reverses high fat‐induced proinflammatory macrophage polarization [181]. Palmitoleate and PAHSAs
are synthesized endogenously in the adipose tissue from fatty acids [180, 182, 183] and n‐3 fatty
acids are derived from diet, although palmitoleate is also present in food [183]. Interestingly,
adipocyte‐specific LPL knockout mice have increased de novo lipogenesis, particularly of
palmitoleate. However, this did not increase BAT activation, WAT beiging or amelioration of
inflammation in these mice [184]. PAHSAs and n‐3 fatty acids were shown to exert their beneficial
metabolic and anti‐inflammatory effects via G protein‐coupled receptor 120 (GPR120), a receptor for
long chain fatty acids that is present in the gut, adipose tissue, pancreas and immune cells [180, 185].
Intriguingly, PAHSA levels in mice are the highest in BAT and WAT [180] and GPR120 expression is
abundant in BAT and WAT which increases upon β3‐adrenergic receptor stimulation or cold [186].
Given the abundance of these metabolically advantageous and anti‐inflammatory lipokines in BAT
and WAT, future research is needed to unravel the thermogenic effects of lipokines and lipid‐
activated GPRs.
Concluding remarks and future perspectives
Translational challenges from mice to men
Extensive studies in mouse models have shown that there is a relation between BAT and obesity, and
that the immune system plays an important role in regulating BAT activity and beiging of WAT, but
whether this is the case in humans still needs to be investigated. Although increasing energy
expenditure by activating BAT or inducing beiging of WAT prevents diet‐induced or genetic obesity in
mice, results from different mouse models are not always relevant to the human situation.
Interscapular classical BAT in mice does not regress with age whereas adult humans no longer
possess this classical BAT depot. Another difficulty in extrapolating mouse data to the human is the
difference in thermoneutral temperature and the activity of BAT at room temperature (18‐22°C).
Housing mice in their thermoneutral condition (30°C) would be comparable to the human situation,
which is not always acknowledged.
Many studies have shown that BAT activity in humans is thermoresponsive, and have suggested that
obesity is associated with a decrease in [18F]FDG uptake in BAT, which may be related to
inflammation. As described, BAT activity in humans is mostly measured by [18F]FDG uptake. However,
the signal generated by [18F]FDG in PET‐CT studies likely underestimates the activity of BAT as it
utilizes both glucose and fatty acids as fuel [105, 187]. Furthermore, insulin resistance reduces
glucose uptake by BAT leaving the uptake of fatty acids and oxidative capacity unaffected [105].
Therefore, it has been postulated that the use of the fatty acid tracer [18F]FTHA is preferred over the
use of [18F]FDG [187]. It would even be better to develop a triglyceride with labelled fatty acids as
tracer, since BAT mostly takes up fatty acids derived from TRLs in an LPL‐dependent manner [89].
Besides alternative methods to quantify BAT activity independent of insulin sensitivity, identification
of specific circulating markers for activated human BAT is highly desired.
Interestingly, mechanisms of beiging of WAT display more similarities between both species. Mice
have a large beiging capacity, especially of the subcutaneous WAT. Although beiging capacity and its
Chapter 2
38
contribution to energy expenditure in humans still needs to be determined in detail, studies support
a similar mechanism. A 6‐weeks 2‐hour daily cold exposure procedure results in the reduction of
body fat [2] and beiging of WAT in patients with pheochromocytoma, a catecholamine‐secreting
tumor, leads to increased metabolism [188]. Hormones, immune cells or cytokines that induce beige
adipocytes in mice and are also present in humans, suggesting that beiging via those mechanisms can
also occur in humans. For example, Brestoff et al. confirm presence of ILC2s in human adipose tissue,
suggesting that the circuit of ILC2s, eosinophils, type 2 cytokines and M2 macrophages that induce
beiging in mice, is also operational in human beiging [6].
Speculation on human BAT from an evolutionary perspective
BAT evolved as a natural defence system against hypothermia in mammals, increasing the
adaptability to explore colder environments [189]. Major threats besides cold in the course of
evolution include limited food supply and infections. Cold challenges were accompanied by a limited
food supply, making BAT a possible survival organ, enabling efficient food hunting in a subthermal
environment [113].
Formerly common helminth and parasite infections elicit an M2 immune response which appear to
be important regulators of beige adipocytes [6]. Along with the migration to colder areas, both the
helminth and host could potentially benefit from increased WAT beiging by promoting host survival
in cold climates [190]. Immunity and cold adaptation are therefore connected. Furthermore, as
starvation and infection co‐occurred, chemokine secretion by adipose tissue could be an evolutionary
advantage in which a beneficial genotype to combat infection and higher amounts of adipose tissue
promotes survival. Another adaptive conservative trait is infection induced insulin resistance,
enabling nutrient supply to immune cells [191].
Although BAT seems to have lost part of its function for evolutionary reasons, young people and
possibly also adults may still benefit from it. BAT participates in glucose and fatty acid clearance and
may still serve as a nutrient buffering system [105, 113]. The role of thermogenesis in handling
excessive energy was already shown in a study in 1979, during which diet‐induced thermogenesis
limited weight gain after a high caloric meal [192].
Therapeutic potential
Potential human drugs that induce beiging of WAT are currently being tested in the clinic, as
reviewed by Giordano et al [193]. A possible target would be the β3‐adrenergic receptor, although it
is not specific for brown adipocytes and is expressed in a variety of organs. Unfortunately, β‐
adrenergic receptor agonists have so far not been shown to have major effects on energy balance. A
latest generation β‐adrenergic receptor agonist, mirabegron (approved to treat the overactive
bladder), increases the metabolic activity of BAT evidenced by increased [18F]FDG uptake in healthy
male subjects [99]. In mice, mirabegron also has anti‐inflammatory effects [76]. Aside from directly
activating brown adipocytes or inducing beiging of WAT with β3‐adrenergic receptor agonists, anti‐
inflammatory drugs may induce expansion of BAT and beiging of WAT and promote energy
expenditure as an anti‐obesity treatment, including salsalate or amlexanox [116, 193]. A few drugs
with high potential for human drug development target factors downstream of the β3‐adrenergic
receptor, such as C/EPBβ and PRDM16 and have extensively been studied in mice, and proven to
induce differentiation of beige and brown adipocytes in mice [193].
Immune regulation in adipose tissue
39
Besides receptor targeted drugs, diet and nutritional components as ways to modulate
thermogenesis and inflammation can be considered as an alternative strategy. Although it is known
that brown, beige and white adipocytes are fuelled by glucose and fatty acids, large knowledge gaps
exist. We still do not know whether different types of dietary fatty acids or carbohydrates elicit
distinct effects on thermogenesis and inflammation. Whereas saturated fatty acids promote
inflammation and are detrimental for metabolic health [178], n‐3 fatty acids are anti‐inflammatory
and act beneficially [179]. Whether dietary n‐3 fatty acids activate BAT or promote beiging remains
to be investigated. An interesting target would be PPARγ, which can be activated by these
polyunsaturated fatty acids and plays a role in both beige and brown adipogenesis as in M2 anti‐
inflammatory macrophage activation [194]. As for specific carbohydrates, hardly anything is known
about their effects on BAT activity or beiging. Almost 30 years ago, Walgren et al. showed that
dietary carbohydrates increased noradrenaline turnover in heart and/or BAT in rats, unrelated to the
type of carbohydrate (i.e. fructose, sucrose, dextrose, corn starch) [195]. More recent investigations
have not looked into the effects of specific carbohydrates on BAT activation or beiging. Future
research might shed light on this underexposed aspect of modulating thermogenesis.
Conclusion
Excessive energy intake results in increased storage of lipids in both white and brown adipocytes,
which challenges the function of these cells. Immune cells and signals in WAT and BAT are
indispensable for the homeostasis of the tissue and contribute to the efflux of lipids stored in white
adipocytes and to high rates of oxidation in brown and beige adipocytes.
Immune cells, including eosinophils and alternatively activated macrophages, have regulatory roles in
metabolic homeostasis of both WAT and BAT and research to identify immunological players is
ongoing. If the mechanism is unravelled in detail, immune regulation is an intriguing therapeutic
target in increasing energy expenditure to reduce weight gain. Notably, the numbers of immune cells
in lean as well as obese BAT are much lower than in WAT, indicating that BAT is relatively more
resistant to diet induced inflammation, but increased tissue inflammation in BAT does occur upon a
positive energy balance. The challenge is to identify the metabolic crosstalk between immune cells
and brown, beige and white adipocytes and the order of events that occur during obesity
development. BAT regulation by the immune system will not come down to an individual immune
cell type and will involve significant crosstalk between different cell types. Important questions to
further address include: What are the immune regulatory effector molecules that are secreted by
brown adipocytes (or a pre‐beige adipocyte or a white‐wanting‐to‐become‐beige adipocyte) to
attract or regulate immune cells? How is BAT activity regulated in obesity? What is the role of the
sympathetic nerve system? And how does BAT activity change during aging?
In conclusion, the presence of BAT in humans and the potential activation of resident BAT or
induction of beige adipocytes in WAT is an interesting target to treat or even prevent obesity‐related
disorders. Cold is still by far the strongest sympathetic signal to activate BAT, but the quest for
identifying biochemical and immunological pathways that are responsible for BAT activation, and
thereby can bypass prolonged cold exposure, is ongoing. The recent finding on the role for immune
cells in brown and beige adipocyte development and physiology harbours a great potential to
increase BAT activity and beneficially alter energy metabolism by interfering in immune responses.
Chapter 2
40
Corresponding author Esther Lutgens, Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical Centre, University
of Amsterdam, Meibergdreef 15, 1105 AZ, Amsterdam, The Netherlands. Tel. +31 (0)20 5 66 33 80. [email protected]
Disclosure statement The authors have nothing to disclose.
Acknowledgements We acknowledge the support from the Netherlands Organization for Scientific Research (NWO)(VICI grant to E.L.), the
Rembrandt Institute of Cardiovascular Science (RICS), the Netherlands CardioVascular Research Initiative (CVON2011‐19),
the Deutsche Forschungsgemeinschaft (DFG) (SFB1123‐A5 to E.L) and the European Research Council (ERC consolidator
grant to E.L.).
Immune regulation in adipose tissue
41
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3
Diet‐inducedobesityinducesrapidinflammatorychangesinbrownadiposetissueinmice
Susan M. van den Berg1, Andrea D. van Dam2,3, Tom T.P. Seijkens1, Pascal J.H. Kusters1, Linda
Beckers1, Myrthe den Toom1, Ntsiki M. Held4, Saskia van der Velden1, Jan Van den Bossche1, Mariëtte
R. Boon2,3, Patrick C.N. Rensen2,3, Esther Lutgens*,1,5, Menno P.J. de Winther*,1,5
1Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical Center, University of Amsterdam,
the Netherlands 2Department of Medicine, Division Endocrinology, Leiden University Medical Center, Leiden, the Netherlands 3Einthoven Laboratory for Experimental Vascular Medicine, Leiden, the Netherlands 4Laboratory Genetic Metabolic Diseases, Academic Medical Center, University of Amsterdam, The Netherlands 5Institute for Cardiovascular Prevention (IPEK), Ludwig Maximilian’s University, Munich, Germany
*these authors contributed equally to this work.
Submitted
Chapter 3
50
Abstract
Brown adipose tissue (BAT) contributes to non‐shivering thermogenesis by burning glucose and fat to
produce heat. Obesity is associated with a loss of BAT activity. In obesity, white adipose tissue
inflammation contributes to metabolic disorders. We aimed to investigate if diet‐induced obesity
also induces inflammation in BAT and if this affects brown adipocyte activity. We performed a time
course of diet‐induced obesity ranging from 1 day to 18 weeks by feeding mice a 45% high fat diet
and subsequently analysed BAT biology. Furthermore, we stimulated a brown adipocyte cell line with
the cytokines TNF, IFNγ, and LPS to mimic a pro‐inflammatory environment. BAT rapidly accumulated
lipids after short term high‐fat diet (3 days). Macrophage numbers as well as cytokine and chemokine
gene expression in BAT increased along with this increased lipid storage. Surprisingly, brown
adipocytes in vitro were very capable of producing a variety of chemokines in response to
inflammatory stimuli. BAT is a very plastic tissue, which is rapidly remodelled upon diet‐induced
obesity. This remodelling is characterized by a swift recruitment of macrophages and induction of
inflammatory mediators. Moreover, brown adipocytes themselves may be cytokine producers in the
inflammatory profile of BAT.
Brown adipose tissue in obesity
51
Introduction
Brown adipose tissue (BAT) contributes to adaptive thermogenesis by burning glucose and fat to
produce heat [1]. Brown adipocytes have many small lipid droplets and numerous mitochondria that
contain large amounts of uncoupling protein 1 (UCP1). This protein mediates the thermogenic
function by increasing the permeability of the inner mitochondrial membrane, which causes protons
to flux back into the mitochondrial matrix, bypassing ATP synthesis, ultimately resulting in generation
of heat [2]. Cold activates the hypothalamus in the brain, which stimulates the sympathetic nervous
system (SNS) to produce noradrenaline. In turn, noradrenaline activates brown adipocytes by binding
to β3‐adrenergic receptors on their surface [3].
BAT was previously known to be present in human new‐borns and rodents, but interest in BAT has
rapidly increased since active BAT depots were visualized in human adults. Cold‐activated
metabolically‐active adipose tissue was shown by 18F‐fluorodeodeoxy‐glucose positron emission
tomography computed tomography ([18F]FDG‐PET/CT) imaging, mostly in the supraclavicular region
[4‐6]. Furthermore, repeated cold exposure as well as a single oral dose of mirabegron, a β3‐
adrenergic receptor agonist, increases the uptake of [18F]FDG in the supraclavicular region [4, 7‐9].
Mouse models with genetically decreased BAT activity often show a cold sensitive‐ but also an
obesogenic phenotype [10]. Examples include UCP1 knockout mice, leptin‐deficient ob/ob mice and
mice that lack all three β‐adrenergic receptor knockout mice [11‐13].
White adipose tissue (WAT) stores energy, whereas active BAT increases energy expenditure.
Intriguingly, BAT activity, measured by [18F]FDG uptake, is inversely correlated with body mass index
[4]. Since obesity is one of the most prevalent chronic disorders worldwide and increases the risk of a
variety of comorbidities including insulin resistance and cardiovascular disease, it could be
therapeutically beneficial to increase the amount and activity of BAT, which would result in increased
energy expenditure and reduce body fat.
Adipose tissue contains a variety of immune cells, including macrophages, eosinophils and T cells,
which contribute to tissue homeostasis [14]. In obesity, hypertrophy of white adipocytes leads to
hypoxia, cell death and tissue dysfunction. This causes recruitment of a variety of immune cells and
interferes with tissue homeostasis [15]. Inflammation is a key feature of obesity and insulin
resistance [16]. The recruitment of pro‐inflammatory macrophages is a critical event in obese WAT
inflammation [14, 17, 18]. Macrophages in lean healthy WAT mainly exhibit an anti‐inflammatory
phenotype, whereas in obese WAT, the number of pro‐inflammatory “classically‐activated”
macrophages increases [14]. Adipocytes as well as immune cells in obese WAT produce pro‐
inflammatory cytokines, including TNF, IL‐6, CCL2 (Mcp‐1), CCL4 (Mip‐1b) and CXCL12 (SDF‐1), which
further propagate the ongoing inflammatory response, both within the tissue and systemically [19‐
23].
Whether and how immune cells play a role in (obese) BAT is largely unknown [10]. Similar to WAT,
obese BAT shows decreased insulin signalling and increased gene expression of the macrophage
marker Adgre1 (encoding F4/80), and gene expression of cytokines Tnf, Il6 and Ccl2 increases in
obese BAT in mice [21, 24, 25]. The pro‐inflammatory cytokine TNF suppresses UCP1 in brown
Chapter 3
52
adipocytes, suggesting a decreased activity of BAT in an obese environment [25‐28]. Furthermore,
anti‐inflammatory macrophages were shown to contribute to BAT activation as cold exposure
increased macrophage markers in both WAT and BAT in mice, and absence of anti‐inflammatory
macrophages blunted BAT activity and induced cold intolerance [29]. However, subsequent studies
showed no effect on adaptive thermogenesis upon depletion of macrophages in BAT [30].
As BAT activation can be beneficial in combatting obesity, it is critical to understand what underlies
BAT function in an obese state. Here, we determined whether and when the immune system in BAT
is affected by a high‐fat diet (HFD), by studying a time course of HFD in which 8 groups of mice
received a HFD ranging from 1 day to 18 weeks. We studied short‐term and long‐term immunological
adipose tissue changes upon a HFD and compared WAT and BAT. Additionally, we tested brown
adipocyte functioning in vitro under inflammatory conditions.
Materials and methods
Mice
Age‐matched male wild‐type C57Bl/6 mice (Charles River) received different durations of HFD (45%
kcal fat, 35% kcal carbohydrate, 20% kcal protein, Special Diets Services, Witham, United Kingdom).
All mice were included in the experiments at the age of 7 weeks and were given a HFD in a time
course ranging from 1 day up to 18 weeks before being sacrificed at the age of 25 weeks (8 groups of
n=10‐11). Mice had ad libitum access to food and water and were maintained under a 12 hour light‐
dark cycle. Mice were fasted overnight, weighed and subsequently euthanized using 0.25 mg/g
ketamine and 0.05 mg/g xylazine. Glucose levels were measured from whole blood using a
glucometer (Bayercontour, Basel, Switzerland). Blood was obtained by cardiac puncture using EDTA‐
filled syringes and liver, spleen, interscapular BAT, epididymal WAT (EpAT) and subcutaneous WAT
(ScAT) were dissected and weighed. All experimental procedures were approved by the Animal
Experimentation Ethics Committee of the University of Amsterdam.
Histology
Tissues were collected, fixed in 4% paraformaldehyde and embedded in paraffin. The area of
intracellular lipid vacuoles in BAT was quantified on a haematoxylin and eosin staining using ImageJ
software (NIH, USA). Immunohistochemistry on EpAT and ScAT was performed for CD45 (clone
30F11, BD Biosciences, Breda, the Netherlands). Frozen BAT sections were stained for CD68 (clone
FA‐11, Bio‐Rad, Veenendaal, the Netherlands) and quantified in ImageJ (NIH, USA).
Real‐time PCR
Total RNA was extracted using TRIzol (Invitrogen, Carlsbad, CA, USA) and reverse transcribed with an
iScript cDNA synthesis kit (Bio‐Rad, Veenendaal, the Netherlands). The quantitative PCR was
performed using a SYBR green PCR kit and a ViiA7 RT‐PCR system (Applied Biosystems, Leusden, the
Netherlands). Expression was normalized to the housekeeping genes Rplp0, β‐Actin and cycloA. The
results are expressed as relative to the control group, which was assigned a value of 1.
Culture and differentiation of brown adipocytes
T37i cells were kindly provided by Marc Lombès [31] and cultured in DMEM‐F12 Glutamax, 10% FCS,
penicillin (100 U/ml) and streptomycin (100 µg/ml) (ThermoFisher Scientific, Waltham, MA, USA).
Brown adipose tissue in obesity
53
After growing confluent, cells were differentiated by adding 2 nM Triiodothyronine (T3) (Sigma‐
Aldrich, Zwijndrecht, the Netherlands) and 112 ng/ml insulin (Sigma‐Aldrich) to the media. After 9
days of differentiation, the cells were stimulated with 10 ng/ml TNF (Invitrogen), 100 U/ml IFNγ
(Peprotech, Rocky Hill, NJ, USA) or 20 ng/ml LPS (Sigma) for 24 hours. Supernatant was collected for
ELISA and cells were harvested for RNA analysis.
Transwell migration assay
RAW264.7 macrophages (5x104) were seeded in a 24‐well transwell migration assay with 8 µm pores
(Sigma‐Aldrich) and incubated for 3 hours with supernatant from differentiated T37i brown
adipocytes. Macrophages were fixed with 4% formaldehyde, washed and stained with 5% toluidine
blue. Non‐migrated cells were swiped with a cotton tip and migrated cells counted in ImageJ (NIH,
USA).
ELISA
IL‐6 (Invitrogen) and CXCL1 (R&D systems, Minneapolis, MN, USA) were quantified by ELISA in
accordance to the suppliers’ protocols.
Statistics
Results are presented as mean ± SEM. Analysis between more than two groups in the time course
experiments were done by a one‐way ANOVA with Tukey post‐test analysis and the change per group
was expressed relative to the control group. Analysis between two groups was done by a Student’s t
test. Statistics were calculated in GraphPad Prism 5.0 (GraphPad Software, Inc., La Jolla, CA, USA). P‐
values <0.05 were considered significant.
Results
Adipose tissue changes in a HFD‐time course
Eight groups of mice were fed a HFD for different durations in time (Figure 1A). Body weight
increased after two weeks of HFD and continued to increase throughout the experiment, without
major changes in liver or spleen weight (Table 1). Blood glucose levels already increased after 1 day
of HFD and remained high in all HFD groups (Table 1).
Table 1. General characteristics of mice in the course of DIO Weeks of HFD
Mean ± SEM (p)a
0 1d 3d 1w 2w 4w 10w 18w
Body weight
(g)
27.3 ± 0.6
29.5 ± 0.6
31.1 ± 0.8
31.5 ± 0.5
34.4 ± 0.9 (***)
36.5 ± 1.2 (***)
38.1 ± 0.9 (***)
41.3 ± 2.0 (***)
Glucose
(mg/dl)
69
± 3
130
± 15
(***)
118
± 7
(**)
123
± 4
(***)
104
± 5
(*)
91
± 4
112
± 9
(**)
124
± 7
(***)
Liver (g) 1.21 ±
0.05
1.30 ±
0.05
1.32 ±
0.05
1.24 ±
0.03
1.22 ±
0.02
1.19 ±
0.04
1.11 ±
0.04
1.39 ± 0.09
Spleen (mg) 83.6 ± 5.7 71.7 ± 3.3 87.3 ± 3.4 88.0 ± 7.5 85.3 ± 3.3 94.6 ± 7.2 82.6 ± 5.9 106.2 ± 5.1 aData in this table is analysed by a one‐way ANOVA with Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group. n=10‐11, *=P<0.05, **=P<0.01, ***=P<0.001.
Chapter 3
54
Figure 1. Lipid uptake in BAT occurs early in time after initiation of high‐fat diet feeding. A. Study outline. Eight different groups of mice were subjected to a 45% HFD for different times (1 day (d) to 18 weeks (w)) and sacrificed at the same age. B. Interscapular BAT was removed after sacrifice and its weight was determined. C. The percentage of lipid as measured on an H&E stained BAT section. D. EpAT and (E) ScAT were removed after sacrifice and weighed. n=10‐11, *=P<0.05, **=P<0.01, ***=P<0.001 (1‐way ANOVA with Tukey post‐test analysis).
Upon HFD feeding, an increase in BAT weight was already present after 3 days of HFD (Figure 1B).
Histological analysis showed that the lipid content in brown adipocytes rapidly increased and
plateaued after 3 days of HFD and remained high until 18 weeks of HFD (Figure 1C), indicating that
BAT accumulates lipids very rapidly. In contrast, the increase in EpAT and ScAT weight reached
significance after 2 weeks of HFD and kept rising during the entire duration of the experiment (Figure
1D and E).
High-fat diet
Chow diet
18w 12w 4w 2w 1w 3d 1d
BAT
HFD
gram
0 1d 3d 1w 2w 4w 10w 18w0.00
0.05
0.10
0.15
0.20
**
***
BAT
HFD
% li
pid
drop
lets
0 1d 3d 1w 2w 4w 10w 18w0
20
40
60
80
*** **** *
**
EpAT
HFD
gram
0 1d 3d 1w 2w 4w 10w 18w0
1
2
3
***
****** ***
ScAT
HFD
gram
0 1d 3d 1w 2w 4w 10w 18w0.0
0.5
1.0
1.5
2.0
2.5
*****
***
***
A
B C
D E
Brown adipose tissue in obesity
55
Obesity associated inflammation in BAT
Obese WAT is associated with leukocyte infiltration with a prominent role for macrophages, as the
number of AT macrophages correlates with the extent of metabolic dysfunction [14, 18]. In BAT, we
observed rapid changes in lipid uptake and increased brown adipocyte size after the start of HFD, and
we wondered whether macrophages also infiltrate BAT in the course of diet‐induced obesity (DIO).
We histologically analysed BAT macrophages in situ by CD68 staining and observed increased
macrophage counts again already after three days upon induction of DIO (Figure 2A,B). Gene
expression of the macrophage‐specific marker F4/80 also increased rapidly and reached significance
after 1 week of HFD (Figure 2C). Additional markers for classical pro‐inflammatory (Nos2) vs anti‐
inflammatory (CD301, Arg1) macrophages hinted towards an increase in pro‐inflammatory
macrophages rather than in anti‐inflammatory macrophages (Figure 2C).
Figure 2. Increased macrophage numbers in obese BAT. A. Representative image of a CD68+ macrophage immunohistochemical staining on frozen BAT sections. B. Quantified immunohistochemical staining of relative to the number of adipocytes. C. Gene expression analysis of BAT showing the macrophage gene Adgre1 (encoding F4/80) and in the pro‐inflammatory macrophage marker Nos2 (iNOS) and anti‐inflammatory macrophage marker genes Clec10a (CD301/Mgl1) and Arg1. D. CD45
+ leukocyte counts relative to the number of adipocytes in EpAT after 18 weeks of HFD counted on paraffin immunohistochemical stainings and (E) leukocyte count upon a HFD in ScAT. n=10‐11, *=P<0.05, **=P<0.01, ***=P<0.001 (1‐way ANOVA with Tukey post‐test analysis).
A
C
BAT macrophages
HFD
# CD
68+ /a
dipo
cyte
0 1d 3d 1w 2w 4w 10w 18w0.00
0.01
0.02
0.03
* *** *
***
01d3d1w2w4w10w18wRe
lativ
e ex
pres
sion
Adgre1 Nos2 Clec10a Arg10.0
0.5
1.0
1.5
2.0
2.5
****
** * *
EpAT leukocytes
HFD
# CD
45 /a
dipo
cyte
0 1d 3d 1w 2w 4w 10w 18w0.0
0.1
0.2
0.3
0.4
0.5***
B
ScAT leukocytes
HFD
# CD
45 /a
dipo
cyte
0 1d 3d 1w 2w 4w 10w 18w0.00
0.05
0.10
0.15
*
D E
+ +
100 μm
Chapter 3
56
Total leukocytes (CD45+ cells) in EpAT increased with more established obesity (Figure 2D).
Furthermore, we observed that EpAT was more prone to obesity associated inflammation than ScAT;
EpAT displays an increase in leukocytes after 18 weeks of HFD (Figure 2D), whereas ScAT leukocyte
counts did not increase within this time frame (Figure 2E).
Increased cytokine expression in BAT upon HFD
In obese WAT, adipocytes and immune cells secrete a variety of cytokines that recruit inflammatory
cells and propagate the inflammatory profile of the tissue [15]. To study whether factors secreted by
brown adipocytes can affect macrophage migration, we performed an in vitro macrophage migration
assay in which RAW264.7 macrophages were seeded on a transwell membrane and incubated with
supernatant from a differentiated brown adipocyte cell line (T37i). Interestingly, macrophage
migration massively increased in the presence of T37i supernatant compared to the spontaneous
migration using medium only (Figure 3A), suggesting that chemoattractants are secreted by brown
adipocytes. We then studied alterations in cytokine and chemokine expression in BAT in vivo and
found that the expression of cytokines occurs simultaneously with the increase in lipid uptake and
macrophage numbers in BAT. Gene expression of Tgfb1, Il6, Tnf, Ccl2, Ccl4, Ccl7, Cxcl1 and Cx3cl1
was increased after 18 weeks of HFD, with a rapid increase already starting after 3 days of HFD for
most mediators (Figure 3B‐I). The increase in pro‐inflammatory cytokines, including Tnf, was
relatively higher than the increase in macrophage F4/80 expression, again suggesting a shift to a
more pro‐inflammatory phenotype of the macrophages.
After 18 weeks of HFD we found large increases in gene expression of the cytokines Tnf, Ccl2, Ccl4
and Ccl5 in EpAT. Interestingly, relative to the gene expression in EpAT of chow‐fed mice, obese BAT
also displayed an inflammatory cytokine environment in which Tnf, Ccl2 and Ccl4 increased to the
same extent as in obese EpAT, whereas Il6 and Cx3cl1 specifically increased in BAT and Ccl5 was
specifically induced in EpAT (Figure 3J‐O).
Brown adipocytes are regulated by pro‐ inflammatory cytokines in vitro
We next studied the effects of inflammatory stimuli on brown adipocyte function in vitro. A brown
adipocyte cell line (T37i) was stimulated for 24 hours with pro‐inflammatory cytokines TNF and IFNγ,
and lipopolysaccharide (LPS), which is a surface membrane component of gram‐negative bacteria
and allows cells to recognize bacterial invasion via Toll‐like receptor 4 (TLR4) (Figure 4A). All stimuli
lowered Ucp1 gene expression, which suggests inhibition of thermogenic function (Figure 4B).
Furthermore, the expression of Ppargc1a, a gene regulating mitochondrial biogenesis, was decreased
after TNF and LPS stimulation (Figure 4B). The cytokine stimulations TNF and IFNγ also lowered the
expression of the fatty acid receptor Cd36 and all stimulations decreased gene expression of
lipoprotein lipase (Lpl), suggesting that pro‐inflammatory stimuli also inhibit fatty acid uptake by the
cells (Figure 4B).
The HFD rapidly increased cytokine and chemokine gene expression in BAT (Figure 3) but it is unclear
which cell type is responsible for this production. Although macrophages as immune cells are key
candidates to produce these inflammatory mediators we also examined expression of these genes in
brown adipocytes in vitro. In the brown adipocytes that were stimulated with pro‐inflammatory
cytokines TNF and IFNγ, and LPS, we analysed gene expression of a set of relevant chemokines.
Interestingly, all three conditions were able to induce a variety of chemokines, including Il6, Ccl2,
Ccl3, Ccl4, Ccl5, Ccl7, Cxcl1, Cxcl12 and Cx3cl1 (Figure 4C,DI). Even more intriguing, IL‐6 and CXCL1
were also secreted in the supernatant in nanogram quantities, confirming secretion of relevant levels
Brown adipose tissue in obesity
57
of these inflammatory mediators by brown adipocytes (Figure 4E). Altogether, the brown adipocytes
themselves likely also contribute to the inflammatory signals within BAT upon a HFD challenge.
Figure 3. Increased cytokine expression in obese BAT. A. The migration of RAW264.7 macrophages in vitro when incubated with supernatant from a brown adipocyte cell line (T37i). B. BAT gene expression of Tgfb1, (C) Il6, (D) Tnf, (E) Ccl2, (F) Ccl4, (G) Ccl7, (H) Cxcl1 and (I) Cx3cl1. J‐O. Comparison of gene expression in EpAT and BAT, expression was calculated relative to the chow EpAT group for (J) Tnf, (K) Il‐6, (L) Ccl2, (M) Ccl4, (N) Ccl5 and (O) Cx3cl1. n=10‐11, *=P<0.05, **=P<0.01, ***=P<0.001 (A, J‐O Student t‐test, B‐I 1‐way ANOVA with Tukey post‐test analysis).
Ccl7
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tive
expr
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2
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Cxcl1
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0 1d 3d 1w 2w 4w 10w 18w0
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***
Cx3cl1
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1.5 * ** *
Tnf
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Il6
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5
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BAT
EpAT
Chapter 3
58
Figure 4. Inflammatory stimuli reduce Ucp1 expression and increase chemokine secretion by brown adipocytes in vitro A. In vitro model of brown adipocyte cell line (T37i) stimulations. B. Gene expression of Ucp1, Ppargc1a, Cd36 and Lpl after 24 hours of stimulation with TNF, IFNγ or LPS. C, D. Gene expression of Ccl2, Ccl3, Ccl4, Ccl5, Ccl7, Cxcl12, Cx3cl1, Il6 and Cxcl1 upon pro‐inflammatory stimuli. E. ELISA measurements of the supernatant showing secretion of IL‐6 and CXCL1 upon pro‐inflammatory stimuli. n=4, *=P<0.05, **=P<0.01, ***=P<0.001 (Students t‐test).
A B
D E
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ng/m
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C
Brown adipose tissue in obesity
59
Discussion
Obesity is associated with a chronic state of inflammation in WAT and a reduction in BAT function.
Our study shows that BAT of mice subjected to diet‐induced obesity undergoes tissue expansion
within 3 days of HFD. A rapid increase in BAT weight and lipid droplet size after the start of a HFD
suggests that BAT is an extremely plastic tissue that adapts to changes in dietary conditions. These
increases in BAT weight and lipid content plateaued after 3 days of dietary challenge and was only
further increased in the 18 weeks HFD group. Within the first week of HFD, the expanding BAT
attracted immune cells associated with increased cytokine and chemokine gene expression, although
the highest levels of inflammation were measured in the 18 weeks of HFD group. In vitro data using a
brown adipocyte cell line showed that a pro‐inflammatory environment decreases Ucp1 expression
in brown adipocytes and that brown adipocytes themselves are capable of producing and secreting
inflammatory cytokines and chemokines, and increases macrophage migration.
Within the expanding BAT we observed an increase in macrophages, with a more pro‐inflammatory
phenotype as evidenced by higher expression of Nos2 and increased inflammatory cytokines
including Tnf and Il6. Inflammatory stimuli also enhance the release of TNF and IL‐6 from pro‐
inflammatory macrophages. However, in adipose tissue, IL‐6 is not necessarily pro‐inflammatory and
may mediate anti‐inflammatory macrophage polarization by inducing the IL‐4 receptor [32]. We also
observed increased expression of Ccl2, Ccl4, and Ccl7, which are chemotactic signals for monocytes
and macrophages. Cxcl1, a neutrophil chemokine, and especially Cx3cl1 also rapidly increased in BAT
after the start of HFD. CX3CL1, also known as fractalkine, recruits monocytes and macrophages and
loss of CX3CL1‐CX3CR1 signalling was previously shown to reduce macrophage accumulation in WAT
as well as BAT and to reduces development of DIO [33].
We show that pro‐inflammatory signals associated with obesity affects brown adipocytes. By
mimicking a pro‐inflammatory environment in an in vitro model in which brown adipocytes were
stimulated with TNF, IFNγ and LPS we found decreased Ucp1 and Ppargc1a expression. This is in line
with reports by others who show that TNF suppresses Ucp1 expression and impairs mitochondrial
biogenesis in brown adipocytes [25‐28]. These data thus suggest decreased brown adipocyte activity
upon inflammatory signals, which could be a mechanism behind the well‐established obesity‐
associated decreased BAT activity.
White adipocytes secrete many adipokines, including leptin and adiponectin but also (adipo‐)
cytokines, including IL‐6 and CCL2 [34]. Furthermore, under inflammatory (i.e. LPS stimulated)
conditions, white adipocytes secrete TNF, IL‐6, CCL3, CCL4 and CXCL12 [35]. Brown adipocytes
secrete “BATokines” as well, which can act in a paracrine or autocrine manner [36]. For example,
thermogenic active brown and/or beige adipocytes secrete factors that promote hypertrophy,
hyperplasia, vascularization and innervation including irisin, metrnl, FGF21, neuregulin 4 and also IL‐6
[36]. Interestingly, we observed that, especially upon pro‐inflammatory stimuli in vitro, brown
adipocytes themselves express a variety of cytokines and chemokines including Il‐6, Ccl2, Ccl3, Ccl4,
Ccl5, Ccl7, Cxcl1, Cxcl12 and Cx3cl1 and probably others. These data indicate that the brown
adipocytes themselves may be an important source of inflammatory signals within BAT upon a HFD
challenge.
Chapter 3
60
Our study further shows that BAT stores excessive lipids, in which the accumulation of lipids gives the
tissue a ‘whitened’ phenotype. Others have shown that ‘whitening’ of BAT in mice is characterized by
hypoxia, mitochondrial dysfunction and decreased SNS innervation [37]. One mechanism
contributing to immune regulation in BAT could be the SNS. Surgical denervation of BAT and
treatment with the β‐adrenergic antagonist propranolol upregulate gene expression of Tnf, ll6 and
Ccl2, without a change in F4/80 expression. This shows that the SNS is involved in maintaining the
anti‐inflammatory state in BAT and decreased SNS signalling disturbs the anti‐inflammatory
environment [38]. Future research should further examine the contribution of the SNS to immune
regulation of obese BAT.
In mice, cold exposure increases markers for anti‐inflammatory macrophages in BAT and beiging in
ScAT, and macrophage depletion decreases BAT activity. Anti‐inflammatory macrophages are thus
important stimuli of adaptive thermogenesis. They release noradrenaline, via the production of
tyrosine‐hydroxylase, a catecholamine‐synthesizing enzyme, and IL‐4‐STAT6 signalling [29, 30]. Not
only brown adipocytes express β‐adrenergic receptors; macrophages also express β2‐adrenergic
receptors, enabling a complex auto‐ and paracrine loop [39, 40]. Although literature states an
important link between anti‐inflammatory macrophages and BAT functioning, our data does not
support a loss of anti‐inflammatory macrophages in obese BAT but rather an increase in pro‐
inflammatory macrophages. This would imply that a disturbed mechanism of BAT activation by anti‐
inflammatory macrophages is not the major cause of obesity associated dysfunctional BAT.
Insulin resistance that accompanies obesity is partly attributable to changes in adipokine secretion by
WAT. BAT is highly vascularized to supply the adipocytes with substrate and oxygen for oxidation and
enable efficient distribution of heat [1]. Therefore, the cytokines and chemokines produced by brown
adipocytes in obesity might be transported out of the tissue and thereby also contribute to systemic
inflammation. However, it will be challenging to determine the relative contribution of BAT to
increased circulating cytokine levels and insulin resistance.
We found many inflammatory signals and a rapid increase in macrophages in BAT of HFD‐fed mice,
although we cannot exclude the possibility that other immune cell types play a role as well. Our study
shows that brown adipocytes produce inflammatory mediators, which likely mediate crosstalk
between adipocytes and macrophages. Future studies will be needed to elucidate on the complete
spectrum of BAT regulation by immune cells. Furthermore, the contribution of immune cells to
differentiation, proliferation and activation of beige adipocytes within WAT and possible overlapping
mechanisms with classical BAT will be of great interest. Although dysfunctional BAT in obesity likely
results from combined physiological alterations at multiple sites, we have shown that the
immunological signals are one important regulatory factor.
Brown adipose tissue in obesity
61
Corresponding author Prof. Menno P.J. de Winther. Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical
Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands. +31 (0)20 5 66 6762.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements We acknowledge the support from the Rembrandt Institute of Cardiovascular Science (PR, MdW, EL) and the Netherlands
CardioVascular Research Initiative: “the Dutch Heart Foundation, Dutch Federation of University Medical Centres, the
Netherlands Organisation for Health Research and Development and the Royal Netherlands Academy of Sciences" for the
GENIUS project “Generating the best evidence‐based pharmaceutical targets for atherosclerosis” (CVON2011‐19). This work
was supported by the Netherlands Organization for Scientific Research (NWO) (VENI to JVdV, VICI grant to EL), the
Netherlands Heart Foundation (Dr E. Dekker grant to TS and junior postdoc grant to JVdB), the European Research Council
(ERC con grant to EL) and Horizon 2020 (REPROGRAM to EL and MdW).
Chapter 3
62
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[40] Grailer JJ, Haggadone MD, Sarma JV, Zetoune FS, Ward PA. 2014. Induction of M2 Regulatory Macrophages through the β2‐Adrenergic Receptor with Protection during Endotoxemia and Acute Lung Injury. Journal of Innate Immunity 6(5):607‐618.
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4
Helminthantigenscounteractarapidhigh‐fatdietinduceddropineosinophilsinadepotspecificmannerinmice
Susan M. van den Berg1, Andrea D. van Dam2,3, Pascal J.H. Kusters1, Linda Beckers1, Myrthe den
Toom1, Saskia van der Velden1, Jan Van den Bossche1, Irma van Die4, Mariëtte R. Boon2,3, Patrick C.N.
Rensen2,3, Esther Lutgens*,1,5, Menno P.J. de Winther*,1,5
1Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical Center, University of Amsterdam,
the Netherlands 2Department of Medicine, Division Endocrinology, Leiden University Medical Center, Leiden, the Netherlands 3Einthoven Laboratory for Experimental Vascular Medicine, Leiden, the Netherlands 4Department of Molecular Cell Biology and Immunology, Neuroscience Campus Amsterdam, Vrije Universiteit Medical
Center, Amsterdam, the Netherlands 5Institute for Cardiovascular Prevention (IPEK), Ludwig Maximilian’s University, Munich, Germany
*these authors contributed equally to this work.
Submitted
Chapter 4
66
Abstract
Brown adipose tissue (BAT) activation or white adipose tissue (WAT) beiging can increase energy
expenditure and reduce obesity‐associated diseases. The immune system is a potential target in
mediating brown and beige adipocyte activation. Homeostasis of lean WAT is maintained by type 2
and anti‐inflammatory immune cells, in which eosinophils produce interleukin (IL)‐4 and sustain anti‐
inflammatory macrophage activation. In obesity, WAT shows a decreased number of eosinophils and
an increased infiltration of pro‐inflammatory immune cells. We studied eosinophil numbers in BAT,
epididymal WAT (EpAT) and subcutaneous WAT (ScAT) after 1 day, 3 days or 1 week of high‐fat diet
(HFD) in C57Bl/6 mice. Helminth antigens induce a type 2 immune response and can be protective
against metabolic defects in obesity. We hypothesized that this phenotype involves activation of BAT
or beige adipogenesis. Therefore, 1 week HFD‐fed mice were injected with helminth antigens from
Schistosoma mansoni and Trichuris suis. After 1 day of HFD, eosinophils started to decline in EpAT
and BAT, and after 3 days in ScAT, along with a decrease in the eosinophil chemoattractant Ccl3 in
EpAT and ScAT and Ccl5 and Ccl11 in BAT. Furthermore, macrophages increased after 3 days of HFD
in EpAT. Interestingly, IL‐4 stimulation of brown adipocytes in vitro resulted in increased Ucp1
expression and the production of CCL11 (26‐fold increase, p<0.0001). Treatment with helminth
antigens resulted in high numbers of eosinophils, macrophages and T cells in EpAT. However, there
were almost no effects of the helminth antigens on ScAT and BAT immune cells and no activation of
BAT or beiging of WAT. Adipose tissue eosinophils decline rapidly after starting a HFD in mice. Short‐
term treatment with helminth antigens results in an adipose depot specific type 2 immune response
which does not affect BAT activation or beiging of WAT.
Type 2 immunological effects of HFD and helminth antigens on adipose tissue
67
Introduction
Brown adipose tissue (BAT) contributes to the control of body temperature, by the production of
heat in response to cold. Brown adipocytes have numerous mitochondria that express uncoupling
protein 1 (UCP1). Activation of UCP1 by noradrenaline results in uncoupling of the respiratory chain,
bypassing ATP synthesis and the dissipation of energy into heat [1]. Rodents as well as new‐borns
from larger organisms, including humans, have interscapular BAT depots. In humans, BAT regresses
with age but can still be found supraclavicular and in the neck region, but also periaortic,
paravertebral and perirenal [2]. BAT activity can be detected using a positron‐emission tomographic
and X‐ray computed tomography (PET/CT)‐scan and 18F‐fluorodeoxy‐glucose ([18F]FDG) uptake, in
which cold exposure increases the uptake of glucose [3]. Interestingly, [18F]FDG uptake is negatively
correlated with BMI, suggestive of decreased BAT activity in obesity [3].
Brown‐like or ‘beige/brite’ adipocytes can also be present within white adipose tissue (WAT) [4].
These beige adipocytes display similar characteristics as classical brown adipocytes, including a
multilocular appearance and Ucp1 expression. They are present at low quantities but can be induced
by cold or β3‐adrenergic receptor agonists. The inguinal subcutaneous adipose tissue (ScAT) depot
has the highest beiging potential after cold exposure [5], although the thermogenic capacity is only
10% of that of interscapular BAT [6]. Activation of BAT and the induction of beige adipocytes,
beneficially increases energy expenditure and therefore has the potential to reduce excessive energy
stores in obesity [7].
Homeostasis of adipose tissue is maintained by immune cells, in which lean WAT and BAT are
characterized by an anti‐inflammatory immune cell composition [8]. The stromal vascular fraction
(SVF) has high numbers of anti‐inflammatory macrophages, eosinophils, CD4+ T helper cells and
regulatory T cells. Anti‐inflammatory macrophages are sustained by Th2‐type cytokines, such as IL‐4.
In adipose tissue, the majority of IL‐4 secreting cells are eosinophils [9]. In obesity, increased lipid
uptake, hypertrophic adipocytes and leakage of fatty acids causes recruitment of pro‐inflammatory
immune cells. Obese WAT and BAT to a lower extent, is infiltrated by pro‐inflammatory macrophages
[10, 11]. Interestingly, eosinophil numbers in WAT are inversely correlated with body weight in mice
and absence of eosinophils results in increased body weight, impaired glucose tolerance and less
cold‐induced beige adipogenesis [9, 12]. Anti‐inflammatory macrophages have been implicated in
BAT activation and beiging of WAT, in which they release noradrenaline and activate brown and
beige adipocytes [12, 13]. Furthermore, administration of IL‐4 in mice reduces body weight, improves
insulin sensitivity and promotes beiging of WAT [12]. Thus, BAT activation and WAT beiging is
promoted by eosinophils that produce IL‐4, which stimulates anti‐inflammatory macrophages to
release noradrenaline, resulting in activation and increased expression of UCP1. Decreased BAT
activity in obesity and a decrease of both eosinophils and anti‐inflammatory macrophages in obese
WAT suggests that this immunological circuitry is disturbed in an obese state.
Helminths are parasitic worms that induce a type 2 immune response in their host, including a
massive increase in eosinophils. Interestingly, this type 2 response induced by Schistosoma mansoni‐
soluble egg antigens (SEA) or Nippostrongylus brasiliensis can be protective against metabolic
disorders [9, 14, 15]. Furthermore, soluble products of the whipworm Trichuris suis (TsSP) also induce
Chapter 4
68
a type 2 response and are proven to be beneficial in autoimmune diseases, including multiple
sclerosis [16]. How TsSP affects adipose tissue immune cells is unknown.
The type 2 immune response induced by helminth antigens has already been shown to be
metabolically beneficial and we hypothesized that this phenotype could be related to activation of
BAT and/or beiging of WAT. Therefore, we first determined how a high‐fat diet (HFD) affects
eosinophils in BAT, epididymal WAT (EpAT) and ScAT using a short time course of HFD. We then
studied how 1 week of SEA and TsSP alters the immune composition of these three adipose tissues
and whether this resulted in BAT activation and beiging of EpAT and ScAT.
Materials and methods
Mice
Male wild‐type C57Bl/6 mice (Charles River) were given a HFD (45% kcal fat, 35% kcal carbohydrate,
20% kcal protein, Special Diets Services, Witham, United Kingdom) for 1 day, 3 days or 1 week at the
age of 16 weeks (4 groups of n=11). For the SEA and TsSP studies, male wild‐type C57Bl/6 mice
(Charles River) were given a HFD for 1 week and received 4 intraperitoneal injections every 3 days
with PBS, 50 µg SEA or 50 µg TsSP starting 3 days before the diet (3 groups of n=12). Mice had ad
libitum access to food and water and were maintained under a 12h light‐dark cycle. Mice were
euthanized using 0.25 mg/g ketamine and 0.05 mg/g xylazine. Epididymal adipose tissue (EpAT),
subcutaneous inguinal adipose tissue (ScAT) and interscapular BAT were dissected and weighed. All
experimental procedures were approved by the Animal Experimentation Ethics Committee of the
University of Amsterdam.
SEA and TsSP preparation
Schistosoma mansoni‐soluble egg antigens (SEA) was kindly provided by Fred Lewis (Biomedical
Research Institute, Rockville, MD, USA) and prepared as described previously [17]. Soluble products
of Trichuris suis (TsSP) was kindly provided by Irma van Die and prepared as described in [18].
Flow cytometry
Adipose tissues were collected in PBS, transferred to DMEM‐20 mM HEPES (ThermoFisher Scientific,
Waltham, MA, USA) and minced into small pieces. Digestion was done using 0.25 mg/ml liberase
(Roche, Basal, Switzerland) in DMEM‐20 mM HEPES for 45 minutes at 37°C. Digested tissue was
passed through a 70 µm nylon mesh (BD Biosciences) and centrifuged at 1250 rpm for 6 minutes. The
adipocyte fraction was removed and the pellet containing the SVF was resuspended in FACS buffer
(0.5% BSA in PBS). Cell suspensions were incubated with an Fc‐receptor blocking antibody to prevent
non‐specific binding and a biotin‐PE‐dump antibody mix (BioLegend and eBioscience, San Diego, CA,
USA) followed by a staining using the antibodies: CD45, CD68 (BioLegend), CD11b, SiglecF, Ly6G (BD
Biosciences, Breda, the Netherlands), CD3, MHCII, F4/80 (eBioscience). Stainings were analysed by
FACS (LSR Fortessa, BD Biosciences) and FlowJo software (Tree star).
Histology
Tissues were collected, fixed in 4% paraformaldehyde and embedded in paraffin.
Immunohistochemistry on EpAT and ScAT was performed for UCP1 (Sigma‐Aldrich, Zwijndrecht, the
Netherlands). The presence of beige adipocytes, defined by a multilocular appearance and positive
Type 2 immunological effects of HFD and helminth antigens on adipose tissue
69
for UCP1 was scored by M.T., who was blinded for the experimental conditions. When present, the
quantity was expressed as + being a few cells, ++ a few areas and +++ when the section was full of
beige adipocytes.
Real‐time PCR
Total RNA was extracted using TRIzol (Invitrogen, Carlsbad, CA, USA) and reverse transcribed with an
iScript cDNA synthesis kit (Bio‐Rad, Veenendaal, the Netherlands). The quantitative PCR was
performed using a SYBR green PCR kit and a ViiA7 RT‐PCR system (Applied Biosystems, Leusden, the
Netherlands). The results are expressed as relative to the control group, which was assigned a value
of 1.
Culture and differentiation of brown adipocytes
T37i cells were kindly provided by Marc Lombès [19] and cultured in DMEM‐F12 Glutamax, 10% FCS,
penicillin (100 U/ml) and streptomycin (100 µg/ml) (ThermoFisher Scientific). After growing
confluent, cells were differentiated by adding 2 nM Triiodothyronine (T3) (Sigma‐Aldrich) and 112
ng/ml insulin (Sigma‐Aldrich) to the media. After 9 days of differentiation, the cells were stimulated
with 20 ng/ml IL‐4 (Peprotech, Rocky Hill, NJ, USA) for 24 hours. Supernatant was collected for the
quantification of CCL11 (R&D systems, Minneapolis, MN, USA) by ELISA in accordance to the
suppliers’ protocols.
Statistics
Results are presented as mean ± SEM. Analysis between more than two groups in the time course
experiments were done by a one‐way ANOVA with Tukey post‐test analysis and the change per group
expressed relative to the control group. Analysis between two groups was done by a Student’s t test
or a chi‐square test. Statistics were calculated in GraphPad Prism 5.0 (GraphPad Software, Inc., La
Jolla, CA, USA). P‐values <0.05 were considered significant.
Results
Eosinophils rapidly decrease in adipose tissue upon HFD
We were interested in how a HFD affects eosinophil numbers in adipose tissue. Because we
previously observed how only one week of HFD induces rapid inflammatory changes in BAT related
to cytokine and chemokine expression, we included very short time points of HFD; 1 day, 3 days and
1 week. Furthermore, others have described an important role for eosinophils in adipose tissue in
which they are involved in the regulation of brown adipocyte activity and beige adipogenesis. We
determined eosinophil numbers in age‐matched HFD‐fed C57Bl/6 wild‐type mice and compared
them to mice on a chow diet (Fig.1A). Analysis of adipose tissue immune cells by flow cytometry
revealed that already after 1 day of HFD the numbers of eosinophils decline in EpAT and BAT and
after 3 days in ScAT as well (Fig.1B). Particularly in BAT, eosinophils dramatically dropped by 65%. In
EpAT, this was 35%. In the EpAT, we also observed an increase in macrophages after 3 days of HFD
(Fig.1C). These data show us that before the onset of obesity, a HFD has rapid effects on the immune
cell composition of brown and white adipose tissues.
Chapter 4
70
To determine how a HFD can cause such rapid changes in eosinophil numbers, we started by looking
at the expression of key chemokines that are known to attract eosinophils, Ccl3, Ccl5 and Ccl11.
Although the expression of these chemokines was indeed downregulated, this differed per adipose
tissue depot. In EpAT and ScAT, especially Ccl3 was downregulated in the groups of mice that
received a HFD for 1 day or more (Fig.2A, B), and in BAT the expression of Ccl5 and Ccl11 was lower in
the HFD groups (Fig.2B).
IL‐4 induced brown adipocyte activation and CCL11 production in vitro
The immunological circuit of BAT activation and beiging of WAT is via IL‐4 producing eosinophils that
stimulate anti‐inflammatory macrophages to produce noradrenaline which activates brown
adipocytes and induces beiging of WAT [9, 20]. To study the effects of such a type 2 anti‐
inflammatory cytokine environment in vitro, we stimulated a brown adipocyte cell line (T37i) with
the cytokine IL‐4 for 24 hours. Interestingly, IL‐4 upregulated Ucp1 expression, suggesting that IL‐4
alone already has a direct effect on the activation of brown adipocytes, without the necessity of
noradrenalin‐secreting macrophages (Fig.2C). Furthermore, to have an indication on how CCL11, the
most important chemoattractant for eosinophils, is regulated in BAT, we found that stimulation of
the brown adipocytes with IL‐4 massively increased the expression and secretion of CCL11 (Fig.2D).
Altogether, these data suggest a mechanism in which a HFD suppresses the activation of BAT and
possibly the beiging potential of WAT by a decrease in IL‐4 producing eosinophils.
Figure 1. A rapid decline in adipose tissue eosinophils upon a short time high‐fat diet in mice A. Schematic study outline. B. Flow cytometry analysis of the stromal vascular fraction of EpAT, ScAT and BAT showing a decrease in the percentage of eosinophils (SiglecF
+ leukocytes) after 1 day of HFD in EpAT, 3 days of HFD in ScAT and 1 day of HFD in BAT. C. The percentage of macrophages (F4/80+ leukocytes), which start to increase of 3 days of HFD in EpAT and not in ScAT and BAT. n=11, *=P<0.05, **=P<0.01, ***=P<0.001 (1‐way ANOVA with Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group).
1w 3d 1d 0
A B C
45% HFD
4 groups of mice
- No HFD
- 1 day
- 3 days
- 1 week
Eosinophils
No HFD 1d 3d 1w0
10
20
30
40
*****
***
Sigl
ecF+
(% o
f CD
11b+ )
Macrophages
No HFD 1d 3d 1w0
20
40
60
80
** **
F4/8
0+ (%
of C
D11
b+ )
Macrophages
No HFD 1d 3d 1w0
20
40
60
80
Macrophages
No HFD 1d 3d 1w0
10
20
30
Eosinophils
No HFD 1d 3d 1w0
5
10
15
***** *
Eosinophils
No HFD 1d 3d 1w0
10
20
30
** ***
Sigl
ecF+
(% o
f CD
11b+ )
F4/8
0+ (%
of C
D11
b+ )
Sigl
ecF+
(% o
f CD
11b+ )
F4/8
0+ (%
of C
D11
b+ )
EpAT
ScAT
BAT
Type 2 immunological effects of HFD and helminth antigens on adipose tissue
71
Figure 2. A HFD decreases gene expression of eosinophil chemoattractants in adipose tissues A. In ScAT, the chemoattractant Ccl3 decreased after 1 day of HFD, whereas in (B) BAT the chemoattractant Ccl5 and Ccl11 were downregulated upon a HFD. C. A brown adipocyte cell line (T37i) was stimulated with the cytokine IL‐4 for 24 hours. This upregulated Ucp1, indicating that IL‐4 has a direct effect ‐4 on the activation of brown adipocytes. D. Both gene expression of Ccl11 and the presence of CCL11 in the supernatant as assessed by ELISA was increased by IL‐4 stimulation. n=11, *=P<0.05, **=P<0.01, ***=P<0.001 (A,B: 1‐way ANOVA with Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group. C,D: Student’s t test).
EpAT
Ccl3 Ccl5 Ccl110
1
2
3
* *
**01d3d1w
mR
NA
exp
ress
ion
(AU
)
ScAT
Ccl3 Ccl5 Ccl110.0
0.5
1.0
1.5
2.001d3d1w
***** **
mR
NA
exp
ress
ion
(AU
)
BAT
Ccl3 Ccl5 Ccl110.0
0.5
1.0
1.5
2.0
* **
****
01d3d1w
mR
NA
exp
ress
ion
(AU
)
Ccl110
50
100
150
200-***
mR
NA
exp
ress
ion
(AU
)
Ucp10.0
0.5
1.0
1.5
2.0-IL-4*
mR
NA
exp
ress
ion
(AU
)
D E
CCL110
1000
2000
3000
4000 -IL-4
***
pg/m
l
A
B
C
IL-4
Chapter 4
72
Figure 3. No changes in body weight after short‐term treatment with SEA and TsSP A. Schematic study design. B. Body weight and the weight of three adipose tissue depots, EpAT, ScAT and BAT, did not differ between the groups. n=12
A
B
1 week 45% HFD
Day -3 Day 0 Day 3 Day 6
†
Control SEA TsSP0
20
40
60
80
BA
T (m
g)
Control SEA TsSP0
200
400
600
EpA
T (m
g)
Control SEA TsSP0
100
200
300
400
ScA
T (m
g)
Control SEA TsSP0
10
20
30
Bod
y w
eigh
t (g)
Adipose tissue depot‐specific effects of helminth antigen induced type 2 immune responses
We wondered if we could rescue the HFD‐induced decrease in adipose tissue eosinophils in mice by
the administration of the helminth antigens SEA and TsSP, which are known to induce a type 2
immune response. We subjected C57Bl/6 wild‐type mice (n=12) to a HFD for 1 week and injected
them with the helminth antigens SEA and TsSP (Fig.3A). These treatments did not cause changes in
body weight or BAT, EpAT and ScAT weight (Fig.3B). Flow cytometry of the adipose tissues revealed
major effects on immune cell composition of the EpAT in the SEA and TsSP groups (Fig.4A). Total
leukocyte numbers (CD45+ cells) were 6.4‐fold and 4.0‐fold increased in SEA and TsSP groups,
respectively. High counts of adipose tissue eosinophils (SiglecF+ cells) were observed by SEA, +34‐fold
difference and +23‐fold difference by TsSP. The number of macrophages (CD68+MHCII+ cells)
increased by 5.8‐fold and 5.2‐fold in the SEA and TsSP groups respectively. Furthermore, SEA
increased T cell numbers (CD3+ cells) 12.3‐fold, and TsSP 7.6‐fold. Neutrophil counts (Ly6G+ cells)
were unaffected by the treatments. However, the effects of the helminth antigens on ScAT and BAT
were minor compared to EpAT with slightly more T cells (+3.3‐fold difference) and macrophages
(+2.4‐fold difference) by TsSP in the ScAT and no changes in BAT (Fig.4B,C). Gene expression data
confirmed increased macrophages and eosinophils in EpAT by Cd68 and Ccr3 expression respectively
(Fig.4D). Furthermore, the chemokines responsible for attracting eosinophils Ccl3, Ccl5 and Ccl11
were indeed upregulated in EpAT (Fig.4D). In ScAT and BAT, no alterations in gene expression were
observed (Fig.4E,F).
Helminth antigen treatment does not cause beiging in WAT
As especially ScAT is prone to beiging, we wondered whether the small change in leukocyte
composition in ScAT altered the presence of beige adipocytes. We performed immunohistochemistry
for UCP1 on ScAT and scored the occurrence of UCP1 positive areas in a blinded fashion. This
quantification showed a small trend towards a higher presence of beige adipocytes in ScAT of mice
treated with the helminth antigens SEA, and not in the TsSP group (Table 1, Fig.5A). Furthermore, we
did not observe any beige adipocytes in EpAT and thus the major increase in eosinophils in EpAT did
not result in beige adipogenesis in this adipose tissue depot (data not shown). In BAT, Ucp1
expression was unaffected by either SEA or TsSP treatment (Fig.5B).
Type 2 immunological effects of HFD and helminth antigens on adipose tissue
73
Figure 4. Helminth antigens induce an adipose depot specific type 2 immune response A. Injections with SEA and TsSP resulted in a large increase in the number of total EpAT leukocytes (CD45+), T cells (CD3+), Macrophages (CD68+MHCII+), Eosinophils (SiglecF
+) and not in neutrophils (Ly6G+). B. In ScAT, only TsSP showed a significant induction of T cells and macrophages. C. No changes in immune cell composition in BAT were observed after SEA and TsSP treatment. D. Gene expression analysis showing that macrophage marker Cd68, eosinophil marker Ccr3 and chemokines Ccl3, Ccl5 and Ccl11 were all increased in EpAT of SEA and TsSP treated mice. E,F. No effects of the treatments were observed in ScAT and BAT. n=12, *=P<0.05, **=P<0.01, ***=P<0.001 (Student’s t test between the control group and either SEA or TsSP)
EpAT
Cd68 Ccr3 Ccl3 Ccl5 Ccl110
2
4
6
8 CtrlSEATsSP
*** **
*** ***
***
** *
*** **
mR
NA
exp
ress
ion
(AU
)
ScAT
Cd68 Ccr3 Ccl5 Ccl110
1
2
3CtrlSEATsSP
mR
NA
exp
ress
ion
(AU
)
BAT
Cd68 Ccr3 Ccl3 Ccl5 Ccl110.0
0.5
1.0
1.5
2.0 CtrlSEATsSP
mR
NA
exp
ress
ion
(AU
)
D E
A EpAT
B ScAT
C BAT
F
Leukocytes
Control SEA TsSP0
1000
2000
3000
4000
****
****
CD
45+ (
#/g)
T cells
Control SEA TsSP0
50
100
150
200***
****
CD
3+ (#/
g)
Eosinophils
Control SEA TsSP0
500
1000
1500
****
****
Sigl
ecF+ (
#/g)
Neutrophils
Control SEA TsSP0
2
4
6
8
10
Ly6G
+ (#/
g)
Macrophages
Control SEA TsSP0
100
200
300
400
500
*** **
CD
68+ M
HC
II+ (#/
g)
Leukocytes
Control SEA TsSP0
200
400
600
800
1000
CD
45+ (
#/g)
T cells
Control SEA TsSP0
20
40
60*
CD
3+ (#/
g)
Eosinophils
Control SEA TsSP0
10
20
30
40
50
Sigl
ecF+ (
#/g)
Neutrophils
Control SEA TsSP0
5
10
15
Ly6G
+ (#/
g)
Macrophages
Control SEA TsSP0
20
40
60
80 *
CD
68+ M
HC
II+ (#/
g)
Leukocytes
Control SEA TsSP0
50
100
150
200
250
CD
45+ (
#/g)
T cells
Control SEA TsSP0
5
10
15
20
CD
3+ (#/
g)
Eosinophils
Control SEA TsSP0
5
10
15
Sigl
ecF+ (
#/g)
Neutrophils
Control SEA TsSP0
5
10
15
20
25
Ly6G
+ (#/
g)
Macrophages
Control SEA TsSP0
2
4
6
CD
68+ M
HC
II+ (#/
g)
Chapter 4
74
Figure 5. No effects on thermogenic capacity by helminth antigen treatment A. Scoring of presence of beige adipocytes in ScAT. Beige adipocytes were defined by a multilocular appearance and positive for UCP1. When present, the quantity was expressed as +: a few cells, ++: a few areas and +++ when the section was full of beige adipocytes. B. Gene expression of Ucp1 in BAT was not different upon SEA or TsSP treatment. n=12
Table 1. Presence of beiging in ScAT after helminth treatment.
‐ + ++ +++ p‐value
Ctrl 9 2 0 1
SEA 3 1 4 2 0.056
TsSP 6 2 1 2 0.59
Values represent the number of mice within the groups that were scored on no (‐) or a few beige adipocytes (+), a few beige areas (++) or many beige areas (+++) in ScAT. P‐value as determined by a Chi‐square test. n=10‐12
A ScAT
B BAT
Ucp1
Ctrl SEA TsSP0.0
0.5
1.0
1.5
mR
NA
exp
ress
ion
(AU
)
Control SEA TsSP
Type 2 immunological effects of HFD and helminth antigens on adipose tissue
75
Discussion
Immune cells contribute to activation of BAT and beiging of WAT, via IL‐4 producing eosinophils that
stimulate anti‐inflammatory macrophages to secrete noradrenaline [9, 13]. In the current study, we
show that short‐term HFD causes a rapid decline in eosinophils in EpAT, ScAT and BAT, along with a
decrease in chemotactic signals for eosinophils. We also show that the anti‐inflammatory cytokine IL‐
4 can directly activate brown adipocytes in vitro and that brown adipocytes themselves are
extremely capable of producing and secreting CCL11, the chemokine that recruits eosinophils.
Altogether, these findings indicate that a type 2 and anti‐inflammatory environment in adipose
tissues and the associated beige/brown adipocyte activation or beige adipogenesis could beneficially
alter metabolism in obesity. Therefore, we administrated helminth antigens from SEA and TsSP to
mice on a HFD for 1 week to induce a type 2 immune response. We observed very depot‐specific
effects of SEA and TsSP in which EpAT was massively infiltrated with eosinophils, macrophages and T
cells whereas almost no differences were found in ScAT and BAT. Furthermore, SEA and TsSP
treatments did not result in increased thermogenic energy expenditure, as we did not observe
alterations in BAT activity or beiging of the WAT depots.
Eosinophil numbers depend on signals that affect recruitment, survival and/or cell death. Our data
suggests that the recruitment of eosinophils is indeed altered, as we observe a decrease in gene
expression of the chemotactic signals Ccl3, Ccl5 and Ccl11. Eosinophils greatly depend on survival
signals, including IL‐3, IL‐5 and granulocyte macrophage colony‐stimulating factor (GM‐CSF), of which
IL‐5 is the most important [21]. IL‐5 in turn is produced by group 2 innate lymphoid cells (ILC2s) [22].
Eosinophils have a short half‐life, reports vary between 3 to 18 hours in blood and up to 6 days in
tissues, with a great tissue‐dependent variety [21‐23]. In lean adipose tissue, eosinophils have a low
turnover (10% in 3 days) [22], suggesting that a lean state contains high survival signals. Factors
related to obesity and potentially involved in reducing eosinophil numbers include the pro‐
inflammatory cytokines TNF, which can induce eosinophil apoptosis [21] and IFNγ, which can inhibit
differentiation and migration of eosinophils [24]. Furthermore, TGFβ has been suggested to block the
survival effects of IL‐5 on eosinophils [25], although TGFβ expression also correlates with an increase
in eosinophils in airways of asthma patients [26]. TGFβ is a multifunctional cytokine that possibly has
diverse effects in different tissues. Eosinophil degranulation can also result in cell death [27].
As eosinophils have a short half‐life, a withdrawal of survival signals and/or increased apoptotic
factors while chemoattractants are decreased can result in a rapid decline. To unravel a genuine
mechanism behind this fast drop in adipose tissue eosinophils upon a HFD, future studies should
address which signals cause chemokines to decrease, determine whether IL‐5 and ILC2s are affected
by short‐term HFD and find correlations between apoptotic factors and eosinophil numbers in
adipose tissues. Eosinophils migrate via vascular cell adhesion molecule‐1 (VCAM‐1) and intercellular
cell adhesion molecule‐1 (ICAM‐1) through α4 and αL integrins expressed on eosinophils, as integrin
antibodies block eosinophil accumulation in adipose tissue [9]. This makes VCAM‐1 and ICAM‐1 also
interesting targets to study if eosinophil migration is affected in obese adipose tissues.
Our results are in line with others who found that WAT of obese mice has a reduction in eosinophils
[9]. We further state that eosinophils immediately decline shortly after the start of a HFD. The next
Chapter 4
76
important step would be to confirm these immunological changes in human adipose tissue, as the
physiological resemblance for a shift from chow food to 1 day of HFD in mice is difficult to
conceptualize. To further improve the translational relevance of these findings, it would be very
interesting to determine whether increased lipid uptake of adipocytes or a specific dietary
component is responsible for the rapid immune response in adipose tissue.
Others have shown that chronic SEA treatment and the associated increase in type 2 immunity can
be protective in the pathogenesis of metabolic disorders [14]. We hypothesized that these metabolic
improvements were related to BAT activity and beiging of WAT. In our study, the type 2 immune
response induced by helminth antigens from both SEA as well as TsSP was very depot specific and
was only full blown present in EpAT. Surprisingly, an extreme increase in EpAT eosinophils (+34‐fold
by SEA and +23‐fold by TsSP) did not cause beige adipogenesis. This might be explained by a low
beiging capacity of EpAT [5]. Therefore, we speculate that BAT activation or WAT beiging is not the
mechanism that is causing the metabolic effects that others have seen upon a chronic infection with
SEA. Beiging predominantly occurs in ScAT, possibly due to differences in a precursor population for
beige adipocytes [28], which is an interesting topic for future research.
Administration of IL‐4 for 10 days in vivo induces beiging of ScAT [12], indicating that this mechanism
could take place in this time frame. In our study, although 10 days of treatment with SEA and TsSP
was enough to induce a massive immune response in EpAT, we did not observe effects on body
weight or adipose tissue weight. Therefore, this short‐term study might not have been long enough
to observe either metabolic effects or BAT activation and beige adipogenesis.
With the use of helminth antigens, we wanted to reverse the effects of a drop in eosinophils in
adipose tissue caused by a HFD. The mice in this experiment were therefore given a HFD during the
treatment. In ScAT and BAT, the effect of HFD might have caused a larger decline in eosinophils then
the SEA and TsSP could restore. Therefore, with the HFD, we might have decreased the capacity to
activate BAT and the beiging potential of ScAT and we could have seen effects when performing the
study under chow conditions.
Helminths express a wide variety of protein‐ and lipid linked glycans and it is unknown which
molecular mechanisms are responsible for the immune modulating effects [29]. In our study, we
observe similar effects from SEA and TsSP, indicating that they have a common parasitic signal that
exerts the effects in adipose tissue. Before helminth antigens could be a therapeutically applied, it
will be needed to identify which parasitic signal causes the metabolically beneficial type 2 immune
response.
In healthy adipose tissue, a type 2 immunological circuitry contributes to tissue homeostasis, by IL‐4
producing eosinophils that sustain anti‐inflammatory macrophages [9]. Our data shows that Il‐4
probably has a bigger role in adipose tissue than only the polarization of macrophages with also a
direct effect on brown adipocytes. A HFD disturbs the homeostatic type 2 immunological circuitry,
and cannot be reversed by a helminth‐induced type 2 response. The crosstalk between immune cells
and adipocytes and how they regulate tissue homeostasis is more complex than previously thought
with many questions that remain.
Type 2 immunological effects of HFD and helminth antigens on adipose tissue
77
Corresponding author Prof. Menno P.J. de Winther. Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical
Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands. +31 (0)20 5 66 6762.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements We acknowledge the support from the Rembrandt Institute of Cardiovascular Science (PR, MW, EL) and the Netherlands
CardioVascular Research Initiative: “the Dutch Heart Foundation, Dutch Federation of University Medical Centres, the
Netherlands Organisation for Health Research and Development and the Royal Netherlands Academy of Sciences" for the
GENIUS project “Generating the best evidence‐based pharmaceutical targets for atherosclerosis” (CVON2011‐19). This work
was supported by the Netherlands Organization for Scientific Research (NWO) (VICI grant to EL), the Netherlands Heart (Dr
E. Dekker grant to TS) and the European Research Council (ERC con grant to EL).
Chapter 4
78
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5
Diet‐inducedobesityinmicediminisheshematopoieticstemandprogenitorcellsinthebonemarrow
Susan M. van den Berg1,*, Tom T.P. Seijkens1,*, Pascal J.H. Kusters1, Linda Beckers1, Myrthe den
Toom1, Esther Smeets1, Johannes Levels2, Menno P.J. de Winther1 and Esther Lutgens1,3
1Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical Centre, University of Amsterdam,
the Netherlands 2Department of Experimental Vascular Medicine, Academic Medical Centre, University of Amsterdam, the Netherlands 3Institute for Cardiovascular Prevention, Ludwig Maximilians University, Munich, Germany
*theseauthorscontributedequallytothiswork.
The FASEB Journal 2016; 30(5):1779‐1788
Chapter 5
82
Abstract
Obesity is associated with chronic low‐grade inflammation, characterized by leukocytosis and
inflammation in the adipose tissue. Continuous activation of the immune system is a stressor for
hematopoietic stem‐ and progenitor cells (HSPCs) in the bone marrow (BM). Here we studied how
diet‐induced obesity (DIO) affects HSPC population dynamics in the BM. Eight groups of age‐matched
C57Bl/6 mice received a high fat diet (HFD; 45% kcal from fat) ranging from 1 day up to 18 weeks.
The obesogenic diet caused decreased proliferation of lineage‐Sca‐1+c‐Kit+ (LSK) cells in the BM and a
general suppression of progenitor cell populations including common lymphoid progenitors and
common myeloid progenitors. Within the LSK population, DIO induced a shift in stem cells that are
capable of self‐renewal towards maturing multipotent progenitor cells. The higher differentiation
potential resulted in increased lymphoid and myeloid ex vivo colony forming capacity. In a
competitive BM transplantation, BM from obese animals showed impaired multilineage
reconstitution when transplanted into chow fed mice. Our data demonstrate that obesity stimulates
the differentiation and reduces proliferation of HSPCs in the BM, leading to a decreased HSPC
population. This implies that the effects of obesity on HSPCs hampers proper functioning of the
immune system.
DIO in mice decreases HSPCs in the BM
83
Introduction
Obesity and obesity‐associated complications such as cardiovascular diseases and type 2 diabetes are
responsible for high morbidity and mortality rates worldwide. Obesity is characterized by a pro‐
inflammatory activation status of immune cells within the adipose tissue, but also in liver, pancreas,
muscle and hypothalamus [1]. Cells from both the innate and adaptive immune system, especially
adipose tissue macrophages, but also CD8+, CD4+ effector and regulatory T cells, are crucial in the
pathogenesis of obesity and its metabolic complications [1, 2]. Interestingly, in experimental‐ as well
as human obesity, an increase in the circulating pool of these (activated) immune cells has been
observed [3‐5].
The primary site of immune cell production is the bone marrow (BM), where the most primitive
precursors, the hematopoietic stem and progenitor cells (HSPCs), reside. These cells, defined as
lineage‐Sca‐1+c‐Kit+ (LSK) cells, are located in a specialized BM microenvironment, which critically
regulates self‐renewal, quiescence, differentiation and mobilization of the HSPCs via growth factors
(including stem cell factor; SCF, macrophage‐, granulocyte macrophage‐, granulocyte‐, colony
stimulating factor; M‐CSF, GM‐CSF, G‐CSF), chemokines, cytokines (including TNF, IFNγ, IL‐6), cell
cycle regulators (cyclins and tumor suppressor genes) and adhesion signals [6, 7]. Hence, HSPC
quiescence and proliferation is tightly regulated to prevent stem cell exhaustion, thereby ensuring
lifelong hematopoiesis [8].
The most primitive hematopoietic stem cells are defined by the cell surface marker CD150 and can
give rise to an active self‐renewing HSPC population [9]. The acquisition of CD34 is one of the earliest
events during activation of HSPCs from their dormant state. Further differentiation is marked by loss
of CD150 and gain of CD135, resulting in differentiating progenitor cells [9, 10]. Loss of CD150
expressing cells is therefore functionally associated with less self‐renewal capacity.
Hematopoietic stressors, such as inflammation, affect HSPC homeostasis. HSPCs directly respond to
cytokines, such as IFNγ, TNF, IL‐1 and IL‐6 [11‐16]. For example, Escherichia coli infection leads to an
expansion of the HSPC population via LPS‐induced TNF and NF‐κB signaling [17, 18]. Furthermore,
pseudomonas aeruginosa and mycobacterium avium infections are associated with defective stem
cell activity and BM from infected mice does not engraft well after transplantation [14, 19]. These
studies suggest that immune‐mediated BM exhaustion can result from persistent activation of
quiescent HSPCs, thus depleting the total HSPC population over time.
Besides acute inflammation during infection, chronic inflammatory conditions also alter HSPC
biology. We and others have reported that atherosclerosis, a chronic inflammatory disease of the
arteries, is associated with hypercholesterolemia‐driven expansion of the HSPC population in BM of
LDLr‐/‐ mice, especially of the myeloid population [20, 21]. Mouse models of extreme obesity,
including leptin deficient Ob/Ob mice, as well as C57Bl/6 mice on a 60% high fat diet (HFD), also
present with an increase in progenitor cells in the BM [5, 22]. However, the precise effects of obesity
on the HSPC population remain elusive. We therefore investigated how an obesogenic diet affects
the HSPC population in the BM over the course of 18 weeks, using a relatively mild model (45% kcal
from fat) of diet‐induced obesity (DIO) in mice.
Chapter 5
84
Materials and methods
Animals Age‐matched male C57Bl/6 mice (Charles River) were included in the experiments at the age of 7
weeks, at which time the first group of mice received a HFD (45% kcal fat, 35% kcal carbohydrate,
20% kcal protein, Special Diets Services, Witham, United Kingdom) and all other groups were kept on
a chow diet. All mice were given a HFD in a time course ranging from 1 day up to 18 weeks before
sacrifice. All mice were sacrificed at the age of 25 weeks, regardless of the length of obesogenic diet.
Mice had ad libitum access to food and water and were maintained under a 12h light‐dark cycle.
Mice were fasted overnight and subsequently euthanized. Glucose levels were measured from whole
blood using a glucometer (Bayercontour, Basel, Switzerland). Blood was obtained by cardiac
puncture using EDTA‐filled syringes and organs (BM and epididymal adipose tissue; EpAT) were
dissected and processed for flow cytometric analysis. All experimental procedures were approved by
the Animal Experimentation Ethics Committee of the University of Amsterdam.
Hematological and lipoprotein measurements
Hematological analysis was performed on a ScilVet abc plus+ (ScilVet, Oostelbeers, The Netherlands).
Plasma triglyceride and cholesterol levels were measured by fast‐performance liquid
chromatography, as described previously [20].
Flow cytometry
BM was harvested in cold PBS. BM cell suspension was passed through a 70 µm nylon mesh (BD
Biosciences, Breda, the Netherlands). Lineage depletion was performed for the HSPC analysis by
magnetic bead isolation according to the manufacturer’s instructions (Lineage Cell Depletion Kit;
Miltenyi Biotec, Teterow, Germany). EpAT was rinsed in PBS, minced into small pieces, digested in a
collagenase mixture (DMEM‐20 mM HEPES, Collagenase I and XI, Sigma‐Aldrich, Zwijndrecht, the
Netherlands) for 45 minutes at 37°C, passed through a 70 µm nylon mesh (BD Biosciences) and
centrifuged at 1,250 rpm for 6 minutes. The pelleted SVF was resuspended in FACS buffer (0.5% BSA
in PBS). Blood and BM cell suspensions were incubated with hypotonic lysis buffer (8.4 g NH4Cl and
0.84 g NaHCO3 per liter distilled water) to remove erythrocytes. Cell suspensions were incubated
with an Fc‐receptor blocking antibody to prevent non‐specific binding. CD3, CD8, CD25, FoxP3, F4/80,
CD11b, CD11c, Gr‐1, CD45.1, B220 (eBioscience, San Diego, CA, USA), CD4 (BD Biosciences) and
CD45, Ly6G, CD206 (Biolegend, San Diego, CA, USA) antibodies were incubated with the indicated
tissues. BM cells were characterized using antibodies for CD5, CD11b, Ter119, Sca‐1, CD34, CD16/32,
CD127 (eBioscience), B220 (BD Biosciences) and Ly6G, c‐Kit, CD150, CD135 (Biolegend). Staining was
analyzed by FACS (FACSCanto II, BD Biosciences) and FlowJo software version 7.6.5. (Tree star).
FACS analysis on mature hematopoietic cells in blood started by first gating on the total CD45+
leukocyte population. Myeloid cells were selected on CD11b+. Within the CD11b+ population,
monocytes were selected by excluding Ly6G+ neutrophils and pro‐inflammatory monocytes were
then defined by the remaining Gr1+ population, as Gr1 binds both Ly6C as Ly6G. Lymphoid cells were
selected on CD3+, then gated for CD8+CD4‐ (cytotoxic T cells) and CD8‐CD4+ (T helper cells). Within
this T helper cell population we characterized regulatory T cells by CD25+FoxP3+.
DIO in mice decreases HSPCs in the BM
85
BrdU labelling
Mice were injected intraperitoneally with 0.2 mg/g BrdU. BM was collected 16 hours later, and Lin‐
cells were isolated. BrdU incorporation was determined by intracellular staining with anti‐BrdU
antibodies, using the FITC BrdU Flow Kit (BD Biosciences).
Propidium iodide (PI) staining
For cell‐cycle analysis, Lin‐ BM cells were stained with the primary antibodies, fixed in 70% ethanol
for 48 hours, and treated with PI RNase buffer (BD Biosciences).
Colony‐forming unit (CFU) assays
BM was isolated and 1x104 BM cells were cultured in 2 ml semisolid methylcellulose medium
supplemented with growth factors (MethoCult; Stem Cell Technologies, Grenoble, France) at 37°C in
98% humidity and 5% CO2 for 7 days. The amount of lymphoid‐, myeloid‐, granulocyte‐ and monocyte
colonies was counted by T.S., who was blinded for the experimental conditions.
Competitive Bone Marrow Transplantation (cBMT)
C57Bl/6 CD45.2 recipient mice were housed in filter‐top cages and received antibiotics in their
drinking water (60 U/ml polymyxin B sulfate, Invitrogen, Carlsbad, CA, USA and 100 g/ml neomycin,
Sigma) 1 week pre‐BMT and 5 weeks post‐BMT. The mice received 2x6 Gy total body irradiation on
two consecutive days. BM was isolated from age‐matched donor C57Bl/6 CD45.1 mice fed either
chow or 45% HFD for 10 weeks, and mixed 2:1 with BM from donor C57Bl/6 CD45.2 chow fed mice.
Donor BM was injected intravenously. Starting 4 weeks post‐BMT, blood samples were taken every 3
weeks to analyze the reconstitution of peripheral blood leukocytes by flow cytometry.
Quantitative PCR
Total RNA was extracted using TRIzol (Invitrogen) and reverse transcribed with an iScript cDNA
synthesis kit (Bio‐Rad, Veenendaal, the Netherlands). The quantitative PCR was performed using a
SYBR green PCR kit and a ViiA7 RT‐PCR system (Applied Biosystems, Leusden, the Netherlands). The
result is expressed as relative to the control group, which was assigned a value of 1.
Statistics
Results are presented as mean ± SEM. Analysis between two groups was done by a Student’s t test,
more than two groups by a one‐way ANOVA with Tukey post‐test analysis and the change per group
expressed relative to the control group. The repeated measures on blood leukocytes in the cBMT
experiments were analyzed by a two‐way ANOVA with matched values and a bonferroni multiple
comparison test. Statistics were calculated in GraphPad Prism 5.0 (GraphPad Software, Inc., La Jolla,
CA, USA). P‐values <0.05 were considered significant.
Results
A 45% obesogenic diet induces a pro‐inflammatory immune cell profile in the peripheral blood
Eight groups of age‐matched C57Bl/6 mice were subjected to different durations of obesogenic diet
(Fig. 1A for a graphic study design). Body weight increased after 2 weeks of the obesogenic diet
(Table 1). Blood glucose levels increased in the HFD groups (Table 1). Total plasma cholesterol levels
(in VLDL, LDL and HDL) increased after 3 days, and plasma triglyceride levels (in VLDL) were elevated
after 2 weeks of HFD feeding (Table 1) compared to the 0 days HFD group.
Chapter 5
86
Table 1. General characteristics of mice in the course of DIO Weeks of HFD
Mean ± SEM (p)a
0 1/7 3/7 1 2 4 10 18
Body weight, g 27.3 ± 0.6
29.5 ± 0.6
31.1 ± 0.8
31.5 ± 0.5
34.4 ± 0.9(***)
36.5 ± 1.2(***)
38.1 ± 0.9(***)
41.3 ± 2.0 (***)
Glucose, mg/dl 69 ± 3
130 ± 15 (***)
118 ± 7 (**)
123 ± 4 (***)
104 ± 5 (*)
91 ± 4
112 ± 9 (**)
124 ± 7 (***)
Triglycerides, µM 203 ± 19
253 ± 51
405 ± 87
343 ± 60
508 ± 71 (**)
810 ± 82 (***)
535 ± 28 (**)
603 ± 47 (***)
VLDL 100 ± 15
133 ± 17
283 ± 56
253 ± 61
408 ± 55 (**)
668 ± 90 (***)
423 ± 24 (***)
473 ± 46 (***)
LDL 90 ± 10
110 ± 30
103 ± 27
75 ± 3
88 ± 14
128 ± 26
95 ± 10
112 ± 6
HDL 13 ± 2
10 ± 4
20 ± 7
15 ± 5
13 ± 3
15 ± 3
18 ± 5
18 ± 2
Cholesterol, mM 1.75 ± 0.13
2.40 ± 0.17
2.92 ± 0.18 (***)
2.88 ± 0.26 (***)
2.83 ± 0.13 (**)
2.87 ± 0.27 (***)
3.27 ± 0.08 (***)
3.32 ± 0.08 (***)
VLDL 0.02 ± 0.004
0.02 ± 0.003
0.03 ± 0.009
0.03 ± 0.008
0.05 ± 0.007
0.09 ± 0.020 (***)
0.06 ± 0.005 (*)
0.06 ± 0.006 (*)
LDL 0.27 ± 0.03
0.42 ± 0.03
0.81 ± 0.11 (**)
0.68 ± 0.08 (*)
0.74 ± 0.13 (**)
0.76 ± 0.14 (**)
0.89 ± 0.09 (***)
1.00 ± 0.06 (***)
HDL 1.46 ± 0.13
1.97 ± 0.14
2.07 ± 0.16 (*)
2.17 ± 0.19 (**)
2.05 ± 0.08 (*)
2.03 ± 0.15 (*)
2.32 ± 0.08 (***)
2.26 ± 0.04 (***)
aData in this table is analyzed by a one‐way ANOVA with Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group. n=10‐11, *=P<0.05, **=P<0.01, ***=P<0.001.
Total blood leukocyte counts did not differ between the groups (Fig. 1B). The percentage of
CD11b+Ly6G‐ monocytes as well as the percentage of CD3+ T cells was not affected by the obesogenic
diet (Fig. 1C,D). However, both the myeloid and lymphoid fraction showed a pro‐inflammatory
profile. Starting at 4 weeks of HFD, the number of pro‐inflammatory Gr1high monocytes (Fig. 1E) and
number of cytotoxic CD8+ T cells, the T cell fraction considered to induce AT inflammation and insulin
resistance [23], had increased (Fig. 1F). This was accompanied by a relative decrease in CD4+ T helper
cells (Fig. 1G). The regulatory T cell fraction showed a slight increase during the first few weeks of
HFD, but decreased again with more advanced obesity (Fig. 1H). Similar results were obtained in
spleen (Supplemental Fig. 1).
An obesogenic diet causes a switch from quiescent to differentiating LSK cells
Since the obesogenic diet affects the mature immune cell populations, we investigated the effects of
an obesogenic diet on HSPC biology. HSPCs were defined as Lin‐Sca‐1+c‐Kit+ (LSK) cells and within this
LSK population we studied different subsets; long‐term hematopoietic stem cells (LT‐HSCs; Lin‐Sca‐
1+c‐Kit+CD150+CD34‐), short‐term hematopoietic stem cells (ST‐HSCs; Lin‐Sca‐1+c‐Kit+CD150+CD34+),
as well as the early multipotent progenitors (E‐MPPs; Lin‐Sca‐1+c‐Kit+CD150‐CD135‐), and late
multipotent progenitors (L‐MPPs; Lin‐Sca‐1+c‐Kit+CD150‐CD135+)(Fig. 2A for an overview) [20].
Interestingly, within the LSK population, the obesogenic diet relatively decreased the ST‐HSCs after 4
weeks whereas the percentage of LT‐HSC fraction was not affected (Fig. 2B,C). Remarkably, the
percentage of E‐MPPs was increased already after 3 days of HFD feeding and remained increased
DIO in mice decreases HSPCs in the BM
87
over the course of obesity (Fig. 2D). We also observed a small decrease in percentage of L‐MPP after
10 weeks of HFD (Fig. 2E). LT‐ and ST‐HSCs are capable of self‐renewal, however, this characteristic is
lost when they mature and differentiate to multipotent progenitors. Our data thus indicate that an
obesogenic diet shifts the characteristics of early HSPCs from a quiescent population capable of self‐
renewal, towards a more mature HSPC population with increased differentiation potential.
Figure 1. Pro‐inflammatory immune cell profile in peripheral blood by FACS in a time course of DIO. (a) Study design. (b) No changes in total blood leukocyte counts or (c) percentage of CD11b+Ly6G‐ monocytes and (d) CD3+ T cells of the total leukocyte population (CD45+). (e) After 4 weeks of the obesogenic diet, pro‐inflammatory Gr1high monocytes increased, as well as (f) CD8+ cytotoxic T cells. (g) CD4+ T helper cells decreased, whereas (h) the regulatory CD4
+CD25+FoxP3+ T cells increased after 1 week of HFD but stabilized again in the course of DIO. n=10‐11, *=P<0.05, **=P<0.01 (1‐way ANOVA with Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group).
AWeeks of HFD
0
1/7
3/7
1
2
4
10
18
HFD
Chow
†
7 weeks 25 weeksAge
Monocytes
0 1/7 3/7 1 2 4 10 180
5
10
15
20
Weeks of HFD
% C
D11
b+ Ly6
G- (
of C
D45
+ )
Pro-inflammatory monocytes
0 1/7 3/7 1 2 4 10 180
2
4
6
8
* *
Weeks of HFD
% G
r1hi
gh (o
f CD
45+ )
Blood leukocyte count
0 1/7 3/7 1 2 4 10 180
1
2
3
4
5
Weeks of HFD
WB
C 1
09 /l
T cells
0 1/7 3/7 1 2 4 10 180
10
20
30
40
Weeks of HFD
% C
D3+ (
of C
D45
+ )
Helper T cells
0 1/7 3/7 1 2 4 10 180
10
20
30
40
50
**
** **
Weeks of HFD
% C
D4+ (
of C
D45
+ CD
3+ )
Cytotoxic T cells
0 1/7 3/7 1 2 4 10 180
10
20
30
40 * *
Weeks of HFD
% C
D8+ (
of C
D45
+ CD
3+ )
Regulatory T cells
0 1/7 3/7 1 2 4 10 180
5
10
15
**
Weeks of HFD
% C
D25
+ Fox
P3+ (
of C
D4+ )
C D
E F
G
B
H
Chapter 5
88
When culturing BM from the obesogenic fed mice in a CFU assay ex vivo, the increase in
differentiation potential further pushed the cells towards differentiation, as indicated by an increase
in lymphoid colony forming potential of the BM of mice receiving an obesogenic diet which was
already increased after 3 days and continued to increase over the course of obesity (Fig. 2F). The
myeloid colony forming potential of BM cells increased after a more prolonged duration of the
obesogenic diet, resulting in an increase in myeloid and monocyte forming colonies after 18 weeks of
HFD feeding, when obesity was established (Fig. 2G‐I).
HSPCs of obesogenic mice exhibit decreased proliferation
Differentiating LSK cells loose Sca‐1 (LK cells) and/or c‐Kit (LS cells) and start to express markers for
common myeloid progenitors (CMPs; Lin‐Sca‐1‐c‐Kit+CD34+CD16/32low) or common lymphoid
progenitors (CLP; Lin‐Sca‐1+c‐Kit‐CD127+) (Fig. 3A for an overview) [10, 20].
Granulocytes
0 1/7 3/7 1 2 4 10 180
5
10
15
20
Weeks of HFD
Colo
nies
(n)
Monocytes
0 1/7 3/7 1 2 4 10 180
20
40
60 *
Weeks of HFDCo
loni
es (n
)
Myeloid
0 1/7 3/7 1 2 4 10 180
20
40
60
80
100*
Weeks of HFD
Colo
nies
(n)
Lymphoid
0 1/7 3/7 1 2 4 10 180
50
100
150
*
*****
*
Weeks of HFD
Colo
nies
(n)
L-MMPs in BM
0 1/7 3/7 1 2 4 10 180
10
20
30
40
*
Weeks of HFD
% C
D150
- CD13
5+ (of L
SK)
E-MMPs in BM
0 1/7 3/7 1 2 4 10 180
20
40
60
80
* *
******
***
Weeks of HFD
% C
D150
- CD13
5- (of L
SK)
ST-HSCs in BM
0 1/7 3/7 1 2 4 10 180
1
2
3
4
5
****** **
Weeks of HFD
% C
D150
+ CD34
+ (o
f LSK
)
LT-HSCs in BM
0 1/7 3/7 1 2 4 10 180
10
20
30
40
Weeks of HFD
% C
D150
+ CD34
- (of L
SK)
A B C
D E
F G IH
LT- HSC
ST-HSC
E-MPP
L-MPP
CD150+CD34-
CD150+CD34+
CD150-CD135-
CD150-CD135+
Prim
itivity
Figure 2. DIO shifts early HSPCs towards a more mature HSPC population with higher differentiation potential. (a) Schematic overview of HSPC maturation in the BM. (b) FACS data of LT‐HSC, (c) ST‐HSC, (d) E‐MMP, (e) L‐MMP of all LSK cells showing a decrease of ST‐HSC after 4, 10 and 18 weeks of the obesogenic diet, together with an increase in E‐MMP cells after 3 days, and 2 to 18 weeks of HFD. (f‐i) CFU assays of BM from mice on an obesogenic diet showing increased potential to form lymphoid and myeloid colonies. n=10‐11, *=P<0.05, **=P<0.01, ***=P<0.001 (1‐way ANOVA with Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group).
DIO in mice decreases HSPCs in the BM
89
Figure 3. LSK cell maturation in BM of obesogenic mice. (a) Schematic overview of LSK cell maturation. (b) The percentage LSK cells of all Lin‐ cells in the BM did not differ in the course of DIO. (c) Decreased myeloid progenitors (LK cells) after 4, 10 and 18 weeks and (d) a decrease in lymphoid progenitors (LS cells) after 18 weeks of obesogenic diet. (e) A decrease in CMP was seen after 18 weeks and (f) a decrease in CLP after 10 weeks. n=10‐11, *=P<0.05, **=P<0.01, ***=P<0.001 (1‐way ANOVA followed by Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group).
LSK cells in BM
0 1/7 3/7 1 2 4 10 180.0
0.5
1.0
1.5
2.0
Weeks of HFD
% S
ca-1
+ c-Ki
t+ (of l
in- )
LS cells in BM
0 1/7 3/7 1 2 4 10 180
1
2
3
4
*
Weeks of HFD
% S
ca-1
+ (of l
in- )
LK cells in BM
0 1/7 3/7 1 2 4 10 180
10
20
30
40
50
** *
***
Weeks of HFD
% c
-Kit+ (o
f lin
- )
CMP cells in BM
0 1/7 3/7 1 2 4 10 180
10
20
30
*
Weeks of HFD
% C
D34+ CD
16/3
2+ (of L
K ce
lls)
CLP cells in BM
0 1/7 3/7 1 2 4 10 180
5
10
15
***
Weeks of HFD
% C
D127
+ (of L
SK c
ells
)
A B
C D
E F
LSK
CLPCMP
LSLK
Lin-Sca-1+c-Kit+
Lin-Sca-1+Lin-c-Kit+
CD127+CD34+CD16/32+
Prim
itivity
Chapter 5
90
Although we observed a shift within the total LSK population from early HSPC populations towards a
more differentiation‐ capable progenitor cell, which we confirmed by the increased lymphoid and
myeloid CFU potential, the fraction of LSK, LK and LS cells, as well as the fraction of CLP and CMP
remained stable or even decreased over the course of obesity (Fig. 3B‐F). This observation made us
hypothesize that although the HSPCs show an increased differentiation into mature immune cells,
they may suffer from exhaustion in an obesogenic environment. We therefore analyzed the
proliferative and reconstitution capacity of HSPCs primed by an obesogenic diet.
BrdU uptake in LSK cells in the BM was measured in mice that received the 45% obesogenic diet or a
chow diet for 18 weeks. Interestingly, Lin‐, LSK as well as LK cells exhibited a lower proliferation rate
in the HFD group compared to chow diet (Fig. 4A‐D). Furthermore, PI staining of Lin‐ cells in the BM
showed that in the obesogenic diet group, a higher percentage of cells was in the G0/G1 phase of the
cell cycle (Fig. 4e), and a lower percentage of cells was in the M phase (Fig. 4F,G).
Figure 4. Decreased proliferation of HSPCs in BM of obese mice after 18 weeks of obesogenic diet. FACS data showing a decrease in BrdU labelled (a) Lin‐ cells, (b) LSK cells and (c) LK cells, but no differences in (d) LS cells between obesogenic and chow fed mice. (e) Schematic overview of different phases in cell division. (f) Cell cycle analysis of the BM by DNA staining showing a higher percentage of cells in the G0/G1 resting phase and a lower percentage of cells in the G2/M dividing phase of the cell cycle in the HFD group compared to a chow diet. (g) A representative PI staining of the two experimental groups. n=9‐10, *=P<0.05, **=P<0.01 (Student’s t tests).
% proliferating LK cells
Chow 18w HFD0
10
20
30
40
50
**
% B
rdU+ c
ells
(of L
K ce
lls)
% proliferating LS cells
Chow 18w HFD0
20
40
60
80
100
% B
rdU+ c
ells
(of L
S ce
lls)
% proliferating lin- cells
Chow 18w HFD0
20
40
60
80
**
% B
rdU+
cel
ls (o
f Lin
- )
% proliferating LSK cells
Chow 18w HFD0
5
10
15
20
25
*
% B
rdU+ c
ells
(of L
SK)
A B C
D E
F
S G1
G2
M
Gap 1
DNA
synthesis
Gap 2
Mitosis
GGGChow
18w HFD
G0/G1 S/G2 M0
20
40
60
80Chow18w HFD
*
*
% o
f cel
ls
DIO in mice decreases HSPCs in the BM
91
Gene expression of cell cycle regulators, Cyclin D2, E1 and A2, which regulate G1 progression, G1‐S
transition and S phase progression respectively [24‐27], were decreased in obesogenic BM cells (Fig.
5A‐C). In line with this, tumor suppressor genes p15 and p27, which prevent cdk4 and cdk2 activation
via cyclin D and cyclin E interactions, were upregulated in the obesogenic BM, all reflecting
suppression of proliferation (Fig. 5D,E). Furthermore, cytokines Tnf, Il‐1β and Il‐6 (but not Ifnγ),
factors that have been shown to suppress HSPC proliferation [16], were increased in the HFD BM,
indicating alterations in the BM microenvironment that affect the proliferation of the HSPCs (Fig. 5F‐
I). We did not observe differences in Scf, M‐csf, Gm‐csf or G‐csf or their receptors (Fig. 5J,K), although
our data does suggest a slight increase in these growth factors upon a HFD, which may be sufficient
to stimulate differentiation of BM cells.
TNF
Chow 18w HFD0.0
0.5
1.0
1.5
2.0p=0.07
rela
tive
gene
exp
ress
ion
IL-1b
Chow 18w HFD0.0
0.5
1.0
1.5 *
rela
tive
gene
exp
ress
ion
IL-6
Chow 18w HFD0.0
0.5
1.0
1.5
2.0*
rela
tive
gene
exp
ress
ion
IFNy
Chow 18w HFD0
1
2
3
rela
tive
gene
exp
ress
ion
Cyclin D2
Chow 18w HFD0.0
0.5
1.0
1.5
**
rela
tive
gene
exp
ress
ion
Cyclin E1
Chow 18w HFD0.0
0.5
1.0
1.5
**
rela
tive
gene
exp
ress
ion
Cyclin A2
Chow 18w HFD0.0
0.5
1.0
1.5
**re
lativ
e ge
ne e
xpre
ssio
n
p15
Chow 18w HFD0
1
2
3 *
rela
tive
gene
exp
ress
ion
p27
Chow 18w HFD0.0
0.5
1.0
1.5 *
rela
tive
gene
exp
ress
ion
F G IH
A B C
D E
SCF M-CSF M-CSFr0.0
0.5
1.0
1.5Chow18w HFD
rela
tive
gene
exp
ress
ion
GM-CSF GM-CSFr G-CSF G-CSFr0
2
4
6
8Chow18w HFD
p=0.05
rela
tive
gene
exp
ress
ion
KJ
Figure 5. An obesogenic diet for 18 weeks induced alterations in cell cycle regulator and cytokine gene expression in the BM. (a‐c) Gene expression of cyclins was increased in obesogenic BM compared to chow fed mice. (d,e) Gene expression of tumor suppressor genes p15 and p27 was decreased in obesogenic BM. (f‐i) Cytokine gene expression of TNF, IL‐1β and IL‐6, but not IFNγ, was increased in BM from HFD mice. (j,k) Gene expression of SCF, M‐CSF, GM‐CSF or G‐CSF or their receptors were not significantly increased in HFD BM. n=16‐19 *=P<0.05, **=P<0.01 (Student’s t tests).
Chapter 5
92
Obesogenic‐primed HSPCs have decreased reconstitution capacity
The increase in differentiation potential of HSPCs in vivo in mice fed an obesogenic diet and a
decreased proliferation capacity led to a decrease in LK and LS cells. Interestingly, the decrease in
proliferation was lower than the decrease in cells. As the percentage of LSK cells decreased by 29%,
the proliferation in this population was decreased by 25%. For the LK and LS cells, the decrease in
cells was 52% and 23%, whereas the decrease in proliferation was 37% and 2.5% respectively. This
indicates that the decrease in these progenitor cell populations is not solely due to a decrease in
proliferation. Therefore, we hypothesized that an obesogenic environment can prime HSPCs
affecting their potential to initiate multilineage reconstitution. To test this hypothesis, a cBMT was
performed with BM obtained from either HFD or chow fed donor CD45.1 mice, which was mixed with
BM obtained from chow fed donor CD45.2 mice, and then transplanted in chow fed CD45.2
recipients (Fig. 6A). Reconstitution of immune cells was analyzed every three weeks starting 4 weeks
post‐BMT. We determined the percentage of CD45.1 cells within different leukocyte subsets, which
had reconstituted from the BM of mice fed either an obesogenic or chow diet. We observed that the
percentage of CD45.1 T cells, B cells, neutrophils, monocytes, Gr1high and Gr1low monocytes on all
time points was lower in mice that received BM from an obesogenic donor than mice that received
BM from a chow fed donor (Fig. 6B‐G). Furthermore, upon sacrifice, the percentage of CD8+ cytotoxic
T cells, CD4+CD25+FoxP3+ regulatory T cells, monocytes and both M1‐ and M2‐like macrophages
originating from the obesogenic donor BM was lower in EpAT compared to the chow donor BM (Fig.
6H,I). This indicates that obesogenic‐primed BM is less capable of immune cell reconstitution in
blood and tissues.
As the recipient mice did not receive a HFD, these results show that HSPCs subjected to an
obesogenic diet are primed and have long‐term central effects on HSPC biology. The shift we
observed from an early quiescent self‐renewing HSPC population towards mature differentiating
HSPCs, and a decreased potential to initiate multilineage reconstitution implies impaired self‐renewal
capacity.
DIO in mice decreases HSPCs in the BM
93
Figure 6. Less immune cell reconstitution of obesogenic‐primed BM in a cBMT. (a) Schematic overview of a cBMT in which CD45.1 BM from age‐matched 10 weeks HFD or 10 weeks chow fed donor mice was mixed with CD45.2 BM from chow donor mice and transplanted into chow CD45.2 recipients. (b) Blood FACS data showing that transplanting obesogenic‐primed BM decreased the reconstitution of CD45.1
+ T cells, (c) CD45.1+ B cells, (d) CD45.1+ monocytes, (e) CD45.1
+ neutrophils, (f) CD45.1+ inflammatory Gr1high monocytes and (g) CD45.1+ Gr1low monocytes compared to transplantation of chow BM. (h,i) Reconstitution of CD45.1 lymphoid and myeloid cells in EpAT. Mo=monocytes (CD11b
+Ly6G
‐), Ma=macrophages (CD11b
+F4/80
+), M1=M1‐like macrophages (CD11b
+F4/80
+CD11c
+CD206
‐), M2=M2‐like macrophages
(CD11b+F4/80
+CD11c
‐CD206
+). n =14 *=P<0.05, **=P<0.01, ***=P<0.001, ****=P<0.0001 (b‐g: two‐way ANOVA with matched values
followed by bonferroni multiple comparison test. h,i: Student’s t tests).
EpAT
CD4+ CD8+ CD25+FoxP3+0
20
40
60
**
*% C
D45
.1
EpAT
Mo Ma M1 M20
20
40
60
80ChowHFD
* * **
% C
D45
.1
4w 6w 9w 12w0
20
40
60
80
* ** ***
**
Time
% C
D45
.1 (o
f CD
3+ B22
0- )
4w 6w 9w 12w0
20
40
60
80ChowHFD
******* ***
**
Time
% C
D45
.1 (o
f CD
11b+ L
y6G
+ )
4w 6w 9w 12w0
20
40
60
80ChowHFD
** ** ***
Time
% C
D45
.1 (o
f CD
11b+ G
r1lo
w)
F
H
A B
D
DonorsBone marrow
mixed 2:1
Recipients
Lean
CD45.1
Lean
CD45.2Lean
CD45.2
Obese
CD45.1
Lean
CD45.2
Lean
CD45.2
+
+
I
4w 6w 9w 12w0
20
40
60
80ChowHFD
************
****
Time
% C
D45
.1 (o
f B22
0+ CD
3- )
4w 6w 9w 12w0
20
40
60
80****
** ***
Time
% C
D45
.1 (o
f CD
11b+ L
y6G
- )
4w 6w 9w 12w0
20
40
60
80
***** ** **
Time
% C
D45
.1 (o
f CD
11b+ G
r1+ )
G
C
E
Chapter 5
94
Discussion
Mild chronic inflammation associated with obesity is a stressor for HSPCs in the BM. Here we show
that a 45% obesogenic diet causes a switch from quiescent to differentiating HSPCs. The loss of self‐
renewing characteristics and a decrease in proliferation results in impaired multilineage
reconstitution, revealing that an obesogenic environment has large effects on HSPC dynamics.
In addition to the traditional comorbidities of obesity, studies in humans as well as animal models
have demonstrated that obesity causes impaired immune function, leading to an increased
susceptibility to infectious diseases [28‐32]. Signals in the BM microenvironment regulate HSPC
dynamics. The composition of the BM is therefore essential for a correct functioning of the
hematopoietic system. An increase in visceral fat in obesity is positively correlated with BM adiposity
[33]. Consequently, adipocyte infiltration and accumulation in the BM can affect hematopoietic
maintenance and differentiation [34]. Mesenchymal stem cells (MSC) in the BM are also involved in
hematopoietic niches and microenvironments. A HFD induces alterations in MSCs, including
increased production of cytokines IL‐1, IL‐6, and TNF. These signals may influence the BM
microenvironment and modulate hematopoiesis, and consequently hamper proper functioning of
the immune system [35].
Besides dyslipidemia, cholesterol and glucose metabolism appear to be critical for proper
maintenance of the HSPC population. Mice with defective cholesterol efflux pathways due to a
deficiency of apolipoprotein E or abca1/abcg1 have increased levels of growth factors in the BM,
which causes the HSPCs to proliferate and expand, leading to leukocytosis [36‐38]. Interestingly, in a
previous study by our group using a model of hypercholesterolemia in LDLR‐/‐ mice, we also
observed an increased expression of inflammatory cytokines in the BM, including Tnf, Il‐1β and Il‐6
[20] . However, here we show alterations in the HSPC population in DIO that opposite to our findings
in hypercholesterolemic LDLR‐/‐ mice. Where the 45% HFD caused decreased proliferation of the LSK
cells and decreased reconstitution in a cBMT, hypercholesterolemia resulted in increased
proliferation and increased reconstitution in a cBMT [20]. Using a highly similar experimental setup in
the same lab, the major differences are the diet and mouse model with 45% kcal from fat in C57Bl/6
mice vs a hypercholesterolemic diet, containing 0.15% cholesterol, in LDLR‐/‐ mice. Full knock out
models for defective cholesterol efflux pathways have a different lipoprotein metabolism which
causes systemic hypercholesterolemia but can also directly affect the HSPCs. Our data show that an
obesogenic diet results in signals that causes HSPCs to differentiate; factors that are independent
from the genotype of the HSPCs and that are dominant over signals that could stimulate
proliferation. Furthermore, hypercholesterolemia alone is potentially a less severe stressor
compared to the systemic inflammation seen in obesity, which does stimulate leukocytosis but does
not deplete the HSPC pool.
Two models of obesity, leptin deficient Ob/Ob mice, as well as C57Bl/6 mice on a 60% HFD for 20
weeks, show a relative increase in CMP and GMPs in the BM, with Ob/Ob mice also having an
expanded HSPC population [5]. The 45% HFD in our milder obesity model resulted in a relative
decrease in CLP and CMP cells and their lymphoid and myeloid progenitors (LK and LS cells). One
week of 60% HFD has been shown to trigger a transient depletion of LT‐HSCs followed by a later
increase in LT‐HSCs and MPPs [22]. The 45% HFD may not have been as robust as a 60% HFD to
DIO in mice decreases HSPCs in the BM
95
deplete the LT‐HSC pool early in the course of HFD [22]. Furthermore, leukocytosis in our study is less
severe than reported in Ob/Ob mice or in C57Bl/6 on a 60% HFD. However, in our model, after 4
weeks of HFD, we did observe a decrease in hematopoietic stem cells, possibly due to a gradual
depletion of the HSPC pool. Nagareddy et al also show that lowering glucose with a sodium glucose
cotransporter 2 inhibitor in type 2 diabetic obese mice does not correct leukocytosis, indicating that
hyperglycemia by itself is not the major driver of leukocyte production [21]. Another difference in
these diets is the carbohydrate content. As more kcal are coming from fat, the relative carbohydrate
content is less (35% in our 45% diet versus 20% in the 60% HFD). To distinguish to contribution of
different nutrients on HSPC dynamics would be interesting for future studies.
When we transplanted BM from either lean or obese mice, competitively mixed with BM from lean
mice, into lean recipient mice, the HFD‐primed BM was less capable of systemic as well as tissue
leukocyte reconstitution. This indicates that priming of the HSPCs by a HFD cannot be reversed by a
lean BM niche. In weight loss experiments, returning mice from a HFD to a chow diet results in
normalization of body weight and glucose tolerance and normalizes the number of LT‐HSPCs [22].
However, the quantity of granulocyte‐, macrophages progenitors and Lin‐ cells remained elevated in
the BM, further suggesting long term, persisting, dietary effects on the HSPCs [22].
Within the LSK population, DIO induces a switch in cells that are capable of self‐renewal (the LT‐ and
ST‐HSCs) towards immune progenitor cells with a higher differentiation potential (E‐MPP and L‐
MPPs), as demonstrated by a loss of CD150 expression. Our data on a relative increase in LSK cells
with a higher differentiation potential is in line with data from Singer et al [22] who transplanted
4000 sorted LSK cells from either lean or obese donors in lean recipient mice, which increased
myelopoiesis in mice that received obese donor LSK cells. This shows that when transplanting an
absolute number of LSK cells, the population also includes relatively more cells with maturation
potential leading to an increased mature leukocytes in the circulation [22].
We observed an increase in ex vivo lymphoid and myeloid colony forming potential in the course of
DIO. Taking cells out of their obesogenic BM environment possibly removes the suppressive effects
on proliferation leaving the increased potential to differentiation, resulting in an increased number of
colonies in our CFU assays. This increased differentiation combined with decreased proliferation in
vivo, leads to a decreased supply and a reduction in progenitor cell populations. Interestingly,
transplantation of obesogenic BM into a normolipidemic environment revealed a decrease in
multilineage reconstitution of obesogenic primed HSPCs, most likely resulting from the increased
quiescence of the HSPC. However, as we observed that the decrease in progenitor cell populations
was not solely due to a decrease in proliferation, the additional effects observed after obesogenic
diet priming may still result in an increased differentiation potential of the HSPCs.
Our results and that of others show that the BM pool is altered in the low‐grade inflammatory state
associated with obesity. Different nutritional and/or genetic models can induce a variety and even
opposing effects on the HSPCs. Signals from different tissues act on the hematopoietic system, which
are influenced by hypercholesterolemia, dyslipidemia and hyperglycemia. Which model or dietary
component contains the most dominant signals remains to be determined and should be taken into
account when speculating on a potential therapy in humans.
Chapter 5
96
The present study demonstrates that an obesogenic diet induces long‐term alterations in the
hematopoietic system including loss of stemcellness and loss of self‐renewal, which may result in a
depletion of the most primitive HSPCs. This may consequently cause disturbed immunological
responses to infections and contribute to the persistence and/or progression of chronic
inflammatory diseases. Furthermore, as an obesogenic diet induces long‐term, cell‐intrinsic
alterations in HSPCs, obesity may hamper proper functioning of the immune system, even after
successful weight loss.
DIO in mice decreases HSPCs in the BM
97
Corresponding author Prof. Esther Lutgens, Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical Centre,
University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands. +31 (0)20 5 66 33 80.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements We acknowledge the support from the Netherlands Organization for Scientific Research (NWO)(VICI grant to E.L.), the
Rembrandt foundation (S.B., M.W., E.L.), the Dutch Heart Foundation (Dr. E. Dekker MD‐grant to T.S.), the Netherlands
CardioVascular Research Initiative (CVON2011‐19) and the Deutsche Forschungsgemeinschaft (DFG) (SFB1054‐B04 to E.L.
and SFB1123‐A5 to E.L.).
Chapter 5
98
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Chapter 5
100
Monocytes
0 1/7 3/7 1 2 4 10 180
20
40
60
Weeks of HFD
% C
D11
b+ Ly6
G- (
of C
D45
+ )
Pro-inflammatory monocytes
0 1/7 3/7 1 2 4 10 180
2
4
6
8
Weeks of HFD
% G
r1hi
gh (o
f CD
45+ )
T cells
0 1/7 3/7 1 2 4 10 180
20
40
60
Weeks of HFD
% C
D3+ (
of C
D45
+ )
Helper T cells
0 1/7 3/7 1 2 4 10 180
20
40
60
Weeks of HFD
% C
D4+ (
of C
D45
+ CD
3+ )
Cytotoxic T cells
0 1/7 3/7 1 2 4 10 180
10
20
30
40**
Weeks of HFD
% C
D8+ (
of C
D45
+ CD
3+ )
Regulatory T cells
0 1/7 3/7 1 2 4 10 180
5
10
15
* * *
Weeks of HFD
% C
D25
+ Fox
P3+ (
of C
D4+ )
A
C D
E F
B
Supplementary data
Supplementary Figure 1. Spleen immune cell profile by FACS in a time course of DIO. (a) No changes in percentage of CD11b+Ly6G‐ monocytes and (b) CD3+ T cells of the total leukocyte population (CD45+). (c) The pro‐inflammatory Gr1high monocytes were unchanges in the spleen after an obesogenic diet. (d) After 18 weeks of the obesogenic diet CD8
+ cytotoxic T cells were increased. (e) CD4
+ T helper cells were unaltered, whereas (f) the regulatory CD4+CD25+FoxP3+ T cells decreased only after short term HFD. n=10‐11, *=P<0.05 (1‐way ANOVA with Tukey post‐test analysis and the change per group expressed relative to the 0 weeks of HFD group).
6
BlockingCD40‐TRAF6interactionsbysmallmoleculeinhibitor6860766amelioratesthecomplicationsofdiet‐inducedobesityinmice
Susan M. van den Berg*,1, Tom T.P. Seijkens*,1, Pascal J.H. Kusters1, Barbara Zarzycka2, Linda
Beckers1, Myrthe den Toom1, Marion J.J. Gijbels1,3,4, Antonios Chatzigeorgiou5, Christian Weber2,6,
Menno P.J. de Winther1, Triantafyllos Chavakis5, Gerry A.F. Nicolaes2, Esther Lutgens1,6
1Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical Centre, University of Amsterdam,
The Netherlands. email: [email protected] 2Department of Biochemistry, University of Maastricht, Maastricht, The Netherlands 3Department of Pathology, Maastricht University, Maastricht, The Netherlands 4Department of Molecular Genetics, Maastricht University, The Netherlands 5Department of Clinical Pathobiochemistry and Institute for Clinical Chemistry and Laboratory Medicine, Medical Faculty,
Technische Universität Dresden, Dresden, Germany 6Institute for Cardiovascular Prevention (IPEK), Ludwig Maximilian’s University, Munich, Germany
* these authors contributed equally to this work.
International Journal of Obesity 2015; 39(5):782‐790
Chapter 6
102
Abstract Immune processes contribute to the development of obesity and its complications, such as insulin
resistance, type 2 diabetes mellitus and cardiovascular disease. Approaches that target the
inflammatory response are promising therapeutic strategies for obesity. In this context, we recently
demonstrated that the interaction between the co‐stimulatory protein CD40 and its downstream
adaptor protein Tumor necrosis factor Receptor Associated Factor (TRAF) 6 promotes adipose tissue
inflammation, insulin resistance and hepatic steatosis in mice in the course of diet‐induced obesity.
Here, we evaluated the effects of a small‐molecule inhibitor (SMI) of the CD40‐TRAF6 interaction,
SMI 6860766, on the development of obesity and its complications in mice that were subjected to
diet‐induced obesity (DIO). Treatment with SMI 6860766 did not result in differences in weight gain,
but improved glucose tolerance. Moreover, SMI 6860766 treatment reduced the amount of CD45+
leukocytes in the epididymal adipose tissue by 69%. Especially the number of adipose tissue CD4+
and CD8+ T cells and macrophages were significantly decreased. Our results indicate that small‐
molecule mediated inhibition of the CD40‐TRAF6 interaction is a promising therapeutic strategy for
the treatment of metabolic complications of obesity by improving glucose tolerance, by reducing the
accumulation of immune cells to the adipose tissue as well as by skewing of the immune response
towards a more anti‐inflammatory profile.
SMI‐mediated CD40‐TRAF6 inhibition in obesity
103
Introduction
Obesity and its associated conditions, including insulin resistance, type 2 diabetes mellitus, and
cardiovascular diseases (CVD), affect more than 1000 million people worldwide, a number that is
increasing and is expected to do so for the next decades [1, 2]. Chronic caloric excess may shorten
healthy lifespan of humans by 5‐20 years, which results in a tremendous socio‐economic burden [1].
Hence the development of novel therapeutic strategies for obesity and its related disorders is a
public health priority.
Over the past decades, chronic inflammation has been identified as the pathological substrate of
obesity and its complications [1, 3, 4]. Elevated plasma levels of cytokines, such as TNF and IL‐6,
characterize obese subjects and show a positive correlation with the extent of metabolic dysfunction
[5‐9]. Obese adipose tissue (AT) exhibits hallmarks of an ongoing inflammatory response, such as
immune cell infiltration and activation, as well as the presence of pro‐inflammatory cytokines and
adipokines including TNF, IL‐6, and leptin [5‐9]. Moreover, chronic inflammation has a pivotal role in
the development of obesity‐associated diseases, such as hepatic steatosis, type 2 diabetes mellitus
and CVD [5‐10]. Strategies that modulate the inflammatory response are therefore promising
therapeutic modalities for obesity and its complications.
The co‐stimulatory dyad CD40 and its ligand CD40L (CD154) have a well‐known role in immune cell
activation and inflammation [11]. After binding of CD40L, CD40 recruits adaptor proteins, the Tumor
necrosis factor Receptor Associated Factors (TRAFs) [11]. The intracellular domain of CD40 contains
two binding sites for these proteins: a proximal site that binds TRAF2/3/5 and a distal site, which
binds TRAF6 [11]. After binding of the TRAF proteins to CD40, signal transduction is initiated, which
eventually results in the expression of inflammatory mediators [11]. Mice with a genetic deficiency in
the CD40‐TRAF6 interaction, but not the CD40‐TRAF2/3/5 interaction, are protected against the
development of neointima formation and atherosclerosis, both exponents of an ongoing
inflammatory condition of the arterial wall [12, 13].
Accumulating evidence indicates that the CD40‐CD40L dyad has an important role in obesity [6, 14‐
17]. For example, plasma levels of soluble CD40L correlate with the body mass index in obese
subjects and decrease after bariatric surgery [14]. CD40‐CD40L‐mediated interactions between
adipocytes and AT immune cells, including T cells and macrophages, promote the expression of pro‐
inflammatory cytokines and chemokines that increase the recruitment of other inflammatory cells to
the AT [15, 16]. Ligation of CD40 on adipocytes promotes insulin resistance by decreasing IRS‐1 and
GLUT4 expression [15, 16]. Genetic deficiency or antibody‐mediated inhibition of CD40L reduced the
inflammatory and metabolic complications of diet‐induced obesity (DIO) [15, 16]. In contrast,
deficiency of CD40 aggravates the complications of DIO [18‐20]. To understand these opposing
effects of CD40L and CD40, we previously investigated the role of different downstream adaptor
proteins of the CD40‐CD40L pathway. We found that deficiency of CD40‐TRAF2/3/5 interactions
increased metabolic and inflammatory complications of DIO, whereas deficiency of CD40‐TRAF6
interactions rather improved AT inflammation and insulin resistance in DIO mice. Based on these
data, we developed a small molecule inhibitor (SMI) against CD40‐TRAF6 interactions, and we could
show that treatment with SMI 6877002 improved insulin resistance, hepatic steatosis and M1
Chapter 6
104
macrophage accumulation into the epididymal AT (EpAT), when treatment was initiated 6 weeks
after the beginning of the diet [19]. Thus, inhibition of the CD40‐TRAF6 interaction is a promising
therapeutic strategy for obesity.
Here we performed a more thorough analysis of the effects of SMI‐mediated CD40‐TRAF6 inhibition
in DIO. We investigated the effects of SMI 6860766, an analogue of 6877002 with a lower IC50 [19],
in DIO. In addition, we here administered the CD40‐TRAF6 inhibitor after 12 weeks of high fat diet
(HFD), when obesity was much more progressed and continued diet and treatment until week 18.
Our data suggest that SMI‐mediated CD40‐TRAF6 inhibition is a promising novel therapeutic
approach in DIO‐related metabolic dysregulation.
Materials and methods
TRAF 6 C‐domain expression, purification and binding analysis
His‐tagged TRAF6 C‐domain (residues 346‐504) was expressed in E. Coli using the pET21d expression
vector (Novagen). Protein was purified by affinity chromatography, followed by gel filtration in
running buffer (25 mM TRIS, 200mM NaCl and 0.5 mM TCEP). The direct binding between the TRAF6
C‐domain and SMI 6860766 was measured by Surface Plasmon Resonance (SPR) (Biacore T200, GE
Healthcare). TRAF6 C‐domain was immobilized on Sensor Chip CM5 using the amine coupling
method. This reached a density of approximately 12,000 and 7,500 RU. SMI 6860766 was dissolved in
PBS buffer containing 5% v/v DMSO. All measurements were carried out at 25°C and with a flow rate
of 50 ul min‐1 in SPR running buffer (PBS, 0.05% Tween20, 5% DMSO, pH=7.4). Sensorgrams were
corrected by subtracting the initial level of SPR signal before injection of the SMI or the TRAF6 C‐
domain. Data were analysed using the BIAevaluation software. Equilibrium dissociation constants
(Kd) were determined from a model of the steady state affinity (3 independent runs were averaged).
In vitro macrophage culture
Bone marrow (BM) cells were isolated from CD40+/+, CD40‐/‐ mice (C57Bl/6 background)[21] as well
as mice containing site directed mutagenesis for the respective CD40‐TRAF binding domains (CD40‐
Twt, CD40‐TRAF2/3/5‐/‐ and CD40‐TRAF6‐/‐) [22] and cultured in RPMI supplemented with 15% L929
conditioned medium to generate BM‐derived macrophages. BM‐derived macrophages were
activated with an agonistic CD40 antibody cocktail FGK45 and 5D12 (both 25 µg/ml, Bioceros BV)
overnight, incubated with SMI 6860766 for 1 hour and frozen for real time PCR analysis.
Mice
Male C57Bl/6 mice (n=24) were purchased from Charles River and maintained at the animal facility of
the University of Amsterdam. Mice received a HFD (35% kcal carbohydrate, 45% kcal fat, 20% kcal
protein, Special Diets Services, Witham, United Kingdom) for 18 weeks from the age of 7 weeks.
After 12 weeks of HFD, mice were treated daily with SMI 6860766 (10µmol kg‐1)(n=12), or vehicle
(0.05% tween80, 5% DMSO in PBS) [19] (n=12) for 6 weeks. Mice had ad libitum access to food and
water and were maintained under a 12 hour light‐dark cycle. Food intake and body weight were
measured weekly. After the experimental procedure, mice were fasted overnight and subsequently
euthanized. Blood was collected and organs were dissected and processed for flow cytometric and
histological analysis. All experimental procedures were approved by the Animal Experimentation
Ethics Committee of the University of Amsterdam.
SMI‐mediated CD40‐TRAF6 inhibition in obesity
105
Haematology and biochemical measurements
Blood was obtained by cardiac puncture with EDTA‐filled syringes. Haematological analysis was
performed on a ScilVet abc plus+ (ScilVet, Oostelbeers, The Netherlands).
Insulin assay and calculation of HOMA‐IR
Fasting insulin was measured in plasma by enzyme‐linked immunoabsorbent assay (Mercodia,
Uppsala, Sweden) following manufacturers' protocol. Glucose levels were measured from whole
blood upon sacrifice using a glucometer (Bayercontour, Basel, Switzerland). The homeostasis model
assessment of insulin resistance (HOMA‐IR) was calculated using the following formula: HOMA‐IR =
fasting glucose (mmol/l) × fasting insulin (mU/l)/22.5.
Glucose tolerance test
One week before and 3 and 6 weeks after the initiation of SMI 6860766 or vehicle treatment, a
glucose tolerance test (GTT) was performed. 12h fasted mice were injected i.p. with glucose (1mg g‐1,
Sigma‐Aldrich, Zwijndrecht, the Netherlands). Glucose levels were measured from whole blood using
a glucometer (Bayercontour, Basel, Switzerland) at times indicated in the figures.
Real‐time PCR
Total RNA was extracted using TRIzol (Invitrogen, Carlsbad, CA, USA) and reverse transcribed with an
iScript cDNA synthesis kit (Bio‐Rad, Veenendaal, the Netherlands). qPCR was performed using a SYBR
green PCR kit (Applied Biosystems, Leusden, the Netherlands) on a ViiA7 real‐time PCR system
(Applied Biosystems). The result is expressed as relative to the control group, which was assigned a
value of 1.
Flow cytometry
EpAT and subcutaneous AT (ScAT) were removed, rinsed in PBS and minced into small pieces. Tissues
were digested in a collagenase mixture (DMEM‐20mM HEPES, Collagenase I and Collagenase XI,
Sigma‐Aldrich, Zwijndrecht, the Netherlands) for 45 minutes at 37°C. The digested samples were
passed through a 70 µm nylon mesh (BD Biosciences, Breda, the Netherlands). The suspension was
centrifuged at 1250 rpm for 6 minutes and the pelleted SVF was resuspended in FACS buffer.
Erythrocytes in blood were removed by incubation with hypotonic lysis buffer (8.4 g of NH4Cl and
0.84 g of NaHCO3 per liter of distilled water). To prevent non‐specific binding of antibodies to the Fc
receptor, all cell suspensions were incubated with a CD16/32 antibody prior to labelling. CD3, CD8,
CD25, FoxP3, F4/80, CD11b, CD11c, Gr‐1 (eBioscience, San Diego, CA, USA) CD4, Ly6G (BD, Breda, the
Netherlands) and CD206 (Biolegend, San Diego, CA, USA) antibodies were incubated with the
indicated tissues. Staining was analysed by flow cytometry (FACSCanto II, BD Biosciences, Breda, The
Netherlands) and FlowJo software version 7.6.5. (Tree star).
Histology
Tissues were collected, fixed in 4% paraformaldehyde and embedded in paraffin. Liver steatosis and
inflammation were graded on 4 μm thick haematoxylin‐eosin (H&E) stained sections.
Immunohistochemistry on liver, EpAT and ScAT was performed for CD45 (BD, Breda, the
Netherlands) and on EpAT for CD68 (Bio‐rad). Five μm frozen sections of the liver were stained with
Oil red O (Sigma‐Aldrich, Zwijndrecht, the Netherlands). Organs were analysed by H&E staining.
Morphometric analyses were performed using the Las4.1 software (Leica, Rijswijk, the Netherlands)
Chapter 6
106
and ImageJ software. Analyses were performed by an observer who was blinded for the
experimental conditions.
Statistical analysis
Results are presented as mean ± SEM. Data were analysed by a Student’s t test or Chi‐squared test
using GraphPad Prism 5.0 software (GraphPad Software, Inc., La Jolla, CA, USA). P‐values <0.05 were
considered significant.
SMI‐mediated CD40‐TRAF6 inhibition in obesity
107
Results
Characterization of SMI 6860766
The CD40‐TRAF6 SMI 6860766 is specifically modelled for the CD40‐TRAF6 binding site on the TRAF6
molecule, the molecular structure of SMI 6860766 is shown in Figure 1a. Biacore analysis shows that
SMI 6860766 binds well to the part of the TRAF6 molecule in which the CD40‐binding site is present
(Figure 1b). In vitro, SMI 6860766 dose‐dependently suppressed CD40‐induced gene expression of IL‐
1β and IL‐6 cytokines in BM‐derived macrophages (Figure 1c). Further functional specificity can be
deducted from the following experiment: BM‐derived macrophages from CD40+/+, CD40‐/‐, CD40‐Twt,
CD40‐TRAF2/3/5‐/‐ and CD40‐TRAF6‐/‐ mice (mice that contain a site directed mutagenesis in the
CD40 gene for the respective TRAF binding domain on CD40) were activated with the CD40‐agonistic
antibodies 5D12 and FGK45. When stimulating these macrophages overnight, CD40+/+, CD40‐Twt and
CD40‐TRAF2/3/5‐/‐ macrophages display a high expression of CCL2, whereas CCL2 was significantly
reduced in CD40‐/‐ and CD40‐TRAF6‐/‐ macrophages, proving that CD40‐TRAF6 interactions are
responsible for the decrease in CCL2 levels. When adding SMI 6860766, together with the CD40‐
activating antibody, CCL2 levels were also reduced in CD40+/+, CD40‐Twt and CD40‐TRAF2/3/5‐/‐
macrophages, suggesting that the CD40‐TRAF6 inhibiting SMI 6860766 is specific for CD40‐TRAF6
interactions (Figure 1d).
Figure 1. Characterization of SMI 6860766. (a) Molecular structure of SMI 6860766. (b) Surface plasmon resonance sensogram of SMI 6860766 which confirmed the direct binding of SMI 6860766 to immobilized TRAF6 C‐domain. Data represent the average of three independent experiments. (c) Dose dependent inhibition of IL‐1β and IL‐6 gene expression in FGK45 (agonistic CD40‐antibody) stimulated bone marrow derived macrophages. (d) CCL2 gene expression in bone marrow derived macrophages of the respective genotypes stimulated with the CD40 agonistic antibody FGK45 and 5D12. n =6 per group, **=p<0.01, ***=p<0.001.
A
D
B
C
IL1
0.001 0.01 0.1 1 10 1000.0
0.5
1.0
1.5
Concentration (μM)
rela
tive
gene
exp
ress
ion
IL6
0.001 0.01 0.1 1 10 1000.0
0.2
0.4
0.6
0.8
1.0
Concentration (μM)
rela
tive
gene
exp
ress
ion
CCL2
C57Bl6
CD40-/-
CD40-T
wt
CD40-T
2/2/5-
/-
CD40-T
6-/-
0
5
10
15 ***** *** Ctrl
6860766
rela
tive
gene
exp
ress
ion
6860766
Chapter 6
108
Small molecule treatment did not result in side effects
We previously developed small molecule inhibitors of the CD40‐TRAF6 interaction [19], and found no
side effects for SMI 6877002. In the present study, we administered SMI 6860766, an analogue of
6877002 with a lower IC50, in a more progressed model of DIO where treatment with the CD40‐
TRAF6 inhibitor was started after 12 weeks of HFD and continued until week 18 of HFD. We did not
observe any side effects. Daily treatment with SMI 6860766 for the last 6 weeks of 18 weeks of HFD
did not result in any changes in peripheral blood leukocyte counts or cell‐composition
(Supplementary figure 1a‐f). Macroscopic and histopathological analysis revealed no abnormalities of
the SMI treatment in more than 20 organs analysed, which included spleen, colon, small intestine,
stomach, kidney, lung, heart and muscle.
Alterations in adipocyte size in the adipose tissue
During the course of DIO, both groups showed a significant gain in body weight (Figure 2a). However,
there were no differences in body weight gain or food intake between the experimental groups
(Figure 2a, Table 1). SMI 6860766 treatment did not affect EpAT, ScAT or BAT weights (Table 1).
Because SMI 6860766 binds to CD40‐TRAF6 we confirmed presence of CD40 in the adipose tissues
and found an 11 fold higher expression of CD40 in EpAT compared to ScAT in the control group,
indicating that the SMI can exert larger effects in EpAT (Figure 2b). Interestingly, although the weight
of adipose depots did not differ, the number of adipocytes per field in EpAT of mice treated with SMI
6860766 was increased by 15.3% revealing that SMI treatment decreased adipocyte size, suggesting
less lipid storage and improved metabolic function (Figure 2c,d) [23]. A slight increase in number of
adipocytes per field in ScAT did not reach statistical significance (Figure 2c).
SMI 6860766 improved glucose tolerance in DIO
Next, we evaluated the effect of SMI 6860766 treatment on glucose sensitivity. Basal levels of
glucose and plasma insulin did not differ between the experimental groups and the HOMA‐IR indices
were not different (Table 1). Glucose tolerance tests (GTT) were performed before and after 3 and 6
weeks of treatment. No differences between the groups were observed before treatment was
initiated (data not shown). However, three weeks of treatment with SMI 6860766 resulted in
improved glucose sensitivity, compared with vehicle treated mice and an even more pronounced
effect on glucose sensitivity was seen after 6 weeks of treatment (Figure 2e). These data indicate that
SMI 6860766 improves glucose sensitivity in DIO.
CTRL 6860766
Food intake (g/wk) 20.9 ± 0.6 22.5 ± 1.1
EpAT (g) 1.8 ± 0.2 1.4 ± 0.2
ScAT (g) 1.1 ± 0.2 0.8 ± 0.1
BAT (mg) 142 ± 10 136 ± 14
Glucose (mg/dl) 153 ± 6.6 157 ± 5.4
Insulin (mU/l) 11.0 ± 1.0 11.1 ± 1.0
HOMA‐IR 4.2 ± 0.4 4.2 ± 0.4
Table 1. Characteristics of mice who received a HFD
for 18 weeks and were treated with vehicle or SMI
6860766 after 12 weeks of HFD and continued
treatment until week 18.
n=12, *=p<0.05.
SMI‐mediated CD40‐TRAF6 inhibition in obesity
109
Figure 2. Daily treatment with SMI 6860766 during the last 6 weeks of 18 weeks HFD reduced adipocyte size and improved glucose sensitivity in mice. (a) Treatment with SMI 6860766 did not result in differences in body weight gain. (b) Gene expression of CD40 showing a higher expression of CD40 in EpAT compared to ScAT. (c) Adipocyte size of EpAT and ScAT showing a 15.3% decrease in epididymal adipocyte size of SMI 6860766 treated mice as indicated by an increase in adipocyte numbers per field. n=12 per group. (d) H&E staining illustrating that epididymal adipocyte size was decreased in SMI 6860766 treated mice compared to vehicle. (e) GTT of mice fed a HFD for 15 weeks with 3 weeks of SMI 6860766 treatment and GTT of mice fed a HFD for 18 weeks with 6 weeks of treatment showing improved glucose sensitivity in mice treated with SMI 6860766 compared to vehicle. n =6 per group, *=p<0.05, **=p<0.01, ***=p<0.001.
D
C
2 4 6 8 10 12 14 16 1820
25
30
35
40CTRL6860766
Weeks of HFD
Bod
y w
eigh
t (g)
3 weeks of SMI treatment
0 15 30 45 75 1200
100
200
300
400
*
time (min)
% o
f ini
tial b
lood
glu
cose
6 weeks of SMI treatment
0 15 30 45 75 1200
100
200
300
400CTRL6860766
** ******
***
time (min)
% o
f ini
tial b
lood
glu
cose
E
0
20
40
60
80
100*
# ad
ipoc
ytes
/fie
ld
EpAT ScAT
0
50
100
150CTRL6860766
# ad
ipoc
ytes
/fie
ld
CD40
CTRL 6860766 CTRL 68607660
5
10
15
20
***
ScAT EpAT
rela
tive
gene
exp
ress
ion
A B
Ctrl 686076650 μm50 μm
Chapter 6
110
SMI 6860766 does not induce differences in hepatosteatosis
As liver steatosis is an important metabolic complication, we assessed liver weights, hepatic
inflammation and degree of steatosis. However, in this relatively mild model of DIO (45% calories
from fat) we did not observe differences in liver weight between the groups (Figure 3a) or severe
inflammation in either group (Figure 3b,c). Furthermore, there were no differences in hepatic lipid
content and the degree of steatosis (Figure 3d‐f).
Amelioration of adipose tissue inflammation after compound treatment
As CD40 is important in inflammation and AT inflammation has a key role in the development of
insulin resistance, we analysed immune cell composition by flow cytometry. We observed a 68.5%
decrease in number of total leukocytes (CD45+ cells) in the EpAT of mice treated with SMI 6860766
(Figure 4a). Leukocyte subset analysis revealed that T helper cell numbers (CD3+CD4+ cells) were
decreased by 50.4% and cytotoxic T cell numbers (CD3+CD8+ cells) were decreased by 61.6%,
whereas the numbers of regulatory T cells (CD3+CD4+CD25+FoxP3+ cells) were not affected (Figure
4b‐d). No differences in ScAT T cell numbers were observed (Supplemental Figure 2a‐d).
Additionally, the number of EpAT macrophages (CD11b+Ly6G‐F4/80+ cells) was decreased by 81.7%
by SMI 6860766 (Figure 4e). Macrophage subset analysis showed that pro‐inflammatory M1
macrophages (CD11b+Ly6G‐F4/80+CD11c+CD206‐ cells) were decreased by 80.8% and anti‐
inflammatory M2 macrophages (CD11b+Ly6G‐F4/80+CD11c‐CD206+ cells) by 70.5% (Figure 4f,g). The
pro‐inflammatory M1‐macrophages showed a larger decrease than the M2‐macrophages, therefore,
the ratio between M1 and M2 macrophages was 2.4 in the control group whereas the treatment
resulted in a ratio 1.5, suggesting a more anti‐inflammatory profile of the epididymal adipose tissue
depot (Figure 4h). No aberrations in ScAT macrophage numbers were observed (Supplemental Figure
2e‐g).
Figure 3. No liver abnormalities were seen in vehicle or SMI 6860766 treated groups. (a) Liver weights did not differ. (b) Immunohistochemical leukocyte (CD45) staining and (c) scoring of hepatic inflammation by H&E staining did not show significant differences between the two experimental groups. (d) Oil red O staining of liver sections and (e,f) scoring of hepatosteatosis did not show differences in degree of steatosis ‐or lipid content. n=12 per group.
CTRL 68607660
500
1000
1500
2000
Liver weight
wei
ght (
mg)
Macro steatosis
CTRL 68607660
50
100-+/-++++++
% o
f mic
e
Micro steatosis
CTRL 68607660
50
100- +/-++++++
% o
f mic
e
Leukocytes
0
5
10
15
20
# C
D45
+ ce
lls /f
ield
CTRL 6860766
Lipid content
0.0
0.2
0.4
0.6
0.8
1.0
Oil
red
O-s
tain
ed a
rea
(%)
CTRL 6860766
Inflammation
CTRL 68607660
50
100- +/-++++++
% o
f mic
e
CA B
D E F
SMI‐mediated CD40‐TRAF6 inhibition in obesity
111
Furthermore, immunohistochemical analysis of the immune cells in EpAT showed decreased
amounts of leukocytes (CD45+) in the SMI 6860766 treated group, which was not seen in ScAT (Figure
5a,b). Additionally, we observed a reduction in macrophage number (CD68+) relative to the number
of adipocytes in EpAT of the SMI 6860766 treated group (Figure 5c,d). Together our data
demonstrate that treatment with SMI 6860766 ameliorated diet‐induced AT inflammation.
Figure 4. Flow cytometric analysis demonstrates that SMI 6860766 treatment reduces AT leukocyte count. (a) Flow cytometry showed decreased leukocyte numbers (CD45
+ cells) in the EpAT of SMI 6860766 treated mice. (b) Lymphocyte subset analysis revealed that especially T helper (CD3
+CD4+) cells and (c) cytotoxic (CD3+CD4‐CD8+) T cells were decreased, whereas (d) regulatory (CD4+CD25+FoxP3+) T cells were not affected. (e) The number of total macrophages (CD11b+F4/80+), as well as (f) pro‐inflammatory M1 (CD11b+F4/80+CD11c+CD206‐) and (g) anti‐inflammatory M2 (CD11b+F4/80+CD11c‐CD206+) macrophages was decreased by treatment with SMI 6860766. (h) The ratio between M1‐ and M2 macrophages. n=12 per group, *=p<0.05, **=p<0.01.
EpAT Leukocytes
0
100000
200000
300000
400000
*#
of C
D45
+ ce
lls
Macrophages
0
100000
200000
300000
**
# of
CD
11b+
F4/8
0+ c
ells
M1 macrophages
0
20000
40000
60000
80000
100000
*
# of
F4/
80+C
D11
c+C
D20
6- c
ells
M2 macrophages
0
10000
20000
30000
40000
50000
*
# of
F4/
80+C
D11
c-C
D20
6+ c
ells
Ratio M1:M2 macrophages
0
1
2
3
4
M1:
M2
ratio
Regulatory T cells
0
500
1000
1500
2000
# of
CD
4+C
D25
+Fox
P3+
cells
T helper cells
0
10000
20000
30000
40000
50000
**
CTRL6860766
# of
CD
3+C
D4+
cel
ls
Cytotoxic T cells
0
2000
4000
6000
8000
10000
*
# of
CD
3+C
D8+
cel
ls
C
A B
D
E F
G H
Chapter 6
112
Figure 5. Immunohistochemical assessment of adipose tissue leukocytes demonstrating reduced infiltration of total leukocytes and macrophages into AT of SMI 6860766 treated mice. (a) Immunohistochemical analysis of EpAT and ScAT by leukocyte (CD45) staining showing a decreased amount of leukocytes in the EpAT of SMI 6860766 treated mice. No differences were seen in leukocyte counts in ScAT of SMI 6860766 treated mice. (b) CD45 staining of EpAT illustrating the decreased amount of CD45
+ cells in SMI 6860766 treated mice compared to vehicle. (c) Immunohistochemical analysis by macrophage (CD68) staining showing a decreased amount of macrophages in the EpAT of SMI 6860766 treated mice. (d) Pictures of a CD68 staining of EPAT to illustrate the decrease in CD68+ cells in SMI 6860766 treated mice. n=12 per group,**=p<0.01.
SMI‐mediated CD40‐TRAF6 inhibition in obesity
113
Discussion
Immune cell activation and inflammation within the visceral adipose tissue play a pivotal role in the
pathogenesis of obesity and obesity‐associated insulin resistance [1, 3, 4]. During the development of
obesity, the number of inflammatory cells, especially T cells and macrophages, increases and
correlates with the extent of metabolic dysfunction [5, 24, 25]. Interactions between adipocytes, T
cells and macrophages result in the secretion of pro‐inflammatory mediators, such as TNF and IL‐6,
which directly interfere with insulin signalling and promote insulin resistance [5, 24, 25]. Strategies
that deplete AT immune cells restore insulin sensitivity [5, 24, 25]. Skewing of immune cells towards
their pro‐inflammatory subsets, such as M1 macrophages, is known to aggravate AT inflammation
and insulin resistance [25, 26]. These events change the AT into an insulin resistant, pro‐
inflammatory environment that propagates ongoing local and systemic metabolic deterioration.
Co‐stimulatory molecules are known to efficiently propagate and mediate immune reactions, and
emerging evidence obtained from experiments in gene‐deficient and transgenic mouse models
suggests that co‐stimulatory molecules are an interesting therapeutic target to combat obesity and
the metabolic syndrome [15, 17‐20, 27‐30]. The therapeutic propensity of co‐stimulatory molecules
has been explored by treating DIO mice with antibodies against the CD28‐CD80/86 and CD40L‐CD40
system [15, 20, 30].
Simultaneous blocking of CD80 and CD86 by antibodies in DIO mice resulted in an improved glucose
tolerance and in an increase in insulin sensitivity. Moreover, AT inflammation and hepatic
inflammation were significantly reduced upon antibody treatment [30]. Similar results were obtained
when CD40L was blocked using an anti‐CD40L antibody in DIO mice. Weight gain was only slightly
reduced, but glucose tolerance and insulin sensitivity did improve upon treatment, as did AT
inflammation: less CD4+ T cells and macrophages were present in the visceral adipose tissue [15]. As
genetic deficiency of CD40, the receptor for CD40L, aggravates obesity [18‐20, 28], Wolf et al
administered agonistic CD40 antibodies to DIO mice, and found that HFD induced weight gain was
abolished, glucose tolerance and insulin sensitivity had improved, and inflammatory cell recruitment
(CD4+ T cells and M1 macrophages) in the visceral adipose tissue was halted [20]. These data show
that antibody‐mediated targeting of co‐stimulation in obesity is a powerful approach to improve
inflammation‐associated metabolic deterioration. However, although the results of antibody
treatment are positive, long‐term use of antibodies blocking co‐stimulatory receptors or ligands is
likely to result in unwanted side effects as long‐term dysfunction of the CD40L‐CD40 pathway or
CD80/86 pathway will result in immune‐system derangements.
In the present study we therefore chose to block the CD40‐CD40L system with a novel SMI that was
generated to target the interaction between CD40 and TRAF6, and, we were able to successfully
improve AT inflammation and metabolic dysfunction. The small molecule inhibitor approach has
several advantages. By blocking only parts of the CD40 signal transduction pathway, and not CD40 or
CD40L itself, severe immune suppression will be prevented. Moreover, our SMIs are selected using
the FAF‐Drugs2 filter and the ADME/tox filter to obtain the most optimal administration, delivery,
metabolism, extraction and toxicology profile [31, 32], and in addition, compounds containing
reactive or toxic groups were rejected. This screening process selected lead SMIs that successfully
blocked inflammation and are very suitable for further drug development [19]. Although the
Chapter 6
114
modelling and structure‐based virtual ligand screening and validation was performed with great care,
it cannot be ruled out that our SMI is able to bind to other proteins.
In a previous paper, we reported that we could improve insulin sensitivity, AT inflammation and
hepatosteatosis by administering our lead compound 6877002 (IC50= 15.9 μM) during the last 6
weeks of the experiment in a model of DIO in which mice received a 60% HFD for 12 weeks [19].
Here, we used an optimized SMI, derived from our lead compound, with an IC50 of 0.3 μM (SMI
6860766) [19], and used a milder, but a more prolonged model of DIO (45% HFD for 18 weeks),
which is more representative for the clinical problem of obesity induced metabolic complications.
Treatment was started after 12 weeks of diet, when obesity was already evident, and was continued
for 6 weeks. Remarkably, we saw a massive decrease in adipose tissue inflammation, with less influx
of CD4+ and CD8+ T cells, and macrophages. This decrease was more pronounced than we observed
after SMI 6877002 treatment, where we only found a decrease in the accumulation of M1
macrophages. Because obese visceral adipose tissue exhibits increased inflammation compared to
the subcutaneous adipose depot [33] and CD40 has a low expression in ScAT, the CD40‐TRAF6 SMI
did not affect ScAT to the same extend as EpAT. SMI 6860766 treatment prevented adipocyte
hypertrophy, suggesting less adipocyte dysfunction, improved glucose tolerance, and slightly
ameliorated hepatic inflammation. However, we did not observe an effect on hepatic steatosis, as
we observed with SMI 6877002. These differences could be due to the experimental set‐up, where
45% HFD used in the present study did not induce hepatic steatosis.
The beneficial effects of small molecule mediated inhibition of the CD40‐TRAF6 interaction in DIO
appears to be more pronounced than the effects observed in mice with a genetic deficiency of the
CD40‐TRAF6 interaction, which may be explained by the genetic constitution of the transgenic mice.
In a previous study we investigated the role of CD40‐TRAF6 interactions in the development of DIO
on leukocytes, and not in non‐hematopoietic cells such as adipocytes and hepatocytes. We
demonstrated that deficiency of CD40‐TRAF6 interactions in MHCII+ cells modestly improved AT
inflammation and metabolic and hepatic complications of DIO. However, CD40‐induced signalling in
non‐hematopoietic cells may also be involved in DIO. For example, ligation of CD40 on adipocytes
results in the secretion of pro‐inflammatory cytokines and decreased insulin sensitivity [16]. In the
present study we inhibited the CD40‐TRAF6 interaction in both hematopoietic and non‐
hematopoietic cells, which may explain the more pronounced beneficial effects of small molecule‐
mediated inhibition compared to the effects that we previously reported in the transgenic mice.
Another explanation could be that our SMI has other, not yet identified, beneficial effects on the
inhibition of inflammation.
In conclusion, we have shown that SMI 6860766 improves obesity‐associated glucose sensitivity by
reducing AT inflammation. SMIs that block the pro‐inflammatory CD40‐TRAF6 interaction are a
promising therapeutic strategy. Additional development and refinement of the inhibitors, as well as
optimized delivery methods such as nanoparticle mediated drug delivery are required before these
SMIs can be utilized in humans.
SMI‐mediated CD40‐TRAF6 inhibition in obesity
115
Corresponding author Prof. Esther Lutgens, Department of Medical Biochemistry, Experimental Vascular Biology, Academic Medical Centre,
University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands. +31 (0)20 5 66 33 80.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements We acknowledge the support from the Netherlands CardioVascular Research Initiative: “the Dutch Heart Foundation, Dutch
Federation of University Medical Centres, the Netherlands Organisation for Health Research and Development and the
Royal Netherlands Academy of Sciences" for the GENIUS project “Generating the best evidence‐based pharmaceutical
targets for atherosclerosis” (CVON2011‐19). This work was supported by the Netherlands Organization for Scientific
Research (NWO)(VICI grant to EL, and CW, medium investment grant to GN), the Netherlands Heart Foundation
(Established investigator grant to EL, MW, Dr E. Dekker grant to TS), the Rembrandt foundation (SB, MW, EL), the Deutsche
Forschungsgemeinschaft (DFG FOR809: LU1643/1‐2, SO876/3‐1, WE1913/11‐2, SFB914‐B08 and SFB 1054 and SFB‐1123 to
EL, CW, CH279/5‐1 to TC), European Research Council Grants (N°281296 to TC and ERC AdG °249929 to CW), a grant from
the Else‐Kroner‐Fresenius‐Stiftung (to TC) and by a grant from the German Federal Ministry of Education and Research to
the German Center for Diabetes Research (DZD e.V.) (to TC) and DZHK (German Centre for Cardiovascular Research, MHA
VD1.2 to CW), the Cardiovascular Research Institute Maastricht (to GN), the EU (grant KBBE‐2011‐5 289350 to GN), the
Transnational University Limburg (to GN) and Cyttron II (FES0908 to GN).
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[14] Baena‐Fustegueras JA, Pardina E, Balada E, et al. 2013. Soluble cd40 ligand in morbidly obese patients: Effect of body mass index on recovery to normal levels after gastric bypass surgery. JAMA Surg 148(2):151‐156.
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[16] Poggi M, Jager J, Paulmyer‐Lacroix O, Peiretti F, Gremeaux T, Verdier M et al. 2009. The inflammatory receptor CD40 is expressed on human adipocytes: contribution to crosstalk between lymphocytes and adipocytes. Diabetologia 52(6):1152‐1163.
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[22] Ahonen C. 2002. The CD40‐TRAF6 axis controls affinity maturation and the generation of long‐lived plasma cells. Nat Immunol 3:451–456.
[23] Lafontan M. 2014. Adipose tissue and adipocyte dysregulation. Diabetes Metab 40(1):16‐28. [24] Osborn O, Olefsky JM. 2012. The cellular and signaling networks linking the immune system and metabolism in
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[29] Zhong J, Rao X, Braunstein Z, Taylor A, Narula V, Hazey J et al. 2014. T‐cell costimulation protects obesity‐induced adipose inflammation and insulin resistance. Diabetes 63(4):1289‐1302.
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[31] Lagorce D, Sperandio O, Galons H, Miteva M, Villoutreix B. 2008. FAF‐Drugs2: Free ADME/tox filtering tool to assist drug discovery and chemical biology projects. BMC Bioinformatics 9(1):396.
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[33] Nishimura S, Manabe I, Naga iR. 2009. Adipose tissue inflammation in obesity and metabolic syndrome. Discov Med 8(41):55‐60.
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Supplementary data
Supplementary figure 1. SMI 6860766 treatment did not cause hematologic abnormalities. (a) Peripheral blood total leukocyte counts, (b) lymphocyte, (c) monocyte, (d) granulocyte, (e) erythrocyte and (f) platelet numbers. n=12 per group.
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Supplementary figure 2. Flow cytometry showed no significant changes in ScAT leukocyte count after SMI 6860766 treatment. (a) The number of (CD3
+) T cells,(b) (CD3+CD4+) T helper cells, (c) cytotoxic (CD3+CD8+) T cells and (d regulatory (CD4+CD25+FoxP3+) T cells were not different in ScAT of SMI 6860766 treated mice. (e) The number of total macrophages (CD11b+F4/80+), as well as (f) pro‐inflammatory M1 (CD11b+F4/80+CD11c+CD206‐) and (g) anti‐inflammatory M2 (CD11b+F4/80+CD11c‐
CD206+) macrophages were not significantly different after treatment with SMI 6860766. n=12 per group.
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7
Generaldiscussion
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General discussion
123
What to do against obesity‐associated diseases?
The obesity ‘epidemic’, with 39% of the world’s adult population overweight and 13% obese, is
increasing at an alarming rate with numbers that have doubled in the last 30 years (WHO). Although
it is known that limiting energy intake and engaging in regular physical activity prevents obesity, the
numbers are still increasing and leading to higher incidences of type 2 diabetes, cardiovascular
diseases, fatty liver disease (NASH) and cancer. We approached the problem by studying the immune
system, as chronic inflammation is an important underlying mechanism in the pathogenesis of
obesity. This thesis describes three targets to battle obesity‐associated diseases. First, we studied the
link between inflammation, adaptive thermogenesis and obese white adipose tissue (WAT) and
brown adipose tissue (BAT). Second, we evaluated how obesity affects hematopoietic stem‐ and
progenitor cells in the bone marrow. Third, we studied the effects of administration of a CD40‐TRAF6
blocking compound (SMI 6860766) in obesity.
The crosstalk between brown adipocytes and immune cells
An important reason that obesity leads to insulin resistance and cardiovascular disease is chronic
inflammation. Pro‐inflammatory immune cells infiltrate obese WAT which affects steady‐state
homeostasis of insulin signalling and adipogenesis [1]. The immune system in WAT is therefore
widely studied. The other hot topic of interest in obesity research is BAT. BAT uses glucose and fat as
fuel to produce heat and provides a potential counterbalance against excessive energy stores in
obesity [2]. Brown‐like or beige adipocytes can also be induced within WAT, called beiging [3]. This
occurs upon cold exposure, β3‐adrenergic receptor agonist treatment or by immunological signals [4‐
8]. Our data in chapter 3 provides evidence that inflammation also plays a role in obese BAT. In an
18‐week time course of diet‐induced obesity in mice, 3 days of high‐fat diet (HFD) already resulted in
the accumulation of lipids in BAT. Interestingly, within this time frame, BAT macrophages as well as
cytokine and chemokine gene expression increased. Together with the HFD‐induced increased pro‐
inflammatory environment in BAT we also found a rapid decrease in WAT and BAT eosinophils. This
confirms previous results by others who have shown that eosinophils play a large role in maintaining
immune homeostasis in WAT [9]. Eosinophils are part of an immunological circuitry of BAT activation
and beiging of WAT as IL‐4 producing cells that stimulate anti‐inflammatory macrophages to secrete
noradrenaline, which activates brown adipocytes and induces beiging of WAT thereby increasing
energy expenditure [9, 10]. HFD‐induced tissue remodelling and a shift from an anti‐inflammatory
towards a pro‐inflammatory environment already occurs before the onset of weight gain. These
rapid changes show that a HFD disturbs the homeostatic adipose tissue environment, which suggests
that the thermogenic capacity of adipose tissues is decreased.
In vitro experiments in chapter 3 and 4 further unravel a crosstalk between immune cells and brown
adipocytes. Both pro‐ and anti‐inflammatory cytokines affect brown adipocytes and regulate their
activity as well as the production of chemoattractants for a variety of immune cells. A direct effect of
IL‐4 on brown adipocytes implies an important role for IL‐4 and therefore also for IL‐4 producing
eosinophils.
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When we established that immune cells and signals were involved in obese BAT and that short‐term
HFD caused a decline in eosinophils, we decided to increase eosinophils by using a model of helminth
antigens, known to induce a type 2 immune response [11]. Chapter 4 describes how administration
of Schistosoma mansoni‐soluble egg antigens (SEA) and soluble products of Trichuris suis (TsSP) in
HFD‐fed mice resulted in a massive induction of eosinophils, macrophages and T cells in epididymal
WAT. However, we observed fat‐depot specific responses in which subcutaneous WAT and BAT were
almost unaffected by the helminth antigens. Others have shown that a helminth infection in obesity
is protective against body weight gain and insulin resistance via a type 2 immune response [12].
However, within the 10 days of our study we did not observe changes in body weight, nor did any of
the adipose tissues that we studied have alterations in their thermogenic activation. These data
suggest that different adipose tissue depots have differential roles in adaptive thermogenesis.
Should we promote helminth infections?
In the course of evolution, helminth infections have shaped our immune system. Helminths have
been able to induce an immune response that promotes their survival and prevents elimination. This
includes a mode of tolerance and the induction of immune‐regulatory pathways in a Th2 immune
response, characterized by a type 2 anti‐inflammatory environment. Current hygienic standards and
treatments reduce the exposure to parasites [13], which is suggested to lead to a higher incidence of
allergy, autoimmune disease, cardiovascular disease and diabetes [14]. Skewing the immune system
towards type 2 immunity can be protective in all these diseases. A link between type 2 immunity in
thermogenic adipose tissue might evolutionary have been beneficial when helminth’s host travelled
and had to adjust to colder climates.
A harmonic co‐existence between helminths and their hosts does not often result in severe clinical
symptoms. However, as helminths do live of their host, an infection can cause nutritional and vitamin
deficiencies, anaemia, growth delays, increased intestinal mobility and an increased risk of other
infectious diseases [11]. Therefore, if we want to use the helminth‐induced type 2 immune response
therapeutically, we should either find a helminth that does not cause side‐effects or identify the
immunomodulatory components. The helminth Trichuris suis is presumably transient, self‐limiting
and does not cause adverse effects in healthy subjects [13]. Small scale human trials in allergy,
multiple sclerosis and inflammatory bowel disease are already performed with Trichuris suis and
shown to improve remission [13]. In our study, we did not use live parasitic worms but the soluble
egg antigens from Schistosoma mansoni and soluble products of Trichuris suis. This is still a complex
mixture of protein‐ and lipid linked glycans and knowing which components and which molecular
mechanisms are responsible for the immune modulating effects will provide insight for future
therapeutic possibilities.
Remaining questions on the immune modulation of brown(ing) adipose tissue
To improve the translational value of our data, there are still a few questions that need be
addressed. In humans, the presence of metabolically active BAT is confirmed. However, how this
metabolic active tissue resembles pre‐existing classical brown or inducible beige adipocytes in mice is
still under investigation. To improve our possibilities of increasing energy expenditure in humans via
General discussion
125
BAT activation we need to determine the nature of these cells by identifying reliable markers and
then confirm them in tissue biopsies. Defining the cell types is important as different precursors
might require a different stimulation to maximally promote their thermogenic capacity.
That the immunological circuitry of ILC2s, eosinophils and anti‐inflammatory macrophages is also
involved in adaptive thermogenesis in humans needs to be confirmed as well as how these immune
cells are affected by the human diet. Our data shows a rapid decrease in eosinophil numbers in
adipose tissue and it will be interesting to extrapolate the effects of 1 day of HFD in mice to the
effects of a great variety in the human diet. Therefore, we need to determine the physiological
relevance of the immunological circuitry in human adipose tissue homeostasis, how this changes
over time after a high‐fat meal and whether different dietary components affect eosinophil numbers.
When we administered helminth antigens to mice we observed adipose tissue depot‐specific effects
on immune cell infiltration. Although helminth antigens were known to induce a type 2 response in
WAT, we highlight that this effect is adipose depots specific, and that the immune cell populations in
ScAT and BAT are far less affected by SEA and TsSP than in EpAT. Adipose depots differ in
morphological, cellular and molecular signature, as well as in differences in innervation by the
sympathetic nervous system, vascularisation and immune cell composition [15]. Literature suggests
that a type 2 immune response would induce beiging of WAT. In our study, the induction of a type 2
immune response in EpAT did not result in beige adipogenesis and, as we did not observe a type 2
immune response in ScAT or BAT, we also did not observe effects on thermogenesis there. These
data suggest that not just any white adipocyte can transdifferentiate into a beige adipocyte upon
anti‐inflammatory stimuli. As ScAT is close to the body surface, it could be physiologically beneficial
to have thermogenic capacities that can function as a layer of isolation. Knowing how to make a
beige adipocyte from a white adipocyte and how to specifically stimulate the adipose depots with
the most energy‐dissipating potential would have a high impact on energy metabolism.
A divergent immune cell composition between lean and obese WAT is well described, and we further
unraveled HFD‐induced changes in BAT. However, it is still unclear how alterations in immune cell
composition are regulated and how it affects tissue homeostasis.
If immune cells are involved in the activation of BAT, one can hypothesize that cold‐exposed BAT
contains a different composition of immune signals. We would therefore benefit from a comparison
between the immune cell components in BAT or beiging WAT at thermoneutrality versus cold
exposed subjects.
In humans, 18F‐fluorodeoxy‐glucose ([18F]FDG) uptake, as measured with a positron‐emission
tomographic and X‐ray computed tomography (PET/CT)‐scan, is negatively correlated with BMI,
which suggests that BAT activity is decreased in obesity [16]. However, aside from glucose, an
important substrate for brown adipocytes is fatty acids. One study has already shown that cold‐
induced fatty acid uptake by BAT is similar in individuals with type 2 diabetes, age‐matched controls
and in healthy young controls [17], suggesting that previous conclusions based on [18F]FDG uptake
may also be due to the insulin resistant state of obese individuals. Therefore, studies using a fatty
acid tracer are needed to confirm a relation between BAT activity and obesity. It has been shown
that the insulin receptor is essential for adipogenesis and it would be interesting to investigate if the
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beiging potential of insulin resistant adipocytes is reduced in obesity [18]. Furthermore, in vitro
substrate studies could explore how and which fuel sources brown adipocytes use exactly.
Surgical denervation of BAT results in a ‘whitened’ phenotype of BAT [19]. Furthermore, surgical
denervation of BAT or treatment with the β‐adrenergic antagonist propranolol upregulates gene
expression of Tnf, Il6 and Ccl2, suggesting that the sympathetic nervous system (SNS) contributes to
an anti‐inflammatory homeostatic environment within BAT [20]. It would therefore be interesting to
study the contribution of the SNS in immune regulation of BAT.
It would also be interesting to study whether different types of fatty acids or other dietary
components can activate brown adipocytes. Different lipids can also have opposing inflammatory
signalling properties, for example, saturated fatty acids can promote inflammation whereas
unsaturated n‐3 fatty acids have anti‐inflammatory effects [21‐23]. Furthermore, an example of a
dietary component that can affect thermogenesis is capsaicin, a component of chili peppers, which
can increase sympathetic nerve activity, thermogenesis and energy expenditure [24].
Diet‐induced chronic inflammation affects hematopoietic stem cells
The most primitive precursors for immune cells, the hematopoietic stem‐ and progenitor cells
(HSPCs), reside in the bone marrow [25]. The bone marrow is the primary site of immune cell
production and activation of HSPCs can generate the desired immune response during acute
infection. However, chronic inflammatory conditions can cause inappropriate activation. Disturbed
HSPC homeostasis can affect disease development or progression. In chapter 5, we studied how the
chronic low‐grade inflammation caused by obesity affects HSPC population dynamics in the BM using
a time‐course of HFD ranging from 1 day up to 18 weeks. The HFD stimulated the differentiation and
reduces proliferation of HSPCs in the BM, leading to a decreased HSPC population, including loss of
stemcellness and loss of self‐renewal characteristics. In a competitive BM transplantation, chow fed
mice received BM from obese animals which resulted in impaired multi‐lineage reconstitution. This
suggests long‐term alterations in HSPCs that would persist even after weight loss.
Our data indicate that the chronic inflammatory state in obesity continuously recruits immune cells
from the bone marrow and disturbs HSPC homeostasis. If obesity results in a dysregulation of the
earliest hematopoietic stem cells, this can have major effects on later lineages and potentially cause
disturbed immunological responses to future infections. To study whether these changes hamper
proper functioning of the immune system, it would be interesting to study different infection models
in obesity.
Studies using varying nutritional mouse models of hypercholesterolemia, dyslipidemia and
hyperglycemia indicate that different dietary components can promote alternative immune
responses and can therefore also differentially alter HSPC homeostasis [26‐28]. To improve the
translational value of our data, it is important to distinguish the contribution of different nutrients on
HSPC dynamics.
General discussion
127
Inhibition of CD40‐TRAF6 interactions improves metabolism in obesity
CD40 is a co‐stimulatory molecule and involved in mediating a wide variety of immune responses.
Co‐stimulatory molecules have a central role in inflammation in which they provide an additional
stimuli to activate antigen presenting cells and T cells [29]. They can also activate monocytes and
granulocytes as well as adipocytes or endothelial cells [30, 31]. CD40 recruits adaptor proteins, TNFR‐
associated factors (TRAFs) for signalling transduction. The different binding sites for TRAF2/3/5 and
for TRAF6 allows CD40 to activate different signalling pathways depending on which adaptor protein
binds, which cell type and other local conditions [31].
In obesity, CD40 deficiency increases metabolic and inflammatory complications in mice [32, 33].
Interestingly, CD40‐TRAF2/3/5 deficient mice resemble this phenotype, whereas CD40‐TRAF6
deficiency improves WAT inflammation, insulin resistance and protects against obesity and
atherosclerosis [32, 33]. This reveals a differential involvement of TRAF adapter proteins in the
actions of CD40, making inhibition of CD40 and TRAF6 interaction a potential target in inflammatory
diseases. The Lutgens lab previously designed a small molecule inhibitor (SMI) for the interaction
between CD40 and TRAF6. In chapter 6, we applied this CD40‐TRAF6 SMI in a model of diet‐induced
obesity in mice and successfully improved WAT inflammation and metabolic dysfunction.
Our data show that small‐molecule mediated inhibition of the CD40‐TRAF6 interaction is a promising
therapeutic strategy against type 2 diabetes and adipose tissue inflammation in diet‐induced obesity.
A major advantage of this approach is that using a small molecule inhibitor which blocks only part of
the CD40 signal transduction pathway prevents severe immune suppression.
The beneficial effects on inflammation and metabolism of the CD40‐TRAF6 SMI seems to be even
more pronounced then the genetic models of CD40‐TRAF6 deficiency. This is possibly due to the
genetic model, which was specific for MHCII+ cells, and the SMI might also target other cell types.
Therefore, future studies could further identify whether and how other cell types use CD40‐TRAF6
interactions and if the SMI effects on those cell types also contribute to a metabolic beneficial
phenotype. In general, insight in cell type specific CD40 signalling will contribute to future
improvements to CD40 targeted therapies.
The SMI was virtually modelled to have a well drug‐like profile, based on absorption, distribution,
metabolism, excretion and toxicity values using the FAF‐Drugs2 filter and the ADME/tox filter [34,
35]. However, we cannot exclude the possibility that the SMI is able to bind to other proteins.
Further pharmacological analysis is needed before pre‐clinical trials and a safe application to
humans.
Three potential targets in battling obesity
This thesis describes three targets to battle chronic inflammation in obesity‐associated diseases.
First, we studied inflammation in obese white‐ and brown adipose tissue and whether the immune
system contributes to adaptive thermogenesis. Next, we explored how obesity affects hematopoietic
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stem‐ and progenitor cells in mice. Third, we studied CD40‐TRAF6 inhibition in diet‐induced obese
mice.
Our main findings are:
‐ Short‐term HFD leads to an accumulation of lipids, macrophages infiltration and pro‐
inflammatory cytokines and chemokines in brown adipose tissue
‐ White‐ and brown adipose tissue eosinophils rapidly decline upon a HFD
‐ A crosstalk between immune cell cytokines and brown adipocyte derived chemokines is
involved in the regulation of brown adipocyte activity
‐ Short‐term treatment with helminth antigens of HFD‐fed mice induces an adipose depot‐
specific type 2 immune response which does not contribute to thermogenic activation
‐ Obesity induces long‐term alterations on hematopoietic stem cells including loss of
stemcellness, increased differentiation potential and reduced proliferation
‐ Small molecule inhibition of CD40‐TRAF6 improves glucose tolerance and reduces adipose
tissue immune cells in diet‐induced obese mice
As discussed in previous sections, all three approaches require additional research before humans
can profit from it.
Different cold‐exposure protocols have been successful in increasing BAT activity in humans. In
theory, every individual has the tool to stimulate their energy expenditure, contributing to a
metabolically beneficial phenotype. Easy, non‐invasive ways include taking a cold shower every day,
swim in the ocean all year round or just turn down the thermostat. Using biochemical compounds
would avoid uncomfortable methods.
Although our data on the interaction between immune cells and BAT in obesity does not lead to a
direct therapeutic application, we show that BAT is affected by a pro‐inflammatory state in diet‐
induced obesity and that there is an important crosstalk between brown adipocytes and immune
cells. When aiming to stimulate beiging of WAT, our experiments suggest that it is essential to induce
an immunological stimulus in adipose depots with the most energy‐dissipating potential. Before we
start manipulating the immune system to induce brown or beige adipocyte activation, further studies
are needed to clarify the exact relation between the immune system and loss of BAT activity in
obesity.
An important conclusion from our data on hematopoietic stem cells is that diet‐induced obesity
results in long‐term effects on the immune system, which highlights the importance of preventing
obesity instead of treating the consequences. It will be even more interesting to determine how
different dietary components affect the immune system and HSPCs.
In terms of therapeutic applications, our research showing metabolic beneficial effects of inhibiting
CD40‐TRAF6 interactions without finding any side effects has the highest potential. Long‐term effects
and pre‐clinical research to block CD40‐TRAF6 interactions is ongoing and seems very promising.
General discussion
129
Altogether, this thesis describes three targets in obesity associated chronic inflammation, including
immuno‐regulation of BAT, alterations in immune cell progenitors in the bone marrow and beneficial
metabolic effects of a CD40‐TRAF6 blocking compound in obesity.
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General discussion
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AppendixI:Summary
Appendix I
134
Summary
135
Summary
Brown adipose tissue (BAT) is involved in adaptive thermogenesis. Especially rodents and new‐borns
of larger organisms, including humans, have large BAT depots, which decrease with age.
Furthermore, brown adipocytes have also been identified within white adipose tissue (WAT), so‐
called beige adipocytes in a process of ‘beiging’ or beige adipogenesis. In humans, glucose uptake in
BAT is negatively correlated with BMI, suggesting that BAT activity is decreased under obese
circumstances. Obesity is characterized by a chronic state of low‐grade inflammation, especially
within white adipose tissue (WAT). Chapter 2 provides an overview of how BAT is regulated,
summarizes the immunological adipose tissue alterations in obesity and describes what is currently
known about immune regulation of BAT and beige adipogenesis.
Chapter 3 describes how the inflammatory status of BAT changes in mice that were given a high‐fat
diet (HFD) for different durations in time, varying from 1 day to 18 weeks. Comparable to WAT,
obese BAT accumulates lipids and displays a pro‐inflammatory environment with an increase in
macrophage markers and transcripts of cytokines and chemokines. Interestingly, where this increase
in WAT is observed after 18 weeks of HFD, the morphology and immune cell composition of BAT
changes after 3 days of HFD. In vitro data on a brown adipocyte cell line shows that brown adipocytes
are highly potent in producing chemokines in response to pro‐inflammatory cytokines and provides
evidence that brown adipocytes affect macrophage migration. The rapid diet‐induced remodelling of
BAT shows that it is an extremely plastic tissue and that brown adipocytes themselves contribute to
the inflammatory profile of obese BAT.
In chapter 4 we further study the link between obese adipose tissue, immune cells and thermogenic
metabolism. The first important finding was a rapid decline in WAT & BAT eosinophils after a HFD in
mice. Others have shown that eosinophils are the main IL‐4 producing cells in lean WAT, and
therefore contribute to tissue homeostasis by sustaining anti‐inflammatory macrophages. We also
observe a direct role for IL‐4 on brown adipocyte activation and their production of eosinophil
chemoattractant CCL11. To restore this type 2 immunological circuitry in adipose tissue of mice on a
HFD, we injected helminth antigens from Schistosoma mansoni and Trichuris suis. This induced a
massive increase in type 2 immune cells in the epididymal WAT depot, but not in subcutaneous WAT
or BAT. We also did not observe any effects on thermogenic capacity of the adipose tissues. This
chapter concludes that a HFD disturbs the homeostatic type 2 immunological circuitry adipose
tissues which cannot be reversed by a helminth‐induced type 2 response.
Hematopoietic stem‐ and progenitor cells (HSPCs) reside in the bone marrow. In chapter 5 we found
that obesity induces long‐term alterations on HSPCs including a decreased HSPC population,
increased differentiation potential and reduced proliferation. Transplantation of bone marrow from
HFD‐fed mice to chow fed mice resulted in impaired multi‐lineage reconstitution, which suggests that
long‐term diet‐induced alterations in HSPCs would persist even after weight loss. This chapter
therefore highlights the importance of preventing obesity.
In chapter 6 we applied a small molecule inhibitor (SMI) that targets the co‐stimulatory molecule
CD40 and its adaptor protein TRAF6 in diet‐induced obese mice. Inhibiting CD40‐TRAF6 signalling
Appendix I
136
using this SMI, improved glucose tolerance and reduced adipose tissue immune cells in diet‐induced
obese mice with no side‐effects. Our data show that small‐molecule mediated inhibition of the CD40‐
TRAF6 interaction is a promising therapeutic strategy against type 2 diabetes and adipose tissue
inflammation in diet‐induced obesity.
Altogether, this thesis describes three targets in obesity associated chronic inflammation. These
targets were the immune system in BAT, immune cell progenitors in the bone marrow and inhibition
of the interaction between the co‐stimulatory molecule CD40 and its adaptor protein TRAF6. A
general discussion of our data can be found in chapter 7.
AppendixII:Samenvatting
Appendix II
138
Samenvatting
139
Samenvatting
Chocola, koekjes en snoepgoed liggen te lonken bij elke kassa, een hamburger met frietjes en cola is
het goedkoopste menu en regelmatig sporten past niet in ons drukke schema; het is zo makkelijk om
dik te worden. Overgewicht en obesitas komt steeds meer voor wat ertoe leidt dat cardiovasculaire
ziektes, diabetes type 2, vervetting van de lever en kanker vaker voorkomen. Door de opeenstapeling
van vetten in obesitas wordt het immuunsysteem geactiveerd. Inflammatie ligt aan de grondslag van
obesitas geassocieerde ziekten. In dit proefschrift hebben we vanuit drie verschillende invalshoeken
onderzocht wat de effecten zijn van een hoog vet dieet op het immuunsysteem en metabolisme.
De eerste benadering was het onderzoeken van de rol van het immuunsysteem in bruin vet. Obesitas
zorgt voor een verhoging van pro‐inflammatoire immuuncellen in wit vet en dat draagt bij aan het
ontwikkelen van insulineresistentie. Wit vet slaat overtollige energie op terwijl bruin vet juist energie
verstookt tot warmte. Bruin vet is betrokken bij de regulatie van lichaamstemperatuur en gebruikt
vet en glucose om warmte te produceren. In hoofdstuk 2 bespraken we hoe bruin vet werkt, hoe
obesitas de samenstelling van immuun cellen in vetweefsel verandert en hoe het immuunsysteem
betrokken is bij de activatie van bruin vet. In hoofdstuk 3 hebben we onderzocht of obesitas ook
zorgt voor inflammatie in bruin vet door 8 verschillende groepen muizen een vetrijk dieet te geven in
een tijdsreeks variërend van 1 dag tot 18 weken. Evenals wit vet gaat bruin vet overtollig vet opslaan
en vertoont een pro‐inflammatoir immunologisch profiel. Echter treden deze veranderingen in wit
vet na 18 weken dieet op terwijl in bruin vet de veranderingen al na 3 dagen aanwezig zijn. Ook
hebben we in zowel hoofdstuk 3 als 4 met behulp van een bruin vet cellijn in kweek aangetoond dat
bruin vetcellen allerlei chemotactische signalen voor immuuncellen uitscheiden en de migratie van
macrofagen kunnen beïnvloeden. In hoofdstuk 4 hebben we verder gekeken naar de link tussen
obees vetweefsel, immuun cellen en thermogenese. Een belangrijke bevinding in deze studie was dat
eosinofielen snel na de start van een vetrijk dieet verminderen in alle vetdepots. Dit is belangrijk
omdat eosinofielen betrokken zijn bij een homeostatische type 2 immuunomgeving in gezond
vetweefsel door het produceren van het cytokine IL‐4. Daarmee houden ze anti‐inflammatoire
macrofagen in stand die weer noradrenaline produceren dat bruin vetcellen activeert. Om de
verstoring van het type 2 immuunprofiel in vetweefsel na een vetrijk dieet te herstellen hebben we
bij muizen parasitaire producten van Schistosoma mansoni en Trichuris suis toegediend waarvan
bekend is dat die zorgen voor een type 2 immuunrespons. Helaas vonden we dat de type 2 respons
alleen in het epididymale vet plaatsvond en dat in dit vetdepot geen verhoogde thermogenese
plaatsvond wat niet bijdroeg aan een metabool gunstig effect.
De voorloper cellen van immuun cellen, hematopoëtische stamcellen, bevinden zich in het
beenmerg. Omdat obesitas leidt tot een chronische ontsteking worden er continu immuuncellen
geproduceerd en geactiveerd. Daarom hebben we in hoofdstuk 5 onderzocht wat het effect is van
obesitas op hematopoëtische stamcellen. Hierin vonden we dat obesitas lange termijneffecten heeft
op de stamcellen in het beenmerg. De hoeveelheid vermindert, hun differentiatie wordt
gestimuleerd en de proliferatie vermindert. Het transplanteren van beenmerg vanuit een muis op
een vetrijk dieet naar een gezonde muis gaf een verstoorde reconstitutie van beenmergcellen. De
stamcellen die aan een vetrijke omgeving waren blootgesteld waren nog steeds verstoord in een
Appendix II
140
gezonde omgeving, wat laat zien dat lange termijneffecten van obesitas op de stamcellen niet
zouden verdwijnen na bijvoorbeeld succesvol gewichtsverlies.
Het co‐stimulatoire molecuul CD40 speelt een belangrijke rol in de activatie van immuun cellen en is
betrokken bij metabole ziektes. CD40 signaleert o.a. via TRAF6 en in hoofdstuk 6 hebben we muizen
een vetrijk dieet gegeven en behandeld met een CD40‐TRAF6‐‘small molecule inhibitor’, een stof die
de interactie tussen CD40 en TRAF6 blokkeert. Deze behandeling zorgde voor een vermindering van
ontstekingscellen in het vetweefsel en ook een verbetering in glucosetolerantie. Hiermee laten we
zien dat deze aanpak een veelbelovende therapie zou kunnen zijn voor het verminderen van
obesitas‐geassocieerde inflammatie om daarmee metabole complicaties van obesitas te verbeteren.
AppendixIII:PhDPortfolio
Appendix III
142
PhD Portfolio
143
Curriculum Vitae
Susanna Maria van den Berg was born on August 31, 1987 in Leiden, the Netherlands. She obtained
her Bachelor in Biomedical Sciences at Leiden University in 2009. Before starting her Research
Master Biomedical Sciences at Leiden University in 2010, she took care of the race‐rowers at student
rowing club Asopos de Vliet for one year, as vice‐president and commissioner race‐rowing. During
her master’s program she studied the contribution of epithelial mesenchymal transition to the
resistance to angiogenesis inhibitors at the Johns Hopkins University in Baltimore, USA. In a second
internship at the Leiden University Medical Center (LUMC) she studied structural MRI in patients
after long‐term remission of Cushing’s disease. She then became a PhD candidate at the department
of medical biochemistry in the Academic Medical Center, Amsterdam in 2013. The great supervision
of Prof. Esther Lutgens and Prof. Menno de Winther together with an extensive collaboration with
Andrea van Dam in the group of Prof. Patrick Rensen at the division of Endocrinology in the LUMC
resulted in this thesis.
PhD training
Courses
‐ Advanced Immunology (VUmc & Sanquin) 2014 3.0 ECTS
‐ Anatomy of the house mouse (Tytgat Institute) 2013 1.5 ECTS
‐ Basic Laboratory Safety (AMC) 2013 0.4 ECTS
Seminars, workshops
‐ Medical Biochemistry weekly department seminars 2013‐2017 4.0 ECTS
‐ EVB weekly seminars 2013‐2017 4.0 ECTS
‐ Monthly EVB Journal club 2013‐2017 3.0 ECTS
‐ Ruysch lectures 2013‐2017 1.0 ECTS
‐ Joseph Tager lectures 2013‐2017 1.0 ECTS
Presentations and conferences
‐ Keystone symposium: ‘Obesity and Adipose Tissue Biology’, Banff, 2016 1.0 ECTS
Canada poster presentation
‐ Keystone symposium: ‘Beige and Brown fat: basic biology and 2015 1.0 ECTS
novel therapeutics’, Snowbird, USA poster presentation
‐ Keystone symposium: ‘Innate Immunity, Metabolism and Vascular 2014 1.0 ECTS
Injury’, Whistler, Canada poster presentation
‐ Rembrandt Institute of Cardiovascular Science (RICS), 2014 0.5 ECTS
Noordwijkerhout, the Netherlands oral presentation
‐ Cardiovascular Research Conference, Noordwijkerhout 2013 0.5 ECTS
Supervising student internships
‐ Lars Larsen 2015 2.0 ECTS
‐ Gerdien van den Heuvel 2014 2.5 ECTS
‐ Johanna Sierra 2013 1.0 ECTS
Appendix III
144
Publications
1. van den Berg, S.M., van Dam, A.D., Seijkens, T.T.P., Kusters, P.J.H., Beckers, L., den Toom, M., Held,
N.M., van der Velden, S., Van den Bossche, J., Boon, M.R., Rensen, P.C.N., Lutgens, E., and de
Winther, M.P.J., Diet‐induced obesity induces rapid inflammatory changes in brown adipose tissue in
mice. Submitted
2. van den Berg, S.M., van Dam, A.D., Kusters, P.J.H., Beckers, L., den Toom, M., van der Velden, S.,
Van den Bossche, J., van Die, I., Boon, M.R., Rensen, P.C.N., Lutgens, E., and de Winther, M.P.J.,
Helminth antigens counteract a rapid high‐fat diet induced drop in eosinophils in a depot specific
manner in mice. Submitted
3. Hoeke, G., Wang, Y., van Dam, A.D., Mol, I.M., van den Berg, S.M., Groen, A.K., Rensen, P.C.N.,
Berbée, J.F.P., and Boon, M.R., Statin treatment potentiates the lipid‐lowering and anti‐atherogenic
effect of BAT activation by accelerating lipoprotein remnant clearance. Submitted
4. van Dam, A.D., van Beek, L., Pronk, A.C.M., van den Berg, S.M., Janssen, G., van Veelen, P., Koning,
F., van Kooten, C., Rensen, P.C.N., Boon, M.R., Verbeek, S., Willems van Dijk, K., and van Harmelen,
V., IgG is elevated in obese white adipose tissue but this does not mediate glucose intolerance via
Fcγ‐receptor or complement activation. Under Revision
5. van den Berg, S.M., van Dam, A.D., Rensen, P.C.N., de Winther, M.P.J., and Lutgens, E., Immune
modulation of brown(ing) adipose tissue in obesity. Endocrine Reviews, 2017; 38: 46‐69.
6. Van den Bossche, J., Baardman, J., Otto, N.A., van der Velden, S., Neele, A.E., van den Berg, S.M.,
Luque‐Martin, R., Chen, H., Boshuizen, M.C.S., Ahmed, M., Hoeksema, M.A., de Vos, A.F., and
de Winther, M.P.J., Mitochondrial Dysfunction Prevents Repolarization of Inflammatory
Macrophages. Cell Reports, 2016; 17: 684‐696.
7. van Dam, A.D., Bekkering, S., Crasborn, M., van Beek, L., van den Berg, S.M., Vrieling, F., Joosten,
S.A., van Harmelen, V., de Winther, M.P.J., Lütjohann, D., Lutgens, E., Boon, M.R., Riksen, N.P.,
Rensen, P.C.N., and Berbée, J.F.P., BCG lowers plasma cholesterol levels and delays atherosclerotic
lesion progression in mice. Atherosclerosis, 2016; 251: 6‐14.
8. van den Berg, S.M.*, Seijkens, T.T.P.*, Kusters, P.J.H., Beckers, L., den Toom, M., Smeets, E., Levels,
J., de Winther, M.P.J., and Lutgens, E., Diet‐induced obesity in mice diminishes hematopoietic stem
and progenitor cells in the bone marrow. The FASEB Journal, 2016; 30: 1779‐1788. (*Authors
contributed equally)
9. van Dam, A.D., Nahon, K.J., Kooijman, S., van den Berg, S.M., Kanhai, A.A., Kikuchi, T., Heemskerk,
M.M., van Harmelen, V., Lombès, M., van den Hoek, A.M., de Winther, M.P.J., Lutgens, E., Guigas, B.,
Rensen, P.C.N., and Boon, M.R., Salsalate Activates Brown Adipose Tissue in Mice. Diabetes, 2015;
64: 1544‐1554.
PhD Portfolio
145
10. van den Berg, S.M.*, Seijkens, T.T.P.*, Kusters, P.J.H., Zarzycka, B., Beckers, L., den Toom, M.,
Gijbels, M.J.J., Chatzigeorgiou, A., Weber, C., de Winther, M.P.J., Chavakis, T., Nicolaes, G.A.F., and
Lutgens, E., Blocking CD40‐TRAF6 interactions by small‐molecule inhibitor 6860766 ameliorates the
complications of diet‐induced obesity in mice. International Journal of Obesity, 2015; 39: 782‐790.
(*Authors contributed equally)
11. Leonhard, W.N., Zandbergen, M., Veraar, K., van den Berg, S., van der Weerd, L., Breuning, M., de
Heer, E., and Peters, D.J.M., Scattered Deletion of PKD1 in Kidneys Causes a Cystic Snowball Effect
and Recapitulates Polycystic Kidney Disease. Journal of the American Society of Nephrology, 2015; 26:
1322‐1333.
12. Chatzigeorgiou, A., Seijkens, T., Zarzycka, B., Engel, D., Poggi, M., van den Berg, S., van den Berg,
S., Soehnlein, O., Winkels, H., Beckers, L., Lievens, D., Driessen, A., Kusters, P., Biessen, E., Garcia‐
Martin, R., Klotzsche‐von Ameln, A., Gijbels, M., Noelle, R., Boon, L., Hackeng, T., Schulte, K.‐M., Xu,
A., Vriend, G., Nabuurs, S., Chung, K.‐J., Willems van Dijk, K., Rensen, P.C.N., Gerdes, N., de Winther,
M., Block, N.L., Schally, A.V., Weber, C., Bornstein, S.R., Nicolaes, G., Chavakis, T., and Lutgens, E.,
Blocking CD40‐TRAF6 signaling is a therapeutic target in obesity‐associated insulin resistance.
Proceedings of the National Academy of Sciences of the United States of America, 2014; 111: 2686‐
2691.
13. Andela, C.D., van der Werff, S.J.A., Pannekoek, J.N., van den Berg, S., Meijer, O.C., van Buchem,
M.A., Rombouts, S.A.R.B., van der Mast, R.C., Romijn, J.A., Tiemensma, J., Biermasz, N.R., van der
Wee, N.J.A., and Pereira, A.M., Smaller grey matter volumes in the anterior cingulate cortex and
greater cerebellar volumes in patients with long‐term remission of Cushing's disease: a case–control
study. European Journal of Endocrinology, 2013; 169: 811‐819.
14. van der Werff, S., van den Berg, S., Pannekoek, J., Elzinga, B., and Van Der Wee, N., Neuroimaging
resilience to stress: a review. Frontiers in Behavioral Neuroscience, 2013; 7: 39.
15. Fu, C., van der Zwan, A., Gerber, S., van den Berg, S., No, E., C.H.Wang, W., Sheibani, N., Carducci,
M.A., Kachhap, S., and Hammers, H.J., Screening assay for blood vessel maturation inhibitors.
Biochemical and Biophysical Research Communications, 2013; 438: 364‐369.
Appendix III
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Dankwoord
Dankwoord
148
Dankwoord
149
Met dit laatste hoofdstuk wil ik graag een hele rits collega’s, familie en vrienden bedanken voor alles
wat mij heeft geholpen dit proefschrift te schrijven. Van intellectuele discussies, vroege ochtenden
op het lab tot zeer gewaardeerde gezelligheid in de kroeg, het was een groot avontuur!
Mijn grootste dank gaat uit naar Prof. Dr. Menno de Winther en Prof. Dr. Esther Lutgens, het ideale
duo dat mij direct onder hun hoede nam. Binnen onze groep was ik het gelukkige biomedische
onderzoekertje dat mocht genieten van de waardevolle combinatie tussen jullie biologische en
medische achtergrond en daarmee the‐best‐of‐both‐worlds kon meepakken. Jullie vullen elkaar
perfect aan en de juiste experimentele condities werden meteen uitgevoerd in uitgebreide in vivo
experimenten. We hadden gezellige werkbesprekingen die met jullie creativiteit en beslisvaardigheid
altijd weer voor vele nieuwe plannen zorgden. Ik besef me goed hoeveel geluk ik heb gehad met
twee deuren die altijd open stonden en twee promotoren waar ik altijd op kon rekenen.
Verder wil ik ook mijn copromotor Prof. Dr. Patrick Rensen bedanken voor zijn begeleiding. Jij,
Mariëtte en Andrea vormen een energieke en creatieve combinatie die ik heel erg waardeer, alle
drie; heel erg bedankt! Ik ben heel blij met ons grote samenwerkingsproject, dat ik aanspraak kon
maken op jullie uitgebreide bruin vet kennis, kon genieten van onze woest interessante Rembrandt
meetings met koekjes en thee (of bubbels) en ook zeker toen we na een kleine crisis in het AMC
grote experimenten bij jullie konden doen. Lieve hardwerkende Andrea, hoe je het doet weet ik niet
maar telkens als ik met jou heb gepraat of zelfs maar gemaild, ben ik weer helemaal gerustgesteld en
enthousiast!
Ook wil ik graag alle leden van de promotie commissie hartelijk danken voor het beoordelen van mijn
proefschrift: Prof. Dr. Noam Zelcer, Prof. Dr. Max Nieuwdorp, Prof. Dr. Ronald Oude Elferink, Prof. Dr.
Ronit Shiri‐Sverdlov, Dr. Bruno Guigas en Dr. Riekelt Houtkooper.
Ik had niet durven dromen dat ik in zo’n hechte, sociale groep als bij de experimentele vasculaire
biologie terecht zou komen. Onze vele lab activiteiten met borrels, bbq’s, bingo, curling, karten,
quizzen en natuurlijk de skivakantie waren een groot feest. Annette, jij bent de perfecte collega;
behulpzaam, vol ervaring, energiek, slim, ik kan nog wel even doorgaan. Met jou kun je alles delen en
je vindt het altijd leuk om op Twan te passen, superfijn! Heel erg bedankt dat je mijn paranimf wilt
zijn! Marieke, samen trokken we ten strijde om ons stadje te verdedigen. We deelden lief en leed in
de trein, bij total‐body‐work‐out of waar het ook uitkwam. Zonder jou was ik nooit bij de EVB terecht
gekomen, dus ik heb heel veel aan jou te danken! Sassefras, je beweert een ochtendhumeur te
hebben maar ik heb er nooit wat van gemerkt! Zelfs niet toen we vroeg in de ochtend met onze
koelboxen naar Leiden scheurden. Je zorgzame aard houd je vaak verborgen maar kwam goed van
pas bij de opvoeding van onze visjes. Tom, wat heb ik van jou veel geleerd; praktisch in het lab, in rap
tempo onderzoek doen, protocollen goedgekeurd krijgen, alles verwerken en tegelijkertijd weer door
met het volgende experiment, heerrlijk! Linda en Myrthe, het was altijd genieten als we een
obesitas‐opoffering hadden en twerkend de dagen door gingen. Zeker ook onmisbaar bij het vetjes
knippen waren natuurlijk gezellig kamergenootje Suzanne, de getalenteerde Pascal en vele
studenten. Marion, jouw gezelligheid, directheid en roze wangetjes geven borrels interessante
wendingen. Je hebt een gave om andere mensen borrels te laten plannen wanneer het jou uitkomt
en ook om zoveel mensen te enthousiasmeren mee te gaan. Verder wil ik ook Marten, Jeroen,
Claudia, Annelie, Quinte, Carlos, Helene, Marnix, Thijs, Ewelina, Oliver, Charo en Koen bedanken voor
Dankwoord
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alle gezelligheid en hulp op en rond het lab. En zeker ook de studenten die ik heb mogen begeleiden:
Lars, Gerdien, Johanna en Pascal; bedankt voor jullie bijdrage aan de data in dit proefschrift.
Ntsiki, met verbazing geef ik je een proefschrift zonder seahorse data of western blots.. Toch wil ik je
bedanken voor je hulp en geduld als ik voor de zoveelste keer langskwam. We hebben veel geleerd
maar helaas geen figuur kunnen maken, wie weet mogen we voor een revisie nog een keer los.
Lieve Barbara, Michelle, Emily, Yordi en Annefieke, EJD’08; voor altijd mijn ploeggenootjes. Het is niet
meer samen knallen in de boot maar gelukkig wel samen knallen in de kroeg of bij een etentje. Ik ben
heel blij dat wij ooit samen in een boot zijn beland en dat jullie nog steeds altijd voor me klaar staan!
Emily, bedankt voor het lenen van je camera en fotografie‐skills voor de cover!
Lieve Femke, van bus‐vakanties naar Zuid‐Europa tot avonturen in Maleisië en Cuba, van
zaterdagavond biertjes drinken met de band tot babykleertjes uitzoeken, tijden veranderen en wij
veranderen gewoon mee! Een hele waardevolle vriendschap van voor‐, tijdens‐ en na mijn promotie!
BW‐buddies, Marieke, Sarah, Jessica, Jasmijn, Sabrina, Kimberly en Madelon, als we samen hebben
afgesproken, ben ik daarna weer helemaal opgeladen om hardcore de wetenschap aan te kunnen.
Eén keer per jaar is er zelfs een heel weekend voor nodig, die moeten we er zeker in houden! Met
onze vele raakvlakken is het altijd één en al gezelligheid en als we ons vaste lijstje onderwerpen langs
zijn gegaan is het tijd voor pepernoten, kadootjes en buikpijn van het lachen (of van de kaasfondue).
Laurens, Yordi, Leonard, Lilian, Ruud en Sam; bedankt dat jullie het gnerk‐gehalte in mijn leven hoog
houden. Hoe een avond met jullie verloopt is altijd een verrassing, maar gegarandeerd een avontuur!
Lieve papa en mama, van huis uit kregen wij een grote wetenschappelijke mind‐set mee waardoor ik
me helemaal thuis voelde in het doen van onderzoek. We zijn opgevoed met het gevoel alsof de
wereld aan onze voeten ligt en we alles konden worden wat we wilden. Jullie hebben ons subtiel en
met veel vrijheid de goede kant op gestuurd. Heel erg bedankt voor alle steun en goede zorgen. Chris
en Andrea, heel erg bedankt voor jullie interesse, de oppas‐sessies en de grote gezelligheid met zijn
allen! Rietske, ik ben heel blij dat papa zo’n leuke vrouw heeft gevonden. Je maakt iedereen gelukkig
en met jouw energie wordt jullie pensioen één groot feest. Annie en Luc, wat heb ik een geluk met
zulke lieve schoonouders, bedankt voor het vele oppassen tijdens het afronden van dit proefschrift.
Lieve Jan, je hebt mij onbeschrijflijk veel hulp, advies en steun gegeven de afgelopen jaren, zowel op
het lab als thuis. Je zit altijd bomvol goede ideeën en zo belanden we bij de lekkerste restaurantjes,
de nieuwste brouwerijen, in de mooiste natuur en op het meest afgelegen eiland. Verliefd worden
op jou is het beste wat me ooit is overkomen. Ik vind ons een geweldig team en ik hoop dat we nog
veel moois gaan meemaken samen!
Mijn kleine Twan, je hebt nog net niet met een pipet in je handje je eerste experiment gedaan maar
bent wel al helemaal geïntegreerd op het lab. Terwijl wij lekker aan het borrelen waren lag jij te
sjansen vanuit de kinderwagen of liet je je trucjes zien op tafel; je sociale vaardigheden zitten er al
goed in! Ik kan niet zeggen dat je een bijdrage hebt geleverd aan dit proefschrift, integendeel.. Maar
het is het allemaal waard en je maakt mij intens gelukkig, wat zeker veel dank verdient!
De afgelopen vier jaar zijn oprecht fantastisch geweest, maar ook aan dit goede komt een eind,
bedankt allemaal!