Recent Advances in Acne Pathogenesis

Implications for Therapy

Shinjita Das, Rachel V. Reynolds

Am J Clin Dermatol. 2014;15(6):479-488.

Abstract and Introduction

Abstract

Acne pathogenesis is a multifactorial process that occurs at the level of the pilosebaceous unit.

While acne was previously perceived as an infectious disease, recent data have clarified it as an

inflammatory process in which Propionibacterium acnes and innate immunity play critical roles in

propagating abnormal hyperkeratinization and inflammation. Alterations in sebum composition, and

increased sensitivity to androgens, also play roles in the inflammatory process. A stepwise approach

to acne management utilizes topical agents for mild to moderate acne (topical retinoid as mainstay ±

topical antibiotics) and escalation to oral agents for more resistant cases (oral antibiotics or

hormonal agents in conjunction with a topical retinoid or oral isotretinoin alone for severe acne).

Concerns over antibiotic resistance and the safety issues associated with isotretinoin have prompted

further research into alternative medications and devices for the treatment of acne. Radiofrequency,

laser, and light treatments have demonstrated modest improvement for inflammatory acne (with

blue-light photodynamic therapy being the only US FDA-approved treatment). However,

limitations in study design and patient follow-up render these modalities as adjuncts rather than

standalone options. This review will update readers on the latest advancements in our understanding

of acne pathogenesis and treatment, with emphasis on emerging treatment options that can help

improve patient outcomes.

Introduction

Acne vulgaris is a chronic inflammatory skin disease that develops around the pilosebaceous

apparatus and manifests as open and closed comedones as well as inflammatory papules, pustules,

and nodules. Commonly involved sites are the face, chest, upper back, and upper arms.

Inflammatory acne can result in potentially disfiguring scarring and post-inflammatory

hyperpigmentation, highlighting the importance of treatment.

Acne pathogenesis begins with abnormal keratinization that causes impaction and distension of the

lower portion of the infundibulum, forming the comedo.[1–3] Other factors include a complex

interplay among sebum production, with changes in lipid composition, hypersensitivity to androgen

stimulation, Propionibacterium acnes, and local inflammatory cytokines elaborated by the innate

immune system.[4–10]

The general principles of acne treatment involve targeting the various pathogenic factors. The

therapeutic ladder ranges from topical therapies for mild to moderate acne (retinoids and

antimicrobials) to oral agents for moderate to severe acne (antibiotics, hormonal agents, and

isotretinoin; of note, all oral treatments [except isotretinoin] should be prescribed in conjunction

with topical treatment). More recently, physical modalities, such as radiofrequency, photodynamic

therapy (PDT), and laser therapies have been studied.

Acne Pathogenesis

Overview of Innate Immunity

The innate immune system (rapid response but no memory to specific pathogens) underlies the

inflammatory basis for acne pathogenesis.[11] Through the physical barrier and acidic environment

of the stratum corneum (which limits bacterial colonization), skin provides first-line defense within

the innate immune system.[12–14] Skin also elaborates soluble factors (including complement factors,

antimicrobial peptides, chemokines, and cytokines) and expresses pattern recognition receptors

(PRRs) that mediate inflammatory responses against pathogen-associated molecular patterns

(PAMPs).[15–17] For ease of reference, Table 1 lists key inflammatory mediators of acne

pathogenesis.

Toll-like Receptors

Toll-like receptors (TLRs) are a subtype of PRR that can activate innate immune responses through

keratinocytes, neutrophils, monocytes/macrophages, natural killer cells, and dendritic cells

(including Langerhans cells). There are nearly a dozen different TLRs, but TLR2 and TLR4 appear

to be specific for acne pathogenesis. Microbial ligands (such as P. acnes) can activate several

pathways that ultimately converge to activate nuclear factor (NF)-κB transcription factor.

Downstream release of inflammatory cytokines (such as interleukin [IL]-1, IL-6, IL-8, IL-10, IL-12,

and tumor necrosis factor [TNF]-α) mediate pathogen destruction via effector cells.[18–21] TLR

activation also leads to release of antimicrobial peptides (such as human β defensin 1 [HβD1] and

human β defensin 2 [HβD2]) that play an important role in innate immune defenses.[21,23] TLR-

mediated cytokines also induce matrix metalloproteinases (MMPs) that play roles in acne

inflammation, dermal matrix destruction, and scar formation.[17,23]

Innate Immune System-mediated Inflammation

Inflammation both initiates and propagates acne pathogenesis. IL-1α is the critical comedogenic

cytokine, and one hypothesis for IL-1α stimulation is increased sebum production and decreased

linoleic acid (which contributes to loss of the skin barrier).[24,25] Other factors that drive IL-1

expression include TLR2 induction by P. acnes and NF-κB mediated transcription (stimulated by

oxidized squalene). IL-1 secretion by keratinocytes is stimulated by activation of TLRs (2 and 4) by

P. acnes, which promotes further keratinocyte proliferation, migration, and production of IL-

1.[1,26,27] In vitro studies have demonstrated that IL-1 is necessary for comedo formation.[26] Studies

demonstrating that IL-1α receptor blockade inhibits follicular hyperkeratinization support the notion

that an inflammatory milieu is necessary for acne pathogenesis and that the process is IL-1

specific.[2,28] Follicular IL-1α stimulates surrounding vascular endothelial cells to elaborate further

inflammatory markers (E-selectin, vascular cell adhesion molecule-1, intercellular adhesion

molecule-1, and human leukocyte antigen-DR).[1]

In the following sections, we discuss the role of acne mediators and treatment strategies.

Hyperkeratinization in Acne

Subclinical microcomedones are the initial acne lesions that mature into clinically apparent

comedones and inflammatory lesions. Acne lesions express higher levels of inflammatory factors

than normal skin, with notable inflammation and follicular epithelial hyperproliferation, even at the

level of the microcomedo. While it was previously thought that abnormal keratinization occurred

first, recent studies have demonstrated that expression of IL-1α (found within open comedones)

precedes abnormal keratinization.[1,19] Hyperkeratinization results from both follicular epithelial

hyperproliferation and retention of keratinocytes, thereby forming a keratin plug at the follicular

infundibulum. Epithelial hyperproliferation (comedo formation) is driven by rises in and sensitivity

to androgens, sebum lipid composition, P. acnes overgrowth, and local cytokines. Biofilm produced

by P. acnes also contributes to comedone formation by limiting desquamation of ductal

keratinocytes and propagating the infundibular plug.[1,29]

Sebum Abnormalities in Acne

Sebum is composed of triglycerides and free fatty acids (57.5 %), wax esters (26 %), squalene (12

%), and cholesterol and cholesterol esters (4.5 %) and provides lubrication and hydration, UVB

photoprotection, and lipophilic antioxidants for skin/hair. Sebum oleic and palmitoleic acids are

also antibacterial.[7]

Sebum production is at least partly regulated by androgen and retinoids. Androgen receptors are

located within the basal layer of sebaceous glands and keratinocytes, and androgens promote

sebaceous gland growth and sebum secretion.[30] On the other hand, systemic retinoids inhibit

sebocyte differentiation and cause sebaceous gland shrinkage.[31,32] Peroxisome proliferator-

activated proteins (PPARs) are nuclear transcription factors that dimerize with retinoid receptors to

regulate sebum production and keratinocyte differentiation.[33,34] New findings suggest that

sebaceous glands are also involved in neuroendocrine function and the stress response. For

example, melanocortins (melanocyte-stimulating hormone and adrenocorticotropic hormone) and

corticotropin-releasing hormone (in response to physiologic stress) bind to their respective receptors

within sebaceous glands to stimulate sebum production.[35–39] Insulin-like growth factor-1 can

induce sterol response binding protein-1 (SREBP-1), which stimulates sebaceous gland

lipogenesis.[40]

Sebum production is necessary but not sufficient for acne pathogenesis, and the composition of

sebum lipids can influence inflammation. Alterations in sebum lipid composition (such as increased

desaturation of free fatty acids, squalene, and squalene peroxide, or reduced levels of linoleic acid)

have been associated with follicular hypercornification through direct and indirect modulation of

the innate immune system.[34,41–44] Lipid peroxidation products can increase inflammatory cytokines

(including IL-1α) and activate PPARs, particularly PPARα. Oxidized squalenes also upregulate 5-

lipoxygenase (5-LOX), which promotes conversion of arachidonic acid to leukotriene B4 and

subsequent recruitment of inflammatory cells via PPARα.[45–47] PPARs activate T cells through

activator protein 1 (AP-1) and NFκB-mediated transcriptional regulation.[27,48]

Sebum-mediated inflammation is linked to P. acnes proliferation. Sebaceous glands and sebum

lipids provide an anaerobic setting for P. acnes growth. As sebum passes through the follicular duct,

lipases produced by P. acnes hydrolyze triglycerides into pro-inflammatory free fatty acids.[12–14] P.

acnes also binds TLR2 and TLR4 on sebaceous glands to stimulate sebocyte production of

antimicrobial peptides (HβD1 and HβD2) and inflammatory cytokines (TNFα, IL-1α, and IL-

8).[22,49–51] These observations suggest a role of sebaceous glands in pathogen recognition and

stimulation of the innate immune system.

Androgens in Acne

While produced in larger quantities by adrenal glands and gonads, androgens are also made locally

within sebaceous glands, where they promote keratinocyte and sebaceous gland proliferation.[52,53]

Adrenal and gonadal androgens are converted to testosterone and dihydrotestosterone by type 1 5α-

reductase found within the follicular infundibulum. Puberty has been associated with the onset of

acne vulgaris, as the rise in androgens during this period stimulates sebum production through

binding of receptors on sebaceous glands and pilosebaceous ducts. In fact, acneprone skin prone has

higher levels of androgen receptors and increased 5α-reductase activity.[54] Clinically, patients with

polycystic ovarian syndrome, congenital adrenal hyperplasia, and hormonal tumors (androgen

excess states) have higher rates of acne, while those with androgen deficiency or insensitivity do not

tend to develop acne.[55–57] Androgens also promote comedogenesis through regulation of growth

factors and IL-1α, which stimulate hyperkeratinization within the follicular duct and

infundibulum.[28]

Propionibacterium Acnes

P. acnes is a Gram-positive rod that thrives in the anaerobic environment of the pilosebaceous

apparatus. This commensal bacterium drives inflammatory responses that lead to acne pathogenesis.

P. acnes-secreted lipases digest sebum triglycerides into free fatty acids that stimulate antimicrobial

peptides (HβD1 and 2, cathelicidin, and granulysin) and downstream inflammatory responses.[58] P.

acnes also directly engages TLR2 and TLR4 found on keratinocytes and inflammatory cells to

propagate cytokines and chemokines (IL-1, IL-6, IL-8, and TNFα) that recruit neutrophils (through

IL-8) and macrophages to the follicle.[18,49] The ensuing inflammatory response causes follicular

wall rupture. Macrophages propagate the cycle by releasing more IL-8 (for neutrophil recruitment)

and IL-12 (for T-helper 1 cell [Th1] response).[28,49–51] TLRmediated cytokines also stimulate AP-1,

which induces MMPs that lead to tissue destruction and scar formation.[59,60] TLRs can also

stimulate expression of antimicrobial peptides.[22,27,61]

P. acnes promotes follicular hyperkeratinization by inducing integrin (cell adhesion protein) and

filaggrin (found in higher concentrations in sebaceous duct and infundibulum of acne-prone

skin).[18,62] Biofilm (a polysaccharide lining produced by P. acnes that surrounds microbes and

improves adherence to the follicle) further promotes hyperkeratinization and increases P. acnes

resistance to antibiotics.[29]

Figures 1 and 2 summarize the new data on acne pathophysiology.

Figure 1.

Propionibacterium acnes mediates acne pathogenesis through innate immune activation. AP

activator protein, FFA free fatty acid, IL interleukin, MMP matrix metellaproteinases, NF nuclear

factor, PMNs polymorphonuclear leukocytes, TLR toll-like receptor, TNF tumor necrosis factor

Figure 2.

Hyperkeratinization (which underlies microcomedo formation) is promoted by sebum production,

androgen stimulation, and Propionibacterium acnes via innate immune mechanisms. IL interleukin,

NF nuclear factor, TLR toll-like receptor

Acne Treatment

The therapeutic ladder for acne management ranges from topical to oral agents, with more recent

advances in the development of physical modalities, including radiofrequency, PDT, and laser

treatments.

Topical Retinoids

Topical retinoids (first-generation all-trans retinoic acid [tretinoin] and third-generation adapalene

and tazarotene) are the therapeutic cornerstone for acne comprising mild comedonal and

inflammatory papules.[63] By binding to nuclear membrane retinoic acid receptors (RARs), they

normalize follicular keratinocyte differentiation and cohesion of corneocytes to promote

comedolysis and inhibit comedogenesis.[64] Specifically, tretinoin binds all three RARs with

moderate affinity (RAR-α, β, γ); adapalene preferentially binds RAR-β, γ; and tazarotene has

highest affinity for RAR-β. Furthermore, adapalene also downregulates 5-lipoxygenase, leukotriene

production, and AP-1 transcription factor (thus inhibiting MMP-mediated tissue destruction and

scarring).[65] Adapalene also plays an anti-inflammatory role by inhibiting neutrophil chemotaxis.

Both tretinoin and adapalene inhibit monocyte expression of TLR2 and have been demonstrated to

prevent release of oxygen free radicals by neutrophils.[66,67] Topical retinoids can facilitate

percutaneous absorption of benzoyl peroxide and topical antibiotics.

Topical Antibacterial Agents

Benzoyl peroxide causes bacterial oxidation and is bactericidal. Of note, benzoyl peroxide

does not induce bacterial resistance.

Clindamycin and erythromycin are the two most commonly prescribed topical antibiotics

with coverage against Staphylococcus aureus and P. acnes. They function by binding the

50S ribosomal subunit and inhibit protein synthesis. Concomitant use with benzoyl peroxide

can limit development of bacterial resistance.

Azelaic acid inhibits mitochondrial activity and DNA synthesis.

Dapsone inhibits dihydropteroate synthetase and nucleic acid synthesis.

Sodium sulfacetamide also inhibits bacterial dihydropteroate synthetase.

Oral Antibiotics

Oral antibiotics are useful for moderate to severe inflammatory acne refractory to topical therapy.

They exhibit their effects via both antimicrobial and direct anti-inflammatory properties.

Specifically, antibiotics have been shown to inhibit bacterial lipase, down-regulate inflammatory

cytokines, prevent neutrophil chemotaxis, and inhibit MMPs.[68–70]

Tetracyclines (doxycycline and minocycline, in particular, which bind 30S ribosomal

subunit) are the most frequently prescribed oral antibiotics for acne. Lowdose tetracyclines

(e.g. doxycycline 20 mg twice daily) are recommended for anti-inflammatory effect and to

minimize the risk of antibiotic resistance.[71]

Macrolides, such as erythromycin and azithromycin, are also utilized, as they suppress P.

acnes proliferation within the follicle. They are implemented as secondline agents after

tetracycline antibiotics.

Oral Hormonal Treatment

Hormonal therapy may benefit women with signs of hyperandrogenism (premenstrual flares,

jawline distribution, hirsutism, and presentation or worsening of acne in adulthood). Oral

contraceptives inhibit ovarian and adrenal production of androgens and most contain both estrogen

and progestin to reduce the risk of endometrial cancer. The three US FDA-approved oral

contraceptives for acne treatment include norgestimate-ethinyl estradiol, ethinylestradiol with

norethindrone, and ethinyl estradiol with drospirenone.[72,73] Newer formulations also show

antiandrogen effects. Spironolactone, which has dual function through androgen receptor blockade

and inhibition of 5αreductase, can reduce sebum production and improve acne.[73,74] Flutamide

(FDA-approved for the treatment of prostate cancer) is a non-steroidal androgen receptor blocker

that has also been effective, but cost and hepatotoxicity significantly limit its use.[75] Outside of the

USA, clinicians can prescribe anti-androgen cyproterone acetate at low dosage (2 mg/day) or higher

dosage (50 mg/day) combined with estrogen; of note, patients on higher-dosage cyproterone acetate

should be monitored for hepatotoxicity.

Oral Retinoids

Isotretinoin (13-cis-retinoic acid) is FDA-approved for treatment-resistant severe, nodulocystic acne

and exerts an in vitro anti-androgen effect through inhibition of the 3αhydroxysteroid activity of

retinol dehydrogenase. While 13-cis-retinoic acid does not directly bind any of the nuclear retinoic

acid receptors, its metabolites are thought to bind RAR and retinoid X receptor (RXR). The RAR-

RXR heterodimer then binds the retinoid ligand and transcriptionally regulates genes involved in

inhibition of inflammation, promotion of follicular keratinocyte differentiation, and decrease in

sebaceous gland activity. RAR-RXR can also antagonize AP-1, which limits activation of MMPs

that cause tissue destruction and acne scarring.[76–79]

Isotretinoin exerts anti-sebocyte effect through a unique non-retinoid receptor mechanism that

recruits neutrophil gelatinase-associated lipocalin (NGAL), which can induce apoptosis in

sebocytes during isotretinoin treatment. Interestingly, an increase in NGAL after isotretinoin

treatment occurs before decreases in sebum production and P. acnes. Ongoing research will

elucidate mechanisms by which NGAL mediates the isotretinoin effect.[80] Another recent study

noted that isotretinoin impacts the antimicrobial peptide expression in acne lesions. Specifically,

isotretinoin therapy was associated with reduced expression of cathelicidin, HβD2, lactoferrin,

psoriasin, and koebnerisin; however, only cathelicidin and koebnerisin levels normalized after a full

6-month course of treatment. Isotretinoin was noted to have no impact on other antimicrobial

peptides, including granulysin, perforin, and dermcidin, among others. These findings raise the

possibility of specifically targeting antimicrobial peptides in the treatment of acne.[81]

Devices for Acne Treatment

Most acne treatments provide short-term clearance, and isotretinoin is the only medication that

offers the best chance at long-term remission. However, the burden of monitoring programs for

teratogenicity and the risk of adverse effects have prompted continued research to identify other

treatments. Increasing antibiotic resistance is another important factor in seeking new treatment

approaches.[82,83] Radiofrequency, light, and laser devices are active areas of research and

development. Of note, randomized double-blinded clinical trials with sufficient numbers of patients

and with comparisons of devices with standard therapies (topical and systemic drugs) are lacking.

Furthermore, data are limited regarding frequency of relapses after treatment with devices. It is not

possible to evaluate the real benefit of medical devices, and at this time these devices should be

viewed as useful adjuncts to established therapies rather than as standalone treatment options.

Radiofrequency Devices. Non-ablative radiofrequency devices use radio waves to heat the dermis

and subcutaneous tissue while sparing the epidermis. Initially designed to improve skin laxity,

radiofrequency devices have recently been studied in the treatment of inflammatory acne. High

temperatures are thought to kill bacteria and shrink sebaceous glands. While preliminary studies

show promise, the study samples were small.[84–87] More recently, fractional radiofrequency

treatment with insulated microneedles targeting the middermis has shown promise in treating

inflammatory acne lesions.[86] Common side effects include pinpoint bleeding at the sites of

treatment, pain, and redness. Unlike lasers, radiofrequency treatments do not cause

hyperpigmentation (no epidermal heating), and thus may be a viable option for patients with darker

skin types.

Light and Laser Treatments for Acne. Non-Coherent Light Sources: Non-coherent light sources

harness the photochemical effect of visible or ultraviolet light on endogenous bacterial porphyrins

to produce free radicals and reactive oxygen species that can cause membrane damage to P. acnes

(major porphyrin is protoporphyrin IX). The optimal wavelengths for killing P.acnes in this manner

are in the blue light region (400–420 nm).

Intense pulse light (IPL) provides non-coherent pulses of visible light (longer wavelength than blue

light) that are thought to penetrate deeper into the follicle to photoactivate P. acnes porphyrins.[88]

Efficacy data are conflicting, and newer devices combine IPL source with suction devices

('photopneumatic' devices) with some improved efficacy.[89] Major limitations include lack of

properly controlled evaluation.

Blue light treatment is FDA-approved for acne treatment. Initial studies demonstrated that 8 weeks

of blue light therapy for acne can reduce the number of inflammatory, but not comedonal, acne

lesions (patients should continue a topical retinoid).[90] Others have reported improvement of acne

on the face and trunk with broadspectrum green and violet wavelengths.[91] The FDA recently

approved the use of narrow-band light sources for home-use laser devices.

PDT utilizes exogenous photosensitizers that preferentially absorb into the pilosebaceous unit and

enhance the effect of light treatments. Because aminolevulinic acid (ALA) and methyl-

aminolevulinate (M-ALA) are only FDA-approved for treatment of actinic keratoses, their

application in acne treatment is considered off-label. PDT requires at least 70–90 min incubation of

photosensitizer on the skin, followed by exposure to either blue (415 nm, ALA) or red (635 nm, M-

ALA) light. The longer wavelength of red light penetrates deeper into the dermis where sebaceous

glands reside (500–1,000 μm) and thus offers better efficacy.[92,93]

Variations of PDT include 'high dose' (optimized to target the sebaceous glands but with more side

effects) and 'low dose' (gentler parameters with fewer side effects but less effective). There can be

40–70 % improvement in inflammatory lesions lasting 3–6 months, with further improvement noted

on repeat treatment. PDT can cause significant inflammatory side effects, including pain, post-

procedure photosensitivity, and phototoxic effect (edema, blistering—downtime for 7–10 days post-

procedure).[94] This side effect profile may be tolerable for patients with moderate to severe acne

who have failed or want to avoid isotretinoin, do not want to receive an oral contraceptive, or are

planning to become pregnant.

Lasers for Acne Treatment: Pulsed-dye lasers (PDL) cause selective photothermolysis of dilated

blood vessels within acne lesions. While there is no direct effect on P. acnes or sebum production,

the soluble factor transforming growth factor (TGF)-β (a cytokine involved in wound healing) is

up-regulated after non-ablative PDL therapy. TGF-β seems to mediate an anti-inflammatory effect

that manifests as a global improvement in acne appearance rather than limited to the treated site.

The lack of high-quality controlled trials have made it difficult to interpret conflicting reports of

PDL efficacy.[95,96]

Mid-infrared lasers gained attention when the 1,450 nm diode laser was shown to damage

sebaceous glands in a rabbit ear model and in ex vivo human skin.[97] Photothermolysis at the level

of the sebaceous gland and alteration in follicular hyperkeratinization may play a role. A

preliminary split-back randomized controlled trial showed significant decrease in acne lesion counts

compared with control sites.[97] In follow-up studies, most of the patients were concomitantly

receiving either an oral or topical acne regimen during the laser treatment.[98] A subsequent splitface

study in 38 subjects showed no benefit using the Revised Acne Grading scale. However, global

improvement in acne was observed, again suggesting the role of a soluble factor (not identified in

the paper).[99]

More recently, Sakamoto et al.[101] have spear-headed efforts to utilize selective photothermolysis of

lipids within sebaceous glands. Anderson et al.[100] have identified 1,210 and 1,720 nm as

wavelengths where the absorption coefficient of lipid exceeds that of water. In vitro studies of fresh

porcine skin samples have demonstrated selective thermal damage to fat but not overlying skin near

the 1,210 nm wavelength. Furthermore, artificially prepared sebum has absorption peaks near

1,210, 1,728, 1,760, 2,306, and 2,346 nm.[101] These data suggest the potential for sebum to be

targeted as a chromophore in the selective photothermolysis of sebaceous glands. Preliminary

studies (ex vivo specimens of skin) have shown that 1,700 nm for 100–135 ms pulse durations

selectively target sebaceous glands without damage to the epidermis or dermis. Studies are

underway to further characterize these observations.

Conclusions

Acne is an inflammatory disease that results from multiple factors within the pilosebaceous

apparatus. New data about the physiopathology of acne show that acne is an inflammatory (rather

than infectious) disease in which innate immunity and P. acnes play a crucial role.

While acne treatment is multimodal, the mainstay of therapy remains prevention of comedone

formation. Topical agents are used for the treatment of mild to moderate acne, whereas oral agents

are generally reserved for moderate to severe disease. Of the currently available treatments,

isotretinoin offers the most sustained improvement and even cure. However, the side effect profile,

government-mandated monitoring process, and teratogenicity, render it cumbersome to prescribe.

This has prompted further research into both medical and device options. While radiofrequency,

laser, and light treatments offer some improvement for inflammatory acne (with blue-light PDT

being the only FDA-approved treatment), these options are not effective standalone modalities.

Future studies will offer further insights into acne pathogenesis and, in turn, spark the development

of novel targeted treatments.

References

1. Jeremy AHT, Holland DB, Roberts SG, Thomson KF, Cunliffe WJ. Inflammatory events

are involved in acne lesion initiation. J Invest Dermatol. 2003;121(1):20–7.

2. Guy R, Kealey T. Modelling the infundibulum in acne. Dermatol Basel Switz.

1998;196(1):32–7.

3. Guy R, Kealey T. The effects of inflammatory cytokines on the isolated human sebaceous

infundibulum. J Invest Dermatol. 1998;110(4):410–5.

4. Thiboutot D. Hormones and acne: pathophysiology, clinical evaluation, and therapies.

Semin Cutan Med Surg. 2001;20(3):144–53.

5. Thiboutot D, Jabara S, McAllister JM, Sivarajah A, Gilliland K, Cong Z, et al. Human skin

is a steroidogenic tissue: steroidogenic enzymes and cofactors are expressed in epidermis,

normal sebocytes, and an immortalized sebocyte cell line (SEB-1). J Invest Dermatol.

2003;120(6):905–14.

6. Thiboutot D. Acne: hormonal concepts and therapy. Clin Dermatol. 2004;22(5):419–28.

7. Thiboutot D. Regulation of human sebaceous glands. J Invest Dermatol. 2004;123(1):1–12.

8. Plewig G, Fulton JE, Kligman AM. Cellular dynamics of comedo formation in acne

vulgaris. Arch Für Dermatol Forsch. 1971;242(1):12–29.

9. Vowels BR, Yang S, Leyden JJ. Induction of proinflammatory cytokines by a soluble factor

of Propionibacterium acnes: implications for chronic inflammatory acne. Infect Immun.

1995;63(8):3158–65.

10. Beylot C, Auffret N, Poli F, Claudel J-P, Leccia M-T, Del Giudice P, et al.

Propionibacterium acnes: an update on its role in the pathogenesis of acne. J Eur Acad

Dermatol Venereol. 2014;28(3):271–8.

11. Parkin J, Cohen B. An overview of the immune system. Lancet. 2001;357(9270):1777–89.

12. Fluhr JW, Kao J, Jain M, Ahn SK, Feingold KR, Elias PM. Generation of free fatty acids

from phospholipids regulates stratum corneum acidification and integrity. J Invest Dermatol.

2001;117(1):44–51.

13. Elias PM. Stratum corneum defensive functions: an integrated view. J Invest Dermatol.

2005;125(2):183–200.

14. Elias PM. The skin barrier as an innate immune element. Semin Immunopathol.

2007;29(1):3–14.

15. Braff MH, Bardan A, Nizet V, Gallo RL. Cutaneous defense mechanisms by antimicrobial

peptides. J Invest Dermatol. 2005;125(1):9–13.

16. Schauber J, Gallo RL. Antimicrobial peptides and the skin immune defense system. J

Allergy Clin Immunol. 2008;122(2):261–6.

17. Walport MJ. Complement. First of two parts. N Engl J Med. 2001;344(14):1058–66.

18. Jugeau S, Tenaud I, Knol AC, Jarrousse V, Quereux G, Khammari A, et al. Induction of

toll-like receptors by Propionibacteriumacnes. Br J Dermatol. 2005;153(6):1105–13.

19. Kim J. Review of the innate immune response in acne vulgaris: activation of toll-like

receptor 2 in acne triggers inflammatory cytokine responses. Dermatol Basel Switz.

2005;211(3):193–8.

20. McInturff JE, Modlin RL, Kim J. The role of toll-like receptors in the pathogenesis and

treatment of dermatological disease. J Invest Dermatol. 2005;125(1):1–8.

21. McInturff JE, Kim J. The role of toll-like receptors in the pathophysiology of acne. Semin

Cutan Med Surg. 2005;24(2):73–8.

22. Nagy I, Pivarcsi A, Kis K, Koreck A, Bodai L, McDowell A, et al. Propionibacterium acnes

and lipopolysaccharide induce the expression of antimicrobial peptides and proinflammatory

cytokines/chemokines in human sebocytes. Microbes Infect Inst Pasteur. 2006;8(8):2195–

205.

23. Selway JL, Kurczab T, Kealey T, Langlands K. Toll-like receptor 2 activation and

comedogenesis: implications for the pathogenesis of acne. BMC Dermatol. 2013;13:10.

24. Perisho K, Wertz PW, Madison KC, Stewart ME, Downing DT. Fatty acids of

acylceramides from comedones and from the skin surface of acne patients and control

subjects. J Invest Dermatol. 1988;90(3):350–3.

25. Elias PM, Brown BE, Ziboh VA. The permeability barrier in essential fatty acid deficiency:

evidence for a direct role for linoleic acid in barrier function. J Invest Dermatol.

1980;74(4):230–3.

26. Freedberg IM, Tomic-Canic M, Komine M, Blumenberg M. Keratins and the keratinocyte

activation cycle. J Invest Dermatol. 2001;116(5):633–40.

27. Kang S, Cho S, Chung JH, Hammerberg C, Fisher GJ, Voorhees JJ. Inflammation and

extracellular matrix degradation mediated by activated transcription factors nuclear factor-

kappaB and activator protein-1 in inflammatory acne lesions in vivo. Am J Pathol.

2005;166(6):1691–9.

28. Guy R, Green MR, Kealey T. Modeling acne in vitro. J Invest Dermatol. 1996;106(1):176–

82.

29. Burkhart CG, Burkhart CN. Expanding the microcomedone theory and acne therapeutics:

Propionibacterium acnes biofilm produces biological glue that holds corneocytes together to

form plug. J Am Acad Dermatol. 2007;57(4):722–4.

30. Kariya Y, Moriya T, Suzuki T, Chiba M, Ishida K, Takeyama J, et al. Sex steroid hormone

receptors in human skin appendage and its neoplasms. Endocr J. 2005;52(3):317–25.

31. Zouboulis CC, Korge B, Akamatsu H, Xia LQ, Schiller S, Gollnick H, et al. Effects of 13-

cis-retinoic acid, all-trans-retinoic acid, and acitretin on the proliferation, lipid synthesis and

keratin expression of cultured human sebocytes in vitro. J Invest Dermatol. 1991;96(5):792–

7.

32. Nelson AM, Gilliland KL, Cong Z, Thiboutot DM. 13-cis Retinoic acid induces apoptosis

and cell cycle arrest in human SEB-1 sebocytes. J Invest Dermatol. 2006;126(10):2178–89.

33. Schoonjans K, Staels B, Auwerx J. The peroxisome proliferator activated receptors

(PPARS) and their effects on lipid metabolism and adipocyte differentiation. Biochim

Biophys Acta. 1996;1302(2):93–109.

34. Ottaviani M, Camera E, Picardo M. Lipid mediators in acne. Mediators Inflamm.

2010;2010:858176.

35. Zhang L, Anthonavage M, Huang Q, Li W-H, Eisinger M. Proopiomelanocortin peptides

and sebogenesis. Ann N Y Acad Sci. 2003;994:154–61.

36. Ganceviciene R, Graziene V, Böhm M, Zouboulis CC. Increased in situ expression of

melanocortin-1 receptor in sebaceous glands of lesional skin of patients with acne vulgaris.

Exp Dermatol. 2007;16(7):547–52.

37. Ganceviciene R, Graziene V, Fimmel S, Zouboulis CC. Involvement of the corticotropin-

releasing hormone system in the pathogenesis of acne vulgaris. Br J Dermatol.

2009;160(2):345–52.

38. Thiboutot D, Sivarajah A, Gilliland K, Cong Z, Clawson G. The melanocortin 5 receptor is

expressed in human sebaceous glands and rat preputial cells. J Invest Dermatol.

2000;115(4):614–9.

39. Zouboulis CC, Seltmann H, Hiroi N, Chen W, Young M, Oeff M, et al. Corticotropin-

releasing hormone: an autocrine hormone that promotes lipogenesis in human sebocytes.

Proc Natl Acad Sci USA. 2002;99(10):7148–53.

40. Smith TM, Gilliland K, Clawson GA, Thiboutot D. IGF-1 induces SREBP-1 expression and

lipogenesis in SEB-1 sebocytes via activation of the phosphoinositide 3-kinase/Akt

pathway. J Invest Dermatol. 2008;128(5):1286–93.

41. Kligman AM, Katz AG. Pathogenesis of acne vulgaris. I. Comedogenic properties of human

sebum in external ear canal of the rabbit. Arch Dermatol. 1968;98(1):53–7.

42. Mills OH, Kligman AM. A human model for assessing comedogenic substances. Arch

Dermatol. 1982;118(11):903–5.

43. Motoyoshi K. Enhanced comedo formation in rabbit ear skin by squalene and oleic acid

peroxides. Br J Dermatol. 1983;109(2):191–8.

44. Downing DT, Stewart ME, Wertz PW, Strauss JS. Essential fatty acids and acne. J Am Acad

Dermatol. 1986;14(2 Pt 1):221–5.

45. Zouboulis CC. Acne and sebaceous gland function. Clin Dermatol. 2004;22(5):360–6.

46. Ottaviani M, Alestas T, Flori E, Mastrofrancesco A, Zouboulis CC, Picardo M. Peroxidated

squalene induces the production of inflammatory mediators in HaCaT keratinocytes: a

possible role in acne vulgaris. J Invest Dermatol. 2006;126(11):2430–7.

47. Alestas T, Ganceviciene R, Fimmel S, Müller-Decker K, Zouboulis CC. Enzymes involved

in the biosynthesis of leukotriene B4 and prostaglandin E2 are active in sebaceous glands. J

Mol Med Berl Ger. 2006;84(1):75–87.

48. Weindl G, Schäfer-Korting M, Schaller M, Korting HC. Peroxisome proliferator-activated

receptors and their ligands: entry into the post-glucocorticoid era of skin treatment? Drugs.

2005;65(14):1919–34.

49. Kim J, Ochoa MT, Krutzik SR, Takeuchi O, Uematsu S, Legaspi AJ, et al. Activation of

toll-like receptor 2 in acne triggers inflammatory cytokine responses. J Immunol.

2002;169(3):1535–41.

50. Kuwahara K, Kitazawa T, Kitagaki H, Tsukamoto T, Kikuchi M. Nadifloxacin, an antiacne

quinolone antimicrobial, inhibits the production of proinflammatory cytokines by human

peripheral blood mononuclear cells and normal human keratinocytes. J Dermatol Sci.

2005;38(1):47–55.

51. Nagy I, Pivarcsi A, Koreck A, Széll M, Urbán E, Kemény L. Distinct strains of

Propionibacterium acnes induce selective human beta-defensin-2 and interleukin-8

expression in human keratinocytes through toll-like receptors. J Invest Dermatol.

2005;124(5):931–8.

52. Deplewski D, Rosenfield RL. Role of hormones in pilosebaceous unit development. Endocr

Rev. 2000;21(4):363–92.

53. Gilliver SC, Ashworth JJ, Ashcroft GS. The hormonal regulation of cutaneous wound

healing. Clin Dermatol. 2007;25(1):56–62.

54. Thiboutot D, Harris G, Iles V, Cimis G, Gilliland K, Hagari S. Activity of the type 1 5

alpha-reductase exhibits regional differences in isolated sebaceous glands and whole skin. J

Invest Dermatol. 1995;105(2):209–14.

55. Merke DP, Bornstein SR. Congenital adrenal hyperplasia. Lancet. 2005;365(9477):2125–36.

56. Lolis MS, Bowe WP, Shalita AR. Acne and systemic disease. Med Clin North Am.

2009;93(6):1161–81.

57. Maluki AH. The frequency of polycystic ovary syndrome in females with resistant acne

vulgaris. J Cosmet Dermatol. 2010;9(2):142–8.

58. Dessinioti C, Katsambas AD. The role of Propionibacteriumacnes in acne pathogenesis:

facts and controversies. Clin Dermatol. 2010;28(1):2–7.

59. Jalian HR, Liu PT, Kanchanapoomi M, Phan JN, Legaspi AJ, Kim J. All-trans retinoic acid

shifts Propionibacterium acnesinduced matrix degradation expression profile toward matrix

preservation in human monocytes. J Invest Dermatol. 2008;128(12):2777–82.

60. Choi J-Y, Piao MS, Lee J-B, Oh JS, Kim I-G, Lee S-C. Propionibacteriumacnes stimulates

pro-matrix metalloproteinase-2 expression through tumor necrosis factor-alpha in human

dermal fibroblasts. J Invest Dermatol. 2008;128(4):846–54.

61. Chronnell CM, Ghali LR, Ali RS, Quinn AG, Holland DB, Bull JJ, et al. Human beta

defensin-1 and -2 expression in human pilosebaceous units: upregulation in acne vulgaris

lesions. J Invest Dermatol. 2001;117(5):1120–5.

62. Jarrousse V, Castex-Rizzi N, Khammari A, Charveron M, Dréno B. Modulation of integrins

and filaggrin expression by Propionibacteriumacnes extracts on keratinocytes. Arch

Dermatol Res. 2007;299(9):441–7.

63. Thielitz A, Abdel-Naser MB, Fluhr JW, Zouboulis CC, Gollnick H. Topical retinoids in

acne–an evidence-based overview. J Dtsch Dermatol Ges. 2008;6(12):1023–31.

64. Kurlandsky SB, Xiao JH, Duell EA, Voorhees JJ, Fisher GJ. Biological activity of all-trans

retinol requires metabolic conversion to all-trans retinoic acid and is mediated through

activation of nuclear retinoid receptors in human keratinocytes. J Biol Chem.

1994;269(52):32821–7.

65. Waugh J, Noble S, Scott LJ. Adapalene: a review of its use in the treatment of acne vulgaris.

Drugs. 2004;64(13):1465–78.

66. Thielitz A, Krautheim A, Gollnick H. Update in retinoid therapy of acne. Dermatol Ther.

2006;19(5):272–9.

67. Bikowski JB. Mechanisms of the comedolytic and anti-inflammatory properties of topical

retinoids. J Drugs Dermatol. 2005;4(1):41–7.

68. Monk E, Shalita A, Siegel DM. Clinical applications of nonantimicrobial tetracyclines in

dermatology. Pharmacol Res Off J Ital Pharmacol Soc. 2011;63(2):130–45.

69. Mays RM, Gordon RA, Wilson JM, Silapunt S. New antibiotic therapies for acne and

rosacea. Dermatol Ther. 2012;25(1):23–37.

70. Sapadin AN, Fleischmajer R. Tetracyclines: nonantibiotic properties and their clinical

implications. J Am Acad Dermatol. 2006;54(2):258–65.

71. Skidmore R, Kovach R, Walker C, Thomas J, Bradshaw M, Leyden J, et al. Effects of

subantimicrobial-dose doxycycline in the treatment of moderate acne. Arch Dermatol.

2003;139(4):459–64.

72. Lam C, Zaenglein AL. Contraceptive use in acne. Clin Dermatol. 2014;32(4):502–15.

73. Goodfellow A, Alaghband-Zadeh J, Carter G, Cream JJ, Holland S, Scully J, et al. Oral

spironolactone improves acne vulgaris and reduces sebum excretion. Br J Dermatol.

1984;111(2):209–14.

74. Shaw JC. Spironolactone in dermatologic therapy. J Am Acad Dermatol. 1991;24(2 Pt

1):236–43.

75. Adalatkhah H, Pourfarzi F, Sadeghi-Bazargani H. Flutamide versus a cyproterone acetate-

ethinyl estradiol combination in moderate acne: a pilot randomized clinical trial. Clin

Cosmet Investig Dermatol. 2011;4:117–21.

76. Kang S, Li XY, Voorhees JJ. Pharmacology and molecular action of retinoids and vitamin D

in skin. J Investig Dermatol Symp Proc. 1996;1(1):15–21.

77. Strauss JS, Rapini RP, Shalita AR, Konecky E, Pochi PE, Comite H, et al. Isotretinoin

therapy for acne: results of a multicenter dose-response study. J Am Acad Dermatol.

1984;10(3):490–6.

78. Layton AM, Knaggs H, Taylor J, Cunliffe WJ. Isotretinoin for acne vulgaris–10 years later:

a safe and successful treatment. Br J Dermatol. 1993;129(3):292–6.

79. DiGiovanna JJ. Systemic retinoid therapy. Dermatol Clin. 2001;19(1):161–7.

80. Lumsden KR, Nelson AM, Dispenza MC, Gilliland KL, Cong Z, Zaenglein AL, et al.

Isotretinoin increases skin-surface levels of neutrophil gelatinase-associated lipocalin in

patients treated for severe acne. Br J Dermatol. 2011;165(2):302–10.

81. Borovaya A, Dombrowski Y, Zwicker S, Olisova O, Ruzicka T, Wolf R, et al. Isotretinoin

therapy changes the expression of antimicrobial peptides in acne vulgaris. Arch Dermatol

Res. 2014;306(8):689–700.

82. Patel M, Bowe WP, Heughebaert C, Shalita AR. The development of antimicrobial

resistance due to the antibiotic treatment of acne vulgaris: a review. J Drugs Dermatol.

2010;9(6):655–64.

83. Hoover WD, Davis SA, Fleischer AB, Feldman SR. Topical antibiotic monotherapy

prescribing practices in acne vulgaris. J Dermatol Treat. 2014;25(2):97–9.

84. Ruiz-Esparza J, Gomez JB. Nonablative radiofrequency for active acne vulgaris: the use of

deep dermal heat in the treatment of moderate to severe active acne vulgaris

(thermotherapy): a report of 22 patients. Dermatol Surg. 2003;29(4):333–9 (discussion 339).

85. Taub AF. Procedural treatments for acne vulgaris. Dermatol Surg. 2007;33(9):1005–26.

86. Lee SJ, Goo JW, Shin J, Chung WS, Kang JM, Kim YK, et al. Use of fractionated

microneedle radiofrequency for the treatment of inflammatory acne vulgaris in 18 Korean

patients. Dermatol Surg. 2012;38(3):400–5.

87. Munavalli GS, Weiss RA. Evidence for laser- and light-based treatment of acne vulgaris.

Semin Cutan Med Surg. 2008;27(3):207–11.

88. Wat H, Wu DC, Rao J, Goldman MP. Application of intense pulsed light in the treatment of

dermatologic disease: a systematic review. Dermatol Surg. 2014;40(4):359–77.

89. Lee EJ, Lim HK, Shin MK, Suh D-H, Lee S-J, Kim NI. An open-label, split-face trial

evaluating efficacy and safty of photopneumatic therapy for the treatment of acne. Ann

Dermatol. 2012;24(3):280–6.

90. Elman M, Slatkine M, Harth Y. The effective treatment of acne vulgaris by a high-intensity,

narrow band 405–420 nm light source. J Cosmet Laser Ther. 2003;5(2):111–7.

91. Huh SY, Na J-I, Huh C-H, Park K-C. The effect of photodynamic therapy using indole-3-

acetic acid and green light on acne vulgaris. Ann Dermatol. 2012;24(1):56–60.

92. Wan MT, Lin JY. Current evidence and applications of photodynamic therapy in

dermatology. Clin Cosmet Investig Dermatol. 2014;7:145–63.

93. Ma L, Xiang L-H, Yu B, Yin R, Chen L, Wu Y, et al. Low-dose topical 5-aminolevulinic

acid photodynamic therapy in the treatment of different severity of acne vulgaris.

Photodiagnosis Photodyn Ther. 2013;10(4):583–90.

94. Sakamoto FH, Torezan L, Anderson RR. Photodynamic therapy for acne vulgaris: a critical

review from basics to clinical practice: part II. Understanding parameters for acne treatment

with photodynamic therapy. J Am Acad Dermatol. 2010;63(2):195–211 (quiz 211–2).

95. Seaton ED, Charakida A, Mouser PE, Grace I, Clement RM, Chu AC. Pulsed-dye laser

treatment for inflammatory acne vulgaris: randomised controlled trial. Lancet.

2003;362(9393):1347–52.

96. Orringer JS, Kang S, Hamilton T, Schumacher W, Cho S, Hammerberg C, et al. Treatment

of acne vulgaris with a pulsed dye laser: a randomized controlled trial. JAMA.

2004;291(23):2834–9.

97. Paithankar DY, Ross EV, Saleh BA, Blair MA, Graham BS. Acne treatment with a 1,450

nm wavelength laser and cryogen spray cooling. Lasers Surg Med. 2002;31(2):106–14.

98. Friedman PM, Jih MH, Kimyai-Asadi A, Goldberg LH. Treatment of inflammatory facial

acne vulgaris with the 1450-nm diode laser: a pilot study. Dermatol Surg. 2004;30(2 Pt

1):147–51.

99. Darné S, Hiscutt EL, Seukeran DC. Evaluation of the clinical efficacy of the 1,450 nm laser

in acne vulgaris: a randomized split-face, investigator-blinded clinical trial. Br J Dermatol.

2011;165(6):1256–62.

100. Anderson RR, Farinelli W, Laubach H, Manstein D, Yaroslavsky AN, Gubeli J, et al.

Selective photothermolysis of lipid-rich tissues: a free electron laser study. Lasers Surg

Med. 2006;38(10):913–9.

101. Sakamoto FH, Doukas AG, Farinelli WA, Tannous Z, Shinn M, Benson S, et al.

Selective photothermolysis to target sebaceous glands: theoretical estimation of parameters

and preliminary results using a free electron laser. Lasers Surg Med. 2012;44(2):175–83.