Marc S. Penn and Megan KamathNovel Mechanisms for Maintaining Endothelial Barrier Function in Sepsis

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is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231Circulation doi: 10.1161/CIRCULATIONAHA.112.146100

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Editorial

Novel Mechanisms for Maintaining Endothelial BarrierFunction in Sepsis

Marc S. Penn, MD, PhD; Megan Kamath, MD

The pathophysiology of multiple disease states, includingsepsis,1,2 hypertension,1 atherosclerosis,35 and ischemia-reperfusion injury,6 is characterized by loss of the endothelialbarrier function leading to the accumulation of plasma-borneproteins, tissue edema, and cell death. Importantly therapeuticstrategies that inhibit loss of endothelial barrier functionhave been associated with strategies that improve out-comes.7 There is growing understanding of the role ofadherens in maintaining the integrity of the endothelialbarrier function8 and the molecular pathways that regulateadheren expression.9

Article see p 2728Treatment strategies that have been shown to enhance

endothelial barrier function have typically focused on pre-venting endothelial activation,10 inhibiting endothelial cellfree radical generation,6 and targeting intracellular kinases.11Many studies demonstrate that upregulation or prevention ofRac1 inhibition during endothelial cell activation leads topreserved endothelial barrier function.11

In addition to edema, sepsis is characterized by endothelialactivation, and the generation of microemboli attributed to theupregulation of the tissue factormediated clotting cascadefurther complicates the clinical course of sepsis.12 Preclinicalstudies have demonstrated that the cytokine cascade inducedby sepsis adversely affects endothelial and intimal barrierfunctions.1 In addition to decreasing endothelial barrier func-tion, the cytokine cascades have been shown to activate theclotting cascade through endothelial cell tissue factor expres-sion.10 This upregulation of tissue factor can lead activationof the extrinsic coagulation cascade and, because of thegeneration of factors Xa and Va, the generation of thrombin(Figure). The potential therapeutic benefit of downregulatingendothelial activation and thrombin generation has beendemonstrated through the early use of activated protein C(Xigris) in patients with evidence of worsening sepsis.13

In this issue of Circulation, Aman et al14 further ourunderstanding of the molecular mechanisms regulating theRho family GTPase Rac1 and endothelial barrier function

through their studies on the molecular effects of imatinibmesylate (Gleevec). Imatinib was first approved in 2001 andhas shown clinical efficacy in the treatment of chronicmyelogenous leukemia and gastrointestinal stromal tumors.15It was designed to inhibit tyrosine kinases, including c-ablkinase, platelet-derived growth factor receptors, c-KIT, anddiscoid domain receptors 1 and 2.16 In an excellent exampleof translation medicine, Aman et al14 initiated their studies onthe effects of imatinib and endothelial barrier function basedon the observations of Overbeek et al,17 who reportedobserving resolution of interstitial edema after the adminis-tration of imatinib in a patient with suspected pulmonaryveno-occlusive disease.

In their studies, Aman et al14 investigated through in vitroand in vivo models the effects of imatinib on microvascularperfusion, extravasation of fluid, and vascular leakage, con-cluding that imatinib has a direct protective effect on theendothelial barrier through its previously unrecognized effectof imatinib, inhibition of the abl-related gene (Arg) kinase,and the subsequent preservation of Rac1.14 In vitro studiesusing human umbilical vein endothelial cells demonstratedthat imatinib inhibited of endothelial leakage in response tovascular endothelial growth factor in a dose-dependent man-ner attributed to the preservation of cell-cell junctions andprevention of gap formation. Their studies identified a novelmolecular effect of imatinib on endothelial cells, because theobserved effects were mediated through inhibition of Argkinase and preservation of Rac1 activity and endothelialbarrier function. Small-interfering RNA knockdowns ofknown tyrosine kinases, platelet-derived growth factorreceptor- and -, c-Abl, and discoid domain receptor 1, didnot mimic the effects of imatinib on thrombin-inducedendothelial barrier dysfunction, whereas knockdown of Argdid.

Murine models were used to demonstrate the protectiveeffect of imatinib in vivo. Extravasation of fluid was evalu-ated with Evan blue. Of note, however, vascular leakage andpulmonary edema were compared between mice pretreatedwith imatinib or saline. Although significant benefits werenoted in these experiments in mice pretreated with imatinib,the potential clinical relevance of this finding for patientswith ongoing sepsis remained unclear. To address the clinicalrelevance, the investigators assessed the effects of imatinibtreatment in animals with ongoing sepsis induced by cecalligation and puncture. In this model, imatinib was adminis-tered 6 and 18 hours after induction of sepsis with assessmentof vascular leakage by Evan blue dye extravasation. Thestudy results demonstrated a significant attenuation of vascu-lar leakage in the kidneys and the lungs 24 hours after theinduction of sepsis.14

The opinions expressed in this article are not necessarily those of theeditors or of the American Heart Association.

From the Department of Integrative Medical Sciences, Northeast OhioMedical University, Rootstown, OH (M.S.P.); and Summa Cardiovascu-lar Institute (M.S.P.) and Department of Medicine (M.K.), Summa HealthSystem, Akron, OH.

Correspondence to Marc S. Penn, MD, PhD, Northeast Ohio MedicalUniversity, Summa Cardiovascular Institute, Summa Health System, 525E Market St, Akron, OH 44304. E-mail mpenn2@neomed.edu

(Circulation. 2012;126:2677-2679.) 2012 American Heart Association, Inc.Circulation is available at http://circ.ahajournals.orgDOI: 10.1161/CIRCULATIONAHA.112.146100

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The data from this study further define the molecularmechanisms associated with loss of endothelial barrier func-tion in sepsis and the potential for imatinib inhibition ofendothelial dysfunction through preservation of rac1 (Figure).The data further demonstrate a novel role for Arg kinase ininducing endothelial barrier dysfunction in sepsis. Beyond thespecific effects of imatinib, these data further define the thera-peutic potential of inhibiting thrombin in sepsis. The inhibitioncan either be through inhibition of thrombin generation (eg,with activated protein C) or specific downstream effects ofthrombin, such as Arg kinase, as seen in the study by Amanet al 14 with imatinib (Figure).

The improvements observed in response to imatinib are incontrast to those obtained with 3-hydroxy-3-methylglutaryl co-enzyme A reductase inhibitors. 3-Hydroxy-3-methylglutaryl co-enzyme A reductase inhibitors are known to aid in themaintenance of endothelial barrier function, yet the inhibitionof 3-hydroxy-3-methylglutaryl coenzyme A reductase blocksgeranylgeranylation and downstream translocation of RhoAand Rac1 to the cell membrane. Despite the downregulationof Rac1 activity, 3-hydroxy-3-methylglutaryl coenzyme Areductase has been shown to inhibit thrombin-mediated de-creases in endothelial barrier function in vitro5,18 and in vivo.5Whether this implies an as yet to be identified off-target effectof statin therapy or the inhibition of superoxide through theparallel inhibition of reduced nicotinamide-adenine dinucle-otide phosphate oxidase remains to be determined.

On a cellular level, imatinib protects the endothelial barrierand may prevent edema formation in the setting of thrombingeneration; however, it is interesting to note that periorbitaland pedal edemas are 2 of the most common adverse effectsof chronic imatinib treatment.19,20 Clearly whether these andother adverse effects represent confounding or problematic

consequences of off-target effects of imatinib on preservationof endothelial barrier function awaits further investigation.The authors hypothesize that duration of treatment andspecific inhibition of Arg and c-abl in individual vascularbeds may account for these findings. Given that treatment ofsepsis with imatinib could be achieved with short-term use, itmay lessen the potential for significant adverse effects attrib-uted to inhibition of off-target kinases. Given the limitedtreatments options and the devastating consequences of sig-nificant loss of endothelial barrier function in sepsis, it wouldappear that further investigation may be warranted to deter-mine whether this interesting preclinical study has identifiedreal clinical potential.

Sources of FundingThis work was supported by the generous support of the SkirballFoundation and the Corbin Foundation.

DisclosuresNone.

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Figure. Schematic representation of activation of the extrinsiccoagulation cascade through the endothelial upregulation of tis-sue factor (TF) and tissue factor VIIa (FVIIa)-mediated conver-sion of factor X (FX) to activated factor X (FXa). Combined FXawith activated factor V (FVa) cleaves prothrombin to thrombin.Activated protein C inhibits FVa function (as well as factor VIIIa,not shown), leading to decreased generation of thrombin. Ima-tinib inhibits the downstream effects of thrombin through theinhibition of Arg kinase and preservation of Rac1 activity.

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KEY WORDS: Editorials edema endothelial cells shock

Penn and Kamath Imatinib Effects on Sepsis 2679

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