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Pharmacological Research 58 (2008) 148–151

Contents lists available at ScienceDirect

Pharmacological Research

journa l homepage: www.e lsev ier .com/ locate /yphrs

ndothelial progenitor cells in vascular repair and remodeling

ihail Hristov, Christian Weber ∗

nstitute for Molecular Cardiovascular Research (IMCAR), RWTH Aachen University, Germany

r t i c l e i n f o

rticle history:Accepted 26 July 2008

eywords:tem cellsngiogenesisrterial injuryoronary artery disease

a b s t r a c t

Postnatal bone marrow contains a subtype of unique progenitor cells that have the capacity to differen-tiate into functional endothelial cells. Hence, these cells have been termed endothelial progenitor cells(EPCs). In general, circulating EPCs were characterized by the expression of CD133, CD34 and the vascularendothelial growth factor receptor-2 (VEGFR2). Recent data have additionally described some CD14+/low

myeloid subsets as functional endothelial precursors. Convincing evidence in vivo has further emergedthat the vascular homing of EPCs contributes to endothelial regeneration thereby limiting neointimalhyperplasia after arterial injury. However, in the context of primary atherosclerosis, plaque progression

and destabilization, injection of EPCs as well as application of stem-cell mobilizing factors have beenshown to correlate with conversion to unstable plaque phenotype. Clinically, the number and function ofEPCs have been positively linked with an improved endothelial function or regeneration but frequentlyinversely correlated with cardiovascular risk (factors). Thus, considering the dual contribution of EPCs invascular repair and remodeling in primary atherosclerosis versus arterial injury and identifying mech-anisms for selective control of their recruitment appears crucial to improve prediction and to directly

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. Introduction

The endothelial monolayer represents a dynamic physiologicalorder between circulating blood and the surrounding tissue thatrovides a non-adhesive surface for platelets and leukocytes butlso produces a variety of important vasoregulatory factors such asndothelin, prostaglandins and nitric oxide [1]. In healthy subjects,low basal level of endothelial turnover has been described [1,2].owever, acute injury or chronic immuno-inflammatory endothe-

ial dysfunction leads to the loss of anti-thrombotic propertiesarallel to enhanced arrest and transmigration of circulating leuko-ytes. This pathological vessel remodeling gradually results inxcessive sub-endothelial accumulation of lipids and immune cells,ncontrolled proliferation of smooth muscle cells (SMCs), matrixeposition and foam cell formation [3]. Consequently, occlusivetherosclerotic plaques with luminal narrowing of the arterial wallevelop and clinically result in chronic distal tissue ischemia often

omplicated by acute stroke or myocardial infarction [3,4]. Beyondhe vascular complications above, an adequate endothelial regen-ration is also crucial for diminishing arterial stenosis secondaryollowing injury (balloon angioplasty or stent placement). This

∗ Corresponding author at: IMCAR, University Hospital Aachen, Pauwelsstr. 30,-52074 Aachen, Germany. Tel.: +49 241 80 80580; fax: +49 241 80 82716.

E-mail address: cweber@ukaachen.de (C. Weber).

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043-6618/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.phrs.2008.07.008

homeostasis.© 2008 Elsevier Ltd. All rights reserved.

ndothelial repair can occur by migration and proliferation of sur-ounding mature endothelial cells. However, mature endothelialells are terminally differentiated with a low proliferative potentialnd their capacity to substitute damaged endothelium is altogetherimited. Therefore, the endothelial regeneration may need the sup-ort of other cell types. Accumulating evidence in the past decade

ndicates that adult peripheral blood contains a unique subtypef bone marrow-derived cells with properties similar to those ofmbryonic angioblasts [5–7]. These cells have the potential to dif-erentiate into mature endothelial cells and have therefore beenermed endothelial progenitor cells (EPCs). Recent studies in ani-

als and humans unveiled the ability of EPCs to ameliorate theunction of ischemic organs possibly by both induction and modu-ation of angiogenesis (incorporation and paracrine action) and toupport the re-endothelialization of injured arteries by replacinghe dysfunctional endothelial cells [8,9].

. Origin and characterization of EPCs

EPCs represent the most widely studied adult human progen-tor cell subpopulation up to now. These cells can be localized

n the bone marrow and peripheral blood or can reside in therterial wall [5,6,10]. Despite impressive amount of publishedata, the exact definition of EPCs remains rather controversialnd not yet consistent. This is related to the fact that differentubsets (CD34+/− and CD14+) of circulating cells were reported

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M. Hristov, C. Weber / Pharmac

o acquire endothelial phenotype [5,6,11,12]. Nevertheless, markerombinations most commonly used for identifying the putative cir-ulating EPC comprise CD133+CD34+VEGFR2+ or CD34+VEGFR2+

ell subsets [5,6]. Recent reports have highlighted also circulatingyeloid subpopulations (CD14+CD34low, CD14+VEGFR2+CXCR2+/−

nd CD14lowCD16+Tie-2+) as functional angiogenic cells with con-ributions to endothelial repair and ischemic or tumor angiogenesis12–14].

EPCs were cultured by plating of separated mononuclearells on fibronectin-coated dishes in endothelial-specific growthedium. This in vitro approach results in obtaining of two cell

ypes. First, adherent spindle-shaped cells develop after 4–7ays described as “early” endothelial outgrowth from peripherallood [8,14–16]. Notably, these “early” endothelial-like cells shareommon morphological properties of monocytes and endothe-ial cells (expression of CD11b, CD11c, CD14, CD31, VE-cadherinogether with LDL-uptake and lectin-binding) and secret angio-enic cytokines [14–16]. Second, after 3–4 weeks in culture a “late”roliferative outgrowth with characteristics of mature endothelialells arises and these cells possess the ability to form functionalessels and to attenuate neointimal hyperplasia in vivo [17–20].

Accordingly, EPCs correspond to a rather heterogeneous popu-ation of multiple origins and phenotype. Their common featuresncompass expression of progenitor, myeloid and endothelialarkers (CD133, CD34, vascular endothelial growth factor receptor-(VEGFR2), CD14, CD31, VE-cadherin, etc.), clonal expansion

apacity, proliferative potential and differentiation towards thendothelial lineage. Moreover, cultured endothelial outgrowthrom bone marrow or peripheral blood could be successfullymployed for molecular analysis and functional assessment in vitros well as for transplantation in terms of pro-angiogenic therapy inivo.

. Physiological and pathological complexity of EPCobilization and homing

Under steady-state conditions, progenitor cells were maintainednactive and contact the bone marrow stroma. During mobiliza-ion, the major pool of bone marrow progenitor cells, including forxample the c-kit+ population, becomes activated and shifts fromhe quiescent stromal niche into the bone marrow sinusoids [21,22].his process is initiated by matrix metalloproteinase-9-dependentleavage of membrane bound c-kit and undergoes activation by dis-inct peripheral signals [22]. One of the most essential triggers for

obilization of stem/progenitor cells is the CXC chemokine stromalell-derived factor-1� (SDF-1�/CXCL12), which specifically binds tohe CXC chemokine receptor-4 (CXCR4) [21,23]. CXCL12 is expressedr surface-immobilized not only at the bone marrow stromal cellsut also at endothelial cells, injured SMCs and activated platelets

n the periphery [21,23]. Clinically, CXCL12 has been describeds homeostatic and anti-inflammatory chemokine with reducedlasma levels being correlated with unstable coronary artery dis-ase [24]. The gene expression of CXCL12 is mainly regulated by theranscription factor hypoxia-inducible factor-1� (HIF-1�) [21,23].he expression of HIF-1� is up regulated in injured arteries and inypoxic tissue, thus mediating the CXCL12-dependent recruitmentf regenerative CXCR4+ progenitor cells [25,26].

In addition to CXCR4, EPCs also express the CXC chemokineeceptor-2 (CXCR2), and ligands (CXCL1 and CXCL7) for those recep-

ors were consecutively up regulated during inflammation but alsofter arterial injury, which may ameliorate endothelial recoveryy selectively recruiting myeloid CD14+/low EPC subsets [14,27].ccordingly, blockade of the angiogenic CXCR2 ligand keratinocyte-erived chemokine (KC/CXCL1) inhibited endothelial recovery and

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ncreased neointimal growth in Apoe−/− mice after wire-injury ofhe carotid artery [27]. Moreover, blocking the pro-inflammatory

acrophage migration inhibitory factor (MIF) associated with aonversion into a more stable plaque phenotype and severelyeduced macrophage infiltration [28]. This could be explained byhe finding that MIF acts as a dual agonist of both, CXCR2 andXCR4, resulting in recruitment of macrophages but possibly alsoascular progenitor cells, which may balance their contributiono neointimal area [29]. In contrast, MIF is expected to retain aotent and pre-dominant CXCR2 activity in primary atherogene-is, amounting to regression of established plaques. Parallel to CXChemokine/receptor pairs, the CC chemokine/receptor couples alsolay a central role in the homing of EPCs after arterial injury [30,31].or instance, infused bone marrow myeloid cells over-expressingCL2 significantly accelerated endothelial healing and reduced theeointima thus unveiling the importance of the CCL2/CCR2 axis31]. Constitutively active beta-integrins and the P-selectin glyco-rotein ligand-1 were also crucially involved in the recruitment ofPCs in vitro as well as in vivo [32–34]. Last but not least, activatedlatelets at the site of arterial injury have been shown to supportrrest and differentiation of circulating EPCs [30].

Nitric oxide was identified as another powerful and specificobilizing factor for EPCs and their recruitment was consequently

mpaired in eNos−/− mice [35]. In addition, hypoxia and someytokines (e.g. VEGF, erythropoietin and G-CSF) have been showno increase the number of EPCs [15,36,37]. Similar to hypoxia inhe context of vascular damage, burn injury or surgical bypassntervention rapidly mobilized circulating CD133+VEGFR2+ EPCs38]. CD34+ cell subsets were differentially affected in patientsith heart failure, acute myocardial infarction or chronic coro-ary endothelial dysfunction [39–42]. Even more physiologically,strogens and physical training were reported to associate withlevated EPC counts [9]. Therapeutically, statins and thiazolidin-iones transiently mobilized circulating EPCs but also facilitatedheir neo-endothelial incorporation, thus suppressing neointi-

al hyperplasia [43–45]. Conversely, long-term treatment withtatins dose dependently decreased the number of circulatingD34+VEGFR2+ EPCs but increased another subpopulation oftill more mature CD34+CD144+ cells in the systemic circulation46–48]. Recent evidence further revealed positive effects of somenti-hypertensive therapeutics such as calcium antagonists andngiotensin-converting enzyme inhibitors on number and functionf EPCs [49,50].

. Involvement of EPCs in vascular repair and remodeling

Endothelial dysfunction is an early hallmark of atheroscleroticisease and may correlate to ischemic events even in the absence ofrterial obstruction. In the context of regeneration, published datarom animal studies have unveiled that EPCs effectively contributen restoring endothelial function and diminishing neointimal for-

ation after arterial injury [14,20,30,44]. EPCs have also beenhown to participate in regeneration of damaged endothelial cellsn atherosclerosis-prone Apoe−/− mice and a large percentage ofenewed endothelial cells in vascular grafts originated from circu-ating progenitors [51,52]. Parallel to these findings EPCs were alsorucially involved in neovascularization of ischemic tissue by cre-ting new vessels and by delivering of angiogenic growth factors8,9].

Translating this basic knowledge to the clinical setting has intro-uced the therapeutic application of autologous EPCs or bonearrow mononuclear cells for adjuvant cell-based treatment of

cute as well chronic myocardial ischemia [53]. The commonlinical concept currently claims an even protective role of EPCs

150 M. Hristov, C. Weber / Pharmacologica

Table 1Involvement of EPCs in primary atherosclerosis and injury repair

Stage of arterial disease Cellular effect and mechanism

Primary atherosclerosisEarly lesions (endothelial dysfunction) Protective endothelial recoveryPlaque progression Incorporation at predeliction

sitesAdvanced rupture-prone plaques Neovascularization,

destabilization

Secondary atherosclerosis after injuryRepair after plaque rupture Possible contribution to

regenerationBalloon injury/stent and transplant Reendothelialization and

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reduced neointimaMyocardial ischemia and infarction Mobilization and contribution

to tissue repair

uring the progression of atherosclerosis and further suggests thathese cells may serve as reliable biomarker for endogenous vascularepair [54]. Reduced numbers of EPCs have been associated with theramingham risk factor score, peripheral endothelial dysfunctionnd the incidence of future cardiovascular events [55,56]. Of note,he number of EPCs significantly increased in patients with unsta-le angina, while their function did not differ as compared to thoseith stable coronary artery disease [57]. Some recent studies, how-

ver, revealed that the number of circulating EPCs was not affectedy cardiovascular risk factors but obviously associated with thextent of vessel disease and long-term statin therapy [41,46,58,59].urthermore, published data in animal models have shown thatnfusion of EPCs increased lipid content and decreased collagenmount in atherosclerotic plaques of Apoe−/− mice [60]. In the sameine elevated levels of plasma CXCR2 receptor ligands such as CXCL1nd CXCL7 were clinically related to plaque destabilization [61,62].hese findings may be explained in part by influx of CXCR2+ mono-yte subsets that include also putative endothelial precursors withnflammatory, proteolytic and angiogenic properties [14,63]. Thus,he dual contribution of EPC subsets to vascular remodeling dur-ng generalized advanced atherosclerosis versus local endothelialysfunction after arterial injury need a critical re-evaluation.

. Conclusions

Further to chronic inflammation and immunological interac-ions, vascular progenitor cells have been recognized as pivotallymplicated players in the pathogenesis of atherosclerosis. In par-icular, EPCs obviously participate in endothelial maintenancend their number is affected during coronary artery disease. Aidespread EPC mobilization may associate with plaque destabi-

ization, whereas local stimulation of EPC recruitment after arterialnjury may even effectively support endothelial healing. Basedn this ambivalence, selectively controlling the mobilization andoming of EPCs will help to regulate the endogenous arterialegenerative potential (Table 1). This knowledge appears crucialn the clinical application of EPCs in terms of reliable prognos-ic/diagnostic biomarker but also of devising therapeutic targetsor cardiovascular regenerative medicine.

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