Endothelial progenitor cells in cirrhosis: The more, the merrier?
Authors: Pierre-Emmanuel Rautou
Journal: Journal of Hepatology (2012)

Increased number and function of endothelial progenitor cells stimulate angiogenesis by resident liver sinusoidal endothelial cells (SECs) in cirrhosis through paracrine factorsJournal of HepatologyVol. 57Issue 6PreviewRecent studies have shown a pathological role of angiogenesis in the progression of chronic liver diseases (CLDs). The present study focused on numbers and angiogenic functions of circulating endothelial progenitor cells (EPCs) in patients with cirrhosis. Full-Text PDF Portal hypertension is a severe and frequent complication of chronic liver diseases. Its consequences represent a leading cause of death or of liver transplantation in patients with cirrhosis. Its pathogenesis mainly originates from an elevation in hepatic vascular resistance to portal blood flow due to structural and dynamic factors. Structural factors include fibrosis, vascular remodeling with capillarization of sinusoids, vascular occlusion, and regenerative nodule formation. The dynamic component is due to increased hepatic vascular tone, related to reduced bioavailability of intrahepatic vasodilators, particularly of nitric oxide, and increased activity of several endogenous vasoconstrictors [[1]Garcia-Pagan J.C. Gracia-Sancho J. Bosch J. Functional aspects on the pathophysiology of portal hypertension in cirrhosis.J Hepatol. 2012; 57: 458-461Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar]. Secondarily to the increased hepatic vascular resistance, there is a progressive splanchnic arterial vasodilatation, that increments portal blood flow, and aggravates and perpetuates portal hypertension. In recent years, angiogenesis, a dynamic process leading to the formation of new vessels from pre-existing blood vessels [[2]Thabut D. Shah V. Intrahepatic angiogenesis and sinusoidal remodeling in chronic liver disease: new targets for the treatment of portal hypertension?.J Hepatol. 2010; 53: 976-980Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar], has been linked to both intrahepatic and extrahepatic components of portal hypertension [2Thabut D. Shah V. Intrahepatic angiogenesis and sinusoidal remodeling in chronic liver disease: new targets for the treatment of portal hypertension?.J Hepatol. 2010; 53: 976-980Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 3Bosch J. Abraldes J.G. Fernandez M. Garcia-Pagan J.C. Hepatic endothelial dysfunction and abnormal angiogenesis: new targets in the treatment of portal hypertension.J Hepatol. 2010; 53: 558-567Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar]. Within the liver, angiogenesis is postulated to contribute to portal hypertension by promoting fibrogenesis. Indeed, angiogenesis is a typical feature of liver fibrosis. Moreover, pharmacologic interventions that inhibit angiogenesis, especially using receptor tyrosine kinase inhibitors such as sorafenib or sunitinib, or molecules targeting vascular endothelial growth factor (VEGF) receptor type 2 or Tie2, induce a decrease in hepatic fibrosis [[2]Thabut D. Shah V. Intrahepatic angiogenesis and sinusoidal remodeling in chronic liver disease: new targets for the treatment of portal hypertension?.J Hepatol. 2010; 53: 976-980Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar]. Outside of the liver, angiogenesis contributes to the formation of portosystemic collaterals and the increase in splanchnic blood flow [[3]Bosch J. Abraldes J.G. Fernandez M. Garcia-Pagan J.C. Hepatic endothelial dysfunction and abnormal angiogenesis: new targets in the treatment of portal hypertension.J Hepatol. 2010; 53: 558-567Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar]. Portal hypertension is characterized by an extensive network of portosystemic collateral vessels, which include the esophageal and gastric varices. Evidence supporting a role for extrahepatic angiogenesis in the pathogenesis of portal hypertension comprises recent investigations demonstrating that VEGF, a potent angiogenic factor, is overexpressed in splanchnic organs from portal hypertensive animals. The expression of VEGF receptor-2 and the endothelial cell marker CD31 is also increased in the splanchnic territory in experimental models of portal hypertension. Importantly, blockade of VEGF-derived signaling markedly decreases splanchnic vascularization and portal venous inflow in rodent models of portal hypertension [1Garcia-Pagan J.C. Gracia-Sancho J. Bosch J. Functional aspects on the pathophysiology of portal hypertension in cirrhosis.J Hepatol. 2012; 57: 458-461Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar, 3Bosch J. Abraldes J.G. Fernandez M. Garcia-Pagan J.C. Hepatic endothelial dysfunction and abnormal angiogenesis: new targets in the treatment of portal hypertension.J Hepatol. 2010; 53: 558-567Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar]. In this issue of the Journal of Hepatology, Kaur and colleagues reported increased levels of circulating endothelial progenitor cells (EPC) in cirrhosis patients and showed that these cells stimulated angiogenesis in vitro (Fig. 1) [[4]Kaur S. Tripathi D. Dongre K. Garg V. Rooge S. Mukopadhyay A. et al.Increased number and function of endothelial progenitor cells stimulate angiogenesis by resident liver sinusoidal endothelial cells (SECs) in cirrhosis through paracrine factors.J Hepatol. 2012; 57: 1193-1198Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar]. This study represents an important step forward in the understanding of the pathogenesis of intrahepatic angiogenesis in cirrhosis and suggests a specificity of cirrhotic EPCs. EPCs are immature precursor cells, smaller than 15 μm that are detectable in blood and bone marrow [5Fadini G.P. Losordo D. Dimmeler S. Critical reevaluation of endothelial progenitor cell phenotypes for therapeutic and diagnostic use.Circ Res. 2012; 110: 624-637Crossref PubMed Scopus (543) Google Scholar, 6Sabatier F. Camoin-Jau L. Anfosso F. Sampol J. Dignat-George F. Circulating endothelial cells, microparticles and progenitors: key players towards the definition of vascular competence.J Cell Mol Med. 2009; 13: 454-471Crossref PubMed Scopus (186) Google Scholar]. They are rare since their concentration in blood is around 0.005–0.01% of the white blood cells [[5]Fadini G.P. Losordo D. Dimmeler S. Critical reevaluation of endothelial progenitor cell phenotypes for therapeutic and diagnostic use.Circ Res. 2012; 110: 624-637Crossref PubMed Scopus (543) Google Scholar]. In vitro, EPCs are characterized by their ability to form adherent colonies that proliferate and differentiate into endothelial lineage. In vivo, they contribute to angiogenesis within ischemic sites or to vascular repair after vessel wall injury [[6]Sabatier F. Camoin-Jau L. Anfosso F. Sampol J. Dignat-George F. Circulating endothelial cells, microparticles and progenitors: key players towards the definition of vascular competence.J Cell Mol Med. 2009; 13: 454-471Crossref PubMed Scopus (186) Google Scholar]. Nevertheless, accumulating data demonstrate that these properties identify a heterogeneous pool of cells with regard to their origin, differentiation and functional characteristics, as reviewed elsewhere [[5]Fadini G.P. Losordo D. Dimmeler S. Critical reevaluation of endothelial progenitor cell phenotypes for therapeutic and diagnostic use.Circ Res. 2012; 110: 624-637Crossref PubMed Scopus (543) Google Scholar]. In the absence of an unambiguous and consistent definition of EPCs, these cells are typically identified either by surface antigens using flow cytometry, or by in vitro colony-forming assays. Using flow cytometry, the minimal antigenic profile should include one or more marker of immaturity (usually CD34 and/or CD133 in humans; CD34, c-kit, or Sca-1 in mice), and one or more marker of endothelial commitment (usually VEGF receptor-2). The alternative approach, i.e., colony formation in culture, measures not only absolute numbers of EPCs, but also their overall proliferative capacity and may be more reflective of EPC capacity for endothelial repair. Kaur and colleagues used both assays and observed higher circulating EPC levels in chronic liver disease patients without hepatocellular carcinoma (HCC) than in controls. This increase was more pronounced in cirrhosis patients and inversely correlated with prothrombin time. Likewise, another group of investigators previously found a borderline significant increase in circulating EPC levels in cirrhosis patients without HCC [[7]Sieghart W. Fellner S. Reiberger T. Ulbrich G. Ferlitsch A. Wacheck V. et al.Differential role of circulating endothelial progenitor cells in cirrhotic patients with or without hepatocellular carcinoma.Dig Liver Dis. 2009; 41: 902-906Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar]. Interestingly, EPC levels correlated with hepatic venous pressure gradient [[7]Sieghart W. Fellner S. Reiberger T. Ulbrich G. Ferlitsch A. Wacheck V. et al.Differential role of circulating endothelial progenitor cells in cirrhotic patients with or without hepatocellular carcinoma.Dig Liver Dis. 2009; 41: 902-906Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar]. Three other studies reported either no difference or lower circulating EPC levels in cirrhosis patients as compared to controls [8Chen C.H. Chang L.T. Tung W.C. Chen Y.L. Chang C.L. Leu S. et al.Levels and values of circulating endothelial progenitor cells, soluble angiogenic factors, and mononuclear cell apoptosis in liver cirrhosis patients.J Biomed Sci. 2012; 19: 66Crossref PubMed Scopus (20) Google Scholar, 9Ho J.W. Pang R.W. Lau C. Sun C.K. Yu W.C. Fan S.T. et al.Significance of circulating endothelial progenitor cells in hepatocellular carcinoma.Hepatology. 2006; 44: 836-843Crossref PubMed Scopus (124) Google Scholar, 10Lemoli R.M. Catani L. Talarico S. Loggi E. Gramenzi A. Baccarani U. et al.Mobilization of bone marrow-derived hematopoietic and endothelial stem cells after orthotopic liver transplantation and liver resection.Stem Cells. 2006; 24: 2817-2825Crossref PubMed Scopus (79) Google Scholar]. Several factors can account for this discrepancy. First, controls were not matched for cardio-vascular risk factors in any of the above mentioned studies. It limits the interpretation of the results since absolute number and functional capacity of EPCs are reduced in individuals with diabetes, smoking, arterial hypertension, hypercholesterolemia, obesity or aging [5Fadini G.P. Losordo D. Dimmeler S. Critical reevaluation of endothelial progenitor cell phenotypes for therapeutic and diagnostic use.Circ Res. 2012; 110: 624-637Crossref PubMed Scopus (543) Google Scholar, 6Sabatier F. Camoin-Jau L. Anfosso F. Sampol J. Dignat-George F. Circulating endothelial cells, microparticles and progenitors: key players towards the definition of vascular competence.J Cell Mol Med. 2009; 13: 454-471Crossref PubMed Scopus (186) Google Scholar]. Some of these risk factors, such as diabetes and smoking, are frequent in cirrhosis patients and may mask the impact of cirrhosis on EPC levels. For instance, in the study by Chen and colleagues, 52% and 31% of cirrhosis patients were smokers and had diabetes, respectively. This prevalence was not mentioned in controls but we can assume that it was lower [[8]Chen C.H. Chang L.T. Tung W.C. Chen Y.L. Chang C.L. Leu S. et al.Levels and values of circulating endothelial progenitor cells, soluble angiogenic factors, and mononuclear cell apoptosis in liver cirrhosis patients.J Biomed Sci. 2012; 19: 66Crossref PubMed Scopus (20) Google Scholar]. Second, these three studies likely suffer from a limitation in power since they included only 10–16 cirrhosis patients without HCC [8Chen C.H. Chang L.T. Tung W.C. Chen Y.L. Chang C.L. Leu S. et al.Levels and values of circulating endothelial progenitor cells, soluble angiogenic factors, and mononuclear cell apoptosis in liver cirrhosis patients.J Biomed Sci. 2012; 19: 66Crossref PubMed Scopus (20) Google Scholar, 9Ho J.W. Pang R.W. Lau C. Sun C.K. Yu W.C. Fan S.T. et al.Significance of circulating endothelial progenitor cells in hepatocellular carcinoma.Hepatology. 2006; 44: 836-843Crossref PubMed Scopus (124) Google Scholar, 10Lemoli R.M. Catani L. Talarico S. Loggi E. Gramenzi A. Baccarani U. et al.Mobilization of bone marrow-derived hematopoietic and endothelial stem cells after orthotopic liver transplantation and liver resection.Stem Cells. 2006; 24: 2817-2825Crossref PubMed Scopus (79) Google Scholar]. HCC patients should be analyzed separately as HCC is associated with an increase in circulating EPC levels [7Sieghart W. Fellner S. Reiberger T. Ulbrich G. Ferlitsch A. Wacheck V. et al.Differential role of circulating endothelial progenitor cells in cirrhotic patients with or without hepatocellular carcinoma.Dig Liver Dis. 2009; 41: 902-906Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar, 9Ho J.W. Pang R.W. Lau C. Sun C.K. Yu W.C. Fan S.T. et al.Significance of circulating endothelial progenitor cells in hepatocellular carcinoma.Hepatology. 2006; 44: 836-843Crossref PubMed Scopus (124) Google Scholar, 11Yu D. Sun X. Qiu Y. Zhou J. Wu Y. Zhuang L. et al.Identification and clinical significance of mobilized endothelial progenitor cells in tumor vasculogenesis of hepatocellular carcinoma.Clin Cancer Res. 2007; 13: 3814-3824Crossref PubMed Scopus (79) Google Scholar]. Third, these studies did not mention the severity of cirrhosis in patients without HCC, while circulating EPC levels seem to be increased primarily in advanced cirrhosis patients [8Chen C.H. Chang L.T. Tung W.C. Chen Y.L. Chang C.L. Leu S. et al.Levels and values of circulating endothelial progenitor cells, soluble angiogenic factors, and mononuclear cell apoptosis in liver cirrhosis patients.J Biomed Sci. 2012; 19: 66Crossref PubMed Scopus (20) Google Scholar, 9Ho J.W. Pang R.W. Lau C. Sun C.K. Yu W.C. Fan S.T. et al.Significance of circulating endothelial progenitor cells in hepatocellular carcinoma.Hepatology. 2006; 44: 836-843Crossref PubMed Scopus (124) Google Scholar, 10Lemoli R.M. Catani L. Talarico S. Loggi E. Gramenzi A. Baccarani U. et al.Mobilization of bone marrow-derived hematopoietic and endothelial stem cells after orthotopic liver transplantation and liver resection.Stem Cells. 2006; 24: 2817-2825Crossref PubMed Scopus (79) Google Scholar]. Altogether, these data suggest that circulating EPC levels are increased in patients with advanced cirrhosis. Their potential interest as biomarkers of fibrosis and/or portal hypertension remains to be assessed in large cohorts of patients with controls matched for cardiovascular risk factors. The mechanism responsible for this increase in circulating EPCs levels in cirrhosis is unknown. If acute ischemia generally increases circulating EPC levels, reduced levels and/or function are reported in patients with chronic cardiovascular diseases, such as stable coronary artery disease, congestive heart failure, cerebrovascular disease, transplant vasculopathy or stent restenosis [[6]Sabatier F. Camoin-Jau L. Anfosso F. Sampol J. Dignat-George F. Circulating endothelial cells, microparticles and progenitors: key players towards the definition of vascular competence.J Cell Mol Med. 2009; 13: 454-471Crossref PubMed Scopus (186) Google Scholar]. This suggests that chronic liver ischemia potentially associated with cirrhosis may not account for elevated circulating EPC levels in cirrhosis [[12]Valla D.C. Thrombosis and anticoagulation in liver disease.Hepatology. 2008; 47: 1384-1393Crossref PubMed Scopus (83) Google Scholar]. Mobilization of EPCs depends on the nitric oxide-dependent upregulation of matrix metalloproteinase-9 expression by bone marrow stromal cells [[13]Aicher A. Heeschen C. Mildner-Rihm C. Urbich C. Ihling C. Technau-Ihling K. et al.Essential role of endothelial nitric oxide synthase for mobilization of stem and progenitor cells.Nat Med. 2003; 9: 1370-1376Crossref PubMed Scopus (1216) Google Scholar]. A large body of evidence unmasks extrahepatic overproduction of nitric oxide as one of the hallmarks of cirrhosis [[14]Hennenberg M. Trebicka J. Sauerbruch T. Heller J. Mechanisms of extrahepatic vasodilation in portal hypertension.Gut. 2008; 57: 1300-1314Crossref PubMed Scopus (123) Google Scholar]. We can speculate that this high nitric oxide production could contribute to EPC mobilization and to elevated EPC levels in cirrhosis. Kaur and colleagues demonstrated that circulating EPCs from cirrhosis patients stimulated proliferation of sinusoidal endothelial cells and their organization in tubular structures. This effect was dependent upon the release by cirrhotic EPCs of two proangiogenic growth factors: platelet-derived growth factor and VEGF. Given the above mentioned postulated contribution of hepatic angiogenesis to fibrogenesis, the authors suggested that circulating EPCs may contribute to cirrhosis progression and its complications. In contrast, several previous studies demonstrated that injection of healthy rat EPCs into cirrhosis rats decreased liver fibrosis and inflammation, increased hepatocyte proliferation and improved liver function [15Nakamura T. Torimura T. Sakamoto M. Hashimoto O. Taniguchi E. Inoue K. et al.Significance and therapeutic potential of endothelial progenitor cell transplantation in a cirrhotic liver rat model.Gastroenterology. 2007; 133 (91–107.e101)Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar, 16Sakamoto M. Nakamura T. Torimura T. Iwamoto H. Masuda H. Koga H. et al.Transplantation of EPCs ameliorates vascular dysfunction and portal hypertension in CCl(4)-induced rat liver cirrhotic model.J Gastroenterol Hepatol. 2012; Google Scholar, 17Taniguchi E. Kin M. Torimura T. Nakamura T. Kumemura H. Hanada S. et al.Endothelial progenitor cell transplantation improves the survival following liver injury in mice.Gastroenterology. 2006; 130: 521-531Abstract Full Text Full Text PDF PubMed Scopus (90) Google Scholar, 18Liu F. Liu Z.D. Wu N. Cong X. Fei R. Chen H.S. et al.Transplanted endothelial progenitor cells ameliorate carbon tetrachloride-induced liver cirrhosis in rats.Liver Transpl. 2009; 15: 1092-1100Crossref PubMed Scopus (33) Google Scholar]. Importantly, EPC injection also induced intrahepatic angiogenesis, decreased portal pressure and increased hepatic blood flow [[16]Sakamoto M. Nakamura T. Torimura T. Iwamoto H. Masuda H. Koga H. et al.Transplantation of EPCs ameliorates vascular dysfunction and portal hypertension in CCl(4)-induced rat liver cirrhotic model.J Gastroenterol Hepatol. 2012; Google Scholar]. If these data confirm the proangiogenic effect of EPCs in the cirrhotic liver, they question the view that liver angiogenesis induced by EPCs contributes to fibrosis and portal hypertension. Two hypotheses can reconcile these observations. First, the effect of EPCs on liver fibrosis and portal hypertension may depend upon their source. Kaur and colleagues found a two to threefold higher proangiogenic activity with circulating EPCs from cirrhosis patients than from healthy controls [[4]Kaur S. Tripathi D. Dongre K. Garg V. Rooge S. Mukopadhyay A. et al.Increased number and function of endothelial progenitor cells stimulate angiogenesis by resident liver sinusoidal endothelial cells (SECs) in cirrhosis through paracrine factors.J Hepatol. 2012; 57: 1193-1198Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar]. All studies reporting a favorable effect of EPC transplantation used EPCs derived from the bone marrow of healthy rats. This suggests that the proangiogenic activity of cirrhotic EPCs may be excessive and induce the formation of abnormal vessels, thus worsening liver fibrosis and portal hypertension. Alternatively, liver angiogenesis induced by cirrhotic EPCs may be beneficial and the increase in EPC levels in cirrhosis may be a compensatory mechanism. In support of this statement, EPC are generally implicated in repair to injury response [[6]Sabatier F. Camoin-Jau L. Anfosso F. Sampol J. Dignat-George F. Circulating endothelial cells, microparticles and progenitors: key players towards the definition of vascular competence.J Cell Mol Med. 2009; 13: 454-471Crossref PubMed Scopus (186) Google Scholar]. Moreover, new intrahepatic vessels could theoretically act as portal hypertension decompressing shunts [[2]Thabut D. Shah V. Intrahepatic angiogenesis and sinusoidal remodeling in chronic liver disease: new targets for the treatment of portal hypertension?.J Hepatol. 2010; 53: 976-980Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar]. Finally, beside the studies mentioned above showing that therapeutic approaches targeting aberrant vasculature structure in cirrhosis could have a beneficial effect on portal hypertension, few others did not. For example, in a study performed using two complementary models of cirrhosis, the administration of cilengitide, an inhibitor of the αvβ3 integrin that plays an important role in liver angiogenesis, promoted hepatic fibrosis and inflammation despite its anti-angiogenic effects [[19]Patsenker E. Popov Y. Stickel F. Schneider V. Ledermann M. Sagesser H. et al.Pharmacological inhibition of integrin alphavbeta3 aggravates experimental liver fibrosis and suppresses hepatic angiogenesis.Hepatology. 2009; 50: 1501-1511Crossref PubMed Scopus (141) Google Scholar]. Assessing the effect of the transplantation of cirrhotic EPCs into cirrhosis rats would help determine the correct hypothesis. In conclusion, this study not only highlights a new mechanism for intrahepatic angiogenesis but also provides new prospects. The proangiogenic effect of EPCs in cirrhosis may not be restricted to liver sinusoidal endothelial cells. EPCs may also contribute to splanchnic angiogenesis, which plays a role in the development and maintenance of splanchnic hyperemia in portal hypertension [[1]Garcia-Pagan J.C. Gracia-Sancho J. Bosch J. Functional aspects on the pathophysiology of portal hypertension in cirrhosis.J Hepatol. 2012; 57: 458-461Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar]. Moreover, in different population of subjects, circulating EPC levels correlated with endothelial function [5Fadini G.P. Losordo D. Dimmeler S. Critical reevaluation of endothelial progenitor cell phenotypes for therapeutic and diagnostic use.Circ Res. 2012; 110: 624-637Crossref PubMed Scopus (543) Google Scholar, 20Hill J.M. Zalos G. Halcox J.P. Schenke W.H. Waclawiw M.A. Quyyumi A.A. et al.Circulating endothelial progenitor cells, vascular function, and cardiovascular risk.N Engl J Med. 2003; 348: 593-600Crossref PubMed Scopus (3146) Google Scholar]. Whether EPC could contribute to the arterial vasodilation associated with portal hypertension in cirrhosis patients remains to be determined. Further studies are thus needed to delineate how EPCs may modulate portal hypertension. The author declared that he does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. P.-E.R. was supported by "Association Française pour l'Étude du Foie", by "bourse Robert Tournut" from the "Société Nationale Française de Gastroentérologie" and by the Philippe Foundation. The author declared that he does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.