The ability of organisms spontaneously to develop collateral vessels is an important response to vascular occlusions and restores perfusion of ischaemic tissues. Pathological conditions such as hyperlipidaemia, diabetes, and hypertension lead to endothelial dysfunction, impairment of neovascularization, and insufficient tissue perfusion. Thus, in most degenerative cardiovascular diseases, these natural adaptive responses to a compromised perfusion are insufficient to block the progression of the disease and ischaemic complications. Circulating endothelial progenitor cells (EPCs) play a role in the repair of damage to blood vessels under ischaemic conditions. To date, EPC numbers have been inversely correlated with the number of cardiovascular risk factors, extent of coronary disease, and future cardiovascular events. Interestingly, hypertension is associated with a decrease in the number of circulating EPCs parallel to the increase in cardiovascular risk score. Treatments with perindopril [angiotensin-converting enzyme (ACE) inhibitor] and losartan [angiotensin receptor blocker (ARB)] both improved blood flow recovery in a model of hind limb ischaemia in spontaneously hypertensive rats. Perindopril but not losartan increased the number of circulating EPCs. Although the decrease in blood pressure is similar with ACE inhibitors and ARBs, normalization of the number of circulating EPCs by ACE inhibitors could account for the differential benefits observed for organ protection, especially for myocardial infarction protection.
Assessment of the propensity for vascular events has been based on measurement of risk factors predisposing to vascular injury. These assessments are based on the strong associations between risk factors such as hypertension, cholesterol levels, smoking, and diabetes which were first described almost a half century ago. The ability of organisms to spontaneously develop collateral vessels is an important response to tissue hypoxia and operates to restore perfusion in ischaemic tissues. Pathological conditions such as hyperlipidaemia, diabetes, and hypertension lead to endothelial dysfunction, impairment of neovascularization, and insufficient tissue perfusion. Thus, in most degenerative cardiovascular diseases, these natural adaptive responses to a compromised perfusion are insufficient to block the progression of the disease and the ischaemic complications. Vascular health is now better represented as a balance between ongoing injury and resultant vascular repair, mediated at least in part by circulating endothelial progenitor cells (EPCs). To date, risk of vascular events has been exclusively assessed in terms of propensity for vascular damage, either by assessing conventional risk factors or more recently by assaying markers of inflammation and other circulating factors related to subsequent clinical events.1
Accumulating evidence suggests that bone marrow-derived EPCs promote endothelial repair and contribute to ischaemia-induced neovascularization. Endothelial progenitor cells play an important role in accelerating re-endothelialization at areas of vascular damage, and EPC count is a viable strategy for assessing repair capacity. To date, EPC counts have been inversely correlated with the number of cardiovascular risk factors, extent of coronary disease, and future cardiovascular events.
Roles of the endothelial progenitor cells in health and disease
Progenitor cells participate in ischaemia-induced angiogenesis and are thought to be mobilized from the bone marrow and recruited by hypoxic tissues where they are incorporated into blood vessels and physically contribute to vascular endothelium. Alternatively, EPCs can facilitate angiogenesis in a paracrine fashion by secretion of angiogenic factors to mobilize bone marrow progenitors and to activate mature endothelial cells.2,3 Precise characterization of bone marrow-derived cells in experimental animals shows that the cells are recruited to sites of ischaemia, but remain in a perivascular location where they might participate in tissue revascularization through the production of paracrine factors corresponding to the ‘vasculogenic growth factors’ of the second phase and the ‘angiogenic growth factors’ of the first phase (see review in Kinnaird et al.4). The functional properties of EPCs and peripheral blood mononuclear cells show similarities.5 An increasing body of evidence suggests that cardiovascular risk factors affect the number and properties of EPCs. An inverse correlation is found between the number (and functional activity) of EPCs and cardiovascular risk factors among apparently healthy people and in patients with CAD.6,7 The number of EPCs correlates with endothelial function and is a better predictor of this than the patient's combined Framingham risk factor score7 (Figure 1). Several clinical trials have evaluated the effect of autologous transplants of bone marrow mononuclear cells (BM-MNCs) or circulating progenitor cells in ischaemic myocardium, but the results are still the subject of controversy.8,9 Double-blind, placebo-controlled studies have not yet definitely established whether this approach is effective.
Circulating endothelial progenitor cells and cardiovascular risk (from Hill et al.7).
Cardiovascular risk factors and endothelial progenitor cells10
Specific risk factors, such as diabetes and hypercholesterolaemia, inhibit vessel growth. Interestingly, hypertension also hampers angiogenesis and collateral growth.7,11–14 Alteration in the process of angiogenic tissue repair is probably mediated by a reduction in the protein levels of key pro-angiogenic growth factors such as vascular endothelial growth factor (VEGF) and hepatocyte growth factor in hypertensive animals.15,16 Endothelial dysfunction may also contribute to impaired angiogenesis in human and experimental hypertension.17–19
Multiple studies have consistently reported an association between lipid metabolism and the number and activity of human EPCs. Endothelial progenitor cell colony-forming units are significantly reduced in number in relatively healthy subjects with elevated serum cholesterol.7 The functional activities of isolated EPCs, such as proliferation, migration, adhesion, and in vitro vasculogenic capacity, are also impaired in patients with hypercholesterolaemia.20 In patients with coronary artery disease, LDL cholesterol inversely correlates with the number of circulating EPCs.6
Among risk factors, hypertension is the strongest predictor of EPC migratory impairment.7 It is interesting to remark that in adult subjects without a history of cardiovascular disease, the number of circulating EPCs is inversely correlated with the Framingham risk score, which includes systolic blood pressure as a major component. One possible mechanism explaining the lower number of circulating EPCs in hypertensives is the accelerated senescence of EPCs demonstrated both in hypertensive animals and humans.21 Alternatively, it may be that progenitor cell dysfunction plays a role in the pathogenesis of hypertension. Hypertension is associated with an increase in oxidative stress constituting an underlying pathogenic mechanism that affects both angiogenesis and vasculogenesis. In parallel, ROS levels are higher in BM-MNCs isolated from hypertensive rats than from normotensive controls, and different subunits of NADPH oxidase are up-regulated. Increases in ROS levels decrease bone marrow cell differentiation into cells of endothelial phenotype in vitro and hamper their therapeutic in vivo effect.22,23 Moreover, angiotensin II diminishes telomerase activity in EPCs, accelerates the onset of EPC senescence through an increase in oxidative stress, and affects in vitro EPC proliferation.24 Angiotensin II also potentiates VEGF-induced network formation by EPCs, probably by up-regulation of the VEGF receptor in the endothelial cells.
Ischaemia-induced neovascularization is impaired in diabetes mellitus.25,26 The number of EPCs is reduced in both type 1 and type 2 diabetes.27 Furthermore, marked EPC dysfunction may underlie mechanisms involved in the pathogenesis of vascular complications in diabetic patients. Endothelial progenitor cell proliferation, adhesion, and angiogenic properties are impaired in this setting. A decreased EPC count and EPC dysfunction are inversely related to the levels of haemoglobin A1c, implying that the degree of glycaemic dysregulation is associated with EPC pathophysiology.28 Diabetic EPCs adhere normally to fibronectin, collagen, and quiescent endothelial cells, but show decreased adherence to human umbilical vein endothelial cells activated by tumour necrosis factor-α. Notably, thrombospondin-1 mRNA expression is significantly up-regulated in diabetic EPCs, and EPC adhesion is decreased in vitro and in vivo.29 Several other functions of progenitor cells might be impaired by cardiovascular risk factors. Hence, impairment of progenitor cell adhesion to endothelium might also participate in progenitor cell dysfunction in hypertension. The clonogenic and adhesion capacity of cultured EPCs is significantly lower in diabetic patients with peripheral arterial disease than in diabetic patients without.30
Other risk factors
Smoking is a significant predictor of reduced circulating and cultured endothelial progenitors. Moreover, EPCs from heavy smokers die prematurely during the early phase of culture.31 Homocysteine, which is another common cardiovascular risk factor, was shown to decrease the number and to impair the activity of EPCs from human peripheral blood.32 Asymmetric dimethylarginine (ADMA), an endogenous NO synthase inhibitor, contributes to endothelial dysfunction and inhibition of angiogenesis and is an independent biomarker of future major adverse cardiovascular events or death.33 Circulating ADMA levels inversely correlate with the number of progenitor cells, and ADMA inhibits EPC function, at least in vitro.34
Pharmacological improvement of ischaemia-induced neovascularization in hypertension
In experimental models of hypertension, as well as in patients with essential hypertension, endothelial function is impaired and may contribute to reduced vessel regeneration in hypoxic under-perfused tissues. Several experimental and clinical studies have suggested that statins, angiotensin-converting enzyme (ACE) inhibitors, angiotensin II type 1 receptor blockers (ARBs), PPAR-γ agonists, and erythropoietin increase the number and functional activity of EPCs and might improve the microcirculatory network. The underlying mechanisms remain largely to be defined, but likely include activation of the PI3-kinase/Akt pathway and endothelial nitric oxide synthase, as well as inhibition of NAD(P)H oxidase activity of progenitor cells.35
Blockers of the renin–angiotensin system, specifically ACE inhibitors and ARBs, are well-established antihypertensive drugs. Both classes ameliorate vascular function in the spontaneously hypertensive rat (SHR)36,37 and in hypertensive patients, suggesting that restoration of endothelial function may contribute to their beneficial effects. ACE inhibitors are the most effective antihypertensive drug class in improving endothelium-dependent vasodilation in large arteries of patients with essential hypertension, suggesting that ACE inhibitors and ARBs have differential effects.38–40 Administration of ACE inhibitors is associated with high levels of circulating EPCs in patients with coronary artery disease.41 Interestingly, patients treated with standard antihypertensives, i.e. combination therapy with hydrochlorothiazide and atenolol, show no effect on the number of circulating EPCs.17,42 ACE inhibitors and ARBs suppress ROS generation in the heart of hypertensive animals with diastolic heart failure.43 In addition, both ARBs and ACE inhibitors inhibit vascular remodelling and reduce ROS in stroke-prone SHRs, partially via a reduction in NAD(P)H oxidase. Treatment with ACE inhibitors, ARBs, or antioxidants (NAC, apocynin) reduces ROS levels and restores the pro-angiogenic effects of BM-MNCs isolated from SHRs.44
We found that chronic treatment with an ACE inhibitor (perindopril) or an ARB (losartan) similarly decreased blood pressure levels in SHRs with experimental ischaemia-induced neovascularization.22,23 Perindopril and losartan restored ischaemia-induced neovascularization in this setting. Interestingly, bone marrow cells from untreated hypertensive rats have reduced pro-angiogenic potential, probably because of reduced ability to move into the peripheral circulation and to differentiate into EPCs in vitro.
We next sought to investigate the pro-angiogenic potential of BM-MNCs in our model of hind limb ischaemia. Angiography score, capillary density, and foot perfusion were significantly decreased in ischaemic SHRs receiving BM-MNCs isolated from untreated SHRs, compared with those transplanted with BM-MNCs isolated from normotensive control rats. Bone marrow cells from SHRs chronically treated with perindopril or losartan displayed a restored pro-angiogenic potential when injected in untreated ischaemic SHRs. Interestingly, treatment of SHRs with perindopril raised the number of circulating EPCs; losartan did not significantly affect the number of EPCs in the peripheral circulation (Figure 2).22 Perindopril restored progenitor cell-related function and subsequently post-ischaemic revascularization in non-hypertensive diabetic animals.45
Quantitative evaluation of circulating endothelial progenitor cells (c-kit and CD34-positive cells) in the blood of Wistar–Kyoto rats (WKY) and spontaneously hypertensive rats with hind limb ischaemia. Per cent values of the numbers of circulating endothelial progenitor cells in non-ischaemic untreated normotensive rats. **P < 0.01, ***P < 0.001 vs. WKY; †P < 0.05, †††P < 0.001 vs. untreated SHR.22
In conclusion, the decrease in the number of circulating EPCs and in their pro-angiogenic capacity likely participates in the hypertension-induced abrogation of post-ischaemic vessel growth and thus in the ischaemic complications of hypertension. The ACE inhibitor perindopril and the AT1 receptor blocker losartan restored post-natal vasculogenesis in this setting. However, despite their similar lowering of blood pressure, only perindopril increased the number of circulating progenitor cells. This difference might account for the differential benefits observed in ischaemic protection.
This work was partially funded by a grant from Servier.
Conflict of interest: Dr Silvestre and Dr Lévy received advisory board or speaker's fees from Novartis, Pfizer, Roche and Servier.
. Combination of the angiotensin-converting enzyme inhibitor perindopril and the diuretic indapamide activate postnatal vasculogenesis in spontaneously hypertensive rats. J Pharmacol Exp Ther 2008;325:766-773.
. Treatment of spontaneously hypertensive rats with rosiglitazone and/or enalapril restores balance between vasodilator and vasoconstrictor actions of insulin with simultaneous improvement in hypertension and insulin resistance. Diabetes 2006;55:3594-3603.
. Angiotensin II type 1 receptor antagonist and angiotensin-converting enzyme inhibitor altered the activation of Cu/Zn-containing superoxide dismutase in the heart of stroke-prone spontaneously hypertensive rats. Hypertens Res 2005;28:67-77.