Infusion of different hematopoietic stem cell populations and ex vivo expanded endothelial progenitor cells augments neovascularization of tissue after ischemia and contributes to reendothelialization after endothelial injury, thereby, providing a novel therapeutic option. However, controversy exists with respect to the identification and the origin of endothelial progenitor cells. Overall, there is consensus that endothelial progenitor cells can derive from the bone marrow and that CD133/VEGFR2 cells represent a population with endothelial progenitor capacity. However, increasing evidence suggests that there are additional bone marrow-derived cell populations (eg, myeloid cells, “side population” cells, and mesenchymal cells) and non-bone marrow-derived cells, which also can give rise to endothelial cells. The characterization of the different progenitor cell populations and their functional properties are discussed. Mobilization and endothelial progenitor cell-mediated neovascularization is critically regulated. Stimulatory (eg, statins and exercise) or inhibitory factors (risk factors for coronary artery disease) modulate progenitor cell levels and, thereby, affect the vascular repair capacity. Moreover, recruitment and incorporation of endothelial progenitor cells requires a coordinated sequence of multistep adhesive and signaling events including adhesion and migration (eg, by integrins), chemoattraction (eg, by SDF-1/CXCR4), and finally the differentiation to endothelial cells. This review summarizes the mechanisms regulating endothelial progenitor cell-mediated neovascularization and reendothelialization.

This Review is part of a thematic series on Angiogenesis, which includes the following articles: Endothelial Progenitor Cells: Characterization and Role in Vascular Biology Bone Marrow–Derived Cells for Enhancing Collateral Development: Mechanisms, Animal Data, and Initial Clinical Experiences Arteriogenesis Innate Immunity and Angiogenesis Syndecans Growth Factors and Blood Vessels: Differentiation and Maturation Ralph Kelly Guest Editor

Endothelial Progenitor Cells: Characterization and Role in Vascular Biology

Atherosclerosis begins in childhood, progresses silently through a long preclinical stage, and eventually manifests clinically, usually from middle age. Over the last 30 years, it has become clear that the initiation and progression of disease, and its later activation to increase the risk of morbid events, depends on profound dynamic changes in vascular biology.1 The endothelium has emerged as the key regulator of vascular homeostasis, in that it has not merely a barrier function but also acts as an active signal transducer for circulating influences that modify the vessel wall phenotype.2 Alteration in endothelial function precedes the development of morphological atherosclerotic changes and can also contribute to lesion development and later clinical complications.3

Appreciation of the central role of the endothelium throughout the atherosclerotic disease process has led to the development of a range of methods to test different aspects of its function, which include measures of both endothelial injury and repair. These have provided not only novel insights into pathophysiology, but also a clinical opportunity to detect early disease, quantify risk, judge response to interventions designed to prevent progression of early disease, and reduce later adverse events in patients.

The present review summarizes current understanding of endothelial biology in health and disease, the strengths and weaknesses of current testing strategies, and their potential applications in clinical research and patient care.

Although only a simple monolayer, the healthy endothelium is optimally placed and is able to respond to physical and chemical signals by production of a wide range of factors that regulate vascular tone, cellular adhesion, thromboresistance, smooth muscle cell proliferation, and vessel wall inflammation. The importance of the endothelium was first recognized by its effect on vascular tone. This is achieved by production and release of several vasoactive molecules that relax or constrict the vessel, as …

Endothelial Function and Dysfunction Testing and Clinical Relevance

Background Endothelial progenitor cells derived from bone marrow are believed to support the integrity of the vascular endothelium. The number and function of endothelial progenitor cells correlate inversely with cardiovascular risk factors, but the prognostic value associated with circulating endothelial progenitor cells has not been defined. Methods The number of endothelial progenitor cells positive for CD34 and kinase insert domain receptor (KDR) was determined with the use of flow cytometry in 519 patients with coronary artery disease as confirmed on angiography. After 12 months, we evaluated the association between baseline levels of endothelial progenitor cells and death from cardiovascular causes, the occurrence of a first major cardiovascular event (myocardial infarction, hospitalization, revascularization, or death from cardiovascular causes), revascularization, hospitalization, and death from all causes. Results A total of 43 participants died, 23 from cardiovascular causes. A first major cardi...

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Circulating endothelial progenitor cells and cardiovascular outcomes.

Background— The maintenance of endothelial integrity plays a critical role in preventing atherosclerotic disease progression. Endothelial progenitor cells (EPCs) were experimentally shown to incorporate into sites of neovascularization and home to sites of endothelial denudation. Circulating EPCs may thus provide an endogenous repair mechanism to counteract ongoing risk factor–induced endothelial injury and to replace dysfunctional endothelium. Methods and Results— In 120 individuals (43 control subjects, 44 patients with stable coronary artery disease, and 33 patients with acute coronary syndromes), circulating EPCs were defined by the surface markers CD34+KDR+ and analyzed by flow cytometry. Cardiovascular events (cardiovascular death, unstable angina, myocardial infarction, PTCA, CABG, or ischemic stroke) served as outcome variables over a median follow-up period of 10 months. Patients suffering from cardiovascular events had significantly lower numbers of EPCs (P<0.05). Reduced numbers of EPCs were as...

Reduced Number of Circulating Endothelial Progenitor Cells Predicts Future Cardiovascular Events Proof of Concept for the Clinical Importance of Endogenous Vascular Repair

BACKGROUND\nEndothelial progenitor cells derived from bone marrow are believed to support the integrity of the vascular endothelium. The number and function of endothelial progenitor cells correlate inversely with cardiovascular risk factors, but the prognostic value associated with circulating endothelial progenitor cells has not been defined.\n\n\nMETHODS\nThe number of endothelial progenitor cells positive for CD34 and kinase insert domain receptor (KDR) was determined with the use of flow cytometry in 519 patients with coronary artery disease as confirmed on angiography. After 12 months, we evaluated the association between baseline levels of endothelial progenitor cells and death from cardiovascular causes, the occurrence of a first major cardiovascular event (myocardial infarction, hospitalization, revascularization, or death from cardiovascular causes), revascularization, hospitalization, and death from all causes.\n\n\nRESULTS\nA total of 43 participants died, 23 from cardiovascular causes. A first major cardiovascular event occurred in 214 patients. The cumulative event-free survival rate increased stepwise across three increasing baseline levels of endothelial progenitor cells in an analysis of death from cardiovascular causes, a first major cardiovascular event, revascularization, and hospitalization. After adjustment for age, sex, vascular risk factors, and other relevant variables, increased levels of endothelial progenitor cells were associated with a reduced risk of death from cardiovascular causes (hazard ratio, 0.31; 95 percent confidence interval, 0.16 to 0.63; P=0.001), a first major cardiovascular event (hazard ratio, 0.74; 95 percent confidence interval, 0.62 to 0.89; P=0.002), revascularization (hazard ratio, 0.77; 95 percent confidence interval, 0.62 to 0.95; P=0.02), and hospitalization (hazard ratio, 0.76; 95 percent confidence interval, 0.63 to 0.94; P=0.01). Endothelial progenitor-cell levels were not predictive of myocardial infarction or of death from all causes.\n\n\nCONCLUSIONS\nThe level of circulating CD34+KDR+ endothelial progenitor cells predicts the occurrence of cardiovascular events and death from cardiovascular causes and may help to identify patients at increased cardiovascular risk.

Circulating Endothelial Progenitor Cells and Cardiovascular Outcomes

Background— The molecular mechanisms by which physical training improves peripheral and coronary artery disease are poorly understood. Bone marrow–derived endothelial progenitor cells (EPCs) are thought to exert beneficial effects on atherosclerosis, angiogenesis, and vascular repair. Methods and Results— To study the effect of physical activity on the bone marrow, EPCs were quantified by fluorescence-activated cell sorter analysis in mice randomized to running wheels (5.1±0.8 km/d, n=12 to 16 per group) or no running wheel. Numbers of EPCs circulating in the peripheral blood of trained mice were enhanced to 267±19%, 289±22%, and 280±25% of control levels after 7, 14, and 28 days, respectively, accompanied by a similar increase of EPCs in the bone marrow and EPCs expanded from spleen-derived mononuclear cells. eNOS −/− mice and wild-type mice treated with N G -nitro-l-arginine methyl ester showed lower EPC numbers at baseline and a significantly attenuated increase of EPC in response to physical activity. Exercise NO dependently increased serum levels of vascular endothelial growth factor and reduced the rate of apoptosis in spleen-derived EPCs. Running inhibited neointima formation after carotid artery injury by 22±2%. Neoangiogenesis, as assessed in a subcutaneous disc model, was increased by 41±16% compared with controls. In patients with stable coronary artery disease (n=19), moderate exercise training for 28 days led to a significant increase in circulating EPCs and reduced EPC apoptosis. Conclusions— Physical activity increases the production and circulating numbers of EPCs via a partially NO-dependent, antiapoptotic effect that could potentially underlie exercise-related beneficial effects on cardiovascular diseases.

Eckart Miche

Papers

Physical Training Increases Endothelial Progenitor Cells, Inhibits Neointima Formation, and Enhances Angiogenesis