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(Circulation Research. 2004;95:343.)
© 2004 American Heart Association, Inc.

Reviews

Endothelial Progenitor Cells

Characterization and Role in Vascular Biology

Carmen Urbich, Stefanie Dimmeler

From Molecular Cardiology, Department of Internal Medicine IV, University of Frankfurt, Frankfurt, Germany.

Correspondence to Stefanie Dimmeler, PhD, Molecular Cardiology, Dept of Internal Medicine IV, University of Frankfurt, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany. E-mail Dimmeler{at}em.uni-frankfurt.de

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

	Abstract

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 
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.

Key Words: progenitor cells • neovascularization • vasculogenesis • angiogenesis • endothelial cells

	Introduction

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 
Differentiation of mesodermal cells to angioblasts and subsequent endothelial differentiation was believed to exclusively occur in embryonic development. This dogma was overturned in 1997, when Asahara and colleagues¹ published that purified CD34⁺ hematopoietic progenitor cells from adults can differentiate ex vivo to an endothelial phenotype. These cells were named "endothelial progenitor cells" (EPCs), showed expression of various endothelial markers, and incorporated into neovessels at sites of ischemia. Rafii’s group in 1998² also reported the existence of "circulating bone marrow-derived endothelial progenitor cells" (CEPCs) in the adult. Again, a subset of CD34⁺ hematopoietic stem cells was shown to differentiate to the endothelial lineage and express endothelial marker proteins such as vWF and incorporated Dil-Ac-LDL. Most convincingly, bone marrow-transplanted genetically tagged cells were covering implanted Dacron grafts.² These pioneering studies suggested the presence of circulating hemangioblasts in the adult. According to the initial discovery, EPCs or CEPCs were defined as cells positive for both hematopoietic stem cell markers such as CD34 and an endothelial marker protein as VEGFR2. Because CD34 is not exclusively expressed on hematopoietic stem cells but, albeit at a lower level, also on mature endothelial cells, further studies used the more immature hematopoietic stem cell marker CD133³ and demonstrated that purified CD133⁺ cells can differentiate to endothelial cells in vitro.⁴ CD133, also known as prominin or AC133, is a highly conserved antigen with unknown biological activity, which is expressed on hematopoietic stem cells but is absent on mature endothelial cells and monocytic cells (see review).⁵ Thus, CD133⁺VEGFR2⁺ cells more likely reflect immature progenitor cells, whereas CD34⁺VEGFR2⁺ may also represent shedded cells of the vessel wall. At present, it is unclear whether CD133 only represents a surface marker or has a functional activity involved in regulation of neovascularization.

Overall, controversy exists with respect to the identification and the origin of endothelial progenitor cells, which are isolated from peripheral blood mononuclear cells by cultivation in medium favoring endothelial differentiation. In peripheral blood mononuclear cells, several possible sources for endothelial cells may exist: (1) the rare number of hematopoietic stem cells, (2) myeloid cells, which may differentiate to endothelial cells under the cultivation selection pressure, (3) other circulating progenitor cells (eg, "side population" cells), and (4) circulating mature endothelial cells, which are shed off the vessel wall⁶ and adhere to the culture dishes. First evidence that there is more than one endothelial progeny within the circulating blood was provided by Hebbel and colleagues, who showed that morphological and functional distinct endothelial cell populations can be grown out of peripheral blood mononuclear cells.⁷ They stratified the different circulating endothelial cells according to their growth characteristics and morphological appearance as "spindle-like cells," which have a low proliferative capacity, and outgrowing cells. Because the outgrowing cells showed a high proliferative potential and originated predominantly from the bone marrow donors, they were considered as circulating angioblasts.⁷ The authors speculated that the spindle-like cells may likely represent mature endothelial cells, which are shed off the vessel wall. However, this hypothesis is difficult to test and has not yet been proven thus far.

Experimentally, preplating may be a way to reduce the heterogeneity of the cultivated EPCs, because this excludes rapidly adhering cells such as differentiated monocytic or possible mature endothelial cells.² However, these protocols do not eliminate myeloid and nonhematopoietic progenitor cells, which may contribute to the ex vivo cultivated cells. There is increasing evidence that myeloid cells can give rise to endothelial cells as well. Specifically, CD14⁺/CD34^– myeloid cells can coexpress endothelial markers and form tube-like structures ex vivo.⁸ Additionally, ex vivo expansion of purified CD14⁺ mononuclear cells yielded cells with an endothelial characteristic, which incorporated in newly formed blood vessels in vivo.⁹ These data would suggest that myeloid cells can differentiate (or transdifferentiate) to the endothelial lineage. Interestingly, lineage tracking showed that myeloid cells are the hematopoietic stem cell-derived intermediates, which contribute to muscle regeneration,¹⁰ suggesting that myeloid intermediates may be part of the repair capacity after injury. Moreover, a subset of human peripheral blood monocytes acts as pluripotent stem cells.¹¹

Of note, a specific problem arises when cells are ex vivo expanded and cultured, because the culture conditions (culture supplements such as FCS and cytokines, plastic) rapidly changes the phenotype of the cells. For example, supplementation of the medium with statins increased the number of endothelial cell colonies isolated from mononuclear cells.¹² Moreover, continuous cultivation was shown to increase endothelial marker protein expression.¹³ This may explain why different groups may obtain cells with different surface factor profile and functional activity although similar protocols were used for cultivation.^9,14–16 Moreover, the interaction of cells within a heterogeneous mixture of cells such as the mononuclear cells from the blood may impact the yield and the functional activity of the cultivated cells.¹⁷

Generally, several studies suggested that other cell populations beside hematopoietic stem cells also can give rise to endothelial cells (Figure 1). Thus, non-bone marrow-derived cells have been shown to replace the endothelial cells in grafts.¹⁸ In addition, adult bone marrow-derived stem/progenitor cells such as the side population cells and multipotent adult progenitor cells, which are distinct from hematopoietic stem cells, have also been shown to differentiate to the endothelial lineage.^19,20 Recently, tissue-resident stem cells have been isolated from the heart, which are capable to differentiate to the endothelial lineage.²¹ These data support the notion that it will be difficult to define the "true" endothelial progenitor cells. Overall, the field is reminiscent to immunology, where T-cells initially were defined as one cell population. However, the functional characterization (eg, cytokine release and response to stimuli) helped to identify novel T-cell subpopulations with distinct functions and capacities. Hopefully, better profiling of distinct cell populations and fate mapping studies will help to identify markers, which distinguish the circulating endothelial precursor within the blood and bone marrow/non-bone marrow-derived endothelial cells.

View larger version (42K): [in this window] [in a new window]   Figure 1. Origin and differentiation of endothelial progenitor cells. Scheme depicts the potential origin and differentiation of endothelial progenitor cells from hematopoietic stem cells and nonhematopoietic cells.

	Role of EPCs in Neovascularization

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 
Improvement of neovascularization is a therapeutic option to rescue tissue from critical ischemia.²² The finding that bone marrow-derived cells can home to sites of ischemia and express endothelial marker proteins has challenged the use of isolated hematopoietic stem cells or EPCs for therapeutic vasculogenesis. Infusion of various distinct cell types either isolated from the bone marrow or by ex vivo cultivation was shown to augment capillary density and neovascularization of ischemic tissue (Table 1 and Figure 2). In animal models of myocardial infarction, the injection of ex vivo expanded EPCs or stem and progenitor cells significantly improved blood flow and cardiac function and reduced left ventricular scarring.^23,24 Similarly, infusion of ex vivo expanded EPCs deriving from peripheral blood mononuclear cells in nude mice or rats improved the neovascularization in hind limb ischemia models.^9,15,23,25 Correspondingly, initial pilot trials indicate that bone marrow-derived or circulating blood-derived progenitor cells are useful for therapeutically improving blood supply of ischemic tissue.^26,27 Autologous implantation of bone marrow mononuclear cells in patients with ischemic limbs significantly augmented ankle-brachial index and reduced rest pain.²⁶ In addition, transplantation of ex vivo expanded endothelial progenitor cells significantly improved coronary flow reserve and left ventricular function in patients with acute myocardial infarction.²⁷

View this table: [in this window] [in a new window]   Table 1. Neovascularization Induced by Injection of Progenitor Cells: Experimental and Clinical Studies

View larger version (35K): [in this window] [in a new window]   Figure 2. Role of EPCs in vascular biology. Injection of EPCs significantly improve reendothelialization and neovascularization after injury.

Besides models of peripheral ischemia (hind limb ischemia), the angiogenic potential of EPCs was also investigated in animal models of tumor angiogenesis. Thereby, the inhibition of VEGF-responsive bone marrow-derived endothelial and hematopoietic precursor cells blocks tumor angiogenesis and growth.²⁸ The use of various different models, cell numbers, and species limits the comparability of the efficiency of distinct cell populations. However, the overall functional improvement appear similar, when isolated human CD34⁺, CD133⁺, EPC, MAPC, or murine Sca-1⁺ cells were used.^{4,9,15,20,23,25,29–32} Likewise, early spindle-like cells and late outgrowing EPCs showed comparable in vivo vasculogenic capacity.³³ These results suggest that the functional activity of the cells to augment neovascularization is rather independent of the type of (endothelial) progenitor cell used. However, the CD34^– fractions of freshly isolated bone marrow- or blood-derived mononuclear cells showed a reduced incorporation and functional activity.^24,29 These data indicate that the number of cells capable to augment neovascularization is low in this crude fraction of freshly isolated uncultivated CD34^– cells. Remarkably, terminally differentiated mature endothelial cells (HMVECs, GEAECs, and SVECs) did not improve neovascularization^15,24,33 suggesting that a not-yet-defined functional characteristic (eg, chemokine or integrin receptors mediating homing) is essential for EPC-mediated augmentation of blood flow after ischemia.

The functional capacity of EPCs to augment blood flow further does not appear to be solely attributable to a monocytic phenotype. Ex vivo cultivated EPCs from CD14⁺ mononuclear cells or CD14^– mononuclear cell starting population improved neovascularization to a similar extent, whereas the same number of freshly isolated mononuclear cells taken from the starting culture did not.⁹ Interestingly, these experimental data are supported by first clinical trials showing that freshly isolated mononuclear cells are not well suited to improve neovascularization in patients with peripheral vascular diseases.²⁶ However, monocytic cells may play a crucial role in collateral growth (arteriogenesis). Thus, the attraction of monocytic cells by monocyte chemoattractant protein-1 (MCP-1) enhanced arteriogenesis.³⁴ Moreover, depletion of the monocytes reduced PlGF-induced arteriogenesis.³⁵ A therapeutic benefit of monocyte infusion on arteriogenesis was demonstrated under conditions of monocyte deficiency induced by chemical depletion.³⁶ These data suggest that monocytic cells are necessary for arteriogenesis and possibly neovascularization. For therapeutic application, the local enhancement of monocyte recruitment might be better suited than systemic infusion of monocytic cells, which only leads to a relatively minor increase in the number of circulating monocytes.

Mechanisms by Which EPC Improve Neovascularization
Although the role of EPCs in neovascularization has been convincingly shown by several groups, the question remains: how do EPCs improve neovascularization?

Bone marrow transplantation of genetically modified cells (rosa-26, GFP, lacZ) was used to assess the incorporation of bone marrow-derived EPC into tissues. The basal incorporation rate of progenitor cells without tissue injury is extremely low.³⁷ In ischemic tissue, the incorporation rate of genetically labeled bone marrow-derived cells, which coexpress endothelial marker proteins, differs from 0% to 90% incorporation.^{19,28,37–41} Likewise, the extent of incorporation of bone marrow-derived cells in cerebral vessels after stroke varies in the literature.^42–44 Whereas two studies reported positive vessels with an average of 34% endothelial marker expressing bone marrow-derived cells,^42,43 other groups could not detect endothelial marker expressing cells.⁴⁴ High amounts (>50%) were predominantly detected in models of tumor angiogenesis.^28,40 Some studies only detected bone marrow-derived cells adjacent to vessels, which do not express endothelial marker proteins.^41,45 A reasonable explanation might be that the model of ischemia (eg, intensity of injury or ischemia)⁴⁶ significantly influences the incorporation rate. A minor ischemia might not as profoundly induce a mobilization of bone marrow-derived endothelial progenitor cells and, thus, may lead to a lower percentage of incorporation of bone marrow-derived progenitor cells. The efficiency of engraftment may additionally differ between distinct progenitor subpopulations (pure hematopoietic stem cells versus complete bone marrow cells). Indeed, therapeutic application of cells by intravenous infusion of ex vivo purified bone marrow mononuclear cells or expanded endothelial progenitor cells led to a higher incorporation rate (7% to 20% incorporation rate) as compared with the endogenously mobilized bone marrow-engrafted cells (2%).^9,47

However, the number of incorporated cells with an endothelial phenotype into ischemic tissues is generally quite low. How can such a small number of cells increase neovascularization? A possible explanation might be that the efficiency of neovascularization may not solely be attributable to the incorporation of EPCs in newly formed vessels, but may also be influenced by the release of proangiogenic factors in a paracrine manner. Indeed, the deletion of Tie-2-positive bone marrow-derived cells through activation of a suicide gene blocked tumor angiogenesis, although these cells are not integrated into the tumor vessels but are detected adjacent to the vessel.⁴¹ Thus, EPCs may act similar to monocytes/macrophages, which can increase arteriogenesis by providing cytokines and growth factors. Indeed, EPCs cultivated from different sources showed a marked expression of growth factors such as VEGF, HGF, and IGF-1 (C.U., unpublished data, 2004). Moreover, adherent monocytic cells, which were cultivated under similar conditions, but do not express endothelial marker proteins, also release VEGF, HGF, and G-CSF.¹⁴ The release of growth factors in turn may influence the classical process of angiogenesis, namely the proliferation and migration as well as survival of mature endothelial cells.⁴⁸ However, EPCs additionally incorporated into the newly formed vessel structures and showed endothelial marker protein expression in vivo. In contrast, infusion of macrophages, which are known to release growth factors,^49,50 but were not incorporated into vessel-like structures, induced only a slight increase in neovascularization after ischemia, indicating—but not proving—that the capacity of EPCs to physically contribute to vessel-like structures may contribute to their potent capacity to improve neovascularization.⁹ Further studies will have to be designed to elucidate the contribution of physical incorporation, paracrine effects and possible effects on vessel remodeling and facilitating vessel branching to EPC-mediated improvement of neovascularization.

	EPCs and Endothelial Regeneration

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 
In the past, the regeneration of injured endothelium has been attributed to the migration and proliferation of neighboring endothelial cells. More recent studies, however, indicate that additional repair mechanisms may exist to replace denuded or injured arteries. Thus, implanted Dacron grafts were shown to be rapidly covered by bone marrow-derived cells deriving from CD34⁺ hematopoietic stem cells in a dog model.² In humans, the surface of ventricular assist devices was covered by even more immature CD133-positive hematopoietic stem cells, which concomitantly express the VEGF-receptor 2.³ Additionally, Walter and coworkers demonstrated that circulating endothelial precursor cells can home to denuded parts of the artery after balloon injury.⁵¹ Bone marrow transplantation experiments revealed that bone marrow-derived cells can contribute to reendothelialization of grafts and denuded arteries.^51–53 However, in a model of transplant arteriosclerosis, bone marrow-derived cells appear to contribute only to a minor extent to endothelial regeneration by circulating cells.¹⁸ These data again indicate that there might be at least two distinct populations of circulating cells that principally are capable to contribute to reendothelialization, namely mobilized cells from bone marrow and non-bone marrow-derived cells. The latter ones may arise from circulating progenitor cells released by non-bone marrow sources (eg, tissue resident stem cells) or represent vessel wall-derived endothelial cells.^18,51–53

A rapid regeneration of the endothelial monolayer may prevent restenosis development by endothelial synthesis of antiproliferative mediators such as nitric oxide. Indeed, enhanced incorporation of ß-galactosidase-positive, bone marrow-derived cells was associated with an accelerated reendothelialization and reduction of restenosis.^51,52 Similar results were reported by Griese et al, who demonstrated that infused peripheral blood monocyte-derived EPC home to bioprosthetic grafts and to balloon-injured carotid arteries, the latter being associated with a significant reduction in neointima deposition.⁵⁴ Likewise, infusion of bone marrow-derived CD34^–/CD14⁺ mononuclear cells, which are not representing the population of the "classical hemangioblast," contributed to endothelial regeneration.¹³ The regenerated endothelium was functionally active as shown by the release of NO,¹³ which is supposed to be one of the major vasculoprotective mechanisms. Consistently, neointima development was significantly reduced after cell infusion.¹³ Whereas the regeneration of the endothelium by EPCs protects lesion formation, bone marrow-derived stem/progenitor cells may also contribute to plaque angiogenesis, thereby potentially facilitating plaque instability.⁵⁵ However, in a recent study, no influence of bone marrow cell infusion on plaque composition was detected in nonischemic mice.⁵⁶ An increase in plaque size was only detected in the presence of ischemia, suggesting that ischemia-induced release of growth factors predominantly accounts for this effect.⁵⁶

Overall, these studies implicate that regardless of the origin of circulating endothelial progenitor cells, this pool of circulating endothelial cells may exert an important function as an endogenous repair mechanism to maintain the integrity of the endothelial monolayer by replacing denuded parts of the artery (Figure 2). One can speculate that these cells may also regenerate the low grade endothelial damage by ongoing induction of endothelial cell apoptosis induced by risk factors for coronary artery disease (see review).⁵⁷ The maintenance of the endothelial monolayer may prevent thrombotic complications and atherosclerotic lesion development. Although this concept has not yet been proven, several hints from recently presented data are supportive. Thus, transplantation of ApoE^–/– mice with wild-type bone marrow reduced atherosclerotic lesion formation.⁵⁸ Moreover, various risk factors for coronary artery disease, such as diabetes, hypercholesterolemia, hypertension, and smoking, affect the number and functional activity of EPCs in healthy volunteers⁵⁹ and in patients with coronary artery disease.⁶⁰ Likewise, diabetic mice and patients were characterized by reduced functional activity of EPCs.^61–63 In addition, factors that reduce cardiovascular risk such as statins^38,51,52,64 or exercise⁶⁵ elevate EPC levels, which contribute to enhanced endothelial repair. The balance of atheroprotective and proatherosclerotic factors, thus, may influence EPC levels and subsequently reendothelialization capacity.

	Mobilization of EPCs

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 
Because EPCs contribute to reendothelialization and neovascularization, increasing the number of these cells may be an attractive therapeutic tool. The mobilization of stem cells in the bone marrow is determined by the local microenvironment, the so-called "stem cell niche," which consists of fibroblasts, osteoblasts, and endothelial cells (see review).⁶⁶ Basically, mobilizing cytokines hamper the interactions between stem cells and stromal cells, which finally allow stem cells to leave the bone marrow via transendothelial migration. Thereby, activation of proteinases such as elastase, cathepsin G, and matrix metalloproteinases (MMPs) cleave adhesive bonds on stromal cells, which interact with integrins on hematopoietic stem cells. MMP-9 was additionally shown to cleave the membrane-bound Kit ligand (mKitL) and induces the release of soluble Kit ligand (KitL; also known as stem cell factor, SCF).⁶⁷ Physiologically, ischemia is believed to be the predominant signal to induce mobilization of EPCs from the bone marrow. Ischemia thereby is believed to upregulate VEGF or SDF-1,^68,69 which in turn are released to the circulation and induce mobilization of progenitor cells from the bone marrow via a MMP-9-dependent mechanism.^30,46,67,70 Furthermore, clinical studies using gene therapy with plasmids encoding for VEGF demonstrated an augmentation of EPC levels in humans.⁷¹ Additional factors inducing mobilization of progenitor cells from the bone marrow have been initially discovered in hematology to harvest hematopoietic stem cells from the peripheral blood for bone marrow transplantation. For instance, granulocyte-colony stimulating factor (G-CSF), a cytokine, which is typically used for mobilization of CD34⁺ cells in patients, also increased the levels of circulating endothelial progenitor cells. A related cytokine, the granulocyte monocyte-colony stimulating factor (GM-CSF), augments EPC levels.³⁰ Moreover, erythropoietin (EPO), which stimulates proliferation and maturation of erythroid precursors, also increased peripheral blood endothelial progenitor cells in mice⁷² and in men.⁷³ The correlation between EPO serum levels and the number of CD34⁺ or CD133⁺ hematopoietic stem cells in the bone marrow in patients with ischemic coronary artery disease further supports an important role of endogenous EPO levels as a physiologic determinant of EPC mobilization.⁷² At present, it is not clear which of the mobilizing factors most potently elevates the levels of EPCs. SDF-1 and VEGF165 showed similar effects and rapidly mobilize hematopoietic stem cells and circulating endothelial precursor cells in animal models, whereas angiopoietin-1 induced a delayed and less pronounced mobilization of endothelial and hematopoietic progenitors.^74,75 Whereas a similar increase in white blood cell counts was achieved by G-CSF application, endothelial colonies (CFU-EC) were significantly lower in G-CSF- compared with VEGF- or SDF-1-treated mice. Of note, these data should be interpreted with caution, because the responsiveness toward cytokines may vary between different mice strains and side-by-side comparisons in humans are lacking. Moreover, the extent of increasing neutrophil and lymphocyte levels, which may provoke proinflammatory responses, has to be considered for a potential therapeutic application.

First evidence for potential pharmacological modulation of systemic EPC levels by atheroprotective drugs came from studies using HMG-CoA reductase inhibitors (statins). Statins were shown to increase the number and the functional activity of EPCs in vitro,^38,76 in mice,^38,76 and in patients with stable coronary artery disease.⁶⁴ The increase in EPC numbers was associated with increased bone marrow-derived cells after balloon injury and accelerated endothelial regeneration.^51,52 Although statins were shown to increase the number of stem cells within the bone marrow, the mechanism for enhancing EPC numbers and function may additionally include an increase in proliferation, mobilization, and prevention of EPC senescence and apoptosis.^12,38,76 Interestingly, recent studies additionally demonstrated that estrogen increased the levels of circulating EPCs.^77,78 Moreover, exercise augmented EPC levels in mice and in men.⁶⁵ The molecular signaling pathways have not been identified thus far. However, several studies indicate that the activation of the PI3K/Akt pathway, which has first been shown to be activated in mature endothelial cells by statins,⁷⁹ may also play an important role in statin-induced increase in EPC levels.^12,76 Likewise, VEGF, EPO, estrogen, and exercise are well known to augment the PI3K/Akt-pathway. Thus, these factors may share some common signaling pathways. Given that recent data showed that eNOS is essential for mobilization of bone marrow-derived stem and progenitor cells,⁴⁷ one may speculate that these stimuli may increase progenitor cell mobilization by PI3K/Akt-dependent activation of the NO-synthase within the bone marrow stromal cells. Indeed, exercise and VEGF-stimulated EPC mobilization was blunted in eNOS^–/– mice.^47,65

	Mechanism of Homing and Differentiation

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 
Although the improvement of adult neovascularization is currently under intensive investigations, the mechanism of homing and differentiation of endothelial progenitor cells is poorly understood. In a previous study assessing in vivo homing of embryonic endothelial progenitor cells derived from cord blood, the circulating cells arrested within tumor microvessels, extravasated into the interstitium, and incorporated into neovessels, suggesting that adhesion and transmigration are involved in the recruitment of endothelial progenitor cells to sites of tumor angiogenesis.⁸⁰ Thus, it is conceivable that ex vivo expanded adult EPCs and hematopoietic stem/progenitor cells may engage similar pathways for recruitment to sites of ischemia and incorporation in newly forming vessels. Recruitment and incorporation of EPCs requires a coordinated sequence of multistep adhesive and signaling events including chemoattraction, adhesion, and transmigration, and finally the differentiation to endothelial cells (Figure 3).

View larger version (47K): [in this window] [in a new window]   Figure 3. Mechanism of EPC homing and differentiation. Recruitment and incorporation of EPCs into ischemic tissue requires a coordinated multistep process including mobilization, chemoattraction, adhesion, transmigration, migration, tissue invasion, and in situ differentiation. Factors that are proposed to regulate the distinct steps are indicated.

Adhesion and Transendothelial Migration
The initial step of homing of progenitor cells to ischemic tissue involves adhesion of progenitor cells to endothelial cells activated by cytokines and ischemia and the transmigration of the progenitor cells through the endothelial cell monolayer.⁸⁰ Integrins are known to mediate the adhesion of various cells including hematopoietic stem cells and leukocytes to extracellular matrix proteins and to endothelial cells.^81–83 Integrins capable of mediating cell-cell interactions are the ß₂-integrins and the ₄ß₁-integrin. ß₁-Integrins are expressed by various cell types including endothelial cells and hematopoietic cells, whereas ß₂-integrins are found preferentially on hematopoietic cells.⁸⁴ Because adhesion to endothelial cells and transmigration events are involved in the in vivo homing of stem cells to tissues with active angiogenesis,⁸⁰ integrins such as the ß₂-integrins and the ₄ß₁-integrin may be involved in the homing of progenitor cells to ischemic tissues. Consistent with the high expression of ß₂-integrins on hematopoietic stem/progenitor cells, ß₂-integrins mediate adhesion and transmigration of hematopoietic stem/progenitor cells.^85,86 ß₂-Integrins (CD18/CD11) are expressed on peripheral blood-derived EPCs and are required for EPC-adhesion to endothelial cells and transendothelial migration in vitro (S.D., personal communication, 2004). Moreover, hematopoietic stem cells (Sca-1⁺/lin^–) lacking ß₂-integrins showed reduced homing and a lower capacity to improve neovascularization after ischemia (S.D., personal communication, 2004). Interestingly, the homing of inflammatory cells during pneumonia or myocardial ischemia in ß₂-integrin-deficient mice is mediated by the ₄ß₁-integrin^87,88 suggesting that deficiency of ß₂-integrins can in part be compensated by the ₄ß₁-integrin. Moreover, conditional deletion of the ₄-integrin selectively inhibited the homing of hematopoietic stem/progenitor cells to the bone marrow but not to the spleen,⁸⁹ suggesting that the homing of progenitor cells to different tissues is dependent on distinct adhesion molecules. Furthermore, in vitro studies showed that MCP-1 stimulated adhesion of bone marrow-derived CD34^–/CD14⁺ monocytes to the endothelium was blocked by anti-ß₁-integrin antibodies.¹³ Interestingly, in this study, adhesion of CD34^–/CD14⁺ monocytes isolated from the peripheral blood to endothelial cells was less affected by MCP-1 and was not blocked by anti-ß₁-integrin antibodies.¹³ Finally, the initial cell arrest of embryonic progenitor cell homing during tumor angiogenesis was suggested to be mediated by E- and P-selectin and P-selectin glycoprotein ligand-1.⁸⁰ Yet, it is important to underscore that this study was performed with embryonic endothelial progenitor cells. It is conceivable that different cell types may use distinct mechanisms for homing to sites of angiogenesis.

Cell-cell contacts and transmigration events might be less important for the reendothelialization of denuded arteries (in contrast to homing of progenitor cells to ischemic tissues). With respect to endothelial progenitor cells, studies investigated the contribution of integrins to reendothelialization, which is mainly driven by adhesion to extracellular matrix proteins. Adhesion of EPCs to denuded vessels appears to be mediated by vitronectin-receptors (_vß₃- and _vß₅-integrins). Thus, inhibition of _vß₃- and _vß₅-integrins with cyclic RGD peptides blocked reendothelialization of denuded arteries in vivo, suggesting that _vß₃- and _vß₅-integrins are involved in the reendothelialization of injured carotid arteries.⁵¹ However, other integrins such as the ß₁-integrins may also mediate adhesion of progenitor cells to extracellular matrix proteins during reendothelialization of denuded arteries.¹³

Chemotaxis, Migration, and Invasion
Given the low numbers of circulating progenitor cells, chemoattraction may be of utmost importance to allow for recruitment of reasonable numbers of progenitor cells to the ischemic or injured tissue. Various studies examined the factors influencing hematopoietic stem cell engraftment to the bone marrow. These factors include chemokines such as SDF-1,^90,91 lipid mediators (sphingosine-1-phosphate),⁹² as well as factors released by heterologous cells.⁹³ The factors attracting circulating EPCs to the ischemic tissue may be similar. Indeed, SDF-1 has been proven to stimulate recruitment of progenitor cells to the ischemic tissue.³¹ SDF-1 protein levels were increased during the first days after induction of myocardial infarction.⁹⁴ Moreover, overexpression of SDF-1 augmented stem cell homing and incorporation into ischemic tissues.^31,94 Interestingly, hematopoietic stem cells were shown to be exquisitely sensitive to SDF-1 and did not react to G-CSF or other chemokines (eg, IL-8 and RANTES).⁹¹ Moreover, VEGF levels are increased during ischemia and capable to act as a chemoattractive factor to EPCs.^68,70,71 Interestingly, the migratory capacity of EPCs or bone marrow cells toward VEGF and SDF-1, respectively, determined the functional improvement of patients after stem cell therapy.⁹⁵ Beside genes, which are directly upregulated by hypoxia, the invasion of immune competent cells to the ischemic tissue may further augment the levels of various chemokines within the ischemic tissue, such as MCP-1 or interleukins, which can attract circulating progenitor cells.¹³ Whereas several studies shed some light on the mechanisms regulating attraction of EPCs to ischemic tissue, less is known with respect to migration and tissue invasion. One may speculate that proteases such as cathepsins or metalloproteases may mediate the tissue invasion of EPCs.

Differentiation
Finally, maturation of EPCs to a functional endothelial cell may be important for functional integration in vessels. The genetic cascades that regulate differentiation in the adult system are largely unknown; however, several studies determined the differentiation of the common mesodermal precursor, the hemangioblasts, during embryonic development. Clearly, VEGF and its receptors play a crucial role for stimulating endothelial differentiation in the embryonic development.^96–98 Likewise, VEGF induces differentiation of endothelial cells in ex vivo culture assays using a variety of adult progenitor populations (CD34⁺,¹ CD133⁺,⁴ peripheral blood mononuclear cells).^15,76 Studies with embryonic stem cells further revealed that the temporal regulation of Homeobox (Hox) genes might play an important role. Thus, the orphan Hox gene termed Hex (also named Prh) is required for differentiation of the hemangioblast into the definitive hematopoietic progenitors and also affected endothelial differentiation.⁹⁹ Additionally, the serine/threonine kinase Pim-1 was recently discovered as a VEGF-responsive gene, which contributes to endothelial differentiation out of embryonic stem cells.¹⁰⁰

	Conclusion

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 
Taken together, infusion of different hematopoietic stem cell populations and ex vivo expanded EPCs augmented neovascularization of tissue after ischemia, thereby providing a novel therapeutic option. However, a variety of unresolved questions remain to be answered (Table 2). The crucial question is how to define an endothelial progenitor cell? 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 suggest that there are additional bone marrow-derived cell populations (eg, myeloid cells) within the blood, which also can give rise to endothelial cells. Moreover, non-bone marrow-derived cells with endothelial characteristic were isolated from the peripheral blood. This might represent shed mature endothelial cells or other endothelial cells deriving from other progenitor cell populations. Clearly, one functional assay to define endothelial progenitor cells independent of their progeny is the demonstration of clonal expansion activity. Possibly, functional assays will gain additional increasing importance, because recent studies suggest that endothelial progenitor cells have a favorable survival and a better response toward angiogenic growth factors compared with mature endothelial cells.¹⁰¹ From a therapeutic point of view, these functional activities might be more important than the source of the progenitor cell. Another open question is which mechanism underlies the improvement of neovascularization by infused EPCs? Likely, paracrine effects contribute in addition to the physical incorporation of EPC into newly formed capillaries. The influence of the incorporation of a rather small number of circulating cells on remodeling and vessel maturation has to be further elucidated.

View this table: [in this window] [in a new window]   Table 2. Unresolved Questions

	Acknowledgments

 
This study is supported by the DFG (FOR 501: Di 600/6-1). We thank A. Aicher, E. Chavakis, C. Heeschen, and A.M. Zeiher for helpful discussions.

	Footnotes

 
Original received March 8, 2004; revision received May 27, 2004; accepted May 28, 2004.

	References

Top Abstract Introduction Role of EPCs in... EPCs and Endothelial... Mobilization of EPCs Mechanism of Homing and... Conclusion References

 

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