Circulating Endothelial Progenitors — Cells as Biomarkers
http://www.100md.com
《新英格兰医药杂志》
Endothelial injury and dysfunction are thought to be critical events in the pathogenesis of atherosclerosis. Thus, understanding the mechanisms that maintain and restore endothelial function may have important clinical implications. A series of clinical and basic studies prompted by the discovery of bone marrow–derived endothelial progenitor cells1 have provided insights into these processes and opened a door to the development of new therapeutic approaches.
Growing evidence suggests that bone marrow–derived endothelial progenitor cells circulate in the blood and play an important role in the formation of new blood vessels as well as contribute to vascular homeostasis in the adult. Circulating endothelial progenitor cells were initially identified through their expression of CD34 (a surface marker common to hematopoietic stem cells and mature endothelial cells) and vascular endothelial cell growth-factor receptor 2 (VEGFR2 or kinase-domain–related [KDR] receptor), but not of other markers seen on fully differentiated endothelial cells.1 Subsequent studies have also used other identifiers, such as the stem-cell marker CD133, and functional assays, including the ability to form endothelial colonies. Endothelial progenitor cells defined in these ways probably represent a heterogeneous population, which, in combination with the lack of a consensual definition, complicates the interpretation of work in this field.
Nevertheless, numerous studies in animals have shown that endothelial progenitor cells can integrate into new and existing blood vessels.2,3,4 Intravenous injection of cytokine-mobilized human endothelial progenitor cells improved myocardial neoangiogenesis and the recovery of functioning in a rat model of infarction.3 Repeated injection of bone marrow–derived cells in a mouse model of atherosclerosis reduced the rate of plaque formation without altering serum lipids levels, and donor endothelial progenitor cells could subsequently be identified in the recipient's blood vessels.4 Previous clinical studies have shown that traditional risk factors for coronary atherosclerosis are associated with low levels of circulating endothelial progenitor cells,5 whereas protective interventions, including statin therapy6 and exercise,7 appear to increase the supply of these cells. Hill et al. found that even in healthy volunteers, levels of endothelial progenitor cells were inversely correlated with the Framingham risk score and actually appeared to predict vascular function better than the Framingham risk score.5 Together, these data suggest that circulating endothelial progenitor cells may participate not only in forming new blood vessels but also in maintaining the integrity and function of vascular endothelium, thereby mitigating disease processes such as atherosclerosis.
In this issue of the Journal, Werner and colleagues have further advanced our understanding of the clinical implications of endothelial progenitor cells.8 Endothelial progenitor cells were quantitated in 519 patients with coronary artery disease who were followed for one year after undergoing catheterization. Patients with higher levels of endothelial progenitor cells had a reduced risk of death from cardiovascular causes and of the composite end point of major cardiovascular events. These relationships were preserved even after adjustment for traditional risk factors and prognostic variables. A similar relationship was seen whether endothelial progenitor cells were identified by virtue of expression either of CD34 and KDR or of CD133 or because of their ability to form endothelial colonies, further strengthening the authors' conclusions. Repeated catheterization was not performed in this cohort, so we do not know whether the reduction in clinical events reflected a slowed progression of atherosclerosis or some other clinical effect. A dissociation between anatomical measures of atherosclerosis and clinical events has been well documented in other settings.
Although this study is consistent with prior work suggesting that circulating endothelial progenitor cells may play a protective role in vascular homeostasis, other explanations for the association between endothelial progenitor number and outcome remain possible. Changes in the number of endothelial progenitor cells and in clinical events might reflect a common underlying etiology, rather than a causal relation. For example, a defect in the production of nitric oxide, which plays an important role in both the mobilization of endothelial progenitor cells9 and blood-vessel function, might account for both observations. Similarly, the number of endothelial progenitor cells may mirror a person's regenerative capacity more broadly and predict clinical events on that basis. Even if endothelial progenitor cells are mechanistically linked to clinical cardiovascular events, such clinical studies do not distinguish between the possibility that the protection is mediated through the integration of endothelial progenitor cells into blood vessels and its possible mediation by other mechanisms, such as the paracrine benefits of endothelial progenitor cell–secreted products.
Although such questions will undoubtedly continue to provide fertile ground for fundamental investigation, the report by Werner and colleagues has more immediate clinical implications. First, it suggests that circulating cell populations may represent a new class of biomarkers that naturally integrate diverse genetic and environmental effects, thereby providing robust physiological and prognostic insights. Second, in the context of coronary disease, the study shows that the number of endothelial progenitor cells is an independent predictor of hard clinical outcomes. As with other biomarkers, a demonstration of clinical usefulness will ultimately require the examination of other patient populations, as well as a demonstration that clinical therapy can be guided and enhanced by this information. Finally, the increased risk associated with reduced levels of endothelial progenitor cells supports the growing interest in the therapeutic potential of enhancing the level of these cells.
The most dramatic extension of this line of reasoning involves transferring bone marrow or peripheral blood cells that are likely to include endothelial progenitor cells to patients with coronary artery disease. Although it would be premature to judge the clinical success of these strategies, early trials, including one randomized (though incompletely blinded) trial, have suggested at least short-term functional benefits of intracoronary infusion of bone marrow cells after acute infarction.10 Trials are planned to address more definitively the potential benefits of such cells in the settings of acute infarction and chronic ischemic cardiomyopathy. Such efforts would be aided substantially by the identification of specific markers as well as an improved understanding of the role of subtypes of endothelial progenitor cells and of the mechanisms by which they work. Ironically, the data presented by Werner and colleagues in combination with work showing the impaired functioning of endothelial progenitor cells in high-risk patients5 suggest that the patients most in need of endothelial progenitor cells may be those who are least able to donate them for autologous transplantation.
Whether these limitations can be overcome through ex vivo expansion or genetic modification of endothelial progenitor cells is unclear. In addition to possible cell-based therapies, work on endothelial progenitor cells provides yet another rationale for redoubling efforts to comply with established therapeutic guidelines, including lifestyle modifications and the use of statin therapy, both of which appear to enhance the number of circulating endothelial progenitor cells.
Whether there will be a downside to enhancing the number and function of endothelial progenitor cells remains unclear, although obvious concerns include exacerbating conditions that are characterized by adverse vessel formation, such as diabetic retinopathy and tumor angiogenesis. Small studies have suggested an association between high levels of circulating endothelial progenitor cells and the risk of certain cancers, such as multiple myeloma.11 Moreover, studies in animals show that bone marrow–derived endothelial progenitors participate in tumor angiogenesis, thereby enhancing tumor growth.12 In the study by Werner and colleagues, the number of deaths from cardiovascular causes among patients with high levels of endothelial progenitor cells was substantially lower than that among patients with lower levels of these cells, without a reduction in the risk of death overall.8 Although this finding could raise the specter of a counterbalancing adverse effect of endothelial progenitor cells, there was no apparent pattern in the deaths due to other causes, and no deaths from cancer were noted in this population.
It is possible that as we learn more about the biology of endothelial progenitor cells, there may be opportunities to target vessel formation more specifically. In addition, therapeutic strategies tailored to individualized risk will undoubtedly help in practice. For example, in the study by Werner et al., patients in the group with the lowest baseline levels of endothelial progenitor cells had a risk of death from cardiovascular causes of 8.3 percent during one year of follow-up, suggesting that the benefits of enhancing the function and number of endothelial progenitor cells may well outweigh the risks in such high-risk populations.
Additional studies will be necessary to address these questions definitively. Larger studies of longer duration performed in different cohorts will be required to determine fully the clinical usefulness of endothelial progenitor cells as a biomarker. Rigorous interventional studies will indicate whether levels of endothelial progenitor cells can be used to guide therapy and whether cell transfer has a role in augmenting the levels of these cells. Basic-science studies should help guide these clinical efforts by further defining the desirable subpopulations of endothelial progenitor cells and the mechanisms by which they mediate their effects. By establishing a connection between circulating endothelial progenitor cells and hard clinical end points, Werner and colleagues provide a potent stimulus for clinical and basic studies to address these important issues.
Source Information
From the Program in Cardiovascular Gene Therapy, Massachusetts General Hospital, and Harvard Medical School — both in Boston.
References
Asahara T, Murohara T, Sullivan A, et al. Isolation of putative progenitor endothelial cells for angiogenesis. Science 1997;275:964-967.
Takahashi T, Kalka C, Masuda H, et al. Ischemia- and cytokine-induced mobilization of bone marrow-derived endothelial progenitor cells for neovascularization. Nat Med 1999;5:434-438.
Kocher AA, Schuster MD, Szabolcs MJ, et al. Neovascularization of ischemic myocardium by human bone-marrow-derived angioblasts prevents cardiomyocyte apoptosis, reduces remodeling and improves cardiac function. Nat Med 2001;7:430-436.
Rauscher FM, Goldschmidt-Clermont PJ, Davis BH, et al. Aging, progenitor cell exhaustion, and atherosclerosis. Circulation 2003;108:457-463.
Hill JM, Zalos G, Halcox JPJ, et al. Circulating endothelial progenitor cells, vascular function, and cardiovascular risk. N Engl J Med 2003;348:593-600.
Vasa M, Fichtlscherer S, Adler K, et al. Increase in circulating endothelial progenitor cells by statin therapy in patients with stable coronary artery disease. Circulation 2001;103:2885-2890.
Laufs U, Werner N, Link A, et al. Physical training increases endothelial progenitor cells, inhibits neointima formation, and enhances angiogenesis. Circulation 2004;109:220-226.
Werner N, Kosiol S, Schiegl T, et al. Circulating endothelial progenitor cells and cardiovascular outcomes. N Engl J Med 2005;353:999-1007.
Aicher A, Heeschen C, Mildner-Rihm C, et al. Essential role of endothelial nitric oxide synthase for mobilization of stem and progenitor cells. Nat Med 2003;9:1370-1376.
Wollert KC, Meyer GP, Lotz J, et al. Intracoronary autologous bone-marrow cell transfer after myocardial infarction: the BOOST randomised controlled clinical trial. Lancet 2004;364:141-148.
Zhang H, Vakil V, Braunstein M, et al. Circulating endothelial progenitor cells in multiple myeloma: implications and significance. Blood 2005;105:3286-3294.
Lyden D, Hattori K, Dias S, et al. Impaired recruitment of bone-marrow-derived endothelial and hematopoietic precursor cells blocks tumor angiogenesis and growth. Nat Med 2001;7:1194-1201.(Anthony Rosenzweig, M.D.)
Growing evidence suggests that bone marrow–derived endothelial progenitor cells circulate in the blood and play an important role in the formation of new blood vessels as well as contribute to vascular homeostasis in the adult. Circulating endothelial progenitor cells were initially identified through their expression of CD34 (a surface marker common to hematopoietic stem cells and mature endothelial cells) and vascular endothelial cell growth-factor receptor 2 (VEGFR2 or kinase-domain–related [KDR] receptor), but not of other markers seen on fully differentiated endothelial cells.1 Subsequent studies have also used other identifiers, such as the stem-cell marker CD133, and functional assays, including the ability to form endothelial colonies. Endothelial progenitor cells defined in these ways probably represent a heterogeneous population, which, in combination with the lack of a consensual definition, complicates the interpretation of work in this field.
Nevertheless, numerous studies in animals have shown that endothelial progenitor cells can integrate into new and existing blood vessels.2,3,4 Intravenous injection of cytokine-mobilized human endothelial progenitor cells improved myocardial neoangiogenesis and the recovery of functioning in a rat model of infarction.3 Repeated injection of bone marrow–derived cells in a mouse model of atherosclerosis reduced the rate of plaque formation without altering serum lipids levels, and donor endothelial progenitor cells could subsequently be identified in the recipient's blood vessels.4 Previous clinical studies have shown that traditional risk factors for coronary atherosclerosis are associated with low levels of circulating endothelial progenitor cells,5 whereas protective interventions, including statin therapy6 and exercise,7 appear to increase the supply of these cells. Hill et al. found that even in healthy volunteers, levels of endothelial progenitor cells were inversely correlated with the Framingham risk score and actually appeared to predict vascular function better than the Framingham risk score.5 Together, these data suggest that circulating endothelial progenitor cells may participate not only in forming new blood vessels but also in maintaining the integrity and function of vascular endothelium, thereby mitigating disease processes such as atherosclerosis.
In this issue of the Journal, Werner and colleagues have further advanced our understanding of the clinical implications of endothelial progenitor cells.8 Endothelial progenitor cells were quantitated in 519 patients with coronary artery disease who were followed for one year after undergoing catheterization. Patients with higher levels of endothelial progenitor cells had a reduced risk of death from cardiovascular causes and of the composite end point of major cardiovascular events. These relationships were preserved even after adjustment for traditional risk factors and prognostic variables. A similar relationship was seen whether endothelial progenitor cells were identified by virtue of expression either of CD34 and KDR or of CD133 or because of their ability to form endothelial colonies, further strengthening the authors' conclusions. Repeated catheterization was not performed in this cohort, so we do not know whether the reduction in clinical events reflected a slowed progression of atherosclerosis or some other clinical effect. A dissociation between anatomical measures of atherosclerosis and clinical events has been well documented in other settings.
Although this study is consistent with prior work suggesting that circulating endothelial progenitor cells may play a protective role in vascular homeostasis, other explanations for the association between endothelial progenitor number and outcome remain possible. Changes in the number of endothelial progenitor cells and in clinical events might reflect a common underlying etiology, rather than a causal relation. For example, a defect in the production of nitric oxide, which plays an important role in both the mobilization of endothelial progenitor cells9 and blood-vessel function, might account for both observations. Similarly, the number of endothelial progenitor cells may mirror a person's regenerative capacity more broadly and predict clinical events on that basis. Even if endothelial progenitor cells are mechanistically linked to clinical cardiovascular events, such clinical studies do not distinguish between the possibility that the protection is mediated through the integration of endothelial progenitor cells into blood vessels and its possible mediation by other mechanisms, such as the paracrine benefits of endothelial progenitor cell–secreted products.
Although such questions will undoubtedly continue to provide fertile ground for fundamental investigation, the report by Werner and colleagues has more immediate clinical implications. First, it suggests that circulating cell populations may represent a new class of biomarkers that naturally integrate diverse genetic and environmental effects, thereby providing robust physiological and prognostic insights. Second, in the context of coronary disease, the study shows that the number of endothelial progenitor cells is an independent predictor of hard clinical outcomes. As with other biomarkers, a demonstration of clinical usefulness will ultimately require the examination of other patient populations, as well as a demonstration that clinical therapy can be guided and enhanced by this information. Finally, the increased risk associated with reduced levels of endothelial progenitor cells supports the growing interest in the therapeutic potential of enhancing the level of these cells.
The most dramatic extension of this line of reasoning involves transferring bone marrow or peripheral blood cells that are likely to include endothelial progenitor cells to patients with coronary artery disease. Although it would be premature to judge the clinical success of these strategies, early trials, including one randomized (though incompletely blinded) trial, have suggested at least short-term functional benefits of intracoronary infusion of bone marrow cells after acute infarction.10 Trials are planned to address more definitively the potential benefits of such cells in the settings of acute infarction and chronic ischemic cardiomyopathy. Such efforts would be aided substantially by the identification of specific markers as well as an improved understanding of the role of subtypes of endothelial progenitor cells and of the mechanisms by which they work. Ironically, the data presented by Werner and colleagues in combination with work showing the impaired functioning of endothelial progenitor cells in high-risk patients5 suggest that the patients most in need of endothelial progenitor cells may be those who are least able to donate them for autologous transplantation.
Whether these limitations can be overcome through ex vivo expansion or genetic modification of endothelial progenitor cells is unclear. In addition to possible cell-based therapies, work on endothelial progenitor cells provides yet another rationale for redoubling efforts to comply with established therapeutic guidelines, including lifestyle modifications and the use of statin therapy, both of which appear to enhance the number of circulating endothelial progenitor cells.
Whether there will be a downside to enhancing the number and function of endothelial progenitor cells remains unclear, although obvious concerns include exacerbating conditions that are characterized by adverse vessel formation, such as diabetic retinopathy and tumor angiogenesis. Small studies have suggested an association between high levels of circulating endothelial progenitor cells and the risk of certain cancers, such as multiple myeloma.11 Moreover, studies in animals show that bone marrow–derived endothelial progenitors participate in tumor angiogenesis, thereby enhancing tumor growth.12 In the study by Werner and colleagues, the number of deaths from cardiovascular causes among patients with high levels of endothelial progenitor cells was substantially lower than that among patients with lower levels of these cells, without a reduction in the risk of death overall.8 Although this finding could raise the specter of a counterbalancing adverse effect of endothelial progenitor cells, there was no apparent pattern in the deaths due to other causes, and no deaths from cancer were noted in this population.
It is possible that as we learn more about the biology of endothelial progenitor cells, there may be opportunities to target vessel formation more specifically. In addition, therapeutic strategies tailored to individualized risk will undoubtedly help in practice. For example, in the study by Werner et al., patients in the group with the lowest baseline levels of endothelial progenitor cells had a risk of death from cardiovascular causes of 8.3 percent during one year of follow-up, suggesting that the benefits of enhancing the function and number of endothelial progenitor cells may well outweigh the risks in such high-risk populations.
Additional studies will be necessary to address these questions definitively. Larger studies of longer duration performed in different cohorts will be required to determine fully the clinical usefulness of endothelial progenitor cells as a biomarker. Rigorous interventional studies will indicate whether levels of endothelial progenitor cells can be used to guide therapy and whether cell transfer has a role in augmenting the levels of these cells. Basic-science studies should help guide these clinical efforts by further defining the desirable subpopulations of endothelial progenitor cells and the mechanisms by which they mediate their effects. By establishing a connection between circulating endothelial progenitor cells and hard clinical end points, Werner and colleagues provide a potent stimulus for clinical and basic studies to address these important issues.
Source Information
From the Program in Cardiovascular Gene Therapy, Massachusetts General Hospital, and Harvard Medical School — both in Boston.
References
Asahara T, Murohara T, Sullivan A, et al. Isolation of putative progenitor endothelial cells for angiogenesis. Science 1997;275:964-967.
Takahashi T, Kalka C, Masuda H, et al. Ischemia- and cytokine-induced mobilization of bone marrow-derived endothelial progenitor cells for neovascularization. Nat Med 1999;5:434-438.
Kocher AA, Schuster MD, Szabolcs MJ, et al. Neovascularization of ischemic myocardium by human bone-marrow-derived angioblasts prevents cardiomyocyte apoptosis, reduces remodeling and improves cardiac function. Nat Med 2001;7:430-436.
Rauscher FM, Goldschmidt-Clermont PJ, Davis BH, et al. Aging, progenitor cell exhaustion, and atherosclerosis. Circulation 2003;108:457-463.
Hill JM, Zalos G, Halcox JPJ, et al. Circulating endothelial progenitor cells, vascular function, and cardiovascular risk. N Engl J Med 2003;348:593-600.
Vasa M, Fichtlscherer S, Adler K, et al. Increase in circulating endothelial progenitor cells by statin therapy in patients with stable coronary artery disease. Circulation 2001;103:2885-2890.
Laufs U, Werner N, Link A, et al. Physical training increases endothelial progenitor cells, inhibits neointima formation, and enhances angiogenesis. Circulation 2004;109:220-226.
Werner N, Kosiol S, Schiegl T, et al. Circulating endothelial progenitor cells and cardiovascular outcomes. N Engl J Med 2005;353:999-1007.
Aicher A, Heeschen C, Mildner-Rihm C, et al. Essential role of endothelial nitric oxide synthase for mobilization of stem and progenitor cells. Nat Med 2003;9:1370-1376.
Wollert KC, Meyer GP, Lotz J, et al. Intracoronary autologous bone-marrow cell transfer after myocardial infarction: the BOOST randomised controlled clinical trial. Lancet 2004;364:141-148.
Zhang H, Vakil V, Braunstein M, et al. Circulating endothelial progenitor cells in multiple myeloma: implications and significance. Blood 2005;105:3286-3294.
Lyden D, Hattori K, Dias S, et al. Impaired recruitment of bone-marrow-derived endothelial and hematopoietic precursor cells blocks tumor angiogenesis and growth. Nat Med 2001;7:1194-1201.(Anthony Rosenzweig, M.D.)