Erythropoietin regulates endothelial progenitor cells

Endothelial progenitor cell therapy in atherosclerosis: a double-edged sword?

Atherosclerosis, an inflammatory process that selectively affects arteries, is highly prevalent in human. Thrombo-occlusive complications of atherosclerosis, including stroke and myocardial infarction, are becoming major causes of morbidity and mortality in the industrialized world. Atherosclerosis develops in response to local endothelial injuries. Endothelial dysfunction and cell loss are prominent features in atherosclerosis. Restoring the endothelial lining to normal is critical for slowing or reversing the progression of atherosclerosis. Increasing data suggest that endothelial progenitor cells (EPCs) play a significant role in reendothelialization of the injured blood vessels. This review focuses on the effects of EPC mobilization and transfusion in the condition of atherosclerosis. The aim of the review is to provide an update on the progress in this research field, highlight the role of EPCs in atherosclerosis and discuss the possible mechanisms and potential risks of progenitor cell-based therapy in atherosclerosis.

From bone marrow to the arterial wall: the ongoing tale of endothelial progenitor cells

Several physiological and pathophysiological stimuli or drugs modulate endothelial progenitor cell (EPC) mobilization. Moreover, levels of circulating EPCs predict cardiovascular risk and left ventricular remodelling after myocardial infarction. Nevertheless, our understanding in this field is complicated by lack of an unequivocal definition of EPCs, thus limiting their clinical applications. This review summarizes current knowledge and uncertainties on EPC characterization and mobilization in the attempt to define their role in the management of cardiovascular diseases.

Physiological basis for the use of erythropoietin in critically ill patients at risk for acute kidney injury

PURPOSE OF REVIEW

Acute kidney injury (AKI) frequently occurs in critically ill patients and is an independent risk factor for poor outcome. The prevention of kidney injury in intensive care remains a great challenge as specific nephroprotective therapies are still lacking. The present review summarizes recent evidence for the use of erythropoietin as a promising candidate to provide protection from AKI.

RECENT FINDINGS

Beyond the known hematopoietic actions of erythropoietin, a number of preclinical studies demonstrated that erythropoietin possesses pleiotropic, organ-protecting properties. Preconditional and postconditional erythropoietin treatment was shown to protect from ischemic, toxic and septic AKI. Despite heterogeneities in study design and dose, erythropoietin consistently ameliorated renal injury. The mechanisms of protection remain largely unclear but may involve reduction of apoptosis, induction of cellular proliferation and tissue repair as well as mobilization of stem cells.

SUMMARY

Animal studies revealed a physiological basis for the use of erythropoietin in AKI, which may be clinically applicable to prevent AKI in critically ill patients, but clinical studies are still lacking.

Potential use of endothelial progenitor cells for regeneration of the vasculature

Over the past decade, interest has been generated in the study of endothelial progenitor cells (EPCs). EPCs have been studied for their role in endogenous maintenance and for their therapeutic potential in vascular regenerative medicine. Despite their obvious potential in clinical practice, there still remain many controversies regarding how EPCs actually enhance endothelial repair and neovascularization. In addition, because of the limited expansion ability of EPCs, expansion of sufficient EPC populations for therapeutic angiogenesis remains a major task. On the other hand, embryonic stem (ES) cells have an extended self-renewal activity and can be expanded without limit, thus ES-cell-derived endothelial cells could be feasible as a novel cell source for therapeutic angiogenesis. In this review, we discuss recent experimental and clinical findings of EPCs and human ES-cell-derived endothelial cells for the treatment of ischemic cardiovascular diseases.

Erythropoietin promotes the growth of tumors lacking its receptor and decreases survival of tumor-bearing mice by enhancing angiogenesis

Erythropoietin (Epo), a known hematopoietic growth factor, has been reported to promote tumor growth and angiogenesis in Epo receptor (EpoR)-positive tumors, but its effects on EpoR-negative tumors have not been clearly shown. Here, we show that Epo accelerates the growth of EpoR-negative tumors by promoting tumor angiogenesis. Mice were inoculated with Lewis lung carcinoma cells and treated with Epo. Erythropoietin accelerated tumor growth and increased intratumoral microvessel density, although it did not accelerate Lewis lung carcinoma cell tumor proliferation in vitro. To observe the direct effect of Epo on endothelial cells, we examined human dermal microvascular endothelial cells (HMVECs) that expressed EpoR. Erythropoietin induced the proliferation of HMVECs and protected them from H2O2-induced cell death. Erythropoietin activated the extracellular signal-regulated kinase signaling pathway and up-regulated the expression of the downstream antiapoptotic protein Bcl-xL in HMVECs. Moreover, in both the absence and presence of tumors, in vivo treatment of mice with Epo increased circulating endothelial progenitor cells. To investigate the role of Epo in a primary tumor model, we inoculated the chemical carcinogen methylcholanthrene (MCA) subcutaneously into mice at two doses, a high or a low dose, which induced fibrosarcoma, and treated them with Epo. Erythropoietin promoted tumor growth after MCA inoculation at both doses and decreased the overall survival of the mice inoculated with the high-dose MCA. However, Epo did not increase the incidence of fibrosarcoma at either dose. Lewis lung carcinoma cells and MCA-induced fibrosarcomas did not express EpoR. These results suggest that Epo accelerates the growth of tumors that lack EpoR expression by promoting tumor angiogenesis.

Elevated levels of circulating endothelial progenitor cells in head and neck cancer patients

BACKGROUND AND OBJECTIVES

Measurement of circulating endothelial cells (CECs) and progenitor cells (EPCs) has potential as a surrogate marker for monitoring anticancer treatment. This study evaluated the significance of CECs and EPCs in the blood of patients with head and neck squamous cell carcinoma.

METHODS

In a prospective trial fresh blood samples from 22 tumor patients and 18 controls were tested using multiparametric flow-cytometry. CECs were defined as CD31(+)/CD146(+) and CD45(-)/7AAD(-). EPCs were defined as CD133(+)/KDR(+) and CD3(-)/CD19(-)/CD33(-)/7AAD(-).

RESULTS

Median levels (min/max) of CECs in the tumor group were 2 (0/5) at the time of diagnosis, 1 (0/5) 1 year after therapy and 2 (0/6) in the control cohort. Median levels of EPCs were 5 (1/41) before and 10 (0/21) after treatment in the tumor group compared to 2 (0/7) in the control cohort (P < 0.001 and P = 0.03). CEC and EPC levels showed no apparent correlation with tumor size and response to radiotherapy after 18 months of observation.

CONCLUSIONS

In this pilot study CD133(+)/KDR(+) EPCs were significantly elevated in head and neck tumor patients before and after therapy. Our results warrant further studies on the use of EPCs as a surrogate marker for anticancer therapies in these patients.

Hemodynamic effects of recombinant human erythropoietin on the central nervous system after subarachnoid hemorrhage: reduction of microcirculatory impairment and functional deficits in a rabbit model

OBJECT

The authors investigated the hemodynamic effects of recombinant human erythropoietin (rhEPO) after subarachnoid hemorrhage (SAH) in rabbits.

METHODS

The authors used male New Zealand White rabbits in this study divided into the following groups: SAH plus saline (16 rabbits), SAH plus low-dose rhEPO (16 rabbits; 1500 IU/kg on Day 0 and 500 IU/kg on Days 2 and 4), SAH plus high-dose rhEPO (10 rabbits; 1500 IU/kg on Days 0, 2, 4, and 6), and sham (6 rabbits). Computed tomography perfusion studies and CT angiography were performed for 1 hour after SAH on Day 0, and once each on Days 2, 4, 7, 9, and 16 after SAH. Assessments of neurological function and tissue histology were also performed.

RESULTS

The mortality rate was significantly lower after rhEPO treatment (12%) than after saline treatment (44%) (p < 0.05). Neurological outcomes in the low-dose and high-dose rhEPO groups were better than in the saline group after SAH (p < 0.05), and the cerebral blood flow in the high-dose rhEPO group was greater than that in the saline group (p < 0.05). The mean transit time was significantly lower on Days 2 and 4 in the low-dose and high-dose rhEPO groups than in the saline group, but increased significantly on Day 7 in both groups (p < 0.05). The hematocrit increased significantly from baseline values in the high-dose and low-dose rhEPO groups on Days 4 and 7, respectively (p < 0.05).

CONCLUSIONS

Treatment with rhEPO after experimental SAH is associated with improved cerebral blood flow and microcirculatory flow as reflected by lower mean transit times. Improved tissue perfusion correlated with reduced mortality and improved neurological outcomes. Further investigation of the impact of increasing hematocrit on hemodynamic changes is needed.

Stem cells for the treatment of skeletal muscle injury

Skeletal muscle injuries are extremely common, accounting for up to 35%-55% of all sports injuries and quite possibly affecting all musculoskeletal traumas. These injuries result in the formation of fibrosis, which may lead to the development of painful contractures, increases patients' risk for repeat injuries, and limits their ability to return to a baseline or pre-injury level of function. The development of successful therapies for these injuries must consider the pathophysiology of these musculoskeletal conditions. We discuss the direct use of muscle-derived stem cells and some key cell population dynamics as well as the use of clinically applicable modalities that may enhance the local supply of stem cells to the zone of injury by promoting angiogenesis.

Pharmacological approaches to improve endothelial repair mechanisms

Endothelial injury is thought to play a pivotal role in the development and progression of vascular diseases, such as atherosclerosis, hypertension or restenosis, as well as their complications, including myocardial infarction or stroke. Accumulating evidence suggests that bone marrow-derived endothelial progenitor cells (EPCs) promote endothelial repair and contribute to ischemia-induced neovascularization. Coronary artery disease and its risk factors, such as diabetes, hypercholesterolemia, hypertension and smoking, are associated with a reduced number and impaired functional activity of circulating EPCs. Moreover, initial data suggest that reduced EPC levels are associated with endothelial dysfunction and an increased risk of cardiovascular events, compatible with the concept that impaired EPC-mediated vascular repair promotes progression of vascular disease. In this review we summarize recent data on the effects of pharmacological agents on mobilization and functional activity of EPCs. In particular, several experimental and clinical studies have suggested that statins, angiotensin-converting enzyme inhibitors, angiotensin II type 1 receptor blockers, PPAR-gamma agonists and erythropoietin increase the number and functional activity of EPCs. The underlying mechanisms remain largely to be defined; however, they 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.

Prolyl hydroxylase inhibition during hyperoxia prevents oxygen-induced retinopathy

Oxygen-induced retinopathy (OIR) in the mouse, like the analogous human disease retinopathy of prematurity, is an ischemic retinopathy dependent on oxygen-induced vascular obliteration. We tested the hypothesis that chemically overriding the oxygen-induced downregulation of hypoxia-inducible factor (HIF) activity would prevent vascular obliteration and subsequent pathologic neovascularization in the OIR model. Because the degradation of HIF-1alpha is regulated by prolyl hydroxylases, we examined the effect of systemic administration of a prolyl hydroxylase inhibitor, dimethyloxalylglycine, in the OIR model. Our results determine that stabilizing HIF activity in the early phase of OIR prevents the oxygen-induced central vessel loss and subsequent vascular tortuosity and tufting that is characteristic of OIR. Overall, these findings imply that simulating hypoxia chemically by stabilizing HIF activity during the causative ischemia phase (hyperoxia) of retinopathy of prematurity may be of therapeutic value in preventing progression to the proliferative stage of the disease.

The potential role of erythropoietin in chronic heart failure: from the correction of anemia to improved perfusion and reduced apoptosis?

Besides stimulating erythropoiesis, erythropoietin (EPO) exerts powerful proangiogenic and antiapoptotic effects. These erythropoiesis-independent effects are potentially useful as a supplement for the treatment of chronic heart failure (CHF). EPO may improve microvascular capacity of ischemic myocardial tissue and could thereby (partially) restore myocardial function. In addition, EPO could protect cardiomyocytes from hypoxic damage and prevent them from apoptosis. However, the clinical value of these erythropoiesis-independent effects for the treatment of CHF remains to be elucidated. Small-sized trials evaluating the effects of EPO treatment on surrogate endpoints in patients with CHF showed positive effects in general; however, their mutual results are not always unambiguous. Moreover, increasing hematocrit levels with EPO has been associated with increased blood viscosity and an inherent risk of thromboembolic events. A currently running multicenter phase III trial is designed to provide clarity concerning the effects of EPO on outcome and safety in patients with CHF. Focusing primarily on outcome, however, does not provide insight into the mode of action and isolated benefits of the erythropoiesis-independent effects of EPO. Further exploration of these effects is a key issue to gain knowledge of the full potential of EPO for the treatment of CHF.

Darbepoetin-alpha prevents progressive left ventricular dysfunction and remodeling in nonanemic dogs with heart failure

In anemic patients with heart failure (HF), erythropoietin-type drugs can elicit clinical improvement. This study examined the effects of chronic monotherapy with darbepoetin-alpha (DARB) on left ventricular (LV) function and remodeling in nonanemic dogs with advanced HF. HF [LV ejection fraction (EF) approximately 25%] was produced in 14 dogs by intracoronary microembolizations. Dogs were randomized to once a week subcutaneous injection of DARB (1.0 microg/kg, n=7) or to no therapy (HF, n=7). All procedures were performed during cardiac catheterization under general anesthesia and under sterile conditions. LV end-diastolic volume (EDV), end-systolic volume (ESV), and EF were measured before the initiation of therapy and at the end of 3 mo of therapy. mRNA and protein expression of caspase-3, hypoxia inducible factor-1alpha, and the bone marrow-derived stem cell marker c-Kit were determined in LV tissue. In HF dogs, EDV and ESV increased and EF decreased after 3 mo of followup. Treatment with DARB prevented the increase in EDV, decreased ESV, and increased EF. DARB therapy also normalized the expression of HIF-1alpha and active caspase-3 and enhanced the expression of c-Kit. We conclude that chronic monotherapy with DARB prevents progressive LV dysfunction and dilation in nonanemic dogs with advanced HF. These results suggest that DARB elicits beneficial effects in HF that are independent of the presence of anemia.

Survival and proliferative roles of erythropoietin beyond the erythroid lineage

Since the isolation and purification of erythropoietin (EPO) in 1977, the essential role of EPO for mature red blood cell production has been well established. The cloning of the EPO gene and production of recombinant human EPO led to the widespread use of EPO in treating patients with anaemia. However, the biological activity of EPO is not restricted to regulation of erythropoiesis. EPO receptor (EPOR) expression is also found in endothelial, brain, cardiovascular and other tissues, although at levels considerably lower than that of erythroid progenitor cells. This review discusses the survival and proliferative activity of EPO that extends beyond erythroid progenitor cells. Loss of EpoR expression in mouse models provides evidence for the role of endogenous EPO signalling in nonhaematopoietic tissue during development or for tissue maintenance and/or repair. Determining the extent and distribution of receptor expression provides insights into the potential protective activity of EPO in brain, heart and other nonhaematopoietic tissues.

Aging and hypertension are independent risk factors for reduced number of circulating endothelial progenitor cells

BACKGROUND

Recent studies have revealed the existence of bone marrow-derived endothelial progenitor cells (EPCs). The number of circulating EPCs might reflect the pathogenesis of atherosclerosis and progression of cardiovascular diseases (CVDs). The purpose of this study was to evaluate the relationship between the number of EPCs and cardiovascular risk factors.

METHODS

Flow cytometry analysis was used to quantify the number of EPCs (CD34(+)AC133(+)CD45(low)) in 135 consecutive hospitalized patients with CVD and 25 healthy subjects.

RESULTS

The number of EPCs was less in the patients than in the healthy subjects (1,047.4 +/- 521.1 vs. 612.8 +/- 461.6/ml, P < 0.0001). The number of EPCs significantly correlated with the number of risk factors (r = 0.424, P < 0.0001). The numbers of EPCs in patients with hypertension and diabetes mellitus were less than those in patients without those diseases (762.6 +/- 579.5 vs. 495.2 +/- 297.7/ml, P < 0.01 and 666.8 +/- 505.5 vs. 477.0 +/- 290.4/ml, P < 0.05, respectively). In healthy subjects a reduced number of EPCs was found in smokers compared with nonsmokers (833.3 +/- 347.5 vs. 1,274.6 +/- 560.9/ml, P < 0.05), whereas smoking did not alter the number of EPCs in the patients group. In multivariate analysis, hypertension and age were independent predictors of reduced number of EPCs. Renin-angiotensin system (RAS) inhibitors increased the number of EPCs (464.7 +/- 252.1/ml vs. 617.5 +/- 343.5/ml, P < 0.05), while calcium antagonists, diuretics, and beta-blockers did not alter the number of EPCs in patients with hypertension.

CONCLUSIONS

These findings suggest that both aging and hypertension are risk factors for reduced number of EPCs and that RAS inhibitors increase the number of EPCs.

New insights in vascular development: vasculogenesis and endothelial progenitor cells

In the course of new blood vessel formation, two different processes--vasculogenesis and angiogenesis--have to be distinguished. The term vasculogenesis describes the de novo emergence of a vascular network by endothelial progenitors, whereas angiogenesis corresponds to the generation of vessels by sprouting from pre-existing capillaries. Until recently, it was thought that vasculogenesis is restricted to the prenatal period. During the last decade, one of the most fascinating innovations in the field of vascular biology was the discovery of endothelial progenitor cells and vasculogenesis in the adult. This review aims at introducing the concept of adult vasculogenesis and discusses the efforts to identify and characterize adult endothelial progenitors. The different sources of adult endothelial progenitors like haematopoietic stem cells, myeloid cells, multipotent progenitors of the bone marrow, side population cells and tissue-residing pluripotent stem cells are considered. Moreover, a survey of cellular and molecular control mechanisms of vasculogenesis is presented. Recent advances in research on endothelial progenitors exert a strong impact on many different disciplines and provide the knowledge for functional concepts in basic fields like anatomy, histology as well as embryology.

Regenerative cell therapy and pharmacotherapeutic intervention in heart failure: Part 2: Pharmacological targets, agents and intervention perspectives

Regenerative medicine represents a promising perspective on therapeutic angiogenesis in patients with cardiovascular disease, including heart failure. However, previous or ongoing clinical trials show ambiguous outcomes with respect to the benefit of regenerative therapy by means of bone marrow stem cell infusion in myocardial infarction patients. Therefore, it is necessary to set up a rational therapeutic strategy in the treatment of congestive heart failure. Chemokines, cytokines and growth factors, as well as pharmaceutical agents, may have an impact on endothelial progenitor cell (EPC) physiology and thus can provide targets for pharmacological intervention. Indeed, EPCs and stem cell niches both in bone marrow and myocardial tissue can be treated as an integral target for recruitment of EPCs from the bone marrow to the cardiac ischaemic niche. In this article, we individually place the signalling factors in their specified context, and explain their roles in the various phases of neovascularisation (see Part 1). (Neth Heart J 2008;16:337-43.).

Ambivalence of progenitor cells in vascular repair and plaque stability

PURPOSE OF REVIEW

To discuss crucial cues (chemokines, adhesion molecules and pharmacological means) that guide and control the context-specific mobilization, recruitment and fate of circulating progenitor cells in arterial repair and plaque stability.

RECENT FINDINGS

The mobilization and recruitment of bone marrow derived or resident progenitor cells giving rise to smooth muscle cells have been implicated in accelerated forms of primary plaque formation and neointimal hyperplasia after arterial injury. By contrast, convincing evidence has emerged that the arterial homing of endothelial progenitor cells contributes to endothelial recovery and thereby limits neointimal growth after endothelial denudation. In the chronic context of primary atherosclerosis, plaque progression and destabilization, a more complex picture has become apparent. Clinically, the number and function of endothelial progenitor cells have been linked with an improved endothelial function or regeneration and have been frequently inversely correlated with cardiovascular risk (factors). In animal models, however, the injection of bone marrow cells or endothelial progenitor cells, as well as the application of stem-cell mobilizing factors, have been associated with an exacerbation of atherosclerosis and unstable plaque phenotype, whereas the contribution of smooth muscle progenitors to primary atherosclerosis appears to be more confined to supporting plaque stability.

SUMMARY

Considering the balance between distinct circulating vascular progenitor cells and identifying mechanisms for selective control of their mobilization and homing appears crucial to improve prediction and to directly modulate endogenous vascular remodeling processes.

Erythropoietin administration after myocardial infarction in mice attenuates ischemic cardiomyopathy associated with enhanced homing of bone marrow-derived progenitor cells via the CXCR-4/SDF-1 axis

Mobilization of bone marrow-derived stem cells (BMCs) was shown to have protective effects after myocardial infarction (MI). However, the classical mobilizing agent, granulocyte-colony stimulating factor (G-CSF) relapsed after revealing an impaired homing capacity. In the search for superior cytokines, erythropoietin (EPO) appears to be a promising agent. Therefore, we analyzed in a murine model of surgically induced MI the influence of EPO treatment on survival and functional parameters as well as BMC mobilization, homing, and effect on resident cardiac stem cells (CSCs). Human EPO was injected intraperitoneally after ligation of the left anterior descendens (LAD) for 3 days with a total dose of 5000 IU/kg 6 and 30 days after MI, and pressure volume relationships were investigated in vivo. Cardiac tissues were analyzed by histology. To show the effect on BMCs and CSCs, FACS analyses were performed. Homing factors were analyzed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and ELISA. EPO-treated animals showed a significant improvement of survival post-MI (62 vs. 36%). At days 6 and 30, all hemodynamic parameters associated with attenuated remodeling, enhanced neovascularization, and diminished apoptotic cells in the peri-infarct area were improved. BMC subpopulations (CD31(+), c-kit(+), and Sca-1(+) cells) were mobilized, and homing of Sca-1(+) and CXCR4(+) BMCs toward an SDF-1 gradient into the ischemic myocardium was enhanced. However, there was no beneficial effect on CSCs. We have shown that EPO application after MI shows cardioprotective effects. This may be explained by mobilization of BMCs, which are homing via the CXCR-4/SDF-1 axis. However, EPO has no beneficial effects on resident CSCs. Therefore, new treatment regimes using EPO together with other agents may combine complementary beneficial effects preventing ischemic cardiomyopathy.

Adult progenitor cells in vascular remodeling during atherosclerosis

The mobilization and recruitment of bone marrow-derived, circulating or tissue resident progenitor cells giving rise to smooth muscle-like cells have been implicated in neointima hyperplasia after arterial injury and in accelerated forms of arterial lesion formation, e.g., transplant arteriopathy or graft vasculopathy. By contrast, convincing evidence has emerged that the vascular homing of endothelial progenitor cells (EPCs) contributes to endothelial recovery, thus limiting neointima formation after arterial injury. In the chronic context of primary atherosclerosis, plaque progression and destabilization, a more complex picture has become apparent. In patients with coronary artery disease, the number and function of EPCs have been linked with an improved endothelial function or regeneration, but have been inversely correlated with cardiovascular risk. In animal models, however, the injection of bone marrow cells or EPCs, or the application of stem-cell mobilizing factors, have been associated with an exacerbation of atherosclerosis and unstable plaque phenotypes, whereas the contribution of bone marrow-derived smooth muscle progenitors to primary atherosclerosis appears to be rather confined. Here, we discuss crucial biochemical cues, namely chemokines, adhesion molecules, growth factors and pharmacological means that guide and control the context-specific mobilization, recruitment and fate of vascular progenitor cells in arterial remodeling during atherosclerosis.

Circulating endothelial cells, bone marrow-derived endothelial progenitor cells and proangiogenic hematopoietic cells in cancer: From biology to therapy

Vascularization, a hallmark of tumorigenesis, is classically thought to occur exclusively through angiogenesis (i.e. endothelial sprouting). However, there is a growing body of evidence that endothelial progenitor cells (EPCs) and proangiogenic hematopoietic cells (HCs) are able to support the vascularization of tumors and may therefore play a synergistic role with angiogenesis. An additional cell type being studied in the field of tumor vascularization is the circulating endothelial cell (CEC), whose presence in elevated numbers reflects vascular injury. Levels of EPCs and CECs are reported to correlate with tumor stage and have been evaluated as biomarkers of the efficacy of anticancer/antiangiogenic treatments. Furthermore, because EPCs and subtypes of proangiogenic HCs are actively participating in capillary growth, these cells are attractive potential vehicles for delivering therapeutic molecules. The current paper provides an update on the biology of CECs, EPCs and proangiogenic HCs, and explores the utility of these cell populations for clinical oncology.

Lesion-targeted thrombopoietin potentiates vasculogenesis by enhancing motility and enlivenment of transplanted endothelial progenitor cells via activation of Akt/mTOR/p70S6kinase signaling pathway

Thrombopoietin (TPO), a physiological regulator of megakaryocyte and platelet development, is a multifunctional positive regulator in early hematopoiesis by hematopoietic stem cells. In this study, we investigated the effect of TPO on endothelial progenitor cells (EPCs) for therapeutic vasculogenesis in vitro and in vivo, and the intracellular signaling mechanism exerting the activity of EPCs. 7-day culture-expanded EPCs derived from human peripheral blood mononuclear cells were applied to each assay. Flow cytometry demonstrated the expression of c-Mpl, the receptor of TPO, in cultured EPCs. In vitro experiments revealed enhanced migration and survival of cultured EPCs by TPO. In vivo, TPO was intramuscularly administered into the foci of ischemic hindlimbs in athymic nude mice, immediately followed by intravenous injection of cultured EPCs, to assess the booster effect of TPO on vascular regeneration. At day 4 post-transplantation, transplanted EPCs were 1.7-fold higher in TPO-treated animals compared to control. At day 28, blood perfusion was recovered in the TPO-treated group, accompanied by an increase in microvascular density. The signaling transduction pathway underlying TPO-mediated activities of cultured EPCs was assessed by Western blotting. TPO induced sequential phosphorylations of Akt to p70S6kinase through mTOR. Inhibition of the PI3-kinase/Akt/mTOR/p70S6kinase signaling pathway negated the biological functions of cultured EPCs, either migration (by LY294002 for PI3-kinase and Rapamycin for mTOR) or survival and tubulogenesis (by Rapamycin). These findings provide evidence that TPO possesses booster potential for therapeutic vasculogenesis, by activating the PI3-kinase/Akt/mTOR/p70S6kinase pathway crucial to the biological activities of EPCs.

Activation of endothelial nitric oxide synthase is critical for erythropoietin-induced mobilization of progenitor cells

The present study aimed to define the ability of erythropoietin (EPO) to mobilize hematopoietic stem cells (c-kit(+)/sca-1(+)/lin-1(-); KSL-cells) and hematopoietic progenitor cells (CD34(+) cells), including vascular endothelial growth factor receptor 2 expressing hematopoietic progenitor cells (CD34(+)/Flk-1(+) cells). We also sought to determine the role of endothelial nitric oxide synthase (eNOS) in EPO-induced mobilization. Wild type (WT) and eNOS(-/-) mice were injected bi-weekly with recombinant erythropoietin (EPO, 1000U/kg, s.c.) for 14 days. EPO increased the number of KSL, CD34(+), CD34(+)/Flk-1(+) cells in circulating blood of wild type mice. These effects of EPO were abolished in eNOS(-/-) mice. Our results demonstrate that, EPO stimulates mobilization of hematopoietic stem and progenitor cells. This effect of EPO is critically dependent on activation of eNOS.

Coupling erythropoietin secretion to mesenchymal stromal cells enhances their regenerative properties

AIMS

Mesenchymal stromal cells (MSCs) possess intrinsic features that identify them as useful for treating ischaemic syndromes. Poor in vivo survival/engraftment of MSCs, however, limits their overall effectiveness. In this work, we tested whether genetically engineering MSCs to secrete erythropoietin (Epo) could represent a better therapeutic platform than MSCs in their native form.

METHODS AND RESULTS

MSCs from C57Bl/6 mice were retrovirally transduced with either an empty vector or one that causes the production of Epo and were then analysed for the alterations in angiogenic and survival potential. Using a mouse model of myocardial infarction (MI), the regenerative potential of null MSCs and Epo-overexpressing MSCs (Epo+MSCs) was assessed using serial echocardiogram and invasive haemodynamic measurements. Infarct size, capillary density and neutrophil influx were assessed using histologic techniques. Using in vitro assays coupled with an in vivo Matrigel plug assay, we demonstrate that engineering MSCs to express Epo does not alter their immunophenotype or plasticity. However, relative to mock-modified MSCs [wild-type (WT)-MSCs], Epo+MSCs are more resilient to apoptotic stimuli and initiate a more robust host-derived angiogenic response. We also identify and characterize the autocrine loop established on MSCs by having them secrete Epo. Furthermore, in a murine model of MI, animals receiving intracardiac injections of Epo+MSCs exhibited significantly enhanced cardiac function compared with WT-MSCs and saline-injected control animals post-MI, owing to the increased myocardial capillary density and the reduced neutrophilia.

CONCLUSION

Epo overexpression enhances the cellular regenerative properties of MSCs by both autocrine and paracrine pathways.

Immune system in renal injury and repair: burning the candle from both ends?

This brief overview is focused on the inherent duality of immune responses: on the one hand they may induce inflammation and precipitate injury, on the other, a number of examples of participation in organ and tissue repair are growing. These processes will be reviewed from the nephrological stand-point. Specifically, the role of different leukocyte subsets, innate immunity, nitric oxide production, and stem cells, among others, are presented.

Biological approaches to ischemic tissue repair: gene- and cell-based strategies

Gene therapy is a potential therapeutic strategy for treatment of ischemic vascular diseases; however, the clinical application of gene therapy has met some anticipated challenges. Recent randomized, controlled trials suggest that patients with cardiovascular disease may also benefit from cell-based therapies, and the optimal treatment regimen may combine both approaches to take advantage of potential synergy between the underlying therapeutic mechanisms. This review discusses recent research into both gene and cell therapy and considers the potential application of a combined treatment approach for cardiovascular and cerebrovascular ischemic diseases.

G-CSF- and erythropoietin-based cell therapy: a promising strategy for angiomyogenesis in myocardial infarction

Granulocyte colony-stimulating factor (G-CSF) and erythropoietin are two cytokines that have been demonstrated to improve cardiac function and perfusion in myocardial infarction. G-CSF was initially evaluated as a stem cell mobilizer and erythropoietin as a cytoprotective agent. However, both cytokines have direct cytoprotective effects and stem cell-mobilizing ability. Direct cytoprotective effects of both cytokines are commonly mediated by the Jak-STAT pathway. In preclinical study, G-CSF and erythropoietin improved cardiac function and perfusion by angiomyogenesis and protection of cardiomyocytes in myocardial infarction. However, results from recent clinical trials did not support beneficial effects of cytokine therapy with G-CSF or erythropoietin alone in patients with myocardial infarction. Further studies are required to elucidate the mechanism of action and to improve therapeutic efficacy by employing novel strategies, such as combined cytokines.

Erythropoietin and the endothelium - a promising link?

The major physiological function of erythropoietin (EPO) is thought to be the induction of erythropoiesis. However, a growing body of evidence indicates that EPO as well as recombinant human erythropoietin (rHuEPO) and its analogues have tissue-protective effects and prevent tissue damage during ischaemia. This mini review summarizes the present knowledge on protective vascular effects of EPO and discusses the potential underlying mechanisms.

The non-haematopoietic biological effects of erythropoietin

In the haematopoietic system, the principal function of erythropoietin (Epo) is the regulation of red blood cell production, mediated by its specific cell surface receptor (EpoR). Following the cloning of the Epo gene (EPO) and characterization of the selective haematopoietic action of Epo in erythroid lineage cells, recombinant Epo forms (epoetin-alfa, epoetin-beta and the long-acting analogue darbepoetin-alfa) have been widely used for treatment of anaemia in chronic kidney disease and chemotherapy-induced anaemia in cancer patients. Ubiquitous EpoR expression in non-erythroid cells has been associated with the discovery of diverse biological functions for Epo in non-haematopoietic tissues. During development, Epo-EpoR signalling is required not only for fetal liver erythropoiesis, but also for embryonic angiogenesis and brain development. A series of recent studies suggest that endogenous Epo-EpoR signalling contributes to wound healing responses, physiological and pathological angiogenesis, and the body's innate response to injury in the brain and heart. Epo and its novel derivatives have emerged as major tissue-protective cytokines that are being investigated in the first human studies involving neurological and cardiovascular diseases. This review focuses on the scientific evidence documenting the biological effects of Epo in non-haematopoietic tissues and discusses potential future applications of Epo and its derivatives in the clinic.

Low-dose erythropoietin improves cardiac function in experimental heart failure without increasing haematocrit

BACKGROUND

Erythropoietin (EPO) may improve cardiac function and induce neovascularisation in experimental models of chronic heart failure (CHF). However, the increased haematocrit associated with EPO treatment might exert concomitant deleterious effects.

AIM

To investigate the haematocrit independent effects of EPO on cardiac function.

METHODS AND RESULTS

Rats underwent permanent coronary artery ligation to induce myocardial infarction (MI) or sham surgery. Three weeks after MI, rats were randomly allocated to treatment with vehicle (MI) or the long-acting EPO analogue darbepoetin alfa administered in a high (40 microg/kg/3 weeks, MI-EPO-high) or a low-dose (0.4 microg/kg/3 weeks, MI-EPO-low). After 9 weeks, haemodynamic parameters, myocardial histology and Myosin Heavy Chain (MHC) isoforms were determined. High-dose EPO resulted in a significant increase in haematocrit (p<0.01) while low-dose EPO had no effect on haematocrit levels. EPO significantly improved cardiac function in both EPO groups, reflected by increased left ventricular (LV)-developed pressure and improved contractility (dP/dt(max)) and relaxation (dP/dt(min)) indices of the LV at 9-weeks (all p<0.05 compared to MI). The improved cardiac function was associated with increased capillary growth (38% in MI-EPO-high (p<0.01) and 27% in MI-EPO-low (p<0.05)) and an attenuated switch to slow beta-MHC isoforms in both EPO groups.

CONCLUSIONS

EPO improves cardiac function and induces neovascularisation at a dose that does not increase haematocrit, thereby circumventing the possible deleterious effects of increased erythropoiesis.

Low-dose growth hormone administration mobilizes endothelial progenitor cells in healthy adults

OBJECTIVE

Endothelial progenitor cells (EPCs) mobilize from the bone marrow secondary to a stimulus and home to sites of injury, where they differentiate into endothelial cells and contribute to the repair of damaged vasculature. We hypothesized that growth hormone (GH) administration would increase the number of circulating EPCs in adults and thereby represent a mechanism to enhance vascular health.

DESIGN

A prospective trial of low-dose GH (0.03mg/kg/week for 4 weeks followed by 0.06mg/kg/week for a maximum of four additional weeks) in 10 healthy adults (6 males and 4 females; mean age 37 years, range 26-65). Primary outcomes measured included the number of circulating EPCs as assessed by colony-forming unit (CFU) assay and flow cytometry. Secondary outcomes included plasma measurements of known mediators of EPC mobilization and indices of nitric oxide (NO). Outcomes were measured at baseline and at study completion.

RESULTS

GH administration increased serum IGF-1 (143ng/mL [IQR 121-164] to 222 [IQR 194-244]; P=0.005). The increase in early-outgrowth EPCs (13 CFU per high-power field [IQR 6-24] to 19 [IQR 13-40]; P=0.005) correlated with the peak IGF-1 after adjustment for the baseline number of early-outgrowth EPCs (r=0.719 [95% CI 0.06, 0.93]; P=0.027). The number of late-outgrowth EPCs as well as CD34+, VEGFR2(KDR)+, and AC133+ cells did not significantly change. Other mediators of EPC mobilization were stable while plasma nitrite trended upwards (1.3micromol/L [IQR 0-2.5] to 3.7 [IQR 2.2-8.9]; P=0.052).

CONCLUSIONS

GH administration selectively augments the early-outgrowth EPC population in healthy individuals. These findings both support GH replacement in the setting of GH deficiency to maintain vascular integrity and have implications for the use of GH in future regenerative cell-based therapies. Furthermore, the decrease in EPCs observed with aging may in part be explained by the declining somatotropic axis, and thereby contribute to cardiovascular senescence.

Circulating bone marrow-derived endothelial progenitor cells: characterization, mobilization, and therapeutic considerations in malignant disease

Until recently, tumor vascularization was thought to occur exclusively through angiogenesis. However, recent studies using different animal models of cancer suggested the importance of bone marrow-derived endothelial progenitor cells (EPCs) (i.e. postnatal vasculogenesis) in tumor vascularization and growth. EPCs are present in the peripheral blood, their levels are increased in response to certain signals/cytokines, and they home into the neovascular bed of malignant tissues. Furthermore, at the clinical level, evidence is emerging that changes in EPC levels might predict the efficacy of anticancer drug combinations that include antiangiogenic agents. On the basis of these observations, EPCs have attractive potential diagnostic and therapeutic applications for malignant diseases. In this paper, we review biological features of EPCs and speculate on the utility of these progenitor cells for medical oncology.

The erythropoietin receptor in normal and cancer tissues

The hormone erythropoietin (EPO) is essential for the survival, proliferation and differentiation of the erythrocytic progenitors. The EPO receptor (EPO-R) of erythrocytic cells belongs to the cytokine class I receptor family and signals through various protein kinases and STAT transcription factors. The EPO-R is also expressed in many organs outside the bone marrow, suggesting that EPO is a pleiotropic anti-apoptotic factor. The controversial issue as to whether the EPO-R is functional in tumor tissue is critically reviewed. Importantly, most studies of EPO-R detection in tumor tissue have provided falsely positive results because of the lack of EPO-R specific antibodies. However, endogenous EPO appears to be necessary to maintain the viability of endothelial cells and to promote tumor angiogenesis. Although there is no clinical proof that the administration of erythropoiesis stimulating agents (ESAs) promotes tumor growth and mortality, present recommendations are that (i) ESAs should be administered at the lowest dose sufficient to avoid the need for red blood cell transfusions, (ii) ESAs should not be used in patients with active malignant disease not receiving chemotherapy or radiotherapy, (iii) ESAs should be discontinued following the completion of a chemotherapy course, (iv) the target Hb should be 12 g/dL and not higher and (v) the risks of shortened survival and tumor progression have not been excluded when ESAs are dosed to target Hb <12 g/dL.

Diabetic nephropathy

Perspectives on the News commentaries are part of a free monthly CME activity. The Mount Sinai School of Medicine, New York, New York, designates this activity for 2.0 AMA PRA Category 1 credits. If you wish to participate, review this article and visit www.diabetes.procampus.net to complete a posttest and receive a certificate. The Mount Sinai School of Medicine is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to provide continuing medical education for physicians.

Erythropoietin: when liability becomes asset in neurovascular repair

Erythropoietin (Epo) leads to the proliferation and differentiation of erythroid precursors, but is also involved in diverse nonhematopoietic biological functions. In this issue of the JCI, Chen, Smith, and colleagues demonstrate that the temporal expression of Epo is critical for determining whether physiological or pathological repair occurs following neurovascular retinal injury in the oxygen-induced retinopathy neonatal mouse model (see the related article beginning on page 526). The pleiotrophic properties of Epo make it a likely novel therapy for treatment of neurovascular damage, but the timing of its use must be carefully considered to prevent untoward effects.

Erythropoietin deficiency decreases vascular stability in mice

Erythropoietin (Epo), a hormone known to stimulate bone marrow erythrocyte production, is widely used to treat anemia in patients at risk for vascular disease. However, the effects of Epo on angiogenesis are not well defined. We studied the role of Epo in a mouse model of retinopathy characterized by oxygen-induced vascular loss followed by hypoxia-induced pathological neovascularization. Without treatment, local retinal Epo levels were suppressed during the vessel loss phase. Administration of exogenous Epo prevented both vessel dropout and subsequent hypoxia-induced neovascularization. Early use of Epo also protected against hypoxia-induced retinal neuron apoptosis. In contrast, retinal Epo mRNA levels were highly elevated during the retinopathy neovascular phase. Exogenous late Epo treatment did not protect the retina, but rather enhanced pathological neovascularization. Epo's early protective effect occurred through both systemic retinal recruitment of proangiogenic bone marrow-derived progenitor cells and activation of prosurvival NF-kappaB via Epo receptor activation on retinal vessels and neurons. Thus early retinal Epo suppression contributed to retinal vascular instability, and elevated Epo levels during the proliferation stage contributed to neovascularization and disease. Understanding the role of Epo in angiogenesis is critical to timing its intervention in patients with retinopathy or other diseases in which pathological angiogenesis plays a significant role.

Therapeutic potential of erythropoietin in cardiovascular disease: Erythropoiesis and beyond

Erythropoietin (EPO) is a glycoprotein hormone implicated in the regulation of red blood cell production. Anemia is common in chronic heart failure (CHF) patients and associated with an inappropriately low EPO-production, suggesting a role for its recombinant human form (rhEPO) in treatment. Although safety concerns have been raised regarding treatment with rhEPO in patients with chronic kidney disease, treatment with rhEPO in patients with CHF has so far been safe and well tolerated. The effect of rhEPO on outcome in anemic CHF patients is under investigation in a phase III clinical trial. In addition to its erythropoietic effects, EPO has been detected in the cardiovascular system, fueling intense research into possible non-hematopoietic effects. EPO has been shown to exert protective effects on the heart during acute myocardial ischemia and improve cardiac function in experimental CHF. Acute protection is mediated through reduction of apoptotic cell death. Improvement of cardiac function in CHF is related to myocardial neovascularization. EPO exhibits a vast array of beneficial effects in cardiovascular disease. In addition to the correction of anemia in CHF, rhEPO might benefit patients with cardiovascular disease.

Erythropoietin in cardiac disease: new features of an old drug

Erythropoietin is a haematopoietic hormone with extensive non-haematopoietic effects. The discovery of an erythropoietin receptor outside the haematopoietic system has fuelled the research into the beneficial effects of erythropoietin for various conditions, predominantly in cardiovascular disease. Experimental evidence has revealed the cytoprotective and angiogenic properties of erythropoietin and it seems that the erythropoietin-erythropoietin receptor system provides a powerful backbone against acute and chronic myocardial ischemia, each gaining from the different properties of erythropoietin. Clinical trials in which erythropoietin was titrated to achieve certain haematocrit levels have generated equivocal results. It has been suggested that a (too) high haematocrit is undesirable in cardiovascular disease. We have shown that intermittent (low-dose) erythropoietin administration, that does not increase haematocrit substantially, suffices to activate the beneficial downstream pathways of erythropoietin. We postulate that intermittent administration or a lower than conventional dose of erythropoietin, not only aimed at increasing haemoglobin at high levels, will provide powerful cellular protection and will improve cardiac outcome, without the side effects of erythropoietin associated with increased haematocrit.

Stem cells for cardiovascular repair - the challenges of the aging heart

The discovery of extracardiac progenitor cells and resident cardiac stem cells in recent years has led to a great deal of interest in the development of therapeutic strategies that target these endogenous cell sources for promotion of cardiovascular repair mechanisms in the diseased heart. Cardiovascular risk increases with age and among many factors, the age-associated decline in cardiac and vascular regenerative capacity may contribute to the progressive deterioration of cardiovascular health. Thus, understanding the mechanisms which underlie the dysregulation of cardiac stem and progenitor cells may lead to the identification of novel targets and approaches to reverse this decline. In this review, we outline the current knowledge about cardiac stem and progenitor cells, their contribution to cardiovascular regenerative processes and factors that may affect their decreased function in aging individuals. Moreover, we describe the therapeutic strategies that are currently being tested in clinical trials as well as potential new avenues of investigation for the future.

Endothelial progenitor cell therapy for the treatment of coronary disease, acute MI, and pulmonary arterial hypertension: current perspectives

Since their identification in 1997, bone marrow derived endothelial progenitor cells (EPCs) have been studied for their role in the endogenous maintenance and repair of endothelium and their potential regenerative capacity beyond the endothelium. In particular, EPCs have been tested in cell therapy approaches with the aim of developing novel therapies for conditions currently lacking effective treatment options. In this review, we discuss the scientific background and clinical experience using EPC delivery or mobilization for the treatment of post-angioplasty restenosis, acute myocardial infarction and pulmonary arterial hypertension. Although these approaches are safe, efficacy has yet to be proven in large randomized clinical trials. Unfortunately, the biology of EPCs is still poorly understood. The success of future clinical trials depends on a better understanding of EPC biology and intelligent design.

Progenitor cells and vascular disease

Vascular progenitor cells have been the focus of much attention in recent years; both from the point of view of their pathophysiological roles and their potential as therapeutic agents. However, there is as yet no definitive description of either endothelial or vascular smooth muscle progenitor cells. Cells with the ability to differentiate into mature endothelial and vascular smooth muscle reportedly reside within a number of different tissues, including bone marrow, spleen, cardiac muscle, skeletal muscle and adipose tissue. Within these niches, vascular progenitor cells remain quiescent, until mobilized in response to injury or disease. Once mobilized, these progenitor cells enter the circulation and migrate to sites of damage, where they contribute to the remodelling process. It is generally perceived that endothelial progenitors are reparative, acting to restore vascular homeostasis, while smooth muscle progenitors contribute to pathological changes. Indeed, the number of circulating endothelial progenitor cells inversely correlates with exposure to cardiovascular risk factors and numbers of animal models and human studies have demonstrated therapeutic roles for endothelial progenitor cells, which can be enhanced by manipulating them to overexpress vasculo-protective genes. It remains to be determined whether smooth muscle progenitor cells, which are less well studied than their endothelial counterparts, can likewise be manipulated to achieve therapeutic benefit. This review outlines our current understanding of endothelial and smooth muscle progenitor cell biology, their roles in vascular disease and their potential as therapeutic agents.

Do non-hemopoietic effects of erythropoietin play a beneficial role in heart failure?

Erythropoietin (EPO) is not solely a hormone charged with regulating the proliferation and differentiation of erythroid cells. Indeed, EPO is synthesized locally by many cells, especially under conditions of stress or injury. In these paracrine/autocrine settings, EPO plays a crucial protective-restorative role, activating cytoprotection (e.g., in the brain, heart, and kidney), reducing inflammatory responses, preserving vascular integrity, and mobilizing stem cells, including proliferation and differentiation of endothelial progenitor cells. EPO administration prevents cardiac myocyte apoptosis and decreases infarct size in several studies using rodent models of myocardial infarction. Recently, some key steps of the signaling pathways by which EPO confers cardioprotection have been identified. The striking finding distilled from work by numerous independent investigators is that EPO mediates protection in the heart (as well as other tissues) by multiple pathways that are not redundant. The following actions proven to play a role in protection from acute cardiac injury can exert a beneficial effect in chronic heart failure (HF): (a) antiapoptotic effect, (b) mobilization of endothelial progenitor cells from bone marrow, and (c) anti-hypertrophic effects. The evidences discussed herein provide a strong basis for the ongoing clinical trials testing EPO in chronic HF.

Patient characteristics and risk factors for nephrogenic systemic fibrosis following gadolinium exposure

Nephrogenic systemic fibrosis (NSF) is a systemic illness, which only affects patients with kidney failure. NSF risk increases with progressively lower levels of kidney function. It is characterized by red skin areas or plaques that develop over several weeks to painful thickened skin with a "woody" texture, resembling "peau d'orange." It may ultimately cause flexion contractures of joints. Skin biopsy reveals thickened collagen bundles, mucin deposition, proliferation of fibroblasts and elastic fibers, without inflammation. Originally described as nephrogenic fibrosing dermopathy (NFD), because of its primarily cutaneous manifestation, it was renamed NSF because of the involvement of various organs like the lungs, myocardium, or striated muscles. The pathogenesis of the disease is not known yet, but recently we suggested a strong association between development of NSF and exposure to gadolinium-based contrast (GBC) agents, thereafter confirmed by other authors. As a consequence of our recent observations, medical authorities imposed restrictions that exclude patients with advanced levels of renal insufficiency from potentially important magnetic resonance imaging studies with gadolinium. Unfortunately, the only alternatives in many situations (examination of brain, lungs, vasculature) are imaging modalities using iodinated radiocontrast agents. Thus, clinicians are faced with weighing the potential risk of NSF from GBC exposure against the risk of acute kidney injury-associated with radiocontrast media. In this dilemma, clinicians must identify patients at high-risk to develop NSF. Known risk factors critical for the development of NSF after exposure to GBC agents (certain chelates and higher doses) are end-stage renal disease requiring dialysis, especially those with little or no residual renal function, and advanced kidney disease not on dialysis. Other potential risk factors include metabolic acidosis, iron overload/intravenous iron, divalent ion disturbances, endothelial/vascular injury, and high erythropoietin doses. Further studies are required.