Inactivation of erythropoietin leads to defects in cardiac morphogenesis

Epicardial-myocardial signaling directing coronary vasculogenesis

The establishment of the coronary circulation is critical for the development of the embryonic heart. Over the last several years, there has been tremendous progress in elucidating the pathways that control coronary development. Interestingly, many of the pathways that regulate the development of the coronary vasculature are distinct from those governing vasculogenesis in the rest of the embryo. It is becoming increasingly clear that coronary development depends on a complex communication between the epicardium, the subepicardial mesenchyme, and the myocardium mediated in part by secreted growth factors. This communication coordinates the growth of the myocardium with the formation of the coronary vasculature. This review summarizes our present understanding of the role of these growth factors in the regulation of coronary development. Continued progress in this field holds the potential to lead to novel therapeutics for the treatment of patients with coronary artery disease.

Erythropoietin promotes spinal cord-derived neural progenitor cell proliferation by regulating cell cycle

Erythropoietin (EPO) regulates the proliferation and differentiation of erythroid cells by binding to its specific transmembrane receptor (EPOR). The presence of EPO and its receptor in the CNS suggests a different function for EPO other than erythropoiesis. The purpose of the present study was to examine EPOR expression and the role of EPO in the proliferation of neonatal spinal cord-derived neural progenitor cells. The effect of EPO on cell cycle progression was also examined, as well as the signaling cascades involved in this process. Our results showed that EPOR was present in the neural progenitor cells and EPO significantly enhanced their proliferation. Cell cycle analysis of EPO-treated neural progenitor cells indicated a reduced percentage of cells in G0/G1 phase, whereas the cell proliferation index (S phase plus G2/M phase) was increased. EPO also increased the proportion of 5-bromo-2-deoxyuridine (BrdU)-positive cells. With respect to the cell cycle signaling, we examined the cyclin-dependent kinases D1, D2 and E, and cyclin-dependent kinase inhibitors, p21cip1, p27kip1 and p57kip2. No significant differences were observed in the expression of these transcripts after EPO administration. Interestingly, the anti-apoptotic factors, mcl-1 and bcl-2 were significantly increased twofold. Moreover, these specific effects of EPO were eliminated by incubation of the progenitor cells with anti-EPO neutralizing antibody. Those observations suggested that EPO may play a role in normal spinal cord development by regulating cell proliferation and apoptosis.

Discovery and basic pharmacology of erythropoiesis-stimulating agents (ESAs), including the hyperglycosylated ESA, darbepoetin alfa: an update of the rationale and clinical impact

Cloning of the human erythropoietin (EPO) gene and development of the first recombinant human erythropoietin (rHuEPO) drug were truly breakthroughs. This allowed a deeper understanding of the structure and pharmacology of rHuEpo, which in turn inspired the discovery and development of additional erythropoiesis-stimulating agents (ESAs). In vivo specific activity and serum half-life of rHuEPO are influenced by the amount and structure of the attached carbohydrate. Increased numbers of sialic acids on carbohydrate attached to rHuEPO correlated with a relative increase in in-vivo-specific activity and increased serum half-life. The effect of increasing the number of sialic-acid-containing carbohydrates on in-vivo-specific activity was explored. Initial research focused on solving the problem of how the protein backbone could be engineered so a cell would add more carbohydrate to it. Additional work resulted in darbepoetin alfa, a longer-acting molecule with two additional carbohydrate chains.

Is there a role for erythropoietin in cardiovascular disease?

IMPORTANCE OF THE FIELD

Despite the advances in the cardiovascular field, cardiovascular diseases remain an important health problem with a high mortality rate. Novel therapeutic attempts that target myocardial ischemia and heart failure offer attractive adjuncts and/or alternatives to commonly employed regimens. The development of novel laboratory technologies over the last decade has led to substantial progress in bringing new therapies to the bedside.

AREAS COVERED IN THIS REVIEW

Current experimental and clinical trials in the use of erythropoietin (EPO) in cardiovascular diseases are reviewed.

WHAT THE READER WILL GAIN

This review will widen knowledge of the therapeutic potential of EPO's non-erythropoietic beneficial effects in a clinical cardiovascular setting.

TAKE HOME MESSAGE

Results from preclinical trials regarding the non-erythropoietic effects of erythropoietin are really encouraging. Further clinical studies are warranted to define the beneficial role of EPO in the clinical setting of coronary artery disease, heart failure and peripheral artery disease.

Genetic fate mapping demonstrates contribution of epicardium-derived cells to the annulus fibrosis of the mammalian heart

The annulus fibrosis electrically insulates the atria and ventricles, allowing the timed sequential beating of these structures that is necessary for efficient heart function. Abnormal development of the annulus fibrosis leads to persistence of accessory electrical pathways from atria to ventricles, providing the anatomical substrate for re-entrant cardiac arrhythmias such as Wolff-Parkinson-White syndrome. To better understand the development of the annulus fibrosis and the etiology of these cardiac arrhythmias, we used Cre-LoxP technology to assess the contribution of epicardium derived cells (EPDCs) to the annulus fibrosis. We found that EPDCs migrated into the region of the forming annulus fibrosis, marked by the protein periostin. These EPDCs also stained positive for procollagen I, suggesting that the EPDCs themselves synthesize proteins of the annulus fibrosis. To further test the hypothesis that EPDCs contribute to cells that synthesize the annulus fibrosis, we purified genetically marked EPDCs from the atrioventricular region and measured gene expression by quantitative PCR. These EPDCs were highly enriched for mRNAs encoding periostin, procollagen I, fibronectin I, vimentin, discoidin domain receptor 2, and tenascin C, markers of fibroblasts and components of the annulus fibrosis. In addition, these EPDCs were highly enriched for Snail, Smad1, Slug, and Twist1, markers for epithelial-to-mesenchymal transition (EMT), and a metalloprotease, Mmp2, that contributes to cellular migration. Our work provides for the first time definitive evidence that epicardium contributes to formation of the mammalian annulus fibrosis through EMT. Abnormalities of this differentiation process may underlie development of some forms of re-entrant atrioventricular tachycardia.

Origin of cardiac fibroblasts and the role of periostin

Cardiac fibroblasts are the most populous nonmyocyte cell type within the mature heart and are required for extracellular matrix synthesis and deposition, generation of the cardiac skeleton, and to electrically insulate the atria from the ventricles. Significantly, cardiac fibroblasts have also been shown to play an important role in cardiomyocyte growth and expansion of the ventricular chambers during heart development. Although there are currently no cardiac fibroblast-restricted molecular markers, it is generally envisaged that the majority of the cardiac fibroblasts are derived from the proepicardium via epithelial-to-mesenchymal transformation. However, still relatively little is known about when and where the cardiac fibroblasts cells are generated, the lineage of each cell, and how cardiac fibroblasts move to reside in their final position throughout all four cardiac chambers. In this review, we summarize the present understanding regarding the function of Periostin, a useful marker of the noncardiomyocyte lineages, and its role during cardiac morphogenesis. Characterization of the cardiac fibroblast lineage and identification of the signals that maintain, expand and regulate their differentiation will be required to improve our understanding of cardiac function in both normal and pathophysiological states.

Cardiogenesis: an embryological perspective

Cardiogenesis, considered as the formation of new heart tissue from embryonic, postnatal, or adult cardiac progenitors, is a pivotal concept to understand the rationale of advanced therapies to repair the damaged heart. In this review, we focus on the cellular and molecular regulation of cardiogenesis in the developing embryo, and we dissect the complex interactions that control the diversification and maturation of a variety of cardiac cell lineages. Our aim is to show how the sophisticated anatomical structure of the adult four-chambered heart strongly depends on the fine regulation of the differentiation of cardiac progenitor cells. These events are shown to be progressive and dynamic as well as plastic, so that the patterned differentiation of distinct heart domains is highly dependent on signals provided by nonmyocardial heart components and extracardiac tissues. Finally, we present the core of our knowledge on cardiac embryogenesis in a biomedical context to provide a critical analysis on the logic of cell therapies designed to treat the failing heart.

The erythropoietin and regenerative medicine: a lesson from fish

BACKGROUND

Erythropoietin (EPO), the main haematopoietic growth factor for the proliferation and differentiation of erythroid progenitor cells, is also known for its angiogenic and regenerative properties.

MATERIALS AND METHODS

In this study, we aimed to test the regenerative effects of EPO administration in an experimental model of Sea bass (Dicentrarchus labrax) subjected to amputation of the caudal fin.

RESULTS

Erythropoietin-treated fishes (3000 UI of human recombinant EPO-alpha immediately after cutting and after 15 days) showed an increased growth rate of their fins compared with those untreated (anova variance: P: 0.01 vs. P: 0.04). By analysing fin length at established times (15 and 30 days after cut), EPO-treated fishes always showed an increased length compared with untreated ones (T-15: 1.1 +/- 0.2 vs. 0.7 +/- 0.2 cm, P: 0.03; T-30: 1.9 +/- 0.3 vs. 1.2 +/- 0.2 cm, P: 0.01). Moreover, exogenous EPO administration induced an enormous increase in EPO-blood levels at each observation time (T-15: 2240 +/- 210 vs. 16.7 +/- 1.8 mU mL(-1), P < 0.001; T-30: 2340 +/- 190 vs. 17.1 +/- 1.9 mU mL(-1), P < 0.001), whereas these levels remained quite unmodified in untreated fishes. Immunochemical analyses performed by confocal laser scanning microscopic observations showed an increased expression of EPO-receptors and PECAM-1 (an endothelial surface marker of vessels sprout) in the regenerating tissue, whereas no signs of inflammation or fibrosis were recognisable.

CONCLUSIONS

All these findings confirm EPO as a new factor involved in regenerative processes, also suggesting a potential, future utility for new therapeutical applications in the field of human regenerative medicine.

Erythropoietin: a multimodal neuroprotective agent

The tissue protective functions of the hematopoietic growth factor erythropoietin (EPO) are independent of its action on erythropoiesis. EPO and its receptors (EPOR) are expressed in multiple brain cells during brain development and upregulated in the adult brain after injury. Peripherally administered EPO crosses the blood-brain barrier and activates in the brain anti-apoptotic, anti-oxidant and anti-inflammatory signaling in neurons, glial and cerebrovascular endothelial cells and stimulates angiogenesis and neurogenesis. These mechanisms underlie its potent tissue protective effects in experimental models of stroke, cerebral hemorrhage, traumatic brain injury, neuroinflammatory and neurodegenerative disease. The preclinical data in support of the use of EPO in brain disease have already been translated to first clinical pilot studies with encouraging results with the use of EPO as a neuroprotective agent.

Fog2 is critical for cardiac function and maintenance of coronary vasculature in the adult mouse heart

Aberrant transcriptional regulation contributes to the pathogenesis of both congenital and adult forms of heart disease. While the transcriptional regulator friend of Gata 2 (FOG2) is known to be essential for heart morphogenesis and coronary development, its tissue-specific function has not been previously investigated. Additionally, little is known about the role of FOG2 in the adult heart. Here we used spatiotemporally regulated inactivation of Fog2 to delineate its function in both the embryonic and adult mouse heart. Early cardiomyocyte- restricted loss of Fog2 recapitulated the cardiac and coronary defects of the Fog2 germline murine knockouts. Later cardiomyocyte-restricted loss of Fog2 (Fog2MC) did not result in defects in cardiac structure or coronary vessel formation. However, Fog2MC adult mice had severely depressed ventricular function and died at 8-14 weeks. Fog2MC adult hearts displayed a paucity of coronary vessels, associated with myocardial hypoxia, increased cardiomyocyte apoptosis, and cardiac fibrosis. Induced inactivation of Fog2 in the adult mouse heart resulted in similar phenotypes, as did ablation of the FOG2 interaction with the transcription factor GATA4. Loss of the FOG2 or FOG2-GATA4 interaction altered the expression of a panel of angiogenesis-related genes. Collectively, our data indicate that FOG2 regulates adult heart function and coronary angiogenesis.

Epicardial control of myocardial proliferation and morphogenesis

The epicardium is a critical tissue that directs several aspects of heart development, particularly via the secretion of soluble factors. This review summarizes recent approaches that implicate the epicardium as the source of mitogenic factors promoting cardiomyocyte proliferation, as the source of instructive signals that direct compact zone organization (morphogenesis), and as the tissue that directs formation of the coronary vasculature.

Recombinant human erythropoietin prevents lipopolysaccharide-induced vascular hyporeactivity in the rat

Erythropoietin (EPO) is a hypoxia-inducible hormone that is essential for normal erythropoiesis in the bone marrow. Administration of recombinant human-EPO is currently being used for the therapy of anemia associated with chronic renal failure and cancer. Moreover, EPO reduces organ injury in experimental hemorrhagic as well as in splanchnic artery occlusion shock and preserves cardiac function after experimental cardiac I/R. Erythropoietin receptors are widely distributed in the cardiovascular system, including endothelial, smooth muscle, cardiac, and other cell types, and nonhematopoietic effects of EPO are increasingly recognized. Thus, the vasculature may be a biological target of EPO. Therefore, the aim of our study was to investigate whether EPO exerts a protective effect in septic shock by modulating vascular dysfunction and hyporeactivity. Rats received EPO (300 U/kg, i.v.) or vehicle 30 min before and 1 and 3 h after LPS (8 x 10 U/kg, i.v.). In vivo and ex vivo (aortic rings) experiments were performed to evaluate the vascular response to contracting and vasodilating agents. The expression of iNOS, intercellular adhesion molecule 1, poly(ADP)ribose polymerase, Bcl-xl, and Bcl-2 was evaluated by Western blot analysis in the rat aorta. We demonstrate that EPO significantly prevents LPS-induced vascular hyporeactivity and endothelial dysfunction. Interestingly, EPO inhibits the increase in iNOS, poly(ADP)ribose polymerase, and intercellular adhesion molecule 1 expression in the aorta of endotoxemic rats and attenuated the decline in the expression of both Bcl-xl and Bcl-2 caused by LPS. In conclusion, our data support the view that EPO has important nonerythropoietic effects protecting organ and tissue against injury and indicate that EPO may be useful in the therapy of patients with septic shock.

Angiogenesis after cerebral ischemia

Though the vascular system of the adult brain is extremely stable under normal baseline conditions, endothelial cells start to proliferate in response to brain ischemia. The induction of angiogenesis, primarily in the ischemic boundary zone, enhances oxygen and nutrient supply to the affected tissue. Additionally, the generation of new blood vessels facilitates highly coupled neurorestorative processes including neurogenesis and synaptogenesis which in turn lead to improved functional recovery. To take advantage of angiogenesis as a therapeutic concept for stroke treatment, the knowledge of the precise molecular mechanisms is mandatory. Especially, since a couple of growth factors involved in post-ischemic angiogenesis may have detrimental adverse effects in the brain by increasing vascular permeability. This article summarizes the knowledge of molecular mechanisms of angiogenesis following cerebral ischemia. Finally, experimental pharmacological and cellular approaches to stimulate and enhance post-ischemic angiogenesis are discussed.

Erythropoietin and renoprotection

In the haematopoietic system, the principal function of erythropoietin (EPO) is the regulation of RBC production. Consequently, following the cloning of the EPO gene, recombinant human EPO (rHuEPO) forms have been widely used for treatment of anaemia in chronic kidney disease and chemotherapy-induced anaemia in cancer patients. However, a steadily growing body of evidence indicates that the therapeutic benefits of rHuEPO could be far beyond the correction of anaemia. Several articles have been recently published on the tissue-protective, nonhaematological effects of rHuEPO that prevent ischaemia-induced tissue damage in several organs including the kidney.In this review, we focus on nonhaematological effects of rHuEPO in various experimental settings of acute and chronic kidney injury. Because this tissue-protective action of rHuEPO is not the result of correction of anaemia-related tissue hypoxia, we will also discuss potential molecular pathways involved. Finally, we will review the current literature on clinical studies with rHuEPO or analogous substances and progression of chronic kidney disease, and propose possible clinical renoprotective strategies.

Cytokines and myocardial regeneration: a novel treatment option for acute myocardial infarction

The regenerative capacity of the myocardium and its blood vessels has now been well demonstrated. The cytokines granulocyte colony-stimulating factor, erythropoietin, and stem cell factor may play a role in helping to stimulate cell regeneration under normal physiologic conditions and in patients with myocardial injury. After an ischemic insult, cytokines are released into the peripheral circulation and signal for the mobilization of stem cells. In experimental cardiac injury models, the addition of cytokines has been shown to improve myocardial function with and without the concurrent use of stem cell therapy. Preliminary studies in humans using cytokine therapy alone for treating myocardial infarction have been disappointing. Future studies in patients with myocardial injury need to examine the use of various combinations of cytokines, with and without the addition of intravascular stem cell infusions or direct stem cell injections.

Association of matrix metalloproteinase-3 with cardiogenic activity during Noggin-induced differentiation of mouse embryonic stem cells

BACKGROUND

Despite the pluripotency of embryonic stem (ES) cells, their clinical applications have been hindered due to the lack of reliable differentiation methods. Recently, it was shown that Noggin could effectively induce cardiomyocyte differentiation by transient treatment of ES cells.

METHODS

To determine how Noggin may induce cardiac differentiation, we compared differentially expressed genes during Noggin-induced differentiation of ES cells using microarray analysis. We found Matrix metalloproteinase-3 (Mmp-3) expression was highly up-regulated by Noggin treatment. To understand the role of Mmp-3 in the cardiac differentiation of ES cells, we inhibited Mmp-3 activity by treating with a specific Mmp-3 inhibitor during Noggin-induced cardiac differentiation of ES cells. We also analyzed the expression levels of cardiac markers and the ratio of spontaneously beating embryoid bodies (EBs) in the presence of the Mmp-3 inhibitor.

RESULTS

We analyzed EB samples from zero, two, and four days with or without Noggin treatment, and found that the expression levels of 2 (0 day), 56 (2 days), and 805 (4 days) genes were altered with Noggin treatment. Up-regulation of Mmp-3 was closely associated with relative increases of cardiogenic, vasculogenic, and hematopoietic genes in EB treated with Noggin. By inhibiting Mmp-3 activity, we verified that Mmp-3 activation is partly responsible for both the expression of cardiac markers and the elevated ratio of spontaneously beating to non-beating EBs.

CONCLUSIONS

The concurrent expression of Mmp-3 with many cardiogenic genes and the specific inhibition of Mmp-3 revealed a critical role for Mmp-3 in Noggin-induced cardiac differentiation of ES cells.

Human embryonic stem cells and cardiac repair

The muscle lost after a myocardial infarction is replaced with noncontractile scar tissue, often initiating heart failure. Whole-organ cardiac transplantation is the only currently available clinical means of replacing the lost muscle, but this option is limited by the inadequate supply of donor hearts. Thus, cell-based cardiac repair has attracted considerable interest as an alternative means of ameliorating cardiac injury. Because of their tremendous capacity for expansion and unquestioned cardiac potential, pluripotent human embryonic stem cells (hESCs) represent an attractive candidate cell source for obtaining cardiomyocytes and other useful mesenchymal cell types for such therapies. Human embryonic stem cell-derived cardiomyocytes exhibit a committed cardiac phenotype and robust proliferative capacity, and recent testing in rodent infarct models indicates that they can partially remuscularize injured hearts and improve contractile function. Although the latter successes give good reason for optimism, considerable challenges remain in the successful application of hESCs to cardiac repair, including the need for preparations of high cardiac purity, improved methods of delivery, and approaches to overcome immune rejection and other causes of graft cell death. This review will describe the phenotype of hESC-derived cardiomyocytes and preclinical experience with these cells and will consider strategies to overcoming the aforementioned challenges.

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.

Hypoxic preconditioning protects rat hearts against ischaemia-reperfusion injury: role of erythropoietin on progenitor cell mobilization

Preconditioning, such as by brief hypoxic exposure, has been shown to protect hearts against severe ischaemia. Here we hypothesized that hypoxic preconditioning (HPC) protects injured hearts by mobilizing the circulating progenitor cells. Ischaemia-reperfusion (IR) injury was induced by left coronary ligation and release in rats kept in room air or preconditioned with 10% oxygen for 6 weeks. To study the role of erythropoietin (EPO), another HPC + IR group was given an EPO receptor (EPOR) antibody via a subcutaneous mini-osmotic pump 3 weeks before IR induction. HPC alone gradually increased haematocrit, cardiac and plasma EPO, and cardiac vascular endothelial growth factor (VEGF) only in the first two weeks. HPC improved heart contractility, reduced ischaemic injury, and maintained EPO and EPOR levels in the infarct tissues of IR hearts, but had no significant effect on VEGF. Interestingly, the number of CD34(+)CXCR4(+) cells in the peripheral blood and their expression in HPC-treated hearts was higher than in control. Preconditioning up-regulated cardiac expression of stromal derived factor-1 (SDF-1) and prevented its IR-induced reduction. The EPOR antibody abolished HPC-mediated functional recovery, and reduced SDF-1, CXCR4 and CD34 expression in IR hearts, as well as the number of CD34(+)CXCR4(+) cells in blood. The specificity of neutralizing antibody was confirmed in an H9c2 culture system. In conclusion, exposure of rats to moderate hypoxia leads to an increase in progenitor cells in the heart and circulation. This effect is dependent on EPO, which induces cell homing by increased SDF-1/CXCR4 and reduces the heart susceptibly to IR injury.

Erythropoietic agents and the elderly

Erythropoietin (Epo) is a peptide hormone that stimulates erythropoiesis. There are several agents in clinical use and in development that either act as ligands for the cell surface receptors of Epo or promote Epo production, which stimulates erythropoiesis. These are known as erythropoietic agents. The agents already in use include epoetin alfa, epoetin beta, and darbepoetin alfa. Newer agents under active investigation include continuous erythropoietin receptor activator (CERA) or proline hydroxylase inhibitors that increase hypoxia-inducible factor-1 (HIF-1), thereby stimulating Epo production and iron availability and supply. Erythropoietic agents have been shown to promote neuronal regeneration and to decrease post-stroke infarct size in mouse models. They have also been reported to shorten survival when used to treat anemia in many cancer patients and to increase thromboembolism. In contrast, rapid decrease of Epo levels as observed in astronauts and high-altitude dwellers upon rapid descent to sea level leads to the decrease of erythroid mass, a phenomenon known as "neocytolysis." The relative decrease in the serum Epo level is known to occur in some subjects with otherwise unexplained anemia of aging. Anemia by itself is a predictor of poor physical function in the elderly and is a significant economic burden on society. One out of every five persons in the United States will be elderly by 2050. Erythropoietic agents, by preventing and treating otherwise unexplained anemias of the elderly and anemia associated with other disease conditions of the elderly, have the potential to improve the functional capacity and to decrease the morbidity and mortality in the elderly, thereby alleviating the overall burden of medical care in society.

High oxygen prevents fetal lethality due to lack of catecholamines

The catecholamine norepinephrine is required for fetal survival, but its essential function is unknown. When catecholamine-deficient [tyrosine hydroxylase (Th) null] mouse fetuses die at embryonic day (E)13.5-14.5, they resemble wild-type (wt) fetuses exposed to hypoxia. They exhibit bradycardia (28% reduction in heart rate), thin ventricular myocardium (20% reduction in tissue), epicardial detachment, and death with vascular congestion, hemorrhage, and edema. At E12.5, before the appearance of morphological deficits, catecholamine-deficient fetuses are preferentially killed by experimentally induced hypoxia and have lower tissue Po(2) levels than wt siblings. By microarray analysis (http://www.ncbi.nlm.nih.gov/geo; accession no. GSE10341), hypoxia-inducible factor-1 target genes are induced to a greater extent in null fetuses than in wt siblings, supporting the notion that mutants experience lower oxygen tension or have an enhanced response to hypoxia. Hypoxia induces a 13-fold increase in plasma norepinephrine levels, which would be expected to increase heart rate, thereby improving oxygen delivery in wt mice. Surprisingly, increasing maternal oxygen (inspired O(2) 33 or 63%) prevents the effects of catecholamine deficiency, restoring heart rate, myocardial tissue, and survival of Th null fetuses to wt levels. We suggest that norepinephrine mediates fetal survival by maintaining oxygen homeostasis.

Extrinsic regulation of cardiomyocyte differentiation of embryonic stem cells

Cardiovascular disease is one of leading causes of death throughout the U.S. and the world. The damage of cardiomyocytes resulting from ischemic injury is irreversible and leads to the development of progressive heart failure, which is characterized by the loss of functional cardiomyocytes. Because cardiomyocytes are unable to regenerate in the adult heart, cell-based therapy of transplantation provides a potential alternative approach to replace damaged myocardial tissue and restore cardiac function. A major roadblock toward this goal is the lack of donor cells; therefore, it is urgent to identify the cardiovascular cells that are necessary for achieving cardiac muscle regeneration. Pluripotent embryonic stem (ES) cells have enormous potential as a source of therapeutic tissues, including cardiovascular cells; however, the regulatory elements mediating ES cell differentiation to cardiomyocytes are largely unknown. In this review, we will focus on extrinsic factors that play a role in regulating different stages of cardiomyocyte differentiation of ES cells.

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.

Early fetal hypoxia leads to growth restriction and myocardial thinning

Hypoxia is necessary for fetal development; however, excess hypoxia is detrimental. Hypoxia has been extensively studied in the near-term fetus, but less is known about earlier fetal effects. The purpose of this study was to determine the window of vulnerability to severe hypoxia, what organ system(s) is most sensitive, and why hypoxic fetuses die. We induced hypoxia by reducing maternal-inspired O2 from 21% to 8%, which decreased fetal tissue oxygenation assessed by pimonidazole binding. The mouse fetus was most vulnerable in midgestation: 24 h of hypoxia killed 89% of embryonic day 13.5 (E13.5) fetuses, but only 5% of E11.5 and 51% of E17.5 fetuses. Sublethal hypoxia at E12.5 caused growth restriction, reducing fetal weight by 26% and protein by 45%. Hypoxia induced HIF-1 target genes, including vascular endothelial growth factor (Vegf), erythropoietin, glucose transporter-1 and insulin-like growth factor binding protein-1 (Igfbp-1), which has been implicated in human intrauterine growth restriction (IUGR). Hypoxia severely compromised the cardiovascular system. Signs of heart failure, including loss of yolk sac circulation, hemorrhage, and edema, were caused by 18-24 h of hypoxia. Hypoxia induced ventricular dilation and myocardial hypoplasia, decreasing ventricular tissue by 50% and proliferation by 21% in vivo and by 40% in isolated cultured hearts. Epicardial detachment was the first sign of hypoxic damage in the heart, although expression of epicardially derived mitogens, such as FGF2, FGF9, and Wnt9b was not reduced. We propose that hypoxia compromises the fetus through myocardial hypoplasia and reduced heart rate.

Coronary development is regulated by ATP-dependent SWI/SNF chromatin remodeling component BAF180

Dissecting the molecular mechanisms that guide the proper development of epicardial cell lineages is critical for understanding the etiology of both congenital and adult forms of human cardiovascular disease. In this study, we describe the function of BAF180, a polybromo protein in ATP-dependent SWI/SNF chromatin remodeling complexes, in coronary development. Ablation of BAF180 leads to impaired epithelial-to-mesenchymal-transition (EMT) and arrested maturation of epicardium around E11.5. Three-dimensional collagen gel assays revealed that the BAF180 mutant epicardial cells indeed possess significantly compromised migrating and EMT potentials. Consequently, the mutant hearts form abnormal surface nodules and fail to develop the fine and continuous plexus of coronary vessels that cover the entire ventricle around E14. PECAM and *-SMA staining assays indicate that these nodules are defective structures resulting from the failure of endothelial and smooth muscle cells within them to form coronary vessels. PECAM staining also reveal that there are very few coronary vessels inside the myocardium of mutant hearts. Consistent with this, quantitative RT-PCR analysis indicate that the expression of genes involved in FGF, TGF, and VEGF pathways essential for coronary development are down-regulated in mutant hearts. Together, these data reveal for the first time that BAF180 is critical for coronary vessel formation.

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.

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.

Erythropoietin protects against doxorubicin-induced cardiomyopathy via a phosphatidylinositol 3-kinase-dependent pathway

Doxorubicin (DOX) is an effective antineoplastic agent whose use has been limited by its cardiotoxic side effects. Recent studies have established that erythropoietin (EPO), a cytokine essential for red blood cell production, protects against ischemic injury in the heart and other organs. The purpose of this study was to assess whether EPO protects the heart against cardiotoxicity induced by DOX. We found that DOX-induced apoptosis and impaired heart function in mice were largely prevented by EPO administration. To investigate the mechanism of protection by EPO, cultured neonatal mouse ventricular myocytes were treated with EPO at therapeutic levels (i.e., 1 U/ml), before application of DOX (0.1-1.0 microM). EPO protected against DOX-induced cardiomyocyte death (by approximately 50%) and apoptosis assessed by annexin-V labeling, DNA fragmentation, and caspase-3 activity. DOX-mediated increases in reactive oxygen species, which trigger cardiotoxicity, were also reversed by preconditioning with EPO. These functional effects of EPO correlated with increased Akt/protein kinase B ( approximately 2-fold) and glycogen synthase kinase 3 (GSK-3; approximately 1.3-fold) phosphorylations, suggesting protection by EPO was mediated by phosphatidylinositol 3-kinase activation. Indeed, preventing Akt and GSK-3beta phosphorylations by phosphatidylinositol 3-kinase (PI3K) inhibition abolished protection by EPO against cardiomyocyte loss, apoptosis, and oxidative stress. Thus, pretreatment with therapeutic levels of EPO can protect the myocardium against DOX-induced impaired heart function and cardiomyocyte apoptosis by activating PI3K-Akt cell survival pathways.

Characterization of developing bowel transplanted from transgenic erythropoietin receptor-deficient mouse embryos

BACKGROUND

Erythropoietin (Epo) receptors (EpoR) are present in embryonic and postnatal mammalian bowel, and activation of EpoR signaling with recombinant Epo (rEpo) has trophic effects. Transgenic mice with absent Epo function are embryonic lethal, so it is not known whether Epo function is required for bowel development.

OBJECTIVE

To characterize bowel structure in the absence of EpoR signaling.

METHODS

Heterozygous EpoR knockout mice were mated. Bowel segments from their embryos were dissected and transplanted beneath the renal capsule of adult wild-type mice and residual embryo tissue was excised for genotyping. Transplants were harvested at 7, 14 or 21 days. The transplanted bowel segments were immunostained to identify proliferation (BrdU+), as well as neuronal (PGP9.5+), endothelial (vWF+), and neuroendocrine (synaptophysin+) cells. Gross and microscopic characteristics of intestinal differentiation were evaluated.

RESULTS

50 transplants were performed: bowel from 49 embryos survived to harvest and 43 showed evidence of bowel development with appropriate small or large intestinal features. No differences in morphology, immunolabeling, or BrdU incorporation were observed between homozygous-null, heterozygote or wild-type bowel. Smooth muscle and mucosal cells were present, along with neuronal, endothelial, and neuroendocrine cells in all genotypes.

CONCLUSIONS

Enteric EpoR signaling is not essential for intestinal morphogenesis.

Mouse models of congenital cardiovascular disease

Congenital heart defects occur in nearly 1% of human live births and many are lethal if not surgically repaired. In addition, the genetic contribution to congenital or acquired cardiovascular diseases that are silent at birth, but progress to cause significant disease in later life is being increasingly appreciated. Heart development and structure are highly conserved between mouse and human. The discoveries that are being made in this model system are highly relevant to understanding the pathogenesis of human heart defects whether they occus in isolation, or in the context of a syndrome. Many of the genes required for cardiovascular development were discovered fortuitously when early lethality or structural defects were observed in mouse mutants generated for other purposes, and relevant genes continue to be defined in this manner. Candidate genes for this process are being identified by their roles other species, or by their expression in pertinent tissues in mice. In this review, I will briefly summarize heart development as currently understood in the mouse, and then discuss how complementary studies in mouse and human have identified genes and pathways that are critical for normal cardiovascular development, and for maintaining the structure and function of this organ system throughout life.

Fibroblast growth factors and Hedgehogs: at the heart of the epicardial signaling center

Over the past several years, increasing attention has been focused on understanding signaling pathways that control key events during midgestational heart development. During this period of development, the heart tube transforms into a functioning organ that must maintain its own blood supply and grow and respond to the physiologic needs of the organism. A critical event that occurs during midgestational heart development is the formation of the epicardium, which functions as a source of cells and as a signaling center that regulates myocardial growth and coronary vascular development. This review will describe our understanding of the role and the mechanism by which the epicardium governs these developmental events, primarily as a result of studies in the mouse. We focus on two key growth factor pathways: fibroblast growth factor and Hedgehog signaling.

Functional conservation of erythropoietin signaling in zebrafish

Erythropoietin (Epo) and its cognate receptor (EpoR) are required for maintaining adequate levels of circulating erythrocytes during embryogenesis and adulthood. Here, we report the functional characterization of the zebrafish epo and epor genes. The expression of epo and epor was evaluated by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization, revealing marked parallels between zebrafish and mammalian gene expression patterns. Examination of the hypochromic mutant, weissherbst, and adult hypoxia-treated hearts indicate that zebrafish epo expression is induced by anemia and hypoxia. Overexpression of epo mRNA resulted in severe polycythemia, characterized by a striking increase in the number of cells expressing scl, c-myb, gata1, ikaros, epor, and betae1-globin, suggesting that both the erythroid progenitor and mature erythrocyte compartments respond to epo. Morpholino-mediated knockdown of the epor caused a slight decrease in primitive and complete block of definitive erythropoiesis. Abrogation of STAT5 blocked the erythropoietic expansion by epo mRNA, consistent with a requirement for STAT5 in epo signaling. Together, the characterization of zebrafish epo and epor demonstrates the conservation of an ancient program that ensures proper red blood cell numbers during normal homeostasis and under hypoxic conditions.

High-dose erythropoietin inhibits apoptosis and stimulates proliferation in neonatal rat intestine

BACKGROUND

Erythropoietin (Epo) receptors are widely expressed in the small bowel of neonatal rats and evidence suggests Epo has important trophic effects in developing bowel.

OBJECTIVE

To compliment in vitro data, we directly examine in vivo the hypotheses that systemic Epo treatment can promote cell division and enterocyte migration, and arrest apoptosis in the ileum of neonatal rats.

DESIGN

Epo (5000 U/kg s.c.) or vehicle treatments were given to one week old Sprague-Dawley rats (n = 86) along with timed injections of the thymidine analog 5-bromo-2-deoxyuridine (BrdU, 50mg/kg s.c.) to label DNA synthesis and track newly proliferating cells. To characterize the time course of effects, animals were killed at scheduled times from 30 min to 24 h after treatment. BrdU-containing cells were immunostained and counted in intestinal crypts, villi, and muscle wall of ileum. Effects of Epo on apoptosis were analyzed by TUNEL staining. Calibrated measurements were made to determine the density or relative proportion of BrdU- and TUNEL-positive cells.

RESULTS

Systemic high-dose Epo promoted cell division in intestinal smooth muscle and enterocytes, stimulated migration of intestinal epithelial cells, and arrested apoptosis of enterocytes at the villous tips.

CONCLUSION

These data provide in vivo evidence that Epo functions trophically in developing intestine tissues.

Epicardium-derived cells in cardiogenesis and cardiac regeneration

During cardiogenesis, the epicardium grows from the proepicardial organ to form the outermost layer of the early heart. Part of the epicardium undergoes epithelial-mesenchymal transformation, and migrates into the myocardium. These epicardium- derived cells differentiate into interstitial fibroblasts, coronary smooth muscle cells, and perivascular fibroblasts. Moreover, epicardium-derived cells are important regulators of formation of the compact myocardium, the coronary vasculature, and the Purkinje fiber network, thus being essential for proper cardiac development. The fibrous structures of the heart such as the fibrous heart skeleton and the semilunar and atrioventricular valves also depend on a contribution of these cells during development. We hypothesise that the essential properties of epicardium-derived cells can be recapitulated in adult diseased myocardium. These cells can therefore be considered as a novel source of adult stem cells useful in clinical cardiac regeneration therapy.

Promotion of neurite outgrowth and protective effect of erythropoietin on the retinal neurons of rats

OBJECTIVE

To clarify the effect of erythropoietin (EPO) on neurite outgrowth of the cultured retinal neurocytes, and investigate whether EPO might potentially be beneficial in protecting cultured retinal neurocytes suffering from glutamate-induced cytotoxity.

METHODS

After the retinal neurocytes were cultured for 48 hours, the culture media was replaced with serum-free media, and the cultured retinal cells were exposed to 1.0 U/ml, 3.0 U/ml and 6.0 U/ml EPO for another 48 hours; then the cells were stained with Sudan Black B, and the neurite outgrowth of those cells were evaluated by an image-analysis system. After the retinal neurocytes were cultured for 48 hours, the cells were cultured in serum-free media containing 5 mM or 10 mM glutamate, and the cells were incubated in the presence or absence of Epo (1.0 U/ml, 3.0 U/ml, 6.0 U/ml respectively) for another 48 hours. The survival and apoptosis rates of those cells were estimated by MTT assay and fluorescein isothiocyanate (FITC)-annexin V/propidium Iodide (PI) flow cytometry respectively.

RESULTS

EPO induced a stable improvement of neurite outgrowth of retinal neurocytes in a dose-dependent manner. Compared with the control group, the neurite outgrowth length increased to 162.8% at 6.0 U/ml EPO exposure. EPO had no any significant effect on the survival and apoptosis rates of the retinal neurocytes cultured in serum-free media, but it was beneficial in promoting the survival and decreasing the early and total apoptosis rates of the cultured retinal neurocytes suffering from glutamate-induced cytotoxicity.

CONCLUSION

EPO had a significant biological effect on neurite outgrowth of the dissociated retinal neurocytes in vitro. EPO was beneficial in promoting the survival and decreasing the apoptosis rates of the cultured retinal neurocytes suffering from glutamate-induced cytotoxicity.

Epo and other hematopoietic factors

The growth factors erythropoietin and granulocyte-colony stimulating factor have hematopoietic and non-hematopoietic functions. Both are used clinically in their recombinant forms. Both also have interesting tissue-protective effects in other organs, which are unrelated to their hematopoietic functions. They have clinical hematopoietic uses in neonatal populations and in experimental non-hematopoietic research, and clinical potential as neuroprotective or tissue-protective agents.

Survival and invasiveness of astrocytomas promoted by erythropoietin

OBJECT

The hypoxia-inducible pleiotropic hormone, erythropoietin (EPO), has recently been found to promote the development and survival of neurons and astrocytes. Since hypoxia has been implicated in the malignant progression of some human cancers, the authors investigated whether EPO signaling influenced the malignant properties of human astrocytoma cells.

METHODS

Reverse transcriptase-polymerase chain reaction, Western blot analysis, and immunohistochemical studies were used to measure EPO and its receptor (EPOR). Cell viability, Matrigel invasion assays, metalloprotease assays, EPO neutralizing antibodies, and EPOR overexpression were used to study the biological actions of EPO. Expression of both EPO and EPOR was observed in the hypoxic regions and invasive margins of glioma specimens obtained at biopsy, and expression of EPOR correlated with the stage of the tumor. The EPOR was also functionally upregulated by hypoxia in cultured glioblastoma multiforme (GBM) cells. Both hypoxia and EPO protected cultured GBM cells from cisplatin cytotoxicity and promoted the invasiveness of GBM cells through Matrigel by potentiating metalloprotease activity. Hypoxia-enhanced cell invasion was attenuated in cells that overexpressed a nonfunctional EPOR.

CONCLUSIONS

Hypoxia-inducible autocrine and paracrine EPO signaling participates in the malignant progression of GBMs.

Early haemoglobin-independent increase of plasma erythropoietin levels in patients with acute myocardial infarction

AIMS

We studied plasma erythropoietin (EPO) levels and their relation with CD34(+)VEGFR-2(+) (mature and progenitor endothelial cells) and CD34(+) CD133(+)VEGFR-2(+), or CD34(+) CD117(+)VEGFR-2(+) (early/immature endothelial progenitors) cells in patients with acute myocardial infarction (AMI).

METHODS AND RESULTS

Fifty AMI patients undergoing percutaneous coronary intervention (PCI) within 6 h of symptom onset were enrolled. EPO, measured by ELISA, and cell subsets, by cytofluorimetric analysis, were evaluated before PCI, 24 h and 7 days afterwards. Forty-five healthy subjects (CTRLs) were studied. Plasma EPO levels were higher in AMI patients at admission, 24 h, and 7 days (P = 0.04, P = 0.0001, P = 0.001, respectively) than in CTRLs. No correlation was evidenced between EPO and haemoglobin (Hb) or haematocrit at admission or 24 h after AMI. Differently, both Hb and haematocrit inversely correlated with EPO at day 7 (P = 0.0016, P = 0.029, respectively). Plasma EPO levels correlated with CD34(+)CD133(+)VEGFR-2(+) cells at day 7 (P = 0.03).

CONCLUSION

AMI patients have increased plasma EPO levels until day 7. In the early phase, plasma EPO levels are Hb-independent; at day 7, an Hb-modulated increase of EPO correlates with the percentage of CD34(+)CD133(+)VEGFR-2(+) cells.

Erythropoietin, a hypoxia-regulated factor, elicits a pro-angiogenic program in human mesenchymal stem cells

OBJECTIVE

The ability of erythropoietin (EPO) to elicit a pro-angiogenic effect on human mesenchymal stem cells (hMSC) was tested. hMSC are currently under study as therapeutic delivery agents that target tumor vessels. Hypoxia favors the differentiation of hMSC towards a pro-angiogenic program. However, the classical angiogenic factors, vascular endothelial growth factor and basic fibroblast growth factor, are not fully capable of restoring this effect. The hypoxia-regulated factor, EPO, induces angiogenesis in endothelial cells. Here, EPO's pro-angiogenic effect on hMSC was analyzed.

METHODS

hMSC were tested for EPO receptor expression by western blot, immunofluorescence, and flow cytometry assays. Downstream receptor signaling components JAK and STAT were measured by standard assays. Pro-angiogenesis effects mediated by EPO treatment of hMSC were measured by proliferation, cytokine, or pro-angiogenesis factor secretion, metalloprotease activation, migration, invasion, wound healing, and tubule formation assays.

RESULTS

hMSC express the cognate EPO receptor and are capable of promoting angiogenesis following EPO treatment in all the angiogenesis assays tested. EPO-treated hMSC proliferate and secrete pro-angiogenesis factors more readily than untreated hMSC. EPO leads to increased hMSC chemotaxis, migration, and activation of matrix metalloprotease-2. This treatment causes greater recruitment of vessels as measured in an in vivo angiogenesis assay.

CONCLUSION

EPO is capable of eliciting a pro-angiogenesis program in hMSC that instigates secretion of angiogenic factors and the subsequent recruitment of endothelium. This study defines a novel mechanism for tumor cell recruitment of blood vessels that is important to consider in the design of stem cell-based therapies.

Hypoxia-inducible factor-2 (HIF-2) regulates hepatic erythropoietin in vivo

Erythropoiesis is critically dependent on erythropoietin (EPO), a glycoprotein hormone that is regulated by hypoxia-inducible factor (HIF). Hepatocytes are the primary source of extrarenal EPO in the adult and express HIF-1 and HIF-2, whose roles in the hypoxic induction of EPO remain controversial. In order to define the role of HIF-1 and HIF-2 in the regulation of hepatic EPO expression, we have generated mice with conditional inactivation of Hif-1alpha and/or Hif-2alpha (Epas1) in hepatocytes. We have previously shown that inactivation of the von Hippel-Lindau tumor suppressor pVHL, which targets both HIFs for proteasomal degradation, results in increased hepatic Epo production and polycythemia independent of Hif-1alpha. Here we show that conditional inactivation of Hif-2alpha in pVHL-deficient mice suppressed hepatic Epo and the development of polycythemia. Furthermore, we found that physiological Epo expression in infant livers required Hif-2alpha but not Hif-1alpha and that the hypoxic induction of liver Epo in anemic adults was Hif-2alpha dependent. Since other Hif target genes such phosphoglycerate kinase 1 (Pgk) were Hif-1alpha dependent, we provide genetic evidence that HIF-1 and HIF-2 have distinct roles in the regulation of hypoxia-inducible genes and that EPO is preferentially regulated by HIF-2 in the liver.

Erythropoietin contributes to implantation: ectopic hemoglobin synthesis in decidual cells of mice

Erythropoietin, by binding to its receptor, stimulates definitive erythroblasts to accumulate hemoglobin (Hb) by up-regulating erythroid-specific genes and causes differentiation of erythroblasts into erythrocytes. In mouse decidua we have found the expression of transcripts for the erythropoietin receptor, the function of which has not yet been elucidated. Erythropoietin signaling was inhibited by the injection of a soluble form of the erythropoietin receptor capable of binding with erythropoietin into the mouse uterine cavity on day 4 of gestation, and pale and defective decidual bodies appeared three days later. These pale decidual bodies contained defective embryos without extension to the ectoplacental region, while normal reddish decidual bodies contained normal developing embryos and expressed embryonic and adult Hb with characteristic location of the respective hemoglobins in which an epsilon- or beta-globin signal was confirmed. Furthermore, blocking of erythropoietin signaling destroyed Hb-containing cells and resulted in apoptosis that caused embryonic death. Thus, erythropoietin-mediated Hb synthesis is essential for the survival of decidual cells. In addition, although no transcripts for GATA-1 and erythroid heme enzymes could be detected, genes for beta-globin, as well as non-specific delta-aminolevulinate synthase, were expressed and regulated in an erythropoietin-dependent manner. This is the first evidence that ectopic Hb synthesis exists and that erythropoietin coregulates erythroid (globin) and nonerythroid (delta-aminolevulinate synthase) genes.