The role of erythropoietin as an inhibitor of tissue ischemia

Erythropoietin is a hypoxia-induced cytokine that stimulates erythropoiesis through the promotion of erythroid precursor cell proliferation and differentiation. Recent evidence supports that erythropoietin has a broad spectrum of tissue protecting actions affecting other systems than hemopoietic. Lately, research has focused on the nonhemopoietic effects of erythropoietin against tissue ischemia due to the unexpected observations of erythropoietin receptor expression by various cells, such as endothelial cells, neuronal cells, cardiac myocytes, and vascular smooth muscle cells. It has been shown that erythropoietin exerts its cardioprotective action during cardiac ischemic injury through reducing the infract size and enhancing new vessel formation over a longer time frame. Erythropoietin plays a crucial role in neuroprotection in many types of ischemic injury in the central and the peripheral nervous system. It is also strongly believed that erythropoietin exhibits a critical role in many other disorders that are pathogenetically related to acute tissue ischemia. This article reviews the proposed implications of erythropoietin in tissue ischemia and discusses the possible mechanisms for this action along with its potential therapeutic applications.     

Cardioprotective effects of erythropoietin in the reperfused ischemic heart: a potential role for cardiac fibroblasts

Erythropoietin has recently been shown to have effects beyond hematopoiesis such as prevention of neuronal and cardiac apoptosis secondary to ischemia. In this study, we evaluated the in vivo protective potential of erythropoietin in the reperfused rabbit heart following ventricular ischemia. We show that "preconditioning" with erythropoietin activates cell survival pathways in myocardial tissue in vivo and adult rabbit cardiac fibroblasts in vitro. These pathways, activated by erythropoietin in both whole hearts and cardiac fibroblasts, are also activated acutely by ischemia/reperfusion injury. Moreover, in vivo studies indicate that erythropoietin treatment either prior to or during ischemia significantly enhances cardiac function and recovery, including left ventricular contractility, following myocardial ischemia/reperfusion. Our data indicate that a contributing in vivo cellular mechanism of this protection is mitigation of myocardial cell apoptosis. This results in decreased infarct size as evidenced by area at risk studies following in vivo ischemia/reperfusion injury, translating into more viable myocardium and less ventricular dysfunction. Therefore, erythropoietin treatment may offer novel protection against ischemic heart disease and may act, at least in part, by direct action on cardiac fibroblasts and myocytes to alter survival and ventricular remodeling.     

The receptor that tames the innate immune response

Tissue injury, hypoxia and significant metabolic stress activate innate immune responses driven by tumor necrosis factor (TNF)-α and other proinflammatory cytokines that typically increase damage surrounding a lesion. In a compensatory protective response, erythropoietin (EPO) is synthesized in surrounding tissues, which subsequently triggers antiinflammatory and antiapoptotic processes that delimit injury and promote repair. What we refer to as the sequelae of injury or disease are often the consequences of this intentionally discoordinated, primitive system that uses a "scorched earth" strategy to rid the invader at the expense of a serious lesion. The EPO-mediated tissue-protective system depends on receptor expression that is upregulated by inflammation and hypoxia in a distinctive temporal and spatial pattern. The tissue-protective receptor (TPR) is generally not expressed by normal tissues but becomes functional immediately after injury. In contrast to robust and early receptor expression within the immediate injury site, EPO production is delayed, transient and relatively weak. The functional EPO receptor that attenuates tissue injury is distinct from the hematopoietic receptor responsible for erythropoiesis. On the basis of current evidence, the TPR is composed of the β common receptor subunit (CD131) in combination with the same EPO receptor subunit that is involved in erythropoiesis. Additional receptors, including that for the vascular endothelial growth factor, also appear to be a component of the TPR in some tissues, for example, the endothelium. The discoordination of the EPO response system and its relative weakness provide a window of opportunity to intervene with the exogenous ligand. Recently, molecules were designed that preferentially activate only the TPR and thus avoid the potential adverse consequences of activating the hematopoietic receptor. On administration, these agents successfully substitute for a relative deficiency of EPO production in damaged tissues in multiple animal models of disease and may pave the way to effective treatment of a wide variety of insults that cause tissue injury, leading to profoundly expanded lesions and attendant, irreversible sequelae.     

Cloning and expression pattern of the Xenopus erythropoietin receptor

Cytokine signaling plays an important role in the survival and differentiation of vertebrate hematopoietic cells. In red blood cells, erythropoietin is a key component of the differentiation program and maintains the homeostasis of the erythroid compartment. In the adult, anemia stimulates high levels of circulating erythropoietin that drives erythropoiesis to restore normal levels of red blood cells in circulation. Erythropoietin activates the erythropoietin receptor on immature red blood cell precursors to promote their survival and differentiation. Although extensively studied in mammalian systems, a complete understanding of the function of the erythropoietin receptor during primitive erythropoiesis has been lacking. To address this problem, we have cloned the Xenopus laevis erythropoietin receptor in order to further understand the development of primitive erythropoiesis. The amphibian erythropoietin receptor shares 33% amino acid sequence identity with the mammalian erythropoietin receptors and contains the conserved extracellular ligand binding and fibronectin domains, the WSXWS motif common to cytokine receptors, and several tyrosine phosphorylation sites located on the intracellular domain of the receptor. Expression of the erythropoietin receptor is first detected by in situ hybridization in the ventral blood island during tailbud stages.     

Pharmacological preconditioning with erythropoietin reduces ischemia–reperfusion injury in the small intestine of rats

AimsConsidering the implications that arose from several recent experimental studies using recombinant human erythropoietin in rodents, erythropoietin has been regarded as a pharmacological preconditioning agent. The purpose of the present study was to evaluate whether erythropoietin has a preconditioning effect against ischemia and reperfusion injury in the small intestine of the rat.Main methodsIntestinal ischemia was induced in male Wistar rats by clamping the superior mesenteric artery for 30 min, followed by reperfusion for 180 min. Recombinant human erythropoietin (1000 or 3000 U/kg) or vehicle was administered intraperitoneally 24 h prior to ischemia. After collection of ileal tissue, evaluation of damage was based on measurements of the accumulation of polymorphonuclear neutrophils by technetium-99m-labeled leukocyte uptake, content of malondialdehyde, reduced glutathione, contractile responses to agonists, and an evaluation of histopathological features in intestinal tissue.Key findingsTreatment with erythropoietin 24 h before ischemia significantly reduced the tissue content of malondialdehyde and increased that of reduced glutathione. Pretreatment also significantly suppressed leukocyte infiltration into the postischemic tissue, as evidenced by the lower content of myeloperoxidase and technetium-99m-labeled leukocytes. Physiological and histopathological improvements were also significant with the rHuEpo treatment.SignificanceResults of the present study indicate that rHuEpo is an effective preconditioning agent in ischemic injury of the small intestine. Protection provided by recombinant human erythropoietin is closely related to the inhibition of oxidative stress and leukocyte infiltration, which might be among the possible protective mechanisms of erythropoietin in intestinal ischemia and reperfusion.     

An extra high dose of erythropoietin fails to support the proliferation of erythropoietin dependent cell lines

Erythropoietin is responsible for the red blood cell formation by stimulating the proliferation and the differentiation of erythroid precursor cells. Erythropoietin triggers the conformational change in its receptor thereby induces the phosphorylation of JAK2. In this study, we show that an extra high dose of erythropoietin, however, fails to activate the erythropoietin receptor, to stimulate the phosphorylation of JAK2 and to support the cell proliferation of Ep-FDC-P2 cell. Moreover, high dose of EPO also inhibited the proliferation of various erythropoietin-dependent cell lines, suggesting that excess amount of EPO could not trigger the conformational change of the receptor. In the presence of an extra high dose of erythropoietin as well as in the absence of erythropoietin, the cells caused the DNA fragmentation, a typical symptom of apoptosis. The impairment of cell growth and the DNA fragmentation at the extremely high concentration of EPO was rescued by the addition of erythropoietin antibody or soluble form of erythropoietin receptor by titrating the excess erythropoietin. These results suggest that two erythropoietin binding sites on erythropoietin receptor dimer should be occupied by a single erythropoietin molecule for the proper conformational change of the receptor and the signal transduction of erythropoietin, instead, when two erythropoietin binding sites on the receptor are shared by two erythropoietin molecules, it fails to evoke the conformational change of erythropoietin receptor adequate for signal transduction.     

Evaluation of functional erythropoietin receptor status in skeletal muscle in vivo: acute and prolonged studies in healthy human subjects

Background

Erythropoietin receptors have been identified in human skeletal muscle tissue, but downstream signal transduction has not been investigated. We therefore studied in vivo effects of systemic erythropoietin exposure in human skeletal muscle.

Methodology/Principal Findings

The protocols involved 1) acute effects of a single bolus injection of erythropoietin followed by consecutive muscle biopsies for 1–10 hours, and 2) a separate study with prolonged administration for 16 days with biopsies obtained before and after. The presence of erythropoietin receptors in muscle tissue as well as activation of Epo signalling pathways (STAT5, MAPK, Akt, IKK) were analysed by western blotting. Changes in muscle protein profiles after prolonged erythropoietin treatment were evaluated by 2D gel-electrophoresis and mass spectrometry. The presence of the erythropoietin receptor in skeletal muscle was confirmed, by the M20 but not the C20 antibody. However, no significant changes in phosphorylation of the Epo-R, STAT5, MAPK, Akt, Lyn, IKK, and p70S6K after erythropoietin administration were detected. The level of 8 protein spots were significantly altered after 16 days of rHuEpo treatment; one isoform of myosin light chain 3 and one of desmin/actin were decreased, while three isoforms of creatine kinase and two of glyceraldehyd-3-phosphate dehydrogenase were increased.

Conclusions/Significance

Acute exposure to recombinant human erythropoietin is not associated by detectable activation of the Epo-R or downstream signalling targets in human skeletal muscle in the resting situation, whereas more prolonged exposure induces significant changes in the skeletal muscle proteome. The absence of functional Epo receptor activity in human skeletal muscle indicates that the long-term effects are indirect and probably related to an increased oxidative capacity in this tissue.     

The potency of erythropoietin-mimic antibodies correlates inversely with affinity

Preclinical animal studies have shown that Ab12.6, an agonistic human Ab targeting the erythropoietin receptor (EPOR), exhibits several potential dosing and safety features that make it an attractive clinical candidate for the treatment of anemia. Ab12.6 was derived by yeast display affinity maturation of parental Ab12, a strategy initially intended to improve off-rate and affinity for EPOR, thereby enhancing erythropoietic activity. Analysis of full-length IgGs derived from yeast clones identified sequences within Ab12 CDRH2 that independently influenced both affinity and potency. The Ab12.6 derivative displayed improved in vitro potency and in vivo efficacy, although its binding affinity to the EPOR was lower than that of the parent Ab12. Additional Ab12 derivatives also exhibited an inverse correlation between affinity and potency. These results suggest that for this class of agonistic Abs, faster off-rates may permit continuous receptor stimulation and more efficient erythropoiesis.     

Inactivation of erythropoietin leads to defects in cardiac morphogenesis.

Erythropoietin is an essential growth factor that promotes survival, proliferation, and differentiation of mammalian erythroid progenitor cells. Erythropoietin(-/-) and erythropoietin receptor(-/-) mouse embryos die around embryonic day 13.5 due, in part, to failure of erythropoiesis in the fetal liver. In this study, we demonstrated a novel role of erythropoietin and erythropoietin receptor in cardiac development in vivo. We found that erythropoietin receptor is expressed in the developing murine heart in a temporal and cell type-specific manner: it is initially detected by embryonic day 10.5 and persists until day 14.5. Both erythropoietin(-/-) and erythropoietin receptor(-/-) embryos suffered from ventricular hypoplasia at day 12-13 of gestation. This defect appears to be independent from the general state of hypoxia and is likely due to a reduction in the number of proliferating cardiac myocytes in the ventricular myocardium. Cell proliferation assays revealed that erythropoietin acts as a mitogen in cells isolated from erythropoietin(-/-) mice, while it has no effect in hearts from erythropoietin receptor(-/-) animals. Erythropoietin(-/-) and erythropoietin receptor(-/-) embryos also suffered from epicardium detachment and abnormalities in the vascular network. Finally, through a series of chimeric analysis, we provided evidence that erythropoietin acts in a manner which is non-cell-autonomous. Our results elucidate a novel role of erythropoietin in cardiac morphogenesis and suggest a combination of anemia and cardiac failure as the cause of embryonic lethality in the erythropoietin(-/-) and erythropoietin receptor(-/-) animals.     

Mechanism of leukemogenesis induced by mink cell focus-forming murine leukemia viruses.

The Friend or Moloney mink cell focus-forming (MCF) virus encodes a recombinant-type envelope glycoprotein, gp70, that is closely related to the membrane glycoprotein, gp55, of Friend spleen focus-forming virus (SFFV). We have shown previously that gp55 has the ability to activate cell growth by binding to the cellular receptor for erythropoietin. Here we show that gp70 encoded by either the Friend or Moloney MCF virus also binds to the erythropoietin receptor and that coexpression of the receptor and gp70 in an interleukin-3 (IL-3)-dependent cell line can activate IL-3-independent growth. Furthermore, when the cDNA for the human IL-2 receptor beta chain, which is related by sequence to the erythropoietin receptor, was introduced into this cell line, it became growth factor independent after infection either with SFFV or with one of the two MCF viruses but not with an ecotropic virus. Based on these observations, we propose a mechanism for the early stage of leukemogenesis induced by the MCF-type murine leukemia viruses.     

Topiramate treatment is neuroprotective and reduces oligodendrocyte loss after cervical spinal cord injury

Excess glutamate release and associated neurotoxicity contributes to cell death after spinal cord injury (SCI). Indeed, delayed administration of glutamate receptor antagonists after SCI in rodents improves tissue sparing and functional recovery. Despite their therapeutic potential, most glutamate receptor antagonists have detrimental side effects and have largely failed clinical trials. Topiramate is an AMPA-specific, glutamate receptor antagonists that is FDA-approved to treat CNS disorders. In the current study we tested whether topiramate treatment is neuroprotective after cervical contusion injury in rats. We report that topiramate, delivered 15-minutes after SCI, increases tissue sparing and preserves oligodendrocytes and neurons when compared to vehicle treatment. In addition, topiramate is more effective than the AMPA-receptor antagonist, NBQX. To the best of our knowledge, this is the first report documenting a neuroprotective effect of topiramate treatment after spinal cord injury.     

The interface between self-assembling erythropoietin receptor transmembrane segments corresponds to a membrane-spanning leucine zipper

Structural and functional studies recently indicated that the erythropoietin receptor exists as a preassembled homodimer whose activation by ligand binding requires self-interaction of its transmembrane segment. Here, we probed the interface formed by the transmembrane segments by asparagine-scanning mutagenesis in a natural membrane. We show that this interface is based on a leucine zipper-like heptad repeat pattern of amino acids. The strongest impact of asparagine was observed at position 241, suggesting the highest packing density around this position, which is in agreement with results obtained upon mutation to alanine. Interestingly, the same face of the transmembrane helix had previously been shown to enter a heterophilic interaction with the transmembrane segment of gp55-P, a viral membrane protein that leads to ligand-independent receptor activation in infected cells. Further, functional characterization of an erythropoietin receptor mutant with asparagine at position 241 in a hematopoietic cell line showed that this protein could still be activated by erythropoietin yet was not constitutively active. This suggests that forced self-interaction of the transmembrane segments does not suffice to induce signaling of the erythropoietin receptor.     

Delayed administration of pyroglutamate helix B surface peptide (pHBSP), a novel nonerythropoietic analog of erythropoietin, attenuates acute kidney injury

In preclinical studies, erythropoietin (EPO) reduces ischemia-reperfusion-associated tissue injury (for example, stroke, myocardial infarction, acute kidney injury, hemorrhagic shock and liver ischemia). It has been proposed that the erythropoietic effects of EPO are mediated by the classic EPO receptor homodimer, whereas the tissue-protective effects are mediated by a hetero-complex between the EPO receptor monomer and the β-common receptor (termed "tissue-protective receptor"). Here, we investigate the effects of a novel, selective-ligand of the tissue-protective receptor (pyroglutamate helix B surface peptide [pHBSP]) in a rodent model of acute kidney injury/dysfunction. Administration of pHBSP (10 μg/kg intraperitoneally [i.p.] 6 h into reperfusion) or EPO (1,000 IU/kg i.p. 4 h into reperfusion) to rats subjected to 30 min ischemia and 48 h reperfusion resulted in significant attenuation of renal and tubular dysfunction. Both pHBSP and EPO enhanced the phosphorylation of Akt (activation) and glycogen synthase kinase 3β (inhibition) in the rat kidney after ischemia-reperfusion, resulting in prevention of the activation of nuclear factor-κB (reduction in nuclear translocation of p65). Interestingly, the phosphorylation of endothelial nitric oxide synthase was enhanced by EPO and, to a much lesser extent, by pHBSP, suggesting that the signaling pathways activated by EPO and pHBSP may not be identical.     

Nonhematopoietic erythropoietin derivatives prevent motoneuron degeneration in vitro and in vivo

Chronic treatment with asialo erythropoietin (ASIALO-EPO) or carbamylated erythropoietin (CEPO) improved motor behavior and reduced motoneuron loss and astrocyte and microglia activation in the cervical spinal cord of wobbler mice, an animal model of amyotrophic lateral sclerosis, but had no effect on hematocrit values. ASIALO-EPO and CEPO, like the parent compound EPO, protected primary motoneuron cultures from kainate-induced death in vitro. Both EPO receptor and the common CD131 beta chain were expressed in cultured motoneurons and in the anterior horn of wobbler mice spinal cord. Our results strongly support a role for the common beta chain CD131 in the protective effect of EPO derivatives on motoneuron degeneration. Thus CEPO, which does not bind to the classical homodimeric EPO receptor and is devoid of hematopoietic activity, could be effective in chronic treatment aimed at reducing motoneuron degeneration.     

Mechanism of erythropoietin action on the erythroid progenitor cells induced from murine erythroleukemia cells (TSA8)

Erythropoietin is a well-known erythroid differentiation and growth factor, but the mechanism of its action is not well understood. In this work, we have examined its mechanism of action on the erythropoietin-responsive murine erythroleukemia cells (TSA8). TSA8 cells become responsive to erythropoietin after induction with DMSO. Stimulatory effects on erythropoietin response are observed with the addition of compounds affecting the cAMP level such as forskolin, phosphodiesterase inhibitor and cholera toxin only in the presence of erythropoietin. cAMP analogues themselves show no stimulatory effect on TSA8 cells, nor does erythropoietin increase cAMP level in the cells. Thus, it is suggested that cAMP does not act as a direct second messenger for signal transduction through erythropoietin receptors, but as a stimulator of the erythropoietin receptor pathway and/or as a second messenger in combination with the receptor pathway. The mechanism for acquisition of responsiveness to growth and differentiation factors of progenitor cells is discussed.     

An activating mutation in the murine erythropoietin receptor induces erythroleukemia in mice: a cytokine receptor superfamily oncogene.

A point mutation at codon 129 of the murine erythropoietin receptor (cEpoR) results in constitutive activation. We have generated a recombinant spleen focus-forming retrovirus in which the env gene is replaced by the cEpoR cDNA. Mice infected with this virus (but not by viruses expressing the wild-type EpoR) develop erythrocytosis and splenomegaly. From the spleen of infected animals we have isolated clonal, growth factor-independent, proerythroblast cell lines that express cEpoR, do not express the putative oncogene spi-1, and have rearranged and inactivated expression of the p53 suppressor oncogene. These cells induce erythroleukemia upon injection into mice. This demonstrates that oncogenic point mutations exist in a member of the cytokine receptor superfamily. The activated erythropoietin receptor does not transform cultured fibroblasts, suggesting why oncogenic mutations in other members of this receptor superfamily have not been detected.     

Physicochemical and biological characterization of asialoerythropoietin. Suppressive effects of sialic acid in the expression of biological activity of human erythropoietin in vitro

Various partially or fully desialylated human erythropoietins were obtained by neuraminidase digestion of the hormone, without non-specific proteolysis and degradation of carbohydrates. Asialoerythropoietin showed a specific activity of 220-IU/mg protein in vivo, although that of the intact erythropoietin was 2.2 x 10(5) IU/mg. A linear relationship was found between the logarithm of the specific activity in vivo and the number of sialic acids. The asialoerythropoietin showed a four-times-higher specific activity in vitro compared with intact erythropoietin using mouse bone marrow cells. It also showed an approximately six-times-higher specific activity in a colony-forming assay for the erythroid colony-forming unit and the erythroid burst-forming unit. Partially or fully de-N-glycosylated erythropoietin derivatives also showed lower in vivo activity but higher in vitro activity than the intact erythropoietin, dependent on the number of sialic acids. To clarify the reason for the enhanced biological activity of asialoerythropoietin in vitro, the binding of intact 125I-erythropoietin or 125I-asialoerythropoietin to cells containing specific receptors for the hormone was analyzed. 125I-asialoerythropoietin bound to spleen cells from anemic mice approximately five times faster than did intact 125I-erythropoietin. The amount of 125I-asialoerythropoietin internalized by target cells, measured in the absence of NaN3, was four times higher than that of intact erythropoietin. These results demonstrate that asialoerythropoietin binds to its receptor faster than the intact form. This may be the main reason for the increased activity of asialoerythropoietin in vitro.