CHBP induces stronger immunosuppressive CD127+ M-MDSC via erythropoietin receptor

Erythropoietin (EPO) is not only an erythropoiesis hormone but also an immune-regulatory cytokine. The receptors of EPO (EPOR)2 and tissue-protective receptor (TPR), mediate EPO's immune regulation. Our group firstly reported a non-erythropoietic peptide derivant of EPO, cyclic helix B peptide (CHBP), which could inhibit macrophages inflammation and dendritic cells (DCs) maturation. As a kind of innate immune regulatory cell, myeloid-derived suppressor cells (MDSCs) share a common myeloid progenitor with macrophages and DCs. In this study, we investigated the effects on MDSCs differentiation and immunosuppressive function via CHBP induction. CHBP promoted MDSCs differentiate toward M-MDSCs with enhanced immunosuppressive capability. Infusion of CHBP-induced M-MDSCs significantly prolonged murine skin allograft survival compared to its counterpart without CHBP stimulation. In addition, we found CHBP increased the proportion of CD11b+Ly6G-Ly6Chigh CD127+ M-MDSCs, which exerted a stronger immunosuppressive function compared to CD11b+Ly6G-Ly6Chigh CD127- M-MDSCs. In CHBP induced M-MDSCs, we found that EPOR downstream signal proteins Jak2 and STAT3 were upregulated, which had a strong relationship with MDSC function. In addition, CHBP upregulated GATA-binding protein 3 (GATA-3) protein translation level, which was an upstream signal of CD127 and regulator of STAT3. These effects of CHBP could be reversed if Epor was deficient. Our novel findings identified a new subset of M-MDSCs with better immunosuppressive capability, which was induced by the EPOR-mediated Jak2/GATA3/STAT3 pathway. These results are beneficial for CHBP clinical translation and MDSC cell therapy in the future.

Erythropoietin and small molecule agonists of the tissue-protective erythropoietin receptor increase FXN expression in neuronal cells in vitro and in Fxn-deficient KIKO mice in vivo

Friedreich's ataxia (FA) is a progressive neurodegenerative disease caused by reduced levels of the mitochondrial protein frataxin (FXN). Recombinant human erythropoietin (rhEPO) increased FXN protein in vitro and in early clinical studies, while no published reports evaluate rhEPO in animal models of FA. STS-E412 and STS-E424 are novel small molecule agonists of the tissue-protective, but not the erythropoietic EPO receptor. We find that rhEPO, STS-E412 and STS-E424 increase FXN expression in vitro and in vivo. RhEPO, STS-E412 and STS-E424 increase FXN by up to 2-fold in primary human cortical cells and in retinoic-acid differentiated murine P19 cells. In primary human cortical cells, the increase in FXN protein was accompanied by an increase in FXN mRNA, detectable within 4 h. RhEPO and low nanomolar concentrations of STS-E412 and STS-E424 also increase FXN in normal and FA patient-derived PBMC by 20%-40% within 24 h, an effect that was comparable to that by HDAC inhibitor 4b. In vivo, STS-E412 increased Fxn mRNA and protein in wild-type C57BL6/j mice. RhEPO, STS-E412, and STS-E424 increase FXN expression in the heart of FXN-deficient KIKO mice. In contrast, FXN expression in the brains of KIKO mice increased following treatment with STS-E412 and STS-E424, but not following treatment with rhEPO. Unexpectedly, rhEPO-treated KIKO mice developed severe splenomegaly, while no splenomegaly was observed in STS-E412- or STS-E424-treated mice. RhEPO, STS-E412 and STS-E424 upregulate FXN expression in vitro at equal efficacy, however, the effects of the small molecules on FXN expression in the CNS are superior to rhEPO in vivo.

Long-acting erythropoietin stimulating agents for persistent anemia after kidney transplant: risk factors and outcome

OBJECTIVES

Posttransplant anemia is associated with an increased risk of congestive heart failure, left ventricular hypertrophy, and death. The purpose of this study was to evaluate the effect of long-acting erythropoietin-stimulating agents on anemia after kidney transplant.

MATERIALS AND METHODS

In 2306 kidney transplant recipients, 250 anemic patients (11%) with stable graft function were followed at the Hamed Al-Essa Organ Transplant Centre (Kuwait) and were assessed for anemia. We enrolled 120 patients into this open-label study in 2 groups: group 1 had treatment with darbepoetin alfa (86 patients) and group 2 had continuous erythropoietin receptor activator (34 patients).

RESULTS

Patient age correlated negatively with serum iron level. Serum ferritin correlated negatively with hemoglobin level 6 months after transplant but not at time of transplant. Most patients were women who received their grafts from male donors. The 2 groups were comparable in frequency of rejection and mean hemoglobin and serum albumin levels at 3, 6, 9, and 12 months after transplant. There was no difference between the 2 groups in renal function (estimated glomerular filtration rate); posttransplant complications such as new-onset diabetes after transplant, hypertension, serious bacterial infections, or patient and graft outcomes.

CONCLUSIONS

Anemia is an important problem after kidney transplant, and iron use is suboptimal in kidney transplant recipients. Darbepoetin alfa and continuous erythropoietin receptor activator had comparable positive results.

Erythropoietin promotes oligodendrogenesis and myelin repair following lysolecithin-induced injury in spinal cord slice culture

Here, we sought to delineate the effect of EPO on the remyelination processes using an in vitro model of demyelination. We report that lysolecithin-induced demyelination elevated EPO receptor (EpoR) expression in oligodendrocyte progenitor cells (OPCs), facilitating the beneficial effect of EPO on the formation of oligodendrocytes (oligodendrogenesis). In the absence of EPO, the resultant remyelination was insufficient, possibly due to a limiting number of oligodendrocytes rather than their progenitors, which proliferate in response to lysolecithin-induced injury. By EPO treatment, lysolecithin-induced proliferation of OPCs was accelerated and the number of myelinating oligodendrocytes and myelin recovery was increased. EPO also enhanced the differentiation of neural progenitor cells expressing EpoR at high level toward the oligodendrocyte-lineage cells through activation of cyclin E and Janus kinase 2 pathways. Induction of myelin-forming oligodendrocytes by high dose of EPO implies that EPO might be the key factor influencing the final differentiation of OPCs. Taken together, our data suggest that EPO treatment could be an effective way to enhance remyelination by promoting oligodendrogenesis in association with elevated EpoR expression in spinal cord slice culture after lysolecithin-induced demyelination.

Erythropoietin induces positive inotropic and lusitropic effects in murine and human myocardium

Initial clinical studies indicate a potential beneficial effect of erythropoietin (EPO) in patients with anemia and heart failure. Here, we investigate the direct contractile effects of erythropoietin on myocardial tissue. Treatment with EPO (50U/mL) using excitable murine and human left ventricular muscle preparations resulted in a 37% and 62% increase in twitch tension, respectively (P<0.05). Isolated murine cardiomyocytes exposed to EPO demonstrated a 41% increase in peak sarcomere shortening (P=0.012). Using compounds that specifically stimulate a non-erythropoietic EPO receptor yielded similar increases in contractile dynamics. Cardiomyocyte Ca(2+)dynamics showed an 18% increase in peak calcium in EPO treated cardiomyocytes over controls (P=0.03). Studies in muscle strips skinned after EPO treatment demonstrated a phosphorylation dependant increase in the viscous modulus as well as an increase in oscillatory work. The EPO mediated increase in peak sarcomere shortening was abrogated by PI3-K blockade via wortmannin and by non-isozyme specific PKC blockade by chelerythrine. Finally, EPO treatment resulted in an increase in PKCε in the particulate cellular fraction, indicating activation of this isoform. EPO exhibits direct positive inotropic and lusitropic effects in cardiomyocytes and ventricular muscle preparation. These effects are mediated through PI3-K and PKCε isoform signaling to directly affect both calcium release dynamics and myofilament function.

A peptide-based erythropoietin-receptor agonist for pure red-cell aplasia

BACKGROUND

We investigated whether a novel, synthetic, peptide-based erythropoietin-receptor agonist (Hematide, Affymax) can stimulate erythropoiesis in patients with anemia that is caused by antierythropoietin antibodies.

METHODS

In this open-label, single-group trial, we enrolled patients with chronic kidney disease who had pure red-cell aplasia or hypoplasia due to antierythropoietin antibodies and treated them with a synthetic peptide-based erythropoietin-receptor agonist. The agonist was administered by subcutaneous injection at an initial dose of 0.05 mg per kilogram of body weight every 4 weeks. The primary end point was a hemoglobin concentration above 11 g per deciliter without the need for transfusions.

RESULTS

We treated 14 patients with the peptide agonist for a median of 28 months. The median hemoglobin concentration increased from 9.0 g per deciliter (with transfusion support in the case of 12 patients) before treatment to 11.4 g per deciliter at the time of the last administration of the agonist; transfusion requirements diminished within 12 weeks after the first dose, after which 13 of the 14 patients no longer required regular transfusions. Peak reticulocyte counts increased from a median of 10x10(9) per liter before treatment to peak counts of greater than 100x10(9) per liter. The level of antierythropoietin antibodies declined over the course of the study and became undetectable in six patients. One patient who initially responded to treatment had a diminished hematologic response a few months later despite increased doses of the agonist and required transfusions again; this patient was found to have antibodies against the agonist. One patient died 4 months after the last dose of the agonist, and a grade 3 or 4 adverse event occurred in seven other patients during the study period.

CONCLUSIONS

This novel agonist of the erythropoietin receptor can correct anemia in patients with pure red-cell aplasia caused by antierythropoietin antibodies. (ClinicalTrials.gov number, NCT00314795.).

Erythropoietin receptor signaling regulates both erythropoiesis and megakaryopoiesis in vivo

Transgenic expression of a gain-of-function truncated mouse erythropoietin receptor gene (EpoR) leads to expansion of the HSC pool in response to human erythropoietin (Epo). We have re-examined this observation using a knock-in mouse model, wherein the mouse EpoR gene was replaced in its proper genetic locus by a single copy of either a wild-type human or a polycythemia-inducing truncated human EPOR gene. Bone marrow cells obtained from knock-in mice were transplanted together with competitor bone marrow cells in a model that allows tracking of erythroid, platelet, and leukocyte contributions by each genotype. Secondary transplants were also performed. Stem/progenitor cells were identified phenotypically and isolated for colony-forming assays to evaluate cytokine responsiveness by cells with the wild-type human or truncated human EPOR gene. Augmented Epo signaling increased erythroid repopulation post-transplant as expected, but had no effect on short-term or long-term leukocyte repopulation. However, the wild-type human EPOR knock-in mouse showed decreases in both erythroid and platelet repopulation compared to marrow cells from the mutant human EPOR knock-in mouse or normal B6 animals. These results provide evidence supporting a role for Epo signaling in megakaryopoiesis in vivo and suggest a role for Epo signaling early in hematopoietic development.

Hematopoietic growth factor mimetics: from concept to clinic

Hematopoietic growth factor (HGF) mimetics offer a number of attractive advantages as therapeutic agents. Small chemical compounds, in particular, provide reduced cost and oral availability. As many of these mimetics are unrelated in structure to the normal cytokine the immunogenic response is not a significant issue. Isolation of small peptide agonists for erythropoietin (EPO) and thrombopoietin (TPO) receptors has been associated with significant translational challenges and here we summarize approaches used to achieve the potency and stability required for clinical utility. We also compare and contrast the initial screening approaches, and the translational and clinical issues associated with two recently approved TPO mimetics, romiplostim and the orally available eltrombopag. Finally we summarize the development and clinical findings for the EPO mimetic, Hematide, consider alternative approaches, and discuss the future potential for isolation of growth factor (GF) mimetics.

Erythropoietin-receptor agonists in critically ill patients: a meta-analysis of randomized controlled trials

INTRODUCTION

Anemia and the need for red blood cell transfusions are common among patients admitted to intensive care units. Erythropoietin has been used to decrease the need for transfusions; however, its ability to improve clinical outcomes is unknown. We evaluated the effect of erythropoietin-receptor agonists on clinically important outcomes, including mortality, length of stay in hospital or intensive care unit, ventilator use, transfusion requirements and major adverse events.

METHODS

To identify relevant studies, we searched electronic databases covering 1950 to 2007 (MEDLINE, EMBASE, the Cochrane Central Register of Controlled Trials and the Scopus database). We also searched conference proceedings and grey literature sources. We selected all randomized controlled trials involving critically ill patients that compared an erythropoietin-receptor agonist with a placebo or no intervention. No language restrictions were considered. Data were extracted using a standardized extraction template. We used a fixed effects model to calculate all summary measures of treatment effects.

RESULTS

Of 673 identified records, 9 studies that investigated erythropoietin alpha met the eligibility criteria and were included in our analysis. Erythropoietin, compared with placebo or no intervention, had no statistically significant effect on overall mortality (odds ratio [OR] 0.86, 95% confidence interval [CI] 0.71-1.05, I2 = 0%). The treatment and control groups did not differ in the length of stay in hospital or intensive care unit, or in the duration of mechanical ventilation, in the 3 studies that reported these outcomes. Erythropoietin, compared with placebo, significantly reduced the odds of a patient receiving at least 1 transfusion (OR 0.73, 95% CI 0.64-0.84, I2 = 54.7%). The mean number of units of blood transfused per patient was decreased by 0.41 units in the erythropoietin group (95% CI 0.10-0.74, I2 = 79.2%). Most of the included studies were performed before the widespread adoption of a restrictive transfusion strategy. Only 1 study provided detailed reports of adverse events, and none of the studies systematically evaluated all patients for venous thromboembolism.

INTERPRETATION

At this time, we do not recommend the routine use of erythropoietin-receptor agonists in critically ill patients. The reduction in red blood cell transfusions per patient was very small, and there is insufficient evidence to determine whether this intervention results in clinically important benefits with acceptable risks.

Neuroprotective effects of erythropoietin in the rat hippocampus after pilocarpine-induced status epilepticus

Neuroprotective functions of erythropoietin (Epo) are thought to involve a heteroreceptor composed of both Epo receptor (Epo-R) and common beta chain (betac). Here, we measured the response of hippocampal Epo system components (Epo, Epo-R and betac) during neurodegenerative processes following pilocarpine-induced status epilepticus (SE), and examined whether recombinant human Epo (rHuEpo) could support neuronal survival. We evidence that Epo is induced in astroglia following SE, in particular within areas displaying delayed neuronal death. In addition, we demonstrate for the first time that rHuEpo reduces considerably hippocampal neurodegeneration following SE. rHuEpo may thus supplement astroglial induction of Epo to promote enhanced hippocampal neuronal survival following SE. We also show that Epo-R is expressed by neurons and astrocytes mainly, while betac is barely detectable in basal conditions and induced in reactive microglia exclusively following SE. Altogether, our results suggest that Epo/rHuEpo exerts neuroprotection, through Epo-R signaling and independently of betac, and, therefore, may be anti-epileptogenic.

Feedback inhibition of leptin receptor/Jak2 signaling via Tyr1138 of the leptin receptor and suppressor of cytokine signaling 3

Leptin is an adipocyte-derived hormone that communicates the status of body energy stores to the brain to regulate feeding and energy balance. The inability of elevated leptin levels to adequately suppress feeding in obesity suggests attenuation of leptin action under these conditions; the activation of feedback circuits due to high leptin levels could contribute to this leptin resistance. Using cultured cells exogenously expressing the long form of the leptin receptor (LRb) or an erythropoietin receptor/LRb chimera, we show that chronic stimulation results in the attenuation of LRb signaling and the establishment of a state in which the receptor is refractory to reactivation. Mutation of LRb Tyr1138 (the site that recruits signal transducer and activator of transcription 3) alleviated this feedback inhibition, suggesting that signal transducer and activator of transcription 3 mediates the induction of a feedback inhibitor, such as suppressor of cytokine signaling 3 (SOCS3), during chronic LRb stimulation. Indeed, manipulation of the expression or activity of the LRb-binding tyrosine phosphatase, SH2-domain containing phosphatase-2, by overexpression of wild-type and dominant negative isoforms or RNA interference-mediated knockdown did not alter the attenuation of LRb signals. In contrast, SOCS3 overexpression repressed LRb signaling, whereas RNA interference-mediated knockdown of SOCS3 resulted in increased LRb signaling that was not attenuated during chronic ligand stimulation. These data suggest that Tyr1138 of LRb and SOCS3 represent major effector pathways for the feedback inhibition of LRb signaling. Furthermore, we show that mice expressing an LRb isoform mutant for Tyr1138 display increased activity of the leptin-dependent growth and immune axes, suggesting that Tyr1138-mediated feedback inhibition may regulate leptin sensitivity in vivo.

Erythropoietin-dependent autocrine secretion of tumor necrosis factor-alpha in hematopoietic cells modulates proliferation via MAP kinase–ERK-1/2 and does not require tyrosine docking sites in the EPO receptor

Primary erythroid cells and erythroid cell lines may synthesize and secrete tumor necrosis factor-alpha (TNF-alpha) following stimulation with erythropoietin (EPO). The effect of triggering TNF-alpha synthesis and secretion was investigated in erythroleukemia and myeloid cell lines: HCD57, DA3-EPOR, and BAF3-EPOR. The EPO-induced, membrane-bound form of autocrine TNF-alpha seemed to enhance proliferation of HCD57 and DA3-EPOR cells; however, the concentration of secreted autocrine/paracrine TNF-alpha was never sufficient to have an effect. Autocrine TNF-alpha acts through TNFRII receptors to stimulate proliferation. Modulation of mitogen-activated protein kinase (MAPK)/extracellular signal-related kinase (ERK-1/2) activity by the membrane-bound form of autocrine TNF-alpha apparently played a central role in the control of EPO-dependent proliferation of HCD57 and DA3-EPOR cells. Primary erythroid cells and DA3-EPOR cells were found to express similar, high levels of both TNFRI and TNFRII, showing that differential expression of TNF-alpha receptors does not explain why primary cells are inhibited and DA3-EPOR cells are stimulated by autocrine TNF-alpha. BAF3 cells expressing a mutant EPOR with no cytoplasmic tyrosine residues were capable of triggering EPO-dependent TNF-alpha synthesis and secretion, indicating that tyrosine-docking sites in the EPOR were not required for EPO-dependent TNF-alpha secretion.

Darbepoetin alfa: in patients with chemotherapy-related anaemia

Darbepoetin alfa, novel erythropoiesis stimulating protein closely related to human erythropoietin, has been developed for the treatment of chemotherapy-related anaemia in patients with non-myeloid malignancies. In three 12-week, phase II studies in patients with cancer and chemotherapy-related anaemia, subcutaneous darbepoetin alfa, administered in once-weekly or 2-, 3- or 4-weekly regimens, dose-dependently increased the mean haemoglobin levels. In a randomised, double-blind, phase III study in 320 patients with lung cancer and chemotherapy-related anaemia, recipients of subcutaneous darbepoetin alfa 2.25 micro g/kg once weekly, received red blood cell (RBC) transfusion approximate, equals 2-fold less frequently than placebo recipients (p < 0.001). In the same study, patients receiving darbepoetin alfa also received fewer standard units of RBC for transfusion and had greater haematopoietic response rate than placebo recipients (both p < 0.001). Subcutaneous darbepoetin alfa 2.25 micro g/kg once weekly also reduced patient-reported fatigue (assessed by a quality-of-life questionnaire) [p = 0.019 vs placebo]. black triangle Darbepoetin alfa was generally well tolerated in clinical trials. The most frequent darbepoetin alfa-related adverse events were: body oedema, arthralgia and skin rash.

Solution-phase synthesis of combinatorial libraries designed to modulate protein-protein or protein-DNA interactions

A short personal perspective on the development of an approach to the solution-phase synthesis of combinatorial libraries for modulating cellular signaling by inhibiting, promoting, or mimicking protein-protein or protein-DNA interactions is provided.

Amino acid determinants of beta-hairpin conformation in erythropoeitin receptor agonist peptides derived from a phage display library

Display of peptide libraries on filamentous phage has led to the identification of peptides of the form X(2-5)CX(2)GPXTWXCX(2-5) (where X is a variable residue) that bind to the extra-cellular portion of the erythropoietin receptor (EPO-R). These peptides adopt beta-hairpin conformations when co-crystallized with EPO-R. Solution NMR studies reveal that the peptide is conformationally heterogeneous in the absence of receptor due to cis-trans isomerization about the Gly-Pro peptide bond. Replacement of the conserved threonine residue with glycine at the turn i+3 position produces a stable beta-hairpin conformation in solution, although this peptide no longer has activity in an EPO-R-dependent cell proliferation assay. A truncated form of the EPO-R-binding peptide (containing the i+3 glycine residue) also forms a highly populated, monomeric beta-hairpin. In contrast, phage-derived peptide antagonists of insulin-like growth factor binding protein 1 (IGFBP-1) have a high level of sequence identity with the truncated EPO-R peptide (eight of 12 residues) yet adopt a turn-alpha-helix conformation in solution. Peptides containing all possible pairwise amino acid substitutions between the EPO-R and IGFBP-1 peptides have been analyzed to assess the degree to which the non-conserved residues stabilize the hairpin or helix conformation. All four residues present in the original sequence are required for maximum population of either the beta-hairpin or alpha-helix conformation, although some substitutions have a more dominant effect. The results demonstrate that, within a given sequence, the observed conformation can be dictated by a small subset of the residues (in this case four out of 12).

A high-throughput assay to identify compounds that can induce dimerization of the erythropoietin receptor

Erythropoietin induces dimerization of the erythropoietin receptor on the surface of erythroid progenitor cells, promoting the differentiation of these cells into mature red blood cells. To facilitate screening of large chemical collections for identification of compounds that can dimerize erythropoietin receptor, we have developed a novel, high-throughput in vitro assay to detect compounds that can cause dimerization of the erythropoietin receptor in solution. To develop this assay, amino acid sequences corresponding to the extracellular domain of erythropoietin receptor were expressed in Escherichia coli as erythropoietin-binding protein (rEBP). A modified version of this protein ((33)P-rEBP) containing a protein kinase A substrate site incorporated into the rEBP was also expressed in E. coli and labeled in vitro using protein kinase A and ¿gamma-(33)PATP. An erythropoietin mimetic peptide (EMP-1), that induces dimerization of rEBP in solution was used to demonstrate dimerization of (33)P-rEBP and rEBP in a 96-well microtiter plate format. EMP-1 induced dimerization of rEBP in this assay with an EC(50) of approximately 245 nM and had a maximal effect at 0.5-2 microM and required the presence of rEBP immobilized on the plate capable of binding EMP-1. EMP-1-induced dimerization of (33)P-rEBP and rEBP was reversed by excess unlabeled rEBP and was not masked by complex mixtures such as whole cell fungal extracts. These data demonstrate the ability of (33)P-rEBP to dimerize with rEBP in vitro in a format that is fully compatible with high-throughput screening.

The structure, organization, activation and plasticity of the erythropoietin receptor

Dimerization of the erythropoietin receptor has long been accepted as the singular step in its mechanism of activation. Recent studies have revealed a regulator process for activation that is dependent on the actual configuration of the receptor-ligand dimer assembly. This aspect of the receptor subunit assembly appears to extend to the unliganded receptor, which can dimerize on the cell surface and diminish any spontaneous background signaling in the absence of ligand. This self-recognition, as well as the multiple ligand binding capabilities of the receptor binding site, is consistent with an emerging theme of plasticity in protein-protein and ligand-receptor interactions.

Activation of erythropoietin receptor in the absence of hormone by a peptide that binds to a domain different from the hormone binding site

Applying a homology search method previously described, we identified a sequence in the extracellular dimerization site of the erythropoietin receptor, distant from the hormone binding site. A peptide identical to that sequence was synthesized. Remarkably, it activated receptor signaling in the absence of erythropoietin. Neither the peptide nor the hormone altered the affinity of the other for the receptor; thus, the peptide does not bind to the hormone binding site. The combined activation of signal transduction by hormone and peptide was strongly synergistic. In mice, the peptide acted like the hormone, protecting against the decrease in hematocrit caused by carboplatin.

Dominant negative effect of a truncated erythropoietin receptor (EPOR-T) on erythropoietin-induced erythroid differentiation: possible involvement of EPOR-T in ineffective erythropoiesis of myelodysplastic syndrome

We isolated a human leukemic cell line UT-7/GM from UT-7, which can differentiate into mature erythroid cells with erythropoietin (EPO) treatment. Using this cell line, we examined the effect of a truncated human EPO receptor (EPOR-T) on EPO-induced erythroid differentiation. Transfection studies revealed that UT-7/GM cells expressing exogenous EPOR-T were likely to undergo apoptosis even in the presence of EPO. In addition, EPOR-T-transfected cells could not differentiate into hemoglobin-positive cells after administration of EPO. These results suggest that EPOR-T is a negative regulator of EPO-induced anti-apoptosis and EPO-induced erythroid differentiation. The EPOR-T form was expressed in seven of nine cases of myelodysplastic syndrome but not in normal controls. In patients with myelodysplastic syndrome, dysregulated expression of EPOR-T may cause apoptosis and blockage of erythroid differentiation, resulting in ineffective erythropoiesis.

An antagonist peptide-EPO receptor complex suggests that receptor dimerization is not sufficient for activation

Dimerization of the erythropoietin (EPO) receptor (EPOR), in the presence of either natural (EPO) or synthetic (EPO-mimetic peptides, EMPs) ligands is the principal extracellular event that leads to receptor activation. The crystal structure of the extracellular domain of EPOR bound to an inactive (antagonist) peptide at 2.7 A resolution has unexpectedly revealed that dimerization still occurs, but the orientation between receptor molecules is altered relative to active (agonist) peptide complexes. Comparison of the biological properties of agonist and antagonist EMPs with EPO suggests that the extracellular domain orientation is tightly coupled to the cytoplasmic signaling events and, hence, provides valuable new insights into the design of synthetic ligands for EPOR and other cytokine receptors.

Isolation of erythropoietin receptor agonist peptides using evolved phage libraries

Cyclic peptides capable of activating the erythropoietin receptor (EPOR) were isolated from phage display libraries by screening with a novel EPOR-IgG fusion protein reagent. A parental clone ERB1 (EPO Receptor Binder 1) was first isolated from a phage display library displaying 38 random amino acids as an N-terminal fusion with the M13 minor capsid protein, pill. An evolved library was then produced from the parental sequence using an oligonucleotide saturation mutagenesis strategy which yielded EPOR binding sequences with 20 times the relative affinity of ERB1. Two synthetic peptides were constructed from these sequences both of which bind the EPO receptor in specific ELISA, and act as full agonists in EPO dependent cell proliferation assays. These peptides are 18 amino acids in length, disulfide-bonded, and have a minimum consensus sequence of CXXGWVGXCXXW, where X represents positions tolerant of several amino acids.

The hyperresponsiveness of cells expressing truncated erythropoietin receptors is contingent on insulin-like growth factor-1 in fetal calf serum

We demonstrate herein that the well documented hyperresponsiveness to erythropoietin (Epo) of Ba/F3 cells expressing C-terminal truncated erythropoietin receptors (EpoRs) is contingent on these cells being in fetal calf serum (FCS). In the absence of FCS, their Epo-induced proliferation is far poorer than Ba/F3 cells expressing wild-type (WT) EpoRs. This hyporesponsiveness in the absence of serum is also seen in DA-3 cells expressing these truncated EpoRs. In fact, long-term proliferation studies performed in the absence of serum show that even at saturating concentrations of Epo, Ba/F3 cells expressing these truncated receptors die via apoptosis, while cells bearing WT EpoRs do not, and this programmed cell death correlates with an inability of Epo-stimulated Ba/F3 cells expressing truncated EpoRs to induce the tyrosine phosphorylation of MAPK and the activation of p70(S6K). Using neutralizing antibodies to insulin-like growth factor (IGF)-1, we show that a major non-Epo factor in FCS that contributes to the hyperresponsive phenotype of Ba/F3 cells expressing truncated EpoRs is IGF-1. Our results suggest that the Epo-hypersensitivity of truncated EpoR expressing Ba/F3 cells is due to the combined effects of these EpoRs not possessing a binding site for the negative regulator, SHP-1, and the triggering of proliferation-inducing/apoptosis-inhibiting cascades, lost through EpoR truncation, by IGF-1.

Amino-terminal dimerization of an erythropoietin mimetic peptide results in increased erythropoietic activity

BACKGROUND

Erythropoietin (EPO), the hormone involved in red blood cell production, activates its receptor by binding to the receptor's extracellular domain and presumably dimerizing two receptor monomers to initiate signal transduction. EPO-mimetic peptides, such as EMP1, also bind and activate the receptor by dimerization. These mimetic peptides are not as potent as EPO, however. The crystal structure of the EPO receptor (EBP) bound to EMP1 reveals the formation of a complex consisting of two peptides bound to two receptors, so we sought to improve the biological activity of EPO-mimetic peptides by constructing covalent dimers of EMP1 and other peptide mimetics linked by polyethylene glycol (PEG).

RESULTS

The potency of the PEG-dimerized EPO peptide mimetics both in vitro and in vivo was improved up to 1,000-fold compared to the corresponding peptide monomers. The dimers were constructed using peptide monomers which have only one reactive amine per molecule, allowing us to conclude that the increase in potency can be attributed to a structure in which two peptides are linked through their respective amino termini to the difunctional PEG molecule. In addition, an inactive peptide was converted into a weak agonist by PEG-induced dimerization.

CONCLUSIONS

The potency of previously isolated peptides that are modest agonists of the EPO receptor was dramatically increased by PEG-induced dimerization. The EPO receptor is thought to be dimerized during activation, so our results are consistent with the proposed 2:2 receptor : peptide stoichiometry. The conversion of an inactive peptide into an agonist further supports the idea that dimerization can mediate receptor activation.

Activation of the erythropoietin (EPO) receptor by bivalent anti-EPO receptor antibodies

Oligomerization of cytokine receptors including the erythropoietin (EPO) receptor has been advanced as a model for activation. If homodimerization of the EPO receptor activates it, then bivalent antibodies raised to the extracellular domain of the EPO receptor should also homodimerize and activate. Mouse monoclonal antibodies (IgG) raised to the soluble, extracellular domain of the human EPO receptor (EPOR) were found that would stimulate thymidine uptake of an human EPO-dependent cell line, UT-7/EPO. Dose response curves showed bell shapes where activity was low at low and high concentrations. Monovalent (Fab) fragments bound to the receptor but did not stimulate thymidine uptake, which indicates that two antibody binding sites are required for activation. The anti-EPOR antibodies stimulated the formation of burst forming unit erythroid colonies from human CD34(+) cells purified from peripheral blood. This indicates that homodimerization of the EPO receptor by anti-EPOR antibodies is sufficient for both proliferation and differentiation of erythroid progenitor cells and that the constraints on dimerization necessary for activation are rather loose.

Small peptides as potent mimetics of the protein hormone erythropoietin

Random phage display peptide libraries and affinity selective methods were used to isolate small peptides that bind to and activate the receptor for the cytokine erythropoietin (EPO). In a panel of in vitro biological assays, the peptides act as full agonists and they can also stimulate erythropoiesis in mice. These agonists are represented by a 14- amino acid disulfide-bonded, cyclic peptide with the minimum consensus sequence YXCXXGPXTWXCXP, where X represents positions allowing occupation by several amino acids. The amino acid sequences of these peptides are not found in the primary sequence of EPO. The signaling pathways activated by these peptides appear to be identical to those induced by the natural ligand. This discovery may form the basis for the design of small molecule mimetics of EPO.

Functional mimicry of a protein hormone by a peptide agonist: the EPO receptor complex at 2.8 A

The functional mimicry of a protein by an unrelated small molecule has been a formidable challenge. Now, however, the biological activity of a 166-residue hematopoietic growth hormone, erythropoietin (EPO), with its class 1 cytokine receptor has been mimicked by a 20-residue cyclic peptide unrelated in sequence to the natural ligand. The crystal structure at 2.8 A resolution of a complex of this agonist peptide with the extracellular domain of EPO receptor reveals that a peptide dimer induces an almost perfect twofold dimerization of the receptor. The dimer assembly differs from that of the human growth hormone (hGH) receptor complex and suggests that more than one mode of dimerization may be able to induce signal transduction and cell proliferation. The EPO receptor binding site, defined by peptide interaction, corresponds to the smaller functional epitope identified for hGH receptor. Similarly, the EPO mimetic peptide ligand can be considered as a minimal hormone, and suggests the design of nonpeptidic small molecule mimetics for EPO and other cytokines may indeed be achievable.