Hematopoietic cell survival signals are elicited through non-tyrosine-containing sequences in the membrane-proximal region of the erythropoietin receptor (EPOR) by a Stat5-dependent pathway

STAT5 as a Key Protein of Erythropoietin Signalization

Erythropoietin (EPO) acts on multiple tissues through its receptor EPOR, a member of a cytokine class I receptor superfamily with pleiotropic effects. The interaction of EPO and EPOR triggers the activation of several signaling pathways that induce erythropoiesis, including JAK2/STAT5, PI3K/AKT, and MAPK. The canonical EPOR/JAK2/STAT5 pathway is a known regulator of differentiation, proliferation, and cell survival of erythroid progenitors. In addition, its role in the protection of other cells, including cancer cells, is under intense investigation. The involvement of EPOR/JAK2/STAT5 in other processes such as mRNA splicing, cytoskeleton reorganization, and cell metabolism has been recently described. The transcriptomics, proteomics, and epigenetic studies reviewed in this article provide a detailed understanding of EPO signalization. Advances in this area of research may be useful for improving the efficacy of EPO therapy in hematologic disorders, as well as in cancer treatment.

Transmembrane Protein Aptamer Induces Cooperative Signaling by the EPO Receptor and the Cytokine Receptor β-Common Subunit

The erythropoietin receptor (EPOR) plays an essential role in erythropoiesis and other cellular processes by forming distinct signaling complexes composed of EPOR homodimers or hetero-oligomers between the EPOR and another receptor, but the mechanism of heteroreceptor assembly and signaling is poorly understood. We report here a 46-residue, artificial transmembrane protein aptamer, designated ELI-3, that binds and activates the EPOR and induces growth factor independence in murine BaF3 cells expressing the EPOR. ELI-3 requires the transmembrane domain and JAK2-binding sites of the EPOR for activity, but not the cytoplasmic tyrosines that mediate canonical EPOR signaling. Instead, ELI-3-induced proliferation and activation of JAK/STAT signaling requires the transmembrane and cytoplasmic domains of the cytokine receptor β-common subunit (βcR) in addition to the EPOR. Moreover, ELI-3 fails to induce erythroid differentiation of primary human hematopoietic progenitor cells but inhibits nonhematopoietic cell death induced by serum withdrawal.

Delayed hemoglobin switching and perinatal neocytolysis in mice with gain-of-function erythropoietin receptor

Mutations of the truncated cytoplasmic domain of human erythropoietin receptor (EPOR) result in gain-of-function of erythropoietin (EPO) signaling and a dominantly inherited polycythemia, primary familial and congenital polycythemia (PFCP). We interrogated the unexplained transient absence of perinatal polycythemia observed in PFCP patients using an animal model of PFCP to examine its erythropoiesis during embryonic, perinatal, and early postnatal periods. In this model, we replaced the murine EpoR gene (mEpoR) with the wild-type human EPOR (wtHEPOR) or mutant human EPOR gene (mtHEPOR) and previously reported that the gain-of-function mtHEPOR mice become polycythemic at 3~6 weeks of age, but not at birth, similar to the phenotype of PFCP patients. In contrast, wtHEPOR mice had sustained anemia. We report that the mtHEPOR fetuses are polycythemic, but their polycythemia is abrogated in the perinatal period and reappears again at 3 weeks after birth. mtHEPOR fetuses have a delayed switch from primitive to definitive erythropoiesis, augmented erythropoietin signaling, and prolonged Stat5 phosphorylation while the wtHEPOR fetuses are anemic. Our study demonstrates the in vivo effect of excessive EPO/EPOR signaling on developmental erythropoiesis switch and describes that fetal polycythemia in this PFCP model is followed by transient correction of polycythemia in perinatal life associated with low Epo levels and increased exposure of erythrocytes' phosphatidylserine. We suggest that neocytolysis contributes to the observed perinatal correction of polycythemia in mtHEPOR newborns as embryos leaving the hypoxic uterus are exposed to normoxia at birth.

KEY MESSAGE

Human gain-of-function EPOR (mtHEPOR) causes fetal polycythemia in knock-in mice. Wild-type human EPOR causes fetal anemia in knock-in mouse model. mtHEPOR mice have delayed switch from primitive to definitive erythropoiesis. Polycythemia of mtHEPOR mice is transiently corrected in perinatal life. mtHEPOR newborns have low Epo and increased exposure of erythrocytes' phosphatidylserine.

Combining erythropoeitin and bone marrow stromal cell therapy after stroke

Both Erythropoietin (EPO) and bone marrow stromal cells (BMSCs) have been shown to improve outcome after stroke. EPO may improve outcome after stroke through its actions on blood progenitor cells, angiogenesis, or direct action in the CNS. BMSCs may improve outcome after stroke by regeneration, altering plasticity of viable cells, or prevention of cell death. Sorting out these potential modes of actions for EPO and BMSCs has been difficult using in vivo models of stroke. This study investigated neuroprotection afforded by EPO, BMSCs and the combination of these modalities in mouse hippocampal slice cultures after oxygen glucose deprivation (OGD). Significant neuroprotection was observed following post-injury treatment of slice cultures with BMSCs and neuroprotection was augmented by treating BMSCs with EPO. EPO alone did not protect neurons from OGD when given after injury, but was effective when given prior to OGD. The failure of EPO to protect when given after injury did not appear to result from its inability to activate EPO signaling pathways involving phosphorylation of Akt. This study supports the implication that BMSCs may rescue dying neurons after ischemia by providing trophic support. The data also show that EPO’s actions as a neuroprotective agent following stroke may be mediated by its actions on BMSCs.

Theoretical and experimental analysis links isoform-specific ERK signalling to cell fate decisions

Cell fate decisions are regulated by the coordinated activation of signalling pathways such as the extracellular signal-regulated kinase (ERK) cascade, but contributions of individual kinase isoforms are mostly unknown. By combining quantitative data from erythropoietin-induced pathway activation in primary erythroid progenitor (colony-forming unit erythroid stage, CFU-E) cells with mathematical modelling, we predicted and experimentally confirmed a distributive ERK phosphorylation mechanism in CFU-E cells. Model analysis showed bow-tie-shaped signal processing and inherently transient signalling for cytokine-induced ERK signalling. Sensitivity analysis predicted that, through a feedback-mediated process, increasing one ERK isoform reduces activation of the other isoform, which was verified by protein over-expression. We calculated ERK activation for biochemically not addressable but physiologically relevant ligand concentrations showing that double-phosphorylated ERK1 attenuates proliferation beyond a certain activation level, whereas activated ERK2 enhances proliferation with saturation kinetics. Thus, we provide a quantitative link between earlier unobservable signalling dynamics and cell fate decisions.

Erythropoiesis in polycythemia vera is hyper-proliferative and has accelerated maturation

Polycythemia vera (PV) is an acquired myeloproliferative clonal disorder, characterized by augmented erythropoiesis. To better define PV pathogenesis, we performed an in vitro erythroid expansion from peripheral blood mononuclear cells of controls and PV patients and evaluated the cells for proliferation, apoptosis, erythroid differentiation, and morphology at the defined time points. PV erythroid progenitors exhibited increased proliferation at days 9-14 and accelerated maturation at days 7-14, with a larger S-phase population (40%) than controls (20%) at day 11; however, the proportion of apoptotic cells was comparable to controls. Previously, we have identified PV-specific dysregulation of several microRNAs (i.e. miR-150, 451, 222, 155, 378). We had analyzed expression profiles of selected target genes of these microRNAs based on in silico prediction and their known function pertinent to the observed PV-specific erythropoiesis differences. p27, cMYB and EPOR showed differential expression in PV erythroid progenitors at the specific stages of erythroid differentiation. In this study, we identified accelerated maturation and hyper-proliferation at early stages of PV erythropoiesis. We speculate that aberrant expression of p27, c-MYB, and EPOR may contribute to these abnormal features in PV erythropoiesis.

Imatinib effect on growth and signal transduction in polycythemia vera

OBJECTIVE

An activating mutation of Janus kinase 2 (JAK2) in majority of polycythemia vera (PV) and other myeloproliferative disorders was reported. As imatinib inhibits several tyrosine kinases, we studied its effect in PV.

PATIENTS AND METHODS

We employed FDCP reporter cells expressing wild-type JAK2 and mutant JAK2(V617F) to study the efficacy of imatinib by cell proliferation assay and its effect on several cell-signaling events. Imatinib's efficacy was also examined on in vitro expanded native human erythroid progenitors. In addition, analysis of the percent JAK2 T-allele and phospho-signal transducer and activator of transcription-5 (STAT5) in granulocytes of PV patients following imatinib therapy was assessed.

RESULTS

Imatinib showed a specific time- and dose-dependent growth inhibitory effect on FDCP cells expressing JAK2(V617F), wherein we observed imatinib's inactivation of JAK2, STAT5 and cKIT proteins. In vitro expanded human PV erythroid progenitors were more sensitive to imatinib than normal erythroid progenitors and FDCP cells expressing JAK2(V617F), with growth inhibition at concentrations attainable in vivo. In an ongoing clinical study, a PV patient showed strong correlation between the percent JAK2 T-allele and his responsiveness to imatinib therapy.

CONCLUSION

Our data elucidate the therapeutic benefit of imatinib seen in some PV patients. Our data suggest that JAK2/STAT5 and cKIT activation may be integrated. To our knowledge, this is the first report demonstrating imatinib's effect on PV erythroid progenitors. These studies underscore the limitation of experiments using cell lines expressing the gene of interest.

Profiling of early gene expression induced by erythropoietin receptor structural variants

The development of erythroid progenitor cells is triggered via the expression of the erythropoietin receptor (EPOR) and its activation by erythropoietin. The function of the resulting receptor complex depends critically on the presence of activated JAK2, and the complex contains a large number of signaling molecules recruited to eight phosphorylated tyrosine residues. Studies using mutant receptor forms have demonstrated that truncated receptors lacking all tyrosines are able to support red blood cell development with low efficiency, whereas add-back mutants containing either Tyr343 or Tyr479 reconstitute EPOR signaling and erythropoiesis in vivo. To study the contribution of tyrosines to receptor function, we analyzed the activation of essential signaling pathways and early gene induction promoted by different receptor structural variants using human epidermal growth factor receptor/murine EPOR hybrids. In our experiments, receptors lacking all tyrosine residues or the JAK2-binding site did not induce mitogenic and anti-apoptotic signaling, whereas add-back mutant receptors containing single tyrosine residues (Try343 and Tyr479) supported the activation of these functions efficiently. Profiling of early gene expression using cDNA array hybridization revealed that (i) the high redundancy in the activation of signaling pathways is continued at the level of transcription; (ii) the expression of many genes targeted by the wild-type receptor is not supported by add-back mutants; and (iii) a small set of genes are exclusively induced by add-back receptors. We report the identification of several early genes that have not been implicated in the EPOR-dependent response so far.

Cytokine signals through STAT3 promote expression of granulocyte secondary granule proteins in 32D cells

OBJECTIVE

In a previous study, we showed that activation of a transfected human erythropoietin receptor (EPOR) in the murine myeloid cell line 32D resulted in the development of morphologic features of granulocytic differentiation and expression of the neutrophil primary granule protein myeloperoxidase. We now studied if EPOR signaling could also mediate secondary granule protein gene expression and investigated the signal transduction requirements for induction of secondary granule gene expression in 32D cells.

MATERIALS AND METHODS

Wild-type and variant 32D cells expressing normal or chimeric EPORs or receptors for granulocyte colony-stimulating factor (G-CSFRs) were stimulated with EPO or G-CSF and the expression of granulocyte-specific genes was analyzed by Northern blot analysis. To determine the signaling mechanisms required for secondary granule protein gene induction, the activation of STAT pathways following growth factor stimulation was studied by Western blot analysis.

RESULTS

We found that EPO treatment of 32D cells engineered to express EPOR did not result in induction of the secondary granule protein genes encoding lactoferrin and 24p3 lipocalin, the mouse homolog of human N-Gal, or the myeloid transcription factor C/EBPepsilon. Replacement of the intracellular domain of EPOR with the intracellular domain of G-CSFR in a chimeric receptor was associated with EPO-mediated induction of lactoferrin, 24p3 lipocalin, and C/EBPepsilon genes. We found that STAT3 phosphorylation was mediated by the intracellular domain of G-CSFR, but not EPOR. Replacement of one or two of the STAT5 binding sites in the intracytoplasmic domain of the EPOR with STAT3 binding sites resulted in EPO-mediated STAT3 activation and a marked increase in the expression of the 24p3 lipocalin gene. Knockdown of STAT3 protein levels with siRNA caused significant decrease in 24p3 lipocalin gene induction.

CONCLUSION

These results indicate that EPOR signaling cannot substitute for G-CSFR signaling to stimulate secondary granule protein gene expression in 32D cells. In addition, STAT3 is a critical mediator of 24p3 lipocalin gene expression in these cells.

Bax-inhibiting peptide derived from mouse and rat Ku70

Bax is a proapoptotic protein that plays a key role in the induction of apoptosis. Ku70 has activities to repair DNA damage in the nucleus and to suppress apoptosis by inhibiting Bax in the cytosol. We previously designed peptides based on the amino acid sequence of Bax-binding domain of human Ku70, and showed that these peptides bind Bax and inhibit cell death in human cell lines. In the present report, we examined the biological activities of other pentapeptides, VPTLK and VPALR, derived from mouse and rat Ku70. Cells in culture accumulated FITC-labeled VPTLK and VPALR, indicating that these peptides are cell permeable (human, mouse, rat, and porcine cells were examined). These peptides bound to Bax and suppressed cell death in various cell types including primary cultured cells. These data suggest that such Bax inhibiting peptides from three mammalian species may be used to protect healthy cells from apoptotic injury under pathological conditions.