EXPRESSION REGULATION AND FUNCTION OF THE ERYTHROPOIETIN RECEPTOR

Diabetes, Digestive, Kidney Diseases

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Abstract

Erythropoietin is required for the production of mature red blood cells. Targeted deletion of murine erythropoietin receptor blocks proliferation and differentiation of definitive erythroid progenitor cells and results in death around embryonic day 13.5. By day 11.5, other defects including hypoplasia of endocardium, myocardium and neuroepithelium can be noted. We observe increase apoptotic cells in fetal liver, heart and brain of the erythropoietin receptor null animals. Introducing the human erythropoietin receptor transgene reduces the increased apoptosis in embryonic tissue, and adult animals survive with no gross morphologic defect. The human erythropoietin receptor transgene is sufficient to drive erythropoietin receptor expression in a temporal and tissue specific manner, and is sufficiently homologous to the murine receptor to restore erythropoiesis and other erythropoietin/erythropoietin receptor response. The requirement for erythropoietin and its receptor for normal heart development and the response of vascular endothelium and cells of neural origin to erythropoietin provide evidence that the function of erythropoietin as a growth factor or cytokine to protect cells from apoptosis extends beyond the hematopoietic lineage. Screening of a human brain cDNA library and quantitative analysis of erythropoietin receptor transcripts indicate that the erythropoietin receptor gene locus is transcriptionally active in adult brain with a significant proportion of transcripts originating far upstream from the erythropoietin receptor coding region. Unlike erythroid cells, brain erythropoietin receptor transcripts are inefficiently or alternately processed with a bias towards the 3' coding region. In human erythropoietin receptor transgenic mice, anemic stress induces expression of the transgene and endogenous erythropoietin receptor gene in hematopoietic tissue and brain. In culture of neuronal cells, hypoxia induces erythropoietin receptor expression and increases sensitivity to erythropoietin. Induction of erythropoietin receptor expression appears to be a consequence of increased transcription from the upstream region and proximal promoter, and a shift towards increased processing efficiency. These data suggest that the neuroprotective effect of erythropoietin and its receptor may require two molecular events: the induction of erythropoietin production by hypoxia and an increase in erythropoietin receptor expression in neuronal cells resulting in increased sensitivity to erythropoietin. We have also observed that erythropoietin receptor is expressed on myoblasts and can mediate a biological response of these cells to treatment with erythropoietin. Primary murine satellite cells and myoblast C2C12 cells exhibit a proliferative response to erythropoietin and a marked decrease in terminal differentiation to form myotubes. As with erythroid progenitor cells, erythropoietin stimulation activates Jak2/Stat5 signal transduction and increases cytoplasmic calcium, which is dependent on tyrosine phosphorylation. The decrease in differentiation of C2C12 cells is concomitant with an increase in Myf-5 and MyoD expression and inhibition of myogenin induction during differentiation, altering the pattern of expression of the MyoD family of transcription factors during muscle differentiation. These data suggest that rather than acting in an instructive or specific mode for differentiation, erythropoietin can stimulate proliferation of myoblasts to expand the progenitor population during differentiation and may also have a potential role in muscle development or repair.