Pregnancy is characterized by increased erythropoiesis within maternal and fetal compartments. The placenta has been shown to produce factors that stimulate erythropoiesis but convincing evidence for placental production of erythropoietin (EPO) is still lacking. Prolactin-like protein E (PLP-E) was recently found to stimulate expression of the adult beta major globin gene in mouse erythroleukemia cells. Here we demonstrate that PLP-E transiently expressed in COS-7 cells stimulates proliferation

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... and erythroid differentiation of murine and human erythroid progenitor cell lines. Electrophoretic mobility shift assays were used to show the activation of STAT5 by PLP-E in the human erythroid cell line TF1. Furthermore, we compared the effects of PLP-E on murine myeloid FDCP1 cells which do not express EPO receptors (EPORs) with effects on cells genetically engineered to express functional EPORs. We provide evidence that PLP-E-dependent proliferation and STAT5 activation is independent of the expression of the EPOR. Taken together, these data suggest that PLP-E acts on specific receptors of erythroid-committed murine and human cells by the activation of intracellular signaling pathways promoting cell growth and differentiation.