THROMBOPOIETIN ACTIVATES A STAT5-LIKE FACTOR IN HEMATOPOIETIC-CELLS

Thrombopoietin (TPO) is a newly cloned cytokine which is the major reg ulator of circulating platelet levels, acting on both proliferation an d differentiation of megakaryocytes. We have investigated the ability of TPO to activate the JAK/STAT pathway in megakaryocytic cell lines, We used either the granulocyte-macrophage colony-stimulating factor (G M-CSF)- and/or erythropoietin (EPO)-dependent UT7 cell line in which t he murine TPO receptor (mumpl) had been transfected (mumpl-UT7 transfe ctants) or the MO7E and DAMI cells which express endogenous human TPO receptors. We demonstrated that TPO activates the kinase JAK2 and a ST AT5-like transcriptional factor but not STAT1, STAT2, STAT3 or STAT4, in a very rapid and transient manner, In order to better ascertain the specificity of the activation of STAT5-related factor by TPO, we inve stigated the effect of other cytokines/growth factors, Both GM-CSF and EPO activated the STAT5-like factor, In contrast, neither interferon (IFN)-gamma nor the mitogenic stem cell factor (SCF) activated STAT5, although IFN-gamma did activate STAT1 in those cells, The hematopoieti c DNA binding activity related to STAT5 was identified as a p97 tyrosi ne-phosphorylated protein band which exhibited identical gel mobility to the mammary STAT5. Because v-mpl, a truncated form of the TPO recep tor c-mpl, was shown to be oncogenic, we tested the activity of v-mpi on STAT5 and found STAT5 constitutively activated in two different v-m pl expressing cells, the transiently transfected Cos7 cells and the st able v-mpl-UT7 transfectants, Overall, our data indicate that STAT5 is widely expressed in hematopoietic cells and activated by a number of cytokines, including TPO, GM-CSF and EPO, but not by IFN-gamma or SCF.