Erythropoietin can induce the expression of Bcl-x(L) through Stat5 in erythropoietin-dependent progenitor cell lines

Erythropoietin (Epo) initiates its cellular response by binding to the Epo receptor, which triggers the activation of signal transducer and activator of transcription (Stat) 5 protein. Cell culture studies of erythroid progen itors have suggested that Epo functions as a survival factor by repressing apoptosis at least in part through Bcl-x(L), an anti-apoptotic protein of t he Bcl-2 family. In this report, we examine whether Stat5 can induce transa ctivation of the bcl-x gene in response to Epo, Two Epo-responsive progenit or cell lines, HCD-57 and Bcl-2-transfected Ba/F3-Epo receptor (Ba/F3-EpoR- Bcl-2), were used in this study. After Epo stimulation, we observed a corre lation between expression of bcl-x, and activation of Stat5 as assessed by the expression of oncostatin M, a direct target of Stat5, and the phosphory lation and nuclear translocation of Stat5, Moreover, a Stat binding element in the bcl-x promoter was found to be active in response to Epo, a finding that was further confirmed because mutagenesis of this sequence motif abro gated its promoter activity and overexpression of a dominant negative Stat5 protein blocked transactivation, When DNA-protein binding analyses were pe rformed, we found that Stat5, not Stat1 or Stat3, was the protein bound to the bcl-x promoter in response to Epo, These data suggest that Epo-dependen t activation of Stat5 is a transcriptional pathway that can be used by Epo- responsive progenitor cells to induce the expression of bcl-x(L) and conseq uently to inhibit apoptosis.