IL-3 dependent regulation of Bcl-x(L) gene expression by STAT5 in a bone marrow derived cell line

Activation of the Jak/STAT pathway by cytokines has been shown to regulate differentiation, proliferation or apoptosis in hematopoeitic cells. Among t he Stat proteins, STAT5 is activated by a broad range of cytokines. In orde r to study the role of STAT5 in hematopoietic cells, we stably expressed a dominant negative form of STAT5 (STAT5A Delta 749) in the IL-3 dependent bo ne marrow derived Ba/F3 cell line. Ba/F3 cells expressing STAT5A Delta 749 were found to be more sensitive to apoptosis than parental or control Ba/F3 cells after IL-3 withdrawal. Analysis of the expression of the cell death regulators, Bcl-2 and Bcl-x, revealed that the level of Bcl-x was lower in Ba/F3 cells expressing STAT5A Delta 749 than in control cells. IL-3 regulat ion of Bcl-x expression at protein and mRNA levels was impaired in these ce lls while that of Bcl-2 expression was unaffected. We further demonstrated that the Bcl-x gene promoter contained a proximal STAT consensus sequence t hat bound STAT5. Transactivation of a Bcl-x gene promoter reporter construc t by STAT5 was observed in Ba/F3 cells. Introduction of a mutation in the S TAT binding site abolished this transactivation. These data indicate that B cl-x is probably a STAT5 target gene. They also support the involvement of STAT5 in hematopoietic cell survival.