Cooperative and redundant effects of STAT5 and Ras signaling in BCR/ABL transformed hematopoietic cells

The Akt, Ras and STAT5 signaling pathways have each been linked to transfor mation of hematopoietic cells by BCR/ABL. However the relative contribution s of these signaling pathways to BCR/ABL mediated cytokine-independent surv ival, proliferation and resistance to DNA damage-induced apoptosis have not been systematically defined. Here we report that activation of either Akt, Ras or STAT5 confers cytokine-independent survival to IL-3 dependent BaF3 cells. Ras or STAT5, but not Akt, also drives cytokine-independent prolifer ation and imparts sustained resistance to DNA damage-induced apoptosis. We also show that dominant negative (DN) inhibition of STAT5, but not Ras or A kt, significantly reduces resistance to DNA damage-induced apoptosis in BCR /ABL transformed BaF3 cells. Whereas inhibition of STAT5 or Ras alone does not compromise cytokine-independent proliferation of BaF3-BCR/ABL cells, si multaneous blockade of both STAT5 and Ras reduces proliferation and maximal ly sensitizes BaF3-BCR/ABL cells to DNA damage induced by gamma -irradiatio n, suggesting a cooperative role for these two signaling pathways in BCR/AB L transformation. The anti-apoptotic properties of BCR/ABL can be partly ex plained by an increase in the expression of Pim-1 and Bcl-XL, as ectopic ex pression of these STAT5 target genes imparts both cytokine-independent surv ival and partial gamma -radiation resistance. These data illustrate both co operative and redundant effects of STAT5 and Ras signaling in BCR/ABL trans formed cells, with STAT5 playing a dominant role in resistance to DNA damag e-induced apoptosis.