The survival function of the Bcr-Abl oncogene is mediated by Bad-dependentand -independent pathways: Roles for phosphatidylinositol 3-kinase and Raf

The Bcr-Abl tyrosine kinase constitutively activates cytokine signal transd uction pathways that stimulate growth and prevent apoptosis in hematopoieti c cells. The antiapoptotic action of interleukin3 (IL-3) has been linked to a signaling pathway which inactivates the proapoptotic protein Bad by phos phorylation through kinases such as Akt and Raf. Here we report also that e xpression of Bcr-Abl leads to phosphorylation of Bad in hematopoietic cells . Bad phosphorylation induced by Bcr-Abl is kinase dependent, requires phos phatidyl inositol 3-kinase (PI3-kinase), and mitochondrial targeting of Raf , and occurs independently of Erk. The ability of Bcr-Abl to confer cytokin e-independent survival to hematopoietic cells was compromised by inhibitors of PI3-kinase, as well as by a dominant negative form of Raf targeted to t he mitochondria. Furthermore, when the capacity of Bcr-Abl to phosphorylate Bad was completely blocked by dominant negative Raf, a subpopulation of ce lls remained viable, providing evidence for Bad-independent survival pathwa ys. This alternative survival pathway remained PI3-kinase dependent. Finall y, Bcr-Abl, but not IL-3, inhibited the proapoptotic activity of overexpres sed Bad. We conclude that the antiapoptotic function of Bcr-Abl is mediated through pathways involving PI3-kinase and Raf and that survival can occur in the absence of Bad phosphorylation.