The Survival Function of the Bcr-Abl Oncogene Is Mediated by Bad-Dependent and -Independent Pathways: Roles for Phosphatidylinositol 3-Kinase and Raf

Abstract

The Bcr-Abl tyrosine kinase constitutively activates cytokine signal transduction pathways that stimulate growth and prevent apoptosis in hematopoietic cells. The antiapoptotic action of interleukin-3 (IL-3) has been linked to a signaling pathway which inactivates the proapoptotic protein Bad by phosphorylation through kinases such as Akt and Raf. Here we report also that expression of Bcr-Abl leads to phosphorylation of Bad in hematopoietic cells. Bad phosphorylation induced by Bcr-Abl is kinase dependent, requires phosphatidylinositol 3-kinase (PI3-kinase), and mitochondrial targeting of Raf, and occurs independently of Erk. The ability of Bcr-Abl to confer cytokine-independent survival to hematopoietic cells was compromised by inhibitors of PI3-kinase, as well as by a dominant negative form of Raf targeted to the mitochondria. Furthermore, when the capacity of Bcr-Abl to phosphorylate Bad was completely blocked by dominant negative Raf, a subpopulation of cells remained viable, providing evidence for Bad-independent survival pathways. This alternative survival pathway remained PI3-kinase dependent. Finally, Bcr-Abl, but not IL-3, inhibited the proapoptotic activity of overexpressed 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.

ACKNOWLEDGMENTS

We thank Michael Greenberg for antibody, Elizabeth Major for help with early phases of this work, Xinyi Wu for assistance with cloning, Lisa Dove for manuscript preparation, and David Chang, Chris Denny, Ke Shuai, and Owen Witte for comments.

This work was supported by grants from the American Cancer Society (C.L.S.), National Institutes of Health (C.L.S. and J.C.R.) and the James S. McDonnell Foundation (C.L.S.). H.G.W. is an AACR Research Fellow in Basic or Translational Research sponsored by the Sidney Kimmel Foundation for Cancer Research. C.L.S. is a Scholar of the Leukemia Society of America.