Mechanism of Cell Survival Mediated by the B-Raf Kinase

Boston Biomedical Research Institute, Watertown MA

Investigators

Abstract

Many types of mammalian cells are dependent upon growth factors for survival. In a variety of cell types, growth factors prevent apoptosis by stimulation of the phosphatidylinositol 3-kinase (PI 3- kinase)/Akt pathway, which directly links growth factor signaling to the central apoptotic pathways by blocking both mitochondrial cytochrome c release and apoptotic protease (caspase) activation. Growth factors are also known to regulate programmed cell death by activating members of the Raf kinase family, although the mechanism of this alternative Raf-dependent survival is not well understood. Recent studies from this laboratory have identified a survival pathway mediated by the B-Raf protein kinase. Although other regulatory steps may also be involved, the predominant effect of B-Raf appears to be downstream of cytochrome c release from mitochondria, and may be to inhibit the cytosolic activation of caspases. Therefore, investigations are planned to elucidate the mechanism by which B-Raf prevents programmed cell death by identifying the relevant downstream targets. The studies will include the following specific aims: (1) to investigate the effects of B-Raf signaling on phosphorylation and inactivation of caspase-9, including, if appropriate, the identification of the kinase that phosphorylates caspase-9, (2) to analyze the effects of B-Raf signaling on the caspase-9 activation complex containing Apaf-1 and its interactions with Bcl-xL, and (3) to study the potential role of B-Raf in transcriptional regulation of caspase inhibitors acting downstream of cytochrome c, such as members of the IAP and heat shock protein families. As an alternative approach to identify the relevant targets of B-Raf signaling, systematic gene expression studies will also be undertaken. The results of these studies will advance the basic understanding of signaling involved growth factor-mediated cell survival.