Kinase regulation of cytochrome c-induced apoptosis in oncogenesis

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One of the most critical events in the intrinsic apoptosis program is the release of cytochrome c from the intermembrane space of the mitochondria. Numerous kinases have been shown to phosphorylate proteins in apoptotic pathways that modulate cytochrome c release, leading to either promotion or suppression of cell death. However, little is known about the manner in which kinases regulate apoptosis at the level of caspase activation and the apoptosome, which are critical to apoptotic progression after mitochondrial release of cytochrome c. Our lab has previously shown that the oncogenic kinase Bcr-Abl can inhibit caspase activation in cytosolic extracts devoid of mitochondria after addition of cytochrome c. We have discovered other candidate kinases, which appear to have roles in apoptotic regulation at the apoptosome. For example, the kinase Pim-2 has recently been demonstrated to be a potent inhibitor of apoptosis prior to cytochrome c release from the mitochondria. Interestingly, we have observed that Pim-2 can also strongly inhibit caspase activation downstream of cytochrome c release at the level of the apoptosome. Additionally, both Pim-2 and Bcr-Abl have been demonstrated to be involved in various aspects of tumorigenesis (prostate and leukemia, respectively). In working to characterize cytochrome c-induced apoptosis in other cancers, our lab has recently shown that breast cancer cells have a hyperactive apoptosome, caused by enhanced binding of caspase-9 to the Apaf-1 CARD. While breast cancer is the first example of a tumor having a hyperactive apoptosome, other cancers have been demonstrated to have defects in proper apoptosome formation and activation. For example, melanoma and carcinomas of the ovary and lung have been reported to be specifically deficient in apoptosis downstream of cytochrome c release from the mitochondria. Hence, we are also working to identify potential apoptosomal defects (or sensitivity) in other types of cancers, such as prostate (where Pim-2 or other kinases may play a role). Cytosolic lysates from these cell types show a low level of caspase-3 activation compared to the hyperactive malignant mammary epithelial cells, and we are examining the apoptotic defects in these prostate cancer cell lines, compared to the appropriate normal counterpart. Understanding how various kinases regulate the apoptosome will allow us to have a better idea of how tumors evade the apoptotic program through disregulation of the apoptotic pathway downstream of cytochrome c release from the mitochondria and could potentially lead to the identification of novel chemotherapeutic targets.