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We have previously shown that treatment of human androgen-responsive prostate cancer cell lines (LNCaP and CWR22-Rv1) with 12-O-tetradecanoylphorbol 13-acetate (TPA), a known activator of both “classical” (calcium-dependent) and “novel” (calcium-independent) protein kinase C (PKC) isoforms, decreases ATM protein levels and induces apoptosis. Co-treatment of these cells with TPA and radiation increased their apoptotic response, while radiation alone failed to induce apoptosis in these cells (Garzotto et. al. 1999; Truman et al., 2003). Garcia-Bermejo ML et al. recently reported that specific activation of PKCα with diacylglycerol (DAG)-lactone, HK654, (Garcia-Bermejo ML et al., 2001) induced apoptosis in LNCaP cells. They also showed that both TPA and HK654 induced marked redistribution of PKCα to the plasma membrane. Our laboratory has now shown that PKCα activation is responsible for decreasing ATM protein levels in response to either TPA or HK654 in LNCaP cells that are undergoing apoptosis. Catalytically competent PKCα is required for this event, since LNCaP cells that are transfected with a kinase-dead PKCα dominant-negative mutant showed no decrease in ATM protein levels and significantly reduced apoptosis. Treatment of LNCaP cells with HK654 leads to increased ceramide generation, which is significantly inhibited by fumonisin B1, a specific inhibitor of ceramide synthase (CS), thus identifying CS activation as a downstream event. These findings support a model in which activation of PKCα by either TPA or HK654 leads to down-regulation of ATM protein levels that in turn accelerates ceramide generation during apoptosis in LNCaP cells. (Supported by NCI Prostate Spore Grant # CA92629-01to A H-F.)

[Proc Amer Assoc Cancer Res, Volume 45, 2004]