PKC alpha activation downregulates ATM and radio-sensitizes androgen-sensitive human prostate cancer cells in vitro and in vivo

We previously demonstrated that treatment of human androgen-responsive prostate cancer cell lines LNCaP and CWR22-Rvl with 12-O-tetradecanoylphorbol 13-acetate (TPA), a known protein kinase C (PKC) activator, decreases ATM protein levels, thus de-repressing the enzyme ceramide synthase (CS) and promoting apoptosis as well as radio-sensitizing these cefls.(1) Here we show that PKC(x mediates the TPA effect on ATM expression, since ATM suppression and apoptosis induced by either TPA or diacylglycerol-lactone (DAG-lactone), both inducing PKC alpha activation,(2) are abrogated in LNCaP cells following transfection of a kinase-dead PKC alpha mutant (KD-PKC alpha). Similarly, KD-PKC alpha blocks the apoptotic response elicited by combination of TPA and radiation, whereas expression of constitutively active PKC alpha is sufficient to sensitize cells to radiation alone, without a need to pre-treat the cells with TPA. These findings identify CS activation as a downstream event of PKC alpha activity in LNCaP cells. Similar results were obtained in CWR22-Rvl cells with DAG-lactone treatment. Using the LNCaP orthotopic prostate model it is shown that treatment with TPA or DAG-lactone induces significant reduction in tumor ATM levels coupled with tumor growth delay. Furthermore, while fractionated radiation alone produces significant tumor growth delay, pretreatment with TPA or DAG-lactone significantly potentiates tumor cure. These findings support a model in which activation of PKC alpha downregulates ATM, thus relieving CS repression by ATM and enhancing apoptosis via ceramide generation. This model may provide a basis for the design of new therapies in prostate cancer.