We previously demonstrated that treatment of human
androgen-responsive
prostate cancer cell lines LNCaP and CWR22-Rv1 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 cells.(1) Here we show that PKCalpha mediates the TPA effect on ATM expression, since ATM suppression and apoptosis induced by either TPA or
diacylglycerol-
lactone (
DAG-lactone), both inducing PKCalpha activation,(2) are abrogated in LNCaP cells following transfection of a
kinase-dead PKCalpha mutant (KD-PKCalpha). Similarly, KD-PKCalpha blocks the apoptotic response elicited by combination of TPA and radiation, whereas expression of constitutively active PKCalpha 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 PKCalpha activity in LNCaP cells. Similar results were obtained in CWR22-Rv1 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 PKCalpha 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.