Downregulation of protein kinase C suppresses induction of apoptosis in human prostatic carcinoma cells.

Protein kinase C (PKC) has been implicated in propagating signals for apoptosis. We have investigated the effect of pharmacological modulation of PKC activity in DU-145 human androgen-independent prostatic carcinoma cells. The apoptotic death of these cells is characterized by the acquisition of classical apoptotic morphology and generation of > or = 1-mbp and 450- to 600-kbp DNA fragments in attached preapoptotic cell populations prior to cellular detachment and accrual of 30- to 50-kbp DNA fragments. We found that induction of apoptosis was arrested by downregulation of PKC activity and not by transient activation or inhibition of the enzyme. Concentrations and durations of exposure to phorbol esters that downregulated PKC activity correlated with inhibition of VP-16 or melphalan-induced morphological apoptosis and generation of the 30-to-50-kbp DNA fragments. Chronic exposure to phorbol-12,13-dibutyrate (PDBu) did not, however, suppress production of the > or = 1-mbp and 450- to 600-kbp DNA fragments found in preapoptotic cell populations, suggesting that PKC downregulation may interfere with the transition between a preapoptotic cell and an apoptotic cell. PKC isozyme analysis revealed that chronic PDBu treatment caused downregulation of PKC-alpha and -epsilon in DU-145 cells. Using concentrations of the PKC inhibitor UCN-01 that were consistent with PKC-alpha inhibition (but not PKC-epsilon inhibition), however, did not mimic the effects of chronic PDBu treatment, implying that downregulation of PKC-epsilon may be of particular importance. Together, these findings suggest that phorbol esters may act as tumor promoters by suppressing apoptosis.