DOWN-REGULATION OF PROTEIN-KINASE-C SUPPRESSES INDUCTION OF APOPTOSISIN HUMAN PROSTATIC-CARCINOMA CELLS

Protein kinase C (PKC) has been implicated in propagating signals for apoptosis, We have investigated the effect of pharmacological modulati on of PKC activity in DU-145 human androgen-independent prostatic carc inoma cells. The apoptotic death of these cells is characterized by th e acquisition of classical apoptotic morphology and generation of grea ter than or equal to 1-mbp and 450- to 600-kbp DNA fragments in attach ed preapoptotic cell populations prior to cellular detachment and accr ual of 30- to 50-kbp DNA fragments. We found that induction of apoptos is 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 correlat ed with inhibition of VP-16 or melphalan-induced morphological apoptos is and generation of the 30- to 50-kbp DNA fragments. Chronic exposure to phorbol-12,13-dibutyrate (PDBu) did not, however, suppress product ion of the greater than or equal to 1-mbp and 450- to 600-kbp DNA frag ments found in preapoptotic cell populations, suggesting that PKC down regulation may interfere with the transition between a preapoptotic ce ll and an apoptotic cell. PKC isozyme analysis revealed that chronic P DBu treatment caused downregulation of PKC LY and -epsilon in DU-145 c ells. Using concentrations of the PKC inhibitor UCN-01 that were consi stent with PKC-cu inhibition (but not PKC-E inhibition), however, did not mimic the effects of chronic PDBu treatment, implying that downreg ulation of PKC-epsilon may be of particular importance. Together, thes e findings suggest that phorbol esters may act as tumor promoters by s uppressing apoptosis. (C) 1996 Academic Press, Inc.