Distinct patterns of phorbol ester-induced downregulation of protein kinase C activity in adriamycin-selected multidrug resistant and parental murine fibrosarcoma cells.

Specific activators of protein kinase C (PKC), including the phorbol-ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), can reduce the chemosensitivities of a variety of mammalian tumor cell lines and their cytotoxic drug-selected multidrug resistant (MDR) variants to MDR-linked drugs, thus implicating PKC in the MDR phenotype. Previously, we reported that the adriamycin-selected MDR murine fibrosarcoma cell line UV-2237M-ADRR has approximately twice as much PKC activity as the parental UV-2237M line. In this report, we show that the level of [3H]phorbol-12,13-dibutyrate specific binding activity was elevated 3.5-fold in the MDR cells, thus establishing that phorbol-ester responsive PKC is overexpressed in the MDR line. Phorbol esters mediate downregulation of PKC by stimulating proteolysis of the enzyme, without altering the rate of PKC synthesis. We report that the kinetics of TPA-induced downregulation of PKC activity differ markedly in parental and MDR UV-2237M cells, providing evidence that the overexpression of phorbol-ester responsive PKC in adriamycin-selected MDR UV-2237M-ADRR cells results, at least in part, from a reduced rate of PKC degradation in the cells.