Reduced daunomycin accumulation in drug-sensitive and multidrug-resistant human carcinoma KB cells following phorbol ester treatment: a potential role for protein kinase C in reducing drug influx.

We have investigated the role of protein kinase C (PKC) in the multidrug resistance (MDR) phenotype of human KB carcinoma cell lines. The PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) reduced daunomycin accumulation in both drug-sensitive KB-3-1 and MDR KB-C1 cells in a time-dependent manner. The inactive phorbol ester 4 alpha TPA did not reduce daunomycin accumulation, and the PKC inhibitor, Ro 31-8220, reversed the TPA effect. TPA had no effect on daunomycin efflux and did not induce Pgp expression in KB-3-1 cells or alter Pgp levels in KB-C1 cells. Linear, short-term daunomycin accumulation was reduced by pretreatment with TPA, an effect that could be reversed by Ro 31-8220. The effects of TPA on PKC subspecies localisation and downregulation were also examined. TPA initially induced translocation of PKCs alpha and delta, and to a lesser extent, PKC epsilon to the membrane fraction; 8 h after TPA treatment, differential effects on downregulation of PKCs alpha and delta were observed between cell lines, although PKC epsilon was not reduced in either cell line. We therefore propose that the TPA-induced reduction in daunomycin accumulation in KB cells is due to a PKC-mediated process, which is maintained after depletion of certain PKC subspecies or is due to activation of downregulation insensitive PKC subspecies. These results suggest that PKC may regulate drug resistance by reducing drug influx in a Pgp-independent manner in KB cells. This may represent a mechanism of drug-resistance independent of, or in addition, to, Pgp-mediated drug efflux.