Inhibition of protein kinase C in multidrug-resistant cells by modulators of multidrug resistance.

We have evaluated the protein kinase C (PKC) activity in two series of cultured cell lines presenting the multidrug-resistance (MDR) phenotype and in the corresponding wild-type cells: the human KB 3.1, KB A1 and KB 8.5 cell lines, and the rat C6, C6 0.5 and C6 1V cell lines. We have observed an increase in PKC activity in the MDR cell lines of the KB cell lineage, proportional to their degree of resistance to doxorubicin. In contrast, the MDR cell lines of the C6 cell lineage presented no change (C6 0.5) or even decrease (C6 1V) in PKC activity; the basal level of PKC activity in C6 cells was, however, 50-fold higher than in KB 3.1 cells. We have tested, in these lines, the effect of four modulators of MDR: verapamil, cyclosporin A, quinine and S-9788, on doxorubicin acytotoxicity and on PKC activity. We observed that cyclosporin A and S-9788, which were the most active on MDR reversal, were able to inhibit PKC activity in the KB resistant lines as well as in all C6 lines, whereas verapamil and quinine had only marginal effects on PKC activity. The distribution of PKC isoenzymes was studied by Western blots. The PKC alpha, gamma and delta isoforms were increased in the KB resistant lines as compared to wild-type cells, which could account for the increase PKC activity we observed. In contrast, PKC alpha and gamma were decreased in C6 1V cells, as expected from the results obtained for total PKC activity, but we also noticed an important decrease in PKC delta in the C6 0.5 line. Our results suggest that an increase in PKC activity is not an absolute requirement for expression of MDR, provided that the basal level be high enough; and that some modulators may act on MDR, not only through direct P-glycoprotein interaction, but also through P-glycoprotein phosphorylation or expression. The distribution of PKC isoenzymes revealed that the modifications encountered between sensitive and resistant cells mainly concerned alpha, gamma and delta isoenzymes of PKC.