DIFFERENTIAL MODULATION OF P-GLYCOPROTEIN TRANSPORT BY PROTEIN-KINASEINHIBITION

Previous studies of P-glycoprotein have demonstrated that its function can be modulated by phosphorylation. In the present study, inhibition of protein kinase C with calphostin C or stauroporine or prolonged tr eatment with the phorbol ester TPA decreased phosphorylation of P-glyc oprotein, and impaired transport of vinblastine. Calphostin C also inh ibited transport of actinomycin D, vincristine, rhodamine, and azidopi ne in SW620 Ad300 multidrug-resistant human colon carcinoma cells. Pho toaffinity labeling of P-glycoprotein with azidopine was decreased by calphostin C, suggesting that dephosphorylation alters the affinity of P-glycoprotein for its substrates. Impaired transport of rhodamine in normal T lymphocytes treated with staurosporine demonstrates that mod ulation of P-glycoprotein function is not limited to cells selected fo r drug resistance in vitro. Transport of P-glycoprotein antagonists in SW620 Ad300 cells was also affected by calphostin C. Cyclosporin A tr ansport decreased, while verapamil transport increased. Cyclosporin A in calphostin C-treated cells resulted in additive P-glycoprotein anta gonism, while no additive effect could be demonstrated with verapamil, suggesting that the increase in verapamil transport makes it a poorer P-glycoprotein antagonist. These studies suggest that transport by P- glycoprotein is a dynamic process which can be modulated by phosphoryl ation, and that antagonists may block P-glycoprotein differently in di fferent phosphorylation states.