title: Circumvention of P-glycoprotein-mediated multiple drug resistance by phosphorylation modulators is independent of protein kinases.
abstract: Expression of P-glycoprotein by tumor cells confers resistance to multiple natural product drugs because of its ability to export these compounds. This transporter is a substrate for several protein kinases; however, the functional significance of its phosphorylation is not defined. We examined the effects of many activators and inhibitors of protein kinases on the activity of P-glycoprotein in drug-resistant human breast carcinoma cells (MCF-7/ADR). Several phorbol esters sensitized these cells to P-glycoprotein substrate drugs; however, there was no correlation with activation of protein kinase C. The 4 alpha- and 4 beta-isomers of phorbol 12-myristate 13-acetate were equally potent in sensitizing the cells to actinomycin D and daunomycin and in increasing the intracellular accumulation of [3H]vinblastine. These effects of 4 beta-phorbol myristate acetate required much higher concentrations than were needed to increase P-glycoprotein phosphorylation and were not antagonized by staurosporine. Similar to verapamil, the phorbol esters did not sensitize MCF-7/ADR cells to cisplatin, nor parental MCF-7 cells to any of the anticancer drugs. Mezerein, K-252a, and H-89 sensitized MCF-7/ADR cells, increased intracellular accumulation of [3H]vinblastine, and antagonized photolabeling of P-glycoprotein by [3H]azidopine. Therefore, phosphorylation does not appear to play a significant role in regulating P-glycoprotein activity in MCF-7/ADR cells.
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000494:pharmacology*; D001943:Breast Neoplasms; D002460:Cell Line; D003609:Dactinomycin / Q000494:pharmacology; D004317:Doxorubicin / Q000633:toxicity*; D018432:Drug Resistance, Multiple; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D006801:Humans; D007700:Kinetics; D010703:Phorbol Esters / Q000737:chemistry / Q000494:pharmacology*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000737:chemistry / Q000494:pharmacology; D014407:Tumor Cells, Cultured; D014747:Vinblastine / Q000378:metabolism
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000970:Antineoplastic Agents; D004791:Enzyme Inhibitors; D010703:Phorbol Esters; D047428:Protein Kinase Inhibitors; D003609:Dactinomycin; D014747:Vinblastine; D004317:Doxorubicin; D011494:Protein Kinases; D010749:Phosphoprotein Phosphatases; D013755:Tetradecanoylphorbol Acetate