title: Mechanism of inhibition of P-glycoprotein-mediated drug transport by protein kinase C blockers.
abstract: P-glycoprotein is a membrane ATPase that transports drugs out of cells and confers resistance to a variety of chemically unrelated drugs (multidrug resistance). P-glycoprotein is phosphorylated by protein kinase C (PKC), and PKC blockers reduce P-glycoprotein phosphorylation and increase drug accumulation. These observations suggest that phosphorylation of P-glycoprotein stimulates drug transport. However, there is evidence that PKC inhibitors directly interact with P-glycoprotein, and therefore the mechanism of their effects on P-glycoprotein-mediated drug transport and the possible role of phosphorylation in the regulation of P-glycoprotein function remain unclear. In the present work, we studied the effects of different kinds of PKC inhibitors on drug transport in cells expressing wild-type human P-glycoprotein and a PKC phosphorylation-defective mutant. We demonstrated that PKC blockers inhibit drug transport hy mechanisms independent of P-glycoprotein phosphorylation. Inhibition by the blockers occurs by (i) direct competition with transported drugs for binding to P-glycoprotein, and (ii) indirect inhibition through a pathway that involves PKC inhibition, but is independent of P-glycoprotein phosphorylation. The effects of the blockers on P-glycoprotein phosphorylation do not seem to play an important role, but the PKC-signaling pathway regulates P-glycoprotein-mediated drug transport.
authors.affiliation: Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77555-0641, USA.
authors.forename: A F
authors.initials: AF
authors.lastname: Castro
authors.forename: J K
authors.initials: JK
authors.lastname: Horton
authors.forename: C G
authors.initials: CG
authors.lastname: Vanoye
authors.forename: G A
authors.initials: GA
authors.lastname: Altenberg
mesh_terms: D016475:3T3 Cells; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D000345:Affinity Labels / Q000494:pharmacology; D000818:Animals; D001386:Azides / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D004095:Dihydropyridines / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006801:Humans; D051379:Mice; D010766:Phosphorylation; D011493:Protein Kinase C / Q000037:antagonists & inhibitors* / Q000235:genetics; D015398:Signal Transduction; D013110:Sphingosine / Q000031:analogs & derivatives / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000345:Affinity Labels; D001386:Azides; D004095:Dihydropyridines; D004791:Enzyme Inhibitors; C041130:azidopine; D011493:Protein Kinase C; D013110:Sphingosine; C005682:safingol