Mechanism of inhibition of P-glycoprotein-mediated drug transport by protein kinase C blockers

P-glycoprotein is a membrane ATPase that transports drugs out of cells and confers resistance to a variety of chemically unrelated drugs (multidrug re sistance). P-glycoprotein is phosphorylated by protein kinase C (PKC), and PKC blockers reduce P-glycoprotein phosphorylation and increase drug accumu lation. These observations suggest that phosphorylation of P-glycoprotein s timulates drug transport. However, there is evidence that PKC inhibitors di rectly interact with P-glycoprotein, and therefore the mechanism of their e ffects on P-glycoprotein-mediated drug transport and the possible role of p hosphorylation in the regulation of P-glycoprotein function remain unclear. In the present work, we studied the effects of different kinds of PKC inhi bitors on drug transport in cells expressing wild-type human P-glycoprotein and a PKC phosphorylation defective mutant. We demonstrated that PKC block ers inhibit drug transport by mechanisms independent of P-glycoprotein phos phorylation. Inhibition by the blockers occurs by (i) direct competition wi th transported drugs for binding to P-glycoprotein, and (ii) indirect inhib ition through a pathway that involves PKC inhibition, but is independent of P-glycoprotein phosphorylation, The effects of the blockers on P-glycoprot ein phosphorylation do not seem to play an important role, but the PKC-sign aling pathway regulates P-glycoprotein-mediated drug transport. BIOCHEM PHA RMACOL 58;11:1723-1733, 1999. (C) 1999 Elsevier Science Inc.