Regulation of volume activated chloride channels by protein kinase C-mediated phosphorylation of P-glycoprotein

The multidrug resistance P-glycoprotein (Pgp) transports hydrophobic drugs out of cells and has been recently associated with volume-activated chloride channels. Activation of these channels by hypotonic swelling was seen to be prevented by protein kinase C (PKC) in cells expressing high levels of Pgp by transfection. HeLa cells possess equivalent chloride currents yet they are not regulated by PKC. HeLa cells do not express Pgp as assessed by Western blotting. Following transfection of HeLa cells with cDNA encoding for Pgp, PKC-dependent suppression of volume activated chloride currents was observed. PKC regulation in transiently transfected HeLa cells was abolished by alanine replacement of the serine/threonine residues in the consensus phosphorylation sites of the linker region of Pgp. Replacement of these residues with glutamate, to mimic the effect of phosphorylation, mimicked the effects of PKC on channel activation. These results indicate that overexpression of Pgp confers PKC-regulation of endogenous volume-activated chloride channels. More generally they favour a model in which Pgp acts as a regulator of volume-activated chloride channels.