PROTEIN-KINASE C-MEDIATED PHOSPHORYLATION OF THE HUMAN MULTIDRUG-RESISTANCE P-GLYCOPROTEIN REGULATES CELL VOLUME-ACTIVATED CHLORIDE CHANNELS

The multidrug resistance P-glycoprotein (P-gp), which transports hydro phobic drugs out of cells, is also associated with volume-activated ch loride currents. It is not yet clear whether P-gp is a channel itself, or whether it is a channel regulator. Activation of chloride currents by hypotonicity in cells expressing P-gp was shown to be regulated by protein kinase C (PKC). HeLa cells exhibited volume-activated chlorid e currents indistinguishable from those obtained in P-gp-expressing ce lls except that they were insensitive to PKC. HeLa cells did not expre ss detectable P-gp but, following transient transfection with cDNA enc oding P-gp, the volume-activated channels acquired PKC regulation. PKC regulation was abolished when serine/threonine residues in the consen sus phosphorylation sites of the linker region of P-gp were replaced w ith alanine. Replacement of these residues with glutamate, in order to mimic the charge of the phosphorylated protein, also mimicked the eff ects of PKC on channel activation. These data demonstrate that PKC-med iated phosphorylation of P-gp regulates the activity of an endogenous chloride channel and thus indicate that P-gp is a channel regulator.