Phosphorylation of P-glycoprotein by PKA and PKC modulates swelling-activated Cl- currents

Several proteins belonging to the ATP-binding cassette superfamily can affe ct ion channel function. These include the cystic fibrosis transmembrane co nductance regulator, the sulfonylurea receptor, and the multidrug resistanc e protein P-glycoprotein (MDR1). We measured whole cell swelling-activated Cl- currents (I-CI,I-swell) in parental cells and cells expressing wild-typ e MDR1 or a phosphorylation-defective mutant (Ser-661, Ser-667, and Ser-671 replaced by Ala). Stimulation of protein kinase C (PKC) with a phorbol est er reduced the rate of increase in I-Cl,I-smell only in cells that express MDR1. PKC stimulation had no effect on steady-state I-Cl,I-swell. Stimulati on of protein kinase A (PKA) with 8-bromoadenosine 3',5'-cyclic monophospha te reduced steady-state I-Cl,I-swell only in MDR1-expressing cells. PKA sti mulation had no effect on the rate of I-Cl,I-swell activation. The effects of stimulation of PKA and PKC on I-Cl,I-swell were additive (i.e., decrease in the rate of activation and reduction in steady-state I-Cl,I-swell). The effects of PKA and PKC stimulation mere absent in cells expressing the pho sphorylation-defective mutant. In summary, it is Likely that phosphorylatio n of MDR1 by PKA and by PKC alters swelling-activated Cl- channels by indep endent mechanisms and that Ser-661, Ser-667, and Ser-671 are involved in th e responses of I-Cl,I-swell to stimulation of PKA. and PKC. These results s upport the notion that MDR1 phosphorylation affects I-Cl,I-swell.