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currents
1 Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555-0641; and 2 Laboratoire Jean Maetz, Commissariat a l'Energie Atomique-Centre National de la Recherche Scientifique, 06230 Villefranche Sur Mer, France
Several proteins belonging to the ATP-binding cassette
superfamily can affect ion channel function. These include the cystic fibrosis transmembrane conductance regulator, the sulfonylurea receptor, and the multidrug resistance protein P-glycoprotein (MDR1).
We measured whole cell swelling-activated
Cl
currents
(ICl,swell) in
parental cells and cells expressing wild-type 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
ester reduced the rate of increase in
ICl,swell only in
cells that express MDR1. PKC stimulation had no effect on steady-state
ICl,swell.
Stimulation of protein kinase A (PKA) with 8-bromoadenosine
3',5'-cyclic monophosphate reduced steady-state ICl,swell only in
MDR1-expressing cells. PKA stimulation had no effect on the rate of
ICl,swell
activation. The effects of stimulation of PKA and PKC on
ICl,swell were
additive (i.e., decrease in the rate of activation and reduction in
steady-state
ICl,swell). The effects of PKA and PKC stimulation were absent in cells expressing the
phosphorylation-defective mutant. In summary, it is likely that
phosphorylation of MDR1 by PKA and by PKC alters swelling-activated Cl
channels by independent
mechanisms and that Ser-661, Ser-667, and Ser-671 are involved in the
responses of
ICl,swell to
stimulation of PKA and PKC. These results support the notion that MDR1
phosphorylation affects
ICl,swell.
multidrug resistance protein; adenosine 5'-triphosphate-binding cassette proteins; cystic fibrosis transmembrane conductance regulator; sulfonylurea receptor; chloride channels
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