Selective modulation of P-glycoprotein's ATPase and anion efflux regulation activities with PKC alpha and PKC epsilon in Sf9 cells

The modulation of P-glycoprotein's (Pgp) ATPase activity and its ability to regulate swelling-activated I-125 efflux, by PKC alpha and PKC epsilon, wa s examined in insect cells. Recombinant baculovirus was used to express hum an Pgp in Sf9 cells and Pgp was also coexpressed with either PKC alpha or P KC epsilon. ATPase assays showed the enzyme activity of Pgp to be elevated during co-expression with the Ca2+ dependent isoform PKC alpha, but not wit h the Ca2+ independent variant PKC epsilon. Furthermore, neither isoform, w hen co-expressed with Pgp, altered the swelling-activated efflux of I-125 f rom Sf9 cells. However, in cells co-expressing Pgp/PKC (alpha or epsilon), pre-treatment with the phorbol ester TPA significantly reduced the swelling -activated I-125 efflux with both PKC isoforms. Our results suggest that ph osphorylation with the Ca2+ independent variant PKC epsilon does not regula te the ATPase activity of Pgp and that stimulation of PKC with TPA alters t he swelling-activated efflux of anions from insect cells expressing Pgp.