Effects of Protein Kinase C Modulators on Multidrug Resistance in Human Glioma Cells

TO IDENTIFY THE role of protein kinase C (PKC) in multidrug resistance, the effects of phorbol-12-myristate-13-acetate (PMA), a PKC activator, or calphostin C, a PKC inhibitor, on intracellular vincristine accumulation and expression of P-glycoprotein phosphorylation were studied in one multidrug-resistant and three multidrug-sensitive human glioma cell lines. Basal PKC activities and immunoreactivities of PKC-α and −ζ were higher in multidrug-resistant cells than in multidrug-sensitive cells. There was no significant difference in the immunoreactivity of PKC-δ between multidrug-resistant and -sensitive cells, and immunoreactive PKC-β, −γ, and −ɛ were not detected in either multidrug-resistant or -sensitive cells. The treatment of multidrug-resistant cells with 100 nM PMA for 2 hours resulted in the activation not of PKC-ζ but of PKC-α, with concomitant decrease in vincristine accumulation and increase in P-glycoprotein phosphorylation. The exposure of multidrug-resistant cells to 100 nM PMA for 24 hours induced down-regulation not of PKC-ζ but of PKC-α, with concurrent decrease in vincristine accumulation, and reduced but still increased P-glycoprotein phosphorylation. The treatment of multidrug-resistant cells with 100 nM calphostin C for 2 hours decreased immunoreactive PKC-ζ and not immunoreactive PKC-α, inducing increase in vincristine accumulation, with concomitant decrease in P-glycoprotein phosphorylation. There was no evidence of significant change in vincristine accumulation in multidrug-sensitive cells treated with PMA or calphostin C. This may suggest that at least two isozymes of PKC, PKC-α and -ζ, are involved in P-glycoprotein phosphorylation and that vincristine efflux function in multidrug-resistant human glioma cells is closely associated with P-glycoprotein phosphorylation and is decreased by PKC inhibitor.