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 activato r, or calphostin C, a PKC inhibitor, on intracellular vincristine accu mulation 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-alpha and -zeta were higher in multidrug-resistant cells than in multidrug-s ensitive cells. There was no significant difference in the immunoreact ivity of PKC-delta between multidrug-resistant and -sensitive cells, a nd immunoreactive PKC-beta, -gamma, and -epsilon were not detected in either multidrug-resistant or -sensitive cells. The treatment of multi drug-resistant cells with 100 nM PMA for 2 hours resulted in the activ ation not of PKC-zeta but of PKC-alpha, with concomitant decrease in v incristine 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-zeta but of PKC-alpha, with concur rent decrease in vincristine accumulation, and reduced but still incre ased P-glycoprotein phosphorylation. The treatment of multidrug-resist ant cells with 100 nM calphostin C for 2 hours decreased immunoreactiv e PKC-zeta and not immunoreactive PKC-alpha, inducing increase in vinc ristine accumulation, with concomitant decrease in P-glycoprotein phos phorylation. There was no evidence of significant change in vincristin e accumulation in multidrug-sensitive cells treated with PMA or calpho stin C. This may suggest that at least two isozymes of PKC, PKC-alpha and -zeta, are involved in P-glycoprotein phosphorylation and that vin cristine efflux function in multidrug-resistant human glioma cells is closely associated with P-glycoprotein phosphorylation and is decrease d by PKC inhibitor.