Effect of Calphostin C (PKC inhibitor) on daunorubicin resistance in P388/ADR and HL60/AR cells: reversal of drug resistance possibly via P-glycoprotein.

Calphostin C is a potent and specific inhibitor of protein kinase C (PKC). In this investigation we examined the effect of Calphostin C (without prior exposure to light) on daunorubicin (DNR) accumulation and sensitivity to DNR in multidrug-resistant (MDR) murine leukemia P388/ADR and human myeloid leukemia HL60/AR cells. P388/ADR cells overexpress P-glycoprotein, whereas HL60/AR cells lack any expression of P-glycoprotein (both at mRNA and protein levels). Calphostin C, in a concentration-dependent manner, increased the accumulation of DNR in P388/ADR cells and partially reversed (threefold) the DNR resistance in P388/ADR cells but had no effect on either of the parameters in HL60/AR cells. Calphostin C-induced increased accumulation of DNR in P388/ADR cells was due to increased uptake and decreased efflux of DNR. Furthermore, Calphostin C increased the uptake and decreased the efflux of rhodamine 123 (a substrate for P-gp) in P388/ADR cells but had no such effect in P388 cells. In addition, Calphostin C without exposure to light did not inhibit PKC activity in any of the cell lines studied. Taken together, these data suggest that Calphostin C may reverse drug resistance via P-glycoprotein independently of its effect on PKC activity. Therefore, any data regarding the effect of Calphostin C on the reversal of MDR should be interpreted in the light of these findings.