Protein kinase C (PKC) has been considered for a potential target of anticancer chemotherapy. PKC-alpha has been associated with growth and metastasis of some cancer cells. However, the role of PKC-alpha in human breast cancer cell proliferation and anticancer chemotherapy remains unclear. In this study, we examined whether alterations of PKC-alpha by phorbol esters and PKC inhibitors could affect proliferation of human breast cancer MCF-7 cells and the cytotoxic effect of chemotherapeutic agents. Exposure for 24 h to doxorubicin (DOX) and vinblastine (VIN) caused a concentration-dependent reduction in proliferation of MCF-7 cells. However, these two anticancer drugs altered cellular morphology and growth pattern in distinct manners. Phorbol 12,13-dibutyrate (PDBu, 100 nM), which enhanced activities of PKC-alpha, increased cancer cell proliferation and attenuated VIN (1 microM)-induced cytotoxicity. These effects were not affected in the presence of 10 nM staurosporine. Phorbol myristate acetate (PMA, 100 nM) that completely depleted PKC-alpha also enhanced cancer cell proliferation and attenuated VIN-induced cytotoxicity. Three potent PKC inhibitors, staurosporine (10 nM), chelerythrine (5 microM) and bisindolylmaleimide-I (100 nM), had no significant effect on MCF-7 cell proliferation; staurosporine and chelerythrine, but not bisindolylmaleimide-I, attenuated VIN-induced cytotoxicity. Moreover, neither phorbol esters nor PKC inhibitors had an effect on cytotoxic effects of DOX (1 microM) on MCF-7 cell proliferation. Thus, these data suggest that MCF-7 cell proliferation or the anti-cancer action of DOX and VIN on breast cancer cells is independent of PKC-alpha.