Mechanisms responsible for broad-based resistance to antitumor drugs derived from natural products (multidrug resistance) are incompletely understood. Agents known to reverse the multidrug-resistant phenotype (verapamil and trifluoperazine) can also inhibit the activity of protein kinase C. When we assayed human breast cancer cell lines for protein kinase C activity, we found that enzyme activity was 7-fold higher in the multidrug-resistant cancer cells compared with the control, sensitive parent cells. Exposure of drug-sensitive cells to the phorbol ester phorbol 12,13-dibutyrate [P(BtO)2] led to an increase in protein kinase C activity and induced a drug-resistance phenotype, whereas exposure of drug-resistant cells to P(BtO)2 further increased drug resistance. In sensitive cells, this increased resistance was accompanied by a 3.5-fold increased phosphorylation of a 20-kDa particulate protein and a 35-40% decreased intracellular accumulation of doxorubicin and vincristine. P(BtO)2 induced resistance to agents involved in the multidrug-resistant phenotype (doxorubicin and vincristine) but did not affect sensitivity to an unrelated alkylating agent (melphalan). The increased resistance was partially or fully reversible by the calcium channel blocker verapamil and by the calmodulin-antagonist trifluoperazine. These data suggest that stimulation of protein kinase C plays a role in the drug-transport changes in multidrug-resistant cells. This may occur through modulation of an efflux pump by protein phosphorylation.

Fine, R. L., Patel, J., & Chabner, B. A. (1988). Phorbol esters induce multidrug resistance in human breast cancer cells. Proceedings of the National Academy of Sciences, 85(2), 582–586.