Potent induction of human colon cancer cell uptake of chemotherapeutic drugs by N-myristoylated protein kinase C-alpha (PKC-alpha) pseudosubstrate peptides through a P-glycoprotein-independent mechanism.

Phorbol ester protein kinase C (PKC) activators and PKC isozyme over-expression have been shown to significantly reduce intracellular accumulation of chemotherapeutic drugs, in association with the induction of multidrug resistance (MDR) in drug-sensitive cancer cells and enhancement of drug resistance in MDR cancer cells. These observations constitute solid evidence that PKC plays a significant role in the MDR phenotype of cancer cells. PKC-catalyzed phosphorylation of the drug-efflux pump P-glycoprotein was recently ruled out as a contributing factor in MDR. At present, the sole drug transport-related event that has been identified as a component of the role of PKC in MDR is PKC-induced expression of the P-glycoprotein-encoding gene mdr1. The objective of this study was to test the hypothesis that PKC can modulate the uptake of chemotherapeutic drugs in cancer cells independently of P-glycoprotein. We analyzed the effects of selective PKC activators/inhibitors on the uptake of radiolabelled cytotoxic drugs by cultured human colon cancer cells that lacked P-glycoprotein activity and did not express the drug efflux pump at the level of message (mdr1) or protein. We found that the selective PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) significantly reduced uptake of [14C] Adriamycin and [3H] vincristine in human colon cancer cells devoid of P-glycoprotein activity, and that PKC-inhibitory N-myristoylated PKC-alpha pseudosubstrate synthetic peptides potently and selectively induced uptake of the cytotoxic drugs in the phorbol ester-treated and non-treated colon cancer cells. TPA treatment of the cells did not induce expression of either P-glycoprotein or its message mdr1. In contrast with [14C]Adriamycin and [3H] vincristine uptake, [3H] 5-fluorouracil uptake by the cells was unaffected by TPA and reduced by the PKC-inhibitory peptides. These results indicate that PKC activation can significantly reduce the uptake of multiple cytotoxic drugs by cancer cells independently of P-glycoprotein, and that N-myristoylated PKC-alpha pseudosubstrate peptides potently and selectively induce uptake of multiple cytotoxic drugs in cultured human colon cancer cells by a novel mechanism that does not involve P-glycoprotein and may involve PKC isozyme inhibition. Thus, N-myristoylated PKC-alpha pseudosubstrate peptides may offer a basis for the development of agents that reverse intrinsic drug resistance in human colon cancer.