PROTEIN-KINASES AND MULTIDRUG-RESISTANCE

Background - Multidrug resistance is a phenomenon in which cancer cells become resistant to a number of natural product cancer chemotherapeutic agents. Multidrug resistant cells are characterized by the overexpression of a membrane protein, the P-glycoprotein. It is believed that this protein is involved in promoting drug efflux from cells. The P-glycoprotein is phosphorylated in vivo but the functional consequences of this phosphorylation are not known. Methods - We have tested the effects of activators and inhibitors of protein kinase C on the accumulation of cationic rhodamine dyes into a clonally selected, multidrug resistant CHO cell line, CH(R)C5. Results - Phorbol dibutyrate (PDBu) promotes dye uptake, but only at high concentrations. Another protein kinase C activator, dioctylglycerol, does not promote dye uptake. Furthermore, the effect of the 4-alpha isomer of phorbol dibutyrate, which is less potent than PDBu in activating protein kinase C, has comparable potency in promoting drug uptake in CH(R)C5 cells. In addition, preincubation of the cells with PDBu for 24 hours, which would be expected to cause the down-regulation of protein kinase C, has no effect on the ability of PDBu to promote dye uptake. The inhibitors of protein kinase C, staurosporine and H-7, have similar effects to the activator PDBu and promote dye uptake. Forskolin, an activator of adenylate cyclase, also promotes dye uptake. Conclusions - The results demonstrate that modulators of protein kinase cannot be indiscriminantly used directly as tools to elucidate the effect of signal transduction pathways on multidrug resistance. At least for CH(R)C5 cells, protein kinase C-catalyzed phosphorylation is not of major importance in the regulation of drug accumulation.