CHARACTERIZATION OF PRENYLCYSTEINES THAT INTERACT WITH P-GLYCOPROTEINAND INHIBIT DRUG TRANSPORT IN TUMOR-CELLS

Prenyleysteine methyl esters that represent the C-terminal structures of prenylated proteins demonstrate specific substrate-like interaction s with P-glycoprotein (Zhang, L., Sachs, C. W., Fine, R. L., and Casey , P. J. (1994) J. Biol. Chem. 269, 15973-15976). The simplicity of the se compounds provides a unique system for probing the structural speci ficity of P-glycoprotein substrates. We have further assessed the stru ctural elements of prenylcysteines involved in the interaction with P- glycoprotein. Carboxyl group methylation, a modification in many preny lated proteins, plays an essential role of blocking the negative charg e at the free carboxylate. Substitution of the methyl ester with a met hyl amide or simple amide does not change the ability of the molecule to stimulate P-glycoprotein ATPase activity, but substitution with a g lycine is not tolerated unless the carboxyl group of glycine is methyl ated. The presence of a nitrogen atom, which is found in many P-glycop rotein substrates and modifiers, is also essential for prenylcysteines to interact with P-glycoprotein. The structure at the nitrogen atom c an, however, influence the type of interaction. Acetylation of the fre e amino group of prenylcysteine results in a significant loss in the a bility of prenylcysteines to stimulate P-glycoprotein ATPase activity. Instead, certain acetylated prenylcysteines behave as inhibitors of t his activity. In studies using MDR1-transfected human breast cancer ce lls, the acetylated prenylcysteine analogs inhibit P-glycoprotein-medi ated drug transport and enhance the steady-state accumulation of [H-3] vinblastine, [H-3]colchicine, and [H-3]taxol. These inhibitors do not, however, affect drug accumulation in parental cells. These studies pr ovide a novel approach for designing P-glycoprotein inhibitors that co uld prove effective in reversing the phenotype of multidrug resistance in tumor cells.