Modulation of P-glycoprotein-mediated drug transport by alterations in lipid fluidity of rat liver canalicular membrane vesicles.

P-glycoprotein (P-gp) is believed to function as an ATP-dependent efflux pump for natural product anti-cancer drugs in multidrug-resistant (MDR) tumor cells and in certain normal tissues. P-gp has been localized to the apical plasma membrane of the bile canaliculus where it has been shown to transport 3Hdaunomycin. In this study, we investigated whether alterations in membrane lipid fluidity of canalicular membrane vesicles (CMV) could modulate the P-gp-mediated accumulation of 3Hdaunomycin and 3Hvinblastine. Accumulation of both cytotoxic agents was stimulated by ATP, exhibited temperature dependence and osmotic sensitivity, and followed Michaelis-Menten kinetics. Alterations in CMV lipid fluidity were induced by the known fluidizers, 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)octanoate (A2C) and benzyl alcohol, and were assessed by fluorescence polarization techniques using the fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene (DPH). Both A2C (2.5-5.0 microM) and benzyl alcohol (10-20 mM) produced a dose-dependent increase in CMV lipid fluidity. Moreover, both fluidizers, at the above doses, significantly inhibited (p < 0.05) the ATP-dependent accumulation of 3Hdaunomycin. 3HVinblastine accumulation was also inhibited by A2C (p < 0.05). Lower doses of A2C (0.6 microM) and benzyl alcohol (1 mM) failed to influence either lipid fluidity or P-gp-mediated drug accumulation. Kinetic analysis revealed that A2C (5.0 microM) noncompetitively inhibited 3Hdaunomycin accumulation and uncompetitively inhibited 3Hvinblastine accumulation with apparent Ki values of approximately 1.5 and approximately 1.2 microM, respectively. Verapamil competitively inhibited P-gp-mediated accumulation of 3Hdaunomycin but failed to alter the fluidity of CMV. Taken together, the present results demonstrate that while increases in membrane fluidity of CMV are not necessarily required to inhibit P-gp-mediated drug accumulation, they can inhibit these processes, at least in CMV. Alterations in the physical state of CMV, therefore, appear to be at least one important modulator of P-gp function.