P-glycoprotein (P-gp) is believed to function as an ATP-dependent efflux pump for natural product anticancer 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 [3H]daunomycin. In this study, we investigated whether alterations in membrane lipid fluidity of canalicular membrane vesicles (CMV) could modulate the P-gp-mediated accumulation of [3H]daunomycin and [3H]vinblastine. 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 μM) 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 [3H]daunomycin. [3H]Vinblastine accumulation was also inhibited by A2C (p < 0.05). Lower doses of A2C (0.6 μM) and benzyl alcohol (1 mM) failed to influence either lipid fluidity or P-gp-mediated drug accumulation. Kinetic analysis revealed that A2C (5.0 μM) noncompetitively inhibited [3H]daunomycin accumulation and uncompetitively inhibited [3H]vinblastine accumulation with apparent K(i) values of ≃1.5 and ≃1.2 μM, respectively. Verapamil competitively inhibited P-gp-mediated accumulation of [3H]daunomycin 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.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - 1992|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology