Effects of the membrane dipole potential on the interaction of saquinavir with phospholipid membranes and plasma membrane receptors of Caco-2 cells

The combined use of the membrane surface potential fluorescent sensor fluor escein phosphatidylethanolamine (FPE) and the membrane dipole potential flu orescent sensor di-8-ANEPPS to characterize the interaction of molecules wi th model and cellular membranes and to asses the influence of the dipole po tential on the interaction is reported. The study of the human immunodefici ency virus protease inhibitor saquinavir with Caco-2 cells and phospholipid membranes reveals that the compound interacts with the lipidic bilayer of model membranes with a simple hyperbolic binding profile but with Caco-2 ce lls in a cooperative way involving membrane receptors. Additional studies i ndicated that colchicine acts as a competitor ligand to saquinavir and sugg ests, in agreement with other reports, that the identity of the saquinavir "receptor" could be P-glycoprotein or the multiple drug resistance-associat ed protein. The modification of the magnitude of the membrane dipole potent ial using compounds such as cholesterol, phloretin, and 6-ketocholestanol i nfluences the binding capacity of saquinavir. Furthermore, removal of chole sterol from the cell membrane using methyl-beta -cyclodextrin significantly decreases the binding capacity of saquinavir. Because removal of cholester ol from the cell membrane has been reported to disrupt membrane domains kno wn as "rafts," our observations imply that the membrane dipole potential pl ays an important role as a modulator of molecule-membrane interactions in t hese membrane structures. Such a role is suggested to contribute to the alt ered behavior of receptor-mediated signaling systems in membrane rafts.