Probing the interaction of the multidrug-resistance phenotype with the polypeptide ionophore gramicidin D via functional channel formation.

It has been proposed that the multidrug resistance (MDR) transporter, P-glycoprotein (P-170), may be physiologically involved in the transport of polypeptides. As a step towards understanding the interaction of P-170 with polypeptides, we isolated various gramicidin-D-resistant mammalian cell lines. Gramicidin D is a hydrophobic pentadecapeptide ionophore that forms proton and alkali metal cation-permeable channels in lipid bilayers. Gramicidin-D-resistant cells displayed a prominent MDR gene amplification, P-170 overexpression, reduced drug accumulation, and consequent resistance to MDR-type cytotoxic agents. Modulators of the MDR phenotype, including verapamil, reserpine and quinidine, rendered these cells sensitive to gramicidin D. Using these cell lines, we established an assay that probes for the intra-membranal interaction between P-170 and gramicidin D. Gramicidin-D channel formation was followed by cellular accumulation of 86Rb+. Ionophore-resistant cells, and other MDR cells, did not show an appreciable increase in 86Rb+ influx rates, in the presence of increasing gramicidin-D concentrations. In contrast, parental cells displayed a dose-dependent increase in the 86Rb+ influx rates. Interestingly, in the absence of serum, gramicidin-D-resistant cells resumed the wild-type, ionophore-dose-dependent increase in 86Rb+ influx rates. MDR modulators caused a resumption of channel formation in ionophore-resistant cells. We conclude that acquisition of the MDR phenotype is an efficient means of cellular protection against gramicidin D. Hence, a new approach is offered in which P-170 interaction with gramicidin D is quantitatively followed by a rapid assessment of the biological activity (i.e. channel formation) of the substrate itself. Possible mechanisms of P-170 interaction with free ionophore monomers, and membrane-associated gramicidin D are discussed.