INTERACTION OF MULTIDRUG-RESISTANT CHINESE-HAMSTER OVARY CELLS WITH THE PEPTIDE IONOPHORE GRAMICIDIN-D

A major form of multidrug resistance results from the overexpression o f P-glycoprotein, a 170 kDa membrane protein. Multidrug resistant (MDR ) Chinese hamster ovary (CHO) cells and mdr1 transfectants displayed c ross-resistance to the channel-forming peptide ionophore gramicidin D, which was reversed by various chemosensitizers, thus directly implica ting P-glycoprotein as the mediator of resistance. However, gramicidin D was not able to inhibit [H-3]azidopine photolabelling of P-glycopro tein. MDR cells were not resistant to other pore-forming ionophores, b ut showed a modest level of cross-resistance to the mobile ionophore v alinomycin. There was no difference in I-125-gramicidin D uptake by re sistant and sensitive cells. Resistant cells showed lower Rb-86(+) upt ake, relative to the drug-sensitive parent. Addition of GmD increased both the rate and the level of Rb-86(+) uptake in sensitive cells, but had no effect on MDR cells. MDR cells also showed much lower rates of gramicidin D-dependent Rb-86(+) efflux than sensitive cells, and this was greatly increased by verapamil. These results suggest that P-glyc oprotein interferes with the formation of ion-conducting gramicidin D channels. In contrast, valinomycin had the same effect on gramicidin D -dependent cation efflux in MDR and sensitive cells. Gramicidin D is t hus unique among the ionophores in being a substrate for P-glycoprotei n, which appears to greatly reduce the formation of active dimeric cha nnels in the plasma membrane of MDR cells.