ALLOSTERIC REGULATION OF [H-3] VINBLASTINE BINDING TO P-GLYCOPROTEIN OF MCF-7 ADR CELLS BY DEXNIGULDIPINE

Plasma membranes were prepared from the P-glycoprotein expressing huma n breast cancer cell line MCF-7 ADR. [H-3]Vinblastine bound to these m embranes saturably with a B-max of 24 pmol/ mg of protein and K-D of 2 3 nM. In contrast, membranes from the parent cells MCF-7 WT, which do not express P-glycoprotein, did not bind [3H]vinblastine with high aff inity. Cytotoxics known to be transported by P-glycoprotein inhibited the binding of [3H]vinblastine, as did multidrug reversing agents incl uding the 1,4-dihydropyridine, dexniguldipine-HCl (K-i, 15 nM). In dis sociation kinetic experiments, dexniguldipine-HCl accelerated the diss ociation of [H-3]vinblastine from P-glycoprotein, indicating a negativ e heterotropic allosteric mechanism of action through a drug binding s ite distinct from that of vinblastine. Other 1,4-dihydropyridines test ed also accelerated [H-3]vinblastine dissociation from P-glycoprotein, however, multidrug reversing drugs of different chemical classes, inc luding quinidine, verapamil and cyclosporin A did not. These results s uggest that P-glycoprotein of MCF-7 ADR cell membranes possesses at le ast two drug acceptor sites which are allosterically coupled: receptor site-1 which binds vinca alkaloids, and receptor site-2 which binds 1 ,4-dihydropyridines such as dexniguldipine-HCl, which had the highest affinity of the tested derivatives.