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Differential Effects of P-Glycoprotein Inhibitiors on NIH3T3 Cells Transfected with Wild-Type (G185) or Mutant (V185) Multidrug Transporters

Laboratory of Molecular Biology [C. O. C., I. P.] and Laboratory of Cell Biology [I. A., M. M. G.], Division of Cancer Biology, Diagnosis and Centers, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-4255

Multidrug resistance (MDR) may be associated with the expression of the MDR1 gene which encodes the 170-kDa cell surface P-glycoprotein (PGP) acting as an energy-dependent multidrug efflux pump. This pump can be inhibited by a variety of drugs including cyclosporin A, quinidine, and verapamil. Substrate specificity of the MDR1 gene product can be altered by a point mutation at amino acid residue 185 in which valine is substituted for glycine, but the effect of this mutation on inhibition of PGP is unknown. Multidrug-resistant NIH3T3 cells transfected with the MDR1 retroviral vector pHaMDR-1/A (G185) or pHaMDR1/A (V185) expressing comparable levels of PGP were compared for patterns of drug resistance and inhibition of drug resistance by MDR reversing agents. The NIH-MDR-G185 transfectants were somewhat preferentially resistant to daunorubicin, taxol, and vinblastine. The mutant (V185) conferred increased resistance to colchicine. This MDR phenotype in both NIH-MDR-G185- and NIH-MDR-V185-transfected NIH3T3 cells was overcome by the addition of cyclosporin A, quinidine, or verapamil. Verapamil was the most potent of the three agents affecting wild-type PGP. However, specific inhibitors showed different potency with wild-type or mutant transporters, depending on the cytotoxic drug whose resistance was being reversed. For example, cyclosporin A at a concentration of 1 µg/ml was a powerful reverser of taxol and colchicine resistance for the mutant drug transporter, but was much less effective for the wild-type transporter. In contrast, verapamil reversed resistance to vinblastine more efficiently for the wild-type transporter than for the mutant transporter. These results suggest that the sensitivity of a multidrug transporter to a reversing agent will depend on the reversing agent, the cytotoxic drug, and the presence or absence of mutations which alter substrate specificity.

¹ To whom requests for reprints should be addressed, at Laboratory of Cell Biology, Building 37, Room 1B22, National Cancer Institute, National Institutes of Health, 37 Convent Drive MSC 4255, Bethesda, MD 20892-4255.

Received 9/ 1/94. Accepted 1/ 5/95.

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