P-GLYCOPROTEIN CONFERS METHOTREXATE RESISTANCE IN 3T6 CELLS WITH DEFICIENT CARRIER-MEDIATED METHOTREXATE UPTAKE

P-glycoprotein (Pgp), a transmembrane efflux pump encoded by the MDR1 gene, transports various lipophilic drugs that enter the cell by passi ve diffusion through the lipid bilayer. Pgp-expressing multidrug-resis tant cell lines are not usually cross-resistant to a hydrophilic antif olate methotrexate (MTX). MTX enters cells primarily through a folate carrier, but passive diffusion becomes the primary mode of MTX uptake in carrier-deficient cells, To test if a deficiency in MTX carrier wou ld allow Pgp to confer resistance to MTX, a MTX carrier-deficient cell line (3T6-C26) was infected with a recombinant retrovirus expressing the human MDR1 gene. The infected 3T6-C26 cells showed increased survi val in MTX relative to uninfected cells, Multistep selection of the in fected cells with vinblastine led to increased Pgp expression and a co ncomitant increase in resistance to MTX, MTX resistance of Pgp-express ing 3T6-C26 cells was reduced by Pgp inhibitors, including a Pgp-speci fic monoclonal antibody UIC2, In contrast, the expression and the inhi bition of Pgp had no effect on MTX resistance in 3T6 cells with normal carrier-mediated MTX uptake, Thus, a deficiency in the MTX carrier en ables Pgp to confer resistance to MTX, suggesting that hydrophilic com pounds may become Pgp substrates when such compounds enter cells by pa ssive diffusion.