Verapamil stimulates glutathione transport by the 190-kDa multidrug resistance protein 1 (MRP1)

Multidrug resistance in tumor cells is often associated with reduced drug a ccumulation resulting from increased expression of the 190-kDa multidrug re sistance protein 1 (MRP1) or the 170-kDa P-glycoprotein. However, unlike P- glycoprotein, MRP1 is a primary active transporter of many conjugated organ ic anions, including the cysteinyl leukotriene LTC4. Moreover, agents such as verapamil that reverse P-glycoprotein-mediated resistance are often poor ly, or not at all, effective in MRP1-overexpressing cells. In the present s tudy, we investigated the effects of verapamil on MRP1-mediated transport p rocesses. We found that verapamil inhibited LTC4 transport into inside-out membrane vesicles prepared from MRP1-transfected cells in a competitive man ner, but only in the presence of reduced glutathione (GSH) or its nonreduci ng S-methyl derivative. In the presence of 1 mM GSH, the apparent K-i for v erapamil was 1.2 mu M, and in the presence of 100 mu M verapamil, the appar ent K-i for GSH was 77 mu M. Verapamil itself was not transported by MRP1 i n either intact cells or membrane vesicles. However, verapamil strongly sti mulated MRP1-mediated GSH uptake by membrane vesicles in a concentration-de pendent and osmotically sensitive manner that was inhibitable by MRP1-speci fic monoclonal antibodies. In the presence of 100 mM verapamil, the apparen t K-m and V-max for GSH uptake were 83 mu M and 55 pmol mg(-1) min(-1), res pectively. It is proposed that the variable ability of verapamil to modulat e MRP1-mediated resistance in different cell lines may be more closely link ed to its effect on the GSH status of the cells than on its ability to inhi bit the MRP1 transporter itself.