PHARMACOLOGICAL CHARACTERIZATION OF MULTIDRUG-RESISTANT MRP-TRANSFECTED HUMAN TUMOR-CELLS

We have previously identified and characterized a novel member of the ATP-binding cassette superfamily of transport proteins, multidrug resi stance protein (MRP), and subsequently demonstrated that its overexpre ssion is sufficient to confer multidrug resistance on previously sensi tive cells (Cole et al., Science (Washington DC), 258: 1650-1654, 1992 ; Grant et al., Cancer Res. 54: 357-361, 1994). In the present study, we have transfected two different eukaryotic expression vectors contai ning MRP complementary DNA into HeLa cells to study the pharmacologica l phenotype produced exclusively by overexpression of human MRP. The d rug resistance patterns of the two MRP-transfected cell populations we re similar. They were characterized by a moderate (5- to 15-fold) leve l of resistance to doxorubicin, daunorubicin, epirubicin, vincristine, and etoposide, and a low (less than or equal to 3-fold) level of resi stance to taxol, vinblastine, and colchicine. The transfectants were n ot resistant to 9-alkyl anthracyclines, mitoxantrone, or cisplatin. Th e MRP-transfected cells mere also resistant to some heavy metal anions including arsenite, arsenate, and trivalent and pentavalent antimonia ls but mere not resistant to cadmium chloride. Accumulation of radiola beled vincristine was reduced by 45% in the MRP-transfected cells and could be restored to the levels found in sensitive cells by depletion of ATP. Rates of vincristine efflux did not differ greatly in the sens itive and resistant cells. The cytotoxic effects of vincristine and do xorubicin could be enhanced in a dose-dependent fashion by coadministr ation of verapamil. Cyclosporin A also increased vincristine toxicity but had less effect on doxorubicin toxicity. The degree of chemosensit ization by verapamil and cyclosporin A was similar in MRP-transfected cells and in cells transfected with the vector alone, suggesting that sensitization involved mechanisms independent of MRP expression. Verap amil and cyclosporin A caused a modest increase in vincristine accumul ation in the resistant cells but did not restore levels to those of th e sensitive cells. Taken together, these data indicate that drug-resis tant cell lines generated by transfection with MRP complementary DNA d isplay some but not all of the characteristics of MRP-overexpressing c ell lines produced by drug selection in vitro. They further demonstrat e that the multidrug resistance phenotype conferred by MRP is similar but not identical to that conferred by P-glycoprotein and includes res istance to arsenical and antimonial oxyanions.