The human multidrug-resistance protein (MRP) gene family contains at least six members: MRP1, encoding the multidrug-resistance protein; MRP2 or cMOAT, encoding the canalicular multispecific organic anion transporter; and four homologs, called MRP3, MRP4, MRP5, and MRP6. In this report, we characterize MRP3, the closest homolog of MRP1. Cell lines were retrovirally transduced with MRP3 cDNA, and new monoclonal antibodies specific for MRP3 were generated. We show that MRP3 is an organic anion and multidrug transporter, like the GS-X pumps MRP1 and MRP2. In Two members of the large family of ABC transporters are known thus far to confer multidrug resistance in human cancer cells. These are the MDR1 P-glycoprotein (1) and the multidrugresistance protein MRP1 (2). Both membrane proteins transport a wide range of drugs with different cellular targets and confer resistance by decreasing the intracellular concentration of drugs. P-glycoprotein transports these drugs in unmodified form, whereas MRP1 can transport drugs either conjugated to anionic ligands such as glutathione (GSH), glucuronide, or sulfate, or in an unmodified form, possibly together with GSH. Well known substrates for MRP1 are, for example, cysteinyl leukotrienes, glutathione disulfide, S-(2,4-dinitrophenyl-)glutathione, ethacrynic acid S-glutathione, etoposide glucuronide, certain steroid glucuronides, and bile salt derivatives (3-6). Transporters with the characteristics of MRP1 are known as GS-X pumps (7) or multispecific organic anion transporters (8).Another GS-X pump is MRP2, a homolog of MRP1. Unlike MRP1, which is nearly ubiquitously expressed (9), MRP2 is present mainly in the canalicular membrane of hepatocytes (10), but is also present in other apical domains of polarized cells such as the epithelial cells of the proximal tubules of the kidney (11). Studies with mutant rats (TR Ϫ ͞GY or EHBR), which lack the MRP2 protein in the canalicular membrane of hepatocytes, have shown that the substrate specificity of MRP2 is very similar to that of MRP1 (12, 13). MRP2 also contributes to transport of anticancer drugs and some metals. The mutant rats showed a reduced biliary clearance of methotrexate (14), of the topoisomerase I inhibitor CPT-11 and its metabolites (15), and of mercury, cadmium, and arsenite (refs. 16 and 17; R.O.E., unpublished observation). Cells transduced with an MRP2 cDNA construct transport the cytostatic drug vinblastine (18). Moreover, overexpression of the MRP2 gene has been found in several cisplatinresistant cell lines (19,20), and transfection of an MRP2-antisense construct into liver cells was reported to confer an increased sensitivity to cytotoxic drugs (21). All these observations strongly suggest that MRP2 may confer multidrug resistance, but whether it does so in cancer patients remains to be established.Besides MRP1 and MRP2, there are at least four MRP homologs expressed in humans, called MRP3, MRP4, MRP5, and MRP6 (20,22). Not much is known about the substrate specificity of these putative new transporters...
