Selenium-dependent and -independent transport of arsenic by the human multidrug resistance protein 2 (MRP2/ABCC2): implications for the mutual detoxification of arsenic and selenium

Simultaneous exposure of lab animals to toxic doses of the human carcinogen arsenic (As) and the essential trace element selenium (Se) results in a remarkable mutual detoxification. A likely basis for this is the in vivo formation and biliary excretion of seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-); however, the transport protein responsible for the biliary efflux of [(GS)(2)AsSe](-) has not been identified. The multidrug resistance protein 2 (MRP2/ABCC2) is an adenosine triphosphate (ATP)-binding cassette transporter expressed at the canalicular membrane of hepatocytes. Rat Mrp2 is known to excrete the As glutathione (GSH/GS-) conjugates arsenic triglutathione [As(GS)(3)] and monomethyl arsenic diglutathione [CH(3)As(GS)(2)] into bile, and in vitro studies have established As(GS)(3) as a substrate for human MRP2. In the present study, membrane vesicles prepared from human embryonic kidney (HEK293T) cells transfected with human MRP2 were used to demonstrate that MRP2 transports [(GS)(2)AsSe](-). In addition, the characteristics of MRP2 transport of As(GS)(3) and [(GS)(2)AsSe](-) were investigated. As(GS)(3) and [(GS)(2)AsSe](-) are chemically labile and have the potential to dissociate. However, arsenite (As(III)) +/- selenite (Se(IV)) transport was not detected in the absence of GSH or in the presence of the non-reducing GSH analog, ophthalmic acid, suggesting that the conjugates are the transported forms. The apparent K(m) values for [(GS)(2)AsSe](-) and As(GS)(3) were 1.7 and 4.2 mu M, respectively, signifying high relative affinities. Membrane vesicles prepared from human erythrocytes, which express the MRP2-related MRP1/ABCC1, MRP4/ABCC4 and MRP5/ABCC5, transported As(GS)(3) in an MRP1- and ATP-dependent manner but did not transport [(GS)(2)AsSe](-). These results have important implications for the Se-dependent and -independent disposition of As.