A Novel Pathway for Arsenic Elimination: Human Multidrug Resistance Protein 4 (MRP4/ABCC4) Mediates Cellular Export

Hundreds of millions of people worldwide are exposed to unacceptable levels of arsenic in drinking water. This is a public health crisis because arsenic is a Group I (proven) human carcinogen. Human cells methylate arsenic to monomethylarsonous acid (MMA), monomethylarsonic acid (MMA), dimethylarsinous acid (DMA), and dimethylarsinic acid (DMA). Although the liver is the predominant site for arsenic methylation, elimination occurs mostly in urine. The protein(s) responsible for transport of arsenic from the liver (into blood), ultimately for urinary elimination, are unknown. Human multidrug resistance protein 1 (MRP1/ABCC1) and MRP2 (ABCC2) are established arsenic efflux pumps, but unlike the related MRP4 (ABCC4) are not present at the basolateral membrane of hepatocytes. MRP4 is also found at the apical membrane of renal proximal tubule cells, making it an ideal candidate for urinary arsenic elimination. In the current study, human MRP4 expressed in HEK293 cells reduced the cytotoxicity and cellular accumulation of arsenate, MMA, MMA, DMA, and DMA while two other hepatic basolateral MRPs (MRP3 and MRP5) did not. Transport studies with MRP4-enriched membrane vesicles revealed that the diglutathione conjugate of MMA, monomethylarsenic diglutathione [MMA(GS)2], and DMA V were the transported species. MMA(GS)2 and DMA V transport was osmotically sensitive, allosteric (Hill coefficients of 1.46 0.2 and 2.96 1.2, respectively), and high affinity (K0.5 of 0.706 0.16 and 0.226 0.15mM, respectively). DMA transport was pH-dependent, with highest affinity and capacity at pH 5.5. These results suggest that human MRP4 could be a major player in the elimination of arsenic.