Abstract

The ATP-binding cassette (ABC) transporter protein multidrug resistance protein 1 (MRP1; ABCC1) plays an important role in the cellular efflux of the high-priority environmental carcinogen arsenic as a triglutathione conjugate [As(GS)₃]. Most mammalian cells can methylate arsenic to monomethylarsonous acid (MMA^III), monomethylarsonic acid (MMA^V), dimethylarsinous acid (DMA^III), and dimethylarsinic acid (DMA^V). The trivalent forms MMA^III and DMA^III are more reactive and toxic than their inorganic precursors, arsenite (As^III) and arsenate (As^V). The ability of MRP1 to transport methylated arsenicals is unknown and was the focus of the current study. HeLa cells expressing MRP1 (HeLa-MRP1) were found to confer a 2.6-fold higher level of resistance to MMA^III than empty vector control (HeLa-vector) cells, and this resistance was dependent on GSH. In contrast, MRP1 did not confer resistance to DMA^III, MMA^V, or DMA^V. HeLa-MRP1 cells accumulated 4.5-fold less MMA^III than HeLa-vector cells. Experiments using MRP1-enriched membrane vesicles showed that transport of MMA^III was GSH-dependent but not supported by the nonreducing GSH analog, ophthalmic acid, suggesting that MMA^III(GS)₂ was the transported form. MMA^III(GS)₂ was a high-affinity, high-capacity substrate for MRP1 with apparent K_m and V_max values of 11 μM and 11 nmol mg⁻¹min⁻¹, respectively. MMA^III(GS)₂ transport was osmotically sensitive and inhibited by several MRP1 substrates, including 17β-estradiol 17-(β-d-glucuronide) (E₂17βG). MMA^III(GS)₂ competitively inhibited the transport of E₂17βG with a K_i value of 16 μM, indicating that these two substrates have overlapping binding sites. These results suggest that MRP1 is an important cellular protective pathway for the highly toxic MMA^III and have implications for environmental and clinical exposure to arsenic.