Abstract

The ATP-binding cassette (ABC) transporter multidrug resistance protein 1 (MRP1/ABCC1) protects cells from arsenic (a proven human carcinogen) through the cellular efflux of arsenic triglutathione [As(GS)₃] and the diglutathione conjugate of monomethylarsonous acid [MMA(GS)₂]. Previously, differences in MRP1 phosphorylation (at Y920/S921) and N-glycosylation (at N19/N23) were associated with marked differences in As(GS)₃ transport kinetics between HEK293 and HeLa cell lines. In the current study, cell line differences in MRP1-mediated cellular protection and transport of other arsenic metabolites were explored. MRP1 expressed in HEK293 cells reduced the toxicity of the major urinary arsenic metabolite dimethylarsinic acid (DMA^V), and HEK-WT-MRP1-enriched vesicles transported DMA^V with high apparent affinity and capacity (K_m 0.19 µM, V_max 342 pmol⋅mg⁻¹protein⋅min⁻¹). This is the first report that MRP1 is capable of exporting DMA^V, critical for preventing highly toxic dimethylarsinous acid formation. In contrast, DMA^V transport was not detected using HeLa-WT-MRP1 membrane vesicles. MMA(GS)₂ transport by HeLa-WT-MRP1 vesicles had a greater than threefold higher V_max compared with HEK-WT-MRP1 vesicles. Cell line differences in DMA^V and MMA(GS)₂ transport were not explained by differences in phosphorylation at Y920/S921. DMA^V did not inhibit, whereas MMA(GS)₂ was an uncompetitive inhibitor of As(GS)₃ transport, suggesting that DMA^V and MMA(GS)₂ have nonidentical binding sites to As(GS)₃ on MRP1. Efflux of different arsenic metabolites by MRP1 is likely influenced by multiple factors, including cell and tissue type. This could have implications for the impact of MRP1 on both tissue-specific susceptibility to arsenic-induced disease and tumor sensitivity to arsenic-based therapeutics.