Regulation of Arsenic Triglutathione [As(GS) 3] Transport by the Human Multidrug Resistance Protein 1 (MRP1/ABCC1) Through Post‐translational Modification

Abstract

The 190‐kDa ATP‐binding cassette transporter, MRP1, is responsible for exporting a diverse array of substrates from the cell including glutathione conjugates. Arsenic, a high priority environmental carcinogen and clinical anti‐cancer agent, is a substrate of MRP1 in its trigluathione form [As(GS) 3]. We initially characterized As(GS) 3 transport by MRP1 using membrane vesicles isolated from a doxorubicin‐selected small cell lung cancer cell line (H69AR) and found it to be a high apparent affinity substrate (Km 0.3 μM). In the current study, we show that the Km of MRP1 for As(GS) 3 depends upon the cell line it is expressed in. Thus, wild‐type MRP1‐enriched membrane vesicles exhibit a >10‐fold difference in Km for As(GS) 3 (0.3 μM vs 4.4 μM) when prepared from transfected HeLa vs HEK293 cells, respectively. On the other hand, mutant MRP1 lacking all N‐linked glycosylation (sugar‐free) expressed in either cell line exhibits a low Km (0.3 – 0.6 μM). Furthermore, when prepared in the presence of phosphatase inhibitors, both wild‐type and sugar‐free MRP1‐enriched membrane vesicles from either HeLa or HEK293 cells exhibit a high Km for As(GS) 3 (3 – 6 μM). Together, these results suggest that the affinity of MRP1 for As(GS) 3 is influenced by cell type, phosphorylation, and glycosylation and elucidate potential implications for the relative detoxification of arsenic by different cell types and tissues.