Role of MRP2 and GSH in intrahepatic cycling of toxins

MRP2 is a canalicular transporter in hepatocytes mediating the transport of a wide spectrum of amphipathic compounds. This includes organic anions but also compounds complexed with GSH as, e.g. alpha -naphthylisothiocyanate ( ANIT) and arsenite. These reversible complexes may fall apart in bile after MRP2-mediated transport, which induces high concentrations of the toxic co mpound in the biliary tree. To further investigate the role of MRP2 in tran sport and toxicity of both compounds, we conducted experiments in transduce d polarized epithelial cells and in vivo, using the Mrp2-deficient TR- rat as a model. Our results show, that in MRP2-transduced MDCK II cells both co mpounds induce disproportionally strong apical GSH secretion. This inductio n of GSH secretion was not observed in the parent cells lacking MRP2 expres sion. This indicated that after transport via MRP2 both complexes released GSH upon which the compound could re-enter the cells. The resulting cycling of both toxins led to concentration dependent GSH depletion of the cells. To further test our hypothesis we administered arsenite (12.5 mu mol absolu te i.v.) to Wistar and Mrp2-deficient TR- rats and collected bile. While bo th arsenite and GSH secretion were absent in TR- rats, the total secretion of arsenite into Wistar bile (2.91 mu mol) was accompanied by a excess secr etion of 24 mu mol GSH, indicating that arsenite undergoes multiple cycles of GSH complexation. We also administered ANIT to both animal models and co uld show that TR- rats are protected from ANIT induced cholestasis. This in dicates that Mrp2-mediated biliary secretion of GS-ANIT is a prerequisite f or development of cholestasis in rats. We hypothesize that the toxic parent compound ANIT is regenerated in the biliary tree where it can exert its to xic properties on bile duct epithelial cells. (C) 2001 Elsevier Science Ire land Ltd. All rights reserved.