Plasma membrane glutathione transporters and their roles in cell physiology and pathophysiology

Abstract

Reduced glutathione (GSH) is critical for many cellular processes, and both its intracellular and extracellular concentrations are tightly regulated. Intracellular GSH levels are regulated by two main mechanisms: by adjusting the rates of synthesis and of export from cells. Some of the proteins responsible for GSH export from mammalian cells have recently been identified, and there is increasing evidence that these GSH exporters are multispecific and multifunctional, regulating a number of key biological processes. In particular, some of the multidrug resistance-associated proteins (Mrp/Abcc) appear to mediate GSH export and homeostasis. The Mrp proteins mediate not only GSH efflux, but they also export oxidized glutathione derivatives (e.g., glutathione disulfide (GSSG), S-nitrosoglutathione (GS-NO), and glutathione-metal complexes), as well as other glutathione S-conjugates. The ability to export both GSH and oxidized derivatives of GSH, endows these transporters with the capacity to directly regulate the cellular thiol-redox status, and therefore the ability to influence many key signaling and biochemical pathways. Among the many processes that are influenced by the GSH transporters are apoptosis, cell proliferation, and cell differentiation. This report summarizes the evidence that Mrps contribute to the regulation of cellular GSH levels and the thiol-redox state, and thus to the many biochemical processes that are influenced by this tripeptide.

Figure 1. Glutathione homeostasis in hepatocytes

GSH is synthesized in the cell cytosol from its precursor amino acids, glutamate, cysteine and glycine. Within the cell, it exists mainly (>98%) in the thiol-reduced form (GSH), but some is also present in the thiol-oxidized (GSSG) and as glutathione S-conjugates. After its synthesis, some of the GSH is delivered into specific intracellular compartments, including mitochondria and endoplasmic reticulum, but much of the GSH is delivered to extracellular spaces, namely blood and bile. Transport of GSH and its conjugates into bile is mediated largely by Mrp2, whereas Mrp1 and Oatp1 may contribute to GSH efflux into blood, although this is still poorly defined. In contrast to GSH synthesis, which occurs intracellularly, GSH degradation occurs exclusively in the extracellular space, and only on the surface of cells that express the ectoenzyme gamma-glutamyl transpeptidase (γGT). In the liver this enzyme is most abundant on the canalicular membrane of hepatocytes and on the apical membrane of bile duct cells. Once GSH and GSH-containing compounds are released from liver cells there is an efficient intrahepatic cycle of glutathione degradation and utilization consisting of: (a) extensive catabolism within biliary spaces, as well as within sinusoidal compartments of some species; (b) direct hepatic reabsorption of some of the breakdown products; and (c) intracellular utilization of the amino acids, or conversion of cysteine S-conjugates to mercapturic acids, i.e., N-acetylcysteine S-conjugates.

Figure 2. Four possible mechanisms by which GSH interacts with the Mrp proteins

(1) GSH itself is a substrate for the Mrp protein; (2) GSH is co-transport with another substrate; (3) Transport of some substrates is stimulated by or is dependent on GSH, but GSH itself is not transported; and (4) GSH transport is stimulated by the presence of drugs that are not themselves transported by the Mrp protein.