Product Toxins Functions as a Co-Transporter of Glutathione and Natural Evidence That the Multidrug Resistance Protein (MRP)

The MRP (multidrug resistance protein) gene, a member of the ubiq ultous superfamily of ATP-binding cassette transporters, is associated with the multidrug resistance of mammalian cells to natural product anticancer agents. We have previously shown that abrogation of MRP expression by gene targeting leads to hypersensitivity to several drugs. In two independently produced MB? double knockout clones, the baseline export of glutathione (GSH) was one-half that of wild-type embryonic stem (ES) ceHs.The export of GSH from wild-type ES cells, but not from the MRP double knockout clones, increased in the presence of etoposide (VP.16) and sodium arsenite, accompanied by equivalent decreases in intracellular levels of GSH. In the two MRP double knockout clones, the intracellular steady-state concentration of etoposide was twofold greater than that in wild-type cells. Depletion of intracellular GSH by D,L-buthi onine sulfoximine increased the intracellular accumulation of radiolabeled etoposide in parental ES cells up to the level present in the two MRP knockout clones but did not change etoposide levels in the MRP knockout clones. These observations provide evidence that: (a) MRP exports GSH physiologically, presumably in association with an endogenous com pound(s); (b) baseline MRP expression protects cells from the toxic effects of xenobiotics by effluxing the xenobiotics and GSH from the intracellular compartment into the extracellular medium by a co-transport mocha nism; and (c) disruption of the gene encoding MRP abrogates the co transport of xenobiotics and GSH.