EVIDENCE THAT THE MULTIDRUG-RESISTANCE PROTEIN (MRP) FUNCTIONS AS A COTRANSPORTER OF GLUTATHIONE AND NATURAL PRODUCE TOXINS

The MRP (multidrug resistance protein) gene, a member of the ubiquitou s superfamily of ATP-binding cassette transporters, is associated with the multidrug resistance of mammalian cells to natural product antica ncer agents, We have previously shown that abrogation of MRP expressio n by gene targeting leads to hypersensitivity to several drugs, In two independently produced MRP double knockout clones, the baseline expor t of glutathione (GSH) was one-bah that of wild-type embryonic stem (E S) cells, 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 intra cellular levels of GSH. In the two MRP double knockout clones, the int racellular steady-state concentration of etoposide was twofold greater than that in wild-type cells, Depletion of intracellular GSH by D,L-b uthionine sulfoximine increased the intracellular accumulation of radi olabeled etoposide in parental ES cells up to the level present in the two MRP knockout clones but did not change etoposide levels in the MR P knockout clones, These observations provide evidence that: (a) MRP e xports GSH physiologically, presumably in association with an endogeno us compound(s); (b) baseline MRP expression protects cells from the to xic effects of xenobiotics by effluxing the xenobiotics and GSH from t he intracellular compartment into the extracellular medium by a co-tra nsport mechanism; and (c) disruption of the gene encoding MRP abrogate s the co-transport of xenobiotics and GSH.