Several members of different families of the ATP-binding cassette (ABC) superfamily of transport proteins are capable of transporting an extraordinarily structurally diverse array of endo- and xenobiotics and their metabolites across cell membranes. Together, these transporters play an important role in the absorption, disposition and elimination of these chemicals in the body. In tumor cells, increased expression of these drug transporters is associated with resistance to multiple chemotherapeutic agents. In this review, current knowledge of the biochemical, physiological and pharmacological properties of nine members of the multidrug resistance protein (MRP)-related ABCC family (MRP1, MRP2, MRP3, MRP4, MRP5, MRP6, MRP7, ABCC11 and ABCC12) as well as the G family member, ABCG2/BCRP, are summarized. A focus is placed on the structural similarities and differences of these drug transporters as well as the molecular determinants of their substrate specificities and transport activities. Factors that regulate expression of the MRP-related proteins and ABCG2/BCRP are also reviewed.

Inorganic arsenic is an established human carcinogen, but its metabolism is incompletely defined. The ATP binding cassette protein, multidrug resistance protein (MRP1/ABCC1), transports conjugated organic anions (e.g. leukotriene C 4 ) and also co-transports certain unmodified xenobiotics (e.g. vincristine) with glutathione (GSH). MRP1 also confers resistance to arsenic in association with GSH; however, the mechanism and the species of arsenic transported are unknown. Using membrane vesicles prepared from the MRP1-overexpressing lung cancer cell line, H69AR, we found that MRP1 transports arsenite (As III ) only in the presence of GSH but does not transport arsenate (As V ) (with or without GSH). The non-reducing GSH analogs L-␥-glutamyl-L-␣-aminobutyryl glycine and S-methyl GSH did not support As III transport, indicating that the free thiol group of GSH is required. GSH-dependent transport of As III was 2-fold higher at pH 6.5-7 than at a more basic pH, consistent with the formation and transport of the acidstable arsenic triglutathione (As(GS) 3 ). Immunoblot analysis of H69AR vesicles revealed the unexpected membrane association of GSH S-transferase P1-1 (GSTP1-1). Membrane vesicles from an MRP1-transfected HeLa cell line lacking membrane-associated GSTP1-1 did not transport As III even in the presence of GSH but did transport synthetic As(GS) 3 . The addition of exogenous GSTP1-1 to HeLa-MRP1 vesicles resulted in GSH-dependent As III transport. The apparent K m of As(GS) 3 for MRP1 was 0.32 M, suggesting a remarkably high relative affinity. As(GS) 3 transport by MRP1 was osmotically sensitive and was inhibited by several conjugated organic anions (MRP1 substrates) as well as the metalloid antimonite (K i 2.8 M). As(GS) 3 transport experiments using MRP1 mutants with substrate specificities differing from wild-type MRP1 suggested a commonality in the substrate binding pockets of As(GS) 3 and leukotriene C 4 . Finally, human MRP2 also transported As(GS) 3 . In conclusion, MRP1 transports inorganic arsenic as a tri-GSH conjugate, and GSTP1-1 may have a synergistic role in this process.

The Leishmania ATP-binding cassette (ABC) transporter PGPA is involved in metal resistance (arsenicals and antimony), although the exact mechanism by which PGPA confers resistance to antimony, the first line drug against Leishmania, is unknown. The results of co-transfection experiments, transport assays, and the use of inhibitors suggest that PGPA recognizes metals conjugated to glutathione or trypanothione, a glutathionespermidine conjugate present in Leishmania. The HA epitope tag of the influenza hemagglutinin as well as the green fluorescent protein were fused at the COOH terminus of PGPA. Immunofluorescence, confocal, and electron microscopy studies of the fully functional tagged molecules clearly indicated that PGPA is localized in membranes that are close to the flagellar pocket, the site of endocytosis and exocytosis in this parasite. Subcellular fractionation of Leishmania tarentolae PG-PAHA transfectants was performed to further characterize this ABC transporter. The basal PGPA ATPase activity was determined to be 115 nmol/mg/min. Transport experiments using radioactive arsenite-glutathione conjugates clearly showed that PGPA recognizes and actively transports thiol-metal conjugates. Overall, the results are consistent with PGPA being an intracellular ABC transporter that confers arsenite and antimonite resistance by sequestration of the metal-thiol conjugates.

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SummaryThe levels of trypanothione, a glutathione±spermidine conjugate, are increased in the protozoan parasite Leishmania selected for resistance to the heavy metal arsenite. The levels of putrescine and spermidine were increased in resistant mutants. This increase is mediated by overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis. Gene overexpression is generally mediated by gene ampli®cation in Leishmania but, here, the mRNA and the enzymatic activity of ODC are increased without gene ampli®cation. This RNA overexpression is stable when cells are grown in the absence of the drug and does not result from gene rearrangements or from an increased rate of RNA synthesis. Transient transfections suggest that mutations in the revertant cells contribute to these elevated levels of RNA. Stable transfection of the ODC gene increases the level of trypanothione, which can contribute to arsenite resistance. In addition to ODC overexpression, the gene for the ABC transporter PGPA is ampli®ed in the mutants. The co-transfection of the ODC and PGPA genes confers resistance in a synergistic fashion in partial revertants, also suggesting that PGPA recognizes metals conjugated to trypanothione.