The MRP-related and BCRP/ABCG2 multidrug resistance proteins: biology, substrate specificity and regulation

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.

The Leishmania ATP-binding cassette protein PGPA is an intracellular metal-thiol transporter ATPase

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 glutathione-spermidine 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 PGPAHA 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.

Elevated levels of polyamines and trypanothione resulting from overexpression of the ornithine decarboxylase gene in arsenite-resistant Leishmania

The 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 amplification in Leishmania but, here, the mRNA and the enzymatic activity of ODC are increased without gene amplification. 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 amplified 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.

Increased transport of pteridines compensates for mutations in the high affinity folate transporter and contributes to methotrexate resistance in the protozoan parasite Leishmania tarentolae

Functional cloning led to the isolation of a novel methotrexate (MTX) resistance gene in the protozoan parasite Leishmania. The gene corresponds to orfG, an open reading frame (ORF) of the LD1/CD1 genomic locus that is frequently amplified in several Leishmania stocks. A functional ORF G-green fluorescence protein fusion was localized to the plasma membrane. Transport studies indicated that ORF G is a high affinity biopterin transporter. ORF G also transports folic acid, with a lower affinity, but does not transport the drug analog MTX. Disruption of both alleles of orfG led to a mutant strain that became hypersensitive to MTX and had no measurable biopterin transport. Leishmania tarentolae MTX-resistant cells without their high affinity folate transporters have a rearranged orfG gene and increased orfG RNA levels. Overexpression of orfG leads to increased biopterin uptake and, in folate-rich medium, to increased folate uptake. MTX-resistant cells compensate for mutations in their high affinity folate/MTX transporter by overexpressing ORF G, which increases the uptake of pterins and selectively increases the uptake of folic acid, but not MTX.

Gene amplification in Leishmania tarentolae selected for resistance to sodium stibogluconate

Leishmania tarentolae promastigotes were selected step by step for resistance to sodium stibogluconate (Pentostam). Mutants resistant to antimony-containing drugs and cross-resistant to arsenite were therefore obtained. Amplification of one common locus was observed in several independent sodium stibogluconate-resistant mutants, and the locus amplified was novel. The copy number of the amplified locus was related to the level of resistance to pentavalent antimony. The gene responsible for antimony resistance was isolated by transfection and was shown to correspond to an open reading frame coding for 770 amino acids. The putative gene product did not exhibit significant homology with sequences present in data banks, and the putative role of this protein in antimony resistance is discussed.

ABC transporters in Leishmania and their role in drug resistance

ABC transporters have been found in several parasitic protozoa including Leishmania. At least two Leishmania ABC transporters are involved in drug resistance. One is PgpA, which is involved in resistance to arsenic and antimony-containing compounds. Antimonials are the drug of choice against Leishmania infections. Transfection and biochemical studies suggest that PgpA recognizes metals conjugated to thiols. The second ABC transporter is closely related to mammalian P-glycoproteins and confers resistance to anticancer drugs by a mechanism that remains to be elucidated. Additional ABC transporters are likely to be present in Leishmania and these are discussed in relation to the phenomenon of antimony resistance.

Efflux systems and increased trypanothione levels in arsenite-resistant Leishmania

The mechanism of resistance to the metal arsenite has been studied and compared in L. mexicana, L. tropica, and L. tarentolae selected in a step by step manner for arsenite resistance. Amplification of the ABC transporter gene pgpA was found to be a frequent resistance mechanism in all species. Transfection of pgpA genes into different species indicated that both the origin of the pgpA gene and the recipient strain into which the gene is transfected seem important for resistance. An increase in the levels of trypanothione was also correlated with metal resistance in different Leishmania species. The mechanism used to increase the levels of trypanothione seems to differ, however, between the different species. This study points to a key role of transporters and thiol levels in metal resistance in Leishmania.

Co-amplification of the gamma-glutamylcysteine synthetase gene gsh1 and of the ABC transporter gene pgpA in arsenite-resistant Leishmania tarentolae

Resistance to the oxyanion arsenite in the parasite Leishmania is multifactorial. We have described previously the frequent amplification of the ABC transporter gene pgpA, the presence of a non-PgpA thiol-metal efflux pump and increased levels of glutathione and trypanothione in resistant cells. Other loci are also amplified, although their role in resistance is unknown. By gene transfection, we have characterized one of these novel genes. It corresponds to gsh1, which encodes gamma-glutamylcysteine synthetase, an enzyme involved in the rate-limiting step of glutathione biosynthesis. Transfection of gsh1 in wild-type cells increased the levels of glutathione and trypanothione to levels found in resistant mutants. These transfectants were not resistant to metals. However, when gsh1 was transfected in partial revertants, it conferred resistance. As pgpA is frequently co-amplified with gsh1, we co-transfected the two genes into both wild-type and partial revertants. Arsenite resistance levels in wild-type cells could be accounted for by the contribution of PgpA alone. In the partial revertant, the gsh1 and pgpA gene product acted synergistically. These results support our previous suggestion that PgpA recognizes metals conjugated to thiols. Furthermore, amplification of gsh1 overcomes the rate-limiting step in the synthesis of trypanothione, contributing to resistance. In addition, the results suggest that at least one more factor acts synergistically with the gsh1 gene product.

High level arsenite resistance in Leishmania tarentolae is mediated by an active extrusion system

Leishmania tarentolae cells selected for resistance to the oxyanions pentavalent or trivalent antimonials or to trivalent arsenicals exhibited cross-resistance to the other oxyanions. The basis for resistance in these mutants was studied by transport experiments using radioactive arsenite. All mutants exhibiting high level resistance to arsenite showed a marked decrease in the steady-state accumulation of arsenite. Decreased accumulation was also observed in antimonials-resistant mutants cross-resistant to various concentrations of arsenite. Cells depleted of endogenous energy reserves with metabolic inhibitors were loaded with radioactive arsenite; following addition of glucose, rapid efflux of arsenite was observed from arsenite mutant cells. Mutants resistant to high levels of arsenicals exhibited amplification of the P-glycoprotein related gene ltpgpA or of a linear amplicon of unknown function. However, the efflux-mediated arsenite resistance did not correlate with the amplification of the ltpgpA gene or with the presence of the linear amplicon. The calcium channel blocker verapamil and arsenite act in synergy in cells exhibiting the efflux system. Overall the oxyanion efflux system in Leishmania shares several properties with other resistance efflux systems mediated by transporters.