Contribution of the Leishmania P-glycoprotein-related gene ltpgpA to oxyanion 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.

Active efflux by multidrug transporters as one of the strategies to evade chemotherapy and novel practical implications of yeast pleiotropic drug resistance

Mankind is faced by the increasing emergence of resistant pathogens, including cancer cells. An overview of the different strategies adopted by a variety of cells to evade chemotherapy is presented, with a focus on the mechanisms of multidrug transport. In particular, we analyze the yeast network for pleiotropic drug resistance and assess the potentiality of this system for further understanding of the mechanism of broad specificity and for development of novel practical applications.

Reversal of heavy metal resistance in multidrug-resistant human KB carcinoma cells

Human KB carcinoma C-A120 cells that express multidrug resistance-associated protein (MRP) were cross-resistant to trivalent and pentavalent antimonials and arsenicals. Intracellular glutathione (GSH) content was higher in C-A120 than its parental KB-3-1 cell line. Glutathione-S-transferase (GST) was similar in both cell lines. Depletion of cellular GSH by treatment of the cells with the inhibitor of gamma-glutamylcysteine synthetase (gamma-GCS), buthione sulfoximine (BSO), significantly increased the sensitivity of both KB-3-1 and C-A120 cells to heavy metals. A pyridine analog, PAK-104P, almost completely reversed the resistance to antimonials and arsenicals in C-A120 cells. BSO at 100 microM or PAK-104P at 10 microM enhanced the accumulation of antimony potassium tartrate in C-A120 cells to the level of that in KB-3-1 cells without the agents. PAK-104P inhibited the ATP-dependent efflux of antimony potassium tartrate. These findings suggest that MRP transports antimony conjugated with GSH ATP-dependently outside the cells and PAK-104P inhibits the transporting activity of MRP.

Coordinated induction of MRP/GS-X pump and gamma-glutamylcysteine synthetase by heavy metals in human leukemia cells

We recently reported that GS-X pump activity, as assessed by ATP-dependent transport of the glutathione-platinum complex and leukotriene C4, and intracellular glutathione (GSH) levels were remarkably enhanced in cis-diamminedichloroplatinum(II) (cisplatin)-resistant human leukemia HL-60 cells (Ishikawa, T., Wright, C. D., and Ishizuka, H. (1994) J. Biol. Chem. 269, 29085-29093). Now, using Northern hybridization and RNase protection assay, we provide evidence that the multidrug resistance-associated protein (MRP) gene, which encodes a human GS-X pump, is expressed at higher levels in cisplatin-resistant (HL-60/R-CP) cells than in sensitive cells, whereas amplification of the MRP gene is not detected by Southern hybridization. Culturing HL-60/R-CP cells in cisplatin-free medium resulted in reduced MRP mRNA levels, but these levels could be induced to rise within 30 h by cisplatin and heavy metals such as arsenite, cadmium, and zinc. The increased levels of MRP mRNA were closely related with enhanced activities of ATP-dependent transport of leukotriene C4 (LTC4) in plasma membrane vesicles. The glutathione-platinum (GS-Pt) complex, but not cisplatin, inhibited ATP-dependent LTC4 transport, suggesting that the MRP/GS-X pump transports both LTC4 and the GS-Pt complex. Expression of gamma-glutamylcysteine synthetase in the cisplatin-resistant cells was also co-induced within 24 h in response to cisplatin exposure, resulting in a significant increase in cellular GSH level. The resistant cells exposed to cisplatin were cross-resistant to melphalan, chlorambucil, arsenite, and cadmium. These observations suggest that elevated expression of the MRP/GS-X pump and increased GSH biosynthesis together may be important factors in the cellular metabolism and disposition of cisplatin, alkylating agents, and heavy metals.

Inhibition by arsenite of anticancer drug cis-diamminedichloroplatinum(II) induced DNA repair and drug resistance in HeLa cells

We have previously reported a cisplatin-resistant HeLa variant cell line (HeLa/CPR) which exhibited an enhancement in repairing cisplatin-DNA adducts (Chao, 1994, Mol. Pharmacol. 45, 1137-1144). In this study, using this cell line, we investigated the modification, by arsenite, of cisplatin-induced cytotoxicity and DNA repair in the resistant cell line. By a sublethal dose of arsenite, cytotoxicity of the resistant cells was enhanced by 2.5-fold, compared to 1.62-fold in the parental cells. Using enzyme-linked immunosorbent assay (ELISA) and a monoclonal antibody specific for cisplatin-DNA adducts, we found that the resistant cells showed a 5.15-fold decrease in the adduct formation compared to the parental cells. However, in the presence of arsenite, the resistant cells showed only a 1.47-fold decrease in the adduct formation, indicating a more than 3-fold modification. Using host cell reactivation of transfected plasmid DNA carrying cisplatin damage (an indirect detection of DNA repair), arsenite also revealed a ∼2-fold modification of adduct formation in the resistant cells. In addition, the time-dependent potentiation of cytotoxicity by arsenite in both cell lines was parallel to the increase of adduct formation. These results indicate that arsenite is an effective modifier of cisplatin-induced resistance and enhanced DNA repair in HeLa/CPR cells. The results are consistent with the notion that the cisplatin-resistant phenotype in HeLa cells is mainly mediated by enhancement of DNA repair.

Molecular characterization of a P-glycoprotein-related tcpgp2 gene in Trypanosoma cruzi

We have cloned, sequenced and characterized a gene from Trypanosoma cruzi (Y strain), termed tcpgp2, which encodes a member of the ABC (ATP-binding cassette) superfamily of evolutionarily conserved transport proteins. The nucleotide sequence of the tcpgp2 gene was determined. It presents a 4602-bp open reading frame, coding for a 1534-amino acid protein, with a predicted molecular mass of 169,470 Da. The deduced amino acid sequence of tcpgp2 exhibited a remarkable homology with the P-glycoprotein-related genes of Leishmania tarentolae, the yeast cadmium factor (YCF1) and the human multidrug resistance-associated protein (MRP). Southern blot analysis using a specific probe indicated that the Tcpgp2 P-glycoprotein is encoded by a single copy gene which maps to a chromosome of about 900 kb. Northern blot analysis revealed that tcpgp2 gene is expressed as a polyadenylated transcript of approximately 5 kb in dividing amastigote and epimastigote forms; we did not detect the transcript in the non-dividing trypomastigote forms of the parasite. Gene transfection experiments in Leishmania tropica indicated that, under the conditions tested, tcpgp2 gene is not involved in drug resistance.

Trypanosomatid shuttle vectors: new tools for the functional dissection of parasite genomes

In the past five years, gene-transfer systems have been established for each of the medically important trypanosomatids: Leishmania sp, Trypanosoma brucei and T. cruzi. Transformation can be mediated by integration, which occurs exclusively by homologous recombination, or by episomal shuttle vectors. In this article, John Kelly will focus on recent progress in the development and applications of trypanosomatid shuttle vectors, ie. vectors which are maintained extrachromosomally and which are capable of autonomous replication in both trypanosomatid and bacterial hosts.

Multidrug resistance and P-glycoproteins in parasitic protozoa

Drug resistance has emerged as a devasting impediment to the treatment and control of diseases of parasitic origin. The underlying mechanisms that contribute to this drug resistance in field isolates, however, are poorly understood. Members of the P-glycoprotein gene (pgp) family have been identified, cloned, and sequenced in Plasmodia, Leishmania, and Entamoeba, and variations in pgp copy number and/or expression have been implicated as a basis for drug resistance in each of these genera. The spectrum of drugs to which parasitic protozoa containing amplified pgp genes and/or transcripts are refractory range from a phenotype similar to that observed with multidrug-resistant mammalian cells to those that are completely distinct. The availability of molecular probes to pgp genes provides valuable reagents to dissect the role of pgp gene amplification and overexpression in mediating drug resistance in parasitic protozoa and to determine the physiological function of P-glycoproteins in this clinically consequential group of human pathogens.

Resistance mechanisms to arsenicals and antimonials

Salts and organic derivatives of arsenic and antimony are quite toxic. Living organisms have adapted to this toxicity by the evolution of resistance mechanisms. Both prokaryotic and eukaryotic cells develop resistance when exposed to arsenicals or antimonials. In the case of bacteria resistance is conferred by plasmid-encoded arsenical resistance (ars) operons. The genes and gene products of the ars operon of the clinically-isolated conjugative R-factor R773 have been identified and their mechanism of action elucidated. The operon encodes an ATP-driven pump that extrudes arsenite and antimonite from the cells. The lowering of their intracellular concentration results in resistance. Arsenate resistance results from the action of the plasmid-encoded arsenate reductase that reduces arsenate to arsenite, which is then pumped out of the cell.

Resistance to arsenic compounds in microorganisms

Arsenic ions, frequently present as environmental pollutants, are very toxic for most microorganisms. Some microbial strains possess genetic determinants that confer resistance. In bacteria, these determinants are often found on plasmids, which has facilitated their study at the molecular level. Bacterial plasmids conferring arsenic resistance encode specific efflux pumps able to extrude arsenic from the cell cytoplasm thus lowering the intracellular concentration of the toxic ions. In Gram-negative bacteria, the efflux pump consists of a two-component ATPase complex. ArsA is the ATPase subunit and is associated with an integral membrane subunit, ArsB. Arsenate is enzymatically reduced to arsenite (the substrate of ArsB and the activator of ArsA) by the small cytoplasmic ArsC polypeptide. In Gram-positive bacteria, comparable arsB and arsC genes (and proteins) are found, but arsA is missing. In addition to the wide spread plasmid arsenic resistance determinant, a few bacteria confer resistance to arsenite with a separate determinant for enzymatic oxidation of more-toxic arsenite to less-toxic arsenate. In contrast to the detailed information on the mechanisms of arsenic resistance in bacteria, little work has been reported on this subject in algae and fungi.

The P-glycoprotein-related gene family in Leishmania

P-glycoprotein gene amplification has been described in several drug-resistant parasitic protozoa. The first P-glycoprotein related gene described in Leishmania was ltpgpA, a gene frequently amplified in arsenite resistant Leishmania. Hybridization experiments indicated that ltpgpA was part of a gene family. In addition to ltpgpA, four novel genes were cloned that are present in two loci: ltpgpB and ltpgpC tandemly linked to ltpgpA on a 800-kb chromosome; and ltpgpD and ltpgpE closely linked on a chromosome ranging from 950 kb to 1400 kb, depending on the Leishmania species. Another P-glycoprotein gene, homologous to the more recently described ldmdr1, was linked to ltpgpD and ltpgpE. Nucleotide sequencing of ltpgpB and ltpgpE revealed that the Leishmania P-glycoprotein-related genes have diverged considerably from the main branch of P-glycoproteins and are more homologous to the recently described multidrug resistance-associated protein found in multidrug-resistant human lung cancer cell lines. Cross-resistance studies and gene transfection experiments indicated that under the conditions tested only ltpgpA and ldmdr1 are involved in resistance to arsenite and antimonials or hydrophobic drugs such as vinblastine respectively.

Microbial multidrug-resistance ABC transporters

Multidrug resistance in tumor cells is often caused by the increased efflux of a wide variety of drugs, mediated by P glycoprotein, a member of the superfamily of ATP-binding cassette (ABC) transporters. The genes encoding members of this superfamily have also been isolated from drug-resistant microorganisms, and the role of microbial ABC transporters in drug resistance is being investigated.

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.