Overexpression of the gene encoding the multidrug resistance-associated protein results in increased ATP-dependent glutathione S-conjugate transport

Active efflux of the free acid form of the fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in multidrug-resistance-protein-overexpressing murine and human leukemia cells

Murine and human cell lines overexpressing the multidrug-resistance protein (MRP) showed a marked decreased accumulation of the fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). In contrast, less altered accumulation was seen in the P-glycoprotein(P-gp)-overexpressing cell lines. The decreased drug accumulation was reversed by the energy inhibitors sodium azide/2-deoxyglucose and by the vinca alkaloid, vincristine, but not by the chemotherapeutic agents, etoposide and adriamycin. Decreased accumulation was linked to active efflux of the hydrophilic free acid form of BCECF from the MRP-overexpressing cell lines, indicating that dye extrusion occurs after the dye ester has been converted to the free acid form in the cytoplasm. The finding suggests that MRP mediates removal of substrates from a cytoplasmic location. Buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis, decreased the vincristine and etoposide resistance displayed by the MRP-expressing murine cell lines, but did not affect the accumulation of BCECF. Thus, while glutathione may be involved in MRP-mediated resistance to some chemotherapeutic agents, it is not necessary for effiux of substrates such as BCECF.

A new pathway for vacuolar cadmium sequestration in Saccharomyces cerevisiae: YCF1-catalyzed transport of bis(glutathionato)cadmium

The yeast cadmium factor (YCF1) gene encodes an MgATP-energized glutathione S-conjugate transporter responsible for the vacuolar sequestration of organic compounds after their S-conjugation with glutathione. However, while YCF1 was originally isolated according to its ability to confer resistance to cadmium salts, neither its mode of interaction with Cd2+ nor the relationship between this process and organic glutathione-conjugate transport are known. Here we show through direct comparisons between vacuolar membrane vesicles purified from Saccharomyces cerevisiae strain DTY167, harboring a deletion of the YCF1 gene, and the isogenic wild-type strain DTY165 that YCF1 mediates the MgATP-energized vacuolar accumulation of Cd-glutathione complexes. The substrate requirements, kinetics and Cd2+/glutathione stoichiometry of cadmium uptake and the molecular weight of the transport-active complex demonstrate that YCF1 selectively catalyzes the transport of bis(glutathionato)cadmium (Cd x +GS2). On the basis of these results--the Cd2+ hypersensitivity of DTY167, versus DTY165, cells, the inducibility of YCF1-mediated transport, and the rapidity and spontaneity of Cd-GS2 formation--this new pathway is concluded to contribute substantially to Cd2+ detoxification.

Regulation of organic anion transport in the liver

In several liver diseases the biliary transport is disturbed, resulting in, for example, jaundice and cholestasis. Many of these symptoms can be attributed to altered regulation of hepatic transporters. Organic anion transport, mediated by the canalicular multispecific organic anion transporter (cmoat), has been extensively studied. The regulation of intracellular vesicular sorting of cmoat by protein kinase C and protein kinase A, and the regulation of cmoat-mediated transport in endotoxemic liver disease, have been examined. The discovery that the multidrug resistance protein (MRP), responsible for multidrug resistance in cancers, transports similar substrates as cmoat led to the cloning of a MRP homologue from rat liver, named mrp2. Mrp2 turned out to be identical to cmoat. At present there is evidence that at least two mrp's are present in hepatocytes, the original mrp (mrp1) on the lateral membrane, and mrp2 (cmoat) on the canalicular membrane. The expression of mrp1 and mrp2 in hepatocytes appears to be cell-cycle-dependent and regulated in a reciprocal fashion. These findings show that biliary transport of organic anions and possibly other canalicular transport is influenced by the entry of hepatocytes into the cell cycle. The cloning of the gene for cmoat opens up new possibilities to study the regulation of hepatic organic anion transport.

Establishment by adriamycin exposure of multidrug-resistant rat ascites hepatoma AH130 cells showing low DT-diaphorase activity and high cross resistance to mitomycins

A resistant subline (AH130/5A) selected from rat hepatoma AH130 cells after exposure to adriamycin (ADM) showed remarkable resistance to multiple antitumor drugs, including mitomycin C (MMC) and porfiromycin (PFM). PFM, vinblastine (VLB), and ADM accumulated in AH130/5A far less than in the parent AH130 (AH130/P) cells. AH130/5A cells showed overexpression of P-glycoprotein (PGP), an increase in glutathione S-transferase activity, and a decrease in DT-diaphorase and glutathione peroxidase activity. The resistance to MMC and VLB of AH130/5A cells was partly reversed by H-87, an inhibitor of PGP. Buthionine sulfoximine, an inhibitor of glutathione synthase, did not affect the action of MMC. tert-Butylhydroquinone induced DT-diaphorase activity, increased PFM uptake, and enhanced the growth-inhibitory action of MMC in AH130/5A cells. Dicumarol, an inhibitor of DT-diaphorase, decreased PFM uptake and reduced the growth-inhibitory action of MMC in AH130/P cells. These results indicated that the adriamycin treatment of hepatoma cells caused multifactorial multidrug resistance involving a decrease in DT-diaphorase activity.

Hepatic glutathione and glutathione S-conjugate transport mechanisms

Glutathione (GSH) plays a critical role in many cellular processes, including the metabolism and detoxification of oxidants, metals, and other reactive electrophilic compounds of both endogenous and exogenous origin. Because the liver is a major site of GSH and glutathione S-conjugate biosynthesis and export, significant effort has been devoted to characterizing liver cell sinusoidal and canalicular membrane transporters for these compounds. Glutathione S-conjugates synthesized in the liver are secreted preferentially into bile, and recent studies in isolated canalicular membrane vesicles indicate that there are multiple transport mechanisms for these conjugates, including those that are energized by ATP hydrolysis and those that may be driven by the electrochemical gradient. Glutathione S-conjugates that are relatively hydrophobic or have a bulky S-substituent are good substrates for the canalicular ATP-dependent transporter mrp2 (multidrug resistance-associated protein 2, also called cMOAT, the canalicular multispecific organic anion transporter, or cMrp, the canalicular isoform of mrp). In contrast with the glutathione S-conjugates, hepatic GSH is released into both blood and bile. GSH transport across both of these membrane domains is of low affinity and is energized by the electrochemical potential. Recent reports describe two candidate GSH transport proteins for the canalicular and sinusoidal membranes (RcGshT and RsGshT, respectively); however, some concerns have been raised regarding these studies. Additional work is needed to characterize GSH transporters at the functional and molecular level.

Expression of multidrug-resistance-associated protein (MRP) and chemosensitivity in human gastric cancer

Evidence has accumulated that, in addition to the MDR1 gene-coded P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP) also mediates the multidrug resistance (MDR) of various human tumors. In the case of gastric cancer, there is little or no involvement of P-glycoprotein, and the mechanisms of MDR remain to be understood. To search for a possible relationship between expression of MRP and sensitivity to anti-cancer agents in gastric cancer, 4 gastric cancer cell lines, 43 human gastric carcinomas and 17 adjacent normal gastric tissue samples were analyzed. Expression of MRP mRNA was evaluated using reverse transcription PCR (RT-PCR) and Southern hybridization. Sensitivity of the test samples to the anti-cancer drugs cisplatin (CDDP), doxorubicin (DXR) and etoposide (VP-16) was examined using the MTT¿3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl [2H]-tetrazolium bromide¿ assay. Immunohistochemical staining with the use of the MRP antibody (MRPr1) was done to confirm the findings regarding the expression of mRNA levels. The MRP expression evaluated with RT-PCR and Southern hybridization as well as with immunohistochemical staining revealed that 23 of 43 gastric-cancer tissues (53.5%), 15 of 17 normal gastric tissues (88%) and 3 of 4 gastric-cancer cell lines (75%) were positive. The MTT assay showed that DXR was significantly more sensitive (p < 0.01) in gastric carcinoma tissues lacking MRP expression than in those with positive expression. The same tendency was seen with the other agents used. Of the cell lines, one which showed no MRP expression also had a higher sensitivity to CDDP, DXR and VP-16 than the other positive cases. These results show that MRP expression is involved in MDR of human gastric cancer and is inversely related to the chemosensitivity of tumor cells against some anticancer drugs.

The multidrug resistance-associated protein gene confers drug resistance in human gastric and colon cancers

To determine the expression of multidrug resistance-associated protein (MRP) gene and its role in gastric and colon cancers, we analyzed 10 gastric and 10 colon non-drug-selected cell lines and a similar number of tissue samples of these cancers. We compared the expression of MRP and mdrl mRNA in cell lines and tissues using reverse-transcriptase polymerase chain reaction. In mdrl-negative cells, the relationship between the level of MRP gene expression and sensitivity to anticancer drugs was examined. The effect of verapamil, an MRP-modulating agent, was also examined in these cells. The expression of MRP gene in gastric cancer cell lines varied from a low to a high level, but mdrl was not detected in any of these cell lines. Colon cancer cell lines expressed low to intermediate levels of MRP gene, and half of the cells co-expressed low to high levels of mdrl. In tissue samples, the expression pattern of the two multidrug resistance (MDR) genes was broadly similar to that described for the cell lines, except that most of the gastric cancer tissue samples did express low levels of mdrl. No significant correlation was observed between the level of MRP gene expression and sensitivity to anticancer drugs in gastric and colon cell lines. However, verapamil significantly increased the sensitivity to etoposide, doxorubicin and vincristine in cells highly expressing MRP gene. Our results indicate that MRP gene may be important in conferring MDR in gastric and colon cancer cells.

Acceleration of MRP-associated efflux of rhodamine 123 by genistein and related compounds

Multidrug resistance (MDR), caused by overexpression of either P-glycoprotein or the multidrug resistance protein (MRP), is characterised by a decreased cellular drug accumulation due to an enhanced drug efflux. In this study, we examined the effects of genistein and structurally related (iso)flavonoids on the transport of rhodamine 123 (Rh123) and daunorubicin in the MRP-overexpressing MDR lung cancer cell lines COR-L23/R and MOR/R. Genistein, genistin, daidzein and quercetin showed major differences in effects on Rh123 vs daunorubicin transport in the MRP-mediated MDR cell lines: the accumulation of daunorubicin was increased, whereas the accumulation of Rh123 was decreased by the flavonoids. The depolarisation of the membrane potential caused by genistein might be involved in the acceleration of the efflux of Rh123 measured in the MRP-overexpressing cell lines. These observations should be taken into account when using fluorescent dyes as probes for determination of transporter activity as a measure of MDR.

Differential expression of DNA topoisomerase II alpha and -beta in P-gp and MRP-negative VM26, mAMSA and mitoxantrone-resistant sublines of the human SCLC cell line GLC4

Sublines of the human small-cell lung carcinoma (SCLC) cell line GLC4 with acquired resistance to teniposide, amsacrine and mitoxantrone (GLC4/VM20x, GLC4/AM3x and GLC4/MIT60x, respectively) were derived to study the contribution of DNA topoisomerase II alpha and -beta (TopoII alpha and -beta) to resistance for TopoII-targeting drugs. The cell lines did not overexpress P-glycoprotein or the multidrug resistance-associated protein but were cross-resistant to other TopoII drugs. GLC4/VM20x showed a major decrease in TopoII alpha protein (54%; for all assays presented in this paper the GLC4 level was defined to be 100%) without reduction in TopoII beta protein; GLC4/AM3x showed only a major decrease in TopoII beta protein (to 18%) and not in TopoII alpha. In GLC4/MIT60x, the TopoII alpha and -beta protein levels were both decreased (TopoII alpha to 31%; TopoII beta protein was undetectable). The decrease in TopoII alpha protein in GLC4/VM20x and GLC4/MIT60x, was mediated by decreased TopoII alpha mRNA levels. Loss of TopoII alpha gene copies contributed to the mRNA decrease in these cell lines. Only in the GLC4/MIT60x cell line was an accumulation defect observed for the drug against which the cell line was made resistant. In conclusion, TopoII alpha and -beta levels were decreased differentially in the resistant cell lines, suggesting that resistance to these drugs may be mediated by a decrease in a specific isozyme.

Identification of the biosynthetic leukotriene C4 export pump in murine mastocytoma cells as a homolog of the multidrug-resistance protein

A membrane glycoprotein of 190 kDa has been identified previously by photoaffinity labeling as a candidate for the ATP-dependent export pump for leukotriene C4 in mastocytoma cells [Leier, I., Jedlitschky, G., Buchholz, U. & Keppler, D. (1994) Eur. J. Biochem. 220, 599-606]. The present study indicates that this protein represents the murine homolog of the human multidrug resistance protein (MRP). In immunoblot analyses several polyclonal anti-MRP antibodies and one monoclonal antibody recognized the protein of 190-kDa in plasma membranes of mastocytoma cells. Immunoprecipitation after photoaffinity labeling with [3H]leukotriene C4 precipitated the labeled 190-kDa glycoprotein. Deglycosylation by glycopeptide N-glycosidase F of mastocytoma membrane proteins was performed in comparison with membranes from MRP-overexpressing cells and resulted in a reduction of the molecular mass of 190 kDa by about 20 kDa in all membrane preparations. The expression of the murine mrp gene in the mastocytoma cells was analyzed by amplification and sequencing of two mrp cDNA fragments in the first nucleotide binding domain (182 bp) and in a domain proximal to the 3'-end (291 bp). The deduced amino acid sequences of these fragments were identical with murine Mrp and 86.7% and 89.7% identical with the corresponding sequences of human MRP. These results indicate that the ATP-dependent release of leukotriene C4 by murine mastocytoma cells is mediated by murine Mrp.

Homologues of the human multidrug resistance genes MRP and MDR contribute to heavy metal resistance in the soil nematode Caenorhabditis elegans

Acquired resistance of mammalian cells to multiple chemotherapeutic drugs can result from enhanced expression of the multidrug resistance-associated protein (MRP), which belongs to the ABC transporter superfamily. ABC transporters play a role in the protection of organisms against exogenous toxins by cellular detoxification processes. We have identified four MRP homologues in the soil nematode Caenorhabditis elegans, and we have studied one member, mrp-1, in detail. Using an mrp::lacZ gene fusion, mrp-l expression was found in cells of the pharynx, the pharynx-intestinal valve and the anterior intestinal cells, the rectum-intestinal valve and the epithelial cells of the vulva. Targeted inactivation of mrp-l resulted in increased sensitivity to the heavy metal ions cadmium and arsenite, to which wild-type worms are highly tolerant. The most pronounced effect of the mrp-1 mutation is on the ability of animals to recover from temporary exposure to high concentrations of heavy metals. Nematodes were found to be hypersensitive to heavy metals when both the MRP homologue, mrp-1, and a member of the P-glycoprotein (Pgp) gene family, pgp-1, were deleted. We conclude that nematodes have multiple proteins, homologues of mammalian proteins involved in the cellular resistance to chemotherapeutic drugs, that protect them against heavy metals.

Modulation of doxorubicin cytotoxicity by ethacrynic acid

Energy-dependent membrane efflux pumps have been implicated in mediating resistance to doxorubicin (DOX). Membrane-transport mechanisms distinct from P-glycoprotein, capable of transporting DOX and glutathione conjugates have been reported in human cells. Since glutathione-conjugate-forming compounds may be candidates for modulating the cytotoxicity of certain anti-neoplastic agents transported by such transport mechanism, the present studies were performed (i) to determine whether ethacrynic acid, a glutathione-conjugate-forming diuretic, can increase DOX cytotoxicity, and (ii) to study the kinetics of DOX transport and its inhibition by the glutathione conjugate of ethacrynic acid (EA-SG) in the H69 human small-cell-lung-cancer cell line and 2 derived DOX-resistant sublines. Our results indicate that more than one DOX transport mechanism may exist in these cell lines, and that glutathione conjugates may be useful for modulating the cytotoxic effects of DOX.

Expression of multidrug-resistance-associated protein (MRP) and chemosensitivity in human gastric cancer

Evidence has accumulated that, in addition to the MDR1 gene-coded P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP) also mediates the multidrug resistance (MDR) of various human tumors. In the case of gastric cancer, there is little or no involvement of P-glycoprotein, and the mechanisms of MDR remain to be understood. To search for a possible relationship between expression of MRP and sensitivity to anti-cancer agents in gastric cancer, 4 gastric cancer cell lines, 43 human gastric carcinomas and 17 adjacent normal gastric tissue samples were analyzed. Expression of MRP mRNA was evaluated using reverse transcription PCR (RT-PCR) and Southern hybridization. Sensitivity of the test samples to the anti-cancer drugs cisplatin (CDDP), doxorubicin (DXR) and etoposide (VP-16) was examined using the MTT¿3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl [2H]-tetrazolium bromide¿ assay. Immunohistochemical staining with the use of the MRP antibody (MRPr1) was done to confirm the findings regarding the expression of mRNA levels. The MRP expression evaluated with RT-PCR and Southern hybridization as well as with immunohistochemical staining revealed that 23 of 43 gastric-cancer tissues (53.5%), 15 of 17 normal gastric tissues (88%) and 3 of 4 gastric-cancer cell lines (75%) were positive. The MTT assay showed that DXR was significantly more sensitive (p < 0.01) in gastric carcinoma tissues lacking MRP expression than in those with positive expression. The same tendency was seen with the other agents used. Of the cell lines, one which showed no MRP expression also had a higher sensitivity to CDDP, DXR and VP-16 than the other positive cases. These results show that MRP expression is involved in MDR of human gastric cancer and is inversely related to the chemosensitivity of tumor cells against some anticancer drugs.

The anthracycline resistance-associated (ara) gene, a novel gene associated with multidrug resistance in a human leukaemia cell line

Multidrug resistance (MDR) in cancer cells is a major contributor to the failure of chemotherapy treatment. This paper describes a novel protein named the anthracycline resistance associated (ARA) protein. The ara gene is amplified in the MDR leukaemia line CCRF-CEM/E1000 and its mRNA is overexpressed. ARA belongs to the ATP binding cassette (ABC) family of proteins. Another ABC protein, the multidrug resistance-associated protein (MRP), has previously been reported to be overexpressed in the CEM/E1000 subline. The primary amino acid sequence of ARA indicates that it is 49.5 kDa without glycosylation, and that it has one potential glycosylation site. ARA has one ATP binding site and associated transmembrane regions. This is in contrast to MRP (190 kDa, 172 kDa deglycosylated) and most other higher eukaryote ABC proteins, which consist of two similar halves, each having one ATP binding site. In addition to ARA being coexpressed with MRP, comparison of amino acid sequences showed that, among known proteins, ARA is most similar to the C-terminal half of MRP.

Reconstitution of ATP-dependent leukotriene C4 transport by Co-expression of both half-molecules of human multidrug resistance protein in insect cells

Multidrug resistance protein (MRP) confers a multidrug resistance phenotype similar to that associated with overexpression of P-glycoprotein. Unlike P-glycoprotein, MRP has also been shown to be a primary active ATP-dependent transporter of conjugated organic anions. The mechanism(s) by which MRP transports these compounds and increases resistance to natural product drugs is unknown. To facilitate studies on the structure and function of MRP, we have determined whether a baculovirus expression system can be used to produce active protein. Full-length MRP as well as molecules corresponding to either the NH2- or COOH-proximal halves of the protein were expressed individually and in combination in Spodoptera frugiperda Sf21 cells. High levels of intact and half-length proteins were detected in membrane vesicles from infected cells. Although underglycosylated, the full-length protein transported leukotriene C4 (LTC4) with kinetic parameters very similar to those of MRP produced in transfected HeLa cells. Neither half-molecule was able to transport LTC4. However, a functional transporter with characteristics similar to those of intact protein could be reconstituted when both half-molecules were co-expressed. Transport of LTC4 by Sf21 membrane vesicles containing either intact or reconstituted MRP was competitively inhibited by both S-decylglutathione and 17beta-estradiol 17-(beta-D-glucuronide), with Ki values similar to those reported previously for MRP expressed in HeLa cells (Loe, D. W., Almquist, K. C., Deeley, R. G., and Cole, S. P. C. (1996) J. Biol. Chem. 271, 9675-9682; Loe, D. W., Almquist, K. C., Cole, S. P. C., and Deeley, R. G. (1996) J. Biol. Chem. 271, 9683-9689). These studies demonstrate that human MRP produced in insect cells can function as an active transporter of LTC4 and that the NH2- and COOH-proximal halves of the protein can assemble efficiently to form a transporter with functional characteristics similar to those of the intact protein.

Identification of the multidrug-resistance protein (MRP) as the glutathione-S-conjugate export pump of erythrocytes

The identification of the multidrug resistance protein (MRP) as a conjugate export pump in several cell types suggested its involvement in the long-known glutathione-S-conjugate transport across erythrocyte membranes. We investigated the ATP-dependent transport of glutathione S-conjugates in human erythrocyte and erythroleukemia cell membrane vesicles using the endogenous conjugate leukotriene C4 (LTC4), known to be a high-affinity substrate for MRP, in addition to S-(2,4-dinitrophenyl)glutathione. The kinetic parameters, including the Km value for LTC4 of 118 +/- 5 nM and the inhibition constants for transport of both substrates for the quinoline-based inhibitor MK 571, were similar to those obtained for transport mediated by recombinant MRP. Direct photoaffinity labeling of human erythrocyte membranes with [3H]LTC4 revealed a major binding protein of about 190 kDa which was immunoprecipitated by an anti-MRP serum. The radiolabeling of this protein was specifically suppressed by the transport inhibitor MK 571. Several additional anti-MRP sera detected the protein of about 190 kDa in human erythrocyte and erythroleukemia cell membranes. These data identify for the first time the glutathione-S-conjugate transporting protein in erythrocyte membranes.

Multiple Pdr1p/Pdr3p binding sites are essential for normal expression of the ATP binding cassette transporter protein-encoding gene PDR5

Saccharomyces cerevisiae has large number of genes that can be genetically altered to produce a multiple or pleiotropic drug resistance phenotype. The homologous zinc finger transcription factors Pdr1p and Pdr3p both elevate resistance to many drugs, including cycloheximide. This elevation in cycloheximide tolerance only occurs in the presence of an intact copy of the PDR5 gene that encodes a plasma membrane-localized ATP binding cassette transporter protein. Previously, we have found that a single binding site for Pdr3p present in the PDR5 promoter is sufficient to provide Pdr3p-responsive gene expression. In this study, we have found that there are three sites in the PDR5 5'-noncoding region that are closely related to one another and are bound by both Pdr1p and Pdr3p. These elements have been designated Pdr1p/Pdr3p response elements (PDREs), and their role in the maintenance of normal PDR5 expression has been analyzed. Mutations have been constructed in each PDRE and shown to eliminate Pdr1p/Pdr3p binding in vitro. Analysis of the effect of these mutant PDREs on normal PDR5 promoter function indicates that each element is required for wild-type expression and drug resistance. A single PDRE placed upstream of a yeast gene lacking its normal upstream activation sequence is sufficient to confer Pdr1p responsiveness to this heterologous promoter.

Trypanothione overproduction and resistance to antimonials and arsenicals in Leishmania

Leishmania resistant to arsenicals and antimonials extrude arsenite. Previous results of arsenite uptake into plasma membrane-enriched vesicles suggested that the transported species is a thiol adduct of arsenite. In this paper, we demonstrate that promastigotes of arsenite-resistant Leishmania tarentolae have increased levels of intracellular thiols. High-pressure liquid chromatography of the total thiols showed that a single peak of material was elevated almost 40-fold. The major species in this peak was identified by matrix-assisted laser desorption/ionization mass spectrometry as N1,N8-bis-(glutathionyl)spermidine (trypanothione). The trypanothione adduct of arsenite was effectively transported by the As-thiol pump. No difference in pump activity was observed in wild type and mutants. A model for drug resistance is proposed in which Sb(V)/As(V)-containing compounds, including the antileishmanial drug Pentostam, are reduced intracellularly to Sb(III)/As(III), conjugated to trypanothione, and extruded by the As-thiol pump. The rate-limiting step in resistance is proposed to be formation of the metalloid-thiol pump substrates, so that increased synthesis of trypanothione produces resistance. Increased synthesis of the substrate rather than an increase in the number of pump molecules is a novel mechanism for drug resistance.

Induction of vanadium accumulation and nuclear sequestration causing cell suicide in human Chang liver cells

Very little is known about the modulation of vanadium accumulation in cells, although this ultratrace element has long been seen as an essential nutrient in lower life forms, but not necessarily in humans where factors modulating cellular uptake of vanadium seem unclear. Using nuclear microscopy, which is capable of the direct evaluation of free and bound (total) elemental concentrations of single cells we show here that an NH4Cl acidification prepulse causes distinctive accumulation of vanadium (free and bound) in human Chang liver cells, concentrating particularly in the nucleus. Vanadium loaded with acidification but leaked away with realkalinization, suggests proton-dependent loading. Vanadyl(4), the oxidative state of intracellular vanadium ions, is known to be a potent source of hydroxyl free radicals (OH). The high oxidative state of nuclei after induction of vanadyl(4) loading was shown by the redox indicator methylene blue, suggesting direct oxidative damage to nuclear DNA. Flow cytometric evaluation of cell cycle phase-specific DNA composition showed degradation of both 2N and 4N DNA phases in G1, S and G2/M cell cycle profiles to a solitary IN DNA peak, in a dose-dependent manner, effective from micromolar vanadyl(4) levels. This trend was reproduced with microccocal nuclease digestion in a time response, supporting the notion of DNA fragmentation effects. Several other approaches confirmed fragmentation occurring in virtually all cells after 4mM V(4) loading. Ultrastructural profiles showed various stages of autophagic autodigestion and well defined plasma membrane outlines, consistent with programmed cell death but not with necrotic cell death. Direct intranuclear oxidative damage seemed associated with the induction of mass suicide in these human Chang liver cells following vanadium loading and nuclear sequestration.

Transport of the glutathione conjugate of ethacrynic acid by the human multidrug resistance protein MRP

The multidrug resistance protein MRP has been shown to mediate the transport of glutathione S-conjugates across membranes. In this study we demonstrate that the glutathione S-conjugate of the diuretic drug ethacrynic acid, which is an efficient inhibitor of glutathione S-transferases, is a high-affinity substrate and inhibitor of the glutathione S-conjugate pump associated with MRP. This implies that ethacrynic acid may modulate drug resistance of tumor cells not only by inhibiting glutathione S-transferase activity, but also by inhibiting the export of drug conjugates from the cell by MRP.

Selection of a subpopulation with fewer DNA topoisomerase II alpha gene copies in a doxorubicin-resistant cell line panel

A panel of doxorubicin-resistant sublines of the human small-cell lung carcinoma cell line GLC4 displays decreasing DNA topoisomerase II alpha (TopoII alpha) mRNA levels with increasing resistance. In the present study we describe how this decrease may be regulated. No significant differences in TopoII alpha mRNA stability or gene arrangement were found, using mRNA slot-blotting and Southern blotting, in the most resistant cell line compared with the parental cell line. To investigate if TopoII alpha gene copy loss contributed to the mRNA decrease, fluorescence in situ hybridisation using a TopoII alpha-specific probe was performed. During doxorubicin resistance development, the composition of the population in each cell line shifted with increasing resistance, from a population in which most cells contain three TopoII alpha gene copies (GLC4) to a population in which most cells contain only two copies. A partial revertant of the most resistant cell line displayed a shift back to the original situation. We conclude that the TopoII alpha gene copy number decrease per cell line is in good agreement with the decreased TopoII alpha mRNA and protein levels, and TopoII activity levels in these cell lines which were described previously.

Intracellular metabolism of the orally active platinum drug JM216: influence of glutathione levels

JM216 (bis-acetato ammine dichloro cyclohexylamine Pt IV) is an oral platinum complex presently undergoing phase II clinical trials. Previous studies have identified some of its biotransformation products in clinical materials. This study evaluated the nature of JM216 biotransformation products intracellularly in two different human ovarian carcinoma cell lines, one relatively sensitive to platinum agents (CH1: JM216 4 h IC50 of 5.8 microM) and the other relatively resistant (SKOV3: JM216 4 h IC50 of 60.7 microM). Metabolic profiles were also evaluated at different growth status and in cells pretreated with buthionine sulphoximine (BSO), an agent known to decrease intracellular glutathione levels. Results showed that JM216 enters the cells and that the nature and percentage of biotransformation products was dependent upon glutathione levels. Furthermore, results support the view that the previously reported peak A biotransformation product contains a glutathione adduct. In exponentially growing SKOV3 cells which contain higher glutathione levels than CH1, (82.5 vs 37.8 nmol mg-1 protein), peak A represented 89% of total platinum 4 h after JM216 exposure compared with only 24% in CH1. Moreover, 60-70% depletion of glutathione achieved by 24 h pretreatment of cells with BSO resulted in a significant decrease in peak A in both cell lines and increased the cytotoxicity of JM216 in both CH1 and SKOV3 by approximately 2-fold. Following a 4 h exposure of exponentially growing SKOV3 cells to JM216, only peak A (89%) and JM216 (11%) could be detected whereas in CH1 cells, peak A (24%), JM216 (73%) and JM118 [cis-ammine dichloro (cyclohexylamine) platinum II] (3%) were detected. However, in CH1 cells at confluence, where glutathione is lower (8 nmol mg-1 protein) four metabolites (plus JM216 itself) were detected following exposure to 50 microM JM216; peak A, JM118, JM383 (bis-acetato ammine (cyclohexylamine) dihydroxy platinum IV) and an unidentified metabolite (D), also observed in patient's plasma ultrafiltrate. In confluent SKOV3 cells exposed to 50 microM JM216, peak A, JM216 and JM118 were detected. A further unidentified metabolite observed in patients receiving JM216 (metabolite F) was not formed inside these tumour cells. Overall, these data suggest that glutathione conjugation represents a major deactivation pathway for JM216.

ATP-dependent uptake of natural product cytotoxic drugs by membrane vesicles establishes MRP as a broad specificity transporter

MRP is a recently isolated ATP-binding cassette family transporter. We previously reported transfection studies that established that MRP confers multidrug resistance [Kruh, G. D., Chan, A., Myers, K., Gaughan, K., Miki, T. & Aaronson, S. A. (1994) Cancer Res. 54, 1649-1652] and that expression of MRP is associated with enhanced cellular efflux of lipophilic cytotoxic agents [Breuninger, L. M., Paul, S., Gaughan, K., Miki, T., Chan, A., Aaronson, S. A. & Kruh, G. D. (1995) Cancer Res. 55, 5342-5347]. To examine the biochemical mechanism by which MRP confers multidrug resistance, drug uptake experiments were performed using inside-out membrane vesicles prepared from NIH 3T3 cells transfected with an MRP expression vector. ATP-dependent transport was observed for several lipophilic cytotoxic agents including daunorubicin, etoposide, and vincristine, as well as for the glutathione conjugate leukotriene C4 (LTC4). However, only marginally increased uptake was observed for vinblastine and Taxol. Drug uptake was osmotically sensitive and saturable with regard to substrate concentration, with Km values of 6.3 microM, 4.4 microM, 4.2 microM, 35 nM, and 38 microM, for daunorubicin, etoposide, vincristine, LTC4, and ATP, respectively. The broad substrate specificity of MRP was confirmed by the observation that daunorubicin transport was competitively inhibited by reduced and oxidized glutathione, the glutathione conjugates S-(p-azidophenacyl)-glutathione (APA-SG) and S-(2,4-dinitrophenyl)glutathione (DNP-SG), arsenate, and the LTD4 antagonist MK571. This study establishes that MRP pumps unaltered lipophilic cytotoxic drugs, and suggests that this activity is an important mechanism by which the transporter confers multidrug resistance. The present study also indicates that the substrate specificity of MRP is overlapping but distinct from that of P-glycoprotein, and includes both the neutral or mildly cationic natural product cytotoxic drugs and the anionic products of glutathione conjugation. The widespread expression of MRP in tissues, combined with its ability to transport both lipophilic xenobiotics and the products of phase II detoxification, indicates that the transporter represents a widespread and remarkably versatile cellular defense mechanism.

Overexpression of multidrug resistance protein gene in human cancer cell lines selected for drug resistance to epipodophyllotoxins

Overexpression of either the multidrug resistance 1 (MDR1) gene or multidrug resistance protein (MRP) gene is involved in acquisition of multidrug-resistant phenotypes in human cancer cells. In this study we examined whether selection for resistance to the epipodophyllotoxins, etoposide/teniposide (VP16/VM26), could induce overexpression of MDR1 or MRP. We have previously isolated two VP16/VM26-resistant KB cell lines. Two VP16/VM26-resistant KB cell lines, KB/VM-1 and KB/ VM-4, which were selected by stepwise exposure to VM26 had decreased accumulation of [3H]VP16 and increased levels of MRP, but no apparent expression of MDR1 gene was observed. Another VP16/VM26-resistant KB cell line, KB/VP-4, which was further isolated from a VP16-resistant KB cell line, KB/VP-2, had decreased accumulation of [3H]VP16 and showed overexpression of MRP gene, but not that of MDR1 gene. We also isolated a VP16-resistant cell line, IN157/VP-1, from a human glioma cell line IN157. IN157/VP-1 cells showed decreased accumulation of [3H]VP16 and overexpression of MRP gene, but not of MDR1. These findings suggest that selection for resistance to VP16/VM26, preferentially induces overexpression of MRP gene.

Molecular mechanisms of multidrug resistance in cancer chemotherapy

The occurrence of multidrug resistance (MDR) is one of the main obstacles in the successful chemotherapeutic treatment of cancer. MDR cell lines are resistant to the so-called naturally occurring anti-cancer drugs, such as anthracyclines, Vinca alkaloids and epipodophyllotoxins, but are not cross-resistant to alkylating agents, antimetabolites and cisplatin. So far, three separate forms of MDR have been characterized in more detail: classical MDR, non-Pgp MDR and atypical MDR. Although all three MDR phenotypes have much in common with respect to cross-resistance patterns, the underlying mechanisms certainly differ. Atypical MDR is associated with quantitative and qualitative alterations in topoisomerase II alpha, a nuclear enzyme that actively participates in the lethal action of cytotoxic drugs. Atypical MDR cells do not overexpress P-glycoprotein, and are unaltered in their ability to accumulate drugs. In this review we will focus on classical and non-Pgp MDR. The molecular mechanism of classical and non-Pgp MDR is transcriptional activation of membrane-bound transport proteins. These transport proteins belong to the ATP-binding cassette (ABC) superfamily of transport systems. The classical MDR phenotype is characterized by a reduced ability to accumulate drugs, due to activity of an energy-dependent uni-directional, membrane-bound, drug-efflux pump with broad substrate specificity. The classical MDR drug pump is composed of a transmembrane glycoprotein (P-glyco-protein-Pgp) with a molecular weight of 170 kD, and is, in man, encoded by the so-called multidrug resistance (MDR1) gene. Typically, non-Pgp MDR has no P-gly-coprotein expression, yet has about the same cross-resistance pattern as classical MDR. This non-Pgp MDR phenotype is caused by overexpression of the multidrug resistance-associated protein (MRP) gene, which encodes a 190 kD membrane-bound glycoprotein (MRP). MRP probably works by direct extrusion of cytotoxic drugs from the cell and/or by mediating sequestration of the drugs into intracellular compartments, both leading to a reduction in effective intracellular drug concentrations. For the classical MDR phenotype, evidence is accumulating that it plays a role indeed, in clinical drug resistance, especially in some hematological malignancies (acute myeloid leukemia, multiple myeloma and non-Hodgkin's lymphoma) and solid tumors (soft tissue sarcomas and neuroblastoma). The association of MRP with clinical drug resistance has not been elaborated, yet, and studies on MRP expression in human cancer have just begun. We found that overexpression of MRP, as determined by RNase protection assay as well as by immunohistochemistry, occurs in several human cancers, among which are cancer of the lung, esophagus, breast and ovary, and leukemias. Further studies are indicated to establish whether elevated MRP expression at diagnosis is an unfavorable prognostic factor for clinical outcome of chemotherapy.

The application of pH-sensitive fluorescent dyes in lactic acid bacteria reveals distinct extrusion systems for unmodified and conjugated dyes

Intracellular pH in bacteria can be measured efficiently between internal pH values of 6.5 and 8.5 with the fluorescent pH indicator 2',7'-bis-(2-carboxyethyl)-5[and-6]-carboxyfluorescein (BCECF). A new fluorescent pH probe with a lower pKa(app) than BCECF was synthesized from fluorescein isothiocyanate and glutamate. The new probe, N-(fluorescein thio-ureanyl)-glutamate (FTUG), was much less sensitive to changes in concentrations of KCl than was BCECF. Similar to BCECF, an efflux of FTUG independent of the proton motive force, but dependent on ATP, was observed both in Lactobacillus plantarum and Lactococcus lactis. Corrections for probe efflux allowed accurate measurements of the pHin. Similar intracellular pH values were determined with FTUG and BCECF, in the range where both probes can be applied, and the pH values correlated well with those estimated from the distribution of radio-labelled benzoic acid. Since FITC can easily be coupled to substrates containing an amino group, it is possible to develop other FITC derivatives as well. The mechanisms of probe excretion and the nature of the excreted product(s) were studied in further detail for BCECF and FTUG. BCECF was excreted from wild-type L. lactis in an unmodified form as was determined by chromatographic and mass spectrometry analysis. In the case of FTUG, the excreted product was a conjugated derivative. Unmodified FTUG was not excreted, although it was present in cellular extracts from L. lactis. Exit of BCECF was completely inhibited in a BCECF efflux mutant (Bef-) of L. lactis, whereas FTUG-conjugate efflux in this mutant was similar to the wild-type. Addition of indomethacin, a known inhibitor of BCECF efflux in human epithelial cells, resulted in complete inhibition of BCECF efflux in wild-type L. lactis, whereas FTUG-conjugate exit was only slightly affected. The results of the mutant and inhibitor studies suggest that FTUG-conjugate and BCECF efflux in L. lactis are mediated by different ATP-driven extrusion systems for organic anions.

Rapid and specific efflux of reduced glutathione during apoptosis induced by anti-Fas/APO-1 antibody

Although human JURKAT T lymphocytes induced to undergo apoptosis with anti-Fas/APO-1 antibody were observed to rapidly lose reduced glutathione (GSH), increased concentrations of oxidized products were not detectable. Unexpectedly, the reduced tripeptide was instead quantitatively recovered in the incubation medium of the cells. As GSH loss was blocked by bromosulfophthalein and dibromosulfophthalein, known inhibitors of hepatocyte GSH transport, a specific export rather than nonspecific leakiness through plasma membranes is proposed to be responsible. Apoptosis was delayed when GSH-diethylesters were used to elevate intracellular GSH, although the high capacity of the activated efflux system quickly negated the benefit of this treatment. Stimulation of GSH efflux provides a novel mechanism whereby Fas/APO-1 ligation can deplete GSH. We speculate that it enhances the oxidative tonus of a responding cell without requiring an increase in the production of reactive oxygen species.

The human multidrug resistance-associated protein functionally complements the yeast cadmium resistance factor 1

A Saccharomyces cerevisiae strain with a disrupted yeast cadmium resistance factor (YCF1) gene (DTY168) is hypersensitive to cadmium. YCF1 resembles the human multidrug resistance-associated protein MRP (63% amino acid similarity), which confers resistance to various cytotoxic drugs by lowering the intracellular drug concentration. Whereas the mechanism of action of YCF1 is not known, MRP was recently found to transport glutathione S-conjugates across membranes. Here we show that expression of the human MRP cDNA in yeast mutant DTY168 cells restores cadmium resistance to the wild-type level. Transport of S-(2,4-dinitrobenzene)-glutathione into isolated yeast microsomal vesicles is strongly reduced in the DTY168 mutant and this transport is restored to wild-type level in mutant cells expressing MRP cDNA. We find in cell fractionation experiments that YCF1 is mainly localized in the vacuolar membrane in yeast, whereas MRP is associated both with the vacuolar membrane and with other internal membranes in the transformed yeast cells. Our results indicate that yeast YCF1 is a glutathione S-conjugate pump, like MRP, and they raise the possibility that the cadmium resistance in yeast involves cotransport of cadmium with glutathione derivatives.

cDNA cloning of the hepatocyte canalicular isoform of the multidrug resistance protein, cMrp, reveals a novel conjugate export pump deficient in hyperbilirubinemic mutant rats

ATP-dependent transport of glutathione and glucuronate conjugates from hepatocytes into bile is mediated by a distinct member of the ATP-binding cassette superfamily. We have cloned and sequenced the canalicular isoform of the multidrug resistance protein from rat liver, and termed it cMrp. This membrane glycoprotein is composed of 1541 amino acids with an identity of 47.8% with the human multidrug resistance protein (MRP) and of 41.9% with the yeast cadmium factor (YCF1). The carboxyl-terminal 130 amino acids of the human hepatocyte canalicular isoform of MRP (cMRP) were 80.2% identical with rat cMrp. cMrp was not expressed in the liver of two mutant rat strains, the Eisai hyperbilirubinemic rat and the GY/TR- mutant, which are deficient in the ATP-dependent transport of conjugates across the canalicular membrane. Immunoblotting using an antibody raised against the carboxyl terminus of cMrp detected the glycoprotein of about 190 kDa only in the canalicular membrane from normal liver. Double immunofluorescence and confocal laser scanning microscopy localized cMrp exclusively to the canalicular membrane domain of hepatocytes and demonstrated its loss in the hyperbilirubinemic mutant rat. The results identify cMrp as a canalicular transport protein with a novel sequence and with a function similar to the one of the 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.

Mechanisms of resistance of human small cell lung cancer lines selected in VP-16 and cisplatin

BACKGROUND

The combination of VP-16 and cisplatin is one of the most active regimens available for the treatment of small cell lung cancer (SCLC), however, most tumors eventually become resistant to these drugs.

METHODS

To investigate the problem of resistance to VP-16 and cisplatin in patients with SCLC, we established two resistant sublines from the drug sensitive human SCLC line, NCI-H209, by in vitro selection in VP-16 and cisplatin.

RESULTS

The VP-16-selected cell line, H209/VP, was more than 100-fold resistant to VP-16, and displayed cross-resistance to VM-26 and other topoisomerase II interactive drugs, but not to vinca alkaloids. There was no difference in accumulation of VP-16 in H209/VP compared with its parent cell line. The level of topoisomerase II-alpha was reduced to 8% of that in the parent cell line, and there was an altered form of this enzyme with a molecular weight of 160 kilodaltons (kDa), in addition to the normal 170 kDa protein. The cisplatin-selected cell line, H209/CP, was 11.5-fold resistant to cisplatin, with only a low level of cross-resistance to other platinum compounds including carboplatin, tetraplatin, iproplatin, and lobaplatin. This line was highly cross-resistant to vinca alkaloids, but not to anthracyclines or epipodophyllotoxins. The H209/CP cell line was not resistant to cadium chloride, suggesting that alterations in metallothionein are unlikely to be a cause of resistance. Although glutathione (GSH) levels were increased nearly 2-fold in H209/CP, there was no difference in levels of the GSH-related enzymes glutathione-S-transferase, glutathione peroxidase, and glutathione reductase, compared with the parent line. The H209/CP line had a 1.4-fold elevation of topoisomerase II-alpha. The accumulation of cisplatin was reduced in this cell line, and there were fewer DNA-interstrand cross links formed in the presence of cisplatin in H209/CP, compared with the parent line. Neither H209/VP nor H209/CP expressed MDR1, the gene for P-glycoprotein. The MRP gene was expressed at a slightly higher level in the H209/VP cell line, but there was no significant increase in expression of this gene in the H209/CP cell line.

CONCLUSIONS

The resistance of the H209/VP cell line is associated with an alteration of topoisomerase II-alpha, whereas the resistance in the H209/CP line is associated with reduced drug accumulation.

Multidrug resistance protein (MRP)-mediated transport of leukotriene C4 and chemotherapeutic agents in membrane vesicles. Demonstration of glutathione-dependent vincristine transport

The 190-kDa multidrug resistance protein (MRP) has recently been associated with the transport of cysteinyl leukotrienes and several glutathione (GSH) S-conjugates. In the present study, we have examined the transport of leukotriene C4 (LTC4) in membrane vesicles from MRP-transfected HeLa cells (T14), as well as drug-selected H69AR lung cancer cells which express high levels of MRP. V(max) and K(m) values for LTC4 transport by membrane vesicles from T14 cells were 529 +/- 176 pmol mg(-1) min(-1) and 105 +/- 31 nM, respectively. At 50 nM LTC4, the K(m) (ATP) was 70 micron. Transport in T14 vesicles was osmotically-sensitive and was supported by various nucleoside triphosphates but not by non- or slowly-hydrolyzable ATP analogs. LTC4 transport rates in membrane vesicles derived from H69AR cells and their parental and revertant variants were consistent with their relative levels of MRP expression. A 190-kDa protein in T14 membrane vesicles was photolabeled by [3H]LTC4 and immunoprecipitation with MRP-specific monoclonal antibodies (mAbs) confirmed that this protein was MRP. LTC4 transport was inhibited by an MRP-specific mAb (QCRL-3) directed against an intracellular conformational epitope of MRP, but not by a mAb (QCRL-1) which recognizes a linear epitope. Photolabeling with [3H]LTC4 was also inhibitable by mAb QCRL-3 but not mAb QCRL-1. GSH did not inhibit LTC4 transport. However, the ability of alkylated GSH derivatives to inhibit transport increased markedly with the length of the alkyl group. S-Decylglutathione was a potent competitive inhibitor of [3H]LTC4 transport (K(i(app)) 116 nM), suggesting that the two compounds bind to the same, or closely related, site(s) on MRP. Chemotherapeutic agents including colchicine, doxorubicin, and daunorubicin were poor inhibitors of [3H]LTC4 transport. Taxol, VP-16, vincristine, and vinblastine were also poor inhibitors of LTC4 transport but inhibition by these compounds was enhanced by GSH. Uptake of [3H]vincristine into T14 membrane vesicles in the absence of GSH was low and not dependent on ATP. However, in the presence of GSH, ATP-dependent vincristine transport was observed. Levels of transport increased with concentrations of GSH up to 5 mM. The identification of an MRP-specific mAb that inhibits LTC4 transport and prevents photolabeling of MRP by LTC4, provides conclusive evidence of the ability of MRP to transport cysteinyl leukotrienes. Our studies also demonstrate that MRP is capable of mediating ATP-dependent transport of vincristine and that transport is GSH-dependent.

ATP-dependent 17 beta-estradiol 17-(beta-D-glucuronide) transport by multidrug resistance protein (MRP). Inhibition by cholestatic steroids

In addition to its ability to confer resistance to a range of natural product type chemotherapeutic agents, multidrug resistance protein (MRP) has been shown to transport the cysteinyl leukotriene, LTC4, and several other glutathione (GSH) S-conjugates. We now demonstrate that its range of potential physiological substrates also includes cholestatic glucuronidated steroids. ATP dependent, osmotically sensitive transport of the naturally occurring conjugated estrogen, 17 beta-estradiol 17-(beta-D-glucuronide) (E(2)17 beta G), was readily demonstrable in plasma membrane vesicles from populations of MRP-transfected HeLa cells (Vmax 1.4 nmol mg-1 min-1, K(m) 2.5 micron). The involvement of MRP was confirmed by demonstrating that transport was completely inhibited by a monoclonal antibody specific for an intracellular conformational epitope of the protein. MRP-mediated transport of LTC4, was competitively inhibited by E(2)17 beta G (K(i(app)) 22 micron), despite the lack of structural similarity between these two substrates. Competitive inhibition of [3H]E(2)17 beta G transport was also observed with a number of other cholestatic conjugated steroids. All of these compounds prevented photolabeling of MRP with [3H]LTC4, demonstrating that the cholestatic steroid and leukotriene conjugates compete either for the same or possibly overlapping sites on the protein. Consistent with the presence of overlapping but non-identical sites, studies using chemotherapeutic drugs to inhibit MRP-mediated E(2)17 beta G transport indicated that daunorubicin had the highest relative potency of the drugs tested, whereas it was the least potent inhibitor of LTC4 transport. Non-cholestatic steroids glucuronidated at the 3 position of the steroid nucleus, such as 17 beta-estradiol 3-(beta-D-glucuronide), did not compete for transport of E(2)17 beta G by MRP, nor did they inhibit photolabeling of the protein with [3H]LTC4. These data identify MRP as a potential transporter of cholestatic conjugated estrogens and demonstrate site-specific requirements for glucuronidation of the steroid nucleus.

Multidrug resistance-associated protein expression in clinical gastric carcinoma

BACKGROUND

We examined the relationship between the expression of a multi-drug resistance-associated protein (MRP) and the biologic factors regarding invasion and metastasis of human gastric cancer.

METHODS

In 75 patients with gastric cancer, the expression of MRP was immunohistochemically investigated and the expression of MRP mRNA was also detected using reverse transcription PCR (RT-PCR). Sensitivity to the anticancer agents, cisplatin (CDDP), doxorubicin (DXR), etoposide (VP-16), and mitomycin C (MMC) was examined using the MTT {3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl[2H]-tetra-zolium bromide} assay. The relation between MRP expression and development, invasion, and metastasis of cancer was analyzed, and overexpression of the tumor suppressor gene p53 was investigated, immunohistochemically.

RESULTS

Immunohistochemically detected MRP positive tumors were noted in 34 of 75 excised tumors (45%), and confirmed by RT-PCR. There was no significant relation between MRP expression and clinicopathologic features or prognosis. Positive p53 staining was evident in 16 of 34 MRP positive tumors (47%) and 18 of 41 negative ones (44%), and there was no significant correlation between MRP and abnormal p53 expression. The MTT assay showed that MRP positive gastric cancer tissue was less sensitive to CDDP, DXR, and MMC compared with MRP negative ones. A similar tendency was noted with VP-16.

CONCLUSIONS

MRP expression relates to the chemosensitivity of tumor cells against some anticancer drugs and is independent of known factors related to the development, invasion, and metastasis of human gastric cancers.

Tissue distribution of the multidrug resistance protein

The human multidrug resistance protein (MRP) is a 190 kd membrane glycoprotein that can cause resistance of human tumor cells to various anticancer drugs, by extruding these drugs out of the cell. Three different monoclonal antibodies, directed against different domains of MRP, allowed us to determine the localization of MRP in a panel of normal human tissues and malignant tumors. Whereas in malignant tumors strong plasma membrane MRP staining was frequently observed, in normal human tissues MRP staining was predominantly cytoplasmatic. Here, MRP was detected in several types of epithelia, muscle cells, and macrophages. From the presence of MRP in many epithelia we infer that MRP, like MDR1 P-glycoprotein, may have an excretory function in protecting the organism against xenobiotics. Recent studies indicate a role for MRP as a carrier for transport of glutathione-conjugated endo- and xenobiotics. The presence of MRP in bronchiolar epithelium, heart muscle, and macrophages would agree with the glutathione S-conjugate carrier activity previously detected in these cells. Furthermore, in 46 of 119 untreated tumors from various histogenetic origins MRP staining was seen. In these tumors MRP may contribute to the intrinsic resistance against treatment with chemotherapeutic drugs.

Recent advances in carrier-mediated hepatic uptake and biliary excretion of xenobiotics

PURPOSE

Besides renal excretion, hepatic metabolism and biliary excretion are the major pathways involved in the removal of xenobiotics. Recently, for many endogenous and exogenous compounds (including drugs), it has been reported that carrier-mediated transport contributes to hepatic uptake and/or biliary excretion. In particular, primary active transport mechanisms have been shown to be responsible for the biliary excretion of anticancer drugs, endogenous bile acids and organic anions including glutathione and glucuronic acid conjugates. Primary active excretion into bile means the positive removal of xenobiotics from the body, and this elimination process is now designated as "Phase III" (T. Ishikawa, Trends Biochem. Sci., 17, 1992) in the detoxification mechanisms for xenobiotics in addition to Phase I by P-450 and Phase II by conjugation.

METHODS

The transporters, which have been called P-glycoprotein (MDR), multidrug resistance related protein (MRP) and GS-X pump and which are believed to be involved in the primary active pumping of xenobiotics from the cells, are now known as the ATP-binding cassette (ABC) transporters. In this review, we first describe the HMG-CoA reductase inhibitor, pravastatin, as a typical case of a carrier-mediated active transport system that contributes to the liver-specific distribution in the body.

RESULTS

Regarding biliary excretion, we have summarized recent results suggesting the possible contribution of the ABC transporters to the biliary excretion of xenobiotics. We also focus on the multiplicities in both hepatic uptake and biliary excretion mechanisms. Analyzing these multiplicities in transport is necessary not only from a biochemical point of view, but also for our understanding of the physiological adaptability of the living body in terms of the removal (detoxification) of xenobiotics.

CONCLUSIONS

Clarification of these transport mechanism may provide important information for studying the pharmacokinetics of new therapeutic drugs and furthermore, leads to the development of the drug delivery systems.

The yeast cadmium factor protein (YCF1) is a vacuolar glutathione S-conjugate pump

The yeast cadmium factor gene (YCF1) from Saccharomyces cerevisiae, which was isolated according to its ability to confer cadmium resistance, encodes a 1,515 amino acid ATP-binding cassette (ABC) protein with extensive sequence homology to the human multidrug resistance-associated protein (MRP1) (Szczypka, M., Wemmie, J. A., Moye-Rowley, W. S., and Thiele, D. J. (1994) J. Biol. Chem. 269, 22853-22857). Direct comparisons between S. cerevisiae strain DTY167, harboring a deletion of the YCF1 gene, and the isogenic wild type strain, DTY165, demonstrate that YCF1 is required for increased resistance to the toxic effects of the exogenous glutathione S-conjugate precursor, 1-chloro-2,4-di-nitrobenzene, as well as cadmium. Whereas membrane vesicles isolated from DTY165 cells contain two major pathways for transport of the model compound S-(2,4-dinitrophenyl)glutathione (DNP-GS), an MgATP-dependent, uncoupler-insensitive pathway and an electrically driven pathway, the corresponding membrane fraction from DTY167 cells is more than 90% impaired for MgATP-dependent, uncoupler-insensitive DNP-GS transport. Of the two DNP-GS transport pathways identified, only the MgATP-dependent, uncoupler-insensive pathway is subject to inhibition by glutathione disulfide, vanadate, verapamil, and vinblastine. The capacity for MgATP-dependent, uncoupler-insensitive conjugate transport in vitro strictly copurifies with the acuolar membrane fraction. Intact DTY165 cells, but not DTY167 cells, mediate vacuolar accumulation of the quorescent glutathione-conjugate, monochlorobimane-GS. Introduction of plasmid borne, epitope-tagged gene encoding functional YCF1 into DTY167 cells alleviates the 1-chloro-2,4-dinitrobenzene-hypersensitive phenotype concomitant with restoration of the capacity of vacuolar membrane vesicles isolated from these cells for MgATP-dependent, uncoupler-insensitive DNP-GS transport. On the basis of these findings, the YCF1 gene of S. cerevisiae is inferred to encode an MgATP-energized, uncoupler-insensitive vacuolar glutathione S-conjugate transporter. The energy requirements, kinetics, substrate specificity, and inhibitor profile of YCF1-mediated transport demonstrate that the vacuolar glutathione conjugate pump of yeast bears a strong mechanistic resemblance to the MRP1-encoded transporter of mammalian cells and the cognate, but as yet molecularly undefined, function of plant cells.

An ATP-dependent As(III)-glutathione transport system in membrane vesicles of Leishmania tarentolae

Membrane preparations enriched in plasma membrane vesicles prepared from promastigotes of Leishmania tarentolae were shown to accumulate thiolate derivatives of 73As(III). Free arsenite was transported at a low rate, but rapid accumulation was observed after reaction with reduced glutathione (GSH) conditions that favor the formation of As(GS)3. Accumulation required ATP but not electrochemical energy, indicating that As(GS)3 is transported by an ATP-coupled pump. Pentostam, a Sb(V)-containing drug that is one of the first-line therapeutic agents for treatment of leishmaniasis, inhibited uptake after reaction with GSH. Vesicles prepared from a strain in which both copies of the pgpA genes were disrupted accumulated As(GS)3 at wild-type levels, demonstrating that the PgpA protein is not the As(GS)3 pump. These results have important implications for the mechanism of drug resistance in the trypanosomatidae, suggesting that a plasma membrane As(GS)3 pump catalyzes active extrusion of metal thiolates, including the Pentostam-glutathione conjugate.

Multidrug resistance in pediatric oncology

Cancer survival among children and adolescents has improved markedly due to evolution of multimodal treatment that incorporates combination chemotherapy, radiation therapy and/or surgery. However, 20-30% of children with malignancies will succumb to their disease or complications associated with their disease or treatment. A major limiting factor to improvement in survival among these patients is the occurrence of intrinsic and/or acquired resistance to our treatment interventions, chemotherapy and radiotherapy. Among these mechanisms, multidrug resistance, the focus of this review, is a well-documented phenomenon whose biochemistry, pharmacology and molecular biology has been extensively studied. A role for multidrug resistance in chemoresistance and therapeutic failure in childhood malignancies is suggested by the observation of clinical resistance to treatment regimes containing agents that are known substrates of multidrug resistance mechanisms. With the current results from studies in rhabdomyosarcoma, neuroblastoma, osteosarcoma, Ewing's sarcoma, leukemia and retinoblastoma, the role of multidrug resistance is still unclear. Earlier studies attempted to define a role for P-glycoprotein-mediated multidrug resistance; however, a limited number of reports suggest that the multidrug-associated resistance protein may play an active role in neuroblastoma. Further studies will be necessary using standardized and uniform approaches for the analyses of these mechanisms. Clinical trials directed toward reversal of multidrug resistance are premature since the exact role of P-glycoprotein is controversial in pediatric malignancies, the role of other mechanisms of multidrug resistance must be assessed and selective inhibitors of multidrug resistance have yet to be developed.

Basolateral localization and export activity of the human multidrug resistance-associated protein in polarized pig kidney cells

The human multidrug resistance-associated protein MRP confers resistance to various cytotoxic drugs by lowering the intracellular drug concentration. Recent evidence indicates that MRP can also transport glutathione S-conjugates across membranes. To study the transport properties of MRP in intact cells, we have expressed human MRP cDNA in the polarized pig kidney epithelial cell line LLC-PK1. MRP mainly localized to the basolateral plasma membrane of these cells, and not to the apical membrane, as determined by immunocytochemistry using confocal laser scanning and electron microscopy. In accordance with this localization, MRP caused increased transport of the glutathione S-conjugate S-(2, 4-dinitrophenyl)-glutathione and of the anticancer drug daunorubicin to the basal side of the epithelial cell layer. Sulfinpyrazone and probenecid, known inhibitors of multispecific organic anion transport, inhibited this basolateral transport, but not the apical transport of daunorubicin mediated by the apically localized human MDR1 P-glycoprotein in MDR1-transfected LLC-PK1 cells. Probenecid and sulfinpyrazone may therefore be useful lead compounds for the development of clinical reversal agents specific for MRP-mediated drug resistance.

ATP-dependent glutathione disulphide transport mediated by the MRP gene-encoded conjugate export pump

We have previously shown that the multidrug resistance protein (MRP) mediates the ATP-dependent membrane transport of the endogenous glutathione conjugate leukotriene C4 (LTC4) and of structurally related anionic conjugates of lipophilic compounds [Jedlitschky, Leier, Buchholz, Center and Keppler (1994) Cancer Res. 54, 4833-4836; Leier, Jedlitschky, Buchholz, Cole, Deeley and Keppler (1994) J. Biol. Chem. 269, 27807-27810]. We demonstrate in the present study that MRP also mediates the ATP-dependent transport of GSSG, as shown in membrane vesicles from human leukaemia cells overexpressing MRP (HL60/ADR cells) or HeLa cells transfected with an MRP expression vector (HeLa T5 cells) in comparison with the respective parental or control cells. The Km value for ATP-dependent transport of GSSG was 93 +/- 26 microM (mean value +/- S.D., n=5) in membrane vesicles from HeLa T5 cells. GSH, at a concentration of 100 microM, was not a substrate for any significant ATP-dependent MRP-mediated transport. The transport of GSSG was competitively inhibited by LTC4, by the leukotriene D4 receptor antagonist 3-([{3-(2-[7-chloro-2-quinolinyl]ethenyl)phenyl}-{(3-dimethylamino-3- oxopropyl)-thio}-methyl]thio)propanoic acid (MK 571) and by S-decylglutathione, with K1 values of 0.3, 0.6 and 0.7 microM respectively. These studies identify MRP as the membrane glycoprotein which mediates the ATP-dependent export of GSSG from these cells.

Alterations in expression of the multidrug resistance-associated protein (MRP) gene in high-grade transitional cell carcinoma of the bladder

Expression of the MRP gene has been demonstrated in vitro to be a casual factor in non-P-glycoprotein-mediated multidrug resistance, and is implicated in resistance to a number of the chemotherapeutic agents currently used in the treatment of high-grade transitional cell carcinoma (TCC) of the bladder (doxorubicin, epirubicin and vinblastine). Using a sensitive RT-PCR-based technique, we have quantified MRP mRNA levels in a series of untreated TCC (n=24), normal bladder (n=5) and control tissue and cell line samples. MRP mRNA was widely expressed and detectable in all samples analysed, with considerable (up to 190-fold) variation observed between individual tumour samples. MRP mRNA levels found in TCC samples were lower than those determined for normal peripheral mononucleocyte (2.3-fold) and testis (4.1-fold) samples, previously reported to be high-expressing tissues, and varied over a similar range to that observed in normal bladder samples. Results indicate that MRP mRNA levels in a greater proportion of high-grade (G3) bladder tumours (55%, 6/11) are significantly reduced (P=0.018) compared with low- and moderate-grade (G1/2) bladder tumours (8%, 1/13), and suggest that MRP mRNA levels frequently become reduced as a consequence of tumour progression to advanced, poorly differentiated disease. No correlation was apparent between MRP and MDR1 mRNA levels, thus providing no evidence to suggest common regulation of the two genes. In a limited number of patients, no evidence was found to support a role for MRP mRNA levels as a determinant of response to chemotherapy in patients being uniformly treated with either cisplatin-methotrexate-vinblastine (n=6) or epirubicin-cisplatin-methotrexate (n=4) regimens. Similarly, no overall pattern of altered MRP mRNA expression was observed following chemotherapy in four patients from whom post chemotherapy biopsies were taken. This study provides a useful pilot investigation regarding the level, variation and pattern of MRP mRNA expression in TCC of the bladder, and suggests that further studies to establish the clinical significance of these variations are required.

Congenital jaundice in rats with a mutation in a multidrug resistance-associated protein gene

The human Dubin-Johnson syndrome and its animal model, the TR(-) rat, are characterized by a chronic conjugated hyperbilirubinemia. TR(-) rats are defective in the canalicular multispecific organic anion transporter (cMOAT), which mediates hepatobiliary excretion of numerous organic anions. The complementary DNA for rat cmoat, a homolog of the human multidrug resistance gene (hMRP1), was isolated and shown to be expressed in the canalicular membrane of hepatocytes. In the TR(-) rat, a single-nucleotide deletion in this gene resulted in a reduced messenger RNA level and absence of the protein. It is likely that this mutation accounts for the TR(-) phenotype.

Functional expression of the multidrug resistance-associated protein in the yeast Saccharomyces cerevisiae

The multidrug resistance-associated protein (MRP) is a member of the ATP binding cassette superfamily of transporters which includes the mammalian P-glycoproteins (P-gp) family. In order to facilitate the biochemical and genetic analyses of MRP, we have expressed human MRP in the yeast Saccharomyces cerevisiae and have compared its functional properties to those of the mouse Mdr3 P-gp isoform. Expression of both MRP and Mdr3 in the anthracycline hypersensitive mutant VASY2563 restored cellular resistance to Adriamycin in this mutant. MRP and Mdr3 expression produced pleiotropic effects on drug resistance in this mutant, as corresponding VASY2563 transformants also acquired resistance to the anti-fungal agent FK506 and to the K+/H+ ionophore valinomycin. The appearance of increased cellular resistance to the toxic effect of Adriamycin (ADM) in MRP and Mdr3 transformants was concomitant with a reduced intracellular accumulation of [14C]ADM in spheroplasts prepared from these cells. Moreover, MRP and Mdr3, but not control spheroplasts, could mediate a time-dependent reduction in the overall cell-associated [14C]ADM from preloaded cells, suggesting the presence of an active ADM transport mechanism in MRP and Mdr3 transformants. Finally, human MRP was found to complement the biological activity of the yeast peptide pheromone transporter Ste6 and partially restored mating in a sterile ste6 null mutant. These findings suggest that despite their relatively low level of structural homology, MRP and P-gp share similar functional aspects, since both proteins can mediate transport of chemotherapeutic drugs and the a mating peptide pheromone in yeast.

Multidrug resistance-associated protein (MRP) in haematological malignancies

The presence of multidrug resistant cells, either acquired or de novo, severely limits treatment outcome in haematological malignancies. Although expression of the Mr 170,000 P-glycoprotein drug pump is likely to play a role in multidrug resistance (MDR) in haematological malignancies, it is now evident that other MDR mechanisms may be operational as well in leukaemias, lymphomas, and multiple myeloma. We determined the expression of a newly recognised drug resistance gene, the Multidrug Resistance-associated Protein (MRP) gene, in peripheral blood cells from healthy volunteers and from patients with haematological malignancies. Expression of MRP mRNA and its Mr 190,000 glycoprotein were estimated by RNase protection assay and immunocytochemistry, respectively. MRP appeared to be ubiquitously expressed at low levels in all nonmalignant haemopoietic cell types. However, some leukaemias showed elevated levels of MRP, probably due to transcriptional activation or increased mRNA stability. High to very high MRP expression levels were frequently found in chronic lymphocytic leukaemia and prolymphocytic leukaemia. Acute myelocytic leukemia often exhibited low but occasionally high MRP expression levels, while in the other acute and chronic leukaemias, lymphomas, and multiple myeloma, predominantly low, basal levels of MRP were found. We conclude that hyperexpression of MRP is observed in leukaemias, and that further studies are needed to assess the clinical relevance of MRP.