Transport of glutathione prostaglandin A conjugates by the multidrug resistance protein 1

Efflux of Depsipeptide FK228 (FR901228, NSC-630176) Is Mediated by P-Glycoprotein and Multidrug Resistance-Associated Protein 1

Depsipeptide FK228 [(E)-(1S,4S,10S,21R)-7[(Z)-ethylideno]-4,21-diisopropyl-2-oxa-12,13-dithia-5,8,20,23-tetraazabicyclo[8,7,6]-tricos-16-ene-3,6,9,22-pentanone], a novel histone deacetylase (HDAC) inhibitor, previously was reported to be a P-glycoprotein (Pgp) substrate. We now expand the investigation to demonstrate that FK228 is a substrate for Pgp and multidrug resistance-associated protein 1 (MRP1). Transport of FK228 across the Caco-2 cell monolayer in apical to basolateral (AP-->BL) and basolateral to apical (BL-->AP) directions in the absence and presence of Pgp and MRP inhibitors were investigated. An in vitro uptake study in human red blood cells (RBCs) and a cytotoxicity assay in MRP1(-) HL60 and MRP1(+) HL60Adr cells were conducted to show that FK228 is an MRP1 substrate. An FK228-resistant cell line (HCT15R) was developed from HCT15 colon carcinoma and characterized using a 70-oligomer cDNA microarray, reverse transcription-polymerase chain reaction, Western blot analysis, histone acetyltransferase (HAT) and HDAC activity assays, and cytotoxicity assays. FK228 showed a nearly unidirectional flux across the Caco-2 cell monolayer, with the BL-->AP apparent permeability coefficient (P(app)) 32 times that of AP-->BL without apparent saturation. Pgp inhibition decreased the BL-->AP P(app) and increased the AP-->BL P(app). RBC showed a concentration-dependent uptake and saturable efflux of FK228. HL60Adr cells were 4-fold more resistant to FK228 than HL60 cells, and the resistance was reversed by MRP inhibition. Up-regulation of Pgp, but not changes of MRPs or HAT/HDAC enzymatic activities, was the major mechanism for the acquired FK228 resistance. These studies demonstrate that FK228 is a substrate for Pgp and MRP1, and reversible Pgp up-regulation is predominantly involved in FK228 resistance in vitro.

Pharmacogenomics of MDR and MRP subfamilies

Drug-metabolizing enzymes, drug transporters and drug targets play significant roles as determinants of drug efficacy and toxicity. Their genetic polymorphisms often affect the expression and function of their products and are expected to become surrogate markers to predict the response to drugs in individual patients. With the sequencing of the human genome, it has been estimated that approximately 500–1200 genes code for drug transporters and, recently, there have been significant and rapid advances in the research on the relationships between genetic polymorphisms of drug transporters and interindividual variation of drug disposition. At present, the clinical studies of multi-drug resistance protein 1 (MDR1, P-glycoprotein, ABCB1), which belongs to the ATP-binding cassette (ABC) superfamily, are the most comprehensive among the ABC transporters, but clinical investigations on other drug transporters are currently being performed around the world. MDR1 can be said to be the most important drug transporter, since clinical reports have suggested that it regulates the disposition of various types of clinically important drugs, but in vitro investigations or animal experiments have strongly suggested that the members of the multi-drug resistance-associated protein (MRP) subfamily can also become key molecules for pharmacotherapy. In addition to those, breast cancer resistance protein (BCRP, ABCG2), another ABC transporter, is well known as a key molecule of multi-drug resistance to several anticancer agents. However, this review focuses on the latest information on the pharmacogenetics of the MDR and MRP subfamilies, and its impact on pharmacotherapy is discussed.

Substrates and inhibitors of efflux proteins interfere with the MTT assay in cells and may lead to underestimation of drug toxicity

The MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay is a widely used method in assessment of cytotoxicity and cell viability, and also in anti-cancer drug studies with tumour cells. These cells often express efflux proteins, such as P-glycoprotein (MDR1) or multidrug resistance (MDR) protein 1 (MRP1). MDCKII cells that overexpress these proteins (MDCKII-MDR1 or MDCKII-MRP1) and normal cells (MDCKII-wt) were used to investigate the effects of efflux pump activity on the results of MTT assay. Efflux protein activity was confirmed with calcein-AM efflux assay, and MTT assay was compared to another cytotoxicity test, the LDH release assay. Inhibition of MRP and MDR1 efflux proteins in MDCKII cell lines was associated paradoxically with increased reduction of MTT, implying an apparent increase in cell viability. This effect was seen when MK 571 (MRP1 and MRP2 inhibitor) or verapamil (MRP1 and MDR1 inhibitor) were used to block efflux protein activity. The calcein-AM efflux assay also showed that the MTT reagent inhibits the function of MDR1 in the MDCKII-MDR1 cell line. This study shows that MDR1 and possibly MRP proteins interfere with the MTT assay. Due to wide substrate specificity of efflux proteins and popularity of the MTT assay this interference is not trivial. Presence of any efflux protein substrate may therefore lead to underestimated results in MTT assay, thereby causing potential bias and erroneous conclusions in cytotoxicity studies.

The genetic polymorphism of drug transporters: functional analysis approaches

Evidence is accumulating to strongly suggest that drug transporters are one of the determining factors governing the pharmacokinetic profile of drugs. To date, a variety of drug transporters have been cloned and classified as solute carriers and ATP-binding cassette transporters. Such drug transporters are expressed in various tissues such as the intestine, brain, liver, and kidney, and play critical roles in the absorption, distribution and excretion of drugs. However, at the present time, information is limited regarding the genetic polymorphism of drug transporters and its impact on their function. In this context, we have undertaken the functional analyses of the polymorphisms identified in drug transporter genes. This article aims to provide an overview on the functional aspects of the non-synonymous polymorphisms of drug transporters and to present standard methods for the evaluation of the effect of polymorphisms on their function.

Interplay between MRP Inhibition and Metabolism of MRP Inhibitors: The Case of Curcumin

The multidrug resistance proteins MRP1 and MRP2 are efflux transporters with broad substrate specificity, including glutathione, glucuronide, and sulfate conjugates. In the present study, the interaction of the dietary polyphenol curcumin with MRP1 and MRP2 and the interplay between curcumin-dependent MRP inhibition and its glutathione-dependent metabolism were investigated using two transport model systems. In isolated membrane vesicles of MRP1- and MRP2-expressing Sf9 cells, curcumin clearly inhibited both MRP1- and MRP2-mediated transport with IC(50) values of 15 and 5 microM, respectively. In intact monolayers of MRP1 overexpressing Madin-Darby canine kidney (MDCKII-MRP1) cells, curcumin also inhibited MRP1-mediated activity, although with a 3-fold higher IC(50) value than the one observed in the vesicle model. Interestingly, MRP2-mediated activity was hardly inhibited in intact monolayers of MRP2-overexpressing MDCKII (MDCKII-MRP2) cells upon exposure to curcumin, whereas the IC(50) value in the vesicle incubations was 5 microM. The difference in extent of inhibition of the MRPs by curcumin in isolated vesicles as compared to intact cells, observed especially for MRP2, was shown to be due to a swift metabolism of curcumin to two glutathione conjugates in the MDCKII cells. Formation of both glutathione conjugates was about six times higher in the MDCKII-MRP2 cells as compared with the MDCKII-MRP1 cells, a phenomenon that could be ascribed to the significantly lower glutathione levels in the cell line. The efflux of both conjugates, identified in the present study as monoglutathionyl curcumin conjugates, was demonstrated to be mediated by both MRP1 and MRP2. From dose-response curves with Sf9 membrane vesicles, glutathionylcurcumin conjugates appeared to be less potent inhibitors of MRP1 and MRP2 than their parent compound curcumin. In conclusion, curcumin clearly inhibits both MRP1- and MRP2-mediated transport, but the glutathione-dependent metabolism of curcumin plays a crucial role in the ultimate level of inhibition of MRP-mediated transport that can be achieved in a cellular system. This complex interplay between MRP inhibition and metabolism of MRP inhibitors, the latter affecting the ultimate potential of a compound for cellular MRP inhibition, may exist not only for a compound like curcumin but also for many other MRP inhibitors presently or previously developed on the basis of vesicle studies.

Basal Membrane Localization of MRP1 in Human Placental Trophoblast

The placental trophoblast is considered to act as a barrier between mother and fetus, mediating the exchange of various materials across the placenta. ATP-binding cassette (ABC) transporters such as P-glycoprotein (P-gp) and multidrug-resistance protein (MRP) are expressed in the placenta and function as efflux transport systems for xenobiotics. In the present study, we aimed to determine the localization of MRP1 in the human placenta in comparison with that of P-gp. Western blotting analysis with human placental membrane vesicles indicated that P-gp and MRP1 are localized on the brush-border membranes and basal membranes, respectively. Immunohistochemical analysis with human normal full-term placenta showed that anti-P-gp monoclonal antibody F4 stained the brush-border side of the trophoblast cells, whereas anti-MRP1 monoclonal antibody MRPr1 stained the basal side. These results confirm that P-gp and MRP1 are located on the brush-border membranes and basal membranes, respectively, of human full-term placental trophoblast. MRP1 was also detected on the abluminal side of blood vessels in the villi. Accordingly, MRP1 may play a role distinct from that of P-gp, which is considered to restrict the influx of xenobiotics into the fetus.

The Effect of Cell Culture Conditions on Saquinavir Transport Through, and Interactions with, MDCKII Cells Overexpressing hMDR1

MDCK cells are cultured using wide-ranging conditions and can produce variable results. To develop a standard protocol for studying saquinavir transport using MDCKII cells, stably transfected MDCKII cells overexpressing human Pgp (MDCKII-PGP) and MDCKII wild-type cells (MDCKII/wt) were used to evaluate the combined effects of seeding density (6.9 x 10(5) or 5 x 10(4) cells/cm2), substratum (polycarbonate +/- collagen coating) and saquinavir presence on monolayer integrity, Pgp expression, and saquinavir transport. The saquinavir efflux ratio (ratio of BL --> AP/AP --> BL permeability) for MDCKII-PGP cells (6.9 x 10(5) cells/cm2) was 57 with variable mannitol permeabilities. Consistent mannitol permeabilities and higher saquinavir efflux ratios were obtained with 5 x 10(4) cells/cm2 on polycarbonate (78) or collagen-coated polycarbonate (126). The MDCKII/wt saquinavir efflux ratio was 9. Saquinavir presence increased paracellular permeability for all treatments relative to cells seeded onto collagen-coated membranes. Collagen coating caused increased Pgp expression and saquinavir efflux ratios correlated (r2 = 0.96) with Pgp expression levels [MDCKII-PGP (on collagen-coated polycarbonate) > MDCKII-PGP (on polycarbonate) > MDCKII/wt (on collagen-coated polycarbonate)]. These results directly and quantitatively link interrelated differences in cell culture conditions to changes in monolayer integrity, transporter expression, and active transport; and emphasize the critical application of controls in cell culture models.

The human multidrug resistance protein MRP4 functions as a prostaglandin efflux transporter and is inhibited by nonsteroidal antiinflammatory drugs

Prostaglandins are involved in a wide variety of physiological and pathophysiological processes, but the mechanism of prostaglandin release from cells is not completely understood. Although poorly membrane permeable, prostaglandins are believed to exit cells by passive diffusion. We have investigated the interaction between prostaglandins and members of the ATP-binding cassette (ABC) transporter ABCC [multidrug resistance protein (MRP)] family of membrane export pumps. In inside-out membrane vesicles derived from insect cells or HEK293 cells, MRP4 catalyzed the time- and ATP-dependent uptake of prostaglandin E1 (PGE1) and PGE2. In contrast, MRP1, MRP2, MRP3, and MRP5 did not transport PGE1 or PGE2. The MRP4-mediated transport of PGE1 and PGE2 displayed saturation kinetics, with Km values of 2.1 and 3.4 microM, respectively. Further studies showed that PGF1alpha, PGF2alpha, PGA1, and thromboxane B2 were high-affinity inhibitors (and therefore presumably substrates) of MRP4. Furthermore, several nonsteroidal antiinflammatory drugs were potent inhibitors of MRP4 at concentrations that did not inhibit MRP1. In cells expressing the prostaglandin transporter PGT, the steady-state accumulation of PGE1 and PGE2 was reduced proportional to MRP4 expression. Inhibition of MRP4 by an MRP4-specific RNA interference construct or by indomethacin reversed this accumulation deficit. Together, these data suggest that MRP4 can release prostaglandins from cells, and that, in addition to inhibiting prostaglandin synthesis, some nonsteroidal antiinflammatory drugs might also act by inhibiting this release.

Antifolate resistance associated with loss of MRP1 expression and function in Chinese hamster ovary cells with markedly impaired export of folate and cholate

Export of folates from a Chinese hamster ovary PyrR100 cell line is markedly impaired, resulting in expansion of cellular folate pools and high-level antifolate resistance. We now report that MRP1 expression is absent in PyrR100 cells along with a marked decrease in MRP5 expression with 3-fold cross-resistance to thiopurines. PyrR100 and wild-type cells had comparable low levels of MRP2 expression; both lacked the breast cancer resistance protein. PyrR100 cells showed a 4-fold decrease in cholate (an MRP substrate) efflux with a 6-fold increase in cellular cholate accumulation compared with wild-type cells. Prostaglandin A1 increased cholate accumulation in wild-type cells to levels comparable with PyrR100 cells. Calcein (an MRP1 substrate) fluorescence increased 5-fold in PyrR100 cells; probenecid increased the intracellular calcein level in wild-type cells to that of PyrR100 cells. Consistent with the loss of MRP1 expression, PyrR100 cells showed modest collateral sensitivity to cholate, etoposide, doxorubicin, and vincristine. Transfection of MRP5 into PyrR100 cells did not alter sensitivity to pyrimethamine or MTX but restored sensitivity to mercaptopurines, indicating that decreased MRP5 expression did not play a role in antifolate resistance. Hence, although MRP-mediated anticancer drug resistance has been associated with gain of function (i.e., overexpression), this is the first report that loss of MRP1 efflux function can expand intracellular folate pools to result in acquired antifolate resistance. The data also suggest that MRP1, and possibly other MRPs that transport folates, can play a role in the maintenance of cellular folate homeostasis.

Characterization of the MRP4- and MRP5-mediated transport of cyclic nucleotides from intact cells

Cyclic nucleotides are known to be effluxed from cultured cells or isolated tissues. Two recently described members of the multidrug resistance protein family, MRP4 and MRP5, might be involved in this process, because they transport the 3',5'-cyclic nucleotides, cAMP and cGMP, into inside-out membrane vesicles. We have investigated cGMP and cAMP efflux from intact HEK293 cells overexpressing MRP4 or MRP5. The intracellular production of cGMP and cAMP was stimulated with the nitric oxide releasing compound sodium nitroprusside and the adenylate cyclase stimulator forskolin, respectively. MRP4- and MRP5-overexpressing cells effluxed more cGMP and cAMP than parental cells in an ATP-dependent manner. In contrast to a previous report we found no glutathione requirement for cyclic nucleotide transport. Transport increased proportionally with intracellular cyclic nucleotide concentrations over a calculated range of 20-600 microm, indicating low affinity transport. In addition to several classic inhibitors of organic anion transport, prostaglandins A(1) and E(1), the steroid progesterone and the anti-cancer drug estramustine all inhibited cyclic nucleotide efflux. The efflux mediated by MRP4 and MRP5 did not lead to a proportional decrease in the intracellular cGMP or cAMP levels but reduced cGMP by maximally 2-fold over the first hour. This was also the case when phosphodiesterase-mediated cyclic nucleotide hydrolysis was inhibited by 3-isobutyl-1-methylxanthine, conditions in which efflux was maximal. These data indicate that MRP4 and MRP5 are low affinity cyclic nucleotide transporters that may at best function as overflow pumps, decreasing steep increases in cGMP levels under conditions where cGMP synthesis is strongly induced and phosphodiesterase activity is limiting.

Binding of a photoaffinity analogue of glutathione to MRP1 (ABCC1) within two cytoplasmic regions (L0 and L1) as well as transmembrane domains 10-11 and 16-17

MRP1 (or ABCC1) is an ABC membrane protein that transports a wide range of natural products as well as glutathione (GSH)-, glucuronate-, and sulfate-conjugated metabolites. In addition, free GSH is required for MRP1 to transport several chemotherapeutic drugs. However, the mechanisms regulating the influence of GSH on MRP1 is poorly understood, and the location of GSH binding site(s) within MRP1 have yet to be determined. To address these issues, we have synthesized a [(125)I] labeled azido-derivative of GSH (IAAGSH) to photoaffinity label MRP1. Our results revealed that IAAGSH labeled MRP1 with high specificity, and binding was inhibited by MRP1 substrates leukotriene C(4) and MK571. Interestingly, verapamil and vincristine enhanced IAAGSH photolabeling of MRP1, in agreement with observations that both drugs enhance GSH transport. We observed GSH to be the best inhibitor of photoaffinity labeling, as compared to oxidized glutathione (GSSG) and four different GSH alkyl derivatives. These observations indicate that IAAGSH interacted with MRP1 in a similar manner as unmodified GSH. Moreover, using eight MRP1-HA variants, each containing hemagglutinin A (HA) epitopes inserted at different sites in MRP1, we mapped the GSH binding sites in MRP1. Our GSH analogue photoaffinity labeled four MRP1 polypeptides that were located within two cytoplasmic domains in linker sequences (L0 and L1) as well as transmembrane domains 10-11 and 16-17. The photoaffinity labeling of polypeptides within L0 and L1 domains is further confirmed using two MRP1-specific monoclonal antibodies (MRPr1 and QCRL1) with epitopes within the linker domains. Taken together, this study provides the most precise information to date on the location of GSH binding sites in MRP1.

Isolation and characterization of Caco-2 subclones expressing high levels of multidrug resistance protein efflux transporter

PURPOSE

The purpose of this study was to isolate Caco-2 subclones that express high levels of multidrug resistance protein (MDR1) and to characterize their kinetics and affinity parameters for MDR1 substrate/inhibitors.

METHODS

The subclones were selected by a dilution cloning technique. The polarized efflux of [3H]-vinblastine across subclone cell monolayers was quantified by measuring the apparent permeability coefficients (Papp) of [3H]-vinblastine in the basolateral (BL)-to-apical (AP) direction and in the AP-to-BL direction (Papp BL-to-AP/Papp AP-to-BL) across the cell monolayers. The expression of MDR1 in the Caco-2 subclones compared with the parental Caco-2 cells was confirmed by Western blotting analysis. The kinetics parameters (Km, Vmax) of [3H]-vinblastine and the inhibitory constants (KI) of several known MDR1 substrates/inhibitors on the transport of [3H]-digoxin determined in the parental Caco-2 cells and Caco-2 subclones were also compared.

RESULTS

Three subclones (#1, #20, #21) were selected based on their polarized efflux of [3H]-vinblastine. The Papp BL-to-AP/Papp AP-to-BL ratios for #1, #20, and #21 were 110, 140, and 112, respectively, and were about 6-fold higher than the ratio observed for the parental Caco-2 cells. In the presence of GF-120918 (2 microM), a known MDR1-specific inhibitor, the Papp BL-to-AP/Papp AP-to-BL ratios were significantly decreased, suggesting that these cells were overexpressing MDRI. The Km values observed for vinblastine in the Caco-2 subclones were nearly identical to the value observed in the parental Caco-2 cells. In contrast, the Vmax values observed in the subclones were approximate 26-69% higher. The KI values observed for various known MDR1 substrates/inhibitors on [3H]-digoxin transport were nearly identical to those in the parental Caco-2 cells and Caco-2 subclones. The high functional efflux activities of these subclones were stable up to 6 months.

CONCLUSIONS

Subclones #1, #20, #21 express high levels of MDR1. These Caco-2 subclones may be useful models for profiling drugs for their MDR1 substrate activity and for establishing structure-transport relationships for this efflux transporter.

Drug efflux transporters in the CNS

The central nervous system (CNS) contains important cellular barriers that maintain homeostasis by protecting the brain from circulating toxins and through the elimination of toxic metabolites generated in the brain. The barriers that limit the concentration of toxins and xenobiotics in the interstitial fluids of the CNS are the capillary endothelial cells of the blood-brain barrier (BBB) and the epithelial cells of the blood-cerebrospinal fluid barrier (BCSFB). Both of these barriers have cellular tight junctions and express transport systems which serve to actively transport nutrients into the brain, and actively efflux toxic metabolites and xenobiotics out of the brain. This review will focus on the expression and function of selected drug efflux transporters in these two barriers, specifically the multidrug resistance transporter, p-glycoprotein, and various organic anion transporters, such as multidrug resistance-associated proteins, organic anion transporter polypeptides, and organic anion transporters. These transport systems are increasingly recognized as important determinants of drug distribution to, and elimination from, different compartments of the CNS. Consequences of drug efflux transporters in barriers of the CNS include limiting the distribution of substrates that are beneficial to treat CNS diseases, and increasing the possibility of drug-drug interactions that may lead to untoward toxicities. Therefore, the study of these transporters is important in examining the various determinants of drug delivery to the CNS.

Multiple membrane-associated tryptophan residues contribute to the transport activity and substrate specificity of the human multidrug resistance protein, MRP1

The multidrug resistance protein, MRP1, is a clinically important ATP-binding cassette transporter in which the three membrane-spanning domains (MSDs), which contain up to 17 transmembrane (TM) helices, and two nucleotide binding domains (NBDs) are configured MSD1-MSD2-NBD1-MSD3-NBD2. In tumor cells, MRP1 confers resistance to a broad spectrum of drugs, but in normal cells, it functions as a primary active transporter of organic anions such as leukotriene C(4) and 17beta-estradiol 17beta-(D-glucuronide). We have previously shown that mutation of TM17-Trp(1246) eliminates 17beta-estradiol 17beta-(D-glucuronide) transport and drug resistance conferred by MRP1 while leaving leukotriene C(4) transport intact. By mutating the 11 remaining Trp residues that are in predicted TM segments of MRP1, we have now determined that five of them are also major determinants of MRP1 function. Ala substitution of three of these residues, Trp(445) (TM8), Trp(553) (TM10), and Trp(1198) (TM16), eliminated or substantially reduced transport levels of five organic anion substrates of MRP1. In contrast, Ala substitutions of Trp(361) (TM7) and Trp(459) (TM9) caused a more moderate and substrate-selective reduction in MRP1 function. More conservative substitutions (Tyr and Phe) of the Trp(445), Trp(553), and Trp(1198) mutants resulted in substrate selective retention of transport in some cases (Trp(445) and Trp(1198)) but not others (Trp(553)). Our findings suggest that the bulky polar aromatic indole side chain of each of these five Trp residues contributes significantly to the transport activity and substrate specificity of MRP1.

Multidrug resistance protein 2 (MRP2) transports HIV protease inhibitors, and transport can be enhanced by other drugs

BACKGROUND

Various drug transporters of the ATP-binding cassette (ABC) family restrict the oral bioavailability and cellular, brain, testis, cerebrospinal fluid and fetal penetration of substrate drugs. MDRI P-glycoprotein (P-gp) has been demonstrated to transport most HIV protease inhibitors (HPI) and to reduce their oral bioavailability and lymphocyte, brain, testis and fetal penetration, possibly resulting in major limiting effects on the therapeutic efficacy of these drugs.

OBJECTIVES

To investigate whether the ABC transporters MRP1, MRP2, MRP3, MRP5 and breast cancer resistance protein 1 (Bcrp1) are efficient transporters of the HPI saquinavir, ritonavir and indinavir.

METHODS

Polarized epithelial non-human (canine) cell lines transduced with human or murine complementary DNA (cDNA) for each of the transporters were used to study transepithelial transport of the HPI.

RESULTS

MRP2 efficiently transported saquinavir, ritonavir and indinavir and this transport could be enhanced by probenecid. Sulfinpyrazone was also able to enhance MRP2-mediated saquinavir transport. In contrast, MRP1, MRP3, MRP5, or Bcrp1 did not efficiently transport the HPI tested.

CONCLUSIONS

Human MRP2 actively transports several HPI and could, based on its known and assumed tissue distribution, therefore reduce HPI oral bioavailability. It may also limit brain and fetal penetration of these drugs and increase their hepatobiliary, intestinal and renal clearance. MRP2 function and enhancement of its activity could adversely affect the therapeutic efficacy, including the pharmacological sanctuary penetration, of HPI. In vivo inhibition of MRP2 function might, therefore, improve HIV/AIDS therapy.

Direct evidence that saquinavir is transported by multidrug resistance-associated protein (MRP1) and canalicular multispecific organic anion transporter (MRP2)

To determine if saquinavir mesylate (saquinavir) is a substrate of human multidrug resistance-associated protein 1 (hMRP1 [ABCC1]) or hMRP2 (cMOAT, or ABCC2), MDCKII cells that overexpress either hMRP1 (MDCKII-MRP1) or hMRP2 (MDCKII-MRP2) were used to investigate saquinavir's cytotoxicity and transport in comparison with those of control MDCKII wild-type (MDCKII/wt) cells. Cytotoxicity was assessed with the mitochondrial marker MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium], and saquinavir transport was measured directly through the cell monolayers. GF120918 (an inhibitor of P glycoprotein, but not of the MRP family) and MK-571 (an MRP family inhibitor) were used to delineate the specific contributions of these transporters to saquinavir cytotoxicity and transport. In the presence of GF120918 and increasing saquinavir concentrations, the MDCKII-MRP1 (50% lethal dose [LD(50)] = 10.5 micro M) and MDCKII-MRP2 (LD(50) = 27.1 micro M) cell lines exhibited statistically greater viability than the MDCKII/wt cells (LD(50) = 7.8 micro M). Saquinavir efflux was directional, not saturable, and was inhibited by MK-571 (35 and 75 micro M) in all cell lines. The ratios of saquinavir (3 micro M) basolateral to apical permeability (i.e., efflux ratios) for the MDCKII/wt, MDCKII-MRP1, and MDCKII-MRP2 cell monolayers were 2.6, 1.8, and 6.8, respectively. The MDCKII-MRP1 cells have a significantly reduced saquinavir efflux ratio relative to MDCKII/wt cells, due to basolaterally directed transport by hMRP1 competing with endogenous, apically directed canine MRP2. The MDCKII-MRP2 cells have a significantly increased saquinavir efflux ratio relative to MDCKII/wt cells, due to the additive effects of the apically directed transport by hMRP2 and endogenous MRP2. Collectively, the cytotoxicity and transport results provide direct evidence that saquinavir is transported by MRP1 and MRP2.

Role of the N-terminal transmembrane region of the multidrug resistance protein MRP2 in routing to the apical membrane in MDCKII cells

In polarized cells, the multidrug resistance protein MRP2 is localized in the apical plasma membrane, whereas MRP1, another multidrug resistance protein (MRP) family member, is localized in the basolateral membrane. MRP1 and MRP2 are thought to contain an N-terminal region of five transmembrane segments (TMD(0)) coupled to 2 times six transmembrane segments via an intracellular loop (L(0)). We previously demonstrated for MRP1 that a mutant lacking TMD(0) but still containing L(0), called L(0)DeltaMRP1, was functional and routed to the lateral plasma membrane. To investigate the role of the TMD(0)L(0) region of MRP2 in routing to the apical membrane, we generated mutants similar to those made for MRP1. In contrast to L(0)DeltaMRP1, L(0)DeltaMRP2 was associated with an intracellular compartment, most likely endosomes. Co-expression with TMD(0), however, resulted in apical localization of L(0)DeltaMRP2 and transport activity. Uptake experiments with vesicles containing L(0)DeltaMRP2 demonstrated that the molecule is able to transport LTC(4). An MRP2 mutant without TMD(0)L(0), DeltaMRP2, was only core-glycosylated and localized intracellularly. Co-expression of DeltaMRP2 with TMD(0)L(0) resulted in an increased protein level of DeltaMRP2, full glycosylation of the protein, routing to the apical membrane, and transport activity. Our results suggest that the TMD(0) region is required for routing to or stable association with the apical membrane.

Prostaglandin transport

Newly synthesized prostaglandins (PGs) efflux from cells by simple diffusion, driven by pH and the membrane potential. Metabolic clearance requires energy-dependent uptake across the plasma membrane, followed by cytoplasmic oxidation. Several PG carriers have been cloned and characterized. PGT is broadly expressed in cyclooxygenase (COX)-positive cells, appears to be a lactate/PG exchanger, and is coordinately regulated with COX. By analogy with neurotransmitter release and re-uptake, PGT may regulate pericellular PG levels via re-uptake. PGT may also direct PGs towards and/or away from specific sets of PG receptors. Other members of the OATP transporter family also catalyze PG uptake; these are variably expressed and have variable affinities for PGs. The OATs are alpha-ketoglutarate/organic anion exchangers that accept PGs; these probably represent the uptake step in renal and hepatic PG degradation and excretion. Finally, certain glutathione-conjugated leukotrienes and PGs are actively extruded from cells by the MRPs; these may also play a role in metabolic clearance of PGs.

Characterization of the efflux transporter(s) responsible for restricting intestinal mucosa permeation of an acyloxyalkoxy-based cyclic prodrug of the opioid peptide DADLE

PURPOSE

To elucidate the efflux transporter(s) responsible for restricting the permeation of an acyloxyalkoxy-based cyclic prodrug of the opioid peptide DADLE (AD) through Caco-2 cell monolayers.

METHODS

The cellular permeation characteristics of AD were investigated using Caco-2 cells, Madin-Darby canine kidney wild-type II cells (MDCK-WT), MDCK cells transfected with the human MDR1 gene (MDCK-MDR1), and MDCK cells transfected with the human MRP2 gene (MDCK-MRP2). These cells were grown as monolayers onto microporous membranes. The disappearance of AD from the donor side and its appearance on the receiver side were monitored by high-performance liquid chromatography. The substrate activity of AD for P-glycoprotein (P-gp) was determined using GF120918, a known P-gp specific inhibitor. The substrate activity of AD for MRP2 was determined by using cyclosporin A, a known MRP2 and P-gp inhibitor.

RESULTS

In Caco-2 cells, the ratio of the apparent permeability coefficients (Papp) of AD flux measured in the basolateral (BL) to apical (AP) direction vs. the flux in the AP-to-BL direction (Papp BL-to-AP/ Papp AP-to-BL) was 99. In the presence of 2 microM GF120918 or 25 microM cyclosporin A. the Papp BL-to-AP/Papp AP-to-BL ratio was decreased to 11. In MDCK-WT, MDCK-MDR1, and MDCK-MRP2 cells, the Papp BL-to-AP/Papp AP-to-BL ratios of AD were 4.7, 10, and 5.8, respectively. A mixture of GF120918 (2 microM) and cyclosporin A (25 microM) decreased the Papp BL-to-AP/Papp AP-to-BL ratios of AD in MDCK-WT, MDCK-MDR1, and MDCK-MRP2 cells to 1.2,1.8, and 2.3, respectively.

CONCLUSIONS

These data suggest that AD is a much better substrate for P-gp than MRP2 and that the restricted permeation of this cyclic prodrug in Caco-2 cells and in the intestinal mucosa probably is due primarily to its substrate activity for P-gp.

A naturally occurring mutation in MRP1 results in a selective decrease in organic anion transport and in increased doxorubicin resistance

The human 190 kDa multidrug resistance protein, MRP1, is a polytopic membrane glycoprotein that confers resistance to a wide range of chemotherapeutic agents. It also transports structurally diverse conjugated organic anions, as well as certain unconjugated and conjugated compounds, in a reduced glutathione-stimulated manner. In this study, we characterized a low-frequency (<1%) naturally occurring mutation in MRP1 expected to cause the substitution of a conserved arginine with serine at position 433 in a predicted cytoplasmic loop of the protein. Transport experiments with membrane vesicles prepared from transfected human embryonic kidney cells and HeLa cells revealed a two-fold reduction in the ATP-dependent transport of the MRP1 substrates, leukotriene C4 (LTC4) and oestrone sulphate. Kinetic analysis showed that this reduction was due to a decrease in Vmax for both substrates but Km was unchanged. In contrast, 17beta-oestradiol-17beta-(D-glucuronide) transport by the Arg433Ser mutant MRP1 was similar to that by wild-type MRP1. Fluorescence confocal microscopy showed that the mutant MRP1 was routed correctly to the plasma membrane. In contrast to the reduced LTC4 and oestrone sulphate transport, stably transfected HeLa cells expressing Arg433Ser mutant MRP1 were 2.1-fold more resistant to doxorubicin than cells expressing wild-type MRP1, while resistance to VP-16 and vincristine was unchanged. These results provide the first example of a naturally occurring mutation predicted to result in an amino acid substitution in a cytoplasmic region of MRP1 that shows an altered phenotype with respect to both conjugated organic anion transport and drug resistance.

Are MDCK cells transfected with the human MDR1 gene a good model of the human intestinal mucosa?

PURPOSE

To investigate whether Madin-Darby canine kidney cells transfected with the human MDR1 gene (MDCK-MDR1) are a good model of the human intestinal mucosa.

METHODS

P-glycoprotein (P-gp) expression in Caco-2 cells was compared with P-gp expression in MDCK wild- type (MDCK-WT) and MDCK-MDR1 cells using Western blotting methods. The polarized efflux activities of P-gp(s) in MDCK-MDRI cells, MDCK-WT cells, and Caco-2 cells were compared using digoxin as a substrate. Apparent Michaelis-Menten constants (K(M),Vmax) for the efflux of vinblastine in these three cell lines were determined. Apparent inhibition constants (K(I)) of known substrates/inhibitors of P-gp were determined by measuring their effects on the efflux of digoxin in Caco-2 or MDCK-MDR1 cell monolayers.

RESULTS

MDCK-MDR1 cells expressed higher levels of P-gp compared to Caco-2 and MDCK-WT cells, as estimated by Western blots. Two isoforms of P-gp were expressed in Caco-2 and MDCK cells migrating with molecular weights of 150 kDa and 170 kDa. In MDCK-MDR1 cells, the 150 kDa isoforms appeared to be overexpressed. The MDCK-MDR1 cells exhibited higher polarized efflux of [3H]-digoxin than did Caco-2 and MDCK-WT cells. K(M) values of vinblastine in Caco-2. MDCK-WT, and MDCK-MDR1 cells were 89.2+/-26.1, 24.5+/-1.1, and 252.8+/-134.7 microM, respectively, whereas Vmax values were 1.77+/-0.22, 0.42+/-0.01, and 2.43+/-0.86 pmolcm(-2)s(-1), respectively. Known P-gp substrates/inhibitors showed, in general, lower K(I) values for inhibition of digoxin efflux in Caco-2 cells than in MDCK-MDR1 cells.

CONCLUSIONS

These data suggest that the MDCK-MDR1 cells overexpress the 150 kDa isoform of P-gp. MDCK-MDR1 cells are a useful model for screening the P-gp substrate activity of drugs and drug candidates. However, the apparent kinetics constants and affinities of substrates determined in the MDCK-MDR1 cell model may be different than the values obtained in Caco-2 cells. These differences in substrate activity could result from differences in the relative expression levels of total P-gp in Caco-2 and MDCK-MDR1 cells and/or differences in the partitioning of substrates into these two cell membrane bilayers.

Characterization of the efflux transporter(s) responsible for restricting intestinal mucosa permeation of the coumarinic acid-based cyclic prodrug of the opioid peptide DADLE

PURPOSE

To elucidate the efflux transporter(s) responsible for restricting the permeation of a coumarinic acid-based cyclic prodrug of the opioid peptide DADLE (CD) thorough Caco-2 cell monolayers.

METHODS

The cellular permeability characteristics of CD were investigated using Caco-2 cells, Madin-Darby canine kidney-wild type II cells (MDCK-WT). MDCK cells transfected with the human MDR1 gene (MDCK-MDR1), and MDCK cells transfected with human MRP2 gene (MDCK-MRP2). These cells were grown as monolayers onto microporous membranes. The disappearance from the donor side and appearance on the receiver side of CD were monitored by HPLC. The substrate activity of CD for P-gp was determined by using GF120918. a known P-gp specific inhibitor. The substrate activity of CD for MRP2 was determined by using cyclosporin A (CsA), a known MRP2 and P-gp inhibitor.

RESULTS

In Caco-2 cells, the ratio of the apparent permeability coefficients (Papp) of CD flux in the basolateral (BL) to apical (AP) direction vs. the flux in the AP-to-BL direction (Papp-BL-to-AP/Papp AP-to-BL) was 71. In the presence of GF120918 (2 microM), the Papp BL-to-AP/Papp AP-to-BL ratio was decreased to 16. In the presence of CsA (25 microM), the ratio was decreased to 5.6. In MDCK-WT. MDCK-MDR1, and MDCK-MRP2 cells, the Papp BL-AP/Papp AP-to-BL ratios of CD were 13, 35, and 22, respectively. CsA (25 microM) greatly decreased the Papp BL-P-AP/Papp AP-to-BL ratios in MDCK-WT and MDCK-MDR1 cells to 1.5 and 3.2, respectively. However, in MDCK-MRP2 cells. CsA (25 microM) decreased the ratio only to 11. A mixture of GF120918 (2 microM) and CsA (25 microM) decreased the Papp BL-to-AP/Papp AP-to-BL ratios of CD in MDCK-WT, MDCK-MDR1, and MDCK-MRP2 cells to 1.4, 2.7, and 5.4. respectively.

CONCLUSIONS

These data suggest that CD is a good substrate for both P-gp and MRP2 and that the restricted permeation of this cyclic prodrug in Caco-2 cells and in the intestinal mucosa is probably due to its substrate activities for both of these efflux transporters.

Are MDCK cells transfected with the human MRP2 gene a good model of the human intestinal mucosa?

PURPOSE

To investigate whether Madin-Darby canine kidney cells transfected with the human MRP2 gene (MDCK-MRP2) are a good model of the human intestinal mucosa.

METHODS

MRP2 expression in Caco-2 cells was compared with the expression of this efflux transporter in MDCK-wild type (MDCK-WT) and MDCK-MRP2 cells using Western blotting methods. The polarized efflux activities of MRP2 in the MDCK-MRP2, MDCK-WT. MDCK cells transfected with the human MDR1 gene (MDCK-MDR1), and Caco-2 cells were compared using vinblastine as a substrate. Apparent Michaelis-Menten constants (K(M), Vmax) for the efflux of vinblastine in Caco-2 and MDCK-MRP2 cells were determined in the presence of GF120918 (2 microM), which inhibits P-glycoprotein but does not affect MRP2. Apparent inhibitory constants (K(I)) of known substrates/inhibitors of MRP2 were determined by measuring their effects on the efflux of vinblastine in these cell lines.

RESULTS

MDCK-MRP2 cells expressed higher levels of MRP2 than MDCK-WT and Caco-2 cells as measured by Western blotting technique. Two isoforms of MRP2 expressed in Caco-2 and MDCK cells migrated at molecular weights of 150 kD and 190 kD. In MDCK-MRP2 cells, the 150 kD isoform appeared to be overexpressed. MDCK-MRP2 cell monolayers exhibited higher polarized efflux of vinblastine than Caco-2 and MDCK-WT cell monolayers. K(M) values for vinblastine in Caco-2 and MDCK-MRP2 cells were determined to be 71.8+/-11.6 and 137.3+/-33.6 microM. respectively, and Vmax values were determined to be (0.54+/-0.03 and 2.45+/-0.31 pmolcm(-2)s(-1), respectively. Known substrates/inhibitors of MRP2 showed differences in their ability to inhibit vinblastine efflux in Caco-2 cells as compared to MDCK-MRP2 cells

CONCLUSIONS

These data suggest that MDCK-MRP2 cells overexpress only the 150 kD isoform of MRP2. The 190 kD isoform, which was also found in Caco-2 cells and MDCK-WT cells, was present in MDCK-MRP2 cells but not over expressed. The apparent kinetics constants and affinities of some MRP2 substrates were different in Caco-2 cells and MDCK-MRP2 cells. These differences in substrate activity could result from differences in the relative expression levels of the MRP2 isoforms present in Caco-2 cells and MDCK-MRP2 cells and/or differences in the partitioning of substrates in these two cell membrane bilayers.

Transport of leukotriene C4 and structurally related conjugates

Transport proteins control the release of the endogenous glutathione conjugate leukotriene C4 (LTC4) from leukotriene-synthesizing cells as well as its hepatobiliary and renal elimination. The photolabile conjugated triene structure of LTC4 has enabled direct photoaffinity labeling of the multidrug resistance protein 1 (MRP1, symbol ABC C1) in membranes from mastocytoma cells, leading to the identification of the function of this protein as an ATP-dependent export pump for LTC4 and structurally related conjugates. MRP1 is assigned to the C branch of the superfamily of ATP-binding cassette (ABC) transporters and was originally identified by virtue of its association with drug resistance in tumor cells. Besides LTC4, which is a high-affinity substrate, a variety of conjugates of hydrophobic endogenous or xenobiotic substances with glutathione, glucuronate, or sulfate are transported by MRP1. In addition, hydrophobic compounds may undergo cotransport with glutathione. Effective inhibitors of MRP1-mediated transport include structural analogs of LTC4 and of other cysteinyl leukotrienes. The ATP-dependent transport system which transports cysteinyl leukotrienes across the hepatocyte canalicular membrane into bile was cloned and characterized as the second isoform or paralog of the mammalian MRP family, MRP2 (ABC C2). MRP2 is localized to the apical membrane of polarized cells. The overall substrate specificities of MRP1 and MRP2 are similar, despite an amino acid identity of only 48%. The transport proteins mediating the uptake of LTC4 into hepatocytes across the basolateral membrane are members of the organic anion transporter (OATP) branch of the solute carrier (SLC) superfamily and are thus distinct from the ATP-dependent export pumps of the MRP family.

Delineating the contribution of secretory transporters in the efflux of etoposide using Madin-Darby canine kidney (MDCK) cells overexpressing P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP1), and canalicular multispecific organic anion transporter (cMOAT)

Multidrug resistance conferred to cancer cells is often mediated by the expression of efflux transporter "pumps". It is also believed that many of the same transporters are involved in drug efflux from numerous normal endothelial and epithelial cell types in the intestine, brain, kidney, and liver. Etoposide transport kinetics were characterized in Caco-2 cells and in well established Madin-Darby canine kidney (MDCKII) cell lines that were stably-transfected with a human cDNA encoding P-glycoprotein (Pgp), human multidrug resistance protein (MRP1), or the canalicular multispecific organic anion (cMOAT) transporters to determine the roles of these transporters in etoposide efflux. Etoposide transport kinetics were concentration-dependent in the MDCKII-MDR1 and MDCKII-cMOAT cells. The apparent secretory Michaelis constant (Km) and carrier-mediated permeability (Pc) values for Pgp and cMOAT were 254.96 +/- 94.39 microM and 5.96 +/- 0.41 x 10(-6) cm/s and 616.54 +/- 163.15 microM and 1.87 +/- 0.10 x 10(-5) cm/s, respectively. The secretory permeability of etoposide decreased significantly in the basal to apical (B to A) (i.e., efflux) direction, whereas the permeability increased 2.3-fold in the apical to basal (A to B) direction in MDCKII-MDR1 cells in the presence of elacridar (GF120918). Moderate inhibition of etoposide efflux by leukotriene C4 (LTC4) was observed in MDCKII-cMOAT cells. Furthermore, etoposide inhibited LTC4 efflux, confirming the involvement of cMOAT. The flux of etoposide in MDCKII-MRP1 cells was similar to that in MDCKII/wt control cells. The current results demonstrate that the secretory transport mechanism of etoposide involves multiple transporters, including Pgp and cMOAT but not MRP1. These results demonstrate that Pgp and cMOAT are involved in the intestinal secretory transport of etoposide. Since the intestinal secretion of etoposide was previously reported in the literature, it also suggests that they may be involved in the in vivo intestinal secretion of etoposide; however, mechanistic in vivo studies are required to confirm this.

The MRP4/ABCC4 gene encodes a novel apical organic anion transporter in human kidney proximal tubules: putative efflux pump for urinary cAMP and cGMP

The cyclic nucleotides cAMP and cGMP play key roles in cellular signaling and the extracellular regulation of fluid balance. In the kidney, cAMP is excreted across the apical proximal tubular membrane into urine, where it reduces phosphate reabsorption through a dipyridamole-sensitive mechanism that is not fully understood. It has long been known that this cAMP efflux pathway is dependent on ATP and is inhibited by probenecid. However, its identity and whether cGMP shares the same transporter have not been established. Here the expression, localization, and functional properties of human multidrug resistance protein 4 (MRP4) are reported. MRP4 is localized to the proximal tubule apical membrane of human kidney, and membrane vesicles from Sf9 cells expressing human MRP4 exhibit ATP-dependent transport of [(3)H]cAMP and [(3)H]cGMP. Both probenecid and dipyridamole are potent MRP4 inhibitors. ATP-dependent [(3)H]methotrexate and [(3)H]estradiol-17beta-D-glucuronide transport by MRP4 and interactions with the anionic conjugates S-(2,4-dinitrophenyl)-glutathione, N-acetyl-(2,4-dinitrophenyl)-cysteine, alpha-naphthyl-beta-D-glucuronide, and p-nitrophenyl-beta-D-glucuronide are also demonstrated. In kidneys of rats deficient in the apical anionic conjugate efflux pump Mrp2, Mrp4 expression is maintained at the same level. It is concluded that MRP4 is a novel apical organic anion transporter and the putative efflux pump for cAMP and cGMP in human kidney proximal tubules.

Mutation of Trp1254 in the multispecific organic anion transporter, multidrug resistance protein 2 (MRP2) (ABCC2), alters substrate specificity and results in loss of methotrexate transport activity

The ATP-binding cassette (ABC) proteins comprise a large superfamily of transmembrane transporters that utilize the energy of ATP hydrolysis to translocate their substrates across biological membranes. Multidrug resistance protein (MRP) 2 (ABCC2) belongs to subfamily C of the ABC superfamily and, when overexpressed in tumor cells, confers resistance to a wide variety of anticancer chemotherapeutic agents. MRP2 is also an active transporter of organic anions such as methotrexate (MTX), estradiol glucuronide (E217betaG), and leukotriene C4 and is located on the apical membrane of polarized cells including hepatocytes where it acts as a biliary transporter. We recently identified a highly conserved tryptophan residue in the related MRP1 that is critical for the substrate specificity of this protein. In the present study, we have examined the effect of replacing the analogous tryptophan residue at position 1254 of MRP2. We found that only nonconservative substitutions (Ala and Cys) of Trp1254 eliminated [3H]E217betaG transport by MRP2, whereas more conservative substitutions (Phe and Tyr) had no effect. In addition, only the most conservatively substituted mutant (W1254Y) transported [3H]leukotriene C4, whereas all other substitutions eliminated transport of this substrate. On the other hand, all substitutions of Trp1254 eliminated transport of [3H]MTX. Finally, we found that sulfinpyrazone stimulated [3H]E217betaG transport by wild-type MRP2 4-fold, whereas transport by the Trp1254 substituted mutants was enhanced 6-10-fold. In contrast, sulfinpyrazone failed to stimulate [3H]MTX transport by either wild-type MRP2 or the MRP2-Trp1254 mutants. Taken together, our results demonstrate that Trp1254 plays an important role in the ability of MRP2 to transport conjugated organic anions and identify this amino acid in the putative last transmembrane segment (TM17) of this ABC protein as being critical for transport of MTX.

Transport of fluorescein in MDCKII-MRP1 transfected cells and mrp1-knockout mice

The multidrug resistant-associated protein 1 (MRP1) is a membrane-bound transport protein that is involved in the efflux of organic anions and has been implicated in multidrug resistance in cancer. MRP1 has also been reported to be ubiquitously expressed in normal tissues, including the brain. The presence of functional organic anion transporters in the blood-brain and blood-CSF barriers that influence the distribution of various compounds to the brain has long been known. The purpose of this study was to examine the role of MRP1 in the brain distribution of a model organic anion, fluorescein. The substrate specificity of MRP1 for fluorescein was initially determined by examining the accumulation of fluorescein in MDCKII MRP1-transfected cells. The distribution of fluorescein in the brain was then examined in wild-type and mrp1 gene knockout mice. The results show that in MDCKII MRP1-transfected cells, the accumulation of fluorescein was significantly lower (about 40% lower) than that in wild-type MDCKII cells. MRP1 inhibitors such as probenecid, MK-571, and LY402913 enhanced fluorescein accumulation in MDCKII MRP1-transfected cells to a greater extent than in wild-type MDCKII cells. In an in vivo study, after intravenous injection of fluorescein, the fluorescein brain-to-plasma concentration ratio in mrp1 knockout mice was not significantly different than that in wild-type mice. However, when probenecid was co-administered with fluorescein in wild-type mice, the fluorescein brain-to-plasma ratio was significantly increased (1.5-fold). These findings suggest that fluorescein is a substrate for MRP1. Furthermore, the in vivo study also suggests that MRP1 has a limited role in the transport and distribution of fluorescein in the brain. Therefore, other organic anion transport proteins, including the various isoforms of the MRP family, may be responsible for the accumulation and transport of organic anions in the brain.

Transport of the beta -O-glucuronide conjugate of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) by the multidrug resistance protein 1 (MRP1). Requirement for glutathione or a non-sulfur-containing analog

Nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) play a crucial role in the induction of lung cancer, and NNAL-O-glucuronide formation and elimination are important steps in detoxification of these compounds. In the present study, we investigated the ATP-binding cassette (ABC) protein, MRP1 (ABCC1), as a candidate transporter responsible for NNAL-O-glucuronide export. MRP1 mediates the active transport of numerous GSH-, sulfate-, and glucuronide-conjugated organic anions and can transport certain xenobiotics by a mechanism that may involve co-transport with GSH. Using membrane vesicles prepared from transfected cells, we found that MRP1 transports [3H]NNAL-O-glucuronide but is dependent on the presence of GSH (Km 39 microm, Vmax 48 pmol x mg(-1) x min(-1)). We also found that the sulfur atom in GSH was dispensable because transport was supported by the GSH analog, gamma-glutamyl-alpha-aminobutyryl-glycine. Despite stimulation of NNAL-O-glucuronide transport by GSH, there was no detectable reciprocal stimulation of [3H]GSH transport. Moreover, whereas the MRP1 substrates leukotriene C4 (LTC4) and 17beta-estradiol 17beta-(d-glucuronide) (E(2)17betaG) inhibited GSH-dependent uptake of [3H]NNAL-O-glucuronide, only [3H]LTC4 transport was inhibited by NNAL-O-glucuronide (+GSH) and the kinetics of inhibition were complex. A mutant form of MRP1, which transports LTC4 but not E(2)17betaG, also did not transport NNAL-O-glucuronide suggesting a commonality in the binding elements for these two glucuronidated substrates, despite their lack of reciprocal transport inhibition. Finally, the related MRP2 transported NNAL-O-glucuronide with higher efficiency than MRP1 and unexpectedly, GSH inhibited rather than stimulated uptake. These studies provide further insight into the complex interactions of the MRP-related proteins with GSH and their conjugated organic anion substrates, and extend the range of xenotoxins transported by MRP1 and MRP2 to include metabolites of known carcinogens involved in the etiology of lung and other cancers.

Cyclopentenone prostaglandins: new insights on biological activities and cellular targets

The cyclopentenone prostaglandins PGA2, PGA1, and PGJ2 are formed by dehydration within the cyclopentane ring of PGE2, PGE1, and PGD2. PGJ2 is metabolized further to yield Delta(12)-PGJ(2) and 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)). Various compounds within the cyclopentenone prostaglandin family possess potent anti-inflammatory, anti-neoplastic, and anti-viral activity. Most actions of the cyclopentenone prostaglandins do not appear to be mediated by binding to G-protein coupled prostanoid receptors. Rather, the bioactivity of these compounds results from their interaction with other cellular target proteins. 15-deoxy-Delta(12,14)-PGJ(2) is a high affinity ligand for the nuclear receptor PPARgamma and modulates gene transcription by binding to this receptor. Other activities of the cyclopentenone prostaglandins are mediated by the reactive alpha,beta-unsaturated carbonyl group located in the cyclopentenone ring. The transcription factor NF-kappaB and its activating kinase are key targets for the anti-inflammatory activity of 15d-PGJ2, which inhibits NF-kappaB-mediated transcriptional activation by PPARgamma-dependent and independent molecular mechanisms. Other cyclopentenone prostaglandins, such as Delta(7)-PGA1 and Delta(12)-PGJ2, have strong anti-tumor activity. These compounds induce cell cycle arrest or apoptosis of tumor cells depending on the cell type and treatment conditions. We review here recent progress in understanding the mechanisms of action of the cyclopentenone prostaglandins and their possible use as therapeutic agents.

ABC drug transporters: hereditary polymorphisms and pharmacological impact in MDR1, MRP1 and MRP2

Transport by ATP-dependent efflux pumps, such as P-glycoprotein (PGP) and multi-drug resistance related proteins (MRPs), influences bioavailability and disposition of drugs. These efflux pumps serve as defence mechanisms and determine bioavailability and CNS concentrations of many drugs. However, despite the fact that substantial data have been accumulated on the structure, function and pharmacological role of ABC transporters and even though modification of PGP function is an important mechanism of drug interactions and adverse effects in humans, there is a striking lack of data on variability of the underlying genes. This review focuses on the human drug transporter proteins PGP (MDR1) and the multi-drug resistance proteins MRP1 and MRP2. An overview is provided of pharmacologically relevant genetic, structural and functional data as well as on hereditary polymorphisms, their phenotypical consequences and pharmacological implications.

Influence of passive permeability on apparent P-glycoprotein kinetics

PURPOSE

The objectives of this work were to evaluate the importance of moderate passive permeability on apparent P-glycoprotein (P-gp) kinetics, and demonstrate that inspection of basolateral to apical and apical to basolateral (BL-AP/AP-BL) permeability ratios may result in a compound being overlooked as a P-gp substrate and inhibitor of another drug's transport via P-gp inhibition.

METHODS

The permeability ratios of nicardipine, vinblastine, cimetidine, and ranitidine were determined across Caco-2 monolayers that express P-gp, in the presence and absence of the specific P-gp inhibitor, GF120918. In addition, the permeability ratio of vinblastine was studied after pretreatment of Caco-2 monolayers with nicardipine, ranitidine, or cimetidine. Similar studies were repeated with hMDRI-MDCK monolayers.

RESULTS

The permeability ratios for cimetidine and vinblastine were >2. The permeability ratios for nicardipine and ranitidine were close to unity, and were not affected by the addition of GF120918. Based solely on ratios, only compounds with moderate transcellular permeability (vinblastine and cimetidine) would be identified as P-gp substrates. Although the permeability ratios appeared to be unity for nicardipine and ranitidine, both compounds affected the permeability of vinblastine, and were identified as substrates and inhibitors of P-gp. Studies performed in hMDR1-MDCK cells confirmed these experimental results. Data were explained in the context of a kinetic model, where passive permeability and P-gp efflux contribute to overall drug transport.

CONCLUSIONS

Moderate passive permeability was necessary for P-gp to reduce the AP-BL drug permeability. Inspection of the permeability ratio after directional transport studies did not effectively identify P-gp substrates that affected the P-gp kinetics of vinblastine. Because of the role of passive permeability, drug interaction studies with known P-gp substrates, rather than directional permeability studies, are needed to elucidate a more complete understanding of P-gp kinetics.

The (patho)physiological functions of the MRP family

The identification of certain members of the large superfamily of ATP binding cassette transport proteins such as MDR1 -P-glycoprotein and the multidrug resistance protein MRP1 as ATP-dependent drug efflux pumps has been a major contribution in our understanding of the multidrug resistance phenotype of cancer cells. Importantly, both transport proteins that exhibit only low structural homology have a very different substrate specificity but confer resistance to a similar spectrum of natural product chemotherapeutic drugs. In contrast to the drug transporter MDR1, MRP1 mainly transports anionic Phase II-conjugates. In addition MRP1-mediated drug resistance is highly dependent on high intracellular glutathione levels which may be linked to the apparent physiological involvement of MRP1 in glutathione-related cellular processes. This review summarizes the current knowledge about functional aspects of MRP1 and its five recently cloned homologues MRP2-MRP6 and discusses their substrate specificities and cellular localization with emphasis on drug resistance. Copyright 2000 Harcourt Publishers Ltd.

Molecular pharmacology of renal organic anion transporters

Renal organic anion transport systems play an important role in the elimination of drugs, toxic compounds, and their metabolites, many of which are potentially harmful to the body. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine of a wide variety of anionic substrates. Recent studies have shown that organic anion secretion in renal proximal tubule is mediated by distinct sodium-dependent and sodium-independent transport systems. Knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past few years by cloning of various carrier proteins. However, a number of fundamental questions still have to be answered to elucidate the participation of the cloned transporters in the overall tubular secretion of anionic xenobiotics. This review summarizes the latest knowledge on molecular and pharmacological properties of renal organic anion transporters and homologs, with special reference to their nephron and plasma membrane localization, transport characteristics, and substrate and inhibitor specificity. A number of the recently cloned transporters, such as the p-aminohippurate/dicarboxylate exchanger OAT1, the anion/sulfate exchanger SAT1, the peptide transporters PEPT1 and PEPT2, and the nucleoside transporters CNT1 and CNT2, are key proteins in organic anion handling that possess the same characteristics as has been predicted from previous physiological studies. The role of other cloned transporters, such as MRP1, MRP2, OATP1, OAT-K1, and OAT-K2, is still poorly characterized, whereas the only information that is available on the homologs OAT2, OAT3, OATP3, and MRP3-6 is that they are expressed in the kidney, but their localization, not to mention their function, remains to be elucidated.

Inhibitory effect of the reversal agents V-104, GF120918 and Pluronic L61 on MDR1 Pgp-, MRP1- and MRP2-mediated transport

The human multidrug transporter MDR1 P-glycoprotein and the multidrug resistance proteins MRP1 and MRP2 transport a range of cytotoxic drugs, resulting in multidrug resistance in tumour cells. To overcome this form of drug resistance in patients, several inhibitors (reversal agents) of these transporters have been isolated. Using polarized cell lines stably expressing human MDR1, MRP1 or MRP2cDNA, and 2008 ovarian carcinoma cells stably expressing MRP1 cDNA, we have investigated in this study the specificity of the reversal agents V-104 (a pipecolinate derivative), GF120918 (an acridone carboxamide derivative also known as GG918), and Pluronic L61 (a (poly)oxypropethylene and (poly)oxypropylene block copolymer). Transport experiments with cytotoxic drugs with polarized cell lines indicate that all three compounds efficiently inhibit MDR1 Pgp. Furthermore, V-104 partially inhibits daunorubicin transport by MRP1 but not vinblastine transport by MRP2. V-104 reverses etoposide resistance of 2008/MRP1 cells, whereas GF120918 does not reverse resistance due to MRP1. V-104 partially inhibits the export of the organic anion dinitrophenyl S-glutathione by MDCKII-MRP1 but not by MDCKII-MRP2 cells. Unexpectedly, export of the organic anion calcein by MDCKII-MRP1 and MDCKII-MRP2 cells is stimulated by Pluronic L61, probably because it relieves the block on entry of calcein AM into the cell by endogenous MDR1 Pgp.

Conjugate export pumps of the multidrug resistance protein (MRP) family: localization, substrate specificity, and MRP2-mediated drug resistance

The membrane proteins mediating the ATP-dependent transport of lipophilic substances conjugated to glutathione, glucuronate, or sulfate have been identified as members of the multidrug resistance protein (MRP) family. Several isoforms of these conjugate export pumps with different kinetic properties and domain-specific localization in polarized human cells have been cloned and characterized. Orthologs of the human MRP isoforms have been detected in many different organisms. Studies in mutant rats lacking the apical isoform MRP2 (symbol ABCC2) indicate that anionic conjugates of endogenous and exogenous substances cannot exit from cells at a sufficient rate unless an export pump of the MRP family is present in the plasma membrane. Several mutations in the human MRP2 gene have been identified which lead to the absence of the MRP2 protein from the hepatocyte canalicular membrane and to the conjugated hyperbilirubinemia of Dubin-Johnson syndrome. Overexpression of recombinant MRP2 confers resistance to multiple chemotherapeutic agents. Because of its function in the terminal excretion of cytotoxic and carcinogenic substances, MRP2 as well as other members of the MRP family, play an important role in detoxification and chemoprevention.

Structural, mechanistic and clinical aspects of MRP1

The cDNA encoding ATP-binding cassette (ABC) multidrug resistance protein MRP1 was originally cloned from a drug-selected lung cancer cell line resistant to multiple natural product chemotherapeutic agents. MRP1 is the founder of a branch of the ABC superfamily whose members (from species as diverse as plants and yeast to mammals) share several distinguishing structural features that may contribute to functional and mechanistic similarities among this subgroup of transport proteins. In addition to its role in resistance to natural product drugs, MRP1 (and related proteins) functions as a primary active transporter of structurally diverse organic anions, many of which are formed by the biotransformation of various endo- and xenobiotics by Phase II conjugating enzymes, such as the glutathione S-transferases. MRP1 is involved in a number of glutathione-related cellular processes. Glutathione also appears to play a key role in MRP1-mediated drug resistance. This article reviews the discovery of MRP1 and its relationships with other ABC superfamily members, and summarizes current knowledge of the structure, transport functions and relevance of this protein to in vitro and clinical multidrug resistance.

ATPase activity of purified and reconstituted multidrug resistance protein MRP1 from drug-selected H69AR cells

The ATP-binding cassette transporter protein, multidrug resistance protein MRP1, was purified from doxorubicin-selected H69AR lung tumor cells which express high levels of this protein. A purification procedure comprised of a differential two-step solubilization of MRP1 from plasma membranes with 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate followed by immunoaffinity chromatography using the MRP1-specific monoclonal antibody QCRL-1 was developed. Approximately 300 microgram of MRP1 was obtained from 6 mg of plasma membranes at 80-90% purity, as indicated by silver staining of protein gels. After reconstitution of purified MRP1 into proteoliposomes, kinetic analyses indicated that its K(m) for ATP hydrolysis was 104+/-22 microM with maximal activity of 5-10 nmol min(-1) mg(-1) MRP1. MRP1 ATPase activity was further characterized with various inhibitors and exhibited an inhibition profile that distinguishes it from P-glycoprotein and other ATPases. The ATPase activity of reconstituted MRP1 was stimulated by the conjugated organic anion substrates leukotriene C(4) (LTC(4)) and 17beta-estradiol 17-(beta-D-glucuronide) with 50% maximal stimulation achieved at concentrations of 150 nM and 1.6 microM, respectively. MRP1 ATPase was also stimulated by glutathione disulfide but not by reduced glutathione or unconjugated chemotherapeutic agents. This purification and reconstitution procedure is the first to be described in which the ATPase activity of the reconstituted MRP1 retains kinetic characteristics with respect to ATP-dependence and substrate stimulation that are very similar to those deduced from transport studies using MRP1-enriched plasma membrane vesicles.

Glutathione and glutathione-dependent enzymes represent a co-ordinately regulated defence against oxidative stress

Increases in the intracellular levels of reactive oxygen species (ROS), frequently referred to as oxidative stress, represents a potentially toxic insult which if not counteracted will lead to membrane dysfunction, DNA damage and inactivation of proteins. Chronic oxidative stress has numerous pathological consequences including cancer, arthritis and neurodegenerative disease. Glutathione-associated metabolism is a major mechanism for cellular protection against agents which generate oxidative stress. It is becoming increasingly apparent that the glutathione tripeptide is central to a complex multifaceted detoxification system, where there is substantial inter-dependence between separate component members. Glutathione participates in detoxification at several different levels, and may scavenge free radicals, reduce peroxides or be conjugated with electrophilic compounds. Thus, glutathione provides the cell with multiple defences not only against ROS but also against their toxic products. This article discusses how glutathione biosynthesis, glutathione peroxidases, glutathione S-transferases and glutathione S-conjugate efflux pumps function in an integrated fashion to allow cellular adaption to oxidative stress. Co-ordination of this response is achieved, at least in part, through the antioxidant responsive element (ARE) which is found in the promoters of many of the genes that are inducible by oxidative and chemical stress. Transcriptional activation through this enhancer appears to be mediated by basic leucine zipper transcription factors such as Nrf and small Maf proteins. The nature of the intracellular sensor(s) for ROS and thiol-active chemicals which induce genes through the ARE is described. Gene activation through the ARE appears to account for the enhanced antioxidant and detoxification capacity of normal cells effected by many cancer chemopreventive agents. In certain instances it may also account for acquired resistance of tumours to cancer chemotherapeutic drugs. It is therefore clear that determining the mechanisms involved in regulation of ARE-driven gene expression has enormous medical implications.

Localization of a substrate specificity domain in the multidrug resistance protein

Multidrug resistance protein (MRP) confers resistance to a number of natural product chemotherapeutic agents. It is also a high affinity transporter of some physiological conjugated organic anions such as cysteinyl leukotriene C(4) and the cholestatic estrogen, 17beta-estradiol 17(beta-D-glucuronide) (E(2)17betaG). We have shown that the murine orthologue of MRP (mrp), unlike the human protein, does not confer resistance to common anthracyclines and is a relatively poor transporter of E(2)17betaG. We have taken advantage of these functional differences to identify region(s) of MRP involved in mediating anthracycline resistance and E(2)17betaG transport by generating mrp/MRP hybrid proteins. All hybrid proteins conferred resistance to the Vinca alkaloid, vincristine, when transfected into human embryonic kidney cells. However, only those in which the COOH-terminal third of mrp had been replaced with the corresponding region of MRP-conferred resistance to the anthracyclines, doxorubicin, and epirubicin. Exchange of smaller segments of the COOH-terminal third of the mouse protein by replacement of either amino acids 959-1187 or 1188-1531 with those of MRP produced proteins capable of conferring some level of resistance to the anthracyclines tested. All hybrid proteins transported cysteinyl leukotriene C(4) with similar efficiencies. In contrast, only those containing the COOH-terminal third of MRP transported E(2)17betaG with an efficiency comparable with that of the intact human protein. The results demonstrate that differences in primary structure of the highly conserved COOH-terminal third of mrp and MRP are important determinants of the inability of the murine protein to confer anthracycline resistance and its relatively poor ability to transport E(2)17betaG.

Interactions of prostaglandin A2 with the glutathione-mediated biotransformation system

The cyclopentenone prostaglandin A2 (PGA2) is known to inhibit cell proliferation, and metabolism of this compound thus might be important in controlling its ultimate function. The glutathione-related metabolism of PGA2 was therefore investigated both with purified glutathione S-transferase P1-1 (GSTP1-1) and with IGR-39 human melanoma cells. Firstly, the irreversible inhibition of human GSTP1-1 and its mutants C47S, C101S, and C47S/C101S was studied. PGA2 appeared to inhibit GSTP1-1 mainly by binding to the cysteine 47 moiety of the enzyme. This binding was reversed by a molar excess of GSH, indicating that retro-Michael cleavage occurs. Secondly, after exposing IGR-39 human melanoma cells to PGA2, both diastereoisomers of the PGA2-glutathione conjugate are excreted into the medium, although with a clear excess of the S-form, due to its preferential formation by the GSTP1-1 present in the cells. Thirdly, the effect of PGA2 on intracellular GST activity was determined by quantification of the excreted glutathione conjugate S-(2,4-dinitrophenyl)glutathione (DNPSG) after exposure to 1-chloro-2,4-dinitrobenzene. DNPSG excretion was inhibited after incubation with 10 or 20 microM PGA2 for 1 or 4 hr, as a result of glutathione depletion, reversible GST inhibition, and covalent modification of intracellular GST. Furthermore, PGA2 also inhibited transport of DNPSG by the multidrug resistance-associated protein, an effect that was reversible and competitive. In conclusion, PGA2 modulates all three aspects of the glutathione-mediated biotransformation system, i.e. GSH levels, GSTP1-1 activity, and transport of GSH conjugates. A role for GSTP1-1 as a specific transport protein inside the cell is indicated.

Canalicular multispecific organic anion transporter/multidrug resistance protein 2 mediates low-affinity transport of reduced glutathione

The canalicular multispecific organic anion transporter (cMOAT), a member of the ATP-binding cassette transporter family, mediates the transport of a broad range of non-bile salt organic anions from liver into bile. cMOAT-deficient Wistar rats (TR-) are mutated in the gene encoding cMOAT, leading to defective hepatobiliary transport of a whole range of substrates, including bilirubin glucuronide. These mutants also have impaired hepatobiliary excretion of GSH and, as a result, the bile flow in these animals is reduced. In the present work we demonstrate a role for cMOAT in the excretion of GSH both in vivo and in vitro. Biliary GSH excretion in rats heterozygous for the cMOAT mutation (TR/tr) was decreased to 63% of controls (TR/TR) (114+/-24 versus 181+/-20 nmol/min per kg body weight). Madin-Darby canine kidney (MDCK) II cells stably expressing the human cMOAT protein displayed >10-fold increase in apical GSH excretion compared with wild-type MDCKII cells (141+/-6.1 pmol/min per mg of protein versus 13.2+/-1.3 pmol/min per mg of protein in wild-type MDCKII cells). Similarly, MDCKII cells expressing the human multidrug resistance protein 1 showed a 4-fold increase in GSH excretion across the basolateral membrane. In several independent cMOAT-transfectants, the level of GSH excretion correlated with the expression level of the protein. Furthermore, we have shown, in cMOAT-transfected cells, that GSH is a low-affinity substrate for the transporter and that its excretion is reduced upon ATP depletion. In membrane vesicles isolated from cMOAT-expressing MDCKII cells, ATP-dependent S-(2,4-dinitrophenyl)glutathione uptake is competitively inhibited by high concentrations of GSH (Ki approximately 20 mM). We concluded that cMOAT mediates low-affinity transport of GSH. However, since hepatocellular GSH concentrations are high (5-10 mM), cMOAT might serve an important physiological function in maintenance of bile flow in addition to hepatic GSH turnover.

Anti-cancer-prostaglandin-induced cell-cycle arrest and its modulation by an inhibitor of the ATP-dependent glutathione S-conjugate export pump (GS-X pump)

The A and J series of prostaglandins (PGs) accumulate in the nuclei to suppress the proliferation of cancer cells. Here we report that Delta7-PGA1 methyl ester, a synthetic anti-cancer PG, increased the level of mRNA for the cyclin-dependent kinase inhibitor p21 in human leukaemia HL-60 cells. The induction of p21 was associated with the accumulation of hypophosphorylated retinoblastoma protein (pRB) and the suppression of c-myc gene expression. Since the p53 gene is deleted in HL-60 cells, the anti-cancer PG is suggested to inhibit cancer cell growth by inducing p21 via a p53-independent pathway. Unlike HL-60 cells, cisplatin-resistant HL-60/R-CP cells were insensitive to Delta7-PGA1 methyl ester. While c-myc expression was transiently suppressed, neither G1 arrest nor hypophosphorylation of pRB was observed with the anti-cancer PG. Plasma membrane vesicles from HL-60/R-CP cells showed an enhanced level of GS-X pump (ATP-dependent glutathione S-conjugate export pump) activity towards the glutathione S-conjugate of Delta7-PGA1 methyl ester (Km 110 nM). GIF-0019 ¿N-carbomethoxy-S-[5-(4-benzoylphenyl)pentyl]glutathione dimethyl ester¿, a specific inhibitor of the GS-X pump, dose-dependently enhanced the cellular sensitivity of HL-60/R-CP cells to Delta7-PGA1 methyl ester and induced G1 arrest. The GS-X pump is suggested to play a pivotal role in modulating the biological action of the anti-cancer PG.

GSTP1-1 stereospecifically catalyzes glutathione conjugation of ethacrynic acid

Using 1H NMR two diastereoisomers of the ethacrynic acid glutathione conjugate (EASG) as well as ethacrynic acid (EA) could be distinguished and quantified individually. Chemically prepared EASG consists of equal amounts of both diastereoisomers. GSTP1-1 stereospecifically catalyzes formation of one of the diastereoisomers (A). The GSTP1-1 mutant C47S and GSTA1-1 preferentially form the same diastereoisomer of EASG as GSTP1-1. Glutathione conjugation of EA by GSTA1-2 and GSTA2-2 is not stereoselective. When human melanoma cells, expressing GSTP1-1, were exposed to ethacrynic acid, diastereoisomer A was the principal conjugate formed, indicating that even at physiological pH the enzyme catalyzed reaction dominates over the chemical conjugation.

Functional multidrug resistance protein (MRP1) lacking the N-terminal transmembrane domain

The human multidrug resistance protein (MRP1) causes drug resistance by extruding drugs from tumor cells. In addition to an MDR-like core, MRP1 contains an N-terminal membrane-bound region (TMD0) connected to the core by a cytoplasmic linker (L0). We have studied truncated MRP1 versions containing either the MDR-like core alone or the core plus linker L0, produced in the baculovirus-insect (Sf9) cell system. Their function was examined in isolated membrane vesicles. Full-length MRP1 showed ATP-dependent, vanadate-sensitive accumulation of leukotriene C4 and N-ethylmaleimide glutathione. In addition, leukotriene C4-stimulated, vanadate-dependent nucleotide occlusion was detected. The MDR-like core was virtually inactive. Co-expression of the core with the N-terminal region including L0 fully restored MRP1 function. Unexpectedly, a truncated MRP1 mutant lacking the entire TMD0 region but still containing L0 behaved like wild-type MRP1 in vesicle uptake and nucleotide trapping experiments. We also expressed the MRP1 constructs in polarized canine kidney derived MDCKII cells. Like wild-type MRP1, the MRP1 protein without the TMD0 region was routed to the lateral plasma membrane and transported dinitrophenyl glutathione and daunorubicin. The TMD0L0 and the MRP1 minus TMD0L0 remained in an intracellular compartment. Taken together, these experiments strongly suggest that the TMD0 region is neither required for the transport function of MRP1 nor for its proper routing to the plasma membrane.

Stimulation of ATPase activity of purified multidrug resistance-associated protein by nucleoside diphosphates

Membrane vesicles prepared from cells expressing the multidrug resistance-associated protein (MRP) transport glutathione S-conjugates of hydrophobic substrates in an ATP dependent manner. Purified MRP possesses ATPase activity which can be further stimulated by anticancer drugs or leukotriene C4. However, the detailed relationship between ATP hydrolysis and drug transport has not been established. How the ATPase activity of MRP is regulated in the cell is also not known. In this report, we have examined the effects of different nucleotides on the ATPase activity of purified MRP. We have found that pyrimidine nucleoside triphosphates have little effect on enzymatic activity. In contrast, purine nucleotides dATP, dGTP, and adenosine 5'-(beta,gamma-imido)triphosphate function as competitive inhibitors. Somewhat unexpectedly, low concentrations of all the nucleoside diphosphates (NDPs) tested, except UDP, stimulate the ATPase activity severalfold. ADP or GDP at higher concentrations was inhibitory, reflecting NDP binding to the substrate site. On the other hand, the enhancement of hydrolysis at low NDP concentrations must reflect interactions with a separate site. Therefore, we postulate the presence of at least two types of nucleotide binding sites on the MRP, a catalytic site(s) to which ATP preferentially binds and is hydrolyzed and a regulatory site to which NDPs preferentially bind and stimulate hydrolysis. Interestingly, the stimulatory effects of drugs transported by MRP and NDPs are not additive, i.e. drugs are not able to further stimulate the NDP-activated enzyme. Hence, the two activation pathways intersect at some point. Since both nucleotide binding domains of MRP are likely to be required for drug stimulation of ATPase activity, the two sites that we postulate may also involve both domains.

Renal transport of organic anions

This review deals with the different transport mechanisms mediating the apical and basolateral transport of organic anions, all of which are restricted to the proximal tubule. Several transport mechanisms, such as the para-aminohippurate basolateral transporter and the apical proton coupled di- and tripeptide transporter have been cloned, and their role in renal transport has been well characterized. Other transport proteins have been cloned from the kidney, liver, or intestine, but their role in the renal transport of organic anions needs to be elucidated. This is the case with Mdr2, oatp1 and OAT-K1, which were identified in the apical membrane of the proximal tubule, and with MDR1, the precise localization of which is still uncertain. Other apical transport mechanisms, sodium coupled transports and anion exchangers are involved in organic anion reabsorption.