Estradiol 3-glucuronide is transported by the multidrug resistance-associated protein 2 but does not activate the allosteric site bound by estradiol 17-glucuronide

Hepatic Bile Formation: Canalicular Osmolarity and Paracellular and Transcellular Water Flow

The purpose of this minireview is to show that a new paradigm is developing regarding hepatic bile flow. The focus thus far has been on carrier-mediated transport of bile acids and other solutes, such as glutathione, which create an osmotic gradient for the transcellular and paracellular flow of water into canaliculi. In addition to the physicochemical properties of bile acids, which govern the osmotic gradient, data now exist showing that the tight junctions governing paracellular water flow and Aquaporin-8 water channels governing transcellular water flow are regulated independently. Thus, the rate of water flow into the canaliculus in response to bile acid transport is variable and determines canalicular bile acid concentration, which affects the production and solubilization of cholesterol-lecithin vesicles. These new considerations modify thinking regarding the occurrence of cholestasis and its progression and reorient the design of experimental studies that can distinguish the different determinants of bile flow. SIGNIFICANCE STATEMENT: The paradigm that water flow into the canaliculus is determined only by the rate of carrier-mediated transport has been challenged recently by the changes that occur in hepatic bile composition in the Claudin-2 knockout mouse and with the cholestatic effect of estradiol 17β-d-glucuronide. Thus, a respective reduction in paracellular or transcellular canalicular water flow, probably via Aquaporin 8, has no significant effect on bile acid excretion.

Drug Concentration Asymmetry in Tissues and Plasma for Small Molecule-Related Therapeutic Modalities

The well accepted "free drug hypothesis" for small-molecule drugs assumes that only the free (unbound) drug concentration at the therapeutic target can elicit a pharmacologic effect. Unbound (free) drug concentrations in plasma are readily measurable and are often used as surrogates for the drug concentrations at the site of pharmacologic action in pharmacokinetic-pharmacodynamic analysis and clinical dose projection in drug discovery. Furthermore, for permeable compounds at pharmacokinetic steady state, the free drug concentration in tissue is likely a close approximation of that in plasma; however, several factors can create and maintain disequilibrium between the free drug concentration in plasma and tissue, leading to free drug concentration asymmetry. These factors include drug uptake and extrusion mechanisms involving the uptake and efflux drug transporters, intracellular biotransformation of prodrugs, membrane receptor-mediated uptake of antibody-drug conjugates, pH gradients, unique distribution properties (covalent binders, nanoparticles), and local drug delivery (e.g., inhalation). The impact of these factors on the free drug concentrations in tissues can be represented by K p,uu, the ratio of free drug concentration between tissue and plasma at steady state. This review focuses on situations in which free drug concentrations in tissues may differ from those in plasma (e.g., K p,uu > or <1) and discusses the limitations of the surrogate approach of using plasma-free drug concentration to predict free drug concentrations in tissue. This is an important consideration for novel therapeutic modalities since systemic exposure as a driver of pharmacologic effects may provide limited value in guiding compound optimization, selection, and advancement. Ultimately, a deeper understanding of the relationship between free drug concentrations in plasma and tissues is needed.

MRP2 Inhibition by HIV Protease Inhibitors in Rat and Human Hepatocytes: A Quantitative Confocal Microscopy Study

Hepatic drug transporters play a pivotal role in the excretion of drugs from the body, in drug-drug interactions, as well as in drug-induced liver toxicity. Hepatocytes cultured in sandwich configuration are an advantageous model to investigate the interactions of drug candidates with apical efflux transporters in a biorelevant manner. However, the commonly used "offline" assays (i.e., that rely on measuring intracellular accumulated amounts after cell lysis) are time- and resource-consuming, and the data output is often highly variable. In the present study, we used confocal microscopy to investigate the inhibitory effect of all marketed HIV protease inhibitors (10 μM) on the apical efflux transporter multidrug resistance-associated protein 2 (MRP2; ABCC2) by visualizing the biliary accumulation of the fluorescent substrate 5(6)-carboxy-2',7'-dichlorofluorescein (CDF). This method was applied with sandwich-cultured human and rat hepatocytes. Alterations in the biliary excretion index of CDF were calculated on the basis of quantitative analysis of fluorescence intensities in the confocal images. In human hepatocytes, lopinavir followed by tipranavir, saquinavir, atazanavir, and darunavir were the most potent inhibitors of MRP2-mediated efflux of CDF. In rat hepatocytes, tipranavir inhibited Mrp2-mediated CDF efflux most potently, followed by lopinavir and nelfinavir. In conclusion, a comparison of these findings with previously published data generated in offline transporter inhibition assays indicates that this microscopy-based approach enables investigation of the inhibitory effect of drugs on efflux transporters in a very sensitive but nondestructive manner.

Cellular Pharmacokinetic Model-Based Analysis of Genistein, Glyceollin, and MK-571 Effects on 5 (and 6)-Carboxy-2',7'-Dichloroflourescein Disposition in Caco-2 Cells

Pharmacokinetic modeling was used to describe 5 (and 6)-carboxy-2',7'-dichloroflourescein (CDF) disposition in Caco-2 cells following CDF or CDFDA (CDF diacetate) dosing. CDF transcellular flux was modeled by simple passive diffusion. CDFDA dosing models were based on simultaneous fitting of CDF levels in apical, basolateral, and intracellular compartments. Predicted CDF efflux was 50% higher across the apical versus the basolateral membrane. This difference was similar following apical and basolateral CDFDA dosing, despite intracellular levels being 3-fold higher following basolateral dosing, thus supporting nonsaturable CDF efflux kinetics. A 3-compartment catenary model with intracellular CDFDA hydrolysis described CDF disposition. This model predicted that apical CDF efflux was not altered in the presence of MK-571, and that basolateral membrane clearance was enhanced to account for reduced intracellular CDF in the presence of this multidrug resistance-associated protein (MRP) inhibitor. Similar effects were predicted for glyceollin, while genistein exposure had no predicted effects on CDF efflux. These modulator effects are discussed in the context of model predicted intracellular CDF concentrations relative to reports of CDF affinity (measured by Km) for MRP2 and MRP3. This model-based analysis confirms the complexity of efflux kinetics and suggests that other transporters may have contributed to CDF efflux.

Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter

ATP binding cassette type-C (ABCC) transporters move molecules across cell membranes upon hydrolysis of ATP; however, their coupling of ATP hydrolysis to substrate transport remains elusive. Drosophila multidrug resistance-associated protein (DMRP) is the functional ortholog of human long ABCC transporters, with similar substrate and inhibitor specificity, but higher activity. Exploiting its high activity, we kinetically dissected the catalytic mechanism of DMRP by using E₂-d-glucuronide (E₂G), the physiologic substrate of human ABCC. We examined the DMRP-mediated interdependence of ATP and E₂G in biochemical assays. We observed E₂G-dependent ATPase activity to be biphasic at subsaturating ATP concentrations, which implies at least 2 E₂G binding sites on DMRP. Furthermore, transport measurements indicated strong nonreciprocal cooperativity between ATP and E₂G. In addition to confirming these findings, our kinetic modeling with the Complex Pathway Simulator indicated a 10-fold decrease in the E₂G-mediated activation of ATP hydrolysis upon saturation of the second E₂G binding site. Surprisingly, the binding of the second E₂G allowed for substrate transport with a constant rate, which tightly coupled ATP hydrolysis to transport. In summary, we show that the second E₂G binding-similar to human ABCC2-allosterically stimulates transport activity of DMRP. Our data suggest that this is achieved by a significant increase in the coupling of ATP hydrolysis to transport.-Karasik, A., Ledwitch, K. V., Arányi, T., Váradi, A., Roberts, A., Szeri, F. Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter.

Cysteinyl Leukotriene Receptor 1/2 Antagonists Nonselectively Modulate Organic Anion Transport by Multidrug Resistance Proteins (MRP1-4)

Active efflux of both drugs and organic anion metabolites is mediated by the multidrug resistance proteins (MRPs). MRP1 (ABCC1), MRP2 (ABCC2), MRP3 (ABCC3), and MRP4 (ABCC4) have partially overlapping substrate specificities and all transport 17β-estradiol 17-(β-d-glucuronide) (E217βG). The cysteinyl leukotriene receptor 1 (CysLT1R) antagonist MK-571 inhibits all four MRP homologs, but little is known about the modulatory effects of newer leukotriene modifiers (LTMs). Here we examined the effects of seven CysLT1R- and CysLT2R-selective LTMs on E217βG uptake into MRP1-4-enriched inside-out membrane vesicles. Their effects on uptake of an additional physiologic solute were also measured for MRP1 [leukotriene C4 (LTC4)] and MRP4 [prostaglandin E2 (PGE2)]. The two CysLT2R-selective LTMs studied were generally more potent inhibitors than CysLT1R-selective LTMs, but neither class of antagonists showed any MRP selectivity. For E217βG uptake, LTM IC50s ranged from 1.2 to 26.9 μM and were most comparable for MRP1 and MRP4. The LTM rank order inhibitory potencies for E217βG versus LTC4 uptake by MRP1, and E217βG versus PGE2 uptake by MRP4, were also similar. Three of four CysLT1R-selective LTMs also stimulated MRP2 (but not MRP3) transport and thus exerted a concentration-dependent biphasic effect on MRP2. The fourth CysLT1R antagonist, LY171883, only stimulated MRP2 (and MRP3) transport but none of the MRPs were stimulated by either CysLT2R-selective LTM. We conclude that, in contrast to their CysLTR selectivity, CysLTR antagonists show no MRP homolog selectivity, and data should be interpreted cautiously if obtained from LTMs in systems in which more than one MRP is present.

A multifactorial approach to hepatobiliary transporter assessment enables improved therapeutic compound development

The bile salt export pump (BSEP) is expressed at the canalicular domain of hepatocytes, where it serves as the primary route of elimination for monovalent bile acids (BAs) into the bile canaliculi. The most compelling evidence linking dysfunction in BA transport with liver injury in humans is found with carriers of mutations that render BSEP nonfunctional. Based on mounting evidence, there appears to be a strong association between drug-induced BSEP interference and liver injury in humans; however, causality has not been established. For this reason, drug-induced BSEP interference is best considered a susceptibility factor for liver injury as other host- or drug-related properties may contribute to the development of hepatotoxicity. To better understand the association between BSEP interference and liver injury in humans, over 600 marketed or withdrawn drugs were evaluated in BSEP expressing membrane vesicles. The example of a compound that failed during phase 1 human trials is also described, AMG 009. AMG 009 showed evidence of liver injury in humans that was not predicted by preclinical safety studies, and BSEP inhibition was implicated. For 109 of the drugs with some effect on in vitro BSEP function, clinical use, associations with hepatotoxicity, pharmacokinetic data, and other information were annotated. A steady state concentration (C(ss)) for each of these annotated drugs was estimated, and a ratio between this value and measured IC₅₀ potency values were calculated in an attempt to relate exposure to in vitro potencies. When factoring for exposure, 95% of the annotated compounds with a C(ss)/BSEP IC₅₀ ratio ≥ 0.1 were associated with some form of liver injury. We then investigated the relationship between clinical evidence of liver injury and effects to multidrug resistance-associated proteins (MRPs) believed to play a role in BA homeostasis. The effect of 600+ drugs on MRP2, MRP3, and MRP4 function was also evaluated in membrane vesicle assays. Drugs with a C(ss)/BSEP IC₅₀ ratio ≥ 0.1 and a C(ss)/MRP IC₅₀ ratio ≥ 0.1 had almost a 100% correlation with some evidence of liver injury in humans. These data suggest that integration of exposure data, and knowledge of an effect to not only BSEP but also one or more of the MRPs, is a useful tool for informing the potential for liver injury due to altered BA transport.

Functional analysis of nonsynonymous single nucleotide polymorphisms of multidrug resistance-associated protein 2 (ABCC2)

BACKGROUND

Multidrug resistance-associated protein 2 (MRP2; ABCC2) mediates the biliary excretion of glutathione, glucuronide, and sulfate conjugates of endobiotics and xenobiotics. Single nucleotide polymorphisms (SNPs) of MRP2 contribute to interindividual variability in drug disposition and ultimately in drug response.

OBJECTIVES

To characterize the transport function of human wild-type (WT) MRP2 and four SNP variants, S789F, A1450T, V417I, and T1477M.

METHODS

The four SNP variants were expressed in Sf9 cells using recombinant baculovirus infection. The kinetic parameters [Km, (μmol/l); V(max), (pmol/mg/min); the Hill coefficient] of ATP-dependent transport of leukotriene C(4) (LTC(4)), estradiol-3-glucuronide (E(2)3G), estradiol-17β-glucuronide (E(2)17G), and tauroursodeoxycholic acid (TUDC) were determined in Sf9-derived plasma membrane vesicles. Transport activity was normalized for expression level.

RESULTS

The V(max) for transport activity was decreased for all substrates for S789F, and for all substrates except E(2)17G for A1450T. V417I showed decreased apparent affinity for LTC(4), E(2)3G, and E(2)17G, whereas transport was similar between wild-type (WT) and T1477M, except for a modest increase in TUDC transport. Examination of substrate-stimulated MRP2-dependent ATPase activity of S789F and A1450T, SNPs located in MRP2 nucleotide-binding domains (NBDs), demonstrated significantly decreased ATPase activity and only modestly decreased affinity for ATP compared with WT.

CONCLUSION

SNPs in the NBDs (S789F in the D-loop of NBD1, or A1450T near the ABC signature motif of NBD2) variably decreased the transport of all substrates. V417I in membrane spanning domain 1 selectively decreased the apparent affinity for the glutathione and glucuronide conjugated substrates, whereas the T1477M SNP in the carboxyl terminus altered only TUDC transport.

Quantitative immunofluorescent blotting of the multidrug resistance-associated protein 2 (MRP2)

INTRODUCTION

Quantitation of the expression levels of proteins involved in drug transport and disposition is needed to overcome limitations of film-based detection of chemiluminescent immunoblots.

PURPOSE

The purpose was to describe and validate a quantitative immunofluorescent blotting method for detection of ATP-binding cassette transporter isoform C2/multidrug resistance-associated protein 2 (ABCC2/MRP2).

METHODS

Western blotting was performed by electrophoresis of membrane vesicle protein isolated from Sf9 cells overexpressing MRP2 subsequently blotted with infrared labeled secondary antibody. The bound complex was detected using the Odyssey Infrared Imaging System (Li-Cor; Lincoln, NE). The images were analyzed using the Odyssey Application Software to obtain the integrated intensities, followed by linear regression of the intensity data.

RESULTS

The limits of quantitation for the time-insensitive technique described here were from 0.001 μg to 0.5 μg of total membrane protein, the coefficient of variation of the slope was 8.9%; r2 values were 0.986 ± 0.012. The utility and sensitivity of this technique were demonstrated in quantitating expression of MRP2 in human placental tissue samples, in which MRP2 was present in low abundance.

DISCUSSION

The immunofluorescent blotting technique described provides sensitive, reproducible, and quantitative determinations of large, integral membrane proteins such as MRP2, all with potential long-term cost savings.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Hepatobiliary disposition of 3alpha,6alpha,7alpha,12alpha-tetrahydroxy-cholanoyl taurine: a substrate for multiple canalicular transporters

Tetrahydroxy bile acids become major biliary bile acids in Bsep(-/-) mice and Fxr(-/-) mice fed cholic acid; we characterized disposition of these novel bile acids that also occur in patients with cholestasis. We investigated mouse Mrp2 (mMrp2) and P-glycoprotein [(P-gp) mMdr1a]-mediated transport of a tetrahydroxy bile acid, 6α-OH-taurocholic acid (6α-OH-TC), and its biliary excretion in wild-type and Mrp2(-/-) mice in the presence or absence of N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918), a P-gp and breast cancer resistance protein inhibitor. 6α-OH-TC was rapidly excreted into bile of wild-type mice (78% recovery); coinfusion of GF120918 had no significant effect. In Mrp2(-/-) mice, biliary excretion was decreased (52% recovery) and coinfusion of GF120918 further decreased these values (34% recovery). In wild-type, but not Mrp2(-/-), mice, 6α-OH-TC increased bile flow 2.5-fold. Membrane vesicle transport studies of 6α-OH-TC (0.05-0.75 mM) yielded saturation kinetics with a higher apparent affinity for mMrp2 (K(m) = 0.13 mM) than for mMdr1a (K(m) = 0.33 mM); mBsep transported 6α-OH-TC with positive cooperativity (Hill slope = 2.1). Human multidrug resistance-associated protein (MRP) 2 and P-gp also transported 6α-OH-TC but with positive cooperativity (Hill slope = 3.6 and 1.6, respectively). After intraileal administration, the time course of 6α-OH-TC biliary recovery was similar to that of coinfused taurocholate, implying that 6α-OH-TC can undergo enterohepatic cycling. Thus, Mrp2 plays a key role in 6α-OH-TC biliary excretion, whereas P-glycoprotein plays a secondary role; Bsep likely mediates excretion of 6α-OH-TC in the absence of Mrp2 and P-gp. In Bsep(-/-) mice, efficient synthesis of tetrahydroxy bile acids that are Mrp2 and P-gp substrates can explain the noncholestatic phenotype.

Formation and efflux of ATP-binding cassette transporter substrate 2,4-dinitrophenyl-S-glutathione from cultured human term placental villous tissue fragments

Upon exposure to 1-chloro-2,4-dinitrobenzene (CDNB), the human placental tissue forms its glutathione conjugate 2,4-dinitrophenyl-S-glutathione (DNP-SG). The purpose of this study was to investigate the involvement of human placental ATP-binding cassette (ABC) transporters in the efflux of DNP-SG. Placental tissue samples were obtained from pregnant patients undergoing C-section deliveries following normal pregnancies; villous tissue was cultured in suspension, and DNP-SG formation and efflux upon exposure to 100 microM CDNB were measured by HPLC. DNP-SG efflux decreased by 69.1 (+/-11.3)%, 51.1 (+/-5.4)%, 56.7 (+/-8.3)% and 53.6 (+/-10.8)% (p < 0.05) in the presence of 5 mM sodium orthovanadate (ATPase inhibitor), 100 microM MK571 (MRP-inhibitor), 1 mM dipyridamole (BCRP/P-gp/MRP1-inhibitor) and 100 microM verapamil (P-gp/MRP1 inhibitor) respectively, without any change in DNP-SG formation, total tissue glutathione, GSH/GSSG ratio, tissue integrity or tissue viability. These data clearly established the role of ABC transporters in the human placental efflux of DNP-SG. To investigate the contribution of various ABC transporters toward DNP-SG transport, ATP-dependent transport of 3H-DNP-SG was determined in Sf9 membrane vesicles overexpressing P-gp, BCRP and the MRP proteins. MRP1-mediated DNP-SG transport was inhibited in the presence of sodium orthovanadate, MK571, dipyridamole and verapamil in the presence of glutathione. Furthermore, MRP1-mediated transport [K(t) = 11.3 +/- 1.3 microM and v(max) = 86.7 +/- 1.9 pmol/mg/min] was a high-affinity process compared to MRP2-mediated transport [K(t) = 168 +/- 7 microM and v(max) = 1367 +/- 18 pmol/mg/min]. The inhibition pattern and the kinetics of DNP-SG efflux in the placental villous tissue were consistent with MRP1-mediated DNP-SG efflux, suggesting a functional role and an apical localization for an MRP1-like transporter in the human placental syncytiotrophoblast.

Multidrug resistance protein 2-mediated estradiol-17beta-D-glucuronide transport potentiation: in vitro-in vivo correlation and species specificity

Multidrug resistance protein 2 (MRP2) is a multispecific organic anion transporter expressed at important pharmacological barriers, including the canalicular membrane of hepatocytes. At this location it is involved in the elimination of both endogenous and exogenous waste products, mostly as conjugates, to the bile. Estradiol-17beta-d-glucuronide (E(2)17betaG), a widely studied endogenous substrate of MRP2, was shown earlier to recognize two binding sites of the transporter in vesicular transport assays. MRP2 modulators (substrates and nonsubstrates) potentiate the transport of E(2)17betaG by MRP2. We correlated data obtained from studies of different complexities and investigated the species-specific differences between rat and human MRP2-mediated transport. We used vesicular transport assays, sandwich-cultured primary hepatocytes, and in vivo biliary efflux in rats. Our results demonstrate that the rat Mrp2 transporter, unlike the human MRP2, transports E(2)17betaG according to Michaelis-Menten type kinetics. Nevertheless, in the presence of modulator drugs E(2)17betaG transport mediated by the rat transporter also shows cooperative kinetics as potentiation of E(2)17betaG transport was observed in the vesicular transport assay. We also demonstrated that the potentiation exists both in rat and in human hepatocytes and in vivo in rats.

Transport of diclofenac by breast cancer resistance protein (ABCG2) and stimulation of multidrug resistance protein 2 (ABCC2)-mediated drug transport by diclofenac and benzbromarone

Diclofenac is an important analgesic and anti-inflammatory drug, widely used for treatment of postoperative pain, rheumatoid arthritis, and chronic pain associated with cancer. Consequently, diclofenac is often used in combination regimens and undesirable drug-drug interactions may occur. Because many drug-drug interactions may occur at the level of drug transporting proteins, we studied interactions of diclofenac with apical ATP-binding cassette (ABC) multidrug efflux transporters. Using Madin-Darby canine kidney (MDCK)-II cells transfected with human P-glycoprotein (P-gp; MDR1/ABCB1), multidrug resistance protein 2 (MRP2/ABCC2), and breast cancer resistance protein (BCRP/ABCG2) and murine Bcrp1, we found that diclofenac was efficiently transported by murine Bcrp1 and moderately by human BCRP but not by P-gp or MRP2. Furthermore, in Sf9-BCRP membrane vesicles diclofenac inhibited transport of methotrexate in a concentration-dependent manner. We next used MDCK-II-MRP2 cells to study interactions of diclofenac with MRP2-mediated drug transport. Diclofenac stimulated paclitaxel, docetaxel, and saquinavir transport at only 50 microM. We further found that the uricosuric drug benzbromarone stimulated MRP2 at an even lower concentration, having maximal stimulatory activity at only 2 microM. Diclofenac and benzbromarone stimulated MRP2-mediated transport of amphipathic lipophilic drugs at 10- and 250-fold lower concentrations, respectively, than reported for other MRP2 stimulators. Because these concentrations are readily achieved in patients, adverse drug-drug interactions may occur, for example, during cancer therapy, in which drug concentrations are often critical and stimulation of elimination via MRP2 may result in suboptimal chemotherapeutic drug concentrations. Moreover, stimulation of MRP2 activity in tumors may lead to increased efflux of chemotherapeutic drugs and thereby drug resistance.

Species-dependent transport and modulation properties of human and mouse multidrug resistance protein 2 (MRP2/Mrp2, ABCC2/Abcc2)

Multidrug resistance protein 2 (MRP2/Mrp2) is a transporter that can influence the absorption, distribution, and elimination of many drugs. Mrp2 knockout mice are being used to study Mrp2 functions in vivo, including pharmacokinetics of drugs. To assess possible species-specific differences between human MRP2 and mouse Mrp2, we generated polarized cell lines expressing mouse Mrp2 and used these to investigate transport of clinically important agents. We also tested the ability of other drugs to modulate MRP2/Mrp2-mediated transport, a phenomenon that can lead to drug-drug interactions. In MDCK cells stably expressing human MRP2 or mouse Mrp2, saquinavir and docetaxel were more efficiently transported by mouse Mrp2, whereas vinblastine was transported better by human MRP2. MRP2/Mrp2-mediated transepithelial transport of several drugs could be stimulated by probenecid and sulfanitran, but stimulation was often more pronounced for human MRP2 than for mouse Mrp2. Interestingly, for some drugs the MRP2 modulator sulfinpyrazone had opposite effects on both transporters, stimulating human MRP2 and inhibiting mouse Mrp2 activity. In vesicular transport studies, transport of estradiol-17beta-glucuronide by mouse Mrp2 showed homotropic cooperativity, as previously described for human MRP2. The MRP2 modulators again showed differential effects on estradiol-17beta-glucuronide transport, most notably with sulfinpyrazone stimulating human MRP2 and profoundly inhibiting mouse Mrp2 activity. In conclusion, although human and mouse MRP2/Mrp2 have largely overlapping substrate specificities, there are important species differences in the transport efficiency of MRP2 substrates and in the modulation of transport by other compounds. These differences should be taken into account when results obtained in mice are extrapolated to humans.

Human multidrug resistance protein 2 transports the therapeutic bile salt tauroursodeoxycholate

The multidrug resistance protein 2 (MRP2/ABCC2) mediates the biliary excretion of glucuronide and glutathione conjugates of endogenous and exogenous compounds. We examined the activation of human MRP2-mediated ATP-dependent transport by the choleretic bile salt ursodeoxycholic acid (UDC) and its taurine and glycine amidates in Sf9 cell membranes expressing MRP2 using beta-estradiol 17-(beta-D-glucuronide) (E(2)17G) and beta-estradiol 3-(beta-D-glucuronide) (E(2)3G) as substrates. MRP2 transported E(2)3G via classic Michaelis-Menten kinetics (K(m) = 122 microM; V(max) = 3.0 nmol/mg/min), whereas E(2)17G transport showed positive cooperativity (Hill slope, 2.15; K(m) = 75 microM; V(max) = 3.8 nmol/mg/min). UDC, tauroursodeoxycholate, and glycoursodeoxycholate (80-100 microM) maximally stimulated E(2)3G transport 9-, 7.9-, and 3.6-fold, respectively, whereas higher concentrations (1-2 mM) inhibited transport. At low (0.3 microM) concentrations, tauroursodeoxycholate was transported only in the presence of E(2)17G or E(2)3G, but not other MRP2 substrates such as methotrexate, leukotriene C(4), or S-methylglutathione. Kinetic analysis of higher concentrations of tauroursodeoxycholate transport by MRP2 showed positive cooperativity (Hill slope, 1.84; K(m) = 127 microM; V(max) = 779 pmol/mg/min). Taurocholate (2-100 microM) was not detectably transported by MRP2 either alone or in the presence of E(2)17G but was transported in the presence of E(2)3G. Thus, UDC, tauroursodeoxycholate, and glycoursodeoxycholate activated MRP2 transport. Tauroursodeoxycholate was transported by MRP2 and demonstrated positive cooperativity, identifying it as the second MRP2 substrate able to stimulate its own transport. The data suggest MRP2 binding sites that can require specific complementarities between substrates and modulators of MRP2-mediated transport.

Functional analysis of mouse and monkey multidrug resistance-associated protein 2 (Mrp2)

We investigated the intrinsic transport activity of mouse and monkey Mrp2 and compared it with that of rat and dog Mrp2 reported previously. Mrp2 cDNAs were isolated from BALB/c and Macaca fascicularis liver, respectively, and vesicle transport studies were performed using recombinant Mrp2s expressed in insect Sf9 cells. ATP-dependent transport of [3H]leukotriene C4 (LTC4), [3H]17beta-estradiol 17-(beta-D-glucuronide) (E217betaG), [3H]bromosulfophthalein (BSP), and [3H]cholecystokinin octapeptide (CCK-8) were readily detected for all Mrp2s. A species difference in the intrinsic transport activity was apparent for LTC4 (monkey > mouse, dog > rat) and BSP (rat, dog, monkey > mouse). In addition to the difference in the transport activity, complex kinetic profiles were also evident in CCK-8, where a cooperative transport site was observed. Moreover, the transport of [3H]E217betaG by mouse and monkey Mrp2 was quite different from that of rat and dog Mrp2 in that 1) there was practically only nonsaturable uptake for [3H]E217betaG and 2) 4-methylumbelliferon glucuronide (Mrp2 modulator) showed a concentration-dependent stimulatory effect on the transport of [3H]E217betaG in mouse and monkey Mrp2, whereas rat and dog transport activity was inhibited by the modulator. In conclusion, although the substrate specificity is similar, the intrinsic transport activity differs from one species to another. This is due not only to the difference in the Km and Vmax values, but also the qualitatively different mode of substrate and modulator recognition exhibited by different species.

Increase in Mrp1 expression and 4-hydroxy-2-nonenal adduction in heart tissue of Adriamycin-treated C57BL/6 mice

Multidrug resistance-associated protein 1 (MRP1) mediates the ATP-dependent efflux of endobiotics and xenobiotics, including estradiol 17-(beta-d-glucuronide), leukotriene C(4), and the reduced glutathione conjugate of 4-hydroxy-2-nonenal (HNE), a highly reactive product of lipid peroxidation. Adriamycin is an effective cancer chemotherapeutic drug whose use is limited by cardiotoxicity. Adriamycin induces oxidative stress and production of HNE in cardiac tissue, which may contribute to cardiomyopathy. We investigated the role of Mrp1 in Adriamycin-induced oxidative stress in cardiac tissue. Mice were treated with Adriamycin (20 mg/kg, i.p.), and heart homogenate and sarcolemma membranes were assayed for Mrp1 expression and ATP-dependent transport activity. Expression of Mrp1 was increased at 6 and 24 hours after Adriamycin treatment compared with saline treatment. HNE-adducted proteins were significantly increased (P < 0.001) in the homogenates at 6 hours after Adriamycin treatment and accumulated further with time; HNE adduction of a 190-kDa protein was evident 3 days after Adriamycin treatment. Mrp1 was localized predominately in sarcolemma as shown by confocal and Western blot analysis. Sarcolemma membrane vesicles transported leukotriene C(4) with a K(m) and V(max) of 51.8 nmol/L and 94.1 pmol/min/mg, respectively, and MK571 (10 micromol/L) inhibited the transport activity by 65%. Exposure of HEK(Mrp1) membranes to HNE (10 micromol/L) significantly decreased the V(max) for estradiol 17-(beta-d-glucuronide) transport by 50%. These results show that expression of Mrp1 in the mouse heart is localized predominantly in sarcolemma. Adriamycin treatment increased Mrp1 expression and HNE adduction of Mrp1. Cardiac Mrp1 may play a role in protecting the heart from Adriamycin-induced cardiomyopathy by effluxing HNE conjugates.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

MRP2 and 3 in health and disease

MRPs are membrane proteins transporting organic anions at the expense of ATP hydrolysis. MRP2 is known to be a major transporter of organic anions from the liver into bile. We discuss recent results showing allosteric control of human but not rat MRP2. MRP3 has been considered a major player in bile salt metabolism, but our recent results with Mrp3 KO mice do not support this. Instead, we have found a role for MRP3 in the cellular export of drug-glucuronide conjugates. We discuss problems in extrapolating results obtained for murine MRPs.

Kinetic validation of the use of carboxydichlorofluorescein as a drug surrogate for MRP5-mediated transport

Multidrug resistance protein-5 (MRP5, ABCC5) is a member of the ATP-binding cassette transporter superfamily that effluxes a broad range of natural and xenobiotic compounds such as cyclic GMP, antiviral compounds, and cancer chemotherapeutic agents including nucleoside-based drugs, antifolate agents and platinum compounds. In cellular assays, MRP5 transfectants are less fluorescent after incubation with 5-chloromethylfluorescein diacetate (CMFDA). The present study examines the uptake of a close fluorescent analog, carboxydichlorofluorescein (CDCF), and drug substrates into inside-out membrane vesicles prepared from MRP transfected cells. MRP5-mediated uptake of CDCF was ATP-dependent and GSH-independent and possessed a Km of 12 microM and a Vmax of 56 pmol/min/mg prot. Comparison of kinetic parameters with drug substrates such as methotrexate (MTX), pemetrexed (Alimta), and the metabolite of 5-fluorouracil, 5-fluorodeoxyuridine monophosphate (5-FdUMP) (Km values of 0.3-1.3 mM) indicated that MRP5 has a 25-100-fold higher affinity for CDCF than for these drugs and that they share a common transport binding site. In addition, the potency of MRP5 inhibitors such as probenecid, MK571, and the phosphodiesterase 5 inhibitors correlated well between the uptake of CDCF and MTX. A survey of CDCF uptake by other MRPs revealed that MRP2 (ABCC2) also demonstrated ATP-dependent uptake with a Km of 19 microM and Vmax of 95.5 pmol/min/mg prot, while MRP1 (ABCC1) and MRP4 (ABCC4) had little to no uptake. Taken together, these data indicate that CDCF is a useful fluorescent drug surrogate with which to measure ATP-dependent MRP5-mediated transport.

Variable expression of MRP2 (ABCC2) in human placenta: influence of gestational age and cellular differentiation

MRP2 (ABCC2) is an ATP-binding cassette (ABC)-type membrane protein involved in transport of conjugates of various drugs and endogenous compounds. MRP2 has been localized to the apical membrane of syncytiotrophoblasts and is assumed to be involved in diaplacental transfer of the above substances. It has been shown that both genetic and environmental factors can influence MRP2 expression. We therefore investigated whether gestational age, cellular differentiation, and genetic polymorphisms influence expression and localization of MRP2 in 58 human placenta samples. We detected a significant increase of transporter-mRNA with gestational age by quantitative real-time polymerase chain reaction (MRP2 mRNA/18S rRNA ratio x 1000 +/- S.D.; 0.43 +/- 0.13 in early preterms versus 1.18 +/- 0.44 in late preterms versus 2.1 +/- 0.63 in terms; p < 0.05). MRP2 protein followed the mRNA amount as shown by Western blotting (mean relative band intensity +/- S.D.; 0.56 +/- 0.1 versus 0.7 +/- 0.18 versus 0.92 +/- 0.19; early preterms versus terms p < 0.05). In cultured cytotrophoblasts, MRP2 expression increased with differentiation to syncytiotrophoblasts, with a peak on day 2 (MRP2 mRNA/18S rRNA ratio x 1000 +/- S.D.; 0.06 +/- 0.01 versus 0.88 +/- 0.27 versus 0.24 +/- 0.02 on days 0, 2, and 4). Moreover, we studied the effect of single nucleotide polymorphisms (C-24T; G1249A, and C3972T) in the MRP2 gene on placental expression. One of these polymorphisms (G1249A) resulted in a significantly reduced expression of MRP2 mRNA in preterms. In summary, the expression of MRP2 in human placenta is influenced by gestational age, cellular differentiation, and genetic factors.