Two distinct mechanisms for bilirubin glucuronide transport by rat bile canalicular membrane vesicles. Demonstration of defective ATP-dependent transport in rats (TR-) with inherited conjugated hyperbilirubinemia

Quantitative assessment of the multiple processes responsible for bilirubin homeostasis in health and disease

Serum bilirubin measurements are commonly obtained for the evaluation of ill patients and to screen for liver disease in routine physical exams. An enormous research effort has identified the multiple mechanisms involved in the production and metabolism of conjugated (CB) and unconjugated bilirubin (UB). While the qualitative effects of these mechanisms are well understood, their expected quantitative influence on serum bilirubin homeostasis has received less attention. In this review, each of the steps involved in bilirubin production, metabolism, hepatic cell uptake, and excretion is quantitatively examined. We then attempt to predict the expected effect of normal and defective function on serum UB and CB levels in health and disease states including hemolysis, extra- and intrahepatic cholestasis, hepatocellular diseases (eg, cirrhosis, hepatitis), and various congenital defects in bilirubin conjugation and secretion (eg, Gilbert's, Dubin-Johnson, Crigler-Najjar, Rotor syndromes). Novel aspects of this review include: 1) quantitative estimates of the free and total UB and CB in the plasma, hepatocyte, and bile; 2) detailed discussion of the important implications of the recently recognized role of the hepatic OATP transporters in the maintenance of CB homeostasis; 3) discussion of the differences between the standard diazo assay versus chromatographic measurement of CB and UB; 4) pharmacokinetic implications of the extremely high-affinity albumin binding of UB; 5) role of the enterohepatic circulation in physiologic jaundice of newborn and fasting hyperbilirubinemia; and 6) insights concerning the clinical interpretation of bilirubin measurements.

A product of heme catabolism modulates bacterial function and survival

Bilirubin is the terminal metabolite in heme catabolism in mammals. After deposition into bile, bilirubin is released in large quantities into the mammalian gastrointestinal (GI) tract. We hypothesized that intestinal bilirubin may modulate the function of enteric bacteria. To test this hypothesis, we investigated the effect of bilirubin on two enteric pathogens; enterohemorrhagic E. coli (EHEC), a Gram-negative that causes life-threatening intestinal infections, and E. faecalis, a Gram-positive human commensal bacterium known to be an opportunistic pathogen with broad-spectrum antibiotic resistance. We demonstrate that bilirubin can protect EHEC from exogenous and host-generated reactive oxygen species (ROS) through the absorption of free radicals. In contrast, E. faecalis was highly susceptible to bilirubin, which causes significant membrane disruption and uncoupling of respiratory metabolism in this bacterium. Interestingly, similar results were observed for other Gram-positive bacteria, including B. cereus and S. aureus. A model is proposed whereby bilirubin places distinct selective pressure on enteric bacteria, with Gram-negative bacteria being protected from ROS (positive outcome) and Gram-positive bacteria being susceptible to membrane disruption (negative outcome). This work suggests bilirubin has differential but biologically relevant effects on bacteria and justifies additional efforts to determine the role of this neglected waste catabolite in disease processes, including animal models.

Bilirubin is the terminal breakdown product of heme, which is deposited at high concentrations in the human intestine, where it can come into contact with host cells, the gastrointestinal (GI) microflora, and invading pathogens. Here, we report that bilirubin can act as a protectant for the Gram-negative bacterial pathogen E. coli O157:H7, which causes severe hemorrhagic diarrhea and life-threatening kidney damage. Paradoxically, bilirubin is highly toxic towards another enteric opportunistic pathogen, the Gram-positive bacterium E. faecalis. Whereas the protection of E. coli stems from the neutralization of host reactive oxygen species, bilirubin's toxicity toward E. faecalis is rooted in its lipophilic properties, which drives the rapid association of bilirubin with bacteria, leading to disrupted cell membranes and concomitant death. These results suggest small molecule metabolites can modulate bacterial communities in the intestine, a finding that may have important implications for diseases caused by enteric bacteria and disrupted flora.

Modulation of intestinal transport of 2,4-dinitrophenyl-S-glutathione, a multidrug resistance-associated protein 2 substrate, by bilirubin treatment in rats

The effect of bilirubin treatment on intestinal transport of 2,4-dinitrophenyl-S-glutathione (DNP-SG), a substrate of multidrug resistance-associated protein 2 (MRP2), after application of 1-chloro-2, 4-dinitrobenzene (CDNB), a precursor of DNP-SG, was examined in rat intestine by the in-vitro everted sac, in-situ re-circulating perfusion, and in-situ loop methods. CDNB was taken up rapidly by jejunum and ileum, and the consequent intestinal efflux of DNP-SG, a glutathione conjugated metabolite of CDNB, was significantly higher in jejunum than in ileum in the in-situ and in-vitro studies. Co-administration of bilirubin (100 μM), as well as probenecid (1 mM) or ciclosporin (100 μM), with CDNB decreased the DNP-SG efflux in jejunum significantly, but not in ileum. The suppression of DNP-SG efflux in jejunum was also observed after intravenous administration of bilirubin (85.5 μmol kg−1), in which plasma bilirubin glucuronide levels were approximately 100 μM. In the in-vitro metabolism study, bilirubin exerted no significant effect on CDNB metabolism in the intestinal S9 fraction (supernatant of 9000 g). These results suggested that the diseased states accompanied with hyperbilirubinaemia might have increased the intestinal absorption, or oral bioavailability, of MRP2 substrates by suppressing MRP2 function at the proximal intestinal region.

The apical conjugate efflux pump ABCC2 (MRP2)

ABCC2 is a member of the multidrug resistance protein subfamily localized exclusively to the apical membrane domain of polarized cells, such as hepatocytes, renal proximal tubule epithelia, and intestinal epithelia. This localization supports the function of ABCC2 in the terminal excretion and detoxification of endogenous and xenobiotic organic anions, particularly in the unidirectional efflux of substances conjugated with glutathione, glucuronate, or sulfate, as exemplified by leukotriene C(4), bilirubin glucuronosides, and some steroid sulfates. The hepatic ABCC2 pump contributes to the driving forces of bile flow. Acquired or hereditary deficiency of ABCC2, the latter known as Dubin-Johnson syndrome in humans, causes an increased concentration of bilirubin glucuronosides in blood because of their efflux from hepatocytes via the basolateral ABCC3, which compensates for the deficiency in ABCC2-mediated apical efflux. In this article we provide an overview on the molecular characteristics of ABCC2 and its expression in various tissues and species. We discuss the transcriptional and posttranscriptional regulation of ABCC2 and review approaches to the functional analysis providing information on its substrate specificity. A comprehensive list of sequence variants in the human ABCC2 gene summarizes predicted and proven functional consequences, including variants leading to Dubin-Johnson syndrome.

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.

Transport deficient (TR−) hyperbilirubinemic rats are resistant to acetaminophen hepatotoxicity

The biliary excretion of acetaminophen (APAP) is reduced in transport deficient (TR-) hyperbilirubinemic rats lacking the multidrug resistance-associated protein 2 (Mrp2). This mutant strain of Wistar rats has impaired biliary excretion of organic anions and increased hepatic glutathione. The rational for this study was to determine if there is an altered risk for liver damage by APAP in the absence of Mrp2. Therefore, the susceptibility of TR- rats to APAP hepatotoxicity was investigated. Male Wistar and TR- rats were fasted overnight before APAP treatment (1 g/kg). Hepatotoxicity was assessed 24 h later by plasma sorbitol dehydrogenase activity and histopathology. In other studies, TR- rats received buthionine sulfoximine before APAP to reduce hepatic glutathione to values similar to those in Wistar rats. mRNA expression of APAP metabolizing enzymes was also measured in naïve animals. Wistar rats treated with APAP showed significant elevations in plasma sorbitol dehydrogenase activity, while no increases in enzyme activity were observed in TR- rats. Histopathology was in agreement. Hepatic non-protein sulfhydryls were significantly lower in Wistar rats receiving APAP than in TR- rats. TR- rats treated with buthionine sulfoximine and APAP showed dramatic increases in hepatotoxicity. TR- rats had increased mRNA expression of several APAP metabolizing enzymes. Mrp2 expression not only is important in biliary excretion, but also influences the toxic potential of reactive intermediates by controlling intrahepatic GSH and possibly drug metabolism.

Cyclosporine interacts with mycophenolic acid by inhibiting the multidrug resistance-associated protein 2

In mycophenolate mofetil (MMF)-treated organ transplant recipients, lower mycophenolic acid (MPA) plasma concentrations have been found in cyclosporine (CsA) compared with tacrolimus (Tac)-based immunosuppressive regimens. We previously demonstrated that CsA decreases exposure to MPA and increases exposure to its metabolite MPA-glucuronide (MPAG), possibly by interfering with the biliary excretion of MPAG. To elucidate the role of the multidrug resistance-associated protein (Mrp)-2 in the interaction between MMF and CsA, we treated three groups of 10 Mrp2-deficient rats (TR- rat) for 6 days with either vehicle, CsA (8 mg/kg) or Tac (4 mg/kg) by oral gavage. Hereafter, co-administration with MMF (20 mg/kg) was started in all groups and continued through day 14. The 24-h MPA/MPAG area under the concentration-time curve (AUC) was determined after single (day 7) and multiple MMF doses (day 14). On both study days, there were no significant differences in the mean MPA and MPAG AUC between CsA and Tac-treated animals. We conclude that the pharmacokinetics of MMF are comparable in Mrp2-deficient rats receiving either CsA or Tac as co-medication. This finding suggests that CsA-mediated inhibition of the biliary excretion of MPAG by the Mrp2 transporter is the mechanism responsible for the interaction between CsA and MMF.

Estimation of the Three-Dimensional Pharmacophore of Ligands for Rat Multidrug-Resistance?Associated Protein 2 Using Ligand-Based Drug Design Techniques

PURPOSE

Multidrug-resistance-associated protein 2 (Mrp2) shows a broad substrate specificity toward amphiphilic organic anions. This study identified key functional groups of ligand molecules for binding to rat Mrp2, determined their relative locations, and examined substrate specificity through receptor mapping using three-dimensional (3D) quantitative structure-activity relationship (3D-QSAR) analysis.

METHODS

Ligand-binding conformations were estimated using conformational analysis (CAMDAS) and molecular superposition (SUPERPOSE) methods to clarify the substrate specificity of rat Mrp2 in relation to 3D ligand structures.

RESULTS

Two types of binding conformations of ligands for rat Mrp2 were identified. 3D-QSAR comparative molecular-field analysis (CoMFA) revealed a statistically significant model for one type, in which the steric, electrostatic, and log P contributions to the binding affinity for rat Mrp2 were 63.0%, 33.4%, and 3.6%, respectively (n = 16, q2 = 0.59, n = 3, r2 = 0.99, and s = 0.08).

CONCLUSIONS

The 3D pharmacophore of ligands for rat Mrp2, and the ligand-binding region of rat Mrp2, were estimated. Ligand recognition of rat Mrp2 is achieved through interactions in two hydrophobic and two electrostatically positive sites (primary binding sites). The broad substrate specificity of rat Mrp2 might result from the combination of secondary (two electrostatically positive and two electrostatically negative sites) and primary binding sites.

Metabolism and disposition of gemfibrozil in Wistar and multidrug resistance-associated protein 2-deficient TR- rats

1. The roles of multidrug resistance-associated protein (Mrp) 2 deficiency and Mrp3 up-regulation were evaluated on the metabolism and disposition of gemfibrozil. 2. Results from in vitro studies in microsomes showed that the hepatic intrinsic clearance (CLint) for the oxidative metabolism of gemfibrozil was slightly higher (1.5-fold) in male TR- rats, which are deficient in Mrp2, than in wild-type Wistar rats, whereas CLint for glucuronidation was similar in both strains. 3. The biliary excretion of intravenously administered [14C]gemfibrozil was significantly impaired in TR-) rats compared with Wistar rats (22 versus 93% of the dose excreted as the acyl glucuronides over 72 h). Additionally, the extent of urinary excretion of radioactivity was much higher in TR- than in Wistar rats (78 versus 2.6% of the dose). 4. There were complex time-dependent changes in the total radioactivity levels and metabolite profiles in plasma, liver and kidney, some of which appeared to be related to the up-regulation of Mrp3. 5. Overall, it was demonstrated that alterations in the expression of the transporters Mrp2 and Mrp3 significantly affected the excretion as well as the secondary metabolism and distribution of [14C]gemfibrozil.

Y-700 [1-[3-Cyano-4-(2,2-dimethylpropoxy)phenyl]-1H-pyrazole-4-carboxylic acid]: a potent xanthine oxidoreductase inhibitor with hepatic excretion

Y-700 (1-[3-Cyano-4-(2,2-dimethylpropoxy)phenyl]-1H-pyrazole-4-carboxylic acid) is a newly synthesized inhibitor of xanthine oxidoreductase (XOR). Steady-state kinetics with the bovine milk enzyme indicated a mixed type inhibition with K(i) and K(i) ' values of 0.6 and 3.2 nM, respectively. Titration experiments showed that Y-700 bound tightly both to the active sulfo-form and to the inactive desulfo-form of the enzyme with K(d) values of 0.9 and 2.8 nM, respectively. X-ray crystallographic analysis of the enzyme-inhibitor complex revealed that Y-700 closely interacts with the channel leading to the molybdenum-pterin active site but does not directly coordinate to the molybdenum ion. In oxonate-treated rats, orally administered Y-700 (1-10 mg/kg) dose dependently lowered plasma urate levels. At a dose of 10 mg/kg, the hypouricemic action of Y-700 was more potent and of longer duration than that of 4-hydroxypyrazolo(3,4-d)pyrimidine, whereas its action was approximately equivalent to that of 2-(3-cyano-4-isobutoxyphenyl)-4-methyl-5-thiazolecarboxylic acid, a nonpurine inhibitor of XOR. In normal rats, orally administered Y-700 (0.3-3 mg/kg) dose dependently reduced the urinary excretion of urate and allantoin, accompanied by an increase in the excretion of hypoxanthine and xanthine. Y-700 (1 mg/kg) was absorbed rapidly by the oral route with high bioavailability (84.1%). Y-700 was hardly excreted via the kidneys but was mainly cleared via the liver. These results suggest that Y-700 will be a promising candidate for the treatment of hyperuricemia and other diseases in which XOR may be involved.

The human organic anion transport protein SLC21A6 is not sufficient for bilirubin transport

A recent study (Cui, Y., Konig, J., Leier, I., Buchholz, U., and Keppler, D. (2001) J. Biol. Chem. 276, 9626-9630) suggests that human OATP2 (SLC21A6), also known as OATP-C and LST1, mediates hepatic bilirubin transport. Because of methodologic concerns, this study was designed to examine this issue using a bilirubin transport assay that was validated in overnight cultured rat hepatocytes. These studies showed that cultured rat hepatocytes transported bilirubin with kinetics virtually identical to the transport of sulfobromophthalein. This assay was then used to quantify bilirubin transport by HeLa cells that had been stably transfected with OATP2 under regulation of a metallothionein promoter. Immunoblot analysis revealed abundant expression of OATP2 after incubation of cells for 48 h in zinc, whereas uninduced cells had no expression of this protein. In OATP2-expressing (zinc-induced) HeLa cells at 37 degrees C, the uptake of [35S]sulfobromophthalein was substantial (51.6 +/- 16.5 pmol/15 min/mg protein, n = 5) with little cell-associated ligand in non-expressing (uninduced) cells (0.54 +/- 0.16 pmol/15 min/mg protein, n = 5, p < 0.002). In contrast, there was no difference (p > 0.2) in cell-associated [3H]bilirubin in induced (OATP2-expressing) as compared with uninduced cells (11.25 +/- 3.02 pmol/15 min/mg protein versus 9.15 +/- 1.68 pmol/min/mg protein, respectively, n = 5) We obtained similar results in OATP2-transfected HEK293 cells that were used in the original report. The existence of a bilirubin transporter has been an important field of investigation for many years. Although the current study indicates that a role for OATP2 in hepatocyte bilirubin transport is unlikely, it provides new and sensitive tools that can be adapted to examine the function of putative bilirubin transporters in the future.

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.

Trandolaprilat, an angiotensin-converting enzyme inhibitor, is not excreted in bile via an ATP-dependent active transporter (cMOAT)

Trandolapril is the prodrug of an angiotensin-converting enzyme (ACE) inhibitor. It has been proposed that its active metabolite, trandolaprilat, is mainly excreted in bile, but this has not been clearly demonstrated. Recently it has been reported that temocaprilat, an active metabolite of the ACE inhibitor temocapril, is effectively excreted in bile via an ATP-dependent active transporter (canalicular multispecific organic anion transporter: cMOAT). To investigate whether trandolaprilat has the pharmacological ability to affect the cMOAT system in a manner similar to temocaprilat. The lipophilicity of trandolaprilat and temocaprilat was measured to determine the n-octanol-water partition coefficients. The dose-dependent inhibition of the up-take of [3H]-estradiol-17beta-D-glucuronide and [3H]-2,4-dinitrophenyl-S-glutathione, which are good substrates for cMOAT, in canalicular membrane vesicles (CMVs) prepared from Sprague-Dawley rats was determined in the presence of trandolaprilat and temocaprilat. The partition coefficient of trandolaprilat (log Po/w - 1.1) was about 30 times higher than that of temocaprilat (log Po/w - 2.5). The uptake of [3H]-estradiol-17beta-D-glucuronide and [3H]-2,4-dinitrophenyl-S-glutathione was dose-dependently inhibited by the presence of temocaprilat, but trandolaprilat had no effect on the transport of [3H]-estradiol-17beta-D-glucuronide or [3H]-2,4-dinitrophenyl-S-glutathione into CMVs even at concentrations as high as 200 microM. It could be concluded that trandolaprilat has a higher lipophilicity than temocaprilat. But the hepatobiliary excretion system via cMOAT may not contribute to the excretion of trandolaprilat in bile.

Genetic lesions of bilirubin uridine-diphosphoglucuronate glucuronosyltransferase (UGT1A1) causing Crigler-Najjar and Gilbert syndromes: correlation of genotype to phenotype

Uridine-diphosphoglucuronate glucuronosyltransferases (UGTs) are a family of enzymes that conjugate various endogenous and exogenous compounds with glucuronic acid and facilitate their excretion in the bile. Bilirubin-UGT(1) (UGT1A1) is the only isoform that significantly contributes to the conjugation of bilirubin. Lesions in the gene encoding bilirubin-UGT(1), lead to complete or partial inactivation of the enzyme causing the rare autosomal recessively inherited conditions, Crigler-Najjar syndrome type-1 (CN-1) and type 2 (CN-2), respectively. Inactivation of the enzyme leads to accumulation of unconjugated bilirubin in the serum. Severe hyperbilirubinemia seen in CN-1 can cause bilirubin encephalopathy (kernicterus). Kernicterus can be fatal or may leave behind permanent neurological sequelae. Here, we have compiled more than 50 genetic lesions of UGT1A1 that cause CN-1 (including 9 novel mutations) or CN-2 (including 3 novel mutations) and have presented a correlation of structure to function of UGT1A1. In contrast to Crigler-Najjar syndromes, Gilbert syndrome is a common inherited condition characterized by mild hyperbilirubinemia. An insertional mutation of the TATAA element upstream to UGT1A1 results in a reduced level of expression of the gene. Homozygosity for the variant promoter is required for Gilbert syndrome, but not sufficient for manifestation of hyperbilirubinemia, which is partly dependent on the rate of bilirubin production. Several structural mutations of UGT1A1, for example, a G71R substitution, have been reported to cause mild reduction of UGT activity toward bilirubin, resulting in mild hyperbilirubinemia, consistent with Gilbert syndrome. When the normal allele of a heterozygote carrier for a Crigler-Najjar type structural mutation contains a Gilbert type promoter, intermediate levels of hyperbilirubinemia, consistent with the diagnosis of CN-2, may be observed.

Genetic lesions of bilirubin uridine-diphosphoglucuronate glucuronosyltransferase (UGT1A1) causing Crigler-Najjar and Gilbert syndromes: correlation of genotype to phenotype

Uridine-diphosphoglucuronate glucuronosyltransferases (UGTs) are a family of enzymes that conjugate various endogenous and exogenous compounds with glucuronic acid and facilitate their excretion in the bile. Bilirubin-UGT(1) (UGT1A1) is the only isoform that significantly contributes to the conjugation of bilirubin. Lesions in the gene encoding bilirubin-UGT(1), lead to complete or partial inactivation of the enzyme causing the rare autosomal recessively inherited conditions, Crigler-Najjar syndrome type-1 (CN-1) and type 2 (CN-2), respectively. Inactivation of the enzyme leads to accumulation of unconjugated bilirubin in the serum. Severe hyperbilirubinemia seen in CN-1 can cause bilirubin encephalopathy (kernicterus). Kernicterus can be fatal or may leave behind permanent neurological sequelae. Here, we have compiled more than 50 genetic lesions of UGT1A1 that cause CN-1 (including 9 novel mutations) or CN-2 (including 3 novel mutations) and have presented a correlation of structure to function of UGT1A1. In contrast to Crigler-Najjar syndromes, Gilbert syndrome is a common inherited condition characterized by mild hyperbilirubinemia. An insertional mutation of the TATAA element upstream to UGT1A1 results in a reduced level of expression of the gene. Homozygosity for the variant promoter is required for Gilbert syndrome, but not sufficient for manifestation of hyperbilirubinemia, which is partly dependent on the rate of bilirubin production. Several structural mutations of UGT1A1, for example, a G71R substitution, have been reported to cause mild reduction of UGT activity toward bilirubin, resulting in mild hyperbilirubinemia, consistent with Gilbert syndrome. When the normal allele of a heterozygote carrier for a Crigler-Najjar type structural mutation contains a Gilbert type promoter, intermediate levels of hyperbilirubinemia, consistent with the diagnosis of CN-2, may be observed.

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.

Cisplatin induces renal expression of P-glycoprotein and canalicular multispecific organic anion transporter

The expression of two members of the ATP-binding cassette family of transport proteins, P-glycoprotein (P-gp) and the canalicular multispecific organic anion transporter (cMOAT or Mrp2), was evaluated in renal brush-border membranes (BBM) and various rat tissues after cisplatin treatment. One administration of cisplatin (5 mg/kg) increased P-gp expression by >200-300% in renal BBM and in crude membranes from liver and intestine. The increase in P-gp expression in the kidney was also detected in photolabeling experiments, suggesting the induction of functional P-gp. cMOAT expression was increased by >10-fold in renal BBM after cisplatin administration, although it had no effect on liver cMOAT expression. The increase in the levels of both proteins was maximal at 2 days after cisplatin treatment and lasted for at least 8 days. These results indicate that a single administration of cisplatin induces overexpression of P-gp and cMOAT in specific tissues. This may be of significant relevance to the design of clinical trials using cisplatin as a single chemotherapeutic agent or in combination with other drugs.

Transport of monoglucuronosyl and bisglucuronosyl bilirubin by recombinant human and rat multidrug resistance protein 2

The secretion of bilirubin conjugates from hepatocytes into bile represents a decisive step in the prevention of hyperbilirubinemia. The bilirubin conjugates, monoglucuronosyl bilirubin (MGB) and bisglucuronosyl bilirubin (BGB), were previously suggested to be endogenous substrates for the apical multidrug resistance protein (MRP2), a member of the adenosine triphosphate (ATP)-binding cassette family of transporters (symbol ABCC2), also termed canalicular multispecific organic anion transporter. We have characterized this ATP-dependent transport using membrane vesicles from human embryonic kidney (HEK) cells expressing recombinant rat as well as human MRP2. MGB and BGB, (3)H-labeled in the glucuronosyl moiety, were synthesized enzymatically with recombinant UDP-glucuronosyltransferase 1A1, and stabilized with ascorbate. Rates for ATP-dependent transport of MGB and BGB (0.5 micromol/L each) by human MRP2 were 183 and 104 pmol x mg protein(-1) x min(-1), respectively. K(m) values were 0.7 and 0.9 micromol/L for human MRP2, and 0.8 and 0.5 micromol/L for rat MRP2, with MGB and BGB as substrates, respectively. Leukotriene C(4) and 17beta-glucuronosyl estradiol, which are both known high-affinity substrates for human MRP2, inhibited [(3)H]MGB transport with IC(50) values of 2.3 and 30 micromol/L, respectively. Cyclosporin A competitively inhibited human and rat MRP2-mediated transport of [(3)H]MGB, with K(i) values of 21 and 10 micromol/L, respectively. Our results provide direct evidence that recombinant MRP2, cloned from rat as well as human liver, mediates the primary-active ATP-dependent transport of the bilirubin conjugates MGB and BGB.

Uptake of bromosulfophthalein via SO2-4/OH- exchange increases the K+ conductance of rat hepatocytes

In confluent primary cultures of rat hepatocytes, micromolar concentrations of bromosulfophthalein (BSP) lead to a sizeable hyperpolarization of membrane voltage. The effect is a saturable function of BSP concentration yielding an apparent value of 226 micromol/l and a Vmax of -10.3 mV. The BSP-induced membrane hyperpolarization is inhibited by the K+ channel blocker Ba2+, and in cable-analysis and ion-substitution experiments it becomes evident that the effect is due to a significant increase in cell membrane K+ conductance. Voltage changes were attenuated by the simultaneous administration of SO2-4, succinate, and cholate (cis-inhibition) and increased after preincubation with SO2-4 and succinate (trans-stimulation), suggesting that the effect occurs via BSP uptake through the known SO2-4/OH- exchanger. Microfluorometric measurements reveal that BSP-induced activation of K+ conductance is not mediated by changes in cell pH, cell Ca2+, or cell volume. It is concluded that K+ channel activation by BSP (as well as by DIDS and indocyanine green) may reflect a physiological mechanism linking the sinusoidal uptake of certain anions to their electrogenic canalicular secretion.

Molecular aspects of organic compound transport across the plasma membrane of hepatocytes

Many organic compounds are taken up from the blood by membrane transporters, taken across the sinosuidal membrane of hepatocytes and then excreted into bile via the bile canalicular membrane. The hepatic uptake of conjugated bile acids is mediated by the sodium taurocholate cotransporting polypeptide. Many organic anions and bulky organic cations are incorporated into hepatocytes by the organic anion transporting polypeptide, while small organic cations are transported by the organic cation transporter. At the canalicular membrane, organic compounds are excreted into bile by ATP-binding cassette transporters which hydrolyse ATP to ADP. Excretion of monovalent bile acids is mediated by the canalicular bile salt transporter and that of organic anions, including divalent bile acid, conjugates, are mediated by the multi-drug resistance-associated protein 2, also termed canalicular multi-specific organic anion transporter. Organic cations are excreted into bile by the multi-drug resistance gene product (MDR) 1 and phospholipids are excreted by MDR3 (mdr2 in mice and rats). The clinical syndromes associated with alterations of these transporters are also discussed.

Biochemical and molecular aspects of genetic disorders of bilirubin metabolism

Bilirubin, the oxidative product of heme in mammals, is excreted into the bile after its esterification with glucuronic acid to polar mono- and diconjugated derivatives. The accumulation of unconjugated and conjugated bilirubin in the serum is caused by several types of hereditary disorder. The Crigler-Najjar syndrome is caused by a defect in the gene which encodes bilirubin UDP-glucuronosyltransferase (UGT), whereas the Dubin-Johnson syndrome is characterized by a defect in the gene which encodes the canalicular bilirubin conjugate export pump of hepatocytes. Animal models such as the unconjugated hyperbilirubinemic Gunn rat, the conjugated hyperbilirubinemic GY/TR-, and the Eisai hyperbilirubinemic rat, have contributed to the understanding of the molecular basis of hyperbilirubinemia in humans. Elucidation of both the structure of the UGT1 gene complex, and the Mrp2 (cMoat) gene which encodes the canalicular conjugate export pump, has led to a greater understanding of the genetic basis of hyperbilirubinemia.

Drug export activity of the human canalicular multispecific organic anion transporter in polarized kidney MDCK cells expressing cMOAT (MRP2) cDNA

The canalicular (apical) membrane of the hepatocyte contains an ATP-dependent transport system for organic anions, known as the multispecific organic anion transporter (cMOAT). The deduced amino acid sequence of cMOAT is 49% identical to that of the human multidrug resistance- associated protein (MRP) MRP1, and cMOAT and MRP1 are members of the same sub-family of adenine nucleotide binding cassette transporters. In contrast to MRP1, cMOAT was predominantly found intracellularly in nonpolarized cells, suggesting that cMOAT requires a polarized cell for plasma membrane routing. Therefore, we expressed cMOAT cDNA in polarized kidney epithelial MDCK cell lines. When these cells are grown in a monolayer, cMOAT localizes to the apical plasma membrane. We demonstrate that cMOAT causes transport of the organic anions S-(2,4-dinitrophenyl)-glutathione, the glutathione conjugate of ethacrynic acid, and S-(PGA1)-glutathione, a substrate not shown to be transported by organic anion transporters previously. Transport is inhibited only inefficiently by compounds known to block MRP1. We also show that cMOAT causes transport of the anticancer drug vinblastine to the apical side of a cell monolayer. We conclude that cMOAT is a 5'-adenosine triphosphate binding cassette transporter that potentially might be involved in drug resistance in mammalian cells.

Impaired activity of the bile canalicular organic anion transporter (Mrp2/cmoat) is not the main cause of ethinylestradiol-induced cholestasis in the rat

To test the hypothesis that impaired activity of the bile canalicular organic anion transporting system mrp2 (cmoat) is a key event in the etiology of 17alpha-ethinylestradiol (EE)-induced intrahepatic cholestasis in rats, EE (5 mg/kg subcutaneously daily) was administered to male normal Wistar (NW) and mrp2-deficient Groningen Yellow/Transport-deficient Wistar (GY/TR-) rats. Elevated plasma bilirubin levels in GY/TR- rats increased upon EE-treatment from 65 +/- 8.4 micromol/L to 183 +/- 22.7 micromol/L within 3 days, whereas bilirubin levels remained unaffected in NW rats. Biliary bilirubin secretion was 1.5-fold increased in NW rats but remained unaltered in GY/TR- rats. Plasma bile salt concentrations remained unchanged in both strains, although hepatic levels of the sinusoidal Na+-taurocholate cotransporting protein (ntcp) were markedly reduced. Biliary secretion of endogenous bile salt was not affected in either strain. A clear reduction of mrp2 levels in liver plasma membranes of NW rats was found after 3 days of treatment. The bile salt-independent fraction of bile flow (BSIF) was reduced from 2.6 to 2.0 microL/min/100 g body weight in NW rats with a concomitant 62% reduction of biliary glutathione secretion. The absence of mrp2 and biliary glutathione in GY/TR- rats did not prevent induction of EE-cholestasis; a similar absolute reduction of BSIF, i.e., from 1.1 to 0.6 microL/min/100 g bodyweight, was found in these animals. EE treatment caused a reduction of the maximal biliary secretory rate (S(RM)) of the mrp2 substrate, dibromosulphthalein (DBSP), from 1,040 to 695 nmol/min/100 g body weight (-38%) in NW rats and from 615 to 327 nmol/min/100 g body weight (-46%) in GY/TR- rats. These results demonstrate that inhibition of mrp2 activity and/or biliary glutathione secretion is not the main cause of EE-induced cholestasis in rats. The data indicate that alternative pathways exist for the biliary secretion of bilirubin and related organic anions that are also affected by EE.

Functional analysis of a canalicular multispecific organic anion transporter cloned from rat liver

Transport of many organic anions across the bile canalicular membrane is mediated by the canalicular multispecific organic anion transporter (cMOAT). Previously, we cloned cDNA that may encode cMOAT from Sprague-Dawley rat liver (Ito, K., Suzuki, H., Hirohashi, T., Kume, K., Shimizu, T., and Sugiyama, Y. (1997) Am. J. Physiol. 272, G16-G22). In the present study, the function of this cloned cDNA was investigated by examining the ATP-dependent uptake of S-(2,4-dinitrophenyl)-glutathione (DNP-SG) into membrane vesicles isolated from an NIH/3T3 cell line transfected with an expression vector containing the cloned cDNA. Although the membrane vesicles from the control NIH/3T3 cells exhibited endogenous activity in transporting DNP-SG and leukotriene C4 in an ATP-dependent manner, the transfection of cMOAT cDNA resulted in a significant increase in the transport activity for these ligands. The uptake of DNP-SG into membrane vesicles was osmotically sensitive and was stimulated to some extent by other nucleotide triphosphates (GTP, UTP, and CTP) but not by AMP or ADP. The K(m) and Vmax values for the uptake of DNP-SG by the membrane vesicles were 0.175 +/- 0.031 microM and 11.0 +/- 0.73 pmol/min/mg protein, respectively, for the transfected rat cMOAT and 0.141 +/- 0.036 microM and 3.51 +/- 0.39 pmol/min/mg protein, respectively, for the endogenous transporter expressed on control NIH/3T3 cells. These results suggest that the product of the previously cloned cDNA has cMOAT activity being able to transport organic anions in an ATP-dependent manner. Alternatively, it is possible that the cDNA product encodes an activator of endogenous transporter since the K(m) value for DNP-SG was comparable between the vector- and cMOAT-transfected cells. The transport activity found in the control NIH/3T3 cells may be ascribed to mouse cMOAT since Northern blot analysis indicated the presence of a transcript that hybridyzed to the carboxyl-terminal ATP-binding cassette sequence of the murine protein.

ATP-dependent transport of bilirubin glucuronides by the multidrug resistance protein MRP1 and its hepatocyte canalicular isoform MRP2

Bilirubin is secreted from the liver into bile mainly as monoglucuronosyl and bisglucuronosyl conjugates. We demonstrate for the first time that ATP-dependent transport of both bilirubin glucuronides is mediated by the multidrug resistance protein (MRP1) as well as by the distinct canalicular (apical) isoform MRP2, also termed cMRP or cMOAT (canalicular multispecific organic anion transporter). In membrane vesicles from MRP1-transfected HeLa cells mono[3H]glucuronosylbilirubin and bis[3H]glucuronosylbilirubin (each at 0.5 microM) were transported with rates of 5.3 and 3.1 pmol/min per mg of protein respectively. Rat hepatocyte canalicular membrane vesicles, which contain Mrp2 (the rat equivalent of MRP2), transported mono[3H]glucuronosylbilirubin and bis[3H]glucuronosylbilirubin at rates of 8.9 and 8.5 pmol/min per mg of protein, whereas membrane vesicles from mutant liver lacking Mrp2 showed no transport of the conjugates. In membrane vesicles from human hepatoma Hep G2 cells, which predominantly expressed MRP2, transport rates were 8.3 and 4.4 pmol/min per mg of protein for monoglucuronosylbilirubin and bisglucuronosylbilirubin respectively. ATP-dependent transport of the glutathione S-conjugate -3H-leukotriene C4, an established high-affinity substrate for MRP1 and MRP2, was inhibited by both bilirubin glucuronides with IC50 values between 0.10 and 0.75 microM. The ratios of leukotriene C4 transport and bilirubin glucuronide transport, determined in the same membrane vesicle preparation, indicated substrate specificity differences between MRP1 and MRP2 with a preference of MRP2 for the glucuronides.

Multiple routes and regulation by tyrosine phosphorylation characterize the ATP-dependent transport of 2,4-dinitrophenyl S-glutathione in inside-out vesicles from human erythrocytes

ATP-dependent efflux routes for 2,4-dinitrophenyl S-glutathione (DNP-SG) were investigated using inside-out vesicles from human erythrocytes. Nonlinear double-reciprocal plots of transport at substrate concentrations ranging from 0.07 to 500 micro;m indicated that multiple transport routes were operative. Sensitivity to naphthyl glucuronide separated [3H]DNP-SG transport into two low-affinity components which by computer analysis exhibited Km values of 330 and 1400 micro;m, respectively. At low substrate concentrations, two high-affinity routes were observed. The predominant activity (hMOAT3a) exhibited a Km of 0.18 micro;m (Vmax = 22 pmol/min/mg protein), whereas the second activity (MOAT3b) had a Km of 0.58 micro;m (Vmax = 16 pmol/min/mg protein). High-affinity transport of DNP-SG increased substantially (2.5-fold) in vesicles preincubated with genistein or other tyrosine kinase inhibitors. Kinetic analyses in vesicles pretreated with 50 micro;m genistein showed that increased transport resulted from the appearance of a new activity (hMOAT3c) with a Km of 0.85 micro;m and a substantially elevated Vmax (80 pmol/min/mg protein). At varying concentrations of genistein, a progression was observed that was consistent with the conversion of hMOAT3b to hMOAT3a and hMOAT3a to hMOAT3c. Phenylarsine oxide, a phosphotyrosine phosphatase inhibitor, produced an opposite progression. Specificity studies showed that hMOAT3a exhibited the highest affinity for various anionic conjugates, and had a notable binding preference for glutathione disulfide. The relative effectiveness of the various inhibitors was similar for hMOAT3a, hMOAT3b, and hMOAT3c, as well as for a corresponding mMOAT3 activity from L1210 mouse cells. The results show that human erythrocytes contain multiple ATP-dependent efflux systems for DNP-SG and that separation of these systems can be achieved on the basis of substrate Km value and inhibitor and activator specificity. High-affinity transport can proceed via three activities which appear to be subforms of a single system with differing levels of tyrosine phosphorylation. Multiple hMOAT3 subforms provide flexibility for extruding various anionic conjugates and may have evolved in erythrocytes to expedite the efflux of GS-SG.

Regulation of organic anion transport in the liver

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

Hepatic glutathione and glutathione S-conjugate transport mechanisms

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

Biliary electrolytes and enzymes in patients with and without gallstones

pH, osmolarity, various electrolytes, nine enzymes, and bile acid were determined in hepatic and gallbladder biles from 108 and 100 patients, respectively, relating to various types of gallstones. The pH, osmolarity, and electrolytes were essentially identical in all groups of patients except for slightly higher Ca and Mg in the hepatic bile in patients with muddy pigment stones. The gallbladder bile contained much higher inorganic cations yet remained isosmotic as a result of their sequestration into bile acid micelles. Excluding extremely high values, the activities of nine enzymes in the bile showed only minor differences among four groups of patients except for a high beta-glucuronidase activity in the hepatic bile in patients with muddy pigment stones. The biliary baseline activities of various enzymes and the relation to their serum levels were determined by their sources and subcellular localization in the hepatocytes. We concluded that biliary electrolytes and enzymes were basically similar in patients with and without gallstones except for higher levels of Ca, Mg, and beta-glucuronidase in hepatic bile in patients with muddy pigment stones.

MOAT4, a novel multispecific organic-anion transporter for glucuronides and mercapturates in mouse L1210 cells and human erythrocytes

Glucuronides and mercapturates were examined as possible high-affinity substrates for a low-affinity ATP-dependent transport system for 2,4-dinitrophenyl S-glutathione (DNP-SG) in mouse L1210 cells. Initial inhibitor studies with inside-out vesicles revealed that the low-affinity transport of [3H]DNP-SG (Km 450 microM) exhibits a high sensitivity to N-acetyl 2,4-dinitrophenyl cysteine (NAc-DNP-Cys) (Ki 5.0 microM) and alpha-naphthyl beta-D-glucuronide (naphthyl glucuronide) (Ki 8.5 microM). Direct transport measurements showed the presence of ATP-dependent uptake activities for NAc-DNP-[35S]Cys and naphthyl [14C] glucuronide, and Km values for half-maximal transport were comparable to the Ki values of these compounds for inhibition of [3H]DNP-SG transport. Transport of [3H]DNP-SG, NAc-DNP-[35S]Cys and naphthyl [14C]glucuronide each showed the same sensitivity to various anions and anion conjugates. Inhibition was competitive and was most potent for bilirubin ditaurate, indoprofen, 4-biphenylacetic acid, 4-acridine 4 beta-D-glucuronide, N-acetyl leukotriene E4, 17 beta-oestradiol 3 beta-D-glucuronide and taurolithocholate 3-sulphate. Inside-out vesicles from human erythrocytes contain a comparable ATP-dependent transport system. These results show that NAc-DNP-Cys and naphthyl glucuronide are high-affinity substrates for a single system identified previously as a low-affinity transporter of DNP-SG. Substrate and inhibitor studies identify this system as a novel multispecific organic-anion transport system (MOAT4) that accommodates glucuronides and mercapturates and is distinct from other MOAT transporters. Human erythrocytes contain an additional ATP-dependent system for NAc-DNP-Cys (Km 33 microM) that does not transport monoglucuronides.

Pravastatin transport across the hepatocyte canalicular membrane requires both ATP and a transmembrane pH gradient

Hepatic excretion of non-bile acid organic anions is reported to be ATP-dependent and a defect of this transport has been reported in congenitally jaundiced rats, animal models of human Dubin-Johnson syndrome. To investigate the effect of the transmembrane pH gradient on hepatocyte canalicular membrane transport of ATP-dependent organic anions, uptake of pravastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase-inhibiting organic anion, by hepatocyte canalicular membrane vesicles was observed in the presence or absence of transmembrane pH gradients. Uptake was assessed by a rapid filtration technique. ATP-dependent pravastatin uptake was stimulated in the presence of a transmembrane pH gradient (in > out) in Sprague-Dawley (SD) rats. Uptake was dependent on both pravastatin and ATP concentrations and showed saturation kinetics. After intravenous injection of [14C]-pravastatin (0.3 mumol), 81% of the dose was excreted in the bile within 35 min in SD rats, whereas only 20% was excreted in the bile in Eisai hyperbilirubinuria rats. ATP and the pH gradient also co-stimulated the uptake of pravastatin in Eisai hyperbilirubinuria rats, although the K(m) was much higher and Vmax was much lower than corresponding values in SD rats. This coincided well with the marked reduction in vivo biliary excretion of pravastatin in jaundiced rats.

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

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

Identification of efflux systems for large anions and anionic conjugates as the mediators of methotrexate efflux in L1210 Cells

Two ATP-dependent efflux systems for methotrexate have been identified in inside-out vesicles from an L1210 mouse cell variant with a defective influx carrier for methotrexate. Transport at 40 muM [3H]methotrexate was separated by inhibitors into two components comprising 62 and 38% of total transport activity. The predominant route was inhibited by low concentrations of indoprofen (Ki=2.5 muM, 4-biphenylacetic acid (Ki=5.3 muM), and flurbiprofen (Ki=5.2 muM, whereas the second component showed a high sensitivity to the glutathione conjugates of bromosulfophthalein (Ki=0.08 muM), ethacrynic acid (Ki=0.52 muM, and 1-chloro-2,4-dinitrobenzene (Ki=0.77 muM). Bilirubin ditaurate was a potent inhibitor of both transport components (Ki=1.5 and 0.17 muM, respectively). Separation of transport activities without interference from the other route was achieved by adding an excess (100 muM) of either the glutathione conjugate of ethacrynic acid or biphenylacetic acid. Double-reciprocal plots of transport at various substrate concentrations gave Km values of 170 and 250 muM for methotrexate transport via the anion-sensitive and conjugate-sensitive routes, respectively. A comparison of inhibitor specificities indicated that the anion-sensitive transport activity in vesicles represents efflux system II for methotrexate in intact cells and is the same system identified previously in vesicles as an anion/anion conjugate pump. The conjugate-sensitive activity corresponds to efflux system I for methotrexate in intact cells and is the same system identified in vesicles as the high-affinity glutathione conjugate pump.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

The role of thiols in ATP-dependent transport of S-(2,4-dinitrophenyl)glutathione by rat liver plasma membrane vesicles

The effect of thiol/disulfide exchange on ATP-dependent S-(2,4-dinitrophenyl)glutathione (GS-DNP) transport was studied in sodium nitrate treated rat liver plasma membrane vesicles. Transport followed Michaelis-Menten kinetics with an apparent Km of 9.6 microM for GS-DNP and 124 microM for ATP. 5,5'-Dithiobis(2-nitrobenzoate) (DTNB) and N-ethylmaleimide (NEM) efficiently inactivated GS-DNP transport activity in a dose- and time-dependent manner. Half-maximal inactivation occurred in 10 min at 40 microM for DTNB and 550 microM for NEM. Inactivation by DTNB was reversed by dithiothreitol. S-(N-Ethyl)maleimyl glutathione and/or ATP-Mg2+, but neither S-(N-ethyl)maleimyl cysteinylglycine nor oxidized glutathione could protect transport activity from inactivation by NEM or cystamine. These results suggest that reactive thiols are located near the active site of the transporter and that S-alkyl and the gamma-glutamyl residues of glutathione are important for protection. Biological disulfides which were tested included cystine, oxidized glutathione, oxidized Coenzyme-A, oxidized lipoic acid, and oxidized lipoamide; cystamine was the most potent reversible inactivator. Molecular oxygen also inactivated transport activity, which was recovered on addition of dithiothreitol, suggesting intramolecular disulfide formation by vicinal thiols. We interpret these results to indicate that the ATP-dependent GS-DNP transporter contains two or more thiols which are necessary for the maintenance of transport activity. The reversible inactivation of the activity by biological disulfides suggests that the transporter may be regulated by thiol/disulfide exchange in vivo.

Availability and mean transit times of phenol and its metabolites in the isolated perfused rat liver: normal and retrograde studies using tracer concentrations of phenol

Phenolic compounds are frequently detoxified by the formation of sulphate and glucuronic acid conjugates in the liver. These conjugates are formed in the hepatocytes and then either transported into the bile or back into the blood. In this study, we examined the transport kinetics of phenol and its metabolites in the isolated perfused rat liver by monitoring the outflow profiles of these compounds after a bolus input in a single pass preparation. Phenol was almost exclusively metabolized to phenyl sulphate (97%) at the trace concentrations used, with the amount of phenol and metabolites excreted into the bile being minimal (3.5%). The metabolite formed was rapidly transported back into the perfusate, with mean transit times of 17.4 and 12.3 s anterograde and 24.9 and 24.2 s retrograde at flow rates of 15 and 30 mL min-1 respectively, which were intermediate between those of Evans blue and water. The outflow concentration-time profile for phenyl sulphate formation was unaffected by the addition of another organic anion (bromosulphophthalein). The effect of enzyme zonation on outflow concentration-time profiles was also investigated using retrograde perfusions. The transit time ratios for generated metabolite to water for anterograde perfusions (0.6) was found to be more than twice that for retrograde perfusions (0.23) at 15 mL min-1 and approximately 1.6 times greater at 30 mL min-1, being 0.58 and 0.37 respectively. The relative ratios obtained are consistent with previous findings that normalized variance of solutes in the retrograde perfusions is greater than that for anterograde perfusions.

Kinetic analysis of hepatobiliary transport for conjugated metabolites in the perfused liver of mutant rats (EHBR) with hereditary conjugated hyperbilirubinemia

PURPOSE

Previously, we found that the biliary excretion of the 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole (E3040) glucuronide is severely impaired in Eisai hyperbilirubinemic rats (EHBR), while that of sulfate remains normal (Takenaka et al., J. Pharmacol. Exp. Ther., 274: 1362-1369, 1995). The purpose of the present study is to clarify the mechanisms for impairment of the biliary excretion of E3040 glucuronide in EHBR.

METHODS

We kinetically analyzed the disposition of the conjugates in the perfused liver at steady state. The uptake of the conjugates into the isolated canalicular membrane vesicles (CMVs) was also examined.

RESULTS

At steady state, the bile/liver unbound concentration ratios of the conjugates were 40-400 in both rat strains, indicating a highly concentrated process. The biliary excretion clearance (CLu,bile) of the glucuronide, defined for the unbound concentration in the liver, was decreased in EHBR to 1/30 of that in normal rats, whereas the CLu,bile of the sulfate was comparable between the two rat strains. In vitro, the transport of E3040 glucuronide into CMV prepared from SD rats exhibited the ATP dependency, whereas minimal effect of ATP was observed on the uptake of the glucuronide into CMV from EHBR. In contrast, the uptake of E3040 sulfate was comparable between SD rats and EHBR. Furthermore, ATP did not stimulate the uptake of sulfate into the CMVs.

CONCLUSIONS

It was suggested (1) that the excretion of E3040 glucuronide across the bile canalicular membrane is mediated by the primary active transporter which is defective in EHBR and (2) that the bile canalicular transport system for E3040 sulfate is different from that for the glucuronide in that the former remains normal in EHBR.

Evidence for glucuronide (small molecule) sorting by human hepatic endoplasmic reticulum

The entry of substrates into, and the export of glururonides from, the lumen of hepatic endoplasmic reticulum (ER) in vitro (sealed microsomes) has been measured using radioactivity-labelled materials and a rapid filtration assay. Analysis of liver microsomes from a jaundiced patient showed the accumulation of bilirubin glucuronides within the lumen of the ER. Further analysis of these hepatic microsomes revealed that newly synthesized 1-naphthol glucuronide could exit from the microsomes whereas bilirubin glucuronide was accumulated within the microsomes. These results suggest the existence of mechanisms for the sorting of small molecules, destined for export through bile canalicular or basolateral plasma membranes, by ER. Furthermore, these sorting processes may be regulated by specific transporters within the ER.

An introduction to drug disposition: the basic principles of absorption, distribution, metabolism, and excretion

A knowledge of the fate of a drug, its disposition (absorption, distribution, metabolism, and excretion, known by the acronym ADME) and pharmacokinetics (the mathematical description of the rates of these processes and of concentration-time relationships), plays a central role throughout pharmaceutical research and development. These studies aid in the discovery and selection of new chemical entities, support safety assessment, and are critical in defining conditions for safe and effective use in patients. ADME studies provide the only basis for critical judgments from situations where the behavior of the drug is understood to those where it is unknown: this is most important in bridging from animal studies to the human situation. This presentation is intended to provide an introductory overview of the life cycle of a drug in the animal body and indicates the significance of such information for a full understanding of mechanisms of action and toxicity.

Multiple canalicular transport mechanisms for glutathione S-conjugates. Transport on both ATP- and voltage-dependent carriers

A large number of structurally distinct electrophiles are conjugated to glutathione within hepatocytes, and the resulting glutathione S-conjugates are selectively transported across the canalicular membrane into bile. To test the hypothesis that a single multi-specific, ATP-dependent carrier mediates biliary secretion of glutathione S-conjugates, the present study compared the driving forces and substrate specificity for canalicular transport of S-ethylglutathione (ethyl-SG), a low molecular weight and relatively hydrophilic thioether, and S-(2,4-dinitrophenyl)-glutathione (DNP-SG), a larger and more hydrophobic anion, using isolated rat liver canalicular membrane vesicles. In agreement with previous findings, DNP-SG transport was stimulated by ATP, although there was considerable transport in the absence of ATP. ATP-independent DNP-SG transport was unaffected by a Na+ gradient, was enhanced by a valinomycin-induced K+ diffusion potential, and was saturable, with both high affinity (Km = 8 +/- 2 microM) and low affinity (Km = 0.5 +/- 0.1 mM) components. High affinity ATP-independent DNP-SG uptake was cis-inhibited by GSH, GSH monoethyl ester, glutathione S-conjugates, other gamma-glutamyl compounds, sulfobromophthalein, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). In contrast, ATP-dependent DNP-SG uptake was unaffected by GSH, GSH ester, S-methyl glutathione, or S-carbamidomethyl glutathione, but was strongly inhibited by sulfobromophthalein, DIDS, and by high molecular weight and relatively hydrophobic glutathione S-conjugates. Transport of the low molecular weight ethyl-SG conjugate was only minimally stimulated by ATP (10-20%). ATP-independent ethyl-SG uptake was electrogenic, saturable (Km = 10 +/- 1 microM) and was inhibited by GSH and all glutathione S-conjugates tested. These findings indicate the presence of multiple canalicular transport mechanisms for glutathione S-conjugates and demonstrate that the physicochemical properties of the S moiety are major determinants of transport. Relatively high molecular weight hydrophobic conjugates are substrates for both ATP-dependent and -independent mechanisms, whereas low molecular weight glutathione S-conjugates are transported largely by electrogenic carriers.

The role of the canalicular multispecific organic anion transporter in the disposal of endo- and xenobiotics

Bile is an important excretory route for the elimination of amphiphilic organic anions, and hepatocytes are the primary secretory units of bile formation. The hepatocytic basolateral and canalicular membranes are equipped with various carrier proteins. Transport across the canalicular membrane represents a major concentrative step. Various ATP-dependent transporters have been identified, such as a multispecific organic anion transporter (canalicular multispecific organic ion transporter, cMOAT), a bile acid transporter and several P-glycoproteins. TR- rats, which lack cMOAT activity, have been valuable in defining the substrate specificity of cMOAT. A wide range of glucuronide-, glutathione- and sulfate-conjugates are transported by this system.