Rotor Syndrome: Glucuronidated Bile Acidemia From Defective Reuptake by Hepatocytes.

Organic anion transporting polypeptide (OATP) 1B1 (gene, solute carrier organic anion transporter family member 1B1 [SLCO1B1]) and OATP1B3 (SLCO1B3) serve as transporters for hepatic uptake of important endogenous substances and several commonly prescribed drugs. Inactivation of both proteins together causes Rotor syndrome. How this OATP1B1/1B3 defect disturbs bile acid (BA) metabolism is largely unknown. In this study, we performed detailed BA analysis in 3 patients with genetically diagnosed Rotor syndrome. We found that BAs glucuronidated at the C-3 position (BA-3G) accounted for 50% or more of total BAs in these patients. In contrast but similarly to healthy controls, only trace amounts of BA-3G were detected in patients with constitutional indocyanine green excretory defect (OATP1B3 deficiency) or sodium-taurocholate cotransporting polypeptide (NTCP; gene, solute carrier family 10 member 1 [SLC10A1]) deficiency. Therefore, substantial amounts of BA-3G are synthesized in hepatocytes. The cycling pathway of BA-3G, consisting of excretion from upstream hepatocytes and uptake by downstream hepatocytes by OATP1B1/1B3 may exist to reduce the burden on upstream hepatocytes. Conclusion: Detailed BA analysis revealed glucuronidated bile acidemia in patients with Rotor syndrome. Further exploration of the physiologic role of glucuronidated BAs is necessary.

Inherited disorders of bilirubin transport and conjugation: new insights into molecular mechanisms and consequences.

Inherited disorders of bilirubin metabolism might reduce bilirubin uptake by hepatocytes, bilirubin conjugation, or secretion of bilirubin into bile. Reductions in uptake could increase levels of unconjugated or conjugated bilirubin (Rotor syndrome). Defects in bilirubin conjugation could increase levels of unconjugated bilirubin; the effects can be benign and frequent (Gilbert syndrome) or rare but severe, increasing the risk of bilirubin encephalopathy (Crigler-Najjar syndrome). Impairment of bilirubin secretion leads to accumulation of conjugated bilirubin (Dubin-Johnson syndrome). We review the genetic causes and pathophysiology of disorders of bilirubin transport and conjugation as well as clinical and therapeutic aspects. We also discuss the possible mechanisms by which hyperbilirubinemia protects against cardiovascular disease and the metabolic syndrome and the effects of specific genetic variants on drug metabolism and cancer development.

The roles of MRP2, MRP3, OATP1B1, and OATP1B3 in conjugated hyperbilirubinemia.

Increased concentrations of bilirubin glucuronides in blood plasma indicate hepatocellular dysfunction. Elucidation of the transport processes of bilirubin conjugates across the basolateral (sinusoidal) and the canalicular plasma membrane domains of hepatocytes has decisively contributed to our current understanding of the molecular basis of conjugated hyperbilirubinemia in human liver diseases. Under normal conditions, unconjugated bilirubin is taken up into hepatocytes by transporters of the organic anion-transporting polypeptide (OATP) family, followed by conjugation with glucuronic acid, and ATP-dependent transport into bile. This efflux across the canalicular membrane is mediated by multidrug resistance protein 2 (MRP2 or ABCC2), which is a 190-kDa glycoprotein transporting with high affinity and efficiency monoglucuronosyl bilirubin and bisglucuronosyl bilirubin into bile. MRP2 is hereditarily deficient in human Dubin-Johnson syndrome. Under pathophysiological conditions such as cholestatic liver injury and MRP2 inhibition, the basolateral efflux pump multidrug resistance protein 3 (MRP3 or ABCC3) is responsible for the occurrence of conjugated hyperbilirubinemia. MRP3 is a glycoprotein with a similar molecular mass as MRP2, with 48% amino acid identity, and with overlapping substrate specificity. Human MRP3 is the only basolateral efflux pump shown to transport bilirubin glucuronides. In human and rat hepatocytes, MRP3/Mrp3 is strongly upregulated under conditions of cholestasis and MRP2 deficiency. This is in line with the concept that basolateral efflux pumps of the hepatocyte compensate for impaired canalicular efflux of compounds into bile and contribute to balance the rate of uptake or synthesis of compounds in hepatocytes with the capacity for efflux into bile.

New insights in bilirubin metabolism and their clinical implications.

Bilirubin, a major end product of heme breakdown, is an important constituent of bile, responsible for its characteristic colour. Over recent decades, our understanding of bilirubin metabolism has expanded along with the processes of elimination of other endogenous and exogenous anionic substrates, mediated by the action of multiple transport systems at the sinusoidal and canalicular membrane of hepatocytes. Several inherited disorders characterised by impaired bilirubin conjugation (Crigler-Najjar syndrome type I and type II, Gilbert syndrome) or transport (Dubin-Johnson and Rotor syndrome) result in various degrees of hyperbilirubinemia of either the predominantly unconjugated or predominantly conjugated type. Moreover, disrupted regulation of hepatobiliary transport systems can explain jaundice in many acquired liver disorders. In this review, we discuss the recent data on liver bilirubin handling based on the discovery of the molecular basis of Rotor syndrome. The data show that a substantial fraction of bilirubin conjugates is primarily secreted by MRP3 at the sinusoidal membrane into the blood, from where they are subsequently reuptaken by sinusoidal membrane-bound organic anion transporting polypeptides OATP1B1 and OATP1B3. OATP1B proteins are also responsible for liver clearance of bilirubin conjugated in splanchnic organs, such as the intestine and kidney, and for a number of endogenous compounds, xenobiotics and drugs. Absence of one or both OATP1B proteins thus may have serious impact on toxicity of commonly used drugs cleared by this system such as statins, sartans, methotrexate or rifampicin. The liver-blood cycling of conjugated bilirubin is impaired in cholestatic and parenchymal liver diseases and this impairment most likely contributes to jaundice accompanying these disorders.

[Neonatal hyperbilirubinemia and molecular mechanisms of jaundice]. / Vrozené hyperbilirubinemie a molekulární mechanizmy zloutenky.

The introductory summarises the classical path of heme degradation and classification of jaundice. Subsequently, a description of neonatal types of jaundice is given, known as Crigler Najjar, Gilberts, DubinJohnson and Rotor syndromes, emphasising the explanation of the molecular mechanisms of these metabolic disorders. Special attention is given to a recently discovered molecular mechanism of the Rotor syndrome. The mechanism is based on the inability of the liver to retrospectively uptake the conjugated bilirubin fraction primarily excreted into the blood, not bile. A reduced ability of the liver to uptake the conjugated bilirubin contributes to the development of hyperbilirubinemia in common disorders of the liver and bile ducts and to the toxicity of xenobiotics and drugs using transport proteins for conjugated bilirubin.

Loss of OATP1B3 function causes Rotor syndrome: implications for potential use of inhibitors in cancer.

There has been increasing recognition that organic anion transporter proteins (OATPs) play an important role in the biology of various cancers. De novo expression of OATPs has been identified in breast, colon, pancreatic, gastric and prostate cancer cells, among others. In patients with prostate cancer, polymorphisms encoding decreased functioning OATP1B3 were associated with a longer time to progression on androgen deprivation therapy and a longer overall survival which is likely caused by reduced tumoral testosterone uptake. Because of these findings, therapeutic inhibition targeting OATP1B3 has been proposed. However, any enthusiasm for inhibiting OATP1Bs therapeutically has been tempered by reservations about potential consequences. For instance, inhibitors could interfere with several normal physiological processes mediated by OATP1B3 (i.e., bile acid reuptake, bilirubin uptake, etc) or cause potential, as-yet unknown, drug interactions by barring hepatic uptake, subsequent metabolism and elimination.

Hyperbilirubinemia syndromes (Gilbert-Meulengracht, Crigler-Najjar, Dubin-Johnson, and Rotor syndrome).

Hyperbilirubinemia is an important clinical sign that often indicates severe hepatobiliary disease of different etiologies. Inherited non-haemolytichyperbilirubinemic conditions include Dubin-Johnson, Rotor, and Gilbert-Meulengracht syndromes, which are important differential diagnoses indicating benign disease that require no immediate treatment. Dubin-Johnson and Rotor syndromes are rare, exhibit mixed direct and indirect hyperbilirubinemia as well as typical profiles or urinary coproporphyrin excretion. Gilbert-Meulengracht disease leads to unconjugated hyperbilirubinemia because of impaired glucuronidation activity, and is part of a spectrum of genetic variants also encompassing fatal Crigler-Najjar syndrome. Gilbert-Meulengracht syndrome can be diagnosed by clinical presentation, biochemistry and genotyping, and carries significance regarding the disposition towards drug-associated toxicity. In addition, the precise diagnosis of these inherited hyperbilirubinemic syndromes avoids unnecessary invasive procedures for suspected more severe hepatobiliary disease.

Bilirubin and biliverdin protect rodents against diabetic nephropathy by downregulating NAD(P)H oxidase.

We recently found a markedly lower prevalence of vascular complications, including kidney disease, in diabetic patients with Gilbert syndrome, a congenital form of hyperbilirubinemia, suggesting a beneficial effect of bilirubin (BIL) on diabetic nephropathy. To directly examine this, we determined whether hereditary hyperbilirubinemic Gunn j/j rats and biliverdin (BVD)-treated diabetic db/db mice were resistant to the development of renal disease. Both rodent models had less albuminuria and complete protection against the progression of mesangial expansion accompanied by normalization of transforming growth factor-ß1 and fibronectin expression. Simultaneously, there was normalization of urinary and renal oxidative stress markers, and the expression of nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase subunits in the kidney. In cultured vascular endothelial and mesangial cells, BIL and BVD significantly inhibited NADPH-dependent superoxide production, and both high glucose- and angiotensin II-induced production of reactive oxygen species. Collectively, our findings suggest that BIL and BVD may protect against diabetic nephropathy and may lead to novel antioxidant therapies for diabetic nephropathy.

[Gilbert syndrome]. / Gilbert-szindróma.

Gilbert disease is a benign disorder of the bilirubin conjugation, which affects 7-10% of the average population. The symptoms are usually only mild jaundice and the slightly elevated unconjugated bilirubin level, other laboratory tests and the liver functions are usually normal. In most cases, mutation of the UDP glucuronyltransferase gene leads to impaired bilirubin conjugation. Besides the usual laboratory methods, genetic analyses of the UDP glucuronyltransferase gene can help in the diagnosis. In 80-100% of the patients the (TA)-insertion in the promoter-region of the gene is present in homozygous - (TA) 7 /(TA) 7 - form, and leads to the decrease of the amount of functionally active enzyme. The role of missense mutations localized in the coding region has not been clarified yet, but their co-occurrence with the (TA) 7 promoter-variant might mean an explanation to the elevated bilirubin level, jaundice, and the familiar aggregation of Gilbert disease.

Liver repopulation with bone marrow derived cells improves the metabolic disorder in the Gunn rat.

BACKGROUND: Reversible ischaemia/reperfusion (I/R) liver injury has been used to induce engraftment and hepatic parenchymal differentiation of exogenous beta2-microglubulin(-)/Thy1(+) bone marrow derived cells. AIM: To test the ability of this method of hepatic parenchymal repopulation, theoretically applicable to clinical practice, to correct the metabolic disorder in a rat model of congenital hyperbilirubinaemia. METHODS AND RESULTS: Analysis by confocal laser microscopy of fluorescence labelled cells and by immunohistochemistry for beta2-microglubulin, 72 hours after intraportal delivery, showed engraftment of infused cells in liver parenchyma of rats with I/R, but not in control animals with non-injured liver. Transplantation of bone marrow derived cells obtained from GFP-transgenic rats into Lewis rats resulted in the presence of up to 20% of GFP positive hepatocytes in I/R liver lobes after one month. The repopulation rate was proportional to the number of transplanted cells. Infusion of GFP negative bone marrow derived cells into GFP positive transgenic rats resulted in the appearance of GFP negative hepatocytes, suggesting that the main mechanism underlying parenchymal repopulation was differentiation rather than cell fusion. Transplantation of wild type bone marrow derived cells into hyperbilirubinaemic Gunn rats with deficient bilirubin conjugation after I/R damage resulted in 30% decrease in serum bilirubin, the appearance of bilirubin conjugates in bile, and the expression of normal UDP-glucuronyltransferase enzyme evaluated by polymerase chain reaction. CONCLUSIONS: I/R injury induced hepatic parenchymal engraftment and differentiation into hepatocyte-like cells of bone marrow derived cells. Transplantation of bone marrow derived cells from non-affected animals resulted in the partial correction of hyperbilirubinaemia in the Gunn rat.

Molecular genetics of unconjugated hyperbilirubinemia in Taiwanese.

In bilirubin metabolism, increased destruction of erythrocytes, defect in the function of organic anion transporter polypeptide 2 (OATP2) or UDP-glucuronosyltransferase 1A1 (UGT1A1) may result in unconjugated hyperbilirubinemia. Although glucose-6-phosphate dehydrogenase (G6PD) deficiency is known to be associated with the development of neonatal hyperbilirubinemia, it was observed that in neonates severe hyperbilirubinemia caused by G6PD deficiency, without associated polymorphisms in the UGT1A1 or the OATP2 gene, was preventable. Variations at the nucleotide (nt) 388 of OATP2 gene and nt-211 of UGT1A1 gene, were found to be a risk factor for severe hyperbilirubinemia amongst Taiwanese neonates, respectively. G6PD deficiency, variations at nts 388 and 521 of OATP2 gene, and variations at nt-211 and in the promoter area of UGT1A1 gene were reported to be the risk factors for the occurrence of mild hyperbilirubinemia amongst Taiwanese adults. The status of the haplotypes of G6PD, OATP2, and UGT1A1 genes affected the odds ratio and the bilirubin levels in the hyperbilirubinemic subjects. Moreover, carriage of the variant-211 UGT1A1 allele, as well as UGT1A7*3 allele, was demonstrated to represent a risk factor for the development of, and a determinant for, metastases associated with Taiwanese colorectal-cancer patients. Further investigation is warranted to evaluate this phenomenon.

[Inherited disorders of bilirubin metabolism]. / Disordini ereditari del metabolismo della bilirubina.

Jaundice in an infant or older child may reflect accumulation of either unconjugated or conjugated bilirubin and could be related to inherited bilirubin disorders. Three grades of inherited unconjugated hyperbilirubinemia are recognised in humans. This spectrum of disorders is distinguished primarily on the basis of the plasma bilirubin level, the response to phenobarbital administration, and the presence or absence of bilirubin glucoronides in bile. The enzyme responsible for the conjugation of bilirubin is the bilirubin uridine-diphosphate-glucuronosyltransferase (UGT). Mutations in the gene encoding bilirubin-UGT (UGT1A1), 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). Gilbert syndrome (GS) is due to an insertional mutation at homozygous state of the TATAA element (seven TA repeats) of UGT1A1 producing a reduced level of expression of the gene. The association of GS with haemolytic anemias, e.g., Hereditary Spherocytosis (HS) or Congenital Dyserythropoietic Anemia type 2 (CDA 2), increase the hyperbilirubinemia level and the risk of cholelithiasis. Forms of chronic conjugated hyperbilirubinemia are Dubin-Johnson syndrome, Rotor syndrome, Alagille syndrome or arteriohepatic dysplasia, Wilson disease or hepatolenticular degeneration. Liver or liver cell transplantation is the therapy in some cases.

Umbilical vein and placental vessels from newborns with hereditary haemorrhagic telangiectasia type 1 genotype are normal despite reduced expression of endoglin.

Hereditary haemorrhagic telangiectasia, HHT, is an autosomal dominant disorder that affects approximately 1 in 8000 people. HHT1 is associated with mutations in the ENG (Endoglin) gene and with haploinsufficiency. The disorder is characterized by focally dilated vessels, which can lead to arteriovenous malformations and serious complications even in young children. In the current study, umbilical cord and placenta samples from newborns with ENG mutations were analyzed to estimate the level of corresponding protein and look for potential vascular dysplasia. We confirmed, using metabolic labelling and flow cytometry, that endoglin levels were significantly reduced to median values of 47 per cent (range 32-56 per cent) and 58 per cent (46-90 per cent), respectively, in human umbilical vein endothelial cells derived from newborns with ENG mutations (HHT1 group; n=18) relative to samples from newborns shown not to have the familial mutation (non-HHT group). We also quantified the relative expression of endoglin by estimating the endoglin/PECAM-1 staining ratio in tissue sections. We observed significantly lower values in the HHT1 group, compared to the non-HHT group for the umbilical vein (n=9; median 0.6 vs 0.9; ranges 0.2-1.0 and 0.5-1.5) and for placental stem villus vessels (n=9 and 10; median 0.42 vs 0.93; ranges 0.24-0.58 and 0.56-1.18). No differences in the estimated umbilical vein cross-sectional area and in the proportion of vessels present in placental villi were observed in sections from the HHT1 group relative to the non-HHT group. Thus, blood vessels from HHT1 individuals are maintained intact in the umbilical vein and placenta during pregnancy and delivery, despite a significant reduction in endoglin expression.

Glucuronidation and excretion of nonylphenol in perfused rat liver.

Nonylphenol, an environmental estrogenic chemical, is reported to have adverse effects on the reproductive organs of animals. In this study, the metabolism of nonylphenol and that of other alkylphenols in the rat liver was investigated using liver perfusion. Alkylphenols (nonylphenol, hexylphenol, butylphenol, and ethylphenol) were glucuronidated by rat liver microsomes. Nonylphenol was found to be conjugated with glucuronic acid by an isoform of UDP-glucuronosyltransferase, UGT2B1, expressed in yeast AH22 cells. However, when nonylphenol was perfused into rat liver in situ, it was difficult for free nonylphenol and conjugated metabolite to be excreted into the bile or vein, and most of the perfused nonylphenol remained free and as a glucuronide conjugate in the liver tissue, even after 1 h of perfusion. After 1 h of perfusion of the other alkylphenols, most of them were excreted into the bile as glucuronides. Ethylphenol, which has the shortest alkyl chain, was excreted rapidly into both the bile and vein; however, the excretion rates of alkylphenols having longer alkyl chains tended to be slow. MRP-2-deficient Eisai hyperbilirubinemic rats could not secrete alkylphenol-glucuronides into the bile, indicating that alkylphenol-glucuronides are transported by MRP-2 to the bile in normal Sprague-Dawley rats. The results indicate that the kinetics of excretion of alkylphenol-glucuronides into the bile or vein depends on the length of alkyl chain and suggest that nonylphenol-glucuronide formed in the liver cannot be transported by MRP-2.

Involvement of an organic anion transporter (canalicular multispecific organic anion transporter/multidrug resistance-associated protein 2) in gastrointestinal secretion of glutathione conjugates in rats.

We investigated the role of cMOAT/MRP2 (canalicular multispecific organic anion transporter/multidrug resistance-associated protein 2) in the intestinal secretion of organic anions by comparing the behavior in Sprague-Dawley (SD) rats and Eisai hyperbilirubinemic rat (EHBR) whose cMOAT/MRP2 is hereditarily defective. After i.v. administration of 1-chloro-2,4-dinitrobenzene (30 micromol/kg), the biliary and intestinal excretion of its glutathione conjugate 2, 4-dinitrophenyl-S-glutathione (DNP-SG), a substrate for cMOAT/MRP2, was significantly reduced in EHBR compared with SD rats. This result also was confirmed by Ussing chamber studies; DNP-SG showed 1.5-fold greater serosal-to-mucosal flux compared with the mucosal-to-serosal flux in SD rats, whereas a similar flux was observed in both directions in EHBR. In addition, metabolic inhibitors reduced the preferential serosal-to-mucosal flux of DNP-SG in SD rats. In everted sac studies, intestinal secretion clearance, defined as the efflux rate of DNP-SG into the mucosal side divided by the area under the curve on the serosal side, was significantly lower in the jejunum of EHBR than that in SD rats. Northern blot analyses demonstrated the highest mRNA level of cMOAT/MRP2 in the jejunum, which is in good agreement with the results of the everted sac studies. These results suggest that cMOAT/MRP2 is involved in the secretion of organic anions in the small intestine.

Decreased pancreatic exocrine function in the mutant Eisai hyperbilirubinemic rat (EHBR).

The Eisai hyperbilirubinemic rat (EHBR) is a Sprague-Dawley rat (SDR) mutant with conjugated hyperbilirubinemia as an autosomal recessive trait. EHBRs manifest jaundice from birth, which is permanent except for a transient decrease at 6-8 weeks of age. To investigate whether the hyperbilirubinemia affects pancreatic exocrine function and acinar cell growth, EHBRs at 6 (without jaundice) and 12 weeks (with jaundice) of age were compared, along with SDRs as controls. Pancreatic wet weights did not significantly differ, but amylase content of acini was lower in EHBRs than SDRs. No correlation was found between pancreatic wet weight and serum bilirubin levels in EHBRs. In vitro amylase release in response to 1-100 pM cholecystokinin octapeptide (CCK-8), 1 microM 12-O-tetradecanoylphorbol 13-acetate, and 2.5 microM calcium ionophore A23187, and in vivo pancreatic secretion stimulated by CCK-8 (0.08 microg/kg body weight/h) were significantly lower in the EHBR cases than in the SDRs. At the electron microscopic level, many acinar cell nuclei were pyknotic, most elements of their Golgi complexes were atrophied, some mitochondria demonstrated fusion, and the rough-surfaced endoplasmic reticulum (RER) was dilated in EHBRs. These findings are indicative of a hypofunctional state. However, no differences between EHBRs at 6 and 12 weeks of age were evident, suggesting that the hyperbilirubinemia does not exert any pronounced influence on acinar cell growth or pancreatic exocrine function.

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.

Temocaprilat, a novel angiotensin-converting enzyme inhibitor, is excreted in bile via an ATP-dependent active transporter (cMOAT) that is deficient in Eisai hyperbilirubinemic mutant rats (EHBR).

Temocapril x HCl (alpha-((2S,6R)-6-[(1S)-1-ethoxy-carbonyl-3-phenyl-propyl]amino-5-oxo-2- (2-thienyl)perhydro-1,4-thiazepin-4-yl)acetic acid hydrochloride) is a novel prodrug of an angiotensin-converting enzyme (ACE) inhibitor. Unlike many other ACE inhibitors, its pharmacologically active metabolite,temocaprilat, is excreted predominantly in bile. To investigate the mechanism for the biliary excretion of temocaprilat, we performed in vivo and in vitro experiments using mutant Eisai hyperbilirubinemic rats EHBR) whose canalicular multispecific organic anion transporter (cMOAT) is hereditarily defective. Biliary clearance of temocaprilat after i.v. administration of [14C]temocapril x HCl (1.0 mg/kg) in EHBR was significantly lower than that in Sprague-Dawley rats (5.00 ml/min/kg for Sprague-Dawley rats vs. 0.25 ml/min/kg for EHBR). The uptake of temocaprilat into canalicular membrane vesicles (CMVs) prepared from Sprague-Dawley rats was stimulated in the presence of ATP, whereas little stimulation was observed in CMVs from EHBR. The initial uptake rate of ATP-dependent transport of temocaprilat showed saturation kinetics; we obtained an apparent V(max) value of 1.14 nmol/min/mg protein and a K(m) value 92.5 microM. ATP-dependent transport of temocaprilat was competitively inhibited by 2,4-dinitrophenyl-S-glutathione, a typical substrate for cMOAT with an inhibition constant (K(i)) of 25.8 microM. The K(m) value for the uptake of 2,4-dinitrophenyl-S-glutathione into CMVs (K(m) = 29.6 microM) was consistent with this K(i) value. In addition, the ATP-dependent uptake of 2,4-dinitrophenyl-S-glutathione was inhibited by temocaprilat in a concentration-dependent manner. Active forms of some ACE inhibitors (benazepril, cilazapril, delapril, enalapril and imidapril) did not affect the transport of temocaprilat into CMVs even at concentrations as high as 200 microM. These data suggest that temocaprilat is effectively excreted in bile via cMOAT that is deficient in EHBR and that many of other ACE inhibitors have low affinity for cMOAT.

Effects of organic anions and bile acid conjugates on biliary excretion of LTC4 in the rat.

Biliary organic anion excretion is mediated by an ATP-dependent primary active transporter, so-called canalicular multispecific organic anion transporter (cMOAT). On the other hand, a multiplicity of canalicular organic anion transport has been suggested. Therefore, to examine the substrate specificity of cMOAT using inhibition of excretion of [3H] LTC4-derived radioactive products in the bile as a marker, we examined the effects of various organic anions and bile acid conjugates on the biliary excretion of LTC4 in rats. Biliary excretion of the metabolites of [3H] LTC4, which was injected via the femoral vein, was markedly inhibited by sulfobromophthalein-glutathione, taurolithocholate-3-sulfate, and ursodeoxycholate-3-O-glucuronide. In contrast, dibromosulfophthalein and cefpiramide slightly inhibited, and pravastatin, taurocholate, and 3,7-sul-UDC did not affect biliary LTC4 excretion. Furthermore, vinblastine and phenothiazine, a P-glycoprotein substrate and inducer, did not affect biliary LTC4 excretion. Among various organic anions and bile acid conjugates, LTC4, sulfobromophthalein-glutathione, taurolithocholate-3-sulfate, and ursodeoxycholate-3-O-glucuronide may be good substrates for cMOAT.