Cholestasis caused by inhibition of the adenosine triphosphate-dependent bile salt transport in rat liver

Role of Hepatocyte Transporters in Drug-Induced Liver Injury (DILI)-In Vitro Testing

Bile acids and bile salts (BA/BS) are substrates of both influx and efflux transporters on hepatocytes. Canalicular efflux transporters, such as BSEP and MRP2, are crucial for the removal of BA/BS to the bile. Basolateral influx transporters, such as NTCP, OATP1B1/1B3, and OSTα/β, cooperate with canalicular transporters in the transcellular vectorial flux of BA/BS from the sinusoids to the bile. The blockage of canalicular transporters not only impairs the bile flow but also causes the intracellular accumulation of BA/BS in hepatocytes that contributes to, or even triggers, liver injury. In the case of BA/BS overload, the efflux of these toxic substances back to the blood via MRP3, MRP4, and OST α/β is considered a relief function. FXR, a key regulator of defense against BA/BS toxicity suppresses de novo bile acid synthesis and bile acid uptake, and promotes bile acid removal via increased efflux. In drug development, the early testing of the inhibition of these transporters, BSEP in particular, is important to flag compounds that could potentially inflict drug-induced liver injury (DILI). In vitro test systems for efflux transporters employ membrane vesicles, whereas those for influx transporters employ whole cells. Additional in vitro pharmaceutical testing panels usually include cellular toxicity tests using hepatocytes, as well as assessments of the mitochondrial toxicity and accumulation of reactive oxygen species (ROS). Primary hepatocytes are the cells of choice for toxicity testing, with HepaRG cells emerging as an alternative. Inhibition of the FXR function is also included in some testing panels. The molecular weight and hydrophobicity of the drug, as well as the steady-state total plasma levels, may positively correlate with the DILI potential. Depending on the phase of drug development, the physicochemical properties, dosing, and cut-off values of BSEP IC₅₀ ≤ 25-50 µM or total C_ss,plasma/BSEP IC₅₀ ≥ 0.1 may be an indication for further testing to minimize the risk of DILI liability.

Ursodeoxycholic acid markedly promotes the absorption of microemulsion-formulated cyclosporine A: A case report

WHAT IS KNOWN AND OBJECTIVE

Cyclosporine A (CyA) causes intrahepatic biliary stasis via inhibition of bile acid excretion through the bile salt export pump. We report a case of a patient in whom ursodeoxycholic acid (UDCA) markedly promoted the absorption of microemulsion-formulated CyA.

CASE SUMMARY

The patient was a 22-year-old Japanese man diagnosed with stage 3 aplastic anaemia. He was treated with CyA, and 2 h post-dose (C2) was increased by UDCA.

WHAT IS NEW AND CONCLUSION

A remarkable interaction was observed between CyA and UDCA. This is a valuable finding for improving the treatment strategies for haematological disorders.

Effects of psoralen on hepatic bile acid transporters in rats

Fructus Psoraleae (FP), widely used in traditional medicine, is increasingly reported to cause serious hepatotoxicity in recent years. However, the main toxic constituents responsible for hepatotoxicity and the underlying mechanisms are poorly understood. In the present study, psoralen, a main and quality-control constituent of FP, was intragastrically administered to Sprague-Dawley rats at a dose of 60 mg/kg for 1, 3 and 7 days. Blood and selected tissue samples were collected and analyzed for biochemistry and histopathology to evaluate hepatotoxicity. The results showed that psoralen could induce hepatotoxicity by enhanced liver-to-body weight ratio and alterations of serum alanine aminotransferase (ALT), aspartate aminotransferase (AST) and total cholesterol after administration for 3 days. In addition, histopathological examinations also indicated the hepatotoxicity induced by psoralen. Furthermore, the mRNA and protein levels of hepatic bile acid transporters were significantly changed, in which MRP4, ABCG5 and ABCG8 were repressed, while the protein level of NTCP tended to increase in the rat liver. Taken together, psoralen caused liver injury possibly through affecting bile acid transporters, leading to the disorder of bile acid transport and accumulation in hepatocytes.

Grape Seed Proanthocyanidin Extract Mitigates Titanium Dioxide Nanoparticle (TiO2-NPs)-Induced Hepatotoxicity Through TLR-4/NF-κB Signaling Pathway

With the progress of nanotechnology, the adverse effects of nanoscale materials are receiving much attention. Inhibition of toll-like receptor 4 (TLR-4)/nuclear factor kappa B (NF-κB) signaling is a hallmark for downregulating the expression of many inflammatory genes implicated in oxidative stress. Therefore, the present study aimed to demonstrate the influence of grape seed proanthocyanidin extract (GSE) on the hepatic TLR-4/ NF-κB signaling pathway in TiO₂-NP-induced liver damage in rats. Forty male Albino rats were divided into 4 groups (n = 10): G1 was used as a control, G2 received TiO₂-NPs (500 mg/kg/day orally) from the 17th to 30th day (acute toxicity), G3 received GSE (75 mg/kg/day orally) for 30 days, and G4 pre- and co-treated with GSE (for 30 days) and TiO₂-NPs (from the 17th to 30th day), with the aforementioned doses. TiO₂-NPs induced severe hepatic injury that was indicated by biochemical alterations in serum liver markers (acetylcholinesterase, ALT, ALP, total proteins, albumin, and direct bilirubin), oxidative stress indicators (MDA, GSH, and catalase), and histopathological alterations as well. Moreover, TiO₂-NPs triggered an inflammatory response via the upregulation of TLR-4, NF-κB, NIK, and TNF-α mRNA expressions. Pre- and co-treatments with GSE alleviated the detrimental effects of TiO₂-NPs which were enforced by the histopathological improvements. These results indicated that GSE effectively protected against TiO₂-NP-induced hepatotoxicity via the inhibition of TLR-4/NF-κB signaling and hence suppressed the production of pro inflammatory cytokines such as TNF-α and improved the antioxidant status of the rats.

Roles of Hepatic Drug Transporters in Drug Disposition and Liver Toxicity

Hepatic drug transporters are mainly distributed in parenchymal liver cells (hepatocytes), contributing to drug's liver disposition and elimination. According to their functions, hepatic transporters can be roughly divided into influx and efflux transporters, translocating specific molecules from blood into hepatic cytosol and mediating the excretion of drugs and metabolites from hepatic cytosol to blood or bile, respectively. The function of hepatic transport systems can be affected by interspecies differences and inter-individual variability (polymorphism). In addition, some drugs and disease can redistribute transporters from the cell surface to the intracellular compartments, leading to the changes in the expression and function of transporters. Hepatic drug transporters have been associated with the hepatic toxicity of drugs. Gene polymorphism of transporters and altered transporter expressions and functions due to diseases are found to be susceptible factors for drug-induced liver injury (DILI). In this chapter, the localization of hepatic drug transporters, their regulatory factors, physiological roles, and their roles in drug's liver disposition and DILI are reviewed.

The in silico identification of human bile salt export pump (ABCB11) inhibitors associated with cholestatic drug-induced liver injury

Drug-induced liver injury (DILI) is one of the major causes of drug attrition and failure. Currently, there is increasing evidence that direct inhibition of the human bile salt export pump (BSEP/ABCB11) by drugs and/or metabolites is one of the most important mechanisms of cholestatic DILI. In the present study, we employ two in silico methods, random forest (RF) and the pharmacophore method, to recognize potential BSEP inhibitors that could cause cholestatic DILI, with the aim of mitigating the risk of cholestatic DILI to some extent. The RF model achieved the best prediction performance, producing AUC (area under receiver operating characteristic curve) values of 0.901, 0.929 and 0.996 for leave-one-out cross-validation, the test set and the external test set, respectively, indicating that the built RF model has a satisfactory identification ability. As a complement to the RF model, the pharmacophore model was also built and was proved to be reliable with good predictive performance based on the internal and external validation results. Further analysis indicates that hydrophobicity, molecular size and polarity are important factors that influence the inhibitory activity of BSEP. Furthermore, the two models are applied to screen FDA-approved small molecule drugs, among which the drugs with the potential risk of cholestatic DILI are reported. In conclusion, the RF and pharmacophore models that we present can be considered as integrated screening tools to indicate the potential risk of cholestatic DILI by inhibition of BSEP.

Inhibition of Human Hepatic Bile Acid Transporters by Tolvaptan and Metabolites: Contributing Factors to Drug-Induced Liver Injury?

Tolvaptan is a vasopressin V(2)-receptor antagonist that has shown promise in treating Autosomal Dominant Polycystic Kidney Disease (ADPKD). Tolvaptan was, however, associated with liver injury in some ADPKD patients. Inhibition of bile acid transporters may be contributing factors to drug-induced liver injury. In this study, the ability of tolvaptan and two metabolites, DM-4103 and DM-4107, to inhibit human hepatic transporters (NTCP, BSEP, MRP2, MRP3, and MRP4) and bile acid transport in sandwich-cultured human hepatocytes (SCHH) was explored. IC(50) values were determined for tolvaptan, DM-4103 and DM-4107 inhibition of NTCP (∼41.5, 16.3, and 95.6 μM, respectively), BSEP (31.6, 4.15, and 119 μM, respectively), MRP2 (>50, ∼51.0, and >200 μM, respectively), MRP3 (>50, ∼44.6, and 61.2 μM, respectively), and MRP4 (>50, 4.26, and 37.9 μM, respectively). At the therapeutic dose of tolvaptan (90 mg), DM-4103 exhibited a C(max)/IC(50) value >0.1 for NTCP, BSEP, MRP2, MRP3, and MRP4. Tolvaptan accumulation in SCHH was extensive and not sodium-dependent; intracellular concentrations were ∼500 μM after a 10-min incubation duration with tolvaptan (15 μM). The biliary clearance of taurocholic acid (TCA) decreased by 43% when SCHH were co-incubated with tolvaptan (15 μM) and TCA (2.5 μM). When tolvaptan (15 μM) was co-incubated with 2.5 μM of chenodeoxycholic acid, taurochenodeoxycholic acid, or glycochenodeoxycholic acid in separate studies, the cellular accumulation of these bile acids increased by 1.30-, 1.68-, and 2.16-fold, respectively. Based on these data, inhibition of hepatic bile acid transport may be one of the biological mechanisms underlying tolvaptan-associated liver injury in patients with ADPKD.

Current Concepts in Drug-Induced Bile Salt Export Pump (BSEP) Interference

Numerous drugs have been shown to inhibit the activity of the Bile Salt Export Pump (BSEP in humans, Bsep in animals), and this is now considered to be one of several mechanisms by which idiosyncratic drug-induced liver injury (DILI) may be initiated in susceptible patients. The potential importance of BSEP inhibition by drugs has been recognized by the European Medicines Agency and the International Transporter Consortium, who have recommended that it should be evaluated during drug development when evidence of cholestatic liver injury has been observed in nonclinical safety studies or in human clinical trials. In addition, some pharmaceutical companies have proposed evaluation and minimization of BSEP inhibition during drug discovery, when there is a chemical choice, to help reduce DILI risk. The methods that can be used to assess and quantify BSEP inhibition, and key gaps in our current understanding of the relationship between this process and DILI, are discussed.

Hepatocellular exposure of troglitazone metabolites in rat sandwich-cultured hepatocytes lacking Bcrp and Mrp2: interplay between formation and excretion

Inhibition of bile acid transport by troglitazone (TGZ) and its major metabolite, TGZ sulfate (TS), may lead to hepatocellular accumulation of toxic bile acids; TS accumulation and hepatotoxicity may be associated with impaired TS biliary excretion. This study evaluated the impact of impaired transport of breast cancer resistance protein (Bcrp) and multidrug resistance-associated protein 2 (Mrp2) on the hepatobiliary disposition of generated metabolites, TS and TGZ glucuronide (TG). Sandwich-cultured hepatocytes (SCH) from Mrp2-deficient (TR(-)) rats in combination with Bcrp knockdown using RNA interference were employed. The biliary excretion index (BEI) of generated TS was not significantly altered by impaired Bcrp (20.9 to 21.1%) and/or Mrp2 function (24.4% and 17.5% in WT and TR(-) rat SCH, respectively). Thus, loss-of-function of Mrp2 and/or Bcrp do not appear to be risk factors for increased hepatocellular TS accumulation in rats, potentially because of a compensatory transporter(s) that excretes TS into bile. Further investigations revealed that the compensatory TS biliary transporter was not the bile salt export pump (Bsep) or P-glycoprotein (P-gp). Interestingly, TGZ sulfation was significantly decreased in TR(-) compared with WT rat SCH (total recovery: 2.8 versus 5.0% of TGZ dose), resulting in decreased hepatocellular TS accumulation, even though sulfotransferase activity in TR(-) rat hepatocyte S9 fraction was similar. Hepatocellular TG accumulation was significantly increased in TR(-) compared with WT rat SCH due to increased glucuronidation and negligible TG biliary excretion. These data emphasize that the interplay between metabolite formation and excretion determines hepatocellular exposure to generated metabolites such as TS and TG.

Biosynthesis and trafficking of the bile salt export pump, BSEP: therapeutic implications of BSEP mutations

The bile salt export pump (BSEP, ABCB11) is the primary transporter of bile acids from the hepatocyte to the biliary system. This rate-limiting step in bile formation is essential to the formation of bile salt dependent bile flow, the enterohepatic circulation of bile acids, and the digestion of dietary fats. Mutations in BSEP are associated with cholestatic diseases such as progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2), drug-induced cholestasis, and intrahepatic cholestasis of pregnancy. Development of clinical therapies for these conditions necessitates a clear understanding of the cell biology of biosynthesis, trafficking, and transcriptional and translational regulation of BSEP. This chapter will focus on the molecular and cell biological aspects of this critical hepatic membrane transporter.

Assessment of gadoxetate DCE-MRI as a biomarker of hepatobiliary transporter inhibition

Drug-induced liver injury (DILI) is a clinically important adverse drug reaction, which prevents the development of many otherwise safe and effective new drugs. Currently, there is a lack of sensitive and specific biomarkers that can be used to predict, assess and manage this toxicity. The aim of this work was to evaluate gadoxetate-enhanced MRI as a potential novel biomarker of hepatobiliary transporter inhibition in the rat. Initially, the volume fraction of extracellular space in the liver was determined using gadopentetate to enable an estimation of the gadoxetate concentration in hepatocytes. Using this information, a compartmental model was developed to characterise the pharmacokinetics of hepatic uptake and biliary excretion of gadoxetate. Subsequently, we explored the impact of an investigational hepatobiliary transporter inhibitor on the parameters of the model in vivo in rats. The investigational hepatobiliary transporter inhibitor reduced both the rate of uptake of gadoxetate into the hepatocyte, k1 , and the Michaelis-Menten constant, Vmax , characterising its excretion into bile, whereas KM values for biliary efflux were increased. These effects were dose dependent and correlated with effects on plasma chemistry markers of liver dysfunction, in particular bilirubin and bile acids. These results indicate that gadoxetate-enhanced MRI provides a novel functional biomarker of inhibition of transporter-mediated hepatic uptake and clearance in the rat. Since gadoxetate is used clinically, the technology has the potential to provide a translatable biomarker of drug-induced perturbation of hepatic transporters that may also be useful in humans to explore deleterious functional alterations caused by transporter inhibition.

An updated review on drug-induced cholestasis: mechanisms and investigation of physicochemical properties and pharmacokinetic parameters

Drug-induced cholestasis is an important form of acquired liver disease and is associated with significant morbidity and mortality. Bile acids are key signaling molecules, but they can exert toxic responses when they accumulate in hepatocytes. This review focuses on the physiological mechanisms of drug-induced cholestasis associated with altered bile acid homeostasis due to direct (e.g., bile acid transporter inhibition) or indirect (e.g., activation of nuclear receptors, altered function/expression of bile acid transporters) processes. Mechanistic information about the effects of a drug on bile acid homeostasis is important when evaluating the cholestatic potential of a compound, but experimental data often are not available. The relationship between physicochemical properties, pharmacokinetic parameters, and inhibition of the bile salt export pump among 77 cholestatic drugs with different pathophysiological mechanisms of cholestasis (i.e., impaired formation of bile vs. physical obstruction of bile flow) was investigated. The utility of in silico models to obtain mechanistic information about the impact of compounds on bile acid homeostasis to aid in predicting the cholestatic potential of drugs is highlighted.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Functional impairment of P-glycoprotein by sodium nitroprusside pretreatment in mouse brain capillary endothelial cells

We examined whether pretreatment of mouse brain blood vessel endothelial cell clone 4 (MBEC4) cells with sodium nitroprusside (SNP), a NO(x) donor, as an in vitro model of the bloodbrain barrier could affect P-glycoprotein (P-gp) functional activity. Uptake into the cells and MBEC4 plasma membrane vesicles (MPMVs) in the presence or absence of SNP pretreatment was used to investigate functional changes. Increased accumulation of [(3)H]vincristine, a widely used substrate for P-gp, into MBEC4 was observed upon SNP pretreatment, likely due to impaired P-gp function. To better understand the mechanism of the impairment, MPMVs were prepared and characterized in terms of purity and Na(+)-dependent glucose uptake. [(3)H]daunomycin uptake into MPMVs was diminished after SNP pretreatment in the presence of an ATP-regenerating system, indicating that the functional activity of P-gp was impaired after exposure to SNP. Under conditions of excess ATP, daunomycin uptake into the vesicles was still decreased after SNP pretreatment, indicating that SNP interacted directly with the transport system, but not with the ATP-regenerating system. Together, these results suggest that NO or NO(x) functionally impairs P-gp in the in vitro blood-brain barrier model with SNP pretreatment.

In vitro inhibition of the bile salt export pump correlates with risk of cholestatic drug-induced liver injury in humans

Inhibition of the activity of the human bile salt export pump (BSEP: ABCB11) has been proposed to play a role in drug-induced liver injury (DILI). To enhance understanding of the relationship between BSEP inhibition and DILI, inhibition of human BSEP (hBSEP) and its rat ortholog (rBsep) by 85 pharmaceuticals was investigated in vitro. This was explored using assays that quantified inhibition of ATP-dependent [(3)H]taurocholate uptake into inverted plasma membrane vesicles from Sf21 insect cells, which expressed the proteins. Of the pharmaceuticals, 40 exhibited evidence of in vitro transporter inhibition and overall a close correlation was observed between potency values for inhibition of hBSEP and rBsep activity (r(2) = 0.94), although 12 drugs exhibited >2-fold more potent inhibition of hBSEP than rBsep. The median potency of hBSEP inhibition was higher among drugs that caused cholestatic/mixed DILI than among drugs that caused hepatocellular or no DILI, as was the incidence of hBSEP inhibition with IC(50) <300 μM. All drugs with hBSEP IC(50) <300 μM had molecular weight >250, ClogP >1.5, and nonpolar surface area >180Å. A clear distinction was not evident between hBSEP IC(50) or unbound plasma concentration (C(max, u)) of the drugs in humans and whether the drugs caused DILI. However, all 17 of the drugs with hBSEP IC(50) <100 μM and C(max, u) >0.002 μM caused DILI. Overall, these data indicate that inhibition of hBSEP/rBsep correlates with the propensity of numerous pharmaceuticals to cause cholestatic DILI in humans and is associated with several of their physicochemical properties.

Drug-induced cholestasis

Recent progress in understanding the molecular mechanisms of bile formation and cholestasis have led to new insights into the pathogenesis of drug-induced cholestasis. This review summarizes their variable clinical presentations, examines the role of transport proteins in hepatic drug clearance and toxicity, and addresses the increasing importance of genetic determinants, as well as practical aspects of diagnosis and management.

Multidrug resistance proteins (MRPs, ABCCs): importance for pathophysiology and drug therapy

The nine multidrug resistance proteins (MRPs) represent the major part of the 12 members of the MRP/CFTR subfamily belonging to the 48 human ATP-binding cassette (ABC) transporters. Cloning, functional characterization, and cellular localization of most MRP subfamily members have identified them as ATP-dependent efflux pumps with a broad substrate specificity for the transport of endogenous and xenobiotic anionic substances localized in cellular plasma membranes. Prototypic substrates include glutathione conjugates such as leukotriene C(4) for MRP1, MRP2, and MRP4, bilirubin glucuronosides for MRP2 and MRP3, and cyclic AMP and cyclic GMP for MRP4, MRP5, and MRP8. Reduced glutathione (GSH), present in living cells at millimolar concentrations, modifies the substrate specificities of several MRPs, as exemplified by the cotransport of vincristine with GSH by MRP1, or by the cotransport of GSH with bile acids or of GSH with leukotriene B(4) by MRP4.The role of MRP subfamily members in pathophysiology may be illustrated by the MRP-mediated release of proinflammatory and immunomodulatory mediators such as leukotrienes and prostanoids. Pathophysiological consequences of many genetic variants leading to a lack of functional MRP protein in the plasma membrane are observed in the hereditary MRP2 deficiency associated with conjugated hyperbilirubinemia in Dubin-Johnson syndrome, in pseudoxanthoma elasticum due to mutations in the MRP6 (ABCC6) gene, or in the type of human earwax and osmidrosis determined by single nucleotide polymorphisms in the MRP8 (ABCC8) gene. The hepatobiliary and renal elimination of many drugs and their metabolites is mediated by MRP2 in the hepatocyte canalicular membrane and by MRP4 as well as MRP2 in the luminal membrane of kidney proximal tubules. Therefore, inhibition of these efflux pumps affects pharmacokinetics, unless compensated by other ATP-dependent efflux pumps with overlapping substrate specificities.

Genetic variations of bile salt transporters as predisposing factors for drug-induced cholestasis, intrahepatic cholestasis of pregnancy and therapeutic response of viral hepatitis

INTRODUCTION

Drug-induced cholestasis, intrahepatic cholestasis of pregnancy and viral hepatitis are acquired forms of liver disease. Cholestasis is a pathophysiologic state with impaired bile formation and subsequent accumulation of bile salts in hepatocytes. The bile salt export pump (BSEP) (ABCB11) is the key export system for bile salts from hepatocytes.

AREAS COVERED

This article provides an introduction into the physiology of bile formation followed by a summary of the current knowledge on the key bile salt transporters, namely, the sodium-taurocholate co-transporting polypeptide NTCP, the organic anion transporting polypeptides (OATPs), BSEP and the multi-drug resistance protein 3. The pathophysiologic consequences of altered functions of these transporters, with an emphasis on molecular and genetic aspects, are then discussed.

EXPERT OPINION

Knowledge of the role of hepatocellullar transporters, especially BSEP, in acquired cholestasis is continuously increasing. A common variant of BSEP (p.V444A) is now a well-established susceptibility factor for acquired cholestasis and recent evidence suggests that the same variant also influences the therapeutic response and disease progression of viral hepatitis C. Studies in large independent cohorts are now needed to confirm the relevance of p.V444A. Genome-wide association studies should lead to the identification of additional genetic factors underlying cholestatic liver disease.

The role of the sodium-taurocholate cotransporting polypeptide (NTCP) and of the bile salt export pump (BSEP) in physiology and pathophysiology of bile formation

Bile formation is an important function of the liver. Bile salts are a major constituent of bile and are secreted by hepatocytes into bile and delivered into the small intestine, where they assist in fat digestion. In the small intestine, bile salts are almost quantitatively reclaimed and transported back via the portal circulation to the liver. In the liver, hepatocytes take up bile salts and secrete them again into bile for ongoing enterohepatic circulation. Uptake of bile salts into hepatocytes occurs largely in a sodium-dependent manner by the sodium taurocholate cotransporting polypeptide NTCP. The transport properties of NTCP have been extensively characterized. It is an electrogenic member of the solute carrier family of transporters (SLC10A1) and transports predominantly bile salts and sulfated compounds, but is also able to mediate transport of additional substrates, such as thyroid hormones, drugs and toxins. It is highly regulated under physiologic and pathophysiologic conditions. Regulation of NTCP copes with changes of bile salt load to hepatocytes and prevents entry of cytotoxic bile salts during liver disease. Canalicular export of bile salts is mediated by the ATP-binding cassette transporter bile salt export pump BSEP (ABCB11). BSEP constitutes the rate limiting step of hepatocellular bile salt transport and drives enterohepatic circulation of bile salts. It is extensively regulated to keep intracellular bile salt levels low under normal and pathophysiologic situations. Mutations in the BSEP gene lead to severe progressive familial intrahepatic cholestasis. The substrates of BSEP are practically restricted to bile salts and their metabolites. It is, however, subject to inhibition by endogenous metabolites or by drugs. A sustained inhibition will lead to acquired cholestasis, which can end in liver injury.

Role of the bile salt export pump, BSEP, in acquired forms of cholestasis

Generation of bile is a key function of the liver. Its impairment leads to accumulation of cytotoxic bile salts in hepatocytes and, consequently, to liver disease. The bile salt export pump, BSEP, is critically involved in the secretion of bile salts into bile. Its function can be disturbed or abolished by inherited mutations. This will lead to progressive intrahepatic cholestais and severe liver disease. In addition to mutations, BSEP can be inhibited by acquired factors, such as xenobiotics or drugs, aberrant bile salt metabolites, or pregnancy. This inhibition will lead to acquired cholestasis. Some drugs are now known to be competitive inhibitors of Bsep. In addition, a polymorphism in the gene coding for BSEP has been identified as a potential susceptibility factor for acquired cholestasis.

Biliary diseases in heart transplanted patients: a comparison between cyclosporine A versus tacrolimus-based immunosuppression

A cyclosporine (CsA)-based immunosuppression is associated with an increased incidence of cholelithiasis after heart transplantation. It is not known if tacrolimus (Tac) has comparable biliary side effects in humans.

We evaluated the incidence of gallbladder sludge and cholelithiasis under Tac-based immunosuppression by ultrasound examinations in 31 cardiac transplants (25 male, 6 female, mean age: 59 ± 11 years). Data were compared to 57 patients (47 male, 10 female, mean age: 58 ± 11 years) who received CsA-based immunosuppression.

6 patients receiving Tac and 6 patients receiving CsA had already gallstones prior to transplantation so that finally 25 patients of the Tac group and 51 patients of the CsA group could be evaluated.

In the Tac group the incidence of biliary sludge was 4% (1 of 25), of gallstones 28% (7 of 25). In comparison, patients receiving CsA developed biliary sludge in also 4% (2 of 51) and gallstones in 25% (13 of 51). Nine of 42 males in the CsA group (21%) and eight of 20 males in the Tac group (40%) developed either gallstones or sludge (n.s). Six of nine females in the CsA group (67%), but none of five females in the Tac group (0%) developed either gallstones or sludge (p = 0.01).

In summary, the incidence of biliary disease in patients with Tac is comparable with CsA-based immunosuppression. We recommend regular sonographical examinations to detect biliary diseases as early as possible. In cases of clinically, laboratory and sonographical signs of cholecystitis cholecystectomy is indicated. It seems that towards lithogenicity female patients benefit more from a Tac-based treatment because the occurrence of gallstones is rare.

Bile acid transporters in health and disease

In recent years the discovery of a number of major transporter proteins expressed in the liver and intestine specifically involved in bile acid transport has led to improved understanding of bile acid homeostasis and the enterohepatic circulation. Sodium (Na(+))-dependent bile acid uptake from portal blood into the liver is mediated primarily by the Na(+) taurocholate co-transporting polypeptide (NTCP), while secretion across the canalicular membrane into the bile is carried out by the bile salt export pump (BSEP). In the ileum, absorption of bile acids from the lumen into epithelial cells is mediated by the apical Na(+) bile salt transporter (ASBT), whereas exit into portal blood across the basolateral membrane is mediated by the organic solute transporter alpha/beta (OSTalpha/beta) heterodimer. Regulation of transporter gene expression and function occurs at several different levels: in the nucleus, members of the nuclear receptor superfamily, regulated by bile acids and other ligands are primarily involved in controlling gene expression, while cell signalling events directly affect transporter function, and subcellular localization. Polymorphisms, dysfunction, and impaired adaptive responses of several of the bile acid transporters, e.g. BSEP and ASBT, results in liver and intestinal disease. Bile acid transporters are now understood to play central roles in driving bile flow, as well as adaptation to various pathological conditions, with complex regulation of activity and function in the nucleus, cytoplasm, and membrane.

Analysis of liver function in renal transplant recipients undergoing C2-monitoring for cyclosporine

There exists no systematic evaluation of liver function in renal allograft recipients undergoing C2-monitoring with Neoral [cyclosporine A (CsA)-microemulsion]. In the present cohort analysis, we compared the hepatic profiles of C2-monitored (n = 80), C0-monitored (n = 81), and non-CsA-treated renal allograft recipients (n = 29), transplanted between 1/1999 and 2/2004 in our institution. While the C2-targets were set in accordance with (n = 72) or below (n = 8) the consensus on Neoral (1500 +/- 200 ng/ml), non-CsA-patients received FK506 (n = 29), partially in combination with rapamycin (n = 13) as primary immunosuppression. Analysis of maximum hepatic laboratory parameters and also repeated measures by anova within 30 days post-transplant revealed highly significant elevations of direct, indirect and total bilirubin, glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, and lactate dehydrogenase (P < 0.001) in the C2-group, in comparison with the C0- and the non-CsA-group. Bilirubin-levels were by far the most affected of all hepatic parameters, and correlated with C2-levels (r2 = 0.62). Seventeen CsA-patients had excessive bilirubin-elevations (>4 mg/dl) and were therefore considered to be 'CsA-sensitive' [14 C2-patients (17% of all C2-patients), 3 C0-patients (4% of all C0-patients)]. Bilirubin- and the other parameter elevations in these patients were reversible upon withdrawal or lowering of CsA. Most 'CsA-sensitive' patients (n = 12, 70%) displayed pre-transplant hepatic impairment, indicating a pre-existing liver instability. Collectively, our data emphasize the need for increased awareness toward individual predispositions for CsA-sensitivity.

Effect of thiazolidinediones on bile acid transport in rat liver

The thiazolidinedione derivatives, troglitazone, rosiglitazone, and pioglitazone, are novel insulin-sensitizing drugs that are useful in the treatment of type 2 diabetes. However, hepatotoxicity associated with troglitazone led to its withdrawal from the market in March 2000. In view of case reports of hepatotoxicity from rosiglitazone and pioglitazone, it is unclear whether thiazolidinediones as a class are associated with hepatotoxicity. Although the mechanism of troglitazone-associated hepatotoxicity has not been elucidated, troglitazone and its major metabolite, troglitazone sulfate, competitively inhibit adenosine triphosphate (ATP)-dependent taurocholate transport in isolated rat canalicular liver plasma membrane vesicles mediated by the canalicular bile salt export pump (Bsep). These results suggest that cholestasis may be a factor in troglitazone-associated hepatotoxicity. To determine whether this effect is 1) limited to canalicular bile acid transport and 2) is specific to troglitazone, the effect of troglitazone, rosiglitazone, and ciglitazone on bile acid transport was examined in rat basolateral (blLPM) and canalicular (cLPM) liver plasma membrane vesicles. In cLPM vesicles, troglitazone, rosiglitazone, and ciglitazone (100 microM) all significantly inhibited ATP-dependent taurocholate transport. In blLPM vesicles, these three thiazolidinediones also significantly inhibited Na(+)-dependent taurocholate transport. Inhibition of bile acid transport was concentration dependent and competitive in both cLPM and blLPM vesicles. In conclusion, these findings are consistent with a class effect by thiazolidinediones on hepatic bile acid transport. If hepatotoxicity is associated with this effect, then hepatotoxicity is not limited to troglitazone. Alternatively, if hepatotoxicity is limited to troglitazone, other mechanisms are responsible for its reported hepatotoxicity.

The bile salt export pump

Canalicular secretion of bile salts mediated by the bile salt export pump Bsep constitutes the major driving force for the generation of bile flow. Bsep is a member of the B-family of the super family of ATP-binding cassette transporters and is classified as ABCB11. Bsep has a narrow substrate specificity, which is largely restricted to bile salts. Bsep is extensively regulated at the transcriptional and posttranscriptional level, which directly modulates canalicular bile formation. Pathophysiological alterations of Bsep by either inherited mutations or acquired processes such as inhibition by drugs or disease-related down regulation may lead to a wide spectrum of mild to severe forms of liver disease. Furthermore, many genetic variants of Bsep are known, some of which potentially render individuals susceptible to acquired forms of liver disease.

Use of canalicular membrane vesicles (CMVs) from rats, dogs, monkeys and humans to assess drug transport across the canalicular membrane

INTRODUCTION

A novel application of a Ultrafree filter cartridge/centrifugation method was evaluated to determine uptake in canalicular membrane vesicles (CMVs) from SD rats, beagle dogs, cynomolgus monkeys (common safety species in the pharmaceutical industry) and humans to assess biliary transport.

METHODS

CMVs prepared from fresh livers of rats, dogs, monkeys and humans (four donors) were characterized for enrichment, basolateral and Golgi contamination and orientation. The presence of MRP2 and p-glycoprotein (P-gp) were confirmed by Western blots. Uptake of [3H]-leukotriene C4 (LTC4) and [3H]-estradiol-17beta-d-glucuronide (E2-Gluc) was determined at a low substrate concentration and/or by kinetic measurements (K(m) and V(max)). Correlation of in vitro data with in vivo findings was achieved by determining the biliary clearance of E2-Gluc in rats after a single i.v. dose and with literature in vivo data for LTC4.

RESULTS

CMVs were highly enriched and minimally contaminated based on marker enzyme activities. Uptake clearance among different species varied by approximately ten-fold (rat > dog = human > monkey) for LTC4 and less than two-fold for E2-Gluc. The lower uptake of LTC4 by human than rat CMVs may be attributed to a higher Km value for human than rat CMVs. Uptake of LTC4 or E2-Gluc by human CMVs showed little inter-subject variability (2-5-fold). Differences in in vitro uptake clearance (10-fold) between LTC4 and E2-Gluc in rat CMVs seemed to correlate with differences in their biliary clearance (4-fold) in rats, consistent with LTC4 and E2-Gluc being a high and a low clearance substrate, respectively.

DISCUSSION

A novel application of a Ultrafree filter cartridge/centrifugation method was developed to determine uptake in CMVs from different preclinical animal safety species and humans, and may represent a useful approach to study the mechanism of biliary excretion during drug discovery and development.

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.

Influence of Cyclosporine on the Serum Concentration and Biliary Excretion of Mycophenolic Acid and 7-O-Mycophenolic Acid Glucuronide

The authors have investigated whether cyclosporine decreases the serum concentration of mycophenolic acid, the active principle of the immunosuppressant mycophenolate mofetil, and increases that of the inactive metabolite 7-O-mycophenolic acid glucuronide by reducing their enterohepatic recirculation. Rats were treated daily with methylcellulose (1.66 mL/kg PO) plus 0.9% NaCl (6 mL/kg IP), mycophenolate mofetil (20 mg/kg PO) plus 0.9% NaCl (6 mL/kg IP), methylcellulose (1.66 mL/kg PO) plus cyclosporine (5 mg/kg IP), and mycophenolate mofetil (20 mg/kg PO) plus cyclosporine (5 mg/kg IP). After 14 days a bile fistula was installed to measure the biliary excretion of the immunosuppressants and their metabolites. After 90 minutes blood was taken to determine their concentrations in blood or serum by HPLC. Cyclosporine significantly decreased the serum concentration of mycophenolic acid by 39% and increased, not significantly, that of 7-O-mycophenolic acid glucuronide by 53%. The biliary excretion of 7-O-mycophenolic acid glucuronide was significantly reduced by cyclosporine by 57%, whereas that of mycophenolic acid was not affected. Mycophenolate mofetil did not show a significant effect on either the blood concentration or the biliary excretion of cyclosporine and its metabolites AM1, AM9, AM1c, and AM4N. Cyclosporine significantly decreased the serum concentration of active mycophenolate acid and increased, not significantly, the serum concentration of inactive 7-O-mycophenolic acid glucuronide, presumably by reducing the biliary excretion of this inactive metabolite.

Co-administration of cyclosporine A alleviates thioacetamide-induced liver injury

AIM: To investigate the effects of cyclosporine A (CsA) on thioacetamide (TAA)-induced liver injury.

METHODS: CsA was co-administrated (7.5 μg/kg body weight per day, i.p.) into rat to investigate the role of CsA on TAA-(200 mg/kg body weight per 3 d for 30 d, i.p.)induced liver injury.

RESULTS: The data show that TAA caused liver fibrosis in rat after 30 d of treatment. CsA alleviates the morphological changes of TAA-induced fibrosis in rat liver. The blood glutamyl oxaloacetic transaminase (GOT)/glutamyl pyruvic transaminase (GPT) in the TAA-injury group is elevated compared to that of the normal rat. Compared with the TAA-injury group, the blood GOT/GPT and TGFβ1 (by RT-PCR analysis) are reduced in the CsA plus TAA-treated rat. The level of the transforming growth factor receptor I (TGFβ-R1) in the CsA plus TAA-treated group shows higher than that in the TAA only group, but shows a lower level of the fibroblast growth factor receptor 4 (FGFR4) in the CsA plus TAA-treated group, when using the Western blot analysis. After immunostaining of the frozen section, TGFβ-R1 and FGFR4 are more concentrated in rat liver after CsA plus TAA injury.

CONCLUSION: This result suggests that CsA has an alleviated effect on TAA-induced liver injury by increasing the multidrug resistance P-glycoprotein and could be through the regulation of TGFβ-R1 and FGFR4.

Expression, localization, and function of MRP5 (ABCC5), a transporter for cyclic nucleotides, in human placenta and cultured human trophoblasts: effects of gestational age and cellular differentiation

The placenta functions both as site for nutrition and protection of the fetus. Transport proteins, including members of the multidrug resistance protein (MRP)/ABCC subfamily, have been recognized to contribute to the latter function. MRP5 (ABCC5) was identified as transmembrane transport protein for cyclic nucleotides, especially 3',5'-cyclic GMP (cGMP), indicating an additional role in signal transduction and a potential role in placenta development. We therefore studied expression, localization, and function of MRP5 in placenta of different gestational ages. Quantitative real-time polymerase chain reaction revealed expression of MRP5 in all 60 samples from pre-term and term placenta, with a decreasing mean expression with gestational age (MRP5/18S-ratio x 1000; < 32 weeks: 2.91 +/- 0.73, n = 15; 32 to 37 weeks: 2.10 +/- 0.87, n = 15; > 37 weeks: 0.46 +/- 0.08, n = 30; P < 0.01). Immunofluorescence microscopy with an anti-MRP5 antibody indicated localization of MRP5 preferentially in the basal membrane of syncytiotrophoblasts and in and around fetal vessels. ATP-dependent [(3)H]cGMP transport as evidence for MRP5 function could be demonstrated in isolated basal membrane vesicles. Moreover, the influence of cellular differentiation on MRP5 expression was studied in isolated trophoblasts, revealing an increase of the MRP5 expression in parallel with the hCG production (MRP5/18S-ratio x 1000 was 2.4 +/- 0.5 at day 5 of culture and 1.45 +/- 0.5 at day 0 of culture, n = 3 preparations, significant difference with P < 0.05). In conclusion, MRP5 expression depends on gestational age and varies throughout the differentiation process. In view of the important role of cGMP for cellular differentiation, MRP5 may play a role in placental development in context with a specific need for cellular cGMP export.

Valspodar: current status and perspectives

Valspodar (Amdray, SDZ PSC 833) is derived from cyclosporin, but lacks the immunosuppressive and most of the collateral activities of cyclosporin A (CsA, Sandimmune, Neoral); it exhibits an enhanced capacity to chemosensitise tumour cells showing the classical type multiple drug-resistance (MDR) associated with MDR1 P-glycoprotein (Pgp) overexpression. This valspodar-mediated chemosensitisation of MDR tumour cells is reviewed with regard to its mechanism of inhibition on Pgp flippase function, and its potential inhibition of anticancer drug (ACD) metabolisation by CYP3A enzymes is discussed. Potent inhibition of the membranous and cytoplasmic detoxification mechanisms expressed by cells at the absorption and clearance borders in the body by valspodar results in the many pharmacokinetic interactions with other drugs that are substrates of either, or both, Pgp and CYP classes of detoxifying enzyme. In view of the present ability to restrict oral bioavailability of valspodar within a narrow range, and to adapt adequately the chemotherapeutic dosages to achieve their equivalent exposure in the presence or absence of valspodar, current clinical data on its efficacy and safety permit optimism for ongoing Phase III trials. The potential of valspodar to increase exposure or to modulate the biodistribution of other chemotherapeutics, such as HIV protease inhibitors to the brain, is further evoked, as this might become another application of the new drug. This evaluation of valspodar compared to CsA attempts to interpret its mechanisms of action, rather than to serve as a complete and comparative repertoire of all published preclinical and clinical data.

EXPRESSION, LOCALIZATION, AND FUNCTION OF THE CARNITINE TRANSPORTER OCTN2 (SLC22A5) IN HUMAN PLACENTA

L-carnitine is assumed to play an important role in fetal development, and there is evidence that carnitine is transported across the placenta. The protein involved in this transfer, however, has not been identified on a molecular level. We therefore characterized localization and function of the carnitine transporter OCTN2 in human placenta. Significant expression of OCTN2 mRNA was detected in human placenta applying real-time polymerase chain reaction technology. Confocal immunofluorescence microscopy using an antibody directed against the carboxy terminus of OCTN2 protein revealed that it is predominantly expressed in the apical membrane of syncytiotrophoblasts. This was confirmed by the costaining of organic anion-transporting polypeptide B and MRP2, which are known to be expressed mainly in the basal and apical syncytiotrophoblasts membrane, respectively. To further support this finding, we performed transport studies using basal and apical placenta membrane vesicles. We could demonstrate that the carnitine uptake into the apical vesicles was about eight times higher compared with the basal ones. Moreover, this uptake was sodium- and pH-dependent with an apparent K(m) value of 21 muM and inhibited by verapamil, which is in line with published data for recombinant OCTN2. Finally, experiments using trophoblasts in cell culture revealed that expression of OCTN2 paralleled human choriogonadotropin production and thus is modulated by cellular differentiation. In summary, we show expression and function of OCTN2 in human placenta. Moreover, several lines of evidence indicate that OCTN2 is localized in the apical membrane of syncytiotrophoblasts, thereby suggesting a major role in the uptake of carnitine during fetal development.

ATP-dependent transport of organic anions into isolated basolateral membrane vesicles from rat intestine

The mechanism for the cellular extrusion of organic anions across the intestinal basolateral membrane was examined using isolated membrane vesicles from rat jejunum, ileum, and colon. It was found that 17beta-estradiol 17beta-D-glucuronide (E217betaG) is taken up in an ATP-dependent manner into the basolateral membrane vesicles (BLMVs) but not into the brush-border or microsomal counterparts. The ATP-dependent uptake of E217betaG into BLMVs from jejunum and ileum was described by a single component with a Km value of 23.5 and 8.31 microM, respectively, whereas that into the BLMVs from colon was described by assuming the presence of high (Km=0.82 microM)- and low-affinity (Km=35.4 microM) components. Taurocholate, 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole glucuronide and taurolithocholate sulfate, but not leukotriene C4, were significantly taken up by the BLMVs. In addition to such substrate specificity, the inhibitor sensitivity of the ATP-dependent transport in BLMVs was similar to that of rat multidrug resistance-associated protein 3 (Mrp3), which is located on the basolateral membrane of enterocytes. Together with the fact that the rank order of the extent of the expression of Mrp3 (jejunum < ileum << colon) is in parallel with that of the extent of the transport of ligands, these results suggest that the ATP-dependent uptake of organic anions into isolated intestinal BLMVs is at least partly mediated by Mrp3.

Cyclosporine A-induced reduction of bile salt synthesis associated with increased plasma lipids in children after liver transplantation

Hyperlipidemia is a common side effect of cyclosporine A (CsA) after solid organ transplantation. CsA also markedly reduces the synthesis rate of bile salts in rats and can inhibit biliary bile salt secretion. It is not known, however, whether CsA inhibits the synthesis of bile salts in humans, and whether the hyperlipidemic effects of CsA are related to bile salt metabolism. Our objective was to assess the effects of CsA on the synthesis rate of bile salts and on plasma triglycerides and cholesterol levels in pediatric liver transplant patients. Before and after discontinuation of CsA treatment after liver transplantation, synthesis rate and pool size of the primary bile salts cholate and chenodeoxycholate were determined using a stable isotope dilution technique and related to plasma lipids. In 6 children (age: 3-16 years) CsA treatment was discontinued at 2 years (median 2.3 years) after liver transplantation. Discontinuation of CsA increased synthesis rate of chenodeoxycholate (+38%, P <.001) and cholate (+21%, P <.05) and the pool size of chenodeoxycholate (+54%, P <.001). Discontinuation of CsA decreased plasma levels of cholesterol (-18%, P <.05) and triglycerides (-23%, P <.05). Bile salt synthesis rate appeared to be inversely correlated with plasma cholesterol (Spearman rank correlation coefficient [r(s)] = -0.82, P <.01) and plasma triglyceride levels (r(s) = -0.62, P <.05). In conclusion, CsA inhibits bile salt synthesis and increases plasma concentration of cholesterol and triglycerides in pediatric liver transplant patients. Suppression of bile salt synthesis by long-term CsA treatment may contribute to hyperlipidemia and thus to increased risk for cardiovascular disease.

Mrp2 is involved in benzylpenicillin-induced choleresis

Benzylpenicillin (PCG; 180 micromol/kg), a classic beta-lactam antibiotic, was intravenously given to Sprague-Dawley (SD) rats and multidrug resistance-associated protein 2 (Mrp2)-deficient Eisai hyperbilirubinemic rats (EHBR). A percentage of the [(3)H]PCG was excreted into the bile of the rats within 60 min (SD rats: 31.7% and EHBR: 4.3%). Remarkably, a transient increase in the bile flow ( approximately 2-fold) and a slight increase in the total biliary bilirubin excretion were observed in SD rats but not in the EHBR after PCG administration. This suggests that the biliary excretion of PCG and its choleretic effect are Mrp2-dependent. Positive correlations were observed between the biliary excretion rate of PCG and bile flow (r(2) = 0.768) and more remarkably between the biliary excretion rate of GSH and bile flow (r(2) = 0.968). No ATP-dependent uptake of [(3)H]PCG was observed in Mrp2-expressing Sf9 membrane vesicles, whereas other forms of Mrp2-substrate transport were stimulated in the presence of PCG. GSH efflux mediated by human MRP2 expressed in Madin-Darby canine kidney II cells was enhanced in the presence of PCG in a concentration-dependent manner. In conclusion, the choleretic effect of PCG is caused by the stimulation of biliary GSH efflux as well as the concentrative biliary excretion of PCG itself, both of which were Mrp2 dependent.

Cotransport of reduced glutathione with bile salts by MRP4 (ABCC4) localized to the basolateral hepatocyte membrane

The liver is the major source of reduced glutathione (GSH) in blood plasma. The transport protein mediating the efflux of GSH across the basolateral membrane of human hepatocytes has not been identified so far. In this study we have localized the multidrug resistance protein 4 (MRP4; ABCC4) to the basolateral membrane of human, rat, and mouse hepatocytes and human hepatoma HepG2 cells. Recombinant human MRP4, expressed in V79 hamster fibroblasts and studied in membrane vesicles, mediated ATP-dependent cotransport of GSH or S-methyl-glutathione together with cholyltaurine, cholylglycine, or cholate. Several monoanionic bile salts and the quinoline derivative MK571 were potent inhibitors of this unidirectional transport. The K(m) values were 2.7 mmol/L for GSH and 1.2 mmol/L for the nonreducing S-methyl-glutathione in the presence of 5 micromol/L cholyltaurine, and 3.8 micromol/L for cholyltaurine in the presence of 5 mmol/L S-methyl-glutathione. Transport of bile salts by MRP4 was negligible in the absence of ATP or without S-methyl-glutathione. These findings identify a novel pathway for the efflux of GSH across the basolateral hepatocyte membrane into blood where it may serve as an antioxidant and as a source of cysteine for other organs. Moreover, MRP4-mediated bile salt transport across the basolateral membrane may function as an overflow pathway during impaired bile salt secretion across the canalicular membrane into bile. In conclusion, MRP4 can mediate the efflux of GSH from hepatocytes into blood by cotransport with monoanionic bile salts.

Regulation of rat organic anion transporters in bile salt-induced cholestatic hepatitis: effect of ursodeoxycholate

Hepatic uptake of organic anions, including bile salts, is mediated by members of the organic anion-transporting polypeptide (Oatp) family. In rat liver, Oatp1 (Slc21a1), Oatp2 (Slc21a5), and Oatp4 (Slca10) are expressed at the basolateral membrane of hepatocytes and may be differentially regulated under pathophysiologic conditions such as cholestasis. The aim of this study was to determine the effects of cholic acid (CA) and ursodeoxycholic acid (UDCA) on the expression of Oatp4 compared with Ntcp, Oatp1, and Oatp2. Wistar rats were fed with CA (0.5%) or both CA (0.5%) and UDCA (0.25%) for 3 weeks. Oatp expression was studied by Northern and Western blot analysis as well as immunofluorescence analysis. Transport function was compared measuring biliary secretion of (14)C-CA and (14)C-taurocholic acid (TCA). In CA-fed animals, biliary secretion of (14)C-CA and (14)C-TCA was markedly delayed over 40 minutes compared with controls. Accordingly, Oatp4 protein was significantly down-regulated in CA-fed animals together with Oatp1 and Ntcp. Cofeeding of CA plus UDCA prevented the impairment of (14)C-CA and (14)C-TCA secretion and the down-regulation of Oatp4. Oatp4 messenger RNA (mRNA) levels did not differ significantly between bile salt-fed groups, suggesting a posttranscriptional effect of CA on Oatp4 expression. In contrast to Oatp1 and Oatp4, Oatp2 protein expression was increased by CA feeding, indicating a differential regulation of Oatp transporters. In conclusion, we show that CA feeding may cause cholestasis associated with a posttranscriptional down-regulation of Oatp4. UDCA may prevent impairment of hepatic function by restoring hepatic transporter expression.

Identification of domains participating in the substrate specificity and subcellular localization of the multidrug resistance proteins MRP1 and MRP2

The human multidrug resistance protein MRP1 and its homolog, MRP2, are both thought to be involved in cancer drug resistance and the transport of a wide variety of organic anions, including the cysteinyl leukotriene C4 (LTC4) (Km = 0.1 and 1 microm). To determine which domain of these proteins is associated with substrate specificity and subcellular localization, we constructed various chimeric MRP1/MRP2 molecules and expressed them in polarized mammalian LLC-PK1 cells. We examined the kinetic properties of each chimeric protein by measuring LTC4 and methotrexate transport in inside-out membrane vesicles, sensitivity to an anticancer agent, etoposide, and subcellular localization by indirect immunofluorescence methods. The following results were determined in these studies: (i) when the NH2-proximal 108 amino acids of MRP2, including transmembrane (TM) helices 1-3, were exchanged with the corresponding region of MRP1, Km(LTC4) values of the chimera decreased approximately 4-fold and Km(methotrexate) values increased approximately 5-fold relative to those of wild-type MRP2 and MRP1, respectively, whereas resistance to etoposide increased approximately 3-fold; (ii) when the NH2-proximal region up to TM9 of MRP2 was exchanged with the corresponding region of MRP1, a further increase in etoposide resistance was observed, and subcellular localization moved from the apical to the lateral membrane; (iii) when two-thirds of MRP2 at the NH2 terminus were exchanged with the corresponding MRP1 region, the chimeric protein transported LTC4 with an efficiency comparable with that achieved by the wild-type MRP1; and (iv) exchange of the COOH-terminal 51 amino acids between MRP1 and MRP2 did not affect the localization of either of the proteins. These results provide a strong framework for further studies aimed at determining the precise domains of MRP1 and MRP2 with affinity for LTC4 and anticancer agents.

Prevention by dietary (n-6) polyunsaturated phosphatidylcholines of intrahepatic cholestasis induced by cyclosporine A in animals

Previous findings showed that dietary (n-6) polyunsaturated phosphatidylcholines (vegetable lecithin) could efficiently prevent intrahepatic cholestasis induced by cyclosporine A in rats. Mechanistic studies showed that expressions in rat liver of Na(+), K(+)-ATPase, Ca(2+), Mg(2+)-ATPase and F-actin were both decreased by drug administration and both enhanced by (n-6) lecithin enriched diet. There is a possible direct effect of phosphatidylcholines, vectors of polyunsaturated fatty acids provided by the metabolism of the dietary lecithin, on the aforesaid hepatic parameters. Such modulations by drug and diet result in reversed modifications of membrane composition and fluidity. Final outcome is decreased and enhanced bile lipid secretion by cyclosporine and vegetable lecithin enriched diet respectively. Moreover, we advance the hypothesis of a bypass process including a separate and functional actin-independent way for the non micellar and phospholipid-dependent secretion of bile lipids. The relationships between the ATPases, the microfilament components such as F-actin and the different transporters still remain to be clarified. Furthermore, one can speculate on beneficial effects in humans of diets enriched in vegetable lecithins that might prevent cholestasis induced by cyclosporine A.

Mechanism of Active Secretion of Phenolsulfonphthalein in the Liver via Mrp2 (abcc2), an Organic Anion Transporter

Phenolsulfonphthalein (PSP) has been selected as a model drug that is eliminated from both the kidney and liver in rats. Although the renal PSP transport system has been studied, few details of the biliary excretion of PSP have been reported. We investigated the biliary excretion system for PSP in rats. It has been reported that the biliary excretion of many organic anions from hepatocytes into bile is mediated by a primary active transporter, referred to as multidrug resistance-associated protein 2 (Mrp2/abcc2). The biliary excretion of PSP in SD rats was significantly decreased in the presence of Mrp2 inhibitors. The biliary excretion of PSP in Eisai hyperbilirubinemic rats (EHBR), hereditarily Mrp2-defective rats, was significantly lower than that in SD rats. Moreover, an efflux experiment using Caco-2 cells was carried out to confirm Mrp2-mediated PSP transport. Mrp2 inhibitors significantly decreased PSP efflux from Caco-2 cells. These results suggest that Mrp2 contributes to the biliary excretion of PSP in SD rats.

Cyclosporin a and enterohepatic circulation of bile salts in rats: decreased cholate synthesis but increased intestinal reabsorption

Cyclosporin A (CsA) has been shown to inhibit synthesis and hepatobiliary transport of bile salts. However, effects of CsA on the enterohepatic circulation of bile salts in vivo are largely unknown. We characterized the effects of CsA on the enterohepatic circulation of cholate, with respect to synthesis rate, pool size, cycling time, intestinal absorption, and the expression of relevant transporters in liver and intestine in rats. CsA (1 mg. 100 g(-1). day(-1) s.c.) or its solvent was administered daily to male rats for 10 days. Cholate synthesis rate and pool size were determined by a 2H4-cholate dilution technique. Bile and feces were collected for determination of cholate and total bile salts, respectively. Cycling time and intestinal absorption of cholate were calculated. The mRNA levels and corresponding transporter protein levels in liver and intestine were assessed by real-time polymerase chain reaction and Western analysis, respectively. CsA treatment decreased cholate synthesis rate by 71%, but did not affect pool size or cycling time. CsA reduced the amount of cholate lost per enterohepatic cycle by approximately 70%. Protein levels of the apical sodium-dependent bile salt transporter (Asbt) were 2-fold increased in distal ileum of CsA-treated rats, due to post-transcriptional events. In conclusion, chronic CsA treatment markedly reduces cholate synthesis rate in rats, but does not affect cholate pool size or cycling time. Our results strongly suggest that CsA enhances efficacy of intestinal cholate reabsorption through increased Asbt protein expression in the distal ileum, which contributes to maintenance of cholate pool size in CsA-treated rats.

Potential Cholestatic Activity of Various Therapeutic Agents Assessed by Bile Canalicular Membrane Vesicles Isolated from Rats and Humans

The active transport of solutes mediated by the bile salt export pump (BSEP/ABCB11) and multidrug resistance associated protein-2 (MRP2/ABCC2) are thought to involve bile acid-dependent and -independent bile formation, respectively. To evaluate the potential of therapeutic agents as inhibitors of such transporters on bile canalicular membranes, we examined the inhibition of the primary active transport of typical substrates by 15 drugs, clinically known to cause cholestasis in canalicular membrane vesicles. The inhibition by most of the compounds in rat canalicular membrane vesicles (CMVs) was minimal or observed at much higher concentrations than obtained in clinical situations. However, cloxacillin, cyclosporin A and midecamycin inhibited BSEP, and cyclosporin A and midecamycin inhibited MRP2 with an inhibition constant close to the clinical concentration. By comparing the inhibition potential between rat and human CMVs, the inhibition of BSEP- and MRP2-mediated transport by midecamycin and cyclosporin A was relatively similar whereas the inhibitory effect on BSEP-mediated transport by cloxacillin and glibenclamide was more marked in humans than in rats. These results suggest that the majority of cholestasis-inducing drugs have a minimal inhibitory effect on rat BSEP and MRP2 although species differences in inhibitory potential should be considered, especially in the case of BSEP.