Taurine conjugate of 3 alpha,6 beta,7 beta-trihydroxy-5 beta,22-cholen-24-oic acid (tauro-delta 22-beta-muricholate): the major bile acid in the serum of female rats treated with alpha-naphthylisothiocyanate and its secretion by liver slices

Serum bile acids as a sensitive biological marker for evaluating hepatic effects of organic solvents

Serum bile acids (SBAs) are suggested as a potentially sensitive and specific indicator of liver function which, accordingly, could provide an early indication of hepatobiliary dysfunction. This offers advantages over more traditional parameters of liver integrity/function. Recent studies have shown that occupational exposure to low levels of halogenated aliphatic or non-halogenated aromatic solvents is associated with significant increases in SBA levels. As this has often been evident in the absence of any effect on conventional parameters of hepatobiliary integrity/function, elevated SBA levels may well be regarded as a sensitive biological marker of exposure/effect of these compounds. In addition, it may be considered that they provide an early indicator of solvent-induced changes in hepatobiliary function. Extensive studies with experimental animals have also provided supporting evidence for these observations in solvent-exposed individuals. Investigations of the mechanisms at cellular and subcellular levels by which these increases occur have suggested that these effects are likely to be the result of selective, dose-related and reversible inhibition of bile acid uptake at the sinusoidal domain of the hepatocyte plasma membrane. Increased concentrations of SBA under low levels of exposure to different solvents have been demonstrated to be a short-lived and reversible effect which is not accompanied by any other evidence of liver damage. Therefore, it could be assumed that it is unlikely that there would be pathological sequelae to these effects, although the longer term ramifications of such effects have not been thoroughly investigated. Nevertheless, the available evidence indicates that investigation of SBA in solvent-exposed workers could provide useful indications of exposure and effect.

Preparation and culture of precision-cut organ slices from human and animal

1.Human and animal precision-cut organ slices are being widely used to obtain drug metabolism and toxicity profiles in vitro. These data are then used to predict what might be seen in human patients. The accuracy of this prediction and extrapolation of the findings based on human or animal in vitro systems to the findings that occur in vivo is dependent on both the quality of the tissue itself and the quality of the in vitro system. 2.The quality of human organs used in research is dependent on procurement methods, warm ischaemia time, preservation solutions, cold ischaemia time, and donor-specific factors. It is important to confirm that the organs being used are highly viable and fully functional before using them in scientific studies. 3.The optimal preparation and incubation of organ slices is also essential in maintaining slice viability and function. It is important to prepare the slices in a cold preservation solution, to prepare the slices at a correct thickness, and to incubate the slices in a system where the slice rotates in out of the oxygen atmosphere and medium. 4.Meeting the criteria outlined here will lead to successful organ slice cultures for investigating drug-induced mechanisms and organ-specific toxicity.

Liver slices in in vitro pharmacotoxicology with special reference to the use of human liver tissue

In the early years of research in in vitro pharmacotoxicology liver slices have been used. After a decline in the application of slices in favour of the use of isolated hepatocytes and the isolated perfused liver preparation, the development of the Krumdieck slicer in the 1980s led to a ;comeback' of the technique. This review will focus on the use of human liver, with special reference to the comparison of slices with isolated hepatocytes in in vitro pharmacotoxicology. In addition, an overview on the predictive value of these in vitro systems for drug disposition and toxicity in vivo will be given. Preservation techniques for liver slices and hepatocytes will also be discussed. These techniques ensure an efficient utilization of the scarce human material. For long-term storage of liver slices and hepatocytes, cryopreservation seems most promising. However, cryopreservation is still in its infancy, and reports mainly deal with drug metabolism studies after cryopreservation. Drug toxicity, metabolism and transport data determined in slices and isolated hepatocytes, from both human and animal liver showed good correlation with the corresponding parameters measured in vivo. Therefore, the results obtained in such studies may give rise to more in-depth research on the mechanisms of pharmactoxicology in the human liver.

Liver fibrosis in vitro: Cell culture models and precision-cut liver slices

Chronic liver injury of various etiologies can cause liver fibrosis, which is characterized by the progressive accumulation of connective tissue in the liver. As no effective treatment for liver fibrosis is available yet, extensive research is ongoing to further study the mechanisms underlying the development of disease- or toxicity-induced liver fibrosis and to identify potential pro- or anti-fibrotic properties of compounds. This review gives an overview of the in vitro methods that are currently available for this purpose. The first focus is on cell culture models, since the majority of in vitro research uses these systems. Both primary cells and cell lines as well as the use of different culture matrices and co-culture models are discussed. Second, the use of precision-cut liver slices, which recently came into attention as in vitro model for the study of fibrosis, is discussed. The overview clearly shows that continuous optimization and adaptation have extended the potential of in vitro models for liver fibrosis during the past years. By combining the use of the different cell and tissue culture models, the mechanisms underlying multicellular fibrosis development can be studied in vitro and potential pro- or anti-fibrotic properties of compounds can be identified both on single liver cell types and in human liver tissue.

Influence of Verapamil and Cyclosporin A on bile acid metabolism and transport in rat liver slices

Verapamil (V) is a specific inhibitor of the P-glycoprotein (mdr1) in the hepatocyte canalicular membrane. Cyclosporin A (CsA) as an essential immunosuppressive drug has potentially cholestatic adverse effects on the liver, but increases the expression of mdr1. In precision-cut liver slices from 34- to 40-day-old male Wistar rats 26 individual free and conjugated bile acids (BAs) as markers of hepatic transport and synthesis function were analysed after 4 h incubation with V (100 microM) or CsA (5 microM) in Krebs-Henseleit buffer. Some slices were loaded with cholic acid (CA 5 microM) or tauro-ursodeoxycholic acid (T-UDCA 5 microM) to investigate the V and CsA effects under conditions of BA supplementation. BAs were determined in tissue and medium by HPLC with postcolumn derivatisation and fluorescence detection. V and CsA, influencing different targets in BA transport, enhanced slice concentrations of T- and glyco- (G-) conjugated CA only when exogenous CA was given additionally. This BA accumulation in tissue is more reflected at decreased medium concentrations of these BAs after V and CsA incubations. Both V and CsA also inhibited CA uptake into the slices. The acidic chenodeoxycholic acid (CDCA) synthesis pathway is disturbed: T- and G-CDCA concentrations are diminished in slices and medium after V and CsA incubations. T-UDCA plus V or CsA enhanced not only its own slice concentration but also the concentration of the trihydroxylated tauro-muricholic acid (T-beta-MCA), reflecting the conversion of the accumulated dihydroxylated T-UDCA into the T-beta-MCA. The similar effects of V and CsA on BA transport and metabolism can be explained by mdr1 mediated disturbances of cellular ATP transport rather than by inhibition of individual BA transporters.

Bile acid transport and metabolism in rat liver slices

To further characterise precision-cut liver slices from 34- to 40-day-old male rats as an in vitro model for bile acid (BA) metabolism and transport, the effect of the primary BAs cholic (CA, 5 microM) and chenodeoxycholic acid (CDCA, 0.15 and 0.75 microM) as well as of the therapeutically used tauroursodeoxycholic acid (T-UDCA, 5 microM) on BA profiles was investigated. After 4 h incubation in 5 ml Krebs-Henseleit buffer (KHB) 26 individual BAs were determined in slices (50 mg liver/5 ml KHB) and medium by HPLC with postcolumn derivatisation and fluorescence detection. In control incubations, mean total BA concentrations were 5.09 nmol/50 mg liver (101.80 nmol/g liver) in slices and 25.71 nmol/5 ml KHB, among them 72% taurine-(T-), 22% glycine-(G-) conjugated and 6% free BAs in tissue and medium. The main BAs were beta-muricholic (beta-MCA and conjugates) and cholic acids (CA and conjugates) in tissue and medium. The following results were obtained after addition of CDCA, CA, and T-UDCA, respectively, to the KHB. The toxic CDCA was quantitatively converted mainly to T-UDCA and taurohyodeoxycholic (T-HDCA) acid. CA was conjugated in equal shares to T- and G-CA, whereas T-UDCA was enriched in slices and hydroxylated half to T-beta-MCA, which is the main BA in rats. In conclusion, rat liver slices are highly effective not only in uptake, conjugation and excretion of BAs but also in conversion of strong detergent into less toxic BAs.

Precision-cut organ slices to investigate target organ injury

Drug-induced organ injury is a multifaceted process, involving numerous cell types and mediators, and remains a significant safety issue in pharmaceutical development and clinical therapy. Organ slices, an in vitro model representing the multicellular, structural and functional features of in vivo tissue, is a promising model for elucidating mechanisms of drug-induced organ injury and for characterising species susceptibilities. Time- and concentration-dependent drug-induced effects on organ slice gene expression, function and morphology are providing insight into the molecular and biochemical pathways leading to organ dysfunction, an altered morphology and the induction of repair pathways. Human organ slice studies are valuable for bridging the extrapolation of animal-derived data and for identifying mechanisms relevant for humans. The liver is the major organ used in organ slice studies; however, the utility of extrahepatic-derived slices, as well as cocultures for investigating multiple organ involvement in tissue injury is increasing. Organ slice investigations can further our understanding of the cell types and cell interactions involved in drug-induced injury and the consequences of drug-induced off-target effects for identifying compound liabilities that will impact safety.

Organ slices for the evaluation of human drug toxicity

Human organ slices, an in vitro model representing the multicellular and functional features of in vivo tissue, is a promising model for characterizing mechanisms of drug-induced organ injury and for identifying biomarkers of organ injury. Target organ injury is a significant clinical issue. In vitro models, which compare human and animal tissue to improve the extrapolation of animal in vivo studies for predicting human outcome, will contribute to improving drug candidate selection and to defining species susceptibilities in drug discovery and development programs. A critical aspect to the performance and outcome of human organ slice studies is the use of high quality tissue, and the use of culture conditions that support optimum organ slice survivability, in order to accurately reproduce mechanisms of organ injury in vitro. The attribute of organ slices possessing various cell types and interactions contributes to the overall biotransformation, inflammatory response and assessment of injury. Regional differences and changes in morphology can be readily evaluated by histology and special stains, similar to tissue obtained from in vivo studies. The liver is the major organ of which slice studies have been performed, however the utility of extra-hepatic derived slices, as well as co-cultures is increasing. Recent application of integrating gene expression, with human organ slice function and morphology demonstrate the increased potential of this model for defining the molecular and biochemical pathways leading to drug-induced tissue changes. By gaining a more detailed understanding of the mechanisms of drug-induced organ injury, and by correlating clinical measurements with drug-induced effects in the in vitro models, the vision of human in vitro models to identify more sensitive and discriminating markers of organ damage is attainable.

Conjugation with taurine prevents side-chain desaturation of ursodeoxycholic and beta-muricholic acids in bile fistula rats

The metabolism of intravenously infused bile salts, tauroursodeoxycholate, tauro-beta-muricholate and their corresponding unconjugated forms in the liver was investigated in bile salt-depleted bile fistula rats. The biliary bile salt composition was determined by gas chromatography-mass spectrometry using chemical positive ionization and electron-impact methods. For an infusion rate of 2 micromol/min/kg, all bile salts were efficiently secreted in bile, inducing similar choleresis. Only tauroconjugated bile salts were recovered; no glucuronide or glyco derivatives were detected. The infusion of free ursodeoxycholate led to the appearance of a metabolite identified as a Delta22 derivative (12%). A similar biotransformation rate (11%) was observed following free beta-muricholate infusion. In contrast, no metabolite was observed after infusion of the tauroconjugated form of ursodeoxycholate and beta-muricholate. The unsaturation process probably depends on the availability of the carboxyl group for the starting step of the beta-oxidation mechanism. In conclusion, the current in vivo study demonstrates a hepatic origin for Delta22 bile salts. It also shows that free bile salts were sensitive to Delta22 formation while conjugation with taurine totally prevented the side-chain oxidation of the two 7beta-hydroxylated bile salts.

Chemical synthesis of (22E)-3 alpha,6 beta,7 beta-trihydroxy-5 beta-chol-22-en-24-oic acid and its taurine and glycine conjugates: a major bile acid in the rat

A method for the synthesis of Delta(22)-beta-muricholic acid (Delta(22)-beta-MCA), (22E)-3 alpha,6 beta,7 beta-trihydroxy-5 beta-chol-22-en-24-oic acid, and its taurine and glycine conjugates (Delta(22)-beta-muricholyltaurine and Delta(22)-beta-muricholylglycine) is described. The key intermediate, 3 alpha,6 beta,7 beta-triformyloxy-23,24-dinor-5 beta-cholan-22-al, was prepared from beta-muricholic acid (beta-MCA) via the 24-nor-22-ene and 24-nor-22,23-diol derivatives. Wittig reaction of the aldehyde with (carbomethoxymethylene) triphenylphosphorane and subsequent hydrolysis gave (unconjugated) Delta(22)-beta-MCA. Condensation reaction of the unconjugated acid with taurine or glycine methyl ester using diethylphosphorocyanide yielded the naturally occurring taurine or glycine conjugate (N-acylamidate) of Delta(22)-beta-MCA. These synthetic reference compounds are now available for investigation of the metabolism of beta-MCA by bacterial and hepatic enzymes in the rat and should also be useful as substrates for reductive deuteration or tritiation to give the 22,23-(2)H or (3)H-beta-MCA.

Adaptive regulation of bile salt transporters in kidney and liver in obstructive cholestasis in the rat

BACKGROUND & AIMS

Cholestasis results in adaptive regulation of bile salt transport proteins in hepatocytes that may limit liver injury. However, it is not known if changes also occur in the expression of bile salt transporters that reside in extrahepatic tissues, particularly the kidney, which might facilitate bile salt excretion during obstructive cholestasis.

METHODS

RNA and protein were isolated from liver and kidney 14 days after common bile duct ligation in rats and assessed by RNA protection assays, Western analysis, and tissue immunofluorescence. Sodium-dependent bile salt transport was also measured in brush border membrane vesicles from the kidney.

RESULTS

After common bile duct ligation, serum bile salts initially rose and then declined to lower levels after 3 days. In contrast, urinary bile salt excretion rose progressively over the 2-week period. By that time, the ileal sodium-dependent bile salt transporter messenger RNA and protein expression in total liver had increased to 300% and 200% of controls, respectively, while falling to 46% and 37% of controls, respectively, in the kidney. Sodium-dependent uptake of (3)H-taurocholate in renal brush border membrane vesicles was decreased. In contrast, the multidrug resistance-associated protein 2 expression in the kidney was increased 2-fold, even 1 day after ligation. Immunofluorescent studies confirmed the changes in the expression of these transporters in liver and kidney.

CONCLUSIONS

These studies show that the molecular expression of bile salt transporters in the kidney and cholangiocytes undergo adaptive regulation after common bile duct obstruction in the rat. These responses may facilitate extrahepatic pathways for bile salt excretion during cholestasis.

The applicability of rat and human liver slices to the study of mechanisms of hepatic drug uptake

In the present study we investigated the applicability of the liver slice model to study mechanisms of drug uptake. Four model compounds were investigated that enter hepatocytes via entirely different membrane transport mechanisms. Rhodamine B (RB), which enters hepatocytes by passive diffusion, was homogeneously distributed throughout the rat liver slice (250 microm thickness) within 5 min, indicating that the penetration rate into the slice and the diffusion rate into the cells are rapid. In contrast, lucigenin (LU), which is taken up by hepatocytes through adsorptive endocytosis, was detected in the inner cell layers after 15 min. Digoxin uptake into the slice showed a temperature-dependent component and was stereoselectively inhibited by quinine, which is compatible with the involvement of a carrier-mediated uptake mechanism. The neo-glycoalbumin Lactose(27)-Human Serum Albumin (Lact(27)-HSA) and the negatively charged Succinylated-Human Serum Albumin (Suc-HSA) entered the slices and were taken up temperature-dependently into hepatocytes and endothelial cells, respectively. The liver slice preparation is a valuable tool to investigate the mechanisms of cellular uptake of drugs. Moreover, the precision-cut liver slices offer the unique possibility to study both hepatocyte and endothelial cell function in human and rat liver.

Simultaneous determination of bile acids in rat liver tissue by high-performance liquid chromatography

A method for the simultaneous determination of bile acids in rat liver tissue by high-performance liquid chromatography was developed. Without prior fractionation and alkaline hydrolysis, 30 unconjugated, glycine- and taurine-conjugated bile acids were detected by post-column enzymatic reaction and fluorescence detection. They were separated on a reversed-phase column using a linear gradient solvent system of 10 mM tribasic ammonium phosphate-acetonitrile-methanol (44:12:5, v/v/v) and 20 mM dibasic ammonium phosphate-acetonitrile-methanol (2:1:2, v/v/v). The limits of detection were 1-5 pmol, and calibration curves were linear for concentrations ranging between 10 and 4000 pmol per 10 microl injection. This rapid and reliable method is effective for measuring bile acid levels in liver tissue not only of rats but also of patients with hepatobiliary and other diseases.

Alpha-naphthylisothiocyanate-induced elevation of serum bile acids: lack of causative effect on bile acid transport

In recent years chemicals including chlorinated solvents have been found to interfere with the transport of bile acids (BA) by hepatocytes, which probably accounts for the raised serum bile acids (SBA) after exposure. However, the known cholestatic agent, alpha-naphthylisothiocyanate (ANIT) has never been fully examined for its effect on these processes. Accordingly, the direct effects in vitro and the effects of in vivo treatment on bile acid transport have been investigated in this study. Direct addition of ANIT (5-100 microns) to hepatocytes isolated from untreated rats did not result in any change in uptake or efflux of taurocholic acid (TC), one of the most obviously elevated SBA in ANIT-treated rats. Additionally, accumulation of TC over an extended incubation period was not affected by ANIT. In vivo treatment with ANIT (50 mumol/kg i.p. on each of 3 consecutive days) resulted in a marked elevation of total serum bile acids (TSBA) and a slight increase in the activity of serum alkaline phosphatase (ALP) and a very mild hyperbilirubinemia, while other markers of liver injury were unaltered. In hepatocytes isolated from these rats, Km and Vmax for uptake and V0 for efflux were no different between ANIT and vehicle-treated animals. In conclusion, ANIT showed no effects on transport of BA on in vitro exposure or after treatment in vivo where SBA were clearly elevated. The lack of effects of ANIT on transport of bile acids is consistent with other postulated mechanisms of action. Furthermore, this indicates that the effects noted with solvents are not necessarily replicated by substances known to cause histopathological cholestasis.

Bile acids in the assessment of hepatocellular function

Bile acids, which are synthesized in the liver from cholesterol, are important in the production of bile flow, excretion of cholesterol, and intestinal digestion and absorption of fats and fat-soluble vitamins. Increases and/or alterations in concentrations of bile acids in serum are specific and sensitive indicators of hepatobiliary disorders. Synthesis of bile acids in hepatocytes involves steps in endoplasmic reticulum, cytosol, mitochondria, and peroxisomes. Other important hepatocellular processes involving bile acids include active uptake by the basolateral membrane, intracellular transport, P-450-mediated conjugations and hydroxylations, and canalicular secretion. Hydrophobic bile acids produce hepatotoxicity in vivo and in vitro. In experimental and epidemiologic studies, some of these forms have been identified as causative agents in the development of colon and liver (experimental only) cancer. Conversely, several hydrophilic forms, primarily ursodeoxycholic acid, have demonstrated cytoprotective properties in a variety of clinical and experimental hepatobiliary diseases and disorders. Because bile acids can have dramatically different properties and effects, determination of mechanisms of action of these compounds has become an active area of research. Primary isolated hepatocytes provide an opportunity to investigate bile acid-related functions and effects in well-designed, carefully controlled studies. Short-term cultures have been used to study a variety of issues related to bile acids, including cytotoxicity, synthesis, and hepatocellular processing. With these systems, however, many functions of mature hepatocytes, including those pertaining to bile acids, can be lost when cultures are maintained for more than several days. Recent developments in culture techniques permit long-term maintenance of functionally stable, differentiated cells. Pertaining to bile acid research, these systems remain to be fully characterized but, in appropriate situations, they should provide important alternatives to in vivo studies and short-term in vitro assays.

Simultaneous determination of bile acids in rat bile and serum by high-performance liquid chromatography

A method for the simultaneous determination of bile acids in rat bile and serum by high-performance liquid chromatography with a post-column enzymic reaction and fluorescence detection has been developed. Without prior fractionation and alkaline hydrolysis, 26 unconjugated, glycine- and taurine-conjugated bile acids were determined. They were separated on a reversed-phase column using a linear gradient solvent system of 200 mM dibasic ammonium phosphate buffer (pH 7.9)-acetonitrile-methanol (73:19:8, v/v/v) and 20 mM dibasic ammonium phosphate buffer (pH 7.9)-acetonitrile-methanol (2:1:2, v/v/v). The limits of detection were 1-5 pmol, and calibration curves were linear for concentrations between 10 and 4000 pmol. This rapid and reliable method is effective for measuring bile acid levels in the bile and serum not only of rats but also of patients with hepatobiliary and other diseases.