Culture density contributes to hepatic functions of fresh human hepatocytes isolated from chimeric mice with humanized livers: Novel, long-term, functional two-dimensional in vitro tool for developing new drugs

Chimeric mice with humanized livers are considered a useful animal model for predicting human (h-) drug metabolism and toxicity. In this study, the characteristics of fresh h-hepatocytes (cFHHs, PXB-cells^®) isolated from chimeric mice (PXB-mice^®) were evaluated in vitro to confirm their utility for drug development. cFHHs cultured at high density (2.13 × 10⁵ cells/cm²) displayed stable production of h-albumin and cytochrome P450 (CYP) 3A activities for at least 21 days. The mRNA expression levels of 10 of 13 CYP, UDP-glucuronosyltransferase (UGT), and transporters were maintained at >10% of the levels of freshly isolated cFHHs after 21 days. From 1 week, many bile canaliculi were observed between cFHHs, and the accumulation of the multidrug resistance-associated protein and bile salt export pump substrates in these bile canaliculi was clearly inhibited by cyclosporin A. Microarray analysis of cFHHs cultured at high density and at low density (0.53 × 10⁵ cells/cm²) revealed that high density culture maintained high expressions of some transcription factors (HNF4α, PXR, and FXR) perhaps involved in the high CYP, UGT and transporter gene expressions of cFHHs. These results strongly suggest that cFHHs could be a novel in vitro tool for drug development studies.

Aberrant activation of atypical protein kinase C in carbon tetrachloride-induced oxidative stress provokes a disturbance of cell polarity and sealing of bile canalicular lumen

Polarized hepatocytes contain tight junctions (TJs), which are among the most important junctions for sealing the bile canalicular lumen from the sinusoidal space. Alterations in TJs are implicated in chronic cholestatic liver diseases, such as primary biliary cirrhosis and primary sclerosing cholangitis, which have lipid peroxidation marker elevations or antioxidant vitamin decreases. However, the effect of oxidative stress on hepatocyte polarity or liver morphology is unknown. We found that carbon tetrachloride (CCl4)-induced oxidative stress resulted in disassembly of TJs. Ultrastructural analysis revealed disruption in TJs, Golgi morphology, and expansion of the bile canalicular lumen size in CCl4-treated hepatocytes. The Par complex [Par-3-atypical protein kinase C (aPKC) and Par-6 ternary complex] regulates TJs and lumen formation, and the Par-3-aPKC complex formation was inhibited by CCl4 treatment. Moreover, the antioxidant compound vitamin E prohibited a CCl4-induced disturbance in TJs and Par-3-aPKC complex formation. aPKC phosphorylates Par-3 and down-regulates its own affinity with Par-3. Importantly, aPKC kinase activity and Par-3 phosphorylation were significantly increased in CCl4-treated rat livers. These results indicate that the Par-3-aPKC complex plays a crucial role in the maintenance of hepatocyte polarity and sealing of the bile canalicular lumen. Our findings suggest that bile canalicular lumen expansion might explain the presence of cholestasis in patients with primary biliary cirrhosis and primary sclerosing cholangitis.

Defective canalicular transport and toxicity of dietary ursodeoxycholic acid in the abcb11-/- mouse: transport and gene expression studies

The bile salt export pump (BSEP), encoded by the abcb11 gene, is the major canalicular transporter of bile acids from the hepatocyte. BSEP malfunction in humans causes bile acid retention and progressive liver injury, ultimately leading to end-stage liver failure. The natural, hydrophilic, bile acid ursodeoxycholic acid (UDCA) is efficacious in the treatment of cholestatic conditions, such as primary biliary cirrhosis and cholestasis of pregnancy. The beneficial effects of UDCA include promoting bile flow, reducing hepatic inflammation, preventing apoptosis, and maintaining mitochondrial integrity in hepatocytes. However, the role of BSEP in mediating UDCA efficacy is not known. Here, we used abcb11 knockout mice (abcb11-/-) to test the effects of acute and chronic UDCA administration on biliary secretion, bile acid composition, liver histology, and liver gene expression. Acutely infused UDCA, or its taurine conjugate (TUDC), was taken up by the liver but retained, with negligible biliary output, in abcb11-/- mice. Feeding UDCA to abcb11-/- mice led to weight loss, retention of bile acids, elevated liver enzymes, and histological damage to the liver. Semiquantitative RT-PCR showed that genes encoding Mdr1a and Mdr1b (canalicular) as well as Mrp4 (basolateral) transporters were upregulated in abcb11-/- mice. We concluded that infusion of UDCA and TUDC failed to induce bile flow in abcb11-/- mice. UDCA fed to abcb11-/- mice caused liver damage and the appearance of biliary tetra- and penta-hydroxy bile acids. Supplementation with UDCA in the absence of Bsep caused adverse effects in abcb11-/- mice.

Cytochrome P450 expression-induction profile and chemically mediated alterations of the WIF-B9 cell line

BACKGROUND INFORMATION

WIF-B9 is a hybrid cell line obtained by fusion of rat hepatoma cells (Fao) and human fibroblasts (WI38). It exhibits the structural and functional characteristics of differentiated hepatocytes, including active bile canaliculi. The aim of the present study was to characterize the WIF-B9 cell line as a model for analysing drug-induced hepatic effects. The drug metabolism potential of WIF-B9 cells was identified by studying the rat and human CYP (cytochrome P450) mRNA constitutive expression profile and induction potential after exposure to reference inducers. The morphological alterations provoked by chemical entities were also characterized.

RESULTS

Competitive reverse transcriptase-PCR revealed that four rat (1A1, 2B1/2, 2E1 and 4A1) and four human (1A1, 2Cs, 2D6 and 2E1) CYP mRNA isoforms were constitutively expressed in WIF-B9 cells. The rat CYP forms were expressed at levels 2-4 orders of magnitude higher than the human forms. Exposure for 20-72 h to increasing concentrations of CYP reference inducers (beta-naphthoflavone, 3-methyl cholanthrene, dexamethasone, phenobarbital, clofibrate and pregnenolone 16alpha-carbonitrile) revealed that the rat CYP 1A1, 1A2, 3A1, 3A2 and 4A1 and human CYP 1A1 and 2Cs mRNAs were inducible. Rat CYP 1A1 and 1A2 were the most inducible isoforms since they were overexpressed up to 100-fold after 20-48 h of treatment with beta-naphthoflavone. Human CYP 1A1 and 2Cs mRNAs were induced 3-fold after 48 h of treatment with phenobarbital. Other mechanisms involved in hepatotoxicity were explored using microscopy and immunofluorescence. The WIF-B9 cell line exhibited fragmentation and dilatation of bile canaliculi upon exposure to erythromycin, and to isoniazid and cytochalasins, respectively. Monensin promoted cell depolarization and cytoplasmic granulation. Ethionine promoted cytoplasmic vacuolation and dilatation of the Golgi structures.

CONCLUSIONS

These results indicate that the CYP expression and induction profiles and the morphological features of WIF-B9 cells allow prediction in vitro of the induction and hepatotoxicity profiles of chemical entities.

Hepatotoxicity of Combined Treatment with Cisplatin and Gentamicin in the Guinea Pig

The damaging effects on the liver tissue of treatment with cisplatin followed by the aminoglycoside antibiotic gentamicin were studied in guinea pigs. The ultrastructural findings revealed foci of damage in the liver parenchyma, including its vascular component. Injurious effects to cytoplasmatic organelles such as mitochondria and endoplasmic reticulum as well as to nuclei were observed. In addition, abundance of lysosomes, autophagic vacuoles, and amorphous-granular bile in the lumina of bile canaliculi was found. Focal sinusoidal lining damage and capillarization of sinusoids were also present. In vascular lumina, some erythrocytes showed a deformed shape ("ropalocytosis"). Taken together, these findings indicate that the combined treatment with cisplatin followed by gentamicin is toxic to components of liver tissue. Since toxic changes have been shown in vessels of the inner ear and in renal-glomerular capillaries, the present observations of hepatotoxicity indicate the potential vascular damage to various tissues. The injurious effects of the cisplatin-aminoglycoside combination should be considered during its use in clinical conditions.

Protective effects of hepatocellular canalicular conjugate export pump (Mrp2) on sodium arsenite-induced hepatic dysfunction in rats

Arsenic is a double-edged sword to human health. The excretion of various organic anions into bile is mediated by an adenosine triphosphate-dependent conjugate export pump, which has been identified as the canalicular isoform of the multidrug resistance protein 2 (Mrp2). It has been proved that Mrp2 can transport arsenite in vitro, but its effects in vivo are not clear. The aim of this study was to investigate whether Mrp2 plays a role in exportation of arsenic in vivo and its protective effects on liver function. Mrp2 protein level in rat liver was determined by Western blot analysis. Total arsenic concentrations in whole blood and bile were measured using hydride generation atomic absorption spectrometry. Alanine aminotransferase (ALT) activity, aspartate aminotransferase activity (AST), glutathione peroxidase (GSH-PX) activity, malon dialdehyde (MDA) and total bilirubin were measured by biochemical assays. The morphological changes were observed by electron microscopy. Total arsenic levels in blood and bile of arsenite-treated rats were significantly higher than those of control rats (P<0.05) at all three different time points. The overexpression of Mrp2 was 36.61%, 32.36% and 12.73% at 2, 4 and 6 weeks, respectively (percentage of controls, P<0.05), which was significantly higher than controls. A positive correlation between Mrp2 expression level and total arsenic concentration in bile indicated that Mrp2 accelerated the transport of arsenic. Electron microscopy showed that microvilli of bile canaliculi became swollen and sparse. ALT and AST activities in serum were markedly raised at 6 weeks. MDA level in serum increased (P<0.05) and GSH-PX activity in serum decreased except for 2 weeks. Damage of liver function became worse following decreased expression of Mrp2. In conclusion, overexpression of Mrp2 may explain increased biliary excretion of arsenic and it may protect liver function.

Vasopressin-induced morphological changes in polarized rat hepatocyte multiplets: dual calcium-dependent effects

Calcium-mobilizing hormones and neurotransmitters are known to affect cell morphology and function including cell differentiation or division. In this study, we examined vasopressin (AVP)-induced morphological changes in a polarized system of rat hepatocytes. Light and electron microscope observations showed that AVP induced microvilli formation and a remodeling of the isolated hepatocyte F-actin submembrane cytoskeleton, these two events being correlated. We showed that these effects were rapid, reversible, observed at nanomolar AVP concentration and mediated by the V(1a) receptor. On polarized multicellular systems of hepatocytes, we observed a rapid reduction of the bile canaliculi lumen at the apical pole and micovilli formation at the basolateral domain with an enlarged F-actin cytoskeleton. Neither activation of protein kinase C nor A via phorbol ester or dibutyryl cAMP induced such rapid morphological changes, at variance with ionomycin, suggesting that AVP-induced intracellular calcium rise plays a crucial role in those effects. By using spectrofluorimetry and cytochemistry, we showed that calcium release from intracellular stores was involved in bile canaliculus contraction, while calcium entry from the extracellular space controlled microvilli formation. Taken together, AVP and calcium-mobilizing agonists differentially regulate physiological hepatocyte plasma membrane events at the basal and the apical domains via topographically specialized calcium-dependent mechanisms.

4-phenylbutyrate enhances the cell surface expression and the transport capacity of wild-type and mutated bile salt export pumps

Progressive familial intrahepatic cholestasis type 2 (PFIC2) is caused by a mutation in the bile salt export pump (BSEP/ABCB11) gene. We previously reported that E297G and D482G BSEP, which are frequently found mutations in European patients, result in impaired membrane trafficking, whereas both mutants retain their transport function. The dysfunctional localization is probably attributable to the retention of BSEP in endoplasmic reticulum (ER) followed by proteasomal degradation. Because sodium 4-phenylbutyrate (4PBA) has been shown to restore the reduced cell surface expression of mutated plasma membrane proteins, in the current study, we investigated the effect of 4PBA treatment on E297G and D482G BSEP. Transcellular transport and cell surface biotinylation studies using Madin-Darby canine kidney (MDCK) II cells demonstrated that 4PBA treatment increased functional cell surface expression of wild-type (WT), E297G, and D482G BSEP. The prolonged half-life of cell surface-resident BSEP with 4PBA treatment was responsible for this result. Moreover, treatment of Sprague-Dawley rats with 4PBA resulted in an increase in BSEP expression at the canalicular membrane, which was accompanied by an increase in the biliary excretion of [(3)H]taurocholic acid (TC).

CONCLUSION

4PBA treatment with a clinically achievable concentration enhances the cell surface expression and the transport capacity of WT, E297G, and D482G BSEP in MDCK II cells, and also induces functional BSEP expression at the canalicular membrane and bile acid transport via canalicular membrane in vivo. 4PBA is a potential pharmacological agent for treating not only PFIC2 patients with E297G and D482G mutations but also other cholestatic patients, in whom the BSEP expression at the canalicular membrane is reduced.

Anchoring of protein kinase A-regulatory subunit IIalpha to subapically positioned centrosomes mediates apical bile canalicular lumen development in response to oncostatin M but not cAMP

Oncostatin M and cAMP signaling stimulate apical surface-directed membrane trafficking and apical lumen development in hepatocytes, both in a protein kinase A (PKA)-dependent manner. Here, we show that oncostatin M, but not cAMP, promotes the A-kinase anchoring protein (AKAP)-dependent anchoring of the PKA regulatory subunit (R)IIalpha to subapical centrosomes and that this requires extracellular signal-regulated kinase 2 activation. Stable expression of the RII-displacing peptide AKAP-IS, but not a scrambled peptide, inhibits the association of RIIalpha with centrosomal AKAPs and results in the repositioning of the centrosome from a subapical to a perinuclear location. Concomitantly, common endosomes, but not apical recycling endosomes, are repositioned from a subapical to a perinuclear location, without significant effects on constitutive or oncostatin M-stimulated basolateral-to-apical transcytosis. Importantly, however, the expression of the AKAP-IS peptide completely blocks oncostatin M-, but not cAMP-stimulated apical lumen development. Together, the data suggest that centrosomal anchoring of RIIalpha and the interrelated subapical positioning of these centrosomes is required for oncostatin M-, but not cAMP-mediated, bile canalicular lumen development in a manner that is uncoupled from oncostatin M-stimulated apical lumen-directed membrane trafficking. The results also imply that multiple PKA-mediated signaling pathways control apical lumen development and that subapical centrosome positioning is important in some of these pathways.

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.

Cholestasis in pregnancy associated with ciclosporin therapy in renal transplant recipients

Obstetric cholestasis (OC) presents with pruritus in the second half of pregnancy and is associated with increased risk of foetal distress, intra-uterine death and premature delivery. From a tertiary referral, renal-obstetric clinic, we report the occurrence of OC in 5/23 pregnancies of women with renal transplants maintained on ciclosporin treatment (European incidence 0.1-1.5% of pregnancies). All required premature delivery for foetal reasons at 33-37/40 (median 34/40). Ciclosporin, at therapeutic concentrations, inhibits bile salt excretion pump (BSEP) function in rats and humans. We propose that OC developed in our patients because the mild inhibition of the canalicular pumps by ciclosporin was only revealed in pregnancy when increases in progesterone metabolites overwhelmed pump function. We suggest that all pregnant women receiving ciclosporin should be closely monitored from the second trimester for the development of OC. If detected, enhanced foetal and maternal monitoring to optimize time of delivery and pregnancy outcome is required.

Light and electron microscopic studies on liver histology in chicks fed aflatoxin

This study was carried out under both light and electron microscopy to investigate the effects on liver carbohydrate and lipid metabolism caused by aflatoxin (AF) fed to chicks. Twenty broiler chicks were used. The birds were housed in electrically heated battery cages and exposed to light for 24 h. Feed and water were provided ad libitum. The animals were allocated to two groups each made up of 10 broilers. Total aflatoxin levels of zero (0) and 5 mg of AF/kg feed (81.05% AFB1, 8.79% AFG1, 6.06% AFB2, and 4.10% AFG2) added to a commercial diet, were fed to chicks from hatching up to 3 weeks of age, when the experiment was terminated. The chicks were executed by cervical dislocation and liver samples were obtained for light and electron microscopy. Oil red 'O', Sudan Black B, periodic acid Schiff (PAS) and Best's carmine stains were used to reveal fat and glycogen in the liver. Histological changes in hepatocytes included increased lipid droplets, high glycogen content, and mild mononuclear cell infiltration in the portal areas. Ultrastructural findings were destruction of rough endoplasmic reticulum (rER), reduction in mitochondrial size, enlargement of bile canaliculi, and cisternal dilatation of the smooth endoplasmic reticulum (sER).

Efficient trafficking of MDR1/P-glycoprotein to apical canalicular plasma membranes in HepG2 cells requires PKA-RIIalpha anchoring and glucosylceramide

In hepatocytes, cAMP/PKA activity stimulates the exocytic insertion of apical proteins and lipids and the biogenesis of bile canalicular plasma membranes. Here, we show that the displacement of PKA-RIIalpha from the Golgi apparatus severely delays the trafficking of the bile canalicular protein MDR1 (P-glycoprotein), but not that of MRP2 (cMOAT), DPP IV and 5'NT, to newly formed apical surfaces. In addition, the direct trafficking of de novo synthesized glycosphingolipid analogues from the Golgi apparatus to the apical surface is inhibited. Instead, newly synthesized glucosylceramide analogues are rerouted to the basolateral surface via a vesicular pathway, from where they are subsequently endocytosed and delivered to the apical surface via transcytosis. Treatment of HepG2 cells with the glucosylceramide synthase inhibitor PDMP delays the appearance of MDR1, but not MRP2, DPP IV, and 5'NT at newly formed apical surfaces, implicating glucosylceramide synthesis as an important parameter for the efficient Golgi-to-apical surface transport of MDR1. Neither PKA-RIIalpha displacement nor PDMP inhibited (cAMP-stimulated) apical plasma membrane biogenesis per se, suggesting that other cAMP effectors may play a role in canalicular development. Taken together, our data implicate the involvement of PKA-RIIalpha anchoring in the efficient direct apical targeting of distinct proteins and glycosphingolipids to newly formed apical plasma membrane domains and suggest that rerouting of Golgi-derived glycosphingolipids may underlie the delayed Golgi-to-apical surface transport of MDR1.

Inhibition of Hepatobiliary Transport as a Predictive Method for Clinical Hepatotoxicity of Nefazodone

Treatment with the antidepressant nefazodone has been associated with clinical idiosyncratic hepatotoxicty. Using membranes expressing human bile salt export pump (BSEP), human sandwich hepatocytes, and intact rats, we compared nefazodone and its marketed analogs, buspirone and trazodone. We found that nefazodone caused a strong inhibition of BSEP (IC(50) = 9 microM), inhibition of taurocholate efflux in human hepatocytes (IC(50) = 14 microM), and a transient increase in rat serum bile acids 1 h after oral drug administration. Buspirone or trazodone had no effect on biliary transport system. Nefazodone produced time- and concentration-dependent toxicity in human hepatocytes with IC(50) = 18 microM and 30 microM measured by inhibition of protein synthesis after 6 h and 24 h incubation, respectively. Toxicity was correlated with the amount of unmetabolized nefazodone. Partial recovery in toxicity by 24 h has been associated with metabolism of nefazodone to sulfate and glucuronide conjugates. The saturation of nefazodone metabolism resulted in sustained decrease in protein synthesis and cell death at 50 microM. The toxicity was not observed with buspirone or trazodone. Addition of 1-aminobenzotriazole (ABT), an inhibitor of CYP450, resulted in enhancement of nefazodone toxicity at 10 microM and was associated with accumulation of unmetabolized nefazodone. In human liver microsomes, ABT also prevented metabolism of nefazodone and formation of glutathione conjugates. We suggest that inhibition of bile acid transport by nefazodone is an indicator of potential hepatotoxicity. Our findings are consistent with the clinical experience and suggest that described methodology can be applied in the selection of nonhepatotoxic drug candidates.

Contribution of canalicular glutathione efflux to bile formation. From cholestasis associated alterations to pharmacological intervention to modify bile flow

At least one third of the bile flow is driven osmotically by the amount of hepatic glutathione excreted into canalicular spaces. Beyond the importance of this secretory mechanism for bile formation, the excretion of glutathione is an important way to discharge toxic anionic compounds deriving from liver metabolism of exogenous and endogenous substances. Thus, biliary secretion of glutathione and its conjugates really works as a major detoxification system for the hepatocytes. Derangement of hepatic and/or biliary glutathione status can occur in several experimental animal models of liver injury and in human diseases. In the present review, we will focus on mechanisms of bile glutathione efflux and changes associated with cholestatic conditions. Novel findings on the role of water channels and of the multidrug resistant proteins in bile salt-independent bile formation, will also be discussed. New routes of intervention to modify bile flow for therapeutic purposes are considered.

MODULATION OF HEPATIC CANALICULAR OR BASOLATERAL TRANSPORT PROTEINS ALTERS HEPATOBILIARY DISPOSITION OF A MODEL ORGANIC ANION IN THE ISOLATED PERFUSED RAT LIVER

This study examined the impact of hepatic transport protein modulation on the hepatobiliary disposition of a nonmetabolized probe substrate, 5- (and 6)-carboxy-2',7'dichlorofluorescein (CDF) in rat isolated perfused livers (IPLs). In vivo treatment with modulators (100 and 200 mg/kg/day clofibric acid, 80 mg/kg/day phenobarbital, and 25 mg/kg/day dexamethasone) was used to alter the expression of hepatic transport proteins [organic anion transporting polypeptide 1a1, multidrug resistance-associated protein (Mrp) 3, and Mrp2] governing the disposition of CDF. The basolateral and biliary excretion of CDF was measured in single-pass IPLs from control and treated rats. Modulators increased the percentage of CDF eliminated into perfusate of IPLs from treated rats ( approximately 20-35%) compared with controls ( approximately 10%); CDF biliary excretion was decreased in the treated groups. These observations are consistent with modulator-associated increases in the first-order rate constant governing CDF excretion from the hepatocytes into perfusate (k(perfusate)) or decreases in the first-order rate constant governing CDF excretion into bile (k(bile)). Pharmacokinetic modeling of the data and subsequent simulations revealed that the routes of CDF excretion were most sensitive to changes in k(perfusate). In contrast, hepatic accumulation of CDF was most sensitive to k(bile). The differential sensitivity of CDF excretory routes and hepatic accumulation to these rate constants is a function of intrahepatic distribution kinetics, which must be taken into consideration in assessing the potential impact of altered hepatobiliary transport processes.

Effect of a new hepatoprotective agent, YH-439, on the hepatobiliary transport of organic cations (OCS): Selective inhibition of sinusoidal OCs uptake without influencing glucose uptake and canalicular OCs excretion

The effect of a new hepatoprotective agent, YH-439, on the hepatobiliary transport of a model organic cation (OC), TBuMA (tributylmethylammonium), was investigated. The area under the plasma concentration-time curve (AUC) from time zero to 4 h following iv administration of TBuMA (6.6 micromol/kg) was increased significantly when YH-439 in corn oil (300 mg/kg) was orally administered to rats 24 h prior to the experiment. Nevertheless, the cumulative biliary excretion of TBuMA remained unchanged. As a consequence, the apparent biliary clearance (CLb) of TBuMA was decreased significantly as a result of YH-439 pretreatment, consistent with the fact that the in vivo excretion clearance of TBuMA across the canalicular membrane (CLexc) was not changed by the pretreatment. The in vitro uptake of TBuMA into isolated hepatocytes was decreased by one half by the pretreatment, owing to a decrease in the apparent Vmax and CLlinear, but the Km for the process remained constant. Most interestingly, however, the sinusoidal uptake of glucose, a nutrient, into hepatocytes was not influenced by the pretreatment, suggesting the YH-439 pretreatment specifically impaired the sinusoidal uptake of OCs. Thus, the OC-specific inhibition of hepatic uptake, without influencing the uptake of glucose, a nutrient, appeared to be associated with the hepatoprotective activity of YH-439.

Mrp2/Abcc2 transport activity is stimulated by protein kinase Calpha in a baculo virus co-expression system

Cholestatic and choleretic effect are well known for protein kinase C activator and inhibitor, respectively. However, post-translational regulation, especially the effect of phosphorylation status of the biliary transporters on their intrinsic transport activity has not been fully understood. In this study, effect of phosphorylation on the transport activity of Mrp2, a biliary organic anion transporter, was examined in membrane vesicles isolated from Sf9 cells co-expressing excess amount of protein kinase Calpha (PKCalpha). Mrp2-mediated transport activity was enhanced to three-fold by co-expressing PKCalpha. At the same time, phosphorylation of Mrp2 was also detected. The Km and Vmax values for the transport of [3H]estradiol-17beta-D-glucuronide exhibited a 1.5-fold decrease and a 1.9-fold increase, respectively. Probenecid (100 microM) and benzylpenicillin (1 mM), both are activator of Mrp2, did not stimulated the transport activity of phosphorylated Mrp2. On the other hand, transport activity was further stimulated by Estron-3-sulfate and taurocholic acid. Similar mechanism that occurred in the presence of probenecid and benzylpenicillin, but different from that occurred in the presence of Estron-3-sulfate and taurocholic acid seems to be involved in the stimulation. Considering the discrepancy between the previous in vivo inhibitory effect of PKC activators and our in vitro stimulatory effect of PKCalpha on Mrp2 transport activity, direct modulation of Mrp2-transport activity may be minor if any under in vivo condition.

Xenobiotics Inhibit Hepatic Uptake and Biliary Excretion of Taurocholate in Rat Hepatocytes

Reports suggest that troglitazone, and to a lesser extent bosentan, may alter bile acid homeostasis by inhibiting the bile salt export pump. The present studies examined the hypothesis that these xenobiotics may modulate multiple hepatic bile acid transport mechanisms. In suspended rat hepatocytes, troglitazone (10 microM) decreased the initial rate of taurocholate uptake approximately 3-fold; the initial uptake rate of estradiol-17beta-D-glucuronide, a substrate of the organic anion transporting polypeptides, also was decreased approximately 4-fold. Bosentan (100 microM) decreased the initial uptake rate of taurocholate and estradiol-17beta-D-glucuronide by approximately 12- and approximately 7-fold, respectively. In sandwich-cultured rat hepatocytes, 10-min accumulation of taurocholate in cells + bile canaliculi (408 +/- 57 pmol/mg protein) was decreased significantly by troglitazone (157 +/- 17 pmol/mg protein, respectively) only in the presence of Na+, the driving force for the sodium taurocholate cotransporting polypeptide. A similar decrease with 10-fold higher concentrations of bosentan was noted. The biliary excretion index of taurocholate (55 +/- 8%) was decreased in the presence of 10 microM troglitazone (27 +/- 2%) and 100 microM bosentan (10 +/- 6%). In conclusion, xenobiotics may alter hepatic bile acid transport by inhibiting both hepatic uptake and biliary excretion.

Biliary excretion of bile acids and organic anions in rats with dichloroethylene-induced bile canalicular injury

BACKGROUND

Hepatocytes in zone 1 of the hepatic lobule play a role in the uptake and biliary excretion of bile acids and organic anions under physiological conditions, and hepatocytes in zone 3 may play a role only when there is a high-dose load. To further elucidate the role of hepatic zonation in the hepatic handling of bile acids and organic anions, the biliary excretion of these compounds was studied in rats with dichloroethylene (DCE)-induced selective zone 3 bile canalicular injury.

METHODS

Biliary excretion of various bile acids and organic anions was studied in rats 1 h after oral administration of DCE (5 mg/100 g). The effect of DCE on the immunostaining of multidrug resistance protein 2 (Mrp2; an important canalicular organic anion transporter) in the liver was also examined.

RESULTS

The biliary excretory maximum of taurocholate and tauroursodeoxycholate was decreased in DCE-treated rats, whereas the biliary excretion of taurolithocholate-sulfate and phenolphthalein-glucuronide was unchanged in DCE-treated rats, and DCE treatment decreased the biliary excretion of sulfobromophthalein and pravastatin. DCE decreased Mrp2 staining in the canalicular membrane of zone 3 hepatocytes on immunohistochemistry.

CONCLUSIONS

These findings indicate that canalicular transport in zone 3 hepatocytes is important in the biliary excretion of bile acids and organic anions, when they are administered at high doses.

Biliary Secretory Function in Rats Chronically Intoxicated with Aluminum

The effects of a chronic aluminum (Al) exposure on biliary secretory function, with special emphasis on hepatic handling of non-bile salt organic anions, was investigated. Male Wistar rats received, intraperitoneally, either 27 mg/kg body weight of Al, as Al hydroxide [Al (+) rats], or the vehicle saline [Al (-) rats] three times a week for 3 months. Serum and hepatic Al levels were increased by the treatment (approximately 9- and 4-fold, respectively). This was associated with enhanced malondialdehyde formation (+110%) and a reduction in GSH content (-17%) and in the activity of the antioxidant enzymes catalase (-84%) and GSH peroxidase (-46%). Bile flow (-23%) and the biliary output of bile salts (-39%), cholesterol (-43%), and proteins (-38%) also decreased. Compartmental analysis of the plasma decay of the model organic anion bromosulphophthalein revealed that sinusoidal uptake and canalicular excretion of the dye were significantly decreased in Al (+) rats (-53 and -43%, respectively). Expression of multidrug resistance-associated protein 2 (Mrp2), the main, multispecific transporter involved in the canalicular excretion of organic anions, was also decreased (-40%), which was associated with a significant decrease in the cumulative biliary excretion of the Mrp2 substrate, dinitrophenyl-S-glutathione (-50%). These results show that chronic Al exposure leads to oxidative stress, cholestasis, and impairment of the hepatic handling of organic anions by decreasing both sinusoidal uptake and canalicular excretion. The alteration of the latter process seems to be causally related to impairment of Mrp2 expression. We have addressed some possible mechanisms involved in these deleterious effects.

Pseudoductules in the rat liver in experimental adriamycin-induced nephrotic syndrome

Pseudoductules proliferation in the liver is the condition observed in some liver diseases in human and experimental animals. It is observed in viral and alcoholic hepatitis, in hepatic cirrhosis and toxical liver damage. In that study pseudoductules were observed after i.p. single dose of adriamycin (5 mg/kg of body weight) administered to rats. Adriamycin given in this way is used to induce experimental nephrotic syndrome.

Synergistic role of 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7alpha-hydroxylase in the pathogenesis of manganese-bilirubin-induced cholestasis in rats

Manganese (Mn) and bilirubin (BR) induce intrahepatic cholestasis when injected sequentially. It was suggested that accumulation of newly synthesized cholesterol in the canalicular membrane is an initial step in the development of cholestasis. To clarify the involvement of cholesterol in the pathogenesis of Mn-BR-induced cholestasis, we examined biliary secretion and liver subcellular distribution of lipids in the cholestatic conditions (Mn-BR combination). We also examined hepatic metabolism of cholesterol under cholestatic and noncholestatic (Mn or BR given alone) conditions. The Mn-BR combination reduced bile flow by 50%, and bile acid, phospholipids, and cholesterol output by 42, 75, and 90%, respectively. There was a dramatic impairment of cholate excretion but not chenodeoxycholate excretion. Phosphatidylcholine species secreted into bile were unchanged, and microsomal total phospholipid content was significantly increased. Although there was no changes in liver membrane phospholipid content, the cholesterol/phospholipid ratio was increased by more than 50% in the canalicular fraction. Despite the increased concentration of cholesterol in canalicular membrane the activities of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, key enzyme in cholesterol synthesis, and cholesterol 7alpha-hydroxylase, key enzyme in cholesterol conversion to bile acids, were significantly reduced. However, the injection of Mn alone significantly increased both enzymes, while BR alone inhibited cholesterol 7alpha-hydroxylase by 62%, without affecting HMG-CoA reductase. In conclusion, the critical cholestatic events in Mn-Br-induced cholestasis appear to be mediated through the synergistic effects of Mn and BR acting on synthesis and degradation of cholesterol.

Discovery of the hepatic canalicular and intestinal cholesterol transporters. New targets for treatment of hypercholesterolemia

Arteriosclerosis and cholesterol cholelithiasis are characterized by abnormal regulation of cholesterol trafficking and solubilization, and subsequent development of the arteriosclerotic plaque in the artery walls and gallstone formation in the gallbladder, respectively. Cholesterol metabolism is controlled by many complex polygenetic - environmental interactions that contribute to the regulation of serum lipoprotein cholesterol levels and biliary cholesterol and bile acids secretion, which constitute the only pathway for sterol elimination from the organism. Much of our understanding of cholesterol metabolism has arisen from studies of the pathways controlling cholesterol synthesis and the uptake and degradation of LDL and HDL lipoproteins. Recently, two new members of the ABC transporter family (ABCG5 and ABCG8 heterodimers) have been discovered in the apical pole of the enterocyte and in the canalicular membrane of hepatocytes. Experiments in genetically engineered mice have demonstrated that ABCG5/G8 represent the physiological canalicular transporter of biliary cholesterol and the intestinal secretory mechanism of absorbed dietary plant sterols. Interestingly, mutation of ABCG5 and or ABCG8 genes in man causes sitosterolemia, a rare genetic disease characterized by massive absorption of plant sterols and premature arteriosclerosis. The potential pharmacological manipulation of biliary cholesterol secretion represents another important therapeutic target to treat hypercholesterolemia, if this manipulation is simultaneously accompanied by measures aimed to avoid gallbladder cholesterol crystallization. The best theoretical drug should decrease serum lipoprotein cholesterol levels, increase biliary cholesterol secretion and fecal elimination and favoring at the same time gallbladder emptying to prevent gallstone formation.

Cyclosporin A induced internalization of the bile salt export pump in isolated rat hepatocyte couplets

Isolated rat hepatocyte couplets were used to perform the comparative study of two widely used immunosuppressors, cyclosporin A (CsA) and tacrolimus (FK506) on hepatocanalicular function. We assessed canalicular function by counting the percentage of couplets that were able to accumulate the fluorescent cholephile, cholyl-lysyl-fluorescein (CLF), into the canalicular vacuole between the two cells, i.e., canalicular vacuole accumulation (CVA) of CLF. Compared to controls (DMSO-treated cells), CsA, in the approximate range of concentrations used therapeutically, caused inhibition of CVA of CLF, disorganization of the bile salt export pump (Bsep) localization at canalicular level resulting in its relocation into the cell, and disruption of the pericanalicular F-actin cytoskeleton. In contrast, FK506, at both approximately therapeutic and supratherapeutic concentrations, had no deleterious effect upon CVA of CLF, upon the localization of the bile salt transporter at the canalicular membrane, or on the organization of the pericanalicular F-actin cytoskeleton. These results point to transporter and cytoskeletal disorganization as contributors or determinants of CsA-induced cholestasis at canalicular level, whereas FK506 does not appear to produce these cholestasis-determining responses even at supratherapeutic concentrations.

Prevention of taurolithocholate-induced hepatic bile canalicular distortions by HPLC-characterized extracts of artichoke (Cynara scolymus) leaves

The effects of water-soluble extracts of artichoke (Cynara scolymus L.) leaves on taurolithocholate-induced cholestatic bile canalicular membrane distortions were studied in primary cultured rat hepatocytes using electron microscopy. Artichoke extracts at concentrations between 0.08 and 0.5 mg/ml were able to prevent the formation of bizarre canalicular membrane transformations in a dose-dependent manner when added simultaneously with the bile acid. However, prevention also occurred when the hepatocytes were preincubated with the extracts, indicating that absorption of the bile acid to components of the extracts was not involved. These results demonstrate that artichoke leaf extracts exert a potent anticholestatic action at least in the case of taurolithocholate. This effect may contribute to the overall hepatoprotective influence of this herbal formulation.

Contribution of mrp2 in alterations of canalicular bile formation by the endothelin antagonist bosentan

BACKGROUND/AIMS

Bosentan, a dual endothelin ET(A/B) receptor antagonist, may cause dose-dependent reversible cholestatic liver injury. We herein tested whether bosentan or metabolites, both eliminated in bile, induce alterations in bile secretion.

METHODS

Bile flow and output of bile constituents were monitored in pentobarbital-anesthetized rats with biliary fistulas. Normal and TR(-) rats with a genetic defect in mrp2, received bosentan intravenous injections.

RESULTS

Bosentan bolus intravenous injections of 0.1-10mg/kg triggered a dose-dependent increase in biliary bilirubin excretion. In addition, doses (> or =10mg/kg) caused a sustained increase in canalicular bile salt-independent bile flow, combined with significant increases in the concentration and output of glutathione and of bicarbonate in bile. In rats receiving bosentan (> or =10mg/kg), both under basal conditions and under intravenous taurocholate perfusion (2micromol/min/kg), phospholipid and cholesterol secretions were profoundly inhibited and uncoupled from bile salt secretion. In TR(-) rats, the choleretic effect of bosentan was reduced to non-significant levels. The stimulation of bilirubin secretion and the uncoupling of phospholipid from bile salt secretion were absent, whereas that of cholesterol was maintained.

CONCLUSIONS

Bosentan alters canalicular bile formation in major part via mrp2-mediated mechanisms. Intermittent uncoupling of lipid from bile salt secretion may contribute to bosentan hepatic adverse reaction.

In vivo perturbation of rat hepatocyte canalicular membrane function by diclofenac

Clinical use of diclofenac is associated with a small but significant incidence of hepatotoxicity. It has been reported that in vivo diclofenac treatment results in decreased activity of the extracellular canalicular membrane protein dipeptidylpeptidase IV in rats as a consequence of protein adduct formation by its electrophilic metabolite diclofenac acyl glucuronide. The present study has investigated the effects of in vivo diclofenac treatment (15 mg/kg/day for 7 days) on the activity of an another four rat extracellular canalicular membrane proteins. Animals administered diclofenac (n = 6) had 47.9, 60.4, and 51.6% lower (p < 0.05) canalicular activities of gamma-glutamyltransferase, Mg(2+)-ATPase, and leucine aminopeptidase, respectively, compared with controls (n = 6), but there was no difference in alkaline phosphatase activity. In general, protein adduct formation by acyl glucuronides has been associated with decreased protein function, and the lower canalicular enzyme activities in diclofenac-treated rats may suggest that gamma-glutamyltransferase, Mg(2+)-ATPase, and leucine aminopeptidase are also targets of adduct formation by acyl glucuronide metabolites of diclofenac. However, intracellular redistribution and/or decreased synthesis of these enzymes would also be consistent with our results. The ability of diclofenac acyl glucuronide (200 microg/ml) to form covalently bound adducts with gamma-glutamyltransferase (10 mg/ml) was demonstrated following in vitro incubations (16 h, pH 7.4, and 37 degrees C) in which 20.7 +/- 2.1 ng of diclofenac were covalently bound per milligram of protein. In these in vitro studies, the low concentration of protein adducts formed was not associated with any significant change in gamma-glutamyltransferase activity.

Modifying hepatic phospholipid synthesis associates with biliary phospholipid secretion rate in a transporter-independent manner in rats: relation to canalicular membrane fluidity

Biliary phospholipid secretion is mediated by a multidrug resistance gene product, and its molecular subselection occurs at the site of secretion to modulates bile metastability. The aim of this study was to determine the effect of modifying hepatic phospholipid synthesis on canalicular phospholipid transporter expression and membrane fluidity. Bile-duct cannulation was performed in male Sprague-Dawley rats pretreated with or without intravenous infusion of dimethylethanolamine, an intermediate phospholipid metabolite along the pathway of phosphatidylcholine synthesis of phosphatidylethanolamine N-methylation (0.01 mg/min/100 g body wt) for 15 hr, followed by sodium taurocholate infusion (50 nmol/min/100 g body wt) with or without sulfobromophthalein (50 nmol/min/100 g body wt). Dimethylethanolamine enhanced biliary phospholipid secretion in association with a decrease in biliary phospholipid hydrophobicity. Dimethylethanolamine also increased canalicular membrane fluidity defined by 1,6-diphenyl-1,3,5-hexatriene fluorescence depolarization, whereas the expression of multidrug resistance gene product and multidrug resistance associated protein was unchanged. In contrast, a disproportionate reduction of biliary phospholipid secretion caused by sulfobromophthalein (uncoupling) was enhanced by under the treatment with dimethylethanolamine. In conclusion, the increase in biliary phospholipid secretion and canalicular membrane fluidity without a drastic change of its canalicular transporter by dimethylethanolamine suggests that such a canalicular membrane fluidity facilitates the transporter activity and/or phospholipid molecular movement from the canalicular outer membrane into the bile. A more drastic reduction in phospholipid secretion under sulfobromophthalein-caused uncoupling indicates the possibility of a preferential distribution of relatively hydrophilic phosphatidylcholine molecules to bile salt micelles since sulfobromophthalein is known to reduce the micellar capacity to extract membrane lipids for biliary secretion.

The endothelin antagonist bosentan inhibits the canalicular bile salt export pump: a potential mechanism for hepatic adverse reactions

BACKGROUND

During clinical trials bosentan, the first orally active endothelin receptor antagonist, caused asymptomatic transaminase elevations in some patients. In this study we investigated whether inhibition of the hepatocanalicular bile salt export pump (rodents, Bsep; humans, BSEP ABCB11) could account for bosentan-induced liver injury.

METHODS

We reanalyzed the safety database of the bosentan trials for cholestatic liver injury, determined the cholestatic potency of bosentan in the rat, and studied the effects of bosentan and its metabolites on Bsep-mediated taurocholate transport in vitro.

RESULTS

Bosentan caused dose-dependent and reversible liver injury in 2% to 18% of patients and caused a significant increase of serum bile salt levels (P <.01). Concomitant administration of glyburide (INN, glibenclamide) enhanced the cholestatic potency of bosentan. Similar effects were seen in rats, in which serum bile salt levels were increased by glyburide less than by bosentan, which increased the levels less than a combination of bosentan and glyburide. In vitro, Bsep-mediated taurocholate transport was inhibited by bosentan (inhibition constant, approximately 12 micromol/L) and metabolites (inhibition constant, approximately 8.5 micromol/L for metabolite Ro 47-8634).

CONCLUSIONS

These results indicate that bosentan-induced liver injury is mediated, at least in part, by inhibition of Bsep/BSEP-causing intracellular accumulation of cytotoxic bile salts and bile salt induced liver cell damage. The data further emphasize the pathophysiologic importance of drug-Bsep interactions in acquired forms of cholestatic liver injury.

Cyclosporin A reduces canalicular membrane fluidity and regulates transporter function in rats

Changes of the biliary canalicular membrane lipid content can affect membrane fluidity and biliary lipid secretion in rats. The immunosuppressant cyclosporin A is known to cause intrahepatic cholestasis. This study investigated whether cyclosporin A influenced canalicular membrane fluidity by altering membrane phospholipids or transporter expression. In male Sprague-Dawley rats, a bile-duct cannula was inserted to collect bile, and sodium taurocholate was infused (100 nmol/min per 100 g) for 60 min. During steady-state taurocholate infusion, cyclosporin A (20 mg/kg) or vehicle was injected intravenously and then bile was collected for 80 min. After killing the rats, canalicular membrane vesicles were prepared. Expression of canalicular membrane transporters was assessed by Western blotting and canalicular membrane vesicle fluidity was estimated by fluorescence polarization. Cyclosporin A reduced biliary lipid secretion along with a disproportionate reduction of lipids relative to bile acids. Cyclosporin A significantly decreased canalicular membrane fluidity along with an increase of the cholesterol/phospholipid molar ratio. Only expression of the transporter P-glycoprotein was increased by cyclosporin A. Because canalicular membrane transporter expression was largely unchanged by cyclosporin A despite a marked decrease of biliary lipid secretion, transporter activity may partly depend upon canalicular membrane fluidity.

Cholestatic potential of troglitazone as a possible factor contributing to troglitazone-induced hepatotoxicity: in vivo and in vitro interaction at the canalicular bile salt export pump (Bsep) in the rat

Troglitazone is a thiazolidinedione insulin sensitizer drug for the treatment of type 2 non-insulin-dependent diabetes mellitus (NIDDM). Based on an increasing number of reports on troglitazone-associated liver toxicity, the cholestatic potential of troglitazone has been investigated. Rapid and dose-dependent increases in the plasma bile acid concentrations were observed in rats after a single intravenous administration of troglitazone. A radiolabeled taurocholic acid tracer accumulated in liver tissue, indicating an interference with the hepatobiliary export of bile acids. In isolated canalicular rat liver plasma membrane preparations, troglitazone competitively inhibited the ATP-dependent taurocholate transport (apparent K(i) value, 1.3 microM), mediated by the canalicular bile salt export pump (Bsep). Troglitazone sulfate, the main troglitazone metabolite eliminated into bile, also showed competitive Bsep inhibition with an apparent K(i) value of 0.23 microM. A comparable inhibition was observed for both compounds in canalicular plasma membrane vesicles prepared from Mrp2-deficient (TR(-)) rats, suggesting a direct (cis-) inhibition of Bsep by troglitazone and troglitazone sulfate. A high accumulation potential was observed for troglitazone sulfate in rat liver tissue, indicating that the hepatobiliary export of this conjugated metabolite might represent a rate-limiting step in the overall elimination process of troglitazone. This accumulation in combination with the high Bsep inhibition potential suggested that mainly troglitazone sulfate was responsible for the interaction with the hepatobiliary export of bile acids at the level of the canalicular Bsep in rats. Such an interaction might lead to a troglitazone-induced intrahepatic cholestasis in humans as well, contributing to the formation of a troglitazone-induced liver toxicity.

Manganese-bilirubin effect on cholesterol accumulation in rat bile canalicular membranes

Manganese-bilirubin (Mn-BR)-induced cholestasis in rats is associated with altered lipid composition of various hepatic subcellular fractions. Increased bile canalicular (BCM) cholesterol content in Mn-BR cholestasis and the intracellular source of the accumulating cholesterol were investigated. To label the total hepatic cholesterol pool, male Sprague-Dawley rats were given ip 3H-cholesterol, followed 18 h later by 2-14C-mevalonic acid (a precursor of cholesterol synthesis). To induce cholestasis, manganese (Mn, 4.5 mg/kg) and bilirubin (BR, 25 mg/kg) were injected iv; animals were killed 30 min after BR injection; canalicular and sinusoidal membranes, microsomes, mitochondria, and cytosol were isolated. Total cholesterol content of each fraction was determined by spectrophotometric techniques as well as radiolabeled techniques. In Mn-BR cholestasis, the total cholesterol concentrations of BCM and cytosol were significantly increased. Also, the contribution of 14C-labeled cholesterol (newly synthesized cholesterol) was enhanced in all isolated cellular fractions. The results are consistent with the hypothesis that accumulation of newly synthesized cholesterol in BCM is involved in Mn-BR cholestasis. An enhanced rate of synthesis of cholesterol, however, does not appear to be the causal event, as the activity of HMG-CoA reductase (rate-limiting enzyme in cholesterol synthesis), assessed in vitro, was decreased following Mn-BR treatment. Treatment with the Mn-BR combination may affect other aspects of intracellular cholesterol dynamics.

Effects of organic anions and vinblastine on biliary excretion of erythromycin in rats

Since little is known about the mechanism of biliary excretion of cationic drugs, biliary excretion of erythromycin was studied in rats. Infusion of sulfobromophthalein and taurocholate significantly decreased biliary erythromycin excretion, whereas infusion of dibromosulfophthalein, cefpiramide, ursodeoxycholate-3-O-glucuronide and taurolithocholate-3-sulfate had no effect on biliary excretion of erythromycin. Vinblastine significantly inhibited biliary erythromycin excretion. Phenothiazine treatment significantly increased biliary erythromycin excretion. However, erythromycin infusion did not affect biliary vinblastine excretion. These findings indicate a multiplicity of biliary excretory pathways for organic cations; at least one additonal pathway may exist for organic cations apart from P-glycoprotein.

Dexamethasone- and osmolarity-dependent expression of the multidrug-resistance protein 2 in cultured rat hepatocytes

Expression of the conjugate export pump multidrug-resistance protein 2 (MRP2) in liver is regulated by endotoxin and anti-tumour agents. This paper reports on the effects of dexamethasone and osmolarity on MRP2 expression. MRP2 expression was studied at the protein, mRNA, immunocytochemical and functional levels in cultured rat hepatocytes. Protein and mRNA expression of MRP2 in rat hepatocytes 24 and 48 h after isolation were largely dependent on the presence of dexamethasone (100 nmol/l) in the culture medium. MRP2 was localized at the pseudocanalicular membrane and increased expression of MRP2 was accompanied by a widening of the pseudocanaliculi. In presence of dexamethasone, hypo-osmolarity (205 mosmol/l) led to a strong induction of MRP2 mRNA and protein, whereas expression was decreased by hyperosmolarity (405 mosmol/l). Also, a decay of MRP2 protein and mRNA following dexamethasone withdrawal was osmosensitive. Expression of dipeptidylpeptidase IV, another canalicular protein, was unaffected by dexamethasone and osmolarity. It is concluded that glucocorticoids are strong inducers of MRP2 in liver. Besides short-term carrier insertion/retrieval, osmoregulation of MRP2 also involves a long-term effect on MRP2 expression.

Modification of Ca2+, Mg2+-ATPase and F-actin distribution in hepatocytes of cyclosporine A treated rats. Effect of soyabean lecithin and triacylglycerol

We studied the effect of cyclosporine A on hepatic Ca2+, Mg2+-ATPase and F-actin on bile canalicular and basolateral membranes in rats fed either soyabean lecithin, or triacylglycerol enriched diet, or low fat diet. Ca2+, Mg2+-ATPase histochemical activity was not modified in lecithin-cyclosporine A group, whereas the activity was decreased in the other groups. The triacylglycerol-cyclosporine A group had the lower activity. The histochemical staining of F-actin was quite normal in lecithin-cyclosporine group but decreased in the other cyclosporine A treated groups. The lower staining was observed in the triacylglycerol-cyclosporine group. The alteration of Ca2+, Mg2+-ATPase and F-actin by cyclosporine A, related to cholestasis evidenced by a decrease in bile salt secretion, were prevented by dietary soyabean lecithin and amplified by dietary soyabean triacylglycerol.

Vasopressin-induced disruption of actin cytoskeletal organization and canalicular function in isolated rat hepatocyte couplets: possible involvement of protein kinase C

The effect of vasopressin (VP) on canalicular function and hepatocellular morphology, with particular regard to actin cytoskeletal organization and the concomitant plasma membrane bleb formation, was studied in isolated rat hepatocyte couplets. VP induced the concentration-dependent formation of multiple plasma membrane blebs as well as simultaneous impairment in both canalicular vacuolar accumulation (cVA) and retention (cVR) of the fluorescent bile acid, cholyl-lysyl-fluorescein (CLF), which evaluate couplet secretory function and tight-junction integrity, respectively. These effects were mimicked by the protein kinase C (PKC) activator, phorbol dibutyrate (PDB), but not by the protein kinase A (PKA) activator, dibutyryl-cAMP. VP-induced bleb formation and canalicular dysfunction were fully prevented by the protein kinase inhibitor, H-7, but not by the PKA inhibitor, KT5720, further suggesting a specific role of PKC. VP-induced alterations were also prevented by pretreatment with the Ca2+-buffering agent, BAPTA/AM, but not with the calmodulin-dependent protein kinase II antagonist, calmidazolium. Neither the Ca2+-activated neutral protease inhibitor, leupeptin, nor the antioxidants, alpha-tocopherol or deferoxamine, were able to prevent either VP-induced plasma membrane blebbing or canalicular dysfunction. The Ca2+-ionophore, A23187, mimicked the VP-induced alterations, but its harmful effects were completely prevented by H-7. Bleb formation induced by VP and PDB was accompanied by an extensive redistribution of filamentous actin from the pericanalicular area to the cell body, and this effect was fully prevented by H-7. These results suggest that VP-induced canalicular and cytoskeletal dysfunction is mediated by PKC and that classical (Ca2+-dependent) PKC appear to be involved because intracellular Ca2+ is required for VP to induce its harmful effects.

Taurocholate induces preferential release of phosphatidylcholine from rat liver canalicular vesicles

AIMS/BACKGROUND

Biliary phospholipid secretion involves predominant segregation of canalicular phosphatidylcholine into bile. We tested the hypothesis that micellar concentrations of the major physiologic bile salt taurocholate can preferentially solubilize phosphatidylcholine from the canalicular rat liver plasma membrane.

METHODS

Subcellular fractions from rat liver and kidney were isolated with standardized procedures, incubated in vitro with taurocholate or 3-[(3-cholamidopropyl)dimethylammonio]-propane-1-sulphonate (CHAPS) and released phospholipids determined after centrifugation.

RESULTS

After incubation of canalicular (cLPM) and basolateral (blLPM) rat liver plasma membrane vesicles with 6 and 8 mM taurocholate, the proportion of phosphatidylcholine released was about two-fold higher as compared with its relative contribution to the overall lipid composition of the membranes. Quantitatively, this taurocholate-induced preferential phosphatidylcholine release was about four-fold higher in cLPM (117 nmol) as compared with blLPM (28 nmol). Comparison of membranes from different organs showed that increased sphingomyelin content reduced taurocholate-induced phosphatidylcholine release. Furthermore, phosphatidylcholine release from cLPM did not fit an inverse exponential relationship between membrane sphingomyelin content and phosphatidylcholine release from different starting material, indicating that cLPM is especially prone to taurocholate-induced phosphatidylcholine release. In contrast, in rat liver microsomes and kidney brush border membranes, taurocholate released phospholipids in proportion of their membrane contents, indicating an unspecific membrane solubilizing effect only. Similarly, CHAPS had an unselective lipid solubilizing effects in cLPM and blLPM.

CONCLUSION

These results support the concept that the very last step of canalicular phospholipid secretion is mediated in vivo by bile salt-induced vesiculation of phosphatidylcholine-enriched microdomains from the outer leaflet of cLPM.

Decreased biliary secretion of proteins and phospholipids by rats with 1,1-dichloroethylene-induced bile canalicular injury

1,1-Dichloroethylene (DCE, 50 mg/kg) rapidly and selectively injures the bile canalicular membrane of zone 3 hepatocytes. Thus, DCE is of value as a tool to assess the consequences of alterations in canalicular membrane integrity on bile formation. Our objective was to characterize the effects of DCE on the biliary secretion of proteins and phospholipids in freely moving rats. DCE treatment caused a rapid and sustained decrease in total biliary protein output. In contrast, canalicular membrane-localized enzyme activities more slowly increased to 8- to 15-fold in bile from DCE-treated rats. Biliary output of lysosomal enzymes was altered in a biphasic manner. Specifically, there was a transient fivefold increase within 30 min of DCE treatment and then a progressive decrease to approximately 10% basal levels by 4 h. Secretion of phospholipids into bile decreased rapidly in a striking and sustained manner, after DCE. Our findings of diminished lysosomal protein and phospholipid secretion following DCE treatment are consistent with an important role for canalicular membrane integrity in their entry into bile.

Hepatobiliary effects of tertiary-butylhydroperoxide (tBOOH) in isolated rat hepatocyte couplets

The organic hydroperoxide, tertiary-butylhydroperoxide (tBOOH), causes oxidative damage in a number of cell types. It is used here in an isolated rat hepatocyte couplet preparation to study adverse hepatobiliary effects of peroxidative damage in vitro. At subcytotoxic concentrations (as determined by lactate dehydrogenase release and maintenance of cytoplasmic ATP concentrations) tBOOH caused decreased accumulation of a fluorescent bile acid analogue, cholyl-lysyl-fluorescein (CLF), in the canalicular vacuole of couplets (a hepatobiliary effect; cholestasis). This was dose dependent in the range 100-200 microM. At the same concentrations it brought about release of preaccumulated CLF, suggesting that its effect was more likely to be on sealing properties of the vacuole than processes of uptake, transcytosis, and secretion. Pretreatment of tBOOH-treated couplets with the antioxidants deferoxamine mesylate (iron chelator) and dimethyl sulfoxide (free radical scavenger) resulted in the prevention of both canalicular vacuolar accumulation (cVA, which assesses canalicular function) and canalicular vacuolar retention (cVR, which assesses the retaining ability of couplets) depression at 100 microM tBOOH but not at higher concentrations. This indicates that the cholestatic effect of tBOOH has a preventable and nonpreventable phase and that free radicals are involved in these processes. By selectively generating the two types of tBOOH radical, peroxyl (tBOO.) and alkoxyl (tBO.), using suitable catalysts, we were able to determine that the peroxyl radical was most probably involved in tBOOH-induced cholestasis. This was further supported by experiments employing specific peroxyl and alkoxyl radical scavengers; only the peroxyl scavenger reduced the effect of tBOOH upon canalicular function under the conditions studied.

Carbon monoxide as a regulator of bile canalicular contractility in cultured rat hepatocytes

This study aimed to examine the mechanism(s) by which carbon monoxide (CO), a product of heme oxygenase reaction, controls the contractility of bile canaliculus (BC) in hepatocytes. When BCs associated with the couplet cells in cultured rat hepatocyte suspension were observed using time-lapse video microscopy, they exhibited periodical contractions with a most-probable interval of 6 minutes under our experimental conditions. The addition of 1 micromol/L zinc protoporphyrin IX (ZnPP), a potent inhibitor of heme oxygenase, to the culture medium elicited a 40% shortening of the interval time together with an increase in intracellular calcium concentrations, while the same concentration of iron protoporphyrin IX did not induce such changes. The production of CO, which was 0.5 nmol/h/10(8) cells in the absence of ZnPP, diminished to less than 0.1 nmol/h/10(8) cells upon application of ZnPP. The ZnPP-elicited increases in both contractile frequency and intracellular calcium concentrations were attenuated by the addition of 1 micromol/L CO or 50 micromol/L 1,2-bis(2-aminophenoxy) ethane-tetraacetate, a calcium chelator. Clotrimazole or metyrapone, inhibitors of cytochrome P450-dependent monooxygenase activities, also attenuated the ZnPP-induced elevation of the contractile frequency. On the other hand, intracellular cyclic guanosine monophosphate (cGMP) contents were not altered significantly by the application of ZnPP or by CO. These results indicate that CO generated by heme oxygenase controls the BC function by changing intracellular calcium concentrations presumably through a mechanism involving the cytochrome P450 reaction.

Actin filaments and microtubules are involved in different membrane traffic pathways that transport sphingolipids to the apical surface of polarized HepG2 cells

In polarized HepG2 hepatoma cells, sphingolipids are transported to the apical, bile canalicular membrane by two different transport routes, as revealed with fluorescently tagged sphingolipid analogs. One route involves direct, transcytosis-independent transport of Golgi-derived glucosylceramide and sphingomyelin, whereas the other involves basolateral to apical transcytosis of both sphingolipids. We show that these distinct routes display a different sensitivity toward nocodazole and cytochalasin D, implying a specific transport dependence on either microtubules or actin filaments, respectively. Thus, nocodazole strongly inhibited the direct route, whereas sphingolipid transport by transcytosis was hardly affected. Moreover, nocodazole blocked "hyperpolarization," i.e., the enlargement of the apical membrane surface, which is induced by treating cells with dibutyryl-cAMP. By contrast, the transcytotic route but not the direct route was inhibited by cytochalasin D. The actin-dependent step during transcytotic lipid transport probably occurs at an early endocytic event at the basolateral plasma membrane, because total lipid uptake and fluid phase endocytosis of horseradish peroxidase from this membrane were inhibited by cytochalasin D as well. In summary, the results show that the two sphingolipid transport pathways to the apical membrane must have a different requirement for cytoskeletal elements.

ATP-dependent transport of glutathione conjugate of 7beta, 8alpha-dihydroxy-9alpha,10alpha-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene in murine hepatic canalicular plasma membrane vesicles

Glutathione (GSH) S-transferases (GSTs) have an important role in the detoxification of (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9, 10-tetrahydrobenzo[a]pyrene [(+)-anti-BPDE], which is the ultimate carcinogen of benzo[a]pyrene. However, the fate and/or biological activity of the GSH conjugate of (+)-anti-BPDE [(-)-anti-BPD-SG] is not known. We now report that (-)-anti-BPD-SG is a competitive inhibitor (Ki 19 microM) of Pi-class isoenzyme mGSTP1-1, which among murine hepatic GSTs is most efficient in the GSH conjugation of (+)-anti-BPDE. Thus the inhibition of mGSTP1-1 activity by (-)-anti-BPD-SG might interfere with the GST-catalysed GSH conjugation of (+)-anti-BPDE unless one or more mechanisms exist for the removal of the conjugate. The results of the present study indicate that (-)-anti-BPD-SG is transported across canalicular liver plasma membrane (cLPM) in an ATP-dependent manner. The ATP-dependent transport of (-)-anti-[3H]BPD-SG followed Michaelis-Menten kinetics (Km 46 microM). The ATP dependence of the (-)-anti-BPD-SG transport was confirmed by measuring the stimulation of ATP hydrolysis (ATPase activity) by the conjugate in the presence of cLPM protein, which also followed Michaelis-Menten kinetics. In contrast, a kinetic analysis of ATP-dependent uptake of the model conjugate S-[3H](2,4-dinitrophenyl)-glutathione ([3H]DNP-SG) revealed the presence of a high-affinity and a low-affinity transport system in mouse cLPM, with apparent Km values of 18 and 500 microM respectively. The ATP-dependent transport of (-)-anti-BPD-SG was inhibited competitively by DNP-SG (Ki 1.65 microM). Likewise, (-)-anti-BPD-SG was found to be a potent competitive inhibitor of the high-affinity component of DNP-SG transport (Ki 6.3 microM). Our results suggest that GST-catalysed conjugation of (+)-anti-BPDE with GSH, coupled with ATP-dependent transport of the resultant conjugate across cLPM, might be the ultimate detoxification pathway for this carcinogen.

Large intravenous bilirubin loads increase the cytotoxicity of bile and lower the resistance of the canalicular membrane to cytotoxic injury and cause cholestasis in pigs

BACKGROUND

Large intravenous bilirubin loads cause loss of hepatic canalicular membrane microvilli and cholestasis. This study examines whether these untoward effects might be due to canalicular membrane injury from cytotoxic bile.

METHODS

The cytotoxicity of bile was assayed against pig erythrocytes before and throughout 4.5-h intravenous infusion of 170 microg kg(-1) body weight of bilirubin in anaesthetized pigs. The capacity to generate canalicular bile flow was tested before and after bilirubin infusion by means of short-term intraportal cholic acid infusion.

RESULTS

Bilirubin infusion increased the cytotoxicity of hepatic bile, reduced biliary phospholipid secretion by 90%, and caused cholestasis. Cholic acid infusion before bilirubin also increased the cytotoxicity of bile but increased bile flow and doubled biliary phospholipid output.

CONCLUSION

Large intravenous bilirubin infusions increase the cytotoxicity of bile, suppress biliary phospholipid secretion, and render hepatic canalicular membrane microvilli susceptible to injury from cytotoxic bile so that cholestasis occurs.

BAPS prize lecture: New insight into mechanisms of parenteral nutrition-associated cholestasis: role of plant sterols. British Association of Paediatric Surgeons

BACKGROUND PURPOSE

Infants on long-term parenteral nutrition frequently will have progressive cholestatic liver disease, the cause of which remains largely unknown. The aim of this study is to establish a possible role for plant sterols (phytosterols) in the pathogenesis of parenteral nutrition-associated cholestasis (PNAC).

METHODS

Two experimental studies were used: (1) A study on neonatal piglets involved the daily injection of plant sterols; measurement of their concentrations in serum, liver, and bile during a 14-day period; and determination of serum bile acid concentrations, bile acid secretion, and bile flow at the end of the 14-day period. (2) Isolated rat hepatocyte couplets were used to study the effects of sterols on canalicular secretion.

RESULTS

The daily injection of phytosterols (in amounts similar to those given to infants who receive parenteral nutrition) led to their progressive accumulation in the piglets' serum, liver, and bile. Serum bile acid levels were significantly higher in the sterol-treated piglets. Maximal bile acid excretion was significantly lower in the sterol group. Phytosterols caused a significant inhibition of secretory function in isolated rat hepatocyte couplets.

CONCLUSION

Important contaminants of commercial lipid emulsions (phytosterols) have been identified as a possible cause of PNAC.

Canalicular retention as an in vitro assay of tight junctional permeability in isolated hepatocyte couplets: effects of protein kinase modulation and cholestatic agents

A simple, fast method to evaluate acute changes of tight junctional permeability in isolated hepatocyte couplets is proposed. The method consists of the recording of the number of canalicular vacuoles able to retain the previously accumulated fluorescent bile acid analogue cholyl-lysyl-fluorescein (CLF), as visualized by inverted fluorescent microscopy, following acute exposure to the compounds under study. The method was validated by (i) making a systematic documentation of the effect on CLF retention of a variety of hormonal modulators (vasopressin and phorbol esters), as well as several cholestatic (taurolithocholic acid, cyclosporin A, and estradiol 17 beta-glucuronide) and hepatotoxic agents (menadione, A23187, and t-butyl hydroperoxide), all known to affect biliary permeability in intact liver, and (ii) carrying out a comparative analysis of the results obtained with those recorded using rapid canalicular access of horseradish peroxidase (HRP) as an alternative procedure. The compounds tested all decreased canalicular vacuolar retention of CLF in a dose-dependent manner. Vasopressin- and phorbol ester-induced decline in CLF retention were prevented by pretreatment with the protein kinase C inhibitors H-7 and staurosporine, thus confirming a role for this enzyme in canalicular permeability regulation. A significant direct correlation (r = 0.934, p < 0.001) was obtained when the decrease in canalicular retention of CLF was compared with the increment in the canalicular access of HRP. Image analysis revealed that cellular fluorescence was not increased following exposure to these compounds, suggesting a paracellular rather than transcellular route for CLF egress. These results all support canalicular vacuolar retention of CLF as a suitable method to readily evaluate acute changes in tight junctional permeability in isolated hepatocyte couplets induced by physiological modulators or hepatotoxic agents.

Disruption of actin organization by cytochalasin D does not impair biliary secretion of organic anions in the rat

The bile canaliculi of hepatocytes contract spontaneously, and it is hypothesized that this canalicular motility provides a propulsive force for normal intrahepatic bile flow. Cytochalasin disrupts actin polymerization, inhibits contraction, and decreases bile flow. We investigated whether this cholestasis was associated with impaired canalicular secretion. Isolated rat hepatocyte doublets, with and without incubation with 2 mumol/L cytochalasin D (cytD), were superfused, under first-order conditions, to steady state with fluorescein isothiocyanate-labeled glycocholic acid (FITC-GC) and carboxy-4',5'-dimethylfluorescein diacetate (CMFD), which are fluorescent substrates for the bile acid and the nonbile acid organic anion transport pathways, respectively. Fluorescent microscopic images were quantified and the data analyzed by noncompartmental and compartmental kinetic methods. cytD dilated the canalicular spaces fivefold but did not change the proportion of doublets that secreted either probe. Cytochalasin did not affect the mean cellular transit times of FITC-GC (2.8 and 2.5 minutes for control and cytochalasin-treated groups, respectively) and of carboxy-4',5'-dimethylfluorescein (3.8 and 3.7 minutes, respectively). Analysis with a three-compartment model gave estimates of the rate constants for canalicular secretion: 0.21 +/- 0.04 and 0.22 +/- 0.03 min-1 in control and treated cells, respectively, for FITC-GC, and 0.14 +/- 0.01 and 0.16 +/- 0.02 min-1, respectively, for carboxy-dimethylfluorescein. When kinetics are first-order, the canalicular secretion of organic anions is not altered by actin disruptive agents, suggesting that actin filaments do not modulate the function or distribution of these transporters. This suggests that impaired contractility rather than impaired canalicular secretion is the mechanism of cytD-induced cholestasis.

Large intravenous loads of bilirubin photoconversion products, in contrast to bilirubin, do not cause cholestasis in bile acid-depleted pigs

BACKGROUND

Large intravenous bilirubin infusions in bile acid-depleted pigs (BADP) destroy hepatocyte canalicular membrane microvilli (CMV) and cause cholestasis. This study examines whether bilirubin photoconversion product infusions do the same.

METHODS

The effects of systemic infusion of 135 mumol.kg-1 body weight bilirubin photoconversion products on CMV density and choleretic response to intraportal bile acid infusion were studied in BADP. Furthermore, the effects of 135 mumol.kg-1 b.w. bilirubin infusion, either through an arteriovenous bilirubin photoconversion shunt device (PCD) or intravenously, were measured in PCD-connected BADP.

RESULTS

Intravenous bilirubin photoconversion product infusions affected neither the CMV density nor the choleretic response to cholic acid infusion, and neither did bilirubin infusion through the PCD. In contrast, intravenous bilirubin infusion caused canalicular injury and cholestasis in four of six PCD-connected BADP.

CONCLUSION

Bilirubin photoconversion products do not destroy CMV or cause cholestasis in BADP. A bilirubin photoconversion shunt device can confer cholestasis protection to bilirubin-loaded BADP.

Effects of cholestatic agents on the structure and function of bile canaliculi in neonatal rat hepatocytes in primary culture

The effects of cytochalasin B and colchicine on the structure and function of bile canaliculi were studied in neonatal rat hepatocytes in primary culture. Cellular contacts of neonatal hepatocytes were not as tight as those of adult hepatocytes. There was no remarkable difference in the ultrastructure of bile canaliculi between neonatal and adult hepatocytes. Neonatal hepatocytes treated with cytochalasin B were round in shape and aggregated in groups of several cells. Actin filaments stained by rhodamine-phalloidin were disrupted and condensed at the cell periphery or around dilated bile canaliculi. Markedly-dilated bile canaliculi with less microvilli were observed by transmission electron microscopy while the secretory function of horseradish peroxidase, which was used as a marker for uptake, transport and secretion into bile canaliculi, were maintained. The lumen of dilated bile canaliculi was found close to the undersurfaces of hepatocytes by scanning electron microscopy after turning over the cultured cells. By colchicine treatment, the filamentous structure of microtubules in neonatal hepatocytes disappeared. The ultrastructure of the bile canaliculi was not affected by the treatment, but transport and secretion of horseradish peroxidase into bile canaliculi were inhibited. The development of strict cellular polarity in neonatal hepatocytes may be suppressed in neonatal hepatocytes; however, cholestatic agents which rearrange the cytoskeleton caused the same morphological or functional changes of bile canaliculi as in adult hepatocytes.

Mechanisms of hepatic transport of cyclosporin A: an explanation for its cholestatic action?

The hepatic transport of the immunosuppressive Cyclosporin A (CyA) was studied using liposomal phospholipid membranes, freshly isolated rat hepatocytes and bile canalicular plasma membrane vesicles from rat liver. The Na(+)-dependent, saturable uptake of the bile acid 3H-taurocholate into isolated rat liver cells was apparently competitively inhibited by CyA. However, the uptake of CyA into the cells was neither saturable, nor temperature-dependent nor Na(+)-dependent, nor could it be inhibited by bile salts or CyA-derivatives, indicating passive diffusion. In steady state depolarization fluorescence studies, CyA caused a concentration-dependent decrease of anisotropy, indicating a membrane fluidizing effect. Ion flux experiments demonstrated that CyA dramatically increases the permeability of Na+ and Ca2+ across phospholipid membranes in a dose- and time-dependent manner, suggesting a iontophoretic activity that might have a direct impact on cellular ion homeostasis and regulation of bile acid uptake. Photoaffinity labeling with a [3H]-labeled photolabile CyA-derivative resulted in the predominant incorporation of radioactivity into a membrane polypeptide with an apparent molecular weight of 160,000 and a minor labeling of polypeptides with molecular weights of 85,000-90,000. In contrast, use of a photolabile bile acid resulted in the labeling of a membrane polypeptide with an apparent molecular weight of 110,000, representing the bile canalicular bile acid carrier. The photoaffinity labeling as well as CyA transport by canalicular membrane vesicles were inhibited by CyA and the p-glycoprotein substrates daunomycin and PSC-833, but not by taurocholate, indicating that CyA is excreted by p-glycoprotein. CyA uptake by bile canalicular membrane vesicles was ATP-dependent and could not be inhibited by taurocholate. CyA caused a decrease in the maximum amount of bile salt accumulated by the vesicles with time. However, initial rates of [3H]-taurocholate uptake within the first 2.5 min remained unchanged at increasing CyA concentrations. In summary, the data indicate that CyA does not directly interact with the hepatic bile acid transport systems. Its cholestatic action may rather be the result of alterations in membrane fluidity, intracellular effects and an interaction with p-glycoprotein.