Inhibition of the renal apical sodium dependent bile acid transporter prevents cholemic nephropathy in mice with obstructive cholestasis

BACKGROUND & AIMS

Cholemic nephropathy (CN) is a severe complication of cholestatic liver diseases for which there is no specific treatment. We revisited its pathophysiology with the aim of identifying novel therapeutic strategies.

METHODS

Cholestasis was induced by bile duct ligation (BDL) in mice. Bile flux in kidneys and livers was visualized by intravital imaging, supported by MALDI mass spectrometry imaging and liquid chromatography-tandem mass spectrometry. The effect of AS0369, a systemically bioavailable apical sodium-dependent bile acid transporter (ASBT) inhibitor, was evaluated by intravital imaging, RNA-sequencing, histological, blood, and urine analyses. Translational relevance was assessed in kidney biopsies from patients with CN, mice with a humanized bile acid (BA) spectrum, and via analysis of serum BAs and KIM-1 (kidney injury molecule 1) in patients with liver disease and hyperbilirubinemia.

RESULTS

Proximal tubular epithelial cells (TECs) reabsorbed and enriched BAs, leading to oxidative stress and death of proximal TECs, casts in distal tubules and collecting ducts, peritubular capillary leakiness, and glomerular cysts. Renal ASBT inhibition by AS0369 blocked BA uptake into TECs and prevented kidney injury up to 6 weeks after BDL. Similar results were obtained in mice with humanized BA composition. In patients with advanced liver disease, serum BAs were the main determinant of KIM-1 levels. ASBT expression in TECs was preserved in biopsies from patients with CN, further highlighting the translational potential of targeting ASBT to treat CN.

CONCLUSIONS

BA enrichment in proximal TECs followed by oxidative stress and cell death is a key early event in CN. Inhibiting renal ASBT and consequently BA enrichment in TECs prevents CN and systemically decreases BA concentrations.

IMPACT AND IMPLICATIONS

Cholemic nephropathy (CN) is a severe complication of cholestasis and an unmet clinical need. We demonstrate that CN is triggered by the renal accumulation of bile acids (BAs) that are considerably increased in the systemic blood. Specifically, the proximal tubular epithelial cells of the kidney take up BAs via the apical sodium-dependent bile acid transporter (ASBT). We developed a therapeutic compound that blocks ASBT in the kidneys, prevents BA overload in tubular epithelial cells, and almost completely abolished all disease hallmarks in a CN mouse model. Renal ASBT inhibition represents a potential therapeutic strategy for patients with CN.

Exploiting Apical Sodium-Dependent Bile Acid Transporter (ASBT)-Mediated Endocytosis with Multi-Functional Deoxycholic Acid Grafted Alginate Amide Nanoparticles as an Oral Insulin Delivery System

OBJECTIVE

Oral administration of insulin is a potential candidate for managing diabetes. However, it is obstructed by the gastrointestinal tract barriers resulting in negligible oral bioavailability.

METHODS

This investigation presents a novel nanocarrier platform designed to address these challenges. In this regard, the process involved amination of sodium alginate by ethylene diamine, followed by its conjugation with deoxycholic acid.

RESULTS

The resulting DCA@Alg@INS nanocarrier revealed a significantly high insulin loading content of 63.6 ± 1.03% and encapsulation efficiency of 87.6 ± 3.84%, with a particle size of 206 nm and zeta potentials of -3 mV. In vitro studies showed sustained and pH-dependent release profiles of insulin from nanoparticles. In vitro cellular studies, confocal laser scanning microscopy and flow cytometry analysis confirmed the successful attachment and internalization of DCA@Alg@INS nanoparticles in Caco-2 cells. Furthermore, the DCA@Alg@INS demonstrated a superior capacity for cellular uptake and permeability coefficient relative to the insulin solution, exhibiting sixfold and 4.94-fold enhancement, respectively. According to the uptake mechanism studies, the results indicated that DCA@Alg@INS was mostly transported through an energy-dependent active pathway since the uptake of DCA@Alg@INS by cells was significantly reduced in the presence of NaN3 by ~ 92% and at a low temperature of 4°C by ~ 94%.

CONCLUSIONS

Given the significance of administering insulin through oral route, deoxycholic acid-modified alginate nanoparticles present a viable option to surmount various obstacles presented by the gastrointestinal.

A3907, a systemic ASBT inhibitor, improves cholestasis in mice by multiorgan activity and shows translational relevance to humans

BACKGROUND AND AIMS

Cholestasis is characterized by intrahepatic accumulation of bile constituents, including bile acids (BAs), which promote liver damage. The apical sodium-dependent BA transporter (ASBT) plays an important role in BA reabsorption and signaling in ileum, bile ducts, and kidneys. Our aim was to investigate the pharmacokinetics and pharmacological activity of A3907, an oral and systemically available ASBT inhibitor in experimental mouse models of cholestasis. In addition, the tolerability, pharmacokinetics, and pharmacodynamics of A3907 were examined in healthy humans.

APPROACH AND RESULTS

A3907 was a potent and selective ASBT inhibitor in vitro. In rodents, orally administered A3907 distributed to the ASBT-expressing organs, that is, ileum, liver, and kidneys, and dose dependently increased fecal BA excretion. A3907 improved biochemical, histological, and molecular markers of liver and bile duct injury in Mdr2-/- mice and also had direct protective effects on rat cholangiocytes exposed to cytotoxic BA concentrations in vitro . In bile duct ligated mice, A3907 increased urinary BA elimination, reduced serum BA levels, and prevented body weight loss, while improving markers of liver injury. A3907 was well tolerated and demonstrated target engagement in healthy volunteers. Plasma exposure of A3907 in humans was within the range of systemic concentrations that achieved therapeutic efficacy in mouse.

CONCLUSIONS

The systemic ASBT inhibitor A3907 improved experimental cholestatic disease by targeting ASBT function at the intestinal, liver, and kidney levels, resulting in marked clearance of circulating BAs and liver protection. A3907 is well tolerated in humans, supporting further clinical development for the treatment of cholestatic liver diseases.

Identification of Novel HBV/HDV Entry Inhibitors by Pharmacophore- and QSAR-Guided Virtual Screening

The hepatic bile acid transporter Na⁺/taurocholate co-transporting polypeptide (NTCP) was identified in 2012 as the high-affinity hepatic receptor for the hepatitis B and D viruses (HBV/HDV). Since then, this carrier has emerged as promising drug target for HBV/HDV virus entry inhibitors, but the synthetic peptide Hepcludex^® of high molecular weight is the only approved HDV entry inhibitor so far. The present study aimed to identify small molecules as novel NTCP inhibitors with anti-viral activity. A ligand-based bioinformatic approach was used to generate and validate appropriate pharmacophore and QSAR (quantitative structure–activity relationship) models. Half-maximal inhibitory concentrations (IC₅₀) for binding inhibition of the HBV/HDV-derived preS1 peptide (as surrogate parameter for virus binding to NTCP) were determined in NTCP-expressing HEK293 cells for 150 compounds of different chemical classes. IC₅₀ values ranged from 2 µM up to >1000 µM. The generated pharmacophore and QSAR models were used for virtual screening of drug-like chemicals from the ZINC¹⁵ database (~11 million compounds). The 20 best-performing compounds were then experimentally tested for preS1-peptide binding inhibition in NTCP-HEK293 cells. Among them, four compounds were active and revealed experimental IC₅₀ values for preS1-peptide binding inhibition of 9, 19, 20, and 35 µM, which were comparable to the QSAR-based predictions. All these compounds also significantly inhibited in vitro HDV infection of NTCP-HepG2 cells, without showing any cytotoxicity. The best-performing compound in all assays was ZINC000253533654. In conclusion, the present study demonstrates that virtual compound screening based on NTCP-specific pharmacophore and QSAR models can predict novel active hit compounds for the development of HBV/HDV entry inhibitors.

Hesperidin alleviates cholestasis via activation of the farnesoid X receptor in vitro and in vivo

Cholestasis causes the intrahepatic accumulation of bile acids leading to hepatobiliary injury. Recently obeticholic acid, a farnesoid X receptor (FXR) agonist, was FDA-approved to treat cholestatic liver diseases, providing a new therapeutic strategy for cholestasis. The purpose of the current study was to characterize a novel FXR agonist and verify the anti-cholestatic effect of hesperidin (HP) in vivo and in vitro. Based on a molecular docking study that predicted that HP would bind to FXR, the hepatoprotective effect of HP against cholestasis and hepatotoxicity was evaluated in mice and in normal and FXR-suppressed HepaRG cells. HP prevented bile acid toxicity in HepaRG cells, and this effect was blocked by FXR silencing. HP appears to activate FXR to prevent cholestatic liver injury. Dynamic change analysis of bile acids revealed that HP promoted bile acid excretion into feces and reduced hepatic accumulation via the regulation of the FXR-target genes bile salt export pump, multi-drug resistance-associated protein 2, and Na+-taurocholate cotransporting polypeptide. Furthermore, HP down-regulated enzymes involved in bile acid synthesis including cholesterol 7α-hydroxylase and sterol 27-hydroxylase. HP produced a protective effect against cholestasis via FXR activation, and may be an effective approach for the prevention and treatment of cholestatic liver diseases.

Effect of the Dimeric Bile Acid Analogue S 0960, a Specific Inhibitor of the Apical Sodium-dependent Bile Salt Transporter in the Ileum, on the Renal Handling of Taurocholate

The effect of the dimeric bile acid analogue S 0960 (CAS 142974-51-4), a specific inhibitor of the apical sodium-dependent bile salt transporter (ASBT) in the ileum, on kidney function was studied by clearance experiments in anesthetized rats. Additional experiments were performed on proximal tubular cells freshly isolated from rat kidney cortex and enriched by nycodenz density gradient centrifugation. The clearance studies, which were performed after a 5 h bile duct ligation, revealed a marked rise of the 3H-taurocholate clearance (from 85.4 +/- 15.7 to 371.1 +/- 86.0 microliters/min 100 g b.w., p < 0.05) and a considerable fall of the fractional tubular 3H-taurocholate reabsorption (from 90.2 +/- 1.72 to 68.2 +/- 7.50%, p < 0.05) after S 0960 at a dose of 10 mg/kg i.v. whereas the glomerular filtration rate did not significantly change (from 919 +/- 165 to 1055 +/- 162 microliters/min/100 g b.w.). Isolated proximal tubular cells showed a significant accumulation of 3H-taurocholate. The 3H-taurocholate cell/bath concentration ratio amounted to 3.34 +/- 0.17 at a 3H-taurocholate bath concentration of 3 x 10(-7) mol/l. LiCl (10(-3) mol/l), which is known to inhibit sodium-dependent transport processes in the kidney, markedly diminished cellular 3H-taurocholate uptake (by 65.8%) whereas probenecid (CAS 57-66-9, 10(-4) mol/l), the classical inhibitor of the basolateral organic acid transporter in the kidney, did not significantly affect 3H-taurocholate uptake. This finding indicates that transport of taurocholate by the basolaterally located organic acid transporter is not involved in the uptake process. The kinetic studies revealed an apparent K(m) value of 31 mumol/l and a Vmax value of 6.7 mumol/l cell water/min for tubular 3H-taurocholate uptake. At concentrations > 30 mumol/l S 0960 virtually completely inhibited cellular 3H-taurocholate uptake. 3H-taurocholate uptake was half-maximally inhibited at a S 0960 concentration of 5.8 mumol/l. The results of this functional study are in line with recent molecular evidence that the apical sodium-dependent bile salt transporters in kidney and ileum are identical and demonstrate that S 0960 is a potent inhibitor of the apical sodium-dependent taurocholate transporter in the kidney which augments the renal clearance of 3H-taurocholate. Compounds such as S 0960 may be of special therapeutical value in patients with extrahepatic cholestasis and elevated levels of plasma bile acids.

Why we should be vigilant: drug cytotoxicity observed with in vitro transporter inhibition studies

From routine in vitro drug-transporter inhibition assays, observed inhibition is typically assumed from direct interaction with the transporter. Other mechanisms that possibly reduce substrate uptake are not frequently fully examined. The objective of this study was to investigate the association of transporter inhibition with drug cytotoxicity. From a pool of drugs that were identified as known ASBT or OCTN2 inhibitors, 21 drugs were selected to screen inhibitory potency of their prototypical substrate and cytotoxicity against three human sodium-dependent solute carrier (SLC) transporters: apical sodium-dependent bile acid transporter (ASBT), organic cation/carnitine transporter (OCTN2), and the excitatory amino acid transporter 4 (EAAT4) in stable cell lines. Twenty drugs showed apparent inhibition in OCTN2-MDCK and ASBT-MDCK. Four dihydropyridine calcium channel blockers were cytotoxic to MDCK cells, and the observed cytotoxicity of three of them accounted for their apparent OCTN2 inhibition, and consequently were classified as non-OCTN2 inhibitors. Meanwhile, since their cytotoxicity only moderately contributed to ASBT inhibition, these three were still considered ASBT inhibitors. Four other drugs showed apparent inhibition in EAAT4-HEK cells, and cytotoxicity of three drugs corresponded with their inhibition of this transporter. Therefore, cytotoxicity significantly affected EAAT4 observations. Results showed the potential of cytotoxicity as a mechanism that can account for apparent in vitro transporter inhibition. Drug cytotoxicity varied in different cell lines, which could increase false positives for pharmacophore development.

Coordinated induction of bile acid detoxification and alternative elimination in mice: role of FXR-regulated organic solute transporter-alpha/beta in the adaptive response to bile acids

The bile acid receptor farnesoid X receptor (FXR) is a key regulator of hepatic defense mechanisms against bile acids. A comprehensive study addressing the role of FXR in the coordinated regulation of adaptive mechanisms including biosynthesis, metabolism, and alternative export together with their functional significance is lacking. We therefore fed FXR knockout (FXR(-/-)) mice with cholic acid (CA) and ursodeoxycholic acid (UDCA). Bile acid synthesis and hydroxylation were assessed by real-time RT-PCR for cytochrome P-450 (Cyp)7a1, Cyp3a11, and Cyp2b10 and mass spectrometry-gas chromatography for determination of bile acid composition. Expression of the export systems multidrug resistance proteins (Mrp)4-6 in the liver and kidney and the recently identified basoalteral bile acid transporter, organic solute transporter (Ost-alpha/Ost-beta), in the liver, kidney, and intestine was also investigated. CA and UDCA repressed Cyp7a1 in FXR(+/+) mice and to lesser extents in FXR(-/-) mice and induced Cyp3a11 and Cyp2b10 independent of FXR. CA and UDCA were hydroxylated in both genotypes. CA induced Ost-alpha/Ost-beta in the liver, kidney, and ileum in FXR(+/+) but not FXR(-/-) mice, whereas UDCA had only minor effects. Mrp4 induction in the liver and kidney correlated with bile acid levels and was observed in UDCA-fed and CA-fed FXR(-/-) animals but not in CA-fed FXR(+/+) animals. Mrp5/6 remained unaffected by bile acid treatment. In conclusion, we identified Ost-alpha/Ost-beta as a novel FXR target. Absent Ost-alpha/Ost-beta induction in CA-fed FXR(-/-) animals may contribute to increased liver injury in these animals. The induction of bile acid hydroxylation and Mrp4 was independent of FXR but could not counteract liver toxicity sufficiently. Limited effects of UDCA on Ost-alpha/Ost-beta may jeopardize its therapeutic efficacy.

Tienilic acid enhances hyperbilirubinemia in Eisai hyperbilirubinuria rats through hepatic multidrug resistance-associated protein 3 and heme oxygenase-1 induction

We demonstrated that tienilic acid, a diuretic drug withdrawn from the market because of hepatic failure, enhanced hyperbilirubinemia in Eisai hyperbilirubinuria rats (EHBR) with a defect of canalicular multidrug resistance-associated protein 2 (Mrp2). In contrast, no remarkable changes were noted in Sprague-Dawley (SD) rats, the parent strain for EHBR. To investigate a mechanism underlying this enhanced hyperbilirubinemia, we focused on comprehensive effects of tienilic acid on clinicopathological aspects and expression of hepatic transporters. Other than eventual hyperbilirubinemia with slightly increased biliary bilirubin, a single oral treatment of EHBR with tienilic acid at 300 mg/kg caused no changes in serum alanine aminotransferase and alkaline phosphatase, bile flow rate and biliary bile acid secretion, or hepatic morphology. In analyses of mRNA expression of the hepatic transporters, elevated Mrp3 expression in EHBR correlated with an increase in serum total bilirubin, suggesting increased bilirubin transport from the liver into the peripheral blood flow. Hepatic heme oxygenase-1 (Ho-1) mRNA, a stress-induced isoform of the rate-limiting enzyme in the catabolism of heme to bilirubin, was markedly upregulated in EHBR at the same dose at which increased serum bilirubin was seen. A time-course study revealed that marked induction of Ho-1 occurred earlier than that of Mrp3, followed by an increase in serum bilirubin. These results suggest that hepatic Mrp3 and Ho-1 may contribute to tienilic acid-enhanced hyperbilirubinemia in EHBR by inducing increased bilirubin transport from the liver into the blood stream, preceded by potentiation of bilirubin formation in the liver.

Carrier-mediated hepatic uptake of a novel nonrenal excretion type uric acid generation inhibitor, Y-700

Y-700, a novel xanthine oxidase inhibitor, was recently developed for the treatment of hyperuricemia and gout. Since the major elimination route of this compound is hepatic metabolism and excretion, the aim of the present study was to characterize the uptake mechanism of Y-700 in the liver, which is also the pharmacological target of Y-700. Efficient uptake of Y-700 was observed both in the liver in vivo and in isolated rat hepatocytes. The uptake was Na(+)-dependent, saturable and inhibited both by ATP-depressants and various organic anions. Indomethacin competitively inhibited Y-700 uptake, whereas the inhibitory effect of organic cations and nucleosides was not so remarkable. Saturable and Na(+)-dependent uptake of Y-700 was also observed in freshly isolated human hepatocytes. Uptake of Y-700 by sinusoidal membrane transporters, such as organic anion transporter (Oat) 2 and organic anion transporting polypeptide (OATP)-B, OATP-C, OATP-8, and Oatp1, could not be detected although uptake of Y-700 in the oocytes expressing sodium/taurocholate cotransporting polypeptide (NTCP) was slightly observed. In conclusion, active transport system(s), which specifically recognize certain types of anionic compounds, are involved in the hepatic uptake of Y-700 and, at least partially, relevant to its elimination from the circulation as well as delivery to pharmacological target.

Role of nuclear receptors and hepatocyte-enriched transcription factors for Ntcp repression in biliary obstruction in mouse liver

Expression of the main hepatic bile acid uptake system, the Na+-taurocholate cotransporter (Ntcp), is downregulated during cholestasis. Bile acid-induced, farnesoid X receptor (FXR)-mediated induction of the nuclear repressor short heterodimer partner (SHP) has been proposed as a key mechanism reducing Ntcp expression. However, the role of FXR and SHP or other nuclear receptors and hepatocyte-enriched transcription factors in mediating Ntcp repression in obstructive cholestasis is unclear. FXR knockout (FXR-/-) and wild-type (FXR+/+) mice were subjected to common bile duct ligation (CBDL). Cholic acid (CA)-fed and LPS-treated FXR-/- and FXR+/+ mice were studied for comparison. mRNA levels of Ntcp and SHP and nuclear protein levels of hepatocyte nuclear factor (HNF)-1alpha, HNF-3beta, HNF-4alpha, retinoid X receptor (RXR)-alpha, and retinoic acid receptor (RAR)-alpha and their DNA binding were assessed. Hepatic cytokine mRNA levels were also measured. CBDL and CA led to Ntcp repression in FXR+/+, but not FXR-/-, mice, whereas LPS reduced Ntcp expression in both genotypes. CBDL and LPS but not CA induced cytokine expression and reduced levels of HNF-1alpha, HNF-3beta, HNF-4alpha, RXRalpha, and RARalpha to similar extents in FXR+/+ and FXR-/-. DNA binding of these transactivators was unaffected by CA in FXR+/+ mice but was markedly reduced in FXR-/- mice. In conclusion, Ntcp repression by CBDL and CA is mediated by accumulating bile acids via FXR and does not depend on cytokines, whereas Ntcp repression by LPS is independent of FXR. Reduced levels of HNF-1alpha, RXRalpha, and RARalpha in CBDL FXR-/- mice and reduced DNA binding in CA-fed FXR-/- mice, despite unchanged Ntcp levels, indicate that these factors may have a minor role in regulation of mouse Ntcp during cholestasis.

Dephosphorylation of Ser-226 facilitates plasma membrane retention of Ntcp

Ntcp is a phosphoprotein, and its translocation by cAMP to the plasma membrane is associated with dephosphorylation. However, the phosphorylation site(s) of Ntcp is not known. Thus, the aim of the present study was to determine the potential Ntcp phosphorylation sites and whether any of these phosphorylation sites is involved in Ntcp translocation. To determine the potential phosphorylation sites, metabolically labeled [32P]Ntcp isolated from hepatocytes was digested with clostripain and then subjected to SDS-PAGE followed by autoradiography. Clostripain digestion resulted in two phosphorylated peptides, and cAMP decreased phosphorylation of one of the peptides (7.8 K(d)), which contains the putative third cytoplasmic loop with three serine (Ser-213, Ser-226, and Ser-227) and two threonine (Thr-219 and Thr-225) residues. To determine whether any one of these serine/threonine residues is phosphorylated and/or is involved in Ntcp translocation, each of these serine/threonine residues were mutated to alanine. HuH-7 cells were transiently transfected with the wild-type and the mutated Ntcps followed by determination of taurocholate uptake and Ntcp expression, translocation and phosphorylation. Mutation of only Ser-226 resulted in 30% decrease in Ntcp phosphorylation and in 2.5 and 3.2-fold increases in taurocholate uptake and Ntcp retention in the plasma membrane, respectively. Cyclic AMP failed to further decrease phosphorylation and increase translocation of S226A-Ntcp. Taken together, these results suggest that the Ser-226 in the third cytoplasmic loop of Ntcp is phosphorylated and cAMP may increase Ntcp translocation to the plasma membrane by dephosphorylating Ntcp at this site.

Expression and localization of hepatobiliary transport proteins in progressive familial intrahepatic cholestasis

Mutations of the bile salt export pump (BSEP) or the multidrug resistance P-glycoprotein 3 (MDR3) are linked to impaired bile salt homeostasis and lead to progressive familial intrahepatic cholestasis (PFIC)-2 and -3, respectively. The regulation of bile salt transporters in PFIC is not known. Expression of hepatobiliary transporters in livers of ten patients with a PFIC phenotype was studied by quantitative reverse transcription polymerase chain reaction, Western blotting, and immunofluorescence microscopy. PFIC was diagnosed by clinical and laboratory findings. All patients could be assigned to PFIC-2 or PFIC-3 by the use of BSEP- and MDR3-specific antibodies and by MDR3 gene-sequencing. Whereas in all PFIC-2 patients, BSEP immunoreactivity was absent from the canalicular membrane, in three PFIC-3 livers, canalicular MDR3 immunoreactivity was detectable. Serum bile salts were elevated to 276 +/- 233 and to 221 +/- 109 micromol/L in PFIC-2 and PFIC-3, respectively. Organic anion transporting polypeptide OATP1B1, OATP1B3, and MRP2 mRNA and protein levels were reduced, whereas sodium taurocholate cotransporting polypeptide (NTCP) was only reduced at the protein level, suggesting a posttranscriptional NTCP regulation. Whereas MRP3 mRNA and protein were not significantly altered, MRP4 messenger RNA and protein were significantly increased in PFIC. In conclusion, PFIC-2 may be reliably diagnosed by immunofluorescence, whereas the diagnosis of PFIC-3 requires gene-sequencing. Several mechanisms may contribute to elevated plasma bile salts in PFIC: reduced bile salt uptake via NTCP, OATP1B1, and OATP1B3, decreased BSEP-dependent secretion into bile, and increased transport back into plasma by MRP4. Upregulation of MRP4, but not of MRP3, might represent an important escape mechanism for bile salt extrusion in PFIC.

Bile acids induce hepatic stellate cell proliferation via activation of the epidermal growth factor receptor

BACKGROUND & AIMS

Hepatic stellate cell (HSC) proliferation is a key event in the development of liver fibrosis. In many liver diseases, HSCs are exposed to inflammatory cytokines, reactive oxygen species, and bile acids. Although inflammatory cytokines and reactive oxygen species are known to promote proliferation of HSCs, nothing is known about the effects of bile acids on HSC proliferation or apoptosis. The aim of this study was to investigate the effects of bile acids on HSC proliferation.

METHODS

HSCs were exposed to bile acids with different hydrophobicity (5-200 micromol/L). HSC proliferation and cell cycle-related events were assessed by bromodeoxyuridine incorporation, cell counting and proliferating cell nuclear antigen and cyclin E expression, apoptosis by caspase-3 activity assay, immunocytochemistry for active caspase-3 and acridine orange staining, and activation of signal transduction pathways by Western blot using phospho-specific antibodies. Uptake of bile acids was investigated using fluorescent bile acids.

RESULTS

All bile acids, at concentrations >25 micromol/L, induce a 2.5- to 3-fold increase in HSC proliferation via activation of the epidermal growth factor receptor. Bile acid-induced proliferation is mediated by activation of a protein kinase C/extracellular signal-regulated kinase/p70S6K-dependent pathway. Bile acids did not induce apoptosis in HSCs. HSCs do not take up fluorescent bile acids and do not express the bile acid importer ntcp.

CONCLUSIONS

Bile acids at levels reached in cholestatic conditions are an independent profibrogenic factor. Bile acids induce HSC proliferation via the activation of the epidermal growth factor receptor, whereas HSCs are protected against bile acid-induced apoptosis by excluding bile acids.

Therapeutic effects and possible mechanisms of a snake venom preparation in the fibrotic rat liver

The effects of a Chinese snake venom preparation from Agkistrodon halys pallas, used for treatment of hepatic fibrosis/cirrhosis in China, was investigated in an in vivo rat model and using in situ hepatic perfusion. Four groups were used in the experiments: (i) healthy, (ii) healthy/venom-treated, (iii) carbon tetrachloride (CCl4)-treated, and (iv) CCl4/venom-treated. Treatment effects were assessed by determining hepatic histopathology, biochemistry and fibrosis index parameters, bile production, biliary taurocholate recovery, hepatic mRNA expression of four bile salt transporters (Ntcp, Bsep, Oatp-1, and Oatp-3), comparison of hepatic microcirculation, fibrinolytic activity, and antithrombotic effects. Liver histopathology, biochemistry, and fibrosis index showed a dramatic improvement in venom-treated animals. There were significant differences in bile production between healthy/venom-treated and all other experimental groups and between CCl4/venom-treated and CCl4-treated animals, but no significant differences were found between CCl4/venom-treated and healthy animals. Biliary taurocholate recovery was significantly increased in healthy/venom-treated and CCl4/venom-treated animals. The expression of mRNA levels of the four bile salt transporters showed an increase after venom treatment. The hepatic microcirculation studies showed normalized sinusoidal beds in CCl4/venom-treated animals compared to healthy animals, whereas CCl4-treated animals showed abnormal profiles to the healthy and the CCl4/AHPV-treated animals. The fibrinogen and plasma thromboxane B2 levels of healthy rats decreased with increasing dose after venom treatment. It was concluded that snake venom treatment may be therapeutic in treatment of hepatic fibrosis/cirrhosis by possibly a combination of increased bile flow and improved hepatic microcirculation, changes in bile salt transporter expression, and fibrinolytic and antithrombotic effects of the snake venom preparation.

Cytokine-independent repression of rodent Ntcp in obstructive cholestasis

Cholestatic liver injury is associated not only with accumulation of bile acids but also with activation of proinflammatory cytokines. Common bile duct ligation (CBDL) induces sustained downregulation of the Na(+)/taurocholate cotransporter (Ntcp) in rodent liver. Although repression of Ntcp during endotoxemia is cytokine mediated, it is unclear whether inflammatory cytokines contribute to this downregulation in obstructive cholestasis. Cytokine inactivation in CBDL rats and mice was either performed directly with tumor necrosis factor alpha (etanercept) or interleukin 1 beta inactivation (anakinra/AMG 719) or indirectly Kupffer cell depletion via intraperitoneal administration of liposome-encapsulated dichloromethylene bisphosphonate. Protein and messenger RNA (mRNA) expression of Ntcp and short heterodimer partner (SHP) were analyzed via Western and Northern blotting. Key regulators of Ntcp (hepatocyte nuclear factor 1 alpha [HNF-1alpha], HNF-4alpha, retinoid X receptor alpha [RXRalpha]:retinoic acid receptor alpha [RARalpha]) were studied via electrophoretic mobility shift analysis and nuclear Western blot analysis. Both methods of cytokine inactivation failed to maintain Ntcp protein or mRNA expression within 3 days after CBDL in either rats or mice (20%-40% of sham controls), while SHP mRNA expression increased three- to five-fold. Decreased nuclear HNF-1alpha and HNF-4alpha protein levels (45% and 60% of sham controls, respectively) and HNF-1alpha binding activity (32% of sham controls) were not restored during cytokine inactivation after CBDL, indicating cytokine-independent mechanisms of Ntcp regulation. RXRalpha:RARalpha binding remained unchanged in all experimental conditions. In conclusion, during obstructive cholestasis accumulating bile acids per se, without major contribution of cytokines, leads to downregulation of Ntcp via repression of HNF-1alpha and HNF-4alpha.

Biliary Transport Systems: Short‐Term Regulation

Bile secretion is conveyed by a large set of transporter proteins. Their activity is controlled on long- and short-term timescales. Short-term regulation of transcellular transport has to guarantee intra- and extracellular molecular homeostasis and has to meet the actual cellular metabolic needs. As transport activity depends not only on transporter expression, measurements of mRNA or protein levels will not fully predict functionality. Transporter activity is also determined by covalent modifications (e.g., phosphorylation), substrate competition, and subcellular transporter localization. The latter is a major target of short-term regulation of bile secretion and involves rapid endo- and exocytosis of transporter-bearing vesicles from and into the respective cell membrane. In liver parenchymal cells, several signaling pathways were identified that govern these processes; however, the underlying molecular mechanisms still need to be characterized. Different techniques have been employed in studies on transporter retrieval and insertion, which are discussed in this chapter.

Degradation of the sodium taurocholate cotransporting polypeptide (NTCP) by the ubiquitin-proteasome system

The sodium taurocholate cotransporting polypeptide (Ntcp, Slc10a1) is the major uptake system for bile acids into liver cells. This study investigated the degradation of rat Ntcp and human NTCP by the ubiquitin-proteasome system (UPS). In stably transfected HepG2 cells, rat Ntcp was complex-glycosylated and localized at the plasma membrane. Inhibition of proteasomes by MG-132 or lactacystin led to the accumulation of intracellular Ntcp, a process dependent on de novo protein synthesis. Intracellular Ntcp was core-glycosylated, indicating an endoplasmic reticulum (ER) origin. Core-glycosylated Ntcp was found in cytosolic, detergent-insoluble deposits with characteristics of aggresomes: they co-localized with ubiquitin at the microtubule organization center and Ntcp from these deposits was polyubiquitinated. Transient transfections of Ntcp/NTCP induced intracellular deposits that co-localized with ubiquitin, even in the absence of proteasome inhibitors. Similarly, in livers of patients with progressive familial intrahepatic cholestasis, NTCP could be detected co-localized with ubiquitin in hepatocytes. We conclude that maturing Ntcp/NTCP is degraded by the ubiquitin-proteasome system at the level of ER-associated degradation (ERAD). An imbalance in the synthesis and degradation of NTCP at the level of the ER or alterations in the ERAD machinery might be the cause of intracellular NTCP deposits in transient transfections and in cholestatic livers.

Vectorial transport of bile salts across MDCK cells expressing both rat Na+-taurocholate cotransporting polypeptide and rat bile salt export pump

Bile salts are predominantly taken up by hepatocytes via the basolateral Na(+)-taurocholate cotransporting polypeptide (NTCP/SLC10A1) and secreted into the bile by the bile salt export pump (BSEP/ABCB11). In the present study, we transfected rat Ntcp and rat Bsep into polarized Madin-Darby canine kidney cells and characterized the transport properties of these cells for eight bile salts. Immunohistochemical staining demonstrated that Ntcp was expressed at the basolateral domains, whereas Bsep was expressed at the apical domains. Basal-to-apical transport of taurocholate across the monolayer expressing only Ntcp and that coexpressing Ntcp/Bsep was observed, whereas the flux across the monolayer of control and Bsep-expressing cells was symmetrical. Basal-to-apical transport of taurocholate across Ntcp/Bsep-coexpressing monolayers was significantly higher than that across monolayers expressing only Ntcp. Kinetic analysis of this vectorial transport of taurocholate gave an apparent K(m) value of 13.9 +/- 4.7 microM for cells expressing Ntcp alone, which is comparable with 22.2 +/- 4.5 microM for cells expressing both Ntcp and Bsep and V(max) values of 15.8 +/- 4.2 and 60.8 +/- 9.0 pmol.min(-1).mg protein(-1) for Ntcp alone and Ntcp and Bsep-coexpressing cells, respectively. Transcellular transport of cholate, glycocholate, taurochenodeoxycholate, chenodeoxycholate, glycochenodeoxycholate, tauroursodeoxycholate, ursodeoxycholate, and glycoursodeoxycholate, but not that of lithocholate was also observed across the double transfectant. This double-expressing system can be used as a model to clarify vectorial transport of bile salts across hepatocytes under physiological conditions.

Kupffer cell depletion with liposomal clodronate prevents suppression of Ntcp expression in endotoxin-treated rats

BACKGROUND/AIMS

In sepsis-associated cholestasis, expression of many genes involved in bile acid transport, including Ntcp, is suppressed by cytokines. Kupffer cells (KC) are an important source of cytokines in sepsis. To assess the consequences of KC depletion on hepatic Ntcp expression in endotoxemic rats.

METHODS

Sprague-Dawley rats received liposomal clodronate (CLO) or vehicle (PBS) to deplete KC prior to lipopolysaccharide (LPS) exposure. Plasma and liver samples were taken 1 and 16 h after LPS exposure.

RESULTS

Complete CLO-depletion of KC by was demonstrated by immunohistochemistry. Hepatic gene expression of IL-1beta and TNFalpha as well as TNFalpha plasma levels in CLO/LPS-injected animals were significantly reduced to a mean of 41, 36 and 23% of controls injected with LPS only. Ntcp RNA- and protein expression was significantly higher whereas plasma bile salt concentration was lower in CLO/LPS animals vs. animals injected with LPS only. Binding activity of transcription factors RXR:RAR and HNF1alpha was decreased in LPS only controls but preserved in CLO/LPS treated animals.

CONCLUSIONS

Clodronate-depletion of KC blocks cytokine-mediated Ntcp suppression upon endotoxin exposure. KC may represent pharmacological targets for treatment of sepsis-associated cholestasis.

Thyroid hormone transporters

Thyroid hormone is essential for the development of the brain and the nervous system. Cellular entry is required for conversion of thyroid hormones by the intracellular deiodinases and for binding of T(3) to its nuclear receptors. Several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have so far identified two categories of transporters involved in thyroid hormone uptake (i.e., organic anion transporters and amino acid transporters). Among the organic anion transporters, both Na(+) taurocholate cotransporting polypeptide (NTCP) and various members of the organic anion transporting polypeptide (OATP) family mediate transport of iodothyronines. Because iodothyronines are a particular class of amino acids derived from tyrosine residues, it is no surprise that some amino acid transporters have been shown to be involved in thyroid hormone transport. We have characterized monocarboxylate transporter 8 (MCT8) as a very active and specific thyroid hormone transporter, the gene of which is located on the X chromosome. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. MCT8 shows 50% amino acid identity with a system T amino acid transporter 1 (TAT1). TAT1, also called MCT10, has been characterized to transport aromatic amino acids but no iodothyronines. We have also found that mutations in MCT8 are associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels in young boys.

Disrupted coordinate regulation of farnesoid X receptor target genes in a patient with cerebrotendinous xanthomatosis

Cerebrotendinous xanthomatosis (CTX), sterol 27-hydroxylase (CYP27A1) deficiency, is associated with markedly reduced chenodeoxycholic acid (CDCA), the most powerful activating ligand for farnesoid X receptor (FXR). We investigated the effects of reduced CDCA on FXR target genes in humans. Liver specimens from an untreated CTX patient and 10 control subjects were studied. In the patient, hepatic CDCA concentration was markedly reduced but the bile alcohol level exceeded CDCA levels in control subjects (73.5 vs. 37.8 +/- 6.2 nmol/g liver). Cholesterol 7alpha-hydroxylase (CYP7A1) and Na+/taurocholate-cotransporting polypeptide (NTCP) were upregulated 84- and 8-fold, respectively. However, small heterodimer partner (SHP) and bile salt export pump were normally expressed. Marked CYP7A1 induction with normal SHP expression was not explained by the regulation of liver X receptor alpha (LXRalpha) or pregnane X receptor. However, another nuclear receptor, hepatocyte nuclear factor 4alpha (HNF4alpha), was induced 2.9-fold in CTX, which was associated with enhanced mRNA levels of HNF4alpha target genes, CYP7A1, 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase, CYP27A1, and NTCP. In conclusion, the coordinate regulation of FXR target genes was lost in CTX. The mechanism of the disruption may be explained by a normally stimulated FXR pathway attributable to markedly increased bile alcohols with activation of HNF4alpha caused by reduced bile acids in CTX liver.

Multihormonal regulation of hepatic sinusoidal Ntcp gene expression

Bile acids are efficiently removed from sinusoidal blood by a number of transporters including the Na+-taurocholate-cotransporting polypeptide (Ntcp). Na+-dependent bile salt uptake, as well as Ntcp, are expressed twofold higher in male compared with female rat livers. Also, estrogen administration to male rats decreases Ntcp expression. The aims of this study were to determine the hormonal mechanism(s) responsible for this sexually dimorphic expression of Ntcp. We examined castrated and hypophysectomized rats of both sexes. Sex steroid hormones, growth hormone, thyroid, and glucocorticoids were administered, and livers were examined for changes in Ntcp messenger RNA (mRNA). Ntcp mRNA and protein content were selectively increased in males. Estradiol selectively decreased Ntcp expression in males, whereas ovariectomy increased Ntcp in females, confirming the importance of estrogens in regulating Ntcp. Hypophysectomy decreased Ntcp mRNA levels in males and prevented estrogen administration from decreasing Ntcp, indicating the importance of pituitary hormones. Although constant infusion of growth hormone to intact males reduced Ntcp, its replacement alone after hypophysectomy did not restore the sex differences. In contrast, thyroid hormone and corticosterone increased Ntcp mRNA in hypophysectomized rats. Sex differences in Ntcp mRNA levels were produced only when the female pattern of growth hormone was administered to animals also receiving thyroid and corticosterone. Thyroid and dexamethasone also increased Ntcp mRNA in isolated rat hepatocytes, whereas growth hormone decreased Ntcp. These findings demonstrate the essential role that pituitary hormones play in the sexually dimorphic control of Ntcp expression in adult rat liver and in the mediation of estrogen effects.

Small hepatocytes in culture develop polarized transporter expression and differentiation

Rat small hepatocytes have been shown to proliferate in culture and to form organoids with differentiated hepatocytes in vitro. To evaluate the degree of polarized transporter differentiation of rat small hepatocytes during 9 weeks of culturing, we studied the time-dependent expression and subcellular localization of the major bile salt and organic anion transport systems of hepatocytes [i.e. the basolateral sodium-taurocholate co-transporting protein (Ntcp), organic-anion-transporting polypeptide 1b2 (Oatp1b2), the canalicular bile-salt export pump (Bsep) and multidrug-resistance-associated protein 2 (Mrp2)]. Small hepatocytes proliferated and differentiated in culture and formed sharply demarcated colonies as assessed by morphology, α-fetoprotein, albumin and Mrp1 expression. Polarized surface transporter expression was evident after 5 weeks of culturing for Ntcp, Oatp1b2 and Mrp2, and after 7 weeks for Bsep. After 9 weeks in culture, the vast majority of matured hepatocytes expressed Ntcp/Oatp1b2 at the basolateral and Bsep/Mrp2 at the canalicular plasma-membrane domains. This polarized transporter expression was accompanied by canalicular secretion of fluorescein-diacetate and cholylglycyl-fluorescein. Furthermore, an anastomizing three-dimensional network of bile canaliculi developed within piling-up colonies. These data demonstrate that cultured rat small hepatocytes acquire a fully differentiated transporter expression phenotype during their development into hepatic `organoid-like' clusters of mature hepatocytes. Thereby, the time-dependent sequence of transporter expression mirrored the ontogenesis of transporter expression in developing rat liver, supporting the concept that small hepatocytes correspond to the hepatocyte lineage derived from embryonic hepatoblasts and/or from a different pool of `committed hepatocyte progenitor cells'.

Differential expression of bile salt and organic anion transporters in developing rat liver

BACKGROUND/AIMS

Differentiated hepatocytes express distinct transport systems at their basolateral and canalicular membrane domains. Here, we investigated the ontogenesis of the polar expression of hepatocellular organic anion and bile salt transport systems in rat liver.

METHODS

mRNA levels (real time PCR) and protein expression (immunofluorescence microscopy) were investigated for the Na(+)-taurocholate cotransport protein (Ntcp), the organic anion transporting polypeptides (Oatp1a1, Oatp1a4, Oatp1b2), the multidrug resistance associated proteins (Mrp2, Mrp6) and the bile salt export pump (Bsep).

RESULTS

Expression of mRNA and protein was detected first for Oatp1b2, Mrp2 and Mrp6 at embryonic day 16 (E16), followed by Ntcp, Oatp1a1 and Bsep at E20 and by Oatp1a4 at postnatal day 5 (P5). Intracellular localization of Oatps (e.g. Oatp1b2) preceded expression at the plasma membrane. Approximate adult phenotypes of polarized expression were achieved for Ntcp by P5, for Bsep, Mrp2 and Mrp6 by P12 and for Oatp1a1, Oatp1a4 and Oatp1b2 by P29.

CONCLUSIONS

The data demonstrate that full maturation of polarized transporter expression in rat liver requires several weeks. The findings provide a molecular explanation for the previously observed chronology of the functional maturation of bile salt-independent and dependent bile formation and of hepatic detoxification functions in developing rat liver.

Tauroursodeoxycholic acid protects rat hepatocytes from bile acid-induced apoptosis via activation of survival pathways

Ursodeoxycholic acid (UDCA) is used in the treatment of cholestatic liver diseases, but its mechanism of action is not yet well defined. The aim of this study was to explore the protective mechanisms of the taurine-conjugate of UDCA (tauroursodeoxycholic acid [TUDCA]) against glycochenodeoxycholic acid (GCDCA)-induced apoptosis in primary cultures of rat hepatocytes. Hepatocytes were exposed to GCDCA, TUDCA, the glyco-conjugate of UDCA (GUDCA), and TCDCA. The phosphatidylinositol-3 kinase pathway (PI3K) and nuclear factor-kappaB were inhibited using LY 294002 and adenoviral overexpression of dominant-negative IkappaB, respectively. The role of p38 and extracellular signal-regulated protein kinase mitogen-activated protein kinase (MAPK) pathways were investigated using the inhibitors SB 203580 and U0 126 and Western blot analysis. Transcription was blocked by actinomycin-D. Apoptosis was determined by measuring caspase-3, -9, and -8 activity using fluorimetric enzyme detection, Western blot analysis, immunocytochemistry, and nuclear morphological analysis. Our results demonstrated that uptake of GCDCA is needed for apoptosis induction. TUDCA, but not TCDCA and GUDCA, rapidly inhibited, but did not delay, apoptosis at all time points tested. However, the protective effect of TUDCA was independent of its inhibition of caspase-8. Up to 6 hours of preincubation with TUDCA before addition of GCDCA clearly decreased GCDCA-induced apoptosis. At up to 1.5 hours after exposure with GCDCA, the addition of TUDCA was still protective. This protection was dependent on activation of p38, ERK MAPK, and PI3K pathways, but independent of competition on the cell membrane, NF-kappaB activation, and transcription. In conclusion, TUDCA contributes to the protection against GCDCA-induced mitochondria-controlled apoptosis by activating survival pathways.

Role of liver-enriched transcription factors and nuclear receptors in regulating the human, mouse, and rat NTCP gene

Hepatic uptake of bile acids is mediated by the Na(+)-taurocholate cotransporting polypeptide (NTCP; SLC10A1) of the basolateral hepatocyte membrane. Several cis-acting elements in the rat Ntcp gene promoter have been characterized. However, little is known about the mechanisms that control the expression of the human or mouse NTCP/Ntcp. We, therefore, compared the transcriptional regulation of the human and mouse NTCP/Ntcp gene with that of the rat. By computer alignment, a sequence in the 5'-regulatory region that is conserved between species was identified near the transcription start site. Huh7 cells were transfected with luciferase constructs containing the conserved region from each species. The hepatocyte nuclear factors (HNF)1alpha and -4alpha and the retinoid X receptor/retinoic acid receptor dimer (RXRalpha/RARalpha) bound and transactivated the rat but not the human or mouse NTCP/Ntcp promoters. In contrast, activation by the CCAAT/enhancer binding protein-beta was specific for human and mouse NTCP/Ntcp. The only consensus motif present in all three species was HNF3beta. HNF3beta formed a specific DNA-protein complex in electrophoretic mobility shift assays and inhibited NTCP/Ntcp promoter activity in cotransfection assays. Finally, a minor repressive effect of bile acids was only found for rat Ntcp. The transcriptional repressor small heterodimer partner (SHP) did not affect NTCP/Ntcp promoter activity. We conclude that 1) the transcriptional regulation of the conserved NTCP/Ntcp 5'-regulatory region differs considerably among human, mouse, and rat; and 2) the conserved NTCP/Ntcp regulatory region is not directly regulated by SHP. Bile acids may regulate NTCP/Ntcp indirectly by modulating the capacity of nuclear factors to activate gene expression.

Estradiol represses prolactin-induced expression of Na+/taurocholate cotransporting polypeptide in liver cells through estrogen receptor-alpha and signal transducers and activators of transcription 5a

Na(+)/taurocholate cotransporting polypeptide (ntcp) mediates the uptake of bile salts from plasma across the basolateral domain of the hepatocyte. We have demonstrated that ntcp expression can be induced by prolactin (PRL) and placental lactogen via the PRL receptor and signal transducers and activators of transcription (Stat)5a pathway. However, elevated levels of placental lactogen do not increase the expression of ntcp in pregnant rats. Because plasma estradiol (E(2)) levels are also elevated in pregnancy, we investigated the inhibitory effects of E(2) on PRL-induced ntcp activation. E(2) treatment inhibited the PRL-induced increase in liver ntcp mRNA to the same levels as in rats treated with E(2) alone. Estrogen receptor-alpha (ERalpha) mRNA and protein expression in liver were increased 2.6-fold and 2.2-fold, respectively, in pregnancy relative to controls. In HepG2 cells, E(2) repressed PRL-induced ntcp reporter gene expression in a dose-dependent manner in the presence of cotransfected ERalpha. The ERalpha antagonist ICI 182,780 reversed E(2)-induced repression, indicating specificity of inhibition by E(2). Overexpression of coactivator p300 did not reverse the inhibitory effects of E(2) and ERalpha. Western and gel shift analysis revealed that E(2)-bound ERalpha decreased the tyrosine phosphorylation and DNA-binding activity of Stat5a, indicating that the inhibitory effect of E(2) was mediated, at least in part, by interfering with PRL-mediated signal transduction. The present studies demonstrate the physiological significance of cross-talk between ERalpha and Stat5a in liver, in which both proteins are expressed. These data also establish a novel mechanism by which expression of ntcp, an important hepatic bile acid transporter, can be regulated by multiple hormones.

Progressive familial intrahepatic cholestasis, type 1, is associated with decreased farnesoid X receptor activity

BACKGROUND & AIMS

The mechanisms by which mutations in the familial intrahepatic cholestasis-1 gene cause Byler's disease (progressive familial intrahepatic cholestasis type 1) are unknown.

METHODS

Interactions among the apical sodium-dependent bile acid transporter, the farnesoid X receptor (FXR), and familial intrahepatic cholestasis-1 were studied in the ileum of children with progressive familial intrahepatic cholestasis type 1 and in Caco-2 cells.

RESULTS

Increased ileal apical sodium-dependent bile acid transporter messenger RNA (mRNA) expression was detected in 3 patients with progressive familial intrahepatic cholestasis type 1. Paradoxically, ileal lipid-binding protein mRNA expression was repressed, suggesting a central defect in bile acid response. Ileal FXR and short heterodimer partner mRNA levels were reduced in the same 3 patients. In Caco-2 cells, antisense-mediated knock-down of endogenous familial intrahepatic cholestasis-1 led to up-regulation of apical sodium-dependent bile acid transporter and down-regulation of FXR, ileal lipid-binding protein, and short heterodimer partner mRNA. In familial intrahepatic cholestasis-1-negative Caco-2 cells, the activity of the human apical sodium-dependent bile acid transporter promoter was enhanced, whereas the human FXR and bile salt excretory pump promoters' activities were reduced. Overexpression of short heterodimer partner but not of the FXR abrogated the effect of familial intrahepatic cholestasis-1 antisense oligonucleotides. FXR cis-element binding and FXR protein were reduced primarily in nuclear but not cytoplasmic extracts from familial intrahepatic cholestasis-1-negative Caco-2 cells.

CONCLUSIONS

Loss of familial intrahepatic cholestasis-1 leads to diminished nuclear translocation of the FXR, with the subsequent potential for pathologic alterations in intestinal and hepatic bile acid transporter expression. Marked hypercholanemia and cholestasis are predicted to develop, presumably because of both enhanced ileal uptake of bile salts via up-regulation of the apical sodium-dependent bile acid transporter and diminished canalicular secretion of bile salts secondary to down-regulation of the bile salt excretory pump.

Sequence and structural analysis of cellular retinoic acid-binding proteins reveals a network of conserved hydrophobic interactions

Proteins in the intracellular lipid-binding protein (iLBP) family show remarkably high structural conservation despite their low-sequence identity. A multiple-sequence alignment using 52 sequences of iLBP family members revealed 15 fully conserved positions, with a disproportionately high number of these (n=7) located in the relatively small helical region. The conserved positions displayed high structural conservation based on comparisons of known iLBP crystal structures. It is striking that the beta-sheet domain had few conserved positions, despite its high structural conservation. This observation prompted us to analyze pair-wise interactions within the beta-sheet region to ask whether structural information was encoded in interacting amino acid pairs. We conducted this analysis on the iLBP family member, cellular retinoic acid-binding protein I (CRABP I), whose folding mechanism is under study in our laboratory. Indeed, an analysis based on a simple classification of hydrophobic and polar amino acids revealed a network of conserved interactions in CRABP I that cluster spatially, suggesting a possible nucleation site for folding. Significantly, a small number of residues participated in multiple conserved interactions, suggesting a key role for these sites in the structure and folding of CRABP I. The results presented here correlate well with available experimental evidence on folding of CRABPs and their family members and suggest future experiments. The analysis also shows the usefulness of considering pair-wise conservation based on a simple classification of amino acids, in analyzing sequences and structures to find common core regions among homologues.

Regulation of renal tubular bile acid transport in the early phase of an obstructive cholestasis in the rat

BACKGROUND/AIMS

In obstructive liver diseases, urinary excretion of bile acids is markedly enhanced. The mechanism of this effect is not entirely clear. The aim of the present study was to assess the glomerular and tubular factors involved in the renal handling of bile acids during the early phase of an obstructive cholestasis induced by a 24-hour bile duct ligation in rats.

METHODS

In addition to conventional clearance experiments, taurocholate transport was studied on proximal tubular cells freshly isolated from rat kidneys. The expression of the taurocholate transport protein was determined in these cells by immunoblotting.

RESULTS

Glomerular filtration rate and arterial blood pressure were not significantly affected by the cholestasis induced. The (3)H-taurocholate and (3)H-cholate clearances significantly increased whereas the fractional tubular reabsorption rates of these bile acids significantly fell. Probenecid did not affect the (3)H-taurocholate and (3)H-cholate clearances. By employing S0960, a specific inhibitor of the sodium-dependent bile salt transporter ASBT, it could be shown that this transporter mediates (3)H-taurocholate uptake into the proximal tubular cells. The bile duct ligation caused a significant decrease of the V(max) value of the taurocholate transporter indicating a downregulation of this transporter in cholestasis. The downregulation occurred without a change of the ASBT protein content of the proximal tubular cells.

CONCLUSION

The results demonstrate that there is a functional adaptive downregulation of renal tubular bile acid transport enhancing renal clearance of bile acids during the early phase of an obstructive cholestasis.

Fluorescent dyes as probes to study lipid-binding proteins

We studied the equilibrium binding of two hydrophobic fluorescent dyes, ANS and bisANS, to four members of a family of intracellular lipid-binding proteins: IFABP, CRABP I, CRABP II, and ILBP. The spectral and binding parameters for the probes bound to the proteins were determined. Typically, there was a single binding site on each protein for the ligands. However, IFABP cooperatively bound a second bisANS molecule in the binding pocket. Comparative analysis of affinities and spectral characteristics for the two probes allowed us to examine the contributions of electrostatic and hydrophobic interactions to the binding process, and to address some aspects of the internal structure of the studied proteins.

Human ileal bile acid transporter gene ASBT (SLC10A2) is transactivated by the glucocorticoid receptor

BACKGROUND

Patients with Crohn's disease suffer from intestinal bile acid malabsorption. Intestinal bile acid absorption is mediated by the apical sodium dependent bile acid transporter ASBT/IBAT (SLC10A2). In rats, ASBT is induced by glucocorticoids.

AIMS

To study whether human ASBT is activated by glucocorticoids and to elucidate the mechanism of regulation.

PATIENTS AND METHODS

ASBT expression in ileal biopsies from patients with Crohn's disease and from healthy subjects was quantified by western blot. ASBT promoter function was studied in luciferase assays and by electrophoretic mobility shift assay.

RESULTS

In 16 patients with Crohn's disease, ASBT expression was reduced to 69 (7.5)% compared with healthy controls (mean (SEM); p = 0.01). In 10 healthy male volunteers, ASBT protein expression was increased 1.34 (0.11)-fold (mean (SEM); p<0.05) after 21 days' intake of budesonide (9 mg/day) whereas expression of the peptide transporter 1 was unaffected. Reporter constructs of the human ASBT promoter were activated 15-20-fold by coexpression of the glucocorticoid receptor (GR) and exposure to the GR ligands dexamethasone or budesonide. Two glucocorticoid response elements in the ASBT promoter, arranged as inverted hexanucleotide repeats (IR3 elements), conferred inducibility by GR and dexamethasone in a heterologous promoter context and were shown to bind GR in mobility shift assays.

CONCLUSIONS

Human ASBT is induced by glucocorticoids in vitro and in vivo. Induction of ASBT by glucocorticoids could be beneficial in patients with Crohn's disease who exhibit reduced ASBT expression. This study identifies ASBT as a novel target of glucocorticoid controlled gene regulation in the human intestine.

Ethnicity-dependent polymorphism in Na+-taurocholate cotransporting polypeptide (SLC10A1) reveals a domain critical for bile acid substrate recognition

The key transporter responsible for hepatic uptake of bile acids from portal circulation is Na+-taurocholate cotransporting polypeptide (NTCP, SLC10A1). This transporter is thought to be critical for the maintenance of enterohepatic recirculation of bile acids and hepatocyte function. Therefore, functionally relevant polymorphisms in this transporter would be predicted to have an important impact on bile acid homeostasis/liver function. However, little is known regarding genetic heterogeneity in NTCP. In this study, we demonstrate the presence of multiple single nucleotide polymorphisms in NTCP in populations of European, African, Chinese, and Hispanic Americans. Specifically four nonsynonymous single nucleotide polymorphisms associated with a significant loss of transport function were identified. Cell surface biotinylation experiments indicated that the altered transport activity of T668C (Ile223-->Thr), a variant seen only in African Americans, was due at least in part to decreased plasma membrane expression. Similar expression patterns were observed when the variant alleles were expressed in HepG2 cells, and plasma membrane expression was assessed using immunofluorescence confocal microscopy. Interestingly the C800T (Ser267-->Phe) variant, seen only in Chinese Americans, exhibited a near complete loss of function for bile acid uptake yet fully normal transport function for the non-bile acid substrate estrone sulfate, suggesting this position may be part of a region in the transporter critical and specific for bile acid substrate recognition. Accordingly, our study indicates functionally important polymorphisms in NTCP exist and that the likelihood of being carriers of such polymorphisms is dependent on ethnicity.

Unique inhibition of bile salt-induced apoptosis by lecithins and cytoprotective bile salts in immortalized mouse cholangiocytes

Bile duct epithelium is physiologically exposed to high concentrations of bile salts, suggesting the presence of a cytoprotective mechanism(s). The aim of this study was to clarify whether bile salts cause bile duct cell damage and to elucidate the mechanism(s) providing protection against such an action of bile salts. Immortalized mouse cholangiocytes were incubated with taurocholate, taurochenodeoxycholate, glycochenodeoxycholate (GCDC), taurodeoxycholate, and tauroursodeoxycholate (TUDC), followed by flow-cytometric analysis and caspase activity assay to evaluate the induction of apoptosis. GCDC time-dependently induced caspase 3 (3.4-fold)- and caspase 9 (1.4-fold)-mediated apoptosis of cholangiocytes, but this was inhibited by lecithins and TUDC. Further, expression of cholangiocyte bile salt transporters (apical sodium-dependent bile salt transporter [Asbt] and multidrug resistance protein 3 [Mrp3]) was examined by RT-PCR and western blotting, and cholangiocyte bile salt uptake was determined using radiolabeled bile salts. Expression of cholangiocyte Asbt and Mrp3 was increased by bile salts, whereas lecithins interestingly reduced bile salt uptake to inhibit cholangiocyte apoptosis. In conclusion, bile salts themselves cause cholangiocyte apoptosis when absorbed by and retained inside the cell, but this is inhibited by washing out cytotoxic bile salts according to Mrp3, a rescue exporting molecule. Biliary lecithin is seemingly another cytoprotective player against cytotoxic bile salts, reducing their uptake, and this is associated with a reduced expression of Mrp3.

Inhibition of apolipoprotein B secretion by taurocholate is controlled by the N-terminal end of the protein in rat hepatoma McArdle-RH7777 cells

Bile salts (BS) inhibit the secretion of apolipoprotein B (apoB) and triacylglycerol (TG) in primary rat, mouse and human hepatocytes and in mice in vivo. We investigated whether lipidation of apoB into a lipoprotein particle is required for this inhibitory action of BS. The sodium/taurocholate co-transporting polypeptide (Ntcp) was co-expressed in McArdle-RH7777 (McA-RH7777) cells stably expressing the full-length human apoB100 (h-apoB100, secreted as TG-rich lipoprotein particles) or carboxyl-truncated human apoB18 (h-apoB18, secreted in lipid-free form). The doubly transfected cell lines (h-apoB/r-Ntcp) effectively accumulated taurocholic acid (TC). TC incubation decreased the secretion of endogenous rat apoB100 (-50%) and h-apoB18 (-35%), but did not affect secretion of rat apoA-I. Pulse-chase experiments (35S-methionine) indicated that the impaired secretion of radiolabeled h-apoB18 and h-apoB100 was associated with accelerated intracellular degradation. The calpain protease inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) partially inhibited intracellular apoB degradation but did not affect the amount of either h-apoB18 or h-apoB100 secreted into the medium, indicating that inhibition of apoB secretion by TC is not due to calpain-dependent proteasomal degradation. We conclude that TC does not inhibit apoB secretion by interference with its lipidation, but rather involves a mechanism dependent on the N-terminal end of apoB.

Hepatoprotection by the farnesoid X receptor agonist GW4064 in rat models of intra- and extrahepatic cholestasis

Farnesoid X receptor (FXR) is a bile acid-activated transcription factor that is a member of the nuclear hormone receptor superfamily. Fxr-null mice exhibit a phenotype similar to Byler disease, an inherited cholestatic liver disorder. In the liver, activation of FXR induces transcription of transporter genes involved in promoting bile acid clearance and represses genes involved in bile acid biosynthesis. We investigated whether the synthetic FXR agonist GW4064 could protect against cholestatic liver damage in rat models of extrahepatic and intrahepatic cholestasis. In the bile duct-ligation and alpha-naphthylisothiocyanate models of cholestasis, GW4064 treatment resulted in significant reductions in serum alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase, as well as other markers of liver damage. Rats that received GW4064 treatment also had decreased incidence and extent of necrosis, decreased inflammatory cell infiltration, and decreased bile duct proliferation. Analysis of gene expression in livers from GW4064-treated cholestatic rats revealed decreased expression of bile acid biosynthetic genes and increased expression of genes involved in bile acid transport, including the phospholipid flippase MDR2. The hepatoprotection seen in these animal models by the synthetic FXR agonist suggests FXR agonists may be useful in the treatment of cholestatic liver disease.

Substrate specificities of rat oatp1 and ntcp: implications for hepatic organic anion uptake

Transport of a series of 3H-radiolabeled C23, C24, and C27 bile acid derivatives was compared and contrasted in HeLa cell lines stably transfected with rat Na+/taurocholate cotransporting polypeptide (ntcp) or organic anion transporting polypeptide 1 (oatp1) in which expression was under regulation of a zinc-inducible promoter. Similar uptake patterns were observed for both ntcp and oatp1, except that unconjugated hyodeoxycholate was a substrate of oatp1 but not ntcp. Conjugated bile acids were transported better than nonconjugated bile acids, and the configuration of the hydroxyl groups (alpha or beta) had little influence on uptake. Although cholic and 23 norcholic acids were transported by ntcp and oatp1, other unconjugated bile acids (chenodeoxycholic, ursodeoxycholic) were not. In contrast to ntcp, oatp1-mediated uptake of the trihydroxy bile acids taurocholate and glycocholate was four- to eightfold below that of the corresponding dihydroxy conjugates. Ntcp mediated high affinity, sodium-dependent transport of [35S]sulfobromophthalein with a Km similar to that of oatp1-mediated transport of [35S]sulfobromophthalein (Km = 3.7 vs. 3.3 muM, respectively). In addition, for both transporters, uptake of sulfobromophthalein and taurocholic acid showed mutual competitive inhibition. These results indicate that the substrate specificity of ntcp is considerably broader than previously suspected and caution the extrapolation of transport data obtained in vitro to physiological function in vivo.

Role of passive and adaptive immunity in influencing enterocyte-specific gene expression

Numerous genes expressed by intestinal epithelial cells are developmentally regulated, and the influence that adaptive (AI) and passive (PI) immunity have in controlling their expression has not been evaluated. In this study, we tested the hypothesis that both PI and AI influenced enterocyte gene expression by developing a breeding scheme that used T and B cell-deficient recombination-activating gene (RAG) mice. RNA was isolated from the liver and proximal/distal small intestine at various ages, and the steady-state levels of six different transcripts were evaluated by RNase protection assay. In wild-type (WT) pups, all transcripts [Fc receptor of the neonate (FcRn), polymeric IgA receptor (pIgR), GLUT5, lactase-phlorizin hydrolase (lactase), apical sodium-dependent bile acid transporter (ASBT), and Na+/glucose cotransporter (SGLT1)] studied were developmentally regulated at the time of weaning, and all transcripts except ASBT had the highest levels of expression in the proximal small intestine. In WT suckling pups reared in the absence of PI, pIgR mRNA levels were increased 100% during the early phase of development. In mice lacking AI, the expression of pIgR and lactase were significantly attenuated, whereas FcRn and GLUT5 levels were higher compared with WT mice. Finally, in the absence of both passive and active immunity, expression levels of pIgR and lactase were significantly lower than similarly aged WT mice. In summary, we report that the adaptive and passive immune status of mice influences steady-state mRNA levels of several important, developmentally regulated enterocyte genes during the suckling and weaning periods of life.

Targeted deletion of the ileal bile acid transporter eliminates enterohepatic cycling of bile acids in mice

The ileal apical sodium bile acid cotransporter participates in the enterohepatic circulation of bile acids. In patients with primary bile acid malabsorption, mutations in the ileal bile acid transporter gene (Slc10a2) lead to congenital diarrhea, steatorrhea, and reduced plasma cholesterol levels. To elucidate the quantitative role of Slc10a2 in intestinal bile acid absorption, the Slc10a2 gene was disrupted by homologous recombination in mice. Animals heterozygous (Slc10a2+/-) and homozygous (Slc10a2-/-) for this mutation were physically indistinguishable from wild type mice. In the Slc10a2-/- mice, fecal bile acid excretion was elevated 10- to 20-fold and was not further increased by feeding a bile acid binding resin. Despite increased bile acid synthesis, the bile acid pool size was decreased by 80% and selectively enriched in cholic acid in the Slc10a2-/- mice. On a low fat diet, the Slc10a2-/- mice did not have steatorrhea. Fecal neutral sterol excretion was increased only 3-fold, and intestinal cholesterol absorption was reduced only 20%, indicating that the smaller cholic acid-enriched bile acid pool was sufficient to facilitate intestinal lipid absorption. Liver cholesteryl ester content was reduced by 50% in Slc10a2-/- mice, and unexpectedly plasma high density lipoprotein cholesterol levels were slightly elevated. These data indicate that Slc10a2 is essential for efficient intestinal absorption of bile acids and that alternative absorptive mechanisms are unable to compensate for loss of Slc10a2 function.

Growth hormone modulation of the rat hepatic bile transporter system in endotoxin-induced cholestasis

Treatment with high dose human GH, although an effective anabolic agent, has been associated with increased incidence of sepsis, inflammation, multiple organ failure, and death in critically ill patients. We hypothesized that GH might increase mortality by exacerbating cholestasis through modulation of bile acid transporter expression. High dose GH was continuously infused over 4 d into rats, and on the final day lipopolysaccharides were injected. Hepatic bile acid transporter expression was measured by Northern analysis and immunoblotting and compared with serum markers of cholestasis and endotoxinemia. Compared with non-GH-treated controls, GH increased endotoxin-induced markers of cholestasis and liver damage as well as augmented IL-6 induction. In endotoxinemia, GH treatment significantly induced multidrug resistance-associated protein 1 mRNA and protein and suppressed organic anion transporting polypeptides, Oatp1 and Oatp4, mRNA, suggesting impaired uptake of bilirubin and bile acids at the basolateral surface of the hepatocyte, which could contribute to the observed worsening of cholestasis by GH. This study of endotoxinemia may thus provide a mechanistic link between GH treatment and exacerbation of cholestasis through modulation of basolateral bile acid transporter expression in the rat hepatocyte.

Effects of proinflammatory cytokines on rat organic anion transporters during toxic liver injury and cholestasis

Hepatobiliary transporters are down-regulated in toxic and cholestatic liver injury. Cytokines such as tumor necrosis factor alpha (TNF-alpha) and interleukin 1 beta (IL-1 beta) are attributed to mediate this regulation, but their particular contribution in vivo is still unknown. Thus, we studied the molecular mechanisms by which Ntcp, Oatp1, Oatp2, and Mrp2 are regulated by proinflammatory cytokines during liver injury. Rats were injected intraperitoneally with either carbon tetrachloride or endotoxin. Inactivation of TNF-alpha and IL-1 beta was achieved by repetitive intraperitoneal injection of etanercept and anakinra, respectively. Messenger RNA (mRNA) levels of transporters and binding activities as well as nuclear protein levels of Ntcp, Oatp2, and Mrp2 transactivators were determined 20 to 24 hours later. In contrast to IL-1 beta, TNF-alpha inactivation alone fully prevented down-regulation of Ntcp, Oatp1, and Oatp2 mRNA as well as reduced binding activity of hepatocyte nuclear factor 1 (HNF-1) in CCl(4)-induced toxic injury. In endotoxemia, down-regulation of Mrp2, and partially in case of Ntcp, could be prevented by IL-1 beta but not TNF-alpha blockade. However, inactivation of either cytokine led to preservation of HNF1 and partially of retinoid X receptor/retinoic acid receptor (RXR/RAR) binding activity. No effect of anticytokines was seen on pregnane X receptor (PXR) and constitutive androstane receptor (CAR) binding activity as well as nuclear protein mass. In conclusion, TNF-alpha represents the master cytokine responsible for HNF1-dependent down-regulation of Ntcp, Oatp1, and Oatp2 in CCl(4)-induced toxic liver injury. IL-1 beta predominates in a complex signaling network of Ntcp and Mrp2 regulation in cholestatic liver injury. In contrast to in vitro studies, HNF1 and RXR/RAR-independent mechanisms appear to be more important in regulation of Mrp2 and Ntcp gene expression in endotoxemia.

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

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

Intestinal resection- and steroid-associated alterations in gene expression were not accompanied by changes in lipid uptake

BACKGROUND/AIMS

Glucocorticosteroids alter the morphology and transport function of the intestine of adult rats. This study was undertaken to assess the possible effect on intestinal lipid uptake of the locally acting steroid budesonide, or the systemically active prednisone or dexamethasone.

METHODS

Sprague-Dawley rats underwent intestinal transection or 50% intestinal resection. Budesonide, prednisone, dexamethasone, or control vehicle was given for 2 weeks from the time of surgery. Uptake was measured using ring uptake technique.

RESULTS

Resection had no effect on the mRNA expression for the early response genes, for proglucagon, or for the ileal lipid binding protein (ILBP), but was associated with reduced jejunal ornithine decarboxylase (ODC) mRNA and with reduced jejunal mRNA for the liver fatty acid binding protein (L-FABP). All three steroids reduced jejunal mRNA for proglucagon and c-jun, and did not affect the mRNA for L-FABP or for ILBP. These resection- and steroid-associated changes in gene expression were not associated with alterations in the intestinal uptake of long chain fatty acids or cholesterol.

CONCLUSIONS

The resection-associated alterations in the RNA expression of ODC and L-FABP and the steroid-associated changes in mRNA expression of c-jun and proglucagon were not accompanied by variations in lipid uptake.

Liver receptor homologue-1 mediates species- and cell line-specific bile acid-dependent negative feedback regulation of the apical sodium-dependent bile acid transporter

Intestinal reclamation of bile salts is mediated in large part by the apical sodium-dependent bile acid transporter (ASBT). The bile acid responsiveness of ASBT is controversial. Bile acid feeding in mice results in decreased expression of ASBT protein and mRNA. Mouse but not rat ASBT promoter activity was repressed in Caco-2, but not IEC-6, cells by chenodeoxycholic acid. A potential liver receptor homologue-1 (LRH-1) cis-acting element was identified in the bile acid-responsive region of the mouse but not rat promoter. The mouse, but not rat, promoter was activated by LRH-1, and this correlated with nuclear protein binding to the mouse but not rat LRH-1 element. The short heterodimer partner diminished the activity of the mouse promoter and could partially offset its activation by LRH-1. Interconversion of the potential LRH-1 cis-elements between the mouse and rat ASBT promoters was associated with an interconversion of LRH-1 and bile acid responsiveness. LRH-1 protein was found in Caco-2 cells and mouse ileum, but not IEC-6 cells or rat ileum. Bile acid response was mediated by the farnesoid X receptor, as shown by the fact that overexpression of a dominant-negative farnesoid X-receptor eliminated the bile acid mediated down-regulation of ASBT. In addition, ASBT expression in farnesoid X receptor null mice was unresponsive to bile acid feeding. In summary cell line- and species-specific negative feedback regulation of ASBT by bile acids is mediated by farnesoid X receptor via small heterodimer partner-dependent repression of LRH-1 activation of the ASBT promoter.

Identification of optimal harvest sites of ileal stem cells for treatment of bile acid malabsorption in a dog model

Ileal mucosal stem cells expressing the sodium-dependent ileal bile acid transporter (IBAT) have been successfully transplanted into the jejunum of rodents in projects aimed at creating a "neoileum" to treat bile acid malabsorption. To find optimal harvest sites for a dog model of stem cell transplantation, the exact location of peak IBAT expression in the donor ileum needs to be known. We therefore mapped IBAT function, IBAT mRNA, and IBAT protein in the ileum of Beagle dogs (N=3). Mucosal samples were taken every 5 cm in the ileum and every 20 cm in the jejunum of each dog. Sodium-dependent (active) and sodium-independent (passive) taurocholate uptake rates were measured using a standardized everted sleeve technique. IBAT mRNA concentrations were determined by semiquantitative reverse transcriptase-polymerase chain reaction and IBAT protein concentrations by fluorometric immunohistochemical analysis. The small bowel measured 208+/-17 cm (mean+/-standard error of the mean). Active and passive uptake rates were found to follow distinct distribution curves. Significant active uptake was seen only at the terminal 50 cm and peaked at 479+/-176 pM/mm(2). Depending on location, active uptake accounted for approximately half of the total uptake. IBAT mRNA and protein distributions corroborated uptake curves. The terminal 10 to 50 cm of ileum has the highest bile acid uptake capacity. This short segment appears to be the most promising donor site for ileal stem cell transplants to create a "neoileum" in dogs.

Inhibition of both the apical sodium-dependent bile acid transporter and HMG-CoA reductase markedly enhances the clearance of LDL apoB

Discovery of the ileal apical sodium-dependent bile acid transporter (ASBT) permitted development of specific inhibitors of bile acid reabsorption, potentially a new class of cholesterol-lowering agents. In the present study, we tested the hypothesis that combining the novel ASBT inhibitor, SC-435, with the HMG-CoA reductase inhibitor, atorvastatin, would potentiate reductions in LDL cholesterol (LDL-C) and LDL apolipoprotein B (apoB). ApoB kinetic studies were performed in miniature pigs fed a typical human diet and treated with the combination of SC-435 (5 mg/kg/day) plus atorvastatin (3 mg/kg/day) (SC-435+A) or a placebo. SC-435+A decreased plasma total cholesterol by 23% and LDL-C by 40%. Multicompartmental analysis (SAAM II) demonstrated that LDL apoB significantly decreased by 35% due primarily to a 45% increase in the LDL apoB fractional catabolic rate (FCR). SC-435+A significantly decreased hepatic concentrations of free cholesterol and cholesteryl ester, and increased hepatic LDL receptor mRNA consequent to increased cholesterol 7alpha-hydroxylase expression and activity. In comparison, SC-435 (10 mg/kg/day) monotherapy decreased LDL apoB by 10% due entirely to an 18% increase in LDL apoB FCR, whereas atorvastatin monotherapy (3 mg/kg/day) decreased LDL apoB by 30% due primarily to a 22% reduction in LDL apoB production. We conclude that SC-435+A potentiates the reduction of LDL-C and LDL apoB due to complementary mechanisms of action.