Topological photoaffinity labeling of the rabbit ileal Na+/bile-salt-cotransport system

Postnatal Deletion of Fat Storage-inducing Transmembrane Protein 2 (FIT2/FITM2) Causes Lethal Enteropathy

Lipid droplets (LDs) are phylogenetically conserved cytoplasmic organelles that store neutral lipids within a phospholipid monolayer. LDs compartmentalize lipids and may help to prevent cellular damage caused by their excess or bioactive forms. FIT2 is a ubiquitously expressed transmembrane endoplasmic reticulum (ER) membrane protein that has previously been implicated in LD formation in mammalian cells and tissue. Recent data indicate that FIT2 plays an essential role in fat storage in an in vivo constitutive adipose FIT2 knock-out mouse model, but the physiological effects of postnatal whole body FIT2 depletion have never been studied. Here, we show that tamoxifen-induced FIT2 deletion using a whole body ROSA26CreER(T2)-driven FIT2 knock-out (iF2KO) mouse model leads to lethal intestinal pathology, including villus blunting and death of intestinal crypts, and loss of lipid absorption. iF2KO mice lose weight and die within 2 weeks after the first tamoxifen dose. At the cellular level, LDs failed to form in iF2KO enterocytes after acute oil challenge and instead accumulated within the ER. Intestinal bile acid transporters were transcriptionally dysregulated in iF2KO mice, leading to the buildup of bile acids within enterocytes. These data support the conclusion that FIT2 plays an essential role in regulating intestinal health and survival postnatally.

The SLC10 carrier family: transport functions and molecular structure

The SLC10 family represents seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids. Although termed solute carriers (SLCs), only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na(+)-taurocholate cotransporting polypeptide (NTCP)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT)). The other members of the family are orphan carriers not all localized in the cell membrane. The name "bile acid transporter family" arose because the first two SLC10 members (NTCP and ASBT) are carriers for bile salts that establish their enterohepatic circulation. In recent years, information has been obtained on their 2D and 3D membrane topology, structure-transport relationships, and on the ligand and sodium-binding sites. For SLC10A2, the putative 3D morphology was deduced from the crystal structure of a bacterial SLC10A2 analog, ASBT(NM). This information was used in this chapter to calculate the putative 3D structure of NTCP. This review provides first an introduction to recent knowledge about bile acid synthesis and newly found bile acid hormonal functions, and then describes step-by-step each individual member of the family in terms of expression, localization, substrate pattern, as well as protein topology with emphasis on the three functional SLC10 carrier members.

Crystal structure of head-to-head dimers of cholic and deoxycholic acid derivatives with different symmetric bridges

The crystal structure of three head-to-head dimers (having two cholic acid or deoxycholic acid units) linked at carbon atoms C3 by aromatic or alkyl bridges is studied. An internal coordinates system is necessary for describing the relative orientation in the space of the two bile acid residues. Five angles (three torsion and two common ones) are necessary for defining the relative position of both steroid residues in space. Carbon atoms C3 (which always carries a α-hydroxy group in natural bile acids), and C10 and C13 (which always carry β-methyl groups) of each steroid residue are suitable for this purpose. Furthermore, the distance between each C3 carbon atoms of both steroid residues will allow one to locate the steroids in space. The three dimers selected provide a large range of values for these angles. The packing, hydrogen bond network, and location of guest in the three crystals are discussed.

Homo- and hetero-dimeric architecture of the human liver Na+-dependent taurocholate co-transporting protein

The NTCP (Na+–taurocholate co-transporting protein)/SLC10A [solute carrier family 10 (Na+/bile acid co-transporter family)] 1 is tightly controlled to ensure hepatic bile salt uptake while preventing toxic bile salt accumulation. Many transport proteins require oligomerization for their activity and regulation. This is not yet established for bile salt transporters. The present study was conducted to elucidate the oligomeric state of NTCP. Chemical cross-linking revealed the presence of NTCP dimers in rat liver membranes and U2OS cells stably expressing NTCP. Co-immunoprecipitation of tagged NTCP proteins revealed a physical interaction between subunits. The C-terminus of NTCP was not required for subunit interaction, but was essential for exit from the ER (endoplasmic reticulum). NTCP without its C-terminus (NTCP Y307X) retained full-length wtNTCP (wild-type NTCP) in the ER in a dominant fashion, suggesting that dimerization occurs early in the secretory pathway. FRET (fluorescence resonance energy transfer) using fluorescently labelled subunits further demonstrated that dimerization persists at the plasma membrane. NTCP belongs to the SLC10A protein family which consists of seven members. NTCP co-localized in U2OS cells with SLC10A4 and SLC10A6, but not with SLC10A3, SLC10A5 or SLC10A7. SLC10A4 and SLC10A6 co-immunoprecipitated with NTCP, demonstrating that heteromeric complexes can be formed between SLC10A family members in vitro. Expression of SLC10A4 and NTCP Y307X resulted in a reduction of NTCP abundance at the plasma membrane and NTCP-mediated taurocholate uptake, whereas expression of SLC10A6 or NTCP E257N, an inactive mutant, did not affect NTCP function. In conclusion, NTCP adopts a dimeric structure in which individual subunits are functional. Bile salt uptake is influenced by heterodimerization when this impairs NTCP plasma membrane trafficking.

Role of the intestinal bile acid transporters in bile acid and drug disposition

Membrane transporters expressed by the hepatocyte and enterocyte play critical roles in maintaining the enterohepatic circulation of bile acids, an effective recycling and conservation mechanism that largely restricts these potentially cytotoxic detergents to the intestinal and hepatobiliary compartments. In doing so, the hepatic and enterocyte transport systems ensure a continuous supply of bile acids to be used repeatedly during the digestion of multiple meals throughout the day. Absorption of bile acids from the intestinal lumen and export into the portal circulation is mediated by a series of transporters expressed on the enterocyte apical and basolateral membranes. The ileal apical sodium-dependent bile acid cotransporter (abbreviated ASBT; gene symbol, SLC10A2) is responsible for the initial uptake of bile acids across the enterocyte brush border membrane. The bile acids are then efficiently shuttled across the cell and exported across the basolateral membrane by the heteromeric Organic Solute Transporter, OSTα-OSTβ. This chapter briefly reviews the tissue expression, physiology, genetics, pathophysiology, and transport properties of the ASBT and OSTα-OSTβ. In addition, the chapter discusses the relationship between the intestinal bile acid transporters and drug metabolism, including development of ASBT inhibitors as novel hypocholesterolemic or hepatoprotective agents, prodrug targeting of the ASBT to increase oral bioavailability, and involvement of the intestinal bile acid transporters in drug absorption and drug-drug interactions.

Aggregation behavior of tetracarboxylic surfactants derived from cholic and deoxycholic acids and ethylenediaminetetraacetic acid

The reaction of 3beta-aminoderivatives of cholic and deoxycholic acids (steroid residues) with dimethyl ester of ethylenediaminetetraacetic acid (bridge) leads to the formation of dimers carrying four carboxylic organic functions, two of them located on the side chain of each steroid residue and the other two on the bridge. As tetrasodium salts, these new compounds behave as surfactants and have been characterized by surface tension, fluorescence intensity of pyrene (as a probe), and static and dynamic light scattering measurements. Thermodynamic parameters for micellization were obtained from the dependence of the critical micelle concentration (cmc) with temperature. For both surfactants, the fraction of bound counterions is close to 0.5. The aggregation behavior is similar to one of their bile salt residues [i.e., sodium cholate (NaC) and sodium deoxycholate (NaDC)] and can be summarized as follows: (i) molecular areas at the interface for the new surfactants are fairly close to twice the value for a single molecule in a monolayer of natural bile salts; (ii) the environment where pyrene is solubilized is very apolar, as in natural bile salt aggregates; (iii) Gibbs free energies (per steroid residue) for micellization are not far from published values for NaC and NaDC, and the differences can be understood on the basis of less hydrophobicity of the new surfactants due to the charges in the bridge; and (iv) as for NaC and NaDC, aggregates have rather low aggregation numbers (which depend on the amount of added inert salt, NaCl). A structure based on the disklike model accepted for small bile salt aggregates is proposed.

Intestinal bile acid physiology and pathophysiology

Bile acids (BAs) have a long established role in fat digestion in the intestine by acting as tensioactives, due to their amphipathic characteristics. BAs are reabsorbed very efficiently by the intestinal epithelium and recycled back to the liver via transport mechanisms that have been largely elucidated. The transport and synthesis of BAs are tightly regulated in part by specific plasma membrane receptors and nuclear receptors. In addition to their primary effect, BAs have been claimed to play a role in gastrointestinal cancer, intestinal inflammation and intestinal ionic transport. BAs are not equivalent in any of these biological activities, and structural requirements have been generally identified. In particular, some BAs may be useful for cancer chemoprevention and perhaps in inflammatory bowel disease, although further research is necessary in this field. This review covers the most recent developments in these aspects of BA intestinal biology.

The novel putative bile acid transporter SLC10A5 is highly expressed in liver and kidney

Here we report the identification, cloning, and characterization of SLC10A5, which is a new member of Solute Carrier Family 10 (SLC10), also known as the "sodium/bile acid cotransporter family". Expression of SLC10A5/Slc10a5 was examined by quantitative real-time PCR and revealed its highest expression levels in liver and kidney in humans, rat and mouse. In rat liver and kidney, Slc10a5 expression was localized by in situ hybridization to hepatocytes and proximal tubules, respectively. A SLC10A5-FLAG fusion protein was expressed in HEK293 cells and showed an apparent molecular weight of 42 kDa after immunoprecipitation. When expressed in Xenopus laevis oocytes, the SLC10A5-FLAG protein was detected in the oocyte's plasma membrane but showed no transport activity for taurocholate, cholate, estrone-3-sulfate, or dehydroepiandrosterone sulfate. As bile acid carriers are the most related carriers to SLC10A5 though, we strongly suppose that SLC10A5 is an orphan carrier with yet non-identified substrates.

Exploitation of bile acid transport systems in prodrug design

The enterohepatic circulation of bile acids is one of the most efficient recycling routes in the human body. It is a complex process involving numerous transport proteins, which serve to transport bile acids from the small intestine into portal circulation, from the portal circulation into the hepatocyte, from the hepatocyte into the bile, and from the gall bladder to the small intestine. The tremendous transport capacity and organ specificity of enterohepatic circulation combined with versatile derivatization possibilities, rigid steroidal backbone, enantiomeric purity, availability, and low cost have made bile acids attractive tools in designing pharmacological hybrid molecules and prodrugs with the view of improving intestinal absorption, increasing the metabolic stability of pharmaceuticals, specifically targeting drugs to organs involved in enterohepatic circulation, as well as sustaining therapeutically reasonable systemic concentrations of active agents. This article briefly describes bile acid transport proteins involved in enterohepatic circulation, summarizes the key factors affecting on the transport by these proteins, and reviews the use of bile acids and their derivatives in designing prodrugs capable of exploiting the bile acid transport system.

Apical sodium dependent bile acid transporter (ASBT, SLC10A2): a potential prodrug target

A major hurdle impeding the successful clinical development of drug candidates can be poor intestinal permeability. Low intestinal permeability may be enhanced by a prodrug approach targeting membrane transporters in the small intestine. Transporter specificity, affinity, and capacity are three factors in targeted prodrug design. The human apical sodium dependent bile acid transporter (SLC10A2) belongs to the solute carrier family (SLC) of transporters and is an important carrier protein expressed in the small intestine. In spite of its appearing to be an excellent target for prodrug design, few studies have targeted human apical sodium dependent bile acid transporter (hASBT) to improve oral bioavailability. This review discusses bile acids including their chemistry and their absorptive disposition. Additionally, hASBT-mediated prodrug targeting is discussed, including QSAR, in vitro models for hASBT assay, and the current progress in utilizing hASBT as a drug delivery target.

The solute carrier family SLC10: more than a family of bile acid transporters regarding function and phylogenetic relationships

The solute carrier family 10 (SLC10) comprises two sodium-dependent bile acid transporters, i.e. the Na(+)/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2). These carriers are essentially involved in the maintenance of the enterohepatic circulation of bile acids mediating the first step of active bile acid transport through the membrane barriers in the liver (NTCP) and intestine (ASBT). Recently, four new members of the SLC10 family were described and referred to as P3 (SLC10A3), P4 (SLC10A4), P5 (SLC10A5) and sodium-dependent organic anion transporter (SOAT; SLC10A6). Experimental data supporting carrier function of P3, P4, and P5 is currently not available. However, as demonstrated for SOAT, not all members of the SLC10 family are bile acid transporters. SOAT specifically transports steroid sulfates such as oestrone-3-sulfate and dehydroepiandrosterone sulfate in a sodium-dependent manner, and is considered to play an important role for the cellular delivery of these prohormones in testes, placenta, adrenal gland and probably other peripheral tissues. ASBT and SOAT are the most homologous members of the SLC10 family, with high sequence similarity ( approximately 70%) and almost identical gene structures. Phylogenetic analyses of the SLC10 family revealed that ASBT and SOAT genes emerged from a common ancestor gene. Structure-activity relationships of NTCP, ASBT and SOAT are discussed at the amino acid sequence level. Based on the high structural homology between ASBT and SOAT, pharmacological inhibitors of the ASBT, which are currently being tested in clinical trials for cholesterol-lowering therapy, should be evaluated for their cross-reactivity with SOAT.

Regulation of taurocholate transport in freshly isolated proximal tubular cells of the rat kidney by protein kinases

BACKGROUND/AIMS

The bile acids filtered through the glomeruli nearly completely escape urinary excretion due to an efficient tubular reabsorption process. Reabsorption is mediated mainly by the sodium-dependent bile acid transporter (ASBT) which is located in the brush border membranes of proximal tubular cells. The present study addresses the question whether this transporter is subject to short-term regulation by protein kinases.

METHODS

The effects of specific activators or inhibitors of eight different protein kinases (PKs) on 3H-taurocholate uptake of proximal tubular cells were investigated. The cells were freshly isolated from rat kidneys by nycodenz density gradient centrifugation.

RESULTS

Activation of the cAMP/PKA system by forskolin, 8-Br-cAMP, or the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine significantly diminished cellular 3H-taurocholate uptake whereas 8-Br-cGMP had no effect. Also the MEK1/2 inhibitors PD98059 and U0126, and the p38 mitogen-activated protein (MAP) kinase inhibitor SB203580 decreased 3H-taurocholate uptake. Phorbol myristate acetate and dioctanolglycerol, activators of PKC, and chelerythrine, a selective inhibitor of PKC, did not affect 3H-taurocholate uptake. Likewise the phosphatidylinositol-3 kinase inhibitor wortmannin and the tyrosine kinase inhibitor genistein induced no significant change of cellular 3H-taurocholate uptake. In a sodium-free medium forskolin and PD98059 did not affect 3H-taurocholate uptake but SB203580 significantly decreased it.

CONCLUSION

It is concluded that PKA and MAP kinases are involved in the regulation of the ASBT-mediated taurocholate uptake into proximal tubular cells. p38 MAP kinase may have an additional effect on a sodium-independent tubular taurocholate transporter.

Weanling, but not adult, rabbit colon absorbs bile acids: flux is linked to expression of putative bile acid transporters

Intestinal handling of bile acids is age dependent; adult, but not newborn, ileum absorbs bile acids, and adult, but not weanling or newborn, distal colon secretes Cl(-) in response to bile acids. Bile acid transport involving the apical Na(+)-dependent bile acid transporter (Asbt) and lipid-binding protein (LBP) is well characterized in the ileum, but little is known about colonic bile acid transport. We investigated colonic bile acid transport and the nature of the underlying transporters and receptors. Colon from adult, weanling, and newborn rabbits was screened by semiquantitative RT-PCR for Asbt, its truncated variant t-Asbt, LBP, multidrug resistance-associated protein 3, organic solute transporter-alpha, and farnesoid X receptor. Asbt and LBP showed maximal expression in weanling and significantly less expression in adult and newborn rabbits. The ileum, but not the colon, expressed t-Asbt. Asbt, LBP, and farnesoid X receptor mRNA expression in weanling colon parallel the profile in adult ileum, a tissue designed for high bile acid absorption. To examine their functional role, transepithelial [(3)H]taurocholate transport was measured in weanling and adult colon and ileum. Under short-circuit conditions, weanling colon and ileum and adult ileum showed net bile acid absorption: 1.23 +/- 0.62, 5.53 +/- 1.20, and 11.41 +/- 3.45 nmol x cm(-2) x h(-1), respectively. However, adult colon secreted bile acids (-1.39 +/- 0.47 nmol x cm(-2) x h(-1)). We demonstrate for the first time that weanling, but not adult, distal colon shows net bile acid absorption. Thus increased expression of Asbt and LBP in weanling colon, which is associated with parallel increases in taurocholate absorption, has relevance in enterohepatic conservation of bile acids when ileal bile acid recycling is not fully developed.

NMR dynamic studies suggest that allosteric activation regulates ligand binding in chicken liver bile acid-binding protein

Apo chicken liver bile acid-binding protein has been structurally characterized by NMR. The dynamic behavior of the protein in its apo- and holo-forms, complexed with chenodeoxycholate, has been determined via (15)N relaxation and steady state heteronuclear (15)N((1)H) nuclear Overhauser effect measurements. The dynamic parameters were obtained at two pH values (5.6 and 7.0) for the apoprotein and at pH 7.0 for the holoprotein, using the model free approach. Relaxation studies, performed at three different magnetic fields, revealed a substantial conformational flexibility on the microsecond to millisecond time scales, mainly localized in the C-terminal face of the beta-barrel. The observed dynamics are primarily caused by the protonation/deprotonation of a buried histidine residue, His(98), located on this flexible face. A network of polar buried side chains, defining a spine going from the E to J strand, is likely to provide the long range connectivity needed to communicate motion from His(98) to the EF loop region. NMR data are accompanied by molecular dynamics simulations, suggesting that His(98) protonation equilibrium is the triggering event for the modulation of a functionally important motion, i.e. the opening/closing at the protein open end, whereas ligand binding stabilizes one of the preexisting conformations (the open form). The results presented here, complemented with an analysis of proteins belonging to the intracellular lipid-binding protein family, are consistent with a model of allosteric activation governing the binding mechanism. The functional role of this mechanism is thoroughly discussed within the framework of the mechanism for the enterohepatic circulation of bile acids.

Ileal Bile Acid-binding Protein, Functionally Associated with the Farnesoid X Receptor or the Ileal Bile Acid Transporter, Regulates Bile Acid Activity in the Small Intestine

Bile acids secreted in the small intestine are reabsorbed in the ileum where they activate the nuclear farnesoid X receptor (FXR), which in turn stimulates expression of the ileal bile acid-binding protein (I-BABP). We first hypothesized that I-BABP may negatively regulate the FXR activity by competing for the ligands, bile acids. Reporter assays using stable HEK293 cell lines expressing I-BABP revealed that I-BABP enhances rather than attenuates FXR activity. In these cells I-BABP localizes predominantly in the cytosol and partially in the nucleus, a distribution that does not shift in response to FXR expression. In vitro binding assays reveal that recombinant I-BABP is able to bind 35S-labeled FXR and that chenodeoxycholic acid (CDCA) stimulates this interaction modestly. When FLAG-tagged FXR was expressed in stable cells, the FXR.I-BABP complex in the nuclear extracts was more efficiently immunoprecipitable with anti-FLAG antibodies in the presence of CDCA. These results indicate that I-BABP stimulates FXR activity through a mutual interaction augmented by bile acids. When stable cells were transfected with an expression plasmid of the ileal bile acid transporter 14(IBAT) essential for the reabsorption of conjugated bile acids, the C-labeled conjugated bile acid, glycocholic acid, was more efficiently imported via IBAT in the presence than absence of I-BABP, whereas no change was observed in 14C-labeled CDCA uptake, which is independent of IBAT. Immunofluorescent staining analysis revealed that these two proteins co-localize in the vicinity of the plasma membrane in stable cells. Taken together, the current data provide the first evidence that I-BABP is functionally associated with FXR and IBAT in the nucleus and on the membrane, respectively, stimulating FXR transcriptional activity and the conjugated bile acid uptake mediated by IBAT in the ileum.

Bile duct proliferation associated with bile salt-induced hypercholeresis in Mdr2 P-glycoprotein-deficient mice

BACKGROUND/AIMS

Bile flow consists of bile salt-dependent bile flow (BSDF), generated by canalicular secretion of bile salts, and bile salt-independent flow (BSIF), probably of combined canalicular and ductular origin. Bile salt transport proteins have been identified in cholangiocytes, suggesting a role in control of BSDF and/or in control of bile salt synthesis through cholehepatic shunting.

METHODS

We studied effects of bile duct proliferation under non-cholestatic conditions in multidrug resistance-2 P-glycoprotein (Abcb4)-deficient multidrug resistance gene-2 (Mdr2(-/-)) mice. BSDF and BSIF were determined in wild-type and Mdr2(-/-) mice during infusion of step-wise increasing dosages of tauroursodeoxycholate (TUDC). Cholate synthesis rate was determined by 2H4-cholate dilution. Results were related to expression of transport proteins in liver and intestine.

RESULTS

During TUDC infusion, BSDF was increased by approximately 50% and BSIF by approximately 100% in Mdr2(-/-) mice compared with controls. Cholate synthesis rate was unaffected in Mdr2(-/-) mice. Hepatic expression of the apical sodium-dependent bile salt transporter (Asbt), its truncated form (tAsbt) and the multidrug resistance-related protein 3 were upregulated in Mdr2(-/-) mice.

CONCLUSIONS

Bile duct proliferation in Mdr2(-/-) mice enhances cholehepatic shunting of bile salts, which is associated with a disproportionally high bile flow but does not affect bile salt synthesis.

Intestinal cholesterol absorption: identification of different binding proteins for cholesterol and cholesterol absorption inhibitors in the enterocyte brush border membrane

Absorption of cholesterol from the intestine is a central part of body cholesterol homeostasis. The molecular mechanisms of intestinal cholesterol absorption and the proteins mediating membrane transport are not known. We therefore aimed to identify the proteins involved in intestinal cholesterol absorption across the luminal brush border membrane of small intestinal enterocytes. By photoaffinity labeling using photoreactive derivatives of cholesterol and 2-azetidinone cholesterol absorption inhibitors, an 80-kDa and a 145-kDa integral membrane protein were identified as specific binding proteins for cholesterol and cholesterol absorption inhibitors, respectively, in the brush border membrane of small intestinal enterocytes. The 80-kDa cholesterol-binding protein did not interact with cholesterol absorption inhibitors and vice versa; cholesterol or plant sterols did not interfere with the 145-kDa molecular target for cholesterol absorption inhibitors. Both proteins showed an identical tissue distribution and were exclusively found at the anatomical sites of cholesterol absorption-duodenum, jejunum and ileum. Neither stomach, cecum, colon, rectum, kidney, liver nor fat tissue expressed the 80- or 145-kDa binding proteins for cholesterol and cholesterol absorption inhibitors. Both proteins are different from the hitherto described candidate proteins for the intestinal cholesterol transporter,-SR-BI, ABC G5/ABC G8 or ABC A1. Our data strongly suggest that intestinal cholesterol absorption is not facilitated by a single transporter protein but occurs by a complex machinery. Two specific binding proteins for cholesterol (80 kDa) and cholesterol absorption inhibitors (145 kDa) of the enterocyte brush border membrane are probable protein constituents of the mechanism responsible for the intestinal absorption of cholesterol.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

The relative ligand binding preference of the murine ileal lipid binding protein

The ileal lipid binding protein (ILBP), a member of the intracellular lipid binding protein family, is a 14-kDa protein that has bile and fatty acids as possible physiological ligands. The ligand binding specificity of this protein is not well characterized. Therefore, we studied the lipid binding activity of purified recombinant murine ILBP (mILBP) in vitro. These studies demonstrated by direct analysis the interaction of mILBP with naturally occurring bile and fatty acids. The rank order of binding preference for fatty acids, or unconjugated and conjugated bile acids, was assessed. Among fatty acids, mILBP preferred species that had longer chain length and increased saturation, similar to other members of the intracellular lipid binding protein family. Among the bile acids, mILBP showed the greatest preference for conjugated species that contained a doubly hydroxylated steroid moiety. The results demonstrate that mILBP exhibits a preference for certain species of bile and fatty acids.

Insights into the bile acid transportation system: the human ileal lipid-binding protein-cholyltaurine complex and its comparison with homologous structures

Bile acids are generated in vivo from cholesterol in the liver, and they undergo an enterohepatic circulation involving the small intestine, liver, and kidney. To understand the molecular mechanism of this transportation, it is essential to gain insight into the three-dimensional (3D) structures of proteins involved in the bile acid recycling in free and complexed form and to compare them with homologous members of this protein family. Here we report the solution structure of the human ileal lipid-binding protein (ILBP) in free form and in complex with cholyltaurine. Both structures are compared with a previously published structure of the porcine ILBP-cholylglycine complex and with related lipid-binding proteins. Protein structures were determined in solution by using two-dimensional (2D)- and 3D-homo and heteronuclear NMR techniques, leading to an almost complete resonance assignment and a significant number of distance constraints for distance geometry and restrained molecular dynamics simulations. The identification of several intermolecular distance constraints unambiguously determines the cholyltaurine-binding site. The bile acid is deeply buried within ILBP with its flexible side-chain situated close to the fatty acid portal as entry region into the inner ILBP core. This binding mode differs significantly from the orientation of cholylglycine in porcine ILBP. A detailed analysis using the GRID/CPCA strategy reveals differences in favorable interactions between protein-binding sites and potential ligands. This characterization will allow for the rational design of potential inhibitors for this relevant system.

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

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

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Identification of a ligand-binding site in the Na+/bile acid cotransporting protein from rabbit ileum

Reabsorption of bile acids occurs in the terminal ileum by a Na(+)-dependent transport system composed of several subunits of the ileal bile acid transporter (IBAT) and the ileal lipid-binding protein. To identify the bile acid-binding site of the transporter protein IBAT, ileal brush border membrane vesicles from rabbit ileum were photoaffinity labeled with a radioactive 7-azi-derivative of cholyltaurine followed by enrichment of IBAT protein by preparative SDS gel electrophoresis. Enzymatic fragmentation with chymotrypsin yielded IBAT peptide fragments in the molecular range of 20.4-4 kDa. With epitope-specific antibodies generated against the C terminus a peptide of molecular mass of 6.6-7 kDa was identified as the smallest peptide fragment carrying both the C terminus and the covalently attached radiolabeled bile acid derivative. This clearly indicates that the ileal Na(+)/bile acid cotransporting protein IBAT contains a bile acid-binding site within the C-terminal 56-67 amino acids. Based on the seven-transmembrane domain model for IBAT, the bile acid-binding site is localized to a region containing the seventh transmembrane domain and the cytoplasmic C terminus. Alternatively, assuming the nine-transmembrane domain model, this bile acid-binding site is localized to the ninth transmembrane domain and the C terminus.

Identification of the bile acid-binding site of the ileal lipid-binding protein by photoaffinity labeling, matrix-assisted laser desorption ionization-mass spectrometry, and NMR structure

The ileal lipid-binding protein (ILBP) is the only physiologically relevant bile acid-binding protein in the cytosol of ileocytes. To identify the bile acid-binding site(s) of ILBP, recombinant rabbit ILBP photolabeled with 3-azi- and 7-azi-derivatives of cholyltaurine was analyzed by a combination of enzymatic fragmentation, gel electrophoresis, and matrix-assisted laser desorption ionization (MALDI)-mass spectrometry. The attachment site of the 3-position of cholyltaurine was localized to the amino acid triplet His(100)-Thr(101)-Ser(102) using the photoreactive 3,3-azo-derivative of cholyltaurine. With the corresponding 7,7-azo-derivative, the attachment point of the 7-position could be localized to the C-terminal part (position 112-128) as well as to the N-terminal part suggesting more than one binding site for bile acids. By chemical modification and NMR structure of ILBP, arginine residue 122 was identified as the probable contact point for the negatively charged side chain of cholyltaurine. Consequently, bile acids bind to ILBP with the steroid nucleus deep inside the protein cavity and the negatively charged side chain near the entry portal. The combination of photoaffinity labeling, enzymatic fragmentation, MALDI-mass spectrometry, and NMR structure was successfully used to determine the topology of bile acid binding to ILBP.

Identification of binding proteins for cholesterol absorption inhibitors as components of the intestinal cholesterol transporter

To identify protein components of the intestinal cholesterol transporter, rabbit small intestinal brush border membrane vesicles were submitted to photoaffinity labeling using photoreactive derivatives of 2-azetidinone cholesterol absorption inhibitors. An integral membrane protein of M(r) 145.3+/-7.5 kDa was specifically labeled in brush border membrane vesicles from rabbit jejunum and ileum. Its labeling was concentration-dependently inhibited by the presence of cholesterol absorption inhibitors whereas bile acids, D-glucose, fatty acids or cephalexin had no effect. The inhibitory potency of 2-azetidinones to inhibit photolabeling of the 145 kDa protein correlated with their in vivo activity to inhibit intestinal cholesterol absorption. These results suggest that an integral membrane protein of M(r) 145 kDa is (a component of) the cholesterol absorption system in the brush border membrane of small intestinal enterocytes.

Perfused rat intrahepatic bile ducts secrete and absorb water, solute, and ions

BACKGROUND & AIMS

We report a novel approach to study biliary water, bile acid, and HCO(3)(-) transport: the microperfusion of intrahepatic bile duct units (IBDUs) isolated from normal rat liver.

METHODS

To study water transport, IBDUs were perfused in vitro with a membrane-impermeant fluorescent volume marker, fluorescein sulfonate; net water movement (J(v)) and osmotic water permeability (P(f)) were then calculated. To study solute transport, IBDUs were perfused with taurocholic acid (TCA) and bile acid uptake was calculated from the concentrations of TCA in the perfused and collected solutions. To study ion transport, IBDUs were perfused with the cell-impermeant pH-sensitive dye BCECF dextran; luminal pH was determined from fluorescence excitation ratios.

RESULTS

When inward (secretory) or outward (absorptive) osmotic gradients were established across IBDUs, water movement was observed from bath to lumen (i.e., secretion) and from lumen to bath (i.e., absorption). The perfused IBDUs absorbed TCA in a saturable, sodium-dependent manner; in addition, TCA absorption was blocked in a dose-dependent fashion by S0960, a specific inhibitor of the Na(+)/bile acid cotransporter. Addition of forskolin to HCO(3)(-)-containing (but not HCO(3)(-)-free) bath buffer resulted in lumen alkalinization reflecting HCO(3)(-) transport into the lumen of perfused IBDUs.

CONCLUSIONS

The results provide direct functional evidence that intrahepatic bile ducts both secrete and absorb water in response to osmotic gradients, actively absorb bile acid, and transport HCO(3)(-).

Downregulation of ileal bile acid absorption in bile-duct-ligated rats

BACKGROUND/AIMS

Accumulation of toxic bile acids in cholestasis contributes to liver injury and depends on their synthesis, secretion and intestinal absorption. In the present study, we investigated the effect of cholestasis on the active ileal absorption of bile acids in vivo and the adaptation of transporters involved in ileal bile acid absorption.

METHODS

Male Wistar rats underwent ligation of the common bile duct or biliary diversion. Sham-operated rats served as controls. Active ileal bile acid absorption of taurocholate was measured by an intestinal perfusion technique. Transporter mRNA levels of the Na+/bile acid cotransporting protein (IBAT), ileal lipid binding protein (ILBP) and organic anion transporter subtype 3 (Oatp3) and protein expression of IBAT and ILBP were determined in the distal ileum.

RESULTS

After bile duct ligation the intestinal absorption rates of taurocholate were lower (p<0.05) and after biliary diversion absorption rates were higher compared to sham-operated animals (p<0.05). The absorption rates were inversely correlated to serum bile acid concentrations. Levels of IBAT-, ILBP- and Oatp3- mRNA were not different between the groups. However, in cholestatic rats, the expression of the 99-kDa dimer of IBAT was decreased compared to controls (p<0.05), whereas the 46-kDa monomeric protein of IBAT and the expression of ILBP was unchanged. After biliary diversion a similar pattern of protein expression was observed, despite an increased absorption rate.

CONCLUSIONS

Cholestasis leads to a decreased active ileal absorption of taurocholate. The changes in protein expression may not account for the different absorption rates. The intestinal absorption of bile acids seems to be regulated by their systemic concentration.

Intracellular transport of bile acids

Bile acids originate from the liver and are transported via bile to the intestines where they perform an important role in the absorption of lipids and lipid-soluble nutrients. Most of the bile acids are reclaimed from the terminal ileum and returned to the liver via portal blood for reuse. The transport of bile acids is vectorial in both liver and intestinal cells, originating and terminating at opposite poles. Bile acids enter through the basolateral pole in liver cells, and through the apical pole in intestinal cells. During the past decade, much has been learned about the mechanisms by which bile acids enter and exit liver and intestinal cells. By contrast, the mechanisms by which bile acids are transported across cells remain poorly understood. The current body of evidence suggests that bile acids do not traverse the cell by vesicular transport. Although a carrier-mediated mechanism is a likely alternative, only a handful of intracellular proteins capable of binding bile acids have been described. The significance of these proteins in the intracellular transport of bile acids remains to be tested.

Biliary fibrosis associated with altered bile composition in a mouse model of erythropoietic protoporphyria

BACKGROUND & AIMS

Reduced activity of ferrochelatase in erythropoietic protoporphyria (EPP) results in protoporphyrin (PP) accumulation in erythrocytes and liver. Liver disease may occur in patients with EPP, some of whom develop progressive liver failure that necessitates transplantation. We investigated the mechanisms underlying EPP-associated liver disease in a mouse model of EPP.

METHODS

Liver histology, indicators of lipid peroxidation, plasma parameters of liver function, and bile composition were studied in mice homozygous (fch/fch) for a point mutation in the ferrochelatase gene and in heterozygous (fch/+) and wild-type (+/+) mice.

RESULTS

Microscopic examination showed bile duct proliferation and biliary fibrosis with portoportal bridging in fch/fch mice. PP content was 130-fold increased, and thiobarbituric acid-reactive substances (+30%) and conjugated dienes (+75%) were slightly higher in fch/fch than in fch/+ and +/+ livers. Levels of hepatic thiols (-12%) and iron (-52%) were reduced in fch/fch livers. Liver enzymes and plasma bilirubin were markedly increased in the homozygotes. Plasma bile salt levels were 80 times higher in fch/fch than in fch/+ and +/+ mice, probably related to the absence of the Na(+)-taurocholate cotransporting protein (Ntcp) in fch/fch liver. Paradoxically, bile flow was not impaired and biliary bile salt secretion was 4 times higher in fch/fch mice than in controls. Up-regulation of the intestinal Na(+)-dependent bile salt transport system in fch/fch mice may enhance efficiency of bile salt reabsorption. The bile salt/lipid ratio and PP content of fch/fch bile were increased 2-fold and 85-fold, respectively, compared with +/+, whereas biliary glutathione was reduced by 90%. Similar effects on bile formation were caused by griseofulvin-induced inhibition of ferrochelatase activity in control mice.

CONCLUSIONS

Bile formation is strongly affected in mice with impaired ferrochelatase activity. Rather than peroxidative processes, formation of cytotoxic bile with high concentrations of bile salts and PP may cause biliary fibrosis in fch/fch mice by damaging bile duct epithelium.

Intestinal absorption of bile acids: paradoxical behaviour of the 14 kDa ileal lipid-binding protein in differential photoaffinity labelling

Photoaffinity labelling of brush border membrane vesicles from rabbit ileum with radiolabelled 3,3-azo and 7,7-azo derivatives of taurocholate identified integral membrane proteins of molecular masses 93 and 46 kDa, as well as a 14 kDa peripheral membrane protein, as components of the ileal Na+/bile acid transport system [Kramer, Girbig, Gutjahr, Kowalewski, Jouvenal, Müller, Tripier and Wess (1993) J. Biol. Chem. 268, 18035-18046]. Differential photoaffinity labelling in the presence of non-radiolabelled bile acid derivatives led, as expected, to a concentration-dependent decrease in the extent of labelling of the 93 and 46 kDa transmembrane proteins, which are the monomeric and dimeric forms of the ileal bile acid transporter protein. The extent of labelling of the 14 kDa ileal lipid-binding protein (ILBP), however, increased on the addition of unlabelled bile acids, the increase being dependent on the structure of the bile acid added. The possibility of artifacts was excluded by photoaffinity labelling experiments in the frozen state as well as by model calculations. The experimental results suggest that the binding of bile acids to ILBP can increase the affinity of ILBP for bile acids. These results would be in accordance with a substrate-load modification of transport activity and a positive-feedback regulation mechanism for active uptake of bile acid in the ileum.

New insights into bile acid transport

The enterohepatic circulation of bile acids is maintained by a series of membrane transport proteins. Recent studies of the cloned sodium bile acid cotransporters have provided new insights into their tissue expression, regulation, and their relationship to cholesterol homeostasis and human diseases such as primary bile acid malabsorption.