Characterization of the bile acid transport system in normal and transformed hepatocytes. Photoaffinity labeling of the taurocholate carrier protein

Enhancing thermal stability and lytic activity of phage lysin PlyAB1 from Acinetobacter baumannii

With the increasingly serious drug resistance of Acinetobacter baumannii, it is urgent to find new antibacterial drugs. Phage lysin PlyAB1 has a bactericidal effect on drug-resistant Acinetobacter baumannii, which has the potential to replace antibiotics to fight infection caused by Acinetobacter baumannii. However, its application is limited by its thermal stability and lytic activity. To solve these problems, molecular dynamics (MD) simulations combined with Hotspot wizard 3.0 were used to identify key residue sites affecting thermal stability, and evolutionary analysis combined with multiple sequence alignment was used to identify key residue sites affecting lytic activity. Four single-point variants with significantly increased thermal stability and four single-point variants with significantly lytic activity were obtained, respectively. Furthermore, by superimposing mutations, we obtained three double-point variants G100Q/K69R, G100R/K69R, and G100K/K69R with significantly improved thermal stability and improved lytic activity. At 45℃, the lytic activity and half-life of the optimal variant G100Q/K69R were 1.51 folds and 24 folds higher than those of the wild PlyAB1, respectively. These results deepen our understanding of the structure and function of phage lysin and contribute to the application of phage lysin in antibiotic substitution. This article is protected by copyright. All rights reserved.

Current advances of carbene-mediated photoaffinity labeling in medicinal chemistry

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites. In particular, carbene-mediated photoaffinity labeling (cmPAL) has been widely used in drug target identification owing to its excellent photolabeling efficiency, minimal steric interference and longer excitation wavelength. Specifically, diazirines, which are among the precursors of carbenes and have higher carbene yields and greater chemical stability than diazo compounds, have proved to be valuable photolabile reagents in a diverse range of biological systems. This review highlights current advances of cmPAL in medicinal chemistry, with a focus on structures and applications for identifying small molecule–protein and macromolecule–protein interactions and ligand-gated ion channels, coupled with advances in the discovery of targets and inhibitors using carbene precursor-based biological probes developed in recent decades.

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites.

Transcription of the Human Microsomal Epoxide Hydrolase Gene (EPHX1) Is Regulated by PARP-1 and Histone H1.2. Association with Sodium-Dependent Bile Acid Transport

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in the metabolism of numerous xenobiotics as well as mediating the sodium-dependent transport of bile acids into hepatocytes. These compounds are involved in cholesterol homeostasis, lipid digestion, excretion of xenobiotics and the regulation of several nuclear receptors and signaling transduction pathways. Previous studies have demonstrated the critical role of GATA-4, a C/EBPα-NF/Y complex and an HNF-4α/CAR/RXR/PSF complex in the transcriptional regulation of the mEH gene (EPHX1). Studies also identified heterozygous mutations in human EPHX1 that resulted in a 95% decrease in mEH expression levels which was associated with a decrease in bile acid transport and severe hypercholanemia. In the present investigation we demonstrate that EPHX1 transcription is significantly inhibited by two heterozygous mutations observed in the Old Order Amish population that present numerous hypercholanemic subjects in the absence of liver damage suggesting a defect in bile acid transport into the hepatocyte. The identity of the regulatory proteins binding to these sites, established using biotinylated oligonucleotides in conjunction with mass spectrometry was shown to be poly(ADP-ribose)polymerase-1 (PARP-1) bound to the EPHX1 proximal promoter and a linker histone complex, H1.2/Aly, bound to a regulatory intron 1 site. These sites exhibited 71% homology and may represent potential nucleosome positioning domains. The high frequency of the H1.2 site polymorphism in the Amish population results in a potential genetic predisposition to hypercholanemia and in conjunction with our previous studies, further supports the critical role of mEH in mediating bile acid transport into hepatocytes.

Antibacterial properties of Acinetobacter baumannii phage Abp1 endolysin (PlyAB1)

BACKGROUND

Acinetobacter baumannii has emerged as one of the most important hospital-acquired pathogens in the world, because of its resistance to almost all available antibiotic drugs. Endolysins from phages are attracting increasing interest as potential antimicrobial agents, especially for drug-resistant bacteria. We previously isolated and characterized Abp1, a virulent phage targeting the multidrug-resistant A. baumannii strain, AB1.

METHODS

To evaluate the antimicrobial potential of endolysin from the Abp1 phage, the endolysin gene plyAB1 was cloned and over-expressed in Escherichia coli, and the lytic activity of the recombinant protein (PlyAB1) was tested by turbidity assessment and bacteria counting assays.

RESULTS

PlyAB1 exhibits a marked lytic activity against A. baumannii AB1, as shown by a decrease in the number of live bacteria following treatment with the enzyme. Moreover, PlyAB1 displayed a highly specific lytic effect against all of the 48 hospital-derived pandrug-resistant A. baumannii isolates that were tested. These isolates were shown to belong to different ST clones by multilocus sequence typing.

CONCLUSIONS

The results presented here show that PlyAB1 has potential as an antibiotic against drug-resistant A. baumannii.

Transcription of the human microsomal epoxide hydrolase gene (EPHX1) is regulated by an HNF-4α/CAR/RXR/PSF complex

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in the metabolism of numerous xenobiotics as well as mediating the sodium-dependent transport of bile acids into hepatocytes where they are involved in cholesterol excretion and metabolism, lipid digestion and regulating numerous signaling pathways. Previous studies have demonstrated the critical role of GATA-4 and a C/EBPα-NF/Y complex in the regulation of the mEH gene (EPHX1). In this study we show that HNF-4α and CAR/RXR also bind to the proximal promoter region and regulate EPHX1 expression. Bile acids, which inhibit the expression of HNF-4α also decrease the expression of EPHX1. Studies also established that the binding of HNF-4α was essential for the activation of EPHX1 activity by CAR suggesting the formation of a complex between these adjacent factors. The nature of this regulatory complex was further explored using a biotinylated oligonucleotide of this region in conjunction with BioMag beads and mass spectrometric analysis which demonstrated the presence of an additional inhibitory factor (PSF), confirmed by co-immunoprecipitation and ChIP analyses, which interacted with DNA-bound CAR/RXR/HNF-4α forming a 4-component regulatory complex.

Development and application of diazirines in biological and synthetic macromolecular systems

Many different reagents and methodologies have been utilised for the modification of synthetic and biological macromolecular systems. In addition, an area of intense research at present is the construction of hybrid biosynthetic polymers, comprised of biologically active species immobilised or complexed with synthetic polymers. One of the most useful and widely applicable techniques available for functionalisation of macromolecular systems involves indiscriminate carbene insertion processes. The highly reactive and non-specific nature of carbenes has enabled a multitude of macromolecular structures to be functionalised without the need for specialised reagents or additives. The use of diazirines as stable carbene precursors has increased dramatically over the past twenty years and these reagents are fast becoming the most popular photophors for photoaffinity labelling and biological applications in which covalent modification of macromolecular structures is the basis to understanding structure-activity relationships. This review reports the synthesis and application of a diverse range of diazirines in macromolecular systems.

CCAAT/enhancer-binding protein alpha (C/EBPalpha) activates transcription of the human microsomal epoxide hydrolase gene (EPHX1) through the interaction with DNA-bound NF-Y

Microsomal epoxide hydrolase (mEH) plays a central role in xenobiotic metabolism as well as mediating the sodium-dependent uptake of bile acids into the liver, where these compounds regulate numerous biological processes such as cholesterol metabolism and hepatocyte signaling pathways. Little is known, however, about the factors that control the constitutive and inducible expression of the mEH gene (EPHX1) that is altered during development and in response to numerous xenobiotics. In previous studies we have established that GATA-4 binding to the EPHX1 core promoter is critical for EPHX1 expression. The -80/+25 bp core promoter also contained a reversed CCAAT box (-5/-1 bp), integrity of which was required for maximal basal EPHX1 transcription in HepG2 cells. Transient transfection of CCAAT/enhancer-binding protein alpha (C/EBPalpha) substantially stimulated EPHX1 promoter activity. Electrophoretic mobility shift assays, however, revealed that nuclear factor Y (NF-Y), but not C/EBPalpha, directly bound to this site although increased expression of NF-Y had no effect on EPHX1 promoter activity. These results suggested that C/EBPalpha activated EPHX1 expression through its interaction with NF-Y bound to the CCAAT box. The existence of a C/EBPalpha[NF-Y] complex was supported by electrophoretic mobility shift assays using antibodies against NF-Y and C/EBPalpha as well as by the ability of a dominant-negative NF-Y expression vector to inhibit promoter activity. The interaction between these transcription factors was established by co-immunoprecipitation analysis and glutathione S-transferase pull-down assays, whereas the association of the two factors and the interaction of NF-Y with the CCAAT box in vivo was confirmed by chromatin immunoprecipitation assays. C/EBPalpha-dependent EPHX1 activation was also supported by reconstitution studies in HeLa cells that lack this protein. These results establish that EPHX1 expression is regulated by C/EBPalpha interacting with DNA-bound NF-Y.

Regulation of human microsomal epoxide hydrolase gene (EPHX1) expression by the transcription factor GATA-4

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a crucial role in the metabolism of numerous xenobiotics as well as in mediating the hepatic sodium-dependent uptake of bile acids that are involved in numerous physiological processes including the regulation of cholesterol metabolism. The transcription factors and nuclear receptors that control the constitutive and inducible expression of the mEH gene (EPHX1), however, have not been described. To characterize these factors, a series of 5'-deletion constructs have been transfected into human liver-derived HepG2 cells as well as non-hepatic HeLa cells. Promoter activity analysis indicated the presence of a positive regulatory element in the -80/-70 bp region. Sequence analysis revealed a putative GATA site at -79/-74 bp as well as an additional site at -31/-26 bp. Electrophoretic mobility shift assays with an anti-GATA-4 antibody confirmed that GATA-4 bound to these two sites with a dissociation constant of 1.56 nM (-79 site) and 0.65 nM (-31 site). Coexpression of GATA-4 stimulated EPHX1 promoter activity up to 7.5-fold in a dose-dependent manner. Endogenous EPHX1 message in HepG2 cells was also significantly increased by overexpression of GATA-4. Mutating the -79 element resulted in a 65% loss of promoter activity, while mutating the -31 element had no effect on basal activity but greatly reduced the response to additional GATA-4. In HeLa cells, which do not express GATA-4, EPHX1 activity was negligible; however, activity could be reconstituted by the addition of exogenous GATA-4. These results demonstrate that GATA-4 plays a critical role in regulating EPHX1 expression.

Cell surface expression and bile acid transport function of one topological form of m-epoxide hydrolase

The bifunctional hepatic protein, microsomal epoxide hydrolase (mEH), plays a central role in the metabolism of many xenobiotics as well as mediating the Na(+)-dependent uptake of bile acids in parallel with the Na(+)-taurocholate co-transporting protein (ntcp). Previous studies have established that mEH is expressed in the endoplasmic reticulum with two topological orientations, where the type II form is targeted to the plasma membrane. In this report the topology and transport properties of mEH as a function of plasma membrane expression in cultured hepatocytes, transfected Madin-Darby canine kidney cells expressing mEH (MDCK[mEH]), and the human hepatoma cell line, HepG2, were studied using confocal fluorescence microscopy and substrate uptake measurements. Analysis of mEH localization with an anti-mEH monoclonal antibody demonstrated the expression of one topological form on the plasma membrane of hepatocytes and MDCK[mEH] cells where both systems exhibited Na(+)-dependent bile acid uptake. In contrast, Na(+)-dependent bile acid transport in HepG2 cells and hepatocytes in culture (72 h) was substantially reduced as was the expression of ntcp. Although the total mEH level was undiminished, the decrease of bile acid transport was associated with the loss of mEH surface expression possibly resulting from an alteration in mEH endoplasmic reticulum topology and/or the plasma membrane protein targeting system in these de-differentiated cells.

Inhibition of human m-epoxide hydrolase gene expression in a case of hypercholanemia

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in carcinogen metabolism and is also able to mediate the sodium-dependent uptake of bile acids into hepatocytes. Studies have identified a subject (S-1) with extremely elevated serum bile salt levels in the absence of observable hepatocellular injury, suggesting a defect in bile acid uptake. In this individual, mEH protein and mEH mRNA levels were reduced by approximately 95% and 85%, respectively, whereas the expression and amino acid sequence of another bile acid transport protein (NTCP) was unaffected. Sequence analysis of the mEH gene (EPHX1) revealed a point mutation at an upstream HNF-3 site (allele I) and in intron 1 (allele II), which resulted in a significant decrease in EPHX1 promoter activity in transient transfection assays. Gel shift assays using a radiolabeled oligonucleotide from each region resulted in specific transcription factor binding patterns, which were altered in the presence of the mutation. These studies demonstrate that the expression of mEH is greatly reduced in a patient with hypercholanemia, suggesting that mEH participates in sodium-dependent bile acid uptake in human liver where its absence may contribute to the etiology of this disease.

Membrane topology and cell surface targeting of microsomal epoxide hydrolase. Evidence for multiple topological orientations

Microsomal epoxide hydrolase (mEH) is a bifunctional membrane protein that plays a central role in the metabolism of xenobiotics and in the hepatocyte uptake of bile acids. Numerous studies have established that this protein is expressed both in the endoplasmic reticulum and at the sinusoidal plasma membrane. Preliminary evidence has suggested that mEH is expressed in the endoplasmic reticulum (ER) membrane with two distinct topological orientations. To further characterize the membrane topology and targeting of this protein, an N-glycosylation site was engineered into mEH to serve as a topological probe for the elucidation of the cellular location of mEH domains. The cDNAs for mEH and this mEH derivative (mEHg) were then expressed in vitro and in COS-7 cells. Analysis of total expressed protein in these systems indicated that mEHg was largely unglycosylated, suggesting that expression in the ER was primarily of a type I orientation (Ccyt/Nexo). However, analysis, by biotin/avidin labeling procedures, of mEHg expressed at the surface of transfected COS-7 cells, showed it to be fully glycosylated, indicating that the topological form targeted to this site originally had a type II orientation (Cexo/Ncyt) in the ER. The surface expression of mEH was also confirmed by confocal fluorescence scanning microscopy. The sensitivity of mEH topology to the charge at the N-terminal domain was demonstrated by altering the net charge over a range of 0 to +3. The introduction of one positive charge led to a significant inversion in mEH topology based on glycosylation site analysis. A truncated form of mEH lacking the N-terminal hydrophobic transmembrane domain was also detected on the extracellular surface of transfected COS-7 cells, demonstrating the existence of at least one additional transmembrane segment. These results suggest that mEH may be integrated into the membrane with multiple transmembrane domains and is inserted into the ER membrane with two topological orientations, one of which is targeted to the plasma membrane where it mediates bile acid transport.

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

For the investigation of the topology of the rabbit ileal Na+/bile-salt-cotransport system, composed of a 93-kDa integral membrane protein and a peripheral 14-kDa bile-acid-binding protein (ILBP), we have synthesized photolabile dimeric bile-salt-transport inhibitors (photoblockers), G1-X-G2, where two bile acid moieties (G1 and G2) are tethered together via a spacer, X, and where one of the two bile acid moieties carries a photoactivatable group. These photoblockers specifically interact with the ileal Na+/bile-salt-cotransport system as demonstrated by a concentration-dependent inhibition of [3H]cholyltaurine uptake by rabbit ileal brush-border membrane vesicles and by inhibition of photolabeling of the 93-kDa and 14-kDa bile-salt-binding proteins by 7,7-azo and 3,3-azo derivatives of cholyltaurine. Ileal bile-salt uptake was specifically inhibited by the photoblockers, which were not taken up themselves by the small intestine as demonstrated by in vivo ileal perfusion. Dependent on the photoblocker used several polypeptides in the molecular-mass range of 14-130 kDa were labeled. The cytoplasmically attached 14-kDa ILBP was significantly labeled only by inhibitors that are photoactivatable in bile acid moiety G1, suggesting that during binding and translocation of a bile-salt molecule by the ileal bile-salt-transport system the steroid nucleus gets access to the cytoplasmic site of the ileal brush-border membrane first. Photoaffinity labeling in the frozen state with the transportable 3,3-azo and 7,7-azo derivatives of cholyltaurine revealed a time-dependent increase in the extent of labeling of the 14-kDa and 93-kDa proteins, suggesting a labeling of these proteins from the cytoplasmic site of the ileal brush-border membrane. By photoaffinity labeling in the frozen state with the various photoblockers time-dependent changes in the extent of photoaffinity labeling of bile-salt-binding proteins were observed, demonstrating the possibility of topological analysis of the rabbit ileal Na+/bile-salt-cotransport system.

Relationship between structure and intestinal absorption of bile acids with a steroid or side-chain modification

A structure-activity relationship for bile acid (BA) intestinal absorption is known to exist. To better understand the BA structural requirements for optimal BA intestinal absorption, rabbit ileal perfusion studies were performed. Unconjugated BA: Ursodeoxycholic (UDCA) and ursocholic acid (UCA) with methyl (6MUDCA and 6MUCA) or fluoro group (6FUDCA and 6FUCA) in the 6 position and UCA with a methyl group in 23 position (23MUCA) were compared with unconjugated UDCA, UCA, deoxycholic (DCA), chenodeoxycholic (CDCA), hyocholic (HCA), and hyodeoxycholic (HDCA) acid. BA lipophilicity was evaluated by their octanol-water partition coefficient. Conjugated BA: Taurine-conjugated UDCA and UCA with a methyl group in the 23 position (T23MUDCA and T23MUCA) were compared with the corresponding taurine-conjugated natural analogs. An analog of glycine-conjugated UDCA with the C24 amide bond replaced by a -CO-CH2- in the 24 position (24PUDCA) was studied and results were compared with the natural form (GUDCA). Unconjugated BA absorption was dose dependent (i.e., passive) and followed their lipophilicity: DCA > 6MUDCA > CDCA > HDCA > UDCA > HCA > 6FUDCA > 6MUCA > 6FUCA > UCA. Conjugated BA absorption was active, and Vmax was in the following order: TCA > TUDCA > TUCA > T23MUCA > T23MUDCA > 24PUDCA > GUDCA. 24PUDCA transport was also active and higher than GUDCA.

CONCLUSION

Passive transport is dependent on BA lipophilicity. Conjugated BAs are actively transported, and the presence of a 23-C methyl group does not improve transport when compared with the natural analogs. The substitution of the C24 amide bond with a -CO-CH2-still affords interaction of the BA with the intestinal transport carrier.

The rabbit ileal lipid-binding protein. Gene cloning and functional expression of the recombinant protein

A bile-acid-binding protein of Mr 14000 has been previously identified by photoaffinity labeling in rabbit ileal brush border membrane vesicles [Kramer et al. (1993) J. Biol. Chem. 268, 18035-18046]. This peripheral membrane-associated protein was purified and identified as an ileal lipid-binding protein. It was further shown to be identical to the cytosolic 14-kDa bile-acid-binding protein from the same tissue. Starting with sequence information from tryptic fragments, we cloned and sequenced the gene and its transcript. It has four exons (123, 176, 90, 115 bp) and three introns (1372, 2291, 3137 bp) and a similar structure as the genes from other members of the fatty-acid-binding protein family. The deduced protein has 128 amino acid residues and a calculated molecular mass of 14404 Da. It exhibits high similarity to its human (83%), mouse (77%), rat (76%) and porcine (72%) counterparts. Furthermore, the recombinant protein was produced in Escherichia coli and shown to be identical to native protein from ileal tissue. Functionality of the recombinant protein was demonstrated by labeling with various photoaffinity derivatives of bile acids. Ranking of the photolabeling efficiency of these probes towards the recombinant protein was comparable to the respective ranking towards the native protein. Polyclonal antibodies that were raised in hens against the recombinant protein, specifically recognized the ileal lipid-binding protein in the brush border membrane and cytosol from rabbit ileum. In contrast, no labeling was observed with jejunal tissue. Our results suggest a specific role of the membrane-associated ileal lipid-binding protein for the process of ileal bile acid uptake.

The functional expression of sodium-dependent bile acid transport in Madin-Darby canine kidney cells transfected with the cDNA for microsomal epoxide hydrolase

Previous studies have suggested that the enzyme microsomal epoxide hydrolase (mEH) is able to mediate sodium-dependent transport of bile acids such as taurocholate into hepatocytes (von Dippe, P., Amoui, M., Alves, C., and Levy, D.(1993) Am. J. Physiol. 264, G528-G534). In order to characterize directly the putative transport properties of the enzyme, a pCB6 vector containing the cDNA for this protein (pCB6-mEH) was transfected into Madin-Darby canine kidney (MDCK) cells, and stable transformants were isolated that could express mEH at levels comparable with the levels expressed in hepatocytes. Sodium-dependent transport of taurocholate was shown to be dependent on the expression of mEH and to be inhibited by the bile acid transport inhibitor 4,4'-diisothiocyanostilbene-2,2'disulfonic acid (DIDS), as well as by other bile acids. Kinetic analysis of this system indicated a Km of 26.3 microM and a Vmax of 117 pmol/mg protein/min. The Km value is essentially the same as that observed in intact hepatocytes. The transfected MDCK cells also exhibited sodium-dependent transport of cholate at levels 150% of taurocholate in contrast to hepatocytes where cholate transport is only 30% of taurocholate levels, suggesting that total hepatocyte bile acid transport is a function of multiple transport systems with different substrate specificities, where mEH preferentially transports cholate. This hypothesis is further supported by the observation that a monoclonal antibody that partially protects (26%) taurocholate transport from inhibition by DIDS in hepatocytes provides almost complete protection (88%) from DIDS inhibition of hepatocyte cholate transport, suggesting that taurocholate is also taken up by an alternative system not recognized by this antibody. Additional support for this concept is provided by the observation that the taurocholate transport system is almost completely protected (92%) from DIDS inhibition by this antibody in MDCK cells that express mEH as the only bile acid transporter. These results demonstrate that mEH is expressed on the surface of hepatocytes as well as on transfected MDCK cells and is able to mediate sodium-dependent transport of taurocholate and cholate.

First-pass elimination of a peptidomimetic thrombin inhibitor is due to carrier-mediated uptake by the liver. Interaction with bile acid transport systems

CRC 220 (4-methoxy-2, 3, 6-trimethylphenylsulfonyl-L-aspartyl-D-4-amidinophenylalanyl -piperidide) is a competitive peptide-based trombin inhibitor with high affinity to human alpha-thrombin (Ki 2.5 nM). The amphiphilic compound exhibits virtually no systemic bioavailability despite proteolytic stability and proven enteral absorption. After intravenous application (V. jejunalis) in rats CRC 220 is almost completely excreted into bile. Simultaneous administration of bile acids considerably decreases this first-pass elimination. CRC 220 is extensively taken up in isolated rat hepatocytes by a saturable carrier-mediated transport with Km 23.7 microM and Vmax 775 pmol x mg-1 x min-1. A large part of this transport is energy-dependent. At temperatures above 20 degrees C, the uptake is accelerated exponentially. The activation energy amounts to 82 kj/mol. A minor portion of CRC 220 uptake occurs by physical diffusion with a permeability coefficient of 7.83 x 10(-7) cm/sec at 12 degrees C. Sodium ions energize CRC 220 uptake. Replacement of sodium by choline or lithium decreases the transport rate of 23-40%. In addition, a negative membrane potential facilitates the uptake. CRC 220 transport is only observed in hepatocytes: it is absent in BHK, FAO, HepG2, HPCT 1E3, and HPCT 1E3-TC cells. In the presence of 4-amidinophenylalanine derivatives, CRC 220 uptake is considerably decreased. Inhibition also occurs with bile acids and bromosulfophthalein, but less with bumetanide. Because CRC 220 inhibits bile acid uptake into hepatocytes and vice versa, the results suggest that the first-pass elimination of this amphiphilic thrombin inhibitor is due to an active carrier-mediated transport process in the basolateral plasma membrane of rat hepatocytes, and that this transport occurs via a bile acid transport system.

Current concepts of hepatic uptake, intracellular transport and biliary secretion of bile acids: physiological basis and pathophysiological changes in cholestatic liver dysfunction

Hepatic sinusoidal uptake of bile acids is mediated by defined carrier proteins against unfavourable concentration and electrical gradients. Putative carrier proteins have been identified using bile acid photoaffinity labels and more recently using immunological probes, such as monoclonal antibodies. At the sinusoidal domain, proteins with molecular weights of 49 and 54 kDa have been shown to be carriers for bile acid transport. The 49 kDa protein has been associated with the Na(+)-dependent uptake of conjugated bile acids, while the 54 kDa carrier has been involved in the Na(+)-independent bile acid uptake process. Within the hepatocyte, cytosolic proteins, such as the glutathione S-transferase (also designated the Y protein), the Y binders and the fatty acid binding proteins, are able to bind bile acids and possibly facilitate their movement to the canalicular domain. At the canalicular domain a 100 kDa carrier protein has been isolated and it has been shown by several laboratories that this particular protein is concerned with canalicular bile acid transport. The system is ATP-dependent and follows Michaelis-Menten kinetics. Interference with bile acid transport has been demonstrated by several chemicals. The mechanisms by which these chemicals inhibit bile acid transport may explain the apparent cholestatic properties observed in patients and experimental animals treated with these agents. Several studies have shown that Na+/K(+)-ATPase activity is markedly decreased in cholestasis induced by ethinyloestradiol, taurolithocholate and chlorpromazine. However, other types of interference have been described and the cholestatic effects may be the result of several mechanisms. Cholestasis is associated with several adaptive changes that may be responsible for the accumulation of bile acids and other cholephilic compounds in the blood of these patients. It may be speculated that the nature of these changes is to protect liver parenchymal cells from an accumulation of bile acids to toxic levels. However, more detailed quantitative experiments are necessary to answer questions with regard to the significance of these changes and the effect of various hepatobiliary disorders in modifying these mechanisms. It is expected that the mechanisms by which bile acid transport is regulated and efforts to understand the molecular basis for these processes will be among the areas of future research.

In situ localization of the hepatocytic Na+/Taurocholate cotransporting polypeptide in rat liver

BACKGROUND/AIMS

An Na+/taurocholate cotransporting polypeptide (Ntcp) has recently been cloned from rat liver. The aim of this study was to directly characterize the native Ntcp on the protein level and study its in situ distribution in rat liver.

METHODS

A rabbit antiserum was raised against a fusion protein containing the maltose-binding protein and the C terminus of Ntcp. Native Ntcp was localized in situ by immunofluorescent techniques. Expression of Ntcp was directly correlated with taurocholate uptake measurements in stably transfected Chinese hamster ovary cells.

RESULTS

Native Ntcp showed an apparent molecular weight of 51,000. After deglycosylation of isolated basolateral rat liver plasma membranes, the apparent molecular weight of Ntcp decreased to 33,500. In intact rat liver, Ntcp was selectively localized at the basolateral surface domain of hepatocytes. In short-term cultured hepatocytes, a positive surface immunoreaction was only obtained in detergent-permeabilized cell cultures. In stably transfected Chinese hamster ovary cells, the surface expression of immunopositive Ntcp was associated with Na(+)-dependent taurocholate uptake activity.

CONCLUSIONS

Native Ntcp represents a glycoprotein of the basolateral hepatocyte plasma membrane with its C-terminal end facing the intracellular compartment. Furthermore, surface expression of Ntcp is a prerequisite for Na(+)-dependent taurocholate uptake to occur, thus providing further proof for its bile acid transport function in rat liver.

Competition of bile acids on the sulfobromophthalein uptake in basolateral rat liver plasma membrane vesicles

The effect of different bile acids (BA) on the hepatic uptake of sulfobromophthalein (BSP) was investigated in liver plasma membrane vesicles enriched in basolateral fraction. BSP uptake was measured either in the absence (electroneutral component) or in the presence of a membrane potential (electrogenic component) induced by the addition of valinomycin in the presence of an inwardly-directed potassium gradient. BSP uptake was also measured in the presence of different BA [cholate (C), taurocholate (TC), ursodeoxycholate (UDC) and tauroursodeoxycholate (TUDC)]. Electrogenic BSP uptake was not affected by BA. Conversely, the electroneutral portion of the BSP uptake was inhibited with an inhibition constant (Ki, microM) of 230 +/- 40 for C, 103 +/- 33 for TC, 99 +/- 34 for UDC and 120 +/- 39 for TUDC, respectively (means +/- SD, N = 4). The Dixon and Cornish-Bowden plot of the data revealed an uncompetitive type of inhibition for each BA. These data indicate that the electroneutral, but not the electrogenic, BSP transport system is modulated by BA.

A membrane-bound form of protein disulfide isomerase (PDI) and the hepatic uptake of organic anions

Protein disulfide isomerase (PDI) was considered to be involved in the hepatic uptake of certain organic anions because the protein is photoaffinity labeled by photolabile derivatives of the bile acid taurocholate. Several lines of evidences including photoaffinity labeling experiments indicated a close relationship between the uptake of bile acids and the organic anion bumetanide. The possible involvement of PDI in hepatic transport processes of these organic anions was tested with polyclonal antibodies raised against a PDI-beta-galactosidase fusion protein. Western blot analysis and immunofluorescence of intact hepatocytes showed that protein disulfide isomerase is located in sinusoidal rat liver plasma membranes. This protein is immunologically identical with microsomal PDI prepared from bovine liver. The plasma membrane form of PDI is, however, not labeled by photoactivated bumetanide as revealed by two-dimensional gel electrophoresis. These results indicate that, although a membrane-bound form of the PDI is present in the sinusoidal plasma membrane of rat hepatocytes, this protein is not involved in the hepatocellular uptake of the organic anion bumetanide.

Purification and partial sequence of proteins involved in the cholic acid transport into rat liver hepatocytes

Two proteins, in previous work labeled by affinity markers derived from taurocholic acid, were purified and partially sequenced. Antibodies were raised against purified proteins, and cross-reactions were carefully checked. The influence of these antibodies upon taurocholic acid import into vesicles from rat liver plasma membranes was measured, and showed a distinct inhibition of transport in the case of the 54 kD protein.

Two different mRNAs from rat liver code for the transport of bumetanide and taurocholate in Xenopus laevis oocytes

The aim of this study was to elucidate whether bumetanide, which is a competitive inhibitor of carrier mediated bile acid uptake in liver cells, is transported by bile acid carriers. The expression of hepatocellular transport proteins for bile acid uptake and the uptake of the loop diuretic bumetanide was therefore studied in Xenopus laevis oocytes by injection of rat liver poly(A)(+)-RNA. Three hours after injection, a 70% increase in [3H]taurocholate uptake versus noninjected oocytes was accompanied by an increase in only 24% in the uptake of [3H]bumetanide. Size fractionation of the poly(A)(+)-RNA yielded 33 mRNA fractions of which fraction 21 accounted for an 800% increase of taurocholate transport with only a slight increase in bumetanide uptake. Bumetanide transport was coded by mRNA-fraction 18, which stimulated uptake by 160-200% with a concomitant small increase in taurocholate uptake. Uptake of cholate was induced by both mRNA fractions with almost 2.5 fold greater expression by the bumetanide fraction. Oocyte transport of taurocholate (expressed by fraction 21) and bumetanide transport (expressed by fraction 18) were characterized in terms of Na+ dependency, inhibition by 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid (DIDS) and mutual competition. The results indicate that the bumetanide transporter mRNA is clearly different from the mRNA for the taurocholate transport protein. The mRNA fraction 18 was used for the construction of a cDNA library.

Characterization and chemical modification of the Na(+)-dependent bile-acid transport system in brush-border membrane vesicles from rabbit ileum

The Na(+)-dependent uptake system for bile acids in the ileum from rabbit small intestine was characterized using brush-border membrane vesicles. The uptake of [3H]taurocholate into vesicles prepared from the terminal ileum showed an overshoot uptake in the presence of an inwardly-directed Na(+)-gradient ([Na+]out > [Na+]in), in contrast to vesicles prepared from the jejunum. The Na(+)-dependent [3H]taurocholate uptake was cis-inhibited by natural bile acid derivatives, whereas cholephilic organic compounds, such as phalloidin, bromosulphophthalein, bilirubin, indocyanine green or DIDS - all interfering with hepatic bile-acid uptake - did not show a significant inhibitory effect. Photoaffinity labeling of ileal membrane vesicles with 3,3-azo- and 7,7-azo-derivatives of taurocholate resulted in specific labeling of a membrane polypeptide with apparent molecular mass 90 kDa. Bile-acid derivatives inhibiting [3H]taurocholate uptake by ileal vesicles also inhibited labeling of the 90 kDa polypeptide, whereas compounds with no inhibitory effect on ileal bile-acid transport failed to show a significant effect on the labeling of the 90 kDa polypeptide. The involvement of functional amino-acid side-chains in Na(+)-dependent taurocholate uptake was investigated by chemical modification of ileal brush-border membrane vesicles with a variety of group-specific agents. It was found that (vicinal) thiol groups and amino groups are involved in active ileal bile-acid uptake, whereas carboxyl- and hydroxyl-containing amino acids, as well as tyrosine, histidine or arginine are not essential for Na(+)-dependent bile-acid transport activity. The irreversible inhibition of [3H]taurocholate transport by DTNB or NBD-chloride could be partially reversed by thiols like 2-mercaptoethanol or DTT. Furthermore, increasing concentrations of taurocholate during chemical modification with NBD-chloride were able to protect the ileal bile-acid transporter from inactivation. These findings suggest that a membrane polypeptide of apparent M(r) 90,000 is a component of the active Na(+)-dependent bile-acid reabsorption system in the terminal ileum from rabbit small intestine. Vicinal thiol groups and amino groups of the transport system are involved in Na(+)-dependent transport activity, whereas other functional amino acids are not essential for transport activity.

Leukotriene uptake by hepatocytes and hepatoma cells

The uptake of tritiated cysteinyl leukotrienes (LTC4, LTD4, LTE4) and LTB4 was investigated in freshly isolated rat hepatocytes and different hepatoma cell lines under initial-rate conditions. Leukotriene uptake by hepatocytes was independent of an Na+ gradient and a K+ diffusion potential across the hepatocyte membranes as established in experiments with isolated hepatocytes and plasma membrane vesicles. Kinetic experiments with isolated hepatocytes indicated a low-Km system and a non-saturable system for the uptake of cysteinyl leukotrienes as well as LTB4 under the conditions used. AS-30D hepatoma cells and human Hep G2 hepatoma cells were deficient in the uptake of cysteinyl leukotrienes, but showed significant accumulation of LTB4. Moreover, only LTB4 was metabolized in Hep G2 hepatoma cells. Competition studies on the uptake of LTE4 and LTB4 (10 nM each) indicated inhibition by the organic anions bromosulfophthalein, S-decyl glutathione, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonate, probenecid, docosanedioate, and hexadecanedioate (100 microM each), but not by taurocholate, the amphiphilic cations verapamil and N-propyl ajmaline, and the neutral glycoside ouabain. Cholate and the glycoside digitoxin were inhibitors of LTB4 uptake only. Bromosulfophthalein, the strongest inhibitor of leukotriene uptake by hepatocytes, did not inhibit LTB4 uptake by Hep G2 hepatoma cells under the same experimental conditions. Leukotriene-binding proteins were analyzed by comparative photoaffinity labeling of human hepatocytes and Hep G2 hepatoma cells using [3H]LTE4 and [3H]LTB4 as the photolabile ligands. Predominant leukotriene-binding proteins with apparent molecular masses in the ranges of 48-58 kDa and 38-40 kDa were labeled by both leukotrienes in the particulate and in the cytosolic fraction of hepatocytes, respectively. In contrast, no labeling was obtained with [3H]LTE4 in Hep G2 cells. With [3H]LTB4 a protein with a molecular mass of about 48 kDa was predominantly labeled in the particulate fraction of the hepatoma cells, whereas in the cytosolic fraction a labeled protein in the range of 40 kDa was detected. Our results provide evidence for the existence of distinct uptake systems for cysteinyl leukotrienes and LTB4 at the sinusoidal membrane of hepatocytes; however, some of the inhibitors tested interfere with both transport systems. Only LTB4, but not cysteinyl leukotrienes, is taken up and metabolized by the transformed hepatoma cells.

Separation and purification by two-dimensional gel electrophoresis of a 52-54 kDa bumetanide binding protein from rat liver plasma membranes

By affinity labeling with photolabile [3H]bumetanide, a 52-54 kDa bumetanide binding protein was identified in the sinusoidal plasma membrane fraction from rat liver. The protein is assumed to represent the carrier for hepatic uptake of loop diuretics. By two-dimensional (2D) gel electrophoresis we have purified this protein from hepatocytes, sinusoidal plasma membranes and subfractions of associated and integral plasma membrane proteins. Amongst more than 20 protein spots, a single integral plasma membrane protein was detected. The apparent pI of this molecule is 6.7. Specific labeling of this protein was not found in the fraction of associated plasma membrane proteins. To detect possible binding of radioactive bumetanide to microsomal cytochrome P450s, photolabeling experiments with integral plasma membrane proteins were performed under nitrogen/carbon monoxide atmosphere and in the presence of piperonyl butoxide. Labeling of the 52-54 kDa protein was not affected by these inhibitors of P450 enzymes. Taken together, these results indicate that the bumetanide binding protein is very likely to be a non-microsomal integral plasma membrane protein.

Characterization of the transport of a synthetic bile salt, iodinated cholyl-glycyl-tyrosine, in isolated cultured rat hepatocytes

The uptake of tri-hydroxy conjugated bile salts by hepatocytes is principally by a sodium-dependent carrier. We examined the uptake kinetics of the high-specific-activity, hydroxylated, conjugated bile salt 125I-labeled cholyl-glycyl-tyrosine, to determine whether this synthetic bile salt was transported by the sodium-dependent bile salt system. 125I-labeled cholyl-glycyl-tyrosine was synthesized, and its transport kinetics were studied in freshly cultured rat hepatocytes. Uptake into hepatocytes was time and temperature dependent and was decreased by the inhibitors diisothiocyanodisulfonic acid stilbene, probenecid and carbonyl cyanide chlorophenyl hydrazone, demonstrating carrier mediation and energy dependence. At concentrations of iodinated cholyl-glycyl-tyrosine less than 10 mumol/L, uptake was 27% +/- 5% sodium dependent, whereas at concentrations from 10 mumol/L to 40 mumol/L uptake was 52% +/- 4% sodium dependent. The apparent affinity for uptake of 125I-labeled cholyl-glycyl-tyrosine was 8 +/- 2 mumol/L, and the maximal velocity was 50 +/- 20 pmol/micrograms DNA/min. Both taurocholate and indocyanine green inhibited uptake of 125I-labeled cholyl-glycyl-tyrosine. Indocyanine green inhibited the uptake of 125I-labeled cholyl-glycyl-tyrosine (Ki = 10 microns) more effectively than taurocholate (Ki = 20 microns). We conclude that 125I-labeled cholyl-glycyl-tyrosine is not a specific probe for either sodium-dependent bile salt or sodium-independent organic anion carriers, but appears to use both systems in a concentration-dependent manner in cultured rat hepatocytes.

Interaction of sulfonylureas with the transport of bile acids into hepatocytes

The sulfonylurea compounds glisoxepide and glibenclamide inhibit the uptake of bile acids into isolated rat hepatocytes. The Ki values for the inhibition of cholate uptake was 9 microM with glibenclamide and 200 microM with glisoxepide. The inhibition of cholate uptake by both sulfonylureas was noncompetitive. Uptake of the conjugated bile acid taurocholate was inhibited by glibenclamide, Ki = 75 microM. Again the inhibition was noncompetitive. Glisoxepide inhibited taurocholate uptake only in the absence of sodium ions. Under sodium-free conditions glisoxepide also strongly inhibited cholate uptake. The inhibition was competitive, Ki = 42 microM. Both bile acids interfered with the hepatocellular uptake of [3H]glisoxepide, with IC50 values of 375 and 467 microM for cholate and taurocholate, respectively. The uptake of [3H]glibenclamide was inhibited by cholate, IC50 = 328 microM, but not by taurocholate. Glisoxepide uptake was further inhibited by blockers of the hepatocellular monocarboxylate transporter, by the loop diuretic bumetanide, by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) and by sulfate. Glibenclamide uptake was weakly inhibited by DIDS and by anthracene-9-carboxylic acid (A-9-C) but not by bumetanide and sulfate. Neither bromosulfophthalein nor the fatty acid oleate inhibited glisoxepide or glibenclamide uptake. These results are consistent with the transport of glisoxepide via the transport system for the unconjugated bile acid cholate. Glibenclamide uptake is mediated by a still unknown hepatocellular transport system.

Uptake of taurocholate, a vecuronium-like organic cation, ORG 9426, and ouabain into carcinogen-induced diploid and polyploid hepatocytes obtained by centrifugal elutriation

Bile acid uptake, an important function of differentiated hepatocytes, is decreased in hepatocellular carcinomas and gamma-glutamyltranspeptidase-positive, putatively preneoplastic hepatocytes. Whether hepatic uptake is also changed in carcinogen-induced diploid hepatocytes versus polyploid hepatocytes is unknown. The present study has determined whether the hepatic uptake of three model compounds, an anionic bile acid, an organic cation and a neutral organic compound, into diploid cells is different from that in polyploid hepatocytes. These two hepatocyte populations were separated from the parent freshly isolated hepatocyte suspension by centrifugal elutriation. Flow cytometric analysis indicated that the diploid fraction contained approximately 83% diploid cells and that the polyploid fraction had about 84% polyploid hepatocytes. Initial uptake velocity was determined for taurocholate (1-50 microM), ORG 9426 (20-400 microM), a vecuronium-like cation, and ouabain (20-500 microM). Apparent Km was not different between diploid and polyploid cells for the three tested substrates, whereas apparent Vmax was decreased in diploid hepatocytes for taurocholate and ouabain by 42 and 55%, respectively. There were no changes in the hepatic uptake of ORG 9426. These data indicate that uptake by the bile acid/multispecific carrier is compromised in carcinogen-induced diploid cells.

Functional molecular mass of the 14C-azidobenzamidotaurocholic acid binding proteins in hepatocellular bile acid transport systems

The apparent target size of 14C-azidobenzamidotaurocholate binding proteins in basolateral rat liver plasma membranes (blPm) was determined by analysis of the radiation induced decrease of the binding of this photoreactive taurocholate analog to blPm. Radiation causes a dose-dependent mono-exponential reduction of binding of ABATC to the protein subunits with molecular masses of 48-50 and 52-54 kDa in SDS-PAGE. The minimal functional molecular mass of the 48-50 and 52-54 kDa ABATC binding proteins was determined to be 99 +/- 8.2 and 93.2 +/- 7 kDa, respectively.

Photoaffinity labeling of plasma membrane proteins involved in the transport of loop diuretics into hepatocytes

To identify proteins involved in the hepatocellular uptake of loop diuretics, [3H]bumetanide was photoactivated by light flash in the presence of either intact isolated rat hepatocytes, rat liver basolateral plasma membranes or integral membrane proteins extracted from the basolateral plasma membranes. Proteins of 52-54, 48, 33, 27, 25 and 23 kDa in sodium dodecyl sulfate (SDS) gel electrophoresis were radiolabeled on intact hepatocytes. On liver basolateral plasma membranes a 50-52 kDa protein was the most intensely labeled protein. After separation into integral and associated membrane proteins by extraction with Triton X-114, radioactive labeling was only found in integral membrane proteins with a molecular weight of 50-52 kDa. Photoactivated bumetanide irreversibly inhibited the hepatocellular uptake of cholate, taurocholate but not of serine. Binding proteins for photoactivated bumetanide were absent on AS 30-D ascites hepatoma cells. Labeling of all proteins was sodium dependent in intact hepatocytes but was sodium independent in plasma membranes. Labeling was prevented by non-labeled bumetanide and by the loop diuretics piretanide and furosemide. Labeling protection was further achieved with organic anions such as bromosulfophthalein, rifampicin, probenecid and by the bile acids taurocholate, deoxycholate and dehydrocholate. The radiolabeled proteins did not belong to the bumetanide-sensitive NaCl/KCl co-transport system which apparently does not occur in intact isolated rat hepatocytes.

Characterization of a novel 14 kDa bile acid-binding protein from rat ileal cytosol

A 14 kDa polypeptide in rat ileal cytosol has been identified as the major intestinal cytosolic bile acid-binding protein (I-BABP) by photoaffinity labeling with the radiolabeled 7,7-azo derivative of taurocholate (7,7-azo-TC). To further characterize I-BABP, the protein was purified by lysylglycocholate Sepharose 4B affinity and DE-52 anion-exchange chromatography. The purified I-BABP contained a single 14 kDa band on SDS-PAGE. The 14 kDa protein showed a 26-fold increase in binding affinity for [3H]7,7-azo-TC compared to cytosolic protein. Immunoblotting of protein fractions separated by affinity chromatography showed that neither liver fatty acid binding protein (L-FABP) nor intestinal fatty acid binding protein (I-FABP) bind to the affinity column and that the 14 kDa protein which bound to the column and was subsequently eluted with detergent did not cross-react with anti-L-FABP or anti-I-FABP. The 14 kDa protein labeled with [3H]7,7-azo-TC was radioimmunoprecipitated from cytosol by rabbit antiserum raised against purified I-BABP. I-BABP was shown to have a blocked N-terminus; however, its mixed internal sequence generated from cyanogen bromide-cleaved protein and amino acid composition indicated that it was related to (although clearly distinct from) both I-FABP and L-FABP. These studies have isolated a 14 kDa bile acid-binding protein from rat ileal cytosol which is immunologically and biochemically distinct from I-FABP and L-FABP.

Residence time distributions of solutes in the perfused rat liver using a dispersion model of hepatic elimination: 1. Effect of changes in perfusate flow and albumin concentration on sucrose and taurocholate

The residence time distributions of sucrose and taurocholate have been determined from the outflow concentration-time profiles after bolus input into an in situ perfused rat liver preparation. The normalized variance (and the dispersion number) appeared to be independent of perfusate flow rate (10 to 37 ml/min) and perfusate albumin concentration (0.5%). The apparent volume of distribution for sucrose appeared to increase with flow rate but was unaffected by the concentration of albumin (0-5%) present in the perfusate. The changes in taurocholate availability with flow rate were adequately accounted for by the dispersion model, whereas taurocholate availability-protein binding changes required an albumin-mediated transport model to be used in conjunction with the dispersion model.

Defective drug uptake contributing to multidrug resistance in hepatoma cells can be evaluated in vitro

In clinical practice the acquired or de novo resistance of tumors to antitumor chemotherapy remains a big problem. However, in the past few years some progress has been made in understanding the two principal mechanisms: metabolic alterations leading to a reduced cytostatic or cytotoxic effect of drugs, and reduced accumulation of drugs within the tumor cells [15, 34, 35]. The second phenomenon is usually attributed to the ability of tumor cells to accelerate the efflux of various xenobiotics. This phenomenon is considered primarily responsible for the development of multidrug resistance (MDR). However, loss or impairment of drug uptake by the tumor cells may also contribute to resistance to antitumor drugs. This paper focuses on recent findings with hepatoma cells, which support this view.