Organic solute transporter, OSTalpha-OSTbeta: its role in bile acid transport and cholestasis

Bile salt signaling and bile salt-based therapies in cardiometabolic disease

Bile salts have an established role in the emulsification and intestinal absorption of dietary lipids, and their homeostasis is tightly controlled by various transporters and regulators in the enterohepatic circulation. Notably, emerging evidence points toward bile salts as major modulators of cardiometabolic disease (CMD), an umbrella disease of disorders affecting the heart and blood vessels that is caused by systemic metabolic diseases such as Type 2 diabetes mellitus (T2DM) and metabolic dysfunction-associated steatotic liver disease (MASLD), the latter encompassing also metabolic dysfunction-associated steatohepatitis (MASH). The underlying mechanisms of protective effects of bile salts are their hormonal properties, enabling them to exert versatile metabolic effects by activating various bile salt-responsive signaling receptors with the nuclear farnesoid X receptor (FXR) and the Takeda G-protein-coupled receptor 5 (TGR5) as most extensively investigated. Activation of FXR and TGR5 is involved in the regulation of glucose, lipid and energy metabolism, and inflammation. Bile salt-based therapies directly targeting FXR and TGR5 signaling have been evaluated for their therapeutic potential in CMD. More recently, therapeutics targeting bile salt transporters thereby modulating bile salt localization, dynamics, and signaling, have been developed and evaluated in CMD. Here, we discuss the current knowledge on the contribution of bile salt signaling in the pathogenesis of CMD and the potential of bile salt-based therapies for the treatment of CMD.

The regulation of tissue-specific farnesoid X receptor on genes and diseases involved in bile acid homeostasis

Bile acids (BAs) facilitate the absorption of dietary lipids and vitamins and have also been identified as signaling molecules involved in regulating their own metabolism, glucose and lipid metabolism, as well as immunity. Disturbances in BA homeostasis are associated with various enterohepatic and metabolic diseases, such as cholestasis, nonalcoholic steatohepatitis, inflammatory bowel disease, and obesity. As a key regulator, the nuclear orphan receptor farnesoid X receptor (FXR, NR1H4) precisely regulates BA homeostasis by transcriptional regulation of genes involved in BA synthesis, metabolism, and enterohepatic circulation. FXR is widely regarded as the most potential therapeutic target. Obeticholic acid is the only FXR agonist approved to treat patients with primary biliary cholangitis, but its non-specific activation of systemic FXR also causes high-frequency side effects. In recent years, developing tissue-specific FXR-targeting drugs has become a research highlight. This article provides a comprehensive overview of the role of tissue-specific intestine/liver FXR in regulating genes involved in BA homeostasis and briefly discusses tissue-specific FXR as a therapeutic target for treating diseases. These findings provide the basis for the development of tissue-specific FXR modulators for the treatment of enterohepatic and metabolic diseases associated with BA dysfunction.

Pharmacological inhibition of MEK1/2 signaling disrupts bile acid metabolism through loss of Shp and enhanced Cyp7a1 expression

The RAS-MAPK signaling pathway is one of the most frequently dysregulated pathways in human cancer. Small molecule inhibitors directed against this pathway have clinical activity in patients with various cancer types and can improve patient outcomes. However, the use of these drugs is associated with adverse effects, which can result in dose reduction or treatment interruption. A better molecular understanding of on-target, off-tumor effects may improve toxicity management. In the present study, we aimed to identify early initiating biological changes in the liver upon pharmacological inhibition of the RAS-MAPK signaling pathway. To this end, we tested the effect of MEK inhibitor PD0325901 using mice and human hepatocyte cell lines. Male C57BL/6 mice were treated with either vehicle or PD0325901 for six days, followed by transcriptome analysis of the liver and phenotypic characterization. Pharmacological MEK inhibition altered the expression of 423 genes, of which 78 were upregulated and 345 were downregulated. We identified Shp, a transcriptional repressor, and Cyp7a1, the rate-limiting enzyme in converting cholesterol to bile acids, as the top differentially expressed genes. PD0325901 treatment also affected other genes involved in bile acid regulation, which was associated with changes in the composition of plasma bile acids and composition and total levels of fecal bile acids and elevated predictive biomarkers of early liver toxicity. In conclusion, short-term pharmacological MEK inhibition results in profound changes in bile acid metabolism, which may explain some of the clinical adverse effects of pharmacological inhibition of the RAS-MAPK pathway, including gastrointestinal complications and hepatotoxicity.

Transcriptomic responses to cytotoxic drug cisplatin in water flea Daphnia magna

Cytotoxic drugs have been recognized by the European Union as the potential threat in the aquatic environment. As a typical cytotoxic drug, effects of long-term exposure to cisplatin at the environmentally relevant concentrations on the crustacean health and its molecular mechanism remain undetermined. In this study, the growth and reproduction of Daphnia magna resulting from cisplatin exposure were initially assessed. While the phenotypes were not altered in 2 μg L^-1, 20 μg L^-1, and 200 μg L^-1 treatment groups, cisplatin at 500 µg L^-1 significantly reduced the offspring number to 8-13 neonates in each brood, which was lower than 13-27 neonates in the control group. In addition to the delay in the time of first pregnancy, the body length was decreased by approximate 12.13% at day 7. Meanwhile, all daphnids died after exposure to 500 µg L^-1 cisplatin for 17 days. Transcriptome profiling bioassays were performed for 10 days to explore the alternation at the molecular level. Briefly, 980 (257 up- and 723 down-regulated), 429 (182 up- and 247 down-regulated) and 1984 (616 up-regulated and 1368 down-regulated) genes were differentially expressed (adj p < 0.05) in low (2 μg L^-1), medium (200 μg L^-1) and high (500 μg L^-1) cisplatin treatment groups, respectively. Differentially expressed genes were primarily enriched in the digestion and absorption, nerve conduction, endocrine interference, and circulatory related pathways. Specifically, the down-regulated digestive secretion and nutrient absorption and neuronal conduction pathways may lead to insufficient energy supply involved in growth and reproduction, and hinder ovarian development and cell growth. Down-regulation of ovarian steroids and relaxin signaling pathways may be related to the reduction of offspring number and delayed pregnancy, and reduced body length of D. magna may attribute to the enrichment of insulin secretion pathway. In addition, the death of D. magna may result from the reduced expression of genes in cardiomyocyte contraction and apoptosome processes. Taken together, this study revealed the potential toxic mechanism of cisplatin in a model water flea.

Versatile Triad Alliance: Bile Acid, Taurine and Microbiota

Taurine is the most abundant free amino acid in the body, and is mainly derived from the diet, but can also be produced endogenously from cysteine. It plays multiple essential roles in the body, including development, energy production, osmoregulation, prevention of oxidative stress, and inflammation. Taurine is also crucial as a molecule used to conjugate bile acids (BAs). In the gastrointestinal tract, BAs deconjugation by enteric bacteria results in high levels of unconjugated BAs and free taurine. Depending on conjugation status and other bacterial modifications, BAs constitute a pool of related but highly diverse molecules, each with different properties concerning solubility and toxicity, capacity to activate or inhibit receptors of BAs, and direct and indirect impact on microbiota and the host, whereas free taurine has a largely protective impact on the host, serves as a source of energy for microbiota, regulates bacterial colonization and defends from pathogens. Several remarkable examples of the interaction between taurine and gut microbiota have recently been described. This review will introduce the necessary background information and lay out the latest discoveries in the interaction of the co-reliant triad of BAs, taurine, and microbiota.

Bile Acids Transporters of Enterohepatic Circulation for Targeted Drug Delivery

Bile acids (BAs) are important steroidal molecules with a rapidly growing span of applications across a variety of fields such as supramolecular chemistry, pharmacy, and biomedicine. This work provides a systematic review on their transport processes within the enterohepatic circulation and related processes. The focus is laid on the description of specific or less-specific BA transport proteins and their localization. Initially, the reader is provided with essential information about BAs′ properties, their systemic flow, metabolism, and functions. Later, the transport processes are described in detail and schematically illustrated, moving step by step from the liver via bile ducts to the gallbladder, small intestine, and colon; this description is accompanied by descriptions of major proteins known to be involved in BA transport. Spillage of BAs into systemic circulation and urine excretion are also discussed. Finally, the review also points out some of the less-studied areas of the enterohepatic circulation, which can be crucial for the development of BA-related drugs, prodrugs, and drug carrier systems.

Bile acid metabolism dysregulation associates with cancer cachexia: roles of liver and gut microbiome

BACKGROUND

Cancer cachexia is a multifactorial metabolic syndrome in which bile acid (BA) metabolism might be involved. The aim of the present study was to clarify the contribution of liver and gut microbiota to BA metabolism disturbance in cancer cachexia and to check the possibility of targeting BA metabolism using agents such as tauroursodeoxycholic acid (TUDCA) for cancer cachexia therapy.

METHODS

The BA profiles in liver, intestine, and serum of mice with cancer cachexia induced by inoculation of colon C26 tumour cells were analysed using metabolomics methods and compared with that of control mice. Proteomic analysis of liver protein expression profile and 16S rRNA gene sequencing analysis of gut microbiota composition in cancer cachexia mice were conducted. Expression levels of genes related to farnesoid X receptor (FXR) signalling pathway in the intestine and liver tissues were analysed using RT-PCR analysis. The BA profiles in serum of clinical colon cancer patients with or without cachexia were also analysed and compared with that of healthy volunteers. The effects of TUDCA in treating cancer cachexia mice were observed.

RESULTS

In the liver of cancer cachexia mice, expression of BA synthesis enzymes was inhibited while the amount of total BAs increased (P < 0.05). The ratios of conjugated BAs/un-conjugated BAs significantly increased in cancer cachexia mice liver (P < 0.01). Gut microbiota dysbiosis such as decrease in Lachnospiraceae and increase in Enterobacteriaceae was observed in the intestine of cancer cachexia mice, and microbial metabolism of BAs was reduced. Increase in expression of FGF15 in intestine (P < 0.01) suggested the activation of FXR signalling pathway which might contribute to the regulation of BA synthesis enzymes, transporters, and metabolic enzymes. Increase in the BA conjugation was observed in the serum of cancer cachexia mice. Results of clinical patients showed changes in BA metabolism, especially the increase in BA conjugation, and also suggested compensatory mechanism in BA metabolism regulation. Oral administration of 50 mg/kg TUDCA could significantly ameliorate the decrease in body weight (P < 0.001), muscle loss (P < 0.001), and atrophy of heart and liver (P < 0.05) in cancer cachexia mice without influence on tumour growth.

CONCLUSIONS

Bile acid metabolism dysregulation such as decrease in BA synthesis, increase in BA conjugation, and decrease in BA microbial metabolism was involved in development of cancer cachexia in mice. Targeting BA metabolism using agents such as TUDCA might be helpful for cancer cachexia therapy.

miRNA-382-5p Suppresses the Expression of Farnesoid X Receptor to Promote Progression of Liver Cancer

Background

The dysregulation of microRNAs (miRNAs) and hepatotoxicity due to the aberrant accumulation of bile acids (BAs) are notorious causes that predispose an individual to the development of hepatocellular carcinoma (HCC). Farnesoid X receptor (FXR), encoded by NR1H4 gene, has been identified as a crucial BA receptor to maintain the homeostasis of BA pool and its expression is decreased in HCC. miR-382-5p plays an important role in the pathogenesis of many human malignancies and was reported to promote the proliferation and differentiation of normal liver cells and liver regeneration. However, there is still some controversy about its role in HCC microenvironment. This study aims to explore the expression pattern of miR-382-5p in HCC and its role in regulating FXR during the development of HCC.

Methods

Tissues collected from 30 HCC patients were subjected to extraction of total RNA and quantitative real-time PCR (qRT-PCR) for the analyses of miR-382-5p expression and NR1H4 mRNA levels, and their expressions were verified by analyzing the online HCC-related GSE datasets. The role of miR-382-5p in regulating cellular proliferation and expression of FXR in different HCC cell lines was analyzed by qRT-PCR, Western Blot, real-time cellular analysis (RTCA) and luciferase reporter assays. The role of miR-382-5p in regulating downstream genes of FXR in HCC cells was also analyzed.

Results

miR-382-5p was upregulated in HCC tissues and inversely associated with the downregulation of NR1H4 mRNA levels. The luciferase reporter assay proved that miR-382-5p directly targeted the 3ʹ-untranslated region (3ʹ-UTR) of human NR1H4 mRNA. Overexpression of miR-382-5p led to a malignant proliferation of HCC cells by suppressing the expression of FXR. In contrast, blocking the endogenous miR-382-5p was sufficient to suppress the cellular proliferation rate of HCC through increasing FXR expression. Additionally, miR-382-5p inhibited the expression of some target genes of FXR, including SHP, FGF19 and SLC51A, and this inhibitory effect was FXR-dependent.

Conclusion

Therefore, miR-382-5p promotes the progression of HCC in vitro by suppressing FXR and could serve as a valuable therapeutic target for HCC treatment.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Lipid Emulsion Use in Pediatric Patients Requiring Long-Term Parenteral Nutrition

The ability to deliver nutrients via parenteral nutrition (PN) has markedly improved the prognosis of infants and children with intestinal failure. Technical refinements and advances in knowledge have led to the development of highly sophisticated PN solutions that are tailored to meet the needs of pediatric patients. However, children who require long-term PN have an increased risk of complications such as catheter-related sepsis, liver disease, and bone disease. Although the pathogenesis of intestinal failure associated liver disease (IFALD) is multifactorial, studies have identified a possible link between the dose of lipid emulsions based on soybean oil and cholestasis, shown to occur with a significantly higher frequency in patients receiving >1 g lipids/kg/d. Potential contributing factors include oxidative stress, high ω-6 polyunsaturated fatty acid (PUFA) and phytosterol content, and relatively low α-tocopherol levels. Lipid emulsions containing fish oil offer potential advantages compared with traditional emulsions with a high soybean oil content, such as decreased ω-6 and increased ω-3 PUFA concentrations, high concentrations of α-tocopherol, and reduced phytosterol content. Studies in PN-dependent children at risk for IFALD have shown that lipid emulsions containing fish oil reduce the risk of cholestasis and improve biochemical measures of hepatobiliary function compared with pure soybean oil emulsions. This review summarizes evidence regarding the role of lipid emulsions in the management of pediatric patients with intestinal failure requiring long-term PN, with a particular focus on the prevention and treatment of IFALD.

Early pathogenesis of cystic fibrosis gallbladder disease in a porcine model

Hepatobiliary disease causes significant morbidity in people with cystic fibrosis (CF), yet this problem remains understudied. We previously found that newborn CF pigs have microgallbladders with significant luminal obstruction in the absence of infection and consistent inflammation. In this study, we sought to better understand the early pathogenesis of CF pig gallbladder disease. We hypothesized that loss of CFTR would impair gallbladder epithelium anion/liquid secretion and increase mucin production. CFTR was expressed apically in non-CF pig gallbladder epithelium but was absent in CF. CF pig gallbladders lacked cAMP-stimulated anion transport. Using a novel gallbladder epithelial organoid model, we found that Cl- or HCO3- was sufficient for non-CF organoid swelling. This response was absent for non-CF organoids in Cl-/HCO3--free conditions and in CF. Single-cell RNA-sequencing revealed a single epithelial cell type in non-CF gallbladders that coexpressed CFTR, MUC5AC, and MUC5B. Despite CF gallbladders having increased luminal MUC5AC and MUC5B accumulation, there was no significant difference in the epithelial expression of gel-forming mucins between non-CF and CF pig gallbladders. In conclusion, these data suggest that loss of CFTR-mediated anion transport and fluid secretion contribute to microgallbladder development and luminal mucus accumulation in CF.

Bile acid metabolism and circadian rhythms

Circadian rhythms are biological systems that synchronize cellular circadian oscillators with the organism's daily feeding-fasting or rest-activity cycles in mammals. Circadian rhythms regulate nutrient absorption and utilization at the cellular level and are closely related to obesity and metabolic disorders. Bile acids are important modulators that facilitate nutrient absorption and regulate energy metabolism. Here, we provide an overview of the current connections and future perspectives between the circadian clock and bile acid metabolism as well as related metabolic diseases. Feeding and fasting cycles influence bile acid pool size and composition, and bile acid signaling can respond to acute lipid and glucose utilization and mediate energy balance. Disruption of circadian rhythms such as shift work, irregular diet, and gene mutations can contribute to altered bile acid metabolism and heighten obesity risk. High-fat diets, alcohol, and gene mutations related to bile acid signaling result in desynchronized circadian rhythms. Gut microbiome also plays a role in connecting circadian rhythms with bile acid metabolism. The underlying mechanism of how circadian rhythms interact with bile acid metabolism has not been fully explored. Sustaining bile acid homeostasis based on circadian rhythms may be a potential therapy to alleviate metabolic disturbance.

Bile acid transporter-mediated oral drug delivery

Bile acids are synthesized in the liver, stored in the gallbladder, and secreted into the duodenum at meals. Apical sodium-dependent bile acid transporter (ASBT), an ileal Na⁺-dependent transporter, plays the leading role of bile acid absorption into enterocytes, where bile acids are delivered to basolateral side by ileal bile acid binding protein (IBABP) and then released by organic solute transporter OSTα/β. The absorbed bile acids are delivered to the liver via portal vein. In this process called "enterohepatic recycling", only 5% of the bile acid pool (~3 g in human) is excreted in feces, indicating the large recycling capacity and high transport efficacy of ASBT-mediated absorption. Therefore, bile acid transporter-mediated oral drug delivery has been regarded as a feasible and potential strategy to improve the oral bioavailability. This review introduces the key factors in enterohepatic recycling, especially the mechanism of bile acid uptake by ASBT, and the development of bile acid-based oral drug delivery for ASBT-targeting, including bile acid-based prodrugs, bile acid/drug electrostatic complexation and bile acid-containing nanocarriers. Furthermore, the specific transport pathways of bile acid in enterocytes are described and the recent finding of lymphatic delivery of bile acid-containing nanocarriers is discussed.

Novel insights into the organic solute transporter alpha/beta, OSTα/β: From the bench to the bedside

Organic solute transporter alpha/beta (OSTα/β) is a heteromeric solute carrier protein that transports bile acids, steroid metabolites and drugs into and out of cells. OSTα/β protein is expressed in various tissues, but its expression is highest in the gastrointestinal tract where it facilitates the recirculation of bile acids from the gut to the liver. Previous studies established that OSTα/β is upregulated in liver tissue of patients with extrahepatic cholestasis, obstructive cholestasis, and primary biliary cholangitis (PBC), conditions that are characterized by elevated bile acid concentrations in the liver and/or systemic circulation. The discovery that OSTα/β is highly upregulated in the liver of patients with nonalcoholic steatohepatitis (NASH) further highlights the clinical relevance of this transporter because the incidence of NASH is increasing at an alarming rate with the obesity epidemic. Since OSTα/β is closely linked to the homeostasis of bile acids, and tightly regulated by the nuclear receptor farnesoid X receptor, OSTα/β is a potential drug target for treatment of cholestatic liver disease, and other bile acid-related metabolic disorders such as obesity and diabetes. Obeticholic acid, a semi-synthetic bile acid used to treat PBC, under review for the treatment of NASH, and in development for the treatment of other metabolic disorders, induces OSTα/β. Some drugs associated with hepatotoxicity inhibit OSTα/β, suggesting a possible role for OSTα/β in drug-induced liver injury (DILI). Furthermore, clinical cases of homozygous genetic defects in both OSTα/β subunits resulting in diarrhea and features of cholestasis have been reported. This review article has been compiled to comprehensively summarize the recent data emerging on OSTα/β, recapitulating the available literature on the structure-function and expression-function relationships of OSTα/β, the regulation of this important transporter, the interaction of drugs and other compounds with OSTα/β, and the comparison of OSTα/β with other solute carrier transporters as well as adenosine triphosphate-binding cassette transporters. Findings from basic to more clinically focused research efforts are described and discussed.

Dietary intake of bioactive ingredients impacts liver and adipose tissue transcriptomes in a porcine model of prepubertal early obesity

Global prevalence of obesity has increased to epidemic proportions over the past 40 years, with childhood obesity reaching alarming rates. In this study, we determined changes in liver and adipose tissue transcriptomes of a porcine model for prepubertal early obesity induced by a high-calorie diet and supplemented with bioactive ingredients. A total of 43 nine-weeks-old animals distributed in four pens were fed with four different dietary treatments for 10 weeks: a conventional diet; a western-type diet; and a western-type diet with Bifidobacterium breve and rice hydrolysate, either adding or not omega-3 fatty acids. Animals fed a western-type diet increased body weight and total fat content and exhibited elevated serum concentrations of cholesterol, whereas animals supplemented with bioactive ingredients showed lower body weight gain and tended to accumulate less fat. An RNA-seq experiment was performed with a total of 20 animals (five per group). Differential expression analyses revealed an increase in lipogenesis, cholesterogenesis and inflammatory processes in animals on the western-type diet while the supplementation with bioactive ingredients induced fatty acid oxidation and cholesterol catabolism, and decreased adipogenesis and inflammation. These results reveal molecular mechanisms underlying the beneficial effects of bioactive ingredient supplementation in an obese pig model.

Roles of Hepatic Drug Transporters in Drug Disposition and Liver Toxicity

Hepatic drug transporters are mainly distributed in parenchymal liver cells (hepatocytes), contributing to drug's liver disposition and elimination. According to their functions, hepatic transporters can be roughly divided into influx and efflux transporters, translocating specific molecules from blood into hepatic cytosol and mediating the excretion of drugs and metabolites from hepatic cytosol to blood or bile, respectively. The function of hepatic transport systems can be affected by interspecies differences and inter-individual variability (polymorphism). In addition, some drugs and disease can redistribute transporters from the cell surface to the intracellular compartments, leading to the changes in the expression and function of transporters. Hepatic drug transporters have been associated with the hepatic toxicity of drugs. Gene polymorphism of transporters and altered transporter expressions and functions due to diseases are found to be susceptible factors for drug-induced liver injury (DILI). In this chapter, the localization of hepatic drug transporters, their regulatory factors, physiological roles, and their roles in drug's liver disposition and DILI are reviewed.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Cenicriviroc, a cytokine receptor antagonist, potentiates all-trans retinoic acid in reducing liver injury in cholestatic rodents

BACKGROUND & AIMS

Cholestatic liver injury is mediated by bile acid-induced inflammatory responses. We hypothesized that superior therapeutic effects might be achieved by combining treatments that reduce the bile acid pool size with one that blocks inflammation.

METHODS

Bile duct-ligated (BDL) rats and Mdr2(Abcb4)-/- mice were treated with all-trans retinoic acid (atRA), a potent inhibitor of bile acid synthesis, 5 mg/kg/d by gavage, or Cenicriviroc (CVC), a known antagonist of CCR2 and CCR5, 50 mg/kg/d alone or in combination for 14 days and 1 month respectively.

RESULTS

All-trans retinoic acid alone reduced bile acid pool size and liver necrosis in BDL rats. However, the combination with CVC further reduced liver to body weight ratio, bile acid pool size, plasma liver enzyme, bilirubin, liver necrosis and fibrosis when compared to the atRA treatment. The assessment of hepatic hydroxyproline content further confirmed the reduced liver injury concurrent with reduction of pro-inflammatory cytokines emphasizing the synergistic effects of these two agents. Profiling of hepatic inflammatory cells revealed that combination therapy reduced neutrophils and T cells but not macrophages. The superior therapeutic effects of combination treatment were also confirmed in Mdr2-/- mice where a significant reduction in plasma liver enzymes, bilirubin, liver fibrosis, bile duct proliferation and hepatic infiltration of neutrophils and T cells and expression of cytokines were found.

CONCLUSIONS

Multitargeted therapy is an important paradigm for treating cholestatic liver injury. The combination of CVC with atRA or other FXR activators may warrant a clinical trial in patients with cholestatic liver disease.

Characterization of a novel organic solute transporter homologue from Clonorchis sinensis

Clonorchis sinensis is a liver fluke that can dwell in the bile ducts of mammals. Bile acid transporters function to maintain the homeostasis of bile acids in C. sinensis, as they induce physiological changes or have harmful effects on C. sinensis survival. The organic solute transporter (OST) transports mainly bile acid and belongs to the SLC51 subfamily of solute carrier transporters. OST plays a critical role in the recirculation of bile acids in higher animals. In this study, we cloned full-length cDNA of the 480-amino acid OST from C. sinensis (CsOST). Genomic analysis revealed 11 exons and nine introns. The CsOST protein had a 'Solute_trans_a' domain with 67% homology to Schistosoma japonicum OST. For further analysis, the CsOST protein sequence was split into the ordered domain (CsOST-N) at the N-terminus and disordered domain (CsOST-C) at the C-terminus. The tertiary structure of each domain was built using a threading-based method and determined by manual comparison. In a phylogenetic tree, the CsOST-N domain belonged to the OSTα and CsOST-C to the OSTβ clade. These two domains were more highly conserved with the OST α- and β-subunits at the structure level than at sequence level. These findings suggested that CsOST comprised the OST α- and β-subunits. CsOST was localized in the oral and ventral suckers and in the mesenchymal tissues abundant around the intestine, vitelline glands, uterus, and testes. This study provides fundamental data for the further understanding of homologues in other flukes.

Farnesoid X Receptor Agonists Obeticholic Acid and Chenodeoxycholic Acid Increase Bile Acid Efflux in Sandwich-Cultured Human Hepatocytes: Functional Evidence and Mechanisms

The farnesoid X receptor (FXR) is a nuclear receptor that regulates genes involved in bile acid homeostasis. FXR agonists, obeticholic acid (OCA) and chenodeoxycholic acid (CDCA), increase mRNA expression of efflux transporters in sandwich-cultured human hepatocytes (SCHH). This study evaluated the effects of OCA and CDCA treatment on the uptake, basolateral efflux, and biliary excretion of a model bile acid, taurocholate (TCA), in SCHH. In addition, changes in the protein expression of TCA uptake and efflux transporters were investigated. SCHH were treated with 1 µM OCA, 100 µM CDCA, or vehicle control for 72 hours followed by quantification of deuterated TCA uptake and efflux over time in Ca²⁺-containing and Ca²⁺-free conditions (n = 3 donors). A mechanistic pharmacokinetic model was fit to the TCA mass-time data to obtain estimates for total uptake clearance (CL_Uptake), total intrinsic basolateral efflux clearance (CL_int,BL), and total intrinsic biliary clearance (CL_int,Bile). Modeling results revealed that FXR agonists significantly increased CL_int,BL by >6-fold and significantly increased CL_int,Bile by 2-fold, with minimal effect on CL_Uptake Immunoblotting showed that protein levels of the basolateral transporter subunits organic solute transporter α and β (OSTα and OSTβ) in FXR agonist-treated SCHH were significantly induced by >2.5- and 10-fold, respectively. FXR agonist-mediated changes in the expression of other TCA transporters in SCHH were modest. In conclusion, this is the first report demonstrating that OCA and CDCA increased TCA efflux in SCHH, which contributed to reduced intracellular TCA concentrations. Increased basolateral efflux of TCA was consistent with increased OSTα/β protein expression in OCA- and CDCA-treated SCHH.

Organic Solute Transporter α-β Protects Ileal Enterocytes From Bile Acid-Induced Injury

BACKGROUND & AIMS

Ileal bile acid absorption is mediated by uptake via the apical sodium-dependent bile acid transporter (ASBT), and export via the basolateral heteromeric organic solute transporter α-β (OSTα-OSTβ). In this study, we investigated the cytotoxic effects of enterocyte bile acid stasis in Ostα-/- mice, including the temporal relationship between intestinal injury and initiation of the enterohepatic circulation of bile acids.

METHODS

Ileal tissue morphometry, histology, markers of cell proliferation, gene, and protein expression were analyzed in male and female wild-type and Ostα-/- mice at postnatal days 5, 10, 15, 20, and 30. Ostα-/-Asbt-/- mice were generated and analyzed. Bile acid activation of intestinal Nrf2-activated pathways was investigated in Drosophila.

RESULTS

As early as day 5, Ostα-/- mice showed significantly increased ileal weight per length, decreased villus height, and increased epithelial cell proliferation. This correlated with premature expression of the Asbt and induction of bile acid-activated farnesoid X receptor target genes in neonatal Ostα-/- mice. Expression of reduced nicotinamide adenine dinucleotide phosphate oxidase-1 and Nrf2-anti-oxidant responsive genes were increased significantly in neonatal Ostα-/- mice at these postnatal time points. Bile acids also activated Nrf2 in Drosophila enterocytes and enterocyte-specific knockdown of Nrf2 increased sensitivity of flies to bile acid-induced toxicity. Inactivation of the Asbt prevented the changes in ileal morphology and induction of anti-oxidant response genes in Ostα-/- mice.

CONCLUSIONS

Early in postnatal development, loss of Ostα leads to bile acid accumulation, oxidative stress, and a restitution response in ileum. In addition to its essential role in maintaining bile acid homeostasis, Ostα-Ostβ functions to protect the ileal epithelium against bile acid-induced injury. NCBI Gene Expression Omnibus: GSE99579.

Role of farnesoid X receptor in cholestasis

The nuclear receptor farnesoid X receptor (FXR) plays an important role in physiological bile acid synthesis, secretion and transport. Defects of FXR regulation in these processes can cause cholestasis and subsequent pathological changes. FXR regulates the synthesis and uptake of bile acid via enzymes. It also increases bile acid solubility and elimination by promoting conjugation reactions and exports pump expression in cholestasis. The changes in bile acid transporters are involved in cholestasis, which can result from the mutations of transporter genes or acquired dysfunction of transport systems, such as inflammation-induced intrahepatic cholestasis. The modulation function of FXR in extrahepatic cholestasis is not identical to that in intrahepatic cholestasis, but the discrepancy may be reduced over time. In extrahepatic cholestasis, increasing biliary pressure can induce bile duct proliferation and bile infarcts, but the absence of FXR may ameliorate them. This review provides an update on the function of FXR in the regulation of bile acid metabolism, its role in the pathophysiological process of cholestasis and the therapeutic use of FXR agonists.

A Novel Di-Leucine Motif at the N-Terminus of Human Organic Solute Transporter Beta Is Essential for Protein Association and Membrane Localization

The heteromeric membrane protein Organic Solute Transporter alpha/beta is the major bile acid efflux transporter in the intestine. Physical association of its alpha and beta subunits is essential for their polarized basolateral membrane localization and function in the transport of bile acids and other organic solutes. We identified a highly conserved acidic dileucine motif (-EL20L21EE) at the extracellular amino-tail of organic solute transporter beta from multiple species. To characterize the role of this protein interacting domain in the association of the human beta and alpha subunits and in membrane localization of the transporter, Leu20 and Leu21 on the amino-tail of human organic solute transporter beta were replaced with alanines by site-directed mutagenesis. Co-immunoprecipitation study in HEK293 cells demonstrated that substitution of the leucine residues with alanines prevented the interaction of the human beta mutant with the alpha subunit. Membrane biotinylation demonstrated that the LL/AA mutant eliminated membrane expression of both subunits. Computational-based modelling of human organic solute transporter beta suggested that the LL/AA mutation substantially alters both the structure and lipophilicity of the surface, thereby not only affecting the interaction with the alpha subunit but also possibly impacting the capacity of the beta subunit to traffick through the cell and interact with the membrane. In summary, our findings indicate that the dileucine motif in the extracellular N-terminal region of human organic solute transporter beta subunit plays a critical role in the association with the alpha subunit and in its polarized plasma membrane localization.

Nuclear factor-κB regulates the expression of multiple genes encoding liver transport proteins

In this study we identified the mechanisms underlying the inhibitory effects of NF-κB on the expression of genes encoding multiple liver transport proteins. Well-conserved NF-κB binding sites were found in the promoters of farnesoid X receptor (FXR)-target genes. An electromobility shift assay (EMSA) demonstrated the specific interaction between the NF-κB p65 protein and a (32)P-labeled BSEP NF-κB response element (NF-κBE). Chromatin immunoprecipitation (ChIP) analysis confirmed binding of NF-κB p65 to the BSEP locus but not the FXRE in vitro. NF-κB p65 overexpression in Huh-7 cells markedly repressed FXR/RXR transactivation of the BSEP, ABCG5/G8, MRP2, and FXR promoters, which was totally reversed by expression of the IκBα super-repressor. NF-κB interacted directly with FXR on coimmunoprecipitation, suggesting another level for the inhibitory effects of NF-κB on FXR-target genes. In vivo ChIP analysis with liver nuclei obtained from mice after 3 days of common bile duct ligation (BDL) or 6 h post-lipopolysaccharide (LPS) injection showed a markedly increased recruitment of NF-κB p65 to the Bsep promoter compared with controls. There was also increased recruitment of the corepressor silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) and histone deacetylase (HDAC)3 and HDAC2 to the NF-κB sites. We also found that NF-κB p65 was recruited to NF-κB binding sites in the promoters of organic solute transporter, OSTα and OSTβ, and unexpectedly activated rather than repressed gene expression. In mouse liver after BDL NF-κB recruitment to Ostα and Ostβ promoters was associated with increased binding of the potent coactivator cAMP response element binding protein (CREB)-binding protein (CBP)/p300 to the NF-κBE and depletion of CBP/p300 at the FXR element. Overall, these studies demonstrate a novel role for NF-κB in adaptation to obstructive and LPS-induced cholestasis acting through recruitment to specific NF-κB binding sites in the promoters of FXR-target genes and possibly through direct interaction with FXR.

Chlorpromazine-induced perturbations of bile acids and free fatty acids in cholestatic liver injury prevented by the Chinese herbal compound Yin-Chen-Hao-Tang

BACKGROUNDS

Yin-Chen-Hao-Tang (YCHT), a commonly used as a traditional chinese medicine for liver disease. Several studies indicated that YCHT may improving hepatic triglyceride metabolism and anti-apoptotic response as well as decreasing oxidative stress .However, little is known about the role of YCHT in chlorpromazine (CPZ) -induced chlolestatic liver injury. Therefore, we aimed to facilitate the understanding of the pathogenesis of cholestatic liver injury and evaluate the effect of Yin-Chen-Hao-Tang (YCHT) on chlorpromazine (CPZ)-induced cholestatic liver injury in rats based on the change of bile acids (BAs) and free fatty acids (FFAs) alone with the biochemical indicators and histological examination.

METHODS

We conducted an experiment on CPZ-induced cholestatic liver injury in Wistar rats with and without YCHT for nine consecutive days. Serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), albumin (ALB), total bilirubin (TBIL), total cholesterol (TC), triglycerides (TG), low density lipoprotein-cholesterol (LDL-C) were measured to evaluate the protective effect of YCHT against chlorpromazine (CPZ)-induced cholestatic liver injury. Histopathology of the liver tissue showed that pathological injuries were relieved after YCHT pretreatment. In addition, ultra-performance lipid chromatography coupled with quadrupole mass spectrometry (UPLC-MS) and gas chromatography coupled with mass spectrometry (GC-MS) was applied to determine the content of bile acids, free fatty acids, respectively.

RESULTS

Obtained data showed that YCHT attenuated the effect of CPZ-induced cholestatic liver injury, which was manifested by the serum biochemical parameters and histopathology of the liver tissue. YCHT regulated the lipid levels as indicated by the reversed serum levels of TC, TG, and LDL-C. YCHT also regulated the disorder of BA and FFA metabolism by CPZ induction.

CONCLUSIONS

Results indicated that YCHT exerted a protective effect on CPZ-induced cholestasis liver injury. The variance of BA and FFA concentrations can be used to evaluate the cholestatic liver injury caused by CPZ and the hepatoprotective effect of YCHT.

Metabolic effects of cholecystectomy: gallbladder ablation increases basal metabolic rate through G-protein coupled bile acid receptor Gpbar1-dependent mechanisms in mice

Background & Aims

Bile acids (BAs) regulate energy expenditure by activating G-protein Coupled Bile Acid Receptor Gpbar1/TGR5 by cAMP-dependent mechanisms. Cholecystectomy (XGB) increases BAs recirculation rates resulting in increased tissue exposure to BAs during the light phase of the diurnal cycle in mice. We aimed to determine: 1) the effects of XGB on basal metabolic rate (BMR) and 2) the roles of TGR5 on XGB-dependent changes in BMR.

Methods

BMR was determined by indirect calorimetry in wild type and Tgr5 deficient (Tgr5^-/-) male mice. Bile flow and BAs secretion rates were measured by surgical diversion of biliary duct. Biliary BAs and cholesterol were quantified by enzymatic methods. BAs serum concentration and specific composition was determined by liquid chromatography/tandem mass spectrometry. Gene expression was determined by qPCR analysis.

Results

XGB increased biliary BAs and cholesterol secretion rates, and elevated serum BAs concentration in wild type and Tgr5^-/- mice during the light phase of the diurnal cycle. BMR was ~25% higher in cholecystectomized wild type mice (p <0.02), whereas no changes were detected in cholecystectomized Tgr5^-/- mice compared to wild-type animals.

Conclusion

XGB increases BMR by TGR5-dependent mechanisms in mice.

Mechanism underlying an elevated serum bile acid level in chronic renal failure patients

INTRODUCTION

Bile acids play an important role in the digestion of dietary lipids. Bile acid metabolism is regulated by the digestive system. The kidney is an important organ of the urinary system and is believed to play a minor role in bile acid excretion; however, many recent studies have reported an increased serum bile acid level and alterations in bile acid homeostasis in both clinical and animal model studies on chronic renal failure. The existing research findings on the mechanisms underlying this phenomenon were mostly derived from animal model studies, but clinical investigations have been limited.

MATERIALS AND METHODS

Kidney tissues and serum and urine samples from CRF patients and normal controls were studied.

RESULTS

We found increased serum bile acid levels and decreased urine bile acid output levels in chronic renal failure patients. Mesangial cell and endothelial cell proliferation, glomerular sclerosis, renal interstitial fibrosis, and intrarenal vascular sclerosis were observed based on hematoxylin-eosin and Masson trichrome staining pathology analysis. Scatter diagram and Pearson correlation analysis showed that in chronic renal failure patients, the estimated glomerular filtration rate and serum bile acid level were interrelated. Reverse transcription polymerase chain reaction and Western blotting results indicated that reabsorption and secretion of bile acid at the apical surface of the proximal renal tubular did not contribute to the elevated serum BA level.

CONCLUSION

The increase in plasma bile acid is due to decreased bile acid filtration through the kidneys in CRF patients.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Deleterious effect of oltipraz on extrahepatic cholestasis in bile duct-ligated mice

BACKGROUND & AIMS

Oltipraz (4-methyl-5(pyrazinyl-2)-1-2-dithiole-3-thione), a promising cancer preventive agent, has an antioxidative activity and ability to enhance glutathione biosynthesis, phase II detoxification enzymes and multidrug resistance-associated protein-mediated efflux transporters. Oltipraz can protect against hepatotoxicity caused by carbon tetrachloride, acetaminophen and alpha-naphthylisothiocyanate. Whether oltipraz has hepato-protective effects on obstructive cholestasis is unknown.

METHODS

We administered oltipraz to mice for 5 days prior to bile duct ligation (BDL) for 3 days. Liver histology, liver function markers, bile flow rates and hepatic expression of profibrogenic genes were evaluated.

RESULTS

Mice pretreated with oltipraz prior to BDL demonstrated higher levels of serum aminotransferases and more severe liver damage than in control mice. Higher bile flow and glutathione secretion rates were observed in unoperated mice treated with oltipraz than in control mice, suggesting that liver necrosis in oltipraz-treated BDL mice may be related partially to increased bile-acid independent flow and biliary pressure. Oltipraz treatment in BDL mice enhanced α-smooth muscle actin expression, consistent with activation of hepatic stellate cells and portal fibroblasts. Matrix metalloproteinases (Mmp) 9 and 13 and tissue inhibitors of metalloproteinases (Timp) 1 and 2 levels were increased in the oltipraz-treated BDL group, suggesting that the secondary phase of liver injury induced by oltipraz might be due to excessive Mmp and Timp secretions, which induce remodeling of the extracellular matrix.

CONCLUSIONS

Oltipraz treatment exacerbates the severity of liver injury following BDL and should be avoided as therapy for extrahepatic cholestatic disorders due to bile duct obstruction.

SUMOylation of the farnesoid X receptor (FXR) regulates the expression of FXR target genes

BACKGROUND

Small ubiquitin-like modifiers (SUMO) are covalently conjugated to other proteins including nuclear receptors leading to modification of various cellular processes.

RESULTS

Ligand-dependent SUMOylation of farnesoid X receptor (FXR) negatively regulates the expression of its target genes.

CONCLUSION

SUMO modification attenuates the capacity of FXR to function as a transcriptional activator.

SIGNIFICANCE

Defining post-translation modification of FXR bySUMOis important to understanding how this nuclear receptor functions in health and disease. The farnesoid X receptor (FXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metabolism including bile acid homeostasis. Here we show that FXR is covalently modified by the small ubiquitin-like modifier (Sumo1), an important regulator of cell signaling and transcription. Well conserved consensus sites at lysine 122 and 275 in the AF-1 and ligand binding domains, respectively, of FXR were subject to SUMOylation in vitro and in vivo. Chromatin immunoprecipitation (ChIP) analysis showed that Sumo1 was recruited to the bile salt export pump (BSEP), the small heterodimer partner (SHP), and the OSTα-OSTβ organic solute transporter loci in a ligand-dependent fashion. Sequential chromatin immunoprecipitation (ChIP-ReChIP) verified the concurrent binding of FXR and Sumo1 to the BSEP and SHP promoters. Overexpression of Sumo1 markedly decreased binding and/or recruitment of FXR to the BSEP and SHP promoters on ChIP-ReChIP. SUMOylation did not have an apparent effect on nuclear localization of FXR. Expression of Sumo1 markedly inhibited the ligand-dependent, transactivation of BSEP and SHP promoters by FXR/retinoid X receptor α (RXRα) in HepG2 cells. In contrast, mutations that abolished SUMOylation of FXR or siRNA knockdown of Sumo1 expression augmented the transactivation of BSEP and SHP promoters by FXR. Pathways for SUMOylation were significantly altered during obstructive cholestasis with differential Sumo1 recruitment to the promoters of FXR target genes. In conclusion, FXR is subject to SUMOylation that regulates its capacity to transactivate its target genes in normal liver and during obstructive cholestasis.

β-Subunit of the Ostα-Ostβ organic solute transporter is required not only for heterodimerization and trafficking but also for function

The organic solute transporter, Ost/Slc51, is composed of two distinct proteins that must heterodimerize to generate transport activity, but the role of the individual subunits in mediating transport activity is unknown. The present study identified regions in Ostβ required for heterodimerization with Ostα, trafficking of the Ostα-Ostβ complex to the plasma membrane, and bile acid transport activity in HEK293 cells. Bimolecular fluorescence complementation analysis revealed that a 25-amino acid peptide containing the Ostβ transmembrane (TM) domain heterodimerized with Ostα, although the resulting complex failed to reach the plasma membrane and generate cellular [(3)H]taurocholate transport activity. Deletion of the single TM domain of Ostβ abolished interaction with Ostα, demonstrating that the TM segment is necessary and sufficient for formation of a heteromeric complex with Ostα. Mutation of the highly conserved tryptophan-asparagine sequence within the TM domain of Ostβ to alanines did not prevent cell surface trafficking, but abolished transport activity. Removal of the N-terminal 27 amino acids of Ostβ resulted in a transporter complex that reached the plasma membrane and exhibited transport activity at 30 °C. Complete deletion of the C terminus of Ostβ abolished [(3)H]taurocholate transport activity, but reinsertion of two native arginines immediately C-terminal to the TM domain rescued this defect. These positively charged residues establish the correct N(exo)/C(cyt) topology of the peptide, in accordance with the positive inside rule. Together, the results demonstrate that Ostβ is required for both proper trafficking of Ostα and formation of the functional transport unit, and identify specific residues of Ostβ critical for these processes.

Bile acids and liver regeneration

Bile acids possess many important physiological functions. They have been shown to play pivotal roles in the absorption of dietary lipids and fat soluble vitamins as well as in regulating bile acid homeostasis, lipoprotein and glucose metabolism. Recent evidence suggests that bile acid signaling pathway plays an important role in normal liver regeneration. This review aims to elucidate the potential role of the bile acid signaling pathway in liver regeneration and to highlight possible mechanisms involved.

Pleiotropic functions of the organic solute transporter Ostα-Ostβ

The heteromeric organic solute transporter alpha-beta (Ostα-Ostβ) is expressed at relatively high levels on the basolateral membrane of enterocytes, where it plays a critical role in the intestinal absorption of bile acids and the enterohepatic circulation. However, this transporter is also expressed in nearly all human tissues, including those that are not normally thought to be involved in bile acid homeostasis, indicating that Ostα-Ostβ may have additional roles beyond bile acid transport in these other tissues, or that bile acids and their derivatives are more pervasive than currently envisioned. Emerging data from different laboratories provide support for both of these hypotheses. In particular, recent studies indicate that tissues such as brain and ovary have the capacity to synthesize bile acids or bile acid precursors. In addition, studies examining Ostα-Ostβ substrate specificity have revealed that this transporter can also accept conjugated steroids, including some neurosteroids, and that the transporter is selectively expressed in steroidogenic cells of the brain and adrenal gland, suggesting a novel function for Ostα-Ostβ. The broad tissue expression of Ostα-Ostβ is also consistent with the emerging concept that bile acids and their derivatives act as signaling molecules in diverse tissues. Bile acids activate nuclear receptors such as the farnesoid X receptor (FXR/NR1H4), the pregnane X receptor and the vitamin D receptor, are ligands for a G-protein-coupled bile acid receptor (GPBAR1/TGR5), and can also activate protein kinases A and C as well as mitogen-activated protein kinase pathways. These signaling pathways are present in many tissues and regulate processes such as triglyceride, glucose and energy homeostasis. Note that although FXR and TGR5 are thought to function primarily as bile acid receptors, they are modulated by some other sterols and select lipid metabolites, and are also widely expressed in tissues, indicating a complex interplay among diverse regulatory networks that impact critical cell and organ functions. The present report summarizes the evidence for a pleiotropic role of Ostα-Ostβ in different tissues.

Ductal plates in hepatic ductular reactions. Hypothesis and implications. III. Implications for liver pathology

This article discusses on the basis of the ductal plate hypothesis the implication of the concept for several liver abnormalities. The occurrence of ductal plates (DP) during liver growth in childhood would explain the paraportal and parenchymal localizations of von Meyenburg complexes in postnatally developed parts of the liver, and their higher incidence in adulthood versus childhood. It partly clarifies the lack of postnatal intrahepatic bile duct development in Alagille syndrome and the reduced number of portal tracts in this disease. Ductular reactions (DRs) in DP configuration are the predominant type of progenitor cell reaction in fulminant necro-inflammatory liver disease, when lack of sufficient parenchymal regeneration results in liver failure. The concept of dissecting DRs explains the micronodular pattern of advanced biliary and alcoholic cirrhosis. The concept explains the DP patterns of bile ducts in several cases of biliary atresia, with implications for diagnosis and prognosis. The hypothesis also has an impact on concepts about stem/progenitor cells and their niche.

Ductal plates in hepatic ductular reactions. Hypothesis and implications. III. Implications for liver pathology

This article discusses on the basis of the ductal plate hypothesis the implication of the concept for several liver abnormalities. The occurrence of ductal plates (DP) during liver growth in childhood would explain the paraportal and parenchymal localizations of von Meyenburg complexes in postnatally developed parts of the liver, and their higher incidence in adulthood versus childhood. It partly clarifies the lack of postnatal intrahepatic bile duct development in Alagille syndrome and the reduced number of portal tracts in this disease. Ductular reactions (DRs) in DP configuration are the predominant type of progenitor cell reaction in fulminant necro-inflammatory liver disease, when lack of sufficient parenchymal regeneration results in liver failure. The concept of dissecting DRs explains the micronodular pattern of advanced biliary and alcoholic cirrhosis. The concept explains the DP patterns of bile ducts in several cases of biliary atresia, with implications for diagnosis and prognosis. The hypothesis also has an impact on concepts about stem/progenitor cells and their niche.

Differential disposition of chenodeoxycholic acid versus taurocholic acid in response to acute troglitazone exposure in rat hepatocytes

Inhibition of bile acid (BA) transport may contribute to the hepatotoxicity of troglitazone (TRO), a peroxisome proliferator-activated receptor gamma agonist. Typically, studies use taurocholic acid (TCA) as a model substrate to investigate effects of xenobiotics on BA disposition. However, TRO may differentially affect the transport of individual BAs, potentially causing hepatocyte accumulation of more cytotoxic BAs. The effects of TRO on the disposition of [(14)C]-labeled chenodeoxycholic acid ([(14)C]CDCA), an unconjugated cytotoxic BA, were determined in suspended hepatocytes and sandwich-cultured hepatocytes (SCH) from rats. (E)-3-[[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid (MK571), a multidrug resistance-associated protein (MRP) inhibitor, was included to evaluate involvement of MRPs in CDCA disposition. Accumulation in cells + bile of total [(14)C]CDCA species in SCH was sixfold greater than [(3)H]TCA and unaffected by 1 and 10μM TRO; 100μM TRO and 50μM MK571 ablated biliary excretion and significantly increased intracellular accumulation of total [(14)C]CDCA species. Results were similar in Mrp2-deficient TR(-) rat hepatocytes. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed that taurine- and glycine-conjugated CDCA, in addition to unconjugated CDCA, accumulated in hepatocytes during the 10-min incubation. In suspended rat hepatocytes, initial [(14)C]CDCA uptake was primarily Na(+)-independent, whereas initial [(3)H]TCA uptake was primarily Na(+)-dependent; TRO and MK571 decreased [(14)C]CDCA uptake to a lesser extent than [(3)H]TCA. Unexpectedly, MK571 inhibited Na(+)-taurocholate cotransporting polypeptide and bile salt export pump. Differential effects on uptake and efflux of individual BAs may contribute to TRO hepatotoxicity. Although TCA is the prototypic BA used to investigate the effects of xenobiotics on BA transport, it may not be reflective of other BAs.

Trafficking and transporter disorders in pediatric cholestasis

This article describes the uses of immunostaining in the diagnosis of cholestasis. To immunostain for bile salt export pump (BSEP) and multidrug resistance protein 3 in severe hepatobiliary disease manifest early in life can rapidly identify whether sequencing of ABCB11 or ABCB4 is likely to yield a genetic diagnosis. To immunostain for canalicular ectoenzymes as well as transporters, with transmission electron microscopy, can suggest whether sequencing of ATP8B1 is likely to yield a genetic diagnosis. Demonstrating BSEP expression can direct attention to bile acid synthesis disorders. Immunostaining for multidrug resistance-associated protein 2 serves principally as a control for adequacy of processing.

The role of inflammation in cholestasis: clinical and basic aspects

Hepatobiliary transport systems are essential for the uptake and excretion of a variety of compounds including bile acids. Disruption and dysregulation of this excretory pathway result in cholestasis, leading to the intrahepatic accumulation of bile acids and other toxic compounds with progression of liver pathology. Cholestasis induced by inflammation is a common complication in patients with extrahepatic infections or inflammatory processes, generally referred to as sepsis-associated cholestasis. Microbial products, including endotoxin, induce signaling pathways within hepatocytes either directly, or through activation of proinflammatory cytokines, leading to rapid and profound reductions in bile flow. The expression and function of key hepatobiliary transporters are suppressed in response to inflammatory signaling. These proinflammatory signaling cascades lead to repressed expression and activity of a large number of nuclear transcriptional regulators, many of which are essential for maintenance of hepatobiliary transporter gene expression. Interestingly, recently discovered molecular crosstalk between bile acid activated nuclear receptors and proinflammatory nuclear mediators may provide new means of understanding adaptive processes within liver. Inflammation-induced cholestasis and the effects of retained molecules in cholestasis on inflammatory signals are interwoven in the liver, providing potential opportunities for research and therapeutics.