Bile acids modulate the interferon signalling pathway

Bile acid-microbiota crosstalk in hepatitis B virus infection

Hepatitis B virus (HBV) is a hepatotropic non-cytopathic virus characterized by liver-specific gene expression. HBV infection highjacks bile acid metabolism, notably impairing bile acid uptake via sodium taurocholate cotransporting polypeptide (NTCP), which is a functional receptor for HBV entry. Concurrently, HBV infection induces changes in bile acid synthesis and the size of the bile acid pool. Conversely, bile acid facilitates HBV replication and expression through the signaling molecule farnesoid X receptor (FXR), a nuclear receptor activated by bile acid. However, in HepaRG cells and primary hepatocytes, FXR agonists suppress HBV RNA expression and the synthesis and secretion of DNA. In the gut, the size and composition of the bile acid pool significantly influence the gut microbiota. In turn, the gut microbiota impacts bile acid metabolism and innate immunity, potentially promoting HBV clearance. Thus, the bile acid-gut microbiota axis represents a complex and evolving relationship in the context of HBV infection. This review explores the interplay between bile acid and gut microbiota in HBV infection and discusses the development of HBV entry inhibitors targeting NTCP.

Regulation of Immune Homeostasis, Inflammation, and HIV Persistence by the Microbiome, Short-Chain Fatty Acids, and Bile Acids

Despite antiretroviral therapy (ART), people living with human immunodeficiency virus (HIV) (PLWH) continue to experience chronic inflammation and immune dysfunction, which drives the persistence of latent HIV and prevalence of clinical comorbidities. Elucidating the mechanisms that lead to suboptimal immunity is necessary for developing therapeutics that improve the quality of life of PLWH. Although previous studies have found associations between gut dysbiosis and immune dysfunction, the cellular/molecular cascades implicated in the manifestation of aberrant immune responses downstream of microbial perturbations in PLWH are incompletely understood. Recent literature has highlighted that two abundant metabolite families, short-chain fatty acids (SCFAs) and bile acids (BAs), play a crucial role in shaping immunity. These metabolites can be produced and/or modified by bacterial species that make up the gut microbiota and may serve as the causal link between changes to the gut microbiome, chronic inflammation, and immune dysfunction in PLWH. In this review, we discuss our current understanding of the role of the microbiome on HIV acquisition and latent HIV persistence despite ART. Further, we describe cellular/molecular cascades downstream of SCFAs and BAs that drive innate or adaptive immune responses responsible for promoting latent HIV persistence in PLWH. This knowledge can be used to advance HIV cure efforts.

Immunomodulatory functions of FXR

The Farnesoid-x-receptor (FXR) is a bile acids sensor activated in humans by primary bile acids. FXR is mostly expressed in liver, intestine and adrenal glands but also by cells of innate immunity, including macrophages, liver resident macrophages, the Kupffer cells, natural killer cells and dendritic cells. In normal physiology and clinical disorders, cells of innate immunity mediate communications between liver, intestine and adipose tissues. In addition to FXR, the G protein coupled receptor (GPBAR1), that is mainly activated by secondary bile acids, whose expression largely overlaps FXR, modulates chemical communications from the intestinal microbiota and the host's immune system, integrating epithelial cells and immune cells in the entero-hepatic system, providing a mechanism for development of a tolerogenic state toward the intestinal microbiota. Disruption of FXR results in generalized inflammation and disrupted bile acids metabolism. While FXR agonism in preclinical models provides counter-regulatory signals that attenuate inflammation-driven immune dysfunction in a variety of liver and intestinal disease models, the clinical relevance of these mechanisms in the setting of FXR-related disorders remain poorly defined.

Roles of bile acids in enteric virus replication

Bile acids (BAs) are evolutionally conserved molecules synthesized in the liver from cholesterol to facilitating the absorption of fat-soluble nutrients. In the intestines, where enteric viruses replicate, BAs also act as signaling molecules that modulate various biological functions via activation of specific receptors and cell signaling pathways. To date, BAs present either pro-viral or anti-viral effects for the replication of enteric viruses in vivo and in vitro. In this review, we summarized current information on biosynthesis, transportation and metabolism of BAs and the role of BAs in replication of enteric caliciviruses, rotaviruses, and coronaviruses. We also discussed the application of BAs for cell culture adaptation of fastidious enteric caliciviruses and control of virus infection, which may provide novel insights into the development of antivirals and/or disinfectants for enteric viruses.

Na+-Taurocholate Co-Transporting Polypeptide (NTCP) in Livers, Function, Expression Regulation, and Potential in Hepatitis B Treatment

Chronic hepatitis B virus (HBV) infection has become one of the leading causes of liver cirrhosis and hepatocellular carcinoma globally. The discovery of sodium taurocholate co-transporting polypeptide (NTCP), a solute carrier, as a key receptor for HBV and hepatitis D virus (HDV) has opened new avenues for HBV treatment. Additionally, it has led researchers to generate hepatoma cell lines (including HepG2-NTCP and Huh-7-NTCP) susceptible to HBV infection in vitro, hence, paving the way to develop and efficiently screen new and novel anti-HBV drugs. This review summarizes the history, function and critical findings regarding NTCP as a viral receptor for HBV/HDV, and it also discusses recently developed drugs targeting NTCP.

Taurocholic acid inhibits the response to interferon-α therapy in patients with HBeAg-positive chronic hepatitis B by impairing CD8+ T and NK cell function

Pegylated interferon-alpha (PegIFNα) therapy has limited effectiveness in hepatitis B e-antigen (HBeAg)-positive chronic hepatitis B (CHB) patients. However, the mechanism underlying this failure is poorly understood. We aimed to investigate the influence of bile acids (BAs), especially taurocholic acid (TCA), on the response to PegIFNα therapy in CHB patients. Here, we used mass spectrometry to determine serum BA profiles in 110 patients with chronic HBV infection and 20 healthy controls (HCs). We found that serum BAs, especially TCA, were significantly elevated in HBeAg-positive CHB patients compared with those in HCs and patients in other phases of chronic HBV infection. Moreover, serum BAs, particularly TCA, inhibited the response to PegIFNα therapy in HBeAg-positive CHB patients. Mechanistically, the expression levels of IFN-γ, TNF-α, granzyme B, and perforin were measured using flow cytometry to assess the effector functions of immune cells in patients with low or high BA levels. We found that BAs reduced the number and proportion and impaired the effector functions of CD3+CD8+ T cells and natural killer (NK) cells in HBeAg-positive CHB patients. TCA in particular reduced the frequency and impaired the effector functions of CD3+CD8+ T and NK cells in vitro and in vivo and inhibited the immunoregulatory activity of IFN-α in vitro. Thus, our results show that BAs, especially TCA, inhibit the response to PegIFNα therapy by impairing the effector functions of CD3+CD8+ T and NK cells in HBeAg-positive CHB patients. Our findings suggest that targeting TCA could be a promising approach for restoring IFN-α responsiveness during CHB treatment.

Association of serum total cholesterol with pegylated interferon-α treatment in HBeAg-positive chronic hepatitis B patients

BACKGROUND

Recent studies suggest that serum lipids are associated with pegylated interferon-alpha (PEG-IFN-α) treatment response in chronic hepatitis C patients. However, the role of serum lipids in influencing the outcome of HBV treatment is not well understood. This study aims to investigate the association of serum lipids with the response to interferon-alpha treatment for chronic hepatitis B (CHB) patients.

METHODS

We dynamically measured 11 clinical serum lipid parameters of 119 hepatitis B e antigen (HBeAg)-positive CHB patients, including 53 patients who achieved sustained response (SR) and 66 patients who achieved non-response (NR) induced by PEG-IFN-α treatment for 48 weeks.

RESULTS

The dynamic analysis showed that the baseline serum total cholesterol (TCHO) level was higher in the NR group than that in the SR group (P=0.004). Moreover, the correlation analysis demonstrated a significant positive correlation between TCHO and hepatitis B surface antigen (HBsAg) at baseline (P=0.009). In addition, CHB patients with high baseline TCHO levels exhibited higher HBV DNA, HBsAg, HBeAg and hepatitis B e antibody (HBeAb) levels during early treatment periods (weeks 0, 4, 12 and 24) than those with the low TCHO levels. Furthermore, the logistic regression analysis identified that baseline serum TCHO was a risk factor for NR achievement (OR=4.94; P=0.047).

CONCLUSIONS

Our results indicated that serum TCHO was associated with PEG-IFN-α therapeutic response in HBeAg-positive CHB patients which suggested that serum TCHO could be useful as an auxiliary clinical factor to predict poor efficacy of PEG-IFN-α therapy.

A Systems Vaccinology Approach Reveals the Mechanisms of Immunogenic Responses to Hantavax Vaccination in Humans

Hantavax is an inactivated vaccine for hemorrhagic fever with renal syndrome (HFRS). The immunogenic responses have not been elucidated yet. Here we conducted a cohort study in which 20 healthy subjects were administered four doses of Hantavax during 13-months period. Pre- and post- vaccinated peripheral blood mononuclear cells (PBMCs) and sera were analysed by transcriptomic and metabolomic profilings, respectively. Based on neutralizing antibody titers, subjects were subsequently classified into three groups; non responders (NRs), low responders (LRs) and high responders (HRs). Post vaccination differentially expressed genes (DEGs) associated with innate immunity and cytokine pathways were highly upregulated. DEG analysis revealed a significant induction of CD69 expression in the HRs. High resolution metabolomics (HRM) analysis showed that correlated to the antibody response, cholesteryl nitrolinoleate, octanoyl-carnitine, tyrosine, ubiquinone-9, and benzoate were significantly elevated in HRs, while chenodeoxycholic acid and methyl palmitate were upregulated in NRs and LRs, compared with HRs. Additionally, gene-metabolite interaction revealed upregulated gene-metabolite couplings in, folate biosynthesis, nicotinate and nicotinamide, arachidonic acid, thiamine and pyrimidine metabolism in a dose dependent manner in HR group. Collectively, our data provide new insight into the underlying mechanisms of the Hantavax-mediated immunogenicity in humans.

The functional role of sodium taurocholate cotransporting polypeptide NTCP in the life cycle of hepatitis B, C and D viruses

Chronic hepatitis B, C and D virus (HBV, HCV and HDV) infections are a major cause of liver disease and cancer worldwide. Despite employing distinct replication strategies, the three viruses are exclusively hepatotropic, and therefore depend on hepatocyte-specific host factors. The sodium taurocholate co-transporting polypeptide (NTCP), a transmembrane protein highly expressed in human hepatocytes that mediates the transport of bile acids, plays a key role in HBV and HDV entry into hepatocytes. Recently, NTCP has been shown to modulate HCV infection of hepatocytes by regulating innate antiviral immune responses in the liver. Here, we review the current knowledge of the functional role and the molecular and cellular biology of NTCP in the life cycle of the three major hepatotropic viruses, highlight the impact of NTCP as an antiviral target and discuss future avenues of research.

Gut microbiota-mediated protection against influenza virus subtype H9N2 in chickens is associated with modulation of the innate responses

Commensal gut microbiota plays an important role in health and disease. The current study was designed to assess the role of gut microbiota of chickens in the initiation of antiviral responses against avian influenza virus. Day-old layer chickens received a cocktail of antibiotics for 12 (ABX-D12) or 16 (ABX-D16) days to deplete their gut microbiota, followed by treatment of chickens from ABX-12 with five Lactobacillus species combination (PROB), fecal microbial transplant suspension (FMT) or sham treatment daily for four days. At day 17 of age, chickens were challenged with H9N2 virus. Cloacal virus shedding, and interferon (IFN)-α, IFN-β and interleukin (IL)-22 expression in the trachea, lung, ileum and cecal tonsils was assessed. Higher virus shedding, and compromised type I IFNs and IL-22 expression was observed in ABX-D16 chickens compared to control, while PROB and FMT showed reduced virus shedding and restored IL-22 expression to levels comparable with undepleted chickens. In conclusion, commensal gut microbiota of chickens can modulate innate responses to influenza virus subtype H9N2 infection in chickens, and modulating the composition of the microbiome using probiotics- and/or FMT-based interventions might serve to promote a healthy community that confers protection against influenza virus infection in chickens.

Development of a cell-based assay to identify hepatitis B virus entry inhibitors targeting the sodium taurocholate cotransporting polypeptide

Sodium taurocholate cotransporting polypeptide (NTCP) is a major entry receptor of hepatitis B virus (HBV) and one of the most attractive targets for anti-HBV drugs. We developed a cell-mediated drug screening method to monitor NTCP expression on the cell surface by generating a HepG2 cell line with tetracycline-inducible expression of NTCP and a monoclonal antibody that specifically detects cell-surface NTCP. Using this system, we screened a small molecule library for compounds that protected against HBV infection by targeting NTCP. We found that glabridin, a licorice-derived isoflavane, could suppress viral infection by inducing caveolar endocytosis of cell-surface NTCP with an IC₅₀ of ~40 μM. We also found that glabridin could attenuate the inhibitory effect of taurocholate on type I interferon signaling by depleting the level of cell-surface NTCP. These results demonstrate that our screening system could be a powerful tool for discovering drugs targeting HBV entry.

Solute Carrier NTCP Regulates Innate Antiviral Immune Responses Targeting Hepatitis C Virus Infection of Hepatocytes

Chronic hepatitis B, C, and D virus (HBV, HCV, and HDV) infections are the leading causes of liver disease and cancer worldwide. Recently, the solute carrier and sodium taurocholate co-transporter NTCP has been identified as a receptor for HBV and HDV. Here, we uncover NTCP as a host factor regulating HCV infection. Using gain- and loss-of-function studies, we show that NTCP mediates HCV infection of hepatocytes and is relevant for cell-to-cell transmission. NTCP regulates HCV infection by augmenting the bile-acid-mediated repression of interferon-stimulated genes (ISGs), including IFITM3. In conclusion, our results uncover NTCP as a mediator of innate antiviral immune responses in the liver, and they establish a role for NTCP in the infection process of multiple viruses via distinct mechanisms. Collectively, our findings suggest a role for solute carriers in the regulation of innate antiviral responses, and they have potential implications for virus-host interactions and antiviral therapies.

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NTCP is involved in hepatocyte infection by multiple viruses via distinct mechanisms

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NTCP facilitates HCV infection by modulating innate antiviral responses

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Solute carrier NTCP is a regulator of antiviral immune responses in the liver

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This function is relevant for infection and therapies for hepatotropic viruses

Shaping macrophages function and innate immunity by bile acids: mechanisms and implication in cholestatic liver diseases

The liver is selectively enriched in innate immune cells, macrophages (Kupffer cells), natural killer, and natural killer T cells. These cells release an array of mediators with cytotoxic, pro- and anti-inflammatory, angiogenic, fibrogenic, and mitogenic activity that function to fight infections, limit tissue injury, and promote wound healing. The diverse activity of macrophages is mediated by distinct subpopulations that develop in response to signals within their microenvironment. Understanding the mechanisms and role of the microenvironment contributing to modulation of macrophage populations is crucial for comprehension of the pathophysiology of liver injury in diverse conditions. Several studies initiated in the 1990s have shown that bile acids modulate innate and adaptive immunity. In the last decade, bile acids turned into hormones and signalling molecules involved in many metabolic and inflammatory processes. Biological properties of bile acids are thought to be mediated mainly through activation of the nuclear receptor FXR, the membrane receptor TGR5, as well as PK, ERK, MAP kinases signalling pathways. FXR and TGR5 agonists are currently under development for clinical purpose. This review analyses the mechanisms involved in the immunomodulatory effects of bile acids on the macrophage and discuss their implications in the pathophysiology of cholestasis, primary biliary cirrhosis and primary sclerosing cholangitis.

Modulation of fatty acid and bile acid metabolism by peroxisome proliferator-activated receptor α protects against alcoholic liver disease

BACKGROUND

Chronic alcohol intake affects liver function and causes hepatic pathological changes. It has been shown that peroxisome proliferator-activated receptor α (PPARα)-null mice developed more pronounced hepatic changes than wild-type (WT) mice after chronic exposure to a diet containing 4% alcohol. The remarkable similarity between the histopathology of alcoholic liver disease (ALD) in Ppara-null model and in humans, and the fact that PPARα expression and activity in human liver are less than one-tenth of those in WT mouse liver make Ppara-null a good system to investigate ALD.

METHODS

In this study, the Ppara-null model was used to elucidate the dynamic regulation of PPARα activity during chronic alcohol intake. Hepatic transcriptomic and metabolomic analyses were used to examine alterations of gene expression and metabolites associated with pathological changes. The changes triggered by alcohol consumption on gene expression and metabolites in Ppara-null mice were compared with those in WT mice.

RESULTS

The results showed that in the presence of PPARα, 3 major metabolic pathways in mitochondria, namely the fatty acid β-oxidation, the tricarboxylic acid cycle, and the electron transfer chain, were induced in response to a 2-month alcohol feeding, while these responses were greatly reduced in the absence of PPARα. In line with the transcriptional modulations of these metabolic pathways, a progressive accumulation of triglycerides, a robust increase in hepatic cholic acid and its derivatives, and a strong induction of fibrogenesis genes were observed exclusively in alcohol-fed Ppara-null mice.

CONCLUSIONS

These observations indicate that PPARα plays a protective role to enhance mitochondrial function in response to chronic alcohol consumption by adaptive transcriptional activation and suggest that activation of this nuclear receptor may be of therapeutic value in the treatment for ALD.

Bile acids PKA-dependently induce a switch of the IL-10/IL-12 ratio and reduce proinflammatory capability of human macrophages

That cholestatic conditions are accompanied by an enhanced susceptibility to bacterial infection in human and animal models is a known phenomenon. This correlates with the observation that bile acids have suppressive effects on cells of innate and adaptive immunity. The present study provides evidence that in human macrophages, bile acids inhibit the LPS-induced expression of proinflammatory cytokines without affecting the expression of the anti-inflammatory cytokine IL-10. This results in a macrophage phenotype that is characterized by an increased IL-10/IL-12 ratio. Correspondingly, bile acids suppress basal phagocytic activity of human macrophages. These effects of bile acids can be mimicked by cAMP, which is presumably induced TGR5-dependently. The data provided further suggest that in primary human macrophages, modulation of the macrophage response toward LPS by bile acids involves activation of CREB, disturbed nuclear translocation of NF-κB, and PKA-dependent enhancement of LPS-induced cFos expression. The increase in cFos expression is paralleled by an enhanced formation of a protein complex comprising cFos and the p65 subunit of NF-κB. In summary, the data provided suggest that in human macrophages, bile acids induce an anti-inflammatory phenotype characterized by an increased IL-10/IL-12 ratio via activation of PKA and thereby, prevent their activation as classically activated macrophages. This bile acid-induced modulation of macrophage function may also be responsible for the experimentally and clinically observed anti-inflammatory and immunosuppressive effects of bile acids.

Ursodeoxycholic acid and bile-acid mimetics as therapeutic agents for cholestatic liver diseases: an overview of their mechanisms of action

Chronic cholestasis and liver inflammation are the two main pathophysiological components of the two major classes of disease - primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC) - leading to bile duct destruction and ultimately to cirrhosis and liver failure. Ursodeoxycholic acid (UDCA), initially introduced as a therapeutic approach to counteract the cholestatic components to PBC and PSC, was subsequently shown to exhibit unexpected anti-inflammatory and immunomodulatoty properties. The use of farnesoid X receptor (FXR) and TGR5 agonists in various animal models have confirmed early observations indicating that bile acids are not only toxicants and inflammagens, but also repressors of innate and adaptive immunity. Obeticholic acid is a bile-acid mimetic, with no toxic or inflammagen behavior, that strongly activates FXR to combat the toxic effects of high concentrations of bile acid. Because UDCA is not an FXR agonist, its combination with obeticholic acid could be a promising tool for the treatment of PBC and PSC. In this overview, the biological properties of UDCA, NorUDCA and FXR agonists are highlighted, as well as their overlapping mechanisms of action in inflammatory biliary disorders.

The role of bile acid retention and a common polymorphism in the ABCB11 gene as host factors affecting antiviral treatment response in chronic hepatitis C

The outcome of hepatitis C virus (HCV) infection and the likelihood of a sustained virological response (SVR) to antiviral therapy depends on both viral and host characteristics. In vitro studies demonstrated that bile acids (BA) interfere with antiviral interferon effects. We investigate the influence of plasma BA concentrations and an ABCB11 polymorphism associated with lower transporter expression on viral load and SVR. Four hundred and fifty-one Caucasian HCV-patients treated with PEG-interferon and ribavirin were included in the study. ABCB11 1331T>C was genotyped, and plasma BA levels were determined. The 1331C allele was slightly overrepresented in HCV-patients compared to controls. In HCV-patients, a significant difference between patients achieving SVR vs non-SVR was observed for HCV-2/3 (5 vs 9 μm; P=0.0001), while median BA levels in HCV-1 were marginally elevated. Normal BA levels <8 μm were significantly associated with SVR (58.3%vs 36.3%; OR 2.48; P=0.0001). This difference was significant for HCV-2/3 (90.7%vs 67.6%; P=0.002) but marginal in HCV-1 (38.7%vs 27.8%; P=0.058). SVR rates were equivalent between ABCB11 genotypes for HCV-1, but increased for HCV-2/3 (TT 100%vs CC 78%; OR 2.01; P=0.043). IL28B genotype had no influence on these associations. No correlation between BA levels and HCV RNA was detected for any HCV genotype. The higher allelic frequency of ABCB11 1331C in HCV-patients compared to controls may indirectly link increased BA to HCV chronicity. Our data support a role for BA as host factor affecting therapy response in HCV-2/3 patients, whereas a weaker association was found for HCV-1.

Nuclear receptors as new perspective for the management of liver diseases

Nuclear receptors (NRs) are ligand-activated transcription factors that act as sensors for a broad range of natural and synthetic ligands and regulate several key hepatic functions including bile acid homeostasis, bile secretion, lipid and glucose metabolism, as well as drug deposition. Moreover, NRs control hepatic inflammation, regeneration, fibrosis, and tumor formation. Therefore, NRs are key for understanding the pathogenesis and pathophysiology of a wide range of hepatic disorders. Finally, targeting NRs and their alterations offers exciting new perspectives for the treatment of liver diseases.

A common polymorphism in the ABCB11 gene is associated with advanced fibrosis in hepatitis C but not in non-alcoholic fatty liver disease

Chronic HCV (hepatitis C virus)-associated cirrhosis represents a major indication for liver transplantation. Bile acids contribute to hepatic stellate cell activation as a key event in fibrogenesis. The aim of the present study was to investigate the role of bile acids and polymorphisms in bile acid level-regulating genes on fibrosis progression. A total of 206 subjects with chronic HCV infection were included for ABCB11 (ATP-binding cassette, subfamily B, member II) 1331T>C and NR1H4 (nuclear receptor) -1G>T genotyping, 178 of which were analysed for fibrosis stage. Exclusion criteria were HBV (hepatitis B virus) or HIV coinfection, alcohol >40 g/day and morbid obesity. A total of 358 patients with NAFLD (non-alcoholic fatty liver disease) were genotyped for comparison with a non-viral liver disease. Caucasian individuals (n = 110), undergoing liver resection for focal hepatic metastasis, served as controls. The ABCB11 1331C allele was significantly overrepresented in HCV patients compared with controls {allelic frequency 62.9%; OR (odds ratio), 1.41 [95% CI (confidence interval), 1.012-1.965]}. Median plasma bile acid levels were not significantly increased in the CC compared with TT genotype [7.2 (1-110) μmol/l compared with 3.5 (1-61) μmol/l; values are medians (range). A significant association between the presence of cirrhosis and ABCB11 genotype (CC compared with CT or TT, P=0.047) was observed in the χ2 test and independent of other risk factors of age, gender, body mass index and disease duration in multivariate analysis (P = 0.010). No such association could be observed in fatty liver patients with regard to advanced fibrosis (F ≥ 2). The common ABCB11 1331CC genotype, which is present in 40% of HCV patients and renders the carrier susceptible to increased bile acid levels, is associated with cirrhosis.

Genetic variations of bile salt transporters as predisposing factors for drug-induced cholestasis, intrahepatic cholestasis of pregnancy and therapeutic response of viral hepatitis

INTRODUCTION

Drug-induced cholestasis, intrahepatic cholestasis of pregnancy and viral hepatitis are acquired forms of liver disease. Cholestasis is a pathophysiologic state with impaired bile formation and subsequent accumulation of bile salts in hepatocytes. The bile salt export pump (BSEP) (ABCB11) is the key export system for bile salts from hepatocytes.

AREAS COVERED

This article provides an introduction into the physiology of bile formation followed by a summary of the current knowledge on the key bile salt transporters, namely, the sodium-taurocholate co-transporting polypeptide NTCP, the organic anion transporting polypeptides (OATPs), BSEP and the multi-drug resistance protein 3. The pathophysiologic consequences of altered functions of these transporters, with an emphasis on molecular and genetic aspects, are then discussed.

EXPERT OPINION

Knowledge of the role of hepatocellullar transporters, especially BSEP, in acquired cholestasis is continuously increasing. A common variant of BSEP (p.V444A) is now a well-established susceptibility factor for acquired cholestasis and recent evidence suggests that the same variant also influences the therapeutic response and disease progression of viral hepatitis C. Studies in large independent cohorts are now needed to confirm the relevance of p.V444A. Genome-wide association studies should lead to the identification of additional genetic factors underlying cholestatic liver disease.

Inhibition of interferon-α-induced signaling by hyperosmolarity and hydrophobic bile acids

Apart from viral conditions, host factors such as elevated bile acid concentrations are determinants of successful interferon-α (IFN-α) treatment in patients with chronic hepatitis C or B. The present study demonstrates that hydrophobic bile acids inhibit Jak1- and Tyk2-phosphorylation, which lead to blockade of STAT1-mediated IFN-α-signaling in the sodium-taurocholate cotransporting peptide (NTCP)-transfected human hepatoma cell line HepG2, resulting in a decreased mRNA and protein expression of IFN-stimulated genes such as myxovirus resistance protein A (MxA) or dsRNA-activated protein kinase (PKR). In addition, hyperosmotic stress leads to an inhibition of IFN-α-induced Jak1- and Tyk2-phosphorylation, and STAT1/STAT2-phosphorylation and gene expression. This inhibitory effect of hydrophobic bile acids or hyperosmolarity is not due to caspase-mediated cleavage or lysosomal degradation of the cognate receptors or to the generation of oxidative stress, activation of p38- or Erk-mediated MAPK pathways or phosphatase activity. Preincubation with the organic osmolyte betaine blocked the inhibitory effect of bile acids or hyperosmolarity on MxA protein expression, but had no effect on transcript levels or activation of STAT1, suggesting that betaine mediates its effects on MxA expression at a translational or post-translational level. Our findings could provide a rationale for betaine use in cholestatic HBV/HCV patients undergoing interferon therapy.

Targeting host factors: a novel rationale for the management of hepatitis C virus

Hepatitis C is recognized as a major threat to global public health. The current treatment of patients with chronic hepatitis C is the addition of ribavirin to interferon-based therapy which has limited efficacy, poor tolerability, and significant expense. New treatment options that are more potent and less toxic are much needed. Moreover, more effective treatment is an urgent priority for those who relapse or do not respond to current regimens. A major obstacle in combating hepatitis C virus (HCV) infection is that the fidelity of the viral replication machinery is notoriously low, thus enabling the virus to quickly develop mutations that resist compounds targeting viral enzymes. Therefore, an approach targeting the host cofactors, which are indispensable for the propagation of viruses, may be an ideal target for the development of antiviral agents because they have a lower rate of mutation than that of the viral genome, as long as they have no side effects to patients. Drugs targeting, for example, receptors of viral entry, host metabolism or nuclear receptors, which are factors required to complete the HCV life cycle, may be more effective in combating the viral infection. Targeting host cofactors of the HCV life cycle is an attractive concept because it imposes a higher genetic barrier for resistance than direct antiviral compounds. However the principle drawback of this strategy is the greater potential for cellular toxicity.

Enhancement of genotype 1 hepatitis C virus replication by bile acids through FXR

BACKGROUND/AIMS

Hepatitis C virus (HCV) infected patients with high serum levels of bile acids (BAs) usually fail to respond to antiviral therapy. Besides, BAs are essential factors for replication of the porcine enteric calicivirus by inhibiting interferon signaling. The role of BAs on HCV RNA replication was thus assessed.

METHODS

BAs and other compounds were tested using an HCV-replication model containing a luciferase reporter gene.

RESULTS

BAs, especially chenodeoxycholate and deoxycholate, up-regulated genotype 1 HCV RNA replication by more than tenfold. Only free but not conjugated BAs were active, suggesting that their effect was mediated by a nuclear receptor. Only farnesoid X receptor (FXR) ligands stimulated HCV replication while FXR silencing and FXR antagonism by guggulsterone blocked the up-regulation induced by BAs. Furthermore, guggulsterone alone inhibited basal level of HCV replication by tenfold. Modulation of HCV replication by FXR ligands occurred in the same proportion in presence or absence of type I interferon, suggesting a mechanism of action independent of this control of viral replication. However, BAs or guggulsterone did not affect replication of genotype 2a-JFH1.

CONCLUSIONS

Exposure to routinely measured concentrations of BAs increases HCV replication by a novel mechanism involving activation of the nuclear receptor FXR.

Taurolithocholic acid-3 sulfate impairs insulin signaling in cultured rat hepatocytes and perfused rat liver

BACKGROUND/AIMS

The role of bile acids for insulin resistance in cholestatic liver disease is unknown.

METHODS

The effect of taurolithocholic acid-3 sulfate (TLCS) on insulin signaling was studied in cultured rat hepatocytes and perfused rat liver.

RESULTS

TLCS induced insulin resistance at the level of insulin receptor (IR) beta Tyr(1158) phosphorylation, phosphoinositide (PI) 3-kinase activity and protein kinase (PK)B Ser(473) phosphorylation in cultured hepatocytes. Consistently, the insulin stimulation of the PI 3-kinase-dependent K(+) uptake, hepatocyte swelling and proteolysis inhibition was blunted by TLCS in perfused rat liver. The PKC inhibitor Go6850 and tauroursodeoxycholate (TUDC) counteracted the suppression of insulin-induced IRbeta and PKB phosphorylation by TLCS. Rapamycin and dibutyryl-cAMP, which inhibited basal signaling via mammalian target of rapamycin (mTOR), restored insulin-induced PKB- but not IRbeta phosphorylation. In livers from 7 day bile duct-ligated rats PKB Ser(473) phosphorylation was decreased by about 50%.

CONCLUSION

TLCS induces insulin resistance by a PKC-dependent suppression of insulin-induced IRbeta phosphorylation and the PI 3-kinase/PKB path. This can in part be compensated by a decrease of mTOR activity, which may release insulin-sensitive components downstream of the insulin receptor from tonic inhibition. The data suggest that retention of hydrophobic bile acids confers insulin resistance on the cholestatic liver.

Bile acids promote the expression of hepatitis C virus in replicon-harboring cells

Hepatitis C virus (HCV) is a cause of chronic liver disease, with more than 170 million persistently infected individuals worldwide. Although the combination therapy of alpha interferon (IFN-alpha) and ribavirin is effective for chronic HCV infection, around half of all patients infected with HCV genotype 1 fail to show sustained virologic responses and remain chronically infected. Previously, we demonstrated that bile acids were essential for growth of porcine enteric calicivirus in cell culture in association with down-regulation of IFN responses. Because hepatocytes are exposed to high concentrations of bile acids in the liver, we hypothesized that bile acids have similar effects on HCV replication. We incubated HCV replicon-harboring cells (genotype 1b, Con1) in the presence of various bile acids and monitored the expression of HCV RNA and protein (NS5B). The addition of an individual bile acid (deoxycholic acid, chenodeoxycholic acid, ursodeoxycholic acid, or glycochenodeoxycholic acid) in the medium increased the levels of HCV RNA and proteins up to fivefold at 48 h of incubation. An antagonist of bile acid receptor farnesoid X receptor (FXR), Z-guggulsterone, reduced the bile acid-mediated increase of HCV RNA. When IFN (alpha or gamma) and each bile acid were incubated together, we observed that bile acid significantly reduced the anti-HCV effect of IFN. These results indicated that bile acids are factors in the failure of IFN treatment for certain patients infected with HCV genotype 1. Our finding may also contribute to the establishment of better regimens for treatment of chronic HCV infections by including agents altering the bile acid-mediated FXR pathway.

Bile acid interactions with cholangiocytes

Cholangiocytes are exposed to high concentrations of bile acids at their apical membrane. A selective transporter for bile acids, the Apical Sodium Bile Acid Cotransporter (ASBT) (also referred to as Ibat; gene name Slc10a2) is localized on the cholangiocyte apical membrane. On the basolateral membrane, four transport systems have been identified (t-ASBT, multidrug resistance (MDR)3, an unidentified anion exchanger system and organic solute transporter (Ost) heteromeric transporter, Ostα-Ostβ. Together, these transporters unidirectionally move bile acids from ductal bile to the circulation. Bile acids absorbed by cholangiocytes recycle via the peribiliary plexus back to hepatocytes for re-secretion into bile. This recycling of bile acids between hepatocytes and cholangiocytes is referred to as the cholehepatic shunt pathway. Recent studies suggest that the cholehepatic shunt pathway may contribute in overall hepatobiliary transport of bile acids and to the adaptation to chronic cholestasis due to extrahepatic obstruction. ASBT is acutely regulated by an adenosine 3', 5’-monophosphate (cAMP)-dependent translocation to the apical membrane and by phosphorylation-dependent ubiquitination and proteasome degradation. ASBT is chronically regulated by changes in gene expression in response to biliary bile acid concentration and inflammatory cytokines. Another potential function of cholangiocyte ASBT is to allow cholangiocytes to sample biliary bile acids in order to activate intracellular signaling pathways. Bile acids trigger changes in intracellular calcium, protein kinase C (PKC), phosphoinositide 3-kinase (PI3K), mitogen-activated protein (MAP) kinase and extracellular signal-regulated protein kinase (ERK) intracellular signals. Bile acids significantly alter cholangiocyte secretion, proliferation and survival. Different bile acids have differential effects on cholangiocyte intracellular signals, and in some instances trigger opposing effects on cholangiocyte secretion, proliferation and survival. Based upon these concepts and observations, the cholangiocyte has been proposed to be the principle target cell for bile acids in the liver.

Expression of the interferon-induced Mx proteins in biliary atresia

Biliary atresia (BA) is a rare disease of the newborn for which the Kasai procedure is curative only for a few of the patients. The dilemma is that all therapeutic attempts to cure the disease are symptomatic because the etiology is still unclear. One theory suggests a progressive inflammatory process, possibly induced by a viral infection. The aim of the present study was to investigate the activity of type I interferons (IFNs) in the livers of patients with BA. Mx proteins, which mediate an early innate immune response, are a very sensitive marker for type I IFN activity (eg, to viral infection). Liver biopsies were taken during the Kasai procedure from 13 newborns with BA who were serologically negative for hepatotropic viruses. Age-matched controls originated from 7 patients with neonatal cholestasis (eg, inspissated bile syndrome), 3 aborted fetuses, and a 10-year-old child. The immunostaining procedure (alkaline phosphatase anti-alkaline phosphatase) was performed with Mx-specific monoclonal antibody. Immunostaining for Mx proteins was positive in the hepatocytes of all newborns with BA, whereas the intrahepatic bile ducts were positive in all but one. In the control group, 8 of 11 liver samples were Mx-negative. This is the first study dealing with the detection of type I IFN activity in the liver of patients with BA. This observation supports the etiologic consideration of type I IFN-mediated immune response. Although positive findings of viruses in patients with BA are still inconsistent, the present study retraces the progressive inflammatory process in BA one more step toward its beginning.

SOCS-1 and SOCS-3 inhibit IFN-alpha-induced expression of the antiviral proteins 2,5-OAS and MxA

Although the use of IFN-alpha in combination with ribavirin has improved the treatment efficacy of chronic hepatitis C virus (HCV) infection, 20-50% of patients still fail to eradicate the virus depending on the HCV genotype. Recently, overexpression of HCV core protein has been shown to inhibit IFN signaling and induce SOCS-3 expression. Aim of this study was to examine the putative role of SOCS proteins in IFN resistance. By Western blot analysis, a 4-fold induction of STAT-1/3 phosphorylation by IFN-alpha was observed in mock-transfected HepG2 clones. In contrast, IFN-induced STAT-1/3 phosphorylation was considerably downregulated by SOCS-1/3 overexpression. In mock-transfected cells, IFN-alpha induced 2',5'-OAS and myxovirus resistance A (MxA) promoter activity 40- to 80-fold and 10- to 35-fold, respectively, and this effect was abrogated in SOCS-1/3 overexpressing cells. As detected by Northern blot technique, IFN-alpha potently induced 2',5'-OAS and MxA mRNA expression in the control clones. Overexpression of SOCS-1 completely abolished both 2',5'-OAS and MxA mRNA expression, whereas SOCS-3 mainly inhibited 2',5'-OAS mRNA expression. Our results demonstrate that SOCS-1 and SOCS-3 proteins inhibit IFN-alpha-induced activation of the Jak-STAT pathway and expression of the antiviral proteins 2',5'-OAS and MxA. These data suggest a potential role of SOCS proteins in IFN resistance during antiviral treatment.

Bile acids are essential for porcine enteric calicivirus replication in association with down-regulation of signal transducer and activator of transcription 1

A porcine enteric calicivirus (PEC), strain Cowden in the family Caliciviridae (genus Sapovirus), can be propagated in a continuous cell line, LLC-PK cells, but only in the presence of an intestinal content fluid filtrate from gnotobiotic pigs. This cell culture system is presently the only in vitro model among caliciviruses that cause gastrointestinal disease, including members of the genera Sapovirus and Norovirus. We report here the identification of bile acids as active factors in intestinal content fluid essential for PEC growth. Bile acids that allowed PEC growth induced an increase in cAMP concentration in LLC-PK cells that was associated with down-regulation of IFN-mediated signal transducer and activator of transcription 1 phosphorylation, a key element in innate immunity. In addition, cAMP/protein kinase A pathway inhibitors, suramin, MDL12330A, or H89 suppressed bile acid-mediated PEC replication. We propose a mechanism for enteric calicivirus growth dependent on bile acids, ubiquitous molecules present in the intestine at the site of the virus replication that involves the protein kinase A cell-signaling pathway and a possible down-regulation of innate immunity.

Suppression of renal disease and mortality in the female NZB x NZW F1 mouse model of systemic lupus erythematosus (SLE) by chenodeoxycholic acid

The objective of this study was to examine the effects of ursodeoxycholic (UDCA) and chenodeoxycholic acid (CDCA) on autoimmune disease in the NZB x NZW F1 (B/W) mouse model of systemic lupus erythematosus (SLE). The development of murine lupus was assessed in female B/W mice given UDCA or CDCA. At 6 week intervals mice were examined for weight change, albuminuria, anti-DNA antibody and total IgG levels. Morbidity and mortality were assessed daily. UDCA- and CDCA-treated mice were examined at 24 weeks of age for serum cytokines, lymphocyte phenotype, and in vitro cytokine production after immunization with DNP-KLH. Liver and kidneys were examined histopathologically. The administration of UDCA and CDCA was tolerated without side effects. Weight gain in UDCA- or CDCA-treated and control mice was identical through 24 weeks of age. CDCA, but not UDCA, suppressed the development of renal disease. CDCA-treated B/W mice also had improved survival compared to UDCA-treated or control B/W mice. There were no significant effects of CDCA on anti-DNA antibodies, serum total IgG, or other immunologic parameters. CDCA-treated mice had lower serum IFN-gamma concentrations compared to control and UDCA-treated mice. The bile acid, CDCA, significantly inhibited the development of renal disease and modestly prolonged lifespan in the female B/W mouse model of SLE. Suppression of glomerulonephritis was associated with lower serum IFNgamma concentrations. Further investigation is needed to verify potential mechanisms of action, but these findings suggest that bile acids may alter the development or progression of autoimmunity.

Apoptosis in diseases of the liver

Apoptosis, or programmed cell death, and the elimination of apoptotic cells are crucial factors in the maintenance of liver health Apoptosis allows hepatocytes to die without provoking a potentially harmful inflammatory response In contrast to necrosis, apoptosis is tightly controlled and regulated via several mechanisms, including Fas/Fas ligand interactions, the effects of cytokines such as tumor necrosis factor alpha (TNF-alpha) and transforming growth factor beta (TGF-beta), and the influence of pro- and antiapoptotic mitochondria-associated proteins of the B-cell lymphoma-2 (Bcl-2) family. Efficient elimination of apoptotic cells in the liver relies on Kupffer cells and endothelial cells and is thought to be regulated by the expression of certain cell surface receptors. Liver disease is often associated with enhanced hepatocyte apoptosis, which is the case in viral and autoimmune hepatitis, cholestatic diseases, and metabolic disorders. Disruption of apoptosis is responsible for other diseases, for example, hepatocellular carcinoma. Use and abuse of certain drugs, especially alcohol, chemotherapeutic agents, and acetaminophen, have been associated with increased apoptosis and liver damage. Apoptosis also plays a role in transplantation-associated liver damage, both in ischemia/reperfusion injury and graft rejection. The role of apoptosis in various liver diseases and the mechanisms by which apoptosis occurs in the liver may provide insight into these diseases and suggest possible treatments.

Geranylgeranylacetone induces antiviral gene expression in human hepatoma cells

Geranylgeranylacetone (GGA), an isoprenoid compound, is used clinically as an anti-ulcer drug. Since some isoprenoids including retinoids have anti-tumor and anti-viral activities in a variety of cell types, we investigated whether GGA could induce anti-viral proteins in human hepatoma cells. The HuH-7 and HepG2 cells were treated with GGA, and expression of anti-viral proteins such as 2'5'-oligoadenylate synthetase (2'5'-OAS) and double-stranded RNA-dependent protein kinase (PKR) in these cells was analyzed. GGA stimulated 2'5'-OAS and PKR gene expression at the transcriptional level through the formation of interferon-stimulated gene factor 3 (ISGF3), which regulates both gene transcription. By Western blotting, GGA induced expression of signal transducers and activators of transcription 1, 2 (STAT1, STAT2) and p48 proteins, components of ISGF3, together with the phosphorylation of STAT1. These results suggest that GGA acts as a potent inducer of anti-viral gene expression by stimulating the ISGF3 formation in human hepatoma cells.

Apoptotic cell death does not parallel other indicators of liver damage in chronic hepatitis C patients

The mechanisms of hepatocyte damage and the events that lead to high rates of chronic liver disease in hepatitis C virus (HCV) infection remain unclear. Recent in vitro studies have suggested that the HCV core protein may disrupt specific signalling pathways of apoptosis. This prompted us to study patients with chronic HCV infection to: determine the extent of apoptosis in the liver; evaluate whether clinical and biochemical data are correlated with histological findings; and to investigate if apoptosis is related to the histological activity of the disease. Twelve patients with chronic hepatitis C were included in the study. Liver histology was scored by using the histological activity index (HAI) of Knodell et al. DNA fragmentation was assessed in liver tissue by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labelling (TUNEL) assay. Routine methods were used to determine serum markers of liver disease. Bile acids were measured in serum and liver by gas chromatography. Patients were placed, according to their HAI score, into group A (3.8 +/- 0.3) or group B (7.8 +/- 0.8) (P < 0.01). Liver enzymes tended to be higher in group B patients than in patients of group A. Levels of toxic bile acids in serum were greater in patients than in controls (P < 0.01). Chenodeoxycholic acid values were slightly higher in serum and liver of patients in group A. Liver biopsies with low HAI scores showed an increased rate of apoptosis (18.0 +/- 4.0 apoptotic cells per field) compared to those with higher HAI scores (6.6 +/- 2.1, P < 0.05) or to controls (3.5 +/- 0.4, P < 0.01). Hence, less severe liver disease, associated with lower histological grades and biochemistries, as well as increased levels of chenodeoxycholic acid, induces an expanded apoptotic response. The lower apoptotic rate in advanced liver disease may be associated with the high incidence of hepatocellular dysplasia/neoplasia.