Bile salt-induced hepatocyte apoptosis involves epidermal growth factor receptor-dependent CD95 tyrosine phosphorylation

Recent Advances in the Digestive, Metabolic and Therapeutic Effects of Farnesoid X Receptor and Fibroblast Growth Factor 19: From Cholesterol to Bile Acid Signaling

Bile acids (BA) are amphiphilic molecules synthesized in the liver (primary BA) starting from cholesterol. In the small intestine, BA act as strong detergents for emulsification, solubilization and absorption of dietary fat, cholesterol, and lipid-soluble vitamins. Primary BA escaping the active ileal re-absorption undergo the microbiota-dependent biotransformation to secondary BA in the colon, and passive diffusion into the portal vein towards the liver. BA also act as signaling molecules able to play a systemic role in a variety of metabolic functions, mainly through the activation of nuclear and membrane-associated receptors in the intestine, gallbladder, and liver. BA homeostasis is tightly controlled by a complex interplay with the nuclear receptor farnesoid X receptor (FXR), the enterokine hormone fibroblast growth factor 15 (FGF15) or the human ortholog FGF19 (FGF19). Circulating FGF19 to the FGFR4/β-Klotho receptor causes smooth muscle relaxation and refilling of the gallbladder. In the liver the binding activates the FXR-small heterodimer partner (SHP) pathway. This step suppresses the unnecessary BA synthesis and promotes the continuous enterohepatic circulation of BAs. Besides BA homeostasis, the BA-FXR-FGF19 axis governs several metabolic processes, hepatic protein, and glycogen synthesis, without inducing lipogenesis. These pathways can be disrupted in cholestasis, nonalcoholic fatty liver disease, and hepatocellular carcinoma. Thus, targeting FXR activity can represent a novel therapeutic approach for the prevention and the treatment of liver and metabolic diseases.

The many facets of bile acids in the physiology and pathophysiology of the human liver

Bile acids perform vital functions in the human liver and are the essential component of bile. It is therefore not surprising that the biology of bile acids is extremely complex, regulated on different levels, and involves soluble and membrane receptors as well as transporters. Hereditary disorders of these proteins manifest in different pathophysiological processes that result in liver diseases of varying severity. In this review, we summarize our current knowledge of the physiology and pathophysiology of bile acids with an emphasis on recently established analytical approaches as well as the molecular mechanisms that underlie signaling and transport of bile acids. In this review, we will focus on ABC transporters of the canalicular membrane and their associated diseases. As the G protein-coupled receptor, TGR5, receives increasing attention, we have included aspects of this receptor and its interaction with bile acids.

Cumulative damage effect of jaundice may be an effective predictor of complications in patients undergoing radical resection of Bismuth type II or above hilar cholangiocarcinoma

Background

There is currently no preoperative risk assessment system for predicting complications after radical resection of hilar cholangiocarcinoma. This study examined the association between the cumulative damage effect of jaundice (CDEJ) and the complications of radical resection of Bismuth II or above hilar cholangiocarcinoma.

Methods

Patients who underwent radical resection of hilar cholangiocarcinoma at the Department of Hepatobiliary Surgery, West China Hospital of Sichuan University, from April 2010 to January 2018 were retrospectively included.

Results

Of the 171 included patients, 115 (67.3%) patients experienced complications. Multivariate analysis found that CDEJ [odds ratio (OR) =1.0001, 95% confidence interval (95% CI) =1.000027–1.000239, P=0.014], cholangitis (OR =9.638, 95% CI =2.683–34.622, P=0.001), and preoperative bilirubin (OR =1.006, 95% CI =1.002–1.01, P=0.004) were independently associated with the incidence of complications. CDEJ (OR =1.0001, 95% CI =1.00001–1.00019, P=0.024), age (OR =1.083, 95% CI =1.029–1.14, P=0.002), preoperative bilirubin (OR =1.083, 95% CI =1.029–1.14, P=0.002), and future liver remnant (FLR) (OR =0.963, 95% CI =0.941–0.986, P=0.002) were independently associated with hepatic failure. To predict the incidence of complications, the following criteria were used. For the CDEJ cutoff of 2,151, the area under the receiver operating characteristic curve (AUC) was 0.69 (95% CI =0.615–0.759), the sensitivity was 66.09%, and the specificity was 69.64%. For the preoperative bilirubin cutoff of 111.7 µmol/L, the AUC was 0.65 (95% CI =0.573–0.721), the sensitivity was 84.35%, and the specificity was 42.86%. To predict hepatic failure, the following criteria were used. For the CDEJ cutoff of 3,931.95, the AUC was 0.605 (95% CI =0.582–0.679), the sensitivity was 51.28%, and the specificity was 70.45%. For the preoperative bilirubin cutoff of 115.9 µmol/L, the AUC was 0.638 (95% CI =0.561–0.71), the sensitivity was 92.31%, and the specificity was 32.58%. For the FLR cutoff of 50, the AUC was 0.638 (95% CI =0.515–0.667), the sensitivity was 48.72%, and the specificity was 78.79%.

Conclusions

CDEJ was independently associated with complications and can moderately predict complications after surgical resection of hilar cholangiocarcinoma.

Bile Acid Toxicity and Protein Kinases

If the bile acids reach to pathological concentrations due to cholestasis, accumulation of hydrophobic bile acids within the hepatocyte may result in cell death. Thus, hydrophobic bile acids induce apoptosis in hepatocytes, while hydrophilic bile acids increase intracellular adenosine 3',5'-monophosphate (cAMP) levels and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways to protect hepatocytes from apoptosis.Two apoptotic pathways have been described in bile acids-induced death. Both are controlled by multiple protein kinase signaling pathways. In mitochondria-controlled pathway, caspase-8 is activated with death domain-independent manner, whereas, Fas-dependent classical pathway involves ligand-independent oligomerization of Fas.Hydrophobic bile acids dose-dependently upregulate the inflammatory response by further stimulating production of inflammatory cytokines. Death receptor-mediated apoptosis is regulated at the cell surface by the receptor expression, at the death-inducing signaling complex (DISC) by expression of procaspase-8, the death receptors Fas-associated death domain (FADD), and cellular FADD-like interleukin 1-beta (IL-1β)-converting enzyme (FLICE) inhibitory protein (cFLIP). Bile acids prevent cFLIP recruitment to the DISC and thereby enhance initiator caspase activation and lead to cholestatic apoptosis. At mitochondria, the expression of B-cell leukemia/lymphoma-2 (Bcl-2) family proteins contribute to apoptosis by regulating mitochondrial cytochrome c release via Bcl-2, Bcl-2 homology 3 (BH3) interacting domain death agonist (Bid), or Bcl-2 associated protein x (Bax). Fas receptor CD95 activation by hydrophobic bile acids is initiated by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) signaling. However, activation of necroptosis by ligands of death receptors requires the kinase activity of receptor interacting protein1 (RIP1), which mediates the activation of RIP3 and mixed lineage kinase domain-like protein (MLKL). In this chapter, mainly the effect of protein kinases signal transduction on the mechanisms of hydrophobic bile acids-induced inflammation, apoptosis, necroptosis and necrosis are discussed.

Bile Acids and GPBAR-1: Dynamic Interaction Involving Genes, Environment and Gut Microbiome

Bile acids (BA) are amphiphilic molecules synthesized in the liver from cholesterol. BA undergo continuous enterohepatic recycling through intestinal biotransformation by gut microbiome and reabsorption into the portal tract for uptake by hepatocytes. BA are detergent molecules aiding the digestion and absorption of dietary fat and fat-soluble vitamins, but also act as important signaling molecules via the nuclear receptor, farnesoid X receptor (FXR), and the membrane-associated G protein-coupled bile acid receptor 1 (GPBAR-1) in the distal intestine, liver and extra hepatic tissues. The hydrophilic-hydrophobic balance of the BA pool is finely regulated to prevent BA overload and liver injury. By contrast, hydrophilic BA can be hepatoprotective. The ultimate effects of BA-mediated activation of GPBAR-1 is poorly understood, but this receptor may play a role in protecting the remnant liver and in maintaining biliary homeostasis. In addition, GPBAR-1 acts on pathways involved in inflammation, biliary epithelial barrier permeability, BA pool hydrophobicity, and sinusoidal blood flow. Recent evidence suggests that environmental factors influence GPBAR-1 gene expression. Thus, targeting GPBAR-1 might improve liver protection, facilitating beneficial metabolic effects through primary prevention measures. Here, we discuss the complex pathways linked to BA effects, signaling properties of the GPBAR-1, mechanisms of liver damage, gene-environment interactions, and therapeutic aspects.

CD95 Structure, Aggregation and Cell Signaling

CD95 is a pre-ligand-associated transmembrane (TM) receptor. The interaction with its ligand CD95L brings to a next level its aggregation and triggers different signaling pathways, leading to cell motility, differentiation or cell death. This diversity of biological responses associated with a unique receptor devoid of enzymatic property raises the question of whether different ligands exist, or whether the fine-tuned control of CD95 aggregation and conformation, its distribution within certain plasma membrane sub-domains or the pattern of post-translational modifications account for this such broad-range of cell signaling. Herein, we review how the different domains of CD95 and their post-translational modifications or the different forms of CD95L can participate in the receptor aggregation and induction of cell signaling. Understanding how CD95 response goes from cell death to cell proliferation, differentiation and motility is a prerequisite to reveal novel therapeutic options to treat chronic inflammatory disorders and cancers.

Hepatoprotective impact of the bile acid receptor TGR5

During liver repair after injury, bile secretion has to be tightly modulated in order to preserve liver parenchyma from bile acid (BA)-induced injury. The mechanisms allowing the liver to maintain biliary homeostasis during repair after injury are not completely understood. Besides their historical role in lipid digestion, bile acids (BA) and their receptors constitute a signalling network with multiple impacts on liver repair, both stimulating regeneration and protecting the liver from BA overload. BA signal through nuclear (mainly Farnesoid X Receptor, FXR) and membrane (mainly G Protein-coupled BA Receptor 1, GPBAR-1 or TGR5) receptors to elicit a wide array of biological responses. While a great number of studies have been dedicated to the hepato-protective impact of FXR signalling, TGR5 is by far less explored in this context. Because the liver has to face massive and potentially harmful BA overload after partial ablation or destruction, BA-induced protective responses crucially contribute to spare liver repair capacities. Based on the available literature, the TGR5 BA receptor protects the remnant liver and maintains biliary homeostasis, mainly through the control of inflammation, biliary epithelial barrier permeability, BA pool hydrophobicity and sinusoidal blood flow. Mouse experimental models of liver injury reveal that in the lack of TGR5, excessive inflammation, leaky biliary epithelium and hydrophobic BA overload result in parenchymal insult and compromise optimal restoration of a functional liver mass. Translational perspectives are thus opened to target TGR5 with the aim of protecting the liver in the context of injury and BA overload.

Tauroursodeoxycholate protects from glycochenodeoxycholate-induced gene expression changes in perfused rat liver

Tauroursodeoxycholate (TUDC) is well known to protect against glycochenodeoxycholate (GCDC)-induced apoptosis in rat hepatocytes. In the present study, we analyzed whether TUDC also exerts protective effects by modulating GCDC-induced gene expression changes. For this, gene array-based transcriptome analysis and quantitative polymerase chain reaction (qPCR) were performed on RNA isolated from rat livers perfused with GCDC, TUDC or a combination of both (each 20 μm for 2 h). GCDC led to a significant increase of lactate dehydrogenase (LDH) into the effluent perfusate, which was prevented by TUDC. GCDC, TUDC and co-perfusion induced distinct gene expression changes. While GCDC upregulated the expression of several pro-inflammatory genes, co-perfusion with TUDC increased the expression of pro-proliferative and anti-apoptotic p53 target genes. In line with this, levels of serine20-phosphorylated p53 and of its target gene p21 were elevated by GCDC in a TUDC-sensitive way. GCDC upregulated the oxidative stress surrogate marker 8OH(d)G and the pro-apoptotic microRNAs miR-15b/16 and these effects were prevented by TUDC. The upregulation of miR-15b and miR-16 in GCDC-perfused livers was accompanied by a downregulation of several potential miR-15b and miR-16 target genes. The present study identified changes in the transcriptome of the rat liver which suggest, that TUDC is hepatoprotective by counteracting GCDC-induced gene expression changes.

Role of epidermal growth factor receptor in liver injury and lipid metabolism: Emerging new roles for an old receptor

Epidermal growth factor receptor (EGFR) is conventionally known to play a crucial role in hepatocyte proliferation, liver regeneration and is also associated with hepatocellular carcinogenesis. In addition to these proliferative roles, EGFR has also implicated in apoptotic cell death signaling in various hepatic cells, mitochondrial dysfunction and acute liver necrosis in a clinically relevant murine model of acetaminophen overdose, warranting further comprehensive exploration of this paradoxical role of EGFR in hepatotoxicity. Apart from ligand dependent activation, EGFR can also be activated in ligand-independent manner, which is mainly associated to liver injury. Recent evidence has also emerged demonstrating important role of EGFR in lipid and fatty acid metabolism in quiescent and regenerating liver. Based on these findings, EGFR has also been shown to play an important role in steatosis in clinically relevant murine NAFLD models via regulating master transcription factors governing fatty acid synthesis and lipolysis. Moreover, several lines of evidences indicate that EGFR is also involved in hepatocellular injury, oxidative stress, inflammation, direct stellate cell activation and fibrosis in chronic liver injury models, including repeated CCl₄ exposure, high-fat diet and fast-food diet models. In addition to briefly summarizing role of EGFR in liver regeneration, this review comprehensively discusses all these non-conventional emerging roles of EGFR. Considering evidences of multi-facet role of EGFR at various levels in these pathophysiological process, EGFR can be a promising therapeutic target for various liver diseases, including acute liver failure and NAFLD, requiring further exploration. These roles of EGFR are relevant for alcoholic liver diseases (ALD) as well, thus providing a valid rationale for future investigations exploring a role of EGFR in ALD.

Assessment of biological functions for C3A cells interacting with adverse environments of liver failure plasma

BACKGROUND

For its better differentiated hepatocyte phenotype, C3A cell line has been utilized in bioartificial liver system. However, up to now, there are only a few of studies working at the metabolic alternations of C3A cells under the culture conditions with liver failure plasma, which mainly focus on carbohydrate metabolism, total protein synthesis and ureagenesis. In this study, we investigated the effects of acute liver failure plasma on the growth and biological functions of C3A cells, especially on CYP450 enzymes.

METHODS

C3A cells were treated with fresh DMEM medium containing 10% FBS, fresh DMEM medium containing 10% normal plasma and acute liver failure plasma, respectively. After incubation, the C3A cells were assessed for cell viabilities, lactate dehydrogenase leakage, gene transcription, protein levels, albumin secretion, ammonia metabolism and CYP450 enzyme activities.

RESULTS

Cell viabilities decreased 15%, and lactate dehydrogenase leakage had 1.3-fold elevation in acute liver failure plasma group. Gene transcription exhibited up-regulation, down-regulation or stability for different hepatic genes. In contrast, protein expression levels for several CYP450 enzymes kept constant, while the CYP450 enzyme activities decreased or remained stable. Albumin secretion reduced about 48%, and ammonia accumulation increased approximately 41%.

CONCLUSIONS

C3A cells cultured with acute liver failure plasma showed mild inhibition of cell viabilities, reduction of albumin secretion, and increase of ammonia accumulation. Furthermore, CYP450 enzymes demonstrated various alterations on gene transcription, protein expression and enzyme activities.

Glycochenodeoxycholate Promotes Liver Fibrosis in Mice with Hepatocellular Cholestasis

Hydrophobic bile salts are considered to promote liver fibrosis in cholestasis. However, evidence for this widely accepted hypothesis remains scarce. In established animal models of cholestasis, e.g., by Mdr2 knockout, cholestasis and fibrosis are both secondary to biliary damage. Therefore, to test the specific contribution of accumulating bile salts to liver fibrosis in cholestatic disease, we applied the unique model of inducible hepatocellular cholestasis in cholate-fed Atp8b1^G308V/G308V mice. Glycochenodeoxycholate (GCDCA) was supplemented to humanize the murine bile salt pool, as confirmed by HPLC. Biomarkers of cholestasis and liver fibrosis were quantified. Hepatic stellate cells (HSC) isolated from wild-type mice were stimulated with bile salts. Proliferation, cell accumulation, and collagen deposition of HSC were determined. In cholestatic Atp8b1^G308V/G308V mice, increased hepatic expression of αSMA and collagen1a mRNA and excess hepatic collagen deposition indicated development of liver fibrosis only upon GCDCA supplementation. In vitro, numbers of myofibroblasts and deposition of collagen were increased after incubation with hydrophobic but not hydrophilic bile salts, and associated with EGFR and MEK1/2 activation. We concluded that chronic hepatocellular cholestasis alone, independently of biliary damage, induces liver fibrosis in mice in presence of the human bile salt GCDCA. Bile salts may have direct pro-fibrotic effects on HSC, putatively involving EGFR and MEK1/2 signaling.

Inflammation and Cell Death During Cholestasis: The Evolving Role of Bile Acids

Cholestasis results in blockage of bile flow whether the point of obstruction occurs extrahepatically or intrahepatically. Bile acids are a primary constituent of bile, and thus one of the primary outcomes is acute retention of bile acids in hepatocytes. Bile acids are normally secreted into the biliary tracts and then released into the small bowel before recirculating back to the liver. Retention of bile acids has long been hypothesized to be a primary cause of the associated liver injury that occurs during acute or chronic cholestasis. Despite this, a surge of papers in the last decade have reported a primary role for inflammation in the pathophysiology of cholestatic liver injury. Furthermore, it has increasingly been recognized that both the constituency of individual bile acids that make up the greater pool, as well as their conjugation status, is intimately involved in their toxicity, and this varies between species. Finally, the role of bile acids in drug-induced cholestatic liver injury remains an area of increasing interest. The purpose of this review is to critically evaluate current proposed mechanisms of cholestatic liver injury, with a focus on the evolving role of bile acids in cell death and inflammation.

Bile Acid-Activated Receptors: GPBAR1 (TGR5) and Other G Protein-Coupled Receptors

The BA-responsive GPCRs S1PR2 and TGR5 are almost ubiquitously expressed in human and rodent tissues. In the liver, S1PR2 is expressed in all cell types, while TGR5 is predominately found in non-parenchymal cells. In contrast to S1PR2, which is mainly activated by conjugated bile acids (BAs), all BAs serve as ligands for TGR5 irrespective of their conjugation state and substitution pattern.Mice with targeted deletion of either S1PR2 or TGR5 are viable and develop no overt phenotype. In liver injury models, S1PR2 exerts pro-inflammatory and pro-fibrotic effects and thus aggravates liver damage, while TGR5 mediates anti-inflammatory, anti-cholestatic, and anti-fibrotic effects. Thus, inhibitors of S1PR2 signaling and agonists for TGR5 have been employed to attenuate liver injury in rodent models for cholestasis, nonalcoholic steatohepatitis, and fibrosis/cirrhosis.In biliary epithelial cells, both receptors activate a similar signaling cascade resulting in ERK1/2 phosphorylation and cell proliferation. Overexpression of both S1PR2 and TGR5 was found in human cholangiocarcinoma tissue as well as in CCA cell lines, where stimulation of both GPCRs resulted in transactivation of the epidermal growth factor receptor and triggered cell proliferation as well as increased cell migration and invasiveness.This chapter will focus on the function of S1PR2 and TGR5 in different liver cell types and summarizes current knowledge on the role of these receptors in liver disease models.

Measuring Apoptosis and Necrosis in Cholestatic Liver Injury

Cholestasis can be induced by obstruction of bile ducts or intrahepatic toxicity of drugs and chemicals. However, the mode of cell death during cholestasis, i.e., apoptosis or necrosis, has been controversial. There are fundamental reasons for the controversies, both of which are discussed here, namely the design of experiments and the use of parameters with limited specificity for a certain mode of cell death. Based on the assumption that cholestatic liver injury is caused by accumulation of bile acids, rodent (mainly rat) hepatocytes have been exposed to hydrophobic, glycine-conjugated bile acids, which resulted in apoptotic cell death. The problems with this experimental design are that in rodents bile acids are predominantly taurine conjugated and rodent hepatocytes are never exposed to these levels of glycine-conjugated bile acids. In contrast, taurine-conjugated bile acids trigger inflammatory gene activation in rodent hepatocytes and a necro-inflammatory injury in vivo. On the other hand, human hepatocytes are more resistant to glycine-conjugated bile acids and die by necrosis when exposed to high biliary levels of these bile acids. In this chapter, we describe multiple assays including the caspase activity assay, which is specific for apoptosis, and the general cell death assays alanine aminotransferase or lactate dehydrogenase activities in cell culture medium or plasma. An increase in these enzyme activities without caspase activity indicates necrotic cell death. Thus, both the experimental design and the selection of cell death parameters are critical for the relevance of the experiments for the human pathophysiology.

Bile acids and their receptors during liver regeneration: "Dangerous protectors"

Tissue repair is orchestrated by a finely tuned interplay between processes of regeneration, inflammation and cell protection, allowing organisms to restore their integrity after partial loss of cells or organs. An important, although largely unexplored feature is that after injury and during liver repair, liver functions have to be maintained to fulfill the peripheral demand. This is particularly critical for bile secretion, which has to be finely modulated in order to preserve liver parenchyma from bile-induced injury. However, mechanisms allowing the liver to maintain biliary homeostasis during repair after injury are not completely understood. Besides cytokines and growth factors, bile acids (BA) and their receptors constitute an insufficiently explored signaling network during liver regeneration and repair. BA signal through both nuclear (mainly Farnesoid X Receptor, FXR) and membrane (mainly G Protein-coupled BA Receptor 1, GPBAR-1 or TGR5) receptors which distributions are large in the organism, and which activation elicits a wide array of biological responses. While a number of studies have been dedicated to FXR signaling in liver repair processes, TGR5 remains poorly explored in this context. Because of the massive and potentially harmful BA overload that faces the remnant liver after partial ablation or destruction, both BA-induced adaptive and proliferative responses may stand in a central position to contribute to the regenerative response. Based on the available literature, both BA receptors may act in synergy during the regeneration process, in order to protect the remnant liver and maintain biliary homeostasis, otherwise potentially toxic BA overload would result in parenchymal insult and compromise optimal restoration of a functional liver mass.

Dual Role of Epidermal Growth Factor Receptor in Liver Injury and Regeneration after Acetaminophen Overdose in Mice

Epidermal growth factor receptor (EGFR) plays a crucial role in hepatocyte proliferation. Its role in acetaminophen (APAP)-mediated hepatotoxicity and subsequent liver regeneration is completely unknown. Role of EGFR after APAP-overdose in mice was studied using pharmacological inhibition strategy. Rapid, sustained and dose-dependent activation of EGFR was noted after APAP-treatment in mice, which was triggered by glutathione depletion. EGFR-activation was also observed in primary human hepatocytes after APAP-treatment, preceding elevation of toxicity markers. Treatment of mice with an EGFR-inhibitor (EGFRi), Canertinib, 1h post-APAP resulted in robust inhibition of EGFR-activation and a striking reduction in APAP-induced liver injury. Metabolic activation of APAP, formation of APAP-protein adducts, APAP-mediated JNK-activation and its mitochondrial translocation were not altered by EGFRi. Interestingly, EGFR rapidly translocated to mitochondria after APAP-treatment. EGFRi-treatment abolished mitochondrial EGFR activity, prevented APAP-mediated mitochondrial dysfunction/oxidative-stress and release of endonucleases from mitochondria, which are responsible for DNA-damage/necrosis. Treatment with N-acetylcysteine (NAC), 4h post-APAP in mice did not show any protection but treatment of EGFRi in combination with NAC showed decrease in liver injury. Finally, delayed treatment with EGFRi, 12-h post-APAP, did not alter peak injury but caused impairment of liver regeneration resulting in sustained injury and decreased survival after APAP overdose in mice. Impairment of regeneration was due to inhibition of cyclinD1 induction and cell cycle arrest. Our study has revealed a new dual role of EGFR both in initiation of APAP-injury and in stimulation of subsequent compensatory regeneration after APAP-overdose.

Fas Versatile Signaling and Beyond: Pivotal Role of Tyrosine Phosphorylation in Context-Dependent Signaling and Diseases

The Fas/FasL system is known, first and foremost, as a potent apoptosis activator. While its proapoptotic features have been studied extensively, evidence that the Fas/FasL system can elicit non-death signals has also accumulated. These non-death signals can promote survival, proliferation, migration, and invasion of cells. The key molecular mechanism that determines the shift from cell death to non-death signals had remained unclear until the recent identification of the tyrosine phosphorylation in the death domain of Fas as the reversible signaling switch. In this review, we present the connection between the recent findings regarding the control of Fas multi-signals and the context-dependent signaling choices. This information can help explain variable roles of Fas signaling pathway in different pathologies.

AP-1 Inhibition by SR 11302 Protects Human Hepatoma HepG2 Cells from Bile Acid-Induced Cytotoxicity by Restoring the NOS-3 Expression

The harmful effects of bile acid accumulation occurring during cholestatic liver diseases have been associated with oxidative stress increase and endothelial nitric oxide synthase (NOS-3) expression decrease in liver cells. We have previously reported that glycochenodeoxycholic acid (GCDCA) down-regulates gene expression by increasing SP1 binding to the NOS-3 promoter in an oxidative stress dependent manner. In the present study, we aimed to investigate the role of transcription factor (TF) AP-1 on the NOS-3 deregulation during GCDCA-induced cholestasis. The cytotoxic response to GCDCA was characterized by 1) the increased expression and activation of TFs cJun and c-Fos; 2) a higher binding capability of these at position -666 of the NOS-3 promoter; 3) a decrease of the transcriptional activity of the promoter and the expression and activity of NOS-3; and 4) the expression increase of cyclin D1. Specific inhibition of AP-1 by the retinoid SR 11302 counteracted the cytotoxic effects induced by GCDCA while promoting NOS-3 expression recovery and cyclin D1 reduction. NOS activity inhibition by L-NAME inhibited the protective effect of SR 11302. Inducible NOS isoform was no detected in this experimental model of cholestasis. Our data provide direct evidence for the involvement of AP-1 in the NOS-3 expression regulation during cholestasis and define a critical role for NOS-3 in regulating the expression of cyclin D1 during the cell damage induced by bile acids. AP-1 appears as a potential therapeutic target in cholestatic liver diseases given its role as a transcriptional repressor of NOS-3.

Na(+) /H(+) exchanger regulatory factor 1 knockout mice have an attenuated hepatic inflammatory response and are protected from cholestatic liver injury

The intercellular adhesion molecule 1 (ICAM-1) is induced in mouse liver after bile duct ligation (BDL) and plays a key role in neutrophil-mediated liver injury in BDL mice. ICAM-1 has been shown to interact with cytoskeletal ezrin-radixin-moesin (ERM) proteins that also interact with the PDZ protein, Na(+) /H(+) exchanger regulatory factor 1 (NHERF-1/EBP50). In NHERF-1(-/-) mice, ERM proteins are significantly reduced in brush-border membranes from kidney and small intestine. ERM knockdown reduces ICAM-1 expression in response to tumor necrosis factor alpha. Here we show that NHERF-1 assembles ERM proteins, ICAM-1 and F-actin into a macromolecule complex that is increased in mouse liver after BDL. Compared to wild-type (WT) mice, both sham-operated and BDL NHERF-1(-/-) mice have lower levels of activated ERM and ICAM-1 protein in the liver accompanied by significantly reduced hepatic neutrophil accumulation, serum alanine aminotransferase, and attenuated liver injury after BDL. However, total bile acid concentrations in serum and liver of sham and BDL NHERF-1(-/-) mice were not significantly different from WT controls, although hepatic tetrahydroxylated bile acids and Cyp3a11 messenger RNA levels were higher in NHERF-1(-/-) BDL mice.

CONCLUSION

NHERF-1 participates in the inflammatory response that is associated with BDL-induced liver injury. Deletion of NHERF-1 in mice leads to disruption of the formation of ICAM-1/ERM/NHERF-1 complex and reduction of hepatic ERM proteins and ICAM-1, molecules that are up-regulated and are essential for neutrophil-mediated liver injury in cholestasis. Further study of the role of NHERF-1 in the inflammatory response in cholestasis and other forms of liver injury should lead to discovery of new therapeutic targets in hepatic inflammatory diseases.

Regulation of plasma membrane localization of the Na+-taurocholate cotransporting polypeptide (Ntcp) by hyperosmolarity and tauroursodeoxycholate

In perfused rat liver, hepatocyte shrinkage induces a Fyn-dependent retrieval of the bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2) from the canalicular membrane (Cantore, M., Reinehr, R., Sommerfeld, A., Becker, M., and Häussinger, D. (2011) J. Biol. Chem. 286, 45014-45029) leading to cholestasis. However little is known about the effects of hyperosmolarity on short term regulation of the Na(+)-taurocholate cotransporting polypeptide (Ntcp), the major bile salt uptake system at the sinusoidal membrane of hepatocytes. The aim of this study was to analyze hyperosmotic Ntcp regulation and the underlying signaling events. Hyperosmolarity induced a significant retrieval of Ntcp from the basolateral membrane, which was accompanied by an activating phosphorylation of the Src kinases Fyn and Yes but not of c-Src. Hyperosmotic internalization of Ntcp was sensitive to SU6656 and PP-2, suggesting that Fyn mediates Ntcp retrieval from the basolateral membrane. Hyperosmotic internalization of Ntcp was also found in livers from wild-type mice but not in p47(phox) knock-out mice. Tauroursodeoxycholate (TUDC) and cAMP reversed hyperosmolarity-induced Fyn activation and triggered re-insertion of the hyperosmotically retrieved Ntcp into the membrane. This was associated with dephosphorylation of the Ntcp on serine residues. Insertion of Ntcp by TUDC was sensitive to the integrin inhibitory hexapeptide GRGDSP and inhibition of protein kinase A. TUDC also reversed the hyperosmolarity-induced retrieval of bile salt export pump from the canalicular membrane. These findings suggest a coordinated and oxidative stress- and Fyn-dependent retrieval of sinusoidal and canalicular bile salt transport systems from the corresponding membranes. Ntcp insertion was also identified as a novel target of β1-integrin-dependent TUDC action, which is frequently used in the treatment of cholestatic liver disease.

The Bile Acid Receptor TGR5 and Liver Regeneration

BACKGROUND

Most of the literature on the bile acid (BA) membrane receptor TGR5 is dedicated to its potential role in the metabolic syndrome, through its regulatory impact on energy expenditure, insulin and GLP-1 secretion, and inflammatory processes. While the receptor was cloned in 2002, very little data are available on TGR5 functions in the normal and diseased liver. However, TGR5 is highly expressed in Kupffer cells and liver endothelial cells, and is particularly enriched in the biliary tract [cholangiocytes and gallbladder (GB) smooth muscle cells]. We recently demonstrated that TGR5 has a crucial protective impact on the liver in case of BA overload, including after partial hepatectomy.

KEY MESSAGES

TGR5-KO mice after PH exhibited periportal bile infarcts, excessive hepatic inflammation and defective adaptation of biliary composition (bicarbonate and chloride). Most importantly, TGR5-KO mice had a more hydrophobic BA pool, with more secondary BA than WT animals, suggesting that TGR5-KO bile may be harmful for the liver, mainly in situations of BA overload. As GB is both the tissue displaying the highest level of TGR5 expression and a crucial physiological site for the regulation of BA pool hydrophobicity by reducing secondary BA, we investigated whether TGR5 may control BA pool composition through an impact on GB. Preliminary data suggest that in the absence of TGR5, reduced GB filling dampens the cholecystohepatic shunt, resulting in more secondary BA, more hydrophobic BA pool and extensive liver injury in case of BA overload.

CONCLUSIONS

In the setting of BA overload, TGR5 is protective of the liver through the regulation of not only secretory and inflammatory processes, but also through the control of BA pool composition, at least in part by targeting the GB. Thereby, TGR5 appears to be crucial for protecting the regenerating liver from BA overload.

Genetic variations of bile salt transporters

Bile salt transporters directly or indirectly influence biological processes through physicochemical or signalling properties of bile salts. The coordinated action of uptake and efflux transporters in polarized epithelial cells of the liver, biliary tree, small intestine and kidney determine bile salt concentrations in different compartments of the body. Genetic variations of bile salt transporters lead to clinical relevant phenotypes of varying severity ranging from a predisposition for drug-induced liver injury to rapidly progressing end-stage liver disease. This review focuses on the impact of genetic variations of bile salt transporters including BSEP, NTCP, ASBT and OSTα/β and discusses approaches for transporter analysis.

Protein kinase Cδ protects against bile acid apoptosis by suppressing proapoptotic JNK and BIM pathways in human and rat hepatocytes

Retained bile acids, which are capable of inducing cell death, activate protein kinase Cδ (PKC-δ) in hepatocytes. In nonhepatic cells, both pro- and antiapoptotic effects of PKC-δ are described. The aim of this study was to determine the role of PKC-δ in glycochenodeoxycholate (GCDC)-induced apoptosis in rat hepatocytes and human HUH7-Na-taurocholate-cotransporting polypeptide (Ntcp) cells. Apoptosis was monitored morphologically by Hoechst staining and biochemically by immunoblotting for caspase 3 cleavage. The role of PKC-δ was evaluated with a PKC activator (phorbol myristate acetate, PMA) and PKC inhibitors (chelerythrine, H-7, or calphostin), PKC-δ knockdown, and wild-type (WT) or constitutively active (CA) PKC-δ. PKC-δ activation was monitored by immunoblotting for PKC-δ Thr505 and Tyr311 phosphorylation or by membrane translocation. JNK and Akt phosphorylation and the amount of total bisindolylmaleimide (BIM) were determined by immunoblotting. GCDC induced the translocation of PKC-δ to the mitochondria and/or plasma membrane in rat hepatocytes and HUH7-Ntcp cells and increased PKC-δ phosphorylation on Thr505, but not on Tyr311, in HUH7-Ntcp cells. GCDC-induced apoptosis was attenuated by PMA and augmented by PKC inhibition in rat hepatocytes. In HUH-Ntcp cells, transfection with CA or WT PKC-δ attenuated GCDC-induced apoptosis, whereas knockdown of PKC-δ increased GCDC-induced apoptosis. PKC-δ silencing increased GCDC-induced JNK phosphorylation, decreased GCDC-induced Akt phosphorylation, and increased expression of BIM. GCDC translocated BIM to the mitochondria in rat hepatocytes, and knockdown of BIM in HUH7-Ntcp cells decreased GCDC-induced apoptosis. Collectively, these results suggest that PKC-δ does not mediate GCDC-induced apoptosis in hepatocytes. Instead PKC-δ activation by GCDC stimulates a cytoprotective pathway that involves JNK inhibition, Akt activation, and downregulation of BIM.

Decreasing mitochondrial fission prevents cholestatic liver injury

Mitochondria frequently change their shape through fission and fusion in response to physiological stimuli as well as pathological insults. Disrupted mitochondrial morphology has been observed in cholestatic liver disease. However, the role of mitochondrial shape change in cholestasis is not defined. In this study, using in vitro and in vivo models of bile acid-induced liver injury, we investigated the contribution of mitochondrial morphology to the pathogenesis of cholestatic liver disease. We found that the toxic bile salt glycochenodeoxycholate (GCDC) rapidly fragmented mitochondria, both in primary mouse hepatocytes and in the bile transporter-expressing hepatic cell line McNtcp.24, leading to a significant increase in cell death. GCDC-induced mitochondrial fragmentation was associated with an increase in reactive oxygen species (ROS) levels. We found that preventing mitochondrial fragmentation in GCDC by inhibiting mitochondrial fission significantly decreased not only ROS levels but also cell death. We also induced cholestasis in mouse livers via common bile duct ligation. Using a transgenic mouse model inducibly expressing a dominant-negative fission mutant specifically in the liver, we demonstrated that decreasing mitochondrial fission substantially diminished ROS levels, liver injury, and fibrosis under cholestatic conditions. Taken together, our results provide new evidence that controlling mitochondrial fission is an effective strategy for ameliorating cholestatic liver injury.

Clinical application of transcriptional activators of bile salt transporters

Hepatobiliary bile salt (BS) transporters are critical determinants of BS homeostasis controlling intracellular concentrations of BSs and their enterohepatic circulation. Genetic or acquired dysfunction of specific transport systems causes intrahepatic and systemic retention of potentially cytotoxic BSs, which, in high concentrations, may disturb integrity of cell membranes and subcellular organelles resulting in cell death, inflammation and fibrosis. Transcriptional regulation of canalicular BS efflux through bile salt export pump (BSEP), basolateral elimination through organic solute transporters alpha and beta (OSTα/OSTβ) as well as inhibition of hepatocellular BS uptake through basolateral Na(+)-taurocholate cotransporting polypeptide (NTCP) represent critical steps in protection from hepatocellular BS overload and can be targeted therapeutically. In this article, we review the potential clinical implications of the major BS transporters BSEP, OSTα/OSTβ and NTCP in the pathogenesis of hereditary and acquired cholestatic syndromes, provide an overview on transcriptional control of these transporters by the key regulatory nuclear receptors and discuss the potential therapeutic role of novel transcriptional activators of BS transporters in cholestasis.

Taurocholic Acid Does Not Induce Apoptosis in HCT116 Cells Regardless of Its Intracellular Concentration

Hydrophobic bile acids but not hydrophilic bile acids induce apoptosis in HCT116 cells. We expressed sodium-dependent bile acid transporters in HCT116 cells, and the intracellular concentration of hydrophilic bile acids increased to that of the hydrophobic bile acids. But no sign of apoptosis was observed, which suggests a hydrophobic-bile acid-specific mechanism for the induction of apoptosis in HCT116 cells.

Metformin protects rat hepatocytes against bile acid-induced apoptosis

BACKGROUND

Metformin is used in the treatment of Diabetes Mellitus type II and improves liver function in patients with non-alcoholic fatty liver disease (NAFLD). Metformin activates AMP-activated protein kinase (AMPK), the cellular energy sensor that is sensitive to changes in the AMP/ATP-ratio. AMPK is an inhibitor of mammalian target of rapamycin (mTOR). Both AMPK and mTOR are able to modulate cell death.

AIM

To evaluate the effects of metformin on hepatocyte cell death.

METHODS

Apoptotic cell death was induced in primary rat hepatocytes using either the bile acid glycochenodeoxycholic acid (GCDCA) or TNFα in combination with actinomycin D (actD). AMPK, mTOR and phosphoinositide-3 kinase (PI3K)/Akt were inhibited using pharmacological inhibitors. Apoptosis and necrosis were quantified by caspase activation, acridine orange staining and Sytox green staining respectively.

RESULTS

Metformin dose-dependently reduces GCDCA-induced apoptosis, even when added 2 hours after GCDCA, without increasing necrotic cell death. Metformin does not protect against TNFα/ActD-induced apoptosis. The protective effect of metformin is dependent on an intact PI3-kinase/Akt pathway, but does not require AMPK/mTOR-signaling. Metformin does not inhibit NF-κB activation.

CONCLUSION

Metformin protects against bile acid-induced apoptosis and could be considered in the treatment of chronic liver diseases accompanied by inflammation.

Osmotic regulation of hepatic betaine metabolism

Betaine critically contributes to the control of hepatocellular hydration and provides protection of the liver from different kinds of stress. To investigate how the hepatocellular hydration state affects gene expression of enzymes involved in the metabolism of betaine and related organic osmolytes, we used quantitative RT-PCR gene expression studies in rat hepatoma cells as well as metabolic and gene expression profiling in primary hepatocytes of both wild-type and 5,10-methylenetetrahydrofolate reductase (MTHFR)-deficient mice. Anisotonic incubation caused coordinated adaptive changes in the expression of various genes involved in betaine metabolism, in particular of betaine homocysteine methyltransferase, dimethylglycine dehydrogenase, and sarcosine dehydrogenase. The expression of betaine-degrading enzymes was downregulated by cell shrinking and strongly induced by an increase in cell volume under hypotonic conditions. Metabolite concentrations in the culture system changed accordingly. Expression changes were mediated through tyrosine kinases, cyclic nucleotide-dependent protein kinases, and JNK-dependent signaling. Assessment of hepatic gene expression using a customized microarray chip showed that hepatic betaine depletion in MTHFR(-/-) mice was associated with alterations that were comparable to those induced by cell swelling in hepatocytes. In conclusion, the adaptation of hepatocytes to changes in cell volume involves the coordinated regulation of betaine synthesis and degradation and concomitant changes in intracellular osmolyte concentrations. The existence of such a well-orchestrated response underlines the importance of cell volume homeostasis for liver function and of methylamine osmolytes such as betaine as hepatic osmolytes.

microRNA-199a-5p protects hepatocytes from bile acid-induced sustained endoplasmic reticulum stress

Sustained endoplasmic reticulum (ER) stress has been linked to cell death and the pathogenesis of many liver diseases, including toxic liver, cholestasis, and infectious liver disease. The cellular pathways that attenuate hepatic ER stress have been the focus of many recent studies, but the role of microRNAs (miRNA) in this process remains unknown. Here, we report that one of the most abundant miRNAs in hepatocytes, miR-199a-5p, was elevated in both bile acid- and thapsigargin (TG)-stimulated cultured hepatocytes, as well as in the liver of bile duct-ligated mice. We identify the misfolded protein chaperone GRP78, as well as the unfolded protein response transducers endoplasmic reticulum to nucleus signaling 1 and activating transcription factor 6 as direct targets of miR-199a-5p, and show that endogenous miR-199a-5p represses the 3′ untranslated regions (UTRs) of their mRNAs. Through gain-of-function and loss of function approaches, we demonstrate that the elevated miR-199-5p disrupts sustained ER stress and prevents hepatocytes from undergoing bile acid- or TG-induced cell death. Furthermore, we reveal that the transcription factor AP-1 is a strong positive regulator of miR-199a-5p. In brief, our study demonstrates that AP-1/miR-199a-5p and ER stress mediators form a feedback loop, which shields hepatocytes from sustained ER stress and protects the liver from injury. On the basis of these findings, we also suggest that the miRNA miR-199a-5p is a potential target for clinical approaches aiming to protect hepatocytes in liver disease.

The bile salt export pump (BSEP) in health and disease

The bile salt export pump (BSEP) is the major transporter for the secretion of bile acids from hepatocytes into bile in humans. Mutations of BSEP are associated with cholestatic liver diseases of varying severity including progressive familial intrahepatic cholestasis type 2 (PFIC-2), benign recurrent intrahepatic cholestasis type 2 (BRIC-2) and genetic polymorphisms are linked to intrahepatic cholestasis of pregnancy (ICP) and drug-induced liver injury (DILI). Detailed analysis of these diseases has considerably increased our knowledge about physiology and pathophysiology of bile secretion in humans. This review focuses on expression, localization, and function, short- and long-term regulation of BSEP as well as diseases association and treatment options for BSEP-associated diseases.

Perspective: TGR5 (Gpbar-1) in liver physiology and disease

Bile acids are signaling molecules with diverse endocrine functions. Bile acid effects are mediated through the nuclear receptor farnesoid X receptor (FXR), the G-protein coupled receptor TGR5 (Gpbar-1) and various other bile acid sensing molecules. TGR5 is almost ubiquitously expressed and has been detected in different non-parenchymal cells of human and rodent liver. Here, TGR5 has anti-inflammatory, anti-apoptotic and choleretic functions. Mice with targeted deletion of TGR5 are protected from the development of cholesterol gallstones. Administration of specific TGR5 agonists lowers serum and liver triglyceride levels thereby reducing liver steatosis. Furthermore, activation of TGR5 promotes intestinal glucagon-like peptide-1 (GLP-1) release, thereby modulating glucose homeostasis and energy expenditure in brown adipose tissue and skeletal muscle. Additionally, TGR5 exerts anti-inflammatory actions resulting in decreased liver injury in animal models of sepsis. These beneficial effects make TGR5 an attractive therapeutic target for metabolic diseases, such as diabetes, obesity, atherosclerosis and steatohepatitis.

Effect of bile duct ligation on bile acid composition in mouse serum and liver

BACKGROUND

Cholestatic liver diseases can be caused by genetic defects, drug toxicities, hepatobiliary malignancies or obstruction of the biliary tract. Cholestasis leads to accumulation of bile acids (BAs) in hepatocytes. Direct toxicity of BAs is currently the most accepted hypothesis for cholestatic liver injury. However, information on which bile acids are actually accumulating during cholestasis is limited.

AIM

To assess the BA composition in liver and serum after bile duct ligation (BDL) in male C57Bl/6 mice between 6 h and 14 days and evaluate toxicity of the most abundant BAs.

RESULTS

Bile acid concentrations increased in liver (27-fold) and serum (1400-fold) within 6 h after surgery and remained elevated up to 14 days. BAs in livers of BDL mice became more hydrophilic than sham controls, mainly because of increased 6β-hydroxylation and taurine conjugation. Among the eight unconjugated and 16 conjugated BAs identified in serum and liver, only taurocholic acid (TCA), β-muricholic acid (βMCA) and TβMCA were substantially elevated representing >95% of these BAs over the entire time course. Although glycochenodeoxycholic acid and other conjugated BAs increased in BDL animals, the changes were several orders of magnitude lower compared with TCA, βMCA and TβMCA. A mixture of these BAs did not cause apoptosis or necrosis, but induced inflammatory gene expression in cultured murine hepatocytes.

CONCLUSION

The concentrations of cytotoxic BAs are insufficient to cause hepatocellular injury. In contrast, TCA, βMCA and TβMCA are able to induce pro-inflammatory mediators in hepatocytes. Thus, BAs act as inflammagens and not as cytotoxic mediators after BDL in mice.

CD95 death receptor and epidermal growth factor receptor (EGFR) in liver cell apoptosis and regeneration

Recent evidence suggests that signaling pathways towards cell proliferation and cell death are much more interconnected than previously thought. Whereas not only death receptors such as CD95 (Fas, APO-1) can couple to both, cell death and proliferation, also growth factor receptors such as the epidermal growth factor receptor (EGFR) are involved in these opposing kinds of cell fate. EGFR is briefly discussed as a growth factor receptor involved in liver cell proliferation during liver regeneration. Then the role of EGFR in activating CD95 death receptor in liver parenchymal cells (PC) and hepatic stellate cells (HSC), which represent a liver stem/progenitor cell compartment, is described summarizing different ways of CD95- and EGFR-dependent signaling in the liver. Here, depending on the hepatic cell type (PC vs. HSC) and the respective signaling context (sustained vs. transient JNK activation) CD95-/EGFR-mediated signaling ends up in either liver cell apoptosis or cell proliferation.

Bile salt export pump inhibitors are associated with bile acid-dependent drug-induced toxicity in sandwich-cultured hepatocytes

Drug-induced liver injury (DILI) is a major reason for the dropout of candidate compounds from drug testing and the withdrawal of pharmaceuticals from clinical use. Among the various mechanisms of liver injury, the accumulation of bile acids (BAs) within hepatocytes is thought to be a primary mechanism for the development of DILI. Although bile salt export pump (BSEP) dysfunction is considered a susceptibility factor for DILI, little is known about the relationship between drug-induced BSEP dysfunction and BA-dependent hepatotoxicity. Furthermore, few methods are at hand for the systematic and quantitative evaluation of BA-dependent DILI. This study aimed to construct a model of DILI by employing sandwich-cultured hepatocytes (SCHs). SCHs can be used to assess functions of canalicular transporters such as BSEP and the activity of metabolic enzymes. Here, the impact of 26 test compounds (ritonavir, troglitazone, etc.) was investigated on BA-dependent cytotoxicity in SCHs. SCHs were exposed to each compound for 24h with or without BAs (glycochenodeoxycholic acid, deoxycholic acid, etc.). As a result, BA-dependent toxicity was observed for 11 test compounds in SCHs treated in the presence of BAs, while no signs of toxicity were observed for SCHs treated in the absence of BAs. Of the 11 compounds, nine were known BSEP inhibitors. Moreover, for some compounds, an increase in the severity of BA-dependent toxicity was observed in SCHs that were co-treated with 1-aminobenzotriazole, a non-selective inhibitor of cytochrome P450 (CYP450)-mediated drug metabolism. These results indicate that the SCH-based model is likely to prove useful for the evaluation of BA-dependent DILI, including the effects of drug metabolism and BSEP inhibition on liver injury.

A placental phenotype for intrahepatic cholestasis of pregnancy

Intrahepatic cholestasis of pregnancy (ICP) is a pregnancy specific liver disease associated with significant risk of fetal complications. It is hypothesised that the risk of adverse fetal outcomes relates to the toxic effects of bile acids, the levels of which are increased in both maternal and fetal serum. Human and rodent studies have shown that transplacental transfer of bile acids is impaired in ICP. Furthermore, the morphology of placentas from the rodent model of ICP is markedly abnormal, and is associated with increased expression of apoptotic markers and oxidative stress. Using placental tissue from ICP cases and normal pregnancies and cultured placental explant fragments we investigated the histological and molecular effects of cholestasis. We also examined the influence of ursodeoxycholic acid (UDCA) administration on these parameters. Here we report that ICP is associated with several morphological abnormalities of the placenta, including an increase in the number of syncytial knots, and that these can be reproduced in an in vitro (explant) model exposed to the bile acids taurocholic acid and taurochenodoexycholic acid. Furthermore, we demonstrate that ursodeoxycholic acid, a drug commonly used in the management of ICP, has a protective effect on placental tissue both in vivo and in vitro.

Hepatocyte IKK2 protects Mdr2-/- mice from chronic liver failure

Mice lacking the Abc4 protein encoded by the multidrug resistance-2 gene (Mdr2(-/-)) develop chronic periductular inflammation and cholestatic liver disease resulting in the development of hepatocellular carcinoma (HCC). Inhibition of NF-κB by expression of an IκBα super-repressor (IκBαSR) transgene in hepatocytes was shown to prevent HCC development in Mdr2(-/-) mice, suggesting that NF-κB acts as a tumour promoter in this model of inflammation-associated carcinogenesis. On the other hand, inhibition of NF-κB by hepatocyte specific ablation of IKK2 resulted in increased liver tumour development induced by the chemical carcinogen DEN. To address the role of IKK2-mediated NF-κB activation in hepatocytes in the pathogenesis of liver disease and HCC in Mdr2(-/-) mice, we generated Mdr2-deficient animals lacking IKK2 specifically in hepatocytes using the Cre-loxP system. Mdr2(-/-) mice lacking IKK2 in hepatocytes developed spontaneously a severe liver disease characterized by cholestasis, major hyperbilirubinemia and severe to end-stage fibrosis, which caused muscle wasting, loss of body weight, lethargy and early spontaneous death. Cell culture experiments showed that primary hepatocytes lacking IKK2 were more sensitive to bile acid induced death, suggesting that hepatocyte-specific IKK2 deficiency sensitized hepatocytes to the toxicity of bile acids under conditions of cholestasis resulting in greatly exacerbated liver damage. Mdr2(-/-)IKK2(Hep-KO) mice remarkably recapitulate chronic liver failure in humans and might be of special importance for the study of the mechanisms contributing to the pathogenesis of end-stage chronic liver disease or its implications on other organs.

CONCLUSION

IKK2-mediated signaling in hepatocytes protects the liver from damage under conditions of chronic inflammatory cholestasis and prevents the development of severe fibrosis and liver failure.

Lithocholic acid disrupts phospholipid and sphingolipid homeostasis leading to cholestasis in mice

Lithocholic acid (LCA) is an endogenous compound associated with hepatic toxicity during cholestasis. LCA exposure in mice resulted in decreased serum lysophosphatidylcholine (LPC) and sphingomyelin levels due to elevated lysophosphatidylcholine acyltransferase (LPCAT) and sphingomyelin phosphodiesterase (SMPD) expression. Global metabolome analysis indicated significant decreases in serum palmitoyl-, stearoyl-, oleoyl-, and linoleoyl-LPC levels after LCA exposure. LCA treatment also resulted in decreased serum sphingomyelin levels and increased hepatic ceramide levels, and induction of LPCAT and SMPD messenger RNAs (mRNAs). Transforming growth factor-β (TGF-β) induced Lpcat2/4 and Smpd3 gene expression in primary hepatocytes and the induction was diminished by pretreatment with the SMAD3 inhibitor SIS3. Furthermore, alteration of the LPCs and Lpcat1/2/4 and Smpd3 expression was attenuated in LCA-treated farnesoid X receptor-null mice that are resistant to LCA-induced intrahepatic cholestasis.

CONCLUSION

This study revealed that LCA induced disruption of phospholipid/sphingolipid homeostasis through TGF-β signaling and that serum LPC is a biomarker for biliary injury.

Bile acids induce inflammatory genes in hepatocytes: a novel mechanism of inflammation during obstructive cholestasis

Inflammation contributes to liver injury during cholestasis. The mechanism by which cholestasis initiates an inflammatory response in the liver, however, is not known. Two hypotheses were investigated in the present studies. First, activation of Toll-like receptor 4 (TLR4), either by bacterial lipopolysaccharide or by damage-associated molecular pattern molecules released from dead hepatocytes, triggers an inflammatory response. Second, bile acids act as inflammagens, and directly activate signaling pathways in hepatocytes that stimulate production of proinflammatory mediators. Liver inflammation was not affected in lipopolysaccharide-resistant C3H/HeJ mice after bile duct ligation, indicating that Toll-like receptor 4 is not required for initiation of inflammation. Treatment of hepatocytes with bile acids did not directly cause cell toxicity but increased the expression of numerous proinflammatory mediators, including cytokines, chemokines, adhesion molecules, and other proteins that influence immune cell levels and function. Up-regulation of several of these genes in hepatocytes and in the liver after bile duct ligation required early growth response factor-1, but not farnesoid X receptor. In addition, early growth response factor-1 was up-regulated in the livers of patients with cholestasis and correlated with levels of inflammatory mediators. These data demonstrate that Toll-like receptor 4 is not required for the initiation of acute inflammation during cholestasis. In contrast, bile acids directly activate a signaling network in hepatocytes that promotes hepatic inflammation during cholestasis.

Anti-oxidants do not prevent bile acid-induced cell death in rat hepatocytes

BACKGROUND

Bile acids, reactive oxygen species (ROS) and inflammatory cytokines are crucial regulators of cell death in acute and chronic liver diseases. The contribution of each factor to hepatocyte death, either apoptosis or necrosis, has not been clarified as yet. It has been suggested that the generation of oxidative stress by bile acids contributes to hepatocyte death during cholestasis and bile acid toxicity, although the beneficial role of ROS prevention in bile acid-mediated cell death is not fully understood.

AIM

Study the effects of anti-oxidants in bile acid-induced cell death in vitro.

METHODS

Primary rat hepatocytes were exposed to the bile acids glycochenodeoxycholic acid (GCDCA) or taurolithocholic acid-3 sulphate in the absence or presence of ROS scavengers or anti-oxidants. Haeme oxygenase (HO)-1 mRNA levels were analysed by quantitative polymerase chain reaction. Apoptosis was quantified by acridine orange staining and caspase-3 activity assay. Necrosis was detected by Sytox green staining.

RESULTS

Anti-oxidants do not attenuate bile acid-induced cell death. Furthermore, bile acid exposure does not enhance the mRNA expression of the oxidative stress-responsive gene HO-1. The Src-kinase inhibitor, SU6656, does reduce GCDCA-induced apoptosis and necrosis.

CONCLUSIONS

In hepatocytes, bile acid-induced toxicity is not prevented by scavengers of oxidative stress. The beneficial effects observed in patients might be because of the contribution of ROS and cytokines rather than bile acid-mediated oxidative stress. However, the use of specific Src kinase inhibitors might be a useful tool to prevent bile acid-induced injury in liver diseases.

Protein kinase C induces endocytosis of the sodium taurocholate cotransporting polypeptide

Bile salts influence signaling and metabolic pathways. In hepatocytes, the sodium taurocholate cotransporting polypeptide (Ntcp) is a major determinant of intracellular bile salt levels. Short-term downregulation of Ntcp is not well characterized to date. FLAG and enhanced green fluorescent protein (EGFP) tags were cloned to the extra- and intracellular termini of Ntcp. Endocytosis of Ntcp in transfected HepG2 cells was visualized by fluorescence of EGFP, and membrane surface expression of Ntcp was quantified by flow cytometry with fluorochrome-labeled FLAG antibodies. Activation of protein kinase C (PKC) by phorbolester or thymeleatoxin an activator of Ca(2+)-dependent conventional PKCs (cPKCs), induced endocytosis of Ntcp, whereas the Na(+)-K(+)-ATPase remained in the plasma membrane. The PKC inhibitor BIM I and the cPKC-selective inhibitor Gö6976 abolished PMA-induced endocytosis. Because of this internalization, cell surface expression of Ntcp was reduced by 36 +/- 7%, bile salt uptake was decreased by 25%, and taurolithocholate sulfate-induced cell toxicity was prevented. In conclusion, Ca(2+)-dependent PKCs induce vesicular retrieval of Ntcp, thereby reducing bile salt uptake. This mechanism may protect hepatocytes from toxic intracellular bile salt concentrations.

Bile salts and cholestasis

Bile salts have a crucial role in hepatobiliary and intestinal homeostasis and digestion. Primary bile salts are synthesized by the liver from cholesterol, and may be modified by the intestinal flora to form secondary and tertiary bile salts. Bile salts are efficiently reabsorbed from the intestinal lumen to undergo enterohepatic circulation. In addition to their function as a surfactant involved in the absorption of dietary lipids and fat-soluble vitamins bile salts are potent signaling molecules in both the liver and intestine. Under physiological conditions the bile salt pool is tightly regulated, but the adaptive capacity may fall short under cholestatic conditions. Elevated serum and tissue levels of potentially toxic hydrophobic bile salts during cholestasis may cause mitochondrial damage, apoptosis or necrosis in susceptible cell types. Therapeutic nontoxic bile salts may restore impaired hepatobiliary secretion in cholestatic disorders. The hydrophilic bile salt ursodeoxycholate is today regarded as the effective standard treatment of primary biliary cirrhosis and intrahepatic cholestasis of pregnancy, and is implicated for use in various other cholestatic conditions. Novel therapeutic bile salts that are currently under evaluation may also prove valuable in the treatment of these diseases.

Inhibition of nitric oxide synthesis during induced cholestasis ameliorates hepatocellular injury by facilitating S-nitrosothiol homeostasis

Cholestatic liver injury following extra- or intrahepatic bile duct obstruction causes nonparenchymal cell proliferation and matrix deposition leading to end-stage liver disease and cirrhosis. In cholestatic conditions, nitric oxide (NO) is mainly produced by a hepatocyte-inducible NO synthase (iNOS) as a result of enhanced inflow of endotoxins to the liver and also by accumulation of bile salts in hepatocytes and subsequent hepatocellular injury. This study was aimed to investigate the role of NO and S-nitrosothiol (SNO) homeostasis in the development of hepatocellular injury during cholestasis induced by bile duct ligation (BDL) in rats. Male Wistar rats (200-250 g) were divided into four groups (n=10 each), including sham-operated (SO), bile duct-ligated (BDL), tauroursodeoxycholic acid (TUDCA, 50 mg/kg) and S-methylisothiourea (SMT, 25 mg/kg) treated. After 7 days, BDL rats showed elevated serum levels of gamma-glutamiltranspeptidase, aspartate aminotransferase, alanine aminotransferase, LDH, and bilirubin, bile duct proliferation and fibrosis, compared with the SO group. TUDCA treatment did not significantly alter these parameters, but the iNOS inhibitor SMT ameliorated hepatocellular injury, as shown by lower levels of circulating hepatic enzymes and bilirubin, and a decreased grade of bile duct proliferation and fibrosis. Both TUDCA and SMT treatments reversed Mrp2 canalicular pump expression to control levels. However, only SMT treatment significantly lowered the increased levels of plasma NO and S-nitrosation (S-nitrosylation) of liver proteins in BDL rats. Moreover, BDL resulted in a reduction of the S-nitrosoglutathione reductase (GSNOR/Adh5) enzymatic activity and a downregulation of the GSNOR/Adh5 mRNA expression that was reverted by SMT, but not TUDCA, treatment. A total of 25 liver proteins, including S-adenosyl methionine synthetase, betaine-homocysteine S-methyltransferase, Hsp90 and protein disulfide isomerase, were found to be S-nitrosated in BDL rats. In conclusion, the inhibition of NO production during induced cholestasis ameliorates hepatocellular injury. This effect is in part mediated by the improvement of cell proficiency in maintaining SNO homeostasis.

Overexpression of mcl-1 attenuates liver injury and fibrosis in the bile duct-ligated mouse

Hepatocyte apoptosis contributes to liver injury and fibrosis after cholestatic injury. Our aim was to ascertain if the anti-apoptotic protein Mcl-1 alters liver injury or fibrosis in the bile duct-ligated mouse. Markers of apoptosis and fibrosis were compared in wild-type and transgenic mice expressing human Mcl-1 after bile duct ligation. Compared to hMcl-1 transgenic animals, ligated wild-type mice displayed a significant increase in TUNEL-positive cells and in caspase 3/7-positive hepatocytes. Consistent with apoptotic injury, the pro-apoptotic protein Bak underwent a conformational change to an activated form upon cholestatic injury, a change mitigated by hMcl-1 overexpression. Likewise, liver histology, number of bile infarcts, serum ALT values, markers of hepatic fibrosis, and animal survival were improved in bile duct-ligated mice transgenic for hMcl-1 as compared to wild-type mice. In conclusion, increased Mcl-1 expression plays a role in hepatoprotection upon cholestatic liver injury.