Role of bile acids in liver injury and regeneration following acetaminophen overdose

Sintilimab combined with acetaminophen aggravates liver injury through apoptotic and disturbed bile acid pathways

Sintilimab, an immune checkpoint inhibitor, and acetaminophen (APAP), a common analgesic, have been implicated in hepatotoxicity. However, their combined effect on liver injury remains understudied. This study investigated the exacerbating hepatotoxic effects of sintilimab in combination with APAP in mice, focusing on the apoptotic markers and bile acids disruptions. We found that their combination significantly increased serum liver enzymes (aminotransferase and alanine aminotransferase), induced histological liver damage, and boosted hepatic oxidative stress markers (reactive oxygen species, 8-hydroxy-2'-deoxyguanosine, malondialdehyde) while depleting glutathione. Furthermore, the co-treatment also heightened apoptotic markers (cytochrome C, caspase-9, caspase-3, Bax) compared to the control and APAP group alone, indicating a more pronounced apoptotic response. Additionally, it downregulated CYP7A1 expression and disrupted bile acid metabolism, characterized by decreased levels of primary bile acids and increased levels of secondary bile acids, suggesting a role in the aggravated liver injury. This highlights the potential mechanism by which sintilimab and APAP interact, leading to increased oxidative stress, apoptosis and disruption of bile acid homeostasis, which together contribute to the exacerbation of liver injury. The study underscores the need for caution when co-administering sintilimab and APAP, emphasizing the importance of monitoring liver injury to mitigate the risk of combined drug use.

Christensenella minuta Alleviates Acetaminophen-Induced Hepatotoxicity by Regulating Phenylalanine Metabolism

Acetaminophen (APAP)-induced liver injury (AILI), even liver failure, is a significant challenge due to the limited availability of therapeutic medicine. Christensenella minuta (C. minuta), as a probiotic therapy, has shown promising prospects in metabolism and inflammatory diseases. Our research aimed to examine the influence of C. minuta on AILI and explore the molecular pathways underlying it. We found that administration of C. minuta remarkably alleviated AILI in a mouse model, as evidenced by decreased levels of alanine transaminase (ALT) and aspartate aminotransferase (AST) and improvements in the histopathological features of liver sections. Additionally, there was a notable decrease in malondialdehyde (MDA), accompanied by restoration of the reduced glutathione/oxidized glutathione (GSH/GSSG) balance, and superoxide dismutase (SOD) activity. Furthermore, there was a significant reduction in inflammatory markers (IL6, IL1β, TNF-α). C. minuta regulated phenylalanine metabolism. No significant difference in intestinal permeability was observed in either the model group or the treatment group. High levels of phenylalanine aggravated liver damage, which may be linked to phenylalanine-induced dysbiosis and dysregulation in cytochrome P450 metabolism, sphingolipid metabolism, the PI3K-AKT pathway, and the Integrin pathway. Furthermore, C. minuta restored the diversity of the microbiota, modulated metabolic pathways and MAPK pathway. Overall, this research demonstrates that supplementing with C. minuta offers both preventive and remedial benefits against AILI by modulating the gut microbiota, phenylalanine metabolism, oxidative stress, and the MAPK pathway, with high phenylalanine supplementation being identified as a risk factor exacerbating liver injury.

The Role of Mechanistic Biomarkers in Understanding Acetaminophen Hepatotoxicity in Humans

Our understanding of the fundamental molecular mechanisms of acetaminophen (APAP) hepatotoxicity began in 1973 to 1974, when investigators at the US National Institutes of Health published seminal studies demonstrating conversion of APAP to a reactive metabolite that depletes glutathione and binds to proteins in the liver in mice after overdose. Since then, additional groundbreaking experiments have demonstrated critical roles for mitochondrial damage, oxidative stress, nuclear DNA fragmentation, and necrotic cell death as well. Over the years, some investigators have also attempted to translate these mechanisms to humans using human specimens from APAP overdose patients. This review presents those studies and summarizes what we have learned about APAP hepatotoxicity in humans so far. Overall, the mechanisms of APAP hepatotoxicity in humans strongly resemble those discovered in experimental mouse and cultured hepatocyte models, and emerging biomarkers also suggest similarities in liver repair. The data not only validate the first mechanistic studies of APAP-induced liver injury performed 50 years ago but also demonstrate the human relevance of numerous studies conducted since then. SIGNIFICANCE STATEMENT: Human studies using novel translational, mechanistic biomarkers have confirmed that the fundamental mechanisms of acetaminophen (APAP) hepatotoxicity discovered in rodent models since 1973 are the same in humans. Importantly, these findings have guided the development and understanding of treatments such as N-acetyl-l-cysteine and 4-methylpyrazole over the years. Additional research may improve not only our understanding of APAP overdose pathophysiology in humans but also our ability to predict and treat serious liver injury in patients.

Perturbations in human bile acid profiles following drug-induced liver injury investigated using semitargeted high-resolution mass spectrometry

RATIONALE

Acetaminophen (APAP) overdose is the leading cause of acute liver failure (ALF) in North America. To investigate the effect of drug-induced liver injury (DILI) on circulating bile acid (BA) profiles, serum from ALF patients and healthy controls were analyzed using a semitargeted high-resolution mass spectrometry approach to measure BAs in their unconjugated and amidated forms and their glucuronide and sulfate conjugates.

METHODS

Human serum samples from 20 healthy volunteers and 34 ALF patients were combined with deuterated BAs and extracted, prior to liquid chromatography high-resolution tandem mass spectrometry analysis. A mix of 46 standards helped assign 26 BAs in human serum by accurate mass and retention time matching. Moreover, other isomers of unconjugated and amidated BAs, as well as glucuronide and sulfate conjugates, were assigned by accurate mass filtering. In vitro incubations of standard BAs provided increased information for certain peaks of interest.

RESULTS

A total of 275 BA metabolites, with confirmed or putative assignments, were measured in human serum samples. APAP overdose significantly influenced the levels of most BAs, promoting glycine conjugation, and, to a lesser extent, taurine conjugation. When patient outcome was considered, 11 BAs were altered significantly, including multiple sulfated species. Although many of the BAs measured did not have exact structures assigned, several putatively identified BAs of interest were further characterized using in vitro incubations.

CONCLUSION

An optimized chromatographic separation tailored to BAs of ranging polarities was combined with accurate mass measurements to investigate the effect that DILI has on their complex profiles and metabolism to a much wider extent than previously possible. The analysis of complex BA profiles enabled in-depth analysis of the BA metabolism perturbations in ALF, including certain metabolites related to patient outcomes.

Deletion of hepatocyte cysteine dioxygenase type 1, a bile acid repressed gene, enhances glutathione synthesis and ameliorates acetaminophen hepatotoxicity

Liver is a major organ that metabolizes sulfur amino acids cysteine, which is the substrate for the synthesis of many essential cellular molecules including GSH, taurine, and coenzyme A. Bile acid-activated farnesoid x receptor (FXR) inhibits cysteine dioxygenase type 1 (CDO1), which mediates hepatic cysteine catabolism and taurine synthesis. To define the impact of bile acid inhibition of CDO1 on hepatic sulfur amino acid metabolism and antioxidant capacity, we developed hepatocyte-specific CDO1 knockout mice (Hep-CDO1 KO) and hepatocyte specific CDO1 transgenic mice (Hep-CDO1 Tg). Liver metabolomics revealed that genetic deletion of hepatic CDO1 reduced de novo taurine synthesis but had no impact on hepatic taurine abundance or bile acid conjugation. Consistent with reduced cysteine catabolism, Hep-CDO1 KO mice showed increased hepatic cysteine abundance but unaltered methionine cycle intermediates and coenzyme A synthesis. Upon acetaminophen overdose, Hep-CDO1 KO mice showed increased GSH synthesis capacity and alleviated liver injury. In contrast, hepatic CDO1 overexpression in Hep-CDO1 Tg mice stimulated hepatic cysteine to taurine conversion, resulting in reduced hepatic cysteine abundance. However, Hep-CDO1 Tg mice and WT showed similar susceptibility to acetaminophen-induced liver injury. Hep-CDO1 Tg mice showed similar hepatic taurine and coenzyme A compared to WT mice. In summary, these findings suggest that bile acid and FXR signaling inhibition of CDO1-mediated hepatic cysteine catabolism preferentially modulates hepatic GSH synthesis capacity and antioxidant defense, but has minimal effect on hepatic taurine and coenzyme A abundance. Repression of hepatic CDO1 may contribute to the hepatoprotective effects of FXR activation under certain pathologic conditions.

Cholangiocyte organoids to study drug-induced injury

BACKGROUND

Drug induced bile duct injury is a frequently observed clinical problem leading to a wide range of pathological features. During the past decades, several agents have been identified with various postulated mechanisms of bile duct damage, however, mostly still poorly understood.

METHODS

Here, we investigated the mechanisms of chlorpromazine (CPZ) induced bile duct injury using advanced in vitro cholangiocyte cultures. Intrahepatic cholangiocyte organoids (ICOs) were driven into mature cholangiocyte like cells (CLCs), which were exposed to CPZ under cholestatic or non-cholestatic conditions through the addition of a bile acid cocktail.

RESULTS

CPZ caused loss of monolayer integrity by reducing expression levels of tight junction protein 1 (TJP1), E-cadherin 1 (CDH1) and lysyl oxidase homolog 2 (LOXL2). Loss of zonula occuludens-1 (ZO-1) and E-cadherin was confirmed by immunostaining after exposure to CPZ and rhodamine-123 leakage further confirmed disruption of the cholangiocyte barrier function. Furthermore, oxidative stress seemed to play a major role in the early damage response by CPZ. The drug also decreased expression of three main basolateral bile acid transporters, ABCC3 (ATP binding cassette subfamily C member 3), SLC51A/B (solute carrier family 51 subunit alpha/beta) and multidrug resistance transporter ABCB1 (ATP binding cassette subfamily B member 1), thereby contributing to bile acid accumulation. CPZ did not induce an inflammatory response by itself, but addition of TNFα revealed a synergistic effect.

CONCLUSION

These results show that ICOs present a model to identify toxic drugs affecting the bile ducts while providing mechanistic insights into hepatotoxicity.

Oral fecal transplantation enriches Lachnospiraceae and butyrate to mitigate acute liver injury

While fecal microbiota transplantation (FMT) shows promise in treating human diseases, oral capsule FMT is more accepted and accessible to patients. However, microbe selection in the upper gastrointestinal tract (UGIT) through oral administration remains unclear. Here, we demonstrate that short-term oral fecal gavage (OFG) alleviates acetaminophen-induced acute liver injury (AILI) in mice, regardless of the divergent effects of commensal gut microbes. Pasteurized fecal gavage yields similar therapeutic effects. OFG enriches gut Lachnospiraceae and butyrate compared to donor feces. Butyrate mitigates AILI-induced ferroptosis via AMPK-ULK1-p62 signaling to simultaneously induce mitophagy and Nrf2 antioxidant responses. Combined N-acetylcysteine and butyrate administration significantly improves AILI mouse survival rates. These observations indicate the significance of the UGIT in modulating the implanted fecal microbes through oral administration and its potential biological and clinical impacts. Our findings also highlight a possible strategy for applying microbial metabolites to treat acute liver injury.

Hepatocyte-specific deletion of small heterodimer partner protects mice against acetaminophen-induced hepatotoxicity via activation of Nrf2

Acetaminophen (APAP) overdose stands as the primary cause of acute liver failure in the United States. APAP hepatotoxicity involves hepatic glutathione (GSH) depletion and mitochondrial damage. To counteract the toxicity of APAP, the nuclear factor erythroid 2 like 2 (Nrf2) activates the expression of genes responsible for drug detoxification and GSH synthesis. In this study, we present evidence that the elimination of hepatocyte small heterodimer partner, a critical transcriptional repressor for liver metabolism, results in Nrf2 activation and protects mice from APAP-induced acute liver injury. Initial investigations conducted on wildtype (WT) mice revealed a swift downregulation of Shp mRNA within the first 24 h after APAP administration. Subsequent treatment of hepatocyte-specific Shp knockout (ShpHep-/-) mice with 300 mg/kg APAP for 2 h exhibited comparable bioactivation of APAP with that observed in the WT controls. However, a significant reduction in liver injury was observed in ShpHep-/- after APAP treatment for 6 and 24 h. The decreased liver injury correlated with a faster recovery of GSH, attributable to heightened expression of Nrf2 target genes involved in APAP detoxification and GSH synthesis. Moreover, in vitro studies revealed that SHP protein interacted with NRF2 protein, inhibiting the transcription of Nrf2 target genes. These findings hold relevance for humans, as overexpression of SHP hindered APAP-induced NRF2 activation in primary human hepatocytes. In conclusion, our studies have unveiled a novel regulatory axis involving SHP and NRF2 in APAP-induced acute liver injury, emphasizing SHP as a promising therapeutic target in APAP overdose-induced hepatotoxicity.

The Evolution of Circulating Biomarkers for Use in Acetaminophen/Paracetamol-Induced Liver Injury in Humans: A Scoping Review

Acetaminophen (APAP) is a widely used drug, but overdose can cause severe acute liver injury. The first reports of APAP hepatotoxicity in humans were published in 1966, shortly after the development of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) as the first biomarkers of liver injury as opposed to liver function. Thus, the field of liver injury biomarkers has evolved alongside the growth in APAP hepatotoxicity incidence. Numerous biomarkers have been proposed for use in the management of APAP overdose patients in the intervening years. Here, we comprehensively review the development of these markers from the 1960s to the present day and briefly discuss possible future directions.

EGR1 is crucial for the chlorogenic acid-provided promotion on liver regeneration and repair after APAP-induced liver injury

Improper use of acetaminophen (APAP) will induce acute liver failure. This study is designed to investigate whether early growth response-1 (EGR1) participated in the promotion on liver repair and regeneration after APAP-induced hepatotoxicity provided by natural compound chlorogenic acid (CGA). APAP induced the nuclear accumulation of EGR1 in hepatocytes regulated by extracellular-regulated protein kinase (ERK)1/2. In Egr1 knockout (KO) mice, the liver damage caused by APAP (300 mg/kg) was more severe than in wild-type (WT) mice. Results of chromatin immunoprecipitation and sequencing (ChIP-Seq) manifested that EGR1 could bind to the promoter region in Becn1, Ccnd1, and Sqstm1 (p62) or the catalytic/modify subunit of glutamate-cysteine ligase (Gclc/Gclm). Autophagy formation and APAP-cysteine adduct (APAP-CYS) clearance were decreased in Egr1 KO mice administered with APAP. The EGR1 deletion reduced hepatic cyclin D1 expression at 6, 12, or 18 h post APAP administration. Meanwhile, the EGR1 deletion also decreased hepatic p62, Gclc and Gclm expression, GCL enzymatic activity, and glutathione (GSH) content and decreased nuclear factor erythroid 2-related factor 2 (Nrf2) activation and thus aggravated oxidative liver injury induced by APAP. CGA increased EGR1 nuclear accumulation; enhanced hepatic Ccnd1, p62, Gclc, and Gclm expression; and accelerated the liver regeneration and repair in APAP-intoxicated mice. In conclusion, EGR1 deficiency aggravated liver injury and obviously delayed liver regeneration post APAP-induced hepatotoxicity through inhibiting autophagy, enhancing liver oxidative injury, and retarding cell cycle progression, but CGA promoted the liver regeneration and repair in APAP-intoxicated mice via inducing EGR1 transcriptional activation.

Regeneration and Recovery after Acetaminophen Hepatotoxicity

Liver regeneration is a compensatory response to tissue injury and loss. It is known that liver regeneration plays a crucial role in recovery following acetaminophen (APAP)-induced hepatotoxicity, which is the major cause of acute liver failure (ALF) in the US. Regeneration increases proportional to the extent of liver injury upon APAP overdose, ultimately leading to regression of injury and spontaneous recovery in most cases. However, severe APAP overdose results in impaired liver regeneration and unchecked progression of liver injury, leading to failed recovery and mortality. Inter-communication between various cell types in the liver is important for effective regenerative response following APAP hepatotoxicity. Various non-parenchymal cells such macrophages, stellate cells, and endothelial cells produce mediators crucial for proliferation of hepatocytes. Liver regeneration is orchestrated by synchronized actions of several proliferative signaling pathways involving numerous kinases, nuclear receptors, transcription factors, transcriptional co-activators, which are activated by cytokines, growth factors, and endobiotics. Overt activation of anti-proliferative signaling pathways causes cell-cycle arrest and impaired liver regeneration after severe APAP overdose. Stimulating liver regeneration by activating proliferating signaling and suppressing anti-proliferative signaling in liver can prove to be important in developing novel therapeutics for APAP-induced ALF.

The pharmacology and mechanisms of traditional Chinese medicine in promoting liver regeneration: A new therapeutic option

BACKGROUND

The liver is renowned for its remarkable regenerative capacity to restore its structure, size and function after various types of liver injury. However, in patients with end-stage liver disease, the regenerative capacity is inhibited and liver transplantation is the only option. Considering the limitations of liver transplantation, promoting liver regeneration is suggested as a new therapeutic strategy for liver disease. Traditional Chinese medicine (TCM) has a long history of preventing and treating various liver diseases, and some of them have been proven to be effective in promoting liver regeneration, suggesting the therapeutic potential in liver diseases.

PURPOSE

This review aims to summarize the molecular mechanisms of liver regeneration and the pro-regenerative activity and mechanism of TCM formulas, extracts and active ingredients.

METHODS

We conducted a systematic search in PubMed, Web of Science and the Cochrane Library databases using "TCM", "liver regeneration" or their synonyms as keywords, and classified and summarized the retrieved literature. The PRISMA guidelines were followed.

RESULTS

Forty-one research articles met the themes of this review and previous critical studies were also reviewed to provide essential background information. Current evidences indicate that various TCM formulas, extracts and active ingredients have the effect on stimulating liver regeneration through modulating JAK/STAT, Hippo, PI3K/Akt and other signaling pathways. Besides, the mechanisms of liver regeneration, the limitation of existing studies and the application prospect of TCM to promote liver regeneration are also outlined and discussed in this review.

CONCLUSION

This review supports TCM as new potential therapeutic options for promoting liver regeneration and repair of the failing liver, although extensive pharmacokinetic and toxicological studies, as well as elaborate clinical trials, are still needed to demonstrate safety and efficacy.

Semi-Targeted Profiling of Bile Acids by High-Resolution Mass Spectrometry in a Rat Model of Drug-Induced Liver Injury

Using a semi-targeted approach, we have investigated the effect of acetaminophen on circulating bile acid profiles in rats, including many known bile acids and potential isomeric structures, as well as glucuronide and sulfate conjugates. The chromatographic separation was based on an optimized reverse-phase method exhibiting excellent resolution for a complex mix of bile acids using a solid-core C18 column, coupled to a high-resolution quadrupole time-of-flight system. The semi-targeted workflow consisted of first assigning all peaks detectable in samples from 46 known bile acids contained in a standard mix, as well as additional peaks for other bile acid isomers. The presence of glucuronide and sulfate conjugates was also examined based on their elemental formulae and detectable peaks with matching exact masses were added to the list of features for statistical analysis. In this study, rats were administered acetaminophen at four different doses, from 75 to 600 mg/kg, with the highest dose being a good model of drug-induced liver injury. Statistically significant changes were found by comparing bile acid profiles between dosing levels. Some tentatively assigned conjugates were further elucidated using in vitro metabolism incubations with rat liver fractions and standard bile acids. Overall, 13 identified bile acids, 23 tentatively assigned bile acid isomers, and 9 sulfate conjugates were found to increase significantly at the highest acetaminophen dose, and thus could be linked to drug-induced liver injury.

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.

Hepatocyte-Specific Deletion of Yes-Associated Protein Improves Recovery From Acetaminophen-Induced Acute Liver Injury

Overdose of acetaminophen (APAP) is the major cause of acute liver failure (ALF) in the Western world with very limited treatment options. Previous studies from our groups and others have shown that timely activation of liver regeneration is a critical determinant of transplant-free survival of APAP-induced ALF patients. Here, we report that hepatocyte-specific deletion of Yes-associated protein (Yap), the downstream mediator of the Hippo Kinase signaling pathway results in faster recovery from APAP-induced acute liver injury. Initial studies performed with male C57BL/6J mice showed a rapid activation of Yap and its target genes within first 24 h after APAP administration. Treatment of hepatocyte-specific Yap knockout (Yap-KO) mice with 300 mg/kg APAP resulted in equal initial liver injury but a significantly accelerated recovery in Yap-KO mice. The recovery was accompanied by significantly rapid hepatocyte proliferation supported by faster activation of Wnt/β-catenin pathway. Furthermore, Yap-KO mice had significantly earlier and higher pro-regenerative inflammatory response following APAP overdose. Global gene expression analysis indicated that Yap-KO mice had a robust activation of transcription factors involved in response to endoplasmic reticulum stress (XBP1) and maintaining hepatocyte differentiation (HNF4α). In conclusion, these data indicate that inhibition of Yap in hepatocytes results in rapid recovery from APAP overdose due to an earlier activation of liver regeneration.

Integration of transcriptomics and metabolomics confirmed hepatoprotective effects of steamed shoot extracts of ginseng (Panax ginseng C.A. Meyer) on toxicity caused by overdosed acetaminophen

The study aimed, by integrating transcriptomics and metabolomics, to reveal novel biomarkers caused by overdosed acetaminophen (APAP) and liver protection substances procured by pre-administration of ginseng shoots extract (GSE). Totally 4918 genes and 127 metabolites were identified as differentially expressed genes and differential metabolites, respectively. According to KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment, such pathways as primary bile acid biosynthesis, bile secretion, retinol metabolism, histidine and several other amino-related metabolism were significantly altered by GSE and disturbed by subsequent overdosed APAP at the transcriptomic as well as metabolomic levels. Fifteen key biomarker metabolites related to these pathways were up-regulated in APAP-treated vs GSE-pretreated liver tissues, and were reported exerting anti-oxidant, anti-inflammatory, anti-apoptotic and/or immunomodulate functions, three of which even possessed direct hepatoprotection effects. Twenty five vital unigenes modulating these metabolites were further verified by correlation analysis and expression levels of fifteen of them were examined by qRT-PCR. Our findings indicate that GSE may be an effective dietary supplement for preventing the liver damage caused by the overdosed APAP.

Progress in research on the roles of TGR5 receptor in liver diseases

TGR5 (G protein-coupled bile acid receptor 1, GPBAR-1) is a G protein-coupled receptor with seven transmembrane domains and is widely distributed in various organs and tissues. As an important bile acid receptor, TGR5 can be activated by primary and secondary bile acids. Increased expression of TGR5 is a risk factor for polycystic liver disease and hepatobiliary cancer. However, there is evidence that the anti-inflammatory effect of the TGR5 receptor and its regulatory effect on hydrophobic bile acid confer protective effects against most liver diseases. Recent studies have shown that TGR5 receptor activation can alleviate the development of diabetic liver fibrosis, regulate the differentiation of natural killer T cells into NKT10 cells, increase the secretion of anti-inflammatory factors, inhibit the invasion of hepatitis B virus, promote white adipose tissue browning, improve arterial vascular dynamics, maintain tight junctions between bile duct cells, and protect against apoptosis. In portal hypertension, TGR5 receptor activation can inhibit the contraction of hepatic stellate cells and improve intrahepatic microcirculation. In addition, the discovery of the regulatory relationship between the TGR5 receptor and miRNA-26a provides a new direction for further studies of the molecular mechanism underlying the effects of TGR5. In this review, we describe recent findings linking TGR5 to various liver diseases, with a focus on the mechanisms underlying its effects and potential therapeutic implications.

Sleep disorders are associated with acetaminophen-induced adverse reactions and liver injury

Risk factors related to the development of acetaminophen (APAP)-induced adverse reactions and liver injury remain uncertain. Sleep disorders have been linked to some health outcomes. This study examined the associations of sleep disorders with APAP-induced adverse reactions or liver injury and the possible mechanisms. From NIS database, adverse reactions, liver injury and sleep disorders were identified. Factors associated with the risk of the total adverse effects or liver injury were examined with logistic regression. From Gene Expression Omnibus database, datasets GSE111828, containing transcriptome data based on RNA-seq analysis from liver samples extracted from mice post APAP administration, and GSE92913, containing transcriptome data based on microarray analysis from liver samples extracted from mice with sleep deprivation, were analyzed. A total of 4372754 patients without and 91314 patients with sleep disorders were eligible for analyses. Both before and after propensity score matching, APAP-induced adverse reactions were higher in patients with sleep disorders than in patients without. In multivariate regression, sleep disorders were associated with higher odds of APAP-induced adverse reactions (adjusted OR [aOR] 2.005, 95 % CI 1.343-2.995) and liver injury (aOR 2.788, 95 % CI 1.310-5.932). Genes that were enriched in bile secretion and retinol metabolism and PPAR signaling pathways were basically down-regulated in livers of mice after APAP administration and livers of mice with sleep deprivation. This study shows that sleep disorders may be novel independent risk factors for APAP-associated adverse reactions and liver injury and provides bioinformation linking sleep disorders to increased risk of APAP-induced liver injury.

Liver regeneration: biological and pathological mechanisms and implications

The liver is the only solid organ that uses regenerative mechanisms to ensure that the liver-to-bodyweight ratio is always at 100% of what is required for body homeostasis. Other solid organs (such as the lungs, kidneys and pancreas) adjust to tissue loss but do not return to 100% of normal. The current state of knowledge of the regenerative pathways that underlie this 'hepatostat' will be presented in this Review. Liver regeneration from acute injury is always beneficial and has been extensively studied. Experimental models that involve partial hepatectomy or chemical injury have revealed extracellular and intracellular signalling pathways that are used to return the liver to equivalent size and weight to those prior to injury. On the other hand, chronic loss of hepatocytes, which can occur in chronic liver disease of any aetiology, often has adverse consequences, including fibrosis, cirrhosis and liver neoplasia. The regenerative activities of hepatocytes and cholangiocytes are typically characterized by phenotypic fidelity. However, when regeneration of one of the two cell types fails, hepatocytes and cholangiocytes function as facultative stem cells and transdifferentiate into each other to restore normal liver structure. Liver recolonization models have demonstrated that hepatocytes have an unlimited regenerative capacity. However, in normal liver, cell turnover is very slow. All zones of the resting liver lobules have been equally implicated in the maintenance of hepatocyte and cholangiocyte populations in normal liver.

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.

Acetaminophen Test Battery (ATB): A Comprehensive Method to Study Acetaminophen-Induced Acute Liver Injury

Acetaminophen (APAP) overdose is the major cause of acute liver failure (ALF) in the Western world. Extensive research is ongoing to identify the mechanisms of APAP-induced ALF. APAP-induced acute liver injury is also one of the most commonly studied drug-induced liver injury models in the field of hepatotoxicity. APAP toxicity is triphasic and includes three mechanistically interlinked but temporally distinct phases of initiation, progression, and recovery/regeneration. Despite how commonly it is studied, the methods to study APAP toxicity differ significantly, often leading to confusing and contradictory data. There are number of reviews on mechanisms of APAP toxicity, but a detailed mechanism-based comprehensive method and list of assays that covers all phases of APAP hepatotoxicity are missing. The goal of this review is to provide a standard protocol and guidelines to study APAP toxicity in mice including a test battery that can help investigators to comprehensively analyze APAP toxicity in the specific context of their hypothesis. Further, we will identify the major roadblocks and common technical problems that can significantly affect the results. This acetaminophen test battery (ATB) will be an excellent guide for scientists studying this most common and clinically relevant drug-induced liver injury and will also be helpful as a roadmap for hypothesis development to study novel mechanisms.

Transcriptomic analysis across liver diseases reveals disease-modulating activation of constitutive androstane receptor in cholestasis

Background & Aims

Liver diseases are caused by many factors, such as genetics, nutrition, and viruses. Therefore, it is important to delineate transcriptomic changes that occur in various liver diseases.

Methods

We performed high-throughput sequencing of mouse livers with diverse types of injuries, including cholestasis, diet-induced steatosis, and partial hepatectomy. Comparative analysis of liver transcriptome from mice and human samples of viral infections (HBV and HCV), alcoholic hepatitis (AH), non-alcoholic steatohepatitis (NASH), and biliary atresia revealed distinct and overlapping gene profiles associated with liver diseases. We hypothesised that discrete molecular signatures could be utilised to assess therapeutic outcomes. We focused on cholestasis to test and validate the hypothesis using pharmacological approaches.

Results

Here, we report significant overlap in the expression of inflammatory and proliferation-related genes across liver diseases. However, cholestatic livers were unique and displayed robust induction of genes involved in drug metabolism. Consistently, we found that constitutive androstane receptor (CAR) activation is crucial for the induction of the drug metabolic gene programme in cholestasis. When challenged, cholestatic mice were protected against zoxazolamine-induced paralysis and acetaminophen-induced hepatotoxicity. These protective effects were diminished upon inhibition of CAR activity. Further, drug metabolic genes were also induced in the livers from a subset of biliary atresia patients, but not in HBV and HCV infections, AH, or NASH. We also found a higher expression of CYP2B6, a CAR target, in the livers of biliary atresia patients, underscoring the clinical importance of our findings.

Conclusions

Comparative transcriptome analysis of different liver disorders revealed specific induction of phase I and II metabolic genes in cholestasis. Our results demonstrate that CAR activation may lead to variations in drug metabolism and clinical outcomes in biliary atresia.

Lay summary

Transcriptomic analysis of diverse liver diseases revealed alterations in common and distinct pathways. Specifically, in cholestasis, we found that detoxification genes and their activity are increased. Thus, cholestatic patients may have an unintended consequence on drug metabolism and not only have a beneficial effect against liver toxicity, but also may require adjustments to their therapeutic dosage.

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Cell cycle, inflammation, and glucose homeostasis are some of the common pathways altered in a variety of liver disorders.

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Phase I and II metabolic genes are induced in Fxr^−/−Shp^−/− double knockouts (DKOs) and bile-acid-fed control mice.

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Activation of xeno-sensor, constitutive androstane receptor (CAR), is observed in cholestasis.

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Inhibiting CAR activity in DKO mice exacerbates zoxazolamine-induced paralysis and acetaminophen-induced hepatotoxicity.

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A subset of patients with biliary atresia display increased expression of CAR target protein CYP2B6.

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.

Comparison of liver regeneration after partial hepatectomy and acetaminophen-induced acute liver failure: A global picture based on transcriptome analysis

Liver regenerates following surgical removal and after drug-induced liver injury (DILI). However, most of the mechanisms of liver regeneration were identified using partial hepatectomy (PHX) model rather than using DILI models. We compared mechanisms of liver regeneration following PHX and after acetaminophen (APAP) overdose, a DILI model, using transcriptomic approach. Kinetics of hepatocyte proliferation and global gene expression profiles were studied in male C57BL/6J mice either subjected to PHX or following APAP overdose. Liver regeneration was much more synchronized after PHX as compared to APAP overdose. Transcriptomics analysis revealed activation of common upstream regulators in both models including growth factors HGF, EGF and VEGF; and cytokines IL6 and TNFα. However, magnitude of activation and temporality was significantly differed between the two models. HGF and VEGF showed similar activation between PHX and APAP but activation of EGF was significantly stronger in the APAP model. Activation of IL6 and TNFα transcriptional programs was delayed but remarkably higher in APAP. These dissimilarities could be attributed to inherent differences in the two models including significant injury and inflammation exclusively in the APAP model. This study highlights need to study mechanisms of liver regeneration after DILI separately from the mechanisms of regeneration PHX.

Targeted Analysis of 46 Bile Acids to Study the Effect of Acetaminophen in Rat by LC-MS/MS

Bile acids represent a large class of steroid acids synthesized in the liver and further metabolized by many bacterial and mammalian enzymes. Variations in bile acid levels can be used as a measure of liver function. There still exists, however, a need to study the variation of individual circulating bile acids in the context of hepatotoxity or liver disease. Acetaminophen (APAP), a drug commonly taken to relieve pain and decrease fever, is known to cause acute liver failure at high doses. We have developed a targeted liquid chromatography-tandem mass spectrometry method to monitor the effects of different doses of APAP on the bile acid plasma profile in a rat model. The analysis method was optimized to ensure chromatographic resolution of isomeric species using a mixture of 46 standard bile acids, and 14 isotopically-labeled internal standard (IS) compounds detected in multiple reaction monitoring (MRM) mode on a triple quadrupole mass spectrometer. Four doses of acetaminophen were studied, the highest of which shows signs of hepatotoxicity in rats. This targeted method revealed that high dose APAP has an important effect on bile acid profiles. Changes were seen in several unconjugated bile acids as well as glycine conjugates; however, no obvious changes were apparent for taurine-conjugated species.

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.

Metabolic Comorbidities and Risk of Development and Severity of Drug-Induced Liver Injury

The incidence and rates of diagnosis of drug-induced liver injury (DILI) have been increasing in recent years as findings from basic research and the examination of clinical databases reveal information about the clinical course, etiology, and prognosis of this complex disease. The prevalence of metabolic comorbidities (e.g., diabetes mellitus, fatty liver, obesity, and metabolic syndrome (MetS)) has been increasing during the same period. The results of preclinical and clinical research studies indicate that characteristics of metabolic comorbidities are also factors that affect DILI phenotype and progression. The objective of this review is to present the evidence for DILI and hepatotoxicity mechanisms, incidence, and outcomes in patients with MetS and nonalcoholic fatty liver disease. Moreover, we also summarize the relationships between drugs used to treat metabolic comorbidities and DILI.

Mechanistic identification of biofluid metabolite changes as markers of acetaminophen-induced liver toxicity in rats

Acetaminophen (APAP) is the most commonly used analgesic and antipyretic drug in the world. Yet, it poses a major risk of liver injury when taken in excess of the therapeutic dose. Current clinical markers do not detect the early onset of liver injury associated with excess APAP-information that is vital to reverse injury progression through available therapeutic interventions. Hence, several studies have used transcriptomics, proteomics, and metabolomics technologies, both independently and in combination, in an attempt to discover potential early markers of liver injury. However, the casual relationship between these observations and their relation to the APAP mechanism of liver toxicity are not clearly understood. Here, we used Sprague-Dawley rats orally gavaged with a single dose of 2 g/kg of APAP to collect tissue samples from the liver and kidney for transcriptomic analysis and plasma and urine samples for metabolomic analysis. We developed and used a multi-tissue, metabolism-based modeling approach to integrate these data, characterize the effect of excess APAP levels on liver metabolism, and identify a panel of plasma and urine metabolites that are associated with APAP-induced liver toxicity. Our analyses, which indicated that pathways involved in nucleotide-, lipid-, and amino acid-related metabolism in the liver were most strongly affected within 10 h following APAP treatment, identified a list of potential metabolites in these pathways that could serve as plausible markers of APAP-induced liver injury. Our approach identifies toxicant-induced changes in endogenous metabolism, is applicable to other toxicants based on transcriptomic data, and provides a mechanistic framework for interpreting metabolite alterations.

Mechanisms and biomarkers of liver regeneration after drug-induced liver injury

Liver, the major metabolic organ in the body, is known for its remarkable capacity to regenerate. Whereas partial hepatectomy (PHx) is a popular model for the study of liver regeneration, the liver also regenerates after acute injury, but less is known about the mechanisms that drive it. Recent studies have shown that liver regeneration is critical for survival in acute liver failure (ALF), which is usually due to drug-induced liver injury (DILI). It is sometimes assumed that the signaling pathways involved are similar to those that regulate regeneration after PHx, but there are likely to be critical differences. A better understanding of regeneration mechanisms after DILI and hepatotoxicity in general could lead to development of new therapies for ALF patients and new biomarkers to predict patient outcome. Here, we summarize what is known about the mechanisms of liver regeneration and repair after hepatotoxicity. We also review the literature in the emerging field of liver regeneration biomarkers.

Liver Regeneration after Acetaminophen Hepatotoxicity: Mechanisms and Therapeutic Opportunities

Acetaminophen (N-acetyl-para-aminophenol; APAP) overdose is the most common cause of acute liver failure in the Western world, with limited treatment opportunities. For years, research on APAP overdose has been focused on investigating the mechanisms of hepatotoxicity, with limited success in advancing therapeutic strategies. Acute liver injury after any insult, including APAP overdose, is followed by compensatory liver regeneration, which promotes recovery and is a crucial determinant of the final outcome. Liver regeneration after APAP-induced liver injury is dose dependent and impaired after severe APAP overdose. Although robust regenerative response is associated with spontaneous recovery and survival, impaired regeneration results in faster progression of injury and death after APAP overdose. APAP hepatotoxicity-induced liver regeneration involves a complex time- and dose-dependent interplay of several signaling mediators, including growth factors, cytokines, angiogenic factors, and other mitogenic pathways. Compared with the liver injury, which is established before most patients seek medical attention and has proved difficult to manipulate, liver regeneration can be potentially modulated even in late-stage APAP-induced acute liver failure. Despite recent efforts to study the mechanisms of liver regeneration after APAP-induced liver injury, more comprehensive research in this area is required, especially regarding factors that contribute to impaired regenerative response, to develop novel regenerative therapies for APAP-induced acute liver failure.

Bile acids regulate cysteine catabolism and glutathione regeneration to modulate hepatic sensitivity to oxidative injury

Bile acids are signaling molecules that critically control hepatocellular function. Disrupted bile acid homeostasis may be implicated in the pathogenesis of chronic liver diseases. Glutathione is an important antioxidant that protects the liver against oxidative injury. Various forms of liver disease share the common characteristics of reduced cellular glutathione and elevated oxidative stress. This study reports a potentially novel physiological function of bile acids in regulating hepatic sulfur amino acid and glutathione metabolism. We found that bile acids strongly inhibited the cysteine dioxygenase type-1-mediated (CDO1-mediated) cysteine catabolic pathway via a farnesoid X receptor-dependent mechanism. Attenuating this bile acid repressive effect depleted the free cysteine pool and reduced the glutathione concentration in mouse liver. Upon acetaminophen challenge, cholestyramine-fed mice showed impaired hepatic glutathione regeneration capacity and markedly worsened liver injury, which was fully prevented by N-acetylcysteine administration. These effects were recapitulated in CDO1-overexpressing hepatocytes. Findings from this study support the importance of maintaining bile acid homeostasis under physiological and pathophysiological conditions, as altered hepatic bile acid signaling may negatively impact the antioxidant defense mechanism and sensitivity to oxidative injury. Furthermore, this finding provides a possible explanation for the reported mild hepatotoxicity associated with the clinical use of bile acid sequestrants in human patients.

Pleiotropic Role of p53 in Injury and Liver Regeneration after Acetaminophen Overdose

p53 is the major cellular gatekeeper involved in proliferation, cell death, migration, and homeostasis. The role of p53 in pathogenesis of drug-induced liver injury is unknown. We investigated the role of p53 in liver injury and regeneration after acetaminophen (APAP) overdose, the most common cause of acute liver failure in the Western world. Eight-week-old male wild-type (WT) and p53 knockout (p53KO) mice were treated with 300 mg/kg APAP, and the dynamics of liver injury and regeneration were studied over a time course of 0 to 96 hours. Deletion of p53 resulted in a threefold higher liver injury than in WT mice. Interestingly, despite higher liver injury, p53KO mice recovered similarly as the WT mice because of faster liver regeneration. Deletion of p53 did not affect APAP bioactivation and initiation of injury. Microarray analysis revealed that p53KO mice had disrupted metabolic homeostasis and induced inflammatory and proliferative signaling. p53KO mice showed prolonged steatosis correlating with prolonged liver injury. Initiation of liver regeneration in p53KO mice was delayed, but once initiated, cell cycle was significantly faster than WT mice because of sustained AKT, extracellular signal-regulated kinase, and mammalian target of rapamycin signaling. These studies show that p53 plays a pleotropic role after APAP overdose, where it prevents progression of liver injury by maintaining metabolic homeostasis and also regulates initiation of liver regeneration through proliferative signaling.

Identification of an Oleanane-Type Triterpene Hedragonic Acid as a Novel Farnesoid X Receptor Ligand with Liver Protective Effects and Anti-inflammatory Activity

Farnesoid X receptor (FXR) and G-protein-coupled bile acid receptor 1 (GPBAR1) are two important bile acid (BA) receptors. As non-BAs drug template for GPBAR1, none of the natural oleanane-type triterpenes have been reported as FXR ligands, despite FXR and GPBAR1 having similar binding pockets for BAs. Here, we report the natural triterpene hedragonic acid that has been isolated from the stem and root of Celastrus orbiculatus Thunb. (COT) as an effective agonist for FXR. Both biochemical amplified luminescent proximity homogeneous assay and cell-based reporter assays showed that hedragonic acid regulated the transcriptional activity of FXR. Circular dichroism spectroscopy further suggested the conformational changes of FXR upon the binding of hedragonic acid. Interestingly, the crystal structure of hedragonic acid-bound FXR revealed a unique binding mode with hedragonic acid occupying a novel binding pocket different from the classic binding position. The structural comparison between hedragonic acid-bound FXR and oleanolic acid-bound GPBAR1 explained the molecular basis for the selectivity of oleanane-type triterpenes for FXR. Moreover, hedragonic acid treatment protected mice from liver injury induced by acetaminophen overdose and decreased hepatic inflammatory responses in an FXR-dependent manner, suggesting that hedragonic acid might be one of the major components of COT for its multifunctional pharmaceutical uses. In conclusion, our results provide novel structure templates for drug design based on natural triterpenes by targeting FXR and/or GPBAR1 with pharmaceutical values.

Engineered fibroblast growth factor 19 protects from acetaminophen-induced liver injury and stimulates aged liver regeneration in mice

The liver displays a remarkable regenerative capacity triggered upon tissue injury or resection. However, liver regeneration can be overwhelmed by excessive parenchymal destruction or diminished by pre-existing conditions hampering repair. Fibroblast growth factor 19 (FGF19, rodent FGF15) is an enterokine that regulates liver bile acid and lipid metabolism, and stimulates hepatocellular protein synthesis and proliferation. FGF19/15 is also important for liver regeneration after partial hepatectomy (PH). Therefore recombinant FGF19 would be an ideal molecule to stimulate liver regeneration, but its applicability may be curtailed by its short half-life. We developed a chimaeric molecule termed Fibapo in which FGF19 is covalently coupled to apolipoprotein A-I. Fibapo retains FGF19 biological activities but has significantly increased half-life and hepatotropism. Here we evaluated the pro-regenerative activity of Fibapo in two clinically relevant models where liver regeneration may be impaired: acetaminophen (APAP) poisoning, and PH in aged mice. The only approved therapy for APAP intoxication is N-acetylcysteine (NAC) and no drugs are available to stimulate liver regeneration. We demonstrate that Fibapo reduced liver injury and boosted regeneration in APAP-intoxicated mice. Fibapo improved survival of APAP-poisoned mice when given at later time points, when NAC is ineffective. Mechanistically, Fibapo accelerated recovery of hepatic glutathione levels, potentiated cell growth-related pathways and increased functional liver mass. When Fibapo was administered to old mice prior to PH, liver regeneration was markedly increased. The exacerbated injury developing in these mice upon PH was attenuated, and the hepatic biosynthetic capacity was enhanced. Fibapo reversed metabolic and molecular alterations that impede regeneration in aged livers. It reduced liver steatosis and downregulated p21 and hepatocyte nuclear factor 4 α (Hnf4α) levels, whereas it stimulated Foxm1b gene expression. Together our findings indicate that FGF19 variants retaining the metabolic and growth-promoting effects of this enterokine may be valuable for the stimulation of liver regeneration.

Bile Acid-Mediated Sphingosine-1-Phosphate Receptor 2 Signaling Promotes Neuroinflammation during Hepatic Encephalopathy in Mice

Hepatic encephalopathy (HE) is a neuropsychiatric complication that occurs due to deteriorating hepatic function and this syndrome influences patient quality of life, clinical management strategies and survival. During acute liver failure, circulating bile acids increase due to a disruption of the enterohepatic circulation. We previously identified that bile acid-mediated signaling occurs in the brain during HE and contributes to cognitive impairment. However, the influences of bile acids and their downstream signaling pathways on HE-induced neuroinflammation have not been assessed. Conjugated bile acids, such as taurocholic acid (TCA), can activate sphingosine-1-phosphate receptor 2 (S1PR2), which has been shown to promote immune cell infiltration and inflammation in other models. The current study aimed to assess the role of bile-acid mediated S1PR2 signaling in neuroinflammation and disease progression during azoxymethane (AOM)-induced HE in mice. Our findings demonstrate a temporal increase of bile acids in the cortex during AOM-induced HE and identified that cortical bile acids were elevated as an early event in this model. In order to classify the specific bile acids that were elevated during HE, a metabolic screen was performed and this assay identified that TCA was increased in the serum and cortex during AOM-induced HE. To reduce bile acid concentrations in the brain, mice were fed a diet supplemented with cholestyramine, which alleviated neuroinflammation by reducing proinflammatory cytokine expression in the cortex compared to the control diet-fed AOM-treated mice. S1PR2 was expressed primarily in neurons and TCA treatment increased chemokine ligand 2 mRNA expression in these cells. The infusion of JTE-013, a S1PR2 antagonist, into the lateral ventricle prior to AOM injection protected against neurological decline and reduced neuroinflammation compared to DMSO-infused AOM-treated mice. Together, this identifies that reducing bile acid levels or S1PR2 signaling are potential therapeutic strategies for the management of HE.

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.

Inhibition of Glycogen Synthase Kinase 3 Accelerated Liver Regeneration after Acetaminophen-Induced Hepatotoxicity in Mice

Overdose of acetaminophen (APAP) is the leading cause of acute liver failure (ALF) in the United States. Timely initiation of compensatory liver regeneration after APAP hepatotoxicity is critical for final recovery, but the mechanisms of liver regeneration after APAP-induced ALF have not been extensively explored yet. Previous studies from our laboratory have demonstrated that activation of β-catenin signaling after APAP overdose is associated with timely liver regeneration. Herein, we investigated the role of glycogen synthase kinase 3 (GSK3) in liver regeneration after APAP hepatotoxicity using a pharmacological inhibition strategy in mice. Treatment with specific GSK3 inhibitor (L803-mts), starting from 4 hours after 600 mg/kg dose of APAP, resulted in early initiation of liver regeneration in a dose-dependent manner, without modifying the peak regenerative response. Acceleration of liver regeneration was not secondary to alteration of APAP-induced hepatotoxicity, which remained unchanged after GSK3 inhibition. Early cell cycle initiation in hepatocytes after GSK3 inhibition was because of rapid induction of cyclin D1 and phosphorylation of retinoblastoma protein. This was associated with increased activation of β-catenin signaling after GSK3 inhibition. Taken together, our study has revealed a novel role of GSK3 in liver regeneration after APAP overdose and identified GSK3 as a potential therapeutic target to improve liver regeneration after APAP-induced ALF.

Tropisetron Protects Against Acetaminophen-Induced Liver Injury via Suppressing Hepatic Oxidative Stress and Modulating the Activation of JNK/ERK MAPK Pathways

Objectives. To investigate the protective effects of tropisetron on acetaminophen- (APAP-) induced liver injury in a mice model. Methods. C57BL/6 male mice were given tropisetron (0.3 to 10 mg/kg) 30 minutes before a hepatotoxic dose of acetaminophen (300 mg/kg) intraperitoneally. Twenty hours after APAP intoxication, sera alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels, hepatic myeloperoxidase (MPO), malondialdehyde (MDA), glutathione (GSH), and superoxide dismutase (SOD) activities, and liver histopathological changes were examined. The MAP kinases were also detected by western blotting. Results. Our results showed that tropisetron pretreatment significantly attenuated the acute elevations of the liver enzyme ALT level, hepatic MPO activity, and hepatocytes necrosis in a dose-dependent manner (0.3-10 mg/kg) in APAP-induced hepatotoxicity mice. Tropisetron (1 and 3 mg/kg) suppressed APAP-induced hepatic lipid peroxidation expression and alleviated GSH and SOD depletion. Administration of tropisetron also attenuated the phosphorylation of c-Jun-NH2-terminal protein kinase (JNK) and extracellular signal-regulated kinase (ERK) caused by APAP. Conclusion. Our data demonstrated that tropisetron's hepatoprotective effect was in part correlated with the antioxidant, which were mediated via JNK and ERK pathways on acetaminophen-induced liver injury in mice.

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.

Hepatic 3D spheroid models for the detection and study of compounds with cholestatic liability

Drug-induced cholestasis (DIC) is poorly understood and its preclinical prediction is mainly limited to assessing the compound's potential to inhibit the bile salt export pump (BSEP). Here, we evaluated two 3D spheroid models, one from primary human hepatocytes (PHH) and one from HepaRG cells, for the detection of compounds with cholestatic liability. By repeatedly co-exposing both models to a set of compounds with different mechanisms of hepatotoxicity and a non-toxic concentrated bile acid (BA) mixture for 8 days we observed a selective synergistic toxicity of compounds known to cause cholestatic or mixed cholestatic/hepatocellular toxicity and the BA mixture compared to exposure to the compounds alone, a phenomenon that was more pronounced after extending the exposure time to 14 days. In contrast, no such synergism was observed after both 8 and 14 days of exposure to the BA mixture for compounds that cause non-cholestatic hepatotoxicity. Mechanisms behind the toxicity of the cholestatic compound chlorpromazine were accurately detected in both spheroid models, including intracellular BA accumulation, inhibition of ABCB11 expression and disruption of the F-actin cytoskeleton. Furthermore, the observed synergistic toxicity of chlorpromazine and BA was associated with increased oxidative stress and modulation of death receptor signalling. Combined, our results demonstrate that the hepatic spheroid models presented here can be used to detect and study compounds with cholestatic liability.

From hepatoprotection models to new therapeutic modalities for treating liver diseases: a personal perspective

A variety of rodent models of hepatoprotection have been developed in which tolerance to acetaminophen-induced hepatotoxicity occurs. Autoprotection/heteroprotection is a phenomenon where prior exposure to a mildly toxic dose of toxicant confers protection against a subsequently administered higher dose of the same toxicant (as in the case of autoprotection) or to a different toxicant (referred to as heteroprotection). Multiple mechanisms regulate this adaptive response, including hepatocellular proliferation, proteostasis, enhanced expression of cytoprotective genes, and altered tissue immune response. In this review, we will discuss recent findings that highlight the complexity of these adaptive mechanisms and we also outline the usefulness of these findings to devise therapeutic and/or diagnostic tools for acetaminophen-induced liver damage in patients.

Bile acids promote diethylnitrosamine-induced hepatocellular carcinoma via increased inflammatory signaling

Hepatocellular carcinoma (HCC) is the most common hepatic malignancy and the third leading cause of cancer related deaths. Previous studies have implicated bile acids in pathogenesis of HCC, but the mechanisms are not known. We investigated the mechanisms of HCC tumor promotion by bile acids the diethylnitrosamine (DEN)-initiation-cholic acid (CA)-induced tumor promotion protocol in mice. The data show that 0.2% CA treatment resulted in threefold increase in number and size of DEN-induced liver tumors. All tumors observed in DEN-treated mice were well-differentiated HCCs. The HCCs observed in DEN-treated CA-fed mice exhibited extensive CD3-, CD20-, and CD45-positive inflammatory cell aggregates. Microarray-based global gene expression studies combined with Ingenuity Pathway Analysis revealed significant activation of NF-κB and Nanog in the DEN-treated 0.2% CA-fed livers. Further studies showed significantly higher TNF-α and IL-1β mRNA, a marked increase in total and phosphorylated-p65 and phosphorylated IκBα (degradation form) in livers of DEN-treated 0.2% CA-fed mice. Treatment of primary mouse hepatocytes with various bile acids showed significant induction of stemness genes including Nanog, KLF4, Sox2, and Oct4. Quantification of total and 20 specific bile acids in liver, and serum revealed a tumor-associated bile acid signature. Finally, quantification of total serum bile acids in normal, cirrhotic, and HCC human samples revealed increased bile acids in serum of cirrhotic and HCC patients. Taken together, these data indicate that bile acids are mechanistically involved pathogenesis of HCC and may promote HCC formation via activation of inflammatory signaling.

Liver-Specific Deletion of Integrin-Linked Kinase in Mice Attenuates Hepatotoxicity and Improves Liver Regeneration After Acetaminophen Overdose

Acetaminophen (APAP) overdose is the major cause of acute liver failure in the US. Prompt liver regeneration is critical for recovery after APAP hepatotoxicity, but mechanisms remain elusive. Extracellular matrix (ECM)-mediated signaling via integrin-linked kinase (ILK) regulates liver regeneration after surgical resection. However, the role of ECM signaling via ILK in APAP toxicity and compensatory regeneration is unknown, which was investigated in this study using liver-specific ILK knockout (KO) mice. ILK KO and wild-type (WT) mice were treated with 300 mg/kg APAP, and injury and regeneration were studied at 6 and 24 h after APAP treatment. ILK KO mice developed lower liver injury after APAP overdose, which was associated with decreased JNK activation (a key mediator of APAP toxicity). Further, higher glutathione levels after APAP treatment and lower APAP protein adducts levels, along with lower levels of CYP2E1, suggest decreased metabolic activation of APAP in ILK KO mice. Interestingly, despite lower injury, ILK KO mice had rapid and higher liver regeneration after APAP overdose accompanied with increased β-catenin signaling. In conclusion, liver-specific deletion of ILK improved regeneration, attenuated toxicity after APAP overdose, and decreased metabolic activation of APAP. Our study also indicates that ILK-mediated ECM signaling plays a role in the regulation of CYP2E1 and may affect toxicity of several centrilobular hepatotoxicants including APAP.

TGR5 is essential for bile acid-dependent cholangiocyte proliferation in vivo and in vitro

OBJECTIVE

Cholestatic liver diseases in humans as well as bile acid (BA)-feeding and common bile duct ligation (CBDL) in rodents trigger hyperplasia of cholangiocytes within the portal fields. Furthermore, elevation of BA levels enhances proliferation and invasiveness of cholangiocarcinoma (CCA) cells in animal models, thus promoting tumour progression. TGR5 is a G-protein coupled BA receptor, which is highly expressed in cholangiocytes and postulated to mediate the proliferative effects of BA.

DESIGN

BA-dependent cholangiocyte proliferation was examined in TGR5-knockout and wild type mice following cholic acid (CA)-feeding and CBDL. TGR5-dependent proliferation and protection from apoptosis was studied in isolated cholangiocytes and CCA cell lines following stimulation with TGR5 ligands and kinase inhibitors. TGR5 expression was analysed in human CCA tissue.

RESULTS

Cholangiocyte proliferation was significantly reduced in TGR5-knockout mice in response to CA-feeding and CBDL. Taurolithocholic acid and TGR5-selective agonists induced cholangiocyte proliferation through elevation of reactive oxygen species and cSrc mediated epidermal growth factor receptor transactivation and subsequent Erk1/2 phosphorylation only in wild type but not in TGR5-knockout-derived cells. In human CCA tissue TGR5 was overexpressed and the pathway of TGR5-dependent proliferation via epidermal growth factor receptor and extracellular signal-regulated kinase (ERK)1/2 activation also translated to CCA cell lines. Furthermore, apoptosis was inhibited by TGR5-dependent CD95 receptor serine phosphorylation.

CONCLUSIONS

TGR5 is an important mediator of BA-induced cholangiocyte proliferation in vivo and in vitro. Furthermore, TGR5 protects cholangiocytes from death receptor-mediated apoptosis. These mechanisms may protect cholangiocytes from BA toxicity under cholestatic conditions, however, they may trigger proliferation and apoptosis resistance in malignantly transformed cholangiocytes, thus promoting CCA progression.