Farnesoid X Receptor Activation Promotes Hepatic Amino Acid Catabolism and Ammonium Clearance in Mice

Bile acid metabolism and signaling in liver disease

Bile acids (BAs) have signaling functions efficiently regulating their own metabolism and transport as well as key aspects of lipid and glucose homeostasis. BAs shape the gut microbial flora and conversely are metabolized by microbiota. Disruption of BA transport, metabolism and physiological signaling function contribute to the pathogenesis and progression of a wide range of liver diseases including cholestatic disorders and metabolic dysfunction-associated steatotic liver disease (MASLD) as well as hepatocellular and cholangiocellular carcinoma. Additionally, impaired BA signaling may also affect the intestine and kidney, thereby contributing to failure of gut integrity driving the progression and complications of portal hypertension, cholemic nephropathy and the development of extrahepatic malignancies such as colorectal cancer. This review will summarize recent advances in the understanding of BA signaling, metabolism and transport focusing on transcriptional regulation and novel BA-focused therapeutic strategies for cholestatic and metabolic liver diseases.

Ammonia-induced stress response in liver disease progression and hepatic encephalopathy

Ammonia levels are orchestrated by a series of complex interrelated pathways in which the urea cycle has a central role. Liver dysfunction leads to an accumulation of ammonia, which is toxic and is strongly associated with disruption of potassium homeostasis, mitochondrial dysfunction, oxidative stress, inflammation, hypoxaemia and dysregulation of neurotransmission. Hyperammonaemia is a hallmark of hepatic encephalopathy and has been strongly associated with liver-related outcomes in patients with cirrhosis and liver failure. In addition to the established role of ammonia as a neurotoxin in the pathogenesis of hepatic encephalopathy, an increasing number of studies suggest that it can lead to hepatic fibrosis progression, sarcopenia, immune dysfunction and cancer. However, elevated systemic ammonia levels are uncommon in patients with metabolic dysfunction-associated steatotic liver disease. A clear causal relationship between ammonia-induced immune dysfunction and risk of infection has not yet been definitively proven. In this Review, we discuss the mechanisms by which ammonia produces its diverse deleterious effects and their clinical relevance in liver diseases, the importance of measuring ammonia levels for the diagnosis of hepatic encephalopathy, the prognosis of patients with cirrhosis and liver failure, and how our knowledge of inter-organ ammonia metabolism is leading to the development of novel therapeutic approaches.

Increased intestinal bile acid absorption contributes to age-related cognitive impairment

Cognitive impairment in the elderly is associated with alterations in bile acid (BA) metabolism. In this study, we observe elevated levels of serum conjugated primary bile acids (CPBAs) and ammonia in elderly individuals, mild cognitive impairment, Alzheimer's disease, and aging rodents, with a more pronounced change in females. These changes are correlated with increased expression of the ileal apical sodium-bile acid transporter (ASBT), hippocampal synapse loss, and elevated brain CPBA and ammonia levels in rodents. In vitro experiments confirm that a CPBA, taurocholic acid, and ammonia induced synaptic loss. Manipulating intestinal BA transport using ASBT activators or inhibitors demonstrates the impact on brain CPBA and ammonia levels as well as cognitive decline in rodents. Additionally, administration of an intestinal BA sequestrant, cholestyramine, alleviates cognitive impairment, normalizing CPBAs and ammonia in aging mice. These findings highlight the potential of targeting intestinal BA absorption as a therapeutic strategy for age-related cognitive impairment.

Bile acid metabolism and signaling in health and disease: molecular mechanisms and therapeutic targets

Bile acids, once considered mere dietary surfactants, now emerge as critical modulators of macronutrient (lipid, carbohydrate, protein) metabolism and the systemic pro-inflammatory/anti-inflammatory balance. Bile acid metabolism and signaling pathways play a crucial role in protecting against, or if aberrant, inducing cardiometabolic, inflammatory, and neoplastic conditions, strongly influencing health and disease. No curative treatment exists for any bile acid influenced disease, while the most promising and well-developed bile acid therapeutic was recently rejected by the FDA. Here, we provide a bottom-up approach on bile acids, mechanistically explaining their biochemistry, physiology, and pharmacology at canonical and non-canonical receptors. Using this mechanistic model of bile acids, we explain how abnormal bile acid physiology drives disease pathogenesis, emphasizing how ceramide synthesis may serve as a unifying pathogenic feature for cardiometabolic diseases. We provide an in-depth summary on pre-existing bile acid receptor modulators, explain their shortcomings, and propose solutions for how they may be remedied. Lastly, we rationalize novel targets for further translational drug discovery and provide future perspectives. Rather than dismissing bile acid therapeutics due to recent setbacks, we believe that there is immense clinical potential and a high likelihood for the future success of bile acid therapeutics.

Understanding the role of ursodeoxycholic acid and gut microbiome in non-alcoholic fatty liver disease: current evidence and perspectives

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, resulting in a huge medical burden worldwide. Accumulating evidence suggests that the gut microbiome and bile acids play pivotal roles during the development of NAFLD. Patients with NAFLD exhibit unique signatures of the intestinal microbiome marked by the priority of Gram-negative bacteria, decreased ratio of Firmicutes/Bacteroidetes (F/B), and increased Prevotella and Lachnospiraceae. The intestinal microbiota is involved in the metabolism of bile acids. Ursodeoxycholic acid (UDCA) is a key determinant in maintaining the dynamic communication between the host and gut microbiota. It generally shows surprising therapeutic potential in NAFLD with several mechanisms, such as improving cellular autophagy, apoptosis, and mitochondrial functions. This action is based on its direct or indirect effect, targeting the farnesoid X receptor (FXR) and various other nuclear receptors. This review aims to discuss the current studies on the involvement of the microbiome-UDCA interface in NAFLD therapy and provide prospective insights into future preventative and therapeutic approaches for NAFLD.

Farnesoid X receptor agonist tropifexor detoxifies ammonia by regulating the glutamine metabolism and urea cycles in cholestatic livers

Hyperammonemia refers to elevated levels of ammonia in the blood, which is an important pathological feature of liver cirrhosis and hepatic failure. Preclinical studies suggest tropifexor (TXR), a novel non-bile acid agonist of Farnesoid X Receptor (FXR), has shown promising effects on reducing hepatic steatosis, inflammation, and fibrosis. This study evaluates the impact of TXR on hyperammonemia in a piglet model of cholestasis. We here observed blood ammonia significantly elevated in patients with biliary atresia (BA) and was positively correlated with liver injury. Targeted metabolomics and immunblotting showed glutamine metabolism and urea cycles were impaired in BA patients. Next, we observed that TXR potently suppresses bile duct ligation (BDL)-induced injuries in liver and brain with improving the glutamine metabolism and urea cycles. Within the liver, TXR enhances glutamine metabolism and urea cycles by up-regulation of key regulatory enzymes, including glutamine synthetase (GS), carbamoyl-phosphate synthetase 1 (CPS1), argininosuccinate synthetase (ASS1), argininosuccinate lyase (ASL), and arginase 1 (ARG1). In primary mice hepatocytes, TXR detoxified ammonia via increasing ureagenesis. Mechanically, TXR activating FXR to increase express enzymes that regulating ureagenesis and glutamine synthesis through a transcriptional approach. Together, these results suggest that TXR may have therapeutic implications for hyperammonemic conditions in cholestatic livers.

Circulating concentrations of bile acids and prevalent chronic kidney disease among newly diagnosed type 2 diabetes: a cross-sectional study

BACKGROUND

The relationship between circulating bile acids (BAs) and kidney function among patients with type 2 diabetes is unclear. We aimed to investigate the associations of circulating concentrations of BAs, particularly individual BA subtypes, with chronic kidney disease (CKD) in patients of newly diagnosed type 2 diabetes.

METHODS

In this cross-sectional study, we included 1234 newly diagnosed type 2 diabetes who participated in an ongoing prospective study, the Dongfeng-Tongji cohort. Circulating primary and secondary unconjugated BAs and their taurine- or glycine-conjugates were measured using ultraperformance liquid chromatography-tandem mass spectrometry. CKD was defined as eGFR < 60 ml/min per 1.73 m2. Logistic regression model was used to compute odds ratio (OR) and 95% confidence interval (CI).

RESULTS

After adjusting for multiple testing, higher levels of total primary BAs (OR per standard deviation [SD] increment: 0.78; 95% CI: 0.65-0.92), cholate (OR per SD: 0.78; 95% CI: 0.66-0.92), chenodeoxycholate (OR per SD: 0.81; 95% CI: 0.69-0.96), glycocholate (OR per SD: 0.81; 95% CI: 0.68-0.96), and glycochenodeoxycholate (OR per SD: 0.82; 95% CI: 0.69-0.97) were associated with a lower likelihood of having CKD in patients with newly diagnosed type 2 diabetes. No significant relationships between secondary BAs and odds of CKD were observed.

CONCLUSIONS

Our findings showed that higher concentrations of circulating unconjugated primary BAs and their glycine-conjugates, but not taurine-conjugates or secondary BAs, were associated with lower odds of having CKD in patients with type 2 diabetes.

Decreased FXR Agonism in the Bile Acid Pool Is Associated with Impaired FXR Signaling in a Pig Model of Pediatric NAFLD

The objective of this study was to investigate whether the impairment of farnesoid X receptor (FXR)-fibroblast growth factor 19 (FGF19) signaling in juvenile pigs with non-alcoholic fatty liver disease (NAFLD) is associated with changes in the composition of the enterohepatic bile acid pool. Eighteen 15-day-old Iberian pigs, pair-housed in pens, were allocated to receive either a control (CON) or high-fructose, high-fat (HFF) diet. Animals were euthanized in week 10, and liver, blood, and distal ileum (DI) samples were collected. HFF-fed pigs developed NAFLD and had decreased FGF19 expression in the DI and lower FGF19 levels in the blood. Compared with the CON, the HFF diet increased the total cholic acid (CA) and the CA to chenodeoxycholic acid (CDCA) ratio in the liver, DI, and blood. CA and CDCA levels in the DI were negatively and positively correlated with ileal FGF19 expression, respectively, and blood levels of FGF19 decreased with an increasing ileal CA to CDCA ratio. Compared with the CON, the HFF diet increased the gene expression of hepatic 12-alpha-hydrolase, which catalyzes the synthesis of CA in the liver. Since CA species are weaker FXR ligands than CDCA, our results suggest that impairment of FXR-FGF19 signaling in NAFLD pigs is associated with a decrease in FXR agonism in the bile acid pool.

Biotransformed bear bile powder ameliorates diet-induced nonalcoholic steatohepatitis in mice through modulating arginine biosynthesis via FXR/PXR-PI3K-AKT-NOS3 axis

NASH is a highly prevalent metabolic syndrome that has no specific approved agents up to now. BBBP, which mainly contains bile acids, possess various pharmacological properties and some bile acids are available for NASH treatment. Herein, the therapeutic effects and underlying mechanisms of BBBP against NASH were systemically evaluated. In this study, mice received an HFHS diet over a 20-week period to induce NASH with or without BBBP intervention were used to evaluate the effect and underlying mechanisms of BBBP against NASH. Our results demonstrated that BBBP attenuated hepatic steatosis, reduced body weight gain and lipid concentrations, and improved sensitivity to insulin and tolerance to glucose in mice fed an HFHS diet. Metabolomics and transcriptomic analysis revealed that BBBP suppressed the arginine biosynthesis by up-regulating NOS3 expression and the PI3K-Akt signaling pathway was also regulated by BBBP, as indicated by 55 DEGs. Bioinformatic analysis predicted the regulatory effect of the FXR/PXR-PI3K-AKT-NOS3 axis on arginine biosynthesis-related metabolites. These results were further confirmed by the significantly increased mRNA and protein levels of NOS3, PI3K (Pik3r2), and AKT1. And the increased levels of arginine biosynthesis related-metabolites, such as urea, aspartic acid, glutamic acid, citrulline, arginine, and ornithine, were confirmed accurately based on targeted metabolomics analysis. Together, our study uncoded the complicated mechanisms of anti-NASH activities of BBBP, and provided critical evidence inspiring the discovery of innovative therapies based on BBBP in the treatment of NASH.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Genetic alterations and molecular mechanisms underlying hereditary intrahepatic cholestasis

Hereditary cholestatic liver disease caused by a class of autosomal gene mutations results in jaundice, which involves the abnormality of the synthesis, secretion, and other disorders of bile acids metabolism. Due to the existence of a variety of gene mutations, the clinical manifestations of children are also diverse. There is no unified standard for diagnosis and single detection method, which seriously hinders the development of clinical treatment. Therefore, the mutated genes of hereditary intrahepatic cholestasis were systematically described in this review.

FXR agonist GW4064 enhances anti-PD-L1 immunotherapy in colorectal cancer

Colorectal cancer (CRC) is one of the top three malignant tumors in terms of morbidity, and the limited efficacy of existing therapies urges the discovery of potential treatment strategies. Immunotherapy gradually becomes a promising cancer treatment method in recent decades; however, less than 10% of CRC patients could really benefit from immunotherapy. It is pressing to explore the potential combination therapy to improve the immunotherapy efficacy in CRC patients. It is reported that Farnesoid X receptor (FXR) is deficiency in CRC and associated with immunity. Herein, we found that GW4064, a FXR agonist, could induce apoptosis, block cell cycle, and mediate immunogenic cell death (ICD) of CRC cells in vitro. Disappointingly, GW4064 could not suppress the growth of CRC tumors in vivo. Further studies revealed that GW4064 upregulated PD-L1 expression in CRC cells via activating FXR and MAPK signaling pathways. Gratifyingly, the combination of PD-L1 antibody with GW4064 exhibited excellent anti-tumor effects in CT26 xenograft models and increased CD8+ T cells infiltration, with 33% tumor bearing mice cured. This paper illustrates the potential mechanisms of GW4064 to upregulate PD-L1 expression in CRC cells and provides important data to support the combination therapy of PD-L1 immune checkpoint blockade with FXR agonist for CRC patients.

Treatment of neurometabolic epilepsies: Overview and recent advances

The rarity and heterogeneity of neurometabolic diseases make it challenging to reach evidence-based principles for their specific treatments. Indeed, current treatments for many of these diseases remain symptomatic and supportive. However, an ongoing scientific and medical revolution has led to dramatic breakthroughs in molecular sciences and genetics, revealing precise pathophysiologic mechanisms. Accordingly, this has led to significant progress in the development of novel therapeutic approaches aimed at treating epilepsy resulting from these conditions, as well as their other manifestations. We overview recent notable treatment advancements, from vitamins, trace minerals, and diets to unique medications targeting the elemental pathophysiology at a molecular or cellular level, including enzyme replacement therapy, enzyme enhancing therapy, antisense oligonucleotide therapy, stem cell transplantation, and gene therapy.

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.

Activation of Pancreatic Acinar FXR Protects against Pancreatitis via Osgin1-Mediated Restoration of Efficient Autophagy

Pancreatitis is the leading cause of hospitalization in gastroenterology, and no medications are available for treating this disease in current clinical practice. FXR plays an anti-inflammatory role in diverse inflammatory diseases, while its function in pancreatitis remains unknown. In this study, we initially observed a marked increase of nuclear FXR in pancreatic tissues of human patients with pancreatitis. Deleting the FXR in pancreatic acinar cells (FXR^acinarΔ/Δ ) led to more severe pancreatitis in mouse models of caerulein-induced acute and chronic pancreatitis, while the FXR agonist GW4064 significantly attenuated pancreatitis in caerulein or arginine-induced acute pancreatitis and caerulein-induced chronic pancreatitis. FXR deletion impaired the viability and stress responses of pancreatic exocrine organoids (PEOs) in vitro. Utilizing RNA-seq and ChIP-seq of PEOs, we identified Osgin1 as a direct target of FXR in the exocrine pancreas, which was also increasingly expressed in human pancreatitis tissues compared to normal pancreatic tissues. Pancreatic knockdown of Osgin1 by AAV-pan abolished the therapeutic effects of FXR activation on pancreatitis, whereas pancreatic overexpression of Osgin1 effectively alleviated caerulein-induced pancreatitis. Mechanistically, we found that the FXR-OSGIN1 axis stimulated autophagic flux in the pancreatic tissues and cell lines, which was considered as the intrinsic mechanisms through which FXR-OSGIN1 protecting against pancreatitis. Our results highlight the protective role of the FXR-OSGIN1 axis in pancreatitis and provided a new target for the treatment of this disease.

O-GlcNAcylation enhances CPS1 catalytic efficiency for ammonia and promotes ureagenesis

Life-threatening hyperammonemia occurs in both inherited and acquired liver diseases affecting ureagenesis, the main pathway for detoxification of neurotoxic ammonia in mammals. Protein O-GlcNAcylation is a reversible and nutrient-sensitive post-translational modification using as substrate UDP-GlcNAc, the end-product of hexosamine biosynthesis pathway. Here we show that increased liver UDP-GlcNAc during hyperammonemia increases protein O-GlcNAcylation and enhances ureagenesis. Mechanistically, O-GlcNAcylation on specific threonine residues increased the catalytic efficiency for ammonia of carbamoyl phosphate synthetase 1 (CPS1), the rate-limiting enzyme in ureagenesis. Pharmacological inhibition of O-GlcNAcase, the enzyme removing O-GlcNAc from proteins, resulted in clinically relevant reductions of systemic ammonia in both genetic (hypomorphic mouse model of propionic acidemia) and acquired (thioacetamide-induced acute liver failure) mouse models of liver diseases. In conclusion, by fine-tuned control of ammonia entry into ureagenesis, hepatic O-GlcNAcylation of CPS1 increases ammonia detoxification and is a novel target for therapy of hyperammonemia in both genetic and acquired diseases.

FXR deficiency in hepatocytes disrupts the bile acid homeostasis and inhibits autophagy to promote liver injury in Schistosoma japonicum-infected mice

Background

Schistosomiasis, with 250 million people affected, is characterized by its serious hepatic inflammatory response and fibrosis formation, which could lead to dangerous complications, such as portal hypertension, splenomegaly and even ascites. But until now, the pathogenesis of schistosomiasis remains largely unknown. Farnesoid X Receptor (FXR), a bile acid-activated nuclear transcription factor mainly expresses in hepatocytes in the liver, can regulate liver diseases by controlling bile acid metabolism.

Methodology/Principal findings

In this study, we found that the expression of FXR was decreased in the liver of infected mice as shown by western blot and RT-qPCR assays. Furthermore, hepatocyte-specific FXR-deficient mice (FXR^flox/floxAlb^Cre, FXR-HKO) were generated and infected with ~16 cercariae of S. japonicum for five weeks. We found that FXR deficiency in hepatocytes promoted the progression of liver injury, aggravated weight loss and death caused by infection, and promoted inflammatory cytokines production, such as IL-6, IL-1β, TNF-α, IL-4, IL-10, and IL-13. Surprisingly, hepatic granulomas and fibrosis were not affected. In addition, using UPLC-MS/MS spectrometry, it was found that S. japonicum infection resulted in elevated bile acids in the liver of mice, which was more obvious in FXR-deficient mice. Meanwhile, autophagy was induced in littermate control mice due to the infection, but it was significantly decreased in FXR-HKO mice.

Conclusions/Significance

All these findings suggest that FXR deficiency in hepatocytes disrupts bile acid homeostasis and inhibits autophagy, which may aggravate the damages of hepatocytes caused by S. japonicum infection. It highlights that FXR in hepatocytes plays a regulatory role in the progression of schistosomiasis.

The liver, a critical metabolic organ, consists of approximately 80% parenchymal hepatocytes. Hepatic schistosomiasis results in inflammatory granulomas response and fibrosis formation that inevitably affects hepatocytes. However, whether and how hepatocytes involved in the progression of liver injury caused by S. japonicum is not clear. Here, we found that, the level of FXR, a key regulator of bile acid metabolism in hepatocytes, was clearly decreased in the liver of mice with this worm infection. Further, we found that FXR specifically deficient in hepatocytes increased bile acids toxicity and inhibited hepatocellular autophagy induced by worm infection, which therefore accelerated the progression of schistosomiasis by promoting hepatocyte injuries but not the formation of egg granulomas and hepatic fibrosis. Our work provides a regulatory loop of FXR/bile acids-autophagy in schistosomiasis, which suggests a role of hepatic FXR in the protection from hepatic damages caused by infection with S. japonicum.

Recent advances on FXR-targeting therapeutics

The bile acid receptor FXR has emerged as a bona fide drug target for chronic cholestatic and metabolic liver diseases, ahead of all non-alcoholic fatty liver disease (NAFLD). FXR is highly expressed in the liver and intestine and activation at both sites differentially contributes to its desired metabolic effects. Unrestricted FXR activation, however, also comes along with undesired effects such as a pro-atherogenic lipid profile, pruritus and hepatocellular toxicity under certain conditions. Several pre-clinical studies have confirmed the potency of FXR activation for cholestatic and metabolic liver diseases, but overall it remains still open whether selective activation of intestinal FXR is advantageous over pan-FXR activation and whether restricted or modulated FXR activation can limit some of the side effects. Even more, FXR antagonist also bear the potential as intestinal-selective drugs in NAFLD models. In this review we will discuss the molecular prerequisites for FXR activation, pan-FXR activation and intestinal FXR in/activation from a therapeutic point of view, different steroidal and non-steroidal FXR agonists, ways to restrict FXR activation and finally what we have learned from pre-clinical models and clinical trials with different FXR therapeutics.

Amino acid metabolism, transport and signalling in the liver revisited

The liver controls the systemic exposure of amino acids entering via the gastro-intestinal tract. For most amino acids except branched chain amino acids, hepatic uptake is very efficient. This implies that the liver orchestrates amino acid metabolism and also controls systemic amino acid exposure. Although many amino acid transporters have been identified, cloned and investigated with respect to substrate specificity, transport mechanism, and zonal distribution, which of these players are involved in hepatocellular amino acid transport remains unclear. Here, we aim to provide a review of current insight into the molecular machinery of hepatic amino acid transport. Furthermore, we place this information in a comprehensive overview of amino acid transport, signalling and metabolism.

Glutaminolysis-ammonia-urea Cycle Axis, Non-alcoholic Fatty Liver Disease Progression and Development of Novel Therapies

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing worldwide, reflecting the current epidemics of obesity, insulin resistance, type 2 diabetes mellitus, and metabolic syndrome. NAFLD is characterized by the accumulation of fat in the liver, and is known to be a cause of cirrhosis. Although many pathways have been proposed, the cause of NAFLD-linked fibrosis progression is still unclear, which posed challenges for the development of new therapies to prevent NASH-related cirrhosis and hepatocellular carcinoma. Cirrhosis is associated with activation of hepatic stellate cells (HSC) and accumulation of excess extracellular matrix proteins, and inhibiting the activation of HSCs would be expected to slow the progression of NAFLD-cirrhosis. Multiple molecular signals and pathways such as oxidative stress and glutaminolysis have been reported to promote HSC activation. Both mechanisms are plausible antifibrotic targets in NASH, as the activation of HSCs the proliferation of myofibroblasts depend on those processes. This review summarizes the role of the glutaminolysis-ammonia-urea cycle axis in the context of NAFLD progression, and shows how the axis could be a novel therapeutic target.

Recent Advances in the Medicinal Chemistry of Farnesoid X Receptor

Farnesoid X receptor (FXR) is an important regulator of bile acid, lipid, amino acid, and glucose homeostasis, hepatic inflammation, regeneration, and fibrosis. FXR has been recognized as a promising drug target for various metabolic diseases such as lipid disorders, nonalcoholic fatty liver disease (NAFLD), nonalcoholic steatohepatitis (NASH), and chronic kidney disease. A large number of FXR ligands have been developed by pharmaceutical companies and academic institutions, and several candidates have progressed into clinical trials in the past decade. However, it is continually a challenge to discover drugs targeting FXR due to side effects associated with long-term administration. In this perspective, we summarize the research progress on medicinal chemistry of FXR modulators from 2018 to the present by discussing the diverse structures of synthetic FXR modulators including steroidal and non-steroidal ligands, their structure-activity relationships (SARs), and their therapeutic applications.

Ginsenoside Rb1 ameliorates Glycemic Disorder in Mice With High Fat Diet-Induced Obesity via Regulating Gut Microbiota and Amino Acid Metabolism

Accumulating evidences suggested an association between gut microbiome dysbiosis and impaired glycemic control. Ginsenoside Rb1 (Rb1) is a biologically active substance of ginseng, which serves anti-diabetic effects. However, its working mechanism especially interaction with gut microbes remains elusive in detail. In this study, we investigated the impact of Rb1 oral supplementation on high fat diet (HFD) induced obesity mice, and explored its mechanism in regulating blood glucose. The results showed that higher liver weight and lower cecum weight were observed in HFD fed mice, which was maintained by Rb1 administration. In addition, Rb1 ameliorated HFD induced blood lipid abnormality and improved insulin sensitivity. Several mRNA expressions in the liver were measured by quantitative real-time PCR, of which UCP2, Nr1H4, and Fiaf were reversed by Rb1 treatment. 16S rRNA sequencing analysis indicated that Rb1 significantly altered gut microbiota composition and increased the abundance of mucin-degrading bacterium Akkermansia spp. compared to HFD mice. As suggested via functional prediction, amino acid metabolism was modulated by Rb1 supplementation. Subsequent serum amino acids investigation indicated that several diabetes associated amino acids, like branched-chain amino acids, tryptophan and alanine, were altered in company with Rb1 supplementation. Moreover, correlation analysis firstly implied that the circulation level of alanine was related to Akkermansia spp.. In summary, Rb1 supplementation improved HFD induced insulin resistance in mice, and was associated with profound changes in microbial composition and amino acid metabolism.

Plasma Exchange in Acute and Acute on Chronic Liver Failure

Liver failure in the context of acute (ALF) and acute on chronic liver failure (ACLF) is associated with high mortality in the absence of a liver transplant. For decades, therapeutic plasma exchange (TPE) is performed for the management of immune-mediated diseases. TPE has emerged as an attractive extracorporeal blood purification technique in patients with ALF and ACLF. The basic premise of using TPE is to remove the toxic substances which would allow recovery of native liver functions by facilitating liver regeneration. In recent years, encouraging data have emerged, suggesting the benefits of TPE in patients with liver failure. TPE has emerged as an attractive liver support device for the failing liver until liver transplantation or clinical recovery. The data in patients with ALF suggest routine use of high-volume TPE, while the data for such a strategy are less robust for patients with ACLF.

A naturally occurring FXR agonist, alisol B 23-acetate, protects against renal ischemia-reperfusion injury

The ligand-activated nuclear receptor, farnesoid X receptor (FXR), plays a pivotal role in regulating renal function. Activation of FXR by its specific agonists exerts renoprotective action in animals with acute kidney injury (AKI). In the present study, we aimed to identify naturally occurring agonists of FXR with potential as therapeutic agents in renal ischemia-reperfusion injury. In vitro and in vivo FXR activation was determined by a dual-luciferase assay, docking analysis, site-directed mutagenesis, and whole kidney transcriptome analysis. Wild-type (WT) and FXR knockout (FXR-/-) mice were used to determine the effect of potential FXR agonist on renal ischemia-reperfusion injury (IRI). We found that alisol B 23-acetate (ABA), a major active triterpenoid extracted from Alismatis rhizoma, a well-known traditional Chinese medicine, can activate renal FXR and induce FXR downstream gene expression in mouse kidney. ABA treatment significantly attenuated renal ischemia-reperfusion-induced AKI in WT mice but not in FXR-/- mice. Our results demonstrate that ABA can activate renal FXR to exert renoprotection against ischemia-reperfusion injury-induced AKI. Therefore, ABA may represent a potential therapeutic agent in the treatment of ischemic AKI.NEW & NOTEWORTHY In the present study, we found that alisol B 23-acetate (ABA), an identified natural farnesoid X receptor (FXR) agonist from the well-known traditional Chinese medicine Alismatis rhizoma, protects against ischemic acute kidney injury (AKI) in an FXR-dependent manner, as reflected by improved renal function, reduced renal tubular apoptosis, ameliorated oxidative stress, and suppressed inflammatory factor expression. Therefore, ABA may have great potential as a novel therapeutic agent in the treatment of AKI in the future.

Emodin palliates high-fat diet-induced nonalcoholic fatty liver disease in mice via activating the farnesoid X receptor pathway

BACKGROUND

Cassia mimosoides Linn (CMD) is a traditional Chinese herb that clears liver heat and dampness. It has been widely administered in clinical practice to treat jaundice associated with damp-heat pathogen and obesity. Emodin (EMO) is a major bioactive constituent of CMD that has apparent therapeutic efficacy against obesity and fatty liver. Here, we investigated the protective effects and underlying mechanisms of EMO against high-fat diet (HFD)-induced nonalcoholic fatty liver disease (NAFLD).

OBJECTIVE

We aimed to investigate whether EMO activates farnesoid X receptor (FXR) signaling to alleviate HFD-induced NAFLD.

MATERIALS AND METHODS

In vivo assays included serum biochemical indices tests, histopathology, western blotting, and qRT-PCR to evaluate the effects of EMO on glucose and lipid metabolism disorders in wild type (WT) and FXR knockout mice maintained on an HFD. In vitro experiments included intracellular triglyceride (TG) level measurement and Oil Red O staining to assess the capacity of EMO to remove lipids induced by oleic acid and palmitic acid in WT and FXR knockout mouse primary hepatocytes (MPHs). We also detected mRNA expression of FXR signaling genes in MPHs.

RESULTS

After HFD administration, body weight and serum lipid and inflammation levels were dramatically increased in the WT mice. The animals also presented with impaired glucose tolerance, insulin resistance, and antioxidant capacity, liver tissue attenuation, and pathological injury. EMO remarkably reversed the foregoing changes in HFD-induced mice. EMO improved HFD-induced lipid accumulation, insulin resistance, inflammation, and oxidative stress in a dose-dependent manner in WT mice by inhibiting FXR expression. EMO also significantly repressed TG hyperaccumulation by upregulating FXR expression in MPHs. However, it did not improve lipid accumulation, insulin sensitivity, or glucose tolerance in HFD-fed FXR knockout mice.

CONCLUSIONS

The present study demonstrated that EMO alleviates HFD-induced NAFLD by activating FXR signaling which improves lipid accumulation, insulin resistance, inflammation, and oxidative stress.

Identification and characterization of novel splice variants of human farnesoid X receptor

Farnesoid X receptor (FXR, NR1H4) is a ligand-activated nuclear receptor, which regulates bile acid, lipid and glucose metabolism. Due to these functions, FXR has been investigated as a potential drug target for the treatment of liver diseases, such as primary biliary cholangitis and non-alcoholic steatohepatitis. Based on the previously described four splice variants, it has been suggested that alternative promoter usage and splicing may have an impact on total FXR activity as a result of encoding functionally diverse variants. Here we aimed for a systematic analysis of human hepatic FXR splice variants. In addition to the previously described FXRα1-4, we identified four novel splice variants (FXRα5-8) in human hepatocytes, which resulted from previously undetected exon skipping events. These newly identified isoforms displayed diminished DNA binding and impaired transactivation activities. Isoform FXRα5, which suppressed the transactivation activity of the functional isoform FXRα2, was further characterized as deficient in heterodimerization, coactivator recruitment and ligand binding. These findings were further supported by molecular dynamics simulations, which offered an explanation for the behavior of this isoform on the molecular level. FXRα5 exhibited low uniform expression levels in nearly all human tissues. Our systematic analysis of FXR splice variants in human hepatocytes resulted in the identification of four novel FXR isoforms, which all proved to be functionally deficient, but one novel variant, FXRα5, also displayed dominant negative activity. The possible associations with and roles of these novel isoforms in human liver diseases require further investigation.

Obeticholic acid ameliorates obesity and hepatic steatosis by activating brown fat

Obeticholic acid (OCA) is exemplified as a potent drug for treating primary biliary cirrhosis and nonalcoholic fatty liver disease by inhibiting bile acid synthesis. However, it remains unclear whether the effect of OCA is mediated by the function of brown adipose tissue (BAT). In the present study, brown adipogenesis differentiation in vitro and db/db mouse model treated with OCA were used to assess the anti-obesity function by body weight tracking, O₂ consumption, food intake, physical activity, glucose tolerance tests. In addition, uncoupling protein 1 (Ucp1) protein expression in brown adipose tissue was measured by western blotting, morphometry of brown adipose tissue was analyzed by hematoxylin and eosin staining. Hepatic steatosis was detected by Oil-Red O staining and serological analysis was performed to assess the effect of OCA on hyperlipidemia. OCA treatment enhanced brown adipocyte cell differentiation and upregulated the expression of the BAT-specific gene Ucp1) in C3H10T1/2 cells in vitro. Consistent with these findings, OCA increased whole-body energy metabolism and glucose homeostasis by enhancing BAT activity in vivo, and ultimately decreased body weight gain in db/db mice. In addition, the results demonstrated that spontaneous hepatic steatosis in db/db mice was ameliorated following OCA treatment. In summary, OCA functioned as a BAT activator to help ameliorate obesity and maintain glucose homeostasis in db/db mice. The present results may provide a novel potential therapeutic approach to activate brown fat in patients with obesity and other metabolic disorders.

An Integrative Transcriptomic and Methylation Approach for Identifying Differentially Expressed Circular RNAs Associated with DNA Methylation Change

Recently, accumulating evidence has supported that circular RNA (circRNA) plays important roles in tumorigenesis by regulating gene expression at transcriptional and post-transcriptional levels. Expression of circRNAs can be epigenetically silenced by DNA methylation; however, the underlying regulatory mechanisms of circRNAs by DNA methylation remains largely unknown. We explored this regulation in hepatocellular carcinoma (HCC) using genome-wide DNA methylation and RNA sequencing data of the primary tumor and matched adjacent normal tissues from 20 HCC patients. Our pipeline identified 1012 upregulated and 747 downregulated circRNAs (collectively referred to as differentially expressed circRNAs, or DE circRNAs) from HCC RNA-seq data. Among them, 329 DE circRNAs covered differentially methylated sites (adjusted p-value < 0.05, |ΔM| > 0.5) in circRNAs’ interior and/or flanking regions. Interestingly, the corresponding parental genes of 46 upregulated and 31 downregulated circRNAs did not show significant expression change in the HCC tumor versus normal samples. Importantly, 34 of the 77 DE circRNAs (44.2%) had significant correlation with DNA methylation change in HCC (Spearman’s rank-order correlation, p-value < 0.05), suggesting that aberrant DNA methylation might regulate circular RNA expression in HCC. Our study revealed genome-wide differential circRNA expression in HCC. The significant correlation with DNA methylation change suggested that epigenetic regulation might act on both mRNA and circRNA expression. The specific regulation in HCC and general view in other cancer or disease requires further investigation.

Splice variants of metabolic nuclear receptors: Relevance for metabolic disease and therapeutic targeting

Metabolic nuclear receptors are ligand-activated transcription factors which control a wide range of metabolic processes and signaling pathways in response to nutrients and xenobiotics. Targeting these NRs is at the forefront of our endeavours to generate novel treatment options for diabetes, metabolic syndrome and fatty liver disease. Numerous splice variants have been described for these metabolic receptors. Structural changes, as a result of alternative splicing, lead to functional differences among NR isoforms, resulting in the regulation of different metabolic pathways by these NR splice variants. In this review, we describe known splice variants of FXR, LXRs, PXR, RXR, LRH-1, CAR and PPARs. We discuss their structure and functions, and elaborate on the regulation of splice variant abundance by nutritional signals. We conclude that NR splice variants pose an intriguing new layer of complexity in metabolic signaling, which needs to be taken into account in the development of treatment strategies for metabolic diseases.

FXR in liver physiology: Multiple faces to regulate liver metabolism

The liver is the central metabolic hub which coordinates nutritional inputs and metabolic outputs. Food intake releases bile acids which can be sensed by the bile acid receptor FXR in the liver and the intestine. Hepatic and intestinal FXR coordinately regulate postprandial nutrient disposal in a network of interacting metabolic nuclear receptors. In this review we summarize and update the "classical roles" of FXR as a central integrator of the feeding state response, which orchestrates the metabolic processing of carbohydrates, lipids, proteins and bile acids. We also discuss more recent and less well studied FXR effects on amino acid, protein metabolism, autophagic turnover and inflammation. In addition, we summarize the recent understanding of how FXR signaling is affected by posttranslational modifications and by different FXR isoforms. These modifications and variations in FXR signaling might be considered when FXR is targeted pharmaceutically in clinical applications.

Targeted Metabolomics Identifies Differential Serum and Liver Amino Acids Biomarkers in Rats with Alcoholic Liver Disease

To investigate changes in serum and hepatic levels of amino acids in ALD and to provide novel evidence and approaches for the prevention and treatment of ALD. Twenty specific pathogen-free SD male rats were devided into two groups, ten for the control group, and ten for the model group. Serum biochemical markers, including alanine aminotransferase, aspartate aminotransferase, laminin and hyaluronidase were measured. Histological analysis of liver tissues was performed. Serum and liver amino acids levels were quantitatively determined by ultra-high-performance liquid chromatography-tandem quadrupole mass spectrometry (UPLC-TQMS)-based targeted metabolomics. Compared with the normal group, ALD rats showed an obvious increase in the levels of β-alanine, alanine, serine, ornithine, tyrosine and the tyrosine ratio, while there was a decrease in arginine levels, the BTR ratio and Fischer's ratio in serum. Additionally, ALD rats exhibited a significant increase in the levels of cysteine and putrescine, while there was a decrease in sarcosine, β-alanine, serine, proline, valine, threonine, ornithine, lysine, histidine, tyrosine, symmetric dimethylarginine, methionine, isoleucine and methionine-sulfoxide levels in liver tissues compared with the normal group. The serum and liver amino acids showed significant changes in ALD rats and can be considered as potential specific diagnostic biomarkers for ALD.

Circulating liver function markers and colorectal cancer risk: A prospective cohort study in the UK Biobank

Evidence links the liver to development of colorectal cancer (CRC). However, it remains unknown how liver function may influence CRC risk in the general population. We conducted a prospective cohort study in the UK Biobank of 375 693 participants who provided blood samples in 2006 to 2010. Circulating levels of liver function markers (alanine transaminase [ALT], aspartate transaminase [AST], total bilirubin [TBIL], gamma glutamyltransferase [GGT], alkaline phosphatase [ALP], total protein [TP] and albumin [ALB]) were measured. Incident cancer cases were identified through linkage to the national cancer registry up to 2019. Repeated biomarker measurements were available from a subset of 11 320 participants who were re-assessed in 2012 to 2013. After a median follow-up of 10.0 years, we documented 2662 cases of CRC. Circulating levels of ALT, AST, TBIL, GGT, TP and ALB at baseline were inversely associated with CRC risk (P < .01), with multivariable hazard ratio (95% confidence interval) comparing decile 10 vs 1 of 0.62 (0.51-0.75), 0.63 (0.53-0.75), 0.85 (0.72-1.02), 0.74 (0.61-0.89), 0.70 (0.59-0.84) and 0.66 (0.55-0.79), respectively. Strengthened associations were found after recalibration for repeated measurements. The associations appeared stronger for proximal colon cancer than distal colon cancer and rectal cancer, but consistent for early-, mid- and late-onset CRC. In a large cohort of general population, the UK Biobank, higher circulating levels of ALT, AST, TBIL, GGT, TP and ALB, largely within the normal range, were associated with a lower risk of CRC. The findings support a link between liver function and CRC, and may spur future research on the gut-microbiota-liver axis.

FXR Isoforms Control Different Metabolic Functions in Liver Cells via Binding to Specific DNA Motifs

BACKGROUND & AIMS

The nuclear receptor subfamily 1 group H member 4 (NR1H4, also called FXR) is a ligand-activated transcription factor that, upon binding of bile acids, regulates the expression of genes involved in bile acid, fat, sugar, and amino acid metabolism. Transcript variants encode the FXR isoforms alpha 1, alpha 2, alpha 3, and alpha 4, which activate different genes that regulate metabolism. Little is known about the mechanisms by which the different isoforms regulate specific genes or how the expression of these genes affects the outcomes of patients given drugs that target FXR.

METHODS

We determined genome-wide binding of FXR isoforms in mouse liver organoids that express individual FXR isoforms using chromatin immunoprecipitation, followed by sequencing analysis and DNA motif discovery. We validated regulatory DNA sequences by mobility shift assays and with luciferase reporters using mouse and human FXR isoforms. We analyzed mouse liver organoids and HepG2 cells that expressed the FXR isoforms using chromatin immunoprecipitation, quantitative polymerase chain reaction, and immunoblot assays. Organoids were analyzed for mitochondrial respiration, lipid droplet content, and triglyceride excretion. We used the FXR ligand obeticholic acid to induce FXR activity in organoids, cell lines, and mice. We collected data on the binding of FXR in mouse liver and the expression levels of FXR isoforms and gene targets in human liver tissue and primary human hepatocytes from the Gene Expression Omnibus.

RESULTS

In mouse liver cells, 89% of sites that bound FXR were bound by only FXRα2 or FXRα4, via direct interactions with the DNA sequence motif ER-2. Via DNA binding, these isoforms regulated metabolic functions in liver cells, including carbon metabolism and lipogenesis. Incubation with obeticholic acid increased mitochondrial pyruvate transport and reduced insulin-induced lipogenesis in organoids that expressed FXRα2 but not FXRα1. In human liver tissues, levels of FXRα2 varied significantly and correlated with expression of genes predicted to be regulated via an ER-2 motif.

CONCLUSIONS

Most metabolic effects regulated by FXR in mouse and human liver cells are regulated by the FXRα2 isoform via specific binding to ER-2 motifs. The expression level of FXRα2 in liver might be used to predict responses of patients to treatment with FXR agonists.

The Status of Bile Acids and Farnesoid X Receptor in Brain and Liver of Rats with Thioacetamide-Induced Acute Liver Failure

Acute liver failure (ALF) leads to neurological symptoms defined as hepatic encephalopathy (HE). Although accumulation of ammonia and neuroinflammation are generally accepted as main contributors to HE pathomechanism, a buildup of bile acids (BA) in the blood is a frequent component of liver injury in HE patients. Recent studies have identified the nuclear farnesoid X receptor (FXR) acting via small heterodimer partner (SHP) as a mediator of BA-induced effects in the brain of ALF animals. The present study investigated the status of the BA–FXR axis in the brain and the liver, including selective changes in pertinent genes in thioacetamide (TAA)-induced ALF in Sprague–Dawley rats. FXR was found in rat neurons, confirming earlier reports for mouse and human brain. BA accumulated in blood but not in the brain tissue. Expression of mRNAs coding for Fxr and Shp was reduced in the hippocampus and of Fxr mRNA also in the cerebellum. Changes in Fxr mRNA levels were not followed by changes in FXR protein. The results leave open the possibility that mobilization of the BA–FXR axis in the brain may not be necessarily pathognomonic to HE but may depend upon ALF-related confounding factors.

Molecular Physiology of Bile Acid Signaling in Health, Disease, and Aging

Over the past two decades, bile acids (BAs) have become established as important signaling molecules that enable fine-tuned inter-tissue communication from the liver, their site of production, over the intestine, where they are modified by the gut microbiota, to virtually any organ, where they exert their pleiotropic physiological effects. The chemical variety of BAs, to a large extent determined by the gut microbiome, also allows for a complex fine-tuning of adaptive responses in our body. This review provides an overview of the mechanisms by which BA receptors coordinate several aspects of physiology and highlights new therapeutic strategies for diseases underlying pathological BA signaling.

An Overview of Lipid Metabolism and Nonalcoholic Fatty Liver Disease

The occurrence of nonalcoholic fatty liver disease (NAFLD) is associated with major abnormalities of hepatic lipid metabolism. We propose that lipid abnormalities directly or indirectly contribute to NAFLD, especially fatty acid accumulation, arachidonic acid metabolic disturbance, and ceramide overload. The effects of lipid intake and accumulation on NAFLD and NAFLD treatment are explained with theoretical and experimental details. Overall, these findings provide further understanding of lipid metabolism in NAFLD and may lead to novel therapies.

Gypenosides regulate farnesoid X receptor-mediated bile acid and lipid metabolism in a mouse model of non-alcoholic steatohepatitis

Background

Gypenosides (Gyp) are the main ingredient of the Chinese medicine, Gynostemma pentaphyllum. They are widely used in Asia as a hepatoprotective agent. Here, we elucidated the mechanism of Gyp in non-alcoholic steatohepatitis (NASH) with a focus on farnesoid X receptor (FXR)-mediated bile acid and lipid metabolic pathways.

Methods

NASH was induced in mice by high-fat diet (HFD) feeding, while mice in the control group were given a normal diet. At the end of week 10, HFD-fed mice were randomly divided into HFD, HFD plus Gyp, and HFD plus obeticholic acid (OCA, FXR agonist) groups and were given the corresponding treatments for 4 weeks. Next, we analyzed the histopathological changes as well as the liver triglyceride (TG) level and serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), fasting blood glucose (FBG), fasting insulin (FINS), TG, total cholesterol (TC), and low-density lipoprotein cholesterol (LDL-C) levels as well as the bile acid profile. We carried out RT-PCR and western blotting to detect HFD-induced alterations in gene/protein expression related to bile acid and lipid metabolism.

Results

The HFD group had histopathological signs of hepatic steatosis and vacuolar degeneration. The liver TG and serum ALT, AST, FBG, FINS, TC, and LDL-C levels as well as the total bile acid level were significantly higher in the HFD group than in the control group (P < 0.01). In addition, we observed significant changes in the expression of proteins involved in bile acid or lipid metabolism (P < 0.05). Upon treatment with Gyp or OCA, signs of hepatic steatosis and alterations in different biochemical parameters were significantly improved (P < 0.05). Further, HFD-induced alterations in the expression genes involved in bile acid and lipid metabolism, such as CYP7A1, BSEP, SREBP1, and FASN, were significantly alleviated.

Conclusions

Gyp can improve liver lipid and bile acid metabolism in a mouse model of NASH, and these effects may be related to activation of the FXR signaling pathway.

Epithelial splicing regulatory protein 2-mediated alternative splicing reprograms hepatocytes in severe alcoholic hepatitis

Severe alcoholic hepatitis (SAH) is a deadly liver disease without an effective medical therapy. Although SAH mortality is known to correlate with hepatic accumulation of immature liver cells, why this occurs and how it causes death are unclear. Here, we demonstrate that expression of epithelial splicing regulatory protein 2 (ESRP2), an RNA-splicing factor that maintains the nonproliferative, mature phenotype of adult hepatocytes, was suppressed in both human SAH and various mouse models of SAH in parallel with the severity of alcohol consumption and liver damage. Inflammatory cytokines released by excessive alcohol ingestion reprogrammed adult hepatocytes into proliferative, fetal-like cells by suppressing ESRP2. Sustained loss of ESRP2 permitted reemergence of a fetal RNA-splicing program that attenuates the Hippo signaling pathway and thus allows fetal transcriptional regulators to accumulate in adult liver. We further showed that depleting ESRP2 in mice exacerbated alcohol-induced steatohepatitis, enabling surviving hepatocytes to shed adult hepatocyte functions and become more regenerative, but threatening overall survival by populating the liver with functionally immature hepatocytes. Our findings revealed a mechanism that explains why liver failure develops in patients with the clinical syndrome of SAH, suggesting that recovery from SAH might be improved by limiting adult-to-fetal reprogramming in hepatocytes.

Progress and challenges in development of new therapies for urea cycle disorders

Urea cycle disorders (UCD) are inborn errors of metabolism caused by deficiency of enzymes required to transfer nitrogen from ammonia into urea. Current paradigms of treatment focus on dietary manipulations, ammonia scavenger drugs, and orthotopic liver transplantation. In the last years, there has been intense preclinical research aiming at developing more effective treatments for UCD, and as a result, several novel approaches based on new knowledge of the disease pathogenesis, cell and gene therapies are currently under clinical investigation. We provide an overview of the latest advances for the development of novel therapies for UCD.

Salidroside improves high-fat diet-induced non-alcoholic steatohepatitis by regulating the gut microbiota-bile acid-farnesoid X receptor axis

BACKGROUND

Our previous studies found that salidroside can effectively treat non-alcoholic steatohepatitis (NASH). Here, we discuss the mechanism of salidroside in the treatment of NASH with a focus on the gut microbiota-bile acid-farnesoid X receptor axis.

METHODS

A NASH mouse model was created by providing mice with a high-fat diet (HFD) for 14 weeks. Mice were randomly divided into the HFD group, HFD + salidroside treatment group, and HFD + obeticholic acid treatment group (n = 8 in each group) and were intragastrically administered corresponding drugs for 4 weeks. Hematoxylin-eosin staining was performed to evaluate the histopathological changes associated with the various treatments. In addition, liver triglyceride (TG) content, serum alanine aminotransferase (ALT) activity, serum inflammatory factors, gut microbiota diversity, and the bile acid profile were evaluated. Western blotting and RT-PCR were performed to detect the expressions of FXR and fibroblast growth factor 15 (FGF15).

RESULTS

The HFD group displayed obvious signs of hepatic steatosis. The liver TG, serum ALT, and IL-1a, IL-12, MCP-1, KC, MIP-1a, and MIP-1β were significantly higher in the HFD group than the control group (P < 0.01). Intestinal bacteria and bile acid profiles changed significantly in the HFD group (P < 0.05). Further, the expressions of FXR and FGF15 decreased significantly in the HFD group (P < 0.05). After treatment with salidroside, liver steatosis, TG content, and serum inflammatory factors significantly improved and HFD-induced intestinal bacteria, bile acid disorder, and FXR deficiency were significantly alleviated (P < 0.05).

CONCLUSION

Salidroside can improve NASH via the gut microbiota-bile acid-FXR axis.

Farnesoid X receptor and bile acids regulate vitamin A storage

The nuclear receptor Farnesoid X Receptor (FXR) is activated by bile acids and controls multiple metabolic processes, including bile acid, lipid, carbohydrate, amino acid and energy metabolism. Vitamin A is needed for proper metabolic and immune control and requires bile acids for efficient intestinal absorption and storage in the liver. Here, we analyzed whether FXR regulates vitamin A metabolism. Compared to control animals, FXR-null mice showed strongly reduced (>90%) hepatic levels of retinol and retinyl palmitate and a significant reduction in lecithin retinol acyltransferase (LRAT), the enzyme responsible for hepatic vitamin A storage. Hepatic reintroduction of FXR in FXR-null mice induced vitamin A storage in the liver. Hepatic vitamin A levels were normal in intestine-specific FXR-null mice. Obeticholic acid (OCA, 3 weeks) treatment rapidly reduced (>60%) hepatic retinyl palmitate levels in mice, concurrent with strongly increased retinol levels (>5-fold). Similar, but milder effects were observed in cholic acid (12 weeks)-treated mice. OCA did not change hepatic LRAT protein levels, but strongly reduced all enzymes involved in hepatic retinyl ester hydrolysis, involving mostly post-transcriptional mechanisms. In conclusion, vitamin A metabolism in the mouse liver heavily depends on the FXR and FXR-targeted therapies may be prone to cause vitamin A-related pathologies.

Steroidogenic control of liver metabolism through a nuclear receptor-network

Objective

Coupling metabolic and reproductive pathways is essential for the survival of species. However, the functions of steroidogenic enzymes expressed in metabolic tissues are largely unknown.

Methods and results

Here, we show that in the liver, the classical steroidogenic enzyme Cyp17a1 forms an essential nexus for glucose and ketone metabolism during feed-fast cycles. Both gain- and loss-of-function approaches are used to show that hepatic Cyp17a1 is induced by fasting, catalyzes the production of at least one hormone-ligand (DHEA) for the nuclear receptor PPARα, and is ultimately required for maintaining euglycemia and ketogenesis during nutrient deprivation. The feedback-loop that terminates Cyp17a1-PPARα activity, and re-establishes anabolic liver metabolism during re-feeding is mapped to postprandial bile acid-signaling, involving the receptors FXR, SHP and LRH-1.

Conclusions

Together, these findings represent a novel paradigm of homeostatic control in which nutritional cues feed-forward on to metabolic pathways by influencing extragonadal steroidogenesis.

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The classical steroidogenic enzyme, Cyp17a1, is upregulated in liver during fasting.

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CYP17a1 produces a hormone-ligand for the nuclear receptor PPARα and affects glucose and lipid handling in the liver.

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Hepatic Cyp17a1 is essential for maintaining glycaemia and ketones during fasting.

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Bile acids, via a nuclear receptor cascade, repress hepatic Cyp17a1 as part of the re-feeding response.

Ursodeoxycholic acid exerts hepatoprotective effects by regulating amino acid, flavonoid, and fatty acid metabolic pathways

INTRODUCTION

Ursodeoxycholic acid (UDCA) is an intestinal bacterial metabolite with hepatoprotective effects. However, molecular mechanisms underlying its effects remain unclear.

OBJECTIVES

The aim of this study was to investigate the mechanisms underlying the therapeutic effects of UDCA by using global metabolomics analyses in healthy subjects.

METHODS

Healthy Korean men were administered UDCA at dosage of 400, 800, or 1200 mg daily for 2 weeks. Serum samples were collected and used for liver function tests and to determine miR-122 expression levels. Urinary and plasma global metabolomics analyses were conducted using a liquid chromatography system coupled with quadrupole-time-of-flight mass spectrometry (LC/QTOFMS) and gas chromatography-TOFMS (GC/TOFMS). Unsupervised multivariate analysis (principal component analysis) was performed to identify discriminative markers before and after treatment.

RESULTS

Alanine transaminase score and serum miR-122 levels decreased significantly after 2 weeks of treatment. Through LC- and GC-based metabolomic profiling, we identified 40 differential metabolites in plasma and urine samples.

CONCLUSIONS

Regulation of liver function scores and metabolic alternations highlight the potential hepatoprotective action of UDCA, which were primarily associated with amino acid, flavonoid, and fatty acid metabolism in healthy men.

Developments in bile salt based therapies: A critical overview

Bile acids, amphipathic molecules known for their facilitating role in fat absorption, are also recognized as signalling molecules acting via nuclear and membrane receptors. Of the bile acid-activated receptors, the Farnesoid X Receptor (FXR) and the G protein-coupled bile acid receptor-1 (Gpbar1 or TGR5) have been studied most extensively. Bile acid signaling is critical in the regulation of bile acid metabolism itself, but it also plays a significant role in glucose, lipid and energy metabolism. Activation of FXR and TGR5 leads to reduced hepatic bile salt load, improved insulin sensitivity and glucose regulation, increased energy expenditure, and anti-inflammatory effects. These beneficial effects render bile acid signaling an interesting therapeutic target for the treatment of diseases such as cholestasis, non-alcoholic fatty liver disease, and diabetes. Here, we summarize recent findings on bile acid signaling and discuss potential and current limitations of bile acid receptor agonist and modulators of bile acid transport as future therapeutics for a wide-spectrum of diseases.

Noncanonical farnesoid X receptor signaling inhibits apoptosis and impedes liver fibrosis

Background

Hepatocyte is particularly vulnerable to apoptosis, a hallmark of many liver diseases. Although pro-apoptotic mechanisms have been extensively explored, less is known about the hepatocyte-specific anti-apoptotic molecular events and it lacks effective approach to combat hepatocyte apoptosis. We investigated the anti-apoptotic effect and mechanism of farnesoid X receptor (FXR), and strategies of how to target FXR for inhibiting apoptosis implicated in liver fibrosis.

Methods

Sensitivity to apoptosis was compared between wild type and Fxr^−/− mice and in cultured cells. Cell-based and cell-free assays were employed to identify the binding protein of FXR and to uncover the mechanism of its anti-apoptotic effect. Overexpression of FXR by adenovirus-FXR was employed to determine its anti-fibrotic effect in CCl₄-treated mice. Specimens from fibrotic patients were collected to validate the relevance of FXR on apoptosis/fibrosis.

Findings

FXR deficiency sensitizes hepatocytes to death receptors (DRs)-engaged apoptosis. FXR overexpression, but not FXR ligands, inhibits apoptosis both in vitro and in vivo. Apoptotic stimuli lead to drastic reduction of FXR protein levels, a prerequisite for DRs-engaged apoptosis. Mechanistically, FXR interacts with caspase 8 (CASP8) in the cytoplasm, thus preventing the formation of death-inducing signaling complex (DISC) and activation of CASP8. Adenovirus-FXR transfection impedes liver fibrosis in CCl₄-treated mice. Specimens from fibrotic patients are characterized with reduced FXR expression and compromised FXR/CASP8 colocalization.

Interpretation

FXR represents an intrinsic apoptosis inhibitor in hepatocytes and can be targeted via restoring its expression or strengthening FXR/CASP8 interaction for inhibiting hepatocytes apoptosis in liver fibrosis.

Fund

National Natural Science Foundation of China.

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FXR physically interacts with CASP8 in cytoplasm.

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FXR inhibits death receptors-engaged apoptosis independent of transactivation.

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Reduction of cytosolic FXR is a prerequisite initiating apoptosis cascade.

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Forced overexpression of FXR impedes liver fibrosis.

Systemic Inflammation and Acute-on-Chronic Liver Failure: Too Much, Not Enough

ACLF is a specific, but complex and multifactorial form of acute decompensation of cirrhosis and is characterized by an extraordinary dynamic natural course, rapidly evolving organ failure, and high short-term mortality. Dysbalanced immune function is central to its pathogenesis and outcome with an initial excessive systemic inflammatory response that drives organ failure and mortality. Later in its course, immuno-exhaustion/immunoparalysis prevails predisposing the patient to secondary infectious events and reescalation in end-organ dysfunction and mortality. The management of patients with ACLF is still poorly defined. However, as its pathophysiology is gradually being unravelled, potential therapeutic targets emerge that warrant further study such as restoring or substituting albumin via plasma exchange or via albumin dialysis and evaluating usefulness of TLR4 antagonists, modulators of gut dysbiosis (pre- or probiotics), and FXR-agonists.

Mammalian Sulfur Amino Acid Metabolism: A Nexus Between Redox Regulation, Nutrition, Epigenetics, and Detoxification

SIGNIFICANCE

Transsulfuration allows conversion of methionine into cysteine using homocysteine (Hcy) as an intermediate. This pathway produces S-adenosylmethionine (AdoMet), a key metabolite for cell function, and provides 50% of the cysteine needed for hepatic glutathione synthesis. The route requires the intake of essential nutrients (e.g., methionine and vitamins) and is regulated by their availability. Transsulfuration presents multiple interconnections with epigenetics, adenosine triphosphate (ATP), and glutathione synthesis, polyol and pentose phosphate pathways, and detoxification that rely mostly in the exchange of substrates or products. Major hepatic diseases, rare diseases, and sensorineural disorders, among others that concur with oxidative stress, present impaired transsulfuration. Recent Advances: In contrast to the classical view, a nuclear branch of the pathway, potentiated under oxidative stress, is emerging. Several transsulfuration proteins regulate gene expression, suggesting moonlighting activities. In addition, abnormalities in Hcy metabolism link nutrition and hearing loss.

CRITICAL ISSUES

Knowledge about the crossregulation between pathways is mostly limited to the hepatic availability/removal of substrates and inhibitors. However, advances regarding protein-protein interactions involving oncogenes, identification of several post-translational modifications (PTMs), and putative moonlighting activities expand the potential impact of transsulfuration beyond methylations and Hcy.

FUTURE DIRECTIONS

Increasing the knowledge on transsulfuration outside the liver, understanding the protein-protein interaction networks involving these enzymes, the functional role of their PTMs, or the mechanisms controlling their nucleocytoplasmic shuttling may provide further insights into the pathophysiological implications of this pathway, allowing design of new therapeutic interventions. Antioxid. Redox Signal. 29, 408-452.

Update on FXR Biology: Promising Therapeutic Target?

Farnesoid X receptor (FXR), a metabolic nuclear receptor, plays critical roles in the maintenance of systemic energy homeostasis and the integrity of many organs, including liver and intestine. It regulates bile acid, lipid, and glucose metabolism, and contributes to inter-organ communication, in particular the enterohepatic signaling pathway, through bile acids and fibroblast growth factor-15/19 (FGF-15/19). The metabolic effects of FXR are also involved in gut microbiota. In addition, FXR has various functions in the kidney, adipose tissue, pancreas, cardiovascular system, and tumorigenesis. Consequently, the deregulation of FXR may lead to abnormalities of specific organs and metabolic dysfunction, allowing the protein as an attractive therapeutic target for the management of liver and/or metabolic diseases. Indeed, many FXR agonists have been being developed and are under pre-clinical and clinical investigations. Although obeticholic acid (OCA) is one of the promising candidates, significant safety issues have remained. The effects of FXR modulation might be multifaceted according to tissue specificity, disease type, and/or energy status, suggesting the careful use of FXR agonists. This review summarizes the current knowledge of systemic FXR biology in various organs and the gut–liver axis, particularly regarding the recent advancement in these fields, and also provides pharmacological aspects of FXR modulation for rational therapeutic strategies and novel drug development.