Xenobiotic Nuclear Receptor Signaling Determines Molecular Pathogenesis of Progressive Familial Intrahepatic Cholestasis

The Hepatokine Orosomucoid 2 Mediates Beneficial Metabolic Effects of Bile Acids

Bile acids (BAs) are pleiotropic regulators of metabolism. Elevated levels of hepatic and circulating BAs improve energy metabolism in peripheral organs, but the precise mechanisms underlying the metabolic benefits and harm still need to be fully understood. In the current study, we identified orosomucoid 2 (ORM2) as a liver-secreted hormone (i.e., hepatokine) induced by BAs and investigated its role in BA-induced metabolic improvements in mouse models of diet-induced obesity. Contrary to our expectation, under a high-fat diet (HFD), our Orm2 knockout (Orm2-KO) exhibited a lean phenotype compared with C57BL/6J control, partly due to the increased energy expenditure. However, when challenged with a HFD supplemented with cholic acid, Orm2-KO eliminated the antiobesity effect of BAs, indicating that ORM2 governs BA-induced metabolic improvements. Moreover, hepatic ORM2 overexpression partially replicated BA effects by enhancing insulin sensitivity. Mechanistically, ORM2 suppressed interferon-γ/STAT1 activities in inguinal white adipose tissue depots, forming the basis for anti-inflammatory effects of BAs and improving glucose homeostasis. In conclusion, our study provides new insights into the molecular mechanisms of BA-induced liver-adipose cross talk through ORM2 induction.

ARTICLE HIGHLIGHTS

Molecular insights into experimental models and therapeutics for cholestasis

Cholestatic liver disease (CLD) is a range of conditions caused by the accumulation of bile acids (BAs) or disruptions in bile flow, which can harm the liver and bile ducts. To investigate its pathogenesis and treatment, it is essential to establish and assess experimental models of cholestasis, which have significant clinical value. However, owing to the complex pathogenesis of cholestasis, a single modelling method can merely reflect one or a few pathological mechanisms, and each method has its adaptability and limitations. We summarize the existing experimental models of cholestasis, including animal models, gene-knockout models, cell models, and organoid models. We also describe the main types of cholestatic disease simulated clinically. This review provides an overview of targeted therapy used for treating cholestasis based on the current research status of cholestasis models. In addition, we discuss the respective advantages and disadvantages of different models of cholestasis to help establish experimental models that resemble clinical disease conditions. In sum, this review not only outlines the current research with cholestasis models but also projects prospects for clinical treatment, thereby bridging basic research and practical therapeutic applications.

Microbial products linked to steatohepatitis are reduced by deletion of nuclear hormone receptor SHP in mice

Deletion of the nuclear hormone receptor small heterodimer partner (Shp) ameliorates the development of obesity and nonalcoholic steatohepatitis (NASH) in mice. Liver-specific SHP plays a significant role in this amelioration. The gut microbiota has been associated with these metabolic disorders, and the interplay between bile acids (BAs) and gut microbiota contributes to various metabolic disorders. Since hepatic SHP is recognized as a critical regulator in BA synthesis, we assessed the involvement of gut microbiota in the antiobesity and anti-NASH phenotype of Shp-/- mice. Shp deletion significantly altered the levels of a few conjugated BAs. Sequencing the 16S rRNA gene in fecal samples collected from separately housed mice revealed apparent dysbiosis in Shp-/- mice. Cohousing Shp-/- mice with WT mice during a Western diet regimen impaired their metabolic improvement and effectively disrupted their distinctive microbiome structure, which became indistinguishable from that of WT mice. While the Western diet challenge significantly increased lipopolysaccharide and phenylacetic acid (PAA) levels in the blood of WT mice, their levels were not increased in Shp-/- mice. PAA was strongly associated with hepatic peroxisome proliferator-activated receptor gamma isoform 2 (Pparg2) activation in mice, which may represent the basis of the molecular mechanism underlying the association of gut bacteria and hepatic steatosis. Shp deletion reshapes the gut microbiota possibly by altering BAs. While lipopolysaccharide and PAA are the major driving forces derived from gut microbiota for NASH development, Shp deletion decreases these signaling molecules via dysbiosis, thereby partially protecting mice from diet-induced metabolic disorders.

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.

The long and the small collide: LncRNAs and small heterodimer partner (SHP) in liver disease

Long non-coding RNAs (lncRNAs) are a large and diverse class of RNA molecules that are transcribed but not translated into proteins, with a length of more than 200 nucleotides. LncRNAs are involved in gene expression and regulation. The abnormal expression of lncRNAs is associated with disease pathogenesis. Small heterodimer partner (SHP, NR0B2) is a unique orphan nuclear receptor that plays a pivotal role in many biological processes by acting as a transcriptional repressor. In this review, we present the critical roles of SHP and summarize recent findings demonstrating the regulation between lncRNAs and SHP in liver disease.

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.

Phase 0 of the Xenobiotic Response: Nuclear Receptors and Other Transcription Factors as a First Step in Protection from Xenobiotics

This mini-review examines the crucial importance of transcription factors as a first line of defense in the detoxication of xenobiotics. Key transcription factors that recognize xenobiotics or xenobiotic-induced stress such as reactive oxygen species (ROS), include AhR, PXR, CAR, MTF, Nrf2, NF-κB, and AP-1. These transcription factors constitute a significant portion of the pathways induced by toxicants as they regulate phase I-III detoxication enzymes and transporters as well as other protective proteins such as heat shock proteins, chaperones, and anti-oxidants. Because they are often the first line of defense and induce phase I-III metabolism, could these transcription factors be considered the phase 0 of xenobiotic response?

Constitutive Androstane Receptor Differentially Regulates Bile Acid Homeostasis in Mouse Models of Intrahepatic Cholestasis

Bile acid (BA) homeostasis is tightly regulated by multiple transcription factors, including farnesoid X receptor (FXR) and small heterodimer partner (SHP). We previously reported that loss of the FXR/SHP axis causes severe intrahepatic cholestasis, similar to human progressive familial intrahepatic cholestasis type 5 (PFIC5). In this study, we found that constitutive androstane receptor (CAR) is endogenously activated in Fxr:Shp double knockout (DKO) mice. To test the hypothesis that CAR activation protects DKO mice from further liver damage, we generated Fxr;Shp;Car triple knockout (TKO) mice. In TKO mice, residual adenosine triphosphate (ATP) binding cassette, subfamily B member 11 (ABCB11; alias bile salt export pump [BSEP]) function and fecal BA excretion are completely impaired, resulting in severe hepatic and biliary damage due to excess BA overload. In addition, we discovered that pharmacologic CAR activation has different effects on intrahepatic cholestasis of different etiologies. In DKO mice, CAR agonist 1,4‐bis[2‐(3,5‐dichloropyridyloxy)]benzene (TCPOBOP; here on TC) treatment attenuated cholestatic liver injury, as expected. However, in the PFIC2 model Bsep knockout (BKO) mice, TC treatment exhibited opposite effects that reflect increased BA accumulation and liver injury. These contrasting results may be linked to differential regulation of systemic cholesterol homeostasis in DKO and BKO livers. TC treatment selectively up‐regulated hepatic cholesterol levels in BKO mice, supporting de novo BA synthesis. Conclusion: CAR activation in DKO mice is generally protective against cholestatic liver injury in these mice, which model PFIC5, but not in the PFIC2 model BKO mice. Our results emphasize the importance of the genetic and physiologic background when implementing targeted therapies to treat intrahepatic cholestasis.

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.