Farnesoid X receptor alleviates age-related proliferation defects in regenerating mouse livers by activating forkhead box m1b transcription

Farnesoid X receptor‑driven metabolic plasticity: Bridging physiological adaptation and malignant transformation in lipid handling (Review)

Metabolic reprogramming represents a hallmark of malignant tumors, manifested through progressive alterations in nutrient utilization patterns during oncogenesis. As fundamental constituents of biological membranes, essential components of signaling pathways, and critical energy substrates, lipids undergo comprehensive metabolic restructuring in neoplastic cells. This lipid remodeling confers enhanced adaptability to sustain uncontrolled proliferation while promoting aggressive migratory phenotypes. Farnesoid X receptor (FXR), a ligand‑activated nuclear receptor responsive to bile acid (BA) derivatives and cholesterol metabolites, orchestrates key aspects of lipid homeostasis. Its regulatory network encompasses cholesterol/BA metabolism, fatty acid (FA) metabolism and plasma lipoprotein trafficking pathways. Emerging evidence positions FXR as a pleiotropic modulator in oncogenesis, with dysregulated expression patterns documented across multiple tumor lineages and premalignant lesions. This mechanistic understanding has propelled FXR‑targeted therapeutics into the forefront of precision oncology development. The present review critically examines the FXR‑lipid axis in lipid‑enriched malignancies, with particular emphasis on its regulatory circuitry governing BA flux and FA turnover.

Mapping of mitogen and metabolic sensitivity in organoids defines requirements for human hepatocyte growth

Mechanisms underlying human hepatocyte growth in development and regeneration are incompletely understood. In vitro, human fetal hepatocytes (FH) can be robustly grown as organoids, while adult primary human hepatocyte (PHH) organoids remain difficult to expand, suggesting different growth requirements between fetal and adult hepatocytes. Here, we characterize hepatocyte organoid outgrowth using temporal transcriptomic and phenotypic approaches. FHs initiate reciprocal transcriptional programs involving increased proliferation and repressed lipid metabolism upon initiation of organoid growth. We exploit these insights to design maturation conditions for FH organoids, resulting in acquisition of mature hepatocyte morphological traits and increased expression of functional markers. During PHH organoid outgrowth in the same culture condition as for FHs, the adult transcriptomes initially mimic the fetal transcriptomic signatures, but PHHs rapidly acquire disbalanced proliferation-lipid metabolism dynamics, resulting in steatosis and halted organoid growth. IL6 supplementation, as emerged from the fetal dataset, and simultaneous activation of the metabolic regulator FXR, prevents steatosis and promotes PHH proliferation, resulting in improved expansion of the derived organoids. Single-cell RNA sequencing analyses reveal preservation of their fetal and adult hepatocyte identities in the respective organoid cultures. Our findings uncover mitogen requirements and metabolic differences determining proliferation of hepatocytes changing from development to adulthood.

Nuclear Receptor-Mediated Hepatomegaly and Liver Regeneration: An Update

Nuclear receptors (NRs), a superfamily of ligand-activated transcription factors, are critical in cell growth, proliferation, differentiation, metabolism, and numerous biologic events. NRs have been reported to play important roles in hepatomegaly (liver enlargement) and liver regeneration by regulating target genes or interacting with other signals. In this review, the roles and involved molecular mechanisms of NRs in hepatomegaly and liver regeneration are summarized and the future perspectives of NRs in the treatment of liver diseases are discussed. SIGNIFICANCE STATEMENT: NRs play critical roles in hepatomegaly and liver regeneration, indicating the potential of NRs as targets to promote liver repair after liver injury. This paper reviews the characteristics and molecular mechanisms of NRs in regulating hepatomegaly and liver regeneration, providing more evidence for NRs in the treatment of related liver diseases.

Liver regeneration biology: Implications for liver tumour therapies

The liver has remarkable regenerative potential, with the capacity to regenerate after 75% hepatectomy in humans and up to 90% hepatectomy in some rodent models, enabling it to meet the challenge of diverse injury types, including physical trauma, infection, inflammatory processes, direct toxicity, and immunological insults. Current understanding of liver regeneration is based largely on animal research, historically in large animals, and more recently in rodents and zebrafish, which provide powerful genetic manipulation experimental tools. Whilst immensely valuable, these models have limitations in extrapolation to the human situation. In vitro models have evolved from 2-dimensional culture to complex 3 dimensional organoids, but also have shortcomings in replicating the complex hepatic micro-anatomical and physiological milieu. The process of liver regeneration is only partially understood and characterized by layers of complexity. Liver regeneration is triggered and controlled by a multitude of mitogens acting in autocrine, paracrine, and endocrine ways, with much redundancy and cross-talk between biochemical pathways. The regenerative response is variable, involving both hypertrophy and true proliferative hyperplasia, which is itself variable, including both cellular phenotypic fidelity and cellular trans-differentiation, according to the type of injury. Complex interactions occur between parenchymal and non-parenchymal cells, and regeneration is affected by the status of the liver parenchyma, with differences between healthy and diseased liver. Finally, the process of termination of liver regeneration is even less well understood than its triggers. The complexity of liver regeneration biology combined with limited understanding has restricted specific clinical interventions to enhance liver regeneration. Moreover, manipulating the fundamental biochemical pathways involved would require cautious assessment, for fear of unintended consequences. Nevertheless, current knowledge provides guiding principles for strategies to optimise liver regeneration potential.

Changes of farnesoid X receptor and Takeda G‑protein coupled receptor 5 following biliary tract external drainage in hemorrhagic shock

Since biliary tract external drainage (BTED) is increasingly used to treat patients with shock, it is necessary to clarify pathophysiological changes following BTED in hemorrhagic shock (HS). The present study aimed to investigate the effect of BTED on farnesoid X receptor (FXR) and Takeda G-protein coupled receptor 5 (TGR-5) expression in HS. A total of 24 Sprague-Dawley rats were randomly allocated to sham, BTED, HS and HS + BTED groups. Rat models of HS were induced by drawing blood from the femoral artery until a mean arterial pressure of 40±5 mmHg was achieved and maintained for 60 min. Rat models of BTED were induced by inserting a catheter into the bile duct. The distal end of the bile duct was ligated, and the catheter was passed through the rat flank to allow external collection of bile. Reverse transcription-quantitative PCR, western blotting and immunohistochemistry were performed to detect changes in expression levels of FXR and TGR-5 in the jejunum, ileum and liver. Expression levels of FXR and TGR-5 increased significantly in jejunum and liver following HS (P<0.05). BTED significantly decreased expression levels of FXR in the liver (P<0.05) and TGR-5 in the jejunum, ileum and liver (P<0.05). In conclusion, expression levels of FXR and TGR-5 increased in HS but BTED decreased expression levels of FXR and TGR-5 in HS.

Nuclear receptors: a bridge linking the gut microbiome and the host

Background

The gut microbiome is the totality of microorganisms, bacteria, viruses, protozoa, and fungi within the gastrointestinal tract. The gut microbiome plays key roles in various physiological and pathological processes through regulating varieties of metabolic factors such as short-chain fatty acids, bile acids and amino acids. Nuclear receptors, as metabolic mediators, act as a series of intermediates between the microbiome and the host and help the microbiome regulate diverse processes in the host. Recently, nuclear receptors such as farnesoid X receptor, peroxisome proliferator-activated receptors, aryl hydrocarbon receptor and vitamin D receptor have been identified as key regulators of the microbiome-host crosstalk. These nuclear receptors regulate metabolic processes, immune activity, autophagy, non-alcoholic and alcoholic fatty liver disease, inflammatory bowel disease, cancer, obesity, and type-2 diabetes.

Conclusion

In this review, we have summarized the functions of the nuclear receptors in the gut microbiome-host axis in different physiological and pathological conditions, indicating that the nuclear receptors may be the good targets for treatment of different diseases through the crosstalk with the gut microbiome.

A modified animal model of hepatic regeneration induced by hilar bile duct ligation

Mechanisms of the proliferation of liver are mainly studied in animal model of liver regeneration after partial hepatectomy (PH). However, the PH model involves complex regeneration mechanisms, including hemodynamic factors, cytokines, growth factors, and metabolites. Among liver metabolites, bile acid (BA) is a key signaling molecule that regulates liver regeneration. This study aimed to establish a new type of rapid liver hyperplasia model induced mainly by bile acid pathway through hepatoenteral circulation with hilar bile duct ligation (HBDL). We first established the HBDL model by ligating the bile duct of all hepatic lobes but the right lateral lobe in rabbits and compared with the PVL model and sham operation group. Changes in the liver lobe and hemodynamics were observed. Liver function and the bile acid level were also analyzed. Then we verified the HBDL model in mice. Liver function and the levels of bile acids and cytokines were tested. The protein and mRNA levels of FXR, FGF15, CYP7A1 and FoxM1b in liver tissue were also analyzed. After hilar ligation of the biliary tract, the unligated liver lobes proliferated significantly. Compared with those in the sham group, the volume and weight of the unligated right lateral lobe of the liver in the HBDL group and the PVL group increased significantly (P < 0.05). Transient liver function impairment occurred both in the HBDL group and PVL group in the rabbit model as well as the mouse models. The bile acid levels in the HBDL groups of the rabbit model and mouse model increased significantly within first week after surgery (P < 0.05). The immunohistochemistry results confirmed the proliferation of hepatocytes in the unligated liver lobe. Compared with those in the sham group, the levels of FXR, FGF15 and FoxM1b in the HBDL group were significantly increased (P < 0.05), while the expression of CYP7A1 was inhibited. Compared with those in the HBDL group, the postoperative hemodynamic changes in the PVL group were significant (P < 0.05). The levels of IL-6 and TNF-α in the HBDL group were higher than those in the sham group. The HBDL model is simple to establish and exhibits good surgical tolerance. The model has definite proliferative effect and strong specificity of bile acid pathway. This is an ideal animal model to study the mechanism of liver regeneration through bile acid pathway.

Aged-related Function Disorder of Liver is Reversed after Exposing to Young Milieu via Conversion of Hepatocyte Ploidy

Previous study showed that senescent hepatocytes from aged liver could be rejuvenated after repopulated in the young recipient liver. The proliferative capacity of hepatocytes was restored with the senescence reversal. However, it is unknown whether metabolic and homeostatic function of aged liver, as well as age-dependent liver steatosis could be rejuvenated or alleviated. Here, we found that senescent hepatocytes from aged liver were rejuvenated after exposing to young blood. An autonomous proliferation of senescent hepatocytes which resulting in ploidy reversal might be the underlying mechanism of senescent reversal. After performing 2/3 partial hepatectomy (2/3PHx) in young blood exposed old liver, delayed DNA synthesis of senescent hepatocytes was rescued and the number of BrdU positive hepatocytes was restored from 4.39±2.30% to 17.85±3.21%, similarly to that in the young mice at 36 hours post 2/3PHx. Moreover, Cyclin A2 and Cyclin E1 overexpression of hepatocytes in aged liver facilitating the G1/S phase transition was contributed to enhance liver regeneration. Furthermore, lipid droplet spread widely in the elderly human liver and old mouse liver, but this aged-associated liver steatosis was alleviated as senescence reversal. Collectively, our study provides new thoughts for effectively preventing age-related liver diseases.

Unaltered Liver Regeneration in Post-Cholestatic Rats Treated with the FXR Agonist Obeticholic Acid

In a previous study, obeticholic acid (OCA) increased liver growth before partial hepatectomy (PHx) in rats through the bile acid receptor farnesoid X-receptor (FXR). In that model, OCA was administered during obstructive cholestasis. However, patients normally undergo PHx several days after biliary drainage. The effects of OCA on liver regeneration were therefore studied in post-cholestatic Wistar rats. Rats underwent sham surgery or reversible bile duct ligation (rBDL), which was relieved after 7 days. PHx was performed one day after restoration of bile flow. Rats received 10 mg/kg OCA per day or were fed vehicle from restoration of bile flow until sacrifice 5 days after PHx. Liver regeneration was comparable between cholestatic and non-cholestatic livers in PHx-subjected rats, which paralleled liver regeneration a human validation cohort. OCA treatment induced ileal Fgf15 mRNA expression but did not enhance post-PHx hepatocyte proliferation through FXR/SHP signaling. OCA treatment neither increased mitosis rates nor recovery of liver weight after PHx but accelerated liver regrowth in rats that had not been subjected to rBDL. OCA did not increase biliary injury. Conclusively, OCA does not induce liver regeneration in post-cholestatic rats and does not exacerbate biliary damage that results from cholestasis. This study challenges the previously reported beneficial effects of OCA in liver regeneration in cholestatic rats.

Farnesoid X receptor: a potential therapeutic target in multiple organs

Farnesoid X receptor (FXR), a member of the nuclear receptor family, is a common receptor found in the intestine and liver, and helps to maintain systemic metabolic homeostasis through regulating bile acid, glucose, lipid metabolism, and energy homeostatsis. In addition, FXR regulates the functions of various organs, such as liver, intestine, kidney, breast, pancreas, cardiovascular system and brain. FXR also plays a key role in regulation of gut-microbiota through mediating the various signaling pathways. Accordingly, FXR has become an attractive therapeutic target in a variety of diseases. This review combines classical and recent research reports to introduce the basic information about FXR and its important roles in various organs of the body.

The Role of FGF19 and MALRD1 in Enterohepatic Bile Acid Signaling

Bile acids are the catabolic end products of cholesterol metabolism that are best known for their role in the digestion of lipids. In the last two decades, extensive investigation has shown bile acids to be important signaling molecules in metabolic processes throughout the body. Bile acids are ligands that can bind to several receptors, including the nuclear receptor farnesoid X receptor (FXR) in ileal enterocytes. FXR activation induces the expression of fibroblast growth factor (FGF) 15/19, a hormone that can modulate bile acid levels, repress gluconeogenesis and lipogenesis, and promote glycogen synthesis. Recent studies have described a novel intestinal protein, MAM and LDL Receptor Class A Domain containing 1 (MALRD1) that positively affects FGF15/19 levels. This signaling pathway presents an exciting target for treating metabolic disease and bile acid-related disorders.

Etiopathogenesis, Challenges and Remedies Associated With Female Genital Tuberculosis: Potential Role of Nuclear Receptors

Extra-pulmonary tuberculosis (EPTB) is recognized mainly as a secondary manifestation of a primary tuberculosis (TB) infection in the lungs contributing to a high incidence of morbidity and mortality. The TB bacilli upon reactivation maneuver from the primary site disseminating to other organs. Diagnosis and treatment of EPTB remains challenging due to the abstruse positioning of the infected organs and the associated invasiveness of sample acquisition as well as misdiagnosis, associated comorbidities, and the inadequacy of biomarkers. Female genital tuberculosis (FGTB) represents the most perilous form of EPTB leading to poor uterine receptivity (UR), recurrent implantation failure and infertility in females. Although the number of TB cases is reducing, FGTB cases are not getting enough attention because of a lack of clinical awareness, nonspecific symptoms, and inappropriate diagnostic measures. This review provides an overview for EPTB, particularly FGTB diagnostics and treatment challenges. We emphasize the need for new therapeutics and highlight the need for the exaction of biomarkers as a point of care diagnostic. Nuclear receptors have reported role in maintaining UR, immune modulation, and TB modulation; therefore, we postulate their role as a therapeutic drug target and biomarker that should be explored in FGTB.

Prolonged oxidative stress and delayed tissue repair exacerbate acetaminophen-induced liver injury in aged mice

The liver gradually loses its regenerative capabilities with aging. However, it remains unknown whether aging affects drug-induced liver injury. Here, we used acetaminophen induced acute liver injury model to compare tissue injury and regeneration of aged mice (>80 weeks old) with young ones (8-10 weeks old). The mortality of aged mice after acetaminophen injury was higher than that of young mice. Transient increase of serum GOT and decrease of reduced glutathione (GSH) were not returned to original levels in aged mice even at 48 hours. In addition, Foxm1b and its targets Ccnd1 and Cdk1 were upregulated in young but not in aged mice after 48 hours. Moreover, an apoptosis-related gene, Cidea, was upregulated specifically in aged livers, which was consistent with increased number of TUNEL⁺ hepatocytes. Unexpectedly, damaged hepatocytes were retained in aged liver tissue, which may be caused by impaired recruitment of macrophages to the damaged area, without increases in Ccl2 after acetaminophen injury. Collectively, prolonged oxidative stress due to delayed recovery of GSH and the retention of damaged hepatocytes may suppress tissue repair and hepatocyte proliferation, resulting in exacerbation of acetaminophen injury in aged mice. Thus, aging is a risk factor conferring susceptibility against drug-induced liver injury.

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.

Transcriptional regulation of microRNA-126a by farnesoid X receptor in vitro and in vivo

OBJECTIVES

Recent research has indicated the microRNA-126a (miR-126a) is an endothelial cell-specific and highly conserved endogenous small non-coding RNA molecule. It contributes to the vascular integrity and angiogenesis, but the molecular regulation mechanism of miR-126a remains unknown.

RESULTS

Herein, quantitative real-time polymerase chain reaction (qRT-PCR) results showed that Farnesoid X Receptor (FXR) activation promoted miR-126a expression in HepG2, LO2, and Hep1-6 cells. Furthermore, FXR was found to transcriptionally regulate the miR-126a by binding to its DR8 site. The binding site of FXR was confirmed on intron 6 or 7 of miR-126a host gene epidermal growth factor-like domain 7 (EGFL7) by luciferase reporter assays, electrophoretic mobility shift assays (EMSAs) and chromatin immunoprecipitation (ChIP) assays.

CONCLUSIONS

All these data collectively suggest that FXR regulates transcripts of miR-126a by binding to DR8 in miR-126a gene promoter. This study may provide a molecular therapeutic target for angiogenic disorders, aging, and liver failure.

Long-term impact and clinical significance of living donor liver transplantation with respect to donor liver restoration and spleen size: A prospective study

This study aimed to evaluate postoperative long-term liver restoration and splenic enlargement and their clinical significance in living donor liver transplantation. One hundred and sixteen donors who had donated livers more than 5 years previously accepted the invitation to participate in this study. The liver restoration rate and the splenic enlargement rate were calculated as the rate with respect to the original volume. The mean liver restoration rate was 0.99 ± 0.12 and older age was associated with a higher incidence for liver restoration rate <0.95 (P = .005), whereas type of donor operation was not. The donors with liver restoration rate <0.95 showed lower serum albumin levels than those with liver restoration rate ≥0.95. The mean splenic enlargement rate was 1.10 ± 0.16. Right lobe donors demonstrated higher splenic enlargement rate (1.14 ± 0.18) than left lobe/lateral segment donors (1.06 ± 0.13). In the donors with splenic enlargement rate ≥1.10, platelet count was not fully restored to the preoperative level. In conclusion, older age increases the risk for incomplete postoperative liver restoration, which may be associated with a decrease in albumin more than 5 years after donation. Right lobe donation poses a risk of splenic enlargement, which is associated with incomplete restoration of platelet count.

A Change in Bile Flow: Looking Beyond Transporter Inhibition in the Development of Drug-induced Cholestasis

BACKGROUND

Drug-induced Liver Injury (DILI) has received increasing attention over the past decades, as it represents the leading cause of drug failure and attrition. One of the most prevalent and severe forms of DILI involves the toxic accumulation of bile acids in the liver, known as Drug-induced Cholestasis (DIC). Traditionally, DIC is studied by exploring the inhibition of hepatic transporters such as Bile Salt Export Pump (BSEP) and multidrug resistance-associated proteins, predominantly through vesicular transport assays. Although this approach has identified numerous drugs that alter bile flow, many DIC drugs do not demonstrate prototypical transporter inhibition, but rather are associated with alternative mechanisms.

METHODS

We undertook a focused literature search on DIC and biliary transporters and analyzed peer-reviewed publications over the past two decades or so.

RESULTS

We have summarized the current perception regarding DIC, biliary transporters, and transcriptional regulation of bile acid homeostasis. A growing body of literature aimed to identify alternative mechanisms in the development of DIC has been evaluated. This review also highlights current in vitro approaches used for prediction of DIC.

CONCLUSION

Efforts have continued to focus on BSEP, as it is the primary route for hepatic biliary clearance. In addition to inhibition, drug-induced BSEP repression or the combination of these two has emerged as important alternative mechanisms leading to DIC. Furthermore, there has been an evolution in the approaches to studying DIC including 3D cell cultures and computational modeling.

Therapeutic targets for liver regeneration after acute severe injury: a preclinical overview

Introduction: Liver transplantation is the only viable treatment with a proven survival benefit for acute liver failure (ALF). Donor organ shortage is, however, a major hurdle; hence, alternative approaches that enable liver regeneration and target acute severe hepatocellular damage are necessary.Areas covered: This article sheds light on therapeutic targets for liver regeneration and considers their therapeutic potential. ALF following extensive hepatocyte damage and small-for-size syndrome (SFSS) are illuminated for the reader while the molecular mechanisms of liver regeneration are assessed in accordance with relevant therapeutic strategies. Furthermore, liver background parameters and predictive biomarkers that might associate with liver regeneration are reviewed.Expert opinion: There are established and novel experimental strategies for liver regeneration to prevent ALF resulting from SFSS. Granulocyte-colony stimulating factor (G-CSF) is a promising agent targeting liver regeneration after acute severe injury. Autophagy and hepatocyte senescence represent attractive new targets for liver regeneration in acute severe hepatic injury. Liver support strategies, including tissue engineering, constitute novel regenerative means; the success of this is dependent on stem cell research advances. However, there is no firm clinical evidence that these supportive strategies may alleviate hepatocellular damage until liver transplantation becomes available or successful self-liver regeneration occurs.

Chronic activation of FXR-induced liver growth with tissue-specific targeting Cyclin D1

The nuclear receptor (FXR) plays essential roles in maintaining bile acid and lipid homeostasis by regulating diverse target genes. And its agonists were promising agents for treating various liver diseases. Nevertheless, the potential side effect of chronic FXR activation by specific agonists is not fully understood. In this study, we investigated the mechanism of FXR agonist WAY-362450 induced liver enlargement during treating liver diseases. We demonstrated that chronic ingestion of WAY-362450 induced liver hypertrophy instead of hyperplasia in mouse. Global transcriptional pattern was also examined in mouse livers after treatment with WAY-362450 by RNA-seq assay. Through GO and KEGG enrichment analyses, we demonstrated that the expression of Cyclin D1 (Ccnd1) among the cell cycle-regulating genes was notably increased in WAY-362450-treated mouse liver. Activation of FXR-induced Ccnd1 expression in hepatocyte in a time-dependent manner in vivo and in vitro. Through bioinformatics analysis and ChIP assay, we identified FXR as a direct transcriptional activator of Ccnd1 through binding to a potential enhancer, which was specifically active in livers. We also found active histone acetylation was essential for Ccnd1 induction by FXR. Thus, our study indicated that activation of FXR-induced harmless liver hypertrophy with spatiotemporal modulation of Ccnd1. With a better understanding of the mechanism of tissue-specific gene regulation by FXR, it is beneficial for development and appropriate application of its specific agonist in preventing hepatic diseases.

Deletion of miR-126a Promotes Hepatic Aging and Inflammation in a Mouse Model of Cholestasis

MicroRNAs (miRNAs) act as regulators of aging at the tissue or organism level or as regulators of cellular senescence. Targeted deletion of miR-126 in mice causes partial embryonic lethality, but its biological function in the liver is still largely unknown. Here, we deleted miR-126a, using the CRISPR/Cas9 system in vitro and in vivo. miR-126a was reduced in the aging livers, and disruption of miR-126a in bone mesenchymal stem cells (BMSCs) induced age-associated telomere shortening, DNA damage responses, and proinflammatory cytokines. Moreover, disruption of miR-126a in mice caused hepatocyte senescence, inflammation, and metabolism deficiency. In addition, disruption of miR-126a via BMSC transplantation aggravated the severity of liver defects induced by cholestasis compared with that in the functional miR-126a BMSC group. Mechanistically, we identified versican (VCAN) as a novel direct miR-126a-5p target that induces telomere shortening, BMSC senescence, and nuclear factor κB (NF-κB) pathway activation. This study identified aging-related reduced expression of miR-126a and promotion of its target VCAN as a key mechanism in the regulation of hepatic metabolic function during aging and hepatic damage by inducing NF-κB pathway activation, DNA repair function disorder, and telomere attrition. The findings indicate that miR-126a may be a drug target for the treatment of hepatic failure.

Novel and emerging therapies for cholestatic liver diseases

While bile acids are important for both digestion and signalling, hydrophobic bile acids can be harmful, especially when in high concentrations. Mechanisms for the protection of cholangiocytes against bile acid cytotoxicity include negative feedback loops via farnesoid X nuclear receptor (FXR) activation, the bicarbonate umbrella, cholehepatic shunting and anti-inflammatory signalling, among others. By altering or overwhelming these defence mechanisms, cholestatic diseases such as primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) can further progress to biliary cirrhosis, end-stage liver disease and death or liver transplantation. While PBC is currently treated with ursodeoxycholic acid (UDCA) and obeticholic acid (OCA), many fail treatment, and we have yet to find an effective therapy for PSC. Novel therapies under evaluation target nuclear and surface receptors including FXR, transmembrane G-protein-coupled receptor 5 (TGR5), peroxisome proliferator-activated receptor (PPAR) and pregnane X receptor (PXR). Modulation of these receptors leads to altered bile composition, decreased cytotoxicity, decreased inflammation and improved metabolism. This review summarizes our current understanding of the role of bile acids in the pathophysiology of cholestatic liver diseases, presents the rationale for already approved medical therapies and discusses novel pharmacologic therapies under investigation.

Farnesoid X Receptor Activation Enhances Transforming Growth Factor β-Induced Epithelial-Mesenchymal Transition in Hepatocellular Carcinoma Cells

Farnesoid X receptor (FXR) is a receptor for bile acids and plays an important role in the regulation of bile acid metabolism in the liver. Although FXR has been shown to affect hepatocarcinogenesis through both direct and indirect mechanisms, potential roles of FXR in epithelial–mesenchymal transition (EMT) in hepatocellular carcinoma (HCC) remain unclear. We examined the effect of several FXR ligands on EMT-related morphological changes in HCC cell lines, such as HuH-7 and Hep3B cells. FXR agonists (chenodeoxycholic acid, GW4064, and obeticholic acid)—but not an antagonist (guggulsterone)—induced actin polymerization and expression of N-cadherin and phosphorylated focal adhesion kinase, although they were less effective than transforming growth factor β (TGF-β). FXR agonist treatment enhanced TGF-β-induced EMT morphologic changes and FXR antagonist inhibited the effect of TGF-β. Thus, FXR activation enhances EMT in HCC and FXR antagonists may be EMT-suppressing drug candidates.

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.

Dose-related liver injury of Geniposide associated with the alteration in bile acid synthesis and transportation

Fructus Gardenia (FG), containing the major active constituent Geniposide, is widely used in China for medicinal purposes. Currently, clinical reports of FG toxicity have not been published, however, animal studies have shown FG or Geniposide can cause hepatotoxicity in rats. We investigated Geniposide-induced hepatic injury in male Sprague-Dawley rats after 3-day intragastric administration of 100 mg/kg or 300 mg/kg Geniposide. Changes in hepatic histomorphology, serum liver enzyme, serum and hepatic bile acid profiles, and hepatic bile acid synthesis and transportation gene expression were measured. The 300 mg/kg Geniposide caused liver injury evidenced by pathological changes and increases in serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) and γ-glutamytransferase (γ-GT). While liver, but not sera, total bile acids (TBAs) were increased 75% by this dose, dominated by increases in taurine-conjugated bile acids (t-CBAs). The 300 mg/kg Geniposide also down-regulated expression of Farnesoid X receptor (FXR), small heterodimer partner (SHP) and bile salt export pump (BSEP). In conclusion, 300 mg/kg Geniposide can induce liver injury with associated changes in bile acid regulating genes, leading to an accumulation of taurine conjugates in the rat liver. Taurocholic acid (TCA), taurochenodeoxycholic acid (TCDCA) as well as tauro-α-muricholic acid (T-α-MCA) are potential markers for Geniposide-induced hepatic damage.

MicroRNA-149* suppresses hepatic inflammatory response through antagonizing STAT3 signaling pathway

Chronic inflammation is increasingly recognized as an important component of tumorigenesis and metabolic diseases. The roles of microRNA149* (miRNA149*) in inflammation remain poorly understood. Here, we demonstrate that miR-149* is a suppressor of STAT3-mediated inflammation. MiR-149*^−/− mice were generated with CRISPR/CAS9 technique. In a lipopolysaccharide (LPS)-induced inflammation model, miR-149*^−/− mice show more severe liver injury and inflammation, compared with wild-type (WT) mice. MiR-149*^−/− mice also displayed elevated messenger RNA (mRNA) levels of interleukin (IL)-6, inducible nitric oxide synthase (iNOS), complement C3 (C3) and IL-4 in response to LPS. Then miR-149* agomir administration is largely able to alleviate the LPS-induced some inflammatory gene expression in WT mouse liver. In vitro, miR-149* mimics inhibited expression of STAT3-meidated inflammatory mediators induced by LPS and suppresses the phosphorylation of STAT3 and its transcription activity in HepG2 cells. These findings identify miR-149* as a negative mediator of inflammation that may serve as an attractive therapeutic tool for immune and inflammatory liver diseases.

Farnesoid X receptor, a novel proto-oncogene in non-small cell lung cancer, promotes tumor growth via directly transactivating CCND1

Farnesoid X receptor (FXR), a nuclear receptor for maintaining bile acid homeostasis, has been recognized as a tumor suppressor in enterohepatic tissues. However, its expression and functional role in non-small cell lung cancer (NSCLC) remain unclear. We report that FXR is significantly increased in NSCLC and that it predicts poor clinical outcomes in NSCLC patients. FXR knockdown in NSCLC cells inhibited in vitro cell proliferation, blocked xenograft growth in nude mice, and delayed the G1/S transition of the cell cycle, whereas ectopic overexpression of FXR promoted NSCLC cell proliferation. Mechanistic analysis demonstrated that FXR could directly bind to an inverted repeat-0 sequence in the CCND1 promoter and activate its transcription. Cyclin D1 overexpression rescued NSCLC cells from the delayed G1/S transition and the impaired cell proliferation induced by FXR knockdown. Importantly, a positive correlation between the expression of FXR and cyclin D1 was confirmed in NSCLC samples, and patients with high expression of both FXR and cyclin D1 had the worst prognosis. In summary, our results suggest that FXR has oncogenic potential in NSCLC development, providing mechanistic insights that could be exploited for both prognostic and therapeutic purposes.

TGR5, Not Only a Metabolic Regulator

G-protein-coupled bile acid receptor, Gpbar1 (TGR5), is a member of G-protein-coupled receptor (GPCR) superfamily. High levels of TGR5 mRNA were detected in several tissues such as small intestine, stomach, liver, lung, especially in placenta and spleen. TGR5 is not only the receptor for bile acids, but also the receptor for multiple selective synthetic agonists such as 6α-ethyl-23(S)-methyl-cholic acid (6-EMCA, INT-777) and a series of 4-benzofuranyloxynicotinamde derivatives to regulate different signaling pathways such as nuclear factor κB (NF-κB), AKT, and extracellular signal-regulated kinases (ERK). TGR5, as a metabolic regulator, is involved in energy homeostasis, bile acid homeostasis, as well as glucose metabolism. More recently, our group and others have extended the functions of TGR5 to more than metabolic regulation, which include inflammatory response, cancer and liver regeneration. These findings highlight TGR5 as a potential drug target for different diseases. This review summarizes the basic information of TGR5 and its new functions.

Farnesoid X Receptor Agonists and Other Bile Acid Signaling Strategies for Treatment of Liver Disease

The intracellular nuclear receptor farnesoid X receptor (FXR) and the transmembrane G protein-coupled receptor 5 (TGR5) respond to bile acids (BAs) by activating transcriptional networks and/or signaling cascades. These cascades affect the expression of a great number of target genes relevant for BA, cholesterol, lipid and carbohydrate metabolism, as well as genes involved in inflammation, fibrosis and carcinogenesis. FXR activation in the liver tissue and beyond, such as the gut-liver axis, kidney and adipose tissue, plays a role in metabolic diseases. These BA receptors activators hold promise to become a new class of drugs to be used in the treatment of chronic liver disease, hepatocellular cancer and extrahepatic inflammatory and metabolic diseases. This review discusses the relevant BA receptors, the new drugs that target BA transport and signaling and their possible applications.

The role of bile salts in liver regeneration

A growing body of evidence has demonstrated that bile salts are important for liver regeneration following partial hepatectomy. The relative bile salt overload after partial liver resection causes activation of bile salt receptors in non-parenchymal (viz. the plasma membrane receptor TGR5) and parenchymal (viz. the intracellular receptor FXR) cells in the liver, thus, providing signals to the regenerative process. Impaired bile salt signaling in mice with genetic deficiency of Tgr5 or Fxr results in delayed liver regeneration after partial hepatectomy, and is accompanied by mortality in case of Fxr knock-out mice. Conversely, compensatory liver re-growth in hepatectomized mice can be stimulated by feeding of bile salts or alisol B 23-acetate, a natural triterpenoid agonist of Fxr. A large number of animal studies underscore the importance of strict maintenance of bile salt homeostasis for proper progression of liver regeneration. Both ileal and hepatic Fxr play a key role in regulation of bile salt homeostasis and, thus, preventing hepatotoxicity caused by excessive levels of bile salts. They further contribute to liver regeneration by induction of mitogenic factors. Agents that target bile salt receptors hold promise as drugs to stimulate liver regeneration in selected patients.

Selective targeting of nuclear receptor FXR by avermectin analogues with therapeutic effects on nonalcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) has become a predictive factor of death from many diseases. Farnesoid X receptor (FXR) is an ideal target for NAFLD drug development due to its crucial roles in lipid metabolism. The aim of this work is to examine the molecular mechanisms and functional roles of FXR modulation by avermectin analogues in regulating metabolic syndromes like NAFLD. We found that among avermectin analogues studied, the analogues that can bind and activate FXR are effective in regulating metabolic parameters tested, including reducing hepatic lipid accumulation, lowering serum cholesterol and glucose levels, and improving insulin sensitivity, in a FXR dependent manner. Mechanistically, the avermectin analogues that interact with FXR exhibited features as partial agonists, with distinctive properties in modulating coregulator recruitment. Structural features critical for avermectin analogues to selectively bind to FXR were also revealed. This study indicated that in addition to antiparasitic activity, avermectin analogues are promising drug candidates to treat metabolism syndrome including NAFLD by directly targeting FXR. Additionally, the structural features that discriminate the selective binding of FXR by avermectin analogues may provide a unique safe approach to design drugs targeting FXR signaling.

FXR agonists as therapeutic agents for non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome and a risk factor for both cardiovascular and hepatic related morbidity and mortality. The increasing prevalence of this disease requires novel therapeutic approaches to prevent disease progression. Farnesoid X receptors are bile acid receptors with roles in lipid, glucose, and energy homeostasis. Synthetic farnesoid X receptor (FXR) agonists have been developed to specifically target these receptors for therapeutic use in NAFLD patients. Here, we present a review of bile acid physiology and how agonism of FXR receptors has been examined in pre-clinical and clinical NAFLD. Early evidence suggests a potential role for synthetic FXR agonists in the management of NAFLD; however, additional studies are needed to clarify their effects on lipid and glucose parameters in humans.

Fibroblast Growth Factor Signaling Controls Liver Size in Mice With Humanized Livers

BACKGROUND & AIMS

The ratio of liver size to body weight (hepatostat) is tightly controlled, but little is known about how the physiologic functions of the liver help determine its size. Livers of mice repopulated with human hepatocytes (humanized livers) grow to larger than normal; the human hepatocytes do not recognize the fibroblast growth factor (FGF)-15 produced by mouse intestine. This results in up-regulation of bile acid synthesis in the human hepatocytes and enlargement of the bile acid pool. We investigated whether abnormal bile acid signaling affects the hepatostat in mice.

METHODS

We crossed Fah(-/-), Rag2(-/-), Il2r(-/-) mice with nonobese diabetic mice to create FRGN mice, whose livers can be fully repopulated with human hepatocytes. We inserted the gene for human FGF19 (ortholog to mouse Fgf15), including regulatory sequences, into the FRGN mice to create FRGN19(+) mice. Livers of FRGN19(+) mice and their FRGN littermates were fully repopulated with human hepatocytes. Liver tissues were collected and bile acid pool sizes and RNA sequences were analyzed and compared with those of mice without humanized livers (controls).

RESULTS

Livers were larger in FRGN mice with humanized livers (13% of body weight), compared with control FRGN mice; they also had much larger bile acid pools and aberrant bile acid signaling. Livers from FRGN19(+) normalized to 7.8% of body weight, and their bile acid pool and signaling more closely resembled that of control FRGN19(+) mice. RNA sequence analysis showed activation of the Hippo pathway, and immunohistochemical and transcription analyses revealed increased hepatocyte proliferation, but not apoptosis, in the enlarged humanized livers of FRGN mice. Cell sorting experiments showed that although healthy human liver does not produce FGF19, nonparenchymal cells from cholestatic livers produce FGF19.

CONCLUSIONS

In mice with humanized livers, expression of an FGF19 transgene corrects bile acid signaling defects, resulting in normalization of bile acid synthesis, the bile acid pool, and liver size. These findings indicate that liver size is, in part, regulated by the size of the bile acid pool that the liver must circulate.

Application of elderly donor for liver transplantation

Recently, much more attention has been paid on application of elderly donor due to the shortage of organs. Although liver quality of elderly donors may be sub-optimal comparing with that from younger donors, primary non-function of a liver graft is a rare event. On the other hand, long-term graft and recipient survival for usage of elderly grafts has become a major concern and focus of research. Many transplant centers have changed the upper limit of donor age from previous 50 to 70 or even 75-year-old and achieved good graft function. Although some scholars believed that liver transplant using elderly grafts was associated with high probability of delayed liver function recovery, graft loss and hepatitis C recurrence, reports from several transplant centers document that long-term survival of grafts and recipients may be significantly improved through certain screening of donors and recipients before transplant. In conclusion, it is very important and relatively safe to use grafts from elderly donors to expand the donor pool. However, elderly donors and corresponding recipients must be carefully selected before transplant. The long-term effect of advanced age on grafts and recipients need to be evaluated through a comprehensive and long-term in-depth observation.

Role and molecular mechanism of farnesoid X receptor in obstructive jaundice

Obstructive jaundice is a common and frequently occurring disease, which can result in multiple organ dysfunction syndrome in serious conditions due to the abnormal accumulation of bile acids in blood. Farnesoid X receptor (FXR), a nuclear receptor for bile acid, plays a significant role in bile acid metabolism. Recent research demonstrates that FXR also participates in the regulation of the pathological and physiological processes during obstructive jaundice. In this article, we review the latest research about the role and molecular mechanism of FXR in obstructive jaundice, in order to explore new methods and strategies for curing the disease.

Nuclear bile acid signaling through the farnesoid X receptor

Bile acids (BAs) are amphipathic molecules produced from cholesterol by the liver. Expelled from the gallbladder upon meal ingestion, BAs serve as fat solubilizers in the intestine. BAs are reabsorbed in the ileum and return via the portal vein to the liver where, together with nutrients, they provide signals to coordinate metabolic responses. BAs act on energy and metabolic homeostasis through the activation of membrane and nuclear receptors, among which the nuclear receptor farnesoid X receptor (FXR) is an important regulator of several metabolic pathways. Highly expressed in the liver and the small intestine, FXR contributes to BA effects on metabolism, inflammation and cell cycle control. The pharmacological modulation of its activity has emerged as a potential therapeutic strategy for liver and metabolic diseases. This review highlights recent advances regarding the mechanisms by which the BA sensor FXR contributes to global signaling effects of BAs, and how FXR activity may be regulated by nutrient-sensitive signaling pathways.

Guo G. Farnesoid X receptor, the bile acid sensing nuclear receptor, in liver regeneration

The liver is unique in regenerative potential, which could recover the lost mass and function after injury from ischemia and resection. The underlying molecular mechanisms of liver regeneration have been extensively studied in the past using the partial hepatectomy (PH) model in rodents, where 2/3 PH is carried out by removing two lobes. The whole process of liver regeneration is complicated, orchestrated event involving a network of connected interactions, which still remain fully elusive. Bile acids (BAs) are ligands of farnesoid X receptor (FXR), a nuclear receptor of ligand-activated transcription factor. FXR has been shown to be highly involved in liver regeneration. BAs and FXR not only interact with each other but also regulate various downstream targets independently during liver regeneration. Moreover, recent findings suggest that tissue-specific FXR also contributes to liver regeneration significantly. These novel findings suggest that FXR has much broader role than regulating BA, cholesterol, lipid and glucose metabolism. Therefore, these researches highlight FXR as an important pharmaceutical target for potential use of FXR ligands to regulate liver regeneration in clinic. This review focuses on the roles of BAs and FXR in liver regeneration and the current underlying molecular mechanisms which contribute to liver regeneration.

Chronic activation of FXR in transgenic mice caused perinatal toxicity and sensitized mice to cholesterol toxicity

The nuclear receptor farnesoid X receptor (FXR) (nuclear receptor subfamily 1, group H, member 4, or NR1H4) is highly expressed in the liver and intestine. Previous reports have suggested beneficial functions of FXR in the homeostasis of bile acids, lipids, and glucose, as well as in promoting liver regeneration and inhibiting carcinogenesis. To investigate the effect of chronic FXR activation in vivo, we generated transgenic mice that conditionally and tissue specifically express the activated form of FXR in the liver and intestine. Unexpectedly, the transgenic mice showed several intriguing phenotypes, including partial neonatal lethality, growth retardation, and spontaneous liver toxicity. The transgenic mice also displayed heightened sensitivity to a high-cholesterol diet-induced hepatotoxicity but resistance to the gallstone formation. The phenotypes were transgene specific, because they were abolished upon treatment with doxycycline to silence the transgene expression. The perinatal toxicity, which can be rescued by a maternal vitamin supplement, may have resulted from vitamin deficiency due to low biliary bile acid output as a consequence of inhibition of bile acid formation. Our results also suggested that the fibroblast growth factor-inducible immediate-early response protein 14 (Fn14), a member of the proinflammatory TNF family, is a FXR-responsive gene. However, the contribution of Fn14 induction in the perinatal toxic phenotype of the transgenic mice remains to be defined. Because FXR is being explored as a therapeutic target, our results suggested that a chronic activation of this nuclear receptor may have an unintended side effect especially during the perinatal stage.

Bile acid signaling and liver regeneration

The liver is able to regenerate itself in response to partial hepatectomy or liver injury. This is accomplished by a complex network of different cell types and signals both inside and outside the liver. Bile acids (BAs) are recently identified as liver-specific metabolic signals and promote liver regeneration by activating their receptors: Farnesoid X Receptor (FXR) and G-protein-coupled BA receptor 1 (GPBAR1, or TGR5). FXR is a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. FXR promotes liver regeneration after 70% partial hepatectomy (PHx) or liver injury. Moreover, activation of FXR is able to alleviate age-related liver regeneration defects. Both liver- and intestine-FXR are activated by BAs after liver resection or injury and promote liver regeneration through distinct mechanism. TGR5 is a membrane-bound BA receptor and it is also activated during liver regeneration. TGR5 regulates BA hydrophobicity and stimulates BA excretion in urine during liver regeneration. BA signaling thus represents a novel metabolic pathway during liver regeneration. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

FXR and liver carcinogenesis

Farnesoid X receptor (FXR) is a member of the nuclear receptor family and a ligand-modulated transcription factor. In the liver, FXR has been considered a multi-functional cell protector and a tumor suppressor. FXR can suppress liver carcinogenesis via different mechanisms: 1) FXR maintains the normal liver metabolism of bile acids, glucose and lipids; 2) FXR promotes liver regeneration and repair after injury; 3) FXR protects liver cells from death and enhances cell survival; 4) FXR suppresses hepatic inflammation, thereby preventing inflammatory damage; and 5) FXR can directly increase the expression of some tumor-suppressor genes and repress the transcription of several oncogenes. However, inflammation and epigenetic silencing are known to decrease FXR expression during tumorigenesis. The reactivation of FXR function in the liver may be a potential therapeutic approach for patients with liver cancer.

Farnesoid X receptor antagonizes JNK signaling pathway in liver carcinogenesis by activating SOD3

The farnesoid X receptor (FXR) is a key metabolic and homeostatic regulator in the liver. In the present work, we identify a novel role of FXR in antagonizing c-Jun N-terminal kinase (JNK) signaling pathway in liver carcinogenesis by activating superoxide dismutase 3 (SOD3) transcription. Compared with wild-type mouse liver, FXR(-/-) mouse liver showed elevated JNK phosphorylation. JNK1 deletion suppressed the increase of diethylnitrosamine-induced tumor number in FXR(-/-) mice. These results suggest that JNK1 plays a key role in chemical-induced liver carcinogenesis in FXR(-/-) mice. We found that ligand-activated FXR was able to alleviate H₂O₂or tetradecanoylphorbol acetate-induced JNK phosphorylation in human hepatoblastoma (HepG2) cells or mouse primary hepatocytes. FXR ligand decreased H₂O₂-induced reactive oxygen species (ROS) levels in wild-type but not FXR(-/-) mouse hepatocytes. FXR knockdown abolished the inhibition of 3-[2-[2-chloro-4-[[3-(2,6-dichlorophenyl)-5-(1-methylethyl)-4-isoxazolyl]methoxy]phenyl]ethenyl]-Benzoic acid (GW4064) on JNK phosphorylation and ROS production induced by H₂O₂in HepG2 cells. The gene expression of SOD3, an antioxidant defense enzyme, was increased by FXR activation in vitro and in vivo. An FXR-responsive element, inverted repeat separated by 1 nucleotide in SOD3 promoter, was identified by a combination of transcriptional reporter assays, EMSAs, and chromatin immunoprecipitation assays, which indicated that SOD3 could be a direct FXR target gene. SOD3 knockdown abolished the inhibition of GW4064 on JNK phosphorylation induced by H₂O₂in HepG2 cells. In summary, FXR may regulate SOD3 expression to suppress ROS production, resulting in decreasing JNK activity. These results suggest that FXR, as a novel JNK suppressor, may be an attractive therapeutic target for liver cancer treatment.

Clustering nuclear receptors in liver regeneration identifies candidate modulators of hepatocyte proliferation and hepatocarcinoma

Background & Aims

Liver regeneration (LR) is a valuable model for studying mechanisms modulating hepatocyte proliferation. Nuclear receptors (NRs) are key players in the control of cellular functions, being ideal modulators of hepatic proliferation and carcinogenesis.

Methods & Results

We used a previously validated RT-qPCR platform to profile modifications in the expression of all 49 members of the NR superfamily in mouse liver during LR. Twenty-nine NR transcripts were significantly modified in their expression during LR, including fatty acid (peroxisome proliferator-activated receptors, PPARs) and oxysterol (liver X receptors, Lxrs) sensors, circadian masters RevErbα and RevErbβ, glucocorticoid receptor (Gr) and constitutive androxane receptor (Car). In order to detect the NRs that better characterize proliferative status vs. proliferating liver, we used the novel Random Forest (RF) analysis to selected a trio of down-regulated NRs (thyroid receptor alpha, Trα; farsenoid X receptor beta, Fxrβ; Pparδ) as best discriminators of the proliferating status. To validate our approach, we further studied PPARδ role in modulating hepatic proliferation. We first confirmed the suppression of PPARδ both in LR and human hepatocellular carcinoma at protein level, and then demonstrated that PPARδ agonist GW501516 reduces the proliferative potential of hepatoma cells.

Conclusions

Our data suggest that NR transcriptome is modulated in proliferating liver and is a source of biomarkers and bona fide pharmacological targets for the management of liver disease affecting hepatocyte proliferation.

Non-viral FoxM1 gene delivery to hepatocytes enhances liver repopulation

Hepatocyte transplantation as a substitute strategy of orthotopic liver transplantation is being studied for treating end-stage liver diseases. Several technical hurdles must be overcome in order to achieve the therapeutic liver repopulation, such as the problem of insufficient expansion of the transplanted hepatocytes in recipient livers. In this study, we analyzed the application of FoxM1, a cell-cycle regulator, to enhance the proliferation capacity of hepatocytes. The non-viral sleeping beauty (SB) transposon vector carrying FoxM1 gene was constructed for delivering FoxM1 into the hepatocytes. The proliferation capacities of hepatocytes with FoxM1 expression were examined both in vivo and in vitro. Results indicated that the hepatocytes with FoxM1 expression had a higher proliferation rate than wild-type (WT) hepatocytes in vitro. In comparison with WT hepatocytes, the hepatocytes with FoxM1 expression had an enhanced level of liver repopulation in the recipient livers at both sub-acute injury (fumaryl acetoacetate hydrolase (Fah)^–/– mice model) and acute injury (2/3 partial hepatectomy mice model). Importantly, there was no increased risk of tumorigenicity with FoxM1 expression in recipients even after serial transplantation. In conclusion, expression of FoxM1 in hepatocytes enhanced the capacity of liver repopulation without inducing tumorigenesis. FoxM1 gene delivered by non-viral SB vector into hepatocytes may be a viable approach to promote therapeutic repopulation after hepatocyte transplantation.

Identification of trisubstituted-pyrazol carboxamide analogs as novel and potent antagonists of farnesoid X receptor

Farnesoid X receptor (FXR, NRIH4) plays a major role in the control of cholesterol metabolism. This suggests that antagonizing the transcriptional activity of FXR is a potential means to treat cholestasis and related metabolic disorders. Here we describe the synthesis, biological evaluation, and structure-activity relationship (SAR) studies of trisubstituted-pyrazol carboxamides as novel and potent FXR antagonists. One of these novel FXR antagonists, 4j has an IC50 of 7.5 nM in an FXR binding assay and 468.5 nM in a cell-based FXR antagonistic assay. Compound 4j has no detectable FXR agonistic activity or cytotoxicity. Notably, 4j is the most potent FXR antagonist identified to date; it has a promising in vitro profile and could serve as an excellent chemical tool to elucidate the biological function of FXR.

Role of nuclear receptor FXR in liver regeneration

Liver regeneration is a practical compensatory re-growth in response to the loss of hepatic tissue. The mechanism of liver regeneration is very complex and many cytokines, transcription factors and signaling pathways are involved in this process. The farnesoid X receptor (FXR) is a member of metabolic nuclear receptors of intracellular ligand-activated transcription factors and plays an important role in metabolism of bile acids, lipid and glucose. In addition, it has been recently reported that FXR is crucial for liver regeneration. FXR activation directly promotes liver regeneration by regulating hepatocyte proliferation and regulates synthesis and transport of bile acids to prevent the liver from injury by increased bile acids after hepatectomy. The metabolic regulation of FXR is beneficial to liver regeneration. This review focuses on the mechanism of FXR regulation of liver regeneration and targeted drugs.

Poly(ADP-ribose) polymerase 1 promotes oxidative-stress-induced liver cell death via suppressing farnesoid X receptor α

Farnesoid X receptor α (FXR) is highly expressed in the liver and regulates the expression of various genes involved in liver repair. In this study, we demonstrated that activated poly(ADP-ribose) polymerase 1 (PARP1) promoted hepatic cell death by inhibiting the expression of FXR-dependent hepatoprotective genes. PARP1 could bind to and poly(ADP-ribosyl)ate FXR. Poly(ADP-ribosyl)ation dissociated FXR from the FXR response element (FXRE), present in the promoters of target genes, and suppressed FXR-mediated gene transcription. Moreover, treatment with a FXR agonist attenuated poly(ADP-ribosyl)ation of FXR and promoted FXR-dependent gene expression. We further established the CCl4-induced acute liver injury model in wild-type and FXR-knockout mice and identified an essential role of FXR poly(ADP-ribosyl)ation in CCl4-induced liver injury. Thus, our results identified poly(ADP-ribosyl)ation of FXR by PARP1 as a key step in oxidative-stress-induced hepatic cell death. The molecular association between PARP1 and FXR provides new insight into the mechanism, suggesting that inhibition of PARP1 could prevent liver injury.

Development of time resolved fluorescence resonance energy transfer-based assay for FXR antagonist discovery

FXR (farnesoid X receptor, NRIH4), a nuclear receptor, plays a major role in the control of cholesterol metabolism. FXR ligands have been investigated in preclinical studies for targeted therapy against metabolic diseases, but have shown limitations. Therefore, there is a need for new agonist or antagonist ligands of FXR, both for potential clinical applications, as well as to further elucidate its biological functions. Here we describe the use of the X-ray crystal structure of FXR complexed with the potent small molecule agonist GW4064 to design and synthesize a novel fluorescent, high-affinity probe (DY246) for time resolved fluorescence resonance energy transfer (TR-FRET) assays. We then used the TR-FRET assay for high throughput screening of a library of over 5000 bioactive compounds. From this library, we identified 13 compounds that act as putative FXR transcriptional antagonists.

Nuclear receptors as drug targets in cholestatic liver diseases

Cholestatic liver diseases encompass a wide spectrum of disorders with different causes, resulting in impaired bile flow and accumulation of bile acids and other potentially hepatotoxic cholephils. The understanding of the molecular mechanisms of bile formation and cholestasis has recently improved significantly through new insights into nuclear receptor (patho)biology. Nuclear receptors are ligand-activated transcription factors, which act as central players in the regulation of genes responsible for elimination and detoxification of biliary constituents accumulating in cholestasis. They also control other pathophysiologic processes such as inflammation, fibrogenesis, and carcinogenesis involved in the pathogenesis and disease progression of cholestasis liver diseases.

Deficiency of G-protein-coupled bile acid receptor Gpbar1 (TGR5) enhances chemically induced liver carcinogenesis

Gpbar1 (TGR5), a membrane-bound bile acid receptor, is well known for its roles in regulation of energy homeostasis and glucose metabolism. TGR5 activation also inhibits nuclear factor κB (NF-κB)-mediated inflammation. Here we show that TGR5 deficiency enhances chemically induced liver carcinogenesis, and that TGR5 is a negative regulator of signal transducer and activator of transcription 3 (STAT3) signaling. Mice lacking TGR5 were much more susceptible to diethylnitrosamine (DEN)-induced acute liver injury and liver carcinogenesis than wildtype (WT) mice. Consistent with the increasing incidence of liver cancer in TGR5(-/-) mice, hepatocyte death, compensatory proliferation, and gene expression of certain inflammatory cytokines and matrix metalloproteinases were more sensitive to DEN induction in the absence of TGR5 signaling. In vitro, TGR5 activation greatly inhibited proliferation and migration of human liver cancer cells. We then found that TGR5 activation strongly suppressed STAT3 signaling in vitro and in vivo. Furthermore, we observed that TGR5 antagonizes the STAT3 pathway through suppressing STAT3 phosphorylation, its transcription activity, and DNA binding activity, which suggests that TGR5 antagonizes liver tumorigenesis at least in part by inhibiting STAT3 signaling.

CONCLUSION

These findings identify TGR5 as a novel liver tumor suppressor that may serve as an attractive therapeutic tool for human liver cancer.

Promotion of liver regeneration/repair by farnesoid X receptor in both liver and intestine in mice

Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily and is the primary bile acid receptor. We previously showed that FXR was required for the promotion of liver regeneration/repair after physical resection or liver injury. However, the mechanism by which FXR promotes liver regeneration/repair is still unclear. Here we show that both hepatic-FXR and intestine-FXR contributed to promote liver regeneration/repair after either 70% partial hepatectomy or carbon tetrachloride-induced liver injury. Hepatic FXR, but not intestine FXR, is required for the induction of Foxm1b gene expression in liver during liver regeneration/repair. In contrast, intestine FXR is activated to induce FGF15 expression in intestine after liver damage. Ectopic expression of FGF15 was able to rescue the defective liver regeneration/repair in intestine-specific FXR null mice.

CONCLUSION

These results demonstrate that, in addition to the cell-autonomous effect of hepatic FXR, the endocrine FGF15 pathway activated by FXR in intestine also participates in the promotion of liver regeneration/repair.

Downregulation of nuclear receptor FXR is associated with multiple malignant clinicopathological characteristics in human hepatocellular carcinoma

The nuclear receptor farnesoid X receptor (FXR) acts as a liver protector by regulating normal liver homeostasis. Spontaneously developed liver tumors have been found in FXR-null mice. However, the role of FXR in the tumorigenesis of human hepatocellular carcinoma (HCC) is still poorly understood. In this study, we measured the expression of FXR and its primary target gene, small heterodimer partner, and analyzed the clinical significance of FXR expression in HCC patients. A lentiviral vector that selectively overexpresses FXR was used to investigate the function of FXR in HCC cell proliferation both in vitro and in vivo. Our data showed that in human HCC, FXR expression was significantly reduced and was positively correlated with multiple malignant clinicopathological characteristics. Lentivirus-mediated exogenous FXR expression resulted in a marked increase of small heterodimer partner expression, significant repression of liver cancer cell proliferation, and tumor growth in nude mice. These results suggest that FXR may be of clinical and pharmacological importance as a promising biomarker of HCC.