Targeting nuclear receptors for the treatment of fatty liver disease

Triggering and resolution of inflammation in NASH

Nonalcoholic steatohepatitis (NASH) is considered the progressive form of nonalcoholic fatty liver disease (NAFLD) and is characterized by liver steatosis, inflammation, hepatocellular injury and different degrees of fibrosis. A central issue in this field relates to the identification of those factors that trigger inflammation, thus fuelling the transition from nonalcoholic fatty liver to NASH. These triggers of liver inflammation might have their origins outside the liver (such as in adipose tissue or the gut) as well as inside the organ (for instance, lipotoxicity, innate immune responses, cell death pathways, mitochondrial dysfunction and endoplasmic reticulum stress), both of which contribute to NASH development. In this Review, we summarize the currently available information on the key upstream triggers of inflammation in NASH. We further delineate the mechanisms by which liver inflammation is resolved and the implications of a defective pro-resolution process. A better knowledge of these mechanisms should help to design targeted therapies able to halt or reverse disease progression.

Fat nucleosome: Role of lipids on chromatin

Structural changes in chromatin regulate gene expression and define phenotypic outcomes. Molecules that bind to the nucleosome, the complex of DNA and histone proteins, are key modulators of chromatin structure. Most recently, the formation of condensed chromatin regions based on phase-separation in the cell, a basic physical mechanism, was proposed. Increased understanding of the mechanisms of interaction between chromatin and lipids suggest that small lipid molecules, such as cholesterol and short-chain fatty acids, can regulate important nuclear functions. New biophysical data has suggested that cholesterol interacts with nucleosome through multiple binding sites and affects chromatin structure in vitro. Regardless of the mechanism of how lipids bind to chromatin, there is currently little awareness that lipids may be stored in chromatin and influence its state. Focusing on lipids that bind to nuclear receptors, clinically relevant transcription factors, we discuss the potential interactions of the nucleosome with steroid hormones, bile acids and fatty acids, which suggest that other lipid chemotypes may also impact chromatin structure through binding to common sites on the nucleosome. Herein, we review the main impacts of lipids on the nuclear environment, emphasizing its role on chromatin architecture. We postulate that lipids that bind to nucleosomes and affect chromatin states are likely to be worth investigating as tools to modify disease phenotypes at a molecular level.

Mild drinking habit is a risk factor for hepatocarcinogenesis in non-alcoholic fatty liver disease with advanced fibrosis

AIM

The impact of mild drinking habit (less than 20 g/d of ethanol) on the clinical course of non-alcoholic fatty liver disease (NAFLD) has not been determined. We examined the influence of a mild drinking habit on liver carcinogenesis from NAFLD.

METHODS

A total of 301 patients who had been diagnosed as having NAFLD by liver biopsy between 2003 and 2016 [median age: 56 years, 45% male, 56% with non-alcoholic steatohepatitis, 26% with advanced fibrosis (F3-4)] were divided into the mild drinking group with ethanol consumption of less than 20 g/d (mild drinking group, n = 93) and the non-drinking group (n = 208). Clinicopathological features at the time of liver biopsy and factors related to hepatocellular carcinoma (HCC) occurrence were compared between the groups.

RESULTS

We observed significant differences in male prevalence (P = 0.01), platelet count (P = 0.04), and gamma-glutamyl transpeptidase (P = 0.02) between the test groups. Over 6 years of observation, the HCC appearance rate was significantly higher in the mild drinking group (6.5% vs 1.4%, P = 0.02). Multivariate survival analysis using Cox's regression model revealed that hepatic advanced fibrosis (F3-4) (P < 0.01, risk ratio: 11.60), diabetes mellitus (P < 0.01, risk ratio: 89.50), and serum triglyceride (P = 0.04, risk ratio: 0.98) were factors significantly related to HCC in all NAFLD patients, while the effect of a drinking habit was marginal (P = 0.07, risk ratio: 4.43). In patients with advanced fibrosis (F3-4), however, a drinking habit (P = 0.04, risk ratio: 4.83), alpha-fetoprotein (P = 0.01, risk ratio: 1.23), and diabetes mellitus (P = 0.03, risk ratio: 12.00) were identified as significant contributors to HCC occurrence.

CONCLUSION

A mild drinking habit appears to be a risk factor for hepatocarcinogenesis in NAFLD patients, especially those with advanced fibrosis.

Role of the pregnane X receptor in binge ethanol-induced steatosis and hepatotoxicity

The pregnane X receptor (PXR, NR1I2) is a xenobiotic-sensing nuclear receptor that defends against toxic agents. We have shown that PXR promotes chronic ethanol (EtOH)-induced steatosis. Therefore, we examined the role of PXR in binge EtOH-induced hepatotoxicity. Male wild type (WT) and Pxr-null mice were orally administered three binge doses of EtOH (4.5 g/kg, every 12 hours) and euthanized four hours after the final dose. Pxr-null mice displayed higher basal mRNA levels of hepatic lipogenic transcription factor sterol regulatory element binding protein 1c (Srebp-1c) and its target stearoyl-CoA desaturase 1 (Scd1) and the lipid peroxide detoxifying aldo-keto reductase 1b7 (Akr1b7) and higher protein levels of EtOH-metabolizing alcohol dehydrogenase 1 (ADH1). In both genotypes, binge EtOH-induced triglyceride accumulation was associated with inhibition of fatty acid β-oxidation and upregulation of Srebp-1c- regulated lipogenic genes and hepatic CYP2E1 protein. Unexpectedly, gene expression of Cyp2b10, a constitutive androstane receptor target gene, implicated in EtOH hepatotoxicity, was PXR-dependent upregulated by binge EtOH. Also, PXR-dependent was the binge EtOH-induced inhibition of hepatic Akr1b8 mRNA, and protein levels of aldehyde dehydrogenase (ALDH) 1A1 and anti-apoptotic Bcl-2, but increased pro-apoptotic Bax protein expression, leading to increases in residual EtOH concentration and the cellular oxidative stress marker, malondialdehyde. In contrast, Pxr-null mice displayed increased Akr1b7 gene and ADH1 protein expression and hypertriglyceridemia following binge EtOH exposure. Taken together, this study demonstrates that PXR ablation prevents EtOH induced upregulation of Cyp2b10 and that PXR potentiates binge EtOH-induced oxidative stress and inhibition of EtOH catabolism, but protects against alcoholic hyperlipidemia.

Mechanisms of Action of Dehydroepiandrosterone

Dehydroepiandrosterone (3β-hydroxy-5-androsten-17-one, DHEA) and its sulfated metabolite DHEA-S are the most abundant steroids in circulation and decline with age. Rodent studies have shown that DHEA has a wide variety of effects on liver, kidney, adipose, reproductive tissues, and central nervous system/neuronal function. The mechanisms by which DHEA and DHEA-S impart their physiological effects may be direct actions on plasma membrane receptors, including a DHEA-specific, G-protein-coupled receptor in endothelial cells; various neuroreceptors, e.g., aminobutyric-acid-type A, N-methyl-d-aspartate (NMDA), and sigma-1 (S1R) receptors; by binding steroid receptors: androgen and estrogen receptors (ARs, ERα, or ERβ); or by their metabolism to more potent sex steroid hormones, e.g., testosterone, dihydrotestosterone, and estradiol, which bind with higher affinity to ARs and ERs. DHEA inhibits voltage-gated T-type calcium channels. DHEA activates peroxisome proliferator-activated receptor (PPARα) and CAR by a mechanism apparently involving PP2A, a protein phosphatase dephosphorylating PPARα and CAR to activate their transcriptional activity. We review our recent study showing DHEA activated GPER1 (G-protein-coupled estrogen receptor 1) in HepG2 cells to stimulate miR-21 transcription. This chapter reviews some of the physiological, biochemical, and molecular mechanisms of DHEA and DHEA-S activity.

Serum autotaxin levels are correlated with hepatic fibrosis and ballooning in patients with non-alcoholic fatty liver disease

AIM

To examine the relationship between serum autotaxin (ATX) concentrations and clinicopathological findings in non-alcoholic fatty liver disease (NAFLD) patients.

METHODS

One hundred eighty-six NAFLD patients who had undergone liver biopsy between 2008 and 2017 were retrospectively enrolled. Serum samples were collected at the time of biopsy and ATX was measured by enzyme immunoassays. Sera obtained from 160 healthy, non-obese individuals were used as controls. Histological findings were graded according to an NAFLD scoring system and correlations with serum ATX were calculated by Spearman's test. Diagnostic accuracy was evaluated using the area under the receiver operating characteristic curve (AUC). Cut-off values were identified by the Youden index, and the nearest clinically applicable value to the cutoff was considered the optimal threshold for clinical convenience.

RESULTS

Serum ATX levels were significantly higher in NAFLD patients than in controls (0.86 mg/L vs 0.76 mg/L, P < 0.001) and correlated significantly with ballooning score and fibrosis stage (r = 0.36, P < 0.001 and r = 0.45, P < 0.001, respectively). Such tendencies were stronger in female patients. There were no remarkable relationships between ATX and serum alanine aminotransferase, lipid profiles, or steatosis scores. The AUC values of ATX for predicting the presence of fibrosis (≥ F1), significant fibrosis (≥ F2), severe fibrosis (≥ F3), and cirrhosis (F4), were all more than 0.70 in respective analyses.

CONCLUSION

Serum ATX levels may at least partially reflect histological severity in NAFLD.

Nonalcoholic fatty liver disease, cholesterol gallstones, and cholecystectomy: new insights on a complex relationship

PURPOSE OF REVIEW

Gallstone disease (GSD) and nonalcoholic fatty liver disease (NAFLD often coexist in a given patient and both conditions are associated to obesity and insulin resistance. The relationship between GSD and NAFLD is complex and bidirectional. In the present review, we summarize the existing information on the complex link between GSD and NAFLD and the potential implications for patient care.

RECENT FINDINGS

Several clinical studies and systematic reviews have addressed the association between NAFLD and GSD underscoring that NAFLD is an independent risk factor for GSD. Conversely, GSD has been found also to be an independent risk factor for NAFLD with GSD potentially being linked to greater disease severity. In addition to the data showing association of NAFLD and GSD, recent evidence has also showed that cholecystectomy may itself be a risk factor for NAFLD development. The complex and bidirectional relationship between these diseases is partially explained by a number of common pathogenic links but the precise underlying mechanisms of the association of GSD and NAFLD need to be better delineated. Also, although the mechanisms of the promotional effect of cholecystectomy on NAFLD development are unknown, recent findings unveiling new aspects of gallbladder physiology and endocrine actions of bile acids provide a framework to advance research in this field.

SUMMARY

In this review, we address the different aspects of the complex association between NAFLD and GSD. The potential underlying mechanisms and recent information on endocrine actions of bile acids and the gallbladder are reviewed.

Liver X Receptor Inverse Agonist SR9243 Suppresses Nonalcoholic Steatohepatitis Intrahepatic Inflammation and Fibrosis

Abnormal metabolism of cholesterol may be a contributing factor in nonalcoholic steatohepatitis (NASH) pathogenesis. Accumulating evidence has shown that liver X receptor (LXR) is closely related to intrahepatic inflammation and fibrosis. In this study, we evaluated the effects of a novel liver-specific LXR inverse agonist, SR9243, on antifibrosis in NASH mice. A high-cholesterol diet was employed to induce NASH in BALB/c mice by either carbon tetrachloride (CCL4) administration or bile-duct ligation (BDL). Once NASH was induced, mice were treated with SR9243 for one month by intraperitoneal (i.p.) injection. Liver tissues were collected to determine the degree of fibrosis and intrahepatic inflammation via pathological examination and QPCR; serum was collected to analyze the plasma lipid levels and liver function by clinical biochemistry. The mice developed hepatic steatosis, severe hepatic inflammation, and fibrosis by BDL or CCL4. Treatment with SR9243 significantly reduced the severity of hepatic inflammation and ameliorated hepatic fibrosis; simultaneously, body weight, serum glucose, and plasma lipid levels were controlled effectively. Our data demonstrate that SR9243 exerts an antifibrotic and anti-inflammatory effect in NASH mice; hence these findings highly suggest that LXR inverse agonist could be therapeutically important in NASH treatment.

Novel alternative splicing variants of <i>ACOX1</i> and their differential expression patterns in goats

Abstract. As the first and rate-limiting enzyme of the peroxisomal β-oxidation pathway, acyl-coenzyme A oxidase 1 (ACOX1), which is regulated by peroxisome proliferator-activated alfa (PPARα), is vital for fatty acid oxidation and deposition, especially in the lipid metabolism of very long-chain fatty acids. Alternative splicing events of ACOX1 have been detected in rodents, Nile tilapia, zebra fish and humans but not in goats. Herein, we identified a novel splice variant of the ACOX1 gene, which was designated as ACOX1-SV1, in addition to the complete transcript, ACOX1, in goats. The length of the ACOX1-SV1 coding sequence was 1983 bp, which presented a novel exon 2 variation owing to alternative 5′-splice site selection in exon 2 and partial intron 1, compared to that in ACOX1. The protein sequence analysis indicated that ACOX1-SV1 was conserved across different species. Reverse-transcription quantitative real-time polymerase chain reaction (RT-qPCR) analysis showed that these two isoforms were expressed spatially and differently in different tissue types. ACOX1 and ACOX1-SV1 were expressed at high levels in liver, spleen, brain and adipose tissue in kid goats, and they were abundantly expressed in the fat, liver and spleen of adults. Interestingly, whether in kids or in adults, in fat, the mRNA level of ACOX1 was considerably higher than that of ACOX1-SV1. In contrast, in the liver, the expression of ACOX1-SV1 was considerably higher than that of ACOX1. This differential expression patterns showed the existence of a tissue-dependent splice regulation. These novel findings for ACOX1 should provide new insights for further studies on the function of ACOX1 and its variants that should aid in the breeding of goats with improved meat quality.

Metabolic and hepatic effects of liraglutide, obeticholic acid and elafibranor in diet-induced obese mouse models of biopsy-confirmed nonalcoholic steatohepatitis

AIM

To evaluate the pharmacodynamics of compounds in clinical development for nonalcoholic steatohepatitis (NASH) in obese mouse models of biopsy-confirmed NASH.

METHODS

Male wild-type C57BL/6J mice (DIO-NASH) and Lep^ob/ob (ob/ob-NASH) mice were fed a diet high in trans-fat (40%), fructose (20%) and cholesterol (2%) for 30 and 21 wk, respectively. Prior to treatment, all mice underwent liver biopsy for confirmation and stratification of liver steatosis and fibrosis, using the nonalcoholic fatty liver disease activity score (NAS) and fibrosis staging system. The mice were kept on the diet and received vehicle, liraglutide (0.2 mg/kg, SC, BID), obeticholic acid (OCA, 30 mg/kg PO, QD), or elafibranor (30 mg/kg PO, QD) for eight weeks. Within-subject comparisons were performed on changes in steatosis, inflammation, ballooning degeneration, and fibrosis scores. In addition, compound effects were evaluated by quantitative liver histology, including percent fractional area of liver fat, galectin-3, and collagen 1a1.

RESULTS

Liraglutide and elafibranor, but not OCA, reduced body weight in both models. Liraglutide improved steatosis scores in DIO-NASH mice only. Elafibranor and OCA reduced histopathological scores of hepatic steatosis and inflammation in both models, but only elafibranor reduced fibrosis severity. Liraglutide and OCA reduced total liver fat, collagen 1a1, and galectin-3 content, driven by significant reductions in liver weight. The individual drug effects on NASH histological endpoints were supported by global gene expression (RNA sequencing) and liver lipid biochemistry.

CONCLUSION

DIO-NASH and ob/ob-NASH mouse models show distinct treatment effects of liraglutide, OCA, and elafibranor, being in general agreement with corresponding findings in clinical trials for NASH. The present data therefore further supports the clinical translatability and utility of DIO-NASH and ob/ob-NASH mouse models of NASH for probing the therapeutic efficacy of compounds in preclinical drug development for NASH.

Circular RNAs in Metabolic Diseases

Metabolic diseases include diabetes mellitus (DM), obesity, metabolic syndrome, and non-alcoholic fatty liver disease (NAFLD). Circular RNA is a new type of RNA that is different from traditional linear RNA and has a closed loop structure. However, the function of circular RNA is not yet well elucidated in metabolic diseases. Only a few studies have reported about the relationship between circular RNA and metabolic diseases such as DM and NAFLD. This chapter presents a brief review of epidemiology, pathophysiology, or treatment of DM and NAFLD and then discusses the relationship between circular RNA and DM or NAFLD. Besides, this chapter further provides an updated discussion of the most relevant discoveries regarding circular RNA and their potential applications in molecular diagnostics, nucleic acid therapy, and biomarkers.

Hepatic regulation of VLDL receptor by PPARβ/δ and FGF21 modulates non-alcoholic fatty liver disease

Objective

The very low-density lipoprotein receptor (VLDLR) plays an important role in the development of hepatic steatosis. In this study, we investigated the role of Peroxisome Proliferator-Activated Receptor (PPAR)β/δ and fibroblast growth factor 21 (FGF21) in hepatic VLDLR regulation.

Methods

Studies were conducted in wild-type and Pparβ/δ-null mice, primary mouse hepatocytes, human Huh-7 hepatocytes, and liver biopsies from control subjects and patients with moderate and severe hepatic steatosis.

Results

Increased VLDLR levels were observed in liver of Pparβ/δ-null mice and in Pparβ/δ-knocked down mouse primary hepatocytes through mechanisms involving the heme-regulated eukaryotic translation initiation factor 2α (eIF2α) kinase (HRI), activating transcription factor (ATF) 4 and the oxidative stress-induced nuclear factor (erythroid-derived 2)-like 2 (Nrf2) pathways. Moreover, by using a neutralizing antibody against FGF21, Fgf21-null mice and by treating mice with recombinant FGF21, we show that FGF21 may protect against hepatic steatosis by attenuating endoplasmic reticulum (ER) stress-induced VLDLR upregulation. Finally, in liver biopsies from patients with moderate and severe hepatic steatosis, we observed an increase in VLDLR levels that was accompanied by a reduction in PPARβ/δ mRNA abundance and DNA-binding activity compared with control subjects.

Conclusions

Overall, these findings provide new mechanisms by which PPARβ/δ and FGF21 regulate VLDLR levels and influence hepatic steatosis development.

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PPARβ/δ deficiency leads to increased levels of hepatic VLDLR levels.

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FGF21 protects against hepatic steatosis by attenuating ER stress-induced VLDLR upregulation.

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Human hepatic steatosis is accompanied by increased levels of VLDLR and reduced expression of PPARβ/δ.

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PPARβ/δ and FGF21 may influence NAFLD development by regulating VLDLR levels.

Pregnane X receptor promotes ethanol-induced hepatosteatosis in mice

The pregnane X receptor (PXR, NR1I2) is a xenobiotic-sensing nuclear receptor that modulates the metabolic response to drugs and toxic agents. Both PXR activation and deficiency promote hepatic triglyceride accumulation, a hallmark feature of alcoholic liver disease. However, the molecular mechanism of PXR-mediated activation of ethanol (EtOH)-induced steatosis is unclear. Here, using male wildtype (WT) and Pxr-null mice, we examined PXR-mediated regulation of chronic EtOH-induced hepatic lipid accumulation and hepatotoxicity. EtOH ingestion for 8 weeks significantly (1.8-fold) up-regulated Pxr mRNA levels in WT mice. The EtOH exposure also increased mRNAs encoding hepatic constitutive androstane receptor (3-fold) and its target, Cyp2b10 (220-fold), in a PXR-dependent manner. Furthermore, WT mice had higher serum EtOH levels and developed hepatic steatosis characterized by micro- and macrovesicular lipid accumulation. Consistent with the development of steatosis, lipogenic gene induction was significantly increased in WT mice, including sterol regulatory element-binding protein 1c target gene fatty-acid synthase (3.0-fold), early growth response-1 (3.2-fold), and TNFα (3.0-fold), whereas the expression of peroxisome proliferator-activated receptor α target genes was suppressed. Of note, PXR deficiency suppressed these changes and steatosis. Protein levels, but not mRNAs levels, of EtOH-metabolizing enzymes, including alcohol dehydrogenase 1, aldehyde dehydrogenase 1A1, and catalase, as well as the microsomal triglyceride transfer protein, involved in regulating lipid output were higher in Pxr-null than in WT mice. These findings establish that PXR signaling contributes to ALD development and suggest that PXR antagonists may provide a new approach for ALD therapy.

A polyphenol-rich cranberry extract reverses insulin resistance and hepatic steatosis independently of body weight loss

Objective

Previous studies have reported that polyphenol-rich extracts from various sources can prevent obesity and associated gastro-hepatic and metabolic disorders in diet-induced obese (DIO) mice. However, whether such extracts can reverse obesity-linked metabolic alterations remains unknown. In the present study, we aimed to investigate the potential of a polyphenol-rich extract from cranberry (CE) to reverse obesity and associated metabolic disorders in DIO-mice.

Methods

Mice were pre-fed either a Chow or a High Fat-High Sucrose (HFHS) diet for 13 weeks to induce obesity and then treated either with CE (200 mg/kg, Chow + CE, HFHS + CE) or vehicle (Chow, HFHS) for 8 additional weeks.

Results

CE did not reverse weight gain or fat mass accretion in Chow- or HFHS-fed mice. However, HFHS + CE fully reversed hepatic steatosis and this was linked to upregulation of genes involved in lipid catabolism (e.g., PPARα) and downregulation of several pro-inflammatory genes (eg, COX2, TNFα) in the liver. These findings were associated with improved glucose tolerance and normalization of insulin sensitivity in HFHS + CE mice. The gut microbiota of HFHS + CE mice was characterized by lower Firmicutes to Bacteroidetes ratio and a drastic expansion of Akkermansia muciniphila and, to a lesser extent, of Barnesiella spp, as compared to HFHS controls.

Conclusions

Taken together, our findings demonstrate that CE, without impacting body weight or adiposity, can fully reverse HFHS diet-induced insulin resistance and hepatic steatosis while triggering A. muciniphila blooming in the gut microbiota, thus underscoring the gut-liver axis as a primary target of cranberry polyphenols.

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CE was shown to prevent obesity and its metabolic complications in DIO-mice.

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CE did not reverse obesity but alleviated liver steatosis and glucose intolerance.

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Liver inflammation was blunted in CE-treated mice.

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CE treatment expanded Akkermansia muciniphila and Barnesiella in the gut microbiota.

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CE targets the gut-liver axis to primarily improve glucose homeostasis.

Gut-Liver Axis Derangement in Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is the most frequent type of chronic liver disease in the pediatric age group, paralleling an obesity pandemic. A “multiple-hit” hypothesis has been invoked to explain its pathogenesis. The “first hit” is liver lipid accumulation in obese children with insulin resistance. In the absence of significant lifestyle modifications leading to weight loss and increased physical activity, other factors may act as “second hits” implicated in liver damage progression leading to more severe forms of inflammation and hepatic fibrosis. In this regard, the gut–liver axis (GLA) seems to play a central role. Principal players are the gut microbiota, its bacterial products, and the intestinal barrier. A derangement of GLA (namely, dysbiosis and altered intestinal permeability) may promote bacteria/bacterial product translocation into portal circulation, activation of inflammation via toll-like receptors signaling in hepatocytes, and progression from simple steatosis to non-alcoholic steato-hepatitis (NASH). Among other factors a relevant role has been attributed to the farnesoid X receptor, a nuclear transcriptional factor activated from bile acids chemically modified by gut microbiota (GM) enzymes. The individuation and elucidation of GLA derangement in NAFLD pathomechanisms is of interest at all ages and especially in pediatrics to identify new therapeutic approaches in patients recalcitrant to lifestyle changes. Specific targeting of gut microbiota via pre-/probiotic supplementation, feces transplantation, and farnesoid X receptor modulation appear promising.