Fibroblast growth factor (Fgf) 21 is a novel target gene of the aryl hydrocarbon receptor (AhR)

Blautia Coccoides is a Newly Identified Bacterium Increased by Leucine Deprivation and has a Novel Function in Improving Metabolic Disorders

Gut microbiota is linked to human metabolic diseases. The previous work showed that leucine deprivation improved metabolic dysfunction, but whether leucine deprivation alters certain specific species of bacterium that brings these benefits remains unclear. Here, this work finds that leucine deprivation alters gut microbiota composition, which is sufficient and necessary for the metabolic improvements induced by leucine deprivation. Among all the affected bacteria, B. coccoides is markedly increased in the feces of leucine-deprived mice. Moreover, gavage with B. coccoides improves insulin sensitivity and reduces body fat in high-fat diet (HFD) mice, and singly colonization of B. coccoides increases insulin sensitivity in gnotobiotic mice. The effects of B. coccoides are mediated by metabolizing tryptophan into indole-3-acetic acid (I3AA) that activates the aryl hydrocarbon receptor (AhR) in the liver. Finally, this work reveals that reduced fecal B. coccoides and I3AA levels are associated with the clinical metabolic syndrome. These findings suggest that B. coccoides is a newly identified bacterium increased by leucine deprivation, which improves metabolic disorders via metabolizing tryptophan into I3AA.

Hepatic FGF21: Its Emerging Role in Inter-Organ Crosstalk and Cancers

Fibroblast growth factor (FGF) 21 is one of the FGF members with special endocrine properties. In the last twenty years, it has attracted intense research and development for its physiological functions that respond to dietary manipulation, pharmacological benefits of improving the macronutrient metabolism, and clinical values as a biomarker of various human diseases. Generally, FGF21 can be produced by major metabolic organs, but only the subgroup from the liver shows canonical endocrine properties, which emphasizes the special value of delineating the unique secretory and functional characteristics of hepatic FGF21. There has been a growth in literature to address the extra-hepatic activities of FGF21, and many striking findings have therefore been published. Yet, they are fragmented and scattered, and controversies are raised from divergent findings. For this reason, there is a need for a systematic and critical evaluation of current research in this aspect. In this review, we focus on the current knowledge about the molecular biology of endocrine FGF21, especially present details on the regulation of circulating levels of FGF21. We also emphasize its emerging roles in inter-organ crosstalk and cancer development.

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

Copy number variation (CNV) of the AHR gene in the Ashidan yak and its association with growth traits

Copy number variation (CNV) is a principal genomic structure variation affecting the gene expression through the dose-effect and change of gene regulatory region. It plays an important role in regulating the various complex traits of vertebrates. The aromatic hydrocarbon receptor (AHR) is a member of ligand-dependent transcription factors which belong to the alkaline helix-loop-helix PASS family. It is used as a conservative environmental sensor during biological evolution. This study, tracked the growth data (body weight, withers height, body length, chest girth) of 332 yaks in four stages (6, 12, 18, and 30 months) were tracked. The CNV of the yaks was analyzed using real-time quantitative PCR, and the correlation between CNV of AHR and yak growth traits was analyzed using the SPSS and R software. The AHR gene expression profiles were assessed in different tissues of the 18-month-old yak. The statistical analysis indicated the AHR-CNV of the Ashidan yak to significantly correlate with the body length (P < 0.05), and was found to be correlated with the withers height at 18 months old (P < 0.01) with extreme significance. To sum up, this study for the first time discussed the relationship between AHR-CNV and the growth traits of the Ashidan yak. The results indicated that the AHR gene might become a new molecular marker in the breeding yak.

Activation of activator protein-1-fibroblast growth factor 21 signaling attenuates Cisplatin hepatotoxicity

Fibroblast growth factor (Fgf/FGF) 21, which plays important roles in sugar, lipid and energy metabolism, has been accepted as a mito-stress marker gene. We recently reported that FGF21 expression can be up-regulated via activation of aryl hydrocarbon receptor (AhR) or glucocorticoid receptor (GR) and that FGF21 plays important cytoprotective roles. Cisplatin (cis-diamminedichloroplatinum, CDDP) is a widely used chemotherapeutic drug. Numerous adverse effects including hepatotoxicity have been noted during CDDP therapy. It is known that CDDP can induce mitochondrial dysfunction. The studies were designed to determine the regulation of Fgf/FGF21 expression by CDDP, and to characterize the underlying mechanisms of its regulation, as well as to determine the impact of gain or loss of Fgf/FGF21 function on the progression of CDDP hepatotoxicity. Our results showed that CDDP and phorbol ester induced mRNA and protein expression of Fgf/FGF21 and β-Klotho, two essential components of Fgf21 signaling, in mouse livers and cultured mouse/human hepatocytes. Luciferase reporter assays and ChIP-qPCR assays demonstrated that the cJun-AP-1 activation is responsible for CDDP- and phorbol ester-induced Fgf/FGF21 expression. Such induction is abolished after cotreated with AP-1 inhibitor SR11302. In addition, CDDP produces more severe liver injury in Fgf21-null than wild-type mice. Pre-treatment of GR activator dexamethasone or AhR activator β-Naphthoflavone, both of which can induce Fgf21 expression, attenuated CDDP-induced hepatotoxicity in vivo and in vitro. In conclusion, Fgf/FGF21-β-Klotho signaling can be activated via AP-1 activation. Gain of Fgf/FGF21 function attenuates the progression of CDDP hepatotoxicity, which may be considered clinically to improve CDDP therapy.

The Aryl Hydrocarbon Receptor in Energy Balance: The Road from Dioxin-Induced Wasting Syndrome to Combating Obesity with Ahr Ligands

The aryl hydrocarbon receptor (AHR) has been studied for over 40 years, yet our understanding of this ligand-activated transcription factor remains incomplete. Each year, novel findings continually force us to rethink the role of the AHR in mammalian biology. The AHR has historically been studied within the context of potent activation via AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), with a focus on how the AHR mediates TCDD toxicity. Research has subsequently revealed that the AHR is actively involved in distinct physiological processes ranging from the development of the liver and reproductive organs, to immune system function and wound healing. More recently, the AHR was implicated in the regulation of energy metabolism and is currently being investigated as a potential therapeutic target for obesity. In this review, we re-trace the steps through which the early toxicological studies of TCDD led to the conceptual framework for the AHR as a potential therapeutic target in metabolic disease. We additionally discuss the key discoveries that have been made concerning the role of the AHR in energy metabolism, as well as the current and future directions of the field.

A Review of the Functional Roles of the Zebrafish Aryl Hydrocarbon Receptors

Over the last 2 decades, the zebrafish (Danio rerio) has emerged as a stellar model for unraveling molecular signaling events mediated by the aryl hydrocarbon receptor (AHR), an important ligand-activated receptor found in all eumetazoan animals. Zebrafish have 3 AHRs-AHR1a, AHR1b, and AHR2, and studies have demonstrated the diversity of both the endogenous and toxicological functions of the zebrafish AHRs. In this contemporary review, we first highlight the evolution of the zebrafish ahr genes, and the characteristics of the receptors including developmental and adult expression, their endogenous and inducible roles, and the predicted ligands from homology modeling studies. We then review the toxicity of a broad spectrum of AHR ligands across multiple life stages (early stage, and adult), discuss their transcriptomic and epigenetic mechanisms of action, and report on any known interactions between the AHRs and other signaling pathways. Through this article, we summarize the promising research that furthers our understanding of the complex AHR pathway through the extensive use of zebrafish as a model, coupled with a large array of molecular techniques. As much of the research has focused on the functions of AHR2 during development and the mechanism of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) toxicity, we illustrate the need to address the considerable knowledge gap in our understanding of both the mechanistic roles of AHR1a and AHR1b, and the diverse modes of toxicity of the various AHR ligands.

Aryl Hydrocarbon Receptor as a Target for Lycopene Preventing DEHP-Induced Spermatogenic Disorders

Di(2-ethylhexyl)phthalate (DEHP) is widely used as a plasticizer to improve product flexibility and workability. Lycopene (LYC) is a natural compound and has promising preventive potentials, especially antireproductive toxicity, but the specific underlying mechanism is yet to be fully defined. Our study investigated the effect of LYC on DEHP-induced spermatogenesis disorders. Male ICR mice were treated with DEHP (500 or 1000 mg/kg BW/day) and/or LYC (5 mg/kg BW/day) for 28 days. Our results indicated that LYC could relieve the DEHP-induced injury of seminiferous tubules and spermatogenic cells, swelling of endoplasmic reticulum (ER), and an increase of mitochondria. LYC prevented increased levels of nuclear damage to DNA and the deformity rate and decreased values of sperm motility, number, and density. Moreover, LYC treatment decreased DEHP-induced nuclear accumulation of aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT), and the expressions of their downstream target genes such as cytochrome P450-dependent monooxygenases (CYP) 1A1, 1A2, and 1B1 were markedly reduced to normal in the LYC treatment group. Our study showed that LYC can prevent DEHP-induced spermatogenic disorders via an AHR/ARNT signaling system. This study provided new evidence of AHR as a target for LYC, which can prevent DEHP-induced toxicity.

Is ursodeoxycholic acid therapy due to pregnant intrahepatic cholestasis change the adiponectin and fibroblast growth factor-21 levels?-Pilot study

The main aim of the study was to assess changes in adiponectin and fibroblast growth factor 21 (FGF21) levels in pregnant women with intrahepatic cholestasis during ursodeoxycholic acid (UDCA) therapy. The study included 40 pregnant women with intrahepatic cholestasis (ICP) treated with UDCA. In the pregnant ICP group, material for further analysis was collected three times: before the first dose of drug T1, 4 weeks after the first dose of drug T2, 8 weeks after the first dose of drug T3, and 1 day after delivery T4 (P < .05). Regarding changes in the adiponectin concentration profile, three statistical significance (P < .05) was found: before the first dose and 8 weeks of treatment and 1 day after delivery, as well as between 4 and 8 weeks of UDCA acid therapy. In the fourth and eighth weeks of treatment, adiponectin levels reached a higher concentration than before the first dose of UDCA, but a decrease was observed 1 day after delivery. It has been confirmed that UDCA therapy has an impact on the dynamics of changes in adiponectin and FGF21 levels as well as indicators characterizing liver function.

Fibroblast growth factor signaling in non-alcoholic fatty liver disease and non-alcoholic steatohepatitis: Paving the way to hepatocellular carcinoma

Metabolic disorders are increasingly leading to non-alcoholic fatty liver disease, subsequent steatohepatitis, cirrhosis and hepatocellular carcinoma. Fibroblast growth factors and their receptors play an important role in maintaining metabolic homeostasis also in the liver and disorders in signaling have been identified to contribute to those pathophysiologic conditions leading to hepatic lipid accumulation and chronic inflammation. While specific and well tolerated inhibitors of fibroblast growth factor receptor activity are currently developed for (non-liver) cancer therapy, treatment of non-alcoholic fatty liver disease and non-alcoholic steatohepatitis is still limited. Fibroblast growth factor-mimicking or restoring approaches have recently evolved as a novel therapeutic option and the impact of such interactions with the fibroblast growth factor receptor signaling network during non-alcoholic fatty liver disease/non-alcoholic steatohepatitis development is reviewed here.

Inducible Loss of the Aryl Hydrocarbon Receptor Activates Perigonadal White Fat Respiration and Brown Fat Thermogenesis via Fibroblast Growth Factor 21

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor highly expressed in hepatocytes. Researchers have employed global and liver-specific conditional Ahr knockout mouse models to characterize the physiological roles of the AHR; however, the gestational timing of AHR loss in these models can complicate efforts to distinguish the direct and indirect effects of post-gestational AHR deficiency. Utilizing a novel tamoxifen-inducible AHR knockout mouse model, we analyzed the effects of hepatocyte-targeted AHR loss in adult mice. The data demonstrate that AHR deficiency significantly reduces weight gain and adiposity, and increases multilocular lipid droplet formation within perigonadal white adipose tissue (gWAT). Protein and mRNA expression of fibroblast growth factor 21 (FGF21), an important hepatokine that activates thermogenesis in brown adipose tissue (BAT) and gWAT, significantly increases upon AHR loss and correlates with a significant increase of BAT and gWAT respiratory capacity. Confirming the role of FGF21 in mediating these effects, this phenotype is reversed in mice concomitantly lacking AHR and FGF21 expression. Chromatin immunoprecipitation analyses suggest that the AHR may constitutively suppress Fgf21 transcription through binding to a newly identified xenobiotic response element within the Fgf21 promoter. The data demonstrate an important AHR-FGF21 regulatory axis that influences adipose biology and may represent a “druggable” therapeutic target for obesity and its related metabolic disorders.

Suppression of Hepatic CYP3A4 Expression and Activity by 3-Methylcholanthrene in Humanized PXR-CAR-CYP3A4/3A7 Mice

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants that activate the aryl hydrocarbon receptor, thereby triggering a range of biologic responses, exemplified by the induction of CYP1A1 PAHs can also regulate the expression of members of the CYP3A subfamily, with reports of mainly suppressive effects on mouse hepatic Cyp3a11 expression, but paradoxically both inductive and suppressive effects on human hepatic CYP3A4 expression. Understanding the regulation of CYP3A4 expression by PAHs is important because of the widespread exposure of humans to these chemicals and the central role of the CYP3A4 enzyme in the metabolism of clinically important drugs and endogenous substances. The present study used 3-methylcholanthrene (MC) as a model PAH to characterize the in vivo regulation of CYP3A4 expression and activity in humanized pregnane X receptor-constitutive androstane receptor-CYP3A4/3A7 mice. Adult mice were treated by intraperitoneal injection with MC (80 mg/kg), or corn oil vehicle, and euthanized 24 or 72 hours later. As a positive control response, pronounced induction of hepatic Cyp1a1 by MC was confirmed at both time points in males and females at the mRNA, protein, and catalytic activity levels. Basal hepatic CYP3A4 expression and activity were significantly higher in female versus male mice. MC treatment suppressed hepatic CYP3A4 in female mice at 72 hours postdosing at the mRNA, protein, and catalytic activity levels. A similar response was observed in male mice, although the suppression of CYP3A4 protein levels did not achieve statistical significance. This mouse model will facilitate further studies of the mechanisms and consequences of CYP3A4 suppression by PAHs.

Negative association between serum aryl hydrocarbon receptor concentrations and β-cell function in patients with no history of diabetes undergoing coronary angiography

BACKGROUND

The aim of the present study was to investigate the association between serum aryl hydrocarbon receptor (AhR) levels and insulin resistance and β-cell function in patients undergoing coronary angiography with no history of diabetes.

METHODS

Patients with no history of diabetes who had undergone coronary angiography underwent an oral glucose tolerance test (OGTT) 2-4 weeks after discharge from hospital; blood samples were collected for measurements of glucose, insulin, and AhR. Patients' glucose regulation status was determined on the basis of the OGTT. Insulin resistance was assessed using the homeostasis model assessment of insulin resistance (HOMA-IR). β-Cell function was assessed using the insulinogenic index (IGI).

RESULTS

The study included 473 patients (mean (±SD) age 61 ±12 years, 81.8% male, mean body mass index 26.1 ±3.6 kg/m² ). Overall, mean serum AhR concentrations were 25.1 ±12.2 pg/mL. Patients with normal glucose tolerance had a lower serum AhR concentrations than patients with prediabetes or newly diagnosed diabetes (23.4 ±10.8 vs 26.2 ±13.2 and 26.9 ±12.3 pg/mL, respectively; P = 0.029). Linear regression analysis revealed that serum AhR concentrations were not associated with HOMA-IR, but were negatively associated with IGI after adjustment for several confounders, including HOMA-IR (β = -0.162; 95% confidence interval - 0.302, -0.022; P = 0.023).

CONCLUSIONS

In patients with no history of diabetes, serum AhR concentrations were negatively associated with β-cell function, independent of several confounders, including insulin resistance.

Epalrestat Stimulated Oxidative Stress, Inflammation, and Fibrogenesis in Mouse Liver

Epalrestat (EPS), an aldose reductase inhibitor, is widely prescribed to manage diabetic neuropathy. It is generally believed that EPS is beneficial to diabetic patients because it can protect endothelial cells, Schwann cells, or other neural cells from oxidative stress. However, several clinical studies revealed that EPS therapy led to liver dysfunction, which limited its clinical applications. Currently, the underlying mechanism by which EPS causes liver dysfunction is unknown. This study aimed to investigate the mechanism responsible for EPS-induced liver injury. In mouse liver, EPS 1) increased oxidative stress, indicated by increased expression of manganese superoxide dismutase, Ho-1, and Nqo1, 2) induced inflammation, indicated by infiltration of inflammatory cells, and induced expression of tumor necrosis factor-alpha, CD11b, and CD11c, as well as 3) predisposed to induce fibrosis, evidenced by increased mRNA and protein expression of early profibrotic biomarker genes procollagen I and alpha-smooth muscle actin, and by increased collagen deposition. In cultured mouse and human hepatoma cells, EPS treatment induced oxidative stress, decreased cell viability, and triggered apoptosis evidenced by increased Caspase-3 cleavage/activation. In addition, EPS increased mRNA and protein expression of cytoglobin in mouse liver, indicating that EPS activated hepatic stellate cells (HSCs). Furthermore, EPS treatment in cultured human HSCs increased cell viability. In summary, EPS administration induced oxidative stress and inflammation in mouse liver, and stimulated liver fibrogenesis. Therefore, cautions should be exercised during EPS therapy.

The Biological Clock: A Pivotal Hub in Non-alcoholic Fatty Liver Disease Pathogenesis

Non-alcoholic fatty liver disease (NAFLD) is the most frequent hepatic pathology in the Western world and may evolve into steatohepatitis (NASH), increasing the risk of cirrhosis, portal hypertension and hepatocellular carcinoma. NAFLD derives from the accumulation of hepatic fat due to discrepant free fatty acid metabolism. Other factors contributing to this are deranged nutrients and bile acids fluxes as well as alterations in nuclear receptors, hormones, and intermediary metabolites, which impact on signaling pathways involved in metabolism and inflammation. Autophagy and host gut-microbiota interplay are also relevant to NAFLD pathogenesis. Notably, liver metabolic pathways and bile acid synthesis as well as autophagic and immune/inflammatory processes all show circadian patterns driven by the biological clock. Gut microbiota impacts on the biological clock, at the same time as the appropriate timing of metabolic fluxes, hormone secretion, bile acid turnover, autophagy and inflammation with behavioural cycles of fasting/feeding and sleeping/waking is required to circumvent hepatosteatosis, indicating significant interactions of the gut and circadian processes in NAFLD pathophysiology. Several time-related factors and processes interplay in NAFLD development, with the biological clock proposed to act as a network level hub. Deranged physiological rhythms (chronodisruption) may also play a role in liver steatosis pathogenesis. The current article reviews how the circadian clock circuitry intimately interacts with several mechanisms involved in the onset of hepatosteatosis and its progression to NASH, thereby contributing to the global NAFLD epidemic.

Loss of liver-specific and sexually dimorphic gene expression by aryl hydrocarbon receptor activation in C57BL/6 mice

The aryl hydrocarbon receptor (AhR) is a highly conserved transcription factor that mediates a broad spectrum of species-, strain-, sex-, age-, tissue-, and cell-specific responses elicited by structurally diverse ligands including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dose-dependent effects on liver-specific and sexually dimorphic gene expression were examined in male and female mice gavaged with TCDD every 4 days for 28 or 92 days. RNA-seq data revealed the coordinated repression of 181 genes predominately expressed in the liver including albumin (3.7-fold), α-fibrinogen (14.5-fold), and β-fibrinogen (17.4-fold) in males with corresponding AhR enrichment at 2 hr. Liver-specific genes exhibiting sexually dimorphic expression also demonstrated diminished divergence between sexes. For example, male-biased Gstp1 was repressed 3.0-fold in males and induced 4.5-fold in females, which were confirmed at the protein level. Disrupted regulation is consistent with impaired GHR-JAK2-STAT5 signaling and inhibition of female specific CUX2-mediated transcription as well as the repression of other key transcriptional regulators including Ghr, Stat5b, Bcl6, Hnf4a, Hnf6, Foxa1/2/3, and Zhx2. Attenuated liver-specific and sexually dimorphic gene expression was concurrent with the induction of fetal genes such as alpha-fetoprotein. The results suggest AhR activation causes the loss of liver-specific and sexually dimorphic gene expression producing a functionally "de-differentiated" hepatic phenotype.

Host-microbiome interactions: the aryl hydrocarbon receptor and the central nervous system

The microbiome located within a given host and its organs forms a holobiont, an intimate functional entity with evolutionarily designed interactions to support nutritional intake and reproduction. Thus, all organs in a holobiont respond to changes within the microbiome. The development and function of the central nervous system and its homeostatic mechanisms are no exception and are also subject to regulation by the gut microbiome. In order for the holobiont to function effectively, the microbiome and host must communicate. The aryl hydrocarbon receptor is an evolutionarily conserved receptor recognizing environmental compounds, including a number of ligands produced directly and indirectly by the microbiome. This review focuses on the microbiome-gut-brain axis in regard to the aryl hydrocarbon receptor signaling pathway and its impact on underlying mechanisms in neurodegeneration.

Hepatic Fgf21 Expression Is Repressed after Simvastatin Treatment in Mice

Fibroblast growth factor 21 (Fgf21) is a hormone with emerging beneficial roles in glucose and lipid homeostasis. The interest in Fgf21 as a potential antidiabetic drug and the factors that regulate its production and secretion is growing. Statins are the most widely prescribed drug for the treatment of dyslipidemia. However, the function of statins is not limited to the lowering of cholesterol as they are associated with pleiotropic actions such as antioxidant, anti-inflammatory and cytoprotective effects. The recently described effect of statins on mitochondrial function and the induction of Fgf21 by mitochondrial stress prompted us to investigate the effect of statin treatment on Fgf21 expression in the liver. To this end, C57BL6J male mice and primary mouse hepatocytes were treated with simvastatin, and Fgf21 expression was subsequently assessed by immunoblotting and quantitative real-time PCR. Hepatic Fgf21 protein and mRNA and circulating levels of FGF21significantly decreased in mice that had received simvastatin in their food (0.1% w/w) for 1 week. This effect was also observed with simvastatin doses as low as 0.01% w/w for 1 week or following 2 intraperitoneal injections within a single day. The reduction in Fgf21 mRNA levels was further verified in primary mouse hepatocytes, indicating that the effect of simvastatin is cell autonomous. In conclusion, simvastatin treatment reduced the circulating and hepatic Fgf21 levels and this effect warrants further investigation with reference to its role in metabolism.

Dose-Dependent Metabolic Reprogramming and Differential Gene Expression in TCDD-Elicited Hepatic Fibrosis

We have previously shown that in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-elicited NAFLD progression, central carbon, glutaminolysis, and serine/folate metabolism are reprogrammed to support NADPH production and ROS defenses. To further investigate underlying dose-dependent responses associated with TCDD-induced fibrosis, female C57BL/6 mice were gavaged with TCDD every 4 days (d) for 28 d or 92 d. RNA-Seq, ChIP-Seq (2 h), and 28 d metabolomic (urine, serum, and hepatic extract) analyses were conducted with complementary serum marker assessments at 92 d. Additional vehicle and 30 µg/kg treatment groups were allowed to recover for 36 d following the 92-d treatment regimen to examine recovery from TCDD-elicited fibrosis. Histopathology revealed dose-dependent increases in hepatic fat accumulation, inflammation, and periportal collagen deposition at 92 days, with increased fibrotic severity in the recovery group. Serum proinflammatory and profibrotic interleukins-1β, -2, -4, -6, and -10, as well as TNF-α and IFN-γ, exhibited dose-dependent induction. An increase in glucose tolerance was observed with a concomitant 3.0-fold decrease in hepatic glycogen linked to increased ascorbic acid biosynthesis and proline metabolism, consistent with increased fibrosis. RNA-Seq identified differential expression of numerous matrisome genes including an 8.8-fold increase in Tgfb2 indicating myofibroblast activation. Further analysis suggests reprogramming of glycogen, ascorbic acid, and amino acid metabolism in support of collagen deposition and the use of proline as a substrate for ATP production via the proline cycle. In summary, we demonstrate that glycogen, ascorbic acid, and amino acid metabolism are also reorganized to support remodeling of the extracellular matrix, progressing to hepatic fibrosis in response to chronic injury from TCDD.

Hepatic Aryl Hydrocarbon Receptor Attenuates Fibroblast Growth Factor 21 Expression

The Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in many physiological processes. Several studies indicate that AHR is also involved in energy homeostasis. Fibroblast growth factor 21 (FGF21) is an important regulator of the fasting and feeding responses. When administered to various genetic and diet-induced mouse models of obesity, FGF21 can attenuate obesity-associated morbidities. Here, we explore the role of AHR in hepatic Fgf21 expression through the use of a conditional, hepatocyte-targeted AHR knock-out mouse model (Cre(Alb)Ahr(Fx/Fx)). Compared with the congenic parental strain (Ahr(Fx/Fx)), non-fasted Cre(Alb)Ahr(Fx/Fx) mice exhibit a 4-fold increase in hepatic Fgf21 expression, as well as elevated expression of the FGF21-target gene Igfbp1 Furthermore, in vivo agonist activation of AHR reduces hepatic Fgf21 expression during a fast. The Fgf21 promoter contains several putative dioxin response elements (DREs). Using EMSA, we demonstrate that the AHR-ARNT heterodimer binds to a specific DRE that overlaps binding sequences for peroxisome proliferator-activated receptor α (PPARα), carbohydrate response element-binding protein (ChREBP), and cAMP response element-binding protein, hepatocyte specific (CREBH). In addition, we reveal that agonist-activated AHR impairs PPARα-, ChREBP-, and CREBH-mediated promoter activity in Hepa-1 cells. Accordingly, agonist treatment in Hepa-1 cells ablates potent ER stress-driven Fgf21 expression, and pre-treatment with AHR antagonist blocks this effect. Finally, we show that pre-treatment of primary human hepatocytes with AHR agonist diminishes PPARα-, glucose-, and ER stress-driven induction of FGF21 expression, indicating the effect is not mouse-specific. Together, our data show that AHR contributes to hepatic energy homeostasis, partly through the regulation of FGF21 expression and signaling.

Roles of FGFs As Paracrine or Endocrine Signals in Liver Development, Health, and Disease

The liver plays important roles in multiple processes including metabolism, the immune system, and detoxification and also has a unique capacity for regeneration. FGFs are growth factors that have diverse functions in development, health, and disease. The FGF family now comprises 22 members. Several FGFs have been shown to play roles as paracrine signals in liver development, health, and disease. FGF8 and FGF10 are involved in embryonic liver development, FGF7 and FGF9 in repair in response to liver injury, and FGF5, FGF8, FGF9, FGF17, and FGF18 in the development and progression of hepatocellular carcinoma. In contrast, FGF15/19 and FGF21 are endocrine signals. FGF15/19, which is produced in the ileum, is a negative regulator of bile acid metabolism and a stimulator of gallbladder filling. FGF15/19 is a postprandial, insulin-independent activator of hepatic protein and glycogen synthesis. It is also required for hepatocellular carcinoma and liver regeneration. FGF21 is a hepatokine produced in the liver. FGF21 regulates glucose and lipid metabolism in white adipose tissue. Serum FGF21 levels are elevated in non-alcoholic fatty liver. FGF21 also protects against non-alcoholic fatty liver. These findings provide new insights into the roles of FGFs in the liver and potential therapeutic strategies for hepatic disorders.

The emerging role of fibroblast growth factor 21 in diabetic nephropathy

Diabetic nephropathy (DN), an important cause of end-stage renal diseases, brings about great social and economic burden. Due to the variable pathological changes and clinical course, the prognosis of DN is very difficult to predict. DN is also usually associated with enhanced genomic damage and cellular injury. Fibroblast growth factor 21 (FGF21), a nutritionally regulated hormone secreted mainly by the liver, plays a critical role in metabolism. Administration of FGF21 decreases blood glucose, triglyceride, and cholesterol levels, and improves insulin sensitivity, which is closely associated with the development and progression of glomerular diseases. In addition, FGF21 level was associated with renal function. However, the precise role of FGF21 in DN remains unclear. This review will give a comprehensive understanding of the underlying role of FGF21 and its possible interaction with other molecules in DN.

Practical prospects for boosting hepatic production of the "pro-longevity" hormone FGF21

Fibroblast growth factor-21 (FGF21), produced mainly in hepatocytes and adipocytes, promotes leanness, insulin sensitivity, and vascular health while down-regulating hepatic IGF-I production. Transgenic mice overexpressing FGF21 enjoy a marked increase in median and maximal longevity comparable to that evoked by calorie restriction - but without a reduction in food intake. Transcriptional factors which promote hepatic FGF21 expression include PPARα, ATF4, STAT5, and FXR; hence, fibrate drugs, elevated lipolysis, moderate-protein vegan diets, growth hormone, and bile acids may have potential to increase FGF21 synthesis. Sirt1 activity is required for optimal responsiveness of FGF21 to PPARα, and Sirt1 activators can boost FGF21 transcription. Conversely, histone deacetylase 3 (HDAC3) inhibits PPARα's transcriptional impact on FGF21, and type 1 deacetylase inhibitors such as butyrate therefore increase FGF21 expression. Glucagon-like peptide-1 (GLP-1) increases hepatic expression of both PPARα and Sirt1; acarbose, which increases intestinal GLP-1 secretion, also increases FGF21 and lifespan in mice. Glucagon stimulates hepatic production of FGF21 by increasing the expression of the Nur77 transcription factor; increased glucagon secretion can be evoked by supplemental glycine administered during post-absorptive metabolism. The aryl hydrocarbon receptor (AhR) has also been reported recently to promote FGF21 transcription. Bilirubin is known to be an agonist for this receptor, and this may rationalize a recent report that heme oxygenase-1 induction in the liver boosts FGF21 expression. There is reason to suspect that phycocyanorubin, a bilirubin homolog that is a metabolite of the major phycobilin in spirulina, may share bilirubin's agonist activity for AhR, and perhaps likewise promote FGF21 induction. In the future, regimens featuring a plant-based diet, nutraceuticals, and safe drugs may make it feasible to achieve physiologically significant increases in FGF21 that promote metabolic health, leanness, and longevity.

Transcriptional and Chromatin Regulation during Fasting - The Genomic Era

An elaborate metabolic response to fasting is orchestrated by the liver and is heavily reliant on transcriptional regulation. In response to hormones (glucagon, glucocorticoids) many transcription factors (TFs) are activated and regulate various genes involved in metabolic pathways aimed at restoring homeostasis: gluconeogenesis, fatty acid oxidation, ketogenesis, and amino acid shuttling. We summarize recent discoveries regarding fasting-related TFs with an emphasis on genome-wide binding patterns. Collectively, the findings we discuss reveal a large degree of cooperation between TFs during fasting that occurs at motif-rich DNA sites bound by a combination of TFs. These new findings implicate transcriptional and chromatin regulation as major determinants of the response to fasting and unravels the complex, multi-TF nature of this response.

Minireview: Roles of Fibroblast Growth Factors 19 and 21 in Metabolic Regulation and Chronic Diseases

Fibroblast growth factor (FGF)19 and FGF21 are hormones that regulate metabolic processes particularly during feeding or starvation, thus ultimately influencing energy production. FGF19 is secreted by the intestines during feeding and negatively regulates bile acid synthesis and secretion, whereas FGF21 is produced in the liver during fasting and plays a crucial role in regulating glucose and lipid metabolism, as well as maintaining energy homeostasis. FGF19 and FGF21 are regarded as late-acting hormones because their functions are only used after insulin and glucagon have completed their actions. Although FGF19 and FGF21 are activated under different conditions, they show extensively functional overlap in terms of improving glucose tolerance, insulin sensitivity, weight loss, and lipid, and energy metabolism, particularly in pathological conditions such as diabetes, obesity, metabolic syndrome, and cardiovascular and renal diseases. Most patients with these metabolic diseases exhibit reduced serum FGF19 levels, which might contribute to its etiology. In addition, the simultaneous increase in serum FGF21 levels is likely a compensatory response to reduced FGF19 levels, and the 2 proteins concertedly maintain metabolic homeostasis. Here, we review the physiological and pharmacological cross talk between FGF19 and FGF21 in relation to the regulation of endocrine metabolism and various chronic diseases.

Inhibition of miR-29 has a significant lipid-lowering benefit through suppression of lipogenic programs in liver

MicroRNAs (miRNAs) are important regulators and potential therapeutic targets of metabolic disease. In this study we show by in vivo administration of locked nucleic acid (LNA) inhibitors that suppression of endogenous miR-29 lowers plasma cholesterol levels by ~40%, commensurate with the effect of statins, and reduces fatty acid content in the liver by ~20%. Whole transcriptome sequencing of the liver reveals 883 genes dysregulated (612 down, 271 up) by inhibition of miR-29. The set of 612 down-regulated genes are most significantly over-represented in lipid synthesis pathways. Among the up-regulated genes are the anti-lipogenic deacetylase sirtuin 1 (Sirt1) and the anti-lipogenic transcription factor aryl hydrocarbon receptor (Ahr), the latter of which we demonstrate is a direct target of miR-29. In vitro radiolabeled acetate incorporation assays confirm that pharmacologic inhibition of miR-29 significantly reduces de novo cholesterol and fatty acid synthesis. Our findings indicate that miR-29 controls hepatic lipogenic programs, likely in part through regulation of Ahr and Sirt1, and therefore may represent a candidate therapeutic target for metabolic disorders such as dyslipidemia.

FGF21 as a mediator of adaptive responses to stress and metabolic benefits of anti-diabetic drugs

Most hormones secreted from specific organs of the body in response to diverse stimuli contribute to the homeostasis of the whole organism. Fibroblast growth factor 21 (FGF21), a hormone induced by a variety of environmental or metabolic stimuli, plays a crucial role in the adaptive response to these stressful conditions. In addition to its role as a stress hormone, FGF21 appears to function as a mediator of the therapeutic effects of currently available drugs and those under development for treatment of metabolic diseases. In this review, we highlight molecular mechanisms and the functional importance of FGF21 induction in response to diverse stress conditions such as changes of nutritional status, cold exposure, and exercise. In addition, we describe recent findings regarding the role of FGF21 in the pathogenesis and treatment of diabetes associated with obesity, liver diseases, pancreatitis, muscle atrophy, atherosclerosis, cardiac hypertrophy, and diabetic nephropathy. Finally, we discuss the current understanding of the actions of FGF21 as a crucial regulator mediating beneficial metabolic effects of therapeutic agents such as metformin, glucagon/glucagon-like peptide 1 analogues, thiazolidinedione, sirtuin 1 activators, and lipoic acid.

Fibroblast growth factor 21 participates in adaptation to endoplasmic reticulum stress and attenuates obesity-induced hepatic metabolic stress

AIMS/HYPOTHESIS

Fibroblast growth factor 21 (FGF21) is an endocrine hormone that exhibits anti-diabetic and anti-obesity activity. FGF21 expression is increased in patients with and mouse models of obesity or nonalcoholic fatty liver disease (NAFLD). However, the functional role and molecular mechanism of FGF21 induction in obesity or NAFLD are not clear. As endoplasmic reticulum (ER) stress is triggered in obesity and NAFLD, we investigated whether ER stress affects FGF21 expression or whether FGF21 induction acts as a mechanism of the unfolded protein response (UPR) adaptation to ER stress induced by chemical stressors or obesity.

METHODS

Hepatocytes or mouse embryonic fibroblasts deficient in UPR signalling pathways and liver-specific eIF2α mutant mice were employed to investigate the in vitro and in vivo effects of ER stress on FGF21 expression, respectively. The in vivo importance of FGF21 induction by ER stress and obesity was determined using inducible Fgf21-transgenic mice and Fgf21-null mice with or without leptin deficiency.

RESULTS

We found that ER stressors induced FGF21 expression, which was dependent on a PKR-like ER kinase-eukaryotic translation factor 2α-activating transcription factor 4 pathway both in vitro and in vivo. Fgf21-null mice exhibited increased expression of ER stress marker genes and augmented hepatic lipid accumulation after tunicamycin treatment. However, these changes were attenuated in inducible Fgf21-transgenic mice. We also observed that Fgf21-null mice with leptin deficiency displayed increased hepatic ER stress response and liver injury, accompanied by deteriorated metabolic variables.

CONCLUSIONS/INTERPRETATION

Our results suggest that FGF21 plays an important role in the adaptive response to ER stress- or obesity-induced hepatic metabolic stress.

FGF21 as a Stress Hormone: The Roles of FGF21 in Stress Adaptation and the Treatment of Metabolic Diseases

Fibroblast growth factor 21 (FGF21) is an endocrine hormone that is primarily expressed in the liver and exerts beneficial effects on obesity and related metabolic diseases. In addition to its remarkable pharmacologic actions, the physiological roles of FGF21 include the maintenance of energy homeostasis in the body in conditions of metabolic or environmental stress. The expression of FGF21 is induced in multiple organs in response to diverse physiological or pathological stressors, such as starvation, nutrient excess, autophagy deficiency, mitochondrial stress, exercise, and cold exposure. Thus, the FGF21 induction caused by stress plays an important role in adaptive response to these stimuli. Here, we highlight our current understanding of the functional importance of the induction of FGF21 by diverse stressors as a feedback mechanism that prevents excessive stress.

FGF21 as a Hepatokine, Adipokine, and Myokine in Metabolism and Diseases

Fibroblast growth factor (FGF) family members are mostly secreted as signaling proteins with diverse functions in development and metabolism. FGF21 is a unique FGF with metabolic, but not proliferative activities. FGF21 is mostly induced by different kinds of stress and acts though FGF receptor 1c with β-Klotho as a cofactor in an endocrine or, in parts, autocrine/paracrine manner. Hepatic FGF21 directly acts on white adipocytes to inhibit lipolysis and acts through the brain to increase systemic glucocorticoid levels and suppress physical activity in response to starvation. It also protects against dioxin toxicity. Adipocytic FGF21 induces the browning of white adipose tissue (WAT) and activates brown adipocytes in response to cold exposure. It also acts as an upstream effector of adiponectin in white adipocytes. Myocytic FGF21 protects against diet-induced obesity and insulin resistance, induces the browning of WAT, and protects against cardiac hypertrophy. In addition, Fgf21 polymorphisms are possibly related with metabolic diseases and FGF21 are biomarker of metabolic diseases. These findings indicate that FGF21 plays roles as a hepatokine, adipokine, and myokine in metabolism, injury protection, and diseases.