Adipose fibroblast growth factor 21 is up-regulated by peroxisome proliferator-activated receptor gamma and altered metabolic states

Nrf2 induces fibroblast growth factor 21 in diabetic mice

Transcription factor Nrf2 (nuclear factor E2-related factor 2) is a master regulator of cellular defense system against oxidative and electrophilic stresses and is negatively regulated by an adaptor protein Keap1 (Kelch-like ECH-associated protein 1). Nrf2 also plays a pivotal role in metabolic homeostasis, such as lipid metabolism and energy expenditure as well as redox homeostasis. FGF21 (fibroblast growth factor 21) is known as a key mediator of glucose and lipid metabolism. Here, we found that Nrf2 is involved in FGF21 regulation in diabetic model mice. Nrf2 induction by genetic knockdown of Keap1 increased plasma FGF21 level and hepatic Fgf21 expression in diabetic db/db mice and high-calorie-diet-induced obesity model mice. Administration of CDDO-Im (oleanolic triterpenoid 1-[2-cyano-3,12-dioxooleane-1, 9(11)-dien-28-oyl] imidazole), a potent Nrf2 inducer, up-regulated plasma FGF21 level and hepatic Fgf21 expression in db/db mice, whereas CDDO-Im did not induce FGF21 in db/db mice with Nrf2 knockout background. Furthermore, in Keap1-knockdown db/db mice, Nrf2 enhanced expression of glucose- and lipid-metabolism-related genes in adipose tissues, which improved plasma lipid profiles. These results show that Nrf2 positively regulates FGF21 expression in diabetic mice. We propose that FGF21 is a potential efficacy biomarker that mediates metabolic regulation by the Keap1-Nrf2 system.

The PPARα-FGF21 hormone axis contributes to metabolic regulation by the hepatic JNK signaling pathway

The cJun NH2-terminal kinase (JNK) stress signaling pathway is implicated in the metabolic response to the consumption of a high-fat diet, including the development of obesity and insulin resistance. These metabolic adaptations involve altered liver function. Here, we demonstrate that hepatic JNK potently represses the nuclear hormone receptor peroxisome proliferator-activated receptor α (PPARα). Therefore, JNK causes decreased expression of PPARα target genes that increase fatty acid oxidation and ketogenesis and promote the development of insulin resistance. We show that the PPARα target gene fibroblast growth factor 21 (Fgf21) plays a key role in this response because disruption of the hepatic PPARα-FGF21 hormone axis suppresses the metabolic effects of JNK deficiency. This analysis identifies the hepatokine FGF21 as a critical mediator of JNK signaling in the liver.

The control of insulin secretion by adipokines: current evidence for adipocyte-beta cell endocrine signalling in metabolic homeostasis

Metabolic homeostasis is maintained by the coordinated action of multiple organ systems. Insulin secretion is often enhanced during obesity or insulin resistance to maintain glucose and lipid homeostasis, whereas a loss of insulin secretion is associated with type 2 diabetes. Adipocytes secrete hormones known as adipokines which act on multiple cell types to regulate metabolism. Many adipokines have been shown to influence beta cell function by enhancing or inhibiting insulin release or by influencing beta cell survival. Insulin, in turn, regulates lipolysis and promotes glucose uptake and lipid storage in adipocytes. As adipokine secretion and action is strongly influenced by obesity, this provides a potential route by which beta cell function is coordinated with adiposity, independently of alterations in blood glucose or lipid levels. In this review, I assess the evidence for the direct regulation of beta cell function by the adipokines leptin, adiponectin, extracellular nicotinamide phosphoribosyltransferase, apelin, resistin, retinol binding protein 4, fibroblast growth factor 21, nesfatin-1 and fatty acid binding protein 4. I summarise in vitro and in vivo data and discuss the influence of obesity and diabetes on circulating adipokine concentrations, along with the potential for influencing beta cell function in human physiology. Finally, I highlight future research questions that are likely to yield new insights into the exciting field of insulinotropic adipokines.

Effects of fibroblast growth factor 21 on cell damage in vitro and atherosclerosis in vivo

Fibroblast growth factor 21 (FGF-21), which is a modulator of glucose and lipid homeostasis, acts as a novel therapeutic reagent for many metabolic perturbations. However, its potential as a treatment for cardiovascular disease, especially atherosclerosis (AS) has not been fully explored. Here, we report that recombinant FGF-21 improves resistance to cell damage from oxidative stress in vitro, and from atherosclerosis in vivo. Human umbilical vein endothelial cells (HUVECs) were induced with H2O2, followed by treatment with high purity recombinant FGF-21. The results indicated that FGF-21 significantly enhanced cell viability and decreased the degree of DNA fragmentation in HUVECs, as caused by H2O2 stress induction. Further studies revealed that FGF-21 inhibited H2O2-induced cell apoptosis by preventing the activation of mitogen-activated protein kinase (MAPK) signaling pathways. In an established rat model, FGF-21 dramatically improved the condition of atherosclerotic rats by decreasing serum levels of total triglyceride (TG), low density lipoprotein cholesterol (LDL-C), and total cholesterol (TC), and by increasing the serum levels of high density lipoprotein cholesterol (HDL-C). FGF-21 also has antioxidant effects in the atherosclerotic rat, such that increased levels of superoxide dismutase, reduced glutathione, and reduced malondialdehyde were observed. These data provide novel insight into the potential use of FGF-21 in the prevention and treatment of human cardiovascular diseases.

The role of fibroblast growth factor 21 in the pathogenesis of liver disease: a novel predictor and therapeutic target

INTRODUCTION

Fibroblast growth factor 21 (FGF21) is one of the FGF family members that is produced mainly by tissues with high metabolic activity such as liver, pancreas, muscle and adipose tissue. The major function of FGF21 is to improve insulin sensitivity, ameliorate hepatic steatosis and enhance energy expenditure. Recently, several studies have reported a correlation between FGF21 and liver disease with numerous cross-sectional studies demonstrating significant correlation. This review will focus on the role of FGF21 in the pathogenesis of liver disease and its potential role as a biomarker and a new target for therapeutic intervention.

AREAS COVERED

This review discusses cross-sectional studies and underlying mechanisms of FGF21 as an endocrine hormone in several liver diseases. Two major theories of 'endoplasmic reticulum stress' and 'FGF21 resistance' in particular are explained. Moreover, early functional detection and pharmacological effect of FGF21 for liver disease are also described.

EXPERT OPINION

FGF21 can be a promising treatment in liver disease. However, still several problems are needed to be answered. The most important are whether different liver disease share common underlying mechanisms and the pharmacological effect in human with limited studies. Further studies are needed to explain the underlying mechanisms and develop potential therapeutic effect for human liver disease.

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

The toxic effects of dioxins, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), mainly through activation of the aryl hydrocarbon receptor (AhR) are well documented. Fibroblast growth factor (Fgf) 21 plays critical roles in metabolic adaptation to fasting by increasing lipid oxidation and ketogenesis in the liver. The present study was performed to determine whether activation of the AhR induces Fgf21 expression. In mouse liver, TCDD increased Fgf21 mRNA in both dose- and time-dependent manners. In addition, TCDD markedly increased Fgf21 mRNA expression in cultured mouse and human hepatocytes. Moreover, TCDD increased mRNA (in liver) and protein levels (in both liver and serum) of Fgf21 in wild-type mice, but not in AhR-null mice. Chromatin immunoprecipitation assays showed that TCDD increased AhR protein binding to the Fgf21 promoter (-105/+1 base pair). Fgf21-null mice administered 200μg/kg of TCDD died within 20days, whereas wild-type mice receiving the same treatment were still alive at one month after administration. This indicates that TCDD-induced Fgf21 expression protects against TCDD toxicity. Diethylhexylphthalate (DEHP) pretreatment attenuated TCDD-induced Fgf21 expression in mouse liver and white adipose tissue, which may explain a previous report that DEHP pretreatment decreases TCDD-induced wasting. In conclusion, Fgf21 appears to be a target gene of AhR-signaling pathway in mouse and human liver.

FGF21-based pharmacotherapy--potential utility for metabolic disorders

Currently available therapies for diabetes or obesity produce modest efficacy and are usually used in combination with agents targeting cardiovascular risk factors. Fibroblast growth factor 21 (FGF21) is a circulating protein with pleiotropic metabolic actions; pharmacological doses of FGF21 produce anti-diabetic, lipid-lowering, and weight-reducing effects in rodents. Several potential benefits have translated to non-human primates and obese humans with type 2 diabetes (T2D). Accumulating results point to a specific receptor complex and actions in adipose tissue, liver, and brain; several pathways lead to enhanced fatty acid oxidation, increased insulin sensitivity, and augmented energy expenditure. A range of strategies are being explored to derive potent, safe, and convenient therapies which could potentially represent novel approaches to prevent and treat a variety of metabolic disorders.

Transcriptional regulation of fibroblast growth factor 21 expression

Fibroblast growth factor 21 (FGF21) is an attractive target for treating metabolic disease due to its wide-ranging beneficial effects on glucose and lipid metabolism. Circulating FGF21 levels are increased in insulin-resistant states; however, endogenous FGF21 fails to improve glucose and lipid metabolism in obesity, suggesting that metabolic syndrome is an FGF21-resistant state. Therefore, transcription factors for FGF21 are potential drug targets that could increase FGF21 expression in obesity and reduce FGF21 resistance. Despite many studies on the metabolic effects of FGF21, the transcriptional regulation of FGF21 gene expression remains controversial and is not fully understood. As the FGF21 transcription factor pathway is one of the most promising targets for the treatment of metabolic syndrome, further investigation of FGF21 transcriptional regulation is required.

Significant gender difference in serum levels of fibroblast growth factor 21 in Danish children and adolescents

Introduction

Fibroblast Growth Factor 21 (FGF21) is a novel metabolic factor with effect on glucose and lipid metabolism, and shown to be elevated in diseases related to metabolic syndrome. Due to the increasing frequency of metabolic syndrome in the pediatric population, and as FGF21 studies in children are limited, we investigated baseline serum levels of FGF21 in healthy children during an oral glucose tolerance test.

Methods

A total of 179 children and adolescents from the COPENHAGEN Puberty Study were included. An OGTT with glucose and insulin measurements, a dual energy X-ray absorptiometry (DXA) scan and a clinical examination including pubertal staging were done on all subjects. Serum levels of FGF21, adiponectin, and leptin were determined by immunoassays at baseline.

Results

The girls had significantly higher levels of FGF21 compared with boys (155 pg/mL vs. 105 pg/mL, P = 0.04). 38 children (21%) had levels below detection limit of assay. Baseline levels of FGF21 showed positive correlation with triglycerides, but no significant correlations were found between FGF21-concentration and body mass index (BMI), DXA-derived fat percentage, LDL- HDL- and non-HDL cholesterol, leptin or adiponectin levels, respectively. Neither was any correlation found between baseline FGF21-levels and the dynamic changes in glucose and insulin levels during the OGTT.

Conclusion

FGF21 is independent of adiposity in children, and the significant metabolic effect seems to be limited to pathological conditions associated with insulin resistance. The higher levels of triglycerides in the girls may explain the significantly higher levels of FGF21 in girls compared with boys.

Systematic review registration

The COPENHAGEN Puberty Study was registered in ClinicalTrials.gov (identifier NCT01411527), and approved by the local ethics committee (reference no. KF 01 282214 and KF 11 2006–2033).

ATF4- and CHOP-dependent induction of FGF21 through endoplasmic reticulum stress

Fibroblast growth factor 21 (FGF21) is an important endogenous regulator involved in the regulation of glucose and lipid metabolism. FGF21 expression is strongly induced in animal and human subjects with metabolic diseases, but little is known about the molecular mechanism. Endoplasmic reticulum (ER) stress plays an essential role in metabolic homeostasis and is observed in numerous pathological processes, including type 2 diabetes, overweight, nonalcoholic fatty liver disease (NAFLD). In this study, we investigate the correlation between the expression of FGF21 and ER stress. We demonstrated that TG-induced ER stress directly regulated the expression and secretion of FGF21 in a dose- and time-dependent manner. FGF21 is the target gene for activating transcription factor 4 (ATF4) and CCAAT enhancer binding protein homologous protein (CHOP). Suppression of CHOP impaired the transcriptional activation of FGF21 by TG-induced ER stress in CHOP-/- mouse primary hepatocytes (MPH), and overexpression of ATF4 and CHOP resulted in FGF21 promoter activation to initiate the transcriptional programme. In mRNA stability assay, we indicated that ER stress increased the half-life of mRNA of FGF21 significantly. In conclusion, FGF21 expression is regulated by ER stress via ATF- and CHOP-dependent transcriptional mechanism and posttranscriptional mechanism, respectively.

Liraglutide suppresses obesity and hyperglycemia associated with increases in hepatic fibroblast growth factor 21 production in KKAy mice

Social isolation contributes to the development of obesity and insulin-independent diabetes in KKA^y mice. Here we show that systemic administration of liraglutide, a long-acting human glucagon-like peptide-1 (GLP-1) analog, significantly decreased food intake, body weight, and blood glucose levels at 24 h after its administration while having no significant effects on plasma insulin and glucagon levels in individually housed KKA^y mice. In addition, the systemic administration of liraglutide significantly increased plasma fibroblast growth factor (Fgf) 21 levels (1.8-fold increase) associated with increases in the expression of hepatic Fgf21 (1.9-fold increase) and Pparγ (1.8-fold increase), while having no effects on the expression of hepatic Pparα and Fgf21 in white adipose tissue. Moreover, systemic administration of liraglutide over 3 days significantly suppressed food intake, body weight gain, and hyperglycemia in KKA^y mice. On the other hand, despite remarkably increased plasma active GLP-1 levels (4.2-fold increase), the ingestion of alogliptin, a selective dipeptidyl peptidase-4 inhibitor, over 3 days had no effects on food intake, body weight, blood glucose levels, and plasma Fgf21 levels in KKA^y mice. These findings suggest that systemic administration of liraglutide induces hepatic Fgf21 production and suppresses the social isolation-induced obesity and diabetes independently of insulin, glucagon, and active GLP-1 in KKA^y mice.

Silencing of FGF-21 expression promotes hepatic gluconeogenesis and glycogenolysis by regulation of the STAT3-SOCS3 signal

Insulin resistance is a metabolic disorder associated with type 2 diabetes. Recent reports have shown that fibroblast growth factor-21 (FGF-21) plays an important role in the progression of insulin resistance. However, the biochemical and molecular mechanisms by which changes in FGF-21 activation result in changes in the rates of hepatic gluconeogenesis and glycogenolysis remain to be elucidated. In this study, we developed adenovirus-mediated shRNA against FGF-21 to inhibit FGF-21 expression in ApoE knockout mice. Using this mouse model, we determined the effects of FGF-21 knockdown in vivo on hepatic glucose production, gluconeogenesis and glycogenolysis, and their relationship with the signal transducer and activator of transcription 3 (STAT3)/suppressor of cytokine signaling 3 (SOCS3) signal pathways. We show that liver-specific knockdown of FGF-21 in high-fat diet-fed ApoE knockout mice resulted in a 39% increase in glycogenolysis and a 75% increase in gluconeogenesis, accompanied by increased hepatic expression of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. Furthermore, FGF-21 knockdown decreased phosphorylation of STAT3 and SOCS3 expression in high-fat diet-fed mice. Our data suggest that hepatic FGF-21 knockdown increases gluconeogenesis and glycogenolysis by activation of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase via the STAT3/SOCS3 pathway, ultimately leading to exacerbation of hepatic insulin resistance.

c9,t11-Conjugated linoleic acid ameliorates steatosis by modulating mitochondrial uncoupling and Nrf2 pathway

Oxidative stress, hepatic steatosis, and mitochondrial dysfunction are key pathophysiological features of nonalcoholic fatty liver disease. A conjugated linoleic acid (CLA) mixture of cis9,trans11 (9,11-CLA) and trans10,cis12 (10,12-CLA) isomers enhanced the antioxidant/detoxifying mechanism via the activation of nuclear factor E2-related factor-2 (Nrf2) and improved mitochondrial function, but less is known about the actions of specific isomers. The differential ability of individual CLA isomers to modulate these pathways was explored in Wistar rats fed for 4 weeks with a lard-based high-fat diet (L) or with control diet (CD), and, within each dietary treatment, two subgroups were daily administered with 9,11-CLA or 10,12-CLA (30 mg/day). The 9,11-CLA, but not 10,12-CLA, supplementation to CD rats improves the GSH/GSSG ratio in the liver, mitochondrial functions, and Nrf2 activity. Histological examination reveals a reduction of steatosis in L-fed rats supplemented with both CLA isomers, but 9,11-CLA downregulated plasma concentrations of proinflammatory markers, mitochondrial dysfunction, and oxidative stress markers in liver more efficiently than in 10,12-CLA treatment. The present study demonstrates the higher protective effect of 9,11-CLA against diet-induced pro-oxidant and proinflammatory signs and suggests that these effects are determined, at least in part, by its ability to activate the Nrf2 pathway and to improve the mitochondrial functioning and biogenesis.

Regulators of human white adipose browning: evidence for sympathetic control and sexual dimorphic responses to sprint interval training

The conversion of white adipose to the highly thermogenic beige adipose tissue has been proposed as a potential strategy to counter the unfavorable consequences of obesity. Three regulators of this conversion have recently emerged but information regarding their control is limited, and contradictory. We present two studies examining the control of these regulators. Study 1: In 10 young men, the plasma concentrations of irisin and fibroblast growth factor 21 (FGF21) were determined prior to and during activation of the sympathetic nervous system via hypoxic gas breathing (FIO2 = 0.11). The measurements were performed twice, once with and once without prior/concurrent sympathetic inhibition via transdermal clonidine administration. FGF21 was unaffected by basal sympathetic inhibition (338±113 vs. 295±80 pg/mL; P = 0.43; mean±SE), but was increased during hypoxia mediated sympathetic activation (368±135); this response was abrogated (P = 0.035) with clonidine (269±93). Irisin was unaffected by sympathetic inhibition and/or hypoxia (P>0.21). Study 2: The plasma concentration of irisin and FGF21, and the skeletal muscle protein content of fibronectin type III domain containing 5 (FNDC5) was determined in 19 young adults prior to and following three weeks of sprint interval training (SIT). SIT decreased FGF21 (338±78 vs. 251±36; P = 0.046) but did not affect FNDC5 (P = 0.79). Irisin was decreased in males (127±18 vs. 90±23 ng/mL; P = 0.045) and increased in females (139±14 vs. 170±18). Collectively, these data suggest a potential regulatory role of acute sympathetic activation pertaining to the browning of white adipose; further, there appears to be a sexual dimorphic response of irisin to SIT.

Skeletal muscle mitochondrial uncoupling drives endocrine cross-talk through the induction of FGF21 as a myokine

UCP1-Tg mice with ectopic expression of uncoupling protein 1 (UCP1) in skeletal muscle (SM) are a model of improved substrate metabolism and increased longevity. Analysis of myokine expression showed an induction of fibroblast growth factor 21 (FGF21) in SM, resulting in approximately fivefold elevated circulating FGF21 in UCP1-Tg mice. Despite a reduced muscle mass, UCP1-Tg mice showed no evidence for a myopathy or muscle autophagy deficiency but an activation of integrated stress response (ISR; eIF2α/ATF4) in SM. Targeting mitochondrial function in vitro by treating C2C12 myoblasts with the uncoupler FCCP resulted in a dose-dependent activation of ISR, which was associated with increased expression of FGF21, which was also observed by treatment with respiratory chain inhibitors antimycin A and myxothiazol. The cofactor required for FGF21 action, β-klotho, was expressed in white adipose tissue (WAT) of UCP1-Tg mice, which showed an increased browning of WAT similar to what occurred in altered adipocyte morphology, increased brown adipocyte markers (UCP1, CIDEA), lipolysis (HSL phosphorylation), and respiratory capacity. Importantly, treatment of primary white adipocytes with serum of transgenic mice resulted in increased UCP1 expression. Additionally, UCP1-Tg mice showed reduced body length through the suppressed IGF-I-GH axis and decreased bone mass. We conclude that the induction of FGF21 as a myokine is coupled to disturbance of mitochondrial function and ISR activation in SM. FGF21 released from SM has endocrine effects leading to increased browning of WAT and can explain the healthy metabolic phenotype of UCP1-Tg mice. These results confirm muscle as an important endocrine regulator of whole body metabolism.

Inventing new medicines: The FGF21 story

Since the discovery of insulin in 1921, protein therapeutics have become vital tools in the treatment of diabetes mellitus. This heritage has been extended with the comparatively recent introduction of recombinant and re-engineered insulins, in addition to the advent of GLP1 agonists. FGF21 represents an example of a novel experimental protein therapy which is able to induce favorable metabolic effects in various species ranging from rodents to man.

The aim of this review is to communicate the story of the FGF21 drug discovery path from identification in a functional in vitro screen, to the eventual evaluation of its utility in patients. Given that the development of FGF21 advanced hand-in-hand with rapidly evolving scientific research around this target, we have also attempted to describe our view of recent developments regarding the mechanistic understanding of FGF21 biology.

Fibroblast growth factor 21, the endocrine FGF pathway and novel treatments for metabolic syndrome

Diabetes and associated metabolic conditions have reached pandemic proportions worldwide, and there is a clear unmet medical need for effective and safe therapies. Fibroblast growth factor (FGF)21 is an atypical member of the FGF family. The ability of FGF21 to normalize glucose, lipid and energy homeostasis has attracted considerable interest as a potential therapeutic for treating diabetes and obesity. Many different engineering approaches have successfully improved the plasma half life, protein stability and solubility, as well as 'manufacturability' of FGF21. Novel approaches such as agonist antibodies to FGF21 receptor complexes have opened new opportunities previously unavailable. This review summarizes recent advances in understanding the functions, target tissues and receptors for FGF21. Furthermore, it provides an up-to-date appraisal of the approaches on therapeutic development targeting this pathway.

Downstream signaling pathways in mouse adipose tissues following acute in vivo administration of fibroblast growth factor 21

FGF21 is a novel secreted protein with robust anti-diabetic, anti-obesity, and anti-atherogenic activities in preclinical species. In the current study, we investigated the signal transduction pathways downstream of FGF21 following acute administration of the growth factor to mice. Focusing on adipose tissues, we identified FGF21-mediated downstream signaling events and target engagement biomarkers. Specifically, RNA profiling of adipose tissues and phosphoproteomic profiling of adipocytes, following FGF21 treatment revealed several specific changes in gene expression and post-translational modifications, specifically phosphorylation, in several relevant proteins. Affymetrix microarray analysis of white adipose tissues isolated from both C57BL/6 (fed either regular chow or HFD) and db/db mice identified over 150 robust potential RNA transcripts and over 50 potential secreted proteins that were changed greater than 1.5 fold by FGF21 acutely. Phosphoprofiling analysis identified over 130 phosphoproteins that were modulated greater than 1.5 fold by FGF21 in 3T3-L1 adipocytes. Bioinformatic analysis of the combined gene and phosphoprotein profiling data identified a number of known metabolic pathways such as glucose uptake, insulin receptor signaling, Erk/Mapk signaling cascades, and lipid metabolism. Moreover, a number of novel events with hitherto unknown links to FGF21 signaling were observed at both the transcription and protein phosphorylation levels following treatment. We conclude that such a combined "omics" approach can be used not only to identify robust biomarkers for novel therapeutics but can also enhance our understanding of downstream signaling pathways; in the example presented here, novel FGF21-mediated signaling events in adipose tissue have been revealed that warrant further investigation.

Amorfrutin B is an efficient natural peroxisome proliferator-activated receptor gamma (PPARγ) agonist with potent glucose-lowering properties

AIMS/HYPOTHESIS

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) is an important gene regulator in glucose and lipid metabolism. Unfortunately, PPARγ-activating drugs of the thiazolidinedione class provoke adverse side effects. As recently shown, amorfrutin A1 is a natural glucose-lowering compound that selectively modulates PPARγ. In this study we aimed to characterise, in vitro, a large spectrum of the amorfrutins and similar molecules, which we isolated from various plants. We further studied in vivo the glucose-lowering effects of the so far undescribed amorfrutin B, which featured the most striking PPARγ-binding and pharmacological properties of this family of plant metabolites.

METHODS

Amorfrutins were investigated in vitro by binding and cofactor recruitment assays and by transcriptional activation assays in primary human adipocytes and murine preosteoblasts, as well as in vivo using insulin-resistant high-fat-diet-fed C57BL/6 mice treated for 27 days with 100 mg kg(-1) day(-1) amorfrutin B.

RESULTS

Amorfrutin B showed low nanomolar binding affinity to PPARγ, and micromolar binding to the isotypes PPARα and PPARβ/δ. Amorfrutin B selectively modulated PPARγ activity at low nanomolar concentrations. In insulin-resistant mice, amorfrutin B considerably improved insulin sensitivity, glucose tolerance and blood lipid variables after several days of treatment. Amorfrutin B treatment did not induce weight gain and furthermore showed liver-protecting properties. Additionally, amorfrutins had no adverse effects on osteoblastogenesis and fluid retention.

CONCLUSIONS/INTERPRETATION

The application of plant-derived amorfrutins or synthetic analogues thereof constitutes a promising approach to prevent or treat complex metabolic diseases such as insulin resistance or type 2 diabetes.

High sugar intake and development of skeletal muscle insulin resistance and inflammation in mice: a protective role for PPAR- δ agonism

Peroxisome Proliferator Activated Receptor (PPAR)-δ agonists may serve for treating metabolic diseases. However, the effects of PPAR-δ agonism within the skeletal muscle, which plays a key role in whole-body glucose metabolism, remain unclear. This study aimed to investigate the signaling pathways activated in the gastrocnemius muscle by chronic administration of the selective PPAR-δ agonist, GW0742 (1 mg/kg/day for 16 weeks), in male C57Bl6/J mice treated for 30 weeks with high-fructose corn syrup (HFCS), the major sweetener in foods and soft-drinks (15% wt/vol in drinking water). Mice fed with the HFCS diet exhibited hyperlipidemia, hyperinsulinemia, hyperleptinemia, and hypoadiponectinemia. In the gastrocnemius muscle, HFCS impaired insulin and AMP-activated protein kinase signaling pathways and reduced GLUT-4 and GLUT-5 expression and membrane translocation. GW0742 administration induced PPAR-δ upregulation and improvement in glucose and lipid metabolism. Diet-induced activation of nuclear factor-κB and expression of inducible-nitric-oxide-synthase and intercellular-adhesion-molecule-1 were attenuated by drug treatment. These effects were accompanied by reduction in the serum concentration of interleukin-6 and increase in muscular expression of fibroblast growth factor-21. Overall, here we show that PPAR-δ activation protects the skeletal muscle against the metabolic abnormalities caused by chronic HFCS exposure by affecting multiple levels of the insulin and inflammatory cascades.

Fibroblast growth factor 21 is not required for the antidiabetic actions of the thiazoladinediones

Fibroblast growth factor 21 is an emerging metabolic regulator that was recently proposed to be a fed-state inducible factor in adipose tissue. As mice lacking FGF21 were refractory to treatment with rosiglitazone, FGF21 was suggested to underlie PPARγ-driven pharmacology and side effect profile (Dutchak et al., 2012 [12]). To evaluate FGF21/PPARγ cross-talk we conducted experiments in control and FGF21 null animals and found that rosiglitazone was equally efficacious in both strains. Specifically, diverse endpoints ranging from enhanced glycemic control, improved lipid homeostasis and side effects such as adipose accumulation were evident in both genotypes. Furthermore, the transcriptional signature and cytokine secretion profile of rosiglitazone action were maintained in our FGF21KO animals. Finally, we found that FGF21 in adipose was expressed at comparable levels in fasted and fed states. Thus, our data present a new viewpoint on the FGF21/PPARγ interplay whereby FGF21 is not necessary for the metabolic events downstream of PPARγ.

Development of a novel long-acting antidiabetic FGF21 mimetic by targeted conjugation to a scaffold antibody

Fibroblast growth factor (FGF)21 improves insulin sensitivity, reduces body weight, and reverses hepatic steatosis in preclinical species. We generated long-acting FGF21 mimetics by site-specific conjugation of the protein to a scaffold antibody. Linking FGF21 through the C terminus decreased bioactivity, whereas bioactivity was maintained by linkage to selected internal positions. In mice, these CovX-Bodies retain efficacy while increasing half-life up to 70-fold compared with wild-type FGF21. A preferred midlinked CovX-Body, CVX-343, demonstrated enhanced in vivo stability in preclinical species, and a single injection improved glucose tolerance for 6 days in ob/ob mice. In diet-induced obese mice, weekly doses of CVX-343 reduced body weight, blood glucose, and lipids levels. In db/db mice, CVX-343 increased glucose tolerance, pancreatic β-cell mass, and proliferation. CVX-343, created by linkage of the CovX scaffold antibody to the engineered residue A129C of FGF21 protein, demonstrated superior preclinical pharmacodynamics by extending serum half-life of FGF21 while preserving full therapeutic functionality.

Increased fibroblast growth factor 21 expression in high-fat diet-sensitive non-human primates (Macaca mulatta)

OBJECTIVE

Fibroblast growth factor 21 (FGF21) is a metabolic regulator of glucose and lipid metabolism. The physiological role of FGF21 is not yet fully elucidated, however, administration of FGF21 lowers blood glucose in diabetic animals. Moreover, increased levels of FGF21 are found in obese and diabetic rodents and humans compared with lean/non-diabetic controls.

METHODS

Adult male rhesus macaque monkeys were chronically maintained on a high-fat diet (HFD) or a standard diet (control, CTR). Plasma levels of FGF21, triglycerides and cholesterol were measured and body weight was record. Glucose-stimulated insulin secretion (GSIS) and glucose clearance was determined during an intravenous glucose tolerance test. Furthermore, expression of FGF21 and its receptors were determined in liver, pancreas, three white adipose tissues (WATs) and two skeletal muscles.

RESULTS

A cohort of the high-fat fed monkeys responded to the HFD with increasing body weight, plasma lipids, total cholesterol, GSIS and decreased glucose tolerance. These monkeys were termed HFD sensitive. Another cohort of monkeys did not become obese and maintained normal insulin sensitivity. These animals were defined as HFD resistant. Plasma FGF21 levels were significantly increased in all HFD fed monkeys compared with the CTR group. The HFD-sensitive monkeys showed a significant increase in FGF21 mRNA expression in all examined tissues compared with CTR, whereas FGF21 expression in the HFD-resistant group was only increased in the liver, pancreas and the retroperitoneal WAT. In the WAT, the co-receptor β-klotho was downregulated in the HFD-sensitive monkeys compared with the HFD-resistant group.

CONCLUSION

This study demonstrates that HFD changes FGF21 and FGF21 receptor expression in a tissue-specific manner in rhesus monkeys; differential regulation is moreover observed between HFD-sensitive and -resistant monkeys. Monkeys that maintain normal levels of the FGF21 co-receptor β-klotho in the WAT on HFD were protected toward development of dyslipidemia and hyperglycemia.

FGF21 mediates the lipid metabolism response to amino acid starvation

Lipogenic gene expression in liver is repressed in mice upon leucine deprivation. The hormone fibroblast growth factor 21 (FGF21), which is critical to the adaptive metabolic response to starvation, is also induced under amino acid deprivation. Upon leucine deprivation, we found that FGF21 is needed to repress expression of lipogenic genes in liver and white adipose tissue, and stimulate phosphorylation of hormone-sensitive lipase in white adipose tissue. The increased expression of Ucp1 in brown adipose tissue under these circumstances is also impaired in FGF21-deficient mice. Our results demonstrate the important role of FGF21 in the regulation of lipid metabolism during amino acid starvation.

Activation of Liver FGF21 in hepatocarcinogenesis and during hepatic stress

Background

FGF21 is a promising intervention therapy for metabolic diseases as fatty liver, obesity and diabetes. Recent results suggest that FGF21 is highly expressed in hepatocytes under metabolic stress caused by starvation, hepatosteatosis, obesity and diabetes. Hepatic FGF21 elicits metabolic benefits by targeting adipocytes of the peripheral adipose tissue through the transmembrane FGFR1-KLB complex. Ablation of adipose FGFR1 resulted in increased hepatosteatosis under starvation conditions and abrogation of the anti-obesogenic action of FGF21. These results indicate that FGF21 may be a stress responsive hepatokine that targets adipocytes and adipose tissue for alleviating the damaging effects of stress on the liver. However, it is unclear whether hepatic induction of FGF21 is limited to only metabolic stress, or to a more general hepatic stress resulting from liver pathogenesis and injury.

Methods

In this survey-based study, we examine the nature of hepatic FGF21 activation in liver tissues and tissue sections from several mouse liver disease models and human patients, by quantitative PCR, immunohistochemistry, protein chemistry, and reporter and CHIP assays. The liver diseases include genetic and chemical-induced HCC, liver injury and regeneration, cirrhosis, and other types of liver diseases.

Results

We found that mouse FGF21 is induced in response to chemical (DEN treatment) and genetic-induced hepatocarcinogenesis (disruptions in LKB1, p53, MST1/2, SAV1 and PTEN). It is also induced in response to loss of liver mass due to partial hepatectomy followed by regeneration. The induction of FGF21 expression is potentially under the control of stress responsive transcription factors p53 and STAT3. Serum FGF21 levels correlate with FGF21 expression in hepatocytes. In patients with hepatitis, fatty degeneration, cirrhosis and liver tumors, FGF21 levels in hepatocytes or phenotypically normal hepatocytes are invariably elevated compared to normal health subjects.

Conclusion

FGF21 is an inducible hepatokine and could be a biomarker for normal hepatocyte function. Activation of its expression is a response of functional hepatocytes to a broad spectrum of pathological changes that impose both cellular and metabolic stress on the liver. Taken together with our recent data, we suggest that hepatic FGF21 is a general stress responsive factor that targets adipose tissue for normalizing local and systemic metabolic parameters while alleviating the overload and damaging effects imposed by the pathogenic stress on the liver. This study therefore provides a rationale for clinical biomarker studies in humans.

Detection of thermogenesis in rodents in response to anti-obesity drugs and genetic modification

Many compounds and genetic manipulations are claimed to confer resistance to obesity in rodents by raising energy expenditure. Examples taken from recent and older literature, demonstrate that such claims are often based on measurements of energy expenditure after body composition has changed, and depend on comparisons of energy expenditure divided by body weight. This is misleading because white adipose tissue has less influence than lean tissue on energy expenditure. Application of this approach to human data would suggest that human obesity is usually due to a low metabolic rate, which is not an accepted view. Increased energy expenditure per animal is a surer way of demonstrating thermogenesis, but even then it is important to know whether this is due to altered body composition (repartitioning), or increased locomotor activity rather than thermogenesis per se. Regression analysis offers other approaches. The thermogenic response to some compounds has a rapid onset and so cannot be due to altered body composition. These compounds usually mimic or activate the sympathetic nervous system. Thermogenesis occurs in, but may not be confined to, brown adipose tissue. It should not be assumed that weight loss in response to these treatments is due to thermogenesis unless there is a sustained increase in 24-h energy expenditure. Thyroid hormones and fibroblast growth factor 21 also raise energy expenditure before they affect body composition. Some treatments and genetic modifications alter the diurnal rhythm of energy expenditure. It is important to establish whether this is due to altered locomotor activity or efficiency of locomotion. There are no good examples of compounds that do not affect short-term energy expenditure but have a delayed effect. How and under what conditions a genetic modification or compound increases energy expenditure influences the decision on whether to seek drugs for the target or take a candidate drug into clinical studies.

Recruitment of histone methyltransferase G9a mediates transcriptional repression of Fgf21 gene by E4BP4 protein

The liver responds to fasting-refeeding cycles by reprogramming expression of metabolic genes. Fasting potently induces one of the key hepatic hormones, fibroblast growth factor 21 (FGF21), to promote lipolysis, fatty acid oxidation, and ketogenesis, whereas refeeding suppresses its expression. We previously reported that the basic leucine zipper transcription factor E4BP4 (E4 binding protein 4) represses Fgf21 expression and disrupts its circadian oscillations in cultured hepatocytes. However, the epigenetic mechanism for E4BP4-dependent suppression of Fgf21 has not yet been addressed. Here we present evidence that histone methyltransferase G9a mediates E4BP4-dependent repression of Fgf21 during refeeding by promoting repressive histone modification. We find that Fgf21 expression is up-regulated in E4bp4 knock-out mouse liver. We demonstrate that the G9a-specific inhibitor BIX01294 abolishes suppression of the Fgf21 promoter activity by E4BP4, whereas overexpression of E4bp4 leads to increased levels of dimethylation of histone 3 lysine 9 (H3K9me2) around the Fgf21 promoter region. Furthermore, we also show that E4BP4 interacts with G9a, and knockdown of G9a blocks repression of Fgf21 promoter activity and expression in cells overexpressing E4bp4. A G9a mutant lacking catalytic activity, due to deletion of the SET domain, fails to inhibit the Fgf21 promoter activity. Importantly, acute hepatic knockdown by adenoviral shRNA targeting G9a abolishes Fgf21 repression by refeeding, concomitant with decreased levels of H3K9me2 around the Fgf21 promoter region. In summary, we show that G9a mediates E4BP4-dependent suppression of hepatic Fgf21 by enhancing histone methylation (H3K9me2) of the Fgf21 promoter.

Fibroblast growth factor 21 predicts the metabolic syndrome and type 2 diabetes in Caucasians

OBJECTIVE

The incidence of the metabolic syndrome and type 2 diabetes mellitus (T2DM) is rising worldwide. Liver-derived fibroblast growth factor (FGF)-21 affects glucose and lipid metabolism. The aim of this study was to analyze the predictive value of FGF-21 on the incidence of T2DM and the metabolic syndrome.

RESEARCH DESIGN AND METHODS

The Metabolic Syndrome Berlin Potsdam (MeSyBePo) recall study includes 440 individuals. Glucose metabolism was analyzed using an oral glucose tolerance test, including insulin measurements. FGF-21 was measured using enzyme-linked immunosorbent assay. Primary study outcome was diabetes and the metabolic syndrome incidence and change of glucose subtraits.

RESULTS

During a mean follow-up of 5.30 ± 0.1 years, 54 individuals developed the metabolic syndrome, 35 developed T2DM, and 69 with normal glucose tolerance at baseline progressed to impaired glucose metabolism, defined as impaired fasting glucose, impaired glucose tolerance, or T2DM. FGF-21 predicted incident metabolic syndrome (lnFGF-21 odds ratio [OR] 2.6 [95% CI 1.5 - 4.5]; P = 0.001), T2DM (2.4 [1.2-4.7]; P = 0.01), and progression to impaired glucose metabolism (2.2 [1.3 - 3.6]; P = 0.002) after adjustment for age, sex, BMI, and follow-up time. Additional adjustment for waist-to-hip ratio, systolic blood pressure, HDL cholesterol, triglycerides, and fasting glucose did not substantially modify the predictive value of FGF-21.

CONCLUSIONS

FGF-21 is an independent predictor of the metabolic syndrome and T2DM in apparently healthy Caucasians. These results may indicate FGF-21 resistance precedes the onset of the metabolic syndrome and T2DM.

Emerging role of fibroblast growth factors 15/19 and 21 as metabolic integrators in the liver

Fibroblast growth factors (FGFs) 15/19 and 21 belong to the FGF endocrine subfamily. They present the intriguing characteristic to be transcribed and secreted in certain tissues and to act as hormones. The insulin-mimetic properties of FGF21 and the regulatory role of FGF15/19 in bile acid and glucose homeostasis endorse these hormones as druggable targets in metabolic disorders. Here, we present details on discoveries, identification, transcriptional regulation, and mechanism of actions of FGF15/19 and FGF21 with a critical perspective view on their putative role as metabolic integrators in the liver.

Fasting-induced FGF21 is repressed by LXR activation via recruitment of an HDAC3 corepressor complex in mice

The liver plays a pivotal role in the physiological adaptation to fasting and a better understanding of the metabolic adaptive responses may give hints on new therapeutic strategies to control the metabolic diseases. The liver X receptors (LXRs) are well-established regulators of lipid and glucose metabolism. More recently fibroblast growth factor 21 (FGF21) has emerged as an important regulator of energy homeostasis. We hypothesized that the LXR transcription factors could influence Fgf21 expression, which is induced in response to fasting. Wild-type, LXRα(-/-), and LXRβ(-/-) mice were treated for 3 d with vehicle or the LXR agonist GW3965 and fasted for 12 h prior to the killing of the animals. Interestingly, serum FGF21 levels were induced after fasting, but this increase was blunted when the mice were treated with GW3965 independently of genotypes. Compared with wild-type mice, GW3965-treated LXRα(-/-) and LXRβ(-/-) mice showed improved insulin sensitivity and enhanced ketogenic response at fasting. Of note is that during fasting, GW3965 treatment tended to reduce liver triglycerides as opposed to the effect of the agonist in the fed state. The LXR-dependent repression of Fgf21 seems to be mainly mediated by the recruitment of LXRβ onto the Fgf21 promoter upon GW3965 treatment. This repression by LXRβ occurs through the recruitment and stabilization of the repressor complex composed of retinoid-related orphan receptor-α/Rev-Erbα/histone deacetylase 3 onto the Fgf21 promoter. Our data clearly demonstrate that there is a cross talk between the LXR and FGF21 signaling pathways in the adaptive response to fasting.

βKlotho is required for fibroblast growth factor 21 effects on growth and metabolism

Fibroblast growth factor 21 (FGF21) is a fasting-induced hepatokine that has potent pharmacologic effects in mice, which include improving insulin sensitivity and blunting growth. The single-transmembrane protein βKlotho functions as a coreceptor for FGF21 in vitro. To determine if βKlotho is required for FGF21 action in vivo, we generated whole-body and adipose tissue-selective βKlotho-knockout mice. All of the effects of FGF21 on growth and metabolism were lost in whole-body βKlotho-knockout mice. Selective elimination of βKlotho in adipose tissue blocked the acute insulin-sensitizing effects of FGF21. Taken together, these data demonstrate that βKlotho is essential for FGF21 activity and that βKlotho in adipose tissue contributes to the beneficial metabolic actions of FGF21.

Biological role, clinical significance, and therapeutic possibilities of the recently discovered metabolic hormone fibroblastic growth factor 21

Fibroblast growth factor 21 (FGF21), a 181 amino acid circulating protein, is a member of the FGF superfamily, with relevant metabolic actions. It acts through the interaction with specific FGF receptors and a cofactor called β-Klotho, whose expression is predominantly detected in metabolically active organs. FGF21 stimulates glucose uptake in adipocytes via the induction of glucose transporter-1. This action is additive and independent of insulin. β-Cell function and survival are preserved, and glucagon secretion is reduced by this protein, thus decreasing hepatic glucose production and improving insulin sensitivity. Lipid profile has been shown to be improved by FGF21 in several animal models. FGF21 increases energy expenditure in rodents and induces weight loss in diabetic nonhuman primates. It also exerts favorable effects on hepatic steatosis and reduces tissue lipid content in rodents. Adaptive metabolic responses to fasting, including stimulation of ketogenesis and fatty acid oxidation, seem to be partially mediated by FGF21. In humans, serum FGF21 concentrations have been found elevated in insulin-resistant states, such as impaired glucose tolerance and type 2 diabetes. FGF21 levels are correlated with hepatic insulin resistance index, fasting blood glucose, HbA1c, and blood glucose after an oral glucose tolerance test. A relationship between FGF21 levels and long-term diabetic complications, such as nephropathy and carotid atheromatosis, has been reported. FGF21 levels decreased in diabetic patients after starting therapy with insulin or oral agents. Increased FGF21 serum levels have also been found to be associated with obesity. In children, it is correlated with BMI and leptin levels, whereas in adults, FGF21 levels are mainly related to several components of the metabolic syndrome. Serum FGF21 levels have been found to be elevated in patients with ischemic heart disease. In patients with renal disease, FGF21 levels exhibited a progressive increase as renal function deteriorates. Circulating FGF21 levels seem to be related to insulin resistance and inflammation in dialysis patients. In summary, FGF21 is a recently identified hormone with antihyperglycemic, antihyperlipidemic, and thermogenic properties. Direct or indirect potentiation of its effects might be a potential therapeutic target in insulin-resistant states.

TNF-α represses β-Klotho expression and impairs FGF21 action in adipose cells: involvement of JNK1 in the FGF21 pathway

Fibroblast growth factor 21 (FGF21) is a member of the FGF family that reduces glycemia and ameliorates insulin resistance. Adipose tissue is a main target of FGF21 action. Obesity is associated with a chronic proinflammatory state. Here, we analyzed the role of proinflammatory signals in the FGF21 pathway in adipocytes, evaluating the effects of TNF-α on β-Klotho and FGF receptor-1 expression and FGF21 action in adipocytes. We also determined the effects of rosiglitazone on β-Klotho and FGF receptor-1 expression in models of proinflammatory signal induction in vitro and in vivo (high-fat diet-induced obesity). Because c-Jun NH(2)-terminal kinase 1 (JNK1) serves as a sensing juncture for inflammatory status, we also evaluated the involvement of JNK1 in the FGF21 pathway. TNF-α repressed β-Klotho expression and impaired FGF21 action in adipocytes. Rosiglitazone prevented the reduction in β-Klotho expression elicited by TNF-α. Moreover, β-Klotho levels were reduced in adipose tissue from high-fat diet-induced obese mice, whereas rosiglitazone restored β-Klotho to near-normal levels. β-Klotho expression was increased in white fat from JNK1(-/-) mice. The absence of JNK1 increased the responsiveness of mouse embryonic fibroblast-derived adipocytes and brown adipocytes to FGF21. In conclusion, we show that proinflammatory signaling impairs β-Klotho expression and FGF21 responsiveness in adipocytes. We also show that JNK1 activity is involved in modulating FGF21 effects in adipocytes. The impairment in the FGF21 response machinery in adipocytes and the reduction in FGF21 action in response to proinflammatory signals may play important roles in metabolic alterations in obesity and other diseases associated with enhanced inflammation.

Cobalt chloride decreases fibroblast growth factor-21 expression dependent on oxidative stress but not hypoxia-inducible factor in Caco-2 cells

Fibroblast growth factor-21 (FGF21) is a potential metabolic regulator with multiple beneficial effects on metabolic diseases. FGF21 is mainly expressed in the liver, but is also found in other tissues including the intestine, which expresses β-klotho abundantly. The intestine is a unique organ that operates in a physiologically hypoxic environment, and is responsible for the fat absorption processes including triglyceride breakdown, re-synthesis and absorption into the portal circulation. In the present study, we investigated the effects of hypoxia and the chemical hypoxia inducer, cobalt chloride (CoCl(2)), on FGF21 expression in Caco-2 cells and the consequence of fat accumulation. Physical hypoxia (1% oxygen) and CoCl(2) treatment decreased both FGF21 mRNA and secreted protein levels. Gene silence and inhibition of hypoxia-inducible factor-α (HIFα) did not affect the reduction of FGF21 mRNA and protein levels by hypoxia. However, CoCl(2) administration caused a significant increase in oxidative stress. The addition of n-acetylcysteine (NAC) suppressed CoCl(2)-induced reactive oxygen species (ROS) formation and completely negated CoCl(2)-induced FGF21 loss. mRNA stability analysis demonstrated that the CoCl(2) administration caused a remarkable reduction in FGF21 mRNA stability. Furthermore, CoCl(2) increased intracellular triglyceride (TG) accumulation, along with a reduction in mRNA levels of lipid lipase, hormone sensitive lipase (HSL) and adipose triglyceride lipase (ATGL), and an increase of sterol regulatory element-binding protein-1c (SREBP1c) and stearoyl-coenzyme A (SCD1). Addition of both NAC and recombinant FGF21 significantly attenuated the CoCl(2)-induced TG accumulation. In conclusion, the decrease of FGF21 in Caco-2 cells by chemical hypoxia is independent of HIFα, but dependent on an oxidative stress-mediated mechanism. The regulation of FGF21 by hypoxia may contribute to intestinal lipid metabolism and absorption.

Metabolic actions of fibroblast growth factor 21

PURPOSE OF REVIEW

FGF21 has emerged as a hormone involved in energy homeostasis. A large number of recent reports have expanded the role of FGF21 from a response factor to prolonged fasting to a key hormone that regulates free fatty acid (FFAs) levels. The therapeutic role of recombinant human FGF21 for type 2 diabetes and dyslipidemia is under study.

RECENT FINDINGS

Recent evidence suggests that supraphysiological concentrations of FFAs induce FGF21 secretion (i.e., starvation and intense physical activity) through the peroxisome proliferator-activated receptor alpha (PPARα) pathway. The rise in FGF21 levels is aimed at improving energy production (ketogenesis) and utilization (oxidation) of FFAs. FGF21 increment may protect against chronic exposure to high concentrations of FFAs, which causes lipotoxicity in muscle, pancreas, and liver. In addition, FGF21 induces appetite and inhibits growth, probably as part of the adaptive starvation response. The autocrine function of FGF21 in adipose tissue increases PPARγ activity and glucose uptake. Increased plasma FGF21 levels have been found in insulin resistance states in humans. However, the reason for this rise in FGF21 values is still under study.

SUMMARY

We propose that FGF21 serves as a defense mechanism against supraphysiological concentrations of FFAs. In addition, FGF21 might have a therapeutic indication in humans.

Impact of short-term high-fat feeding and insulin-stimulated FGF21 levels in subjects with low birth weight and controls

OBJECTIVE

Fibroblast growth factor 21 (FGF21) is a metabolic factor involved in glucose and lipid metabolism. However, little is known about the physiological role of FGF21 during a dietary challenge in humans.

RESEARCH DESIGN AND METHODS

Twenty healthy low birth weight (LBW) with known risk of type 2 diabetes and 26 control (normal birth weight (NBW)) young men were subjected to 5 days of high-fat (HF) overfeeding (+50%). Basal and clamp insulin-stimulated serum FGF21 levels were examined before and after the diet, and FGF21 mRNA expression was measured in muscle and fat biopsies respectively.

RESULTS

Five days of HF overfeeding diet significantly (P<0.001) increased fasting serum FGF21 levels in both the groups (P<0.001). Furthermore, insulin infusion additionally increased serum FGF21 levels to a similar extent in both the groups. Basal mRNA expression of FGF21 in muscle was near the detection limit and not present in fat in both the groups before and after the dietary challenge. However, insulin significantly (P<0.001) increased FGF21 mRNA in both muscle and fat in both the groups during both diets.

CONCLUSION

Short-term HF overfeeding markedly increased serum FGF21 levels in healthy young men with and without LBW but failed to increase muscle or fat FGF21 mRNA levels. This suggests that the liver may be responsible for the rise of serum FGF21 levels during overfeeding. In contrast, the increase in serum FGF21 levels during insulin infusion may arise from increased transcription in muscle and fat. We speculate that increased serum FGF21 levels during HF overfeeding may be a compensatory response to increase fatty acid oxidation and energy expenditure.

Fibroblast growth factor 21 (FGF-21) and its relation to obesity, metabolic syndrome, and nonalcoholic fatty liver in children: a longitudinal analysis

CONTEXT

Fibroblast growth factor 21 (FGF-21), a potent activator of glucose uptake, has been proposed to be related to insulin resistance, metabolic syndrome (MetS), nonalcoholic fatty liver disease (NAFLD), and weight status.

OBJECTIVE

Our objective was to study the relationships between FGF-21, parameters of MetS, and NAFLD before and after weight loss in obese children.

DESIGN AND SETTING

This was a cross-sectional comparison between obese and normal-weight children and longitudinal 1-yr follow-up study in obese children participating in a lifestyle intervention in a primary care setting.

PATIENTS

Patients included 60 obese and 40 lean children of same age, gender, and pubertal stage.

INTERVENTION

The outpatient 1-yr intervention program was based on exercise, behavior, and nutrition therapy.

MAIN OUTCOMES MEASURES

We evaluated fasting serum FGF-21, weight status [body mass index (BMI) expressed as sd score (SDS)], body fat, insulin resistance index (homeostasis model assessment), leptin, transaminases, free fatty acids (FFA), waist circumference, blood pressure, and lipids.

RESULTS

Compared with the normal-weight children, obese children demonstrated significantly (P < 0.001) increased FGF-21, leptin, and homeostasis model assessment levels. FGF-21 was significantly (P < 0.05) correlated to BMI, SDS-BMI, FFA, and leptin both in cross-sectional and longitudinal analyses but not to any additional analyzed parameter. Children with and without MetS or NAFLD did not differ significantly with respect to their FGF-21 concentrations. A decrease of SDS-BMI was associated with a significant (P = 0.038) decrease of FGF-21 levels (mean -34%).

CONCLUSIONS

FGF-21 concentrations are reversibly increased in obese children and are related to leptin and FFA. However, our data do not support a significant relationship between FGF-21, insulin resistance, and features of MetS or NAFLD in children.

FGF21 is increased by inflammatory stimuli and protects leptin-deficient ob/ob mice from the toxicity of sepsis

The acute phase response (APR) produces marked alterations in lipid and carbohydrate metabolism including decreasing plasma ketone levels. Fibroblast growth factor 21 (FGF21) is a recently discovered hormone that regulates lipid and glucose metabolism and stimulates ketogenesis. Here we demonstrate that lipopolysaccharide (LPS), zymosan, and turpentine, which induce the APR, increase serum FGF21 levels 2-fold. Although LPS, zymosan, and turpentine decrease the hepatic expression of FGF21, they increase FGF21 expression in adipose tissue and muscle, suggesting that extrahepatic tissues account for the increase in serum FGF21. After LPS administration, the characteristic decrease in plasma ketone levels is accentuated in FGF21-/- mice, but this is not due to differences in expression of carnitine palmitoyltransferase 1α or hydroxymethyglutaryl-CoA synthase 2 in liver, because LPS induces similar decreases in the expression of these genes in FGF21-/- and control mice. However, in FGF21-/- mice, the ability of LPS to increase plasma free fatty acid levels is blunted. This failure to increase plasma free fatty acid could contribute to the accentuated decrease in plasma ketone levels because the transport of fatty acids from adipose tissue to liver provides the substrate for ketogenesis. Treatment with exogenous FGF21 reduced the number of animals that die and the rapidity of death after LPS administration in leptin-deficient ob/ob mice and to a lesser extent in control mice. FGF21 also protected from the toxic effects of cecal ligation and puncture-induced sepsis. Thus, FGF21 is a positive APR protein that protects animals from the toxic effects of LPS and sepsis.

Activating transcription factor 4-dependent induction of FGF21 during amino acid deprivation

Nutrient deprivation or starvation frequently correlates with amino acid limitation. Amino acid starvation initiates a signal transduction cascade starting with the activation of the kinase GCN2 (general control non-derepressible 2) phosphorylation of eIF2 (eukaryotic initiation factor 2), global protein synthesis reduction and increased ATF4 (activating transcription factor 4). ATF4 modulates a wide spectrum of genes involved in the adaptation to dietary stress. The hormone FGF21 (fibroblast growth factor 21) is induced during fasting in liver and its expression induces a metabolic state that mimics long-term fasting. Thus FGF21 is critical for the induction of hepatic fat oxidation, ketogenesis and gluconeogenesis, metabolic processes which are essential for the adaptive metabolic response to starvation. In the present study, we have shown that FGF21 is induced by amino acid deprivation in both mouse liver and cultured HepG2 cells. We have identified the human FGF21 gene as a target gene for ATF4 and we have localized two conserved ATF4-binding sequences in the 5' regulatory region of the human FGF21 gene, which are responsible for the ATF4-dependent transcriptional activation of this gene. These results add FGF21 gene induction to the transcriptional programme initiated by increased levels of ATF4 and offer a new mechanism for the induction of the FGF21 gene expression under nutrient deprivation.

Serum fibroblast growth factor 21 was elevated in subjects with type 2 diabetes mellitus and was associated with the presence of carotid artery plaques

AIMS

Fibroblast growth factor 21 (FGF21) is an important regulator of glucose/lipid metabolism. Although there are studies examining the relationship between serum FGF21 levels and glucose homeostasis, the role of FGF21 remains unclear. The objective of this study was to examine whether serum FGF21 levels are associated with metabolic parameters in subjects with varying degrees of obesity and glucose tolerance and with complications in subjects with type2 diabetes mellitus (T2DM).

METHODS

The study consisted of 213 subjects who were lean and had normal glucose tolerance (lean NGT), were overweight with NGT, had impaired glucose tolerance (IGT) or had T2DM. Serum FGF21 levels and their associations with the parameters of adiposity, glucose tolerance and the presence of diabetic complications were examined.

RESULTS

The serum FGF21 levels in T2DM were higher than in lean NGT. Serum FGF21 levels showed a positive correlation with the urine albumin-to-creatinine ratio (ACR) in all subjects except for the T2DM subjects, who showed a correlation after adjustment of age, gender and body mass index. Moreover, the subjects with carotid artery plaque showed higher serum FGF21 levels than those without complications.

CONCLUSION

Serum FGF21 levels were associated with the urine ACR and diabetic complications including carotid artery plaque.

Differential specificity of endocrine FGF19 and FGF21 to FGFR1 and FGFR4 in complex with KLB

BACKGROUND

Recent studies suggest that betaKlotho (KLB) and endocrine FGF19 and FGF21 redirect FGFR signaling to regulation of metabolic homeostasis and suppression of obesity and diabetes. However, the identity of the predominant metabolic tissue in which a major FGFR-KLB resides that critically mediates the differential actions and metabolism effects of FGF19 and FGF21 remain unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We determined the receptor and tissue specificity of FGF21 in comparison to FGF19 by using direct, sensitive and quantitative binding kinetics, and downstream signal transduction and expression of early response gene upon administration of FGF19 and FGF21 in mice. We found that FGF21 binds FGFR1 with much higher affinity than FGFR4 in presence of KLB; while FGF19 binds both FGFR1 and FGFR4 in presence of KLB with comparable affinity. The interaction of FGF21 with FGFR4-KLB is very weak even at high concentration and could be negligible at physiological concentration. Both FGF19 and FGF21 but not FGF1 exhibit binding affinity to KLB. The binding of FGF1 is dependent on where FGFRs are present. Both FGF19 and FGF21 are unable to displace the FGF1 binding, and conversely FGF1 cannot displace FGF19 and FGF21 binding. These results indicate that KLB is an indispensable mediator for the binding of FGF19 and FGF21 to FGFRs that is not required for FGF1. Although FGF19 can predominantly activate the responses of the liver and to a less extent the adipose tissue, FGF21 can do so significantly only in the adipose tissue and adipocytes. Among several metabolic and endocrine tissues, the response of adipose tissue to FGF21 is predominant, and can be blunted by the ablation of KLB or FGFR1.

CONCLUSIONS

Our results indicate that unlike FGF19, FGF21 is unable to bind FGFR4-KLB complex with affinity comparable to FGFR1-KLB, and therefore, at physiological concentration less likely to directly and significantly target the liver where FGFR4-KLB predominantly resides. However, both FGF21 and FGF19 have the potential to activate responses of primarily the adipose tissue where FGFR1-KLB resides.

FGF21 and the second coming of PPARγ

Peptide hormone fibroblast growth factor-21 (FGF21) has insulin-mimetic properties. Dutchak et al. now suggest that FGF21 also acts in an autocrine fashion in adipocytes and is required to mediate effects of the PPARγ agonist class of antidiabetic drugs. Does this new property improve FGF21's fledgling clinical prospects or endorse a clinical resuscitation of PPARγ agonists?

Fibroblast growth factor-21 regulates PPARγ activity and the antidiabetic actions of thiazolidinediones

Fibroblast growth factor-21 (FGF21) is a circulating hepatokine that beneficially affects carbohydrate and lipid metabolism. Here, we report that FGF21 is also an inducible, fed-state autocrine factor in adipose tissue that functions in a feed-forward loop to regulate the activity of peroxisome proliferator-activated receptor γ (PPARγ), a master transcriptional regulator of adipogenesis. FGF21 knockout (KO) mice display defects in PPARγ signaling including decreased body fat and attenuation of PPARγ-dependent gene expression. Moreover, FGF21-KO mice are refractory to both the beneficial insulin-sensitizing effects and the detrimental weight gain and edema side effects of the PPARγ agonist rosiglitazone. This loss of function in FGF21-KO mice is coincident with a marked increase in the sumoylation of PPARγ, which reduces its transcriptional activity. Adding back FGF21 prevents sumoylation and restores PPARγ activity. Collectively, these results reveal FGF21 as a key mediator of the physiologic and pharmacologic actions of PPARγ.

Endocrine fibroblast growth factors 15/19 and 21: from feast to famine

We review the physiology and pharmacology of two atypical fibroblast growth factors (FGFs)-FGF15/19 and FGF21-that can function as hormones. Both FGF15/19 and FGF21 act on multiple tissues to coordinate carbohydrate and lipid metabolism in response to nutritional status. Whereas FGF15/19 is secreted from the small intestine in response to feeding and has insulin-like actions, FGF21 is secreted from the liver in response to extended fasting and has glucagon-like effects. FGF21 also acts in an autocrine fashion in several tissues, including adipose. The pharmacological actions of FGF15/19 and FGF21 make them attractive drug candidates for treating metabolic disease.

Fibroblast growth factor 21 promotes bone loss by potentiating the effects of peroxisome proliferator-activated receptor γ

The endocrine hormone fibroblast growth factor 21 (FGF21) is a powerful modulator of glucose and lipid metabolism and a promising drug for type 2 diabetes. Here we identify FGF21 as a potent regulator of skeletal homeostasis. Both genetic and pharmacologic FGF21 gain of function lead to a striking decrease in bone mass. In contrast, FGF21 loss of function leads to a reciprocal high-bone-mass phenotype. Mechanistically, FGF21 inhibits osteoblastogenesis and stimulates adipogenesis from bone marrow mesenchymal stem cells by potentiating the activity of peroxisome proliferator-activated receptor γ (PPAR-γ). Consequently, FGF21 deletion prevents the deleterious bone loss side effect of the PPAR-γ agonist rosiglitazone. Therefore, FGF21 is a critical rheostat for bone turnover and a key integrator of bone and energy metabolism. These results reveal that skeletal fragility may be an undesirable consequence of chronic FGF21 administration.

Various oscillation patterns of serum fibroblast growth factor 21 concentrations in healthy volunteers

BACKGROUND

Fibroblast growth factor 21 (FGF21) was originally identified as a paroxysm proliferator activated receptor-α target gene product and is a hormone involved in metabolic regulation. The purpose of this study was to investigate the diurnal variation of serum FGF21 concentration in obese and non-obese healthy volunteers.

METHODS

Blood samples were collected from five non-obese (body mass index [BMI] ≤23 kg/m(2)) and five obese (BMI ≥25 kg/m(2)) healthy young men every 30 to 60 minutes over 24 hours. Serum FGF21 concentrations were determined by radioimmunoassay. Anthropometric parameters, glucose, free fatty acid, insulin, leptin, and cortisol concentrations were also measured.

RESULTS

The serum FGF21 concentrations displayed various individual oscillation patterns. The oscillation frequency ranged between 6 and 12 times per day. The average duration of oscillation was 2.52 hours (range, 1.9 to 3.0 hours). The peaks and troughs of FGF21 oscillation showed no circadian rhythm. However, the oscillation frequency had a diurnal variation and was lower during the light-off period than during the light-on period (2.4 vs. 7.3 times, P<0.001). There was no difference in the total frequency or duration of oscillations between non-obese and obese subjects, but obese individuals had increased numbers of larger oscillations (amplitude ≥0.19 ng/mL).

CONCLUSION

Various oscillation patterns in serum FGF21 concentration were observed, and reduced oscillation frequencies were seen during sleep. The oscillation patterns of serum FGF21 concentration suggest that FGF21 may be secreted into systemic circulation in a pulsatile manner. Obesity appeared to affect the amplitude of oscillations of serum FGF21.