Fibroblast Growth Factor 21 Production Research Articles

A Specific ChREBP and PPARα Cross-Talk Is Required for the Glucose-Mediated FGF21 Response

SummaryWhile the physiological benefits of the fibroblast growth factor 21 (FGF21) hepatokine are documented in response to fasting, little information is available on Fgf21 regulation in a glucose-overload context. We report that peroxisome-proliferator-activated receptor α (PPARα), a nuclear receptor of the fasting response, is required with the carbohydrate-sensitive transcription factor carbohydrate-responsive element-binding protein (ChREBP) to balance FGF21 glucose response. Microarray analysis indicated that only a few hepatic genes respond to fasting and glucose similarly to Fgf21. Glucose-challenged Chrebp−/− mice exhibit a marked reduction in FGF21 production, a decrease that was rescued by re-expression of an active ChREBP isoform in the liver of Chrebp−/− mice. Unexpectedly, carbohydrate challenge of hepatic Pparα knockout mice also demonstrated a PPARα-dependent glucose response for Fgf21 that was associated with an increased sucrose preference. This blunted response was due to decreased Fgf21 promoter accessibility and diminished ChREBP binding onto Fgf21 carbohydrate-responsive element (ChoRE) in hepatocytes lacking PPARα. Our study reports that PPARα is required for the ChREBP-induced glucose response of FGF21.

Effect of circulating glucagon and free fatty acids on hepatic FGF21 production in dairy cows.

Modern dairy cows meet the energy demand of early lactation by calling on hormonally driven mechanisms to increase the use of lipid reserves. In this context, we recently reported that fibroblast growth factor-21 (FGF21), a hormone required for efficient use of lipid reserves in rodents, is upregulated in periparturient dairy cows. Increased plasma FGF21 in early lactation coincides with elevated circulating concentrations of glucagon (GCG) and nonesterified fatty acids (NEFA). To assess the relative contribution of these factors in regulating FGF21, two experiments were performed in energy-sufficient, nonpregnant, nonlactating dairy cows. In the first study, cows were injected with saline or GCG every 8 h over a 72-h period. GCG increased hepatic FGF21 mRNA by an average of fivefold over matched controls but had no effect on plasma FGF21. In the second study, cows were infused and injected with saline, infused with Intralipid and injected with saline, or infused with Intralipid and injected with GCG. Infusions and injections were administered intravenously over 16 h and subcutaneously every 8 h, respectively. Intralipid infusion increased plasma NEFA from 92 to 550 µM within 3 h and increased plasma FGF21 from 1.3 to >11 ng/ml 6 h later; FGF21 mRNA increased by 34-fold in liver but remained invariant in adipose tissue. GCG injections during the Intralipid infusion had no additional effects on plasma NEFA, liver FGF21 mRNA, or plasma FGF21. These data implicate plasma NEFA as a key factor triggering hepatic production and increased circulating concentrations of FGF21 in early lactation.

Chronic high-sucrose diet increases fibroblast growth factor 21 production and energy expenditure in mice

Excess carbohydrate intake causes obesity in humans. On the other hand, acute administration of fructose, glucose or sucrose in experimental animals has been shown to increase the plasma concentration of anti-obesity hormones such as glucagon-like peptide 1 (GLP-1) and Fibroblast growth factor 21 (FGF21), which contribute to reducing body weight. However, the secretion and action of GLP-1 and FGF21 in mice chronically fed a high-sucrose diet has not been investigated. To address the role of anti-obesity hormones in response to increased sucrose intake, we analyzed mice fed a high-sucrose diet, a high-starch diet or a normal diet for 15 weeks. Mice fed a high-sucrose diet showed resistance to body weight gain, in comparison with mice fed a high-starch diet or control diet, due to increased energy expenditure. Plasma FGF21 levels were highest among the three groups in mice fed a high-sucrose diet, whereas no significant difference in GLP-1 levels was observed. Expression levels of uncoupling protein 1 (UCP-1), FGF receptor 1c (FGFR1c) and β-klotho (KLB) mRNA in brown adipose tissue were significantly increased in high sucrose-fed mice, suggesting increases in FGF21 sensitivity and energy expenditure. Expression of carbohydrate responsive element binding protein (ChREBP) mRNA in liver and brown adipose tissue was also increased in high sucrose-fed mice. These results indicate that FGF21 production in liver and brown adipose tissue is increased in high-sucrose diet and participates in resistance to weight gain.

A low-protein diet induces body weight loss and browning of subcutaneous white adipose tissue through enhanced expression of hepatic fibroblast growth factor 21 (FGF21).

Fibroblast growth factor 21 (FGF21) is considered a promising therapeutic candidate for the treatment of obesity. Since FGF21 production is regulated by various nutritional factors, we analyze the impact of low protein intake on circulating levels of this growth hormone in mice and in a sub cohort of the PREDIMED (Prevención con Dieta Mediterránea) trial. We also describe the role of hepatic FGF21 in metabolic adaptation to a low-protein diet (LPD). We fed control and liver-specific Fgf21 knockout (LFgf21KO) mice a LPD. This diet increased FGF21 production by inducing its overexpression in liver, and this correlated with a body weight decrease without changes in food intake. The LPD also caused FGF21-dependent browning in subcutaneous white adipose tissue (scWAT), as indicated by an increase in the expression of uncoupling protein 1 (UCP1). In a subgroup of 78 individuals from the PREDIMED trial, we observed an inverse correlation between protein intake and circulating FGF21 levels. Our results reinforce the involvement of FGF21 in coordinating energy homeostasis under a range of nutritional conditions. Moreover, here we describe an approach to increase the endogenous production of FGF21, which if demonstrated functional in humans, could generate a treatment for obesity.

α1-Adrenergic receptor downregulates hepatic FGF21 production and circulating FGF21 levels in mice

Fibroblast growth factor 21 (FGF21) is primarily secreted by the liver as an endocrine hormone and is suggested as a promising target for the treatment of metabolic diseases. FGF21 acts centrally to exert its effects on energy expenditure and body weight via the sympathetic nervous system in mice. Here we show that intraperitoneal injection of phentolamine (an α-adrenergic receptor antagonist, 5mg/kg) significantly increased plasma FGF21 levels compared with the saline controls in C57BL6J mice, whereas alprenolol (a β-adrenergic receptor antagonist, 6mg/kg) had no effect. In addition, intraperitoneal injection of prazosin (an α1-adrenergic receptor antagonist, 5mg/kg) significantly increased plasma FGF21 levels compared with the controls, whereas yohimbine (an α2-adrenergic receptor antagonist, 5mg/kg) had no effect. Moreover, the treatment with prazosin significantly increased the expression of hepatic FGF21, while having no effect on the expression of hepatic PPARα and PPARγ. After a 5-h fast, intraperitoneal injection of prazosin significantly increased plasma FGF21 levels and impaired glucose tolerance compared with controls. These findings suggest that α1-adrenergic receptor downregulates the expression of hepatic FGF21 and plasma FGF21 levels independently of feeding and hepatic PPARα and PPARγ expression in mice, and that the increases in circulating FGF21 levels might be related to impaired glucose tolerance.

Low-protein diet induces, whereas high-protein diet reduces hepatic FGF21 production in mice, but glucose and not amino acids up-regulate FGF21 in cultured hepatocytes

Fibroblast growth factor 21 (FGF21) is a polypeptide secreted by the liver and involved in several metabolic processes such as thermogenesis and lipid oxidation. The nutritional mechanisms controlling FGF21 production are poorly understood. This study aimed to investigate how dietary carbohydrates and proteins impact FGF21 production and how in turn, FGF21 is involved in the metabolic adaptation to changes in the carbohydrate and protein contents of the diet. For that purpose, we fed 25 male C57BL/6 mice diets composed of different protein and carbohydrate contents (normal-protein and carbohydrate diet (N=9, NPNC), low-protein high-carbohydrate diet (N=8, LPHC), high-protein low-carbohydrate diet (N=8, HPLC) for 3 weeks. We measured liver Fgf21 gene expression, synthesis and secretion as well as different parameters related to energy and glucose metabolism. We also investigated the direct role of amino acids and glucose in the control of Fgf21 gene expression in hepatocyte primary cultures (n=6).In vivo, FGF21 responds acutely to LPHC intake whereas under an HPLC diet, plasma FGF21 circulating levels are low in the fasted and refed states. In hepatocytes, Fgf21 expression was controlled by glucose but not amino acids. Both diets increased the thermic effect of feeding (TEF) and ketogenesis was increased in fasted HPLC mice. The results presented suggest that dietary glucose, rather than amino acids, directly controls FGF21 secretion, and that FGF21 may be involved in the increased TEF response to LPHC. The effects of the HPLC diet on ketogenesis and TEF are probably controlled by other metabolic pathways.

Dietary Carbohydrates but Not Proteins Are the Main Nutritional Determinant of FGF21 Production in Mice

Fibroblast growth factor 21 (FGF21) is a polypeptide secreted by the liver and involved in various metabolic processes, such as ketogenesis, lipid oxidation, thermogenesis and white adipose tissue browning. Nutritional mechanisms controlling its secretion are poorly understood. The aim of the present study was to investigate the influence of changes in the protein to carbohydrate ratio in the diet on FGF21 synthesis and secretion by the liver and the subsequent impact on energy metabolic pathways.25 male C57BL/6 mice aged 7 weeks were divided into 3 groups (n = 8–9/group) and fed during three weeks with a low‐protein/high‐carb diet (LPHC), a normo‐protein/normo‐carb diet (NPNC) or a high‐protein/low‐carb diet (HPLC). Food intake and body weight were measured throughout the experiment. At the end of the first week, the thermogenic response to feeding (TEF) was followed during 6 hours after ingestion of a 1g‐calibrated meal of their test‐diet given after an overnight fast. At the end of the experiment, the same test‐meal procedure was repeated and mice were killed four hours latr, for measurement of gene expression in liver, brown and epidydymal adipose tissues. In parallel with this in vivo experiment, primary cultures of rat hepatocytes were conducted to measure the specific effects of amino acids, leucine and glucose in the presence or absence of insulin and/or glucagon on the synthesis of FGF21.Plasma FGF21 measured 4 hours after ingestion of the test‐meals was positively correlated with the carbohydrate content of the diet. Consumption of LPHC and HPLC diets increased TEF but with different kinetics compared to the NPNC diet. In LPHC‐fed mice expression of UCP2 in the epididymal adipose was increased while in HPLC‐fed mice, ketogenesis, measured before ingestion of the test‐meal (fasted state), was increased. In primary cultured of hepatocytes high glucose concentrations in the presence of insulin increased FGF21 synthesis. Amino acid or leucine alone had no effect on FGF21 synthesis, glucagon decreased FGF21 synthesis and this last effect was antagonized by insulin.These results indicate that FGF21 synthesis is stimulated by carbohydrate and glucose but not protein and amino acid. Therefore, the increased TEF observed in response to ingestion of the LPHC diet was probably mediated by FGF21 whereas the influence of HPLC diet on fasting ketogenesis and TEF did not involve FGF21.

FGF21 Mediates Endocrine Control of Simple Sugar Intake and Sweet Taste Preference by the Liver

The liver is an important integrator of nutrient metabolism, yet no liver-derived factors regulating nutrient preference or carbohydrate appetite have been identified. Here we show that the liver regulates carbohydrate intake through production of the hepatokine fibroblast growth factor 21 (FGF21), which markedly suppresses consumption of simple sugars, but not complex carbohydrates, proteins, or lipids. Genetic loss of FGF21 in mice increases sucrose consumption, whereas acute administration or overexpression of FGF21 suppresses the intake of both sugar and non-caloric sweeteners. FGF21 does not affect chorda tympani nerve responses to sweet tastants, instead reducing sweet-seeking behavior and meal sizevia neurons in the hypothalamus. This liver-to-brain hormonal axis likely represents a negative feedback loop as hepatic FGF21 production is elevated by sucrose ingestion. We conclude that the liver functions to regulate macronutrient-specificintake by producing an endocrine satiety signal that acts centrally to suppress the intake of "sweets."

Mapping the response of human fibroblast growth factor 21 (FGF21) promoter to serum availability and lipoic acid in HepG2 hepatoma cells

The hormone-like polypeptide, fibroblast growth factor 21 (FGF21), is a major modulator of lipid and glucose metabolism and an exploratory treatment strategy for obesity related metabolic disorders. The costs of recombinant FGF21 and mode of delivery by injection are important constraints to its wide therapeutic use. The stimulation of endogenous FGF21 production through diet is being explored as an alternative approach. To that end, we examined the mechanism(s) by which serum manipulation and lipoic acid (a dietary activator of FGF21) induce FGF21 in human hepatocellular carcinoma HepG2 cells. Serum withdrawal markedly induced FGF21 mRNA levels (88 fold) and FGF21 secreted in the media (19 fold). Lipoic acid induced FGF21 mRNA 7 fold above DMSO-treated control cells and FGF21 secretion 3 fold. These effects were several-fold greater than those of PPARα agonist, Wy14643, which failed to induce FGF21 above and beyond the induction seen with serum withdrawal. The use of transcription inhibitor, actinomycin D, revealed that de novo mRNA synthesis drives FGF21 secretion in response to serum starvation. Four previously unrecognized loci in FGF21 promoter were nucleosome depleted and enriched in acetylated histone H3 revealing their role as transcriptional enhancers and putative transcription factor binding sites. FGF21 did not accumulate to a significant degree in induced HepG2 cells, which secreted FGF21 time dependently in media. We conclude that lipoic acid cell signaling connects with the transcriptional upregulation of FGF21 and it may prove to be a safe and affordable means to stimulate FGF21 production.

Short-Term Curcumin Gavage Sensitizes Insulin Signaling in Dexamethasone-Treated C57BL/6 Mice1–3

Background: Long-term dietary curcumin (>12 wk) improves metabolic homeostasis in obese mice by sensitizing insulin signaling and reducing hepatic gluconeogenesis. Whether these occur only secondary to its chronic anti-inflammatory and antioxidative functions is unknown.Objective: In this study, we assessed the insulin sensitization effect of short-term curcumin gavage in a rapid dexamethasone-induced insulin resistance mouse model, in which the chronic anti-inflammatory function is eliminated.Methods: Six-week-old male C57BL/6 mice received an intraperitoneal injection of dexamethasone (100 mg/kg body weight) or phosphate-buffered saline every day for 5 d, with or without simultaneous curcumin gavage (500 mg/kg body weight). On day 7, insulin tolerance tests were performed. After a booster dexamethasone injection and curcumin gavage on day 8, blood glucose and insulin concentrations were measured. Liver tissues were collected on day 10 for quantitative polymerase chain reaction and Western blotting to assess gluconeogenic gene expression, insulin signaling, and the expression of fibroblast growth factor 21 (FGF21). Primary hepatocytes from separate, untreated C57BL/6 mice were used for testing the in vitro effect of curcumin treatment.Results: Dexamethasone injection impaired insulin tolerance (P < 0.05) and elevated ambient plasma insulin concentrations by ∼2.7-fold (P < 0.01). Concomitant curcumin administration improved insulin sensitivity and reduced hepatic gluconeogenic gene expression. The insulin sensitization effect of curcumin was demonstrated by increased stimulation of S473 phosphorylation of protein kinase B (P < 0.01) in the dexamethasone-treated mouse liver, as well as the repression of glucose production in primary hepatocytes (P < 0.001). Finally, curcumin gavage increased FGF21 expression by 2.1-fold in the mouse liver (P < 0.05) and curcumin treatment increased FGF21 expression in primary hepatocytes.Conclusion: These observations suggest that the early beneficial effect of curcumin intervention in dexamethasone-treated mice is the sensitization of insulin signaling, involving the stimulation of FGF21 production, a known insulin sensitizer.

R‐α‐lipoic acid potentiates fasting‐induced transcription and secretion of hepatic fibroblast growth factor 21 (FGF21)

Recombinant FGF21 (rFGF21) is an experimental polypeptide therapy against type‐2 diabetes and lipid anomalies. However, the high costs of producing rFGF21 and the mode of delivery by injection are important limitations to its wide therapeutic use. The stimulation of endogenous FGF21 production through diet is being explored as an alternative approach. Our research showed that naturally occurring dietary organosulfur compound R‐α‐lipoic acid (LA) stimulated liver Fgf21 expression (+175%) and blood FGF21 levels (+330%) and replicated the adaptive metabolic response to fasting evoked by FGF21 in vivo. In the present study, we examined the mechanism(s) by which serum manipulation and LA induce FGF21 in HepG2 human hepatocellular carcinoma cells. Serum starvation markedly induced FGF21 mRNA levels (50 fold) and FGF21 secreted in the media (19 fold). LA induced FGF21 mRNA 7 fold above DMSO‐treated control cells and FGF21 secretion 3 fold. These effects were several‐fold greater than those of PPARα agonist, Wy14643, an inducer of FGF21. The use of transcription inhibitor, actinomycin D, revealed that de novo mRNA synthesis drives FGF21 secretion in response to serum starvation and LA, and FGF21 does not accumulate in HepG2 cells to a significant degree. Previously unrecognized loci of FGF21 distal promoter were nucleosome depleted and transcriptionally active revealing new putative transcription factors. We conclude that LA activates FGF21 distal promoter resulting in FGF21 secretion by liver cells. LA may prove to be a safe and affordable means to stimulate FGF21 production. Supported by UNL ARD Hatch, and Layman Seed Award.

FGF21 attenuates pathological myocardial remodeling following myocardial infarction through the adiponectin-dependent mechanism

Ischemic heart disease is one of the leading causes of death. Fibroblast growth factor 21 (FGF21) is a circulating factor with an anti-diabetic property. Skeletal muscle is an important source of FGF21 production. Here, we investigated whether skeletal muscle-derived FGF21 modulates cardiac remodeling in a murine model of myocardial infarction. Myocardial infarction was produced in C57BL/6J wild-type (WT) mice by the permanent ligation of the left anterior descending coronary artery (LAD). Adenoviral vectors expressing FGF21 (Ad-FGF21) or control β-galactosidase were intramuscularly injected into mice at 3 days before permanent LAD ligation. Intramuscular injection of Ad-FGF21 increased plasma FGF21 levels in WT mice compared with control. Treatment of WT mice with Ad-FGF21 led to improvement of left ventricular systolic dysfunction and dilatation at 2 weeks after LAD ligation. Ad-FGF21 administration to WT mice also led to enhancement of capillary density in the infarct border zone, and reduction of myocyte apoptosis in the remote zone, which were accompanied by decreased expression of pro-inflammatory cytokines. Furthermore, treatment of WT mice with Ad-FGF21 increased plasma levels of adiponectin, which is a cardioprotective adipokine. The beneficial effects of Ad-FGF21 on cardiac dysfunction and inflammatory response after myocardial infarction were diminished in adiponectin-knockout mice. These data suggest that muscle-derived FGF21 ameliorates adverse cardiac remodeling after myocardial infarction, at least in part, through an adiponectin-dependent mechanism.

The good, the bad, and the unknown: Fructose and FGF21.

In this issue of Molecular Metabolism, Dushay et al. report new results demonstrating fructose ingestion in humans acutely and robustly raises circulating levels of fibroblast growth factor 21 (FGF21) [1], a recently discovered hormone that has been proposed to have beneficial effects on metabolic health, including reduction of body weight and improvements of glucose and lipid metabolism [2]. People suffering from the metabolic syndrome have higher baseline circulating FGF21 levels and exhibit a larger increase of plasma FGF21 concentrations following an oral fructose load. In contrast, the increase of FGF21 after ingestion of the same amount of glucose was delayed and much more modest. These results are paradoxical in that the metabolic effects of FGF-21 appear to be mainly beneficial while those of dietary fructose are generally deleterious [3]. The authors suggest the possibility that obese subjects with metabolic syndrome are resistant to the actions of FGF21 (akin to augmentation of circulating insulin and leptin concentrations and responses in insulin and leptin resistance) and that the exaggerated response to fructose ingestion may be a compensatory response [1]. The increases of FGF-21 in response to fructose ingestion across normal weight and overweight/obese subjects was significantly correlated with circulating insulin and glucose levels in response to oral glucose loads suggesting a relationship between FGF21 and insulin senstivity/glucose tolerance. Together, these novel results shed light on a potential mechanism related to the adverse metabolic effects of dietary fructose and raise new questions about the nutritional regulation of metabolic responses to dietary macronutrients, specifically sugars.

FGF21 maintains glucose homeostasis by mediating the cross talk between liver and brain during prolonged fasting.

Hepatic gluconeogenesis is a main source of blood glucose during prolonged fasting and is orchestrated by endocrine and neural pathways. Here we show that the hepatocyte-secreted hormone fibroblast growth factor 21 (FGF21) induces fasting gluconeogenesis via the brain-liver axis. Prolonged fasting induces activation of the transcription factor peroxisome proliferator-activated receptor α (PPARα) in the liver and subsequent hepatic production of FGF21, which enters into the brain to activate the hypothalamic-pituitary-adrenal (HPA) axis for release of corticosterone, thereby stimulating hepatic gluconeogenesis. Fasted FGF21 knockout (KO) mice exhibit severe hypoglycemia and defective hepatic gluconeogenesis due to impaired activation of the HPA axis and blunted release of corticosterone, a phenotype similar to that observed in PPARα KO mice. By contrast, intracerebroventricular injection of FGF21 reverses fasting hypoglycemia and impairment in hepatic gluconeogenesis by restoring corticosterone production in both FGF21 KO and PPARα KO mice, whereas all these central effects of FGF21 were abrogated by blockage of hypothalamic FGF receptor-1. FGF21 acts directly on the hypothalamic neurons to activate the mitogen-activated protein kinase extracellular signal-related kinase 1/2 (ERK1/2), thereby stimulating the expression of corticotropin-releasing hormone by activation of the transcription factor cAMP response element binding protein. Therefore, FGF21 maintains glucose homeostasis during prolonged fasting by fine tuning the interorgan cross talk between liver and brain.

FGF21 expression and release in muscle cells: involvement of MyoD and regulation by mitochondria-driven signalling.

Although the liver is generally considered the main site of production of FGF21 (fibroblast growth factor-21), high FGF21 levels have been found to be associated with neuromuscular mitochondrial genetic diseases, and there are indications that the muscle may be a relevant site of FGF21 production under conditions of muscular mitochondrial stress. In the present study, we found that expression and release of FGF21 was associated with myogenic differentiation, and we identified MyoD as a major controller of FGF21 gene transcription. Mimicking mitochondrial dysfunction using respiratory chain/oxidative phosphorylation inhibitors resulted in enhanced expression and release of FGF21 by muscle cells. The increased production of reactive oxygen species, subsequent induction of p38 MAPK (mitogen-activated protein kinase) and activation of an ATF2 (activating transcription factor 2)-binding site at the proximal promoter region of the FGF21 gene was found to be a major mechanism linking mitochondrial dysfunction with enhanced FGF21 gene transcription in myogenic cells. The myogenic factor MyoD was required for the induction of FGF21 gene transcription by mitochondrial dysfunction, thus explaining the preferential response of muscle cells to mitochondrial dysfunction-induced FGF21 expression and secretion. FGF21 release by muscle cells in response to mitochondrial alterations may represent a physiological mechanism by which the sensing of internal energetic status by muscles results in the release of FGF21 to favour systemic metabolic adaptations.

Hepatic FGF21 production is increased in late pregnancy in the mouse.

Female mammals call on hormonally driven metabolic adaptations to meet the energy demand of late pregnancy and lactation. These maternal adaptations preserve limiting nutrients and promote their transfer to the uterus during pregnancy or mammary gland during lactation. The novel metabolic hormone fibroblast growth factor-21 (FGF21) was recently shown to increase suddenly at the onset of lactation in dairy cows, but whether FGF21 is induced during the reproductive cycle of other mammals is unknown. To start addressing this question, we studied subsets of mice when virgin (V), on day 18 of pregnancy (P18) and on lactation day 1 (L1), L5 and L14. Plasma FGF21 increased from nearly undetectable levels to over 8 ng/ml between V and P18 and returned to V levels by L1. Gene expression studies showed that liver was the major source of plasma FGF21 at P18 with little or no contribution from other known expressing tissues or from the developing placenta and mammary epithelial cells. The increased FGF21 production at P18 was dissociated from plasma nonesterified fatty acids and liver lipids, unlike that seen in fasted V mice. Changes in FGF21 signaling components in target tissues were modest except for reduced β-Klotho and FGFR1c expression in P18 adipose tissue. The placenta expressed both β-Klotho and FGFR1c, raising the possibility that it responds to FGF21. In conclusion, maternal FGF21 is increased when products of conception account for ∼ 40% of maternal weight, suggesting that FGF21 orchestrates some of the adaptations needed to meet the energy demand of late pregnancy.

Glucagon stimulates hepatic FGF21 secretion through a PKA- and EPAC-dependent posttranscriptional mechanism.

Previous studies have shown that whole body deletion of the glucagon receptor suppresses the ability of starvation to increase hepatic fibroblast growth factor 21 (FGF21) expression and plasma FGF21 concentration. Here, we investigate the mechanism by which glucagon receptor activation increases hepatic FGF21 production. Incubating primary rat hepatocyte cultures with glucagon, dibutyryl cAMP or forskolin stimulated a 3-4-fold increase in FGF21 secretion. The effect of these agents on FGF21 secretion was not associated with an increase in FGF21 mRNA abundance. Glucagon induction of FGF21 secretion was additive with the stimulatory effect of a PPARα activator (GW7647) on FGF21 secretion. Inhibition of protein kinase A (PKA) and downstream components of the PKA pathway [i.e. AMP-activated protein kinase and p38 MAPK] suppressed glucagon activation of FGF21 secretion. Incubating hepatocytes with an exchange protein directly activated by cAMP (EPAC)-selective cAMP analog [i.e. 8-(4-chlorophenylthio)-2'-O-methyladenosine-3', 5'-cyclic monophosphate (cpTOME)], stimulated a 3.9-fold increase FGF21 secretion, whereas inhibition of the EPAC effector, Rap1, suppressed glucagon activation of FGF21 secretion. Treatment of hepatocytes with insulin also increased FGF21 secretion. In contrast to glucagon, insulin activation of FGF21 secretion was associated with an increase in FGF21 mRNA abundance. Glucagon synergistically interacted with insulin to stimulate a further increase in FGF21 secretion and FGF21 mRNA abundance. These results demonstrate that glucagon increases hepatic FGF21 secretion via a posttranscriptional mechanism and provide evidence that both the PKA branch and EPAC branch of the cAMP pathway play a role in mediating this effect. These results also identify a novel synergistic interaction between glucagon and insulin in the regulation of FGF21 secretion and FGF21 mRNA abundance. We propose that this insulin/glucagon synergism plays a role in mediating the elevation in FGF21 production during starvation and conditions related to metabolic syndrome.

Mapping the response of hepatic human fibroblast growth factor 21 promoter to nutrient availability (1045.10)

The pleiotropic hormone‐like polypeptide, fibroblast growth factor 21 (FGF21), is a major modulator of lipid and glucose metabolism and an exploratory treatment strategy for obesity related metabolic disorders. Recombinant FGF21 (rFGF21) is currently being investigated as therapeutic against high blood lipids and type‐2 diabetes. However, the high costs of producing rFGF21 and the mode of delivery by injection are important limitations to its wide therapeutic use. A cost effective and practical alternative is to stimulate endogenous FGF21 production through diet. Our research shows that naturally occurring dietary compound R‐α‐lipoic acid (LA) stimulates liver Fgf21 expression (+175%) and blood FGF21 levels (+330%) and replicates the adaptive metabolic response to fasting supported by FGF21 in vivo. In the present study, we interrogate FGF21 promoter regulation from HepG2 human hepatocellular carcinoma cells under fed and starved states and in the presence of LA. We employed Formaldehyde‐Assisted Isolation of Regulatory Elements (FAIRE)‐coupled qPCR to quantify nucleosome‐depleted regions in the promoter. Our results provide new insights into how nutrient‐responsive transcription factor binding sites regulate FGF21. The potential impact of this work is that LA may prove to be a safe and affordable means to stimulate FGF21 production.Grant Funding Source: Supported by UNL Faculty Seed Grant, and Layman Seed Award.

Retinoic Acid Receptor β Stimulates Hepatic Induction of Fibroblast Growth Factor 21 to Promote Fatty Acid Oxidation and Control Whole-body Energy Homeostasis in Mice

Activation of retinoic acid receptor (RAR) with all-trans-retinoic acid (RA) ameliorates glucose intolerance and insulin resistance in obese mice. The recently discovered fibroblast growth factor 21 (FGF21) is a hepatocyte-derived hormone that restores glucose and lipid homeostasis in obesity-induced diabetes. However, whether hepatic RAR is linked to FGF21 in the control of lipid metabolism and energy homeostasis remains elusive. Here we identify FGF21 as a direct target gene of RARβ. The gene transcription of Fgf21 is increased by the RAR agonist RA and by overexpression of RARα and RARβ, but it is unaffected by RARγ in HepG2 cells. Promoter deletion analysis characterizes a putative RA-responsive element (RARE) primarily located in the 5'-flanking region of the Fgf21 gene. Disruption of the RARE sequence abolishes RA responsiveness. In vivo adenoviral overexpression of RARβ in the liver enhances production and secretion of FGF21, which in turn promotes hepatic fatty acid oxidation and ketogenesis and ultimately leads to increased energy expenditure in mice. The metabolic effects of RA or RARβ are mimicked by FGF21 overexpression and largely abolished by FGF21 knockdown. Moreover, hepatic RARβ is bound to the putative RAREs of the Fgf21 promoter in a fasting-inducible manner in vivo, which contributes to FGF21 induction and the metabolic adaptation to prolonged fasting. In addition to other nuclear receptors, such as peroxisome proliferator-activated receptor α and retinoic acid receptor-related receptor α, RAR may act as a novel component to induce hepatic FGF21 in the regulation of lipid metabolism. The hepatic RAR-FGF21 pathway may represent a potential drug target for treating metabolic disorders.

FGF21: The Center of a Transcriptional Nexus in Metabolic Regulation

Fibroblast growth factor 21 (FGF21) is emerging as a key regulator of energy homeostasis and presents a novel target for the development of therapies for the treatment of diabetes, cardiovascular disease and obesity. Recent publications have demonstrated that FGF21 resides downstream of a complex network of transcriptional regulators which modulate its expression in response to a wide array of physiological stimuli or pharmacologic agents. The manner in which these mechanisms are integrated to regulate FGF21 transcription, production and subsequent secretion is poorly understood. While FGF21 is detected in many metabolically active tissues and is regulated by several of the known transcription factors involved in metabolic control little is known about how these pathways are integrated. In this review, we discuss the data presented to date on regulation of FGF21 by a wide array of transcription factors and explore how it relates to metabolic regulation in vivo.