FGF21 suppresses hepatic glucose production through the activation of atypical protein kinase Cι/λ

Synergistic effect of canine FGF-21 combined with insulin in the treatment of canine diabetes

Previous studies have shown that FGF-21 can ameliorate hyperglycemia and improve the level of oxidative stress in vivo in diabetic mice. The hypoglycemic effect is safe and lasting, but it takes a longer time to exert its effect. Insulin treatment of canine diabetes takes effect quickly; however, its action time is short, and it is prone to cause hypoglycemia. In the present study, we investigated the synergistic effect of cFGF-21 combined with insulin in the treatment of canine diabetes. In the short and long-term treatment of diabetic dogs, cFGF-21 combined with insulin showed an obvious synergistic effect, and its hypoglycemic effect was significantly better than that of cFGF-21 or insulin injection alone. The combination, exhibited the advantage of the fast onset of insulin and the long-term hypoglycemic effect of cFGF-21. Additionally, cFGF-21 combined with insulin effectively relieved the oxidative stress in diabetic dogs. Studies of the synergistic mechanism showed that cFGF-21 combined with insulin could effectively inhibit liver gluconeogenesis and the synthesis of long-chain fatty acids and promote the phosphorylation of the common factor AKT in the pathway to better regulate hyperglycemia in diabetic dogs. In conclusion, cFGF-21 combined with insulin is a promising candidate for canine diabetes therapeutics.

Gestational Exercise Antagonises the Impact of Maternal High-Fat High-Sucrose Diet on Liver Mitochondrial Alterations and Quality Control Signalling in Male Offspring

Maternal high-caloric nutrition and related gestational diabetes mellitus (GDM) are relevant modulators of the intrauterine environment, increasing the risk of liver metabolic alterations in mothers and offspring. In contrast, as a non-pharmacological approach against metabolic disorders, exercise is highly recommended in GDM treatment. We analysed whether gestational exercise (GE) protects mothers from diet-induced GDM metabolic consequences and mitigates liver mitochondrial deleterious alterations in their 6-week-old male offspring. Female Sprague Dawley rats were fed with control or high-fat high-sucrose (HFHS) diet and kept sedentary or submitted to GE. Male offspring were sedentary and fed with control diet. Sedentary HFHS mothers and their offspring showed impaired hepatic mitochondrial biogenesis and morphological evidence of mitochondrial remodelling. In contrast, GE-related beneficial effects were demonstrated by upregulation of mitochondrial biogenesis signalling markers and mitochondrial fusion proteins and downregulation of mitochondrial fission protein. Alterations in miR-34a, miR-130b, and miR-494, associated with epigenetic regulation of mitochondrial biogenesis, suggested that GE is a more critical modulator of intergenerational changes in miRs expression than the maternal diet. Our data showed that GE positively modulated the altered hepatic mitochondrial biogenesis and dynamics markers and quality control signalling associated with maternal HFHS-diet-related GDM in mothers and offspring.

Exercise performed during pregnancy positively modulates liver metabolism and promotes mitochondrial biogenesis of female offspring in a rat model of diet-induced gestational diabetes

Gestational diabetes mellitus (GDM) is associated with a high-risk for metabolic complications in offspring. However, exercise is recognized as a non-pharmacological strategy against metabolic disorders and is recommended in GDM treatment. This study aimed to investigate whether gestational exercise (GE) could modulate maternal high-fat high-sucrose (HFHS) diet-related hepatic metabolic and mitochondrial outcomes in female offspring of mothers with HFHS-induced GDM. Female Sprague-Dawley rats were fed with control or HFHS diet and kept sedentary or submitted to GE. Their female offspring were fed with control diet and kept sedentary. Hepatic lipid accumulation, lipid metabolism regulators, mitochondrial biogenesis and dynamics markers, and microRNAs associated to the regulation of these markers were evaluated. Female offspring of GDM mothers showed increased body weight at early age, whereas GE prevented this effect of maternal HFHS-feeding and reduced hepatic lipid accumulation. GE stimulated hepatic mRNA transcription and protein expression of mitochondrial biogenesis markers (peroxisome proliferator-activated receptor-gamma co-activator-1alpha and mitochondrial transcription factor A) and mRNA transcription of mitochondrial dynamics markers (mitofusin-1, mitofusin-2, and dynamin-related protein-1) that were altered by maternal GDM, while mitochondrial dynamics markers protein expression was not affected by maternal diet/GE except for optic atrophy-1. MicroRNAs associated with these processes (miR-122, miR-34a, miR-130b, miR-494), and the expression of auto/mitophagy- and apoptosis-related proteins were not substantially influenced by altered intrauterine environment. Our findings suggest that GE is an important regulator of the intrauterine environment positively affecting liver metabolism and promoting liver mitochondrial biogenesis in female offspring despite eventual effects of maternal HFHS-feeding and related GDM.

Prenatal dexamethasone exposure induced pancreatic β-cell dysfunction and glucose intolerance of male offspring rats: Role of the epigenetic repression of ACE2

The prevalence of diabetes in children and adolescents has been rising gradually, which is relevant to adverse environment during development, especially prepartum. We aimed to explore the effects of prenatal dexamethasone exposure (PDE) on β-cell function and glucose homeostasis in juvenile offspring rats. Pregnant Wistar rats were subcutaneously administered with dexamethasone [0.1, 0.2, 0.4mg/(kg.d)] from gestational day 9 to 20. PDE impaired glucose tolerance in the male offspring rather than the females. In male offspring, PDE impaired the development and function of β-cells, accompanied with lower H3K9ac, H3K14ac and H3K27ac levels in the promoter region of angiotensin-converting enzyme 2 (ACE2) as well as suppressed ACE2 expression. Meanwhile, PDE increased expression of glucocorticoid receptor (GR) and histone deacetylase 3 (HDAC3) in fetal pancreas. Dexamethasone also inhibited ACE2 expression and insulin production in vitro. Recombinant expression of ACE2 restored insulin production inhibited by dexamethasone. In addition, dexamethasone activated GR and HDAC3, increased protein interaction of GR with HDAC3, and promoted the binding of GR-HDAC3 complex to ACE2 promoter region. Both RU486 and TSA abolished dexamethasone-induced decline of histone acetylation and ACE2 expression. In summary, suppression of ACE2 is involved in PDE induced β-cell dysfunction and glucose intolerance in juvenile male offspring rats.

Advances in Biological Functions and Clinical Studies of FGF21

Fibroblast growth factor 21 (FGF21) regulates many crucial biological processes in human and mammals, particularly metabolic modulation and protective effect after injury. Therefore, determining complex regulatory mechanisms and elucidating the signaling pathway may greatly promote the prevention, diagnosis, and treatment of related injury and metabolic diseases. This review focused on the metabolic modulation and protective effect of FGF21 and summarized the molecular mechanisms and clinical research developments.

Fibroblast growth factor 21: a novel long-acting hypoglycemic drug for canine diabetes

Currently, insulin is commonly used in the clinical management of canine diabetes. However, it must be injected preprandially causing much inconvenience to the owners. Therefore, the development of long-acting hypoglycemic agents has attracted much attention in the scientific community. This study aimed to investigate the long-acting hypoglycemic effect of canine fibroblast growth factor 21 (cFGF-21) in diabetic dogs. Diabetic dogs were administered with cFGF-21, polyethylene glycol-modified cFGF-21 (PEG-cFGF-21), or insulin once a day, once every 2, 3, or 4 days subcutaneously. The results showed that cFGF-21 and PEG-cFGF-21 maintained blood glucose comparable to normal levels for 2 and 3 days respectively while insulin maintained the blood glucose for only 2 h after a single injection. After treatment with cFGF-21, oral glucose tolerance test (OGTT) was significantly improved with glycosylated hemoglobin (HbA1c) close to the normal levels. In addition, cFGF-21 significantly repaired islet β cells, increased insulin content, and protected the pancreas from streptozotocin-induced injury. Furthermore, cFGF-21 exhibited both antioxidant and anti-inflammatory properties in the pancreas. We conclude, therefore, that cFGF-21 and PEG-cFGF-21 can maintain blood glucose comparable to normal levels for 2 and 3 days respectively after a single dose. The long-acting efficacy of cFGF-21 can be attributed to improvement in oxidative stress and the reduction of inflammation in the pancreas.

Maresin 1 Regulates Hepatic FGF21 in Diet-Induced Obese Mice and in Cultured Hepatocytes

SCOPE

To study the effects of Maresin 1 (MaR1), a docosahexaenoic-acid-derived lipid mediator, on fibroblast growth factor 21 (FGF21) production and to characterize the tissue-specific regulation of Fgf21 and its signaling pathway in liver, skeletal muscle, and white adipose tissue (WAT).

METHODS AND RESULTS

Diet-induced obese (DIO) mice are treated with MaR1 (50 µg kg^-1 , 10 days, oral gavage) and serum FGF21 levels and liver, muscle and WAT Fgf21, β-Klotho, Fgfr1, Egr1, and cFos mRNA expression are evaluated. Additionally, MaR1 effects are tested in mouse primary hepatocytes, HepG2 human hepatocytes, C2C12 myotubes, and 3T3-L1 adipocytes. In DIO mice, MaR1 decreases circulating FGF21 levels and HFD-induced hepatic Fgf21 mRNA expression. MaR1 increases hepatic β-Klotho, Egr1, and cFos in DIO mice. In WAT, MaR1 counteracts the HFD-induced downregulation of Fgf21, Fgfr1, and β-Klotho. In muscle, MaR1 does not modify Fgf21 but promoted Fgfr1 expression. In mouse primary hepatocytes, MaR1 decreases Fgf21 expression and downregulated Pparα mRNA levels. In HepG2 cells, MaR1 reverses the increased production of FGF21 and the downregulation of FGFR1, Β-KLOTHO, EGR1, and cFOS induced by palmitate. Preincubation with a PPARα antagonist prevents MaR1 effects on FGF21 secretion.

CONCLUSION

The ability of MaR1 to modulate FGF21 can contribute to its beneficial metabolic effects.

Going Back to the Biology of FGF21: New Insights

Fibroblast growth factor 21 (FGF21) is a protein highly synthesized in the liver that exerts paracrine and endocrine control of many aspects of energy homeostasis in multiple tissues. In preclinical models of obesity and type 2 diabetes, treatment with FGF21 improves glucose homeostasis and promotes weight loss, and, as a result, FGF21 has attracted considerable attention as a therapeutic agent for the treatment of metabolic syndrome in humans. An improved understanding of the biological role of FGF21 may help to explain why its therapeutic potential in humans has not been fully realized. This review will cover the complexities in FGF21 biology in rodents and humans, with emphasis on its role in protection from central and peripheral facets of obesity.

A study of the circulating fibroblast growth factor 21 as a novel noninvasive biomarker of hepatic injury in genotype-4 chronic hepatitis C: Egyptian patients and their response to direct-acting antiviral agents

Background

Fibroblast growth factor (FGF) 21 was reported to be induced by different injurious agents, including chronic hepatitis C (CHC) virus, affecting the liver. The aims of this study were to evaluate the FGF21 levels in CHC patients before and after the treatment with direct-acting antiviral agents (DAAs) in comparison to that in control subjects and to correlate these levels with insulin resistance (IR), lipid profile, and fibrosis stages.

Patients and methods

We studied 75 naive CHC patients and 40 age- and gender-matched healthy control subjects. Patients were divided into five groups based on the severity of fibrosis as detected by Fibroscan as follows: F0, n=2; F1, n=13; F2, n=23; F3, n=16; F4, n=21. We estimated the FGF21 levels at the start of the study for all the participants and for the patients only at the end of treatment with simisipivir (SIM) and sofosbuvir (SOF). These levels were compared between the patients and the control subjects and also for the patients before and after the treatment with DAAs. The FGF21 levels were correlated to IR, lipid profile, and stages of liver fibrosis.

Results

The FGF21, fasting blood sugar (FBS), fasting insulin, and homeostasis model of IR (HOMA-IR) were significantly higher in CHC patients compared to control (5.04±0.75 vs 4.7±0.52, 20.15±5.13 vs 13.15±4.2, 4.49±1.28 vs 2.72±0.87, and 123.7±52.6 vs 21.8±8.8; P≤0.01, P≤0.001, P≤0.001, and P≤0.001, respectively). The posttreatment FGF21 levels were significantly reduced when compared to the pretreatment levels (123.7±52.5 vs 60.5±32.7, P≤0.001). FGF21 levels showed significant negative correlation with FBS and positive correlation with serum albumin (P≤0.05 and P≤0.003, respectively). The multiple linear regression analysis revealed that serum albumin, high-density lipoprotein cholesterol (HDL-c), and the stage of liver fibrosis were independent risk factors for FGF21.

Conclusion

Besides its metabolic modulator role, FGF21 strongly introduced itself as a novel biomarker of hepatic injury in Egyptian, genotype-4, CHC patients.

Links Between Obesity-Induced Brain Insulin Resistance, Brain Mitochondrial Dysfunction, and Dementia

It is widely recognized that obesity and associated metabolic changes are considered a risk factor to age-associated cognitive decline. Inflammation and increased oxidative stress in peripheral areas, following obesity, are patently the major contributory factors to the degree of the severity of brain insulin resistance as well as the progression of cognitive impairment in the obese condition. Numerous studies have demonstrated that the alterations in brain mitochondria, including both functional and morphological changes, occurred following obesity. Several studies also suggested that brain mitochondrial dysfunction may be one of underlying mechanism contributing to brain insulin resistance and cognitive impairment in the obese condition. Thus, this review aimed to comprehensively summarize and discuss the current evidence from various in vitro, in vivo, and clinical studies that are associated with obesity, brain insulin resistance, brain mitochondrial dysfunction, and cognition. Contradictory findings and the mechanistic insights about the roles of obesity, brain insulin resistance, and brain mitochondrial dysfunction on cognition are also presented and discussed. In addition, the potential therapies for obese-insulin resistance are reported as the therapeutic strategies which exert the neuroprotective effects in the obese-insulin resistant condition.

A mathematical model of the impact of insulin secretion dynamics on selective hepatic insulin resistance

Physiological insulin secretion exhibits various temporal patterns, the dysregulation of which is involved in diabetes development. We analyzed the impact of first-phase and pulsatile insulin release on glucose and lipid control with various hepatic insulin signaling networks. The mathematical model suggests that atypical protein kinase C (aPKC) undergoes a bistable switch-on and switch-off, under the control of insulin receptor substrate 2 (IRS2). The activation of IRS1 and IRS2 is temporally separated due to the inhibition of IRS1 by aPKC. The model further shows that the timing of aPKC switch-off is delayed by reduced first-phase insulin and reduced amplitude of insulin pulses. Based on these findings, we propose a sequential model of postprandial hepatic control of glucose and lipid by insulin, according to which delayed aPKC switch-off contributes to selective hepatic insulin resistance, which is a long-standing paradox in the field.

Dysregulation of insulin secretion dynamics plays a role in diabetes development. Here, the authors build a mathematical model of hepatic insulin signaling and propose a sequential model of post-meal control of glucose and lipids, according to which delayed aPKC suppression would contribute to selective hepatic insulin resistance.

Alterations in 3-Hydroxyisobutyrate and FGF21 Metabolism Are Associated With Protein Ingestion-Induced Insulin Resistance

Systemic hyperaminoacidemia, induced by either intravenous amino acid infusion or protein ingestion, reduces insulin-stimulated glucose disposal. Studies of mice suggest that the valine metabolite 3-hydroxyisobutyrate (3-HIB), fibroblast growth factor 21 (FGF21), adiponectin, and nonesterified fatty acids (NEFAs) may be involved in amino acid-mediated insulin resistance. We therefore measured in 30 women the rate of glucose disposal, and plasma 3-HIB, FGF21, adiponectin, and NEFA concentrations, under basal conditions and during a hyperinsulinemic-euglycemic clamp procedure (HECP), with and without concomitant ingestion of protein (n = 15) or an amount of leucine that matched the amount of protein (n = 15). We found that during the HECP without protein or leucine ingestion, the grand mean ± SEM plasma 3-HIB concentration decreased (from 35 ± 2 to 14 ± 1 µmol/L) and the grand median [quartiles] FGF21 concentration increased (from 178 [116, 217] to 509 [340, 648] pg/mL). Ingestion of protein, but not leucine, decreased insulin-stimulated glucose disposal (P < 0.05) and prevented both the HECP-mediated decrease in 3-HIB and increase in FGF21 concentration in plasma. Neither protein nor leucine ingestion altered plasma adiponectin or NEFA concentrations. These findings suggest that 3-HIB and FGF21 might be involved in protein-mediated insulin resistance in humans.

Impact of FGF21 on glycemic control

Fibroblast growth factor 21 (FGF21) plays a role in regulating adaptation to various metabolic abnormalities. In addition, FGF21 is involved in controlling glucose and lipid homeostasis. The regulation of FGF21 is a complex process and depends upon multiple metabolic factors and hormones. Humans and animals with obesity or type 2 diabetes have abnormal expression and changes of FGF21 in the circulation. Interventional studies in rodents and monkeys with obesity, insulin resistance or type 2 diabetes revealed a potential therapeutic relevance of FGF21 in correcting these abnormalities. This review summarizes the current knowledge about the regulation of FGF21 by distinct metabolic and endogenous factors, considering the most relevant studies. In this context, the results of interventional studies in humans and various animal models of diseases, such as diabetes and obesity, are discussed. In addition, potential mechanisms of the molecular regulation of FGF21 expression and secretion are reviewed.

Fibroblast growth factor 21 and its novel association with oxidative stress

Fibroblast growth factor 21 (FGF21) is an endocrine-member of the FGF family. It is synthesized mainly in the liver, but it is also expressed in adipose tissue, skeletal muscle, and many other organs. It has a key role in glucose and lipid metabolism, as well as in energy balance. FGF21 concentration in plasma is increased in patients with obesity, insulin resistance, and metabolic syndrome. Recent findings suggest that such increment protects tissue from an increased oxidative stress environment. Different types of physical stress, such as strenuous exercising, lactation, diabetic nephropathy, cardiovascular disease, and critical illnesses, also increase FGF21 circulating concentration. FGF21 is now considered a stress-responsive hormone in humans. The discovery of an essential response element in the FGF21 gene, for the activating transcription factor 4 (ATF4), involved in the regulation of oxidative stress, and its relation with genes such as NRF2, TBP-2, UCP3, SOD2, ERK, and p38, places FGF21 as a key regulator of the oxidative stress cell response. Its role in chronic diseases and its involvement in the treatment and follow-up of these diseases has been recently the target of new studies. The diminished oxidative stress through FGF21 pathways observed with anti-diabetic therapy is another clue of the new insights of this hormone.

FGF21 improves cognition by restored synaptic plasticity, dendritic spine density, brain mitochondrial function and cell apoptosis in obese-insulin resistant male rats

Fibroblast growth factor 21 (FGF21) is an endocrine hormone which exerts beneficial effects on metabolic regulation in obese and diabetic models. However, the effect of FGF21 on cognition in obese-insulin resistant rats has not been investigated. We hypothesized that FGF21 prevented cognitive decline in obese-insulin resistant rats by improving hippocampal synaptic plasticity, dendritic spine density, brain mitochondrial function and brain FGF21 signaling as well as decreasing brain cell apoptosis. Eighteen male Wistar rats were divided into two groups, and received either a normal diet (ND) (n=6) or a high fat diet (HFD) (n=12) for 12weeks. At week 13, the HFD-fed rats were subdivided into two subgroups (n=6/subgroup) to receive either vehicle or recombinant human FGF21 (0.1mg/kg/day) for four weeks. ND-fed rats were given vehicle for four weeks. At the end of the treatment, cognitive function, metabolic parameters, pro-inflammatory markers, brain mitochondrial function, cell apoptosis, hippocampal synaptic plasticity, dendritic spine density and brain FGF21 signaling were determined. The results showed that vehicle-treated HFD-fed rats developed obese-insulin resistance and cognitive decline with impaired hippocampal synaptic plasticity, decreased dendritic spine density, brain mitochondrial dysfunction and increased brain cell apoptosis. Impaired brain FGF 21 signaling was found in these obese-insulin resistant rats. FGF21-treated obese-insulin resistant rats had improved peripheral insulin sensitivity, increased hippocampal synaptic plasticity, increased dendritic spine density, restored brain mitochondrial function, attenuated brain cells apoptosis and increased brain FGF21 signaling, leading to a prevention of cognitive decline. These findings suggest that FGF21 treatment exerts neuroprotection in obese-insulin resistant rats.

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.

Fibroblast Growth Factor 21 As an Emerging Therapeutic Target for Type 2 Diabetes Mellitus

Fibroblast growth factor (FGF) 21 is a distinctive member of the FGF family that functions as an endocrine factor. It is expressed predominantly in the liver, but is also found in adipose tissue and the pancreas. Pharmacological studies have shown that FGF21 normalizes glucose and lipid homeostasis, thereby preventing the development of metabolic disorders, such as obesity and diabetes. Despite growing evidence for the therapeutic potential of FGF21, paradoxical increases of FGF21 in different disease conditions point to the existence of FGF21 resistance. In this review, we give a critical appraisal of recent advances in the understanding of the regulation of FGF21 production under various physiological conditions, its antidiabetic actions, and the clinical implications. We also discuss recent preclinical and clinical trials using engineered FGF21 analogs in the management of diabetes, as well as the potential side effects of FGF21 therapy.

The emerging role of fibroblast growth factor 21 in diabetic nephropathy

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

FGF21 is not required for glucose homeostasis, ketosis or tumour suppression associated with ketogenic diets in mice

AIMS/HYPOTHESIS

Ketogenic diets (KDs) have increasingly gained attention as effective means for weight loss and potential adjunctive treatment of cancer. The metabolic benefits of KDs are regularly ascribed to enhanced hepatic secretion of fibroblast growth factor 21 (FGF21) and its systemic effects on fatty-acid oxidation, energy expenditure (EE) and body weight. Ambiguous data from Fgf21-knockout animal strains and low FGF21 concentrations reported in humans with ketosis have nevertheless cast doubt regarding the endogenous function of FGF21. We here aimed to elucidate the causal role of FGF21 in mediating the therapeutic benefits of KDs on metabolism and cancer.

METHODS

We established a dietary model of increased vs decreased FGF21 by feeding C57BL/6J mice with KDs, either depleted of protein or enriched with protein. We furthermore used wild-type and Fgf21-knockout mice that were subjected to the respective diets, and monitored energy and glucose homeostasis as well as tumour growth after transplantation of Lewis lung carcinoma cells.

RESULTS

Hepatic and circulating, but not adipose tissue, FGF21 levels were profoundly increased by protein starvation, independent of the state of ketosis. We demonstrate that endogenous FGF21 is not essential for the maintenance of normoglycaemia upon protein and carbohydrate starvation and is therefore not needed for the effects of KDs on EE. Furthermore, the tumour-suppressing effects of KDs were independent of FGF21 and, rather, driven by concomitant protein and carbohydrate starvation.

CONCLUSIONS/INTERPRETATION

Our data indicate that the multiple systemic effects of KD exposure in mice, previously ascribed to increased FGF21 secretion, are rather a consequence of protein malnutrition.

Hepatic insulin resistance and increased hepatic glucose production in mice lacking Fgf21

Fibroblast growth factor 21 (FGF21) is an important regulator of hepatic glucose and lipid metabolism and represents a potential pharmacological agent for the treatment of type 2 diabetes and obesity. Mice fed a ketogenic diet (KD) develop hepatic insulin resistance in association with high levels of FGF21, suggesting a state of FGF21 resistance. To address the role of FGF21 in hepatic insulin resistance, we assessed insulin action in FGF21 whole-body knock-out (FGF21 KO) male mice and their littermate WT controls fed a KD. Here, we report that FGF21 KO mice have hepatic insulin resistance and increased hepatic glucose production associated with an increase in plasma glucagon levels. FGF21 KO mice are also hypometabolic and display increased fat mass compared with their WT littermates. Taken together, these findings support a major role of FGF21 in regulating energy expenditure and hepatic glucose and lipid metabolism, and its potential role as a candidate in the treatment of diseases associated with insulin resistance.

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

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