Cellular mechanisms by which FGF21 improves insulin sensitivity in male mice

Metabolic Effects of FGF-21: Thermoregulation and Beyond

Fibroblast growth factor (FGF)-21, a member of the FGF family, is a novel hormone involved in the control of metabolism by modulating glucose homeostasis, insulin sensitivity, ketogenesis, and promoting adipose tissue "browning." Recent studies demonstrated that brown adipose tissue is not only a target for FGF-21, but is also a potentially important source of systemic FGF-21. These findings support the hypothesis that FGF-21 plays a physiologic role in thermogenesis and thermogenic recruitment of white adipose tissue by an autocrine-paracrine axis. This review examines the role of FGF-21 in thermogenesis from the perspective of cell-based, animal model, and human studies. We also present recent advances in the characterization of FGF-21's regulation of metabolism.

Unlock the Thermogenic Potential of Adipose Tissue: Pharmacological Modulation and Implications for Treatment of Diabetes and Obesity

Brown adipose tissue (BAT) is considered an interesting target organ for the treatment of metabolic disease due to its high metabolic capacity. Non-shivering thermogenesis, once activated, can lead to enhanced partitioning and oxidation of fuels in adipose tissues, and reduce the burden of glucose and lipids on other metabolic organs such as liver, pancreas, and skeletal muscle. Sustained long-term activation of BAT may also lead to meaningful bodyweight loss. In this review, we discuss three different drug classes [the thiazolidinedione (TZD) class of PPARγ agonists, β3-adrenergic receptor agonists, and fibroblast growth factor 21 (FGF21) analogs] that have been proposed to regulate BAT and beige recruitment or activation, or both, and which have been tested in both rodent and human. The learnings from these classes suggest that restoration of functional BAT and beige mass as well as improved activation might be required to fully realize the metabolic potential of these tissues. Whether this can be achieved without the undesired cardiovascular side effects exhibited by the TZD PPARγ agonists and β3-adrenergic receptor agonists remains to be resolved.

ApoA5 knockdown improves whole-body insulin sensitivity in high-fat-fed mice by reducing ectopic lipid content

ApoA5 has a critical role in the regulation of plasma TG concentrations. In order to determine whether ApoA5 also impacts ectopic lipid deposition in liver and skeletal muscle, as well as tissue insulin sensitivity, we treated mice with an antisense oligonucleotide (ASO) to decrease hepatic expression of ApoA5. ASO treatment reduced ApoA5 protein expression in liver by 60–70%. ApoA5 ASO-treated mice displayed approximately 3-fold higher plasma TG concentrations, which were associated with decreased plasma TG clearance. Furthermore, ApoA5 ASO-treated mice fed a high-fat diet (HFD) exhibited reduced liver and skeletal muscle TG uptake and reduced liver and muscle TG and diacylglycerol (DAG) content. HFD-fed ApoA5 ASO-treated mice were protected from HFD-induced insulin resistance, as assessed by hyperinsulinemic-euglycemic clamps. This protection could be attributed to increases in both hepatic and peripheral insulin responsiveness associated with decreased DAG activation of protein kinase C (PKC)-ε and PKCθ in liver and muscle, respectively, and increased insulin-stimulated AKT2 pho­sphory­lation in these tissues. In summary, these studies demonstrate a novel role for ApoA5 as a modulator of susceptibility to diet-induced liver and muscle insulin resistance through regulation of ectopic lipid accumulation in liver and skeletal muscle.

Fibroblast growth factor 21 limits lipotoxicity by promoting hepatic fatty acid activation in mice on methionine and choline-deficient diets

BACKGROUND & AIMS

Nonalcoholic fatty liver disease is a common consequence of human and rodent obesity. Disruptions in lipid metabolism lead to accumulation of triglycerides and fatty acids, which can promote inflammation and fibrosis and lead to nonalcoholic steatohepatitis. Circulating levels of fibroblast growth factor (FGF)21 increase in patients with nonalcoholic fatty liver disease or nonalcoholic steatohepatitis; therefore, we assessed the role of FGF21 in the progression of murine fatty liver disease, independent of obesity, caused by methionine and choline deficiency.

METHODS

C57BL/6 wild-type and FGF21-knockout (FGF21-KO) mice were placed on methionine- and choline-deficient (MCD), high-fat, or control diets for 8-16 weeks. Mice were weighed, and serum and liver tissues were collected and analyzed for histology, levels of malondialdehyde and liver enzymes, gene expression, and lipid content.

RESULTS

The MCD diet increased hepatic levels of FGF21 messenger RNA more than 50-fold and serum levels 16-fold, compared with the control diet. FGF21-KO mice had more severe steatosis, fibrosis, inflammation, and peroxidative damage than wild-type C57BL/6 mice. FGF21-KO mice had reduced hepatic fatty acid activation and β-oxidation, resulting in increased levels of free fatty acid. FGF21-KO mice given continuous subcutaneous infusions of FGF21 for 4 weeks while on an MCD diet had reduced steatosis and peroxidative damage, compared with mice not receiving FGF21. The expression of genes that regulate inflammation and fibrosis were reduced in FGF21-KO mice given FGF21, similar to those of wild-type mice.

CONCLUSIONS

FGF21 regulates fatty acid activation and oxidation in livers of mice. In the absence of FGF21, accumulation of inactivated fatty acids results in lipotoxic damage and increased steatosis.

Brown adipose tissue and its therapeutic potential

Obesity and related diseases are a major cause of human morbidity and mortality and constitute a substantial economic burden for society. Effective treatment regimens are scarce, and new therapeutic targets are needed. Brown adipose tissue, an energy-expending tissue that produces heat, represents a potential therapeutic target. Its presence is associated with low body mass index, low total adipose tissue content and a lower risk of type 2 diabetes mellitus. Knowledge about the development and function of thermogenic adipocytes in brown adipose tissue has increased substantially in the last decade. Important transcriptional regulators have been identified, and hormones able to modulate the thermogenic capacity of the tissue have been recognized. Intriguingly, it is now clear that humans, like rodents, possess two types of thermogenic adipocytes: the classical brown adipocytes found in the interscapular brown adipose organ and the so-called beige adipocytes primarily found in subcutaneous white adipose tissue after adrenergic stimulation. The presence of two distinct types of energy-expending adipocytes in humans is conceptually important because these cells might be stimulated and recruited by different signals, raising the possibility that they might be separate potential targets for therapeutic intervention. In this review, we will discuss important features of the energy-expending brown adipose tissue and highlight those that may serve as potential targets for pharmacological intervention aimed at expanding the tissue and/or enhancing its function to counteract obesity.

Long-term low carbohydrate diet leads to deleterious metabolic manifestations in diabetic mice

We investigated long-term effects of low carbohydrate diets on wild type mice, streptozotocin-injected and KKAy obese diabetic mice. These mice were pair-fed three different types of diets, standard chow (SC, C∶P∶F = 63∶15∶22), a low carbohydrate (LC, C∶P∶F = 38∶25∶37) diet and a severely carbohydrate restricted (SR, C∶P∶F = 18∶45∶37) diet for 16 weeks. Despite comparable body weights and serum lipid profiles, wild type and diabetic mice fed the low carbohydrate diets exhibited lower insulin sensitivity and this reduction was dependent on the amount of carbohydrate in the diet. When serum fatty acid compositions were investigated, monounsaturation capacity, i.e. C16:1/C16:0 and C18:1/C18:0, was impaired in all murine models fed the low carbohydrate diets, consistent with the decreased expression of hepatic stearoyl-CoA desaturase-1 (SCD1). Interestingly, both the hepatic expressions and serum levels of fibroblast growth factor 21 (FGF21), which might be related to longevity, were markedly decreased in both wild type and KKAy mice fed the SR diet. Taking into consideration that fat compositions did not differ between the LC and SR diets, we conclude that low carbohydrate diets have deleterious metabolic effects in both wild type and diabetic mice, which may explain the association between diets relatively low in carbohydrate and the elevated risk of cardiovascular events observed in clinical studies.

FGF21 is an endocrine signal of protein restriction

Enhanced fibroblast growth factor 21 (FGF21) production and circulation has been linked to the metabolic adaptation to starvation. Here, we demonstrated that hepatic FGF21 expression is induced by dietary protein restriction, but not energy restriction. Circulating FGF21 was increased 10-fold in mice and rats fed a low-protein (LP) diet. In these animals, liver Fgf21 expression was increased within 24 hours of reduced protein intake. In humans, circulating FGF21 levels increased dramatically following 28 days on a LP diet. LP-induced increases in FGF21 were associated with increased phosphorylation of eukaryotic initiation factor 2α (eIF2α) in the liver, and both baseline and LP-induced serum FGF21 levels were reduced in mice lacking the eIF2α kinase general control nonderepressible 2 (GCN2). Finally, while protein restriction altered food intake, energy expenditure, and body weight gain in WT mice, FGF21-deficient animals did not exhibit these changes in response to a LP diet. These and other data demonstrate that reduced protein intake underlies the increase in circulating FGF21 in response to starvation and a ketogenic diet and that FGF21 is required for behavioral and metabolic responses to protein restriction. FGF21 therefore represents an endocrine signal of protein restriction, which acts to coordinate metabolism and growth during periods of reduced protein intake.

Exendin-4 regulates lipid metabolism and fibroblast growth factor 21 in hepatic steatosis

OBJECTIVE

Hepatokine fibroblast growth factor (FGF) 21 takes part in the regulation of lipid metabolism in the liver and adipose tissue. We investigated whether exendin-4 regulates the expression of FGF21 in the liver, and whether the effects of exendin-4 on the regulation of FGF21 expression are mediated via silent mating type information regulation 2 homolog (SIRT) 1 or SIRT6.

MATERIALS/METHODS

The C57BL/6J mice were fed a low fat diet, high fat diet, or high fat diet with 1 nmol/kg/day exendin-4 intraperitoneal injection for 10 weeks. HepG2 used in vitro study was treated with palmitic aicd (0.4 mM) with or without exendin-4 (100 nM) and FGF21 (50 nM) for 24 hours. The change of FGF21 and its receptors expression by exendin-4 were measured using quantitative real-time RT-PCR and Western blot. The intracellular lipid content in HepG2 cells was evaluated by Oil Red O staining. Inhibition of FGF21, SIRT1 and SIRT6, by 10 nM siRNA was performed to establish the signaling pathway of exendin-4 action in hepatic lipid metabolism.

RESULTS

Exendin-4 increased the expression of FGF21 and its receptors in high fat diet-induced obese mice. In addition, recombinant FGF21 treatment reduced lipid content in palmitic acid-treated HepG2 cells. We also observed significantly decreased expression of peroxisomal proliferator-activated receptor (PPAR) α and medium-chain acyl-coenzyme A dehydrogenase (MCAD) in hepatocytes transfected with FGF21 siRNA. In cells treated with exendin-4, inhibition of SIRT1, but not SIRT6, by siRNA significantly repressed the expression of FGF21 mRNA, whereas decreased SIRT1 expression by inhibition of FGF21 was not observed.

CONCLUSIONS

These data suggest that exendin-4 could improve fatty liver by increasing SIRT1-mediated FGF21.

The role of hepatic lipids in hepatic insulin resistance and type 2 diabetes

Non-alcoholic fatty liver disease and its downstream sequelae, hepatic insulin resistance and type 2 diabetes, are rapidly growing epidemics, which lead to increased morbidity and mortality rates, and soaring health-care costs. Developing interventions requires a comprehensive understanding of the mechanisms by which excess hepatic lipid develops and causes hepatic insulin resistance and type 2 diabetes. Proposed mechanisms implicate various lipid species, inflammatory signalling and other cellular modifications. Studies in mice and humans have elucidated a key role for hepatic diacylglycerol activation of protein kinase Cε in triggering hepatic insulin resistance. Therapeutic approaches based on this mechanism could alleviate the related epidemics of non-alcoholic fatty liver disease and type 2 diabetes.

Role of autophagy in the pathophysiology of nonalcoholic fatty liver disease: A controversial issue

Autophagy is a mechanism involved in cellular homeostasis under basal and stressed conditions delivering cytoplasmic content to the lysosomes for degradation to macronutrients. The potential role of autophagy in disease is increasingly recognised and investigated in the last decade. Nowadays it is commonly accepted that autophagy plays a role in the hepatic lipid metabolism. Hence, dysfunction of autophagy may be an underlying cause of non-alcoholic fatty liver disease. However, controversy of the exact role of autophagy in the lipid metabolism exists: some publications report a lipolytic function of autophagy, whereas others claim a lipogenic function. This review aims to give an update of the present knowledge on autophagy in the hepatic lipid metabolism, hepatic insulin resistance, steatohepatitis and hepatic fibrogenesis.

Myokines in insulin resistance and type 2 diabetes

Skeletal muscle represents the largest organ of the body in non-obese individuals and is now considered to be an active endocrine organ releasing a host of so-called myokines. These myokines are part of a complex network that mediates communication between muscle, the liver, adipose tissue, the brain and other organs. Recent data suggest that myokines regulated by muscle contraction may play a key role in mediating the health-promoting effects of regular physical activity. Although hundreds of myokines have recently been described in proteomic studies, we currently have a rather limited knowledge of the specific role these myokines play in the prevention of insulin resistance, inflammation and associated metabolic dysfunction. Several myokines are known to have both local and endocrine functions, but in many cases the contribution of physical activity to the systemic level of these molecules remains as yet unexplored. Very recently, novel myokines such as irisin, which is thought to induce a white to brown shift in adipocytes, have gained considerable interest as potential therapeutic targets. In this review, we summarise the most recent findings on the role of myokines in the regulation of substrate metabolism and insulin sensitivity. We further explore the role of myokines in the regulation of inflammation and provide a critical assessment of irisin and other myokines regarding their potential as therapeutic targets.

Hydrodynamic delivery of FGF21 gene alleviates obesity and fatty liver in mice fed a high-fat diet

FGF21 is a secreted protein that plays critical roles in regulating glucose and lipid metabolism. In this study, we evaluated the effects of FGF21 gene transfer on C57BL/6 mice fed a high fat diet (HFD). We demonstrate that transfer of the FGF21 gene using a hydrodynamics-based procedure increased mRNA levels of FGF21 exclusively in the liver, consequently generating a sustained high level of FGF21 protein in blood that peaked at 500 ng/ml 1 day after injection, leading to a variety of beneficial effects including blockade of HFD-induced obesity, alleviation of fatty liver and improvement in glucose homeostasis. These effects were associated with altered expression of Ucp1, Dio2, Pgc1α, Pparγ2, Mgat1, F4/80, Mcp1 and Tnfα, which are involved in thermogenesis, lipogenesis and chronic inflammation in the liver and adipose tissues. Transfer of the FGF21 gene in HFD-induced obese mice greatly increased the expression of thermogenic genes in adipose tissue, resulting in similar improvements in systemic metabolism including reduction of adiposity, alleviation of fatty liver and attenuation of insulin resistance. Mechanistic studies on the effects of FGF21 gene transfer in lean mice revealed that mice transferred with FGF21 gene displayed suppressed lipogenesis in the liver and enhanced thermogenesis in brown adipose tissue which was coincident with a significant improvement in glucose tolerance. Collectively, our results suggest that transfer of the FGF21 gene could be considered a promising approach for treating obesity and its complications.

Ketosis onset type 2 diabetes had better islet β-cell function and more serious insulin resistance

Diabetic ketosis had been identified as a characteristic of type 1 diabetes mellitus (T1DM), but now emerging evidence has identified that they were diagnosed as T2DM after long time follow up. This case control study was aimed at comparing the clinical characteristic, β-cell function, and insulin resistance of ketosis and nonketotic onset T2DM and providing evidence for treatment selection. 140 cases of newly diagnosed T2DM patients were divided into ketosis (62 cases) and nonketotic onset group (78 cases). After correction of hyperglycemia and ketosis with insulin therapy, plasma C-peptide concentrations were measured at 0, 0.5, 1, 2, and 3 hours after 75 g glucose oral administration. Area under the curve (AUC) of C-peptide was calculated. Homoeostasis model assessment was used to estimate basal β-cell function (HOMA-β) and insulin resistance (HOMA-IR). Our results showed that ketosis onset group had higher prevalence of nonalcoholic fatty liver disease (NAFLD) than nonketotic group (P = 0.04). Ketosis onset group had increased plasma C-peptide levels at 0 h, 0.5 h, and 3 h and higher AUC(0-0.5), AUC₀₋₁, AUC₀₋₃ (P < 0.05). Moreover, this group also had higher HOMA-β and HOMA-IR than nonketotic group (P < 0.05). From these data, we concluded that ketosis onset T2DM had better islet β-cell function and more serious insulin resistance than nonketotic onset T2DM.

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.

Hepatic SIRT1 attenuates hepatic steatosis and controls energy balance in mice by inducing fibroblast growth factor 21

BACKGROUND & AIMS

The hepatocyte-derived hormone fibroblast growth factor 21 (FGF21) is a hormone-like regulator of metabolism. The nicotinamide adenine dinucleotide-dependent deacetylase SIRT1 regulates fatty acid metabolism through multiple nutrient sensors. Hepatic overexpression of SIRT1 reduces steatosis and glucose intolerance in obese mice. We investigated mechanisms by which SIRT1 controls hepatic steatosis in mice.

METHODS

Liver-specific SIRT1 knockout (SIRT1 LKO) mice and their wild-type littermates (controls) were divided into groups that were placed on a normal chow diet, fasted for 24 hours, or fasted for 24 hours and then fed for 6 hours. Liver tissues were collected and analyzed by histologic examination, gene expression profiling, and real-time polymerase chain reaction assays. Human HepG2 cells were incubated with pharmacologic activators of SIRT1 (resveratrol or SRT1720) and mitochondrion oxidation consumption rate and immunoblot analyses were performed. FGF21 was overexpressed in SIRT1 LKO mice using an adenoviral vector. Energy expenditure was assessed by indirect calorimetry.

RESULTS

Prolonged fasting induced lipid deposition in livers of control mice, but severe hepatic steatosis in SIRT1 LKO mice. Gene expression analysis showed that fasting up-regulated FGF21 in livers of control mice but not in SIRT1 LKO mice. Decreased hepatic and circulating levels of FGF21 in fasted SIRT1 LKO mice were associated with reduced hepatic expression of genes involved in fatty acid oxidation and ketogenesis, and increased expression of genes that control lipogenesis, compared with fasted control mice. Resveratrol or SRT1720 each increased the transcriptional activity of the FGF21 promoter (-2070/+117) and levels of FGF21 messenger RNA and protein in HepG2 cells. Surprisingly, SIRT1 LKO mice developed late-onset obesity with impaired whole-body energy expenditure. Hepatic overexpression of FGF21 in SIRT1 LKO mice increased the expression of genes that regulate fatty acid oxidation, decreased fasting-induced steatosis, reduced obesity, increased energy expenditure, and promoted browning of white adipose tissue.

CONCLUSIONS

SIRT1-mediated activation of FGF21 prevents liver steatosis caused by fasting. This hepatocyte-derived endocrine signaling appears to regulate expression of genes that control a brown fat-like program in white adipose tissue, energy expenditure, and adiposity. Strategies to activate SIRT1 or FGF21 could be used to treat fatty liver disease and obesity.

Increased FGF21 plasma levels in humans with sepsis and SIRS

Fibroblast growth factor 21 (FGF21) is a key regulator in glucose and lipid metabolism and its plasma levels have been shown to be increased not only in humans in different situations such as type 2 diabetes, obesity, and nonalcoholic fatty liver disease but also in animal models of sepsis and pancreatitis. FGF21 is considered as a pharmacological candidate in conditions associated with insulin resistance. The aim of this study was to compare FGF21 plasma levels in patients with sepsis, in patients with systemic inflammatory response syndrome (SIRS), and in healthy controls. We measured FGF21 plasma concentrations in 22 patients with established sepsis, in 11 with SIRS, and in 12 healthy volunteers. Here, we show that FGF21 levels were significantly higher in plasma obtained from patients with sepsis and SIRS in comparison with healthy controls. Also, FGF21 levels were significantly higher in patients with sepsis than in those with noninfectious SIRS. FGF21 plasma levels measured at study entry correlated positively with the APACHE II score, but not with procalcitonin levels, nor with C-reactive protein, classical markers of sepsis. Plasma concentrations of FGF21 peaked near the onset of shock and rapidly decreased with clinical improvement. Taken together, these results indicate that circulating levels of FGF21 are increased in patients presenting with sepsis and SIRS, and suggest a role for FGF21 in inflammation. Further studies are needed to explore the potential role of FGF21 in sepsis as a potential therapeutic target.