Endurance Exercise-Induced Fgf21 Promotes Skeletal Muscle Fiber Conversion through TGF-β1 and p38 MAPK Signaling Pathway

Fgf21 has been identified as playing a regulatory role in muscle growth and function. Although the mechanisms through which endurance training regulates skeletal muscle have been widely studied, the contribution of Fgf21 remains poorly understood. Here, muscle size and function were measured, and markers of fiber type were evaluated using immunohistochemistry, immunoblots, or qPCR in endurance-exercise-trained wild-type and Fgf21 KO mice. We also investigated Fgf21-induced fiber conversion in C2C12 cells, which were incubated with lentivirus and/or pathway inhibitors. We found that endurance exercise training enhanced the Fgf21 levels of liver and GAS muscle and exercise capacity and decreased the distribution of skeletal muscle fiber size, and fast-twitch fibers were observed converting to slow-twitch fibers in the GAS muscle of mice. Fgf21 promoted the markers of fiber-type transition and eMyHC-positive myotubes by inhibiting the TGF-β1 signaling axis and activating the p38 MAPK signaling pathway without apparent crosstalk. Our findings suggest that the transformation and function of skeletal muscle fiber types in response to endurance training could be mediated by Fgf21 and its downstream signaling pathways. Our results illuminate the mechanisms of Fgf21 in endurance-exercise-induced fiber-type conversion and suggest a potential use of Fgf21 in improving muscle health and combating fatigue.

Alternate-day fasting alleviates high fat diet induced non-alcoholic fatty liver disease through controlling PPARα/Fgf21 signaling

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a common chronic liver disease that ultimately leads to cirrhosis and hepatocellular carcinoma. Intermittent fasting has been proposed as a nonpharmacological dietary approach against metabolic diseases, including NAFLD. In this study, we aimed to investigate the effect of alternate day fasting (ADF) on high-fat diet (HFD)-induced NAFLD in C57BL/6 mice.

METHODS

A mouse model of fatty liver disease was established by feeding the mice a HFD for 16 weeks. The mice were administered by body weight, lipid accumulation and inflammation. PPARα, FGF21, serum triglycerides (TG), total cholesterol (TC), transaminase and lipogenesis were assessed.

RESULTS

The results showed that long-term ADF can attenuate fatty liver disease by reducing hepatic inflammation and lipid accumulation in a mouse model. Meanwhile, fasting elevated the expression of serum and hepatic fibroblast growth Factor 21 (Fgf21), a circulating hormone produced predominantly in the liver, and could effectively prevent and ameliorate the pathogenesis of NAFLD. Serum starvation also enhanced Fgf21 expression and reduced free fatty acid (FFA)-induced lipid storage in hepatocytes. Moreover, refeeding inhibited the increase in Fgf21 expression induced by fasting. This fasted-to-refed transition is closely related to the expression of Fgf21. Further in vitro and in vivo studies showed that fasting-sensitive PPARα is indispensable for the expression of Fgf21 and its protective effect on NAFLD.

CONCLUSION

These findings indicated that long-term ADF protects mouse livers against HFD induced fatty liver disease through controlling PPARα/Fgf21 signaling. In conclusion, ADF can emerge as a non-pharmacological dietary approach against fatty liver disease.

Effect of nighttime light exposure on glucose metabolism in protein-restricted mice

Disruption of biological rhythms due to exposure to artificial light at night (ALAN) has emerged as a new risk factor for metabolic diseases. However, the effects of ALAN exposure on energy metabolism with concomitant misalignment in the circadian system caused by nutritional imbalance remain largely unexplored. Here, we evaluate whether a low-protein (LP) diet could enhance the effects induced by exposure to ALAN on the energy metabolism and consequently predispose to metabolic disorders. Male C57BL6/J mice were weaned on a normal protein (NP) or a LP diet and housed on 12 h light:12 h darkness (LD) cycle. After 6 weeks, mice maintained on their respective diets were subdivided into normal light/darkness cycle (NP/LD; LP/LD) or exposed to ALAN (NP/LL; LP/LL) for 8 weeks. We observed that exposure to ALAN concomitant to LP diet disrupts the behavioral rhythms, without shifting the timing of food intake. Furthermore, exposure to ALAN leads to increased body and fat pad weights, higher levels of fast and fed glycemia and glucose intolerance independent of the diet consumed. Importantly, the effects of ALAN on circadian regulation of insulin sensitivity were diet-dependent with LP/LL mice showing insulin resistance in an opposite time of day than NP/LL. At the molecular level, exposure to ALAN concurrent with LP diet increased the expression of phosphoenolpyruvate carboxykinase 1 in both periods analyzed and inverted the pattern of fibroblast growth factor 21 (Fgf21) expression in the liver. Our data suggest that dietary protein restriction modulates the effects induced by nighttime light exposure on glucose metabolism, which could be partially related with the dysregulation of hepatic Fgf21 expression.

Distinct roles of UVRAG and EGFR signaling in skeletal muscle homeostasis

Objective

Autophagy is a physiological self-eating process that can promote cell survival or activate cell death in eukaryotic cells. In skeletal muscle, it is important for maintaining muscle mass and function that is critical to sustain mobility and regulate metabolism. The UV radiation resistance-associated gene (UVRAG) regulates the early stages of autophagy and autophagosome maturation and plays a key role in endosomal trafficking. This study investigated the essential in vivo role of UVRAG in skeletal muscle biology.

Methods

To determine the role of UVRAG in skeletal muscle in vivo, we generated muscle-specific UVRAG knockout mice using the Cre-loxP system driven by Myf6 promoter that is exclusively expressed in skeletal muscle. Myf6-Cre⁺ UVRAG^fl/fl (M-UVRAG^−/−) mice were compared to littermate Myf6-Cre⁺ UVRAG^+/+ (M-UVRAG^+/+) controls under basal conditions on a normal chow diet. Body composition, muscle function, and mitochondria morphology were assessed in muscles of the WT and KO mice at 24 weeks of age.

Results

M-UVRAG^−/− mice developed accelerated sarcopenia and impaired muscle function compared to M-UVRAG^+/+ littermates at 24 weeks of age. Interestingly, these mice displayed improved glucose tolerance and increased energy expenditure likely related to upregulated Fgf21, a marker of muscle dysfunction. Skeletal muscle of the M-UVRAG^−/− mice showed altered mitochondrial morphology with increased mitochondrial fission and EGFR accumulation reflecting defects in endosomal trafficking. To determine whether increased EGFR signaling had a causal role in muscle dysfunction, the mice were treated with an EGFR inhibitor, gefitinib, which partially restored markers of muscle and mitochondrial deregulation. Conversely, constitutively active EGFR transgenic expression in UVRAG-deficient muscle led to further detrimental effects with non-overlapping distinct defects in muscle function, with EGFR activation affecting the muscle fiber type whereas UVRAG deficiency impaired mitochondrial homeostasis.

Conclusions

Our results show that both UVRAG and EGFR signaling are critical for maintaining muscle mass and function with distinct mechanisms in the differentiation pathway.

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Deletion of UVRAG in skeletal muscle accelerates muscle wasting with aging.

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UVRAG in skeletal muscle regulates mitochondrial dynamics and function.

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UVRAG deletion leads to EGFR accumulation in skeletal muscle.

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Constitutively active EGFR contributes to muscle fiber type determination.

Metformin Therapy Aggravates Neurodegenerative Processes in ApoE-/- Mice

Epidemiological studies suggest that individuals with diabetes mellitus are at greater risk of developing Alzheimer's disease. A well-known insulin-sensitizing drug and the most widely prescribed oral medication for diabetes is metformin. There is evidence that metformin acts in a neuroprotective manner via the AMPK/mTOR pathway by inhibiting the tau phosphorylation. In addition, it is known that metformin upregulates Fgf21, which in turn activates the AMPK/mTOR pathway and mediates neuroprotection. Thus, metformin-induced Fgf21 release may be involved in AMPK/mTOR activation. However, some studies reported that metformin causes cognition impairment. Due to the controversial data on the neuroprotective properties of metformin, we treated Apolipoprotein E deficient (ApoE- /-) mice, a mouse model of tauopathy, with metformin for 18 weeks. Metformin-treated mice revealed increased expression of lipogenic genes, i.e., lxrα and srebp1c. In line with this, metformin caused an increase in plasma triglyceride leading to enhanced gliosis as indicated by an increase of GFAP-positive cells. Although the systemic Fgf21 concentration was increased, metformin did not activate the FgfR1c/AMPK/mTOR pathway suggesting a Fgf21-resistant state. Further, metformin-treated mice showed increased tau phosphorylation and reduced numbers of NeuN-and PSD95-positive cells. Thus, metformin-associated lipogenesis as well as inflammation aggravated neurodegenerative processes in ApoE- /- mice. Consequently, this study supports previous observations showing that metformin causes impairment of cognition.

Pyruvate kinase M2 represses thermogenic gene expression in brown adipocytes

Utilizing the thermogenic capacity of brown adipose tissue is a potential anti-obesity strategy; therefore, the mechanisms controlling expression of thermogenesis-related genes are of interest. Pyruvate kinase (PK) catalyzes the last step of glycolysis and exists as four isoenzymes: PK, liver, PK, red blood cell, PK, muscle (PKM1 and PKM2). PKM2 has both glycolytic and nuclear functions. Here, we report that PKM2 is enriched in brown adipose compared with white adipose tissue. Specific knockdown of PKM2 in mature brown adipocytes demonstrates that silencing of PKM2 does not lead to a decrease in PK activity, but causes a robust upregulation of thermogenic uncoupling protein 1 (Ucp1) and fibroblast growth factor 21 (Fgf21) gene expression. This increase is not mediated by any of the known mechanisms for PKM2-regulated gene expression, thus implying the existence of a novel mechanism for PKM2-dependent effects on gene expression.

Tanycytes Regulate Lipid Homeostasis by Sensing Free Fatty Acids and Signaling to Key Hypothalamic Neuronal Populations via FGF21 Secretion

The hypothalamus plays a key role in the detection of energy substrates to regulate energy homeostasis. Tanycytes, the hypothalamic ependymo-glia, are located at a privileged position to integrate multiple peripheral inputs. We observed that tanycytes produce and secrete Fgf21 and are located close to Fgf21-sensitive neurons. Fasting, likely via the increase in circulating fatty acids, regulates this central Fgf21 production. Tanycytes store palmitate in lipid droplets and oxidize it, leading to the activation of a reactive oxygen species (ROS)/p38-MAPK signaling pathway, which is essential for tanycytic Fgf21 expression upon palmitate exposure. Tanycytic Fgf21 deletion triggers an increase in lipolysis, likely due to impaired inhibition of key neurons during fasting. Mice deleted for tanycytic Fgf21 exhibit increased energy expenditure and a reduction in fat mass gain, reminiscent of a browning phenotype. Our results suggest that tanycytes sense free fatty acids to maintain body lipid homeostasis through Fgf21 signaling within the hypothalamus.

ChREBP Reciprocally Regulates Liver and Plasma Triacylglycerol Levels in Different Manners

Carbohydrate response element-binding protein (ChREBP) has an important role in the carbohydrate-mediated regulation of hepatic de novo lipogenesis, but the mechanism for how it regulates plasma triacylglycerol (TAG) levels has not been established. This study aimed to clarify the role of ChREBP in regulation of plasma TAG levels. We analyzed the metabolic changes in mice infected with an adenovirus expressing ChREBP Δ196 (Ad-ChREBP). Compared with adenovirus harboring green fluorescent protein infected mice, Ad-ChREBP-infected mice had higher plasma free fatty acid levels and paradoxically lower plasma 3-hydroxybutyrate levels through decreased fatty acid oxidation, rather than ketogenesis. Consistent with their hepatomegaly and increased lipogenic gene expression, the liver TAG contents were much higher. Regarding lipid composition, C16:0 was much lower and C18:1n-9 was much higher, compatible with increased stearoyl CoA desaturase-1 and ELOVL fatty acid elongase 6 expression. Furthermore, Ad-ChREBP-infected mice had decreased plasma TAG and very low density lipoprotein (VLDL)-TAG levels, consistent with decreased Angiopoietin-like protein 3 (Angptl3) and increased fibroblast growth factor (Fgf21) mRNA and protein levels. Finally, Ad-ChREBP infection increased white adipose tissue Ucp1 mRNA levels with increased plasma Fgf21 levels. Because Fgf21 and Angptl3 are known to activate and suppress lipolysis in adipose tissues and oxidative tissues, ChREBP appears to regulate plasma TAG levels by modulating Fgf21 and Angptl3 levels. Thus, ChREBP overexpression led to dissociation of hepatic steatosis from hyperlipidemia.

Short-term methionine deprivation improves metabolic health via sexually dimorphic, mTORC1-independent mechanisms

Obesity and diabetes are major challenges to global health, and there is an urgent need for interventions that promote weight loss. Dietary restriction of methionine promotes leanness and improves metabolic health in mice and humans. However, poor long-term adherence to this diet limits its translational potential. In this study, we develop a short-term methionine deprivation (MD) regimen that preferentially reduces fat mass, restoring normal body weight and glycemic control to diet-induced obese mice of both sexes. The benefits of MD do not accrue from calorie restriction, but instead result from increased energy expenditure. MD promotes increased energy expenditure in a sex-specific manner, inducing the fibroblast growth factor (Fgf)-21-uncoupling protein (Ucp)-1 axis only in males. Methionine is an agonist of the protein kinase mechanistic target of rapamycin complex (mTORC)-1, which has been proposed to play a key role in the metabolic response to amino acid-restricted diets. In our study, we used a mouse model of constitutive hepatic mTORC1 activity and demonstrate that suppression of hepatic mTORC1 signaling is not required for the metabolic effects of MD. Our study sheds new light on the mechanisms by which dietary methionine regulates metabolic health and demonstrates the translational potential of MD for the treatment of obesity and type 2 diabetes.-Yu, D., Yang, S. E., Miller, B. R., Wisinski, J. A., Sherman, D. S., Brinkman, J. A., Tomasiewicz, J. L., Cummings, N. E., Kimple, M. E., Cryns, V. L., Lamming, D. W. Short-term methionine deprivation improves metabolic health via sexually dimorphic, mTORC1-independent mechanisms.

Loss of Hepatic Mitochondrial Long-Chain Fatty Acid Oxidation Confers Resistance to Diet-Induced Obesity and Glucose Intolerance

The liver has a large capacity for mitochondrial fatty acid β-oxidation, which is critical for systemic metabolic adaptations such as gluconeogenesis and ketogenesis. To understand the role of hepatic fatty acid oxidation in response to a chronic high-fat diet (HFD), we generated mice with a liver-specific deficiency of mitochondrial long-chain fatty acid β-oxidation (Cpt2^L-/- mice). Paradoxically, Cpt2^L-/- mice were resistant to HFD-induced obesity and glucose intolerance with an absence of liver damage, although they exhibited serum dyslipidemia, hepatic oxidative stress, and systemic carnitine deficiency. Feeding an HFD induced hepatokines in mice, with a loss of hepatic fatty acid oxidation that enhanced systemic energy expenditure and suppressed adiposity. Additionally, the suppression in hepatic gluconeogenesis was sufficient to improve HFD-induced glucose intolerance. These data show that inhibiting hepatic fatty acid oxidation results in a systemic hormetic response that protects mice from HFD-induced obesity and glucose intolerance.

Long-term caloric restriction in ApoE-deficient mice results in neuroprotection via Fgf21-induced AMPK/mTOR pathway

Caloric restriction (CR) decelerates the aging process, extends lifespan and exerts neuroprotective effects in diverse species by so far unknown mechanisms. Based on known neuroprotective effects of fibroblastic growth factor 21 (Fgf21) we speculate that CR upregulates Fgf21, which phosphorylates neuronal AMP-activated protein kinase (AMPK), leading to a decrease of mammalian target of rapamycin (mTOR) signaling activity and an inhibition of tau-hyperphosphorylation. This in turn reduces the formation of neurofibrillary tangles, a neuropathological hallmark of Alzheimer's disease. ApoE-deficient mice (ApoE−/−), serving as a model of neurodegeneration, showed upon CR vs. ad libitum feeding increased Fgf21 levels in both, plasma and brain as well as higher phosphorylation of fibroblastic growth factor receptor 1c (Fgfr1c), extracellular signal-regulated kinases 1/2 (ERK1/2) and AMPK in brain, lower activity of mTOR and decreased Tau-phosphorylation. Finally, CR in ApoE−/− mice caused neuroprotection as indicated by a higher synaptic plasticity shown by immunohistochemical analysis with increased numbers of PSD95-positive neurons and a better cognitive performance as analyzed with Morris water maze test. These data provide substantial evidence that neuroprotection upon CR seems to be Fgf21-dependent. Further experiments are necessary to evaluate Fgf21 as a therapeutic tool to treat tauopathy for improvement of cognitive performance.

Activation of GR but not PXR by dexamethasone attenuated acetaminophen hepatotoxicities via Fgf21 induction

Glucocorticoid receptor (GR) signaling is indispensable for cell growth and development, and plays important roles in drug metabolism. Fibroblast growth factor (Fgf) 21, an important regulator of glucose, lipid, and energy metabolism, plays a cytoprotective role by attenuating toxicities induced by chemicals such as dioxins, acetaminophen (APAP), and alcohols. The present study investigates the impact of dexamethasone (DEX)-activated GR on Fgf21 expression and how it affects the progression of APAP-induced hepatotoxicity. Our results showed that DEX dose/concentration- and time-dependently increased Fgf21 mRNA and protein expression in mouse liver as well as cultured mouse and human hepatoma cells. By using PXR-null mouse model, we demonstrated that DEX induced Fgf21 expression by a PXR-independent mechanism. In cultured mouse and human hepatoma cells, inhibition of GR signaling, by RU486 (Mifepristone) or GR silencing using GR-specific siRNA, attenuated DEX-induced Fgf21 expression. In addition, DEX increased luciferase reporter activity driven by the 3.0-kb mouse and human Fgf21/FGF21 gene promoter. Further, ChIP-qPCR assays demonstrated that DEX increased the binding of GR to the specific cis-regulatory elements located in the 3.0-kb mouse and human Fgf21/FGF21 gene promoter. Pretreatment of 2mg/kg DEX ameliorated APAP-induced liver injury in wild-type but not Fgf21-null mice. In conclusion, via GR activation, DEX induced Fgf21 expression in mouse liver and human hepatoma cells.

A grape seed procyanidin extract inhibits HDAC activity leading to increased Pparα phosphorylation and target-gene expression

SCOPE

Histone deacetylases (HDACs) have emerged as epigenetic regulators of risk factors associated with the metabolic syndrome (MetS), and certain botanical extracts have proven to be potent HDAC inhibitors. Understanding the role of dietary procyanidins in HDAC inhibition is important in exploring the therapeutic potential of natural products.

METHODS

C57BL/6 mice were gavaged with vehicle (water) or grape seed procyanidin extract (GSPE, 250 mg/kg) and terminated 14 h later. Liver and serum were harvested to assess the effect of GSPE on HDAC activity, histone acetylation, Pparα activity and target-gene expression, and serum lipid levels.

RESULTS

GSPE increased histone acetylation and decreased Class I HDAC activity in vivo, and dose-dependently inhibited recombinant HDAC2 and 3 activities in vitro. Accordingly, Pparα gene and phosphorylated protein expression were increased, as were target genes involved in fatty acid catabolism, suggesting increased Pparα activity. Serum fibroblast growth factor 21 (Fgf21) was elevated, and triglyceride levels were reduced by 28%.

CONCLUSION

GSPE regulates HDAC and Pparα activities to modulate lipid catabolism and reduce serum triglycerides in vivo.

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.