Stressed Liver and Muscle Call on Adipocytes with FGF21

Methods to predict heart failure in diabetes patients

INTRODUCTION

Type 2 diabetes mellitus (T2DM) is one of the leading causes of cardiovascular disease and powerful predictor for new-onset heart failure (HF).

AREAS COVERED

We focus on the relevant literature covering evidence of risk stratification based on imaging predictors and circulating biomarkers to optimize approaches to preventing HF in DM patients.

EXPERT OPINION

Multiple diagnostic algorithms based on echocardiographic parameters of cardiac remodeling including global longitudinal strain/strain rate are likely to be promising approach to justify individuals at higher risk of incident HF. Signature of cardiometabolic status may justify HF risk among T2DM individuals with low levels of natriuretic peptides, which preserve their significance in HF with clinical presentation. However, diagnostic and predictive values of conventional guideline-directed biomarker HF strategy may be non-optimal in patients with obesity and T2DM. Alternative biomarkers affecting cardiac fibrosis, inflammation, myopathy, and adipose tissue dysfunction are plausible tools for improving accuracy natriuretic peptides among T2DM patients at higher HF risk. In summary, risk identification and management of the patients with T2DM with established HF require conventional biomarkers monitoring, while the role of alternative biomarker approach among patients with multiple CV and metabolic risk factors appears to be plausible tool for improving clinical outcomes.

Fibroblast growth factor-21 alleviates proteasome injury via activation of autophagy flux in Parkinson's disease

Parkinson's disease (PD) is one of the most common and complex Neurodegeneration, with an inherited metabolic disorder. Fibroblast growth factor 21 (FGF21), an endocrine hormone that belongs to the fibroblast growth factor superfamily, plays an extensive role in metabolic regulation. However, our understandings of the specific function and mechanisms of FGF21 on PD are still quite limited. Here, we aimed to elucidate the actions and the underlying mechanisms of FGF21 on dopaminergic neurodegeneration using cellular models of parkinsonism. To investigate the effects of FGF21 on dopaminergic neurodegeneration in vitro, proteasome impairment models of PD were utilized. Human dopaminergic neuroblastoma SH-SY5Y cells were treated with the proteasome inhibitor lactacystin (5 μmol/L) for 12 h, then with 50 ng/ml FGF-21 with or without 5 mmol/L of 3-methyladenine.The cells were dissected to assess alterations in autophagy using immunofluorescence, immunoblotting and electron microscopy assays. Our data indicate that FGF21 prevents dopaminergic neuron loss and shows beneficial effects against proteasome impairment induced PD syndrome, indicating it might be a potent candidate for developing novel drugs to deal with PD.

Fibroblast Growth Factor 21: A Fascinating Perspective on the Regulation of Muscle Metabolism

Fibroblast growth factor 21 (FGF21) plays a vital role in normal eukaryotic organism development and homeostatic metabolism under the influence of internal and external factors such as endogenous hormone changes and exogenous stimuli. Over the last few decades, comprehensive studies have revealed the key role of FGF21 in regulating many fundamental metabolic pathways, including the muscle stress response, insulin signaling transmission, and muscle development. By coordinating these metabolic pathways, FGF21 is thought to contribute to acclimating to a stressful environment and the subsequent recovery of cell and tissue homeostasis. With the emphasis on FGF21, we extensively reviewed the research findings on the production and regulation of FGF21 and its role in muscle metabolism. We also emphasize how the FGF21 metabolic networks mediate mitochondrial dysfunction, glycogen consumption, and myogenic development and investigate prospective directions for the functional exploitation of FGF21 and its downstream effectors, such as the mammalian target of rapamycin (mTOR).

Cutting Edge: CD36 Mediates Phagocyte Tropism and Avirulence of Toxoplasma gondii

Resistance and tolerance are vital for survivability of the host-pathogen relationship. Virulence during Toxoplasma infection in mice is mediated by parasite kinase-dependent antagonism of IFN-γ-induced host resistance. Whether avirulence requires expression of parasite factors that induce host tolerance mechanisms or is a default status reflecting the absence of resistance-interfering factors is not known. In this study, we present evidence that avirulence in Toxoplasma requires parasite engagement of the scavenger receptor CD36. CD36 promotes macrophage tropism but is dispensable for the development of resistance mechanisms. Instead CD36 is critical for re-establishing tissue homeostasis and survival following the acute phase of infection. The CD36-binding capacity of T. gondii strains is negatively controlled by the virulence factor, ROP18. Thus, the absence of resistance-interfering virulence factors and the presence of tolerance-inducing avirulence factors are both required for long-term host-pathogen survival.

How does the skeletal muscle communicate with the brain in health and disease?

Endocrine mechanisms have been largely associated with metabolic control and tissue cross talk in mammals. Classically, myokines comprise a class of signaling proteins released in the bloodstream by the skeletal muscle, which mediate physiological and metabolic responses in several tissues, including the brain. Recent exciting evidence suggests that myokines (e.g. cathepsin B, FNDC5/irisin, interleukin-6) act to control brain functions, including learning, memory, and mood, and may mediate the beneficial actions of physical exercise in the brain. However, the intricate mechanisms connecting peripherally released molecules to brain function are not fully understood. Accumulating findings further indicates that impaired skeletal muscle homeostasis impacts brain metabolism and physiology. Here we review recent findings that suggest that muscle-borne signals are essential for brain physiology and discuss perspectives on how these signals vary in response to exercise or muscle diseases. Understanding the complex interactions between skeletal muscle and brain may result in more effective therapeutic strategies to expand healthspan and to prevent brain disease.

PPARα agonist WY-14,643 induces adipose atrophy and fails to blunt chronic ethanol-induced hepatic fat accumulation in mice lacking adipose FGFR1

Fibroblast growth factor 21 (FGF21) is mainly regulated by peroxisome proliferator-activated receptor α (PPARα) in liver. The PPARα-FGF21 axis protects against alcohol-related liver disease (ALD). FGF21 exerts its effect via FGF receptor 1 (FGFR1). However, liver specific FGFR1 abrogation had no effect on ALD. Adipose tissues highly express FGFR1. When adipocyte specific FGFR1 knockout (fgfr1^adipoQ-cre) mice and corresponding normal control (fgfr1^fl/fl) mice were fed with Lieber-DeCarli ethanol liquid diet for 3 weeks, liver triglyceride (TG) accumulation was increased in the fgfr1^fl/fl mice to a greater extent than in the fgfr1^adipoQ-cre mice. When PPARα agonist WY-14,643 was added in the liquid ethanol diet at 10 mg/L, the ethanol-induced liver TG accumulation was blunted in the fgfr1^fl/fl mice but not in the fgfr1^adipoQ-cre mice. There was no significant difference in WY-14,643-induced fatty acid oxidation, ethanol metabolism, and oxidative stress between the fgfr1^fl/fl and fgfr1^adipoQ-cre mice. Interestingly, adipose atrophy was induced by WY-14,643 in the fgfr1^adipoQ-cre mice but not in the fgfr1^fl/fl mice. Serum free fatty acid was also decreased by WY-14,643 in the fgfr1^adipoQ-cre mice but not in the fgfr1^fl/fl mice. These results suggest that WY-14,643 inhibits alcoholic fatty liver and regulates adipose tissue mass and fat mobilization from adipose tissues to liver in an adipocyte FGFR1-dependent manner.

Chronic exposure to PFO4DA and PFO5DoDA, two perfluoroalkyl ether carboxylic acids (PFECAs), suppresses hepatic stress signals and disturbs glucose and lipid metabolism in male mice

Perfluoroalkyl ether carboxylic acids (PFECAs), including PFO4DA and PFO5DoDA, have been found in both surface water and volunteer blood samples from polluted regions. However, little knowledge is available on their potential bioaccumulation and health risk. In the present study, the half-lives of PFO4DA and PFO5DoDA in male mouse serum were 24 h and nearly 43 d, respectively, indicating markedly increased difficulty in eliminating PFO5DoDA from the body. After 140 d daily exposure both PFO4DA and PFO5DoDA (10 μg/kg/d) increased body weight. Hepatomegaly was the most sensitive phenomenon after exposure treatment, with occurrence even in the 2 μg/kg/d exposure groups. RNA-seq analysis supported a similar but stronger effect of PFO5DoDA compared with PFO4DA. A wide array of genes involved in stimulus sensing and response were suppressed. In addition to weight gain, hyperglycemia was also observed after treatment. Increased glucose and decreased pyruvate and lactate levels in the liver supported a reduction in glycolysis, consistent with the reduction in the key regulator Pfkfb3. In conclusion, chronic PFO4DA and PFO5DoDA exposure suppressed stress signals and disturbed glucose and lipid metabolism in the liver. The longer serum half-life and stronger hepatic bioaccumulation of PFO5DoDA, at least partially, contributed to its stronger hepatotoxicity than that of PFO4DA.

Characterization of the gut-liver-muscle axis in cirrhotic patients with sarcopenia

BACKGROUND & AIM

Sarcopenia is frequent in cirrhosis and is associated with unfavourable outcomes. The role of the gut-liver-muscle axis in this setting has been poorly investigated. The aim of this study was to identify gut microbiota, metabolic and inflammatory signatures associated with sarcopenia in cirrhotic patients.

METHODS

Fifty cirrhotic patients assessed for the presence of sarcopenia by the quantification of muscle mass and strength were compared with age- and sex-matched controls. A multiomic analysis, including gut microbiota composition and metabolomics, serum myokines and systemic and intestinal inflammatory mediators, was performed.

RESULTS

The gut microbiota of sarcopenic cirrhotic patients was poor in bacteria associated with physical function (Methanobrevibacter, Prevotella and Akkermansia), and was enriched in Eggerthella, a gut microbial marker of frailty. The abundance of potentially pathogenic bacteria, such as Klebsiella, was also increased, to the detriment of autochthonous ones. Sarcopenia was associated with elevated serum levels of pro-inflammatory mediators and of fibroblast growth factor 21 (FGF21) in cirrhotic patients. Gut microbiota metabolic pathways involved in amino acid, protein and branched-chain amino acid metabolism were up-regulated, in addition to ethanol, trimethylamine and dimethylamine production. Correlation networks and clusters of variables associated with sarcopenia were identified, including one centred on Klebsiella/ethanol/FGF21/Eggerthella/Prevotella.

CONCLUSIONS

Alterations in the gut-liver-muscle axis are associated with sarcopenia in patients with cirrhosis. Detrimental but also compensatory functions are involved in this complex network.

FGF21 in obesity and cancer: New insights

The endocrine FGF21 was discovered as a novel metabolic regulator in 2005 with new functions bifurcating from the canonic heparin-binding FGFs that directly promote cell proliferation and growth independent of a co-receptor. Early studies have demonstrated that FGF21 is a stress sensor in the liver and possibly, several other endocrine and metabolic tissues. Hepatic FGF21 signals via endocrine routes to quench episodes of metabolic derangements, promoting metabolic homeostasis. The convergence of mouse and human studies shows that FGF21 promotes lipid catabolism, including lipolysis, fatty acid oxidation, mitochondrial oxidative activity, and thermogenic energy dissipation, rather than directly regulating insulin and appetite. The white and brown adipose tissues and, to some extent, the hypothalamus, all of which host a transmembrane receptor binary complex of FGFR1 and co-receptor KLB, are considered the essential tissue and molecular targets of hepatic or pharmacological FGF21. On the other hand, a growing body of work has revealed that pancreatic acinar cells form a constitutive high-production site for FGF21, which then acts in an autocrine or paracrine mode. Beyond regulation of macronutrient metabolism and physiological energy expenditure, FGF21 appears to function in forestalling the development of fatty pancreas, steato-pancreatitis, fatty liver, and steato-hepatitis, thereby preventing the development of advanced pathologies such as pancreatic ductal adenocarcinoma or hepatocellular carcinoma. This review is intended to provide updates on these new discoveries that illuminate the protective roles of FGF21-FGFR1-KLB signal pathway in metabolic anomalies-associated severe tissue damage and malignancy, and to inform potential new preventive or therapeutic strategies for obesity-inflicted cancer patients via reducing metabolic risks and inflammation.

Exercise and dietary intervention ameliorate high-fat diet-induced NAFLD and liver aging by inducing lipophagy

Exercise and dietary intervention are currently available strategies to treat nonalcoholic fatty liver disease (NAFLD), while the underlying mechanism remains controversial. Emerging evidence shows that lipophagy is involved in the inhibition of the lipid droplets accumulation. However, it is still unclear if exercise and dietary intervention improve NAFLD through regulating lipophagy, and how exercise of skeletal muscle can modulate lipid metabolism in liver. Moreover, NAFLD is associated with aging, and little is known about the effect of lipid accumulation on aging process. Here in vivo and in vitro models, we found that exercise and dietary intervention reduced lipid droplets formation, decreased hepatic triglyceride in the liver induced by high-fat diet. Exercise and dietary intervention enhanced the lipophagy by activating AMPK/ULK1 and inhibiting Akt/mTOR/ULK1 pathways respectively. Furthermore, exercise stimulated FGF21 production in the muscle, followed by secretion to the circulation to promote the lipophagy in the liver via an AMPK-dependent pathway. Importantly, for the first time, we demonstrated that lipid accumulation exacerbated liver aging, which was ameliorated by exercise and dietary intervention through inducing lipophagy. Our findings suggested a new mechanism of exercise and dietary intervention to improve NAFLD through promoting lipophagy. The study also provided evidence to support that muscle exercise is beneficial to other metabolic organs such as liver. The FGF21-mediated AMPK dependent lipophagy might be a potential drug target for NAFLD and aging caused by lipid metabolic dysfunction.

Differential effects of protein intake versus intake of a defined oligopeptide on FGF-21 in obese human subjects in vivo

BACKGROUND

FGF-21 is described as a powerful metabolic regulator with beneficial effects including glucose-lowering and improvement of insulin sensitivity without hypoglycaemia. On the other hand, FGF-21 is activated when muscle and other tissues are stressed by external effects or internal cellular pathogens that lead to shortcomings in metabolic balance. Previous results suggested that FGF-21 could be a promising target to develop future metabolic therapeutics.

PURPOSE

The present study was performed to gain deeper insight into the regulation of FGF-21 by protein metabolism in obese human subjects.

METHODS

FGF-21 serum concentrations were measured in a cohort of n = 246 obese humans ± type 2 diabetes mellitus (T2DM) (median age 53.0 [46.0; 60.0] years and BMI 40.43 [35.11; 47.24] kg/m²) and related to the nutritional protein intake. In addition, the effect of a novel oligopeptide purified from a β-casein hydrolysate on FGF-21 was examined in vitro in liver cells and in vivo in a human intervention study with the main focus on metabolic inflammation including 40 mainly obese subjects (mean age 41.08 ± 9.76 years, mean BMI 38.29 ± 9.4 kg/m²) in a randomized 20 weeks double-blind cross-over design.

MAIN FINDINGS

In the cohort analysis, FGF-21 serum concentrations were significant lower with higher protein intake in obese subjects without T2DM but not in obese subjects with T2DM. Furthermore, relative methionine intake was inversely related to FGF-21. While global protein intake in obesity was inversely associated with FGF-21, incubation of HepG2 cells with a β-casein oligopeptide increased FGF-21 expression in vitro. This stimulatory effect was also present in vivo, since in the clinical intervention study treatment of obese subjects with the β-casein oligopeptide for 8 weeks significantly increased FGF-21 serum levels from W0 = 23.86 pg/mL to W8 = 30.54 pg/mL (p < 0.001), while no increase was found for placebo.

CONCLUSION

While the total nutritional protein intake is inversely associated with FGF-21 serum levels, a purified and well characterised oligopeptide is able to induce FGF-21 serum levels in humans. These findings suggest a differential role of various components of protein metabolism on FGF-21, rather than this factor being solely a sensor of total nutritional protein intake.

Exercise Alleviates Obesity-Induced Metabolic Dysfunction via Enhancing FGF21 Sensitivity in Adipose Tissues

Exercise promotes adipose remodeling and improves obesity-induced metabolic disorders through mechanisms that remain obscure. Here, we identify the FGF21 signaling in adipose tissues as an obligatory molecular transducer of exercise conferring its metabolic benefits in mice. Long-term high fat diet-fed obese mice exhibit compromised effects of exogenous FGF21 on alleviation of hyperglycemia, hyperinsulinemia, and hyperlipidemia, accompanied with markedly reduced expression of FGF receptor-1 (FGFR1) and β-Klotho (KLB) in adipose tissues. These impairments in obese mice are reversed by treadmill exercise. Mice lacking adipose KLB are refractory to exercise-induced alleviation of insulin resistance, glucose dysregulation, and ectopic lipid accumulation due to diminished adiponectin production, excessive fatty acid release, and enhanced adipose inflammation. Mechanistically, exercise induces the adipose expression of FGFR1 and KLB via peroxisome proliferator-activated receptor-gamma-mediated transcriptional activation. Thus, exercise sensitizes FGF21 actions in adipose tissues, which in turn sends humoral signals to coordinate multi-organ crosstalk for maintaining metabolic homeostasis.

Fibroblast growth factor 21 and autophagy: A complex interplay in Parkinson disease

Parkinson disease (PD) is the second common neurodegenerative disorder after Alzheimer's disease (AD). The predominant pathological hallmark is progressive loss of dopaminergic (DA) neurones in the substantia nigra (SN) complicated by aggregation of misfolded forms of alpha-synuclein (α-syn). α-syn is a cytosolic synaptic protein localized in the presynaptic neuron under normal circumstances. What drives misfolding of this protein is largely unknown. However, recent studies suggest that autophagy might be an important risk factor for contributing towards PD. Autophagy is an evolutionarily conserved mechanism that causes the clearance or degradation of misfolded, mutated and damaged proteins, organelles etc. However, in an aging individual this process might deteriorate which could possibly lead to the accumulation of damaged proteins. Hence, autophagy modulation might provide some interesting cues for the treatment of PD. Additionally, Fibroblast growth factor 21 (FGF21) which is known for its role as a potent regulator of glucose and energy metabolism has also proved to be neuroprotective in various neurodegenerative conditions possibly via mediation of autophagy.

The FGF metabolic axis

Members of the fibroblast growth factor (FGF) family play pleiotropic roles in cellular and metabolic homeostasis. During evolution, the ancestor FGF expands into multiple members by acquiring divergent structural elements that enable functional divergence and specification. Heparan sulfate-binding FGFs, which play critical roles in embryonic development and adult tissue remodeling homeostasis, adapt to an autocrine/paracrine mode of action to promote cell proliferation and population growth. By contrast, FGF19, 21, and 23 coevolve through losing binding affinity for extracellular matrix heparan sulfate while acquiring affinity for transmembrane α-Klotho (KL) or β-KL as a coreceptor, thereby adapting to an endocrine mode of action to drive interorgan crosstalk that regulates a broad spectrum of metabolic homeostasis. FGF19 metabolic axis from the ileum to liver negatively controls diurnal bile acid biosynthesis. FGF21 metabolic axes play multifaceted roles in controlling the homeostasis of lipid, glucose, and energy metabolism. FGF23 axes from the bone to kidney and parathyroid regulate metabolic homeostasis of phosphate, calcium, vitamin D, and parathyroid hormone that are important for bone health and systemic mineral balance. The significant divergence in structural elements and multiple functional specifications of FGF19, 21, and 23 in cellular and organismal metabolism instead of cell proliferation and growth sufficiently necessitate a new unified and specific term for these three endocrine FGFs. Thus, the term "FGF Metabolic Axis," which distinguishes the unique pathways and functions of endocrine FGFs from other autocrine/paracrine mitogenic FGFs, is coined.

CYP2A6 is associated with obesity: studies in human samples and a high fat diet mouse model

BACKGROUND/OBJECTIVES

CYP2A6 (CYP2A5 in mice) is mainly expressed in the liver. Hepatic CYP2A6 expression is increased in patients with non-alcoholic fatty liver disease (NAFLD). In mice, hepatic CYP2A5 is induced by high fat diet (HFD) feeding. Hepatic CYP2A5 is also increased in monosodium glutamate-induced obese mice. NAFLD is associated with obesity. In this study, we examined whether obesity is related to CYP2A6.

SUBJECTS/METHODS

Obesity genetic association study: The SAGE is a comprehensive genome-wide association study (GWAS) with case subjects having a lifetime history of alcohol dependence and control subjects never addicted to alcohol. We used 1030 control individuals with self-reported height and weight. A total of 12 single nucleotide polymorphisms (SNP) within the CYP2A6 gene were available. Obesity was determined as a BMI ≥30: 30-34.9 (Class I obesity) and ≥35 (Class II and III obesity). Animal experiment study: CYP2A5 knockout (cyp2a5-/-) mice and wild type (cyp2a5+/+) mice were fed HFD for 14 weeks. Body weight was measured weekly. After an overnight fast, the mice were sacrificed. Liver and blood were collected for biochemical assays.

RESULTS

Single marker analysis showed that three SNPs (rs8192729, rs7256108, and rs7255443) were associated with class I obesity (p < 0.05). The most significant SNP for obesity was rs8192729 (odds ratio (OR) = 1.94, 95% confidence intervals = 1.21-3.10, p = 0.00582). After HFD feeding, body weight was increased in cyp2a5-/- mice to a greater extent than in cyp2a5+/+ mice, and fatty liver was more pronounced in cyp2a5-/- mice than in cyp2a5+/+ mice. PPARα deficiency in cyp2a5-/- mice developed more severe fatty liver, but body weight was not increased significantly.

CONCLUSION

CYP2A6 is associated with human obesity; CYP2A5 protects against obesity and NAFLD in mice. PPARα contributes to the CYP2A5 protective effects on fatty liver but it opposes to the protective effects on obesity.

Lack of muscle mTOR kinase activity causes early onset myopathy and compromises whole-body homeostasis

BACKGROUND

The protein kinase mechanistic target of rapamycin (mTOR) controls cellular growth and metabolism. Although balanced mTOR signalling is required for proper muscle homeostasis, partial mTOR inhibition by rapamycin has beneficial effects on various muscle disorders and age-related pathologies. Besides, more potent mTOR inhibitors targeting mTOR catalytic activity have been developed and are in clinical trials. However, the physiological impact of loss of mTOR catalytic activity in skeletal muscle is currently unknown.

METHODS

We have generated the mTORmKOKI mouse model in which conditional loss of mTOR is concomitant with expression of kinase inactive mTOR in skeletal muscle. We performed a comparative phenotypic and biochemical analysis of mTORmKOKI mutant animals with muscle-specific mTOR knockout (mTORmKO) littermates.

RESULTS

In striking contrast with mTORmKO littermates, mTORmKOKI mice developed an early onset rapidly progressive myopathy causing juvenile lethality. More than 50% mTORmKOKI mice died before 8 weeks of age, and none survived more than 12 weeks, while mTORmKO mice died around 7 months of age. The growth rate of mTORmKOKI mice declined beyond 1 week of age, and the animals showed profound alterations in body composition at 4 weeks of age. At this age, their body weight was 64% that of mTORmKO mice (P < 0.001) due to significant reduction in lean and fat mass. The mass of isolated muscles from mTORmKOKI mice was remarkably decreased by 38-56% (P < 0.001) as compared with that from mTORmKO mice. Histopathological analysis further revealed exacerbated dystrophic features and metabolic alterations in both slow/oxidative and fast/glycolytic muscles from mTORmKOKI mice. We show that the severity of the mTORmKOKI as compared with the mild mTORmKO phenotype is due to more robust suppression of muscle mTORC1 signalling leading to stronger alterations in protein synthesis, oxidative metabolism, and autophagy. This was accompanied with stronger feedback activation of PKB/Akt and dramatic down-regulation of glycogen phosphorylase expression (0.16-fold in tibialis anterior muscle, P < 0.01), thus causing features of glycogen storage disease type V.

CONCLUSIONS

Our study demonstrates a critical role for muscle mTOR catalytic activity in the regulation of whole-body growth and homeostasis. We suggest that skeletal muscle targeting with mTOR catalytic inhibitors may have detrimental effects. The mTORmKOKI mutant mouse provides an animal model for the pathophysiological understanding of muscle mTOR activity inhibition as well as for mechanistic investigation of the influence of skeletal muscle perturbations on whole-body homeostasis.

Dietary Fructose Consumption and Triple-Negative Breast Cancer Incidence

In the past century the western world has found a way to combat most communicative diseases; however, throughout that time the prevalence of obesity, hyperglycemia, and hyperlipidemia have drastically increased. These symptoms characterize metabolic syndrome-a non-communicable disease which has become one of the greatest health hazards of the world. During this same time period the western diet had dramatically changed. Homecooked meals have been replaced by highly-processed, calorically dense foods. This conversion to the current western diet was highlighted by the incorporation of high-fructose corn syrup (HFCS) into sweetened beverages and foods. The consumption of large amounts of dietary sugar, and fructose in particular, has been associated with an altered metabolic state, both systemically and in specific tissues. This altered metabolic state has many profound effects and is associated with many diseases, including diabetes, cardiovascular disease, and even cancer (1). Specific types of cancer, like triple-negative breast cancer (TNBC), are both responsive to dietary factors and exceptionally difficult to treat, illustrating the possibility for preventative care through dietary intervention in at risk populations. To treat these non-communicable diseases, including obesity, diabetes, and cancer, it is imperative to understand systemic and localized metabolic abnormalities that drive its progression. This review will specifically explore the links between increased dietary fructose consumption, development of metabolic disturbances and increased incidence of TNBC.

Fibroblast Growth Factor Binding Protein 3 (FGFBP3) impacts carbohydrate and lipid metabolism

Secreted FGF binding proteins (FGFBP) mobilize locally-acting paracrine FGFs from their extracellular storage. Here, we report that FGFBP3 (BP3) modulates fat and glucose metabolism in mouse models of metabolic syndrome. BP3 knockout mice exhibited altered lipid metabolism pathways with reduced hepatic and serum triglycerides. In obese mice the expression of exogenous BP3 reduced hyperglycemia, hepatosteatosis and weight gain, blunted de novo lipogenesis in liver and adipose tissues, increased circulating adiponectin and decreased NEFA. The BP3 protein interacts with endocrine FGFs through its C-terminus and thus enhances their signaling. We propose that BP3 may constitute a new therapeutic to reverse the pathology associated with metabolic syndrome that includes nonalcoholic fatty liver disease and type 2 diabetes mellitus.

Myokines as Possible Therapeutic Targets in Cancer Cachexia

Cachexia is an extremely serious syndrome which occurs in most patients with different cancers, and it is characterized by systemic inflammation, a negative protein and energy balance, and involuntary loss of body mass. This syndrome has a dramatic impact on the patient's quality of life, and it is also associated with a low response to chemotherapy leading to a decrease in survival. Despite this, cachexia is still underestimated and often untreated. New research is needed in this area to understand this complex phenomenon and ultimately find treatment methods and therapeutic targets. The skeletal muscle can act as an endocrine organ. Signaling between muscles and other systems is done through myokines, cytokines, and proteins produced and released by myocytes. In this review, we would like to draw attention to some of the most important myokines that could have potential as biomarkers and therapeutic targets: myostatin, irisin, myonectin, decorin, fibroblast growth factor 21, interleukin-6, interleukin-8, and interleukin-15.

Fibroblast growth factor 21 induces lipolysis more efficiently than it suppresses lipogenesis in goat adipocytes

Fibroblast growth factor 21 (FGF21) potentially regulates glucose and lipid metabolism in energy homeostasis. We investigated dynamic changes in goat adipocytes treated with 75 nM FGF21 for 24, 36 and 48 h. Compared to controls, FGF21-treated adipocytes displayed smaller lipid droplets and altered levels of the mRNA transcripts encoding several lipolysis genes. The genes with elevated mRNA levels included: ATGL, HSL, CPT-1, and UCP1, and this was observed mainly at 24 and 36 h (P < 0.05). Some gene expression was attenuated including lipogenesis genes, such as SREBP1, PPARγ, C/EBPα, and ACC. This attenuation was observed mainly at 24 h (P < 0.05). Among the genes that were significantly induced or inhibited, ATGL, PGC1α, and C/EBPα were observed a significant effect at 48 h (P < 0.05). In addition, FGF21 treatment greatly increased number of mitochondria and the expression of genes implicated in mitochondrial biogenesis, such as PGC1α, NRF1, and TFAM. These results suggest that FGF21 treatment induced lipolysis more effectively than it suppressed lipogenesis in goat adipocytes, and that mitochondrial biogenesis plays an important role in these cells.

Perilipin 5 Deletion Unmasks an Endoplasmic Reticulum Stress-Fibroblast Growth Factor 21 Axis in Skeletal Muscle

Lipid droplets (LDs) are critical for the regulation of lipid metabolism, and dysregulated lipid metabolism contributes to the pathogenesis of several diseases, including type 2 diabetes. We generated mice with muscle-specific deletion of the LD-associated protein perilipin 5 (PLIN5, Plin5^MKO ) and investigated PLIN5's role in regulating skeletal muscle lipid metabolism, intracellular signaling, and whole-body metabolic homeostasis. High-fat feeding induced changes in muscle lipid metabolism of Plin5^MKO mice, which included increased fatty acid oxidation and oxidative stress but, surprisingly, a reduction in inflammation and endoplasmic reticulum (ER) stress. These muscle-specific effects were accompanied by whole-body glucose intolerance, adipose tissue insulin resistance, and reduced circulating insulin and C-peptide levels in Plin5^MKO mice. This coincided with reduced secretion of fibroblast growth factor 21 (FGF21) from skeletal muscle and liver, resulting in reduced circulating FGF21. Intriguingly, muscle-secreted factors from Plin5^MKO , but not wild-type mice, reduced hepatocyte FGF21 secretion. Exogenous correction of FGF21 levels restored glycemic control and insulin secretion in Plin5^MKO mice. These results show that changes in lipid metabolism resulting from PLIN5 deletion reduce ER stress in muscle, decrease FGF21 production by muscle and liver, and impair glycemic control. Further, these studies highlight the importance for muscle-liver cross talk in metabolic regulation.

Dietary Manipulations That Induce Ketosis Activate the HPA Axis in Male Rats and Mice: A Potential Role for Fibroblast Growth Factor-21

In response to an acute threat to homeostasis or well-being, the hypothalamic-pituitary-adrenocortical (HPA) axis is engaged. A major outcome of this HPA axis activation is the mobilization of stored energy, to fuel an appropriate behavioral and/or physiological response to the perceived threat. Importantly, the extent of HPA axis activity is thought to be modulated by an individual's nutritional environment. In this study, we report that nutritional manipulations signaling a relative depletion of dietary carbohydrates, thereby inducing nutritional ketosis, acutely and chronically activate the HPA axis. Male rats and mice maintained on a low-carbohydrate high-fat ketogenic diet (KD) exhibited canonical markers of chronic stress, including increased basal and stress-evoked plasma corticosterone, increased adrenal sensitivity to adrenocorticotropin hormone, increased stress-evoked c-Fos immunolabeling in the paraventricular nucleus of the hypothalamus, and thymic atrophy, an indicator of chronic glucocorticoid exposure. Moreover, acutely feeding medium-chain triglycerides (MCTs) to rapidly induce ketosis among chow-fed male rats and mice also acutely increased HPA axis activity. Lastly, and consistent with a growing literature that characterizes the hepatokine fibroblast growth factor-21 (FGF21) as both a marker of the ketotic state and as a key metabolic stress hormone, the HPA response to both KD and MCTs was significantly blunted among mice lacking FGF21. We conclude that dietary manipulations that induce ketosis lead to increased HPA axis tone, and that the hepatokine FGF21 may play an important role to facilitate this effect.

Cytochrome P450s and Alcoholic Liver Disease

Alcohol consumption causes liver diseases, designated as Alcoholic Liver Disease (ALD). Because alcohol is detoxified by alcohol dehydrogenase (ADH), a major ethanol metabolism system, the development of ALD was initially believed to be due to malnutrition caused by alcohol metabolism in liver. The discovery of the microsomal ethanol oxidizing system (MEOS) changed this dogma. Cytochrome P450 enzymes (CYP) constitute the major components of MEOS. Cytochrome P450 2E1 (CYP2E1) in MEOS is one of the major ROS generators in liver and is considered to be contributive to ALD. Our labs have been studying the relationship between CYP2E1 and ALD for many years. Recently, we found that human CYP2A6 and its mouse analog CYP2A5 are also induced by alcohol. In mice, the alcohol induction of CYP2A5 is CYP2E1-dependent. Unlike CYP2E1, CYP2A5 protects against the development of ALD. The relationship of CYP2E1, CYP2A5, and ALD is a major focus of this review.

To be or not to be cell autonomous? Autophagy says both

Although cells are a part of the whole organism, classical dogma emphasizes that individual cells function autonomously. Many physiological and pathological conditions, including cancer, and metabolic and neurodegenerative diseases, have been considered mechanistically as cell-autonomous pathologies, meaning those that damage or defect within a selective population of affected cells suffice to produce disease. It is becoming clear, however, that cells and cellular processes cannot be considered in isolation. Best known for shuttling cytoplasmic content to the lysosome for degradation and repurposing of recycled building blocks such as amino acids, nucleotides, and fatty acids, autophagy serves a housekeeping function in every cell and plays key roles in cell development, immunity, tissue remodeling, and homeostasis with the surrounding environment and the distant organs. In this review, we underscore the importance of taking interactions with the microenvironment into consideration while addressing the cell autonomous and non-autonomous functions of autophagy between cells of the same and different types and in physiological and pathophysiological situations.

Fibroblast Growth Factor 21 Promotes C2C12 Cells Myogenic Differentiation by Enhancing Cell Cycle Exit

Fibroblast growth factor 21 (FGF21), a secretion protein, functions as a pivotal regulator of energy metabolism and is being considered as a therapeutic candidate in metabolic syndromes. However, the roles of FGF21 in myogenic differentiation and cell cycle remain obscure. In this study, we investigated the function of FGF21 in myogenesis and cell cycle exit using C2C12 cell line. Our data showed that the expression of myogenic genes as well as cell cycle exit genes was increased after FGF21 overexpression, and FGF21 overexpression induces cell cycle arrest. Moreover, cell cycle genes were decreased in FGF21 overexpression cells while they were increased in FGF21 knockdown cells. Further, FGF21/P53/p21/Cyclin-CDK has been suggested as the key pathway for cell cycle exit mediated by FGF21 in C2C12 cells. Also, we deduce that FGF21 promotes the initiation of myogenic differentiation mainly through enhancing cell cycle exit of C2C12 cells. Taken together, our results demonstrated that FGF21 promotes cell cycle exit and enhances myogenic differentiation of C2C12 cells. This study provided new evidence that FGF21 promotes myogenic differentiation, which could be useful for better understanding the roles of FGF21 in myogenesis.

FGF21 Is an Insulin-Dependent Postprandial Hormone in Adult Humans

CONTEXT

Fibroblast growth factor 21 (FGF21) secretion has been shown to respond directly to carbohydrate consumption, with glucose, fructose, and sucrose all reported to increase plasma levels of FGF21 in rodents and humans. However, carbohydrate consumption also results in secretion of insulin.

OBJECTIVE

The aim of this study was to examine the combined and independent effects of hyperglycemia and hyperinsulinemia on total and bioactive FGF21 in the postprandial period in humans, and determine whether this effect is attenuated in conditions of altered insulin secretion and action.

METHODS

Circulating glucose, insulin, total and bioactive FGF21, and fibroblast activation protein were measured in adults with and without type 2 diabetes (T2D) following an oral glucose tolerance test (OGTT), and under a series of insulin and glucose clamp conditions and following high-fat diet in healthy adults.

RESULTS

Circulating total and bioactive FGF21 levels responded acutely to OGTT, and their ratio was attenuated in T2D patients with reduced postprandial insulin response. The clamp studies revealed that insulin but not glucose accounts for the postprandial rise in FGF21. Finally, there was an attenuated rise in FGF21 in response to a high-fat dietary intervention that is known to alter insulin-stimulated substrate utilization in metabolically active tissues.

CONCLUSIONS

Insulin rather than glucose per se increases total and bioactive FGF21 in the postprandial period in adult humans. Understanding the impact of T2D on bioactive FGF21 will have a significant effect upon the efficacy of therapeutic agents designed to target the FGF21 pathway.

Rush to the fire: FGF21 extinguishes metabolic stress, metaflammation and tissue damage

FGF21 is a master regulator of homeostasis of local and systemic lipid, glucose and energy metabolism. Since its discovery a decade ago, significant progress has been made in understanding the basic molecular, cellular and physiological mechanisms underlying its metabolic roles, and characterizing its beneficial pharmacological activities and possible pathological roles in obesity, diabetes, dyslipidemia, fatty liver disease and their collateral complications and tissue damage. Under basal or normal conditions, FGF21 appears to play a dispensable role in metabolism. However, in response to a variety of cellular and metabolic stress, FGF21 is significantly upregulated to serve as a potent catabolic factor leading to the clearance of excessive lipids and glucose, and therefore, antagonizes metabolic and energy imbalance in a negative fashion. Furthermore, FGF21 treatment ameliorates tissue damage resulted from the harmful effects of metabolic abnormalities, which often ensue an oxidative, pro-inflammatory, inflammatory and/or immune stress state, the so-called metaflammation. Most notably, studies focusing on the liver, pancreas, cardio-vasculature and kidney have revealed its significant protective effects against the structural and functional damages induced by the obese, diabetic or other abnormal metabolic conditions. In this review, we will summarize the current progress on the roles of FGF21 against metaflammation and metabolic tissue damage.

Use of FGF-21 as a Biomarker of Mitochondrial Disease in Clinical Practice

Recent work has suggested that fibroblast growth factor-21 (FGF-21) is a useful biomarker of mitochondrial disease (MD). We routinely measured FGF-21 levels on patients who were investigated at our centre for MD and evaluated its diagnostic performance based on detailed genetic and other laboratory findings. Patients’ FGF-21 results were assessed by the use of age-adjusted z-scores based on normalised FGF-21 values from a healthy population. One hundred and fifty five patients were investigated. One hundred and four of these patients had molecular evidence for MD, 27 were deemed to have disorders other than MD (non-MD), and 24 had possible MD. Patients with defects in mitochondrial DNA (mtDNA) maintenance (n = 32) and mtDNA rearrangements (n = 17) had the highest median FGF-21 among the MD group. Other MD patients harbouring mtDNA point mutations (n = 40) or mutations in other autosomal genes (n = 7) and those with partially characterised MD had lower FGF-21 levels. The area under the receiver operating characteristic curve for distinguishing MD from non-MD patients was 0.69. No correlation between FGF-21 and creatinine, creatine kinase, or cardio-skeletal myopathy score was found. FGF-21 was significantly associated with plasma lactate and ocular myopathy. Although FGF-21 was found to have a low sensitivity for detecting MD, at a z-score of 2.8, its specificity was above 90%. We suggest that a high serum concentration of FGF-21 would be clinically useful in MD, especially in adult patients with chronic progressive external ophthalmoplegia, and may enable bypassing muscle biopsy and directly opting for genetic analysis. Availability of its assay has thus modified our diagnostic pathway.

Alcoholic fatty liver is enhanced in CYP2A5 knockout mice: The role of the PPARα-FGF21 axis

BACKGROUND & AIMS

Cytochrome P450 2A5 (CYP2A5) is induced by ethanol, and the ethanol induction of CYP2A5 is regulated by nuclear factor-erythroid 2-related factor 2 (NRF2). Cyp2a5 knockout (Cyp2a5^-/-) mice develop more severe alcoholic fatty liver than Cyp2a5^+/+ mice. Fibroblast growth factor 21 (FGF21), a PPARα-regulated liver hormone, is involved in hepatic lipid metabolism. Alcoholic and non-alcoholic fatty liver are enhanced in Pparα knockout (Pparα^-/-) mice. This study investigates the relationship between the PPARα-FGF21 axis and the enhanced alcoholic fatty liver in Cyp2a5^-/- mice.

METHODS

Mice were fed the Lieber-Decarli ethanol diet to induce alcoholic fatty liver.

RESULTS

More severe alcoholic fatty liver disease was developed in Cyp2a5^-/- mice than in Cyp2a5^+/+ mice. Basal FGF21 levels were higher in Cyp2a5^-/- mice than in Cyp2a5^+/+ mice, but ethanol did not further increase the elevated FGF21 levels in Cyp2a5^-/- mice while FGF21 was induced by ethanol in Cyp2a5^+/+ mice. Basal levels of serum FGF21 were lower in Pparα^-/- mice than in Pparα^+/+ mice; ethanol induced FGF21 in Pparα^+/+ mice but not in Pparα^-/- mice, whereas ethanol induced hypertriglyceridemia in Pparα^-/- mice but not in Pparα^+/+ mice. Administration of recombinant FGF21 normalized serum FGF21 and triglyceride in Pparα^-/- mice. Alcoholic fatty liver was enhanced in liver-specific Fgf21 knockout mice. Pparα and Cyp2a5 double knockout (Pparα^-/-/Cyp2a5^-/-) mice developed more severe alcoholic fatty liver than Pparα^+/+/Cyp2a5^-/- mice.

CONCLUSIONS

These results suggest that CYP2A5 protects against the development of alcoholic fatty liver disease, and the PPARα-FGF21 axis contributes to the protective effects of CYP2A5 on alcoholic fatty liver disease.

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 combined docosahexaenoic acid–thyroid hormone protocol upregulates rat liver β-Klotho expression and downstream components of FGF21 signaling as a potential novel approach to metabolic stress conditions

We study the mechanism of how liver preconditioning by a DHA and triiodothyronine combined protocol underlies peroxisome-proliferator activated receptor α (PPARα)-fibroblast growth factor 21 (FGF21) upregulation.

Metabolic Adaptation in Obesity and Type II Diabetes: Myokines, Adipokines and Hepatokines

Obesity and type II diabetes are characterized by insulin resistance in peripheral tissues. A high caloric intake combined with a sedentary lifestyle is the leading cause of these conditions. Whole-body insulin resistance and its improvement are the result of the combined actions of each insulin-sensitive organ. Among the fundamental molecular mechanisms by which each organ is able to communicate and engage in cross-talk are cytokines or peptides which stem from secretory organs. Recently, it was reported that several cytokines or peptides are secreted from muscle (myokines), adipose tissue (adipokines) and liver (hepatokines) in response to certain nutrition and/or physical activity conditions. Cytokines exert autocrine, paracrine or endocrine effects for the maintenance of energy homeostasis. The present review is focused on the relationship and cross-talk amongst muscle, adipose tissue and the liver as secretory organs in metabolic diseases.

Hepatic Fgf21 Expression Is Repressed after Simvastatin Treatment in Mice

Fibroblast growth factor 21 (Fgf21) is a hormone with emerging beneficial roles in glucose and lipid homeostasis. The interest in Fgf21 as a potential antidiabetic drug and the factors that regulate its production and secretion is growing. Statins are the most widely prescribed drug for the treatment of dyslipidemia. However, the function of statins is not limited to the lowering of cholesterol as they are associated with pleiotropic actions such as antioxidant, anti-inflammatory and cytoprotective effects. The recently described effect of statins on mitochondrial function and the induction of Fgf21 by mitochondrial stress prompted us to investigate the effect of statin treatment on Fgf21 expression in the liver. To this end, C57BL6J male mice and primary mouse hepatocytes were treated with simvastatin, and Fgf21 expression was subsequently assessed by immunoblotting and quantitative real-time PCR. Hepatic Fgf21 protein and mRNA and circulating levels of FGF21significantly decreased in mice that had received simvastatin in their food (0.1% w/w) for 1 week. This effect was also observed with simvastatin doses as low as 0.01% w/w for 1 week or following 2 intraperitoneal injections within a single day. The reduction in Fgf21 mRNA levels was further verified in primary mouse hepatocytes, indicating that the effect of simvastatin is cell autonomous. In conclusion, simvastatin treatment reduced the circulating and hepatic Fgf21 levels and this effect warrants further investigation with reference to its role in metabolism.

Expression of fibroblast growth factor 21 in patients with biliary atresia

Fibroblast growth factor 21 is a critical circulating adipokine involving in metabolic disorders and various liver diseases. This study was performed to investigate whether FGF21 is also associated with the pathophysiology of biliary atresia. Serum FGF21 levels were measured in 57 BA patients and 20 age matched healthy controls. We also examined hepatic FGF21 mRNA expression and FGF21 protein levels in liver tissues obtained from 15 BA patients undergoing liver transplantation and 5 cases of pediatric donation after cardiac death donor without liver diseases by RT-PCR and Western blotting. Patients with BA showed significantly higher serum FGF21 levels than those without BA (554.7pg/mL [83-2300] vs. 124.5pg/mL [66-270], P<0.05). Patients with BA also had significantly higher FGF21 mRNA and protein levels in hepatic tissues than control subjects. Serum FGF21 expression increased corresponding to the severity of liver fibrosis. Furthermore, serum FGF21 levels dropped significantly in BA patients within 6months after liver transplantation and approached baseline in healthy controls (P>0.05). In vivo, FXR knockout could significantly abrogate cholestasis induced FGF21 expression. FGF21 levels in serum and liver tissue increased significantly in BA patients. In vivo, cholestasis could induce FGF21 expression in FXR dependent manner.

Circulating Fibroblast Growth Factor 21 (Fgf21) as Diagnostic and Prognostic Biomarker in Renal Cancer

Background

The finding of new biomarkers is needed to have a better sub-classification of primary renal tumors (RCC) as well as more reliable predictors of outcome and therapy response. In this study, we evaluated the role of circulating FGF21, an endocrine factor, as a diagnostic and prognostic biomarker for ccRCC.

Materials and Methods

Serum samples from healthy controls (HC), clear cell and chromophobe RCC cancer patients were obtained from the serum biobank “Biobanco Público de Muestras Séricas Oncológicas” (BPMSO) of the “Instituto de Oncología “Ángel H. Roffo”. Serum FGF21 and leptin were measured by ELISA while other metabolic markers were measured following routinely clinical procedures.

Results

One of our major findings was that FGF21 levels were significantly increased in ccRCC patients compared with HC. Moreover, we showed an association between the increased serum FGF21 levels and the shorter disease free survival in a cohort of 98 ccRCC patients, after adjustment for other predictors of outcome.

Conclusion

Our results suggest that higher FGF21 serum level is an independent prognostic biomarker, associated with worse free-disease survival.

Expression profile and overexpression outcome indicate a role for βKlotho in skeletal muscle fibro/adipogenesis

Regeneration of skeletal muscles is required throughout life to ensure optimal performance. Therefore, a better understanding of the resident cells involved in muscle repair is essential. Muscle repair relies on satellite cells (SCs), the resident myogenic progenitors, but also involves the contribution of interstitial cells including fibro/adipocyte progenitors (FAPs). To elucidate the role of the fibroblast growth factor (FGF) signaling in these two cell populations, we previously analyzed freshly isolated cells for their FGF receptor (FGFR) signature. Transcript analysis of the four Fgfr genes revealed distinct expression profiles for SCs and FAPs, raising the possibility that these two cell types have different FGF-mediated processes. Here, we pursued this hypothesis exploring the role of the Klotho genes, whose products are known to function as FGFR co-receptors for the endocrine FGF subfamily. Isolated SC and FAP populations were analyzed in culture, exhibiting spontaneous myogenic or adipogenic differentiation, respectively. αKlotho expression was not detected in either population. βKlotho expression, while not detected in SCs, was strongly upregulated in FAPs entering adipogenic differentiation, coinciding with expression of a panel of adipogenic genes and preceding the appearance of intracellular lipid droplets. Overexpression of βKlotho in mouse cell line models enhanced adipogenesis in NIH3T3 fibroblasts but had no effect on C2C12 myogenic cells. Our study supports a pro-adipogenic role for βKlotho in skeletal muscle fibro/adipogenesis and calls for further research on involvement of the FGF-FGFR-βKlotho axis in the fibro/adipogenic infiltration associated with functional deterioration of skeletal muscle in aging and muscular dystrophy.

Overexpression of β-Klotho in Adipose Tissue Sensitizes Male Mice to Endogenous FGF21 and Provides Protection From Diet-Induced Obesity

The endocrine hormone fibroblast growth factor 21 (FGF21) is induced in the adaptive response to nutrient deprivation, where it serves to regulate the integrated response to fasting via its primary receptor complex, FGF receptor 1 coupled with the cofactor β-klotho (KLB) in target tissues. Curiously, endogenous FGF21 levels are also elevated in preclinical models of obesity and in obese/diabetic individuals. In addition to higher FGF21 levels, reduced KLB expression in liver and adipose tissue has been noted in these same individuals, suggesting that obesity may represent an FGF21 resistant state. To explore the contribution of tissue-specific KLB levels to endogenous FGF21 activity, in both fasting and high-fat diet feeding conditions, we generated animals overexpressing KLB in liver (LKLBOE) or adipose (ATKLBOE). Supportive of tissue-specific partitioning of FGF21 action, after chronic high-fat feeding, ATKLBOE mice gained significantly less weight than WT. Reduced weight gain was associated with elevated caloric expenditure, accompanied by a reduced respiratory exchange ratio and lower plasma free fatty acids levels, suggestive of augmented lipid metabolism. In contrast, LKLBOE had no effect on body weight but did reduce plasma cholesterol. The metabolic response to fasting was enhanced in LKLBOE mice, evidenced by increased ketone production, whereas no changes in this were noted in ATKLBOE mice. Taken together, these data provide further support that specific effects of FGF21 are mediated via engagement of distinct target organs. Furthermore, enhancing KLB expression in adipose may sensitize to endogenous FGF21, thus representing a novel strategy to combat metabolic disease.

Fibroblast growth factors, old kids on the new block

The fibroblast growth factors (FGFs) are a family of cell intrinsic regulatory peptides that control a broad spectrum of cellular activities. The family includes canonic FGFs that elicit their activities by activating the FGF receptor (FGFR) tyrosine kinase and non-canonic members that elicit their activities intracellularly and via FGFR-independent mechanisms. The FGF signaling axis is highly complex due to the existence of multiple isoforms of both ligands and receptors, as well as cofactors that include the chemically heterogeneous heparan sulfate (HS) cofactors, and in the case of endocrine FGFs, the Klotho coreceptors. Resident FGF signaling controls embryonic development, maintains tissue homeostasis, promotes wound healing and tissue regeneration, and regulates functions of multiple organs. However, ectopic or aberrant FGF signaling is a culprit for various diseases, including congenital birth defects, metabolic disorder, and cancer. The molecular mechanisms by which the specificity of FGF signaling is achieved remain incompletely understood. Since its application as a druggable target has been gradually recognized by pharmaceutical companies and translational researchers, understanding the determinants of FGF signaling specificity has become even more important in order to get into the position to selectively suppress a particular pathway without affecting others to minimize side effects.

Muscle mitochondrial stress adaptation operates independently of endogenous FGF21 action

Objective

Fibroblast growth factor 21 (FGF21) was recently discovered as stress-induced myokine during mitochondrial disease and proposed as key metabolic mediator of the integrated stress response (ISR) presumably causing systemic metabolic improvements. Curiously, the precise cell-non-autonomous and cell-autonomous relevance of endogenous FGF21 action remained poorly understood.

Methods

We made use of the established UCP1 transgenic (TG) mouse, a model of metabolic perturbations made by a specific decrease in muscle mitochondrial efficiency through increased respiratory uncoupling and robust metabolic adaptation and muscle ISR-driven FGF21 induction. In a cross of TG with Fgf21-knockout (FGF21^−/−) mice, we determined the functional role of FGF21 as a muscle stress-induced myokine under low and high fat feeding conditions.

Results

Here we uncovered that FGF21 signaling is dispensable for metabolic improvements evoked by compromised mitochondrial function in skeletal muscle. Strikingly, genetic ablation of FGF21 fully counteracted the cell-non-autonomous metabolic remodeling and browning of subcutaneous white adipose tissue (WAT), together with the reduction of circulating triglycerides and cholesterol. Brown adipose tissue activity was similar in all groups. Remarkably, we found that FGF21 played a negligible role in muscle mitochondrial stress-related improved obesity resistance, glycemic control and hepatic lipid homeostasis. Furthermore, the protective cell-autonomous muscle mitohormesis and metabolic stress adaptation, including an increased muscle proteostasis via mitochondrial unfolded protein response (UPR^mt) and amino acid biosynthetic pathways did not require the presence of FGF21.

Conclusions

Here we demonstrate that although FGF21 drives WAT remodeling, the adaptive pseudo-starvation response under elevated muscle mitochondrial stress conditions operates independently of both WAT browning and FGF21 action. Thus, our findings challenge FGF21 as key metabolic mediator of the mitochondrial stress adaptation and powerful therapeutic target during muscle mitochondrial disease.

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Muscle mitochondrial stress-induced browning of white adipose tissue fully requires FGF21.

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Negligible role of myokine FGF21 on whole body metabolic adaptations.

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Muscle mitohormesis and starvation-like response operates independently of FGF21 action.

Skeletal muscle mitochondrial uncoupling prevents diabetes but not obesity in NZO mice, a model for polygenic diabesity

Induction of skeletal muscle (SM) mitochondrial stress by expression of uncoupling protein 1 (UCP1) in mice results in a healthy metabolic phenotype associated with increased secretion of FGF21 from SM. Here, we investigated whether SM mitochondrial uncoupling can compensate obesity and insulin resistance in the NZO mouse, a polygenic diabesity model. Male NZO mice were crossed with heterozygous UCP1 transgenic (tg) mice (mixed C57BL/6/CBA background) and further backcrossed to obtain F1 and N2 offspring with 50 and 75 % NZO background, respectively. Male F1 and N2 progeny were fed a high-fat diet ad libitum for 20 weeks from weaning. Blood glucose was reduced, and diabetes (severe hyperglycemia >300 mg/dl) was fully prevented in both F1- and N2-tg progeny compared to a diabetes prevalence of 15 % in F1 and 42 % in N2 wild type. In contrast, relative body fat content and plasma insulin were decreased, and glucose tolerance was improved, in F1-tg only. Both F1 and N2-tg showed decreased lean body mass. Accordingly, induction of SM stress response including FGF21 expression and secretion was similar in both F1 and N2-tg mice. In white adipose tissue, expression of FGF21 target genes was enhanced in F1 and N2-tg mice, whereas lipid metabolism genes were induced in F1-tg only. There was no evidence for induction of browning in either UCP1 backcross. We conclude that SM mitochondrial uncoupling induces FGF21 expression and prevents diabetes in mice with a 50-75 % NZO background independent of its effects on adipose tissue.

Therapeutic potential of the endocrine fibroblast growth factors FGF19, FGF21 and FGF23

The endocrine fibroblast growth factors (FGFs), FGF19, FGF21 and FGF23, are critical for maintaining whole-body homeostasis, with roles in bile acid, glucose and lipid metabolism, modulation of vitamin D and phosphate homeostasis and metabolic adaptation during fasting. Given these functions, the endocrine FGFs have therapeutic potential in a wide array of chronic human diseases, including obesity, type 2 diabetes, cancer, and kidney and cardiovascular disease. However, the safety and feasibility of chronic endocrine FGF administration has been challenged, and FGF analogues and mimetics are now being investigated. Here, we discuss current knowledge of the complex biology of the endocrine FGFs and assess how this may be harnessed therapeutically.

Metabolic fibroblast growth factors (FGFs): Mediators of energy homeostasis

The metabolic fibroblast growth factors (FGFs), FGF1, FGF15/19, and FGF21 differ from classic FGFs in that they modulate energy homeostasis in response to fluctuating nutrient availability. These unique mediators of metabolism regulate a number of physiological processes which contribute to their potent pharmacological properties. Administration of pharmacological doses of these FGFs causes weight loss, increases energy expenditure, and improves carbohydrate and lipid metabolism in obese animal models. However, many questions remain regarding the precise molecular and physiological mechanisms governing the effects of individual metabolic FGFs. Here we review the metabolic actions of FGF1, FGF15/19, and FGF21 while providing insights into their pharmacological effects by examining known biological functions.

Gene expression in WAT from healthy humans and monkeys correlates with FGF21-induced browning of WAT in mice

OBJECTIVE

Identify a gene expression signature in white adipose tissue (WAT) that reports on WAT browning and is associated with a healthy phenotype.

METHODS

RNA from several different adipose depots across three species were analyzed by whole transcriptome profiling, including 1) mouse subcutaneous white fat, brown fat, and white fat after in vivo treatment with FGF21; 2) human subcutaneous and omental fat from insulin-sensitive and insulin-resistant patients; and 3) rhesus monkey subcutaneous fat from healthy and dysmetabolic individuals.

RESULTS

A "browning" signature in mice was identified by cross-referencing the FGF21-induced signature in WAT with the brown adipose tissue (BAT) vs. WAT comparison. In addition, gene expression levels in WAT from insulin-sensitive/healthy vs. insulin-resistant/dysmetabolic humans and rhesus monkeys, respectively, correlated with the gene expression levels in mouse BAT vs. WAT. A subset of 49 genes were identified that were consistently regulated or differentially expressed in the mouse and human data sets that could be used to monitor browning of WAT across species.

CONCLUSIONS

Gene expression profiles of WATs from healthy insulin-sensitive individuals correlate with those of BAT and FGF21-induced browning of WAT.

Skeletal muscle increases FGF21 expression in mitochondrial disorders to compensate for energy metabolic insufficiency by activating the mTOR-YY1-PGC1α pathway

Fibroblast growth factor 21 (FGF21) is a growth factor with pleiotropic effects on regulating lipid and glucose metabolism. Its expression is increased in skeletal muscle of mice and humans with mitochondrial disorders. However, the effects of FGF21 on skeletal muscle in response to mitochondrial respiratory chain deficiency are largely unknown. Here we demonstrate that the increased expression of FGF21 is a compensatory response to respiratory chain deficiency. The mRNA and protein levels of FGF21 were robustly raised in skeletal muscle from patients with mitochondrial myopathy or MELAS. The mammalian target of rapamycin (mTOR) phosphorylation levels and its downstream targets, Yin Yang 1 (YY1) and peroxisome proliferator-activated receptor γ, coactivator 1α (PGC-1α), were increased by FGF21 treatment in C2C12 myoblasts. Activation of the mTOR-YY1-PGC1α pathway by FGF21 in myoblasts regulated energy homeostasis as demonstrated by significant increases in intracellular ATP synthesis, oxygen consumption rate, activity of citrate synthase, glycolysis, mitochondrial DNA copy number, and induction of the expression of key energy metabolic genes. The effects of FGF21 on mitochondrial function required phosphoinositide 3-kinase (PI3K), which activates mTOR. Inhibition of PI3K, mTOR, YY1, and PGC-1α activities attenuated the stimulating effects of FGF21 on intracellular ATP levels and mitochondrial gene expression. Our findings revealed that mitochondrial respiratory chain deficiency elicited a compensatory response in skeletal muscle by increasing the FGF21 expression levels in muscle, which resulted in enhanced mitochondrial function through an mTOR-YY1-PGC1α-dependent pathway in skeletal muscle.

Circulating levels of fibroblast growth factor-21 increase with age independently of body composition indices among healthy individuals

BACKGROUND

Circulating FGF21 levels are commonly elevated in disease states. There is limited information regarding concentrations of circulating FGF21 in the absence of disease, as well as age-related differences in body composition that may contribute to FGF21 regulation across groups.

OBJECTIVE

The objectives of this study were to assess FGF21 levels across age groups (childhood to elder adulthood), and investigate whether body composition indices are associated with age-related differences in circulating FGF21.

MATERIALS AND METHODS

We cross-sectionally analyzed serum concentrations of FGF21 in 184 healthy subjects aged 5-80y (45% male). Multiple linear regression was performed to assess the independent association of categorical age (children: 5-12y, young adults: 20-29y, adults: 30-50y, older adults: 55-64y, elder adults: 65-80y) with FGF21 concentration taking into account DXA-measured body composition indices [bone mineral density (BMD) and percent lean, trunk, and fat mass]. We also stratified analysis by tertile of FGF21.

RESULTS

Incremental increases in FGF21 levels were observed across age groups (youngest to highest). Age group was positively associated with FGF21 level independent of body composition indices (age group variable: β=0.25, 0.24, 0.24, 0.23, all P<0.0001, controlling for percent lean, BMD, percent fat, and percent trunk fat, respectively). By FGF21 tertile, age group was associated with FGF21 in the lowest tertile only (β=13.1, 0.19, 0.18, all P≤0.01, accounting for percent lean, fat and trunk fat, respectively), but not when accounting for BMD.

CONCLUSIONS

Our findings in a healthy population display an age-related increase in serum FGF21, highlighting a potential age effect in response to metabolic demand over the lifecourse. FGF21 levels increase with age independently of body composition. At lower levels of FGF21, BMD, but not other body composition parameters, attenuates the association between FGF21 level and age, suggesting the metabolic demand of the skeleton may provide a link between FGF21 and energy metabolism.

Loss of FGF21 in diabetic mouse during hepatocellular carcinogenetic transformation

Diabetes associated metabolic syndrome has been shown to be an independent risk factor for the development of hepatocellular carcinoma (HCC). Cirrhosis, in fact, was not always a prerequisite of HCC development and this might particularly apply to the metabolic abnormality associated HCC. This study was to investigate diabetes associated HCC and the potential role of FGF21 during carcinogenetic transformation of HCC. Dimethylnitrosamine (DEN) was used to induce HCC in the diabetic OVE26 mice. Pronounced damage characterized by steatohepatitis was found in the liver of diabetic mice. Steatohepatitis accompanied by constant cell proliferation and tumor cell growth were also found in the hepatic tissues of diabetic OVE26 mice when DEN being administrated. FGF21 protein level increased in liver tissues at an early stage along with steatohepatitis in diabetic OVE26 mice, but decreased in liver tissues later when HCC was developed. In addition, decreased FGF21 protein level was associated with cancerous hyper-proliferation and aberrant p53 and TGF-β/Smad signaling during HCC development. Loss of FGF21 may play an important role in HCC carcinogenetic transformation during metabolic liver injury in diabetic animals. The present finding calls attention to the need to control metabolic disorders associated with diabetes and may further develop a protective strategy against HCC.

Hepatic PTEN deficiency improves muscle insulin sensitivity and decreases adiposity in mice

BACKGROUND & AIMS

PTEN is a dual lipid/protein phosphatase, downregulated in steatotic livers with obesity or HCV infection. Liver-specific PTEN knockout (LPTEN KO) mice develop steatosis, inflammation/fibrosis and hepatocellular carcinoma with aging, but surprisingly also enhanced glucose tolerance. This study aimed at understanding the mechanisms by which hepatic PTEN deficiency improves glucose tolerance, while promoting fatty liver diseases.

METHODS

Control and LPTEN KO mice underwent glucose/pyruvate tolerance tests and euglycemic-hyperinsulinemic clamps. Body fat distribution was assessed by EchoMRI, CT-scan and dissection analyses. Primary/cultured hepatocytes and insulin-sensitive tissues were analysed ex vivo.

RESULTS

PTEN deficiency in hepatocytes led to steatosis through increased fatty acid (FA) uptake and de novo lipogenesis. Although LPTEN KO mice exhibited hepatic steatosis, they displayed increased skeletal muscle insulin sensitivity and glucose uptake, as assessed by euglycemic-hyperinsulinemic clamps. Surprisingly, white adipose tissue (WAT) depots were also drastically reduced. Analyses of key enzymes involved in lipid metabolism further indicated that FA synthesis/esterification was decreased in WAT. In addition, Ucp1 expression and multilocular lipid droplet structures were observed in this tissue, indicating the presence of beige adipocytes. Consistent with a liver to muscle/adipocyte crosstalk, the expression of liver-derived circulating factors, known to impact on muscle insulin sensitivity and WAT homeostasis (e.g. FGF21), was modulated in LPTEN KO mice.

CONCLUSIONS

Although steatosis develops in LPTEN KO mice, PTEN deficiency in hepatocytes promotes a crosstalk between liver and muscle, as well as adipose tissue, resulting in enhanced insulin sensitivity, improved glucose tolerance and decreased adiposity.