Identification of a Domain within Peroxisome Proliferator-Activated Receptor γ Regulating Expression of a Group of Genes Containing Fibroblast Growth Factor 21 That Are Selectively Repressed by SIRT1 in Adipocytes

Fibroblast growth factor 21 promotes bone loss by potentiating the effects of peroxisome proliferator-activated receptor γ

The endocrine hormone fibroblast growth factor 21 (FGF21) is a powerful modulator of glucose and lipid metabolism and a promising drug for type 2 diabetes. Here we identify FGF21 as a potent regulator of skeletal homeostasis. Both genetic and pharmacologic FGF21 gain of function lead to a striking decrease in bone mass. In contrast, FGF21 loss of function leads to a reciprocal high-bone-mass phenotype. Mechanistically, FGF21 inhibits osteoblastogenesis and stimulates adipogenesis from bone marrow mesenchymal stem cells by potentiating the activity of peroxisome proliferator-activated receptor γ (PPAR-γ). Consequently, FGF21 deletion prevents the deleterious bone loss side effect of the PPAR-γ agonist rosiglitazone. Therefore, FGF21 is a critical rheostat for bone turnover and a key integrator of bone and energy metabolism. These results reveal that skeletal fragility may be an undesirable consequence of chronic FGF21 administration.

Sirtuins mediate mammalian metabolic responses to nutrient availability

Metabolic diseases are an increasing threat in developed countries. Dysregulation of metabolic pathways, caused by imbalances in energy homeostasis, leads to obesity, diabetes and cardiovascular disease with devastating results for both individuals and societies. Sirtuins, a conserved family of NAD(+)-dependent deacetylase enzymes found in many species, regulate various metabolic pathways and have emerged as important sensors of energy status in mammals. The nuclear sirtuins, SIRT1, SIRT6 and SIRT7, regulate the activity of key transcription factors and cofactors of numerous metabolic pathways in almost all tissues by linking nutrient signals with the cellular responses to energy demands. The mitochondrial sirtuins, SIRT3, SIRT4 and SIRT5, regulate the activity of important mitochondrial enzymes and drive metabolic cycles in response to fasting and calorie restriction. Accumulating evidence indicates that sirtuins can be beneficial in the prevention of metabolic and age-related diseases and suggests that they can be pharmacologically activated to ameliorate such diseases. This Review describes the latest advances in the understanding of the function of sirtuins as regulators of mammalian metabolism and focuses on the role of these enzymes as mediators of nutrient availability.

Adiponectin and reproduction

Adiponectin is an adipocyte-derived cytokine that acts as a major regulator of insulin sensitivity. Adiponectin deficiency can result in severe diabetes and metabolic disorders in humans. Since its discovery, our understanding of adiponectin's biological functions has expanded from insulin sensitization properties to new effects on inflammation, immunology, and human reproduction. Indeed, both obesity and excessive leanness are associated with reproductive dysfunction. The objective of this chapter is to review such biological actions and the potential roles of adiponectin on human reproduction. There is accumulative evidence for direct effects of this adipokine on the late stages of folliculogenesis and on the development of a functional placenta. In addition, clinical and genomic studies associate hypoadiponectinemia with pregnancy-related disorders, including polycystic ovarian syndrome.

Circulating fibroblast growth factor 21 levels are closely associated with hepatic fat content: a cross-sectional study

Background and Aims

Fibroblasts growth factor 21 (FGF21), a liver-secreted endocrine factor involved in regulating glucose and lipid metabolism, has been shown to be elevated in patients with non-alcoholic fatty liver disease (NAFLD). This study aimed to evaluate the quantitative correlation between serum FGF21 level and hepatic fat content.

Methods

A total of 138 subjects (72 male and 66 female) aged from 18 to 65 years with abnormal glucose metabolism and B-ultrasonography diagnosed fatty liver were enrolled in the study. Serum FGF21 levels were determined by an in-house chemiluminescence immunoassay and hepatic fat contents were measured by proton magnetic resonance spectroscopy.

Results

Serum FGF21 increased progressively with the increase of hepatic fat content, but when hepatic fat content increased to the fourth quartile, FGF21 tended to decline. Serum FGF21 concentrations were positively correlated with hepatic fat content especially in subjects with mild/moderate hepatic steatosis (r = 0.276, p = 0.009). Within the range of hepatic steatosis from the first to third quartile, FGF21 was superior to any other traditional clinical markers including ALT to reflect hepatic fat content. When the patients with severe hepatic steatosis (the fourth quartile) were included, the quantitative correlation between FGF21 and hepatic fat content was weakened.

Conclusions

Serum FGF21 was a potential biomarker to reflect the hepatic fat content in patients with mild or moderate NAFLD. In severe NAFLD patients, FGF21 concentration might decrease due to liver inflammation or injury.

SIRT1 as a potential therapeutic target for treatment of nonalcoholic fatty liver disease

Sirtuins are members of the silent information regulator 2 (Sir2) family, a group of Class III histone/protein deacetylases. There are 7 different sirtuins in mammals (SIRT1-7), of which SIRT1 is the best known and most studied. SIRT1 is responsible for the regulation of protein activation by means of deacetylating a variety of proteins that play important roles in the pathophysiology of metabolic diseases. Recently, it has been shown that SIRT1 plays key roles in the regulation of lipid and glucose homeostasis, control of insulin secretion and sensitivity, antiinflammatory effects, control of oxidative stress and the improvements in endothelial function that result due to increased mitochondrial biogenesis and β-oxidation capacity.

Nonalcoholic fatty liver disease (NAFLD) is currently the most common liver disease, and it has been accepted as the hepatic component of metabolic syndrome. Recent studies have shown that SIRT expression in the liver is significantly decreased in an NAFLD model of rats fed a high-fat diet, and moderate SIRT1 overexpression protects mice from developing NAFLD. In addition to resveratrol, a natural SIRT1 activator, small-molecule pharmacologic SIRT1 activators have positive effects on metabolic diseases. These effects are particularly promising in the case of diabetes mellitus, for which phase studies are currently being performed. With this information, we hypothesized that the pharmacologic activation of SIRT1, which has been implicated in the pathogenesis of NAFLD, will be a potential therapeutic target for treating NAFLD. In this paper, we review the metabolic effects of SIRT1 and its association with the pathophysiology of NAFLD.

Fibroblast growth factor 21 induces glucose transporter-1 expression through activation of the serum response factor/Ets-like protein-1 in adipocytes

Fibroblast growth factor 21 (FGF21) is a liver-secreted endocrine factor with multiple beneficial effects on obesity-related disorders. It enhances glucose uptake by inducing the expression of glucose transporter-1 (GLUT1) in adipocytes. Here we investigated the signaling pathways that mediate FGF21-induced GLUT1 expression and glucose uptake in vitro and in animals. Quantitative real-time PCR and a luciferase reporter assay showed that FGF21 induced GLUT1 expression through transcriptional activation. The truncation of the GLUT1 promoter from -3145 to -3105 bp, which contains two highly conserved serum response element (SRE) and E-Twenty Six (ETS) binding motif, dramatically decreased FGF21-induced promoter activity of the GLUT1 gene. A chromatin immunoprecipitation assay demonstrated that the transcription factors serum response factor (SRF) and Ets-like protein-1 (Elk-1) were recruited to the GLUT1 promoter upon FGF21 stimulation. The siRNA-mediated knockdown of either SRF or Elk-1 resulted in a marked attenuation in FGF21-induced GLUT1 expression and glucose uptake in adipocytes. In C57 lean mice, a single intravenous injection of FGF21 induced phosphorylation of Elk-1 at Ser(383) and SRF at Ser(103) and also led to the recruitment of Elk-1 and SRF to the GLUT1 promoter in epididymal fats. By contrast, such effects of in vivo FGF21 administration were blunted in high fat diet-induced obese mice. In conclusion, FGF21 induces GLUT1 expression and glucose uptake through sequential activation of ERK1/2 and SRF/Elk-1, which in turn triggers the transcriptional activation of GLUT1 in adipocytes. The impairment in this signaling pathway may contribute to FGF21 resistance in obese mice.

SIRT1 controls lipolysis in adipocytes via FOXO1-mediated expression of ATGL

Recent studies have established SIRT1 as an important regulator of lipid metabolism, although the mechanism of its action at the molecular level has not been revealed. Here, we show that knockdown of SIRT1 with the help of small hairpin RNA decreases basal and isoproterenol-stimulated lipolysis in cultured adipocytes. This effect is attributed, at least in part, to the suppression of the rate-limiting lipolytic enzyme, adipose triglyceride lipase (ATGL), at the level of transcription. Mechanistically, SIRT1 controls acetylation status and functional activity of FoxO1 that directly binds to the ATGL promoter and regulates ATGL gene transcription. We have also found that depletion of SIRT1 decreases AMP-dependent protein kinase (AMPK) activity in adipocytes. To determine the input of AMPK in regulation of lipolysis, we have established a stable adipose cell line that expresses a dominant-negative α1 catalytic subunit of AMPK under the control of the inducible TET-OFF lentiviral expression vector. Reduction of AMPK activity does not have a significant effect on the rates of lipolysis in this cell model. We conclude, therefore, that SIRT1 controls ATGL transcription primarily by deacetylating FoxO1.

Fibroblast growth factor 21: effects on carbohydrate and lipid metabolism in health and disease

PURPOSE OF REVIEW

The review summarizes recent findings examining the effects of fibroblast growth factor (FGF) 21 on carbohydrate and lipid metabolism with emphasis on publications from 2010.

RECENT FINDINGS

FGF21 is considered to be a metabolic hormone rather than a traditional growth factor. Recent studies identified the important role of FGF21 in regulation of energy balance. Thus, the protein regulates hepatic metabolism and improves insulin sensitivity. Furthermore, central FGF21 action in the brain increases energy expenditure and insulin sensitivity in rodents. Interestingly, FGF21 expression is elevated in the adaptive response to fasting but also regulated by feeding-induced mechanisms. Moreover, FGF21 levels are elevated in obese animals and positively correlate with BMI in humans suggesting obesity as a FGF21-resistant state.

SUMMARY

FGF21 is a metabolic hormone that is regulated by nutritional status and influences glucose and lipid metabolism by central and peripheral mechanisms. Future research is needed to expand our understanding of the diagnostic and therapeutic relevance of FGF21-dependent pathways in humans.

The Ras inhibitors caveolin-1 and docking protein 1 activate peroxisome proliferator-activated receptor γ through spatial relocalization at helix 7 of its ligand-binding domain

Peroxisome proliferator-activated receptor γ (PPARγ) is a transcription factor that promotes differentiation and cell survival in the stomach. PPARγ upregulates and interacts with caveolin-1 (Cav1), a scaffold protein of Ras/mitogen-activated protein kinases (MAPKs). The cytoplasmic-to-nuclear localization of PPARγ is altered in gastric cancer (GC) patients, suggesting a so-far-unknown role for Cav1 in spatial regulation of PPARγ signaling. We show here that loss of Cav1 accelerated proliferation of normal stomach and GC cells in vitro and in vivo. Downregulation of Cav1 increased Ras/MAPK-dependent phosphorylation of serine 84 in PPARγ and enhanced nuclear translocation and ligand-independent transcription of PPARγ target genes. In contrast, Cav1 overexpression sequestered PPARγ in the cytosol through interaction of the Cav1 scaffolding domain (CSD) with a conserved hydrophobic motif in helix 7 of PPARγ's ligand-binding domain. Cav1 cooperated with the endogenous Ras/MAPK inhibitor docking protein 1 (Dok1) to promote the ligand-dependent transcriptional activity of PPARγ and to inhibit cell proliferation. Ligand-activated PPARγ also reduced tumor growth and upregulated the Ras/MAPK inhibitors Cav1 and Dok1 in a murine model of GC. These results suggest a novel mechanism of PPARγ regulation by which Ras/MAPK inhibitors act as scaffold proteins that sequester and sensitize PPARγ to ligands, limiting proliferation of gastric epithelial cells.

Caveolins/caveolae protect adipocytes from fatty acid-mediated lipotoxicity

Mice and humans lacking functional caveolae are dyslipidemic and have reduced fat stores and smaller fat cells. To test the role of caveolins/caveolae in maintaining lipid stores and adipocyte integrity, we compared lipolysis in caveolin-1 (Cav1)-null fat cells to that in cells reconstituted for caveolae by caveolin-1 re-expression. We find that the Cav1-null cells have a modestly enhanced rate of lipolysis and reduced cellular integrity compared with reconstituted cells as determined by the release of lipid metabolites and lactic dehydrogenase, respectively, into the media. There are no apparent differences in the levels of lipolytic enzymes or hormonally stimulated phosphorylation events in the two cell lines. In addition, acute fasting, which dramatically raises circulating fatty acid levels in vivo, causes a significant upregulation of caveolar protein constituents. These results are consistent with the hypothesis that caveolae protect fat cells from the lipotoxic effects of elevated levels fatty acids, which are weak detergents at physiological pH, by virtue of the property of caveolae to form detergent-resistant membrane domains.

Epigenetic codes of PPARγ in metabolic disease

Peroxisome proliferator-activated receptor gamma (PPARγ), a ligand-regulated nuclear hormone receptor, plays critical roles in metabolism and adipogenesis. PPARγ ligands such as thiazolidinediones (TZDs) exert insulin sensitizing and anti-inflammatory effects primarily through action on adipocytes, and are thus widely used to treat metabolic syndrome, especially type II diabetes. A number of PPARγ interacting partners have been identified, many of which are known epigenetic regulators, including enzymes for histone acetylation/deacetylation and histone methylation/demethylation. However, their functional roles in the PPARγ transcriptional pathway are not well defined. Recent advances in ChIP-based and deep sequencing technology are revealing previously underappreciated epigenomic mechanisms and therapeutic potentials of this nuclear receptor pathway.

Hormone-like fibroblast growth factors and metabolic regulation

The family of fibroblast growth factors (FGFs) consisting now of 22 members is generally considered to control a wide range of biological functions such as development, differentiation and survival. However, research during the past decade provided substantial evidence that a so called "hormone-like" subgroup of FGFs, comprised of FGF19, FGF21 and FGF23, is involved in the regulation of diverse metabolic pathways to control glucose, lipid, bile acid, phosphate and vitamin D metabolism. The unique properties of these FGFs include predominant production of the factors in selective tissues, their abundance in the blood due to the lack of extracellular heparin-mediated sequestration, and highly specific tissue-targeted action via engagement of their respective co-receptors. The important metabolic context of FGF19, FGF21, and FGF23 actions has revealed important novel roles for FGFs and provided significant means to explore an opportunity for therapeutic targeting of these factors and their corresponding pathways.

Fibroblast growth factor 21 is a metabolic regulator that plays a role in the adaptation to ketosis

Fibroblast growth factor 21 (FGF21) was originally identified as a member of the FGF family in homology studies and is a member of the endocrine FGF subfamily that lacks heparin binding domains and is released into the circulation. A potential role as a metabolic regulator emerged when FGF21 was shown to increase glucose uptake in adipocytes. Subsequently, marked elevations in FGF21 expression were observed in mice that ate a ketogenic diet and when fasting, which suggests that FGF21 expression plays a role in the adaptation to metabolic states that require increased fatty acid oxidation. Consistent with this evidence, FGF21 knockout mice were not able to respond appropriately to consumption of a ketogenic diet. FGF21 expression is downstream of peroxisome proliferator-activated receptor (PPAR) α in the liver and PPARγ in adipose tissue. FGF21 concentrations are higher in both rodent and human obesity, and recent data suggest that obesity may be an FGF21-resistant state. Recent data increasingly suggest that FGF21 is an important metabolic regulator that may have potential clinical implications.

Transcription coactivator mediator subunit MED1 is required for the development of fatty liver in the mouse

Peroxisome proliferator-activated receptor-γ (PPARγ), a nuclear receptor, when overexpressed in liver stimulates the induction of adipocyte-specific and lipogenesis-related genes and causes hepatic steatosis. We report here that Mediator 1 (MED1; also known as PBP or TRAP220), a key subunit of the Mediator complex, is required for high-fat diet-induced hepatic steatosis as well as PPARγ-stimulated adipogenic hepatic steatosis. Mediator forms the bridge between transcriptional activators and RNA polymerase II. MED1 interacts with nuclear receptors such as PPARγ and other transcriptional activators. Liver-specific MED1 knockout (MED1(ΔLiv) ) mice, when fed a high-fat (60% kcal fat) diet for up to 4 months failed to develop fatty liver. Similarly, MED1(ΔLiv) mice injected with adenovirus-PPARγ (Ad/PPARγ) by tail vein also did not develop fatty liver, whereas mice with MED1 (MED1(fl/fl) ) fed a high-fat diet or injected with Ad/PPARγ developed severe hepatic steatosis. Gene expression profiling and northern blot analyses of Ad/PPARγ-injected mouse livers showed impaired induction in MED1(ΔLiv) mouse liver of adipogenic markers, such as aP2, adipsin, adiponectin, and lipid droplet-associated genes, including caveolin-1, CideA, S3-12, and others. These adipocyte-specific and lipogenesis-related genes are strongly induced in MED1(fl/fl) mouse liver in response to Ad/PPARγ. Re-expression of MED1 using adenovirally-driven MED1 (Ad/MED1) in MED1(ΔLiv) mouse liver restored PPARγ-stimulated hepatic adipogenic response. These studies also demonstrate that disruption of genes encoding other coactivators such as SRC-1, PRIC285, PRIP, and PIMT had no effect on hepatic adipogenesis induced by PPARγ overexpression.

CONCLUSION

We conclude that transcription coactivator MED1 is required for high-fat diet-induced and PPARγ-stimulated fatty liver development, which suggests that MED1 may be considered a potential therapeutic target for hepatic steatosis. (HEPATOLOGY 2011;).

FGF21 reloaded: challenges of a rapidly growing field

Fibroblast growth factor 21 (FGF21) is a pleiotropic hormone-like protein and a major metabolic regulator. However, several key aspects of FGF21 biology remain poorly understood. Indeed, the list of controversies in the FGF21 field spans a variety of topics, from basic matters such as the anatomic distribution of FGF21 expression and the molecular composition of the FGF21 receptor, to the ultimate question of therapeutic relevance of FGF21-dependent pathways in humans. In this paper, we focus on current challenges in the field in an attempt to provide a balanced overview of FGF21 biology and guide future research into this exciting metabolic target.

The role of mammalian sirtuins in the regulation of metabolism, aging, and longevity

Ever since the discovery of sirtuins a decade ago, interest in this family of NAD-dependent deacetylases has exploded, generating multiple lines of evidence implicating sirtuins as evolutionarily conserved regulators of lifespan. In mammals, it has been established that sirtuins regulate physiological responses to metabolism and stress, two key factors that affect the process of aging. Further investigation into the intimate connection among sirtuins, metabolism, and aging has implicated the activation of SIRT1 as both preventative and therapeutic measures against multiple age-associated disorders including type 2 diabetes and Alzheimer's disease. SIRT1 activation has clear potential to not only prevent age-associated diseases but also to extend healthspan and perhaps lifespan. Sirtuin activating compounds and NAD intermediates are two promising ways to achieve these elusive goals.

Serum fibroblast growth factor 21 levels in polycystic ovary syndrome

AIM

This study was designed to measure serum fibroblast growth factor 21 (FGF21) levels in patients with polycystic ovary syndrome (PCOS) and healthy subjects.

METHODS

A total of 37 women were evaluated. Serum levels FGF21, glucose, lipid profile, hormones (follicle-stimulating hormone, luteinising hormone, oestradiol, testosterone, thyroid stimulating hormone, prolactin and insulin) were determined in 24 PCOS (15 subjects of PCOS BMI < 25 kg/m2, 9 subjects of PCOS BMI ≥ kg/m2) and 13 control group (BMI < 25 kg/m2).

RESULTS

Serum FGF21 levels were higher in the PCOS group [99.5 (173.7) pg/ml] than in the control group [52.0 (88.0) pg/ml]. LH and T are significantly higher in PCOS cases (respectively; p < 0.05, p < 0.01). A positive correlation was found between FGF21 and luteinising hormone and testosterone (respectively; r = 0.43 p = 0.007, r = 0.38, p = 0.02). Multivariate discriminant analysis showed that BMI, triglyceride, HOMA-IR, fasting glucose with rise of FGF21 were found significant in PCOS.

CONCLUSION

Our study indicates that FGF21 in cases with PCOS exhibit an increase along with the increase of BMI and also has a positive correlation with LH and T. Further studies are required to clarify the aetiology and effects of FGF21 in women with PCOS.

At the stem of youth and health

Cellular senescence is a specialized form of growth arrest, confined to mitotic cells, induced by various stressful stimuli and characterized by a permanent growth arrest, resistance to apoptosis, an altered pattern of gene expression and the expression of some markers that are characteristic, although not exclusive, to the senescent state. Senescent cells profoundly modify neighboring and remote cells through the production of an altered secretome, eventually leading to inflammation, fibrosis and possibly growth of neoplastic cells. Mammalian aging has been defined as a reduction in the capacity to adequately maintain tissue homeostasis or to repair tissues after injury. Tissue homeostasis and regenerative capacity are nowadays considered to be related to the stem cell pool present in every tissue. For this reason, pathological and patho-physiological conditions characterized by altered tissue homeostasis and impaired regenerative capacity can be viewed as a consequence of the reduction in stem cell number and/or function. Last, cellular senescence is a double-edged sword, since it may inhibit the growth of transformed cells, preventing the occurrence of cancer, while it may facilitate growth of preneoplastic lesions in a paracrine fashion; therefore, interventions targeting this cell response to stress may have a profound impact on many age-related pathologies, ranging from cardiovascular disease to oncology. Aim of this review is to discuss both molecular mechanisms associated with stem cell senescence and interventions that may attenuate or reverse this process.

PPAR modulation of kinase-linked receptor signaling in physiology and disease

Kinase-linked receptors and nuclear receptors connect external cues to gene transcription. Among nuclear receptors, peroxisome proliferator-activated receptors (PPARs) are of special interest in relation to widespread human diseases. Mapping out connections between PPARs and kinase-linked receptor signaling is central to better understand physiological and pathophysiological processes and to better define therapeutic strategies. This is the aim of the present review.

Ablation of the very-long-chain fatty acid elongase ELOVL3 in mice leads to constrained lipid storage and resistance to diet-induced obesity

Although saturated and monounsaturated very-long-chain fatty acids (VLCFAs) have long been associated with undesirable effects on health, including obesity, heart failure, and atherosclerosis, the physiological role of endogenous synthesis is largely unknown. The fatty acid elongase ELOVL3 is involved in the synthesis of C20-C24 saturated and monounsaturated VLCFAs mainly in liver, brown and white adipose tissue, and triglyceride-rich glands such as the sebaceous and meibomian glands. Here we show that ablation of ELOVL3 leads to reduced adiponectin levels, constrained expansion of adipose tissue, and resistance against diet-induced obesity, a situation that is more exaggerated in female mice. Both female and male knockout mice show reduced hepatic lipogenic gene expression and triglyceride content, a situation that is associated with reduced de novo fatty acid synthesis and uptake. As a consequence, the VLDL-triglyceride level in serum is significantly reduced. Remarkably, despite increased energy expenditure, markedly reduced serum levels of leptin, and increased expression of orexigenic peptides in the hypothalamus, the Elovl3(-/-) mice do not compensate by increased food intake. Thus, these results reveal that C20-C22 saturated and monounsaturated VLCFAs produced by ELOVL3 are indispensable for appropriate synthesis of liver triglycerides, fatty acid uptake, and storage in adipose tissue.

Distinct association of serum FGF21 or adiponectin levels with clinical parameters in patients with type 2 diabetes

Fibroblast growth factor 21 (FGF21) has been identified as a novel metabolic regulator. This cross-sectional study was performed to clarify how serum FGF21 levels were associated with clinical parameters in Japanese subjects with type 2 diabetes (n=139). Anthropometric and blood biochemical parameters, uses of drugs for diabetes, hypertension and dyslipidemia were examined regarding associations with fasting serum FGF21 concentrations. FGF21 levels were 6-times higher in those subjects taking fibrates. However, a use of thiazolidinediones did not affect serum FGF21 levels while it induced higher serum adiponectin levels. In univariate analyses, FGF21 levels showed associations with a use of fibrates, triglyceride levels, creatinine levels, waist circumference, and BMI. Multiple regression analyses adjusted for age, gender and BMI showed that a use of fibrates, triglyceride levels and creatinine levels were strong contributors to serum FGF21 levels. In contrast, a use of thiazolidinediones, HDL-cholesterol levels and fasting insulin levels were strong contributors to serum adiponectin levels. This study revealed that serum FGF21 levels were biochemical indicators correlating to a set of essential metabolic parameters, which was distinct from that correlating to serum adiponectin levels in subjects with type 2 diabetes.

Fibroblast growth factor 21 regulates energy metabolism by activating the AMPK-SIRT1-PGC-1alpha pathway

Fibroblast growth factor 21 (FGF21) has been identified as a potent metabolic regulator. Administration of recombinant FGF21 protein to rodents and rhesus monkeys with diet-induced or genetic obesity and diabetes exerts strong antihyperglycemic and triglyceride-lowering effects and reduction of body weight. Despite the importance of FGF21 in the regulation of glucose, lipid, and energy homeostasis, the mechanisms by which FGF21 functions as a metabolic regulator remain largely unknown. Here we demonstrate that FGF21 regulates energy homeostasis in adipocytes through activation of AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1), resulting in enhanced mitochondrial oxidative function. AMPK phosphorylation levels were increased by FGF21 treatment in adipocytes as well as in white adipose tissue from ob/ob mice. FGF21 treatment increased cellular NAD(+) levels, leading to activation of SIRT1 and deacetylation of its downstream targets, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) and histone 3. Activation of AMPK and SIRT1 by FGF21 in adipocytes enhanced mitochondrial oxidative capacity as demonstrated by increases in oxygen consumption, citrate synthase activity, and induction of key metabolic genes. The effects of FGF21 on mitochondrial function require serine/threonine kinase 11 (STK11/LKB1), which activates AMPK. Inhibition of AMPK, SIRT1, and PGC-1alpha activities attenuated the effects of FGF21 on oxygen consumption and gene expression, indicating that FGF21 regulates mitochondrial activity and enhances oxidative capacity through an AMPK-SIRT1-PGC1alpha-dependent mechanism in adipocytes.

Ten years of NAD-dependent SIR2 family deacetylases: implications for metabolic diseases

Since the discovery of NAD-dependent deacetylase activity of the silent information regulator-2 (SIR2) family ('sirtuins'), many exciting connections between protein deacetylation and energy metabolism have been revealed. The importance of sirtuins in the regulation of many fundamental biological responses to various nutritional and environmental stimuli has been firmly established. Sirtuins have also emerged as critical regulators for aging and longevity in model organisms. Their absolute requirement of NAD has revived an enthusiasm in the study of mammalian biosynthesis of NAD. Sirtuin-targeted pharmaceutical and nutriceutical interventions against age-associated diseases are also on the horizon. This review summarizes the recent progress in sirtuin research (particularly in mammalian sirtuin biology) and re-evaluates the connection between sirtuins, metabolism, and age-associated diseases (e.g., type-2 diabetes) to set a basis for the next ten years of sirtuin research.

SIRT6 protects against pathological damage caused by diet-induced obesity

The NAD+-dependent SIRT6 deacetylase is a therapeutic candidate against the emerging metabolic syndrome epidemic. SIRT6, whose deficiency in mice results in premature aging phenotypes and metabolic defects, was implicated in a calorie restriction response that showed an opposite set of phenotypes from the metabolic syndrome. To explore the role of SIRT6 in metabolic stress, wild type and transgenic (TG) mice overexpressing SIRT6 were fed a high fat diet. In comparison to their wild-type littermates, SIRT6 TG mice accumulated significantly less visceral fat, LDL-cholesterol, and triglycerides. TG mice displayed enhanced glucose tolerance along with increased glucose-stimulated insulin secretion. Gene expression analysis of adipose tissue revealed that the positive effect of SIRT6 overexpression is associated with down regulation of a selective set of peroxisome proliferator-activated receptor-responsive genes, and genes associated with lipid storage, such as angiopoietin-like protein 4, adipocyte fatty acid-binding protein, and diacylglycerol acyltransferase 1, which were suggested as potential targets for drugs to control metabolic syndrome. These results demonstrate a protective role for SIRT6 against the metabolic consequences of diet-induced obesity and suggest a potentially beneficial effect of SIRT6 activation on age-related metabolic diseases.

Mechanisms regulating repression of haptoglobin production by peroxisome proliferator-activated receptor-gamma ligands in adipocytes

Obesity leads to inflammation of white adipose tissue involving enhanced secretion of cytokines and acute-phase proteins in response in part to the accumulation of excess lipids in adipocytes. Haptoglobin is an acute-phase reactant secreted by white adipose tissue and induced by inflammatory cytokines such as TNFalpha. In this study, we investigated the mechanisms regulating haptoglobin expression in adipocytes. Peroxisome proliferator-activated receptor (PPAR)-gamma agonists such as thiazolidinediones (TZDs) as well as non-TZD ligands can repress in vitro and in vivo haptoglobin expression in adipocytes and also prevent its induction by TNFalpha. This action requires direct involvement of PPAR gamma in regulating haptoglobin gene transcription because mutation of critical amino acids within helix 7 of the ligand-binding domain of PPAR gamma prevents repression of the haptoglobin gene by the synthetic ligands. Chromatin immunoprecipitation analysis shows active binding of PPAR gamma to a distal region of the haptoglobin promoter, which contains putative PPAR gamma binding sites. Additionally, PPAR gamma induces transcription of a luciferase reporter gene when driven by the distal promoter region of the haptoglobin gene, and TZD treatment significantly reduces the extent of this induction. Furthermore, the mutated PPAR gamma is incapable of enhancing luciferase activity in these in vitro reporter gene assays. In contrast to other adipokines repressed by TZDs such as resistin and chemerin, repression of haptoglobin does not require either CCAAT/enhancer-binding protein C/EBP alpha or the corepressors C-terminal binding protein 1 or 2. These data are consistent with a model in which synthetic PPAR gamma ligands selectively activate PPAR gamma bound to the haptoglobin gene promoter to arrest haptoglobin gene transcription.

Fibroblast growth factor 21: from pharmacology to physiology

Fibroblast growth factor 21 (FGF21) is an atypical member of the FGF family that functions as an endocrine hormone. Pharmacologic studies show that FGF21 has broad metabolic actions in obese rodents and primates that include enhancing insulin sensitivity, decreasing triglyceride concentrations, and causing weight loss. In lean rodents, FGF21 expression is strongly induced in liver by prolonged fasting through a mechanism that involves the nuclear receptor peroxisome proliferator-activated receptor alpha. FGF21, in turn, induces the transcriptional coactivator protein peroxisome proliferator-activated receptor gamma coactivator protein 1alpha and stimulates hepatic gluconeogenesis, fatty acid oxidation, and ketogenesis. FGF21 also blocks somatic growth and sensitizes mice to a hibernation-like state of torpor. Thus, FGF21 plays a key role in eliciting and coordinating the adaptive starvation response. Interestingly, FGF21 expression is induced in white adipose tissue by peroxisome proliferator-activated receptor gamma, which suggests that it also regulates metabolism in the fed state. This article highlights recent advances in our understanding of FGF21's pharmacologic and physiologic actions.

Fibroblast growth factor 21-deficient mice demonstrate impaired adaptation to ketosis

Fibroblast growth factor 21 (FGF21) is a key metabolic regulator. Expressed primarily in liver and adipose tissue, FGF21 is induced via peroxisome proliferator-activated receptor (PPAR) pathways during states requiring increased fatty acid oxidation including fasting and consumption of a ketogenic diet. To test the hypothesis that FGF21 is a physiological regulator that plays a role in lipid oxidation, we generated mice with targeted disruption of the Fgf21 locus (FGF21 knockout). Mice lacking FGF21 had mild weight gain and slightly impaired glucose homeostasis, indicating a role in long-term energy homeostasis. Furthermore, FGF21KO mice tolerated a 24-h fast, indicating that FGF21 is not essential in the early stages of starvation. In contrast to wild-type animals in which feeding KD leads to dramatic weight loss, FGF21KO mice fed KD gained weight, developed hepatosteatosis, and showed marked impairments in ketogenesis and glucose control. This confirms the physiological importance of FGF21 in the adaptation to KD feeding. At a molecular level, these effects were accompanied by lower levels of expression of PGC1alpha and PGC1beta in FGF21KO mice, strongly implicating these key transcriptional regulators in the action of FGF21. Furthermore, within the liver, the maturation of the lipogenic transcription factor sterol regulatory element-binding protein-1c was increased in FGF21KO mice, implicating posttranscriptional events in the maladaptation of FGF21KO mice to KD. These data reinforce the role of FGF21 is a critical regulator of long-term energy balance and metabolism. Mice lacking FGF21 cannot respond appropriately to a ketogenic diet, resulting in an impaired ability to mobilize and utilize lipids.

Fibroblast growth factor 21 reduces the severity of cerulein-induced pancreatitis in mice

BACKGROUND & AIMS

Fibroblast growth factor 21 (FGF21) acts as a hormonal regulator during fasting and is involved in lipid metabolism. Fgf21 gene expression is regulated by peroxisome proliferator-activated receptor (PPAR)-dependent pathways, which are enhanced during pancreatitis. Therefore, the aim of this study was to investigate FGF21's role in pancreatic injury.

METHODS

Fgf21 expression was quantified during cerulein-induced pancreatitis (CIP) or following mechanical or thapsigargin-induced stress through Northern blot analysis, in situ hybridization, and quantitative reverse transcription polymerase chain reaction. FGF21 protein was quantified by Western blot analysis. Isolated acinar cells or AR42J acinar cells were treated with recombinant FGF21 protein, and extracellular regulated kinase 1/2 activation was examined. The severity of CIP was compared between wild-type mice and mice overexpressing FGF21 (FGF21Tg) or harboring a targeted deletion of Fgf21 (Fgf21(-/-)).

RESULTS

Acinar cell Fgf21 expression markedly increased during CIP and following injury in vitro. Purified FGF21 activated the extracellular regulated kinase 1/2 pathway in pancreatic acinar cells. The severity of CIP is inversely correlated to FGF21 expression because FGF21Tg mice exhibited decreased serum amylase and decreased pancreatic stellate cell activation, whereas Fgf21(-/-) mice had increased serum amylase and tissue damage. The expression of Fgf21 was also inversely correlated to expression of Early growth response 1, a proinflammatory and profibrotic transcription factor.

CONCLUSIONS

These studies suggest a novel function for Fgf21 as an immediate response gene protecting pancreatic acini from overt damage.

Function of sirtuins in biological tissues

Sirtuins are protein deacetylases, which are dependent on nicotine adenine dinucleotide. They are phylogenetically conserved from bacteria to humans. Seven sirtuin proteins localized in a wide variety of subcellular locations have been identified in the human genome. The most important known function of sirtuins is their regulation of transcriptional repression, mediated through binding of a complex containing sirtuins and other proteins. Studies have shown that sirtuins have pathophysiological relevance to neurodegeneration, muscle differentiation, inflammation, obesity, and cancer. In addition, sirtuin activity extends the lifespan of several organisms. In this review, we discuss the mode(s) of action of sirtuins, and their biological role(s) in health and disease.

C/EBPalpha and the corepressors CtBP1 and CtBP2 regulate repression of select visceral white adipose genes during induction of the brown phenotype in white adipocytes by peroxisome proliferator-activated receptor gamma agonists

White adipose tissue (WAT) stores energy in the form of triglycerides, whereas brown tissue (BAT) expends energy, primarily by oxidizing lipids. WAT also secretes many cytokines and acute-phase proteins that contribute to insulin resistance in obese subjects. In this study, we have investigated the mechanisms by which activation of peroxisome proliferator-activated receptor gamma (PPARgamma) with synthetic agonists induces a brown phenotype in white adipocytes in vivo and in vitro. We demonstrate that this phenotypic conversion is characterized by repression of a set of white fat genes ("visceral white"), including the resistin, angiotensinogen, and chemerin genes, in addition to induction of brown-specific genes, such as Ucp-1. Importantly, the level of expression of the "visceral white" genes is high in mesenteric and gonadal WAT depots but low in the subcutaneous WAT depot and in BAT. Mutation of critical amino acids within helix 7 of the ligand-binding domain of PPARgamma prevents inhibition of visceral white gene expression by the synthetic agonists and therefore shows a direct role for PPARgamma in the repression process. Inhibition of the white adipocyte genes also depends on the expression of C/EBPalpha and the corepressors, carboxy-terminal binding proteins 1 and 2 (CtBP1/2). The data further show that repression of resistin and angiotensinogen expression involves recruitment of CtBP1/2, directed by C/EBPalpha, to the minimal promoter of the corresponding genes in response to the PPARgamma ligand. Developing strategies to enhance the brown phenotype in white adipocytes while reducing secretion of stress-related cytokines from visceral WAT is a means to combat obesity-associated disorders.

Effects of rosiglitazone on fasting plasma fibroblast growth factor-21 levels in patients with type 2 diabetes mellitus

OBJECTIVE

Fibroblast growth factor-21 (FGF-21) has recently been characterized as a potent metabolic regulator, but its pathophysiologic roles in humans remain unknown. This study aimed to investigate the effects of rosiglitazone on plasma FGF-21 levels in patients with type 2 diabetes mellitus (T2DM).

DESIGN AND METHODS

Thirty patients with new-onset T2DM (nT2DM), 34 type 2 diabetic patients with poor glycemic control (pT2DM) after the treatment with single hypoglycemic agent metformin, and 30 sex- and age-matched normal glycaemic controls (NGT) participated in the study. The pT2DM group was treated with rosiglitazone for 12 weeks. Plasma FGF-21 levels were measured with a RIA. The relationship between plasma FGF-21 levels and metabolic parameters was also analyzed.

RESULTS

Fasting plasma FGF-21 levels were higher in nT2DM and pT2DM groups than in the control (1.81+/-0.64 vs 1.87+/-0.63 vs 1.52+/-0.61 microg/l, P<0.05), but there was no difference between nT2DM and pT2DM groups. Fasting plasma FGF-21 levels were decreased significantly in pT2DM group after the treatment with rosiglitazone compared with pre-treatment (1.59+/-0.63 vs 1.87+/-0.64 micro/l, P<0.05). In all diabetic patients, multiple regression analysis showed that HbA1c, fasting insulin, and homeostasis model assessment-insulin resistance index were independently associated with plasma FGF-21 levels.

CONCLUSIONS

In pT2DM patients, plasma FGF-21 levels are increased, but significantly decreased after the treatment with rosiglitazone on top of ongoing metformin therapy. These data suggest that rosiglitazone may play a role in lowering FGF-21 levels in T2DM patients.

Glucose induces FGF21 mRNA expression through ChREBP activation in rat hepatocytes

Fibroblast growth factor 21 (FGF21) has beneficial effects of improving the plasma glucose and lipid profiles in diabetic rodents. Here, we investigated carbohydrate response element binding protein (ChREBP) involvement in the regulation of FGF21 mRNA expression in liver. Glucose stimulation and adenoviral overexpression of dominant active ChREBP increased FGF21 mRNA. Consistently, adenoviral expression of dominant negative Mlx inhibited glucose induction of FGF21 mRNA. Furthermore, deletion studies of mouse FGF21 gene promoter (-2000 to +65 bp) revealed a glucose responsive region between -74 and -52 bp. These findings suggest that FGF21 expression is regulated by ChREBP.

FGF21: a novel prospect for the treatment of metabolic diseases

The increasing prevalence of metabolic diseases is alarming and highlights the need for more effective and safer therapies to treat these diseases. Recent evidence from several animal studies indicates that FGF21 induces numerous beneficial metabolic changes without apparent adverse effects. These results suggest that FGF21 could be a novel and attractive drug candidate for the treatment of cardiovascular disease, obesity and type 2 diabetes. The pharmacology of FGF21, molecular mechanisms contributing to the actions of this compound, and knowledge gaps to be addressed to allow further exploration of the therapeutic potential of this molecule are discussed in this review.

Fibroblast growth factor 21: an overview from a clinical perspective

Fibroblast growth factor 21 (FGF21) has been proposed as a novel putative therapeutic agent in type 2 diabetes. A large amount of data, predominantly obtained from murine models but also from non-human primates, suggest that FGF21 ameliorates obesity-associated hyperglycemia and hyperlipidemia primarily via effects on adipose tissue and the pancreas. In addition, FGF21 has been reported to play a pivotal regulatory role in starvation and ketosis. However, while it is clear that FGF21 has potent effects in vivo in several animal models, the exact mechanisms remain elusive. Moreover, very recent results from different human cohort studies have shown a paradoxical regulation of plasma FGF21 in obesity and type 2 diabetes as well as other important qualitative differences in the effects and regulation of FGF21 between rodents and humans. This review focuses on the most recently published data on FGF21 with emphasis on results obtained in humans.

SIRT1 and insulin resistance

Sirtuin 1 (SIRT1), the mammalian homolog of SIR2, was originally identified as a NAD-dependent histone deacetylase, the activity of which is closely associated with lifespan under calorie restriction. Growing evidence suggests that SIRT1 regulates glucose or lipid metabolism through its deacetylase activity for over two dozen known substrates, and has a positive role in the metabolic pathway through its direct or indirect involvement in insulin signaling. SIRT1 stimulates a glucose-dependent insulin secretion from pancreatic beta cells, and directly stimulates insulin signaling pathways in insulin-sensitive organs. Furthermore, SIRT1 regulates adiponectin secretion, inflammatory responses, gluconeogenesis, and levels of reactive oxygen species, which together contribute to the development of insulin resistance. Moreover, overexpression of SIRT1 and several SIRT1 activators has beneficial effects on glucose homeostasis and insulin sensitivity in obese mice models. These findings suggest that SIRT1 might be a new therapeutic target for the prevention of disease related to insulin resistance, such as metabolic syndrome and diabetes mellitus, although direct evidence from clinical studies in humans is needed to prove this possibility. In this Review, we discuss the potential role and therapeutic promise of SIRT1 in insulin resistance on the basis of the latest experimental studies.

The NAD World: a new systemic regulatory network for metabolism and aging--Sirt1, systemic NAD biosynthesis, and their importance

For the past several years, it has been demonstrated that the NAD-dependent protein deacetylase Sirt1 and nicotinamide phosphoribosyltransferase (Nampt)-mediated systemic NAD biosynthesis together play a critical role in the regulation of metabolism and possibly aging in mammals. Based on our recent studies on these two critical components, we have developed a hypothesis of a novel systemic regulatory network, named "NAD World", for mammalian aging. Conceptually, in the NAD World, systemic NAD biosynthesis mediated by intra- and extracellular Nampt functions as a driver that keeps up the pace of metabolism in multiple tissues/organs, and the NAD-dependent deacetylase Sirt1 serves as a universal mediator that executes metabolic effects in a tissue-dependent manner in response to changes in systemic NAD biosynthesis. This new concept of the NAD World provides important insights into a systemic regulatory mechanism that fundamentally connects metabolism and aging and also conveys the ideas of functional hierarchy and frailty for the regulation of metabolic robustness and aging in mammals.

The FGF family: biology, pathophysiology and therapy

The family of fibroblast growth factors (FGFs) regulates a plethora of developmental processes, including brain patterning, branching morphogenesis and limb development. Several mitogenic, cytoprotective and angiogenic therapeutic applications of FGFs are already being explored, and the recent discovery of the crucial roles of the endocrine-acting FGF19 subfamily in bile acid, glucose and phosphate homeostasis has sparked renewed interest in the pharmacological potential of this family. This Review discusses traditional applications of recombinant FGFs and small-molecule FGF receptor kinase inhibitors in the treatment of cancer and cardiovascular disease and their emerging potential in the treatment of metabolic syndrome and hypophosphataemic diseases.

From heterochromatin islands to the NAD World: a hierarchical view of aging through the functions of mammalian Sirt1 and systemic NAD biosynthesis

For the past couple of decades, aging science has been rapidly evolving, and powerful genetic tools have identified a variety of evolutionarily conserved regulators and signaling pathways for the control of aging and longevity in model organisms. Nonetheless, a big challenge still remains to construct a comprehensive concept that could integrate many distinct layers of biological events into a systemic, hierarchical view of aging. The "heterochromatin island" hypothesis was originally proposed 10 years ago to explain deterministic and stochastic aspects of cellular and organismal aging, which drove the author to the study of evolutionarily conserved Sir2 proteins. Since a surprising discovery of their NAD-dependent deacetylase activity, Sir2 proteins, now called "sirtuins," have been emerging as a critical epigenetic regulator for aging. In this review, I will follow the process of conceptual development from the heterochromatin island hypothesis to a novel, comprehensive concept of a systemic regulatory network for mammalian aging, named "NAD World," summarizing recent studies on the mammalian NAD-dependent deacetylase Sirt1 and nicotinamide phosphoribosyltransferase (Nampt)-mediated systemic NAD biosynthesis. This new concept of the NAD World provides critical insights into a systemic regulatory mechanism that fundamentally connects metabolism and aging and also conveys the ideas of functional hierarchy and frailty for the regulation of aging in mammals.

The Klotho gene family as a regulator of endocrine fibroblast growth factors

The Klotho gene encodes a single-pass transmembrane protein and functions as an aging-suppressor gene, which extends lifespan when overexpressed and accelerates the development of aging-like phenotypes when disrupted in mice. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates phosphate and vitamin D homeostasis. It has been shown that Klotho-deficient mice and Fgf23 knockout mice exhibit identical phenotypes. This observation led to the identification of Klotho as a cofactor essential for interactions between FGF23 and FGF receptors. In addition to the Klotho-FGF23 axis, recent studies has shown that betaKlotho, a Klotho family protein, also functions as a cofactor required for FGF19 and FGF21 signaling and determines the tissue-specific metabolic activities of FGF19 and FGF21. This review summarizes recent progress in understanding of Klotho and betaKlotho function in the regulation of tissue-specific metabolic activity of the endocrine fibroblast growth factors (FGF19, FGF21, and FGF23).

Therapeutic potential of SIRT1 and NAMPT-mediated NAD biosynthesis in type 2 diabetes

Both genetic and environmental factors contribute to the pathogenesis of type 2 diabetes, and it is critical to understand the interplay between these factors in the regulation of insulin secretion and insulin sensitivity to develop effective therapeutic interventions for type 2 diabetes. For the past several years, studies on the mammalian NAD-dependent protein deacetylase SIRT1 and systemic NAD biosynthesis mediated by nicotinamide phosphoribosyltransferase (NAMPT) have demonstrated that these two regulatory components together play a critical role in the regulation of glucose homeostasis, particularly in the regulation of glucose-stimulated insulin secretion in pancreatic beta cells. These components also contribute to the age-associated decline in beta cell function, which has been suggested to be one of the major contributing factors to the pathogenesis of type 2 diabetes. In this review article, the roles of SIRT1 and NAMPT-mediated systemic NAD biosynthesis in glucose homeostasis and the pathophysiology of type 2 diabetes will be summarized, and their potential as effective targets for the treatment and prevention of type 2 diabetes will be discussed.

Endocrine fibroblast growth factors as regulators of metabolic homeostasis

Endocrine fibroblast growth factors (FGFs) function as hormones that maintain specific metabolic states by controlling homeostasis of bile acid, glucose, fatty acid, phosphate, and vitamin D. Endocrine FGFs exert their biological activity through a common design of coreceptor system consisting of the Klotho gene family of transmembrane proteins and cognate FGF receptors. Moreover, expression of endocrine FGFs is regulated by nuclear receptors whose lipophilic ligands are generated under the control of these hormones in the target organs. Thus, novel endocrine axes have emerged that regulate diverse metabolic processes through feedback loops composed of the FGF, Klotho, FGF receptor, and nuclear receptor gene families. This review summarizes the role of Klotho family proteins in the regulation of metabolic activity and expression of the endocrine FGFs.

Fibroblast growth factor 21 corrects obesity in mice

Fibroblast growth factor 21 (FGF21) is a metabolic regulator that provides efficient and durable glycemic and lipid control in various animal models. However, its potential to treat obesity, a major health concern affecting over 30% of the population, has not been fully explored. Here we report that systemic administration of FGF21 for 2 wk in diet-induced obese and ob/ob mice lowered their mean body weight by 20% predominantly via a reduction in adiposity. Although no decrease in total caloric intake or effect on physical activity was observed, FGF21-treated animals exhibited increased energy expenditure, fat utilization, and lipid excretion, reduced hepatosteatosis, and ameliorated glycemia. Transcriptional and blood cytokine profiling studies revealed effects consistent with the ability of FGF21 to ameliorate insulin and leptin resistance, enhance fat oxidation and suppress de novo lipogenesis in liver as well as to activate futile cycling in adipose. Overall, these data suggest that FGF21 exhibits the therapeutic characteristics necessary for an effective treatment of obesity and fatty liver disease and provides novel insights into the metabolic determinants of these activities.

FGF21 is an Akt-regulated myokine

Fibroblast growth factor-21 (FGF21) functions as a metabolic regulator. The FGF21 transcript is reported to be abundantly expressed in liver, but little is known about the regulation of FGF21 expression in other tissues. In this study, we show that levels of FGF21 protein expression were similar in skeletal muscle and liver from fasted mice. FGF21 transcript and protein expression were upregulated in gastrocnemius muscle of skeletal muscle-specific Akt1 transgenic mice. Serum concentration of FGF21 was also increased by Akt1 transgene activation. In cultured skeletal muscle cells, FGF21 expression and secretion was regulated by insulin, Akt transduction and LY294002. These data indicate that skeletal muscle is a source of FGF21 and that its expression is regulated by a phosphatidylinosistol 3-kinase (PI3-kinase)/Akt1 signaling pathway-dependent mechanism.

Transcriptional targets of sirtuins in the coordination of mammalian physiology

Sirtuins (Sirts) compose a family of NAD(+)-dependent deacetylases and/or ADP-ribosyltransferases, which have been implicated in aging, metabolism, and tolerance to oxidative stress. Many of the biological processes regulated by Sirts result from the adaptation of complex gene-expression programs to the energetic state of the cell, sensed through NAD(+) levels. To that respect, Sirts, and particularly the founding member of the family Sirt1, have emerged as important regulators of transcription, which they modulate both positively and negatively by targeting histones and transcriptional complex regulatory proteins. This review will focus on recent advances that have started deciphering how mammalian Sirts regulate transcriptional networks and thereby control physiology.

Adipose fibroblast growth factor 21 is up-regulated by peroxisome proliferator-activated receptor gamma and altered metabolic states

Adipose tissue is a metabolically responsive endocrine organ that secretes a myriad of adipokines. Antidiabetic drugs such as peroxisome proliferator-activated receptor (PPAR) gamma agonists target adipose tissue gene expression and correct hyperglycemia via whole-body insulin sensitization. The mechanism by which altered gene expression in adipose tissue affects liver and muscle insulin sensitivity (and thus glucose homeostasis) is not fully understood. One possible mechanism involves the alteration in adipokine secretion, in particular the up-regulation of secreted factors that increase whole-body insulin sensitivity. Here, we report the use of transcriptional profiling to identify genes encoding for secreted proteins the expression of which is regulated by PPARgamma agonists. Of the 379 genes robustly regulated by two structurally distinct PPARgamma agonists in the epididymal white adipose tissue (EWAT) of db/db mice, 33 encoded for known secreted proteins, one of which was FGF21. Although FGF21 was recently reported to be up-regulated in cultured adipocytes by PPARgamma agonists and in liver by PPARalpha agonists and induction of ketotic states, we demonstrate that the protein is transcriptionally up-regulated in adipose tissue in vivo by PPARgamma agonist treatment and under a variety of physiological conditions, including fasting and high fat diet feeding. In addition, we found that circulating levels of FGF21 protein were increased upon treatment with PPARgamma agonists and under ketogenic states. These results suggest a role for FGF21 in mediating the antidiabetic activities of PPARgamma agonists.