FGF21 is a novel metabolic regulator involved in the control of glucose homeostasis, insulin sensitivity, and ketogenesis. The liver has been considered the main site of production and release of FGF21 into the blood. Here, we show that, after thermogenic activation, brown adipose tissue becomes a source of systemic FGF21. This is due to a powerful cAMP-mediated pathway of regulation of FGF21 gene transcription. Norepinephrine, acting via ␤-adrenergic, cAMP-mediated, mechanisms and subsequent activation of protein kinase A and p38 MAPK, induces FGF21 gene transcription and also FGF21 release in brown adipocytes. ATF2 binding to the FGF21 gene promoter mediates cAMP-dependent induction of FGF21 gene transcription. FGF21 release by brown fat in vivo was assessed directly by analyzing arteriovenous differences in FGF21 concentration across interscapular brown fat, in combination with blood flow to brown adipose tissue and assessment of FGF21 half-life. This analysis demonstrates that exposure of rats to cold induced a marked release of FGF21 by brown fat in vivo, in association with a reduction in systemic FGF21 half-life. The present findings lead to the recognition of a novel pathway of regulation the FGF21 gene and an endocrine role of brown fat, as a source of FGF21 that may be especially relevant in conditions of activation of thermogenic activity.Fibroblast growth factor 21 (FGF21) is a metabolic regulator involved in the control of glucose homeostasis, insulin sensitivity, and ketogenesis (1-4). Treatment with FGF21 corrects metabolic disturbances such as hyperglycemia and insulin resistance in rodent models of obesity and diabetes (1, 5-7). It also has been reported that FGF21 exerts autocrine and paracrine actions on livers that promote ketogenesis (2-4). Two recent studies in FGF21 gene-ablated mice have demonstrated that FGF21 is required for the physiological response of mice to fasting and to ketogenic diets (8, 9), although a third study did not confirm these observations (10). Moreover, FGF21 favors glucose utilization in white adipose tissue (WAT), 2 and there are conflicting data on to whether FGF21 activates or does not activate lipolysis in white fat (3, 10, 11). Recently, FGF21 has been reported to promote thermogenic activity in neonatal brown adipose tissue (BAT) and in isolated brown adipocytes (12); there are indications that FGF21 may also promote BAT thermogenic activation in adult mice (1, 6, 7). The liver is considered the main site of production and release of FGF21 into the blood. Expression of the FGF21 gene in the liver is under the control of PPAR␣, and fatty acid availability, acting via PPAR␣, seems to be the main determinant of hepatic FGF21 gene expression and release (2, 3,12,13). Extra-hepatic tissues, including white and brown adipose tissues and skeletal muscle, also express the FGF21 gene (14), and PPAR␥ activation has been reported to induce FGF21 gene expression in white adipocytes (14,15). On the basis of cell culture studies, muscle cells have been proposed to be capabl...

Fibroblast growth factor 21 is an endocrine factor, secreted mainly by the liver, that exerts metabolic actions that favour glucose metabolism. Its role in the heart is unknown. Here we show that Fgf21 À / À mice exhibit an increased relative heart weight and develop enhanced signs of dilatation and cardiac dysfunction in response to isoproterenol infusion, indicating eccentric hypertrophy development. In addition, Fgf21 À / À mice exhibit enhanced induction of cardiac hypertrophy markers and pro-inflammatory pathways and show greater repression of fatty acid oxidation. Most of these alterations are already present in Fgf21 À / À neonates, and treatment with fibroblast growth factor 21 reverses them in vivo and in cultured cardiomyocytes. Moreover, fibroblast growth factor 21 is expressed in the heart and is released by cardiomyocytes. Fibroblast growth factor 21 released by cardiomyocytes protects cardiac cells against hypertrophic insults. Therefore, the heart appears to be a target of systemic, and possibly locally generated, fibroblast growth factor 21, which exerts a protective action against cardiac hypertrophy.

SUMMARY Plasma FGF21 levels and hepatic FGF21 gene expression increase dramatically after birth in mice. This induction is initiated by suckling, requires lipid intake, is impaired in PPARα null neonates, and is mimicked by treatment with the PPARα activator, Wy14,643. Neonates exhibit reduced FGF21 expression in response to fasting, in contrast to the upregulation occurring in adults. Changes in FGF21 expression due to suckling or nutritional manipulations were associated with circulating free fatty acid and ketone body levels. We mimicked the FGF21 postnatal rise by injecting FGF21 into fasting neonates, and found that this enhanced the expression of genes involved in thermogenesis within brown fat, and increased body temperature. Brown adipocytes treated with FGF21 exhibited increased expression of thermogenic genes, higher total and uncoupled respiration, and enhanced glucose oxidation. We propose that the induction of FGF21 production by the liver mediates direct activation of brown fat thermogenesis during the fetal-to-neonatal transition.

The thermogenic activity of brown adipose tissue (BAT) and browning of white adipose tissue are important components of energy expenditure. Here we show that GPR120, a receptor for polyunsaturated fatty acids, promotes brown fat activation. Using RNA-seq to analyse mouse BAT transcriptome, we find that the gene encoding GPR120 is induced by thermogenic activation. We further show that GPR120 activation induces BAT activity and promotes the browning of white fat in mice, whereas GRP120-null mice show impaired cold-induced browning. Omega-3 polyunsaturated fatty acids induce brown and beige adipocyte differentiation and thermogenic activation, and these effects require GPR120. GPR120 activation induces the release of fibroblast growth factor-21 (FGF21) by brown and beige adipocytes, and increases blood FGF21 levels. The effects of GPR120 activation on BAT activation and browning are impaired in FGF21-null mice and cells. Thus, the lipid sensor GPR120 activates brown fat via a mechanism that involves induction of FGF21.

Background: PPAR␣ is a distinctive marker of the brown-versus-white fat phenotype. Results: PPAR␣ induces PGC-1␣ gene transcription in brown adipocytes through mechanisms involving PRDM16. Conclusion: PPAR␣ regulates brown fat thermogenesis via induction of PGC-1␣ and PRDM16 gene expression. Significance: Activation of PGC-1␣ by PPAR␣ provides a molecular mechanism for concerted induction of thermogenic genes (UCP1, mitochondrial genes, and lipid oxidation genes) in brown fat.

Collectively, these findings reveal a major involvement of the Sirt1-PPARα interaction in the protective role of Sirt1 against cardiac hypertrophy.

Sirt3 (silent mating type information regulation 2, homolog 3), a member of the sirtuin family of protein deacetylases with multiple actions on metabolism and gene expression is expressed in association with brown adipocyte differentiation. Using Sirt3-null brown adipocytes, we determined that Sirt3 is required for an appropriate responsiveness of cells to noradrenergic, cAMP-mediated activation of the expression of brown adipose tissue thermogenic genes. The transcriptional coactivator Pgc-1␣ (peroxisome proliferator-activated receptor-␥ coactivator-1␣) induced Sirt3 gene expression in white adipocytes and embryonic fibroblasts as part of its overall induction of a brown adipose tissue-specific pattern of gene expression. In cells lacking Sirt3, Pgc-1␣ failed to fully induce the expression of brown fat-specific thermogenic genes. Pgc-1␣ activates Sirt3 gene transcription through coactivation of the orphan nuclear receptor Err (estrogen-related receptor)-␣, which bound the proximal Sirt3 gene promoter region. Err␣ knockdown assays indicated that Err␣ is required for full induction of Sirt3 gene expression in response to Pgc-1␣. The present results indicate that Pgc-1␣ controls Sirt3 gene expression and this action is an essential component of the overall mechanisms by which Pgc-1␣ induces the full acquisition of a brown adipocyte differentiated phenotype.Brown adipose tissue plays a major role in the control of energy expenditure in mammals. The specific mitochondrial uncoupling that is characteristic of brown adipocytes creates a specialized cell type adapted to promoting energy expenditure in response to cold or overfeeding. In contrast, white adipocytes are specialized in the accumulation of metabolic energy in the form of lipids. Brown adipocytes respond to noradrenergic stimulation through ␤-adrenoreceptors in the cell surface, which bind norepinephrine and signal through adenylate cyclase to increase cAMP and activate protein kinase A. Ultimately, this signaling cascade lead to activation of hormonesensitive lipase, induction of the expression of the gene for uncoupling protein-1 (Ucp1) 2 and other genes involved in thermogenesis, and activation of the Ucp1-dependent uncoupling of mitochondria (1). Acquisition of the cellular machinery typical of brown adipocyte thermogenic function is a highly plastic process. In fact, pre-adipocytes or even white adipocytes may acquire brown adipocyte properties in response to developmental or environment regulators. Several molecular agents have been reported to promote brown adipocyte differentiation, central among them is Pgc-1␣.Pgc-1␣ is a transcriptional coactivator that plays a major role in the acquisition of the specific brown adipocyte phenotype. Pgc-1␣ coactivates nuclear receptors and transcription factors and thereby activates genes involved in thermogenesis (e.g. Ucp1), lipid oxidation, and mitochondrial oxidation, which are associated with the specific thermogenic function of brown adipose tissue (2). For instance, when white adipocytes are forced to expres...

Peroxisome proliferator activated receptor-␥ co-activator-1␣ (PGC-1␣) is a transcriptional co-activator that coordinately regulates the expression of distinct sets of metabolism-related genes in different tissues. Here we show that PGC-1␣ expression is reduced in skeletal muscles from mice lacking the sirtuin family deacetylase SIRT1. Conversely, SIRT1 activation or overexpression in differentiated C2C12 myotubes increased PGC-1␣ mRNA expression. The transcription-promoting effects of SIRT1 occurred through stimulation of PGC-1␣ promoter activity and were enhanced by co-transfection of myogenic factors, such as myocyte enhancer factor 2 (MEF2) and, especially, myogenic determining factor (MyoD). SIRT1 bound to the proximal promoter region of the PGC-1␣ gene, an interaction potentiated by MEF2C or MyoD, which also interact with this region. In the presence of MyoD, SIRT1 promoted a positive autoregulatory PGC-1␣ expression loop, such that overexpression of PGC-1␣ increased PGC-1␣ promoter activity in the presence of co-expressed MyoD and SIRT1. Chromatin immunoprecipitation showed that SIRT1 interacts with PGC-1␣ promoter and increases PGC-1␣ recruitment to its own promoter region. Immunoprecipitation assays further showed that SIRT1-PGC-1␣ interactions are enhanced by MyoD. Collectively, these data indicate that SIRT1 controls PGC-1␣ gene expression in skeletal muscle and that MyoD is a key mediator of this action. The involvement of MyoD in SIRT1-dependent PGC-1␣ expression may help to explain the ability of SIRT1 to drive musclespecific gene expression and metabolism. Autoregulatory control of PGC-1␣ gene transcription seems to be a pivotal mechanism for conferring a transcription-activating response to SIRT1 in skeletal muscle.Peroxisome proliferator activated receptor-␥ co-activator-1␣ (PGC-1␣) 2 is a transcriptional co-activator that is recognized as a master controller of the expression of genes involved in metabolic regulation. PGC-1␣ exerts differential effects on metabolism in different tissues. In brown adipose tissue, PGC-1␣ increases the expression of uncoupling protein 1 and genes involved in mitochondrial oxidative pathways. In the liver, PGC-1␣ induces the expression of genes involved in gluconeogenesis and the metabolic response to starvation. In skeletal muscle, PGC-1␣ expression is rapidly induced by exercise in vivo (1), a response considered to be a mechanism for modulating metabolic flux in response to decreased ATP levels (2). Chronic exercise also increases PGC-1␣ expression in association with fiber-type switching toward the more oxidative and high endurance type IIa and type I fibers. The fiber-type switch promoted by PGC-1␣ is characterized by increased mitochondrial density and function, increased oxidative metabolism, increased expression of myofibrillar proteins characteristic of type I and type IIa muscle fibers and a switch in substrate fuel usage (3). Furthermore, greater levels of PGC-1␣ are found in oxidative fibers compared with glycolytic fibers, even in a rested state (3). Mo...