FGF21 acts centrally to induce sympathetic nerve activity, energy expenditure, and weight loss

FGF21 improves cognition by restored synaptic plasticity, dendritic spine density, brain mitochondrial function and cell apoptosis in obese-insulin resistant male rats

Fibroblast growth factor 21 (FGF21) is an endocrine hormone which exerts beneficial effects on metabolic regulation in obese and diabetic models. However, the effect of FGF21 on cognition in obese-insulin resistant rats has not been investigated. We hypothesized that FGF21 prevented cognitive decline in obese-insulin resistant rats by improving hippocampal synaptic plasticity, dendritic spine density, brain mitochondrial function and brain FGF21 signaling as well as decreasing brain cell apoptosis. Eighteen male Wistar rats were divided into two groups, and received either a normal diet (ND) (n=6) or a high fat diet (HFD) (n=12) for 12weeks. At week 13, the HFD-fed rats were subdivided into two subgroups (n=6/subgroup) to receive either vehicle or recombinant human FGF21 (0.1mg/kg/day) for four weeks. ND-fed rats were given vehicle for four weeks. At the end of the treatment, cognitive function, metabolic parameters, pro-inflammatory markers, brain mitochondrial function, cell apoptosis, hippocampal synaptic plasticity, dendritic spine density and brain FGF21 signaling were determined. The results showed that vehicle-treated HFD-fed rats developed obese-insulin resistance and cognitive decline with impaired hippocampal synaptic plasticity, decreased dendritic spine density, brain mitochondrial dysfunction and increased brain cell apoptosis. Impaired brain FGF 21 signaling was found in these obese-insulin resistant rats. FGF21-treated obese-insulin resistant rats had improved peripheral insulin sensitivity, increased hippocampal synaptic plasticity, increased dendritic spine density, restored brain mitochondrial function, attenuated brain cells apoptosis and increased brain FGF21 signaling, leading to a prevention of cognitive decline. These findings suggest that FGF21 treatment exerts neuroprotection in obese-insulin resistant rats.

Hormonal and nutritional signalling in the control of brown and beige adipose tissue activation and recruitment

Recent research has revealed that the activity of adipose tissue (BAT) in adult humans is higher than previously thought, and that obese patients show abnormally low levels of brown fat activity. Studies in experimental animals have shown that BAT is a site of energy expenditure, and that BAT activity protects against obesity and associated metabolic diseases. The action of the sympathetic nervous activity on BAT depots is considered the main regulator of BAT activity in rodent models and possibly also in humans. However, recent research has revealed the existence of additional hormonal factors, produced by distinct peripheral tissues or present in the diet, that influence the amount and activity of BAT. These hormonal factors may act on BAT directly, but also indirectly by targeting the brain and determining the intensity of sympathetic action upon BAT. Identification and characterization of novel factors that control BAT may provide clues for the development of new strategies to treat obesity and metabolic diseases.

PF-05231023, a long-acting FGF21 analogue, decreases body weight by reduction of food intake in non-human primates

PF-05231023, a long-acting FGF21 analogue, is a promising potential pharmacotherapy for the treatment of obesity and associated comorbidities. Previous studies have shown the potential of FGF21 and FGF21-like compounds to decrease body weight in mice, non-human primates, and humans; the precise mechanisms of action remain unclear. In particular, there have been conflicting reports on the degree to which FGF21-induced weight loss in non-human primates is attributable to a decrease in food intake versus an increase in energy expenditure. Here, we present a semi-mechanistic mathematical model of energy balance and body composition developed from similar work in mice. This model links PF-05231023 administration and washout to changes in food intake, which in turn drives changes in body weight. The model is calibrated to and compared with recently published data from cynomolgus macaques treated with PF-05231023, demonstrating its accuracy in describing pharmacotherapy-induced weight loss in these animals. The results are consistent with the hypothesis that PF-05231023 decreases body weight in cynomolgus macaques solely by a reduction in food intake, with no direct effect on energy expenditure.

A BAT-Centric Approach to the Treatment of Diabetes: Turn on the Brain

The marked (18)F-flurodeoxyglucose uptake by brown adipose tissue (BAT) enabled its identification in human positron emission tomography imaging studies. In this Perspective, we discuss how glucose extraction by BAT and beige adipose tissue (BeAT) sufficiently impacts on glycemic control. We then present a unique overview of the central circuits modulated by gluco-regulatory hormones, temperature, and glucose itself, which converge on sympathetic preganglionic neurons and whose activation syphon circulating glucose into BAT/BeAT. Targeted stimulation of the sympathetic nervous system at specific nodes to selectively recruit BAT/BeAT may represent a safe and effective means of treating diabetes.

FGF21 ameliorates the neurocontrol of blood pressure in the high fructose-drinking rats

Fibroblast growth factor-21 (FGF21) is closely related to various metabolic and cardiovascular disorders. However, the direct targets and mechanisms linking FGF21 to blood pressure control and hypertension are still elusive. Here we demonstrated a novel regulatory function of FGF21 in the baroreflex afferent pathway (the nucleus tractus solitarii, NTS; nodose ganglion, NG). As the critical co-receptor of FGF21, β-klotho (klb) significantly expressed on the NTS and NG. Furthermore, we evaluated the beneficial effects of chronic intraperitoneal infusion of recombinant human FGF21 (rhFGF21) on the dysregulated systolic blood pressure, cardiac parameters, baroreflex sensitivity (BRS) and hyperinsulinemia in the high fructose-drinking (HFD) rats. The BRS up-regulation is associated with Akt-eNOS-NO signaling activation in the NTS and NG induced by acute intravenous rhFGF21 administration in HFD and control rats. Moreover, the expressions of FGF21 receptors were aberrantly down-regulated in HFD rats. In addition, the up-regulated peroxisome proliferator-activated receptor-γ and -α (PPAR-γ/-α) in the NTS and NG in HFD rats were markedly reversed by chronic rhFGF21 infusion. Our study extends the work of the FGF21 actions on the neurocontrol of blood pressure regulations through baroreflex afferent pathway in HFD rats.

Neuroinflammatory and autonomic mechanisms in diabetes and hypertension

Interdisciplinary studies in the research fields of endocrinology and immunology show that obesity-associated overnutrition leads to neuroinflammatory molecular changes, in particular in the hypothalamus, chronically causing various disorders known as elements of metabolic syndrome. In this process, neural or hypothalamic inflammation impairs the neuroendocrine and autonomic regulation of the brain over blood pressure and glucose homeostasis as well as insulin secretion, and elevated sympathetic activation has been appreciated as a critical mediator. This review describes the involved physiology and mechanisms, with a focus on glucose and blood pressure balance, and suggests that neuroinflammation employs the autonomic nervous system to mediate the development of diabetes and hypertension.

Modulation of energy balance by fibroblast growth factor 21

Fibroblast growth factors (FGFs) are a superfamily of 22 proteins related to cell proliferation and tissue repair after injury. A subgroup of three proteins, FGF19, FGF21, and FGF23, are major endocrine mediators. These three FGFs have low affinity to heparin sulfate during receptor binding; in contrast they have a strong interaction with the cofactor Klotho/β-Klotho. FGF21 has received particular attention because of its key role in carbohydrate, lipids, and energy balance regulation. FGF21 improves glucose and lipids metabolism as well as increasing energy expenditure in animal models and humans. Conditions that induce human physical stress such as exercise, lactation, obesity, insulin resistance, and type 2 diabetes influence FGF21 circulating levels. FGF21 also has an anti-oxidant function in human metabolic diseases which contribute to understanding the FGF21 compensatory increment in obesity, the metabolic syndrome, and type 2 diabetes. Interestingly, energy expenditure and weight loss is induced by FGF21. The mechanism involved is through "browning" of white adipose tissue, increasing brown adipose tissue activity and heat production. Therefore, clinical evaluation of therapeutic action of exogenous FGF21 administration is warranted, particularly to treat diabetes and obesity.

CRFR1 in AgRP Neurons Modulates Sympathetic Nervous System Activity to Adapt to Cold Stress and Fasting

Signaling by the corticotropin-releasing factor receptor type 1 (CRFR1) plays an important role in mediating the autonomic response to stressful challenges. Multiple hypothalamic nuclei regulate sympathetic outflow. Although CRFR1 is highly expressed in the arcuate nucleus (Arc) of the hypothalamus, the identity of these neurons and the role of CRFR1 here are presently unknown. Our studies show that nearly half of Arc-CRFR1 neurons coexpress agouti-related peptide (AgRP), half of which originate from POMC precursors. Arc-CRFR1 neurons are innervated by CRF neurons in the hypothalamic paraventricular nucleus, and CRF application decreases AgRP(+)CRFR1(+) neurons' excitability. Despite similar anatomy in both sexes, only female mice selectively lacking CRFR1 in AgRP neurons showed a maladaptive thermogenic response to cold and reduced hepatic glucose production during fasting. Thus, CRFR1, in a subset of AgRP neurons, plays a regulatory role that enables appropriate sympathetic nervous system activation and consequently protects the organism from hypothermia and hypoglycemia.

The regulation of FGF21 gene expression by metabolic factors and nutrients

Fibroblast growth factor 21 (FGF21) gene expression is altered by a wide array of physiological, metabolic, and environmental factors. Among dietary factors, high dextrose, low protein, methionine restriction, short-chain fatty acids (butyric acid and lipoic acid), and all-trans-retinoic acid were repeatedly shown to induce FGF21 expression and circulating levels. These effects are usually more pronounced in liver or isolated hepatocytes than in adipose tissue or isolated fat cells. Although peroxisome proliferator-activated receptor α (PPARα) is a key mediator of hepatic FGF21 expression and function, including the regulation of gluconeogenesis, ketogenesis, torpor, and growth inhibition, there is increasing evidence of PPARα-independent transactivation of the FGF21 gene by dietary molecules. FGF21 expression is believed to follow the circadian rhythm and be placed under the control of first order clock-controlled transcription factors, retinoic acid receptor-related orphan receptors (RORs) and nuclear receptors subfamily 1 group D (REV-ERBs), with FGF21 rhythm being anti-phase to REV-ERBs. Key metabolic hormones such as glucagon, insulin, and thyroid hormone have presumed or clearly demonstrated roles in regulating FGF21 transcription and secretion. The control of the FGF21 gene by glucagon and insulin appears more complex than first anticipated. Some discrepancies are noted and will need continued studies. The complexity in assessing the significance of FGF21 gene expression resides in the difficulty to ascertain (i) when transcription results in local or systemic increase of FGF21 protein; (ii) if FGF21 is among the first or second order genes upregulated by physiological, metabolic, and environmental stimuli, or merely an epiphenomenon; and (iii) whether FGF21 may have some adverse effects alongside beneficial outcomes.

Targeting adipose tissue in the treatment of obesity-associated diabetes

Adipose tissue regulates numerous physiological processes, and its dysfunction in obese humans is associated with disrupted metabolic homeostasis, insulin resistance and type 2 diabetes mellitus (T2DM). Although several US-approved treatments for obesity and T2DM exist, these are limited by adverse effects and a lack of effective long-term glucose control. In this Review, we provide an overview of the role of adipose tissue in metabolic homeostasis and assess emerging novel therapeutic strategies targeting adipose tissue, including adipokine-based strategies, promotion of white adipose tissue beiging as well as reduction of inflammation and fibrosis.

Role of GCN2-Independent Signaling Through a Noncanonical PERK/NRF2 Pathway in the Physiological Responses to Dietary Methionine Restriction

Restricting availability of essential amino acids (EAAs) limits aminoacylation of tRNAs by their cognate EAAs and activates the nutrient-sensing kinase, general control nonderepressible 2 (GCN2). Activated GCN2 phosphorylates eukaryotic initiation factor 2 (eIF2), altering gene-specific translation and initiating a transcriptional program collectively described as the integrated stress response (ISR). Central GCN2 activation by EAA deprivation is also linked to an acute aversive feeding response. Dietary methionine restriction (MR) produces a well-documented series of physiological responses (increased energy intake and expenditure, decreased adiposity, and increased insulin sensitivity), but the role of GCN2 in mediating them is unknown. Using Gcn2(-/-) mice, we found that the absence of GCN2 had no effect on the ability of MR to reduce body weight or adiposity, increase energy intake and expenditure, increase hepatic transcription and release of fibroblast growth factor 21, or improve insulin sensitivity. Interestingly, hepatic eIF2 phosphorylation by MR was uncompromised in Gcn2(-/-) mice. Instead, protein kinase R-like endoplasmic reticulum (ER) kinase (PERK) was activated in both intact and Gcn2(-/-) mice. PERK activation corresponded with induction of the ISR and the nuclear respiratory factor 2 antioxidant program but not ER stress. These data uncover a novel glutathione-sensing mechanism that functions independently of GCN2 to link dietary MR to its metabolic phenotype.

Adipose Tissue and Energy Expenditure: Central and Peripheral Neural Activation Pathways

Increasing energy expenditure is an appealing therapeutic target for the prevention and reversal of metabolic conditions such as obesity or type 2 diabetes. However, not enough research has investigated how to exploit pre-existing neural pathways, both in the central nervous system (CNS) and peripheral nervous system (PNS), in order to meet these needs. Here, we review several research areas in this field, including centrally acting pathways known to drive the activation of sympathetic nerves that can increase lipolysis and browning in white adipose tissue (WAT) or increase thermogenesis in brown adipose tissue (BAT), as well as other central and peripheral pathways able to increase energy expenditure of these tissues. In addition, we describe new work investigating the family of transient receptor potential (TRP) channels on metabolically important sensory nerves, as well as the role of the vagus nerve in regulating energy balance.

Fibroblast growth factor 21 has no direct role in regulating fertility in female mice

Objective

Reproduction is an energetically expensive process. Insufficient calorie reserves, signaled to the brain through peripheral signals such as leptin, suppress fertility. Recently, fibroblast growth factor 21 (FGF21) was implicated as a signal from the liver to the hypothalamus that directly inhibits the hypothalamic–gonadotropin axis during fasting and starvation. However, FGF21 itself increases metabolic rate and can induce weight loss, which suggests that the effects of FGF21 on fertility may not be direct and may reflect changes in energy balance.

Methods

To address this important question, we evaluated fertility in several mouse models with elevated FGF21 levels including ketogenic diet fed mice, fasted mice, mice treated with exogenous FGF21 and transgenic mice over-expressing FGF21.

Results

We find that ketogenic diet fed mice remain fertile despite significant elevation in serum FGF21 levels. Absence of FGF21 does not alter transient infertility induced by fasting. Centrally infused FGF21 does not suppress fertility despite its efficacy in inducing browning of inguinal white adipose tissue. Furthermore, a high fat diet (HFD) can restore fertility of female FGF21-overexpressing mice, a model of growth restriction, even in the presence of supraphysiological serum FGF21 levels.

Conclusions

We conclude that FGF21 is not a direct physiological regulator of fertility in mice. The infertility observed in FGF21 overexpressing mice is likely driven by the increased energy expenditure and consequent excess calorie requirements resulting from high FGF21 levels.

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Ketogenic diet fed mice remain fertile despite significant elevation in serum FGF21.

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Central infusion of FGF21 does not suppress fertility in female mice.

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Mice lacking FGF21 have similar post-fasting delay of cycling as control mice.

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High fat diet restores fertility in FGF21-Tg mice despite supra physiological serum FGF21.

Metabolic interplay between white, beige, brown adipocytes and the liver

In mammalian evolution, three types of adipocytes have developed, white, brown and beige adipocytes. White adipocytes are the major constituents of white adipose tissue (WAT), the predominant store for energy-dense triglycerides in the body that are released as fatty acids during catabolic conditions. The less abundant brown adipocytes, the defining parenchymal cells of brown adipose tissue (BAT), internalize triglycerides that are stored intracellularly in multilocular lipid droplets. Beige adipocytes (also known as brite or inducible brown adipocytes) are functionally very similar to brown adipocytes and emerge in specific WAT depots in response to various stimuli including sustained cold exposure. The activation of brown and beige adipocytes (together referred to as thermogenic adipocytes) causes both the hydrolysis of stored triglycerides as well as the uptake of lipids and glucose from the circulation. Together, these fuels are combusted for heat production to maintain body temperature in mammals including adult humans. Given that heating by brown and beige adipocytes is a very-well controlled and energy-demanding process which entails pronounced shifts in energy fluxes, it is not surprising that an intensive interplay exists between the various adipocyte types and parenchymal liver cells, and that this influences systemic metabolic fluxes and endocrine networks. In this review we will emphasize the role of hepatic factors that regulate the metabolic activity of white and thermogenic adipocytes. In addition, we will discuss the relevance of lipids and hormones that are secreted by white, brown and beige adipocytes regulating liver metabolism in order to maintain systemic energy metabolism in health and disease.

Forced expression of fibroblast growth factor 21 reverses the sustained impairment of liver regeneration in hPPARα(PAC) mice due to dysregulated bile acid synthesis

Peroxisome proliferator activated receptor α (PPARα) stimulates hepatocellular proliferation is species-specific. Activation of mouse, but not human, PPARα induces hepatocellular proliferation, hepatomegaly, and liver cancer. Here we tested the hypothesis that human and mouse PPARα affects liver regeneration differentially. PPARα-humanized mice (hPPARα(PAC)) were similar to wild type mice in responding to fasting-induced PPARα signaling. However, these mouse livers failed to regenerate in response to partial hepatectomy (PH). The liver-to-body weight ratios did not recover even 3 months after PH in hPPARα(PAC). The mouse PPARα-mediated down-regulation of let-7c was absent in hPPARα(PAC), which might partially be responsible for impaired proliferation. After PH, hPPARα(PAC) displayed steatosis, necrosis, and inflammation mainly in periportal zone 1, which suggested bile-induced toxicity. Quantification of hepatic bile acids (BA) revealed BA overload with increased hydrophobic BA in hPPARα(PAC). Forced FGF21 expression in partial hepatectomized hPPARα(PAC) reduced hepatic steatosis, prevented focal necrosis, and restored liver mass. Compared to mouse PPARα, human PPARα has a reduced capacity to regulate metabolic pathways required for liver regeneration. In addition, FGF21 can compensate for the reduced ability of human PPARα in stimulating liver regeneration, which suggests the potential application of FGF21 in promoting hepatic growth in injured and steatotic livers in humans.

Roles of FGFs As Paracrine or Endocrine Signals in Liver Development, Health, and Disease

The liver plays important roles in multiple processes including metabolism, the immune system, and detoxification and also has a unique capacity for regeneration. FGFs are growth factors that have diverse functions in development, health, and disease. The FGF family now comprises 22 members. Several FGFs have been shown to play roles as paracrine signals in liver development, health, and disease. FGF8 and FGF10 are involved in embryonic liver development, FGF7 and FGF9 in repair in response to liver injury, and FGF5, FGF8, FGF9, FGF17, and FGF18 in the development and progression of hepatocellular carcinoma. In contrast, FGF15/19 and FGF21 are endocrine signals. FGF15/19, which is produced in the ileum, is a negative regulator of bile acid metabolism and a stimulator of gallbladder filling. FGF15/19 is a postprandial, insulin-independent activator of hepatic protein and glycogen synthesis. It is also required for hepatocellular carcinoma and liver regeneration. FGF21 is a hepatokine produced in the liver. FGF21 regulates glucose and lipid metabolism in white adipose tissue. Serum FGF21 levels are elevated in non-alcoholic fatty liver. FGF21 also protects against non-alcoholic fatty liver. These findings provide new insights into the roles of FGFs in the liver and potential therapeutic strategies for hepatic disorders.

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.

Potential roles of fibroblast growth factor 21 in the brain

Fibroblast growth factor 21 (FGF21) is an endocrine hormone, playing an important role in the regulation of metabolism. FGF21 is primarily expressed by several tissues, including liver, pancreas, thymus, heart, muscle, adipose tissue, and brain. In addition to the effects of FGF21 in lowering glucose and lipid levels, increasing insulin sensitivity and regulating energy homeostasis in rodents and non-human primate models of diabetes and obesity, previous reports have demonstrated that FGF21 also plays an important role in the brain involving it in potential effects in metabolic regulation, neuroprotection and cognition. In this review, the current available evidence from both in vitro and in vivo investigations regarding the roles of FGF21 and its function in the brain are comprehensively summarized. In addition, the mechanistic insights regarding the roles of FGF21 in the brain and its potential neuroprotective benefits are also presented and discussed.

The FGF21-adiponectin axis in controlling energy and vascular homeostasis

Whole-body energy metabolism and cardiovascular homeostasis are tightly controlled processes that involve highly coordinated crosstalk among distal organs. This is mainly achieved by a large number of hormones released from each organ. Among them, fibroblast growth factor 21 (FGF21) and adiponectin have recently gained considerable attention, since both of them possess multiple profound protective effects against a myriad of cardio-metabolic disorders. Despite their distinct structures and production sites, these two hormones share striking functional similarity. This dichotomy is recently reconciled by the demonstration of the FGF21-adiponectin axis. In adipocytes, both transcription and secretion of adiponectin are strongly induced by FGF21, which is partially dependent on PPARγ activity. Furthermore, the glucose-lowering, lipid-clearing, and anti-atherosclerotic functions of FGF21 are diminished in adiponectin-null mice, suggesting that adiponectin serves as an obligatory mediator of FGF21-elicited metabolic and vascular benefits. However, in both animals and human subjects with obesity, circulating FGF21 levels are increased whereas plasma adiponectin concentrations are reduced, perhaps due to FGF21 resistance, suggesting that dysfunctional FGF21-adiponectin axis is an important contributor to the pathogenesis of obesity-related cardio-metabolic syndrome. The FGF21-adiponectin axis protects against a cluster of cardio-metabolic disorders via mediating multi-organ communications, and is a promising target for therapeutic interventions of these chronic diseases.

Fibroblast Growth Factor 21 Mediates Glycemic Regulation by Hepatic JNK

The cJun NH2-terminal kinase (JNK)-signaling pathway is implicated in metabolic syndrome, including dysregulated blood glucose concentration and insulin resistance. Fibroblast growth factor 21 (FGF21) is a target of the hepatic JNK-signaling pathway and may contribute to the regulation of glycemia. To test the role of FGF21, we established mice with selective ablation of the Fgf21 gene in hepatocytes. FGF21 deficiency in the liver caused marked loss of FGF21 protein circulating in the blood. Moreover, the protective effects of hepatic JNK deficiency to suppress metabolic syndrome in high-fat diet-fed mice were not observed in mice with hepatocyte-specific FGF21 deficiency, including reduced blood glucose concentration and reduced intolerance to glucose and insulin. Furthermore, we show that JNK contributes to the regulation of hepatic FGF21 expression during fasting/feeding cycles. These data demonstrate that the hepatokine FGF21 is a key mediator of JNK-regulated metabolic syndrome.

FGF21 Lowers Plasma Triglycerides by Accelerating Lipoprotein Catabolism in White and Brown Adipose Tissues

FGF21 decreases plasma triglycerides (TGs) in rodents and humans; however, the underlying mechanism or mechanisms are unclear. In the present study, we examined the role of FGF21 in production and disposal of TG-rich lipoproteins (TRLs) in mice. Treatment with pharmacological doses of FGF21 acutely reduced plasma non-esterified fatty acids (NEFAs), liver TG content, and VLDL-TG secretion. In addition, metabolic turnover studies revealed that FGF21 facilitated the catabolism of TRL in white adipose tissue (WAT) and brown adipose tissue (BAT). FGF21-dependent TRL processing was strongly attenuated in CD36-deficient mice and transgenic mice lacking lipoprotein lipase in adipose tissues. Insulin resistance in diet-induced obese and ob/ob mice shifted FGF21 responses from WAT toward energy-combusting BAT. In conclusion, FGF21 lowers plasma TGs through a dual mechanism: first, by reducing NEFA plasma levels and consequently hepatic VLDL lipidation and, second, by increasing CD36 and LPL-dependent TRL disposal in WAT and BAT.

A Long-Acting FGF21 Molecule, PF-05231023, Decreases Body Weight and Improves Lipid Profile in Non-human Primates and Type 2 Diabetic Subjects

FGF21 plays a central role in energy, lipid, and glucose homeostasis. To characterize the pharmacologic effects of FGF21, we administered a long-acting FGF21 analog, PF-05231023, to obese cynomolgus monkeys. PF-05231023 caused a marked decrease in food intake that led to reduced body weight. To assess the effects of PF-05231023 in humans, we conducted a placebo-controlled, multiple ascending-dose study in overweight/obese subjects with type 2 diabetes. PF-05231023 treatment resulted in a significant decrease in body weight, improved plasma lipoprotein profile, and increased adiponectin levels. Importantly, there were no significant effects of PF-05231023 on glycemic control. PF-05231023 treatment led to dose-dependent changes in multiple markers of bone formation and resorption and elevated insulin-like growth factor 1. The favorable effects of PF-05231023 on body weight support further evaluation of this molecule for the treatment of obesity. Longer studies are needed to assess potential direct effects of FGF21 on bone in humans.

Bile acids induce uncoupling protein 1-dependent thermogenesis and stimulate energy expenditure at thermoneutrality in mice

It has been proposed that diet-induced obesity at thermoneutrality (TN; 29°C) is reduced by a UCP1-dependent thermogenesis; however, it has not been shown how UCP1-dependent thermogenesis can be activated in the absence of sympathetic activity. A recent study provides such a mechanism by showing that dietary bile acids (BAs) suppress obesity in mice fed a high-fat diet (HFD) by a mechanism dependent on type 2 deiodinase (DIO2); however, neither a role for UCP1 nor the influence of sympathetic activity was properly assessed. To test whether the effects of BAs on adiposity are independent of Ucp1 and cold-activated thermogenesis, obesity phenotypes were determined in C57BL6/J.(+)/(+) (WT) and C57BL6/J.Ucp1.(-)/(-) mice (Ucp1-KO) housed at TN and fed a HFD with or without 0.5% (wt/wt) cholic acid (CA) for 9 wk. CA in a HFD reduced adiposity and hepatic lipogenesis and improved glucose tolerance in WT but not in Ucp1-KO mice and was accompanied by increases in food intake and energy expenditure (EE). In iBAT, CA increased Ucp1 mRNA and protein levels 1.5- and twofold, respectively, and increased DIO2 and TGR5 protein levels in WT mice. Despite enhanced Dio2 expression in Ucp1-KO and Ucp1-KO-CA treated mice, this did not enhance the ability of BAs to reduce obesity. By comparing the effects of BAs on WT and Ucp1-KO mice at TN, our study showed that BAs suppress diet-induced obesity by increasing EE through a mechanism dependent on Ucp1 expression, which is likely independent of adrenergic signaling.

FGF21 Regulates Sweet and Alcohol Preference

Fibroblast growth factor 21 (FGF21) is a hormone induced by various metabolic stresses, including ketogenic and high-carbohydrate diets, that regulates energy homeostasis. In humans, SNPs in and around the FGF21 gene have been associated with macronutrient preference, including carbohydrate, fat, and protein intake. Here we show that FGF21 administration markedly reduces sweet and alcohol preference in mice and sweet preference in cynomolgus monkeys. In mice, these effects require the FGF21 co-receptor β-Klotho in the central nervous system and correlate with reductions in dopamine concentrations in the nucleus accumbens. Since analogs of FGF21 are currently undergoing clinical evaluation for the treatment of obesity and type 2 diabetes, our findings raise the possibility that FGF21 administration could affect nutrient preference and other reward behaviors in humans.

Fibroblast Growth Factor 21 Protects against Atherosclerosis via Fine-Tuning the Multiorgan Crosstalk

Fibroblast growth factor 21 (FGF21) is a metabolic hormone with pleiotropic effects on energy metabolism and insulin sensitivity. Besides its antiobese and antidiabetic activity, FGF21 also possesses the protective effects against atherosclerosis. Circulating levels of FGF21 are elevated in patients with atherosclerosis, macrovascular and microvascular complications of diabetes, possibly due to a compensatory upregulation. In apolipoprotein E-deficient mice, formation of atherosclerotic plaques is exacerbated by genetic depletion of FGF21, but is attenuated upon replenishment with recombinant FGF21. However, the blood vessel is not the direct target of FGF21, and the antiatherosclerotic activity of FGF21 is attributed to its actions in adipose tissues and liver. In adipocytes, FGF21 promotes secretion of adiponectin, which in turn acts directly on blood vessels to reduce endothelial dysfunction, inhibit proliferation of smooth muscle cells and block conversion of macrophages to foam cells. Furthermore, FGF21 suppresses cholesterol biosynthesis and attenuates hypercholesterolemia by inhibiting the transcription factor sterol regulatory element-binding protein-2 in hepatocytes. The effects of FGF21 on elevation of adiponectin and reduction of hypercholesterolemia are also observed in a phase-1b clinical trial in patients with obesity and diabetes. Therefore, FGF21 exerts its protection against atherosclerosis by fine-tuning the interorgan crosstalk between liver, brain, adipose tissue, and blood vessels.

Acyl CoA synthetase 5 (ACSL5) ablation in mice increases energy expenditure and insulin sensitivity and delays fat absorption

Objective

The family of acyl-CoA synthetase enzymes (ACSL) activates fatty acids within cells to generate long chain fatty acyl CoA (FACoA). The differing metabolic fates of FACoAs such as incorporation into neutral lipids, phospholipids, and oxidation pathways are differentially regulated by the ACSL isoforms. In vitro studies have suggested a role for ACSL5 in triglyceride synthesis; however, we have limited understanding of the in vivo actions of this ACSL isoform.

Methods

To elucidate the in vivo actions of ACSL5 we generated a line of mice in which ACSL5 expression was ablated in all tissues (ACSL5^−/−).

Results

Ablation of ACSL5 reduced ACSL activity by ∼80% in jejunal mucosa, ∼50% in liver, and ∼37% in brown adipose tissue lysates. Body composition studies revealed that ACSL5^−/−, as compared to control ACSL5^loxP/loxP, mice had significantly reduced fat mass and adipose fat pad weights. Indirect calorimetry studies demonstrated that ACSL5^−/− had increased metabolic rates, and in the dark phase, increased respiratory quotient. In ACSL5^−/− mice, fasting glucose and serum triglyceride were reduced; and insulin sensitivity was improved during an insulin tolerance test. Both hepatic mRNA (∼16-fold) and serum levels of fibroblast growth factor 21 (FGF21) (∼13-fold) were increased in ACSL5^−/− as compared to ACSL5^loxP/loxP. Consistent with increased FGF21 serum levels, uncoupling protein-1 gene (Ucp1) and PPAR-gamma coactivator 1-alpha gene (Pgc1α) transcript levels were increased in gonadal adipose tissue. To further evaluate ACSL5 function in intestine, mice were gavaged with an olive oil bolus; and the rate of triglyceride appearance in serum was found to be delayed in ACSL5^−/− mice as compared to control mice.

Conclusions

In summary, ACSL5^−/− mice have increased hepatic and serum FGF21 levels, reduced adiposity, improved insulin sensitivity, increased energy expenditure and delayed triglyceride absorption. These studies suggest that ACSL5 is an important regulator of whole-body energy metabolism and ablation of ACSL5 may antagonize the development of obesity and insulin resistance.

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Role of acyl CoA synthetase 5 (ACSL5) in systemic metabolism was studied in an ACSL5 deficient mouse.

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ACSL5 deficiency reduced total ACSL activity in liver, intestine, and brown adipose tissue.

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ACSL5 deficient mice had increased hepatic and circulating FGF21 expression and energy expenditure.

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ACSL5 deficient mice demonstrated delayed triglyceride absorption.

Fibroblast Growth Factor 21-Null Mice Do Not Exhibit an Impaired Response to Fasting

Fibroblast growth factor 21 (FGF21) is a pleotropic metabolic regulator, expression of which is elevated during fasting. To this end, the precise role played by FGF21 in the biology of fasting has been the subject of several recent studies, which have demonstrated contributions to the regulation of both lipid and carbohydrate metabolism. In the present study, we compared wild-type (WT) and FGF21-null (FGF21KO) mice, demonstrating that, despite the significant induction of FGF21 during fasting in the WT animals, our strain of FGF21-null mice exhibits only limited impairments in their adaptation to nutrient deprivation. Specifically, fasted FGF21KO mice display a mild attenuation of gluconeogenic transcriptional induction in the liver accompanied by partially blunted glucose production in response to a pyruvate challenge. Furthermore, FGF21KO mice displayed only minor impairments in lipid metabolism in the fasted state, limited to accumulation of hepatic triglycerides and a reduction in expression of genes associated with fatty acid oxidation. To address the possibility of compensation to germline deletion of FGF21, we further interrogated the role of endogenous FGF21 via acute pharmacological blockade of FGF21 signaling. At the transcriptional level, we show that FGF21 signaling is required for full induction of gluconeogenic and oxidative genes in the liver. However, corroborating our findings in FGF21KO mice, pharmacological blockade of the FGF21 axis did not profoundly disrupt the physiological response to fasting. Taken as a whole, these data demonstrate that, while FGF21 is partially required for appropriate gene expression during the fed to fasted transition, its absence does not significantly impact the downstream physiology of the fasted state.

Fibroblast growth factor-21, energy balance and obesity

Fibroblast growth factor (FGF)-21 is an endocrine member of the FGF family with healthy effects on glucose and lipid metabolism. FGF21 reduces glycemia and lipidemia in rodent models of obesity and type 2 diabetes. In addition to its effects improving insulin sensitivity, FGF21 causes weight loss by increasing energy expenditure. Activation of the thermogenic activity of brown adipose tissue and promotion of the appearance of the so-called beige/brite type of brown adipocytes in white fat are considered the main mechanisms underlying the leaning effects of FGF21. Paradoxically, however, obesity in rodents and humans is characterized by high levels of FGF21 in the blood. Some degree of resistance to the actions of FGF21 has been proposed as part of the endocrine alterations in obesity. The resistance in adipose tissue from obese rodents and patients is likely attributable to abnormally low levels of the FGF co-receptor β-Klotho, required for FGF21 cellular action. However, native FGF21 and FGF21 derivatives retain their healthy metabolic and weight-loss effects when used as pharmacological agents to treat obese rodents and humans. FGF21 derivatives or molecules mimicking FGF21 action appear to be interesting candidates for the development of novel anti-obesity drugs designed to increase energy expenditure.

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.

Understanding the Physiology of FGF21

Fibroblast growth factor 21 (FGF21) is a peptide hormone that is synthesized by several organs and regulates energy homeostasis. Excitement surrounding this relatively recently identified hormone is based on the documented metabolic beneficial effects of FGF21, which include weight loss and improved glycemia. The biology of FGF21 is intrinsically complicated owing to its diverse metabolic functions in multiple target organs and its ability to act as an autocrine, paracrine, and endocrine factor. In the liver, FGF21 plays an important role in the regulation of fatty acid oxidation both in the fasted state and in mice consuming a high-fat, low-carbohydrate ketogenic diet. FGF21 also regulates fatty acid metabolism in mice consuming a diet that promotes hepatic lipotoxicity. In white adipose tissue (WAT), FGF21 regulates aspects of glucose metabolism, and in susceptible WAT depots, it can cause browning. This peptide is highly expressed in the pancreas, where it appears to play an anti-inflammatory role in experimental pancreatitis. It also has an anti-inflammatory role in cardiac muscle. Although typically not expressed in skeletal muscle, FGF21 is induced in situations of muscle stress, particularly mitochondrial myopathies. FGF21 has been proposed as a novel therapeutic for metabolic complications such as diabetes and fatty liver disease. This review aims to interpret and delineate the ever-expanding complexity of FGF21 physiology.

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.

Hypothalamic-autonomic control of energy homeostasis

Regulation of energy homeostasis is tightly controlled by the central nervous system (CNS). Several key areas such as the hypothalamus and brainstem receive and integrate signals conveying energy status from the periphery, such as leptin, thyroid hormones, and insulin, ultimately leading to modulation of food intake, energy expenditure (EE), and peripheral metabolism. The autonomic nervous system (ANS) plays a key role in the response to such signals, innervating peripheral metabolic tissues, including brown and white adipose tissue (BAT and WAT), liver, pancreas, and skeletal muscle. The ANS consists of two parts, the sympathetic and parasympathetic nervous systems (SNS and PSNS). The SNS regulates BAT thermogenesis and EE, controlled by central areas such as the preoptic area (POA) and the ventromedial, dorsomedial, and arcuate hypothalamic nuclei (VMH, DMH, and ARC). The SNS also regulates lipid metabolism in WAT, controlled by the lateral hypothalamic area (LHA), VMH, and ARC. Control of hepatic glucose production and pancreatic insulin secretion also involves the LHA, VMH, and ARC as well as the dorsal vagal complex (DVC), via splanchnic sympathetic and the vagal parasympathetic nerves. Muscle glucose uptake is also controlled by the SNS via hypothalamic nuclei such as the VMH. There is recent evidence of novel pathways connecting the CNS and ANS. These include the hypothalamic AMP-activated protein kinase-SNS-BAT axis which has been demonstrated to be a key modulator of thermogenesis. In this review, we summarize current knowledge of the role of the ANS in the modulation of energy balance.

Metabolic responses to dietary leucine restriction involve remodeling of adipose tissue and enhanced hepatic insulin signaling

Dietary leucine was incrementally restricted to test whether limiting this essential amino acid (EAA) would fully reproduce the beneficial responses produced by dietary methionine restriction. Restricting leucine by 85% increased energy intake and expenditure within 5 to 7 days of its introduction and reduced overall accumulation of adipose tissue. Leucine restriction (LR) also improved glucose tolerance, increased hepatic release of fibroblast growth factor 21 into the blood stream, and enhanced insulin-dependent activation of Akt in liver. However, LR had no effect on hepatic lipid levels and failed to lower lipogenic gene expression in the liver. LR did affect remodeling of white and brown adipose tissues, increasing expression of both thermogenic and lipogenic genes. These findings illustrate that dietary LR reproduces many but not all of the physiological responses of methionine restriction. The primary differences occur in the liver, where methionine and LR cause opposite effects on tissue lipid levels and expression of lipogenic genes. Altogether, these findings suggest that the sensing systems which detect and respond to dietary restriction of EAAs act through mechanisms that both leucine and methionine are able to engage, and in the case of hepatic lipid metabolism, may be unique to specific EAAs such as methionine.

Neuronal Control of Brown Fat Activity

Brown adipose tissue (BAT) activation reduces body fat and metabolic disorders by the enhanced combustion of lipids and glucose into heat. The thermogenic activity of brown adipocytes is primarily driven by the sympathetic nervous system (SNS) and controlled by the brain. In this review, we present recent advances in understanding how cues, such as temperature, light, and proteins, modulate the activity of brown fat by acting on the various hypothalamic nuclei. Given that activated BAT has a high capacity to take up and burn fatty acids (FAs) and glucose, pharmacological stimulation of brown fat in humans by either targeting the hypothalamus or mimicking outflow of the sympathetic nervous system might help improve glucose metabolism and insulin sensitivity, and also lower body fat.

Hepatocyte β-Klotho regulates lipid homeostasis but not body weight in mice

β-Klotho (β-Kl), a transmembrane protein expressed in the liver, pancreas, adipose tissues, and brain, is essential for feedback suppression of hepatic bile acid synthesis. Because bile acid is a key regulator of lipid and energy metabolism, we hypothesized potential and tissue-specific roles of β-Kl in regulating plasma lipid levels and body weight. By crossing β-kl(-/-) mice with newly developed hepatocyte-specific β-kl transgenic (Tg) mice, we generated mice expressing β-kl solely in hepatocytes (β-kl(-/-)/Tg). Gene expression, metabolomic, and in vivo flux analyses consistently revealed that plasma level of cholesterol, which is over-excreted into feces as bile acids in β-kl(-/-), is maintained in β-kl(-/-) mice by enhanced de novo cholesterogenesis. No compensatory increase in lipogenesis was observed, despite markedly decreased plasma triglyceride. Along with enhanced bile acid synthesis, these lipid dysregulations in β-kl(-/-) were completely reversed in β-kl(-/-)/Tg mice. In contrast, reduced body weight and resistance to diet-induced obesity in β-kl(-/-) mice were not reversed by hepatocyte-specific restoration of β-Kl expression. We conclude that β-Kl in hepatocytes is necessary and sufficient for lipid homeostasis, whereas nonhepatic β-Kl regulates energy metabolism. We further demonstrate that in a condition with excessive cholesterol disposal, a robust compensatory mechanism maintains cholesterol levels but not triglyceride levels in mice.

Metabolic roles of endocrine fibroblast growth factors

Considerable effort is currently being devoted to understanding the physiological and pharmacological action of the endocrine fibroblast growth factors (FGFs). These three proteins (FGF15/19, FGF21 and FGF23) act in a tissue-specific manner through a membrane-complex consisting of an FGF-receptor and α/βKlotho. FGF15/19 is produced in the intestine and regulates postprandial liver metabolism and gallbladder filling. FGF21 is largely liver-derived and co-ordinates adaptive changes in response to nutritional and physiological stresses. FGF23 signals from the bone to the kidney to maintain phosphate homeostasis. In pharmacological settings, FGF15/19, FGF21, and the prototypical FGF1, potentially represent novel treatments for obesity and diabetes. This review summarises the recent advances in our understanding of the biology of these important metabolic regulators.

FGF21 is not required for glucose homeostasis, ketosis or tumour suppression associated with ketogenic diets in mice

AIMS/HYPOTHESIS

Ketogenic diets (KDs) have increasingly gained attention as effective means for weight loss and potential adjunctive treatment of cancer. The metabolic benefits of KDs are regularly ascribed to enhanced hepatic secretion of fibroblast growth factor 21 (FGF21) and its systemic effects on fatty-acid oxidation, energy expenditure (EE) and body weight. Ambiguous data from Fgf21-knockout animal strains and low FGF21 concentrations reported in humans with ketosis have nevertheless cast doubt regarding the endogenous function of FGF21. We here aimed to elucidate the causal role of FGF21 in mediating the therapeutic benefits of KDs on metabolism and cancer.

METHODS

We established a dietary model of increased vs decreased FGF21 by feeding C57BL/6J mice with KDs, either depleted of protein or enriched with protein. We furthermore used wild-type and Fgf21-knockout mice that were subjected to the respective diets, and monitored energy and glucose homeostasis as well as tumour growth after transplantation of Lewis lung carcinoma cells.

RESULTS

Hepatic and circulating, but not adipose tissue, FGF21 levels were profoundly increased by protein starvation, independent of the state of ketosis. We demonstrate that endogenous FGF21 is not essential for the maintenance of normoglycaemia upon protein and carbohydrate starvation and is therefore not needed for the effects of KDs on EE. Furthermore, the tumour-suppressing effects of KDs were independent of FGF21 and, rather, driven by concomitant protein and carbohydrate starvation.

CONCLUSIONS/INTERPRETATION

Our data indicate that the multiple systemic effects of KD exposure in mice, previously ascribed to increased FGF21 secretion, are rather a consequence of protein malnutrition.

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.

Brown adipose tissue: a potential target in the fight against obesity and the metabolic syndrome

BAT (brown adipose tissue) is the main site of thermogenesis in mammals. It is essential to ensure thermoregulation in newborns. It is also found in (some) adult humans. Its capacity to oxidize fatty acids and glucose without ATP production contributes to energy expenditure and glucose homoeostasis. Brown fat activation has thus emerged as an attractive therapeutic target for the treatment of obesity and the metabolic syndrome. In the present review, we integrate the recent advances on the metabolic role of BAT and its relation with other tissues as well as its potential contribution to fighting obesity and the metabolic syndrome.

Molecular and neuroendocrine mechanisms of cancer cachexia

Cancer and its morbidities, such as cancer cachexia, constitute a major public health problem. Although cancer cachexia has afflicted humanity for centuries, its underlying multifactorial and complex physiopathology has hindered the understanding of its mechanism. During the last few decades we have witnessed a dramatic increase in the understanding of cancer cachexia pathophysiology. Anorexia and muscle and adipose tissue wasting are the main features of cancer cachexia. These apparently independent symptoms have humoral factors secreted by the tumor as a common cause. Importantly, the hypothalamus has emerged as an organ that senses the peripheral signals emanating from the tumoral environment, and not only elicits anorexia but also contributes to the development of muscle and adipose tissue loss. Herein, we review the roles of factors secreted by the tumor and its effects on the hypothalamus, muscle and adipose tissue, as well as highlighting the key targets that are being exploited for cancer cachexia treatment.

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.

Compromised responses to dietary methionine restriction in adipose tissue but not liver of ob/ob mice

OBJECTIVE

Dietary methionine restriction (MR) reduces adiposity and hepatic lipids and increases overall insulin sensitivity in part by reducing lipogenic gene expression in liver, inducing browning of white adipose tissue (WAT), and enhancing the lipogenic and oxidative capacity of the remodeled WAT.

METHODS

Ob/ob mice have compromised β-adrenergic receptor expression in adipose tissue and were used to test whether MR could ameliorate obesity, insulin resistance, and disordered lipid metabolism.

RESULTS

In contrast to responses in wild-type mice, MR failed to slow accumulation of adiposity, increase lipogenic and thermogenic gene expression in adipose tissue, reduce serum insulin, or increase serum adiponectin in ob/ob mice. However, MR produced comparable reductions in hepatic lipids and lipogenic gene expression in both genotypes. In addition, MR was fully effective in increasing insulin sensitivity in adiponectin(-/-) mice.

CONCLUSIONS

These findings show that diet-induced changes in hepatic lipid metabolism are independent of weight loss and remodeling of WAT and are not required for insulin sensitization. In contrast, the failure of ob/ob mice to mount a normal thermogenic response to MR suggests that the compromised responsiveness of adipose tissue to SNS input is an important component of the inability of the diet to correct their obesity and insulin resistance.

Hepatic CEACAM1 Over-Expression Protects Against Diet-Induced Fibrosis and Inflammation in White Adipose Tissue

CEACAM1 promotes insulin extraction, an event that occurs mainly in liver. Phenocopying global Ceacam1 null mice (Cc1(-/-) ), C57/BL6J mice fed a high-fat (HF) diet exhibited reduced hepatic CEACAM1 levels and impaired insulin clearance, followed by hyperinsulinemia, insulin resistance, and visceral obesity. Conversely, forced liver-specific expression of CEACAM1 protected insulin sensitivity and energy expenditure, and limited gain in total fat mass by HF diet in L-CC1 mice. Because CEACAM1 protein is barely detectable in white adipose tissue (WAT), we herein investigated whether hepatic CEACAM1-dependent insulin clearance pathways regulate adipose tissue biology in response to dietary fat. While HF diet caused a similar body weight gain in L-CC1, this effect was delayed and less intense relative to wild-type (WT) mice. Histological examination revealed less expansion of adipocytes in L-CC1 than WT by HF intake. Immunofluorescence analysis demonstrated a more limited recruitment of crown-like structures, and qRT-PCR analysis showed no significant rise in TNFα mRNA levels in response to HF intake in L-CC1 than WT mice. Unlike WT, HF diet did not activate TGF-β in WAT of L-CC1 mice, as assessed by Western analysis of Smad2/3 phosphorylation. Consistently, HF diet caused relatively less collagen deposition in L-CC1 than WT mice, as shown by Trichrome staining. Coupled with reduced lipid redistribution from liver to visceral fat, lower inflammation and fibrosis could contribute to protected energy expenditure against HF diet in L-CC1 mice. The data underscore the important role of hepatic insulin clearance in the regulation of adipose tissue inflammation and fibrosis.

Forced Hepatic Overexpression of CEACAM1 Curtails Diet-Induced Insulin Resistance

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) regulates insulin sensitivity by promoting hepatic insulin clearance. Liver-specific inactivation or global null-mutation of Ceacam1 impairs hepatic insulin extraction to cause chronic hyperinsulinemia, resulting in insulin resistance and visceral obesity. In this study we investigated whether diet-induced insulin resistance implicates changes in hepatic CEACAM1. We report that feeding C57/BL6J mice a high-fat diet reduced hepatic CEACAM1 levels by >50% beginning at 21 days, causing hyperinsulinemia, insulin resistance, and elevation in hepatic triacylglycerol content. Conversely, liver-specific inducible CEACAM1 expression prevented hyperinsulinemia and markedly limited insulin resistance and hepatic lipid accumulation that were induced by prolonged high-fat intake. This was partly mediated by increased hepatic β-fatty acid oxidation and energy expenditure. The data demonstrate that the high-fat diet reduced hepatic CEACAM1 expression and that overexpressing CEACAM1 in liver curtailed diet-induced metabolic abnormalities by protecting hepatic insulin clearance.

FGF21 does not require interscapular brown adipose tissue and improves liver metabolic profile in animal models of obesity and insulin-resistance

FGF21 is a key metabolic regulator modulating physiological processes and its pharmacological administration improves metabolic profile in preclinical species and humans. We used native-FGF21 and a long-acting FGF21 (PF-05231023), to determine the contribution of liver and brown adipose tissue (BAT) towards metabolic improvements in Zucker rats and DIO mice (DIOs). FGF21 improved glucose tolerance and liver insulin sensitivity in Zuckers without affecting BW and improved liver function by decreased lipogenesis, increased fatty acid oxidation and improved insulin signaling. Through detailed lipidomic analyses of liver metabolites in DIOs, we demonstrate that FGF21 favorably alters liver metabolism. We observed a dose-dependent increase of [(18)F]-FDG-glucose uptake in interscapular BAT (iBAT) of DIOs upon FGF21 administration. Upon excision of iBAT (X-BAT) and administration of FGF21 to mice housed at 80 °F or 72 °F, the favorable effects of FGF21 on BW and glucose excursion were fully retained in both sham and X-BAT animals. Taken together, we demonstrate the liver as an organ that integrates the actions of FGF21 and provide metabolic benefits of FGF21 in Zucker rats and DIOs. Finally, our data demonstrates iBAT does not play a role in mediating favorable metabolic effects of FGF21 administration in DIOs housed at 80 °F or 72 °F.