Going Back to the Biology of FGF21: New Insights

© 2019 Elsevier Ltd Fibroblast growth factor 21 (FGF21) is a protein highly synthesized in the liver that exerts paracrine and endocrine control of many aspects of energy homeostasis in multiple tissues. In preclinical models of obesity and type 2 diabetes, treatment with FGF21 improves glucose homeostasis and promotes weight loss, and, as a result, FGF21 has attracted considerable attention as a therapeutic agent for the treatment of metabolic syndrome in humans. An improved understanding of the biological role of FGF21 may help to explain why its therapeutic potential in humans has not been fully realized. This review will cover the complexities in FGF21 biology in rodents and humans, with emphasis on its role in protection from central and peripheral facets of obesity

Antibody mediated targeting of the FGFR1c isoform increases glucose uptake in white and brown adipose tissue in male mice

The increased prevalence of obesity and its cardiometabolic implications demonstrates the imperative to identify novel therapeutic targets able to effect meaningful metabolic changes in this population. Antibody-mediated targeting of fibroblast growth factor receptor 1c isoform (FGFR1c) has been shown to ameliorate hyperglycaemia and protect from diet- and genetically-induced obesity in rodents and non-human primates. However, it is currently unknown which tissue(s) contribute to this glucose lowering effect. Thus, to elucidate this effect we treated euglycaemic mice with H7, a monoclonal antibody which selectively targets the FGFR1c isoform, and employed whole body positron emission computed tomography with a glucose tracer (18F-flurodeoxyglucose). Treatment with H7 increased basal glucose uptake in white and brown adipose tissues (WAT and BAT respectively), the brain and liver, but reduced it in the quadricep muscles. Consequentially, blood glucose was significantly reduced in response to treatment. Under insulin-stimulated conditions, the effects of H7 were maintained in WAT, BAT, liver and muscle. Treatment with H7 decreased triglyceride content and increased adipose triglyceride lipase content in white adipose tissue, whilst increasing activation of acetyl coenzyme A carboxylase, suggesting futile cycling of triglycerides, albeit favouring net hydrolysis. We demonstrated, in vitro, this is a direct effect of treatment in adipose tissue as basal cellular respiration and glucose uptake were increased in response to treatment. Taken together, these data suggest that antibody-mediated targeting of FGFR1c exerts its powerful glucose-lowering efficacy primarily due to increased glucose uptake in adipose tissue

Photoperiodic regulation of FGF21 production in the Siberian hamster

FGF21 is an endocrine member of the fibroblast growth factor superfamily that has been shown to play an important role in the physiological response to nutrient deprivation. Food restriction enhances hepatic FGF21 production, which serves to engage an integrated response to energy deficit. Specifically, elevated FGF21 levels lead to reduced gluconeogenesis and increased hepatic ketogenesis. However, circulating FGF21 concentrations also paradoxically rise in states of metabolic dysfunction such as obesity. Furthermore, multiple peripheral tissues also produce FGF21 in addition to the liver, raising questions as to its endocrine and paracrine roles in the control of energy metabolism. The objectives of this study were to measure plasma FGF21 concentrations in the Siberian hamster, a rodent which undergoes a seasonal cycle of fattening and body weight gain in the long days (LD) of summer, followed by reduction of appetite and fat catabolism in the short days (SD) of winter. Groups of adult male hamsters were raised in long days, and then exposed to SD for up to 12 weeks. Chronic exposure of LD animals to SD led to a significant increase in circulating FGF21 concentrations. This elevation of circulating FGF21 was preceded by an increase in liver FGF21 protein production evident as early as 4 weeks of exposure to SD. FGF21 protein abundance was also increased significantly in interscapular brown adipose tissue, with a positive correlation between plasma levels of FGF21 and BAT protein abundance throughout the experimental period. Epididymal white adipose tissue and skeletal muscle (gastrocnemius) also produced FGF21, but levels did not change in response to a change in photoperiod. In summary, a natural programmed state of fat catabolism was associated with increased FGF21 production in the liver and BAT, consistent with the view that FGF21 has a role in adapting hamsters to the hypophagic winter state

Dual effects of fibroblast growth factor 21 on hepatic energy metabolism

The aim of this study was to investigate the mechanisms by which FGF21 affects hepatic integration of carbohydrate and fat metabolism in Siberian hamsters, a natural model of adiposity. Twelve aged matched adult male Siberian hamsters maintained in their long day fat state since birth were randomly assigned to one of two treatment groups and were continuously infused with either vehicle (saline; n=6) or recombinant human FGF21 protein (1 mg/kg/day; n=6) for 14 days. FGF21 administration caused a 40% suppression (P<0.05) of hepatic pyruvate dehydrogenase complex (PDC), the rate-limiting step in glucose oxidation, a 34% decrease (P<0.05) in hepatic acetylcarnitine accumulation, an index of reduced PDC flux, a 35% increase (P<0.05) in long-chain acylcarnitine content (an index of flux through β-oxidation) and a 47% reduction (P<0.05) in hepatic lipid content. These effects were underpinned by increased protein abundance of PD kinase-4 (a negative regulator of PDC), the phosphorylated (inhibited) form of acetyl-CoA carboxylase (ACC, a negative regulator of delivery of fatty acids into the mitochondria), and the transcriptional co-regulators of energy metabolism peroxisome proliferator activated receptor gamma co-activator alpha (PGC1 and sirtuin-1 (SIRT-1). These findings provide novel mechanistic basis to support the notion that FGF21 exerts profound metabolic benefits in the liver by modulating nutrient flux through both carbohydrate (mediated by a PDK4-mediated suppression of PDC activity) and fat (mediated by deactivation of ACC) metabolism, and therefore may be an attractive target for protection from increased hepatic lipid content and insulin resistance that frequently accompany obesity and diabetes

Whole-body and adipose tissue-specific mechanisms underlying the metabolic effects of fibroblast growth factor 21 in the Siberian hamster.

OBJECTIVE: Fibroblast growth factor 21 (FGF21) has been shown to rapidly lower body weight in the Siberian hamster, a preclinical model of adiposity. This induced negative energy balance mediated by FGF21 is associated with both lowered caloric intake and increased energy expenditure. Previous research demonstrated that adipose tissue (AT) is one of the primary sites of FGF21 action and may be responsible for its ability to increase the whole-body metabolic rate. The present study sought to determine the relative importance of white (subcutaneous AT [sWAT] and visceral AT [vWAT]), and brown (interscapular brown AT [iBAT]) in governing FGF21-mediated metabolic improvements using the tissue-specific uptake of glucose and lipids as a proxy for metabolic activity. METHODS: We used positron emission tomography-computed tomography (PET-CT) imaging in combination with both glucose (18F-fluorodeoxyglucose) and lipid (18F-4-thiapalmitate) tracers to assess the effect of FGF21 on the tissue-specific uptake of these metabolites and compared responses to a control group pair-fed to match the food intake of the FGF21-treated group. In vivo imaging was combined with ex vivo tissue-specific functional, biochemical, and molecular analyses of the nutrient uptake and signaling pathways. RESULTS: Consistent with previous findings, FGF21 reduced body weight via reduced caloric intake and increased energy expenditure in the Siberian hamster. PET-CT studies demonstrated that FGF21 increased the uptake of glucose in BAT and WAT independently of reduced food intake and body weight as demonstrated by imaging of the pair-fed group. Furthermore, FGF21 increased glucose uptake in the primary adipocytes, confirming that these in vivo effects may be due to a direct action of FGF21 at the level of the adipocytes. Mechanistically, the effects of FGF21 are associated with activation of the ERK signaling pathway and upregulation of GLUT4 protein content in all fat depots. In response to treatment with FGF21, we observed an increase in the markers of lipolysis and lipogenesis in both the subcutaneous and visceral WAT depots. In contrast, FGF21 was only able to directly increase the uptake of lipid into BAT. CONCLUSIONS: These data identify brown and white fat depots as primary peripheral sites of action of FGF21 in promoting glucose uptake and also indicate that FGF21 selectively stimulates lipid uptake in brown fat, which may fuel thermogenesis

Reduced adiposity attenuates FGF21 mediated metabolic improvements in the Siberian hamster

FGF21 exerts profound metabolic effects in Siberian hamsters exposed to long day (LD) photoperiods that increase appetite and adiposity, however these effects are attenuated in short day (SD) animals that display hypophagia and reduced adiposity. The aim of this study was to investigate whether the beneficial effects of a novel mimetic of FGF21 in the LD state are a consequence of increased adiposity or of the central photoperiodic state. This was achieved by investigating effects of FGF21 in aged hamsters, which is associated with reduced adiposity. In LD hamsters with increased adiposity, FGF21 lowered body weight as a result of both reduced daily food intake and increased caloric expenditure, driven by an increase in whole-body fat oxidation. However, in LD animals with reduced adiposity, the effect of FGF21 on body weight, caloric intake and fat oxidation were significantly attenuated or absent when compared to those with increased adiposity. These attenuated/absent effects were underpinned by the inability of FGF21 to increase the expression of key thermogenic genes in interscapular and visceral WAT. Our study demonstrates the efficacy of a novel FGF21 mimetic in hamsters, but reveals attenuated effects in the animal model where adiposity is reduced naturally independent of photoperiod

Hypothalamic and brainstem glucose-dependent insulinotropic polypeptide receptor neurons employ distinct mechanisms to affect feeding

Central glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) signaling is critical in GIP-based therapeutics’ ability to lower body weight, but pathways leveraged by GIPR pharmacology in the brain remain incompletely understood. We explored the role of Gipr neurons in the hypothalamus and dorsal vagal complex (DVC) — brain regions critical to the control of energy balance. Hypothalamic Gipr expression was not necessary for the synergistic effect of GIPR/GLP-1R coagonism on body weight. While chemogenetic stimulation of both hypothalamic and DVC Gipr neurons suppressed food intake, activation of DVC Gipr neurons reduced ambulatory activity and induced conditioned taste avoidance, while there was no effect of a short-acting GIPR agonist (GIPRA). Within the DVC, Gipr neurons of the nucleus tractus solitarius (NTS), but not the area postrema (AP), projected to distal brain regions and were transcriptomically distinct. Peripherally dosed fluorescent GIPRAs revealed that access was restricted to circumventricular organs in the CNS. These data demonstrate that Gipr neurons in the hypothalamus, AP, and NTS differ in their connectivity, transcriptomic profile, peripheral accessibility, and appetite-controlling mechanisms. These results highlight the heterogeneity of the central GIPR signaling axis and suggest that studies into the effects of GIP pharmacology on feeding behavior should consider the interplay of multiple regulatory pathways

FGF21 is an insulin-dependent postprandial hormone in adult humans

Context: Fibroblast growth factor 21 (FGF21) secretion has been shown to respond directly to carbohydrate consumption, with glucose, fructose and sucrose all reported to increase plasma levels of FGF21 in rodents and humans. However, carbohydrate consumption also results in secretion of insulin. Objective: The aim of this study was to examine the combined and independent effects of hyperglycemia and hyperinsulinemia on total and bioactive FGF21 in the postprandial period in humans, and determine whether this effect is attenuated in conditions of altered insulin secretion and action. Methods: Circulating glucose, insulin, total and bioactive FGF21 and fibroblast activation protein (FAPα) were measured in adults with and without type 2 diabetes (T2D) following an oral glucose tolerance test (OGTT), and under a series of insulin and glucose clamp conditions and following high fat diet in healthy adults. Results: Circulating total and bioactive FGF21 levels responded acutely to OGTT, and their ratio was attenuated in T2D patients with reduced postprandial insulin response. The clamp studies revealed that insulin but not glucose accounts for the postprandial rise in FGF21. Finally, there was an attenuated rise in FGF21 in response to a high fat dietary intervention that is known to alter insulin-stimulated substrate utilization in metabolically active tissues. Conclusions: Insulin rather than glucose per se increases total and bioactive FGF21 in the postprandial period in adult humans. Understanding the impact of T2D on bioactive FGF21 will have a significant effect upon the efficacy of therapeutic agents designed to target the FGF21 pathway

Antibody-mediated inhibition of the FGFR1c isoform induces a catabolic lean state in Siberian hamsters

Hypothalamic tanycytes are considered to function as sensors of peripheral metabolism [1]. To facilitate this role, they express a wide range of receptors, including fibroblast growth factor receptor 1 (FGFR1). Using a monoclonal antibody (IMC-H7) that selectively antagonizes the FGFR1c isoform [2], we investigated possible actions of FGFR1c in a natural animal model of adiposity, the Siberian hamster. Infusion of IMC-H7 into the third ventricle suppressed appetite and increased energy expenditure. Likewise, peripheral treatment with IMC-H7 decreased appetite and body weight and increased energy expenditure and fat oxidation. A greater reduction in body weight and caloric intake was observed in response to IMC-H7 during the long-day fat state as compared to the short-day lean state. This enhanced response to IMC-H7 was also observed in calorically restricted hamsters maintained in long days, suggesting that it is the central photoperiodic state rather than the peripheral adiposity that determines the response to FGFR1c antagonism. Hypothalamic thyroid hormone availability is controlled by deiodinase enzymes (DIO2 and DIO3) expressed in tanycytes and is the key regulator of seasonal cycles of energy balance [3 and 4]. Therefore, we determined the effect of IMC-H7 on hypothalamic expression of these deiodinase enzymes. The reductions in food intake and body weight were always associated with decreased expression of DIO2 in the hypothalamic ependymal cell layer containing tanycytes. These data provide further support for the notion the tanycytes are an important component of the mechanism by which the hypothalamus integrates central and peripheral signals to regulate energy intake and expenditure

Hypothalamic and brainstem glucose-dependent insulinotropic polypeptide receptor neurons employ distinct mechanisms to affect feeding

Central glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) signaling is critical in GIP-based therapeutics’ ability to lower body weight, but pathways leveraged by GIPR pharmacology in the brain remain incompletely understood. We explored the role of Gipr neurons in the hypothalamus and dorsal vagal complex (DVC) — brain regions critical to the control of energy balance. Hypothalamic Gipr expression was not necessary for the synergistic effect of GIPR/GLP-1R coagonism on body weight. While chemogenetic stimulation of both hypothalamic and DVC Gipr neurons suppressed food intake, activation of DVC Gipr neurons reduced ambulatory activity and induced conditioned taste avoidance, while there was no effect of a short-acting GIPR agonist (GIPRA). Within the DVC, Gipr neurons of the nucleus tractus solitarius (NTS), but not the area postrema (AP), projected to distal brain regions and were transcriptomically distinct. Peripherally dosed fluorescent GIPRAs revealed that access was restricted to circumventricular organs in the CNS. These data demonstrate that Gipr neurons in the hypothalamus, AP, and NTS differ in their connectivity, transcriptomic profile, peripheral accessibility, and appetite-controlling mechanisms. These results highlight the heterogeneity of the central GIPR signaling axis and suggest that studies into the effects of GIP pharmacology on feeding behavior should consider the interplay of multiple regulatory pathways