Antibody-Mediated Targeting of the FGFR1c Isoform Increases Glucose Uptake in White and Brown Adipose Tissue in Male Mice

Metabolic Messengers: fibroblast growth factor 1

While fibroblast growth factor (FGF) 1 is expressed in multiple tissues, only adipose-derived and brain FGF1 have been implicated in the regulation of metabolism. Adipose FGF1 production is upregulated in response to dietary stress and is essential for adipose tissue plasticity in these conditions. Similarly, in the brain, FGF1 secretion into the ventricular space and the adjacent parenchyma is increased after a hypercaloric challenge induced by either feeding or glucose infusion. Potent anorexigenic properties have been ascribed to both peripheral and centrally injected FGF1. The ability of recombinant FGF1 and variants with reduced mitogenicity to lower glucose, suppress adipose lipolysis and promote insulin sensitization elevates their potential as candidates in the treatment of type 2 diabetes mellitus and associated comorbidities. Here, we provide an overview of the known metabolic functions of endogenous FGF1 and discuss its therapeutic potential, distinguishing between peripherally or centrally administered FGF1.

Beta-klotho in type 2 diabetes mellitus: From pathophysiology to therapeutic strategies

Type 2 diabetes mellitus (T2DM) has become a global health problem with no cure. Despite lifestyle modifications and various pharmaceutical options, the achievement of stable and durable glucose control along with effective prevention of T2DM-related cardiovascular complications remains a challenging task in clinical management. With its selective high abundance in metabolic tissues (adipose tissue, liver, and pancreas), β-Klotho is the essential component of fibroblast growth factor (FGF) receptor complexes. It is essential for high-affinity binding of endocrine FGF19 and FGF21 to evoke the signaling cascade actively involved in homeostatic maintenance of glucose metabolism and energy expenditure. In this Review, we discuss the biological function of β-Klotho in the regulation of glucose metabolism and offer mechanistic insights into its involvement in the pathophysiology of T2DM. We review our current understanding of the endocrine axis comprised of β-Klotho and FGFs (FGF19 and FGF21) and its regulatory effects on glucose metabolism under physiological and T2DM conditions. We also highlight advances in the development and preclinical validation of pharmacological compounds that target β-Klotho and/or the β-Klotho-FGFRs complex for the treatment of T2DM. Given the remarkable advances in this field, we also discuss outstanding research questions and the many challenges in the clinical development of β-Klotho-based therapies.

Adipose-derived exosomal miR-210/92a cluster inhibits adipose browning via the FGFR-1 signaling pathway in high-altitude hypoxia

Cold and hypoxia are critical drivers of adaptation to high altitudes. Organisms at high altitudes have adapted to maximize the efficiency of oxygen utilization and are less prone to obesity and diabetes than those at low altitudes. Brown adipose tissue (BAT) dissipates energy in the form of heat in both humans and rodents; it also serves to regulate metabolism to curb obesity. However, the role of BAT in high-altitude populations is poorly understood. Serum exosomes can be easily obtained, enabling the study of BAT functions and identification of biomarkers in serum exosomes, both of which contribute to understanding the role of BAT in high-altitude populations. ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography integrated with computed tomography (PET/CT) is the gold standard for studying BAT in human adults. Here, we studied BAT in healthy high-altitude populations via PET/CT and serum exosomal microRNAs (miRNAs). The observations were validated in mouse tissues and demonstrated that high-altitude hypoxia activated BAT through attenuated white adipose tissue (WAT) secreted exosomal miR-210/92a, which enhanced the FGFR-1 expression in BAT.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

Progress in promising anti-fibrotic therapies

Introduction: Liver cirrhosis is the most deleterious consequence of chronic liver diseases of different etiologies. Progression of liver diseases to cirrhosis, irrespective of etiology, involves chronic parenchymal injury, persistent activation of inflammatory response, sustained activation of fibrogenesis and wound-healing response. Despite intensive research on antifibrotic drugs, novel therapeutics specifically for liver have not been yet licensed. This review will examine compounds currently under development and key challenges in specific settings as for example that of NAFLD associated fibrosis.Areas covered: Results of the main phase II and III trial, including those with negative results, are presented and discussed. The endpoints selected and their limitations highlighted in order to suggest potential options to move forward.Expert opinion: Strategies based on single-molecule targets, associated so far with some disappointing results, may be unlikely to succeed in the context of such complex pathogenesis. Blocking at the same time different pathways that drive fibrosis progression may be required to provide significant benefit.

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.

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FGF21 increases glucose and lipid uptake in adipose tissue.

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The selective FGF21-induced increase in lipid uptake in BAT may fuel thermogenesis.

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Unlike BAT, glucose uptake in WAT may be used for lipogenesis.

Photoperiodic changes in adiposity increase sensitivity of female Siberian hamsters to systemic VGF derived peptide TLQP-21

TLQP-21, a peptide encoded by the highly conserved vgf gene, is expressed in neuroendocrine cells and has been the most prominent VGF-derived peptide studied in relation to control of energy balance. The recent discovery that TLQP-21 is the natural agonist for the complement 3a receptor 1 (C3aR1) has revived interest in this peptide as a potential drug target for obesity. We have investigated its function in Siberian hamsters (Phodopus sungorus), a rodent that displays natural seasonal changes in body weight and adiposity as an adaptation to survive winter. We have previously shown that intracerebroventricular administration of TLQP-21 reduced food intake and body weight in hamsters in their long-day fat state. The aim of our current study was to determine the systemic actions of TLQP-21 on food intake, energy expenditure and body weight, and to establish whether adiposity affected these responses. Peripheral infusion of TLQP-21 (1mg/kg/day for 7 days) in lean hamsters exposed to short photoperiods (SP) reduced cumulative food intake in the home cage (p<0.05), and intake when measured in metabolic cages (P<0.01). Energy expenditure was significantly increased (p<0.001) by TLQP-21 infusion, this was associated with a significant increase in uncoupling protein 1 mRNA in brown adipose tissue (BAT) (p<0.05), and body weight was significantly reduced (p<0.05). These effects of systemic TLQP-21 treatment were not observed in hamsters exposed to long photoperiod (LP) with a fat phenotype. C3aR1 mRNA and protein were abundantly expressed in the hypothalamus, brown and white adipose tissue in hamsters, but changes in expression cannot explain the differential response to TLQP-21 in lean and fat hamsters.

Peptide-Based Regulation of Metabolism as Related to Obesity

Currently there are few, if any, approved and effective medicines for the attenuation of obesity, diabetes, and insulin resistance. This commentary addresses a communication describing the effect on glucose dynamics and obesity of a peptide monoclonal antibody for fibroblast growth factor (FGF) receptor 1c isoform (FGFR1c). The general lack of suitable, effective drugs is discussed, as is the treatment potential of FGF family receptors. The FGFR1c monoclonal antibody developed by the authors lowers body weight gain, blood glucose, and adipose tissue weight. It also enhances glucose uptake by fat cells [white adipose tissue (WAT) and 3T3-L1]. The robust weight loss, fat loss, and lower blood glucose were attributed to an observed potential futile cycle of continuous lipogenesis and lipolysis and adenosine triphosphate use in WAT.