The autocrine role of FGF21 in cultured adipocytes

Characterization of FGF21 Sites of Production and Signaling in Mice

Fibroblast growth factor (FGF) 21 is an endocrine hormone that signals to multiple tissues to regulate metabolism. FGF21 and another endocrine FGF, FGF15/19, signal to target tissues by binding to the co-receptor β-klotho (KLB), which then facilitates the interaction of these different FGFs with their preferred FGF receptor. KLB is expressed in multiple metabolic tissues, but the specific cell types and spatial distribution of these cells are not known. Furthermore, while circulating FGF21 is primarily produced by the liver, recent publications have indicated that brain-derived FGF21 impacts memory and learning. Here we use reporter mice to comprehensively assess KLB and FGF21 expression throughout the body. These data provide an important resource for guiding future studies to identify important peripheral and central targets of FGFs and to determine the significance of nonhepatic FGF21 production.

Critical role of FGF21 in diabetic kidney disease: from energy metabolism to innate immunity

Diabetic kidney disease (DKD) stands as the predominant cause of chronic kidney disease (CKD) on a global scale, with its incidence witnessing a consistent annual rise, thereby imposing a substantial burden on public health. The pathogenesis of DKD is primarily rooted in metabolic disorders and inflammation. Recent years have seen a surge in studies highlighting the regulatory impact of energy metabolism on innate immunity, forging a significant area of research interest. Within this context, fibroblast growth factor 21 (FGF21), recognized as an energy metabolism regulator, assumes a pivotal role. Beyond its role in maintaining glucose and lipid metabolism homeostasis, FGF21 exerts regulatory influence on innate immunity, concurrently inhibiting inflammation and fibrosis. Serving as a nexus between energy metabolism and innate immunity, FGF21 has evolved into a therapeutic target for diabetes, nonalcoholic steatohepatitis, and cardiovascular diseases. While the relationship between FGF21 and DKD has garnered increased attention in recent studies, a comprehensive exploration of this association has yet to be systematically addressed. This paper seeks to fill this gap by summarizing the mechanisms through which FGF21 operates in DKD, encompassing facets of energy metabolism and innate immunity. Additionally, we aim to assess the diagnostic and prognostic value of FGF21 in DKD and explore its potential role as a treatment modality for the condition.

Brown adipose tissue-derived metabolites and their role in regulating metabolism

The discovery and rejuvenation of metabolically active brown adipose tissue (BAT) in adult humans have offered a new approach to treat obesity and metabolic diseases. Beyond its accomplished role in adaptive thermogenesis, BAT secretes signaling molecules known as "batokines", which are instrumental in regulating whole-body metabolism via autocrine, paracrine, and endocrine action. In addition to the intrinsic BAT metabolite-oxidizing activity, the endocrine functions of these molecules may help to explain the association between BAT activity and a healthy systemic metabolic profile. Herein, we review the evidence that underscores the significance of BAT-derived metabolites, especially highlighting their role in controlling physiological and metabolic processes involving thermogenesis, substrate metabolism, and other essential biological processes. The conversation extends to their capacity to enhance energy expenditure and mitigate features of obesity and its related metabolic complications. Thus, metabolites derived from BAT may provide new avenues for the discovery of metabolic health-promoting drugs with far-reaching impacts. This review aims to dissect the complexities of the secretory role of BAT in modulating local and systemic metabolism in metabolic health and disease.

FGF21 alleviates adipose stem cell senescence via CD90 glycosylation-dependent glucose influx in remodeling healthy white adipose tissue

The senescence of adipose stem cells (ASCs) impairs healthy adipose tissue remodeling, causing metabolic maladaptation to energy surplus. The intrinsic molecular pathways and potential therapy targets for ASC senescence are largely unclear. Here, we showed that visceral ASCs were prone to senescence that was caused by reactive oxygen species (ROS) overload, especially mitochondrial ROS. These senescent ASCs failed to sustain efficient glucose influx, pentose phosphate pathway (PPP) and redox homeostasis. We showed that CD90 silence restricted the glucose uptake by ASCs and thus disrupted their PPP and anti-oxidant system, resulting in ASC senescence. Notably, fibroblast growth factor 21 (FGF21) treatment significantly reduced the senescent phenotypes of ASCs by augmenting CD90 protein via glycosylation, which promoted glucose influx via the AKT-GLUT4 axis and therefore mitigated ROS overload. For diet-induced obese mice, chronic administration of low-dose FGF21 relieved their visceral white adipose tissue (VAT) dysfunction and systemic metabolic disorders. In particular, VAT homeostasis was restored in FGF21-treated obese mice, where ASC repertoire was markedly recovered, accompanied by CD90 elevation and anti-senescent phenotypes in these ASCs. Collectively, we reveal a molecular mechanism of ASC senescence by which CD90 downregulation interferes glucose influx into PPP and redox homeostasis. And we propose a FGF21-based strategy for healthy VAT remodeling, which targets CD90 glycosylation to correct ASC senescence and therefore combat obesity-related metabolic dysfunction.

Beige Adipocyte as the Flame of White Adipose Tissue: Regulation of Browning and Impact of Obesity

Beige adipocyte, the third and relatively new type of adipocyte, can emerge in white adipose tissue (WAT) under thermogenic stimulations that is termed as browning of WAT. Recent studies suggest that browning of WAT deserves more attention and therapies targeting browning of WAT can be helpful for reducing obesity. Beyond the major inducers of browning, namely cold and β 3-adrenergic stimulation, beige adipocytes are affected by several factors, and excess adiposity per se may also influence the browning process. The objective of the present review is to provide an overview of recent clinical and preclinical studies on the hormonal and nonhormonal factors that affect the browning of WAT. This review further focuses on the role of obesity per se on browning process.

Endocrine Fibroblast Growth Factors in Relation to Stress Signaling

Fibroblast growth factors (FGFs) play important roles in various growth signaling processes, including proliferation, development, and differentiation. Endocrine FGFs, i.e., atypical FGFs, including FGF15/19, FGF21, and FGF23, function as endocrine hormones that regulate energy metabolism. Nutritional status is known to regulate the expression of endocrine FGFs through nuclear hormone receptors. The increased expression of endocrine FGFs regulates energy metabolism processes, such as fatty acid metabolism and glucose metabolism. Recently, a relationship was found between the FGF19 subfamily and stress signaling during stresses such as endoplasmic reticulum stress and oxidative stress. This review focuses on endocrine FGFs and the recent progress in FGF studies in relation to stress signaling. In addition, the relevance of the stress-FGF pathway to disease and human health is discussed.

FGF21 promotes thermogenic gene expression as an autocrine factor in adipocytes

The contribution of adipose-derived FGF21 to energy homeostasis is unclear. Here we show that browning of inguinal white adipose tissue (iWAT) by β-adrenergic agonists requires autocrine FGF21 signaling. Adipose-specific deletion of the FGF21 co-receptor β-Klotho renders mice unresponsive to β-adrenergic stimulation. In contrast, mice with liver-specific ablation of FGF21, which eliminates circulating FGF21, remain sensitive to β-adrenergic browning of iWAT. Concordantly, transgenic overexpression of FGF21 in adipocytes promotes browning in a β-Klotho-dependent manner without increasing circulating FGF21. Mechanistically, we show that β-adrenergic stimulation of thermogenic gene expression requires FGF21 in adipocytes to promote phosphorylation of phospholipase C-γ and mobilization of intracellular calcium. Moreover, we find that the β-adrenergic-dependent increase in circulating FGF21 occurs through an indirect mechanism in which fatty acids released by adipocyte lipolysis subsequently activate hepatic PPARα to increase FGF21 expression. These studies identify FGF21 as a cell-autonomous autocrine regulator of adipose tissue function.

Abu-Odeh et al. demonstrate that autocrine action of FGF21 is a required second signal promoting thermogenic gene expression in catecholamine-stimulated adipocytes. Hepatic FGF21 secretions, secondary to catecholamine-stimulated adipocyte lipolysis, are dispensable for adipose tissue browning. These studies identify FGF21 as a cell-autonomous autocrine regulator of adipose tissue function.

Leptin, Acting at Central Level, Increases FGF21 Expression in White Adipose Tissue via PPARβ/δ

The altered function of adipose tissue can result in obesity, insulin resistance, and its metabolic complications. Leptin, acting on the central nervous system, modifies the composition and function of adipose tissue. To date, the molecular changes that occur in epididymal white adipose tissue (eWAT) during chronic leptin treatment are not fully understood. Herein we aimed to address whether PPARβ/δ could mediate the metabolic actions induced by leptin in eWAT. To this end, male 3-month-old Wistar rats, infused intracerebroventricularly (icv) with leptin (0.2 μg/day) for 7 days, were daily co-treated intraperitoneally (ip) without or with the specific PPARβ/δ receptor antagonist GSK0660 (1 mg/kg/day). In parallel, we also administered GSK0660 to control rats fed ad libitum without leptin infusion. Leptin, acting at central level, prevented the starvation-induced increase in circulating levels of FGF21, while induced markedly the endogenous expression of FGF21 and browning markers of eWAT. Interestingly, GSK0660 abolished the anorectic effects induced by icv leptin leading to increased visceral fat mass and reduced browning capacity. In addition, the pharmacological inhibition of PPARβ/δ alters the immunomodulatory actions of central leptin on eWAT. In summary, our results demonstrate that PPARβ/δ is involved in the up-regulation of FGF21 expression induced by leptin in visceral adipose tissue.

Levels of β-klotho determine the thermogenic responsiveness of adipose tissues: involvement of the autocrine action of FGF21

Fibroblast growth factor-21 (FGF21) is a hormonal regulator of metabolism; it promotes glucose oxidation and the thermogenic capacity of adipose tissues. The levels of β-klotho (KLB), the co-receptor required for FGF21 action, are decreased in brown (BAT) and white (WAT) adipose tissues during obesity, diabetes, and lipodystrophy. Reduced β-klotho levels have been proposed to account for FGF21 resistance in these conditions. In this study, we explored whether downregulation of β-klotho affects metabolic regulation and the thermogenic responsiveness of adipose tissues using mice with total (KLB-KO) or partial (KLB-heterozygotes) ablation of β-klotho. We herein show that KLB gene dosage was inversely associated with adiposity in mice. Upon cold exposure, impaired browning of subcutaneous WAT and milder alterations in BAT were associated with reduced KLB gene dosage in mice. Cultured brown and beige adipocytes from mice with total or partial ablation of the KLB gene showed reduced thermogenic responsiveness to β3-adrenergic activation by treatment with CL316,243, indicating that these effects were cell-autonomous. Deficiency in FGF21 mimicked the KLB-reduction-induced impairment of thermogenic responsiveness in brown and beige adipocytes. These results indicate that the levels of KLB in adipose tissues determine their thermogenic capacity to respond to cold and/or adrenergic stimuli. Moreover, an autocrine action of FGF21 in brown and beige adipocytes may account for the ability of the KLB level to influence thermogenic responsiveness.NEW & NOTEWORTHY Reduced levels of KLB (the obligatory FGF21 co-receptor), as occurring in obesity and type 2 diabetes, reduce the thermogenic responsiveness of adipose tissues in cold-exposed mice. Impaired response to β3-adrenergic activation in brown and beige adipocytes with reduced KLB occurs in a cell-autonomous manner involving an autocrine action of FGF21.

Metabolic Messengers: FGF21

As a non-canonical fibroblast growth factor, fibroblast growth factor 21 (FGF21) functions as an endocrine hormone that signals to distinct targets throughout the body. Interest in therapeutic applications for FGF21 was initially sparked by its ability to correct metabolic dysfunction and decrease body weight associated with diabetes and obesity. More recently, new functions for FGF21 signalling have emerged, thus indicating that FGF21 is a dynamic molecule capable of regulating macronutrient preference and energy balance. Here, we highlight the major physiological and pharmacological effects of FGF21 related to nutrient and energy homeostasis and summarize current knowledge regarding FGF21’s pharmacodynamic properties. In addition, we provide new perspectives and highlight critical unanswered questions surrounding this unique metabolic messenger.

The Nuanced Metabolic Functions of Endogenous FGF21 Depend on the Nature of the Stimulus, Tissue Source, and Experimental Model

Fibroblast growth factor 21 (FGF21) is a hormone that is involved in the regulation of lipid, glucose, and energy metabolism. Pharmacological FGF21 administration promotes weight loss and improves insulin sensitivity in rodents, non-human primates, and humans. However, pharmacologic effects of FGF21 likely differ from its physiological effects. Endogenous FGF21 is produced by many cell types, including hepatocytes, white and brown adipocytes, skeletal and cardiac myocytes, and pancreatic beta cells, and acts on a diverse array of effector tissues such as the brain, white and brown adipose tissue, heart, and skeletal muscle. Different receptor expression patterns dictate FGF21 function in these target tissues, with the primary effect to coordinate responses to nutritional stress. Moreover, different nutritional stimuli tend to promote FGF21 expression from different tissues; i.e., fasting induces hepatic-derived FGF21, while feeding promotes white adipocyte-derived FGF21. Target tissue effects of FGF21 also depend on its capacity to enter the systemic circulation, which varies widely from known FGF21 tissue sources in response to various stimuli. Due to its association with obesity and non-alcoholic fatty liver disease, the metabolic effects of endogenously produced FGF21 during the pathogenesis of these conditions are not well known. In this review, we will highlight what is known about endogenous tissue-specific FGF21 expression and organ cross-talk that dictate its diverse physiological functions, with particular attention given to FGF21 responses to nutritional stress. The importance of the particular experimental design, cellular and animal models, and nutritional status in deciphering the diverse metabolic functions of endogenous FGF21 cannot be overstated.