Time-imposed daily restricted feeding induces rhythmic expression of Fgf21 in white adipose tissue of mice

Exploring endocrine FGFs - structures, functions and biomedical applications

The family of fibroblast growth factors (FGFs) consists of 22 members with diverse biological functions in cells, from cellular development to metabolism. The family can be further categorized into three subgroups based on their three modes of action. FGF19, FGF21, and FGF23 are endocrine FGFs that act in a hormone-like/endocrine manner to regulate various metabolic activities. However, all three members of the endocrine family require both FGF receptors (FGFRs) and klotho co-receptors to elicit their functions. α-klotho and β-klotho act as scaffolds to bring endocrine FGFs closer to their receptors (FGFRs) to form active complexes. Numerous novel studies about metabolic FGFs' structures, mechanisms, and physiological insights have been published to further understand the complex molecular interactions and physiological activities of endocrine FGFs. Herein, we aim to review the structures, physiological functions, binding mechanisms to cognate receptors, and novel biomedical applications of endocrine FGFs in recent years.

Circadian Regulation of Endocrine Fibroblast Growth Factors on Systemic Energy Metabolism

The circadian clock is an endogenous biochemical timing system that coordinates the physiology and behavior of organisms to earth's ∼24-hour circadian day/night cycle. The central circadian clock synchronized by environmental cues hierarchically entrains peripheral clocks throughout the body. The circadian system modulates a wide variety of metabolic signaling pathways to maintain whole-body metabolic homeostasis in mammals under changing environmental conditions. Endocrine fibroblast growth factors (FGFs), namely FGF15/19, FGF21, and FGF23, play an important role in regulating systemic metabolism of bile acids, lipids, glucose, proteins, and minerals. Recent evidence indicates that endocrine FGFs function as nutrient sensors that mediate multifactorial interactions between peripheral clocks and energy homeostasis by regulating the expression of metabolic enzymes and hormones. Circadian disruption induced by environmental stressors or genetic ablation is associated with metabolic dysfunction and diurnal disturbances in FGF signaling pathways that contribute to the pathogenesis of metabolic diseases. Time-restricted feeding strengthens the circadian pattern of metabolic signals to improve metabolic health and prevent against metabolic diseases. Chronotherapy, the strategic timing of medication administration to maximize beneficial effects and minimize toxic effects, can provide novel insights into linking biologic rhythms to drug metabolism and toxicity within the therapeutical regimens of diseases. Here we review the circadian regulation of endocrine FGF signaling in whole-body metabolism and the potential effect of circadian dysfunction on the pathogenesis and development of metabolic diseases. We also discuss the potential of chrononutrition and chronotherapy for informing the development of timing interventions with endocrine FGFs to optimize whole-body metabolism in humans. SIGNIFICANCE STATEMENT: The circadian timing system governs physiological, metabolic, and behavioral functions in living organisms. The endocrine fibroblast growth factor (FGF) family (FGF15/19, FGF21, and FGF23) plays an important role in regulating energy and mineral metabolism. Endocrine FGFs function as nutrient sensors that mediate multifactorial interactions between circadian clocks and metabolic homeostasis. Chronic disruption of circadian rhythms increases the risk of metabolic diseases. Chronological interventions such as chrononutrition and chronotherapy provide insights into linking biological rhythms to disease prevention and treatment.

A ketogenic diet containing medium-chain triglycerides reduces REM sleep duration without significant influence on mouse circadian phenotypes

Ketogenic diets (KDs) affect the circadian rhythms of behavior and clock gene expression in experimental animals. However, these diets were designed to simulate a fasting state; thus, whether these effects are caused by diet-induced ketogenesis or persistent starvation is difficult to distinguish. The present study aimed to define the effects of a KD containing medium-chain triglycerides (MCT-KD) that increase blood ketone levels without inducing carbohydrate starvation, on circadian rhythms and sleep regulation. Mice were fed with a normal diet (CTRL) or MCT-KD for 2 weeks. Blood β-hydroxybutyrate levels were significantly increased up to 2 mM by the MCT-KD, whereas body weight gain and blood glucose levels were identical between the groups, suggesting that ketosis accumulated without carbohydrate starvation in the MCT-KD mice. Circadian rhythms of wheel-running activity and core body temperature were almost identical, although wheel-running was slightly reduced in the MCT-KD mice. The circadian expression of the core clock genes, Per1, Per2, Bmal1, and Dbp in the hypothalamus, heart, liver, epididymal adipose tissues, and skeletal muscle were almost identical between the CTRL and MCT-KD mice, whereas the amplitude of hepatic Per2 and adipose Per1 expression was increased in MCT-KD mice. The MCT-KD reduced the duration of rapid-eye-movement (REM) sleep without affecting the duration of non-REM sleep and the duration of wakefulness. These findings suggested that the impact of ketone bodies on circadian systems are limited, although they might reduce locomotor activity and REM sleep duration.

The potential function and clinical application of FGF21 in metabolic diseases

As an endocrine hormone, fibroblast growth factor 21 (FGF21) plays a crucial role in regulating lipid, glucose, and energy metabolism. Endogenous FGF21 is generated by multiple cell types but acts on restricted effector tissues, including the brain, adipose tissue, liver, heart, and skeletal muscle. Intervention with FGF21 in rodents or non-human primates has shown significant pharmacological effects on a range of metabolic dysfunctions, including weight loss and improvement of hyperglycemia, hyperlipidemia, insulin resistance, cardiovascular disease, and non-alcoholic fatty liver disease (NAFLD). Due to the poor pharmacokinetic and biophysical characteristics of native FGF21, long-acting FGF21 analogs and FGF21 receptor agonists have been developed for the treatment of metabolic dysfunction. Clinical trials of several FGF21-based drugs have been performed and shown good safety, tolerance, and efficacy. Here we review the actions of FGF21 and summarize the associated clinical trials in obesity, type 2 diabetes mellitus (T2DM), and NAFLD, to help understand and promote the development of efficient treatment for metabolic diseases via targeting FGF21.

Plasma FGF21 Levels Are Not Associated with Weight Loss or Improvements in Metabolic Health Markers upon 12 Weeks of Energy Restriction: Secondary Analysis of an RCT

Recent studies suggest that circulating fibroblast growth factor 21 (FGF21) may be a marker of metabolic health status. We performed a secondary analysis of a 12-week randomized controlled trial to investigate the effects of two energy restriction (ER) diets on fasting and postprandial plasma FGF21 levels, as well as to explore correlations of plasma FGF21 with metabolic health markers, (macro)nutrient intake and sweet-taste preference. Abdominally obese subjects aged 40-70 years (n = 110) were randomized to one of two 25% ER diets (high-nutrient-quality diet or low-nutrient-quality diet) or a control group. Plasma FGF21 was measured in the fasting state and 120 min after a mixed meal. Both ER diets did not affect fasting or postprandial plasma FGF21 levels despite weight loss and accompanying health improvements. At baseline, the postprandial FGF21 response was inversely correlated to fasting plasma glucose (ρ = -0.24, p = 0.020) and insulin (ρ = -0.32, p = 0.001), HOMA-IR (ρ = -0.34, p = 0.001), visceral adipose tissue (ρ = -0.24, p = 0.046), and the liver enzyme aspartate aminotransferase (ρ = -0.23, p = 0.021). Diet-induced changes in these markers did not correlate to changes in plasma FGF21 levels upon intervention. Baseline higher habitual polysaccharide intake, but not mono- and disaccharide intake or sweet-taste preference, was related to lower fasting plasma FGF21 (p = 0.022). In conclusion, we found no clear evidence that fasting plasma FGF21 is a marker for metabolic health status. Circulating FGF21 dynamics in response to an acute nutritional challenge may reflect metabolic health status better than fasting levels.

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.

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.

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.

Time-restricted feeding alleviates cardiac dysfunction induced by simulated microgravity via restoring cardiac FGF21 signaling

Dietary restriction has been well-described to improve health metrics, but whether it could benefit pathophysiological adaptation to extreme environment, for example, microgravity, remains unknown. Here, we investigated the effects of a daily rhythm of fasting and feeding without reducing caloric intake on cardiac function and metabolism against simulated microgravity. Male rats under ad libitum feeding or time-restricted feeding (TRF; food access limited to 8 hours every day) were subjected to hindlimb unloading (HU) to simulate microgravity. HU for 6 weeks led to left ventricular dyssynchrony and declined cardiac function. HU also lowered pyruvate dehydrogenase (PDH) activity and impaired glucose utilization in the heart. All these were largely preserved by TRF. TRF showed no effects on HU-induced loss of cardiac mass, but significantly improved contractile function of cardiomyocytes. Interestingly, TRF raised liver-derived fibroblast growth factor 21 (FGF21) level and enhanced cardiac FGF21 signaling as manifested by upregulation of FGF receptor-1 (FGFR1) expression and its downstream markers in HU rats. In isolated cardiomyocytes, FGF21 treatment improved PDH activity and glucose utilization, consequently enhancing cell contractile function. Finally, both liver-specific knockdown (KD) of FGF21 and cardiac-specific FGFR1 KD abrogated the cardioprotective effects of TRF in HU rats. These data demonstrate that TRF improves cardiac glucose utilization and ameliorates cardiac dysfunction induced by simulated microgravity, at least partially, through restoring cardiac FGF21 signaling, suggesting TRF as a potential countermeasure for cardioprotection in long-term spaceflight.

Synchronization of the circadian clock by time-restricted feeding with progressive increasing calorie intake. Resemblances and differences regarding a sustained hypocaloric restriction

Circadian rhythms are the product of the interaction of molecular clocks and environmental signals, such as light-dark cycles and eating-fasting cycles. Several studies have demonstrated that the circadian rhythm of peripheral clocks, and behavioural and metabolic mediators are re-synchronized in rodents fed under metabolic challenges, such as hyper- or hypocaloric diets and subjected to time-restricted feeding protocols. Despite the metabolic challenge, these approaches improve the metabolic status, raising the enquiry whether removing progressively the hypocaloric challenge in a  time-restricted feeding protocol leads to metabolic benefits by the synchronizing effect. To address this issue, we compared the effects of two time-restricted feeding protocols, one involved hypocaloric intake during the entire protocol (HCT) and the other implied a progressive intake accomplishing a normocaloric intake at the end of the protocol (NCT) on several behavioural, metabolic, and molecular rhythmic parameters. We observed that the food anticipatory activity (FAA) was driven and maintained in both HCT and NCT. Resynchronization of hepatic molecular clock, free fatty acids (FFAs), and FGF21 was elicited closely by HCT and NCT. We further observed that the fasting cycles involved in both protocols promoted ketone body production, preferentially beta-hydroxybutyrate in HCT, whereas acetoacetate was favoured in NCT before access to food. These findings demonstrate that time-restricted feeding does not require a sustained calorie restriction for promoting and maintaining the synchronization of the metabolic and behavioural circadian clock, and suggest that metabolic modulators, such as FFAs and FGF21, could contribute to FAA expression.

Synchronization of the circadian clock by time-restricted feeding with progressive increasing calorie intake. Resemblances and differences regarding a sustained hypocaloric restriction

Circadian rhythms are the product of the interaction of molecular clocks and environmental signals, such as light-dark cycles and eating-fasting cycles. Several studies have demonstrated that the circadian rhythm of peripheral clocks, and behavioural and metabolic mediators are re-synchronized in rodents fed under metabolic challenges, such as hyper- or hypocaloric diets and subjected to time-restricted feeding protocols. Despite the metabolic challenge, these approaches improve the metabolic status, raising the enquiry whether removing progressively the hypocaloric challenge in a  time-restricted feeding protocol leads to metabolic benefits by the synchronizing effect. To address this issue, we compared the effects of two time-restricted feeding protocols, one involved hypocaloric intake during the entire protocol (HCT) and the other implied a progressive intake accomplishing a normocaloric intake at the end of the protocol (NCT) on several behavioural, metabolic, and molecular rhythmic parameters. We observed that the food anticipatory activity (FAA) was driven and maintained in both HCT and NCT. Resynchronization of hepatic molecular clock, free fatty acids (FFAs), and FGF21 was elicited closely by HCT and NCT. We further observed that the fasting cycles involved in both protocols promoted ketone body production, preferentially beta-hydroxybutyrate in HCT, whereas acetoacetate was favoured in NCT before access to food. These findings demonstrate that time-restricted feeding does not require a sustained calorie restriction for promoting and maintaining the synchronization of the metabolic and behavioural circadian clock, and suggest that metabolic modulators, such as FFAs and FGF21, could contribute to FAA expression.

Fibroblast Growth Factor 21 and the Adaptive Response to Nutritional Challenges

The Fibroblast Growth Factor 21 (FGF21) is considered an attractive therapeutic target for obesity and obesity-related disorders due to its beneficial effects in lipid and carbohydrate metabolism. FGF21 response is essential under stressful conditions and its metabolic effects depend on the inducer factor or stress condition. FGF21 seems to be the key signal which communicates and coordinates the metabolic response to reverse different nutritional stresses and restores the metabolic homeostasis. This review is focused on describing individually the FGF21-dependent metabolic response activated by some of the most common nutritional challenges, the signal pathways triggering this response, and the impact of this response on global homeostasis. We consider that this is essential knowledge to identify the potential role of FGF21 in the onset and progression of some of the most prevalent metabolic pathologies and to understand the potential of FGF21 as a target for these diseases. After this review, we conclude that more research is needed to understand the mechanisms underlying the role of FGF21 in macronutrient preference and food intake behavior, but also in β-klotho regulation and the activity of the fibroblast activation protein (FAP) to uncover its therapeutic potential as a way to increase the FGF21 signaling.

Fibroblast growth factors in control of lipid metabolism: from biological function to clinical application

PURPOSE OF REVIEW

Several members of the fibroblast growth factor (FGF) family have been identified as key regulators of energy metabolism in rodents and nonhuman primates. Translational studies show that their metabolic actions are largely conserved in humans, which led to the development of various FGF-based drugs, including FGF21-mimetics LY2405319, PF-05231023, and pegbelfermin, and the FGF19-mimetic NGM282. Recently, a number of clinical trials have been published that examined the safety and efficacy of these novel therapeutic proteins in the treatment of obesity, type 2 diabetes (T2D), nonalcoholic steatohepatitis (NASH), and cholestatic liver disease. In this review, we discuss the current understanding of FGFs in metabolic regulation and their clinical potential.

RECENT FINDINGS

FGF21-based drugs induce weight loss and improve dyslipidemia in patients with obesity and T2D, and reduce steatosis in patients with NASH. FGF19-based drugs reduce steatosis in patients with NASH, and ameliorate bile acid-induced liver damage in patients with cholestasis. In contrast to their potent antidiabetic effects in rodents and nonhuman primates, FGF-based drugs do not appear to improve glycemia in humans. In addition, various safety concerns, including elevation of low-density lipoprotein cholesterol, modulation of bone homeostasis, and increased blood pressure, have been reported as well.

SUMMARY

Clinical trials with FGF-based drugs report beneficial effects in lipid and bile acid metabolism, with clinical improvements in dyslipidemia, steatosis, weight loss, and liver damage. In contrast, glucose-lowering effects, as observed in preclinical models, are currently lacking.

Impact of aging and caloric restriction on fibroblast growth factor 21 signaling in rat white adipose tissue

Caloric restriction (CR) suppresses age-related pathophysiology and extends lifespan. We recently reported that metabolic remodeling of white adipose tissue (WAT) plays an important role in the beneficial actions of CR; however, the detailed molecular mechanisms of this remodeling remain to be established. In the present study, we aimed to identify CR-induced alterations in the expression of fibroblast growth factor 21 (FGF21), a regulator of lipid and glucose metabolism, and of its downstream signaling mediators in liver and WAT, across the lifespan of rats. We evaluated groups of rats that had been either fed ad libitum or calorie restricted from 3 months of age and were euthanized at 3.5, 9, or 24 months of age, under fed and fasted conditions. The expression of FGF21 mRNA and/or protein increased with age in liver and WAT. Interestingly, in the WAT of 9-month-old fed rats, CR further upregulated FGF21 expression and eliminated the aging-associated reductions in the expression of FGFR1 and beta-klotho (KLB; FGF21 receptor complex). It also enhanced the expression of FGF21 targets, including glucose transporter 1 and peroxisome proliferator-activated receptor (PPAR)γ coactivator-1α. The analysis of transcriptional regulators of Fgf21 suggested that aging and CR might upregulate Fgf21 expression via different mechanisms. In adipocytes in vitro, constitutive FGF21 overexpression upregulated the FGF21 receptor complex and FGF21 targets at the mRNA or protein level. Thus, both aging and CR induced FGF21 expression in rat WAT; however, only CR activated FGF21 signaling. Our results suggest that FGF21 signaling contributes to the CR-induced metabolic remodeling of WAT, likely activating glucose uptake and mitochondrial biogenesis.

An Ultradian Feeding Schedule in Rats Affects Metabolic Gene Expression in Liver, Brown Adipose Tissue and Skeletal Muscle with Only Mild Effects on Circadian Clocks

Restricted feeding is well known to affect expression profiles of both clock and metabolic genes. However, it is unknown whether these changes in metabolic gene expression result from changes in the molecular clock or in feeding behavior. Here we eliminated the daily rhythm in feeding behavior by providing 6 meals evenly distributed over the light/dark-cycle. Animals on this 6-meals-a-day feeding schedule retained the normal day/night difference in physiological parameters including body temperature and locomotor activity. The daily rhythm in respiratory exchange ratio (RER), however, was significantly phase-shifted through increased utilization of carbohydrates during the light phase and increased lipid oxidation during the dark phase. This 6-meals-a-day feeding schedule did not have a major impact on the clock gene expression rhythms in the master clock, but did have mild effects on peripheral clocks. In contrast, genes involved in glucose and lipid metabolism showed differential expression. In conclusion, eliminating the daily rhythm in feeding behavior in rats does not affect the master clock and only mildly affects peripheral clocks, but disturbs metabolic rhythms in liver, skeletal muscle and brown adipose tissue in a tissue-dependent manner. Thereby, a clear daily rhythm in feeding behavior strongly regulates timing of peripheral metabolism, separately from circadian clocks.

Regulation of fibroblast growth factor 15/19 and 21 on metabolism: in the fed or fasted state

Fibroblast growth factor (FGF) 15/19 and FGF21 are two atypical members of FGF19 subfamily that function as hormones. Exogenous FGF15/19 and FGF21 have pharmacological effects, and endogenous FGF15/19 and FGF21 play vital roles in the maintenance of energy homeostasis. Recent reports have expanded the effects of FGF15/19 and FGF21 on carbohydrate and lipid metabolism. However, the regulations of FGF15/19 and FGF21 on metabolism are different. FGF15/19 is mainly secreted from the small intestine in response to feeding, and FGF21 is secreted from the liver in response to extended fasting and from the liver and adipose tissue in response to feeding. In this work, we reviewed the regulatory effects of FGF15/19 and FGF21 on metabolism in the fast and fed states. This information may provide some insight into the metabolic regulation of FGF15/19 and FGF21 in different physiological condition.

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.

Free access to a running-wheel advances the phase of behavioral and physiological circadian rhythms and peripheral molecular clocks in mice

Behavioral and physiological circadian rhythms are controlled by endogenous oscillators in animals. Voluntary wheel-running in rodents is thought to be an appropriate model of aerobic exercise in humans. We evaluated the effects of chronic voluntary exercise on the circadian system by analyzing temporal profiles of feeding, core body temperature, plasma hormone concentrations and peripheral expression of clock and clock-controlled genes in mice housed under sedentary (SED) conditions or given free access to a running-wheel (RW) for four weeks. Voluntary wheel-running activity advanced the circadian phases of increases in body temperature, food intake and corticosterone secretion in the mice. The circadian expression of clock and clock-controlled genes was tissue- and gene-specifically affected in the RW mice. The temporal expression of E-box-dependent circadian clock genes such as Per1, Per2, Nr1d1 and Dbp were slightly, but significantly phase-advanced in the liver and white adipose tissue, but not in brown adipose tissue and skeletal muscle. Peak levels of Per1, Per2 and Nr1d1 expression were significantly increased in the skeletal muscle of RW mice. The circadian phase and levels of hepatic mRNA expression of the clock-controlled genes that are involved in cholesterol and fatty acid metabolism significantly differed between SED and RW mice. These findings indicated that endogenous clock-governed voluntary wheel-running activity provides feedback to the central circadian clock that systemically governs behavioral and physiological rhythms.

The role of fibroblast growth factor 21 in the pathogenesis of liver disease: a novel predictor and therapeutic target

INTRODUCTION

Fibroblast growth factor 21 (FGF21) is one of the FGF family members that is produced mainly by tissues with high metabolic activity such as liver, pancreas, muscle and adipose tissue. The major function of FGF21 is to improve insulin sensitivity, ameliorate hepatic steatosis and enhance energy expenditure. Recently, several studies have reported a correlation between FGF21 and liver disease with numerous cross-sectional studies demonstrating significant correlation. This review will focus on the role of FGF21 in the pathogenesis of liver disease and its potential role as a biomarker and a new target for therapeutic intervention.

AREAS COVERED

This review discusses cross-sectional studies and underlying mechanisms of FGF21 as an endocrine hormone in several liver diseases. Two major theories of 'endoplasmic reticulum stress' and 'FGF21 resistance' in particular are explained. Moreover, early functional detection and pharmacological effect of FGF21 for liver disease are also described.

EXPERT OPINION

FGF21 can be a promising treatment in liver disease. However, still several problems are needed to be answered. The most important are whether different liver disease share common underlying mechanisms and the pharmacological effect in human with limited studies. Further studies are needed to explain the underlying mechanisms and develop potential therapeutic effect for human liver disease.

Stressed Liver and Muscle Call on Adipocytes with FGF21

Fibroblast growth factor 21 (FGF21) is an emerging regulator of local and systemic metabolic homeostasis. Treatment with pharmacological levels of FGF21 alleviates obesity and associated metabolic diseases including diabetes. However, beyond anti-obesogenic effects, the normal roles and underlying mechanisms of FGF21 as an endocrine hormone remain unclear. A recent wave of studies has revealed that FGF21 is a stress-induced endocrine factor in liver, muscle, and other tissues that targets adipose tissue and adipocytes through the FGFR1-betaKlotho complex. Adipose tissues and adipocytes within diverse tissues respond with metabolites and adipokine signals that affect functions of body tissues systemically and cells within the local microenvironment adjacent to adipocytes. Normally this is to prevent impaired tissue-specific function and damage to diverse tissues secreting FGF21 in response to chronic stress. Therefore, diverse stressed tissues and the adipose tissue and adipocytes constitute a beneficial endocrine and paracrine communication network through FGF21. Here we attempt to unify these developments with beneficial pharmacological effects of FGF21 on obesity in respect to inter-organ stress communication and mechanisms.

Fibroblast growth factor 21, the endocrine FGF pathway and novel treatments for metabolic syndrome

Diabetes and associated metabolic conditions have reached pandemic proportions worldwide, and there is a clear unmet medical need for effective and safe therapies. Fibroblast growth factor (FGF)21 is an atypical member of the FGF family. The ability of FGF21 to normalize glucose, lipid and energy homeostasis has attracted considerable interest as a potential therapeutic for treating diabetes and obesity. Many different engineering approaches have successfully improved the plasma half life, protein stability and solubility, as well as 'manufacturability' of FGF21. Novel approaches such as agonist antibodies to FGF21 receptor complexes have opened new opportunities previously unavailable. This review summarizes recent advances in understanding the functions, target tissues and receptors for FGF21. Furthermore, it provides an up-to-date appraisal of the approaches on therapeutic development targeting this pathway.

Increased fibroblast growth factor 21 expression in high-fat diet-sensitive non-human primates (Macaca mulatta)

OBJECTIVE

Fibroblast growth factor 21 (FGF21) is a metabolic regulator of glucose and lipid metabolism. The physiological role of FGF21 is not yet fully elucidated, however, administration of FGF21 lowers blood glucose in diabetic animals. Moreover, increased levels of FGF21 are found in obese and diabetic rodents and humans compared with lean/non-diabetic controls.

METHODS

Adult male rhesus macaque monkeys were chronically maintained on a high-fat diet (HFD) or a standard diet (control, CTR). Plasma levels of FGF21, triglycerides and cholesterol were measured and body weight was record. Glucose-stimulated insulin secretion (GSIS) and glucose clearance was determined during an intravenous glucose tolerance test. Furthermore, expression of FGF21 and its receptors were determined in liver, pancreas, three white adipose tissues (WATs) and two skeletal muscles.

RESULTS

A cohort of the high-fat fed monkeys responded to the HFD with increasing body weight, plasma lipids, total cholesterol, GSIS and decreased glucose tolerance. These monkeys were termed HFD sensitive. Another cohort of monkeys did not become obese and maintained normal insulin sensitivity. These animals were defined as HFD resistant. Plasma FGF21 levels were significantly increased in all HFD fed monkeys compared with the CTR group. The HFD-sensitive monkeys showed a significant increase in FGF21 mRNA expression in all examined tissues compared with CTR, whereas FGF21 expression in the HFD-resistant group was only increased in the liver, pancreas and the retroperitoneal WAT. In the WAT, the co-receptor β-klotho was downregulated in the HFD-sensitive monkeys compared with the HFD-resistant group.

CONCLUSION

This study demonstrates that HFD changes FGF21 and FGF21 receptor expression in a tissue-specific manner in rhesus monkeys; differential regulation is moreover observed between HFD-sensitive and -resistant monkeys. Monkeys that maintain normal levels of the FGF21 co-receptor β-klotho in the WAT on HFD were protected toward development of dyslipidemia and hyperglycemia.

Activation of Liver FGF21 in hepatocarcinogenesis and during hepatic stress

Background

FGF21 is a promising intervention therapy for metabolic diseases as fatty liver, obesity and diabetes. Recent results suggest that FGF21 is highly expressed in hepatocytes under metabolic stress caused by starvation, hepatosteatosis, obesity and diabetes. Hepatic FGF21 elicits metabolic benefits by targeting adipocytes of the peripheral adipose tissue through the transmembrane FGFR1-KLB complex. Ablation of adipose FGFR1 resulted in increased hepatosteatosis under starvation conditions and abrogation of the anti-obesogenic action of FGF21. These results indicate that FGF21 may be a stress responsive hepatokine that targets adipocytes and adipose tissue for alleviating the damaging effects of stress on the liver. However, it is unclear whether hepatic induction of FGF21 is limited to only metabolic stress, or to a more general hepatic stress resulting from liver pathogenesis and injury.

Methods

In this survey-based study, we examine the nature of hepatic FGF21 activation in liver tissues and tissue sections from several mouse liver disease models and human patients, by quantitative PCR, immunohistochemistry, protein chemistry, and reporter and CHIP assays. The liver diseases include genetic and chemical-induced HCC, liver injury and regeneration, cirrhosis, and other types of liver diseases.

Results

We found that mouse FGF21 is induced in response to chemical (DEN treatment) and genetic-induced hepatocarcinogenesis (disruptions in LKB1, p53, MST1/2, SAV1 and PTEN). It is also induced in response to loss of liver mass due to partial hepatectomy followed by regeneration. The induction of FGF21 expression is potentially under the control of stress responsive transcription factors p53 and STAT3. Serum FGF21 levels correlate with FGF21 expression in hepatocytes. In patients with hepatitis, fatty degeneration, cirrhosis and liver tumors, FGF21 levels in hepatocytes or phenotypically normal hepatocytes are invariably elevated compared to normal health subjects.

Conclusion

FGF21 is an inducible hepatokine and could be a biomarker for normal hepatocyte function. Activation of its expression is a response of functional hepatocytes to a broad spectrum of pathological changes that impose both cellular and metabolic stress on the liver. Taken together with our recent data, we suggest that hepatic FGF21 is a general stress responsive factor that targets adipose tissue for normalizing local and systemic metabolic parameters while alleviating the overload and damaging effects imposed by the pathogenic stress on the liver. This study therefore provides a rationale for clinical biomarker studies in humans.

Bone as an endocrine organ

OBJECTIVE

To review the recent evidence that has emerged supporting the role of bone as an endocrine organ.

METHODS

This review will detail how bone has emerged as a bona fide endocrine "gland," and with that, the potential therapeutic implications that could be realized for this hormone-secreting tissue by detailing the evidence in the literature supporting this view.

RESULTS

The recent advances point to the skeleton as an endocrine organ that modulates glucose tolerance and testosterone production by secretion of the bone-specific protein osteocalcin.

CONCLUSIONS

Bone has classically been viewed as an inert structure that is necessary for mobility, calcium homeostasis, and maintenance of the hematopoietic niche. Recent advances in bone biology using complex genetic manipulations in mice have highlighted the importance of bone not only as a structural scaffold to support the human body, but also as a regulator of a number of metabolic processes that are independent of mineral metabolism.

Continuous exposure to a novel stressor based on water aversion induces abnormal circadian locomotor rhythms and sleep-wake cycles in mice

Psychological stressors prominently affect diurnal rhythms, including locomotor activity, sleep, blood pressure, and body temperature, in humans. Here, we found that a novel continuous stress imposed by the perpetual avoidance of water on a wheel (PAWW) affected several physiological diurnal rhythms in mice. One week of PAWW stress decayed robust circadian locomotor rhythmicity, while locomotor activity was evident even during the light period when the mice are normally asleep. Daytime activity was significantly upregulated, whereas nighttime activity was downregulated, resulting in a low amplitude of activity. Total daily activity gradually decreased with increasing exposure to PAWW stress. The mice could be exposed to PAWW stress for over 3 weeks without adaptation. Furthermore, continuous PAWW stress enhanced food intake, but decreased body weight and plasma leptin levels, indicating that sleep loss and PAWW stress altered the energy balance in these mice. The diurnal rhythm of corticosterone levels was not severely affected. The body temperature rhythm was diurnal in the stressed mice, but significantly dysregulated during the dark period. Plasma catecholamines were elevated in the stressed mice. Continuous PAWW stress reduced the duration of daytime sleep, especially during the first half of the light period, and increased nighttime sleepiness. Continuous PAWW stress also simultaneously obscured sleep/wake and locomotor activity rhythms compared with control mice. These sleep architecture phenotypes under stress are similar to those of patients with insomnia. The stressed mice could be entrained to the light/dark cycle, and when they were transferred to constant darkness, they exhibited a free-running circadian rhythm with a timing of activity onset predicted by the phase of their entrained rhythms. Circadian gene expression in the liver and muscle was unaltered, indicating that the peripheral clocks in these tissues remained intact.

Emerging role of fibroblast growth factors 15/19 and 21 as metabolic integrators in the liver

Fibroblast growth factors (FGFs) 15/19 and 21 belong to the FGF endocrine subfamily. They present the intriguing characteristic to be transcribed and secreted in certain tissues and to act as hormones. The insulin-mimetic properties of FGF21 and the regulatory role of FGF15/19 in bile acid and glucose homeostasis endorse these hormones as druggable targets in metabolic disorders. Here, we present details on discoveries, identification, transcriptional regulation, and mechanism of actions of FGF15/19 and FGF21 with a critical perspective view on their putative role as metabolic integrators in the liver.

βKlotho is required for fibroblast growth factor 21 effects on growth and metabolism

Fibroblast growth factor 21 (FGF21) is a fasting-induced hepatokine that has potent pharmacologic effects in mice, which include improving insulin sensitivity and blunting growth. The single-transmembrane protein βKlotho functions as a coreceptor for FGF21 in vitro. To determine if βKlotho is required for FGF21 action in vivo, we generated whole-body and adipose tissue-selective βKlotho-knockout mice. All of the effects of FGF21 on growth and metabolism were lost in whole-body βKlotho-knockout mice. Selective elimination of βKlotho in adipose tissue blocked the acute insulin-sensitizing effects of FGF21. Taken together, these data demonstrate that βKlotho is essential for FGF21 activity and that βKlotho in adipose tissue contributes to the beneficial metabolic actions of FGF21.

Endocrine fibroblast growth factors 15/19 and 21: from feast to famine

We review the physiology and pharmacology of two atypical fibroblast growth factors (FGFs)-FGF15/19 and FGF21-that can function as hormones. Both FGF15/19 and FGF21 act on multiple tissues to coordinate carbohydrate and lipid metabolism in response to nutritional status. Whereas FGF15/19 is secreted from the small intestine in response to feeding and has insulin-like actions, FGF21 is secreted from the liver in response to extended fasting and has glucagon-like effects. FGF21 also acts in an autocrine fashion in several tissues, including adipose. The pharmacological actions of FGF15/19 and FGF21 make them attractive drug candidates for treating metabolic disease.