The FGF family has 22 members and is divided into 7 subfamilies based on their sequence similarity and phylogeny (So and Leung 2016). FGF21 belongs to an atypical subfamily, which lacks the conventional FGF heparin-binding domain, and thus FGF21 can diffuse away from the tissues of production and function as an endocrine factor (Ogawa et al. 2007). Nishimura’s team first cloned the human and mouse FGF21 genes in 2000 by means of PCR technique (Tetsuya Nishimura et al. 2000). Human FGF21 is located on chromosome 19 and mouse FGF21 is located on chromosome 7. Interestingly, the mammalian FGF21 gene is highly conserved; for example, mature FGF21 consists of 210 and 209 amino acids in mice and humans, respectively, and the two proteins share 75% of similarity of amino acid sequences (Tetsuya Nishimura et al. 2000).
FGF21 Signaling and Regulation
It was previously reported that FGF21 secretion was regulated by several critical metabolic pathways such as PPARα and PPARγ (Kharitonenkov and Adams 2014). Liver is the main source of circulating FGF21, of which its production is controlled by PPARα. In this regard, starvation increases hepatic FGF21 expression, which requires PPARα to execute this process. In fact, PPARα is a nuclear receptor highly expressed in liver, and it can induce FGF21 transcription by binding to the FGF21 gene promoter (Inagaki et al. 2007). PPARα activation increases gluconeogenesis, fatty acid oxidation, and ketogenesis (Bae et al. 2014). On the other hand, PPARγ is a nuclear receptor and plays a key role in adipose tissues, which regulate lipid synthesis and storage, glucose metabolism, and differentiation of adipocyte (Bae et al. 2014). Activation of PPARγ stimulates the production of adipokines and FGF21 and thus improves insulin sensitization (Dutchak et al. 2012).
Apart from PPARα and PPARγ, several studies have demonstrated that hepatic FGF21 expression may be regulated by other signaling molecule; for example, thyroid hormone receptor β induces hepatic FGF21 during lipolysis and fatty acid oxidation. Moreover, hepatic retinoic acid receptor β (RARβ) can bind to retinoic acid (RA)-responsive elements in the FGF21 promoter after fasting (Bae et al. 2014). As such, FGF21 is subject to regulation by various stimuli or signaling pathways.
Role of FGF21 in Liver
As discussed, the liver is known to be one of the main target sites of FGF21 action and production. Killing of the hepatocytes by albumin Cre dramatically reduces serum FGF21 and fasting-stimulated FGF21 induction, and thus it is believed that the hepatocytes are the major source of circulating FGF21 (Markan et al. 2014). FGF21 has been shown to phosphorylate hepatic FRS2 and ERK1/2 in both lean and obese animals (Fisher et al. 2010). Similar results were obtained in human hepatoma HepG2 cells, in which FGF21 stimulates ERK1/2 phosphorylation but suppresses apolipoprotein(a) expression (Fisher and Maratos-Flier 2016).
Under the conditions of starvation or ketogenic diet, fatty acids are able to activate PPARα with concomitant expression of hepatic FGF21 which, in turn, affects fatty acid oxidation and ketosis (Badman et al. 2007). Moreover, FGF21 knockout mice increased body weight and accumulation of liver triglycerides, which are associated with inflammation and fibrosis of fatty liver. These diseased states may be due, in part, to impairments of fatty acid oxidation, fatty acids uptake, and diacylglycerol synthesis as well as energy expenditure (Fisher et al. 2014). Furthermore, hepatic expression and circulating levels of FGF21 are increased in fatty liver disease, which is associated with the increase in PPARα expression (Fisher and Maratos-Flier 2016).
In addition, the liver plays a role in controlling blood glucose homeostasis by striking a balance of glucose uptake/storage and glucose output. Previous studies have demonstrated that FGF21 suppresses hepatic glucose output in rat hepatoma cells, via the downregulation of glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK), the two major genes being critical for gluconeogenesis (So and Leung 2016). Conversely, FGF21 can stimulate hepatic gluconeogenesis to speedup glucose output so as to maintain the blood glucose levels during fasting states, probably via the involvement of PPARα. Surprisingly, FGF21 administration has dual functions under normal and fasting conditions; FGF2 normally suppresses hepatic glucose output while it also induces gluconeogenesis during fasting, indicating that FGF21 regulates differentially gluconeogenesis based on the specified conditions (So and Leung 2016).
In light of these findings, the demonstrated benefits of FGF21 on the liver may be regulated directly or indirectly via secondary mediators that are responsive to hepatic FGF21. Notwithstanding the existence of this evidence, the mechanism of action by which FGF21 regulates fatty acids oxidation and glucose homeostasis remains elusive and controversial that requires further investigations.
Role of FGF21 in Adipose Tissues
Both KLB and FGFR1c, which are the two indispensable constituents of FGF21 receptor complex, are highly expressed in white adipose tissues (Ogawa et al. 2007; Kliewer and Mangelsdorf 2010). Importantly, PPARγ activation upregulates FGF21 expression in adipocytes, while not affecting circulating levels of FGF21; this observation may be explained by the fact that FGF21 cannot be secreted out of the extracellular matrix of adipocytes (Bae et al. 2014). Nevertheless, the metabolic role of FGF21 was first identified in adipocytes and that glucose uptake in 3T3L1 adipocytes was promoted independently of the action of insulin (Kharitonenkov et al. 2005). In such case, FGF21 activates Erk1/2 signaling and increases the expression of glucose transporter 1 (GLUT1) for glucose uptake. Later on, it was found that FGF21 could stimulate glucose uptake in both white adipose tissue and brown adipose tissue in normal mice (Fisher and Maratos-Flier 2016).
FGF21 has been found to modulate fatty acid homeostasis; for example, a short-term administration of FGF21 reduces serum levels of nonesterified free fatty acids (NEFAs) and a long-term treatment leads to low NEFAs (Fisher et al. 2010; Xu et al. 2009). Moreover, PPARγ coactivator protein-1 (PGC-1α) mediates the metabolic actions of FGF21, such as activation of fatty acid oxidation, gluconeogenesis, and tricarboxylic acid cycle (Zhang et al. 2015). FGF21 also participates in the transition of fatty acids from WAT to the liver during fasting state, as evidenced by increased expression of hormone-sensitive as well as adipocyte triacylglycerol lipases in WAT of mice (Fisher and Maratos-Flier 2016).
In addition, FGF21of BAT increases thermogenesis via the induction of uncoupling protein 1 (Ucp1) gene expression. The increased thermogenic gene expression in BAT shows that FGF21 may help to maintain body temperature. It is plausible to propose that this increase in nonshivering thermogenesis may, at least partly, explain the demonstrated metabolic effects of FGF21, such as increases in weight loss, energy expenditure, and glucose tolerance (Xu et al. 2009). This proposition is further supported by a study that FGF21 administration reduces body weight and improves glucose tolerance in diet-induced obese mice, through promoting browning of WAT and glucose uptake in BAT. In this respect, adiponectin is one of the adipokines secreted from the adipose tissues, which is highly linked to the improved insulin sensitivity in mice and humans. Interestingly, FGF21 increases the levels of circulating adiponectin, thereby offering an explanation of how adipose tissue-derived adiponectin mediates the beneficial effects of FGF21 on glucose and lipid metabolism.
FGF21 as a Potential Drug Target in Metabolic Disorders
Metabolic disorders are classified as a group of diseases such as obesity, diabetes, hyperlipidemia that are caused by dysregulated metabolism of carbohydrates, lipids, and proteins. The prevalence of obesity and obesity-related diabetes is on a year-on-year rising rate globally, leading to high incidence of mortality and morbidity as well as a major socioeconomic burden and issue. It is, therefore, in desperate need to explore better option of preventative and therapeutic measures for metabolic diseases (Zhang et al. 2015).
Emerging research findings have shown the close relationship between FGF21 and obesity. In this regard, FGF21 was reported to promote lipolysis, and elevated serum levels of FGF21were found in both obese diabetic db/db mice and obese patients. High fat diet-induced obese mice are irresponsive to their own increase in serum FGF21 levels, suggestive of an FGF21-resistant state in obesity. This phenomenon can be explained by the downregulation of FGFR1 and KLB (Fisher et al. 2010). However, administration of exogenous FGF21 has a beneficial impact on diet-induced obesity. In addition, a similar effect was observed in FGF21 transgenic mice; the previous results suggest the potential use of FGF21 on the prevention and treatment of obesity (Zhang et al. 2015).
On the other hand, the expression levels of FGF21 are changed during the progression of diabetes. Circulating FGF21 and hepatic FGF21 expression are higher in type 2 diabetes (Kharitonenkov et al. 2005). In contrast, serum FGF21 levels were decreased in type 1 diabetic patients. It is possibly due to the reduction in insulin production from pancreatic beta cells, as insulin acts as an inducer of hepatic FGF21 (Zhang et al. 2015). Moreover, FGF21 can reverse glucose intolerance and insulin insensitivity, and even reduce blood glucose levels. Importantly, similar regulatory effects have been observed in diabetic rhesus monkeys, which are more physiologically akin to diabetic humans (Kharitonenkov et al. 2007). In addition, FGF21 administration improves β-cell function, thus giving rise to the potent beneficial glycemic effects of FGF21. For example, treatments with FGF21 preserve β-cell mass and function, as demonstrated by increases in islet cell number and insulin expression from diabetic mice (Wente et al. 2006). Several studies have suggested that FGF21 reduces β-cell apoptosis and glucolipotoxicity via the Akt pathway (Wente et al. 2006; Woo et al. 2013). Besides, FGF21 can be protective against HFD-induced inflammation and islet hyperplasia in the pancreas (So and Leung 2016). In this regard, abnormality of FGF21’s action is critically attributed to the pancreatic islet dysfunction and eventually leads to the development of T2DM pathogenesis. Notwithstanding these study findings, how FGF21 functions in the pancreatic β-cells and whether FGF21 regulates glucose homeostasis by positively modulating islet autophagy and protecting islets from glucolipotoxicity remain to be elucidated. Given that progressive β-cell loss is a culprit for the development of type 2 diabetes, these solid evidence points to the protective role of FGF21 that can be used to prevent or slow down the progression of type 2 diabetes.
Nevertheless, the potential clinical application of FGF21 is greatly hindered because the pharmacokinetic characteristics of native FGF21 protein are poor. For instance, FGF21 protein has a short half-life (0.5–2 h) and thus it is readily degraded or aggregated. In addition, the native FGF21 protein with a relatively small molecular size (22 kDa) passes easily through the glomerulus in the kidney and then eliminated into the urine (Kharitonenkov et al. 2005, 2007); in fact, it has been reported that serum levels of FGF21 are higher in patients with impaired renal function. To address this issue, some modified FGF21 mimetics or analogues have been recently designed and developed to improve the stability and potency of native FGF21; they include PEGylated FGF21, monoclonal antibody-FGF21 fusion protein, and FGF21 mimetic antibodies (So and Leung 2016). Some of them have been proven in animal models that can improve glucose tolerance and promote weight loss. In spite of promising benefits, there are some concerns on the potential side effect of FGF21 therapy. Recent animal studies demonstrated in mice that FGF21 leads to female infertility by suppressing the release of luteinizing hormone. Moreover, FGF21 treatment is associated with low bone density, in which FGF21 prevents the binding of growth hormone to chondrocytes and thus both proliferation and differentiation of chondrocytes are reduced. However, these harmful effects have not been observed in humans (So and Leung 2016). To this end, extra basic science and clinical studies are needed to further investigate into the safety and efficacy for the potential use of FGF21 mimetics/analogues as therapeutic option.
FGF21 is a distinctive member of FGF family and systemic peptide hormone that has myriad physiological functions: FGF21 action is neither restricted to a single tissue nor participate in a single physiological process. FGF21 production is triggered by various physiological conditions, such as stressful conditions and adaptive responses during the progression of metabolic disorders. FGF21 can act as an endocrine hormone or as an autocrine/paracrine regulator, depending on the site of production and action. Numerous studies have proven the beneficial effects of FGF21 in the management of obesity and its associated disease such as diabetes, which are mediated by promoting energy expenditure and restoring glucose and lipid homeostasis. If confirmed, FGF21 represents a promising therapeutic agent for the preventative and therapeutic measure for metabolic diseases. To this end, more basic science and clinical studies warrant to be undertaken in order to optimize the pharmacokinetic properties, safety, and efficacy as well as the potential side effects of FGF21 therapy.