Introduction
Fibroblast growth factor21 (FGF21) was classified as a fibroblast growth factor based on its structure, as it contains a common domain and shares 10%–30% sequence identity with other FGFs [
1]. The mammalian fibroblast growth factor family currently consists of 22 members divided into seven subfamilies based on their structural similarities and modes of action [
2]. Most FGFs act as paracrine factors regulating cell growth and differentiation [
3]. However, members of the FGF19 subfamily, which also includes FGF21 [
1] and FGF23 [
4-
6], differ in two important aspects from all other FGF proteins. First, they have no or very small mitogenic effects. Second, they exert hormone-like effects. Thus, FGF19 (the human ortholog of murine FGF15) [
7] is primarily expressed in the intestine but regulates bile acid synthesis in the liver in both rodents [
8] and humans [
9]. FGF23 is produced in bone tissue and regulates phosphate and vitamin D metabolism via effects on the kidney [
10], while FGF21 is predominantly expressed in the liver and has beneficial effects on several metabolic parameters in different animal models of obesity [
11].
Structure and signaling pathway
The human
FGF21 gene is located on chromosome 19 and encodes a 209-amino acid long protein with an N-terminal signaling peptide which after cleavage results in a mature protein of 181 amino acids. FGF21 in human and mice share 75% identity at the amino acid level. FGFs mediate their action via a set of membrane-bound FGF receptors (FGFRs) that in turn are expressed in multiple splice variants. FGFR1–4 contain an intracellular tyrosine kinase domain that is activated upon ligand binding. They lead to the activation of a number of downstream signals, including MAPKs, RAF1, AKT1 and STATs [
12]. However, FGFs cannot interact with FGFRs directly since they require a co-factor to bind and activate FGFR signaling efficiently. The presence of specific transmembrane protein, β-Klotho, from the Klotho-family involves in the binding and activation of FGFR [
13,
14]. Several reports suggest that the c-receptor splice isoforms of FGFR1–3 exhibit a particular affinity to β-Klotho and could thus act as endogenous receptors for FGF21 [
15-
17]. Studies showed that all the effects of FGF21 on growth and metabolism were lost in whole-body β-Klotho-knockout (KO) mice and the acute insulin-sensitizing effects of FGF21 were lost in adipose tissue-selective β-Klotho-KO mice. β-Klotho is required for FGF21 effects on growth and metabolism [
18].
Pharmaceutical action
The role of FGF21 in regulating metabolism was first reported in 2005 that FGF21 stimulated glucose uptake in mouse 3T3-L1 adipocytes and in primary cultures of human adipocytes [
11]. Transgenic mice overexpressing FGF21 in liver had improved insulin sensitivity and glucose clearance, reduced plasma triglyceride concentrations, and were resistant to weight gain when fed a high-fat diet [
11]. Administration of recombinant FGF21 to obese, insulin-resistant ob/ob or db/db mice or to Zucker diabetic fatty rats caused similar effects, which include reduction in plasma glucose and hepatic triglyceride concentration, increase in energy expenditure and insulin sensitivity [
11,
19,
20]. In similar studies performed with diabetic rhesus monkeys, FGF21 caused significant decreases in fasting plasma glucose, insulin, and triglycerides [
21]. Importantly, FGF21 did not cause hypoglycemia either in this primate model or in any of the rodent models. There is evidence that FGF21 exerts some of its effects directly on the endocrine pancreas. Short-term treatment of normal or db/db mice with FGF21 lowered plasma insulin concentrations [
22]. In addition, FGF21 suppressed glucagon secretion from isolated rat islets and reduced plasma glucagon concentrations in mice [
11]. Treatment of diet-induced obese mice with FGF21 for longer periods (3-6 weeks) reversed hepatic steatosis, decreased hepatic glucose production, and increased insulin-stimulated glucose uptake in the heart, adipose tissue, and skeletal muscle [
20]. Likewise, administration of FGF21 to ob/ob mice for 8 days improved hepatic insulin sensitivity and also increased liver glycogen content [
23]. In summary, FGF21 has profound effects on carbohydrate and lipid metabolism in rodents and primates. An adverse consequence of the effect of FGF21 on adipocytes occurs in bone, where pharmacological levels of FGF21 decrease bone mass. FGF21 causes bone loss in part by enhancing the differentiation of bone marrow mesenchymal stem cells into adipocytes instead of osteoblasts. Bone loss is a potential clinical concern as FGF21 is developed as a drug for treating metabolic disease [
24] (Table 1).
Physiological role
FGF21 in fasting and feeding
FGF21 is strongly induced in the mouse liver by fasting [
25-
27]. Fasting-mediated induction of FGF21 requires the peroxisome proliferator-activated receptor α (PPARα), a nuclear receptor activated by fatty acids and the fibrate class of hypolipidemic drugs [
25-
27]. PPARα binds directly to the
FGF21 gene promoter to induce its transcription [
26]. FGF21 is also strongly induced in the mouse liver by ketogenic diet and by suckling in mouse neonates, conditions that mimic starvation in forcing the body to burn fatty acids rather than carbohydrates [
25,
28]. FGF21 has effects in lean mice on metabolism, growth, and the phenomenon of torpor that are all consistent with an important role for FGF21 in coordinating the adaptive starvation response. Fasting is detected by the brain, leading to lipolysis and contributing to other adaptations such as torpor. Fatty acids are released from adipose tissue, taken up by the liver, and either oxidized or converted to ketones. Ketones released by the liver are used as fuel by the brain. Activation of PPARα, presumably via activation by fatty acids, increases transcription of
FGF21. FGF21 contributes to ketogenesis and gluconeogensis in liver, and adaptation such as torpor by the brain [
25,
26,
29 ] (Fig. 1A).
Surprisingly, FGF21 is induced in white adipose tissue (WAT) by fasting and refeeding regimens [
30]. It was recently showed that the full insulin sensitizing effects of the thiazolidinedione drug (TZD) rosiglitazone, a potent PPARγ agonist, require FGF21 [
30]. FGF21-KO mice are refractory to both the beneficial, insulin-sensitizing effects and side effects of TZD such as weight gain and fluid retention. However, unlike the fasting response that elicits FGF21 release from the liver into circulation, feeding and pharmacological induction of FGF21 in WAT do not cause a corresponding increase in circulating levels of FGF21 [
30]. These results reveal that FGF21 acts in an autocrine or paracrine fashion in WAT and is a part of feed-forward regulatory pathway that contributes to the fed-state response in WAT (Fig. 1B).
FGF21 and growth hormone axis
The anabolic actions of growth hormone (GH), including the induction of its downstream effector, insulin-like growth factor 1 (IGF-1), are lost in starving animals [
31]. This phenomenon of dissociating the catabolic from the anabolic effects of GH is referred to as “growth hormone resistance.” A remarkable phenotype of FGF21 transgenic mice is their diminutive size. FGF21 transgenic mice weigh substantially less than wild-type mice, while retaining their appropriate body proportions [
32]. Growth retardation is not due to a decrease in GH concentrations. Rather, basal GH concentrations are modestly increased. Notably, circulating IGF-1 concentrations are reduced in FGF21 transgenic mice, as are hepatic levels of the active form of the transcription factor STAT5, a major regulator of IGF-1 transcription [
32]. In addition to an effect on GH signaling, FGF21 can also lead to GH resistance through counteracting the action of GH on lipolysis. It is demonstrated that in FGF21-KO mice, the magnitude of GH-stimulated elevation of circulating glycerol is much higher than that in wild type mice. And FGF21 insufficiency enhances GH-induced lipolysis in mice. These studies suggest that FGF21 acts as a negative feedback signal to block GH-stimulated lipolysis in adipocytes [
33]. FGF21 inhibits growth as part of its broader role in promoting energy conservation during starvation (Fig. 1C).
FGF21 and brown adipose tissue
Some studies showed that FGF21 is induced by cold in brown adipose tissue (BAT) [
34,
35]. Injection of FGF21 into mice stimulated the expression of thermogenic genes such as uncoupling protein-1 and deiodinase-2 in BAT [
19]. However, more recent work has focused on expression and action of FGF21 in BAT itself. Treatment with exogenous FGF21 has been reported to promote thermogenic activity in neonatal BAT and in isolated brown adipocytes [
35]. The first report to study cold-induced activation demonstrated that short-term exposure of mice to low ambient temperatures led to a marked induction of FGF21 mRNA levels in BAT [
35]. This increase in FGF21 message is specific to BAT as FGF21 was not elevated in liver or WAT in response to cold exposure. It is known that low ambient temperatures activate BAT via the sympathetic nervous system [
36]. In particular, β3-adrenergic receptors have been shown to be critical in propagating the thermogenic signal to BAT [
37,
38]. Selective β3-agonist treatment of WT mice led to a highly significant fold increase in FGF21 mRNA levels in BAT to levels comparable to those observed in WAT and liver. In the same cohort, peroxisome proliferator-activated receptor γ coactivator1α (PGC-1 α) mRNA levels were found to be induced by both cold and β3-agonist, while no differences in the expression of PPARα were reported [
34]. However, recent researches have showed that when exposed to cold or β-adrenergic compounds, certain WAT depots can convert to a “brown-like” state, and FGF21 plays a physiological role in this procedure by regulating PGC-1α. FGF21 acts to activate and expand the thermogenic machinery to provide a robust defense against hypothermia [
39] (Fig. 1D).
Clinical relevance of FGF21 in humans
Circulating FGF21 levels are induced by fibrates and other PPARα agonists in humans [
40,
41]. Circulating FGF21 concentrations are also increased in obese individuals fed a very low-calorie diet for 3 weeks [
40]. While these data suggest similarities in the way that FGF21 is regulated across species, the magnitude of FGF21 induction by PPARα agonists and fasting is modest in humans compared with mice. Notably, circulating FGF21 levels are not increased in humans by either shorter-term fasts or ketogenic diets [
40-
42] or in subjects with anorexia nervosa [
43,
44], suggesting that there may be important differences in the regulation and function of FGF21 between rodents and humans. Interestingly, circulating FGF21 concentrations are increased in human subjects who either are overweight or have type 2 diabetes, impaired glucose tolerance, or nonalcoholic fatty liver disease [
45-
52]. It seems likely that this circulating FGF21 is derived from the liver, perhaps due to the induction of FGF21 by elevated hepatic lipid and carbohydrate levels. While the human findings appear to be at odds with the insulin-sensitizing actions of FGF21 in rodents and monkeys, hepatic FGF21 mRNA levels and plasma FGF21 concentrations are similarly increased in diet-induced and genetically obese mice [
53,
54]. Importantly, these mice still respond to pharmacological doses of FGF21 with improved insulin sensitivity. One possibility is that obesity and insulin resistance cause “FGF21 resistance” in rodents and humans. While a study from one group supports this hypothesis [
54], another study does not [
55]. The basis and consequences of increased FGF21 in metabolic disease remain to be determined.
Conclusions and perspectives
FGF21 can be considered late-acting fed- and fasted-state hormones, acting on the heels of insulin and glucagon to regulate metabolism in response to nutritional status. The understanding of FGF21 as a major metabolic regulator is rapidly evolving. In spite of this accelerated pace of investigation, the scientific appreciation of FGF21 biology is still fragmented and controversial, and these knowledge gaps are yet to be filled through prioritized research focused on the most fundamental questions. With the therapeutic relevance of FGF21 pathway in humans being the primary inquiry, FGF21 has remarkable pharmacological effects on carbohydrate and lipid metabolism, particularly in the context of obese animals. The pharmacological actions of FGF21 make it attractive as future drugs for treating metabolic disease. Indeed, FGF21 is already in clinical trials. However, an adverse consequence of the effect of FGF21 on adipocytes occurs in bone, where pharmacological levels of FGF21 decrease bone mass [
24]. Even though FGF21 resistance has been demonstrated in rodents, this does not preclude FGF21 inducing a robust pharmacological response in these species [
23,
54]. Thus, it seems plausible that chronically delivered pharmacological doses of more potent FGF21 agonists will be capable of overcoming FGF21 resistance in humans, if this is indeed the case. Anyway, based on their profound pharmacological and physiological effects on metabolism, future studies in humans should explore the therapeutic potential of FGF21 and provide a better understanding of its mechanism of action.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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