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

FGF15/19 is expressed in small intestine under the regulation of bile acid (BA) nuclear receptor farnesoid X receptor (FXR) [40, 41], and is a negative feedback regulator of BA synthesis and gallbladder filling (Fig. 2).

In response to fasting, BAs are stored in the gallbladder until they are needed for digestion normally [15]. In fed state, BAs are released from the gallbladder into the intestine, bind to and activate FXR, thereby induce expression of FGF19 [42]. In humans, serum FGF19 levels exhibited a rhythm with peaks occurring 90–120 min after the postprandial rise in serum BAs, and the FGF19 peaks in turn preceded the declining phase of BA synthesis [43].

In small intestine, BA induces FGF15/19 expression by activating FXR. FXR induces the expression of small heterodimer partner (SHP) in liver [44, 45]. However, unlike most nuclear receptors, SHP lacks a DNA-binding domain and binds indirectly to the CYP7A1 promoter [44‐46]. Knockout studies in FXR-KO and SHP-KO mice has been demonstrated the significance of the FXR–SHP pathway in bile acid homeostasis, both of which increase CYP7A1 expression [47‐49], the rate-limiting enzyme in the classical BA biosynthetic pathway [14]. FGF15/19 represses CYP7A1 by binding to the FGFR4/KLB receptor complex to activate downstream signaling cascade [43, 50]. A more recent study showed that an uncharacterized gene, Diet1, transcriptionally and post-transcriptionally influences FGF15/19 level as well as CYP7A1 level, and it co-localize with FGF19 in cultured intestinal cells [51]. This suggests that Diet1 plays a role in FGF15/19 intestine-liver axis involved in the BA synthesis. FGF15/19, a hormone made by the distal small intestine in response to BAs, also promotes relaxation and refilling of the gallbladder after a meal. Cholecystokinin (CCK) is a hormone secreted by duodenum causing gallbladder contraction to release bile, which facilitates lipid digestion. Bile acids travel to ileum, where they induce FGF15 synthesis. FGF15 in turn stimulates gallbladder filling by relaxing smooth muscle in gallbladder [15].

After a meal, besides regulation of BA synthesis and gallbladder filling, FGF15/19 has an effect on glycogen synthesis. FGF15/19 acts on FGFR/KLB receptor complexes to represses cholesterol 7a-hydroxylase (CYP7A1) through small heteromer partner [41, 52], and then increase hepatic glycogen synthase (GS) activity and glycogen synthesis in an insulin-independent manner by inducing the phosphorylation and inactivation of GSK3s [53]. Serum FGF19 levels peak approximately 3 h after a meal [43] and increase glycogen synthesis by activation of the Ras/ERK pathway; in contrast, serum insulin levels peak within 1 h after a meal and stimulate glycogen synthesis by the phosphoinositide 3-kinase (Akt) pathway [42].

In addition to glycogen synthesis, FGF15/19 also has an effect on gluconeogenesis. To date, gluconeogenesis inhibition is also differently mediated by FGF19 and insulin by dephosphorylation and inactivation of cAMP response element-binding protein (CREB) and Akt-dependent phosphorylation and FoxO1 degradation, respectively [54]. Unlike insulin, FGF15/19 represses gluconeogenesis gene expressions by promoting protein kinase B(Akt) dependent FOXO1 phosphorylation and dephosphorylation, FGF15/19 cannot activate the PI3K/Akt pathway [42]. The mechanism by which FGF15/19 blocks the expression of gluconeogenesis genes involves dephosphorylation and inactivation of the transcription factor CREB [13]. This in turn down-regulates peroxisome proliferator-activated receptor-1α (PGC1α) transcription, which subsequently decreases its binding to glucose-6-phosphatase catalytic subunit gene and phosphoenolpyruvate carborykinase gene promoters [13]. Therefore, FGF15/19 inhibits gluconeogenesis via regulating the expression of genes involved in gluconeogenesis.

In fasted status, FGF19 increased the phosphorylation of ERK1 and ERK2 in liver. FGF19 induced phosphorylation of both glycogen synthase kinase (GSK) 3α and GSK3β in animals fasted overnight, which correlated with decreased phosphorylation of Ser⁶⁴¹ and Ser⁶⁴⁵ on glycogen synthase and increased glycogen synthase activity [53]. However, the effects of FGF15/19 and insulin on metabolism in fasted status are noticeable. Although both of them can stimulate glycogen and repress gluconeogenesis, there still are important differences as insulin acts through the insulin receptor-PI3K-Akt pathway, and FGF15/19 mediates its effects through the FGFR/KLB-ERK pathway. In addition, there are significant temporal differences. In humans, insulin is released minutes after a meal, and in rodent experiments, serum insulin concentrations and its downstream Akt phosphorylation in liver peak approximately 15 min after a high-carbohydrate or high-fat diet. In contrast, FGF15 mRNA levels in ileum and downstream hepatic ERK1/2 phosphorylation peak about 1 h after feeding [13]. Likewise, FGF19 serum levels peak near 2 h after a meal [43], and accordingly, circulating FGF19 levels in humans negatively correlate with fasting glucose levels and metabolic syndrome [55‐57]. Thus, FGF15/19 acts after insulin in the transition from the fed to the fasted state.

FGF21 is an important regulator of metabolism. A larger number of recent reports have expanded that FGF21 expression is induced in various tissues in response to fasting and feeding. The physiological function of FGF21 in the maintenance of nutritional homeostasis has been suggested (Fig. 3).

Emerging evidences have shown that fasting increases hepatic FGF21 mRNA expression and plasma FGF21 level in mice. Fasting mediated induction of FGF21 requires the peroxisome proliferator-activated receptor a (PPARα) [18, 58, 59]. PPARα can bind directly to the FGF21 gene promoter to induce its transcription [18]. It has been shown that fasting-induced FGF21 in liver increases gluconeogenesis, but does not increase glycogenolysis [60]. Gluconeogenesis is controlled by several key enzymes including fructose-1,6-bisphosphatase, glucose-6-phosphatase and phosphoenopyruvate carboxykinase. An acute FGF21 treatment leads to the gene expressions of hepatic glucose-6-phosphatase and phosphoenopyruvate carborykinase [61]. PGC-lα as a transcriptional coactivator regulating gluconeogenic gene is increased after FGF21 treatment [62]. PGC-1α knockout mice fail to induce glucose-6-phosphatase catalytic subunit and phosphoenopyruvate carboxykinase mRNA expression after FGF21 injection [60]. Taking together, PGC-1α may play an important role in FGF21 promoting hepatic gluconeogenesis. In contract to those findings, a study using mice with liver-specific deletion of PGC-1α reveals that liver PGC-1α is unnecessary in the effects of FGF21 on the gluconeogenesis [61]. The difference results from those two studies can be explained by the mouse models. One is a whole-body PGC-1α knockout and the other is a liver-specific knockout. It suggests that PGC-1α expressed in tissues other than liver affects FGF21 regulatory effects on the gluconeogenesis. For example, a study has shown that FGF21 induces PGC-1α via an indirect mechanism of central nervous system [63]. So the importance of PGC-1α induction for FGF21 action remains in question [60, 61].

Ketone bodies are produced in the liver and delivered to the brain as the major source of energy during fasting period. FGF21 is a molecule regulating lipid metabolism in response to fasting [18, 19]. Increased FGF21 promotes adipose lipolysis in white adipose tissue in an endocrine manner, and increases the fatty acids transport to the liver where they are directly oxidized for energy production or utilized as a source for ketone body formation [18]. FGF21 transgenic mice have higher serum concentrations of β-hydroxybutyrate even in the fed state [18]. PPARα is responsible for coordinating lipid oxidation and ketogenesis in the liver during starvation. Although FGF21 is one of PPARα target genes, FGF21 induces ketone body production through a mechanism distinct from that previously described for PPARα. As we mentioned above, FGF21 induces ketogenesis by stimulating lipolysis, thereby increasing the supply of free fatty acids to the liver [18, 24]. Both carnitine palmitoyl transferase-1a (CPT-1a) and hydroxymethyl glutaryl-CoA synthase-2 (HMGCS2) are rate-limiting enzymes in ketogenesis [64]. Their genes are directly changed by PPARα [65]. However, FGF21 cannot regulate their gene expressions, but increases their protein levels through a posttranscriptional mechanism [18].

Mice fed a high-fat, low-carbohydrate ketogenic diet exhibit marked increases in FGF21 expression in the liver [20, 58, 66] and in white adipose tissue (WAT) by fasting–refeeding regimens [67, 68]. These responses in the liver and WAT are likely mediated by carbohydrate response element-binding protein and PPARγ, respectively [20‐23]. Notably, unlike the fasting response that elicits FGF21 release from the liver into circulation, feeding induction of FGF21 in WAT do not cause a corresponding increase in circulating levels of FGF21, so FGF21 secreted by adipose tissue promotes fatty acid synthesis in adipose tissue in a paracrine or autocrine manner [68], and liver generated FGF21 promotes adipose lipolysis in white adipose tissue in an endocrine manner [18]. So FGF21 regulates lipogenesis and lipolysis by distinct modes. Recent studies found that FGF21 acts as a negative feedback signal to terminate GH-stimulated lipolysis in adipocytes and hepatocytes. In the liver, GH stimulates transcription of the FGF21 through the signal transducer and activator of transcription 5 (STAT 5) signaling pathway [69, 70].