Minireview: Roles of Fibroblast Growth Factors 19 and 21 in Metabolic Regulation and Chronic Diseases

Abstract

Fibroblast growth factor (FGF)19 and FGF21 are hormones that regulate metabolic processes particularly during feeding or starvation, thus ultimately influencing energy production. FGF19 is secreted by the intestines during feeding and negatively regulates bile acid synthesis and secretion, whereas FGF21 is produced in the liver during fasting and plays a crucial role in regulating glucose and lipid metabolism, as well as maintaining energy homeostasis. FGF19 and FGF21 are regarded as late-acting hormones because their functions are only used after insulin and glucagon have completed their actions. Although FGF19 and FGF21 are activated under different conditions, they show extensively functional overlap in terms of improving glucose tolerance, insulin sensitivity, weight loss, and lipid, and energy metabolism, particularly in pathological conditions such as diabetes, obesity, metabolic syndrome, and cardiovascular and renal diseases. Most patients with these metabolic diseases exhibit reduced serum FGF19 levels, which might contribute to its etiology. In addition, the simultaneous increase in serum FGF21 levels is likely a compensatory response to reduced FGF19 levels, and the 2 proteins concertedly maintain metabolic homeostasis. Here, we review the physiological and pharmacological cross talk between FGF19 and FGF21 in relation to the regulation of endocrine metabolism and various chronic diseases.

The fibroblast growth factor (FGF) family comprises 22 members that are further classified into 7 subfamilies based on their structural similarities and mechanisms of action (1, 2). Most FGFs bind to and activate cell surface tyrosine kinase FGF receptors (FGFRs) (1–4) via a high-affinity interaction with heparin, and then function in a paracrine/autocrine manner to induce cell proliferation and differentiation (3, 4). Members of the FGF19 subfamily, including FGF19, FGF21, and FGF23, also function by activating FGFRs. FGF19 activates mainly FGFR4, whereas FGF21 and FGF23 signal predominantly via FGFR1c and FGFR2c (5). Unlike conventional FGFs, FGF19 subfamily members lack a classic heparin-binding domain (6). This characteristic allows these proteins to avoid capture by local cells; therefore, they are secreted into the bloodstream and function as hormones. However, the FGF19 subfamily has a low affinity for heparin sulfate, another coreceptor named Klotho is required to allow the FGF19 subfamily to exert their biological functions (1, 6). The klotho gene, named after the spinner, was identified in 1997 as a gene mutated in a mouse strain that exhibited short life span and complex phenotypes resembling human premature aging syndrome. Klotho is a transmembrane protein family that contains α-Klotho, β-Klotho, and lactase-like Klotho (7, 8). α-Klotho is primarily expressed in the distal convoluted tubules of the kidney and is required for the biological actions of FGF23 (9). β-Klotho shares 41% amino acid identity with α-Klotho and is required for the biological effects of FGF21 and FGF19 (3, 10). Members of the FGF19 subfamily function in various biological activities such as regulating the enterohepatic circulation of bile acid, regulating glucose and lipid metabolism, and maintaining phosphate/vitamin D homeostasis (5). Although FGF19 and FGF21 show only approximately 35% sequence homology, their functions considerably overlap, including maintaining bodyweight and regulating carbohydrate and lipid homeostases (3).

The interaction between carbohydrate and lipid metabolism allows the body to maintain its energy resources, particularly during nutritional stress. Generally, after feeding, glucose may enter any of 4 main metabolic pathways: 1) glucose is decomposed into H₂O and CO₂ or lactate associated with production of ATP through aerobic or anaerobic metabolism pathways, respectively; 2) glucose is decomposed into ribose and reduced nicotinamide adenine dinucleotide phosphate via the phosphopentose pathway; 3) excessive glucose is polymerized into glycogen and stored in the liver as an energy source; 4) excessive glucose is transformed to fat and stored in adipose tissues through their common metabolic intermediate product, acetyl-coenzyme A (CoA), which is subsequently transformed into palmitic acid via a 4-step reaction of condensation, reduction, dehydration, and reduction. However, during fasting or starvation, the glucose requirement of the body increases. Glucose is thus obtained from food decomposition, glycogenolysis, and glucogenolysis. Gluconeogenesis is derived from fat catabolism. Triglyceride (TG), which is a blood lipid, can be decomposed into glycerol and 3 fatty acids. These products are then transformed into acetyl-CoA, which is the key molecule of gluconeogenesis. In this review, we focus mainly on the metabolic activities of FGF19 and FGF21, with particular emphasis on the similarities and differences between these 2 hormones in terms of maintaining energy homeostasis (glucose and lipid metabolism) and metabolic disorders.

Identification of FGF19 and Its Effect on Bile Acid Hemostasis

In 1999, FGF19 was first discovered in the human brain during embryonic and fetal development, which indicated that it plays an important role in brain development during embryogenesis (11). Similarly, FGF15, which is an ortholog of FGF19, is expressed in the central nervous system of rodents, where it plays an important role in stimulating the differentiation, but not proliferation, of mature neural cells (12, 13). Actually, FGF19 and FGF15 are the same protein function in human and rodent, respectively. The nucleotide sequence revealed a complete amino acid sequence of the protein (216 amino acids). The amino sequences of human FGF19 are very similar (∼51% amino acid identity) to those of mouse FGF15 (218 amino acids). In adults, however, FGF15/19 is inactive in the neural system and instead, plays an important role in the regulation of bile acid homeostasis (14).

Bile acid is produced by the liver, secreted into the circulation, and then stored mainly in the gallbladder. After each meal, the gallbladder contracts and squeezes bile acids into the intestinal tract to facilitate intestinal absorption and lipid transportation via the enterohepatic circulation, which functions not only in feedback inhibition of bile acid synthesis, but also in lipid homeostasis (15). FGF19 is a negative regulator of bile acid synthesis and transportation. Liver-derived bile acid can bind to farnesoid X receptor in enterocytes, and the resulting heterodimer functions as a transcription factor that induces intestinal FGF19 expression (16). Then, FGF19 is transported into the liver, where it activates Src homology 2 domain domain-containing protein tyrosine phosphatase-2 signaling in both an autocrine and paracrine manner to inhibit the expression of Cyp7a1, which is the rate-limiting enzyme in bile acid synthesis (10). A number of studies have confirmed the therapeutic potential of FGF19 in the treatment of metabolic disorders using rodent and primate models of obesity and diabetes (3, 10, 17). However, one major concern relating to the potential use of FGF19 is its mitogenic function, which may facilitate in tumorigenesis (18).

FGF21 Identification and Its General Actions

The human fgf21 gene was first identified by PCR analysis in the liver (19). Mature FGF21 consists of 209 and 210 amino acids in humans and mice, respectively; the 2 proteins are 75% homologous (19). Previous studies have shown that FGF21 is predominantly expressed in the liver and adipose tissues, and is also expressed at lower levels in other organs such as skeletal muscle, heart, kidneys, and testes (19–21). FGF21 had attracted the attention of researchers in 2005, when its metabolic regulatory effects were identified. Specifically, Kharitonenkov et al demonstrated that FGF21 increased glucose uptake in both 3T3-L1 adipocytes and human primary adipocytes, and that these effects were mainly due to the up-regulation of glucose transporter (GLUT)-1 but not GLUT-4 (22). Moreover, FGF21 improves glucose tolerance and insulin sensitivity, as well as decreases blood glucose levels (22). Similar regulatory effects have been observed in carbohydrates in primate (23). Unlike insulin, an overdose of FGF21 does not result in hypoglycemia (22). In addition, FGF21 is not carcinogenic because it lacks a mitogenic function (22). In the liver, peroxisome proliferator-activated receptor (PPAR)α is the main mediator of FGF21 expression and function, including the regulation of gluconeogenesis, ketogenesis, torpor, and growth inhibition (3, 24, 25). Similarly, PPARα-deficiency results in the failure to up-regulate FGF21 expression in the liver, whereas FGF21 deficiency inhibits the biological actions of PPARα (25).

In adipocytes, FGF21 expression is induced by feeding and follows a PPARγ-dependent manner. Mice with the deletion of fgf21 gene (FGF21-knockout [KO]) fail to display PPARγ activities such decreasing body fat and lipidmia, improving insulin sensitivity, and increasing lipogenesis (26). In addition, a mechanistic study demonstrated that FGF21 stimulates the transcriptional activity of PPARγ mainly by preventing the sumoylation of PPARγ at K107 (26, 27). The administration of a PPARα activator induces FGF21 expression in the liver, which is associated with an increase in circulating FGF21 levels. Activation of PPARγ also up-regulates FGF21 expression in adipocytes tissue, although an increase in circulating FGF21 levels was not observed. One possible explanation is that secreted FGF21 from adipocyte tissues is constrained by the white adipocyte tissue (WAT) extracellular matrix (28). This raises an important question regarding the mechanism by which FGF21 induces adaptive responses to fasting or starvation. Potthoff et al demonstrated that PPARγ coactivator protein-1 (PGC-1α) is a key regulator of the biological functions of FGF21, including increased fatty acid oxidation, tricarboxylic acid cycle flux, and gluconeogenesis without increasing glycogenolysis (29). In addition, Chau et al demonstrated that FGF21 regulates mitochondrial activity and enhances the oxidative capacity via an AMP-activated protein kinase (AMPK)-sirtuin type 1-PGC-1α-dependent mechanism in adipocytes (30).

Additionally, a large body of studies have indicated that FGF21 expression is up-regulated during different kinds of stresses, including chemicals such as acetaminophen, dioxin, cerulein, and phenylephrine (31–33), environmental stress such as cold, nutritional stress such as starvation (fasting) and overnutrition (obesity), endoplasmic reticulum (ER) stress (34, 35), mitochondria stress (36, 37), and oxidative stress (38, 39). These facts suggest that FGF21 acts as a key regulator in the adaptation of the body or organs to various kinds of stress and may function as a preventive response to limit the progression of stress in these altered conditions (Figure 1).

Figure 1.

Summary of FGF21 stimuli. FGF21 is predominantly expressed in the liver, adipocyte tissues, and pancreas in response to multiple stimuli, including chemical toxicity, such as acetaminophen, dioxin, cerulean, and phenylephrine; nutritional stress, such as energy deprivation or overload; activation of PPARα/γ, upstream activators of FGF21; as well as oxidative stress, mitochondrial stress, and ER stress.

Role of FGF19 and FGF21 in Regulating Glucose Metabolism

Nutritional status is regulated by multiple factors, including insulin and glucagon, which are early-acting hormones that respond to nutritional stress (40, 41). In the fed state, insulin production and secretion rapidly increase to maintain carbohydrate homeostasis by enhancing lipogenesis and glycogen synthesis and suppressing gluconeogenesis and protein synthesis (42, 43). Conversely, in the fasted state, glucagon is the early-acting hormone that induces glucose metabolism, which is characterized by increased glycogenolysis and decreased gluconeogenesis and ketogenesis (3). In contrast, FGF15/19 and FGF21 are late-acting hormones of the fed and fasted states, respectively; these induce the secretion of insulin and glucagon to regulate glucose homeostasis (Table 1) (3). A functional study has shown that FGF19 transgenic mice are resistant to high-fat diet (HFD)-induced glucose intolerance and insulin resistance (44). Similarly, the administration of exogenous FGF19 prevents the development of glucose metabolic disorders in either HFD-fed or ob/ob mice (45). In addition, FGF15-KO mice exhibit impaired glucose tolerance and reduced insulin sensitivity, which is reversed by treatment with recombinant FGF19 (40, 46). Mechanistic studies have shown that FGF19-maintained glucose homeostasis was attributable not only to the enhancement of glycogen synthesis by increasing glycogen synthase (GS) activity, but also to the suppression of gluconeogenesis via activation of the cAMP-response element-binding protein (CREB)/PGC-1α signaling pathway (47). Although FGFR4 is generally the main receptor of FGF19, a truncated FGF19 variant lacking the β-Klotho-binding domain could still activate FGFR4 and subsequently regulate bile acid homeostasis, but failed to stimulate glucose uptake in 3T3-L1 adipocytes, suggesting that FGFR4 is not required for the glucose tolerance-improving effects of FGF19 (48). Additionally, metabolic disorders in FGFR4-KO mice could not be corrected by up-regulating hepatic FGFR4 expression (46), suggesting that other tissues contribute to FGF19-induced glucose homeostasis. In parallel, recent studies further demonstrated that the intracerebroventricular administration of FGF19 induces insulin-independent glucose lowering in ob/ob mice and improves insulin sensitivity in HFD-fed rats (49, 50), indicating that the FGF19-induced blood glucose regulatory effect might be due to the action of a central system. The summary of FGF19's function and associated mechanisms are presented in Figure 2. FGF21 is a metabolic regulator that maintains glucose homeostasis. Unlike FGF19, FGF21 mainly functions at the late stage of fasting (22). A previous in vitro study indicated that FGF21 induces glucose uptake in various cell lines, possibly by enhancing the expression and activity of GLUT-1 (51). Unlike insulin, FGF21 induces glucose uptake via a slow-onset pathway that likely requires protein synthesis (22). However, the combination treatment using insulin and FGF21 induced synergistic effects that further enhanced glucose clearance by activating both GLUT-4 and GLUT-1 in adipocytes (22, 42). FGF21, which is predominantly expressed in the liver and regulated by hepatic PPARα, induces lipid oxidation, TG clearance, glycolysis, and ketogenesis in the liver (24). In addition, FGF21 also maintains metabolic homeostasis in other peripheral organs. For example, administration of FGF21 increased the number of islets and insulin content in type 2 diabetic mice, suggesting that the FGF21-induced glucose regulatory effects could be mainly attributed to the preservation of pancreatic β-cells (52). Moreover, sc administration of FGF21 in diabetic mice significantly decreased fasting insulin and postprandial glucose levels (53). FGF21 also prevents lipotoxicity- or diabetes-induced insulin resistance in skeletal muscle cells or in kidney tissues (54, 55). Interestingly, some of these organs mentioned above lack FGFR1 or β-Klotho, which suggests the existence of an indirect action of FGF21. Two recent studies have indicated that FGF21 stimulates the expression of adiponectin, which plays an important role in maintaining glucose and lipid metabolism and homeostasis (56, 57). Meanwhile, adiponectin-KO mice were refractory to several therapeutic benefits of FGF21 (56, 57). Therefore, it is possible that adiponectin mediates at least some of the functions of FGF21. Recent studies have demonstrated that FGF21 induces the metabolic regulatory or protective effect on its target organs by fine-tuning cross talk among multiple organs. Xu and coworkers reported that ip administrated FGF21 could be detected in the cerebrospinal fluid, which activates the hypothalamic-pituitary-adrenal axis via the FGFR1c/β-Klotho-ERK-CREB signaling pathway to release corticosterone and stimulate hepatic gluconeogenesis (58), implying that the central system plays a key role in indirectly mediating FGF21's function (Figure 3). In addition, we recently demonstrated that FGF21 prevents atherosclerosis via integration of the liver, adipocytes tissue, and blood vessels (Figure 3). Mechanistically, the protective effect of FGF21 on blood vessels against atherosclerosis is attributable to the reduction in hypercholesterolemia via the induction of adiponectin in adipocytes tissues and suppression of hepatic sterol regulatory element-binding protein (Srebp)-2 in the liver (59).

Table 1.

Function of Insulin, FGF19, Glucagon, and FGF21 at Different Nutritional Conditions

Nutritional status	Main regulator	Half-life	Functions
Fed	Insulin	≈4 min	Glucose catabolism ↑
			Gluconeogenesis ↓
		Reaches its peak serum level at 1 h after a meal	Lipogenesis ↑
			Protein synthesis ↑
			Glycogen synthesis ↑
After feeding	FGF19	≈30 min	Bile acid synthesis ↓
			Bile acid transportation ↓
		Reaches its peak serum level at 3 h after a meal	Bile acid gallbladder filling ↑
			Glucose catabolism ↑
			Gluconeogenesis ↓
			Protein synthesis ↑
			Glycogen synthesis ↑
Fasted	Glucagon	≈5 min	Glycogenolysis ↑
		Reaches its peak serum level at 3–5 d after a meal	Gluconeogenesis ↑
			Ketogenesis ↑
			Glucose catabolism ↓
Starved	FGF21	≈30 min	Gluconeogenesis ↑
		Reaches its peak serum level at 7 d after a meal	Ketogenesis ↑
			Glucose catabolism ↓
			Lipolysis ↑
			Energy expenditure ↓

Figure 2.

The functions of FGF19 and the corresponding mechanisms. Endogenous FGF19 expression is induced in intestinal enterocytes by the activation of FXR/RXR heterodimers. Secreted FGF19 mediates multiple metabolic processes. On one hand, FGF19 can bind to FGFR4 in the presence of β-Klotho and activation of cAMP, followed by stimulation of bile acid filling into the gallbladder. In addition, liver is another key target organ of FGF19 that activates the SHP/CYP7A1 pathway by binding with FGFR4/β-Klotho that negatively regulates bile acid synthesis. On the other hand, by binding with FGFR1/β-Klotho, FGF19 mainly regulates glucose metabolism, including suppression of gluconeogenesis and enhancement of glucose catabolism via the inhibition of the CREB/PGC-1α pathway, and improvement of glycogen synthesis via inhibition of GS kinase (GSK) pathway. CYP7A1, cholesterol 7-a-monooxygenase; FXR, farnesoid X receptor; GSK3, GS kinase-3; RXR, retinoid X receptor; SHP, protein tyrosine phosphatase.

Figure 3.

Pharmacological effect of FGF21 is partly attributed to the interactions among multiple organs. FGF21 was initially regarded as a metabolic regulator that directly acts on the target organs. However, recent studies have revealed that FGF21 also indirectly acts upon target organs. FGF21-induced gluconeogenesis, ketogenesis, and lipolysis are mediated by the activation of hippocampus-pituitary-adrenal gland aix. In addition, FGF21 prevents atherosclerosis by reducing CHO, which is induced by an increase in the production of adiponectin from adipocytes tissue and suppression of SRPBP-2 in the liver.

Unlike FGF19, FGF21 maintains glucose homeostasis in different nutritional states. In the fasted state, FGF21 induces hyperglycemic effects by stimulating lipolysis, ketogenesis, gluconeogenesis, and increasing insulin sensitivity (24). Interestingly, Inagaki et al indicated that ketogenic effects are comparably enhanced in fgf21 gene overexpressing transgenic (FGF21-TG) mice and their wild-type (WT) mice during fasting because endogenous FGF21 is sufficient in imparting its biological effects (25). A similar effect was also observed in FGF21-KO mice, which suggests that most of the effects of FGF21 on glucose metabolism in the liver are due to the pharmacological but not physiological effects of FGF21 (60). On the other hand, FGF21 induces hypoglycemic effects by stimulating lipogenesis and adipocyte differentiation in the fed state (26). Therefore, the physiological conditions of these studies assessing the relationship between FGF21 and glucose metabolism should be carefully selected. The summary of the functions and the associated mechanisms of FGF21 are presented in Figure 4.

Figure 4.

The functions of FGF21 and the possible mechanism. Secreted FGF21 functions as a metabolic regulator in either endocrine or autocrine manner in multiple organs, including blood vessels, testis, kidney, heart, skeletal muscle, and brain. FGF21 acts on the above organs not only via directly binding to FGFRs of these organs in the presence of β-Klotho but is also mediated by adiponectin or central neural system. AKT, protein kinase B; AS, atherosclorosis; eNOS, endothelial nitric oxide synthase; NF-κB, nuclear factor-κB; R1, FGFR1; R2, FGFR2; Sirt-1, sirtuin type 1.

Role of FGF19 and FGF21 in Regulating Lipid Metabolism

Bile acids are physiological detergents that mediate the intestinal absorption and transport of lipids. FGF19, a negative regulator of bile acid synthesis and transport, suppresses lipid absorption and hyperlipidemia (15), which indicates that FGF19 plays an important role in regulating lipid metabolism. Clinically, decreased serum FGF19 levels during fasting are associated with the development of nonalcoholic fatty liver disease (NAFLD) in obese adolescents. Tomlinson et al (44) demonstrated that the bodyweight of FGF19 transgenic mice fed on a standard diet significantly decreased and was mainly due to reduced adiposity rather than food intake (44). In addition, FGF19 transgenic mice are also resistant to HFD-induced obesity and increased fat content. A study conducted by a Japanese group suggested that treating obese mice with recombinant FGF19 decreased the transcription of a series of genes that were closely associated with lipogenesis, including acetyl-CoA carboxylase (ACC), Cd36, Srebp-1c, stearoyl-CoA desaturase 1 (SCD1), and Cyp7a1 (61). A similar negative regulation of lipogenesis was also observed in obese FGF19-TG mice (44). A mechanistic study revealed that FGF19 inhibits the expression of lipogenic enzymes by activating downstream kinase STAT3, which is an inhibitor of SREBP-1c expression, and decreasing the expression of PGC-1β instead of altering ERK, P38 mitogen-activated protein kinase, or AMPK activity (62). Interestingly, Wu et al reported that treatment using FGF19 increases serum TG and cholesterol levels in diet induced-obese mice (63). The above findings suggest that FGF19 exerts both lipid-raising and -lowering effects under different conditions. The dual functions of FGF19 could be attributed to different binding receptors and target tissues (63). For instance, FGF19 induces lipolysis through the activation of FGFR1c, primarily in adipose and other tissues except liver. On the other hand, it induces lipogenesis through the activation of FGFR4 and negatively regulates hepatic bile acid synthesis (48, 63). Under fasting or starved conditions, the lipolysis-induced production of fatty acids compensates for carbohydrate depletion to provide nearly half of the energy required by the entire body. An in vitro study demonstrated that incubating hepatocytes with fatty acids stimulates the secretion of FGF21 (64). Clinical studies have suggested that serum FGF21 levels are positively correlated with obesity and fatty liver (65, 66). In addition, a randomize controlled trial showed that lipid infusion increases FGF21 levels (64). These studies suggest that increased FGF21 levels might be an adaptive protective response to lipotoxicity. Inagaki et al reported that the levels of serum and hepatic TGs significantly decreased in FGF21 transgenic mice fed on standard chow compared with WT mice (25). On the other hand, FGF21 knockdown mice fed on a ketogenic diet showed further enhancement of KD-induced excessive lipid accumulation in the liver (24). In addition, the size of adipocytes in FGF21 transgenic mice was notably smaller than those in WT mice (25). A mechanism study demonstrated that FGF21-induced lipolytic effect was due to the up-regulation of various lipases, particularly hormone-sensitive lipase and adipose TG lipase (25). Besides induction of lipolysis, FGF21 also induces lipid β-oxidation in the liver, as characterized by the increased expression of hydroxyacyl-CoA dehydrogenase, carnitine palmitoyltransferase 1 α, acyl-CoA oxidase, and cluster of differentiation 36 (CD36) via the AMPK-sirtuin type 1-PGC-1α signaling pathway, which might be mediated by adiponectin (24, 30). Conversely, another study indicated that FGF21 induces adipocyte differentiation and lipogenesis in obese mice (26). The observed opposite effect of FGF21 on lipid metabolism suggests that FGF21 has diverse regulatory roles in various nutritional states to maintain lipid homeostasis (67). FGF21 playing a role in the regulation of the TG/fatty acid cycle might explain why FGF21 could both stimulate and repress lipolysis in white adipocytes (26).

Metabolic Regulatory Effect of FGF21 on Carbohydrate and Lipid Is Independent of UCP-1-Mediated Browning of WAT

Mammalian adipocyte tissue consists of WAT and brown adipocyte tissue (BAT). WAT serves as the main storage tissue of neutral fats. On the other hand, BAT preserves body temperature under cold condition by releasing intracellular BAT lipids during lipolysis and generating heat in mitochondria via uncoupling protein (UCP)-1 (68). UCP-1 imparts a thermogenic effect by eliminating the voltage difference across the mitochondrial membrane and transforming energy to heat instead of generating ATP (69). However, recent studies have shown that during prolonged exposure to cold condition, WAT can also convert to a “browning-like” state that is characterized by an expansion of its multilocular structure and a decrease in lipid storage (70). Fisher et al demonstrated that FGF21-KO mice displayed an impairment of the ability to adapt to low temperatures with reduced WAT browning. Administration of exogenous FGF21 remarkably enhances the body's defense to chronic cold exposure by inhibiting its regulatory effect on the expressions of UCP-1 and other thermogenic genes in fat tissues (70). Another mechanistic study demonstrated that FGF21 regulates this process, at least in part, by enhancing adipose tissue PGC-1α protein levels independent of mRNA expression (70). Do FGF21-induced UCP-1 activity and the browning effect of WAT contribute to the regulation of lipid and carbohydrate metabolism? Two recent publications in Cell Metabolism and Cell Reports examined the relationship between WAT browning and pharmacological effects of FGF21 on metabolic regulation, and concluded that the therapeutic effects of FGF21 for correcting metabolic disorders of carbohydrate and lipid are, to a large extent, independent of WAT browning (71, 72). Their studies confirmed that after administration of an FGF21 mimetic, BAT activity and BAT-derived UCP-1 expression were significantly enhanced at both 21°C and 30°C, whereas the browning effect of WAT was only observed at 21°C. However, exogenous FGF21 or the FGF21 mimetic induced similar results that included an increase in energy expenditure without altering food intake, weight loss, and improvement in glycemic/lipid levels at both temperatures or in both UCP-1-KO and WT mice. These results suggest that WAT browning and UCP-1 are not required in the pharmacological effects of FGF21 treatment on metabolic regulation (71, 72).

Effects of FGF19 and FGF21 on Metabolic Diseases

Metabolic disease pertains to a group of diseases that are caused by metabolic disorders involving carbohydrates, lipids, proteins, and nucleic acids such as obesity, diabetes, hyperlipidemia, gout, and osteoporosis. The prevalence of obesity and diabetes continue to increase around the world, resulting in higher incidence and mortality rates. An epidemiological investigation has shown that more than 600 million people from around the world will be obese in 2013 (73). The study also confirmed that China has the second largest obese population in the world. Compared with obesity, the development of diabetes is more severe in China. It has been reported that approximately 11.6% of the Chinese population are diabetic, whereas nearly 50% of the population are prediabetic (74). Furthermore, about 80% of all diabetic patients die from cardiovascular events (75). Each diabetic patient has as much as a 40% lifetime risk of developing diabetic kidney disease, and it is the single most common cause of end-stage renal disease and diabetic nephropathy. Therefore, finding an ideal preventative measure against metabolic disorders without subsequently generating severe complications is warranted (76).

FGF19 and FGF21 in obesity

The evidence described above suggests that FGF19 and FGF21 play important roles in regulating glucose and lipid metabolism. Therefore, extensive research studies have been conducted on the relationship between FGF19/21 and metabolic diseases, especially obesity. A clinical study indicated that plasma FGF19 levels significantly decrease in obese patients compared with healthy control subjects (77). Similarly, serum FGF19 levels were also lower in obese adolescents with NAFLD compared with healthy control subjects, and are inversely correlated with the probability of nonalcoholic steatohepatitis and fibrosis in children with NAFLD (78). However, Schreuder et al showed that insulin resistance did not influence the expression of intestinal FGF19 production in NAFLD patients, although the negative regulatory effects of FGF19 on bile acid synthesis were impaired in NAFLD patients with insulin resistance (79).

The negative correlation between FGF19 and obesity was further confirmed in experimental studies involving animal models. Administration of human recombinant FGF19 to HFD-induced obese mice induced a significant dose-dependent decrease in body mass and blood glucose levels, which were associated with a decrease in the concentrations of TG, as well as increased fatty acid oxidation, brown tissue mass, and insulin sensitivity (45). FGF15-KO mice exhibited the glucose intolerance and impaired capability of hepatic glycogen storage compared with WT mice (80).

In contrast to FGF19, FGF21 stimulates lipolysis. Obese diabetic db/db mice and obese persons have elevated, rather than reduced, serum FGF21 levels. Similarly, elevated serum FGF21 levels were also observed in NAFLD patients and were positively correlated with intrahepatic TG levels (81). HFD-induced obese mice also have increased serum FGF21 levels and are insensitive to exogenous FGF21 due to the down-regulation of FGFR1 and β-Klotho, suggesting that obesity is an FGF21-resistant state (82). However, FGF21 resistance can be reversed by weight loss and lowering blood glucose therapy (83). Although endogenous FGF21 elevation had no beneficial impact on obesity, administration of exogenous FGF21 resulted in resistance to diet-induced obesity, which was indicative of the potential use of FGF21 as an antiobesity molecule. A similar effect was also observed in FGF21 transgenic mice. The above findings implied that the elevated levels of endogenous FGF21 induced by obesity is still relatively insufficient to induce an antiobesity effect.

FGF19 and FGF21 in diabetes

Unlike the defined findings in animal studies, alteration in FGF19 levels in diabetic patients is controversial. Brufau et al reported similar plasma FGF19 levels between diabetic patients and normal subjects (84). However, other studies observed reduced plasma FGF19 levels in patients with type 2 diabetes (41). Similarly, FGF19 levels were also significantly lower in patients with gestational diabetes compared with healthy pregnant females (85). In addition, plasma FGF19 levels were negatively associated with body mass index, TG/high density lipoprotein-cholesterol (HDL-c), high sensitive-C-reactive protein (CRP), and Hemoglobin A1c in diabetic patients (86). A functional study demonstrated the antidiabetic effect of exogenous human FGF19 in the ob/ob mice, which was characterized by a reduction in hepatic gluconeogenesis and an improvement in glucose utility (50). A similar antidiabetic effect was also observed in FGF15 transgenic mice (3).

Plasma FGF21 levels are significantly higher in type 2 diabetic patients and are positively correlated with hypertension, hyperglycemia, Hemoglobin A1c insulin resistance, and high sensitive-CRP levels (87). The above phenomena suggest that FGF21 might be a potential marker for the diagnosis of type 2 diabetes. In a large prospective study involving Chinese subjects, increased plasma FGF21 levels were positively correlated to worsening hyperglycemia and dyslipidemia in prediabetic subjects displaying a normal phenotype (88). In contrast, serum FGF21 levels were significantly lower in type 1 diabetic and latent autoimmune diabetes in adult patients compared with age- and sex-matched healthy subjects (89). The lowering effect of circulating FGF21 in type 1 diabetic and latent autoimmune diabetes in adult patients was probably due to the lack of insulin, which acted as an inducer of hepatic FGF21 (90). Therefore, circulating FGF21 can be regarded as a biomarker not only for subtyping diabetes, but also for predicting the risk of diabetes. A functional study indicated that the administration of exogenous FGF21 to diabetic mice resulted in the amelioration of hyperglycemia and hyperlipidemia, improvement of insulin sensitivity, reduction of body mass, and increase in fat use and energy expenditure (19). Because reduced plasma FGF19 levels contribute to gestational diabetes mellitus (GDM), significant attention has been paid toward determining the role of FGF21 in these findings. Stein et al reported similar plasma FGF21 levels between GDM patients and healthy pregnant females, but were positively correlated with markers for insulin resistance and dyslipidemia, including TGs, leptin, adiponectin, and HDL (91). Therefore, the above studies suggest that reduced serum FGF19 levels might play a role in the pathophysiology of GDM, whereas increased serum FGF21 levels might be a compensatory response to this disease. To date, several studies have focused on the relationship between FGF21 and type 2 diabetes.

FGF19 and FGF21 in Cardiovascular Diseases

A recent study involving a Chinese population observed reduced plasma FGF19 levels in patients with coronary artery disease (CAD), which were negatively associated with biomarkers that determine the severity of CAD (92). An animal study revealed that fgf15-mutant mice exhibited a disorder in the cardiac outflow tract that was likely caused by the aberrant behavior of cardiac neural crest cells (93). However, studies that have assessed the relationship between FGF19 and cardiovascular disease are limited; therefore, the usefulness of FGF19 as a potential marker for the diagnosis of cardiovascular diseases remains uncertain. Unlike FGF19, patients with CAD exhibit significantly higher plasma FGF21 levels, which were positively associated with serum total cholesterol, TG, and HDL levels (94, 95). Another study revealed that serum FGF21 levels were higher in patients with carotid atherosclerosis, and were positively correlated with risk factors such as adverse lipid profiles and CRP (96). An in vitro study demonstrated that FGF21 was up-regulated in oxidized LDL-treated cardiac endothelial cells, which was induced by bezafibrate and resulted in the inhibition of oxidized LDL-induced apoptosis in cardiac endothelial cells (97). In addition, our previous study showed that FGF21 prevents palmitate (a lipotoxic agent)-induced apoptosis in both H9C2 and primary cardiomyocytes via ERK1/2-mediated P38 mitogen-activated protein kinase-AMPK signaling, which also mediates the cardiac protection of FGF21 against diabetes-induce cardiac cell death at the early stage and cardiac dysfunction and fibrosis at the late stage (98). Our research group has also recently revealed that FGF21 deletion-aggravated cardiac lipid accumulation is likely mediated by cardiac nuclear factor erythroid 2 p45 related factor 2-driven CD36 up-regulation, which may contribute to the increase in cardiac oxidative stress and remodeling, and eventual development of diabetic cardiomyopathy (99). We also showed that FGF21 protected H9c2 cells from ischemia/reperfusion injury via an protein kinase B/GS kinase-3β/caspase-3-dependent pathway, as well as prevented oxidative stress and recovered energy supplies (100). In an ex vivo Langendorff system, Patel et al showed that FGF21 imparted a cardioprotective effect and restored cardiac function via autocrine/paracrine pathways; however, the protective effects were reduced in obese mice (101). In addition, FGF21-KO mice were more sensitive to isoproterenol-induced cardiac hypertrophy than WT mice, as characterized by an increased heart weight, ventricular dilation, and cardiac dysfunction. These pathological changes were reversed by the administration of recombinant FGF21. We recently revealed that FGF21 provided beneficial effects on the hearts of type 1 diabetic mice. We found that FGF21 prevented type 1 diabetes-induced cardiac cell apoptosis mainly by up-regulating AMPK-mediated pathways (98).

FGF19/21 and Renal Diseases

Reiche et al reported that the serum FGF19 and FGF21 levels were 1.5- and 15-fold higher, respectively, in patients receiving chronic hemodialysis compared with healthy subjects (102). In addition, circulating FGF19 levels were negatively correlated with circulating adiponectin and CRP in patients with chronic hemodialysis (103). Circulating FGF19 levels are potential predictors of end-stage renal disease (103). Impaired FGF19 response in mice after feeding was also observed in oxidative stress-associated late-stage chronic kidney disease (104). The study also confirmed that the abnormal plasma FGF19 levels were corrected by antioxidative therapy (104). Conversely, recent studies have revealed that serum FGF21 levels were higher in the patients with both chronic and acute renal dysfunction (105). In addition, Lin et al demonstrated that serum FGF21 levels gradually increased with the development of renal disease from the early to late stage (106). An animal study indicated that FGF21 prevented diabetic nephropathy by improving systematic alterations, including insulin resistance, hyperglycemia, and dyslipidemia, and it had antifibrotic effects (55). Our previous study also confirmed that FGF21 exhibited beneficial effects on the kidneys of type 1 diabetic mice by preventing oxidative stress, inflammation, apoptosis, and fibrosis (76).

Summary

FGF15/19 and FGF21 are key members of the FGF19 subfamily. The expression of both FGF19 and FGF21 is induced by multiple stimuli such as chemical stress, nutritional stress, mitochondrial stress, PPAR activators, oxidative, and ER stress. Due to the absence of a heparin-binding domain, both FGF19 and FGF21 are secreted into the bloodstream and function as endocrine factors that regulate glucose/lipid metabolism and energy homeostasis in multiple target organs, including the liver, heart, skeletal muscle, testis, kidney, blood vessel, and pancreas. A recent study demonstrated that besides its direct action on the target organs, FGF21 can also indirectly induce beneficial or therapeutic effects on target organs by fine-tuning the cross talk among multiple organs. FGF19 acts as a fed-state hormone, whereas FGF21 acts as a fasted-state hormone. Both animal and clinical studies revealed reduced and increased serum FGF19 and FGF21 levels, respectively, in patients with metabolic diseases, including metabolic syndrome, obesity, or type 2 diabetes. The above evidence suggests that FGF19 and FGF21 complement each other and synergistically maintain carbohydrate and lipid metabolism and homeostasis.

Based on the regulatory effects of FGF19 and FGF21, a number of studies have assessed its potential therapeutic value in the treatment of metabolic diseases, which demonstrated that both FGF19 and FGF21 can induce preventive effects on obesity and diabetes, as well as diabetes-induced macrovascular and microvascular complications, specifically the cardiovascular complications and renal complication. However, several bottlenecks have to be resolved before these molecules could be applied to the clinics. One major concern regarding the potential use of FGF19 is its mitogenic function, which may increase the risk of tumorigenesis (18). Therefore, additional investigations that explore a modified FGF19 to reduce or eradicate its mitogenic ability or analogues that only mimic FGF19-induced metabolic regulatory effect are warranted. Native FGF21 also possesses a few shortcomings, including poor stability and short half-life in vivo. To overcome these issues, an analog of FGF21, LY2405319, was designed to induce glucose-, bodyweight-, and lipid-lowering effects that are indistinguishable from native FGF21 (107). This peptide is suitable for larger-scale production and also for oral administration. For instance, a pharmaceutic agency, Amgen, has developed a modified FGF21 molecules, named fragment crystallizable-FGF21 and polyethylene glycol-modified FGF21, both which can prolong the half-life of FGF21 to 12–30 hours (108, 109). Alternative analogs such as CVX-343 (Pfizer), mimAb1 (Amgen), and C3201-HAS (Amgen) were also developed as β-Klotho activators (109–111). These promising analogs or mimetics will eventually be administered to patients with metabolic syndromes such as obesity, diabetes, and even diabetic complications.

Acknowledgments

This work was supported in part by National Science Foundation of China Grants 81370917 (to C.Z.) and 81471045 (to X.Lin), the Research Development Fund of Wenzhou Medical University Grant QTJ13005 (to C.Z.), the National Institutes of Health Grant 1R01 DK091338-01A1 (to L.C.), the Natural Science Foundation of Zhejiang Province Grant Y14H070033 (to H.Y.), the Medical and Healthy Technological of Zhejiang Province Grant 201472233 (to C.Z.), the Project of Public Welfare of Wenzhou Grant 2014Y0416 (to C.Z.), and Project for Selected Overseas Chinese supported by Zhejiang Technology Foundation (to C.Z.).

Disclosure Summary: The authors have nothing to disclose.

Abbreviations

 
• AMPK

  AMP-activated protein kinase

 
• BAT

  brown adipocyte tissue

 
• CAD

  coronary artery disease

 
• CoA

  coenzyme A

 
• CD36

  cluster of differentiation 36

 
• CREB

  cAMP-response element-binding protein

 
• CRP

  C-reactive protein

 
• ER

  endoplasmic reticulum

 
• FGF

  fibroblast growth factor

 
• FGFR

  FGF receptor

 
• GDM

  gestational diabetes mellitus

 
• GLUT

  glucose transporter

 
• GS

  glycogen synthase

 
• HDL-c

  high density lipoprotein-cholesterol

 
• HFD

  high-fat diet

 
• KO

  knockout

 
• NAFLD

  nonalcoholic fatty liver disease

 
• P38 MAPK

  P38 mitogen-activated protein kinase

 
• PGC-1α

  PPARγ coactivator protein-1

 
• PPAR

  peroxisome proliferator-activated receptor

 
• SH2

  Src homology 2

 
• Srebp

  sterol regulatory element-binding protein

 
• TG

  triglyceride

 
• UCP

  uncoupling protein

 
• WAT

  white adipocyte tissue

 
• WT

  wild-type.

Author notes

^*

F.Z., L.Y., and X.Lin contributed equally to this work.