Fibroblast growth factor 19 secretion and function in perinatal development

Abstract

Limited work has focused on fibroblast growth factor-19 (FGF19) secretion and function in the perinatal period. FGF19 is a potent growth factor that coordinates development of the brain, eye, inner ear, and skeletal system in the embryo, but after birth, FGF19 transitions to be an endocrine regulator of the classic pathway of hepatic bile acid synthesis. FGF19 has emerged as a mediator of metabolism and bile acid synthesis in aged animals and adults in the context of liver disease and metabolic dysfunction. FGF19 has also been shown to have systemic insulin-sensitizing and skeletal muscle hypertrophic effects when induced or supplemented at supraphysiological levels in adult rodent models. These effects could be beneficial to improve growth and nutritional outcomes in preterm infants, which are metabolically resistant to the anabolic effects of enteral nutrition. Existing clinical data on FGF19 secretion and function in the perinatal period in term and preterm infants has been equivocal. Studies in pigs show that FGF19 expression and secretion are upregulated with gestational age and point to molecular and endocrine factors that may be involved. Work focused on FGF19 in pediatric diseases suggests that augmentation of FGF19 secretion by activation of gut FXR signaling is associated with benefits in diseases such as short bowel syndrome, parenteral nutrition-associated liver disease, and biliary atresia. Future work should focus on characterization of FGF19 secretion and the mechanism underpinning the transition of FGF19 function as an embryological growth factor to metabolic and bile acid regulator.

INTRODUCTION

The perinatal period is a time of substantial change in the physiological function of organ systems of the neonate. These changes span multiple organ systems, where the lungs are needed to inflate and exchange gasses; the brain needs to regulate temperature and begin processing sensory input; and the gastrointestinal tract must appropriately digest and absorb enteral nutrition while facilitating immune exclusion and tolerance to environmental factors, especially the microbiota (1). In addition, cellular and metabolic processes change to support the acclimation and slower growth rate characteristic of neonates in the extrauterine environment (2). The transition of the neonate from fetal to extrauterine life is closely regulated by endocrine and paracrine growth factors and hormones, among other signaling proteins (2). Fibroblast growth factor-19 (FGF19) functions as a growth factor during fetal development but transitions to an endocrine, bile acid-regulating hormone in postnatal life, yet the signals that coordinate this transition in function at birth are poorly understood.

FGF19 IN FETAL DEVELOPMENT

In the developing embryo, FGF19 plays a significant role in the cellular organization, patterning, and development of brain, heart, inner ear, eye, and skeleton (3). Nishimura et al. (4) first established FGF19 in the brain of the developing human fetus. Ladher et al. (5) established that FGF19 is transiently expressed in the mesoderm of the developing embryo and subsequently directs the development of the inner ear in chickens. Nakayama et al. (6) demonstrated that FGF19 plays an important role in the growth and patterning of the lens and retina in zebrafish. In addition, Vincentz et al. (7) found that fibroblast growth factor-15 (FGF15), the FGF19 murine ortholog plays a significant role in the development of the cardio outflow tract, meaning dysfunction in the expression of FGF19 may result in congenital heart defects. Kurose et al. (3) showed that the expression of FGF19 in the developing optical tract, limb buds, and neural cells of the mid- and hindbrain of chick embryos differed from the expression of the murine ortholog, FGF15 in mouse embryos, which was more limited in the developing brain and retina, establishing the first of a long list of differences in the expression and activity of FGF19 and FGF15. Finally, the development of congenital skeletal abnormalities and disorders from FGF19 dysfunction highlights its importance in bone, including endochondral ossification, craniofacial patterning, and long bone development, and FGF19 dysfunction plays a role in the pathogenesis of congenital skeletal abnormalities and disorders (8).

FGF19 POSTNATAL ACTIVITY

FGF19 is one of the three endocrine fibroblast growth factors and is a signaling mechanism that has historically been studied in the context of regulation of bile acid synthesis, but more recent work has demonstrated extrahepatic metabolic regulatory activity. FGF19 is transcribed in enterocytes of the distal ileum in response to bile acid interaction with the nuclear hormone receptor farnesoid X receptor (FXR) (9). The most extensively studied function of FGF19 is as a negative regulator of hepatic bile acid synthesis. Briefly, FGF19 is secreted as a ∼25-kDa protein and transported to the liver via portal circulation where it interacts with the extracellular domains of fibroblast growth factor receptor-4 (FGFR4) and coreceptor β-klotho (KLB) located on the cell membranes of hepatocytes. This signal is propagated in the hepatocyte to downregulate the transcription of cytochrome P450 section 7 family A member 1 (CYP7A1), an enzyme that catalyzes the rate-limiting step in the classic pathway of bile acid synthesis (10). Therefore, FGF19 serves as a negative feedback mechanism to regulate the synthesis of bile acids via the classic pathway of bile acid synthesis in hepatocytes in mature individuals. The alternative pathway of bile acid synthesis, also known as the acidic pathway, is not responsive to negative feedback by FGF19. Work done in infants and piglets suggest, based on hepatic and urinary oxysterol profiles, that the acidic pathway is more active in the perinatal period than in adults (10). Therefore, FGF19 may have a reduced capacity to downregulate bile acid synthesis in neonates or preterm infants because of the higher relative activity of the alternative pathway of bile acid synthesis in fetal and early neonatal life (11–14).

In addition to the function in bile acid homeostasis, research done primarily in rodents shows that FGF19 has additional intra- and extrahepatic roles mediating metabolism. FGF19, when administered at supraphysiological levels or overexpressed in transgenic rodent models, functions as a metabolic hormone that enhances insulin sensitivity, promotes protein synthesis in the liver, and protects against steatosis in rodent models of nonalcoholic steatohepatitis (NASH) and obesity (Fig. 1) (15, 16). FGF19 suppresses endogenous hepatic glucose production, gluconeogenesis, and lipolysis but increases insulin sensitivity and glycogen synthesis (17–19). When FGF15 or KLB is knocked out in the liver of mouse models, there is diminished protein synthesis and limited regenerative capacity, suggesting these actions occur at physiological levels (10). In extrahepatic tissues, when administered or expressed at supraphysiological levels, FGF19 signals through fibroblast growth factor receptor-1C (FGFR1c) and KLB in the central nervous system, particularly in the hypothalamus, to regulate systemic glycemia and metabolic rate (10, 20). Benoit et al. demonstrated that human FGF19 (hFGF19) signals through FGFR1 or FGFR4 and KLB to phosphorylate the mammalian target of rapamycin (mTOR) to result in myofibril hypertrophy and increased grip strength in rodent models. In addition, hFGF19 was protective against dexamethasone-induced sarcopenia in aged mice (21). Other work by Guo et al. (22) showed that FGF19 was protective against obesity-induced muscle loss, caused reduced weight gain, and restored glucose tolerance in rodent models fed high-fat diets by signaling through AMP-activated protein kinase (AMPK) and sirtuin-1 (SIRT-1). Other work has focused on FGF19 as a target and oncogene in the pathogenesis of hepatocellular carcinoma (23).

Figure 1.

Fibroblast growth factor-19 pre- and postnatal secretion and target tissues at physiological and supraphysiological levels. FGFR4, fibroblast growth factor receptor-4; FGFR1, fibroblast growth factor receptor-1. (Created with BioRender.com).

The potential for FGF19 to alleviate metabolic diseases such as nonalcoholic fatty liver disease (NAFLD) has made it a popular pharmaceutical target. For example, an FGF19 analog has been extensively studied as a bariatric surgery-mimetic in patients, resulting in type II diabetes remission in rodent models and improvements in NASH histology score, independent of weight loss in human patients (24). Given this, FGF19’s insulin-sensitizing, bile acid, and skeletal muscle accretion actions could make it an intriguing target to improve growth and metabolism in premature neonates given the inherent metabolic limitation in their capacity for digestion and absorption, the cellular metabolic responses to nutrition (25).

FGF19 CLINICAL STUDIES IN INFANTS

A number of recent studies have examined the function of FGF19 in infants, especially those born premature (26, 27). Given the evidence of metabolic effects of FGF19 on glucose homeostasis and insulin-like actions in adult humans and mice, current knowledge about FGF19 function in infants represents an important gap in our understanding of the factors that may impact infant nutrient metabolism and growth. Sanchez-Infantes et al. were the first to measure FGF19 levels in infants born appropriate for gestational age (AGA) and small for gestational age (SGA) at birth, 2 days, and 4 and 12 mo of age. FGF19 levels in AGA infants increased 10-fold by 4 mo of age to twice the adult concentrations, where the FGF19 levels were 50% lower in SGA than AGA infants at 4 mo of age (27). In contrast, Yang et al. found that FGF19 levels in cord blood decreased with advancing gestational age and were lower in vaginally born babies compared with cesarean-born babies. In addition, FGF19 levels in cord blood positively correlated with birth weight Z-score in females, but not in males (28).

Memon et al. (26) examined the correlation between plasma FGF19 and CYP7A1 activity [plasma levels of 25-OH cholesterol (C4)] in preterm and term infants in the early perinatal period before the first enteral feed and at approximately the same time on subsequent days. The investigators hypothesized that FGF19 levels would be lowest in preterm infants at birth because FGF19 is secreted in response to intestinal bile acid stimulation, and work by Watkins (29) showed that the bile acid pool in preterm infants is approximately one-third the size of term infants. In addition, the authors expected a negative relationship between FGF19 level and CYP7A1 activity since FGF19 inhibits hepatic expression of CYP7A1 thereby reducing the plasma C4 level. Surprisingly, they found that FGF19 was not correlated with plasma C4 level, and FGF19 levels were inversely related to enteral feeding and gestational age; FGF19 was highest in preterm infants and lowest in term infants (26). This data suggest that the FGF19-CYP7A1 feedback loop is not functional in the perinatal period, particularly in preterm infants. More recently, Memon et al. confirmed this disruption in FGF19-CYP7A1 signaling with a case study comparing an infant that was on prolonged parenteral nutrition and healthy preterm infants. In both cases, and control infants, FGF19 was high at birth and gradually fell as infants aged. In addition, FGF19 failed to correlate with CYP7A1 activity as measured by plasma C4 (30). These infant studies raise important questions about both the regulation of FGF19 secretion and metabolic function in the perinatal period.

FGF19 IN NEONATES: IN SEARCH OF A FUNCTION

Rodent models, especially mice, are useful in the discovery and elucidation of molecular mechanisms because of their ability to generate genetic models of targeted gene disruption and overexpression. However, the use of rodent models in the investigation of the FGF15/19 pathway and its relevance to human has some limitations. First, the expression of the rodent ortholog, FGF15, is limited to the intestine, whereas, in humans and pigs, FGF19 is expressed in the intestine, gallbladder, and liver (31). Moreover, FGF19 can exert glucose-sensitizing effects in the brain, liver, adipose, and skeletal muscle by binding to FGFR1c and regulating bile acid homeostasis through FGFR4 (present in the liver) (17). In contrast, FGF15 only binds to FGFR4 and thus the effects are localized in the liver and not systemic (10). The timeline of perinatal development of the gastrointestinal tract in the pig is more similar to the human infant than in mice (32, 33). The pig is an excellent model for perinatal development, as neonatal pigs show a close homology to human infants, and show a similar pathological response to premature birth (31). Pigs also express FGF19, rather than FGF15, in the liver and small intestine (14, 34). Gavalda-Navarro et al. (35) reported in pigs that plasma FGF19 levels were low at birth and increased with postnatal development into adulthood. More recently, our group showed that pigs born 10 days preterm (analogous to 30 wk gestation in human infants) had lower plasma FGF19 at birth compared with pigs born at term gestational age (34). We recently extended this observation and demonstrated that the lower plasma FGF19 levels in preterm than term newborn pigs are associated with reduced hepatic bile acid pool size and suppressed ileal responsiveness to bile acids (14). Using ex vivo model of cultured intestinal explants, we showed that induction of ileal tissue FXR-FGF19 signaling was significantly blunted in preterm compared with term pigs. We also showed that the blunted ileal FXR-FGF19 response to bile acids was linked to developmental pattern of GATA4 expression. Thompson et al. (36) demonstrated that the transcription factor, GATA4 coordinates the higher FGF15 expression in the distal ileum by negatively regulating FGF15 in the proximal jejunum. GATA4 is also developmentally regulated in different regions of the small intestine of pigs, where it is higher in the distal ileum at birth compared with term pigs (14). These results confirm observations that the localization of GATA4 expression is developmentally regulated, thus the localization of FGF19 expression is also developmentally regulated (14, 35, 36).

Additional factors that may be involved in the perinatal regulation of plasma FGF19 are hormones that are well known to coordinate the initiation of the birth process and labor, such as glucocorticoids, estrogen and progesterone, and oxytocin. Wang et al. (37) demonstrated that sulfated progesterone metabolites can antagonize FGF19 secretion in porcine hepatocytes. Al-Aqil et al. (38) and Jia et al. (39) found that pharmacological levels of a glucocorticoid receptor agonist (dexamethasone) inhibit FGF15 expression in a dose-dependent manner, and this inhibition is independent of the glucocorticoid receptor. Wang et al. found that birth modality (planned cesarean vs. vaginal birth) did not affect FGF19 levels in cord blood, indicating that the interaction between exposure to the changes in plasma hormones associated with labor and birth may be complex or specific to the endocrine status of the infant, rather than the mother.

CLINICAL APPLICATIONS

Additional research examining FGF19 secretion and regulation in a pediatric population has focused on the role of FGF19 in the pathogenesis of gastrointestinal and liver disease states (Fig. 2). Parenteral nutrition is a commonly used clinical practice to deliver nutrition in cases of severe illness or gastrointestinal dysfunction where enteral nutrition is not tolerated. However, prolonged parenteral nutrition can result in cholestatic liver disease, termed parenteral nutrition-associated liver disease (PNALD), where the lack of intestinal-liver signaling triggered by enteral nutrition disrupts normal regulatory mechanisms in bile flow, as well as adversely affecting the gastrointestinal tract by reducing mucosal integrity, and creating a proinflammatory environment in the mucosa and lamina propria, which may be exacerbated by alterations in the gut microbiome (40). Hepatic accumulation of bile acids leads to inflammation, steatosis, and tissue injury, and can result in fibrosis and cirrhosis (40). The combination of injury, inflammation, and loss of functional mass in the gut and liver adversely impacts FXR-FGF19 signaling, disrupting bile acid synthesis regulation, and exacerbating bile acid accumulation in the liver, resulting in further damage (40). We showed that total parenteral nutrition reduces plasma FGF19 and enteral feeding of small doses of bile acid can restore FGF19 levels and prevent cholestasis (41). More recently, others have shown that targeting intestinal FXR-FGF19 signaling with enteral FXR agonists can prevent PNALD. These studies strongly support the concept that reducing bile acid production through FGF19-mediated reduction in CYP7A1 may improve cholestasis in animal models (40, 42).

Figure 2.

A: fibroblast growth factor-19 (FGF19) signaling in the distal ileal enterocyte and in hepatocytes in healthy, physiological conditions. Bile acids are transported into the enterocyte via apical sodium-dependent bile acid transporter (ASBT) where it signals the farnesoid X receptor (FXR), which facilitates the transcription of FGF19. FGF19 is transported to the liver via enterohepatic circulation where it interacts with fibroblast growth factor receptor-4 (FGFR4) and coreceptor β-Klotho (KLB) on the membrane of the hepatocyte. The signal is transduced via phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) small heterodimeric partner (SHP), resulting in a downregulation of the transcription of cytochrome P450 7A1 (CYP7A1), resulting in a downregulation of bile acid synthesis. B: disruption of enterohepatic FGF19-Bile acid signaling by biliary atresia, parenteral nutrition-associated liver disease (PNALD), and short bowel syndrome. Biliary atresia and PNALD limit FGF19 transduction and therefore negative feedback on bile acid synthesis in hepatocytes. Short bowel syndrome limits FGF19 production because of a loss of ileal functional mass.

Intestinal failure and short bowel syndrome are often treated with parenteral nutrition. Parenteral nutrition, coupled with a lack of intestinal activity can result in cholestatic liver disease. Mutanen et al. (43) found that when the ileum is lost in cases of pediatric intestinal failure, this results in lower plasma FGF19, which in turn, may exacerbate hepatic inflammation and fibrosis because of bile acid accumulation. Studies in pigs with short bowel syndrome (SBS) suggest that gut dysbiosis and altered bile acid metabolism may lead to blunted activation of intestinal FXR and FGF19 secretion, resulting in increased CYP7A1 expression and bile acid synthesis (44). The impact of targeting FXR and FGF19 in models of short-bowel syndrome to prevent associated liver disease has been mixed. Villarroya and colleagues (35) found that targeting FXR with enteral bile acid treatment improved gut growth but failed to alleviate liver injury in pigs with SBS. Studies in rats with SBS showed that FXR agonist treatment improved intestinal mucosal growth and reduced liver injury (45). In contrast, Pereira-Fantini et al. (44) showed that an FXR agonist (obeticholic acid) exacerbated liver damage in a porcine model of short bowel syndrome.

Biliary atresia is a devastating developmental disease that is the most common cause of neonatal cholestasis and can lead to liver failure and death within two years if left untreated (46). Approximately 70%–80% of patients with biliary atresia will require liver transplants (46). Hasegawa et al. (47) found that hepatocytes collected from pediatric patients with biliary atresia and treated with FGF19 failed to reduce their expression of CYP7A1, suggesting that an underlying pathology of biliary atresia is an uncoupling of hepatic FGF19-CYP7A1 signaling. Johansson et al. (46) expanded on this observation, finding that circulating levels of FGF19 were not correlated with bile acid levels or CYP7A1 activity but decreased FGF19 levels after Kasai portoenterostomy were indicative of increased likelihood of native liver survival. Recently, Xiao et al. (48) found that targeting FXR with a potent agonist in a bile-duct ligation-induced model of obstructive cholestasis ameliorated liver injury and fibrosis.

CONCLUSIONS

There is an overall dearth of knowledge about the fundamental regulation, secretion, and action of FGF19 in the pediatric populations and especially in the perinatal period. Key questions remain about the factors that upregulate FGF19 production at birth, particularly regarding the functionality of the FGF19-CYP7A1 negative feedback loop and the potential systemic, extra-hepatic metabolic effects of FGF19 signaling. Data collected from animal models have demonstrated that FGF19 may be an intriguing target to improve growth, metabolic, and hepatic outcomes in neonates, particularly preterm neonates, but our lack of understanding of the precise mechanisms of regulation of FGF19 limits its current utility as a nutritional or pharmaceutical target.

GRANTS

This paper was supported by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) Grants T32 DK007664 (to C. Vonderohe), K01 DK129408 (to G. Guthrie), and DK094616 (to D. G. Burrin) and by the United States Department of Agriculture-Agricultural Research Service Grant 3092-51000-060-01 (to D. G. Burrin).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

C.V. prepared figures; C.V. and D.G.B. drafted manuscript; C.V., G.G., and D.G.B. edited and revised manuscript; C.V., G.G., and D.G.B. approved final version of manuscript.