Developmental regulation of the gut-liver (FGF19-CYP7A1) axis in neonates.

Introduction: Fibroblast growth factor 19 (FGF19) is a gut-derived hormone that regulates the expression of CYP7A1, the rate-limiting enzyme in bile acid (BA) synthesis pathway. Dysregulation of the FGF19-CYP7A1 (gut-liver) axis is associated with cholestatic liver disease. Infants, especially preterm infants and those with intestinal failure are at high risk for developing cholestatic liver disease. The activity of the gut-liver axis has not been characterized in this population. Our objective was to assess relationships between circulating FGF19 concentrations and CYP7A1 activity in neonates.Materials and methods: Plasma samples were obtained longitudinally from term and preterm infants (22-41-week gestation) hospitalized in a neonatal intensive care unit. Infants with congenital and acquired gastrointestinal disorders were excluded. Plasma levels of 7α-hydroxy-4-cholesten-3-one (C4), a marker of CYP7A1 activity, were quantified using HPLC-MS/MS. Plasma FGF19 concentrations were quantified by ELISA. Data were analyzed using linear regression models and structural equation modeling.Results: One hundred eighty-one plasma samples were analyzed from 62 infants. C4 concentrations were undetectable prior to 30 weeks' gestation and, thereafter, increased with advancing gestational age and with volume of enteral feeds. They did not correlate with serum FGF19 concentrations, which decreased with advancing gestational age and volume of enteral feeds.Discussion: The activity of CYP7A1, the rate-limiting BA synthetic enzyme in adults, is developmentally regulated and undetectable in newborns less than 30 weeks' gestation. FGF19 concentrations do not correlate with CYP7A1 activity, suggesting that the gut-liver axis is not functional in infants. High FGF19 concentrations at birth in infants less than 37 weeks' gestation is a novel finding, and its source and role in preterm infants warrants further investigation.Rationale: The intestinal hormone, fibroblast growth factor 19 (FGF19), is a major regulator of CYP7A1, the rate limiting enzyme in bile acid (BA) synthesis. Recently, dysregulation of the gut-liver (FGF19-CYP7A1) axis has been implicated in adult cholestatic liver disease, and animal studies have shown that exogenous FGF19 protects against liver injury. Given the therapeutic potential related to this signaling pathway, we sought to characterize the association between CYP7A1 and FGF19 in term and preterm infants. We conducted a prospective, observational study that measured in vivo CYP7A1 activity and FGF19 concentrations in 62 term and preterm infants (n = 181 samples). We found that CYP7A1 activity is developmentally regulated; its activity is undetectable prior to 30 weeks' gestation and increases with advancing gestational age and volume of enteral feeds. Contrary to expectation, we demonstrated that FGF19 is expressed at birth in preterm infants and decreases over time, even as enteral feeds increase. Using structural equation modeling, we were able to show that CYP7A1 activity does not correlate with FGF19 concentrations. Our results suggest that the gut-liver axis is not upregulated in preterm and term infants and that neonates with cholestatic liver disease will unlikely benefit from supplemental FGF19. We also report novel findings of elevated FGF19 concentrations in preterm infants at birth and speculate that there may be an extra-intestinal source of FGF19 that is developmentally expressed in these infants.