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.引言：成纤维细胞生长因子19（Fibroblast growth factor 19, FGF19）是一种肠道来源激素，可调控胆汁酸（BA）合成通路中的限速酶细胞色素P450 7A1（CYP7A1）的表达。FGF19-CYP7A1（肠肝）轴的调控异常与胆汁淤积性肝病相关。婴儿群体，尤其是早产儿及肠功能衰竭患儿，罹患胆汁淤积性肝病的风险较高。目前该群体的肠肝轴活性尚未得到明确表征。本研究旨在评估新生儿循环FGF19浓度与CYP7A1活性之间的关联。 材料与方法：本研究纵向采集了新生儿重症监护病房（neonatal intensive care unit）住院的足月儿与早产儿（孕周22~41周）的血浆样本。排除合并先天性及获得性胃肠道疾病的婴儿。采用高效液相色谱-串联质谱法（HPLC-MS/MS）定量检测CYP7A1活性标志物7α-羟基-4-胆甾烯-3-酮（C4）的血浆水平，通过酶联免疫吸附试验（ELISA）定量检测血浆FGF19浓度。采用线性回归模型（linear regression models）与结构方程模型（structural equation modeling）对数据进行分析。 结果：本研究共纳入62名婴儿，分析其181份血浆样本。孕30周前无法检测到C4浓度，此后C4浓度随孕周增加及肠内喂养量增加而升高。C4浓度与血清FGF19浓度无相关性；而FGF19浓度随孕周增加及肠内喂养量增加而降低。 讨论：作为成人胆汁酸合成的限速酶，CYP7A1的活性受发育调控，孕30周前的新生儿无法检测到其活性。FGF19浓度与CYP7A1活性无相关性，提示婴儿的肠肝轴尚未具备功能。孕37周前的新生儿出生时FGF19浓度较高，这是一项全新的发现，其来源及在早产儿中的作用有待进一步研究。 研究依据：肠道激素成纤维细胞生长因子19（Fibroblast growth factor 19, FGF19）是胆汁酸（BA）合成限速酶细胞色素P450 7A1（CYP7A1）的主要调控因子。近期研究显示，肠肝（FGF19-CYP7A1）轴调控异常与成人胆汁淤积性肝病相关，且动物实验表明外源性FGF19可减轻肝损伤。鉴于该信号通路具备治疗潜力，我们旨在表征足月儿与早产儿体内CYP7A1活性与FGF19浓度之间的关联。本研究为一项前瞻性观察性研究，共纳入62名足月儿与早产儿（共181份样本），检测其体内CYP7A1活性与FGF19浓度。研究发现，CYP7A1活性受发育调控：孕30周前无法检测到其活性，且随孕周增加及肠内喂养量增加而升高。与预期相反，我们证实早产儿出生时即可检测到FGF19表达，且其浓度随时间推移逐渐降低，即便肠内喂养量增加亦是如此。通过结构方程模型分析，我们证实CYP7A1活性与FGF19浓度无相关性。研究结果提示，早产儿与足月儿的肠肝轴均未出现上调，因此胆汁淤积性肝病新生儿或无法从补充性FGF19治疗中获益。我们还首次报道了早产儿出生时FGF19浓度升高这一全新发现，并推测此类婴儿体内可能存在发育阶段特异性的肠外FGF19来源。