Gut bile acids, Fxr, & Fgf15 in total parenteral nutrition-associated cholestasis

描述（由申请人提供）：全肠外营养（Total parenteral nutrition， TPN）广泛用于不能耐受肠内喂养的患者，包括早产儿、小肠手术患者或胰腺疾病患者。长期TPN与严重并发症TPN相关性胆汁淤积（TPN- ac）相关。TPN-AC的发病机制尚不清楚。任何深入了解TPN-AC病因的分子机制的努力都可能使我们在未来找到一种预防其发生的方法。胆汁酸的过量产生对肝脏是有害的，胆汁酸的稳态需要严格调节。最重要的机制是调节胆汁酸的稳态是由一个配体激活的转录因子，属于核受体超家族，farnesoid X受体（Fxr）。我们和其他人已经证明，在小鼠中，肠道Fxr的激活诱导小肠成纤维细胞生长因子15 （Fgf15）通过抑制编码胆汁酸合成限速酶的Cyp7a1基因表达来抑制胆汁酸合成。相反，肝脏中Fxr的激活诱导小异源二聚体伴侣（Shp），其在抑制Cyp7a1基因表达中仅起次要作用。这种理解肠道因子介导的肝脏胆汁酸合成调节的关键作用的范式转变也已在人类肝细胞中得到证实。因此，肠道胆汁酸/Fxr/Fgf15通路可能是TPN-AC病因和治疗的根本依据。本研究的目的是确定肠道胆汁酸/Fxr/Fgf15通路在小鼠TPN-AC发育和治疗中的作用，从而为人类提供新的治疗策略。我推测TPN- ac的机制是TPN增加胆汁酸合成和破坏肠肝循环，导致胆汁淤积，失去肠胆汁酸/Fxr/Fgf15途径的激活；通过向肠道补充胆汁酸、重新激活肠道Fxr或在TPN期间给予外源性Fgf15来重建这一途径可能预防和/或治疗TPN- ac。提出了三个独立但相互关联的目标，以在小鼠中验证这一新的假设。目的1。全面表征TPN对胆汁酸稳态的破坏，确定胆汁酸合成对TPN- ac发展的贡献，并测试补充肠道胆汁酸是否能预防和/或治疗TPN- ac。目标2。确定TPN对肠道Fxr功能的影响，并检测肠道Fxr再激活在多大程度上预防或治疗TPN- ac。目标3。确定Fgf15在预防和治疗TPN-AC中的作用。我们之前的工作为理解通过肠道胆汁酸/Fxr/Fgf15途径抑制肝脏胆汁酸合成的机制提供了范式转变。该建议一旦完成，将为了解TPN-AC的病因机制提供新的关键科学基础，并可能对未来设计新的治疗策略预防和/或治疗TPN-AC产生巨大影响。

DESCRIPTION (provided by applicant): Total parenteral nutrition (TPN) is widely used to supply nutrients to patients who cannot tolerate enteral feeding, including premature infants, patients with small-bowel surgery, or patients with pancreatic diseases. Long-term TPN is associated with a severe complication, TPN-associated cholestasis (TPN-AC). The mechanism underlying the cause of TPN-AC is poorly understood. Any effort to gain insight into the molecular mechanism of TPN-AC cause may allow us to develop a way to prevent its occurrence in future. Overt production of bile acids is toxic to livers and bile acid homeostasis needs to be tightly regulated. The most important mechanism is regulating bile acid homeostasis is medicated by a ligand-activated transcription factor belonging to the nuclear receptor superfamily, farnesoid X receptor (Fxr). We and others have shown that in mice, activation of intestinal Fxr induces fibroblast growth factor 15 (Fgf15) in the small intestine to suppress bile acid synthesis by inhibiting the gene expression of Cyp7a1 that encodes the rate-limiting enzyme in bile acid synthesis. In contrast, activation of Fxr in the liver induces small heterodimer partner (Shp), which only plays a minor role in inhibiting Cyp7a1 gene expression. This paradigm shift in understanding critical roles of gut factor-mediated regulation of bile acid synthesis in the liver has also been confirmed in human hepatocytes. Therefore, the intestinal bile acids/Fxr/Fgf15 pathway may be a fundamental basis for TPN-AC cause and treatment. The objective of this proposal is to determine the role of the intestinal bile acids/Fxr/Fgf15 pathway in TPN-AC development and treatment in mice, in order to provide a novel therapeutic strategy in humans. I hypothesize that the mechanism of TPN-AC cause is that increased bile acid synthesis and disruption of enterohepatic circulation by TPN leads to cholestasis, following loss of activation of the intestine bile acids/Fxr/Fgf15 pathway; re-establishing this pathway by replenishing bile acids into the gut, re-activation of intestinal Fxr or administering exogenous Fgf15 during TPN may prevent and/or treat TPN-AC. Three independent but inter-related aims are proposed to test this novel hypothesis in mice. Aim 1. Comprehensively characterize the disruption of bile acid homeostasis by TPN, determine the contribution of bile acid synthesis to TPN-AC development, and test whether replenishing gut bile acids prevent and/or treat TPN-AC. Aim 2. Determine the effects of TPN on intestinal Fxr function, and test to what degree re-activation of gut Fxr prevents or treats TPN-AC. Aim 3. Determine the role of Fgf15 in the prevention and treatment of TPN-AC. Our previous work has provided a paradigm shift in understanding the mechanism of suppressing bile acid synthesis in the liver by the gut bile acids/Fxr/Fgf15 pathway. This proposal, once completed, will provide a novel and critical scientific basis in understanding the mechanism(s) of TPN-AC cause, and likely have a huge impact on designing novel therapeutic strategies in future prevention and/or treatment of TPN-AC.