Molecular Biology of Bile Acid Synthesis

描述（申请人提供）：胆汁酸信号在肝脏代谢和炎症的控制中起关键作用。有毒胆汁酸的积累引起肝脏炎症，并有助于慢性非酒精性脂肪性肝病（NAFLD）、糖尿病和肥胖的发病机制。胆汁酸的合成速率是由第一和限速酶——胆固醇7？-羟化酶（CYP7A1），而胆汁酸组成由甾醇12？-羟化酶（CYP8B1）在胆酸合成中的作用。胆汁酸激活核受体FXR和G蛋白偶联受体TGR5，调节脂质、葡萄糖和能量代谢。研究表明，FXR在胆汁酸反馈抑制胆汁酸合成中起关键作用。胆汁酸的抗炎作用最近才被认识到。然而，胆汁酸调节肝脏代谢稳态和炎症的潜在分子机制尚不清楚。2型糖尿病患者血清12？-羟基化胆汁酸，这与胰岛素抵抗和血脂异常有关。我们设计了三个特定目的来验证FXR和TGR5信号调节胆汁酸的合成和组成，从而影响肝脏炎症、胰岛素抵抗、NAFLD和肥胖的假设。具体目的1是研究FXR和TGR5信号在胆汁酸代谢中的作用机制。FXR-/-、TGR5-/-和FXR/TGR5双敲除（DK）小鼠将验证FXR/TGR5/CYP8B1通路可能在胆汁酸合成和脂质代谢调控中发挥作用的假设。特异性目的2是研究TGR5和FXR信号在肝细胞抗炎中的作用。FXR和TGR5信号的激活可能影响巨噬细胞极化，从而减轻肝脏炎症。采用TGR5-/-、FXR-/-和FXR/TGR5 DK小鼠研究FXR和TGR5的抗炎作用机制。具体目的3是研究NAFLD、胰岛素抵抗和肥胖患者的昼夜节律和禁食/限食对胆酸合成的营养调节。禁食限饲和高脂饮食影响胆汁酸合成和能量代谢的昼夜节律。本实验旨在研究CYP8B1在血脂异常、胰岛素抵抗和NAFLD中的作用。本研究对于阐明胆汁酸合成和脂质稳态调控的分子机制，以及肝脏相关代谢性疾病的发病机制具有重要意义。这项研究的结果将有潜力开发药物治疗，以改善胰岛素敏感性，肝脏炎症，肝脂肪变性，糖尿病和肥胖。

DESCRIPTION (provided by applicant): Bile acid signaling plays a critical role in control of liver metabolism and inflammation. Accumulation of toxic bile acids causes liver inflammation and contributes to pathogenesis of chronic non-alcoholic fatty liver disease (NAFLD), diabetes and obesity. The rate of bile acid synthesis is regulated by the first and rate- limiting enzyme, cholesterol 7?-hydroxylase (CYP7A1), whereas bile acid composition is regulated by sterol 12?-hydroxylase (CYP8B1) in cholic acid synthesis. Bile acids activate a nuclear receptor FXR and a G protein coupled receptor TGR5 to regulate lipid, glucose and energy metabolism. It has been proposed that FXR plays a key role in bile acid feedback inhibition of bile acid synthesis. The anti-inflammatory action of bile acids has been recognized recently. However, the underlying molecular mechanisms of bile acid regulation of hepatic metabolic homeostasis and inflammation are not understood. Type 2 diabetes patients have higher serum 12?-hydroxylated bile acids, which is correlated to insulin resistance and dyslipidemia. Three specific aims are designed to test the hypothesis that FXR and TGR5 signaling regulate bile acid synthesis and composition, which affect hepatic inflammation, insulin resistance, NAFLD and obesity. Specific aim 1 is to study the mechanisms of FXR and TGR5 signaling in bile acid metabolism. FXR-/-, TGR5-/- and FXR/TGR5 double knockout (DK) mice will be used to test the hypothesis that the FXR/TGR5/CYP8B1 pathway may play a role in regulation of bile acid synthesis and lipid metabolism. Specific aim 2 is to study TGR5 and FXR signaling in anti-inflammation in hepatocytes. Activation of FXR and TGR5 signaling may affect macrophage polarization to reduce hepatic inflammation. TGR5-/-, FXR-/- and FXR/TGR5 DK mice will be used to study the mechanisms of anti-inflammatory action of FXR and TGR5. Specific aim 3 is to study nutrient regulation of the diurnal rhythm and fasting/restricted feeding on bile acid synthesis in NAFLD, insulin resistance and obesity. Fasting and restricted feeding and high fat diet affect the circadian rhythm of bile acid synthesis and energy metabolism. Experiments are designed to study the role of CYP8B1 in dyslipidemia, insulin resistance and NAFLD. This study is highly significant in elucidating the molecular mechanisms of regulation of bile acid synthesis and lipid homeostasis, and pathogenesis of liver-related metabolic diseases. Results from this study will have potential for developing drug therapies to improve insulin sensitivity, liver inflammation, hepatic steatosis, diabetes and obesity.