FXR/RXR异二聚体调控下游信号通路机制的结构生物学研究

Bile acid, an important ingredient of bile, is the metabolite of cholesterol degradation in the liver. It plays vital roles in the absorption, clearance and transportation of hydrophobic compounds in liver and gut. The normal metabolism of bile acid is imperative for maintaining physiological function in human body. Obesity, Type Ⅱ diabetes and high cholesterol can raise the free fatty acids level in the liver, leading to NAFLD or even NASH. Recent study confirmed FXR as a valid drug target for treating these diseases. FXR functions through dimerization with RXR. However, the mechanism of how FXR/RXR heterodimer regulate the downstream gene remains to be fully understood. We have already obtained the complex structure of FXR-LBD/RXR-LBD with their co-activators and ligands. In this proposal, we aim to determine the full-length FXR/RXR heterodimer complexed with their DNA response element, cofactors and ligands. This will help reveal the mechanism of FXR/RXR regulating signal pathway and provide a better understanding of the structure function relationship for nuclear receptor family in general.

胆汁酸是胆固醇在肝中降解的代谢产物，也是胆汁的重要成分，对肝肠内疏水性化合物的吸收、清除和转运起着重要作用。胆汁酸代谢对维持人体正常生理功能意义重大。肥胖、Ⅱ型糖尿病及高胆固醇都能导致肝脏中自由脂肪酸的升高，从而造成非酒精性脂肪肝,进而发展为脂肪性肝炎。近来，作为体内胆汁酸感应器的核受体FXR已经成为治疗此类疾病重要的药物靶点。FXR通过与RXR形成异二聚体行使其生物学功能，目前对于该异二聚体调控下游信号的机制并不清楚。我们已经解析了FXR-LBD/RXR-LBD与辅因子及配体的复合物结构，本课题将进一步解析全长FXR/RXR蛋白与DNA反应元件及辅因子、配体的多元复合物晶体结构，揭示其信号通路调节机制的结构基础，进一步完善对核受体家族功能的认识。

核受体FXR作为主要的胆汁酸受体在胆固醇代谢，脂类平衡及膳食中脂肪和维生素的吸收中起到重要作用。由于其在代谢中的核心作用, 所以FXR是代谢类疾病的重要药物靶标, 如原发性胆汁性肝硬化和非酒精性肝炎。因此FXR调节剂的开发具有重要意义。FXR在体内和RXR一起形成异二聚体来调控下游多种基因的表达。而到目前为止，FXR/RXR异二聚体的结构基础及功能并不是很清楚。我们解析了四个FXR与HNC化合物的复合物晶体结构，发现小分子的不同构象会诱导FXR配体结合口袋构象的变化。然后我们进一步解析了人源FXR/RXR配体结合区的异二聚体结构。通过荧光偏振实验，我们发现异二聚体中的FXR和RXR比它们各自为单体时对辅因子（SRC1）的亲和力高。此外，通过比对不同小分子结合的FXR单体和FXR/RXR异二聚体结构发现结合小分子及异二聚化使得FXR的螺旋11的羧基末端发生构象变化，进而影响辅因子结合界面的稳定性，最终影响FXR上的辅因子结合。我们的研究结果揭示了FXR/RXR异二聚体间的信号转导途径，为针对FXR的药物研发提供了可能的新方向。