Tissue specific mechanism of FXR in suppressing bile-acid synthesis

描述（申请人提供）：胆汁酸主要在肝脏中以胆固醇7a羟化酶（cyp7a1）为限速酶产生。胆汁酸与肝脏、胆道、肠道和心血管疾病有关。FXR是一种胆汁酸激活的核受体，在维持胆汁酸稳态中起重要作用。FXR功能障碍有助于胆汁淤积、胆结石、脂肪肝疾病和肝脏肿瘤的发展。新出现的证据表明，在小鼠中，FXR通过肝脏和肠道中启动的途径抑制cyp7a1基因的表达。然而，为了理解FXR在抑制胆红酸合成中的作用，在明确抑制途径的起始位置、SHP和FGF15的参与程度以及肝脏中有哪些信号分子抑制cyp7a1基因表达方面存在明显的空白。本应用的目的是建立FXR以组织特异性方式调节胆汁酸合成的潜在分子机制。我的假设是，肠道启动的FGF15- FGR4途径是主要的，肝脏启动的SHP-LRH-1级联是FXR激活抑制cyp7a1的次要潜在机制，此外，Egr-1和cJun都参与FGFR4途径抑制cyp7a1基因表达。该假设是基于我们令人信服的初步数据，这些数据共同表明，FXR，而不是SHP，肠道FXR，而不是肝脏FXR，是抑制cyp7a1基因表达的主要原因。我们计划通过追求以下具体目标来检验假设并实现目标。(1)建立FXR抑制cyp7a1基因表达的组织特异性功能。(2)确定SHP和FGF15在多大程度上参与抑制cyp7a1和调节胆酸稳态。(3)阐明FGFR4激活后抑制cyp7a1基因表达的信号通路。提出的研究是创新的，代表了理解胆汁酸合成调节的范式转变。我们是第一个将组织特异性FXR KO小鼠的使用与细胞和分子技术相结合，以确定调节胆汁酸反馈抑制的基本途径。这些研究将确定FXR在组织中的特定作用，这将为研究胆汁酸、胆固醇和甘油三酯的稳态提供基础的理解。此外，该研究的完成将有助于在未来设计组织特异性FXR调节剂，以更好地预防和治疗与胆汁酸紊乱相关的人类疾病。公共卫生相关性：胆汁酸是胆汁的主要成分，对调节胆固醇和甘油三酯的稳态很重要。FXR在调节胆汁酸稳态中是必不可少的，并有助于胆汁淤积、胆结石、脂肪肝疾病、肝脏肿瘤和动脉粥样硬化的发展。该研究的完成将有助于未来设计组织特异性FXR调节剂，以更好地预防和治疗与胆汁酸紊乱相关的人类疾病。

DESCRIPTION (provided by applicant): Bile acids are mainly produced in the liver with cholesterol 7a hydroxylase (cyp7a1) the rate-limiting enzyme. Bile acids are involved in liver, biliary, intestinal, and cardiovascular diseases. FXR is a bile-acid activated nuclear receptor and is essential in maintaining bile-acid homeostasis. FXR dysfunction contributes to the development of cholestasis, gallstones, fatty-liver disease, and liver tumor. Emerging evidence suggest that in mice FXR suppresses cyp7a1 gene expression via pathways initiated both in the liver and the intestine. However, to understand the roles of FXR in suppressing bile-acid synthesis, clear gaps exist in consolidating where the suppressive pathway initiates, to what extent SHP and FGF15 are involved, and what signaling molecules are in the liver to suppress cyp7a1 gene expression. The objective of this application is to establish the underlying molecular mechanism by which FXR regulates bile-acid synthesis in a tissue-specific manner. My hypothesis is that intestine-initiated FGF15- FGR4 pathway is the major and the liver-initiated SHP-LRH-1 cascade is a minor underlying mechanism for suppressing cyp7a1 with FXR activation, in addition, both Egr-1 and cJun are involved in FGFR4 pathway to suppress cyp7a1 gene expression. The hypothesis is based on our compelling preliminary data that collectively showed that FXR, but not SHP, intestinal FXR, but not hepatic FXR, is mainly responsible for the suppression of cyp7a1 gene expression. We plan to test the hypothesis and accomplish the objective by pursuing the following specific aims. (1) Establish the tissue-specific function of FXR in suppressing cyp7a1 gene expression. (2) Determine to what extent that SHP and FGF15 are involved in suppressing cyp7a1 and regulating bile-acid homeostasis. (3) Elucidate the signaling pathways for suppressing cyp7a1 gene expression following FGFR4 activation. The proposed research is innovative and represents a paradigm shift in understanding the regulation of bile-acid synthesis. We are the first to combine the usage of tissue-specific FXR KO mice with cellular and molecular techniques to identify the fundamental pathways in regulating bile-acid feedback inhibition. These studies will identify the tissue specific roles of FXR, which will provide a fundamental understanding for the study of bile-acid, cholesterol, and triglyceride homeostasis. Furthermore, completion of the proposed study will shed light on designing tissue-specific FXR modulators in the future to better prevent and treat human diseases associated with bile-acid disorders. Public Health Relevance: Bile acids are the main components in bile and are important for regulating cholesterol and triglyceride homeostasis. FXR is essential in regulating bile-acid homeostasis, and contributes to the development of cholestasis, gallstones, fatty-liver disease, liver tumors, and atherosclerosis. Completion of the proposed study will shed light on designing tissue-specific FXR modulators in the future to better prevent and treat human diseases associated with bile-acid disorders.