New pathways regulating bile acid homeostasis and cholestatic liver diseases

描述（由申请人提供）：转化为胆汁酸是体内胆固醇消除的主要途径，因此对维持胆固醇稳态至关重要。哺乳动物肝脏中至少存在两条胆汁酸生物合成途径。主要的中性途径由Cyp 7a 1启动，然后是Cyp 8b 1。另外的酸性途径由Cyp 27启动。胆汁形成和/或胆汁流动的损害将 引起细胞毒性胆汁酸的肝内滞留，导致胆管和肝细胞的肝损伤，并导致胆汁淤积性肝病。虽然核受体是胆汁酸合成酶的重要转录调节因子， 一个关键的未满足的需求，以确定新的球员，控制胆汁酸稳态，以更好地 了解胆汁淤积性肝病的分子基础。 基于我们广泛的体外和体内的初步结果，在目前的赠款周期中产生的，我们已经发现了一个新的作用，转录因子E2 F1在调节胆汁酸代谢。以前E2 F1已被证明在细胞周期进程，细胞凋亡，脂肪细胞和β细胞功能中发挥作用。根据我们的初步结果，有必要探讨E2 F1在胆汁淤积性肝纤维化中的关键作用。这项研究意义重大，因为它揭示了 首次发现E2 F1作为一个看门人调控胆汁酸信号通路中的关键基因。 本提案的总体目标是了解E2 F1转录的关键作用 在胆汁淤积性肝纤维化的发展中，通过两种不同的机制：1）Egr-1（早期生长反应1）表达的独特双相调节以控制胆汁淤积性肝脏炎症和损伤;和2）SHP（小异二聚体伴侣）和Cyp 7a 1（胆固醇7 α-羟化酶）/Cyp 8b 1（12 α-羟化酶）表达的直接调节以控制胆汁酸生物合成和代谢。基于我们的初步结果，本申请的中心假设是E2 F1是一个新的参与者，通过胆汁酸信号通路中关键基因的转录调节来控制胆汁酸稳态。我们认为E2 F1是协调控制胆汁酸水平的调节网络的中心。具体地说， E2 F1作为Egr-1和Cyp 7a 1/Cyp 8b 1表达的正调节剂，以及SHP基因表达的负调节剂。E2 F1对SHP的抑制减弱了SHP对Egr-1和Cyp 7a 1/8b 1的抑制，这增强了E2 F1的活性。通过E2 F1/Cyp 7a 1/8b 1级联诱导胆汁酸进一步增强E2 F1通过FXR激活Egr-1。具体目标是：1）通过Egr-1的双相调节阐明E2 F1在胆汁淤积性肝纤维化中的作用; 2）通过与SHP和Cyp 7a 1/Cyp 8b 1的相互作用阐明E2 F1在胆汁酸稳态中的作用。充分确定胆汁酸代谢的调节，包括胆汁酸合成和catalysts，是必不可少的进一步发展，在诊断，管理和预防胆汁淤积性肝病。

DESCRIPTION (provided by applicant): The conversion to bile acids represents a major pathway for cholesterol elimination from the body, which is therefore of critical importance in maintaining cholesterol homeostasis. At least two pathways of bile acid biosynthesis exist in mammalian liver. The major neutral pathway is initiated by Cyp7a1, followed by Cyp8b1. The additional acidic pathway is initiated by Cyp27. Impairment of bile formation and/or bile flow will cause intrahepatic retention of cytotoxic bile acids, leading to liver injury to bile ducts and hepatocytes, and resulting in cholestatic liver diseases. Although it is well established that nuclear receptors are important transcriptional regulators of bile acid synthetic enzymes, there is a critical unmet need to identify new players that control bile acid homeostasis in order to better understand the molecular basis of cholestatic liver diseases. Based on our extensive in vitro and in vivo preliminary results generated during the current grant cycle, we have uncovered a novel role for the transcription factor E2F1 in the regulation of bile acid metabolism. Previously E2F1 has been shown to play a role in cell cycle progression, apoptosis, and adipocyte and beta cell function. In light of our preliminary results, it is imperative to explore the critical fnction of E2F1 in cholestatic liver fibrosis. The proposed research is significant because it uncovers for the first time E2F1 as a gate keeper that regulates key genes in the bile acid signaling pathway. The overall objective of this proposal is to understand the critical role of the E2F1 transcription factor in the development of cholestatic liver fibrosis through two distinct mechanisms: 1) a unique biphasic regulation of Egr-1 (early growth response 1) expression to control cholestatic liver inflammation and injury; and 2) a direct regulation of SHP (small heterodimer partner) and Cyp7a1 (cholesterol 7 alpha-hydroxylase)/Cyp8b1 (12- alpha-hydroxylase) expression to control bile acid biosynthesis and metabolism. The central hypothesis of this application, based on our Preliminary Results, is that E2F1 is a new player that controls bile acid homeostasis by transcriptional regulation of key genes in the bile acid signaling pathway. We propose that E2F1 is at the center of a regulatory network that coordinately controls bile acid levels. Specifically, E2F1 functions as both a positive regulator of Egr-1 and Cyp7a1/Cyp8b1 expression, and a negative regulator of SHP gene expression. E2F1 repression of SHP diminishes SHP inhibition of Egr-1 and Cyp7a1/8b1, which reinforces E2F1 activity. Induction of bile acids by the E2F1/Cyp7a1/8b1 cascade further enhances Egr-1 activation by E2F1 via FXR. The Specific Aims are: 1) To elucidate the role of E2F1 in cholestatic liver fibrosis via a biphasic regulation f Egr-1, and 2) To elucidate the role of E2F1 in bile acid homeostasis via cross-talk with SHP and Cyp7a1/Cyp8b1. Fully defining the regulation of bile acid metabolism, including bile acid synthesis and catabolism, is essential for further advances in diagnosis, management, and prevention of cholestatic liver diseases.