Novel transcriptional regulators of bile acid metabolism and hepatotoxicity

描述（由申请方提供）：类法尼醇-X-核受体（FXR）的激活诱导影响多种代谢途径调节的多种基因的转录。FXR在正常生理学中的重要性可见于动物中，其中FXR的失调已显示与异常胆汁酸代谢、高脂血症、动脉粥样硬化、高血糖症、对肝毒素的易感性增加、肝癌和肠屏障的通透性增加相关。尽管有如此多的作用，但FXR在调节胆汁酸代谢和肝肠循环的几乎所有方面中的核心作用是特别重要的。鹅去氧胆酸（CDCA）和胆酸（CA）的肝脏合成分别由酶Cyp 7a 1和Cyp 8b 1调节。许多研究支持肝和肠FXR激活导致Cyp 7a 1和/或Cyp 8b 1抑制的模型。目前的理论表明，这两种基因的抑制依赖于肝脏中SHP水平的增加和/或来源于肝脏的Fgf 15/19与肝细胞质膜上其同源受体的相互作用。然而，SHP和Fgf 15/19的相对重要性以及Cyp 7a 1和Cyp 8b 1的急性或长期抑制机制仍有待确定。事实上，目前的数据表明，额外的未知途径也可能参与这两个基因的抑制。在此，我们确定了两个新的途径，参与Cyp 7a 1和Cyp 8b 1的镇压。首先，我们确定了两个转录抑制因子作为新的FXR靶基因。这两个基因都不知道参与胆汁酸或脂质代谢。然后，我们使用功能获得和功能丧失的研究，以证明这些阻遏物的功能，以抑制Cyp 8b 1和Cyp 7a 1，分别。重要的是，这种抑制改变了胆汁酸库中特定胆汁酸的比例。此外，我们证明了谷胱甘肽转移酶和相关基因的数量的调节，其功能是保护肝脏免受毒性挑战。总之，这些初步研究i）挑战了SHP和Fgf 15/19是Cyp 7a 1和Cyp 8b 1的主要/唯一调节剂的当前范式，ii）提供了解释FXR激活可能导致的肝脏保护的机制。拟议的实验将扩展这些新的发现，并确定所涉及的分子机制。更好地了解FXR激活的作用显得尤为重要，因为FXR激动剂目前正在临床试验中进行测试。

DESCRIPTION (provided by applicant): Activation of the farnesoid-X-nuclear receptor (FXR) induces the transcription of numerous genes that affect the regulation of multiple metabolic pathways. The importance of FXR in normal physiology can be seen in animals where dysregulation of FXR has been shown to be associated with abnormal bile acid metabolism, hyperlipidemia, atherosclerosis, hyperglycemia, increased susceptibility to hepatotoxins, hepatocarcinoma and increased permeability of the intestinal barrier. Despite this plethora of effects, the central role of FXR in regulating virtually all aspects of bile acid metabolism and th enterohepatic circulation is particularly important. The hepatic synthesis of chenodeoxycholic acid (CDCA) and cholic acid (CA) is regulated by the enzymes Cyp7a1 and Cyp8b1, respectively. Numerous studies have supported a model in which activation of hepatic and intestinal FXR results in repression of Cyp7a1 and/or Cyp8b1. Current dogma suggests that repression of these two genes is dependent upon increased levels of SHP in the liver and/or the interaction of intestinally- derived Fgf15/19 with its cognate receptor on the hepatocyte plasma membrane. However, the relative importance of SHP and Fgf15/19, and the mechanisms involved in acute or long-term repression of Cyp7a1 and Cyp8b1 remains to be established. Indeed, the current data suggest that additional as yet unknown, pathways may also be involved in repression of these two genes. Herein, we identify two novel pathways that are involved in the repression of Cyp7a1 and Cyp8b1. First, we identified two transcriptional repressors as novel FXR target genes. Neither gene was known to be involved in bile acid or lipid metabolism. We then used gain-of-function and loss-of-function studies to demonstrate that these repressors function to repress Cyp8b1 and Cyp7a1, respectively. Importantly, such repression altered the ratios of specific bile acids in the bile acid pool. In addition, we demonstrate the regulation of number of glutathione transferases, and related genes, that function to protect the liver from toxic challenges. Together, these preliminary studies i) challenge the current paradigm that SHP and Fgf15/19 are the major/sole regulators of Cyp7a1 and Cyp8b1 and ii) provide a mechanism that explains the hepato-protection that can result from FXR activation. The proposed experiments will extend these novel findings and identify the molecular mechanisms involved. A better understanding of the effects of FXR activation appears particularly important, as FXR agonists are currently being tested in clinical trials.