Novel transcriptional regulators of bile acid metabolism and hepatotoxicity

描述(由申请人提供)：激活法尼醇-X-核受体(FXR)诱导许多基因的转录，这些基因影响多个代谢途径的调节。FXR在正常生理中的重要性可以在动物身上看到，在这些动物中，FXR的失调被证明与胆汁酸代谢异常、高脂血症、动脉粥样硬化、高血糖、对肝毒素的易感性增加、肝癌和肠道屏障通透性增加有关。尽管有过多的影响，FXR在调节胆汁酸代谢和肠肝循环的几乎所有方面的核心作用尤其重要。肝脏鹅去氧胆酸(CDCA)和胆酸(CA)的合成分别受Cyp7a1和Cyp8b1酶的调节。大量研究支持肝脏和肠道FXR激活导致Cyp7a1和/或Cyp8b1抑制的模型。目前的学说认为，这两个基因的抑制依赖于肝脏中SHP水平的增加和/或肠道来源的Fgf15/19与其肝细胞质膜上的同源受体的相互作用。然而，SHP和Fgf15/19的相对重要性以及参与急性或长期抑制Cyp7a1和Cyp8b1的机制仍有待确定。事实上，目前的数据表明，其他未知的途径也可能参与抑制这两个基因。在这里，我们发现了两条参与Cyp7a1和Cyp8b1抑制的新途径。首先，我们确定了两个转录抑制因子为新的FXR靶基因。目前尚不清楚这两个基因与胆汁酸或脂肪代谢有关。然后，我们使用功能增益和功能损失研究来证明这些抑制子分别抑制Cyp8b1和Cyp7a1的功能。重要的是，这种抑制改变了胆酸池中特定胆汁酸的比例。此外，我们还展示了谷胱甘肽转移酶和相关基因数量的调节，这些基因具有保护肝脏免受毒物攻击的功能。综上所述，这些初步研究i)挑战了当前的范式，即SHP和Fgf15/19是Cyp7a1和Cyp8b1的主要/唯一调节因子；ii)提供了一种解释FXR激活所导致的肝脏保护的机制。拟议中的实验将扩展这些新的发现，并确定涉及的分子机制。更好地了解FXR激活的影响似乎特别重要，因为FXR激动剂目前正在进行临床试验。

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.