LXR-Dependent Cholesterol Sensing

项目总结： 据估计，非酒精性脂肪性肝病(NAFLD)影响了美国25%的成年人 美国和大约25%的NAFLD患者进展为非酒精性脂肪性肝炎(NASH)， 以肝脏炎症、肝细胞肿胀和纤维化为特征。重要的是，纳什增加了风险 用于肝硬变、肝细胞癌和肝功能衰竭。触发NAFLD进展的过程 对于纳什，或者即使有从一种病理状态到另一种病理状态的真正的循序渐进，仍然是 下定决心。非酒精性脂肪肝与肝脏甘油三酯升高和脂肪酸合成速率升高有关。 然而，由于大多数非酒精性脂肪肝患者没有NASH，目前还不清楚是否只有升高的脂肪酸才是 足以促进炎症和纤维化。NASH患者肝脏中的胆固醇也会增加 非酯化胆固醇与疾病严重程度相关。我们认为，一种独特的方法 解开胆固醇在慢性肝病中的作用将可逆地扰乱胆固醇的感知。 肝X受体α(LxRα)是一种重要的胆固醇感受器，调节肝脏脂肪和 胆固醇代谢在转录水平上响应胆固醇衍生物的直接结合。 为了干扰胆固醇的感知，我们已经产生了将lxrα的色氨酸441改变为 苯丙氨酸(W441F)。W441F干扰内源性胆固醇衍生LXR配体的结合 允许由有效的合成激动剂进行转录调节，提供了一种独特的工具，可以阻止 LXRα能够感知胆固醇，同时仍允许进行药物控制。 当喂饲高脂/高胆固醇饲料时，αW441F小鼠发育迅速(在4周内) 与NASH相关的病理学。令人惊讶的是，W441F小鼠肝脏甘油三酯降低，但 胆固醇的增加。因此，在没有肝脏升高的情况下，可能会出现纳什样表型。 甘油三酯(“胆固醇依赖型NASH”)。我们假设W441F的LXR活性降低 肝细胞导致胆固醇积聚作为启动事件的促进 炎症和纤维化。此外，我们认为升高的肝脏胆固醇促进NASH，在 至少部分是通过依赖胆固醇激活带有PDZ结合基序的转录辅助激活因子 (TAZ，WWTR1)。TAZ在人类NASH患者的肝脏中过度表达，并促进炎症和 在小鼠模型中过度表达时的纤维化。最后，我们在W441F中重新建立LXR活动的能力 使用药理学试剂的小鼠提供了一个独特的机会来确定胆固醇- 从属纳什可以反转。综上所述，我们建议的研究将为 胆固醇水平升高在NASH和其他慢性疾病发病机制中的作用 疾病。

Project Summary: Non-alcoholic fatty liver disease (NAFLD) is estimated to affect 25% of the adult population in the United States and approximately 25% of NAFLD patients progress to non-alcoholic steatohepatitis (NASH), characterized by liver inflammation, hepatocyte ballooning, and fibrosis. Importantly, NASH increases the risks for cirrhosis, hepatocellular carcinoma, and liver failure. The processes that trigger the progression of NAFLD to NASH or even if there is a true stepwise progression from one pathological state to the other remain to be determined. NAFLD is associated with increases in liver triglycerides and elevated rates of fatty acid synthesis. Nevertheless, since most NAFLD patients do not have NASH, it is not clear if elevated fatty acids alone are sufficient to promote inflammation and fibrosis. Cholesterol is also increased in the livers of patients with NASH and non-esterified cholesterol correlates with disease severity. We reasoned that a unique approach to unraveling the roles of cholesterol in chronic liver diseases would be to reversibly disrupt cholesterol sensing. Liver x receptor alpha (LXRα) functions as an important cholesterol sensor that regulates hepatic fat and cholesterol metabolism at the transcriptional level in response to the direct binding of cholesterol derivatives. To disrupt cholesterol sensing we have generated mouse lines that change tryptophan 441 of LXRα to phenylalanine (W441F). W441F disrupts binding of endogenous cholesterol derived LXR ligands while still allowing transcription regulation by potent synthetic agonists providing a unique tool that blocks the ability of LXRα to sense cholesterol while still allowing pharmacological control. When fed a high fat/high cholesterol diet LXRα W441F mice rapidly (within 4 weeks) develop pathologies associated with NASH. Strikingly W441F mice have decreased hepatic triglycerides but large increases in cholesterol. Therefore, NASH-like phenotypes can arise in the absence of elevated hepatic triglycerides (“cholesterol-dependent NASH”). We hypothesize that decreased LXR activity in W441F hepatocytes leads to cholesterol accumulation which serves as the initiating event promoting inflammation and fibrosis. Furthermore, we propose that elevated hepatic cholesterol promotes NASH, at least in part, via cholesterol-dependent activation of the transcriptional coactivator with PDZ binding motif (TAZ, Wwtr1). TAZ is over expressed in livers of human NASH patients and promotes inflammation and fibrosis when over expressed in mouse models. Finally, our ability to re-establish LXR activity in W441F mice using pharmacological agents provides a unique opportunity to determine if cholesterol- dependent NASH can be reversed. Taken together our proposed studies will provide new insights into the mechanisms by which elevated cholesterol levels contribute to the pathogenesis of NASH and other chronic diseases.