Role of Cholesterol in Non Alcoholic Fatty Liver

描述(由申请人提供)： 非酒精性脂肪性肝病(NAFLD)是一种肝脏脂代谢紊乱，胆固醇和脂肪均积聚在肝细胞内。很大一部分NAFLD患者进展为肝脏炎症、坏死和进行性肝纤维化。非酒精性脂肪肝现在被认为是美国和退伍军人人群中导致肝硬变的主要原因；而且很快就会成为肝移植最常见的适应症。除了进展为终末期肝病外，它还经常与糖尿病(胰岛素抵抗)和心血管疾病有关。这些临床表型的聚集现在被称为代谢综合征。目前治疗NAFLD的方法包括降胆固醇和甘油三酯、胰岛素增敏剂、抗氧化剂和生活方式改变等，但尚未发现对逆转NAFLD有效。目前还没有好的治疗方法。为了开发更有效的治疗方法，更好地了解肝细胞胆固醇、脂肪、葡萄糖和胆汁酸代谢的相互作用是至关重要的。肝脏是维持全身脂肪平衡的中心。我们的实验室已经确认，将胆固醇输送到细胞线粒体进行氧化是调节肝脏脂肪代谢的关键步骤。我们发现，线粒体胆固醇传递蛋白StARD1在肝细胞中的表达增加，下调了胆固醇的生物合成途径，而上调了胆固醇的降解和分泌途径。肝脏StARD1表达的增加通过线粒体酶CYP27A1启动的途径增加了胆固醇的氧化；由此产生的氧固醇产物是能够调节脂质代谢变化的调节分子。此外，胆汁酸是CYP27A1启动的胆固醇代谢途径的最终产物，本身就是重要的调节分子；能够通过激活肠道和肝脏的核受体FXR来调节胆固醇和脂肪的肠道吸收和代谢。本应用的目的是阐明胆固醇氧化的StARD1/CyP27A1途径在调节肝脏脂质和血糖稳态中的作用。这些研究的完成：1)将确定在接近模拟典型美国生活方式的条件下，在2个NAFL动物模型中，增加细胞内胆固醇降解为调节性氧固醇和FXR激活胆汁酸是否能够逆转脂肪肝；2)将被用于将FXR介导的途径的激活从调节性氧固醇增加的影响中分离出来；3)寻找通过增加StARD1/CYP27A1途径的表达而产生的未被发现的调节性氧固醇，这有助于面对西方饮食所注意到的显著的肝脏降脂；4)寻找控制细胞内调节器氧固醇水平的新的调节位点；以及，5)确定在存在和不存在高脂肪/胆固醇肠道内容物的情况下，自然产生的FXR激动剂(即胆汁酸)和合成的FXR激动剂如何在Cyp27a1基因敲除小鼠模型中以不同的方式改变肝脏的胆固醇、脂肪和葡萄糖代谢；缺乏CYP27A1的自然发生的脂肪肝遗传模型产生氧固醇。根据我们的初步发现，我们认为，联合使用增加选择性激活FXR和调节氧固醇激活的通路可以用来逆转NAFL，并可能逆转动脉粥样硬化。这一提议带来了基础科学发现，导致了新的治疗方法。正如提案中概述的那样，这些研究有可能立即为NAFLD的治疗提供临床前景，不仅是在我们的退伍军人群体中，而且在整个美国人口中。

DESCRIPTION (provided by applicant): Non-alcoholic fatty liver disease (NAFLD) is a disorder of liver lipid metabolism in which both cholesterol and fat accumulate in liver cells. A significant fraction of patients with NAFLD progress to liver inflammation, necrosis, and progressive liver fibrosis. NAFLD is now recognized as a leading cause of cirrhosis in the U.S. and in the Veteran's population; and, is poised to soon represent the most common indication for liver transplantation. In addition to its progression to end-stage liver disease, it is frequently associated with diabetes (insulin resistance) and cardiovascular disease. The clustering of these clinical phenotypes is now known as Metabolic Syndrome. Current therapeutic approaches for the treatment of NAFLD inclusive of cholesterol and triglyceride lowering agents, insulin sensitizing agents, anti-oxidant, and life-style modifications have not been found effective in reversing NAFLD. No good treatment approach currently exists. A better understanding of the interplay of liver cell cholesterol, fat, glucose, and bile acid metabolism is crucial in order to develop more effective therapies. The liver is central to total body lipid homeostasis. Our laboratory has identified delivery of cholesterol into the cell's mitochondria for oxidation as a crucial step in the regulaton of liver lipid metabolism. We have found that increased expression of the mitochondrial cholesterol delivery protein, StARD1, in hepatocytes (liver cells), down-regulates pathways of cholesterol biosynthesis while up-regulating pathways of cholesterol degradation and secretion. Increasing liver StARD1 expression increases cholesterol oxidation via pathways initiated by mitochondrial enzyme, CYP27A1; and, the resulting oxysterol products are regulatory molecules capable of mediating changes in lipid metabolism. Furthermore, bile acids, the end product of the CYP27A1 initiated pathway of cholesterol metabolism, are important regulatory molecules in their own right; capable of mediating cholesterol and fat intestinal absorption and metabolism through activation of an intestinal and liver nuclear receptor, FXR. The objective of this application is to elucidate the role of the StARD1/CYP27A1 pathway of cholesterol oxidation in the regulation of hepatic lipid and glucose homeostasis. The completion of these studies: 1) will determine if increasing intracellular cholesterol degradation into regulatory oxysterols and FXR activating bile acids is capable of reversing fatty liver in 2 NAFL animal models under conditions which closely simulate a typical American lifestyle; 2) will be used to pull apart the activation o FXR mediated pathways by bile acids from the effects of increased regulatory oxysterols; 3) to search for undiscovered regulatory oxysterols generated by increasing expression of the StARD1/CYP27A1 pathway that contribute to the profound liver lipid lowering noted in the face of a Western diet; 4) to search for novel sites of regulation which control intracellular regulator oxysterol levels; and, 5) to determine in the presence and absence of high fat/cholesterol intestinal content how naturally occurring FXR agonists (i.e bile acids) vs. synthetic FXR agonists may differentially alter hepatic cholesterol, fat, and glucose metabolism in a Cyp27a1 knockout mouse model; a naturally occurring genetic model of fatty liver lacking CYP27A1 generated oxysterols. Based on our preliminary findings, we believe that used in concert increased selective activation of the FXR and regulatory oxysterol activated pathways can be utilized to reverse NAFL, and, likely, atherogenesis. This proposal brings forth basic science discoveries that lead to novel treatment approaches. As outlined in the proposal, these studies have the potential to offer immediate clinical promise for treatment of NAFLD not only in our Veterans population, but in the U.S. population as a whole.