微小RNA介导的甲状腺激素对肝脏胆固醇代谢及胆汁酸合成途径的调控作用及机制研究

Maintaining cholesterol homeostasis is important for our health. Elevated or abnormal cholesterol levels are associated with an increased risk for cardiovascular and cerebrovascular diseases. Thyroid hormone (TH) has a very potent cholesterol-lowering effect. Increased synthesis of bile acid from cholesterol (biliary cholesterol excretion) by TH has been proposed to explain the TH-mediated reduction of serum cholesterol levels. However, the detailed mechanisms involved and whether TH plays a role in classic and alternative bile acid synthetic pathways are not clear. In this study, to further understand the cholesterol-lowering effect of TH, we try to discover novel miRNA-mediated epigenetic mechanisms or pathways underlying the regulation of hepatic cholesterol metabolism by TH. We have investigated the miRNA expression profiles in the liver of hypo- and hyperthyroid mice and identified a group of miRNA that might be involved in the regulation of cholesterol metabolism by TH. Based on our preliminary data, we speculated that TH-regulated miR-378 might mediate the effect of TH on cholesterol metabolism and bile acid synthesis through targeting MAF-G. Here, we propose to continue and complete the study of the role of TH-regulated miRNAs in cholesterol metabolism, explore whether TH is able to control whole-body metabolic homeostasis through regulating the synthesis and metabolism of bile acids, and identify potential drug targets for hypercholesterolemia and other metabolic diseases.

胆固醇代谢稳态的维持对于健康至关重要。胆固醇代谢异常会增加患心脑血管疾病风险。甲状腺激素（TH）具有很强的降血清胆固醇作用，其调控胆固醇代谢的重要一环就是促进胆固醇向胆汁酸的转化，即促进胆固醇的排泄。但相关的机制，特别是TH对胆汁酸合成经典途径和旁路途径的调控还知之甚少。本项目将尝试从表观遗传学的角度，以微小RNA（miRNA）为切入点，探索TH调控肝脏胆固醇代谢的新机制及相关的降脂机制。申请人前期研究比较了甲亢甲减小鼠肝脏的miRNA表达谱，筛选并鉴定了一批可能参与TH调控胆固醇代谢的miRNA。我们推测，受TH调控的miR-378及其靶基因MAF-G可能介导了TH对肝脏胆固醇代谢及胆汁酸合成途径的调控作用。本项目将完成对参与TH调控胆固醇代谢的miRNA的鉴定和机制研究工作；探索TH是否可通过调节胆汁酸代谢影响机体代谢稳态；发现潜在药物靶点，为防治高胆固醇血症及相关代谢性疾病提供依据。

甲状腺激素（TH）具有很强的降血清胆固醇作用，其调控胆固醇代谢的重要一环就是促进胆固醇向胆汁酸的转化，即促进胆固醇的排泄。但相关的机制，特别是TH对胆汁酸合成经典途径和旁路途径的调控还知之甚少。本项目以微小RNA（miRNA）为切入点，探索TH调控肝脏胆固醇代谢的新机制及相关的降脂机制。本项目比较了甲亢甲减小鼠肝脏的miRNA表达谱，筛选并鉴定了一批可能参与TH调控胆固醇代谢的miRNA，比如肝脏miR-378，过表达肝脏miR-378能够显著降低血清总胆固醇水平。后续研究发现，肝脏miR-378能够调控胆汁酸合成经典途径和旁路途径中关键酶CYP7B1，CYP8B1和CYP27A1的表达，增加胆囊和粪便中胆汁酸水平，提示miR-378可通过增加肝脏胆固醇向胆汁酸的转化及外排，加快体内胆固醇的清除，进而降低血清总胆固醇水平。机制研究发现，胆汁酸合成负调控因子MAFG是miR-378的直接靶基因，在肝脏中敲减MAFG可模拟肝脏过表达miR-378对胆固醇和胆汁酸代谢的调控作用，而在肝脏中过表达MAFG可以逆转过表达miR-378对胆固醇和胆汁酸代谢的调控作用，提示肝脏MAFG介导了miR-378对小鼠胆固醇和胆汁酸代谢的调控作用。利用敲除miR-378的小鼠和肝脏特异性适度表达miR-378的小鼠，本研究还验证了肝脏miR-378对胆固醇和胆汁酸代谢的调控作用及机制，并发现只需上调肝脏miR-378的水平两倍，就能让小鼠维持较低的血清总胆固醇水平，并抵抗饮食诱导的高胆固醇血症。本项目的研究阐明了由miR-378/MAFG介导的TH对肝脏胆汁酸合成的调控网络，提示受TH调控的miR-378及其靶基因MAF-G可能介导了TH对肝脏胆固醇代谢及胆汁酸合成途径的调控作用，提高了对甲状腺激素复杂调控作用的认识水平，发现了潜在药物靶点，为防治高胆固醇血症及相关代谢性疾病提供依据。