肝脏内源性硫化氢调节胆固醇代谢的机制

Sirtuin1(SIRT1), a nicotinamide adenine dinucleotide-dependent class Ⅲ protein deacetylase, is a key protein responsible for energy homeostasis. SIRT1 regulates cholesterol metabolism by deacetylating SREBP(sterol response element binding protein)、LXR(liver X receptor) and FXR(farnesoid X receptor). SREBP transcription factor family is a critical regulator of lipid and sterol homeostasis in mammals. During fasting, SIRT1 can directly deacetylate SREBP. And chemical activators of SIRT1 inhibit SREBP target gene expression, correlating with decreased liver lipid and cholesterol levels. LXRs regulate reverse cholesterol transport by stimulating expression of the ATP-binding cassette transporter A1. SIRT1 interacts with LXR and promotes deacetylation and activity. Moreover, FXR is a nuclear bile acid receptor and a target of SIRT1. Deacetylation of FXR by SIRT1 decreases its stability and promotes transactivation ability. ..Hydrogen sulfide, biologically synthesized by three enzymes, cystathionine γ-lyase(CSE), cystathionine β-synthetase(CBS) and 3-mercaptopyruvate sulfurtransferase(3-MST), functions as a signaling molecule in an array of physiological processes including adipogenesis, inflammation, apoptosis and so on. Over the last decade research about hydrogen sulfide has increased our understanding of this gasotransmitter. Increasing evidence indicates that hydrogen sulfide plays a significant role in cholesterol metabolism. But the mechanism has remained incompletely understood. We have found that hydrogen sulfide increases SIRT1 expression, and decreases total cholesterol in serum. On the basis, the study is designed to investigate the changes of endogenous hydrogen sulfide system and SIRT1 activity in Paigen Diet fed mice. On the other hand, we will observe the cholesterol metabolism and SIRT1 activity by overexpressing or knockdown hydrogen sulfide system in hypercholesterolemic mice. Then we will prove whether hydrogen sulfide regulates cholesterol metabolism by activating SIRT1. Finally, we intend to research the mechanism by which hydorgen sulfide activates SIRT1. We hypothesize that hydrogen sulfide decreases total cholesterol by increasing SIRT1 expression and enhancing SIRT1 deacetylase activity.

SIRT1是烟酰胺腺嘌呤二核苷酸依赖的Ⅲ型组蛋白去乙酰化酶，在能量代谢过程中具有重要作用。它通过使胆固醇代谢过程中关键转录调节因子固醇反应元件结合蛋白-1、肝X受体和法尼酯X受体去乙酰化，参与调节胆固醇代谢。硫化氢是一个新型的气体信号分子，我们的工作发现硫化氢可提高小鼠肝脏内SIRT1的表达，并降低血浆中的总胆固醇。在此基础上，本项目拟在高脂高胆固醇喂养的小鼠模型上，研究胆固醇代谢紊乱后肝脏内源性硫化氢系统和SIRT1活性的改变；反之，在高胆固醇血症小鼠模型上，过表达或敲低肝脏中硫化氢系统，观察胆固醇代谢和SIRT1活性的变化；此外，采用工具药抑制SIRT1的活性，观察硫化氢对胆固醇代谢的影响；最后，以SIRT1的翻译后修饰为切入点，研究硫化氢活化SIRT1信号的机制。我们推测，硫化氢可通过促进SIRT1的表达并提高SIRT1活性调节肝脏胆固醇代谢，从而降低血浆中总胆固醇。

硫化氢（H2S）是继一氧化氮、一氧化碳之后的新型气体信号分子，在人体生理活动中发挥重要作用。CSE、CBS、3-MST是体内产硫化氢的关键酶。SIRT1是Ⅲ型组蛋白去乙酰化酶，以NAD+依赖的方式促进底物蛋白去乙酰化，影响底物蛋白的活性，从而参与机体各项活动。我们发现以高脂高胆固醇饮食喂养的ApoE基因敲除小鼠为动物模型，给予H2S供体NaHS、GYY4137可降低血浆和肝脏中的总胆固醇，甘油三酯无明显变化。肝脏中合成胆固醇相关的蛋白SREBP1、SREBP2、HMGCR表达降低，而SIRT1表达增多。SREBP1和SREBP2是SIRT1的靶蛋白。肝脏中乙酰化的SREBP1和SREBP2减少，提示SIRT1去乙酰化活性增强。在肝细胞过表达CSE，SIRT1表达增多，SREBP1和SREBP2表达降低，乙酰化的SREBP1和SREBP2减少，细胞内胆固醇降低。利用siRNA敲低CSE，结果相反。接着，我们发现H2S可通过硫氢化修饰第371、374、395、398这四个半胱氨酸提高SIRT1的去乙酰化活性。H2S可通过PI3K/AKT通路上调SIRT1的表达。以上结果表明，H2S可通过提高SIRT1活性及表达抑制肝脏内胆固醇合成，为降低血脂提供新的思路。