SREBP－1c和ChREBP依赖的染色质重塑与非酒精性脂肪肝

Abnormal intrahepatic lipid accumulation is the basis for the development of non-alcoholic fatty liver(NAFL). The signaling network composed of insulin, glucose, sterol regulatory element binding protein-1c(SREBP-1c) and carbohydrate response element binding protein(ChREBP) has been shown to be involved in fine-tuning hepatic lipid metabolism. There were little reports about how SREBP-1c and ChREBP regulated the transcription of their downstream target gene-fatty acid synthase(FAS). That the chromatin remodeling, as the final part of signal transduction participates in SREBP-1c and ChREBP-dependent transcriptional regulation of FAS is still an unknown field. By methods of chromatin immunoprecipitation and Micrococcal nuclease digestion assay, this project is to deeply and systematicly investigate chromatin remodelings that occur in the course of FAS transcription regulated by SREBP-1c and ChREBP, under the action of insulin and glucose. And so we can find synergistic chromatin remodeling events, in order to provide the initial basis for searching inhibitors or activators of chromatin remodeling factors that can be applied in the treatment of NAFL. Thus we can supply new ideas and foundations for the prevention and treatment of fatty liver disease through this study.

肝内脂质的异常积聚是非酒精性脂肪肝发生发展的基础，胰岛素、葡萄糖与SREBP-1c、ChREBP组成的信号网络已被证实参与肝脏脂质代谢的精细调节。但鲜见有关SREBP-1c、ChREBP对下游靶基因FAS转录调控的报道。染色质重塑作为信号转导的最终环节，参与SREBP-1c和ChREBP依赖FAS靶基因转录调控也是未知领域。本课题拟采用染色质免疫沉淀和MNase酶切分析等技术，对胰岛素和葡萄糖作用下SREBP-1c与ChREBP依赖的FAS基因转录过程中发生的染色质重塑事件进行深入系统的研究，在SREBP-1c和ChREBP调控FAS基因转录过程中发现具有协同作用的染色质重塑事件，为进一步寻找可用于脂肪肝治疗的染色质重塑因子抑制剂或激活剂提供前期研究基础。

碳水化合物反应元件结合蛋白（Carbohydrate responsive element binding protein，ChREBP）是葡萄糖依赖脂质合成的关键转录因子，固醇调节元件结合蛋白-1c（Sterol regulatory element-binding protein-1c）是胰岛素依赖的关键转录因子，它们在糖脂代谢中发挥枢纽作用。ChREBP或SREBP-1c诱导脂质从头合成关键酶-脂肪酸合酶（Fatty acid synthase，FASN）转录。FASN基因转录的过度激活是导致肝细胞脂肪变性的重要分子事件。由于基因结构高度稳定且转录过程极其复杂，信号分子通常是在不影响基因DNA序列的前提下，通过影响组蛋白与DNA双链之间的亲和性,进而影响转录因子与靶基因之间的亲和性，但并不改变DNA序列，达到调控基因转录的目的。ChREBP或SREBP-1c通过与FASN基因启动子区碳水化合物反应元件（Carbohydrate response element，ChoRE）、固醇结合原件(srerol response element，SRE）结合，调控FASN基因的选择性转录。然而，有关ChREBP或SREBP-1c调控FASN基因转录过程的组蛋白修饰事件仍不清楚。该研究对因FASN基因转录过度激活而引起的肝细胞糖脂代谢紊乱以及脂肪变性具有重要意义。.结果：葡萄糖或胰岛素均能诱导FASN基因启动子区域H3K4三甲基化、H3磷酸化H3、H4乙酰化水平升高，而H3K9、H4K20三甲基化水平降低。在葡萄糖刺激下抑制肝细胞ChREBP表达，FASN基因ChREBP-ChoRE结合水平、H3、H4乙酰化、H3K4三甲基化及H3S10磷酸化水平均下降，而H3K9和H4K20三甲基化水平则上升，导致FASN表达水平下降。在胰岛素刺激下抑制肝细胞ChREBP或SREBP-1c，FASN基因ChREBP-ChoRE，SREBP-1c-SRE结合水平、H3、H4乙酰化、水平下降，导致FASN表达水平下降。在胰岛素或葡萄糖刺激下抑制FASN基因HAT活性，并不能抑制ChREBP和SREBP-1c的蛋白表达，却通过阻遏ChREBP、SREBP-1c分别与FASN基因启动区ChoRE、SRE结合导致FASN基因转录下调。