肥胖基因产物瘦素调控肝星状细胞基因甲基化相关酶MATⅡ表达的分子机制及SREBP-1c在其中的作用

Leptin, the product of the obese (ob) gene，promotes hepatic stellate cell (HSC) activation, the key step in liver fibrogenesis. Epigenetic regulation is the important way to precisely regulation of gene expression in HSCs. DNA methylation, one of the epigenetic regulation, is broadly used for controlling gene expression. Up to 85% of all transmethylation occur in liver. S-adenosylmethionine (SAM) is the principal methyl donor. Methionine adenosyltransferaseⅡ (MATⅡ), a key enzyme for SAM synthesis, is encoded by genes of MAT2A and MAT2B and promote HSC activation. Our preliminary experiments showed that leptin regulated the expressions of genes of MAT2A and MAT2B in HSC. Therefore, the program aims to elucidate the underlying mechanisms in vivo (ob/ob mice) and in vitro and experimental techniques include gene mutation and chromatin immunoprecipitation. We previously demonstrated that leptin downregulated sterol regulatory element binding protein-1c (SREBP-1c) expression, the key transcription factor for inhibition of HSC activation, but the downstream mechanisms have not been elucidated, thus the effects of SREBP-1c on gene expressions of MAT2A and MAT2B and the role of SREBP-1c in leptin regulation of genes of MAT2A and MAT2B will be furtherly investigated. The data from the researches will contribute to deep mechanisms underlying the promotion roles of leptin in HSC activation and liver fibrogenesis.

肥胖基因产物瘦素促肝星状细胞(HSC)激活。肝纤维化发生关键步骤HSC激活的重要基础之一是表观遗传学调控HSC实现对相关基因精确调控。基因甲基化是表观遗传学调控中广泛而重要的机制。体内85%的转甲基化反应在肝中进行。影响甲基化主要供体S-腺苷甲硫氨酸合成的关键酶蛋氨酸腺苷转移酶Ⅱ（MATⅡ）由MAT2A及MAT2B基因表达，显著促进HSC激活。我们初步显示瘦素调控HSC中MAT2A及MAT2B基因表达。因而该项目将利用ob/ob肥胖鼠，启动子突变，CHIP等技术，阐明瘦素在HSC中调控MAT2A及MAT2B基因表达的分子机制。该课题组先前成果表明瘦素调控着抑制HSC激活的关键转录因子SREBP-1c表达，但下游机制不明，所以SREBP-1c对MAT2A及MAT2B基因表达影响及在瘦素调控两基因中所扮的角色将得到阐明。该研究揭示瘦素促HSC激活及肥胖症患者高肝纤维化发生率的深层机制。

肝星状细胞HSC激活为肝纤维化发生关键，需对相关基因表达的精确调控。基因的甲基化为基因表达调控重要途径。S-腺苷甲硫氨酸（SAM)是细胞中甲基主要供体，影响基因甲基化，在肝脏中最丰富。影响SAM生成的关键酶为蛋氨酸腺苷转移酶（MAT），如MATⅡ。其由催化亚基MAT2A及调节亚基MAT2B组成。体外实验提示MAT2A及MAT2B促进HSC激活。肥胖症患者易发生肝纤维化，瘦素发挥独特作用。研究显示瘦素促HSC激活及肝纤维化。.该课题研究了瘦素对MAT2A及MAT2B的调控。因为MAT2A及MAT2B处于基因的表观调控的上游，因而瘦素对MAT2A及MAT2B的调控具有更广泛生物学意义。结果提示瘦素通过beta-Catenin影响转录因子E2F4，E2F4可结合于MAT2A启动子，抑制MAT2A表达。瘦素需要PI3K/Akt激活c-Jun而结合于MAT2B1启动子，促进MAT2B的表达。MAT2A及MAT2B的敲除减低瘦素促纤维化。c-Jun为AP-1的组成成分。beta-Catenin信号通路能够促进许多不同组织的纤维化发生。AP-1影响众多炎症因子的转录因子。结合以上结果提示：这两种蛋白可作为治疗肥胖症患者肝纤维化的靶点。.SREBP1c是抑制HSC激活的关键转录因子。研究结果提示SREBP1c不影响瘦素调控MAT2A及MAT2B，故未继续该部分。但显示SREBP1c可结合于MAT2B1启动子抑制其表达，且几种重要的与肝纤维化极为相关的蛋白受其影响，包括两种最强促肝纤维化因子：PDGFbeta受体及TGFbeta受体。.同时在肝纤维化模型中显示促细胞分裂蛋白CyclinD1以及CyclinE1在聚集的激活态HSC中表达弱。提示：已经迁移聚集在炎症处的HSC的分裂已经完成。聚集炎症处后，HSC功能可能发生转变，即转变为大量合成与分泌胞外基质。.该课题揭示了瘦素促肝纤维化发生新机制。瘦素通过此机制，影响甲基供体，从而影响HSC基因表观调控。该研究为高血清瘦素的肥胖症患者的治疗提供基础。