MG53通过转录调控影响心肌组织的氧化还原稳态

MG53 plays a vital role in metabolism. In the patients with metabolic syndrome and the animal models with metabolic disorders, the expression levels of MG53 are significantly increased. Importantly, transgenic mice with cardiac specific overexpression of MG53 develop the phenotype that resembles diabetic cardiomyopathy (DCM), suggesting that increased expression of MG53 may induce DCM. Mechanistically, MG53 acts as an E3 ligase to facilitate the degradation of insulin receptor and insulin receptor substrate 1 via ubiquitination, which impairs insulin signaling and induces insulin resistance of cardiomyocytes. Moreover, MG53 regulates the expression of PPARalpha at transcriptional level and enhances myocardial lipid uptake and usage, and subsequently causes lipid accumulation and toxicity. In order to further elucidate the function of MG53 as a transcriptional regulator, ChIP-seq was performed to identify the target genes of MG53 in the whole genome. Preliminary results showed that many MG53 target genes are related to mitochondrial oxidative phosphorylation (OXPHOS). Indeed, the oxygen consumption rate of the cardiomyocytes isolated from the cardiac specific MG53 transgenic mice is significantly increased as compared to that of the wild type controls, indicating MG53 overexpression causes dysregulation of mitochondrial function. Therefore, we will study the transcriptional regulation of mitochondria OXPHOS genes by MG53. Furthermore, we will investigate if MG53 overexpression induces ROS generation and disturbs redox homeostasis in cardiac myocytes, which in turn, contribute to the dysfunction of diabetic heart.

MG53是参与代谢调控的关键分子。在代谢综合征患者和代谢紊乱的动物模型中，MG53的表达均有显著增加。MG53心肌特异转基因小鼠表现出糖尿病心肌病的症状，提示MG53表达增高可能诱发糖尿病心肌病。研究显示，MG53可以作为E3泛素化连接酶促进胰岛素受体和胰岛素受体底物1的降解，使心肌细胞产生胰岛素抵抗；也可以在转录水平调节PPARalpha的表达，导致心肌细胞过度摄取和利用脂肪酸，造成心肌组织中脂质沉积和脂毒性。为了进一步研究MG53作为转录调控因子的作用，我们采用ChIP-seq的方法寻找MG53的靶基因。初步实验结果显示，MG53的很多靶基因与线粒体氧化磷酸化相关，而MG53过表达转基因小鼠心肌细胞的耗氧量确实明显高于野生型小鼠，显示线粒体功能异常。因此，我们将研究MG53如何通过转录调控调节线粒体功能并影响心肌细胞ROS的产生和氧化还原稳态，从而导致糖尿病心肌病。

本课题研究了MG53作为转录调控因子调节细胞能量代谢的作用。我们采用ChIP-seq的方法寻找MG53的靶基因，发现带有MG53结合位点的基因很多是细胞核中编码线粒体氧化呼吸链复合体组分的基因，而且其DNA结合位点与调节线粒体功能的重要转录因子NRF1（nuclear respiratory factor 1）的保守序列几乎完全一致。进一步我们发现MG53和NRF1有直接的相互作用，仅过表达NRF1对于呼吸链复合体组分的表达影响很小，而MG53促进线粒体氧化呼吸的功能也依赖于NRF1，提示二者可能是通过形成异源二聚体实现对于线粒体功能的调节。转录组和代谢组学的分析结果支持ChIP-seq的发现，即MG53过表达增强线粒体的氧化磷酸化功能和活性氧（ROS）的产生，而敲低MG53的表达则具有相反作用。此外，MG53促进线粒体的融合(fussion)，但是并不影响线粒体的裂变(fission)和生成(biogenesis)。因此在长期高表达MG53的细胞中，线粒体氧化呼吸增强，伴随ROS生成增加，导致线粒体损伤，但是细胞不能以新生的线粒体替代。因此，过表达MG53的转基因小鼠心肌细胞的耗氧量高于野生型小鼠，线粒体功能异常，心肌组织氧化还原稳态被破坏，从而导致糖尿病心肌病。