The Mediator complex in the coordinate regulation of lipogenic gene expression

摘要 在人类和啮齿动物的胰岛素抵抗模型中，糖异生的调节发生了改变，以至于在禁食状态下肝脏葡萄糖产量增加，而在进食状态下抑制减少。同时，肝脏新生脂肪生成在禁食状态下增加，在进食状态下进一步增加。胰岛素不能抑制肝脏葡萄糖输出，但仍能激活新生脂肪生成，被称为选择性胰岛素抵抗。许多研究已经研究了DNA结合转录因子、转录因子共激活因子和共抑制因子在控制肝脏造脂基因表达中的调控。尽管对这些反式因子进行了深入的调查 这些蛋白都不直接与DNA依赖的RNA聚合酶II相互作用，通常认为所有的调控都发生在反式因子的水平上。一种称为介体的关键复合体将多种反式因子连接到依赖DNA的RNA聚合酶II上。在哺乳动物中，介体由至少30个单独的亚基组成，这些亚基由四个亚复合体组装而成，即头、中、尾和激动子模块。在酵母中，最初认为介体是表达机制的一个组成部分。然而，我们最近在果蝇幼虫、肝细胞培养细胞和活体肝脏中证明了 CDK8/CycC复合体是激活子模块(CDK8/CycC、MED12和Med13)的组成部分，受胰岛素和营养状态的动态调节。根据mTORC1调节介体激活子模块(CDK8/CycC、MED12和MED13)蛋白水平的先前和我们的新的初步数据，我们假设介体复合体经历了结构重组，从禁食状态下的生脂转录抑制状态(即所谓的大介体复合体)到摄食状态下的生脂转录激活状态(即小介体复合体)。胰岛素抵抗状态下介体复合体的失调至少部分是导致禁食状态下造脂基因表达增加的原因。在这项建议中，我们将确定生理和病理- 介体复合体的生理变化、营养信号及其失调导致介体复合体的重组，以及这些变化对成脂基因表达的影响。

ABSTRACT In human and rodent models of insulin resistance, the regulation of gluconeogenesis is altered such that hepatic glucose production is enhanced in the fasted state with reduced suppression in the fed state. In parallel, hepatic de novo lipogenesis is elevated in fasted state and further increased in the fed state. The inability of insulin to suppress hepatic glucose output but still to activate de novo lipogenesis has been referred to as selective insulin resistance. Numerous studies have examined the regulation of DNA binding transcription factors, transcription factor co-activators and co-repressors in the control of liver lipogenic gene expression. Despite the intensive investigation of these trans-factors that control lipogenic gene expression, none of these proteins directly interacts with DNA-dependent RNA polymerase II and it has generally been believed that all of the regulation occurs at the level of the trans-factors. One critical complex termed the Mediator connects multiple trans-factors to the DNA-dependent RNA polymerase II. In mammalians, Mediator is composed of at least 30 individual subunits that are assembled from four sub-complexes, head, middle, tail and kinase sub-modules. In yeast, it was originally suggested that the Mediator is a constitutive component of the expression machinery. However, in drosophila larvae, hepatocyte cultured cells, and liver in vivo we recently demonstrated that the CDK8/CycC complex a component of the kinase sub-module (CDK8/CycC, Med12 and Med13) undergoes dynamic regulation by insulin and nutritional states. Based upon these previous and our new preliminary data that mTORC1 regulates the protein levels of Mediator kinase submodule (CDK8/CycC, MED12 and MED13), we hypothesize that the Mediator complex undergoes structural reorganization from a lipogenic transcriptional repressing state (so called large Mediator complex) in the fasted state to a lipogenic transcriptional activating state (so called small Mediator complex) in the fed state. The dysregulation of the Mediator complex in states of insulin resistance accounts, at least in part, for the elevation of lipogenic gene expression in the fasted state. In this proposal we will determine the physiologic and patho- physiologic alterations of the Mediator complex, the nutritional signals and their dysregulation resulting in the Mediator complex re-organization, and the consequences of these changes on lipogenic gene expression.