The Mediator complex in the coordinate regulation of lipogenic gene expression

摘要 在人类和啮齿类动物胰岛素抵抗模型中，肝葡萄糖生成的调节发生改变，使得空腹状态下的肝葡萄糖生成增强，进食状态下的抑制减少。同时，肝脏新生脂肪生成在禁食状态下升高，并在进食状态下进一步增加。胰岛素不能抑制肝葡萄糖输出但仍激活从头脂肪生成被称为选择性胰岛素抵抗。许多研究已经检测了DNA结合转录因子、转录因子共激活子和共抑制子在控制肝脏脂肪生成基因表达中的调节。尽管对这些反式因子进行了深入的研究， 这些蛋白质都不直接与DNA依赖性RNA聚合酶II相互作用，一般认为所有的调节都发生在反式因子水平。一种称为介体的关键复合物将多个反式因子连接到DNA依赖性RNA聚合酶II。在大肠杆菌中，介体由至少30个单独的亚基组成，这些亚基由四个亚复合物（头部、中部、尾部和激酶子模块）组装而成。在酵母中，最初认为介体是表达机制的组成成分。然而，在果蝇幼虫、肝细胞培养细胞和体内肝脏中，我们最近证明， CDK 8/CycC复合物是激酶亚模块（CDK 8/CycC、Med 12和Med 13）的组分，其经历胰岛素和营养状态的动态调节。基于mTORC 1调节介体激酶亚模块（CDK 8/CycC、MED 12和MED 13）蛋白水平的这些先前和我们新的初步数据，我们假设介体复合物经历了从禁食状态下的脂肪生成转录抑制状态（所谓的大介体复合物）到进食状态下的脂肪生成转录激活状态（所谓的小介体复合物）的结构重组。胰岛素抵抗状态下的介体复合物的失调至少部分地解释了禁食状态下脂肪生成基因表达的升高。在本建议中，我们将确定生理和病理- 介体复合物的生理改变、营养信号及其失调导致介体复合物重组，以及这些变化对脂肪生成基因表达的后果。

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.