mTORC1-dependent regulation of the CycC/CDK8 complex

描述（由申请方提供）：肝脏新生脂肪生成失调在肥胖症中普遍存在，并与胰岛素抵抗、2型糖尿病和心血管疾病密切相关。 肝脏中从头脂肪生成的调节是一个复杂的过程，其依赖于营养/激素状态、底物前体的可用性、关键酶活性步骤的变构调节以及通过转录因子ChREBP（特别是对于本申请而言，固醇应答元件结合蛋白-1（SREBP-1c）转录因子）对脂肪生成基因表达的转录控制。SREBP-1c是负责脂肪酸和甘油三酯的肝脏生物合成的限速酶的关键激活剂，并且是胰岛素诱导的肝细胞中从头脂肪生成的主要效应。胰岛素和摄食通过以下三个不同水平急性调节SREBP-1c转录活性：1）增加SREBP-1c转录; 2）从最初位于内质网膜中的前体蛋白水解成熟; 3）增加核SREBP-1c蛋白稳定性。最近，我们发现了一种新的胰岛素/营养调节信号通路，主要负责核SREBP-1c的稳定性 蛋白我们已经发现，在基础（禁食状态）中，细胞周期蛋白依赖性激酶8（CDK 8）与其激活剂细胞周期蛋白C（CycC）复合使核SREBP-1c磷酸化，从而诱导核SREBP-1c泛素化和快速蛋白酶体介导的降解。相反，再喂养导致CDK 8/CycC蛋白复合物的下调，从而稳定核SREBP-1c蛋白，以增强脂肪生成基因表达和与胰岛素的其他作用相结合的从头脂肪生成。因此，CDK 8/CycC复合物的营养/激素下调功能上导致核SREBP-1c蛋白的稳定，作为驱动肝脂肪生成基因表达的复杂机制的一部分。最近，我们观察到CDK 8蛋白的稳定性与mTORC 1的激活状态呈负相关。基于这些最新的发现，我们提出了以下工作假设：mTORC 1信号通路的激活通过直接或间接磷酸化迅速下调CDK 8/CycC蛋白复合物，这反过来又导致CDK 8/CycC复合物的解离，随后CDK 8和CycC泛素化和蛋白酶体介导的降解。在这项提案中，我们建议确定初始营养信号传导途径和分子机制，调节CDK 8/CycC蛋白复合物和核SREBP-1c稳定性，控制脂肪生成基因表达和从头脂肪生成。这将通过确定激素/营养物下调CDK 8/CycC复合物来实现，下调通过1）mTORC 1的激活和随后蛋白酶体介导的CDK 8和CycC蛋白的降解; mTORC 1依赖性调节CDK 8/CycC复合物组装状态;和3）CDK 8和CycC中赖氨酸残基的泛素化。然后，我们将通过1）分析CDK 8和/或CycC磷酸化/去磷酸化; 2）鉴定负责CDK 8/CycC下调的mTORC 1依赖性激酶; 3）定位CKD 8和/或CycC中的特异性磷酸化位点;以及4）分析磷酸化缺陷突变对CDK 8/CycC稳定性、核SREB- 1c稳定性和脂肪生成基因表达的影响。最后，我们将通过以下方式确定mTORC 1调节机制是否有助于胰岛素抵抗状态下从头脂肪生成的正常生理和病理生理调节：1）在肝脏特异性Raptor和LKB 1敲除小鼠中检查CDK 8/CycC复合物、核SREBP-1c稳定性、脂肪生成基因表达和从头脂肪生成; 2）确定小鼠肝脏中CDK 8/CycC降解抗性突变体的功能;和3） 分析高脂饮食诱导的胰岛素抵抗和遗传性糖尿病db/db小鼠中CDK 8/CycC复合物的调节。

DESCRIPTION (provided by applicant): Dysregulation of hepatic de novo lipogenesis is prevalent in obesity and closely associates with insulin resistance, Type 2 Diabetes and cardiovascular disease. The regulation of de novo lipogenesis in the liver is a complex process that is dependent upon the nutrient/hormone state, availability of substrate precursors, allosteric regulation of key enzymati activity steps, and the transcriptional control of lipogenic gene expression through the transcripton factors ChREBP and in particular for this application the sterol response element-binding protein-1 (SREBP-1c) transcription factor. SREBP-1c is a pivotal activator of rate-limiting enzymes that are responsible for hepatic biosynthesis of fatty acids and triglycerides and it is the primary effecto of insulin-induced de novo lipogenesis in hepatocytes. Insulin and feeding acutely regulates SREBP-1c transcriptional activity at three distinct levels by 1) increasing SREBP-1c transcription; 2) protelytic maturation from its precursor that is initially located in endoplasmic reticulum membrane, and 3) increased nuclear SREBP-1c protein stability. Recently, we have uncovered a novel insulin/nutrient regulated signaling pathway that is primarily responsible for the stability of the nuclear SREBP-1c protein. We have found that in the basal (fasted state) cyclin- dependent kinase-8 (CDK8) in comple with its activator cyclin C (CycC) phosphorylates nuclear SREBP-1c that induces nuclear SREBP-1c ubiquitination and rapid proteasome mediated degradation. In contrast, refeeding results in the dow-regulation of CDK8/CycC protein complex thereby stabilizing nuclear SREBP-1c protein to enhance lipogenic gene expression and de novo lipogenesis in conjunction with other actions of insulin. Thu, the nutrient/hormone down regulation of the CDK8/CycC complex functionally results in the stabilizaion of the nuclear SREBP-1c protein as part of the complex mechanisms driving hepatic lipogenic gene expression. More recently, we have observed that that CDK8 protein stability is inversely related t the mTORC1 activation state. Based upon these recent findings, we propose the following working hypothesis: Activation of the mTORC1-signaling pathway rapidly down regulates the CDK8/CycC protein complex through either a direct or indirect phosphorylation, that in turn leads to dissociaion of the CDK8/CycC complex, subsequent CDK8 and CycC ubiquitination and proteasome-mediated degradation. In this proposal we propose to determine the initial nutrient signaling pathways and molecular mechanisms regulating the CDK8/CycC protein complex and nuclear SREBP-1c stability in the control of lipogenic gene expression and de novo lipogenesis. This will be accomplished by determining hormone/nutrient down regulation of CDK8/CycC complex occurs through 1) activation of mTORC1 and subsequent proteasome-mediated degradation of the CDK8 and CycC proteins; mTORC1-dependent regulation CDK8/CycC complex assembly state; and 3) ubiquitination of lysine residues in CDK8 and CycC. We will then determine whether CDK8, CycC or both are the targets of hormone/nutrient stimulation of mTORC1 signaling by 1) analyzing CDK8 and/or CycC phosphorylation/dephosphorylation; 2) identifying the mTORC1-dependent kinase(s) responsible for CDK8/CycC down regulation; 3) mapping the specific phosphorylation sites in CKD8 and/or CycC; and 4) analyzing the effect of phosphorylation defective mutations on CDK8/CycC stability, nuclear SREB- 1c stability and lipogenic gene expression. Lastly, we will determine if the mTORC1 regulatory mechanism contributes to the normal physiologic and pathophysiologic regulation of de novo lipogenesis in insulin resistant states by 1) examining the CDK8/CycC complex, nuclear SREBP-1c stability, lipogenic gene expression and de novo lipogenesis in liver-specific Raptor and LKB1 knocout mice; 2) determining the function of CDK8/CycC degradation-resistant mutants in mouse liver; and 3) analyzing the regulation of the CDK8/CycC complex in high fat diet-induced insulin resistant and genetically diabetic db/db mice.