Function and Mechanism of DDB1-CUL4A E3 Ligase in Liver Lipid Metabolism

描述（由申请人提供）：非酒精性脂肪肝，也称为脂肪肝，是由于肝细胞中脂肪过度积聚所致，是影响10-30%一般人群的最常见肝病。脂肪肝也被认为是代谢综合征的一个组成部分，并与胰岛素抵抗、肥胖和糖尿病有关。近35%的脂肪肝患者会发展为慢性炎症、纤维化和最终的肝衰竭，目前尚无有效的治疗方法。因此，必须了解脂肪肝发生的分子机制。我们的初步研究表明，损伤特异性DNA结合蛋白1（DDB 1）E3连接酶在高脂饮食（HFD）诱导的脂肪肝的发展中起着至关重要的作用。特别是，我们发现肝脏DDB 1蛋白水平在肥胖条件下升高。肝脏特异性缺失Ddb 1保护小鼠免受HFD诱导的脂肪积累，并选择性抑制肝脏中的糖酵解和脂肪生成基因表达。在原代小鼠肝细胞中，Ddb 1敲低抑制从头脂肪生成速率，降低甘油三酯含量，并阻断脂肪生成的关键调节因子碳水化合物反应元件结合蛋白（ChREBP）的活性。在机制水平上，我们发现了DDB 1通过泛素化降解O-GlcNAcase（OGA）间接调节ChREBP蛋白O-GlcNAcylation和稳定性的证据。尽管DDB 1作为CUL 4A E3连接酶复合物的关键组分在DNA损伤反应的背景下已经被充分研究，但其代谢作用仍然未知。基于初步数据，我们假设DDB 1 E3连接酶通过激活ChREBP介导的肝脏脂肪生成而促进脂肪肝的发展。为了验证这一假设，我们提出了三个相互关联的具体目标.目的1：确定DDB 1 E3连接酶是否通过ChREBP促进从头脂肪生成并促成肥胖相关性脂肪肝。目的2：检测DDB 1 E3连接酶是否通过降解OGA蛋白促进肥胖大鼠ChREBP的O-GlcNAc化和稳定性。目的3：探讨AMPK通路在肥胖形成过程中对肝脏DDB 1表达的营养调控作用。该提案旨在深入了解DDB 1 E3连接酶的代谢作用。具体而言，我们预期确定DDB 1 E3连接酶如何调节肝脏中的从头脂肪生成并通过ChREBP诱导脂肪肝。这些研究目标的实现应该揭示营养过剩导致脂肪积累的基本机制，并为治疗脂肪肝提供潜在的新治疗途径。

DESCRIPTION (provided by applicant): Non-alcoholic liver steatosis, also called fatty liver, is due to excessive fat accumulation in hepatocytes and is the most common liver disease affecting 10-30% of the general population. Fatty liver is also regarded as a component of metabolic syndrome, and is associated with insulin resistance, obesity, and diabetes. Nearly 35% of patients with fatty liver develop chronic inflammation, fibrosis, and eventual liver failure and there is no effective treatment for fatty liver disease. Therefore, it is imperative to understand the molecular mechanisms underlying the development of fatty liver. Our preliminary studies demonstrate that the damage-specific DNA binding protein 1 (DDB1) E3 ligase plays a crucial role in the development of high-fat diet (HFD)-induced fatty liver. In particular, we discovered that liver DDB1 protein levels were elevated under obese conditions. Liver-specific deletion of Ddb1 protects mice from the HFD-induced fat accumulation and selectively suppresses glycolytic and lipogenic gene expression in the liver. In primary mouse hepatocytes, Ddb1 knockdown represses de novo lipogenesis rate, reduces triglyceride content, and blocks the activity of a key regulator of lipogenesis, carbohydrate responsive- element binding protein (ChREBP). At the mechanistic level, we uncovered evidence that DDB1 indirectly regulates the ChREBP protein O-GlcNAcylation and stability by degrading O-GlcNAcase (OGA) through ubiquitination. Although DDB1 has been well-studied in the context of DNA damage response as a critical component of the CUL4A E3 ligase complex, its metabolic actions remain unknown. Based upon the preliminary data, we hypothesize that DDB1 E3 ligase contributes to the development of fatty liver by activating ChREBP-mediated hepatic lipogenesis. To test this hypothesis, we propose three inter- connected specific aims. Aim 1: Determine whether the DDB1 E3 ligase promotes de novo lipogenesis and contributes to obesity-associated fatty liver via ChREBP. Aim 2: Test whether DDB1 E3 ligase promotes O- GlcNAcylation and stability of ChREBP via degrading OGA protein in obesity. Aim 3: Investigate how the AMPK pathway mediates nutritional regulation of hepatic DDB1 expression during the obesity development. This proposal seeks an in-depth understanding of the metabolic role of DDB1 E3 ligase. Specifically, we anticipate identifying how DDB1 E3 ligase regulates de novo lipogenesis in the liver and induces fatty liver through ChREBP. Accomplishment of these research objectives should reveal fundamental mechanisms by which nutrient excess leads to fat accumulation and provide potential novel therapeutic avenues to treat fatty liver disease.