E3泛素连接酶Ubr3在机体胰岛素抵抗及糖代谢过程中的作用及机制

Ubiquitin-proteasome system caries out a key posttranslational modification, ubiquitination, via a three-enzyme cascade catalyzed by E1, E2 and E3 enzymes. E3 ubiquitin ligases have been found to be essential to the develpoment of insulin resistance and diabetes. Recently, Ubr3 is identified as a RING finger E3 ubiquitin ligase, though the roles of Ubr3 in the development of insulin resistance and diabetes remain elusive. Herein, this study puts forward for the first time that ablation of Ubr3 in mice leads to an elevation of the body weight, yet, paradoxically, it results in an improvement of insulin sensitivity and glucose tolerance. Traditional view links obesity to insulin resistance. However, we challenges this view that we identify a novel pathway that increases weight gain but shows unaffected, or rather improved insulin sensitivity. This phenomenon is also known as "healthy obesity", which suggests that there are other mechanisms apart from "causality" between obesity and insulin resistance. Therefore, in-depth study of "healthy obesity" phenomena will help to reveal the pathogenesis of insulin resistance. Ubr3-KO mice that exhibit almost the same phenotype as a human “healthy obesity” in this project are valuable mouse models to study relation between obesity and insulin resistance. In this study, Ubr3-KO mouse model and cell culture model will be used to investigate the role and mechanisms by which Ubr3-dependent ubiquitination regulates glucose metabolism and insulin sensitivity and thereby provide novel perspectives and directions for understanding the development as well as identifying effective treatments of insulin resistance and diabetes mellitus.

泛素-蛋白酶体系是细胞内蛋白降解主要途径，其中E3泛素连接酶是关键调控蛋白。据报道，E3酶参与到代谢调控和胰岛素抵抗的发生发展过程。Ubr3属于新型环指E3酶，它与糖尿病之间的关系尚不清楚。申请人前期研究发现，与对照鼠相比，Ubr3基因敲除小鼠体重显著增加，而其胰岛素敏感性却明显改善。传统观点认为“肥胖与胰岛素抵抗紧密相关”，但是临床上也会遇到体重增加但胰岛素敏感性并不受影响的个体，我们俗称“健康型肥胖”。 这一现象提示我们，肥胖和胰岛素抵抗之间的关系还有区别于“因果”的其它可能。因此，深入研究 “健康型肥胖”的相关问题，有助于更加清晰地揭示胰岛素抵抗的发病机制。本项目中Ubr3基因敲除小鼠呈现出几乎相同的表型，是研究该问题有价值的模型。本研究拟将利用Ubr3基因敲除小鼠模型和细胞模型揭示Ubr3相关泛素化调控糖代谢的作用和机制，为研究胰岛素抵抗和糖尿病的发生发展提供新的研究视角和方向。

肥胖是造成胰岛素抵抗和2型糖尿病的重要诱因。1、泛素-蛋白酶体系是细胞内蛋白降解主要途径，其中E3泛素连接酶是关键调控蛋白。据报道，E3酶参与到代谢调控和胰岛素抵抗的发生发展过程。Ubr3属于新型环指E3酶，它与肥胖及糖尿病之间的关系尚不清楚。本项目首次发现Ubr3基因敲除小鼠呈现出肥胖、系统性糖脂代谢紊乱等特征，肝脏特异性敲除而非脂肪能够引起胰岛素抵抗，结合质谱和siRNA筛选提示GSTM3是Ubr3的潜在底物；2、内质网应激是导致肝脏胰岛素抵抗和糖脂代谢紊乱的元凶之一，XBP1s是内质网的重要稳定器，提高XBP1s水平有利于缓解肝脏内质网应激。而XBP1s蛋白稳定性低，严重阻碍了其改善代谢的功能。本项目首次发现通过突变泛素化位点（K60/K77）操纵XBP1s的表达和/或活性可以有效解决因XBP1s自身不稳定而功能受限的问题，进行通过缓解内质网应激改善胰岛素敏感性；3、近年来，激活产热性脂肪组织在抵抗肥胖、改善血糖、增进胰岛素敏感性方面的效用已被广泛证实，因此，增加 UCP1+的产热性脂肪作为肥胖治疗的靶点，为潜在减肥药物的发现提供了新的方向。基于这一思路，本项目采用了先进的关联图谱CMap法进行小分子转录组学筛选，首次发现了贯叶金丝桃素（HPF）具有潜在的诱导产热作用，进一步在体内外对HPF进行药效学考察，验证了HPF的抗肥胖、改善机体代谢等作用，并深入研究了HPF激活脂肪细胞产热的机制。在此基础上，本项目开发了有限水解质谱法，筛选到HPF的直接作用靶蛋白Dlat,敲低Dlat能够有效逆转HPF的促产热作用。此项研究旨在加深了解泛素化如何影响肥胖、糖尿病等相关代谢性疾病，进而为阐明糖代谢平衡发生紊乱的病理学基础提供新的视角，为2型糖尿病的治疗提供新的干预靶点，并为临床研发新型减肥药提供全新的研究方向。