肥胖作为代谢综合征、非酒精性脂肪肝、2型糖尿病和动脉粥样硬化性心脏病和癌症等多种慢性代谢疾病的重要诱因，对人体健康造成了极大的威胁。肥胖和这些慢性代谢性疾病之间的密切联系促使人们对脂肪细胞的功能进行更深入的了解。小鼠双微体2（Murine double minute 2, MDM2）是一种E3泛素连接酶，能够泛素化降解肿瘤抑制因子p53从而在肿瘤病理学中发挥重要作用，但是关于其在脂肪细胞生物学中发挥的调控作用知之甚少。在本论文中，通过体外分离C57小鼠的脂肪前体细胞并将其诱导分化成熟，我们发现随着脂肪前体细胞的分化程度的增加，MDM2的蛋白表达水平也逐渐增加，这意味着MDM2在脂肪组织的发育过程中发挥着重要作用。另一方面，MDM2在脂肪组织中的表达受到营养状态的调节，高脂饮食（High fat diet, HFD）明显上调了MDM2在脂肪组织中的表达水平。为了进一步探讨MDM2如何调控脂肪组织的生物学功能，我们利用CRISPR-cas9技术构建了Mdm2脂肪细胞特异性过表达（Mdm2 adipocyte-specific knock-in, Mdm2-AKI）小鼠。在正常饮食（Normal chow diet, NCD）喂养的条件下，Mdm2-AKI不同部位的脂肪组织重量都表现出明显的增加，并且表现出体重增加、胰岛素抵抗和能量消耗减少等代谢异常的表型。然而，令人意外地是，长期HFD的喂养导致Mdm2-AKI小鼠不同部位的白色脂肪组织出现了不同的表型。HFD促进了Mdm2-AKI小鼠腹股沟脂肪组织（inguinal White adipose tissue, iWAT）的重量增加，而附睾白色脂肪组织（epididymal White adipose tissue, eWAT）的重量并没有增加，并且eWAT发生了明显的脂肪组织功能障碍，如衰老、细胞凋亡和慢性炎症，从而导致Mdm2-AKI小鼠发生明显的胰岛素抵抗和肝脏脂肪变性。在机制上，我们利用HFD喂养的Mdm2-AKI小鼠以及对照组WT（Wild type）小鼠的eWAT对MDM2的下游靶标蛋白进行了蛋白质组、泛素化修饰组学检测，发现MDM2在脂质代谢过程中具有广泛的潜在作用，其中，前列腺六跨膜上皮抗原4（Six-transmembrane epithelial antigen of prostate 4，STEAP4）可能是MDM2的一个潜在底物。我们进行了一系列的体外实验证实了MDM2可以与STEAP4相互作用，并通过泛素化修饰STEAP4的K18和K161位点降解抑制STEAP4蛋白的表达。最后，恢复STEAP4在Mdm2-AKI小鼠的eWAT中的表达可以改善MDM2诱导的脂肪功能障碍、胰岛素抵抗和肝脏脂肪变性。总之，我们的研究表明eWAT中的MDM2-STEAP4轴在维持健康的脂肪组织功能和改善肝脏脂肪变性方面起着重要作用。

     Obesity is an important cause of many chronic metabolic diseases such as metabolic syndrome, non-alcoholic fatty liver disease, type 2 diabetes, atherosclerotic heart disease, and cancer, posing a great threat to human health. The strong link between obesity and these chronic metabolic diseases has prompted a greater understanding of the function of adipocytes. Murine double minute 2 (MDM2), an E3 ubiquitin ligase, has been highlighted in oncopathology as an upstream regulatory factor of the tumor suppressor p53, but little is known about its regulatory role in adipocyte biology. Here, by isolating adipose precursor cells from C57 mice in vitro and inducing them to differentiate and mature, we found that MDM2 was strongly induced during adipocyte differentiation, which indicated that MDM2 acts as an important part to the development of adipose tissue. On the other hand, the expression of MDM2 in adipose tissue could be regulated by nutritional status, and high-fat diet (HFD) significantly up-regulated the expression level of MDM2 in the adipose tissue. To further explore how MDM2 regulates the biological functions of adipose tissue, we constructed MDM2 adipocyte-specific knock-in (Mdm2-AKI) mice using CRISPR-cas9 technology. Under the condition of normal chow diet (NCD) feeding, Mdm2-AKI mice exhibited a significant increased adipose tissue mass of different parts and a series of metabolic abnormal phenotypes, such as weight gain, insulin resistance, and decreased energy expenditure. Interestingly, chronic HFD feeding resulted in distinct phenotypes of white adipose tissue in different sites of Mdm2-AKI mice. Under HFD-fed condition, epididymal white adipose tissue (eWAT), not inguinal adipose tissue (iWAT), was significantly reduced in weight and developed marked adipose tissue dysfunction, such as senescence, apoptosis, and chronic inflammation, resulting in marked insulin resistance and hepatic steatosis in Mdm2-AKI mice. Mechanistically, we used eWAT from HFD-fed Mdm2-AKI mice and control wild type (WT) mice to analyze the downstream target proteins of MDM2 by label-free quantitative proteome and ubiquitinome. Bioinformatics analysis revealed a wide range of potential roles for MDM2 in lipid and carbohydrate metabolism, and among these potential substrates, we identified the Six-transmembrane epithelial antigen of prostate 4 (STEAP4) as a true substrate of MDM2. We carried out a series of in vitro experiments to confirm that MDM2 could interact with STEAP4 and inhibit the expression of STEAP4 by ubiquitinating the K18 and K161 sites of STEAP4 for degradation. Finally, restoration of STEAP4 in eWAT of Mdm2-AKI mice rescued MDM2-induced adipose dysfunction, insulin resistance, and hepatic steatosis. In conclusion, the MDM2-STEAP4 axis in the adipose tissue was crucial for maintaining healthy adipose tissue function and ameliorating hepatic steatosis.