肝癌的发生发展伴随着机体蛋白质稳态的改变——致癌蛋白激活或抑癌蛋白失活导致的蛋白质稳态破坏往往是细胞癌变的导火索[1]。泛素-蛋白酶体系统具有及时清除异常蛋白质、维持蛋白质内稳态的重要作用，从而保证细胞生命活动的有序进行[2]。S期激酶相关蛋白2 （S-phase kinase-associated protein 2，SKP2）是最早被发现的泛素连接酶之一，特异性催化底物的泛素化修饰。SKP2在多种肿瘤中显著高表达，与肿瘤进展密切相关。以往的研究发现蛋白质翻译后修饰，如磷酸化、泛素化和乙酰化修饰等，能够调控SKP2的分子生物学功能。但是否还有其他的翻译后修饰参与了SKP2功能的调控，目前尚不明确。
己糖胺生物合成通路（hexosamine biosynthesis pathway，HBP）是糖酵解的重要分支途径，其终产物UDP-GlcNAc是发生O-GlcNAc修饰所必要的供体。研究显示O-GlcNAc修饰在多种肿瘤中显著上调，与肿瘤发生发展密切相关[3]。O-GlcNAc糖基化可以调节蛋白质稳定性、亚细胞定位、活性和蛋白质-蛋白质相互作用等，从而参与调控肿瘤增殖和转移等过程[4]。研究显示O-GlcNAc修饰与泛素化修饰之间存在着交互作用与级联反应，协同调控昼夜节律和肿瘤进展等生理/病理生理过程[5,6]。但O-GlcNAc修饰直接作用于泛素连接酶并调控其酶活性或功能的研究报道较少。肝癌中SKP2的促癌特性是否受到O-GlcNAc修饰调控，有待进一步研究。
2024年2月23日，重庆医科大学感染性疾病分子生物学教育部重点实验室汪凯、唐霓和黄露义课题组在Oncogene在线发表了题为O-GlcNAcylation of E3 ubiquitin ligase SKP2 promotes hepatocellular carcinoma proliferation的研究论文[7]。该研究首次发现E3泛素连接酶SKP2存在O-GlcNAc修饰，其修饰水平在肝癌中显著上调；重点解析了O-GlcNAc修饰对SKP2生物学功能的影响及其促进肝癌进展的分子机制，证实靶向SKP2的O-GlcNAc修饰能够抑制肝癌细胞增殖，为拓展肝癌的治疗策略提供了潜在的新靶点。
该团队首先通过多种体内外实验明确SKP2蛋白可以发生O-GlcNAc修饰，质谱鉴定到SKP2的第34位丝氨酸（Ser34）是其O-GlcNAc修饰的关键位点。随后在多种细胞模型和异种小鼠移植瘤模型中，观察到SKP2的O-GlcNAc修饰促进了肝癌细胞的增殖。机制解析发现Ser34位点的O-GlcNAc修饰能够增强SKP2蛋白的稳定性，增强SKP2与SKP1的结合能力，促进SCF-SKP2 E3泛素连接酶复合体的形成，进而促进细胞周期蛋白依赖性激酶抑制因子p27和p21的泛素化降解，从而解除p27和p21对细胞周期G1-S期的限制，促进肝癌细胞增殖。
SKP2的O-GlcNAc修饰促进肝癌增殖的机制图
上述研究首次发现E3泛素连接酶SKP2能够直接被O-GlcNAc糖基化修饰，揭示了SKP2的O-GlcNAc修饰能够促进肝癌细胞增殖，并解析了O-GlcNAc修饰调控SKP2分子生物学功能的具体机制。该研究揭示了E3泛素连接酶SKP2的O-GlcNAc修饰在肝癌进展的作用，提示泛素化修饰与O-GlcNAc修饰之间的潜在联系精细调控目标蛋白的降解和生物学功能。同时，也证实了蛋白翻译后修饰的级联反应在细胞周期调控中的重要作用；此外靶向SKP2的O-GlcNAc修饰为肝癌的治疗提供了新的策略。
重庆医科大学感染性疾病分子生物学教育部重点实验室汪凯副研究员、唐霓教授和黄露义老师为论文的共同通讯作者。冯中奇硕士、印家鑫博士、张芷榕硕士为论文的并列第一作者。
原文链接：https://www.nature.com/articles/s41388-024-02977-7
O-GlcNAcylation of E3 ubiquitin ligase SKP2 promotes hepatocellular carcinoma proliferation
Zhongqi Feng, Jiaxin Yin, Zhirong Zhang, Zhen Chen, Luyi Huang*, Ni Tang*, Kai Wang*
Abstract
O-linked-β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) and ubiquitination are critical posttranslational modifications that regulate tumor development and progression. The continuous progression of the cell cycle is the fundamental cause of tumor proliferation. S-phase kinase-associated protein 2 (SKP2), an important E3 ubiquitin ligase, assumes a pivotal function in the regulation of the cell cycle. However, it is still unclear whether SKP2 is an effector of O-GlcNAcylation that affects tumor progression. In this study, we found that SKP2 interacted with O-GlcNAc transferase (OGT) and was highly O-GlcNAcylated in hepatocellular carcinoma (HCC). Mechanistically, the O-GlcNAcylation at Ser34 stabilized SKP2 by reducing its ubiquitination and degradation mediated by APC-CDH1. Moreover, the O-GlcNAcylation of SKP2 enhanced its binding ability with SKP1, thereby enhancing its ubiquitin ligase function. Consequently, SKP2 facilitated the transition from the G1-S phase of the cell cycle by promoting the ubiquitin degradation of cell cycle-dependent kinase inhibitors p27 and p21. Additionally, targeting the O-GlcNAcylation of SKP2 significantly suppressed the proliferation of HCC. Altogether, our findings reveal that O-GlcNAcylation, a novel posttranslational modification of SKP2, plays a crucial role in promoting HCC proliferation, and targeting the O-GlcNAcylation of SKP2 may become a new therapeutic strategy to impede the progression of HCC.
DOI: https://doi.org/10.1038/s41388-024-02977-7
主要参考文献
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7 Feng Z, Yin J, Zhang Z, Chen Z, Huang L, Tang N, et al. O-GlcNAcylation of E3 ubiquitin ligase SKP2 promotes hepatocellular carcinoma proliferation. Oncogene. 2024 Feb DOI: 10.1038/s41388-024-02977-7
