Glycosylation Network Research Center, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea.
Department of Molecular Medicine and Biopharmaceutical Sciences, Graduate School of Convergence Science and Technology and College of Medicine Or College of Pharmacy, Seoul National University, 28 Yeongeon-dong, Jongno-gu, Seoul, 03080, Republic of Korea.
Sci Rep. 2020 Nov 16;10(1):19908. doi: 10.1038/s41598-020-76862-0.
O-linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification which occurs on the hydroxyl group of serine or threonine residues of nucleocytoplasmic proteins. It has been reported that the presence of this single sugar motif regulates various biological events by altering the fate of target proteins, such as their function, localization, and degradation. This study identified SMAD4 as a novel O-GlcNAc-modified protein. SMAD4 is a component of the SMAD transcriptional complex, a major regulator of the signaling pathway for the transforming growth factor-β (TGF-β). TGF-β is a powerful promoter of cancer EMT and metastasis. This study showed that the amount of SMAD4 proteins changes according to cellular O-GlcNAc levels in human lung cancer cells. This observation was made based on the prolonged half-life of SMAD4 proteins. The mechanism behind this interaction was that O-GlcNAc impeded interactions between SMAD4 and GSK-3β which promote proteasomal degradation of SMAD4. In addition, O-GlcNAc modification on SMAD4 Thr63 was responsible for stabilization. As a result, defects in O-GlcNAcylation on SMAD4 Thr63 attenuated the reporter activity of luciferase, the TGF-β-responsive SMAD binding element (SBE). This study's findings imply that cellular O-GlcNAc may regulate the TGF-β/SMAD signaling pathway by stabilizing SMAD4.
O-连接β-N-乙酰氨基葡萄糖（O-GlcNAc）是一种翻译后修饰，发生在核细胞质蛋白丝氨酸或苏氨酸残基的羟基上。据报道，这个单一糖基模体的存在通过改变靶蛋白的命运，如它们的功能、定位和降解，来调节各种生物事件。本研究鉴定出 SMAD4 是一种新型的 O-GlcNAc 修饰蛋白。SMAD4 是 SMAD 转录复合物的一个组成部分，是转化生长因子-β（TGF-β）信号通路的主要调节剂。TGF-β 是癌症 EMT 和转移的强大促进剂。本研究表明，人肺癌细胞中细胞 O-GlcNAc 水平的变化会导致 SMAD4 蛋白的数量发生变化。这一观察结果是基于 SMAD4 蛋白半衰期延长得出的。这种相互作用的机制是 O-GlcNAc 阻碍了 SMAD4 与 GSK-3β 之间的相互作用，GSK-3β 促进 SMAD4 的蛋白酶体降解。此外，SMAD4 Thr63 上的 O-GlcNAc 修饰负责稳定。因此，SMAD4 Thr63 上 O-GlcNAcylation 的缺陷会减弱 luciferase 的报告活性，即 TGF-β 反应性 SMAD 结合元件（SBE）。本研究的结果表明，细胞内的 O-GlcNAc 可能通过稳定 SMAD4 来调节 TGF-β/SMAD 信号通路。
