糖基转移酶异常介导的蛋白质糖链结构变化与恶性肿瘤发生发展互为因果。项目申请人前期研究发现，作为介导Mucin样O-糖基化修饰的N-乙酰基半乳糖胺转移酶家族的重要成员，N-乙酰基半乳糖胺转移酶5（GALNT5）在胃癌中的低表达与肿瘤进展相关，并可作为独立预后因子增强TNM分期预测患者术后生存的准确性（Brit J Cancer，2014），然而其发挥肿瘤抑制作用的分子机制及功能意义尚不清楚。胃粘膜高表达的黏蛋白Mucin4可通过类表皮生长因子域与表皮生长因子受体2(HER2)结合抑制其二聚化及信号转导，GALNT5可能在调控Mucin4的O-糖链合成及功能发挥中起重要作用。本申请项目拟在前期预实验基础上，研究GALNT5调控Mucin4的O-糖链合成进而影响HER2信号的分子机制及功能意义，建立其参与胃癌发生发展的分子调控和干预治疗模型，为胃癌术后生存评估分子分型和靶向治疗药物开发奠定基础。

Aberrant O-linked glycosylation status catalyzed by polypeptide N-acetygalactosaminyl transferases has been reported to be associated with the development of many malignant tumors. Our previous study has identified that the polypeptide N-acetygalactosaminyl transferase 5 (GALNT5) was reduced in gastric cancer tissues compared with non-tumor mucosa and correlated with deteriorated tumor cell differentiation, aggravated tumor invasion, lymph node metastasis ,advanced TNM stage and reduced overall survival. However, the precise mechanisms underlying the role of GALNT5 in pathogenesis of gastric cancer remain unknown. The ErbB family is composed by ErbB1/EGF receptor (EGFR), ErbB2/HER2/Neu, ErbB3 and ErbB4 which are closely related to tumor differentiation, proliferation, survival, migration and anti-apoptosis etc. Following ligand binding and receptor activation, these receptors are endocytosed and transported to lysomes where the receptor is degraded. The ability of receptors retention on cell membrane and formation of homo-dimers or hetero-dimers are key determinants of signal activity. HER2 mainly acts as coreceptor with other receptors for lacking specific ligand, and HER3 also need heterdimered with other receptor to transmit signaling for owning a kinase dead c-tetmainl. MUC4 is a membrane binding mucin modified by GalNAc-Ts. Muc4 can interact with HER2 via EGF domain in the extracellular section. GALNT5 play important role in modifying the O-glycosylation of Muc4 to affect their conformation and reducing constitutive endocytosis. GALNT5 mediated Muc4 O-glycosylation blocked the formation of HER2 homodimers and/or HER2-HER3 heterodimers by competitive interation with HER2. Reduction of GALNT5 expression facilitated the interaction of HER2 and other receptors, enhancing downstream pathway activation, including ras/raf/MEK/Erk and PI3K/AKT signaling. This project proposal for grant application is designed to elucidate the regulatory effect and functional role of the mechanism of glycosylation of MUC4 post-translation and its function on HER2 signal transduction based on human gastric cancer tissues and cells . From the point of view in GALNT5 prompting cell differentiation and cancer glycobiology, our present research project will focus on the molecular mechanism and functional significance for O-glycosylation-mediated alteration of membrane HER2 heter/homo dimer formation by abnormally expressed GALNT5 and will establish a model of molecular regulation and therapeutic intervention of HER2 receptor O-glycosylation change on gastric cancer cells membrane, which paves the way to clinical investigation and drug discovery based on molecular therapy targeting aberrant O-glycosylation and HER2 signal activation in gastric cancer patients，and constructs a GALNT5 based molecular prognosis model.