远处转移是乳癌的主要死因，但转移的具体机制尚不清楚。我们利用前期建立的基于表面抗原的循环肿瘤细胞（CTCs）捕获技术发现了一个转移性乳癌CTCs中高表达的新分子N4BP3，前期实验发现N4BP3能促进乳癌细胞增殖和转移能力，且能抑制PTEN的mRNA转录水平和蛋白表达。TCGA数据库提示N4BP3与乳癌远处转移和生存密切相关。文献报道N4BP3是PTEN重要泛素化连接酶NEDD4的潜在结合蛋白之一，但目前N4BP3在乳癌转移中的作用机制尚不清楚。本项目拟在验证N4BP3抑制PTEN从而促进乳癌转移作用的基础上，从N4BP3结合NEDD4而催化PTEN的多泛素化降解，或抑制PTEN启动子转录而下调PTEN产生这两方面来研究N4BP3抑制PTEN的分子机制，进而通过临床大样本和小鼠移植瘤模型验证。项目从CTCs这个新视点探究乳癌转移的关键分子机制，为靶向治疗转移性乳癌奠定扎实的研究基础。

Metastasis is the main cause of death in breast cancer, but the molecular mechanism of metastasis remains largely unknown. Based on our established CTCs isolation technique, we found that N4BP3 was highly expressed in the CTCs of metastatic breast cancer. Our pilot experiment results showed that N4BP3 can target PTEN which is important in PI3K/AKT signaling pathway, and significantly regulate the ability of proliferation, invasion and metastasis in breast cancer cells. Analysis of TCGA public databases indicated that N4BP3 was inversely correlated to the distant metastasis free survival of breast cancer. N4BP3 was known as a potential adaptor protein of NEDD4, which is the key E3 ubiquitin protein ligase for PTEN. Until now, the function of N4BP3 on breast cancer metastasis has not been reported in the literature. Thus, in this project, we will investigate the role of N4BP3 in the hematogenous metastasis of breast cancer by in vitro and in vivo experiments, and explore the mechanism of N4BP3 inhibiting PTEN from two aspects: One is that N4BP3 bound to the NEDD4 and caused the PTEN ubiquitination degradation, the other is that N4BP3 functioned on the PTEN promoter and inhibited the PTEN transcription. Finally, validation will be performed in clinical breast cancer patients and mouse model. This work aims to explore the molecular mechanism of breast cancer hematogenous metastasis from aspect of CTCs and would contribute a solid foundation for the coming research into the target treatment for metastatic breast cancer.