APOBEC3B介导肝癌起始细胞形成及免疫逃逸的分子机制研究

Liver tumor initiating cell is stem-like cell that can escape from immune surveillance and progress into liver cancer. However, the mechanism of formation of tumor initiating cell and escaping of the cell from immune surveillance remains elusive. Based on analysis of clinical samples and comparison of corresponding pathologic specimen, our previous work has found that high expression of APOBEC3B is related to poor prognosis of liver cancer patients. We has also found that overexpression of APOBEC3B can promote formation of CD133+ liver cancer stem cell but meanwhile suppress expression of cytokines related to immune escape such as CXCL9 and CLCL10. It is speculated that APOBEC3B takes part in formation of liver tumor initiating cell and its immune escape. This project will focus on the following contents. (1)Systematically investigate the mechanism of how APOBEC3B promotes dedifferentiation of liver cell to become tumor initiating cell by taking part into liver cell epigenetic change mediated by PRC2 complex based on the strategies of inducing tumor stem cells ex vivo.(2) Clarify the molecular mechanism of how APOBEC3B induces immune escape of liver tumor initiating cell by suppressing expression of CXCL10 and MHC-I molecules.(3) Develop small molecular inhibitors of APOBEC3B based on the mechanism discovered in this project. In sum, this project will clarify key molecular event during liver cancer initiation to provide principle and experimental basis for establishment of new drugs for liver cancer treatment.

肝癌起始细胞具有干细胞样特征，可脱离免疫监视进展为肝癌，而其形成过程及免疫逃逸的具体机制尚未阐明。本课题组前期分析临床病例及对应病理标本发现，APOBEC3B（A3B）高表达与患者预后负相关，过表达AB可促进CD133+的肝癌干细胞形成，同时可抑制CXCL9、CLCL10等免疫逃逸相关趋化因子的表达，推测A3B可促进肝癌起始细胞的形成及免疫逃逸。本项目主要研究内容：（1）利用体内外诱导肿瘤干细胞形成的研究策略，系统阐述A3B参与PRC2复合物的组装介导肝细胞表观遗传修饰改变，促进去分化重编程形成肝癌起始细胞的分子机制；（2）探索A3B降低CXCL10，MHC-I类分子介导肝癌起始细胞免疫逃逸的分子机制；（3）设计、合成并评价基于A3B功能抑制的小分子先导化合物，用于HCC的早期预防与治疗。通过上述研究不仅阐明肝癌发生发展的关键分子事件，也为研发新型肝癌预防和治疗药物提供理论基础和实验依据。

肝细胞癌（Hepatocellular carcinoma，HCC）是全球常见的一种肝脏恶性肿瘤。慢性炎症微环境是肝细胞癌（HCC）发生及进展的重要病理特征之一，肝癌细胞与微环境中免疫细胞之间的相互作用可塑造免疫抑制微环境。肿瘤相关巨噬细胞（Tumor-associated macrophages，TAM）及髓源抑制细胞（myeloid-derived suppressor cells，MDSC）是肿瘤微环境中主要的免疫抑制细胞亚群，通过分泌负向调节因子，增加精氨酸酶（ARG1）和免疫检测点分子的表达等多种方式介导肿瘤免疫逃逸。本项目系统阐述机体固有免疫分子，DNA胞嘧啶脱氨基酶APOBEC3B（A3B） 介导HCC免疫抑制微环境形成的具体分子机制。本项目发现：1）HCC患者中A3B是唯一在癌巢中高表达的APOBEC3家族分子，导致肝癌患者的不良预后；2）炎症微环境通过RelB转录因子结合A3B启动子区域-116位的GGGGAAAACC序列促进A3B表达；3）A3B通过募集TAM及MDSC细胞介导肝癌免疫抑制微环境；4）酶活性位点突变的A3B分子依然具有募集TAM及MDSC细胞介导肝癌免疫抑制微环境的功能；5）APOBEC3B抑制H3K27me3甲基化水平，促进CCL2、IL-34、BMP7表达，募集TAM和MDSC，促进巨噬细胞M2表型转化，介导免疫逃逸微环境；6）A3B通过其C端序列与PRC2复合物相互作用，抑制H3K27me3的甲基化水平，促进趋化因子表达。本项目系统阐述A3B和PRC2复合物相互作用，从抑制PRC2复合物结合DNA及抑制PRC2甲基转移酶活性两个方面改变PRC2复合物的功能，促进整体H3K27me3水平降低，导致多个促进单核抑制细胞（MDSC）和肿瘤相关巨噬细胞（TAM）募集的趋化因子及细胞因子的表达量增加，促进CD8 T细胞免疫耗竭，介导HCC的恶性进展。 本项目阐明了HCC表观遗传机制介导免疫抑制微环境形成的新机制，对该机制的干预可能成为肝癌免疫治疗的新靶点。