抑癌蛋白p53乙酰化修饰调控肿瘤免疫的分子机制研究

Whether C-terminal domain (CTD) acetylation regulates p53-dependent selective transcription of a subset of p53 downstream targets to determine p53-mediated context-dependent biological functions is a critical scientific question in the field of p53 cancer biology. To address this question, we generated a p53KQ/- knock-in mouse model to specifically mimic CTD acetylated status of p53 in vivo. By employing a high-throughput screening to compare the alteration of gene expression in peripheral lymphoid organs derived from p53+/-, p53-/- and p53KQ/- mice treated with or without γ-radiation, we found that the wild-type p53, but not the CTD acetylation mimicking p53KQ mutant, significantly up-regulated PD-1 (encoded by gene PDCD1) expression. Based on these observations, we hypothesized that p53 participates in the regulation of cancer immunity in a CTD acetylation-dependent manner. We further plan to investigate the following key points in this project: 1) the manners and conditions that p53 activates PD-1; 2) the mechanism by which CTD acetylation selectively modulates p53-dependent PD-1 expression; 3) whether p53 directly contributes to cancer immune regulation via transactivation of PD-1. This study aims to establish a relationship among p53 per se, CTD acetylation status and cancer immune regulation, by which we are looking forward to raising a potential strategy by using p53 agonists and/or HDAC inhibitors to enhance cancer immune efficacy and, finally, providing a pre-clinical evidence of this strategy for cancer therapy.

羧基末端乙酰化修饰是否调控抑癌蛋白p53对靶基因的选择性转录进而决定p53特定生物学功能的发挥是p53肿瘤生物学领域的关键问题。我们建立了模拟羧基末端乙酰化修饰的p53KQ/-小鼠模型；高通量分析γ-射线照射前后p53+/-、p53-/-和p53KQ/-小鼠外周淋巴器官基因表达的变化，发现野生型p53，而非p53KQ，显著激活肿瘤免疫检查点PD-1（由PDCD1基因编码）的表达。据此提出：羧基末端乙酰化修饰可能通过调节p53对下游靶基因的选择性转录参与肿瘤免疫的调控。本研究拟探讨：p53激活PD-1的具体方式及所需条件；羧基末端乙酰化修饰选择性调控p53依赖的PD-1表达的分子机制；p53是否通过调节PD-1直接参与肿瘤免疫的调控。本研究旨在建立p53及其羧基末端乙酰化修饰与肿瘤免疫调控的关系，为p53小分子激活剂、去乙酰基酶抑制剂在肿瘤免疫疗法中的潜在应用提供基础医学的证据。

乙酰化修饰是调节抑癌蛋白p53生物学功能的重要方式；但是位点特异性乙酰化修饰调控p53转录活性或转录选择性，进而调节p53肿瘤抑制功能的分子机制尚不完全清楚。PD-1是关键的免疫检查点，T细胞表达的PD-1在肿瘤免疫的调节中起重要作用；近年来发现部分肿瘤细胞中也存在PD-1的表达，但其生物学功能及调控机制尚不明确。我们前期研究发现p53乙酰化修饰参与PD-1的表达调控；据此，本项目对p53及其位点特异性乙酰化修饰调节PD-1表达的分子机制及其肿瘤生物学意义展开研究。我们发现：在肿瘤细胞中普遍存在p53依赖的PD-1的转录激活；PD-1是p53直接下游靶基因；DNA结合结构域（DNA-binding Domain, DBD）的乙酰化修饰通过促进p53对转录共激活因子p300、CBP和TIP60在PD-1启动子区的招募，进而激活PD-1的转录；羧基末端结构域（Carboxyl-terminal Domain, CTD）的乙酰化修饰则通过抑制p53与转录共激活因子BRD4的相互作用，进而对PD-1的转录起负性调控作用；在肺癌来源的H1299细胞中重新表达PD-1，可抑制肿瘤细胞的增殖和成瘤，其潜在机制是PD-1抑制了AKT-mTOR信号通路的激活；阻碍肿瘤细胞中PD-1的转录激活显著下调p53依赖的肿瘤抑制功能，表明肿瘤细胞中p53-PD-1轴的激活在抗肿瘤过程中起重要作用。本项目的科学意义在于证实了肿瘤细胞固有PD-1（cancer cell-intrinsic PD-1）可通过免疫非依赖的方式调节肿瘤发生发展的生物学作用；揭示了肿瘤细胞中p53及其乙酰化修饰调控PD-1转录的多重分子机制；提出了靶向肿瘤细胞p53-PD-1轴的潜在肿瘤干预策略。