PPM1D suppresses p53-dependent transactivation and cell death by inhibiting the HRI-eIF2alpha-ATF4 pathway [ChIP-seq]

The p53 transcription factor is a master regulator of cellular stress responses restrained by repressors such as MDM2 and the phosphatase PPM1D. Activation of p53 with pharmacological inhibitors of its repressors is being tested in clinical trials for cancer therapy, but efficacy has been limited by poor induction of tumor cell death. We demonstrate that dual inhibition of MDM2 and PPM1D induces cell death in multiple cancer cell types via amplification of the p53 transcriptional program through the eIF2alpha -ATF4 pathway. PPM1D inhibition induces phosphorylation of eIF2alpha, ATF4 accumulation, and ATF4-dependent enhancement of p53-dependent transactivation upon MDM2 inhibition. Dual inhibition of p53 repressors depletes heme and induces HRI-dependent eIF2alpha phosphorylation. Pharmacological induction of eIF2alpha phosphorylation synergizes with MDM2 inhibition to induce cell death and halt tumor growth in mice. These results demonstrate that PPM1D inhibits both the p53 network and the integrated stress response controlled by eIF2alpha, with clear therapeutic implications. Effects of MDM2 and PPM1D inhibition on p53 occupancy in TPC1 and K1 cell lines.p53转录因子（p53 transcription factor）是细胞应激反应的主调控因子，其活性受MDM2、磷酸酶PPM1D（phosphatase PPM1D）等阻遏蛋白的抑制。采用靶向其阻遏蛋白的药理学抑制剂激活p53的癌症治疗方案已进入临床试验，但因肿瘤细胞死亡诱导效率低下，治疗疗效始终受限。本研究证实，同时抑制MDM2与PPM1D可通过eIF2α-ATF4通路（eIF2alpha-ATF4 pathway）扩增p53转录程序，进而在多种癌细胞系中诱导细胞死亡。PPM1D抑制可诱导eIF2α磷酸化、ATF4积累，并在MDM2抑制时增强ATF4依赖的p53反式激活过程。双重抑制p53的阻遏蛋白会耗竭细胞内血红素，并诱导HRI依赖的eIF2α磷酸化。在小鼠模型中，通过药理学手段诱导eIF2α磷酸化可与MDM2抑制产生协同效应，诱导细胞死亡并抑制肿瘤生长。上述结果表明，PPM1D可同时抑制p53信号通路与eIF2α调控的整合应激反应（integrated stress response），该发现具备明确的治疗应用价值。MDM2与PPM1D抑制对TPC1及K1细胞系中p53染色质结合情况的影响。