PARP1 depletion induces RIG-I-dependent signaling in human cancer cells

DNA Damage Response (DDR) and DNA repair pathways are emerging as potent, ubiquitous suppressors of innate immune signaling in human cells. Here, we show that human cells surviving depletion of the Single Strand Break (SSB) repair protein PARP1 undergo p21-dependent senescence or cell cycle checkpoint activation in the context of activation of innate immune signaling, or viral mimicry. Specifically, we observe induction of a large number of interferon-stimulated genes (ISGs) and multiple pattern recognition receptors (PRRs; including RIG-I, MDA-5, MAVS, TLR3 and STING) and increased nuclear IRF3 staining. Mechanistically, depletion of the double-stranded RNA (dsRNA) helicase RIG-I or its downstream effector MAVS specifically rescues ISG induction in PARP1-depleted cells, suggesting that the RIG-I/MAVS pathway is required for sustained ISG expression in this context. Experiments with conditioned media or a neutralizing antibody to the α/β-IFN receptor revealed that persistent ISG expression additionally requires an autocrine/paracrine loop. Finally, loss of PARP1 and radiation-induced DNA damage strongly synergize in the induction of p21 and ISGs. Overall, these findings increase our understanding of how PARP1 may suppress deleterious phenotypes associated to aging, inflammation and cancer in humans.DNA损伤应答（DNA Damage Response, DDR）与DNA修复通路，正逐渐被证实为人类细胞内天然免疫信号通路的强效、广谱抑制因子。本研究发现，在天然免疫信号激活（即病毒模拟现象）的背景下，经单链断裂（Single Strand Break, SSB）修复蛋白PARP1敲低后存活的人类细胞，会发生p21依赖的细胞衰老或细胞周期检查点激活。具体而言，本研究观察到大量干扰素刺激基因（interferon-stimulated genes, ISGs）、多种模式识别受体（pattern recognition receptors, PRRs，包括RIG-I、MDA-5、MAVS、TLR3及STING）的诱导表达，以及细胞核内IRF3染色信号增强。机制研究显示，敲减双链RNA（double-stranded RNA, dsRNA）解旋酶RIG-I或其下游效应蛋白MAVS，可特异性逆转PARP1敲低细胞中ISGs的诱导表达，提示在此场景下RIG-I/MAVS通路是维持ISGs持续表达所必需的。通过条件培养基或针对α/β干扰素受体的中和抗体开展的实验进一步证实，ISGs的持续表达还依赖自分泌/旁分泌环路。最后，PARP1缺失与辐射诱导的DNA损伤在诱导p21及ISGs表达方面具有显著协同效应。综上，本研究结果加深了我们对PARP1如何抑制人类衰老、炎症与癌症相关有害表型的理解。