Brestovitsky Anna, Nebenzahl-Sharon Keren, Kechker Peter, Sharf Rakefet, Kleinberger Tamar
Department of Microbiology, the Rappaport Faculty of Medicine and Research Institute, Technion-Israel Institute of Technology, Haifa, Israel.
PLoS Pathog. 2016 Feb 11;12(2):e1005420. doi: 10.1371/journal.ppat.1005420. eCollection 2016 Feb.
The DNA damage response (DDR) is a conglomerate of pathways designed to detect DNA damage and signal its presence to cell cycle checkpoints and to the repair machinery, allowing the cell to pause and mend the damage, or if the damage is too severe, to trigger apoptosis or senescence. Various DDR branches are regulated by kinases of the phosphatidylinositol 3-kinase-like protein kinase family, including ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR). Replication intermediates and linear double-stranded genomes of DNA viruses are perceived by the cell as DNA damage and activate the DDR. If allowed to operate, the DDR will stimulate ligation of viral genomes and will inhibit virus replication. To prevent this outcome, many DNA viruses evolved ways to limit the DDR. As part of its attack on the DDR, adenovirus utilizes various viral proteins to cause degradation of DDR proteins and to sequester the MRN damage sensor outside virus replication centers. Here we show that adenovirus evolved yet another novel mechanism to inhibit the DDR. The E4orf4 protein, together with its cellular partner PP2A, reduces phosphorylation of ATM and ATR substrates in virus-infected cells and in cells treated with DNA damaging drugs, and causes accumulation of damaged DNA in the drug-treated cells. ATM and ATR are not mutually required for inhibition of their signaling pathways by E4orf4. ATM and ATR deficiency as well as E4orf4 expression enhance infection efficiency. Furthermore, E4orf4, previously reported to induce cancer-specific cell death when expressed alone, sensitizes cells to killing by sub-lethal concentrations of DNA damaging drugs, likely because it inhibits DNA damage repair. These findings provide one explanation for the cancer-specificity of E4orf4-induced cell death as many cancers have DDR deficiencies leading to increased reliance on the remaining intact DDR pathways and to enhanced susceptibility to DDR inhibitors such as E4orf4. Thus DDR inhibition by E4orf4 contributes both to the efficiency of adenovirus replication and to the ability of E4orf4 to kill cancer cells.
DNA损伤反应（DDR）是一组旨在检测DNA损伤并将其存在信号传递给细胞周期检查点和修复机制的通路，使细胞能够暂停并修复损伤，或者在损伤过于严重时触发凋亡或衰老。DDR的各个分支由磷脂酰肌醇3激酶样蛋白激酶家族的激酶调控，包括共济失调毛细血管扩张症突变基因（ATM）和ATM及Rad3相关蛋白（ATR）。DNA病毒的复制中间体和线性双链基因组被细胞视为DNA损伤并激活DDR。如果DDR发挥作用，它将刺激病毒基因组的连接并抑制病毒复制。为了防止这种结果，许多DNA病毒进化出了限制DDR的方法。作为其对DDR攻击的一部分，腺病毒利用多种病毒蛋白导致DDR蛋白降解，并将MRN损伤传感器隔离在病毒复制中心之外。在这里，我们表明腺病毒进化出了另一种抑制DDR的新机制。E4orf4蛋白与其细胞伴侣PP2A一起，可降低病毒感染细胞和经DNA损伤药物处理的细胞中ATM和ATR底物的磷酸化，并导致经药物处理的细胞中受损DNA的积累。E4orf4抑制其信号通路并不需要ATM和ATR相互配合。ATM和ATR缺陷以及E4orf4的表达均能提高感染效率。此外，E4orf4此前报道称单独表达时可诱导癌症特异性细胞死亡，它使细胞对亚致死浓度的DNA损伤药物杀伤更加敏感；这可能是因为它抑制了DNA损伤修复。这些发现为E4orf4诱导的细胞死亡的癌症特异性提供了一种解释，因为许多癌症存在DDR缺陷，导致对其余完整的DDR通路的依赖性增加，并对DDR抑制剂（如E4orf4）的敏感性增强。因此，E4orf4对DDR的抑制作用既有助于腺病毒的复制效率，也有助于E4orf4杀死癌细胞的能力。
