DNA-PK binds microbial dsDNA

DNA-dependent serine/threonine protein kinase DNA-PK is a DNA damage sensor, which is composed of a large catalytic subunit DNA-PKcs and a heterodimer of Ku70 & Ku80 subunits. DNA-PK was found both in the nucleus and in the cytosol (Lucero H et al. 2003). While in the nucleus DNA-PK is critical for the repair of double-stranded DNA breaks during the lymphocyte development, in the cytosol it can also bind DNA fragments to transmit stress signals (Dip R & Naegeli H 2005; Yotsumoto S et al. 2008; Dragoi AM et al. 2004; Ferguson BJ et al. 2012).<p>This Reactome event presents DNA-PK as a holoenzyme, however it remains unclear whether all DNA-PK subunits are critical for exogenous DNA recognition, whether they function as a DNA-PK complex or each subunit acts independently in certain circumstances (Zhang X et al. 2011; Ferguson BJ et al. 2012).<p> Studies involving different human and mouse cell lines yielded variable results regarding to DNA-PK signaling functions. The catalytic subunit DNA-PKcs has been shown to associate with Akt upon CpG-OND-stimulation triggering transient nuclear translocation of Akt in mouse bone marrow-derived macrophages (BMDMs)(Dragoi AM et al. 2004). DNA-PKcs has been also reported to induce ERK activation and production of anti-inflammatory cytokine IL-10 in CpG-ODN-stimulated mouse monocyte/macrophage cell line RAW264.7, while production of pro-inflammatory cytokine IL-12 was negatively regulated (Yotsumoto S et al. 2008). In addition, endosomal translocation of CpG-ODN was found to regulate DNA-PKcs-mediated responses to CpG-OND (Yotsumoto S et al. 2008; Hazeki K et al. 2011). Moreover, DNA-PK subunits have been implicated in IFN regulatory factor (IRF)-dependent innate immune responses. Ku-70 was shown to induce production of type III IFN (IFN -lamda 1) in human embryonic kidney HEK293 cells transfected with DNA. The Ku70-mediated IFN-lamda 1 activation required a longer size of DNA (>500 bp DNA) (Zhang X et al. 2011). Whether DNA-PK mediates activation of IFN-beta production is debatable. Ku70- or DNA-PKcs-deficient mouse bone marrow-derived macrophages cells mounted an identical IFN-beta response when compared to their wild-type controls (Stetson DB & Medzhitov R 2006). However, the other group demonstrated that DNA-PK induced IRF3-dependent production of IFN-beta in DNA-stimulated mouse embryonic fibroblast(MEF) and human HEK293 cells (Ferguson BJ et al. 2012). Thus, the molecular mechanism behind DNA-PK activation by cytosolic DNA remains to be clarified.<p>It's interesting to note that in the nucleus DNA-PK may regulate IRF3 transcriptional activity in response to viral infection. DNA-PK was found to bind and phosphorylate IRF-3 at Thr-135 in Sendai virus (SV)-treated human endometrial adenocarcinoma HEC1B cells. DNA-PK-dependent phosphorylation at Thr-135 is thought to retain transcriptionally active IRF-3 in the nucleus (Karpova AY et al. 2002).

DNA依赖性丝氨酸/苏氨酸蛋白激酶DNA-PK作为一种DNA损伤感受器，由一个大型的催化亚基DNA-PKcs以及由Ku70与Ku80亚基组成的异源二聚体构成。DNA-PK既存在于细胞核中也存在于细胞质中（Lucero H等，2003年）。在细胞核中，DNA-PK对于淋巴细胞发育过程中双链DNA断裂的修复至关重要；而在细胞质中，它能够结合DNA片段以传递应激信号（Dip R与Naegeli H，2005年；Yotsumoto S等，2008年；Dragoi AM等，2004年；Ferguson BJ等，2012年）。《Reactome》事件将DNA-PK呈现为一个全酶，然而，关于所有DNA-PK亚基是否对外源性DNA识别至关重要，它们是作为DNA-PK复合体发挥作用，还是在某些情况下各亚基独立作用，目前尚不明确（Zhang X等，2011年；Ferguson BJ等，2012年）。<p>涉及不同人类和鼠细胞系的研究在DNA-PK信号传导功能方面产生了不同的结果。催化亚基DNA-PKcs在CpG-OND刺激后与Akt结合，触发小鼠骨髓来源的巨噬细胞（BMDMs）中Akt的短暂核转位（Dragoi AM等，2004年）。DNA-PKcs也被报道在CpG-ODN刺激的小鼠单核细胞/巨噬细胞系RAW264.7中诱导ERK激活和抗炎细胞因子IL-10的产生，同时负调节促炎细胞因子IL-12的产生（Yotsumoto S等，2008年）。此外，CpG-ODN的内吞体转位被发现可以调节DNA-PKcs介导的CpG-OND反应（Yotsumoto S等，2008年；Hazeki K等，2011年）。此外，DNA-PK亚基还与IFN调节因子（IRF）依赖的先天免疫反应有关。Ku-70被发现可以诱导人类胚胎肾HEK293细胞中转染DNA后产生III型干扰素（IFN-λ1）。Ku70介导的IFN-λ1激活需要较长的DNA长度（>500 bp的DNA）（Zhang X等，2011年）。DNA-PK是否介导IFN-β的产生尚有争议。与野生型对照相比，Ku70或DNA-PKcs缺陷型小鼠骨髓来源的巨噬细胞细胞表现出相同的IFN-β反应（Stetson DB与Medzhitov R，2006年）。然而，另一组研究显示，DNA-PK在DNA刺激的小鼠胚胎成纤维细胞（MEF）和人类HEK293细胞中诱导IRF3依赖的IFN-β产生（Ferguson BJ等，2012年）。因此，细胞质DNA激活DNA-PK的分子机制尚待阐明。<p>值得注意的是，在细胞核中，DNA-PK可能调节病毒感染引起的IRF3转录活性。研究发现，DNA-PK可以与Sendai病毒（SV）处理的人类子宫内膜腺癌HEC1B细胞中的IRF-3结合并磷酸化Thr-135。DNA-PK依赖的Thr-135磷酸化被认为可以保持转录活性的IRF-3在细胞核中（Karpova AY等，2002年）。