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).
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).
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.
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).
