NEMO, also known as IKKγ, is a regulatory subunit of the complex involved in activation of nuclear factor κB (NF-κB). Zhao et al. now show that in addition this function in NF-κB activation, NEMO participates in the activation of transcription factors called interferon regulatory factors (IRFs), specifically IRF3 and IRF7, which, in combination with NF-kB and activator protein 1, stimulate the production of type I interferons (IFN-α and -β) in virally infected cells. IRF3 and IRF7 are especially important for the initial production of IFN-α that occurs in virally infected cells. Experiments with mouse embryo fibroblasts (MEFs) in which the gene encoding NEMO (Ikgkb) or IKKβ (a kinase required for NF-κB activation) that were infected with virus showed that efficient production of IFN-α and the subsequent expression of signal transducer and activator of transcription 1 (STAT1) required NEMO, but not IKKβ, suggesting that NEMO was not acting through the NF-κB pathway. Reporter gene assays also demonstrated the dependence of IRF3- and IRF7-mediated gene expression on NEMO. Phosphorylation and dimerization is required for activation of IRF3 and IRF7, and NEMO was required as the knockout cells exhibited a lack of phosphorylation, dimerization, and DNA binding in response to viral infection. In the NEMO knockout cells, overexpression of TBK1 (an IKK-related kinase that is upstream of IRF3 and IRF7 activation) restored reporter gene activation, and NEMO was also required for activation of the kinase activity of TBK1, placing NEMO upstream of TBK1 in the path to IRF3 and IRF7 activation. However, NEMO did not interact directly with TBK1 (or the related kinase IKKε). Instead, immunoprecipitation studies with transfected cells showed that TANK (TRAF family member-associated NF-κB activator) was required for the formation of a trimeric complex of TANK, NEMO, and IKKε or TBK1. Mutational analysis and deletion analysis suggest that different regions of NEMO participate in NF-κB or IRF activation and may explain some of the diseases associated with various point mutations in the Ikgkb in humans.
T. Zhao, L. Yang, Q. Sun, M. Arguello, D. W. Ballard, J. Hiscott, R. Lin, The NEMO adaptor bridges the nuclear factor-κB and interferon regulatory factor signaling pathways. Nat. Immunol. 8, 592-600 (2007). [PubMed]