Editors' ChoiceImmunology

IKKβ: Commanding the Inflammatory Response

Sci. Signal.  16 Dec 2014:
Vol. 7, Issue 356, pp. ec349
DOI: 10.1126/scisignal.aaa4744

Monocytes are immune cells in the blood that are key responders to injury and infection, and they differentiate into macrophages and dendritic cells when they enter tissues. Members of the nuclear factor κB (NF-κB) family of transcription factors are activated in response to the recognition of danger- or pathogen-associated molecular patterns (DAMPs, PAMPs), which stimulate the kinases of the IKK family. Lopez-Pelaez et al. and Ren et al. showed that IKKβ (which is involved in the activation of NF-κB) also phosphorylated and stimulated the activity of the transcription factor interferon regulatory factor 5 (IRF5), a protein important for proinflammatory gene expression in response to activation of Toll-like receptors (TLRs) by PAMPs or DAMPs. As with NF-κB, IRF5 is kept inactive in the cytosol in unstimulated cells. In studies with a human dendritic cell line (Gen2.2) and mouse macrophage-like cells (RAW264.7), Lopez-Pelaez et al. found that knockdown or pharmacological inhibition of IKKβ or its upstream activating kinase TAK1 blocked nuclear translocation of IRF5 in response to stimulation with a TLR7 agonist. Knockdown of IRF5 specifically compromised the induction of interferon β (IFN-β) mRNA and secretion from Gen2.2 cells. Mass spectrometry analysis, followed by in vitro kinase assays, identified a specific serine in IRF5 as the site of phosphorylation by IKKβ. Mutation of this site blocked the nuclear translocation of IRF5 in response to TLR7 agonists. Ren et al. found in the human monocytic cell line THP1 that knockdown of IRF5 compromised the induction of mRNAs for multiple proinflammatory cytokines, including IFN-β, the chemokine CXCL10, and interleukin 12 (IL-12), in response to the TLR4 ligand lipopolysaccharide (LPS). Overexpression of IRF5 in the human embryonic kidney cell line HEK 293T, which does not have IRF5, enhanced the induction of mRNA for tumor necrosis factor–α (TNF-α) and IFN-β when the cells were infected with an RNA virus, which activates the intracellular PAMP recognition pathway mediated by RIG-I. Addition of IKKβ to cytosolic extracts from HEK 293T cells expressing FLAG-tagged IRF5 stimulated the formation of IRF5 dimers; dimerization is necessary for IRF transcriptional activity. Knockdown of components of the NF-κB–activating pathway in the FLAG-IRF5–expressing HEK 293T cells indicated that IKKβ, TRAF6, and NEMO were components common to the activation of both NF-κB and IRF5. Mass spectrometry analysis identified several serines as phosphorylated in samples of partially purified tagged mouse IRF5 incubated with IKKβ. The same serine residue as the one identified by Lopez-Pelaez et al. was necessary for IKKβ-induced dimerization and activation of IRF5. Stimulation of THP1 cells with LPS induced IRF5 phosphorylation at this site, which was detected with a phosphorylation site–specific antibody, and pharmacological inhibition of IKKβ blocked this phosphorylation. Thus, as emphasized by Hayden and Ghosh, IKKβ appears to be a master activator of both the antiviral and proinflammatory responses mediated by IRF5 and NF-κB, respectively.

M. Lopez-Pelaez, D. J. Lamont, M. Peggie, N. Shpiro, N. S. Gray, P. Cohen, Protein kinase IKKβ-catalyzed phosphorylation of IRF5 at Ser462 induces its dimerization and nuclear translocation in myeloid cells. Proc. Natl. Acad. Sci. U.S.A. 111, 17432–17437 (2014). [Abstract] [Full Text]

J. Ren, X. Chen, Z. J. Chen, IKKβ is an IRF5 kinase that instigates inflammation. Proc. Natl. Acad. Sci. U.S.A. 111, 17438–17443 (2014). [Abstract] [Full Text]

M. S. Hayden, S. Ghosh, Innate sense of purpose for IKKβ. Proc. Natl. Acad. Sci. U.S.A. 111, 17348–17349 (2014). [Abstract] [Full Text]