Cellular FLIP long isoform (cFLIPL)–IKKα interactions inhibit IRF7 activation, representing a new cellular strategy to inhibit IFNα expression

Abstract

Interferon α (IFNα) is important for antiviral and anticancer defenses. However, overproduction is associated with autoimmune disorders. Thus, the cell must precisely up- and down-regulate IFNα to achieve immune system homeostasis. The cellular FLICE-like inhibitory protein (cFLIP) is reported to inhibit IFNα production. However, the mechanism for this antagonism remained unknown. The goal here was to identify this mechanism. Here we examined the signal transduction events that occur during TLR9-induced IRF7 activation. The cFLIP long isoform (cFLIP) inhibited the expression of IRF7-controlled natural or synthetic genes in several cell lines, including those with abundant IRF7 protein levels ( dendritic cells). cFLIP inhibited IRF7 phosphorylation; however, cFLIP-IRF7 interactions were not detectable, implying that cFLIP acted upstream of IRF7 dimerization. Interestingly, cFLIP co-immunoprecipitated with IKKα, and these interactions correlated with a loss of IKKα-IRF7 interactions. Thus, cFLIP appears to bind to IKKα to prevent IKKα from phosphorylating and activating IRF7. To the best of our knowledge, this is the first report of a cellular protein that uses this approach to inhibit IRF7 activation. Perhaps this cFLIP property could be engineered to minimize the deleterious effects of IFNα expression that occur during certain autoimmune disorders.

Epigenomic and transcriptomic analyses reveal differences between low-grade inflammation and severe exhaustion in LPS-challenged murine monocytes

“…In addition, phosphorylation of TRIM28 is not necessarily equivalent to reduced gene expression. FOXO3 164 and CFLAR 165 have also been shown to inhibit IRF7 but neither had notable differences in gene expression.…”

C6orf106 is a novel inhibitor of the interferon-regulatory factor 3–dependent innate antiviral response

“…We propose that this may be a result of inadvertent interactions of nuclear C6orf106 or cFLIP L with inactive IRF3, which has been shown to shuttle between the cytosol and nucleus in unstimulated cells (38). Most recently, cFLIP L was also shown to modulate IRF7 activity by preferentially interacting with the activating kinase IKK␣, pre- venting downstream IFN-␣ transcription (39). Thus, it is possible that C6orf106 may also have other roles in the regulation of cytosolic effectors in response to other PAMPs (such as CpG DNA), and we are currently investigating the extent of PAMPinduced signaling affected by C6orf106.…”

Molecular characterisation of ILRUN, a novel inhibitor of proinflammatory and antimicrobial cytokines

“…To our knowledge, the only cellular protein comparable to ILRUN from a mechanistic perspective is cFLIPL (cellular FLICE-like inhibitory protein, long isoform), which shares no molecular homology with ILRUN but binds IRF3 to inhibit interactions between the transcriptional coactivator CBP and the IFN-β promoter (Gates and Shisler, 2016). Interestingly, cFLIPL also inhibits IRF7-dependent IFNα transcription by binding IκB kinase-α (IKKα) and preventing IKKα from phosphorylating and activating IRF7 (Gates-Tanzer and Shisler, 2018). The effect of ILRUN on steady-state CBP/p300 protein levels raises the possibility that, similar to cFLIPL, ILRUN regulates IFN signalling more broadly than only via IRF3.…”