Recent evidence indicates a new role for histone deacetylases (HDACs) in the activation of genes governing the host immune response. Virus, along with other pathogenic stimuli, triggers an antiviral defense mechanism through the induction of IFN, IFN-stimulated genes, and other proinflammatory cytokines. Many of these genes have been shown to be regulated by transcription factors of the IFN regulatory factor (IRF) family. Recent studies from IRF5 knockout mice have confirmed a critical role for IRF5 in virus-induced type I IFN expression and proinflammatory cytokines IL-6, IL-12, and TNF-α; yet, little is known of the molecular mechanism of IRF5-mediated proinflammatory cytokine expression. In this study, we show that both HDACs and histone acetyltransferases (HATs) associate with IRF5, leading to alterations in its transactivation ability. Using the HDAC inhibitor trichostatin A, we demonstrate that ISRE, IFNA, and IL6 promoters require HDAC activity for transactivation and transcription, whereas TNFα does not. Mapping the interaction of corepressor proteins (HDAC1, silencing mediator of retinoid and thyroid receptor/nuclear corepressor of retinoid receptor, and Sin3a) and HATs to IRF5 revealed distinct differences, including the dependence of IRF5 phosphorylation on HAT association resulting in IRF5 acetylation. Data presented in this study support a mechanism whereby virus triggers the dynamic conversion of an IRF5-mediated silencing complex to that of an activating complex on promoters of target genes. These data provide the first evidence, to our knowledge, of a tightly controlled transcriptional mechanism whereby IRF5 regulates proinflammatory cytokine expression in conjunction with HATs and HDACs.
IRF5 is a member of the Interferon Regulatory Factor (IRF) family of transcription factors activated downstream of the Toll-Like receptors (TLRs). Polymorphisms in IRF5 have been shown to be associated with the autoimmune disease Systemic Lupus Erythematosus (SLE) and other autoimmune conditions, suggesting a central role for IRF5 in the regulation of the immune response. Four different IRF5 isoforms originate due to alternative splicing and to the presence or absence of a 30 nucleotide insertion in IRF5 exon 6. Since the polymorphic region disturbs a PEST domain, a region associated with protein degradation, we hypothesized that the isoforms bearing the insertion might have increased stability, thus explaining the association of individual IRF5 isoforms with SLE. As the E3 ubiquitin ligase TRIpartite Motif 21 (TRIM21) has been shown to regulate the stability and hence activity of members of the IRF family, we investigated whether IRF5 is subjected to regulation by TRIM21 and whether dysregulation of this mechanism could explain the association of IRF5 with SLE. Our results show that IRF5 is degraded following TLR7 activation and that TRIM21 is involved in this process. Comparison of the individual IRF5 variants demonstrates that isoforms generated by alternative splicing are resistant to TRIM21-mediated degradation following TLR7 stimulation, thus providing a functional link between isoforms expression and stability/activity which contributes to explain the association of IRF5 with SLE.
The transcription factor interferon regulatory factor 5 (IRF5) exerts crucial functions in the regulation of host immunity against extracellular pathogens, DNA damage-induced apoptosis, death receptor signaling, and macrophage polarization. Tight regulation of IRF5 is thus warranted for an efficient response toward extracellular stressors and for limiting autoimmune and inflammatory responses. Here we report that the COP9 signalosome (CSN), a general modulator of diverse cellular and developmental processes, associates constitutively with IRF5 and promotes its protein stability. The constitutive CSN/IRF5 interaction was identified using proteomics and confirmed by endogenous immunoprecipitations. The CSN/IRF5 interaction occurred on the carboxyl and amino termini of IRF5; a single internal deletion from amino acids 455 to 466 (⌬455-466) was found to significantly reduce IRF5 protein stability. CSN subunit 3 (CSN3) was identified as a direct interacting partner of IRF5, and knockdown of this subunit with small interfering RNAs resulted in enhanced degradation. Degradation was further augmented by knockdown of CSN1 and CSN3 together. The ubiquitin E1 inhibitor UBEI-41 or the proteasome inhibitor MG132 prevented IRF5 degradation, supporting the idea that its stability is regulated by the ubiquitin-proteasome system. Importantly, activation of IRF5 by the death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) resulted in enhanced degradation via loss of the CSN/IRF5 interaction. This study defines CSN to be a new interacting partner of IRF5 that controls its stability.
Repeated low-dose γ-irradiation (IR) induces thymic lymphoma in mice because of oncogenic mutations propagating from a primitive hematopoietic stem/progenitor cell (HSC) in the bone marrow. It is well known that IR-induced thymic lymphomagenesis is markedly enhanced by p53 deficiency, yet data also indicate that p53-dependent apoptosis can actively drive tumor formation in this model. The latter was recently expounded on by findings from Puma-deficient mice, indicating that loss of this proapoptotic p53 target gene results in protection from IR-induced lymphomagenesis rather than enhanced susceptibility to. Similar to Puma, the transcription factor interferon regulatory factor 5 (Irf5) has been reported as a p53 target gene and is required for DNA damage-induced apoptosis. To date, no studies have been performed to elucidate the in vivo role of IRF5 in tumorigenesis. Given its essential role in DNA damage-induced apoptosis, we explored the tumor suppressor function of IRF5 in IR-induced thymic lymphomagenesis. Somewhat surprisingly, we found that thymic lymphoma development was significantly suppressed in Irf5(-/-) mice as compared with wild-type littermates. Suppression was due, in part, to reduced thymocyte and HSC apoptosis, resulting in reduced compensatory proliferation, and reduced replication stress-associated DNA damage. The observed effects were independent of p53 or Puma as these proteins were upregulated in Irf5(-/-) mice in response to IR. This study demonstrates an important new role for IRF5 in maintaining HSC homeostasis after IR and supports the non-redundant functions of IRF5, p53 and PUMA in DNA damage-induced lymphomagenesis. We propose that IRF5 may be an attractive target for developing therapeutic agents to ameliorate radiation-induced bone marrow injury.
The transcription factor interferon regulatory factor 5 (IRF5) exerts crucial functions in the regulation of host immunity against extracellular pathogens, DNA damage-induced apoptosis, death receptor signaling, and macrophage polarization. Tight regulation of IRF5 is thus warranted for an efficient response to extracellular stressors and for limiting autoimmune and inflammatory responses. Here we report that the COP9 signalosome (CSN), a general modulator of diverse cellular and developmental processes, associates constitutively with IRF5 and promotes its protein stability. The constitutive CSN/IRF5 interaction was identified using proteomics and confirmed by endogenous immunoprecipitations. The CSN/IRF5 interaction occurred on the carboxyl and amino termini of IRF5; a single internal deletion (Δ455-466) was found to significantly reduce IRF5 protein stability. CSN3 was identified as a direct interacting partner of IRF5, and knockdown of this subunit with small interfering RNAs (siRNAs) resulted in enhanced degradation. Degradation was further augmented by knockdown of CSN1 and CSN3 together. The ubiquitin E1 inhibitor UBEI-41 or the proteasome inhibitor MG132 prevented IRF5 degradation, supporting that its stability is regulated by the ubiquitin-proteasome system. Importantly, activation of IRF5 by the death receptor ligand tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) resulted in enhanced degradation via loss of the CSN/IRF5 interaction. This study defines the CSN as a new interacting partner of IRF5 that controls its stability. KEYWORDS COP9, IRF5T he interferon regulatory factor (IRF) family consists of nine cellular IRFs, each with pleiotropic biological functions (1). IRF5 has an important role in the induction of type I interferons (IFNs) and proinflammatory cytokines and is thus a critical mediator of innate and adaptive immunity (2-4). More recent studies have shown that Irf5 is an autoimmune susceptibility gene associated with increased risk of Citation Korczeniewska J, Barnes BJ. 2018. Corrected and republished from: The COP9 signalosome interacts with and regulates interferon regulatory factor 5 protein stability.
It was recently brought to our attention that this article contains erroneous image duplications of some Western blots in four figures. Image duplications were found to occur primarily in loading controls, such as -actin and GAPDH (glyceraldehyde-3-phosphate dehydrogenase), but were not limited to these. We deeply apologize to the readership of MCB for these errors, and we retract the article. A corrected article is in progress.
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