Systematic characterization by mass spectrometric analysis of phosphorylation sites in IRF-3 regulatory domain activated by IKK-i

Characterization of distinct molecular interactions responsible for IRF3 and IRF7 phosphorylation and subsequent dimerization

IRF3 and IRF7 are critical transcription factors in the innate immune response. Their activation is controlled by phosphorylation events, leading to the formation of homodimers that are transcriptionally active. Phosphorylation occurs when IRF3 is recruited to adaptor proteins via a positively charged surface within the regulatory domain of IRF3. This positively charged surface also plays a crucial role in forming the active homodimer by interacting with the phosphorylated sites stabilizing the homodimer. Here, we describe a distinct molecular interaction that is responsible for adaptor docking and hence phosphorylation as well as a separate interaction responsible for the formation of active homodimer. We then demonstrate that IRF7 can be activated by both MAVS and STING in a manner highly similar to that of IRF3 but with one key difference. Regulation of IRF7 appears more tightly controlled; while a single phosphorylation event is sufficient to activate IRF3, at least two phosphorylation events are required for IRF7 activation.

Advances in IKBKE as a potential target for cancer therapy

IKBKE (inhibitor of nuclear factor kappa-B kinase subunit epsilon), a member of the nonclassical IKK family, plays an important role in the regulation of inflammatory reactions, activation and proliferation of immune cells, and metabolic diseases. Recent studies have demonstrated that IKBKE plays a crucial regulatory role in malignant tumor development. In recent years, IKBKE, an important oncoprotein in several kinds of tumors, has been widely found to regulate a variety of cytokines and signaling pathways. IKBKE promotes the growth, proliferation, invasion, and drug resistance of various cancers. This paper makes a detailed review that focuses on the recent discoveries of IKBKE in the malignant tumors, and puts forward that IKBKE is becoming an important therapeutic target for clinical treatment, which has been more and more realized.

DUSP1 regulates apoptosis and cell migration, but not the JIP1-protected cytokine response, during Respiratory Syncytial Virus and Sendai Virus infection

The host antiviral response involves the induction of interferons and proinflammatory cytokines, but also the activation of cell death pathways, including apoptosis, to limit viral replication and spreading. This host defense is strictly regulated to eliminate the infection while limiting tissue damage that is associated with virus pathogenesis. Post-translational modifications, most notably phosphorylation, are key regulators of the antiviral defense implying an important role of protein phosphatases. Here, we investigated the role of the dual-specificity phosphatase 1 (DUSP1) in the host defense against human respiratory syncytial virus (RSV), a pathogenic virus of the Pneumoviridae family, and Sendai virus (SeV), a model virus being developed as a vector for anti-RSV vaccine. We found that DUSP1 is upregulated before being subjected to proteasomal degradation. DUSP1 does not inhibit the antiviral response, but negatively regulates virus-induced JNK/p38 MAPK phosphorylation. Interaction with the JNK-interacting protein 1 scaffold protein prevents dephosphorylation of JNK by DUSP1, likely explaining that AP-1 activation and downstream cytokine production are protected from DUSP1 inhibition. Importantly, DUSP1 promotes SeV-induced apoptosis and suppresses cell migration in RSV-infected cells. Collectively, our data unveils a previously unrecognized selective role of DUSP1 in the regulation of tissue damage and repair during infections by RSV and SeV.

Transfected poly(I:C) activates different dsRNA receptors, leading to apoptosis or immunoadjuvant response in androgen-independent prostate cancer cells

Despite the effectiveness of surgery or radiation therapy for the treatment of early-stage prostate cancer (PCa), there is currently no effective strategy for late-stage disease. New therapeutic targets are emerging; in particular, dsRNA receptors Toll-like receptor 3 (TLR3) and cytosolic helicases expressed by cancer cells, once activated, exert a pro-apoptotic effect in different tumors. We previously demonstrated that the synthetic analog of dsRNA poly(I:C) induces apoptosis in the androgen-dependent PCa cell line LNCaP in a TLR3-dependent fashion, whereas only a weak apoptotic effect is observed in the more aggressive and androgen-independent PCa cells PC3 and DU145. In this paper, we characterize the receptors and the signaling pathways involved in the remarkable apoptosis induced by poly(I:C) transfected by Lipofectamine (in-poly(I:C)) compared with the 12-fold higher free poly(I:C) concentration in PC3 and DU145 cells. By using genetic inhibition of different poly(I:C) receptors, we demonstrate the crucial role of TLR3 and Src in in-poly(I:C)-induced apoptosis. Therefore, we show that the increased in-poly(I:C) apoptotic efficacy is due to a higher binding of endosomal TLR3. On the other hand, we show that in-poly(I:C) binding to cytosolic receptors MDA5 and RIG-I triggers IRF3-mediated signaling, leading uniquely to the up-regulation of IFN-β, which likely in turn induces increased TLR3, MDA5, and RIG-I proteins. In summary, in-poly(I:C) activates two distinct antitumor pathways in PC3 and DU145 cells: one mediated by the TLR3/Src/STAT1 axis, leading to apoptosis, and the other one mediated by MDA5/RIG-I/IRF3, leading to immunoadjuvant IFN-β expression.

Proteomics and NF-κB: an update

The transcription factor NF-κB was discovered in 1986 and since then has been extensively studied in relation to cancer research and inflammatory or autoimmune diseases due to its important roles in the regulation of apoptosis and inflammation as well as innate and adaptive immunity. Although much is known about NF-κB signaling, novel NF-κB functions in different diseases are still being uncovered, together with its target proteins, interaction partners and regulators of its activation cascade. Proteomic approaches are particularly suited to the discovery of new proteins involved in distinct signal transduction cascades. This review provides an update on and extension of a recent review that summarized a number of proteomic approaches to NF-κB signaling. The studies discussed here utilized innovative techniques and offer several new hypotheses on the role of NF-κB in physiological and pathophysiological processes, which open new avenues for research on NF-κB in the future.

Structural insights into the functions of TBK1 in innate antimicrobial immunity

Tank-binding kinase 1 (TBK1) is a serine/threonine protein-kinase mediating innate antimicrobial immunity. TBK1 is involved in the signaling of TLRs, RLRs, and STING-mediated sensing of cytosolic DNA. Stimulation of these receptors results in the activation of TBK1, which phosphorylates interferon regulatory factor (IRF)-3. Phosphorylated IRF-3 translocates into the nucleus to initiate the transcription of the interferon (IFN)-β gene. Here, we show that TBK1 is activated by autophosphorylation at residue Ser172. Structures of TBK1 bound to two inhibitors showed that TBK1 has the IκB kinase fold with three distinct domains: the kinase domain, the ubiquitin-like domain, and the scaffold and dimerization domain. However, the overall structures of the TBK1 monomer and its dimer are different from IKKβ in the arrangements of the three domains and in dimer formation. Phosphorylation of IRF-3 by TBK1 in vitro results in its oligomerization, and phosphorylation of residue Ser386 plays a key role in IRF-3 activation.

Interferon regulation factor-3 is a critical regulator of the mature of dendritic cells from mice

Interferon regulatory factor-3 (IRF-3) plays an important role in virus and double-stranded RNA-mediated induction of type I interferon and RANTES (regulated on activation normal T cell expressed and secreted), DNA damage signalling, tumour suppression and virus-induced apoptosis. IRF-3 had recently been shown to contribute to T-cell activation in response to pathogens, which implicated an extensive immunological role for IRF-3. Dendritic cells (DCs) played critical roles as professional APCs in the development of immune responses. However, it was unclear whether IRF-3 had any effect on phenotype or function of DCs. In this study, it was shown that IRF-3 acted as a promoter of DC maturation. The level of IRF-3 expression was transiently upregulated and accumulated in the nucleus in TNF-α-induced immune maturation of mice DC cells. Knockdown of IRF-3 by small interfering RNA in DC cells resulted in both phenotypic and functional immaturation, even without TNF-α treatment. Overall, our data demonstrated for the first time that IRF-3 was a critical regulator of mice DC maturation.

IκB kinase ε (IKKε): a therapeutic target in inflammation and cancer

The innate immune system forms our first line of defense against invading pathogens and relies for a major part on the activation of two transcription factors, NF-κB and IRF3. Signaling pathways that activate these transcription factors are intertwined at the level of the canonical IκB kinases (IKKα, IKKβ) and non-canonical IKK-related kinases (IKKε, TBK1). Recently, significant progress has been made in understanding the function and mechanism of action of IKKε in immune signaling. In addition, IKKε impacts on cell proliferation and transformation, and is thereby also classified as an oncogene. Studies with IKKε knockout mice have illustrated a key role for IKKε in inflammatory and metabolic diseases. In this review we will highlight the mechanisms by which IKKε impacts on signaling pathways involved in disease development and discuss its potential as a novel therapeutic target.

Ser386 phosphorylation of transcription factor IRF-3 induces dimerization and association with CBP/p300 without overall conformational change

The transcription factor IRF-3 is activated by microbial invasions and produces a variety of cytokines including type-I interferon. Upon microbial infection, IRF-3 is phosphorylated at its C-terminal regulatory domain, then oligomerized, translocated into the nucleus, and here it binds to CBP/p300. Although a number of studies have been reported investigating the activation mechanism of IRF-3, there are a number of unresolved issues, especially on the phosphorylation sites, the oligomerization process and the binding mechanism with CBP/p300. In this report, the phosphorylated IRF-3 regulatory domain (IRF-3 RD) was prepared using the kinase IKK-i, and the active form of phosphorylated IRF-3 RD was identified. The paper also reports the crystal structure of the active form of the phosphorylated IRF-3 RD. Furthermore, the phosphorylation of Ser386 was found to be essential for its dimerization and binding with CBP/p300 using mutational analysis and mass spectrometry. Thus, we conclude that the phosphorylation of Ser386 is essential for activation of IRF-3.