Interferon Regulatory Factor
IRF1 : Interferon regulatory factor 1; MAR
IRF2 : Interferon regulatory factor 2
IRF3 : Interferon regulatory factor 3
IRF4 : Interferon regulatory factor 4; Lymphocyte-specific IRF (LSIRF); PU.1-interaction partner (PIP); ICSBP in adult T-cell leukemia cell lines or activated T cells (ICSAT); Multiple myeloma oncogene 1(MUM1); SHEP8; NF-EM5
IRF5 : Interferon regulatory factor 5; Systemic lupus erythematosus, susceptibility to, 10 (SLEB10)
IRF6 : Interferon regulatory factor 6; Van Der Woude Syndrome (VWS1); OFC6; PPS1
IRF7 : Interferon regulatory factor 7; IMD39
IRF8 : Interferon regulatory factor 8; IFN-consensus-sequence-binding protein (ICSBP)
IRF9 : Interferon regulatory factor 9; IFN-stimulated gene factor 3, Gamma (ISGF3G); ISGF3 P48 Subunit (P48)
Interferon regulatory factors (IRFs) are a family of type-I interferon transcriptional regulators, consisting of nine members (IRF1-9). The first IRF was identified in 1998 (Miyamoto et al. 1988), and since then their role in immunity has been extensively studied. IRF1 was first identified as IFNβ gene-binding protein, which is induced by viruses (Miyamoto et al. 1988). The following year, IRF2 was identified through cross-hybridization with IRF1 cDNA (Harada et al. 1989). After the identification of IRF1 and IRF2, numerous experimental approaches were adopted to explore this family and additional members were identified, including IRF3, IRF4 (also known as ICSAT, LSIRF, MUMI, or PIP), IRF5, IRF6, IRF7, IRF8 (also known as ICSBP), and IRF9 (also known as p48 or ISGF3γ) (Honda and Taniguchi 2006).
Structure, Mechanism, and Function of Interferon Regulatory Factors (IRFs)
A full-length crystal structure has not yet been resolved for any IRF; however, many partial structures have been submitted to the Protein Data Bank (PDB). From the known crystal structures of IRFs bound to DNA, the IRF protein is composed of four β-sheets, three helices, and a loop, bound to the ISRE in the promoter region, with a GAAA consensus motif (Escalante et al. 1998; Panne et al. 2004). The IAD containing the C-terminal domain is composed of a β-sandwich core flanked by loops and helices (Takahasi et al. 2003) (Fig. 1). After their activation and subsequent translocation to the nucleus, IRFs bind to the enhancer region of IFNs and aid in transcription. The IAD domain plays a defined role in the regulation of IRFs. However, the H1 and H5 helices present in the IAD domain of IRF3 and IRF7 are involved in an autoinhibitory mechanism (Fig. 1) (Takahasi et al. 2003). Kinase-dependent phosphorylation dislocates this autoinhibitory domain, leading to dimerization and nuclear translocation. The IAD domain is conserved in all IRFs, except IRF1 and IRF2, but the autoinhibition mechanism has only been reported in IRF3 and IRF7 (Qin et al. 2003).
Role of IRFs in IFN Signaling
Interferons (IFNs) are the central mediators of the innate immune system and can be grouped into two categories: type I (IFNα and β) and type II (IFNγ). Type I IFNs are primarily regulated by IRFs in the host after pathogen invasion and are considered the crucial mediators of an antiviral response. Type II IFNs are regulated and produced by other immune cell types, including NK cells and T-cells (Honda and Taniguchi 2006).
IRF4 and IRF8 are expressed in cells that play a key role in immune responses (Kanno et al. 2005). Levels of IRF4 have been shown to be increased with elevated levels of IFNα, whereas IRF8 expression is mainly regulated by IFNγ (Wang and Morse 2009). IRF4 and IRF8 directly bind to IRF-E and ISRE motifs and regulate cellular immunity (Lu 2008). This interaction between IRF4/IRF8 and the ISRE motif is mediated by the synergistic effect of the PU.1 transcription factor. In addition, Spi-B and several other factors have been reported to regulate IRF4 and IRF8 function during IFN signaling (Escalante et al. 2002).
The precise mechanism of IRF5 activation is not yet fully understood. However, it has been suggested that TRAF6-mediated ubiquitination is required for IRF5 nuclear translocation, following toll-like receptor (TLR)7 and TLR9 activation (Balkhi et al. 2008). Like IRF3 and IRF7, the C-terminus of IRF5 also contains a phosphorylation site, which helps to dislocate the auto-inhibitory region in the IAD domain, leading to homodimerization and subsequent nuclear translocation of IRF5 (Chen and Royer 2010). IRF9 was identified as a DNA-binding subunit of the interferon-stimulated gene factor 3 (ISGF3) and can induce many IFN-inducible genes. Induction of IRF9 is triggered by activation of the IFNα/β receptors (Chen et al. 2014). After binding to IFNAR or IFNϒR receptors, type I and type II IFNs induce phosphorylation of STAT proteins through JAK1/TYK2 and JAK1/JAK2, respectively (Fig. 2). The phosphorylated STAT factor then binds to IRF9, which translocates to the nucleus and induces expression of IFN-inducible genes through ISRE-containing promoters (Dussurget et al. 2014).
Role of IRFs in TLR Signaling
TLR signaling can be divided into two types, depending on the type of TLR and the adaptor protein involved: myeloid differentiation primary-response protein 88 (MyD88)-dependent and MyD88-independent (TRIF-dependent). The MyD88-dependent pathway is shared by almost all TLRs, whereas the TRIF-dependent pathway is limited to TLR3 and TLR4 (Tamura et al. 2008). IRF-related molecular events are mainly activated through TLR3, TLR4, TLR7, and TLR9 in MyD88-dependent and/or TRIF-dependent pathways. In response to invading pathogens, a stimulus, such as dsRNA or the pathogen itself, initiates the TLR pathways and activates the IRF kinase, such as TANK-binding kinase 1 (TBK1), through MyD88- and TRIF-dependent pathways (Fig. 2). This activated kinase then phosphorylates IRF3 and IRF7, leading to their dimerization and nuclear translocation (Sasai and Yamamoto 2013).
A substantial amount of evidence exists regarding the induction and activation of IRF4 (Banerjee et al. 2013; Wang and Ning 2013). It was recently reported that IRF4 is activated through c-Src-mediated phosphorylation; however, this kinase-dependent activation has not been fully investigated. Furthermore, the latent membrane protein 1 (LMP1) pathway induced by the Epstein-Barr (EBV) virus leads to activation of IRF4 (Wang and Ning 2013). Upon activation, the IRF4 protein localizes mainly to the nucleus; however, a considerable amount also resides in the cytoplasm. Cytoplasmic IRF4 binds to the IRF5 binding region on MyD88 and inhibits the sustained activity of IRF5 but not IRF7 (Negishi et al. 2005). Expression of pro-inflammatory cytokines was markedly enhanced in peritoneal macrophages derived from IRF4-/- mice, which was inhibited by ectopic expression of IRF4 in these cells (Negishi et al. 2005). IRF4 has also been reported as a negative regulator of TLR-induced NF-κB (Honma et al. 2005).
Unlike TLR3 and endosomal TLR4, TLR7 and TLR9 regulate IFN signaling via the MyD88 adaptor protein. Following the activation of TLR7 or TLR9, IRF7 is activated either through the kinase cascade or directly through the death domain of MyD88 (Fig. 2) (O’Neill and Bowie 2007). A robust production of type I IFN signaling can be achieved via a positive feedback loop during TLR7-MyD88-IRF7 pathway activation (Colonna 2007). Like IRF7, IRF5 is also activated by direct binding to MyD88 and TRAF6; activated IRF5 then dimerizes, translocates to the nucleus, and participates in pro-inflammatory cytokine induction (Brown et al. 2011).
IRF8 has been reported to be involved in TLR9 signaling and augmenting the type I IFN response. Direct activation of pro-inflammatory cytokines has also been attributed to TLR9-dependent activation of IRF8 (Lande and Gilliet 2010). In addition to being a key regulator of TLR signaling, IRF8 has been shown to suppress TLR3 expression by suppressing IRF1 and also by inhibiting polyinosinic-polycytidylic acid (poly(I:C))-mediated TLR3 signaling in human monocyte-derived DCs (Fragale et al. 2011).
Role of IRFs in Cytosolic PRR Signaling
Regulation of IRFs During Viral Infection
Role of IRFs in Disease
IRFs are critical regulators of the immune system, and therefore any abnormality in their expression or function can predispose the host to numerous diseases. There is much evidence to indicate that IRFs have key functions in the regulation of cellular responses linked to oncogenesis (Tamura et al. 2005). Moreover, IRFs have been reported to be involved in the pathogenesis of cardiovascular, neurological, and metabolic diseases (Zhao et al. 2015). Chronic and inappropriate production of type I IFN has been linked to a predisposition to many neurological diseases (Zhao et al. 2015). New discoveries have shown that IRFs can function alone, independent of their immune-related effects, suggesting that IRF-related diseases could be due to their role in immune system as well as their autonomous functions. The involvement of multiple IRFs can lead to the progression of a single pathophysiological condition. As discussed above, IRFs share a high degree of sequence and functional homology. Therefore, when investigating a disease or immune disorder it is important to consider the possibility of cross talk between multiple activated IRFs.
The IRF family of proteins plays diverse roles in immunity, both innate and adaptive, and provides vital mechanisms to defend the host against pathogens. In addition, IRFs also play a critical role in the development of immune cells. Accumulating evidence indicates that IRFs undergo posttranslational modification to activate or attenuate the transcriptional process, and any abnormality in IRF expression or function could predispose the host to numerous diseases. Future studies will elucidate the active, genome-wide behavior of individual IRFs before, during, and after transcription, which could uncover new ways to overcome IRF-related immune disorders.
This work was supported by the National Research Foundation of Korea (NRF-2015R1A2A2A09001059).
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