Abstract
PLK (Polo-like kinase) inhibitors, such as BI-2536, have been reported to suppress IFNB (encoding IFNβ, interferon β) gene transcription induced by ligands that activate TLR3 (Toll-like receptor 3) and TLR4. In the present study, we found that BI-2536 is likely to exert this effect by preventing the interaction of the transcription factors IRF3 (interferon-regulatory factor 3) and c-Jun with the IFNB promoter, but without affecting the TBK1 {TANK [TRAF (tumour-necrosis-factor-receptor-associated factor)-associated nuclear factor κB activator]-binding kinase 1}-catalysed phosphorylation of IRF3 at Ser³⁹⁶, the dimerization and nuclear translocation of IRF3 or the phosphorylation of c-Jun and ATF2 (activating transcription factor 2). Although BI-2536 inhibits few other kinases tested, it interacts with BET (bromodomain and extra-terminal) family members and displaces them from acetylated lysine residues on histones. We found that BET inhibitors that do not inhibit PLKs phenocopied the effect of BI-2536 on IFNB gene transcription. Similarly, BET inhibitors blocked the interaction of IRF5 with the IFNB promoter and the secretion of IFNβ induced by TLR7 or TLR9 ligands in the human plasmacytoid dendritic cell line GEN2.2, but without affecting the nuclear translocation of IRF5. We found that the BET family member BRD4 (bromodomain-containing protein 4) was associated with the IFNB promoter and that this interaction was enhanced by TLR3- or TLR4-ligation and prevented by BI-2536 and other BET inhibitors. Our results establish that BET family members are essential for TLR-stimulated IFNB gene transcription by permitting transcription factors to interact with the IFNB promoter. They also show that the interaction of the IFNB promoter with BRD4 is regulated by TLR ligation and that BI-2536 is likely to suppress IFNB gene transcription by targeting BET family members.
Highlights
The interaction of viral dsRNA or bacterial LPS with TLR3 (Toll-like receptor 3) or TLR4 respectively activates a signalling network that induces the production of interferon β (IFNβ)
Similar to Ifnb gene transcription, we found that BI-2536 and JQ1 suppressed the transcription of Il6 (Supplementary Figures S5A and S5B), suggesting that BET family members are important in regulating the accessibility of transcription factors to the Il6 promoter
We confirmed that BI-2536 suppressed Ifnb gene transcription induced by poly(I:C) or LPS, but found that it occurred independently of the classical TRIFdependent pathway in which the activation of TBK1 is followed by the phosphorylation and dimerization of the transcription factor interferon-regulatory factor 3 (IRF3) (Figure 1 and Supplementary Figure S1)
Summary
The interaction of viral dsRNA or bacterial LPS (lipopolysaccharide) with TLR3 (Toll-like receptor 3) or TLR4 respectively activates a signalling network that induces the production of IFNβ (interferon β, encoded by the IFNB gene). The first traces of IFNβ formed by the TLR3 pathway bind to the Type interferon receptor (IFNAR), activating the JAK (Janus kinase) family members JAK1 and TYK2 (tyrosine kinase 2), which phosphorylate STAT1 (signal transducer and activator of transcription 1) and STAT2 [11] These proteins form heterodimers that associate with IRF9 to form the ISGF3 (interferon-stimulated gene factor 3) complex, which binds to ISREs (interferon-stimulated response elements) in the promoters of ISGs (interferon-stimulated genes). IRF7 drives a positive-feedback loop that amplifies IFNβ production after prolonged exposure to viral dsRNA [14,15]
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