A20 Is a Negative Regulator of IFN Regulatory Factor 3 Signaling

Abstract

IFN regulatory factor 3 (IRF-3) is a critical transcription factor that regulates an establishment of innate immune status following detection of viral pathogens. Recent studies have revealed that two IκB kinase (IKK)-like kinases, NF-κB-activating kinase/Traf family member-associated NF-κB activator-binding kinase 1 and IKK-i/IKKε, are responsible for activation of IRF-3, but the regulatory mechanism of the IRF-3 signaling pathway has not been fully understood. In this study, we report that IRF-3 activation is suppressed by A20, which was initially identified as an inhibitor of apoptosis and inducibly expressed by dsRNA. A20 physically interacts with NF-κB-activating kinase/Traf family member-associated NF-κB activator-binding kinase 1 and IKK-i/IKKε, and inhibits dimerization of IRF-3 following engagement of TLR3 by dsRNA or Newcastle disease virus infection, leading to suppression of the IFN stimulation response element- and IFN-β promoter-dependent transcription. Importantly, knocking down of A20 expression by RNA interference results in enhanced IRF-3-dependent transcription triggered by the stimulation of TLR3 or virus infection. Our study thus demonstrates that A20 is a candidate negative regulator of the signaling cascade to IRF-3 activation in the innate antiviral response.

Protein Kinase PKR Amplification of Interferon β Induction Occurs through Initiation Factor eIF-2α-mediated Translational Control

“…Because A20 is induced by NF-B signaling in a manner similar to IB-␣ resulting in inhibition of NF-B signaling upstream of IB-␣ (59), it is reasonable to speculate that A20 would represent an additional potential target for PKR-dependent translational control of NF-B signaling. Since A20 regulates NEMO activity, and NEMO activity is required for the activation of IRF3 through TBK1 and IK (60), then PKR-dependent inhibition of A20 translation presumably could also affect IRF activation in some instances, perhaps explaining the PKR-dependent activation of IRF3 observed following mutant vaccinia virus infection (32). In fact, the absence of negative regulators of NF-B signaling may also explain the PKR-dependence of IB phosphorylation and activation of IK shown by others (20,21).…”

Rip1 Mediates the Trif-dependent Toll-like Receptor 3- and 4-induced NF-κB Activation but Does Not Contribute to Interferon Regulatory Factor 3 Activation

“…Consistently, we find Traf6 and Tak1 recruited to TLR3, and we find TLR3/4-induced cytokine production ablated in traf6Ϫ/Ϫ cells. Additionally, TLR3 and TLR4 NF-B and IRF-3 responses have been shown to be negatively regulated by the ubiquitin-modifying enzyme A20 (27)(28)(29).…”

A20: Central Gatekeeper in Inflammation and Immunity

“…Hiscott and co-workers (15) showed that virusinduced expression of A20 efficiently blocks RIG-I-mediated activation of NF-B and IRF3 but only weakly interferes with the response initiated by TLR3. On the other hand, two other studies using A20 overexpression or knockdown showed that A20 can also interfere with TLR3-induced IRF and NF-B activation (16,17). These results suggest that virusinducible expression of A20 negatively regulates RIG-I-and TLR3-mediated induction of an antiviral state.…”

“…2) (23). Zinc finger 4 is crucial for the ubiquitin ligase activity of A20 (16), but also zinc finger 7 seems to be important for NF-B inhibition (23,38). Some of the zinc fingers in A20 might act as ubiquitin receptors, which is suggested by the observation that Rabex-5 is able to directly bind ubiquitin via an A20-like zinc finger motif (39).…”