Identification of Caspase-mediated Decay of Interferon Regulatory Factor-3, Exploited by a Kaposi Sarcoma-associated Herpesvirus Immunoregulatory Protein

Abstract

Upon virus infection, the cell mounts an innate type I interferon (IFN) response to limit the spread. This response is orchestrated by the constitutively expressed IFN regulatory factor (IRF)-3 protein, which becomes post-translationally activated. Although the activation events are understood in detail, the negative regulation of this innate response is less well understood. Many viruses, including Kaposi sarcoma-associated herpesvirus (KSHV), have evolved defense strategies against this IFN response. Thus, KSHV encodes a viral IRF (vIRF)-2 protein, sharing homology with cellular IRFs and is a known inhibitor of the innate IFN response. Here, we show that vIRF-2 mediates IRF-3 inactivation by a mechanism involving caspase-3, although vIRF-2 itself is not pro-apoptotic. Importantly, we also show that caspase-3 participates in normal IRF-3 turnover in the absence of vIRF-2, during the antiviral response induced by poly(I:C) transfection. These data provide unprecedented insight into negative regulation of IRF-3 following activation of the type I IFN antiviral response and the mechanism by which KSHV vIRF-2 inhibits this innate response.

Human Bocavirus NP1 Inhibits IFN-β Production by Blocking Association of IFN Regulatory Factor 3 with IFNB Promoter

“…Several viral proteins were also reported to modulate the function of activated IRF-3. For instance, ICP0 of bovine herpes virus 1, ORF 61 of varicella-zoster virus, and vIRF-2 of Kaposi's sarcoma-associated herpesvirus (KSHV) cause IRF-3 degradation (39,42,44); E1A of adenovirus 5 and vIRF-1/vIRF-2 of KSHV inhibit recruitment of CBP/p300 coactivators by IRF-3, decreasing the transcription efficiency (38,40,46,49); IE1 of HSV-1 disrupts IRF-3 dimer in the nucleus (41); NSs of La Crosse encephalitis virus (a bunyavirus) targets downstream of IRF-3 by specifically inducing proteasomal degradation of RNA polymerase II subunit RPB1 293T cells were cotransfected with HA-tagged NP1 and Flag-tagged IRF-3 or deletion mutant expression plasmids for 24 h. Cells were lysed and subjected to IP using mouse anti-FLAG tag (upper panel) or mouse anti-HA tag (middle panel). Mouse IgG was used as negative control.…”

“…36,37). Only a limited number of viral proteins was reported to interfere with the function of activated IRF-3 in the nucleus (38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51).…”

Cooperation between Viral Interferon Regulatory Factor 4 and RTA To Activate a Subset of Kaposi's Sarcoma-Associated Herpesvirus Lytic Promoters

“…For in vitro translation, the coding regions of RTA and vIRF4 were subcloned into derivatives of the pCITE2a ϩ vector (Novagen). Expression plasmids encoding IRF7 (32), vIRF2 (2), and truncated vIRF4 (24) were kind gifts of Isabelle Marié (New York University School of Medicine), David Blackbourn (Cancer Research UK Cancer Centre, University of Birmingham), and Hye-Ra Lee and Jae Jung (University of Southern California), respectively.…”

Distinct Roles of Kaposi's Sarcoma-Associated Herpesvirus-Encoded Viral Interferon Regulatory Factors in Inflammatory Response and Cancer

“…Although interaction between the short form of vIRF-2 and IRF-3 was not observed (77), full-length vIRF-2 was found to associate with IRF-3 (79). vIRF2 was shown to recruit caspase 3 to IRF-3 and thus accelerate the caspase-dependent process of IRF-3 turnover, leading to an inefficient antiviral response (79). Recently, it was also shown that vIRF2 inhibits type I IFN signaling by targeting components of the interferon-stimulated gene factor 3 (ISGF3) complex, STAT1 and IRF-9, which results in the inhibition of ISG expression (81).…”

“…Moreover, this spliced form of vIRF-2 was characterized as an inducible gene from microarray and quantitative PCR studies (16,78). vIRF2 is present in the cytoplasm and the nucleus of infected cells, and its expression can be induced by IFN treatment (79).…”