Pivotal role for the ESCRT-II complex subunit EAP30/SNF8 in IRF3-dependent innate antiviral defense

The activation of interferon (IFN)-regulatory factor-3 (IRF3), characterized by phosphorylation and nuclear translocation of the latent transcription factor, is central to initiating innate antiviral responses. Whereas much has been learned about the upstream pathways and signaling mechanisms leading to IRF3 activation, how activated IRF3 operates in the nucleus to control transcription of IFNs remains obscure. Here we identify EAP30 (a.k.a, SNF8/VPS22), an endosomal sorting complex required for transport (ESCRT)-II subunit, as an essential factor controlling IRF3-dependent antiviral defense. Depletion of EAP30, but not other ESCRT-II subunits, compromised IRF3-dependent induction of type I and III IFNs, IFN-stimulated genes (ISGs) and chemokines by double-stranded RNA or viruses. EAP30, however, was dispensable for the induction of inflammatory mediators of strict NF-κB target. Significantly, knockdown of EAP30 also impaired the establishment of an antiviral state against vesicular stomatitis virus and hepatitis C virus, which are of distinct viral families. Mechanistically, EAP30 was not required for IRF3 activation but rather acted at a downstream step. Specifically, a fraction of EAP30 localized within the nucleus, where it formed a complex with IRF3 and its transcriptional co-activator, CREB-binding protein (CBP), in a virus-inducible manner. These interactions promoted IRF3 binding to target gene promoters such as IFN-β, IFN-λ1 and ISG56. Together, our data describe an unappreciated role for EAP30 in IRF3-dependent innate antiviral response in the nucleus.

干扰素调节因子-3（interferon-regulatory factor-3, IRF3）的激活，以潜伏转录因子的磷酸化及核转位为典型特征，是启动先天抗病毒应答的核心事件。尽管学界已对介导IRF3激活的上游通路与信号机制积累了大量研究成果，但激活态IRF3如何在细胞核内调控干扰素的转录仍未明晰。本研究鉴定出EAP30（又名SNF8/VPS22）——一种内体分选转运复合体-II（endosomal sorting complex required for transport, ESCRT-II）亚基——作为调控IRF3依赖型抗病毒防御的必需因子。敲低EAP30而非其他ESCRT-II亚基，会削弱双链RNA或病毒诱导的I型、III型干扰素、干扰素刺激基因（IFN-stimulated genes, ISGs）及趋化因子的IRF3依赖型表达。然而，EAP30对于严格依赖NF-κB的炎症介质的诱导并非必需。值得注意的是，敲低EAP30还会削弱机体对两类分属不同病毒科的病毒——水疱性口炎病毒（vesicular stomatitis virus）与丙型肝炎病毒（hepatitis C virus）——的抗病毒状态建立。从机制层面来看，EAP30并非IRF3激活所必需，而是作用于下游信号环节。具体而言，有一部分EAP30定位于细胞核内，可通过病毒诱导的方式与IRF3及其转录共激活因子CREB结合蛋白（CREB-binding protein, CBP）形成复合物。这些相互作用可促进IRF3结合IFN-β、IFN-λ1及ISG56等靶基因启动子。综上，本研究的数据揭示了EAP30在细胞核内IRF3依赖型先天抗病毒应答中此前未被认知的重要作用。