DDX3 as well as IPS-1 were expressed even without any stimulation (Fig. 2C and 4A and B) and bound each other in the cytoplasm (Fig. 2C). Hence, DDX3 is a cytoplasmic molecule that can detect viral RNA produced in infected cells. Knockdown studies suggested that polyI:C-mediated IFN promoter activation was abrogated in DDX3-deficient cells even in the presence of overexpressed RIG-I or MDA5 (Fig. 5). DDX3 silencing happened with two different siRNA. Thus, DDX3

may enable RIG-I and IPS-1 to confer activation of the cytoplasmic RNA-sensing pathway on virus-infected cells. The IFN-β-inducing pathway involves IRF-3 kinases TBK1 and IKKε, which may be targets of DDX3 15, 16. By in vitro reporter analysis, increasing amounts of DDX3 barely

affected IFN-β promoter buy PF-02341066 activation by TBK1 and IKKε (Fig. 6A and B). Slight TBK1-enhancing activity could manage to be detected with DDX3 when decreasing amounts of TBK1 was used in the assay (Fig. 6C and D). HeLa cells induced the mRNA of RIG-I and IFN-β in response to polyI:C RO4929097 cost stimulation within 1 h (Fig. 4A). More exactly, IFN-β induction was ∼30 min faster than RIG-I induction in response to polyI:C. IFN-β mRNA induction was peaked around 3 h post stimulation, while RIG-I induction continued to increase>3 h (Fig. 4A). When HEK293 cells were infected with vesicular stomatitis virus (VSV) (a RIG-I-stimulating virus), the IFN-β mRNA was induced from 6 h, and by that time no RIG-I

message was generated (Fig. 4B–D). The RIG-I message began to appear>8 h and was markedly increased (Fig. 4B and D). In either case, no up-regulation was observed with DDX3 but sufficiently present in the cytoplasm (Fig. 4C). buy ZD1839 Furthermore, overexpression of DDX3 in HeLa cells resulted in potential prevention of VSV propagation (Fig. 7). However, the distribution profiles of DDX3 and IPS-1 were barely altered in response to polyI:C stimulation (Fig. 2C). The results allow us to interpret that when viral RNA enter the cytoplasm of infected cells, the RNA first induce a small amount of IFN-β in conjunction with the complex containing trace RIG-I and then the induced IFN-β fosters intensive RIG-I/MDA5 induction. The complex is reconstituted together with upcoming RIG-I/MDA5 to amplify the cytoplasmic IFN-inducing pathway. Although the molecular reconstitution was not visible with overexpressed proteins by confocal analysis, DDX3 may act as an enhancing factor for initial RNA-sensing by the IPS-1 complex and conducts the rapid response to viral RNA to facilitate the IPS-1 signaling. We identified DDX3 as a protein that bound to the IPS-1 CARD region, duplexed RNA and RLR. Although the DDX3 helicase domain is a DEAD box type similar to those of RIG-I and MDA5, DDX3 does not have a signaling domain corresponding to the CARD domain.