SHORT REPORTS The exonuclease ISG20 is directly induced by synthetic dsRNA via NF-jB and IRF1 activation Lucile Espert 1 , Cle´mence Rey 1 , Laure Gonzalez 1 , Genevie`ve Degols 1 , Mounira Kmar Chelbi-Alix 2 , Nadir Mechti* ,1 and Ce´line Gongora* ,1 1 CNRS UMR 5160, EFS, 240 avenue Emile Jeanbrau, 34094 Montpellier, Cedex 5, France; 2 CNRS UPR 9045, 7, Rue Guy Moquet, Institut Andre´Lwoff, 94801 Villejuif, Cedex, France Many interferon (IFN)-stimulated genes are also induced by double-stranded RNA (dsRNA), a component closely associated with the IFN system in the context of virus– host interactions. Recently, we demonstrated that the IFN-induced 3 0 -5 0 exonuclease ISG20 possesses anti- viral activities against RNA viruses. Here we show that ISG20 induction by synthetic dsRNA (pIpC) is stronger and faster than its induction by IFN. Two families of transcription factors are implicated in the transcriptional activation of ISG20 by dsRNA. Initially, the NF-kB factors p50 and p65 bind and activate the kB element of the Isg20 promoter. This is followed by IRF1 binding to the ISRE. As pIpC often induces protein movements in the cells, we questioned whether it could influence ISG20 localization. Interestingly and contrary to IFN, dsRNA induces a nuclear matrix enrichment of the ISG20 protein. dsRNA induction of ISG20 via NF-kB and its antiviral activity led us to suggest that ISG20 could participate in the cellular response to virus infection. Oncogene (2004) 23, 4636–4640. doi:10.1038/sj.onc.1207586 Published online 5 April 2004 Keywords: ISG20; dsRNA; NF-kB; IRF1; transcrip- tion; interferon Double-stranded RNA (dsRNA) is an important component of viral infection that stimulates host antiviral responses, including the production of cyto- kines like type I interferon (IFN) (Jacobs and Langland, 1996). Recent studies show that a member of the interleukine-1/Toll receptor super-family, Toll-like re- ceptor 3 (TLR3), recognizes dsRNA and induces the synthesis of more than 100 genes called dsRNA- stimulated genes (DSGs) (Alexopoulou et al., 2001). Some of them are shown to be involved in the response of cells to viral infection (Geiss et al., 2001). Interest- ingly, several DSGs are also induced by IFNs, indicating that IFNs and dsRNA stimulate the transcription of partially overlapping set of genes. dsRNA is also the activator of two enzymes of the IFN system: the dsRNA-activated serine–threonine proteine kinase (PKR) and the 2 0 ,5 0 oligoadenylate synthetase (2,5 OAS) that activates the dormant endoribonuclease RNaseL. When activated, PKR and RNaseL inhibit translation and activate apoptosis, leading to a strong antiviral effect (Samuel, 2001). The dsRNA signalling pathway activates several transcription factors such as NF-kB and IRF-1, -3 and -7 (Williams, 1999). Activa- tion of these factors is mediated by protein kinases including PKR, p38, JNK and IKK, but the activation pathways are not completely understood. Moreover, evidence for a role of PKR in NF-kB activation is controversial. Iordanov et al. have suggested that PKR is only required for dsRNA-mediated inhibition of host- cell protein synthesis but not for NF-kB activation in response to dsRNA or virus infection (Iordanov et al., 2000). On the other hand, dsRNA and viruses can also activates a ‘virus activated kinase’ (VAK), which is responsible for phosphorylation of the transcription factors IRF-3 and IRF-7. This modification permits IRF activation and dimerization, leading to their nuclear translocation and direct activation of target genes carrying IFN-stimulated responsive element (ISRE) on their promoters. Two groups recently reported that the IkB kinase (IKK)-related kinases IKKe and TANK-binding kinase 1 are components of VAK (Fitzgerald et al., 2003; Sharma et al., 2003). IRF-3 and NF-kB can also be concomitantly activated in response to the binding of extracellular dsRNA to TLR3 (Doyle et al., 2002). ISG20 is a 3 0 -5 0 exonuclease whose gene is transcriptionally induced by both types I and II IFN (Gongora et al., 1997, 2000; Nguyen et al., 2001). Its induction by IFN is strictly dependent upon the activation and the binding of IRF1 to a specific ISRE on the Isg20 promoter. Moreover, the TATA-less Isg20 promoter contains one E-box and putative NF-kB and Sp1 binding sites suggesting that it could be induced by other stimuli. We have recently shown that ISG20 possesses high antiviral activity against VSV infection, even in a PKR/RNaseL/Mx null background (Espert et al., 2003). This result indicates that ISG20 partici- pates in the IFN-induced antiviral state. As ISG20 has a putative NF-kB site in its promoter, we asked whether it Received 30 October 2003; revised 16 December 2003; accepted 29 January 2004; Published online 5 April 2004 *Correspondence: C Gongora; E-mail: celine.gongora@ibph. pharma.univ-montp1.fr and N Mechti; E-mail: nadir.mechti@ibph. pharma.univ-montp1.fr Oncogene (2004) 23, 4636–4640 & 2004 Nature Publishing Group All rights reserved 0950-9232/04 $30.00 www.nature.com/onc