No abstract

The cellular antiviral state mediated by type I interferon (IFN) is the most important host defense mechanism occurring at the early stage of virus infection (15,42,45). IFN binds to the IFN-␣ receptor (IFNAR), which consists of two subunits, IFNAR1 and IFNAR2 (34). The binding of IFN leads to the heterodimerization of the two subunits and the subsequent phosphorylation of two tyrosine kinases, Janus kinase 1 (Jak1) and tyrosine kinase 2 (Tyk2), which are associated with the intracellular domains of the IFNAR (42, 43). Phosphorylated Jak1 and Tyk2, in turn, phosphorylate signal transducer and activator of transcription 1 (STAT1) and STAT2, which are downstream transcriptional factors located in the cytoplasm (9). Once phosphorylated, STAT1 and STAT2 form a trimeric complex with the DNA binding protein, IFN regulatory factor 9, termed IFN-stimulated gene factor 3 (ISGF3) (20,27). ISGF3 then translocates to the nucleus, where it binds to specific promoter elements of IFN-inducible genes (the IFNstimulated response element) and induces the expression of hundreds of IFN-inducible genes that have antiviral and immunoregulatory functions (10,14). However, IFN does not always induce the antiviral response effectively. The efficacy of IFN can be limited by anti-IFN proteins encoded in viral genomes or by host cellular suppressors regulating IFN signaling (24,28,50). Even IFN-sensitive viruses (not armed with anti-

Certain oncolytic viruses exploit activated Ras signaling in order to replicate in cancer cells. Constitutive activation of the Ras/MEK pathway is known to suppress the effectiveness of the interferon (IFN) antiviral response, which may contribute to Ras-dependent viral oncolysis. Here, we identified 10 human cancer cell lines (out of 16) with increased sensitivity to the anti-viral effects of IFN-α after treatment with the MEK inhibitor U0126, suggesting that the Ras/MEK pathway underlies their reduced sensitivity to IFN. To determine how Ras/MEK suppresses the IFN response in these cells, we used DNA microarrays to compare IFN-induced transcription in IFN-sensitive SKOV3 cells, moderately resistant HT1080 cells, and HT1080 cells treated with U0126. We found that 267 genes were induced by IFN in SKOV3 cells, while only 98 genes were induced in HT1080 cells at the same time point. Furthermore, the expression of a distinct subset of IFN inducible genes, that included RIGI, GBP2, IFIT2, BTN3A3, MAP2, MMP7 and STAT2, was restored or increased in HT1080 cells when the cells were co-treated with U0126 and IFN. Bioinformatic analysis of the biological processes represented by these genes revealed increased representation of genes involved in the anti-viral response, regulation of apoptosis, cell differentiation and metabolism. Furthermore, introduction of constitutively active Ras into IFN sensitive SKOV3 cells reduced their IFN sensitivity and ability to activate IFN-induced transcription. This work demonstrates for the first time that activated Ras/MEK in human cancer cells induces downregulation of a specific subset of IFN-inducible genes.