PRIMARY ACTIVATION OF INTERFERON-A AND INTERFERON-B GENE-TRANSCRIPTION BY INTERFERON REGULATORY FACTOR-3

The family of interferon (IFN) regulatory factors (IRFs) encodes DNA-b inding transcription factors, some of which function as modulators of virus-induced signaling. The IRF-3 gene is constitutively expressed in many tissues and cell types, and neither virus infection nor IFN trea tment enhances its transcription. In infected cells, however, IRF-3 pr otein is phosphorylated at the carboxyl terminus, which facilitates it s binding to the CBP/p300 coactivator. In the present study, we demons trate that overexpression of IRF-3 significantly enhances virus-mediat ed transcription of the IFNA and IFNB genes in infected cells as well as IFN synthesis. IRF-3-mediated activation of IFN genes depends in pa rt on carboxyl-terminal phosphorylation of a cluster of Ser/Thr residu es, because a mutant with Ser/Thr to Ala substitutions activates the I FN promoter less efficiently. However, overexpression of IRF-3 in huma n 2FTGH cells alone results in the induction of an antiviral state, wh ich depends on functional IFN signaling, because IRF-3 does not induce an antiviral state in mutant 2FTGH cells defective in either JAK-1 or p48 functions; also no antiviral effect of IRF-3 could be demonstrate d in Vero cells that lack the IFNA and IFNB genes. This finding indica tes that the observed antiviral activity of IRF-3 in 2FTGH cells resul ts mainly from the induction of IFNs. Furthermore, E1A protein inhibit ed IRF-3-mediated stimulation of the IFNA4 promoter in transient expre ssion assays; this inhibition could be reversed partially by overexpre ssion of CBP/p300 and was not demonstrated with the mutant of E1A that does not bind p300. These results identify IRF-3 and CBP/p300 as inte gral components of the virus-induced complex that stimulates type 1 IF N gene transcription. The observation that adenovirus E1A antagonizes IRF-3 mediated activation suggests that E1A and IRF-3 may compete for binding to CBP/p300 and implicates a novel mechanism by which adenovir us may overcome the antiviral effects of the IFN pathway.