STAT-Phosphorylation–Independent Induction of Interferon Regulatory Factor-9 by Interferon-β

Type I interferon (IFN)-dependent STAT1 and STAT2 activation requires specific tyrosine residues (337Y and 512Y) located in the cytoplasmic domain of IFNAR-2c, the β-subunit of the human type I IFN receptor. To identify STAT activation-independent induction of ISGs, we used a mutant cell line in which both 337Y and 512Y were substituted with phenylalanine (337F512F or FF mutant). In these cells, type I IFN failed to activate STAT1, STAT2, and STAT3 did not induce well-characterized ISGs and did not exert antiviral or antiproliferative effects. Using Oligonucleotide array (Affymetrix™) analysis, we showed that interferon regulatory factor-9 (IRF-9) was the only gene induced by IFN-β in FF cells. Transient transfection analysis using an IRF-9 promoter–reporter luciferase construct in FF cells confirmed induction of the IRF-9 transcription unit by IFN-β. EMSA analysis using an IFN-stimulated response element (ISRE)-like sequence on the IRF-9 promoter detected 2 novel DNA-binding complexes induced in nuclear ...