IFN-gamma-induced TNF-alpha expression is regulated by interferon regulatory factors 1 and 8 in mouse macrophages

We have previously described that IFN-gamma induces cyclooxygenase 2 and inducible NO synthase expression by a mechanism that involved endogenously produced TNF-alpha. In this study, we report that TNF-alpha production is induced by IFN-gamma treatment in the murine macrophage cell line RAW 264.7. TNF-alpha mRNA levels are increased in cells treated with IFN-gamma in a time-dependent manner and IFN-gamma also increased human TNF-alpha promoter-dependent transcription. Two regions in the TNF-alpha promoter seem to be responsible for the IFN-gamma response: a distal region between -1311 and -615 bp of the human TNF-alpha promoter, and a proximal region located between -95 and -36 bp upstream of the transcriptional start. In contrast, IFN-gamma stimulation induces the expression of the transcription factors IRF-1 and IRF-8. Overexpression of these transcription factors produces an increase in the transcriptional activity of the human TNF-alpha promoter. There is a correlation between the regions of the TNF-alpha promoter responsible of the transcriptional activation elicited by IRF-1 and IRF-8 and those required for IFN-gamma response. In addition, IRF-1 and IRF-8 are recruited to the TNF-alpha promoter in IFN-gamma-treated RAW 264.7 cells, as demonstrated by chromatin immunoprecipitation assays. Moreover, overexpression of IRF-1 and IRF-8 induces TNF-alpha production in unstimulated RAW 264.7 macrophages, comparable to the production of TNF-alpha elicited by IFN-gamma stimulation, and silencing of IRF-1 and/or IRF-8 with specific small interfering RNAs, decreases IFN-gamma-elicited TNF-alpha production. In summary, IFN-gamma treatment induces TNF-alpha expression at transcriptional level requiring the coordinate action of IRF-1 and IRF-8.