IFN-γ regulation of class II transactivator promoter IV in macrophages and microglia:: Involvement of the suppressors of cytokine signaling-1 protein

The discovery of the class II transactivator (CIITA) transcription factor, and its IFN-gamma -activated promoter (promoter IV), have provided new opportunities to understand the molecular mechanisms of IFN-gamma -induced class II MHC expression. Here, we investigated the molecular regulation of IFN-gamma -induced murine CIITA promoter IV activity in microglia/macrophages. In the macrophage tell line RAW261.7, IFN-gamma inducibility of CIITA promoter IV is dependent on an IFN-gamma activation sequence (GAS) element and adjacent E-Box, and an IFN response factor (IRF) element, all within 196 bp of the transcription start site. In both RAW cells and the microglia cell line EOC20, two IFN-gamma -activated transcription factors, STAT-1 alpha and IRF-1, bind the GAS and IRF elements, respectively. The E-Box binds upstream stimulating factor-1 (USF-1), a constitutively expressed transcription factor. Functionally, the GAS, E-Box, and IRF elements are each essential for IFN-gamma -induced CIITA promoter IV activity. The effects of the suppressors of cytokine signaling-1 (SOCS-1) protein an IFN-gamma -induced CIITA and class II MHC expression were examined. Ectopic expression of SOCS-1 inhibits IFN-gamma -induced activation of CIITA promoter IV and subsequent class II MHC protein expression. Interestingly, SOCS-1 inhibits the constitutive expression of STAT-1 alpha and its IFN-gamma -induced tyrosine phosphorylation and binding to the GAS element in CIITA promoter IV. AS well, IFN-gamma -induced expression of IRE-I and its binding to the IRF element is inhibited. These results indicate that SOCS-1 may be responsible for attenuating IFN-gamma -induced CIITA and class II MBC expression in macrophages.