IFN-gamma regulation of the type IV class II transactivator promoter in astrocytes

The transcriptional activation of class II MHC genes requires the class II transactivator (CIITA) protein, a regulator that is essential for both constitutive and IFN-gamma-inducible class II MHC expression. The CIITA gene is controlled by multiple independent promoters; two promoters direct constitutive expression, while another, the type IV CIITA promoter, mediates IFN-gamma-induced expression. We investigated the molecular regulation of IFN-gamma-induced type IV CIITA promoter activity in astrocytes. IFN-gamma inducibility of the type IV CIITA promoter is dependent on three cis-acting elements contained within a 154-bp fragment of the promoter; the proximal IFN-gamma activation sequence (GAS) element, the E box, and the proximal IFN regulatory factor (IRF) element. Two IFN-gamma-activated transcription factors, STAT-1alpha and IRF-1, bind the proximal GAS and IRF elements, respectively. The E box binds upstream stimulating factor-1 (USF-1), a constitutively expressed transcription factor. Furthermore, STAT-1alpha binding to the proximal GAS element is dependent on the binding of USF-1 to the adjacent E box. Functionally, the proximal IRF element is essential for IFN-gamma induction of type IV CIITA promoter activity, while the proximal GAS and E box elements contribute to the IFN-gamma inducibility of this promoter. In astrocytes, TNF-alpha enhances IFN-gamma-induced class II MHC transcription. Our results demonstrate that TNF-alpha does not enhance IFN-gamma-induced transcriptional activation of the type IV CIITA promoter, indicating that the enhancing effect of TNF-alpha is mediated downstream of CIITA transcription. These results define the molecular basis of IFN-gamma activation of the type IV CIITA promoter in astrocytes.