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Nuclear factor of activated T cells (NFAT) plays an important role in expression of many cytokine genes including interleukin-2 and interleukin-4. However, its role in interferon-gamma (IFN-gamma) expression is not well understood. In the current studies, two strong NFAT-binding sites in the IFN-gamma promoter were identified by DNase I footprint analysis at positions -280 to -270 and -163 to -155. NFATp bound independently to both sites and was required for the formation of a composite element with AP-1 spanning position -163 to -147. In Jurkat T cells and primary lymphocytes, activation-induced expression of IFN-gamma reporter constructs containing point mutations in either NFAT site or the AP-1 component of the composite site was decreased by approximately 40-65%. Despite elimination of both strong NFAT-binding sites, the IFN-gamma promoter remained completely sensitive to inhibition by cyclosporin. This suggests that other elements in the IFN-gamma promoter, such as the IFN-gamma proximal element, are sufficient for cyclosporin sensitivity of this gene. Ying-Yang 1 (YY1), a potential inhibitor of IFN-gamma expression, binds to sites located between the two NFAT sites. Mutation of the YY1 sites alone had little effect on IFN-gamma promoter activity. However, mutation of both the NFAT and YY1-binding sites abolished activation-induced expression in primary murine splenocytes but not in Jurkat T cells. This suggests that under some conditions, YY1 may play a positive role in activation-induced transcription of IFN-gamma.

The specificity of the various STAT SH2 domains for different tyrosine-containing peptides enables cytokines to activate different signaling pathways and to induce distinct patterns of gene expression. We show that STAT4 has a unique peptide specificity and binds to the peptide sequence pYLPSNID (where pY represents phosphotyrosine). This motif is found at tyrosine residue 800 in the beta2 subunit of the interleukin-12 receptor and is required for DNA binding and transcriptional activity of STAT4. Our data demonstrate that transfection of interleukin-12 receptor beta1 and beta2 subunits is sufficient for STAT4 activation but not for STAT1 or STAT3 activation.