Regulation of IL-12 p40 promoter activity in primary human monocytes: roles of NF-kappaB, CCAAT/enhancer-binding protein beta, and PU.1 and identification of a novel repressor element (GA-12) that responds to IL-4 and prostaglandin E(2).

Appropriate regulation of IL-12 expression is critical for cell-mediated immune responses. In the present study, we have analyzed the regulation of IL-12 p40 promoter activity in primary human monocytes in vivo. Accordingly, we analyzed the p40 promoter by in vivo footprinting in resting and activated primary human blood CD14(+) monocytes. Interestingly, footprints at binding sites for trans-activating proteins such as C/EBP, NF-kappaB, and ETS were only found upon stimulation with LPS and IFN-gamma. In contrast, a footprint over a purine-rich sequence at -155, termed GA-12 (GATA sequence in the IL-12 promoter), was observed in resting, but not activated, cells. Further characterization of this site revealed specific complex formation at a protected GATA core motif in unstimulated primary monocytes and RAW264.7 macrophages. Mutagenesis within the GA-12 sequence caused a significant up-regulation of inducible IL-12 p40 promoter activity in both transient and stable transfection systems, suggesting a repressor function of this site. Furthermore, binding activity of the GA-12 binding protein GAP-12 was increased by treatment with two potent inhibitors of IL-12 expression, IL-4 and PGE(2). Finally, we observed that IL-4-mediated repression of IL-12 p40 promoter activity is critically dependent on an intact GA-12 sequence. In summary, our data underline the complex regulation of the human IL-12 p40 promoter and identify GA-12 as a potent, novel repressor element that mediates IL-4-dependent suppression of inducible promoter activity in monocytes. Regulation of GAP-12 binding may thus modulate IL-12 p40 gene expression.