TNF-α Gene Expression in Macrophages: Regulation by NF-κB Is Independent of c-Jun or C/EBPβ

The interaction of transcription factors is critical in the regulation of gene expression. This study characterized the mechanism by which NF-kappa B family members interact to regulate the human TNF-alpha gene. A 120-bp TNF-alpha promoter-reporter, possessing binding sites for NF-kappa B (kappa B3), C/EBP beta (CCAAT/enhancer binding protein beta), and c-Jun, was activated by cotransfection of plasmids expressing the wild-type version of each of these transcription factors. Employing adenoviral vectors, dominant-negative versions of NF-kappa B p65, and c-Jun, but not C/EBP beta, suppressed (p < 0.05-0.001) LPS-induced TNF-alpha secretion in primary human macrophages. Following LPS stimulation, NF-kappa B p50/p65 heterodimers bound to the kappa B3 site and c-Jun to the -103 AP-1 site of the TNF-alpha promoter. By transient transfection, NF-kappa B p65 and p50 synergistically activated the TNF-alpha promoter. In contrast, no synergy was observed between NF-kappa B p65, with or without NF-kappa B p50, and c-Jun or C/EBP beta, even in the presence of the coactivator p300. The contribution of the upstream kappa B binding sites was also examined. Following LPS stimulation, the kappa B1 site bound both NF-kappa B p50/p65 heterodimers and p50 homodimers. The binding by NF-kappa B p50 homodimers to the kappa B1, but not to the kappa 3, site contributed to the inability of macrophages to respond to a second LPS challenge. In summary, adjacent kappa B3 and AP-1 sites in the human TNF-alpha promoter contribute independently to LPS-induced activation. Although both the kappa B1 and kappa B3 sites bound transcriptionally active NF-kappa B p50/p65 heterodimers, only the kappa B1 site contributed to down-regulation by NF-kappa B p50 homodimers.