Regulation of c-Fos Gene Expression by NF-κB: A p65 Homodimer Binding Site in Mouse Embryonic Fibroblasts but Not Human HEK293 Cells

The immediate early gene c-Fos is reported to be regulated by Elk-1 and cAMP response element-binding protein (CREB), but whether nuclear factor (NF)-kappa B is also required for controlling c-Fos expression is unclear. In this study, we determined how NF-kappa B's coordination with Elk/serum response factor (SRF) regulates c-fos transcription. We report that PMA strongly induced c-Fos expression, but tumor necrosis factor (TNF)-alpha did not. In mouse embryonic fibroblasts, the PMA induction of c-Fos was suppressed by a deficiency in IKK alpha, IKK beta, IKK gamma, or p65. By contrast, in human embryonic kidney 293 cells, PMA induced c-Fos independently of p65. In accordance with these results, we identified an NF-kappa B binding site in the mouse but not human c-fos promoter. Under PMA stimulation, IKK alpha/beta mediated p65 phosphorylation and the binding of the p65 homodimer to the NF-kappa B site in the mouse c-fos promoter. Furthermore, our studies demonstrated independent but coordinated functions of the IKK alpha/beta-p65 and extracellular signal-regulated kinase (ERK)-Elk-1 pathways in the PMA induction of c-Fos. Collectively, these results reveal the distinct requirement of NF-kappa B for mouse and human c-fos regulation. Binding of the p65 homodimer to the kappa B site was indispensable for mouse c-fos expression, whereas the kappa B binding site was not present in the human c-fos promoter. Because of an inability to evoke sufficient ERK activation and Elk-1 phosphorylation, TNF-alpha induces c-Fos more weakly than PMA does in both mouse and human cells.