Activation of endothelial cell NF-kappaB by interleukin (IL)-1 constitutes an event critical to the progression of the innate immune response. In this context, oxidants have been associated with NF-kappaB activation, although the molecular source and mechanism of targeting have remained obscure. We found that RelA, essential for NF-kappaB activation by IL-1, was associated with the NADPH oxidase adapter protein p47(phox) in yeast two-hybrid, coprecipitation, and in vitro binding studies. RelA and p47-GFP also colocalized in endothelial cells in focal submembranous dorsoventral protrusions. Overexpression of p47(phox) synergized with IL-1beta in the activation of an artificial kappaB-luciferase reporter and specifically augmented IL-1beta-induced RelA transactivation activity. p47(phox) overexpression also greatly increased IL-1beta-stimulated RelA phosphorylation, whereas it had no effect on I-kappaB degradation or on RelA nuclear translocation or kappaB binding. The tandem SH3 domains of p47(phox) were found to associate with a proline-rich mid-region of RelA (RelA-PR) located between the Rel homology and transactivation domains. The RelA-PR peptide blocked interaction of p47(phox) and RelA, and ectopic expression of RelA-PR abrogated IL-1beta-induced transactivation of the NF-kappaB-dependent E-selectin promoter. Further, suppression of NADPH oxidase function through the inhibitor diphenylene iodonium, the superoxide dismutase mimetic Mn(III) tetrakis(4-benzoic acid)porphyrin (MnTBAP), or expression of a dominant interfering mutant of a separate NADPH oxidase subunit (p67(V204A)) decreased IL-1beta-induced E-selectin promoter activation, suggesting that p47(phox) facilitates NF-kappaB activation through linkage with the NADPH oxidase. IL-1beta rapidly increased tyrosine phosphorylation of IL-1 type I receptor-associated proteins, suggesting that oxidants may operate through inactivation of local protein-tyrosine phosphatases in the proximal IL-1beta signaling pathway leading to RelA activation.