Interleukin-1 receptor-associated kinase (IRAK)-1 plays an essential role in Toll-like receptor/interleukin-1 receptor (TLR/IL-1R) -associated NF-B activation through its involvement in IKK activation, which then leads to subsequent IB degradation and NF-B nuclear translocation. In the present studies, we demonstrate a novel pathway in which IRAK-1 present in the nucleus participates in NF-B-dependent gene expression. Nuclear localization of IRAK-1 is increased on cellular stimulation with IL-1 and LPS, or CRM-1-dependent nuclear export blockade. Induction of IRAK-1 produces enhanced NF-B transcriptional activity that precedes IB- degradation and nuclear translocation of NF-B. IRAK-1 binds to the promoter of NF-B-regulated gene, IB-, and enhances binding of the NF-B p65 subunit to NF-B responsive elements within the IB- promoter. IRAK-1 phosphorylates histone H3 in vitro and is required for IL-1-induced phosphorylation of histone H3 at serine 10 in vivo. These data indicate that both cytosolic and nuclear actions of IRAK-1 participate in the activation of NF-B-dependent transcriptional events.—Liu, G., Park, Y.-J., Abraham, E. Interleukin-1 receptor-associated kinase (IRAK)-1-mediated NF-B activation requires cytosolic and nuclear activity.