The insulin receptor substrate (IRS) proteins are major substrates of both insulin receptor and insulin-like growth factor (IGF)-I receptor tyrosine kinases. Previously, we reported that IRS-3 is localized to both cytosol and nucleus, and possesses transcriptional activity In the present study, we identified Bcl-3 as a novel binding protein to IRS-3 Bcl-3 is a nuclear protein, which forms a complex with the homodimer of p50 NF-kappa B, leading to enhancement of transcription through p50 NF-kappa B We found that Bcl-3 interacts with the pleckstrin homology domain and the phosphotyrosine binding domain of IRS-3, and that IRS-3 interacts with the ankyrin repeat domain of Bcl-3 In addition, IRS-3 augmented the binding activity of 1)50 to the NF-kappa B DNA binding site, as well as the tumor necrosis factor (TNF)-alpha-induced transcriptional activity of NW-kappa B Lastly. IRS-3 enhanced NF-kappa B-dependent anti-apoptotic gene induction and consequently inhibited TNF-alpha-induced cell death This series of results proposes a novel function for IRS-3 as a transcriptional regulator in TNF-alpha signaling, distinct from its function as a substrate of insulin/IGF receptor kinases (C) 2010 Elsevier Inc All rights reserved