Identification of optineurin as an interleukin-1 receptor-associated kinase 1-binding protein and its role in regulation of MyD88-dependent signaling

Upon stimulation of toll-like receptors with various microbial ligands, induction of a variety of inflammatory genes is elicited by activation of a myeloid differentiation primary-response protein 88 (MyD88)-dependent signaling pathway. Interleukin-1 (IL-1) receptor-associated kinase 1 (IRAK1) plays an essential role in this pathway by activating nuclear factor kappa B (NF-kappa B) and mitogen-activated kinases (MAPKs). Here, we identified optineurin (OPTN) as an IRAK1-binding protein by yeast two-hybrid screening using IRAK1 as bait. A C-terminal fragment of OPTN harboring a ubiquitin-binding domain was co-immunoprecipitated with IRAK1. In reporter analyses, overexpression of OPTN inhibited IL-1 beta-, IRAK1-, and LPS-induced NF-kappa B activation. Consistently, OPTN deficiency resulted in increased NF-kappa B activation in response to IL-1 beta/LPS stimulation. To address the mechanisms underlying the inhibitory effect of OPTN on NF-kappa B signaling, we focused on tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), which is an adaptor protein of IRAK1 and upon polyubiquitination plays a crucial role during NF-kappa Bactivation. Overexpression of OPTN prevented TRAF6 polyubiquitination. Furthermore, OPTN H486R mutant, which is unable to recruit the deubiquitinase CYLD, failed to inhibit IRAK1-induced NF-kappa Bactivation. These results suggest that the IRAK1-binding protein OPTN negatively regulates IL-1 beta/LPS-induced NF-kappa Bactivation by preventing polyubiquitination of TRAF6.