Optineurin Negatively Regulates TNFα- Induced NF-κB Activation by Competing with NEMO for Ubiquitinated RIP

NF-kB essential modulator (NEMO), the regulatory subunit of the IkB kinase (IKK) that activates NF-kB, is essential for NF-kB activation [1]. NEMO was recently found to contain a region that preferentially binds Lys (K)63-linked but not K48-linked polyubiquitin (polyUb) chains, and the ability of NEMO to bind to K63-linked polyUb RIP (receptor-interacting protein) is necessary for efficient tumor necrosis factor a (TNFa)-induced NF-kB activation [2, 3]. Optineurin is a homolog of NEMO, and mutations in the optineurin gene are found in a subset of patients with glaucoma [4], a neurodegenerative disease involving the loss of retinal ganglion cells [5]. Although optineurin shares considerable homology with NEMO, in resting cells, it is not present in the high-molecular-weight complex containing IKKa and IKKb, and optineurin cannot substitute for NEMO in lipopolysaccharide (LPS)-induced NF-kB activation [6]. On the other hand, the overexpression of optineurin blocks the protective effect of E3-14.7K on cell death caused by the overexpression of TNFa receptor 1 (TNFR1) [7]. Here we show that optineurin has a K63-linked polyUbbinding region similar to that of NEMO, and like NEMO, it bound K63but not K48-linked polyUb. Optineurin competitively antagonized NEMO’s binding to polyUb RIP, and its overexpression inhibited TNFainduced NF-kB activation. This competition occurs at physiologic protein levels because microRNA silencing of optineurin resulted in markedly enhanced TNFa-induced NF-kB activity. These results reveal a physiologic role for optineurin in dampening TNFa signaling, and this role might provide an explanation for its association with glaucoma.