Regulation of IκB Kinase-related Kinases and Antiviral Responses by Tumor Suppressor CYLD

Abstract

The IB kinase (IKK)-related kinases, IKK⑀ and TBK1, participate in the induction of type I interferons (IFNs) during viral infections. Deregulated activation of IKK⑀ and TBK1 also contributes to the abnormal cell survival and transformation. However, how these kinases are negatively regulated remains unclear. We show here that the tumor suppressor CYLD has an essential role in preventing aberrant activation of IKK⑀/TBK1. CYLD deficiency causes constitutive activation of IKK⑀/TBK1, which is associated with hyper-induction of IFNs in virus-infected cells. We further show that CYLD targets a cytoplasmic RNA sensor, RIG-I, and inhibits the ubiquitination of this IKK⑀/ TBK1 stimulator. Consistent with the requirement of ubiquitination in RIG-I function, CYLD potently inhibits RIG-I-mediated activation of the IFN-␤ promoter. These findings establish CYLD as a key negative regulator of IKK⑀/TBK1 and suggest a role for CYLD in the control of RIG-I ubiquitination.Innate immunity serves as the first line of host defense against microbial infections. A central step in antiviral innate immunity involves the production of type I interferons (IFNs), 3 which is triggered when host pattern recognition receptors (PRRs) detect viral products (1, 2). Two recently identified cytoplasmic PRRs, retinoic acid-induced gene I (RIG-I) and melanoma differentiation-associated gene 5, play critical roles in mediating antiviral innate immune responses (3). These PRRs signal for IFN production through common downstream signaling pathways that involve activation of two homologous protein kinases, IKK⑀ and TBK1, as well as the typical IB kinase (IKK) (4 -9).IKK⑀ (also named IKKi) and TBK1 (also named T2K and NAK) are known as IKK-related kinases because of their structural homology to IKK (10 -14), a central component of the NF-B signaling pathway (15,16). Unlike the typical IKK, which phosphorylates NF-B inhibitors (IBs) and mediates NF-B activation, IKK⑀ and TBK1 phosphorylate IRF3 and IRF7 to trigger their dimerization and nuclear translocation, thereby mediating induction of IFN gene expression (9, 16). In addition, emerging evidence suggests that IKK⑀ and TBK1 also regulate cell survival and oncogenesis via an IFN-independent mechanism (17-20). However, how IKK⑀/TBK1 is negatively regulated is currently not well understood.An emerging mechanism that regulates signal transduction in various biological processes is protein ubiquitination (21). Analogous to protein phosphorylation, which is reversibly controlled by protein kinases and phosphatases, protein ubiquitination is a reversible event mediated by the counteractive actions of ubiquitin-conjugating enzymes and deubiquitinating enzymes (DUBs) (22). Although ubiquitination is traditionally known as a process that mediates protein degradation by the proteasome, it is now evident that specific ubiquitin chains facilitate protein/protein interactions that lead to the activation of signaling molecules (23). Notably, ubiquitination of RIG-I, a PRR that recognizes double-stranded and 5Ј-phospho...

The ubiquitin-specific protease 17 is involved in virus-triggered type I IFN signaling

“…For example, two ubiquitin E3 enzymes, TRIM25 and REUL (also called RNF135 and Riplet), have been demonstrated to catalyze K63-linked polyubiquitinations of RIG-I, which are critical for activation of RIG-I and signaling to downstream components [30][31][32]. Two DUBs, A20 and CYLD, have been demonstrated to remove the K63-linked polyubiquitin chains from RIG-I and thereby negatively regulate virus-induced type I IFN signaling [36,37]. On the other hand, RNF125 catalyzes K48-linked polyubiquitination of RIG-I and MDA5 on their N-terminal caspase recruitment domains (CARD) and these ubiquitinations cause proteasome-dependent degradation of RIG-I and MDA5, thereby negatively regulating virus-induced type I IFN signaling [28].…”

“…Several ubiquitin ligase enzymes have been identified to regulate these processes [28][29][30][31][32][33][34][35]. However, only a few DUBs are known to regulate these pathways: A20 was shown to negatively regulate the RIG-I-induced antiviral state [36], DUBA was identified to be required for efficient deubiquitination of TRAF3 and to function as a negative regulator of innate immune responses [25], and CYLD, known as a tumor suppressor, was recently reported to negatively regulate the activation of TBK1/ IKKε [37].…”

FLN29 Deficiency Reveals Its Negative Regulatory Role in the Toll-like Receptor (TLR) and Retinoic Acid-inducible Gene I (RIG-I)-like Helicase Signaling Pathway

“…SOCS1, IRAK-M, and A20 have been identified as such negative regulators of the TLR-NF-B pathway (12)(13)(14). Additionally, DUBA, RNF125, SHP-2, Cyld, and A20 have been reported as negative regulators of the RLH-IRF3 pathway (15)(16)(17)(18)(19)(20)(21). In our previous study, we demonstrated that FLN29 protein induced by type I or II IFNs acts as a negative regulator for the TLR4-TRAF6 signaling pathway in macrophages.…”

Regulation of Virus-triggered Signaling by OTUB1- and OTUB2-mediated Deubiquitination of TRAF3 and TRAF6

“…In contrast, the E3 ubiquitin ligase RNF125 catalyzes Lys 48 -linked ubiquitination of RIG-I and negatively regulates RIG-I-mediated signaling (24). In addition, an OTU (ovarian tumor domain) containing deubiquitinating enzyme, CYLD, dissociates Lys 63 -linked ubiquitin moieties from RIG-I, and therefore negatively regulates RIG-Imediated signaling (25,26). The ubiquitination/deubiquitination systems also target other components in the virus-triggered pathways.…”