Regulation of NF-κB by deubiquitinases

Abstract

The nuclear factor-κB (NF-κB) pathway is a critical regulator of innate and adaptive immunity. Noncanonical K63-linked polyubiquitination plays a key regulatory role in NF-κB signaling pathways by functioning as a scaffold to recruit kinase complexes containing ubiquitin-binding domains. Ubiquitination is balanced by deubiquitinases that cleave polyubiquitin chains and oppose the function of E3 ubiquitin ligases. Deubiquitinases therefore play an important role in the termination of NF-κB signaling and the resolution of inflammation. In this review, we focus on NF-κB regulation by deubiquitinases with an emphasis on A20 and CYLD. Deubiquitinases and the ubiquitin/proteasome components that regulate NF-κB may serve as novel therapeutic targets for inflammatory diseases and cancer.

Fig. 1. Mechanisms of A20 inhibition of NF-κB

(A) The ubiquitin-editing function of A20. In response to TNF stimulation, A20 expression is induced and inhibits NF-κB in a negative feedback loop in a two-step manner. (1) A20 first hydrolyzes K63-linked polyubiquitin chains on RIP1 in an OTU-dependent manner to inhibit IKK and NF-κB signaling. (2) A20 then conjugates K48-linked polyubiquitin chains onto RIP1 to trigger its proteasomal degradation. (B) Disruption of E2:E3 ubiquitin enzyme complexes by A20. The E3 ligase TRAF6 inducibly interacts with the E2 enzymes Ubc13 and UbcH5c upon IL-1R/TLR4 stimulation. (1) A20 interacts with Ubc13, UbcH5c and TRAF6 and disrupts the binding between TRAF6 and the E2 enzymes. (2) A20 then conjugates K48-linked polyubiquitin chains on Ubc13 (and UbcH5c) to trigger its proteasomal degradation.

Fig. 2. Signaling pathways regulated by A20, CYLD, and other DUBs

(A) TNF binding to TNFR1 triggers the K63-linked polyubiquitination of RIP1 which is negatively regulated by A20, CYLD and possibly other DUBs (Cezanne, USP21, etc.). (B) IL-1 binding to the IL-1R or LPS binding to TLR4 triggers the activation and K63-linked polyubiquitination of TRAF6 which is negatively regulated by A20, CYLD and USP7. (C) Sensing of MDP by NOD2 facilitates the K63-linked polyubiquitination of RIP2 which is counteracted by A20. (D) T-cell receptor engagement by antigen and costimulation activates PKCθ and the CBM complex leading to MALT1 K63-linked polyubiquitination. A20 inhibits MALT1 ubiquitination, however MALT1 cleaves A20 to inactivate its function. CYLD also hydrolyzes K63-linked polyubiquitin chains from TAK1 in peripheral T cells. (E) RIG-I/MDA5 sense viral nucleic acid and inducibly interact with the mitochondrial adaptor molecule MAVS which assembles a signaling complex containing the E3 ligase TRAF3 and the kinases TBK1/IKKi. CYLD inhibits antiviral signaling by removing K63-linked polyubiquitin chains from RIG-I. DUBA removes K63-linked polyubiquitin chains from TRAF3. A20, together with ABIN1 and TAX1BP1, remove K63-linked polyubiquitin chains from TBK1/IKKi. (F) RANK ligand/RANK ligation in osteoclasts triggers TRAF6 activation and polyubiquitination. CYLD cleaves K63-linked polyubiquitin chains from TRAF6 and requires the adaptor molecule p62 to interact with TRAF6.

Fig. 3. Activation and assembly of the A20 ubiquitin-editing complex

TNF stimulation activates NF-κB and induces A20 expression as part of a negative feedback loop. IKKα phosphorylates TAX1BP1 on Ser593 and Ser624 which nucleates the A20 ubiquitin-editing complex and is required for interactions between TAX1BP1, Itch, RNF11 and A20. The A20 ubiquitin-editing complex inhibits RIP1 K63-linked polyubiquitination to terminate NF-κB signaling downstream of TNFR1.