The tumour suppressor CYLD is a negative regulator of RIG-I-mediated antiviral response.

On detecting viral RNAs, the RNA helicase retinoic acid-inducible gene I (RIG-I) activates the interferon regulatory factor 3 (IRF3) signalling pathway to induce type I interferon (IFN) gene transcription. How this antiviral signalling pathway might be negatively regulated is poorly understood. Microarray and bioinformatic analysis indicated that the expression of RIG-I and that of the tumour suppressor CYLD (cylindromatosis), a deubiquitinating enzyme that removes Lys 63-linked polyubiquitin chains, are closely correlated, suggesting a functional association between the two molecules. Ectopic expression of CYLD inhibits the IRF3 signalling pathway and IFN production triggered by RIG-I; conversely, CYLD knockdown enhances the response. CYLD removes polyubiquitin chains from RIG-I as well as from TANK binding kinase 1 (TBK1), the kinase that phosphorylates IRF3, coincident with an inhibition of the IRF3 signalling pathway. Furthermore, CYLD protein level is reduced in the presence of tumour necrosis factor and viral infection, concomitant with enhanced IFN production. These findings show that CYLD is a negative regulator of RIG-I-mediated innate antiviral response.

Negative regulation of TCR signaling by NF-kappaB2/p100.

The positive regulation of the NF-kappaB-signaling pathway in response to TCR stimulation has been well-studied. However, little is known about the negative regulation of this pathway in T cells. This negative regulation is crucial in controlling the duration of TCR signaling and preventing abnormal lymphocyte activation and proliferation. Therefore, understanding the negative regulation of TCR-mediated NF-kappaB signaling is essential in understanding the mechanisms involved in T cell function and homeostasis. TCR stimulation of human CD4+ T cells resulted in an increase in NF-kappaB2/p100 expression with no appreciable increase in p52, its cleavage product. Due to the presence of inhibitory ankyrin repeats in the unprocessed p100, this observation suggests that p100 may function as a negative regulator of the NF-kappaB pathway. Consistent with this hypothesis, ectopic expression of p100 inhibited TCR-mediated NF-kappaB activity and IL-2 production in Jurkat T cells. Conversely, knockdown of p100 expression enhanced NF-kappaB transcriptional activity and IL-2 production upon TCR activation. p100 inhibited the pathway by binding and sequestering Rel transcription factors in the cytoplasm without affecting the activity of the upstream IkappaB kinase. The kinetics and IkappaB kinase gamma/NF-kappaB essential modulator dependency of p100 induction suggest that NF-kappaB2/p100 acts as a late-acting negative-feedback signaling molecule in the TCR-mediated NF-kappaB pathway.

Ubiquitination of RIP1 regulates an NF-kappaB-independent cell-death switch in TNF signaling.

TNF receptor 1 (TNFR1) can trigger opposing responses within the same cell: a prosurvival response or a cell-death pathway [1, 2]. Cell survival requires NF-kappaB-mediated transcription of prosurvival genes [3-9]; apoptosis occurs if NF-kappaB signaling is blocked [5, 7-9]. Hence, activation of NF-kappaB acts as a cell-death switch during TNF signaling. This study demonstrates that the pathway includes another cell-death switch that is independent of NF-kappaB. We show that lysine 63-linked ubiquitination of RIP1 on lysine 377 inhibits TNF-induced apoptosis first through an NF-kappaB-independent mechanism and, subsequently, through an NF-kappaB-dependent mechanism. In contrast, in the absence of ubiquitination, RIP1 serves as a proapoptotic signaling molecule by engaging CASPASE-8. Therefore, RIP1 is a dual-function molecule that can be either prosurvival or prodeath depending on its ubiquitination state, and this serves as an NF-kappaB-independent cell-death switch early in TNF signaling. These results provide an explanation for the conflicting reports on the role of RIP1 in cell death; this role was previously suggested to be both prosurvival and prodeath [10-12]. Because TRAF2 is the E3 ligase for RIP1 [13], these observations provide an explanation for the NF-kappaB-independent antiapoptotic function previously described for TRAF2 [14-16].

B cell maturation antigen, the receptor for a proliferation-inducing ligand and B cell-activating factor of the TNF family, induces antigen presentation in B cells.

B cell maturation Ag (BCMA), a member of the TNFR superfamily expressed on B cells, binds to a proliferation-inducing ligand (APRIL) and B cell-activating factor of the TNF family (BAFF) but the specific B cell responses regulated by BCMA remain unclear. This study demonstrates that ligation of A20 B cells transfected with BCMA induces the expression of CD40, CD80/B7-1, CD86/B7-2, MHC class II, and CD54/ICAM-1, which subsequently enhances the presentation of OVA peptide Ag to DO11.10 T cells. BCMA expression in murine splenic B cells can be induced with IL-4 and IL-6, allowing subsequent treatment with APRIL or agonist anti-BCMA to similarly induce Ag presentation. A comparative analysis of hybrid receptors of TNFR2 fused to the cytoplasmic domains of APRIL/BAFF receptors found that only BCMA, but not transmembrane activator and calcium-modulator and cyclophilin ligand interactor or BAFF-R, is capable of activating Ag presentation. Although all three receptors can trigger NF-kappaB signaling, only BCMA activates the JNK pathway conferring on BCMA the specific ability to activate this Ag presentation response.