The inhibitor of apoptosis (IAP) proteins are a family of anti-apoptotic regulators found in viruses and metazoans. c-IAP1
and c-IAP2 are recruited to tumor necrosis factor receptor 1 (TNFR1)-associated complexes where they can regulate receptor-mediated
signaling. Both c-IAP1 and c-IAP2 have been implicated in TNFα-stimulated NF-κB activation. However, individual c-IAP1 and c-IAP2 gene knock-outs in mice did not reveal changes in TNF signaling pathways, and the phenotype of a combined deficiency of c-IAPs
has yet to be reported. Here we investigate the role of c-IAP1 and c-IAP2 in TNFα-stimulated activation of NF-κB. We demonstrate
that TNFα-induced NF-κB activation is severely diminished in the absence of both c-IAP proteins. In addition, combined absence
of c-IAP1 and c-IAP2 rendered cells sensitive to TNFα-induced cell death. Using cells with genetic ablation of c-IAP1 or cells
where the c-IAP proteins were eliminated using IAP antagonists, we show that TNFα-induced RIP1 ubiquitination is abrogated
in the absence of c-IAPs. Furthermore, we reconstitute the ubiquitination process with purified components in vitro and demonstrate that c-IAP1, in collaboration with the ubiquitin conjugating enzyme (E2) enzyme UbcH5a, mediates polymerization
of Lys-63-linked chains on RIP1. Therefore, c-IAP1 and c-IAP2 are required for TNFα-stimulated RIP1 ubiquitination and NF-κB
"cIAP1 and cIAP2 are also RING domain-containing E3s reported to catalyze RIP1 polyubiquitination (Mahoney et al., 2008), but a recent study also suggests that cIAP1, along with UbcH5, generates K11-linked polyubiquitin chains on RIP1 in the TNFR1 signaling complex to activate NF-κB in a nondegradative manner (Dynek et al., 2010). Although several studies clearly demonstrate the pivotal roles of cIAP1/2 in RIP1 polyubiquitination (Mahoney et al., 2008; Varfolomeev et al., 2008), the detailed mechanisms underlying cIAP1/2 and TRAF2/5 interplay remain elusive. "
[Show abstract][Hide abstract] ABSTRACT: Ubiquitination, also denoted ubiquitylation, is a posttranslational modification that has been implicated in the regulation of both innate and adaptive immune responses. Ubiquitination plays crucial roles in innate immune signaling by ensuring the proper orchestration of several signaling mediators that constitute a functional immune response. Herein, we briefly summarize the latest discoveries concerning the molecular ubiquitination-related machinery that senses, assembles, and disassembles innate immune signaling mediators. Introduction The mammalian immune system, which involves a complex yet tightly regulated network of interactions among different types of cells, cell receptors, and signaling pathways, constantly battles invading pathogens. In addition to the aforementioned immune-system participants, its specificity and complexity also depend on posttranslational modifications of proteins involved in the initiation, maintenance, and termination of immune responses. These posttranslational modifications involve the addition of a chemical group or another protein(s) at one or more site of substrate. To date, more than 200 types of posttranslational modifications have been reported (Kho et al., 2004); with phosphorylation, ubiquitination, and sumoylation being the most extensively studied and well characterized. Ubiquitination is a key posttranslational modification regulating numerous biological processes at various cellular levels, e.g., protein trafficking, the cell cycle, and immune responses. The addition of ubiquitin to a substrate protein usually involves three main steps: activation by a ubiquitin-activating enzyme (E1), conjugation by a
Current issues in molecular biology 07/2016; 18(1):1-10. · 5.75 Impact Factor
"UbcH5 was originally found as an E2 family that can support IKK activation in crude cell extracts (Chen et al., 1996). Recently, UbcH5s are further identified as critical partners of c-IAPs to specifically mediate ubiquitination of RIP1 in TNF-a signaling (Dynek et al., 2010; Varfolomeev et al., 2008; Xu et al., 2009). UbcH5 can also cooperate with LUBAC to generate a linear polyUb chain on NEMO (Kirisako et al., 2006; Tokunaga et al., 2009). "
[Show abstract][Hide abstract] ABSTRACT: UbcH5 is the key ubiquitin-conjugating enzyme catalyzing ubiquitination during TNF-α-triggered NF-κB activation. Here, we identified an herb-derived sesquiterpene lactone compound IJ-5 as a preferential inhibitor of UbcH5 and explored its therapeutic value in inflammatory and autoimmune disease models. IJ-5 suppresses TNF-α-induced NF-κB activation and inflammatory gene transcription by inhibiting the ubiquitination of receptor-interacting protein 1 and NF-κB essential modifier, which is essential to IκB kinase activation. Mechanistic investigations revealed that IJ-5 preferentially binds to and inactivates UbcH5 by forming a covalent adduct with its active site cysteine and thereby preventing ubiquitin conjugation to UbcH5. In preclinical models, pretreatment of IJ-5 exhibited potent anti-inflammatory activity against TNF-α- and D-galactosamine-induced hepatitis and collagen-induced arthritis. These findings highlight the potential of UbcH5 as a therapeutic target for anti-TNF-α interventions and provide an interesting lead compound for the development of new anti-inflammation agents.
"Since TNFα is a transcriptional target of NFκB this signaling cascade constitutes an autoregulatory feed-back loop. In the absence of cIAP1/2 RIP1 is not ubiquitylated (24), which induces its interaction with the FADD/Caspase-8 complex and triggers apoptotic cell death (33). "
[Show abstract][Hide abstract] ABSTRACT: Defects in apoptosis regulation are one main cause of cancer development and may result from overexpression of anti-apoptotic proteins such as inhibitor of apoptosis proteins (IAPs). IAPs are cell death regulators that, among other functions, bind caspases, and interfere with apoptotic signaling via death receptors or intrinsic cell death pathways. All IAPs share one to three common structures, the so called baculovirus-IAP-repeat (BIR)-domains that allow them to bind caspases and other proteins. X-linked inhibitor of apoptosis protein (XIAP) is the most potent and best-defined anti-apoptotic IAP family member that directly neutralizes caspase-9 via its BIR3 domain and the effector caspases-3 and -7 via its BIR2 domain. A natural inhibitor of XIAP is SMAC/Diablo, which is released from mitochondria in apoptotic cells and displaces bound caspases from the BIR2/BIR3 domains of XIAP thereby reactivating cell death execution. The central apoptosis-inhibitory function of XIAP and its overexpression in many different types of advanced cancers have led to significant efforts to identify therapeutics that neutralize its anti-apoptotic effect. Most of these drugs are chemical derivatives of the N-terminal part of SMAC/Diablo. These "SMAC-mimetics" either specifically induce apoptosis in cancer cells or act as drug-sensitizers. Several "SMAC-mimetics" are currently tested by the pharmaceutical industry in Phase I and Phase II trials. In this review, we will discuss recent advances in understanding the function of IAPs in normal and malignant cells and focus on approaches to specifically neutralize XIAP in cancer cells.
Frontiers in Oncology 07/2014; 4:197. DOI:10.3389/fonc.2014.00197
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