c-IAP1 and c-IAP2 Are Critical Mediators of Tumor Necrosis Factor α (TNFα)-induced NF-κB Activation

Abstract

Cellular IAP12 and IAP2 (c-IAP1 and c-IAP2) were identified in a search for proteins associated with TNF receptors (TNFRs) (1). Through binding to TNFR-associated factor 2 (TRAF2), c-IAP1 and c-IAP2 are recruited to TNFR signaling complexes, where they regulate the activation of caspase-8 (1, 2). c-IAP1 and c-IAP2 were also proposed to modulate activation of the canonical NF-B pathway, although most of these studies relied on overexpression (3, 4). In contrast, however, targeted deletion of c-IAP1 or c-IAP2 genes in mice did not reveal any abnormalities in TNF␣-induced NF-B (nuclear factor B) activation (5, 6). The absence of any appreciable phenotype in single c-IAP knock-out mice has been attributed to the putative redundancy of c-IAP1 and c-IAP2 due to their high level of sequence and functional similarities (7). Thus, combined deficiency of cellular IAPs might expose their role in this signaling pathway. In support of this possibility, a null mutation in the sole cellular IAP in zebrafish results in severe defects in NF-B activation (8). c-IAP1 and c-IAP2 are also RING domain-containing ubiquitin ligases capable of promoting ubiquitination of several of their binding partners, including TRAF2 and SMAC (second mitochondrial activator of caspases) (4, 5, 9 -12).TNFR1 mediates activation of several signaling pathways, among them the canonical NF-B pathway (13). Binding of TNF␣ to TNFR1 induces recruitment of the adaptor protein TNFR-associated death domain (TRADD) to the death domain of the receptor (14). Through its death domain and amino-terminal region, TRADD recruits RIP1 (receptor-interacting protein), TRAF2, and through its interaction with TRAF2, c-IAP1 and c-IAP2 (13). Following binding to TRADD, TRAF2 was thought to mediate non-degradative Lys-63-linked polyubiquitination of RIP1 via its RING E3 ligase domain (15,16). This RIP1 modification induces assembly of two RIP1-associated kinase complexes, TAK1-TABs (transforming growth factor ␤-activated kinase 1-TAK1-binding proteins) and IB kinase (IKK) (17)(18)(19). Binding of these two complexes to Lys-63-linked polyubiquitin chains on RIP1 leads to phosphorylation of IKK␤ and subsequent phosphorylation and proteasomal degradation of IB (20). Loss of IB allows translocation of p50/RelA dimer to the nucleus and induction of gene expression (20).In the present study, we investigate the role of c-IAP1 and c-IAP2 in TNF␣-induced NF-B activation. We discover that c-IAP proteins are important mediators of canonical NF-B signaling and demonstrate that the absence of c-IAPs severely attenuates TNF␣-induced NF-B activation. Finally, we show that c-IAPs are ubiquitin ligases capable of promoting polymerization of Lys-63-linked polyubiquitin chains on the critical adapter in the canonical NF-B signaling pathway, RIP1. was from Genentech, Inc. The primary antibodies against mouse c-IAP1 were kindly provided by Drs. John Silke and David Vaux; anti-human c-IAP1 antibodies were purchased from R&D (affinity-purified goat antibody) or Protein Tech Group Inc.; pan c-IAP1/2 ...

Targeting inhibitor of apoptosis proteins by Smac mimetic elicits cell death in poor prognostic subgroups of chronic lymphocytic leukemia

“…35,36,44 This conclusion is supported by the following lines of evidence: First, the TNFacatching antibody Enbrel fails to prevent BV6-induced cell death in primary CLL cells. Similarly, we recently described that BV6/Dexamethasone-induced cell death in childhood ALL occurs in a TNFa-independent manner.…”

USP11-dependent selective cIAP2 deubiquitylation and stabilization determine sensitivity to Smac mimetics

“…39 In contrast, the absence of both cIAP1 and cIAP2 is required for complete inhibition of TNFα-mediated survival, suggesting that these inhibitors have redundant functions. 20,21 Evidence also supports that cIAP2 has independent roles from cIAP1 in suppressing apoptosis. For examples, we and others have found that the levels of cIAP2, but not cIAP1, are greatly increased after TNFα stimulation.…”

“…18,19 Because cIAP1 and cIAP2 show functional redundancy in TNFα-mediated survival, the depletion of both proteins is usually required for effective induction of cell death upon TNFα treatment. 20,21 However, there are several reports showing that cIAP2 expression, but not cIAP1 expression, renders cells resistant to Smac mimetic-induced cell death. 20,21 For example, cIAP2 upregulation via phosphoinositide 3-kinase (PI3K) upon compound 3 treatment in certain cell lines was shown to facilitate apoptosis evasion.…”

“…20,21 However, there are several reports showing that cIAP2 expression, but not cIAP1 expression, renders cells resistant to Smac mimetic-induced cell death. 20,21 For example, cIAP2 upregulation via phosphoinositide 3-kinase (PI3K) upon compound 3 treatment in certain cell lines was shown to facilitate apoptosis evasion. 22 In addition, treatment with compounds A and C led to cIAP1 dimerization, without cIAP2 dimerization, resulting in the autoubiquitylation and subsequent degradation of cIAP1.…”

RIP kinases: key decision makers in cell death and innate immunity

“…This is followed by the recruitment of a number of E3 ubiquitin ligases to RIP1, including TNF receptor-associated factor 2 (TRAF2) or TRAF5 and the cellular inhibitor of apoptosis proteins (cIAPs) cIAP1 and cIAP2 resulting in the formation of Complex I. [21][22][23][24] TRAF2 25,26 and cIAPs [27][28][29][30] catalyse the polyubiquitination of RIP1. The ubiquitin-decorated RIP1 is recognized by ubiquitin-binding domain containing proteins in the IkB kinase (IKK) 31 and TAK1 kinase complexes [32][33][34] thus facilitating TAK-1-mediated phosphorylation and activation of IKKs.…”