Involvement of linear polyubiquitylation of NEMO in NF-κB activation

Abstract

Nuclear factor-kappaB (NF-kappaB) is a key transcription factor in inflammatory, anti-apoptotic and immune processes. The ubiquitin pathway is crucial in regulating the NF-kappaB pathway. We have found that the LUBAC ligase complex, composed of the two RING finger proteins HOIL-1L and HOIP, conjugates a head-to-tail-linked linear polyubiquitin chain to substrates. Here, we demonstrate that LUBAC activates the canonical NF-kappaB pathway by binding to NEMO (NF-kappaB essential modulator, also called IKKgamma) and conjugates linear polyubiquitin chains onto specific Lys residues in the CC2-LZ domain of NEMO in a Ubc13-independent manner. Moreover, in HOIL-1 knockout mice and cells derived from these mice, NF-kappaB signalling induced by pro-inflammatory cytokines such as TNF-alpha and IL-1beta was suppressed, resulting in enhanced TNF-alpha-induced apoptosis in hepatocytes of HOIL-1 knockout mice. These results indicate that LUBAC is involved in the physiological regulation of the canonical NF-kappaB activation pathway through linear polyubiquitylation of NEMO.

TAK1 control of cell death

“…Upon TNFa stimulation, adaptor molecules including TNFa receptor type-1-associated death domain protein (TRADD), TNFa receptorassociated factor 2 and 5 (TRAF2 and TRAF5), cellular inhibitor of apoptosis 1 and 2 (cIAP1/2) and RIPK1 are recruited to the receptor complex (TNF receptor 1 (TNFR1) Complex I) (Figure 1), in which RIPK1 acquires a K63-linked or linear polyubiquitin chain by E3 ligases, TRAF2/5 cIAP1/2 or linear ubiquitin chain assembly complex containing two E3 ligases HOIL-1 and HOIP. [18][19][20][21][22] TAK1 is recruited and activated through TAK1-binding protein 2 (TAB2) binding to the RIPK1 polyubiquitin chain. 23,24 Upon binding the polyubiquitin chain, TAK1 phosphorylates and activates the IKK complex composed of IKKa, IKKb and NEMO (also called IKKg), which leads to phosphorylation and degradation of IkB resulting in activation of NF-kB ( Figure 1).…”

“…Upon TNFa stimulation, TNFR1 forms Complex I, in which RIPK1 acquires a polyubiquitin chain. [18][19][20][21] TAK1 binds to the polyubiquitin chain though TAB2, and activates the IKK complex, leading to the activation of NF-kB. TAK1 also activates MAPK cascades.…”

A single NFκB system for both canonical and non-canonical signaling

“…In the context of TNF signaling, the receptorassociated proteins TRADD, tumor necrosis factor receptor-associated factor (TRAF) 2 and cIAP1/2 recruit LUBAC, an ubiquitin ligase complex consisting of HOIP and HOIL-1L [14]. LUBAC conjugates linear-linked ubiquitin chains to NEMO, which results in IKK activation, possibly through trans-autophosphorylation [15]. In the third pathway of IKK2 activation, ligand engagement of the IL-1R and TLRs leads to the recruitment of the E2/E3 ligase complex made up of Ubc13 and TRAF6 [16].…”

NIK promotes tissue destruction independently of the alternative NF-κB pathway through TNFR1/RIP1-induced apoptosis

“…3,4 The formation of this complex is initiated by the interaction of ligand-bound TNFR1 with TNF receptor-associated death domain (TRADD) and by the recruitment of the kinase receptor interacting protein kinase 1 (RIP1), the adaptor proteins TNF receptor-associated factor 2/5 and the ubiquitin ligase cellular inhibitor of apoptosis 1/2 (c-IAP1/2) and linear ubiquitin chain assembly complex (LUBAC). [5][6][7][8][9] c-IAP1/2 and LUBAC target RIP1 for K11, K63 and linear polyubiquitination priming RIP1 for the recruitment of two kinase-containing complexes, namely, TAK1/TAK1-binding protein 1 (TAB1)/TAB2 and NEMO/ inhibitor of NF-κB kinase α (IKKα)/IKKβ. 5,[10][11][12][13] Ultimately, activation of IKKβ leads to the phosphorylation and proteasomal degradation of IκBα and the nuclear translocation of NF-κB, a pathway defined as the classical, or the canonical, NF-κB pathway.…”