Ubiquitin-Mediated Regulation of RIPK1 Kinase Activity Independent of IKK and MK2

Abstract

SummaryTumor necrosis factor (TNF) can drive inflammation, cell survival, and death. While ubiquitylation-, phosphorylation-, and nuclear factor κB (NF-κB)-dependent checkpoints suppress the cytotoxic potential of TNF, it remains unclear whether ubiquitylation can directly repress TNF-induced death. Here, we show that ubiquitylation regulates RIPK1’s cytotoxic potential not only via activation of downstream kinases and NF-kB transcriptional responses, but also by directly repressing RIPK1 kinase activity via ubiquitin-dependent inactivation. We find that the ubiquitin-associated (UBA) domain of cellular inhibitor of apoptosis (cIAP)1 is required for optimal ubiquitin-lysine occupancy and K48 ubiquitylation of RIPK1. Independently of IKK and MK2, cIAP1-mediated and UBA-assisted ubiquitylation suppresses RIPK1 kinase auto-activation and, in addition, marks it for proteasomal degradation. In the absence of a functional UBA domain of cIAP1, more active RIPK1 kinase accumulates in response to TNF, causing RIPK1 kinase-mediated cell death and systemic inflammatory response syndrome. These results reveal a direct role for cIAP-mediated ubiquitylation in controlling RIPK1 kinase activity and preventing TNF-mediated cytotoxicity.

Cell Death and Liver Disease

“…RIPK1 can be directly inhibited via ubiquitination by anti-apoptotic cIAPs and this prevents TNF-mediated apoptosis or necroptosis. 68 In contrast, CYPD required for MPT- driven necrosis deubiquitinates RIPK1 to facilitate necrosome formation to enforce the cell death pathway. 69 Active RIPK3 and MLKL activates NLRP3 inflammasome, 29 serving as a direct link between necroptosis and inflammation besides DAMP-mediated inflammasome activation.…”

TNF receptor signalling in autoinflammatory diseases

“…This differential offers up several possibilities for fine-tuning ubiquitin dependent signalling mediated by TNF and also for IL-1 mediated activation of NF-B via the IL-1 receptor (IL-1R) (for detailed review see (7)). A recent study shows that ubiquitination also 'IPK potential, not only through activation of the NK-B pathway but also by directly supressing RIPK1 kinase activity, via ubiquitin-dependent inactivation (27).…”

Post-translational Modification of OTULIN Regulates Ubiquitin Dynamics and Cell Death

“…Tumor necrosis factor (TNF) is a pleiotropic cytokine implicated in an array of physiological processes, notably inflammation, cell survival, and cell death (Kalliolias and Ivashkiv, 2016). The outcome of TNF signaling is tightly controlled by dynamic post-translational modifications (PTMs) of various substrates in the TNF receptor 1 (TNFR1) pathway, with proteolytic cleavage, phosphorylation, and ubiquitination playing particularly significant roles (Annibaldi et al, 2018). In response to TNF binding, TNFR1 trimerization triggers the assembly of a membrane-bound complex called complex I, consisting of TNFR1associated death domain (TRADD), TNFR-associated factor 2 (TRAF2), cellular inhibitor of apoptosis proteins 1/2 (cIAP1/2), and receptor interacting protein kinase 1 (RIPK1).…”

“…cIAP1/2-mediated ubiquitination of RIPK1 scaffolds the recruitment of the linear ubiquitin assembly complex (LUBAC), composed of HOIP, HOIL-1L and Sharpin, which in turn modifies RIPK1 and nuclear factor kB (NF-kB) essential modulator (NEMO) with linear ubiquitin (Met1-Ub) chains (Haas et al, 2009). Ubiquitinated RIPK1 within complex I recruits the TAK1/TAB2/TAB3 and IKK1/2/NEMO complexes, which promote mitogen-activated protein kinase (MAPK) and NF-kB activation and the transcription of several pro-survival and pro-inflammatory genes (Annibaldi et al, 2018). Reduced ubiquitination of RIPK1 promotes complex I destabilization and cytosolic translocation of TRADD and RIPK1, which assemble with FADD, c-FLIP L , and procaspase-8 in a pro-apoptotic complex called complex IIa (Bertrand et al, 2008).…”