M1‐linked ubiquitination facilitates NF‐κB activation and survival during sterile inflammation

Abstract

Methionine 1 (M1)-linked ubiquitination plays a key role in the regulation of inflammatory nuclear factor-jB (NF-jB) signalling and is important for clearance of pathogen infection in Drosophila melanogaster. M1-linked ubiquitin (M1-Ub) chains are assembled by the linear ubiquitin E3 ligase (LUBEL) in flies. Here, we have studied the role of LUBEL in sterile inflammation induced by different types of cellular stresses. We have found that the LUBEL catalyses formation of M1-Ub chains in response to hypoxic, oxidative and mechanical stress conditions. LUBEL is shown to be important for flies to survive low oxygen conditions and paraquat-induced oxidative stress. This protective action seems to be driven by stress-induced activation of the NF-jB transcription factor Relish via the immune deficiency (Imd) pathway. In addition to LUBEL, the intracellular mediators of Relish activation, including the transforming growth factor activating kinase 1 (Tak1), Drosophila inhibitor of apoptosis (IAP) Diap2, the IjB kinase c (IKKc) Kenny and the initiator caspase Death-related ced-3/Nedd2-like protein (Dredd), but not the membrane receptor peptidoglycan recognition protein (PGRP)-LC, are shown to be required for sterile inflammatory response and survival. Finally, we showed that the stress-induced upregulation of M1-Ub chains in response to hypoxia, oxidative and mechanical stress is also induced in mammalian cells and protects from stress-induced cell death. Taken together, our results suggest that M1-Ub chains are important for NF-jB signalling in inflammation induced by stress conditions often observed in chronic inflammatory diseases and cancer.Abbreviations AMP, antimicrobial peptide; daGal4, daughterless-Gal4; DAMP, danger-associated molecular pattern; Diap, Drosophila inhibitor of apoptosis; Dif, Dorsal-related immunity factor; Dredd, death-related ced-3/Nedd2-like protein; DUB, deubiquitinating enzyme; Fadd, fas-associated death domain; HIF, hypoxia-inducible factor; HOIL-1L, heme-oxidised iron-responsive element-binding protein 2 ubiquitin ligase-1L; HOIP, HOIL-1interacting protein; IKK, IjB kinase; Imd, immune deficiency; LDD, linear ubiquitin chain-determining domain; LUBAC, linear ubiquitin chain assembly complex; LUBEL, linear ubiquitin E3 ligase; NF-jB, nuclear factor j-light-chain enhancer of activated B cells; NZF, npl4 zinc finger; PAMP, pathogen-associated molecular pattern; PGRP, peptidoglycan recognition protein; PIM, PUB-interacting motif; PNG, peptide N-glycanase; PRR, pattern-recognition receptor; PUB, UBA or UBX-containing protein; RBR, RING in between RING; RING, really interesting new gene; SAMP, stress-associated molecular pattern; SHARPIN, SH3 and multiple ankyrin repeat domains protein (SHANK)-associated RBCK1 homology (RH) domain-interacting protein; Sima, similar; Tab2, transforming growth factor-b-activated kinase 1 binding protein 2; Tak1, transforming growth factor activating kinase; TGF, transforming growth factor; TNF, tumour necrosis factor; TRAIL, TNF-related apoptosis-inducing ligand; TUB...

Linear ubiquitin as a common regulator of cellular stress

“…In addition, it is well known that obesity induces the production of cytokines, such as TNF and IL-1b, which activate immune responses [13]. Hence, the findings described by Aalto et al [5] may have important implications for understanding the mechanisms behind obesity-associated inflammation. Could it be possible that M1-Ub is implicated in fine-tuning stress response and immune signalling during adipose tissue expansion?…”

“…The authors observed increased M1-Ub levels in response to hypoxia and other stresses in CaCo2 cells and the inhibition of HOIP, using HOIPINs, prevented this [5]. However, no changes were observed during hypoxia or mechanical stress at the level of NF-jB activation in these cells.…”

“…Aalto et al showed that hypoxia induced an increase in LUBEL-dependent M1-Ub [5]. However, this event was neither required for oxygen sensing nor for the activation of HIF.…”

“…There is, unfortunately, no evidence of a functional death receptor signalling complex in the context of OTULIN deficiency. Yet, based on the findings presented by Aalto et al [5], it may be possible that M1-Ub aberrantly generated by the loss of OTU-LIN and that is not associated with death receptors is inducing the promiscuous activation of signalling platforms that cause a constitutively activated NF-jBmediated inflammatory response. Both the differences and similarities between loss of HOIP, OTULIN or its catalytic inactivation are puzzling, and it will be extremely interesting to see what the future research will teach us about how linear ubiquitination regulates immune and stress responses at the level of immune receptor complexes and/or cytoplasmic signalling platforms.…”

“…However, little is known about the role of M1-Ub in response to other type of stresses. In the study by Aalto et al, the authors show that hypoxia, oxidative stress and mechanical stress induce M1-Ub in flies resulting in Relish (homolog for NF-jB)-mediated gene activation and protection from stress [5]. LUBEL loss or inactivation resulted in reduced survival of the flies upon these stresses in a process mediated by Imd pathway activation.…”