The linear ubiquitin chain assembly complex regulates TRAIL‐induced gene activation and cell death

Abstract

The linear ubiquitin chain assembly complex (LUBAC) is the only known E3 ubiquitin ligase which catalyses the generation of linear ubiquitin linkages de novo. LUBAC is a crucial component of various immune receptor signalling pathways. Here, we show that LUBAC forms part of the TRAIL‐R‐associated complex I as well as of the cytoplasmic TRAIL‐induced complex II. In both of these complexes, HOIP limits caspase‐8 activity and, consequently, apoptosis whilst being itself cleaved in a caspase‐8‐dependent manner. Yet, by limiting the formation of a RIPK1/RIPK3/MLKL‐containing complex, LUBAC also restricts TRAIL‐induced necroptosis. We identify RIPK1 and caspase‐8 as linearly ubiquitinated targets of LUBAC following TRAIL stimulation. Contrary to its role in preventing TRAIL‐induced RIPK1‐independent apoptosis, HOIP presence, but not its activity, is required for preventing necroptosis. By promoting recruitment of the IKK complex to complex I, LUBAC also promotes TRAIL‐induced activation of NF‐κB and, consequently, the production of cytokines, downstream of FADD, caspase‐8 and cIAP1/2. Hence, LUBAC controls the TRAIL signalling outcome from complex I and II, two platforms which both trigger cell death and gene activation.

Nonapoptotic functions of Fas/CD95 in the immune response

“…Interestingly, the nonenzymatic activity of caspase‐8 is essential for the formation of the complex because catalytically inactive caspase‐8 was able to restore TRAIL‐induced nonapoptotic signaling in caspase‐8 knockout cells. This complex was further confirmed by work from the Walczak group showing that this complex also contained TRAF2, c‐IAP1/2, and LUBAC . Surprisingly, this study also demonstrated that this complex is formed at the plasma membrane and that the LUBAC complex is a critical determinant of cell fate by restricting caspase‐8 activation through ubiquitination while activating the NF‐κB pathway.…”

A Dual Role of Caspase-8 in Triggering and Sensing Proliferation-Associated DNA Damage, a Key Determinant of Liver Cancer Development

“…Since the linear ubiquitin chain assembly complex (LUBAC) plays a role in preventing cell-death-inducing complex formation in various cell types including hepatocytes (Lafont et al., 2017, Shimizu et al., 2017), it was considered a candidate signaling event. Indeed, LUBAC components HOIP, HOIL-1, and SHARPIN, as well as the inhibitor of apoptosis proteins (IAP) cIAP1, cIAP2, and XIAP, known to negatively regulate formation of the ripoptosome (Tenev et al., 2011), were transiently reduced 15 to 30 min post-doxorubicin treatment of U2OS cells (Figure 6H).…”

Ubiquitin ligases in oncogenic transformation and cancer therapy

“…177). Recent studies indicate that RIPK1 (REFS 178,179) and LUBAC 179 can be directly recruited to membrane-bound TRAILR, suggesting that NF-κB activation can occur at the plasma membrane. Understanding the signalling circuits that are activated downstream of TRAILR is of special interest, as several TRAILR agonists have been evaluated as putative cancer therapeutics with limited clinical success (reviewed in REF.…”

“…The finding that TRAILR can induce migratory 180 and inflammatory responses that promote tumorigenesis 181 sheds light on the complex signalling circuits that respond to TRAIL. Therefore, fine tuning of LUBAC-mediated NF-κB activation may constitute one step to improve therapeutic effects of TRAILR agonists 179 and possible pathways that have been associated with deregulated LUBAC components.…”

“…As indicated by the lighter shaded complex, RIPK1 and LUBAC may be directly recruited to complex I, allowing MAPK and NF-κB activation independent of complex II formation. Understanding the ubiquitin-mediated mechanisms that result in pro-tumorigenic NF-κB activation upon TRAILR stimulation is one of the important steps towards improving the therapeutic effect of TRAILR agonists, which are currently being evaluated as anticancer drugs 179 . APC/C, anaphase-promoting complex; also known as the cyclosome; BCL-xL, also known as BCL2L1; BID, BH3, interacting domain death agonist; FBXW7, F-box/WD repeat-containing protein 7; HOIL1, haeme-oxidized IRP2 ubiquitin ligase 1 (also known as RBCK1); HOIP, HOIL1-interacting protein (also known as RNF31); NEMO, NF-κB essential modulator; P, phosphorylation; PUMA, p53 up-regulated modulator of apoptosis (also known as BBC3); SHARPIN, shank-associated RH domain-interacting protein; Ub, ubiquitin.…”