NEMO regulates a cell death switch in TNF signaling by inhibiting recruitment of RIPK3 to the cell death-inducing complex II

Abstract

Incontinentia Pigmenti (IP) is a rare X-linked disease characterized by early male lethality and multiple abnormalities in heterozygous females. IP is caused by NF-κB essential modulator (NEMO) mutations. The current mechanistic model suggests that NEMO functions as a crucial component mediating the recruitment of the IκB-kinase (IKK) complex to tumor necrosis factor receptor 1 (TNF-R1), thus allowing activation of the pro-survival NF-κB response. However, recent studies have suggested that gene activation and cell death inhibition are two independent activities of NEMO. Here we describe that cells expressing the IP-associated NEMO-A323P mutant had completely abrogated TNF-induced NF-κB activation, but retained partial antiapoptotic activity and exhibited high sensitivity to death by necroptosis. We found that robust caspase activation in NEMO-deficient cells is concomitant with RIPK3 recruitment to the apoptosis-mediating complex. In contrast, cells expressing the ubiquitin-binding mutant NEMO-A323P did not recruit RIPK3 to complex II, an event that prevented caspase activation. Hence NEMO, independently from NF-κB activation, represents per se a key component in the structural and functional dynamics of the different TNF-R1-induced complexes. Alteration of this process may result in differing cellular outcomes and, consequently, also pathological effects in IP patients with different NEMO mutations.

The in vivo evidence for regulated necrosis

“…177 SHARPIN was demonstrated to be part of the linear ubiquitination complex (LUBAC), which connects linear ub-linkages to RIPK1 and NEMO (which requires RIPK1 ubiquitination to initiate NF-κB signaling). 162 In line with this idea, chronic inflammation of the skin is triggered upon keratinocyte-specific deletion of FADD, a phenomenon that depends on RIPK3-dependent necroptosis and partially on TNFR1 and CYLD (an enzyme deubiquitinating RIPK1). 162 In line with this idea, chronic inflammation of the skin is triggered upon keratinocyte-specific deletion of FADD, a phenomenon that depends on RIPK3-dependent necroptosis and partially on TNFR1 and CYLD (an enzyme deubiquitinating RIPK1).…”

“…178,179 Therefore, in the absence of SHARPIN, NF-κB signaling is inhibited 180 and aberrant induction of cell death can emerge. 162 In line with this idea, chronic inflammation of the skin is triggered upon keratinocyte-specific deletion of FADD, a phenomenon that depends on RIPK3-dependent necroptosis and partially on TNFR1 and CYLD (an enzyme deubiquitinating RIPK1). 181 Additionally, conditional loss of caspase-8 within the skin leads to a similar inflammatory phenotype, 182 and RIPK3-deficiency is protective for FADD and caspase-8 deficiency in the skin.…”

“…156 Deletion of the deubiquitinase A20 leads to a partly TNFR1-dependent TUNELpositivity, which was interpreted as apoptosis. 162 It is worth mentioning that the alarmin IL-33, which is actively released from necroptotic cells, promotes Treg function in the intestine, which control intestinal inflammation. 158,159 This phenotype can be rescued by the RIPK1 kinase-inactive knock-in.…”

“…161 Mechanistically, this was shown to occur by the inhibition of recruitment of RIPK3 to the necrosome. 162 It is worth mentioning that the alarmin IL-33, which is actively released from necroptotic cells, promotes Treg function in the intestine, which control intestinal inflammation. 163…”

Life, death, and autophagy in cancer: NF-κB turns up everywhere

“…Although previous studies demonstrated that NEMO induces necroptosis by promoting RIPK1/RIPK3 necrosome 139,140 , further studies are needed to better understand the IKK/NF-κΒ-independent role of NEMO in controlling PCD in the liver. Nevertheless, based on these studies, the use of small molecule inhibitors of RIPK1 Kinase, also blocking necroptosis, would be an effective therapeutic option for patients with liver damages and HCC.…”