TNF-α Induces Two Distinct Caspase-8 Activation Pathways

Abstract

The inflammatory response of mammalian cells to TNF-alpha can be switched to apoptosis either by cotreatment with a protein synthesis inhibitor, cycloheximide, or Smac mimetic, a small molecule mimic of Smac/Diablo protein. Cycloheximide promotes caspase-8 activation by eliminating endogenous caspase-8 inhibitor, c-FLIP, while Smac mimetic does so by triggering autodegradation of cIAP1 and cIAP2 (cIAP1/2), leading to the release of receptor interacting protein kinase (RIPK1) from the activated TNF receptor complex to form a caspase-8-activating complex consisting of RIPK1, FADD, and caspase-8. This process also requires the action of CYLD, a RIPK1 K63 deubiquitinating enzyme. RIPK1 is critical for caspase-8 activation-induced by Smac mimetic but dispensable for that triggered by cycloheximide. Moreover, Smac mimetic-induced caspase-8 activation is not blocked by endogenous c-FLIP. These findings revealed that TNF-alpha is able to induce apoptosis via two distinct caspase-8 activation pathways that are differentially regulated by cIAP1/2 and c-FLIP.

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

“…Indeed, as previously reported, RIP1-independent (TRADDdependent) and RIP1-dependent apoptotic pathways can be activated when cells are stimulated with TNFα and CHX or TNFα and SMs, respectively. 17 We found that NIK +/+ and NIK − / − MEFs equally died following exposure to TNFα/CHX, indicating that the RIP1-independent pro-apoptotic machinery was intact in NIK-deficient cells (Figure 2a Figures S2b-d). We recently showed that the RIP1 kinase inhibitor necrostatin-1 (Nec-1) prevented SM/TNFα-induced caspase activation.…”

“…Caspase-8 is either activated 1 within a TRADD/FADD-containing complex (complex IIa) or in a RIP1/FADD-containing complex (complex IIb). 17,18 The former assembles when cells are deficient in p65 or pretreated with cycloheximide (CHX), whereas the latter forms in the absence of c-IAP1/2, in TAK1 kinase-inhibited conditions or in NEMO-depleted cells. 19,20 In addition, under caspase-repressed conditions RIP1 associates with RIP3 to mediate necroptosis, a form of programmed necrosis that takes place during viral infection or TNF alpha (TNFα)-induced systemic inflammatory response syndrome (SIRS).…”

RIP kinases: key decision makers in cell death and innate immunity

“…[43][44][45][46] In this unmodified form, RIP can leave the TNF-R1 complex to associate with Fas-associated death domain (FADD) and procaspase 8 and form death-inducing signalling complex also known as Complex II. 27,43,[47][48][49] The use of IAP antagonists or loss of cIAP proteins generates a similar ripoptosome complex consisting of RIP1, FADD and caspase 8, with components of this complex being subject to ubiquitination and inactivation by cIAPs. [49][50][51] Complex II and the ripoptosome can promote processing of procaspase 8 to its active form resulting in triggering of the caspase cascade that culminates in cell death by apoptosis.…”

“…27,43,[47][48][49] The use of IAP antagonists or loss of cIAP proteins generates a similar ripoptosome complex consisting of RIP1, FADD and caspase 8, with components of this complex being subject to ubiquitination and inactivation by cIAPs. [49][50][51] Complex II and the ripoptosome can promote processing of procaspase 8 to its active form resulting in triggering of the caspase cascade that culminates in cell death by apoptosis. 45 Recently, we have demonstrated that a member of the Pellino E3 ubiquitin ligase family, Pellino3, targets RIP1 and impairs Complex II formation in the TNF signalling pathway to suppress cell apoptosis.…”

dsDNA ASCs for caspase 8-mediated apoptosis

“…Similarly, cellular inhibitors of apoptosis (cIAPs) positively and negatively regulate inflammasome activation and caspase 1 processing, 12 and also compromises caspase 8 activation at the death receptor DISC. 13 Thus, via ubiquitination of specific signaling components cIAPs may shift the inflammasome response towards inflammation and pyroptosis, or immunologically silent apoptosis. 14 At the end, it remains to be asked why nature has invented a system that allows simultaneous induction of cell death by pyroptosis and associated inflammation, and apoptosis.…”