NFκB activation by Fas is mediated through FADD, caspase-8, and RIP and is inhibited by FLIP

Abstract

Fas (APO-1/CD95) is the prototypic death receptor, and the molecular mechanisms of Fas-induced apoptosis are comparably well understood. Here, we show that Fas activates NFκB via a pathway involving RIP, FADD, and caspase-8. Remarkably, the enzymatic activity of the latter was dispensable for Fas-induced NFκB signaling pointing to a scaffolding-related function of caspase-8 in nonapoptotic Fas signaling. NFκB was activated by overexpressed FLIPL and FLIPS in a cell type–specific manner. However, in the context of Fas signaling both isoforms blocked FasL-induced NFκB activation. Moreover, down-regulation of both endogenous FLIP isoforms or of endogenous FLIPL alone was sufficient to enhance FasL-induced expression of the NFκB target gene IL8. As NFκB signaling is inhibited during apoptosis, FasL-induced NFκB activation was most prominent in cells that were protected by Bcl2 expression or caspase inhibitors and expressed no or minute amounts of FLIP. Thus, protection against Fas-induced apoptosis in a FLIP-independent manner converted a proapoptotic Fas signal into an inflammatory NFκB-related response.

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

“…Although RIP1 mediates the activation of NFkB in response to a number of death receptors, including TNF-receptor 1 (TNF-R1), 15,16 TNF-related apoptosis-inducing ligand receptor 1 (TRAIL1) 17 and Fas, 18 the greatest appreciation of the function of RIP1 has emerged from exploring its role in TNF-mediated inflammation and cell death. The stimulation of TNF-R1 with TNF leads to the interaction of TNF-R1-associated death domain protein (TRADD) 19 and RIP1 20 with the TNF-R1 signalling complex (Figure 2).…”

RIP1, a kinase on the crossroads of a cell's decision to live or die

“…25 When the proapoptotic Fas signal is impeded by high levels of FLIP L or FLIP S , overexpression of Bcl-2 or addition of zVAD-fmk, then Fas induces the activation of NF-kB. 26 FasL-induced activation of NF-kB is impaired in RIP1 À/À , caspase 8 À/À or FADD À/À Jurkat cells, demonstrating that Fas-induced NF-kB signalling depends on FADD, caspase-8 and RIP1 26 ( Figure 5). Whereas active caspase-8 counteracts NF-kB activation by cleaving RIP1, 14 inhibition of caspase-8 by zVAD-fmk prevents RIP1 cleavage, thereby augmenting the activation of NF-kB.…”

“…27 Fas-mediated upregulation of cFos and cJun, which most likely occurs via the ERK and JNK pathway, respectively, still occurs in RIP1-deficient Jurkat cells, indicating that this kinase is dispensable in these pathways. 26 TRAIL-R-induced apoptosis and activation of NF-jB and MAP kinases. TRAIL forms a homotrimer, and upon binding to the receptor, the TRAIL receptor/ligand complex is internalized through the endosomal pathway.…”

Current insights into the regulation of programmed cell death by NF-κB

“…Recent work has uncovered that A20 promotes the elimination of RIP1 in the TNF-R1 complex by replacing K63 ubiquitin chains with K48 polyubiquitin, thereby acting as both a deubiquitinating enzyme and as an E3 ligase to downregulate NF-kB signaling to suppress chronic inflammation and cell death . cFLIP potently suppresses death receptor-induced apoptosis, as it blunts activation of pro-caspase-8 and -10 (Micheau et al, 2001;Kreuz et al, 2004). It has recently been shown that cFLIP also participates along with caspase-8 in promoting NF-kB activation via BCL-10, mucosa-associated-lymphoidtissue lymphoma-translocation gene 1 (MALT1) and RIP1 and thus plays a key role in T-cell survival and proliferation following T-cell-receptor stimulation (Abraham and Shaham, 2004;Zhou et al, 2004;Launay et al, 2005, reviewed in Budd et al, 2006.…”