Participation of c-FLIP in NLRP3 and AIM2 inflammasome activation

Abstract

Cellular FLICE-inhibitory protein (c-FLIP) is an inhibitor of caspase-8 and is required for macrophage survival. Recent studies have revealed a selective role of caspase-8 in noncanonical IL-1b production that is independent of caspase-1 or inflammasome. Here we demonstrated that c-FLIP L is an unexpected contributor to canonical inflammasome activation for the generation of caspase-1 and active IL-1b. Hemizygotic deletion of c-FLIP impaired ATP-and monosodium uric acid (MSU)-induced IL-1b production in macrophages primed through Toll-like receptors (TLRs). Decreased IL-1b expression was attributed to a reduced activation of caspase-1 in c-FLIP hemizygotic cells. In contrast, the production of TNF-a was not affected by downregulation in c-FLIP. c-FLIP L interacted with NLRP3 or procaspase-1. c-FLIP is required for the full NLRP3 inflammasome assembly and NLRP3 mitochondrial localization, and c-FLIP is associated with NLRP3 inflammasome. c-FLIP downregulation also reduced AIM2 inflammasome activation. In contrast, c-FLIP inhibited SMAC mimetic-, FasL-, or Dectin-1-induced IL-1b generation that is caspase-8-mediated. Our results demonstrate a prominent role of c-FLIP L in the optimal activation of the NLRP3 and AIM2 inflammasomes, and suggest that c-FLIP could be a valid target for treatment of inflammatory diseases caused by overactivation of inflammasomes.

Cellular FLIP Inhibits Myeloid Cell Activation by Suppressing Selective Innate Signaling

“…4I), suggesting that the increased activation in c-FLIP-deficient BMDCs is dissociated from necrosis and activation of RIP-1. We also examined the activation of c-FLIP f/2 BMDCs, as we have previously found that the presence of hemizygotic c-FLIP prevents cell death in myeloid cells (29). Increase in dectin-1-induced TNF-a production and IL-2 secretion was similarly observed in c-FLIP f/2 BMDCs (Fig.…”

“…Additionally, TLR3, TLR4, and dectin-1 have been shown to trigger IL-1b production in the caspase-8-dependent pathway (41,42). We recently found that c-FLIP suppresses dectin-1-induced IL-1b production by direct inhibition of caspase-8 (29). We thus examined whether the increased expression of TNF-a, IL-2, and G-CSF in c-FLIP-deficient DCs was due to enhanced activation of caspase-8.…”

“…Anti-tubulin was purchased from Millipore, and anti-lamin B was from Invitrogen. The constructs of c-FLIP, MAPK kinase (MKK)3, MKK6, and p38a were previously described (29,31,32).…”

“…Notably, c-FLIP is essential for the survival of macrophages (27,28). In mice with c-FLIP conditional KO (CKO) in macrophages, no c-FLIP 2/2 macrophages are detectable (28,29). In contrast, an earlier study has shown that deletion of caspase-8, the c-FLIP-binding partner, impairs macrophage development but exhibits little effect on DC differentiation (30), suggesting a difference between macrophages and DCs in their sensitivity to caspase-8/c-FLIP deficiency.…”

The Inflammasome Adaptor ASC Induces Procaspase-8 Death Effector Domain Filaments

“…cFLIP L is also a tandem DEDcontaining protein but has a catalytically inactive caspase-like domain (59) and has recently been shown to promote activation of caspase-1 by the AIM2 and NLRP3 inflammasomes (60). Its function was attributed to interaction of the caspase-like domain of cFLIP L with NLRP3 and AIM2.…”