Differential activation of the inflammasome by caspase-1 adaptors ASC and Ipaf

Abstract

Specific adaptors regulate the activation of initiator caspases; for example, FADD and Apaf-1 engage caspases 8 and 9, respectively. The adaptors ASC, Ipaf and RIP2 have each been proposed to regulate caspase-1 (also called interleukin (IL)-1 converting enzyme), which is activated within the 'inflammasome', a complex comprising several adaptors. Here we show the impact of ASC-, Ipaf- or RIP2-deficiency on inflammasome function. ASC was essential for extracellular ATP-driven activation of caspase-1 in toll-like receptor (TLR)-stimulated macrophages. Accordingly, ASC-deficient macrophages exhibited defective maturation of IL-1beta and IL-18, and ASC-null mice were resistant to lipopolysaccharide-induced endotoxic shock. Furthermore, activation of caspase-1 in response to an intracellular pathogen (Salmonella typhimurium) was abrogated severely in ASC-null macrophages. Unexpectedly, Ipaf-deficient macrophages activated caspase-1 in response to TLR plus ATP stimulation but not S. typhimurium. Caspase-1 activation was not compromised by loss of RIP2. These data show that whereas ASC is key to caspase-1 activation within the inflammasome, Ipaf provides a special conduit to the inflammasome for signals triggered by intracellular pathogens. Notably, cell death triggered by stimuli that engage caspase-1 was ablated in macrophages lacking either ASC or Ipaf, suggesting a coupling between the inflammatory and cell death pathways.

GSDMD membrane pore formation constitutes the mechanism of pyroptotic cell death

“…Infection of primary murine bone marrow‐derived macrophages (BMDMs) with Salmonella enterica serovar Typhimurium ( S. typhimurium ) activates the NLRC4 inflammasome (Mariathasan et al , 2004) and results in caspase‐1‐ and GSDMD‐dependent pyroptosis and cytokine release (Appendix Fig S1C and D). To estimate the size of the GSDMD‐dependent plasma membrane pore in BMDMs, we measured cell lysis as a function of time in the presence of PEGs of increasing size (Fig 1E and F).…”

Methylation-induced silencing of ASC and the effect of expressed ASC on p53-mediated chemosensitivity in colorectal cancer

“…The significance of ASC suppression in patients with malignant diseases has not yet been well documented. Since ASC-deficient mice were born and grew normally and had no cancer development, it is suspected that ASC suppression might thus correlate with neither normal development nor carcinogenesis (Mariathasan et al, 2004). We speculated that the absence of ASC might cause resistance to p53-mediated chemosensitivity based on the findings of our in vitro experiments.…”

The estrogen-responsive B box protein: a novel enhancer of interleukin-1β secretion

“…Although most of our experiments were performed with transfected cells, several findings suggest that these results are also relevant for the endogenous proteins: (i) Asc is an essential activator of procaspase-1 in macrophages, 15 and Asc is also expressed by COS-1 cells and human primary keratinocytes (our own unpublished results); (ii) NALP1 belongs to a family of 14 members 32 and at least NALP1 is expressed by human primary keratinocytes (results not shown); (iii) proIL-1b secretion and activation is strictly dependent on the expression of procaspase-1 in vivo 8 and also in our experiments; (iv) endogenous EBBP is also secreted by primary keratinocytes and macrophages (results not shown); and (v) endogenous EBBP and IL-1b colocalize in keratinocytes and macrophages (results not shown) and interact with each other, at least in monocytes (Figure 3). Finally, downregulation of endogenous EBBP in COS-1 cells or human primary keratinocytes using siRNA reduced the level of secreted IL-1b ( Figures 5 and 6), demonstrating that also endogenous EBBP is involved in proIL-1b maturation.…”

“…This secretion-promoting effect of EBBP for caspase-1 was much stronger after cotransfection of pro-and mature IL-1b expression plasmids (lanes 13-16, 21-24, supernatant). Highest secretion was observed after coexpression of the cytokine with either EBBP or individual EBBP domains (lanes [14][15][16][22][23][24]. Surprisingly, forced expression of EBBP and pro-and to a lesser extent mature IL-1b resulted in degradation of procaspase-1 (lanes 14, 15, 22, 23, lysate and supernatant).…”

“…9 In addition, it is unclear if there is a link between procaspase-1 activation and IL-1b secretion. However, caspase-1 and the procaspase-1 activator Asc are secreted together with IL-1b from activated macrophages, 9,15 suggesting that procaspase-1 and proIL-1b activation and secretion are coupled. 5 Several inhibitors of the inflammasome are known.…”