Apoptosis-Associated Speck-Like Protein Containing a Caspase Recruitment Domain Is a Regulator of Procaspase-1 Activation

Abstract

Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)/target of methylation-induced silencing/PYCARD represents one of only two proteins encoded in the human genome that contains a caspase recruitment domain (CARD) together with a pyrin, AIM, ASC, and death domain-like (PAAD)/PYRIN/DAPIN domain. CARDs regulate caspase family proteases. We show here that ASC binds by its CARD to procaspase-1 and to adapter proteins involved in caspase-1 activation, thereby regulating cytokine pro-IL-1β activation by this protease in THP-1 monocytes. ASC enhances IL-1β secretion into the cell culture supernatants, at low concentrations, while suppressing at high concentrations. When expressed in HEK293 cells, ASC interferes with Cardiak/Rip2/Rick-mediated oligomerization of procaspase-1 and suppresses activation this protease, as measured by protease activity assays. Moreover, ASC also recruits procaspase-1 into ASC-formed cytosolic specks, separating it from Cardiak. We also show that expression of the PAAD/PYRIN family proteins pyrin or cryopyrin/PYPAF1/NALP3 individually inhibits IL-1β secretion but that coexpression of ASC with these proteins results in enhanced IL-1β secretion. However, expression of ASC uniformly interferes with caspase-1 activation and IL-1β secretion induced by proinflammatory stimuli such as LPS and TNF, suggesting pathway competition. Moreover, LPS and TNF induce increases in ASC mRNA and protein expression in cells of myeloid/monocytic origin, revealing another level of cross-talk of cytokine-signaling pathways with the ASC-controlled pathway. Thus, our results suggest a complex interplay of the bipartite adapter protein ASC with PAAD/PYRIN family proteins, LPS (Toll family receptors), and TNF in the regulation of procaspase-1 activation, cytokine production, and control of inflammatory responses.

Dual role of TMS1/ASC in death receptor signaling

“…Previous studies indicate that TMS1 is upregulated in response to cytokines such as IL-1b, IFN-g, TNFa and LPS in immune cells (Shiohara et al, 2002;Stehlik et al, 2003), however, the mechanism of this regulation was not determined. We found here that TMS1 is also regulated by TNFa in breast epithelial cells.…”

“…Previous studies have shown that TMS1 is induced in these cell types in response to proinflammatory stimuli including IL-1b, LPS and TNFa (Shiohara et al, 2002;Stehlik et al, 2003), although the exact mechanism of this upregulation has not been addressed. TMS1 is also expressed in many epithelial cell types (Masumoto et al, 2001).…”

TIR, CARD and PYRIN: three domains for an antimicrobial triad

“…It is well established that ASC is an important mediator of caspase-1 activation, and an homophilic CARD-CARD interaction between ASC and procaspase-1 has been reported. 118 Accordingly, macrophages from ASC-deficient mice are defective in caspase-1 activation (and IL-1b secretion) in response to TLR stimulation or activation by the Gram-negative bacterial pathogen, S. typhimurium. 67 The role of ASC in the activation of the NF-kB pathway remains somewhat controversial.…”

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

“…The inflammasome component Asc has been described as an activator of procaspase-1 21 or as a modulator, leading to activation of procaspase-1 at low and inhibition at higher concentrations. 30 As the EBBP homolog Pyrin binds to Asc, 20 we tested if this is also the case for EBBP. Therefore, we performed co-IPs of HA-tagged EBBP and Asc fusion proteins tagged with an epitope of the vesicular stomatitis virus (VSV) or with a Flag tag.…”