The PAAD/PYRIN-Family Protein ASC Is a Dual Regulator of a Conserved Step in Nuclear Factor κB Activation Pathways

Abstract

Apoptosis-associated speck-like protein containing a Caspase recruitment domain (ASC) belongs to a large family of proteins that contain a Pyrin, AIM, ASC, and death domain-like (PAAD) domain (also known as PYRIN, DAPIN, Pyk). Recent data have suggested that ASC functions as an adaptor protein linking various PAAD-family proteins to pathways involved in nuclear factor (NF)-κB and pro-Caspase-1 activation. We present evidence here that the role of ASC in modulating NF-κB activation pathways is much broader than previously suspected, as it can either inhibit or activate NF-κB, depending on cellular context. While coexpression of ASC with certain PAAD-family proteins such as Pyrin and Cryopyrin increases NF-κB activity, ASC has an inhibitory influence on NF-κB activation by various proinflammatory stimuli, including tumor necrosis factor (TNF)α, interleukin 1β, and lipopolysaccharide (LPS). Elevations in ASC protein levels or of the PAAD domain of ASC suppressed activation of IκB kinases in cells exposed to pro-inflammatory stimuli. Conversely, reducing endogenous levels of ASC using siRNA enhanced TNF- and LPS-induced degradation of the IKK substrate, IκBα. Our findings suggest that ASC modulates diverse NF-κB induction pathways by acting upon the IKK complex, implying a broad role for this and similar proteins containing PAAD domains in regulation of inflammatory responses.

Dual role of TMS1/ASC in death receptor signaling

“…This is achieved through binding of TNFa to the TNFa receptor (TNFR), which in turn recruits TRADD and RIP or TRAF2 to induce activation of the IKK complex (Natoli et al, 1997;Kelliher et al, 1998) and JNK (Yeh et al, 1997) respectively. Previous reports have suggested that TMS1 can either promote or inhibit NF-kB signaling depending on cell type, the coexpression of specific adaptor proteins and/or TMS1 expression levels Manji et al, 2002;Stehlik et al, 2002;Wang et al, 2002). The vast majority of these studies have utilized ectopic expression of TMS1.…”

“…Thus, it seems that TMS1 is dispensable for NF-kB signaling. Whether it functions in a feedback mechanism to inhibit NF-kB, as suggested by some studies (Stehlik et al, 2002), remains to be determined. However, in our hands even high-level expression of TMS1 in MTMS22 cells had no effect on TNFa-or Fas-induced activation of NF-kB.…”

“…Previous studies addressing the impact of TMS1 on NF-kB activity are largely conflicting and have shown that TMS1 can promote or inhibit NF-kB activity depending on the stimulus and the co-expression of other adaptor proteins such as the PYPAFs or members of the pyrin family. For example, co-expression of TMS1 with the adaptor proteins PYPAF1/cryopyrin, PYPAF5 or PYPAF7 stimulates NF-kB activity Manji et al, 2002;Wang et al, 2002), whereas other reports indicate that overexpression of either a full-length TMS1 or the pyrin domain of TMS1 inhibits NF-kB activity induced by TNFa, Bcl-10 or Nod-1 (Stehlik et al, 2002). Most of these studies utilize overexpression of TMS1 or forced interactions between TMS1 and other adaptor proteins, which may not be physiologically relevant.…”

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

“…One possible explanation for this is the heterozygosity of cells; there are cells expressing and non-expressing ASC in cancer lesions, or there exists another mechanism for the expression of ASC, despite the methylation of its promoter region. In addition, a variety of expression levels of ASC protein, as seen in Figure 1a, might be caused by circumstances around the cells, because ASC can be induced by various kinds of stimulation, including p53 activation (Stehlik et al, 2002;Ohtsuka et al, 2004). The significance of ASC suppression in patients with malignant diseases has not yet been well documented.…”

Cryopyrin and pyrin activate caspase-1, but not NF-κB, via ASC oligomerization

“…Cryopyrin is primarily expressed in peripheral blood leukocytes. 28,29 Recent reports suggest that cryopyrin is involved in the regulation of NF-kB activation, [29][30][31][32][33] and this in turn regulates the expression of genes involved in the immune response and inflammation. 34 Cryopyrin also regulates IL-1b generation by assembling an inflammasome complex with ASC and caspase-1.…”