Mechanism and Repertoire of ASC-Mediated Gene Expression

Abstract

Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) is an adaptor molecule that mediates inflammatory and apoptotic signals. Although the role of ASC in caspase-1-mediated IL-1β and IL-18 maturation is well known, ASC also induces NF-κB activation and cytokine gene expression in human cells. In this study, we investigated the molecular mechanism and repertoire of ASC-induced gene expression in human cells. We found that the specific activation of ASC induced AP-1 activity, which was required for optimal IL8 promoter activity. ASC activation also induced STAT3-, but not STAT1-, IFN-stimulated gene factor 3- or NF-AT-dependent reporter gene expression. The ASC-mediated AP-1 activation was NF-κB-independent and primarily cell-autonomous response, whereas the STAT3 activation required NF-κB activation and was mediated by a factor that can act in a paracrine manner. ASC-mediated AP-1 activation was inhibited by chemical or protein inhibitors for caspase-8, caspase-8-targeting small-interfering RNA, and p38 and JNK inhibitors, but not by a caspase-1 inhibitor, caspase-9 or Fas-associated death domain protein (FADD) dominant-negative mutants, FADD- or RICK-targeting small-interfering RNAs, or a MEK inhibitor, indicating that the ASC-induced AP-1 activation is mediated by caspase-8, p38, and JNK, but does not require caspase-1, caspase-9, FADD, RICK, or ERK. DNA microarray analyses identified 75 genes that were induced by ASC activation. A large proportion of them was related to transcription (23%), inflammation (21%), or cell death (16%), indicating that ASC is a potent inducer of inflammatory and cell death-related genes. This is the first report of ASC-mediated AP-1 activation and the repertoire of genes induced downstream of ASC activation.

AIM2/ASC triggers caspase-8-dependent apoptosis in Francisella-infected caspase-1-deficient macrophages

“…36 An ASC/caspase-8-dependent pathway has been described in immortalised cell lines 37 not expressing caspase-1. 38 We have detected both ASC and caspase-1 in CD4 and CD8 T cells as well as in B cells (data not shown) indicating that caspase-1 expression is not restricted to myeloid cells. At this time, we do not know if such cells exist in a physiological context.…”

Inflammasome is a central player in the induction of obesity and insulin resistance

“…Whereas ASC may mediate effects on adipocyte hypertrophy, as observed in Casp-1 −/− animals, macrophage influx in the ASC −/− animals may partly be regulated independently of caspase-1. In this regard, recent studies reported that ASC directly mediates gene expression (28) and has a critical role in the development of experimental autoimmune encephalomyelitis and arthritis independently of the inflammasome (29, 30). However, in obesity, several of the parameters tested suggest that ASC and caspase-1 work in the same pathway.…”

The NLR Adaptor ASC/PYCARD Regulates DUSP10, Mitogen-activated Protein Kinase (MAPK), and Chemokine Induction Independent of the Inflammasome

“…It is noteworthy that a connection between ASC and the transcription factor AP1 has recently been established using a reconstituted cell system that is engineered to respond to the bacterial cell wall component muramyl dipeptide (25). AP1 is among a large number of transcriptional regulators that are downstream targets of MAPKs (26).…”

“…ASC has been proposed to regulate cytokine transcription through activation of NF-B (23,24). AP1, STAT3, ISGF3, and NF-AT have also been identified as transcriptional ASC targets in a reconstituted cell system with exogenously expressed ASC and a chimeric CARD12/NOD2 protein (25). Given the complexity of inflammatory signaling, it is likely that additional signaling pathways contribute to inflammasome-independent ASC function in macrophages.…”