ASC is a pro-apoptotic protein containing a pyrin domain (PD) and a caspase-recruitment domain (CARD). A previous study suggests that ASC interacts with Ipaf, a member of the Apaf-l/Nodl protein family. However, the functional relevance of the interaction has not been determined. Here, we report that co-expression of ASC with Ipaf or oligomerization of ASC induces both apoptosis and NF-kappaB activation. Apoptosis induced through ASC was inhibited by a mutant form of Caspase-8 but not by that of Caspase-1. The PD of ASC physically interacted with Caspase-8 as well as with pyrin, the familial Mediterranean fever gene product. Caspase-8 deficiency rescued mouse fibroblasts from apoptosis induced by ASC oligomerization. Pyrin disrupted the interaction between ASC and Caspase-8, and inhibited both apoptosis and NF-kappaB activation induced by ASC. These findings suggest that ASC is a mediator of NF-kappaB activation and Caspase-8-dependent apoptosis in an Ipaf signaling pathway. (C) 2003 Elsevier Science (USA). All rights reserved.