cIAPs Block Ripoptosome Formation, a RIP1/Caspase-8 Containing Intracellular Cell Death Complex Differentially Regulated by cFLIP Isoforms

TLDR

The role of cIAPs in regulating intracellular cell death pathways remains unclear. The Ripoptosome, a 2 MDa complex centered on RIP1 and containing FADD, caspase‑8/10, and cFLIP isoforms, is necessary but not sufficient for cell death. Loss of cIAPs triggers spontaneous Ripoptosome assembly, which, depending on cFLIP isoforms, acts as a caspase rheostat to determine whether cells undergo RIP3‑dependent necroptosis or caspase‑dependent apoptosis, thereby influencing receptor‑triggered cell death and inflammatory outcomes.

Abstract

The intracellular regulation of cell death pathways by cIAPs has been enigmatic. Here we show that loss of cIAPs promotes the spontaneous formation of an intracellular platform that activates either apoptosis or necroptosis. This 2 MDa intracellular complex that we designate "Ripoptosome" is necessary but not sufficient for cell death. It contains RIP1, FADD, caspase-8, caspase-10, and caspase inhibitor cFLIP isoforms. cFLIP(L) prevents Ripoptosome formation, whereas, intriguingly, cFLIP(S) promotes Ripoptosome assembly. When cIAPs are absent, caspase activity is the "rheostat" that is controlled by cFLIP isoforms in the Ripoptosome and decides if cell death occurs by RIP3-dependent necroptosis or caspase-dependent apoptosis. RIP1 is the core component of the complex. As exemplified by our studies for TLR3 activation, our data argue that the Ripoptosome critically influences the outcome of membrane-bound receptor triggering. The differential quality of cell death mediated by the Ripoptosome may cause important pathophysiological consequences during inflammatory responses.