Abstract

A role for cellular inhibitors of apoptosis (IAPs [cIAPs])
in preventing CD95 death has been suspected but
not previously explained mechanistically. In this
study, we find that the loss of cIAPs leads to a dramatic
sensitization to CD95 ligand (CD95L) killing. Surprisingly,
this form of cell death can only be blocked by a combination
of RIP1 (receptor-interacting protein 1) kinase and caspase
inhibitors. Consistently, we detect a large increase in
RIP1 levels in the CD95 death-inducing signaling complex
(DISC) and in a secondary cytoplasmic complex (complex II)
in the presence of IAP antagonists and loss of RIP1-
protected cells from CD95L/IAP antagonist–induced death.
Cells resistant to CD95L/IAP antagonist treatment could
be sensitized by short hairpin RNA–mediated knockdown
of cellular FLICE-inhibitory protein (cFLIP). However, only
cFLIPL and not cFLIPS interfered with RIP1 recruitment to the
DISC and complex II and protected cells from death. These
results demonstrate a fundamental role for RIP1 in CD95
signaling and provide support for a physiological role of
caspase-independent death receptor–mediated cell death.