The Ripoptosome, a Signaling Platform that Assembles in Response to Genotoxic Stress and Loss of IAPs

Abstract

A better understanding of the mechanisms through which anticancer drugs exert their effects is essential to improve combination therapies. While studying how genotoxic stress kills cancer cells, we discovered a large ∼2MDa cell death-inducing platform, referred to as "Ripoptosome." It contains the core components RIP1, FADD, and caspase-8, and assembles in response to genotoxic stress-induced depletion of XIAP, cIAP1 and cIAP2. Importantly, it forms independently of TNF, CD95L/FASL, TRAIL, death-receptors, and mitochondrial pathways. It also forms upon Smac-mimetic (SM) treatment without involvement of autocrine TNF. Ripoptosome assembly requires RIP1's kinase activity and can stimulate caspase-8-mediated apoptosis as well as caspase-independent necrosis. It is negatively regulated by FLIP, cIAP1, cIAP2, and XIAP. Mechanistically, IAPs target components of this complex for ubiquitylation and inactivation. Moreover, we find that etoposide-stimulated Ripoptosome formation converts proinflammatory cytokines into prodeath signals. Together, our observations shed new light on fundamental mechanisms by which chemotherapeutics may kill cancer cells.

Neisseria gonorrhoeae Modulates Cell Death in Human Endocervical Epithelial Cells through Export of Exosome-Associated cIAP2

“…We also observed increased production of IL-1␤ in the presence of BV6 following N. gonorrhoeae stimulation. These results are consistent with literature describing cIAP inhibition of the ripoptosome complex, a complex that processes IL-1␤ and leads to a caspase-independent cell death known as necroptosis (37,39,49,50). We speculate that increased expression of intracellular cIAP2 induced by gonococcal infection in epithelial cells may thus serve to prevent a highly inflammatory cell death pathway.…”

“…This suggests that N. gonorrhoeae-induced depletion of intracellular cIAP2 may lead to caspase-8 and RIP1 complex formation. One consequence of an active caspase-8/RIP1/RIP3 complex is the induction of IL-1␤ cleavage (39) As an indirect measure of complex formation, changes in IL-1␤ production due to BV6 were also measured. Treatment of BV6 alone did not affect IL-1␤ production.…”

Using natural products to promote caspase-8-dependent cancer cell death

“…This may be due to the difference in overall shape of the molecule, where the 17-beta hydroxy group of withanolide E and the alpha orientation of the lactone side chain result in a change in shape that seems crucial for TRAIL apoptosis sensitizing activity by association of FADD, RIP1, caspase-8 and c-FLIP also known as the ripoptosome [31]. Spontaneous formation of ripoptosome has been attributed to genotoxic stress induced by chemotherapies such as etopsides or downregulation of IAPs by Smac (Second mitochondria-derived activator of caspase) mimetics [32,33]. Once the ripoptosome is formed, it will trigger caspase-8 activation, resulting in apoptotic cell death initiation.…”

Necrostatin-1 blocks both RIPK1 and IDO: consequences for the study of cell death in experimental disease models

“…Indeed, despite its name, Nec-1/1-MTH-Trp can also target RIPK1-mediated apoptosis when cIAPs are blocked by Smac mimetics or are downregulated. 12,13 Conditions of caspase-8 deficiency favor necroptosis induction [14][15][16][17] in which RIPK1/RIPK3 are implicated (Figure 1b). Therefore, in pathological conditions it cannot a priori be said whether Nec-1 mediated inhibition targets RIPK1-mediated apoptosis or RIPK1-mediated necroptosis.…”