Caspase-8 can be activated by interchain proteolysis without receptor-triggered dimerization during drug-induced apoptosis

Proteases of the Caspase family are thought to be activated by proteolytic processing of their inactive zymogens. However, although proteolytic cleavage is sufficient for executioner caspases, a different mechanism has been recently proposed for initiator caspases, such as Caspase-8, which are believed to be activated by proximity-induced dimerization. According to this model, dimerization rather than proteolytic processing is considered as the critical event for Caspase-8 activation. Such a mechanism would suggest that in the absence of a dimerization platform such as the death-inducing signaling complex, Caspase-8 proteolytic cleavage would result in an inactive Enzyme. As several studies have described Caspase-8 cleavage during mitochondrial Apoptosis, we now investigated whether Caspase-8 becomes indeed catalytically active in this pathway. Using an in vivo affinity labeling approach, we demonstrate that Caspase-8 is activated in etoposide-treated cells in vivo in the absence of the receptor-induced death-inducing signaling complex formation. Furthermore, we show that both Caspase-3 and -6 are required for the efficient activation of Caspase-8. Our data therefore indicate that interchain cleavage of Caspase-8 in the mitochondrial pathway is sufficient to produce an active Enzyme even in the absence of receptor-driven procaspase-8 dimerization.