Genetics of Apoptosis

protection from tBID-induced apoptosis. Cells from caspase-9-/-, Apaf-/-, or
caspase-3-/- mice remain viable for 24 h after retroviral expression of tBID, but are
killed after 48 h (Cheng et al., 2001). In line with these observations, cells from
caspase-9-/- (Hakem et al., 1998; Kuida et al., 1998), Apaf-/- (Cecconi et al., 1998;
Yoshida et al., 1998), or caspase-3-/- (Kuida et al., 1996; Woo et al., 1998) mice remain
susceptible to CD95-induced apoptosis. One possible mechanism by which tBID-
mediated mitochondrial depolarization can promote death in the absence of either
cyto c/Apaf–l/caspase-9 or caspase-3 may involve the activation of redundant effector
caspases via release of Smac/DIABLO (second mitochondria-derived activator of
caspase). Smac/DIABLO promotes caspase activation by binding and antagonizing
members of the IAP (inhibitor of apoptosis) family of proteins (Du et al., 2000;
Verhagen et al., 2000). The human IAP family comprises six members which inhibit
the effector caspases-9, -3, and -7 (Deveraux et al., 1997; Deveraux et al., 1998; Goyal,
2001). The mitochondrial release of Smac/DIABLO into the cytoplasm via the
caspase-8-BID-BAX/BAK pathway may sequester IAPs and allow activation of
multiple effector caspases. The simultaneous activation of multiple redundant effector
caspases may explain why deficiency of any single caspase (caspase-9 or caspase-3) is
insufficient to block death receptor-induced apoptosis. It is also possible that
irreversible damage to the mitochondria may itself be sufficient to induce apoptosis
in a caspase-independent manner.
While these observations indicate that mitochondrial disruption via the BID-BAX/
BAK pathway is essential for death receptor-induced apoptosis of certain cell types
(hepatocytes, cancer cells), embryonic fibroblasts and thymocytes from BID-deficient
or BAX/BAK double-knockout (DKO) mice remain sensitive to CD95/Fas-induced
apoptosis (Lindsten et al., 2000; Wei et al., 2001). The precise reasons for the
differential requirement of the cross-talk between the extrinsic and intrinsic pathways
in type I and type II cells have yet to be elucidated. These may involve biochemical
differences at the receptor level and/or differences in the expression of initiator
caspases and/or antiapoptotic proteins that determine the threshold that must be
crossed for death receptors to activate downstream caspases.

Role of caspase-10 in death receptor-induced apoptosis

In addition to caspase-8, death receptors can also induce recruitment and activation
of the structurally related protein, caspase-10. In cells with endogenous expression of
both caspase-8 and caspase-10, CD95L and Apo2L/TRAIL can recruit either protein
to their DISC, where both enzymes are proteolytically activated with similar kinetics
(Kischkel et al., 2001). Caspase-10 recruitment and cleavage requires the adapter
FADD/Mort1 and DISC assembly. Cells expressing either caspase-8 or caspase-10
can undergo ligand-induced apoptosis, indicating that each caspase can initiate
apoptosis independently of the other (Kischkel et al., 2001). Thus, apoptosis signaling
by death receptors involves not only caspase-8 but also caspase-10, and both caspases
may have equally important roles in apoptosis initiation. However, caspase-8 plays

8 GENETICS OF APOPTOSIS