Genetics of Apoptosis

mitochondrial disruption and release of prodeath cofactors (cytochrome c, Smac/

DIABLO). The interaction of cytochrome c with Apaf-1 results in a nucleotide-

dependent conformational change that allows binding of procaspase-9. The

formation of the procaspase-9/Apaf-1/cytochrome c complex promotes the

transactivation of caspase-9. Caspase-9 activates downstream caspases (caspase-3

and caspase-7), thereby amplifying the caspase cascade and promoting apoptosis.

The death receptor-induced signaling pathway is regulated at multiple levels: (1)

Decoy receptors interfere with the interaction of death ligands with their cognate

death receptors. DcR1/TRAIL-R3 and DcR2/TRAIL-R4 compete with TRAIL-

R1/TRAIL-R2 for Apo2L/TRAIL; DcR3 binds to CD95L and competitively

inhibits the interaction of CD95 with CD95L. (2) Silencer of death domains

(SODD) inhibit the intrinsic self-aggregation of the death domain of TNFR1. (3)

The recruitment and activation of caspase-8 are inhibited by FLICE-inhibitory

protein (FLIPs). (4) Phosphorylation of BID by casein kinases (CKI and CKII)

renders BID resistant to caspase-8-mediated cleavage. (5) Sequestration of tBID

by the Bcl-2 homolog, Bcl-xL, curtails its ability to promote the allosteric activation

of BAX or BAK. (6) Inhibitor-of-apoptosis proteins (IAPs) inhibit effector caspases

(caspase-3, caspase-7, caspase-9, and caspase-6), until they are themselves

sequestered by Smac/DIABLO. NF-κB promotes the expression of c-FLIP, Bcl-xL,

and members of the IAP family (c-IAP1, C-IAP2, XIAP). By inducing the

concurrent expression of multiple antiapoptotic proteins that interrupt different

steps along the death receptor-signaling pathway, NF-κB exerts a multipronged

inhibition of death-receptor signals.

and caspase-8 to DR4 and DR5 (Kischkel et al., 2000). Regardless of the specific
mechanism employed to activate caspase-8, experiments with embryonic fibroblasts
from caspase-8-deficient mice confirm that caspase-8 is essential for initiation of
apoptosis by CD95/Fas, TNFR1, DR3, DR4, and DR5 (Varfolomeev et al., 1998).

Caspase-8-mediated activation of downstream effector caspases

Caspase-8-mediated activation of caspase-3. In some cell types (termed ‘type I’), robust
activation of caspase-8 by formation of the DISC results in the direct cleavage and
activation of the downstream effector caspase-3, which in turn, cleaves other caspases
(such as caspase-6) and vital substrates, leading to the terminal events of apoptosis
(Scaffidi et al., 1998). Such cell types can undergo apoptosis via death receptor-
induced activation of the caspase cascade independently of the mitochondria.
However, experiments with caspase-3 knockout mice indicate that while caspase-3
may serve an important role in internucleosomal DNA fragmentation, it is not
required for CD95- or TNF-induced apoptosis (Woo et al., 1998).
Caspase-8-mediated cleavage of BID—mitochondrial disruption by BAX or BAK.
Caspase-8 cleaves and activates BID (p22), a ‘BH-3 domain only’ prodeath member
of the Bcl-2 family (Wang, C.Y. et al., 1996; Li, J.H. et al., 1998; Luo et al., 1998;
Gross et al., 1999b; Roy and Nicholson, 2000). The active truncated form of BID
(tBID; p15) translocates to the outer mitochondrial membrane, where it binds and
homooligomerizes BAX or BAK, two multidomain proapoptotic members of the
Bcl-2 family (Eskes et al., 2000; Wei, M.C. et al., 2000). tBID-induced
homooligomerization of BAX or BAK results in an allosteric conformational change

6 GENETICS OF APOPTOSIS