sensitizes cells to TNF-α-induced apoptosis (Tang et al., 2001b). Conversely, over-
expression of a caspase-resistant mutant of IKKβ promotes the sustained activation
of NF-κB and prevents TNF-α-induced apoptosis. Therefore, caspase-mediated
cleavage of IKKβ may be a mechanism by which caspases terminate the activation of
NF-κB and remove the key obstacle to their own activity. Caspases may also achieve
this end by proteolytic removal of the NH 2 -terminal domain (containing the Ser^32
and Ser^36 phosphorylation residues) of iκBα, thereby generating a IκB fragment that
is resistant to TNF-α-induced degradation and functions as a super-repressor of NF-
κB activation (Barkett et al., 1997; Reuther and Baldwin, 1999).
While caspases may prevent activation of NF-κB via proteolytic inactivation of the
upstream signals (described above), ligation of death receptors can directly induce
caspase-mediated cleavage of RelA (Ravi et al., 1998a). The truncation of the
transactivation domain generates a transcriptionally inactive dominant-negative
fragment of RelA that serves as an efficient proapoptotic feedback mechanism
between caspase activation and NF-κB inactivation (Levkau et al., 1999).
These observations suggest that the protection conferred by NF-κB against death
receptor-induced apoptosis may be eliminated by caspase-mediated proteolysis of the
RIP/TRAF-IKK-IκBα-RelA pathway, thereby tilting the dynamic balance between
death receptors and NF-κB-induced survival proteins in favor of cell death. The direct
cleavage of RelA ensures the irreversible loss of NF-κB activity, resulting in the rapid
amplification of caspase activity and inevitable cell death.
Caspase-mediated cleavage of Bcl-2, Bcl-xL, and IAPs—amplification of caspase
activity. Many of the key antiapoptotic proteins that inhibit caspases are themselves
targets of caspases. Antiapoptotic members of the Bcl-2 family (Bcl-2 and Bcl-xL)
compete with proapoptotic multidomain members (BAX, BAK) for binding to tBID.
As such, Bcl-2 and Bcl-xL inhibit tBID-mediated activation of BAX/BAK, thereby
limiting mitochondrial disruption. However, both Bcl-2 and Bcl-xL are themselves
substrates for caspases. The loop domain of Bcl-2 is cleaved at Asp^34 by caspase-3 in
vitro, in cells overexpressing caspase-3, and during induction of apoptosis by death
receptors (CD95/Fas, TRAIL-R1/R2) (Cheng et al., 1997; Ravi et al., 2001). Death
receptor-induced caspase-mediated proteolytic cleavage of Bcl-2 inactivates its
survival function by removal of the BH4 domain. The carboxyl-terminal Bcl-2
cleavage product, which retains the BH3 homology and transmembrane regions,
behaves as a BAX-like death effector and potentiates apoptosis (Cheng et al., 1997).
Cleavage of Bcl-2 contributes to amplification of the caspase cascade, and cleavage-
resistant mutants of Bcl-2 confer increased protection against apoptosis. Akin to
Bcl-2, Bcl-xL is cleaved by caspases during induction of apoptosis by diverse stressful
stimuli (Clem et al., 1998; Fujita et al., 1998). Likewise, proteolytic cleavage converts
Bcl-xL into two prodeath fragments. However, it is not yet known whether Bcl-xL is
cleaved during death receptor-induced apoptosis or whether cleavage-resistant
mutants of Bcl-xL offer better protection against death receptor-induced apoptosis.
Akin to Bcl-2 family members, both c-IAP1 and XIAP are also caspase substrates
(Deveraux et al., 1999; Clem et al., 2001). Overexpression of the caspase-induced
cleavage product of c-IAP1 induces apoptosis (Clem et al., 2001). Likewise, caspase-
20 GENETICS OF APOPTOSIS