Genetics of Apoptosis

6.1

Regulation of caspase-8 activation by FLIPs

Many viruses encode apoptosis inhibitors to avoid the host’s apoptotic response.
These include baculovirus caspase inhibitors p35 and IAPs, and gamma-her-pesvirus
encoded v-FLIPs (for viral FLICE-inhibitory proteins), which interfere with apoptosis
signaled through death receptors (reviewed in Ekert et al., 1999). Although cellular
homologs of p35 have not been found, IAP and v-FLIP-like proteins are present in
mammals. Both viral and cellular FLIPs contain two DEDs that interact with the
DED in FADD, thus inhibiting the recruitment and activation of caspase-8 by TNFR
family members (Irmler et al., 1997; Thome et al., 1997). The cellular FLIP can be
upregulated in some cell lines under critical involvement of the NF-kB pathway
(Michaeu et al., 2001 ). flip-/- embryos do not survive past day 10.5 of embryogenesis,
and they exhibit impaired heart development, a phenotype similar to that reported
for fadd-/-^ and casp8-/-^ embryos (Yeh et al., 2000; Varfolomeev et al., 1998; Zhang, J.
et al., 1998). However, unlike fadd-/- and casp8-/- cells, flip-/-embryonic fibroblasts are
highly sensitive to FasL- or TNF-induced apoptosis and show rapid induction of
caspase activities, suggesting that while FLIP cooperates with FADD and caspase-8
during embryonic development, it is also necessary for protecting cells against
apoptosis mediated by TNFR family members (Yeh et al., 2000).

6.2

Regulation of caspases by IAPs

As mentioned above, genes encoding IAP proteins were originally cloned from
baculoviruses through their ability to inhibit virus-induced apoptosis in lepidopteran
cells (Crook et al., 1993). Subsequently, IAP-like proteins have been found in yeast,
C.elegans, Drosophila, and mammals (see Chapter 3 for details). There are two IAP-
like proteins in Drosophila (DIAP-1 and -2), and several in mammals, including XIAP,
cIAP-1, cIAP-2, NAIP, and survivin (Deveraux and Reed, 1999). Each IAP protein
contains in its N-terminal region one to three copies of the baculovirus IAP repeats
(BIRs), which are required for the function of all IAPs (Deveraux and Reed, 1999;
Hay, 2000). In addition, some IAPs, including cIAP-1 and XIAP, have a ring-finger
domain that act as ubiquitin-protein ligases. Although not all IAPs act as inhibitors
of apoptosis, XIAP, and cIAP-1 and -2 can physically interact with caspases and
inhibit mature caspase-3, -7, and -9 (Chapter 3; Deveraux and Reed, 1999). The
recent structural studies suggest that the N-terminal BIR2 linker region of XIAP
occupies the catalytic site in the active caspase-3 and–7, whereas the adjacent BIR2
may help stabilize the interaction (Chai et al., 2001b; Huang et al., 2001; Riedl et
al., 2001b). In contrast, the XIAP BIR3 domain is required for inhibiting caspase-9
(Sun et al., 2000). These data suggest that distinct mechanisms govern IAP-mediated
inhibition of initiator and effector caspases.
In Drosophila, the cell death activators reaper (RPR), head involution defective
(HID), and GRIM physically interact with DIAP1 (Hay, 2000). Genetic and

THE ROLE OF CASPASES IN APOPTOSIS 41