mice are hypersensitive to TNF-α- or CD95/Fas-induced apoptosis (Yeh et al.,
2000). Akin to v-FLIP, c-FLIP appears to function as a physiologic inhibitor of death
receptor-induced apoptosis via homotypic DED-mediated interactions with FADD
and procaspase-8 (Irmler et al., 1997; Srinivasula et al., 1997). c-FLIP also interacts
with TRAF2 and receptor-interacting protein (RIP), which are responsible for
TNFR1-induced activation of NF-κB and JNK. However, c-FLIP-deficient cells do
not exhibit any change in TNF-α-induced activation of NF-κB (Yeh et al., 2000).
Conversely, NF-κB activation is required for TNF-α-induced expression of c-FLIP
(Kreuz et al., 2001). These observations indicate that NF-κB-induced expression of
c-FLIP protects cells from death receptor-induced apoptosis by preventing initiation
of the caspase cascade.
4.4
Regulation of BID cleavage and functionCaspase-8-induced cleavage of BID is required for mitochondrial amplification of
downstream caspases in response to death receptor engagement in type II cells.
Therefore, regulation of BID cleavage or activity is a mechanism of controlling death
receptor-induced apoptosis in type II cells. These regulatory mechanisms are
described below.
Regulation of BID phosphorylation and cleavage by casein kinases
I and IIBID is cleaved by caspase-8 at Asp 59, which resides in a large flexible loop between
the second and third a helices (Li.H. et al., 1998; Luo et al., 1998). This cleavage site
is located between the Thr and Ser residues which are phosphorylated by casein
kinases I and II (CKI and CKII) (Desagher et al., 2001). CKI exists as monomers of
seven isoforms encoded by distinct genes (α, β, γ1, γ2, γ3, δ, and ε) (Tuazon and
Traugh, 1991). CKII is an evolutionarily conserved holoenzyme composed of two
catalytic α (and/or α’) subunits and two regulatory β subunits (Tuazon and Traugh,
1991). Phosphorylation of BID by CKI and CKII has been reported to render BID
resistant to caspase-8-mediated cleavage (Desagher et al., 2001). Conversely, a mutant
of BID that cannot be phosphorylated at these residues is apparently more sensitive
to caspase-8-induced cleavage and more effective than wild-type BID in promoting
apoptosis. Consistent with these observations, activation of CKI and CKII reportedly
delays CD95/Fas-induced apoptosis, whereas CK inhibitors potentiate death
receptor-induced apoptosis (Desagher et al., 2001).
While these observations suggest that the protection conferred by CKs is mediated
by phosphorylation of BID, CKs may also target other proteins, such as NF-κB, that
regulate the caspase-8-BID-BAX/BAK death pathway. It is also possible that other
kinases, such as protein kinase C (PKC), may be involved in the phosphorylation of
BID. Activation of PKC by phorbol esters prevents CD95-induced cleavage of BID
and apoptosis, and this protective effect is reversed by inhibition of PKC (Holmstrom
12 GENETICS OF APOPTOSIS