Genetics of Apoptosis

Expression of death receptors

Cell-surface expression of CD95/Fas is elevated by immune activation of lymphocytes
or in response to cytokines such as interferon-γ, TNF, and CD40 ligand (CD40L)
(Leithauser et al., 1993; Krammer, 2000a) [Q4]. Likewise, immune activation of
lymphocytes results in an elevation of DR5 (Screaton et al., 1997a) and concomitant
reduction of DcR1 levels (Mongkolsapaya et al., 1998). Since immune activation and
clonal lymphoid expansion must be followed by cellular demise to preserve
homeostasis, the immune system must employ molecular mechanisms to couple
immune activation (and expression of survival proteins) with the induction of death
receptors that mediate the decay of the immune response. One mechanism by which
the immune system accomplishes this task is by employing NF-κB. While activation
of NF-κB by costimulation of lymphocytes induces death receptors (CD95, DR5)
(Ravi et al., 2001; Zheng et al., 2001), the activity of the induced receptors is held
in check by the concurrent NF-κB-mediated induction of antiapoptotic proteins
(described earlier) (Ravi et al., 2001). Engagement of the induced receptors by their
cognate ligands (expressed by activated T cells or DCs) may become capable of
inducing apoptosis following decay of NF-κB activity at the termination of immune
stimulation or by caspase-mediated inactivation of NF-κB (see below).

Caspase-mediated cleavage of antiapoptotic proteins

Once death receptors are induced and engaged, the activation of caspases is amplified
by caspase-induced proteolytic cleavage of several key antiapoptotic proteins,
including NF-κB and NF-κB-induced survival proteins. The proteins targeted by
caspases and the functional effects of such cleavage are described below.
Inactivation of NF-κB by caspase-mediated proteolysis—loss of survival gene expression.
The proteins responsible for mediating TNF-α-induced activation of NF-κB are
themselves substrates of caspases. Caspase-8-mediated cleavage of RIP at Asp^32
destroys its ability to activate IKK (Lin et al., 1999). In addition to inhibiting NF-
κB activation, the NH 2 -terminal-deficient fragment (RIPc) generated by such
cleavage promotes the assembly of TNFR1-TRADD-FADD complex and potentiates
TNF-α-induced apoptosis (Lin et al., 1999). Proteolysis of TRAF1 and TRAF2 also
results in increased sensitivity to death receptor-induced apoptosis (Duckett and
Thompson, 1997; Arch et al., 2000; Leo et al., 2001). Caspase-8-mediated cleavage
of TRAF1 at Asp^163 during TNF-α—or CD95L-induced apoptosis generates a
COOH-terminal fragment that inhibits TRAF2—or TNFR1-mediated activation
of NF-κB (Leo et al., 2001). Since the truncated protein contains a TRAF domain,
it may act as a dominant negative inhibitor of interactions of TRAF1 with either
TRAF2 or c-IAPs (Schwenzer et al., 1999).
IKKβ, the catalytic subunit responsible for the canonical pathway of NF-κB
activation, is itself inactivated by caspase-3-mediated proteolysis (at Asp^78 , Asp^214 ,
Asp^373 , and Asp^546 ) during TNF-α—or CD95-induced apoptosis (Tang et al.,
2001b). Expression of the IKKβ 1–546) fragment inhibits endogenous IKK and

DEATH RECEPTORS IN APOPTOSIS 19