The receptor pathway (see Chapter 1) is activated through ligand binding to
receptors in the plasma membrane (Schulze-Osthoff et al., 1998; Schmitz et al.,
2000). Activation of the Fas/CD95, TNFα, and TRAIL receptors have been shown
to trigger induction of apoptosis. Upon ligand binding, the receptors oligomerize,
presumably forming trimers. This results in the recruitment of adapter molecules to
the intracellular domains of the receptors. Inactive procaspases, such as caspase-8 or
-10, form a complex with the adapter molecules, which is followed by the proteolytic
activation of the caspases into their active forms. The active initiator caspases
subsequently activate downstream executioner caspases, such as caspase-3 or –7.
These, in turn, cleave cytosolic, structural, and nuclear proteins, triggering cellular,
structural and morphologic changes ultimately leading to cell death.
The second pathway is the mitochondrial pathway. The Bcl-2 family of proteins
controls this pathway. This group of proteins contains members with either pro- or
antiapoptotic activity. The proapoptotic members can be further subdivided into
multidomain and ‘BH3-domain-only’ proteins. The multidomain proteins, such as
Bax and Bak, exert their function through permeabilization of the outer
mitochondrial membrane, resulting in the release of proteins, including cytochrome
c, apoptosis-inducing factor (AIF), adenylate kinase, endonuclease G, and Smac/
Diablo from the intermembrane space of the mitochondria (Kluck et al., 1997; Kohler
et al., 1999; Daugas et al., 2000; Du et al., 2000; Verhagen et al., 2000; Li et al.,
2001). In the cytosol, cytochrome c forms a complex with Apaf1, dATP, and
procaspase-9, referred to as the apoptosome (Li et al., 1997; Zou et al., 1999). The
procaspase is processed into its active form, which subsequently activates downstream
caspases such as caspase-3 and -7. Thus, at the lower level, the two apoptosis pathways
converge into a common pathway. However, the upstream events regulating the
activation of the proapoptotic Bcl-2 proteins are still largely unknown. Several stimuli,
including, for example, hypoxia, production of reactive oxygen species, kinase
inhibitors such as staurosporine, UV or gamma irradiation, growth factor deprivation,
and several cytotoxic compounds, have all been shown to activate the mitochondrial
pathway. However, how the individual stimuli, which are very different in nature,
initiate and activate the signaling cascade remains unknown. In addition, the specific
function of the individual multidomain members is elusive; it is possible that various
family members are activated by specific stimuli.
The receptor and the mitochondrial pathways long appeared to be two
independent pathways. However, it is now clear that cross-talk between the two
pathways exists. This is mediated through the ‘BH3-domain-only’ protein, Bid. In
some cell types, the so-called type 2 cells, only a small amount of caspase-8 is activated
by the receptor pathway. The active caspase cleaves Bid into a C-terminal and an N-
terminal fragment (Li, H. et al., 1998; Schmitz et al., 1999). The C-terminal fragment
of Bid (tcBid) activates multidomain proteins, such as Bax and Bak (Gross et al.,
1999b; Eskes et al., 2000). In type 2 cells, the mitochondrial pathway functions as
amplification for the receptor pathway. On the contrary, in type 1 cells, a large amount
of caspase-8 is activated by the receptor pathway. The active caspase-8 then directly
124 GENETICS OF APOPTOSIS