The BH3-only proteins share sequence homology with Bcl-2 only in the BH3
domain, which is required to interact with other Bcl-2 family members (Huang and
Strasser, 2000). These proteins appear to act as key molecules that link the upstream
signaling to the apoptotic machinery. Like EGL-1, mammalian BH3 proteins interact
with the prosurvival members of the Bcl-2 family, and perhaps inhibit their function.
These proteins are regulated in a number of different ways. For example, Noxa and
Puma are transcriptionally regulated by p53 (Oda et al., 2000; Nakano et al., 2001;
Yu et al., 2001), Bid is regulated by caspase-8-mediated cleavage (Li et al., 1998a;
Luo et al., 1998), Bad is regulated by phosphorylation-dependent binding to 14–3-3
(Datta et al., 1997), whereas Bim and Bmf are regulated by sequestration to the
cellular motor proteins (Putalakath et al., 1999; 2001). In all cases, the activation and
interaction of the BH3-only proteins with other Bcl-2 family members initiates the
caspase activation events (Chapter 4).
6.4
Regulation by transcriptionApoptosis has been generally considered as a process whereby pre-existing proteins
involved in cell death are activated by proteolytic cleavage or protein modification in
response to an apoptotic signal, resulting in the activation of the caspase cascade. In
the last few years, however, transcriptional regulation of many proteins of the
apoptotic machinery, including caspases and Bcl-2 family members, has been
demonstrated both in mammals and in Drosophila. Many mammalian caspases are
transcriptionally upregulated under certain conditions (e.g., Kinoshita et al., 1996;
Chen, M. et al., 2000; von Mering et al., 2001). However, the clearest example of
transcriptional regulation of caspase activation comes from studies in Drosophila.
During the larval/pupal metamorphosis in Drosophila, obsolete larval organs
undergo stage-specific, programmed cell death in response to steroid hormone
ecdysone (reviewed in Riddiford, 1993; Thummel, 1996; Baehrecke, 2000). At the
end of the third larval instar, a high-titer ecdysone pulse triggers puparium formation,
initiating metamorphosis and the prepupal stage of development. This is followed
by a second pulse of ecdysone, approximately 10 h after puparium formation, which
signals eversion of the adult head and defines the prepupal to pupal transition. In
response to the ecdysone pulse at the late third instar stage, apoptosis is initiated in
the larval midgut, and the larval salivary glands die in early pupae immediately after
the prepupal pulse of ecdysone. Recent studies have shown that ecdysone mediates
cell death by transcriptionally upregulating a number of death effectors, including
the caspase dronc and the Apaf-1 homologs dark, rpr, and hid, and downregulating
the apoptosis inhibitors, diap1 and diap2 (reviewed in Baehrecke, 2000).
A network of transcriptional regulators spatially and temporally control ecdysone-
mediated cell death. Ecdysone binds to its heterodimeric EcR/UsP receptor and
transcriptionally regulates a set of transcription factors. These include the Broad-
Complex (BR-C), which are zinc finger transcription factors; an Ets-like transcription
factor E74; and the orphan nuclear receptor, E75 (Burtis et al., 1990; Segraves and
THE ROLE OF CASPASES IN APOPTOSIS 43