Hogness, 1990; DiBello et al., 1991). These transcription factors then regulate
secondary response genes. Ten hours after puparium formation, a second pulse of
ecdysone triggers salivary gland cell death, which is also accompanied by the
expression of BR-C, E74, and the additional transcription factors E93 and βFTZ-F1
(reviewed in Thummel, 1996; Baehrecke, 2000). These transcription factors have
been shown to play a role in ecdysone-mediated cell death in the salivary gland and
midgut. For example, rpr transcription in salivary glands is directly regulated by the
EcR/Usp complex (Jiang et al., 2000). BR-C is required for maximal rpr and dronc
expression (Cakouros et al., 2002; Jiang et al., 2000). BR-C and E74 are needed for
optimal induction of hid in salivary glands, whereas βFTZ-F1 is required for the
induction of diap2, and E75 represses this death inhibitor immediately before the
induction of rpr and hid (Jiang et al., 2000). In the salivary glands of E93 mutants,
rpr, hid, dark, and dronc mRNA levels are severely reduced (Lee et al., 2000). Thus,
during development and metamorphosis, upregulation of caspase RNA, along with
the regulation of other death effectors and inhibitors, may play an important role in
the synchronous removal of unwanted cells.
7.
Caspase targetsOver 100 cellular proteins are now known to be cleaved by effector caspases in cells
undergoing apoptosis. These proteins include several functional classes, such as
cytoskeletal proteins, proteins involved in DNA and RNA metabolism, protein
kinases, proinflammatory cytokines, cell-cycle regulators, apoptosis regulators, signal
transducers, transcription factors, and proteins involved in neurodegenerative
disorders (see reviews by Nicholson, 1999; Chang and Yan, 2000). Depending on
the target, the cleavage by caspases can functionally inactivate or activate a protein.
Although the functional significance of all caspase cleavage events is not known, many
of the proteins inactivated by caspase-mediated cleavage play a role in cellular
homeostasis, and their cleavage may simply result in shutting down cell-cycle and
repair machinery, while the cleavage of others is more directly associated with
morphologic changes characteristic of apoptosis. A comprehensive description of the
growing number of caspase substrates is beyond the scope of this chapter, but a few
need special mention here. Apoptosis has long been known to be associated with
cleavage of DNA in characteristic nucleosomal length fragments (Wyllie, 1980). The
DNase responsible for this fragmentation has been identified and named CAD
(caspase-activated DNase) (Enari et al., 1998). CAD normally remains bound to a
cytoplasmic inhibitor (ICAD), which is degraded by caspase-3 in apoptotic cells (Liu
et al., 1997; Sakahira et al., 1998). The free CAD then enters the nucleus and acts
upon chromatin (Sakahira et al., 1998). Cells that are devoid of caspase-3 fail to show
characteristic DNA fragmentation, although they are still able to undergo apoptosis
(Jänicke et al., 1998; Woo et al., 1998), suggesting that caspase-3 plays an essential
role in activating CAD, but characteristic DNA fragmentation is not essential for
apoptosis to proceed. Cleavage of gelsolin, an actin-associated protein, generates a
44 GENETICS OF APOPTOSIS