Page 6 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

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Encyclopedia of Physical Science and Technology EN002G-90 May 17, 2001 20:42

544 Cell Death (Apoptosis)

TRAIL were used in human cancer therapy. Before it can to staurosporine-induced apoptosis. (3) Caspase-2 is re-
be used in a clinical setting, it would be necessary to under- quired for the formation of female germ cells and loss of
stand the molecular mechanisms by which TRAIL trans- the caspase-2 gene renders B cells resistant to granzyme
duces signals from its receptor in human hepatocytes. B-induced cell death. (4) Patients suffering from au-
toimmune lymphoproliferative syndrome (ALPS) type
II, which display immune regulatory defects, have mis-
III. APOPTOTIC PROTEASES, CASPASES sense mutations in caspase-10. Caspase-10 mutants have
less caspase activity and block Fas- and TRAILR1-
In the execution phase of apoptosis, caspase proteases are mediated apoptotic pathways. (5) Both caspase-1 −/− and
activated by various apoptotic stimuli. Caspases belong caspase-11 −/− mice are resistant to endotoxic shock,
to the cysteine protease family and were originally iden- such as results from LPS treatment, and their gene
tified as homologues of the ced-3 (cell death abnormal) products are required for the production of cytokines:
gene product (an executor of apoptosis in Caenorhabdi- caspase-1 is needed for secretion of IL-1α and caspase-11
tis elegans). Indeed, most caspases induce apoptosis if for secretion of IL-1α and β. As discussed above, each
they are overexpressed in growing cells and cell death can caspase is important for both various developmental and
be blocked by caspase-specific inhibitors. Therefore, cas- pathological processes. Knockout analyses for other cas-
pases are accepted as executors for apoptosis in mammals pase genes are still under investigation and will reveal
and this pathway appears to be conserved between species. individual in-vivo functions for each caspase in the near
Caspases are synthesized as zymogens and the active future.
enzymes are generated by proteolytic cleavage of these
caspase precursors. X-ray crystallographic studies have
shown that active caspases are heterotetramers consist- IV. SIGNAL TRANSDUCTION OF DEATH
ing of two large subunits (∼20 kDa) and two smaller FACTOR-MEDIATED APOPTOSIS
(∼10 kDa) subunits. So far, 14 caspases have been cloned
from mammalian sources and divided into three sub- As described earlier, Fas- or TNFRI-mediated apoptosis is
groups based on primary structure, phylogenetic analy- triggered by binding of the cognate death ligands (Fig. 2).
sis, and substrate specificity. Caspases cleave proteins on In general, apoptosis induced by most death factors can
the carboxyl side of aspartic acid with sequence speci- still occur in the presence of inhibitors of protein or RNA
ficity. For example, caspase-1 shows preferential cleav- synthesis, suggesting that the death factor-mediated apop-
age at Asp in the Tyr–Val–Ala–Asp sequence, whereas totic processes proceed without de novo synthesis of either
caspase-3 cleaves at latter Asp in the Asp–Glu–Val–Asp. proteins or RNAs unlike the processes involved in growth
Caspases are classified as initiator, effector and cytokine- and differentiation, and that all of components for apop-
releasing caspases, according to their functions. Initiator totic signaling are constitutively present in cells. There is
caspases include caspase-2, -8, -9, and -10. These have an essential domain required for transduction of the death
large prodomains, which interact with specific adapter signals into cells, so-called death domain (DD), found
molecules to convert the precursor to the active form. Ef- in the cytoplasmic region of both Fas and TNFRI. Im-
fector caspases, including caspase-3, -6, -7, and -14, have munoprecipitation analyses have suggested that a protein
short prodomains and are activated by active initiator cas- complex, designated as the death-inducing signaling com-
pases. This regulatory mechanism is known as a protease plex (DISC), is recruited to the cytoplasmic domain of Fas
cascade.Othercaspasesfoundinmammalsappeartoserve following the interaction of Fas with Fas ligand (Fig. 2).
as cytokine-releasing enzymes. Using the yeast-two hybrid technique with the cytoplas-
Gene disruption experiments have revealed the phys- mic region of Fas or TNFRI as baits, several molecules
iological functions of individual caspases in vivo. (1) that specifically bind to the cytoplasmic region of these
Caspase-3- and -9-deficient mice show embryonic lethal receptors have been discovered. FADD (Fas-associating
phenotypes with neuronal hyperplasia, indicating that protein with death domain)/MORT-1 is a small adapter
caspase-3 and -9 play an important role in neu- protein with a molecular mass of 26 kDa and a death do-
ronal development. Moreover, their embryonic fibrob- main at the its C-terminus. FADD is recruited to trimer-
lasts are resistant to staurosporine-, etoposide-, UV-, ized cytoplasmic region of Fas and binds to Fas via in-
and dexamethasone-induced apoptosis but not to Fas- teractions between the death domains (Fig. 2). Nuclear
mediated apoptosis. (2) Caspase-8-null mice are embry- magnetic resonance (NMR) studies have shown that the
onic lethal and established cells from these mice are re- death domain of Fas consists of six antiparallel, amphi-
sistant to cell death through death receptors including pathic α-helices arranged in a novel fold. Because there
Fas, DR3, and TNFRI, whereas these cells are sensitive are many charged groups from amino acids on the protein