Genetics of Apoptosis

2.2

ER stress-induced apoptotic signal transduction pathways

Through its three signaling arms, the UPR stalls synthesis of new proteins and arrests
cells in G1 while concomitantly upregulating chaperones, allowing the refolding or
degradation of malfolded proteins within the ER. These survival signals may enable
mammalian cells to survive physiologic ER stresses that place an excessive workload
on the ER, such as the enormous secretion of immunoglobulins during differentiation
of B cells into plasma cells (Ma and Hendershot, 2001; Calfon et al., 2002).
Experimentally, ER stress is induced by pharmacologic agents that inhibit N-linked
glycosylation (tunicamycin), block ER-to-Golgi transport (brefeldin A), impair
disulfide formation (DTT), or disrupt ER Ca2+ stores (thapsigargin, an inhibitor of
the sacroplasmic/ER Ca2+ ATPase [SERCA] pumps, or A-23187, a Ca2+ ionophore).
All of these agents eventually induce apoptosis within 24–48 h depending on the cell
type, suggesting that if the damage to the ER is too great, or if balance is not restored
within a certain window of time, an apoptotic response is ultimately elicited (Patil
and Walter, 2001). This idea is supported by the observation that PERK-null MEFs
undergo increased apoptosis following ER stress because of their inability to halt
translation and the continuing buildup of unfolded proteins in the lumen (Harding
et al., 2000b). Numerous signaling molecules have been implicated in ER stress-
induced apoptosis, as outlined below.

2.3

CHOP/GADD153

CHOP, also known as GADD153, is a nuclear transcription factor that forms a
heterodimeric complex with members of the C/EBP family of transcription factors.
CHOP is coordinately upregulated with ER chaperones following ER stress,
suggesting that it is regulated by the Ire1/ATF6 pathways (Kaufman, 1999). CHOP
involvement in apoptosis is underscored by the fact that tunicamycin-induced
apoptosis is impaired in CHOP-/- MEFs, and CHOP-/- mice intraperitoneally injected
with tunicamycin show decreased apoptosis in the renal tubular epithelium (Zinszner
et al., 1998). Of note, MEFs deficient in C/EBPβ, CHOP’s main heterodimerizing
partner, show a similar resistance to tunicamycin, consistent with CHOP’s
functioning as a CHOP/C/EBPβ transcriptional complex (Zinszner et al., 1998).
Overexpression of CHOP alone does not induce apoptosis, but it does arrest growth
and sensitize cells to ER stress apoptosis. Recently, CHOP was reported
transcriptionally to downregulate endogenous Bcl-2 in Rat1 cells and MEFs
(McCullough et al., 2001). CHOP could repress a Bcl-2-cat promoter reporter
construct, and Bcl-2 protein expression was found to decrease within 16 h of CHOP
expression. At present, the mechanism by which CHOP regulates the Bcl-2 promoter,
and whether it depends on C/EBPβ, remain to be determined. In any case, since
Bcl-2 potently inhibits ER stress-induced apoptosis (McCormick et al., 1997;
Srivastava et al., 1999; Ferri and Kroemer, 2001; McCullough et al., 2001), this

100 GENETICS OF APOPTOSIS