Due to a variety of upstream and downstream regulatory interactions, CHOP plays an important role in
ER stress-induced
apoptosis caused by a variety of stimuli such as pathogenic microbial or viral
infections, amino acid starvation, mitochondrial stress,
neurological diseases, and
neoplastic diseases. Under normal physiological conditions, CHOP is ubiquitously present at very low levels. However, under overwhelming
ER stress conditions, the expression of CHOP rises sharply along with the activation of
apoptotic pathways in a wide variety of cells. Those processes are mainly regulated by three factors:
protein kinase RNA-like endoplasmic reticulum kinase (PERK),
activating transcription factor 6 (ATF6), and
inositol requiring protein 1 (IRE1α)
Upstream regulatory pathways During
ER stress, CHOP is mainly induced via activation of the
integrated stress response pathways through the subsequent downstream phosphorylation of a translation initiation factor,
eukaryotic initiation factor 2α (eIF2α), and induction of a transcription factor, activation transcription factor 4 (
ATF4), which converges on the
promoters of target genes, including CHOP. Integrated stress response, and thus CHOP expression, can be induced by • amino acid starvation through
general control non-derepressible-2 (
GCN2) • viral infection through the vertebrate-specific kinases -
double-stranded RNA-activated protein kinase (PKR) • iron deficiency through
heme-regulated inhibitor kinase (
HRI) • stress from the accumulation of unfolded or misfolded proteins in the
ER activates the
integrated stress response through
protein kinase RNA-like endoplasmic reticulum kinase (
PERK). Under ER stress, activated transmembrane protein
ATF6 translocates to the nucleus and interacts with ATF/cAMP response elements and ER stress-response elements, binding the promoters and inducing transcription of several genes involved in
unfolded protein response (including CHOP,
XBP1 and others). Thus,
ATF6 activates the transcription of both CHOP and
XBP-1, while
XBP-1 can also upregulate the expression of CHOP. ER stress also stimulates transmembrane protein IRE1α activity. Upon activation, IRE1α splices the XBP-1 mRNA introns to produce a mature and active XBP-1 protein, that upregulates CHOP expression
IRE1α also stimulates the activation of the
apoptotic-signaling kinase-1 (ASK1), which then activates the downstream kinases,
Jun-N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (
p38 MAPK), which participate in
apoptosis induction along with CHOP. The
P38 MAP kinase family phosphorylates Ser78 and Ser81 of CHOP, which induces cell apoptosis. Moreover, research studies found that the
JNK inhibitors can suppress CHOP upregulation, indicating that
JNK activation is also involved in the modulation of CHOP levels.
Downstream apoptotic pathways Mitochondria-dependent As a transcription factor, CHOP can regulate the expression of many anti-apoptotic and
pro-apoptotic genes, including genes encoding the
BCL2-family proteins,
GADD34 and
TRB-3. In the CHOP-induced apoptotic pathway, CHOP regulates the expression of
BCL2 protein family, that includes anti-apoptotic proteins (
BCL2,
BCL-XL,
MCL-1, and
BCL-W) and pro-apoptotic proteins (BAK,
BAX, BOK,
BIM, PUMA and others). Under ER stress, CHOP can function as either a
transcriptional activator or
repressor. It forms
heterodimers with other C/EBP family transcription factors via bZIP-domain interactions to inhibit the expression of genes responsive to C/EBP family transcription factors, while enhancing the expression of other genes containing a specific 12–14 bp DNA
cis-acting element. CHOP can
downregulate the expressions of anti-apoptotic
BCL2 proteins, and
upregulate the expression of proapoptotic proteins (BIM, BAK and BAX expression). BAX-BAK oligomerization causes
cytochrome c and
apoptosis-inducing factor (AIF) release from mitochondria, eventually causing
cell death.
TRB3 pseudokinase is upregulated by the ER stress-inducible transcriptional factor,
ATF4-CHOP. CHOP interacts with TRB3, which contributes to the induction of apoptosis. The expression of TRB3 has a pro-apoptotic capacity. Therefore, CHOP also regulates apoptosis by upregulating the expression of the TRB3 gene.
Death-receptor dependent Death receptor-mediated apoptosis occurs via activation of death ligands (Fas, TNF, and TRAIL) and
death receptors. Upon activation, the receptor protein,
Fas-associated death domain protein, forms a
death-inducing signaling complex, which activates the downstream
caspase cascade to induce
apoptosis. The PERK-ATF4-CHOP pathway can induce
apoptosis by binding to the
death receptors and upregulating the expression of
death receptor 4 (DR4) and
DR5. CHOP also interacts with the phosphorylated transcription factor
JUN to form a complex that binds to the promoter region of
DR4 in lung cancer cells. Under prolonged ER stress conditions, activation of the PERK-CHOP pathway will permit
DR5 protein levels to rise, which accelerates the formation of the
death-inducing signaling complex (DISC) and activates
caspase-8, leading to
apoptosis Other downstream pathways In addition, CHOP also mediates apoptosis through increasing the expression of the
ERO1α (ER reductase) The
overexpression of CHOP can lead to
cell cycle arrest and result in cell apoptosis. At the same time, CHOP-induced apoptosis can also trigger cell death by inhibiting the expression of
cell cycle regulatory protein, p21. The
p21 protein inhibits the
G1 phase of the cell cycle as well as regulates the activity of pre-apoptotic factors. Identified CHOP-p21 relationship may play a role in changing the cell state from adapting to ER stress towards pre-apoptotic activity. Under most conditions, CHOP can directly bind to the
promoters of
downstream related genes. However, under specific conditions, CHOP can cooperate with other
transcription factors to affect apoptosis. Recent studies have shown that
Bcl-2-associated athanogene 5 (Bag5) is over-expressed in
prostate cancer and inhibits ER stress-induced apoptosis. Overexpression of
Bag5 results in decreased CHOP and
BAX expression, and increased
Bcl-2 gene expression.
Bag5 overexpression inhibited ER stress-induced apoptosis in the
unfolded protein response by suppressing
PERK-
eIF2-
ATF4 and enhancing the
IRE1-
Xbp1 activity. In general, the downstream targets of CHOP regulate the activation of apoptotic pathways, however, the molecular interaction mechanisms behind those processes remain to be discovered. == Interactions ==