Ischemia-induced neuronal cell death is mediated by the endoplasmic reticulum stress pathway involving CHOP

Brain ischemia induces apoptosis in neuronal cells, but the mechanism is not well understood. When wild-type mice were subjected to bilateral common carotid arteries occlusion (BCCAO) for 15 min, apoptosis-associated morphological changes and appearance of TUNEL-positive cells were observed in the striatum and in the hippocampus at 48 h after occlusion. RT-PCR analysis revealed that mRNAs for ER stress-associated proapoptotic factor CHOP and an ER chaperone BiP are markedly induced at 12 h after BCCAO. Immunohistochemical analysis showed that CHOP protein is induced in nuclei of damaged neurons at 24 h after occlusion. In contrast, ischemia-associated apoptotic loss of neurons was decreased in CHOP(-/-) mice. Primary hippocampal neurons from CHOP(-/-) mice were more resistant to hypoxia-reoxygenation-induced apoptosis than those from wild-type animals. These results indicate that ischemia-induced neuronal cell death is mediated by the ER stress pathway involving CHOP induction.     

The double-strand RNA-dependent protein kinase PKR plays a significant role in a sustained ER stress-induced apoptosis

Sustained ER stress leads to apoptosis. However, the exact mechanism still remains to be elucidated. Here, we demonstrate that the double strand RNA-dependent protein kinase (PKR) is involved in the ER stress-mediated signaling pathway. ER stress rapidly activated PKR, inducing the phosphorylation of eIF2alpha, followed by the activation of the ATF4/CHOP pathway. ER-stress-mediated eIF2alpha/ATF4/CHOP signaling and associated cell death was markedly reduced by PKR knockdown. We also found that PKR activation was mediated by PACT, the expression of which was elevated by ER-stress. These results indicate that the ER-stress-mediated eIF2alpha/ATF4/CHOP/cell death pathway is, to some degree, dependent on PACT-mediated PKR activation apart from the PERK pathway.     

Amyloid precursor protein promotes endoplasmic reticulum stress-induced cell death via C/EBP homologous protein-mediated pathway

The accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) is known to activate the ER, which is termed ER stress. Here, we demonstrated that amyloid precursor protein (APP) is a novel mediator of ER stress-induced apoptosis through the C/EBP homologous protein (CHOP) pathway. Expression of APP mRNA was elevated by tunicamycin- or dithiothreitol-induced ER stress. The levels of C83 and APP intracellular domain (AICD) fragments, which are cleaved from APP, were significantly increased under ER stress, although the protein level of full-length APP was decreased. Cellular viability was reduced in APP-over-expressing cells, which was attenuated by treatment with a γ-secretase inhibitor, N -[ N -(3,5-difluorophenacetyl)-L-alanyl]- S -phenylglycine t -butyl ester (DAPT). Cellular viability was also reduced in AICD-FLAG-over-expressing cells. The mRNA and protein levels of CHOP, an ER stress-responsive gene, were remarkably increased by APP over-expression, which was attenuated by treatment with DAPT. CHOP mRNA induction was also found in AICD-FLAG-over-expressing cells. Cell death and CHOP up-regulation by ER stress were attenuated by APP knockdown. Data obtained with a luciferase assay and chromatin immunoprecipitation assay indicated that AICD associates with the promoter region of the CHOP gene. In conclusion, ER stress-induced APP undergoes α- and γ-secretase cleavage and subsequently induces CHOP-mediated cell death.     

Activation of the UPR Protects against Cigarette Smoke-induced RPE Apoptosis through Up-Regulation of Nrf2

Recent studies have revealed a role of endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) in the regulation of RPE cell activity and survival. Herein, we examined the mechanisms by which the UPR modulates apoptotic signaling in human RPE cells challenged with cigarette smoking extract (CSE). Our results show that CSE exposure induced a dose- and time-dependent increase in ER stress markers, enhanced reactive oxygen species (ROS), mitochondrial fragmentation, and apoptosis of RPE cells. These changes were prevented by the anti-oxidant NAC or chemical chaperone TMAO, suggesting a close interaction between oxidative and ER stress in CSE-induced apoptosis. To decipher the role of the UPR, overexpression or down-regulation of XBP1 and CHOP genes was manipulated by adenovirus or siRNA. Overexpressing XBP1 protected against CSE-induced apoptosis by reducing CHOP, p-p38, and caspase-3 activation. In contrast, XBP1 knockdown sensitized the cells to CSE-induced apoptosis, which is likely through a CHOP-independent pathway. Surprisingly, knockdown of CHOP reduced p-eIF2α and Nrf2 resulting in a marked increase in caspase-3 activation and apoptosis. Furthermore, Nrf2 inhibition increased ER stress and exacerbated cell apoptosis, while Nrf2 overexpression reduced CHOP and protected RPE cells. Our data suggest that although CHOP may function as a pro-apoptotic gene during ER stress, it is also required for Nrf2 up-regulation and RPE cell survival. In addition, enhancing Nrf2 and XBP1 activity may help reduce oxidative and ER stress and protect RPE cells from cigarette smoke-induced damage.     

Bufotalin-induced apoptosis in osteoblastoma cells is associated with endoplasmic reticulum stress activation

The search for novel and more efficient chemo-agents against malignant osteoblastoma is important. In this study, we examined the potential anti-osteoblastoma function of bufotalin, and studied the underlying mechanisms. Our results showed that bufotalin induced osteoblastoma cell death and apoptosis in dose- and time-dependent manners. Further, bufotalin induced endoplasmic reticulum (ER) stress activation in osteoblastoma cells, the latter was detected by the induction of C/EBP homologous protein (CHOP), phosphorylation of inositol-requiring enzyme 1 (IRE1) and PKR-like endoplasmic reticulum kinase (PERK), as well as caspase-12 activation. Conversely, the ER stress inhibitor salubrinal, the caspase-12 inhibitor z-ATAD-fmk as well as CHOP depletion by shRNA significantly inhibited bufotalin-induced osteoblastoma cell death and apoptosis. Finally, by using a mice xenograft model, we demonstrated that bufotalin inhibited U2OS osteoblastoma cell growth in vivo. In summary, our results suggest that ER stress contributes to bufotalin-induced apoptosis in osteoblastoma cells. Bufotalin might be investigated as a novel anti-osteoblastoma agent.     

Hypothermia Protects the Brain from Transient Global Ischemia/Reperfusion by Attenuating Endoplasmic Reticulum Response-Induced Apoptosis through CHOP

Endoplasmic reticulum (ER) stress has been implicated in the pathology of cerebral ischemia. Apoptotic cell death occurs during prolonged period of stress or when the adaptive response fails. Hypothermia blocked the TNF or Fas-mediated extrinsic apoptosis pathway and the mitochondria pathway of apoptosis, however, whether hypothermia can block endoplasmic reticulum mediated apoptosis is never known. This study aimed to elucidate whether hypothermia attenuates brain cerebral ischemia/reperfusion (I/R) damage by suppressing ER stress-induced apoptosis. A 15 min global cerebral ischemia rat model was used in this study. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) positive cells in hippocampus CA1 were assessed after reperfusion of the brain. The expressions of C/EBP-homolo gous protein (CHOP) and glucose-regulated protein 78 (GRP78) in ischemic hippocampus CA1 were measured at 6, 12, 24 and 48 h after reperfusion. The results showed that hypothermia significantly attenuated brain I/R injury, as shown by reduction in cell apoptosis, CHOP expression, and increase in GRP78 expression. These results suggest that hypothermia could protect brain from I/R injury by suppressing ER stress-induced apoptosis.     

Fanconi DNA repair pathway is required for survival and long-term maintenance of neural progenitors

Although brain development abnormalities and brain cancer predisposition have been reported in some Fanconi patients, the possible role of Fanconi DNA repair pathway during neurogenesis is unclear. We thus addressed the role of fanca and fancg, which are involved in the activation of Fanconi pathway, in neural stem and progenitor cells during brain development and adult neurogenesis. Fanca(-/-) and fancg(-/-) mice presented with microcephalies and a decreased neuronal production in developing cortex and adult brain. Apoptosis of embryonic neural progenitors, but not that of postmitotic neurons, was increased in the neocortex of fanca(-/-) and fancg(-/-) mice and was correlated with chromosomal instability. In adult Fanconi mice, we showed a reduced proliferation of neural progenitor cells related to apoptosis and accentuated neural stem cells exhaustion with ageing. In addition, embryonic and adult Fanconi neural stem cells showed a reduced capacity to self-renew in vitro. Our study demonstrates a critical role for Fanconi pathway in neural stem and progenitor cells during developmental and adult neurogenesis.     

Rottlerin induces pro-apoptotic endoplasmic reticulum stress through the protein kinase C-δ-independent pathway in human colon cancer cells

Rottlerin, a compound reported to be a PKC δ-selective inhibitor, has been shown to induce growth arrest or apoptosis of human cancer cell lines. In our study, rottlerin dose-dependently induced apoptotic cell death in colon carcinoma cells. Treatment of HT29 human colon carcinoma cells with rottlerin was found to induce a number of signature ER stress markers; phosphorylation of eukaryotic initiation factor-2α (eIF-2α), ER stress-specific XBP1 splicing, and up-regulation of glucose-regulated protein (GRP)-78 and CCAAT/enhancer-binding protein-homologous protein (CHOP). However, suppression of PKC δ expression by siRNA or overexpression of WT-PKC δ and DN-PKC δ did not abrogate the rottlerin-mediated induction of CHOP. These results suggest that rottlerin induces up-regulation of CHOP via PKC δ-independent pathway. Furthermore, down-regulation of CHOP expression using CHOP siRNA attenuated rottlerin-induced apoptosis. Taken together, the present study thus provides strong evidence to support an important role of ER stress response in mediating the rottlerin-induced apoptosis.     

Aberrant, differential and bidirectional regulation of the unfolded protein response towards cell survival by 3'-deoxyadenosine

The unfolded protein response (UPR) is involved in a diverse range of pathologies triggered by endoplasmic reticulum (ER) stress. Endeavor to seek selective regulators of the UPR is a promising challenge towards therapeutic intervention in ER stress-related disorders. In the present report, we describe aberrant, differential and bidirectional regulation of the UPR by 3'-deoxyadenosine (cordycepin) towards cell survival. 3'-Deoxyadenosine blocked ER stress-induced apoptosis via inhibiting the IRE1-JNK pro-apoptotic pathway. 3'-Deoxyadenosine also inhibited apoptosis through reinforcement of the pro-survival eIF2α signaling without affecting PERK activity. It was associated with depression of GADD34 that dephosphorylates eIF2α, and dephosphorylation of eIF2α by salubrinal mimicked the anti-apoptotic effect of 3'-deoxyadenosine. Unexpectedly, although 3'-deoxyadenosine caused activation of eIF2α, it inhibited downstream pro-apoptotic events including induction of ATF4 and expression of CHOP. Cooperation of adenosine transporter and A3 adenosine receptor, but not A1/A2 receptors, mediated the pluripotent effects of 3'-deoxyadenosine. In mice, ER stress caused activation of JNK, expression of CHOP and induction of apoptosis in renal tubules. The apoptosis was significantly attenuated by administration with 3'-deoxyadenosine, and it was correlated with blunted induction of JNK and CHOP in the kidney. These results disclosed atypical pro-survival regulation of the UPR by 3'-deoxyadenosine, which may be advantageous for the treatment of intractable, ER stress-related disorders.