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.     

Crystal structures reveal transient PERK luminal domain tetramerization in endoplasmic reticulum stress signaling

Stress caused by accumulation of misfolded proteins within the endoplasmic reticulum (ER) elicits a cellular unfolded protein response (UPR) aimed at maintaining protein‐folding capacity. PERK, a key upstream component, recognizes ER stress via its luminal sensor/transducer domain, but the molecular events that lead to UPR activation remain unclear. Here, we describe the crystal structures of mammalian PERK luminal domains captured in dimeric state as well as in a novel tetrameric state. Small angle X‐ray scattering analysis (SAXS) supports the existence of both crystal structures also in solution. The salient feature of the tetramer interface, a helix swapped between dimers, implies transient association. Moreover, interface mutations that disrupt tetramer formation in vitro reduce phosphorylation of PERK and its target eIF2α in cells. These results suggest that transient conversion from dimeric to tetrameric state may be a key regulatory step in UPR activation.     

Modulation of endoplasmic reticulum chaperone GRP78 by high glucose in hippocampus of streptozotocin-induced diabetic mice and C6 astrocytic cells

Diabetes mellitus is known to increase the risk of neurodegeneration, and both diseases are reported to be linked to dysfunction of endoplasmic reticulum (ER). Astrocytes are important in the defense mechanism of central nervous system (CNS), with great ability of tolerating accumulation of toxic substances and sensitivity in Ca²⁺ homeostasis which are two key functions of ER. Here, we investigated the modulation of the glucose-regulated protein 78 (GRP78) in streptozotocin (STZ)-induced diabetic mice and C6 cells cultured in high glucose condition. Our results showed that more reactive astrocytes were presented in the hippocampus of STZ-induced diabetic mice. Simultaneously, decrease of GRP78 expression was found in the astrocytes of diabetic mice hippocampus.     

Inhibition of PKR protects against tunicamycin-induced apoptosis in neuroblastoma cells

Endoplasmic reticulum (ER) dysfunction is thought to play a significant role in several neurological disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, amyotrophic lateral sclerosis, cerebral ischemia, and the prion diseases. ER dysfunction can be mimicked by cellular stress signals such as disruption of calcium homeostasis, inhibition of protein glycosylation, and reduction of disulfide bonds, which results in accumulation of misfolded proteins in the ER and leads to cell death by apoptosis. Tunicamycin, which is an inhibitor of protein glycosylation, induces ER stress and apoptosis. In this study, we examined the involvement of double stranded (ds) RNA-activated protein kinase PKR in tunicamycin-induced apoptosis. We used overexpression of the trans-dominant negative, catalytically inactive mutant K296R to inhibit PKR activity in neuroblastoma cells. We demonstrate that inhibition of PKR activation in response to tunicamycin protects neuronal cells from undergoing apoptosis. Furthermore, K296R overexpressing cells show defective PKR activation, delayed eIF2α phosphorylation, dramatically delayed ATF4 expression. In addition, both caspase-3 activation and C/EBP homologous protein (CHOP, also known as GADD153) induction, which are markers of apoptotic cells, are absent from K296R overexpression cells in response to tunicamycin. These results establish that PKR activation plays a major regulatory role in induction of apoptosis in response to ER stress and indicates the potential of PKR as possible target for neuroprotective therapeutics.     

Trans-10, cis-12 conjugated linoleic acid induced cell death in human colon cancer cells through reactive oxygen species-mediated ER stress

Dietary conjugated linoleic acids (CLA) are fatty acid isomers with anticancer activities produced naturally in ruminants or from vegetable oil processing. The anticancer effects of CLA differ upon the cancer origin and the CLA isomers. In this study, we carried out to precise the effects of CLA isomers, c9,t11 and t10,c12 CLA, on mechanisms of cell death induction in colon cancer cells. We first showed that only t10,c12 CLA treatment (25 and 50 μM) for 72 h triggered apoptosis in colon cancer cells without affecting viability of normal-derived colon epithelial cells. Exposure of colon cancer cells to t10,c12 CLA activated ER stress characterized by induction of eIF2α phoshorylation, splicing of Xbp1 mRNA and CHOP expression. Furthermore, we evidenced that inhibition of CHOP expression and JNK signaling decreased t10,c12 CLA-mediated cancer cell death. Finally, we showed that CHOP induction by t10,c12 CLA was dependent on ROS production and that the anti-oxidant N-acetyl-cysteine reduced CHOP induction-dependent cell death. These results highlight that t10,c12 CLA exerts its cytotoxic effect through ROS generation and a subsequent ER stress-dependent apoptosis in colon cancer cells.