BAD and Bcl-2 regulation are early events linking neuronal endoplasmic reticulum stress to mitochondria-mediated apoptosis

The Role of Thrombomodulin in Estrogen-Receptor-Positive Breast Cancer Progression, Metastasis, and Curcumin Sensitivity

Estrogen and estrogen receptors (ER) play a key role in breast cancer progression, which can be treated with endocrine therapy. Nevertheless, resistance to endocrine therapies is developed over time. The tumor expression of thrombomodulin (TM) is correlated with favorable prognosis in several types of cancer. However, this correlation has not yet been confirmed in ER-positive (ER⁺) breast cancer. This study aims to evaluate the role of TM in ER⁺ breast cancer. Firstly, we found that lower TM expression correlates to poor overall survival (OS) and relapse-free survival (RFS) rates in ER⁺ breast cancer patients through Kaplan-Meier survival analysis (p < 0.05). Silencing TM in MCF7 cells (TM-KD) increased cell proliferation, migration, and invasion ability. Additionally, TM-KD MCF7 cells showed higher sensitivity (IC₅₀ 15 μM) to the anti-cancer agent curcumin than the scrambled control cells. Conversely, overexpression of TM (TM-over) in T47D cells leads to decreased cell proliferation, migration, and invasion ability. Furthermore, TM-over T47D cells showed more resistance (IC₅₀ > 40 μM) to the curcumin treatment. The PI staining, DAPI, and tunnel assay also confirmed that the curcumin-induced apoptosis in TM-KD MCF7 cells was higher (90.34%) than in the scrambled control cells (48.54%). Finally, the expressions of drug-resistant genes (ABCC1, LRP1, MRP5, and MDR1) were determined by qPCR. We found that the relative mRNA expression levels of ABCC1, LRP1, and MDR1 genes after curcumin treatment were higher in scrambled control cells than in TM-KD cells. In conclusion, our results demonstrated that TM plays a suppressive role in the progression and metastasis of ER⁺ breast cancer, and it regulates curcumin sensitivity by interfering with ABCC1, LRP1, and MDR1 gene expression.

Heat shock protein 27 influences the anti-cancer effect of curcumin in colon cancer cells through ROS production and autophagy activation

The problem of therapeutic resistance and chemotherapeutic efficacy is tricky and critical in the management of colorectal cancer (CRC). Curcumin is a promising anti-cancer agent. Heat shock protein 27 (HSP27) is correlated with CRC progression and is said to affect CRC response to different therapies. However, the role of HSP27 on the therapeutic efficacy of curcumin remains unknown. HSP27 was silenced using small hairpin RNA (shRNA) technique. The cytotoxic and apoptotic effects of curcumin were assessed by sulforhodamine B (SRB) colorimetric assay, flow cytometric cell cycle analysis, and annexin V/propidium iodide (PI) double-labeling assays. Total reactive oxygen species (ROS)/superoxide and autophagy detection were performed, and the levels of apoptosis-related proteins were examined by Western blotting. It was found that the silencing of HSP27 (HSP27-KD) resulted in increased treatment resistance to curcumin in CRC cells. In addition, cell cycle analysis showed that the curcumin treatment caused cell cycle arrest at the G2/M phase in the control group, and apoptosis was reduced in the HSP27-KD group. Curcumin treatment also resulted in a decrease in anti-apoptotic proteins, p-Akt, Akt, Bcl-2 and p-Bad, and increase in pro-apoptotic proteins Bad and c-PARP levels in the control cells but not in the HSP27-KD cells. This was also followed by low reactive oxygen/nitrogen species (ROS/RNS), superoxide and autophagy induction levels in the HSP27-KD cells as compared to the control cells. Therefore, as silencing of HSP27 increases curcumin resistance by reducing apoptosis and reactive oxidative stress production, HSP27 is a potential selective target for curcumin treatment in CRC.

Phytoceramide ameliorates ß-amyloid protein-induced memory impairment and neuronal death in mice

The present study was performed to investigate the protective effect of phytoceramide against ß-amyloid protein (Aβ) (25-35)-induced memory impairment and its underlying mechanisms in mice. Memory impairment in mice was induced by intracerebroventricular injection of 15 nmol Aβ (25-35) and measured by the passive avoidance test and Morris water maze test. Chronic administration of phytoceramide (10, 25 and 50 mg/kg, p.o.) resulted in significantly less Aβ (25-35)-induced memory loss and hippocampal neuronal death in treated mice compared to controls. The decrease of glutathione level and increase of lipid peroxidation in brain tissue following injection of Aβ (25-35) was reduced by phytoceramide. Alteration of apoptosis-related proteins, increase of inflammatory factors, and phosphorylation of mitogen activated proteins kinases (MAPKs) in Aβ (25-35)-administered mice hippocampus were inhibited by phytoceramide. Phosphatidylinositol 3'-kinase (PI3K)/Akt pathway and phosphorylation of cyclic AMP response element-binding protein (CREB) were suppressed, while phosphorylation of tau (p-tau) was increased in Aß (25-35)-treated mice brain; these effects were significantly inhibited by administration of phytoceramide. These results suggest that phytoceramide has a possible therapeutic role in managing cognitive impairment associated with Alzheimer's disease. The underlying mechanism might involve inhibition of p-tau formation via anti-apoptosis and anti-inflammation activity and promotion of PI3K/Akt/CREB signaling process.

A siRNA screen reveals the prosurvival effect of protein kinase A activation in conditions of unresolved endoplasmic reticulum stress

The endoplasmic reticulum (ER) has a crucial role in the proper folding of proteins that are synthesized in the secretory pathway. Physiological and pathological conditions can induce accumulation of mis- or unfolded proteins in the ER lumen and thereby generate a state of cellular stress known as ER stress. The unfolded protein response aims at restoring protein-folding homeostasis, but turns into a toxic signal when ER stress is too severe or prolonged. ER stress-induced cellular dysfunction and death is associated with several human diseases, but the molecular mechanisms regulating death under unresolved ER stress are still unclear. We performed a siRNA-based screen to identify new regulators of ER stress-induced death and found that repression of the Carney complex-associated protein PRKAR1A specifically protected the cells from ER stress-induced apoptosis, and not from apoptosis induced by etoposide or TNF. We demonstrate that the protection results from PKA activation and associate it, at least in part, with the phosphorylation-mediated inhibition of the PKA substrate Drp1 (dynamin-related protein 1). Our results therefore provide new information on the complex regulation of cellular death under ER stress conditions and bring new insights on the conditions that regulate the pro- versus anti-death functions of PKA.

When ER stress reaches a dead end

Endoplasmic reticulum (ER) stress is a common feature of several physiological and pathological conditions affecting the function of the secretory pathway. To restore ER homeostasis, an orchestrated signaling pathway is engaged that is known as the unfolded protein response (UPR). The UPR has a primary function in stress adaptation and cell survival; however, under irreversible ER stress a switch to pro-apoptotic signaling events induces apoptosis of damaged cells. The mechanisms that initiate ER stress-dependent apoptosis are not fully understood. Several pathways have been described where we highlight the participation of the BCL-2 family of proteins and ER calcium release. In addition, recent findings also suggest that microRNAs and oxidative stress are relevant players on the transition from adaptive to cell death programs. Here we provide a global and integrated overview of the signaling networks that may determine the elimination of a cell under chronic ER stress. This article is part of a Special Section entitled: Cell Death Pathways.

Apoptosis-modulating drugs for improved cancer therapy

Resistance to cell death induction has been recognized as a hallmark of cancer. Increasing understanding of the underlying molecular events regulating different cell death mechanisms like apoptosis, endoplasmic reticulum stress, autophagy, necroptosis and others has opened new possibilities for targeted interference with these pathways. While conventional chemotherapeutic agents usually inhibit cell cycle progression, DNA replication or mitosis execution, novel agents like small molecule kinase inhibitors also target survival-related kinases and signaling pathways and contribute to overcome resistance to chemotherapy and apoptosis. Additionally, antibodies targeting cellular death receptors have been described to specifically target tumor cells only. This review briefly highlights the pathways involved in (apoptotic) cell death and summarizes the current state of development of specific modulators of cell death and how they can help to improve the tolerability of chemotherapy regimens and increase survival rates in patients with advanced cancer diseases.

Homeostatic functions of BCL-2 proteins beyond apoptosis

Since its introduction in 1930 by physiologist Walter Bradford Cannon, the concept of homeostasis remains the cardinal tenet of biologic regulation. Cells have evolved a highly integrated network of control mechanisms, including positive and negative feedback loops, to safeguard homeostasis in face of a wide range of stimuli. Such control mechanisms ultimately orchestrate cell death, division and repair in a manner concordant with cellular energy and ionic balance to achieve proper biologic fitness. The interdependence of these homeostatic pathways is also evidenced by shared control points that decode intra- and extracellular cues into defined effector responses. As critical control points of the intrinsic apoptotic pathway, the BCL-2 family of cell death regulators plays an important role in cellular homeostasis. The different anti- and pro-apoptotic members of this family form a highly selective network of functional interactions that ultimately governs the permeabilization of the mitochondrial outer membrane and subsequent release of apoptogenic factors such as cytochrome c. The advent of loss- and gain-of-function genetic models for the various BCL-2 family proteins has not only provided important insights into apoptosis mechanisms but also uncovered unanticipated roles for these proteins in other physiologic pathways beyond apoptosis (Fig. 1). Here, we turn our attention to these alternative cellular functions for BCL-2 proteins. We begin with a brief introduction of the cast of characters originally known for their capacity to regulate apoptosis and continue to highlight recent advances that have shaped and reshaped our views on their physiologic relevance in integration of apoptosis with other homeostatic pathways.

ER-mediated stress induces mitochondrial-dependent caspases activation in NT2 neuron-like cells

Recent studies have revealed that endoplasmic reticulum (ER) disturbance is involved in the pathophysiology of neurodegenerative disorders, contributing to the activation of the ER stress-mediated apoptotic pathway. Therefore, we investigated here the molecular mechanisms underlying the ER-mitochondria axis, focusing on calcium as a potential mediator of cell death signals. Using NT2 cells treated with brefeldin A or tunicamycin, we observed that ER stress induces changes in the mitochondrial function, impairing mitochondrial membrane potential and distressing mitochondrial respiratory chain complex Moreover, stress stimuli at ER level evoked calcium fluxes between ER and mitochondria. Under these conditions, ER stress activated the unfolded protein response by an overexpression of GRP78, and also caspase-4 and-2, both involved upstream of caspase-9. Our findings show that ER and mitochondria interconnection plays a prominent role in the induction of neuronal cell death under particular stress circumstances.

mTOR-dependent signalling in Alzheimer's disease

Neurodegeneration and neurofibrillary degeneration are the two main pathological mechanisms of cognitive impairments in Alzheimer's disease (AD). It is not clear what factors determine the fates of neurons during the progress of the disease. Emerging evidence has suggested that mTOR-dependent signalling is involved in the two types of degeneration in AD brains. This review focuses on the roles of mTOR-dependent signalling in the pathogenesis of AD. It summarizes the recent advancements in the understanding of its roles in neurodegeneration and neurofibrillary degeneration, as well as the evidence achieved when mTOR-related signalling components were tested as potential biomarkers of cognitive impairments in the clinical diagnosis of AD.

The endoplasmic reticulum in apoptosis and autophagy: role of the BCL-2 protein family

Apoptosis is essential for normal development and maintenance of homeostasis, and disruption of apoptotic pathways is associated with multiple disease states, including cancer. Although initially identified as central regulators of apoptosis at the level of mitochondria, an important role for BCL-2 proteins at the endoplasmic reticulum is now well established. Signaling pathways emanating from the endoplasmic reticulum (ER) are involved in apoptosis initiated by stimuli as diverse as ER stress, oncogene expression, death receptor (DR) ligation and oxidative stress, and the BCL-2 family is almost invariably implicated in the regulation of these pathways. This also includes Ca(2+)-mediated cross talk between ER and mitochondria during apoptosis, which contributes to the mitochondrial dynamics that support the core mitochondrial apoptosis pathway. In addition to the regulation of apoptosis, BCL-2 proteins at the ER also regulate autophagy, a survival pathway that limits metabolic stress, genomic instability and tumorigenesis. In cases where apoptosis is inhibited, however, prolonged autophagy can lead to cell death. This review provides an overview of ER-associated apoptotic and autophagic signaling pathways, with particular emphasis on the BCL-2 family proteins.

Closing the phenotypic gap between transformed neuronal cell lines in culture and untransformed neurons

Studies of neuronal dysfunction in the central nervous system (CNS) are frequently limited by the failure of primary neurons to propagate in vitro. Neuronal cell lines can be substituted for primary cells but they often misrepresent normal conditions. We hypothesized that a three-dimensional (3D) cell culture system would drive the phenotype of transformed neurons closer to that of untransformed cells, as has been demonstrated in non-neuronal cell lines. In our studies comparing 3D versus two-dimensional (2D) culture, neuronal SH-SY5Y (SY) cells underwent distinct morphological changes combined with a significant drop in their rate of cell division. Expression of the proto-oncogene N-myc and the RNA-binding protein HuD was decreased in 3D culture as compared to standard 2D conditions. We observed a decline in the anti-apoptotic protein Bcl-2 in 3D culture, coupled with increased expression of the pro-apoptotic proteins Bax and Bak. Moreover, thapsigargin (TG)-induced apoptosis was enhanced in the 3D cells. Microarray analysis demonstrated significantly differing mRNA levels for over 700 genes in the cells of the two culture types, and indicated that alterations in the G1/S cell-cycle progression contributed to the diminished doubling rate in the 3D-cultured SY cells. These results demonstrate that a 3D culture approach narrows the phenotypic gap between neuronal cell lines and primary neurons. The resulting cells may readily be used for in vitro research of neuronal pathogenesis.

Expanded-polyglutamine huntingtin protein suppresses the secretion and production of a chemokine (CCL5/RANTES) by astrocytes

Huntington's disease (HD) is a hereditary neurological disease caused by expended CAG repeats in the HD gene, which codes for a protein called Huntingtin (Htt). The resultant mutant Huntingtin (mHtt) forms aggregates in neurons and causes neuronal dysfunction. In astrocytes, the largest population of brain cells, mHtt also exists. We report herein that astrocyte-conditioned medium (ACM) collected from astrocytes of R6/2 mice (a mouse model of HD) caused primary cortical neurons to grow less-mature neurites, migrate more slowly, and exhibit lower calcium influx after depolarization than those maintained in wild-type (WT) ACM. Using a cytokine antibody array and ELISA assays, we demonstrated that the amount of a chemokine [chemokine (C-C motif) ligand 5 (CCL5)/regulated on activation normal T cell expressed and secreted (RANTES)] released by R6/2 astrocytes was much less than that by WT astrocytes. When cortical neurons were treated with the indicated ACM, supplementation with recombinant CCL5/RANTES ameliorated the neuronal deficiency caused by HD-ACM, whereas removing CCL5/RANTES from WT-ACM using an anti-CCL5/RANTES antibody mimicked the effects evoked by HD-ACM. Quantitative PCR and promoter analyses demonstrated that mHtt hindered the activation of the CCL5/RANTES promoter by reducing the availability of nuclear factor kappaB-p65 and, hence, reduced the transcript level of CCL5/RANTES. Moreover, ELISA assays and immunocytochemical staining revealed that mHtt retained the residual CCL5/RANTES inside R6/2 astrocytes. In line with the above findings, elevated cytosolic CCL5/RANTES levels were also observed in the brains of two mouse models of HD [R6/2 and Hdh((CAG)150)] and human HD patients. These findings suggest that mHtt hinders one major trophic function of astrocytes which might contribute to the neuronal dysfunction of HD.

Nerve growth factor blocks thapsigargin-induced apoptosis at the level of the mitochondrion via regulation of Bim

This study examined how the neurotrophin, nerve growth factor (NGF), protects PC12 cells against endoplasmic reticulum (ER) stress-induced apoptosis. ER stress was induced using thapsigargin (TG) that inhibits the sarcoplasmic/ER Ca2+-ATPase pump (SERCA) and depletes ER Ca2+ stores. NGF pre-treatment inhibited translocation of Bax to the mitochondria, loss of mitochondrial transmembrane potential, cytochrome c release, activation of caspases (-3, -7 and -9) and apoptosis induction by TG. Notably, TG also caused a marked induction of BimEL mRNA and protein, and knockdown of Bim with siRNA protected cells against TG-induced apoptosis. NGF delayed the induction and increased the phosphorylation of BimEL. NGF-mediated protection was dependent on phosphatidylinositol-3 kinase (PI3K) signalling since all above apoptotic events, including expression and phosphorylation status of BimEL protein, could be reverted by the PI3K inhibitor LY294002. In contrast, NGF had no effect on the TG-mediated induction of the unfolded protein response (increased expression of Grp78, GADD34, splicing of XBP1 mRNA) or ER stress-associated pro-apoptotic responses (induction of C/EBP homologous protein [CHOP], induction and processing of caspase-12). These data indicate that NGF-mediated protection against ER stress-induced apoptosis occurs at the level of the mitochondria by regulating induction and activation of Bim and mitochondrial translocation of Bax.

Tauroursodeoxycholic acid inhibits apoptosis induced by Z alpha-1 antitrypsin via inhibition of Bad

Z alpha-1 antitrypsin (AAT) deficiency is a genetic disease associated with accumulation of misfolded AAT in the endoplasmic reticulum (ER) of hepatocytes. ZAAT-expressing cells display ER stress responses including nuclear factor kappaB activation and apoptosis. Using an in vitro model of ZAAT ER accumulation, we investigated the mechanism of ZAAT-mediated ER-induced apoptosis and evaluated methods to inhibit this process. Here we demonstrate that expression of ZAAT, but not normal MAAT, in HEK293 cells leads to cleavage and activation of caspase-4 and induces apoptosis that is characterized by activation of caspase-3 and caspase-7 and DNA fragmentation. Similar effects are also induced using the ER agonist thapsigargin. A caspase-4-specific short interfering RNA (siRNA) does not impair ZAAT-induced caspase-3/7 activation or cell death in these cells. However, inhibition studies performed using tauroursodeoxycholic acid (TUDCA) demonstrate its ability to inhibit caspase-4 and caspase-3/7 activation, mitochondrial cytochrome c release, and caspase-3 cleavage induced by ZAAT and to promote cell survival. The mechanism by which TUDCA (tauroursodeoxycholic acid) promotes cell survival in ZAAT-expressing cells involves phosphorylation and inactivation of the proapoptotic factor Bad. TUDCA is unable to rescue cells from apoptosis or phosphorylate Bad in the presence of LY294002, a selective P-I-3-kinase inhibitor.

CONCLUSION

These data show that caspase-4 is not essential for ZAAT-induced apoptosis in HEK293 cells and implicates P-I-3-kinase and Bad as potential therapeutic targets for the liver disease associated with ZAAT deficiency.

ER stress and diseases

Proteins synthesized in the endoplasmic reticulum (ER) are properly folded with the assistance of ER chaperones. Malfolded proteins are disposed of by ER-associated protein degradation (ERAD). When the amount of unfolded protein exceeds the folding capacity of the ER, human cells activate a defense mechanism called the ER stress response, which induces expression of ER chaperones and ERAD components and transiently attenuates protein synthesis to decrease the burden on the ER. It has been revealed that three independent response pathways separately regulate induction of the expression of chaperones, ERAD components, and translational attenuation. A malfunction of the ER stress response caused by aging, genetic mutations, or environmental factors can result in various diseases such as diabetes, inflammation, and neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, and bipolar disorder, which are collectively known as 'conformational diseases'. In this review, I will summarize recent progress in this field. Molecules that regulate the ER stress response would be potential candidates for drug targets in various conformational diseases.

Nordihydroguaiaretic acid induces astroglial death via glutathione depletion

Nordihydroguaiaretic acid (NDGA) is known to cause cell death in certain cell types that is independent of its activity as a lipoxygenase inhibitor; however, the underlying mechanisms are not fully understood. In the present study, we examined the cellular responses of cultured primary astroglia to NDGA treatment. Continuous treatment of primary astroglia with 30 microM NDGA caused >85% cell death within 24 hr. Cotreatment with the lipoxygenase products 5-HETE, 12-HETE, and 15-HETE did not override the cytotoxic effects of NDGA. In assays employing the mitochondrial membrane potential-sensitive dye JC-1, NDGA was found to induce a rapid and almost complete loss of mitochondrial membrane potential. However, the mitochondrial permeability transition pore inhibitors cyclosporin A and bongkrekic acid did not block NDGA-induced astroglial death. We found that treatment with N-acetyl cysteine (NAC), glutathione (GSH), and GSH ethyl ester (GSH-EE) did inhibit NDGA-induced astroglial death. Consistently, NDGA-induced astroglial death proceeded in parallel with intracellular GSH depletion. Pretreatment with GSH-EE and NAC did not block NDGA-induced mitochondrial membrane potential loss, and there was no evidence that reactive oxygen species (ROS) production was involved in NDGA-induced astroglial death. Together, these results suggest that NDGA-induced astroglial death occurs via a mechanism that involves GSH depletion independent of lipoxygenase activity inhibition and ROS stress.

Beta-amyloid peptides induces neuronal apoptosis via a mechanism independent of unfolded protein responses

Accumulation of beta-amyloid (Abeta) peptides in senile plaques is one of the pathological hallmarks in Alzheimer's disease (AD), which can trigger apoptosis. We have previously demonstrated that Abeta triggered calcium release from the ER. Depletion of ER Ca(2+) ions has been reported leading to unfolded protein responses (UPR). While hypothesis has been made about UPR and neurodegeneration in AD, little is known about the effects of extracellular accumulation of Abeta on UPR. We have shown previously that activation of PKR in Abeta-triggered apoptosis. Since UPR can trigger PKR, our study aims to elucidate whether extracellular accumulation of Abeta peptides induce UPR in cultured neurons. Our results showed that Abeta could not trigger UPR signalings including phosphorylation of PERK, alternative cleavage of xbp-1 mRNA and induction of transcription of xbp-1 and Gadd153. Taken together, our results suggest that extracellular accumulation of Abeta peptides induce apoptosis via a mechanism independent of UPR.

Lipopolysaccharide prevents apoptosis induced by brefeldin A, an endoplasmic reticulum stress agent, in RAW 264.7 cells

The effect of lipopolysaccharide (LPS) on the cell death induced by endoplasmic reticulum (ER) stress agents in RAW 264.7 cells was studied. LPS prevented the cell death by brefeldin A, but not thapsigargin and tunicamycin. CpG DNA as well as LPS prevented brefeldin A-induced cell death whereas tumor necrosis factor-alpha or interferon-gamma did not. Brefeldin A-induced cell death was mediated with apoptotic cell death and it was significantly inhibited by LPS. LPS abolished the activation of ER stress-related caspases, such as caspases 1, 3, and 4. LPS prevented brefeldin A-induced morphological changes in RAW 264.7 cells. Further, LPS prevented brefeldin A-induced Golgi dispersion. Therefore, LPS was suggested to diminish the stress of ER/Golgi complexes induced by brefeldin A and inhibit apoptosis. The preventive action of LPS on brefeldin A-induced apoptosis is discussed.

Expression of B/K protein in the hippocampus of kainate-induced rat seizure model

B/K protein is a newly identified member of double C2-like domain protein family. We examined the expression of B/K protein in the hippocampus of kainate-induced rat seizure model. Intraperitoneal injection of kainate increased the immunoreactivity to B/K protein in the CA1 to CA3 of the hippocampus. B/K protein expression began to increase at 6 h, reached the maximum at 12 h, and then returned nearly to the normal level at 72 h after the injection of kainate (12 mg/kg), and it was also dependent on the dose of kainate between 4 and 16 mg/kg. In electron microscopic and subcellular fractionation studies, B/K protein was localized in the endoplasmic reticulum (ER) of the hippocampus. Kainate also induced the expression of BiP, a typical ER stress marker protein, in the hippocampus and the cortex, and it was coexpressed with B/K protein. Moreover, thapsigargin-induced ER stress caused upregulation of B/K protein expression in PC12 cells. In conclusion, our data showing the induction of both B/K protein expression and ER stress response in the hippocampus of kainate seizure model, and ER-specific expression and ER stress-induced expression of B/K strongly suggest the possible role of B/K protein in epileptogenesis or epilepsy-induced neuronal damage.

Upstream signaling pathways leading to the activation of double-stranded RNA-dependent serine/threonine protein kinase in beta-amyloid peptide neurotoxicity

One of the hallmarks of Alzheimer's disease is extracellular accumulation of senile plaques composed primarily of aggregated beta-amyloid (Abeta) peptide. Treatment of cultured neurons with Abeta peptide induces neuronal death in which apoptosis is suggested to be one of the mechanisms. We have demonstrated previously that Abeta peptide induces activation of double-stranded RNA-dependent serine/threonine protein kinase (PKR) and phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) in neurons in vitro. Degenerating neurons in brain tissues from Alzheimer's disease patients also displayed high immunoreactivity for phosphorylated PKR and eIF2alpha. Our previous data have also indicated that PKR plays a significant role in mediating Abeta peptide-induced neuronal death, because neurons from PKR knockout mice and neuroblastoma SH-SY5Y cells stably transfected with dominant negative mutant of PKR are less susceptible to Abeta peptide toxicity. Therefore, it is important to understand how PKR is activated by Abeta peptide. We report here that inhibition of caspase-3 activity reduces phosphorylation of PKR and to a certain extent, cleavage of PKR and eIF2alpha in neurons exposed to Abeta peptide. Calcium release from the endoplasmic reticulum and activation of caspase-8 are the upstream signals modulating the caspase-3-mediated activation of PKR by Abeta peptide. Although in other systems HSP90 serves as a repressor for PKR, it is unlikely the candidate for caspase-3 to affect PKR activation in neurons after Abeta peptide exposure. Elucidation of the upstream pathways for PKR activation can help us to understand how this kinase participates in Abeta peptide neurotoxicity and to develop effective neuroprotective strategy.

Reduction of calcium release from the endoplasmic reticulum could only provide partial neuroprotection against beta-amyloid peptide toxicity

Beta-amyloid (Abeta) peptide has been suggested to play important roles in the pathogenesis of Alzheimer's disease (AD). Abeta peptide neurotoxicity was shown to induce disturbance of cellular calcium homeostasis. However, whether modulation of calcium release from the endoplasmic reticulum (ER) can protect neurons from Abeta toxicity is not clearly defined. In the present study, Abeta peptide-triggered ER calcium release in primary cortical neurons in culture is modulated by Xestospongin C, 2-aminoethoxydiphenyl borate or FK506. Our results showed that reduction of ER calcium release can partially attenuate Abeta peptide neurotoxicity evaluated by LDH release, caspase-3 activity and quantification of apoptotic cells. While stress signals associated with perturbations of ER functions such as up-regulation of GRP78 was significantly attenuated, other signaling machinery such as activation of caspase-7 transmitting death signals from ER to other organelles could not be altered. We further provide evidence that molecular signaling in mitochondria play also a significant role in determining neuronal apoptosis because Abeta peptide-triggered activation of caspase-9 was not significantly reduced by attenuating ER calcium release. Our results suggest that neuroprotective strategies aiming at reducing Abeta toxicity should include molecular targets linked to ER perturbations associated with ER calcium release as well as mitochondrial stress.