Bcl-2 acts subsequent to and independent of Ca2+ fluxes to inhibit apoptosis in thapsigargin- and glucocorticoid-treated mouse lymphoma cells

Neuroprotective Effect of Antiapoptotic URG7 Protein on Human Neuroblastoma Cell Line SH-SY5Y

Up-regulated Gene clone 7 (URG7) is a protein localized in the endoplasmic reticulum (ER) and overexpressed in liver cells upon hepatitis B virus (HBV) infection. Its activity has been related to the attenuation of ER stress resulting from HBV infection, promoting protein folding and ubiquitination and reducing cell apoptosis overall. While the antiapoptotic activity of URG7 in HBV-infected cells may have negative implications, this effect could be exploited positively in the field of proteinopathies, such as neurodegenerative diseases. In this work, we aimed to verify the possible contribution of URG7 as a reliever of cellular proteostasis alterations in a neuronal in vitro system. Following tunicamycin-induced ER stress, URG7 was shown to modulate different markers of the unfolded protein response (UPR) in favor of cell survival, mitigating ER stress and activating autophagy. Furthermore, URG7 promoted ubiquitination, and determined a reduction in protein aggregation, calcium release from the ER and intracellular ROS content, confirming its pro-survival activity. Therefore, in light of the results reported in this work, we hypothesize that URG7 offers activity as an ER stress reliever in a neuronal in vitro model, and we paved the way for a new approach in the treatment of neurodegenerative diseases.

Calcium signalling pathways in prostate cancer initiation and progression

Cancer cells proliferate, differentiate and migrate by repurposing physiological signalling mechanisms. In particular, altered calcium signalling is emerging as one of the most widespread adaptations in cancer cells. Remodelling of calcium signalling promotes the development of several malignancies, including prostate cancer. Gene expression data from in vitro, in vivo and bioinformatics studies using patient samples and xenografts have shown considerable changes in the expression of various components of the calcium signalling toolkit during the development of prostate cancer. Moreover, preclinical and clinical evidence suggests that altered calcium signalling is a crucial component of the molecular re-programming that drives prostate cancer progression. Evidence points to calcium signalling re-modelling, commonly involving crosstalk between calcium and other cellular signalling pathways, underpinning the onset and temporal progression of this disease. Discrete alterations in calcium signalling have been implicated in hormone-sensitive, castration-resistant and aggressive variant forms of prostate cancer. Hence, modulation of calcium signals and downstream effector molecules is a plausible therapeutic strategy for both early and late stages of prostate cancer. Based on this premise, clinical trials have been undertaken to establish the feasibility of targeting calcium signalling specifically for prostate cancer.

Type I but Not Type II Calreticulin Mutations Activate the IRE1α/XBP1 Pathway of the Unfolded Protein Response to Drive Myeloproliferative Neoplasms

Approximately 20% of patients with myeloproliferative neoplasms (MPN) harbor mutations in the gene calreticulin (CALR), with 80% of those mutations classified as either type I or type II. While type II CALR-mutant proteins retain many of the Ca2+ binding sites present in the wild-type protein, type I CALR-mutant proteins lose these residues. The functional consequences of this differential loss of Ca2+ binding sites remain unexplored. Here, we show that the loss of Ca2+ binding residues in the type I mutant CALR protein directly impairs its Ca2+ binding ability, which in turn leads to depleted endoplasmic reticulum (ER) Ca2+ and subsequent activation of the IRE1α/XBP1 pathway of the unfolded protein response. Genetic or pharmacologic inhibition of IRE1α/XBP1 signaling induces cell death in type I mutant but not type II mutant or wild-type CALR-expressing cells, and abrogates type I mutant CALR-driven MPN disease progression in vivo.

SIGNIFICANCE

Current targeted therapies for CALR-mutated MPNs are not curative and fail to differentiate between type I- versus type II-driven disease. To improve treatment strategies, it is critical to identify CALR mutation type-specific vulnerabilities. Here we show that IRE1α/XBP1 represents a unique, targetable dependency specific to type I CALR-mutated MPNs. This article is highlighted in the In This Issue feature, p. 265.

Granulocyte colony-stimulating factor reduces the endoplasmic reticulum stress in a rat model of diabetic cardiomyopathy

Prolonged endoplasmic reticulum (ER) stress contributes to the apoptosis of cardiomyocytes, which leads to the development of diabetic cardiomyopathy. Previously, we reported that the granulocyte colony-stimulating factor (G-CSF) reduces the cardiomyocyte apoptosis in diabetic cardiomyopathy; however, the precise mechanisms associated with this process are not yet fully understood. Therefore, in this study, we investigated whether the mechanism of the anti-apoptotic effect of G-CSF was associated with ER stress in a rat model of diabetic cardiomyopathy. Diabetic cardiomyopathy was induced in rats using a high-fat diet combined with the administration of a low-dose of streptozotocin. Diabetic rats were treated with G-CSF or saline for 5 days. Cardiac function was evaluated using serial echocardiography before and 4 weeks after treatment. The rate of cardiomyocyte apoptosis and the expression levels of proteins related to ER stress, including glucose-regulated protein 78 (GRP78), caspase-9, and caspase-12 were analyzed in the cardiac tissue. G-CSF treatment significantly reduced cardiomyocyte apoptosis in the diabetic myocardium and downregulated the expression levels of these proteins in diabetic rats treated with low-dose streptozotocin when compared to that in rats treated with saline. In addition, G-CSF treatment significantly downregulated the expression levels of proteins related to ER stress, such as GRP78, inositol-requiring enzyme-1α (IRE-1α), and C/EBP homologous protein (CHOP) in H9c2 cells under high glucose (HG) conditions. Moreover, G-CSF treatment significantly improved the diastolic dysfunction in serial echocardiography assessments. In conclusion, the anti-apoptotic effect of G-CSF may be associated with the downregulation of ER stress.

A comprehensive review on β-lapachone: Mechanisms, structural modifications, and therapeutic potentials

β-Lapachone (β-lap, 1), an ortho-naphthoquinone natural product isolated from the lapacho tree (Tabebuia avellanedae) in many regions of South America, has received extensive attention due to various pharmacological activities, such as antitumor, anti-Trypanosoma cruzi, anti-Mycobacterium tuberculosis, antibacterial, and antimalarial activities. Related mechanisms of β-lap have been widely investigated for a full understanding of its therapeutic potentials. Numerous derivatives of β-lap have been reported with aims to generate new chemical entities, improve the corresponding biological potency, and overcome disadvantages of its physical and chemical properties and safety profiles. This review will give insight into the pharmacological mechanisms of β-lap and provide a comprehensive understanding of its structural modifications with regard to different therapeutic potentials. The available clinical trials related to β-lap and its derivatives are also summarized.

Monocyte Response to Different Campylobacter jejuni Lysates Involves Endoplasmic Reticulum Stress and the Lysosomal⁻Mitochondrial Axis: When Cell Death Is Better Than Cell Survival

Campylobacter jejuni is a Gram-negative spiral-shaped bacterium, commonly associated with gastroenteritis in humans. It explicates its virulence also by the cytolethal distending toxin (CDT), able to cause irreversible cell cycle arrest. Infection by C. jejuni may result in the development of the Guillain⁻Barré Syndrome, an acute peripheral neuropathy. Symptoms of this disease could be caused by CDT-induced cell death and a subsequent inflammatory response. We tested C. jejuni lysates from different strains on donor monocytes: in fact, monocytes are potent producers of both pro- and anti-inflammatory cytokines, playing a major role in innate immunity and in non-specific host responses. We found, by cytometric and confocal analyses, that mitochondria and lysosomes were differently targeted: The C. jejuni strain that induced the most relevant mitochondrial alterations was the ATCC 33291, confirming an intrinsic apoptotic pathway, whereas the C. jejuni ISS 1 wild-type strain mostly induced lysosomal alterations. Lysates from all strains induced endoplasmic reticulum (ER) stress in monocytes, suggesting that ER stress was not associated with CDT but to other C. jejuni virulence factors. The ER data were consistent with an increase in cytosolic Ca2+ content induced by the lysates. On the contrary, the changes in lysosomal acidic compartments and p53 expression (occurring together from time 0, T0, to 24 h) were mainly due to CDT. The loss of p53 may prevent or impede cell death and it was not observable with the mutant strain. CDT not only was responsible for specific death effects but also seemed to promote an apoptotic stimuli-resisting pathway.

Organelle membrane derived patches: reshaping classical methods for new targets

Intracellular ion channels are involved in multiple signaling processes, including such crucial ones as regulation of cellular motility and fate. With 95% of the cellular membrane belonging to intracellular organelles, it is hard to overestimate the importance of intracellular ion channels. Multiple studies have been performed on these channels over the years, however, a unified approach allowing not only to characterize their activity but also to study their regulation by partner proteins, analogous to the patch clamp “golden standard”, is lacking. Here, we present a universal approach that combines the extraction of intracellular membrane fractions with the preparation of patchable substrates that allows to characterize these channels in endogenous protein environment and to study their regulation by partner proteins. We validate this method by characterizing activity of multiple intracellular ion channels localized to different organelles and by providing detailed electrophysiological characterization of the regulation of IP₃R activity by endogenous Bcl-2. Thus, after synthesis and reshaping of the well-established approaches, organelle membrane derived patch clamp provides the means to assess ion channels from arbitrary cellular membranes at the single channel level.

Taurine protects cisplatin induced cardiotoxicity by modulating inflammatory and endoplasmic reticulum stress responses

Oxidative stress, ER stress, inflammation, and apoptosis results in the pathogenesis of cisplatin-induced cardiotoxicity. The present study was designed to investigate the signaling mechanisms involved in the ameliorating effect of taurine, a conditionally essential amino acid, against cisplatin-mediated cardiac ER stress dependent apoptotic death and inflammation. Mice were simultaneously treated with taurine (150 mg kg^-1 body wt, i.p.) and cisplatin (10 mg kg^-1 body wt, i.p.) for a week. Cisplatin exposure significantly altered serum creatine kinase and troponin T levels. In addition, histological studies revealed disintegration in the normal radiation pattern of cardiac muscle fibers. However, taurine administration could abate such adverse effects of cisplatin. Taurine administration significantly mitigated the reactive oxygen species production, alleviated the overexpression of nuclear factor-κB (NF-κB), and inhibited the elevation of proinflammatoy cytokines, adhesion molecules, and chemokines. Cisplatin exposure resulted in the unfolded protein response (UPR)-regulated CCAAT/enhancer binding protein (CHOP) up-regulation, induction of GRP78: a marker of ER stress and eIF2α signaling. Increase in calpain-1 expression level, activation of caspase-12 and caspase-3, cleavage of the PARP protein as well as the inhibition of antiapoptotic protein Bcl-2 were reflected on cisplatin-triggered apoptosis. Taurine could, however, combat against such cisplatin induced cardiac-abnormalities. The above mentioned findings suggest that taurine plays a beneficial role in providing protection against cisplatin-induced cardiac damage by modulating inflammatory responses and ER stress. © 2016 BioFactors, 42(6):647-664, 2016.

Propofol Decreases Endoplasmic Reticulum Stress-Mediated Apoptosis in Retinal Pigment Epithelial Cells

Age-related macular degeneration (AMD) is the major cause of loss of sight globally. There is currently no effective treatment available. Retinal pigment epithelial (RPE) cells are an important part of the outer blood-retina barrier and their death is a determinant of AMD. Propofol, a common clinically used intravenous anesthetic agent, has been shown to act as an efficacious neuroprotective agent with antioxidative and anti-inflammatory properties in vivo and in vitro. However, little is known about its effects on RPE cells. The purpose of our research was to investigate whether propofol could protect RPE cells from apoptosis through endoplasmic reticulum (ER) stress–dependent pathways. To this end, prior to stimulation with thapsigargin (TG), ARPE-19 cells were pretreated with varying concentrations of propofol. A protective effect of propofol in TG-treated ARPE-9 was apparent, TUNEL and flow cytometric assays showed decreased apoptosis. We further demonstrated that propofol pretreatment attenuated or inhibited the effects caused by TG, such as upregulation of Bax, BiP, C/EBP homologous protein (CHOP), active caspase 12, and cleaved caspase 3, and downregulation of Bcl2. It also decreased the TG-induced levels of ER stress–related molecules such as p-PERK, p-eIF2α, and ATF4. Furthermore, it downregulated the expression of nuclear factor κB (NF-κB). This study elucidated novel propofol-induced cellular mechanisms for antiapoptotic activities in RPE cells undergoing ER stress and demonstrated the potential value of using propofol in the treatment of AMD.

BCL-2 modulates the unfolded protein response by enhancing splicing of X-box binding protein-1

Accumulation of unfolded proteins within the endoplasmic reticulum (ER) triggers a highly conserved stress response mechanism termed the unfolded protein response (UPR). The UPR is a complex series of signaling pathways controlled by ER localized transmembrane receptors, PERK, ATF6 and IRE1α. Following activation IRE1α splices XBP-1 mRNA facilitating the formation of a potent transcription factor, spliced XBP-1. The BCL-2 family members, BAX and BAK, in addition to the mitochondrion also localize to the ER and have been demonstrated to directly interact with IRE1α promoting its activity. In this study we show that in addition to BAX and BAK, the anti-apoptotic BCL-2 protein can regulate IRE1α activity. Enhanced splicing of XBP-1 was observed in BCL-2 overexpressing cells implicating BCL-2 in the complex regulation of IRE1α activity.

Translating endoplasmic reticulum biology into the clinic: a role for ER-targeted natural products?

ER stress has been identified as a hallmark, and sometimes trigger, of several pathologies, notably cancer, inflammation and neurodegenerative diseases like Alzheimer's and Parkinson's. Among the molecules described in literature known to affect ER function, the majority are natural products, suggesting that natural molecules may constitute a significant arsenal of chemical entities for modulating this cellular target. In this review, we will start by presenting the current knowledge of ER biology and the hallmarks of ER stress, thus paving the way for presenting the natural products that have been described as being ER modulators, either stress inducers or ER protectors. The chemistry, distribution and mechanism of action of these compounds will be presented and discussed.

Extracellular deposition of mouse senile AApoAII amyloid fibrils induced different unfolded protein responses in the liver, kidney, and heart

Mouse senile amyloidosis is a disorder in which apolipoprotein A-II deposits extracellularly in many organs as amyloid fibrils (AApoAII). In this study, we intravenously injected 1 μg of isolated AApoAII fibrils into R1.P1-Apoa2(c) mice, to induce AApoAII amyloidosis. We observed that the unfolded protein response was induced by deposition of AApoAII amyloid. We found that the mRNA and the protein expression levels of heat shock protein A5 (HSPA5; also known as glucose-regulated protein 78) were increased in the liver with AApoAII amyloid deposits. Immunohistochemistry showed that HSPA5 was only detected in hepatocytes close to AApoAII amyloid deposits. Furthermore, gene transcription of several endoplasmic reticulum (ER) stress-related proteins increased, including eukaryotic translation initiation factor 2 alpha kinase 3 (Eif2ak3), activating transcription factor 6 (Atf6), activating transcription factor 4 (Atf4), X-box-binding protein 1 splicing (Xbp1s), DNA-damage inducible transcript 3 (Ddit3), and autophagy protein 5 (Atg5). Moreover, apoptosis-positive cells were increased in the liver. Similar results were seen in the kidney but not in the heart. Our study indicates that ER stress responses differed among tissues with extracellular AApoAII amyloid fibril deposition. Although upregulated HSPA5 and the activated unfolded protein response might have roles in protecting tissues against aggregated extracellular AApoAII amyloid deposition, prolonged ER stress induced apoptosis in the liver and the kidney.

Silencing of Nicotiana benthamiana Neuroblastoma-Amplified Gene causes ER stress and cell death

BACKGROUND

Neuroblastoma Amplified Gene (NAG) was identified as a gene co-amplified with the N-myc gene, whose genomic amplification correlates with poor prognosis of neuroblastoma. Later it was found that NAG is localized in endoplasmic reticulum (ER) and is a component of the syntaxin 18 complex that is involved in Golgi-to-ER retrograde transport in human cells. Homologous sequences of NAG are found in plant databases, but its function in plant cells remains unknown.

RESULTS

Nicotiana benthamania Neuroblastoma-Amplified Gene (NbNAG) encodes a protein of 2,409 amino acids that contains the secretory pathway Sec39 domain and is mainly localized in the ER. Silencing of NbNAG by virus-induced gene silencing resulted in growth arrest and acute plant death with morphological markers of programmed cell death (PCD), which include chromatin fragmentation and modification of mitochondrial membrane potential. NbNAG deficiency caused induction of ER stress genes, disruption of the ER network, and relocation of bZIP28 transcription factor from the ER membrane to the nucleus, similar to the phenotypes of tunicamycin-induced ER stress in a plant cell. NbNAG silencing caused defects in intracellular transport of diverse cargo proteins, suggesting that a blocked secretion pathway by NbNAG deficiency causes ER stress and programmed cell death.

CONCLUSIONS

These results suggest that NAG, a conserved protein from yeast to mammals, plays an essential role in plant growth and development by modulating protein transport pathway, ER stress response and PCD.

The unfolded protein response in plants: a fundamental adaptive cellular response to internal and external stresses

In eukaryotic cells, proteins that enter the secretory pathway are translated on membrane-bound ribosomes and translocated into the endoplasmic reticulum (ER), where they are subjected to chaperone-assisted folding, post-translational modification and assembly. During the evolution of the eukaryotic cell, a homeostatic mechanism was developed to maintain the functions of the ER in the face of various internal and external stresses. The most severe stresses imposed on eukaryotic cells can induce ER stress that can overwhelm the processing capacity of the ER, leading to the accumulation of unfolded proteins in the ER lumen. To cope with this accumulation of unfolded proteins, the unfolded protein response (UPR) is activated to alter transcriptional programs through inositol-requiring enzyme 1 (IRE1) and bZIP17/28 in plants. In addition to transcriptional induction of UPR genes, quality control (QC), translational attenuation, ER-associated degradation (ERAD) and ER stress-induced apoptosis are also conserved as fundamental adaptive cellular responses to ER stress in plants. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Calcium signalling and the regulation of apoptosis

Apoptotic cell death is characterized by cell shrinkage, chromatin condensation and fragmentation, formation of apoptotic bodies and phagocytosis (Kerr et al., 1972). At the molecular level, activation of a family of cysteine proteases, caspases, related to interleukin-1beta-converting enzyme is believed to be a crucial event in apoptosis. This is associated with the proteolysis of nuclear and cytoskeletal proteins, cell shrinkage, glutathione efflux, exposure of phosphatidylserine on the cell surface, membrane blebbing, etc. In CD95- or TNF-mediated apoptosis, the proteolytic cascade is believed to be triggered directly by caspase binding to the activated plasma membrane receptor complex. In other forms of apoptosis, the mechanisms of activation of the proteolytic cascade are less well established but may involve imported proteases, such as granzyme B, or factors released from the mitochondria and, possibly, other organelles. Recently, the possibility that cytochrome c released from the mitochondria may serve to activate dormant caspases in the cytosol, and thereby to propagate the apoptotic process, has attracted considerable attention. A perturbation of intracellular Ca(2+) homeostasis has been found to trigger apoptosis in many experimental systems, and the apoptotic process has been related to either a sustained increase in cytosolic free Ca(2+) level or a depletion of intracellular Ca(2+) stores. Although many of the biochemical events involved in the apoptotic process are Ca(2+) dependent, the exact mechanism by which Ca(2+) triggers apoptosis remains unknown. The bcl-2 gene family, which includes both inhibitors and inducers of apoptosis, appears to regulate intracellular Ca(2+) compartmentalization. The induction of apoptosis by Ca(2+)-mobilizing agents results in caspase activation, which is similar to what is seen with other inducers of apoptosis. In addition, Ca(2+)-dependent proteases, such as calpain and a Ca(2+)-dependent nuclear scaffold-associated serine protease, are also activated by Ca(2+) signalling in some cell types where they appear to be involved in alpha-fodrin and lamin beta cleavage, respectively. Thus, a spectrum of proteases are activated during apoptosis depending on both cell type and inducer. This proteolytic cascade can involve both caspases and Ca(2+)-dependent proteases, which seem to interact during the apoptotic process.

Participation of endoplasmic reticulum stress in the pathogenesis of spontaneous glomerulosclerosis--role of intra-renal angiotensin system

Endoplasmic reticulum (ER) is the site of synthesis, folding, assembly, and degradation of proteins. Disruption of ER function leads to ER stress, which is marked by accumulation of unfolded proteins in the ER lumen. Detection of unfolded proteins by the ER membrane receptors triggers the "unfolded protein response (UPR)" designed to restore ER function via activation of the adaptive/cytoprotective responses. Failure of UPR or persistent stress triggers activation of ER stress-mediated apoptotic pathway. Several in vivo and in vitro studies have demonstrated the association of ER stress with glomerular diseases. Imai rats develop progressive glomerulosclerosis (GS), which is associated with oxidative stress, inflammation and activation of intra-renal angiotensin system, and can be prevented by AT-1 receptor blockade (ARB). Since persistent oxidative and inflammatory stresses trigger ER stress-induced apoptosis and tissue injury, we hypothesized that kidneys in the Imai rats may exhibit failure of the adaptive and activation of the apoptotic ER stress responses, which could be prevented by ARB. To this end 10-week old Imai rats were randomized to untreated and ARB-treated groups and observed for 24 weeks. At age 34 weeks, untreated rats showed heavy proteinuria, azotemia, advanced GS, impaired ER stress adaptive/cytoprotective responses (depletion of UPR-mediating proteins), and activation of ER stress apoptotic responses. ARB treatment attenuated GS, suppressed intra-renal oxidative stress, restored ER-associated adaptive/cytoprotective system, and prevented the ER stress mediated apoptotic response in this model. Thus, progressive GS in Imai rats is accompanied by activation of ER stress-associated apoptosis, which can be prevented by ARB.

Induction of apoptosis by 11-dehydrosinulariolide via mitochondrial dysregulation and ER stress pathways in human melanoma cells

In this study the isolated compound 11-dehydrosinulariolide from soft coral Sinularia leptoclados possessed anti-proliferative, anti-migratory and apoptosis-inducing activities against A2058 melanoma cells. Anti-tumor effects of 11-dehydrosinulariolide were determined by MTT assay, cell migration assay and flow cytometry. Growth and migration of melanoma cells were dose-dependently inhibited by 2–8 μg/mL 11-dehydrosinulariolide. Flow cytometric data indicated that 11-dehydrosinulariolide induces both early and late apoptosis in melanoma cells. It was found that the apoptosis induced by 11-dehydrosinulariolide is relevant to mitochondrial-mediated apoptosis via caspase-dependent pathways, elucidated by loss of mitochondrial membrane potential (∆Ψm), release of cytochrome C, activation of caspase-3/-9 and Bax as well as suppression of Bcl-2/Bcl-xL. The cleavage of PARP-1 suggested partial involvement of caspase-independent pathways. Immunoblotting data displayed up-regulations of PERK/eIF2α/ATF4/CHOP and ATF6/CHOP coupling with elevation of ER stress chaperones GRP78, GRP94, calnexin, calreticulin and PDI, implicating the involvement of these factors in ER stress-mediated apoptosis induced by 11-dehydrosinulariolide. The abolishment of apoptotic events after pre-treatment with salubrinal indicated that ER stress-mediated apoptosis is also induced by 11-dehydrosinulariolide against melanoma cells. The data in this study suggest that 11-dehydrosinulariolide potentially induces apoptosis against melanoma cells via mitochondrial dysregulation and ER stress pathways.

Stress management at the ER: regulators of ER stress-induced apoptosis

The endoplasmic reticulum (ER) is an elaborate cellular organelle essential for cell function and survival. Conditions that interfere with ER function lead to the accumulation and aggregation of unfolded proteins which are detected by ER transmembrane receptors that initiate the unfolded protein response (UPR) to restore normal ER function. If the ER stress is prolonged, or the adaptive response fails, apoptotic cell death ensues. Many studies have focused on how this failure initiates apoptosis, particularly because ER stress-induced apoptosis is implicated in the pathophysiology of several neurodegenerative and cardiovascular diseases. In this review we aim to shed light on the proteins that are not core components of the UPR signaling pathway but which can influence the course of the ER stress response by regulating the switch from the adaptive phase to apoptosis.

Tl(I) and Tl(III) activate both mitochondrial and extrinsic pathways of apoptosis in rat pheochromocytoma (PC12) cells

Thallium (Tl) is a highly toxic metal though yet its mechanisms are poorly understood. Previously, we demonstrated that rat pheochromocytoma (PC12) cells exposure to thallous (Tl(I)) or thallic (Tl(III)) cations leads to mitochondrial damage and reduced cell viability. In the present work we comparatively characterized the possible pathways involved in Tl(I)- and Tl(III)- (10-100 muM) mediated decrease in PC12 cells viability. We observed that these cations do not cause cell necrosis but significantly increased the number of cells with apoptotic features. Both cations lead to Bax oligomerization and caused apoptosis inducing factor (AIF), endonuclease G (Endo G), and cytochrome c release from mitochondria, but they did not activate caspase dependent DNAse (CAD). Tl(I)- and Tl(III)-dependent caspases 9 and 3 activation followed similar kinetics, with maximal effects at 18 h of incubation. In addition, Tl(I) promoted phosphatidylserine (PS) exposure. Tl(III) induced 2- and 18-fold increase in Fas content and caspase 8 activity, respectively. Together, experimental results show that Tl(I) and Tl(III) induce PC12 cells apoptosis, although differential pathways are involved. While Tl(I)-mediated cell apoptosis was mainly associated with mitochondrial damage, Tl(III) showed a mixed effect triggering both the intrinsic and extrinsic pathways of apoptosis. These findings contribute to a better understanding of the mechanisms underlying Tl-induced loss of cell viability in PC12 cells.

The role for endoplasmic reticulum stress in diabetes mellitus

Accumulating evidence suggests that endoplasmic reticulum (ER) stress plays a role in the pathogenesis of diabetes, contributing to pancreatic beta-cell loss and insulin resistance. Components of the unfolded protein response (UPR) play a dual role in beta-cells, acting as beneficial regulators under physiological conditions or as triggers of beta-cell dysfunction and apoptosis under situations of chronic stress. Novel findings suggest that "what makes a beta-cell a beta-cell", i.e., its enormous capacity to synthesize and secrete insulin, is also its Achilles heel, rendering it vulnerable to chronic high glucose and fatty acid exposure, agents that contribute to beta-cell failure in type 2 diabetes. In this review, we address the transition from physiology to pathology, namely how and why the physiological UPR evolves to a proapoptotic ER stress response and which defenses are triggered by beta-cells against these challenges. ER stress may also link obesity and insulin resistance in type 2 diabetes. High fat feeding and obesity induce ER stress in liver, which suppresses insulin signaling via c-Jun N-terminal kinase activation. In vitro data suggest that ER stress may also contribute to cytokine-induced beta-cell death. Thus, the cytokines IL-1beta and interferon-gamma, putative mediators of beta-cell loss in type 1 diabetes, induce severe ER stress through, respectively, NO-mediated depletion of ER calcium and inhibition of ER chaperones, thus hampering beta-cell defenses and amplifying the proapoptotic pathways. A better understanding of the pathways regulating ER stress in beta-cells may be instrumental for the design of novel therapies to prevent beta-cell loss in diabetes.

ERADicate ER stress or die trying

Stress within the endoplasmic reticulum (ER) induces a sophisticated network of pathways termed the unfolded protein response (UPR), which is mediated through the ER transmembrane sensors PERK, ATF6, and IRE1. The UPR coordinates the temporary downregulation of protein translation, the upregulation of ER chaperones and folding machinery, and the enhanced expression of components necessary for ER-associated degradation (ERAD) essential for decreasing ER stress by clearing terminally misfolded proteins from the ER. Repetitive but futile folding attempts not only prolong ER stress but can also result in reactive oxygen species (ROS) generation, both of which may result in cell death. Additional mechanisms for decreasing stress and the protein load in the ER have been recently revealed. They include a newly identified function of IRE1 in degradation of select secretory protein mRNAs, a "preemptive" quality control responsible for averting translocation of select secretory proteins into the ER, upregulation of forward trafficking to allow misfolded proteins with intact exit signals to exit the ER, and upregulation of autophagy. The saturation or failure of some or all of these mechanisms can result in cell death and disease, including diabetes and a number of late-onset neurologic diseases.

Mediators of endoplasmic reticulum stress-induced apoptosis

The efficient functioning of the endoplasmic reticulum (ER) is essential for most cellular activities and survival. Conditions that interfere with ER function lead to the accumulation and aggregation of unfolded proteins. ER transmembrane receptors detect the onset of ER stress and initiate the unfolded protein response (UPR) to restore normal ER function. If the stress is prolonged, or the adaptive response fails, apoptotic cell death ensues. Many studies have focused on how this failure initiates apoptosis, as ER stress-induced apoptosis is implicated in the pathophysiology of several neurodegenerative and cardiovascular diseases. In this review, we examine the role of the molecules that are activated during the UPR in order to identify the molecular switch from the adaptive phase to apoptosis. We discuss how the activation of these molecules leads to the commitment of death and the mechanisms that are responsible for the final demise of the cell.

Bcl-2 and calcium: controversy beneath the surface

The function of Bcl-2 family members on the endoplasmic reticulum has received increasing attention in recent years. The endoplasmic reticulum is the major organelle involved in intracellular calcium homeostasis and calcium signaling, including calcium signals that mediate apoptosis induction by anticancer drugs. But currently a controversy exists regarding reported effects of Bcl-2 on the calcium concentration within the lumen of the endoplasmic reticulum. Although several prominent reports indicate that Bcl-2 overexpression is associated with a decrease in luminal calcium, there are a large number of reports indicating that Bcl-2 either does not decrease luminal calcium or actually increases luminal calcium. This review summarizes this vast array of conflicting findings, and analyses potential reasons why different conclusions have been reached by different laboratories. Future directions are emphasized that might bring clarity to this important area of apoptosis biology.

Relationship between beta-amyloid degradation and the 26S proteasome in neural cells

Beta-amyloid peptide (Abeta) plays a central role in mediating neurotoxicity and in the formation of senile plaques in Alzheimer's disease (AD). The investigation of the roles of ubiquitin (Ub) in the process underlying the association of abnormal protein with the inclusion bodies that characterize AD is of great importance for the further understanding of this disorder. We have used primary cultures of cortical neurons and astrocytes to investigate the participation of the Ub-proteasome pathway in the degradation of Abeta and the effect of Abeta(1-42) and of the fragment Abeta(25-35) upon neural cells. We have found that Abeta(25-35) and Abeta(1-42) produce a significant increase in Ub-protein conjugates and in the expression of the Ub-activating enzyme E1. On the other hand, beta peptides inhibited the proteolytic activities of the 26S proteasome. When the proteolytic activity of the 26S proteasome was inhibited with lactacystin, there was a marked decrease in Abeta(1-42) degradation, suggesting that the peptide, in both astrocytes and neurons, could be a possible substrate of this enzymatic complex. Treatment of the cultures with lactacystin prior to the exposure to Abeta produced a significant decrease in cell viability, possibly as a consequence of the inhibition of Abeta degradation leading to a persistent exposure of the cells to the amyloidogenic peptide which results in cell death. Alterations in the Ub-proteasome pathway in AD could affect the normal proteolytic removal of Abeta, leading to an abnormal accumulation of Abeta(1-42).

Apoptosis in Schwann cell cultures is closely interrelated with the activity of the ubiquitin-proteasome proteolytic pathway

Although the participation of the ubiquitin-dependent pathway and of the proteasome in apoptosis has been proposed, its role in this process is not yet clearly defined. In previous studies, we have shown that in the central nervous system of the rat, programmed cell death and the ubiquitin-dependent proteolytic pathway are closely related to each other and that different types of neurons and of glial cells, shown different types of correlation between the two phenomena. In this work, we have used lactacystin, a highly specific inhibitor of the proteasome, to explore in Schwann cell cultures the relationship between the activity of the Ub-dependent pathway and apoptosis. Apoptosis was explored analyzing changes in nuclear morphology, using the Annexin V assay and by flow cytometry. Activity of caspase-3 was also measured. Changes in the levels of ubiquitin-protein conjugates and of the ubiquitin activating enzymes, E1, as well as expression of proteins that instruct the cells to apoptosis (p53, NFkappaB-IkappaB, Bcl2), or that participate in the control and regulation of the cell cycle, were also examined. Our results indicate that the decrease in the activity of the proteasome induced by lactacystin in Schwann cells, induces apoptotic cell death through changes in the concentration of certain key proteins that are involved in the apoptosis-signaling pathways.

SERCA1 truncated proteins unable to pump calcium reduce the endoplasmic reticulum calcium concentration and induce apoptosis

By pumping calcium from the cytosol to the ER, sarco/endoplasmic reticulum calcium ATPases (SERCAs) play a major role in the control of calcium signaling. We describe two SERCA1 splice variants (S1Ts) characterized by exon 4 and/or exon 11 splicing, encoding COOH terminally truncated proteins, having only one of the seven calcium-binding residues, and thus unable to pump calcium. As shown by semiquantitative RT-PCR, S1T transcripts are differentially expressed in several adult and fetal human tissues, but not in skeletal muscle and heart. S1T proteins expression was detected by Western blot in nontransfected cell lines. In transiently transfected cells, S1T homodimers were revealed by Western blot using mildly denaturing conditions. S1T proteins were shown, by confocal scanning microscopy, to colocalize with endogenous SERCA2b into the ER membrane. Using ER-targeted aequorin (erAEQ), we have found that S1T proteins reduce ER calcium and reverse elevation of ER calcium loading induced by SERCA1 and SERCA2b. Our results also show that SERCA1 variants increase ER calcium leakage and are consistent with the hypothesis of a cation channel formed by S1T homodimers. Finally, when overexpressed in liver-derived cells, S1T proteins significantly induce apoptosis. These data reveal a further mechanism modulating Ca(2+) accumulation into the ER of nonmuscle cells and highlight the relevance of S1T proteins to the control of apoptosis.

Calcium is a key signaling molecule in beta-lapachone-mediated cell death

beta-Lapachone (beta-Lap) triggers apoptosis in a number of human breast and prostate cancer cell lines through a unique apoptotic pathway that is dependent upon NQO1, a two-electron reductase. Downstream signaling pathway(s) that initiate apoptosis following treatment with beta-Lap have not been elucidated. Since calpain activation was suspected in beta-Lap-mediated apoptosis, we examined alterations in Ca(2+) homeostasis using NQO1-expressing MCF-7 cells. beta-Lap-exposed MCF-7 cells exhibited an early increase in intracellular cytosolic Ca(2+), from endoplasmic reticulum Ca(2+) stores, comparable to thapsigargin exposures. 1,2-Bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester, an intracellular Ca(2+) chelator, blocked early increases in Ca(2+) levels and inhibited beta-Lap-mediated mitochondrial membrane depolarization, intracellular ATP depletion, specific and unique substrate proteolysis, and apoptosis. The extracellular Ca(2+) chelator, EGTA, inhibited later apoptotic end points (observed >8 h, e.g. substrate proteolysis and DNA fragmentation), suggesting that later execution events were triggered by Ca(2+) influxes from the extracellular milieu. Collectively, these data suggest a critical, but not sole, role for Ca(2+) in the NQO1-dependent cell death pathway initiated by beta-Lap. Use of beta-Lap to trigger an apparently novel, calpain-like-mediated apoptotic cell death could be useful for breast and prostate cancer therapy.

First and second messenger role of calcium. Survival versus apoptosis in serum-free cultured granulosa explants

In order to elucidate the causal relationship between increased intracellular free calcium ([Ca2+]i) and induction of apoptosis, serum-free cultured granulosa cell (GC) explants were subjected to various experimental protocols interfering with normal Ca2+ homeostasis. Modulation of apoptotic indices was calculated on DAPI-stained GC explants. In some experiments [Ca2+]i was measured with the Ca2+ probe fura-2 in combination with epifluorescence microscopy. Buffering of [Ca2+]i with BAPTA-AM resulted in inhibition of apoptosis, while increasing extracellular Ca2+ (otherwise called [Ca2+]e load) resulted in a biphasic response characterized by an initial inhibitory effect on apoptosis followed by a delayed phase of increased apoptosis that became apparent at 4 h after withdrawal of the [Ca2+]e load. The initial inhibitory effect of the [Ca2+]e load on apoptosis was dependent on the concentration of the load (range 2-50 mM), was augmented when the [Ca2+]e load was applied in the presence of the Ca2+ channel blocker methoxyverapamil, and was mimicked by applying Mg2+ and Gd3+, two Ca(2+)-receptor agonists. These observations point towards the involvement of an extracellular Ca(2+)-sensing receptor (CaR). Measurements of [Ca2+]i showed that the ion was increased just after [Ca2+]e load, followed by recovery that was complete at 2 h after the load. Collectively these data suggest that a [Ca2+]e load initiates apoptosis, becoming manifest 4 h later, by the provoked [Ca2+]i increase, and this effect is preceded by an apoptosis-inhibiting phase presumably involving CaR activation. We conclude that Ca2+ may act as a first (extracellular) messenger promoting cell survival and as a second (intracellular) messenger activating the cell death pathway.

Mechanisms underlying hypoxia-induced neuronal apoptosis

In vivo models of cerebral hypoxia-ischemia have shown that neuronal death may occur via necrosis or apoptosis. Necrosis is, in general, a rapidly occurring form of cell death that has been attributed, in part, to alterations in ionic homeostasis. In contrast, apoptosis is a delayed form of cell death that occurs as the result of activation of a genetic program. In the past decade, we have learned considerably about the mechanisms underlying apoptotic neuronal death following cerebral hypoxia-ischemia. With this growth in knowledge, we are coming to the realization that apoptosis and necrosis, although morphologically distinct, are likely part of a continuum of cell death with similar operative mechanisms. For example, following hypoxia-ischemia, excitatory amino acid release and alterations in ionic homeostasis contribute to both necrotic and apoptotic neuronal death. However, apoptosis is distinguished from necrosis in that gene activation is the predominant mechanism regulating cell survival. Following hypoxic-ischemic episodes in the brain, genes that promote as well as inhibit apoptosis are activated. It is the balance in the expression of pro- and anti-apoptotic genes that likely determines the fate of neurons exposed to hypoxia. The balance in expression of pro- and anti-apoptotic genes may also account for the regional differences in vulnerability to hypoxic insults. In this review, we will examine the known mechanisms underlying apoptosis in neurons exposed to hypoxia and hypoxia-ischemia.

Bcl-2 decreases the free Ca2+ concentration within the endoplasmic reticulum

The antiapoptotic protein Bcl-2 localizes not only to mitochondria but also to the endoplasmic reticulum (ER). However, the function of Bcl-2 at the level of the ER is poorly understood. In this study, we have investigated the effects of Bcl-2 expression on Ca(2+) storage and release by the ER. The expression of Bcl-2 decreased the amount of Ca(2+) that could be released from intracellular stores, regardless of the mode of store depletion, the cell type, or the species from which Bcl-2 was derived. Bcl-2 also decreased cellular Ca(2+) store content in the presence of mitochondrial inhibitors, suggesting that its effects were not mediated through mitochondrial Ca(2+) uptake. Direct measurements with ER-targeted Ca(2+)-sensitive fluorescent "cameleon" proteins revealed that Bcl-2 decreased the free Ca(2+) concentration within the lumen of the ER, [Ca(2+)](ER). Analysis of the kinetics of Ca(2+) store depletion in response to the Ca(2+)-ATPase inhibitor thapsigargin revealed that Bcl-2 increased the permeability of the ER membrane. These results suggest that Bcl-2 decreases the free Ca(2+) concentration within the ER lumen by increasing the Ca(2+) permeability of the ER membrane. The increased ER Ca(2+) permeability conferred by Bcl-2 would be compatible with an ion channel function of Bcl-2 at the level of the ER membrane.

Lactacystin, a specific inhibitor of the proteasome, induces apoptosis and activates caspase-3 in cultured cerebellar granule cells

The multicatalytic protease complex or proteasome is a fundamental nonlysosomal tool that the cell uses to process or degrade proteins at a fast rate through the ubiquitin and ATP-dependent proteolytic pathway. Examples of these important proteins include the tumor suppressor protein p53, various cyclins, the cyclin-dependent kinase inhibitor p27, NFkappaB, IkappaB, c-fos, and c-jun. The activation of proteolytic enzymes, including certain cystein-proteases of the ced-3/ICE (interleukin-1beta-converting enzyme) family, is a characteristic feature of the apoptotic program. However, the role of the multicatalytic protease complex in apoptosis is not well known. In order to obtain further information regarding the participation of the ubiquitin-mediated pathway in the decision of the cell to execute the cell death program, we have used a specific inhibitor of the multicatalytic protease complex, lactacystin, in cultured cerebellar granule cells. Cells were obtained from the cerebellum of 6- to 8-day-old Wistar rats and cultured in Neurobasal medium supplemented with B-27. Addition of lactacystin to the cultures induced apoptosis of the granule cells in a time-dependent fashion. The morphological changes produced by the proteasome inhibitor included nuclear condensation and DNA fragmentation measured by the diphenylamine test, as well as a positive labeling by the TUNEL (terminal deoxynucleotidyltransferase mediated-dUTP nick end labeling) assay, all of them typical features of apoptosis. Concomitant with apoptosis, there were changes in the expression of the ubiquitin mRNA, a progressive depletion in the free ubiquitin pool, and an increase in the high molecular weight ubiquitin-protein conjugates. Caspase-3, a member of the ced-3/ICE family of cystein-proteases, showed a marked increase in activity in the lactacystin-treated cells. In flow cytometry studies, the amount of cells in the S phase of the cell cycle was smaller in the lactacystin-treated cells than in controls, suggesting that apoptosis could be due, in part, to an alteration of the cell cycle.

HIV-1 protein Tat induces apoptosis of hippocampal neurons by a mechanism involving caspase activation, calcium overload, and oxidative stress

Patients infected with HIV-1 often exhibit cognitive deficits that are related to progressive neuronal degeneration and cell death. The protein Tat, which is released from HIV-1-infected cells, was recently shown to be toxic toward cultured neurons. We now report that Tat induces apoptosis in cultured embryonic rat hippocampal neurons. Tat induced caspase activation, and the caspase inhibitor zVAD-fmk prevented Tat-induced neuronal death. Tat induced a progressive elevation of cytoplasmic-free calcium levels, which was followed by mitochondrial calcium uptake and generation of mitochondrial-reactive oxygen species (ROS). The intracellular calcium chelator BAPTA-AM and the inhibitor of mitochondrial calcium uptake ruthenium red protected neurons against Tat-induced apoptosis. zVAD-fmk suppressed Tat-induced increases of cytoplasmic calcium levels and mitochondrial ROS accumulation, indicating roles for caspases in the perturbed calcium homeostasis and oxidative stress induced by Tat. An inhibitor of nitric oxide synthase, and the peroxynitrite scavenger uric acid, protected neurons against Tat-induced apoptosis, indicating requirements for nitric oxide production and peroxynitrite formation in the cell death process. Finally, Tat caused a delayed and progressive mitochondrial membrane depolarization, and cyclosporin A prevented Tat-induced apoptosis, suggesting an important role for mitochondrial membrane permeability transition in Tat-induced apoptosis. Collectively, our data demonstrate that Tat can induce neuronal apoptosis by a mechanism involving disruption of calcium homeostasis, caspase activation, and mitochondrial calcium uptake and ROS accumulation. Agents that interupt this apoptotic cascade may prove beneficial in preventing neuronal degeneration and associated dementia in AIDS patients.

c-Fos degradation by the proteasome. An early, Bcl-2-regulated step in apoptosis

c-Fos is a transcription factor that promotes cell growth, differentiation, and transformation. We found that c-Fos was degraded when WEHI7.2 mouse lymphoma cells were induced to undergo apoptosis with the calcium ATPase inhibitor, thapsigargin, or the glucocorticoid hormone, dexamethasone. The degradation of c-Fos preceded caspase-3 activation and apoptotic nuclear chromatin condensation and was inhibited by the proteasome inhibitors MG132, N-acetyl-leucyl-leucyl-norleucinal, and lactacystin. Stable transfection of WEHI7.2 cells with a mutant form of c-Fos that was not degraded by the proteasome inhibited apoptosis. Also, overexpression of Bcl-2 in WEHI7.2 cells blocked c-Fos degradation and inhibited apoptosis. The results indicate that proteasome-mediated degradation of c-Fos is an early, Bcl-2-regulated step in apoptosis induction by thapsigargin and dexamethasone. These findings suggest that c-Fos may have a protective action that is eliminated by proteasome-mediated degradation and preserved by Bcl-2.

From calcium signaling to cell death: two conformations for the mitochondrial permeability transition pore. Switching from low- to high-conductance state

The permeability transition pore (PTP) is a channel of the inner mitochondrial membrane that appears to operate at the crossroads of two distinct physiological pathways, i.e., the Ca2+ signaling network during the life of the cell, and the effector phase of the apoptotic cascade during Ca2+-dependent cell death. Correspondingly, two open conformations of the PTP can also be observed in isolated organelles. A low-conductance state, that allows the diffusion of small ions like Ca2+, is pH-operated, promoting spontaneous closure of the channel. A high-conductance state, that allows the unselective diffusion of big molecules, stabilizes the channel in the open conformation, disrupting in turn the mitochondrial structure and causing the release of proapoptotic factors. Our current results indicate that switching from low- to high-conductance state is an irreversible process that is strictly dependent on the saturation of the internal Ca2+-binding sites of the PTP. Thus, the high-conductance state of the PTP, which was shown to play a pivotal role in the course of excitotoxic and thapsigargin-induced cell death, might result from a Ca2+-dependent conformational shift of the low-conductance state, normally participating in the regulation of cellular Ca2+ homeostasis as a pH-operated channel. These observations lead us to propose a simple biophysical model of the transition between Ca2+ signaling and Ca2+-dependent apoptosis.

Apoptosis induction by the glucocorticoid hormone dexamethasone and the calcium-ATPase inhibitor thapsigargin involves Bc1-2 regulated caspase activation

The requirement for caspases (ICE-like proteases) were investigated in mediating apoptosis of WEHI7.2 mouse lymphoma cells in response to two death inducers with different mechanisms of action, the glucocorticoid hormone dexamethasone (DX) and the calcium-ATPase inhibitor thapsigargin (TG). Apoptosis induction by these agents followed different kinetics, and was closely correlated with in vivo activation of caspase-3 (CPP32/Yama/Apopain) and cleavage of the caspase target protein poly(ADP-ribose) polymerase (PARP). Caspase activation and PARP cleavage were inhibited by Bcl-2 overexpression. Cell extracts from DX- and TG-treated cells cleaved the in vitro synthesized baculovirus p35 ICE-like protease target, producing 25 and 10 kDa fragments. p35 cleavage was inhibited by mutating the active site aspartic acid to alanine, and by a panel of protease inhibitors that inhibit caspase-3-like proteases, including iodoacetamide, N-ethylmaleimide, and Ac-DEVD-cho. Treatment of cells in vivo with two cell permeant peptide fluoromethylketone inhibitors of caspase activity, Z-VAD-fmk and Z-DEVD-fmk, inhibited DX- and TG-induced apoptotic nuclear changes and maintained plasma membrane integrity, whereas the cathepsin inhibitor, Z-FA-fmk, and two calpain inhibitors failed to inhibit apoptosis. An unexpected observation was that due to the delayed time course of DX-induced apoptosis, optimal preservation of plasma membrane integrity was achieved by adding caspase inhibitors beginning 8 h after DX addition. In summary, the findings indicate that two diverse apoptosis-inducing signals converge into a common Bcl-2-regulated pathway that leads to caspase activation and apoptosis.

A tool coming of age: thapsigargin as an inhibitor of sarco-endoplasmic reticulum Ca(2+)-ATPases

Thapsigargin is the most widely used inhibitor of the ubiquitous sarco-endoplasmic reticulum Ca(2+)-ATPases in mammalian cells. Over the past ten years, this guaianolide compound of plant origin has become a popular tool in a host of studies directed at elucidating the mechanisms of intracellular Ca2+ signalling. Its remarkable potency and selectivity have been instrumental in widening our view of the function of intracellular Ca2+ stores to include such key aspects as store-operated Ca2+ entry or the involvement of the stores in protein synthesis or cell growth. In this article Marek Treiman, Casper Caspersen and Søren Brøgger Christensen review the key pharmacological features of thapsigargin action; they also discuss some of the ways in which its unique properties have shown to be important for obtaining new insights into the biology of Ca2+ stores, and how these properties might encompass a therapeutic potential. In parallel, attention is drawn to some of the limitations and pitfalls encountered when working with thapsigargin.

Calbindin D28k blocks the proapoptotic actions of mutant presenilin 1: reduced oxidative stress and preserved mitochondrial function

Mutations in the presenilin 1 (PS-1) gene account for many cases of early-onset autosomal dominant inherited forms of Alzheimer's disease. Recent findings suggest that PS-1 mutations may sensitize neurons to apoptosis induced by trophic factor withdrawal and exposure to amyloid beta-peptide (Abeta). We now report that overexpression of the calcium-binding protein calbindin D28k prevents apoptosis in cultured neural cells expressing mutant PS-1 (L286V and M146V missense mutations). Elevations of the intracellular Ca2+ concentration and generation of reactive oxygen species induced by Abeta, and potentiated by mutant PS-1, were suppressed in calbindin-overexpressing cells. Impairment of mitochondrial function by Abeta (which preceded apoptosis) was exacerbated by PS-1 mutations and was largely prevented by calbindin. These findings suggest that PS-1 mutations render neurons vulnerable to apoptosis by a mechanism involving destabilization of cellular calcium homeostasis, which leads to oxidative stress and mitochondrial dysfunction.

Maintenance of calcium homeostasis in the endoplasmic reticulum by Bcl-2

The oncogene bcl-2 encodes a 26-kD protein localized to intracellular membranes, including the ER, mitochondria, and perinuclear membrane, but its mechanism of action is unknown. We have been investigating the hypothesis that Bcl-2 regulates the movement of calcium ions (Ca2+) through the ER membrane. Earlier findings in this laboratory indicated that Bcl-2 reduces Ca2+ efflux from the ER lumen in WEHI7.2 lymphoma cells treated with the Ca2+-ATPase inhibitor thapsigargin (TG) but does not prevent capacitative entry of extracellular calcium. In this report, we show that sustained elevation of cytosolic Ca2+ due to capacitative entry is not required for induction of apoptosis by TG, suggesting that ER calcium pool depletion may trigger apoptosis. Bcl-2 overexpression maintains Ca2+ uptake in the ER of TG-treated cells and prevents a TG-imposed delay in intralumenal processing of the endogenous glycoprotein cathepsin D. Also, Bcl-2 overexpression preserves the ER Ca2+ pool in untreated cells when extracellular Ca2+ is low. However, low extracellular Ca2+ reduces the antiapoptotic action of Bcl-2, suggesting that cytosolic Ca2+ elevation due to capacitative entry may be required for optimal ER pool filling and apoptosis inhibition by Bcl-2. In summary, the findings suggest that Bcl-2 maintains Ca2+ homeostasis within the ER, thereby inhibiting apoptosis induction by TG.

Mouse lymphoma cells destined to undergo apoptosis in response to thapsigargin treatment fail to generate a calcium-mediated grp78/grp94 stress response

grp78/grp94 induction is critical for maintaining the viability of epithelial cells and fibroblasts following treatment with thapsigargin (TG), an inhibitor of Ca2+ uptake into the endoplasmic reticulum. In contrast to these cell types, WEHI7.2 mouse lymphoma cells undergo apoptosis when treated with TG, prompting us to examine the grp78/grp94 stress response in WEHI7.2 cells. TG treatment failed to induce grp78/grp94 transcription in WEHI7.2 cells, measured by Northern hybridization and nuclear run-on assays, even if the cells were protected from apoptosis by overexpressing bcl-2. However, grp78/grp94 transcription was induced by the glycosylation inhibitor tunicamycin, suggesting that there are at least two grp78/grp94 signaling pathways, one in response to TG-induced endoplasmic reticulum Ca2+ pool depletion, which is inoperable in WEHI7.2 cells, and one in response to glycosylation inhibition, which is operable in WEHI7.2 cells. Studies of additional lymphoid lines, as well as several nonlymphoid lines, suggested a correlation between grp78/grp94 induction and resistance to apoptosis following TG treatment. In conclusion, the vulnerability of TG-treated WEHI7.2 cells to apoptosis may be due to failure to signal a grp78/grp94 stress response.

Involvement of extracellular calcium in phosphatidylserine exposure during apoptosis

The appearance of phosphatidylserine (PS) on the outer surface of apoptotic cells is an important signal for their ingestion. In platelets, elevation of intracellular Ca2+ with thapsigargin can trigger large amounts of PS exposure within minutes. We detected PS exposure in U937 promonocytes and Jurkat T-cells after incubation with thapsigargin, but in only 10% of the cells, and it took up to 6 h to occur. Tumor necrosis factor and anti-Fas antibody rapidly trigger apoptosis in these cells, and chelation of extracellular Ca2+ with 5 mM EGTA inhibited PS exposure by 65% and 50%, respectively. Chelation of intracellular Ca2+ with BAPTA-AM had no effect. Other parameters of apoptosis, including cell blebbing, shrinkage, nuclear fragmentation, activation of the ICE-like proteases, and fodrin cleavage, were not inhibited by extracellular EGTA. We conclude that while an elevation of intracellular Ca2+ is an ineffective trigger of apoptosis in the cells investigated, extracellular Ca2+ is required for efficient PS exposure during apoptosis.