An intracellular calcium store regulates protein synthesis in HeLa cells, but it is not the hormone-sensitive store

Interleukin-12 Induces Receptor Activator of Nuclear Factor-Kappa B Ligand Expression by Human Periodontal Ligament Cells

BACKGROUND

Increased level of proinflammatory cytokine interleukin (IL)-12 correlates with the severity of periodontitis. Yet, a possible role of IL-12 in periodontal disease has not been clarified. The aim of this study is to investigate whether IL-12 affects expression of receptor activator of nuclear factor-kappa B (NF-κB) ligand (RANKL), a potent osteoclast-stimulating factor, by human periodontal ligament (hPDL) cells.

METHODS

To determine the effect of IL-12, hPDL cells were incubated with recombinant human IL-12 (p70) in a dose- (0 to 10 ng/mL) and time-dependent manner. Expression of RANKL was evaluated at mRNA and protein levels. Underlying signaling pathways of IL-12 were determined by using specific inhibitors.

RESULTS

Under the influence of IL-12, hPDL cells expressed significantly higher levels of RANKL. Expression was mediated by signal transducer and activator of transcription 4 and NF-κB signaling pathways. Conditioned medium of IL-12-incubated cells proved to contain molecule(s) that induced RANKL expression. Addition of suramin (G protein-coupled receptor inhibitor) and ethylene glycol tetraacetic acid (calcium chelator) suggested existence of intermediate molecule(s) that could activate heterotrimeric G protein signaling in a calcium-dependent pathway.

CONCLUSIONS

Expression of RANKL by hPDL cells significantly increased after IL-12 treatment. Therefore, this study supports a close interrelationship between immune and skeletal systems and suggests an osteolytic role of IL-12 in pathogenesis of periodontal disease.

Physiological and Neural Adaptations to Eccentric Exercise: Mechanisms and Considerations for Training

Eccentric exercise is characterized by initial unfavorable effects such as subcellular muscle damage, pain, reduced fiber excitability, and initial muscle weakness. However, stretch combined with overload, as in eccentric contractions, is an effective stimulus for inducing physiological and neural adaptations to training. Eccentric exercise-induced adaptations include muscle hypertrophy, increased cortical activity, and changes in motor unit behavior, all of which contribute to improved muscle function. In this brief review, neuromuscular adaptations to different forms of exercise are reviewed, the positive training effects of eccentric exercise are presented, and the implications for training are considered.

Disturbed Ca2+ homeostasis increases glutaminyl cyclase expression; connecting two early pathogenic events in Alzheimer's disease in vitro

A major neuropathological hallmark of Alzheimer’s disease (AD) is the deposition of aggregated β amyloid (Aβ) peptide in the senile plaques. Aβ is a peptide of 38–43 amino acids and its accumulation and aggregation plays a key role early in the disease. A large fraction of β amyloid is N-terminally truncated rendering a glutamine that can subsequently be cyclized into pyroglutamate (pE). This makes the peptide more resistant to proteases, more prone to aggregation and increases its neurotoxicity. The enzyme glutaminyl cyclase (QC) catalyzes this conversion of glutamine to pE. In brains of AD patients, the expression of QC is increased in the earliest stages of pathology, which may be an important event in the pathogenesis. In this study we aimed to investigate the regulatory mechanism underlying the upregulation of QC expression in AD. Using differentiated SK-N-SH as a neuronal cell model, we found that neither the presence of Aβ peptides nor the unfolded protein response, two early events in AD, leads to increased QC levels. In contrast, we demonstrated increased QC mRNA levels and enzyme activity in response to another pathogenic factor in AD, perturbed intracellular Ca²⁺ homeostasis. The QC promoter contains a putative binding site for the Ca²⁺ dependent transcription factors c-fos and c-jun. C-fos and c-jun are induced by the same Ca²⁺-related stimuli as QC and their upregulation precedes QC expression. We show that in the human brain QC is predominantly expressed by neurons. Interestingly, the Ca²⁺- dependent regulation of both c-fos and QC is not observed in non-neuronal cells. Our results indicate that perturbed Ca²⁺ homeostasis results in upregulation of QC selectively in neuronal cells via Ca²⁺- dependent transcription factors. This suggests that disruption of Ca²⁺ homeostasis may contribute to the formation of the neurotoxic pE Aβ peptides in Alzheimer’s disease.

Regulation of nuclear envelope permeability in cell death and survival

The nuclear pore complex (NPC) mediates macromolecular exchange between nucleus and cytoplasm. It is a regulated channel whose functional properties are modulated in response to the physiological status of the cell. Identifying the factors responsible for regulating NPC activity is crucial to understand how intracellular signaling cues are integrated at the level of this channel to control nucleocytoplasmic trafficking. For proteins lacking active translocation signals the NPC acts as a molecular sieve limiting passage across the nuclear envelope (NE) to proteins with a MW below ~40 kD. Here, we investigate how this permeability barrier is altered in paradigms of cell death and cell survival, i.e., apoptosis induction via staurosporine, and enhanced viability via overexpression of Bcl-2. We monitor dynamic changes of the NPC's size-exclusion limit for passive diffusion by confocal time-lapse microscopy of cells undergoing apoptosis, and use different diffusion markers to determine how Bcl-2 expression affects steady-state NE permeability. We show that staurosporine triggers an immediate and gradual leakiness of the NE preceding the appearance of apoptotic hallmarks. Bcl-2 expression leads to a constitutive increase in NE permeability, and its localization at the NE is sufficient for the effect, evincing a functional role for Bcl-2 at the nuclear membrane. In both settings, NPC leakiness correlates with reduced Ca²⁺ in internal stores, as demonstrated by fluorometric measurements of ER/NE Ca²⁺ levels. By comparing two cellular models with opposite outcome these data pinpoint ER/NE Ca²⁺ as a general and physiologically relevant regulator of the permeability barrier function of the NPC.

Subversion of antimicrobial calprotectin (S100A8/S100A9 complex) in the cytoplasm of TR146 epithelial cells after invasion by Listeria monocytogenes

Expressed by squamous mucosal keratinocytes, calprotectin is a complex of two EF-hand calcium-binding proteins of the S100 subfamily (S100A8 and S100A9) with significant antimicrobial activity. Calprotectin-expressing cells resist invasion by Porphyromonas gingivalis, Listeria monocytogenes, and Salmonella enterica serovar Typhimurium (S. typhimurium). To understand the interactions between calprotectin and invasive bacteria, we studied the distribution of calprotectin in the cytoplasm of TR146 epithelial cells. In response to L. monocytogenes, calprotectin mobilized from a diffuse cytoplasmic distribution to a filamentous pattern and colocalized with the microtubule network. Listeria more frequently invaded cells with mobilized calprotectin. Calprotectin mobilization was listeriolysin O-dependent and required calcium (extracellular and intracellular) and an intact microtubule network. In the presence of preformed microtubules in vitro, the anti-Listeria activity of calprotectin was abrogated. To facilitate intraepithelial survival, therefore, Listeria mobilizes calprotectin to colocalize with cytoplasmic microtubules, subverting anti-Listeria activity and autonomous cellular immunity.

An increase in [Ca2+]i is sufficient but not necessary for driving mitosis in early mouse embryos

An increase in intracellular Ca2+ concentration ([Ca2+]i) has been shown to drive sea-urchin embryos and some fibroblasts through nuclear-envelope breakdown (NEBD) and the metaphase-to-anaphase transition. Mitotic Ca2+ transients can be pan-cellular global events or localized to the perinuclear region. It is not known whether Ca2+ is a universal regulator of mitosis or whether its role is confined to specific cell types. To test the hypothesis that Ca2+ is a universal regulator of mitosis, we have investigated the role of Ca2+ in mitosis in one-cell mouse embryos. Fertilized embryos generate Ca2+ transients during the first mitotic division. Imposing a Ca2+ transient by photorelease of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] resulted in acceleration of mitosis entry, suggesting that a [Ca2+]i increase is capable of triggering mitosis. Mitotic Ca2+ transients were inhibited using three independent approaches: injection of intracellular Ca2+ buffers; downregulation of Ins(1,4,5)P3 receptors; and removal of extracellular Ca2+. None of the interventions had any effects on the timing of NEBD or cytokinesis. The possibility that NEBD is driven by localized perinuclear Ca2+ transients was examined using two-photon microscopy but no Ca2+-dependent increases in fluorescence were found to precede NEBD. Finally, the second mitotic division took place in the absence of any detectable [Ca2+]i increase. Thus, although an induced [Ca2+]i increase can accelerate mitosis entry, neither cytosolic nor perinuclear [Ca2+] increases appear to be necessary for progression through mitosis in mouse embryos.

A gonadotropin-releasing hormone insensitive, thapsigargin-sensitive Ca2+ store reduces basal gonadotropin exocytosis and gene expression: comparison with agonist-sensitive Ca2+ stores

We examined whether distinct Ca2+ stores differentially control basal and gonadotropin (GTH-II)-releasing hormone (GnRH)-evoked GTH-II release, long-term GTH-II secretion and contents, and GTH-II-beta mRNA expression in goldfish. Thapsigargin (Tg)-sensitive Ca2+ stores mediated neither caffeine-evoked GTH-II release, nor salmon (s)GnRH- and chicken (c)GnRH-II-stimulated secretion; the latter responses were previously shown to involve ryanodine (Ry)-sensitive Ca2+ stores. Surprisingly, Tg decreased basal GTH-II release. This response was attenuated by prior exposure to sGnRH and caffeine, but was insensitive to the phosphatase inhibitor okadaic acid, the inhibitor of constitutive release brefeldin A and cGnRH-II. GTH-II-beta mRNA expression was decreased at 24 h by 2 microm Tg, and by inhibiting (10 microm Ry) and stimulating (1 nm Ry) Ry receptors. Transient increases in GTH-II-beta mRNA were observed at 2 h and 12 h following 10 microm and 1 nm Ry treatment, respectively. Effects of Tg, Ry and GnRH on long-term GTH-II secretion, contents and apparent production differed from one another, and these changes were not well correlated with changes in GTH-II-beta mRNA expression. Our data show that GTH-II secretion, storage and transcription can be independently controlled by distinct Ca2+ stores.

Function-specific calcium stores selectively regulate growth hormone secretion, storage, and mRNA level

Ca(+) stores may regulate multiple components of the secretory pathway. We examined the roles of biochemically independent intracellular Ca(2+) stores on acute and long-term growth hormone (GH) release, storage, and mRNA levels in goldfish somatotropes. Thapsigargin-evoked intracellular Ca(2+) concentration ([Ca(2+)](i)) signal amplitude was similar to the Ca(2+)-mobilizing agonist gonadotropin-releasing hormone, but thapsigargin (2 microM) did not acutely increase GH release, suggesting uncoupling between [Ca(2+)](i) and exocytosis. However, 2 microM thapsigargin affected long-term secretory function. Thapsigargin-treated cells displayed a steady secretion of GH (2, 12, and 24 h), which decreased GH content (12 and 24 h), but not GH mRNA/production (24 h). In contrast to the results with thapsigargin, activating the ryanodine (Ry) receptor (RyR) with 1 nM Ry transiently increased GH release (2 h). Prolonged activation of RyR (24 h) reduced GH release, contents and apparent production, without changing GH mRNA levels. Inhibiting RyR with 10 microM Ry increased GH mRNA levels, production, and storage (2 h). Increasing [Ca(2+)](i) independently of Ca(2+) stores with the use of 30 mM KCl decreased GH mRNA. Collectively, these results suggest that parts of the secretory pathway can be controlled independently by function-specific Ca(2+) stores.

Dependence of vital cell function on endoplasmic reticulum calcium levels: implications for the mechanisms underlying neuronal cell injury in different pathological states

The endoplasmic reticulum (ER) is a subcellular compartment playing a pivotal role in the control of vital calcium-related cell functions, including calcium storage and signalling. In addition, newly synthesized membrane and secretory proteins are folded and processed in the ER, reactions which are strictly calcium dependent. The ER calcium activity is therefore high, being several orders of magnitude above that of the cytoplasm. Depletion of ER calcium stores causes an accumulation of unfolded proteins in the ER lumen, a pathological situation which induces the activation of two highly conserved stress responses, the ER overload response (EOR) and the unfolded protein response (UPR). EOR triggers activation of the transcription factor NF kappa B, which, in turn, activates the expression of target genes. UPR triggers two downstream processes: it activates the expression of genes coding for ER-resident stress proteins, and it causes a suppression of the initiation of protein synthesis. A similar stress response is activated in pathological states of the brain including cerebral ischaemia, implying common underlying mechanisms. Depending on the extent and duration of the disturbance, an isolated impairment of ER function is sufficient to induce cell injury. In this review, evidence is presented that ER function is indeed disturbed in various diseases of the brain, including acute pathological states (e.g. cerebral ischaemia) and degenerative diseases (e.g. Alzheimer's disease). A body of evidence suggests that disturbances of ER function could be a global pathomechanism underlying neuronal cell injury in various acute and chronic disorders of the central nervous system. If that is true, restoration of ER function or attenuation of secondary disturbances induced by ER dysfunction could present a highly promising new avenue for pharmacological intervention to minimize neuronal cell injury in different pathological states of the brain.

Ca(2+) dynamics in synaptosomes isolated from the squid optic lobe

Synaptosomes from the optic lobes of squid (Loligo forbesi) were prepared by homogenization and allowed to settle onto glass coverslips. Synaptosomes were loaded with Ca(2+) sensitive dyes (Fura-2 AM, Calcium Green-1 AM and Calcium Green-5N AM), visualized by light microscopy and Ca(2+) sensitive fluorescence signals recorded and analyzed. With Fura-2, resting Ca(2+) was found to be 80 nM (n = 10, SEM 5.7). Addition of K(+) (30 mM), caffeine (3 mM) and thapsigargin (10 microM) evoked transient increases in cytoplasmic Ca(2+). Addition of BAPTA-AM (20 microM) decreased intrasynaptosomal free Ca(2+). Similar results were obtained with Calcium Green-1 AM but not with Calcium Green-5N AM. We conclude that synaptosomes from the squid optic lobe posses intact membranes and mechanisms to regulate intrasynaptosomal free [Ca(2+)], as well as caffeine sensitive Ca(2+) stores. The results of this study are discussed with respect to the role of Ca(2+) in presynaptic protein synthesis.

Changes in endoplasmic reticulum luminal environment affect cell sensitivity to apoptosis

To test the role of ER luminal environment in apoptosis, we generated HeLa cell lines inducible with respect to calreticulin and calnexin and investigated their sensitivity to drug-dependent apoptosis. Overexpression of calreticulin, an ER luminal protein, resulted in an increased sensitivity of the cells to both thapsigargin- and staurosporine-induced apoptosis. This correlated with an increased release of cytochrome c from the mitochondria. Overexpression of calnexin, an integral ER membrane protein, had no significant effect on drug-induced apoptosis. In contrast, calreticulin-deficient cells were significantly resistant to apoptosis and this resistance correlated with a decreased release of cytochrome c from mitochondria and low levels of caspase 3 activity. This work indicates that changes in the lumen of the ER amplify the release of cytochrome c from mitochondria, and increase caspase activity, during drug-induced apoptosis. There may be communication between the ER and mitochondria, which may involve Ca(2+) and play an important role in conferring cell sensitivity to apoptosis. Apoptosis may depend on both the presence of external apoptosis-activating signals, and, as shown in this study, on an internal factor represented by the ER.

Role of Ca(2+) fluctuations in L6 myotubes in the regulation of the hexokinase II gene

Expression of the hexokinase (HK) II gene in skeletal muscle is upregulated by electrically stimulated muscle contraction and moderate-intensity exercise. However, the molecular mechanism by which this occurs is unknown. Alterations in intracellular Ca(2+) homeostasis accompany contraction and regulate gene expression in contracting skeletal muscle. Therefore, as a first step in understanding the exercise-induced increase in HK II, the ability of Ca(2+) to increase HK II mRNA was investigated in cultured skeletal muscle cells, namely L6 myotubes. Exposure of cells to the ionophore A-23187 resulted in an approximately threefold increase in HK II mRNA. Treatment of cells with the extracellular Ca(2+) chelator EGTA did not alter HK II mRNA, nor was it able to prevent the A-23187-induced increase. Treatment of cells with the intracellular Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA-AM) also resulted in an approximately threefold increase in HK II mRNA in the absence of ionophore, which was similar to the increase in HK II mRNA induced by the combination of BAPTA-AM and A-23187. In summary, a rise in intracellular Ca(2+) is not necessary for the A-23187-induced increase in HK II mRNA, and increases in HK II mRNA occur in response to treatments that decrease intracellular Ca(2+) stores. Depletion of intracellular Ca(2+) stores may be one mechanism by which muscle contraction increases HK II mRNA.

Protein synthesis in presynaptic endings from squid brain: modulation by calcium ions

Previous biochemical, autoradiographic, and ultrastructural data have shown that, in the synaptosomal fraction of the squid optic lobe, protein synthesis is largely due to the presynaptic terminals of the retinal photoreceptor neurons (Crispino et al. [1993a] Mol. Cell. Neurosci. 4:366-374; Crispino et al. [1993b] J. Neurochem. 61:1144-1146; Crispino et al. [1997] J. Neurosci. 17:7694-7702). We now report that this process is close to its maximum at the basal concentration of cytosolic Ca++, and is markedly inhibited when the concentration of this ion is either decreased or increased. This conclusion is supported by the results of experiments with: 1) compounds known to increase the level of cytosolic Ca++, such as A23187, ionomycin, thapsigargin, and caffeine; 2) compounds sequestering cytosolic calcium ions such as BAPTA-AM; and 3) agents that block the role of Ca++ as second messenger, such as TFP and W7, which inhibit calmodulin, and calphostin, which inhibits protein kinase C. We conclude that variations in the level of cytosolic Ca++ induced in presynaptic terminals by neuronal activity may contribute to the modulation of the local synthesis of protein.

Disturbances of the functioning of endoplasmic reticulum: a key mechanism underlying neuronal cell injury?

Cerebral ischemia leads to a massive increase in cytoplasmic calcium activity resulting from an influx of calcium ions into cells and a release of calcium from mitochondria and endoplasmic reticulum (ER). It is widely believed that this increase in cytoplasmic calcium activity plays a major role in ischemic cell injury in neurons. Recently, this concept was modified, taking into account that disturbances occurring during ischemia are potentially reversible: it then was proposed that after reversible ischemia, calcium ions are taken up by mitochondria, leading to disturbances of oxidative phosphorylation, formation of free radicals, and deterioration of mitochondrial functions. The current review focuses on the possible role of disturbances of ER calcium homeostasis in the pathologic process culminating in ischemic cell injury. The ER is a subcellular compartment that fulfills important functions such as the folding and processing of proteins, all of which are strictly calcium dependent. ER calcium activity is therefore relatively high, lying in the lower millimolar range (i.e., close to that of the extracellular space). Depletion of ER calcium stores is a severe form of stress to which cells react with a highly conserved stress response, the most important changes being a suppression of global protein synthesis and activation of stress gene expression. The response of cells to disturbances of ER calcium homeostasis is almost identical to their response to transient ischemia, implying common underlying mechanisms. Many observations from experimental studies indicate that disturbances of ER calcium homeostasis are involved in the pathologic process leading to ischemic cell injury. Evidence also has been presented that depletion of ER calcium stores alone is sufficient to activate the process of programmed cell death. Furthermore, it has been shown that activation of the ER-resident stress response system by a sublethal form of stress affords tolerance to other, potentially lethal insults. Also, disturbances of ER function have been implicated in the development of degenerative disorders such as prion disease and Alzheimer's disease. Thus, disturbances of the functioning of the ER may be a common denominator of neuronal cell injury in a wide variety of acute and chronic pathologic states of the brain. Finally, there is evidence that ER calcium homeostasis plays a key role in maintaining cells in their physiologic state, since depletion of ER calcium stores causes growth arrest and cell death, whereas cells in which the regulatory link between ER calcium homeostasis and protein synthesis has been blocked enter a state of uncontrolled proliferation.

The effects of a Ca2+ chelator and heavy-metal-ion chelators upon Ca2+ oscillations and activation at fertilization in mouse eggs suggest a role for repetitive Ca2+ increases

During fertilization in mouse eggs, the sperm triggers a series of intracellular Ca2+ oscillations that lead to egg activation, as indicated by pronuclear formation. We show that Ca2+ oscillations in fertilized mouse eggs can be inhibited by addition of either the Ca2+ chelator 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM) or the heavy-metal-ion chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)ethylenediamine (TPEN) plus dithiothreitol (DTT). Both treatments inhibited Ca2+ oscillations, but they had different effects upon egg activation. Blocking Ca2+ oscillations with BAPTA-AM after the occurrence of just two Ca2+ spikes resulted in most eggs forming pronuclei. However, we found that BAPTA-AM-treated fertilizing eggs showed a decreased rate of protein synthesis, which by itself can promote egg activation. In contrast, blocking Ca2+ oscillations with TPEN plus DTT was accompanied by the inhibition of egg activation with no significant effect on protein synthesis. In eggs that were fertilized and then treated with TPEN plus DTT, there was a correlation between the number of Ca2+ spikes and the proportion of eggs that formed pronuclei, as well as between the number of Ca2+ spikes and the time taken for pronuclear formation and the first mitosis to occur. The addition of TPEN plus DTT did not block the generation of Ca2+ spikes or pronuclear formation when eggs were artificially stimulated by electroporation pulses. These data suggest that TPEN plus DTT inhibits pronuclear formation in fertilizing eggs via the inhibition of Ca2+ oscillations and that the number of Ca2+ spikes may regulate egg activation.

Calcium and protein kinase C mediate high-glucose-induced inhibition of inducible nitric oxide synthase in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSMC) proliferation is a key feature in diabetes-associated atherosclerotic disease. Since nitric oxide inhibits VSMC tone, migration, adhesion, and proliferation, we examined the effects of high glucose on IL-1beta-induced NO release from VSMCs in culture. Confluent smooth muscle cells, preincubated with either 5 mmol/L (mM) or 20 mmol/L (mM) glucose for 48 hours, were stimulated with IL-1beta. Nitrite was measured in the culture medium after 24 hours. IL-1beta-induced a 15-fold increase in NO production in normal glucose medium. Glucose (10 to 30 mmol/L (mM)) significantly reduced the response to IL-1beta. High glucose (20 mmol/L (mM)) inhibited IL-1beta-evoked NO production by approximately 50%. IL-1beta-stimulated [3H] citrulline-forming activity of the nitric oxide synthase (NOS) was also significantly lower in high-glucose-exposed cells, and this was reflected in diminished cellular levels of NOS protein. To assess the role of protein kinase C (PKC), membrane PKC activity was measured, and glucose (20 mmol/L (mM)) significantly increased it. Immunoblotting of the membranes revealed a glucose-induced increase in the PKC betaII isoform. 1,2-Dioctanoyl-glycerol, a PKC activator, mimicked the high-glucose effect on IL-1beta-induced NO release, while staurosporine, a PKC inhibitor, reversed it. The role of calcium in the glucose-mediated inhibition of cytokine-induced NO release was determined by treatment with BAPTA, an intracellular chelator of calcium. BAPTA partially reversed the inhibitory effects of glucose. Increasing intracellular calcium by A23187, an ionophore or thapsigargin, an inhibitor of endoplasmic reticulum Ca2+-ATPase, significantly decreased IL-1beta-induced NO release and NOS expression. These results indicate that glucose-induced inhibition of IL-1beta-stimulated NO release and NOS expression may be mediated by PKC activation and increased intracellular calcium.

The effects of artificial calcium buffers on calcium responses and glutamate-mediated excitotoxicity in cultured hippocampal neurons

After loading cultured rat hippocampal neurons with teh acetoxymethyl ester of the Ca2+ buffer BAPTA, or its dimethyl analogue DMB, the magnitudes of transient (20-25 s) depolarization- or excitatory amino acid-induced Ca2+ responses were reduced, as were the rates of increase and recovery of [Ca2+]i. In contrast, during prolonged (3-30 min) stimulation, the magnitudes of the Ca2+ responses were not reduced in buffer-loaded neurons, even though the rates of increase and recovery were still much slower compared to neurons loaded with the control molecule half-BAPTA-AM. The potential consequences of this action of BAPTA and DMB were then examined in an in vitro model of excitotoxicity in which we found that, in both fetal and postnatal cultures, glutamate-induced excitotoxicity was enhanced, rather than reduced. An additional and unexpected observation was that during exposure of neurons to solutions containing BAPTA-AM, dimethyl-BAPTA-AM, or half-BAPTA-AM, we observed a rapid but reversible increase in intracellular [Ca2+] that appeared to be mediated via an activation of voltage-operated Ca2+ channels; most probably due to a direct depolarizing effect. We suggest that the presence of artificial Ca2+ buffers interferes with the normal Ca(2+)-dependent mechanisms for limiting Ca2+ entry during stimulation and thereby leads to an enhanced net Ca2+ influx. One consequence of this action is to enhance the potency of glutamate as an excitotoxic agent. These results agree with previous observations that excitotoxicity is better correlated with the total net flux of Ca2+, rather than measurements of intracellular ionic Ca2+. Our results do not support a potential use of artificial Ca2+ buffers as neuroprotective agents.

Regulation of calreticulin gene expression by calcium

We have isolated and characterized a 12-kb mouse genomic DNA fragment containing the entire calreticulin gene and 2.14 kb of the promoter region. The mouse calreticulin gene consists of nine exons and eight introns, and it spans 4.2 kb of genomic DNA. A 1.8-kb fragment of the calreticulin promoter was subcloned into a reporter gene plasmid containing chloramphenicol acetyltransferase. This construct was then used in transient and stable transfection of NIH/ 3T3 cells. Treatment of transfected cells either with the Ca2+ ionophore A23187, or with the ER Ca2+-ATPase inhibitor thapsigargin, resulted in a five- to sevenfold increase of the expression of chloramphenicol acetyltransferase protein. Transactivation of the calreticulin promoter was also increased by fourfold in NIH/3T3 cells treated with bradykinin, a hormone that induces Ca2+ release from the intracellular Ca2+ stores. Analysis of the promoter deletion constructs revealed that A23187- and thapsigargin-responsive regions are confined to two regions (-115 to -260 and -685 to -1,763) in the calreticulin promoter that contain the CCAAT nucleotide sequences. Northern blot analysis of cells treated with A23187, or with thapsigargin, revealed a fivefold increase in calreticulin mRNA levels. Thapsigargin also induced a fourfold increase in calreticulun protein levels. Importantly, we show by nuclear run-on transcription analysis that calreticulin gene transcription is increased in NIH/3T3 cells treated with A23187 and thapsigargin in vivo. This increase in gene expression required over 4 h of continuous incubation with the drugs and was also sensitive to treatment with cycloheximide, suggesting that it is dependent on protein synthesis. Changes in the concentration of extracellular and cytoplasmic Ca2+ did not affect the increased expression of the calreticulin gene. These studies suggest that stress response to the depletion of intracellular Ca2+ stores induces expression of the calreticulin gene in vitro and in vivo.

Calcium depletion from the endoplasmic reticulum activates the double-stranded RNA-dependent protein kinase (PKR) to inhibit protein synthesis

Calcium depletion from the endoplasmic reticulum inhibits protein synthesis and correlates with increased phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) by a mechanism that does not require ongoing protein synthesis. To elucidate whether protein synthesis inhibition requires eIF-2 alpha phosphorylation and whether eIF-2 alpha phosphorylation is mediated by the double-stranded RNA-dependent protein kinase (PKR), we studied protein synthesis in response to calcium depletion mediated by calcium ionophore A23187 in cell lines overexpressing wild-type eIF-2 alpha, a mutant eIF-2 alpha (S51A) that is resistant to phosphorylation, or a dominant negative mutant PKR (K296P in catalytic subdomain II). Expression of either mutant eIF-2 alpha or mutant PKR partially protected NIH3T3 cells from inhibition of protein synthesis upon A23187 treatment. In contrast, overexpression of wild-type PKR increased sensitivity to protein synthesis inhibition mediated by A23187 treatment. In a COS-1 monkey cell transient transfection system, increased eIF-2 alpha phosphorylation in response to A23187 treatment was inhibited by expression of the dominant negative PKR mutant. Overexpression of the PKR regulatory RNA binding domain, independent of the PKR catalytic domain, was sufficient to inhibit increased phosphorylation of eIF-2 alpha upon A23187 treatment. In addition, overexpression of the HIV TAR RNA binding protein also inhibited eIF-2 alpha phosphorylation upon A23187 treatment. Taken together, our data show that calcium depletion activates PKR to phosphorylate eIF-2 alpha, and this activation is likely mediated through the PKR RNA binding domain.

Adenosine inhibits protein synthesis in isolated rat hepatocytes. Evidence for a lack of involvement of intracellular calcium in the mechanism of inhibition

Extracellularly added adenosine and ATP are potent inhibitors of protein synthesis in liver cells. In this study, the possible involvement of Ca2+ in the mechanism of inhibition of protein synthesis by adenosine was investigated. Stimulation of freshly isolated hepatocytes with adenosine or ATP, at concentrations that impaired protein synthesis, induced an increase in the cytosolic free Ca2+ concentration ([Ca2+]i). However, there was no correlation between the increase in [Ca2+]i and inhibition of radiolabelled leucine incorporation into proteins. Thus, the stimulation of hepatocytes with the V1-receptor agonist, vasopressin, or with the nucleotide triphosphates, UTP and GTP, elicited changes in [Ca2+]i similar to those observed after ATP or adenosine addition, but did not affect protein synthesis. ATP produced near complete discharge of Ca2+ from the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool in isolated hepatocytes, whereas adenosine only had a partial effect. Depletion of the hormone-sensitive Ca2+ pool by adenosine was transient. In contrast, prolonged depletion of internal Ca2+ by thapsigargin resulted in the inhibition of protein synthesis in hepatocytes. However, the inhibition of radiolabelled leucine incorporation into proteins by thapsigargin was further augmented by the additional presence of adenosine. These results show that the inhibition of protein synthesis by adenosine in isolated hepatocytes is not mediated by an increase in [Ca2+]i or depletion of internal pool(s) sensitive to inositol 1,4,5-trisphosphate or thapsigargin.

Effects of hypoxanthine-xanthine oxidase on Ca2+ stores and protein synthesis in human endothelial cells

We have investigated the effects of reactive O2 metabolites generated by the hypoxanthine-xanthine oxidase (HX-XO) system on intracellular Ca2+ and its relation with protein synthesis in human umbilical vein endothelial cells (HUVECs). Spectrofluorometry with fura 2 showed that the oxidative stress induced a rapid transient rise in cytosolic [Ca2+], followed by a sustained elevation above the baseline value. In the presence of La3+, which blocks Ca2+ influx from the extracellular medium, a transient [Ca2+] increase was still observed, but the sustained rise was suppressed. The HX-XO-related [Ca2+] changes were completely prevented by pretreatment with thapsigargin, which depletes intracellular Ca2+ stores. Hence, the effects of HX-XO on Ca2+ homeostasis were due to mobilization of Ca2+ from the intracellular stores with subsequent influx of extracellular Ca2+. HX-XO mobilized more of sequestered Ca2+ than did thrombin, a receptor agonist that depletes only a part of the intracellular Ca2+ stores (the hormone-sensitive stores). To determine the relevance of the HX-XO-related depletion of Ca2+ stores for cell function, we investigated the role of Ca2+ mobilization in the regulation of protein synthesis. Overall protein synthesis in HUVECs was markedly reduced by thapsigargin, which depletes both hormone-sensitive and -insensitive stores, but was not substantially affected by thrombin. Manipulation of the refilling of the Ca2+ stores via the availability of extracellular Ca2+ significantly influenced the thapsigargin-related and the HX-XO-related inhibition of overall protein synthesis. A corresponding effect of extracellular [Ca2+] was seen in polyribosome distribution profiles, which reflected an inhibition of translation initiation in both treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of superoxide, hydrogen peroxide, and hydroxyl radical on intracellular calcium in human endothelial cells

Changes in intracellular Ca2+ homeostasis are thought to contribute to cell dysfunction in oxidative stress. The hypoxanthine-xanthine oxidase system (X-XO) mobilizes Ca2+ from intracellular stores and induces a marked rise in cytosolic calcium in different cell types. To identify the reactive O2 species involved in the disruption of calcium homeostasis by X-XO, we studied the effect of X-XO on [Ca2+]i by spectrofluorimetry with fura-2 in human umbilical vein endothelial cells (HUVEC). The [Ca2+]i response to X-XO was essentially diminished by superoxide dismutase (SOD) (200 U/ml) and catalase (CAT) (200 U/ml), which scavenge the superoxide anion, O2-, or H2O2, respectively. The [Ca2+]i increase stimulated by 10 nmol H2O2/ml/min, generated from the glucose-glucose oxidase system, or 10 microM H2O2, given as bolus, was about a third of that induced by X-XO (10 nmol O2-/ml/min) but was comparable to that induced by X-XO in the presence of SOD. The X-XO-stimulated [Ca2+]i increase was significantly reduced by 100 microM o-phenanthroline, which inhibits the iron-catalysed formation of the hydroxyl radical. On the other hand, the [Ca2+]i response to low dose X-XO (1 nmol O2-/ml/min) was markedly enhanced in the presence of 1 microM H2O2, which itself had no effect on [Ca2+]i. More than 50% of this synergistic effect was prevented by o-phenanthroline. These results indicate that the effect of X-XO on calcium homeostasis appears to result from an interaction of O2- and H2O2, which could be explained by the formation of the hydroxyl radical.

Depletion of calcium from the lumen of endoplasmic reticulum reversibly inhibits passive diffusion and signal-mediated transport into the nucleus

Nuclear pore complexes provide channels for molecular transport across the nuclear envelope. Translocation of most proteins and RNAs through the pore complex is mediated by signal- and ATP-dependent mechanisms, while transport of small molecules is accomplished by passive diffusion. We report here that depletion of calcium from the lumen of the endoplasmic reticulum and nuclear envelope with ionophores or the calcium pump inhibitor thapsigargin rapidly and potently inhibits signal mediated transport of proteins into the nucleus. Lumenal calcium depletion also inhibits passive diffusion through the pore complex. Signal-mediated protein import and passive diffusion are rapidly restored when the drugs depleting lumenal calcium are removed and cells are incubated at 37 degrees C in calcium-containing medium. These results indicate that loss of calcium from the lumen of the endoplasmic reticulum and nuclear envelope reversibly affects properties of pore complex components located on the nuclear/cytoplasmic membrane surfaces, and they provide direct functional evidence for conformational flexibility of the pore complex. These methods will be useful for achieving reversible inhibition of nucleocytoplasmic trafficking for in vivo functional studies, and for studying the structure of the passive diffusion channel(s) of the pore complex.

Effects of thapsigargin, an intracellular calcium-mobilizing agent, on synthesis and secretion of cartilage collagen and proteoglycan

The calcium-mobilizing agents thapsigargin and 2,5-di-(tert-butyl)-1,4- benzohydroquinone were shown to markedly elevate the intracellular calcium concentration of chick embryo chondrocytes in a dose-dependent manner. Under these conditions, the metabolism of macromolecules was variably affected. The synthesis and secretion of protein in general, and of collagen in particular, were significantly inhibited; in contrast, proteoglycan synthesis (but not glycosaminoglycan synthesis) was inhibited, whereas secretion was unaffected. Flunarizine, which prevented the thapsigargin-induced intracellular calcium elevation, and EGTA, which caused only a transient thapsigargin-induced intracellular calcium elevation, did not reverse these alterations. It was concluded, therefore, that the observed effects of thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone on chondrocyte macromolecule metabolism were not related to the ability of these drugs to increase the cytosolic free calcium concentration but may have been due to the specific depletion of the calcium sequestered in the endoplasmic reticulum. The differential effect of these drugs on protein and proteoglycan secretion suggests that the intracellular trafficking of these two classes of macromolecules may be controlled independently.

The intracellular Ca(2+)-pump inhibitors thapsigargin and cyclopiazonic acid induce stress proteins in mammalian chondrocytes

Primary cultures of mammalian articular chondrocytes respond to treatment with the intracellular Ca(2+)-pump inhibitors thapsigargin (TG) and cyclopiazonic acid by specific changes in protein synthesis consistent with a stress response. Two-dimensional gel electrophoresis of newly synthesized proteins confirmed that the response was consistent with the induction of glucose-regulated proteins. The effects of low-dose TG (10 nM), measured by changes in [35S]methionine labelling of newly synthesized proteins, can first be observed by 10 h and are maximal by 24 h. The pattern of changes induced by TG is shared with cyclopiazonic acid, but effects of both perturbants differ significantly from changes induced by heat shock. Upon removal of TG, normal protein synthesis is restored by 48 h. Immunoblots showed increased concentrations of the stress proteins HSP90, HSP72/73 and HSP60 in chondrocytes treated with TG, but induction of newly synthesized heat-shock proteins by TG was not apparent on [35S]methionine-labelled gels. The alterations in protein synthesis induced by Ca(2+)-pump inhibitors were unaffected by BAPTA-AM loading, which clamped cytosolic Ca2+ at resting levels. We conclude that inhibition of intracellular Ca(2+)-pump activity can elicit a stress response, which has important implications for the interpretation of chronic use of Ca(2+)-pump inhibitors. In particular, the activation of the cellular shock response should be considered in interpreting the regulation of protein synthesis and cell survival by Ca(2+)-pump inhibitors such as TG.

Calcium as a permissive factor but not an initiation factor in DNA synthesis induction in cultured rat hepatocytes by the peroxisome proliferator ciprofibrate

The non-genotoxic hepatocarcinogen and peroxisome proliferating agent, ciprofibrate, is a liver mitogen both in vivo and in cultured adult rat hepatocytes, but the mechanisms of its mitogenicity have not been elucidated. We previously observed that ciprofibrate rapidly increased hepatocyte free intracellular Ca2+ concentration ([Ca2+]i), suggesting that this effect may play a role in the initiation of DNA synthesis. In the present study, we have identified a relationship between Ca2+ and the stimulation of hepatocyte DNA synthesis by ciprofibrate. Exposure of cultured adult rat hepatocytes to ciprofibrate (200 microM) for 48 hr increased DNA synthesis by approximately 2-fold, and this response was attenuated in a Ca(2+)-deficient medium and by the Ca2+ channel blockers nicardipine and verapamil. To examine the relationship between the stimulation of hepatocyte DNA synthesis and increases in [Ca2+]i by ciprofibrate, the intracellular Ca2+ chelator 5,5'-dimethyl-1,2-bis(2-aminophenoxyethane)-N,N,N',N'-tetraacetic acid (dimethyl-BAPTA) was employed. Pretreatment of hepatocytes with dimethyl-BAPTA blocked ciprofibrate-induced [Ca2+]i increase, but did not block ciprofibrate-induced hepatocyte DNA synthesis. Dimethyl-BAPTA was only effective in reducing ciprofibrate-induced DNA synthesis when present during the latter 24 hr of a 48-hr culture period. These data suggest that the early mobilization of hepatocyte [Ca2+]i by ciprofibrate does not play an initiating role in the induction of hepatocyte DNA synthesis but rather may operate as a permissive factor for the entry of ciprofibrate-treated adult rat hepatocytes into S-phase.

Okadaic acid uncouples calcium entry from depletion of intracellular stores

The mechanism by which the depletion of intracellular Ca2+ stores stimulates Ca2+ influx is poorly understood. However, the coupling of depletion to influx is broken during mitosis [Preston, S.F. et. al., (1991) Cell Regul., 2, 915-925]. Thus, in interphase HeLa cells, activation of the histamine H1 receptor, or incubation with thapsigargin, which inhibits the Ca(2+)-ATPase of storage vesicles and depletes Ca2+ stores, strongly stimulate Ca2+ influx. In mitotic cells, however, neither histamine nor thapsigargin stimulate Ca2+ influx. Since it has been found that okadaic acid treatment of interphase cells induces a mitotic-like state with respect to a number of other membrane processes, we have asked if okadaic acid might also uncouple Ca2+ depletion from stimulated influx. We show that okadaic acid specifically does suppress this coupling: thapsigargin and histamine deplete stores in control and okadaic-acid-treated HeLa cells, but after treatment with okadaic acid, stimulation of Ca2+ influx is barely detectable. This suggests that a protein phosphorylation/dephosphorylation event controls the coupling of Ca2+ stores to influx, and that there may be a physiological mechanism for control of the Ca2+ response to hormonal signals at the level of coupling.