Calcium signaling and cytotoxicity

Cytosolic irradiation of femtosecond laser induces mitochondria-dependent apoptosis-like cell death via intrinsic reactive oxygen cascades

High-intensity femtosecond lasers have recently been used to irreversibly disrupt nanoscale structures, such as intracellular organelles, and to modify biological functions in a reversible manner: so-called nanosurgery and biophotomodulation. Femtosecond laser pulses above the threshold intensity sufficient for reversible biophotomodulation can cause irreversible changes in the irradiated cell, eventually leading to cell death. Here, we demonstrated that cytosolic irradiation with a femtosecond laser produced intrinsic cascades of reactive oxygen species (ROS), which led to rapid apoptosis-like cell death via a caspase and poly (ADP-ribose) polymerase 1 (PARP-1) signaling pathway. We further showed that cells with enhanced mitochondrial fusion activity are more resilient to laser-induced stress compared to those with enforced mitochondrial fission. Taken together, these findings provide fundamental insight into how optical stimulation intervenes in intrinsic cellular signaling pathways and functions.

Downregulation of E2F1 during ER stress is required to induce apoptosis

The endoplasmic reticulum (ER) has recently emerged as an alternative target to induce cell death in tumours, because prolonged ER stress results in the induction of apoptosis even in chemoresistant transformed cells. Here, we show that the DNA-damage-responsive pro-apoptotic factor E2F1 is unexpectedly downregulated during the ER stress-mediated apoptotic programme. E2F1 decline is a late event during the ER response and is mediated by the two unfolded protein response (UPR) sensors ATF6 and IRE1 (also known as ERN1). Whereas ATF6 directly interacts with the E2F1 promoter, IRE1 requires the involvement of the known E2F1 modulator E2F7, through the activation of its main target Xbp-1. Importantly, inhibition of the E2F1 decrease prevents ER-stress-induced apoptosis, whereas E2F1 knockdown efficiently sensitises cells to ER stress-dependent apoptosis, leading to the upregulation of two main factors in the UPR pro-apoptotic execution phase, Puma and Noxa (also known as BBC3 and PMAIP1, respectively). Our results point to a novel key role of E2F1 in the cell survival/death decision under ER stress, and unveil E2F1 inactivation as a valuable novel potential therapeutic strategy to increase the response of tumour cells to ER stress-based anticancer treatments.

NELL2 function in the protection of cells against endoplasmic reticulum stress

Continuous intra- and extracellular stresses induce disorder of Ca(2+) homeostasis and accumulation of unfolded protein in the endoplasmic reticulum (ER), which results in ER stress. Severe long-term ER stress triggers apoptosis signaling pathways, resulting in cell death. Neural epidermal growth factor-like like protein 2 (NELL2) has been reported to be important in protection of cells from cell death-inducing environments. In this study, we investigated the cytoprotective effect of NELL2 in the context of ER stress induced by thapsigargin, a strong ER stress inducer, in Cos7 cells. Overexpression of NELL2 prevented ER stress-mediated apoptosis by decreasing expression of ER stress-induced C/EBP homologous protein (CHOP) and increasing ER chaperones. In this context, expression of anti-apoptotic Bcl-xL was increased by NELL2, whereas NELL2 decreased expression of pro-apoptotic proteins, such as cleaved caspases 3 and 7. This anti-apoptotic effect of NELL2 is likely mediated by extracellular signal-regulated kinase (ERK) signaling, because its inhibitor, U0126, inhibited effects of NELL2 on the expression of anti- and pro-apoptotic proteins and on the protection from ER stress-induced cell death.

Caffeine-induced Ca2+ oscillations in type I horizontal cells of the carp retina and the contribution of the store-operated Ca2+ entry pathway

The mechanisms of release, depletion, and refilling of endoplasmic reticulum (ER) Ca²⁺ were investigated in type I horizontal cells of the carp retina using a fluo-3-based Ca²⁺ imaging technique. Exogenous application of caffeine, a ryanodine receptor agonist, induced oscillatory intracellular free Ca²⁺ concentration ([Ca²⁺]_i) responses in a duration- and concentration-dependent manner. In Ca²⁺-free Ringer’s solution, [Ca²⁺]_i transients could also be induced by a brief caffeine application, whereas subsequent caffeine application induced no [Ca²⁺]_i increase, which implied that extracellular Ca²⁺ was required for ER refilling, confirming the necessity of a Ca²⁺ influx pathway for ER refilling. Depletion of ER Ca²⁺ by thapsigargin triggered a Ca²⁺ influx which could be blocked by the store-operated channel inhibitor 2-APB, which proved the existence of the store-operated Ca²⁺ entry pathway. Taken together, these results suggested that after being depleted by caffeine, the ER was replenished by Ca²⁺ influx via store-operated channels. These results reveal the fine modulation of ER Ca²⁺ signaling, and the activation of the store-operated Ca²⁺ entry pathway guarantees the replenishment of the ER so that the cell can be ready for response to the subsequent stimulus.

Pharmacology of the capsaicin receptor, transient receptor potential vanilloid type-1 ion channel

The capsaicin receptor, transient receptor potential vanilloid type 1 ion channel (TRPV1), has been identified as a polymodal transducer molecule on a sub-set of primary sensory neurons which responds to various stimuli including noxious heat (> -42 degrees C), protons and vanilloids such as capsaicin, the hot ingredient of chilli peppers. Subsequently, TRPV1 has been found indispensable for the development of burning pain and reflex hyperactivity associated with inflammation of peripheral tissues and viscera, respectively. Therefore, TRPV1 is regarded as a major target for the development of novel agents for the control of pain and visceral hyperreflexia in inflammatory conditions. Initial efforts to introduce agents acting on TRPV1 into clinics have been hampered by unexpected side-effects due to wider than expected expression in various tissues, as well as by the complex pharmacology, of TRPV1. However, it is believed that better understanding of the pharmacological properties of TRPV1 and specific targeting of tissues may eventually lead to the development of clinically useful agents. In order to assist better understanding of TRPV1 pharmacology, here we are giving a comprehensive account on the activation and inactivation mechanisms and the structure-function relationship of TRPV1.

DON shares a similar mode of action as the ribotoxic stress inducer anisomycin while TBTO shares ER stress patterns with the ER stress inducer thapsigargin based on comparative gene expression profiling in Jurkat T cells

Previously, we studied the effects of deoxynivalenol (DON) and tributyltin oxide (TBTO) on whole genome mRNA expression profiles of human T lymphocyte Jurkat cells. These studies indicated that DON induces ribotoxic stress and both DON and TBTO induced ER stress which resulted into T-cell activation and apoptosis. The first goal of the present study was to provide final proof for these mode of actions by comparing the effects of 6 h exposure to DON and TBTO on mRNA expression to those of positive controls of ribotoxic stress (anisomycin), ER stress (thapsigargin) and T cell activation (ionomycin). Genes affected by anisomycin and the majority of genes affected by thapsigargin were affected in the same direction by DON and TBTO, respectively, confirming the expected modes of action. Pathway analysis further sustained that DON induces ribotoxic stress and both DON and TBTO induce unfolded protein response (UPR), ER stress, T cell activation and apoptosis. The second goal was to assess whether DON and/or TBTO affect other pathways above those detected before. TBTO induced groups of genes that are involved in DNA packaging and heat shock response that were not affected by thapsigargin. DON did not affect other genes than anisomycin indicating the effect of DON to be restricted to ribotoxic stress. This study also demonstrates that comparative gene expression analysis is a very promising tool for the identification of modes of action of immunotoxic compounds.

Exposure to tributyltin induces endoplasmic reticulum stress and the unfolded protein response in zebrafish

Tributyltin (TBT) is a major marine contaminant and causes endocrine disruption, hepatotoxicity, immunotoxicity, and neurotoxicity. However, the molecular mechanisms underlying the toxicity of TBT have not been fully elucidated. We examined whether exposure to TBT induces the endoplasmic reticulum (ER) stress response in zebrafish, a model organism. Zebrafish-derived BRF41 fibroblast cells were exposed to 0.5 or 1 μM TBT for 0.5-16 h and subsequently lysed and immunoblotted to detect ER stress-related proteins. Zebrafish embryos, grown until 32 h post fertilization (hpf), were exposed to 1 μM TBT for 16 h and used in whole mount in situ hybridization and immunohistochemistry to visualize the expression of ER chaperones and an ER stress-related apoptosis factor. Exposure of the BRF41 cells to TBT caused phosphorylation of the zebrafish homolog of protein kinase RNA-activated-like ER kinase (PERK), eukaryotic translation initiation factor 2 alpha (eIF2α), and inositol-requiring enzyme 1 (IRE1), characteristic splicing of X-box binding protein 1 (XBP1) mRNA, and enhanced expression of activating transcription factor 4 (ATF4) protein. In TBT-exposed zebrafish embryos, ectopic expression of the gene encoding zebrafish homolog of the 78 kDa glucose-regulating protein (GRP78) and gene encoding CCAAT/enhancer-binding protein homologous protein (CHOP) was detected in the precursors of the neuromast, which is a sensory organ for detecting water flow and vibration. Our in vitro and in vivo studies revealed that exposure of zebrafish to TBT induces the ER stress response via activation of both the PERK-eIF2α and IRE1-XBP1 pathways of the unfolded protein response (UPR) in an organ-specific manner.

Strong anticancer potential of nano-triterpenoid from Phytolacca decandra against A549 adenocarcinoma via a Ca(2+)-dependent mitochondrial apoptotic pathway

We isolated a triterpenoid from an ethanolic extract of Phytolacca decandra and nanoencapsulated it with biodegradable nontoxic polymers of poly(lactide-co-glycolide) to examine if the nanoform of this hitherto unexplored betulinic-acid derivative (NdBA) could produce a stronger anticancer effect by rendering better drug bioavailability and targeted delivery than the nonencapsulated betulinic-acid derivative (dBA). The nanoparticles were characterized with the help of physicochemical and morphological studies involving dynamic light scattering and atomic force microscopy. A549 cancer cells were exposed to NdBA and dBA at the IC50 doses of 50 μg/mL and 100 μg/mL, respectively. Mitochondrial dysfunction-mediated apoptosis was determined by examining the changes in the intracellular calcium content, the reactive oxygen species accumulation, the cytochrome c release, the upregulation of Bcl-2-associated-X protein (Bax) and caspase 3, the downregulation of B cell lymphoma 2, and the mitochondrial membrane potential (ΔΨm) depolarization. Apoptosis was also verified by acridine orange staining observed under fluorescence microscopy and annexin V-fluorescein isothiocyanate/propidium iodide staining through flow cytometric studies. The levels of intracellular adenosine triphosphate/adenosine diphosphate ratio decreased, and the ATPase activity increased more strikingly in A549 cells exposed to NdBA than in A549 cells exposed to dBA. Overall results showed that both drugs directly target the mitochondrial oxidative phosphorylation system, with NdBA having a stronger effect, indicating NdBA to be a better candidate for the development of an anticancer drug for use against lung adenocarcinomas.

Over-expression of X-linked inhibitor of apoptosis protein modulates multiple aspects of neuronal Ca2+ signaling

X-linked inhibitor of apoptosis (XIAP) protects and preserves the function of neurons in both in vitro and in vivo models of excitotoxicity. Since calcium (Ca(2+)) overload is a pivotal event in excitotoxic neuronal cell death, we have determined whether XIAP over-expression influences Ca(2+)-signaling in primary cultures of mouse cortical neurons. Using cortical neuron cultures derived from wild-type (Wt) mice transiently transfected with XIAP or from transgenic mice that over-express XIAP, we show that XIAP opposes the rise in intracellular Ca(2+) concentration by a variety of triggers. Relative to control neurons, XIAP over-expression produced a slight, but significant, elevation of resting Ca(2+) concentrations. By contrast, the rise in intracellular Ca(2+) concentrations produced by N-methyl-D-aspartate receptor stimulation and voltage gated Ca(2+) channel activation were markedly attenuated by XIAP over-expression. The release of Ca(2+) from intracellular stores induced by the sarco/endoplasmic reticulum Ca(2+) ATPase inhibitor thapsigargin was also inhibited in neurons transiently transfected with XIAP. The pan-caspase inhibitor zVAD did not, however, diminish the rise in intracellular Ca(2+) concentrations elicited by L-glutamate suggesting that XIAP influences Ca(2+) signaling in a caspase-independent manner. Taken together, these findings demonstrate that the ability of XIAP to block excessive rises in intracellular Ca(2+) by a variety of triggers may contribute to the neuroprotective effects of this anti-apoptotic protein.

The unfolded protein response to endoplasmic reticulum stress in cultured astrocytes and rat brain during experimental diabetes

Oxidative-nitrosative stress and inflammatory responses are associated with endoplasmic reticulum (ER) stress in diabetic retinopathy, raising the possibility that disturbances in ER protein processing may contribute to CNS dysfunction in diabetics. Upregulation of the unfolded protein response (UPR) is a homeostatic response to accumulation of abnormal proteins in the ER, and the present study tested the hypothesis that the UPR is upregulated in two models for diabetes, cultured astrocytes grown in 25mmol/L glucose for up to 4weeks and brain of streptozotocin (STZ)-treated rats with diabetes for 1-7months. Markers associated with translational blockade (phospho-eIF2α and apoptosis (CHOP), inflammatory response (inducible nitric oxide synthase, iNOS), and nitrosative stress (nuclear translocation of glyceraldehyde-3-phosphate dehydrogenase, GAPDH) were not detected in either model. Nrf2 was present in nuclei of low- and high-glucose cultures, consistent with oxidative stress. Astrocytic ATF4 expression was not altered by culture glucose concentration, whereas phospho-IRE and ATF6 levels were higher in low- compared with high-glucose cultures. The glucose-regulated chaperones, GRP78 and GRP94, were also expressed at higher levels in low- than high-glucose cultures, probably due to recurrent glucose depletion between feeding cycles. In STZ-rat cerebral cortex, ATF4 level was transiently reduced at 4months, and p-IRE levels were transiently elevated at 3months. However, GRP78 and GRP94 expression was not upregulated, and iNOS, amyloid-β, and nuclear accumulation of GAPDH were not evident in STZ-diabetic brain. High-glucose cultured astrocytes and STZ-diabetic brain are relatively resistant to diabetes-induced ER stress, in sharp contrast with cultured retinal Müller cells and diabetic rodent retina.

Regulation of inositol 1,4,5-trisphosphate receptors during endoplasmic reticulum stress

The endoplasmic reticulum (ER) performs multiple functions in the cell: it is the major site of protein and lipid synthesis as well as the most important intracellular Ca(2+) reservoir. Adverse conditions, including a decrease in the ER Ca(2+) level or an increase in oxidative stress, impair the formation of new proteins, resulting in ER stress. The subsequent unfolded protein response (UPR) is a cellular attempt to lower the burden on the ER and to restore ER homeostasis by imposing a general arrest in protein synthesis, upregulating chaperone proteins and degrading misfolded proteins. This response can also lead to autophagy and, if the stress can not be alleviated, to apoptosis. The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and IP3-induced Ca(2+) signaling are important players in these processes. Not only is the IP3R activity modulated in a dual way during ER stress, but also other key proteins involved in Ca(2+) signaling are modulated. Changes also occur at the structural level with a strengthening of the contacts between the ER and the mitochondria, which are important determinants of mitochondrial Ca(2+) uptake. The resulting cytoplasmic and mitochondrial Ca(2+) signals will control cellular decisions that either promote cell survival or cause their elimination via apoptosis. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Assessment of the usefulness of the murine cytotoxic T cell line CTLL-2 for immunotoxicity screening by transcriptomics

A toxicogenomics approach was applied to assess the usefulness of the mouse cytotoxic T cell line CTLL-2 for in vitro immunotoxicity testing. CTLL-2 cells were exposed for 6 h to two model immunotoxic compounds: (1) the mycotoxin deoxynivalenol (DON, 1 and 2 μM), a ribotoxic stress inducer, and (2) the organotin compound tributyltin oxide (TBTO, 100 and 200 nM), an endoplasmic reticulum (ER) stress inducer. Effects on whole-genome mRNA expression were assessed by microarray analysis. The biological interpretation of the microarray data indicated that TBTO (200 nM) induced genes involved in T cell activation, ER stress, NFκB activation and apoptosis, which agreed very well with results obtained before on TBTO exposed Jurkat cells and mouse primary thymocytes. Remarkably, DON (2 μM) downregulated genes involved in T cell activation, ER stress and apoptosis, which is opposite to results obtained before for DON-exposed Jurkat cells and mouse primary thymocytes. Furthermore, the results for DON in CTLL-2 cells are also opposite to the results obtained for TBTO in CTLL-2 cells. In agreement with the lack of induction of ER stress and apoptosis, viability assays showed that CTLL-2 cells are much more resistant to the toxicity of DON than Jurkat cells and primary thymocytes. We propose that CTLL-2 cells lack the signal transduction that induces ER stress and apoptosis in response to ribotoxic stress. Based on the results for TBTO and DON, the CTLL-2 cell line does not yield an added value for immunotoxicity compared to the human Jurkat T cell line.

Differential regulation of TRPV4 mRNA levels by acclimation salinity and extracellular osmolality in euryhaline tilapia

Prolactin (PRL) cells of the euryhaline Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors. Hyposmotically-induced PRL release is mediated by the inward movement of extracellular Ca(2+) through a stretch-activated Ca(2+) channel, which has been recently identified as the transient receptor potential vanilloid 4 (TRPV4). In the present study, changes in plasma PRL, as well as PRL and TRPV4 mRNA expression from the rostral pars distalis (RPD), were measured in fish transferred from seawater (SW) to fresh water (FW) and in fish transferred from FW to SW. The in vitro effects of osmolality on PRL release and on PRL and TRPV4 mRNA expression in dispersed PRL cells were compared between fish adapted to SW and FW. Both the release and expression of PRL fell when fish were transferred to SW and rose when fish were transferred to FW. By contrast, TRPV4 expression increased by 48h after fish were transferred from FW to SW and declined as early as 6h after transfer from SW to FW. A similar pattern was observed in vitro where TRPV4 expression responded positively to an increase in medium osmolality while PRL expression declined. Incubation with the Ca(2+) ionophore, A23187, and the phosphodiesterase inhibitor, IBMX, stimulated PRL release. While both IBMX and A23187 inhibited TRPV4 expression, only A23187 reduced PRL expression. Together, these findings indicate that the expression of TRPV4 mRNA is osmosensitive, increasing as extracellular osmolality rises. Furthermore, these data suggest that TRPV4 expression may be regulated through the same second messenger pathways involved in hyposmotically-induced PRL release.

Efflux of glutathione and glutathione complexes from human erythrocytes in response to vanadate

The main objective of the present study was to investigate if vanadate is extruded from the cells in a glutathione dependent manner resulting in the appearance of extracellular glutathione and complexes of glutathione with vanadium. Vanadate significantly depleted intracellular non-protein sulfhydryl (NPSH) levels in a time- and concentration-dependent manner. The intracellular NPSH level was decreased to 0.0 ± 0.0 μmol/ml erythrocyte when exposed to 10 mM of vanadate for 4h. Extracellular NPSH level was increased concomitantly with the intracellular decrease and reached to 0.1410 ± 0.005 μmol/ml erythrocyte in 4h. Intracellular decrease and extracellular increase in NPSH levels were significantly inhibited in the presence of DIDS, a chloride-bicarbonate exchanger which also mediates phosphate and arsenate transport in erythrocytes. In parallel with the increase in extracellular NPSH levels, significant increases in extracellular glutathione levels were detected following exposure to vanadate. Extracellular glutathione levels reached to 0.0150 ± 0.0.001, 0.0330 ± 0.001, and 0.0576 ± 0.002 μmol/ml erythrocyte with 1, 5, and 10 mM of vanadate respectively. Dimercaptosuccinic acid treatment of supernatants significantly increased the glutathione levels measured in the extracellular media. Utilization of MK571 an MRP inhibitor decreased the rate of glutathione efflux from erythrocytes suggesting a role for this membrane transporter in the process. A known methylation inhibitor periodate oxidized adenosine decreased the rate of glutathione efflux from erythrocytes. This observed decrease in extracellular GSH levels suggests that GSH release partly requires a proper cellular methylation process and that part of GSH detected in the extracellular media may arise from GSH-vandium complexes. The results of the present study indicate that human erythrocyte efflux glutathione in reduced free form and in conjugated form/s that can be recovered with dimercaptosuccinic acid when exposed to vanadate.

Role of calcium and mitochondria in MeHg-mediated cytotoxicity

Methylmercury (MeHg) mediated cytotoxicity is associated with loss of intracellular calcium (Ca²⁺) homeostasis. The imbalance in Ca²⁺ physiology is believed to be associated with dysregulation of Ca²⁺ intracellular stores and/or increased permeability of the biomembranes to this ion. In this paper we summarize the contribution of glutamate dyshomeostasis in intracellular Ca²⁺ overload and highlight the mitochondrial dysfunctions induced by MeHg via Ca²⁺ overload. Mitochondrial disturbances elicited by Ca²⁺ may involve several molecular events (i.e., alterations in the activity of the mitochondrial electron transport chain complexes, mitochondrial proton gradient dissipation, mitochondrial permeability transition pore (MPTP) opening, thiol depletion, failure of energy metabolism, reactive oxygen species overproduction) that could culminate in cell death. Here we will focus on the role of oxidative stress in these phenomena. Additionally, possible antioxidant therapies that could be effective in the treatment of MeHg intoxication are briefly discussed.

Terfenadine induces apoptosis and autophagy in melanoma cells through ROS-dependent and -independent mechanisms

Previously we found that terfenadine, an H1 histamine receptor antagonist, acts as a potent apoptosis inducer in melanoma cells through modulation of Ca²⁺ homeostasis. In this report, focusing our attention on the apoptotic mechanisms activated by terfenadine, we show that this drug can potentially activate distinct intrinsic signaling pathways depending on culture conditions. Serum-deprived conditions enhance the cytotoxic effect of terfenadine and caspase-4 and -2 are activated upstream of caspase-9. Moreover, although we found an increase in ROS levels, the apoptosis was ROS independent. Conversely, terfenadine treatment in complete medium induced ROS-dependent apoptosis. Caspase-4, -2, and -9 were simultaneously activated and p73 and Noxa induction were involved. ROS inhibition prevented p73 and Noxa expression but not p53 and p21 expression, suggesting a role for Noxa in p53-independent apoptosis in melanoma cells. Finally, we found that terfenadine induced autophagy, that can promote apoptosis. These findings demonstrate the great potential of terfenadine to kill melanoma cells through different cellular signaling pathways and could contribute to define new therapeutic strategies in melanoma.

Regulation of cytosolic phospholipase A2 (cPLA2alpha) and its association with cell proliferation in human lens epithelial cells

PURPOSE

To investigate the molecular mechanism for cytosolic phospholipase A2 (cPLA(2)α) regulation and its association to platelet-derived growth factor (PDGF)-induced cell proliferation.

METHODS

cPLA(2)α was examined using human lens epithelial (HLE) B3 cells. Reactive oxygen species (ROS) generation induced by PDGF was analyzed by luminescence assay. Cell proliferation was measured by cell counting and by BrdU assay. Human cPLA(2)α gene was cloned via RT-PCR followed by site-directed mutagenesis to construct HLE B3 cells expressing either inactive cPLA(2)α enzyme with S228A mutation (S228A), or cPLA(2)α truncated at the calcium-binding C2 domain (C2D). Activity of cPLA(2)α was measured by arachidonic acid (AA) release from cell membranes using [(3)H]-arachidonic acid prelabeled cells. The effect of intracellular calcium level on cPLA(2)α function was examined by treating cells with ionomycin (calcium influx), thapsgargin (endoplasmic reticulum [ER] calcium store release) or 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (BAPTA; calcium chelator). Activation of extracellular signal-regulated kinases (ERK), JNK, p38, or Akt was detected by Western blot analysis using specific antibodies.

RESULTS

S228A mutant showed suppressed PDGF-induced reactive oxygen species generation, ERK and JNK activation (no effect on p38 or Akt), and cell proliferation in comparison with the vector alone (Vec) control. Calcium-binding C2 domain cells lost the ability of membrane translocation and activation of cPLA(2)α. PDGF cell signaling was calcium-dependent, and the calcium was supplied either from the external flux or endoplasmic reticulum store. However, enrichment of cellular calcium not only augmented PDGF function, but also demonstrated a cPLA(2)α-dependent calcium-signaling cascade that led to cell proliferation.

CONCLUSIONS

cPLA(2)α is regulated by calcium mobilization and mitogen-activated protein kinases (MAPK) activation. Both PDGF mitogenic action and calcium signaling are cPLA(2)α-dependent.

Amplitude-encoded calcium oscillations in fish cells

The reaction of intracellular Ca(2+) to different agonist stimuli in primary hepatocytes from rainbow trout (Oncorhynchus mykiss) as well as the permanent fish cell line RTL-W1 was investigated systematically. In addition to "classical" agonists such as phenylephrine and ATP, model environmental toxicants like 4-nitrophenol and 3,4-dichloroaniline were used to elucidate possible interactions between toxic effects and Ca(2+) signaling. We report Ca(2+) oscillations in response to several stimuli in RTL-W1 cells and to a lesser extent in primary hepatocytes. Moreover, these Ca(2+) oscillations are amplitude-encoded in contrast to their mammalian counterpart. Bioinformatics and computational analysis were employed to identify key players of Ca(2+) signaling in fish and to determine likely causes for the experimentally observed differences between the Ca(2+) dynamics in fish cells compared to those in mammalian liver cells.

Fluoride-induced death of rat erythrocytes in vitro

Although fluoride (F) in low concentrations is essential for teeth and bone development, its excessive consumption causes numerous deleterious abnormalities in cellular metabolism and physiology often leading to cell death. The present study was performed to establish the toxic F effects inducing the death of rat erythrocytes in vitro. The cells were cultured in the presence of 0.5-16 mM NaF for 1, 5 and 24 h. The progression of erythrocyte death was monitored by cell viability (calcein assay), membrane integrity (hemolysis assay), alterations in the cell morphology (light microscopy) and size (flow cytometry forward scatter), plasma membrane scrambling (annexin V binding). To elucidate the molecular mechanisms underlying F-induced cell death, the cytosolic Ca2+ activity (Fluo-3 fluorescence) and ceramide formation (binding of FITC-labeled antibodies) were determined. Exposure of the rat erythrocytes to NaF considerably suppressed their viability and caused partial cell hemolysis within 24 h. The cells underwent dramatic morphological alterations resulted in appearance of shrunken echinocytes after 1h and swollen spherocytes within 24 h. The development of NaF-induced erythrocyte death was accompanied by progressive PS externalization at the outer cell membrane, ∼45% of the cells were annexin V-positive in response to 16 mM NaF within 24 h with a small cell population exhibiting necrotic features. The cell death was preceded by considerable accumulation of the free cytosolic Ca2+, with statistically significant increase in the number of Fluo-3-positive erythrocytes observed as early as during 1-h incubation with 0.5 mM NaF. NaF also induced moderate ceramide formation. Overall, exposure of the rat erythrocytes to NaF triggers rapid progression of their death in a dose- and time-dependent manner, with appearance of apoptotic cells after 1 and 5 h and transition to necrosis within 24 h. An increase in intracellular [Ca2+] appears to be crucial mechanism implicated in development of NaF-induced apoptosis in rat erythrocytes.

Neuroprotective effects of genistein in VSC4.1 motoneurons exposed to activated microglial cytokines

Pro-inflammatory cytokines released from activated microglia may be responsible for neuronal damage and resulting motor deficits associated with CNS disorders such as spinal cord injury, Parkinson's disease, and multiple sclerosis. Estrogen (17β-estradiol) is capable of ameliorating motoneuron death following spinal cord injury, but has a number of deleterious side effects. Genistein (GEN), an estrogen receptor beta agonist and potent antioxidant, may represent an alternative to estrogen in treating neurodegenerative disorders. However, little is known about the neuroprotective effects of GEN. We therefore tested whether GEN would prevent apoptosis in cultured motoneurons following exposure to pro-inflammatory cytokines released from IFN-γ activated microglia. Exposure of ventral spinal cord 4.1 motoneurons to microglial cytokine supernatant in vitro caused significant apoptosis and reduced mitochondrial membrane potential. An increase in reactive oxygen species, intracellular Ca(2+), calpain, caspases, cytochrome c, and the bax:bcl-2 ratio were also noted. GEN treatment reversed apoptotic death and cellular changes following cytokine exposure and was associated with increased expression of estrogen receptor β suggesting that GEN may promote neuroprotection via receptor-mediated pathways. The addition of ICI 182, 780, an estrogen receptor antagonist following GEN treatment attenuated neuroprotection, suggesting that GEN may act mainly via estrogen receptor β to protect VSC4.1 motoneurons. We conclude that GEN protects cultured ventral spinal cord 4.1 cells from inflammatory insult and thus may represent a potential beneficial therapy in the treatment of neurodegenerative disorders.

Calcium regulation of nucleocytoplasmic transport

Bidirectional trafficking of macromolecules between the cytoplasm and the nucleus is mediated by the nuclear pore complexes (NPCs) embedded in the nuclear envelope (NE) of eukaryotic cell. The NPC functions as the sole pathway to allow for the passive diffusion of small molecules and the facilitated translocation of larger molecules. Evidence shows that these two transport modes and the conformation of NPC can be regulated by calcium stored in the lumen of nuclear envelope and endoplasmic reticulum. However, the mechanism of calcium regulation remains poorly understood. In this review, we integrate data on the observations of calciumregulated structure and function of the NPC over the past years. Furthermore, we highlight challenges in the measurements of dynamic conformational changes and transient transport kinetics in the NPC. Finally, an innovative imaging approach, single-molecule superresolution fluorescence microscopy, is introduced and expected to provide more insights into the mechanism of calcium-regulated nucleocytoplasmic transport.

Relationship between membrane permeability and specificity of human secretory phospholipase A(2) isoforms during cell death

During apoptosis, a number of physical changes occur in the cell membrane including a gradual increase in permeability to vital stains such as propidium iodide. This study explored the possibility that one consequence of membrane changes concurrent with early modest permeability is vulnerability to degradation by secretory phospholipase A(2). The activity of this hydrolytic enzyme toward mammalian cells depends on the health of the cell; healthy cells are resistant, but they become susceptible early during programmed death. Populations of S49 lymphoma cells during programmed death were classified by flow cytometry based on permeability to propidium iodide and susceptibility to secretory phospholipase A(2). The apoptotic inducers thapsigargin and dexamethasone caused modest permeability to propidium iodide and increased staining by merocyanine 540, a dye sensitive to membrane perturbations. Various secretory phospholipase A(2) isozymes (human groups IIa, V, X, and snake venom) preferentially hydrolyzed the membranes of cells that displayed enhanced permeability. In contrast, cells exposed briefly to a calcium ionophore showed the increase in cell staining intensity by merocyanine 540 without accompanying uptake of propidium iodide. Under that condition, only the snake venom and human group X enzymes hydrolyzed cells that were dying. These results suggested that cells showing modest permeability to propidium iodide during the early phase of apoptosis are substrates for secretory phospholipase A(2) and that specificity among isoforms of the enzyme depends on the degree to which the membrane has been perturbed during the death process. This susceptibility to hydrolysis may be important as part of the signal to attract macrophages toward apoptotic cells.

Δ²,³-ivermectin ethyl secoester, a conjugated ivermectin derivative with leishmanicidal activity but without inhibitory effect on mammalian P-type ATPases

Looking at a new putative target for the large spectrum antiparasitic drug ivermectin, we recently showed that avermectin-derived drugs are active against promastigote and amastigote forms of Leishmania amazonensis at low micromolar concentrations. However, we then reported that at this concentration range ivermectin is also able to inhibit three important mammalian P-type ATPases so that unacceptable adverse effects could occur if this drug were used at such high doses therapeutically. The present work aimed to test the activity of ten ivermectin analogs on these rat ATPases in search of a compound with similar leishmanicidal activity but with no effect on the mammalian (host) ATPases at effective concentrations. We synthesized three new ivermectin analogs for testing on rat SERCA (1a and 1b), Na+, K+-ATPase (α₁ and α₂/α₃ isoforms) and H+/K+-ATPase activity, along with seven analogs already characterized for their leishmanicidal activity. Our main finding is that one of the prepared derivatives, Δ²,³-ivermectin ethyl secoester 8, is equipotent to ivermectin 1 for the in vitro leishmanicidal effects but is nearly without effect on the rat ATPases, indicating that it could have a better therapeutic index in vivo and could serve as a candidate for hit-to-lead progression. This conclusion is further supported by the fact that compound 8 produced only 6% (vs 77% for ivermectin) inhibition of the human kidney enzyme at 5 μM, a concentration corresponding to the IC₅₀ for the activity against L. amazonensis amastigotes.

Spatio-temporal calcium dynamics in pacemaking units of the interstitial cells of Cajal

The interstitial cells of Cajal (ICC) are responsible for producing pacemaking signals that stimulate rhythmic contractions in the gastro-intestinal system. The pacemaking signals are generated by membrane depolarizations, which are in turn linked to the integrated transport of calcium between the endoplasmic reticulum (ER), through inositol-trisphosphate receptor (IP(3)R) release, and mitochondria, through the uniporter. A non-specific cation channel (NSCC) is associated with the membrane depolarizations, and is inhibited by intracellular calcium. One theory proposes that the integrated calcium transport occurs within specific regions of the ICC called "pacemaker units," and results in localized calcium concentration reductions within these units, which in turn activate the NSCC and depolarize the membrane. We have constructed a model of the spatio-temporal calcium dynamics within an ICC pacemaker unit to determine under what conditions the local calcium concentrations may reduce below baseline. We obtain reductions of calcium concentrations below baseline but only under certain conditions. Without strong and persistent stimulation of the IP(3)R, reductions of calcium below baseline occur only with a non-physiological, time-dependent uniporter. Alternatively, sufficient IP(3)R release leads to reductions of calcium below baseline, due to depletion of the ER calcium store over the time scale of seconds, although these reductions require strong mitochondrial and ER calcium uptake.

Morphology of spermatogenic and accessory cells in the mussel Modiolus kurilensis under environmental pollution

A comparative light- and electron microscopic study of the male gonads of the bivalve mollusk Modiolus kurilensis from the reference and polluted sites in Amursky Bay (Sea of Japan) was conducted. Testicular acini in the mussels from the reference site had well-ordered structure (vertical spermatogenic columns located among the accessory cells bodies) whereas in the testes of the mollusks from the polluted site, the accessory and spermatogenic cell populations were disarranged. Mussels from the polluted station had about 26% of spermatogenic cells with marginal localization of nuclear chromatin, swollen outer nuclear membrane and heavily vacuolated cytoplasm and about 8% of spermatozoa with transformed or destructed acrosome; in mussels from the reference station, these values were close to zero. The accessory cells in the mussels from the polluted site were underdeveloped, and their phagocytic activity was inhibited. Our ultrastructural observations provide evidence that both spermatogenic and accessory cells are targets of environmental pollution in marine mussels.

The dual role of calcium as messenger and stressor in cell damage, death, and survival

Ca(2+) is an important second messenger participating in many cellular activities; when physicochemical insults deregulate its delicate homeostasis, it acts as an intrinsic stressor, producing/increasing cell damage. Damage elicits both repair and death responses; intriguingly, in those responses Ca(2+) also participates as second messenger. This delineates a dual role for Ca(2+) in cell stress, making difficult to separate the different and multiple mechanisms required for Ca(2+)-mediated control of cell survival and apoptosis. Here we attempt to disentangle the two scenarios, examining on the one side, the events implicated in deregulated Ca(2+) toxicity and the mechanisms through which this elicits reparative or death pathways; on the other, reviewing the role of Ca(2+) as a messenger in the transduction of these same signaling events.

High endoplasmic reticulum activity renders multiple myeloma cells hypersensitive to mitochondrial inhibitors

Multiple myeloma (MM) cells continuously secrete large amounts of immunoglobulins that are folded in the endoplasmic reticulum (ER) whose function depend on the Ca(2+) concentration inside its lumen. Recently, it was shown that the ER membrane leaks Ca(2+) that is captured and delivered back by mitochondria in order to prevent its loss. Thus, we hypothesized that the highly active and abundant ER in MM cells results in greater Ca(2+)-regulation by mitochondria which would render them sensitive to mitochondrial inhibitors. Here, we indeed find that Ca(2+) leak is greater in 3 MM, when compared to 2 B-cell leukemia cell lines. Moreover, this greater leak in MM cells is associated with hypersensitivity to various mitochondrial inhibitors, including CCCP. Consistent with our hypothesis, CCCP is more potent in inducing the unfolded protein response marker, CHOP/GADD153 in MM versus B-cell leukemia lines. Additionally, MM cells are found to be significantly more sensitive to clinically used fenofibrate and troglitazone, both of which were recently shown to have inhibitory effects on mitochondrial function. Overall, our results demonstrate that the unusually high ER activity in MM cells may be exploited for therapeutic benefit through the use of mitochondrial inhibitors including troglitazone and fenofibrate.

The pro-apoptotic substance thapsigargin selectively stimulates re-growth of brain capillaries

Thapsigargin is a pro-apoptotic chemical, which has been shown to be useful to study cell death of cholinergic or dopaminergic neurons, or cells, which degenerate in Alzheimer's disease or Parkinson's disease, respectively. The aim of the present work was to study the effects of thapsigargin in the well established organotypic brain co-slice model composed of the basal nucleus of Meynert (nBM), ventral mesencephalon (vMes), dorsal striatum (dStr) and parietal cortex (Ctx). Cholinergic acetyltransferase-positive neurons in the nBM and dStr and dopaminergic tyrosine hydroxylase-positive neurons in the vMes survived, when cultured for 4 weeks with nerve growth factor and glial cell line-derived neurotrophic factor. Nerve fibers of cholinergic nBM neurons grew into the cortex and dopaminergic nerve fibers sprouted into dopamine D2 receptor-positive dStr. The whole co-slice contained a dense laminin-positive capillary network. Treatment of co-cultures with 3 microM thapsigargin for 24 hr significantly decreased the number of cholinergic neurons and dopaminergic neurons. This cell death displayed apoptotic DAPI-positive malformed nuclei and enhanced TUNEL-positive cells. Thapsigargin selectively stimulated the laminin-positive capillary growth between the nBM and Ctx. In conclusion, the induced cell death of cholinergic and dopaminergic neurons may be accompanied by enhanced angiogenic activity.

Expression of TBX2 promotes anchorage-independent growth and survival in the p53-negative SW13 adrenocortical carcinoma

The transcriptional regulator TBX2 is genetically amplified in several cancers and has, in addition, important roles in development. In carcinogenesis, TBX2 regulates the cell cycle by suppressing the expression of cyclin-dependent kinase (CDK) inhibitors and destabilizes p53 by suppressing expression of ARF. In embryogenesis, however, TBX2 appears to act independently of the cell cycle or p53 and is regulated by growth factors. Tumorigenic functions of TBX2 that are independent of p53 or cell cycle regulation remain poorly understood. Here we used SW13 carcinoma cells which express inactive p53 and have no detectable p16 or p21 CDK-inhibitors as a model to study these functions. Expression of TBX2 in SW13 cells had no effect on the cell cycle but promoted anchorage-independence and increased resistance to apoptotic stimuli including UV-irradiation, the cytotoxic drug doxorubicin and lethal endoplasmic-reticulum stress. This is a cell type-dependent effect as TBX2 overexpression in PANC1 pancreatic cancer cells which are p53-negative has no effect on colony formation or survival after irradiation. Mechanistically, in SW13 cells, TBX2 overexpression strongly reduced the activation of caspase 3, 8 and 9 following UV-irradiation but without altering the expression of the corresponding procaspases. There were, however, dramatic and specific decreases in the expression of procaspases 1 and 4. The expression of the inhibitor of apoptosis, cIAP2/BIRC3, increased in TBX2-overexpressing cells. TBX2 was upregulated in a PI3K-dependent manner by growth factors that are tumorigenic for SW13. Inhibition of Akt phosphorylation abrogates upregulation of TBX2 by FGF-4. Our findings identify TBX2 as a cell type-dependent survival factor under a p53-negative background, and are indicative of a potentially wider role for TBX2 in carcinogenesis than hitherto described.

Induction of GADD153 expression by tributyltin in SH-SY5Y human neuroblastoma cells

The effects of tributyltin (TBT) exposure on the expression of growth arrest- and DNA damage-inducible gene 153 (GADD153), also called C/EBP homologous protein (CHOP), were examined in SH-SY5Y human neuroblastoma cells. In response to TBT exposure, the levels of both GADD153 mRNA and GADD153 protein increased significantly. This effect was preceded by phosphorylation of c-Jun NH(2)-terminal kinase (JNK). Treatment with the JNK inhibitor, SP600125, markedly suppressed TBT-induced GADD153 expression. TBT may induce the expression of GADD153, a gene highly responsive to endoplasmic reticulum (ER) stress, in a manner at least partially dependent upon the JNK pathway in SH-SY5Y cells.

TRPV1: contribution to retinal ganglion cell apoptosis and increased intracellular Ca2+ with exposure to hydrostatic pressure

PURPOSE

Elevated hydrostatic pressure induces retinal ganglion cell (RGC) apoptosis in culture. The authors investigated whether the transient receptor potential vanilloid 1 (TRPV1) channel, which contributes to pressure sensing and Ca(2+)-dependent cell death in other systems, also contributes to pressure-induced RGC death and whether this contribution involves Ca(2+).

METHODS

trpv1 mRNA expression in RGCs was probed with the use of PCR and TRPV1 protein localization through immunocytochemistry. Subunit-specific antagonism (iodo-resiniferatoxin) and agonism (capsaicin) were used to probe how TRPV1 activation affects the survival of isolated RGCs at ambient and elevated hydrostatic pressure (+70 mm Hg). Finally, for RGCs under pressure, the authors tested whether EGTA chelation of Ca(2+) improves survival and whether, with the Ca(2+) dye Fluo-4 AM, TRPV1 contributes to increased intracellular Ca(2+).

RESULTS

RGCs express trpv1 mRNA, with robust TRPV1 protein localization to the cell body and axon. For isolated RGCs under pressure, TRPV1 antagonism increased cell density and reduced apoptosis to ambient levels (P <or= 0.05), whereas for RGCs at ambient pressure, TRPV1 agonism reduced density and increased apoptosis to levels for elevated pressure (P <or= 0.01). Chelation of extracellular Ca(2+) reduced RGC apoptosis at elevated pressure by nearly twofold (P <or= 0.01). Exposure to elevated hydrostatic pressure induced a fourfold increase in RGC intracellular Ca(2+) that was reduced by half with TRPV1 antagonism. Finally, in the DBA/2 mouse model of glaucoma, levels of TRPV1 in RGCs increased with elevated IOP.

CONCLUSIONS

RGC apoptosis induced by elevated hydrostatic pressure arises substantially through TRPV1, likely through the influx of extracellular Ca(2+).

The ABCA1 Q597R mutant undergoes trafficking from the ER upon ER stress

Mutations in ATP binding cassette transporter 1 (ABCA1), a membrane protein associated with cellular cholesterol efflux, cause Tangier disease (TD). Previously, we showed that an ABCA1 Q597R mutant (QR) identified in TD is retained in the endoplasmic reticulum. Here, we report that QR trafficking to the plasma membrane was rapidly induced by thapsigargin or DTT, indicating that ER stress-induced QR trafficking. However, pharmacological rescue of ABCA1 activity was not observed. The trafficking was dependent on COPII components and occurred via the ER-Golgi intermediate compartments. Furthermore, we found that QR was more sensitive to ER stress than ATF6, a transcription factor associated with the ER stress response. These results suggest that thapsigargin can be effective in correcting trafficking defects, and raise the possibility that ER stress-induced trafficking is involved in ER quality control.

Role of mitochondria in drug-induced cholestatic injury

Mitochondria have multiple functions in eukaryotic cells and are organized into dynamic tubular networks that continuously undergo changes through coordinated fusion and fission and migration through the cytosol. Mitochondria integrate cell-signaling networks, especially those involving the intracellular messenger Ca(2+), into the regulation of metabolic pathways. Recently, it has become clear that mitochondria are central to the three main cell death pathways, namely necrosis, apoptosis, and autophagic cell death. This article discusses the role of mitochondria in drug-induced cholestatic injury to the liver. The role of mitochondria in the cellular adaptation against the toxic effects of bile acids is discussed also.

Sequential phases of Ca2+ alterations in pre-apoptotic cells

The very early events of the intrinsic, damage-induced apoptotic pathway, i.e., upstream to Bax activation, probably consist of physico-chemical alterations (i.e., redox, pH or Ca2+ changes) rather then subtle molecular interactions, and in spite of many studies they remain unclear. One problem is that cells undergo apoptosis in an asynchronous way, leading to heterogeneity in the cell population that impairs the results of bulk analyses. In this study, we present a flow cytometric approach for studying Ca2+ alteration in apoptosis at the single cell level. By means of a multiparametric analysis, we could discriminate different sub-populations, i.e., viable and apoptotic cells and cells in secondary necrosis, and separately analyse static as well as dynamic Ca2+ parameters in each sub-population. With this approach, we have identified a set of sequential Ca2+ changes; two very early ones occur prior to any other apoptotic alterations, whereas a later change coincides with the appearance of apoptosis. Interestingly, the two pre-apoptotic changes occur simultaneously in all treated cells, i.e., at fixed times post-treatment, whereas the later one occurs at varying times, i.e., within a wide time range, concomitantly with the other apoptotic events.

Differentiation decreased telomerase activity in rat glioblastoma C6 cells and increased sensitivity to IFN-gamma and taxol for apoptosis

Glioblastoma is the deadliest and most prevalent brain tumor, which is not yet amenable to any treatments. Therefore, new and innovative therapeutic strategies need to be developed for treating this deadly disease. We found that all-trans retinoic acid (ATRA) or 13-cis retinoic acid (13-CRA) induced astrocytic differentiation with down regulation of telomerase activity in rat glioblastoma C6 cells and enhanced sensitivity of the cells to interferon-gamma (IFN-gamma) or taxol (TXL) for apoptosis. Sensitivity of differentiated cells to IFN-gamma or TXL was greatly increased for apoptosis with increases in calcineurin expression, Bax:Bcl-2 ratio, mitochondrial release of cytochrome c, and expression and activity of calpain and caspases. Treatment with IFN-gamma activated caspase-8 indicating induction of apoptosis via the receptor-mediated pathway. Notably, IFN-gamma activated the signal transducer and activator of transcription-1 (STAT-1) for signaling via binding to gamma activator sequence (GAS), whereas TXL activated Raf-1 kinase for inactivation of Bcl-2 by its phosphorylation. We confirmed involvement of different proteolytic mechanisms in cell death by pretreating the cells with caspase-8 inhibitor II, calpeptin (calpain inhibitor), and caspase-9 inhibitor I, and caspase-3 inhibitor IV. Results demonstrated that retinoids induced astrocytic differentiation with down regulation of telomerase activity and worked synergistically to enhance sensitivity of cells to the cytotoxic agent IFN-gamma and the cytostatic agent TXL for apoptosis. This combination therapy for differentiation and apoptosis could be highly effective for controlling the malignant growth of glioblastoma.

Receptor activated C kinase is down-regulated in the male gonad of the marine bivalve mollusc Mya arenaria exposed to tributyltin (TBT)

This study was aimed at investigating the molecular mechanisms by which tributyltin (TBT) impairs the reproductive processes in the marine bivalve Mya arenaria. The suppression polymerase chain reaction subtractive hybridization (SSH) method was used to identify differentially expressed transcripts in the gonads of adult M. arenaria 72 h after a single injection of 160 ng TBT in the adductor muscle. Subtractive cDNA libraries comprising 322 clones were obtained. These clones were sequenced and corresponded to 55 female and 26 single male non-redundant cDNAs. Following similarity searches in genome databases, some of the transcripts could be assigned to cellular functions including mitochondrial respiration, structural proteins, structure of cytoskeleton, nucleic acid regulation, general metabolism and signal transduction. Among the potentially differentially regulated transcripts, Receptor for activated C kinase 1 (RACK1) represented 6% of the total down-regulated clones in males and the corresponding protein exhibited a high degree of similarity (80%) with the human polypeptide. The RACK1 cDNA from M. arenaria consists of 1085 bp, encoding a 318 deduced polypeptide which contains five internal tryptophan-aspartate (WD) repeats, six putative PKC phosphorylation sites, one tyrosine kinase site, four putative N-myristoylation sites as well as a transmembrane segment spanning amino acid 228-251. A significant down-regulation (by approximately 30% (p<0.05)) of RACK1 expression in male gonads exposed to TBT was confirmed by quantitative real-time RT-PCR. Transcript levels of RACK1 were higher in the female gonads than in the mantle, gills and male gonads. Gene expression as detected by in situ hybridization was strong in mature oocytes comparatively to primary germ cells. RACK1 may be a useful biomarker for TBT exposure in the reproductive system of bivalve molluscs.

The quercetin paradox

Free radical scavenging antioxidants, such as quercetin, are chemically converted into oxidation products when they protect against free radicals. The main oxidation product of quercetin, however, displays a high reactivity towards thiols, which can lead to the loss of protein function. The quercetin paradox is that in the process of offering protection, quercetin is converted into a potential toxic product. In the present study, this paradox is evaluated using rat lung epithelial (RLE) cells. It was found that quercetin efficiently protects against H(2)O(2)-induced DNA damage in RLE cells, but this damage is swapped for a reduction in GSH level, an increase in LDH leakage as well as an increase of the cytosolic free calcium concentration. To our knowledge, this is the first study that indicates that the quercetin paradox, i.e. the exchange of damage caused by quercetin and its metabolites, also occurs in living lung cells. Following depletion of GSH in the cells by BSO pre-treatment, this quercetin paradox becomes more pronounced, confirming that the formation of thiol reactive quercetin metabolites is involved in the quercetin paradox. The quercetin paradox in living cells implies that the anti-oxidant directs oxidative damage selectively to thiol arylation. Apparently, the potential toxicity of metabolites formed during the actual antioxidant activity of free radical scavengers should be considered in antioxidant supplementation.

Arsenic trioxide (As2O3) induced calcium signals and cytotoxicity in two human cell lines: SY-5Y neuroblastoma and 293 embryonic kidney (HEK)

Arsenic trioxide (As(2)O(3)) has anticancer properties; however, its use also leads to neuro-, hepato- or nephro-toxicity, and therefore, it is important to understand the mechanism of As(2)O(3) toxicity. We studied As(2)O(3) influence on intracellular calcium ([Ca(2+)](i)) homeostasis of human neuroblastoma SY-5Y and embryonic kidney cells (HEK 293). We also relate the As(2)O(3) induced [Ca(2+)](i) modifications with cytotoxicity. We used Ca(2+) sensitive dyes (fluo-4 and rhod-2) combined with laser scanning microscopy or fluorescence activated cell sorting to measure Ca(2+) changes during the application of As(2)O(3) and we approach evaluation of cytotoxicity. As(2)O(3) (1 microM) increased [Ca(2+)](i) in SY-5Y and HEK 293 cells. Three forms of [Ca(2+)](i)-elevations were found: (1) steady-state increases, (2) transient [Ca(2+)](i)-elevations and (3) Ca(2+)-spikes. [Ca(2+)](i) modifications were independent from extracellular Ca(2+) but dependent on internal calcium stores. The effect was not reversible. Inositol triphosphate (IP(3)) and ryanodine (Ry) receptors are involved in regulation of signals induced by As(2)O(3). 2-APB and dantrolene significantly reduced the [Ca(2+)](i)-rise (p<0.001, t-test) but did not completely abolish [Ca(2+)](i)-elevation or spiking. This indicates that other Ca(2+) regulating mechanisms are involved. In cytotoxicity tests As(2)O(3) significantly reduced cell viability in both cell types. Staining with Hoechst 33342 showed occurrence of apoptosis and DNA damage. Our data suggest that [Ca(2+)](i) is an important messenger in As(2)O(3) induced cell death.

Targeting homeostatic mechanisms of endoplasmic reticulum stress to increase susceptibility of cancer cells to fenretinide-induced apoptosis: the role of stress proteins ERdj5 and ERp57

Endoplasmic reticulum (ER) malfunction, leading to ER stress, can be a consequence of genome instability and hypoxic tissue environments. Cancer cells survive by acquiring or enhancing survival mechanisms to counter the effects of ER stress and these homeostatic responses may be new therapeutic targets. Understanding the links between ER stress and apoptosis may be approached using drugs specifically to target ER stress responses in cancer cells. The retinoid analogue fenretinide [N-(4-hydroxyphenyl) retinamide] is a new cancer preventive and chemotherapeutic drug, that induces apoptosis of some cancer cell types via oxidative stress, accompanied by induction of an ER stress-related transcription factor, GADD153. The aim of this study was to test the hypothesis that fenretinide induces ER stress in neuroectodermal tumour cells, and to elucidate the role of ER stress responses in fenretinide-induced apoptosis. The ER stress genes ERdj5, ERp57, GRP78, calreticulin and calnexin were induced in neuroectodermal tumour cells by fenretinide. In contrast to the apoptosis-inducing chemotherapeutic drugs vincristine and temozolomide, fenretinide induced the phosphorylation of eIF2α, expression of ATF4 and splicing of XBP-1 mRNA, events that define ER stress. In these respects, fenretinide displayed properties similar to the ER stress inducer thapsigargin. ER stress responses were inhibited by antioxidant treatment. Knockdown of ERp57 or ERdj5 by RNA interference in these cells increased the apoptotic response to fenretinide. These data suggest that downregulating homeostatic ER stress responses may enhance apoptosis induced by oxidative stress-inducing drugs acting through the ER stress pathway. Therefore, ER-resident proteins such as ERdj5 and ERp57 may represent novel chemotherapeutic targets.

Role of intracellular calcium and phospholipase A2 in arachidonic acid-induced toxicity in liver cells overexpressing CYP2E1

Liver cells (HepG2 and primary hepatocytes) overexpressing CYP2E1 and exposed to arachidonic acid (AA) were previously shown to lose viability together with enhanced lipid peroxidation. These events were blocked in cells pre-incubated with antioxidants (alpha-tocopherol, glutathione ethyl ester), or in HepG2 cells not expressing CYP2E1. The goal of the current study was to evaluate the role of calcium and calcium-activated hydrolases in these CYP2E1-AA interactions. CYP2E1-expressing HepG2 cells treated with AA showed an early increase in cytosolic calcium and partial depletion of ionomycin-sensitive calcium stores. These changes in calcium were blocked by alpha-tocopherol. AA activated phospholipase A2 (PLA2) in CYP2E1-expressing liver cells, and this was inhibited by PLA2 inhibitors or alpha-tocopherol. PLA2 inhibitors prevented the cell death caused by AA, without affecting CYP2E1 activity or lipid peroxidation. AA toxicity and PLA2 activation were inhibited in calcium-depleted cells, but not by removal of extracellular calcium alone. Removal of extracellular calcium inhibited the early increase in cytosolic calcium caused by AA. CYP2E1 overexpressing HepG2 cells exposed to AA showed a decrease in mitochondrial membrane potential, which was prevented by the PLA2 inhibitors. These results suggest that AA-induced toxicity to CYPE1-expressing cells: (i) is associated with release of Ca2+ from intracellular stores that depends mainly on oxidative membrane damage; (ii) is associated with activation of PLA2 that depends on intracellular calcium and lipid peroxidation; (iii) does not depend on increased influx of extracellular calcium, and (iv) depends on the effect of converging events (lipid peroxidation, intracellular calcium, activation of PLA2) on mitochondria to induce bioenergetic failure and necrosis. These interactions may play a role in alcohol liver toxicity, which requires polyunsaturated fatty acids, and involves induction of CYP2E1.

Effect of n-propylthiouracil or thyroxine on arsenic trioxide toxicity in the liver of rat

Involvement of thyroid gland in the hepatotoxic manifestations of arsenic trioxide (As(III)) has been studied in rat. The effects of n-propylthiouracil (PTU) (a thyrotoxic compound) and L-thyroxine (a thyroid hormone) have been studied with reference to T(3) and T(4) values in the serum, arsenic concentration in the liver, Ca(2+) accumulation in the liver, aspartate transaminase, alanine transaminase and bilirubin values as the indicators of liver function, histopathological observations and finally the ultrastructural studies. It is concluded that hypothyroid condition protects against As(III) toxicity. Scavenging of reactive oxygen species (ROS) that significantly contribute in As(III) toxicity, by high intracellular concentration of reduced glutathione, as a consequence of PTU treatment is proposed as the plausible protective mechanism.