Oxidative damage to sarcoplasmic reticulum Ca2+-ATPase AT submicromolar iron concentrations: evidence for metal-catalyzed oxidation

Acclimation history modulates effect size of calcareous algae (Halimeda opuntia) to herbicide exposure under future climate scenarios

Tropical marine habitat-builders such as calcifying green algae can be susceptible to climate change (warming and acidification). This study evaluated the cumulative effects of ocean warming (OW), ocean acidification (OA) and the herbicide diuron on the calcifying green algae Halimeda opuntia. We also assessed the influence of acclimation history to experimental climate change conditions on physiological responses. H. opuntia were exposed for 15 days to orthogonal combinations of three climate scenarios [ambient (28 °C, pCO₂ = 378 ppm), 2050 (29 °C, pCO₂ = 567 ppm) and 2100 (30 °C, pCO₂ = 721 ppm)] and to six diuron concentrations (up to 29 μg L^-1). Half of the H. opuntia had been acclimated for eight months to the climate scenarios in a mesocosm approach, while the remaining half were not pre-acclimated, as is current practice in most experiments. Climate effects on quantum yield (ΔF/Fm'), photosynthesis and calcification in future climate scenarios were significantly stronger (by -24, -46 and +26%, respectively) in non-acclimated algae, suggesting experimental bias may exaggerate effects in organisms not appropriately acclimated to future-climate conditions. Thus, full analysis was done on acclimated plants only. Interactive effects of future climate scenarios and diuron were observed for ΔF/Fm', while the detrimental effects of climate and diuron on net photosynthesis and total antioxidant capacity (TAC) were additive. Calcification-related enzymes were negatively affected only by diuron, with inhibition of Ca-ATPase and upregulation of carbonic anhydrase. The combined and consistent physiological and biochemical evidence of negative impacts (across six indicators) of both herbicide and future-climate conditions on the health of H. opuntia highlights the need to address both climate change and water quality. Guideline values for contaminants may also need to be lowered considering 'climate adjusted thresholds'. Importantly, this study highlights the value of applying substantial future climate acclimation periods in experimental studies to avoid exaggerated organism responses to OW and OA.

Oxidative stress and endoplasmic stress in calcium oxalate stone disease: the chicken or the egg?

Crystal modulators play a significant role in the formation of calcium oxalate stone disease. When renal cells are subjected to oxalate stress, the loss in cell integrity leads to exposure of multiple proteins that assist and/or inhibit crystal attachment and retention. Contact between oxalate and calcium oxalate with urothelium proves fatal to cells as a result of reactive oxygen species generation and onset of oxidative stress. Hence, as a therapeutic strategy it was hypothesised that supplementation of antioxidants would suffice. On the contrary to popular belief, the detection of oxalate induced endoplasmic reticulum mediated apoptosis proved the ineffectiveness of antioxidant therapy alone. Thus, the inadequacy of antioxidant supplementation in oxalate stress invoked the presence of an alternative pathway for the induction of kidney fibrosis in hyperoxaluric rats. In addition to settling this query, the link between oxidative stress and ER stress is not well understood, especially in urolithiasis.

Iron overload: Effects on cellular biochemistry

Iron is an essential element for human life. However, it is a pro-oxidant agent capable of reacting with hydrogen peroxide. An iron overload can cause cellular changes, such as damage to the plasma membrane leading to cell death. Effects of iron overload in cellular biochemical processes include modulating membrane enzymes, such as the Na, K-ATPase, impairing the ionic transport and inducing irreversible damage to cellular homeostasis. To avoid such damage, cells have an antioxidant system that acts in an integrated manner to prevent oxidative stress. In addition, the cells contain proteins responsible for iron transport and storage, preventing its reaction with other substances during absorption. Moreover, iron is associated with cellular events coordinated by iron-responsive proteins (IRPs) that regulate several cellular functions, including a process of cell death called ferroptosis. This review will address the biochemical aspects of iron overload at the cellular level and its effects on important cellular structures.

Iron overload impact on P-ATPases

Iron is a chemical element that is active in the fundamental physiological processes for human life, but its burden can be toxic to the body, mainly because of the stimulation of membrane lipid peroxidation. For this reason, the action of iron on many ATPases has been studied, especially on P-ATPases, such as the Na⁺,K⁺-ATPase and the Ca²⁺-ATPase. On the Fe²⁺-ATPase activity, the free iron acts as an activator, decreasing the intracellular Fe²⁺ and playing a protection role for the cell. On the Ca²⁺-ATPase activity, the iron overload decreases the enzyme activity, raising the cytoplasmic Ca²⁺ and decreasing the sarco/endoplasmic reticulum and the Golgi apparatus Ca²⁺ concentrations, which could promote an enzyme oxidation, nitration, and fragmentation. However, the iron overload effect on the Na⁺,K⁺-ATPase may change according to the tissue expressions. On the renal cells, as well as on the brain and the heart, iron promotes an enzyme inactivation, whereas its effect on the erythrocytes seems to be the opposite, directly stimulating the ATPase activity, or stimulating it by signaling pathways involving ROS and PKC. Modulations in the ATPase activity may impair the ionic transportation, which is essential for cell viability maintenance, inducing irreversible damage to the cell homeostasis. Here, we will discuss about the iron overload effect on the P-ATPases, such as the Na⁺,K⁺-ATPase, the Ca²⁺-ATPase, and the Fe²⁺-ATPase.

Dietary ascorbic acid reduced micronucleus and nuclear abnormalities in Clarias gariepinus (Burchell 1822) exposed to hospital effluent

Hospital effluents contain myriad of mutagens and genotoxins capable of increasing DNA damage in aquatic biota. African mudfish, Clarias gariepinus, are exposed to genotoxins when cultured in swamps and derelict water bodies often contaminated by effluents. Moreover, its DNA is susceptible to xenobiotic-induced lesions since it lacks L-gulonolactone oxidase and hence cannot synthesize L-ascorbic acid. This study investigated 96-h acute toxicity and protective effects of dietary ascorbic acid (AA) against micronucleus (MN) and abnormal nuclear (NAs) formation in C. gariepinus exposed to sub-lethal concentrations of hospital effluent. Six concentrations (0.5-3.0%) of the effluent were selected to determine the 96-h acute toxicity of the effluent in C. gariepinus, after range finding test. Fish were exposed to sub-lethal concentrations (0.08-1.30%) of the 96 h LC₅₀. Two other groups were exposed to the 96 h LC₅₀ (1.30%) of the effluent +50 and +100 mg/kg of dietary ascorbic for 7 days, and MN and NAs assessed in peripheral erythrocytes. The 96 h LC₅₀ (1.30%) was 1.18 times more toxic than the 24 h LC₅₀ (1.54%), indicating that the toxicity of the effluent increased with exposure duration. MN, nuclear bud, enucleated, fragmented nucleus (apoptosis), and necrotic erythrocytes significantly increase in effluent treated fish. Dietary AA reduced MN from 6.35-fold (1.30% treated group) to 3.72-fold (1.30% + 50 mg AA) and 3.54-fold (1.30% + 100 mg AA). Also, AA reduced total NAs from 2.26-fold (1.30%) to 1.40-fold (1.30% + 50 mg AA) and 1.06-fold (1.30% + 100 mg AA) compared to the control. Heavy metals and physicochemical parameters analyzed in the tested effluent possibly induced the mortality and cytogenotoxicity in C. gariepinus, and this was ameliorated by dietary AA.

Altered Ca(2+) signaling in skeletal muscle fibers of the R6/2 mouse, a model of Huntington's disease

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat within the gene encoding the protein huntingtin. The resulting elongated glutamine (poly-Q) sequence of mutant huntingtin (mhtt) affects both central neurons and skeletal muscle. Recent reports suggest that ryanodine receptor-based Ca(2+) signaling, which is crucial for skeletal muscle excitation-contraction coupling (ECC), is changed by mhtt in HD neurons. Consequently, we searched for alterations of ECC in muscle fibers of the R6/2 mouse, a mouse model of HD. We performed fluorometric recordings of action potentials (APs) and cellular Ca(2+) transients on intact isolated toe muscle fibers (musculi interossei), and measured L-type Ca(2+) inward currents on internally dialyzed fibers under voltage-clamp conditions. Both APs and AP-triggered Ca(2+) transients showed slower kinetics in R6/2 fibers than in fibers from wild-type mice. Ca(2+) removal from the myoplasm and Ca(2+) release flux from the sarcoplasmic reticulum were characterized using a Ca(2+) binding and transport model, which indicated a significant reduction in slow Ca(2+) removal activity and Ca(2+) release flux both after APs and under voltage-clamp conditions. In addition, the voltage-clamp experiments showed a highly significant decrease in L-type Ca(2+) channel conductance. These results indicate profound changes of Ca(2+) turnover in skeletal muscle of R6/2 mice and suggest that these changes may be associated with muscle pathology in HD.

Mitochondrial damage: an important mechanism of ambient PM2.5 exposure-induced acute heart injury in rats

Epidemiological studies suggested that ambient fine particulate matter (PM2.5) exposure was associated with cardiovascular disease. However, the underlying mechanism, especially the mitochondrial damage mechanism, of PM2.5-induced heart acute injury is still unclear. In this study, the alterations of mitochondrial morphology and mitochondrial fission/fusion gene expression, oxidative stress, calcium homeostasis and inflammation in hearts of rats exposed to PM2.5 with different dosages (0.375, 1.5, 6.0 and 24.0mg/kg body weight) were investigated. The results indicated that the PM2.5 exposure induced pathological changes and ultra-structural damage in hearts such as mitochondrial swell and cristae disorder. Furthermore, PM2.5 exposure significantly increased specific mitochondrial fission/fusion gene (Fis1, Mfn1, Mfn2, Drp1 and OPA1) expression in rat hearts. These changes were accompanied by decreases of activities of superoxide dismutase (SOD), Na(+)K(+)-ATPase and Ca(2+)-ATPase and increases of levels of malondialdehyde (MDA), inducible nitric oxide synthase (iNOS) and nitric oxide (NO) as well as levels of pro-inflammatory mediators including TNF-α, IL-6 and IL-1β in rat hearts. The results implicate that mitochondrial damage, oxidative stress, cellular homeostasis imbalance and inflammation are potentially important mechanisms for the PM2.5-induced heart injury, and may have relations with cardiovascular disease.

Biometals and their therapeutic implications in Alzheimer's disease

No disease modifying therapy exists for Alzheimer's disease (AD). The growing burden of this disease to our society necessitates continued investment in drug development. Over the last decade, multiple phase 3 clinical trials testing drugs that were designed to target established disease mechanisms of AD have all failed to benefit patients. There is, therefore, a need for new treatment strategies. Changes to the transition metals, zinc, copper, and iron, in AD impact on the molecular mechanisms of disease, and targeting these metals might be an alternative approach to treat the disease. Here we review how metals feature in molecular mechanisms of AD, and we describe preclinical and clinical data that demonstrate the potential for metal-based drug therapy.

The potential for transition metal-mediated neurodegeneration in amyotrophic lateral sclerosis

Modulations of the potentially toxic transition metals iron (Fe) and copper (Cu) are implicated in the neurodegenerative process in a variety of human disease states including amyotrophic lateral sclerosis (ALS). However, the precise role played by these metals is still very much unclear, despite considerable clinical and experimental data suggestive of a role for these elements in the neurodegenerative process. The discovery of mutations in the antioxidant enzyme Cu/Zn superoxide dismutase 1 (SOD-1) in ALS patients established the first known cause of ALS. Recent data suggest that various mutations in SOD-1 affect metal-binding of Cu and Zn, in turn promoting toxic protein aggregation. Copper homeostasis is also disturbed in ALS, and may be relevant to ALS pathogenesis. Another set of interesting observations in ALS patients involves the key nutrient Fe. In ALS patients, Fe loading can be inferred by studies showing increased expression of serum ferritin, an Fe-storage protein, with high serum ferritin levels correlating to poor prognosis. Magnetic resonance imaging of ALS patients shows a characteristic T₂ shortening that is attributed to the presence of Fe in the motor cortex. In mutant SOD-1 mouse models, increased Fe is also detected in the spinal cord and treatment with Fe-chelating drugs lowers spinal cord Fe, preserves motor neurons, and extends lifespan. Inflammation may play a key causative role in Fe accumulation, but this is not yet conclusive. Excess transition metals may enhance induction of endoplasmic reticulum (ER) stress, a system that is already under strain in ALS. Taken together, the evidence suggests a role for transition metals in ALS progression and the potential use of metal-chelating drugs as a component of future ALS therapy.

Effects of cadmium alone and in combination with low molecular weight chitosan on metallothionein, glutathione-S-transferase, acid phosphatase, and ATPase of freshwater crab Sinopotamon yangtsekiense

Cadmium (Cd) is an environmental contaminant showing a variety of deleterious effects, including the potential threat for the ecological environment and human health via food chains. Low molecular weight chitosan (LMWC) has been demonstrated to be an effective antioxidant. Metallothionein (MT) mRNA levels and activities of glutathione-S-transferase (GST), superoxide dismutase (SOD), acid phosphatase (ACP), Na(+),K(+)-ATPase, and Ca(2+)-ATPase as well as malondialdehyde (MDA) contents in the gills of the freshwater crab Sinopotamon yangtsekiense were analyzed in vivo in order to determine the injury of Cd exposure on the gill tissues as well as the protective effect of LMWC against this injury. The results showed that there was an apparent accumulation of Cd in the gills, which was lessened by the presence of LMWC. Moreover, Cd(2+) significantly increased the gill MT mRNA levels, ACP activity and MDA content while decreasing the activities of SOD, GST, Na(+),K(+)-ATPase, and Ca(2+)-ATPase in the crabs relative to the control. Cotreatment with LMWC reduced the levels of MT mRNA and ACP but raised the activities of GST, Na(+),K(+)-ATPase, and Ca(2+)-ATPase in gill tissues compared with the crabs exposed to Cd(2+) alone. These results suggest that LMWC may exert its protective effect through chelating Cd(2+) to form LMWC-Cd(2+) complex, elevating the antioxidative activities of GST, Na(+),K(+)-ATPase, and Ca(2+)-ATPase as well as alleviating the stress pressure on MT and ACP, consequently protecting the cell from the adverse effects of Cd.

Bidirectional regulation of NF-κB by reactive oxygen species: a role of unfolded protein response

Nuclear factor-κB (NF-κB) is a transcription factor that plays a crucial role in coordinating innate and adaptive immunity, inflammation, and apoptotic cell death. NF-κB is activated by various inflammatory stimuli including peptide factors and infectious microbes. It is also known as a redox-sensitive transcription factor activated by reactive oxygen species (ROS). Over the past decades, various investigators focused on the role of ROS in the activation of NF-κB by cytokines and lipopolysaccharides. However, recent studies also suggested that ROS have the potential to repress NF-κB activity. Currently, it is not well addressed how ROS regulate activity of NF-κB in a bidirectional fashion. In this paper, we summarize evidence for positive and negative regulation of NF-κB by ROS, possible redox-sensitive targets for NF-κB signaling, and mechanisms underlying biphasic and bidirectional influences of ROS on NF-κB, especially focusing on a role of ROS-mediated induction of endoplasmic reticulum stress.

A delicate balance: Iron metabolism and diseases of the brain

Iron is the most abundant transition metal within the brain, and is vital for a number of cellular processes including neurotransmitter synthesis, myelination of neurons, and mitochondrial function. Redox cycling between ferrous and ferric iron is utilized in biology for various electron transfer reactions essential to life, yet this same chemistry mediates deleterious reactions with oxygen that induce oxidative stress. Consequently, there is a precise and tightly controlled mechanism to regulate iron in the brain. When iron is dysregulated, both conditions of iron overload and iron deficiencies are harmful to the brain. This review focuses on how iron metabolism is maintained in the brain, and how an alteration to iron and iron metabolism adversely affects neurological function.

Expression in yeast links field polymorphisms in PfATP6 to in vitro artemisinin resistance and identifies new inhibitor classes

BACKGROUND

The mechanism of action of artemisinins against malaria is unclear, despite their widespread use in combination therapies and the emergence of resistance.

RESULTS

Here, we report expression of PfATP6 (a SERCA pump) in yeast and demonstrate its inhibition by artemisinins. Mutations in PfATP6 identified in field isolates (such as S769N) and in laboratory clones (such as L263E) decrease susceptibility to artemisinins, whereas they increase susceptibility to unrelated inhibitors such as cyclopiazonic acid. As predicted from the yeast model, Plasmodium falciparum with the L263E mutation is also more susceptible to cyclopiazonic acid. An inability to knockout parasite SERCA pumps provides genetic evidence that they are essential in asexual stages of development. Thaperoxides are a new class of potent antimalarial designed to act by inhibiting PfATP6. Results in yeast confirm this inhibition.

CONCLUSIONS

The identification of inhibitors effective against mutated PfATP6 suggests ways in which artemisinin resistance may be overcome.

The unfolded protein response triggered by environmental factors

Endoplasmic reticulum (ER) stress and consequent unfolded protein response (UPR) are involved in a diverse range of pathologies including ischemic diseases, neurodegenerative disorders, and metabolic diseases, such as diabetes mellitus. The UPR is also triggered by various environmental factors; e.g., pollutants, infectious pathogens, therapeutic drugs, alcohol, physical stress, and malnutrition. This review summarizes current knowledge on environmental factors that induce ER stress and describes how the UPR is linked to particular pathological states after exposure to environmental triggers.

Attenuation of endoplasmic reticulum stress and mitochondrial injury in kidney with ischemic postconditioning application and trimetazidine treatment

Background

Endoplasmic reticulum (ER) and mitochondria have been implicated in the pathology of renal ischemia/reperfusion (I/R). In the present study, we investigated whether the use of ischemic postconditioning (IPostC) and trimetazidine (TMZ) separately or combined could reduce ER stress and mitochondria damage after renal ischemia.

Methods

Kidneys of Wistar rats were subjected to 60-min of warm ischemia followed by 120-min of reperfusion (I/R group, n = 6), or to 6 cycles of ischemia/reperfusion (10-s each cycle) just after 60-min of warm ischemia (IPostC group, n = 6), or to i.p. injection of TMZ (3 mg/kg) 30-min before ischemia (TMZ group, n = 6), or to the combination of both treatments (IPostC+TMZ group, n = 6). The results of these experimental groups were compared to those of a sham-operated group in which rat renal pedicles were only dissected. Sodium reabsorption rate, creatinine clearance lactate deshydrogenase (LDH) activity in plasma, and concentration of malonedialdehyde (MDA) in tissue were determined. In addition, Western blot analysis was performed to identify the amounts of cytochrome c, c-JunNH2-terminal kinase (JNK), voltage-dependent anion channel (VDAC), glycogen synthase kinase 3-beta (GSK3-β), and ER stress parameters.

Results

IPostC or/and TMZ significantly decreased cytolysis, oxidative stress and improved renal function in comparison to I/R group. IPostC but not TMZ significantly attenuated ER stress parameters versus I/R group. Indeed, it down-regulated the glucose-regulated protein 78 (GRP78), the activating transcription factor 4 (ATF4), the RNA activated protein kinase (PKR)-like ER kinas (PERK), the X box binding protein-1 (XBP-1) and the caspase12 protein levels. TMZ treatment significantly augmented GSK3-β phosphorylation and reduced levels of cytochrome c and VDAC phosphorylation in comparison to IPostC application. The combination of both treatments gave a synergetic effect. It significantly improved the survival rate, attenuated cytolysis, oxidative stress and improved renal function.

Conclusion

This study revealed that IPostC protects kidney from I/R injury by suppressing ER stress while the beneficial effects of TMZ are mediated by mitochondria protection. The combination of both treatments ameliorated functional recovery.

HOCl causes necrotic cell death in human monocyte derived macrophages through calcium dependent calpain activation

The abundance of dead macrophages in close proximity to HOCl-modified proteins in advanced atherosclerotic plaques implicates HOCl in the killing of macrophages and the formation of the necrotic core region. The mechanism of HOCl mediated death of macrophages was unknown, so using human monocyte derived macrophages (HMDM) we here have shown that HOCl causes a rapid necrotic cell death characterized by loss of MTT reduction, cellular ATP and cell lysis without caspase-3 activation in HMDM cells. The HOCl causes a rise in cytosolic calcium level via the plasma membrane L- and T-type calcium channels and endoplasmic reticulum RyR channel. Blocking of the calcium channels or the addition of calpain inhibitors prevents the HOCl mediated loss of mitochondrial potential, lysosome failure and HMDM cell death. Blocking MPT-pore formation with cyclosporin A also prevents the loss of mitochondrial membrane potential, lysosomal destabilization and HMDM cell death. Blocking the calcium mitochondrial uniporter with ruthenium red also blocks the loss of mitochondrial potential but only at high concentrations. HOCl appears to cause HMDM cell death through destabilization of cytosolic calcium control resulting in the failure of both the mitochondria and lysosomes.

Induction of CCAAT/enhancer-binding protein-homologous protein by cigarette smoke through the superoxide anion-triggered PERK-eIF2α pathway

Cigarette smoke triggers apoptosis through oxidative stress- and endoplasmic reticulum (ER) stress-dependent induction of CCAAT/enhancer-binding protein-homologous protein (CHOP) (Tagawa et al., 2008. Free Radic. Biol. Med. 45, 50-59). We investigated roles of individual reactive oxygen/nitrogen species in the transcriptional induction of CHOP by cigarette smoke. Exposure of bronchial epithelial cells to O(2)(-), ONOO(-) or H(2)O(2) induced expression of CHOP, whereas NO alone did not. Induction of CHOP mRNA by cigarette smoke extract (CSE) was attenuated by scavengers for O(2)(-), ONOO(-) or NO, whereas scavenging H(2)O(2) did not affect the induction of CHOP. Like CSE, O(2)(-) and ONOO(-) caused activation of the CHOP gene promoter. Scavengers for O(2)(-), ONOO(-) or NO attenuated CSE-triggered activation of the CHOP gene promoter. CSE, O(2)(-) and ONOO(-) induced phosphorylation of protein kinase-like ER kinase (PERK) and eukaryotic translation initiation factor 2α (eIF2α) and caused induction of downstream activating transcription factor 4 (ATF4). Scavengers for O(2)(-), ONOO(-) or NO attenuated induction of ATF4 by CSE. Furthermore, dominant-negative inhibition of the PERK-eIF2α pathway exclusively suppressed CSE-triggered induction of CHOP and consequent apoptosis. These results suggest that O(2)(-) and ONOO(-) are selectively involved in CSE-triggered induction of CHOP and that the PERK-eIF2α pathway plays a crucial role in the induction of CHOP and apoptosis downstream of the particular reactive oxygen species.

Low-molecular-weight-chitosan ameliorates cadmium-induced toxicity in the freshwater crab, Sinopotamon yangtsekiense

Cadmium (Cd) has been shown to induce oxidative stress. Low-molecular-weight-chitosan (LMWC) has been demonstrated to exhibit potent antioxidant effects. We investigated the regulation role in Cd²⁺-induced oxidative damage in the hepatopancreas of the freshwater crab Sinopotamon yangtsekiense and the protective effect of LMWC. The results showed that Cd²⁺ significantly increased the hepatopancreatic metallothionein (MT) mRNA levels and protein kinase C (PKC) activity while decreasing the activities of Na⁺,K⁺-ATPase and Ca²⁺-ATPase in crabs relative to the control group. Co-treatment with LMWC suppressed the levels of MT and PKC but raised the activities of Na⁺,K+-ATPase and Ca²⁺-ATPase in hepatopancreatic tissues compared with the crabs exposed to Cd²⁺ alone. We postulate that LMWC may exert its protective effect through regulating the expressions of MT, PKC, Na⁺,K⁺-ATPase and Ca²⁺-ATPase, thereby enhancing antioxidant defense. These observations suggest that LMWC may be beneficial because of its ability to alleviate the Cd²⁺-induced damages to the crabs.

Control of NF-κB and inflammation by the unfolded protein response

Under inflammatory situations, endoplasmic reticulum (ER) stress occurs at local sites and modulates inflammatory processes. NF-κB is a key regulator for immune and inflammatory responses, and its activity is influenced by ER stress positively or negatively. Recent investigation suggested that ER stress induces activation of NF-κB in the early phase, whereas in the later phase, consequent unfolded protein response (UPR) inhibits NF-κB. This review summarizes current knowledge on potential mechanisms underlying the biphasic, bidirectional regulation of NF-κB by the UPR and possible roles for ER stress in the regulation of inflammation.

Glutathione peroxidase-3 deficiency promotes platelet-dependent thrombosis in vivo

BACKGROUND

Glutathione peroxidase-3 (GPx-3) is a selenocysteine-containing plasma protein that scavenges reactive oxygen species in the extracellular compartment. A deficiency of this enzyme has been associated with platelet-dependent thrombosis, and a promoter haplotype with reduced function has been associated with stroke risk.

METHODS AND RESULTS

We recently developed a genetic mouse model to assess platelet function and thrombosis in the setting of GPx-3 deficiency. The GPx-3((-/-)) mice showed an attenuated bleeding time and an enhanced aggregation response to the agonist ADP compared with wild-type mice. GPx-3((-/-)) mice displayed increased plasma levels of soluble P-selectin and decreased plasma cyclic cGMP compared with wild-type mice. ADP infusion-induced platelet aggregation in the pulmonary vasculature produced a more robust platelet activation response in the GPx-3((-/-)) than wild-type mice; histological sections from the pulmonary vasculature of GPx-3((-/-)) compared with wild-type mice showed increased platelet-rich thrombi and a higher percentage of occluded vessels. Cremaster muscle preparations revealed endothelial dysfunction in the GPx-3((-/-)) compared with wild-type mice. With a no-flow ischemia-reperfusion stroke model, GPx-3((-/-)) mice had significantly larger cerebral infarctions compared with wild-type mice and platelet-dependent strokes. To assess the neuroprotective role of antioxidants in this model, we found that manganese(III) meso-tetrakis(4-benzoic acid)porphyrin treatment reduced stroke size in GPx-3((-/-)) mice compared with vehicle-treated controls.

CONCLUSIONS

These findings demonstrate that GPx-3 deficiency results in a prothrombotic state and vascular dysfunction that promotes platelet-dependent arterial thrombosis. These data illustrate the importance of this plasma antioxidant enzyme in regulating platelet activity, endothelial function, platelet-dependent thrombosis, and vascular thrombotic propensity.

Activation of store-operated I(CRAC) by hydrogen peroxide

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) play a role in both innate immunity as well as cellular injury. H2O2 induces changes in intracellular calcium ([Ca(2+)]i) in many cell types and this seems to be at least partially mediated by transient receptor potential melastatin 2 (TRPM2) in cells that express this channel. Here we show that low concentrations of H2O2 induce the activation of the Ca(2+)-release activated Ca(2+) current I(CRAC). This effect is not mediated by direct CRAC channel activation, since H2O2 does not activate heterologously expressed CRAC channels independently of stromal interaction molecule (STIM). Instead, I(CRAC) activation is partially mediated by store depletion through activation of inositol 1,4,5 trisphosphate receptors (IP3R), since pharmacological inhibition of IP3 receptors by heparin or molecular knock-out of all IP3 receptors in DT40 B cells strongly reduce H2O2-induced I(CRAC). The remainder of H2O2-induced I(CRAC) activation is likely mediated by IP3R-independent store-depletion. Our data suggest that H2O2 can activate Ca(2+) entry through TRPM2 as well as store-operated CRAC channels, thereby adding a new facet to ROS-induced Ca(2+) signaling.

Endoplasmic reticulum calcium release potentiates the ER stress and cell death caused by an oxidative stress in MCF-7 cells

Increase in cytosolic calcium concentration ([Ca2+](c)), release of endoplasmic reticulum (ER) calcium ([Ca2+](er)) and ER stress have been proposed to be involved in oxidative toxicity. Nevertheless, their relative involvements in the processes leading to cell death are not well defined. In this study, we investigated whether oxidative stress generated during ascorbate-driven menadione redox cycling (Asc/Men) could trigger these three events, and, if so, whether they contributed to Asc/Men cytoxicity in MCF-7 cells. Using microspectrofluorimetry, we demonstrated that Asc/Men-generated oxidative stress was associated with a slow and moderate increase in [Ca2+](c), largely preceding permeation of propidium iodide, and thus cell death. Asc/Men treatment was shown to partially deplete ER calcium stores after 90 min (decrease by 45% compared to control). This event was associated with ER stress activation, as shown by analysis of eIF2 phosphorylation and expression of the molecular chaperone GRP94. Thapsigargin (TG) was then used to study the effect of complete [Ca2+](er) emptying during the oxidative stress generated by Asc/Men. Surprisingly, the combination of TG and Asc/Men increased ER stress to a level considerably higher than that observed for either treatment alone, suggesting that [Ca2+](er) release alone is not sufficient to explain ER stress activation during oxidative stress. Finally, TG-mediated [Ca2+](er) release largely potentiated ER stress, DNA fragmentation and cell death caused by Asc/Men, supporting a role of ER stress in the process of Asc/Men cytotoxicity. Taken together, our results highlight the involvement of ER stress and [Ca2+](er) decrease in the process of oxidative stress-induced cell death in MCF-7 cells.

Biphasic, bidirectional regulation of NF-kappaB by endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress induces an adaptive program called the unfolded protein response (UPR), which affects activity of an array of kinases and transcription factors. Previous reports provided evidence for activation of nuclear factor-kappaB (NF-kappaB), the major transcription factor regulating inflammatory processes, by ER stress. However, recent investigation also suggested that preceding ER stress suppresses activation of NF-kappaB by subsequent exposure to inflammatory stimuli. Although ER stress induces activation of NF-kappaB in the early phase, consequent UPR may inhibit NF-kappaB-dependent cellular activation in the later phase. This article summarizes current knowledge on potential mechanisms underlying the biphasic, bidirectional regulation of NF-kappaB by ER stress.

Antitumor effects of dehydroxymethylepoxyquinomicin, a novel nuclear factor-kappaB inhibitor, in human liver cancer cells are mediated through a reactive oxygen species-dependent mechanism

Activation of the nuclear transcription factor-kappaB (NF-kappaB) has been implicated in liver tumorigenesis. We evaluated the effects of a novel NF-kappaB inhibitor, dehydroxymethylepoxyquinomicin (DHMEQ), in two human liver cancer cell lines HA22T/VGH and HuH-6. DHMEQ treatment dose dependently decreased the DNA-binding capacity of the NF-kappaB p65 subunit, inhibited cell growth and proliferation, and increased apoptosis as shown by caspase activation, release of cytochrome c, poly(ADP-ribose) polymerase cleavage, and down-regulation of survivin. DHMEQ also induced a dose-dependent activation of mitogen-activated protein kinase kinase/extracellular signal-regulated kinase signaling, and inhibition of this pathway significantly reduced cell growth. It is noteworthy that we observed that DHMEQ stimulated reactive oxygen species (ROS) production in a dose-dependent manner and that pretreatment of the cells with the antioxidant N-acetyl-L-cysteine (NAC) significantly reduced DHMEQ-induced ROS generation. Accordingly, NAC completely reversed the DHMEQ-induced growth inhibition, caspase activation, and cell death. DHMEQ-treated cells exhibited DNA damage, as evaluated by accumulation in nuclear foci of phospho-H2AX, which was completely reversed by NAC. Moreover, DHMEQ induced the expression of genes involved in the endoplasmic reticulum stress response (GRP78, CHOP, TRB3) and promoted the splicing of XBP1 mRNA in a dose-dependent fashion in both cell lines, which was reversed in the presence of NAC. Knockdown of TRB3 mRNA expression by small interference RNA significantly decreased DHMEQ-induced cell growth inhibition. These data suggest that DHMEQ antitumor effects are primarily mediated through ROS generation. Thereby, considering that cancer cells are under increased ER stress and oxidative stress conditions, DHMEQ may greatly improve various anticancer strategies.

Trypanosoma brucei brucei: effects of ferrous iron and heme on ecto-nucleoside triphosphate diphosphohydrolase activity

Trypanosoma brucei brucei is the causative agent of animal African trypanosomiasis, also called nagana. Procyclic vector form resides in the midgut of the tsetse fly, which feeds exclusively on blood. Hemoglobin digestion occurs in the midgut resulting in an intense release of free heme. In the present study we show that the magnesium-dependent ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) activity of procyclic T. brucei brucei is inhibited by ferrous iron and heme. The inhibition of E-NTPDase activity by ferrous iron, but not by heme, was prevented by pre-incubation of cells with catalase. However, antioxidants that permeate cells, such as PEG-catalase and N-acetyl-cysteine prevented the inhibition of E-NTPDase by heme. Ferrous iron was able to induce an increase in lipid peroxidation, while heme did not. Therefore, both ferrous iron and heme can inhibit E-NTPDase activity of T. brucei brucei by means of formation of reactive oxygen species, but apparently acting through distinct mechanisms.

Endoplasmic reticulum stress and unfolded protein response in renal pathophysiology: Janus faces

A number of pathophysiological insults lead to accumulation of unfolded proteins in the endoplasmic reticulum (ER) and cause ER stress. In response to accumulation of unfolded/misfolded proteins, cells adapt themselves to the stress condition via the unfolded protein response (UPR). For the cells, UPR is a double-edged sword. It triggers both prosurvival and proapoptotic signals. ER stress and UPR may, therefore, be involved in a diverse range of pathological situations. However, currently, information is limited regarding roles of ER stress and UPR in the renal pathophysiology. This review describes current knowledge on the relationship between ER stress and diseases and summarizes evidence for the link between ER stress/UPR and renal diseases.

Protective effect of antioxidants against sarcoplasmic reticulum (SR) oxidation by Fenton reaction, however without prevention of Ca-pump activity

The Ca(2+)-ATPase of the sarcoplasmic reticulum (SERCA) of rabbit skeletal muscle was oxidized by Fe2+/H2O2/ascorbic acid (AA), a system which generates HO(.) radicals according to the Fenton reaction: (Fe2(+)+H2O2-->HO(.)+OH(-)+Fe(3+)) under conditions similar to the pathological state of inflammation. Under these conditions, when hydroxyl-radicals and/or ferryl-radicals are generated, a 50% decrease of the SERCA activity was observed, a significant decrease of SH groups and an increase of protein carbonyl groups and lipid peroxidation were identified. Two new bands, time dependent in density, appeared in the SERCA protein electrophoresis after incubation with the Fenton system (at approximately 50 and 75kDa), probably due to structural changes as supported also by trypsin digestion. Immunoblotting of DNPH derivatized protein bound carbonyls detected a time dependent increase after incubation of SERCA with the Fenton system. Trolox and the pyridoindole stobadine (50microM) protected SR against oxidation induced via the Fenton system by preventing SH group oxidation and lipid peroxidation. Pycnogenol((R)) and EGb761 (40microg/ml) protected SERCA in addition against protein bound carbonyl formation. In spite of the antioxidant effects, trolox and stobadine were not able to prevent a decrease in the SERCA Ca(2+)-ATPase activity. Pycnogenol and EGb761 even enhanced the decrease of the Ca(2+)-ATPase activity induced by the Fenton system, probably by secondary oxidative reactions.

Induction of apoptosis by cigarette smoke via ROS-dependent endoplasmic reticulum stress and CCAAT/enhancer-binding protein-homologous protein (CHOP)

In this report, we investigated a role of endoplasmic reticulum (ER) stress in cigarette smoke (CS)-induced apoptosis of human bronchial epithelial cells (hBEC). Exposure of hBEC to CS or CS extract (CSE) caused expression of endogenous ER stress markers GRP78 and CHOP and induction of apoptosis evidenced by nuclear condensation, membrane blebbing, and activation of caspase-3 and caspase-4. In vivo exposure of mice to CS also caused induction of GRP78 and CHOP in the lung. Attenuation of ER stress by overexpression of ER chaperone GRP78 or ORP150 significantly attenuated CSE-triggered apoptosis. Exposure of hBEC to CSE caused generation of reactive oxygen species, and treatment with antioxidants inhibited CSE-induced apoptosis. Interestingly, antioxidants including a scavenger of O(2)(*-) blunted induction of CHOP by CSE without affecting the level of GRP78, and dominant-negative inhibition of CHOP abolished CSE-induced apoptosis. Furthermore, a generator of O(2)(*-) selectively induced CHOP and apoptosis in hBEC. Our results revealed that: (1) CS induces ER stress in vitro and in vivo, (2) ER stress mediates CS-triggered apoptosis downstream of oxidative stress, (3) CS-initiated apoptosis is caused through oxidative stress-dependent induction of CHOP, (4) O(2)(*-) may play a dominant role in this process, and (5) oxidative stress-independent induction of GRP78 counterbalances the proapoptotic action of CHOP.

Role of oxidative stress and nitric oxide in atherothrombosis

During the last decade basic and clinical research has highlighted the central role of reactive oxygen species (ROS) in cardiovascular disease. Enhanced production or attenuated degradation of ROS leads to oxidative stress, a process that affects endothelial and vascular function, and contributes to vascular disease. Nitric oxide (NO), a product of the normal endothelium, is a principal determinant of normal endothelial and vascular function. In states of inflammation, NO production by the vasculature increases considerably and, in conjunction with other ROS, contributes to oxidative stress. This review examines the role of oxidative stress and NO in mechanisms of endothelial and vascular dysfunction with an emphasis on atherothrombosis.

Quinolinate-induced rat striatal excitotoxicity impairs endoplasmic reticulum Ca2+-ATPase function

Excessive activation of NMDA glutamate receptors and the resulting loss of intracellular Ca(2+) homeostasis may be lethal (excitotoxic) to neurons. Such excitotoxicity can be induced in vivo by intrastriatal infusion of quinolinate, as this substance selectively activates NMDA receptors. The aim of the present research was to investigate whether the in vivo treatment of striatal tissue with quinolinate would lead to an early impairment of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) activity or mitochondrial Ca(2+) sequestration, two intracellular mechanisms involved in Ca(2+) homeostasis and signaling. Sodium quinolinate was infused intrastriatally into adult rats, and 6 h later the brains were removed and the corpora striata dissected. At this time point, striatal sections stained with Fluoro-Jade, a cellular marker of cell death, showed initial signs of neuronal degeneration. In addition, SERCA activity decreased 39% in relation to the activity observed in the control striata. A corresponding decrease of the same magnitude in (45)Ca(2+) uptake by striatal microsomes was also found in the treated striata. Western blot analysis did not indicate any decrease in SERCA levels in striatal tissue after quinolinate infusion. Mitochondrial Ca(2+) sequestration was still preserved in quinolinate-treated striatal tissue when the assay was carried out in the presence of physiological concentrations of ATP and Mg(2+). These results suggest that impairment of the SERCA function may be an early event in excitotoxicity.

Exercise-induced cardioprotection against myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion (IR) injury is a major contributor to the morbidity and mortality associated with coronary artery disease. Muscular exercise is a countermeasure to protect against IR-induced cardiac injury in both young and old animals. Specifically, regular bouts of endurance exercise protect the heart against all levels of IR-induced injury. Proposed mechanisms to explain the cardioprotective effects of exercise include alterations in coronary circulation, expression of endoplasmic reticulum stress proteins, increased cyclooxygenase-2 activity, induction of myocardial heat shock proteins, improved cardiac antioxidant capacity, and/or elevation of ATP-sensitive potassium channels on both the sarcolemmal and the mitochondrial inner membranes. Moreover, it seems possible that other, yet to be defined, mechanisms of exercise-induced cardioprotection may also exist. Of the known putative cardioprotective mechanisms, current evidence suggests that elevated myocardial levels of antioxidants and increased expression of sarcolemmal ATP-sensitive potassium channels are both contributors to exercise-induced cardioprotection against IR injury. At present, it is unclear if these two protective mediators act independently or interact to contribute to exercise-induced cardioprotection. Understanding the molecular basis for exercise-induced cardioprotection will provide the required knowledge base to develop therapeutic approaches to protect the heart during an IR insult.

Involvement of selective reactive oxygen species upstream of proapoptotic branches of unfolded protein response

Cadmium triggers apoptosis of LLC-PK1 cells through induction of endoplasmic reticulum (ER) stress. We found that cadmium caused generation of reactive oxygen species (ROS) and that cadmium-induced ER stress was inhibited by antioxidants. In contrast, suppression of ER stress did not attenuate cadmium-triggered oxidative stress, suggesting that ER stress occurs downstream of oxidative stress. Exposure of the cells to either O(2)(*), H(2)O(2), or ONOO(-) caused apoptosis, whereas ER stress was induced only by O(2)(*) or ONOO(-). Transfection with manganese superoxide dismutase significantly attenuated cadmium-induced ER stress and apoptosis, whereas pharmacological inhibition of ONOO(-) was ineffective. Interestingly, transfection with catalase attenuated cadmium-induced apoptosis without affecting the level of ER stress. O(2)(*) caused activation of the activating transcription factor 6-CCAAT/enhancer-binding protein-homologous protein (CHOP) and the inositol-requiring ER-to-nucleus signal kinase 1-X-box-binding protein 1 (XBP1) proapoptotic cascades, and overexpression of manganese superoxide dismutase attenuated cadmium-triggered induction of both pathways. Furthermore, phosphorylation of proapoptotic c-Jun N-terminal kinase by O(2)(*) or cadmium was suppressed by dominant-negative inhibition of XBP1. These data elucidated 1) cadmium caused ER stress via generation of ROS, 2) O(2)(*) was selectively involved in cadmium-triggered, ER stress-mediated apoptosis through activation of the activating transcription factor 6-CHOP and inositol-requiring ER-to-nucleus signal kinase 1-XBP1 pathways, and 3) phosphorylation of JNK was caused by O(2)(*)-triggered activation of XBP1.

Trifluoperazine protects brain plasma membrane Ca(2+)-ATPase from oxidative damaging

In the central nervous system (CNS), a number of different pathological processes such as necrosis, Parkinson's and Alzheimer's diseases are related to disturbance in calcium homeostasis associated with oxidative stress. Here we compare the susceptibility of rat brain plasma membrane Ca(2+)-ATPase (PMCA) and sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) isoforms to in vitro oxidative stress, and investigate a putative role of trifluoperazine (TFP), an antipsychotic drug that is also a powerful inhibitor of Ca(2+)-transporter proteins, in protecting these enzymes. It is shown that, in rat brain, PMCA is very sensitive to the damage induced by preincubation with Fe(2+)-ascorbate, or Fe(2+)-ascorbate plus H2O2, while SERCA is resistant. Inhibition of PMCA activity promoted by Fe(2+)/ascorbate medium is fully prevented by the presence of microM concentrations of either butylated hydroxytoluene (BHT) or TFP, but only partially protected, or reversed, by dithiothreitol (DTT), pointing to some protein cysteine(s) as one of the main targets for a lipid peroxidation-dependent damaging mechanism. However, when 0.5-1 mM H2O2 is added together with Fe(2+)/ascorbate, both BHT and TFP only partially prevent ATPase activity inhibition, and DTT does not confer any protection, suggesting two possible additional mechanisms involving both lipid peroxidation and direct damage to PMCA at amino acid residues other than cysteines. A possible use of micromolar concentrations of TFP as a direct antioxidant protector for PMCA under oxidative stress conditions is discussed.

Oxidant-impaired intracellular Ca2+ signaling in pancreatic acinar cells: role of the plasma membrane Ca2+-ATPase

Pancreatitis is an inflammatory disease of pancreatic acinar cells whereby intracellular calcium concentration ([Ca(2+)](i)) signaling and enzyme secretion are impaired. Increased oxidative stress has been suggested to mediate the associated cell injury. The present study tested the effects of the oxidant, hydrogen peroxide, on [Ca(2+)](i) signaling in rat pancreatic acinar cells by simultaneously imaging fura-2, to measure [Ca(2+)](i), and dichlorofluorescein, to measure oxidative stress. Millimolar concentrations of hydrogen peroxide increased cellular oxidative stress and irreversibly increased [Ca(2+)](i), which was sensitive to antioxidants and removal of external Ca(2+), and ultimately led to cell lysis. Responses were also abolished by pretreatment with (sarco)endoplasmic reticulum Ca(2+)-ATPase inhibitors, unless cells were prestimulated with cholecystokinin to promote mitochondrial Ca(2+) uptake. This suggests that hydrogen peroxide promotes Ca(2+) release from the endoplasmic reticulum and the mitochondria and that it promotes Ca(2+) influx. Lower concentrations of hydrogen peroxide (10-100 muM) increased [Ca(2+)](i) and altered cholecystokinin-evoked [Ca(2+)](i) oscillations with marked heterogeneity, the severity of which was directly related to oxidative stress, suggesting differences in cellular antioxidant capacity. These changes in [Ca(2+)](i) also upregulated the activity of the plasma membrane Ca(2+)-ATPase in a Ca(2+)-dependent manner, whereas higher concentrations (0.1-1 mM) inactivated the plasma membrane Ca(2+)-ATPase. This may be important in facilitating "Ca(2+) overload," resulting in cell injury associated with pancreatitis.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Involvement of ryanodine-operated channels in tert-butylhydroperoxide-evoked Ca2+ mobilisation in pancreatic acinar cells

Reactive oxygen species and related oxidative damage have been implicated in the initiation of acute pancreatitis, a disease characterised in its earliest stages by disruption of intracellular Ca2+ homeostasis. The present study was carried out in order to establish the effect of the organic pro-oxidant, tert-butylhydroperoxide (tBHP), on the mobilisation of intracellular Ca2+ stores in isolated rat pancreatic acinar cells and the mechanisms underlying this effect. Cytosolic free Ca2+ concentrations ([Ca2+]c) were monitored using a digital microspectrofluorimetric system in fura-2 loaded cells. In the presence of normal extracellular Ca2+ concentrations ([Ca2+]o), perfusion of pancreatic acinar cells with 1 mmol l-1 tBHP caused a slow sustained increase in [Ca2+]c. This increase was also observed in a nominally Ca2+-free medium, indicating a release of Ca2+ from intracellular stores. Pretreatment of cells with tBHP abolished the typical Ca2+ response of both the physiological agonist CCK-8 (1 nmol l-1) and thapsigargin (TPS, 1 micromol l-1), an inhibitor of the SERCA pump, in the absence of extracellular Ca2+. Similar results were observed with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP, 0.5 micromol l-1), a mitochondrial uncoupler. In addition, depletion of either agonist-sensitive Ca2+ pools by CCK-8 or TPS or mitochondrial Ca2+ pools by FCCP were unable to prevent the tBHP-induced Ca2+ release. By contrast, simultaneous administration of TPS and FCCP clearly abolished the tBHP-induced Ca2+ release. These results show that tBHP releases Ca2+ from agonist-sensitive intracellular stores and from mitochondria. On the other hand, simultaneous application of FCCP and of 2-aminoethoxydiphenylborane (2-APB), a blocker of IP3-mediated Ca2+ release, was unable to suppress the increase in [Ca2+]c induced by tBHP, while the application of 50 micromol l-1 of ryanodine (which is able to block the ryanodine channels) inhibits tBHP-evoked Ca2+ mobilisation. These findings indicate that tBHP releases Ca2+ from non-mitochondrial Ca2+ pools through ryanodine channels.

AT1 receptor antagonist restores cardiac ryanodine receptor function, rendering isoproterenol-induced failing heart less susceptible to Ca2+ -leak induced by oxidative stress

BACKGROUND

The Ca(2+) regulatory proteins in the sarcoplasmic reticulum (SR) play a key role in the pathogenesis of heart failure. In the present study the effect of chronic beta-receptor-stimulation on cardiac and SR functions was assessed, with or without angiotensin-II receptor antagonist treatment recently reported to have anti-beta-adrenergic activity.

METHODS AND RESULTS

Rats were treated with isoproterenol with (+) or without (-) candesartan (CAN) and then SR vesicles were isolated from the left ventricular muscle. Both Ca(2+)-uptake and the amount of SR Ca(2+)-ATPase were significantly lower in the CAN (-) group than in the shams, but those were almost normally restored in the CAN (+). Although the level of the protein kinase A (PKA)-phosphorylation of the SR Ca(2+) release channel, known as the ryanodine receptor (RyR2), was elevated in the CAN (-), no Ca(2+)-leak was detected. However, SIN-1 (O(2) (-) donor) induced Ca(2+)-leak in the CAN (-) at a 10-fold lower dose than in the sham and CAN (+). In cardiomyocytes, SIN-1 decreased cell shortening and the peak Ca(2+) transient and prolonged time from peak to 70% decline in CAN (-), again at 10-fold lower dose than in the sham and CAN (+).

CONCLUSION

Chronic beta-receptor-stimulation did not induce any Ca(2+)-leak from the SR, whereas Ca(2+)-leak was easily induced when oxidative stress was applied to the PKA-phosphorylated RyR2. Candesartan not only improved Ca(2+)-uptake, but also prevented PKA-phosphorylation, rendering the SR less susceptible to Ca(2+)-leak.

Dose-dependent effect of hydrogen peroxide on calcium mobilization in mouse pancreatic acinar cells

We have employed confocal laser scanning microscopy to investigate how intracellular free calcium concentration ([Ca2+]i) is influenced by hydrogen peroxide (H2O2) in collagenase-dispersed mouse pancreatic acinar cells. In the absence of extracellular calcium, treatment of cells with increasing concentrations of H2O2resulted in an increase in [Ca2+]i, indicating the release of calcium from intracellular stores. Micromolar concentrations of H2O2induced an oscillatory pattern, whereas 1 mmol H2O2/L caused a slow and sustained increase in [Ca2+]i. H2O2abolished the typical calcium release stimulated by thapsigargin or by the physiological agonist cholecystokinin octapeptide (CCK-8). Depletion of either agonist-sensitive or mitochondrial calcium pools was unable to prevent calcium release induced by 1 mmol H2O2/L, but depletion of both stores abolished it. Additionally, lower H2O2concentrations were able to release calcium only after depletion of mitochondrial calcium stores. Treatment with either the phospholipase C inhibitor U-73122 or the inhibitor of the inositol 1,4,5-trisphosphate (IP3) receptor xestospongin C did not modify calcium release from the agonist-sensitive pool induced by 100 µmol H2O2/L, suggesting the involvement of a mechanism independent of IP3 generation. In addition, H2O2reduced amylase release stimulated by CCK-8. Finally, either the H2O2-induced calcium mobilization or the inhibitory effect of H2O2on CCK-8-induced amylase secretion was abolished by dithiothreitol, a sulphydryl reducing agent. We conclude that H2O2at micromolar concentrations induces calcium release from agonist- sensitive stores, and at millimolar concentrations H2O2can also evoke calcium release from the mitochondria. The action of H2O2is mediated by oxidation of sulphydryl groups of calcium ATPases independently of IP3 generation.Key words: hydrogen peroxide, pancreatic acinar cells, intracellular calcium stores, amylase secretion.

Baroreflex function in conscious rats submitted to iron overload

Our hypothesis is that iron accumulated in tissue, rather than in serum, may compromise cardiovascular control. Male Fischer 344 rats weighing 180 to 220 g were divided into 2 groups. In the serum iron overload group (SIO, N = 12), 20 mg elemental iron was injected ip daily for 7 days. In the tissue iron overload group (TIO, N = 19), a smaller amount of elemental iron was injected (10 mg, daily) for 5 days followed by a resting period of 7 days. Reflex heart rate responses were elicited by iv injections of either phenylephrine (0.5 to 5.0 microg/kg) or sodium nitroprusside (1.0 to 10.0 microg/kg). Baroreflex curves were determined and fitted to sigmoidal equations and the baroreflex gain coefficient was evaluated. To evaluate the role of other than a direct effect of iron on tissue, acute treatment with the iron chelator deferoxamine (20 mg/kg, iv) was performed on the TIO group and the baroreflex was re-evaluated. At the end of the experiments, evaluation of iron levels in serum confirmed a pronounced overload for the SIO group (30-fold), in contrast to the TIO group (2-fold). Tissue levels of iron, however, were higher in the TIO group. The SIO protocol did not produce significant alterations in the baroreflex curve response, while the TIO protocol produced a nearly 2-fold increase in baroreflex gain (-4.34 +/- 0.74 and -7.93 +/- 1.08 bpm/mmHg, respectively). The TIO protocol animals treated with deferoxamine returned to sham levels of baroreflex gain (-3.7 +/- 0.3 sham vs -3.6 +/- 0.2 bpm/mmHg) 30 min after the injection. Our results indicate an effect of tissue iron overload on the enhancement of baroreflex sensitivity.

Inhibition of Na+/Ca2+ exchanger by peroxynitrite in microsomes of pulmonary smooth muscle: role of matrix metalloproteinase-2

Treatment of bovine pulmonary artery smooth muscle microsomes with peroxynitrite (ONOO-) (100 microM) markedly stimulated matrix metalloproteinase-2 (MMP-2) activity and also enhanced Ca2+ATPase activity and ATP-dependent Ca2+ uptake. Pretreatment of the microsomes with vitamin E (1 mM) and TIMP-2 (50 microg/ml) preserved the increase in MMP-2 activity, Ca2+ATPase activity and also ATP-dependent Ca2+ uptake in the microsomes. In contrast, Na(+)-dependent Ca2+ uptake in the microsomes was inhibited by ONOO- and this was found to be reversed by vitamin E (1 mM) and TIMP-2 (50 microg/ml). However, changes caused by ONOO- in MMP-2 activity, ATP-dependent Ca2+ uptake and Na(+)-dependent Ca2+ uptake were not reversed upon pretreatment of the microsomes with a low concentration of 5 microg/ml of TIMP-2 which, on the contrary, reversed MMP-2 (1 microg/ml)-mediated alteration on these parameters. The inhibition of Na(+)-dependent Ca2+ uptake by ONOO- and MMP-2 overpowered the stimulation of ATP-dependent Ca2+ uptake in the microsomes. Treatment with ONOO- abolished the inhibitory effect of TIMP-2 (5 microg/ml) on MMP-2 (1 microg/ml) causing 14C-gelatin degradation. Overall, the present study suggests that ONOO- inactivated TIMP-2, the ambient inhibitor of MMP-2, leading to activation of the ambient proteinase, MMP-2, and subsequently stimulated Ca2+ATPase activity and ATP-dependent Ca2+ uptake, but inhibited Na(+)-dependent Ca2+ uptake, resulting in a marked decrease in Ca2+ uptake in microsomes of bovine pulmonary artery smooth muscle.

Effect of hydrogen peroxide on Ca2+ mobilisation in human platelets through sulphydryl oxidation dependent and independent mechanisms

Using Fura-2-loaded human platelets we studied the nature of the mechanisms involved in Ca2+ signalling mediated by H2O2. In a Ca2+-free medium, H2O2 (10 microM-100 mM) induced a concentration-dependent increase in [Ca2+]i. Depletion of either agonist-sensitive or mitochondrial Ca2+ pools reduced this effect while depletion of both stores abolished it. Xestospongin C, an inositol 1,3,5-trisphosphate (IP3) receptor inhibitor, reduced Ca2+ release evoked by 1 mM H2O2 by 45%, indicating that H2O2-induced Ca2+ release involves interaction with IP3 receptors. Blockade of the IP3 turnover by lithium or treatment with U-73122 did not modify H2O2-induced Ca2+ release from the agonist-sensitive pool, suggesting the involvement of a mechanism independent of IP3 generation. H2O2 inhibited Ca2+ reuptake into the agonist-sensitive stores mediated by the sarcoendoplasmic reticulum Ca2+ ATPase (SERCA). Thimerosal (5 microM), a sulphydryl reagent, induced Ca2+ release from the agonist-sensitive stores. This event was impaired by treatment with 2 mM DTT, which also inhibited H2O2-induced Ca2+ release from the agonist-sensitive pool but not from mitochondria. H2O2 reduced the ability of the plasma membrane Ca2+ ATPase (PMCA) to extrude Ca2+ by 75%, an effect that was unaffected by DTT. Consistent with this, thimerosal did not modify the PMCA activity. Finally, exposure to H2O2 triggered platelet aggregation, which was slower than that observed after agonist stimulation. We conclude that H2O2 induced Ca2+ release from agonist-sensitive stores by oxidation of sulphydryl groups in SERCA and the IP3 receptors independently of IP3 generation. In addition, H2O2 induced Ca2+ release from mitochondria and inhibited the PMCA activity by different mechanisms in human platelets.

Role of strategic cysteine residues in oxidative damage to the yeast plasma membrane H(+)-ATPase caused by Fe- and Cu-containing Fenton reagents

Damage caused to Saccharomyces cerevisiae SY4 plasma membrane H(+)-ATPase by Fe- and Cu-Fenton reagents was determined in secretory vesicles containing enzyme in which Cys residues were replaced singly or in pairs by Ala. Cys-221 situated in a beta-sheet domain between M2 and M3 segments, phosphorylation domain-located Cys-409 and Cys-532 situated at the ATP-binding site play a role in the inactivation. In the presence of all three residues the enzyme exhibited a certain basic inactivation, which did not change when Cys-532 was replaced with Ala. In mutants having intact Cys-532 but lacking one or both other cysteines, replacement of Cys-221 with Ala led to lower inactivation, suggesting that Cys-221 may serve as a target for metal-catalyzed oxidation and intact Cys-532 promotes this target role of Cys-221. In contrast, the absence of Cys-409 caused higher inactivation by Fe-Fenton. Cys-532 thus seems to serve as a target for Fe-Fenton, intact Cys-409 causing a conformational change that makes Cys-532 less accessible to oxidation. The mutant lacking both Cys-221 and Cys-409 is more sensitive to Fe-Fenton than to Cu-Fenton and the absence of both Cys residues thus seems to expose presumable extra Fe-binding sites. These data and those on protection by ATP, ADP, 1,4-dithiothreitol and deferrioxamine B point to complex interactions between individual parts of the enzyme molecule that determine its sensitivity towards Fenton reagents. ATPase fragmentation caused by the two reagents differed in that the Fe-Fenton reagent produced in Western blot "smears" whereas the Cu-Fenton reagent produced defined fragments.

Effects of cytosolic NADH/NAD(+) levels on sarcoplasmic reticulum Ca(2+) release in permeabilized rat ventricular myocytes

In the heart ischaemic conditions induce metabolic changes known to have profound effects on Ca(2+) signalling during excitation-contraction coupling. Ischaemia also affects the redox state of the cell. However, the role of cytosolic redox couples, such as the NADH/NAD(+) redox system, for the regulation of Ca(2+) homeostasis has remained elusive. We studied the effects of NADH and NAD(+) on sarcoplasmic reticulum (SR) Ca(2+) release in permeabilized rat ventricular myocytes as well as on Ca(2+) uptake by SR microsomes and ryanodine receptor (RyR) single channel activity. Exposure of permeabilized myocytes to NADH (2 mm; [Ca(2+)](cyt)= 100nm) decreased the frequency and the amplitude of spontaneous Ca(2+) sparks by 62% and 24%, respectively. This inhibitory effect was reversed by NAD(+) (2 mm) and did not depend on mitochondrial function. The inhibition of Ca(2+) sparks by NADH was associated with a 52% decrease in SR Ca(2+) load. Some of the effects observed with NADH may involve the generation of superoxide anion (O(2)(-).) as they were attenuated to just a transient decrease of Ca(2+) spark frequency by superoxide dismutase (SOD). O(2)(-). generated in situ from the xanthine/xanthine oxidase reaction caused a slowly developing decrease of Ca(2+) spark frequency and SR Ca(2+) load by 44% and 32%, respectively. Furthermore, in studies with cardiac SR microsomes NADH slowed the rate of ATP-dependent Ca(2+) uptake by 39%. This effect also appeared to depend on O(2)(-). formation. Single channel recordings from RyRs incorporated into lipid bilayers revealed that NADH (2 mm) inhibited the activity of RyR channels by 84%. However, NADH inhibition of RyR activity was O(2)(-).-independent. In summary, an increase of the cytoplasmic NADH/NAD(+) ratio depresses SR Ca(2+) release in ventricular cardiomyocytes. The effect appears to be mediated by direct NADH inhibition of RyR channel activity and by indirect NADH inhibition (O(2)(-). mediated) of SR Ca(2+)-ATPase activity with a subsequent decrease in SR Ca(2+) content.