Sublethal oxidant stress induces a reversible increase in intracellular calcium dependent on NAD(P)H oxidation in rat alveolar macrophages

Enhanced visualization of histological samples with an adjustable RGB contrast system with application for tissue used in photodynamic therapy

The analysis of histological sections has long been a valuable tool in the pathological studies. The interpretation of tissue conditions, however, relies directly on visual evaluation of tissue slides, which may be difficult to interpret because of poor contrast or poor color differentiation. The Chromatic Contrast Visualization System (CCV) combines an optical microscope with electronically controlled light-emitting diodes (LEDs) in order to generate adjustable intensities of RGB channels for sample illumination. While most image enhancement techniques rely on software post-processing of an image acquired under standard illumination conditions, CCV produces real-time variations in the color composition of the light source itself. The possibility of covering the entire RGB chromatic range, combined with the optical properties of the different tissues, allows for a substantial enhancement in image details. Traditional image acquisition methods do not exploit these visual enhancements which results in poorer visual distinction among tissue structures. Photodynamic therapy (PDT) procedures are of increasing interest in the treatment of several forms of cancer. This study uses histological slides of rat liver samples that were induced to necrosis after being exposed to PDT. Results show that visualization of tissue structures could be improved by changing colors and intensities of the microscope light source. PDT-necrosed tissue samples are better differentiated when illuminated with different color wavelengths, leading to an improved differentiation of cells in the necrosis area. Due to the potential benefits it can bring to interpretation and diagnosis, further research in this field could make CCV an attractive technique for medical applications.

Quercus infectoria galls possess antioxidant activity and abrogates oxidative stress-induced functional alterations in murine macrophages

The present study reports the antioxidant activity of ethanolic extract of Quercus infectoria galls. The antioxidant potency of galls was investigated employing several established in vitro model systems. Their protective efficacy on oxidative modulation of murine macrophages was also explored. Gall extract was found to contain a large amount of polyphenols and possess a potent reducing power. HPTLC analysis of the extract suggested it to contain 19.925% tannic acid (TA) and 8.75% gallic acid (GA). The extract potently scavenged free radicals including DPPH (IC(50)~0.5 microg/ml), ABTS (IC(50)~1 microg/ml), hydrogen peroxide (H(2)O(2)) (IC(50)~2.6 microg/ml) and hydroxyl (*OH) radicals (IC(50)~6 microg/ml). Gall extract also chelated metal ions and inhibited Fe(3+) -ascorbate-induced oxidation of protein and peroxidation of lipids. Exposure of rat peritoneal macrophages to tertiary butyl hydroperoxide (tBOOH) induced oxidative stress in them and altered their phagocytic functions. These macrophages showed elevated secretion of lysosomal hydrolases, and attenuated phagocytosis and respiratory burst. Activity of macrophage mannose receptor (MR) also diminished following oxidant exposure. Pretreatment of macrophages with gall extract preserved antioxidant armory near to control values and significantly protected against all the investigated functional mutilations. MTT assay revealed gall extract to enhance percent survival of tBOOH exposed macrophages. These results indicate that Q. infectoria galls possess potent antioxidant activity, when tested both in chemical as well as biological models.

Estrogen exerts a spatial and temporal influence on reactive oxygen species generation that precedes calcium uptake in high-capacity mitochondria: implications for rapid nongenomic signaling of cell growth

Novel findings that emerged from this study underscore the fact that the dynamic nature of mitochondria leads to functional heterogeneity of [Ca(2+)](mito) with respect to estrogen actions in MCF7 cells. We show that estrogen exposure to cells increased [Ca(2+)](mito) in a high-calcium capacity mitochondrial population but not in low-calcium capacity mitochondria. Physiological concentrations of 17beta-estradiol (E2) modulated Ca(2+)(mito) uptake within 90 s. Interestingly, this calcium response lagged behind the induction of mitochondrial reactive oxygen species (mtROS). The rapid induction of Ca(2+)(mito) in response to E2 and its inhibition by mitochondrial blockers suggest that mitochondria are early nongenomic targets of E2. This suggests that a subpopulation of mitochondria is recruited to respond to new metabolic requirements required by estrogen triggers or, as in this case, E2-induced Ca(2+)(mito) and/or mtROS promotes oxidative signaling without involving nuclear estrogen receptor signaling. Although the early E2-induced Ca(2+) did not alter the expression of genes involved in calcium signaling pathways, an intracellular calcium chelator BAPTA-AM and the Ca(2+)(mito) uniporter blocker ruthenium red prevented E2-induced cell growth. We have shown recently that E2-mediated ROS production controls the promoter activity of cyclin D1 by post-translational modification of calcium sensitive transcription factor CREB. The findings of this study offer a new paradigm that rapid E2-induced changes in mtROS and Ca(2+)(mito) are involved in cell cycle progression presumably through the control of early cell cycle genes. Targeting mitochondria to disrupt communication between mitochondria and ROS/Ca(2+) signaling pathways may provide the basis for a novel anticancer strategy for the treatment of estrogen-dependent breast cancer.

Low energy visible light induces reactive oxygen species generation and stimulates an increase of intracellular calcium concentration in cardiac cells

Low energy visible light (LEVL) irradiation has been shown to exert some beneficial effects on various cell cultures. For example, it increases the fertilizing capability of sperm cells, promotes cell proliferation, induces sprouting of neurons, and more. To learn about the mechanism of photobiostimulation, we studied the relationship between increased intracellular calcium ([Ca2+]i) and reactive oxygen species production following LEVL illumination of cardiomyocytes. We found that visible light causes the production of O2. and H2O2 and that exogenously added H2O2 (12 microm) can mimic the effect of LEVL (3.6 J/cm2) to induce a slow and transient increase in [Ca2+]i. This [Ca2+]i elevation can be reduced by verapamil, a voltage-dependent calcium channel inhibitor. The kinetics of [Ca2+]i elevation and morphologic damage following light or addition of H2O2 were found to be dose-dependent. For example, LEVL, 3.6 J/cm2, which induced a transient increase in [Ca2+]i, did not cause any cell damage, whereas visible light at 12 J/cm2 induced a linear increase in [Ca2+]i and damaged the cells. The linear increase in [Ca2+]i resulting from high energy doses of light could be attenuated into a non-linear small rise in [Ca2+]i by the presence of extracellular catalase during illumination. We suggest that the different kinetics of [Ca2+]i elevation following various light irradiation or H2O2 treatment represents correspondingly different adaptation levels to oxidative stress. The adaptive response of the cells to LEVL represented by the transient increase in [Ca2+]i can explain LEVL beneficial effects.

Anti-apoptotic defense of bcl-2 gene against hydroperoxide-induced cytotoxicity together with suppressed lipid peroxidation, enhanced ascorbate uptake, and upregulated Bcl-2 protein

Although it is well known that Bcl-2 can prevent apoptosis, the Bcl-2's anti-apoptotic mechanism is not fully understood. Here, we investigate the mechanism of oxidant-induced cell death and to investigate the role of Bcl-2 in the tert-butyl hydroperoxide (t-BuOOH)-induced oxidant injury in Rat-1 fibroblasts and their bcl-2 transfected counterparts, b5 cells. Treatment with t-BuOOH causes mitochondrial disfunction and induced morphological features consistent with apoptosis more markedly in Rat-1 cells than in b5 cells. The hydroperoxide t-BuOOH at concentrations less than 100 nM for as long as 48 h or with higher concentrations (up to 100 microM) for only 3 h induces death in Rat-1 cells, whereas their bcl-2 transfectants were significantly resistant to cytotoxicity by both time and all concentration other than 100 microM. The similar results were obtained also for DNA strand cleavages as detected by TUNEL stain. The bcl-2 transfectants significantly suppressed t-BuOOH-induced increases in both lipid peroxidation and caspase-3 activation 3 and 1 h after t-BuOOH exposure, respectively, but failed to suppress either caspase-1 activation or an enhanced production of the intracellular reactive oxygen species (ROS). Intracellular uptake of [1-(14)C] ascorbic acid (Asc) into the bcl-2 transfectants was superior to that into the non-transfectants always under examined conditions regardless of serum addition to culture medium and cell density. Upregulation of Bcl-2 proteins was rapidly induced after t-BuOOH exposure in the transfectants, but not in non-transfectants, and restored till 24 h to the normal Bcl-2 level. Thus suppressions of both lipid peroxidation and the subsequent cell death events such as caspase-3 activation and DNA cleavage were concerned with the inhibitory effects of Bcl-2 on the t-BuOOH-induced cytotoxicity. And some of these events may correlate with Bcl-2 expression-induced partial enhanced anti-oxidant cellular ability including enrichment of intracellular Asc and oxidative stress-induced upregulation of Bcl-2 protein. On the other hand, ROS production and caspase-1 activation were not related to cytoprotection by Bcl-2.

Macrophage signaling and respiratory burst

Macrophages are key defenders of the lung and play an essential role in mediating the inflammatory response. Critical to this is the activation of the NADPH oxidase. Through receptor-mediated interactions, extracellular stimuli activate pathways that signal for the phosphorylation and assembly of the NADPH oxidase. Once the NADPH oxidase is activated, it produces superoxide and H2O2 in a process known as the respiratory burst. The involvement of O2.- and H2O2 in the antimicrobicidal function of macrophages has been assumed for many years, but it is now clear that the H2O2 produced by the respiratory burst functions as a second messenger and activates major signaling pathways in the alveolar macrophage. Both the nuclear factor-kappaB and activator protein-1 transcription factors are activated by H2O2 produced by the respiratory burst, and, since these control the inducible expression of genes whose products are part of the inflammatory response, this may be a critical link between the respiratory burst and other inflammatory responses. The c-Jun N-terminal kinase (JNK) and extracellular-regulated kinase (ERK) pathways, two members of the mitogen-activated protein kinase family, are also activated by the respiratory burst. JNK is activated by both exogenous and endogenously produced H2O2. Studies with ERK have shown that specific agonists of the respiratory burst, but not bolus H2O2, can activate this pathway. The ERK pathway also modulates the expression of genes via phosphorylation of the transcription factor Elk-1 that controls the production of the c-Fos transcription factor. Although an understanding of the mechanism of redox signaling is in its infancy, it is becoming clear that the reactive oxygen species produced by the respiratory burst have a profound effect on intracellular signaling pathways and ultimately in modulating gene expression.

tert-Butylhydroperoxide induces peroxynitrite-dependent mitochondrial permeability transition leading PC12 cells to necrosis

A short-term exposure of PC12 cells to tert-butylhydroperoxide, followed by recovery in fresh culture medium, causes cell death and the extent of this response progressively increases during the 120 min of post-treatment incubation. Morphological and biochemical analyses of these cells revealed that the mode of cell death was necrosis. Cell killing induced by the hydroperoxide seems to be in part mediated by peroxynitrite because the lethal response was markedly and similarly reduced by the nitric oxide synthase inhibitor N omega-nitro-L-arginine methylester and by scavengers of nitric oxide or peroxynitrite. This peroxynitrite-dependent mechanism of cytotoxicity was blunted by antioxidants and inhibitors of mitochondrial permeability transition and the onset of cell death was preceded by mitochondrial depolarization and loss of cellular ATP. We conclude that tert-butylhydroperoxide promotes peroxynitrite-dependent PC12 cell necrosis causally linked to peroxidation of membrane lipids and mitochondrial permeability transition.

Different signalling pathways mediate the opposite effects of endogenous versus exogenous nitric oxide on hydroperoxide toxicity in CHP100 neuroblastoma cells

The results presented in this study indicate that the toxic response brought about by increasing concentrations of tert-butylhydroperoxide in CHP100 cells was mitigated significantly by exogenously added nitric oxide donors via a cyclic GMP-independent mechanism. In contrast with these results, endogenous nitric oxide generated by the Ca2+-mobilizing agent caffeine was found to increase hydroperoxide toxicity. Under these conditions, nitric oxide was not directly toxic to the cells. Rather, nitric oxide was found to promote the caffeine-mediated release of Ca2+ from ryanodine-sensitive Ca2+ stores via a cyclic GMP-independent mechanism. Release of the cation from ryanodine-sensitive Ca2+ stores was causally linked with the caffeine/nitric oxide-mediated enhancement of tert-butylhydroperoxide toxicity. It is concluded that endogenous and exogenous nitric oxide activate diverging signalling pathways independent of cyclic GMP formation and causing opposite effects on the toxic response evoked by tert-butylhydroperoxide in CHP100 cells.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Rotenone and pyruvate prevent the tert-butylhydroperoxide-induced necrosis of U937 cells and allow them to proliferate

Exposure of U937 cells to tert-butylhydroperoxide (tB-OOH) led to cyclosporin A-sensitive mitochondrial membrane permeability transition and necrosis. Pyruvate and rotenone, which increase mitochondrial NADH via different mechanisms, prevented these responses and the cells which received these treatments proliferated with kinetics similar to those observed in untreated cells. In contrast with these results, cells rescued by cyclosporin A were unable to proliferate. Thus, mitochondrial NADH plays a pivotal role in preventing upstream events which result in the onset of mitochondrial membrane permeability transition and death in cells exposed to tB-OOH. These events appear to be critical for recovery of the ability of the cells to proliferate.

Mitochondrial formation of hydrogen peroxide is causally linked to the antimycin A-mediated prevention of tert-butylhydroperoxide-induced U937 cell death

Antimycin A and 2-heptyl-4-hydroxyquinoline N-oxide (HQNO), both of which bind to the same site of complex III, prevented U937 cell killing promoted by tert-butylhydroperoxide (tB-OOH). This cytoprotection was not directly caused by inhibition of electron transport or reduced formation of tB-OOH-derived toxic species, but rather appeared to be the consequence of a mechanism involving mitochondrial formation of hydrogen peroxide. Ubisemiquinone was most likely the electron donor allowing the formation of superoxides and, as a consequence, of hydrogen peroxide.

The alveolar macrophage as a model of calcium signaling in oxidative stress

Regulation of the free intracellular calcium concentration, [Ca2+]i, plays a major role in physiological signal transduction. Many of the essential enzymes in signaling cascades are Ca(2+)-dependent, as are numerous proteins that participate in the regulated function. Oxidative stress, which for many years was considered synonymous with cell and tissue injury, has more recently been demonstrated to alter signal transduction in both positive and negative directions. The realization that hydrogen peroxide and lipid hydroperoxides are produced as part of normal metabolism has led to the proposal that these oxidants function as second messengers. Exposure to environmental and other agents that produce hydroperoxides or the addition of exogenous hydroperoxides also causes elevation of [Ca2+]i in some cells. At sublethal exposure to hydroperoxides, the elevation in [Ca2+]i can either alter or mimic physiological stimulation. In addition to endoplasmic reticulum, mitochondria, and the extracellular space, the phospholipid- and Ca(2+)-binding proteins known as annexins constitute a Ca2+ pool from which this ion may be released under situations of oxidative stress. In this article, the source and consequences of Ca2+ elevation are reviewed with an emphasis on studies done with alveolar macrophages. These phagocytes, which modulate much of the physiological and immunological function of the lung, are susceptible targets for environmental oxidants.

Low catalase activity in xeroderma pigmentosum fibroblasts and SV40-transformed human cell lines is directly related to decreased intracellular levels of the cofactor, NADPH

We have previously shown that fibroblasts from ultra-violet (UV) hypersensitive xeroderma pigmentosum patients (XP) are markedly deficient in catalase activity resulting in high intracellular levels of hydrogen peroxide (H2O2) following UV irradiation. No direct correlation between catalase activity and repair ability was found since XP variant cells which are proficient in nucleotide excision repair (NER) showed activities as low as those found in NER deficient classical XP groups A and D. However, in contrast to the skin cancer prone XP patients, another NER deficient syndrome, trichothiodystrophy (TTD), which does not exhibit any cancer predisposition, was found to present normal catalase activity. Moreover, it was found that a variety of SV40 transformed human cell lines also showed decreased catalase activities. Our previous data showed that a molecular analysis of the normal, XP, TTD or transformed human fibroblast cell lines did not reveal any differences in levels of catalase transcription or amount of catalase protein subunits. These results incited us to examine the structure/function relationship of the tetrameric active enzyme form of catalase (which is the only one able to carry out H2O2 dismutation) with its cofactor NADPH. In the present study, we have measured the effects on catalase activity after adding NADPH either to acellular extracts or during cell culture of the different cell types. The NADPH levels were also quantified directly in intact cells using flow cytometry. Our results show a clear relationship between low catalase activity and striking decrease in intracellular NADPH levels.

Transmembrane redox signaling activates NF-kappaB in macrophages

In macrophages, NF-kappaB can be activated by H2O2 generated by the respiratory burst or added exogenously. The mechanism of H2O2 signaling may involve changes in the cellular redox state or a redox reaction at the plasma membrane; however, the site of H2O2 action cannot be readily ascertained because of its membrane permeability. Ferricyanide, a nonpermeable redox active anion, activated NF-kappaB in the macrophage cell line, J774A.1. In contrast with exogenous H2O2, activation by ferricyanide did not correlate with net oxidation of NAD(P)H or glutathione, suggesting that a transplasma membrane redox reaction itself was the first signaling process in NF-kappaB activation.

Mechanism of the antimycin A-mediated enhancement of t-butylhydroperoxide-induced single-strand breakage in DNA

Inhibitors of complex III increased the DNA strand scission induced by t-butylhydroperoxide (tB-OOH) and cumene hydroperoxide but did not affect DNA damage induced by H2O2. The hypothesis that these effects are selectively linked to inhibition of the electron transport from cytochrome b to cytochrome c1 is validated by the following observations: (1) two complex III inhibitors, antimycin A and 2-heptyl-4-hydroxyquinoline N-oxide, enhanced the tB-OOH-induced DNA cleavage over the same concentration range as that in which inhibition of oxygen consumption was observed; (2) the complex III inhibitor-mediated enhancement of tB-OOH-induced DNA damage was abolished by the complex I inhibitor rotenone or by glucose omission, and (3) the enhancing effects of antimycin A were not observed in respiration-deficient cells. The mechanism whereby the complex III inhibitors potentiate DNA cleavage promoted by tB-OOH was subsequently investigated with intact as well as permeabilized cells. H2O2, produced at the level of mitochondria via a Ca2+-dependent process, was found to account for the enhancing effects of antimycin A.

Ca(2+)-dependent p47phox translocation in hydroperoxide modulation of the alveolar macrophage respiratory burst

Oxidative stress produces dual effects on the respiratory burst of rat alveolar macrophages. Preincubation with hydroperoxide concentrations [H2O2 or tert-butyl hydroperoxide (t-BOOH); < 50 microM] enhances stimulation of the respiratory burst, whereas higher concentrations inhibit stimulation. Both the enhancement and inhibition are markedly attenuated by buffering t-BOOH-induced changes in intracellular Ca2+ concentration ([Ca2+]i). Phosphorylation of the NADPH oxidase component p47phox and its translocation from cytoplasm to plasma membrane are essential in respiratory burst activation. Phorbol 12-myristate 13-acetate (PMA)-stimulated p47phox phosphorylation was negligibly affected by 25 or 100 microM t-BOOH. Nonetheless, 25 microM t-BOOH increased PMA-stimulated p47phox translocation, whereas 100 microM t-BOOH decreased PMA-stimulated translocation. In unstimulated cells, however, neither phosphorylation nor translocation of p47phox was affected by t-BOOH. Buffering of the t-BOOH-mediated changes of [Ca2+]i abolished the effects of t-BOOH on PMA-stimulated translocation in parallel to effects upon the respiratory burst. The results suggest that the dual effects of hydroperoxides are mediated, in part, by Ca(2+)-dependent processes affecting the assembly of the respiratory burst oxidase at steps that are separate from p47phox phosphorylation.

Alternative mechanisms for hydroperoxide-induced DNA single strand breakage

The results presented in this study point out the existence of similarities as well as differences in the DNA-damaging effects of organic vs. inorganic hydroper-oxides in human myeloid leukemia U937 cells. On the one hand, the formation of DNA single strand breaks (SSBs) induced by either hydrogen peroxide (H2O2) or tert-butylhydroperoxide (tBu-OOH) was prevented by iron chelators, was not affected by antioxidants or glucose omission before and during peroxide exposure and was enhanced by prior catalase depletion. Furthermore, H2O2- and tBu-OOH-induced DNA strand scission were also detected after treatment at 0 degree C. On the other hand, H2O2, but not tBu-OOH or cumene hydroperoxide (cum-OOH), produced DNA strand scission in isolated nuclei and post-lysed DNA samples. In addition, lowering the basal intracellular calcium concentration with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) markedly reduced the DNA-damaging efficiency of tBu-OOH while promoting only a slight decline in the number of DNA SSBs induced by H2O2. Taken together, these results are consistent with the commonly held theory that DNA damage caused by H2O2 is mediated by the formation of hydroxyl radicals. tBu-OOH-induced DNA single strand breakage appears to involve both the formation of H2O2 and a rise in cytosolic calcium ions.

Oxidants as stimulators of signal transduction

Redox (oxidation-reduction) reactions regulate signal transduction. Oxidants such as superoxide, hydrogen peroxide, hydroxyl radicals, and lipid hydroperoxides (i.e., reactive oxygen species) are now realized as signaling molecules under subtoxic conditions. Nitric oxide is also an example of a redox mediator. Reactive oxygen species induce various biological processes such as gene expression by stimulating signal transduction components such as Ca(2+)-signaling and protein phosphorylation. Various oxidants increase cytosolic Ca2+; however, the exact origin of Ca2+ is controversial. Ca2+ may be released from the endoplasmic reticulum, extracellular space, or mitochondria in response to oxidant-influence on Ca2+ pumps, channels, and transporters. Alternatively, oxidants may release Ca2+ from Ca2+ binding proteins. Various oxidants stimulate tyrosine as well as serine/threonine phosphorylation, and direct stimulation of protein kinases and inhibition of protein phosphatases by oxidants have been proposed as mechanisms. The oxidant-stimulation of the effector molecules such as phospholipase A2 as well as the activation of oxidative stress-responsive transcription factors may also depend on the oxidant-mediated activation of Ca(2+)-signaling and/or protein phosphorylation. In addition to the stimulation of signal transduction by oxidants, the observations that ligand-receptor interactions produce reactive oxygen species and that antioxidants block receptor-mediated signal transduction led to a proposal that reactive oxygen species may be second messengers for transcription factor activation, apoptosis, bone resorption, cell growth, and chemotaxis. Physiological significance of the role of biological oxidants in the regulation of signal transduction as well as the mechanisms of the oxidant-stimulation of signal transduction are discussed.

Hydroperoxide-induced increases in intracellular calcium due to annexin VI translocation and inactivation of plasma membrane Ca2+-ATPase

Oxidative stress can cause changes in intracellular free calcium concentration ([Ca2+]i) that resemble those occurring under normal cell signaling. In the alveolar macrophage, hydroperoxide-induced elevation of [Ca2+]i modulates the respiratory burst and other important physiologic functions. The source of Ca2+ released by hydroperoxide is intracellular but separate from the endoplasmic reticulum pool released by receptor-mediated stimuli (Hoyal, C. R., Gozal, E., Zhou, H., Foldenauer, K., and Forman, H. J. (1996) Arch. Biochem. Biophys. 326, 166-171). Previous studies in other cells have suggested that mitochondria are a potential source of oxidant-induced [Ca2+]i elevation. In this study we have identified another potential source of hydroperoxide-releasable intracellular calcium, that bound to annexin VI on the inner surface of the plasma membrane. Translocation of annexin VI from the membrane during exposure to t-butyl hydroperoxide matched elevation of [Ca2+]i as a function of time and t-butyl hydroperoxide concentration. The translocation was possibly due to a combination of ATP depletion and oxidative modification of membrane lipids and proteins. A sustained increase in [Ca2+]i occurring > 50 pmol/10(6) cells (50 microM under these conditions) appeared to be a consequence of membrane Ca2+-ATPase dysfunction. These results suggest that exposure to oxidative stress results in early alterations to the plasma membrane and concomitant release of Ca2+ into the cytosol. In addition it suggests a mechanism for participation of annexin VI translocation that may underlie the alterations in macrophage function by oxidative stress.

Oxidized glutathione mediates cation channel activation in calf vascular endothelial cells during oxidant stress

1. The oxidant, tert-butylhydroperoxide (tBuOOH) depolarizes calf pulmonary artery endothelial cells by activating a non-selective cation channel. To identify the molecular mediator of channel activation during oxidant stress, the patch-clamp technique was used to compare tBuOOH-induced changes in membrane potential and channel activity with those induced by oxidized glutathione (GSSG), a cytosolic product of oxidant metabolism. 2. When recording pipettes contained GSSG (2 mM), whole-cell zero-current potential measured immediately following pipette break-in was not different from control values (-57 mV). However, within 20 min of break-in, zero-current potential was depolarized to -7 mV. The time course of depolarization was dependent on the concentration of GSSG and was accelerated by inhibition of GSSG metabolism. 3. In excised membrane patches, channels were activated by internal GSSG, but not by internal tBuOOH, reduced glutathione (GSH), or external GSSG. Channels were equal in size (28 pS) and in ionic selectivity to those activated by incubation of intact cells with tBuOOH. As little as 20 microM GSSG was sufficient to maximally activate channels. However, the time course of channel activation was concentration dependent between 20 microM and 2 mM GSSG. 4. Channel activation by GSSG was reversed by GSH and by increasing the [GSH]:[GSSG] ratio. Likewise, channel activation by pre-incubation of intact cells with tBuOOH was reversed by GSH applied after patch excision. 5. These results strongly suggest that GSSG is an endogenous intracellular mediator of channel activation and depolarization during oxidant stress.

Oxidation of pyridine nucleotides is an early event in the lethality of allyl alcohol

The involvement of altered pyridine nucleotide concentrations in the cytolethality of allyl alcohol was studied in isolated rat hepatocytes. NAD+, NADH, NADP+, NADPH and viability loss (leakage of lactate dehydrogenase into the medium) were measured in cells incubated with 0.5 mM allyl alcohol with or without the addition of 2 mM dithiothreitol at 30 min. Exposure to allyl alcohol increased NADH levels in the first 15 min of incubation. A sharp drop in NADH and NADPH with an accumulation of NADP+ occurred between 30 and 60 min of incubation with allyl alcohol, indicating an oxidation and interconversion of pyridine nucleotides. Dithiothreitol prevented the oxidation of pyridine nucleotides, but not their reduction or interconversion, and protected against cell killing by allyl alcohol. The results suggest that pyridine nucleotide oxidation might be important for allyl alcohol-induced cytotoxicity; however, a causal relationship between pyridine nucleotide oxidation and cell killing is yet to be demonstrated.

Activation of NF kappa B by the respiratory burst of macrophages

H2O2 and other reduced oxygen species have been proposed as activators of the transcription factor, NF Kappa B. Stimulated macrophages produce superoxide and H2O2 (the respiratory burst). We tested the hypothesis that production of these species could serve as part of the NF Kappa B activation pathway in rat alveolar macrophages and the J774A.1 mouse monocyte/macrophage cell line. Phorbol myristate acetate (PMA) and ADP, which stimulate the respiratory burst, caused NF Kappa B activation in both cells. Catalase abolished NF kappa B activation, while superoxide dismutase produced little inhibition. Thus, H2O2 was the principal agent of respiratory burst-associated NF kappa B activation. Abolition of NF kappa B activation by catalase also suggested that intermediate signaling pathways, such as protein kinase C activation or intracellular free calcium elevation must not be involved. Exogenous H2O2 added as a bolus > or = 50 microM (> or = 50 nmol/10(6) macrophages) also activated NF kappa B in macrophages. Nevertheless, the maximum endogenous production of H2O2 by stimulated alveolar macrophages during a 30-min incubation was < or = 1.3 nmol H2O2/10(6) cells for PMA stimulation and < or = 0.2 nmol H2O2/10(6) cells for ADP stimulation. Thus, relatively little endogenous H2O2 generation was required to produce NF kappa B activation compared to the required amount of exogenous H2O2. As H2O2 rapidly diffuses and is consumed, these results suggest that the site of action for endogenously generated H2O2 is probably close to its origin, the plasma membrane.

Evidence for the activation of the signal-responsive phospholipase A2 by exogenous hydrogen peroxide

The intracellular events that lead to arachidonic acid release from bovine endothelial cells in culture treated with hydrogen peroxide were characterized. The hydrogen peroxide-stimulated release of arachidonic acid was time- and dose-dependent, with maximal release achieved at 15 minutes after the addition of 100 microM hydrogen peroxide. Hydrogen peroxide-stimulated release of arachidonic acid was blocked with the phospholipase A2 inhibitor quinacrine. Treatment of the cells with hydrogen peroxide did not result in liberation of oleic acid, indicating that hydrogen peroxide exercised its effect on an arachidonate-specific phospholipase. Pretreatment of the cells with antioxidants, transition metal chelators, and hydroxyl radical scavengers did not affect the hydrogen peroxide-stimulated arachidonic acid release, indicating that the response to hydrogen peroxide is not oxygen radical-mediated. The response to hydrogen peroxide does not appear to be calcium-dependent, due to the following two observations: (a) No increase in intracellular calcium was seen upon exposure of the FURA2-loaded cells to hydrogen peroxide at concentrations sufficient to release arachidonic acid, and (b) no change in the release response was detected in cells loaded with the intracellular calcium chelator BAPTA. Significant inhibition of arachidonic acid release was seen when the cells were pretreated with inhibitors of protein kinase C, but not with inhibitors of tyrosine kinase. The results of these studies indicate that hydrogen peroxide-stimulated arachidonic acid release is mediated by a specific signal-responsive phospholipase A2, and that this process is not mediated via the actions of either lipid peroxidation or calcium but, rather, that a stimulation of intracellular kinase activity is necessary for this response.

Experimental liver cirrhosis induced by alcohol and iron

To determine if alcoholic liver fibrogenesis is exacerbated by dietary iron supplementation, carbonyl iron (0.25% wt/vol) was intragastrically infused with or without ethanol to rats for 16 wk. Carbonyl iron had no effect on blood alcohol concentration, hepatic biochemical measurements, or liver histology in control animals. In both ethanol-fed and control rats, the supplementation produced a two- to threefold increase in the mean hepatic non-heme iron concentration but it remained within or near the range found in normal human subjects. As previously shown, the concentrations of liver malondialdehyde (MDA), liver 4-hydroxynonenal (4HNE), and serum aminotransferases (ALT, AST) were significantly elevated by ethanol infusion alone. The addition of iron supplementation to ethanol resulted in a further twofold increment in mean MDA, 4HNE, ALT, and AST. On histological examination, focal fibrosis was found < 30% of the rats fed ethanol alone. In animals given both ethanol and iron, fibrosis was present in all, with a diffuse central-central bridging pattern in 60%, and two animals (17%) developed micronodular cirrhosis. The iron-potentiated alcoholic liver fibrogenesis was closely associated with intense and diffuse immunostaining for MDA and 4HNE adduct epitopes in the livers. Furthermore, in these animals, accentuated increases in procollagen alpha 1(I) and TGF beta 1 mRNA levels were found in both liver tissues and freshly isolated hepatic stellate cells, perisinusoidal cells believed to be a major source of extracellular matrices in liver fibrosis. The dietary iron supplementation to intragastric ethanol infusion exacerbates hepatocyte damage, promotes liver fibrogenesis, and produces evident cirrhosis in some animals. These results provide evidence for a critical role of iron and iron-catalyzed oxidant stress in progression of alcoholic liver disease.

Modulation of the alveolar macrophage respiratory burst by hydroperoxides

Exposure of alveolar macrophages to hydroperoxides (ROOH) inhibits subsequent stimulation of O2.- production (the respiratory burst). Previous studies (under nonoxidant stress conditions) have shown that elevation of intracellular free calcium ([Ca2+]i) participates in both initiation and termination of O2.- production. In this investigation, the effects of sublethal ROOH exposure on [Ca2+]i and the respiratory burst of rat alveolar macrophages were compared. Exposure to a sublethal range of H2O2 or tert-butylhydroperoxide (10-100 pmol/10(6) cells; initially 10-100 microM under the experimental conditions) for 15 min resulted in dose-dependent effects on the respiratory burst stimulated by various agents, ADP, ATP, zymosan-activated serum, and phorbol myristate acetate. Low concentrations of the ROOH (10 or 25 pmol/10(6) cells) were found to enhance stimulation, whereas exposure to 75 or 100 pmol/10(6) cells resulted in significant inhibition for all of the stimuli. All concentrations of ROOH caused a rapid elevation in [Ca2+]i. For those concentrations of ROOH that produced enhancement of subsequent stimulation of the respiratory burst, [Ca2+]i returned to near baseline before the end of the 15-min preincubation. The temporal- and concentration-dependent effects of ROOH on [Ca2+]i correlate with subsequent enhancement or inhibition of stimulated O2.- production. Similarities between the ROOH-induced changes in [Ca2+]i and the effect of [Ca2+]i changes in physiological regulation of the respiratory burst suggest a potential relationship.

Cellular reducing equivalents and oxidative stress

Energy has been proposed to play a role in the ability of cells and tissues to defend against oxidative stress, even though the ultimate antioxidant capacity of a tissue is determined by the supply of reducing equivalents. The pathways involved in supplying reducing equivalents in response to an oxidative stress remain unclear, particularly if competing reactions such as ATP synthesis are active. Glutathione (GSH), a major component of cellular antioxidant systems, is maintained in the reduced form by glutathione reductase. Although this enzyme is specific for NADPH, the ability of intact cells, isolated mitochondria (which are a major source of free radicals and contain antioxidant systems independent of the rest of the cell), and whole tissues to supply reducing equivalents and maintain normal levels of GSH appears to involve NADH. This article reviews available data regarding the source and pathways by which reducing equivalents are made available to reduce exogenous oxidants, and suggests energy is not a factor. An improved understanding of the mechanism by which reducing equivalents are supplied by tissues to respond to an oxidative stress may direct future research toward designing strategies for augmenting the ability of tissues to defend themselves against oxidative stress induced by reperfusion or xenobiotics.