Increased cytotoxicity of 3-morpholinosydnonimine to HepG2 cells in the presence of superoxide dismutase. Role of hydrogen peroxide and iron

Delivery of superoxide dismutase by TAT and abalone peptides for the protection of skin cells against oxidative stress

Trichoderma reesei superoxide dismutase (TrSOD) is a well-characterized enzyme being stable between 30 and 90°C for 1 h with activity at pH between 2.6 and 9.0. This work aimed to clone, express, purify, and evaluate the protective effect antioxidant of this enzyme on skin cells when fused to transactivator of transcription (TAT) protein transduction domain of HIV-1 and abalone (Ab) peptides to allow cell penetration. TrSOD, TAT-TrSOD-Yfp (fused to yellow fluorescent protein), and Ab-TrSOD were expressed in E. coli and purified as soluble proteins. The cytotoxicity of the enzymes, at the concentrations of 1, 3, and 6 μmol/L, was evaluated for a period of 24 and 48 h of incubation, with no cytotoxic effect on 3T3 fibroblasts. The 3T3 cells were exposed to the oxidant agent tert-butyl hydroperoxide and evaluated for reactive oxygen species (ROS) generation, in the presence or not of the recombinant enzymes. TAT-TrSOD-Yfp was able to decrease the generation of ROS by 15% when used in the concentrations of 3 and 6 μmol/L in comparison to the control, but there was no difference in relation to the effect of TrSOD. Ab-TrSOD, when compared to TrSOD, promoted a decrease in the formation of ROS of 19% and 14% at the concentrations of 1 and 6 μmol/L, respectively, indicating that this recombinant form was more effective in reducing oxidative stress compared to SOD without the cell-penetrating peptide (CPP). Together, these results indicate that the fusion of SOD with these CPP increased the antioxidant capacity of fibroblasts, identified by the reduction in the generation of ROS. In addition, such molecules, in the concentrations initially used, were not toxic to the cells, opening perspectives for the development of products for antioxidant protection of the skin that may have therapeutic and cosmetic application.

Optimizing an Antioxidant TEMPO Copolymer for Reactive Oxygen Species Scavenging and Anti-Inflammatory Effects in Vivo

Oxidative stress is broadly implicated in chronic, inflammatory diseases because it causes protein and lipid damage, cell death, and stimulation of inflammatory signaling. Supplementation of innate antioxidant mechanisms with drugs such as the superoxide dismutase (SOD) mimetic compound 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) is a promising strategy for reducing oxidative stress-driven pathologies. TEMPO is inexpensive to produce and has strong antioxidant activity, but it is limited as a drug due to rapid clearance from the body. It is also challenging to encapsulate into micellar nanoparticles or polymer microparticles, because it is a small, water soluble molecule that does not efficiently load into hydrophobic carrier systems. In this work, we pursued a polymeric form of TEMPO [poly(TEMPO)] to increase its molecular weight with the goal of improving in vivo bioavailability. High density of TEMPO on the poly(TEMPO) backbone limited water solubility and bioactivity of the product, a challenge that was overcome by tuning the density of TEMPO in the polymer by copolymerization with the hydrophilic monomer dimethylacrylamide (DMA). Using this strategy, we formed a series of poly(DMA-co-TEMPO) random copolymers. An optimal composition of 40 mol % TEMPO/60 mol % DMA was identified for water solubility and O2•- scavenging in vitro. In an air pouch model of acute local inflammation, the optimized copolymer outperformed both the free drug and a 100% poly(TEMPO) formulation in O2•- scavenging, retention, and reduction of TNFα levels. Additionally, the optimized copolymer reduced ROS levels after systemic injection in a footpad model of inflammation. These results demonstrate the benefit of polymerizing TEMPO for in vivo efficacy and could lead to a useful antioxidant polymer formulation for next-generation anti-inflammatory treatments.

Direct measurement of actual levels of nitric oxide (NO) in cell culture conditions using soluble NO donors

Applying soluble nitric oxide (NO) donors is the most widely used method to expose cells of interest to exogenous NO. Because of the complex equilibria that exist between components in culture media, the donor compound and NO itself, it is very challenging to predict the dose and duration of NO cells actually experience. To determine the actual level of NO experienced by cells exposed to soluble NO donors, we developed the CellNO Trap, a device that allows continuous, real-time monitoring of the level of NO adherent cells produce and/or experience in culture without the need to alter cell culturing procedures. Herein, we directly measured the level of NO that cells grown in the CellNO Trap experienced when soluble NO donors were added to solutions in culture wells and we characterized environmental conditions that effected the level of NO in in vitro culture conditions. Specifically, the dose and duration of NO generated by the soluble donors S-nitroso-N-acetylpenicillamine (SNAP), S-nitrosoglutathione (GSNO), S-nitrosocysteine (CysNO) and the diazeniumdiolate diethyltriamine (DETA/NO) were investigated in both phosphate buffered saline (PBS) and cell culture media. Other factors that were studied that potentially affect the ultimate NO level achieved with these donors included pH, presence of transition metals (ion species), redox level, presence of free thiol and relative volume of media. Then murine smooth muscle cell (MOVAS) with different NO donors but with the same effective concentration of available NO were examined and it was demonstrated that the cell proliferation ratio observed does not correlate with the half-lives of NO donors characterized in PBS, but does correlate well with the real-time NO profiles measured under the actual culture conditions. This data demonstrates the dynamic characteristic of the NO and NO donor in different biological systems and clearly illustrates the importance of tracking individual NO profiles under the actual biological conditions.

The diversity of microbial responses to nitric oxide and agents of nitrosative stress close cousins but not identical twins

Nitric oxide and related nitrogen species (reactive nitrogen species) now occupy a central position in contemporary medicine, physiology, biochemistry, and microbiology. In particular, NO plays important antimicrobial defenses in innate immunity but microbes have evolved intricate NO-sensing and defense mechanisms that are the subjects of a vast literature. Unfortunately, the burgeoning NO literature has not always been accompanied by an understanding of the intricacies and complexities of this radical and other reactive nitrogen species so that there exists confusion and vagueness about which one or more species exert the reported biological effects. The biological chemistry of NO and derived/related molecules is complex, due to multiple species that can be generated from NO in biological milieu and numerous possible reaction targets. Moreover, the fate and disposition of NO is always a function of its biological environment, which can vary significantly even within a single cell. In this review, we consider newer aspects of the literature but, most importantly, consider the underlying chemistry and draw attention to the distinctiveness of NO and its chemical cousins, nitrosonium (NO(+)), nitroxyl (NO(-), HNO), peroxynitrite (ONOO(-)), nitrite (NO(2)(-)), and nitrogen dioxide (NO(2)). All these species are reported to be generated in biological systems from initial formation of NO (from nitrite, NO synthases, or other sources) or its provision in biological experiments (typically from NO gas, S-nitrosothiols, or NO donor compounds). The major targets of NO and nitrosative damage (metal centers, thiols, and others) are reviewed and emphasis is given to newer "-omic" methods of unraveling the complex repercussions of NO and nitrogen oxide assaults. Microbial defense mechanisms, many of which are critical for pathogenicity, include the activities of hemoglobins that enzymically detoxify NO (to nitrate) and NO reductases and repair mechanisms (e.g., those that reverse S-nitrosothiol formation). Microbial resistance to these stresses is generally inducible and many diverse transcriptional regulators are involved-some that are secondary sensors (such as Fnr) and those that are "dedicated" (such as NorR, NsrR, NssR) in that their physiological function appears to be detecting primarily NO and then regulating expression of genes that encode enzymes with NO as a substrate. Although generally harmful, evidence is accumulating that NO may have beneficial effects, as in the case of the squid-Vibrio light-organ symbiosis, where NO serves as a signal, antioxidant, and specificity determinant. Progress in this area will require a thorough understanding not only of the biology but also of the underlying chemical principles.

Adaptive basal phosphorylation of eIF2α is responsible for resistance to cellular stress-induced cell death in Pten-null hepatocytes

The α-subunit of eukaryotic initiation factor 2 (eIF2α) is a key translation regulator that plays an important role in cellular stress responses. In the present study, we investigated how eIF2α phosphorylation can be regulated by a tumor suppressor PTEN (phosphatase and tensin homolog deleted on chromosome 10) and how such regulation is used by PTEN-deficient hepatocytes to adapt and cope with oxidative stress. We found that eIF2α was hyperphosphorylated when Pten was deleted, and this process was AKT dependent. Consistent with this finding, we found that the Pten-null cells developed resistance to oxidative glutamate and H(2)O(2)-induced cellular toxicity. We showed that the messenger level of CReP (constitutive repressor of eIF2α phosphorylation), a constitutive phosphatase of eIF2α, was downregulated in Pten-null hepatocytes, providing a possible mechanism through which PTEN/AKT pathway regulates eIF2α phosphorylation. Ectopic expression of CReP restored the sensitivity of the Pten mutant hepatocytes to oxidative stress, confirming the functional significance of the downregulated CReP and upregulated phospho-eIF2α in the resistance of Pten mutant hepatocytes to cellular stress. In summary, our study suggested a novel role of PTEN in regulating stress response through modulating the CReP/eIF2α pathway.

Peroxynitrite stress is exacerbated by flavohaemoglobin-derived oxidative stress in Salmonella Typhimurium and is relieved by nitric oxide

Oxidative and nitrosative stresses including nitric oxide (NO), superoxide () and peroxynitrite play key roles in determining the outcome of bacterial infections. In order to survive within the host and allow proliferation within immune cells such as macrophages, Salmonella isolates have a number of inducible proteins that are able to detoxify these highly reactive species, notably the anoxically functioning NO reductase NorVW, and the aerobically functioning flavohaemoglobin, Hmp, which catalyses the reaction between oxygen and NO to produce relatively inert nitrate. However, in the absence of NO but in the presence of reducing substrates and oxygen, is generated from Hmp-mediated electron transfer to bound oxygen and may form a variety of further oxidative species. Hence, Hmp expression is under tight negative regulation by the transcription factor NsrR, abolition of which causes an increase in the production of Hmp. In a previous study, this increase in Hmp levels conferred resistance to the nitrosating agent S-nitrosoglutathione but, perhaps surprisingly, the organism became more sensitive to killing by macrophages. Here, we report that an nsrR mutant that constitutively overexpresses Hmp is also hypersensitive to peroxynitrite in vitro. This sensitivity is alleviated by deletion of the hmp gene or pre-incubation of growing bacteria with NO-releasing agents. We hypothesize that Hmp-expressing cells, in the absence of NO, generate reactive oxygen species, the toxicity of which is exacerbated by peroxynitrite in vitro and in macrophages. RT-PCR confirmed that peroxynitrite causes oxidative stress and upregulation of katG and ahpC, whilst hmp and norV expression are affected very little. The katG gene upregulated by peroxynitrite encodes a catalase peroxidase enzyme with well-established roles in detoxifying peroxides. Here, we report that KatG is also able to enhance the breakdown of peroxynitrite, suggesting that the protective role of this enzyme may be wider than previously thought. These data suggest that spatial and temporal fluctuations in the levels of NO and reactive oxygen species will have important consequences for bacterial survival in the macrophage.

The Base Excision Repair system of Salmonella enterica serovar typhimurium counteracts DNA damage by host nitric oxide

Intracellular pathogens must withstand nitric oxide (NO.) generated by host phagocytes. Salmonella enterica serovar Typhimurium interferes with intracellular trafficking of inducible nitric oxide synthase (iNOS) and possesses multiple systems to detoxify NO.. Consequently, the level of NO. stress encountered by S. Typhimurium during infection in vivo has been unknown. The Base Excision Repair (BER) system recognizes and repairs damaged DNA bases including cytosine and guanine residues modified by reactive nitrogen species. Apurinic/apyrimidinic (AP) sites generated by BER glycosylases require subsequent processing by AP endonucleases. S. Typhimurium xth nfo mutants lacking AP endonuclease activity exhibit increased NO. sensitivity resulting from chromosomal fragmentation at unprocessed AP sites. BER mutant strains were thus used to probe the nature and extent of nitrosative damage sustained by intracellular bacteria during infection. Here we show that an xth nfo S. Typhimurium mutant is attenuated for virulence in C3H/HeN mice, and virulence can be completely restored by the iNOS inhibitor L-NIL. Inactivation of the ung or fpg glycosylase genes partially restores virulence to xth nfo mutant S. Typhimurium, demonstrating that NO. fluxes in vivo are sufficient to modify cytosine and guanine bases, respectively. Mutants lacking ung or fpg exhibit NO.-dependent hypermutability during infection, underscoring the importance of BER in protecting Salmonella from the genotoxic effects of host NO.. These observations demonstrate that host-derived NO. damages Salmonella DNA in vivo, and the BER system is required to maintain bacterial genomic integrity.

SIN-1 cytotoxicity to PC12 cells is mediated by thiol-sensitive short-lived substances generated through SIN-1 decomposition in culture medium

As a generator of peroxynitrite (ONOO(-)), 3-morpholinosydnonimine (SIN-1) is widely used in the study of oxidative/nitrosative stress in cultured cells, although controversy exists regarding active species responsible for cytotoxicity. In this study, we report that unstable thiol-sensitive substances, generated from the reaction of SIN-1 with components in culture medium, play a crucial role in SIN-1 cytotoxicity in PC12 cells. Exposure of cells to culture medium obtained after almost complete SIN-1 decomposition at 37 degrees C for 2h demonstrated almost the same degree of cytotoxicity as did fresh SIN-1. The cytotoxicity of SIN-1-decomposed medium largely depended on serum, decayed with time, and could be completely abolished by the addition of thiols. Degradation of synthetic ONOO(-) in the culture medium did not reproduce the unstable cytotoxicity. The presence of superoxide dismutase (SOD) during SIN-1 decomposition prevented the formation of the cytotoxic substances, whereas SOD had no protection against the cytotoxicity itself, suggesting a crucial role of simultaneously generated superoxide and nitric oxide in the formation of the toxicants, but not in their cytotoxic action. The cytotoxicity of fresh SIN-1 is dramatically suppressed in a basal medium (Hanks balanced salt), suggesting that the cytotoxicity of fresh SIN-1 also requires components of culture medium. These results suggest that SIN-1 cytotoxicity in PC12 cells is mediated via the generation of cytotoxic substances in the medium during its decomposition.

Inhibitory and enhancing effects of NO on H(2)O(2) toxicity: dependence on the concentrations of NO and H(2)O(2)

Nitric oxide (NO) has been shown to both enhance hydrogen peroxide (H(2)O(2)) toxicity and protect cells against H(2)O(2) toxicity. In order to resolve this apparent contradiction, we here studied the effects of NO on H(2)O(2) toxicity in cultured liver endothelial cells over a wide range of NO and H(2)O(2) concentrations. NO was generated by spermine NONOate (SpNO, 0.001-1 mM), H(2)O(2) was generated continuously by glucose/glucose oxidase (GOD, 20-300 U/l), or added as a bolus (200 microM). SpNO concentrations between 0.01 and 0.1 mM provided protection against H(2)O(2)-induced cell death. SpNO concentrations >0.1 mM were injurious with low H(2)O(2) concentrations, but protective at high H(2)O(2) concentrations. Protection appeared to be mainly due to inhibition of lipid peroxidation, for which SpNO concentrations as low as 0.01 mM were sufficient. SpNO in high concentration (1 mM) consistently raised H(2)O(2) steady-state levels in line with inhibition of H(2)O(2) degradation. Thus, the overall effect of NO on H(2)O(2) toxicity can be switched within the same cellular model, with protection being predominant at low NO and high H(2)O(2) levels and enhancement being predominant with high NO and low H(2)O(2) levels.

Nitric oxide increases toxicity of hydrogen peroxide against rat liver endothelial cells and hepatocytes by inhibition of hydrogen peroxide degradation

Nitric oxide (NO) and hydrogen peroxide (H(2)O(2)) show cooperativity in their cytotoxic action. The present study was performed to decipher the mechanisms underlying this phenomenon. In cultured liver endothelial cells and in cultured, glutathione-depleted hepatocytes, the combined exposure to NO (released by spermine NONOate, 1 mM) and H(2)O(2) (released by glucose oxidase) induced cell injury that was far higher than the injury elicited by NO or H(2)O(2) alone. In both cell types, the addition of the NO donor increased H(2)O(2) steady-state levels, although with different kinetics: in hepatocytes, the increase in H(2)O(2) levels was already evident at early time points while in liver endothelial cells it became evident after > or =2 h of incubation. NO exposure inhibited H(2)O(2) degradation, assessed after addition of 50 microM, 200 microM, or 4 mM authentic H(2)O(2), significantly in both cell types. However, again, early and delayed inhibition was observed. The late inhibition of H(2)O(2) degradation in endothelial cells was paralleled by a decrease in glutathione peroxidase activity. Glutathione peroxidase inactivation was prevented by hypoxia or by ascorbate, suggesting inactivation by reactive nitrogen oxide species (NO(x)). Early inhibition of H(2)O(2) degradation by NO, in contrast, could be mimicked by the catalase inhibitor azide. Together, these results suggest that the cooperative effect of NO and H(2)O(2) is due to inhibition of H(2)O(2) degradation by NO, namely to inhibition of catalase by NO itself (predominant in hepatocytes) and/or to inhibition of glutathione peroxidase by NO(x) (prevailing in endothelial cells).

Cell type-dependent release of nitric oxide and/or reactive nitrogenoxide species from intracellular SIN-1: effects on cellular NAD(P)H

SIN-1 is frequently used in cell culture studies as an extracellularly operating generator of peroxynitrite. However, little is known about the nature of the reactive species produced intracellulary from SIN-1. SIN-1 can easily penetrate cells as exemplified for both L-929 mouse fibroblasts and bovine aortic endothelial cells (BAECs) by utilizing capillary zone electrophoresis. In L-929 cells, SIN-1 produced nitric oxide (*NO) as monitored by the fluorescent *NO scavenger FNOCT-1 and by means of a *NO electrode, as well as reactive nitrogenoxide species (RNOS, e.g. peroxynitrite, nitrogen dioxide, dinitrogen trioxide), as detected with the fluorescent indicator DAF-2. Laser scanning microscopy revealed that in L-929 cells SIN-1 -derived species initially oxidized the major fraction of the NAD(P)H within the cytosol and the nuclei, whereas the mitochondrial NAD(P)H level was somewhat increased. In marked contrast to this, in BAECs no evidence for *NO formation was found although the intracellular amount of SIN-1 was four-fold higher than in L-929 cells. In BAECs, the level of NAD(P)H was slightly decreased within the first 10 min after administration of SIN-1 in both the cytosol/nuclei and mitochondria. These observations reflect the capability of SIN-1 to generate intracellularly either almost exclusively RNOS as in BAECs, or RNOS and freely diffusing *NO as in L-929 cells. Nitric oxide as well as RNOS may decisively affect cellular metabolism as indicated by the alterations in the NAD(P)H level. Hence, care should be taken when applying SIN-1 as an exclusively peroxynitrite-generating compound in cell culture systems.

Novel SIN-1 reactive intermediates modulate chloride secretion across murine airway cells

We examined the effects of reactive oxygen-nitrogen intermediates on chloride (Cl-) currents across murine tracheal epithelial (MTE) cells isolated from CD-1 mice. MTE cells were cultured on permeable supports until they formed water-tight monolayers with transepithelial resistances (Rt)>500 Omega/cm2 and then were mounted in Ussing chambers. Baseline short-circuit current (ISC) values, prior to and following the addition of 10 microM amiloride (an inhibitor of sodium-transport pathways) into the apical side, were 65 +/- 1.9 microA/cm2 and 7.6 +/- 0.51 microA/cm2, respectively (X +/- 1 SE, n=32). The addition of 3-morpholinosydnominine (SIN-1, 1 mM), which generates both superoxide and nitric oxide anions, to amiloride-treated monolayers resulted in a transient increase of ISC to a peak value of 35 +/- 1.3 microA/cm2 (X +/- SE, n=14) within the next 30-60 min. After this, the ISC decreased gradually and returned to its pre-SIN-1 value. These changes were blocked by glibenclamide (200 microM), an inhibitor of cystic fibrosis transmembrane regulator, or reduced by glutathione (GSH, 5 mM), a scavenger of peroxynitrite. Forskolin (10 microM) augmented the SIN-1 effect when added at the peak of the SIN-1 response but not when ISC had returned to its baseline value. Perfusion of MTE cells with SIN-1 also increased whole cell Cl- currents 4-fold and the open probability of CFTR-type single-channel currents from 0.041 to 0.92 in a transient fashion. Decomposed SIN-1, but not pure SIN-1c (the stable decomposition product of SIN-1), also increased ISC with an EC50 of 5 microM. Electrospray mass spectroscopy revealed several unique and uncharacterized compounds formed during the decomposition of SIN-1 as well as the reaction of SIN-1c with peroxynitrite. Formation of these compounds was inhibited by GSH. We conclude that compounds formed by the reaction of peroxynitrite with by-products of SIN-1, rather than reactive oxygen-nitrogen species per se, were responsible for the modulation of Cl- secretion across primary cultures of MTE cells.

Reactive oxygen species depolymerize hyaluronan: involvement of the hydroxyl radical

We have previously demonstrated that reactive oxygen species (ROS) are involved in cartilage degradation. A decrease in the size of hyaluronan (HA), which is the major macromolecule in synovial fluid and is responsible for imparting viscosity to it, is reported in arthritis patients. The purpose of this study is to determine the ROS that depolymerize HA. The luminol derivative, L-012, was used to determine the generation of ROS. To generate hydroxyl radicals, a mixture of hydrogen peroxide (H(2)O(2)) and ferrous ions (Fe(2+)) was added to HA. The antioxidants and the depolymerization of HA were studied in this system. The hydroxyl radical is one of the ROS, causing the depolymerization of HA, which reacts with L-01. These data suggest that hydroxyl radicals play an important role at the site of inflammation.

Reactive nitrogen species block cell cycle re-entry through sustained production of hydrogen peroxide

Endogenous sources of reactive nitrogen species (RNS) act as second messengers in a variety of cell signaling events, whereas environmental sources of RNS like nitrogen dioxide (NO2) inhibit cell survival and growth through covalent modification of cellular macromolecules. To examine the effects of RNS on cell cycle progression, murine type II alveolar C10 cells arrested in G0 by serum deprivation were exposed to either NO2 or SIN-1, a generator of RNS, during cell cycle re-entry. In serum-stimulated cells, RNS did not prevent the immediate early gene response by AP-1, but rather blocked cyclin D1 gene expression, resulting cell cycle arrest at the boundary between G0 and G1. Dichlorofluorescin diacetate (DCF) fluorescence indicated that RNS induced sustained production of intracellular hydrogen peroxide (H2O2), which normally is produced only transiently in response to serum growth factors. Loading cells with catalase did not diminish the formation of 3-nitrotyrosine on the cell surface, but rather prevented enhanced DCF fluorescence and rescued cyclin D1 expression and S phase entry. These studies indicate environmental RNS interfere with cell cycle re-entry through an H2O2-dependent mechanism that influences expression of cyclin D1 and progression from G0 to the G1 phase of the cell cycle.

Expression of prion protein increases cellular copper binding and antioxidant enzyme activities but not copper delivery

The N-terminal region of the prion protein PrP(C) contains a series of octapeptide repeats. This region has been implicated in the binding of divalent metal ions, particularly copper. PrP(C) has been suggested to be involved in copper transport and metabolism and in cell defense mechanisms against oxidative insult, possibly through the regulation of the intracellular CuZn superoxide dismutase activity (CuZn-SOD) or a SOD-like activity of PrP(C) itself. However, up to now the link between PrP(C) expression and copper metabolism or SOD activity has still to be formally established; particularly because conflicting results have been obtained in vivo. In this study, we report a link between PrP(C), copper binding, and resistance to oxidative stress. Radioactive copper ((64)Cu) was used at a physiological concentration to demonstrate that binding of copper to the outer plasma cell membrane is related to the level of PrP(C) expression in a cell line expressing a doxycycline-inducible murine PrP(C) gene. Cellular PIPLC pretreatment indicated that PrP(C) was not involved in copper delivery at physiological concentrations. We also demonstrated that murine PrP(C) expression increases several antioxidant enzyme activities and glutathione levels. Prion protein may be a stress sensor sensitive to copper and able to initiate, following copper binding, a signal transduction process acting on the antioxidant systems to improve cell defenses.

Dynamic and biphasic modulation of nitrosation reaction by superoxide dismutases

It has been shown that superoxide dismutase (SOD) can both potentiate and attenuate NO-mediated toxicity. This present study investigated the role of SOD and GSH in a sustained nitrosative and oxidative environment simulated by the nitric oxide (NO) and superoxide (O(2)(.-)) donor, 3-morpholinosydnonimine (SIN-1). We describe, for the first time, that SOD modulates nitrosative chemistry in a dynamic fashion that is both concentration and time-dependent. Specifically, our results show that SOD's effects on nitrosation are biphasic in nature i.e., while lower concentrations of SOD are pronitrosative, higher SOD concentrations inhibit nitrosation. However, even those initially inhibitory higher SOD concentrations became pronitrosative over time. In the presence of physiologically relevant levels of GSH, SOD predominantly exhibits a pronitrosative effect, with a complete loss of antinitrosative effects noted at higher levels of GSH. Our findings likely reflect the complex and dynamic nature of SOD interactions with oxidative and nitrosative species.

Thiourea protects against copper-induced oxidative damage by formation of a redox-inactive thiourea-copper complex

Although thiourea has been used widely to study the role of hydroxyl radicals in metal-mediated biological damage, it is not a specific hydroxyl radical scavenger and may also exert antioxidant effects unrelated to hydroxyl radical scavenging. Thus, we investigated the effects of thiourea on copper-induced oxidative damage to bovine serum albumin (1 mg/ml) in three different copper-containing systems: Cu(II)/ascorbate, Cu(II)/H(2)O(2), and Cu(II)/H(2)O(2)/ascorbate [Cu(II), 0.1 mM; ascorbate, 1 mM; H(2)O(2), 1 mM]. Oxidative damage to albumin was measured as protein carbonyl formation. Thiourea (0.1-10 mM) provided marked and dose-dependent protection against protein oxidation in all three copper-containing systems. In contrast, only minor protection was observed with dimethyl sulfoxide and mannitol, even at concentrations as high as 100 mM. Strong protection was also observed with dimethylthiourea, but not with urea or dimethylurea. Thiourea also significantly inhibited copper-catalyzed oxidation of ascorbate, and competed effectively with histidine and 1,10-phenanthroline for binding of cuprous, but not cupric, copper, as demonstrated by both UV-visible and low temperature electron spin resonance measurements. We conclude that the protection by thiourea against copper-mediated protein oxidation is not through scavenging of hydroxyl radicals, but rather through the chelation of cuprous copper and the formation of a redox-inactive thiourea-copper complex.

Genotoxicity, mitochondrial damage, and apoptosis in human lymphoblastoid cells exposed to peroxynitrite generated from SIN-1

SIN-1 (3-morpholinosydnonimine), the active metabolite of the vasodilator drug molsidomine, decomposes spontaneously in solution. In the presence of oxygen, NO* and O(2)(*-) are released, generating peroxynitrite, a potent oxidizing agent, at a constant rate over a 2 h period. We utilized this system to investigate mechanisms of peroxynitrite-induced cytotoxicity, genotoxicity, apoptosis, and mitochondrial damage in two human lymphoblastoid cell lines carrying either wild-type (TK6 cells) or mutant p53 (WTK-1 cells) genes. Treatment of TK6 cells with 5 mM SIN-1 for 1.5 h resulted in 28 +/- 6% survival 24 h later. Exposure in the presence of different radical scavengers significantly increased survival, as follows: cytochrome c, 96 +/- 3%; Tiron, 69 +/- 0%; SOD plus catalase, 83 +/- 5%; carboxy-PTIO, 87 +/- 3%; and uric acid, 87 +/- 2%. D-mannitol was ineffective in reducing lethality, as were SOD and catalase when added individually or in heat-inactivated form. Spontaneous as well as SIN-1-induced mutant fractions (MF) in both HPRT and TK genes were significantly higher in WTK-1 cells than in TK6 cells (p < 0.05-0.01). Exposure to 2.5 mM SIN-1 induced time-dependent apoptosis in TK6 cells, but not in WTK-1 cells. Mitochondrial membrane depolarization was also observed in both cell lines after SIN-1 treatment. Neutral comet assay demonstrated that SIN-1 treatment resulted in higher levels of DNA double-strand breaks in TK6 cells than in WTK-1 cells. Collectively, these data show that SIN-1 can be used as an effective peroxynitrite generator in cell culture experiments under these experimental conditions, in which it induced a greater apoptotic response but was less potent as a mutagen in TK6 cells compared with WTK-1 cells. Thus, p53 status was an important determinant of SIN-1 induced mutagenesis and apoptosis in these two human lymphoblastoid cell lines.

Nitric oxide-mediated chondrocyte cell death requires the generation of additional reactive oxygen species

OBJECTIVE

Chondrocyte cell death, possibly related to increased production of endogenous nitric oxide (NO), has been observed during the pathogenesis of osteoarthritis and rheumatoid arthritis. The purpose of this study was to investigate the potential role of NO in causing chondrocyte cell death and to determine the contribution of other reactive oxygen species (ROS).

METHODS

Cell death and cytotoxicity were evaluated in human articular chondrocytes in response to various NO donor compounds with and without agents that stimulate or inhibit the production of additional ROS using both the alginate bead and the monolayer culture systems. Cell death was quantified by a total cell count with fluorescent labels, and cytotoxicity was measured as a function of cellular NADH- and NADPH-dependent dehydrogenase activity. To determine if the redox status of the chondrocyte could influence the observed effect of NO, cells were preincubated for 24 hours in L-cystine- and glutathione (GSH)-depleted media to reduce intracellular GSH levels, a major defense mechanism against oxidative stress. Apoptosis was analyzed with the quantification of histone-associated DNA fragments.

RESULTS

Treatment of chondrocytes with peroxynitrite (ONOO-), 3-morpholinosydnonimine (SIN-1), and sodium nitroprusside (SNP) resulted in apoptotic cell death at concentrations of 0.5 mM, 1.0 mM, and 0.5 mM, respectively. In contrast, treatment of chondrocytes with diazeniumdiolates (or the "NOC" compounds, NOC-5 and NOC-12) at concentrations as high as 2.0 mM did not cause cell death. Furthermore, NOC-5 and NOC-12, at all concentrations tested (0.125-2.0 mM), could prevent cell death caused by oxidative stress. Selective ROS scavengers protected against cell death caused by either SIN-1 or ONOO-; however, no protection could be afforded against the cytotoxicity of SNP with any of the ROS scavengers tested.

CONCLUSION

These results show that NO by itself is not cytotoxic to cultured chondrocytes and can even be protective under certain conditions of oxidative stress. Chondrocyte cell death from NO occurs under conditions where other ROS are also generated.

SIN-1-induced cytotoxicity in mixed cortical cell culture: peroxynitrite-dependent and -independent induction of excitotoxic cell death

3-Morpholinosyndnomine (SIN-1) has been reported to be a peroxynitrite (OONO(-)) donor because it produces both nitric oxide (NO) and superoxide (O(2)(-).) upon decomposition in aqueous solution. However, SIN-1 can decompose to primarily NO in the presence of electron acceptors, including those found in biological tissues, making it necessary to determine the release product(s) formed in any given biological system. In a mixed cortical cell culture system, SIN-1 caused a concentration-dependent increase in cortical cell injury with a parallel increase in the release of cellular proteins containing 3-nitrotyrosine into the culture medium. The increase in 3-nitrotyrosine immunoreactivity, a footprint of OONO(-) production, was specific for SIN-1 as exposure to neurotoxic concentrations of an NO donor (Z)-1-[2-aminoethyl)-N-(2-ammonioethyl) aminodiazen-1-ium-1,2-diolate (DETA/NO), or NMDA did not result in the nitration of protein tyrosine residues. Both SIN-1-induced injury and 3-nitrotyrosine staining were prevented by the addition of either 5,10,15,20-Tetrakis (4-sulfonatophenyl) prophyrinato iron (III) [FeTPPS], an OONO(-) decomposition catalyst, or uric acid, an OONO(-) scavenger. Removal of NO alone was sufficient to inhibit the formation of OONO(-) from SIN-1 as well as its cytotoxicity. Removal of O(2)(-). and the subsequently formed H(2)O(2) by superoxide dismutase (SOD) plus catalase likewise prevented the nitration of protein-bound tyrosine but actually enhanced the cytotoxicity of SIN-1, indicating that cortical cells can cope with the oxidative but not the nitrosative stress generated. Finally, neural injury induced by SIN-1 in unadulterated cortical cells was prevented by antagonism of AMPA/kainate receptors, while blockade of the NMDA receptor was without effect. In contrast, activation of both NMDA and non-NMDA receptors contributed to the SIN-1-mediated neurotoxicity when cultures were exposed in the presence of SOD plus catalase. Thus, whether SIN-1 initiates neural cell death in an OONO(-)-dependent or -independent manner is determined by the antioxidant status of the cells. Further, the mode of excitotoxicity by which injury progresses is determined by the NO-related species generated.

Anti-adrenergic effects of nitric oxide donor SIN-1 in rat cardiac myocytes

We studied how the nitric oxide (NO*) donor 3-morpholinosydnonimine (SIN-1) alters the response to beta-adrenergic stimulation in cardiac rat myocytes. We found that SIN-1 decreases the positive inotropic effect of isoproterenol (Iso) and decreases the extent of both cell shortening and Ca2+ transient. These effects of SIN-1 were associated with an increased intracellular concentration of cGMP, a decreased intracellular concentration of cAMP, and a reduction in the levels of phosphorylation of phospholamban (PLB) and troponin I (TnI). The guanylyl cyclase inhibitor 1H-8-bromo-1,2,4-oxadiazolo (3,4-d)benz(b)(1,4)oxazin-1-one (ODQ) was not able to prevent the SIN-1-induced reduction of phosphorylation levels of PLB and TnI. However, the effects of SIN-1 were abolished in the presence of superoxide dismutase (SOD) or SOD and catalase. These data suggest that, in the presence of Iso, NO-related congeners, rather than NO*, are responsible for SIN-1 effects. Our results provide new insights into the mechanism by which SIN-1 alters the positive inotropic effects of beta-adrenergic stimulation.

Potential mechanisms for the inhibition of tumor cell growth by manganese superoxide dismutase

Studies from many laboratories have shown that overexpression of manganese superoxide dismutase (MnSOD) inhibits the growth of numerous tumor cell types. The inhibition of tumor cell growth can be attributed to the increase in the steady-state levels of H2O2 as a result of the increased dismuting activity of MnSOD. Here we demonstrate that overexpression of MnSOD enhances the activity of the superoxide (O2*-)-sensitive enzyme aconitase, decreases the intracellular GSH/GSSG ratio, and dose-dependently inhibits pyruvate carboxylase activity. Thus, alterations in the steady-state concentrations of mitochondrial O2*- and H2O2 as a result of MnSOD overexpression can alter the metabolic capacity of the cell leading to inhibition of cell growth. Furthermore, we propose that MnSOD overexpression can modulate the activity of nitric oxide (*NO) by preventing its reaction with O2*-. This hypothesis suggests that the redox environment of the mitochondria can be altered to favor the activity of *NO rather than peroxynitrite (ONOO-) and may explain the enhanced toxicity of *NO-generating compounds toward MnSOD-overexpressing cell lines. These findings indicate that therapeutic strategies targeted at overexpressing MnSOD in tumor tissue may be more effective when used in combination with agents that deplete the oxidant-buffering and enhance the *NO-generating capacity of the tumor and host, respectively.

A redox-based mechanism for the contractile and relaxing effects of NO in the guinea-pig gall bladder

The purpose of this study was to determine the effects of sodium nitroprusside (SNP), 2,2'-(hydroxynitrosohydrazino)bis-ethanamine (DETA/NO) and 3-morpholinosydnonimine (SIN-1), NO donors which yield different NO reactive species (NO+, NO* and peroxynitrite, respectively), as well as exogenous peroxynitrite, on gall bladder contractility. Under resting tone conditions, SNP induced a dose-dependent contraction with a maximal effect (10.3 +/- 0.7 mN, S.E.M.) at 1 mM. Consistent with these findings, SNP caused a concentration-dependent depolarization of gall bladder smooth muscle. The excitatory effects of SNP were dependent on extracellular calcium entry through L-type Ca2+ channels. Furthermore, the contraction and depolarization were sensitive to tyrosine kinase blockade, and an associated increase in tyrosine phosphorylation was detected in Western blot studies. DETA/NO induced dose-dependent relaxing effects. These relaxations were sensitive to the guanylyl cyclase inhibitor 1H-[1,2,4]oxidiazolo[4,3-a]quinoxaline-1-one (ODQ, 2 microM) but they were not altered by treatment with the potassium channel blockers tetraethylammoniun (TEA, 5 mM) and 4-aminopyridine (4-AP, 5 mM). When tested in a reducing environment (created by 2.5 mM 1,4-dithiothreitol, DTT), SNP caused a relaxation of gall bladder muscle strips. Similarly, the SNP-induced contraction was converted to a relaxation, and associated hyperpolarization, when DTT was added during the steady state of an SNP-induced response. SIN-1 (0.1 mM), which has been shown to release peroxynitrite, induced relaxing effects that were enhanced by superoxide dismutase (SOD, 50 U ml(-1)). The relaxations induced by either SIN-1 alone or SIN-1 in the presence of SOD were strengthened by catalase (1000 U ml(-1)) and abolished by ODQ pretreatment. However, exogenous peroxynitrite induced a concentration-dependent contraction, which was dependent on activation of leukotriene (LT) metabolism and extracellular calcium. The peroxynitrite-induced contraction was abolished in the presence of the peroxynitrite scavenger melatonin. These results suggest that SIN-1 behaves as an NO* rather than a peroxynitrite source. We conclude that, depending on the redox state, NO has opposing effects on the motility of the gall bladder, being a relaxing agent when in NO * form and a contracting agent when in NO+ or peroxynitrite redox species form. Knowledge of the contrasting effects of the different redox forms of NO can clarify our understanding of the effects of NO donors on gall bladder and other smooth muscle cell types.

Modulation of catalase peroxidatic and catalatic activity by nitric oxide

Previously, we found that catalase enhanced the protection afforded by superoxide dismutase to Escherichia coli against the simultaneous generation of superoxide and nitric oxide (Brunelli et al., Arch. Biochem. Biophys. 316:327-334, 1995). Hydrogen peroxide itself was not toxic in this system in the presence or absence of superoxide dismutase. We therefore investigated whether catalase might consume nitric oxide in addition to hydrogen peroxide. Catalase rapidly formed a reversible complex stoichiometrically with nitric oxide with the Soret band shifting from 406 to 426 nm and two new peaks appeared at 540 and at 575 nm, consistent with the formation of a ferrous-nitrosyl complex. Catalase consumed more nitric oxide upon the addition of hydrogen peroxide. Conversely, micromolar concentrations of nitric oxide slowed the catalase-mediated decomposition of hydrogen peroxide. Catalase pretreated with nitric oxide and hydrogen peroxide regained full activity after dialysis. Our results suggest that catalase can slowly consume nitric oxide while nitric oxide modestly inhibits catalase-dependent scavenging of hydrogen peroxide. The protective effects of catalase in combination with superoxide dismutase may result from two actions; reducing peroxynitrite formation by scavenging nitric oxide and by scavenging hydrogen peroxide before it reacts with superoxide dismutase to form additional superoxide.

SIN-1-induced DNA damage in isolated human peripheral blood lymphocytes as assessed by single cell gel electrophoresis (comet assay)

Human lymphocytes were exposed to increasing concentrations of SIN-1, which generates superoxide and nitric oxide, and the formation of single-strand breaks (SSB) in individual cells was determined by the single-cell gel electrophoresis assay (comet assay). A dose- and time-dependent increase in SSB formation was observed rapidly after the addition of SIN-1 (0.1-15 mM). Exposure of the cells to SIN-1 (5 mM) in the presence of excess of superoxide dismutase (0.375 mM) increased the formation of SSB significantly, whereas 1000 U/ml catalase significantly decreased the quantity of SSB. The simultaneous presence of both superoxide dismutase and catalase before the addition of SIN-1 brought the level of SSB to that of the untreated cells. Moreover, pretreatment of the cells with the intracellular Ca(2+)-chelator BAPTA/AM inhibited SIN-1-induced DNA damage, indicating the involvement of intracellular Ca(2+) changes in this process. On the other hand, pretreatment of the same cells with ascorbate or dehydroascorbate did not offer any significant protection in this system. The data suggest that H2O2-induced changes in Ca(2+) homeostasis are the predominant pathway for the induction of SSB in human lymphocytes exposed to oxidants.

Effects of reactive oxygen species on proliferation of Chinese hamster lung fibroblast (V79) cells

Reactive oxygen species (ROS) have emerged as important signaling molecules in the regulation of various cellular processes. In our study, we investigated the effect of a wide range of ROS on Chinese hamster lung fibroblast (V79) cell proliferation. Treatment with H2O2 (100 microM), superoxide anion (generated by 1 mM xanthine and 1 mU/ml xanthine oxidase), menadione, and phenazine methosulfate increased the cell proliferation by approximately 50%. Moreover, a similar result was observed after partial inhibition of superoxide dismutase (SOD) and glutathione peroxidase. This upregulation of cell proliferation was suppressed by pretreatment with hydroxyl radical scavengers and iron chelating agents. In addition to ROS, treatment with exogenous catalase and SOD mimic (MnTMPyP) suppressed the normal cell proliferation. Short-term exposure of the cells to 100 microM H2O2 was sufficient to induce proliferation, which indicated that activation of the signaling pathway is important as an early event. Accordingly, we assessed the ability of H2O2 to activate mitogen-activated protein kinases (MAPK). Jun-N-terminal kinase (JNK) and p38 MAPK were both rapidly and transiently activated by 100 microM H2O2, with maximal activation 30 min after treatment. However, the activity of extracellular signal-regulated kinase (ERK) was not changed. Pretreatment with SB203580 and SB202190, specific inhibitors of p38 MAPK, reduced the cell proliferation induced by H2O2. The activation of both JNK and p38 MAPK was also suppressed by pretreatment with hydroxyl radical scavenger and iron chelating agents. Our results suggest that the trace metal-driven Fenton reaction is a central mechanism that underlies cell proliferation and MAPK activation.

Bicarbonate enhances the hydroxylation, nitration, and peroxidation reactions catalyzed by copper, zinc superoxide dismutase. Intermediacy of carbonate anion radical

The effect of bicarbonate anion (HCO(3)(-)) on the peroxidase activity of copper, zinc superoxide dismutase (SOD1) was investigated using three structurally different probes: 5, 5'-dimethyl-1-pyrroline N-oxide (DMPO), tyrosine, and 2, 2'-azino-bis-[3-ethylbenzothiazoline]-6-sulfonic acid (ABTS). Results indicate that HCO(3)(-) enhanced SOD/H(2)O(2)-dependent (i) hydroxylation of DMPO to DMPO-OH as measured by electron spin resonance, (ii) oxidation and nitration of tyrosine to dityrosine, nitrotyrosine, and nitrodityrosine as measured by high pressure liquid chromatography, and (iii) oxidation of ABTS to the ABTS cation radical as measured by UV-visible spectroscopy. Using oxygen-17-labeled water, it was determined that the oxygen atom present in the DMPO-OH adduct originated from H(2)O and not from H(2)O(2). This result proves that neither free hydroxyl radical nor enzyme-bound hydroxyl radical was involved in the hydroxylation of DMPO. We postulate that HCO(3)(-) enhances SOD1 peroxidase activity via formation of a putative carbonate radical anion. This new and different perspective on HCO(3)(-)-mediated oxidative reactions of SOD1 may help us understand the free radical mechanism of SOD1 and related mutants linked to amyotrophic lateral sclerosis.

A discussion of the chemistry of oxidative and nitrosative stress in cytotoxicity

Nitric oxide (NO) has been shown to be a key bioregulatory agent in a wide variety of biological processes, yet cytotoxic properties have been reported as well. This dichotomy has raised the question of how this potentially toxic species can be involved in so many fundamental physiological processes. We have investigated the effects of NO on a variety of toxic agents and correlated how its chemistry might pertain to the observed biology. The results generate a scheme termed the chemical biology of NO in which the pertinent reactions can be categorized into direct and indirect effects. The former involves the direct reaction of NO with its biological targets generally at low fluxes of NO. Indirect effects are reactions mediated by reactive nitrogen oxide species, such as those generated from the NO/O2 and NO/O2- reactions, which can lead to cellular damage via nitrosation or oxidation of biological components. This report discusses several examples of cytotoxicity involved with these stresses.

The peroxynitrite generator, SIN-1, becomes a nitric oxide donor in the presence of electron acceptors

SIN-1 has been used, in vitro, to simultaneously generate nitric oxide (*NO) and superoxide (O*-2). However, the pharmacological activity of SIN-1 resembles that of a *NO donor. SIN-1 decays by a three-step mechanism. After initial isomerization to an open ring form, SIN-1A reduces oxygen by a one-electron transfer reaction to give O*-2 and the SIN-1 cation radical, which decomposes to form SIN-1C and *NO. Here we report that one-electron oxidizing agents, in addition to oxygen, can oxidize SIN-1A, resulting in the release of *NO without the concomitant formation of O*-2. We demonstrate that easily reducible nitroxides, such as the nitronyl and imino nitroxides, are able to oxidize SIN-1. Biological oxidizing agents such as ferricytochrome c also stimulate *NO production from SIN-1. In addition, decomposition of SIN-1 by human plasma or by the homogenate of rat liver, kidney, and heart tissues results in the formation of *NO. Our findings suggest that SIN-1 may react with heme proteins and other electron acceptors in biological systems to produce *NO. Thus, at the relatively low in vivo oxygen concentrations, SIN-1 is likely to behave more like an *NO donor than a peroxynitrite donor. The relevance of this reaction to myocardial protection afforded by SIN-1 in ischemia/reperfusion-induced injury is discussed.

Down-regulation of cytochrome P450 1A1 gene promoter by oxidative stress. Critical contribution of nuclear factor 1

Oxidative stress interferes with several cellular functions, in particular transcriptional regulation. We show here that the human cytochrome P450 1A1 (CYP1A1) is down-regulated at the transcriptional level by oxidative stress. Basal as well as 2,3,7, 8-tetrachloro-p-dioxin-induced promoter activities are strongly impaired by H2O2 treatment or glutathione depletion with L-buthionine-(S,R)-sulfoximine. Tumor necrosis factor alpha inhibits CYP1A1 expression, and this inhibition is prevented by the antioxidant pyrrolidine dithiocarbamate. We show that these regulations depend on the integrity of the nuclear factor 1 (NFI) site located in the proximal promoter. We therefore examined the redox regulation of this transcription factor. Treatment of human HepG2 or rat H4 hepatoma cells with H2O2 or L-buthionine-(S, R)-sulfoximine inactivates the binding of the NFI transcription factor to its DNA consensus sequence. Furthermore, H2O2 treatment leads to a dose-dependent decrease of reporter gene expressions driven by promoters containing NFI binding sites. Glutathione depletion and catalase inhibition also repress a NFI-driven promoter. Under the same conditions, the CP-1 transcription factor activity is not affected by oxidative stress. Thus, NFI seems particularly sensitive to oxidative stress. This accounts, at least partially, for the regulation of cyp1A1 gene expression.

Neuronal death induced by SIN-1 in the presence of superoxide dismutase: protection by cyclic GMP

The nitrovasodilator 3-morpholinosydnonimine (SIN-1) slowly decomposes to release both nitric oxide (NO) and superoxide (O2-) and thereby produces peroxynitrite (ONOO-), a powerful oxidant which has been proposed to mediate the toxic actions caused by NO. Indeed, ONOO has been shown to cause neuronal death and it has been proposed to occur in different disorders of the CNS such as brain ischaemia, AIDS-associated dementia, amyothrophic lateral sclerosis, etc. We have found that SIN-1 was only slightly toxic to 1-week-old rat cortical neurones in primary culture (LC50=2.5+/-0.5 mM). Superoxide dismutase (SOD; 100 U/ml) significantly increased SIN-1-induced toxicity, an effect that was enhanced in the presence of HbO2, abolished by catalase and accompanied by the formation of hydrogen peroxide (H2O2). We have also found that 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ), a selective inhibitor of soluble guanylate cyclase, enhances cell death induced by SIN-1 (0.2-0.5 mM) + SOD (100 U/ml) in a concentration-dependent way (EC50=0.073+/-0.004 microM). Simultaneously, ODQ inhibits the elevation of cyclic GMP concentrations induced by SIN-1 + SOD in cortical cells (IC50=0.022+/-0.014 microM). Finally, we have also shown that the cyclic GMP mimetic, 8-bromo-cyclic GMP reverses the potentiating effect induced by ODQ on SIN-1 + SOD-induced neuronal death and inhibits the neurotoxicity induced by H2O2 (100 microM). Taken together, these data suggest that H2O2 is the species responsible for the potentiation by SOD of SIN-1-induced cell death and that cyclic GMP elevations confer selective cytoprotection against this H2O2-mediated component of cell death.

Use of metabolically competent human hepatoma cells for the detection of mutagens and antimutagens

The human hepatoma line (Hep G2) has retained the activities of various phase I and phase II enzymes which play a crucial role in the activation/detoxification of genotoxic procarcinogens and reflect the metabolism of such compounds in vivo better than experimental models with metabolically incompetent cells and exogenous activation mixtures. In the last years, methodologies have been developed which enable the detection of genotoxic effects in Hep G2 cells. Appropriate endpoints are the induction of 6-TGr mutants, of micronuclei and of comets (single cell gel electrophoresis assay). It has been demonstrated that various classes of environmental carcinogens such as nitrosamines, aflatoxins, aromatic and heterocyclic amines and polycyclic aromatic hydrocarbons can be detected in genotoxicity assays with Hep G2 cells. Furthermore, it has been shown that these assays can distinguish between structurally related carcinogens and non-carcinogens, and positive results have been obtained with rodent carcinogens (such as safrole and hexamethylphosphoramide) which give false negative results in conventional in vitro assays with rat liver homogenates. Hep G2 cells have also been used in antimutagenicity studies and can identify mechanisms not detected in conventional in vitro systems such as induction of detoxifying enzymes, inactivation of endogenously formed DNA-reactive metabolites and intracellular inhibition of activating enzymes.

Hydrogen peroxide formation by reaction of peroxynitrite with HEPES and related tertiary amines. Implications for a general mechanism

Organic amine-based buffer compounds such as HEPES (Good's buffers) are commonly applied in experimental systems, including those where the biological effects of peroxynitrite are studied. In such studies 3-morpholinosydnonimine N-ethylcarbamide (SIN-1), a compound that simultaneously releases nitric oxide (.NO) and superoxide (O-2), is often used as a source for peroxynitrite. Whereas in mere phosphate buffer H2O2 formation from 1.5 mM SIN-1 was low ( approximately 15 microM), incubation of SIN-1 with Good's buffer compounds resulted in continuous H2O2 formation. After 2 h of incubation of 1.5 mM SIN-1 with 20 mM HEPES about 190 microM H2O2 were formed. The same amount of H2O2 could be achieved from 1.5 mM SIN-1 by action of superoxide dismutase in the absence of HEPES. The increased H2O2 level, however, could not be related to a superoxide dismutase or to a NO scavenger activity of HEPES. On the other hand, SIN-1-mediated oxidation of both dihydrorhodamine 123 and deoxyribose as well as peroxynitrite-dependent nitration of p-hydroxyphenylacetic acid were strongly inhibited by 20 mM HEPES. Furthermore, the peroxynitrite scavenger tryptophan significantly reduced H2O2 formation from SIN-1-HEPES interactions. These observations suggest that peroxynitrite is the initiator for the enhanced formation of H2O2. Likewise, authentic peroxynitrite (1 mM) also induced the formation of both O-2 and H2O2 upon addition to HEPES (400 mM)-containing solutions in a pH (4.5-7.5)-dependent manner. In accordance with previous reports it was found that at pH >/=5 oxygen is released in the decay of peroxynitrite. As a consequence, peroxynitrite(1 mM)-induced H2O2 formation ( approximately 80 microM at pH 7.5) also occurred under hypoxic conditions. In the presence of bicarbonate/carbon dioxide (20 mM/5%) the production of H2O2 from the reaction of HEPES with peroxynitrite was even further stimulated. Addition of SIN-1 or authentic peroxynitrite to solutions of Good's buffers resulted in the formation of piperazine-derived radical cations as detected by ESR spectroscopy. These findings suggest a mechanism for H2O2 formation in which peroxynitrite (or any strong oxidant derived from it) initially oxidizes the tertiary amine buffer compounds in a one-electron step. Subsequent deprotonation and reaction of the intermediate alpha-amino alkyl radicals with molecular oxygen leads to the formation of O-2, from which H2O2 is produced by dismutation. Hence, HEPES and similar organic buffers should be avoided in studies of oxidative compounds. Furthermore, this mechanism of H2O2 formation must be regarded to be a rather general one for biological systems where sufficiently strong oxidants may interact with various biologically relevant amino-type molecules, such as ATP, creatine, or nucleic acids.

Nitric oxide-mediated renal epithelial cell injury during hypoxia and reoxygenation

The potent endothelial-derived vasodilator nitric oxide (NO) has been identified as a protective agent in acute renal failure. However, some recent studies have suggested a detrimental effect of NO on rat proximal tubules exposed to hypoxia and reoxygenation. We determined whether NO metabolites cause intracellular oxidation during hypoxia and reoxygenation and whether this oxidative stress is linked to irreversible cell injury. Primary cultures of rat proximal tubular epithelial cells were studied in a subconfluent stage and subjected to 60 min hypoxia and 30 min reoxygenation. Intracellular oxidation was assessed by monitoring the conversion of nonfluorescent dihydrorhodamine 123 (DHR) to fluorescent rhodamine 123 as a probe for the long-lived oxidant peroxynitrite. Hypoxia and reoxygenation produced a marked increase in cellular generation of oxidant species. Intracellular oxidation of DHR was reduced by approximately 40% when cells were also exposed to the NO synthase inhibitor L-NAME. Oxidation of DHR following hypoxia and reoxygenation was not affected by SOD or DMTU. A combination of SOD and L-NAME was no more effective than L-NAME alone. Hypoxia and reoxygenation produced substantial injury (as LDH release). There was a 40% reduction in LDH release when cells were pretreated with a NO synthase inhibitor. In summary, increased generation of NO capable of inducing intracellular oxidizing reactions and cell death occurred during renal hypoxia and reoxygenation.

The critical role of Hepes in SIN-1 cytotoxicity, peroxynitrite versus hydrogen peroxide

The cytotoxicity of the superoxide anion radical- and nitric oxide-releasing compound SIN-1 to L929 cells was studied in Krebs-Henseleit buffer. pH 7.4, in the presence and absence of Hepes. SIN-1 cytotoxicity was significantly higher in the presence of Hepes than in the absence of Hepes. The available amount of peroxynitrite formed from SIN-1, however, was significantly decreased by Hepes as indicated by decreased oxidation of dihydrorhodamine 123. On the other hand, Hepes largely increased the formation of H2O2 from SIN-1. Catalase protected the L929 cells from SIN-1 cytotoxicity in the buffer with Hepes. In the buffer without Hepes catalase did not have any protective effect. In contrast, tyrosine and tryptophan provided significant protection against SIN-1 cytotoxicity independent of the presence of Hepes. These results demonstrate that the immediate toxic agent formed from SIN-1 decisively depends on the presence of Hepes. In its absence cytotoxicity is most likely mediated by peroxynitrite while in the presence of Hepes, cytotoxicity is conveyed by co-operative action of hydrogen peroxide and reactive nitrogen species.

Nitric oxide and peroxynitrite-mediated pulmonary cell death

Nitric oxide (.NO) can be produced within the lung, and recently inhaled nitric oxide has been used as a therapeutic agent. Peroxynitrite1 (ONOO-), the product of the nearly diffusion-limited reaction between .NO and superoxide, may represent the proximal reactive species mediating .NO injury to pulmonary cells. To investigate the physiological and pathological reactivities of .NO and ONOO- at the molecular and cellular levels, bovine pulmonary artery endothelial cells (BPAEC) and rat type II epithelial cells were exposed to .NO (0.01-2.5 microM/min for 2 h) generated by spermine-NONOate and papa-NONOate and to the same fluxes of ONOO- generated by 1,3-morpholinosydnonimine (SIN-1). Exposure to SIN-1 resulted in cellular injury and death in both cell types. Epithelial cells displayed a concentration-dependent loss of cellular viability within 8 h of exposure. In contrast, BPAEC loss of cellular viability was evident after 18 h postexposure. Events preceding cell death in BPAEC include depolarization of the mitochondrial membrane, evident as early as 6 h postexposure, loss of cellular redox activity at 16 h, and DNA fragmentation detected by in situ staining at 18 h after exposure. Exposure of BPAEC to .NO did not affect the cellular viability, but type II cells were injured in a manner similar to ONOO- exposure. .NO-mediated cellular injury within type II cells was reduced by preincubation with N-acetylcysteine. The data imply that the pathological and physiological effects of .NO may be regulated by its reactions with superoxide and reduced thiols.

C/EBP alpha and C/EBPbeta play similar roles in the transcription of the human Cu/Zn SOD gene

Copper/zinc-containing superoxide dismutase (Cu/Zn SOD) is one of the essential enzymes that protects cells in aerobic conditions. To understand the mechanism of transcriptional regulation of the human Cu/Zn SOD gene, the proximal promoter region of Cu/Zn SOD has been investigated. In a previous study, it was found that transcription factors of the Sp1 and C/EBP family are associated with the expression of human Cu/Zn SOD [Seo et al. (1996). Gene, 178, 177-180]. Of the four known C/EBP activators, the effects of C/EBP alpha and C/EBPbeta on the transcription of Cu/Zn SOD were examined. Usually, one of the two C/EBP factors, C/EBP alpha or C/EBPbeta, exclusively stimulates the target gene, but, unexpectedly, C/EBP alpha and C/EBPbeta stimulated the expression of Cu/Zn SOD to a similar extent in our experiments. LIP, a dominant negative regulator of the C/EBP family, inhibited the transcriptional activation of Cu/Zn SOD stimulated by C/EBP alpha and C/EBPbeta. These results suggest that C/EBP alpha and C/EBPbeta can substitute each other and play similar roles on the transcription of Cu/Zn SOD.

Nitroxides increase the detectable amount of nitric oxide released from endothelial cells

Nitroxides are known to exert superoxide dismutase-mimetic properties and to decrease O-2- and H2O2-mediated cytotoxicity. However, the effect of nitroxides on .NO homeostasis has not been studied yet. The present study investigates the effect of nitroxides on the detectable amount of .NO released by 3-morpholinosydnonimine (SIN-1) and cultured endothelial cells. Cultured bovine aortic and atrial endothelial cells stimulated with 10 microM A23187 released a stable flux of .NO, as detected by .NO chemiluminescence. Addition of 100 units/ml SOD or 10 microM of the nitroxides 4-hydroxy-2,2,6, 6-tetramethylpiperidine-N-oxyl (TEMPOL), 3-carboxy-proxyl, and 3-ethoxycarbonyl-proxyl, increased the chemiluminescence signal. The effect of these nitroxides on the amount of .NO released from cell monolayers was dose-dependent, with the highest efficacy between 30 and 100 microM. EPR spin trapping in SIN-1 solutions revealed the formation of .OH adducts from spontaneous dismutation of O-2 and concomitant reaction with H2O2. Both SOD and TEMPOL increased the signal intensity of the .OH adduct by accelerating the dismutation of O-2. The results of this study demonstrate that the SOD-mimetic activity of nitroxides increases the amount of bioavailable .NO in vitro.

Nitric oxide attenuates cellular hexose monophosphate shunt response to oxidants in articular chondrocytes and acts to promote oxidant injury

Nitric oxide (NO) has been implicated in both cartilage degradation and cell survival. Importantly, NO has been shown, in a cell-type-dependent manner, to directly cause cell death or indirectly promote cell death by compromising the ability of cells to detoxify intra- or extracellular oxidants. In this study we examined the role of NO in the survival of bovine chondrocytes exposed to catabolic cytokines (interleukin-1 (IL-1); tumor necrosis factor [TNF]) with or without the addition of an exogenous oxidant stress (e.g., H2O2, HOOCl, etc.). The exposure of chondrocytes to a mixture of IL-1 and TNF (IL-1/TNF) results in the release of NO but did not alter cell viability. However, there was evidence of NO-dependent oxidative responses in the IL-1/TNF group, as we observed an increased level of intracellular oxidants as well as the appearance of a 55 kD nitrated protein which reflects the formation of peroxynitrite. We next analyzed viability with H2O2. The LD50 for IL-1/TNF-treated cells was 0.1 mM (vs. 1 mM for control). The enhanced sensitivity was completely reversed when cells were incubated with the NO synthase inhibitor 1-n5-1-iminoethylornithine (NIO). To test whether cell death was caused by compromising the ability of cells to detoxify extracellular oxidants, we examined the hexose monophosphate shunt (HMPS) response in cells given H2O2. Treatment of control cells with H2O2 resulted in a fourfold increase in HMPS activity. In contrast, IL-1/TNF cells exhibited no increase in HMPS activity. The attenuation of stimulated HMPS activity was reversed by the coaddition of NIO. Thus, these data indicate that 1) endogenous NO mediates cytokine-dependent susceptibility to oxidant injury and 2) this effect is in part due to impaired activation of the HMPS. In inflamed joints replete with cytokines and oxidants, NO may contribute to chondrocyte death and progressive joint destruction.

Role of nitric oxide and superoxide in Giardia lamblia killing

Giardia lamblia trophozoites were incubated for 2 h with activated murine macrophages, nitric oxide (NO) donors or a superoxide anion generator (20 mU/ml xanthine oxidase plus 1 mM xanthine). Activated macrophages were cytotoxic to Giardia trophozoites (approximately 60% dead trophozoites). The effect was inhibited (> 90%) by an NO synthase inhibitor (200 microM) and unaffected by superoxide dismutase (SOD, 300 U/ml). Giardia trophozoites were killed by the NO donors, S-nitroso-acetyl-penicillamine (SNAP) and sodium nitroprusside (SNP) in a dose-dependent manner (LD50 300 and 50 microM, respectively). A dual NO-superoxide anion donor, 3-morpholino-sydnonimine hydrochloride (SIN-1), did not have a killing effect in concentrations up to 1 mM. However, when SOD (300 U/ml) was added simultaneously with SIN-1 to Giardia, a significant trophozoite-killing effect was observed (approximately 35% dead trophozoites at 1 mM). The mixtures of SNAP or SNP with superoxide anion, which yields peroxynitrite, abolished the trophozoite killing induced by NO donors. Authentic peroxynitrite only killed trophozoites at very high concentrations (3 mM). These results indicate that NO accounts for Giardia trophozoites killing and this effect is not mediated by peroxynitrite.

Nitric oxide donors suppress erythropoietin production in vitro

Many inflammatory diseases are associated with a hypoproliferative anaemia. Patients with this anaemia often present with serum erythropoietin (EPO) concentrations that are too low for the degree of their anaemia. Proinflammatory cytokines, in addition to their inhibitory effects on proliferation of erythroid progenitors, could contribute to the pathogenesis of this anaemia by reducing EPO production. Because several cytokines stimulate nitric oxide (NO) synthase we propose that nitric oxide might mediate the suppression of EPO production during inflammation. In order to test this hypothesis we investigated the effects of NO donors on 24-h hypoxia-induced EPO production in the hepatocellular carcinoma cell line HepG2. Following application of the NO donors sodium nitroprusside (SNP), 3-morpholinosydnonimine (SIN-1), and S-nitroso-N-acetyl-D,L-penicillamine (SNAP), EPO production was dose-dependently reduced: compared to the untreated control EPO production was lowered by 89% with SNP (1000 microM), by 66% with SIN-1 (1000 microM), and by 72% with SNAP (500 microM). In contrast, 8-bromo-cGMP did not inhibit EPO formation. Since pyrogallol (300 microM) and H2O2 (250 microM) showed a comparable suppression of EPO synthesis, we propose that NO might affect EPO production either by a similar direct influence on the cellular redox state or via increasing the cellular content of reactive oxygen species.

Ethanol cytotoxicity to a transfected HepG2 cell line expressing human cytochrome P4502E1

The effect of ethanol on the viability of a HepG2 cell model which was developed to constitutively express human CYP2E1 was studied in an attempt to establish a linkage between CYP2E1, reactive oxygen intermediates, and ethanol toxicity. Assays of toxicity included leakage of lactate dehydrogenase, trypan blue uptake, morphology, and formazan production. Ethanol was toxic to HepG2 E9 cells, which express CYP2E1, but not to HepG2 MV5 cells, which do not express CYP2E1. The ethanol toxicity was dependent on the concentration of ethanol, starting with 10 m ethanol, and on the time of incubation with ethanol. Phorbol 12-myristate 13-acetate, which increases the expression of CYP2E1 in this model, increased the toxicity by ethanol. Ethanol toxicity was prevented by 4-methylpyrazole and by diallyl sulfide, inhibitors of CYP2E1. The ethanol toxicity was also prevented by radical trapping agents such as N-acetylcysteine and N-t-butyl-alpha-phenylnitrone, antioxidative agents such as catalase, superoxide dismutase, thiourea, and uric acid, and inhibitors of lipid peroxidation, such as vitamin E phosphate, Trolox, and diphenylphenylenediamine. Besides ethanol, other substrates such as Me2SO, CCl4, isoniazid, and N,N-dimethylnitrosamine were cytotoxic to cells expressing CYP2E1 but not to control cells. These results indicate that ethanol was toxic to HepG2 cells which express human CYP2E1 by a pathway sensitive to inhibitors of CYP2E1 and to a variety of antioxidative agents. This model appears to be useful in efforts to establish a CYP2E1-dependent ethanol hepatotoxicity system and to evaluate the role of oxidative stress and reactive radical species in the toxicity by ethanol.

Cytosolic phospholipase A2 (PLA2), but not secretory PLA2, potentiates hydrogen peroxide cytotoxicity in kidney epithelial cells

Phospholipase A2 (PLA2) and reactive oxygen species have been implicated both individually and synergistically in various forms of cellular injury. The form(s) of PLA2 important for cell injury and the implications of enhanced activity of the enzyme, however, have not been discerned. Previous studies reveal an increase in PLA2 activity associated with cell injury, but this association does not establish a causal relationship between the increase in activity and the injury. LLC-PK1 cell lines were created that express either the cytosolic PLA2 or a group II PLA2. The susceptibility of these cells to hydrogen peroxide toxicity was determined in order to evaluate the relative importance of these two forms of PLA2 in oxidant injury. Expression of cytosolic PLA2 in the LLC-cPLA2 cell line was associated with a 50-fold increase in PLA2 activity in the cytosolic fraction, an increase in agonist-stimulated arachidonate release, and immunodetection of the cytosolic PLA2 protein that was undetectable in control cells. Exposure to hydrogen peroxide or menadione, but not mercuric chloride, resulted in significantly greater lactate dehydrogenase release in LLC-cPLA2 cells when compared with control cells. Exogenous arachidonic acid (150 microM) did not enhance hydrogen peroxide-induced injury. The intracellular calcium chelator, 1,2-bis-(o-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid/tetra(acetoxymethyl) ester, protected the cells against injury, but the calcium ionophore, A23187, did not increase injury. Glycine conferred no protective effect against hydrogen peroxide toxicity. By contrast to these results with cytosolic PLA2-expressing cells, secretory PLA2 expression to very high levels did not increase susceptibility to hydrogen peroxide. Thus, cytosolic PLA2 may an be an important mediator of oxidant damage to renal epithelial cells.

Nitric oxide regulation of tissue free radical injury

We have presented evidence from a broad range of chemical, cell biological, and in vivo studies showing that .NO can mediate tissue-protective reactions during oxidant stress, as well as toxic and tissue prooxidant effects. One predominant factor that has been identified which influences .NO being protective versus toxic is the relative rates of production and concentrations of .NO and the more "traditional" family of reactive oxygen species, including O2.-, H2O2, .OH, LO., LOO., and high valency complexes of iron. Also, since so many anti-neutrophil actions of .NO have been described, it is likely that .NO will serve a protective role in acute inflammatory reactions. One issue is certain--many new truths remain to be revealed, as we continue to develop our understanding of the toxicology of reactive oxygen- and nitrogen-containing species.

Function of the [2FE-2S] cluster in mammalian ferrochelatase: a possible role as a nitric oxide sensor

Ferrochelatase (E.C. 4.99.1.1) is the terminal enzyme of the heme biosynthetic pathway, catalyzing the insertion of ferrous iron into protoporphyrin. In mammals the enzyme contains a labile [2Fe-2S] center. Although this cluster is absent in all prokaryotic, plant, and yeast ferrochelatases, its destruction or elimination from the mammalian enzyme results in loss of enzyme activity. In the current study we present data which clearly demonstrate that mammalian ferrochelatase is strongly inhibited by nitric oxide and that this effect is mediated via destruction of the [2Fe-2S] cluster. Carbon monoxide has no inhibitory effect, and yeast ferrochelatase, which lacks the [2Fe-2S] cluster, is not affected by NO (or CO). EPR and UV-visible absorption of purified recombinant human ferrochelatase provides evidence that NO is targeting the [2Fe-2S] center. UV-visible absorption spectroscopy of both human and murine recombinant ferrochelatase incubated with NO or the NO donor, S-nitroso-N-acetylpenicillamine (SNAP), indicate a rapid loss of the visible absorption spectrum of the [2Fe-2S] cluster. EPR studies of the resulting samples reveal the characteristic axial S = 1/2 resonance, g perpendicular = 2.033, and g parallel = 2.014 of a cysteinyl-coordinated monomeric iron-dinitrosyl cluster degradation product. Parallel spectroscopic studies of spinach ferredoxin, which also contains a [2Fe-2S] cluster, gave no indication of NO-induced cluster degradation under the same experimental conditions. Exposure of DMSO-induced murine erythroleukemia cells exposed to SNAP results in an initial decrease in heme production, suggesting that in vivo the cluster is rapidly destroyed. The potential physiological relevance of these data to the anemias that are found in individuals with chronic infections is discussed.