Paraquat-mediated selection for mutations in the manganese-superoxide dismutase gene sodA

Mitochondrial Oxidative Stress and Antioxidants Balance in Fatty Liver Disease

Fatty liver disease is one of the most prevalent forms of chronic liver disease that encompasses both alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD). Alcoholic steatohepatitis (ASH) and nonalcoholic steatohepatitis (NASH) are intermediate stages of ALD and NAFLD, which can progress to more advanced forms, including cirrhosis and hepatocellular carcinoma. Oxidative stress and particularly alterations in mitochondrial function are thought to play a significant role in both ASH and NASH and recognized to contribute to the generation of reactive oxygen species (ROS), as documented in experimental models. Despite the evidence of ROS generation, the therapeutic efficacy of treatment with antioxidants in patients with fatty liver disease has yielded poor results. Although oxidative stress is considered to be the disequilibrium between ROS and antioxidants, there is evidence that a subtle balance among antioxidants, particularly in mitochondria, is necessary to avoid the generation of ROS and hence oxidative stress. Conclusion: As mitochondria are a major source of ROS, the present review summarizes the role of mitochondrial oxidative stress in ASH and NASH and presents emerging data indicating the need to preserve mitochondrial antioxidant balance as a potential approach for the treatment of human fatty liver disease, which may pave the way for the design of future trials to test the therapeutic role of antioxidants in fatty liver disease.

Superoxide is not the proximate cause of paraquat toxicity

Paraquat toxicity is thought to occur through the production of superoxide O2(.-) and it has been argued that this oxygen radical species is, itself, an important mediator of the toxicity of this drug. If so, a direct relationship should exist between the steady-state amounts of O2(.-) produced and the lethal effects of paraquat. We have therefore examined O2(.-) mediated chemiluminescence and paraquat sensitivity in bacteria with widely varying superoxide dismutase (SOD) activities. As expected, bacteria with high SOD activity exhibit minimal (lucigenin enhanced) chemiluminescence in the presence of paraquat whereas SOD-deficient bacteria show >90-fold higher chemiluminescence compared to parental strains. Nonetheless, high SOD bacteria are more readily killed by paraquat whereas SOD-deficient organisms show no increased susceptibility to this agent. This further supports our earlier conclusions that hypertrophied SOD activity is inadequate defense against paraquat and that O2(.-) is probably not the proximate toxin by which paraquat mediates cellular injury.

Role of antioxidant enzymes and small molecular weight antioxidants in the pathogenesis of age-related macular degeneration (AMD)

Cells in aerobic condition are constantly exposed to reactive oxygen species (ROS), which may induce damage to biomolecules, including proteins, nucleic acids and lipids. In normal circumstances, the amount of ROS is counterbalanced by cellular antioxidant defence, with its main components—antioxidant enzymes, DNA repair and small molecular weight antioxidants. An imbalance between the production and neutralization of ROS by antioxidant defence is associated with oxidative stress, which plays an important role in the pathogenesis of many age-related and degenerative diseases, including age-related macular degeneration (AMD), affecting the macula—the central part of the retina. The retina is especially prone to oxidative stress due to high oxygen pressure and exposure to UV and blue light promoting ROS generation. Because oxidative stress has an established role in AMD pathogenesis, proper functioning of antioxidant defence may be crucial for the occurrence and progression of this disease. Antioxidant enzymes play a major role in ROS scavenging and changes of their expression or/and activity are reported to be associated with AMD. Therefore, the enzymes in the retina along with their genes may constitute a perspective target in AMD prevention and therapy.

Redox control of liver function in health and disease

Reactive oxygen species (ROS), a heterogeneous population of biologically active intermediates, are generated as by-products of the aerobic metabolism and exhibit a dual role in biology. When produced in controlled conditions and in limited quantities, ROS may function as signaling intermediates, contributing to critical cellular functions such as proliferation, differentiation, and cell survival. However, ROS overgeneration and, particularly, the formation of specific reactive species, inflicts cell death and tissue damage by targeting vital cellular components such as DNA, lipids, and proteins, thus arising as key players in disease pathogenesis. Given the predominant role of hepatocytes in biotransformation and metabolism of xenobiotics, ROS production constitutes an important burden in liver physiology and pathophysiology and hence in the progression of liver diseases. Despite the recognized role of ROS in disease pathogenesis, the efficacy of antioxidants as therapeutics has been limited. A better understanding of the mechanisms, nature, and location of ROS generation, as well as the optimization of cellular defense strategies, may pave the way for a brighter future for antioxidants and ROS scavengers in the therapy of liver diseases.

Increasing mitochondrial superoxide dismutase abundance leads to impairments in protein quality control and ROS scavenging systems and to lifespan shortening

The fungal aging model Podospora anserina contains three superoxide dismutases (SODs) in different cellular compartments. While PaSOD1 represents the Cu/Zn isoform located in the cytoplasm and in the mitochondrial inter-membrane space, PaSOD2 localizes to the perinuclear ER. PaSOD3, a protein with a manganese binding domain and a mitochondrial targeting sequence (MTS) is the mitochondrial SOD. Over-expression of PaSod3 leads to lifespan reduction and increased sensitivity against paraquat and hydrogen peroxide. The negative effects of PaSod3 over-expression correlate with a strong reduction in the abundance of mitochondrial peroxiredoxin, PaPRX1, and the matrix protease PaCLPP disclosing impairments of mitochondrial quality control and ROS scavenging pathways in PaSod3 over-expressors. Deletion of PaSod3 leads to increased paraquat sensitivity while hydrogen peroxide sensitivity and lifespan are not significantly changed when compared to the wild-type strain. These latter characteristics are unexpected and challenge the 'mitochondrial free radical theory of aging'.

Contribution of oxidative damage to antimicrobial lethality

A potential pathway linking hydroxyl radicals to antimicrobial lethality was examined by using mutational and chemical perturbations of Escherichia coli. Deficiencies of sodA or sodB had no effect on norfloxacin lethality; however, the absence of both genes together reduced lethal activity, consistent with rapid conversion of excessive superoxide to hydrogen peroxide contributing to quinolone lethality. Norfloxacin was more lethal with a mutant deficient in katG than with its isogenic parent, suggesting that detoxification of peroxide to water normally reduces quinolone lethality. An iron chelator (bipyridyl) and a hydroxyl radical scavenger (thiourea) reduced the lethal activity of norfloxacin, indicating that norfloxacin-stimulated accumulation of peroxide affects lethal activity via hydroxyl radicals generated through the Fenton reaction. Ampicillin and kanamycin, antibacterials unrelated to fluoroquinolones, displayed behavior similar to that of norfloxacin except that these two agents showed hyperlethality with an ahpC (alkyl hydroperoxide reductase) mutant rather than with a katG mutant. Collectively, these data are consistent with antimicrobial stress increasing the production of superoxide, which then undergoes dismutation to peroxide, from which a highly toxic hydroxyl radical is generated. Hydroxyl radicals then enhance antimicrobial lethality, as suggested by earlier work. Such findings indicate that oxidative stress networks may provide targets for antimicrobial potentiation.

Prooxidant activity of the superoxide dismutase (SOD)-mimetic EUK-8 in proliferating and growth-arrested Escherichia coli cells

Numerous studies have aimed to alleviate oxidative stress in a wide range of organisms by increasing superoxide dismutase (SOD) activity. However, experimental approaches have yielded contradictory evidence, and kinetics models have shown that increases in SOD activity may increase, decrease, or not change hydrogen peroxide (H2O2) production, depending on the balance of the various processes that produce and consume superoxide (O2-). In this study we tested whether administration of EUK-8, a synthetic mimetic of the SOD enzyme, can protect starving Escherichia coli cells against stasis-induced oxidative stress. Surprisingly, administration of EUK-8 to starving E. coli cells enhances the production of reactive oxygen species (ROS), resulting in a massive increase of oxidative damage and replicative death of the bacteria. Our results confirm that manipulation of ROS levels by increasing SOD activity does not necessarily result in a consequent decline of oxidative stress and can yield opposite results in a relatively simple model system such as starving E. coli cells.

Hydrogen peroxide: a metabolic by-product or a common mediator of ageing signals?

The reactive oxygen species that are generated by mitochondrial respiration, including hydrogen peroxide (H2O2), are potent inducers of oxidative damage and mediators of ageing. It is not clear, however, whether oxidative stress is the result of a genetic programme or the by-product of physiological processes. Recent findings demonstrate that a fraction of mitochondrial H2O2, produced by a specialized enzyme as a signalling molecule in the pathway of apoptosis, induces intracellular oxidative stress and accelerates ageing. We propose that genes that control H2O2 production are selected determinants of lifespan.

The return of the Scarlet Pimpernel: cobalamin in inflammation II - cobalamins can both selectively promote all three nitric oxide synthases (NOS), particularly iNOS and eNOS, and, as needed, selectively inhibit iNOS and nNOS

The up-regulation of transcobalamins [hitherto posited as indicating a central need for cobalamin (Cbl) in inflammation], whose expression, like inducible nitric oxide synthase (iNOS), is Sp1- and interferondependent, together with increased intracellular formation of glutathionylcobalamin (GSCbl), adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), may be essential for the timely promotion and later selective inhibition of iNOS and concordant regulation of endothelial and neuronal NOS (eNOS/nNOS.) Cbl may ensure controlled high output of nitric oxide (NO) and its safe deployment, because: (1) Cbl is ultimately responsible for the synthesis or availability of the NOS substrates and cofactors heme, arginine, BH(4) flavin adenine dinucleotide/flavin mononucleotide (FAD/FMN) and NADPH, via the far-reaching effects of the two Cbl coenzymes, methionine synthase (MS) and methylmalonyl CoA mutase (MCoAM) in, or on, the folate, glutathione, tricarboxylic acid (TCA) and urea cycles, oxidative phosphorylation, glycolysis and the pentose phosphate pathway. Deficiency of any of theNOS substrates and cofactors results in 'uncoupled' NOS reactions, decreasedNO production and increased or excessive O(2) (-), H(2)O(2), ONOO(-) and other reactive oxygen species (ROS), reactive nitric oxide species (RNIS) leading to pathology. (2) Cbl is also the overlooked ultimate determinant of positive glutathione status, which favours the formation of more benign NO species, s-nitrosothiols, the predominant form in which NO is safely deployed. Cbl status may consequently act as a 'back-up disc' that ensures the active status of antioxidant systems, as well as reversing and modulating the effects of nitrosylation in cell signal transduction.New evidence shows that GSCbl can significantly promote iNOS/ eNOS NO synthesis in the early stages of inflammation, thus lowering high levels of tumour necrosis factor-a that normally result in pathology, while existing evidence shows that in extreme nitrosative and oxidative stress, GSCbl can regenerate the activity of enzymes important for eventual resolution, such as glucose 6 phosphate dehydrogenase, which ensures NADPH supply, lactate dehydrogenase, and more; with human clinical case studies of OHCbl for cyanide poisoning, suggesting Cbl may regenerate aconitase and cytochrome c oxidase in the TCA cycle and oxidative phosphorylation. Thus, Cbl may simultaneously promote a strong inflammatory response and the means to resolve it.

Alternative pathways as mechanism for the negative effects associated with overexpression of superoxide dismutase

One of the most important antioxidant enzymes is superoxide dismutase (SOD), which catalyses the dismutation of superoxide radicals to hydrogen peroxide. The enzyme plays an important role in diseases like trisomy 21 and also in theories of the mechanisms of aging. But instead of being beneficial, intensified oxidative stress is associated with the increased expression of SOD and also studies on bacteria and transgenic animals show that high levels of SOD actually lead to increased lipid peroxidation and hypersensitivity to oxidative stress. Using mathematical models we investigate the question how overexpression of SOD can lead to increased oxidative stress, although it is an antioxidant enzyme. We consider the following possibilities that have been proposed in the literature: (i) Reaction of H(2)O(2) with CuZnSOD leading to hydroxyl radical formation. (ii) Superoxide radicals might reduce membrane damage by acting as radical chain breaker. (iii) While detoxifying superoxide radicals SOD cycles between a reduced and oxidized state. At low superoxide levels the intermediates might interact with other redox partners and increase the superoxide reductase (SOR) activity of SOD. This short-circuiting of the SOD cycle could lead to an increased hydrogen peroxide production. We find that only one of the proposed mechanisms is under certain circumstances able to explain the increased oxidative stress caused by SOD. But furthermore we identified an additional mechanism that is of more general nature and might be a common basis for the experimental findings. We call it the alternative pathway mechanism.

Why does SOD overexpression sometimes enhance, sometimes decrease, hydrogen peroxide production? A minimalist explanation

Toxic effects of superoxide dismutase (SOD) overexpression are commonly attributed to increased hydrogen peroxide (H(2)O(2)) production. Still, published experiments yield contradictory evidence on whether SOD overexpression increases or decreases H(2)O(2) production. We analyzed this issue using a minimal mathematical model. The most relevant mechanisms of superoxide consumption are treated as pseudo first-order processes, and both superoxide production and the activity of enzymes other than SOD were considered constant. Even within this simple framework, SOD overexpression may increase, hold constant, or decrease H(2)O(2) production. At normal SOD levels, the outcome depends on the ratio between the rate of processes that consume superoxide without forming H(2)O(2) and the rate of processes that consume superoxide with high (>/= 1) H(2)O(2) yield. In cells or cellular compartments where this ratio is exceptionally low (< 1), a modest decrease in H(2)O(2) production upon SOD overexpression is expected. Where the ratio is higher than unity, H(2)O(2) production should increase, but at most linearly, with SOD activity. The results are consistent with the available experimental observations. According to the minimal model, only where most superoxide is eliminated through H(2)O(2)-free processes does SOD activity have the moderately large influence on H(2)O(2) production observed in some experiments.

Overexpression of superoxide dismutase in Trypanosoma cruzi results in increased sensitivity to the trypanocidal agents gentian violet and benznidazole

The parasitic protozoan Trypanosoma cruzi is exposed to toxic oxygen metabolites which arise from drug metabolism or immune mechanisms, in addition to those produced by endogenous processes. Identification and functional analysis of parasite enzymes which confer protection against oxidative stress is therefore of importance. To investigate the role of T. cruzi superoxide dismutase (SOD) we transfected epimastigotes with an expression vector containing a putative Fe-SOD gene homologue and achieved overexpression of enzyme activity (5-8 fold). Inhibition studies carried out on the partially purified enzyme revealed azide and H2O2 sensitivity and cyanide insensitivity, the profile expected of an Fe-isoform. Phenotypic analysis of transformed parasites showed that they were more susceptible than control cells to growth inhibition by the trypanocidal drug benznidazole and by gentian violet, an agent which can be used to decontaminate blood supplies in endemic areas. These results may reflect an imbalance in the antioxidant defences of the parasite produced as a result of overexpression of Fe-SOD.

The role of the cellular antioxidant defense in oxidant carcinogenesis

Oxidant carcinogens interact with multiple cellular targets including membranes, proteins, and nucleic acids. They cause structural damage to DNA and have the potential to mutate cancer-related genes. At the same time, oxidants activate signal transduction pathways and alter the expression of growth- and differentiation-related genes. Indeed, the carcinogenic action of oxidants results from the superposition of these genetic and epigenetic effects. All cells possess elaborate antioxidant defense systems that consist of interacting low and high molecular weight components. Among them, superoxide dismutases (SOD), glutathione peroxidases (GPx), and catalase (CAT) play a central role. Our studies with mouse epidermal cells demonstrate that the balance between several antioxidant enzymes rather than the activity of a single component determines the degree of protection. Unexpectedly, increased levels of Cu,Zn-SOD alone in stable transfectants resulted in sensitization to oxidative chromosomal aberrations and DNA strand breaks. However, a concomitant increase in CAT or GPx in double transfectants corrected or overcorrected the hypersensitivity of the SOD clones depending on the ratios of activities CAT/SOD or GPx/SOD. The cellular antioxidant capacity also affected oxidant induction of the growth-related immediate early protooncogene c-fos. Increases in CAT or SOD reduced the accumulation of c-fos message, albeit for different reasons. The cellular antioxidant defense also affects the action of UVB light (290-320 nm) that represents the most potent carcinogenic wavelength range of the solar spectrum. UVB light is known to exert its action in part through oxidative mechanisms. Increases in CAT and GPx protected mouse epidermal cells from UVB-induced DNA breakage.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the cellular antioxidant defense in oxidant carcinogenesis

Oxidant carcinogens interact with multiple cellular targets including membranes, proteins, and nucleic acids. They cause structural damage to DNA and have the potential to mutate cancer-related genes. At the same time, oxidants activate signal transduction pathways and alter the expression of growth- and differentiation-related genes. Indeed, the carcinogenic action of oxidants results from the superposition of these genetic and epigenetic effects. All cells possess elaborate antioxidant defense systems that consist of interacting low and high molecular weight components. Among them, superoxide dismutases (SOD), glutathione peroxidases (GPx), and catalase (CAT) play a central role. Our studies with mouse epidermal cells demonstrate that the balance between several antioxidant enzymes rather than the activity of a single component determines the degree of protection. Unexpectedly, increased levels of Cu,Zn-SOD alone in stable transfectants resulted in sensitization to oxidative chromosomal aberrations and DNA strand breaks. However, a concomitant increase in CAT or GPx in double transfectants corrected or overcorrected the hypersensitivity of the SOD clones depending on the ratios of activities CAT/SOD or GPx/SOD. The cellular antioxidant capacity also affected oxidant induction of the growth-related immediate early protooncogene c-fos. Increases in CAT or SOD reduced the accumulation of c-fos message, albeit for different reasons. The cellular antioxidant defense also affects the action of UVB light (290-320 nm) that represents the most potent carcinogenic wavelength range of the solar spectrum. UVB light is known to exert its action in part through oxidative mechanisms. Increases in CAT and GPx protected mouse epidermal cells from UVB-induced DNA breakage.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidants induce transcriptional activation of manganese superoxide dismutase in glomerular cells

Cultured rat glomerular mesangial and epithelial cells and bovine glomerular endothelial cells were exposed to various concentrations of hydrogen peroxide (H2O2). Mesangial cells treated with 10 to 100 microM H2O2 for 24 hours showed a two- to ninefold increase in Mn-SOD mRNA expression associated with significantly (P < 0.005) increased Mn-SOD activity (22.2 +/- 1.2 and 12.2 +/- 0.7 mu/mg protein for H2O2 100 microM treated and untreated cells, respectively). In contrast, expression of Cu-Zn SOD and beta-actin mRNA was not affected by H2O2. Induction of Mn-SOD mRNA by H2O2 was inhibited by actinomycin-D (4 microM) treatment. Glomerular endothelial cells also showed an increase in Mn-SOD mRNA expression following 100 microM H2O2 treatment, as did glomerular epithelial cells following treatment with 500 and 1000 microM H2O2 but not with 100 microM. Transcriptional activity of the Mn-SOD gene was assessed with a fusion reporter gene consisting of a luciferase gene (pGL2P) and a 1.2 kb fragment from the rat Mn-SOD genomic DNA (-806 to +408 bp of the transcription initiation site, -806:+408). The construct was transfected into rat glomerular mesangial and epithelial cells. Mesangial and epithelial cells transfected with pGL2P (-806:+408) and treated with H2O2 (100 microM and 1 mM for mesangial and epithelial cells, respectively) demonstrated some threefold increase in luciferase activity, whereas cells transfected with pGL2P lacking the Mn-SOD fragment did not show changes in luciferase activity following H2O2 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the sensitivities of Salmonella typhimurium oxyR and katG mutants to killing by human neutrophils

The respiratory burst of neutrophils is believed to kill bacteria by generating oxidative species, such as superoxide anion, hydrogen peroxide, and oxidized halogen species. The oxyR gene of Salmonella typhimurium controls a regulon induced by oxidative stress, such as exposure to hydrogen peroxide. Some researchers have suggested that oxyR may play a key role in bacterial survival following phagocytosis. We have tested this possibility by comparing the survival, following exposure to human neutrophils, of isogenic strains bearing different oxyR alleles. Neither inactivation of the oxyR gene nor constitutive overexpression of the oxyR-regulated proteins (oxyR1 allele) greatly alters bacterial resistance to neutrophils. The katG gene, encoding the oxyR-regulated enzyme hydroperoxidase I, was also without effect on survival following exposure to neutrophils. We conclude that the oxyR response does not play a significant role in the resistance of S. typhimurium to phagocytic killing in vitro.

Induction of manganese superoxide dismutase by glucocorticoids in glomerular cells

Our previous in vivo study demonstrated that methylprednisolone (MP) activates glomerular antioxidant enzymes and attenuates glomerular oxidant injuries, including those in experimental nephrosis. The present study investigates the cellular mechanism of the MP-induced activation of antioxidant enzymes and their contribution to the attenuation of cellular oxidant toxicity. When bovine glomerular endothelial cells (GECs) were treated with 10 microM MP, cellular manganese superoxide dismutase (Mn-SOD, 3.95 +/- 0.33 mu/mg protein, M +/- SE) and catalase (1.64 +/- 0.06 k/mg protein) activities were significantly (P < 0.05) elevated above control GECs (2.23 +/- 0.43 mu/mg protein and 1.06 +/- 0.09 k/mg protein, respectively). When GECs pretreated with MP (10 microM 24 hrs) were exposed to xanthine (0.1 mM)+xanthine oxidase (5 mU/ml) for four hours, levels of specific membrane lipid peroxidation products, that is, phosphatidylcholine- and phosphatidylethanolamine-hydroperoxides, remained at levels 10 to 25% of those measured in non-MP-treated (xanthine/xanthine oxidase-exposed) control cells. Moreover, the degree of cell damage following exposure to the superoxide generating system, assessed by 51Cr release, was significantly attenuated in MP-treated cells (approximately 50% of MP-non-treated controls, N = 6). Thus, MP-treated GECs with elevated antioxidant enzyme activities by MP were more resistant to the toxic effect of reactive oxygen metabolites. The mechanism of antioxidant enzyme induction by MP was studied for Mn-SOD. MP was shown to enhance Mn-SOD mRNA in bovine GECs and rat glomerular mesangial cells (GMCs) in dose-dependent manners.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterologous expression of drug-metabolizing enzymes in cellular and whole animal models

In this report we describe the heterologous expression of glutathione S-transferase (GST) and cytochrome P450 reductase (Red) in E. coli and Salmonella typhimurium. The same expression vectors could be applied to both systems and high levels of catalytically active GST and Red were obtained. Interestingly the level of expression was invariably higher in S. typhimurium. The level of the alpha class GST being up to 20% of the total bacterial protein. A further advantage of the salmonella system is that strains were used which can be applied to mutagenicity tests. This system was validated by demonstrating increasing mutation frequency of halogenated hydrocarbons in strains expressing the GST and increased cytotoxicity of mitomycin C in cells expressing P450 reductase.

Effects of overproduction of superoxide dismutases in Escherichia coli on inhibition of growth and on induction of glucose-6-phosphate dehydrogenase by paraquat

Stationary phase inocula were more susceptible to the growth inhibitory effect of paraquat than were log phase inocula and this difference was exacerbated in strains overproducing superoxide dismutases (SOD). Glucose-6-phosphate dehydrogenase (G-6-PD), a member of the soxR regulon, was induced by paraquat promptly in the case of log phase cells; but only after a lag in stationary phase cells and this difference was also exaggerated in strains overproducing SOD. The negative consequences of overproduction of SOD on the adaptation of stationary phase cells to paraquat may be attributed to competition for cellular resources with an attendant delay in biosynthesis of other components of soxR. Since overproduction of SOD did not prevent log phase cells from inducing G-6-PD in response to paraquat, it appears likely that soxR can respond to aspects of redox status other than O2-. This conclusion is in accord with data which is already in the literature.

Regulation of sod genes in Escherichia coli: relevance to superoxide dismutase function

This review is concerned with the effects of environmental perturbations on the expression of the two superoxide dismutase (SOD) genes in Escherichia coli (sodA, MnSOD; sodB, FeSOD). Early studies using SOD activity, showed that MnSOD levels respond to changes in oxygen tension, type of substrate, redox active compounds, iron concentration, the nature of the terminal oxidant, and the redox potential of the medium. FeSOD levels appeared nominally insensitive to these perturbations. More recent molecular genetic studies revealed that sodA expression is subject to regulation by three major regulatory systems: fur (ferric uptake regulation) and arcA arcB (aerobic respiratory control) mediate repression of sodA, while a relatively new system, soxR soxS (superoxide response), mediates activation of sodA expression. By contrast, sodB expression, which is much less studied at this time, appears to be positively activated in trans by fur. A rudimentary gene regulation model is presented which rationalizes past observations, is experimentally testable, and should serve as a guide to future research in this area.

Disparate antioxidant enzyme activities in cultured human cutaneous fibroblasts, keratinocytes, and melanocytes

Antioxidant enzyme activities of cultured human foreskin fibroblasts, keratinocytes, and melanocytes from healthy black and Caucasian donors were measured and compared. Fibroblasts had more (p less than 0.05) peroxidase, catalase, glutathione peroxidase, and superoxide dismutase activity than keratinocytes. Keratinocytes had more (p less than 0.05) peroxidase, catalase, glutathione peroxidase, and superoxide dismutase activity than melanocytes. No differences in antioxidant enzyme activities were observed between the cells of any type taken from black or Caucasian people. Antioxidant enzyme activities may affect resistance to damage by oxidants induced by ultraviolet radiation and inflammation.

Manganese superoxide dismutase can reduce cellular damage mediated by oxygen radicals in transgenic plants

In plants, environmental adversity often leads to the formation of highly reactive oxygen radicals. Since resistance to such conditions may be correlated with the activity of enzymes involved in oxygen detoxification, we have generated transgenic tobacco plants which express elevated levels of manganese superoxide dismutase (MnSOD) within their chloroplasts or mitochondria. Leaf discs of these plants have been analyzed in conditions in which oxidative stress was generated preferentially within one or the other organelle. It was found that high level overproduction of MnSOD in the corresponding subcellular location could significantly reduce the amount of cellular damage which would normally occur. In contrast, small increases in MnSOD activity were deleterious under some conditions. A generally applicable model correlating the consequences of SOD with the magnitude of its expression is presented.

Expression of bovine superoxide dismutase in Drosophila melanogaster augments resistance of oxidative stress

Superoxide dismutases (SOD) play a major role in the intracellular defense against oxygen radical damage to aerobic cells. In eucaryotes, the cytoplasmic form of the enzyme is a 32-kDa dimer containing two copper and two zinc atoms (CuZn SOD) that catalyzes the dismutation of the superoxide anion (O2-) to H2O2 and O2. Superoxide-mediated damage has been implicated in a number of biological processes, including aging and cancer; however, it is not certain whether endogenously elevated levels of SOD will reduce the pathological events resulting from such damage. To understand the in vivo relationship between an efficient dismutation of O2- and oxidative injury to biological structures, we generated transgenic strains of Drosophila melanogaster overproducing CuZn SOD. This was achieved by microinjecting Drosophila embryos with P-elements containing bovine CuZn SOD cDNA under the control of the Drosophila actin 5c gene promoter. Adult flies of the resulting transformed lines which expressed both mammalian and Drosophila CuZn SOD were then used as a novel model for evaluating the role of oxygen radicals in aging. Our data show that expression of enzymatically active bovine SOD in Drosophila flies confers resistance to paraquat, an O2(-)-generating compound. This is consistent with data on adult mortality, because there was a slight but significant increase in the mean lifespan of several of the transgenic lines. The highest level of expression of the active enzyme in adults was 1.60 times the normal value. Higher levels may have led to the formation of toxic levels of H2O2 during development, since flies that died during the process of eclosion showed an unusual accumulation of lipofuscin (age pigment) in some of their cells. In conclusion, our data show that free-radical detoxification has a minor by positive effect on mean longevity for several strains.

Two global regulators repress the anaerobic expression of MnSOD in Escherichia coli::Fur (ferric uptake regulation) and Arc (aerobic respiration control)

The expression of sodA, the Escherichia coli gene encoding manganese superoxide dismutase (MnSOD) is induced by aerobiosis and superoxide generators such as paraquat. Analysis of variants expressing sodA in the absence of oxygen has revealed that mutations in genes for two global regulatory systems, Fur (ferric uptake regulation) and Arc (aerobic respiration control), are simultaneously required for the expression of sodA in anaerobiosis. The Fur protein still represses sodA in an iron-dependent fashion in aerobiosis. Moreover, all mutants remain inducible by paraquat, indicating that the positive control of SoxR, which mediates the response to superoxide in E. coli, is still operative. Thus, in addition to the response to the superoxide-mediated oxidative stress which depends on SoxR, two global controls regulate MnSOD expression: ArcA couples MnSOD expression to respiration, and Fur couples it to the intracellular concentration of iron.

Expression of bovine superoxide dismutase in Drosophila melanogaster augments resistance of oxidative stress

Superoxide dismutases (SOD) play a major role in the intracellular defense against oxygen radical damage to aerobic cells. In eucaryotes, the cytoplasmic form of the enzyme is a 32-kDa dimer containing two copper and two zinc atoms (CuZn SOD) that catalyzes the dismutation of the superoxide anion (O2-) to H2O2 and O2. Superoxide-mediated damage has been implicated in a number of biological processes, including aging and cancer; however, it is not certain whether endogenously elevated levels of SOD will reduce the pathological events resulting from such damage. To understand the in vivo relationship between an efficient dismutation of O2- and oxidative injury to biological structures, we generated transgenic strains of Drosophila melanogaster overproducing CuZn SOD. This was achieved by microinjecting Drosophila embryos with P-elements containing bovine CuZn SOD cDNA under the control of the Drosophila actin 5c gene promoter. Adult flies of the resulting transformed lines which expressed both mammalian and Drosophila CuZn SOD were then used as a novel model for evaluating the role of oxygen radicals in aging. Our data show that expression of enzymatically active bovine SOD in Drosophila flies confers resistance to paraquat, an O2(-)-generating compound. This is consistent with data on adult mortality, because there was a slight but significant increase in the mean lifespan of several of the transgenic lines. The highest level of expression of the active enzyme in adults was 1.60 times the normal value. Higher levels may have led to the formation of toxic levels of H2O2 during development, since flies that died during the process of eclosion showed an unusual accumulation of lipofuscin (age pigment) in some of their cells. In conclusion, our data show that free-radical detoxification has a minor by positive effect on mean longevity for several strains.

Overexpression of Cu-Zn superoxide dismutase in Drosophila does not affect life-span

Aging and disease processes may be due to deleterious and irreversible changes produced by free radical reactions. The enzyme copper-zinc superoxide dismutase (Cu-Zn SOD; superoxide:superoxide oxidoreductase, EC 1.15.1.1) performs a protective function by scavenging superoxide radicals. The Cu-Zn SOD gene (Sod) cloned from Drosophila melanogaster was introduced via P element-mediated transformation into the germ line. Homozygous lines carrying additional copies of the Sod gene were recovered and characterized. Increases in Sod transcripts and enzyme activity were observed in the transformed lines, indicating that all of the sequence information required for gene expression is contained on the inserted gene fragment. The effects of additional SOD on oxygen free radical metabolism and longevity were investigated. Additional SOD did not markedly affect oxygen metabolism or longevity.

Cloned manganese superoxide dismutase reduces oxidative stress in Escherichia coli and Anacystis nidulans

The Mn superoxide dismutase gene of Escherichia coli was subcloned into the E. coli-Anacystis nidulans shuttle vector pSG111 to make the plasmid pMYG1. Transformation of E. coli HB101 with pMYG1 resulted in a 6-fold increase in superoxide dismutase activity. There was also induction of Mn superoxide dismutase in the transformants upon exposure to paraquat, as evidenced by dramatically increased levels of the Mn superoxide dismutase polypeptide in cytoplasmic extracts and a 16-fold further increase in superoxide dismutase activity. As well, the E. coli transformants showed resistance to paraquat-mediated inhibition of growth. Anacystis nidulans, a cyanobacterium that has no detectable Mn superoxide dismutase and is, consequently, very sensitive to oxidative stress, was also transformed with pMYG1. The transformants had detectable levels of Mn superoxide dismutase protein and showed resistance to paraquat-mediated inhibition of growth and photobleaching of pigments. Paraquat is known to promote formation of the superoxide radical anion, O2-., and thus the data have been interpreted as indicating that the cloned Mn superoxide dismutase provides protection in both E. coli and A. nidulans against damage attributable to O2-..

Microbial stress proteins

There is general agreement that a function, perhaps the major function, of stress proteins under normal physiological conditions is to help assembly and disassembly of protein complexes and to catalyse protein-translocation processes. It remains unclear, however, as to what role these processes play in stressed cells. It could be that cells under stress produce abnormal, misfolded or otherwise damaged proteins and that increased synthesis of stress proteins is required to counter protein modifications. A role for stress proteins in recovery of cells from stress, as opposed to a role in helping cells to withstand a lethal stress, is thus suggested. The intracellular location of stress proteins, in the unstressed and stressed cell, is worthy of further studies. Members of the hsp70 family are associated with the cytosol, mitochondria and endoplasmic reticulum. There is evidence, particularly from studies on mammalian cells (Tanguay, 1985; Welch and Mizzen, 1988; Arrigo et al., 1988), that following stress hsps migrate to various cellular compartments and subsequently delocalize after stress. However, there is little comparable data from microbial systems for this phenomenon (e.g. Rossi and Lindquist, 1989). The question as to the role of stress proteins in the transient acquisition of thermotolerance remains to be answered. It is insufficient to equate the kinetics of stress-protein synthesis with acquisition of thermotolerance. Quantitative data on the amount of stress protein present at various times, including the recovery period, is required. The demonstration that microbial stress proteins are important antigenic determinants of micro-organisms causing major debilitating diseases in the world is an exciting observation. Studies on the interplay of pathogen and host, both carrying similar antigenic hsp determinants, will be a challenging area for future research. It is likely that E. coli and Sacch. cerevisiae, with their well-established biochemical and genetic properties, will continue to be the experimental systems of choice for studies on stress proteins. On the other hand, it is encouraging that studies on other micro-organisms have expanded in the past few years and have made substantial contributions towards our understanding of the stress response. The ubiquitous nature of the stress response and the remarkable evolutionary conservation of the stress proteins continue to be attractive areas for research.

General method for site-directed mutagenesis in Escherichia coli O18ac:K1:H7: deletion of the inducible superoxide dismutase gene, sodA, does not diminish bacteremia in neonatal rats

A defined deletion in the Escherichia coli K-12 sodA gene (encoding manganese-superoxide dismutase) linked to a nontransposable selectable marker was generated by transposon Tn5 insertion in combination with in vitro mutagenesis. This mutant allele was used to replace the wild-type sodA gene in an E. coli clinical isolate of serotype O18ac:K1:H7 by bacteriophage P1 transduction. The O18ac:K1:H7 sodA mutant contained no manganese-superoxide dismutase and no hybrid manganese-iron-superoxide dismutase. The sodA mutant was more sensitive to paraquat toxicity than were the parental strain and an isogenic mutant bearing an analogously constructed sodA+ Tn5 insertion allele. In a suckling rat model for bacteremia following oral inoculation of E. coli K1, the sodA mutant was undiminished in its capabilities both to colonize the gastrointestinal tract and, surprisingly, to cause bacteremia. In conjunction with the rat model for E. coli K1 pathogenesis, the method for site-directed mutagenesis described in this paper permits determination of the role played in colonization and bacteremia by any K1 gene which either has a homolog in E. coli K-12 or can be cloned and manipulated therein.

Cloning and characterization of an Anacystis nidulans R2 superoxide dismutase gene

The E. coli iron superoxide dismutase gene (sodB) was utilized as a heterologous probe to isolate a superoxide dismutase (sod) gene from Anacystis nidulans R2. Nucleotide sequence analysis revealed a 603 bp open reading frame with deduced amino acid sequence similar to other sod genes and to cyanobacterial superoxide dismutase amino-terminal sequences. Assuming proteolytic cleavage of the initial methionine residue, the molecular mass of the mature A. nidulans R2 sodB polypeptide is 22,000 daltons. Only a single copy of the superoxide dismutase sequence was detected in the A. nidulans R2 genome using Southern hybridization. Northern hybridization analysis indicated a single, monocistronic RNA transcript of approximately 720 bases. Primer extension mapping localized the transcription start site to 46 bases upstream from the initial methionine residue. A single orientation of a 2.1 kb PstI fragment containing the entire sod gene cloned into pUC18 was able to complement E. coli sodAsodB mutants. Complementation of the E. coli mutants was based on the ability of the cells to grow aerobically on minimal glucose medium. Growth curves of the complemented E. coli sodAsodB mutants showed that these cells exhibited levels of resistance to paraquat comparable to that of the wild-type E. coli phenotype.

Bacterial genes involved in response to near-ultraviolet radiation

A model of the possible pathways of activities following NUV treatment was presented in Section I and in Fig. 1. Some of the components are firmly established, some are speculative, and many are difficult to evaluate because of insufficient experimental information. Perhaps the most relevant experiments, especially concerning ozone depletion, would be to determine the mutational specificity of NUV. By selecting lacI mutants after exposing cells to NUV, and sequencing the bases of this gene, this is now feasible. There are some problems, however. The mutation frequency is normally so low that it might be difficult to distinguish NUV mutants from spontaneous mutants. However, by irradiating cells having a uvrA or uvrB mutation, the frequency of mutation above background can be increased considerably. There remains the problem as to what fraction of the observed mutations results from oxidative damage. Some of this could be clarified by comparing mutation spectra of cells treated with NUV and cells subjected to excess oxidative damage and determining what fraction results from other avenues of lesion formation in DNA. Different species of reactive oxygen could cause different kinds of DNA lesions, and, fortunately, use of appropriate mutants should allow us to sort out any differences in specificity of lesions. Also, by appropriate manipulation of quantities of endogenous photosensitizers, it might be possible to sort out the specific mutations that are caused by photodynamic action. Another avenue of research is to explore the pathways by which NUV lesions are repaired, and whether such repair is error prone or error free. Again, the use of mutants such as xthA, uvr, and polA should assist in our understanding of the specificity of the mutational events. There are now a number of examples of global control mechanisms whereby cells abruptly shift their protein synthesis pattern under environmental stress. It is important to understand whether NUV stress results in induction of one or more of the known regulatory genes, or whether another regulon might be involved. One particular aspect of regulation that remains unsolved is the role of the katF gene, which is known to regulate the xthA and katE, but it may also regulate other genes as well. A number of striking physiological events occur even at very low fluences of NUV irradiation of cells. In part, this may be related to regulon induction. However, some of these events are in need of special exploration, such as changes at the membrane level.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA damage and oxygen radical toxicity

A major portion of the toxicity of hydrogen peroxide in Escherichia coli is attributed to DNA damage mediated by a Fenton reaction that generates active forms of hydroxyl radicals from hydrogen peroxide, DNA-bound iron, and a constant source of reducing equivalents. Kinetic peculiarities of DNA damage production by hydrogen peroxide in vivo can be reproduced by including DNA in an in vitro Fenton reaction system in which iron catalyzes the univalent reduction of hydrogen peroxide by the reduced form of nicotinamide adenine dinucleotide (NADH). To minimize the toxicity of oxygen radicals, the cell utilizes scavengers of these radicals and DNA repair enzymes. On the basis of observations with the model system, it is proposed that the cell may also decrease such toxicity by diminishing available NAD(P)H and by utilizing oxygen itself to scavenge active free radicals into superoxide, which is then destroyed by superoxide dismutase.

An assay for the detection of superoxide dismutase in individual Escherichia coli colonies

A method for detecting superoxide dismutase activity in individual colonies of Escherichia coli was developed. The assay involves the lysis of individual cells in colonies on filter papers by a series of lysozyme, chloroform, and freeze-thaw treatments. Filters are placed on agar plates to allow diffusion of cellular enzymes into a solid matrix. A nitroblue tetrazolium overlay is applied to detect superoxide dismutase activity. Colonies possessing activity produce achromatic zones against a dark Formazan background. The assay can detect the presence of superoxide dismutase and the relative amount of enzyme as well. This assay provides a method for screening a population of cells for mutants deficient in or overproducing superoxide dismutase.

Molecular genetics of superoxide dismutases

Molecular genetics of SOD has been recently developed primarily due to the new biotechnologies. Different types of isoenzymes have now been cloned and sequenced from several species ranging from bacteria to human and plants. Knowledge of the nucleotide sequences permitted refinement of structural models and provided information on subcellular locations. Cloned genes allowed the production of large amounts of SOD. They have been used for physiological and regulation studies, structural and enzymatic analyses, and are vital tools for the isolation of mutants. Isolation of mutants is generally essential to the understanding of the biological function of the gene in question. Indeed, SOD deficient mutants have now been isolated in bacteria and yeast. Their properties support, at numerous levels, a major role of SOD in cellular defense against oxygen toxicity. Few data are presently available on the molecular basis of mechanisms that regulate the expression of SOD.

Mutagenic and lethal effects of near-ultraviolet radiation (290-400 nm) on bacteria and phage

Despite decades of study of the effect of near-ultraviolet radiation (NUV) on bacterial cells, insights into mechanisms of deleterious alterations and subsequent recovery are just now emerging. These insights are based on observations that 1) damage by NUV may be caused by a reactive oxygen molecule, since H2O2 may be a photoproduct of NUV; 2) some, but not all, of the effects of NUV and H2O2 are interchangeable; 3) there is an inducible regulon (oxyR) that responds to oxidative stress and is involved in protection against NUV; 4) a number of NUV-sensitive mutants are defective either in the capacity to detoxify reactive oxygen molecules or to repair DNA damage caused by NUV; and 5) recovery from NUV damage may not directly involve induction of the SOS response. Since several distinctly different photoreceptors and targets are involved, it is unknown whether NUV lethality and mutagenesis result from an accumulation of damages or whether there is a particularly critical photoeffect. To fully understand the mechanisms involved, it is important to identify the chromophore(s) of NUV, the mechanism of toxic oxygen species generation, the role of the oxidative defense regulon (oxyR), the specific lesions in the DNA, and the enzymatic events of subsequent repair.