Insights and controversies on sunscreen safety

Although sunlight provides several benefits, ultraviolet (UV) radiation plays an important role in the development of various skin damages such as erythema, photoaging, and photocarcinogenesis. Despite cells having endogenous defense systems, damaged DNA may not be efficiently repaired at chronic exposure. In this sense, it is necessary to use artificial defense strategies such as sunscreen formulations. UV filters should scatter, reflect, or absorb solar UV radiation in order to prevent direct or indirect DNA lesions. However, the safety of UV filters is a matter of concern due to several controversies reported in literature, such as endocrine alterations, allergies, increased oxidative stress, phototoxic events, among others. Despite these controversies, the way in which sunscreens are tested is essential to ensure safety. Sunscreen regulation includes mandatory test for phototoxicity, but photogenotoxicity testing is not recommended as a part of the standard photosafety testing program. Although available photobiological tests are still the first approach to assess photosafety, they are limited. Some existing tests do not always provide reliable results, mainly due to limitations regarding the nature of the assessed phototoxic effect, cell UV sensitivity, and the irradiation protocols. These aspects bring queries regarding the safety of sunscreen wide use and suggest the demand for the development of robust and efficient in vitro screening tests to overcome the existing limitations. In this way, Saccharomyces cerevisiae has stood out as a promising model to fill the gaps in photobiology and to complete the mandatory tests enabling a more extensive and robust photosafety assessment.

Saccharomyces cerevisiae strains as bioindicators for titanium dioxide sunscreen photoprotective and photomutagenic assessment

Although several short-term assays are available for cosmetic photosafety assessment, cell models are usually highly sensitive to UV radiation, tending to overestimate both phototoxic and photomutagenic risks. In addition, these assays are performed with UV doses/fluences that do not correspond to actual environmental conditions. In this sense, Saccharomyces cerevisiae has already proved to be an interesting tool to predict photomutagenic potential of several compounds, including sunscreens. Yeast can support environmental UVB doses compatible with human daily sunlight exposure, allowing the use of irradiation sources to faithfully mimic the external conditions of ambient sunlight. Herein, we used a set of S. cerevisiae mutant strains sensitive to UVA, UVB and Solar Simulated Light sources in order to evaluate their potential as bioindicators for sunscreen development. The bioindicator potential of the strains was tested with the widely-used titanium dioxide inorganic sunscreen. The AWP001 (yno1) and LPW002 (ogg1yno1) strains obtained in this study stood out as promising experimental tools for the validation of this assay. Overall, our results evidenced a set of S. cerevisiae strains particularly useful for evaluating both photoprotective (efficacy) and photo/antiphotomutagenic (safety) potential of UV filters, meeting the industries and regulatory agencies demand for robust and efficient in vitro screening tests.

Photoprotection assessment of olive (Olea europaea L.) leaves extract standardized to oleuropein: In vitro and in silico approach for improved sunscreens

Olive leaves contain higher amount of polyphenols than olive oil and represent a waste product from olive harvest and pruning of olive trees. The most abundant compound in olive leaves is oleuropein. Benefits of the topical application of olive leaves extract were previously reported, but little information is available on its photoprotective potential and the result of the association of this extract with organic UV filters in topical sunscreen formulations. The olive leaves extract photoprotective potential is less explored for both oral and topical photoprotection in comparison with other plants extracts and polyphenols, such as Polypodium leucotomos extract and resveratrol. There are increasing efforts towards developing more efficient sunscreens and a photoprotection assessement along with a better understanding of the photochemistry of naturally occurring sunscreens could aid the design of new and improved commercial sunscreen formulations. This study was designed to investigate the photoprotective potential of olive leaves extract standardized for oleuropein performing a set of in vitro and in silico tools as an innovative approach, highlighting yeast assays, in vitro Sun Protection Factor (SPF) and molecular modelling studies of UV absorption. This study supports the use of olive leaves extract for photoprotection, as an effective photoprotective, anti-mutagenic and antioxidant active, also showing a synergistic effect in association with UV filters with an improvement on in vitro SPF of sunscreen formulations.

Effect of the FE2+ chelation by 2,2'-dipyridyl in the doxorubicin-induced lethality in breast tumor cell lines

Breast cancer cells may exhibit changes in iron homeostasis, which results in increased labile iron pool (LIP) levels. Several studies highlight the crucial role of high LIP levels in the maintenance of tumor cell physiology. Iron chelators have been tested in anticancer therapy in combination with chemotherapeutic agents, to improve drug efficacy. Thus, the aim of this study was to evaluate the effect of 2,2'-dipyridyl (DIP), a Fe²⁺ chelator, in combination with doxorubicin (DOX) in breast tumor cells. The maximum concentration of DIP that did not significantly reduce the viability of MDA-MB-231 cells was 10μM and for MCF-7 cells was 50μM. We observed that MCF-7 had higher LIP levels than MDA-MB-231 cells. DIP alone increased ROS generation in MCF-7 cells, and DIP pretreatment reduced ROS generation induced by DOX treatment. In conclusion, the increase in MCF-7 cell viability induced by DIP pretreatment in DOX-treated cells seems to be related to an increase in the cellular antioxidant capacity and the iron chelator did not improve drug efficacy in the two breast tumor cell lines analyzed.

Influence of Ogg1 repair on the genetic stability of ccc2 mutant of Saccharomyces cerevisiae chemically challenged with 4-nitroquinoline-1-oxide (4-NQO)

In Saccharomyces cerevisiae, disruption of genes by deletion allowed elucidation of the molecular mechanisms of a series of human diseases, such as in Wilson disease (WD). WD is a disorder of copper metabolism, due to inherited mutations in human copper-transporting ATPase (ATP7B). An orthologous gene is present in S. cerevisiae, CCC2 gene. Copper is required as a cofactor for a number of enzymes. In excess, however, it is toxic, potentially carcinogenic, leading to many pathological conditions via oxidatively generated DNA damage. Deficiency in ATP7B (human) or Ccc2 (yeast) causes accumulation of intracellular copper, favouring the generation of reactive oxygen species. Thus, it becomes important to study the relative importance of proteins involved in the repair of these lesions, such as Ogg1. Herein, we addressed the influence Ogg1 repair in a ccc2 deficient strain of S. cerevisiae. We constructed ccc2-disrupted strains from S. cerevisiae (ogg1ccc2 and ccc2), which were analysed in terms of viability and spontaneous mutator phenotype. We also investigated the impact of 4-nitroquinoline-1-oxide (4-NQO) on nuclear DNA damage and on the stability of mitochondrial DNA. The results indicated a synergistic effect on spontaneous mutagenesis upon OGG1 and CCC2 double inactivation, placing 8-oxoguanine as a strong lesion-candidate at the origin of spontaneous mutations. The ccc2 mutant was more sensitive to cell killing and to mutagenesis upon 4-NQO challenge than the other studied strains. However, Ogg1 repair of exogenous-induced DNA damage revealed to be toxic and mutagenic to ccc2 deficient cells, which can be due to a detrimental action of Ogg1 on DNA lesions induced in ccc2 cells. Altogether, our results point to a critical and ambivalent role of BER mediated by Ogg1 in the maintenance of genomic stability in eukaryotes deficient in CCC2 gene.

The anabolic androgenic steroid nandrolone decanoate disrupts redox homeostasis in liver, heart and kidney of male Wistar rats

The abuse of anabolic androgenic steroids (AAS) may cause side effects in several tissues. Oxidative stress is linked to the pathophysiology of most of these alterations, being involved in fibrosis, cellular proliferation, tumorigenesis, amongst others. Thus, the aim of this study was to determine the impact of supraphysiological doses of nandrolone decanoate (DECA) on the redox balance of liver, heart and kidney. Wistar male rats were treated with intramuscular injections of vehicle or DECA (1 mg.100 g⁻¹ body weight) once a week for 8 weeks. The activity and mRNA levels of NADPH Oxidase (NOX), and the activity of catalase, glutathione peroxidase (GPx) and total superoxide dismutase (SOD), as well as the reduced thiol and carbonyl residue proteins, were measured in liver, heart and kidney. DECA treatment increased NOX activity in heart and liver, but NOX2 mRNA levels were only increased in heart. Liver catalase and SOD activities were decreased in the DECA-treated group, but only catalase activity was decreased in the kidney. No differences were detected in GPx activity. Thiol residues were decreased in the liver and kidney of treated animals in comparison to the control group, while carbonyl residues were increased in the kidney after the treatment. Taken together, our results show that chronically administered DECA is able to disrupt the cellular redox balance, leading to an oxidative stress state.

Bipyridine (2,2'-dipyridyl) potentiates Escherichia coli lethality induced by nitrogen mustard mechlorethamine

Alkylating agents are used in anti-tumor chemotherapy because they bind covalently to DNA and generate adducts that may lead to cell death. Bifunctional (HN2) and monofunctional (HN1) nitrogen are two such agents, and HN2 was the first drug successfully employed in anti-leukemia chemotherapy. Currently, HN2 is used either alone or combined with other drugs to treat Hodgkin's disease. It is well known that several crosslinking agents require metabolic activation via reactive oxygen species (ROS) to exert their lethal effects. The objective of this work was therefore to determine whether the abovementioned mustards would also require metabolic activation to exert lethal action against Escherichia coli. For this purpose, we measured survival following exposure to HN2 in E. coli strains that were deficient in nucleotide excision repair (uvrA NER mutant), base excision repair (xthA nfo nth fpg BER mutant) or superoxide dismutase (sodAB mutant) activity. We also performed the same experiments in cells pretreated with an iron chelator (2,2'-dipyridyl, DIP). The NER and BER mutants were only sensitive to HN2 treatment (survival rates similar to those of the wild-type were achieved with 5-fold lower HN2 doses). However, wild-type and sodAB strains were not sensitive to treatment with HN2. In all tested strains, survival dropped by 2.5-fold following pretreatment with DIP compared to treatment with HN2 alone. Furthermore, DIP treatment increased ROS generation in both wild type and sodAB-deficient strains. Based on these data and on the survival of the SOD-deficient strain, we suggest that the increased production of ROS caused by Fe(2+) chelation may potentiate the lethal effects of HN2 but not HN1. This potentiation may arise as a consequence of enhancement in the number of or modification of the type of lesions formed. No sensitization was observed for the non-crosslinkable HN2 analog, HN1.

Genotoxicity and mutagenicity of Echinodorus macrophyllus (chapéu-de-couro) extracts

Echinodorus macrophyllus, commonly known as chapéu-de-couro, is a medicinal plant used in folk medicine to treat inflammation and rheumatic diseases. In this work, we used short-term bacterial assays based on the induction of SOS functions to examine the genotoxicity and mutagenicity of an aqueous extract of E. macrophyllus leaves. Whole extract and an ethyl acetate fraction showed similar genotoxicity and caused an ~70-fold increase in lysogenic induction. The extract also gave a positive result in the SOS chromotest with an increase of 12-fold in β-Galactosidase enzymatic units. There was a strong trend towards base substitutions and frameshifts at purine sites in the mutations induced by the extract in Escherichia coli (CC103 and CC104 strains) and Salmonella typhimurium test strains (22-fold increase in histidine revertants in TA98 strain). Since reactive oxygen species may be implicated in aging process and in degenerative diseases, we used antioxidant compounds as catalase, thiourea and dipyridyl in the lysogenic induction test. All this compounds were able to reduce the induction factor observed in the treatment with chapéu-de-couro, thus suggesting that the genotoxicity and mutagenicity were attributable to the production of reactive oxygen species that targeted DNA purines.

Enzymatic recognition of DNA damage induced by UVB-photosensitized titanium dioxide and biological consequences in Saccharomyces cerevisiae: evidence for oxidatively DNA damage generation

Although titanium dioxide (TiO(2)) has been considered to be biologically inert, finding use in cosmetics, paints and food colorants, recent reports have demonstrated that when TiO(2) is attained by UVA radiation oxidative genotoxic and cytotoxic effects are observed in living cells. However, data concerning TiO(2)-UVB association is poor, even if UVB radiation represents a major environmental carcinogen. Herein, we investigated DNA damage, repair and mutagenesis induced by TiO(2) associated with UVB irradiation in vitro and in vivo using Saccharomyces cerevisiae model. It was found that TiO(2) plus UVB treatment in plasmid pUC18 generated, in addition to cyclobutane pyrimidine dimers (CPDs), specific damage to guanine residues, such as 8-oxo-7,8-dihydroguanine (8-oxoG) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG), which are characteristic oxidatively generated lesions. In vivo experiments showed that, although the presence of TiO(2) protects yeast cells from UVB cytotoxicity, high mutation frequencies are observed in the wild-type (WT) and in an ogg1 strain (deficient in 8-oxoG and FapyG repair). Indeed, after TiO(2) plus UVB treatment, induced mutagenesis was drastically enhanced in ogg1 cells, indicating that mutagenic DNA lesions are repaired by the Ogg1 protein. This effect could be attenuated by the presence of metallic ion chelators: neocuproine or dipyridyl, which partially block oxidatively generated damage occurring via Fenton reactions. Altogether, the results indicate that TiO(2) plus UVB potentates UVB oxidatively generated damage to DNA, possibly via Fenton reactions involving the production of DNA base damage, such as 8-oxo-7,8-dihydroguanine.

Hydrogen peroxide induces a specific DNA base change profile in the presence of the iron chelator 2,2' dipyridyl in Escherichia coli

Pretreatment of Escherichia coli cultures with the iron chelator 2,2'-dipyridyl (1 mM) protects against the lethal effects of low concentrations of hydrogen peroxide (<15 mM). However, at H(2)O(2) concentrations equal to or greater than 15 mM, dipyridyl pretreatment increases lethality and mutagenesis, which is attributed to the formation of different types of DNA lesions. We show here that pretreatment with dipyridyl (1 mM) prior to challenge with high H(2)O(2) concentrations (>or=15 mM) induced mainly G:C-->A:T transitions (more than 100X with 15 mM and more than 250X with 20 mM over the spontaneous mutagenesis rate) in E. coli. In contrast, high H(2)O(2) concentrations in the absence of dipyridyl preferentially induced A:T-->T:A transversions (more than 1800X and more than 300X over spontaneous mutagenesis for 15 and 20 mM, respectively). We also show that in the fpg nth double mutant, the rpoB gene mutation (RifS-RifR) induced by 20 mM H(2)O(2) alone (20X higher) was increased in 20 mM H(2)O(2) and dipyridyl-treated cultures (110X higher), suggesting additional and/or different lesions in cells treated with H(2)O(2) under iron deprivation. It is suggested that, upon iron deprivation, cytosine may be the main damaged base and the origin of the pre-mutagenic lesions induced by H(2)O(2).

Assessment of antimutagenic and genotoxic potential of senna (Cassia angustifolia Vahl.) aqueous extract using in vitro assays

Senna (Cassia angustifolia Vahl.) is widely used as a laxative, although potential side effects, such as toxicity and genotoxicity, have been reported. This study evaluated genotoxic and mutagenic effects of senna aqueous extract (SAE) by means of four experimental assays: inactivation of Escherichia coli cultures; bacterial growth inhibition; reverse mutation test (Mutoxitest) and DNA strand break analysis in plasmid DNA. Our results demonstrated that SAE produces single and double strand breaks in plasmid DNA in a cell free system. On the other hand, SAE was not cytotoxic or mutagenic to Escherichia coli strains tested. In effect, SAE was able to avoid H(2)O(2)-induced mutagenesis and toxicity in Escherichia coli IC203 (uvrA oxyR) and IC205 (uvrA mutM) strains, pointing to a new antioxidant/antimutagenic action of SAE.

Comparative studies of phenotypic and genetic characteristics between two desulfurizing isolates of Rhodococcus erythropolis and the well-characterized R. erythropolis strain IGTS8

Two Rhodococcus erythropolis isolates, named A66 and A69, together with the well-characterized R. erythropolis strain IGTS8 were compared biochemically and genetically. Both isolates, like strain IGTS8, desulfurized DBT to 2-hydroxybiphenyl (2-HBP), following the 4S pathway of desulfurization. Strain IGTS8 showed the highest (81.5%) desulfurization activity in a medium containing DBT at 30 degrees C. Strain A66 showed approximately the same desulfurization activity either when incubated at 30 degrees C or at 37 degrees C, while strain A69 showed an increase of desulfurization efficiency (up to 79%) when incubated at 37 degrees C. Strains A66 and A69 were also able to grow using various organosulfur or organonitrogen-compounds as the sole sulfur or nitrogen sources. The biological responses of A66, A69 and IGTS8 strains to a series of mutagens and environmental agents were evaluated, trying to mimic actual circumstances involved in exposure/handling of microorganisms during petroleum biorefining. The results showed that strains A69 and IGTS8 were much more resistant to UVC treatment than A66. The three desulfurization genes (dszA, dszB and dszC) present in strains A66 and A69 were partially characterized. They seem to be located on a plasmid, not only in the strain IGTS8, but also in A66 and A69. PCR amplification was observed using specific primers for dsz genes in all the strains tested; however, no amplification product was observed using primers for carbazole (car) or quinoline (qor) metabolisms. All this information contributes to broaden our knowledge concerning both the desulfurization of DBT and the degradation of organonitrogen compounds within the R. erythropolis species.

Enhanced sensitivity of Escherichia coli uvrB mutants to mitomycin C points to a UV-C distinct repair for DNA adducts

Nucleotide excision repair (NER) in Escherichia coli repairs DNA by incising the damaged strand on the 3' and 5' sides of the lesion within pyrimidine dimers and DNA cross-linking adducts. Cross-linking adducts belong to a class of chemical damage to DNA that prevent strand separation, and thus, replication and transcription. For this reason, cross-linking agents such as mitomycin C (MC) have been used in cancer chemotherapy. The mechanisms involved in MC binding to DNA have already been defined; however, the repair of these lesions is not fully understood. Our goal was to study the repair of MC DNA lesions in E. coli cells. Several bacterial strains with specific mutations were tested for cellular inactivation and kinetics of DNA repair through analysis of DNA sedimentation profiles in alkaline sucrose gradients. The results obtained show that uvrB mutants are extremely sensitive to MC in contrast to the other isogenic uvrA and uvrC mutant strains. uvrB mutant strains are unable to repair DNA strand breaks produced by MC. Thus, UvrB might play a NER-uncoupled role in the repair of lesions induced by MC in vivo, different from its role on the repair of lesions produced by UV-C. Also it is suggested that a modified NER system is taking place in the repair of MC-adducts.

Characterization of Gordonia sp. strain F.5.25.8 capable of dibenzothiophene desulfurization and carbazole utilization

A dibenzothiophene (DBT)-degrading bacterial strain able to utilize carbazole as the only source of nitrogen was identified as Gordonia sp. F.5.25.8 due to its 16S rRNA gene sequence and phenotypic characteristics. Gas chromatography (GC) and GC-mass spectroscopy analyses showed that strain F.5.25.8 transformed DBT into 2-hydroxybiphenyl (2-HBP). This strain was also able to grow using various organic sulfur or nitrogen compounds as the sole sulfur or nitrogen sources. Resting-cell studies indicated that desulfurization occurs either in cell-associated or in cell-free extracts of F.5.25.8. The biological responses of F.5.25.8 to a series of mutagens and environmental agents were also characterized. The results revealed that this strain is highly tolerant to DNA damage and also refractory to induced mutagenesis. Strain F.5.25.8 was also characterized genetically. Results showed that genes involved in desulfurization (dsz) are located in the chromosome, and PCR amplification was observed with primers dszA and dszB designed based on Rhodococcus genes. However, no amplification product was observed with the primer based on dszC.

Interaction of stannous chloride leads to alteration in DNA, triphosphate nucleotides and isolated bases

Stannous chloride (SnCl2) is a reducing chemical agent used in several man-made products. SnCl2 can generate reactive oxygen species (ROS); therefore, studies have been carried out in order to better understand its damaging action in biological systems. In this work, calf thymus DNA, triphosphate nucleotides and isolated bases were incubated with SnCl2 and the results were analyzed through UV spectrophotometry. The presence of stannous ions altered the absorption spectra of all three isolates. The amount of stannous ions associated to DNA was measured by atomic absorption spectrophotometry. Data showed that more than 40% of the initial SnCl2 concentration was present in the samples. Our results are in accordance with the damaging potential of this salt and present evidence that stannous ions can complex with DNA, inducing ROS in its vicinity, which may be responsible for the observed lesions.

Endonuclease IV and exonuclease III are involved in the repair and mutagenesis of DNA lesions induced by UVB in Escherichia coli

Exonuclease III (Exo III) and endonuclease IV (Endo IV) play a critical role in the base excision repair (BER) of Escherichia coli. Both are endowed with AP endonucleolytic activity, cleaving the 5' phosphodiester bond adjacent to spontaneous or induced abasic sites in DNA. Although mutants defective in Exo III (xthA) are usually hypersensitive to oxidative agents such as hydrogen peroxide, near-UV-light and X-rays, mutants defective in Endo IV (nfo) are not as sensitive as the xthA strain. To further investigate the roles of these AP endonucleases in DNA repair, we evaluated the sensitivity and mutagenesis of xthA and nfo strains after UVB and compared with UVC light. Our results revealed that xthA but not nfo strain was hypersensitive to UVB. The use of Fe(+2) ion chelator (dipyridyl), prior to irradiation, completely protected the xthA mutant against UVB lethal lesions, suggesting the generation of toxic oxidative lesions mediated by transition metal reactions. The nfo strain displayed increased UVB-induced mutagenesis, which was significantly suppressed by pre-treatment with dipyridyl. Although xthA strain did not display increased mutagenesis after UVC and UVB treatments, this phenotype was not related to xthA mutation, but rather to an unknown secondary mutation specifying an antimutator phenotype. After UVB irradiation, the base substitution spectra of nfo strain revealed a bias towards AT-->GC transitions and GC-->CG transversions, which were also suppressed by previous treatment with the iron chelator. Overall, on the basis of the differential sensitivities and mutational spectra displayed after UVC and UVB treatments, we propose a role for Endo IV and Exo III to counteract DNA damage induced by the oxidative counterpart of UVB in E.coli.

Nitrogen mustard- and half-mustard-induced damage in Escherichia coli requires different DNA repair pathways

Bifunctional alkylating agents are used in tumor chemotherapy to induce the death of malignant cells through blockage of DNA replication. Nitrogen mustards are commonly used chemotherapeutic agents that can bind mono- or bifunctionally to guanines in DNA. Mustard HN1 is considered a monofunctional analog of bifunctional mustard HN2 (mechlorethamine). Escherichia coli K12 mutant strains deficient in nucleotide excision repair (NER) or base excision repair (BER) were submitted to increasing concentrations of HN2 or HN1, and the results revealed that damage induced by each chemical demands different DNA repair pathways. Damage induced by HN2 demands the activity of NER with a minor requirement of the BER pathway, while HN1 damage repair depends on BER action, without any requirement of NER function. Taken together, our data suggest that HN1 and HN2 seem to induce different types of damage, since their repair depends on distinct pathways in E. coli.

Reevaluating the role of 1,10-phenanthroline in oxidative reactions involving ferrous ions and DNA damage

It is widely believed that the iron chelator 1,10-phenanthroline (phen) is able to fully block the Fenton reaction by forming a complex (Fe(phen)3(2+), also known as ferroin) that cannot react with H2O2. We observed that phen cannot fully prevent 2-deoxyribose (5 mM) degradation induced by Fenton reagents (30 microM Fe(II) plus 100-500 microM H2O2); protection varied from 55% to 66% when the phen/Fe(II) ratio was 3:1 to 20:1. Inhibition of 2-deoxyribose damage was nearly unchanged if phen was pre-incubated with Fe(II). Moreover, preformed Fe(phen)3(2+) complex added to the solution containing H2O2 was able to induce 2-deoxyribose degradation and methane sulfinic acid formation from the oxidation of 5% DMSO. The partially protective effect of phen was unchanged with the use of either phosphate or HEPES as buffers (5 mM, pH 7.2), or in unbuffered media (pH 5.1). Both DMSO oxidation and 2-deoxyribose degradation correlated with the increase in Fe(phen)3(2+) concentration. Strand breaks in plasmid pTARGETtrade mark DNA induced by Fenton reagents (1 microM Fe(II) plus 25 microM H2O2) in HEPES buffer could only be partially prevented by phen, even when the chelator was 16 times more concentrated than Fe(II). In these experiments, Fe(phen)3(2+) and DNA were pre-incubated from 1 to 10 min before addition of H2O2. Moreover, a high level of DNA strand breakage was observed when iron and phen are added to the reaction immediately before H2O2. On the other hand, phen fully prevented 2-deoxyribose degradation induced by the autoxidation of 30 microM Fe(II) in phosphate-buffered (3 to 30 mM) media. Our data provide evidence that the Fe(phen)3(2+) complex induces in vitro oxidative damage in the presence of H2O2 (possibly by means of Fe(phen)3(2+) dissociation into Fe(phen)2(2+)), but they show that the complex cannot undergo autoxidation.

Role ofOGG1 andNTG2 in the repair of oxidative DNA damage and mutagenesis induced by hydrogen peroxide inSaccharomyces cerevisiae: relationships with transition metals iron and copper

The base excision repair pathway of Saccharomyces cerevisiae possesses three DNA N-glycosylases, viz. Ogg1p, Ngt1p and Ntg2p, involved in the repair of oxidative DNA damage. It was previously reported that inactivation of any of these activities, in most cases, did not generate a sensitive mutant phenotype to a variety of oxidative agents. Only the ntg1 mutant appeared to be more sensitive to hydrogen peroxide (H2O2) than a wild-type (WT) strain. In the present study we evaluated the role of S. cerevisiae OGG1 and NTG2 genes in the repair of oxidative lesions induced by high H2O2 concentrations (5-100 mM for 20 min), followed by catalase treatment (500 IU/ml). In these conditions, the ogg1 mutant was more sensitive than the WT strain to H2O2 (concentration 40-60 mM). Unexpectedly, the inactivation of NTG2 in an ogg1 background was able to suppress both sensitivity and mutagenesis induced by H2O2. Indeed, even the ntg2 single mutant was more resistant than the WT (60-100 mM H2O2). The use of metal ion chelators dipyridyl and neocuproine allowed us to evaluate the participation of iron and copper ions in the production of lethal and mutagenic lesions during H2O2 treatment in different DNA repair-deficient S. cerevisiae strains. The roles of OGG1 and NTG2 genes in the repair of lethal and mutagenic oxidative lesions induced by H2O2 and their relationships with iron and copper ions are discussed.

Does UVB radiation induce SoxS gene expression in Escherichia coli cells?

The SoxRS regulon is induced when bacterial cells are exposed to redox-cycling agents such as menadione or paraquat. In this paper it is shown that a physical agent, such as ultraviolet radiation with a wavelength of 312 nm (UVB) can induce soxS gene expression. The results indicate that this induction involves the RpoS protein. Moreover, an unexpected increase of soxS gene expression independent of a functional soxR gene in UVB-irradiated cells has been verified. This increase could be explained by transcription of soxS gene in a rpoS-dependent pathway.

Dna damage in peripheral blood nuclear cells assessed by comet assay from individuals submitted to scintigraphic examinations

Stannous chloride (SnCl2) is employed as a reducing agent to obtain Technetium-99m-labelled radiophamaceuticals in nuclear medicine kits, being injected endovenously in humans. Toxic effects of these kits were not studied, thus making it important to evaluate their impact in humans. In this study, the toxic effects were evaluated from peripheral blood nuclear cells (PBNC) from patients who received radiopharmaceuticals obtained using such kits. The analyses included results performed by comet assay. DNA damage was visualized in PBNC samples collected within a time up to 2 hr, and 24 hr after radiopharmaceutical injection in the patients. Initially we observed an increase of comet signals, which subsequently were reduced to zero after 24 hr. The diminishing of comet amounts probably is associated with DNA repair of damaged cells or with the elimination by apoptosis of cells whose DNA are not repaired.

Genotoxic effects of stannous chloride (SnCl2) in K562 cell line

The toxic effects of SnCl2 in K562 cells were analyzed in this study. This cell line is resistant to reactive oxygen species (ROS) making it suitable to evaluate the impact of SnCl2 in culture either through ROS or by direct toxicity using Trypan blue dye exclusion, comet and flow cytometry assays. An important loss of viability induced by SnCl2 in a dose-response manner was observed in cells treated in Tris-buffered saline (TBS). This necrotic cell death was further confirmed by flow cytometry. On the other hand, there was no loss of viability when cells were treated in rich medium (RPMI). DNA damage was visualized in SnCl2-treated K562 cells in both tested conditions. The data indicate that SnCl2 induces DNA damage and reduces K562 viability. Both actions seem to be correlated with ROS formation and direct linkage to DNA.

Photosensitization and mutation induced in Escherichia coli and Saccharomyces cerevisiae strains by dorstenin, a psoralen analog isolated from Dorstenia bahiensis

Dorstenin, 5-[3-(4,5-dihydro-5,5-dimethyl-4-oxo-2-furanyl)-butoxy]-7H-furo[3, 2-g] benzopyran-7-one, is a psoralen analog recently isolated from Dorstenia species (Moraceae). In order to characterize its biological activity, its photosensitizing and mutational properties were measured in wild-type E. coli and S. cerevisiae and also in strains carrying mutations which affect DNA repair. Compared to the high activities of psoralen and bergapten, dorstenin showed lower genotoxic effect.

Effects of low iron conditions on the repair of DNA lesions induced by Cumene hydroperoxide in Escherichia coli cells

In the present study, we evaluated the sensitivity of different Escherichia coli strains to Cumene hydroperoxide (CHP) treatment under distinct conditions of Fe2+ availability. Our results showed that the pretreatment with an iron chelator (dipyridyl) protects all the tested strains against CHP toxic effects, but it was not sufficient to abolish the CHP induced mutagenesis. On the other hand, simultaneous pretreatment with both dipyridyl and neocuproine (copper chelator) leads to a complete protection against CHP mutagenic effects. Our data suggest the participation of copper ion in the CHP mutagenesis induced in E. coli.

Nontoxic, mutagenic, and clastogenic activities of Mate-Chimarrão (Ilex paraguariensis)

Aqueous extracts of Ilex paraguarariensis (mate-chimarrão), a species that belongs to the Aquifoliaceae family, were analyzed for the presence of genotoxic, mutagenic, and clastogenic activities through bacterial trials based on the induction of the SOS functions, as well as in human lymphocytes in vitro and in mammalian cells in vivo. The extracts of mate-chimarrão were genotoxic, as assessed by lysogenic induction in Escherichia coli, and they also induced mutagenesis in Salmonella typhimurium. They addition of S9 microsomal fraction, catalase, thiourea, or dipyridyl counteracted the genotoxic activity of mate-chimarrão, suggesting that oxygen reactive species play an essential role in the genotoxicity of mate-chimarrão extracts. The extracts were not clastogenic in vivo (bone marrow cells of rats) in our experimental conditions, but we have observed an increased frequency of chromosomal aberrations in mate-chimarrão-treated human peripheral lymphocytes. Our results suggest that a high consumption of mate-chimarrão can potentiate carcinogenesis in the human oropharynx and esophagus.

Participation of stress-inducible systems and enzymes involved in BER and NER in the protection of Escherichia coli against cumene hydroperoxide

We studied the participation of the stress-inducible systems, as the OxyR, SoxRS and SOS regulons in the protection of Escherichia coli cells against lethal effects of cumene hydroperoxide (CHP). Moreover, we evaluated the participation of BER and NER in the repair of the DNA damage produced by CHP. Our results suggest that the hypersensitivity observed in the oxyR mutants to the lethal effect of CHP does not appear to be due to SOS inducing DNA lesions, but rather to cell membrane damage. On the other hand, DNA damage induced by CHP appears to be repaired by enzymes involved in BER and NER pathways. In this case, Fpg protein and UvrABC complex could be involved cooperatively in the elimination of a specific DNA lesion. Finally, we have detected the requirement for the uvrA gene function in SOS induction by CHP treatment.

Damage induced by stannous chloride in plasmid DNA

Stannous chloride (SnCl(2)) is widely used in daily human life, for example, to conserve soft drinks, in food manufacturing and biocidal preparations. In nuclear medicine, stannous chloride is used as a reducing agent of Technetium-99m, a radionuclide used to label different cells and molecules. In spite of this, stannous chloride is able to generate reactive oxygen species (ROS) which can damage DNA. In this work, plasmid DNA (pUC 9.1) was incubated with SnCl(2) under different conditions and the results analyzed through DNA migration in agarose gel electrophoresis. Our data reinforce the powerful damaging effect induced by stannous ion and suggest that this salt can play a direct role in inducing DNA lesions.

Copper ions mediate the lethality induced by hydrogen peroxide in low iron conditions in Escherichia coli

Iron ions mediate the formation of lethal DNA damage by hydrogen peroxide. However, when cells are depleted of iron ions by the treatment with iron chelators, DNA damage can still be detected. Here we show that the formation of such damage in low iron conditions is due to the participation of copper ions. Copper chelators can inhibit cell inactivation, DNA strand breakage and mutagenesis induced by hydrogen peroxide in cells pre-treated with iron chelators. The Fpg and UvrA proteins play an important role in the repair of DNA lesions formed in these conditions, as suggested by the great sensitivity of the uvrA and fpg mutant strains to the treatment when compared to the wild type strain.

Repair of DNA lesions induced by hydrogen peroxide in the presence of iron chelators in Escherichia coli: participation of endonuclease IV and Fpg

In Escherichia coli, the repair of lethal DNA damage induced by H(2)O(2) requires exonuclease III, the xthA gene product. Here, we report that both endonuclease IV (the nfo gene product) and exonuclease III can mediate the repair of lesions induced by H(2)O(2) under low-iron conditions. Neither the xthA nor the nfo mutants was sensitive to H(2)O(2) in the presence of iron chelators, while the xthA nfo double mutant was significantly sensitive to this treatment, suggesting that both exonuclease III and endonuclease IV can mediate the repair of DNA lesions formed under such conditions. Sedimentation studies in alkaline sucrose gradients also demonstrated that both xthA and nfo mutants, but not the xthA nfo double mutant, can carry out complete repair of DNA strand breaks and alkali-labile bonds generated by H(2)O(2) under low-iron conditions. We also found indications that the formation of substrates for exonuclease III and endonuclease IV is mediated by the Fpg DNA glycosylase, as suggested by experiments in which the fpg mutation increased the level of cell survival, as well as repair of DNA strand breaks, in an AP endonuclease-null background.

Non-coherent visible and infrared radiation increase survival to UV (254 nm) in Escherichia coli K12

Interactions between visible or infrared (IR) and ultraviolet (UV, 254 nm) radiation have been studied in E. coli. Pre-illumination with non-coherent monochromatic 446, 466, 570 and 685 nm radiation, as well as with polychromatic red and IR radiation at room temperature, leads to increased cell survival after a subsequent irradiation with UV light. In the thermic range of the spectrum (red and IR), IR but not red light pre-treatment is able to increase cell survival to a subsequent lethal heat (51 degrees C) challenge, suggesting that increased UV survival may be due to IR-induced heat-shock response. On the other hand, visible-light-induced resistance may be due to a different mechanism, possibly involved with unknown bacterial light receptors.

H2O2-induced cross-protection against UV-C killing in Escherichia coli is blocked in a lexA (Def) background

Pretreatment with 2.5 mM H2O2 protects E. coli cells against UV-C killing, a phenomenon independent of LexA cleavage. In this paper, we observe that this cross-protection response is neither dependent on the dinY gene product nor on the system that controls dinY, since H2O2 is able to induce cross-protection but not to induce the dinY gene in a lexA-noninducible strain [lexA (Ind-)]. Moreover, this response is not induced in a lexA (Def) background, suggesting that the expression of the SOS regulon may inhibit this cross-protection response.

Hydrogen peroxide effects in Escherichia coli cells

We analyzed DNA lesions produced by H2O2 under low iron conditions, the cross adaptive response and the synergistic lethal effect produced by iron chelator-o-phenanthroline, using different Escherichia coli mutants deficient in DNA repair mechanisms. At normal iron levels the lesions produced by H2O2 are repaired mainly by the exonuclease III protein. Under low iron conditions we observed that the Fpg and UvrA proteins as well as SOS and OxyR systems participate in the repair of these lesions. The lethal effect of H2O2 is strengthened by o-phenanthroline if both compounds are added simultaneously to the culture medium. This phenomenon was observed in the wild type cells and in the xthA mutant (hypersensitive to H2O2). E. coli cells treated with low concentrations of H2O2 (micromolar) acquire resistance to different DNA damaging agents. Our results indicate also that pretreatment with high (millimolar) H2O2 concentrations protects cells against killing, by UV and this phenomenon is independent of the SOS system, but dependent on RecA and UvrA proteins. H2O2 induces protection against lethal and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). H2O2 also protects the cells against killing by cumene hydroperoxide, possibly with the participation of Ahp protein.

Synergistic lethal effect between hydrogen peroxide and neocuproine (2,9-dimethyl 1,10-phenanthroline) in Escherichia coli

Despite 2,9-dimethyl 1,10-phenanthroline (NC) has been extensively used as a potential inhibitor of damage due to oxidative stress in biological systems, the incubation of E. coli cultures with the copper ion chelator NC prior to the challenge with hydrogen peroxide caused a lethal synergistic effect. The SOS response seems to be involved in the repair of the synergistic lesions through the recombination pathway. Furthermore, there is evidence for the UvrABC excinuclease participation in the repair of the synergistic lesions, and the base excision repair may also be required for bacterial survival to the synergistic effect mainly at high concentrations of H2O2, being the action of Fpg protein an important event. Incubation of lexA (Ind-) cultures with iron (II) ion chelator 2,2'-dipyridyl simultaneously with NC prevented the lethal synergistic effect. This result suggests an important role of the Fenton reaction on the phenomenon. NC treatment was able to increase the number of DNA strand breaks (DNAsb) induced by 10 mM of H2O2 in lexA (Ind-) strain and the simultaneous treatment with 2,2'-dipyridyl was able to block this effect.

Hydrogen peroxide effects in Escherichia coli cells

We analyzed DNA lesions produced by H2O2 under low iron conditions, the cross adaptive response and the synergistic lethal effect produced by iron chelator-o-phenanthroline, using different Escherichia coli mutants deficient in DNA repair mechanisms. At normal iron levels the lesions produced by H2O2 are repaired mainly by the exonuclease III protein. Under low iron conditions we observed that the Fpg and UvrA proteins as well as SOS and OxyR systems participate in the repair of these lesions. The lethal effect of H2O2 is strengthened by o-phenanthroline if both compounds are added simultaneously to the culture medium. This phenomenon was observed in the wild type cells and in the xthA mutant (hypersensitive to H2O2). E. coli cells treated with low concentrations of H2O2 (micromolar) acquire resistance to different DNA damaging agents. Our results indicate also that pretreatment with high (millimolar) H2O2 concentrations protects cells against killing, by UV and this phenomenon is independent of the SOS system, but dependent on RecA and UvrA proteins. H2O2 induces protection against lethal and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). H2O2 also protects the cells against killing by cumene hydroperoxide, possibly with the participation of Ahp protein.

Mutational potentiality of stannous chloride: an important reducing agent in the Tc-99m-radiopharmaceuticals

Stannous chloride (SnCl2) is frequently used in nuclear medicine as a reducing agent to label many radiopharmaceutical products with technetium-99m (99mTc). The aim of the present paper was to study the role of DNA repair genes in the repair of SnCl2-induced damage, using mutant strains of Escherichia coli lacking one or more DNA repair genes. Our results suggest that the product of the xthA gene, exonuclease III, is required for the repair of lesions induced by SnCl2. We further investigated the mutagenic properties of SnCl2 to a molecular level by using the supF tRNA gene as target in a forward mutational system. We have found that the survival of E. coli cells was strongly reduced with increasing concentrations of SnCl2. Moreover, when the shuttle vector pAC189 carrying the supF gene was treated with SnCl2, and then transfected to E. coli, we observed that its transformation efficiency dropped when compared to the non-treated control, with a parallel increase in mutation frequency after the damaged plasmids have replicated in bacterial cells. The mutation spectrum induced by SnCl2 reveals a high frequency of base substitutions, involving guanines. Sequence analysis of 41 independent supF mutant plasmids revealed that 39 mutants contained base substitutions, with 21 G:C to T:A and 17 G:C to C:G transversions. G to T transversions presumably resulted from 8-oxoG. However, the G to C one may be due to a yet unidentified lesion.

Hydrogen peroxide induces protection against lethal effects of cumene hydroperoxide in Escherichia coli cells: an Ahp dependent and OxyR independent system?

Pretreatment with 2.5 mM H2O2 protects bacterial cells against cumene hydroperoxide killing. This response is independent of the OxyR system, but possibly involves the participation of Ahp protein, since ahp mutants are not protected. Treatment of bacterial cells with high H2O2 concentrations caused an alteration on the electrophoretic profile of the smaller subunit (22-kDa) of Ahp. This alteration does not require novel gene products and is not dependent on the OxyR protein. In this way, we propose that the modification of the 22-kDa subunit of Ahp by high H2O2 concentration may be responsible for the protection against the lethal effects of cumene hydroperoxide.

Effects of 1,10-phenanthroline and hydrogen peroxide in Escherichia coli: lethal interaction

It has been observed that when Escherichia coli cells are treated simultaneously with phenanthroline and H2O2, there is a lethal interaction. In order to analyze the mechanism of this lethal interaction, wild-type and xthA mutant cells of E. coli were treated with 2.5 mM H2O2 and 1 mM phenanthroline. This treatment was preceded by treatments with different metal chelators (dipyridyl for Fe2+, desferal for Fe3+ and neocuproine for Cu2+) or conducted simultaneously to other treatments with chelators and radical scavengers (thiourea, ethanol and sodium benzoate). The lethal interaction was observed in both the E. coli wild-type strain and xthA mutant strain, which is deficient in the exonuclease III repair enzyme. Nevertheless, the mutant strain was much more sensitive than the wild-type one. Dipyridyl pretreatment protected the cells against the lethal interaction, while desferal pretreament was unable to do so. This suggests that the lethal interaction requires Fe2+ and not Fe3+ ions. Ethanol and sodium benzoate were incapable of protecting bacterial cells against the lethal interaction. Even a 20-min pretreatment with benzoate did not confer protection. On the other hand, thiourea protected the cells completely. Based on our results, we propose that the lethal interaction may be caused not only by the reaction kinetics of phenanthroline and Fe, but also by the ability of phenanthroline to intercalate in DNA. After forming the mono and bis complexes, phenanthroline would serve as a shuttle and take the Fe2+ ions to the DNA. So, the Fenton reaction would take its course with the consequent generation of OH. radicals near DNA. This proximity to the DNA would protect the OH. radicals against the scavengers' action, thus optimizing the Fenton reaction.

Role of SOS and OxyR systems in the repair of Escherichia coli submitted to hydrogen peroxide under low iron conditions

There are at least two mechanisms by which H2O2 induces DNA lesions in Escherichia coli: one in the presence of physiological iron levels and the other in low iron conditions. The survival as well as the induction of SOS response in different DNA repair mutant strains of E coli was evaluated after H2O2 treatment under low iron conditions (pretreatment with an iron chelator). Our results indicate that, in normal iron conditions RecA protein has a relevant role in recombination repair events, while in low iron conditions RecA protein is important as a positive regulator of the SOS response. On the other hand, the oxy delta R mutant is sensitive to the lethal effects of H2O2 only in low iron conditions and this sensitivity cannot be correlated with DNA strand breaks.

Hydrogen peroxide induces protection against N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) effects in Escherichia coli

Cross-adaptive response is defined as the capacity of cells to become resistant to a lethal agent when pretreated with a different lethal substance. In the present paper, the cross-adaptive response between hydrogen peroxide and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was studied in Escherichia coli repair mutants. Our results suggest that high doses of H2O2 induces protection against the lethal effects of MNNG in wild-type strain, ada, ogt, ada-ogt, aidB and alkA mutants. On the other hand, the MNNG induced mutagenesis is reduced by H2O2 pretreatment in wild-type and ogt mutant strains, but not in ada mutant. Furthermore, the protecting effect induced by H2O2 is time dependent: it decreases 15 min after the pretreatment and, after 30 min, is almost abolished. This reduction in the protecting effect is followed by an augmentation in the mutation frequency when MNNG is added 30 min after H2O2 pretreatment. This cross-adaptive response may be due to a modification of the MNNG alkylation pattern in the oxidized DNA.

Fpg and UvrA proteins participate in the repair of DNA lesions induced by hydrogen peroxide in low iron level in Escherichia coli

The survival of different DNA repair mutant strains of Escherichia coli treated with H2O2 was evaluated in the presence or absence of an iron chelator (dipyridyl). Our results suggest that Fpg and UvrA proteins participate in vivo in the repair of DNA lesions produced by higher H2O2 concentrations in the presence of an iron chelator while UvrB and UvrC proteins seem to be ineffective in the repair of these lesions.

Lethal interaction between hydrogen peroxide and o-phenanthroline in Escherichia coli

The iron chelator o-phenanthroline enhances the lethal effect of H2O2 about four hundred times in Escherichia coli when both substances are added simultaneously to the culture medium. If o-phenanthroline is added for increasing periods of time prior to the addition of H2O2, there is a shift from this lethal interaction to protection by the chelator about seven hundred times. It is known that the Fe(2+)-o-phenanthroline(I) and Fe(2+)-o-phenanthroline(II) complexes are formed quickly whereas the final and more stable Fe(2+)-o-phenanthroline(III) complex is formed slowly. Moreover, the mono and bis complexes react with H2O2 to produce OH., whereas the tris complex is stable towards H2O2. Therefore, the lethal effect could be explained by the kinetics of reaction of o-phenanthroline with intracellular Fe2+, i.e., the mono and bis complexes are more reactive than intracellular Fe2+.

Membrane permeability and sensitivity to lethal heat are affected by lexA and recA mutations in Escherichia coli K12

Membrane permeability was evaluated in several SOS-deficient strains. Great heat sensitivity was observed in all the lexA (Ind-) strains, which was associated to an increase in membrane permeability (up to 120% increase above the wild-type control), as assayed by the crystal violet (CV) growth inhibition. After irradiation with a single UV dose (75 J.m-2 delivered to wild-type and 2 J.m-2 to the lexA3 strain), survival was followed by plating cells in both nutrient and membrane permeability-selective (nutrient + CV) media and a great lethality due to CV was observed in a lexA mutant, which appeared to be about 100 times more sensitive to CV compared to its wild-type parent strain. The decreased membrane integrity found in the lexA-deficient strains suggests that LexA protein and/or LexA-repressed genes may interact with the bacterial membrane, which could be the location of SOS events.

Porcine immune responses to African swine fever virus (ASFV) infection

Immune responses mediating protection against ASFV are poorly understood. Anti-ASFV antibodies may influence the course of the clinical disease but they have never been found to neutralize the virus. Recent developments on cellular defense mechanisms, using swine protection models, and on the induction and role of some cytokines warrant further investigation on these areas.

Mutagenic and genotoxic effects of mate (Ilex paraguariensis) in prokaryotic organisms

1. The mutagenic and genotoxic effects of mate (Ilex paraguariensis) aqueous solutions were analyzed in bacterial cells. 2. Mate solutions showed mutagenic activity in the Ames test (TA97, TA98, TA100 and TA102 strains) at concentrations of 20 to 50 mg/plate (mutagenic factors of 3.5 to 5.6) and genotoxic activity in the inductest (WP2s (lambda) strain), with a maximal phage induction at concentrations of 10 to 20 mg/plate. Above these concentrations the mate solutions were cytotoxic. 3. Addition of 5 U/ml catalase, 20 microliters/ml S9 rat liver microsomal fraction, 100 mM thiourea or 10 mM dipyridyl completely inhibited the lysogenic induction produced by mate; however, the addition of 1,000 U/ml superoxide dismutase was almost ineffective. 4. Oxygen reactive species may be present in mate solutions playing an essential role in its genotoxicity.

Genotoxic and mutagenic effects of guarana (Paullinia cupana) in prokaryotic organisms

Aqueous extracts of Paullinia cupana (guarana), a species that belongs to the Sapindaceae family, were analyzed for the presence of genotoxic activities in bacterial cells. The extracts of guarana were genotoxic as assessed by lysogenic induction in Escherichia coli and they were also able to induce mutagenesis in Salmonella typhimurium. Addition of S9 microsomal fraction, catalase, superoxide dismutase or thiourea counteracted the genotoxic activity of guarana, suggesting that oxygen reactive species play an essential role in the genotoxicity of aqueous guarana extracts. The genotoxic activity in the extracts was related to the presence of a molecular complex formed by caffeine and a flavonoid (catechin or epicatechin) in the presence of potassium.

Inhibition and induction of SOS responses in Escherichia coli by cobaltous chloride

Mutagenesis induced by several genotoxic agents has been reported to be inhibited by cobaltous chloride. In order to study the effects of this metal in some SOS functions we evaluated mutagenesis, lysogenic induction and phage reactivation in Escherichia coli cells treated with CoCl2. We detected that cobaltous chloride, when present in the plating medium, was able to block mutagenesis and lysogenic induction promoted by UV irradiation. We also found that CoCl2 blocked protein synthesis, so we propose that this effect can be responsible for the antimutagenic and antilysogenic effects of this metal. On the other hand, if the cells were treated for a short period of time with CoCl2, in the absence of Mg, we observed that cobaltous chloride per se was able to promote lysogenic induction as well as to enhance the phage reactivation induced by UV irradiation. We conclude that depending on experimental conditions, cobaltous chloride may act either as an inhibitor or as an inducer of the SOS functions.

Effects of metal ion chelators on DNA strand breaks and inactivation produced by hydrogen peroxide in Escherichia coli: detection of iron-independent lesions

In order to study the role of metallic ions in the H2O2 inactivation of Escherichia coli cells, H2O2-sensitive mutants were treated with metal ion chelators and then submitted to H2O2 treatment. o-Phenanthroline, dipyridyl, desferrioxamine, and neocuproine were used as metal chelators. Cell sensitivity to H2O2 treatment was not modified by neocuproine, suggesting that copper has a minor role in OH production in E. coli. On the other hand, prior treatment with iron chelators protected the cells against the H2O2 lethal effect, indicating that iron participates in the production of OH. However, analysis of DNA sedimentation profiles and DNA degradation studies indicated that these chelators did not completely block the formation of DNA single-strand breaks by H2O2 treatment. Thiourea, a scavenger of OH, caused a reduction in both H2O2 sensitivity and DNA single-strand break production. The breaks observed after treatment with metal chelators and H2O2 were repaired 60 min after H2O2 elimination in xthA but not polA mutant cells. Therefore, we propose that there are at least two pathways for H2O2-induced DNA lesions: one produced by H2O2 through iron oxidation and OH production, in which lesions are repaired by the products of the xthA and polA genes, and the other produced by an iron-independent pathway in which DNA repair requires polA gene products but not those of the xthA gene.