The SOS chromotest: direct assay of the expression of gene sfiA as a measure of genotoxicity of chemicals

In Vitro Modulation of Genotoxicity and Oxidative Stress by Polyphenol-Rich Fraction of Chinese Ladder Brake (Pteris vittata L.)

Pteris vittata L. is a terrestrial genus growing in moist, shady forests and on hillsides. The plant has considerable ethnomedicinal importance. Investigations have been carried out on chemical profiling and antioxidant compounds from some genera of pteridophytes but studies on the biological properties of P. vittata are lacking. Therefore, the present study investigates antioxidant, antigenotoxic, and antiproliferative potential of the aqueous fraction of P. vittata (PWE). A battery of assays were carried out to assess the antioxidant potential of the PWE. SOS chromotest and DNA nicking assay were used to evaluate the antigenotoxicity of the fraction. The cytotoxic effect of PWE was analyzed using MTT and Neutral Single Cell Gel Electrophoresis comet assay. EC50 of 90.188 µg/ml, 80.13 µg/ml, 142.836 µg/ml, and 12.274 µg/ml was obtained in DPPH, superoxide anion scavenging, reducing power and lipid peroxidation assays, respectively. PWE was potent in inhibiting Fenton's reagent-induced nicking of pBR322 plasmid. The fraction significantly inhibited hydrogen peroxide (H2O2) and 4-nitroquinoline-N-oxide (4NQO) induced mutagenicity and a reduction in induction factor was found with increased PWE concentration. GI50 of 147.16 µg/ml was obtained in MTT assay in human MCF-7 breast cancer cell line. PWE induced apoptosis as confirmed from confocal microscopy studies. The protective effects can be attributed to the presence of the phytochemicals in PWE. These results will be helpful in the development of functional food characteristics, as well as unravel the benefits of pteridophytes as promoters of health.

Effect-directed analysis supporting monitoring of aquatic environments--An in-depth overview

Aquatic environments are often contaminated with complex mixtures of chemicals that may pose a risk to ecosystems and human health. This contamination cannot be addressed with target analysis alone but tools are required to reduce this complexity and identify those chemicals that might cause adverse effects. Effect-directed analysis (EDA) is designed to meet this challenge and faces increasing interest in water and sediment quality monitoring. Thus, the present paper summarizes current experience with the EDA approach and the tools required, and provides practical advice on their application. The paper highlights the need for proper problem formulation and gives general advice for study design. As the EDA approach is directed by toxicity, basic principles for the selection of bioassays are given as well as a comprehensive compilation of appropriate assays, including their strengths and weaknesses. A specific focus is given to strategies for sampling, extraction and bioassay dosing since they strongly impact prioritization of toxicants in EDA. Reduction of sample complexity mainly relies on fractionation procedures, which are discussed in this paper, including quality assurance and quality control. Automated combinations of fractionation, biotesting and chemical analysis using so-called hyphenated tools can enhance the throughput and might reduce the risk of artifacts in laboratory work. The key to determining the chemical structures causing effects is analytical toxicant identification. The latest approaches, tools, software and databases for target-, suspect and non-target screening as well as unknown identification are discussed together with analytical and toxicological confirmation approaches. A better understanding of optimal use and combination of EDA tools will help to design efficient and successful toxicant identification studies in the context of quality monitoring in multiply stressed environments.

Type III Secretion-Dependent Sensitivity of Escherichia coli O157 to Specific Ketolides

A subset of Gram-negative bacterial pathogens uses a type III secretion system (T3SS) to open up a conduit into eukaryotic cells in order to inject effector proteins. These modulate pathways to enhance bacterial colonization. In this study, we screened established bioactive compounds for any that could repress T3SS expression in enterohemorrhagic Escherichia coli (EHEC) O157. The ketolides telithromycin and, subsequently, solithromycin both demonstrated repressive effects on expression of the bacterial T3SS at sub-MICs, leading to significant reductions in bacterial binding and actin-rich pedestal formation on epithelial cells. Preincubation of epithelial cells with solithromycin resulted in significantly less attachment of E. coli O157. Moreover, bacteria expressing the T3SS were more susceptible to solithromycin, and there was significant preferential killing of E. coli O157 bacteria when they were added to epithelial cells that had been preexposed to the ketolide. This killing was dependent on expression of the T3SS. Taken together, this research indicates that the ketolide that has accumulated in epithelial cells may traffic back into the bacteria via the T3SS. Considering that neither ketolide induces the SOS response, nontoxic members of this class of antibiotics, such as solithromycin, should be considered for future testing and trials evaluating their use for treatment of EHEC infections. These antibiotics may also have broader significance for treating infections caused by other pathogenic bacteria, including intracellular bacteria, that express a T3SS.

Escherichia coli as a bioreporter in ecotoxicology

Ecotoxicological assessment relies to a large extent on the information gathered with surrogate species and the extrapolation of test results across species and different levels of biological organisation. Bacteria have long been used as a bioreporter for genotoxic testing and general toxicity. Today, it is clear that bacteria have the potential for screening of other toxicological endpoints. Escherichia coli has been studied for years; in-depth knowledge of its biochemistry and genetics makes it the most proficient prokaryote for the development of new toxicological assays. Several assays have been designed with E. coli as a bioreporter, and the recent trend to develop novel, better advanced reporters makes bioreporter development one of the most dynamic in ecotoxicology. Based on in-depth knowledge of E. coli, new assays are being developed or existing ones redesigned, thanks to the availability of new reporter genes and new or improved substrates. The technological evolution towards easier and more sensitive detection of different gene products is another important aspect. Often, this requires the redesign of the bacterium to make it compatible with the novel measuring tests. Recent advances in surface chemistry and nanoelectronics open the perspective for advanced reporter based on novel measuring platforms and with an online potential. In this article, we will discuss the use of E. coli-based bioreporters in ecotoxicological applications as well as some innovative sensors awaited for the future.

New method for evaluation of genotoxicity, based on the use of real-time PCR and lysogenic gram-positive and gram-negative bacteria

A method for the detection of the SOS response as measured by the liberation of resident prophages from the genomes of their hosts is described. It is based on the use of two converging oligonucleotides that flank the attP attachment site of the phage as primers for real-time PCR. Amplification was observed only after the phage DNA became excised. The system responds to both chemicals and physical conditions. Quantitative data on the concentration and/or potency of the genotoxic condition were obtained. Results can be achieved within 1 day and are less susceptible to possible toxic effects than phage generation or other methods that require DNA synthesis. The use of both gram-positive and gram-negative bacteria widens the range of compounds that can be tested because it eliminates impermeability problems derived from the particular composition of each cell wall type.

Gene expression profile analysis: an emerging approach to investigate mechanisms of genotoxicity

The response to stress triggers transcriptional activation of genes involved in cell survival and/or cell death. Thus, the monitoring of gene expression levels in large gene sets or whole genomes in response to various agents (toxicogenomics) has been proposed as a tool for investigating mechanisms of toxicity. Although standard in vitro genetic toxicity testing provides relatively simple and accurate hazard detection, interpretation of positive findings, i.e., in vitro chromosome aberrations, in terms of relevant risk to humans is difficult, due to the limited insight into the underlying mechanisms. Therefore, the development of experimental approaches capable of differentiating a wide range of genotoxic mechanisms is expected to significantly improve risk assessment. The goal of this review is to summarize current developments in toxicogenomic analysis of genotoxic stress, and to provide a perspective on the application of gene expression profile analysis in genetic toxicology.

Differentiation of DNA reactive and non-reactive genotoxic mechanisms using gene expression profile analysis

Genotoxic stress triggers a variety of biological responses including the transcriptional activation of genes regulating DNA repair, cell survival and cell death. Here, we investigated whether gene expression profiles can differentiate between DNA reactive and DNA non-reactive mechanisms of genotoxicity. We analyzed gene expression profiles and micronucleus levels in L5178Y cells treated with cisplatin and sodium chloride. The assessment of cisplatin genotoxicity (up to six-fold increase in the number of micronuclei) and gene expression profile (increased expression of genotoxic stress-associated genes) was in agreement with cisplatin mode of action as a DNA adduct-forming agent. The gene expression profile analysis of cisplatin-treated cells identified a number of genes with robust up regulation of mRNA expression including genes associated with DNA damage (i.e. members of GADD45 family), early response (i.e. cFOS), and heat shock protein (i.e. HSP40 homologue). The gene expression changes correlated well with DNA damage as measured by DNA-protein crosslinks and platinum-DNA binding. To differentiate the genotoxic stress-associated expression profile of cisplatin from a general toxic stress, we have compared the gene expression profile of cisplatin-treated cells to cells treated with sodium chloride, which causes osmotic shock and cell lysis. Although the sodium chloride treatment caused a two-fold induction of micronuclei, the gene expression profile at equitoxic concentrations was remarkably distinct from the profile observed with cisplatin. The profile of sodium chloride featured a complete lack of expression changes in genes associated with DNA damage and repair. In summary, the gene expression profiles clearly distinguished between DNA reactive and non-reactive genotoxic mechanisms of cisplatin and sodium chloride. Our results suggest the potential utility of gene expression profile analysis for elucidating mechanism of action of genotoxic agents.

The genotoxicity of priority polycyclic aromatic hydrocarbons in complex mixtures

Risk assessment of complex environmental samples suffers from difficulty in identifying toxic components, inadequacy of available toxicity data, and a paucity of knowledge about the behavior of geno(toxic) substances in complex mixtures. Lack of information about the behavior of toxic substances in complex mixtures is often avoided by assuming that the toxicity of a mixture is simply the sum of the expected effects from each mixture component, i.e. no synergistic or antagonistic interactions. Although this assumption is supported by research investigating non-genotoxic end-points, the literature describing the behavior of genotoxic substances in complex mixtures is sparse and, occasionally, contradictory. In this study, the results of polycyclic aromatic hydrocarbon (PAH) analyses on freshwater bivalves were used to prepare realistic mixtures containing up to 16 PAHs. The SOS genotoxicity of the mixtures and each component were then assessed in an effort to evaluate the additivity of PAH genotoxicity. At nominal PAH concentrations above 1 microg/ml, observed genotoxic responses were far lower than those predicted under the assumption of additivity. At nominal concentrations below 0.75 microg/ml, differences are smaller and occasionally negligible, indicating that the genotoxicity of unsubstituted homocyclic PAHs is additive or slightly less than additive. Other researchers who have investigated the mutagenicity, carcinogenicity, and DNA binding activity of mixtures containing unsubstituted homocyclic PAHs have also reported additive effects. Therefore, the mutagenic risk posed by simple, well-characterized mixtures of priority PAHs can reasonably be estimated as the sum of the risks posed by the mixture components. Current data indicate that less-than-additive effects likely result from saturation of metabolic pathways needed to activate mutagenic PAHs.

SOS induction of selected naturally occurring substances in Escherichia coli (SOS chromotest)

Naturally occurring substances were tested for genotoxicity using a modified laboratory protocol of the Escherichia coli PQ37 genotoxicity assay (SOS chromotest) in the presence and in the absence of an exogenous metabolizing system from rat liver S9-mix. Aristolochic acid I, II, the plant extract aristolochic acid and psoralene were genotoxic; cycasine, emodine, monocrotaline and retrorsine were classified as marginal genotoxic in the SOS chromotest in the absence of S9-mix. In the presence of an exogenous metabolizing system from rat liver S9-mix aristolochic acid I, the plant extract, beta-asarone, cycasin, monocrotaline, psoralen and retrorsine showed genotoxic effects; aristolochic acid II marginal genotoxic effects. Arecoline, benzyl acetate, coumarin, isatidine dihydrate, reserpine, safrole, sanguinarine chloride, senecionine, senkirkine, tannin and thiourea revealed no genotoxicity in the SOS chromotest either in the presence or in the absence of an exogenous metabolizing system from rat liver S9-mix. For 17 of 20 compounds, the results obtained in the SOS chromotest could be compared to those obtained in the Ames test. It was found that 12 (70.6%) of these compounds give similar responses in both tests (6 positive and 6 negative responses). The present investigation and those reported earlier, the SOS chromotest, using E. coli PQ37, was able to detect correctly most of the Salmonella mutagens and non-mutagens.

Impact of dimethyl sulfoxide and examples of combined genotoxicity in the SOS chromotest

The bacterial SOS chromotest with Escherichia coli PQ37 was used for the assessment of genotoxicity of combined xenobiotic treatments. The modulation of test compound genotoxicity by dimethyl sulfoxide (DMSO), a common solvent for test compounds, was assessed as well. It was shown that DMSO modulated SOS chromotest genotoxicity of several xenobiotics: in comparison to test compound dissolution in water, the commonly used addition of 3.2% (v/v) DMSO as solvent lead to a significant increase in the genotoxicity of K(2)RhCl(5) and beta-propiolactone (BPL). However, the effects of cisplatin decreased significantly when DMSO was added. Thus, albeit DMSO is not genotoxic in this test itself, it can interfere with SOS chromotest responses. Further experiments were performed in the absence of DMSO. BPL and cisplatin in combination showed an over-additive synergism in SOS genotoxicity as well as K(2)RhCl(5) and cisplatin did. Addition of Pd(NH(3))(4)Cl(2) and NaAsO(2), which are non-genotoxic in the SOS chromotest, did not enhance the K(2)RhCl(5)- or BPL-mediated SOS sfiA induction. Nevertheless, at the highest subcytotoxic dose of NaAsO(2) tested (200 microM), a slight yet significant suppression of BPL-mediated SOS genotoxicity was observed. These results confirm that the SOS chromotest is a useful tool for the rapid evaluation of the combined genotoxicity of compound mixtures. However, the use of DMSO as test solvent has to be taken with caution.

Saturation mutagenesis of the E. coli RecA loop L2 homologous DNA pairing region reveals residues essential for recombination and recombinational repair

The disordered mobile loop L2 of the Escherichia coli RecA protein is known to play a central role in DNA binding and pairing. To investigate the local chemical environment in relation to function we performed saturation mutagenesis of the loop L2 region (amino acid positions 193-212) using a site-directed mutagenesis procedure, and determined the recombinational proficiency of the 380 mutants using genetic assays for homologous recombination and recombinational repair. Residues Asn193, Gln194, Arg196, Glu207, Thr209, Gly211, and Gly212 were identified as stringently required for recombinational events in bacterial cells. In addition, our findings suggest the involvement of loop L2 in the ATPase activity of RecA, and a role for residues Gln194, Arg196, Lys198 and Thr209 in the DNA-dependent hydrolysis of ATP. Finally, since 20 residue peptides that comprise this region can pair homologous DNAs by forming filamentous beta-structures, we propose how the information from the mutant analysis might facilitate the use of a simplified amino acid alphabet to design beta-structure forming L2 peptides with improved RecA-like activities.

Radioprotective effect of sodium diethyldithiocarbamate (DDC) and S-2-aminoethyl-isothioronicadenosin-5-triphosphate (adeturon) in gamma-irradiated Escherichia coli cells

The effect of sodium diethyldithiocarbamate (DDC) and S-2-aminoethyl-isothiouronicadenosin-5-triphosphate (adeturon) in the induction of Escherichia coli SOS response promoted by gamma-irradiation was studied by measuring the induction of sulA gene and the induction of lambda prophage. Furthermore, as a way of measure the exonuclease activity in gamma-irradiated cells in the presence or absence of both compounds, the DNA degradation was determined. Adeturon did not affected DNA degradation, but inhibited the induction of the SOS functions studied. On the contrary, DDC inhibited DNA degradation as well as the induction of the sulA gene, but enhanced lambda induction in E. coli lysogenic strains. These results indicate that both compounds diminish the DNA damage produced by gamma-irradiation and also suggest that the mechanisms of radioprotection must be different. Thus, radioprotection mediated by DDC should involve free hydroxyl radical scavenging and a minor activity of exonuclease. The enhancement of phage induction in E. coli cells that DDC produces could be attributed to its quelant effect and this would not be not probably directly related to radioprotection. Adeturon, as thiols, may serve also as scavenging agent of free hydroxyl radicals, diminishing indirectly the DNA damage level. In addition, adeturon must interact with DNA in the same form that other aminothiol compounds do it. This interaction, mediated by amino groups of adeturon, may serve to concentrate these compounds near of the DNA damage site, increasing the potential for the thiol portion of the molecule to donate hydrogen, decreasing the damage level on DNA molecule. However, adeturon do not modify the exonuclease activity. Some topic about the possible clinical application of both compounds are discussed.

Genotoxicity of selected metal compounds in the SOS chromotest

Knowledge concerning the genotoxicity of inorganic metal compounds in the SOS chromotest is limited. Up to now, only Cr(VI), Sn(II) and the platinum antitumor compound cisplatin(II) were shown to be genotoxic in this test system. However, for Cr(VI) and Sn(II), a positive reaction could only be achieved in cytotoxic dose ranges. The aim of the present study was to provide additional data concerning metal salt genotoxicity in the SOS chromotest. Therefore, 14 metal/metalloid salt compounds of platinum, palladium, rhodium, arsenic, antimony and chromium were tested. Four platinum salts, K2PtCl4, cis-Pt(NH3)2Cl2 (cisplatin), trans-Pt(NH3)2Cl2 (transplatin) and PtCl4 as well as two rhodium compounds tested, K2RhCl5 and (NH4)3RhCl6, could be shown to be genotoxic in the chromotest using the tester strain Escherichia coli PQ37. A moderate genotoxicity was shown by the two Cr(VI) compounds K2CrO4 and K2Cr2O7. All palladium compounds and all the other metal salts tested were unable to induce a significant SOS response.

The structural basis of the genotoxicity of nitroarenofurans and related compounds

The CASE (Computer-Automated Structure Evaluation) methodology has been applied to an investigation of the basis of the genotoxicity (sfiA induction) of 79 nitroarenofurans and related molecules examined with the E. coli PQ37 genotoxicity assay (SOS chromotest). CASE identified 9 major activating structural fragments (biophores) responsible for the probability of genotoxicity (SAR). With respect to quantitative features, CASE identified 8 major molecular subunits related to the genotoxic potency (QSAR). Both the SAR as well as the QSAR analysis indicate that a nitro group on position 2 of the furan ring is important for activity provided one or more aromatic rings are attached to the furan ring, i.e. 2-nitrobenzofuran, 2-nitronaphthofuran, 2-nitroanthrafuran and 8-nitropyrenofuran. Additionally, a small substituent at position 3 of the furan ring, i.e. the methyl group of R7371, the ethyl group of R7427 and the butyl group of R7429, enhance the activity of 2-nitronaphtho[2,1-b]furan (R6597), whereas longer aliphatic chains decrease activity. Moreover, the activity of the nitro group at position 2 of the furan ring was increased by substitution of a methoxy group at position 7 of the R6597 structure. Additionally the n-octanol/water partition coefficient (log P) was found to be an important descriptor for the genotoxic potency in E. coli PQ37. Using the identified descriptors CASE correctly predicted the probability of genotoxicity of all of the genotoxicants and non-genotoxicants in the data base. The calculated genotoxic potency was equally good: 94% of all predicted results were within plus/minus one order of magnitude of the experimental result. Using CASE in the predictive mode, the program correctly predicted the probability of sfiA induction in E. coli of 95.8% of 24 "unknown" nitroarenofurans which were not part of the learning set (QSAR with r = 0.88-0.97).

Influence of methoxy and nitro groups in the oxidative metabolism of naphtho[2,1-b]furan

In the present study, we have investigated the role of methoxy and nitro groups in the oxidative metabolism of naphtho[2,1-b]furan. Hepatic microsomes were used to investigate the aerobic metabolism of naphtho[2,1-b]furan (compound A), 2-nitro-naphtho[2,1-b]furan (compound B) and 7-methoxy-naphtho [2,1-b]furan (compound C) and comparison of the metabolites formed was made using HPCL analysis and NMR, mass and UV-visible spectrometry. The different metabolic pathways investigated were compared with the previously reported metabolism of 7-methoxy-2-nitro-naphtho[2,1-b]furan (compound D). Naphtho[2,1-b]furan yield metabolites of both the furan and benzene rings, while metabolites formed from 7-methoxy-naphtho[2,1-b]furan and 2-nitro-naphtho [2,1-b]furan were derived entirely as a result of enzymic attack on the first benzene ring.

Structural requirements for the induction of the SOS repair in bacteria by nitrated polycyclic aromatic hydrocarbons and related chemicals

The CASE (computer-automated structure evaluation) methodology was used to investigate the structural basis of the SOS-inducing activity of 56 nitrated polycyclic aromatic hydrocarbons (nitroarenes, nPAH) and the unsubstituted parent PAH molecules. Based upon the presence and/or absence of structural features, CASE identified 5 activating (biophores) and 4 inactivating (biophobes) fragments responsible for the SOS-inducing activity. Based upon these fragments, CASE correctly calculated the genotoxicity of 94.6% of the molecules in the training set (sensitivity = 0.85, specificity = 1.0). Disregarding the questionable experimental results of the unexpected very weak direct-acting activity of the unsubstituted benzo[a]pyrene, dibenzo[a,h]anthracene and 7,12-dimethylbenz[a]anthracene, the concordance of the prediction was 100%, i.e., sensitivity = 1.0, specificity = 1.0. Additionally, the quantitative analysis of the SOS-inducing potency showed a good correlation between the experimental and predicted results. The present analyses indicate an identity in the structural determinants responsible for SOS induction in E. coli PQ37 (SOS chromotest) and mutagenicity in Salmonella typhimurium.

Genotoxicity of polycyclic aromatic hydrocarbons in escherichia coli PQ37

In the present investigation, 32 polycyclic aromatic hydrocarbons (PAHs) were tested for genotoxicity in E. coli PQ37 using the standard tube assay of the SOS chromotest. PAHs such as benzo[ghi]fluoranthene, benzo[j]fluoranthene, benzo[a]pyrene, chrysene, dibenzo[a,l]pyrene, fluoranthene and triphenylene exhibited high genotoxicity when incubated in the presence of an exogenous metabolic activation mixture. The results were compared to those obtained with the Salmonella/microsome test.

Sources of variability of the Escherichia coli PQ37 genotoxicity assay (SOS chromotest)

To determine the variability in test results obtained with the Escherichia coli PQ37 genotoxicity assay (SOS chromotest) when varying the test protocol, we examined the influences of sodium dodecylsulfate (SDS) concentrations, of buffer pH and composition on the enzyme assays, the effects of E. coli PQ37 density and culture conditions on the expression and/or determination of alkaline phosphatase (ap) and beta-galactosidase (beta-g) activities, the calculated induction factors (IF) and the SOS-inducing potentials (SOSIP). Initially, we used 0-190 ng (0-1 nmole) 4-nitroquinoline-1-oxide (4-NQO) as a reference compound for the standard procedure in the absence of metabolic activation. Subsequently, to evaluate the results of protocol variations we examined several mutagenic compounds of differing chemical classes using both the standard and a modified assay procedure. We observed the highest enzyme activities using 1 mg SDS per tube and calibrating the ap buffer to pH 8.05 and the beta-g buffer to pH 7.75. The longer the incubation period, the higher the enzyme activities. However, with respect to IF and SOSIP there is no reason to incubate in excess of 90 min. We found no significant differences in the IF and SOSIP values when varying substrate conversion times. There was, however, a definite decrease in beta-g activity when extended substrate incubation times were used. Higher enzyme activities are obtained when the bacterial count is increased. Using lower bacterial counts the enzyme activities decreased, but the sensitivity of E. coli towards genotoxic compounds increased.

Comparison of the carcinogenic effects of two 2-nitro-naphthofurans injected sub-cutaneously in rats

7-Methoxy-2-nitro-naphtho[2,1-b]furan (R 7000) and its methylated homolog in position 1 (R 7372) are among the most mutagenic agents presently known, as shown by the results obtained both in the Ames test and in the SOS Chromotest. Their carcinogenic effects were tested in rats. We were able to confirm the carcinogenic effects of these nitro-naphthofurans, the presence of a methyl group--while increasing the mutagenic effect of R 7000 10 times--induces a significant decrease of the carcinogenic effects in R 7372. The discrepancy between the mutagenic effects in bacterial assays and the carcinogenic effects of these two 2-nitro-naphthofurans remains to be explained.