Mutagenic activity of promutagens in plants: indirect evidence of their activation

Genotoxic evaluation of an industrial effluent from an oil refinery using plant and animal bioassays

Polycyclic aromatic hydrocarbons (PAHs) are genotoxic chemicals commonly found in effluents from oil refineries. Bioassays using plants and cells cultures can be employed for assessing environmental safety and potential genotoxicity. In this study, the genotoxic potential of an oil refinery effluent was analyzed by means of micronucleus (MN) testing of Alium cepa, which revealed no effect after 24 h of treatment. On the other hand, primary lesions in the DNA of rat (Rattus norvegicus) hepatoma cells (HTC) were observed through comet assaying after only 2 h of exposure. On considering the capacity to detect DNA damage of a different nature and of these cells to metabolize xenobiotics, we suggest the association of the two bioassays with these cell types, plant (Allium cepa) and mammal (HTC) cells, for more accurately assessing genotoxicity in environmental samples.

Genotoxicity testing of Plantago major extracts in somatic cells of Drosophila melanogaster

Plantago major is used in many parts of the world for the treatment of diseases and to promote the healing of wounds. In the present study, the somatic mutation and recombination test (SMART) in Drosophila melanogaster was used to evaluate the genotoxic activity of an aqueous extract of P. major. The following Drosophila crosses were made: standard (ST) cross, in which virgin flare females (flr3/TM3, Bds) were mated with mwh/mwh males, and high-bioactivation (HB) cross, in which virgin ORR females (ORR/ORR; flr3/TM3, Bds) were mated with mwh/mwh males. Each cross produced two types of descendents, marker-transheterozygous (MH) (mwh +/+ flr3) and balancer-heterozygous (BH) (mwh +/+ TM3, Bds) flies. Three-day-old larvae of both types of descendents were treated with undiluted and diluted (1:1 and 1:2 in water) aqueous extracts of P. major. The extracts were genotoxic in both crosses, producing similar induced frequencies in ST and HB flies. Comparison of the frequencies of wing spots in the BH and MH descendents indicated that recombination was a major response. The results indicate that, under these experimental conditions, aqueous extracts of P. major are genotoxic (recombinagenic).

Metabolic activation of three arylamines and two organophosphorus insecticides by coriander (Coriandrum sativum) a common edible vegetable

Organophosphorus insecticides and arylamines, widely distributed in the environment, can be activated into mutagens by plants. Plant activation of three aromatic amines, 4-nitro-o-phenylenediamine (NOP), m-phenylenediamine (m-PDA) and 2-aminofluorene (2AF), and two organophosphorus insecticides, dimethoate and methyl parathion has been the focus of this study. The plant cell/microbe coincubation assay was used employing coriander (Coriandrum sativum) suspended cell cultures as the activating system. Interestingly, this vegetable is included in the Mexican diet and ingested generally uncooked and could have epidemiological consequences. As a genetic end point, the Salmonella typhimurium tester strain TA98 was used. Protein contents, as well as peroxidase activity and peroxidase activity inhibited by diethyldithiocarbamate (DEDTC) of coriander cultures were determined after the coculture. Coriander cells highly activated three aromatic amines, NOP, m-PDA and 2-AF to mutagenic products detected in Salmonella. On the other hand, insecticides were only lightly activated, probably because peroxidase activity of coriander cells was inhibited, corroborated by DEDTC peroxidase inhibition. In all the assays, NOP was the more potent mutagenic compound. The results demonstrated that coriander cells were metabolically competent and suitable for a plant cell microbe coincubation assay, developed to analyze the promutagen activation by plant systems and can be used as a indicator of potential genetic effects.

Mutagenic interactions between X-rays and two promutagens, o-phenylenediamine and N-nitrosodimethylamine, in the stamen hairs of Tradescantia clone BNL 4430

Mutagenic interactions between X-rays and two promutagens, o-phenylenediamine (PDA) and N-nitrosodimethylamine (DMN), were studied in the stamen hairs of Tradescantia clone BNL 4430, a blue/pink heterozygote. The young inflorescence-bearing shoots with roots of this clone cultivated in a nutrient solution circulating growth chamber were used as tester plants. After determining dose-response curves for X-rays. PDA and DMN, combined treatments with PDA or DMN and X-rays were conducted, exposing acutely to X-rays 20 h before starting, at the midpoint of, or 20 h after completing the PDA or DMN treatments for 4 h. Clear synergistic effects in inducing somatic pink mutations were detected when X-rays were irradiated before the PDA or DMN treatments, resembling those confirmed earlier between maleic hydrazide (MH) and X-rays. On the contrary, clear antagonistic effects were observed when X-rays were given after the PDA or DMN treatments, also resembling those between MH and X-rays. When X-rayed at the midpoint of the PDA or DMN treatments, merely additive and synergistic effects were observed, respectively, differing from the antagonistic effects between MH and X-rays. The mutagenic synergisms detected were considered to be the results of interactions between DNA strand breaks (and the resultant chromosome breaks) induced by X-rays and those by PDA or DMN, whereas the mutagenic antagonisms observed were presumed to be due to X-ray-caused inhibition of the activation of PDA and DMN in the stamen-hair cells. The time periods required for penetrations into floral tissues and/or activations into mutagens seem different among PDA, DMN and MH.

Metabolic activation of meta-phenylenediamine by the alga Chlamydomonas reinhardtii

Promutagens/procarcinogens arylamines are widely distributed in the environment. While it is accepted that these compounds can be metabolized to ultimate mutagens in mammals and higher plants, in aquatic plants they have not yet been explored. Intact wild-type and repair-deficient strains of Chlamydomonas reinhardtii and Saccharomyces cerevisiae D7 strain were assayed for their ability to activate meta-phenylenediamine (m-PDA) to an ultimate mutagen. The different responses of the algal wild-type strain and repair-deficient strains to the toxic and mutagenic effects of m-PDA were observed. Recombination repair played an important role in repair of damage induced to C. reinhardtii DNA by this arylamine. The examined isomer of phenylenediamine induced mutations in both algal and yeast cells. m-PDA was activated in the algal cell/microbe coincubation assay in which algal cells were used as an activating system and bacteria Salmonella typhimurium and yeast Saccharomyces cerevisiae as the genetic indicator organisms. This new assay is, in addition to the animal microsome metabolizing system and the plant cell/microbe coincubation assay, suitable for the detection of environmental promutagens and their conversion to mutagens mainly in aquatic environments.

Detection of primary DNA damage in Chlamydomonas reinhardtii by means of modified microgel electrophoresis

The assessment of genotoxic potential in surface water requires test methods, among which are those that detect initial DNA damage in organisms of aquatic biocenosis. The microgel electrophoresis (MGE) "comet assay" was applied to a ubiquitous unicellular green alga (Chlamydomonas reinhardtii) to detect DNA damage caused by genotoxins. For this, the test protocol described by Singh NP et al. [Exp Cell Res 175: 184-191, 1988] was modified. Major modifications were the use of alkaline lysis buffer with ionic detergents and the reduction of preincubation and electrophoresis times. Short-time exposure of Chlamydomonas to the well-known genotoxicants 4-nitroquinoline-1-oxide (4-NQO), N-nitrosodimethylamine, and hydrogen peroxide led to dose-dependent DNA damage. Chlamydomonas responded very sensitively to treatment with increasing doses of 4-NQO. At a concentration of 25 nM, significant DNA damage was observed. At higher 4-NQO doses (> 100 nM), DNA damage was visible as complete DNA fragmentation into fine granules. N-Nitrosodimethylamine caused genotoxic effects at a concentration range from 0.014 to 0.14 mM without producing complete DNA fragmentation at the concentrations tested (highest dose, 140 mM). To evaluate the influence of illumination conditions during exposure, cells were incubated with increasing doses of H2O2 (0.25-1.0 mM) in darkness and in light. Our results indicate that incubation in light enables Chlamydomonas to cope with oxidative stress more efficiently than under dark conditions. To a certain extent, cytotoxic as well as genotoxic effects of H2O2 depend on the illumination condition or repair and anti-oxidative protection mechanisms activated by light, respectively.

Development and validation of alternative metabolic systems for mutagenicity testing in short-term assays

We present here the results obtained within the framework of an EU funded project aimed to develop and validate alternative metabolic activating systems to be used in short-term mutagenicity assays, in order to reduce the use of laboratory animals for toxicology testing. The activating systems studied were established cell lines (Hep G2, CHEL), genetically engineered V79 cell lines expressing specific rat cytochromes P450, erythrocyte-derived systems, CYP-mimetic chemical systems and plant homogenates. The metabolically competent cell lines were used as indicator cells for genotoxic effects as well as for the preparation of external activating systems using other indicator cells. The following endpoints were used: micronuclei, chromosomal aberrations and sister chromatid exchanges, mutations at the hprt locus, gene mutations in bacteria (Ames test), unscheduled DNA synthesis and DNA breaks detected in the comet assay. All metabolic systems employed activated some promutagens. With some of them, promutagens belonging to many different classes of chemicals were activated to genotoxicants, including carcinogens negative in liver S9-mediated assays. In other cases, the use of the new activating systems allowed the detection of mutagens at much lower substrate concentrations than in liver S9-mediated assays. Therefore, the alternative metabolizing systems, which do not require the use of laboratory animals, have a substantial potential in in vitro toxicology, in the basic genotoxicity testing as well as in the elucidation of activation mechanisms. However, since the data basis is much smaller for the new systems than for the activating systems produced from subcellular liver preparations, the overlapping use of both systems is recommended for the present and near future. For example, liver S9 preparations may be used with some indicator systems (e.g., bacterial mutagenicity), and metabolically competent mammalian cell lines may be used with other indicator systems (e.g., a cytogenetic endpoint) in a battery of basic tests.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Induction of somatic mutations in Tradescantia clone 4430 by three phenylenediamine isomers and the antimutagenic mechanisms of diethyldithiocarbamate and ammonium meta-vanadate

Three isomers of the promutagen phenylenediamine at mM concentrations were plant-activated and induced mutation in stamen hairs of Tradescantia clone 4430. The rank order of the mutagenicity of the isomers was: o-phenylenediamine > m-phenylenediamine > p-phenylenediamine with corresponding mutagenic potencies of 5.60, 1.43, and 0.46 mutant stamen hair cells/mumole, respectively. Diethyldithiocarbamate (DEDTC) and ammonium meta-vanadate (vanadate) repressed the mutagenic activity of o-phenylenediamine (o-PDA) in intact plants. Based on inhibition kinetics and reaction rates, the mechanism of DEDTC antimutagenicity was attributed to the inhibition of peroxidases that are required in the plant activation of o-PDA to mutagenic product(s). Spectrophotometric measurements of equimolar concentrations of o-PDA and vanadate demonstrated that the antimutagenic property of vanadate was mainly due to its reactivity with o-PDA.

Activation of 4-nitro-o-phenylenediamine by the S2 fraction of Zea mays to mutagenic product(s)

Studies on plant metabolic activation with the S2 fraction from Zea mays have been developed. The 4-nitro-o-phenylenediamine (NOP) activation by S2 has been analyzed with the Ames test as a short-term assay. The NOP mutagenic potency was increased two-fold by S2, while rat liver S9 produced the contrary effect. The presence of a NADPH-generating system and the treatment of S2 with CO do not modify S2 activation of NOP. In this fraction, neither cytochrome P450 nor some enzymatic activities depending on cyt-P450 (aniline hydroxylase and aminopyrine demethylase) were detected. Therefore, the enhancement of NOP mutagenic potency by S2 is independent of the mixed-function oxidase system. On the other hand, inhibitors of the peroxidase activity such as N-acetyl-p-aminophenol caused a partial inhibition of S2 activation of NOP. Likewise, diethyldithiocarbamate produced both a reduction of the S2 peroxidase activity in biochemical assays and a partial inhibition of S2 activation of NOP. Moreover, it was possible to find a direct correlation between the activity of peroxidase per plate of both the S2 fraction and horseradish peroxidase and the number of revertants induced by NOP in the TA98 strain. On the basis of these results, we report that a HRP-like peroxidase activity must be the main pathway of NOP activation by the plant metabolic activation system studied in this work.

Inhibitory effects of acetaminophen, 7,8-benzoflavone and methimazole towards N-nitrosodimethylamine mutagenesis in Arabidopsis thaliana

The metabolic inhibitors acetaminophen, 7,8-benzoflavone, and methimazole significantly reduced the mutagenicity of the promutagen N-nitrosodimethylamine in the higher plant Arabidopsis thaliana. In contrast, these metabolic inhibitors had no effect on the mutagenicity of the direct-acting mutagen N-methyl-N'-nitro-N-nitrosoguanidine.

Increased resistance to the toxic effects of alkylating agents in tobacco expressing the E. coli DNA repair gene ada

The protein coding region of the E. coli gene ada has been transferred to tobacco plants by a leaf disc transformation procedure involving an Agrobacterium tumefaciens Ti plasmid. Transformed plants were shown to be transgenic for the ada message and had increased levels of O6-alkylguanine DNA alkyltransferase activity. The N-methyl-N-nitrosourea- or taurinechlorethylnitrosourea-induced inhibition of growth of calluses or of cells in suspension was considerably lower in ada-transformed than in non-transformed plants. This indicates that O6-alkylguanine, O4-alkylthymine or phosphotriesters are growth-inhibitory lesions in tobacco.

Induction of chromosome damage by benzo[a]pyrene, 2-aminofluorene and cyclophosphamide in the root cells of Vicia faba

The induction of sister-chromatid exchanges (SCEs) and chromosome aberrations (CAs) by benzo[a]pyrene (BP), 2-aminofluorene (2-AF) and cyclophosphamide (CP) in the root cells of Vicia faba was examined. BP and 2-AF induced CAs, but not SCEs. CP induced both SCEs and CAs.