Evaluation of the in vitro genetic toxicity of 4-(2, 4-dichlorophenoxy)butyric acid

Modification of auxinic phenoxyalkanoic acid herbicides by the acyl acid amido synthetase GH3.15 from Arabidopsis

Herbicide-resistance traits are the most widely used agriculture biotechnology products. Yet, to maintain their effectiveness and to mitigate selection of herbicide-resistant weeds, the discovery of new resistance traits that use different chemical modes of action is essential. In plants, the Gretchen Hagen 3 (GH3) acyl acid amido synthetases catalyze the conjugation of amino acids to jasmonate and auxin phytohormones. This reaction chemistry has not been explored as a possible approach for herbicide modification and inactivation. Here, we examined a set of Arabidopsis GH3 proteins that use the auxins indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) as substrates along with the corresponding auxinic phenoxyalkanoic acid herbicides 2,4-dichlorophenoxylacetic acid (2,4-D) and 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB). The IBA-specific AtGH3.15 protein displayed high catalytic activity with 2,4-DB, which was comparable to its activity with IBA. Screening of phenoxyalkanoic and phenylalkyl acids indicated that side-chain length of alkanoic and alkyl acids is a key feature of AtGH3.15's substrate preference. The X-ray crystal structure of the AtGH3.15·2,4-DB complex revealed how the herbicide binds in the active site. In root elongation assays, Arabidopsis AtGH3.15-knockout and -overexpression lines grown in the presence of 2,4-DB exhibited hypersensitivity and tolerance, respectively, indicating that the AtGH3.15-catalyzed modification inactivates 2,4-DB. These findings suggest a potential use for AtGH3.15, and perhaps other GH3 proteins, as herbicide-modifying enzymes that employ a mode of action different from those of currently available herbicide-resistance traits.

Modulation of the genotoxicity of pesticides reacted with redox-modified smectite clay

Pesticides are toxic agents intentionally released into the environment; their use raises public health and environmental concerns. In recent years there has been much attention to the biotic degradation of pesticides. Abiotic mechanisms in the soil can contribute to pesticide degradation yet the toxicological impact of such degradation is unclear. This study combines for the first time an investigation into abiotic mechanisms of degradation coupled with toxicological endpoints in mammalian cells. The genotoxicity of three commonly used agricultural pesticides was assessed before and after exposure to redox-modified clay minerals. The objectives of the study were to determine the genotoxicity of 2,4-dichlorophenoxy acetic acid (2,4-D), dicamba, and oxamyl, using single cell gel electrophoresis with Chinese hamster ovary (CHO) cells, and to determine the effect of the iron oxidation state in clay minerals (ferruginous smectite SWa-1) on the genotoxic potency of the pesticides. 2,4-D alone or following reaction with redox-modified clays did not induce DNA damage in CHO cells. Oxamyl alone induced a concentration-dependent increase in genomic DNA damage; however, its genotoxicity declined after reaction with reduced clay minerals. Dicamba was not genotoxic when directly analyzed. When dicamba was reacted with reduced clay, a concentration-dependent increase in genomic DNA damage was observed. This is the first reported case of a pesticide being converted into a genotoxin after exposure to redox-modified smectites. These data introduce a new paradigm on the interaction between redox-modified clays and pesticide-related environmental genotoxicity.

Altered gene expression in human hepatoma HepG2 cells exposed to low-level 2,4-dichlorophenoxyacetic acid and potassium nitrate

2,4-dichlorophenoxyacetic acid (2,4-D) and nitrate are agricultural contaminants found in rural ground water. It is not known whether levels found in groundwater pose a human or environmental health risk, nor is the mechanism of toxicity at the molecular/cellular level understood. This study focused on determining whether 2,4-D or nitrate at environmentally realistic levels elicit gene expression changes in exposed cells. cDNA microarray technology was used to determine the impact of 2,4-D and nitrate in an in vitro model of exposure. Human hepatoma HepG2 cells were incubated with 2,4-D or nitrate alone for 24 h. Cell viability (neutral red assay) and proliferation (BrdU incorporation) were assessed following exposure. Total RNA from treated and control cells were isolated, reverse transcribed and reciprocal labelled with Cy3 or Cy5 dyes, and hybridized to a human cDNA microarray. The hybridized microarray chips were scanned, quantified and analyzed to identify genes affected by 2,4-D or nitrate exposure based on a two-fold increase or decrease in gene expression and reproducibility (affected in three or more treatments). Following filtering, normalization and hierarchical clustering initial data indicate that numerous genes were found to be commonly expressed in at least three or more treatments of 2,4-D or nitrate tested. The affected genes indicate that HepG2 cells respond to environmental, low-level exposure and produce a cellular response that is associated with alterations in the expression of many genes. The affected genes were characterized as stress response, cell cycle control, immunological and DNA repair genes. These findings serve to highlight new pathway(s) in which to further probe the effects of environmental levels of 2,4-D and nitrate.

The genotoxicity of ambient outdoor air, a review: Salmonella mutagenicity

Mutagens in urban air pollution come from anthropogenic sources (especially combustion sources) and are products of airborne chemical reactions. Bacterial mutation tests have been used for large, multi-site, and/or time series studies, for bioassay-directed fractionation studies, for identifying the presence of specific classes of mutagens, and for doing site- or source-comparisons for relative levels of airborne mutagens. Early research recognized that although carcinogenic PAHs were present in air samples they could not account for the majority of the mutagenic activity detected. The mutagenicity of airborne particulate organics is due to at least 500 identified compounds from varying chemical classes. Bioassay-directed fractionation studies for identifying toxicants are difficult to compare because they do not identify all of the mutagens present, and both the analytical and bioassay protocols vary from study to study. However, these studies show that the majority of mutagenicity is usually associated with moderately polar/highly polar classes of compounds that tend to contain nitroaromatic compounds, aromatic amines, and aromatic ketones. Smog chamber studies have shown that mutagenic aliphatic and aromatic nitrogen-containing compounds are produced in the atmosphere when organic compounds (even non-mutagenic compounds) are exposed to nitrogen oxides and sunlight. Reactions that occur in the atmosphere, therefore, can have a profound effect on the genotoxic burden of ambient air. This review illustrates that the mutagenesis protocol and tester strains should be selected based on the design and purpose of the study and that the correlation with animal cancer bioassay results depends upon chemical class. Future emphasis needs to be placed on volatile and semi-volatile genotoxicants, and on multi-national studies that identify, quantify, and apportion mutagenicity. Initial efforts at replacing the Salmonella assay for ambient air studies with some emerging technology should be initiated.

Assessment of the ability of Imazaquin herbicide to induce chromosomal aberrations in vitro in cultured Chinese hamster ovary cells and micronuclei in vivo in mice

The agricultural chemicals marketed to increase food production may not only combat pests and weeds but also present toxic properties and cause genetic damage to the fauna and flora. The Imazaquin herbicide (Scepter 70 DG-Cyanamid) has been widely used in soybean fields in Paraná (Brazil), but information on its genotoxicity is scarce. Thus, in vivo and in vitro studies were carried out to assess the possible clastogenic effect of this herbicide on eukaryote cells. In the in vitro studies, the Chinese hamster ovarian cell lines CHO-K1 (wild) and CHO xrs-5 (mutant) were treated at the three phases of the cell cycle (G1, S and G2) for chromosome aberration (CA) analysis. The in vivo assessment was carried out by the micronucleus test (MN) on Swiss mice (Mus musculus) bone marrow cells. The herbicide did not induce a significant increase in the CA frequency in any of the treatments. No statistically significant differences were observed in the MN frequencies among the groups treated with the herbicide and the negative control. From the test system used in this study, we can conclude that the Imazaquin herbicide did not act as a clastogenic agent either in vitro or in vivo.