Improvements in the reliability ofin vitrogenotoxicity testing

Confined Target-Triggered Hot Spots for In Situ SERS Analysis of Intranuclear Genotoxic Markers

The γH2AX is a type of confined target in nuclei which is highly expressed around the damaged DNA during genotoxicity and has therefore been identified as a marker of genotoxicity. Convenient and intuitive in situ real-time detection of γH2AX is crucial for an accurate assessment of genotoxicity. Selective and nondestructive surface-enhanced Raman spectroscopy (SERS) is suitable to achieve this goal. However, the detection of substances in the nucleus by SERS is still limited due to the contradiction of probes between the nuclei entry efficiency and signal enhancement. This study utilized the characteristics of γH2AX as a confined target and constructed a γH2AX immunosensor based on gold nanoprobes with a small size (15 nm), which was modified with the TAT nuclear targeting peptide to ensure high nuclei entry efficiency. Once DNA damage was induced, the local overexpression of γH2AX further recruited the probe through immune recognition, so that hot spots could be assembled in situ to generate strong Raman signals, which were applied to evaluate the genotoxicity of drug impurities. This study proposed a novel SERS detection strategy, characterized by confined target-induced size conversion and hot spot formation, for in situ real-time analysis of intranuclear targets at the single-living-cell level, which intelligently simplified the structure of SERS probes and the operation process.

Do cytotoxicity and cell death cause false positive results in the in vitro comet assay?

The comet assay is used to measure DNA damage induced by chemical and physical agents. High concentrations of test agents may cause cytotoxicity or cell death, which may give rise to false positive results in the comet assay. Systematic studies on genotoxins and cytotoxins (i.e. non-genotoxic poisons) have attempted to establish a threshold of cytotoxicity or cell death by which DNA damage results measured by the comet assay could be regarded as a false positive result. Thresholds of cytotoxicity/cell death range from 20% to 50% in various publications. Curiously, a survey of the latest literature on comet assay results from cell culture studies suggests that one-third of publications did not assess cytotoxicity or cell death. We recommend that it should be mandatory to include results from at least one type of assay on cytotoxicity, cell death or cell proliferation in publications on comet assay results. A combination of cytotoxicity (or cell death) and proliferation (or colony forming efficiency assay) is preferable in actively proliferating cells because it covers more mechanisms of action. Applying a general threshold of cytotoxicity/cell death to all types of agents may not be applicable; however, 25% compared to the concurrent negative control seems to be a good starting value to avoid false positive comet assay results. Further research is needed to establish a threshold value to distinguish between true and potentially false positive genotoxic effects detected by the comet assay.

OUP accepted manuscript

BlueScreen HC is a mammalian cell-based assay for measuring the genotoxicity and cytotoxicity of chemical compounds and mixtures. The BlueScreen HC assay has been utilized at the Research Institute for Fragrance Materials in a safety assessment program as a screening tool to prioritize fragrance materials for higher-tier testing, as supporting evidence when using a read-across approach, and as evidence to adjust the threshold of toxicological concern. Predictive values for the BlueScreen HC assay were evaluated based on the ability of the assay to predict the outcome of in vitro and in vivo mutagenicity and chromosomal damage genotoxicity assays. A set of 371 fragrance materials was assessed in the BlueScreen HC assay along with existing or newly generated in vitro and in vivo genotoxicity data. Based on a weight-of-evidence approach, the majority of materials in the data set were deemed negative and concluded not to have the potential to be genotoxic, while only a small proportion of materials were determined to show genotoxic effects in these assays. Analysis of the data set showed a combination of high positive agreement but low negative agreement between BlueScreen HC results, in vitro regulatory genotoxicity assays, and higher-tier test results. The BlueScreen HC assay did not generate any false negatives, thereby providing robustness when utilizing it as a high-throughput screening tool to evaluate the large inventory of fragrance materials. From the perspective of protecting public health, it is desirable to have no or minimal false negatives, as a false-negative result may incorrectly indicate the lack of a genotoxicity hazard. However, the assay did have a high percentage of false-positive results, resulting in poor positive predictivity of the in vitro genotoxicity test battery outcome. Overall, the assay generated 100% negative predictivity and 3.9% positive predictivity. In addition to the data set of 371 fragrance materials, 30 natural complex substances were evaluated for BlueScreen HC, Ames, and in vitro micronucleus assay, and a good correlation in all three assays was observed. Overall, while a positive result may have to be further investigated, these findings suggest that the BlueScreen HC assay can be a valuable screening tool to detect the genotoxic potential of fragrance materials and mixtures.

In Vitro Three-Dimensional Liver Models for Nanomaterial DNA Damage Assessment

Whilst the liver possesses the ability to repair and restore sections of damaged tissue following acute injury, prolonged exposure to engineered nanomaterials (ENM) may induce repetitive injury leading to chronic liver disease. Screening ENM cytotoxicity using 3D liver models has recently been performed, but a significant challenge has been the application of such in vitro models for evaluating ENM associated genotoxicity; a vital component of regulatory human health risk assessment. This review considers the benefits, limitations, and adaptations of specific in vitro approaches to assess DNA damage in the liver, whilst identifying critical advancements required to support a multitude of biochemical endpoints, focusing on nano(geno)toxicology (e.g., secondary genotoxicity, DNA damage, and repair following prolonged or repeated exposures).

Ruthenium complexes show promise when submitted to toxicological safety tests using alternative methodologies

The number of cancer cases continues to increase worldwide, and unfortunately the main systemic treatments available have numerous of side effects. Ruthenium complexes have shown to be promising chemotherapeutic agents, since they present low toxicity and are more selective for tumor tissues. We report the synthesis, characterization and biological properties of two new ruthenium (II) complexes containing Lapachol and Lawsone as ligands: (1) [Ru(Law)(dppb)(phen)]PF₆ and (2) [Ru(Lap)(dppb)(phen)]PF₆, where Law = Lawsone, Lap = Lapachol, dppb = 1,4-bis(diphenylphosphine)butane and phen = 1,10-phenanthroline. The ability of the complexes (1) and (2) to interact with CT-DNA (Calf Thymus) was investigated, and the results indicate that the complexes have shown a weak interaction with this macromolecule. Complexes (1) and (2) showed a moderate interaction with BSA, via a spontaneous process with the involvement of van der Waals and hydrogen bond interactions. Both complexes were tested against human lung cancer cell lines, chronic human myeloid leukemia, murine melanoma and human cervical and non-tumoral murine fibroblast adenocarcinoma, human lung fibroblasts and monkey kidney epithelia. The potential for cytotoxicity was tested out using the MTT assay and the neutral red test, to calculate inhibitory concentrations (IC50) and selectivity indices (IS). Both complexes showed a higher selectivity index of 1.17 and 10.91, respectively, for the HeLa tumor line. Studies of toxicological evaluation, using the micronucleus test and the comet assay against non-tumor cells, as well as an assessment of the potential for acute toxicity and neurotoxicity in zebrafish (Danio rerio). In the in vitro micronucleus test, complex (1) showed the least genotoxic potential, and in the in vitro comet assay both compounds had revealed a genotoxic potential at 0.5 and 1.0 mg L^-1, with no difference between 24 h and 48 h exposure times. In the acute toxicity tests on zebrafish embryos, complex (1) showed sublethal effects such as decreased blood circulation and heartbeat rate, which were less pronounced than with complex (2). In contrast to complex 2, which caused lethality even before 48h, complex (1) did not cause the death of the embryos at concentrations up to (2.0 mg L^-1). Complex (2) also lead to a delay in the embryo. Cell based in vitro methods thus proved able to provide specific toxicological data, allowing a significant reduction in ∖animal experimentation. Given that in vitro tests cannot completely replace animal tests, the use of less advanced developmental stages such as zebrafish embryos, which - at least in the European Union - are not regarded protected, could be shown to be an excellent alternative for testing with, e.g., mammals.

Distinct Orchestration and Dynamic Processes on γ-H2AX and p-H3 for Two Major Types of Genotoxic Chemicals Revealed by Mass Spectrometry Analysis

Genotoxic chemicals act by causing DNA damage, which, if left unrepaired, can have deleterious consequences for cell survival. DNA damage response (DDR) gets activated to repair or mitigate the effects of DNA damage. Histone H2AX and H3 phosphorylation biomarkers (γ-H2AX and p-H3) have attracted great attention as they play pivotal roles in the DDR. Simultaneous quantitation of γ-H2AX and p-H3 in exposed cells may monitor the toxicity of genotoxic chemicals and to some extent reflect the subsequent DDR process. Reported here is the first comprehensive characterization of distinct orchestration and dynamic processes on cellular γ-H2AX and p-H3 for two major types of genotoxic chemicals, clastogens and aneugens, by stable isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS). We find that clastogens significantly induce an increase in γ-H2AX and a decrease in p-H3; aneugens have no obvious effect on γ-H2AX, whereas induce either an increase or a decrease in p-H3. In addition, the specific profiles of clastogens and aneugens affecting DNA damage may be dynamically observed, which in turn provides insights into the processes involving DNA damage repair as well as transcription. Taken together, these results suggest that robust LC-MS/MS analysis of γ-H2AX and p-H3 can not only quantitatively differentiate mechanistic information on clastogens and aneugens but also dynamically present the detail profiles of DNA damage and repair processes.

The safety evaluation of food flavoring substances: the role of genotoxicity studies

The Flavor and Extract Manufacturers Association (FEMA) Expert Panel relies on the weight of evidence from all available data in the safety evaluation of flavoring substances. This process includes data from genotoxicity studies designed to assess the potential of a chemical agent to react with DNA or otherwise cause changes to DNA, either in vitro or in vivo. The Panel has reviewed a large number of in vitro and in vivo genotoxicity studies during the course of its ongoing safety evaluations of flavorings. The adherence of genotoxicity studies to standardized protocols and guidelines, the biological relevance of the results from those studies, and the human relevance of these studies are all important considerations in assessing whether the results raise specific concerns for genotoxic potential. The Panel evaluates genotoxicity studies not only for evidence of genotoxicity hazard, but also for the probability of risk to the consumer in the context of exposure from their use as flavoring substances. The majority of flavoring substances have given no indication of genotoxic potential in studies evaluated by the FEMA Expert Panel. Examples illustrating the assessment of genotoxicity data for flavoring substances and the consideration of the factors noted above are provided. The weight of evidence approach adopted by the FEMA Expert Panel leads to a rational assessment of risk associated with consumer intake of flavoring substances under the conditions of use.

Short-term and Long-term Exposure of the MucilAir™ Model to Polycyclic Aromatic Hydrocarbons

Cells grown in monocultures are widely used to model lung tissue. As a result of these culture conditions, these cells exhibit poor morphological similarity to those present in in vivo lung tissue. MucilAir™, a 3-D in vitro model comprising human basal, goblet and ciliated cells, represents a fully differentiated respiratory epithelium that can be used as an alternative and a more realistic system. The aim of our study was to compare the effects of short-term and long-term exposure to two polycyclic aromatic hydrocarbons (PAHs) - benzo[a]pyrene (B[a]P) and 3-nitrobenzanthrone (3-NBA) - using MucilAir as a model of human lung tissue. Two concentrations (0.1 μM and 1 μM) were tested at three time points (24 hours, 7 days and 28 days). Several aspects were assessed: cytotoxicity (lactate dehydrogenase (LDH) release), integrity of the cell layer (transepithelial electrical resistance (TEER)), induction of oxidative stress (reactive oxygen species production) and changes in the expression of selected genes involved in PAH metabolism (CYP1A1 and AKR1C2) and the antioxidant response (ALDH3A1, SOD1, SOD2, GPX1, CAT, HMOX1 and TXNRD1). The results showed that exposure to B[a]P caused a spike in LDH release at day 5. Exposure to 3-NBA caused a number of spikes in LDH release, starting at day 5, and a decrease in TEER after 11 days. CYP1A1 gene expression was upregulated after the 7-day and 28-day B[a]P exposures, as well as after the 24-hour and 7-day 3-NBA exposures. HMOX1 and SOD1 were downregulated after both 24-hour PAH treatments. HMOX1 was upregulated after a 1-week exposure to 3-NBA. There were no significant changes in the messenger RNA (mRNA) levels of AKR1C2, ALDH3A1, TXNRD1, SOD2, GPX1 or CAT. These results illustrate the potential use of this 3-D in vitro lung tissue model in studying the effects of chronic exposure to PAHs.

Heated naringin mitigate the genotoxicity effect of Mitomycin C in BALB/c mice through enhancing the antioxidant status

A major problem with cancer chemotherapy is its severe toxic effects on non-target tissues. Assessment of natural products for their protective effect against anticancer drugs induced toxicity is gaining importance in cancer biology. The aim of the present study was to evaluate the effect of native and thermal treated naringin on the protective effect against mitomycin C (MMC) induced genotoxicity. The genotoxicity in liver kidney and brain cells isolated from Balb/C mice were evaluated by performing the comet assay. Antioxidant and lipid peroxidation assays were carried out to understand the protective effects of these compounds. The comet assay showed that heated and native naringin were not genotoxic at the tested dose (40 mg/kg b.w) on liver, kidney and brain cells. A significant decrease in DNA damages was observed, at the tested doses (20 mg/kg b.w and 40 mg/kg b.w) suggesting a protective role of these molecules against the genotoxicity induced by mitomycin C on liver, kidney and brain cells. Moreover, administration of MMC (6 mg/kg b.w.) altered the activities of glutathione peroxidase and superoxide dismutase accompanied by a significant increase of lipid peroxidation. Pretreatment of mouse with heated and native naringin before MMC administration significantly raised the glutathione peroxidase and superoxide dismutase activities followed by a reduced MMC-induced lipid peroxidation. Our study demonstrated that heat treatment of naringin preserve activities of native naringin. The genoprotective properties of heated and native naringin against MMC could be attributed to its antioxidant activities and its inhibitory effect on lipid peroxidation.

Development of an in vitro PIG-A gene mutation assay in human cells

Mutagens can be carcinogens, and traditionally, they have been identified in vitro using the Salmonella 'Ames' reverse mutation assay. However, prokaryotic DNA packaging, replication and repair systems are mechanistically very different to those in the humans we inevitably seek to protect. Therefore, for many years, mammalian cell line genotoxicity assays that can detect eukaryotic mutagens as well as clastogens and aneugens have been used. The apparent lack of specificity in these largely rodent systems, due partly to their mutant p53 status, has contributed to the use of animal studies to resolve data conflicts. Recently, silencing mutations at the PIG-A locus have been demonstrated to prevent glycophosphatidylinositol (GPI) anchor synthesis and consequentially result in loss of GPI-anchored proteins from the cell's extracellular surface. The successful exploitation of this mutant phenotype in animal studies has triggered interest in the development of an analogous in vitro PIG-A mutation screening assay. This article describes the development of a robust assay design using metabolically active human cells. The assay includes viability and cell membrane integrity assessment and conforms to the future ideas of the 21st-century toxicology testing.

Complementarity of phosphorylated histones H2AX and H3 quantification in different cell lines for genotoxicity screening

The in vitro micronucleus assay is broadly used, but is not per se able to discriminate aneugenic from clastogenic compounds, and cytotoxicity can be a confounding factor. In vitro genotoxicity assays generally rely on cell lines with limited metabolic capabilities. Recently, the use of histone H2AX and H3 phosphorylation markers (γH2AX and p-H3) was proposed to discriminate aneugenic from clastogenic chemicals. The aim of the present study was to develop a new genotoxic screening strategy based on the use of the γH2AX and p-H3 biomarkers in combination with cell lines with distinct biotransformation properties. First, we tested a training set of 20 model chemicals comprised of 10 aneugens, five clastogens and five cytotoxics on three human cell lines (HepG2, LS-174T and ACHN). Our data confirm the robustness of these two biomarkers to discriminate efficiently clastogens, aneugens and misleading cytotoxic chemicals in HepG2 cells. Aneugenic compounds induced either an increase or a decrease in p-H3 depending on their mode of action. Clastogens induced γH2AX, and cytotoxic compounds generated a marked decrease in these two biomarkers. Moreover, the use of different cell lines permits to discriminate direct from bioactivated genotoxins without the need of an exogenous metabolic activation system. Finally, we further evaluated this strategy using a test set of 13 chemicals with controversial genotoxic potential. The resulting data demonstrate that the combined analysis of γH2AX and p-H3 is an efficient strategy. Notably, we demonstrated that three compounds (fisetin, hydroquinone and okadaic acid) display both aneugenic and clastogenic properties.

Validation of high-throughput genotoxicity assay screening using γH2AX in-cell western assay on HepG2 cells

In vitro genotoxicity tests used in regulatory toxicology studies are sensitive, but the occurrence of irrelevant positive results is high compared with carcinogenicity studies in rodents. Current in vitro genotoxicity tests are also often limited by relatively low throughput. The aim of this study was to validate an in vitro genotoxic assay in a 96-well plate format that allows the simultaneous examination of cytotoxicity and genotoxicity. The test is based on the quantification of the phosphorylation of the histone H2AX (γH2AX), which reflects a global genotoxic insult, using the In-Cell Western technique. The assay was evaluated on HepG2 cells by testing a list of 61 compounds recommended by the European Center for the Validation of Alternative Methods (ECVAM), whose genotoxic potential has already been characterized. The γH2AX assay on HepG2 cell line was highly sensitive: 75% of the genotoxic compounds gave a positive result, and specific: 90-100% of nongenotoxic compounds gave negative results. Compared with the micronucleus genotoxicity assay using the same cell line and test compounds, the γH2AX assay was more sensitive and specific. In sum, the high-throughput γH2AX assay described here can accurately detect simultaneously the genotoxic and the cytotoxic potential of compounds with different modes of mutagenic action, notably those who required metabolic activation. The use of this assay in the early discovery phase of drug development may prove to be a valuable way to assess the genotoxic potential of xenobiotics.

The essential comet assay: a comprehensive guide to measuring DNA damage and repair

The comet assay (single cell gel electrophoresis) is the most common method for measuring DNA damage in eukaryotic cells or disaggregated tissues. The assay depends on the relaxation of supercoiled DNA in agarose-embedded nucleoids (the residual bodies remaining after lysis of cells with detergent and high salt), which allows the DNA to be drawn out towards the anode under electrophoresis, forming comet-like images as seen under fluorescence microscopy. The relative amount of DNA in the comet tail indicates DNA break frequency. The assay has been modified to detect various base alterations, by including digestion of nucleoids with a lesion-specific endonuclease. We describe here recent technical developments, theoretical aspects, limitations as well as advantages of the assay, and modifications to measure cellular antioxidant status and different types of DNA repair. We briefly describe the applications of this method in genotoxicity testing, human biomonitoring, and ecogenotoxicology.