Coupling cytotoxicity biomarkers with DNA damage assessment in TK6 human lymphoblast cells

Effects of long-term exposure to 50 Hz magnetic fields on cell viability, genetic damage, and sensitivity to mutagen-induced damage

Until today, it remains controversial whether long-term exposure to extremely low-frequency magnetic fields (ELF-MF) below the legislative exposure limits could result in adverse human health effects. In the present study, the effects of long-term in vitro MF exposure on three different study endpoints (cell viability, genetic damage, and sensitivity to damage induced by known mutagens) were investigated in the human B lymphoblastoid (TK6) cell line. Cells were exposed to 50 Hz MF at three selected magnetic flux densities (i.e., 10, 100, and 500 μT) for different exposure periods ranging from 96h up to 6 weeks. Cell viability following MF exposure was assessed using the ATP-based cell viability assay. Effects of MF exposure on cell genetic damage and cell sensitivity to mutagen-induced damage were evaluated using the in vitro alkaline comet assay and the in vitro cytokinesis block micronucleus assay. The results showed that long-term exposure up to 96h to 50 Hz MF at all tested flux densities could significantly increase TK6 cell viability. In contrast, long-term MF exposure did not affect cell genetic damage, and long-term pre-exposure to MF did not change cell sensitivity to damage induced by known mutagens. At certain time points, statistically significant difference in genotoxicity test results were observed between the MF-exposed cells and the control cells. However, these observations could not be confirmed in the repeat experiments, indicating that they are probably not biologically significant.

Impact of experimental design factors on the potency of genotoxicants in in vitro tests

Previous studies have shown that differences in experimental design factors may alter the potency of genotoxic compounds in in vitro genotoxicity tests. Most of these studies used traditional statistical methods based on the lowest observed genotoxic effect levels, whereas more appropriate methods, such as the benchmark dose (BMD) approach, are now available to compare genotoxic potencies under different test conditions. We therefore investigated the influence of two parameters, i.e. cell type and exposure duration, on the potencies of two known genotoxicants [aflatoxin B1 and ethyl methanesulfonate (EMS)] in the in vitro micronucleus (MN) assay and comet assay (CA). Both compounds were tested in the two assays using two cell types (i.e. CHO-K1 and TK6 cells). To evaluate the effect of exposure duration, the genotoxicity of EMS was assessed after 3 and 24 h of exposure. Results were analyzed using the BMD covariate approach, also referred to as BMD potency ranking, and the outcome was compared with that of more traditional statistical methods based on lowest observed genotoxic effect levels. When comparing the in vitro MN results obtained in both cell lines with the BMD covariate approach, a difference in potency was detected only when EMS exposures were conducted for 24 h, with TK6 cells being more sensitive. No difference was observed in the potency of both EMS and aflatoxin B1 in the in vitro CA using both cell lines. In contrast, EMS was more potent after 24 h exposure compared with a 3 h exposure under all tested conditions, i.e. in the in vitro MN assay and CA in both cell lines. Importantly, for several of the investigated factors, the BMD covariate method could not be used to confirm the differences in potencies detected with the traditional statistical methods, thus highlighting the need to evaluate the impact of experimental design factors with adequate approaches.

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.

Comparative Neurodevelopment Effects of Bisphenol A and Bisphenol F on Rat Fetal Neural Stem Cell Models

Bisphenol A (BPA) is considered as one of the most extensively synthesized and used chemicals for industrial and consumer products. Previous investigations have established that exposure to BPA has been linked to developmental, reproductive, cardiovascular, immune, and metabolic effects. Several jurisdictions have imposed restrictions and/or have banned the use of BPA in packaging material and other consumer goods. Hence, manufacturers have replaced BPA with its analogues that have a similar chemical structure. Some of these analogues have shown similar endocrine effects as BPA, while others have not been assessed. In this investigation, we compared the neurodevelopmental effects of BPA and its major replacement Bisphenol F (BPF) on rat fetal neural stem cells (rNSCs). rNSCs were exposed to cell-specific differentiation media with non-cytotoxic doses of BPA or BPF at the range of 0.05 M to 100 M concentrations and measured the degree of cell proliferation, differentiation, and morphometric parameters. Both of these compounds increased cell proliferation and impacted the differentiation rates of oligodendrocytes and neurons, in a concentration-dependent manner. Further, there were concentration-dependent decreases in the maturation of oligodendrocytes and neurons, with a concomitant increase in immature oligodendrocytes and neurons. In contrast, neither BPA nor BPF had any overall effect on cellular proliferation or the cytotoxicity of astrocytes. However, there was a concentration-dependent increase in astrocyte differentiation and morphological changes. Morphometric analysis for the astrocytes, oligodendrocytes, and neurons showed a reduction in the arborization. These data show that fetal rNSCs exposed to either BPA or BPF lead to comparable changes in the cellular differentiation, proliferation, and arborization processes.

Detecting Neurodevelopmental Toxicity of Domoic Acid and Ochratoxin A Using Rat Fetal Neural Stem Cells

Currently, animal experiments in rodents are the gold standard for developmental neurotoxicity (DNT) investigations; however, testing guidelines for these experiments are insufficient in terms of animal use, time, and costs. Thus, alternative reliable approaches are needed for predicting DNT. We chose rat neural stem cells (rNSC) as a model system, and used a well-known neurotoxin, domoic acid (DA), as a model test chemical to validate the assay. This assay was used to investigate the potential neurotoxic effects of Ochratoxin A (OTA), of which the main target organ is the kidney. However, limited information is available regarding its neurotoxic effects. The effects of DA and OTA on the cytotoxicity and on the degree of differentiation of rat rNSC into astrocytes, neurons, and oligodendrocytes were monitored using cell-specific immunofluorescence staining for undifferentiated rNSC (nestin), neurospheres (nestin and A2B5), neurons (MAP2 clone M13, MAP2 clone AP18, and Doublecortin), astrocytes (GFAP), and oligodendrocytes (A2B5 and mGalc). In the absence of any chemical exposure, approximately 46% of rNSC differentiated into astrocytes and neurons, while 40% of the rNSC differentiated into oligodendrocytes. Both non-cytotoxic and cytotoxic concentrations of DA and OTA reduced the differentiation of rNSC into astrocytes, neurons, and oligodendrocytes. Furthermore, a non-cytotoxic nanomolar (0.05 µM) concentration of DA and 0.2 µM of OTA reduced the percentage differentiation of rNSC into astrocytes and neurons. Morphometric analysis showed that the highest concentration (10 μM) of DA reduced axonal length. These indicate that low, non-cytotoxic concentrations of DA and OTA can interfere with the differentiation of rNSC.

2,4-Dichlorophenol induces DNA damage through ROS accumulation and GSH depletion in goldfish Carassius auratus

2,4-Dichlorophenol (2,4-DCP) is one of the most abundant chlorophenols in the aquatic environment and has been frequently detected in surface waters. Although ecological and cellular toxicity of 2,4-DCP has aroused the public concern, few reports focus on the genotoxicity, especially on DNA double strand breaks (DSBs), of 2,4-DCP in fish. The present study aims to explore the genotoxic effect of 2,4-DCP on DSBs in goldfish Carassius auratus and to further elucidate its potential mechanism. The results showed that 2,4-DCP significantly induced DSBs (detected by neutral comet assay) in erythrocytes and hepatocytes of goldfish in a dose-dependent manner, indicating a genotoxicity of 2,4-DCP on fish. The total antioxidant capability and the content of reduced glutathione (GSH) were significantly decreased, while the level of reactive oxygen species (ROS) was significantly increased in a dose-dependent manner in erythrocytes and hepatocytes, suggesting an oxidative stress caused by 2,4-DCP in fish. N-acetyl-l-cysteine, a precursor of GSH and a ROS scavenger, significantly impaired 2,4-DCP-induced ROS overproduction and DSBs, which proves that ROS accumulation and GSH depletion are involved in 2,4-DCP-induced DNA damage in fish. Environ. Mol. Mutagen. 59:798-9, 2018. © 2018 Wiley Periodicals, Inc.

Combinations of genotoxic tests for the evaluation of group 1 IARC carcinogens

Many of the known human carcinogens are potent genotoxins that are efficiently detected as carcinogens in human populations but certain types of compounds such as immunosuppressants, sex hormones, etc. act via non-genotoxic mechanism. The absence of genotoxicity and the diversity of modes of action of non-genotoxic carcinogens make predicting their carcinogenic potential extremely challenging. There is evidence that combinations of different short-term tests provide a better and efficient prediction of human genotoxic and non-genotoxic carcinogens. The purpose of this study is to summarize the in vivo and in vitro comet assay (CMT) results of group 1 carcinogens selected from the International Agency for Research on Cancer and to discuss the utility of the comet assay along with other genotoxic assays such as Ames, in vivo micronucleus (MN), and in vivo chromosomal aberration (CA) test. Of the 62 agents for which valid genotoxic data were available, 38 of 61 (62.3%) were Ames test positive, 42 of 60 (70%) were in vivo MN test positive and 36 of 45 (80%) were positive for the in vivo CA test. Higher sensitivity was seen in in vivo CMT (90%) and in vitro CMT (86.9%) assay. Combination of two tests has greater sensitivity than individual tests: in vivo MN + in vivo CA (88.6%); in vivo MN + in vivo CMT (92.5%); and in vivo MN + in vitro CMT (95.6%). Combinations of in vivo or in vitro CMT with other tests provided better sensitivity. In vivo CMT in combination with in vivo CA provided the highest sensitivity (96.7%).

Interpreting in vitro micronucleus positive results: simple biomarker matrix discriminates clastogens, aneugens, and misleading positive agents

The specificity of in vitro mammalian cell genotoxicity assays is low, as they yield a high incidence of positive results that are not observed in animal genotoxicity and carcinogenicity tests, that is, "misleading" or "irrelevant" positives. We set out to develop a rapid and effective follow-up testing strategy that would predict whether apparent in vitro micronucleus-inducing effects are due to a clastogenic, aneugenic, or secondary irrelevant mode(s) of action. Priority was given to biomarkers that could be multiplexed onto flow cytometric acquisition of micronucleus frequencies, or that could be accomplished in parallel using a homogeneous-type assay. A training set of 30 chemicals comprised of clastogens, aneugens, and misleading positive chemicals was studied. These experiments were conducted with human TK6 cells over a range of closely spaced concentrations in a continuous exposure design. In addition to micronucleus frequency, the following endpoints were investigated, most often at time of harvest: cleaved Parp-positive chromatin, cleaved caspase 3-positive chromatin, ethidium monoazide bromide-positive chromatin, polyploid nuclei, phospho-histone H3-positive (metaphase) cells, tetramethylrhodamine ethyl ester-negative cells, cellular ATP levels, cell cycle perturbation, and shift in γ-H2AX fluorescence relative to solvent control. Logistic regression was used to identify endpoints that effectively predict chemicals' a priori classification. Cross validation using a leave-one-out approach indicated that a promising base model includes γ-H2AX shift and change in phospho-histone H3-positive events (25/30 correct calls). Improvements were realized when one or two additional endpoints were included (26-30/30 correct calls). These models were further evaluated with a test set of 10 chemicals, and also by evaluating 3 chemicals at a collaborating laboratory. The resulting data support the hypothesis that a matrix of high throughput-compatible biomarkers can effectively delineate two important modes of genotoxic action as well as identify cytotoxicity that can lead to irrelevant positive results.

Genotoxicity and DNA repair processes of zinc oxide nanoparticles

Two different sizes of zinc oxide nanoparticles (ZnO NP, ≤ 35 nm and 50-80 nm) were tested in the human lymphoblastoid cell line TK6 to increase our knowledge on their genotoxic potential. The comet assay was the system used, and the results obtained showed that the highest concentration tested (100 μg/ml) for the two selected compounds was genotoxic. The percent DNA in tail obtained after treatment with ZnO NP (≤ 35 nm) was significantly higher than that of ZnO NP (50-80 nm) at all concentrations tested. To investigate the nature of the induced genotoxic damage, specific enzymes recognizing oxidized DNA bases were used. Treatments with endonuclease III (Endo III) and formamidopyrimidine DNA glycosylase (FPG) demonstrated that only ZnO NP (50-80 nm) were able to induce significant levels of net oxidative DNA damage. Further DNA repair kinetics studies revealed that DNA damage initially induced was removed in approximately 5 h. DNA damage induced by ZnO NP was repaired more slowly than damage following microparticulated ZnO exposure. No marked differences in repair kinetics of both forms of ZnO NP were observed. Evidence indicates that a high proportion of DNA damage induced by ZnO NP (50-80 nm) correlated with induction of oxidative damage, and that both forms of ZnO NP interfere with mechanisms involved in DNA damage repair.

A critique of methods to measure cytotoxicity in mammalian cell genotoxicity assays

Various methods have been used to estimate cytotoxicity in mammalian cell genotoxicity assays since their introduction more than four decades ago, and although there is no agreement on whether any single method is optimal, there is now a better appreciation of their limitations. Methods based on aspects of cellular function are inevitably inaccurate unless some estimate of cell number is included, and those using some measure of cell proliferation give different results depending on the mathematical model used. Although it would be desirable, it is not possible to provide a universal measure of cytotoxicity because the phenomenon is so complex. There is some flexibility in the limits of cytotoxicity proposed in regulatory guidelines, and it can be argued these could be even less precise. Also, to make valid comparisons of the performance of different test systems, novel or established, it would seem essential to use similar measures of cytotoxicity.

Quantitative high-throughput screening for chemical toxicity in a population-based in vitro model

A shift in toxicity testing from in vivo to in vitro may efficiently prioritize compounds, reveal new mechanisms, and enable predictive modeling. Quantitative high-throughput screening (qHTS) is a major source of data for computational toxicology, and our goal in this study was to aid in the development of predictive in vitro models of chemical-induced toxicity, anchored on interindividual genetic variability. Eighty-one human lymphoblast cell lines from 27 Centre d'Etude du Polymorphisme Humain trios were exposed to 240 chemical substances (12 concentrations, 0.26nM-46.0μM) and evaluated for cytotoxicity and apoptosis. qHTS screening in the genetically defined population produced robust and reproducible results, which allowed for cross-compound, cross-assay, and cross-individual comparisons. Some compounds were cytotoxic to all cell types at similar concentrations, whereas others exhibited interindividual differences in cytotoxicity. Specifically, the qHTS in a population-based human in vitro model system has several unique aspects that are of utility for toxicity testing, chemical prioritization, and high-throughput risk assessment. First, standardized and high-quality concentration-response profiling, with reproducibility confirmed by comparison with previous experiments, enables prioritization of chemicals for variability in interindividual range in cytotoxicity. Second, genome-wide association analysis of cytotoxicity phenotypes allows exploration of the potential genetic determinants of interindividual variability in toxicity. Furthermore, highly significant associations identified through the analysis of population-level correlations between basal gene expression variability and chemical-induced toxicity suggest plausible mode of action hypotheses for follow-up analyses. We conclude that as the improved resolution of genetic profiling can now be matched with high-quality in vitro screening data, the evaluation of the toxicity pathways and the effects of genetic diversity are now feasible through the use of human lymphoblast cell lines.

Assessment of genotoxicity induced by 7,12-dimethylbenz(a)anthracene or diethylnitrosamine in the Pig-a, micronucleus and Comet assays integrated into 28-day repeat dose studies

As part of the Stage 3 of the Pig-a international trial, we evaluated 7,12-dimethylbenz(a)anthracene (DMBA) for induction of Pig-a gene mutation using a 28-day repeat dose study design in Sprague-Dawley rats. In the same study, chromosomal damage in peripheral blood and primary DNA damage in liver were also investigated by the micronucleus (MN) assay and the Comet assay, respectively. In agreement with previously published data (Dertinger et al., [2010]: Toxicol Sci 115:401-411), DMBA induced dose-dependent increases of CD59-negative erythrocytes/reticulocytes and micronucleated reticulocytes (MN-RETs). However, there was no significant increase in DNA damage in the liver cells when tested up to 10 mg/kg/day, which appears to be below the maximum tolerated dose. When tested up to 200 mg/kg/day in a follow-up 3 dose study, DMBA was positive in the liver Comet assay. Additionally, we evaluated diethylnitrosamine (DEN), a known mutagen/hepatocarcinogen, for induction of Pig-a mutation, MN and DNA damage in a 28-day study. DEN produced negative results in both the Pig-a mutation assay and the MN assay, but induced dose-dependent increases of DNA damage in the liver and blood Comet assay. In summary, our results demonstrated that the Pig-a mutation assay can be effectively integrated into repeat dose studies and the data are highly reproducible between different laboratories. Also, integration of multiple genotoxicity endpoints into the same study not only provides a comprehensive evaluation of the genotoxic potential of test chemicals, but also reduces the number of animals needed for testing, especially when more than one in vivo genotoxicity tests are required.

Genotoxic evaluation of the non-halogenated disinfection by-products nitrosodimethylamine and nitrosodiethylamine

Disinfection by-products (DBPs) are chemicals that are produced as a result of chlorine being added to water for disinfection. As well as the halogenated DBPs, N-nitrosamines have recently been identified as DBPs, especially when amines and ammonia ions are present in raw water. In this work, the genotoxicity of two nitrosamines, namely nitrosodimethylamine (NDMA) and nitrosodiethylamine (NDEA), has been studied in cultured human cells. To evaluate their genotoxic potential two assays were used, the comet assay and the micronucleus test. The comet assay measures the induction of single and double-strand breaks, and also reveals the induced oxidative DNA damage by using endoIII and FPG enzymes. Chromosomal damage was evaluated by means of the cytokinesis-blocked micronucleus test. The results of the comet assay show that both compounds are slightly genotoxic but only at high concentrations, NDEA being more effective than NDMA. Enzyme treatments revealed that only NDEA was able to produce increased levels of oxidized bases, mainly in purine sites. The results obtained in the micronucleus assay, which measures the capacity of the tested agents to induce clastogenic and/or aneugenic effects, are negative for both of the nitrosamines evaluated, either using TK6 cells or human peripheral blood lymphocytes. Taking into account the very high concentrations needed to produce DNA damage, our data suggest a low, if existent, genotoxic risk associated with the presence of these compounds in drinking water.

Chromium-induced genotoxicity and interference in human lymphoblastoid cell (TK6) repair processes

Two model chromium (Cr) compounds, one hexavalent (sodium chromate) and one trivalent (chromium chloride), were investigated in a human lymphoblastoid cell line (TK6) to increase our knowledge regarding Cr-induced genotoxicity mechanisms. Both selected compounds were genotoxic using the comet assay, although the percentage of DNA in tail obtained after treatment with Cr(VI) was significantly higher than that obtained with Cr(III), at the higher concentrations tested. To determine the nature of the induced damage, enzymes recognizing oxidized bases were used. Treatments with formamidopyrimidine (FPG) and endonuclease III (EndoIII) displayed a greater degree of DNA damage, indicating that the induction of oxidized bases accounts for an important proportion of the damage induced by Cr compounds. In addition, the kinetic repair studies showed that generated DNA damage is removed in approximately 8 h, with the damage induced by Cr(III) being removed/repaired more rapidly than damage produced by Cr(VI). To detect Cr interferences with the repair process, a post-treatment was applied after exposure to 2 Gy gamma radiation. Post-treatment significantly delayed the repair kinetics of DNA damage induced by radiation. This interference effect induced by Cr(VI) was more pronounced. In conclusion, evidence indicates that a high proportion of the Cr-induced DNA damage is correlated with oxidative damage, and that both Cr compounds interfere with repair mechanisms involved in repair of DNA damage induced by gamma radiation.

In vitro screening for population variability in chemical toxicity

Immortalized human lymphoblastoid cell lines have been used to demonstrate that it is possible to use an in vitro model system to identify genetic factors that affect responses to xenobiotics. To extend the application of such studies to investigative toxicology by assessing interindividual and population-wide variability and heritability of chemical-induced toxicity phenotypes, we have used cell lines from the Centre d'Etude du Polymorphisme Humain (CEPH) trios assembled by the HapMap Consortium. Our goal is to aid in the development of predictive in vitro genetics-anchored models of chemical-induced toxicity. Cell lines from the CEPH trios were exposed to three concentrations of 14 environmental chemicals. We assessed ATP production and caspase-3/7 activity 24 h after treatment. Replicate analyses were used to evaluate experimental variability and classify responses. We show that variability of response across the cell lines exists for some, but not all, chemicals, with perfluorooctanoic acid (PFOA) and phenobarbital eliciting the greatest degree of interindividual variability. Although the data for the chemicals used here do not show evidence for broad-sense heritability of toxicity response phenotypes, substantial cell line variation was found, and candidate genetic factors contributing to the variability in response to PFOA were investigated using genome-wide association analysis. The approach of screening chemicals for toxicity in a genetically defined yet diverse in vitro human cell-based system is potentially useful for identification of chemicals that may pose a highest risk, the extent of within-species variability in the population, and genetic loci of interest that potentially contribute to chemical susceptibility.