Toxicity characterization of environmental chemicals by the US National Toxicology Program: an overview

The Effects of Lycium chinense, Cuscuta chinensis, Senna tora, Ophiopogon japonicus, and Dendrobium nobile Decoction on a Dry Eye Mouse Model

Background and objective: Dry eye disease (DED) is a relatively common disorder associated with abnormal tear film and the ocular surface that causes ocular irritation, dryness, visual impairment, and damage to the cornea. DED is not a life-threatening disease but causes discomfort and multifactorial disorders in vision that affect daily life. It has been reported that all traditional medicinal plants exhibit anti-inflammatory effects on several diseases. We hypothesized that the decoction ameliorated ocular irritation and decreased cytokine expression in the cornea. This study aimed to investigate the molecular mechanisms of DED and discover a therapeutic strategy to reduce corneal inflammation. Material and Methods: We used a DED mouse model with extraorbital lacrimal gland (ELG) excision and treated the mice with a decoction of five traditional medicines: Lycium chinense, Cuscuta chinensis, Senna tora, Ophiopogon japonicus, and Dendrobium nobile for 3 months. The tear osmolarity and the ocular surface staining were evaluated as indicators of DED. Immunohistochemistry was used to detect the level of inflammation on the cornea. Results: After treatment with the decoction for three months, epithelial erosions and desquamation were reduced, the intact of corneal endothelium was maintained, and tear osmolarity was restored in the eyes. The IL-1β-associated inflammatory response was reduced in the cornea in the DED model. Conclusions: These data suggested that a mixture of traditional medicines might be a novel therapy to treat DED.

Determining the endocrine disruption potential of industrial chemicals using an integrative approach: Public databases, in vitro exposure, and modeling receptor interactions

Environmental and occupational exposure to industrial chemicals has been linked to toxic and carcinogenic effects in animal models and human studies. However, current toxicology testing does not thoroughly explore the endocrine disrupting effects of industrial chemicals, which may have low dose effects not predicted when determining the limit of toxicity. The objective of this study was to evaluate the endocrine disrupting potential of a broad range of chemicals used in the petrochemical sector. Therefore, 139 chemicals were classified for reproductive toxicity based on the United Nations Globally Harmonized System for hazard classification. These chemicals were evaluated in PubMed for reported endocrine disrupting activity, and their endocrine disrupting potential was estimated by identifying chemicals with active nuclear receptor endpoints publicly available databases. Evaluation of ToxCast data suggested that these chemicals preferentially alter the activity of the estrogen receptor (ER). Four chemicals were prioritized for in vitro testing using the ER-positive, immortalized human uterine Ishikawa cell line and a range of concentrations below the reported limit of toxicity in humans. We found that 2,6-di-tert-butyl-p-cresol (BHT) and diethanolamine (DEA) repressed the basal expression of estrogen-responsive genes PGR, NPPC, and GREB1 in Ishikawa cells, while tetrachloroethylene (PCE) and 2,2'-methyliminodiethanol (MDEA) induced the expression of these genes. Furthermore, low-dose combinations of PCE and MDEA produced additive effects. All four chemicals interfered with estradiol-mediated induction of PGR, NPPC, and GREB1. Molecular docking demonstrated that these chemicals could bind to the ligand binding site of ERα, suggesting the potential for direct stimulatory or inhibitory effects. We found that these chemicals altered rates of proliferation and regulated the expression of cell proliferation associated genes. These findings demonstrate previously unappreciated endocrine disrupting effects and underscore the importance of testing the endocrine disrupting potential of chemicals in the future to better understand their potential to impact public health.

The use of high-throughput screening techniques to evaluate mitochondrial toxicity

Toxicologists and chemical regulators depend on accurate and effective methods to evaluate and predict the toxicity of thousands of current and future compounds. Robust high-throughput screening (HTS) experiments have the potential to efficiently test large numbers of chemical compounds for effects on biological pathways. HTS assays can be utilized to examine chemical toxicity across multiple mechanisms of action, experimental models, concentrations, and lengths of exposure. Many agricultural, industrial, and pharmaceutical chemicals classified as harmful to human and environmental health exert their effects through the mechanism of mitochondrial toxicity. Mitochondrial toxicants are compounds that cause a decrease in the number of mitochondria within a cell, and/or decrease the ability of mitochondria to perform normal functions including producing adenosine triphosphate (ATP) and maintaining cellular homeostasis. Mitochondrial dysfunction can lead to apoptosis, necrosis, altered metabolism, muscle weakness, neurodegeneration, decreased organ function, and eventually disease or death of the whole organism. The development of HTS techniques to identify mitochondrial toxicants will provide extensive databases with essential connections between mechanistic mitochondrial toxicity and chemical structure. Computational and bioinformatics approaches can be used to evaluate compound databases for specific chemical structures associated with toxicity, with the goal of developing quantitative structure-activity relationship (QSAR) models and mitochondrial toxicophores. Ultimately these predictive models will facilitate the identification of mitochondrial liabilities in consumer products, industrial compounds, pharmaceuticals and environmental hazards.

A Guidance Manual for the Toxicity Assessment of Traditional Herbal Medicines

Herbal remedies have been used for thousands of years in worldwide traditional medicines for their potential health benefits. Although they are generally presumed safe unless a significant risk has been identified in humans, increasing number of case reports notify acute or chronic intoxications resulting from their use. This study aims to produce a scientific guide for the evaluation of traditional herbal medicines (THMs) in terms of their toxicity risks based on the published regulatory documents. For this purpose recommended in vitro and in vivo toxicity tests on medicinal products for human use issued by the international regulatory bodies are overviewed and they are then adopted to be used for the toxicity assessment of THMs. Accordingly, based on compilation of these issued regulations, the following tests are recommended for the toxicity assessment of THMs; in vitro cytotoxicity, genotoxicity, acute and repeated dose toxicity, carcinogenicity, reproductive and developmental toxicity, local tolerance tests, toxicokinetic studies, and additional toxicity tests including safety pharmacology, immunotoxicity and antigenicity, endocrine system toxicity, gastro-intestinal toxicity, renal and hepatotoxicity, and drug interaction studies. This study describes and discusses the applicability of these tests for the risk assessment in THMs.

Approaches in fostering quality parameters for medicinal botanicals in the Indian context

India is among the important megabiodiversity centers of the world with nearly 45,000 known plant species. This diversity coupled with a rich heritage of traditional knowledge has made India a home to several important time-honored systems of health care such as Ayurveda, Siddha and Unani. Herbal medicines, however, are associated with a number of shortcomings including uniform efficacy and lack of appropriate quality control measures at various stages of product development. The review intends to outline the importance of fostering quality parameters towards standardization and manufacturing of botanicals for India to emerge as a leader in global market of herbal products. Literature survey was carried out on important parameters for processing and manufacturing of botanicals. The review highlights that there have been constant efforts for developing state of the art technologies in the field of herbal research. It also reflects that Government authorities have also taken a number of initiatives to formulate appropriate guidelines from standardization of raw materials to obtaining botanical products. However, in the Indian context, there exist certain lacunae in the current regulatory mechanisms which need to be strengthened and stringently implemented to ensure safety, purity and efficacy of herbal medicines. Towards this the approaches being developed globally can be adopted. Based on the literature reviewed, in our opinion, four areas viz., benefit sharing, investment by industry, standardization and national/international networking structure need immediate attention for strengthening Traditional Systems of Medicine in India.

High-throughput respirometric assay identifies predictive toxicophore of mitochondrial injury

Many environmental chemicals and drugs negatively affect human health through deleterious effects on mitochondrial function. Currently there is no chemical library of mitochondrial toxicants, and no reliable methods for predicting mitochondrial toxicity. We hypothesized that discrete toxicophores defined by distinct chemical entities can identify previously unidentified mitochondrial toxicants. We used a respirometric assay to screen 1760 compounds (5 μM) from the LOPAC and ChemBridge DIVERSet libraries. Thirty-one of the assayed compounds decreased uncoupled respiration, a stress test for mitochondrial dysfunction, prior to a decrease in cell viability and reduced the oxygen consumption rate in isolated mitochondria. The mitochondrial toxicants were grouped by chemical similarity and two clusters containing four compounds each were identified. Cheminformatic analysis of one of the clusters identified previously uncharacterized mitochondrial toxicants from the ChemBridge DIVERSet. This approach will enable the identification of mitochondrial toxicants and advance the prediction of mitochondrial toxicity for both drug discovery and risk assessment.

Paradigm shift in toxicity testing and modeling

The limitations of traditional toxicity testing characterized by high-cost animal models with low-throughput readouts, inconsistent responses, ethical issues, and extrapolability to humans call for alternative strategies for chemical risk assessment. A new strategy using in vitro human cell-based assays has been designed to identify key toxicity pathways and molecular mechanisms leading to the prediction of an in vivo response. The emergence of quantitative high-throughput screening (qHTS) technology has proved to be an efficient way to decompose complex toxicological end points to specific pathways of targeted organs. In addition, qHTS has made a significant impact on computational toxicology in two aspects. First, the ease of mechanism of action identification brought about by in vitro assays has enhanced the simplicity and effectiveness of machine learning, and second, the high-throughput nature and high reproducibility of qHTS have greatly improved the data quality and increased the quantity of training datasets available for predictive model construction. In this review, the benefits of qHTS routinely used in the US Tox21 program will be highlighted. Quantitative structure-activity relationships models built on traditional in vivo data and new qHTS data will be compared and analyzed. In conjunction with the transition from the pilot phase to the production phase of the Tox21 program, more qHTS data will be made available that will enrich the data pool for predictive toxicology. It is perceivable that new in silico toxicity models based on high-quality qHTS data will achieve unprecedented reliability and robustness, thus becoming a valuable tool for risk assessment and drug discovery.

Simulating quantitative cellular responses using asynchronous threshold Boolean network ensembles

BACKGROUND

With increasing knowledge about the potential mechanisms underlying cellular functions, it is becoming feasible to predict the response of biological systems to genetic and environmental perturbations. Due to the lack of homogeneity in living tissues it is difficult to estimate the physiological effect of chemicals, including potential toxicity. Here we investigate a biologically motivated model for estimating tissue level responses by aggregating the behavior of a cell population. We assume that the molecular state of individual cells is independently governed by discrete non-deterministic signaling mechanisms. This results in noisy but highly reproducible aggregate level responses that are consistent with experimental data.

RESULTS

We developed an asynchronous threshold Boolean network simulation algorithm to model signal transduction in a single cell, and then used an ensemble of these models to estimate the aggregate response across a cell population. Using published data, we derived a putative crosstalk network involving growth factors and cytokines - i.e., Epidermal Growth Factor, Insulin, Insulin like Growth Factor Type 1, and Tumor Necrosis Factor α - to describe early signaling events in cell proliferation signal transduction. Reproducibility of the modeling technique across ensembles of Boolean networks representing cell populations is investigated. Furthermore, we compare our simulation results to experimental observations of hepatocytes reported in the literature.

CONCLUSION

A systematic analysis of the results following differential stimulation of this model by growth factors and cytokines suggests that: (a) using Boolean network ensembles with asynchronous updating provides biologically plausible noisy individual cellular responses with reproducible mean behavior for large cell populations, and (b) with sufficient data our model can estimate the response to different concentrations of extracellular ligands. Our results suggest that this approach is both quantitative, allowing statistical verification and calibration, and extensible, allowing modification and revision as guided by experimental evidence. The simulation methodology is part of the US EPA Virtual Liver, which is investigating the effects of everyday contaminants on living tissues. Future models will incorporate additional crosstalk surrounding proliferation as well as the putative effects of xenobiotics on these signaling cascades within hepatocytes.

Incorporating human dosimetry and exposure into high-throughput in vitro toxicity screening

Many chemicals in commerce today have undergone limited or no safety testing. To reduce the number of untested chemicals and prioritize limited testing resources, several governmental programs are using high-throughput in vitro screens for assessing chemical effects across multiple cellular pathways. In this study, metabolic clearance and plasma protein binding were experimentally measured for 35 ToxCast phase I chemicals. The experimental data were used to parameterize a population-based in vitro-to-in vivo extrapolation model for estimating the human oral equivalent dose necessary to produce a steady-state in vivo concentration equivalent to in vitro AC(50) (concentration at 50% of maximum activity) and LEC (lowest effective concentration) values from the ToxCast data. For 23 of the 35 chemicals, the range of oral equivalent doses for up to 398 ToxCast assays was compared with chronic aggregate human oral exposure estimates in order to assess whether significant in vitro bioactivity occurred within the range of maximum expected human oral exposure. Only 2 of the 35 chemicals, triclosan and pyrithiobac-sodium, had overlapping oral equivalent doses and estimated human oral exposures. Ranking by the potencies of the AC(50) and LEC values, these two chemicals would not have been at the top of a prioritization list. Integrating both dosimetry and human exposure information with the high-throughput toxicity screening efforts provides a better basis for making informed decisions on chemical testing priorities and regulatory attention. Importantly, these tools are necessary to move beyond hazard rankings to estimates of possible in vivo responses based on in vitro screens.

In vitro cell-toxicity screening as an alternative animal model for coral toxicology: effects of heat stress, sulfide, rotenone, cyanide, and cuprous oxide on cell viability and mitochondrial function

The logistics involved in obtaining and maintaining large numbers of corals hampers research on the toxicological effects of environmental contaminants for this ecologically and economically important taxon. A method for creating and culturing single-cell suspensions of viable coral cells was developed. Cell segregation/separation was based on specific cell densities and resulting cell cultures were viable for at least 2 mos. Low-density cells lacking symbiotic zooxanthallae and rich in mitochondria were isolated and cultured for toxicity studies. Cells were exposed to differing degrees or concentrations of heat stress, rotenone, cyanide, sulfide, and cuprous oxide. Cells were assayed for mitochondrial membrane potential using the fluorescent probe, JC-9, and for overall viability using the MTT/formazan spectrophotometric viability assay. Significant differences were observed between controls and treatments and the efficacy of this method was validated; only 2 cm(2) of tissue was required for a seven-point concentration-exposure series.

Compound cytotoxicity profiling using quantitative high-throughput screening

BACKGROUND

The propensity of compounds to produce adverse health effects in humans is generally evaluated using animal-based test methods. Such methods can be relatively expensive, low-throughput, and associated with pain suffered by the treated animals. In addition, differences in species biology may confound extrapolation to human health effects.

OBJECTIVE

The National Toxicology Program and the National Institutes of Health Chemical Genomics Center are collaborating to identify a battery of cell-based screens to prioritize compounds for further toxicologic evaluation.

METHODS

A collection of 1,408 compounds previously tested in one or more traditional toxicologic assays were profiled for cytotoxicity using quantitative high-throughput screening (qHTS) in 13 human and rodent cell types derived from six common targets of xenobiotic toxicity (liver, blood, kidney, nerve, lung, skin). Selected cytotoxicants were further tested to define response kinetics.

RESULTS

qHTS of these compounds produced robust and reproducible results, which allowed cross-compound, cross-cell type, and cross-species comparisons. Some compounds were cytotoxic to all cell types at similar concentrations, whereas others exhibited species- or cell type-specific cytotoxicity. Closely related cell types and analogous cell types in human and rodent frequently showed different patterns of cytotoxicity. Some compounds inducing similar levels of cytotoxicity showed distinct time dependence in kinetic studies, consistent with known mechanisms of toxicity.

CONCLUSIONS

The generation of high-quality cytotoxicity data on this large library of known compounds using qHTS demonstrates the potential of this methodology to profile a much broader array of assays and compounds, which, in aggregate, may be valuable for prioritizing compounds for further toxicologic evaluation, identifying compounds with particular mechanisms of action, and potentially predicting in vivo biological response.

The plasma proteome, adductome and idiosyncratic toxicity in toxicoproteomics research

Toxicoproteomics uses the discovery potential of proteomics in toxicology research by applying global protein measurement technologies to biofluids and tissues after host exposure to injurious agents. Toxicoproteomic studies thus far have focused on protein profiling of major organs and biofluids such as liver and blood in preclinical species exposed to model toxicants. The slow pace of discovery for new biomarkers, toxicity signatures and mechanistic insights is partially due to the limited proteome coverage derived from analysis of native organs, tissues and body fluids by traditional proteomic platforms. Improved toxicoproteomic analysis would result by combining higher data density LC-MS/MS platforms with stable isotope labelled peptides and parallel use of complementary platforms. Study designs that remove abundant proteins from biofluids, enrich subcellular structures and include cell specific isolation from heterogeneous tissues would greatly increase differential expression capabilities. By leveraging resources from immunology, cell biology and nutrition research communities, toxicoproteomics could make particular contributions in three inter-related areas to advance mechanistic insights and biomarker development: the plasma proteome and circulating microparticles, the adductome and idiosyncratic toxicity.

90-day repeated inhalation exposure of surfactant Protein-C/tumor necrosis factor-alpha, (SP-C/TNF-alpha) transgenic mice to air pollutants

Tumor necrosis factor (TNF)-alpha, a cytokine present in inflammed lungs, is known to mediate some of the adverse effects of ozone and inhaled particles. The authors evaluated transgenic mice with constitutive pulmonary expression of TNF-alpha under transcriptional regulation of the surfactant protein-C promoter as an animal model of biological susceptibility to air pollutants. To simulate a repeated, episodic exposure to air pollutants, wild-type and TNF mice inhaled air or a mixture of ozone (0.4 ppm) and urban particles (EHC-93, 4.8 mg/m3) for 4 h, once per week, for 12 consecutive weeks and were sacrificed 20 h after last exposure. TNF mice exhibited chronic lung inflammation with septal thickening, alveolar enlargement, and elevated protein and cellularity in bronchoalveolar lavage fluid (genotype main effect, p < .001). Repeated exposure to pollutants did not result in measurable inflammatory changes in wild-type mice and did not exacerbate the inflammation in TNF mice. The pollutants decreased recovery of alveolar macrophages in tavage fluid of both wild-type and TNF mice (exposure main effect, p < .001). Exacerbation of the rate of protein nitration reactions specifically in the lungs of TNF mice was revealed by the high ratio of 3-nitrotyrosine to L-DOPA after exposure to the air pollutants (Genotype x Exposure factor interaction, p = .014). Serum creatine kinase-MM isoform increased in TNF mice exposed to pollutants (Genotype X Exposure factor interaction, p = .043). The marked pollutant-related nitration in the lungs of the TNF mice reveals basic differences in free radical generation and scavenging in the inflamed lungs in response to pollutants. Furthermore, elevation of circulating creatine kinase-MM isoform specifically in TNF mice exposed to pollutants suggests systemic adverse impacts from lung inflammatory mediators, possibly on muscles and the cardiovascular system.

Evaluation of immuno- and reproductive toxicities and association between immunotoxicological and genotoxicological parameters in waste incineration workers

In this study, we investigated immunotoxicity levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin in 31 waste incineration workers and in 84 control subjects, and evaluated the association between immunological and genotoxicological parameters. DNA damage in mononuclear and polynuclear lymphocytes, and the level of the urinary metabolites, 1-OHP and 2-naphthol, were evaluated in both waste incineration workers and control subjects. Significant differences were detected in these values between exposed and control groups. Number of sperms was lower in the waste incineration workers than in the control subjects, as was the percentage of motile sperms, but a significant difference existed only in the number of sperms (p=0.05). DNA damage in the spermatozoa of waste incineration workers and control subjects measured 1.40+/-0.08 and 1.26+/-0.03, respectively (p=0.001). Specific surface antigens of peripheral lymphocytes, namely, CD3, CD4, CD8, CD19, and CD69 were used to measure immune status in waste incineration workers and control subjects. There was no significant difference in T- and B-cell profiles showed between waste incineration workers and control subjects (p=0.684 and 0.157). In addition, the ratio of T helper cells to T cytotoxic cells was also not remarkably different between groups (p=0.174). However, T-cell activation was found to be significantly higher in the waste incineration workers than in the control subjects (p=0.001), although B-cell activation did not exhibit this trend. The levels of two cytokines (IL-4 an INF-gamma) and four immunoglobulins (IgA, IgE, IgG, and IgM) were also measured in the experimental population. All immunoglobulin types were found in lower amounts in the waste incineration workers, but this diaparity was not significant one. In addition, the levels of two cytokines, IL-4 and INF-gamma, were also found to be lower in the waste incineration workers than in the control subjects, and only in IL-4 was a significant difference determined to exist.