Investigation of potential early key events and mode of action for 1,2-dichloroethane-induced mammary tumors in female rats

1,2-dichloroethane (DCE or EDC) is a chlorinated hydrocarbon used as a chemical intermediate, including in the synthesis of polyvinyl chloride. Although DCE has induced tumors in both rats and mice, the overall weight-of-evidence suggests a lack of in vivo mutagenicity. The present study was conducted to explore a potential mode of action further for tumor formation in rat mammary tissue. Fischer 344 rats were exposed to target concentrations of 0 or 200 ppm of DCE vapors (6 hours/day, 7 days/week) for at least 28 days; 200 ppm represents a concentration of ~20% higher than that reported to induce mammary tumors. Endpoints examined included DNA damage (via Comet assay), glutathione (reduced, oxidized and conjugated), tissue DNA adducts, cell proliferation and serum prolactin levels. Exposure to DCE did not alter serum prolactin levels with consistent estrous stage, did not cause cell proliferation in mammary epithelial cells, nor result in histopathological alterations in the mammary gland. DNA adducts were identified, including the N⁷ -guanylethyl glutathione adduct, with higher adduct levels measured in liver (nontumorigenic target) compared with mammary tissue isolated from the same rats; no known mutagenic adducts were identified. DCE did not increase the Comet assay response in mammary epithelial cells, whereas DNA damage in the positive control (N-nitroso-N-methylurea) was significantly increased. Although the result of this study did not identify a specific mode of action for DCE-induced mammary tumors in rats, the lack of any exposure-related genotoxic responses further contributes to the weight-of-evidence suggesting that DCE is a nongenotoxic carcinogen.

Tiered application of the neutral red release and EpiOcular™ assays for evaluating the eye irritation potential of agrochemical formulations

Agrochemical formulations have been underrepresented in validation efforts for implementing alternative eye irritation approaches but represent a significant opportunity to reduce animal testing. This study assesses the utility of the neutral red release assay (NRR) and EpiOcular™ assay (EO) for predicting the eye irritation potential of 64 agrochemical formulations relative to Draize data. In the NRR, formulations with an NRR50 value ≤ 50 mg/mL were categorized as UN GHS Cat 1 and those >250 mg/mL were classified as UN GHS Non Classified (NC). The accuracy, sensitivity, and specificity were 78, 85 and 76% and 73, 85 and 61% for identifying UN GHS 1 and NC formulations, respectively. Specificity was poor for formulations with NRR50 > 50 to ≤250 mg/mL. The EO (ET-40 method) was explored to differentiate formulations that were UN GHS 1/2 and UN GHS NC. The EO resulted in accuracy, sensitivity, and specificity of 65%, 58% and 75% for identifying UN GHS NC formulations. To improve the overall performance, the assays were implemented using a tiered-approach where the NRR was run as a first-tier followed by the EO. The tiered-approach resulted in improved accuracy (75%) and balanced sensitivity (73%) and specificity (77%) for distinguishing between irritating and non-irritating agrochemical formulations.

Predicting the future: opportunities and challenges for the chemical industry to apply 21st-century toxicity testing

Interest in applying 21st-century toxicity testing tools for safety assessment of industrial chemicals is growing. Whereas conventional toxicology uses mainly animal-based, descriptive methods, a paradigm shift is emerging in which computational approaches, systems biology, high-throughput in vitro toxicity assays, and high-throughput exposure assessments are beginning to be applied to mechanism-based risk assessments in a time- and resource-efficient fashion. Here we describe recent advances in predictive safety assessment, with a focus on their strategic application to meet the changing demands of the chemical industry and its stakeholders. The opportunities to apply these new approaches is extensive and include screening of new chemicals, informing the design of safer and more sustainable chemical alternatives, filling information gaps on data-poor chemicals already in commerce, strengthening read-across methodology for categories of chemicals sharing similar modes of action, and optimizing the design of reduced-risk product formulations. Finally, we discuss how these predictive approaches dovetail with in vivo integrated testing strategies within repeated-dose regulatory toxicity studies, which are in line with 3Rs principles to refine, reduce, and replace animal testing. Strategic application of these tools is the foundation for informed and efficient safety assessment testing strategies that can be applied at all stages of the product-development process.

Concurrent evaluation of general, immune, and genetic toxicity endpoints as part of an integrated testing strategy

Integrated testing strategies involve the assessment of multiple endpoints within a single toxicity study and represent an important approach for reducing animal use and streamlining testing. The present study evaluated the ability to combine general, immune, and genetic toxicity endpoints into a single study. Specifically, this study evaluated the impact of sheep red blood cell (SRBC) immunization, as part of the T-cell dependent antibody response (TDAR) assay, on organ weights, micronuclei (MN) formation (bone marrow and peripheral blood), and the Comet assay response in the liver of female F344/DuCrl rats treated with cyclophosphamide (CP) a known immunosuppressive chemical and genotoxicant. For the TDAR assay, treatment with CP resulted in a dose-dependent decrease in the antibody response with a suppression of greater than 95% at the high dose. Injection with SRBC had no impact on evaluated organ weights, histopathology, hematology, and clinical chemistry parameters. Analysis of MN formation in bone marrow and peripheral blood revealed a dose-dependent increase in response to CP treatment. Injection with SRBC had no impact on the level of MN in control animals and did not alter the dose response of CP. There was a slight increase in liver DNA damage in response to CP as measured by the Comet assay; however, injection with SRBCs did not alter this endpoint. Overall these data provide strong support for the concurrent assessment of general, immune, and genetic toxicology endpoints within a single study as part of an integrated testing strategy approach.

Chemical respiratory allergy: reverse engineering an adverse outcome pathway

Allergic sensitisation of the respiratory tract by chemicals is associated with rhinitis and asthma and remains an important occupational health issue. Although less than 80 chemicals have been confirmed as respiratory allergens the adverse health effects can be serious, and in rare instances can be fatal, and there are, in addition, related socioeconomic issues. The challenges that chemical respiratory allergy pose for toxicologists are substantial. No validated methods are available for hazard identification and characterisation, and this is due in large part to the fact that there remains considerable uncertainty and debate about the mechanisms through which sensitisation of the respiratory tract is acquired. Despite that uncertainty, there is a need to establish some common understanding of the key events and processes that are involved in respiratory sensitisation to chemicals and that might in turn provide the foundations for novel approaches to safety assessment. In recent years the concept of adverse outcome pathways (AOP) has gained some considerable interest among the toxicology community as a basis for outlining the key steps leading to an adverse health outcome, while also providing a framework for focusing future research, and for developing alternative paradigms for hazard characterisation. Here we explore application of the same general principles to an examination of the induction by chemicals of respiratory sensitisation. In this instance, however, we have chosen to adopt a reverse engineering approach and to model a possible AOP for chemical respiratory allergy working backwards from the elicitation of adverse health effects to the cellular and molecular mechanisms that are implicated in the acquisition of sensitisation.

An F1-extended one-generation reproductive toxicity study in Crl:CD(SD) rats with 2,4-dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid (2,4-D) was assessed for systemic toxicity, reproductive toxicity, developmental neurotoxicity (DNT), developmental immunotoxicity (DIT), and endocrine toxicity. CD rats (27/sex/dose) were exposed to 0, 100, 300, 600 (female), or 800 (male) ppm 2,4-D in diet. Nonlinear toxicokinetic behavior was shown at high doses; the renal clearance saturation threshold for 2,4-D was exceeded markedly in females and slightly exceeded in males. Exposure was 4 weeks premating, 7 weeks postmating for P1 males and through lactation for P1 females. F1 offspring were examined for survival and development, and at weaning, pups were divided in cohorts, by sex and dose, and by systemic toxicity (10), DNT (10), DIT (20), and reproductive toxicity (≥ 23). Remaining weanlings were evaluated for systemic toxicity and neuropathology (10–12). Body weight decreased during lactation in high-dose P1 females and in F1 pups. Kidney was the primary target organ, with slight degeneration of proximal convoluted tubules observed in high-dose P1 males and in high-dose F1 males and females. A slight intergenerational difference in kidney toxicity was attributed to increased intake of 2,4-D in F1 offspring. Decreased weanling testes weights and delayed preputial separation in F1 males were attributed to decreased body weights. Endocrine-related effects were limited to slight thyroid hormone changes and adaptive histopathology in high-dose GD 17 dams seen only at a nonlinear toxicokinetic dose. 2,4-D did not cause reproductive toxicity, DNT, or DIT. The “No Observed Adverse Effect Level” for systemic toxicity was 300 ppm in both males (16.6mg/kg/day) and females (20.6mg/kg/day), which is approximately 6700- to 93 000-fold higher than that reported for 2,4-D exposures in human biomonitoring studies.

Toxicogenomic evaluation of long-term hepatic effects of TCDD in immature, ovariectomized C57BL/6 mice

Acute exposure to hepatotoxic doses of 2,3,7,8-tetrachloro- dibenzo-p-dioxin (TCDD) in mice is characterized by differential gene expression that can be phenotypically anchored to elevated levels of serum alanine aminotransferase, increased relative liver weights, hepatic steatosis, inflammation, and hepatocellular necrosis. Unlike most studies that focus on acute exposure effects, this study evaluated the long-term effects of a single oral gavage of 30 μg/kg TCDD at 1, 4, 12, 24, 36, and 72 weeks postdose in ovariectomized C57BL/6 mice. Hepatic TCDD levels were almost completely eliminated by 24 weeks with a calculated half-life of 12 days. Hepatic gene expression analysis identified 395 unique differentially expressed genes between 1 and 12 weeks that decreased to ≤ 8 by 72 weeks, consistent with the minimal hepatic TCDD levels. Hepatic vacuolization, characteristic of short-term exposure, subsided by 4 weeks. Similarly, TCDD-elicited hepatic necrosis and inflammation dissipated by 1 week. Collectively, these results suggest that TCDD-elicited histologic and gene expression responses can be correlated to elevated hepatic TCDD levels, which, once eliminated, elicit minimal hepatic gene expression and histologic alterations.

Assessment of the respiratory sensitization potential of proteins using an enhanced mouse intranasal test (MINT)

There remains a need for a simple and predictive animal model to identify potential respiratory sensitizers. The mouse intranasal test (MINT) was developed to assess the relative allergic potential of detergent enzymes, however, the experimental endpoints were limited to evaluation of antibody levels. The present study was designed to evaluate additional endpoints (serum and allergic antibody levels, pulmonary inflammation and airway hyperresponsiveness (AHR)) to determine their value in improving the predictive accuracy of the MINT. BDF1 mice were intranasally instilled on days 1, 3, 10, 17 and 24 with subtilisin, ovalbumin, betalactoglobulin, mouse serum albumin or keyhole limpet hemocyanin; challenged with aerosolized methacholine or the sensitizing protein on day 29 to assess AHR, and sacrificed on day 29 or 30. Under the conditions of this study, evaluation of AHR did not improve the predictive power of this experimental model. Allergic antibody responses and IgG isotype characterization proved to be the most sensitive and reliable indicators of the protein allergenic potential with BAL responses providing additional insight. These data highlight that the evaluation of the respiratory sensitization potential of proteins can be best informed when multiple parameters are evaluated and that further improvements and refinements of the assay are necessary.

Assessment of the immunotoxic potential of trichloroethylene and perchloroethylene in rats following inhalation exposure

The immunotoxic potential of trichloroethylene (TCE) and perchloroethylene (PERC) was assessed after inhalation exposure through the evaluation of the antibody forming cell (AFC) response to sheep red blood cells (SRBC). Female Sprague-Dawley rats were exposed to TCE or PERC vapor at 0, 100, 300, or 1000 ppm for 6 h/day, 5 days/week for 4 weeks (20 exposure days). Additional 0 ppm control groups were included and were dosed with cyclophosphamide via intraperitoneal injection to serve as positive immunosuppressive controls in the SRBC assay. Additional end-points evaluated included liver, kidney, spleen, and thymus weights, hematology, cellular differentials in bronchoalveolar lavage fluid, histopathology of select tissues, and assessment of the phagocytic activity of pulmonary alveolar macrophage (PERC only). Exposure to the high concentration of TCE (1000 ppm) resulted in increases in relative liver and kidney weights and suppression of AFC responses (AFC/spleen and AFC/10(6) spleen cells) by ≈ 70%, while no treatment-related effects were noted at 100 and 300 ppm. Animals exposed to PERC at levels of 300 or 1000 ppm had statistically significant increases in relative liver weights that were accompanied by very slight hypertrophy of the centrilobular hepatocytes. Animals exposed to PERC did not demonstrate a treatment-related effect on the AFC response and no effect was noted on the phagocytic activity of pulmonary alveolar macrophages. The results of these studies indicate that TCE had immunotoxic potential only at high exposure concentrations (1000 ppm), while PERC, at similar exposure concentrations, did not display any evidence of immunotoxicity.

Transgenic animal models in toxicology: historical perspectives and future outlook

Transgenic animal models are powerful tools for developing a more detailed understanding on the roles of specific genes in biological pathways and systems. Applications of these models have been made within the field of toxicology, most notably for the screening of mutagenic and carcinogenic potential and for the characterization of toxic mechanisms of action. It has long been a goal of research toxicologists to use the data from these models to refine hazard identification and characterization to better inform human health risk assessments. This review provides an overview on the applications of transgenic animal models in the assessment of mutagenicity and carcinogenicity, their use as reporter systems, and as tools for understanding the roles of xenobiotic-metabolizing enzymes and biological receptors in the etiology of chemical toxicity. Perspectives are also shared on the future outlook for these models in toxicology and risk assessment and how transgenic technologies are likely to be an integral tool for toxicity testing in the 21st century.

A draining lymph node assay (DLNA) for assessing the sensitizing potential of proteins

There is a need for a simple and predictive model to identify the respiratory sensitization potential of (novel) proteins. The present study examined the use of a mouse draining lymph node assay (DLNA) approach, employing several routes of exposure, as a possible starting point for assessing protein sensitization potential. Consistent with the experimental procedure for the standard local lymph node assay (LLNA), female BALB/c mice were dosed dermally (topical), intranasally (IN) or by oropharyngeal aspiration (OP) on days 1, 2 and 3, and proliferation in the relevant draining lymph nodes was measured on day 6. For each route, the auricular, superficial cervical and tracheobronchial lymph nodes (TBLN) were evaluated following treatment with Subtilisin Carlsberg (SUB; a potent sensitizer/allergen), ovalbumin (OVA; a potent food allergen), beta-lactoglobulin (BLG; a moderate food allergen), and keyhole limpet hemocyanin (KLH; a strong immunogen with no reports of respiratory sensitization). Initial studies with OVA indicated that dermal administration did not stimulate lymph node proliferation. Responses in the tracheobronchial lymph node were most dramatic (stimulation indices up to 100) and reproducible for both the IN and OP routes. In a comparative experiment, all proteins induced lymph node proliferation with a rank order potency of SUB>KLH>OVA>BLG. The influence of the endotoxin content on lymph node proliferation was determined to be minimal, and did not impact the rank order potency. Molecular characterization of the TBLN at an equipotent proliferative dose was conducted for select gene transcripts based on research examining chemical sensitizers. Expression profiles differed among the four proteins, but the relevance of these responses was not clear and they did not further discriminate their allergic potential. These data illustrate both the opportunities and challenges associated with the examination of the draining lymph node proliferative response to assess the allergenic potential of proteins.

Comparative temporal toxicogenomic analysis of TCDD- and TCDF-mediated hepatic effects in immature female C57BL/6 mice

Temporal analyses were performed on hepatic tissue from immature female C57BL/6 mice in order to compare the gene expression profiles for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibzofuran (TCDF). Time course studies conducted with a single oral dose of 300 microg/kg TCDF or 30 microg/kg TCDD were used to compare differential gene expression on complementary DNA microarrays containing 13,361 features, representing 8194 genes at 2, 4, 8, 12, 24, 72, 120, and 168 h. One hundred and ninety-five genes were identified as differentially regulated by TCDF, of which 116 genes were in common with TCDD, with 109 exhibiting comparable expression profiles (correlation coefficients > 0.3). In general, TCDF was less effective in eliciting hepatic vacuolization, and differential gene expression compared with TCDD when given at an equipotent dose based on a toxic equivalence factor (TEF) of 0.1 for TCDF, especially 72-h postadministration. For example, the induction of Cyp1a1 messenger RNA by TCDF was less when compared TCDD. Moreover, TCDF induced less severe hepatocyte cytoplasmic vacuolization consistent with lower lipid accumulations which significantly subsided by 120 and 168 h when compared with TCDD. TCDF-elicited responses correlated with their hepatic tissue levels which gradually decreased between 18 and 168 h. Although both compounds elicited comparable gene expression profiles, especially at early time points, the TCDF responses were generally weaker. Collectively, these results suggest that the weaker TCDF responses could be attributed to differences in pharmacokinetics. However, more comprehensive dose-response studies are required at optimal times for each end point of interest in order to investigate the effect of pharmacokinetic differences on relative potencies that are important in establishing TEFs.

Comparative toxicogenomic examination of the hepatic effects of PCB126 and TCDD in immature, ovariectomized C57BL/6 mice

Polychlorinated biphenyls are persistent environmental pollutants that elicit a wide range of effects in humans and wildlife, mediated by the aryl hydrocarbon receptor. 3,3',4,4',5-pentachlorobiphenyl (PCB126) is the most potent congener with relative effect potencies ranging from 0.0026 to 0.857, and a toxic equivalency factor (TEF) of 0.1 set by an expert panel of the World Health Organization. In this study, the hepatic effects elicited by 300 microg/kg PCB126 were compared with 30 microg/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in immature, ovariectomized female C57BL/6 mice. Comprehensive hepatic gene expression analyses with complementary histopathology, high-resolution gas chromatograph/high-resolution mass spectrometer tissue analysis, and clinical chemistry were examined. For temporal analysis, mice were orally gavaged with PCB126 or sesame oil vehicle and sacrificed after 2, 4, 8, 12, 18, 24, 72, 120, or 168 h. In the dose-response study, mice were gavaged with 0.3, 1, 3, 10, 30, 100, 300, 1000 microg/kg PCB126, 30 or 100 microg/kg TCDD and sacrificed after 72 h. 251 and 367 genes were differentially expressed by PCB126 at one or more time points or doses, respectively, significantly less than elicited by TCDD. In addition, there was less vacuolization and necrosis, and no immune cell infiltration, despite comparable or higher TEF-adjusted hepatic PCB126 levels. The functional annotation of differentially expressed genes was consistent with the observed histopathology. Collectively, the data indicate that 300 microg/kg PCB126 elicited a subset of weaker effects compared with 30 microg/kg TCDD in immature, ovariectomized C57BL/6 mice.

Inhibition of estrogen-mediated uterine gene expression responses by dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exhibits antiestrogenic properties, including the inhibition of estrogen-induced uterine growth and proliferation. The inhibition of estrogen-mediated gene expression through ER/AhR cross-talk has been proposed as a plausible mechanism; however, only a limited number of inhibited responses have been investigated that are unlikely to fully account for the antiuterotrophic effects of TCDD. Therefore, the effects of TCDD on ethynyl estradiol (EE)-mediated uterine gene expression were investigated using cDNA microarrays with complementary physiological and histological phenotypic anchoring. Mice were gavaged with vehicle, 3 daily doses of 10 mug/kg EE, a single dose of 30 mug/kg TCDD, or a combination of EE plus TCDD and sacrificed after 4, 12, 24, and 72 h. TCDD cotreatment inhibited EE-induced uterine wet weight by 37, 23, and 45% at 12, 24, and 72 h, respectively. TCDD cotreatment also reduced EE-mediated stromal edema, hypertrophy, and hyperplasia and induced marked luminal epithelial cell apoptosis. A 2 x 2 factorial microarray design was used to identify EE- and TCDD-specific differential gene expression responses as well as their interactive effects. Only 133 of the 2753 EE-mediated differentially expressed genes were significantly modulated by TCDD cotreatment, indicating a gene-specific inhibitory response. The EE-mediated induction of many genes, including trefoil factor 1 and keratin 14, were inhibited by greater than 90% by TCDD. Functional annotation of inhibited responses was associated with cell proliferation, water and ion transport, and maintenance of cellular structure and integrity. These inhibited responses correlate with the observed histological alterations and may contribute to the antiuterotrophic effects of TCDD.

Comparative toxicogenomic analysis of the hepatotoxic effects of TCDD in Sprague Dawley rats and C57BL/6 mice

In an effort to further characterize conserved and species-specific mechanisms of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated toxicity, comparative temporal and dose-response microarray analyses were performed on hepatic tissue from immature, ovariectomized Sprague Dawley rats and C57BL/6 mice. For temporal studies, rats and mice were gavaged with 10 or 30 microg/kg of TCDD, respectively, and sacrificed after 2, 4, 8, 12, 18, 24, 72, or 168 h while dose-response studies were performed at 24 h. Hepatic gene expression profiles were monitored using custom cDNA microarrays containing 8567 (rat) or 13,361 (mouse) cDNA clones. Affymetrix data from male rats treated with 40 microg/kg TCDD were also included to expand the species comparison. In total, 3087 orthologous genes were represented in the cross-species comparison. Comparative analysis identified 33 orthologous genes that were commonly regulated by TCDD as well as 185 rat-specific and 225 mouse-specific responses. Functional annotation using Gene Ontology identified conserved gene responses associated with xenobiotic/chemical stress and amino acid and lipid metabolism. Rat-specific gene expression responses were associated with cellular growth and lipid metabolism while mouse-specific responses were associated with lipid uptake/metabolism and immune responses. The common and species-specific gene expression responses were also consistent with complementary histopathology, clinical chemistry, hepatic lipid analyses, and reports in the literature. These data expand our understanding of TCDD-mediated gene expression responses and indicate that species-specific toxicity may be mediated by differences in gene expression which may help explain the wide range of species sensitivities and will have important implications in risk assessment strategies.

In vivo-in vitro toxicogenomic comparison of TCDD-elicited gene expression in Hepa1c1c7 mouse hepatoma cells and C57BL/6 hepatic tissue

Background

In vitro systems have inherent limitations in their ability to model whole organism gene responses, which must be identified and appropriately considered when developing predictive biomarkers of in vivo toxicity. Systematic comparison of in vitro and in vivo temporal gene expression profiles were conducted to assess the ability of Hepa1c1c7 mouse hepatoma cells to model hepatic responses in C57BL/6 mice following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Results

Gene expression analysis and functional gene annotation indicate that Hepa1c1c7 cells appropriately modeled the induction of xenobiotic metabolism genes in vivo. However, responses associated with cell cycle progression and proliferation were unique to Hepa1c1c7 cells, consistent with the cell cycle arrest effects of TCDD on rapidly dividing cells. In contrast, lipid metabolism and immune responses, representative of whole organism effects in vivo, were not replicated in Hepa1c1c7 cells.

Conclusion

These results identified inherent differences in TCDD-mediated gene expression responses between these models and highlighted the limitations of in vitro systems in modeling whole organism responses, and additionally identified potential predictive biomarkers of toxicity.

Dioxin induces an estrogen-like, estrogen receptor-dependent gene expression response in the murine uterus

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a ubiquitous environmental contaminant that elicits a broad range of toxicities in a tissue-, sex-, age-, and species-specific manner, including alterations in estrogen signaling. Many, if not all, of these effects involve changes in gene expression mediated via the activation of the aryl hydrocarbon receptor (AhR), a ligand activated transcription factor. Recent data indicate that TCDD may also elicit AhR-mediated estrogenic activity through interactions with the estrogen receptor (ER). In an effort to further characterize the estrogenic activity of TCDD, a comprehensive time-course analysis of uterine gene expression was conducted using ovariectomized C57BL/6 mice. Comparison of the temporal uterine transcriptional response to TCDD with that of ethynyl estradiol (EE) revealed a large proportion of the TCDD-mediated gene expression changes were also responsive to EE. Furthermore, pretreatment of mice with the pure ER antagonist ICI 182 780 (faslodex) inhibited gene expression responses to both EE and TCDD, providing additional evidence that these transcriptional responses involve the ER.

Toxicogenomics in Risk Assessment: Applications and Needs

Since its inception, there have been high expectations for the science of toxicogenomics to decrease the uncertainties associated with the risk assessment process by providing valuable insights into toxic mechanisms of action. However, the application of these data into risk assessment practices is still in the early stages of development, and proof of principle experiments have yet to emerge. The following discusses some potential applications as well as impediments that warrant a concerted investigation from all stakeholders in order to facilitate the acceptance and subsequent incorporation of toxicogenomics into regulatory decision making.

Temporal and dose-dependent hepatic gene expression patterns in mice provide new insights into TCDD-Mediated hepatotoxicity

In an effort to further characterize the mechanisms of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated toxicity, comprehensive temporal and dose-response microarray analyses were performed on hepatic tissue from immature ovariectomized C57BL/6 mice treated with TCDD. For temporal analysis, mice were gavaged with 30 microg/kg of TCDD or vehicle and sacrificed after 2, 4, 8, 12, 18, 24, 72, or 168 h. Dose-response mice were gavaged with 0, 0.001, 0.01, 0.1, 1, 10, 100, or 300 microg/kg of TCDD and sacrificed after 24 h. Hepatic gene expression profiles were monitored using custom cDNA microarrays containing 13,362 cDNA clones. Gene expression analysis identified 443 and 315 features which exhibited a significant change at one or more doses or time points, respectively, as determined using an empirical Bayes approach. Functional gene annotation extracted from public databases associated gene expression changes with physiological processes such as oxidative stress and metabolism, differentiation, apoptosis, gluconeogenesis, and fatty acid uptake and metabolism. Complementary histopathology (H&E and Oil Red O stains), clinical chemistry (i.e., alanine aminotransferase [ALT], triglyceride [TG], free fatty acids [FFA], cholesterol) and high-resolution gas chromatography/mass spectrometry assessment of hepatic TCDD levels were also performed in order to phenotypically anchor changes in gene expression to physiological end points. Collectively, the data support a proposed mechanism for TCDD-mediated hepatotoxicity, including fatty liver, which involves mobilization of peripheral fat and inappropriate increases in hepatic uptake of fatty acids.

Normalization of two-channel microarray experiments: a semiparametric approach

MOTIVATION

An important underlying assumption of any experiment is that the experimental subjects are similar across levels of the treatment variable, so that changes in the response variable can be attributed to exposure to the treatment under study. This assumption is often not valid in the analysis of a microarray experiment due to systematic biases in the measured expression levels related to experimental factors such as spot location (often referred to as a print-tip effect), arrays, dyes, and various interactions of these effects. Thus, normalization is a critical initial step in the analysis of a microarray experiment, where the objective is to balance the individual signal intensity levels across the experimental factors, while maintaining the effect due to the treatment under investigation.

RESULTS

Various normalization strategies have been developed including log-median centering, analysis of variance modeling, and local regression smoothing methods for removing linear and/or intensity-dependent systematic effects in two-channel microarray experiments. We describe a method that incorporates many of these into a single strategy, referred to as two-channel fastlo, and is derived from a normalization procedure that was developed for single-channel arrays. The proposed normalization procedure is applied to a two-channel dose-response experiment.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces suppressor of cytokine signaling 2 in murine B cells

The B cell, a major component of humoral immunity, is a sensitive target for the immunotoxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), possibly by rendering cells less responsive to antigenic or mitogenic stimulation. Potential mechanisms of TCDD action on B cells were examined in murine B cell lymphoma cells (CH12.LX) treated with 3 nM TCDD or dimethyl sulfoxide vehicle using sequence-verified cDNA microarrays. One transcript that was significantly induced by TCDD was suppressor of cytokine signaling 2 (Socs2). Changes in Socs2 mRNA levels paralleled that of Cyp1a1 with a maximal 3-fold induction observed at 4 h, as determined by quantitative real-time polymerase chain reaction. Socs2 induction seems B cell-specific, because no induction was observed in TCDD-responsive mouse hepatoma cells or human breast cancer cells. TCDD-mediated induction of Socs2 mRNA was dose-dependent and exhibited the characteristic structure-activity relationships observed for the aryl hydrocarbon receptor (AhR) ligands 3,3',4,4',5-pentachlorobiphenyl (PCB-126), indolo[3,2-b]-carbazole, and beta-naphthoflavone. Experiments with cycloheximide and AhR-deficient B cells indicated that Socs2 mRNA induction is a primary effect that is AhR-dependent. Western blot analysis confirmed that Socs2 and Cyp1a1 protein levels were also induced in CH12.LX cells. Promoter analysis revealed the presence of four dioxin-response elements within 1000 base pairs upstream of the Socs2 transcriptional start site, and a reporter gene regulated by the Socs2 promoter was inducible by TCDD. Promoter activity was also dependent on a functional AhR signaling pathway. These results indicate that Socs2 is a primary TCDD-inducible gene that may represent a novel mechanism by which TCDD elicits its immunosuppressive effects.

Comparative analysis of dioxin response elements in human, mouse and rat genomic sequences

Comparative approaches were used to identify human, mouse and rat dioxin response elements (DREs) in genomic sequences unambiguously assigned to a nucleotide RefSeq accession number. A total of 13 bona fide DREs, all including the substitution intolerant core sequence (GCGTG) and adjacent variable sequences, were used to establish a position weight matrix and a matrix similarity (MS) score threshold to rank identified DREs. DREs with MS scores above the threshold were disproportionately distributed in close proximity to the transcription start site in all three species. Gene expression assays in hepatic mouse tissue confirmed the responsiveness of 192 genes possessing a putative DRE. Previously identified functional DREs in well-characterized AhR-regulated genes including Cyp1a1 and Cyp1b1 were corroborated. Putative DREs were identified in 48 out of 2437 human-mouse-rat orthologous genes between -1500 and the transcriptional start site, of which 19 of these genes possessed positionally conserved DREs as determined by multiple sequence alignment. Seven of these nineteen genes exhibited 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated regulation, although there were significant discrepancies between in vivo and in vitro results. Interestingly, of the mouse-rat orthologous genes with a DRE between -1500 and +1500, only 37% had an equivalent human ortholog. These results suggest that AhR-mediated gene expression may not be well conserved across species, which could have significant implications in human risk assessment.

Temporal- and dose-dependent hepatic gene expression changes in immature ovariectomized mice following exposure to ethynyl estradiol

Temporal- and dose-dependent changes in hepatic gene expression were examined in immature ovariectomized C57BL/6 mice gavaged with ethynyl estradiol (EE), an orally active estrogen. For temporal analysis, mice were gavaged every 24 h for 3 days with 100 microg/kg EE or vehicle and liver samples were collected at 2, 4, 8, 12, 24 and 72 h. Gene expression was monitored using custom cDNA microarrays containing 3067 genes/ESTs of which 393 exhibited a change at one or more time points. Functional gene annotation extracted from public databases associated temporal gene expression changes with growth and proliferation, cytoskeletal and extracellular matrix responses, microtubule-based processes, oxidative metabolism and stress, and lipid metabolism and transport. In the dose-response study, hepatic samples were collected 24 h following treatment with 0, 0.1, 1, 10, 100 or 250 microg/kg EE. Thirty-nine of the 79 genes identified as differentially regulated at 24 h in the time course study exhibited a dose-response relationship with an average ED50 value of 47 +/- 3.5 microg/kg. Comparative analysis indicated that many of the identified temporal and dose-dependent hepatic responses are similar to EE-induced uterine responses reported in the literature and in a companion study using the same animals. Results from these studies confirm that the liver is a highly estrogen responsive tissue that exhibits a number of common responses shared with the uterus as well as distinct estrogen-mediated profiles. These data will further aid in the elucidation of the mechanisms of action of estrogens in the liver as well as in other classical and non-classical estrogen responsive tissues.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) Alters the Regulation of Pax5 in Lipopolysaccharide-Activated B Cells

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces a profound suppression of the primary immunoglobulin-M (IgM) antibody response. The suppression of IgM production by TCDD can occur through direct interactions with the B cell, is aryl hydrocarbon receptor-dependent, and is mediated through alterations in the differentiation of B cells into plasma cells. The objective of the present investigation was to characterize the effects of TCDD on the regulation of Pax5, a crucial repressor of B-cell differentiation, and four downstream targets that are directly regulated by Pax5 and involved in immunoglobulin regulation, immunoglobulin heavy chain (IgH), kappa light chain (Igkappa), J chain, and X box protein-1 (XBP-1). Lipopolysaccharide (LPS) activation of aryl hydrocarbon receptor-expressing CH12.LX cells induced B cell differentiation and robust immunoglobulin secretion that was markedly (~50%) suppressed in the presence of 10 nM TCDD. Kinetic studies show that LPS-activation induced a time-dependent decrease in Pax5 mRNA levels, protein, and DNA binding activity during a 72-h culture period that was almost completely blocked in the presence of TCDD. Concomitant with the time-dependent down-regulation of Pax5 in LPS-activated control CH12.LX cells, a reciprocal induction of IgH, Igkappa, J chain mRNA levels, and cellular XBP-1 was observed. Conversely, and consistent with the absence of Pax5 down-regulation associated with TCDD treatment, IgH, Igkappa, J chain mRNA, and XBP-1 protein were persistently repressed in LPS-activated CH12.LX cells. Collectively, these studies demonstrate the involvement of altered Pax5 regulation in the suppression of the primary IgM antibody response by TCDD.

Recombinant human P450 forms 1A1, 1A2, and 1B1 catalyze the bioactivation of heterocyclic amine mutagens in Escherichia coli lacZ strains

Three recombinant human P450 enzymes, forms 1A1, 1A2, and 1B1, were coexpressed with NADPH-cytochrome P450 reductase in an E. coli lacZ strain suitable for detection of the mutagenicity of heterocyclic and aromatic amines. The resulting strains expressed the recombinant P450 holoenzymes at high levels. MeIQ (2-amino-3,4-dimethylimidazo[4,5-f]quinoline) was activated effectively by P450 1A2, weakly by P450 1A1, and not detectably by P450 1B1. MeIQx (2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline) and Trp-P-2 (3-amino-1-methyl-5H-pyrido[4,3-b]indole) were activated by all three enzymes, with form 1A2 the most effective. These strains facilitate analysis of the substrate specificity of human P450 forms that participate in the metabolic activation of carcinogens.