Expanded high-throughput screening and chemotype-enrichment analysis of the phase II: e1k ToxCast library for human sodium-iodide symporter (NIS) inhibition.

The sodium-iodide symporter (NIS) mediates the uptake of iodide into the thyroid. Inhibition of NIS function by xenobiotics has been demonstrated to suppress circulating thyroid hormones and perturb related physiological functions. Until recently, few environmental chemicals had been screened for NIS inhibition activity. We previously screened over 1000 chemicals from the ToxCast Phase II (ph1v2 and ph2) libraries using an in vitro radioactive iodide uptake (RAIU) with the hNIS-HEK293T cell line to identify NIS inhibitors. Here, we broaden the chemical space by expanding screening to include the ToxCast e1k library (804 unique chemicals) with initial screening for RAIU at 1 × 10-4 M. Then 209 chemicals demonstrating > 20% RAIU inhibition were further tested in multiple-concentration, parallel RAIU and cell viability assays. This identified 55 chemicals as active, noncytotoxic RAIU inhibitors. Further cytotoxicity-adjusted potency scoring (with NaClO4 having a reference score of 200) revealed five chemicals with moderate to strong RAIU inhibition (scored > 100). These data were combined with our previous PhII screening data to produce binary hit-calls for ~ 1800 unique chemicals (PhII + e1k) with and without cytotoxicity filtering. Results were analyzed with a ToxPrint chemotype-enrichment workflow to identify substructural features significantly enriched in the NIS inhibition hit-call space. We assessed the applicability of enriched PhII chemotypes to prospectively predict NIS inhibition in the e1k dataset. Chemotype enrichments derived for the combined ~ 1800 dataset also identified additional enriched features, as well as chemotypes affiliated with cytotoxicity. These enriched chemotypes provide important new information that can support future data interpretation, structure-activity relationship, chemical use, and regulation.

Evaluation of potential sodium-iodide symporter (NIS) inhibitors using a secondary Fischer rat thyroid follicular cell (FRTL-5) radioactive iodide uptake (RAIU) assay.

The Fischer rat thyroid follicular cell line (FRTL-5) endogenously expresses the sodium-iodide symporter (NIS) and has been used to identify environmental chemicals that perturb thyroid hormone homeostasis by disruption of NIS-mediated iodide uptake. Previously, a high-throughput radioactive iodide uptake (RAIU) screening assay incorporating the hNIS-HEK293T-EPA cell line was used to identify potential human NIS (hNIS) inhibitors in 1028 ToxCast Phase I (ph1_v2) and Phase II chemicals. In this study, the FRTL-5 cell line was evaluated and applied as a secondary RAIU assay coupled with cell viability assays to further prioritize highly active NIS inhibitors from the earlier screening. Assay validation with ten reference chemicals and performance assessment by chemical controls suggest the FRTL-5 based assays are robust and highly reproducible. Top-ranked chemicals from the ToxCast screening were then evaluated in both FRTL-5 and hNIS RAIU assays using newly sourced chemicals to strengthen the testing paradigm and to enable a rat vs. human species comparison. Eighteen of 29 test chemicals showed less than 1 order of magnitude difference in IC50 values between the two assays. Notably, two common perfluorinated compounds, perfluorooctanesulfonic acid (PFOS) and perfluorohexane sulfonate (PFHxS), demonstrated strong NIS inhibitory activity [IC50 - 6.45 (PFOS) and - 5.70 (PFHxS) log M in FRTL-5 RAIU assay]. In addition, several chemicals including etoxazole, methoxyfenozide, oxyfluorfen, triclocarban, mepanipyrim, and niclosamide also exhibited NIS inhibition with minimal cytotoxicity in both assays and are proposed for additional testing using short-term in vivo assays to characterize effects on thyroid hormone synthesis.

Evaluating Chemicals for Thyroid Disruption: Opportunities and Challenges with in Vitro Testing and Adverse Outcome Pathway Approaches.

BACKGROUND: Extensive clinical and experimental research documents the potential for chemical disruption of thyroid hormone (TH) signaling through multiple molecular targets. Perturbation of TH signaling can lead to abnormal brain development, cognitive impairments, and other adverse outcomes in humans and wildlife. To increase chemical safety screening efficiency and reduce vertebrate animal testing, in vitro assays that identify chemical interactions with molecular targets of the thyroid system have been developed and implemented. OBJECTIVES: We present an adverse outcome pathway (AOP) network to link data derived from in vitro assays that measure chemical interactions with thyroid molecular targets to downstream events and adverse outcomes traditionally derived from in vivo testing. We examine the role of new in vitro technologies, in the context of the AOP network, in facilitating consideration of several important regulatory and biological challenges in characterizing chemicals that exert effects through a thyroid mechanism. DISCUSSION: There is a substantial body of knowledge describing chemical effects on molecular and physiological regulation of TH signaling and associated adverse outcomes. Until recently, few alternative nonanimal assays were available to interrogate chemical effects on TH signaling. With the development of these new tools, screening large libraries of chemicals for interactions with molecular targets of the thyroid is now possible. Measuring early chemical interactions with targets in the thyroid pathway provides a means of linking adverse outcomes, which may be influenced by many biological processes, to a thyroid mechanism. However, the use of in vitro assays beyond chemical screening is complicated by continuing limits in our knowledge of TH signaling in important life stages and tissues, such as during fetal brain development. Nonetheless, the thyroid AOP network provides an ideal tool for defining causal linkages of a chemical exerting thyroid-dependent effects and identifying research needs to quantify these effects in support of regulatory decision making. https://doi.org/10.1289/EHP5297.

High-throughput screening and chemotype-enrichment analysis of ToxCast phase II chemicals evaluated for human sodium-iodide symporter (NIS) inhibition.

In support of the Endocrine Disruptor Screening Program (EDSP), the U.S.EPA's Office of Research and Development (ORD) is developing high-throughput screening (HTS) approaches to identify chemicals that alter target sites in the thyroid hormone (TH) pathway. The sodium iodide symporter (NIS) is a transmembrane glycoprotein that mediates iodide uptake into the thyroid as the initial step of TH biosynthesis. Previously, we screened 293 ToxCast chemicals (ph1v2) using a HEK293T cell line expressing human NIS in parallel radioactive iodide uptake (RAIU) and cell viability assays to identify potential environmental NIS inhibitors. Here, we expanded NIS inhibitor screening for a set of 768 ToxCast Phase II (ph2) chemicals, and applied a novel computational toxicology approach based on the ToxPrint chemotype to identify chemical substructures associated with NIS inhibition. Following single-concentration screening (at 1 × 10-4 M with a 20% inhibition cutoff), 235 samples (228 chemicals) were further tested in multiple-concentration (1 × 10-9 - 1 × 10-4 M) format in both RAIU and cell viability assays. The 167 chemicals that exhibited significant RAIU inhibition were then prioritized using combined RAIU and cell viability responses that were normalized relative to the known NIS inhibitor sodium perchlorate. Some of the highest ranked chemicals, such as PFOS, tributyltin chloride, and triclocarban, have been previously reported to be thyroid disruptors. In addition, several novel chemicals were identified as potent NIS inhibitors. The present results were combined with the previous ph1v2 screening results to produce two sets of binary hit-calls for 1028 unique chemicals, consisting of 273 positives exhibiting significant RAIU inhibition, and 63 positives following application of a cell viability filter. A ToxPrint chemotype-enrichment analysis identified >20 distinct chemical substructural features, represented in >60% of the active chemicals, as significantly enriched in each NIS inhibition hit-call space. A shared set of 9 chemotypes enriched in both hit-call sets indicates stable chemotype signals (insensitive to cytotoxicity filters) that can help guide structure-activity relationship (SAR) investigations and inform future research.

High-Throughput Screening and Quantitative Chemical Ranking for Sodium-Iodide Symporter Inhibitors in ToxCast Phase I Chemical Library.

Thyroid uptake of iodide via the sodium-iodide symporter (NIS) is the first step in the biosynthesis of thyroid hormones that are critical for health and development in humans and wildlife. Despite having long been a known target of endocrine disrupting chemicals such as perchlorate, information regarding NIS inhibition activity is still unavailable for the vast majority of environmental chemicals. This study applied a previously validated high-throughput approach to screen for NIS inhibitors in the ToxCast phase I library, representing 293 important environmental chemicals. Here 310 blinded samples were screened in a tiered-approach using an initial single-concentration (100 µM) radioactive-iodide uptake (RAIU) assay, followed by 169 samples further evaluated in multi-concentration (0.001 µM-100 µM) testing in parallel RAIU and cell viability assays. A novel chemical ranking system that incorporates multi-concentration RAIU and cytotoxicity responses was also developed as a standardized method for chemical prioritization in current and future screenings. Representative chemical responses and thyroid effects of high-ranking chemicals are further discussed. This study significantly expands current knowledge of NIS inhibition potential in environmental chemicals and provides critical support to U.S. EPA's Endocrine Disruptor Screening Program (EDSP) initiative to expand coverage of thyroid molecular targets, as well as the development of thyroid adverse outcome pathways (AOPs).

The use of purified rat Leydig cells complements the H295R screen to detect chemical-induced alterations in testosterone production.

Exposure to endocrine disrupting chemicals has been associated with compromised testosterone production leading to abnormal male reproductive development and altered spermatogenesis. In vitro high-throughput screening (HTS) assays are needed to evaluate risk to testosterone production, yet the main steroidogenesis assay currently utilized is a human adrenocortical carcinoma cell line, H295R, which does not synthesize gonadal steroids at the same level as the gonads, thus limiting assay sensitivity. Here, we propose a complementary assay using a highly purified rat Leydig cell assay to evaluate the potential for chemical-induced alterations in testosterone production by the testis. We evaluated a subset of chemicals that failed to decrease testosterone production in the HTS H295R assay. The chemicals examined fit into one of two categories based on changes in substrates upstream of testosterone in the adrenal steroidogenic pathway (17α-hydroxyprogesterone and 11-deoxycorticosterone) that we predicted should have elicited a decrease in testosterone production. We found that 85% of 20 test chemicals examined inhibited Leydig cell testosterone production in our assay. Importantly, we adopted a 96-well format to increase throughput and efficiency of the Leydig cell assay. We identified a selection criterion based on the AC50 values for 17α-hydroxyprogesterone and 11-deoxycorticosterone generated from the HTS H295R assay that will help prioritize chemicals for further testing in the Leydig cell screen. We hypothesize that the greater dynamic range of testosterone production and sensitivity of the Leydig cell assay permits the detection of small, yet significant, chemical-induced changes not detected by the HTS H295R assay.

Development of a screening approach to detect thyroid disrupting chemicals that inhibit the human sodium iodide symporter (NIS).

The U.S. EPA's Endocrine Disruptor Screening Program aims to use high-throughput assays and computational toxicology models to screen and prioritize chemicals that may disrupt the thyroid signaling pathway. Thyroid hormone biosynthesis requires active iodide uptake mediated by the sodium/iodide symporter (NIS). Monovalent anions, such as the environmental contaminant perchlorate, are competitive inhibitors of NIS, yet limited information exists for more structurally diverse chemicals. A novel cell line expressing human NIS, hNIS-HEK293T-EPA, was used in a radioactive iodide uptake (RAIU) assay to identify inhibitors of NIS-mediated iodide uptake. The RAIU assay was optimized and performance evaluated with 12 reference chemicals comprising known NIS inhibitors and inactive compounds. An additional 39 chemicals including environmental contaminants were evaluated, with 28 inhibiting RAIU over 20% of that observed for solvent controls. Cell viability assays were performed to assess any confounding effects of cytotoxicity. RAIU and cytotoxic responses were used to calculate selectivity scores to group chemicals based on their potential to affect NIS. RAIU IC50 values were also determined for chemicals that displayed concentration-dependent inhibition of RAIU (≥50%) without cytotoxicity. Strong assay performance and highly reproducible results support the utilization of this approach to screen large chemical libraries for inhibitors of NIS-mediated iodide uptake.

Approaches for predicting effects of unintended environmental exposure to an endocrine active pharmaceutical, tamoxifen.

Tamoxifen is an endocrine-active pharmaceutical (EAP) that is used world-wide. Because tamoxifen is a ubiquitous pharmaceutical and interacts with estrogen receptors, a case study was conducted with this compound to (1) determine effects on reproductive endpoints in a nontarget species (i.e., a fish), (2) compare biologically-active metabolites across species, (3) assess whether in vitro assays predict in vivo results, and (4) investigate metabolomic profiles in tamoxifen-treated fish to better understand the biological mechanisms of tamoxifen toxicity. In reproductive assays, tamoxifen exposure caused a significant reduction in egg production and significantly increased ovarian aromatase activity in spawning adult cunner fish (Tautogolabrus adspersus). In plasma from tamoxifen-exposed cunner, the predominant metabolite was 4-hydroxytamoxifen, while in rats it was N-desmethyltamoxifen. Because 4-hydroxytamoxifen is a more biologically active metabolite than N-desmethyltamoxifen, this difference could result in a different level of risk for the two species. The results of in vitro assays with fish hepatic microsomes to assess tamoxifen metabolism did not match in vivo results, indicating probable differences in excretion of tamoxifen metabolites in fish compared with rats. For the first time, a complete in vitro characterization of the metabolism of tamoxifen using fish microsomes is presented. Furthermore, a metabolomic investigation of cunner gonad extracts demonstrates that tamoxifen alters the biochemical profile in this nontarget species. Understanding the consequence of tamoxifen exposure in nontarget species, and assessing the discrepancies between sex- and species-mediated endpoints, is a step toward understanding how to accurately assess the risks posed by EAPs, such as tamoxifen, in the aquatic environment. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1834-1850, 2016.

Novel molecular events associated with altered steroidogenesis induced by exposure to atrazine in the intact and castrate male rat.

Toxicology is increasingly focused on molecular events comprising adverse outcome pathways. Atrazine activates the hypothalamic-pituitary adrenal axis, but relationships to gonadal alterations are unknown. We characterized hormone profiles and adrenal (intact and castrate) and testis (intact) proteomes in rats after 3 days of exposure. The adrenal accounted for most of the serum progesterone and all of the corticosterone increases in intact and castrated males. Serum luteinizing hormone, androstenedione, and testosterone in intact males shared a non-monotonic response suggesting transition from an acute stimulatory to a latent inhibitory response to exposure. Eight adrenal proteins were significantly altered with dose. There were unique proteomic changes between the adrenals of intact and castrated males. Six testis proteins in intact males had non-monotonic responses that significantly correlated with serum testosterone. Different dose-response curves for steroids and proteins in the adrenal and testis reveal novel adverse outcome pathways in intact and castrated male rats.

Modulation of aromatase activity as a mode of action for endocrine disrupting chemicals in a marine fish.

The steroidogenic enzyme aromatase catalyzes the conversion of androgens to estrogens and therefore plays a central role in reproduction. In contrast to most vertebrates, teleost fish have two distinct forms of aromatase. Because brain aromatase activity in fish is up to 1000 times that in mammals, fish may be especially susceptible to negative effects from environmental endocrine-disrupting chemicals (EDCs) that impact aromatase activity. In this study, the effects of estradiol (E2), ethynylestradiol (EE2), octylphenol (OP), and androstatrienedione (ATD) on reproduction and aromatase activity in brains and gonads from the marine fish cunner (Tautogolabrus adspersus) was investigated. The purpose of the study was to explore the relationship between changes in aromatase activity and reproductive output in a marine fish, as well as compare aromatase activity to two commonly used indicators of EDC exposure, plasma vitellogenin (VTG) and gonadosomatic index (GSI). Results with E2, EE2, and ATD indicate that aromatase activity in cunner brain and ovary are affected differently by exposure to these EDCs. In the case of E2 and EE2, male brain aromatase activity was signficantly increased by these treatments, female brain aromatase activity was unaffected, and ovarian aromatase activity was significantly decreased. Treatment with the aromatase inhibitor ATD resulted in significantly decreased aromatase activity in male and female brain, but had no significant impact on ovarian aromatase activity. Regardless of test chemical, a decrease or an increase in male brain aromatase activity relative to controls was associated with decreased egg production in cunner and was also correlated with significant changes in GSI in both sexes. E2 and EE2 significantly elevated plasma VTG in males and females, while ATD had no significant effect. Treatment of cunner with OP had no significant effect on any measured endpoint. Overall, results with these exposures indicate EDCs that impact aromatase activity also affect reproductive output in spawning cunner.

In vitro metabolism and bioavailability tests for endocrine active substances: what is needed next for regulatory purposes?

Legislation and prospective legislative proposals internationally (may) require that chemicals be tested for their ability to disrupt the hormonal systems of mammals. Chemicals found to test positive in vitro are considered to be endocrine active substances (EAS) and may be putative endocrine disruptors (EDs) in vivo. While there is a growing body of international in vitro test guidelines addressing EAS mechanisms and modes of action, to date there are still few or no standardized methods to incorporate metabolic and toxicokinetic aspects into these in vitro tests for EAS. In vitro assays for EAS should incorporate metabolic enzyme systems to better address the relevance of EAS tests to in vivo adverse outcome pathways, and a previous OECD review paper indicated how this could be done. This paper revisits those recommendations, addressing where research and funding efforts are needed to expedite the development of suitable in vitro metabolism systems to improve the accuracy of in vitro assays for identifying EAS and EDs. Recommendations are made for projects to support short, medium, and long-term goals. The complexity of in vivo metabolism presents major challenges for the development of predictive models suitable for the extrapolation of data from in silico/in vitro approaches to models that can occur in vivo. Therefore, the long-term recommendations are intended to foster an international harmonization of databases, delineation of metabolic pathways, and development of predictive tools that will provide a fundamental understanding of the processes by which metabolism occurs, increasing the predictive accuracy of in silico/in vitro methods.

Measurement of steroids in rats after exposure to an endocrine disruptor: mass spectrometry and radioimmunoassay demonstrate similar results.

INTRODUCTION: Commercially available radioimmunoassays (RIAs) are frequently used to evaluate the effects of endocrine disrupting chemicals (EDCs) on steroidogenesis in rats. Currently there are limited data comparing steroid concentrations in rats as measured by RIAs to those obtained using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). This study evaluates the concordance of serum and urine steroid concentrations as quantified by select RIA kits and LC-MS/MS following exposure to an EDC, atrazine (ATR). METHODS: Adult male rats were orally dosed with ATR (200 mg/kg/day) or methylcellulose (1%, vehicle control) for 5 days. Serum was collected and separated into aliquots for analysis. Serum was assayed by RIA for androstenedione (ANDRO), corticosterone (CORT), estradiol (E2), estrone (E1), progesterone (P4), and testosterone (T). Serum was extracted prior to LC-MS/MS analysis with positive electrospray ionization in multiple-reaction monitoring mode for ANDRO, CORT, P4, and T. E1 and E2 concentrations were quantified similarly by LC-MS/MS, following derivatization with dansyl chloride. To compare CORT values from urine, pregnant adult rats were orally dosed with either ATR (100 mg/kg/day) or methylcellulose for 5 days (i.e., gestational days 14-18). Urine samples were collected daily and assayed for CORT by RIA and LC-MS/MS as described above. RESULTS: Data analyses demonstrated significant agreement between the two detection methods as assessed by Pearson product-moment correlation coefficient, Bland-Altman analysis, and the interclass correlation coefficient. No statistically significant differences were observed between RIA and LC-MS/MS means for any of the steroids assayed. DISCUSSION: These findings indicate a significant correlation between the measurement of steroids within rat serum and urine using RIA kits and LC-MS/MS. Differences in the absolute measurements existed, but these were not statistically significant. These findings indicate that steroids may be reliably measured in rat biological media using RIAs or LC-MS/MS.

Atrazine does not induce pica behavior at doses that increase hypothalamic-pituitary-adrenal axis activation and cause conditioned taste avoidance.

Previous work has shown that a single oral administration of atrazine (ATR), a chlorotriazine herbicide, causes rapid increases in plasma adrenocorticotropic hormone (ACTH), serum corticosterone (CORT) and progesterone. The mechanism for these effects is unknown. To test whether administration of ATR causes hypothalamic-pituitary-adrenal (HPA) axis activation through the production of a generalized stress response resulting from gastrointestinal distress, we conducted both conditioned taste avoidance (CTA) and pica behavior experiments. Body temperature data were also collected to detect the presence of stress-induced hyperthermia. Adult male Wistar rats were given a single oral dose of ATR (0, 5, 25, 50, 100, or 200 mg/kg) or the primary ATR metabolite diamino-s-chlorotriazine (DACT; 135 mg/kg). Increases were observed in ACTH (LOEL, 12.5 mg/kg), CORT (LOEL, 5 mg/kg) and progesterone (LOEL, 5 mg/kg) 15 min following a single dose of ATR. DACT (135 mg/kg) increased ACTH (1.3-fold), CORT (2.9-fold) and progesterone (1.9-fold) above vehicle control concentrations, but the magnitude of the responses was much lower than that observed for an equal molar dose of ATR (200 mg/kg; 7.0, 9.0 and 11.0-fold above ACTH, CORT, progesterone controls, respectively). CTA results demonstrated conditioned taste avoidance to ATR, with a NOEL of 5 mg/kg. Animals dosed with DACT developed avoidance responses comparable to the highest dose of ATR. In the pica experiment, lower doses (5-50 mg/kg) of ATR had no effect on pica behavior, as measured 6 and 24 h post-dosing, nor did DACT. However, the highest dose of ATR (200 mg/kg) did induce pica behavior at both time points. No differences in body temperature were observed. Overall, results indicate that increases in ACTH and CORT secretion following administration of ATR occur at doses that are without effect on the display of pica behavior, indicating that the HPA-axis activation caused by ATR is not likely the result of gastrointestinal distress.

Understanding the effects of atrazine on steroidogenesis in rat granulosa and H295R adrenal cortical carcinoma cells.

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) was introduced in the 1950s as a broad spectrum herbicide, and remains one of the most widely used herbicides in the United States. Several studies have suggested that atrazine modifies steroidogenesis and may disrupt reproductive function and development in a variety of species. A primary concern has been whether atrazine increases the synthesis of estrogens, perhaps by enhancing aromatase gene expression and activity. In this study, the effect of atrazine was compared in cultures using primary granulosa cells and H295R adrenal cortical carcinoma cells. Atrazine (10 µM), but not its metabolite, 2-chloro-4,6-diamino-1,2,5-triazine (DACT), significantly increased estradiol production and aromatase activity in granulosa cell cultures only when measured for 1-h following 24h of exposure. In H295R cells, atrazine (10 µM) increased estradiol and estrone production. Importantly, atrazine (10 µM) increased progesterone production from both cell types suggesting a broader effect of atrazine on steroidogenesis.

Molecular modeling for screening environmental chemicals for estrogenicity: use of the toxicant-target approach.

There is a paucity of relevant experimental information available for the evaluation of the potential health and environmental effects of many man made chemicals. Knowledge of the potential pathways for activity provides a rational basis for the extrapolations inherent in the preliminary evaluation of risk and the establishment of priorities for obtaining missing data for environmental chemicals. The differential step in many mechanisms of toxicity may be generalized as the interaction between a small molecule (a potential toxicant) and one or more macromolecular targets. An approach based on computation of the interaction between a potential molecular toxicant and a library of macromolecular targets of toxicity has been proposed for preliminary chemical screening. In the current study, the interaction between a series of environmentally relevant chemicals and models of the rat estrogen receptors (ER) was computed and the results compared to an experimental data set of their relative binding affinities. The experimental data set consists of 281 chemicals, selected from the U.S. EPA's Toxic Substances Control Act (TSCA) inventory, that were initially screened using the rat uterine cytosolic ER-competitive binding assay. Secondary analysis, using Lineweaver-Burk plots and slope replots, was applied to confirm that only 15 of these test chemicals were true competitive inhibitors of ER binding with experimental inhibition constants (K(i)) less than 100 microM. Two different rapid computational docking methods have been applied. Each provides a score that is a surrogate for the strength of the interaction between each ligand-receptor pair. Using the score that indicates the strongest interaction for each pair, without consideration of the geometry of binding between the toxicant and the target, all of the active molecules were discovered in the first 16% of the chemicals. When a filter is applied on the basis of the geometry of a simplified pharmacophore for binding to the ER, the results are improved, and all of the active molecules were discovered in the first 8% of the chemicals. In order to obtain no false negatives in the model that includes the pharmacophore filter, only 8 molecules are false positives. These results indicate that molecular docking algorithms that were designed to find the chemicals that act most strongly at a receptor (and therefore are potential pharmaceuticals) can efficiently separate weakly active chemicals from a library of primarily inactive chemicals. The advantage of using a pharmacophore filter suggests that the development of filters of this type for other receptors will prove valuable.

Chlorotriazine herbicides and metabolites activate an ACTH-dependent release of corticosterone in male Wistar rats.

Previously, we reported that atrazine (ATR) alters steroidogenesis in male Wistar rats resulting in elevated serum corticosterone (CORT), progesterone, and estrogens. The increase in CORT indicated that this chlorotriazine herbicide may alter the hypothalamic-pituitary-adrenal axis. This study characterizes the temporal changes in adrenocorticotropic hormone (ACTH), CORT, and P4 in male Wistar rats following a single dose of ATR (0, 5, 50, 100, and 200 mg/kg), simazine (SIM; 188 mg/kg), propazine (PRO; 213 mg/kg), or primary metabolites, deisopropylatrazine (DIA; 4, 10, 40, 80, and 160 mg/kg), deethylatrazine (DEA; 173 mg/kg), and diamino-s-chlorotriazine (DACT; 3.37, 33.7, 67.5, and 135 mg/kg). The maximum dose for each chemical was the molar equivalent of ATR (200 mg/kg). Significant increases in plasma ACTH were observed within 15 min, following exposure to ATR, SIM, PRO, DIA, or DEA. Dose-dependent elevations in CORT and progesterone were also observed at 15 and 30 min post-dosing with these compounds indicating an activation of adrenal steroidogenesis. Measurement of the plasma concentrations of the parent compounds and metabolites confirmed that ATR, SIM, and PRO are rapidly metabolized to DACT. Although DACT had only minimal effects on ACTH and steroid release, dosing with this metabolite resulted in plasma DACT concentrations that were 60-fold greater than that observed following an equimolar dose of ATR and eightfold greater than equimolar doses of DIA or DEA, indicating that DACT is not likely the primary inducer of ACTH release. Thus, the rapid release of ACTH and subsequent activation of adrenal steroidogenesis following a single exposure to ATR, SIM, PRO, DIA, or DEA may reflect chlorotriazine-induced changes at the level of the brain and/or pituitary.

Characterization of the hypothalamic-pituitary-adrenal axis response to atrazine and metabolites in the female rat.

Atrazine (ATR) has recently been shown to activate the hypothalamic-pituitary-adrenal (HPA) axis in rodents. The current study investigated the effect of ATR and two of its chlorinated metabolites, desisopropylatrazine (DIA) and diamino-s-chlorotriazine (DACT), on the HPA axis in the Long-Evans female rat. A single oral gavage administration of 75 mg/kg ATR or 60.2 mg/kg DIA (a dose equimolar to the applied ATR dose) during the morning of proestrus resulted in significant, acute increases in circulating adrenocorticotropic hormone (ACTH), corticosterone, and progesterone. Oral doses of ATR or DIA were given daily over the course of the 4-day ovarian cycle starting on the day of vaginal estrus, resulted in a similar, dose-responsive activation of the HPA axis. The increase in ACTH, corticosterone, and progesterone by these compounds was of a similar magnitude to that produced by 5-min restraint stress. Single or multiple oral exposures to DACT, on the other hand, did not significantly alter pituitary-adrenal hormone release. These results were observed despite plasma levels of DACT being higher than any other metabolite at the time of hormone measurement. Overall, circulating metabolite concentrations following equimolar dosing were much higher than those observed after ATR administration. Additional studies indicated that the activation of the HPA axis by oral exposure to ATR and DIA was not due simply to the stimulation of gastrointestinal afferents. Similar responses were observed in rats which received an oral dose of ATR following bilateral subdiaphramatic vagotomy and following intravenous administration of DIA in jugular vein catheterized animals. We conclude that ATR and the metabolite DIA significantly activate the HPA axis following oral exposure in the female rat. Activation of this endocrine axis by these chlorotriazines could contribute to the induced changes of female reproductive function reported previously.

Effects of altered food intake during pubertal development in male and female wistar rats.

The U.S. Environmental Protection Agency is currently validating assays that will be used in a Tier I Screening Battery to detect endocrine disrupting chemicals. A primary concern with the Protocols for the Assessment of Pubertal Development and Thyroid Function in Juvenile Male and Female Rats is that a nonspecific reduction in body weight (BWT) during the exposure period may potentially confound the interpretation of effects on the endocrine endpoints. Wistar rats were underfed 10, 20, 30, or 40% less than the ad libitum food consumed by controls from postnatal days (PNDs) 22 to 42 (females) or PNDs 23 to 53 (males). Terminal BWT of females and males were 2, 4, 12, and 19% and 2, 6, 9, and 19% lower than controls, respectively. In the females, neither the age of pubertal onset nor any of the thyroid hormone endpoints were affected by food restriction (FR) that led to a 12% decrease in BWT. Similarly, none of the male reproductive endpoints examined were altered by FR that led to a 9% BWT decrease. However, decreased triiodothyronine and thyroxin was observed in FR males with a 9% reduced BWT. While these data support the use of the maximum tolerated dose for BWT (10%) for the female protocol, effects on the male thyroid endpoints indicate that a slightly lower limit (

Atrazine and reproductive function: mode and mechanism of action studies.

Atrazine, a chlorotriazine herbicide, is used to control annual grasses and broadleaf weeds. In this review, we summarize our laboratory's work evaluating the neuroendocrine toxicity of atrazine (and related chlorotriazines) from an historic perspective. We provide the rationale for our work as we have endeavored to determine: 1) the underlying reproductive changes leading to the development of mammary gland tumors in the atrazine-exposed female rat; 2) the cascade of physiological events that are responsible for these changes (i.e., the mode of action for mammary tumors); 3) the potential cellular mechanisms involving adverse effects of atrazine; and 4) the range of reproductive alterations associated with this pesticide.

Nature of the binding interaction for 50 structurally diverse chemicals with rat estrogen receptors.

This study was conducted to characterize the estrogen receptor (ER)-binding affinities of 50 chemicals selected from among the high production volume chemicals under the U.S. EPA's (U.S. Environmental Protection Agency's) Toxic Substances Control Act inventory. The chemicals were evaluated using the rat uterine cytosolic (RUC) ER-competitive binding assay, with secondary analysis using Lineweaver-Burk plots and slope replots to confirm true competitive inhibition and to determine an experimental K(i). Data from these ER-competitive binding assays represent the types of competitive binding curves that can be obtained when screening chemicals with broad structural diversity. True competitive inhibition was observed in 17 of 50 chemicals. Binding affinities were much lower than that of estradiol (E(2)) with K(i) concentrations ranging from 0.6 to 373 microM as compared with that of E(2) (0.77 nM). Other chemicals that appeared to displace radiolabeled E(2) binding to ER were, in fact, found not to be competitive inhibitors in the secondary K(i) experiments. These seven chemicals likely altered the stability of the assay by changing the buffer pH, denaturing ER, or disrupting the ER-binding kinetics. Thus, several conditions that may confound interpretation of RUC ER-binding assay data are illustrated. For another group of eight chemicals, neither an IC(50) nor K(i) could be determined due to solubility constraints. These chemicals exhibited slight (20-40%) inhibition at concentrations of 10-100 microM, suggesting that they could be competitors at very high concentrations, yet K(i) experiments were not possible as the limit of chemical solubility in the aqueous assay buffer was well above the IC(50). An additional 18 of the 50 chemicals were classified as nonbinders because in concentrations up to 100 microM they produced essentially no displacement of radiolabeled E(2). These results show that although the ER-competitive binding assay is a valuable tool for screening chemicals, secondary tests such as a double reciprocal Lineweaver-Burk experiment with slope replot should be used to confirm true competitive inhibition. This information will be useful for the ongoing validation of the RUC ER-competitive binding assay under the U.S. EPA's Endocrine Disruptor Screening Program, as well as to support research efforts to develop computational models designed to identify chemicals with the ability to bind to ER.