Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - Part IV: the ECETOC and CLE Proposal for a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS).

Following the European Commission Endocrine Disruptor Criteria, substances shall be considered as having endocrine disrupting properties if they (a) elicit adverse effects, (b) have endocrine activity, and (c) the two are linked by an endocrine mode-of-action (MoA) unless the MoA is not relevant for humans. A comprehensive, structured approach to assess whether substances meet the Endocrine Disruptor Criteria for the thyroid modality (EDC-T) is currently unavailable. Here, the European Centre for Ecotoxicology and Toxicology of Chemicals Thyroxine Task Force and CropLife Europe propose a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS). In Tier 0, before entering the Thyroid-NDT-TAS, all available in vivo, in vitro and in silico data are submitted to weight-of-evidence (WoE) evaluations to determine whether the substance of interest poses a concern for thyroid disruption. If so, Tier 1 of the Thyroid-NDT-TAS includes an initial MoA and human relevance assessment (structured by the key events of possibly relevant adverse outcome pathways) and the generation of supportive in vitro/in silico data, if relevant. Only if Tier 1 is inconclusive, Tier 2 involves higher-tier testing to generate further thyroid- and/or neurodevelopment-related data. Tier 3 includes the final MoA and human relevance assessment and an overarching WoE evaluation to draw a conclusion on whether, or not, the substance meets the EDC-T. The Thyroid-NDT-TAS is based on the state-of-the-science, and it has been developed to minimise animal testing. To make human safety assessments more accurate, it is recommended to apply the Thyroid-NDT-TAS during future regulatory assessments.

Nervous system development related gene expression regulation in the zebrafish embryo after exposure to valproic acid and retinoic acid: A genome wide approach.

The evaluation of chemical and pharmaceutical safety for humans is moving from animal studies to New Approach Methodologies (NAM), reducing animal use and focusing on mechanism of action, whilst enhancing human relevance. In developmental toxicology, the mechanistic approach is facilitated by the assessment of predictive biomarkers, which allow mechanistic pathways perturbation monitoring at the basis of human hazard assessment. In our search for biomarkers of maldevelopment, we focused on chemically-induced perturbation of the retinoic acid signaling pathway (RA-SP), a major pathway implicated in a plethora of developmental processes. A genome-wide expression screening was performed on zebrafish embryos treated with two teratogens, all-trans retinoic acid (ATRA) and valproic acid (VPA), and a non-teratogen reference compound, folic acid (FA). Each compound was found to have a specific mRNA expression profile with 248 genes commonly dysregulated by both teratogenic compounds but not by FA. These genes were implicated in several developmental processes (e.g., the circulatory and nervous system). Given the prominent response of neurodevelopmental gene sets, and the crucial need to better understand developmental neurotoxicity, our study then focused on nervous system development. We found 62 genes that are potential early neurodevelopmental toxicity biomarker candidates. These results advance NAM-based safety assessment evaluation by highlighting the usefulness of the RA-SP in providing early toxicity biomarker candidates.

Retinoic acid signaling pathway perturbation impacts mesodermal-tissue development in the zebrafish embryo: Biomarker candidate identification using transcriptomics.

The zebrafish embryo (ZE) model provides a developmental model well conserved throughout vertebrate embryogenesis, with relevance for early human embryo development. It was employed to search for gene expression biomarkers of compound-induced disruption of mesodermal development. We were particularly interested in the expression of genes related to the retinoic acid signaling pathway (RA-SP), as a major morphogenetic regulating mechanism. We exposed ZE to teratogenic concentrations of valproic acid (VPA) and all-trans retinoic acid (ATRA), using folic acid (FA) as a non-teratogenic control compound shortly after fertilization for 4 h, and performed gene expression analysis by RNA sequencing. We identified 248 genes specifically regulated by both teratogens but not by FA. Further analysis of this gene set revealed 54 GO-terms related to the development of mesodermal tissues, distributed along the paraxial, intermediate, and lateral plate sections of the mesoderm. Gene expression regulation was specific to tissues and was observed for somites, striated muscle, bone, kidney, circulatory system, and blood. Stitch analysis revealed 47 regulated genes related to the RA-SP, which were differentially expressed in the various mesodermal tissues. These genes provide potential molecular biomarkers of mesodermal tissue and organ (mal)formation in the early vertebrate embryo.

Dynamic regulation of gene expression and morphogenesis in the zebrafish embryo test after exposure to all-trans retinoic acid.

The zebrafish embryotoxicity test (ZET) is widely used in developmental toxicology. The analysis of gene expression regulation in ZET after chemical exposure provides mechanistic information about the effects of chemicals on morphogenesis in the test. The gene expression response magnitude has been shown to change with exposure duration. The objective of this work is to study the effect of the exposure duration on the magnitude of gene expression changes in the all-trans retinoic acid (ATRA) signaling pathway in the ZET. Retinoic acid regulation is a key driver of morphogenesis and is therefore employed here as an indicator for the regulation of developmental genes. A teratogenic concentration of 7.5 nM of ATRA was given at 3 hrs post fertilization (hpf) for a range of exposure durations until 120 hrs of development. The expression of a selection of genes related to ATRA signaling and downstream developmental genes was determined. The highest magnitudes of gene expression regulation were observed after 2-24 hrs exposure with an optimal response after 4 hrs. Longer exposures showed a decrease in the gene expression response, although continued exposure to 120 hpf caused malformations and lethality. This study shows that assessment of gene expression regulation at early time points after the onset of exposure in the ZET may be optimal for the prediction of developmental toxicity. We believe these results could help optimize sensitivity in future studies with ZET.

A rodent thyroid-liver chip to capture thyroid toxicity on organ function level.

Endocrine disruption by environmental chemicals continues to be a concern for human safety. The rat, a widely used model organism in toxicology, is very sensitive to chemical-induced thyroid perturbation, e.g., histopathological alterations in thyroid tissue. Species differences in the susceptibility to thyroid perturbation lead to uncertainty in human safety risk assessments. Hazard identification and characterization of chemically induced thyroid perturbation would therefore benefit from in vitro models addressing different mechanisms of action in a single functional assay, ideally across species. We here introduce a rat thyroid-liver chip that enables simultaneous identification of direct and indirect (liver-mediated) thyroid perturbation on organ-level functions in vitro. A second manuscript describes our work toward a human thyroid-liver chip (Kühnlenz et al., 2022). The presented microfluidic model consisting of primary rat thyroid follicles and liver 3D spheroids maintains a tissue-specific phenotype for up to 21 days. More precisely, the thyroid model exhibits a follicular architecture expressing basolateral and apical markers and secretes T4. Likewise, liver spheroids retain hepatocellular characteristics, e.g., a stable release of albumin and urea, the presence of bile canalicular networks, and the formation of T4-glucuronide. Experiments with reference chemicals demonstrated proficiency to detect direct and indirect mechanisms of thyroid perturbation through decreased thyroid hormone secretion and increased gT4 formation, respectively. Prospectively this rat thyroid-liver chip model, together with its human counterpart, may support a species-specific quantitative in vitro to in vivo extrapolation to improve a data-driven and evidence-based human safety risk assessment with significant contributions to the 3R principles.

A microfluidic thyroid-liver platform to assess chemical safety in humans.

Thyroid hormones (THs) are crucial regulators of human metabolism and early development. During the safety assessment of plant protection products, the human relevance of chemically induced TH perturbations observed in test animals remains uncertain. European regulatory authorities request follow-up in vitro studies to elucidate human-relevant interferences on thyroid gland function or TH catabolism through hepatic enzyme induction. However, human in vitro assays based on single molecular initiating events poorly reflect the complex TH biology and related liver-thyroid axis. To address this complexity, we present human three-dimensional thyroid and liver organoids with key functions of TH metabolism. The thyroid model resembles in vivo-like follicular architecture and a TSH-dependent triiodothyronine synthesis over 21 days, which is inhibited by methimazole. The HepaRG-based liver model, secreting the critical TH-binding proteins albumin and thyroxine-binding globulin, emulates an active TH catabolism via the formation of glucuronidated and sulfated thyroxine (gT4/sT4). Activation of the nuclear receptors PXR and AHR was demonstrated via the induction of specific CYP isoenzymes by rifampicin, pregnenolone-16α-carbonitrile, and ß-naphthoflavone. However, this nuclear receptor activation, assumed to regulate UDP-glucuronosyltransferases and sulfotransferases, appeared to have no effect on gT4 and sT4 formation in this human-derived hepatic cell line model. Finally, established single-tissue models were successfully co-cultured in a perfused two-organ chip for 21 days. In conclusion, this model presents a first step towards a complex multimodular human platform that will help to identify both direct and indirect thyroid disruptors that are relevant from a human safety perspective.

Metribuzin-induced non-adverse liver changes result in rodent-specific non-adverse thyroid effects via uridine 5'-diphospho-glucuronosyltransferase (UDPGT, UGT) modulation.

Metribuzin is a herbicide that inhibits photosynthesis and has been used for over 40 years. Its main target organ is the liver and to some extent the kidney in rats, dogs, and rabbits. Metribuzin shows a specific thyroxine (T4) profile in rat studies with T4 increases at low doses and T4 decreases at higher doses. Only the T4 decreases occur together with histopathological changes in the thyroid and weight changes of liver and thyroid. A set of experiments was conducted to investigate metribuzin's endocrine disruptor potential according to European guidance and regulations. The results indicate that a liver enzyme modulation, i.e. of the uridine 5'-diphospho-glucuronosyltransferase (UDPGT, UGT), is most likely responsible for both increased and decreased plasma thyroxine level and for thyroid histopathological observations. Animals with high T4 levels show low UGT activity, while animals with low T4 levels show high UGT activity. A causal relationship was inferred, since other potentially human-relevant mode of action (MOA) pathways were excluded in dedicated studies, i.e. inhibition of deiodinases (DIO), inhibition of thyroid peroxidase (TPO) or of the sodium importer system (NIS). This liver metabolism-associated MOA is considered not relevant for human hazard assessment, due to species differences in thyroid homeostasis between humans and rats and, more importantly, based on experimental data showing that metribuzin affects UGT activity in rat but not in human hepatocytes. Further, we discuss whether or not increased T4 levels in the rat, in the absence of histopathological changes, should be considered as adverse and therefore used as an appropriate hazard model for humans. Based on a weight of evidence approach, metribuzin should not be classified as an endocrine disruptor with regard to the thyroid modality.

A review of species differences in the control of, and response to, chemical-induced thyroid hormone perturbations leading to thyroid cancer.

This review summarises the current state of knowledge regarding the physiology and control of production of thyroid hormones, the effects of chemicals in perturbing their synthesis and release that result in thyroid cancer. It does not consider the potential neurodevelopmental consequences of low thyroid hormones. There are a number of known molecular initiating events (MIEs) that affect thyroid hormone synthesis in mammals and many chemicals are able to activate multiple MIEs simultaneously. AOP analysis of chemical-induced thyroid cancer in rodents has defined the key events that predispose to the development of rodent cancer and many of these will operate in humans under appropriate conditions, if they were exposed to high enough concentrations of the affecting chemicals. There are conditions however that, at the very least, would indicate significant quantitative differences in the sensitivity of humans to these effects, with rodents being considerably more sensitive to thyroid effects by virtue of differences in the biology, transport and control of thyroid hormones in these species as opposed to humans where turnover is appreciably lower and where serum transport of T4/T3 is different to that operating in rodents. There is heated debate around claimed qualitative differences between the rodent and human thyroid physiology, and significant reservations, both scientific and regulatory, still exist in terms of the potential neurodevelopmental consequences of low thyroid hormone levels at critical windows of time. In contrast, the situation for the chemical induction of thyroid cancer, through effects on thyroid hormone production and release, is less ambiguous with both theoretical, and actual data, showing clear dose-related thresholds for the key events predisposing to chemically induced thyroid cancer in rodents. In addition, qualitative differences in transport, and quantitative differences in half life, catabolism and turnover of thyroid hormones, exist that would not operate under normal situations in humans.

Minimum datasets to establish a CAR-mediated mode of action for rodent liver tumors.

Methods for investigating the Mode of Action (MoA) for rodent liver tumors via constitutive androstane receptor (CAR) activation are outlined here, based on current scientific knowledge about CAR and feedback from regulatory agencies globally. The key events (i.e., CAR activation, altered gene expression, cell proliferation, altered foci and increased adenomas/carcinomas) can be demonstrated by measuring a combination of key events and associative events that are markers for the key events. For crop protection products, a primary dataset typically should include a short-term study in the species/strain that showed the tumor response at dose levels that bracket the tumorigenic and non-tumorigenic dose levels. The dataset may vary depending on the species and the test compound. As examples, Case Studies with nitrapyrin (in mice) and metofluthrin (in rats) are described. Based on qualitative differences between the species, the key events leading to tumors in mice or rats by this MoA are not operative in humans. In the future, newer approaches such as a CAR biomarker signature approach and/or in vitro CAR3 reporter assays for mouse, rat and human CAR may eventually be used to demonstrate a CAR MoA is operative, without the need for extensive additional studies in laboratory animals.

Comparison of early morphological and molecular changes induced by 17-alpha-methyltestosterone and estradiol benzoate in the rat ovary.

Repeated exposure to 17-α-methyltestosterone (17MT) and estradiol benzoate (EB) for 28 or 90 days in rats induce similar ovarian atrophy. The objective of the present work was to identify and compare the early effects induced by 17MT and EB on the ovary using molecular and histopathological tools. Female rats were evaluated after 1, 3 or 7 days following an oral exposure by gavage at a daily dose of 600 mg/kg/day for 17MT and 5 mg/kg/day for EB. All animals were found to be acyclic after 3 or 7 days of treatment with 17MT and EB. Histopathological changes were present in the ovary, uterus, vagina and mammary gland after both treatments. Ovarian atrophy known as the long term effect of 17MT and EB was not yet detected after 7 days of treatment. But non regressive corpora lutea and cystic follicles were identically observed in the ovary of 17MT and EB treated females. Both compounds induced a decrease of LH transcripts together with an increase of plasma progesterone and prolactin levels. Differences in the profile of regulation of the aromatase were noted after 1 and 3 days of treatment in 17MT treated animals (upregulated) when compared to EB treated animals (downregulated). In summary, we have shown that despite the different nature of hormonal activity, EB and 17MT induce very early endocrine perturbation which presents several similarities. Our work indicated that the detection of early key hormonal markers in short term studies can help to predict the adverse long term effects on target tissues.

Potential new targets involved in 1,3-dinitrobenzene induced testicular toxicity.

1,3-Dinitrobenzene (DNB) causes testicular injury, particularly to Sertoli cells, and induces apoptosis in the surrounding germinal cells in rodents; however, the mechanisms causing this toxicity are poorly understood. Our studies, using standard and molecular tools, were conducted to better understand the pathogenesis of the testicular effects. Four daily oral doses of 0.1-8mg/kg/day caused marked testicular lesions in rats from 4mg/kg/day. Global transcriptomics revealed cell cycle and cell death as the major biological processes affected with the expression of genes associated with cell cycle progression ("mitotic roles of polo-like kinase") being particularly altered. In a single dose time course study (4mg/kg), no adverse changes were recorded; however, in contrast to the data from the multiple dose study, plasma testosterone and testicular steroidogenesis-related gene expression were affected. These steroid hormone effects were confirmed in vitro using the H295R steroidogenesis assay. With this global approach we show that DNB not only induces apoptosis and interferes with cell cycle in the testes but that DNB can also modulate steroid hormone biosynthesis, suggesting an interference with the endocrine system. However, the contribution of the endocrine changes to the severe testicular lesions is presently unknown and requires further investigation.

A molecular and phenotypic integrative approach to identify a no-effect dose level for antiandrogen-induced testicular toxicity.

The safety assessment of chemicals for humans relies on identifying no-observed adverse effect levels (NOAELs) in animal toxicity studies using standard methods. With the advent of high information content technologies, especially microarrays, it is pertinent to determine the impact of molecular data on the NOAELs. Consequently, we conducted an integrative study to identify a no-transcriptomic effect dose using microarray analyses coupled with quantitative reverse transcriptase PCR (RT-qPCR) and determined how this correlated with the NOAEL. We assessed the testicular effects of the antiandrogen, flutamide (FM), in a rat 28-day toxicity study using doses of 0.2-30 mg/kg/day. Plasma testosterone levels and testicular histopathology indicated a NOAEL of 1 mg/kg/day. A no-effect dose of 0.2 mg/kg/day was established based on molecular data relevant to the phenotypic changes. We observed differential gene expression starting from 1 mg/kg/day and a deregulation of more than 1500 genes at 30 mg/kg/day. Dose-related changes were identified for the major pathways (e.g., fatty acid metabolism) associated with the testicular lesion (Leydig cell hyperplasia) that were confirmed by RT-qPCR. These data, along with protein accumulation profiles and FM metabolite concentrations in testis, supported the no-effect dose of 0.2 mg/kg/day. Furthermore, the microarray data indicated a dose-dependent change in the fatty acid catabolism pathway, a biological process described for the first time to be affected by FM in testicular tissue. In conclusion, the present data indicate the existence of a transcriptomic threshold, which must be exceeded to progress from a normal state to an adaptative state and subsequently to adverse toxicity.

Standard and molecular NOAELs for rat testicular toxicity induced by flutamide.

An important step in the safety assessment of chemicals for humans is to determine the no observed adverse effect level (NOAEL) in toxicity studies conducted in animal models. With the increasing use of molecular tools in toxicity studies, a question often posed is how a NOAEL derived from molecular data compares to a NOAEL established using standard methods. The objective of the present study was to address this question when considering testicular toxicity. To do this, we assessed the effects of the reference antiandrogen flutamide on rat testes in a standard 28-day toxicity study using doses of 0.04-150 mg/kg/day. At necropsy, blood samples were collected for testosterone measurements. The testes were collected for histopathological assessment as well as for the evaluation of gene expression changes using quantitative PCR analyses. Results showed that increases in plasma testosterone level and Leydig cell hyperplasia were detected from 6 mg/kg/day. An alteration in the level of accumulation of a selection of genes was also detected from 6 mg/kg/day. This was the case for genes functionally associated with the testicular lesion, such as lipid metabolism and cell death/cell proliferation, as well as for genes not functionally associated with the lesion. Contrary to the misgivings, these data show that, using a standard 28-day toxicity study and a well-characterized adverse effect, the NOAEL based on transcript changes is similar to the NOAELs based on testosterone levels and histopathological examination.

Evaluation of the antiandrogenic effects of flutamide, DDE, and linuron in the weanling rat assay using organ weight, histopathological, and proteomic approaches.

The Organization for Economic Cooperation and Development (OECD) is currently funding the validation of the Hershberger assay as a rapid in vivo means of identifying (anti-) androgens. However, as the assay measures weight changes in the androgen-sensitive tissues of castrated rats, the evaluation of the androgen-stimulated intact weanling as a more ethical model to use in the assay has been requested. As part of the OECD validation exercise two weak antiandrogens, 1,1-dichloro-2,2-bis(4 chlorophenyl)ethane (DDE) and linuron (LIN), were investigated in our laboratory at several dose levels in the testosterone propionate (TP)-stimulated weanling using flutamide (FM) as a positive control. In addition to weight measurements (sex accessory tissues [SATs], epididymides, and testes), histopathological assessment of the seminal vesicles, prostate, and testes was conducted for vehicle control, TP-stimulated, and TP-stimulated animals treated with FM or the top dose level of DDE or LIN. The modulation of a novel prostate protein associated with apoptosis, L-amino acid oxidase (LAO), was evaluated in these same treatment groups. Our gravimetric data (supported by the histopathology data) indicated that the weanling assay can detect SAT and epididymal weight changes induced by the antiandrogens evaluated. Inconsistent and variable data were recorded for the testicular weight and histopathological effects, suggesting that the testis is of little value in the identification of antiandrogens using this model. Three isoforms of LAO were identified, and all were regulated by TP. Modulation of LAO by the antiandrogens indicated that this protein could be a biomarker for endocrine disruption in male rodents.

Correlation between protein accumulation profiles and conventional toxicological findings using a model antiandrogenic compound, flutamide.

In conventional rodent toxicity studies the characterization of the adverse effects of a chemical relies primarily on gravimetric, and histopathological data. The aim of this study was to evaluate if the use of two-dimensional gel electrophoresis could generate protein accumulation profiles, which were in accordance with conventional toxicological findings by investigating a model antiandrogen, flutamide (FM), whose toxic effects, as measured using standard approaches, are well characterized. Male Sprague-Dawley rats were orally exposed to FM (0, 6, 30, and 150 mg/kg/day) for 28 days. The expected inhibition of androgen-dependent tissue stimulation, increased luteinizing hormone and testosterone plasma levels, and Leydig cell hyperplasia were observed. Changes in testicular protein accumulation profiles were evaluated in rats exposed to 150 mg/kg/day FM. Several proteins involved in steroidogenesis (e.g., StAR, ApoE, Hmgcs1, Idi1), cell cycle, and cancer (e.g., Ddx1, Hspd1) were modulated by FM, and these data provided molecular evidence for the hormonal and testicular histopathology changes recorded. Changes in proteins associated with spermatogenesis were also recorded, and these are discussed within the context of the testicular phenotype observed following FM treatment (i.e., normal spermatogenesis but Leydig cell hyperplasia). Overall, our data indicate that the combination of conventional toxicology measurements with omic observations has the potential to improve our global understanding of the toxicity of a compound.

Induction of iron homeostasis genes during estrogen-induced uterine growth and differentiation.

We have previously used genome-wide transcript profiling to investigate the relationships between changes in gene expression and physiological alterations during the response of the immature mouse uterus to estrogens. Here we describe the identification of a functionally inter-related group of estrogen-responsive genes associated with iron homeostasis, including the iron-binding protein lactotransferrin, the ferroxidase ceruloplasmin, the iron delivery protein lipocalin 2 and the iron-exporter ferroportin. Quantitative real-time PCR revealed that the expression of these genes increases with time during the uterotrophic response, reaching maximal levels in the post-proliferative phase (between 48 and 72 h). In contrast, the heme biosynthesis genes aminolevulinic acid synthase 1 and 2 were maximally induced by estrogen at 2 and 4 h, respectively, prior to increased cell proliferation. Together, these data reveal that estrogen induces the temporally coordinated expression of iron homeostasis genes in the mouse uterus, and suggest an important role for iron metabolism during sex steroid hormone-induced uterine cell growth and differentiation.

Phenotypic anchoring of gene expression changes during estrogen-induced uterine growth.

A major challenge in the emerging field of toxicogenomics is to define the relationships between chemically induced changes in gene expression and alterations in conventional toxicologic parameters such as clinical chemistry and histopathology. We have explored these relationships in detail using the rodent uterotrophic assay as a model system. Gene expression levels, uterine weights, and histologic parameters were analyzed 1, 2, 4, 8, 24, 48, and 72 hr after exposure to the reference physiologic estrogen 17 beta-estradiol (E2). A multistep analysis method, involving unsupervised hierarchical clustering followed by supervised gene ontology-driven clustering, was used to define the transcriptional program associated with E2-induced uterine growth and to identify groups of genes that may drive specific histologic changes in the uterus. This revealed that uterine growth and maturation are preceded and accompanied by a complex, multistage molecular program. The program begins with the induction of genes involved in transcriptional regulation and signal transduction and is followed, sequentially, by the regulation of genes involved in protein biosynthesis, cell proliferation, and epithelial cell differentiation. Furthermore, we have identified genes with common molecular functions that may drive fluid uptake, coordinated cell division, and remodeling of luminal epithelial cells. These data define the mechanism by which an estrogen induces organ growth and tissue maturation, and demonstrate that comparison of temporal changes in gene expression and conventional toxicology end points can facilitate the phenotypic anchoring of toxicogenomic data.

Cumulative dietary energy intake determines the onset of puberty in female rats.

Laboratory animal diets for studies to determine the endocrine-disrupting potential of chemicals are under scrutiny because they can affect both assay control values and assay sensitivity. Although phytoestrogen content is important, we have previously shown that a phytoestrogen-rich diet and a phytoestrogen-free diet were equally uterotrophic to rats and advanced vaginal opening (VO) when compared with the standard diet RM1. Abolition of the effects by the gonadotrophin-releasing hormone antagonist Antarelix indicated that these effects were mediated through the hypothalamus-pituitary-reproductive organ axis. In the present study, we investigated the relationship between cumulative energy intake and sexual maturation in female rats. Infant formula (IF) at different concentrations and synthetic diets, with a wide range of metabolizable energy (ME) values, were used to modulate energy intake. Increasing energy intake was associated with an increase in uterine weight (absolute and adjusted for body weight) for both IF and the synthetic diets. In both cases, the increased uterine weight was directly proportional to energy intake. Body weight was unaffected by IF consumption but, in the case of the diets, was increased proportionally with energy consumption. Antarelix abolished the uterine weight increases with both formula and the diets, whereas body weight was unaffected. The mean day of VO was also advanced by high-ME diets and IF, whereas body weight at VO was unaffected. VO occurred at an energy intake of approximately 2,300 kJ/rat determined by measuring total food intake from weaning to VO, indicating that this cumulative energy intake was the trigger for puberty. ME is therefore a critical factor in the choice of diets for endocrine disruption studies.

The need to decide if all estrogens are intrinsically similar.

We used gene expression profiling to investigate whether the molecular effects induced by estrogens of different provenance are intrinsically similar. In this article we show that the physiologic estrogen 17-beta-estradiol, the phytoestrogen genistein, and the synthetic estrogen diethylstilbestrol alter the expression of the same 179 genes in the intact immature mouse uterus under conditions where each chemical has produced an equivalent gravimetric and histologic uterotrophic effect, using the standard 3-day assay protocol. Data are also presented indicating the limitations associated with comparison of gene expression profiles for different chemicals at times before the uterotrophic effects are fully realized. We conclude that the case has yet to be made for regarding synthetic estrogens as presenting a unique human hazard compared with phytoestrogens and physiologic estrogens. Key words: diethylstilbestrol, estrogen, gene expression, genistein, microarray, phytoestrogen, toxicogenomics, uterus.

Natural variability and the influence of concurrent control values on the detection and interpretation of low-dose or weak endocrine toxicities.

While defining the no effect level for the 5 alpha-reductase inhibitor finasteride in the Hershberger assay, we encountered an inverted-U low-dose trophic effect on the prostate gland of the rat. Two attempts to confirm this observation were unsuccessful, and we concluded that the positive effect initially observed was associated with normal biologic variability. During the same period we attempted, unsuccessfully, to repeat our own observation of weak uterotrophic activity in the rat for the sunscreen 3-(4-methylbenzylidene)camphor (4MBC). Further evaluation led us to conclude that 4MBC is uterotrophic only when the control uterine weights are at the low end of their normally encountered range. This led us to reevaluate our earlier mouse uterotrophic assay data for bisphenol A (BPA). Originally we had concluded that BPA gave irreproducible evidence of weak uterotrophic activity, but upon ordering the eight experiments we had conducted, according to decreasing control uterine weight, we confirmed reproducible weak uterotrophic activity for BPA when the control uteri were at the low end of their normal range. In this article, we describe these observations, together with a reanalysis of the data associated with several reported instances of weak or low-dose endocrine effects that have proven difficult to confirm in independent laboratories. These include the activity of BPA on the CF1 mouse prostate; the activities of BPA, octylphenol, and nonylphenol on the rat testis; and the effect of polycarbonate caging on control mouse uterine weight. In all of these cases, variability among controls provides a major obstacle to data interpretation and confirmation. Our recommendations on experimental design are also presented, with a view to ending the current impasse on the reality, or otherwise, of low-dose or weak endocrine toxicities.