Building an adverse outcome pathway network for estrogen-, androgen- and steroidogenesis-mediated reproductive toxicity

Introduction: Adverse Outcome Pathways (AOPs) can support both testing and assessment of endocrine disruptors (EDs). There is, however, a need for further development of the AOP framework to improve its applicability in a regulatory context. Here we have inventoried the AOP-wiki to identify all existing AOPs related to mammalian reproductive toxicity arising from disruption to the estrogen, androgen, and steroidogenesis modalities. Core key events (KEs) shared between relevant AOPs were also identified to aid in further AOP network (AOPN) development. Methods: A systematic approach using two different methods was applied to screen and search the entire AOP-wiki library. An AOPN was visualized using Cytoscape. Manual refinement was performed to remove AOPS devoid of any KEs and/or KERs. Results: Fifty-eight AOPs relevant for mammalian reproductive toxicity were originally identified, with 42 AOPs included in the final AOPN. Several of the KEs and KE relationships (KERs) described similar events and were thus merged to optimize AOPN construction. Sixteen sub-networks related to effects on hormone levels or hormone activity, cancer outcomes, male and female reproductive systems, and overall effects on fertility and reproduction were identified within the AOPN. Twenty-six KEs and 11 KERs were identified as core blocks of knowledge in the AOPN, of which 19 core KEs are already included as parameters in current OECD and US EPA test guidelines. Discussion: The AOPN highlights knowledge gaps that can be targeted for further development of a more complete AOPN that can support the identification and assessment of EDs.

Comparison of female rat reproductive effects of pubertal versus adult exposure to known endocrine disruptors

A prevailing challenge when testing chemicals for their potential to cause female reproductive toxicity is the lack of appropriate toxicological test methods. We hypothesized that starting a 28-day in vivo toxicity study already at weaning, instead of in adulthood, would increase the sensitivity to detect endocrine disruptors due to the possibility of including assessment of pubertal onset. We compared the sensitivity of two rat studies using pubertal or adult exposure. We exposed the rats to two well-known human endocrine disruptors, the estrogen diethylstilbestrol (DES; 0.003, 0.012, 0.048 mg/kg bw/day) and the steroid synthesis inhibitor ketoconazole (KTZ; 3, 12, 48 mg/kg bw/day). Specifically, we addressed the impact on established endocrine-sensitive endpoints including day of vaginal opening (VO), estrous cyclicity, weights of reproductive organs and ovarian histology. After 28 days of exposure, starting either at weaning or at 9 weeks of age, DES exposure altered estrous cyclicity, reduced ovary weight as well as number of antral follicles and corpora lutea. By starting exposure at weaning, we could detect advanced day of VO in DES-exposed animals despite a lower body weight. Some endpoints were affected mainly with adult exposure, as DES increased liver weights in adulthood only. For KTZ, no effects were seen on time of VO, but adrenal and liver weights were increased in both exposure scenarios, and adult KTZ exposure also stimulated ovarian follicle growth. At first glance, this would indicate that a pubertal exposure scenario would be preferrable as timing of VO may serve as sensitive indicator of endocrine disruption by estrogenic mode of action. However, a higher sensitivity for other endocrine targets may be seen starting exposure in adulthood. Overall, starting a 28-day study at weaning with inclusion of VO assessment would mainly be recommended for substances showing estrogenic potential e.g., in vitro, whereas for other substances an adult exposure scenario may be recommended.

Benzophenone-3: Comprehensive review of the toxicological and human evidence with meta-analysis of human biomonitoring studies

BACKGROUND

Benzophenone-3 (BP-3) and its major metabolite benzophenone-1 (BP-1) are widely used as UV filters in sunscreens and cosmetics to prevent sunburn and skin damage, or as stabilizers to prevent photodegradation in many commercial products. As a result, their presence is ubiquitous in the environment, wildlife and humans. Based on endocrine disruption concerns, international regulatory agencies are performing a closer evaluation.

OBJECTIVE AND METHODS

This work aimed to comprehensively review the available human relevant evidence for safety issues in MEDLINE/PubMed in order to create a structured database of studies, as well as to conduct an integrative analysis as part of the Human Biomonitoring for Europe (HBM4EU) Initiative.

RESULTS

A total of 1,635 titles and abstracts were screened and 254 references were evaluated and tabulated in detail, and classified in different categories: i) exposure sources and predictors; ii) human biomonitoring (HBM) exposure levels to perform a meta-analysis; iii) toxicokinetic data in both experimental animals and humans; iv) in vitro and in vivo rodent toxicity studies; and v) human data on effect biomarkers and health outcomes. Our integrative analysis showed that internal peak BP-3 concentrations achieved after a single whole-body application of a commercially available sunscreen (4% w/w) may overlap with concentrations eliciting endocrine disrupting effects in vitro, and with internal concentrations causing in vivo adverse female reproductive effects in rodents that were supported by still limited human data. The adverse effects in rodents included prolonged estrous cycle, altered uterine estrogen receptor gene expression, endometrium hyperplasia and altered proliferation and histology of the mammary gland, while human data indicated menstrual cycle hormonal alterations and increased risk of uterine fibroids and endometriosis. Among the modes of action reported (estrogenic, anti-androgenic, thyroid, etc.), BP-3 and especially BP-1 showed estrogenic activity at human-relevant concentrations, in agreement with the observed alterations in female reproductive endpoints. The meta-analysis of HBM studies identified a higher concern for North Americans, showing urinary BP-3 concentrations on average 10 and 20 times higher than European and Asian populations, respectively.

DISCUSSION AND CONCLUSIONS

Our work supports that these benzophenones present endocrine disrupting properties, endorsing recent European regulatory efforts to limit human exposure. The reproducible and comprehensive database generated may constitute a point of departure in future risk assessments to support regulatory initiatives. Meanwhile, individuals should not refrain from sunscreen use. Commercially available formulations using inorganic UV filters that are practically not absorbed into systemic circulation may be recommended to susceptible populations.

Transcriptional profiling of the developing rat ovary following intrauterine exposure to the endocrine disruptors diethylstilbestrol and ketoconazole

Exposure to endocrine-disrupting chemicals (EDCs) during development may cause reproductive disorders in women. Although female reproductive endpoints are assessed in rodent toxicity studies, a concern is that typical endpoints are not sensitive enough to detect chemicals of concern to human health. If so, measured endpoints must be improved or new biomarkers of effects included. Herein, we have characterized the dynamic transcriptional landscape of developing rat ovaries exposed to two well-known EDCs, diethylstilbestrol (DES) and ketoconazole (KTZ), by 3' RNA sequencing. Rats were orally exposed from day 7 of gestation until birth, and from postnatal day 1 until days 6, 14 or 22. Three exposure doses for each chemical were used: 3, 6 and 12 µg/kg bw/day of DES; 3, 6, 12 mg/kg bw/day of KTZ. The transcriptome changed dynamically during perinatal development in control ovaries, with 1137 differentially expressed genes (DEGs) partitioned into 3 broad expression patterns. A cross-species deconvolution strategy based on a mouse ovary developmental cell atlas was used to map any changes to ovarian cellularity across the perinatal period to allow for characterization of actual changes to gene transcript levels. A total of 184 DEGs were observed across dose groups and developmental stages in DES-exposed ovaries, and 111 DEGs in KTZ-exposed ovaries across dose groups and developmental stages. Based on our analyses, we have identified new candidate biomarkers for female reproductive toxicity induced by EDC, including Kcne2, Calb2 and Insl3.