Osteoblasts and osteoclasts are crucial to maintaining bone homeostasis. These specialized cells rely on various environmental signals and cross talk from one another in order to model, remodel and repair bone. Exogenous chemicals such as the therapeutic drug rosiglitazone, a peroxisome proliferator activated receptor gamma (PPARγ) agonist, can interfere with bone-forming and bone-resorbing pathways, causing osteoporosis and increasing the risk of bone fracture. Evidence is emerging that environmental toxicants induce similar toxic endpoints in bone, both through PPARγ-dependent and PPARγ-independent mechanisms. To date, these toxicants have only ever been considered in isolation or in limited co-exposure studies. This comprehensive review relating these toxicants and their effect on bone health will help guide future studies and illuminate gaps in our knowledge. Five toxicant classes (organotins—mainly tributyltin; TBT, heavy metals—lead, cadmium, and arsenic, dioxin-like chemicals, phthalates, and perfluoroalkyl compounds; PFAs) were examined, with emphasis on molecular targets, osteoclast- and osteoblast-specific effects, animal models and epidemiological data. It was concluded that organotins (TBT) act via PPARγ and RXR agonism, phthalates act via PPARγ agonism, heavy metals act at least through ERK-mediated pathways, and dioxin-like chemicals act through aryl hydrocarbon receptor interaction. The molecular targets of PFAs remain unknown. Additional targets are still being investigated. These findings emphasize the importance of co-exposures, as these toxicants act through diverse molecular mechanisms that may share toxic endpoints, making co-exposure consequences particularly severe. While the evidence available strongly suggests that lead, cadmium, and dioxin-like chemicals are negative modulators of human bone health, evidence supporting this conclusion for organotins, phthalates, arsenic, and PFAs is somewhat lacking. There are still significant gaps in our understanding that must be filled to gain a holistic understanding of these threats to human bone health.

Trichloroethylene (TCE) is among the most common water contaminant in the United States and around the world. It is estimated that between 9% and 34% of all drinking water sources contain some TCE. The EPA set a drinking water standard for TCE at 5 parts per billion (ppb) in 1989, however since this date, many studies have shown TCE is dangerous to the health of adults and unborn children, even at low-level exposures. These studies reveal exposure to TCE can cause multi-organ damage, especially for the kidney, liver, reproductive and development systems. We investigated how TCE can effect embryonic heart development by identifing possible target mechanisms changing after exposure. Acute and chronic exposure to rat cardiomyocytes produced altered calcium flow and significant changes with TCE doses as low as 10ppb. Embryonic carcinoma cells, rat cardiomyocytes and fetal heart tissue all showed global changes in gene expression after low-dose TCE exposure, including critical ion channels that drive calcium flux. High levels of folic acid supplementation in combination with 10ppb TCE exposure in maternal diets caused significant genetic modifications in mRNA expression levels of Day 10 embryonic mouse cardiac tissue. We also found both high and low folate maternal diets leads to similar phenotypic outcomes in embryo development.

Endocrine Disrupting Chemicals (EDC) are exogenous agents that mimic endogenous hormone activity in the body. EDC exposure during the critical period of neonatal development can potentially cause life-long neurological, behavioural and physiological disease. This thesis focuses on the EDC Bisphenol A (BPA), a synthetic xenoestrogen widely prevalent in everyday materials that has significant environmental relevance given its ubiquitous presence in humans around the world. The central research question of my thesis is: Does perinatal exposure to BPA affect rat pup development? A rodent model was selected to study the effects of BPA on the adrenal component of the hypothalamic-pituitary-adrenal axis (HPA axis) stress pathway, which has not been extensively studied. Rat dams were divided into five groups (vehicle control (VEH), positive control diethylstilbestrol (DES), BPA 5, BPA 50 and BPA 500 μg/kg bw/day) and dosed daily throughout gestation and for four days of lactation. Rat pups were sacrificed at two time-points at the beginning and the end of the stress hyporesponsive period (SHRP), at postnatal day (PND) 5 and PND 15. Changes in three morphometric parameters (bodyweight, crown-rump (CR) length and anogenital distance (AGD) were assessed based on the factors of Treatment and Sex. Adrenal gland glucocorticoid receptor (GR) and 18SrRNA expression was determined by qPCR in male pups at PND 5 and PND 15. At PND 5, compared to the VEH group, the BPA 50 pups were significantly heavier (ANOVA, Dunnett’s post-hoc) and the DES and BPA 50 pups had significantly longer CR lengths (ANOVA, Dunnetts’ post-hoc). At PND 15, xenoestrogen treatment significantly influenced CR length (ANOVA). At both time-points, males had significantly longer AGD than females, as physiologically expected (ANOVA). Adrenal gland GR expression in male pups was not significantly affected by treatment, but there was an effect of treatment in18SrRNA gene expression at PND 5 (Kruskal-Wallis). Using the Ct method to determine GR and 18SrRNA fold changes, we cautiously suggest that our experimental doses resulted in a non-monotonic dose response to BPA in the PND 5 animals and a monotonic dose response to BPA exposure in the PND 15 animals. This study highly values the importance of investigating the effects of environmentally relevant, low dose BPA exposure during the critical window of development, given the little that is known about potentially permanent alterations to the stress pathway due to exposure during this delicate period of development.