Elucidating the physiological and molecular mechanisms of endocrine-disrupting compounds in breast cancer malignancy

One in eight women will be diagnosed with breast cancer in their lifetime. Breast cancer is the second most diagnosed cancer in women, preceded only by lung cancer. It accounts for a large share of the global disease burden. Approximately 70% of all cases are estrogen receptor (ER) positive, meaning they rely on estrogen signaling throughout cancer progression. Lifetime estrogen exposure is a well-established risk factor for the development of ER-positive breast cancer. It has additionally been postulated that estrogen is involved in ER-negative breast cancer development due to its effects on the tumor microenvironment. Endocrine-disrupting compounds (EDCs) represent a diverse class of chemicals that perturb the body’s endocrine signaling by mimicking endogenous hormones. Estrogenic EDCs are a subset of EDCs that mimic 17β-estradiol (E2). Concerningly, the ability of estrogenic EDCs to bind and activate the ER could confer breast cancer-promoting properties. Bisphenol-A (BPA), a plastic additive, diethyl-hexyl phthalate (DEHP), a phthalate plasticizer, and α-zeranol (αZAL), a mycotoxin, are all EDCs with demonstrated ER-binding affinities that have been routinely detected in human samples. Due to their affinities for the estrogen receptor, we hypothesized that exposure to EDCs such as BPA, DEHP, and αZAL would induce breast cancer progression in both ER-positive and negative breast cancer via activation of distinct cell signaling pathways. In this dissertation, the effects of these EDCs on breast cancer initiation and promotion were explored using diverse in vitro and in vivo systems of both ER-dependent and -independent models of breast cancer. Here, the effects of EDCs on breast cancer cell growth were assessed, and the ER-dependent mechanisms by which these compounds may act in vitro were interrogated. The roles of EDCs in ER-positive breast cancer xenograft tumor growth and in mammary gland proliferation and carcinogenesis in the ACI rat model were explored. Lastly, the mechanisms behind EDC-induced tumor progression in ER-independent in vivo xenograft models were investigated. In the aforementioned studies, it was demonstrated that DEHP and its major metabolite MEHP had no effects in in vitro assays and were not selected for further experimentation. BPA and αZAL induced ER-positive, but not ER-negative, breast cancer cell growth in vitro. This was paired with a global transcriptomic shift, with marked overlap with the E2-induced transcriptome in ER-positive breast cancer cells. In vivo, BPA and αZAL induced mammary hyperplasia at short timepoints in the ACI rat but did not induce carcinogenesis at later timepoints. Additionally, these compounds had no effect on ER-positive tumor growth in a xenograft model. However, BPA and αZAL promoted tumor growth and markers of progression in two distinct ER-negative xenograft models. The research described in this dissertation combined in vitro and in vivo studies to identify mechanisms through which EDCs promote breast cancer and to determine whether EDCs act similarly or differently from estrogen in inducing breast cancer. Together, these findings provide mechanistic insight into the tumor-promoting potential of estrogenic EDCs and inform ongoing efforts to evaluate the public health implications of chronic environmental exposures. Thus, this work advances our understanding of how ubiquitous EDCs may influence breast cancer progression and risk.