ABSTRACT: Estrogenic endocrine disrupting compounds (EDCs) found in plastics, drinking water, and food are able to bind to the estrogen receptor (ER). Excessive estrogen signaling drives proliferation, progression, and the formation of breast cancer stem cells (CSCs) in ER-positive breast cancer. Thus, we hypothesized that estrogenic EDCs mimic estrogen (E2) in breast cancer cell lines. Three estrogenic EDCs routinely found in human biosamples were selected for analysis: bisphenol-A (BPA), diethyl-hexyl phthalate (DEHP), and alpha-zeranol (αZAL). We assessed proliferation, transcriptional reprogramming, and CSC formation in several breast cancer cell lines. E2, BPA, and αZAL significantly increased cell proliferation in ER-positive, but not in ER-negative cell lines. This was reversed with the administration of an ER-antagonist, ICI 182,780. BPA and αZAL upregulated estrogen target genes (PGR, TFF1) and increased levels of cell cycle protein. Whole-transcriptome RNA sequencing analysis revealed that BPA and αZAL altered the expression of genes in pathways related to cell division, DNA repair, and estrogen signaling, with a substantial transcriptional overlap between EDC and estrogen treatments. Additionally, in the ER-positive cell line MCF-7, BPA and αZAL increased the proportion of CSCs, defined as the CD24low/CD44high expressing subpopulation. Overall, these data indicate that BPA and αZAL act as functional estrogen mimics in breast cancer cells, activating canonical estrogen signaling pathways and promoting stem-like characteristics. Exposure to EDCs is nearly unavoidable; therefore, understanding their estrogenic activities is crucial for assessing environmental contributions to breast cancer risk. This work provides mechanistic insight into how environmental EDCs may influence ER-positive breast cancer biology.
