The Endocrine Disrupting Compounds Bisphenol‐A and α‐Zeranol Mimic the Estrogen Transcriptional Program to Promote Proliferation and Stemness in Breast Cancer Cells

ABSTRACT Excessive activation of the estrogen receptor (ER) drives proliferation, progression, and the formation of breast cancer stem cells (CSCs) in ER‐positive breast cancer. Estrogenic endocrine disrupting compounds (EDCs) found in plastics, water, and food are also able to bind to the ER. Thus, we hypothesized that estrogenic EDCs mimic estrogen (E2) in the pathogenesis of breast cancer by promoting their survival and proliferation. 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 breast cancer cell lines. E2, BPA, and αZAL significantly increased cell proliferation in ER‐positive, but not ER‐negative cell lines. This was reversed after administration of the ER‐antagonist, ICI 182,780. BPA and αZAL upregulated estrogen target genes (PGR, TFF1) and increased levels of cell‐cycle protein. RNA sequencing analysis revealed that BPA and αZAL altered expression of genes related to cell division, DNA repair, and estrogen signaling, with a substantial transcriptional overlap between EDCs and estrogen treatments. Additionally, BPA and αZAL increased the proportion of CSCs, defined as the CD24 low /CD44 high 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. Notably, this study provides the first transcriptomic and stem‐associated characterization of αZAL in ER‐positive breast cancer cells, revealing a robust estrogenic mode of action. This work provides mechanistic insight into how environmental EDCs may influence ER‐positive breast cancer biology.