Obesogen co-exposure promotes triple-negative breast cancer growth via adipocyte-mediated mechanisms

Introduction : Bisphenol A (BPA) and dichlorodiphenyldichloroethylene (DDE; the primary metabolite of DDT) are environmental ‘obesogens' that independently associate with altered adipocyte function and increased breast cancer risk. While the effects of obesogens alone are well studied, particularly in hormone receptor (HR)-positive breast cancer, the biological consequences of combined exposure and their impact on HR-negative disease through adipocyte-tumor interactions remains poorly understood. Here, we investigate how obesogen co-exposure alters mammary adipocyte function and promotes tumorigenesis in a mouse model of triple-negative breast cancer (TNBC). Study design : Post-pubertal BALB/c mice (n=10/group) were exposed to obesogens via drinking water for the duration of the study, at environmentally relevant doses (Vehicle (ethanol); BPA (4µg/kg/day); DDE (0.8µg/kg/day); combination (BPA+DDE)). After 2 weeks exposure, murine mammary cancer cells (4T1; 20,000 cells) were injected bilaterally into the fourth inguinal mammary fat-pads, and investigator-blinded tumor growth tracked for 21 days. To assess how obesogens affect adipocyte function in vitro, 3T3-L1 preadipocytes were differentiated in the presence of vehicle (0.03% ethanol), BPA (1nM), DDE (1µM), or combination (BPA+DDE), and adipokine secretion profiles assessed using an adipokine array. Results : Mouse body weight did not differ between treatment groups. In tumor-free mammary glands, obesogen exposure associated with increased adipocyte size (BPA 1029±231µm 2 , DDE 957±350µm 2 , Combo 1011±344µm 2 ) compared to vehicle-treated mice (766±187µm 2 ; p<0.05). Interestingly, while individual exposures induced adipocyte hypertrophy, only combined exposure increased epithelial cell proliferation (Ki67), compared to vehicle-treated mice (2.8-fold, p=0.02). In tumors, combination treatment associated with increased tumor burden (1.4-fold, p=0.02) and proliferation (1.3-fold, p=0.04), compared to vehicle-treated mice. In vitro, obesogen exposure altered adipocyte secretion of 11/38 adipokines (>2-fold change), 6 of which were unique to combination-treated adipocytes (up: LIF; down: IGF-II, VEGF, M-CSF, CCL2, adiponectin). Exposure of 4T1 cells to this conditioned media increased proliferation (EdU), compared to vehicle controls (1.5-fold, p=0.04). Future directions will investigate whether inhibition of LIF/LIFR signaling abrogates obesogen-induced proliferation. Conclusions : Co-exposure to BPA and DDE synergistically promotes TNBC proliferation at environmentally relevant doses, underscoring the need to study obesogens in the context of real-world co-exposures. The effects on HR-negative disease occurred independent of obesity and were mediated, at least in part, through adipocyte-derived mechanisms, highlighting the need to expand research beyond HR-positive breast cancer.