LAUSR

&NA; Hexachlorobenzene (HCB) is a widespread environmental pollutant and a dioxin‐like compound that binds weakly to the aryl hydrocarbon receptor (AhR). Because AhR and transforming growth factor &bgr;1 (TGF‐&bgr;1) converge to regulate common signaling pathways, alterations in this crosstalk might contribute to developing preneoplastic lesions. The aim of this study was to evaluate HCB action on TGF‐&bgr;1 and AhR signaling in mouse mammary gland, through AhR +/+ and AhR −/− models. Results showed a differential effect in mouse mammary epithelial cells (NMuMG), depending on the dose: 0.05 &mgr;M HCB induced cell migration and TGF‐&bgr;1 signaling, whereas 5 &mgr;M HCB reduced cell migration, promoted cell cycle arrest and stimulated the dioxin response element (DRE) ‐dependent pathway. HCB (5 &mgr;M) enhanced &agr;‐smooth muscle actin expression and decreased TGF‐&bgr; receptor II mRNA levels in immortalized mouse mammary fibroblasts AhR +/+, resembling the phenotype of transformed cells. Accordingly, their conditioned medium was able to enhance NMuMG cell migration. Assays in C57/Bl6 mice showed HCB (3 mg/kg body weight) to enhance ductal hyperplasia, cell proliferation, estrogen receptor &agr; nuclear localization, branch density, and the number of terminal end buds in mammary gland from AhR +/+ mice. Primary culture of mammary epithelial cells from AhR +/+ mice showed reduced AhR mRNA levels after HCB exposure (0.05 and 5 &mgr;M). Interestingly, AhR −/− mice exhibited an increase in ductal hyperplasia and mammary growth in the absence of HCB treatment, thus revealing the importance of AhR in mammary development. Our findings show that environmental HCB concentrations modulate AhR and TGF‐&bgr;1 signaling, which could contribute to altered mammary branching morphogenesis, likely leading to preneoplastic lesions and retaining terminal end buds. Highlights0.05 &mgr;M HCB induces migration and TGF‐&bgr;1 signaling in mammary epithelial cells NMuMG.5 &mgr;M HCB reduces migration, and promotes cell cycle arrest and AhR nuclear pathway in NMuMG.HCB enhances &agr;‐SMA and decreases TGF‐&bgr; receptor II expression in fibroblasts AhR +/+.Conditioned medium from fibroblasts AhR +/+ exposed to HCB enhances NMuMG migration.HCB increases hyperplasia and branching morphogenesis in AhR +/+ mice mammary gland.