Nicotine from tobacco smoke is absorbed into the bloodstream and transferred into breast milk in breastfeeding mothers. Smoking causes a decrease in breast milk volume, adverse changes to the milk composition, and a shortened lactation period. Breast milk is produced by mammary epithelial cells (MECs) in mammary glands during lactation. However, it remains unclear whether nicotine directly affects milk production in lactating MECs. To address this issue, we prepared a culture model with high milk production ability and less-permeable tight junctions (TJs) by seeding mouse MECs on a cell culture insert. Lactating MECs showed expression of α2, α3, β2, and β4 of nicotinic acetylcholine receptors. The high concentration of nicotine at 10–100 μM inhibited β-casein secretion and caused abnormal localization of TJ proteins. We subsequently investigated whether nicotine at a physiological concentration could affect lactating MECs. Nicotine at 1.0 μM directly inhibited α- and β-casein secretion in lactating MECs concurrently with inactivation of STAT5 and glucocorticoid receptor without affecting the TJ barrier. Nicotine treatment also induced MEC apoptosis concurrently with inactivation of Akt. These results support the adverse effects of nicotine on breastfeeding in smoking mothers.
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