Smoking-induced suppression of β-casein in milk is associated with an increase in miR-210-5p expression in mammary epithelia

Smoking during lactation harmfully affects the amount and constituents of breast milk. Infants who consume breast milk containing miR-210-5p may have a higher risk of brain-related diseases. We investigated whether smoking during lactation decreases β-casein concentrations in milk and whether miR-210-5p expression is involved in smoking-induced β-casein suppression. During lactation, maternal CD1 mice were exposed to cigarette smoke (1.7 mg of tar and 14 mg of nicotine) in a smoke chamber for 1 h twice/day for five consecutive days. Control mice were placed in an air-filled chamber equivalent in size to the smoke chamber, with maternal separation times identical to those of the smoked mice. Maternal exposure to smoke during lactation significantly decreased β-casein expression in the mammary epithelia of smoked mice compared to that of the control mice. Signal transducer and activator transcription 5 (STAT5) and phosphorylated STAT5 (pSTAT5) are transcription factors involved in β-casein expression. In the mammary epithelia of smoked mice, the pSTAT5 and STAT5 levels were significantly lower, and miR-210-5p expression was significantly higher than that of the control mice. The β-casein, pSTAT5, and STAT5 protein levels of miR-210-5p mimic-transfected human mammary epithelial MCF-12A cells were significantly lower than those of control siRNA-transfected cells. These results indicate that smoke exposure led to an increase in miR-210-5p expression in mammary epithelium and a decrease in pSTAT5 and β-casein protein levels through the inhibition of STAT5 expression. Moreover, nicotine treatment decreased β-casein protein levels and increased miR-210-5p expression in non-malignant human mammary epithelial MCF-12A cells in a concentration-dependent manner, demonstrating that nicotine significantly affects the β-casein and miR-210-5p levels of breast milk. These results highlight the adverse effects of smoking on breast milk, providing essential information for healthcare professionals and general citizens.