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Oxytocin Regulates the Expression of Selected Colostrum-Derived microRNAs. OBJECTIVE: The highly expressed microRNAs (miRNAs) in milk are known as beneficial miRNAs, such as mir148a-3p which is related to immune system development and disease prevention. There is a need to study their expression and secretion regulatory mechanism in breast milk. We hypothesize that oxytocin can be involved in the regulation of expression and secretion of milk-derived miRNAs. METHODS: Initially, oxytocin's effect on miRNA expression in human mammary cells was analyzed. Secondly, the expression of selected miRNAs in mothers' colostrum treated or not with oxytocin before, during, or after labor was compared. MiRNA expression was analyzed by quantitative real-time PCR. RESULTS: The expression of miR-148a was significantly upregulated, and miR-320 downregulated in oxytocin-treated mammary cells as well as their secreted extracellular vesicles to the media, compared to untreated cells. MiR-148a was found to be upregulated, and miR-320 was downregulated in the human colostrum of exogenous oxytocin-treated mothers. Moreover, miR-320 was highly expressed compared to miR-148a in the colostrum of mothers that didn't receive exogenous oxytocin. In contrast, in the milk of mothers that received exogenous oxytocin, the expression of miRNA-148-3p was highly expressed compared to miR-320. CONCLUSIONS: This study shows that oxytocin modulates the expression of main milk-derived miRNAs. Our findings provides a novel insight into oxytocin's role in milk composition by regulating miRNA expression. Our results implicate that oxytocin increases miRNA expression in mammary epithelial cells and human milk, affecting human milk composition and may contribute to further infant health.