A novel role of microRNA146b in promoting mammary alveolar progenitor cell maintenance

In this report, we have shown that miR146b promotes the maintenance of pregnancy-derived mammary luminal alveolar progenitors. MiR146b expression was significantly higher in the mammary glands of pregnant and lactating mice than in virgin mice. Furthermore, miR146b levels were significantly higher in mouse mammary glands exposed to the sex hormones, estrogen and progesterone, compared with those of untreated control animals. Pregnancy-derived primary mouse mammary epithelial cells in which miR146b was knocked down showed a significant reduction in the number of hollow acinar organoid structures formed on three-dimensional Matrigel and in beta-casein expression. This demonstrates that miR146b promotes the maintenance of pregnancy-derived mammary luminal alveolar progenitors. It has been shown that mouse mammary luminal progenitors give rise to hollow organoid structures, whereas solid organoid structures are derived from stem cells. Among several miR146b targets, miR146b knockdown resulted in preferential STAT3 beta overexpression. In the primary mouse mammary epithelial cells, overexpression of STAT3 beta isoform caused mammary epithelial cell death and a significant reduction in beta-casein mRNA expression. Therefore, we conclude that during pregnancy miR146b is involved in luminal alveolar progenitor cell maintenance, at least partially, by regulating STAT3 beta.