Progesterone downregulation of miR-141 contributes to expansion of stem-like breast cancer cells through maintenance of progesterone receptor and Stat5a

Progesterone (P4) has emerged as an important hormone regulating mammary stem cell populations. In breast cancer, P4 and synthetic analogs increase the number of stem-like cells within luminal estrogen receptor (ER) and progesterone receptor (PR) positive breast cancers. These cells gain expression of de-differentiated cell markers CD44 and cytokeratin 5 (CK5), lose luminal markers ER and PR, and are more therapy resistant. We previously described that P4-downregulation of microRNA (miR)-29a contributes to the expansion of CD44(high) and CK5(+) cells. Here we investigated P4-downregulation of miR-141, a member of the miR-200 family of tumor suppressors, in facilitating an increase in stem-like breast cancer cells. miR-141 was the sole member of the miR-200 family P4-downregulated at the mature miRNA level in luminal breast cancer cell lines. Stable inhibition of miR-141 alone increased the CD44(high) population, and potentiated P4-mediated increases in both CD44(high) and CK5(+) cells. Loss of miR-141 enhanced both mammosphere formation and tumor initiation. miR-141 directly targeted both PR and Stat5a, transcription factors important for mammary stem cell expansion. miR-141 depletion increased PR protein levels, even in cells lines where PR expression is estrogen-dependent. Stat5a suppression via siRNA or a small molecule inhibitor reduced the P4-dependent increase in CK5(+) and CD44(high) cells. These data support a mechanism by which P4-triggered loss of miR-141 facilitates breast cancer cell de-differentiation through deregulation of PR and Stat5a, two transcription factors important for controlling mammary cell fate.