Impact of progesterone on stem/progenitor cells in the human breast.

The epithelium of the human breast is made up of a branching ductal-lobular system, which is lined by a single layer of luminal cells surrounded by a contractile basal cell layer. The co-ordinated development of stem/progenitor cells into these luminal and basal cells is fundamentally important for breast morphogenesis. The ovarian steroid hormone, progesterone, is critical in driving proliferation and normal breast development, yet progesterone analogues have also been shown to be a major driver of breast cancer risk. Studies in recent years have revealed an important role for progesterone in stimulating the mammary stem cell compartment in the mouse mammary gland, and growing evidence supports the notion that progesterone also stimulates progenitor cells in both the normal human breast and in breast cancer cells. As changes in cell type composition are one of the hallmark features of breast cancer progression, these observations have critical implications in discerning the mechanisms of how progesterone increases breast cancer risk. This review summarises recent work regarding the impact of progesterone action on the stem/progenitor cell compartment of the human breast.