A new study has reported that the level of progesterone (which varies during different phases of the menstrual cycle) influences the number of breast stem cells in the breast. Reproductive history is the strongest factor predicting breast cancer risk after age, family history and breast density. Risk of breast cancer is known to increase with the number of menstrual cycles, however the basis for this association has not been fully explained. Since mammary stem cells appear to have a role in cancer initiation, it makes sense to examine the role of ovarian hormones in influencing mammary stem cells.

In the study, the authors demonstrated that the mammary stem cell pool increased 14-fold during peak progesterone levels in mice. Progesterone had a key role in causing this expansion. There was also evidence that mammary stem cells entered into stasis upon cessation of the reproductive cycle (corresponding to menopause in women) in older mice. When progesterone reached its maximum level during the second half of the menstrual cycle, cross-talk increased between normal mammary stem cells and neighboring cells that caused the stem cells to expand in number. This activity may in turn trigger an environment where cancer begins. Until now, breast stem cells have been thought to be generally inactive in the adult female breast. The authors conclude that progesterone-driven shifts in the mammary stem cell pool might establish an environment that starts the process of cell transformation leading to breast cancer.