Trifolium pratense L. (red clover) is a popular
botanical supplement used for women’s health. Irilone isolated
from red clover previously demonstrated progestogenic potentiation
activity. In this study, irilone enhanced progesterone signaling was
determined to not occur due to post-translational phosphorylation
or by reducing progesterone receptor (PR) protein levels but instead
increased PR protein levels in T47D breast cancer cells, which could
be blocked by estrogen receptor (ER) antagonists, suggesting an ER
dependent effect. Further, irilone increased luciferase activity from
a hormone responsive element in a cell line that lacked ER and PR
but expressed the glucocorticoid receptor (GR). A siRNA knockdown
of GR in Ishikawa PR-B endometrial cancer cells reduced irilone’s
ability to enhance progesterone signaling. In an ovariectomized CD-1
mouse model, irilone did not induce uterine epithelial cell proliferation.
The mechanism of action of irilone gives insight into PR crosstalk
with other steroid hormone receptors, which can be important for understanding
botanicals that are used for women’s health.