Low-dose mifepristone inhibits endometrial proliferation and up-regulates androgen receptor

Mifepristone in daily low doses has contraceptive potential by inhibiting ovulation. Follicular development is maintained, and although the endometrium is exposed to unopposed estrogen, there are no signs of hyperplasia or atypia. The mechanism of this antiestrogenic action is unknown. We have investigated the effect of daily low-dose mifepristone on proliferation markers and steroid receptors in surface epithelium, glands, and stroma of the endometrium. Endometrial biopsies were collected from 16 women before ( late proliferative) and 60 and 120 d after taking 2 or 5 mg mifepristone daily for 120 d. Endometrial proliferation (H3 mitosis marker) and steroid ( estrogen, progesterone, and androgen) receptor content were studied using standard immunocyotchemistry techniques. There was a significant decrease in the expression of H3 mitosis marker ( P less than or equal to 0.001) and progesterone receptor ( P < 0.05) in endometrial glands and stroma by d 60 of treatment. In contrast, the expression of androgen receptor increased ( P < 0.01) in glands, surface epithelium, and stroma compared with the pretreatment sample. These changes were maintained at 120 d of treatment. The expression of estrogen receptor was unchanged in stroma and surface epithelium; however, there was a significant decrease in expression after 120 d of treatment ( P = 0.034). As androgens can antagonize estrogen action, enhanced glandular androgen receptor expression induced by mifepristone could play a role in its antiproliferative effects.