Estrogenic activity of RU 486 (mifepristone) in rat uterus and cultured uterine myocytes

OBJECTIVE: Our purpose was to determine whether RU 486 (mifepristone) has direct estrogenic activity in uterine myocytes. STUDY DESIGN: Ovariectomized adult rats were treated with RU 486, and its effect on uterine oxytocin receptor concentration, as a marker of estrogenic activity, was measured. Results were compared with th induction by RU 486 of an estrogen-responsive reporter gene in a cultured Syrian hamster uterine myocyte cell line. RESULTS: Baseline oxytocin receptor concentration was 58.8 ± 7.2 fmol/mg protein (mean ± SEM) and increased to 227 ± 49 fmol/mg with 17β-estradiol (2.5 μg/kg) and to 145 ± 18 fmol/mg afte RU 486 (5 mg/kg) treatment, an effect that was inhibited by the antiestrogen ICI 182,780 (1.5 mg/kg). In the cultured Syrian hamster uterine myocyte cell line cells RU 486 (10−6 mol/L) caused a 2.17 ± 0.17-fold increase in the expression of the reporter gene versus 113.0 ± 7.4-fold with 17β-estradiol (10−8 mol/L). The estrogenic activity of RU 486 was dependent on the presence of both estrogen receptor and the promoterʹs estrogen response element. CONCLUSION: RU 486 has a weak estrogen-like activity in uterine myocytes. This activity may partly explain the therapeutic effects of RU 488 on this target organ.