In human endometrium, cell proliferation is regulated by ovarian steroids through heterotypic interactions between stromal and epithelial cells populating this tissue. We tested the proliferative effects of tibolone and its metabolites using endometrial co-cultures that mimic the normal proliferative response to hormones. We found that both the Δ4-tibolone metabolite and the pure progestin ORG2058 counteract estradiol-driven epithelial cell proliferation. Surprisingly, the estrogen receptor binding 3-hydroxyl-metabolites of tibolone also counteracted estradiol-driven proliferation. Inhibition of proliferation by 3β-OH-tibolone was abrogated by low doses of the progesterone receptor antagonist mifepristone, This suggests that 3β-OH-tibolone is converted to a progestagenic metabolite. We found that the stromal cells used in the co-cultures express high levels of the ketosteroid dehydrogenase, AKR1C2, which is able to oxidize 3β-OH-tibolone back to tibolone. Thus the unexpected progestagenic effect of 3β-OH-tibolone in these co-cultures may be due to metabolic activity present in the stromal cells of the co-cultures
