Estetrol is a weak estrogen antagonizing estradiol-dependent mammary gland proliferation

Estetrol (E-4) is a natural estrogen produced exclusively by the human fetal liver during pregnancy. Its physiological activity remains unknown. In contrast to ethinyl estradiol and estradiol (E-2), E-4 has a minimal impact on liver cell activity and could provide a better safety profile in contraception or hormone therapy. The aim of this study was to delineate if E-4 exhibits an activity profile distinct from that of E-2 on mammary gland. Compared with E-2, E-4 acted as a low-affinity estrogen in both human in vitro and murine in vivo models. E-4 was 100 times less potent than E-2 to stimulate the proliferation of human breast epithelial (HBE) cells and murine mammary gland in vitro and in vivo respectively. This effect was prevented by fulvestrant and tamoxifen, supporting the notion that ER alpha (ESR1) is the main mediator of the estrogenic effect of E-4 on the breast. Interestingly, when E-4 was administered along with E-2, it significantly antagonized the strong stimulatory effect of E-2 on HBE cell proliferation and on the growth of mammary ducts. This study characterizes for the first time the impact of E-4 on mammary gland. Our results highlight that E-4 is less potent than E-2 and exhibits antagonistic properties toward the proliferative effect of E-2 on breast epithelial cells. These data support E-4 as a potential new estrogen for clinical use with a reduced impact on breast proliferation.