Tamoxifen decreases the estradiol induced progesterone receptors by interfering with nuclear estrogen receptor accumulation.

The retention time of the estrogen receptor in the nucleus of target cells after antiestrogen treatment has been shown to be longer than after estradiol. This paper describes the accumulation of nuclear estrogen receptors and the obtention of estrogenic responses (i.e. synthesis of cytosolic progesterone receptors and DNA) in the rat uterus after tamoxifen treatment in the presence or absence of estradiol. One-week ovariectomized adult rats were implanted with a silicone elastomer capsule containing corn oil or 25 micrograms estradiol/capsule (0 h). 48 h after implantation rats were injected with corn oil or 2 mg tamoxifen/kg and decapitated at 72, 96 or 120 h after implantation. In parallel experiments the implants were removed just before the injections of tamoxifen or oil. Tamoxifen injected into rats implanted with oil increased both the occupied nuclear receptors and the progesterone receptors at 96 h. In rats implanted with estradiol, tamoxifen did not increase the occupied nuclear receptors and decreased the levels of progesterone receptor and DNA at 96 h. In rats whose estradiol implants were removed at 48 h tamoxifen did not change the level of occupied nuclear receptors at 72 h but it increased them abruptly at 96 and 120 h. In these rats progesterone receptors decreased at 72 h but they increased at 96 and 120 h, and DNA decreased at 120 h to a lower level than before implantation. The results suggest that when estradiol is acting, tamoxifen is not able to increase the level of occupied estrogen receptor and it acts as an antiestrogen by decreasing the high level of progesterone receptors previously induced by estradiol. When estradiol is not acting tamoxifen behaves as a partial estrogen agonist by inducing progesterone receptors. However, the antiestrogenic action of tamoxifen on the rat uterus DNA does not seem to be affected by estradiol.