Action of an estrogen antagonist in intact and ovariectomized rats

Differential antiestrogen action in the immature rat uterus: a comparison of hydroxylated antiestrogens with high affinity for the estrogen receptor

The non-steroidal antiestrogens LY117018 and monohydroxytamoxifen are partial agonists in the 3-day immature rat uterine weight test. Both compounds stimulated increases in uterine progesterone receptor concentration (as determined by DCC assay and SDG analysis) and luminal epithelial cell height. However, LY117018 was much less estrogenic than monohydroxytamoxifen. The study also showed that the antiestrogenic effects of LY117018 and monohydroxytamoxifen could be reversed in vivo by increasing doses of estradiol. The partial uterotrophic effect of monohydroxytamoxifen and full uterotrophic effects of estradiol were both inhibited by high doses of LY117018 at an approximate dosage ratio of 1:24, w/w. This result suggests a common mechanism for the action of estradiol and the hydroxylated antiestrogens. Since LY117018 reversed the binding of [3H]estradiol and estrogen-competable [3H]monohydroxytamoxifen binding in the rat uterus in vivo, the effects of nonsteroidal antiestrogens on gross uterine wet weight can be explained by competitive interaction with estradiol via the estrogen receptor mechanism. However, the weakly estrogenic antiestrogen LY117018 was unable to inhibit estrogen-stimulated rises in luminal epithelium cell height and progesterone receptor levels. These data suggest a differential interaction by LY117018 in the rat uterus so that only selected estrogen stimulated effects i.e.: uterine wet weight, are antagonized by this particular "antiestrogenic" dose of the drug.

The binding of [3H]-oestradiol-17 beta in the immature rat uterus during the sequential administration of non-steroidal anti-oestrogens

1 The binding of [(3)H]-oestradiol-17beta (0.08 mug) in the uterus, vagina, liver and heart of immature female rats has been studied in vivo and the effect of daily administrations of the non-steroidal anti-oestrogens, tamoxifen and monohydroxytamoxifen, on the 2 h accumulation of [(3)H]-oestradiol-17beta in the uterus has been determined.2 Doses of tamoxifen (8 mug daily) or monohydroxytamoxifen (1.28 mug daily), which have previously been found to antagonize completely the uterotrophic activity of oestradiol-17beta (0.08 mug daily), did not significantly reduce the total uterine binding of 0.08 mug [(3)H]-oestradiol-17beta administered on day 4 of a 3-day uterine weight test.3 The simultaneous administration of tamoxifen (8 mug) or monohydroxytamoxifen (1.28 mug) with [(3)H]-oestradiol-17beta (0.08 mug) on day 3 of a uterine weight test did not significantly reduce the total uterine binding of oestradiol-17beta. The binding of [(3)H]-oestradiol-17beta was reduced if monohydroxytamoxifen or tamoxifen was administered 4 h before the oestradiol.4 Tamoxifen (8 mug daily) or monohydroxytamoxifen (1.28 mug daily) did not prevent the translocation of [(3)H]-oestradiol (0.08 mug) to the uterine cell nucleus on day 3 of a 3-day uterine weight test.5 The measurement of total nuclear oestrogen receptors by an exchange assay technique demonstrated a higher concentration of oestrogen receptors in anti-oestrogen-treated animals compared with controls.6 Since the administration of anti-oestrogenic doses of non-steroidal anti-oestrogens during a 3-day uterine weight test did not inhibit the total binding of oestradiol in the uterus, or affect the translocation of the steroid to the nucleus, the mechanism of action of non-steroidal anti-oestrogens over the range of the partial agonist dose-response curve must involve an interaction, or competition of oestradiol-17beta- and anti-oestrogen-oestrogen receptor complexes for sites within the nucleus.

Differential depletion of cytoplasmic high affinity oestrogen receptors after the in vivo administration of the antioestrogens, clomiphene, MER-25 and tamoxifen

1 The in vivo actions of the oestrogen antagonists, MER-25 and tamoxifen upon the cytosol oestrogen receptors prepared from amygdala, hypothalamus, pituitary and uterus of rats were studied 24 h after drug administration. 2 There was a dose-related depletion of cytosol oestrogen receptors. However, the uterine and pituitary receptors were consistently affected at a lower dose than were those from the brain. 3 The ratios of the combined central ED50 to the combined peripheral ED50 were clomiphene 169 greater than MER-25 19.2 greater than tamoxifen 2.13. 4 The receptor changes were not related to biological activity monitored by serum luteinizing hormone levels and uterotrophic response. 5 The possible role of these drug effects in the induction of ovulation and future developments are discussed.

Nonsteroidal antiestrogens: their biological effects and potential mechanisms of action

The uterotropic and antiuterotrapic effects of a variety of structural derivatives of the nonsteroidal antiestrogen tamoxifen have been determined in the rat and the mouse. One derivative, monohydroxytamoxifen, was found to be a potent antiestrogen in the rat, with a high affinity for the estrogen receptor. Various techniques of sucrose density gradient analysis were used to demonstrate that estradiol and tamoxifen bind to the rat uterine cytoplasmic estrogen receptor. Estrogens and antiestrogens provoke the translocation of estrogen receptors to the nucleus and deplete the cytoplasmic estrogen receptor pool for short or long periods depending on the dose administered. Estradiol stimulates endometrial hyperplasia with an increase in total uterine DNA content, whereas tamoxifen stimulates endometrial hypertrophy with only a slight increase in uterine DNA content. It is concluded that the molecular shape of the ligand that binds to the estrogen receptor determines antiestrogenic activity.

Binding of [3-H]estradiol by brain cell nuclei and female rat sexual behavior: inhibition by antiestrogens

The antiestrogens MER-25, CI-628, and nafoxidine inhibit the uptake of [3H]-estradiol in whole homogenates and isolated cell nuclei tissues and the pituitary, and inhibit estradiol-induced female sexual behavior. The antiestrogens were injected intraperitoneally 2 h prior to an intravenous injection of [3H]estradiol, and the animals were killed 2 h after the estradiol. CI-628 reduces radioactivity in whole homogenates and isolated cell nuclei of cerebral cortex, hypothalamus, preoptic area -septum and pituitary. Nafoxidine reduces uptake in cell nuclei of the hypothalamus, preoptic area-septum and pituitary. In this paradigm, MER-25 inhibited uptake only in the pituitary. In the analogous behavioral experiments, with antiestrogens injected 2 h prior to an intravenous injection of unesterified estradiol, CI-628 and nafoxidine totally inhibited lordosis responding. MER-25 shows no inhibition of behavior in this paradigm. However, when MER-25 is injected 12 h prior to the estradiol, it inhibits retention of [3H]estradiol at 2 h in brain and pituitary cell nuclei, and lordosis responding is also inhibited. Additionally, the antiestrogens can apparently displace previously bound [3H]estrdiol. When the antiestrogens are injected 2 h prior to an injection of [3H]estradiol, MER-25, CI-628 and nafoxidine all show greater inhibition of nuclear estradiol retention at 12 h after the [3H]estradiol injection than 2 h. Analogously, when CI-628 is injected 2 h after an intravenous injection of [3H]estradiol, it displaces most of the radioactivity present in hypothalamic-preoptic area nuclei at 12 h after the estradiol injection. These results indicate that antiestrogens can prevent or reverse the nuclear concentration of estradiol in brain cells and are consistent with a role of the cell nucleus in the induction of estrous behavior by estradiol.

Regional and subcellular [3h]estradiol localization in selected brain regions and pituitary of female mice: effects of unlabeled estradiol and various anti-hormones

Groups of ovariectomized mice were pretreated for 30 min with either vehicle, unlabeled estradiol benzoate or one of the following anti-hormones: cyproterone acetate, CI-628 or CN-69, 725-27. One hour after an I.V. injection of [3H]estradiol the levels of toluence extractable radioactivity retained in the crude nuclear and cytosol fractions of selected brain regions, pituitary and plasma were determined by liquid scintillation spectrometry. The major findings were: (1) In vehicle treated animals [3H]estradiol uptake in the pituitary greatly exceeded that in all brain regions. Within the brain uptake was greatest in the preoptic anterior hypothalamus (POA-AH) and medial basal hypothalamus (MBH) samples. Nuclear uptake exceeded cytosol uptake only in the pituitary and POA-AH and MBH samples. (2) Estradiol benzoate pretreatment greatly reduced nuclear, and to a lesser extent cytosol [3H]estradiol uptake in pituitary, POA-AH and MBH samples. Estradiol benzoate pretreatment also reduced nuclear uptake in the dorsal middle hypothalamus (DMH). (3) The anti-androgen cyproterone acetate, previously shown to have anti-estrogenic effects in female sexual behavior tests, was found to have no effect on nuclear or cytosol [3H]estradiol uptake in brain and pituitary. (4) The anti-estrogen CI-628 was found to reduce nuclear [3H]estradiol uptake in the POA-AH and pituitary samples. It also reduced cytosol [3H]estradiol uptake in the pituitary sample. The anti-estrogen CN-69, 725-27 produced a much greater inhibition of nuclear [3H]estradiol uptake than CI-628 in the POA-AH, MBH, DMH, dorsal posterior hypothalamus and pituitary samples. This anti-estrogen also reduced cytosol [3H]estradiol uptake in the POA-AH, MBH and pituitary samples. (5) A preliminary chromatographic analysis of vehicle control samples indicated that 70-98% of the nuclear radioactivity in target regions such as POA-AH, MBH and pituitary was iso-polar with authentic estradiol, while less than 50% of the radioactivity in plasma and cortex behaved as estradiol.