Binding and biological responses of antiestrogens in the fetal and newborn uteri of guinea-pig

Abstract

The antiestrogen tamoxifen and several tamoxifen metabolites bind to distinct cytoplasmic binding sites in the fetal uterus of guinea-pig. The first one (site A) corresponds to the estrogen receptor and binds tamoxifen with a dissociation constant ( K d ) of 1.8 ± 0.4 nM. The dissociation rate constant ( k 1) of this site at 4°C is 8.3 ± 2 × 10 −4 s −1. The second binding site for tamoxifen (site B) appears to be specific for several triphenylethylene derivatives (nafoxidine and different tamoxifen metabolites). In contrast, natural and synthetic estrogens as well as cortisol. testosterone and progesterone do not compete for this second site. In addition, site B shows a higher affinity for tamoxifen ( K d = 0.39 ± 0.01 nM) and a k −1, at 4°C of 0.81 ± 0.14 × 10 −4 s −1. The tamoxifen-site A complex binds to uterine nuclei “ in vitro” by a temperature dependent process, but tamoxifen-site B complex does not. In other fetal organs which contain no estrogen receptors (heart) or very low levels (lung) the concentration of site B is found to be significantly lower than in the fetal uterus. Furthermore, in comparison to fetal uterus a progressive decrease is observed in the levels of both site B and estrogen receptor in neonatal, immature and adult uteri. The biological responses (uterotrophic effect and progesterone receptor stimulation) elicited by both tamoxifen and estradiol (E 2) in the fetus and 6-day-old newborns were also compared. Three daily injections of E 2 or tamoxifen to the pregnant animal, increase fetal uterine weight by 80%. The same treatment provokes a stimulation of uterine progesterone receptors up to 23.6 ± 4.7 pmol/mg DNA in E 2-treated animals, while after tamoxifen treatment progesterone receptors reach only 6.0 ± 1.8 pmol/mg DNA (2.4 ± 0.7 pmol/mg DNA in non-treated animals). Both tamoxifen and E 2 administered to newborns provoke a similar uterotrophic effect (160 and 165% increase, respectively) and a similar stimulation of uterine progesterone receptors (14.9 ± 2.5 and 15.8 ± 1.2 pmol/mg DNA, respectively) (5.6 ± 0.8 pmol/mg DNA in non-treated animals). These responses appear to be correlated with the nuclear translocation of estrogen receptors after both tamoxifen or E 2 administration.