Progestin receptors in human tissues: Concentrations and binding kinetics

Abstract

Progestin receptor level was measured from myometrial cytosol samples obtained from normally menstruating women (group I), postmenopausal women (group II) and postmenopausal women after 1 week's treatment with estrogens (group III). The following receptor levels were found: Group I, 63–722 fmol/mg cytosol protein; group II, below 100 fmol/mg, and group III, 350–3600 fmol/mg. Estrogen treatment of postmenopausal women preferentially promoted formation of the 7S progestin receptor in the myometrial cytosol. By contrast, the binding of [ 3H]-progesterone occurred only to the 4S component in the myometrial cytosol samples from normally menstruating women. During the menstrual cycle, progestin receptor level in the myometrial cytosol correlated directly to the cytosol estradiol-17β content and inversely to progesterone concentration in both the myometrial cytosol and serum. Interaction of the partially purified myometrial progesterone receptor at +4°C with the following [ 3H]-labeled progestins was studied: Progesterone (P), norethisterone (NET), 16α-ethyl-21-hydroxy-19-nor-4-pregnene-3, 20-dione (ORG 2058), 17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione (R 5020) and 13-ethyl-11-methylene-18,19-dinor-17α-4-pregnen-20-yn-17-ol (ORG 2969). The rate of association of ORG 2969, lacking the 3-keto group, was much slower ( k a = 0.23 × 10 5 1 × mol −1 × s −1) than that of other progestins ( k a range 1.9–4.6 × 10 5 1 × mol −1 × s −1). The rate of dissociation was fastest for P ( k d = 13.0 × 10 −5 s −1, t 1 2 90 min) followed by NET and ORG 2969 ( t 1 2 205 and 235 slower rates of dissociation were recorded for R 5020 and ORG 2058 ( t 1 2 620 and 1000 min). The apparent intrinsic dissociation constants ( K D) calculated from the kinetic parameters were: P 0.56, NET 0.3, ORG 2058 0.04, R 5020 0.04 and ORG 2969 2.5 nmol × 1 −1. These values were about 5–15 times lower than those obtained in the equilibrium binding studies. In the myometrial cytosol of estrogen-treated women, the potent progestins were bound to both 4S and 7S components, although they preferred association with the latter. The low rate of dissociation of ORG 2058 and minimal binding to non-receptor sites render it very suitable for progestin receptor measurements also from non-uterine tissues. This is exemplified by results obtained in assays of progestin receptors from human breast cancer tissue.