Mechanism of antiprogestational action of synthetic steroids

This study was designed to investigate the mechanism of antiprogestational action of synthetic steroids, danazol (17α-hydroxy-pregn-4-en-20-yno [2,3-d] isoxazol-17-ol) and R2323 (13β-ethyl-17α-hydroxy-18,19-dinor-17α-pregna-4,9,11-trien-20-yn-3-one). Oestrogen primed rabbit uteri were used for this study. The sucrose gradient analysis in the supernatant of 248 000 x g (max.) of the uterine homogenate showed that progesterone8S binding was inhibited by each of the synthetic steroid, and the kinetic study revealed that each steroid was a competitive inhibitor, where Kd of progesterone-protein binding was 8.1 × 10−10 m, Ki of danazol-protein binding was 2.7 × 10−8 m and Ki of R2323-protein binding was 6.6 × 10−9m. These results indicate that these synthetic steroids bind to progesterone receptor with moderate binding affinities. The modified Anderson's exchange assay of nuclear receptor demontrated that danazol- or R2323-receptor complex can enter into the uterine nuclei in a lesser degree than does progesterone-receptor complex. RNA synthesis by the uterine chromatin was stimulated if the progesterone-receptor complex was added, but this stimulation was inhibited by addition of danazol or R2323. The chromatin RNA transcription was not induced by danazol- or R2323-receptor complex.

These results suggest that such a synthetic steroid binds to a progesterone receptor, and the steroid-receptor complex enters into the nucleus but does not activate the chromatin template, thus resulting in the antiprogesational effect.

Human breast cancer: androgen action mediated by estrogen receptor

Growth of the human breast cancer cell line MCF-7 is enhanced by androgens, but only at pharmacological concentrations. Although physiological concentrations of androgens translocate the androgen receptor into the nucleus, no mitogenic effects are observed. By contrast, pharmacological androgens translocate not only the androgen receptor but also the estrogen receptor, and at these high doses significantly increase both DNA and estrogen-dependent protein synthesis. We therefore propose that androgens stimulate MCF-7 cell growth not through the androgen receptor but rather through the estrogen receptor.

Progesterone receptor in the rat anterior pituitary: effect of estrogen priming and adrenalectomy

The present study was done to determine if a progesterone receptor is present in rat pituitary. Cytosol was labeled with 3H-progesterone (3HP) or 3H-R5020 (3HR) and subjected to sucrose-glycerol density-gradient centrifugation. Serum progesterone was measured for correlation with progesterone receptor levels. Two 3HP-binding peaks (4S + 6S) were evident in uterine and pituitary cytosols. The 4S peak was eliminated by competition with unlabeled cortisol leaving a single 6S peak (progesterone receptor). Estradiol (E) priming of the male or female rat increased progesterone receptor levels in pituitary cytosol as demonstrated using 3HP and 3HR, and pituitary progesterone receptor bound 3HR with a higher affinity than 3HP. Following adrenalectomy of gonadectomized rats, progesterone receptor levels were increased in pituitary and uterine cytosol of both E-primed and unprimed groups. An inverse relationship was established between serum progesterone and progesterone receptor levels in the uterus and pituitary suggesting that stress-induced adrenal progesterone secretion significantly influences progesterone receptor levels in the rat. These results demonstrate an estrogen-inducible progesterone receptor in the rat pituitary with properties similar to those of the uterine progesterone receptor.

Properties of the cytoplasmic progestin-binding protein in the rabbit uterus

An exchange assay for the measurement of total cytoplasmic progestin binding sites has been developed on rabbit uterine cytosol using the highly potent progestin, R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione) labelled to a high specific activity. This compound has several advantages over progesterone: it is not bound by plasma corticosteroid binding globulin; it has high affinity for the progestin receptor; it binds virtually as fast as progesterone to the receptor, but the complex formed dissociated 8 times slower; its binding is not displaced by more than 2% by compounds devoid of progestational activity (estrogens, testosterone, dexamethasone, aldosterone). Bound endogenous progesterone was exchanged by tritiated R 5020 in a time compatible with receptor stability. At 0 C, total exchange of filled sites occurred in less than 4 h; at this temperature the R 5020-receptor complex was stable for at least 28 h. The conformation of the R 5020-receptor complex was investigated in sucrose density gradients under various experimental conditions. Unlike progesterone, it was possible to detect a 7S peak in uterine cytosol obtained from rabbits injected with a tracer dose of [3H]R 5020 1 h prior to sacrifice.

[Estrogen and progestogen cytosol receptors in human breast carcinoma (author's transl)]

The authors have studied Estrogen (RE) and Progestin (RP) cytosol receptors in 379 human breast carcinomas: 281 tumors suitable for surgery, 26 pseudo-inflammatory tumors, 52 metastases and 20 recurrences. They have used an exchange technique with estradiol for RE and a synthetic compound, R 5020, for RP. The results point out that high rates of RE correlate with post-menopausal women and high rates of RP with pre-menopausal women. Tumours are considered receptor-positive when the binding sites concentration exceeds 100 fmoles/g tissue. Using this as a base 32% of the tumors are RE and RP negative. Considering only the positive tumors, 54% contain both receptors, 31% only RE and 15% only RP. The percentages are also given in terms of the hormonal state of the patients. The results are discussed in terms of McGuire's hypothesis that both receptors are necessary to obtain a positive response to hormonal therapy. Correlation between histology and receptors is also discussed.

Contribution of functional groups of 19-nor-progestogens to binding to progesterone and estradiol-17beta receptors in rabbit uterus

The structural elements of 19-norprogestogens which may be essential for binding to progesterone and estradiol-17beta(E2) receptors were investigated in the rabbit uterine cytosol. The kinetic study showed that 19-nor-progestogens are competitive inhibitors of progesterone-receptor (8S) binding and E2-receptor binding. The affinities of steroids for the progesterone receptor were as follows: norethindrone (Ki of 2.3 X 10(-9)M) greater than 5alpha-dihydronorethindrone greater than norethindrone acetate greater than lynestrenol greater than 17alpha-ethynyl-estra-4-ene-3beta, 17beta-diol greater than ethynodiol diacetate (Ki of 1.3 X 10(-7) M). The affinities of steroids for the E2 receptor were as follows: ethynodiol diacetate (Ki of 1.3 X 10(-7)M) greater than 17alpha-ethynyl-estra-4-ene-3beta, 17beta-diol greater than norethindrone acetate greater than norethindrone greater than 5alpha-dihydronorethindrone greater than lynestrenol (Ki of 8.4 X 10(-7)M). The results indicate that 3-ketone and 17beta-hydroxyl groups, and the plane of ring A/B of 19-norprogestogen are important for binding to the progesterone receptor. The affinities of 19-nor-progestogens for the E2 receptor were very weak. Their affinities for the E2 receptor increased with addition of acetate or hydroxyl groups at the 3beta and 17beta positions, and were decreased by the elimination of a 3 oxygen function or the reduction of ring A.

Antagonism of development and growth of 7,12-dimethylbenz(a)anthracene-induced rat mammary tumors by the antiestrogen U 23,469 and effects on estrogen and progesterone receptors

This study analyzed the effectiveness of a nonsteroidal antiestrogen, cis-(3-[p-(1,2,3,4-tetrahydro-6-methoxy-2-phenyl-1-naphthyl)phenoxy]-1,2-propanediol (U 23,469) previously shown to be potent in antagonizing estrogen-induced uterine growth, in preventing the development of 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumors and in eliciting the regression of established tumors; the study also attempts to elucidate the mechanisms of the tumor antagonism of U 23,469. Virgin female Sprague-Dawley rats that receive DMBA at 47 to 50 days of age and then receive U 23,469 (250 micrograms s.c. in 0.15 M NaCl daily) have a greatly reduced number of mammary tumors and a markedly decreased tumor area. Treatment with U 23,469 for increasing time periods (3, 6, or 12 weeks) beginning 2 weeks after DMBA results in a progressive delay in onset of tumor appearance. U 23,469 treatment beginning 1 week after DMBA or given prior to DMBA is even more effective. The time course of tumor regression (3 months after DMBA) by U 23,469 or ovariectomy is similar, with 50% regression in ca. 2 weeks, and both elicit regression of almost all tumors (greater than 90%). After ovariectomy, cytosol progesterone receptor levels are greatly diminished in tumors and uteri, while cytosol estrogen receptor (ER) levels are high; in both tissues little (ca. one-third) of ER is in the nucleus. During U 23,469 treatment, cytosol ER content is very low in regressing tumor and uterus and over 90% of ER is in the nucleus; cytosol progesterone receptor is slightly depressed in the uterus but is at the untreated level in mammary tumor. These receptor studies suggest that the effectiveness of this antiestrogen in antagonizing mammary tumor development and growth may reside in its ability markedly to perturb the distribution of ER, maintaining over 90% of ER in the nucleus with concomitant low levels of cytoplasmic ER, a situation that may render the mammary tissue insensitive to the animal's own endogenous estrogens.

Hormonal control of progesterone receptors

Uterine progesterone receptors are under dual hormonal control. Estrogen increases the concentration of receptor through a mechanism that depends on synthesis of both RNA and protein. Progesterone decreases the concentration of its own receptor, probably by enhancing its inactivation rate. This regulation explains receptor variations during the estrous cycle. In both guinea pig and rat uteri, cytosol receptor concentration is maximal at the preovulatory period and decreases after ovulation. Nuclear receptor was measured in the rat. Its concentration is also maximal at proestrus, but the higher nuclear to cytosol receptor ratio was observed at metestrus. There is a good correlation (r = 0.78) between nuclear receptor concentration, on one hand, and the product of cytosol receptor concentration times the plasma progesterone concentration, on the other hand. Autoradiographic studies show that receptor variations during the estrous cycle occur simultaneously in all cell types of uterine horn, cervix, and vagina, which suggests that similar mechanisms control receptor concentration in all of these cells. Progesterone receptor was also measured during pregnancy in rat uterus. Cytosol receptor concentration is low at the beginning of pregnancy (approximately 6000 binding sites per cell), declines slightly on Day 5 (approximately 4000 binding sites per cell), and then increases progressively during the remainder of pregnancy to attain its highest value on Day 22 (26,000 binding sites per cell). Nuclear receptor concentration is very low on Day 3 (1200 binding sites per cell), increases slightly on Day 5 (1900 binding sites per cell), decreases on Day 6, and then increases again to attain a plateau between Days 9 and 15 (approximately 2600 binding sites per cell). Thereafter, its concentration begins to decrease rapidly. On Day 22, the mean concentration is very low (700 binding sites per cell); in some animals (probably on the verge of parturition), no nuclear receptor can be detected.