Glucocorticoid binding to solubilized components of human placental trophoblast membranes

Effect of estradiol and progesterone on human chorionic gonadotropin secretion in vitro

Many of the substances known to control the secretion of pituitary gonadotropins also modulate the secretion of human chorionic gonadotropin (hCG) by the placenta. In order to study the effect of estrogens and progestins on hCG secretion, term placental explants were cultured in culture media for 144 hours. During the culture period, hCG secretion increased after 48 hours, and a fortyfold increase was observed after 144 hours (p less than 0.001). Compared to concentrations of hCG in control cultures, secretion of hCG was markedly suppressed in the presence of progesterone 2.25 X 10(-5)M (p less than 0.001), a concentration similar to that found in term placental tissue (1.7 +/- 0.2 micrograms/gm of tissue). Suppression of hCG by progesterone occurred in a dose-response manner (r = -0.9100, p less than 0.01). Estradiol, an important steroid modulator of pituitary gonadotropins, did not significantly suppress the secretion of hCG, except in pharmacologic concentrations (10(-4)M), and physiologic concentrations of estradiol had no effect on the suppression of hCG by progesterone. These results suggest that the mechanism by which progesterone suppresses the secretion of hCG differs from the manner in which steroids modulate the secretion of pituitary gonadotropins.

The measurement of glucocorticoid receptors in human placental cytosol

Cytosol preparations of human term placenta in Tris-HCl buffer containing dithiothreitol were incubated with [3H]dexamethasone at 4 degrees C. Under these conditions the specific binding of [3H]dexamethasone was low when compared with cytosol preparations of sheep fetal lung or ovine placenta. The inclusion of sodium molybdate in the homogenization buffer and the removal of endogenous steroids by treating the cytosol with charcoal before incubation led to a faster and increased uptake of [3H]dexamethasone by human placenta. Scatchard plot analysis of the data showed that the placenta possesses a single class of high-affinity (Kd = 13 +/- 2 nM) low-capacity (193 +/- 34 fmol/mg protein) binding sites for [3H]dexamethasone. The binding of [3H]dexamethasone was reversible and was not due to plasma contamination. The specificity of the binding sites was confirmed by competition experiments in which the highest displacement of [3H]dexamethasone from the binding sites was caused by natural and synthetic corticosteroids. These findings indicate that the human placenta at term contains specific glucocorticoid receptors in high concentrations.

Binding of progestins to the glucocorticoid receptor. Correlation to their glucocorticoid-like effects on in vitro functions of human mononuclear leukocytes

A number of physiological and synthetic progestins were tested for their ability to compete with [3H]dexamethasone for the binding to the glucocorticoid receptor of human mononuclear leukocytes and their ability to elicit glucocorticoid-like effects on the same cells. As compared to the reference compound dexamethasone (relative receptor binding affinity defined as 100%), two potent synthetic progestins with a pregnane-type structure, megestrol acetate and medroxyprogesterone acetate, were found to display a considerable binding affinity towards the receptor (46 and 42%, respectively). The relative binding affinity of the naturally occurring ligand, cortisol, to the receptor was clearly lower (25%). The effective binding of medroxyprogesterone acetate to the glucocorticoid receptor was confirmed by direct binding studies utilizing a tritiated derivative of this steroid. No evidence for the existence of a specific progesterone receptor in human mononuclear leukocytes was obtained as judged by the results of competition experiments where a progesterone receptor-specific ligand [3H]Org 2058 was used. Medroxyprogesterone acetate and megestrol acetate also induced glucocorticoid-like effects on the lymphocyte functions. These included inhibition of the proliferative responses to the T-cell mitogens concanavalin A and phytohaemagglutinin and an enhanced accumulation of immunoglobulin secreting cells in pokeweed mitogen-stimulated cultures. The progestin effect appears to be mediated through a radiosensitive (suppressor) subpopulation of T lymphocytes. In contrast, the synthetic progestins related structurally to 19-nortestosterone, norethisterone and d-norgestrel, were virtually devoid of binding affinity towards the glucocorticoid receptor nor did they measurably influence the in vitro lymphocyte functions. These studies demonstrate that certain progestins in common clinical use probably possess inherent glucocorticoid activity and suggest that side effects attributable to this character (e.g. suppression of the pituitary-adrenal axis) might be expected when these compounds are used in pharmacological doses.

Stimulation of human chorionic gonadotropin secretion by glucocorticoids

The effects of glucocorticoids on hormone secretion by human placenta in organ culture were studied. The addition of cortisol resulted in a fourfold increase in human chorionic gonadotropin (hCG) secretion over that in untreated cultures after 144 hours' incubation (P less than 0.05), and a twofold increase in hCG was observed in the presence of cortisone (P less than 0.01). Dexamethasone stimulated hCG secretion in a dose-response manner (r = 0.9542; P less than 0.01). Progesterone, which suppresses hCG under these conditions, decreased the cortisol-enhanced secretion of hCG (r = -0.9794; P less than 0.01). No change in the secretion of human chorionic somatomammotropin was observed, but glucocorticoids increased heat-stable alkaline phosphatase activity (P less than 0.001). The physiologic significance of glucocorticoid effects on placental hormone synthesis is discussed.