Glucocorticoid Receptors and the Role of Glucocorticoids in Fetal Lung Development

Abstract

The cellular mechanism of glucocorticoid effects upon fetal lung was examined in studies of specific binding'activity for corticosteroids. Cytoplasm of fetal rabbit lung contains receptor sites for [3Hjdexamethasone at a concentration of 0.43 + 0.04 'pmol/mg of cytosol protein, and the apparent dissociation constant for the binding reaction is 2.7 4± 0.4 nM. The ability of various steroids to compete with labeled dexamethasone for binding to receptor correlates with their biologic potency. The hormone-receptor complex formed in vitro at 00 binds with high affinity at 200 to isolated lung nuclei. It is estimated that there are 9500 nuclear binding sites and 12,000 cytoplasmic receptor sites per fetal lung cell.During the last 12 days of gestation in a rabbit, the concentration of cytoplasmic receptor in lung is constant and is 2-to 5-times greater than receptor-site concentration in fetal skin, kidney, heart, muscle, gut, liver, brain, thymus, and placenta. These findings demonstrate that the early steps in the mechanism of glucocorticoid action in target tissues are present in lung cells, and suggest that these hormones accelerate fetal lung differentiation and surfactant production in animals by the induction of new protein synthesis mediated by receptor.Adrenal glucocorticoids induce both morphological and enzymic changes in various target tissues. In fetal rabbit and lamb, these steroid hormones cause accelerated lung development and precocious appearance of pulmonary surfactant in whole-lung homogenates and lung washes (1)(2)(3) into cold isotonic phosphate-buffered saline (pH 7.6) and frozen at -20°for 0-2 days. Receptor activity was assayed in a cytosol fraction prepared at 20 by mincing tissue in one volume of medium 1 and then homogenizing with 6 strokes of a motor-driven (3000 rpm) Teflon pestle in a Duall Tissue Grinder (Kontes Glass Co.). The homogenate was centrifuged at 600 X g for 15 min, then at 100,000 X g for 60 min. Aliquots of the supernatant were added immediately to reaction mixtures containing [3H]dexamethasone at concentrations from 1 to 100 nM. Tubes containing in addition an excess of unlabeled dexamethasone (10 MM) were run in parallel for each [3H]dexamethasone concentration to determine "background" binding activity (6). Binding was complete by 90-120 min at 00, and was proportional to cytosol protein concentration between 2 and 20 mg/ml. Macromolecular bound radioactivity was determined by a charcoal assay (6) that utilizes the property of activated charcoal to absorb free, but not bound, steroid. Results are plotted as P3H]dexamethasone specifically bound (total radioactivity excluded from charcoal minus "background" counts) against free dexamethasone (total dexamethasone added minus the amount bound).Fetal lung nuclei were prepared from the 600 X g pellet by washing in medium 1 twice and once in medium 1 containing 0.25 M sucrose. Pellets of nuclei were combined with aliquots of cytosol previously incubated at 00 with a saturating concentration of PH]dexamethasone, with or ...

The effect of prolactin on the lecithin content of fetal rabbit lung.

“…Stress causes a marked rise in serum prolactin levels (16), as well as in glucocorticoid levels (17). The presence of glucocorticoid receptors in fetal lungs has been cited as evidence of a physiological role for steroids in surfactant maturation (18). However, in the human lung these receptors are present from early fetal life, throughout gestation, and even after birth (19).…”

Glucocorticoid levels in maternal and cord serum after prenatal betamethasone therapy to prevent respiratory distress syndrome.

“…Also, tlher-e is little iinformiiationl conicerninig the safety of prenat,al corticosteroid administration. It does appear that the lunlg is a glucocorticoid target tissue sinice both animal (9,10) and lhumiani ('11) fetal luIg contain specific receptors for glucocorticoids. These bindin-proteins are saturated by physiolog,ic levels of corticosteroidls, suggesting that the (lesire(d nmaturational effects slhould he accomplished without the use of pharmacologic doses of steroid.…”

Studies on the nuclear binding of steroid hormone receptor complex; Characteristics of binding of nuclei from fetal rat liver to 3H-dexamethasone-liver cytoplasmic receptor complex.

“…December1979 affinity and unsaturable nuclear binding of hormone-receptor complexes have been reported (Chamnes et al, 1974;Gannon and Gorski, 1976;, but there have been other reports that the interaction between steroid-receptor complexes and nuclei is of high-affinity and saturable (Ballard, 1972;Kalimi et al, 1973;Lippman and Thompson, 1974;Spelsberg et al, 1976). Recently, Simons et al,(1976) found a cytosol factor which inhibits the translocation of hormonereceptor complexes to nuclei, and they claimed that the underestimations of nuclear binding reported by many authors might be attributable to the neglect of this translocation inhibitor.…”