Corticosteroid Stimulation of Phosphatidylcholine Synthesis in Cultured Fetal Rabbit Lung: Evidence for de Novo Protein Synthesis Mediated by Glucocorticoid Receptors*

Abstract

To investigate further the mechanism whereby glucocorticoids accelerate the maturation of the pulmonary surfactant system, we studied both binding of glucocorticoids and their effect on phosphatidylcholine synthesis in organ cultures of fetal rabbit lung grown in serum-free medium. The greatest effect of dexamethasone (100 nM for 48 h) occurred at 24 days gestation when there was a 103% increase in the rate of choline incorporation into phosphatidylcholine and a 24% increase in the tissue content of disaturated phosphatidylcholine. Stimulation by corticosteroid was first observed after 12 h of exposure. Choline incorporation increased in a linear fashion for 36 h and then began to plateau; removal of the steroid after 24 h prevented any further increase in stimulation. The presence of other hormones in the culture medium was not a prerequisite for the corticosteroid action. Fetal sex had no influence on dexamethasone-induced phosphatidylcholine synthesis or on nuclear binding of dexamethasone. There was a striking similarity between the K_d values for specific nuclear binding of dexamethasone and cortisol (0.6 ± 0.1 and 7.3 ± 0.1 nM, respectively) and the concentrations for half-maximal stimulation of phosphatidylcholine synthesis (0.7 ± 0.1 and 6.8 ± 0.5 nM). The relative potencies of a number of steroids (100 nM) for both nuclear binding and stimulation of choline incorporation were the same: dexamethasone > cortisol > cortisone > corticosterone > dehydrocorticosterone, with no effect by progesterone, testosterone, or estradiol at this dose. Actinomycin D and cycloheximide blocked dexamethasone-induced phosphatidylcholine synthesis in a dose-dependent fashion. Actinomycin D had a marked effect if added at the initiation of hormone exposure, but little effect when added after 24 h, whereas cycloheximide was primarily effective between 24–48 h. These findings suggest that glucocorticoid stimulation of phosphatidylcholine synthesis in fetal lung is mediated by binding to specific receptors, with subsequent de novo synthesis of RNA and protein.