Glucocorticoids affect the synthesis of pulmonary fibroblast-pneumonocyte factor at a pretranslational level

Glucocorticoids accelerate fetal lung maturation by acting on the fetal lung fibroblast to induce the synthesis of fibroblast-pneumonocyte factor which in turn stimulates pulmonary surfactant synthesis by the alveolar type II cell. We have studied the site of glucocorticoid regulation of fibroblast-pneumonocyte factor synthesis in primary cultures of fetal rat lung fibroblasts. Conditioned media from fetal rat lung fibroblasts exposed to cortisol stimulate [Me-3H]choline incorporation into saturated phosphatidylcholine by primary cultures of fetal rat lung alveolar type II cells. This effect is blocked by the presence of actinomycin D during the first, but not the second, 24 h of incubation of the fibroblasts with cortisol. Cycloheximide blocks this effect if present during either the first or second 24 h of incubation. We fractionated mRNA from fetal rat lung fibroblasts incubated in the presence or absence of dexamethasone and observed that cell-free translation products from a fraction of approximately 500 bases possess biological activity in the bioassay. Such activity is only present in cell-free translation products of mRNA isolated from fibroblasts treated with dexamethasone. These results suggest that glucocorticoids act at a pretranslational level to induce production of fibroblast-pneumonocyte factor and that the primary translation products are biologically active.