Glucocorticoids increase fatty-acid synthase mRNA stability in fetal rat lung

Fatty-acid synthase (FAS) is a critical enzyme in surfactant biosynthesis. In fetal lung, glucocorticoids increase synthesis of phosphatidylcholine, the principal lipid component of surfactant, and there is evidence that this effect is mediated by increased expression of the FAS gene. Dexamethasone increases FAS activity, mass, mRNA content, and rate of transcription in cultured explants of fetal rat lung. As previous experiments with actinomycin D suggested that dexamethasone may also increase FAS mRNA content by a posttranscriptional mechanism, we examined the effect of the hormone on FAS mRNA stability. Explants of 19-day fetal rat lungs were cultured for 44 h with and without 100 nM dexamethasone. Some explants were harvested at that point, and others were cultured further with 60 mu M 5,6-dichlororibofuranosylbenzimidazole (DRB), an inhibitor of transcription. RNA was then extracted, and FAS mRNA levels were measured by Northern analysis. mRNA stability was assessed by comparing the amount remaining after culture with DRB with the initial level before addition of the inhibitor. The apparent half-life of FAS mRNA was 4 h in control explants cultured without hormone. FAS mRNA stability was increased 84% in the explants cultured with dexamethasone for 44 h and by 40% in those cultured with the hormone for 5 h. We conclude that glucocorticoids enhance expression of the FAS gene in fetal lung by increasing mRNA stability in addition to stimulating transcription.