Posttranscriptional regulation of surfactant protein-A messenger RNA in human fetal lung in vitro by glucocorticoids

Surfactant protein-A (SP-A), the major pulmonary surfactant-associated protein, is a developmentally and hormonally regulated sialoglycoprotein of about 35,000 mol wt. In previous studies we observed that dexamethasone has dose-dependent biphasic effects on the levels of SP-A and its mRNA in human fetal lung in vitro. At concentrations of 10^-10-10^-9 M, dexamethasone increased the levels of SP-A and its mRNA over those of control tissues, whereas at concentrations ≥ 10^-8 M, the steroid was markedly inhibitory. Our findings suggest that the inhibitory action of dexamethasone (>10^-8 M) on SP-A mRNA levels was mediated by an effect to reduce SP-A mRNA stability, since the steroid caused a dose-dependent increase in the rate of transcription; however, an effect to increase transcription with premature termination leading to instability of mRNA transcripts could not be ruled out. In the present investigation we have studied in detail the mechanisms underlying the biphasic effects of glucocorticoids on SP-A mRNA levels in human fetal lung tissues in vitro. Our findings indicate that dexamethasone (10_-7 M) has no adverse effect on the elongation of nascent mRNA transcripts throughout the SP-A gene; elongation of SP-A mRNA transcripts in dexamethasone-treated tissue expiants was similar to that observed in tissues incubated in control medium or medium containing (Bu)₂CAMP. Therefore, premature termination of SP-A mRNA transcription leading to the instability of SP-A mRNA can be ruled out. On the other hand, we found that dexamethasone (10^-7 M) had a pronounced effect to reduce the apparent half-life of SP-A mRNA; in control explants maintained in the presence of actinomycin-D to block gene transcription, the SP-A mRNA half-life was estimated to be 11.4 h, whereas in tissues also treated with dexamethasone, the SP-A mRNA half-life was reduced by more than 60% to 5.0 h. Dexamethasone also was found to have dose-dependent effects on the degradation of SP-A mRNA. After 12 h of incubation in the presence of actinomycin-D and dexamethasone at 10^-9 and 10^-7 M, the levels of SP-A mRNA were reduced by 50% and 80%, respectively, compared to those in tissue incubated with actinomycin-D alone. The inhibitory effects of glucocorticoids on SP-A mRNA levels were completely reversible and were blocked by the glucocorticoid antagonist RU486. Based on these findings, we suggest that the biphasic effects of glucocorticoids on SP-A mRNA levels in human fetal lung in vitro are caused by the differential actions of glucocorticoids on SP-A mRNA transcription and stability.