Developmental and Glucocorticoid Regulation of Surfactant Protein mRNAs in Fetal Sheep † 310

Abstract

We previously found that glucocorticoid treatment of pregnant sheep increases the content of surfactant proteins SP-A and SP-B in lung tissue and lavage of preterm lambs. To investigate the mechanism of this induction process, we quantitated SP mRNA levels after administration of betamethasone(0.5 mg/kg) to pregnant sheep prior to premature delivery of the fetus at 125 days. In the first protocol, 55 sheep were injected weekly with 1-4 doses of either saline (control) or betamethasone beginning at 104 days gestation with the last dose given 24 h prior to delivery. In a second protocol, 39 sheep were injected with 1-2 doses of saline or betamethasone at 24 and 48 h prior to delivery. Total RNA was extracted from fetal lung and hybridized with cDNAs for sheep SP-A, SP-B and SP-C and human β-actin. mRNA levels for control preterm lambs were 21%, 28% and 39% of the level in term lambs for SP-A, -B and -C, respectively. No increases in mRNA levels were demonstrated in sheep given 1-3 weekly doses of betamethasone vs. control. However, 4 doses of betamethasone, as well as both 48h treatment regimens, produced a 2- to 3-fold increase in each SP mRNA (p<0.01 by ANOVA), achieving levels equivalent to 60-75% of term control lambs. The magnitude of the betamethasone-induced increase in SP mRNA is similar to that for tissue SP-A and SP-B after 2-4 betamethasone doses. The observation that SP-A and SP-B proteins are increased 2 wk after betamethasone treatment, whereas mRNAs are not, implies that induction occurs with 2 doses, is reversible, and that the proteins have a longer half-life than the mRNAs. We conclude that in vivo betamethasone treatment rapidly induces a maximal and coordinated increase in SP-A, SP-B, and SP-C mRNAs which is fully reversible within 7 days. Similar increases in SP mRNA and protein content confirm that glucocorticoid regulation of SP genes in vivo is largely transcriptional.