Xu Z X, Rooney S A
Department of Pediatrics, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
Am J Physiol. 1997 May;272(5 Pt 1):L860-4. doi: 10.1152/ajplung.1997.272.5.L860.
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 microM 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.
脂肪酸合酶（FAS）是表面活性剂生物合成中的一种关键酶。在胎儿肺中，糖皮质激素可增加磷脂酰胆碱（表面活性剂的主要脂质成分）的合成，并且有证据表明这种作用是由FAS基因表达增加介导的。地塞米松可增加胎鼠肺培养外植体中的FAS活性、含量、mRNA含量和转录速率。由于先前用放线菌素D进行的实验表明地塞米松也可能通过转录后机制增加FAS mRNA含量，因此我们研究了该激素对FAS mRNA稳定性的影响。将19天龄胎鼠肺的外植体在有和没有100 nM地塞米松的情况下培养44小时。一些外植体在此时收获，其他外植体则用转录抑制剂60μM 5,6-二氯呋喃核糖基苯并咪唑（DRB）进一步培养。然后提取RNA，并通过Northern分析测量FAS mRNA水平，通过比较用DRB培养后剩余的量与添加抑制剂前的初始水平来评估mRNA稳定性。在无激素培养的对照外植体中，FAS mRNA的表观半衰期为4小时。在用44小时地塞米松培养的外植体中，FAS mRNA稳定性增加了84%，在用5小时激素培养的外植体中增加了40%。我们得出结论，糖皮质激素除了刺激转录外，还通过增加mRNA稳定性来增强胎儿肺中FAS基因的表达。
