Corticosteroid receptor in type II pneumocytes of the rat

Glucocorticoids regulate expression of the fatty acid synthase gene in fetal rat type II cells

Fatty acids are integral components of pulmonary surfactant, a mixture of phospholipids and specific proteins that lines the alveolar surface and is essential for normal lung function. There are developmental increases in fatty acid biosynthesis and surfactant production in late-gestation fetal lung, and both processes are accelerated by glucocorticoids. Fatty acid synthase (FAS) is a key enzyme in de novo fatty acid biosynthesis, and increased FAS activity is responsible for the developmental and hormone-induced increases in fatty acid biosynthesis in fetal lung. Using cultured fetal lung explants, it has been reported that dexamethasone (Dex) increases FAS activity, protein content, mRNA content and rate of transcription. However, FAS expression has not been measured in isolated type II cells, the cellular source of surfactant within the lung. In the present study we measured parameters of FAS expression in type II cells isolated from the lungs of Dex-treated rats. Pregnant rats were injected with Dex or saline on days 18 and 19 of gestation and the fetuses delivered on day 20. Type II cells and fibroblasts were then isolated from the fetal lungs. Dex increased FAS activity, protein content, mRNA content and rate of transcription in the type II cells but not in the fibroblasts. Increased FAS expression in fetal type II cells in response to Dex is consistent with a critical role for FAS in the biosynthesis of lung surfactant.

Glucocorticoid enhances the response of type II cells from newborn rats to surfactant secretagogues

There is a developmental increase in agonist-induced surfactant secretion in type II cells. The response to the P2Y(2) agonist UTP is negligible in early newborn cells but increases with age. The response to terbutaline, N-ethylcarboxyamidoadenosine (NECA), and ATP also increases with age. As glucocorticoids are known to accelerate several aspects of lung maturation we examined the effect of dexamethasone (Dex) on the response of 1-day-old rat type II cells to surfactant secretagogues. Freshly isolated cells were cultured +/-10(-6) M Dex for 18--20 h after which phosphatidylcholine secretion was measured. Dex slightly decreased the basal secretion rate. However, it significantly increased the response to terbutaline, NECA, ATP and UTP. This effect was dependent on Dex concentration (EC(50)=2-6 x 10(-9) M) and blocked by the glucocorticoid receptor antagonist RU-486. It is unlikely to be due to increased receptor content as Dex had no effect on adenylate cyclase, phospholipase C or phospholipase D activation and the response to cAMP, forskolin and phorbol ester, secretagogues acting downstream from receptors, was also increased by Dex. These data show that Dex acts directly on the type II cell to enhance the response to surfactant secretagogues, that the effect of the hormone is mediated by the glucocorticoid receptor and suggest induction of a common downstream signaling step(s). Regulation of surfactant secretion may be an important function of glucocorticoids in the developing lung.