Expression of ABCA3, a causative gene for fatal surfactant deficiency, is up-regulated by glucocorticoids in lung alveolar type II cells.

We have shown previously that the ATP-binding cassette transporter ABCA3 is expressed predominantly at the limiting membrane of the lamellar bodies in lung alveolar type II cells. Very recently, an ABCA3 gene mutation was reported in human newborns with fatal surfactant deficiency. In the present study, we have shown in rat lung that expression of the ABCA3 protein is dramatically increased after embryonic day (E) 20.5 just before birth. Expression was also markedly induced even at E18.5 when dexamethasone (Dex), which is known to accelerate surfactant formation, was administered to pregnant female rats for 3 days from E15.5. Since Dex increased the ABCA3 mRNA expression level in human alveolar type II cell line A549 cells 4-fold, we cloned and characterized the promoter region of the human ABCA3 gene. Promoter activity of the 5'-flanking region of the ABCA3 gene, which contains a potential glucocorticoid-responsive element (GRE), was up-regulated about 2-fold. Up-regulation by Dex was not observed when the GRE-containing region was deleted or when a point mutation was introduced into the GRE, and electrophoretic mobility shift assay using Dex-treated A549 nuclear extracts demonstrated specific binding of the glucocorticoid receptor to the GRE. These findings demonstrate that glucocorticoid-induced up-regulation of ABCA3 expression in vivo is mediated by transcriptional activation through the GRE in the promoter, and suggest that ABCA3 plays an important role in the formation of pulmonary surfactant, probably by transporting lipids such as cholesterol.

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