Glucocorticoid responsiveness conferred by a cloned DNA binding protein.

Glucocorticoids stimulate surfactant protein-B (SP-B) (expression in type II alveolar cells) by unknown mechanisms. We identified, cloned, and characterized a protein that binds the SP-B promoter. This protein, D, increases the activity of the SP-B promoter in response to glucocorticoid stimulation. Protein D was identified by its ability to bind the SP-B promoter region, which it binds at an NF1 site from -184 to -198 bp. Its binding was abolished by digestion of promoter DNA with BalI, which cuts at -194. Protein D was cloned and sequenced. It is a new DNA binding protein of 33 kDa whose carboxyl end contains a modified basic leucine zipper-like DNA binding motif (bzip). The effects of D on SP-B promoter activity were studied in H441 cells, using a reporter construct containing 212 bp from the SP-B promoter with a luciferase reporter gene (p2121uc), which was cotransfected with a protein D expression construct in which D expression was controlled by the SV40 early promoter. These two plasmids were cotransfected into H441 cells. Without added glucocorticoids, D did not alter SP-B promoter activity. When dexamethasone was added, D strongly enhanced SP-B promoter activity. Identification of this protein suggests that, at least for SP-B, glucocorticoid responsiveness may involve one or more hitherto unknown gene activators.