TGT3, THYROID TRANSCRIPTION FACTOR-I, AND SP1 ELEMENTS REGULATE TRANSCRIPTIONAL ACTIVITY OF THE 1.3-KILOBASE PAIR PROMOTER OF T1-ALPHA, A LUNG ALVEOLAR TYPE-I CELL GENE

Alveolar type I epithelial cells form the major surface for gas exchan ge in the lung. To explore how type I cells differ in gene expression from their progenitor alveolar type II cells, we analyzed transcriptio nal regulation of T1 alpha, a gene expressed by adult type I but not t ype II cells. In vivo developmental patterns of T1 alpha expression in lung and brain suggest active gene regulation. We cloned and sequence d 1.25 kilobase pairs of the T1 alpha promoter that can drive reporter expression in lung epithelial cell lines. Deletion analyses identifie d regions important for lung cell expression. The base pair (bp) -100 to -170 fragment conferred differential regulation in lung epithelial cells compared with fibroblasts. Sequence alignment of this fragment w ith type II-specific surfactant protein B and C promoters shows simila r consensus elements arranged in a different order, Gel retardation st udies with alveolar epithelial cell line nuclear extracts, thyroid tra nscription factor I (TTF-1) homeodomain, hepatic nuclear factor (HNF)- 3 beta, or Sp1 proteins, and supershift assays were used to characteri ze TTF-1, HNF-3 (TGT3), and Sp1/Sp3 binding sites. The TGT3 site binds factors with binding properties similar to HNF-3/Fkh (hepatic nuclear factor-3/forkhead) proteins but different from HNF-3 alpha or HNF-3 b eta. Co-transfection with a TTF-1 expression vector moderately transac tivated the -170 bp-reporter construct. Mutational analysis of these t hree binding sites showed reduced transcriptional activity of the -170 bp promoter. Therefore, several regulatory sequences involved in type II cell gene regulation are also present in the T1 alpha promoter, su ggesting that genes of the peripheral lung epithelium may be regulated by similar factors.