LUNG CELL-SPECIFIC EXPRESSION OF THE MURINE SURFACTANT PROTEIN-A (SP-A) GENE IS MEDIATED BY INTERACTIONS BETWEEN THE SP-A PROMOTER AND THYROID TRANSCRIPTION FACTOR-I

Cis-acting elements determining lung epithelial cell-selective transcription of the murine surfactant protein A (SP-A) gene were identified between nucleotide positions -255 and -57. This region of the murine SP-A gene contained nucleotide sequences consistent with thyroid transcription factor-1 (TTF-1) binding motifs. An SP-A-CAT plasmid containing the TTF-1 binding sites was transcriptionally active in mouse lung epithelial (MLE-15) cells but not in HeLa, 3T3, or H441 cells. However, transcription of the SP-A-CAT construct was activated after cotransfection of HeLa cells with a vector expressing recombinant TTF-1, pCMV-TTF-1. Recombinant TTF-1 homeodomain protein bound to four distinct binding sites located between nucleotides -231 to -168. Proteins in nuclear extracts of MLE-15 cells bound TTF-1 binding sites and were supershifted by TTF-1 antibody. Mutations of three of the TTF-1 binding sites in this region reduced expression of the SP-A-CAT construct in transfected MLE-IB cells and reduced transactivation in HeLa cells. TTF-1 interacts with complex protein/DNA binding sites located in the 5'-flanking region of the murine SP-A gene enhancing lung epithelial cell-specific expression in vitro.