By transducing an apoptotic signal in immune effector cells, Fas has been directly implicated in the control of immunological activity. Expression and functional results, however, have also suggested a role for Fas in regulating cell turnover in specific epithelial populations. To characterize factors responsible for Fas expression in epithelial cells, approximately 3 kb of the 5´ flanking region of the mouse Fas gene was isolated. By rapid amplification of cDNA ends and primer extension, transcriptional start sites were identified within 50 bp upstream of the translation start site. Transient transfection of promoter–luciferase constructs in a mouse lung epithelial cell line, MLE-15, localized promoter activity to the first 77 bp of upstream sequence. By using a 60 bp DNA probe (-18 to -77) in electrophoretic mobility-shift assays, three shifted complexes were found. Incubation with excess cold Sp1 oligonucleotide or an anti-Sp3 antibody inhibited complex formation. Site-directed mutagenesis of the Sp1 site resulted in 60–70% loss of promoter activity. In Drosophila SL-2 cells, promoter activity was markedly increased by co-transfection of an Sp3 expression construct. These results show that the Sp3 protein is involved in regulating Fas gene expression in lung epithelial cells.

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