Transcriptional regulation of secretin gene expression

Expression of the gene encoding the hormone secretin is restricted to a specific enteroendocrine cell type and to beta-cells in developing pancreatic islets. To characterize regulatory elements in the secretin gene responsible for its expression in secretin-producing cells, we used a series of reporter genes for transient expression assays in transfection studies carried out in secretin-producing islet cell lines. Analysis of the transcriptional activity of deletion mutants identified a positive cis regulatory domain between 174 and 53 base pairs upstream from the transcriptional initiation site which was required for secretin gene expression in secretin-producing HIT insulinoma cells. Within this enhancer were sequences resembling two binding sites for the transcription factor Sp1, as well as a consensus sequence for binding to helix-loop-helix proteins. Analysis of these three elements by site-directed mutagenesis suggests that each is important for full transcriptional activity. The role of proximal enhancer sequences in directing secretin gene expression to appropriate tissues is further supported by studies in transgenic mice revealing that 1.6 kilobases of the secretin gene 5' flanking sequence were sufficient to direct the expression of either human growth hormone or simian virus 40 large T-antigen reporter genes to all major secretin-producing tissues.