Alpha-Cell-Specific Expression of the Glucagon Gene Is Conferred to the Glucagon Promoter Element by the Interactions of DNA-Binding Proteins

Abstract

The glucagon gene is expressed specifically in the alpha cells of the pancreatic islets. We show here that 300 base pairs of the 5′-flanking region of the rat glucagon gene, linked to a chloramphenicol acetyltransferase reporter plasmid transfected into islet cell lines of different hormone-producing phenotypes, directs transcription only in glucagon-producing islet cells. Deletional and linker-scanning mutations and DNase I footprinting assays identify three transcriptional control elements within these 300 base pairs. Two of these elements (G2 and G3) independently display enhancerlike functions on both homologous and heterologous promoters in glucagon (alpha) cells, but only on heterologous promoters in insulin- (beta) and somatostatin- (delta) expressing cells, and not in non-islet cells. The proximal promoter element (G1), characterized by low intrinsic transcriptional activity, is critical for specific expression of the glucagon gene in alpha cells. However, nuclear extracts prepared from all three islet cell phenotypes give similar protection to the three control elements of the glucagon 5′-flanking sequence. We conclude that these phenotypically distinct islet cell lines all contain regulatory DNA-binding proteins interacting with the three control elements of the glucagon gene, but that factors interacting with the glucagon promoter result in transcriptional activation only in alpha cells, to restrict glucagon gene expression to these cells. These observations suggest that interactions of nuclear proteins with cis-control elements are involved in the programmed developmental expression of the islet polypeptide hormone genes.