title: Characterization of a novel Foxa (hepatocyte nuclear factor-3) site in the glucagon promoter that is conserved between rodents and humans.
abstract: The pancreatic islet hormone glucagon stimulates hepatic glucose production and thus maintains blood glucose levels in the fasting state. Transcription factors of the Foxa [Fox (forkhead box) subclass A; also known as HNF-3 (hepatocyte nuclear factor-3)] family are required for cell-specific activation of the glucagon gene in pancreatic islet alpha-cells. However, their action on the glucagon gene is poorly understood. In the present study, comparative sequence analysis and molecular characterization using protein-DNA binding and transient transfection assays revealed that the well-characterized Foxa-binding site in the G2 enhancer element of the rat glucagon gene is not conserved in humans and that the human G2 sequence lacks basal enhancer activity. A novel Foxa site was identified that is conserved in rats, mice and humans. It mediates activation of the glucagon gene by Foxa proteins and confers cell-specific promoter activity in glucagon-producing pancreatic islet alpha-cell lines. In contrast with previously identified Foxa-binding sites in the glucagon promoter, which bind nuclear Foxa2, the novel Foxa site was found to bind preferentially Foxa1 in nuclear extracts of a glucagon-producing pancreatic islet alpha-cell line, offering a mechanism that explains the decrease in glucagon gene expression in Foxa1-deficient mice. This site is located just upstream of the TATA box (between -30 and -50), suggesting a role for Foxa proteins in addition to direct transcriptional activation, such as a role in opening the chromatin at the start site of transcription of the glucagon gene.
authors.affiliation: Department of Molecular Pharmacology, University of Göttingen, D-37099 Göttingen, Germany.
authors.forename: Sanjeev K
authors.initials: SK
authors.lastname: Sharma
authors.forename: Ulrike
authors.initials: U
authors.lastname: Leinemann
authors.forename: Regine
authors.initials: R
authors.lastname: Ratke
authors.forename: Elke
authors.initials: E
authors.lastname: Oetjen
authors.forename: Roland
authors.initials: R
authors.lastname: Blume
authors.forename: Corinna
authors.initials: C
authors.lastname: Dickel
authors.forename: Willhart
authors.initials: W
authors.lastname: Knepel
mesh_terms: D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D002478:Cells, Cultured; D017124:Conserved Sequence; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation; D005934:Glucagon / Q000235:genetics*; D051541:Hepatocyte Nuclear Factor 3-alpha; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D009154:Mutation; D009687:Nuclear Proteins / Q000378:metabolism*; D011401:Promoter Regions, Genetic / Q000235:genetics* / Q000502:physiology; D051381:Rats; D012689:Sequence Homology, Nucleic Acid; D014157:Transcription Factors / Q000378:metabolism*
chemical_list: D004268:DNA-Binding Proteins; C496499:FOXA1 protein, human; C496500:Foxa1 protein, mouse; C496501:Foxa1 protein, rat; D051541:Hepatocyte Nuclear Factor 3-alpha; D009687:Nuclear Proteins; D014157:Transcription Factors; D005934:Glucagon