title: Mutually exclusive interactions between factors binding to adjacent Sp1 and AT-rich elements regulate gastrin gene transcription in insulinoma cells.
abstract: The gastrin gene is transiently expressed in fetal pancreatic islets during islet neogenesis but then switched off after birth when islet cells become fully differentiated. Previous studies identified a cis-regulatory sequence between -109 and -75 in the human gastrin promoter which binds islet cell-specific activators and a nonspecific repressor and thus may act as a molecular switch. The present study identified another cis-regulatory sequence (-163ACACTAAATGAAAGGGCGGGGCAG-140) which bound two islet nuclear proteins in a mutually exclusive manner, as defined by gel shift competition, methylation interference, and DNase I foot-printing assays. The general transactivator Sp1 recognized the downstream GGGCGGGG sequence, but Sp1 binding was prevented when another islet factor bound to the adjacent AT-rich sequence (CTAAATGA). This gastrin AT-rich element is nearly identical to the binding site (ATAAATGA) for the islet-specific transcription factor beta TF-1. However, the gastrin AT-binding factor appeared to differ from beta TF-1 in its gel mobility shift pattern. Transfections of rat insulinoma cells revealed that mutations which blocked binding to the AT-rich element but allowed Sp1 binding up-regulated transcriptional activity. These results suggest that the gastrin AT-binding factor blocks transactivation by Sp1 and may have a role in the repression of gastrin transcription seen at the end of islet differentiation.
authors.affiliation: Gastrointestinal Unit, Massachusetts General Hospital, Boston 02114, USA.
authors.forename: D C
authors.initials: DC
authors.lastname: Chung
authors.forename: S J
authors.initials: SJ
authors.lastname: Brand
authors.forename: L G
authors.initials: LG
authors.lastname: Tillotson
mesh_terms: D000225:Adenine; D000818:Animals; D001483:Base Sequence; D005755:Gastrins / Q000235:genetics*; D005786:Gene Expression Regulation; D006801:Humans; D007340:Insulinoma / Q000235:genetics; D007515:Islets of Langerhans / Q000196:embryology / Q000378:metabolism*; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009838:Oligodeoxyribonucleotides; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D051381:Rats; D012045:Regulatory Sequences, Nucleic Acid; D016329:Sp1 Transcription Factor / Q000378:metabolism*; D013941:Thymine; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
chemical_list: D005755:Gastrins; D009838:Oligodeoxyribonucleotides; D016329:Sp1 Transcription Factor; D000225:Adenine; D013941:Thymine