The Insulin Gene Contains Multiple Transcriptional Elements That Respond to Glucose

The beta cells in the pancreatic islets of Langerhans increase insulin gene transcription in response to increased glucose concentration. We have mapped sequences within the rat insulin I gene 5'-flanking DNA (rInsI promoter) that direct this transcriptional response to glucose. When linked to chloramphenicol acetyltransferase and expressed in cultured beta cells, no single mutation of the rInsI promoter removes its ability to respond to glucose, although several mutations cause marked reductions in basal chloramphenicol acetyltransferase expression. A 50-bp sequence isolated from the rInsI promoter, the Far-FLAT minienhancer, can confer glucose responsiveness to nonresponsive promoters. Fine mapping of this minienhancer further localizes a glucose response to the sequence GGCCATCTGGCC, or the Far element. Nuclear extracts from islets grown in various glucose concentrations demonstrate a glucose-stimulated increase in a protein complex that binds the Far element and contains the transcription factors Pan-1 and Pan-2. Overexpression of intact or partially deleted Pan-1 ablates the Far-directed transcriptional response to glucose. We conclude that the full glucose response of the insulin promoter involves the interaction of multiple sequence elements. Part of this response, however, results from activation of a complex binding at the Far element.