Characterization of a Novel Protein Kinase C Response Element in the Glucagon Gene

Abstract

To maintain glucose levels in blood within narrow limits the synthesis secretion of pancreatic islet hormones are controlled bya variety of neural hormonal metabolic messengers that act through multiple signal transduction pathways. Glucagon gene transcription is stimulated by cyclic AMP depolarizationinduced calcium influx. In this study the effect of protein kinase C on glucagon gene transcription was investigated. After transient transfection of a glucagon-Reporter fusion gene into the glucagon-Producing islet cell line αTC2 activation of protein kinase C by 12-O- tetradecanoylphorbol-13-acetate (TPA) stimulated glucagon gene transcription. By 5' deletions 3' deletions internal deletion oligonucleotide cassette insertion the TPA-Responsive element was mapped to the G2 element (from -165 to -200). Like TPA overexpression of oncogenic Ras (V-12 Ras) stimulated G2- Mediated transcription whereas overexpression of a dominant negative Ras mutant (N-17 Ras) blocked the effect of TPA. A mutational analysis of G2 function nuclear protein binding indicated that protein kinase C Ras responsiveness is conferred to the glucagon gene by HNF-3β functionally interacting with a protein that binds to a closely associated site with sequence similarity to binding sites of Ets family proteins. HNF-3β belongs to the winged-Helix family of transcription factors has been implicated in the control of cell-specific developmental gene expression. The results of the present study show that the cell lineage-specific transcription factor HNF-3β is an essential component of a novel protein kinase C response element in the glucagon gene.