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 and secretion of pancreatic islet hormones are controlled by a variety of neural, hormonal, and metabolic messengers that act through multiple signal transduction pathways. Glucagon gene transcription is stimulated by cyclic AMP and depolarization-induced 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, and 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 and nuclear protein binding indicated that protein kinase C and 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 and has been implicated in the control of cell-specific and 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.