The proglucagon gene upstream enhancer contains positive and negative domains important for tissue-specific proglucagon gene transcription.

Abstract

The gene encoding proglucagon is restricted in expression to the central nervous system, endocrine pancreas, and intestine. Transgenic experiments indicate that the proglucagon gene upstream enhancer (GUE) element is a principal determinant of both the tissue specificity and the relative level of proglucagon gene transcription. We have now sequenced the rat proglucagon GUE and analyzed the transcriptional properties of proglucagon-luciferase fusion genes (that contain 5'- and 3'-deletions in the GUE) after transfection of islet (InR1-G9) and enteroendocrine (STC-1 and GLUTag) cell lines. The GUE contains both positive and negative elements that are recognized differentially in islet vs. intestinal cell lines. The transcriptional properties of the GUE sequences were more similar in cell lines of intestinal (STC-1 and GLUTag) compared with islet (InR1-G9) phenotypes. The electrophoretic mobility shift assay was used to identify specific domains of the GUE that interacted with nuclear proteins from islet and intestinal cells. Several GUE sequences recognized proteins present in both fibroblast and endocrine cell lines. In contrast, electrophoretic mobility shift assay experiments also identified 1) GUE-protein complexes common to both islet and intestinal cell lines and 2) GUE-protein complexes specific to either islet or intestinal lineages. One of the GUE subdomains, designated GLUE1, displayed enhancer-like activity in InR1-G9 and GLUTag, but not BHK, cell lines. Taken together, these observations demonstrate that the proglucagon GUE is comprised of multiple positive and negative domains that likely function in a combinatorial fashion to regulate islet and intestinal-specific proglucagon gene transcription.