Transcriptional Activation of the Proglucagon Gene by Lithium and β-Catenin in Intestinal Endocrine L Cells

Zuyao Ni,Younes Anini,Xianjun Fang,Gordon Mills,Patricia L Brubaker,Tianru Jin

doi:10.1074/jbc.m206006200

Abstract

The proglucagon gene encodes several peptide hormones that regulate blood glucose homeostasis, growth of the small intestine, and satiety. Among them, glucagon-like peptide 1 (GLP-1) lowers blood glucose levels in patients with diabetes and inhibits eating and drinking in fasted rats. Although proglucagon transcription and GLP-1 synthesis were shown to be activated by forskolin and other protein kinase A (PKA) activators, deleting or mutating the cAMP-response element (CRE) only moderately attenuates the proglucagon gene promoter in response to PKA activation. Therefore, PKA may activate proglucagon transcription via a mechanism independent of the CRE motif. Recently, PKA was shown to phosphorylate and inactivate GSK-3beta, a key mediator in the Wnt signaling pathway. We show here that lithium, an inhibitor of GSK-3beta, activates proglucagon gene transcription and stimulates GLP-1 synthesis in an intestinal endocrine L cell line, GLUTag. The activation was also observed in primary fetal rat intestinal cell (FRIC) cultures, but not in a pancreatic A cell line. Co-transfection of beta-catenin, a downstream effector of GSK-3beta activities, activated the proglucagon gene promoter without a CRE. Furthermore, forskolin and 8-Br-cAMP phosphorylated GSK-3beta at serine 9 in intestinal proglucagon-producing cells, and both lithium and forskolin induced the accumulation of free beta-catenin in these cell lines. These observations indicate that the proglucagon gene is among the targets of the Wnt signaling pathway.

Highlights

The gene encoding proglucagon is expressed in pancreatic A cells and enteroendocrine L cells as well as selective neurons of the central nervous system [1]
In this study, using cultivated intestinal endocrine cell lines as well as primary intestinal cells in culture, we examined whether Wnt signaling pathway/molecules mediate the effect of Protein kinase A (PKA) in activating proglucagon gene transcription and glucagon-like peptide 1 (GLP-1) synthesis
Lithium Activates Proglucagon mRNA Expression—Lithium is a notable inhibitor of GSK-3␤

Summary

Introduction

The gene encoding proglucagon is expressed in pancreatic A cells and enteroendocrine L cells as well as selective neurons of the central nervous system [1]. We show here that lithium, an inhibitor of GSK-3␤, activates proglucagon gene transcription and stimulates GLP-1 synthesis in an intestinal endocrine L cell line, GLUTag. The activation was observed in primary fetal rat intestinal cell (FRIC) cultures, but not in a pancreatic A cell line. Protein kinase A (PKA) is able to stimulate proglucagon gene transcription and glucagon or GLP-1 synthesis in both primary and transformed intestinal endocrine cells as well as in rat primary pancreatic islet cultures [11,12,13,14, 17].

Results

Conclusion