Transcriptional activation of the proglucagon gene by lithium and beta-catenin in intestinal endocrine L cells.
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.
Ni
Zuyao
Z
Division of Cell & Molecular Biology, Toronto General Research Institute, University Health Network, Ontario M5G 2M1, Canada.
Anini
Younes
Y
Fang
Xianjun
X
Mills
Gordon
G
Brubaker
Patricia L
PL
Jin
Tianru
T
D002460:Cell Line; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology; D003598:Cytoskeletal Proteins / Q000502:physiology*; D005786:Gene Expression Regulation / Q000187:drug effects*; D005934:Glucagon / Q000096:biosynthesis / Q000235:genetics*; D052216:Glucagon-Like Peptide 1; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D008094:Lithium / Q000494:pharmacology*; D010446:Peptide Fragments / Q000096:biosynthesis; D052336:Proglucagon; D011401:Promoter Regions, Genetic; D011498:Protein Precursors / Q000096:biosynthesis / Q000235:genetics*; D015534:Trans-Activators / Q000502:physiology*; D015533:Transcriptional Activation; D014162:Transfection; D051176:beta Catenin
C495270:CTNNB1 protein, human; D003598:Cytoskeletal Proteins; D010446:Peptide Fragments; D011498:Protein Precursors; D015534:Trans-Activators; D051176:beta Catenin; D052336:Proglucagon; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D008094:Lithium; D017868:Cyclic AMP-Dependent Protein Kinases