Transcription Factor FoxO1 Mediates Glucagon-Like Peptide-1 Effects on Pancreatic β-Cell Mass

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Abstract
The glucoincretin hormone glucagon-like peptide-1 (GLP-1) increases pancreatic β-cell proliferation and survival through sequential activation of the epidermal growth factor receptor (EGFR), phosphatidylinositol-3 kinase (PI 3-kinase), and Akt. We investigated the role of transcription factor FoxO1 in the proliferative and antiapoptotic actions of GLP-1 in β-cells. GLP-1 inhibited FoxO1 through phosphorylation-dependent nuclear exclusion in pancreatic β (INS832/13) cells. The effect of GLP-1 was suppressed by inhibitors of EGFR (AG1478) and PI 3-kinase (LY294002). In contrast, LY294002 but not AG1478 suppressed insulin-induced FoxO1 phosphorylation. Expression of constitutively nuclear FoxO1 in β-cells prevented the proliferative and antiapoptotic actions of GLP-1 in cultured β-cells and the increase in pancreatic β-cell mass in response to Exendin4 in transgenic mice. Gene expression and chromatin immunoprecipitation assays demonstrated that GLP-1 increases pancreatic and duodenal homeobox gene-1 and Foxa2 expression and inhibits FoxO1 binding to both promoters. We propose that FoxO1 mediates the pleiotropic effects of the glucoincretin hormone on cell proliferation and survival.