2125-P: Incretin-Stimulated Insulin Secretion Is Mediated by Gas Dependent and Independent Mechanisms

The incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) stimulate insulin secretion in β-cells by binding to cognate receptors. The GIPR and GLP-1R are family B G-protein coupled receptors generally thought to mediate signaling through G-αs and generation of cAMP. However, the GLP-1R has also been reported to signal through alternative pathways beyond Gαs, and the relative contributions of Gαs vs. non-Gαs are unknown. The GLP-1R is essential for basal levels of cAMP and normal β-cell tone. We hypothesized that elimination of Gαs would render mouse islets insensitive to GLP-1R agonists. Induction of β-cell specific Gnas knockout with tamoxifen inducible cre caused a >90% reduction in Gαs mRNA in purified β-cells (Gnas^βcell-/-). Perifusion of control and knockout islets demonstrated reduced glucose-stimulated insulin secretion, but persistence of an insulin response to GLP-1R agonists that was ∼40% of control. Similarly, insulin secretion in response to either GIP or glucagon was only partially reduced in Gαs knockout islets. A Gq inhibitor partially reduced insulin secretion in control islets, but abolished GLP-1 stimulated insulin secretion in Gnas^βcell-/- islets. These results demonstrate that the incretins can signal through both Gαs and non-Gαs mechanisms. Interestingly, Gnas^βcell-/- mice were severely hyperglycemic (>400 mg/dl) and had no reduction of glycemia in response to GLP-1R or GIPR agonists. Thus, non-Gαs signaling by either GLP-1R or GIPR is unable to stimulate sufficient insulin secretion to maintain euglycemia in vivo.