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Mutations to the Third Cytoplasmic Domain of the Glucagon-Like Peptide 1 (GLP-1) Receptor Can Functionally Uncouple GLP-1-Stimulated Insulin Secretion in HIT-T15 Cells

Departments of Medicine and Physiology University of Toronto Toronto, Ontario, Canada M5S 1A8

Glucagon-like peptide-1 (GLP-1) is an insulinotropic hormone with powerful antidiabetogenic effects that are thought to be mediated by adenylyl cyclase (AC). Recently, we generated two GLP-1 receptor mutant isoforms (IC3–1 and DM-1) that displayed efficient ligand binding and the ability to promote Ca²⁺ mobilization from intracellular stores but lacked the ability to couple to AC. In the present study, the wild-type rat GLP-1 receptor (WT-GLP-1 R) or the IC3–1 and DM-1 mutant forms were expressed for the first time in the insulin-producing HIT-T15 cells. Only cells expressing WT-GLP-1 R displayed dramatically elevated GLP-1-induced cAMP responses and elevated insulin secretion. The increase in GLP-1-stimulated secretion in cells expressing WT-GLP-1 R, however, was not accompanied by differences in glucose-stimulated insulin release. Prolonged exposure to GLP-1 (10 nM, 17 h), not only led to an increase in insulin secretion but also increased insulin mRNA levels, but only in cells expressing the WT-GLP-1R and not the mutant isoforms. Electrophysiological analyses revealed that GLP-1 application enhanced L-type voltage-dependent Ca²⁺ channel (VDCC) currents > 2-fold and caused a positive shift in VDCC voltage-dependent inactivation in WT-GLP-1R cells only, not control or mutant (DM-1) cells. This action on the Ca²⁺ current was further enhanced by the VDCC agonist, BAYK8644, suggesting GLP-1 acts via a distinct mechanism dependent on cAMP. These studies demonstrate that the GLP-1 receptor efficiently couples to AC to stimulate insulin secretion and that receptors lacking critical residues in the proximal region of the third intracellular loop can effectively uncouple the receptor from cAMP production, VDCC activity, insulin secretion, and insulin biosynthesis.

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