Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are important regulators of metabolism, making their receptors (GLP-1R and GIPR) attractive targets in the treatment of type 2 diabetes mellitus (T2DM). GLP-1R agonists are used clinically to treat T2DM but the use of GIPR agonists remains controversial. Recent studies suggest that simultaneous activation of GLP-1R and GIPR with a single peptide provides superior glycemic control with fewer adverse effects than activation of GLP-1R alone. We investigated the signaling properties of a recently reported dual-incretin receptor agonist (P18). GLP-1R, GIPR, and the closely related glucagon receptor (GCGR) were expressed in HEK-293 cells. Activation of adenylate cyclase via Gαs was monitored using a luciferase-linked reporter gene (CRE-Luc) assay. Arrestin recruitment was monitored using a bioluminescence resonance energy transfer (BRET) assay. GLP-1, GIP, and glucagon displayed exquisite selectivity for their receptors in the CRE-Luc assay. P18 activated GLP-1R with similar potency to GLP-1 and GIPR with higher potency than GIP. Interestingly, P18 was less effective than GLP-1 at recruiting arrestin to GLP-1R and was inactive at GCGR. These data suggest that P18 can act as both a dual-incretin receptor agonist, and as a G protein-biased agonist at GLP-1R.

The incretin hormones: glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are important regulators of many aspects of metabolism including insulin secretion. Their receptors (GIPR and GLP‐1R) are closely related members of the secretin class of G‐protein‐coupled receptors. As both receptors are expressed on pancreatic β‐cells there is at least the hypothetical possibility that they may form heteromers. In the present study, we investigated GIPR/GLP‐1R heteromerization and the impact of GIPR on GLP‐1R‐mediated signaling and vice versa in HEK‐293 cells. Real‐time fluorescence resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) saturation experiments confirm that GLP‐1R and GIPR form heteromers. Stimulation with 1 μM GLP‐1 caused an increase in both FRET and BRET ratio, whereas stimulation with 1 μM GIP caused a decrease. The only other ligand tested to cause a significant change in BRET signal was the GLP‐1 metabolite, GLP‐1 (9–36). GIPR expression had no significant effect on mini‐Gs recruitment to GLP‐1R but significantly inhibited GLP‐1 stimulated mini‐Gq and arrestin recruitment. In contrast, the presence of GLP‐1R improved GIP stimulated mini‐Gs and mini‐Gq recruitment to GIPR. These data support the hypothesis that GIPR and GLP‐1R form heteromers with differential consequences on cell signaling.

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The author wishes to make the following correction to this paper [...]