Triple Hormone Agonist Boosts Insulin via GLP-1 Receptor and Gαq Pathway

**Why this matters:** Next-generation diabetes and obesity drugs are moving toward multi-receptor agonism—simultaneously hitting GLP-1, GIP, and glucagon receptors—to maximise metabolic benefit. Yet the molecular machinery underlying their superior efficacy over single-receptor drugs has remained poorly understood. Clarifying these mechanisms could guide smarter drug design and predict which patients benefit most.

**What was studied:** This study examined IUB447, a validated unimolecular GLP-1/GIP/GCG triagonist, in isolated murine pancreatic islets and in vivo mouse models. Using CRISPR-generated single and double receptor knockout (KO) mice (Gipr−/−, Gcgr−/−, Glp-1r/Gipr double-KO) alongside Trpm5−/− mice, researchers systematically dissected which receptor and downstream signalling components are responsible for triagonist-induced glucose-stimulated insulin secretion (GSIS). Pharmacological inhibitors targeting GLP-1R (exendin-3 [9–39]), Gαq (YM254890), adenylate cyclase (MDL-12330A), PKC (calphostin C), and TRPM5 (TPPO) were also employed. In vivo metabolic phenotyping was conducted in high-fat-diet (HFD) WT and Trpm5−/− mice treated with triagonist every other day for 3 weeks.

**Key results:** The triagonist stimulated GSIS to a greater degree than the loose co-administration of all three individual mono-agonists in WT islets, suggesting a qualitative pharmacological difference. Strikingly, knocking out either Gipr or Gcgr alone did not diminish this enhanced GSIS. Only simultaneous absence of both Glp-1r and Gipr, or selective GLP-1R antagonism with exendin-3 (9–39), abolished the triagonist's insulinotropic superiority. Blocking Gαq signalling with YM254890 or TRPM5 channel activity with TPPO also fully suppressed triagonist-driven GSIS enhancement. Calcium imaging and cAMP assays further supported that the triagonist engages a Gαq–TRPM5 intracellular calcium pathway rather than relying solely on the canonical Gαs–cAMP route. In HFD Trpm5−/− mice in vivo, the triagonist failed to improve glycaemic management, confirming that TRPM5 is indispensable for the drug's metabolic effects under obesogenic conditions.

**Implications:** These findings reframe the triagonist's mechanism as predominantly GLP-1R-centric, with GIP and GCG receptor engagement playing a secondary or modulatory role in insulin secretion. The Gαq–TRPM5 axis is particularly relevant under diabetic conditions, where persistent beta cell depolarisation shifts GLP-1R signalling toward Gαq dominance, preserving insulinotropic capacity even when the conventional Gαs–cAMP pathway is compromised. This may explain why multi-agonist drugs retain efficacy in settings where GIP receptor signalling is blunted.

**Caveats:** All mechanistic data are from mouse models, and GLP-1R signalling profiles can differ between rodents and humans. The study does not address central nervous system contributions to glycaemic improvement, and the specific structural features of IUB447 that confer GLP-1R preference require further investigation.