Positive modulation of glucagon-like peptide 1 (GLP-1) receptor by endogenous haemorphins with implications in glucose metabolism and diabetes.

Abstract

Background and purpose: Haemorphins are known to target G protein-coupled receptors and their reduced serum levels were associated with diabetes and obesity. This is consistent with inhibitory action on dipeptidyl peptidase IV, controlling GLP-1 levels. The modulation of the GLP-1 receptor by haemorphins as a mechanism for regulating glucose homeostasis was investigated.

Experimental approach: The effect of LVV-haemorphin 7 (LVV-H7) on GLP-1 receptor was assessed in human embryonic kidney (HEK293) and rat insulinoma (INS-1 832/3) cells using bioluminescence resonance energy transfer, downstream signalling, cell viability, insulin synthesis experimental and computational ligand binding analysis.

Key results: LVV-H7 has positive action on GLP-1 receptor occurring with GLP-1 but not orforglipron (LY3502970), indicating a peptide-peptide interaction, causing a left shift in GLP-1's dose responses. LVV-H7 also potentiated GLP-1-induced insulin expression and its protective effect against glucose-induced cytotoxicity. Moreover, LVV-H7 slightly reduced the GLP-1's association kinetics with no impact on its binding properties. Molecular docking supported by functional analysis showed the existence of a putative 'allosteric' binding site at the junction between the helical and the extracellular domains (ECDs) and highlighted the critical role of Trp6 of LVV-H7 and Ser31^ECD/Glu34^ECD/Arg299^ECL2/Asp372^ECL3/Arg380^ECL3/™⁷of GLP-1 receptor. This also demonstrated that active GLP-1/GLP-1 receptor complex is required for the positive action of LVV-H7.

Conclusions and implications: LVV-H7 is active on the GLP-1 receptor-mediated metabolic pathways in pancreatic β-cells. This is consistent with reduced serum haemorphin levels in diabetes/obesity, suggesting implications in glucose homeostasis. This constitutes the first Class B GPCR modulated by haemorphins, illustrating the role of haemorphin-GPCR axis in physiology/pathophysiology.