Modulation of the Glucagon-Like Peptide-1 Receptor Signaling by Naturally Occurring and Synthetic Flavonoids

Abstract

The glucagon-like peptide 1 receptor (GLP-1R) is a promising target for the treatment of type II diabetes mellitus because of its role in metabolic homeostasis. In recent years, difficulties with peptide therapies have driven the search for small-molecule compounds to modulate the activity of this receptor. We recently identified quercetin, a naturally occurring flavonoid, as a probe-dependent, pathway-selective allosteric modulator of GLP-1R-mediated signaling. Using Chinese hamster ovary cells expressing the human GLP-1R, we have now extended this work to identify the structural requirements of flavonoids to modify GLP-1R binding and signaling (cAMP formation and intracellular Ca 2ϩ mobilization) of each of the GLP-1R endogenous agonists, as well as the clinically used exogenous peptide mimetic exendin-4. This study identified a chemical series of hydroxyl flavonols with the ability to selectively augment calcium (Ca 2ϩ ) signaling in a peptide agonist-specific manner, with effects only on truncated GLP-1 peptides [GLP-1(7-36)NH 2 and GLP-1(7-37)] and exendin-4, but not on oxyntomodulin or full-length GLP-1 peptides [GLP-1(1-36)NH 2 and GLP-1(1-37)]. In addition, the 3-hydroxyl group on the flavone backbone (i.e., a flavonol) was essential for this activity, however insufficient on its own, to produce the allosteric effects. In contrast to hydroxyl flavonols, catechin had no effect on peptide-mediated Ca 2ϩ signaling but negatively modulated peptide-mediated cAMP formation in a probe-dependent manner. These data represent a detailed examination of the action of different flavonoids on peptide agonists at the GLP-1R and may aid in the development of future small molecule compounds targeted at this receptor.

Small Molecule Allosteric Modulation of the Glucagon-Like Peptide-1 Receptor Enhances the Insulinotropic Effect of Oxyntomodulin

“…Whereas discovery of surrogate agonists that use a receptor binding and activation mechanism similar to GLP-1 is probably difficult, we have reported that small molecules acting allosterically may be a more feasible approach (Koole et al, 2010;Wootten et al, 2011). The data presented here explore the hypothesis that a GLP-1 receptor allosteric modulator can potentiate the activity of the endogenous hormone oxyntomodulin on the GLP-1 receptor and thereby offer an additional small molecule approach to enhance GLP-1 receptor signaling.…”

International Union of Basic and Clinical Pharmacology. XC. Multisite Pharmacology: Recommendations for the Nomenclature of Receptor Allosterism and Allosteric Ligands

“…Class B peptide hormone GPCRs have been particularly difficult to target therapeutically because of the diffuse pharmacophore associated with the peptide orthosteric site, which is extracellular and involves multiple points of interaction with the N terminus and top of the transmembrane bundle. However, a number of small molecules have recently emerged that act allosterically, examples of which are found with molecules targeting the corticotrophin releasing factor-1 receptor (Hoare et al, 2008), the calcitonin receptor (Dong et al, 2009), and the glucagon-like peptide-1 receptor (Knudsen et al, 2007;Wootten et al, 2011). These findings suggest that the allosteric approach represents a viable path forward for discovering small molecules directed against these peptide-hormone receptors.…”

Minireview: Signal Bias, Allosterism, and Polymorphic Variation at the GLP-1R: Implications for Drug Discovery

“…The same trend was also evident for 4Ј-hydroxyflavonol and 3Ј4Ј-dihydroxyflavonol, suggesting hydroxyl groups in these positions are an important chemical feature engendering cooperativity. Conversely, the flavanoid catechin negatively modulated GLP-1(7-36)NH 2 , GLP-1(7-37), GLP-1(1-36)NH 2 , and to a lesser extent exendin-4 and GLP-1 (1-37), but not oxyntomodulin in cAMP accumulation assays (59). In both cases, the absence of modulation in agonist binding affinity suggests an efficacy-driven mechanism of action.…”

“…In both cases, the absence of modulation in agonist binding affinity suggests an efficacy-driven mechanism of action. Furthermore, the differential cooperativity in binding and signaling assays for quercetin, 4Ј-hydroxyflavonol, 3Ј4Ј-dihydroxyflavonol, and catechin, and indeed also compound 2 and BETP (59), are clear examples that the action of allosteric modulators cannot be classified solely through measurement of one pathway. This has also been exemplified at other receptors, including the cannabinoid CB 1 receptor, whereby allosteric Organon Research compounds positively modulate agonist binding affinity but negatively modulate agonist-mediated luciferase signaling (60).…”