β-Arrestin Recruitment and Biased Agonism at Free Fatty Acid Receptor 1

FFAR1/GPR40 is a seven-transmembrane domain receptor (7TMR) expressed in pancreatic beta cells and activated by FFAs. Pharmacological activation of GPR40 is a strategy under consideration to increase insulin secretion in type 2 diabetes. GPR40 is known to signal predominantly via the heterotrimeric G proteins G(q/11). However, 7TMRs can also activate functionally distinct G protein-independent signaling via beta-arrestins. Further, G protein- and beta-arrestin-based signaling can be differentially modulated by different ligands, thus eliciting ligand-specific responses (biased agonism). Whether GPR40 engages beta-arrestin-dependent mechanisms and is subject to biased agonism is unknown. Using bioluminescence resonance energy transfer-based biosensors for real-time monitoring of cell signaling in living cells, we detected a ligand-induced GPR40-beta-arrestin interaction, with the synthetic GPR40 agonist TAK-875 being more effective than palmitate or oleate in recruiting beta-arrestins 1 and 2. Conversely, TAK-875 acted as a partial agonist of G(q/11)-dependent GPR40 signaling relative to both FFAs. Pharmacological blockade of G(q) activity decreased FFA-induced insulin secretion. In contrast, knockdown or genetic ablation of beta-arrestin 2 in an insulin-secreting cell line and mouse pancreatic islets, respectively, uniquely attenuated the insulinotropic activity of TAK-875, thus providing functional validation of the biosensor data. Collectively, these data reveal that in addition to coupling to G(q/11), GPR40 is functionally linked to a beta-arrestin 2-mediated insulinotropic signaling axis. These observations expose previously unrecognized complexity for GPR40 signal transduction and may guide the development of biased agonists showing improved clinical profile in type 2 diabetes.