Résumé
Background and aims : The scaffold protein beta-arrestin2 (ARRB2) is known to uncouple G protein coupled receptors (GPCRs) from the G protein and to recruit new signaling pathways (such as the ERK1/2, PI3K, FAK…). In non beta cells, ARRB2 interacts with a wide range of GPCRs, but its interaction with the GIP receptor (GIPR) is still unclear. Our aim is to determine if ARRB2 is involved in the signaling of the GIPR in pancreatic beta cells.Materials and methods : The experiments were carried out in beta cells from five-month-old Arrb2+/+ and Arrb2-/- male mice. cAMP production (CAMPS-EPAC), endogenous PKA (AKAR3) and ERK1/2 (EKAR) activations, [Ca2+] in the cytosol ([Ca2+]c ; Fura2-LR) and in the endoplasmic reticulum ([Ca2+]ER ; D4ER) were assessed by live cell imaging in mouse pancreatic beta cells. EPAC2 (EPAC2-GFP) recruitment beneath the plasma membrane was monitored by total internal reflection fluorescence microscopy. Actin-F depolymerisation was evaluated by phalloidin staining (Alexa Fluor 488-conjugated phalloidin) and the phosphorylation of Focal Adhesion Kinase (FAK) by immunofluorescence.Results: Insulin secretion from Arrb2-/- islets was reduced by 50% compared to Arrb2+/+ islets in response to GIP (100pM-10nM, p<0.01). When ARRB2 (ARRB2-GFP) was re-expressed in Arrb2-/- beta cells, insulin secretion in response to GIP was restored to a similar level than in Arrb2+/+ islets. Surprisingly, upon GIP stimulation (10pM-10nM), the cAMP production, PKA activation and EPAC2 recruitment were similar in Arrb2+/+and Arrb2-/- beta cells. Both [Ca2+]c and [Ca2+]ER remained comparable. Finally, the activation of ERK1/2 was also similar in Arrb2+/+ and Arrb2-/- beta cells. By contrast, the F-actin depolymerisation induced by 10nM GIP was significantly reduced (~25%, p<0.01) in Arrb2-/- beta cells. PI3Kɤ and FAK have been reported to be involved in F-actin depolymerisation in response to GIP and glucose, respectively, and to be required for optimal insulin secretion. As expected, the PI3K inhibitor (AS604850; 1µmol/l) reduced F-actin depolymerisation (~30%, p<0.01) by GIP stimulation in Arrb2+/+ beta cells, but no additional effect was observed in Arrb2-/- beta cells. Moreover, GIP-induced FAK activation was also reduced by 50% in Arrb2-/- beta cells.Conclusion: Our study revealed that ARRB2 is required for the potentiation of insulin secretion by GIP, through F-actin depolymerisation probably via FAK activation and PI3K recruitment, but independently from the canonical cAMP signalling (PKA and EPAC2) and the ERK1/2 pathway. Therefore, any variation in the expression of ARRB2, as observed in diabetic states, should functionally affect the incretin effect produced by GIP.