GPR119, a Major Enteroendocrine Sensor of Dietary Triglyceride Metabolites Coacting in Synergy With FFA1 (GPR40)

Abstract

Triglycerides (TGs) are among the most efficacious stimulators of incretin secretion; however, the relative importance of FFA1 (G Protein-coupled Receptor [GPR] 40), FFA4 (GPR120), and GPR119, which all recognize TG metabolites, ie, long-chain fatty acid and 2-monoacylglycerol, respectively, is still unclear. Here, we find all 3 receptors to be highly expressed and highly enriched in fluorescence-activated cell sorting-purified GLP-1 and GIP cells isolated from transgenic reporter mice. In vivo, the TG-induced increase in plasma GIP was significantly reduced in FFA1-deficient mice (to 34%, mean of 4 experiments each with 8-10 animals), in GPR119-deficient mice (to 24%) and in FFA1/FFA4 double deficient mice (to 15%) but not in FFA4-deficient mice. The TG-induced increase in plasma GLP-1 was only significantly reduced in the GPR119-deficient and the FFA1/FFA4 double deficient mice, but not in the FFA1, and FFA4-deficient mice. In mouse colonic crypt cultures the synthetic FFA1 agonists, TAK-875 stimulated GLP-1 secretion to a similar extent as the prototype GLP-1 secretagogue neuromedin C; this, however, only corresponded to approximately half the maximal efficiency of the GPR119 agonist AR231453, whereas the GPR120 agonist Metabolex-209 had no effect. Importantly, when the FFA1 agonist was administered on top of appropriately low doses of the GPR119 agonist, a clear synergistic, ie, more than additive, effect was observed. It is concluded that the 2-monoacylglycerol receptor GPR119 is at least as important as the long-chain fatty acid receptor FFA1 in mediating the TG-induced secretion of incretins and that the 2 receptors act in synergy, whereas FFA4 plays a minor if any role.

Fig. 1. Triglyceride metabolite receptors and their expression in GIP and GLP-1 cells.

Panel A – overview of the digestion of dietary TGs by pancreatic lipase generating LCFAs, which potentially could be acting through the two receptors GPR40 and GPR120, and 2-MAG which could be acting through GPR119 to stimulate gut hormone secretion. Panel B – Expression of fat metabolite receptors (orange symbols) in FACS purified murine GIP cells isolated from the GIP-Venus reporter mice versus the expression of these receptors in the neighboring cells. Panel C - Expression of fat metabolite receptors (orange symbols) in FACS purified murine GLP-1 cells isolated from the proglucagon-Venus reporter mice. Grey symbols indicate the relative expression of the main 379 non-olfactory GPCRs.

Fig. 2. Plasma GIP and GLP-1 responses to oral triglyceride challenge in mice – dependency on the metabolite receptors FFA1 (GPR40), FFA4 (GPR120) and GPR119.

Panel A - Plasma GIP levels before and 60 minutes after an oral gavage challenge with 10ml/kg of olive oil in FFA4 (GPR120), FFA1 (GPR40) and double deficient mice versus littermate wild type control mice (left panel) and in GPR119 deficient mice versus littermate control mice (right panel). A representative experiment with n=8-10 animals in each group is shown. The FFA1 and FFA4 deficient animals were taken from a breeding to obtain the double deficient animals and accordingly the joint littermate controls. Panel B – Mean GIP levels of four experiments similar to the one shown in panel A left panel (indicated in filled square symbols) except that in one experiment wild type C57BL/6 mice were used as controls (open symbols). In the right panel is shown the mean GIP response from two experiments with GPR119 deficient mice. Panel A - Plasma GLP-1 levels before and 60 minutes after an oral gavage challenge with 10 ml/kg of olive oil in FFA4 (GPR120), FFA1 (GPR40) and double deficient mice versus littermate wild type control mice (left panel) and in GPR119 deficient mice versus littermate control mice (right panel). A representative experiment with n=8-10 animals in each group is shown. The FFA1 and FFA4 deficient animals were taken from a breeding to obtain the double deficient animals and accordingly the joint littermate controls. Panel B – Mean GLP-1 levels of four experiments similar to the one shown in panel A (indicated in filled square symbols) except that in one experiment wild type C57BL/6 mice were used as controls (open symbols). In the right panel is shown the mean GLP-1 responses from two experiments with GPR119 deficient mice.

Fig. 3. Secretion of GLP-1 in response to synthetic selective FFA1, FFA4 and GPR119 agonists administered alone or co-administered to probe for potential synergistic effects in mouse colonic crypt cultures.

Panel A – Comparison of the effect on GLP-1 secretion in mouse colonic crypt cultures of maximal doses of the FFA1 agonist TAK875 (dark blue), the GPR120 agonist Metabolex-209 (light blue) and the GPR119 agonist AR231453 (green) with the prototype GLP-1 secretagouge the bombesin 2 receptor (BB2) agonist, neuromedin C (NMC) (grey). For FFA1 and GPR119 the responses in FFA1 and GPR119 deficient animals are indicated in hatched columns. Mean +/- SEM, numbers of experiments are indicated in brackets in the columns. Panel B – dose-response experiment for the highly efficacious GPR119 agonist AR231453 performed to identify appropriately low doses to be used for co-administration studies (panels C and D). Panel C - GLP-1 secretion in response to the GPR40 agonist TAK-875 alone (10^-5 and 10^-6 M – in dark blue) and in response to the GPR119 agonist AR231453 alone (10^-9 and 10^-10 M – in green) or the GPR40 agonist together with the GPR119 agonist (orange corresponding to response above the sum of the ‘green’ and the ‘blue’ response). Panel D - GLP-1 secretion in response to the GPR120 agonist Metabolix-209 alone (10^-5 and 10^-6 M – in light blue) and in response to the GPR119 agonist AR231453 alone (10^-9 and 10^-10 M – in green) or the GPR120 agonist together with the GPR119 agonist (orange corresponding to response above the sum of the ‘green’ and the ‘light blue’ response). Panel E – Simplified schematic overview of the physiological ligand binding and signal transduction mechanisms for FFA1, FFA4 and GPR120 in a generic enteroendocrine cell. As indicated LCFAs will induce Gαq signaling through FFA1 leading to PLC activation, IP3 accumulation and increase in intracellular Ca++, whereas 2-MAG will induce Gαs signaling through GPR119 leading to adenylate cyclase activation, cAMP accumulation and presumably activation of Epac and PKA which will activate the fusion machinery of the secretory granules. As shown in panel C co-activation of the FFA1/Gαq and the GPR119/Gαs signaling pathway act in synergy to provide a robust GLP-1 secretory response. GPR120 has little or no effect, conceivably due to the fact that the receptor mainly is coupled to Gαi in enteroendocrine cells [19].