Glucagon-like peptide-1 receptor (GLP-1R) agonists reduce lipid accumulation in peripheral tissues, attenuating atherosclerosis and hepatic steatosis in preclinical studies. We examined whether GLP-1R activation decreases atherosclerosis progression in high-fat diet-fed male ApoE(-/-) mice after administration of streptozotocin and treatment with the long-acting GLP-1R agonist taspoglutide administered once monthly vs. metformin in the drinking water for 12 wk. Taspoglutide did not reduce plaque area or lipid content in the aortic arch or abdominal aorta, and no significant change in aortic macrophage accumulation was detected after taspoglutide or metformin. In contrast, hepatic triglyceride levels were significantly reduced in livers from taspoglutide-treated mice. Both peripheral and intracerebroventricular administration of exendin-4 rapidly decreased plasma triglyceride levels in fasted mice, and taspoglutide therapy in ApoE(-/-) mice modulated the expression of hepatic genes controlling fatty acid uptake and oxidation. We were unable to detect expression of the entire Glp1r coding sequence in macrophages isolated from ApoE(-/-), C57BL/6, and IL10(-/-) mice. Similarly, Glp1r mRNA transcripts were not detected in RNA from isolated murine hepatocytes. Using Western blotting and tissue extracts from Glp1r(+/+) and Glp1r(-/-) mice, and cells transfected with a tagged murine GLP-1R cDNA, we could not validate the sensitivity and specificity of three different GLP-1R antisera commonly used for the detection of GLP-1R protein. Taken together, these findings illustrate divergent actions of GLP-1R agonists on atherosclerosis progression and accumulation of ectopic lipid in ApoE(-/-) mice and highlight the importance of indirect GLP-1R actions for the control of hepatic lipid accumulation.

Aims/hypothesis Glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors attenuate postprandial lipaemia through mechanisms that remain unclear. As dyslipidaemia is a contributing risk factor for cardiovascular disease in type 2 diabetes, we examined the mechanisms linking pharmacological and physiological regulation of GLP-1 action to control of postprandial lipid metabolism.Methods Postprandial lipid synthesis and secretion were assessed in normal and fructose-fed hamsters and in wildtype mice that were treated with or without sitagliptin. Apolipoprotein B-48 (ApoB-48) synthesis and secretion were also examined in primary enterocyte cultures. The importance of exogenous vs endogenous GLP-1R signalling for regulation of intestinal lipoprotein synthesis and secretion was assessed in mice and hamsters treated with the GLP-1R agonist exendin-4, the GLP-1R antagonist exendin(9-39) and in Glp1r Electronic supplementary material The online version of this article

Glucagon receptor (Gcgr) signaling maintains hepatic glucose production during the fasting state however the importance of the Gcgr for lipid metabolism is unclear. We show here that fasted Gcgr-/- mice exhibit a significant increase in hepatic triglyceride secretion and fasting increases fatty acid oxidation (FAO) in wild type (WT) but not in Gcgr-/- mice. Moreover fasting upregulated the expression of FAO-related hepatic mRNA transcripts in Gcgr+/+ but not in Gcgr-/- mice. Exogenous glucagon administration reduced plasma triglycerides in WT mice, inhibited TG synthesis and secretion, and stimulated FA beta oxidation in Gcgr+/+ hepatocytes. The actions of glucagon on TG synthesis and FAO were abolished in PPARα-/- hepatocytes. These findings demonstrate that the Gcgr receptor is required for control of lipid metabolism during the adaptive metabolic response to fasting

Glucagon is a critical regulator of glucose homeostasis; however, mechanisms regulating glucagon action and α-cell function and number are incompletely understood. To elucidate the role of the hepatic glucagon receptor (Gcgr) in glucagon action, we generated mice with hepatocyte-specific deletion of the glucagon receptor. GcgrHep−/− mice exhibited reductions in fasting blood glucose and improvements in insulin sensitivity and glucose tolerance compared with wild-type controls, similar in magnitude to changes observed in Gcgr−/− mice. Despite preservation of islet Gcgr signaling, GcgrHep−/− mice developed hyperglucagonemia and α-cell hyperplasia. To investigate mechanisms by which signaling through the Gcgr regulates α-cell mass, wild-type islets were transplanted into Gcgr−/− or GcgrHep−/− mice. Wild-type islets beneath the renal capsule of Gcgr−/− or GcgrHep−/− mice exhibited an increased rate of α-cell proliferation and expansion of α-cell area, consistent with changes exhibited by endogenous α-cells in Gcgr−/− and GcgrHep−/− pancreata. These results suggest that a circulating factor generated after disruption of hepatic Gcgr signaling can increase α-cell proliferation independent of direct pancreatic input. Identification of novel factors regulating α-cell proliferation and mass may facilitate the generation and expansion of α-cells for transdifferentiation into β-cells and the treatment of diabetes.

Strategies based on activating GLP-1 receptor (GLP-1R) are intensively developed for the treatment of type 2 diabetes. The exhaustive knowledge of the signaling pathways linked to activated GLP-1R within the ␤-cells is of major importance. In ␤-cells, GLP-1 activates the ERK1/2 cascade by diverse pathways dependent on either G␣ s /cAMP/cAMP-dependent protein kinase (PKA) or ␤-arrestin 1, a scaffold protein. Using pharmacological inhibitors, ␤-arrestin 1 small interfering RNA, and islets isolated from ␤-arrestin 1 knock-out mice, we demonstrate that GLP-1 stimulates ERK1/2 by two temporally distinct pathways. The PKA-dependent pathway mediates rapid and transient ERK1/2 phosphorylation that leads to nuclear translocation of the activated kinases. In contrast, the ␤-arrestin 1-dependent pathway produces a late ERK1/2 activity that is restricted to the ␤-cell cytoplasm. We further observe that GLP-1 phosphorylates the cytoplasmic proapoptotic protein Bad at Ser-112 but not at Ser-155. We find that the ␤-arrestin 1-dependent ERK1/2 activation engaged by GLP-1 mediates the Ser-112 phosphorylation of Bad, through p90RSK activation, allowing the association of Bad with the scaffold protein 14-3-3, leading to its inactivation. ␤-Arrestin 1 is further found to mediate the antiapoptotic effect of GLP-1 in ␤-cells through the ERK1/2-p90RSK-phosphorylation of Bad. This new regulatory mechanism engaged by activated GLP-1R involving a ␤-arrestin 1-dependent spatiotemporal regulation of the ERK1/2-p90RSK activity is now suspected to participate in the protection of ␤-cells against apoptosis. Such signaling mechanism may serve as a prototype to generate new therapeutic GLP-1R ligands.GLP-1 (glucagon-like peptide-1), produced by post-translational processing of the proglucagon in enteroendocrine L-cells, is a potent gluco-regulatory peptide hormone. GLP-1 is released into the blood stream in response to nutrient ingestion, such as carbohydrates, amino acids, and fats, during the early postprandial period (1, 2). A major target for GLP-1 actions is the pancreatic ␤-cell. One of the main physiological roles of this endocrine hormone is to enhance insulin secretion in a glucose-dependent manner (1-5). Besides its insulinotropic action, GLP-1 also favors the maintenance of a correct ␤-cell glucose sensing, regulates transcriptional synthesis, induces ␤-cell proliferation, and is protective against apoptosis (6 -9). Strategies based on activating GLP-1 receptor 3 are intensively developed for the treatment of type 2 diabetes and studies aiming at a better and exhaustive understanding of GLP-1 actions within the ␤-cells are of great importance (1-5).GLP-1 exerts its intracellular effects through binding to its specific receptor that spans the plasma membrane. The GLP-1R belongs to the class II (or B) secretin/glucagon/vasoactive intestinal peptide superfamily of heptahelical transmembrane G protein-coupled receptors (GPCRs) (10, 11). The GLP-1R is positively coupled to adenylate cyclase, through G␣ s -containing heterotrimeric G-prot...

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Glucagon-like peptide-1 (GLP-1) secreted from enteroendocrine L cells promotes nutrient disposal via the incretin effect. However, the majority of L cells are localized to the distal gut, suggesting additional biological roles for GLP-1. Here, we demonstrate that GLP-1 receptor (GLP-1R) signaling controls mucosal expansion of the small bowel (SB) and colon. These actions did not require the epidermal growth factor (EGF) or intestinal epithelial insulin-like growth factor (IGF1) receptors but were absent in Glp1r(-/-) mice. Polyp number and size were increased in SB of exendin-4-treated Apc(Min/+) mice, whereas polyp number was reduced in SB and colon of Glp1r(-/-):Apc(Min/+) mice. Exendin-4 increased fibroblast growth factor 7 (Fgf7) expression in colonic polyps of Apc(Min/+) mice and failed to increase intestinal growth in mice lacking Fgf7. Exogenous exendin-4 and Fgf7 regulated an overlapping set of genes important for intestinal growth. Thus, gain and loss of GLP-1R signaling regulates gut growth and intestinal tumorigenesis.

The p44/p42 MAPKs (ERK1/2) cascade regulates beta-cell nuclear events, which modulates cell differentiation and gene transcription, whereas its implication in processes occurring in the cytoplasm, such as activation of the exocytotic machinery, is still unclear. Using the MIN6 beta-cell line and isolated rat islets of Langerhans, we investigated whether glucose, by activating the ERK1/2 cascade, induces phosphorylation of cytoplasmic proteins implicated in exocytosis of insulin granules such as synapsin I. We observed that the majority of ERK1/2 activity induced by glucose remains in the cytoplasm and physically interacts with synapsin I, allowing phosphorylation of the substrate. Therefore, we reexamined the potential requirement of ERK1/2 for insulin secretion. Blocking activation of ERK1/2 using MEK1/2, the MAPK kinase inhibitor PD98059 or using small interfering RNA-mediated silencing of ERK1 and ERK2 expressions resulted in partial inhibition of glucose-induced insulin release, indicating that ERK1/2 pathway participates also in the regulation of insulin secretion. Moreover, using the pancreatic islet perifusion model, we found that the ERK1/2 activity participates in the first and second phases of insulin release induced by glucose. Taken together, our results demonstrate new aspects of the glucose-dependent actions of ERK1/2 in beta-cells exerted on cytoplasmic proteins, including synapsin I, and participating in the overall glucose-induced insulin secretion.