Circulation and degradation of GIP and GLP-1.

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted from the intestinal K- and L-cells, respectively, but are immediately subject to rapid degradation. GLP-1 is found in two active forms, amidated GLP-1 (7-36) amide and glycine-extended GLP-1 (7-37), while GIP exists as a single 42 amino acid peptide. The aminopeptidase, dipeptidyl peptidase IV (DPP IV), which is found in the endothelium of the local capillary bed within the intestinal wall, is important for the initial inactivation of both peptides, with GLP-1 being particularly readily degraded. DPP IV cleavage generates N-terminally truncated metabolites (GLP-1 (9-36) amide / (9-37) and GIP (3-42)), which are the major circulating forms. Subsequently, the peptides may be degraded by other enzymes and extracted in an organ-specific manner. However, other endogenous metabolites have not yet been identified, possibly because existing assays are unable either to recognize them or to differentiate them from the primary metabolites. Neutral endopeptidase 24.11 has been demonstrated to be able to degrade GLP-1 in vivo, but its relevance in GIP metabolism has not yet been established. Intact GLP-1 and GIP are inactivated during passage across the hepatic bed by DPP IV associated with the hepatocytes, and further degraded by the peripheral tissues, while the kidney is important for the final elimination of the metabolites.     

GLP-1 and extra-islet effects.

The main target of action of glucagon-like peptide-1 (GLP-1) is the islet, where the hormone stimulates insulin secretion, promotes beta cell proliferation and neogenesis, and inhibits glucagon secretion. However, GLP-1 receptors are also expressed outside the islets, increasing the likelihood that GLP-1 also plays a role in other organs. These functions are mainly the inhibition of gastric emptying, gastric acid secretion and exocrine pancreatic secretion, indicating that the hormone acts as an enterogastrone--a hormone released from the distal portion of the small intestine that inhibits proximal gastrointestinal events. Another important action of GLP-1 is to induce satiety. Other effects of the hormone include cardioprotection, neuroprotection, induction of learning and memory, stimulation of afferent, sensory nerves, stimulation of surfactant production in the lung, dilatation of pulmonary vessels, induction of diuresis, and also under some conditions, induction of antidiabetic actions unrelated to islet function. Thus, GLP-1 clearly has several manifestations of activity. The physiological relevance of these actions and their contribution to the overall antidiabetic action of GLP-1 when used in treatment of type 2 diabetes remains to be established.     

DPP-4 inhibition increases GIP and decreases GLP-1 incretin effects during intravenous glucose tolerance test in Wistar rats

GIP metabolite [GIP (3-42)] and GLP-1 metabolite [GLP-1 (9-36) amide] have been reported to differ with regard to biological actions. Systemic DPP-4 inhibition can therefore reveal different actions of GIP and GLP-1. In catheter wearing Wistar rats, insulinotropic effects of equipotent doses of GIP (2.0 nmol/kg) and GLP-1 (7-36) amide (4.0 nmol/kg) and vehicle were tested in the absence/presence of DPP-4 inhibition. Blood glucose and insulin were frequently sampled. DPP-4 inhibitor was given at -20 min, the incretin at -5 min and the intravenous glucose tolerance test (0.4 g glucose/kg) commenced at 0 min. G-AUC and I-AUC, insulinogenic index and glucose efflux, were calculated from glucose and insulin curves. Systemic DPP-4 inhibition potentiated the acute GIP incretin effects: I-AUC (115±34 vs. 153±39 ng·min/ml), increased the insulinogenic index (0.74±0.24 vs. 0.99±0.26 ng/mmol), and improved glucose efflux (19.8±3.1 vs. 20.5±5.0 min?1). The GLP-1 incretin effects were diminished: I-AUC (124±18 vs. 106±38 ng·min/ml), the insulinogenic index was decreased (0.70±0.18 vs. 0.50±0.19 ng/mmol), and glucose efflux declined (14.9±3.1 vs. 11.1±3.7 min?1). GLP-1 and GIP differ remarkably in their glucoregulatory actions in healthy rats when DPP-4 is inhibited. These previously unrecognized actions of DPP-4 inhibitors could have implications for future use in humans.     

Human duodenal enteroendocrine cells: source of both incretin peptides, GLP-1 and GIP

Among the products of enteroendocrine cells are the incretins glucagon-like peptide-1 (GLP-1, secreted by L cells) and glucose-dependent insulinotropic peptide (GIP, secreted by K cells). These are key modulators of insulin secretion, glucose homeostasis, and gastric emptying. Because of the rapid early rise of GLP-1 in plasma after oral glucose, we wished to definitively establish the absence or presence of L cells, as well as the relative distribution of the incretin cell types in human duodenum. We confirmed the presence of proglucagon and pro-GIP genes, their products, and glucosensory molecules by tissue immunohistochemistry and RT-PCR of laser-captured, single duodenal cells. We also assayed plasma glucose, incretin, and insulin levels in subjects with normal glucose tolerance and type 2 diabetes for 120 min after they ingested 75 g of glucose. Subjects with normal glucose tolerance (n=14) had as many L cells (15+/-1), expressed per 1,000 gut epithelial cells, as K cells (13+/-1), with some containing both hormones (L/K cells, 5+/-1). In type 2 diabetes, the number of L and L/K cells was increased (26+/-2; P<0.001 and 9+/-1; P < 0.001, respectively). Both L and K cells contained glucokinase and glucose transporter-1, -2, and -3. Newly diagnosed type 2 diabetic subjects had increased plasma GLP-1 levels between 20 and 80 min, concurrently with rising plasma insulin levels. Significant coexpression of the main incretin peptides occurs in human duodenum. L and K cells are present in equal numbers. New onset type 2 diabetes is associated with a shift to the L phenotype.     

Comparison of the subchronic antidiabetic effects of DPP IV-resistant GIP and GLP-1 analogues in obese diabetic (ob/ob) mice.

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are the two key incretin hormones released from the gastrointestinal tract that regulate blood glucose homeostasis through potent insulin secretion. The rapid degradation of GIP and GLP-1 by the ubiquitous enzyme dipeptidyl peptidase IV (DPP IV) renders both peptides noninsulinotropic. However, DPP IV stable agonists, such as N-AcGIP and (Val8)GLP-1, have now been developed. The present study has examined and compared the metabolic effects of subchronic administration of daily i.p. injections of N-AcGIP, (Val8) GLP-1 and a combination of both peptides (all at 25 nmol/kg bw) in obese diabetic (ob/ob) mice. Initial in vitro experiments confirmed the potent insulinotropic properties of N-AcGIP and (Val8)GLP-1 in the clonal pancreatic BRIN BD11 cell line. Subchronic administration of N-AcGIP, (Val8)GLP-1 or combined peptide administration had no significant effects on the body weight, food intake and plasma insulin concentrations. However, all treatment groups had significantly (p < 0.05) decreased plasma glucose levels and improved glucose tolerance by day 14. The effectiveness of the peptide groups was similar, and glucose concentrations were substantially reduced following injection of insulin to assess insulin sensitivity compared to control. These results provide evidence for an improvement of glucose homeostasis following treatment with enzyme-resistant GIP and GLP-1 analogues.     

GLP-1 and GIP are colocalized in a subset of endocrine cells in the small intestine

BACKGROUND: The incretin hormones GIP and GLP-1 are thought to be produced in separate endocrine cells located in the proximal and distal ends of the mammalian small intestine, respectively. METHODS AND RESULTS: Using double immunohistochemistry and in situ hybridization, we found that GLP-1 was colocalized with either GIP or PYY in endocrine cells of the porcine, rat, and human small intestines, whereas GIP and PYY were rarely colocalized. Thus, of all the cells staining positively for either GLP-1, GIP, or both, 55-75% were GLP-1 and GIP double-stained in the mid-small intestine. Concentrations of extractable GIP and PYY were highest in the midjejunum [154 (95-167) and 141 (67-158) pmol/g, median and range, respectively], whereas GLP-1 concentrations were highest in the ileum [92 (80-207) pmol/l], but GLP-1, GIP, and PYY immunoreactive cells were found throughout the porcine small intestine. CONCLUSIONS: Our results provide a morphological basis to suggest simultaneous, rather than sequential, secretion of these hormones by postprandial luminal stimulation.     

GIP and GLP-1 as incretin hormones: lessons from single and double incretin receptor knockout mice

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut-derived incretins secreted in response to nutrient ingestion. Both incretins potentiate glucose-dependent insulin secretion and enhance beta-cell mass through regulation of beta-cell proliferation, neogenesis and apoptosis. In contrast, GLP-1, but not GIP, inhibits gastric emptying, glucagon secretion, and food intake. Furthermore, human subjects with Type 2 diabetes exhibit relative resistance to the actions of GIP, but not GLP-1R agonists. The physiological importance of both incretins has been investigated through generation and analysis of incretin receptor knockout mice. Elimination of incretin receptor action in GIPR-/- or GLP-1R-/- mice produces only modest impairment in glucose homeostasis. Similarly, double incretin receptor knockout (DIRKO) mice exhibit normal body weight and normal levels of plasma glucagon and hypoglycemic responses to exogenous insulin. However, glucose-stimulated insulin secretion is significantly decreased following oral but not intraperitoneal glucose challenge in DIRKO mice and the glucose lowering actions of dipeptidyl peptidase-IV (DPP-IV) inhibitors are extinguished in DIRKO mice. Hence, incretin receptor signaling exerts physiologically relevant actions critical for glucose homeostasis, and represents a pharmacologically attractive target for development of agents for the treatment of Type 2 diabetes.     

Immunocytochemical evidence for a paracrine interaction between GIP and GLP-1-producing cells in canine small intestine

Glucagon-like-peptide 1 (GLP-1) released from the intestine is considered to be an important incretin. We have recently demonstrated that glucose-dependent insulinotropic peptide (GIP) stimulated GLP-1 secretion from canine ileal L cells in culture. To investigate further the interplay between GLP-1- and GIP-secreting cells, we set out to determine the exact location and abundance of both cell types throughout the canine intestine. Canine small intestine was subdivided into 15-20 segments and investigated by immunocytochemistry with computer-assisted imaging. The abundance of GIP-, GLP-1- and somatostatin-immunoreactive cells was determined. GIP-secreting K cells were equally distributed in duodenum and jejunum, with the GLP-1-secreting L cells concentrated in the jejunum (5% duodenum, 73% jejunum and 22% ileum). These results indicated that the middle section of the small intestine containing 69% of the K cells also contained 51% of the L cells. Double immunostaining confirmed this overlap and furthermore over 30% of the L cells in this region were found adjacent to K cells. These results suggest the existence of a paracrine interaction between the K and L cells and indicate the importance of the jejunum in the regulation of insulin release by enteric-derived incretins.     

Effect of GIP, GLP-1, insulin and gastrin on ghrelin release in the isolated rat stomach

Ghrelin release in man depends on the macronutrient composition of the test meal. The mechanisms contributing to the differential regulation are largely unknown. To elucidate their potential role, glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), insulin, gastrin and somatostatin were examined on isolated rat stomach ghrelin secretion, which offers the advantage of avoiding systemic interactions. Basal ghrelin secretion was in a range that did not permit to consistently evaluate inhibiting effects. Therefore, the effect of gastrointestinal hormones and insulin was analyzed during vagal prestimulation. GLP-1(7-36)amide 10(-8) and 10(-7) M decreased ghrelin secretion significantly. In contrast, GIP 10(-8) and 10(-7) M augmented not only prestimulated, but also basal ghrelin secretion (p<0.05). Insulin reduced ghrelin at 10(-10), 10(-8) and 10(-6) M (p<0.05). Both gastrin 10(-8) M and somatostatin 10(-6) M also significantly inhibited ghrelin secretion. These data demonstrate that GLP-1(7-36)amide, insulin, gastrin and somatostatin are potential candidates to contribute to the postprandially observed inhibition of ghrelin secretion with insulin being the most effective inhibitor in this isolated stomach model. GIP, on the other hand, could attenuate the postprandial decrease. Because protein-rich meals do not effectively stimulate GIP release, other as yet unknown intestinal factors must be responsible for protein-induced stimulation of ghrelin release.     

The separate and combined impact of the intestinal hormones, GIP, GLP-1, and GLP-2, on glucagon secretion in type 2 diabetes

Type 2 diabetes mellitus (T2DM) is associated with reduced suppression of glucagon during oral glucose tolerance test (OGTT), whereas isoglycemic intravenous glucose infusion (IIGI) results in normal glucagon suppression in these patients. We examined the role of the intestinal hormones glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon-like peptide-2 (GLP-2) in this discrepancy. Glucagon responses were measured during a 3-h 50-g OGTT (day A) and an IIGI (day B) in 10 patients with T2DM [age (mean ± SE), 51 ± 3 yr; body mass index, 33 ± 2 kg/m(2); HbA(1c), 6.5 ± 0.2%]. During four additional IIGIs, GIP (day C), GLP-1 (day D), GLP-2 (day E) and a combination of the three (day F) were infused intravenously. Isoglycemia during all six study days was obtained. As expected, no suppression of glucagon occurred during the initial phase of the OGTT, whereas significantly (P < 0.05) lower plasma levels of glucagon during the first 30 min of the IIGI (day B) were observed. The glucagon response during the IIGI + GIP + GLP-1 + GLP-2 infusion (day F) equaled the inappropriate glucagon response to OGTT (P = not significant). The separate GIP infusion (day C) elicited significant hypersecretion of glucagon, whereas GLP-1 infusion (day D) resulted in enhancement of glucagon suppression during IIGI. IIGI + GLP-2 infusion (day E) resulted in a glucagon response in the midrange between the glucagon responses to OGTT and IIGI. Our results indicate that the intestinal hormones, GIP, GLP-1, and GLP-2, may play a role in the inappropriate glucagon response to orally ingested glucose in T2DM with, especially, GIP, acting to increase glucagon secretion.     

Increased postprandial responses of GLP-1 and GIP in patients with chronic pancreatitis and steatorrhea following pancreatic enzyme substitution

We aimed to investigate how assimilation of nutrients affects the postprandial responses of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) and to evaluate the effect of pancreatic enzyme substitution (PES) on insulin secretion in patients with chronic pancreatitis (CP) and pancreatic exocrine insufficiency (PEI). Eight male patients with CP and PEI were studied. Blood was sampled frequently on two separate days after ingestion of a liquid meal with and without PES, respectively. Eight healthy male subjects served as a control group. beta-Cell responsiveness was estimated as changes in insulin secretion rates in response to changes in postprandial plasma glucose (PG). There was no difference in the PG incremental area under curve (AUC) for patients with and without PES [406 +/- 100 vs. 425 +/- 80 mM.4 h (mean +/- SE), P = 0.8]. The response of total GLP-1 was higher after PES (AUC: 7.8 +/- 1.2 vs. 5.3 +/- 0.6 nM.4 h, P = 0.01), as was the response of total GIP (AUC: 32.7 +/- 7.5 vs. 21.1 +/- 8.3 nM.4 h, P = 0.01). Concurrently, both plasma insulin, plasma C-peptide, and total insulin secretion increased after PES (AUC: 17.7 +/- 4.2 vs. 13.6 +/- 2.9 nM.4 h, P = 0.02; 237 +/- 31.4 vs. 200 +/- 27.4 nM.4 h, P = 0.005; and 595 +/- 82 vs. 497 +/- 80 pmol.kg(-1).4 h, P = 0.01, respectively). beta-Cell responsiveness to glucose was not significantly different on the two study days for patients with CP. These results suggest that the secretion of GLP-1 and GIP is under influence of the digestion and absorption of nutrients in the small intestine and that PES increases insulin secretion.     

Antiapoptotic effects of GLP-1 in murine HL-1 cardiomyocytes

Activation of apoptosis contributes to cardiomyocyte dysfunction and death in diabetic cardiomyopathy. The peptide glucagon-like peptide-1 (GLP-1), a hormone that is the basis of emerging therapy for type 2 diabetic patients, has cytoprotective actions in different cellular models. We investigated whether GLP-1 inhibits apoptosis in HL-1 cardiomyocytes stimulated with staurosporine, palmitate, and ceramide. Studies were performed in HL-1 cardiomyocytes. Apoptosis was induced by incubating HL-1 cells with staurosporine (175 nM), palmitate (135 μM), or ceramide (15 μM) for 24 h. In staurosporine-stimulated HL-1 cardiomyocytes, phosphatidylserine exposure, Bax-to-Bcl-2 ratio, Bad phosphorylation (Ser(136)), BNIP3 expression, mitochondrial membrane depolarization, cytochrome c release, caspase-3 activation, DNA fragmentation, and mammalian target of rapamycin (mTOR)/p70S6K phosphorylation (Ser(2448) and Thr(389), respectively) were assessed. Apoptotic hallmarks were also measured in the absence or presence of low (5 mM) and high (10 mM) concentrations of glucose. In addition, phosphatidylserine exposure and DNA fragmentation were analyzed in palmitate- and ceramide-stimulated cells. Staurosporine increased apoptosis in HL-1 cardiomyocytes. GLP-1 (100 nM) partially inhibited staurosporine-induced mitochondrial membrane depolarization and completely blocked the rest of the staurosporine-induced apoptotic changes. This cytoprotective effect was mainly mediated by phosphatidylinositol 3-kinase (PI3K) and partially dependent on ERK1/2. Increasing concentrations of glucose did not influence GLP-1-induced protection against staurosporine. Furthermore, GLP-1 inhibited palmitate- and ceramide-induced phosphatidylserine exposure and DNA fragmentation. Incretin GLP-1 protects HL-1 cardiomyocytes against activation of apoptosis. This cytoprotective ability is mediated mainly by the PI3K pathway and partially by the ERK1/2 pathway and seems to be glucose independent. It is proposed that therapies based on GLP-1 may contribute to prevent cardiomyocyte apoptosis.     

Differential effects of oxyntomodulin and GLP-1 on glucose metabolism

Glucagon-like peptide-1 (GLP-1) and oxyntomodulin (OXM) are peptide hormones secreted postprandially from the gut that stimulate insulin secretion in a glucose-dependent manner. OXM activates both the GLP-1 receptor (GLP1R) and the glucagon receptor (GCGR). It has been suggested that OXM acutely modulates glucose metabolism solely through GLP1R agonism. Because OXM activates the GLP1R with lower affinity than GLP-1, we generated a peptide analog (Q→E, OXMQ3E) that does not exhibit glucagon receptor agonist activity but retains the same affinity as OXM for GLP1R. We compared the effects of OXM and OXMQ3E in a glucose tolerance test and, to better characterize the effect on glucose metabolism, we performed controlled infusions of OXM or OXMQ3E during a hyperglycemic clamp performed in wild-type, Glp1r(-/-), and Gcgr(-/-) mice. Our findings show that OXM, but not OXMQ3E, activates the GCGR in vivo. Second, OXM and OXMQ3E improve glucose tolerance following an acute glucose challenge and during a hyperglycemic clamp in mice. Finally, OXM infusion during a glucose clamp reduces the glucose infusion rate (GIR) despite a simultaneous increase in insulin levels in Glp1r(-/-) mice, whereas OXM and OXMQ3E increase GIR to a similar extent in Gcgr(-/-) mice. In conclusion, activation of the GCGR seems to partially attenuate the acute beneficial effects on glucose and contributes to the insulinotropic action of oxyntomodulin.     

Insulin secretion-independent effects of GLP-1 on canine liver glucose metabolism do not involve portal vein GLP-1 receptors

Whether glucagon-like peptide (GLP)-1 requires the hepatic portal vein to elicit its insulin secretion-independent effects on glucose disposal in vivo was assessed in conscious dogs using tracer and arteriovenous difference techniques. In study 1, six conscious overnight-fasted dogs underwent oral glucose tolerance testing (OGTT) to determine target GLP-1 concentrations during clamp studies. Peak arterial and portal values during OGTT ranged from 23 to 65 pM and from 46 to 113 pM, respectively. In study 2, we conducted hyperinsulinemic-hyperglycemic clamp experiments consisting of three periods (P1, P2, and P3) during which somatostatin, glucagon, insulin and glucose were infused. The control group received saline, the PePe group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally, the PePo group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then intraportally (P3), and the PeHa group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then through the hepatic artery (P3) to increase the hepatic GLP-1 load to the same extent as in P3 in the PePo group (n = 8 dogs/group). Arterial GLP-1 levels increased similarly in all groups during P2 ( approximately 50 pM), whereas portal GLP-1 levels were significantly increased (2-fold) in the PePo vs. PePe and PeHa groups during P3. During P2, net hepatic glucose uptake (NHGU) increased slightly but not significantly (vs. P1) in all groups. During P3, GLP-1 increased NHGU in the PePo and PeHa groups more than in the control and PePe groups (change of 10.8 +/- 1.3 and 10.6 +/- 1.0 vs. 5.7 +/- 1.0 and 5.4 +/- 0.8 micromol.kg(-1).min(-1), respectively, P < 0.05). In conclusion, physiological GLP-1 levels increase glucose disposal in the liver, and this effect does not involve GLP-1 receptors located in the portal vein.     

Duodenal-jejunal bypass protects GK rats from {beta}-cell loss and aggravation of hyperglycemia and increases enteroendocrine cells coexpressing GIP and GLP-1

Dramatic improvement of type 2 diabetes is commonly observed after bariatric surgery. However, the mechanisms behind the alterations in glucose homeostasis are still elusive. We examined the effect of duodenal-jejunal bypass (DJB), which maintains the gastric volume intact while bypassing the entire duodenum and the proximal jejunum, on glycemic control, β-cell mass, islet morphology, and changes in enteroendocrine cell populations in nonobese diabetic Goto-Kakizaki (GK) rats and nondiabetic control Wistar rats. We performed DJB or sham surgery in GK and Wistar rats. Blood glucose levels and glucose tolerance were monitored, and the plasma insulin, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP) levels were measured. β-Cell area, islet fibrosis, intestinal morphology, and the density of enteroendocrine cells expressing GLP-1 and/or GIP were quantified. Improved postprandial glycemia was observed from 3 mo after DJB in diabetic GK rats, persisting until 12 mo after surgery. Compared with the sham-GK rats, the DJB-GK rats had an increased β-cell area and a decreased islet fibrosis, increased insulin secretion with increased GLP-1 secretion in response to a mixed meal, and an increased population of cells coexpressing GIP and GLP-1 in the jejunum anastomosed to the stomach. In contrast, DJB impaired glucose tolerance in nondiabetic Wistar rats. In conclusion, although DJB worsens glucose homeostasis in normal nondiabetic Wistar rats, it can prevent long-term aggravation of glucose homeostasis in diabetic GK rats in association with changes in intestinal enteroendocrine cell populations, increased GLP-1 production, and reduced β-cell deterioration.     

Comparative effects of GLP-1 and GIP on cAMP production, insulin secretion, and in vivo antidiabetic actions following substitution of Ala8/Ala2 with 2-aminobutyric acid

The two major incretin hormones, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP), are currently being considered as prospective drug candidates for treatment of type 2 diabetes. Interest in these gut hormones was initially spurred by their potent insulinotropic activities, but a number of other antihyperglycaemic actions are now established. One of the foremost barriers in progressing GLP-1 and GIP to the clinic concerns their rapid degradation and inactivation by the ubiquitous enzyme, dipeptidyl peptidase IV (DPP IV). Here, we compare the DPP IV resistance and biological properties of Abu8/Abu2 (2-aminobutyric acid) substituted analogues of GLP-1 and GIP engineered to impart DPP IV resistance. Whereas (Abu8)GLP-1 was completely stable to human plasma (half-life >12 h), GLP-1, GIP, and (Abu2)GIP were rapidly degraded (half-lives: 6.2, 6.0, and 7.1 h, respectively). Native GIP, GLP-1, and particularly (Abu8)GLP-1 elicited significant adenylate cyclase and insulinotropic activity, while (Abu2)GIP was less effective. Similarly, in obese diabetic (ob/ob) mice, GIP, GLP-1, and (Abu8)GLP-1 displayed substantial glucose-lowering and insulin-releasing activities, whereas (Abu2)GIP was only weakly active. These studies illustrate divergent effects of penultimate amino acid Ala8/Ala2 substitution with Abu on the biological properties of GLP-1 and GIP, suggesting that (Abu8)GLP-1 represents a potential candidate for future therapeutic development.     

Protective effects of GLP-1 analogues exendin-4 and GLP-1(9-36) amide against ischemia-reperfusion injury in rat heart

Glucagon-Like Peptide-1 (GLP-1) is an incretin peptide secreted from intestinal L-cells, whose potent plasma glucose-lowering action has prompted intense efforts to develop GLP-1 receptor-targeting drugs for treatment of diabetic hyperglycemia. More recently, GLP-1 and its analogues have been shown to exert cardiovascular effects in a number of experimental models. Here we tested exendin-4 (Exe-4), a peptide agonist at GLP-1 receptors, and GLP-1(9-36) amide, the primary endogenous metabolite of GLP-1 (both in the concentration range 0.03-3.0 nM), for their protective effects against ischemia-reperfusion injury (IRI) in an isolated rat heart preparation. When administered, the agents were only present for the first 15 min of a 120 min reperfusion period (postconditioning protocol). Exe-4, but not GLP-1(9-36) amide, showed a strong infarct-limiting action (from 33.2% +/-2.7% to 14.5% +/-2.2% of the ischemic area, p<0.05). This infarct size-limiting effect of Exe-4 was abolished by exendin(9-39) (Exe(9-39)), a GLP-1 receptor antagonist. In contrast, both Exe-4 and GLP-1(9-36) amide were able to augment left ventricular performance (left ventricular developed pressure and rate-pressure product) during the last 60 min of reperfusion. These effects were only partially antagonized by Exe(9-39). We suggest that Exe-4, in addition to being currently exploited in treatment of diabetes, may present a suitable candidate for postconditioning trials in clinical settings of IRI. The divergent agonist effects of Exe-4 and GLP-1(9-36), along with correspondingly divergent antagonistic efficacy of Exe(9-39), seem consistent with the presence of more than one type of GLP-1 receptor in this system.     

Regulation of adipocyte formation by GLP-1/GLP-1R signaling

Increased nutrient intake leads to excessive adipose tissue accumulation, obesity, and the development of associated metabolic disorders. How the intestine signals to adipose tissue to adapt to increased nutrient intake, however, is still not completely understood. We show here, that the gut peptide GLP-1 or its long-lasting analog liraglutide, function as intestinally derived signals to induce adipocyte formation, both in vitro and in vivo. GLP-1 and liraglutide activate the GLP-1R, thereby promoting pre-adipocyte proliferation and inhibition of apoptosis. This is achieved at least partly through activation of ERK, PKC, and AKT signaling pathways. In contrast, loss of GLP-1R expression causes reduction in adipogenesis, through induction of apoptosis in pre-adipocytes, by inhibition of the above mentioned pathways. Because GLP-1 and liraglutide are used for the treatment of type 2 diabetes, these findings implicate GLP-1 as a regulator of adipogenesis, which could be an alternate pathway leading to improved lipid homeostasis and controlled downstream insulin signaling.