Glucose‐dependent insulinotropic polypeptide and glucagon‐like peptide‐1: Incretin actions beyond the pancreas

Glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are the two primary incretin hormones secreted from the intestine on ingestion of various nutrients to stimulate insulin secretion from pancreatic β‐cells glucose‐dependently. GIP and GLP‐1 undergo degradation by dipeptidyl peptidase‐4 (DPP‐4), and rapidly lose their biological activities. The actions of GIP and GLP‐1 are mediated by their specific receptors, the GIP receptor (GIPR) and the GLP‐1 receptor (GLP‐1R), which are expressed in pancreatic β‐cells, as well as in various tissues and organs. A series of investigations using mice lacking GIPR and/or GLP‐1R, as well as mice lacking DPP‐4, showed involvement of GIP and GLP‐1 in divergent biological activities, some of which could have implications for preventing diabetes‐related microvascular complications (e.g., retinopathy, nephropathy and neuropathy) and macrovascular complications (e.g., coronary artery disease, peripheral artery disease and cerebrovascular disease), as well as diabetes‐related comorbidity (e.g., obesity, non‐alcoholic fatty liver disease, bone fracture and cognitive dysfunction). Furthermore, recent studies using incretin‐based drugs, such as GLP‐1 receptor agonists, which stably activate GLP‐1R signaling, and DPP‐4 inhibitors, which enhance both GLP‐1R and GIPR signaling, showed that GLP‐1 and GIP exert effects possibly linked to prevention or treatment of diabetes‐related complications and comorbidities independently of hyperglycemia. We review recent findings on the extrapancreatic effects of GIP and GLP‐1 on the heart, brain, kidney, eye and nerves, as well as in the liver, fat and several organs from the perspective of diabetes‐related complications and comorbidities.     

Augmentation of glucagon‐like peptide‐1 receptor signalling by neprilysin inhibition: potential implications for patients with heart failure

Augmentation of glucagon‐like peptide‐1 (GLP‐1) receptor signalling is an established approach to the treatment of type 2 diabetes. However, endogenous GLP‐1 and long‐acting GLP‐1 receptor analogues are degraded not only by dipeptidyl peptidase‐4, but also by neprilysin. This observation raises the possibilities that endogenous GLP‐1 contributes to the clinical effects of neprilysin inhibition and that patients concurrently treated with sacubitril/valsartan and incretin‐based drugs may experience important drug–drug interactions. Specifically, potentiation of GLP‐1 receptor signalling may underlie the antihyperglycaemic actions of sacubitril/valsartan. Neprilysin inhibitors may also be able to augment the effects of long‐acting GLP‐1 analogues to increase heart rate and myocardial cyclic AMP, and thus, potentiate these deleterious actions; if so, concomitant treatment with GLP‐1 receptor agonists may limit the efficacy of neprilysin inhibitors in patients with both heart failure and diabetes. For patients not concurrently treated with GLP‐1 analogues, the action of neprilysin to enhance the effects of GLP‐1 may be particularly relevant in the brain, where augmentation of GLP‐1 and other endogenous peptides may act to inhibit amyloid‐induced neuroinflammation and cytotoxicity and improve memory formation and executive functioning. Experimentally, neprilysin inhibitors may also potentiate the effects of endogenous GLP‐1 and GLP‐1 receptor agonists on blood vessels and the kidney. The role of neprilysin in the metabolism of endogenous GLP‐1 and long‐acting GLP‐1 analogues points to a range of potential pathophysiological effects that may be clinically relevant to patients with heart failure, with or without diabetes.     

Enteral supplementation with glutamine,fiber, and oligosaccharide modulates incretin and glucagon‐like peptide‐2 secretion

Aims/Introduction

A dietary supplementation product enriched with glutamine, dietary fiber and oligosaccharide (GFO) is widely applied for enteral nutrition support in Japan. The aim of the present study was to evaluate the effects of GFO ingestion on secretion of incretins, gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and glucagon-like peptide-2 (GLP-2).

Materials and Methods

We carried out a cross-over study involving 20 healthy Japanese volunteers. The participants received GFO or 17 g of glucose, the equivalent carbohydrate in GFO as the control. Plasma glucose, serum insulin, and plasma total GIP, total GLP-1 and total GLP-2 levels during GFO or glucose loading were determined.

Results

GFO loading produced significantly higher plasma GLP-1 levels at 30 min and 60 min, area under the curve-GLP-1 value, and area under the curve-GLP-2 value after administration compared with those by glucose loading. In contrast, plasma GIP levels at both 30 and 60 min, and area under the curve-GIP value after glucose loading were significantly higher than those after GFO loading.

Conclusions

These results show that GFO ingestion stimulates GLP-1 and GLP-2 secretion, and reduces GIP secretion compared with glucose ingestion. Therefore, GFO could have an intestinotrophic effect as well as an ameliorating effect on metabolic disorders through modification of release of gut hormones.     

The effect of glucagon‐like peptide‐1 and glucagon‐like peptide‐2 on microcirculation: A systematic review

GLP‐1 and GLP‐2 are gut‐derived hormones used in the treatment of diabetes type‐2 and short bowel syndrome, respectively. GLP‐1 attenuates insulin resistance and GLP‐2 reduces enterocyte apoptosis and enhances crypt cell proliferation in the small intestine. In addition, both hormones have vasoactive effects and may be useful in situations with impaired microcirculation. The aim of this systematic review was to provide an overview of the potential effects of GLP‐1 and GLP‐2 on microcirculation. A systematic search was performed independently by two authors in the following databases: PubMed, EMBASE, Cochrane library, Scopus, and Web of Science. Of 1111 screened papers, 20 studies were included in this review: 16 studies in animals, three in humans, and one in humans and rats. The studies were few and heterogeneous and had a high risk of bias. However, it seems that GLP‐1 regulates the pancreatic, skeletal, and cardiac muscle flow, indicating a role in the glucose homeostasis, while GLP‐2 acts primarily in the regulation of the microcirculation of the mid‐intestine. These findings may be useful in gastrointestinal surgery and in situations with impaired microcirculation of the gut.     

Ingestion of a moderate high‐sucrose diet results in glucose intolerance with reduced liver glucokinase activity and impaired glucagon‐like peptide‐1 secretion

Aims/Introduction: Excessive intake of sucrose can cause severe health issues, such as diabetes mellitus. In animal studies, consumption of a high‐sucrose diet (SUC) has been shown to cause obesity, insulin resistance and glucose intolerance. However, several in vivo experiments have been carried out using diets with much higher sucrose contents (50–70% of the total calories) than are typically ingested by humans. In the present study, we examined the effects of a moderate SUC on glucose metabolism and the underlying mechanism. Materials and Methods: C57BL/6J mice received a SUC (38.5% sucrose), a high‐starch diet (ST) or a control diet for 5 weeks. We assessed glucose tolerance, incretin secretion and liver glucose metabolism. Results: An oral glucose tolerance test (OGTT) showed that plasma glucose levels in the early phase were significantly higher in SUC‐fed mice than in ST‐fed or control mice, with no change in plasma insulin levels at any stage. SUC‐fed mice showed a significant improvement in insulin sensitivity. Glucagon‐like peptide‐1 (GLP‐1) secretion 15 min after oral glucose administration was significantly lower in SUC‐fed mice than in ST‐fed or control mice. Hepatic glucokinase (GCK) activity was significantly reduced in SUC‐fed mice. During the OGTT, the accumulation of glycogen in the liver was suppressed in SUC‐fed mice in a time‐dependent manner. Conclusions: These results indicate that mice that consume a moderate SUC show glucose intolerance with a reduction in hepatic GCK activity and impairment in GLP‐1 secretion. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00208.x , 2012)     

Response of truncated glucagon-like peptide-1 and gastric inhibitory polypeptide to glucose ingestion in non-insulin dependent diabetes mellitus

Gastric inhibitory polypeptide (GIP) and truncated glucagon like peptide-1 (tGLP-1) are potent gastrointestinal insulinotropic factors (incretin), mostly released after a meal or ingestion of glucose in man and animals. To investigate whether sulfonylurea (SU) affects the secretion of incretin, the modulation of plasma GIP and tGLP-1 levels following glucose ingestion in non-insulin-dependent diabetic type 2 patients with or without SU therapy was studied. A 75-g oral glucose tolerance test (OGTT) was carried out on 9 healthy subjects (controls) and 18 patients with non-obese type 2, 9 of whom were treated by diet alone (NIDDM-diet) and the other 9 with SU (glibenclamide 2.5 mg or gliclazide 40 mg) once a day (NIDDM-SU). Plasma GIP was measured by radioimmunoassay (RIA) with R65 antibody, and GLP-1 was measured by RIA with N-terminal-directed antiserum R1043 (GLP-1NT) and C-terminal-directed antiserum R2337 (GLP-1CT). Following OGTT, plasma glucose, GIP, GLP-1NT, and GLP-1CT in type 2 patients increased more markedly than in controls, despite the lower response of insulin. However, there were no significant differences in plasma levels of these peptides between the NIDDM-diet and NIDDM-SU groups. Therefore, it is unlikely that SU is involved in the high response of GIP and GLP-1s to OGTT in type 2 patients.     

Weekly glucagon‐like peptide‐1 receptor agonist albiglutide as monotherapy improves glycemic parameters in Japanese patients with type 2 diabetes mellitus: A randomized,double‐blind,placebo‐controlled study

Aims/Introduction

The present phase 3, randomized, double‐blind 24‐week study with extension to 1 year assessed the efficacy and safety of albiglutide compared with placebo in Japanese patients with type 2 diabetes mellitus inadequately controlled by diet and exercise with or without a single oral antidiabetic drug.

Materials and Methods

Patients received weekly albiglutide 30 mg (n = 160), albiglutide 50 mg (n = 150) or a placebo switched to albiglutide 30 mg after 24 weeks (n = 77). Open‐label daily liraglutide 0.9 mg (n = 103) was included as a reference. Oral antidiabetic drug use was discontinued before baseline. The primary end‐point was 24‐week change from baseline in glycated hemoglobin (HbA1c). Secondary end‐points included fasting plasma glucose, bodyweight and adverse events.

Results

At 24 weeks, mean HbA1c changes from baseline were ?1.10, ?1.30, and 0.25% for albiglutide 30, 50 mg and placebo, respectively (P vs placebo <0.0001 for both albiglutide doses), ?1.19% for liraglutide. Decreases in HbA1c with albiglutide were sustained through the study. Mean fasting plasma glucose decreased by ≥20 mg/dL, and the mean change in bodyweight was ≤0.5 kg through 1 year across groups. Most commonly reported adverse events were nasopharyngitis, constipation and nausea. The incidence of adverse events was higher in active treatment groups than in the placebo group. Few hypoglycemia events were reported; no patient withdrew as a result of hypoglycemia. No new safety signals were detected.

Conclusions

Albiglutide monotherapy achieved clinically significant decreases in HbA1c and fasting plasma glucose with good tolerability in Japanese patients with type 2 diabetes mellitus inadequately controlled by diet and exercise with or without a single oral antidiabetic drug.

     

Glucagon‐like peptide‐1 analog liraglutide in combination with sulfonylurea safely improves blood glucose measures vs sulfonylurea monotherapy in Japanese patients with type 2 diabetes: Results of a 52‐week,randomized, multicenter trial

Aims/Introduction:  Sulfonylurea (SU) agents are the most effective drugs at lowering blood glucose when used alone. However, their effectiveness declines after a certain period. The addition of liraglutide to existing SU therapy might reverse some of the known drawbacks of SU.Materials and Methods:  This multicenter, randomized, 52‐week study assessed the long‐term efficacy and safety of adding liraglutide at 0.6 or 0.9 mg/day to existing SU therapy in Japanese patients with inadequately controlled type 2 diabetes.Results:  In total, 264 patients were enrolled and received treatment. At week 52, HbA_1c in the liraglutide 0.6 mg, liraglutide 0.9 mg and placebo groups was reduced from 9.00 to 7.91%, from 8.61 to 7.33%, and from 8.85 to 8.79%, respectively. The mean difference of HbA_1c (95% CI) in the liraglutide 0.6 and 0.9 mg groups vs the placebo group was 0.96 (−1.25 to −0.67) and −1.33 (−1.62 to −1.04), respectively. For the liraglutide 0.6 mg, 0.9 mg and placebo groups, the Japanese Diabetes Society target HbA_1c of <6.9% was achieved by 15.1, 39.1 and 4.5% of patients, respectively. Mean fasting plasma glucose at week 52 was lower in the liraglutide groups compared with the placebo group, and mean bodyweight remained unchanged in the liraglutide groups. Most subjects in all three treatment groups reported mild adverse events. No major hypoglycemic episode was reported.Conclusions:  Once‐daily administration of liraglutide in combination with SU for 52 weeks provided favorable metabolic control, a safety profile and did not alter bodyweight. This trial was registered with ClinicalTrial.gov (no. {"type":"clinical-trial","attrs":{"text":"NCT00395746","term_id":"NCT00395746"}}NCT00395746). (J Diabetes Invest,doi: 10.1111/j.2040‐1124.2011.00103.x, 2011)     

Glucagon‐like peptide‐1 secretion by direct stimulation of L cells with luminal sugar vs non‐nutritive sweetener

Aims/Introduction: Oral ingestion of carbohydrate triggers secretion of glucagon‐like peptide (GLP)‐1, which inhibits the postprandial rise in blood glucose levels. However, the mechanism of carbohydrate‐induced GLP‐1 secretion from enteroendocrine L cells remains unclear. In the present study, GLP‐1 secretion was examined by meal tolerance tests of healthy Japanese volunteers. Materials and Methods: Twenty‐one healthy Japanese men participated in the study. The meal tolerance test was performed with modified nutrient compositions, with or without pretreatment with the α‐glucosidase inhibitor acarbose, or with substitution of sucrose with an equivalent dose of sweeteners in the meal. Blood concentrations of glucose, insulin, GLP‐1, and apolipoprotein (Apo) B‐48 were measured. Results: GLP‐1 secretion started concomitant with the increase in blood glucose levels 10 min after meal ingestion. Insulin secretion started at 5 min, before the increase in blood glucose levels, reflecting the contribution of direct nutrient stimulation on the former parameter and neural regulation in the latter. Carbohydrate retention in the gut lumen induced by acarbose pretreatment extended postprandial GLP‐1 secretion and negated the increase in serum ApoB‐48 levels. GLP‐1 secretion was markedly decreased by a reduction in the amount of sucrose in the meal and was not restored by an equivalent dose of sweeteners used to compensate for the sweet taste. Conclusions: The results indicate that direct stimulation of L cells with sugar, but not sweetener, is required for carbohydrate‐induced GLP‐1 secretion. In addition, inhibition of digestion of dietary carbohydrate by α‐glucosidase inhibitors may prevent postprandial hyperglycemia by increasing GLP‐1 secretion and by inhibiting glucose absorption. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2011.00163.x, 2011)     

Glucagon‐like peptide‐1 receptor agonists for type 2 diabetes: A rational drug development

Today, glucagon‐like peptide‐1 (GLP‐1) receptor agonists are established glucose‐lowering drugs used in the management of type 2 diabetes. Their development emerged from the understanding that a combined islet dysfunction comprising of impaired insulin secretion and exaggerated glucagon secretion is the key defect of hyperglycemia. GLP‐1 was shown to target these defects, and after the discovery that dipeptidyl peptidase‐4 inactivates native GLP‐1, several different dipeptidyl peptidase‐4‐resistant GLP‐1 receptor agonists have been developed. They are administered subcutaneously, but show differences in molecular structure, molecular size and pharmacokinetics, the latter allowing twice‐daily, once‐daily or once‐weekly administration. They have been shown to be efficient in reducing both glycated hemoglobin and bodyweight, and to be safe and highly tolerable. Cardiovascular outcomes trials have shown them to be neutral or beneficial. GLP‐1 receptor agonists are positioned as add‐ons to metformin alone or in combination with oral agents in the clinical paradigm. They are also efficient when combined with insulin, and fixed dose combinations with long‐acting insulin have been developed. Recent development includes a very long administration schedule and oral availability. The research from the first demonstration of the antidiabetic action of GLP‐1 in the early 1990s to the enormously accumulated data today represents a successful and rational development, which has been characterized by focused perseverance to establish this therapy in the management of type 2 diabetes.     

Safety and efficacy of the combination of the glucagon‐like peptide‐1 receptor agonist liraglutide with an oral antidiabetic drug in Japanese patients with type 2 diabetes: Post‐hoc analysis of a randomized, 52‐week,open‐label,parallel‐group trial

Aims/Introduction

The aim of the present post‐hoc analysis was to investigate the safety and efficacy of liraglutide in combination with one oral antidiabetic drug (OAD) across different OAD classes.

Materials and Methods

This was a post‐hoc analysis using data from a 52‐week, open‐label, parallel‐group trial, in which patients with type 2 diabetes inadequately controlled with a single OAD (α‐glucosidase inhibitor, glinide, metformin or thiazolidinedione) were randomized to either pretrial OAD in combination with liraglutide 0.9 mg/day (liraglutide group) or pretrial OAD in combination with an additional OAD (additional OAD group). The primary outcome investigated in this post‐hoc analysis was the incidence of adverse events.

Results

The proportions of patients experiencing adverse events across the different groups of pretrial OADs were comparable between liraglutide and additional OAD (α‐glucosidase inhibitor 74.6 vs 70.0%; glinide 93.1 vs 87.1%; metformin 91.8 vs 87.1%; thiazolidinedione 86.2 vs 96.4%, respectively). Minor hypoglycemia was infrequent (seven episodes in two patients randomized to liraglutide, and two episodes in two patients randomized to additional OAD). The mean reduction in glycated hemoglobin appeared greater with liraglutide therapy, with the estimated mean treatment difference (95% confidence interval [CI]) for liraglutide vs additional OAD ranging from ?0.14%, 95% CI: ?0.48 to 0.21 (?1.5 mmol/mol, 95 CI: ?5.2 to 2.3) to ?0.44%, 95% CI:?0.79 to ?0.09 (?4.8 mmol/mol, 95% CI: ?8.6 to ?1.0).

Conclusions

The present analysis suggests that Japanese patients on OAD monotherapy might benefit from a greater improvement in glycemic control, without impacting tolerability, by combining their OAD with liraglutide rather than another OAD, regardless of which OAD monotherapy they are receiving.

     

Suppression of glucagon secretion is lower after oral glucose administration than during intravenous glucose administration in human subjects

Aims/hypothesis

The incretin effect describes the augmentation of postprandial insulin secretion by gut hormones. It is not known whether glucagon secretion is also influenced by an incretin effect. A glucagon suppression deficiency has been reported in some patients with type 2 diabetes, but it is unclear whether this abnormality is present prior to diabetes onset. We therefore addressed the questions: (1) Is glucagon secretion different after oral and during intravenous glucose administration? (2) If so, is this related to the secretion of incretin hormones? (3) Is glucagon secretion abnormal in first-degree relatives of patients with type 2 diabetes?

Materials and methods

We examined 16 first-degree relatives of patients with type 2 diabetes and ten matched control subjects with an oral glucose load (75 g) and with an ‘isoglycaemic’ intravenous glucose infusion.

Results

Glucagon levels were significantly suppressed by both oral and intravenous glucose (p?p?r?=?0.60, p?=?0.001) and glucagon-like peptide (GLP)-1 (r?=?0.46, p?

Conclusions/interpretation

Despite the glucagonostatic actions of GLP-1, the suppression of glucagon secretion by glucose is diminished after oral glucose ingestion, possibly due to the glucagonotropic actions of GIP and GLP-2. Furthermore, in this group of first-degree relatives, abnormalities in glucagon secretion did not precede the development of other defects, such as impaired insulin secretion.

     

Protective effect of glucagon‐like peptide‐2 in experimental corrosive esophagitis

Corrosive esophageal injuries are one of the life‐threatening morbidities leading to esophageal stricture and perforation affecting all age groups but especially children due to accidental ingestions in this age group. Glucagon‐like peptide‐2 (GLP‐2) is an intestinal polypeptide with potent anti‐inflammatory effects. Its effects are studied in various studies but not in corrosive esophagitis. We aimed to investigate whether it has protective effect in experimental corrosive esophagitis, in the absence of existing studies into possible links. Twenty‐four Wistar‐albino rats, weighing 220–240 g, were randomized into three groups (n = 8 in each). First group is control, second one is sham operated, and the third one is treatment group. Median laparotomy was made in all groups. In sham and treatment groups, esophagus was loosened and suspended from 1 cm proximal to the esophageal junction. The esophagus segment between suspenders was exposed to 0.1 mL 5% NaOH for 10 seconds. In the treatment group, rats were given GLP‐2 for 7 days intraperitoneally. After 7 days, all rats were sacrified and esophagi were totally removed. In the histopathologic examination, esophageal tissues were compared in terms of inflammation, muscularis mucosa injury, and collagen deposition of tunica muscularis. Histopathologic changes in the esophageal tissues of groups were compared. Histopathologic injury in the GLP‐2 treated group was significantly less than sham group (P < 0.05). There was statistically significant healing in the GLP‐2 treatment group. It is concluded that GLP‐2 has a preventive effect on inflammation and collagen accumulation in an experimental corrosive esophagitis. In the light of the information that initial lesions in the early phase are predictors of complications, GLP‐2 is a promising agent that has an anti‐inflammatory effect in caustic injuries.