Cardioprotective and vasodilatory actions of glucagon-like peptide 1 receptor are mediated through both glucagon-like peptide 1 receptor-dependent and -independent pathways

Clinical review: The extrapancreatic effects of glucagon-like peptide-1 and related peptides

CONTEXT

Glucagon-like peptide-1 (GLP-1) 7-36 amide, an insulinotropic hormone released from the intestinal L cells in response to nutrient ingestion, has been extensively reviewed with respect to beta-cell function. However GLP-1 receptors are abundant in many other tissues. Thus, the function of GLP-1 is not limited to the islet cells, and it has regulatory actions on many other organs.

EVIDENCE ACQUISITION

A review of published, peer-reviewed medical literature (1987 to September 2008) on the extrapancreatic actions of GLP-1 was performed.

EVIDENCE SYNTHESIS

The extrapancreatic actions of GLP-1 include inhibition of gastric emptying and gastric acid secretion, thereby fulfilling the definition of GLP-1 as an enterogastrone. Other important extrapancreatic actions of GLP-1 include a regulatory role in hepatic glucose production, the inhibition of pancreatic exocrine secretion, cardioprotective and cardiotropic effects, the regulation of appetite and satiety, and stimulation of afferent sensory nerves. The primary metabolite of GLP-1, GLP-1 (9-36) amide, or GLP-1m, is the truncated product of degradation by dipeptidyl peptidase-4. GLP-1m has insulinomimetic effects on hepatic glucose production and cardiac function. Exendin-4 present in the salivary gland of the reptile, Gila monster (Heloderma suspectum), is a high-affinity agonist for the mammalian GLP-1 receptor. It is resistant to degradation by dipeptidyl peptidase-4, and therefore has a prolonged half-life.

CONCLUSION

GLP-1 and its metabolite have important extrapancreatic effects particularly with regard to the cardiovascular system and insulinomimetic effects with respect to glucose homeostasis. These effects may be particularly important in the obese state. GLP-1, GLP-1m, and exendin-4 therefore have potential therapeutic roles because of their diffuse extrapancreatic actions.

GLP-1 agonist-based therapies: an emerging new class of antidiabetic drug with potential cardioprotective effects

Cardiovascular disease is a leading cause of death in the United States and across the world, and better therapies are constantly being sought to improve patient outcomes. Recent studies have brought our attention to the mechanisms of glucagon-like peptide 1 (GLP-1). Not only does it demonstrate beneficial effects in regard to cardiovascular risk factors (i.e., diabetes, lipid management, and weight control), but it also has been shown in animal studies to have positive cardiac effects irrespective of its effects on glucose control and weight loss. This review discusses the biology of GLP-1 and its effects on cardiovascular risk factors, and it also elaborates on the positive direct cardiovascular outcomes of GLP-1 in animal studies.

GPR119 is essential for oleoylethanolamide-induced glucagon-like peptide-1 secretion from the intestinal enteroendocrine L-cell

OBJECTIVE

Intestinal L-cells secrete the incretin glucagon-like peptide-1 (GLP-1) in response to ingestion of nutrients, especially long-chain fatty acids. The Galphas-coupled receptor GPR119 binds the long-chain fatty acid derivate oleoylethanolamide (OEA), and GPR119 agonists enhance GLP-1 secretion. We therefore hypothesized that OEA stimulates GLP-1 release through a GPR119-dependent mechanism.

RESEARCH DESIGN AND METHODS

Murine (m) GLUTag, human (h) NCI-H716, and primary fetal rat intestinal L-cell models were used for RT-PCR and for cAMP and GLP-1 radioimmunoassay. Anesthetized rats received intravenous or intraileal OEA, and plasma bioactive GLP-1, insulin, and glucose levels were determined by enzyme-linked immunosorbent assay or glucose analyzer.

RESULTS

GPR119 messenger RNA was detected in all L-cell models. OEA treatment (10 micromol/l) of mGLUTag cells increased cAMP levels (P < 0.05) and GLP-1 secretion (P < 0.001) in all models, with desensitization of the secretory response at higher concentrations. GLP-1 secretion was further enhanced by prevention of OEA degradation using the fatty acid amide hydrolase inhibitor, URB597 (P < 0.05-0.001 vs. OEA alone), and was abolished by H89-induced inhibition of protein kinase A. OEA-induced cAMP levels and GLP-1 secretion were significantly reduced in mGLUTag cells transfected with GPR119-specific small interfering RNA (P < 0.05). Application of OEA (10 micromol/l) directly into the rat ileum, but not intravenously, increased plasma bioactive GLP-1 levels in euglycemic animals by 1.5-fold (P < 0.05) and insulin levels by 3.9-fold (P < 0.01) but only in the presence of hyperglycemia.

CONCLUSIONS

The results of these studies demonstrate, for the first time, that OEA increases GLP-1 secretion from intestinal L-cells through activation of the novel GPR119 fatty acid derivate receptor in vitro and in vivo.

B-type natriuretic peptide 8-32, which is produced from mature BNP 1-32 by the metalloprotease meprin A, has reduced bioactivity

32-amino acid B-type natriuretic peptide (BNP 1-32) plays an important role in cardiovascular homeostasis. Recently, it was reported that BNP 1-32 is cleaved by the metalloprotease meprin A to BNP 8-32, the bioactivity of which is undefined. We hypothesized that BNP 8-32 has reduced vasodilating and natriuretic bioactivity compared with BNP 1-32 in vivo. Human BNP 8-32 and BNP 1-32 were compared in a crossover study in eight anesthetized normal canines. After a preinfusion clearance, BNP 1-32 was infused at 30 ng.kg(-1) x min(-1) for 45 min followed by a 60-min washout and a second preinfusion clearance. Then, equimolar BNP 8-32 was infused. In half of the studies, the peptide sequence was reversed. Changes with peptides from the respective preinfusion clearance to infusion clearance were compared with paired tests. Mean arterial pressure was reduced by both BNP 8-32 and BNP 1-32 (-8 +/- 3 vs. -6 +/- 2 mmHg, P = 0.48). Changes in right atrial pressure, pulmonary capillary wedge pressure, heart rate, cardiac output, and glomerular filtration rate were similar. However, urinary sodium excretion increased less with BNP 8-32 than with BNP 1-32 (+171 +/- 24 vs. +433 +/- 43 muEq/min; P = 0.008), as did urinary potassium excretion, urine flow, and renal blood flow. While BNP 8-32 has similar vasodilating actions as BNP 1-32, its diuretic and natriuretic actions are reduced, suggesting a role for meprin A in the regulation of BNP 1-32 bioactivity in the kidney. Meprin A inhibition may be a potential strategy to increase the bioactivity of endogenous and exogenous BNP 1-32 in cardiovascular diseases.

The role of incretins in cardiovascular control

Glucagon-like peptide-1 (GLP-1) is an incretin secreted in response to nutrient ingestion. Understanding the incretin effect on diabetes pathophysiology has led to development of a new class of agents termed incretin mimetics. Exenatide is the first GLP-1 agonist approved to treat type 2 diabetes mellitus (T2DM). Clinical studies have demonstrated exenatide's efficacy in improving glycemic control, often coupled with weight loss. Studies are investigating the potential cardiovascular benefits of GLP-1 agonists. Blood pressure, cholesterol levels, C-reactive protein, and insulin resistance may improve in patients treated with exenatide. The direct effect of GLP-1 on cardiac myocytes and vascular smooth muscle has been an active area of investigation. Infusions of GLP-1 in animal models and human subjects with heart failure have demonstrated significantly improved cardia parameters. In patients with T2DM, GLP-1 infusion has been shown to improve endothelial function, irrespective of changes in insulin sensitivity. These pilot studies provide a foundation for developing therapies aimed at modulating incretin physiology for the additional benefit on the cardiovascular system in patients with T2DM and heart disease.

GLP-1R agonist liraglutide activates cytoprotective pathways and improves outcomes after experimental myocardial infarction in mice

OBJECTIVE

Glucagon-like peptide-1 receptor (GLP-1R) agonists are used to treat type 2 diabetes, and transient GLP-1 administration improved cardiac function in humans after acute myocardial infarction (MI) and percutaneous revascularization. However, the consequences of GLP-1R activation before ischemic myocardial injury remain unclear.

RESEARCH DESIGN AND METHODS

We assessed the pathophysiology and outcome of coronary artery occlusion in normal and diabetic mice pretreated with the GLP-1R agonist liraglutide.

RESULTS

Male C57BL/6 mice were treated twice daily for 7 days with liraglutide or saline followed by induction of MI. Survival was significantly higher in liraglutide-treated mice. Liraglutide reduced cardiac rupture (12 of 60 versus 46 of 60; P = 0.0001) and infarct size (21 +/- 2% versus 29 +/- 3%, P = 0.02) and improved cardiac output (12.4 +/- 0.6 versus 9.7 +/- 0.6 ml/min; P = 0.002). Liraglutide also modulated the expression and activity of cardioprotective genes in the mouse heart, including Akt, GSK3beta, PPARbeta-delta, Nrf-2, and HO-1. The effects of liraglutide on survival were independent of weight loss. Moreover, liraglutide conferred cardioprotection and survival advantages over metformin, despite equivalent glycemic control, in diabetic mice with experimental MI. The cardioprotective effects of liraglutide remained detectable 4 days after cessation of therapy and may be partly direct, because liraglutide increased cyclic AMP formation and reduced the extent of caspase-3 activation in cardiomyocytes in a GLP-1R-dependent manner in vitro.

CONCLUSIONS

These findings demonstrate that GLP-1R activation engages prosurvival pathways in the normal and diabetic mouse heart, leading to improved outcomes and enhanced survival after MI in vivo.

The glucagon-like peptide-1 receptor regulates endogenous glucose production and muscle glucose uptake independent of its incretin action

Glucagon-like peptide-1 (GLP-1) diminishes postmeal glucose excursions by enhancing insulin secretion via activation of the beta-cell GLP-1 receptor (Glp1r). GLP-1 may also control glucose levels through mechanisms that are independent of this incretin effect. The hyperinsulinemic-euglycemic clamp (insulin clamp) and exercise were used to examine the incretin-independent glucoregulatory properties of the Glp1r because both perturbations stimulate glucose flux independent of insulin secretion. Chow-fed mice with a functional disruption of the Glp1r (Glp1r(-/-)) were compared with wild-type littermates (Glp1r(+/+)). Studies were performed on 5-h-fasted mice implanted with arterial and venous catheters for sampling and infusions, respectively. During insulin clamps, [3-(3)H]glucose and 2[(14)C]deoxyglucose were used to determine whole-body glucose turnover and glucose metabolic index (R(g)), an indicator of glucose uptake. R(g) in sedentary and treadmill exercised mice was determined using 2[(3)H]deoxyglucose. Glp1r(-/-) mice exhibited increased glucose disappearance, muscle R(g), and muscle glycogen levels during insulin clamps. This was not associated with enhanced muscle insulin signaling. Glp1r(-/-) mice exhibited impaired suppression of endogenous glucose production and hepatic glycogen accumulation during insulin clamps. This was associated with impaired liver insulin signaling. Glp1r(-/-) mice became significantly hyperglycemic during exercise. Muscle R(g) was normal in exercised Glp1r(-/-) mice, suggesting that hyperglycemia resulted from an added drive to stimulate glucose production. Muscle AMP-activated protein kinase phosphorylation was higher in exercised Glp1r(-/-) mice. This was associated with increased relative exercise intensity and decreased exercise endurance. In conclusion, these results show that the endogenous Glp1r regulates hepatic and muscle glucose flux independent of its ability to enhance insulin secretion.

Potential of glucose-lowering drugs to reduce cardiovascular events

Although a clear relationship exists between glycosylated hemoglobin and cardiovascular (CV) disease in individuals with type 2 diabetes mellitus (T2DM) in epidemiologic studies, data from prospective studies are less clear. Earlier prospective studies examining intensive glucose lowering suffered from a lack of statistical power to show CV event reduction, as well as a lack of durable glycemic control and relatively poor control of associated CV risk factors. Although recent CV outcome trials comparing intensive glycemic compared with standard glycemic control have been disappointing, CV event rates appear to be declining substantially in T2DM individuals in the setting of aggressive global CV risk factor modification. No single hypoglycemic agent or combination of agents was associated with increased CV events or mortality. A comprehensive strategy of multifactorial intervention including aggressive and durable glycemic blood pressure, and lipid lowering, aspirin usage, and lifestyle modifications is beneficial in reducing macrovascular and microvascular events in T2DM individuals.

Dipeptidyl-peptidase IV and B-type natriuretic peptide. From bench to bedside

B-type natriuretic peptide (BNP) has emerged as a reliable biomarker in patients with congestive heart failure. The mature, biologically active B-type natriuretic peptide, BNP

Clin Chem Lab Med 2009;47:248–52.

The role of incretins in glucose homeostasis and diabetes treatment

Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'être is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.

Future perspectives on glucagon-like peptide-1, diabetes and cardiovascular risk

AIMS

Glucagon-like peptide-1 (GLP-1), a gastrointestinal hormone mainly produced in the post-prandial state, reduces blood glucose through the stimulation of insulin secretion and the inhibition of glucagon release. Long-acting GLP-1 receptor agonists, and dipeptidyl-peptidase-4 (DPP-4) inhibitors which increase GLP-1 levels, are used as hypoglycemic treatments in type 2 diabetes. This paper aims at reviewing the potential benefit of those treatments in the prevention of cardiovascular risk in type 2 diabetic patients.

DATA SYNTHESIS

Experimental studies have shown that GLP-1 has several potentially beneficial actions on cardiovascular risk. Some of those, such as protection from myocardial ischemic damage and improvement of cardiac function, have also been demonstrated in humans. However, the equivalence of GLP-1 agonists and DPP-4 inhibitors with GLP-1, with respect to cardiovascular risk profile, cannot be assumed or taken for granted. Drugs of those two classes have been shown to effectively reduce glycated hemoglobin and to have a specific effect on post-prandial glucose; furthermore, they seem to reduce blood pressure and to have some favorable effects on lipid profiles. Additionally, GLP-1 agonists induce weight loss in diabetic patients.

CONCLUSION

The profile of action of GLP-1 receptor agonists and DPP-4 inhibitors suggests the possibility of an actual reduction in cardiovascular risk, which needs to be confirmed by large long-term clinical trials.

Treatment of patients with diabetes with GLP-1 analogues or DPP-4- inhibitors: a hot topic for cardiologists?

Novel drugs for the treatment of patients with diabetes are of interest for cardiologists if they reduce the risk of cardiovascular events. However, as documented by the current discussion about the potential benefits of glitazones, high hopes can fail. Initial beneficial cardiovascular effects shown in proof-of-concept studies were muted by the apparent higher mortality in the metaanalysis of studies with rosiglitazone. Having this in mind, how should one judge about new, emerging antidiabetic therapies, in particular those influencing the incretin axis? The rapidly increasing use of GLP-1 analogues and DPP-4 inhibitors for the treatment of type 2 diabetes mellitus may be of major interest for the cardiologist. Potential beneficial actions on the cardiovascular system so far shown in animal experiments and small proof of concept studies may provide the rationale for using these drugs specifically in diabetic patients with secondary complications such as macrovascular disease or diabetic cardiomyopathy. Theoretically, these new therapies could also proof beneficial in patients with heart failure, independently of concomittend diabetes mellitus. However, many unanswered questions need to be addressed in the near future to extend the experimental findings to potential benefits of real life patients. In summary a new class of antidiabetic drugs, which could possibly directly influence cardiovascular effects of diabetes mellitus and thus possibly treat or even prevent life threatening complications has become available. Further studies both assessing surrogate parameters as well as hard endpoint studies are needed to support the hypothesis generated from the summarized experimental studies.

DPP4 inhibitors for diabetes--what next?

With vildagliptin and sitagliptin on the market for the treatment of type 2 diabetes, dipeptidyl peptidase 4 (DPP4, EC 3.4.14.5) research has entered a new era. Scientists aim to uncover the broader pharmacological profile of DPP4 inhibitors and search for therapeutic opportunities outside diabetes. During the pre-clinical and clinical evaluation of vildagliptin and sitagliptin, there has been a growing awareness of the presence of other DPP4-like peptidases in various cells and tissues. This fuelled the development of more inhibitors with defined selectivity for DPP2, 8 and 9 that were used to investigate the expression, distribution and regulation of these peptidases. In turn, these studies increased the insights in the role of DPP4 in the body's response to various insults.

The role of mitochondrial K(ATP) channels in cardioprotection

Pro-coagulant and pro-inflammatory intramyocardial (micro)vasculature plays an important role in acute myocardial infarction (AMI). Currently, inhibition of serine protease dipeptidyl peptidase 4 (DPP4) receives a lot of interest as an anti-hyperglycemic therapy in type 2 diabetes patients. However, DPP4 also possesses anti-thrombotic properties and may behave as an immobilized anti-coagulant on endothelial cells. Here, we studied the expression and activity of endothelial DPP4 in human myocardial infarction in relation to a prothrombogenic endothelial phenotype. Using (immuno)histochemistry, DPP4 expression and activity were found on the endothelium of intramyocardial blood vessels in autopsied control hearts (n = 9). Within the infarction area of AMI patients (n = 73), this DPP4 expression and activity were significantly decreased, coinciding with an increase in Tissue Factor expression. In primary human umbilical vein endothelial cells (HUVECs), Western blot analysis and digital imaging fluorescence microscopy revealed that DPP4 expression was strongly decreased after metabolic inhibition, also coinciding with Tissue Factor upregulation. Interestingly, inhibition of DPP4 activity with diprotin A also enhanced the amount of Tissue Factor encountered and induced the adherence of platelets under flow conditions. Ischemia induces loss of coronary microvascular endothelial DPP4 expression and increased Tissue Factor expression in AMI as well as in vitro in HUVECs. Our data suggest that the loss of DPP4 activity affects the anti-thrombogenic nature of the endothelium.