Brain GLP-1 signaling regulates femoral artery blood flow and insulin sensitivity through hypothalamic PKC-δ

Impacts of different pancreatic resection ranges on endocrine function in Suncus murinus

BACKGROUND

Surgical intervention involving the pancreas can lead to impaired glucose tolerance and other types of endocrine dysfunction. The scope of pancreatectomy and whether it includes the ventral pancreas are the key factors in the development of postoperative diabetes. The ventral and dorsal pancreases are almost separated in Suncus murinus (S. murinus).

AIM

To investigate the effects of different extents of pancreatic resection on endocrine function in S. murinus.

METHODS

Eight-week-old male S. murinus shrews were randomly divided into three experimental groups according to different pancreatic resection ranges as follows: ventral pancreatectomy (VPx) group; partial pancreatectomy (PPx) group; subtotal pancreatectomy (SPx) group; and a sham-operated group. Postprandial serum insulin, glucagon-like peptide-1 (GLP-1), pancreatic polypeptide (PP), and somatostatin (SST) levels, as well as food intake, weight, blood glucose, and glucose tolerance were regularly measured for each animal.

RESULTS

S. murinus treated with PPx and SPx suffered from varying degrees of impaired glucose tolerance, but only a small proportion of the SPx group developed diabetes. Only S. murinus in the SPx group showed a significant decrease in food intake accompanied by severe weight loss, as well as a significant increase in postprandial serum GLP-1 levels. Postprandial serum PP levels decreased in both the VPx and PPx groups, but not in the SPx group. Postprandial serum SST levels decreased in both VPx and PPx groups, but the decrease was marginal.

CONCLUSION

Severe weight loss after pancreatectomy may be related to loss of appetite caused by compensatory elevation of GLP-1. PP and GLP-1 may play a role in resisting blood glucose imbalance.

Restoration of blood vessel regeneration in the era of combination SGLT2i and GLP-1RA therapy for diabetes and obesity

Ischaemic cardiovascular diseases, including peripheral and coronary artery disease, myocardial infarction, and stroke, remain major comorbidities for individuals with type 2 diabetes (T2D) and obesity. During cardiometabolic chronic disease (CMCD), hyperglycaemia and excess adiposity elevate oxidative stress and promote endothelial damage, alongside an imbalance in circulating pro-vascular progenitor cells that mediate vascular repair. Individuals with CMCD demonstrate pro-vascular 'regenerative cell exhaustion' (RCE) characterized by excess pro-inflammatory granulocyte precursor mobilization into the circulation, monocyte polarization towards pro-inflammatory vs. anti-inflammatory phenotype, and decreased pro-vascular progenitor cell content, impairing the capacity for vessel repair. Remarkably, targeted treatment with the sodium-glucose cotransporter-2 inhibitor (SGLT2i) empagliflozin in subjects with T2D and coronary artery disease, and gastric bypass surgery in subjects with severe obesity, has been shown to partially reverse these RCE phenotypes. SGLT2is and glucagon-like peptide-1 receptor agonists (GLP-1RAs) have reshaped the management of individuals with T2D and comorbid obesity. In addition to glucose-lowering action, both drug classes have been shown to induce weight loss and reduce mortality and adverse cardiovascular outcomes in landmark clinical trials. Furthermore, both drug families also act to reduce systemic oxidative stress through altered activity of overlapping oxidase and antioxidant pathways, providing a putative mechanism to augment circulating pro-vascular progenitor cell content. As SGLT2i and GLP-1RA combination therapies are emerging as a novel therapeutic opportunity for individuals with poorly controlled hyperglycaemia, potential additive effects in the reduction of oxidative stress may also enhance vascular repair and further reduce the ischaemic cardiovascular comorbidities associated with T2D and obesity.

Parallel genomic responses to historical climate change and high elevation in East Asian songbirds

Parallel evolution can be expected among closely related taxa exposed to similar selective pressures. However, parallelism is typically stronger at the phenotypic level, while genetic solutions to achieve these phenotypic similarities may differ. For polygenic traits, the availability of standing genetic variation (i.e., heterozygosity) may influence such genetic nonparallelism. Here, we examine the extent to which high-elevation adaptation is parallel-and whether the level of parallelism is affected by heterozygosity-by analyzing genomes of 19 Paridae species distributed across East Asia with a dramatic east-west elevation gradient. We find that western highlands endemic parids have consistently lower levels of heterozygosity-likely the result of late-Pleistocene demographic contraction-than do parids found exclusively in eastern lowlands, which remained unglaciated during the late Pleistocene. Three widespread species (east to west) have high levels of heterozygosity similar to that observed in eastern species, although their western populations are less variable than eastern ones. Comparing genomic responses to extreme environments of the Qinghai-Tibet Plateau, we find that the most differentiated genomic regions between each high-elevation taxon and its low-elevation relative are significantly enriched for genes potentially related to the oxygen transport cascade and/or thermogenesis. Despite no parallelism at particular genes, high similarity in gene function is found among comparisons. Furthermore, parallelism is not higher in more heterozygous widespread parids than in highland endemics. Thus, in East Asian parids, parallel functional response to extreme elevation appears to rely on different genes, with differences in heterozygosity having no effect on the degree of genetic parallelism.

Lessons from bariatric surgery: Can increased GLP-1 enhance vascular repair during cardiometabolic-based chronic disease?

Type 2 diabetes (T2D) and obesity represent entangled pandemics that accelerate the development of cardiovascular disease (CVD). Given the immense burden of CVD in society, non-invasive prevention and treatment strategies to promote cardiovascular health are desperately needed. During T2D and obesity, chronic dysglycemia and abnormal adiposity result in systemic oxidative stress and inflammation that deplete the vascular regenerative cell reservoir in the bone marrow that impairs blood vessel repair and exacerbates the penetrance of CVD co-morbidities. This novel translational paradigm, termed 'regenerative cell exhaustion' (RCE), can be detected as the depletion and dysfunction of hematopoietic and endothelial progenitor cell lineages in the peripheral blood of individuals with established T2D and/or obesity. The reversal of vascular RCE has been observed after administration of the sodium-glucose cotransporter-2 inhibitor (SGLT2i), empagliflozin, or after bariatric surgery for severe obesity. In this review, we explore emerging evidence that links improved dysglycemia to a reduction in systemic oxidative stress and recovery of circulating pro-vascular progenitor cell content required for blood vessel repair. Given that bariatric surgery consistently increases systemic glucagon-like-peptide 1 (GLP-1) release, we also focus on evidence that the use of GLP-1 receptor agonists (GLP-1RA) during obesity may act to inhibit the progression of systemic dysglycemia and adiposity, and indirectly reduce inflammation and oxidative stress, thereby limiting the impact of RCE. Therefore, therapeutic intervention with currently-available GLP-1RA may provide a less-invasive modality to reverse RCE, bolster vascular repair mechanisms, and improve cardiometabolic risk in individuals living with T2D and obesity.

Glucagon-like peptide-1 (GLP-1) receptor agonists and their cardiovascular benefits-The role of the GLP-1 receptor

Cardiovascular outcome trials revealed cardiovascular benefits for type 2 diabetes mellitus patients when treated with long‐acting glucagon‐like peptide‐1 (GLP‐1) receptor agonists. In the last decade, major advances were made characterising the physiological effects of GLP‐1 and its action on numerous targets including brain, liver, kidney, heart and blood vessels. However, the effects of GLP‐1 and receptor agonists, and the GLP‐1 receptor on the cardiovascular system have not been fully elucidated. We compare results from cardiovascular outcome trials of GLP‐1 receptor agonists and review pleiotropic clinical and preclinical data concerning cardiovascular protection beyond glycaemic control. We address current knowledge on GLP‐1 and receptor agonist actions on the heart, vasculature, inflammatory cells and platelets, and discuss evidence for GLP‐1 receptor‐dependent versus independent effects secondary of GLP‐1 metabolites. We conclude that the favourable cardiovascular profile of GLP‐1 receptor agonists might expand their therapeutic use for treating cardiovascular disease even in non‐diabetic populations.

Brain Endothelial Cells Regulate Glucagon-Like Peptide 1 Entry Into the Brain via a Receptor-Mediated Process

Glucagon-like peptide 1 (GLP-1) in addition to regulating glucose-dependent insulin and glucagon secretion exerts anorexic and neuroprotective effects. While brain-derived GLP-1 may participate in these central actions, evidence suggests that peripherally derived GLP-1 plays an important role and GLP-1 analogs are known to cross the blood brain barrier. To define the role of brain microvascular endothelial cells in GLP-1 entry into the brain, we infused labeled GLP-1 or exendin-4 into rats intravenously and examined their appearance and protein kinase A activities in various brain regions. We also studied the role of endothelial cell GLP-1 receptor and its signaling in endothelial cell uptake and transport of GLP-1. Systemically infused labeled GLP-1 or exendin-4 appeared rapidly in various brain regions and this was associated with increased protein kinase A activity in these brain regions. Pretreatment with GLP-1 receptor antagonist reduced labeled GLP-1 or exendin-4 enrichment in the brain. Sub-diaphragmatic vagus nerve resection did not alter GLP-1-mediated increases in protein kinase A activity in the brain. Rat brain microvascular endothelial cells rapidly took up labeled GLP-1 and this was blunted by either GLP-1 receptor antagonism or protein kinase A inhibition but enhanced through adenylyl cyclase activation. Using an artificially assembled blood brain barrier consisting of endothelial and astrocyte layers, we found that labeled GLP-1 time-dependently crossed the barrier and the presence of GLP-1 receptor antagonist blunted this transit. We conclude that GLP-1 crosses the blood brain barrier through active trans-endothelial transport which requires GLP-1 receptor binding and activation.

Central Sfrp5 regulates hepatic glucose flux and VLDL-triglyceride secretion

OBJECTIVE

Secreted frizzled-related protein 5 (Sfrp5) has been shown to be associated with energy homeostasis and insulin resistance in mouse models of obesity and diabetes. However, its central role in glucose and lipid metabolism is unknown.

METHODS

HFD-fed rats received ICV infusions of vehicle or Sfrp5 during a pancreatic euglycemic clamp procedure. To delineate the pathway(s) by which ICV Sfrp5 modulates HGP and VLDL-TG secretion, we inhibited the hypothalamic K_ATP channel using glibenclamide, the DVC NMDA receptor with MK801, and selectively transected the hepatic branch of the vagal nerve while centrally infusing Sfrp5.

RESULTS

ICV Sfrp5 in HFD-fed rats significantly increased the glucose infusion required to maintain euglycemia due to HGP inhibition during the clamp procedure; moreover, hepatic PEPCK and G6Pase expression was decreased, and InsR and Akt phosphorylation was increased in the liver. ICV Sfrp5 also decreased circulating triglyceride levels via inhibiting hepatic VLDL-TG secretion. These changes were accompanied by the inhibition of enzymes related to lipogenesis in the liver. ICV Sfrp5 significantly increased insulin-stimulated phosphorylation of InsR and Akt in the hypothalamus of HFD-fed rats, and insulin-stimulated immunodetectable PIP3 levels were higher in Sfrp5 group than in control group both in vitro and vivo. The glucose- and lipid-lowering effects of ICV Sfrp5 were eliminated by NMDA receptor or DVC K_ATP channel inhibition or HVAG.

CONCLUSIONS

The present study demonstrates that central Sfrp5 signaling activates a previously unappreciated InsR-Akt-PI3k-K_ATP channel pathway in the hypothalamus and brain-hepatic vagus neurocircuitry to decrease HGP and VLDL-TG secretion.

Exendin-4 overcomes cytokine-induced decreases in gap junction coupling via protein kinase A and Epac2 in mouse and human islets

KEY POINTS

The pancreatic islets of Langerhans maintain glucose homeostasis through insulin secretion, where insulin secretion dynamics are regulated by intracellular Ca2+ signalling and electrical coupling of the insulin producing β-cells in the islet. We have previously shown that cytokines decrease β-cell coupling and that compounds which increase cAMP can increase coupling. In both mouse and human islets exendin-4, which increases cAMP, protected against cytokine-induced decreases in coupling and in mouse islets preserved glucose-stimulated calcium signalling by increasing connexin36 gap junction levels on the plasma membrane. Our data indicate that protein kinase A regulates β-cell coupling through a fast mechanism, such as channel gating or membrane organization, while Epac2 regulates slower mechanisms of regulation, such as gap junction turnover. Increases in β-cell coupling with exendin-4 may protect against cytokine-mediated β-cell death as well as preserve insulin secretion dynamics during the development of diabetes.

ABSTRACT

The pancreatic islets of Langerhans maintain glucose homeostasis. Insulin secretion from islet β-cells is driven by glucose metabolism, depolarization of the cell membrane and an influx of calcium, which initiates the release of insulin. Gap junctions composed of connexin36 (Cx36) electrically couple β-cells, regulating calcium signalling and insulin secretion dynamics. Cx36 coupling is decreased in pre-diabetic mice, suggesting a role for altered coupling in diabetes. Our previous work has shown that pro-inflammatory cytokines decrease Cx36 coupling and that compounds which increase cAMP can increase Cx36 coupling. The goal of this study was to determine if exendin-4, which increases cAMP, can protect against cytokine-induced decreases in Cx36 coupling and altered islet function. In both mouse and human islets, exendin-4 protected against cytokine-induced decreases in coupling and preserved glucose-stimulated calcium signalling. Exendin-4 also protected against protein kinase Cδ-mediated decreases in Cx36 coupling. Exendin-4 preserved coupling in mouse islets by preserving Cx36 levels on the plasma membrane. Exendin-4 regulated Cx36 coupling via both protein kinase A (PKA)- and Epac2-mediated mechanisms in cytokine-treated islets. In mouse islets, modulating Epac2 had a greater impact in mediating Cx36 coupling, while in human islets modulating PKA had a greater impact on Cx36 coupling. Our data indicate that PKA regulates Cx36 coupling through a fast mechanism, such as channel gating, while Epac2 regulates slower mechanisms of regulation, such as Cx36 turnover in the membrane. Increases in Cx36 coupling with exendin-4 may protect against cytokine-mediated β-cell dysfunction to insulin secretion dynamics during the development of diabetes.

GLP-1 Improves Adipocyte Insulin Sensitivity Following Induction of Endoplasmic Reticulum Stress

Glucagon-like peptide 1 (GLP-1) improves insulin resistance of adipose tissue in obese humans. However, the mechanism of this effect is unclear. Perturbation of endoplasmic reticulum (ER) homeostasis impairs insulin signaling. We hypothesized that GLP-1 could directly improve insulin signaling in ER-stressed adipocytes. Here, we examined the effects of GLP-1 on ER stress response in fat cells in an obese and insulin-resistant murine model. We found that GLP-1 analog liraglutide reduced ER stress related gene expression in visceral fat cells accompanied by improved systemic insulin tolerance. Consistently, GLP-1 decreased CHOP expression and increased insulin stimulated AKT phosphorylation (p-AKT) in thapsigargin, a ER stress inducer, treated white fat cells differentiated from visceral stromal vascular fraction. We further found blocking CHOP expression increased insulin stimulated p-AKT in ER-stressed fat cells. Of note, we found mTOR signaling pathway contributed to the expression of ATF4 and subsequently the CHOP expression in ER stress response, while GLP-1 inhibited mTOR activity as exemplified by elevated autophagosome formation and increased LC3II/LC3I ratio. These findings suggest that GLP-1 directly modulates the ER stress response partially via inhibiting mTOR signaling pathway, leading to increased insulin sensitivity in adipocytes.

Effects of GIP on regional blood flow during normoglycemia and hyperglycemia in anesthetized rats

The incretin hormone glucose-dependent insulinotropic polypeptide (GIP) potentiates glucose-stimulated insulin secretion, and affects β-cell turnover. This study aimed at evaluating if some of the beneficial effects of GIP on glucose homeostasis can be explained by modulation of islet blood flow. Anesthetized Sprague-Dawley rats were infused intravenously with different doses of GIP (10, 20, or 60 ng/kg*min) for 30 min. Subsequent organ blood flow measurements were performed with microspheres. In separate animals, islets were perfused ex vivo with GIP (10-6 -10-12  mol/L) during normo- and hyperglycemia and arteriolar responsiveness was recorded. The highest dose of GIP potentiated insulin secretion during hyperglycemia, but had no effect in normoglycemic rats. The highest GIP concentration decreased blood perfusion of whole pancreas, pancreatic islets, duodenum, colon, liver and kidneys. The decrease in blood flow was unaffected by ganglion blockade or adenosine receptor inhibition. In contrast to this, in single perfused islets GIP induced a dose-dependent arteriolar dilation. Thus, high doses of GIP exert a direct dilatory effect on islet arterioles in isolated islets, but induce a generalized vasoconstriction in splanchnic organs, including the whole pancreas and islets, in vivo. The latter effect is unlikely to be mediated by adenosine, the autonomic nervous system, or endothelial mediators.

Acute activation of GLP-1-expressing neurons promotes glucose homeostasis and insulin sensitivity

OBJECTIVE

Glucagon-like peptides are co-released from enteroendocrine L cells in the gut and preproglucagon (PPG) neurons in the brainstem. PPG-derived GLP-1/2 are probably key neuroendocrine signals for the control of energy balance and glucose homeostasis. The objective of this study was to determine whether activation of PPG neurons per se modulates glucose homeostasis and insulin sensitivity in vivo.

METHODS

We generated glucagon (Gcg) promoter-driven Cre transgenic mice and injected excitatory hM3Dq-mCherry AAV into their brainstem NTS. We characterized the metabolic impact of PPG neuron activation on glucose homeostasis and insulin sensitivity using stable isotopic tracers coupled with hyperinsulinemic euglycemic clamp.

RESULTS

We showed that after ip injection of clozapine N-oxide, Gcg-Cre lean mice transduced with hM3Dq in the brainstem NTS downregulated basal endogenous glucose production and enhanced glucose tolerance following ip glucose tolerance test. Moreover, acute activation of PPG neurons^NTS enhanced whole-body insulin sensitivity as indicated by increased glucose infusion rate as well as augmented insulin-suppression of endogenous glucose production and gluconeogenesis. In contrast, insulin-stimulation of glucose disposal was not altered significantly.

CONCLUSIONS

We conclude that acute activation of PPG neurons in the brainstem reduces basal glucose production, enhances intraperitoneal glucose tolerance, and augments hepatic insulin sensitivity, suggesting an important physiological role of PPG neurons-mediated circuitry in promoting glycemic control and insulin sensitivity.

A Specific Gut Microbiota Dysbiosis of Type 2 Diabetic Mice Induces GLP-1 Resistance through an Enteric NO-Dependent and Gut-Brain Axis Mechanism

Glucagon-like peptide-1 (GLP-1)-based therapies control glycemia in type 2 diabetic (T2D) patients. However, in some patients the treatment must be discontinued, defining a state of GLP-1 resistance. In animal models we identified a specific set of ileum bacteria impairing the GLP-1-activated gut-brain axis for the control of insulin secretion and gastric emptying. Using prediction algorithms, we identified bacterial pathways related to amino acid metabolism and transport system modules associated to GLP-1 resistance. The conventionalization of germ-free mice demonstrated their role in enteric neuron biology and the gut-brain-periphery axis. Altogether, insulin secretion and gastric emptying require functional GLP-1 receptor and neuronal nitric oxide synthase in the enteric nervous system within a eubiotic gut microbiota environment. Our data open a novel route to improve GLP-1-based therapies.

Duodenal GLP-1 signaling regulates hepatic glucose production through a PKC-δ-dependent neurocircuitry

Intestinal glucagon-like peptide-1 (GLP-1) is a hormone that stimulates insulin secretion and acts as a neuropeptide to control glucose homeostasis, but little is known whether intestinal GLP-1 has any effect in the control of hepatic glucose production (HGP). Here we found that intraduodenal infusion of GLP-1 activated duodenal PKC-δ, lowered HGP and was accompanied by a decrease in hepatic expression of gluconeogenic enzymes and an increase in hepatic insulin signaling in rats. However, gut co-infusion of either the GLP-1 receptor antagonist Ex-9, or the PKC-δ inhibitor rottlerin with GLP-1, negated the ability of gut GLP-1 to lower HGP and to increase hepatic insulin signaling during clamps. The metabolic and molecular signal effects of duodenal GLP-1 were also negated by co-infusion with tetracaine, pharmacologic inhibition of N-methyl-d-aspartate receptors within the dorsalvagal complex, or hepatic vagotomy in rats. In summary, we identified a neural glucoregulatory function of gut GLP-1 signaling.

Vascular Biology of Glucagon Receptor Superfamily Peptides: Mechanistic and Clinical Relevance

Regulatory peptides produced in islet and gut endocrine cells, including glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, and glucose-dependent insulinotropic polypeptide, exert actions with considerable metabolic importance and translational relevance. Although the clinical development of GLP-1 receptor agonists and dipeptidyl peptidase-4 inhibitors has fostered research into how these hormones act on the normal and diseased heart, less is known about the actions of these peptides on blood vessels. Here we review the effects of these peptide hormones on normal blood vessels and highlight their vascular actions in the setting of experimental and clinical vascular injury. The cellular localization and signal transduction properties of the receptors for glucagon, GLP-1, GLP-2, and glucose-dependent insulinotropic polypeptide are discussed, with emphasis on endothelial cells and vascular smooth muscle cells. The actions of these peptides on the control of blood flow, blood pressure, angiogenesis, atherosclerosis, and vascular inflammation are reviewed with a focus on elucidating direct and indirect mechanisms of action. How these peptides traverse the blood-brain barrier is highlighted, with relevance to the use of GLP-1 receptor agonists to treat obesity and neurodegenerative disorders. Wherever possible, we compare actions identified in cell lines and primary cell culture with data from preclinical studies and, when available, results of human investigation, including studies in subjects with diabetes, obesity, and cardiovascular disease. Throughout the review, we discuss pitfalls, limitations, and challenges of the existing literature and highlight areas of controversy and uncertainty. The increasing use of peptide-based therapies for the treatment of diabetes and obesity underscores the importance of understanding the vascular biology of peptide hormone action.

Brain GLP-1 and insulin sensitivity

Type 2 diabetes is often treated with a class of drugs referred to as glucagon-like peptide-1 (GLP-1) analogs. GLP-1 is a peptide secreted by the gut that acts through only one known receptor, the GLP-1 receptor. The primary function of GLP-1 is thought to be lowering of postprandial glucose levels. Indeed, medications utilizing this system, including the long-acting GLP-1 analogs liraglutide and exenatide, are beneficial in reducing both blood sugars and body weight. GLP-1 analogs were long presumed to affect glucose control through their ability to increase insulin levels through peripheral action on beta cells. However, multiple lines of data point to the ability of GLP-1 to act within the brain to alter glucose regulation. In this review we will discuss the evidence for a central GLP-1 system and the effects of GLP-1 in the brain on regulating multiple facets of glucose homeostasis including glucose tolerance, insulin production, insulin sensitivity, hepatic glucose production, muscle glucose uptake, and connections of the central GLP-1 system to the gut. Although the evidence indicates that GLP-1 receptors in the brain are not necessary for physiologic control of glucose regulation, we discuss the research showing a strong effect of acute manipulation of the central GLP-1 system on glucose control and how it is relevant to type 2 diabetic patients.

Dissociation of GLP-1 and insulin association with food processing in the brain: GLP-1 sensitivity despite insulin resistance in obese humans

Objective

Glucagon-like peptide-1 (GLP-1) is released into the bloodstream after food intake. In addition to stimulating insulin release, it causes satiety and contributes to the termination of food intake. In this study, we investigated whether endogenous GLP-1 affects food-related brain activity and hunger.

Methods

Twenty-four volunteers (12 lean; 12 obese) underwent a 75 g oral glucose tolerance test that promotes GLP-1 secretion. Food cue-induced brain activity was assessed by functional magnetic resonance imaging and GLP-1 concentrations were measured before, 30, and 120 min after glucose intake.

Results

The significant increase in GLP-1 levels negatively correlated with a change in the food cue-induced brain activity in the orbitofrontal cortex, a major reward area. This association was independent of simultaneous alterations in insulin and glucose concentrations. The association was present in lean and overweight participants. By contrast, postprandial insulin changes were associated with orbitofrontal activations in lean individuals only.

Conclusions

The postprandial release of GLP-1 might alter reward processes in the orbitofrontal cortex and might thereby support the termination of food intake and reduce hunger. While obese persons showed brain insulin resistance, no GLP-1 resistance was observed. Our study provides novel insight into the central regulation of food intake by the incretin hormone GLP-1.

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GLP-1 levels associate with food cue-induced brain activity in the orbitofrontal cortex, a major reward area.

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While obese persons are brain insulin-resistant in the orbitofrontal cortex, they still respond to GLP-1.

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Postprandial GLP-1 release may alter reward processes in the orbitofrontal cortex to support the termination of food intake.

Blood pressure-lowering effects of incretin-based diabetes therapies

Glucagon-like peptide-1 receptor (GLP-1) agonists and dipeptidyl-peptidase-4 (DPP-4) inhibitors are therapies that are used to treat hyperglycemia in patients with type 2 diabetes mellitus. Although both of these medication types primarily lower prandial and fasting blood glucose levels by enhanced GLP-1 receptor signalling, they have distinct mechanisms of action. Whereas DPP-4 inhibitors boost patient levels of endogenously produced GLP-1 (and glucose-dependent insulinotropic peptide) by preventing its metabolism by DPP-4 enzymatic activity, GLP-1 receptor agonists are either synthetic analogues of human GLP-1 or exendin-4 based molecules. They are tailored to resist hydrolysis by DPP-4 activity and to provide longer durability in the circulation compared with native GLP-1. Several roles for incretin-based diabetes therapies beyond the endocrine pancreas and their glycemic-lowering properties have now been described, including attenuation of cardiac myocyte injury and reduction in post-ischemic infarction size after cardiovascular insult. Favourable outcomes have also been observed on systolic blood pressure reduction, postprandial intestinal lipoprotein metabolism, endothelial cell function, modulation of innate immune-mediated inflammation and surrogate markers of renal function. As hypertension is an independent risk factor for premature death in patients with type 2 diabetes, potential favourable extrapancreatic actions, particularly within the heart, blood vessels and kidney, for this drug class are of considerable clinical interest. Herein, we highlight and provide critical appraisal of the clinical data supporting the antihypertensive effects of GLP-1 receptor agonists and DPP-4 inhibitors and link possible mechanisms of action to clinical outcomes reported for this drug class.

Resistin Promotes Angiogenesis in Endothelial Progenitor Cells Through Inhibition of MicroRNA206: Potential Implications for Rheumatoid Arthritis

Endothelial progenitor cells (EPCs) promote angiogenesis and are therefore key contributors to a wide variety of angiogenesis-related autoimmune diseases such as rheumatoid arthritis (RA). However, the signaling mechanisms through which these progenitor cells influence RA pathogenesis remain unknown. The aim of this study was to examine whether resistin plays a role in the pathogenesis of and angiogenesis associated with RA by circulating EPCs. We found that levels of resistin in synovial fluid and tissue from patients with RA and from mice with collagen-induced arthritis were overexpressed and promoted the homing of EPCs into the synovium, thereby inducing angiogenesis. EPCs isolated from healthy donors were used to investigate the signal transduction pathway underlying EPC migration and tube formation after treatment with resistin. We found that resistin directly induced a significant increase in expression of vascular endothelial growth factor (VEGF) in EPCs. We also found that the expression of microRNA-206 (miR-206) was negatively correlated with the expression of resistin during EPC-mediated angiogenesis. Notably, the increased expression of VEGF was associated with decreased binding of miR-206 to the VEGF-A 3' untranslated region through protein kinase C delta-dependent AMP-activated protein kinase signaling pathway. Moreover, blockade of resistin reduced EPC homing into synovial fluid and angiogenesis in vivo. Taken together, our study is the first to demonstrate that resistin promotes EPCs homing into the synovium during RA angiogenesis via a signal transduction pathway that involves VEGF expression in primary EPCs. These findings provide support for resistin as a therapeutic target for the patients with RA. Stem Cells 2015;33:2243-2255.

SOCS3 deficiency in leptin receptor-expressing cells mitigates the development of pregnancy-induced metabolic changes

Objective

During pregnancy, women normally increase their food intake and body fat mass, and exhibit insulin resistance. However, an increasing number of women are developing metabolic imbalances during pregnancy, including excessive gestational weight gain and gestational diabetes mellitus. Despite the negative health impacts of pregnancy-induced metabolic imbalances, their molecular causes remain unclear. Therefore, the present study investigated the molecular mechanisms responsible for orchestrating the metabolic changes observed during pregnancy.

Methods

Initially, we investigated the hypothalamic expression of key genes that could influence the energy balance and glucose homeostasis during pregnancy. Based on these results, we generated a conditional knockout mouse that lacks the suppressor of cytokine signaling-3 (SOCS3) only in leptin receptor-expressing cells and studied these animals during pregnancy.

Results

Among several genes involved in leptin resistance, only SOCS3 was increased in the hypothalamus of pregnant mice. Remarkably, SOCS3 deletion from leptin receptor-expressing cells prevented pregnancy-induced hyperphagia, body fat accumulation as well as leptin and insulin resistance without affecting the ability of the females to carry their gestation to term. Additionally, we found that SOCS3 conditional deletion protected females against long-term postpartum fat retention and streptozotocin-induced gestational diabetes.

Conclusions

Our study identified the increased hypothalamic expression of SOCS3 as a key mechanism responsible for triggering pregnancy-induced leptin resistance and metabolic adaptations. These findings not only help to explain a common phenomenon of the mammalian physiology, but it may also aid in the development of approaches to prevent and treat gestational metabolic imbalances.

Fatty acid sensing in the gut and the hypothalamus: in vivo and in vitro perspectives

The ability to properly sense both ingested and circulating nutrients is crucial for the maintenance of metabolic homeostasis. As such, both the gastrointestinal tract and the hypothalamus have demonstrated the capacity to sense and effectively respond to nutrients, such as fatty acids, to control food intake and glucose production to regulate energy and glucose homeostasis. In modern, Westernized societies, obesity and diabetes rates continue to rise unabated, due in part to an increase in highly palatable high-fat diet consumption. Thus, our understanding in the ability of the body to successfully monitor lipids is more vital than ever. This review details the current understanding of both the gut and the brain, specifically the hypothalamus, in sensing fatty acids. Highlighting both in vivo and in vitro studies, we explore some of the mechanisms upon which different fatty acids activate enteroendocrine and neural lipid-sensing signaling mechanisms to subsequently lower food intake and glucose production to ultimately regulate metabolic homeostasis. A better understanding of these lipid-sensing pathways could lay the groundwork for successful pharmacological targets for the treatment of obesity and diabetes.

The effect of liraglutide on endothelial function in patients with type 2 diabetes

This single-centre, 12-week, double-blind, placebo-controlled trial assessed how the human glucagon-like-peptide 1 analogue liraglutide impacted endothelial function in adult patients (n = 49) with type 2 diabetes and no overt cardiovascular disease. Patients were randomized to liraglutide, placebo or glimepiride. At baseline and Week 12, venous occlusion plethysmography was used to measure forearm blood flow (FBF) in response to acetylcholine (ACh) and sodium nitroprusside (SNP) before and after (L)-N(G)-monomethyl arginine (L-NMMA) infusion. At Week 12, ACh-mediated FBF increased with liraglutide and decreased with placebo; however, the between-treatment difference was not significant (p = 0.055). Inhibition of ACh-mediated FBF after L-NMMA infusion increased with liraglutide and decreased with placebo; this between-treatment difference was also not significant (p = 0.149). No change in FBF was observed with SNP. Liraglutide did not significantly impact endothelium-dependent vasodilation after 12 weeks; however, additional investigations looking at the effect of liraglutide on endothelial function in alternative vasculature and during the postprandial period are warranted.

Inactivation of SOCS3 in leptin receptor-expressing cells protects mice from diet-induced insulin resistance but does not prevent obesity

Therapies that improve leptin sensitivity have potential as an alternative treatment approach against obesity and related comorbidities. We investigated the effects of Socs3 gene ablation in different mouse models to understand the role of SOCS3 in the regulation of leptin sensitivity, diet-induced obesity (DIO) and glucose homeostasis. Neuronal deletion of SOCS3 partially prevented DIO and improved glucose homeostasis. Inactivation of SOCS3 only in LepR-expressing cells protected against leptin resistance induced by HFD, but did not prevent DIO. However, inactivation of SOCS3 in LepR-expressing cells protected mice from diet-induced insulin resistance by increasing hypothalamic expression of Katp channel subunits and c-Fos expression in POMC neurons. In summary, the regulation of leptin signaling by SOCS3 orchestrates diet-induced changes on glycemic control. These findings help to understand the molecular mechanisms linking obesity and type 2 diabetes, and highlight the potential of SOCS3 inhibitors as a promising therapeutic approach for the treatment of diabetes.

Direct cardiovascular effects of glucagon like peptide-1

Current gold standard therapeutic strategies for T2DM target insulin resistance or β cell dysfunction as their core mechanisms of action. However, the use of traditional anti-diabetic drugs, in most cases, does not significantly reduce macrovascular morbidity and mortality. Among emerging anti-diabetic candidates, glucagon like peptide-1 (GLP-1) based therapies carry special cardiovascular implications, exerting both direct as well as indirect effects. The direct cardiovascular effects of GLP-1 and its analogs remain the focus of this review.

Green tea polyphenols alleviate early BBB damage during experimental focal cerebral ischemia through regulating tight junctions and PKCalpha signaling

Background

It has been supposed that green tea polyphenols (GTPs) have neuroprotective effects on brain damage after brain ischemia in animal experiments. Little is known regarding GTPs’ protective effects against the blood-brain barrier (BBB) disruption after ischemic stroke. We investigated the effects of GTPs on the expression of claudin-5, occludin, and ZO-1, and the corresponding cellular mechanisms involved in the early stage of cerebral ischemia.

Methods

Male Wistar rats were subjected to a middle cerebral artery occlusion (MCAO) for 0, 30, 60, and 120 min. GTPs (400 mg/kg/day) or vehicle was administered by intragastric gavage twice a day for 30 days prior to MCAO. At different time points, the expression of claudin-5, occludin, ZO-1, and PKCα signaling pathway in microvessel fragments of cerebral ischemic tissue were evaluated.

Results

GTPs reduced BBB permeability at 60 min and 120 min after ischemia as compared with the vehicle group. Transmission electron microscopy also revealed that GTPs could reverse the opening of tight junction (TJ) barrier at 60 min and 120 min after MACO. The decreased mRNA and protein expression levels of claudin-5, occludin, and ZO-1 in microvessel fragments of cerebral ischemic tissue were significantly prevented by treatment with GTPs at the same time points after ischemia in rats. Furthermore, GTPs could attenuate the increase in the expression levels of PKCα mRNA and protein caused by cerebral ischemia.

Conclusions

These results demonstrate that GTPs may act as a potential neuroprotective agent against BBB damage at the early stage of focal cerebral ischemia through the regulation of TJ and PKCα signaling.

Pharmacology, physiology, and mechanisms of incretin hormone action

Incretin peptides, principally GLP-1 and GIP, regulate islet hormone secretion, glucose concentrations, lipid metabolism, gut motility, appetite and body weight, and immune function, providing a scientific basis for utilizing incretin-based therapies in the treatment of type 2 diabetes. Activation of GLP-1 and GIP receptors also leads to nonglycemic effects in multiple tissues, through direct actions on tissues expressing incretin receptors and indirect mechanisms mediated through neuronal and endocrine pathways. Here we contrast the pharmacology and physiology of incretin hormones and review recent advances in mechanisms coupling incretin receptor signaling to pleiotropic metabolic actions in preclinical studies. We discuss whether mechanisms identified in preclinical studies have potential translational relevance for the treatment of human disease and highlight controversies and uncertainties in incretin biology that require resolution in future studies.

Neonatal exendin-4 reduces growth, fat deposition and glucose tolerance during treatment in the intrauterine growth-restricted lamb

Background

IUGR increases the risk of type 2 diabetes mellitus (T2DM) in later life, due to reduced insulin sensitivity and impaired adaptation of insulin secretion. In IUGR rats, development of T2DM can be prevented by neonatal administration of the GLP-1 analogue exendin-4. We therefore investigated effects of neonatal exendin-4 administration on insulin action and β-cell mass and function in the IUGR neonate in the sheep, a species with a more developed pancreas at birth.

Methods

Twin IUGR lambs were injected s.c. daily with vehicle (IUGR+Veh, n = 8) or exendin-4 (1 nmol.kg^-1, IUGR+Ex-4, n = 8), and singleton control lambs were injected with vehicle (CON, n = 7), from d 1 to 16 of age. Glucose-stimulated insulin secretion and insulin sensitivity were measured in vivo during treatment (d 12–14). Body composition, β-cell mass and in vitro insulin secretion of isolated pancreatic islets were measured at d 16.

Principal Findings

IUGR+Veh did not alter in vivo insulin secretion or insulin sensitivity or β-cell mass, but increased glucose-stimulated insulin secretion in vitro. Exendin-4 treatment of the IUGR lamb impaired glucose tolerance in vivo, reflecting reduced insulin sensitivity, and normalised glucose-stimulated insulin secretion in vitro. Exendin-4 also reduced neonatal growth and visceral fat accumulation in IUGR lambs, known risk factors for later T2DM.

Conclusions

Neonatal exendin-4 induces changes in IUGR lambs that might improve later insulin action. Whether these effects of exendin-4 lead to improved insulin action in adult life after IUGR in the sheep, as in the PR rat, requires further investigation.

The glucagon-like peptide-1 receptor agonist Exendin 4 has a protective role in ischemic injury of lean and steatotic liver by inhibiting cell death and stimulating lipolysis

Nonalcoholic fatty liver disease is an increasingly prevalent spectrum of conditions characterized by excess fat deposition within hepatocytes. Affected hepatocytes are known to be highly susceptible to ischemic insults, responding to injury with increased cell death, and commensurate liver dysfunction. Numerous clinical circumstances lead to hepatic ischemia. Mechanistically, specific means of reducing hepatic vulnerability to ischemia are of increasing clinical importance. In this study, we demonstrate that the glucagon-like peptide-1 receptor agonist Exendin 4 (Ex4) protects hepatocytes from ischemia reperfusion injury by mitigating necrosis and apoptosis. Importantly, this effect is more pronounced in steatotic livers, with significantly reducing cell death and facilitating the initiation of lipolysis. Ex4 treatment leads to increased lipid droplet fission, and phosphorylation of perilipin and hormone sensitive lipase - all hallmarks of lipolysis. Importantly, the protective effects of Ex4 are seen after a short course of perioperative treatment, potentially making this clinically relevant. Thus, we conclude that Ex4 has a role in protecting lean and fatty livers from ischemic injury. The rapidity of the effect and the clinical availability of Ex4 make this an attractive new therapeutic approach for treating fatty livers at the time of an ischemic insult.

Lipid sensing and insulin resistance in the brain

Lipid sensing and insulin signaling in the brain independently triggers a negative feedback system to lower glucose production and food intake. Here, we discuss the underlying molecular and neuronal mechanisms of lipid sensing and insulin signaling in the hypothalamus and how these mechanisms are affected in response to high-fat feeding. We propose that high-fat feeding concurrently disrupts hypothalamic insulin-signaling and lipid-sensing mechanisms and that experiments aimed to restore both insulin action and lipid sensing in the brain could effectively lower glucose production and food intake to restore metabolic homeostasis in type 2 diabetes and obesity.

Incretin-based therapies and cardiovascular risk

BACKGROUND

Glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase 4 (DPP4) inhibitors are currently used as glucose-lowering agents in type 2 diabetes, due to their effects on insulin and glucagon secretion. These agents, which are effective in improving glucose control, could also have a beneficial effect on the incidence of cardiovascular events. The analysis of major cardiovascular events reported during trials with metabolic endpoints shows a significant reduction of risk with both classes of drugs. Longer-term trials specifically designed to assess the effects of GLP-1 receptor agonists and DPP4 inhibitors on major cardiovascular events are currently ongoing.

SCOPE

In order to elucidate potential mechanisms of cardiovascular protection with incretin-based therapies, a Medline search (any date up to December 15th, 2011) was performed using the terms 'cardiovascular' and ('DPP-4' or 'GLP-1' or any single name of incretin-based drugs); papers which were considered relevant for the aim of this review were selected by the authors, on the basis of their judgment.

FINDINGS

Incretin-based drugs have beneficial effects on cardiovascular risk factors, such as blood pressure and, to a lesser extent, cholesterol and triglyceride. GLP-1 receptor agonists also reduce body weight. A number of experimental studies has suggested that GLP-1 has direct, beneficial effects on myocardial and endothelial cells, but some of these actions could be mediated via GLP-1 receptor-independent pathways. Available experimental evidence, together with a few pilot studies in humans, shows that GLP-1 receptor agonists and DPP4 inhibitors are capable of ameliorating myocardial function and protect myocardiocytes from ischemic damage, independent of their glucose-lowering effects. Furthermore, both classes of drugs enhance endothelial function. In addition, DPP4 inhibitors increase the availability of endothelial progenitor cells, via a GLP-1 receptor-independent pathway.

CONCLUSION

Taken together, available data suggest that incretin-based therapies could prevent cardiovascular disease via multiple mechanisms.

Lipid sensing in the gut, brain and liver

Elevation of lipid levels affects energy and glucose homeostasis. Organs such as the gut, brain and liver detect a rise in lipids and orchestrate a biochemical, molecular, neuronal and physiological network of responses that alters appetite and the rate of hepatic glucose production. The factors involved in these responses are unclear but the formation of esterified lipids (long-chain fatty acyl-CoAs) and subsequent activation of protein kinase Cδ remain a common sensing mechanism in all three organs. In this paper, we discuss the mechanisms underlying lipid sensing within the gut, brain and liver and their physiological impact on the regulation of glucose and energy homeostasis.

GLP-1, the gut-brain, and brain-periphery axes

Glucagon-like peptide 1 (GLP-1) is a gut hormone which directly binds to the GLP-1 receptor located at the surface of the pancreatic β-cells to enhance glucose-induced insulin secretion. In addition to its pancreatic effects, GLP-1 can induce metabolic actions by interacting with its receptors expressed on nerve cells in the gut and the brain. GLP-1 can also be considered as a neuropeptide synthesized by neuronal cells in the brain stem that release the peptide directly into the hypothalamus. In this environment, GLP-1 is assumed to control numerous metabolic and cardiovascular functions such as insulin secretion, glucose production and utilization, and arterial blood flow. However, the exact roles of these two locations in the regulation of glucose homeostasis are not well understood. In this review, we highlight the latest experimental data supporting the role of the gut-brain and brain-periphery axes in the control of glucose homeostasis. We also focus our attention on the relevance of β-cell and brain cell targeting by gut GLP-1 for the regulation of glucose homeostasis. In addition to its action on β-cells, we find that understanding the physiological role of GLP-1 will help to develop GLP-1-based therapies to control glycemia in type 2 diabetes by triggering the gut-brain axis or the brain directly. This pleiotropic action of GLP-1 is an important concept that may help to explain the observation that, during their treatment, type 2 diabetic patients can be identified as 'responders' and 'non-responders'.