GLP-1 amplifies insulin signaling by up-regulation of IRbeta, IRS-1 and Glut4 in 3T3-L1 adipocytes

Do anti-obesity medical treatments have a direct effect on adipose tissue?

During the past years, several drugs have been developed for the treatment of obesity. Some are already used in clinical practice: orlistat, GLP-1 receptor agonists (RA), GLP-1/GIP biagonists and the melanocortin 4 receptor (MC4R) agonist, setmelanotide. Some should be available in the future: GLP-1/glucagon biagonists, GLP-1/GIP/glucagon triagonists. These drugs act mainly by reducing food intake or fat absorption. However, many of them show specific effects on the adipose tissue. All these drugs show significant reduction of fat mass and, more particularly of visceral fat. If most of the drugs, except orlistat, have been shown to increase energy expenditure in rodents with enhanced thermogenesis, this has not yet been clearly demonstrated in humans. However, biagonists or triagonist stimulating glucagon seem to a have a more potent effect to increase thermogenesis in the adipose tissue and, thus, energy expenditure. Most of these drugs have been shown to increase the production of adiponectin and to reduce the production of pro-inflammatory cytokines by the adipose tissue. GLP-1RAs reduce the size of adipocytes and promote their differentiation. GLP-1RAS and GLP-1/GIP biagonists reduce, in the adipose tissue, the expression of several genes involved in lipogenesis. Further studies are still needed to clarify the precise roles, on the adipose tissue, of these drugs dedicated for the treatment of obesity.

Outer Membrane Vesicles Released from Garlic Exosome-like Nanoparticles (GaELNs) Train Gut Bacteria that Reverses Type 2 Diabetes via the Gut-Brain Axis

Gut microbiota function has numerous effects on humans and the diet humans consume has emerged as a pivotal determinant of gut microbiota function. Here, a new concept that gut microbiota can be trained by diet-derived exosome-like nanoparticles (ELNs) to release healthy outer membrane vesicles (OMVs) is introduced. Specifically, OMVs released from garlic ELN (GaELNs) trained human gut Akkermansia muciniphila (A. muciniphila) can reverse high-fat diet-induced type 2 diabetes (T2DM) in mice. Oral administration of OMVs released from GaELNs trained A. muciniphila can traffick to the brain where they are taken up by microglial cells, resulting in inhibition of high-fat diet-induced brain inflammation. GaELNs treatment increases the levels of OMV Amuc-1100, P9, and phosphatidylcholines. Increasing the levels of Amuc-1100 and P9 leads to increasing the GLP-1 plasma level. Increasing the levels of phosphatidylcholines is required for inhibition of cGas and STING-mediated inflammation and GLP-1R crosstalk with the insulin pathway that leads to increasing expression of Insulin Receptor Substrate (IRS1 and IRS2) on OMV targeted cells. These findings reveal a molecular mechanism whereby OMVs from plant nanoparticle-trained gut bacteria regulate genes expressed in the brain, and have implications for the treatment of brain dysfunction caused by a metabolic syndrome.

GLP-1/GLP-1RAs: New Options for the Drug Treatment of NAFLD

Non-alcoholic fatty liver disease (NAFLD) has recently emerged as a global public health concern. Currently, the cornerstone of NAFLD treatment is lifestyle modification and, if necessary, weight loss. However, compliance is a challenge, and this approach alone may not be sufficient to halt and treat the more serious disease development, so medication is urgently needed. Nevertheless, no medicines are approved to treat NAFLD. Glucagon-like peptide-1 (GLP-1) is an enteropeptide hormone that inhibits glucagon synthesis, promotes insulin secretion, and delays gastric emptying. GLP-1 has been found in recent studies to be beneficial for the management of NAFLD, and the marketed GLP-1 agonist drugs have different degrees of effectiveness for NAFLD while lowering blood glucose. In this article, we review GLP-1 and its physiological roles, the pathogenesis of NAFLD, the correlation between NAFLD and GLP-1 signaling, and potential strategies for GLP-1 treatment of NAFLD.

Regulation of Macronutrients in Insulin Resistance and Glucose Homeostasis during Type 2 Diabetes Mellitus

Insulin resistance is an important feature of metabolic syndrome and a precursor of type 2 diabetes mellitus (T2DM). Overnutrition-induced obesity is a major risk factor for the development of insulin resistance and T2DM. The intake of macronutrients plays a key role in maintaining energy balance. The components of macronutrients distinctly regulate insulin sensitivity and glucose homeostasis. Precisely adjusting the beneficial food compound intake is important for the prevention of insulin resistance and T2DM. Here, we reviewed the effects of different components of macronutrients on insulin sensitivity and their underlying mechanisms, including fructose, dietary fiber, saturated and unsaturated fatty acids, and amino acids. Understanding the diet-gene interaction will help us to better uncover the molecular mechanisms of T2DM and promote the application of precision nutrition in practice by integrating multi-omics analysis.

Regulation of Human Sortilin Alternative Splicing by Glucagon-like Peptide-1 (GLP1) in Adipocytes

Type 2 diabetes mellitus is a chronic metabolic disease with no cure. Adipose tissue is a major site of systemic insulin resistance. Sortilin is a central component of the glucose transporter -Glut4 storage vesicles (GSV) which translocate to the plasma membrane to uptake glucose from circulation. Here, using human adipocytes we demonstrate the presence of the alternatively spliced, truncated sortilin variant (Sort_T) whose expression is significantly increased in diabetic adipose tissue. Artificial-intelligence-based modeling, molecular dynamics, intrinsically disordered region analysis, and co-immunoprecipitation demonstrated association of Sort_T with Glut4 and decreased glucose uptake in adipocytes. The results show that glucagon-like peptide-1 (GLP1) hormone decreases Sort_T. We deciphered the molecular mechanism underlying GLP1 regulation of alternative splicing of human sortilin. Using splicing minigenes and RNA-immunoprecipitation assays, the results show that GLP1 regulates Sort_T alternative splicing via the splice factor, TRA2B. We demonstrate that targeted antisense oligonucleotide morpholinos reduces Sort_T levels and improves glucose uptake in diabetic adipocytes. Thus, we demonstrate that GLP1 regulates alternative splicing of sortilin in human diabetic adipocytes.

Glucagon-like peptide 1 receptor agonist, exendin-4, reduces alcohol-associated fatty liver disease

Fatty liver is the earliest response to excessive ethanol consumption, which increases the susceptibility of the liver to develop advanced stage of liver disease. Our previous studies have revealed that chronic alcohol administration alters metabolic hormone levels and their functions. Of current interest to our laboratory is glucagon-like peptide 1 (GLP-1), a widely studied hormone known to reduce insulin resistance and hepatic fat accumulation in patients with metabolic-associated fatty liver disease. In this study, we examined the beneficial effects of exendin-4 (a GLP-1 receptor agonist) in an experimental rat model of ALD. Male Wistar rats were pair-fed the Lieber-DeCarli control or ethanol diet. After 4 weeks of this feeding regimen, a subset of rats in each group were intraperitoneally injected every other day with either saline or exendin-4 at a dose of 3 nmol/kg/day (total 13 doses) while still being fed their respective diet. At the end of the treatment, rats were fasted for 6 h and glucose tolerance test was conducted. The following day, the rats were euthanized, and the blood and tissue samples collected for subsequent analysis. We found that exendin-4 treatment had no significant effect on body weight gain among the experimental groups. Exendin-4-treated ethanol rats exhibited improved alcohol-induced alterations in liver/body weight and adipose/body weight ratio, serum ALT, NEFA, insulin, adiponectin and hepatic triglyceride levels. Reduction in indices of hepatic steatosis in exendin-4 treated ethanol-fed rats was attributed to improved insulin signaling and fat metabolism. These results strongly suggest that exendin-4 mitigates alcohol-associated hepatic steatosis by regulating fat metabolism.

Metabolic remodeling of glycerophospholipids acts as a signature of dulaglutide and liraglutide treatment in recent-onset type 2 diabetes mellitus

Aims

As metabolic remodeling is a pathological characteristic in type 2 diabetes (T2D), we investigate the roles of newly developed long-acting glucagon-like peptide-1 receptor agonists (GLP-1RAs) such as dulaglutide and liraglutide on metabolic remodeling in patients with recent-onset T2D.

Methods

We recruited 52 cases of T2D and 28 control cases in this study. In the patient with T2D, 39 cases received treatment with dulaglutide and 13 cases received treatment with liraglutide. Using untargeted metabolomics analysis with broad-spectrum LC-MS, we tracked serum metabolic changes of the patients from the beginning to the end of follow-up (12th week).

Results

We identified 198 metabolites that were differentially expressed in the patients with T2D, compared to the control group, in which 23 metabolites were significantly associated with fasting plasma glucose. Compared to pre-treatment, a total of 46 and 45 differentially regulated metabolites were identified after treatments with dulaglutide and liraglutide, respectively, in which the most differentially regulated metabolites belong to glycerophospholipids. Furthermore, a longitudinal integration analysis concurrent with diabetes case-control status revealed that metabolic pathways, such as the insulin resistance pathway and type 2 diabetes mellitus, were enriched after dulaglutide and liraglutide treatments. Proteins such as GLP-1R, GNAS, and GCG were speculated as potential targets of dulaglutide and liraglutide.

Conclusions

In total, a metabolic change in lipids existed in the early stage of T2D was ameliorated after the treatments of GLP-1RAs. In addition to similar effects on improving glycemic control, remodeling of glycerophospholipid metabolism was identified as a signature of dulaglutide and liraglutide treatments.

GLP-1 RAs and SGLT-2 Inhibitors for Insulin Resistance in Nonalcoholic Fatty Liver Disease: Systematic Review and Network Meta-Analysis

OBJECTIVE

Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) and sodium-glucose cotransporter-2 (SGLT-2) inhibitors reduce glycaemia and weight and improve insulin resistance (IR) via different mechanisms. We aim to evaluate and compare the ability of GLP-1 RAs and SGLT-2 inhibitors to ameliorate the IR of nonalcoholic fatty liver disease (NAFLD) patients.

DATA SYNTHESIS

Three electronic databases (Medline, Embase, PubMed) were searched from inception until March 2021. We selected randomized controlled trials comparing GLP-1 RAs and SGLT-2 inhibitors with control in adult NAFLD patients with or without T2DM. Network meta-analyses were performed using fixed and random effect models, and the mean difference (MD) with corresponding 95% confidence intervals (CI) were determined. The within-study risk of bias was assessed with the Cochrane collaborative risk assessment tool RoB.

RESULTS

25 studies with 1595 patients were included in this network meta-analysis. Among them, there were 448 patients, in 6 studies, who were not comorbid with T2DM. Following a mean treatment duration of 28.86 weeks, compared with the control group, GLP-1 RAs decreased the HOMA-IR (MD [95%CI]; -1.573[-2.523 to -0.495]), visceral fat (-0.637[-0.992 to -0.284]), weight (-2.394[-4.625 to -0.164]), fasting blood sugar (-0.662[-1.377 to -0.021]) and triglyceride (- 0.610[-1.056 to -0.188]). On the basis of existing studies, SGLT-2 inhibitors showed no statistically significant improvement in the above indicators. Compared with SGLT-2 inhibitors, GLP-1 RAs decreased visceral fat (-0.560[-0.961 to -0.131]) and triglyceride (-0.607[-1.095 to -0.117]) significantly.

CONCLUSIONS

GLP-1 RAs effectively improve IR in NAFLD, whereas SGLT-2 inhibitors show no apparent effect.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO https://www.crd.york.ac.uk/PROSPERO/, CRD42021251704.

Central GLP-1 contributes to improved cognitive function and brain glucose uptake after duodenum-jejunum bypass on obese and diabetic rats

The improvement of cognitive function following bariatric surgery has been highlighted, yet its underlying mechanisms remain elusive. Finding the improved brain glucose uptake of patients after Roux-en-Y gastric bypass (RYGB), duodenum-jejunum bypass (DJB), and sham surgery (Sham) were performed on obese and diabetic Wistar rats, and intracerebroventricular (ICV) injection of glucagon-like peptide-1 (GLP-1) analog liraglutide (Lira), antagonist exendin-(9-39) (Exe-9), and the viral-mediated GLP-1 receptor (Glp-1r) knockdown (KD) were applied on both groups to elucidate the role of GLP-1 in mediating cognitive function and brain glucose uptake assessed with the Morris water maze (MWM) and positron emission tomography (PET). Insulin and GLP-1 in serum and cerebral spinal fluid (CSF) were measured, and the expression of glucose uptake-related proteins including glucose transporter 1 (GLUT-1), GLUT-4, phospho-Akt substrate of 160kDa (pAS160), AS160, Rab10, Myosin-Va as well as the c-fos marker in the brain were examined. Along with augmented glucose homeostasis following DJB, central GLP-1 was correlated with the improved cognitive function and ameliorated brain glucose uptake, which was further confirmed by the enhancive role of Lira on both groups whereas the Exe-9 and Glp-1r KD were opposite. Known to activate insulin-signaling pathways, central GLP-1 contributes to improved cognitive function and brain glucose uptake after DJB.NEW & NOTEWORTHY The improvement of cognitive function following bariatric surgery has been highlighted while its mechanisms remain elusive. The brain glucose uptake of patients was improved after RYGB, and the DJB and sham surgery performed on obese and diabetic Wistar rats revealed that the elevated central GLP-1 contributes to the dramatic improvement of cognitive function, brain glucose uptake, transport, glucose sensing, and neuronal activation.

Direct Effect of the Synthetic Analogue of Glucagon-Like Peptide Type 1, Liraglutide, on Mature Adipocytes Is Realized through Adenylate-Cyclase-Dependent Enhancing of Insulin Sensitivity

Incretin hormones analogues, including glucagon-like peptide type 1 (GLP-1), exhibit complex glucose-lowering, anorexigenic, and cardioprotective properties. Mechanisms of action of GLP-1 and its analogues are well known for pancreatic β-cells, hepatocytes, and other tissues. Nevertheless, local effects of GLP-1 and its analogues in adipose tissue remain unclear. In the present work effects of the GLP-1 synthetic analogue, liraglutide, on adipogenesis and insulin sensitivity of the 3T3-L1 adipocytes were examined. Enhancement of insulin sensitivity of mature adipocytes by the GLP-1 synthetic analogue liraglutide mediated by adenylate cyclase was demonstrated. The obtained results imply existence of the positive direct insulin-sensitizing effect of liraglutide on mature adipocytes.

Antioxidative Potentials of Incretin-Based Medications: A Review of Molecular Mechanisms

Glucagon-like peptide 1 receptor agonists and dipeptidyl-peptidase 4 inhibitors are medications used for managing diabetes, mimicking the metabolic effects of incretin hormones. Recent evidence suggests that these medications have antioxidative potentials in the diabetic milieu. The pathophysiology of most diabetic complications involves oxidative stress. Therefore, if incretin-based antidiabetic medications can alleviate the free radicals involved in oxidative stress, they can potentially provide further therapeutic effects against diabetic complications. However, the molecular mechanisms by which these medications protect against oxidative stress are not fully understood. In the current review, we discuss the potential molecular mechanisms behind these pharmacologic agents' antioxidative properties.

Effects of Glucagon-Like Peptide-1 Analogue and Fibroblast Growth Factor 21 Combination on the Atherosclerosis-Related Process in a Type 2 Diabetes Mouse Model

BACKGROUND

Glucagon-like peptide-1 (GLP-1) analogues regulate glucose homeostasis and have anti-inflammatory properties, but cause gastrointestinal side effects. The fibroblast growth factor 21 (FGF21) is a hormonal regulator of lipid and glucose metabolism that has poor pharmacokinetic properties, including a short half-life. To overcome these limitations, we investigated the effect of a low-dose combination of a GLP-1 analogue and FGF21 on atherosclerosis-related molecular pathways.

METHODS

C57BL/6J mice were fed a high-fat diet for 30 weeks followed by an atherogenic diet for 10 weeks and were divided into four groups: control (saline), liraglutide (0.3 mg/kg/day), FGF21 (5 mg/kg/day), and low-dose combination treatment with liraglutide (0.1 mg/kg/day) and FGF21 (2.5 mg/kg/day) (n=6/group) for 6 weeks. The effects of each treatment on various atherogenesisrelated pathways were assessed.

RESULTS

Liraglutide, FGF21, and their low-dose combination significantly reduced atheromatous plaque in aorta, decreased weight, glucose, and leptin levels, and increased adiponectin levels. The combination treatment upregulated the hepatic uncoupling protein-1 (UCP1) and Akt1 mRNAs compared with controls. Matric mentalloproteinase-9 (MMP-9), monocyte chemoattractant protein-1 (MCP-1), and intercellular adhesion molecule-1 (ICAM-1) were downregulated and phosphorylated Akt (p-Akt) and phosphorylated extracellular signal-regulated kinase (p-ERK) were upregulated in liver of the liraglutide-alone and combination-treatment groups. The combination therapy also significantly decreased the proliferation of vascular smooth muscle cells. Caspase-3 was increased, whereas MMP-9, ICAM-1, p-Akt, and p-ERK1/2 were downregulated in the liraglutide-alone and combination-treatment groups.

CONCLUSION

Administration of a low-dose GLP-1 analogue and FGF21 combination exerts beneficial effects on critical pathways related to atherosclerosis, suggesting the synergism of the two compounds.

Gut-adipose tissue crosstalk: A bridge to novel therapeutic targets in metabolic syndrome?

The gut is one of the main endocrine organs in our body, producing hormones acknowledged to play determinant roles in controlling appetite, energy balance and glucose homeostasis. One of the targets of such hormones is the adipose tissue, a major energetic reservoir, which governs overall metabolism through the secretion of adipokines. Disturbances either in nutrient and metabolic sensing and consequent miscommunication between these organs constitute a key driver to the metabolic complications clustered in metabolic syndrome. Thus, it is essential to understand how the disruption of this crosstalk might trigger adipose tissue dysfunction, a strong characteristic of obesity and insulin resistance. The beneficial effects of metabolic surgery in the amelioration of glucose homeostasis and body weight reduction allowed to understand the potential of gut signals modulation as a treatment for metabolic syndrome-related obesity and type 2 diabetes. In this review, we cover the effects of gut hormones in the modulation of adipose tissue metabolic and endocrine functions, as well as their impact in tissue plasticity. Furthermore, we discuss how the modulation of gut secretome, either through surgical procedures or pharmacological approaches, might improve adipose tissue function in obesity and metabolic syndrome.

Glucagon-like peptide-1 receptor agonist reduces di(2-ethylhexyl) phthalate-induced atherosclerotic processes in vascular smooth muscle cells

Glucagon-like peptide-1 receptor (GLP1R) agonist is an incretin hormone and regulates glucose metabolism. However, phthalates, known as endocrine disruptors, can interfere with hormone homeostasis. In the present study, we aimed to estimate the impact of GLP1R agonist on di(2 ethylhexyl) phthalate (DEHP)-induced atherosclerosis. For this purpose, the effects of GLP1R agonist on various atherogenesis-related cellular processes and pathways were assessed in vascular smooth muscle cells (VSMCs). DEHP-induced cell proliferation and migration were significantly decreased by GLP1R agonist in VSMCs. Protein levels of matrix metalloproteinase (MMP)-2 and MMP-9 were significantly decreased in cells exposed to GLP1R agonist, compared with DEHP-treated cells. Expression levels of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 were also reduced in GLP1R agonist-treated cells. Similarly, DEHP-associated phosphorylation of protein kinase B and extracellular signal-regulated kinase 1/2 was decreased in GLP1R agonist-treated cells, compared with DEHP-treated cells. Our findings suggest that treatment with GLP1R agonist counteracts the activation of pathways related to atherosclerosis.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

EFFECTS OF EXENDIN-4 ON ENDOPLASMIC RETICULUM STRESS-MEDIATED INSULIN RESISTANCE IN 3T3-L1 ADIPOCYTES

Objective

Endoplasmic reticulum stress (ERS) is suspected as an important factor in the initiation of insulin resistance.

Aim

To explore the effects of exendin-4 (Ex-4) on the endoplasmic reticulum stress (ERS)-mediated insulin resistance in 3T3-L1 adipocytes. In our study, 3T3-L1 adipocytes were pre-treated with ERS inhibitors tauroursodeoxycholic acid (TUDCA), Ex-4 and an ERS inducer tunicamycin (TM) then induced insulin resistance. Glucose consumption of the adipocytes was measured. Western blots determined the protein levels of ERS markers and insulin signaling pathway.

Results

TM treatment reduced insulin-stimulated glucose consumption by 19.7% in 3T3-L1 adipocytes. This repression was blunted by 24h pre-treatment with TUDCA or Ex-4. Ex-4 augmented insulin-stimulated glucose consumption in adipocytes by 14.9%. Western blotting showed that TM treatment significantly increased the ER stress markers including p-IRE, p-JNK, p-PERK, p-eIF2a and ATF6 expression, whereas 24h pre-treatment of adipocytes with TUDCA or Ex-4 alleviated the ER stress. Ex-4 alleviates ERS-induced insulin resistance by upregulating the expression of phosphorylated Akt.

Conclusion

ERs mediates insulin resistance in 3T3-L1 adipocytes, and exendin-4 significantly improves this insulin resistance.

Molecular mechanisms by which GLP-1 RA and DPP-4i induce insulin sensitivity

Glucagon-like peptide-1 is a peptide of incretin family which is used in the management of diabetes as glucagon-like peptide-1 receptor agonist (GLP-1RA). Dipeptidyl peptidase-4 enzyme metabolizes glucagon-like peptide-1 and various dipeptidyl peptidase-4 enzyme inhibitors (DPP-4i) are also used in the management of diabetes. These antidiabetic agents provide anti-hyperglycemic effects via several molecular mechanisms including promoting insulin secretion, suppression of glucagon secretion and slowing the gastric emptying. There is some research suggesting that they can induce insulin sensitivity in peripheral tissues. In this study, we review the possible molecular mechanisms by which GLP-1RA and DPP-4i can improve insulin resistance and increase insulin sensitivity in insulin-dependent peripheral tissues.

Role of adipose tissue GLP-1R expression in metabolic improvement after bariatric surgery in patients with type 2 diabetes

We aimed to explore the relationship between GLP-1 receptor (GLP-1R) expression in adipose tissue (AT) and incretin secretion, glucose homeostasis and weight loss, in patients with morbid obesity and type 2 diabetes undergoing bariatric surgery. RNA was extracted from subcutaneous (SAT) and visceral (VAT) AT biopsies from 40 patients randomized to metabolic gastric bypass, sleeve gastrectomy or greater curvature plication. Biochemical parameters, fasting plasma insulin, glucagon and area under the curve (AUC) of GLP-1 following a standard meal test were determined before and 1 year after bariatric surgery. GLP-1R expression was higher in VAT than in SAT. GLP-1R expression in VAT correlated with weight (r = −0.453, p = 0.008), waist circumference (r = −0.494, p = 0.004), plasma insulin (r = −0.466, p = 0.007), and systolic blood pressure (BP) (r = −0.410, p = 0.018). At 1 year, GLP-1R expression in VAT was negatively associated with diastolic BP (r = −0.361, p = 0.039) and, following metabolic gastric bypass, with the increase of GLP-1 AUC, (R² = 0.46, p = 0.038). Finally, GLP-1R in AT was similar independently of diabetes outcomes and was not associated with weight loss after surgery. Thus, GLP-1R expression in AT is of limited value to predict incretin response and does not play a role in metabolic outcomes after bariatric surgery.

Multiple target tissue effects of GLP-1 analogues on non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH)

Accumulating experimental and clinical evidences over the last decade indicate that GLP-1 analogues have a series of central nervous system and peripheral target tissues actions which are able to significantly influence the liver metabolism. GLP-1 analogues pleiotropic effects proved to be efficacious in T2DM subjects not only reducing liver steatosis and ameliorating NAFLD and NASH, but also in lowering plasma glucose and liver inflammation, improving cardiac function and protecting from kidney dysfunction. While the experimental and clinical data are robust, the precise mechanisms of action potentially involved in these protective multi-target effects need further investigation. Here we present a systematic review of the most recent literature data on the multi-target effects of GLP-1 analogues on the liver, on adipose and muscular tissue and on the nervous system, all capable of influencing significant aspects of the fatty liver disease physiopathology. From this analysis, we can conclude that the multi-target beneficial action of the GLP-1 analogues could explain the positive effects observed in animal and human models on progression of NAFLD to NASH.

After the LEADER trial and SUSTAIN-6, how do we explain the cardiovascular benefits of some GLP-1 receptor agonists?

Recent cardiovascular outcome trials - the LEADER with liragutide and SUSTAIN-6 with semaglutide - have shown significant reductions of major cardiovascular (CV) events with these glucagon-like peptide (GLP)-1 receptor agonists. Progressive separation of the treatment and placebo curves, starting clearly between 12 and 18 months of the trial period, and significant reductions in the risk of myocardial infarction and stroke, indicate that the beneficial CV effects observed with GLP-1 receptor agonists could be due to an antiatherogenic effect. So far, the reasons for such an effect of GLP-1 receptor agonists have not been entirely clear, although several hypotheses may be proposed. As the reductions in glycated haemoglobin and systolic blood pressure (SBP) in these trials were modest, and both trials lasted only a short period of time, reductions in hyperglycaemia and SBP are unlikely to be involved in the beneficial CV effects of GLP-1 receptor agonists. On the other hand, their effect on lipids and, in particular, the dramatic decrease in postprandial hypertriglyceridaemia may explain their beneficial CV actions. Reduction of body weight, including a significant decrease in visceral fat in patients using GLP-1 receptor agonists, may also have beneficial CV effects by reducing chronic proatherogenic inflammation. In addition, there are in-vitro data showing a direct anti-inflammatory effect with these agents that could also be involved in their beneficial CV effects. Moreover, studies in humans have shown significant beneficial effects on ischaemic myocardium after a very short treatment period, suggesting a direct effect of GLP-1 receptor agonists on myocardium, although the precise mechanism remains unclear. Finally, as a reduction in insulin resistance has been associated with a decrease in CV risk, it cannot be ruled out that the lowering of insulin resistance induced by GLP-1 receptor agonists might also be involved in their beneficial CV actions.

Exendin-4 reduces food intake via the PI3K/AKT signaling pathway in the hypothalamus

Exendin-4 (EX-4), a glucagon-like peptide-1 (GLP-1) receptor agonist, has been shown to reduce food intake and to increase proopiomelanocortin (POMC) gene expression in the hypothalamus. In this study, we examined the potential neural mechanisms by which these effects occur. Male Sprague Dawley rats were implanted with a cannula in the third ventricle of the brain through which an inhibitor of phosphatidylinositol-3 kinase (PI3K) (wortmannin) was administered, and EX-4 or vehicle was administered via intraperitoneal (IP) injection. The activity of PI3K/protein kinase B (AKT) and insulin receptor substrate-1 (IRS-1) in the hypothalamic arcuate was determined. We found that EX-4 treatment significantly decreased food intake and body weight. However, there were almost no changes in food intake and body weight when wortmannin injection (into the third ventricle) occurred prior to EX-4 IP injection. EX-4 not only increased the activity of PI3K/AKT, but it also increased IRS-1 activity. These results show that EX-4 likely suppresses food intake due to its ability to enhance insulin signaling.

GLP-1 analogue-induced weight loss does not improve obesity-induced AT dysfunction

Liraglutide shows limited impact on adipose tissue (AT) independent of the associated weight loss. However, despite weight loss and better glycaemic control than with diet alone, Liraglutide increases inflammation and creates a pro-fibrotic environment in subcutaneous AT (SCAT).

Is cleaved glucagon-like peptide 1 really inactive? Effects of GLP-1(9-36) on human adipose stem cells

Glucagon-like peptide 1(9-36) [GLP-1(9-36)] is generated by dipeptidyl peptidase-4 (DPP4) cleavage of the gut incretin hormone, GLP-1(7-36). Since GLP-1(9-36) has a very low affinity for the GLP-1 receptor (GLP-1R), it has so far been considered an inactive form of GLP-1. Here we show GLP-1(9-36) activity in human adipose stem cells (ASC) in vitro. GLP-1(9-36) inhibits human ASC proliferation, glucose uptake and adipogenesis, as well as induces cell apoptosis, to a similar extent as GLP-1(7-36) and liraglutide. Since GLP-1(9-36) effects are not reverted by the receptor antagonist exendin(9-39), which conversely reverts the effects of GLP-1(7-36), we hypothesized that the former may be mediated by a GLP-1 receptor different from the classical pancreatic one. This is the first report of GLP-1(9-36) activity in human adipose cells. Nevertheless, these findings deserve further preclinical studies to better elucidate novel and unforeseen GLP-1(9-36) activities, which could allow a better understanding of the clinical profile of DPP4 inhibitors and GLP-1R agonists.

Perspectives in GLP-1 Research: New Targets, New Receptors

The incretin hormone glucagon-like peptide-1 (GLP-1) binds to and activates its G-protein-coupled-receptor GLP-1R to reduce glycaemia through the stimulation of insulin and suppression of pancreatic glucagon secretion. Recently, GLP-1 effects unrelated to glucose homeostasis have been discovered in myocardium, bone, adipose tissue, and other target organs, which appear to be mainly mediated by GLP-1R-independent pathways. Here, we summarize knowledge on GLP-1R agonists (GLP-1RAs) as they relate to the improvement of glucose control, and focus on the most recently described effects, discussing the preclinical evidence of the involvement of alternative receptors and signalling mechanisms. It is now evident that the universe of GLP-1RAs is expanding further from the initial incretin effect, opening new unforeseen avenues for research and clinical applications.

Glucagon-like peptide 1 decreases lipotoxicity in non-alcoholic steatohepatitis

BACKGROUND & AIMS

Insulin resistance and lipotoxicity are pathognomonic in non-alcoholic steatohepatitis (NASH). Glucagon-like peptide-1 (GLP-1) analogues are licensed for type 2 diabetes, but no prospective experimental data exists in NASH. This study determined the effect of a long-acting GLP-1 analogue, liraglutide, on organ-specific insulin sensitivity, hepatic lipid handling and adipose dysfunction in biopsy-proven NASH.

METHODS

Fourteen patients were randomised to 1.8mg liraglutide or placebo for 12-weeks of the mechanistic component of a double-blind, randomised, placebo-controlled trial (ClinicalTrials.gov-NCT01237119). Patients underwent paired hyperinsulinaemic euglycaemic clamps, stable isotope tracers, adipose microdialysis and serum adipocytokine/metabolic profiling. In vitro isotope experiments on lipid flux were performed on primary human hepatocytes.

RESULTS

Liraglutide reduced BMI (-1.9 vs. +0.04kg/m(2); p<0.001), HbA1c (-0.3 vs. +0.3%; p<0.01), cholesterol-LDL (-0.7 vs. +0.05mmol/L; p<0.01), ALT (-54 vs. -4.0IU/L; p<0.01) and serum leptin, adiponectin, and CCL-2 (all p<0.05). Liraglutide increased hepatic insulin sensitivity (-9.36 vs. -2.54% suppression of hepatic endogenous glucose production with low-dose insulin; p<0.05). Liraglutide increased adipose tissue insulin sensitivity enhancing the ability of insulin to suppress lipolysis both globally (-24.9 vs. +54.8pmol/L insulin required to ½ maximally suppress serum non-esterified fatty acids; p<0.05), and specifically within subcutaneous adipose tissue (p<0.05). In addition, liraglutide decreased hepatic de novo lipogenesis in vivo (-1.26 vs. +1.30%; p<0.05); a finding endorsed by the effect of GLP-1 receptor agonist on primary human hepatocytes (24.6% decrease in lipogenesis vs. untreated controls; p<0.01).

CONCLUSIONS

Liraglutide reduces metabolic dysfunction, insulin resistance and lipotoxicity in the key metabolic organs in the pathogenesis of NASH. Liraglutide may offer the potential for a disease-modifying intervention in NASH.

Glucagon-like peptide 1 (GLP-1)-based therapy upregulates LXR-ABCA1/ABCG1 cascade in adipocytes

A promising treatment for obesity involves the use of therapeutic agents that increase the level of the glucagon-like peptide (GLP-1) which reduces appetite and food intake. Native GLP-1 is rapidly metabolized by the dipeptidyl peptidase-4 (DPP-4) enzyme and, as such, GLP-1 mimetics or DPP-4 inhibitors represent promising treatment approaches. Interestingly, obese patient receiving such medications showed improved lipid profiles and cholesterol homeostasis, however the mechanism(s) involved are not known. Members of the ATP-binding cassette (ABC) transporters, including ABCA1 and ABCG1, play essential roles in reverse cholesterol transport and in high density lipoprotein (HDL) formation. These transporters are under the transcriptional regulation of liver X receptor alpha (LXR-α). We hypothesize that GLP-1 mimetics and/or DPP-4 inhibitors modulate ABCA1/ABCG1 expression in adipocytes through an LXR-α mediated process and thus affecting cholesterol homeostasis. 3T3-L1 adipocytes were treated with the DPP-4 inhibitor vildagliptin (2 nM) or the GLP-1 mimetic exendin-4 (5 nM). Gene and protein expression of ABCA1, ABCG1 and LXR-α were determined and correlated with cholesterol efflux. Expression levels of interleukin-6 (IL-6), leptin and the glucose transporter-4 (GLUT-4) were also determined. Treatment with both medications significantly increased the expression of ABCA1, ABCG1, LXR-α and GLUT-4, decreased IL-6 and leptin, and improved cholesterol efflux from adipocytes (P < 0.05). Our data suggest that GLP-1-based therapy modulate ABCA1/ABCG1 expression in adipocytes potentially through an LXR-α mediated process.

The role of type 2 diabetes in neurodegeneration

A growing body of evidence links type-2 diabetes (T2D) with dementia and neurodegenerative diseases such as Alzheimer's disease (AD). AD is the most common form of dementia and is characterised neuropathologically by the accumulation of extracellular beta amyloid (Aβ) peptide aggregates and intracellular hyper-phosphorylated tau protein, which are thought to drive and/or accelerate inflammatory and oxidative stress processes leading to neurodegeneration. Although the precise mechanism remains unclear, T2D can exacerbate these neurodegenerative processes. Brain atrophy, reduced cerebral glucose metabolism and CNS insulin resistance are features of both AD and T2D. Cell culture and animal studies have indicated that the early accumulation of Aβ may play a role in CNS insulin resistance and impaired insulin signalling. From the viewpoint of insulin resistance and impaired insulin signalling in the brain, these are also believed to initiate other aspects of brain injury, including inflammatory and oxidative stress processes. Here we review the clinical and experimental pieces of evidence that link these two chronic diseases of ageing, and discuss underlying mechanisms. The evaluation of treatments for the management of diabetes in preclinical, and clinical studies and trials for AD will also be discussed.

Effect of liraglutide on proliferation and differentiation of human adipose stem cells

Glucagon-Like Peptide-1 (GLP-1) receptor agonists, used as glucose-lowering drugs, also induce weight loss by inhibiting food intake. The present study was aimed at the assessment of the in vitro effects of the GLP-1 receptor agonist liraglutide on proliferation and differentiation of human adipose stem cells (ASC) obtained from subcutaneous adipose tissue of morbidly obese subjects undergoing bariatric surgery. Liraglutide (10-100 nM) significantly inhibited ASC proliferation and viability, with a maximum effect at 6 days of culture (45% and 50%, for liraglutide 10 and 100 nM, respectively); the effect was reverted by exendin 9-39. Glucose uptake was significantly reduced by liraglutide in a dose dependent manner. Treatment with liraglutide reduced intracellular lipid accumulation in differentiating ASC, together with FABP-4 mRNA expression (-18%, -23%, -46%, for 1 nM, 10 nM and 100 nM, respectively), whereas it stimulated adiponectin (APN) expression (1.86-, 2.64-, 2.28-fold increase, for 1 nM, 10 nM and 100 nM, respectively). Liraglutide exerts effects on human adipose cell precursors, inhibiting proliferation and differentiation, while stimulating the expression of the insulin-sensitizing adipokine APN. These effects could contribute to the actions of GLP-1 receptor agonists on body weight and insulin sensitivity.

Resveratrol ameliorates mitochondrial dysfunction but increases the risk of hypoglycemia following hemorrhagic shock

BACKGROUND

Hemorrhagic shock (HS) may contribute to organ failure, by profoundly altering mitochondrial function. Resveratrol (RSV), a naturally occurring polyphenol, has been shown to promote mitochondrial function and regulate glucose homeostasis in diabetes. We hypothesized that RSV during resuscitation would ameliorate HS-induced mitochondrial dysfunction and improve hyperglycemia following acute blood loss.

METHODS

With the use a decompensated HS model, male Long-Evans rats (n = 6 per group) were resuscitated with lactated Ringer's solution with or without RSV (30 mg/kg) and were killed before hemorrhage (sham), at severe shock, following resuscitation, and 18 hours after resuscitation. At each time point, the liver and kidney mitochondria were isolated to assess individual respiratory complexes (CI, CII, and CIV) and the production of reactive oxygen species (ROS). Blood samples were assayed for glucose, insulin, corticosterone, total glucagon-like peptide (GLP-1), glucagon, and serum cytokine levels. The Homeostatic Model Assessment-Insulin Resistance index was used to quantify insulin resistance.

RESULTS

RSV supplementation following HS significantly improved mitochondrial function and decreased mitochondrial ROS production in both liver and kidney. RSV-treated animals had significantly lower blood glucose levels following resuscitation when compared with sham animals (116.0 ± 20.2 mg/dL vs. 227.7 ± 8.3 mg/dL, p < 0.05) or those resuscitated with lactated Ringer's solution (116.0 ± 20.2 mg/dL vs. 359.0 ± 79.5 mg/dL, p < 0.05). RSV supplementation was associated with significantly decreased plasma insulin levels (1.0 ± 0.4 ng/mL vs. 6.5 ± 3.7 ng/mL, p < 0.05), increased total GLP-1 levels (385.8 ± 56.6 ng/mL vs. 187.3 ± 11.1 ng/mL, p < 0.05), and a lower natural Log Homeostatic Model Assessment-Insulin Resistance index (1.30 ± 0.42 vs. 4.18 ± 0.68, p < 0.05) but had minimal effect on plasma corticosterone, glucagon, or cytokine levels.

CONCLUSION

Resuscitation with RSV restores mitochondrial function and decreases insulin resistance but may be associated with increased hypoglycemia. The observed antiglycemic effects of RSV may be mediated by decreased mitochondrial ROS and increased GLP-1 secretion.

Brain insulin resistance in Alzheimer's disease and its potential treatment with GLP-1 analogs

The prevalence of Alzheimer's disease is increasing rapidly in the absence of truly effective therapies. A promising strategy for developing such therapies is the treatment of brain insulin resistance, a common and early feature of Alzheimer's disease, closely tied to cognitive decline and capable of promoting many biological abnormalities in the disorder. The proximal cause of brain insulin resistance appears to be neuronal elevation in the serine phosphorylation of IRS-1, most likely due to amyloid-β-triggered microglial release of proinflammatory cytokines. Preclinically, the first line of defense is behavior-lowering peripheral insulin resistance (e.g., physical exercise and a Mediterranean diet supplemented with foods rich in flavonoids, curcumin and ω-3 fatty acids). More potent remediation is required, however, at clinical stages. Fortunately, the US FDA-approved antidiabetics exenatide (Byetta; Amylin Pharmaceuticals, Inc., CA, USA) and liraglutide (Victoza; Novo Nordisk A/S, Bagsvaerd, Denmark) are showing much promise in reducing Alzheimer's disease pathology and in restoring normal brain insulin responsiveness and cognitive function.

Exendin-4 Improves Nonalcoholic Fatty Liver Disease by Regulating Glucose Transporter 4 Expression in ob/ob Mice

Exendin-4 (Ex-4), a glucagon-like peptide-1 receptor (GLP-1R) agonist, has been known to reverse hepatic steatosis in ob/ob mice. Although many studies have evaluated molecular targets of Ex-4, its mechanism of action on hepatic steatosis and fibrosis has not fully been determined. In the liver, glucose transporter 4 (GLUT4) is mainly expressed in hepatocytes, endothelial cells and hepatic stellate cells (HSCs). In the present study, the effects of Ex-4 on GLUT4 expression were determined in the liver of ob/ob mice. Ob/ob mice were treated with Ex-4 for 10 weeks. Serum metabolic parameters, hepatic triglyceride levels, and liver tissues were evaluated for hepatic steatosis. The weights of the whole body and liver in ob/ob mice were reduced by long-term Ex-4 treatment. Serum metabolic parameters, hepatic steatosis, and hepatic fibrosis in ob/ob mice were reduced by Ex-4. Particularly, Ex-4 improved hepatic steatosis by enhancing GLUT4 via GLP-1R activation in ob/ob mice. Ex-4 treatment also inhibited hepatic fibrosis by decreasing expression of connective tissue growth factor in HSCs of ob/ob mice. Our data suggest that GLP-1 agonists exert a protective effect on hepatic steatosis and fibrosis in obesity and type 2 diabetes.

Liraglutide enhances glucose transporter 4 translocation via regulation of AMP-activated protein kinase signaling pathways in mouse skeletal muscle cells

OBJECTIVE

Liraglutide is an anti-diabetic drug and human glucagon-like peptide-1 (GLP-1) analog that primarily functions in the pancreas. However, its extra-pancreatic functions are not clear. Skeletal muscle tissue is an important determinant of blood glucose and cells take in approximately 80% of dietary glucose via glucose transporter 4 (GLUT4) on the plasma membrane. Insulin and muscle contraction are two physiological stimuli of GLUT4 translocation to the cell membrane from intracellular storage compartments, but the signaling mechanisms that mediate these processes are different. AMP-activated protein kinase (AMPK) and Akt are the key signal molecules mediating the effects of muscle contraction and insulin, respectively, on GLUT4 translocation. Here, we investigate the effect of liraglutide on GLUT4 translocation and the roles of AMPK and Akt in this mechanism in skeletal muscle cells by stably expressing GLUT4myc with an exofacial myc-epitope C(2)C(12)-GLUT4myc.

MATERIALS/METHODS

The cell surface GLUT4myc levels were determined by an antibody-coupled colorimetric assay. The phosphorylation levels of AMPK, Akt, AS160, TBC1D1, and GLUT4 were determined by western blotting. The cAMP levels were measured by an ELISA kit. siRNA was transfected with Lipofectamine 2000. Analysis of variance (ANOVA) was used for data analysis.

RESULTS

Liraglutide stimulated GLUT4 translocation in C(2)C(12)-GLUT4myc myotubes. Liraglutide increased the intracellular cAMP levels and the phosphorylation of AMPK, AS160, and TBC1D1. Akt phosphorylation and GLUT4 expression were not affected. Inhibition of AMPK by siRNA or Compound C reduced liraglutide-induced GLUT4 translocation.

CONCLUSION

Our results suggest that liraglutide may induce GLUT4 translocation by activation of AMPK in muscle cells.

Deletion of hypoxia-inducible factor-1α in adipocytes enhances glucagon-like peptide-1 secretion and reduces adipose tissue inflammation

It is known that obese adipose tissues are hypoxic and express hypoxia-inducible factor (HIF)-1α. Although some studies have shown that the expression of HIF-1α in adipocytes induces glucose intolerance, the mechanisms are still not clear. In this study, we examined its effects on the development of type 2 diabetes by using adipocyte-specific HIF-1α knockout (ahKO) mice. ahKO mice showed improved glucose tolerance compared with wild type (WT) mice. Macrophage infiltration and mRNA levels of monocyte chemotactic protein-1 (MCP-1) and tumor necrosis factor α (TNFα) were decreased in the epididymal adipose tissues of high fat diet induced obese ahKO mice. The results indicated that the obesity-induced adipose tissue inflammation was suppressed in ahKO mice. In addition, in the ahKO mice, serum insulin levels were increased under the free-feeding but not the fasting condition, indicating that postprandial insulin secretion was enhanced. Serum glucagon-like peptide-1 (GLP-1) levels were also increased in the ahKO mice. Interestingly, adiponectin, whose serum levels were increased in the obese ahKO mice compared with the obese WT mice, stimulated GLP-1 secretion from cultured intestinal L cells. Therefore, insulin secretion may have been enhanced through the adiponectin-GLP-1 pathway in the ahKO mice. Our results suggest that the deletion of HIF-1α in adipocytes improves glucose tolerance by enhancing insulin secretion through the GLP-1 pathway and by reducing macrophage infiltration and inflammation in adipose tissue.

Diabetes drugs and neurological disorders: new views and therapeutic possibilities

Type 2 diabetes is a metabolic disease characterised by insulin resistance with hyperglycaemia and dyslipidaemia. It is associated with increased risk of stroke and vascular dementia, and might contribute to the development of Alzheimer's disease. Recent studies have shown that several antidiabetic drugs can promote neuronal survival and lead to a significant clinical improvement of memory and cognition in different clinical settings. We discuss these emerging data, with a focus on metformin, thiazolidinediones, and the more recently developed compounds targeting the glucagon-like peptide-1 receptor. Data show that these antidiabetic drugs affect brain metabolism, neuroinflammation, and regeneration. Evidence thus far strongly indicates that these antidiabetic drugs could be developed as disease-modifying therapies for human brain diseases in patients with and without diabetes.

The nature, significance, and glucagon-like peptide-1 analog treatment of brain insulin resistance in Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease leading over the course of decades to the most common form of dementia. Many of its pathologic features and cognitive deficits may be due in part to brain insulin resistance recently demonstrated in the insulin receptor→insulin receptor substrate-1 (IRS-1) signaling pathway. The proximal cause of such resistance in AD dementia and amnestic mild cognitive impairment (aMCI) appears to be serine inhibition of IRS-1, a phenomenon likely due to microglial release of inflammatory cytokines triggered by oligomeric Aβ. Studies on animal models of AD and on human brain tissue from MCI cases at high risk of AD dementia have shown that brain insulin resistance and many other pathologic features and symptoms of AD may be greatly reduced or even reversed by treatment with FDA-approved glucagon-like peptide-1 (GLP-1) analogs such as liraglutide (Victoza). These findings call attention to the need for further basic, translational, and clinical studies on GLP-1 analogs as promising AD therapeutics.

Pharmacologic agents for type 2 diabetes therapy and regulation of adipogenesis

The close link between type 2 diabetes and excess body weight highlights the need to consider the effects on weight of different treatments used for correction of hyperglycaemia. Indeed, specific currently available diabetes therapies can cause weight gain, including insulin and its analogues, sulphonylureas, and thiazolidinediones, while others, such as metformin and the GLP-1 receptor agonists, can promote weight loss. Excess body weight in patients with diabetes is largely due to expansion of adipose tissue, and these drugs could interfere with the mechanisms underlying the expansion and differentiation of adipocyte precursors. Almost all anti-diabetes drugs could also potentially affect adipocyte metabolism directly, by modulating lipogenesis, lipolysis, and fat oxidation. This review will examine the available evidence for specific effects of various anti-diabetes drugs on adipose tissue development and function with the ultimate goal of increasing our understanding of how pharmacological agents can modulate energy balance and body fat.

Neurodegenerative disease and obesity: what is the role of weight loss and bariatric interventions?

Neurodegenerative diseases are amongst the leading causes of worldwide disability, morbidity and decreased quality of life. They are increasingly associated with the concomitant worldwide epidemic of obesity. Although the prevalence of both AD and PD continue to rise, the available treatment strategies to combat these conditions remain ineffective against an increase in global neurodegenerative risk factors. There is now epidemiological and mechanistic evidence associating obesity and its related disorders of impaired glucose homeostasis, type 2 diabetes mellitus and metabolic syndrome with both AD and PD. Here we describe the clinical and molecular relationship between obesity and neurodegenerative disease. Secondly we outline the protective role of weight loss, metabolic and caloric modifying interventions in the context of AD and PD. We conclude that the application of caloric restriction through dietary changes, bariatric (metabolic) surgery and gut hormone therapy may offer novel therapeutic strategies against neurodegenerative disorders. Investigating the protective mechanisms of weight loss, metabolic and caloric modifying interventions can increase our understanding of these major public health diseases and their management.

Increased glucagon-like peptide-1 secretion may be involved in antidiabetic effects of ginsenosides

Panax ginseng is one of the most popular herbal remedies. Ginsenosides, major bioactive constituents in P. ginseng, have shown good antidiabetic action, but the precise mechanism was not fully understood. Glucagon-like peptide-1 (GLP1) is considered to be an important incretin that can regulate glucose homeostasis in the gastrointestinal tract after meals. The aim of this study was to investigate whether ginseng total saponins (GTS) exerts its antidiabetic effects via modulating GLP1 release. Ginsenoside Rb1 (Rb1), the most abundant constituent in GTS, was selected to further explore the underlying mechanisms in cultured NCI-H716 cells. Diabetic rats were developed by a combination of high-fat diet and low-dose streptozotocin injection. The diabetic rats orally received GTS (150 or 300 mg/kg) daily for 4 weeks. It was found that GTS treatment significantly ameliorated hyperglycemia and dyslipidemia, accompanied by a significant increase in glucose-induced GLP1 secretion and upregulation of proglucagon gene expression. Data from NCI-H716 cells showed that both GTS and Rb1 promoted GLP1 secretion. It was observed that Rb1 increased the ratio of intracellular ATP to ADP concentration and intracellular Ca2+ concentration. The metabolic inhibitor azide (3 mM), the KATP channel opener diazoxide (340 μM), and the Ca2+ channel blocker nifedipine (20 μM) significantly reversed Rb1-mediated GLP1 secretion. All these results drew a conclusion that ginsenosides stimulated GLP1 secretion both in vivo and in vitro. The antidiabetic effects of ginsenosides may be a result of enhanced GLP1 secretion.

Glucagon-like peptide-1 inhibits adipose tissue macrophage infiltration and inflammation in an obese mouse model of diabetes

AIMS/HYPOTHESIS

Obesity and insulin resistance are associated with low-grade chronic inflammation. Glucagon-like peptide-1 (GLP-1) is known to reduce insulin resistance. We investigated whether GLP-1 has anti-inflammatory effects on adipose tissue, including adipocytes and adipose tissue macrophages (ATM).

METHODS

We administered a recombinant adenovirus (rAd) producing GLP-1 (rAd-GLP-1) to an ob/ob mouse model of diabetes. We examined insulin sensitivity, body fat mass, the infiltration of ATM and metabolic profiles. We analysed the mRNA expression of inflammatory cytokines, lipogenic genes, and M1 and M2 macrophage-specific genes in adipose tissue by real-time quantitative PCR. We also examined the activation of nuclear factor κB (NF-κB), extracellular signal-regulated kinase 1/2 and Jun N-terminal kinase (JNK) in vivo and in vitro.

RESULTS

Fat mass, adipocyte size and mRNA expression of lipogenic genes were significantly reduced in adipose tissue of rAd-GLP-1-treated ob/ob mice. Macrophage populations (F4/80(+) and F4/80(+)CD11b(+)CD11c(+) cells), as well as the expression and production of IL-6, TNF-α and monocyte chemoattractant protein-1, were significantly reduced in adipose tissue of rAd-GLP-1-treated ob/ob mice. Expression of M1-specific mRNAs was significantly reduced, but that of M2-specific mRNAs was unchanged in rAd-GLP-1-treated ob/ob mice. NF-κB and JNK activation was significantly reduced in adipose tissue of rAd-GLP-1-treated ob/ob mice. Lipopolysaccharide-induced inflammation was reduced by the GLP-1 receptor agonist, exendin-4, in 3T3-L1 adipocytes and ATM.

CONCLUSIONS/INTERPRETATION

We suggest that GLP-1 reduces macrophage infiltration and directly inhibits inflammatory pathways in adipocytes and ATM, possibly contributing to the improvement of insulin sensitivity.

Glucagon like peptide-1-induced glucose metabolism in differentiated human muscle satellite cells is attenuated by hyperglycemia

BACKGROUND

Glucagon like peptide-1 (GLP-1) stimulates insulin secretion from the pancreas but also has extra-pancreatic effects. GLP-1 may stimulate glucose uptake in cultured muscle cells but the mechanism is not clearly defined. Furthermore, while the pancreatic effects of GLP-1 are glucose-dependent, the glucose-dependency of its extra-pancreatic effects has not been examined.

METHODS

Skeletal muscle satellite cells isolated from young (22.5 ± 0.97 yr), lean (BMI 22.5 ± 0.6 kg/m(2)), healthy males were differentiated in media containing either 22.5 mM (high) or 5 mM (normal) glucose for 7 days in the absence or presence of insulin and/or various GLP-1 concentrations. Myocellular effects of GLP-1, insulin and glucose were assessed by western-blot, glucose uptake and glycogen synthesis.

RESULTS

We firstly show that the GLP-1 receptor protein is expressed in differentiated human muscle satellite cells (myocytes). Secondly, we show that in 5 mM glucose media, exposure of myocytes to GLP-1 results in a dose dependent increase in glucose uptake, GLUT4 amount and subsequently glycogen synthesis in a PI3K dependent manner, independent of the insulin signaling cascade. Importantly, we provide evidence that differentiation of human satellite cells in hyperglycemic (22.5 mM glucose) conditions increases GLUT1 expression, and renders the cells insulin resistant and interestingly GLP-1 resistant in terms of glucose uptake and glycogen synthesis. Hyperglycemic conditions did not affect the ability of insulin to phosphorylate downstream targets, PKB or GSK3. Interestingly we show that at 5 mM glucose, GLP-1 increases GLUT4 protein levels and that this effect is abolished by hyperglycemia.

CONCLUSIONS

GLP-1 increases glucose uptake and glycogen synthesis into fully-differentiated human satellite cells in a PI3-K dependent mechanism potentially through increased GLUT4 protein levels. The latter occurs independently of the insulin signaling pathway. Attenuation of both GLP-1 and insulin-induced glucose metabolism by hyperglycemia is likely to occur downstream of PI3K.

Demonstrated brain insulin resistance in Alzheimer's disease patients is associated with IGF-1 resistance, IRS-1 dysregulation, and cognitive decline

While a potential causal factor in Alzheimer's disease (AD), brain insulin resistance has not been demonstrated directly in that disorder. We provide such a demonstration here by showing that the hippocampal formation (HF) and, to a lesser degree, the cerebellar cortex in AD cases without diabetes exhibit markedly reduced responses to insulin signaling in the IR→IRS-1→PI3K signaling pathway with greatly reduced responses to IGF-1 in the IGF-1R→IRS-2→PI3K signaling pathway. Reduced insulin responses were maximal at the level of IRS-1 and were consistently associated with basal elevations in IRS-1 phosphorylated at serine 616 (IRS-1 pS⁶¹⁶) and IRS-1 pS⁶³⁶/⁶³⁹. In the HF, these candidate biomarkers of brain insulin resistance increased commonly and progressively from normal cases to mild cognitively impaired cases to AD cases regardless of diabetes or APOE ε4 status. Levels of IRS-1 pS⁶¹⁶ and IRS-1 pS⁶³⁶/⁶³⁹ and their activated kinases correlated positively with those of oligomeric Aβ plaques and were negatively associated with episodic and working memory, even after adjusting for Aβ plaques, neurofibrillary tangles, and APOE ε4. Brain insulin resistance thus appears to be an early and common feature of AD, a phenomenon accompanied by IGF-1 resistance and closely associated with IRS-1 dysfunction potentially triggered by Aβ oligomers and yet promoting cognitive decline independent of classic AD pathology.

Comparing the effects of nano-sized sugarcane fiber with cellulose and psyllium on hepatic cellular signaling in mice

AIM

To compare the effects of dietary fibers on hepatic cellular signaling in mice.

METHODS

Mice were randomly divided into four groups (n = 9/group): high-fat diet (HFD) control, cellulose, psyllium, and sugarcane fiber (SCF) groups. All mice were fed a HFD with or without 10% dietary fiber (w/w) for 12 weeks. Body weight, food intake, fasting glucose, and fasting insulin levels were measured. At the end of the study, hepatic fibroblast growth factor (FGF) 21, AMP-activated protein kinase (AMPK) and insulin signaling protein content were determined.

RESULTS

Hepatic FGF21 content was significantly lowered, but βKlotho, fibroblast growth factor receptor 1, fibroblast growth factor receptor 3, and peroxisome proliferator-activated receptor alpha proteins were significantly increased in the SCF group compared with those in the HFD group (P < 0.01). SCF supplementation also significantly enhanced insulin and AMPK signaling, as well as decreased hepatic triglyceride and cholesterol in comparison with the HFD mice. The study has shown that dietary fiber, especially SCF, significantly attenuates lipid accumulation in the liver by enhancing hepatic FGF21, insulin, and AMPK signaling in mice fed a HFD.

CONCLUSION

This study suggests that the modulation of gastrointestinal factors by dietary fibers may play a key role in both enhancing hepatic multiple cellular signaling and reducing lipid accumulation.

Myocardial insulin signaling and glucose transport are up-regulated in Goto-Kakizaki type 2 diabetic rats after ileal transposition

BACKGROUND

Ileal transposition (IT) as one of the effective treatments for non-obese type 2 diabetes mellitus has been widely investigated. However, the mechanisms underlying profound improvements in glucose homeostasis are still uncertain. Our objective was to explore the myocardial insulin signal transduction and glucose disposal in non-obese type 2 diabetes mellitus rats after IT surgery.

METHODS

Adult male Goto-Kakizaki (GK) rats or Sprague-Dawley (SD) rats were randomly assigned to diabetic IT, diabetic sham-IT, and non-diabetic control SD groups. Food intake, body weight, fasting plasma glucose, insulin tolerance, and serum glucagon-like peptide-1 (GLP-1) were measured. Subsequently, the myocardial glucose uptake and the protein levels of insulin receptor-beta (IR-β), phosphorylated IR-β, insulin receptor substrate 1 (IRS-1), phosphorylated IRS-1, and IRS-1-associated phosphatidylinositol-3 kinase (PI3K) from myocardial cell lysates were evaluated. We also assessed the expression of glucose transporter 4 (GLUT4) in both skeletal muscle and myocardial cell lysates.

RESULTS

Compared to sham operations within 6 months, IT surgery for GK rats did (1) result in less food intake and reduced body weight gain over time, (2) improve plasma glucose homeostasis with increased serum GLP-1 secretion and myocardial glucose uptake, (3) increase protein expression of insulin signaling pathway, including IR-β, IRS-1 and their phosphorylation levels, and IRS-1-associated PI3K in the myocardium, and (4) enhance the protein levels of membrane GLUT4 in skeletal muscle and myocardium.

CONCLUSIONS

IT surgery ameliorates glucose disorder in GK type 2 diabetic rats. Meanwhile, IT surgery is effective in up-regulating both myocardial insulin signaling and glucose disposal within 6 months.

Non-glycaemic effects mediated via GLP-1 receptor agonists and the potential for exploiting these for therapeutic benefit: focus on liraglutide

The glucagon-like peptide-1 receptor agonists (GLP-1 RAs) liraglutide and exenatide can improve glycaemic control by stimulating insulin release through pancreatic β-cells in a glucose-dependent manner. GLP-1 receptors are not restricted to the pancreas; therefore, GLP-1 RAs cause additional non-glycaemic effects. Preclinical and clinical trial data suggest a multitude of additional beneficial effects related to GLP-1 RA therapy, including improvements in β-cell function, systolic blood pressure and body weight. These effects are of a particular advantage to patients with type 2 diabetes, as most are affected by β-cell dysfunction, obesity and hypertension. Transient gastrointestinal adverse events, such as nausea and diarrhoea, are also common. To improve gastrointestinal tolerability, an incremental dosing approach is used with liraglutide and exenatide twice daily. A potential protective role for GLP-1 RAs in the cardiovascular and central nervous systems has been suggested from animal studies and short-term clinical trials. These effects and other safety aspects of GLP-1 therapy are currently being investigated in ongoing long-term clinical studies.

Glp-1 analog, liraglutide, ameliorates hepatic steatosis and cardiac hypertrophy in C57BL/6J mice fed a Western diet

The aims of this study were designed to determine whether liraglutide, a long-acting glucagon-like peptide, could reverse the adverse effects of a diet high in fat that also contained trans-fat and high-fructose corn syrup (ALIOS diet). Specifically, we examined whether treatment with liraglutide could reduce hepatic insulin resistance and steatosis as well as improve cardiac function. Male C57BL/6J mice were pair fed or fed ad libitum either standard chow or the ALIOS diet. After 8 wk the mice were further subdivided and received daily injections of either liraglutide or saline for 4 wk. Hyperinsulinemic-euglycemic clamp studies were performed after 6 wk, revealing hepatic insulin resistance. Glucose tolerance and insulin resistance tests were performed at 8 and 12 wk prior to and following liraglutide treatment. Liver pathology, cardiac measurements, blood chemistry, and RNA and protein analyses were performed. Clamp studies revealed hepatic insulin resistance after 6 wk of ALIOS diet. Liraglutide reduced visceral adiposity and liver weight (P < 0.001). As expected, liraglutide improved glucose and insulin tolerance. Liraglutide improved hypertension (P < 0.05) and reduced cardiac hypertrophy. Surprisingly, liver from liraglutide-treated mice had significantly higher levels of fatty acid binding protein, acyl-CoA oxidase II, very long-chain acyl-CoA dehydrogenase, and microsomal triglyceride transfer protein. We conclude that liraglutide reduces the harmful effects of an ALIOS diet by improving insulin sensitivity and by reducing lipid accumulation in liver through multiple mechanisms including, transport, and increase β-oxidation.

Study of the potential association of adipose tissue GLP-1 receptor with obesity and insulin resistance

The increase in glucagon-like peptide-1 (GLP-1) activity has emerged as a useful therapeutic tool for the treatment of type 2 diabetes mellitus. The actions of GLP-1 on β-cells and the nervous and digestive systems are well known. The action of this peptide in adipose tissue (AT), however, is still poorly defined. Furthermore, no relationship has been established between GLP-1 receptor (GLP-1R) in AT and obesity and insulin resistance (IR). We provide evidence for the presence of this receptor in AT and show that its mRNA and protein expressions are increased in visceral adipose depots from morbidly obese patients with a high degree of IR. Experiments with the 3T3-L1 cell line showed the lipolytic and lipogenic dose-dependent effect of GLP-1. Moreover, GLP-1 stimulated lipolysis in 3T3-L1 adipocytes in a receptor-dependent manner involving downstream adenylate cyclase/cAMP signaling. Our data also demonstrate that the expression of the GLP-1R in AT correlated positively with the homeostasis model assessment index in obese IR subjects. Furthermore, prospective studies carried out with patients that underwent biliopancreatic diversion surgery showed that subjects with high levels of GLP-1R expression in AT, which indicates a deficit of GLP-1 in this tissue, were those whose insulin sensitivity improved after surgery, suggesting the potential relationship between AT GLP-1R and insulin sensitivity amelioration in obese subjects. Altogether these results indicate that the GLP-1/GLP-1R system in AT represents another potential candidate for improving insulin sensitivity in obese patients.