Glucagon-like peptide-1: modulator of β-cell dysfunction and death

Berberine enhances the function of db/db mice islet β cell through GLP-1/GLP-1R/PKA signaling pathway in intestinal L cell and islet α cell

Background: The evidence on berberine stimulating the secretion of GLP-1 in intestinal L cell has been studied. However, few research has explored its role on generating GLP-1 of islet α cell. Our experiment aims to clarify the mechanism of berberine promoting the secretion of GLP-1 in intestinal L cell and islet α cell, activating GLP-1R and its downstream molecules through endocrine and paracrine ways, thus improving the function of islet β cell and treating T2DM. Methods: After confirming that berberine can lower blood glucose and improve insulin resistance in db/db mice, the identity maintenance, proliferation and apoptosis of islet cells were detected by immunohistochemistry and immunofluorescence. Then, the activation of berberine on GLP-1/GLP-1R/PKA signaling pathway was evaluated by Elisa, Western blot and PCR. Finally, this mechanism was verified by in vitro experiments on Min6 cells, STC-1 cells and aTC1/6 cells. Results: Berberine ameliorates glucose metabolism in db/db mice. Additionally, it also increases the number and enhances the function of islet β cell. This process is closely related to improve the secretion of intestinal L cell and islet α cell, activate GLP-1R/PKA signaling pathway through autocrine and paracrine, and increase the expression of its related molecule such as GLP-1, GLP-1R, PC1/3, PC2, PKA, Pdx1. In vitro, the phenomenon that berberine enhanced the GLP-1/GLP-1R/PKA signal pathway had also been observed, which confirmed the results of animal experiments. Conclusion: Berberine can maintain the identity and normal function of islet β cell, and its mechanism is related to the activation of GLP-1/GLP-1R/PKA signal pathway in intestinal L cell and islet α cell.

The Role of the Bacterial Muramyl Dipeptide in the Regulation of GLP-1 and Glycemia

The host's intestinal microbiota contributes to endocrine and metabolic responses, but a dysbiosis in this environment can lead to obesity and insulin resistance. Recent work has demonstrated a role for microbial metabolites in the regulation of gut hormones, including the metabolic hormone, glucagon-like peptide-1 (GLP-1). Muramyl dipeptide (MDP) is a bacterial cell wall component which has been shown to improve insulin sensitivity and glucose tolerance in diet-induced obese mice by acting through the nucleotide oligomerization domain 2 (NOD2) receptor. The purpose of this study was to understand the effects of MDP on GLP-1 secretion and glucose regulation. We hypothesized that MDP enhances glucose tolerance by inducing intestinal GLP-1 secretion through NOD2 activation. First, we observed a significant increase in GLP-1 secretion when murine and human L-cells were treated with a fatty acid MDP derivative (L18-MDP). Importantly, we demonstrated the expression of the NOD2 receptor in mouse intestine and in L-cells. In mice, two intraperitoneal injections of MDP (5 mg/kg body weight) caused a significant increase in fasting total GLP-1 in chow-fed mice, however this did not lead to an improvement in oral glucose tolerance. When mice were exposed to a high-fat diet, they eventually lost this MDP-induced GLP-1 release. Finally, we demonstrated in L-cells that hyperglycemic conditions reduce the mRNA expression of NOD2 and GLP-1. Together these findings suggest MDP may play a role in enhancing GLP-1 during normal glycemic conditions but loses its ability to do so in hyperglycemia.

Genomic history of the Italian population recapitulates key evolutionary dynamics of both Continental and Southern Europeans

BACKGROUND

The cline of human genetic diversity observable across Europe is recapitulated at a micro-geographic scale by variation within the Italian population. Besides resulting from extensive gene flow, this might be ascribable also to local adaptations to diverse ecological contexts evolved by people who anciently spread along the Italian Peninsula. Dissecting the evolutionary history of the ancestors of present-day Italians may thus improve the understanding of demographic and biological processes that contributed to shape the gene pool of European populations. However, previous SNP array-based studies failed to investigate the full spectrum of Italian variation, generally neglecting low-frequency genetic variants and examining a limited set of small effect size alleles, which may represent important determinants of population structure and complex adaptive traits. To overcome these issues, we analyzed 38 high-coverage whole-genome sequences representative of population clusters at the opposite ends of the cline of Italian variation, along with a large panel of modern and ancient Euro-Mediterranean genomes.

RESULTS

We provided evidence for the early divergence of Italian groups dating back to the Late Glacial and for Neolithic and distinct Bronze Age migrations having further differentiated their gene pools. We inferred adaptive evolution at insulin-related loci in people from Italian regions with a temperate climate, while possible adaptations to pathogens and ultraviolet radiation were observed in Mediterranean Italians. Some of these adaptive events may also have secondarily modulated population disease or longevity predisposition.

CONCLUSIONS

We disentangled the contribution of multiple migratory and adaptive events in shaping the heterogeneous Italian genomic background, which exemplify population dynamics and gene-environment interactions that played significant roles also in the formation of the Continental and Southern European genomic landscapes.

Neuroprotective Activity of Sitagliptin via Reduction of Neuroinflammation beyond the Incretin Effect: Focus on Alzheimer's Disease

Sitagliptin is a member of a class of drugs that inhibit dipeptidyl peptidase (DPP-4). It increases the levels of the active form of incretins such as GLP-1 (glucagon-like peptide-1) or GIP (gastric inhibitory polypeptide) and by their means positively affects glucose metabolism. It is successfully applied in the treatment of diabetes mellitus type 2. The most recent scientific reports suggest beneficial effect of sitagliptin on diseases in which neuron damage occurs. Result of experimental studies may indicate a reducing influence of sitagliptin on inflammatory response within encephalon area. Sitagliptin decreased the levels of proinflammatory factors: TNF-α (tumor necrosis factor-α), IL-6 (interleukin-6), IL-17 (interleukin-17), and CD-163 (cluster of differentiation 163), and contributed to an increase in levels of anti-inflammatory factors: IL-10 (interleukin-10) and TGF-β (transforming growth factor β). Moreover, sitagliptin demonstrated antioxidative and antiapoptotic properties by modifying glutamate and glutathione levels within the region of hippocampus in mice. It has been observed that sitagliptin decreases accumulation of β-amyloid within encephalon structures in experimental models of Alzheimer's dementia. This effect may be connected with SDF-1α (stromal cell-derived factor 1α) concentration. Administration of sitagliptin caused a significant improvement in MMSE (Mini-Mental State Examination) tests used for assessment of dementias. The paper presents potential mechanisms of sitagliptin activity in conditions connected with neuroinflammation with special emphasis on Alzheimer's disease.

Therapeutic Potential of Ginsenosides as an Adjuvant Treatment for Diabetes

Ginseng, one of the oldest traditional Chinese medicinal herbs, has been used widely in China and Asia for thousands of years. Ginsenosides extracted from ginseng, which is derived from the roots and rhizomes of Panax ginseng C. A. Meyer, have been used in China as an adjuvant in the treatment of diabetes mellitus. Owing to the technical complexity of ginsenoside production, the total ginsenosides are generally extracted. Accumulating evidence has shown that ginsenosides exert antidiabetic effects. In vivo and in vitro tests revealed the potential of ginsenoside Rg1, Rg3, Rg5, Rb1, Rb2, Rb3, compound K, Rk1, Re, ginseng total saponins, malonyl ginsenosides, Rd, Rh2, F2, protopanaxadiol (PPD) and protopanaxatriol (PPT)-type saponins to treat diabetes and its complications, including type 1 diabetes mellitus, type 2 diabetes mellitus, diabetic nephropathy, diabetic cognitive dysfunction, type 2 diabetes mellitus with fatty liver disease, diabetic cerebral infarction, diabetic cardiomyopathy, and diabetic erectile dysfunction. Many effects are attributed to ginsenosides, including gluconeogenesis reduction, improvement of insulin resistance, glucose transport, insulinotropic action, islet cell protection, hepatoprotective activity, anti-inflammatory effect, myocardial protection, lipid regulation, improvement of glucose tolerance, antioxidation, improvement of erectile dysfunction, regulation of gut flora metabolism, neuroprotection, anti-angiopathy, anti-neurotoxic effects, immunosuppression, and renoprotection effect. The molecular targets of these effects mainly contains GLUTs, SGLT1, GLP-1, FoxO1, TNF-α, IL-6, caspase-3, bcl-2, MDA, SOD, STAT5-PPAR gamma pathway, PI3K/Akt pathway, AMPK-JNK pathway, NF-κB pathway, and endoplasmic reticulum stress. Rg1, Rg3, Rb1, and compound K demonstrated the most promising therapeutic prospects as potential adjuvant medicines for the treatment of diabetes. This paper highlights the underlying pharmacological mechanisms of the anti-diabetic effects of ginsenosides.

Butyrate: A Double-Edged Sword for Health?

Butyrate, a four-carbon short-chain fatty acid, is produced through microbial fermentation of dietary fibers in the lower intestinal tract. Endogenous butyrate production, delivery, and absorption by colonocytes have been well documented. Butyrate exerts its functions by acting as a histone deacetylase (HDAC) inhibitor or signaling through several G protein-coupled receptors (GPCRs). Recently, butyrate has received particular attention for its beneficial effects on intestinal homeostasis and energy metabolism. With anti-inflammatory properties, butyrate enhances intestinal barrier function and mucosal immunity. However, the role of butyrate in obesity remains controversial. Growing evidence has highlighted the impact of butyrate on the gut-brain axis. In this review, we summarize the present knowledge on the properties of butyrate, especially its potential effects and mechanisms involved in intestinal health and obesity.

Exenatide treatment increases serum irisin levels in patients with obesity and newly diagnosed type 2 diabetes

OBJECTIVE

Irisin is a myokine secreted by skeletal muscle during exercise. Abnormal serum irisin levels are associated with obesity and type 2 diabetes (T2D). This study investigated the changes in serum irisin in the obese patients with newly diagnosed T2D following glucagon-like peptide-1 (GLP-1) receptor agonist (exenatide) treatment.

METHODS

Fifty-four obese patients with T2D were treated with exenatide for 12weeks. The control group included 54 age-, sex-, and body mass index (BMI)-matched subjects with normal glucose tolerance.

RESULTS

Patients with T2D had lower irisin than the control group (38.06 [29.29-53.79] vs. 58.01 [43.07-87.79] ng/mL, P<0.01]. Serum irisin was negatively associated with BMI (r=-0.178, P<0.05), fasting blood glucose (FBG; r=-0.170, P<0.05), and glycosylated hemoglobin (HbA1c; r=-0.189, P<0.01) in patients with T2D. Exenatide treatment markedly increased serum irisin by 19.28ng/mL (12.59-25.98) compared to baseline (P<0.01). Increased irisin was significantly correlated with decreased FBG and HbA1c after exenatide treatment (FBG: r=-0.35; HbA1c: r=-0.37; both P<0.05).

CONCLUSIONS

Exenatide treatment significantly increased irisin in patients with T2D. Post-treatment changes in irisin were correlated with decreases in FBG and HbA1c. The upregulation of irisin might be a novel mechanism for the beneficial effects of exenatide in type 2 diabetic patients.

The Role of Common Pharmaceutical Agents on the Prevention and Treatment of Pancreatic Cancer

Survival from pancreatic cancer remains poor. Conventional treatment has resulted in only marginal improvements in survival compared with survival in the previous several decades. Thus, considerable interest has emerged regarding the potential use of common pharmaceutical agents as chemopreventative and chemotherapeutic options. Aspirin, metformin, statins, β-blockers, and bisphosphonates have biologically plausible mechanisms to inhibit pancreatic neoplasia, whereas dipeptidyl-peptidase 4 inhibitors may promote it. Regardless, real-world epidemiological data remain inconclusive. This review examines the hypotheses, evidence, and current state of the literature for each of these medications and their potential roles in the prevention and treatment of pancreatic cancer.

TLR ligands and butyrate increase Pyy expression through two distinct but inter-regulated pathways

The intestinal epithelium is an active barrier separating the host from its microbiota. It senses microbial compounds through expression of a wide range of receptors including the Toll-like receptors (TLRs). TLRs have been shown to regulate epithelium permeability or secretion of defensin by Paneth cells. However, the expression and function of TLRs in enteroendocrine L-cells, a specific subtype of intestinal cells secreting PYY and GLP-1, have not yet been assessed. PYY and GLP-1 are implicated in regulation of gut motility, food intake and insulin secretion, and are of great interest regarding obesity and type 2 diabetes. Using a cellular model of human L-cells and a reporter system for NF-κB activation pathway, we reported functional expression of TLRs in these cells. Stimulation with specific TLR-agonists increased expression of Pyy but not Proglucagon in an NF-κB-dependent manner. Moreover, the effect of TLR stimulation was additive to butyrate, a product of bacterial fermentation, on Pyy expression. Additionally, butyrate also increased Tlr expression, including Tlr4, and the NF-κB response to TLR stimulation. Altogether, our results demonstrated a role of TLRs in the modulation of Pyy expression and the importance of butyrate, a product of bacterial fermentation in regulation of microbial TLR-dependent sensing.

Glucagon-like peptide-1 improves beta-cell antioxidant capacity via extracellular regulated kinases pathway and Nrf2 translocation

Oxidative stress plays an important role in the development of beta-cell dysfunction and insulin resistance, two major pathophysiological abnormalities of type 2 diabetes. Expression levels of antioxidant enzymes in beta cells are very low, rendering them more susceptible to damage caused by reactive oxygen species (ROS). Although the antioxidant effects of glucagon-like peptide-1 (GLP-1) and its analogs have been previously reported, the exact mechanisms involved are still unclear. In this study, we demonstrated that GLP-1 was able to effectively inhibit oxidative stress and cell death of INS-1E beta cells induced by the pro-oxidant tert-butyl hydroperoxide (tert-BOOH). Incubation with GLP-1 enhanced cellular levels of glutathione and the activity of its related enzymes, glutathione-peroxidase (GPx) and -reductase (GR) in beta cells. However, inhibition of ERK, but not of the PI3K/AKT pathway abolished, at least in part, the antioxidant effect of GLP-1. Moreover, ERK activation seems to be protein kinase A (PKA)-dependent because inhibition of PKA with H-89 was sufficient to block the GLP-1-derived protective effect on beta cells. GLP-1 likewise increased the synthesis of GR and favored the translocation of the nuclear transcription factor erythroid 2p45-related factor (Nrf2), a transcription factor implicated in the expression of several antioxidant/detoxificant enzymes. Glucose-stimulated insulin secretion was also preserved in beta-cells challenged with tert-BOOH but pre-treated with GLP-1, probably through the down-regulation of the mitochondrial uncoupling-protein2 (UCP2). Thus, our results provide additional mechanisms of action of GLP-1 to prevent oxidative damage in beta cells through the modulation of signaling pathways involved in antioxidant enzyme regulation.

Identification of Human Islet Amyloid Polypeptide as a BACE2 Substrate

Pancreatic amyloid formation by islet amyloid polypeptide (IAPP) is a hallmark pathological feature of type 2 diabetes. IAPP is stored in the secretory granules of pancreatic beta-cells and co-secreted with insulin to maintain glucose homeostasis. IAPP is innocuous under homeostatic conditions but imbalances in production or processing of IAPP may result in homodimer formation leading to the rapid production of cytotoxic oligomers and amyloid fibrils. The consequence is beta-cell dysfunction and the accumulation of proteinaceous plaques in and around pancreatic islets. Beta-site APP-cleaving enzyme 2, BACE2, is an aspartyl protease commonly associated with BACE1, a related homolog responsible for amyloid processing in the brain and strongly implicated in Alzheimer's disease. Herein, we identify two distinct sites of the mature human IAPP sequence that are susceptible to BACE2-mediated proteolytic activity. The result of proteolysis is modulation of human IAPP fibrillation and human IAPP protein degradation. These results suggest a potential therapeutic role for BACE2 in type 2 diabetes-associated hyperamylinaemia.

Exendin-4 reduces tau hyperphosphorylation in type 2 diabetic rats via increasing brain insulin level

Type 2 diabetes (T2D) is a high risk factor for Alzheimer's disease (AD). Our previous study identified that hyperphosphorylation of tau protein, which is one of the pathophysiologic hallmarks of AD, also occurred in T2D rats' brain; while glucagon-like peptide-1 (GLP-1) mimetics, a type of drug used in T2D, could decrease the phosphorylation of tau, probably via augmenting insulin signaling pathway. The purpose of this study was to further explore the mechanisms that underlie the effect of exendin-4 (ex-4, a GLP-1 receptor agonist) in reducing tau phosphorylation. We found that peripheral ex-4 injection in T2D rats reduced hyperphosphorylation of tau protein in rat hippocampus, probably via increasing hippocampal insulin which activated insulin signaling. Furthermore, we found that ex-4 could neither activate insulin signaling, nor reduce tau phosphorylation in HT22 neuronal cells in the absence of insulin. These results suggested that insulin is required in reduction of tau hyperphosphorylation by ex-4 in brain rats with T2D.

Obesity associated with type 2 diabetes mellitus is linked to decreased PC1/3 mRNA expression in the Jejunum

BACKGROUND

Bariatric surgery is the most effective therapeutic option for obesity and its complications, especially in type 2 diabetes. The aim of this study was to investigate the messenger RNA (mRNA) gene expression of proglucagon, glucose-dependent insulinotropic peptide (GIP), prohormone convertase 1/3 (PC1/3), and dipeptidyl peptidase-IV (DPP-IV) in jejunum cells of the morbidly obese (OB) non type 2 diabetes mellitus (NDM2) and type 2 diabetes mellitus (T2DM), to determine the molecular basis of incretin secretion after bariatric surgery.

METHODS

Samples of jejunal mucosa were obtained from 20 NDM2 patients: removal of a section of the jejunum about 60 cm distal to the ligament of Treitz and 18 T2DM patients: removal of a section of the jejunum about 100 cm distal to the ligament of Treitz. Total RNA was extracted using TRIzol. Reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) was carried out. Samples were sequenced to PC1/3 by ACTGene Análises Moleculares Ltd. Immuno content was quantified with a fluorescence microscope.

RESULTS

T2DM showed decreased PC1/3 mRNA expression in the primers tested (primer a, p=0.014; primer b, p=0.048). Many patients (36.5 %) did not express PC1/3 mRNA. NDM2 and T2DM subjects showed nonsignificantly different proglucagon, GIP, and DPP-IV mRNA expression. The immuno contents of glucagon-like peptide-1 and GIP decreased in T2DM jejunum, but incubation with high glucose stimulated the immuno contents.

CONCLUSIONS

The results suggest that bioactivation of pro-GIP and proglucagon could be impaired by the lower expression of PC1/3 mRNA in jejunum cells of obese patients with T2DM. However, after surgery, food could activate this system and improve glucose levels in these patients.

DPP-4 inhibitors: focus on safety

INTRODUCTION

Dipeptidyl peptidase inhibitors (DPP-4-i) are highly selective inhibitors of the enzyme DPP-4. They act by increasing levels of incretin hormones, which have potent effects on insulin and glucagon release, gastric emptying, and satiety. Our goal is to review the safety issues related to DPP-4-i.

AREAS COVERED

This review is based upon a PubMed search of the literature using keywords alogliptin, linagliptin, saxagliptin, sitagliptin and vildagliptin, DPP-4-i, glucagon-like polypeptide-1 agonists, as well as extensive personal clinical trial experience with each of these agents. The current DPP-4-i have very different chemical structures. Saxagliptin has significant cytochrome P450 metabolism and carries a risk of drug interactions. Linagliptin has primarily entero-hepatic excretion, a benefit in renally impaired patients. A concern arose related to congestive heart failure in the SAVOR TIMI trial of saxagliptin. Several major cardiac studies are underway, with two concluded. Despite lingering uncertainty related to pancreatitis and pancreatic cancer, large randomized trials have not shown an increased risk with DPP-4-i treatment. Cutaneous adverse effects occur with a low frequency with some of these agents.

EXPERT OPINION

DPP-4-i are an additional choice in the group of anti-hyperglycemics. Their principal advantage is a low incidence of hypoglycemia, making these agents desirable in patients such as the elderly and those with cardiac disease. Several large trials have hinted at less cardiac risk with DPP-4-i than with sulfonylureas. The CAROLINA Trial comparing linagliptin and glimepiride may provide a conclusive answer to this question.