Glucagon-like peptide-1 triggers protective pathways in pancreatic beta-cells exposed to glycated serum

DPP-4 is expressed in human pancreatic beta cells and its direct inhibition improves beta cell function and survival in type 2 diabetes

It has been reported that the incretin system, including regulated GLP-1 secretion and locally expressed DPP-4, is present in pancreatic islets. In this study we comprehensively evaluated the expression and role of DPP-4 in islet alpha and beta cells from non-diabetic (ND) and type 2 diabetic (T2D) individuals, including the effects of its inhibition on beta cell function and survival. Isolated islets were prepared from 25 ND and 18 T2D organ donors; studies were also performed with the human insulin-producing EndoC-βH1 cells. Morphological (including confocal microscopy), ultrastructural (electron microscopy, EM), functional (glucose-stimulated insulin secretion), survival (EM and nuclear dyes) and molecular (RNAseq, qPCR and western blot) studies were performed under several different experimental conditions. DPP-4 co-localized with glucagon and was also expressed in human islet insulin-containing cells. Furthermore, DPP-4 was expressed in EndoC-βH1 cells. The proportions of DPP-4 positive alpha and beta cells and DPP-4 gene expression were significantly lower in T2D islets. A DPP-4 inhibitor protected ND human beta cells and EndoC-βH1 cells against cytokine-induced toxicity, which was at least in part independent from GLP1 and associated with reduced NFKB1 expression. Finally, DPP-4 inhibition augmented glucose-stimulated insulin secretion, reduced apoptosis and improved ultrastructure in T2D beta cells. These results demonstrate the presence of DPP-4 in human islet alpha and beta cells, with reduced expression in T2D islets, and show that DPP-4 inhibition has beneficial effects on human ND and T2D beta cells. This suggests that DPP-4, besides playing a role in incretin effects, directly affects beta cell function and survival.

A New Treatment Strategy for Parkinson's Disease through the Gut-Brain Axis: The Glucagon-Like Peptide-1 Receptor Pathway

Molecular communications in the gut-brain axis, between the central nervous system and the gastrointestinal tract, are critical for maintaining healthy brain function, particularly in aging. Epidemiological analyses indicate type 2 diabetes mellitus (T2DM) is a risk factor for neurodegenerative disorders including Alzheimer's disease (AD) and Parkinson's diseases (PD) for which aging shows a major correlative association. Common pathophysiological features exist between T2DM, AD, and PD, including oxidative stress, inflammation, insulin resistance, abnormal protein processing, and cognitive decline, and suggest that effective drugs for T2DM that positively impact the gut-brain axis could provide an effective treatment option for neurodegenerative diseases. Glucagon-like peptide-1 (GLP-1)-based antidiabetic drugs have drawn particular attention as an effectual new strategy to not only regulate blood glucose but also decrease body weight by reducing appetite, which implies that GLP-1 could affect the gut-brain axis in normal and pathological conditions. The neurotrophic and neuroprotective effects of GLP-1 receptor (R) stimulation have been characterized in numerous in vitro and in vivo preclinical studies using GLP-1R agonists and dipeptidyl peptidase-4 inhibitors. Recently, the first open label clinical study of exenatide, a long-acting GLP-1 agonist, in the treatment of PD showed long-lasting improvements in motor and cognitive function. Several double-blind clinical trials of GLP-1R agonists including exenatide in PD and other neurodegenerative diseases are already underway or are about to be initiated. Herein, we review the physiological role of the GLP-1R pathway in the gut-brain axis and the therapeutic strategy of GLP-1R stimulation for the treatment of neurodegenerative diseases focused on PD, for which age is the major risk factor.

RhGLP-1 (7-36) protects diabetic rats against cerebral ischemia-reperfusion injury via up-regulating expression of Nrf2/HO-1 and increasing the activities of SOD

The present study aimed to explore the neuroprotective effect and possible mechanisms of rhGLP-1 (7–36) against transient ischemia/reperfusion injuries induced by middle cerebral artery occlusion (MCAO) in type 2 diabetic rats. First, diabetic rats were established by a combination of a high-fat diet and low-dose streptozotocin (STZ) (30 mg/kg, intraperitoneally). Second, they were subjected to MCAO for 2 h, then treated with rhGLP-1 (7–36) (10, 20, 40 µg/kg i.p.) at the same time of reperfusion. In the following 3 days, they were injected with rhGLP-1 (7–36) at the same dose and route for three times each day. After 72 h, hypoglycemic effects were assessed by blood glucose changes, and neuroprotective effects were evaluated by neurological deficits, infarct volume and histomorphology. Mechanisms were investigated by detecting the distribution and expression of the nuclear factor erythroid-derived factor 2 related factor 2 (Nrf2) in ischemic brain tissue, the levels of phospho-PI3 kinase (PI3K)/PI3K ratio and heme-oxygenase-1 (HO-l), as well as the activities of superoxide dismutase (SOD) and the contents of malondialdehyde (MDA). Our results showed that rhGLP-1 (7–36) significantly reduced blood glucose and infarction volume, alleviated neurological deficits, enhanced the density of surviving neurons and vascular proliferation. The nuclear positive cells ratio and expression of Nrf2, the levels of P-PI3K/PI3K ratio and HO-l increased, the activities of SOD increased and the contents of MDA decreased. The current results indicated the protective effect of rhGLP-1 (7–36) in diabetic rats following MCAO/R that may be concerned with reducing blood glucose, up-regulating expression of Nrf2/HO-1 and increasing the activities of SOD.

Advanced Glycation End Products and Risks for Chronic Diseases: Intervening Through Lifestyle Modification

Advanced glycation end products (AGEs) are a family of compounds of diverse chemical nature that are the products of nonenzymatic reactions between reducing sugars and proteins, lipids, or nucleic acids. AGEs bind to one or more of their multiple receptors (RAGE) found on a variety of cell types and elicit an array of biologic responses. In this review, we have summarized the data on the nature of AGEs and issues associated with their measurements, their receptors, and changes in their expression under different physiologic and disease states. Last, we have used this information to prescribe lifestyle choices to modulate AGE-RAGE cycle for better health.

Glucagon-like peptide-1 attenuates advanced oxidation protein product-mediated damage in islet microvascular endothelial cells partly through the RAGE pathway

Advanced oxidation protein products (AOPPs) are knownt to play a role in the pathogenesis of diseases and related complications. However, whether AOPPs affect the survival of islet microvascular endothelial cells (IMECs) has not been reported to date, at least to the best of our knowledge. In this study, we aimed to investigate the mechanisms underlying AOPP-mediated damage in IMECs and the protective role of glucagon-like peptide-1 (GLP-1), which has been suggested to exert beneficial effects on the cardiovascular system. IMECs were treated with AOPPs (0-200 µg/ml) for 0-72 h in the presence or absence of GLP-1 (100 nmol/l). Apoptosis, cell viability and reactive oxygen species (ROS) production were examined, the expression levels of p53, Bax, receptor for advanced glycation end-products (RAGE) and NAD(P)H oxidase subunit were determined, and the activity of NAD(P)H oxidase, caspase-9 and caspase-3 was also determined. The results revealed that AOPPs increased the expression of RAGE, p47phox and p22phox; induced NAD(P)H oxidase-dependent ROS generation, increased p53 and Bax expression, enhanced the activity of caspase-9 and caspase-3, and induced cell apoptosis. Treatment with GLP-1 decreased the expression of RAGE, inhibited NAD(P)H oxidase activity, decreased cell apoptosis and increased cell viability. On the whole, our findings indicate that AOPPs induce the apoptosis of IMECs via the RAGE-NAD(P)H oxidase-dependent pathway and that treatment with GLP-1 effectively reverses these detrimental effects by decreasing AOPP-induced RAGE expression and restoring the redox balance. Our data may indicate that GLP-1 may prove to be beneficial in attenuating the progression of diabetes mellitus.

Activation of Nrf2 contributes to the protective effect of Exendin-4 against angiotensin II-induced vascular smooth muscle cell senescence

Oxidative stress and impaired antioxidant defense are believed to be contributors to the cardiovascular aging process. The transcription factor nuclear factor-E2-related factor 2 (Nrf2) plays a key role in orchestrating cellular antioxidant defenses and maintaining redox homeostasis. Our previous study showed that Exendin-4, a glucagon-like peptide-1 analog, alleviates angiotensin II (ANG II)-induced vascular smooth muscle cell (VSMC) senescence by inhibiting Rac1 activation via cAMP/PKA (Zhao L, Li AQ, Zhou TF, Zhang MQ, Qin XM. Am J Physiol Cell Physiol 307: C1130-C1141, 2014). The objective of this study is to investigate if Nrf2 mediates the antisenescent effect of Exendin-4 in ANG II-induced VSMCs. Here we report that Exendin-4 triggered Nrf2 nuclear translocation, a downstream target of cAMP-responsive element-binding protein (CREB) and expressions of antioxidant genes heme oxygenase-1 (HO-1) and NAD(P)H quinone oxidoreductase-1 (NQO-1) in a dose- and time-dependent manner. In addition, knock-down of Nrf2 attenuated the inhibitory effects of Exendin-4 on ANG II-induced superoxidant generation and VSMC senescence. PKA/CREB pathway participated in the upregulations of HO-1 and NQO-1 induced by Exendin-4. Notably, our study revealed that Exendin-4 dose-dependently increased the acetylation of Nrf2 and the recruitment of transcriptional coactivator CREB binding protein (CBP) to Nrf2. The Exendin-4-induced Nrf2 transactivation was diminished in the presence of CBP small interfering RNA. Microscope imaging of Nrf2, as well as immunoblotting for Nrf2, showed that the Exendin-4-evoked Nrf2 acetylation favored its nuclear retention. Importantly, CBP silencing attenuated the suppressing effects of Exendin-4 on ANG II-induced VSMC senescence and superoxidant production. In conclusion, these results provide a mechanistic insight into how Nrf2 signaling mediates the antisenescent and antioxidative effects induced by Exendin-4 in VSMCs.

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.

Does Glucagon-like Peptide-1 Ameliorate Oxidative Stress in Diabetes? Evidence Based on Experimental and Clinical Studies

Glucagon-like peptide-1 (GLP-1) has shown to influence the oxidative stress status in a number of in vitro, in vivo and clinical studies. Well-known effects of GLP-1 including better glycemic control, decreased food intake, increased insulin release and increased insulin sensitivity may indirectly contribute to this phenomenon, but glucose-independent effects on ROS level, production and antioxidant capacity have been suggested to also play a role. The potential 'antioxidant' activity of GLP-1 along with other proposed glucose-independent modes of action related to ameliorating redox imbalance remains a controversial topic but could hold a therapeutic potential against micro- and macrovascular diabetic complications. This review discusses the presently available knowledge from experimental and clinical studies on the effects of GLP-1 on oxidative stress in diabetes and diabetes-related complications.

Crosstalk between advanced glycation end products (AGEs)-receptor RAGE axis and dipeptidyl peptidase-4-incretin system in diabetic vascular complications

Advanced glycation end products (AGEs) consist of heterogenous group of macroprotein derivatives, which are formed by non-enzymatic reaction between reducing sugars and amino groups of proteins, lipids and nucleic acids, and whose process has progressed at an accelerated rate under diabetes. Non-enzymatic glycation and cross-linking of protein alter its structural integrity and function, contributing to the aging of macromolecules. Furthermore, engagement of receptor for AGEs (RAGE) with AGEs elicits oxidative stress generation and subsequently evokes proliferative, inflammatory, and fibrotic reactions in a variety of cells. Indeed, accumulating evidence has suggested the active involvement of accumulation of AGEs in diabetes-associated disorders such as diabetic microangiopathy, atherosclerotic cardiovascular diseases, Alzheimer's disease and osteoporosis. Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretins, gut hormones secreted from the intestine in response to food intake, both of which augment glucose-induced insulin release, suppress glucagon secretion, and slow gastric emptying. Since GLP-1 and GIP are rapidly degraded and inactivated by dipeptidyl peptidase-4 (DPP-4), inhibition of DPP-4 and/or DPP-4-resistant GLP-1 analogues have been proposed as a potential target for the treatment of diabetes. Recently, DPP-4 has been shown to cleave multiple peptides, and blockade of DPP-4 could exert diverse biological actions in GLP-1- or GIP-independent manner. This article summarizes the crosstalk between AGEs-RAGE axis and DPP-4-incretin system in the development and progression of diabetes-associated disorders and its therapeutic intervention, especially focusing on diabetic vascular complications.

Glucagon-like peptide-1 secreting cell function as well as production of inflammatory reactive oxygen species is differently regulated by glycated serum and high levels of glucose

Glucagon-like peptide-1 (GLP-1), an intestinal hormone contributing to glucose homeostasis, is synthesized by proglucagon and secreted from intestinal neuroendocrine cells in response to nutrients. GLP-1 secretion is impaired in type 2 diabetes patients. Here, we aimed at investigating whether diabetic toxic products (glycated serum (GS) or high levels of glucose (HG)) may affect viability, function, and insulin sensitivity of the GLP-1 secreting cell line GLUTag. Cells were cultured for 5 days in presence or absence of different dilutions of GS or HG. GS and HG (alone or in combination) increased reactive oxygen species (ROS) production and upregulated proglucagon mRNA expression as compared to control medium. Only HG increased total production and release of active GLP-1, while GS alone abrogated secretion of active GLP-1. HG-mediated effects were associated with the increased cell content of the prohormone convertase 1/3 (PC 1/3), while GS alone downregulated this enzyme. HG upregulated Glucokinase (GK) and downregulated SYNTHAXIN-1. GS abrogated SYNTHAXIN-1 and SNAP-25. Finally, high doses of GS alone or in combination with HG reduced insulin-mediated IRS-1 phosphorylation. In conclusion, we showed that GS and HG might regulate different pathways of GLP-1 production in diabetes, directly altering the function of neuroendocrine cells secreting this hormone.