Antiapoptotic actions of exendin-4 against hypoxia and cytokines are augmented by CREB

The Beneficial Role of Apigenin against Cognitive and Neurobehavioural Dysfunction: A Systematic Review of Preclinical Investigations

Apigenin is a flavone widely present in different fruits and vegetables and has been suggested to possess neuroprotective effects against some neurological disorders. In this study, we systematically reviewed preclinical studies that investigated the effects of apigenin on learning and memory, locomotion activity, anxiety-like behaviour, depressive-like behaviour and sensorimotor and motor coordination in rats and mice with impaired memory and behaviour. We searched SCOPUS, Web of Science, PubMed and Google Scholar for relevant articles. A total of 34 studies were included in this review. The included studies revealed that apigenin enhanced learning and memory and locomotion activity, exhibited anxiolytic effects, attenuated depressive-like behaviour and improved sensorimotor and motor coordination in animals with cognitive impairment and neurobehavioural deficit. Some of the molecular and biochemical mechanisms of apigenin include activation of the ERK/CREB/BDNF signalling pathway; modulation of neurotransmitter levels and monoaminergic, cholinergic, dopaminergic and serotonergic systems; inhibition of pro-inflammatory cytokine production; and attenuation of oxidative neuronal damage. These results revealed the necessity for further research using established doses and short or long durations to ascertain effective and safe doses of apigenin. These results also point to the need for a clinical experiment to ascertain the therapeutic effect of apigenin.

Indirect co-culture of islet cells in 3D biocompatible collagen/laminin scaffold with angiomiRs transfected mesenchymal stem cells

Diabetes is an autoimmune disease in which the pancreatic islets produce insufficient insulin. One of the treatment strategies is islet isolation, which may damage these cells as they lack vasculature. Biocompatible scaffolds are one of the efficient techniques for dealing with this issue. The current study is aimed to determine the effect of transfected BM-MSCS with angiomiR-126 and -210 on the survival and functionality of islets loaded into a 3D scaffold via laminin (LMN). AngiomiRs/Poly Ethylenimine polyplexes were transfected into bone marrow-mesenchymal stem cells (BM-MSCs), followed by 3-day indirect co-culturing with islets laden in collagen (Col)-based hydrogel scaffolds containing LMN. Islet proliferation and viability were significantly increased in LMN-containing scaffolds, particularly in the miRNA-126 treated group. Insulin gene expression was superior in Col scaffolds, especially, in the BM-MSCs/miRNA-126 treated group. VEGF was upregulated in the LMN-containing scaffolds in both miRNA-treated groups, specifically in the miRNA-210, leading to VEGF secretion. MiRNAs' target genes showed no downregulation in LMN-free scaffolds; while a drastic downregulation was seen in the LMN-containing scaffolds. The highest insulin secretion was recorded in the Oxidized dextran (Odex)/Col^LMN+ group with miRNA-126. LMN-containing biocompatible scaffolds, once combined with angiomiRs and their downstream effectors, promote islets survival and restore function, leading to enhanced angiogenesis and glycemic status.

Exenatide regulates Th17/Treg balance via PI3K/Akt/FoxO1 pathway in db/db mice

BACKGROUND

The T helper 17 (Th17)/T regulatory (Treg) cell imbalance is involved in the course of obesity and type 2 diabetes mellitus (T2DM). In the current study, the exact role of glucagon-like peptide-1 receptor agonist (GLP-1RA) exenatide on regulating the Th17/Treg balance and the underlying molecular mechanisms are investigated in obese diabetic mice model.

METHODS

Metabolic parameters were monitored in db/db mice treated with/without exenatide during 8-week study period. The frequencies of Th17 and Treg cells from peripheral blood and pancreas in db/db mice were assessed. The phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt)/Forkhead box O1 (FoxO1) pathway in Th17 and Treg cells from the spleens of male C57BL/6J mice was detected by western blotting. In addition, the expression of glucagon-like peptide-1 receptor (GLP-1R) in peripheral blood mononuclear cells (PBMCs) of male C57BL/6J mice was analyzed.

RESULTS

Exenatide treatment improved β-cell function and insulitis in addition to glucose, insulin sensitivity and weight. Increased Th17 and decreased Treg cells in peripheral blood were present as diabetes progressed while exenatide corrected this imbalance. Progressive IL-17 + T cell infiltration of pancreatic islets was alleviated by exenatide intervention. In vitro study showed no significant difference in the level of GLP-1R expression in PBMCs between control and palmitate (PA) groups. In addition, PA could promote Th17 but suppress Treg differentiation along with down-regulating the phosphorylation of PI3K/Akt/FoxO1, which was reversed by exenatide intervention. FoxO1 inhibitor AS1842856 could abrogate all these effects of exenatide against lipid stress.

CONCLUSIONS

Exenatide could restore systemic Th17/Treg balance via regulating FoxO1 pathway with the progression of diabetes in db/db mice. The protection of pancreatic β-cell function may be partially mediated by inhibiting Th17 cell infiltration into pancreatic islets, and the resultant alleviation of islet inflammation.

The Anti-Inflammatory Effect of Novel Antidiabetic Agents

The incidence of type 2 diabetes (T2DM) has been increasing worldwide and remains one of the leading causes of atherosclerotic disease. Several antidiabetic agents have been introduced in trying to regulate glucose control levels with different mechanisms of action. These agents, and sodium-glucose cotransporter-2 inhibitors in particular, have been endorsed by contemporary guidelines in patients with or without T2DM. Their widespread usage during the last three decades has raised awareness in the scientific community concerning their pleiotropic mechanisms of action, including their putative anti-inflammatory effect. In this review, we delve into the anti-inflammatory role and mechanism of the existing antidiabetic agents in the cardiovascular system and their potential use in other chronic sterile inflammatory conditions.

Crosstalk between incretin hormones, Th17 and Treg cells in inflammatory diseases

Intestinal epithelial cells constantly crosstalk with the gut microbiota and immune cells of the gut lamina propria. Enteroendocrine cells, secrete hormones, such as incretin hormones, which participate in host physiological events, such as stimulating insulin secretion, satiety, and glucose homeostasis. Interestingly, evidence suggests that the incretin pathway may influence immune cell activation. Consequently, drugs targeting the incretin hormone signaling pathway may ameliorate inflammatory diseases such as inflammatory bowel diseases, cancer, and autoimmune diseases. In this review, we discuss how these hormones may modulate two subsets of CD4 + T cells, the regulatory T cells (Treg)/Th17 axis important for gut homeostasis: thus, preventing the development and progression of inflammatory diseases. We also summarize the main experimental and clinical findings using drugs targeting the glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (GLP-1) signaling pathways and their great impact on conditions in which the Treg/Th17 axis is disturbed such as inflammatory diseases and cancer. Understanding the role of incretin stimulation in immune cell activation and function, might contribute to new therapeutic designs for the treatment of inflammatory diseases, autoimmunity, and tumors.

Therapeutic Strategies for Diabetes: Immune Modulation in Pancreatic β Cells

Increased incidence of type I and type II diabetes has been prevailed worldwide. Though the pathogenesis of molecular mechanisms remains still unclear, there are solid evidence that disturbed immune homeostasis leads to pancreatic β cell failure. Currently, autoimmunity and uncontrolled inflammatory signaling pathways have been considered the major factors in the pathogenesis of diabetes. Many components of immune system have been reported to implicate pancreatic β cell failure, including helper T cells, cytotoxic T cells, regulatory T cells and gut microbiota. Immune modulation of those components using small molecules and antibodies, and fecal microbiota transplantation are undergoing in many clinical trials for the treatment of type I and type II diabetes. In this review we will discuss the basis of molecular pathogenesis focusing on the disturbed immune homeostasis in type I and type II diabetes, leading to pancreatic β cell destruction. Finally, we will introduce current therapeutic strategies and clinical trials by modulation of immune system for the treatment of type I and type II diabetes patients.

Insights into a possible role of glucagon-like peptide-1 receptor agonists in the treatment of depression

Depression is a highly prevalent mood disorder and one of the major health concerns in modern society. Moreover, it is characterized by a high prevalence of coexistence with many other diseases including metabolic disorders such as type 2 diabetes mellitus (T2DM) and obesity. Currently used antidepressant drugs, which mostly target brain monoaminergic neurotransmission, have limited clinical efficacy. Although the etiology of depression has not been fully elucidated, current scientific data emphasize the role of neurotrophic factors deficiencies, disturbed homeostasis between the nervous system and the immune and endocrine systems, as well as disturbances in brain energy metabolism and dysfunctions in the gut-brain axis as important factors in the pathogenesis of this neuropsychiatric disorder. Therefore, therapeutic options that could work in a way other than classic antidepressants are being sought to increase the effectiveness of the treatment. Interestingly, glucagon-like peptide-1 receptor agonists (GLP-1RAs), used in the treatment of T2DM and obesity, are known to show pro-cognitive and neuroprotective properties, and exert modulatory effects on immune, endocrine and metabolic processes in the central nervous system. This review article discusses the potential antidepressant effects of GLP-1RAs, especially in the context of their action on the processes related to neuroprotection, inflammation, stress response, energy metabolism, gut-brain crosstalk and the stability of the gut microbiota.

Alogliptin Attenuates Lipopolysaccharide-Induced Neuroinflammation in Mice Through Modulation of TLR4/MYD88/NF-κB and miRNA-155/SOCS-1 Signaling Pathways

BACKGROUND

Endotoxin-induced neuroinflammation plays a crucial role in the pathogenesis and progression of various neurodegenerative diseases. A growing body of evidence supports that incretin-acting drugs possess various neuroprotective effects that can improve learning and memory impairments in Alzheimer's disease models. Thus, the present study aimed to investigate whether alogliptin, a dipeptidyl peptidase-4 inhibitor, has neuroprotective effects against lipopolysaccharide (LPS)-induced neuroinflammation and cognitive impairment in mice as well as the potential mechanisms underlying these effects.

METHODS

Mice were treated with alogliptin (20 mg/kg/d; p.o.) for 14 days, starting 1 day prior to intracerebroventricular LPS injection (8 μg/μL in 3 μL).

RESULTS

Alogliptin treatment alleviated LPS-induced cognitive impairment as assessed by Morris water maze and novel object recognition tests. Moreover, alogliptin reversed LPS-induced increases in toll-like receptor 4 and myeloid differentiation primary response 88 protein expression, nuclear factor-κB p65 content, and microRNA-155 gene expression. It also rescued LPS-induced decreases in suppressor of cytokine signaling gene expression, cyclic adenosine monophosphate (cAMP) content, and phosphorylated cAMP response element binding protein expression in the brain.

CONCLUSION

The present study sheds light on the potential neuroprotective effects of alogliptin against intracerebroventricular LPS-induced neuroinflammation and its associated memory impairment via inhibition of toll-like receptor 4/ myeloid differentiation primary response 88/ nuclear factor-κB signaling, modulation of microRNA-155/suppressor of cytokine signaling-1 expression, and enhancement of cAMP/phosphorylated cAMP response element binding protein signaling.

Vildagliptin attenuates acetic acid-induced colitis in rats via targeting PI3K/Akt/NFκB, Nrf2 and CREB signaling pathways and the expression of lncRNA IFNG-AS1 and miR-146a

Inflammatory processes, including ulcerative colitis (UC), are associated with the increase in synthesis and release of pro-inflammatory cytokines. The release of these cytokines is regulated by phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt)/nuclear factor-kappa B (NFκB) and cAMP response element-binding protein (CREB) signaling pathways as well as over expression of microRNA 146a (miR-146a) and long non-coding RNA interferon gamma antisense 1 (lncRNA IFNG-AS1). Vildagliptin (Vilda), a dipeptidyl peptidase IV (DPP-IV) inhibitor, has an anti-inflammatory, antioxidant and anti-apoptotic effects which were established in various models. However, its possible protective effect in UC has not been clarified. Hence, the current study aimed to explore the possible prophylactic effect of different doses of Vilda against acetic acid (AA)-induced colitis in rats. Forty-eight adult Wistar rats were divided into six groups: control, Vilda (10 mg/kg/day; p.o.), AA, AA + Vilda (5 mg/kg/day; p.o.), AA + Vilda (10 mg/kg/day; p.o.) and AA + sulfasalazine (Sulfa) (100 mg/kg/day; p.o.).Low- and high-dose Vilda showed significant improvement in the disease activity index (DAI) and macroscopic assessment markers. Vilda has markedly inhibited the expression of lncRNA IFNG-AS1 and miR-146a, as well as PI3K/Akt/NFκB pathway, while activated CREB and nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathways, and this was reflected in alleviated oxidative stress, inflammation and apoptosis. Such outcomes were more prominent with the high-dose Vilda versus low-dose Vilda and Sulfa. Moreover, the histological examination showed almost intact histological features in Vilda-treated groups when compared to AA group treated with saline. In conclusion, Vilda can be regarded as a new promising therapeutic alternative against UC.

Hypoxia-Preconditioned Wharton's Jelly-Derived Mesenchymal Stem Cells Mitigate Stress-Induced Apoptosis and Ameliorate Human Islet Survival and Function in Direct Contact Coculture System

Protection of isolated pancreatic islets against hypoxic and oxidative damage-induced apoptosis is essential during a pretransplantation culture period. A beneficial approach to maintain viable and functional islets is the coculture period with mesenchymal stem cells (MSCs). Hypoxia preconditioning of MSCs (Hpc-MSCs) for a short time stimulates the expression and secretion of antiapoptotic, antioxidant, and prosurvival factors. The aim of the present study was to evaluate the survival and function of human islets cocultured with Hpc-MSCs. Wharton's jelly-derived MSCs were subjected to hypoxia (5% O2: Hpc) or normoxia (20% O2: Nc) for 24 hours and then cocultured with isolated human islets in direct and indirect systems. Assays of viability and apoptosis, along with the production of reactive oxygen species (ROS), hypoxia-inducible factor 1-alpha (HIF-1α), apoptotic pathway markers, and vascular endothelial growth factor (VEGF) in the islets, were performed. Insulin and C-peptide secretions as islet function were also evaluated. Hpc-MSCs and Nc-MSCs significantly reduced the ROS production and HIF-1α protein aggregation, as well as downregulation of proapoptotic proteins and upregulation of antiapoptotic marker along with increment of VEGF secretion in the cocultured islet. However, the Hpc-MSCs groups were better than Nc-MSCs cocultured islets. Hpc-MSCs in both direct and indirect coculture systems improved the islet survival, while promotion of function was only significant in the direct cocultured cells. Hpc potentiated the cytoprotective and insulinotropic effects of MSCs on human islets through reducing stressful markers, inhibiting apoptosis pathway, enhancing prosurvival factors, and promoting insulin secretion, especially in direct coculture system, suggesting the effective strategy to ameliorate the islet quality for better transplantation outcomes.

Exosomes derived from human mesenchymal stem cells preserve mouse islet survival and insulin secretion function

Islet cell death and loss of function after isolation and before transplantation is considered a key barrier to successful islet transplantation outcomes. Mesenchymal stem cells (MSCs) have been used to protect isolated islets owing to their paracrine potential partially through the secretion of vascular endothelial growth factor (VEGF). The paracrine functions of MSCs are also mediated, at least in part, by the release of extracellular vesicles including exosomes. In the present study, we examined (i) the effect of exosomes from human MSCs on the survival and function of isolated mouse islets and (ii) whether exosomes contain VEGF and the potential impact of exosomal VEGF on the survival of mouse islets. Isolated mouse islets were cultured for three days with MSC-derived exosomes (MSC-Exo), MSCs, or MSC-conditioned media without exosomes (MSC-CM-without-Exo). We investigated the effects of the exosomes, MSCs, and conditioned media on islet viability, apoptosis and function. Besides the expression of apoptotic and pro-survival genes, the production of human and mouse VEGF proteins was evaluated. The MSCs and MSC-Exo, but not the MSC-CM-without-Exo, significantly decreased the percentage of apoptotic cells and increased islet viability following the downregulation of pro-apoptotic genes and the upregulation of pro-survival factors, as well as the promotion of insulin secretion. Human VEGF was observed in the isolated exosomes, and the gene expression and protein production of mouse VEGF significantly increased in islets cultured with MSC-Exo. MSC-derived exosomes are as efficient as parent MSCs for mitigating cell death and improving islet survival and function. This cytoprotective effect was probably mediated by VEGF transfer, suggesting a pivotal strategy for ameliorating islet transplantation outcomes.

Alleviation of Depression by Glucagon-Like Peptide 1 Through the Regulation of Neuroinflammation, Neurotransmitters, Neurogenesis, and Synaptic Function

Depression has emerged as a major cause of mortality globally. Many studies have reported risk factors and mechanisms associated with depression, but it is as yet unclear how these findings can be applied to the treatment and prevention of this disorder. The onset and recurrence of depression have been linked to diverse metabolic factors, including hyperglycemia, dyslipidemia, and insulin resistance. Recent studies have suggested that depression is accompanied by memory loss as well as depressive mood. Thus, many researchers have highlighted the relationship between depressive behavior and metabolic alterations from various perspectives. Glucagon-like peptide-1 (GLP-1), which is secreted from gut cells and hindbrain areas, has been studied in metabolic diseases such as obesity and diabetes, and was shown to control glucose metabolism and insulin resistance. Recently, GLP-1 was highlighted as a regulator of diverse pathways, but its potential as the therapeutic target of depressive disorder was not described comprehensively. Therefore, in this review, we focused on the potential of GLP-1 modulation in depression.

Significant reduction of apoptosis induced via hypoxia and oxidative stress in isolated human islet by resveratrol

BACKGROUND AND AIMS

Successful islet transplantation as a promising treatment of diabetes type 1 is threatened with the loss of islets during the pre-transplant culture due to hypoxia and oxidative stress-induced apoptosis. Therefore, optimization of culture in order to preserve the islets is a critical point. In this study, we investigated the effect of resveratrol, as a cytoprotective agent, on the cultured human islets.

METHODS AND RESULTS

Isolated islets were treated with different concentrations of resveratrol for 24 and 72 h. Islets' viability, apoptosis, apoptosis markers, and insulin and C-peptide secretion, along with the production of reactive oxygen species (ROS), hypoxia inducible factor 1 alpha (HIF-1α), and its target genes in the islets were investigated. Our findings showed that the islets were exposed to hypoxia and oxidative stress after isolation and during culture. This insult induced apoptosis and decreased viability during 72 h. The presence of resveratrol significantly attenuated HIF-1α and ROS production, reduced apoptosis, promoted the VEGF secretion, and increased the insulin and C-peptide secretion. In this regard, resveratrol improved the islet's survival and function in the culture period.

CONCLUSIONS

Using resveratrol can attenuate the stressful condition for the islets in the pre-transplant culture and subsequently ameliorate their viability and functionality that lead to successful outcome after clinical transplantation.

Liraglutide Protects Against Brain Amyloid-β1-42 Accumulation in Female Mice with Early Alzheimer's Disease-Like Pathology by Partially Rescuing Oxidative/Nitrosative Stress and Inflammation

Alzheimer’s disease (AD) is the most common form of dementia worldwide, being characterized by the deposition of senile plaques, neurofibrillary tangles (enriched in the amyloid beta (Aβ) peptide and hyperphosphorylated tau (p-tau), respectively) and memory loss. Aging, type 2 diabetes (T2D) and female sex (especially after menopause) are risk factors for AD, but their crosslinking mechanisms remain unclear. Most clinical trials targeting AD neuropathology failed and it remains incurable. However, evidence suggests that effective anti-T2D drugs, such as the GLP-1 mimetic and neuroprotector liraglutide, can be also efficient against AD. Thus, we aimed to study the benefits of a peripheral liraglutide treatment in AD female mice. We used blood and brain cortical lysates from 10-month-old 3xTg-AD female mice, treated for 28 days with liraglutide (0.2 mg/kg, once/day) to evaluate parameters affected in AD (e.g., Aβ and p-tau, motor and cognitive function, glucose metabolism, inflammation and oxidative/nitrosative stress). Despite the limited signs of cognitive changes in mature female mice, liraglutide only reduced their cortical Aβ_1–42 levels. Liraglutide partially attenuated brain estradiol and GLP-1 and activated PKA levels, oxidative/nitrosative stress and inflammation in these AD female mice. Our results support the earlier use of liraglutide as a potential preventive/therapeutic agent against the accumulation of the first neuropathological features of AD in females.

Anti-inflammatory potentials of incretin-based therapies used in the management of diabetes

GLP-1 receptor agonists (GLP-1RA) and dipeptidyl peptidase 4 inhibitors (DPP-4i) are two classes of antidiabetic agents used in the management of diabetes based on incretin hormones. There is emerging evidence that they have anti-inflammatory effects. Since most long-term complications of diabetes have a background of chronic inflammation, these agents may be beneficial against diabetic complications not only due to their hypoglycemic potential but also via their anti-inflammatory effects. However, the exact molecular mechanisms by which GLP-1RAs and DPP-4i exert their anti-inflammatory effects are not clearly understood. In this review, we discuss the potential molecular pathways by which these incretin-based therapies exert their anti-inflammatory effects.

Assessing Mouse Islet Function

Islets of Langerhans are clusters of endocrine cells embedded within the exocrine pancreas. Islets constitute only approximately 1-2% of the total pancreas mass in all species, so methods have been developed to digest the pancreas and purify islets from the surrounding acinar cells. This chapter provides detailed protocols for isolation of mouse islets and their in vitro functional characterization in terms of assessments of islet viability, hormone content and secretion, second messenger generation and β-cell proliferation.

Anti-oxidant, anti-apoptotic, anti-hypoxic and anti-inflammatory conditions induced by PTY-2 against STZ-induced stress in islets

The earlier assessment of Pueraria tuberosa (PT) has shown anti-diabetic effects through enhancing incretin action and DPP-IV (Dipeptidyl peptidase-IV) inhibition. The aim of this work was to further explore the protective role of aqueous extract of Pueraria tuberosa tuber (PTY-2) against streptozotocin (STZ) induced islet stress in rats. Diabetes was induced by STZ (65 mg/kg body weight) in charles foster male rats. After 60 days of STZ administration, animals with blood glucose levels > 200 g/dL were considered as diabetic. All the rats were later divided into three groups: Group-1 (STZ untreated normal rats), Group-2 (Diabetic control), and Group-3 (PTY-2 [50 mg/100 g bw treatment for next 10 days to diabetic rats). The rats were then sacrificed after the 10^th day of treatment accordingly. STZ treatment led to an increase in expression of Matrix metalloproteinases-9 (MMP-9), Tumour necrosis factor-α (Tnf-α), Hypoxia inducible factor-α (HIF-1α), Vascular endothelial growth factor (VEGF), Interleukin-6 (IL-6), Protein kinase C-ε (PKC-ε), Nuclear factor kappa-light-chain-enhancer of activated B-cells (NFkB), and Caspase-3. Reverse Transcriptase-PCR (RT-PCR), Immunohistochemistry and Western-Blot analysis showed an increase in the expressions of Superoxide Dismutase (SOD) and Nephrin, and a decrease in the expressions of NFkB, PKC-ε, TNF-α, MMP-9, HIF-1α, VEGF, Caspase-3 and IL-6 after 10 days of PTY-2 treatment. The results showed that PTY-2 favorably changed all the expressions via anti-oxidant, anti-apoptotic, anti-hypoxic and anti-inflammatory pathways, making itself as a protective agent against STZ induced islet stress. Further evaluation of PTY-2 might be helpful in establishing its role in the management of diabetes mellitus.

Protective effect of nobiletin on isolated human islets survival and function against hypoxia and oxidative stress-induced apoptosis

Islets transplantation, as a treatment of type 1 diabetes, faces challenges, including the loss of islets in the process of isolation and pre-transplantation due to cellular stresses-induced apoptosis. Accordingly, the optimization of culture plays a decisive role in the transplantation success. In this study, we evaluated the effect of nobiletin on the cultured human islets. Isolated human islets were treated by different concentrations of nobiletin and cultured for 24 and 72 hours. Then, the islets viability, apoptosis, insulin and C-peptide secretion, and apoptosis markers were evaluated. Also, the production of reactive oxygen species (ROS), hypoxia inducible factor 1 alpha (HIF-1α), and its target genes in the islets were examined. Our findings showed that the islets were encountered with hypoxia and oxidative stress after isolation and during culture. These insults induced apoptosis and reduced viability during culture period. Moreover, the secretion of insulin and C-peptide decreased. Nobiletin treatments significantly improved the islets survival through reduction of HIF-1α and ROS production and suppression of apoptosis, along with increased islets function. Islet protective effect of nobiletin might be related to its anti-oxidant, anti-apoptotic and insulinotropic properties. Hence, in order to achieve viable and functional islets for clinical transplantation, the application of nobiletin during pre-transplantation period is useful.

GLP-1 Limits Adipocyte Inflammation and Its Low Circulating Pre-Operative Concentrations Predict Worse Type 2 Diabetes Remission after Bariatric Surgery in Obese Patients

OBJECTIVE

Glucagon-like peptide (GLP)-1 has been proposed as a key candidate in glucose improvements after bariatric surgery. Our aim was to explore the role of GLP-1 in surgically-induced type 2 diabetes (T2D) improvement and its capacity to regulate human adipocyte inflammation.

METHODS

Basal circulating concentrations of GLP-1 as well as during an oral glucose tolerance test (OGTT) were measured in lean and obese volunteers with and without T2D (n = 93). In addition, GLP-1 levels were determined before and after weight loss achieved by Roux-en-Y gastric bypass (RYGB) (n = 77). The impact of GLP-1 on inflammation signalling pathways was also evaluated.

RESULTS

We show that the reduced (p < 0.05) circulating levels of GLP-1 in obese T2D patients increased (p < 0.05) after RYGB. The area under the curve was significantly lower in obese patients with (p < 0.01) and without (p < 0.05) T2D compared to lean volunteers while obese patients with T2D exhibited decreased GLP-1 levels at baseline (p < 0.05) and 120 min (p < 0.01) after the OGTT. Importantly, higher (p < 0.05) pre-operative GLP-1 concentrations were found in patients with T2D remission after RYGB. We also revealed that exendin-4, a GLP-1 agonist, downregulated the expression of inflammation-related genes (IL1B, IL6, IL8, TNF) and, conversely, upregulated the mRNA levels of ADIPOQ in human visceral adipocytes. Furthermore, exendin-4 blocked (p < 0.05) LPS-induced inflammation in human adipocytes via downregulating the expression and secretion of key inflammatory markers.

CONCLUSIONS

Our data indicate that GLP-1 may contribute to glycemic control and exert a role in T2D remission after RYGB. GLP-1 is also involved in limiting inflammation in human visceral adipocytes.

Overexpression of MUC1 predicts poor prognosis in patients with breast cancer

Breast cancer is the most commonly diagnosed cancer in females; thus, there is an urgent requirement to identify precise biomarkers for the diagnosis and treatment of the disease. Mucin 1 (MUC1) is a glycoprotein that has been demonstrated to be involved in the metastasis and invasion of multiple tumor types. Bioinformatics analyses were conducted to indicate the prognostic value of MUC1 in breast cancer. Additionally, the expression level of MUC1 was assessed using Oncomine analysis. Furthermore, PrognoScan was used to analyze the prognostic value of MUC1 in breast cancer. Mutations of MUC1 were analyzed by the Catalogue of Somatic Mutations in Cancer and cBioPortal databases. In addition, University of California, Santa Cruz (UCSC) was used to examine the methylation status of MUC1. Co-expression of MUC1 mRNA was detected with the cBioPortal, UCSC and Breast Cancer Gene-Expression Miner v4.0 datasets. The results demonstrated that MCU1 is frequently overexpressed in breast cancer and is negatively associated with CpG sites. Furthermore, pooled data indicated that abnormally high expression of MUC1 indicates poor prognosis. Additionally, upregulation of MUC1 expression is associated with estrogen receptor- and progesterone receptor-positive disease, aging and increased Scarff, Bloom and Richardson grade, but is not associated with triple-negative and basal-like status. Subsequent data mining across multiple large databases demonstrated a positive association between MUC1 mRNA expression and cyclic AMP-responsive element-binding protein 3-like 4 (CREB3L4) in breast cancer tissues. The present data indicated that the overexpression of MUC1 indicates a poor prognosis in patients with breast cancer and is associated with MUC1 promoter methylation status. Additionally, MUC1 positively correlated with CREB3L4 and may serve as a potential prognostic factor and therapy target for breast cancer.

Preventive treatment with liraglutide protects against development of glucose intolerance in a rat model of Wolfram syndrome

Wolfram syndrome (WS) is a rare autosomal recessive disorder caused by mutations in the WFS1 (Wolframin1) gene. The syndrome first manifests as diabetes mellitus, followed by optic nerve atrophy, deafness, and neurodegeneration. The underlying mechanism is believed to be a dysregulation of endoplasmic reticulum (ER) stress response, which ultimately leads to cellular death. Treatment with glucagon-like peptide-1 (GLP-1) receptor agonists has been shown to normalize ER stress response in several in vitro and in vivo models. Early chronic intervention with the GLP-1 receptor agonist liraglutide starting before the onset of metabolic symptoms prevented the development of glucose intolerance, improved insulin and glucagon secretion control, reduced ER stress and inflammation in Langerhans islets in Wfs1 mutant rats. Thus, treatment with GLP-1 receptor agonists might be a promising strategy as a preventive treatment for human WS patients.

Brain GLP-1/IGF-1 Signaling and Autophagy Mediate Exendin-4 Protection Against Apoptosis in Type 2 Diabetic Rats

Type 2 diabetes (T2D) is a modern socioeconomic burden, mostly due to its long-term complications affecting nearly all tissues. One of them is the brain, whose dysfunctional intracellular quality control mechanisms (namely autophagy) may upregulate apoptosis, leading to cognitive dysfunction and Alzheimer disease (AD). Since impaired brain insulin signaling may constitute the crosslink between T2D and AD, its restoration may be potentially therapeutic herein. Accordingly, the insulinotropic anti-T2D drugs from glucagon-like peptide-1 (GLP-1) mimetics, namely, exendin-4 (Ex-4), could be a promising therapy. In line with this, we hypothesized that peripherally administered Ex-4 rescues brain intracellular signaling pathways, promoting autophagy and ultimately protecting against chronic T2D-induced apoptosis. Thus, we aimed to explore the effects of chronic, continuous, subcutaneous (s.c.) exposure to Ex-4 in brain cortical GLP-1/insulin/insulin-like growth factor-1 (IGF-1) signaling, and in autophagic and cell death mechanisms in middle-aged (8 months old), male T2D Goto-Kakizaki (GK) rats. We used brain cortical homogenates obtained from middle-aged (8 months old) male Wistar (control) and T2D GK rats. Ex-4 was continuously administered for 28 days, via s.c. implanted micro-osmotic pumps (5 μg/kg/day; infusion rate 2.5 μL/h). Peripheral characterization of the animal models was given by the standard biochemical analyses of blood or plasma, the intraperitoneal glucose tolerance test, and the heart rate. GLP-1, insulin, and IGF-1, their downstream signaling and autophagic markers were evaluated by specific ELISA kits and Western blotting. Caspase-like activities and other apoptotic markers were given by colorimetric methods and Western blotting. Chronic Ex-4 treatment attenuated peripheral features of T2D in GK rats, including hyperglycemia and insulin resistance. Furthermore, s.c. Ex-4 enhanced their brain cortical GLP-1 and IGF-1 levels, and subsequent signaling pathways. Specifically, Ex-4 stimulated protein kinase A (PKA) and phosphoinositide 3-kinase (PI3K)/Akt signaling, increasing cGMP and AMPK levels, and decreasing GSK3β and JNK activation in T2D rat brains. Moreover, Ex-4 regulated several markers for autophagy in GK rat brains (as mTOR, PI3K class III, LC3 II, Atg7, p62, LAMP-1, and Parkin), ultimately protecting against apoptosis (by decreasing several caspase-like activities and mitochondrial cytochrome c, and increasing Bcl2 levels upon T2D). Altogether, this study demonstrates that peripheral Ex-4 administration may constitute a promising therapy against the chronic complications of T2D affecting the brain.

Post-treatment with the GLP-1 analogue liraglutide alleviate chronic inflammation and mitochondrial stress induced by Status epilepticus

Glucagon-like peptide-1(GLP-1) is a growth factor that has neuroprotective and anti-inflammatory properties. The protease resistant GLP-1 analogue liraglutide has been shown to be neuroprotective in previous studies in animal models of Alzheimer's disease or Parkinson's disease. Status epilepticus (SE) is a complex disorder, involving many underlying pathological processes, including excitotoxic and chronic inflammatory events. The present pilot study aims to investigate whether liraglutide alleviates the chronic inflammation response and mitochondrial stress induced by SE in the lithium-pilocarpine animal model. We found that treatment with 25nmol/kg. i.p. once-daily after the induction of SE for 7 days reduced chronic inflammation as shown by reduced numbers of activated microglia and astrocytes, and reduced levels of TNF-α and IL-1ß in the hippocampus. The mitochondrial stress marker BAX was reduced and the survival factor Bcl-2 was enhanced by liraglutide. Blood glucose levels were not affected by liraglutide. We show for the first time that liraglutide can reduce the chronic inflammation and mitochondrial stress induced by SE, and the results suggest that GLP-1 receptor agonists such as liraglutide have restorative and protective effects in the brain after SE and could serve as a potential treatment.

NMDAR antagonists for the treatment of diabetes mellitus-Current status and future directions

Diabetes mellitus is characterized by chronically elevated blood glucose levels accelerated by a progressive decline of insulin-producing β-cells in the pancreatic islets. Although medications are available to transiently adjust blood glucose to normal levels, the effects of current drugs are limited when it comes to preservation of a critical mass of functional β-cells to sustainably maintain normoglycemia. In this review, we recapitulate recent evidence on the role of pancreatic N-methyl-D-aspartate receptors (NMDARs) in β-cell physiology, and summarize effects of morphinan-based NMDAR antagonists that are beneficial for insulin secretion, glucose tolerance and islet cell survival. We further discuss NMDAR-mediated molecular pathways relevant for neuronal cell survival, which may also be important for the preservation of β-cell function and mass. Finally, we summarize the literature for evidence on the role of NMDARs in the development of diabetic long-term complications, and highlight beneficial pharmacologic aspects of NMDAR antagonists in diabetic nephropathy, retinopathy as well as neuropathy.

Anti-Inflammatory Effects of GLP-1-Based Therapies beyond Glucose Control

Glucagon-like peptide-1 (GLP-1) is an incretin hormone mainly secreted from intestinal L cells in response to nutrient ingestion. GLP-1 has beneficial effects for glucose homeostasis by stimulating insulin secretion from pancreatic beta-cells, delaying gastric emptying, decreasing plasma glucagon, reducing food intake, and stimulating glucose disposal. Therefore, GLP-1-based therapies such as GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase-4, which is a GLP-1 inactivating enzyme, have been developed for treatment of type 2 diabetes. In addition to glucose-lowering effects, emerging data suggests that GLP-1-based therapies also show anti-inflammatory effects in chronic inflammatory diseases including type 1 and 2 diabetes, atherosclerosis, neurodegenerative disorders, nonalcoholic steatohepatitis, diabetic nephropathy, asthma, and psoriasis. This review outlines the anti-inflammatory actions of GLP-1-based therapies on diseases associated with chronic inflammation in vivo and in vitro, and their molecular mechanisms of anti-inflammatory action.

Liraglutide is neurotrophic and neuroprotective in neuronal cultures and mitigates mild traumatic brain injury in mice

Traumatic brain injury (TBI), a brain dysfunction for which there is no present effective treatment, is often caused by a concussive impact to the head and affects an estimated 1.7 million Americans annually. Our laboratory previously demonstrated that exendin-4, a long-lasting glucagon-like peptide 1 receptor (GLP-1R) agonist, has neuroprotective effects in cellular and animal models of TBI. Here, we demonstrate neurotrophic and neuroprotective effects of a different GLP-1R agonist, liraglutide, in neuronal cultures and a mouse model of mild TBI (mTBI). Liraglutide promoted dose-dependent proliferation in SH-SY5Y cells and in a GLP-1R over-expressing cell line at reduced concentrations. Pre-treatment with liraglutide rescued neuronal cells from oxidative stress- and glutamate excitotoxicity-induced cell death. Liraglutide produced neurotrophic and neuroprotective effects similar to those of exendin-4 in vitro. The cAMP/PKA/pCREB pathway appears to play an important role in this neuroprotective activity of liraglutide. Furthermore, our findings in cell culture were well-translated in a weight drop mTBI mouse model. Post-treatment with a clinically relevant dose of liraglutide for 7 days in mice ameliorated memory impairments caused by mTBI when evaluated 7 and 30 days post trauma. These data cross-validate former studies of exendin-4 and suggest that liraglutide holds therapeutic potential for the treatment of mTBI. Exendin-4, a long-lasting glucagon-like peptide 1 receptor (GLP-1R) agonist, has neuroprotective effects in cellular and animal models of traumatic brain injury (TBI). Here, we demonstrate neurotrophic and neuroprotective effects of a different GLP-1R agonist, liraglutide, in neuronal cultures and a mouse model of mild TBI (mTBI). Liraglutide promoted dose-dependent proliferation in SH-SY5Y cells and in a GLP-1R over-expressing cell line at reduced concentrations. Pretreatment with liraglutide rescued neuronal cells from oxidative stress- and glutamate excitotoxicity-induced cell death. Liraglutide produced neurotrophic and neuroprotective effects similar to those of exendin-4 in vitro, likely involving the cAMP/PKA/pCREB pathway. Our findings in cell culture were well-translated in a weight-drop mTBI mouse model. Post-treatment with a clinically relevant dose of liraglutide for 7 days in mice ameliorated memory impairments caused by mTBI.

The effects of exendin-4 treatment on graft failure: an animal study using a novel re-vascularized minimal human islet transplant model

Islet transplantation has become a viable clinical treatment, but is still compromised by long-term graft failure. Exendin-4, a glucagon-like peptide 1 receptor agonist, has in clinical studies been shown to improve insulin secretion in islet transplanted patients. However, little is known about the effect of exendin-4 on other metabolic parameters. We therefore aimed to determine what influence exendin-4 would have on revascularized minimal human islet grafts in a state of graft failure in terms of glucose metabolism, body weight, lipid levels and graft survival. Introducing the bilateral, subcapsular islet transplantation model, we first transplanted diabetic mice with a murine graft under the left kidney capsule sufficient to restore normoglycemia. After a convalescent period, we performed a second transplantation under the right kidney capsule with a minimal human islet graft and allowed for a second recovery. We then performed a left-sided nephrectomy, and immediately started treatment with exendin-4 with a low (20μg/kg/day) or high (200μg/kg/day) dose, or saline subcutaneously twice daily for 15 days. Blood was sampled, blood glucose and body weight monitored. The transplanted human islet grafts were collected at study end point and analyzed. We found that exendin-4 exerts its effect on failing human islet grafts in a bell-shaped dose-response curve. Both doses of exendin-4 equally and significantly reduced blood glucose. Glucagon-like peptide 1 (GLP-1), C-peptide and pro-insulin were conversely increased. In the course of the treatment, body weight and cholesterol levels were not affected. However, immunohistochemistry revealed an increase in beta cell nuclei count and reduced TUNEL staining only in the group treated with a low dose of exendin-4 compared to the high dose and control. Collectively, these results suggest that exendin-4 has a potential rescue effect on failing, revascularized human islets in terms of lowering blood glucose, maintaining beta cell numbers, and improving metabolic parameters during hyperglycemic stress.

Dipeptidyl peptidase-4 inhibition in diabetic rats leads to activation of the transcription factor CREB in β-cells

Incretin therapies are effective in controlling blood glucose levels in type 2 diabetic patients by improving the survival and function of β-cells. They include dipeptidyl peptidase-4 (DPP-4) inhibitors and long-acting glucagon-like peptide-1 (GLP-1) analogs. We have previously reported that GLP-1 enhances the survival of cultured human islets by activation of the transcription factor CREB. To test the in vivo relevance of these findings, we examined the effects of alogliptin, a DPP-4 inhibitor, in Zucker Diabetic rats, a model for type 2 diabetes. The plasma levels of GLP-1 increased in alogliptin-treated diabetic rats leading to normoglycemia. Pancreatic islets of untreated diabetic rats were characterized by decreased immunostaining for insulin and PDX-1. Elevation of GLP-1 in treated diabetic rats resulted in the improved survival of β-cells. Dual immunofluorescent staining showed phosphorylation/activation of CREB in insulin-positive β-cells of islets. This led to increases in the levels of CREB targets including Bcl-2, an antiapoptotic mitochondrial protein, BIRC3, a caspase inhibitor and IRS-2, an adapter protein needed for insulin signaling. Findings from this study suggest potential activation of cytoprotective CREB by GLP-1 in pancreatic β-cells of diabetic patients undergoing incretin-based therapies.

CRFR1 activation protects against cytokine-induced β-cell death

During the development of diabetes β-cells are exposed to elevated concentrations of proinflammatory cytokines, TNFα and IL1β, which in vitro induce β-cell death. The class B G-protein-coupled receptors (GPCRs): corticotropin-releasing factor receptor 1 (CRFR1) and CRFR2 are expressed in pancreatic islets. As downstream signaling by other class B GPCRs can protect against cytokine-induced β-cell apoptosis, we evaluated the protective potential of CRFR activation in β-cells in a pro-inflammatory setting. CRFR1/CRFR2 ligands activated AKT and CRFR1 signaling and reduced apoptosis in human islets. In rat and mouse insulin-secreting cell lines (INS-1 and MIN6), CRFR1 agonists upregulated insulin receptor substrate 2 (IRS2) expression, increased AKT activation, counteracted the cytokine-mediated decrease in BAD phosphorylation, and inhibited apoptosis. The anti-apoptotic signaling was dependent on prolonged exposure to corticotropin-releasing factor family peptides and followed PKA-mediated IRS2 upregulation. This indicates that CRFR signaling counteracts proinflammatory cytokine-mediated apoptotic pathways through upregulation of survival signaling in β-cells. Interestingly, CRFR signaling also counteracted basal apoptosis in both cultured INS-1 cells and intact human islets.

CREB mediates the insulinotropic and anti-apoptotic effects of GLP-1 signaling in adult mouse β-cells

Objective

Glucagon-like peptide-1 (GLP-1) plays a major role in pancreatic β-cell function and survival by increasing cytoplasmic cAMP levels, which are thought to affect transcription through activation of the basic leucine zipper (bZIP) transcription factor CREB. Here, we test CREB function in the adult β-cell through inducible gene deletion.

Methods

We employed cell type-specific and inducible gene ablation to determine CREB function in pancreatic β-cells in mice.

Results

By ablating CREB acutely in mature β-cells in tamoxifen-treated Creb^loxP/loxP;Pdx1-CreERT2 mice, we show that CREB has little impact on β-cell turnover, in contrast to what had been postulated previously. Rather, CREB is required for GLP-1 to elicit its full effects on stimulating glucose-induced insulin secretion and protection from cytokine-induced apoptosis. Mechanistically, we find that CREB regulates expression of the pro-apoptotic gene p21 (Cdkn1a) in β-cells, thus demonstrating that CREB is essential to mediating this critical aspect of GLP-1 receptor signaling.

Conclusions

In sum, our studies using conditional gene deletion put into question current notions about the importance of CREB in regulating β-cell function and mass. However, we reveal an important role for CREB in the β-cell response to GLP-1 receptor signaling, further validating CREB as a therapeutic target for diabetes.

Islet cell transplantation for the treatment of type 1 diabetes: recent advances and future challenges

Islet transplantation is a well-established therapeutic treatment for a subset of patients with complicated type I diabetes mellitus. Prior to the Edmonton Protocol, only 9% of the 267 islet transplant recipients since 1999 were insulin independent for >1 year. In 2000, the Edmonton group reported the achievement of insulin independence in seven consecutive patients, which in a collaborative team effort propagated expansion of clinical islet transplantation centers worldwide in an effort to ameliorate the consequences of this disease. To date, clinical islet transplantation has established improved success with insulin independence rates up to 5 years post-transplant with minimal complications. In spite of marked clinical success, donor availability and selection, engraftment, and side effects of immunosuppression remain as existing obstacles to be addressed to further improve this therapy. Clinical trials to improve engraftment, the availability of insulin-producing cell sources, as well as alternative transplant sites are currently under investigation to expand treatment. With ongoing experimental and clinical studies, islet transplantation continues to be an exciting and attractive therapy to treat type I diabetes mellitus with the prospect of shifting from a treatment for some to a cure for all.

The stem cell adjuvant with Exendin-4 repairs the heart after myocardial infarction via STAT3 activation

The poor survival of cells in ischaemic myocardium is a major obstacle for stem cell therapy. Exendin-4 holds the potential of cardioprotective effect based on its pleiotropic activity. This study investigated whether Exendin-4 in conjunction with adipose-derived stem cells (ADSCs) could improve the stem cell survival and contribute to myocardial repairs after infarction. Myocardial infarction (MI) was induced by the left anterior descending artery ligation in adult male Sprague-Dawley rats. ADSCs carrying double-fusion reporter gene [firefly luciferase and monomeric red fluorescent protein (fluc-mRFP)] were quickly injected into border zone of MI in rats treated with or without Exendin-4. Exendin-4 enhanced the survival of transplanted ADSCs, as demonstrated by the longitudinal in vivo bioluminescence imaging. Moreover, ADSCs adjuvant with Exendin-4 decreased oxidative stress, apoptosis and fibrosis. They also improved myocardial viability and cardiac function and increased the differentiation rates of ADSCs into cardiomyocytes and vascular smooth muscle cells in vivo. Then, ADSCs were exposed to hydrogen peroxide/serum deprivation (H₂O₂/SD) to mimic the ischaemic environment in vitro. Results showed that Exendin-4 decreased the apoptosis and enhanced the paracrine effect of ADSCs. In addition, Exendin-4 activated signal transducers and activators of transcription 3 (STAT3) through the phosphorylation of Akt and ERK1/2. Furthermore, Exendin-4 increased the anti-apoptotic protein Bcl-2, but decreased the pro-apoptotic protein Bax of ADSCs. In conclusion, Exendin-4 could improve the survival and therapeutic efficacy of transplanted ADSCs through STAT3 activation via the phosphorylation of Akt and ERK1/2. This study suggests the potential application of Exendin-4 for stem cell–based heart regeneration.

Novel fusion of GLP-1 with a domain antibody to serum albumin prolongs protection against myocardial ischemia/reperfusion injury in the rat

Background

Glucagon-like peptide-1 (GLP-1) and its mimetics reduce infarct size in the setting of acute myocardial ischemia/reperfusion (I/R) injury. However, the short serum half-life of GLP-1 and its mimetics may limit their therapeutic use in acute myocardial ischemia. Domain antibodies to serum albumin (AlbudAbs) have been developed to extend the serum half-life of short lived therapeutic proteins, peptides and small molecules. In this study, we compared the effect of a long acting GLP-1 agonist, DPP-IV resistant GLP-1 (7–36, A8G) fused to an AlbudAb (GAlbudAb), with the effect of the GLP-1 mimetic, exendin-4 (short half-life GLP-1 agonist) on infarct size following acute myocardial I/R injury.

Methods

Male Sprague–Dawley rats (8-week-old) were treated with vehicle, GAlbudAb or exendin-4. Myocardial ischemia was induced 2 h following the final dose for GAlbudAb and 30 min post the final dose for exendin-4. In a subgroup of animals, the final dose of exendin-4 was administered (1 μg/kg, SC, bid for 2 days) 6 h prior to myocardial ischemia when plasma exendin-4 was at its minimum concentration (C_min). Myocardial infarct size, area at risk and cardiac function were determined 24 h after myocardial I/R injury.

Results

GAlbudAb and exendin-4 significantly reduced myocardial infarct size by 28% and 23% respectively, compared to vehicle (both p < 0.01 vs. vehicle) after I/R injury. Moreover, both GAlbudAb and exendin-4 markedly improved post-ischemic cardiac contractile function. Body weight loss and reduced food intake consistent with the activation of GLP-1 receptors was observed in all treatment groups. However, exendin-4 failed to reduce infarct size when administered 6 h prior to myocardial ischemia, suggesting continuous activation of the GLP-1 receptors is needed for cardioprotection.

Conclusions

Cardioprotection provided by GAlbudAb, a long acting GLP-1 mimetic, following myocardial I/R injury was comparable in magnitude, but more sustained in duration than that produced by short-acting exendin-4. Very low plasma concentrations of exendin-4 failed to protect the heart from myocardial I/R injury, suggesting that sustained GLP-1 receptor activation plays an important role in providing cardioprotection in the setting of acute myocardial I/R injury. Long-acting GLP-1 agonists such as GAlbudAb may warrant additional evaluation as novel therapeutic agents to reduce myocardial I/R injury during acute coronary syndrome.

Use of glucagon-like peptide-1 agonists to improve islet graft performance

Human islet transplantation is an effective and promising therapy for type I diabetes. However, long-term insulin independence is both difficult to achieve and inconsistent. De novo or early administration of incretin-based drugs is being explored for improving islet engraftment. In addition to its glucose-dependent insulinotropic effects, incretins also lower postprandial glucose excursion by inhibiting glucagon secretion, delaying gastric emptying, and can protect beta-cell function. Incretin therapy has so far proven clinically safe and tolerable with little hypoglycemic risk. The present review aims to highlight the new frontiers in research involving incretins from both in vitro and in vivo animal studies in the field of islet transplant. It also provides an overview of the current clinical status of incretin usage in islet transplantation in the management of type I diabetes.

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

Glucagon-like peptide-1 (GLP-1) is one of the hormones responsible for the incretin effect, a term that refers to the observation that orally administered glucose results in a larger increase in plasma insulin levels and insulin-dependent decrease in blood glucose concentration when compared to the same amount of glucose given intravenously. GLP-1 is secreted mainly by gut endocrine L-cells and is released under the control of carbohydrates, proteins and lipids. Upon secretion, GLP-1 targets different cell types and exerts a wide variety of actions such as potentiation of glucose-stimulated insulin secretion, reduction of appetite, delay of gastric emptying and increase in β-cell mass. These beneficial effects have resulted in the application of GLP-1-based therapies in patients with type 2 diabetes, but also exploitation of its effects in type 1 diabetes is being envisaged. In this review, we focus on the different, short- and long-term action mechanisms of GLP-1 with specific emphasis on its role as a modulator of β-cell function and survival.

Glucagon-like peptide-1 protects human islets against cytokine-mediated β-cell dysfunction and death: a proteomic study of the pathways involved

Glucagon-like peptide-1 (GLP-1) has been shown to protect pancreatic β-cells against cytokine-induced dysfunction and destruction. The mechanisms through which GLP-1 exerts its effects are complex and still poorly understood. The aim of this study was to analyze the protein expression profiles of human islets of Langerhans treated with cytokines (IL-1β and IFN-γ) in the presence or absence of GLP-1 by 2D difference gel electrophoresis and subsequent protein interaction network analysis to understand the molecular pathways involved in GLP-1-mediated β-cell protection. Co-incubation of cytokine-treated human islets with GLP-1 resulted in a marked protection of β-cells against cytokine-induced apoptosis and significantly attenuated cytokine-mediated inhibition of glucose-stimulated insulin secretion. The cytoprotective effects of GLP-1 coincided with substantial alterations in the protein expression profile of cytokine-treated human islets, illustrating a counteracting effect on proteins from different functional classes such as actin cytoskeleton, chaperones, metabolic proteins, and islet regenerating proteins. In summary, GLP-1 alters in an integrated manner protein networks in cytokine-exposed human islets while protecting them against cytokine-mediated cell death and dysfunction. These data illustrate the beneficial effects of GLP-1 on human islets under immune attack, leading to a better understanding of the underlying mechanisms involved, a prerequisite for improving therapies for diabetic patients.

Induction of an inflammatory loop by interleukin-1β and tumor necrosis factor-α involves NF-kB and STAT-1 in differentiated human neuroprogenitor cells

Proinflammatory cytokines secreted from microglia are known to induce a secondary immune response in astrocytes leading to an inflammatory loop. Cytokines also interfere with neurogenesis during aging and in neurodegenerative diseases. The present study examined the mechanism of induction of inflammatory mediators at the transcriptional level in human differentiated neuroprogenitor cells (NPCs). Interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) induced the expression of cytokines and chemokines in differentiated human NPCs as shown by an immune pathway-specific array. Network motif (NM) analysis of these genes revealed 118 three-node NMs, suggesting complex interactions between inflammatory mediators and transcription factors. Immunofluorescent staining showed increases in the levels of IL-8 and CXCL10 proteins in neurons and glial cells. Findings from Taqman low density array suggested the synergistic actions of IL-1β and TNF-α in the induction of a majority of inflammatory genes by a mechanism involving NF-kB and STAT-1. Nuclear localization of these transcription factors in differentiated NPCs was observed following exposure to IL-1α and TNF-α. Further studies on CXCL10, a chemokine known to be elevated in the Alzheimer's brain, showed that TNF-α is a stronger inducer of CXCL10 promoter when compared to IL-1β. The synergy between these cytokines was lost when ISRE or kB elements in CXCL10 promoter were mutated. Our findings suggest that the activation of inflammatory pathways in neurons and astrocytes through transcription factors including NF-kB and STAT-1 play important roles in neuroglial interactions and in sustaining the vicious cycle of inflammatory response.

A novel strategy to increase the proliferative potential of adult human β-cells while maintaining their differentiated phenotype

Our previous studies demonstrated that Wnt/GSK-3/β-catenin and mTOR signaling are necessary to stimulate proliferative processes in adult human β-cells. Direct inhibition of GSK-3, that engages Wnt signaling downstream of the Wnt receptor, increases β-catenin nuclear translocation and β-cell proliferation but results in lower insulin content. Our current goal was to engage canonical and non-canonical Wnt signaling at the receptor level to significantly increase human β-cell proliferation while maintaining a β-cell phenotype in intact islets. We adopted a system that utilized conditioned medium from L cells that expressed Wnt3a, R-spondin-3 and Noggin (L-WRN conditioned medium). In addition we used a ROCK inhibitor (Y-27632) and SB-431542 (that results in RhoA inhibition) in these cultures. Treatment of intact human islets with L-WRN conditioned medium plus inhibitors significantly increased DNA synthesis ∼6 fold in a rapamycin-sensitive manner. Moreover, this treatment strikingly increased human β-cell proliferation ∼20 fold above glucose alone. Only the combination of L-WRN conditioned medium with RhoA/ROCK inhibitors resulted in substantial proliferation. Transcriptome-wide gene expression profiling demonstrated that L-WRN medium provoked robust changes in several signaling families, including enhanced β-catenin-mediated and β-cell-specific gene expression. This treatment also increased expression of Nr4a2 and Irs2 and resulted in phosphorylation of Akt. Importantly, glucose-stimulated insulin secretion and content were not downregulated by L-WRN medium treatment. Our data demonstrate that engaging Wnt signaling at the receptor level by this method leads to necessary crosstalk between multiple signaling pathways including activation of Akt, mTOR, Wnt/β-catenin, PKA/CREB, and inhibition of RhoA/ROCK that substantially increase human β-cell proliferation while maintaining the β-cell phenotype.

Exendin-4 protects hypoxic islets from oxidative stress and improves islet transplantation outcome

Oxidative stress produced during pancreatic islet isolation leads to significant β-cell damage. Homeostatic cytokines secreted subsequently to islet transplantation damage β-cells by generating oxygen free radicals. In this study, exendin-4, a glucagon-like peptide-1 analog improved islet transplantation outcome by increasing the survival of diabetic recipient mice from 58% to 100%. We hypothesized that this beneficial effect was due to the ability of exendin-4 to reduce oxidative stress. Further experiments showed that it significantly reduced the apoptotic rate of cultured β-cells subjected to hypoxia or to IL-1β. Reduction of apoptotic events was confirmed in pancreatic islet grafts of exendin-4-treated mice. Exendin-4 enhanced Akt phosphorylation of β-cells and insulin released from them. It even augmented insulin secretion from islets cultivated at hypoxic conditions. Exposure to hypoxia led to a decrease in the activation of Akt, which was reversed when β-cells were pretreated with exendin-4. Moreover, exendin-4 increased the activity of redox enzymes in a hypoxia-treated β-cell line and reduced reactive oxygen species production in isolated pancreatic islets. Recovery from diabetes in mice transplanted with hypoxic islets was more efficient when they received exendin-4. In conclusion, exendin-4 rescued islets from oxidative stress caused by hypoxia or due to cytokine exposure. It improved the outcome of syngenic and xenogenic islet transplantation.

Exendin-4 improved rat cortical neuron survival under oxygen/glucose deprivation through PKA pathway

Previous studies demonstrated that exendin-4 (Ex-4) may possess neurotrophic and neuroprotective functions in ischemia insults, but its mechanism remained unknown. Here, by using real-time PCR and ELISA, we identified the distribution of active GLP-1Rs in the rat primary cortical neurons. After establishment of an in vitro ischemia model by oxygen/glucose deprivation (OGD), neurons were treated with various dosages of Ex-4. The MTT assay showed that the relative survival rate increased with the dosage of Ex-4 ranging from 0.2 to 0.8 μg/ml (P<0.001, vs. OGD group). The apoptosis rate was reduced from (49.47±2.70)% to (14.61±0.81)% after Ex-4 treatment (0.4 μg/ml) 12h after OGD (P<0.001). Moreover, immunofluorescence staining indicated that Ex-4 increased glucose-regulated proteins 78 (GRP78) and reduced C/EBP-homologous protein (CHOP). Western blot analysis demonstrated that, after neurons were treated with Ex-4, GRP78 was up-regulated over time (P<0.01, vs. OGD group), while CHOP levels rose to a peak 8h after OGD and then decreased (P<0.05, vs. OGD group). This effect was changed by both the protein kinase A (PKA) inhibitor H89 (P<0.01, P<0.05, respectively, vs. Ex-4 group) and the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (P<0.01, P<0.01, respectively, vs. Ex-4 group) but not by the mitogen-activated protein kinase (MAPK) inhibitor U0126. Our study also revealed that, compared with the Ex-4 group, inhibition of the PKA signaling pathway significantly decreased the survival rate of neurons, down-regulated the expression of B-cell lymphoma 2 (Bcl-2) and up-regulated the Bax expression 3h after ODG (P<0.05, P<0.01, respectively), while neither PI3K nor MAPK inhibition exerted such effects. Furthermore, Western blotting exhibited that PKA expression was elevated in the presence or absence of OGD insults (P<0.05). This study indicated that Ex-4 protected neurons against OGD by modulating the unfolded protein response (UPR) through the PKA pathway and may serve as a novel therapeutic agent for stroke.

Induction of beta-cell resistance to hypoxia and technologies for oxygen delivery to transplanted pancreatic islets

Hypoxia is believed to be a crucial factor involved in cell adaptation to environmental stress. Islet transplantation, especially with immunoisolated islets, interrupts vascular connections, resulting in the substantially decreased delivery of oxygen and nutrients to islet cells. Insulin-producing pancreatic beta cells are known to be highly susceptible to oxygen deficiency. Such susceptibility to hypoxia is believed to be one of the main causes of beta-cell death in the post-transplantation period. Different strategies have been developed for the protection of beta cells against hypoxic injury and for oxygen delivery to transplanted islets. The enhancement of beta-cell defense properties against hypoxia has been achieved using various techniques such as gene transfection, drug supplementation, co-culturing with stem cells and cell selection. Technologies for oxygen delivery to transplanted islets include local neovascularization of subcutaneous sites, electrochemical and photosynthetic oxygen generation, oxygen refuelling of bio-artificial pancreas and whole body oxygenation by using hyperbaric therapy. Progress in the field of oxygen technologies for islet transplantation requires a multidisciplinary approach to explore and optimize the interaction between components of the biological system and different technological processes. This review article focuses mainly on the recently developed strategies for oxygenation and protection from hypoxic injury - to achieve stable and long-term normoglycaemia in diabetic patients with transplanted pancreatic islets.