Autophagy in human type 2 diabetes pancreatic beta cells

Transcribing β-cell mitochondria in health and disease

Background

The recent genome-wide association studies (GWAS) of Type 2 Diabetes (T2D) have identified the pancreatic β-cell as the culprit in the pathogenesis of the disease. Mitochondrial metabolism plays a crucial role in the processes controlling release of insulin and β-cell mass. This notion implies that mechanisms controlling mitochondrial function have the potential to play a decisive pathogenetic role in T2D.

Scope of the review

This article reviews studies demonstrating that there is indeed mitochondrial dysfunction in islets in T2D, and that GWAS have identified a variant in the gene encoding transcription factor B1 mitochondrial (TFB1M), predisposing to T2D due to mitochondrial dysfunction and impaired insulin secretion. Mechanistic studies of the nature of this pathogenetic link, as well as of other mitochondrial transcription factors, are described.

Major conclusions

Based on this, it is argued that transcription and translation in mitochondria are critical processes determining mitochondrial function in β-cells in health and disease.

Pancreatic Beta Cell Identity in Humans and the Role of Type 2 Diabetes

Pancreatic beta cells uniquely synthetize, store, and release insulin. Specific molecular, functional as well as ultrastructural traits characterize their insulin secretion properties and survival phentoype. In this review we focus on human islet/beta cells, and describe the changes that occur in type 2 diabetes and could play roles in the disease as well as represent possible targets for therapeutical interventions. These include transcription factors, molecules involved in glucose metabolism and insulin granule handling. Quantitative and qualitative insulin release patterns and their changes in type 2 diabetes are also associated with ultrastructural features involving the insulin granules, the mitochondria, and the endoplasmic reticulum.

Glucagon-Like Peptide 1 Protects Pancreatic β-Cells From Death by Increasing Autophagic Flux and Restoring Lysosomal Function

Studies in animal models of type 2 diabetes have shown that glucagon-like peptide 1 (GLP-1) receptor agonists prevent β-cell loss. Whether GLP-1 mediates β-cell survival via the key lysosomal-mediated process of autophagy is unknown. In this study, we report that treatment of INS-1E β-cells and primary islets with glucolipotoxicity (0.5 mmol/L palmitate and 25 mmol/L glucose) increases LC3 II, a marker of autophagy. Further analysis indicates a blockage in autophagic flux associated with lysosomal dysfunction. Accumulation of defective lysosomes leads to lysosomal membrane permeabilization and release of cathepsin D, which contributes to cell death. Our data further demonstrated defects in autophagic flux and lysosomal staining in human samples of type 2 diabetes. Cotreatment with the GLP-1 receptor agonist exendin-4 reversed the lysosomal dysfunction, relieving the impairment in autophagic flux and further stimulated autophagy. Small interfering RNA knockdown showed the restoration of autophagic flux is also essential for the protective effects of exendin-4. Collectively, our data highlight lysosomal dysfunction as a critical mediator of β-cell loss and shows that exendin-4 improves cell survival via restoration of lysosomal function and autophagic flux. Modulation of autophagy/lysosomal homeostasis may thus define a novel therapeutic strategy for type 2 diabetes, with the GLP-1 signaling pathway as a potential focus.

Reciprocal regulation of mTOR complexes in pancreatic islets from humans with type 2 diabetes

AIMS/HYPOTHESIS

Mechanistic target of rapamycin complex 1 (mTORC1) is a master regulator of nutritional status at the cellular and organismic level. While mTORC1 mediates beta cell growth and expansion, its hyperactivation has been observed in pancreatic islets from animal models of type 2 diabetes and leads to beta cell loss. We sought to determine whether such mTORC1 activation occurs in humans with type 2 diabetes or in metabolically stressed human islets and whether mTORC1 blockade can restore beta cell function of diabetic islets.

METHODS

Human islets isolated from non-diabetic controls and individuals with type 2 diabetes, as well as human islets and INS-1E cells exposed to increased glucose (22.2 mmol/l), were examined for mTORC1/2 activity by western blotting analysis of phosphorylation of mTORC1 downstream targets ribosomal protein S6 kinase 1 (S6K1), S6 and eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1) and mTORC2 downstream targets Akt and N-myc downstream regulated 1 (NDRG1). mTORC1/2 complexes' integrity was assessed by immunoprecipitation and subsequent western blot analysis. Cell-type specific expression of activated mTORC1 in human islets was examined by immunostaining of pS6 (Ser 235/236) in human islet sections. Beta cell function was measured by glucose-stimulated insulin secretion (GSIS).

RESULTS

While mTORC2 signalling was diminished, mTORC1 activity was markedly increased in islets from patients with type 2 diabetes and in islets and beta cells exposed to increased glucose concentrations. Under high-glucose conditions in metabolically stressed human islets, we identified a reciprocal regulation of different mTOR complexes, with functional upregulation of mTORC1 and downregulation of mTORC2. pS6 immunostaining showed beta cell-specific upregulation of mTORC1 in islets isolated from patients with type 2 diabetes. Inhibition of mTORC1-S6K1 signalling improved GSIS and restored mTORC2 activity in islets from patients with type 2 diabetes as well as in islets isolated from diabetic db/db mice and mice fed a high-fat/high-sucrose diet.

CONCLUSIONS/INTERPRETATION

Our data show the aberrant mTORC1 activity in islets from patients with type 2 diabetes, in human islets cultured under diabetes-associated increased glucose conditions and in diabetic mouse islets. This suggests that elevated mTORC1 activation is a striking pathogenic hallmark of islets in type 2 diabetes, contributing to impaired beta cell function and survival in the presence of metabolic stress.

New insight for metformin against bladder cancer

International Agency for Research on Cancer (IARC) estimated that bladder cancer is the ninth most common cancer in the world, with 430,000 new cases and 165,000 deaths in 2012. Bladder cancer represents the fourth most common cancer in men and ninth most common cancer in women. It is the second most prevalent cancer in men 60 years of age or older in United States. Looking further down, continuing advancements in cancer research could potentially offer more choices for clinician and patient with longer survival and better quality of life. Although, bladder cancer represents an ideal tumor model to test and apply cancer prevention strategies; there are limited studies about application of metformin in the management of bladder cancer. Here, I will shed light on the proposed mechanisms of anti-carcinogenic effects of metformin and cohort of these mechanisms with the novel application of metformin as therapy of bladder cancer.

An Overview on Current Issues and Challenges of Endothelial Progenitor Cell-Based Neovascularization in Patients with Diabetic Foot Ulcer

Diabetic foot ulcer's impaired wound healing, which leads to the development of chronic non-healing wounds and ultimately amputation, is a major problem worldwide. Although recently endothelial progenitor cell-derived cell therapy has been used as a therapeutic intervention to treat diabetic wounds, thereby promoting neovascularization, the results, however, are not satisfactory. In this article, we have discussed the several steps that are involved in the neovascularization process, which might be impaired during diabetes. In addition, we have also discussed the reported possible interventions to correct these impairments. Thus, we have summarized neovascularization as a process with a coordinated sequence of multiple steps and thus, there is the need of a combined therapeutic approach to achieve better treatment outcomes.

Delayed apoptosis allows islet β-cells to implement an autophagic mechanism to promote cell survival

Increased β-cell death coupled with the inability to replicate existing β-cells drives the decline in β-cell mass observed in the progression of both major forms of diabetes. Understanding endogenous mechanisms of islet cell survival could have considerable value for the development of novel strategies to limit β-cell loss and thereby promote β-cell recovery. Insulinoma cells have provided useful insight into β-cell death pathways but observations made in cell lines sometimes fail to translate to primary islets. Here, we report dramatic differences in the temporal regulation and engagement of the apoptotic program in primary rodent islets relative to the INS-1 derived 832/13 cell line. As expected, 832/13 cells rapidly induced cell stress markers in response to ER stress or DNA damage and were fully committed to apoptosis, resulting in >80% cell death within 24 h. In contrast, primary rat islets were largely refractory to cell death in response to ER stress and DNA damage, despite rapid induction of stress markers, such as XBP-1(s), CHOP, and PUMA. Gene expression profiling revealed a general suppression of pro-apoptotic machinery, such as Apaf-1 and caspase 3, and sustained levels of pro-survival factors, such as cIAP-1, cIAP-2, and XIAP, in rat islets. Furthermore, we observed sustained induction of autophagy following chronic ER stress and found that inhibition of autophagy rendered islet β-cells highly vulnerable to ER stress-induced cell death. We propose that islet β-cells dampen the apoptotic response to delay the onset of cell death, providing a temporal window in which autophagy can be activated to limit cellular damage and promote survival.

VMP1-related autophagy induced by a fructose-rich diet in β-cells: its prevention by incretins

The aim of the present study was to demonstrate the role of autophagy and incretins in the fructose-induced alteration of β-cell mass and function. Normal Wistar rats were fed (3 weeks) with a commercial diet without (C) or with 10% fructose in drinking water (F) alone or plus sitagliptin (CS and FS) or exendin-4 (CE and FE). Serum levels of metabolic/endocrine parameters, β-cell mass, morphology/ultrastructure and apoptosis, vacuole membrane protein 1 (VMP1) expression and glucose-stimulated insulin secretion (GSIS) were studied. Complementary to this, islets isolated from normal rats were cultured (3 days) without (C) or with F and F + exendin-4 or chloroquine. Expression of autophagy-related proteins [VMP1 and microtubule-associated protein light chain 3 (LC3)], apoptotic/antiapoptotic markers (caspase-3 and Bcl-2), GSIS and insulin mRNA levels were measured. F rats developed impaired glucose tolerance (IGT) and a significant increase in plasma triacylglycerols, thiobarbituric acid-reactive substances, insulin levels, homoeostasis model assessment (HOMA) for insulin resistance (HOMA-IR) and β-cell function (HOMA-β) indices. A significant reduction in β-cell mass was associated with an increased apoptotic rate and morphological/ultrastructural changes indicative of autophagic activity. All these changes were prevented by either sitagliptin or exendin-4. In cultured islets, F significantly enhanced insulin mRNA and GSIS, decreased Bcl-2 mRNA levels and increased caspase-3 expression. Chloroquine reduced these changes, suggesting the participation of autophagy in this process. Indeed, F induced the increase of both VMP1 expression and LC3-II, suggesting that VMP1-related autophagy is activated in injured β-cells. Exendin-4 prevented islet-cell damage and autophagy development. VMP1-related autophagy is a reactive process against F-induced islet dysfunction, being prevented by exendin-4 treatment. This knowledge could help in the use of autophagy as a potential target for preventing progression from IGT to type 2 diabetes mellitus.

Impaired glucose tolerance attenuates bone accrual by promoting the maturation of osteoblasts: Role of Beclin1-mediated autophagy

Patients with type 2 diabetes mellitus (T2DM) experience a 1.5-3.5 fold increase in fracture risk, but the mechanisms responsible for these alterations in bone biomechanical properties remain elusive. Macroautophagy, often referred to as autophagy, is regulated by signaling downstream of the insulin receptor. Metabolic changes associated with the progression of glucose intolerance have been shown to alter autophagy in various tissues, but limited information is available in relation to bone cells. The aim of this study was to (a) investigate whether autophagy is altered in bone tissue during impaired glucose tolerance, and (b) determine how autophagy impacts osteoblast differentiation, activity, and maturation. Four-week-old, male C57BL/6 mice were fed a control (Con) or high fat (HF) diet for 2, 8, or 16 wks. Mice on the HF diet demonstrated elevated fasting blood glucose and impaired glucose tolerance. Reduced trabecular bone in the femoral neck was evident in the mice on the HF diet by 8 wks compared to Con mice. Histological evaluation of the tibia suggested that the high fat diet promoted terminal differentiation of the osteoblast to an osteocyte. This shift of the osteoblasts towards a non-mineralizing, osteocyte phenotype appears to be coordinated by Beclin1-mediated autophagy. Consistent with these changes in the osteoblast in vivo, the induction of autophagy was able to direct MC3T3-E1 cells towards a more mature osteoblast phenotype. Although these data are somewhat observational, further investigation is warranted to determine if Beclin1-mediated autophagy is essential for the terminal differentiation of the osteoblasts and whether autophagy is having a protective or deleterious effect on bone in T2DM.

Cell-specific increased expression of calpastatin prevents diabetes induced by islet amyloid polypeptide toxicity

The islet in type 2 diabetes (T2D) shares many features of the brain in protein misfolding diseases. There is a deficit of β cells with islet amyloid derived from islet amyloid polypeptide (IAPP), a protein coexpressed with insulin. Small intracellular membrane-permeant oligomers, the most toxic form of IAPP, are more frequent in β cells of patients with T2D and rodents expressing human IAPP. β Cells in T2D, and affected cells in neurodegenerative diseases, share a comparable pattern of molecular pathology, including endoplasmic reticulum stress, mitochondrial dysfunction, attenuation of autophagy, and calpain hyperactivation. While this adverse functional cascade in response to toxic oligomers is well described, the sequence of events and how best to intervene is unknown. We hypothesized that calpain hyperactivation is a proximal event and tested this in vivo by β cell-specific suppression of calpain hyperactivation with calpastatin overexpression in human IAPP transgenic mice. β Cell-specific calpastatin overexpression was remarkably protective against β cell dysfunction and loss and diabetes onset. The critical autophagy/lysosomal pathway for β cell viability was protected with calpain suppression, consistent with findings in models of neurodegenerative diseases. We conclude that suppression of calpain hyperactivation is a potentially beneficial disease-modifying strategy for protein misfolding diseases, including T2D.

Metabolic control of the proteotoxic stress response: implications in diabetes mellitus and neurodegenerative disorders

Proteome homeostasis, or proteostasis, is essential to maintain cellular fitness and its disturbance is associated with a broad range of human health conditions and diseases. Cells are constantly challenged by various extrinsic and intrinsic insults, which perturb cellular proteostasis and provoke proteotoxic stress. To counter proteomic perturbations and preserve proteostasis, cells mobilize the proteotoxic stress response (PSR), an evolutionarily conserved transcriptional program mediated by heat shock factor 1 (HSF1). The HSF1-mediated PSR guards the proteome against misfolding and aggregation. In addition to proteotoxic stress, emerging studies reveal that this proteostatic mechanism also responds to cellular energy state. This regulation is mediated by the key cellular metabolic sensor AMP-activated protein kinase (AMPK). In this review, we present an overview of the maintenance of proteostasis by HSF1, the metabolic regulation of the PSR, particularly focusing on AMPK, and their implications in the two major age-related diseases-diabetes mellitus and neurodegenerative disorders.

Environmental Contaminants and Pancreatic Beta-Cells

Despite health policies as well as clinical and research efforts, diabetes prevalence is still rising around the world. A multitude of causes have been suggested for this increase, mostly related to familial background, the occidental diet which is rich in fat/carbohydrates, and sedentary life style. Type 2 diabetes involves malfunctions of the primary pancreatic beta-cells, usually attributed to local damage; however, it can be associated with other stressful environmental agents, such as chemical contaminants from food, plastic and air, among others. Indeed, exposure to these chemical agents during perinatal and adolescent life can increase the risk of developing cardiometabolic diseases later in life. This review explores data showing which environmental chemical agents may produce injury in beta-cells and further impair the insulinotropic process of type 2 diabetes. Additionally, it points the need to also consider unusual causes of metabolic diseases, such as environmental contaminants.

Autophagy protects human podocytes from high glucose-induced injury by preventing insulin resistance

OBJECTIVE

Insulin resistance is correlated with the progress of albuminuria in diabetic patients, and podocytes are crucial for maintaining the normal function of the glomerular filtration barrier. In the present study, we aimed to investigate the high glucose-induced insulin resistance and cell injury in human podocytes and the putative role of autophagy in this process.

METHODS

Human podocytes were cultured in high glucose-supplemented medium and low glucose and high osmotic conditions were used for the controls. Autophagy in the podocytes was regulated using rapamycin or 3-methyladenine stimulation. Next, autophagy markers including LC3B, Beclin-1, and p62 were investigated using western blot and qPCR, and the insulin responsiveness was analyzed based on glucose uptake and by using the phosphorylation of the insulin receptor with Nephrin as a podocyte injury marker.

RESULTS

The basal autophagy level decreased under the high glucose conditions, which was accompanied by a decrease in the glucose uptake and phosphorylation of the insulin receptor in the human podocytes. More interestingly, the glucose uptake and the phosphorylation of the insulin receptor were decreased by 3-MA stimulation and increased by rapamycin, illustrating that the responsiveness of insulin was regulated by autophagy. The activation of autophagy by rapamycin also ameliorated cell injury in the human podocytes.

CONCLUSIONS

The presence or activation of autophagy was found to play a protective role in human podocytes against high glucose-induced insulin resistance and cell injury, which indicates a novel cellular mechanism and provides a potential therapeutic target for diabetic nephropathy (DN).

Autophagy is a major regulator of beta cell insulin homeostasis

AIMS/HYPOTHESIS

We studied the role of protein degradation pathways in the regulation of insulin production and secretion and hypothesised that autophagy regulates proinsulin degradation, thereby modulating beta cell function.

METHODS

Proinsulin localisation in autophagosomes was demonstrated by confocal and electron microscopy. Autophagy was inhibited by knockdown of autophagy-related (ATG) proteins and using the H(+)-ATPase inhibitor bafilomycin-A1. Proinsulin and insulin content and secretion were assessed in static incubations by ELISA and RIA.

RESULTS

Confocal and electron microscopy showed proinsulin localised in autophagosomes and lysosomes. Beta-Atg7 (-/-) mice had proinsulin-containing sequestosome 1 (p62 [also known as SQSTM1])(+) aggregates in beta cells, indicating proinsulin is regulated by autophagy in vivo. Short-term bafilomycin-A1 treatment and ATG5/7 knockdown increased steady-state proinsulin and hormone precursor chromogranin A content. ATG5/7 knockdown also increased glucose- and non-fuel-stimulated insulin secretion. Finally, mutated forms of proinsulin that are irreparably misfolded and trapped in the endoplasmic reticulum are more resistant to degradation by autophagy.

CONCLUSIONS/INTERPRETATION

In the beta cell, transport-competent secretory peptide precursors, including proinsulin, are regulated by autophagy, whereas efficient clearance of transport-incompetent mutated forms of proinsulin by alternative degradative pathways may be necessary to avoid beta cell proteotoxicity. Reduction of autophagic degradation of proinsulin increases its residency in the secretory pathway, followed by enhanced secretion in response to stimuli.

Mitochondrial network regulation and its potential interference with inflammatory signals in pancreatic beta cells

Mitochondria fulfil multiple tasks in nutrient metabolism, energy production, redox homeostasis and stress response, and are essential for pancreatic beta cell function. The dynamism and health of the mitochondrial network is regulated by fission- and fusion-triggering factors and by a quality control system that removes dysfunctional organelles. Alongside the role of mitochondria in regulating apoptotic cell death mediated primarily via production of reactive oxygen species and release of cytochrome c, there is evidence of other links between mitochondria and inflammation that have implications for cell viability. This review briefly outlines two pathways that are potentially vital for pancreatic beta cell function. The first concerns the regulation of Parkin, a protein that acts, not only as a central player in regulating mitophagy, but also as an activator of the NF-ĸB pathway. The fact that expression of optic atrophy protein 1 (OPA1), a mitochondrial fusion inducer and master mitochondrial cristae biogenetic factor, is increased following NF-ĸB activation highlights a point of mitochondrial control that might be influenced by TNFα signalling. A second axis of interest is suggested by IL-6-mediated upregulation of the fission inducer FIS1 alongside downregulation of mitofusin 2 (MFN2), a guard of mitochondrial fusion and metabolism and an inhibitor of apoptosis. This review summarises a presentation given at the 'Islet inflammation in type 2 diabetes' symposium at the 2015 annual meeting of the EASD. It is accompanied two other reviews on topics from this symposium (by Marc Donath, DOI: 10.1007/s00125-016-3873-z , and Jerry Nadler and colleagues, DOI: 10.1007/s00125-016-3890-y ) and a commentary by the Session Chair, Piero Marchetti (DOI: 10.1007/s00125-016-3875-x ).

Insulin decreases autophagy and leads to cartilage degradation

OBJECTIVE

Autophagy, a key homeostasis mechanism, is defective in Osteoarthritis (OA) and Type 2 Diabetes (T2D). T2D has been proposed as a risk factor for OA. We hypothesized that diabetes impairs articular cartilage integrity by decreasing autophagy. Our objective was to investigate the effects of high glucose and insulin, characteristics of T2D, on cartilage homeostasis.

METHODS

Immortalized human chondrocytes (TC28a2) and primary human chondrocytes (HC) were cultured in 25 mM or 0 mM glucose and treated with insulin (10, 100, 500 nM) for 2, 6 or 24 h. Activity of LC3-II, Akt and rpS6 was evaluated by Western blotting (WB). Human cartilage explants were cultivated with 25 mM glucose and insulin (100,1000 nM) for 24 h to evaluate histopathology. MMP-13 and IL-1β expression was determined by immunohistochemistry and WB. Effects of Rapamycin (10 μM) were analyzed by WB. LC3 and rpS6 expression was determined by WB in chondrocytes from Healthy, Non Diabetic-OA and Diabetic-OA patients.

RESULTS

Insulin downregulates autophagy by reducing LC3 II expression and increasing Akt and rpS6 phosphorylation. Loss of proteoglycans and increased MMP-13 and IL-1β expression was observed after insulin treatment. Autophagy activation by rapamycin reversed insulin effects. Importantly, chondrocytes from diabetic-OA patients showed decreased LC3 and increased p-rpS6 expression compared to Healthy and Non-Diabetic OA patients.

CONCLUSIONS

These results suggest that decreased autophagy might be a mechanism by which diabetes influences cartilage degradation. Pharmacological activation of autophagy may be an effective therapeutic approach to prevent T2D-induced cartilage damage.

The clearance of dying cells: table for two

Phagocytic cells of the immune system must constantly survey for, recognize, and efficiently clear the billions of cellular corpses that arise as a result of development, stress, infection, or normal homeostasis. This process, termed efferocytosis, is critical for the prevention of autoimmune and inflammatory disorders, and persistence of dead cells in tissue is characteristic of many human autoimmune diseases, notably systemic lupus erythematosus. The most notable characteristic of the efferocytosis of apoptotic cells is its ‘immunologically silent' response. Although the mechanisms by which phagocytes facilitate engulfment of dead cells has been a well-studied area, the pathways that coordinate to process the ingested corpse and direct the subsequent immune response is an area of growing interest. The recently described pathway of LC3 (microtubule-associated protein 1A/1B-light chain 3)-associated phagocytosis (LAP) has shed some light on this issue. LAP is triggered when an extracellular particle, such as a dead cell, engages an extracellular receptor during phagocytosis, induces the translocation of autophagy machinery, and ultimately LC3 to the cargo-containing phagosome, termed the LAPosome. In this review, we will examine efferocytosis and the impact of LAP on efferocytosis, allowing us to reimagine the impact of the autophagy machinery on innate host defense mechanisms.

Quercetin-Rich Guava (Psidium guajava) Juice in Combination with Trehalose Reduces Autophagy, Apoptosis and Pyroptosis Formation in the Kidney and Pancreas of Type II Diabetic Rats

We explored whether the combination of anti-oxidant and anti-inflammatory guava (Psidium guajava) and trehalose treatment protects the kidney and pancreas against Type II diabetes (T2DM)-induced injury in rats. We measured the active component of guava juice by HPLC analysis. T2DM was induced in Wistar rats by intraperitoneal administration of nicotinamide and streptozotocin and combination with high fructose diets for 8 weeks. The rats fed with different dosages of guava juice in combination with or without trehalose for 4 weeks were evaluated the parameters including OGTT, plasma insulin, HbA1c, HOMA-IR (insulin resistance) and HOMA-β (β cell function and insulin secretion). We measured oxidative and inflammatory degrees by immunohistochemistry stain, fluorescent stain, and western blot and serum and kidney reactive oxygen species (ROS) by a chemiluminescence analyzer. High content of quercetin in the guava juice scavenged H2O2 and HOCl, whereas trehalose selectively reduced H2O2, not HOCl. T2DM affected the levels in OGTT, plasma insulin, HbA1c, HOMA-IR and HOMA-β, whereas these T2DM-altered parameters, except HbA1c, were significantly improved by guava and trehalose treatment. The levels of T2DM-enhanced renal ROS, 4-hydroxynonenal, caspase-3/apoptosis, LC3-B/autophagy and IL-1β/pyroptosis were significantly decreased by guava juice and trehalose. The combination with trehalose and guava juice protects the pancreas and kidney against T2DM-induced injury.

Factors that Affect Pancreatic Islet Cell Autophagy in Adult Rats: Evaluation of a Calorie-Restricted Diet and a High-Fat Diet

Aging may be a risk factor for type 2 diabetes in the elderly. Dietary intervention can affect glucose tolerance in adults, which may be due to body composition and islet cell autophagy. The aim of this study was to determine the effects of various dietary interventions on islet cell autophagy. Pancreatic tissue and blood samples were collected from Sprague Dawley rats (14–16 months old, n = 15 for each group) that received a normal diet (ND), a high-fat diet (HFD), or a calorie-restricted diet (CRD). The body weight (BW), visceral fat, serum lipid levels, fasting serum glucose, insulin levels, and β/α cell area were determined in 14-16-(0-w), 16-18-(8-w), and 18-20(16-w)-month-old rats. Pancreatic islet autophagy (LC3B and LAMP2), AP (Acid Phosphatase) and apoptosis (apoptosis index, AI (TUNEL assay) and cleaved caspase-3) were detected using immunohistochemistry, ELISA and western blot. At 16 weeks, the expressions of LC3B, LAMP2 and AP markedly increased in both the HFD (P<0.01) and CRD (P<0.05) groups; however, an increase in the AI (P<0.05), cleaved caspase-3 and Beclin1 expression and a decrease in the expressions of BCL2 and BCLXL (P<0.05) were observed in only the HFD group. FFA, triglyceride levels, HOMA-IR, insulin levels and glucagon levels were significantly increased in the HFD group but decreased in the CRD group at 16 weeks (P<0.05). The degree of islet cell autophagy was potentially regulated by the levels of FFA and islet cell insulin and glucagon, which may have been due to the effects of Beclin1/BCL2.

Evidence of β-Cell Dedifferentiation in Human Type 2 Diabetes

CONTEXT

Diabetes is associated with a deficit of insulin-producing β-cells. Animal studies show that β-cells become dedifferentiated in diabetes, reverting to a progenitor-like stage, and partly converting to other endocrine cell types.

OBJECTIVE

To determine whether similar processes occur in human type 2 diabetes, we surveyed pancreatic islets from 15 diabetic and 15 nondiabetic organ donors.

DESIGN

We scored dedifferentiation using markers of endocrine lineage, β-cell-specific transcription factors, and a newly identified endocrine progenitor cell marker, aldehyde dehydrogenase 1A3.

RESULTS

By these criteria, dedifferentiated cells accounted for 31.9% of β-cells in type 2 diabetics vs 8.7% in controls, and for 16.8% vs 6.5% of all endocrine cells (P < .001). The number of aldehyde dehydrogenase 1A3-positive/hormone-negative cells was 3-fold higher in diabetics compared with controls. Moreover, β-cell-specific transcription factors were ectopically found in glucagon- and somatostatin-producing cells of diabetic subjects.

CONCLUSIONS

The data support the view that pancreatic β-cells become dedifferentiated and convert to α- and δ-"like" cells in human type 2 diabetes. The findings should prompt a reassessment of goals in the prevention and treatment of β-cell dysfunction.

Phase III Study on Efficacy and Safety of Triple Combination (Exenatide/Metformin/Biphasic Insulin Aspart) Therapy for Type 2 Diabetes Mellitus

Exenatide, metformin (MET), and biphasic insulin aspart 30 (BIA30) have been widely used in the treatment of patients with type 2 diabetes mellitus (T2DM); however, each of these medications has significant adverse effects, which limit their utilization. This study aimed to evaluate the efficacy and safety of triple combination (exenatide/metformin/biphasic insulin aspart) therapy for T2DM. Two hundred patients with poorly controlled T2DM were randomly divided into the low-dose (0.5 μg exenatide, 0.05 U·kg·d BIA30, and 0.01 g MET twice daily) and normal-dose (2 μg exenatide, 0.2 U·kg·d BIA30, and 0.05 g MET twice daily) groups for 48 weeks of treatment. Of note, 82 and 90 individuals from the low-dose and normal-dose groups, respectively, completed the study. The levels of adiponectin, C-reactive protein, tumor necrosis factor-α, and resistin were measured. The normal-dose treatment was more effective at lowering hemoglobin A1c levels than the low-dose therapy (HbA1c changes of -2.5 ± 0.19% and -0.8 ± 0.07%, respectively) after 48 weeks. The maximum weight decrease was 0.9 kg in the low-dose group and 4.0 kg in the normal-dose group. The triple combination therapy increased the levels of insulin sensitivity and adiponectin and reduced the levels of C-reactive protein, resistin, and tumor necrosis factor-α. No significant difference in the adverse effects was found between the low-dose and normal-dose groups (P > 0.05). In conclusion, the investigated triple combination therapy for T2MD is therefore an effective and safe therapeutic strategy.

Therapeutic targeting of autophagy in cardiovascular disease

Autophagy is an evolutionarily ancient process of intracellular catabolism necessary to preserve cellular homeostasis in response to a wide variety of stresses. In the case of post-mitotic cells, where cell replacement is not an option, finely tuned quality control of cytoplasmic constituents and organelles is especially critical. And due to the ubiquitous and critical role of autophagic flux in the maintenance of cell health, it comes as little surprise that perturbation of the autophagic process is observed in multiple disease processes. A large body of preclinical evidence suggests that autophagy is a double-edged sword in cardiovascular disease, acting in either beneficial or maladaptive ways, depending on the context. In light of this, the autophagic machinery in cardiomyocytes and other cardiovascular cell types has been proposed as a potential therapeutic target. Here, we summarize current knowledge regarding the dual functions of autophagy in cardiovascular disease. We go on to analyze recent evidence suggesting that titration of autophagic flux holds potential as a novel treatment strategy.

Nε-(carboxymethyl) lysine-induced mitochondrial fission and mitophagy cause decreased insulin secretion from β-cells

Nε-(carboxymethyl) lysine-conjugated bovine serum albumin (CML-BSA) is a major component of advanced glycation end products (AGEs). We hypothesised that AGEs reduce insulin secretion from pancreatic β-cells by damaging mitochondrial functions and inducing mitophagy. Mitochondrial morphology and the occurrence of autophagy were examined in pancreatic islets of diabetic db/db mice and in the cultured CML-BSA-treated insulinoma cell line RIN-m5F. In addition, the effects of α-lipoic acid (ALA) on mitochondria in AGE-damaged tissues were evaluated. The diabetic db/db mouse exhibited an increase in the number of autophagosomes in damaged mitochondria and receptor for AGEs (RAGE). Treatment of db/db mice with ALA for 12 wk increased the number of mitochondria with well-organized cristae and fewer autophagosomes. Treatment of RIN-m5F cells with CML-BSA increased the level of RAGE protein and autophagosome formation, caused mitochondrial dysfunction, and decreased insulin secretion. CML-BSA also reduced mitochondrial membrane potential and ATP production, increased ROS and lipid peroxide production, and caused mitochondrial DNA deletions. Elevated fission protein dynamin-related protein 1 (Drp1) level and mitochondrial fragmentation demonstrated the unbalance of mitochondrial fusion and fission in CML-BSA-treated cells. Additionally, increased levels of Parkin and PTEN-induced putative kinase 1 protein suggest that fragmented mitochondria were associated with increased mitophagic activity, and ALA attenuated the CML-BSA-induced mitophage formation. Our study demonstrated that CML-BSA induced mitochondrial dysfunction and mitophagy in pancreatic β-cells. The findings from this study suggest that increased concentration of AGEs may damage β-cells and reduce insulin secretion.

The Role of Organelle Stresses in Diabetes Mellitus and Obesity: Implication for Treatment

The type 2 diabetes pandemic in recent decades is a huge global health threat. This pandemic is primarily attributed to the surplus of nutrients and the increased prevalence of obesity worldwide. In contrast, calorie restriction and weight reduction can drastically prevent type 2 diabetes, indicating a central role of nutrient excess in the development of diabetes. Recently, the molecular links between excessive nutrients, organelle stress, and development of metabolic disease have been extensively studied. Specifically, excessive nutrients trigger endoplasmic reticulum stress and increase the production of mitochondrial reactive oxygen species, leading to activation of stress signaling pathway, inflammatory response, lipogenesis, and pancreatic beta-cell death. Autophagy is required for clearance of hepatic lipid clearance, alleviation of pancreatic beta-cell stress, and white adipocyte differentiation. ROS scavengers, chemical chaperones, and autophagy activators have demonstrated promising effects for the treatment of insulin resistance and diabetes in preclinical models. Further results from clinical trials are eagerly awaited.

Mast cells infiltrate pancreatic islets in human type 1 diabetes

AIMS/HYPOTHESIS

Beta cell destruction in human type 1 diabetes occurs through the interplay of genetic and environmental factors, and is mediated by immune cell infiltration of pancreatic islets. In this study, we explored the role of mast cells as an additional agent in the pathogenesis of type 1 diabetes insulitis.

METHODS

Pancreatic tissue from donors without diabetes and with type 1 and 2 diabetes was studied using different microscopy techniques to identify islet-infiltrating cells. The direct effects of histamine exposure on isolated human islets and INS-1E cells were assessed using cell-survival studies and molecular mechanisms.

RESULTS

A larger number of mast cells were found to infiltrate pancreatic islets in samples from donors with type 1 diabetes, compared with those from donors without diabetes or with type 2 diabetes. Evidence of mast cell degranulation was observed, and the extent of the infiltration correlated with beta cell damage. Histamine, an amine that is found at high levels in mast cells, directly contributed to beta cell death in isolated human islets and INS-1E cells via a caspase-independent pathway.

CONCLUSIONS/INTERPRETATION

These findings suggest that mast cells might be responsible, at least in part, for immune-mediated beta cell alterations in human type 1 diabetes. If this is the case, inhibition of mast cell activation and degranulation might act to protect beta cells in individuals with type 1 diabetes.

Bone marrow-derived mesenchymal stem cells ameliorate chronic high glucose-induced β-cell injury through modulation of autophagy

Chronic hyperglycemia causes a progressive decrease of β-cell function and mass in type 2 diabetic patients. Growing evidence suggests that augment of autophagy may be an effective approach to protect β cells against various extra-/intracellular stimuli. In this study, we thus investigated whether bone marrow-derived mesenchymal stem cells (BM-MSCs) could ameliorate chronic high glucose (HG)-induced β-cell injury through modulation of autophagy. Prolonged exposure to HG decreased cell viability, increased cell apoptosis and impaired basal insulin secretion and glucose-stimulated insulin secretion of INS-1 cells, but BM-MSC treatment significantly alleviated these glucotoxic alternations. In addition, western blotting displayed upregulated expression of Beclin1 and LC3-II in INS-1 cells co-cultured with BM-MSCs. Results from immunofluorescence staining and transmission electronic microscope analysis also revealed that BM-MSCs promoted autophagosomes and autolysosomes formation in HG-treated INS-1 cells. However, it should be noted that inhibition of autophagy significantly diminished the protective effects of BM-MSCs on HG-treated INS-1 cells, suggesting that the improvement of β-cell function and survival induced by BM-MSCs was mediated through autophagy. Furthermore, our results showed that BM-MSCs improved mitochondrial function and reduced reactive oxygen species production in HG-treated INS-1 cells, largely owing to autophagic clearance of impaired mitochondria. In vivo study was performed in rats with type 2 diabetes (T2D). BM-MSC infusion not only ameliorated hyperglycemia, but also promoted restoration of pancreatic β cells in T2D rats. Meanwhile, BM-MSC infusion upregulated LAMP2 expression and enhanced formation of autophagosomes and autolysosomes, combined with reduced β-cell apoptosis and increased number of insulin granules. These findings together indicated that BM-MSCs could protect β cells against chronic HG-induced injury through modulation of autophagy in vitro and in vivo. This study unveiled novel evidence of BM-MSCs as an ideal strategy to enhance autophagy for treatment of T2D mellitus.

High-fat diet increases autophagic flux in pancreatic beta cells in vivo and ex vivo in mice

AIMS/HYPOTHESIS

Defective beta cell function during lipid oversupply and type 2 diabetes is associated with dysregulation of lysosomal function and autophagy. Whether this dysregulation represents augmentation or inhibition is unclear because of technical limitations in assaying autophagy. The current aim was to determine the effects of high-fat feeding on true autophagic flux in beta cells in vivo in mice, and to establish the relationship between autophagy, endoplasmic reticulum (ER) stress and apoptosis.

METHODS

Green fluorescent protein-microtubule-associated protein 1 light chain 3 (GFP-LC3) mice were fed chow or high-fat diets for 8-10 weeks and injected with 100 mg kg(-1) day(-1) chloroquine for 5 days, prior to being killed, to block clearance of autophagic markers. Pancreases and livers were fixed and GFP-LC3 aggregates or autophagosomes were detected by fluorescence or electron microscopy, respectively. Independently, islets isolated from chow or high-fat-fed mice were treated for 2 h with chloroquine ex vivo, and immunoblotting was performed for markers of autophagy (LC3 lipidation - LC3II and p62/SQSTM1), ER stress (C/EBP homology protein [CHOP], phosphorylated eukaryotic initiation factor 2α [p-eIFα] and inositol requiring enzyme 1α [p-IRE1α]) and apoptosis (cleaved caspase-3).

RESULTS

Numbers of autophagosomes and GFP puncta were increased in beta cells by combined high-fat feeding and chloroquine injection, indicative of enhanced autophagic flux. By contrast, GFP puncta were attenuated in liver under the same conditions. Relative to chow-fed controls, islets isolated from fat-fed mice exhibited higher LC3II levels when treated ex vivo with chloroquine. The combination of high-fat feeding and acute chloroquine treatment induced CHOP, p-eIF2α and caspase-3, but not either treatment alone.

CONCLUSIONS/INTERPRETATION

We provide the first in vivo demonstrations that high-fat feeding increases autophagic flux in pancreatic beta cells, and that this serves to protect against induction of terminal ER stress. We also highlight an approach for monitoring dietary alterations in autophagic flux using ex vivo manipulation of isolated islets.

Inhibition of SCD1 impairs palmitate-derived autophagy at the step of autophagosome-lysosome fusion in pancreatic β-cells

Autophagy is indispensable for the proper architecture and flawless functioning of pancreatic β-cells. A growing body of evidence indicates reciprocal communication between autophagic pathways, apoptosis, and intracellular lipids. The way in which elevated levels of free saturated or unsaturated FAs contribute to progressive β-cell failure remains incompletely understood. Stearoyl-CoA desaturase (SCD)1, a key regulatory enzyme in biosynthesis of MUFAs, was shown to play an important role in regulation of β-cell function. Here, we investigated whether SCD1 activity is engaged in palmitate-induced pancreatic β-cell autophagy. We found augmented apoptosis and diminished autophagy upon cotreatment of INS-1E cells with palmitate and an SCD1 inhibitor. Furthermore, we found that additional treatment of the cells with monensin, an inhibitor of autophagy at the step of fusion, exacerbates palmitate-induced apoptosis. Accordingly, diminished SCD1 activity affected the accumulation, composition, and saturation status of cellular membrane phospholipids and neutral lipids. Such an effect was accompanied by aberrant endoplasmic reticulum stress, mitochondrial injury, and decreases in insulin secretion and cell proliferation. Our data reveal a novel mechanism by which the inhibition of SCD1 activity affects autophagosome-lysosome fusion because of perturbations in cellular membrane integrity, thus leading to an aberrant stress response and β-cell failure.

Liraglutide protects pancreatic β-cells against free fatty acids in vitro and affects glucolipid metabolism in apolipoprotein E-/- mice by activating autophagy

The aim of the present study was to determine whether liraglutide (LRG), a long acting glucagon-like peptide 1 analogue, exerted a protective effect on free fatty acid (FFA)-treated pancreatic β-cells via activating autophagy. INS-1 insulinoma pancreatic islet cell lines were treated with FFA and the levels of cell necrosis, apoptosis and autophagy were detected using an MTT assay, flow cytometry and electron microscopy (ECM). A type 2 diabetes mellitus mouse model was established through treatment of mice with a high-fat diet for 8 weeks and injection of streptozotocin. LRG and autophagy inhibitors were used to investigate the protective effect of LRG on pancreatic β-cells in vivo. Metabolic indices were measured and pancreatic autophagy was detected. In the INS-1 cells, viability was higher in the FFA + LRG group compared with the FFA group, while the apoptotic rate was lower (P<0.05). The light chain 3B and p62 autophagy-associated proteins were upregulated by LRG, while ATG7 and Beclin1 were downregulated. Autophagy inhibitors reduced the protective effect of LRG in the FFA-treated INS-1 cells. The type 2 diabetes mouse model was successfully established, termed the HF group, in which LRG was observed to reduce body weight and decrease levels of fasting blood glucose, total cholesterol, serum insulin, triglyceride, low density lipoprotein-cholesterol and glycosylated hemoglobin (P<0.05), compared with the HF group. However, chloroquine treatment abrogated these effects (P<0.05, compared with the HF + LRG group; P>0.05, compared with the HF group). Autophagosomes were also observed under ECM in the pancreatic tissues of mice in the HF + LRG group. Therefore, LRG induced autophagy and exerted protective effects on pancreatic β-cells in vitro and in vivo.

Type 2 diabetes as a protein misfolding disease

Type 2 diabetes (T2D) is a highly prevalent and chronic metabolic disorder. Recent evidence suggests that formation of toxic aggregates of the islet amyloid polypeptide (IAPP) might contribute to β-cell dysfunction and disease. However, the mechanism of protein aggregation and associated toxicity remains unclear. Misfolding, aggregation, and accumulation of diverse proteins in various organs is the hallmark of the group of protein misfolding disorders (PMDs), including highly prevalent illnesses affecting the central nervous system (CNS) such as Alzheimer's disease (AD) and Parkinson's disease (PD). In this review we discuss the current understanding of the mechanisms implicated in the formation of protein aggregates in the endocrine pancreas and associated toxicity in the light of the long-standing knowledge from neurodegenerative disorders associated with protein misfolding.

Alzheimer's disease and type 2 diabetes-related alterations in brain mitochondria, autophagy and synaptic markers

We aimed to investigate mitochondrial function, biogenesis and autophagy in the brain of type 2 diabetes (T2D) and Alzheimer's disease (AD) mice. Isolated brain mitochondria and homogenates from cerebral cortex and hippocampus of wild-type (WT), triple transgenic AD (3xTg-AD) and T2D mice were used to evaluate mitochondrial functional parameters and protein levels of mitochondrial biogenesis, autophagy and synaptic integrity markers, respectively. A significant decrease in mitochondrial respiration, membrane potential and energy levels was observed in T2D and 3xTg-AD mice. Also, a significant decrease in the levels of autophagy-related protein 7 (ATG7) and glycosylated lysosomal membrane protein 1 (LAMP1) was observed in cerebral cortex and hippocampus of T2D and 3xTg-AD mice. Moreover, both brain regions of 3xTg-AD mice present lower levels of nuclear respiratory factor (NRF) 1 while the levels of NRF2 are lower in both brain regions of T2D and 3xTg-AD mice. A decrease in mitochondrial encoded, nicotinamide adenine dinucleotide dehydrogenase subunit 1 (ND1) was also observed in T2D and 3xTg-AD mice although only statistically significant in T2D cortex. Furthermore, a decrease in the levels of postsynaptic density protein 95 (PSD95) in the cerebral cortex of 3xTg-AD mice and in hippocampus of T2D and 3xTg-AD mice and a decrease in the levels of synaptosomal-associated protein 25 (SNAP 25) in the hippocampus of T2D and 3xTg-AD mice were observed suggesting synaptic integrity loss. These results support the idea that alterations in mitochondrial function, biogenesis and autophagy cause synaptic damage in AD and T2D.

Molecular mechanisms of autophagy in the cardiovascular system

Autophagy is a catabolic recycling pathway triggered by various intra- or extracellular stimuli that is conserved from yeast to mammals. During autophagy, diverse cytosolic constituents are enveloped by double-membrane vesicles, autophagosomes, which later fuse with lysosomes or the vacuole to degrade their cargo. Dysregulation in autophagy is associated with a diverse range of pathologies including cardiovascular disease, the leading cause of death in the world. As such, there is great interest in identifying novel mechanisms that govern the cardiovascular response to disease-related stress. First described in failing hearts, autophagy within the cardiovascular system has been characterized widely in cardiomyocytes, cardiac fibroblasts, endothelial cells, and vascular smooth muscle cells. In all cases, a window of optimal autophagic activity seems to be critical to the maintenance of cardiovascular homeostasis and function; excessive or insufficient levels of autophagic flux can each contribute to heart disease pathogenesis. Here, we review the molecular mechanisms that govern autophagosome formation and analyze the link between autophagy and cardiovascular disease.

Islet β-cell failure in type 2 diabetes--Within the network of toxic lipids

Obesity-related type 2 diabetes develops in individuals with the onset of β-cell dysfunction. Pancreatic islet lipotoxicity is now recognized as a primary reason for the onset and progression of the disease. Such dysfunction is reflected by the aberrant secretory capacity and detrimental loss of β-cell mass and survival. Elevated circulating serum fatty acid levels and disordered lipid metabolism management are particularly interesting in the search for biologically relevant triggers of β-cell demise. Herein, we review various types of toxic lipid metabolites that may play a significant role in pancreatic islet failure. The lipotoxic effect on β-cells depends on the type of lipid mediator (e.g., long-chain fatty acids, diacylglycerols, ceramides, phospholipids), cellular location of its action (e.g., endoplasmic reticulum, mitochondria), and associated-organelle conditions (e.g., membranes, vesicles). We also discuss various aspects of lipid action in β-cells, including effects on metabolic pathways, stress responses (e.g., oxidative stress, endoplasmic reticulum stress, and autophagy), and gene expression.

Pancreatic α Cells are Resistant to Metabolic Stress-induced Apoptosis in Type 2 Diabetes

Pancreatic α cells are exposed to metabolic stress during the evolution of type 2 diabetes (T2D), but it remains unclear whether this affects their survival. We used electron microscopy to search for markers of apoptosis and endoplasmic reticulum (ER) stress in α and β cells in islets from T2D or non-diabetic individuals. There was a significant increase in apoptotic β cells (from 0.4% in control to 6.0% in T2D), but no α cell apoptosis. We observed, however, similar ER stress in α and β cells from T2D patients. Human islets or fluorescence-activated cell sorting (FACS)-purified rat β and α cells exposed in vitro to the saturated free fatty acid palmitate showed a similar response as the T2D islets, i.e. both cell types showed signs of ER stress but only β cells progressed to apoptosis. Mechanistic experiments indicate that this α cell resistance to palmitate-induced apoptosis is explained, at least in part, by abundant expression of the anti-apoptotic protein Bcl2l1 (also known as Bcl-xL).

Inhibition of autophagic turnover in β-cells by fatty acids and glucose leads to apoptotic cell death

Autophagy, a cellular recycling process responsible for turnover of cytoplasmic contents, is critical for maintenance of health. Defects in this process have been linked to diabetes. Diabetes-associated glucotoxicity/lipotoxicity contribute to impaired β-cell function and have been implicated as contributing factors to this disease. We tested the hypothesis that these two conditions affect β-cell function by modulating autophagy. We report that exposure of β-cell lines and human pancreatic islets to high levels of glucose and lipids blocks autophagic flux and leads to apoptotic cell death. EM analysis showed accumulation of autophagy intermediates (autophagosomes), with abundant engulfed cargo in palmitic acid (PA)- or glucose-treated cells, indicating suppressed autophagic turnover. EM studies also showed accumulation of damaged mitochondria, endoplasmic reticulum distention, and vacuolar changes in PA-treated cells. Pulse-chase experiments indicated decreased protein turnover in β-cells treated with PA/glucose. Expression of mTORC1, an inhibitor of autophagy, was elevated in β-cells treated with PA/glucose. mTORC1 inhibition, by treatment with rapamycin, reversed changes in autophagic flux, and cell death induced by glucose/PA. Our results indicate that nutrient toxicity-induced cell death occurs via impaired autophagy and is mediated by activation of mTORC1 in β-cells, contributing to β-cell failure in the presence of metabolic stress.

New mechanisms of metformin action: Focusing on mitochondria and the gut

The most well-known mechanism of metformin action, one of the most commonly prescribed antidiabetic drugs, is adenosine monophosphate-activated protein kinase activation; however, recent investigations have shown that adenosine monophosphate-activated protein kinase-independent pathways can explain some of metformin's beneficial metabolic effects as well as undesirable side-effects. Such novel pathways include induction of mitochondrial stress, inhibition of mitochondrial shuttles, alteration of intestinal microbiota, suppression of glucagon signaling, activation of autophagy, attenuation of inflammasome activation, induction of incretin receptors and reduction of terminal endoplasmic reticulum stress. Together, these studies have broadened our understanding of the mechanisms of antidiabetic agents as well as the pathogenic mechanism of diabetes itself. The results of such investigations might help to identify new target molecules and pathways for treatment of diabetes and metabolic syndrome, and could also have broad implications in diseases other than diabetes. Accordingly, new antidiabetic drugs with better efficacy and fewer adverse effects will likely result from these studies.

β-cell dysfunction: Its critical role in prevention and management of type 2 diabetes

Type 2 diabetes (T2DM) is characterized by insulin resistance and β-cell dysfunction. Although, in contrast to type 1 diabetes, insulin resistance is assumed to be a major pathophysiological feature of T2DM, T2DM never develops unless β-cells fail to compensate insulin resistance. Recent studies have revealed that a deficit of β-cell functional mass is an essential component of the pathophysiology of T2DM, implying that β-cell deficit is a common feature of both type 1 and type 2 diabetes. β-cell dysfunction is present at the diagnosis of T2DM and progressively worsens with disease duration. β-cell dysfunction is associated with worsening of glycemic control and treatment failure; thus, it is important to preserve or recover β-cell functional mass in the management of T2DM. Since β-cell regenerative capacity appears somewhat limited in humans, reducing β-cell workload appears to be the most effective way to preserve β-cell functional mass to date, underpinning the importance of lifestyle modification and weight loss for the treatment and prevention of T2DM. This review summarizes the current knowledge on β-cell functional mass in T2DM and discusses the treatment strategy for T2DM.

Autophagy regulates insulin resistance following endoplasmic reticulum stress in diabetes

Autophagy is a kind of cell biological process that maintains the cell's energy level under nutrient-poor conditions, regulates the turnover of abnormal or aged proteins, and disposes of dysfunctional organelles. The autophagy system is activated as a novel signaling pathway in response to endoplasmic reticulum stress (ER stress)-induced insulin resistance (IR). Defective autophagy may be closely related to insulin resistance. There are at least three mechanistically distinct arms of ER stress that regulate the expression of key genes which not only function within the secretory pathway but also affect broad aspects of cell fate and the metabolism of proteins, amino acids, and lipids. ER stress-stimulated insulin resistance is mediated by the autophagy-dependent process. In settings of chronic ER stress, the associated autophagy may contribute to pathophysiological processes involved in a number of prevalent diseases, including diabetes. Whether autophagy plays a protective or harmful role in diabetes awaits further analysis. In this review, we will summarize the current knowledge about the emerging role of autophagy in ER stress-induced insulin resistance. Strategies to take advantage of the potential protective effect of autophagy remain important in the overall treatment of insulin resistance and type 2 diabetes.

Minireview: Autophagy in pancreatic β-cells and its implication in diabetes

Autophagy is a conserved system for the degradation of cytoplasmic proteins and organelles. During insulin resistance, in which insulin secretion is enhanced and β-cell mass is increased owing to changes in the expression and function of various proteins in pancreatic β-cells, autophagic activity appears to also be enhanced to adapt to the dynamic changes occurring in β-cells. Indeed, defective autophagy in β-cells recapitulates several features that are observed in islets during the development of type 2 diabetes mellitus. In addition, the dyregulation of autophagic activity appears to occur in the β-cells of type 2 diabetic model mice and type 2 diabetes mellitus patients. These lines of evidence suggest that autophagic failure may be implicated in the pathophysiology of type 2 diabetes mellitus. In this review, we summarized the recent findings regarding how autophagy in β-cells is regulated and how dysfunction of the autophagic machinery may lead to the dysfunction of β-cells.

Autophagy in cardiovascular biology

Cardiovascular disease is the leading cause of death worldwide. As such, there is great interest in identifying novel mechanisms that govern the cardiovascular response to disease-related stress. First described in failing hearts, autophagy within the cardiovascular system has been widely characterized in cardiomyocytes, cardiac fibroblasts, endothelial cells, vascular smooth muscle cells, and macrophages. In all cases, a window of optimal autophagic activity appears to be critical to the maintenance of cardiovascular homeostasis and function; excessive or insufficient levels of autophagic flux can each contribute to heart disease pathogenesis. In this Review, we discuss the potential for targeting autophagy therapeutically and our vision for where this exciting biology may lead in the future.

Autophagy deficiency in β cells blunts incretin-induced suppression of glucagon release from α cells

Incretin-based therapy such as GLP-1 receptor agonists and DPP-4 inhibitors for type 2 diabetes mellitus is characterized by glucose-dependent insulin secretion and glucose-inhibited glucagon secretion. Recently, autophagy deficiency in islet β cells has been shown to contribute to the pathogenesis of type 2 diabetes mellitus however, with the role of incretin has not been established. To evaluate the role of autophagy in incretin effects, 8-week-old male β cell-specific Atg7 knockout (Atg7(Δβ cell)) mice and wild-type mice were administered vildagliptin for 12 weeks. Vildagliptin treatment improved glucose intolerance and hypoinsulinemia; however, it failed to suppress serum glucagon levels after glucose loading in the Atg7(Δβ cell) mice. Ex vivo glucose-induced glucagon suppression was also blunted in the islets from vildagliptin-treated Atg7(Δβ cell) mice. The α cell mass was not affected by β cell autophagy deficiency or vildagliptin. However, glucagon mRNA expression was significantly increased by vildagliptin in the autophagy-deficient islets, and was significantly reduced by vildagliptin in wild-type islets. Pancreatic glucagon contents were not in agreement with the changes in mRNA expression, suggesting a dysregulation in glucagon translation and secretion. In vitro studies revealed that glucose-stimulated cAMP production was impaired in the autophagy-deficient islets exposed to exendin-4. Taken together, the results suggest that the constitutive autophagy in β cells could regulate incretin-induced glucagon expression and release in α cells, and that cAMP may play a role in this process.

The Effects of Lipid Membranes, Crowding and Osmolytes on the Aggregation, and Fibrillation Propensity of Human IAPP

Type 2 diabetes mellitus (T2DM) is an age-related and metabolic disease. Its development is hallmarked, among others, by the dysfunction and degeneration of β-cells of the pancreatic islets of Langerhans. The major pathological characteristic thereby is the formation of extracellular amyloid deposits consisting of the islet amyloid polypeptide (IAPP). The process of human IAPP (hIAPP) self-association, and the intermediate structures formed as well as the interaction of hIAPP with membrane systems seem to be, at least to a major extent, responsible for the cytotoxicity. Here we present a summary and comparison of the amyloidogenic propensities of hIAPP in bulk solution and in the presence of various neutral and charged lipid bilayer systems as well as biological membranes. We also discuss the cellular effects of macromolecular crowding and osmolytes on the aggregation pathway of hIAPP. Understanding the influence of different cellular factors on hIAPP aggregation will provide more insight into the onset of T2DM and help to develop novel therapeutic strategies.

Liraglutide inhibits autophagy and apoptosis induced by high glucose through GLP-1R in renal tubular epithelial cells

Tubular atrophy and dysfunction is a critical process underlying diabetic nephropathy (DN). Understanding the mechanisms underlying renal tubular epithelial cell survival is important for the prevention of kidney failure associated with glucotoxicity. Autophagy is a cellular pathway involved in protein and organelle degradation. It is associated with many types of cellular homeostasis and human diseases. To date, little is known of the association between high concentrations of glucose and autophagy in renal tubular cells. In the present study, we investigated high glucose-induced toxicity in renal tubular epithelial cells by means of several complementary assays, including cell viability, cell death assays and changes in ultrastructure in an immortalized human kidney cell line, HK-2 cells. The extent of apoptosis was significantly increased in the HK-2 cells following treatment with high levels of glucose. In addition, in in vivo experiments using diabetic rats, high glucose exerted harmful effects on the tissue structure of the kidneys in the diabetic rats. Chronic exposure of the HK-2 cells and tubular epithelial cells of nephritic rats to high levels of glucose induced autophagy. Liraglutide inhibited these effects; however, treatment witht a glucagon-like peptide-1 receptor (GLP-1R) antagonist enhanced these effects. Our results also indicated that the exposure of the renal tubular epithelial cells to high glucose concentrations in vitro led to the downregulation of GLP-1R expression. Liraglutide reversed this effect, while the GLP-1R antagonist promoted it, promoting autophagy, suggesting that liraglutide exerts a renoprotective effect in the presence of high glucose, at least in part, by inhibiting autophagy and increasing GLP-1R expression in the HK-2 cells and kidneys of diabetic rats.

Role of islet β cell autophagy in the pathogenesis of diabetes

While the role of autophagy in the physiology of endocrine organs and the development of metabolic disorders or diabetes has been investigated, the relationship between the pancreatic islet and autophagy has not been explored extensively. In this review, studies on the possible involvement of dysregulated autophagy in the pathogenesis of metabolic syndrome and diabetes will be summarized with an emphasis on the autophagic process in pancreatic islet β cells. Novel findings regarding the role of autophagy in human β cell pathology and the development of type 2 diabetes (T2D) characterized by islet amyloid deposition will be discussed. Careful design of agents enhancing autophagic activity in β cells might serve as a novel approach towards therapeutics for the treatment of diabetes.

Quercetin alleviates high glucose-induced Schwann cell damage by autophagy

Quercetin can reverse high glucose-induced inhibition of neural cell proliferation, and therefore may have a neuroprotective effect in diabetic peripheral neuropathy. It is difficult to obtain primary Schwann cells and RSC96 cells could replace primary Schwann cells in studies of the role of autophagy in the mechanism underlying diabetic peripheral neuropathy. Here, we show that under high glucose conditions, there are fewer autophagosomes in immortalized rat RSC96 cells and primary rat Schwann cells than under control conditions, the proliferative activity of both cell types is significantly impaired, and the expression of Beclin-1 and LC3, the molecular markers for autophagy, is significantly lower. After intervention with quercetin, the autophagic and proliferative activity of both cell types is rescued. These results suggest that quercetin can alleviate high glucose-induced damage to Schwann cells by autophagy.