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Pancreatic β-cells produce and secrete insulin to maintain blood glucose levels within a narrow range. Defects in the function and mass of β-cells play a significant role in the development and progression of diabetes. Increased β-cell deficiency and β-cell apoptosis are observed in the pancreatic islets of patients with type 2 diabetes. At an early stage, β-cells adapt to insulin resistance, and their insulin secretion increases, but they eventually become exhausted, and the β-cell mass decreases. Various causal factors, such as high glucose, free fatty acids, inflammatory cytokines, and islet amyloid polypeptides, contribute to the impairment of β-cell function. Therefore, the maintenance of β-cell function is a logical approach for the treatment and prevention of diabetes. In this review, we provide an overview of the role of these risk factors in pancreatic β-cell loss and the associated mechanisms. A better understanding of the molecular mechanisms underlying pancreatic β-cell loss will provide an opportunity to identify novel therapeutic targets for type 2 diabetes. [ABSTRACT FROM AUTHOR]

Type 2 diabetes is caused by chronic insulin resistance and progressive decline in beta-cell function. Optimal beta-cell function and mass is essential for glucose homeostasis and beta-cell impairment leads to the development of diabetes. Elevated levels of circulating fatty acids (FAs) and disturbances in lipid metabolism regulation are associated with obesity, and they are major factors influencing the increase in the incidence of type 2 diabetes. Chronic free FA (FFA) treatment induces insulin resistance and beta-cell dysfunction; therefore, reduction of elevated plasma FFA levels might be an important therapeutic target in obesity and type 2 diabetes. Lipid signals via receptors, and intracellular mechanisms are involved in FFA-induced apoptosis. In this paper, we discuss lipid actions in beta cells, including effects on metabolic pathways and stress responses, to help further understand the molecular mechanisms of lipotoxicity-induced type 2 diabetes. [ABSTRACT FROM AUTHOR]

The beta-cells in the islets of Langerhans in the pancreas secrete insulin and play an important role in glucose homeostasis. Diabetes, characterized by hyperglycemia, results from an absolute or a relative deficiency of the pancreatic beta-cell mass. Islet transplantation has been considered to be a useful therapeutic approach, but it is largely unsuccessful because most of the transplanted islets are lost in the early stage of transplantation. To evaluate the efficacy of intervention methods for the improvement of islet survival, monitoring of the functional islet mass is needed. Various techniques to image and track transplanted islets have been investigated to assess islets after transplantation. In this review, recent progresses in imaging methods to visualize islets are discussed. [ABSTRACT FROM AUTHOR]

Objective: β-Cell dysfunction plays a central role in the pathogenesis of type 2 diabetes (T2D), and the identification of novel approaches to improve β-cell function is essential to treat this disease. Baicalein, a flavonoid originally isolated from the root of Scutellaria Baicalensis, has been shown to have beneficial effects on β-cell function. Here, the authors investigated the molecular mechanism responsible for the protective effects of baicalein against palmitate (PA)-induced impaired β-cell function, and placed focus on the role of heme oxygenase (HO)-1. Methods: Rat pancreatic β-cell line INS-1 cells or mouse pancreatic islets were cultured with PA (500 μM) to induce lipotoxicity in the presence or absence of baicalein (50 μM), and the expressions of the ER stress markers, ATF-3, CHOP and GRP78 were detected by Western blotting and/or qPCR. The involvement of HO-1 was evaluated by HO-1 siRNA transfection and using the HO-1 inhibitor ZnPP. Results: Baicalein reduced PA-induced ER stress and inflammation and enhanced insulin secretion, and these effects were associated with the induction of HO-1. Furthermore, these protective effects were attenuated by ZnPP and by HO-1 siRNA. Pretreatment of PD98059 (an ERK inhibitor) significantly inhibited the protective effects of baicalein and blocked HO-1 induction. On the other hand, CO production by RuCO (a CO donor) ameliorated PA-induced ER stress, suggesting that CO production followed by HO-1 induction may contribute to the protective effects of baicalein against PA-induced β-cell dysfunction. Conclusion: Baicalein protects pancreatic β-cells from PA-induced ER stress and inflammation via an ERK-HO-1 dependent pathway. The authors suggest HO-1 induction in pancreatic β-cells appears to be a promising therapeutic strategy for T2D. [ABSTRACT FROM AUTHOR]

Type 1 diabetes results from autoimmune-mediated pancreatic beta-cell destruction and transforming growth factor-beta (TGF-β) is known to play a preventive role in type 1 diabetes in non-obese diabetic (NOD) mice. In this study, we investigated the role of Smad4, a key molecule for Smad-dependent TGF-β signaling, in T cells of NOD mice in the pathogenesis of autoimmune diabetes. We generated T-cell-specific Smad4 knockout (Smad4 tKO) NOD mice and assessed the pathological and immunological changes. Smad4 tKO showed earlier onset and increased incidence of diabetes than wild type (WT) NOD mice. Pathological features such as insulitis, anti-glutamic acid decarboxylase auto-antibody levels and serum IFN-γ levels were significantly increased in Smad4 tKO compared with WT NOD mice. Proportion and number of activated/memory CD4+ T cell were significantly increased in pancreatic lymph nodes of Smad4 tKO compared with WT NOD mice. However, the proportion and function of regulatory T cells was not different. Effector CD4+ T cells from Smad4 tKO were more resistant to suppression by regulatory T cells than effector cells from WT NOD mice. The proliferative potential of effector T cells from Smad4 tKO was significantly elevated compared with WT NOD mice, and activation of sterol regulatory element binding protein-1c (SREBP-1c) in T cells of Smad4 tKO NOD mice was correlated with this proliferative activity. We conclude that Smad4 deletion in T cells of NOD mice accelerated the development of autoimmune diabetes and increased the incidence of the disease by dysregulation of T cell activation at least in part via SREBP-1c activation. [ABSTRACT FROM AUTHOR]

Abstract: Glucagon-like peptide-1 (GLP-1), an incretin hormone, is released from intestinal L-cells in response to nutrients. GLP-1 lowers blood glucose levels by stimulating insulin secretion from pancreatic beta-cells in a glucose-dependent manner. In addition, GLP-1 slows gastric emptying, suppresses appetite, reduces plasma glucagon, and stimulates glucose disposal, which are beneficial for glucose homeostasis. Therefore, incretin-based therapies such as GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase IV, an enzyme which inactivates GLP-1, have been developed for treatment of diabetes. This review outlines our knowledge of the actions of GLP-1 on insulin secretion and biosynthesis, beta-cell proliferation and regeneration, and protection against beta-cell damage, as well as the involvement of recently discovered signaling pathways of GLP-1 action, mainly focusing on pancreatic beta-cells. [Copyright &y& Elsevier]

Background: Betacellulin (BTC), a member of the epidermal growth factor family, is known to play an important role in regulating growth and differentiation of pancreatic beta cells. Growth-promoting actions of BTC are mediated by epidermal growth factor receptors (ErbBs), namely ErbB-1, ErbB-2, ErbB-3 and ErbB-4; however, the exact mechanism for beta cell proliferation has not been elucidated. Therefore, we investigated which ErbBs are involved and some molecular mechanisms by which BTC regulates beta cell proliferation. Methodology/Principal Findings: The expression of ErbB-1, ErbB-2, ErbB-3, and ErbB-4 mRNA was detected by RT-PCR in both a beta cell line (MIN-6 cells) and C57BL/6 mouse islets. Immunoprecipitation and western blotting analysis showed that BTC treatment of MIN-6 cells induced phosphorylation of only ErbB-1 and ErbB-2 among the four EGF receptors. BTC treatment resulted in DNA synthetic activity, cell cycle progression, and bromodeoxyuridine (BrdU)-positive staining. The proliferative effect was blocked by treatment with AG1478 or AG825, specific tyrosine kinase inhibitors of ErbB-1 and ErbB-2, respectively. BTC treatment increased mRNA and protein levels of insulin receptor substrate-2 (IRS-2), and this was blocked by the ErbB-1 and ErbB-2 inhibitors. Inhibition of IRS-2 by siRNA blocked cell cycle progression induced by BTC treatment. Streptozotocin-induced diabetic mice injected with a recombinant adenovirus expressing BTC and treated with AG1478 or AG825 showed reduced islet size, reduced numbers of BrdU-positive cells in the islets, and did not attain BTC-mediated remission of diabetes. Conclusions/Significance: These results suggest that BTC exerts proliferative activity on beta cells through the activation of ErbB-1 and ErbB-2 receptors, which may increase IRS-2 expression, contributing to the regeneration of beta cells. [ABSTRACT FROM AUTHOR]

Type 1 diabetes mellitus is an autoimmune disease caused by the destruction of pancreatic beta cells. Many patients with type 1 diabetes experience skeletal muscle wasting. Although the link between type 1 diabetes and muscle wasting is not clearly known, insulin insufficiency and hyperglycemia may contribute to decreased muscle mass. In this study, we investigated the therapeutic effect of the ethanolic extract of Schisandrae chinensis Fructus (SFe) on muscle wasting in streptozotocin (STZ)-induced diabetic mice. STZ-diabetic C57BL/6 mice (blood glucose level ≥300 mg/dL) were orally administered SFe (250 or 500 mg/kg/day) for 6 weeks. We observed that SFe administration did not change blood glucose levels but increased gastrocnemius muscle weight, cross-sectional area, and grip strength in STZ-induced diabetic mice. Administration of SFe (500 mg/kg) decreased the expression of atrophic factors, such as MuRF1 and atrogin-1, but did not alter the expression of muscle synthetic factors. Further studies showed that SFe administration decreased the expression of KLF15 and p-CREB, which are upstream molecules of atrophic factors. Examination of the expression of molecules involved in autophagy–lysosomal pathways (e.g., p62/SQSTM1, Atg7, Beclin-1, ULK-1, LC3-I, and LC3-II) revealed that SFe administration significantly decreased the expression of p62/SQSTM1, LC3-I, and LC3-II; however, no changes were observed in the expression of Atg7, Beclin-1, or ULK-1. Our results suggest that SFe ameliorated muscle wasting in STZ-induced diabetic mice by decreasing protein degradation via downregulation of the CREB-KLF15-mediated UPS system and the p62/SQSTM1-mediated autophagy–lysosomal pathway. [ABSTRACT FROM AUTHOR]

More than 10 viruses have been reported to be associated with the development of type 1 diabetes-like symptoms in animals, with the best evidence coming from studies on the D variant of encephalomyocarditis (EMC-D) virus in mice and Kilham rat virus (KRV) in rats. A high titer of EMC-D viral infection results in the development of diabetes within 3 days, primarily due to the rapid destruction of β cells by viral replication within the cells. A low titer of EMC-D viral infection results in the recruitment of macrophages to the islets. Soluble mediators produced by activated macrophages play a critical role in the destruction of residual β cells. A single amino acid at position 776 of the EMC viral genome controls the diabetogenicity of the virus. In contrast, KRV causes autoimmune type 1 diabetes in diabetes-resistant BioBreeding (DR-BB) rats without direct infection of β cells. Macrophages play an important role in the development of diabetes in KRV-infected DR-BB rats. As well, KRV infection preferentially activates effector T cells, such as Th1-like CD45RC+CD4+ T cells and CD8+ T cells, and downregulates regulatory T cells, such as Th2-like CD45RC−CD4+ T cells. This results in the breakdown of the immune balance, contributing to the development of diabetes in KRV-infected DR-BB rats. [ABSTRACT FROM AUTHOR]

Pancreatic beta cells are vulnerable to oxidative stress, which causes beta cell death and dysfunction in diabetes mellitus. Broussonetia kazinoki Siebold (BK) is a widely used herbal medicine, but its potential effects against beta cell death-induced diabetes have not been studied. Therefore, we investigated the protective effect of an ethanolic extract of BK fruit (BKFE) against streptozotocin (STZ)-induced toxicity in pancreatic beta cells. Intraperitoneal injection of STZ in mice induced hyperglycemia; however, oral administration of BKFE significantly decreased the blood glucose level as well as HbA1c levels. BKFE treatment improved glucose tolerance and increased body weight in diabetic mice. Moreover, BKFE treatment resulted in increased serum insulin levels and insulin expression in the pancreas as well as decreased 4-hydroxynonenal levels induced by oxidative stress. Treatment with STZ decreased cell viability of mouse insulinoma cells (MIN6), which was blocked by BKFE pretreatment. BKFE significantly inhibited apoptotic cells and decreased the expression levels of cleaved-caspase-3 and cleaved-poly (ADP-ribose) polymerase (PARP) induced by STZ treatment. Production of reactive oxygen species in STZ-treated MIN6 cells was also significantly decreased by treatment with BKFE. Erk phosphorylation and Nox4 levels increased in STZ-treated MIN6 cells and the pancreas of mice injected with STZ and this increase was inhibited by treatment with BKFE. Inhibition of Erk phosphorylation by treatment with the PD98059 inhibitor or siRNA Erk also blocked the expression of Nox4 induced by STZ treatment. In conclusion, BKFE inhibits Erk phosphorylation, which in turn prevents STZ-induced oxidative stress and beta cell apoptosis. These results suggested that BKFE can be used to prevent or treat beta cell damage in diabetes. [ABSTRACT FROM AUTHOR]

Type 2 diabetes mellitus (T2DM) is a prevalent chronic metabolic disorder accompanied by high blood glucose, insulin resistance, and relative insulin deficiency. Endoplasmic reticulum (ER) stress induced by high glucose and free fatty acids has been suggested as one of the main causes of β-cell dysfunction and death in T2DM. Stem cell-derived insulin-secreting cells were recently suggested as a novel therapy for diabetes. In the present study, we demonstrate the therapeutic potential of tonsil-derived mesenchymal stem cells (TMSCs) to treat high-fat diet (HFD)-induced T2DM. To explore whether TMSC administration can alleviate T2DM, TMSCs were intraperitoneally injected in HFD-induced T2DM mice once every 2 weeks. TMSC injection markedly improved glucose tolerance and glucose-stimulated insulin secretion and prevented HFD-induced pancreatic β-cell hypertrophy and cell death. In addition, TMSC injection relieved the ER-stress response and preserved gene expression related to glucose sensing and insulin secretion. Moreover, administration of TMSC-derived conditioned medium induced similar therapeutic outcomes, suggesting paracrine effects. Finally, proteomic analysis revealed high secretion of insulin-like growth factor-binding protein 5 by TMSCs, and its expression was critical for the protective effects of TMSCs against HFD-induced glucose intolerance and ER-stress response in pancreatic islets. TMSC administration can alleviate HFD-induced-T2DM via preserving pancreatic islets and their function. These results provide novel evidence of TMSCs as an ER-stress modulator that may be a novel, alternative cell therapy for T2DM. [ABSTRACT FROM AUTHOR]

We have previously reported that long-term treatment of beta cells with interleukin-6 (IL-6) is pro-apoptotic. However, little is known about the regulatory mechanisms that are involved. Therefore, we investigated pro-apoptotic changes in mRNA expression in beta cells in response to IL-6 treatment. We analyzed a microarray with RNA from INS-1 beta cells treated with IL-6, and found that TNF-α mRNA was significantly upregulated. Inhibition of TNF-α expression by neutralizing antibodies significantly decreased annexin V staining in cells compared with those treated with a control antibody. We identified three microRNAs that were differentially expressed in INS-1 cells incubated with IL-6. In particular, miR-181c was significantly downregulated in IL-6-treated cells compared with control cells and the decrease of miR-181c was attenuated by STAT-3 signaling inhibition. TNF-α mRNA was a direct target of miR-181c and upregulation of miR-181c by mimics, inhibited IL-6-induced increase in TNF-α mRNA expression. Consequently, reduction of TNF-α mRNA caused by miR-181c mimics enhanced cell viability in IL-6 treated INS-1 cells. These results demonstrated that miR-181c regulation of TNF-α expression plays a role in IL-6-induced beta cell apoptosis. [ABSTRACT FROM AUTHOR]

Stem or progenitor cells are a potential source of insulin-producing cells, if methods can be found to reliably manipulate their differentiation. Glucagon-like peptide-1 (GLP-1) and its analog, exendin-4, axe known to induce differentiation of insulin-producing pancreatic β cells from different types of stem and progenitor cells. We previously found that when nonobese diabetic (NOD) mice were given a single intravenous injection of a recombinant adenovirus expressing GLP-1 (rAd-GLP-1), insulin-positive cells were detected not only in the pancreas, but also in the intestine. The present study investigated methods to promote the differentiation of intestinal stem cells to insulin-producing cells. An increased number of insulin-stained cells were observed in the intestine of NOD mice when rAd-GLP-1 was locally administered by intra-intestinal injection, as compared with systemic administration of rAd-GLP-1, suggesting that the intestinal stem cell pool might be a good source for differentiation into insulin-producing cells. Neurogenin (ngn)-3 is a transcription factor that responds to Notch signaling, and the expression of ngn-3 determines endocrine differentiation. When we examined the expression of ngn-3 and insulin by immunohistochemical staining in rAd-GLP-l-treated NOD mice, we detected expression of ngn-3 at 1 week after virus injection and insulin at 2 weeks after virus injection. As Notch signaling inhibits ngn-3 expression, we treated a segment of the small intestine (jejunum) from 8-week-old NOD mice with the Notch inhibitor, γ-secretase inhibitor, and exendin-4 alone or in combination and examined insulin mRNA expression. Treatment with a combination of γ-secretase inhibitor and exendin-4 showed additive effects on insulin mRNA expression as compared with treatment with either compound alone. We conclude that differentiation of intestinal stem cells into insulin-producing cells by exendin-4 in adult mice is mediated by ngn-3 expression and promoted by inhibition of Notch signaling. [ABSTRACT FROM AUTHOR]

Liquiritigenin (LQ) is a major active component of licorice root, which is a flavone used for treating many diseases, including diabetes. LQ has been shown to exhibit a glucose-lowering effect in diabetic mice. Therefore, we investigated the potential of LQ to protect against lipotoxicity-induced beta-cell apoptosis and the underlying molecular mechanisms. Exposure of INS-1 rat insulinoma cells to LQ significantly increased cell viability and blocked palmitate (PA)-induced apoptosis, as evidenced by the reduction of Annexin-V-stained cells, cleaved caspase-3 levels, and poly (ADP-ribose) polymerase (PARP) activity, as well as upregulation of Bcl-2 expression. Moreover, LQ treatment significantly reduced the endoplasmic reticulum (ER) stress response by reducing phosphorylated protein kinase RNA-like endoplasmic reticulum kinase (PERK), phosphorylated eIF-2a, and CHOP expression in PA-treated INS-1 cells. The anti-apoptotic effect of LQ treatment was reversed through co-treatment with fulvestrant, a specific inhibitor of the estrogen receptor. LQ also increased AKT phosphorylation, and inactivation of this molecular event failed to decrease PERK phosphorylation with LQ treatment in PA-treated INS-1 cells. This effect was further accompanied by an inability to recover cell viability. These results suggest that LQ protects INS-1 cells from lipotoxicity-induced apoptosis by suppressing ER stress. We conclude that estrogen receptor-mediated AKT phosphorylation is one of the mechanisms contributing to the anti-apoptotic effect of LQ. [ABSTRACT FROM AUTHOR]

Oxidative stress in pancreatic beta cells can inhibit insulin secretion and promote apoptotic cell death. Exendin-4 (EX4), a glucagon-like peptide-1 receptor agonist, can suppress beta cell apoptosis, improve beta cell function and protect against oxidative damage. In this study, we investigated the molecular mechanisms for antioxidative effects of EX4 in pancreatic beta cells. INS-1 cells, a rat insulinoma cell line, were pretreated with EX4 and exposed to palmitate or H 2 O 2 . Reactive oxygen species (ROS) production, and glutathione and insulin secretion were measured. The mRNA and protein expression levels of antioxidant genes were examined. The level of nuclear factor erythroid 2-related factor 2 (Nrf2), its binding to antioxidant response element ( ARE ), and its ubiquination in the presence of EX4 were determined. The Nrf2 signaling pathway was determined using rottlerin (protein kinase [PK]Cδ inhibitor), H89 (PKA inhibitor) and LY294002 (phosphatidylinositide 3-kinase [PI3K] inhibitor). EX4 treatment decreased ROS production, recovered cellular glutathione levels and insulin secretion in the presence of oxidative stress in INS-1 cells. The expression levels of glutamate-cysteine ligase catalytic subunit and heme oxygenase-1 were increased by EX4 treatment. EX4 promoted Nrf2 translocation, ARE binding activity and enhanced stabilization of Nrf2 by inhibition of ubiquitination. Knockdown of Nrf2 abolished the effect of EX4 on increased insulin secretion. Inhibition of PKCδ attenuated Nrf2 translocation and antioxidative gene expression by EX4 treatment. We suggest that EX4 activates and stabilizes Nrf2 through PKCδ activation, contributing to the increase of antioxidant gene expression and consequently improving beta cell function in the presence of oxidative stress. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this study is to investigate the effect of mitochondrial metabolism on high glucose/palmitate (HG/PA)-induced INS-1 beta cell death. Long-term treatment of INS-1 cells with HG/PA impaired energy-producing metabolism accompanying with depletion of TCA cycle intermediates. Whereas an inhibitor of carnitine palmitoyl transferase 1 augmented HG/PA-induced INS-1 cell death, stimulators of fatty acid oxidation protected the cells against the HG/PA-induced death. Furthermore, whereas mitochondrial pyruvate carboxylase inhibitor phenylacetic acid augmented HG/PA-induced INS-1 cell death, supplementation of TCA cycle metabolites including leucine/glutamine, methyl succinate/α-ketoisocaproic acid, dimethyl malate, and valeric acid or treatment with a glutamate dehydrogenase activator, aminobicyclo-heptane-2-carboxylic acid (BCH), significantly protected the cells against the HG/PA-induced death. In particular, the mitochondrial tricarboxylate carrier inhibitor, benzene tricarboxylate (BTA), also showed a strong protective effect on the HG/PA-induced INS-1 cell death. Knockdown of glutamate dehydrogenase or tricarboxylate carrier augmented or reduced the HG/PA-induced INS-1 cell death, respectively. Both BCH and BTA restored HG/PA-induced reduction of energy metabolism as well as depletion of TCA intermediates. These data suggest that depletion of the TCA cycle intermediate pool and impaired energy-producing metabolism may play a role in HG/PA-induced cytotoxicity to beta cells and thus, HG/PA-induced beta cell glucolipotoxicity can be protected by nutritional or pharmacological maneuver enhancing anaplerosis or reducing cataplerosis. [ABSTRACT FROM AUTHOR]