H2S, Endoplasmic Reticulum Stress, and Apoptosis of Insulin-secreting Beta Cells

Cystathionine gamma-lyase (CSE) is a key enzyme in the trans-sulfuration pathway, which uses L-cysteine to produce hydrogen sulfide (H2S). Functional changes of pancreatic beta cells induced by endogenous H2S have been reported, but the effect of the CSE/H2S system on pancreatic beta cell survival has not been known. In this study, we demonstrate that H2Sat physiologically relevant concentrations induced apoptosis of INS-1E cells, an insulin-secreting beta cell line. Transfection of INS-1E cells with a recombinant defective adenovirus containing the CSE gene (Ad-CSE) resulted in a significant increase in CSE expression and H2S production. Ad-CSE transfection also stimulated apoptosis. The other two end products of CSE-catalyzed enzymatic reaction, ammonium and pyruvate, had no effects on INS-1E cell apoptosis, indicating that overexpression of CSE may stimulate INS-1E cell apoptosis via increased endogenous production of H2S. Both exogenous H2S (100 microM) and Ad-CSE transfection inhibited ERK1/2 but activated p38 MAPK. Interestingly, BiP and CHOP, two indicators of endoplasmic reticulum (ER) stress, were up-regulated in H2S-and CSE-mediated apoptosis in INS-1E cells. After suppressing CHOP mRNA expression, H2S-induced apoptosis of INS-1E cells was significantly decreased. Inhibition of p38 MAPK, but not of ERK1/2, inhibited the expression of BiP and CHOP and decreased H2S-stimulated apoptosis, suggesting that p38 MAPK activation functions upstream of ER stress to initiate H2S-induced apoptosis. It is concluded that H2S induces apoptosis of insulin-secreting beta cells by enhancing ER stress via p38 MAPK activation. Our findings may help unmask a novel role of CSE/H2S system in regulating pancreatic functions under physiological condition and in diabetes.

Reduced CD4+ T-cell-specific gene expression in human type 1 diabetes mellitus

Type 1 diabetes mellitus (T1DM) in humans is characterized by the T-cell-dependent destruction of the insulin producing pancreatic beta cells; however, the precise pathogenesis of the disease, especially the initiation of pathologic immune response, is still largely unknown. We hypothesized that the function of human CD4+ T cells is altered in T1DM and analyzed unstimulated human peripheral blood CD4+ T-cell gene expression. We used a novel three-way comparison of DNA microarray data of CD4+ T cells isolated from patients with new onset T1DM, patients with long-term Type 2 diabetes (T2DM), and from healthy control subjects in order to eliminate any possible influence of glucose homeostasis on our findings. We analyzed the T1DM specific gene-expression changes and their functional relevance to T1DM autoimmunity. Our genetic and functional data show that T1DM CD4+ T cells are down-regulated specifically affecting key immune functions and cell cycle. Histone deacetylase gene expression, a key regulator of epigenetic modification is also reduced. The CD4+ T cells showed impaired function, including an abnormal immune response, which may be a key element that leads to the breakdown of self-tolerance.

Analysis of compensatory beta-cell response in mice with combined mutations of Insr and Irs2

Type 2 diabetes results from impaired insulin action and beta-cell dysfunction. There are at least two components to beta-cell dysfunction: impaired insulin secretion and decreased beta-cell mass. To analyze how these two variables contribute to the progressive deterioration of metabolic control seen in diabetes, we asked whether mice with impaired beta-cell growth due to Irs2 ablation would be able to mount a compensatory response in the background of insulin resistance caused by Insr haploinsufficiency. As previously reported, approximately 70% of mice with combined Insr and Irs2 mutations developed diabetes as a consequence of markedly decreased beta-cell mass. In the initial phases of the disease, we observed a robust increase in circulating insulin levels, even as beta-cell mass gradually declined, indicating that replication-defective beta-cells compensate for insulin resistance by increasing insulin secretion. These data provide further evidence for a heterogeneous beta-cell response to insulin resistance, in which compensation can be temporarily achieved by increasing function when mass is limited. The eventual failure of compensatory insulin secretion suggests that a comprehensive treatment of beta-cell dysfunction in type 2 diabetes should positively affect both aspects of beta-cell physiology.

Chronic activation of liver X receptor induces beta-cell apoptosis through hyperactivation of lipogenesis: liver X receptor-mediated lipotoxicity in pancreatic beta-cells

Liver X receptor (LXR)alpha and LXRbeta play important roles in fatty acid metabolism and cholesterol homeostasis. Although the functional roles of LXR in the liver, intestine, fat, and macrophages are well established, its role in pancreatic beta-cells has not been clearly defined. In this study, we revealed that chronic activation of LXR contributes to lipotoxicity-induced beta-cell dysfunction. We observed significantly elevated expression of LXR in the islets of diabetic rodent models, including fa/fa ZDF rats, OLETF rats, and db/db mice. In primary pancreatic islets and INS-1 insulinoma cells, activation of LXR with a synthetic ligand, T0901317, stimulated expression of the lipogenic genes ADD1/SREBP1c, FAS, and ACC and resulted in increased intracellular lipid accumulation. Moreover, chronic LXR activation induced apoptosis in pancreatic islets and INS-1 cells, which was synergistically promoted by high glucose conditions. Taken together, we suggest lipid accumulation caused by chronic activation of LXR in beta-cells as a possible cause of beta-cell lipotoxicity, a key step in the development of type 2 diabetes.

Insulin receptors in beta-cells are critical for islet compensatory growth response to insulin resistance

Insulin and insulin-like growth factor 1 (IGF1) are ubiquitous growth factors that regulate proliferation in most mammalian tissues including pancreatic islets. To explore the specificity of insulin receptors in compensatory beta-cell growth, we examined two models of insulin resistance. In the first model, we used liver-specific insulin receptor knockout (LIRKO) mice, which exhibit hyperinsulinemia without developing diabetes due to a compensatory increase in beta-cell mass. LIRKO mice, also lacking functional insulin receptors in beta-cells (beta IRKO/LIRKO), exhibited severe glucose intolerance but failed to develop compensatory islet hyperplasia, together leading to early death. In the second model, we examined the relative significance of insulin versus IGF1 receptors in islet growth by feeding high-fat diets to beta IRKO and beta-cell-specific IGF1 receptor knockout (beta IGFRKO) mice. Although both groups on the high-fat diet developed insulin resistance, beta IRKO, but not beta IGFRKO, mice exhibited poor islet growth consistent with insulin-stimulated phosphorylation, nuclear exclusion of FoxO1, and reduced expression of Pdx-1. Together these data provide direct genetic evidence that insulin/FoxO1/Pdx-1 signaling is one pathway that is crucial for islet compensatory growth response to insulin resistance.

Functional deficiencies of granulocyte-macrophage colony stimulating factor and interleukin-3 contribute to insulitis and destruction of beta cells

The pathogenesis of type 1 diabetes (T1D) involves the immune-mediated destruction of insulin-producing beta cells in the pancreatic islets of Langerhans. Genetic analysis of families with a high incidence of T1D and nonobese diabetic (NOD) mice, a prototypical model of the disorder, uncovered multiple susceptibility loci, although most of the underlying immune defects remain to be delineated. Here we report that aged mice doubly deficient in granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) manifest insulitis, destruction of insulin-producing beta cells, and compromised glucose homeostasis. Macrophages from mutant mice produce increased levels of p40 after LPS stimulation, whereas concurrent ablation of interferon-gamma (IFN-gamma) ameliorates the disease. The administration of antibodies that block cytotoxic T lymphocyte associated antigen-4 (CTLA-4) to young mutant mice precipitates the onset of insulitis and hyperglycemia. These results, together with previous reports of impaired hematopoietic responses to GM-CSF and IL-3 in patients with T1D and in NOD mice, indicate that functional deficiencies of these cytokines contribute to diabetes.

The emerging role of FOXO transcription factors in pancreatic beta cells

FOXO transcription factors critically control fundamental cellular processes, including metabolism, cell differentiation, cell cycle arrest, DNA repair, and other reactions to cellular stress. FOXO factors sense the balance between stimuli promoting growth and differentiation versus stress stimuli signaling damage. Integrated through the FOXO system, these divergent stimuli decide on cell fate, a choice between proliferation, differentiation, or apoptosis. In pancreatic beta cells, most recent evidence highlights complex FOXO-dependent responses to glucose, insulin, or other growth factors, which include regulatory feedback. In the short term, FOXO-dependent mechanisms help beta cells to accomplish their endocrine function, and may increase their resistance to oxidative stress due to transient hyperglycemia. In the long term, FOXO-dependent responses lead to the adaptation of beta cell mass, conditioning the future ability of the organism to produce insulin and cope with changes in fuel abundance. FOXO emerges as a key factor for the maintenance of a functional endocrine pancreas and represents an interesting element in the development of therapeutic approaches to treat diabetes. This review on the role of FOXO transcription factors in pancreatic beta cells has three parts. In Part I, FOXO transcription factors will be presented in general: structure, molecular mechanisms of regulation, cellular functions, and physiological roles. Part II will focus on specific data about FOXO factors in pancreatic beta cells. Lastly in Part III, it will be attempted to combine general and beta cell-specific knowledge with the aim to envisage globally the role of FOXO factors in beta cell-linked physiology and disease.

Chronic fetal hypoglycemia inhibits the later steps of stimulus-secretion coupling in pancreatic beta-cells

We measured the impact of chronic late gestation hypoglycemia on pancreatic islet structure and function to determine the cause of decreased insulin secretion in this sheep model of fetal nutrient deprivation. Late gestation hypoglycemia did not decrease pancreas weight, insulin content, beta-cell area, beta-cell mass, or islet size. The pancreatic islet isolation procedure selected a group of islets that were larger and had an increased proportion of beta-cells compared with islets measured in pancreatic sections, but there were no morphologic differences between islets isolated from control and hypoglycemic fetuses. The rates of glucose-stimulated pancreatic islet glucose utilization (126.2 +/- 25.3 pmol glucose.islet(-1).h(-1), hypoglycemic, vs. 93.5 +/- 5.5 pmol glucose.islet(-1).h(-1), control, P = 0.47) and oxidation (10.5 +/- 1.7 pmol glucose.islet(-1).h(-1), hypoglycemic, vs. 10.6 +/- 1.6 pmol glucose.islet(-1).h(-1), control) were not different in hypoglycemic fetuses compared with control fetuses. Chronic late gestation hypoglycemia decreased insulin secretion in isolated pancreatic islets by almost 70% in response to direct nonnutrient membrane depolarization and in response to increased extracellular calcium entry. beta-Cell ultrastructure was abnormal with markedly distended rough endoplasmic reticulum in three of the seven hypoglycemic fetuses studied, but in vitro analysis of hypoglycemic control islets showed no evidence that these changes represented endoplasmic reticulum stress, as measured by transcription of glucose regulatory protein-78 and processing of X-box binding protein-1. In conclusion, these studies show that chronic hypoglycemia in late gestation decreases insulin secretion by inhibiting the later steps of stimulus-secretion coupling after glucose metabolism, membrane depolarization, and calcium entry.

Acinar plasticity: development of a novel in vitro model to study human acinar-to-duct-to-islet differentiation

OBJECTIVES

The plasticity of pancreatic tissue is demonstrated in many pancreatic diseases. It has previously been shown that pancreatic islet-to-duct transformation and acinoductal metaplasia have been associated with both pancreatic regeneration and adenocarcinoma in various in vivo and in vitro settings. Understanding this inherent morphogenetic plasticity of the adult pancreas could lead to new therapeutic approaches to pancreatic disease.

METHODS

Cadaveric human pancreases (n = 7) were digested, and purified acinar tissue, which was approximately 85% immunoreactive for amylase and approximately 15% immunoreactive for CK-19, was embedded in a type 1 collagen matrix and cultured in a differentiation medium (DM) consisting of Dulbecco modified Eagle/F12 medium supplemented with cholera toxin (100 ng/mL), epidermal growth factor (10 ng/mL), and insulin (24 mU/mL) for 8 days. After this initial period, the resulting tissues were cultured in DM without cholera toxin, supplemented with gastrin (50 nmol/L) and hepatocyte growth factor (HGF; 10 ng/mL), with islet neogenesis-associated protein (INGAP; 167 nmol/L) or with gastrin + HGF + INGAP for 6 days. Tissue samples were then analyzed for amylase, cytokeratin 19, pancreas duodenum homeobox 1, and endocrine hormone immunoreactivity as well as dithizione positivity.

RESULTS

After 8 days of culture, approximately 90% of acini transformed into ductlike structures. This acinoductal transformation was characterized by a complete absence of amylase staining, with virtually all cells CK-19 immunoreactive. Addition of INGAP led to an approximately 18-fold increase in pancreas duodenum homeobox 1 immunoreactivity, although without an observed increase in insulin production as measured by dithizone positivity. However, when acinar-derived ductlike structures were cultured with gastrin + HGF + INGAP, the total incidence of dithizone-positive structures increased approximately 6-fold (10.9 +/- 2.9% vs 1.7 +/- 0.4%, P = 0.037). Treatment with gastrin + HGF alone led to no significant change in any of the measured parameters.

CONCLUSIONS

We have developed a novel in vitro model of adult human acinoductal metaplasia that will aid not only in developing new methods of expanding beta-cell mass but also provide insights into pancreatic carcinogenesis.

Toxic human islet amyloid polypeptide (h-IAPP) oligomers are intracellular, and vaccination to induce anti-toxic oligomer antibodies does not prevent h-IAPP-induced beta-cell apoptosis in h-IAPP transgenic mice

OBJECTIVE

Islets in type 2 diabetes are characterized by a deficit in beta-cells, increased beta-cell apoptosis, and islet amyloid derived from islet amyloid polypeptide (IAPP). The toxic form of amyloidogenic protein oligomers are distinct and smaller than amyloid fibrils and act by disrupting membranes. Using antibodies that bind to toxic IAPP oligomers (but not IAPP monomers or fibrils) and a vaccination-based approach, we sought to establish whether IAPP toxic oligomers form intra- or extracellularly and whether vaccination to induce anti-toxic oligomer antibodies prevents IAPP-induced apoptosis in human IAPP (h-IAPP) transgenic mice.

RESEARCH DESIGN AND METHODS

Pancreas was sampled from two h-IAPP transgenic mouse models and examined by immunohistochemistry for toxic oligomers. The same murine models were vaccinated with toxic oligomers of Alzheimer beta protein (AbetaP(1-40)) and anti-oligomer titers, and blood glucose and islet pathology were monitored.

RESULTS

Toxic oligomers were detected intracellularly in approximately 20-40% of h-IAPP transgenic beta-cells. Vaccine induced high titers of anti-h-IAPP toxic oligomers in both transgenic models, but beta-cell apoptosis was, if anything, further increased in vaccinated mice, so that neither loss of beta-cell mass nor diabetes onset was delayed.

CONCLUSIONS

IAPP toxic oligomers form in h-IAPP transgenic mouse models, and anti-toxic oligomer antibodies do not prevent h-IAPP-induced beta-cell apoptosis. These data suggest that prevention of h-IAPP oligomer formation may be more useful than a vaccination-based approach in the prevention of type 2 diabetes.

Effect of heat shock protein peptide DiaPep277 on beta-cell function in paediatric and adult patients with recent-onset diabetes mellitus type 1: two prospective, randomized, double-blind phase II trials

BACKGROUND

Aim of this trial was to test whether heat shock protein peptide DiaPep277 treatment in adult and paediatric patients with recent-onset type 1 diabetes (T1D) is safe and whether it can preserve endogenous insulin production.

METHODS

Two studies were performed in a prospective, multicentre, double-blind, placebo-controlled trial. Fifty adult (study p520, aged 16-44 years) and 49 paediatric patients (study p521, 4-15 years) with recent-onset T1D were treated subcutaneously at four different time points with 0.2 mg or 1.0 mg DiaPep277 versus placebo and followed for 18 months. Adult patients were treated with 0.2 mg, 1.0 mg or 2.5 mg DiaPep277 versus placebo. Stimulated C-peptide served as readout for functional beta-cell-mass.

RESULTS

DiaPep277-treatment was not associated with severe side effects. No differences were found in placebo and DiaPep277 treated groups. In adults, a modest trend towards better maintenance of beta-cell function was observed in the 0.2 mg and 1.0 mg group, while there was significant loss of stimulated C-peptide in the placebo and 2.5 mg group. Paediatric patients with low HLA risk showed stable C-peptide levels until 13 months upon treatment with 1 mg DiaPep277. Despite similar stimulated C-peptide levels at baseline, children exhibited a more pronounced loss of beta-cell function over 18 months than adults (p = 0.0003).

CONCLUSION

Administration of DiaPep277 seems safe and may have beneficial effects on C-peptide levels over time in some patients with T1D, but this finding was not accompanied by reduced HbA1c or insulin requirement. Studies with more patients and longer follow-up are needed to further study the effect of DiaPep277.

Dominant-negative effects of a novel mutated Ins2 allele causes early-onset diabetes and severe beta-cell loss in Munich Ins2C95S mutant mice

The novel diabetic mouse model Munich Ins2(C95S) was discovered within the Munich N-ethyl-N-nitrosourea mouse mutagenesis screen. These mice exhibit a T-->A transversion in the insulin 2 (Ins2) gene at nucleotide position 1903 in exon 3, which leads to the amino acid exchange C95S and loss of the A6-A11 intrachain disulfide bond. From 1 month of age onwards, blood glucose levels of heterozygous Munich Ins2(C95S) mutant mice were significantly increased compared with controls. The fasted and postprandial serum insulin levels of the heterozygous mutants were indistinguishable from those of wild-type littermates. However, serum insulin levels after glucose challenge, pancreatic insulin content, and homeostasis model assessment (HOMA) beta-cell indices of heterozygous mutants were significantly lower than those of wild-type littermates. The initial blood glucose decrease during an insulin tolerance test was lower and HOMA insulin resistance indices were significantly higher in mutant mice, indicating the development of insulin resistance in mutant mice. The total islet volume, the volume density of beta-cells in the islets, and the total beta-cell volume of heterozygous male mutants was significantly reduced compared with wild-type mice. Electron microscopy of the beta-cells of male mutants showed virtually no secretory insulin granules, the endoplasmic reticulum was severely enlarged, and mitochondria appeared swollen. Thus, Munich Ins2(C95S) mutant mice are considered a valuable model to study the mechanisms of beta-cell dysfunction and death during the development of diabetes.

In vivo expression of GLP-1/IgG-Fc fusion protein enhances beta-cell mass and protects against streptozotocin-induced diabetes

Glucagon-like peptide 1 (GLP-1) and its analogue exendin-4 (Ex4) have displayed potent glucose homeostasis-modulating characteristics in type 2 diabetes (T2D). However, there are few reports of effectiveness in type 1 diabetes (T1D) therapy, where there is massive loss of beta cells. We previously described a novel GLP-1 analogue consisting of the fusion of active GLP-1 and IgG heavy chain constant regions (GLP-1/IgG-Fc), and showed that in vivo expression of the protein, via electroporation-enhanced intramuscular plasmid-based gene transfer, normalized blood glucose levels in T2D-prone db/db mice. In the present study, GLP-1/IgG-Fc and Ex4/IgG-Fc were independently tested in multiple low-dose streptozotocin-induced T1D. Both GLP-1/IgG-Fc and Ex4/IgG-Fc effectively reduced fed blood glucose levels in treated mice and ameliorated diabetes symptoms, where as control IgG-Fc had no effect. Treatment with GLP-1/IgG-Fc or Ex4/IgG-Fc improved glucose tolerance and increased circulating insulin and GLP-1 levels. It also significantly enhanced islet beta-cell mass, which is likely a major factor in the amelioration of diabetes. This suggests that GLP-1/IgG-Fc gene therapy may be applicable to diseases where there is either acute or chronic beta-cell injury.

Histopathological changes in insulin, glucagon and somatostatin cells in the islets of NOD mice during cyclophosphamide-accelerated diabetes: a combined immunohistochemical and histochemical study

The cyclophosphamide model of accelerated diabetes in the NOD mouse is a useful model of insulin-dependent diabetes mellitus (IDDM). Knowledge on the progressive destruction of beta cells and the fate of other islet endocrine cell-types in this model is sparse. We employed immunohistochemistry and histochemistry, to study temporal changes in islet cell populations, insulitis and glucose transporter-2 expression during cyclophosphamide administration. Cyclophosphamide was administered to day 95 female NOD mice and the pancreas studied at days 0 ( = day 95), 4, 7, 11 and 14 after treatment and in age-matched control mice. At day 0, a majority of the endocrine cells were insulin-positive. Glucagon and somatostatin cells were mostly in the islet periphery and also internally. In the cyclophosphamide group, insulitis was moderate at day 0, declined at day 4 but increased progressively from day 7. The extent of insulitis in treated mice which were diabetes-free at day 14 was comparable to age-matched control mice. From day 11, the marked increase in insulitis correlated with a reciprocal decline in the extent of insulin immunostained islet area. At day 14, the mean insulin area per islet was markedly less in diabetic mice than in age-matched non-diabetic treated and controls. At diabetes, some islets showed co-expression of glucagon and insulin. Our studies suggest that the mean number of glucagon or somatostatin cells per islet does not vary during the study. Glucose transporter-2 immunolabelling was restricted to beta cells but declined in those adjacent to immune cells. We conclude that in the cyclophosphamide model, there is specific and augmented destruction of beta cells immediately prior to diabetes onset. We speculate that the selective loss of glucose transporter-2 shown in this study suggests the existence of a deleterious gradient close to the immune cell and beta cell surface boundary.

Glucose mediates the translocation of NeuroD1 by O-linked glycosylation

O-Linked GlcNAc modification of nuclear and cytosolic proteins has been shown to regulate the function of many cellular proteins. Increased O-linked glycosylation, observed under chronic hyperglycemia conditions, has been implicated in the pathogenesis of diabetes. However, the exact role of O-GlcNAc modification in regulating glucose homeostasis remains to be established. We report here that the subcellular localization of the pancreatic beta cell-specific transcription factor NeuroD1 is regulated by O-linked glycosylation in the mouse insulinoma cell line MIN6. Under low glucose conditions, NeuroD1 is mainly in the cytosol. However, treatment of MIN6 cells with high glucose results in O-linked GlcNAc modification of NeuroD1 and its subsequent translocation into the nucleus. Consistent with these data, treatment of MIN6 cells with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)-amino N-phenylcarbamate, an inhibitor of O-GlcNAcase, causes Neuro-D1 localization to the nucleus and induction of insulin gene expression even on low glucose. Furthermore, we demonstrate that NeuroD1 interacts with the O-GlcNAc transferase, OGT only at high concentrations of glucose and depletion of OGT by using small interfering RNA oligos interferes with the nuclear localization of NeuroD1 on high glucose. On low glucose NeuroD1 interacts with the O-GlcNAcase and becomes deglycosylated, which is likely to be important for export of Neuro-D1 into cytosol in the presence of low glucose. In summary, the presented data suggest that glucose regulates the subcellular localization of NeuroD1 in pancreatic beta cells via O-linked GlcNAc modification of NeuroD1 by OGT.

Pharmacodynamics and stability of subcutaneously infused glucagon in a type 1 diabetic Swine model in vivo

BACKGROUND

The objective of this study was to determine the in vivo pharmacodynamics of glucagon and to test its glycemic effect over days by assessing its time course of activity and potency in a type 1 diabetic swine model.

METHODS

Individual experiments were conducted in different pigs using glucagon preparations that were reconstituted on different days and stored at room temperature or near body temperature before usage. All experiments involved a subcutaneous bolus of glucagon to counter impending hypoglycemia induced by an earlier bolus of fast-acting insulin. Frequent blood-glucose measurements, using a standard in vitro hand-held meter, were taken during each experiment to track variations in blood-glucose concentration.

RESULTS

Significant glucagon action was observed as early as 5 min after administration, as evidenced by an effective halt to declining blood glucose and a subsequent twofold rise in blood glucose after approximately 20 min. Results also indicate that the consumption of glucagon from the subcutaneous depot is substantially faster than that of fast-acting insulin. Furthermore, no significant depreciation was observed in glucagon efficacy across aging preparations as old as 7 days.

CONCLUSIONS

These results suggest profound utility of subcutaneous glucagon in a closed-loop glucose control system, especially since glucagon would provide an effective safeguarding measure to stave off impending hypoglycemia, an application where the rapid effect of subcutaneous glucagon is both serendipitous and essential. Despite the notion that the stability of glucagon in solution at room temperature is inferior to that of fast-acting insulin, its subcutaneous administration has promising prospects for long-term closed-loop ambulatory care.

Protection of pancreatic beta-cells by exendin-4 may involve the reduction of endoplasmic reticulum stress; in vivo and in vitro studies

The aim of this study was to investigate the in vivo and in vitro effects of exendin-4, a potent glucagon-like peptide 1 agonist, on the protection of the pancreatic beta-cells against their cell death. In in vivo experiments, we used beta-cell-specific calmodulin-overexpressing mice where massive apoptosis takes place in their beta-cells, and we examined the effects of chronic treatment with exendin-4. Chronic and s.c. administration of exendin-4 reduced hyperglycemia. The treatment caused significant increases of the insulin contents of the pancreas and islets, and retained the insulin-positive area. Dispersed transgenic islet cells lived only shortly, and several endoplasmic reticulum (ER) stress-related molecules such as immunoglobulin-binding protein (Bip), inositol-requiring enzyme-1alpha, X-box-binding protein-1 (XBP-1), RNA-activated protein kinase-like endoplasmic reticulum kinase, activating transcription factor-4, and C/EBP-homologous protein (CHOP) were more expressed in the transgenic islets. We also found that the spliced form of XBP-1, a marker of ER stress, was also increased in beta-cell-specific calmodulin-overexpressing transgenic islets. In the quantitative real-time PCR analyses, the expression levels of Bip and CHOP were reduced in the islets from the transgenic mice treated with exendin-4. These findings suggest that excess of ER stress occurs in the transgenic beta-cells, and the suppression of ER stress and resultant protection against cell death may be involved in the anti-diabetic effects of exendin-4.

Endoplasmic reticulum stress contributes to beta cell apoptosis in type 2 diabetes

AIMS/HYPOTHESIS

Increased lipid supply causes beta cell death, which may contribute to reduced beta cell mass in type 2 diabetes. We investigated whether endoplasmic reticulum (ER) stress is necessary for lipid-induced apoptosis in beta cells and also whether ER stress is present in islets of an animal model of diabetes and of humans with type 2 diabetes.

METHODS

Expression of genes involved in ER stress was evaluated in insulin-secreting MIN6 cells exposed to elevated lipids, in islets isolated from db/db mice and in pancreas sections of humans with type 2 diabetes. Overproduction of the ER chaperone heat shock 70 kDa protein 5 (HSPA5, previously known as immunoglobulin heavy chain binding protein [BIP]) was performed to assess whether attenuation of ER stress affected lipid-induced apoptosis.

RESULTS

We demonstrated that the pro-apoptotic fatty acid palmitate triggers a comprehensive ER stress response in MIN6 cells, which was virtually absent using non-apoptotic fatty acid oleate. Time-dependent increases in mRNA levels for activating transcription factor 4 (Atf4), DNA-damage inducible transcript 3 (Ddit3, previously known as C/EBP homologous protein [Chop]) and DnaJ homologue (HSP40) C3 (Dnajc3, previously known as p58) correlated with increased apoptosis in palmitate- but not in oleate-treated MIN6 cells. Attenuation of ER stress by overproduction of HSPA5 in MIN6 cells significantly protected against lipid-induced apoptosis. In islets of db/db mice, a variety of marker genes of ER stress were also upregulated. Increased processing (activation) of X-box binding protein 1 (Xbp1) mRNA was also observed, confirming the existence of ER stress. Finally, we observed increased islet protein production of HSPA5, DDIT3, DNAJC3 and BCL2-associated X protein in human pancreas sections of type 2 diabetes subjects.

CONCLUSIONS/INTERPRETATION

Our results provide evidence that ER stress occurs in type 2 diabetes and is required for aspects of the underlying beta cell failure.

Compromised beta-cell development and beta-cell dysfunction in weanling offspring from dams maintained on a high-fat diet during gestation

OBJECTIVES

Reported here are the effects of a high-fat diet (HFD) fed to dams during pregnancy on the weight, beta- and alpha-cell development, and beta-cell function of their weanling offspring.

METHODS

Offspring were obtained from dams maintained on an HFD for the first, second, or third week of gestation or throughout gestation and then on a standard laboratory diet for the duration of lactation. Weanling weights and circulating glucose and insulin concentrations were measured on postnatal day 21, after which pancreata were excised and snap-frozen for quantitative polymerase chain reaction of glucokinase (GK) or processed for immunohistochemical examination and image analysis (beta- and alpha-cell volume, number, and size, and GK immunoreactivity).

RESULTS

All of the weanlings had low body weights and were hypoinsulinemic. In weanlings maintained on an HFD for either the first, second, or third week of gestation, hyperglycemia and a reduction in beta-cell volume and number, in beta- and alpha-cell size, and in both GK messenger RNA expression and immunoreactivity were observed. The development of beta and alpha cells was normal in weanlings maintained on an HFD throughout gestation.

CONCLUSIONS

Maintenance of dams on an HFD for any single week of gestation results in weanling offspring with an impairment in beta-cell development and function.

The interplay of insulin resistance and beta-cell dysfunction involves the development of type 2 diabetes in Chinese obeses

Type 2 diabetes mellitus (T2DM) is a heterogeneous disorder characterized by defects in insulin secretion and action and obesity plays an important role in the deterioration of glucose metabolism. In the present study we evaluated the degree of insulin resistance and first-phase insulin secretion of beta-cell in obese subjects with normal glucose tolerance (NGT), impaired glucose tolerance (IGT), and T2DM in Chinese. A total of 220 subjects underwent standard 75 g oral glucose tolerance test (OGTT) and insulin-modified frequently sampled intravenous glucose tolerance test (FSIGT). Insulin sensitivity index (S I) was assessed by the reduced sample number (n = 12) of Bergman's minimal model method with FSIGT. Insulin secretion capacities were determined by the insulinogenic index (I 30 min - I 0 min)/(G 30 min - G 0 min) in OGTT and the acute insulin response to glucose (AIR) in FSIGT. The disposition index (DI), the product of AIR and S I was used to determine whether AIR was adequate to compensate for insulin resistance. The S I in healthy lean control group was significantly higher than that in NGT, IGT, and T2DM group, but there was no significant difference among NGT, IGT, and T2DM group. The AIR in NGT group was significantly greater than that in control group, but then it was progressively decreased in IGT and T2DM group. The value of DI in control group was significantly higher than that in those three abnormal groups, and was decreased from NGT to IGT and T2DM group with significant difference. It indicates that obese subjects with different glucose tolerances have a similar degree of insulin resistance but differ in insulin secretion in Chinese Han population.

Regulation of pancreatic islet cell survival and replication by gamma-aminobutyric acid

AIMS/HYPOTHESIS

Pancreatic islets have evolved remarkable, though poorly understood mechanisms to modify beta cell mass when nutrient intake fluctuates or cells are damaged. We hypothesised that appropriate and timely adjustments in cell number occur because beta cells release proliferative signals to surrounding cells when stimulated by nutrients and 'bleed' these growth factors upon injury.

MATERIALS AND METHODS

In rat pancreatic islets, we measured DNA content, insulin content, insulin secretion after treatment, immunoblots of apoptotic proteins and the uptake of nucleoside analogues to assess the ability of gamma-aminobutyric acid (GABA), which is highly concentrated in beta cells, to act as a growth and survival factor. This focus is supported by work from others demonstrating that GABA increases cell proliferation in the developing nervous system, acts as a survival factor for differentiated neurons and, interestingly, protects plants under stress.

RESULTS

Our results show that DNA, insulin content and insulin secretion are higher in freshly isolated islets treated with GABA or GABA B receptor agonists. Exposure to GABA upregulated the anti-apoptotic protein B-cell chronic lymphocytic leukaemia XL and limited activation of caspase 3 in islets. The cellular proliferation rate in GABA-treated islets was twice that of untreated controls.

CONCLUSIONS/INTERPRETATION

We conclude that GABA serves diverse purposes in the islet, meeting a number of functional criteria to act as an endogenous co-regulator of beta cell mass.

Glucose intolerance and reduced islet blood flow in transgenic mice expressing the FRK tyrosine kinase under the control of the rat insulin promoter

The FRK tyrosine kinase has previously been shown to transduce beta-cell cytotoxic signals in response to cytokines and streptozotocin and to promote beta-cell proliferation and an increased beta-cell mass. We therefore aimed to further evaluate the effects of overexpression of FRK tyrosine kinase in beta-cells. A transgenic mouse expressing kinase-active FRK under control of the insulin promoter (RIP-FRK) was studied with regard to islet endocrine function and vascular morphology. Mild glucose intolerance develops in RIP-FRK male mice of at least 4 mo of age. This effect is accompanied by reduced glucose-stimulated insulin secretion in vivo and reduced second-phase insulin secretion in response to glucose and arginine upon pancreas perfusion. Islets isolated from the FRK transgenic mice display a glucose-induced insulin secretory response in vitro similar to that of control islets. However, islet blood flow per islet volume is decreased in the FRK transgenic mice. These mice also exhibit a reduced islet capillary lumen diameter as shown by electron microscopy. Total body weight and pancreas weight are not significantly affected, but the beta-cell mass is increased. The data suggest that long-term expression of active FRK in beta-cells causes an in vivo insulin-secretory defect, which may be the consequence of islet vascular abnormalities that yield a decreased islet blood flow.

beta-cell failure in diabetes and preservation by clinical treatment

There is a progressive deterioration in beta-cell function and mass in type 2 diabetics. It was found that islet function was about 50% of normal at the time of diagnosis, and a reduction in beta-cell mass of about 60% was shown at necropsy. The reduction of beta-cell mass is attributable to accelerated apoptosis. The major factors for progressive loss of beta-cell function and mass are glucotoxicity, lipotoxicity, proinflammatory cytokines, leptin, and islet cell amyloid. Impaired beta-cell function and possibly beta-cell mass appear to be reversible, particularly at early stages of the disease where the limiting threshold for reversibility of decreased beta-cell mass has probably not been passed. Among the interventions to preserve or "rejuvenate" beta-cells, short-term intensive insulin therapy of newly diagnosed type 2 diabetes will improve beta-cell function, usually leading to a temporary remission time. Another intervention is the induction of beta-cell "rest" by selective activation of ATP-sensitive K+ (K(ATP)) channels, using drugs such as diazoxide. A third type of intervention is the use of antiapoptotic drugs, such as the thiazolidinediones (TZDs), and incretin mimetics and enhancers, which have demonstrated significant clinical evidence of effects on human beta-cell function. The TZDs improve insulin secretory capacity, decrease beta-cell apoptosis, and reduce islet cell amyloid with maintenance of neogenesis. The TZDs have indirect effects on beta-cells by being insulin sensitizers. The direct effects are via peroxisome proliferator-activated receptor gamma activation in pancreatic islets, with TZDs consistently improving basal beta-cell function. These beneficial effects are sustained in some individuals with time. There are several trials on prevention of diabetes with TZDs. Incretin hormones, which are released from the gastrointestinal tract in response to nutrient ingestion to enhance glucose-dependent insulin secretion from the pancreas, aid the overall maintenance of glucose homeostasis through slowing of gastric emptying, inhibition of glucagon secretion, and control of body weight. From the two major incretins, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), only the first one or its mimetics or enhancers can be used for treatment because the diabetic beta-cell is resistant to GIP action. Because of the rapid inactivation of GLP-1 by dipeptidyl peptidase (DPP)-IV, several incretin analogs were developed: GLP-1 receptor agonists (incretin mimetics) exenatide (synthetic exendin-4) and liraglutide, by conjugation of GLP-1 to circulating albumin. The acute effect of GLP-1 and GLP-1 receptor agonists on beta-cells is stimulation of glucose-dependent insulin release, followed by enhancement of insulin biosynthesis and stimulation of insulin gene transcription. The chronic action is stimulating beta-cell proliferation, induction of islet neogenesis, and inhibition of beta-cell apoptosis, thus promoting expansion of beta-cell mass, as observed in rodent diabetes and in cultured beta-cells. Exenatide and liraglutide enhanced postprandial beta-cell function. The inhibition of the activity of the DPP-IV enzyme enhances endogenous GLP-1 action in vivo, mediated not only by GLP-1 but also by other mediators. In preclinical studies, oral active DPP-IV inhibitors (sitagliptin and vildagliptin) also promoted beta-cell proliferation, neogenesis, and inhibition of apoptosis in rodents. Meal tolerance tests showed improvement in postprandial beta-cell function. Obviously, it is difficult to estimate the protective effects of incretin mimetics and enhancers on beta-cells in humans, and there is no clinical evidence that these drugs really have protective effects on beta-cells.