Role of IGFBP-3 in the regulation of ß-cell mass during obesity: adipose tissue/ß-cell cross talk.

In obesity an increase in ß-cell mass occurs to cope with the rise in insulin demand. This ß-cell plasticity is essential to avoid the onset of hyperglycemia, although the molecular mechanisms that regulate this process remain unclear. This study analyzed the role of adipose tissue in the control of ß-cell replication. Using a diet-induced model of obesity, we obtained conditioned media from three different white adipose tissue depots. Only in the adipose tissue depot surrounding the pancreas did the diet induce changes that led to an increase in INS1E cells and the islet replication rate. To identify the factors responsible for this proliferative effect, adipose tissue gene expression analysis was conducted by microarrays and quantitative RT-PCR. Of all the differentially expressed proteins, only the secreted ones were studied. IGF binding protein 3 (Igfbp3) was identified as the candidate for this effect. Furthermore, in the conditioned media, although the blockage of IGFBP3 led to an increase in the proliferation rate, the blockage of IGF-I receptor decreased it. Taken together, these data show that obesity induces specific changes in the expression profile of the adipose tissue depot surrounding the pancreas, leading to a decrease in IGFBP3 secretion. This decrease acts in a paracrine manner, stimulating the ß-cell proliferation rate, probably through an IGF-I-dependent mechanism. This cross talk between the visceral-pancreatic adipose tissue and ß-cells is a novel mechanism that participates in the control of ß-cell plasticity.

BACE2 plays a role in the insulin receptor trafficking in pancreatic ß-cells.

BACE1 (ß-site amyloidogenic cleavage of precursor protein-cleaving enzyme 1) is a ß-secretase protein that plays a central role in the production of the ß-amyloid peptide in the brain and is thought to be involved in the Alzheimer's pathogenesis. In type 2 diabetes, amyloid deposition within the pancreatic islets is a pathophysiological hallmark, making crucial the study in the pancreas of BACE1 and its homologous BACE2 to understand the pathological mechanisms of this disease. The objectives of the present study were to characterize the localization of BACE proteins in human pancreas and determine their function. High levels of BACE enzymatic activity were detected in human pancreas. In normal human pancreas, BACE1 was observed in endocrine as well as in exocrine pancreas, whereas BACE2 expression was restricted to ß-cells. Intracellular analysis using immunofluorescence showed colocalization of BACE1 with insulin and BACE2 with clathrin-coated vesicles of the plasma membrane in MIN6 cells. When BACE1 and -2 were pharmacologically inhibited, BACE1 localization was not altered, whereas BACE2 content in clathrin-coated vesicles was increased. Insulin internalization rate was reduced, insulin receptor ß-subunit (IRß) expression was decreased at the plasma membrane and increased in the Golgi apparatus, and a significant reduction in insulin gene expression was detected. Similar results were obtained after specific BACE2 silencing in MIN6 cells. All these data point to a role for BACE2 in the IRß trafficking and insulin signaling. In conclusion, BACE2 is hereby presented as an important enzyme in ß-cell function.

Molecular mechanisms of tungstate-induced pancreatic plasticity: a transcriptomics approach.

BACKGROUND: Sodium tungstate is known to be an effective anti-diabetic agent, able to increase beta cell mass in animal models of diabetes, although the molecular mechanisms of this treatment and the genes that control pancreas plasticity are yet to be identified. Using a transcriptomics approach, the aim of the study is to unravel the molecular mechanisms which participate in the recovery of exocrine and endocrine function of streptozotocin (STZ) diabetic rats treated with tungstate, determining the hyperglycemia contribution and the direct effect of tungstate. RESULTS: Streptozotocin (STZ)-diabetic rats were treated orally with tungstate for five weeks. Treated (STZ)-diabetic rats showed a partial recovery of exocrine and endocrine function, with lower glycemia, increased insulinemia and amylasemia, and increased beta cell mass achieved by reducing beta cell apoptosis and raising beta cell proliferation. The microarray analysis of the pancreases led to the identification of three groups of differentially expressed genes: genes altered due to diabetes, genes restored by the treatment, and genes specifically induced by tungstate in the diabetic animals. The results were corroborated by quantitative PCR. A detailed description of the pathways involved in the pancreatic effects of tungstate is provided in this paper. Hyperglycemia contribution was studied in STZ-diabetic rats treated with phloridzin, and the direct effect of tungstate was determined in INS-1E cells treated with tungstate or serum from untreated or treated STZ-rats, observing that tungstate action in the pancreas takes places via hyperglycemia-independent pathways and via a combination of tungstate direct and indirect (through the serum profile modification) effects. Finally, the MAPK pathway was evaluated, observing that it has a key role in the tungstate-induced increase of beta cell proliferation as tungstate activates the mitogen-activated protein kinase (MAPK) pathway directly by increasing p42/p44 phosphorylation and indirectly by decreasing the expression of raf kinase inhibitor protein (Rkip), a negative modulator of the pathway. CONCLUSION: In conclusion, tungstate improves pancreatic function through a combination of hyperglycemia-independent pathways and through its own direct and indirect effects, whereas the MAPK pathway has a key role in the tungstate-induced increase of beta cell proliferation.

Inhibitory effects of leptin on pancreatic alpha-cell function.

OBJECTIVE: Leptin released from adipocytes plays a key role in the control of food intake, energy balance, and glucose homeostasis. In addition to its central action, leptin directly affects pancreatic beta-cells, inhibiting insulin secretion, and, thus, modulating glucose homeostasis. However, despite the importance of glucagon secretion in glucose homeostasis, the role of leptin in alpha-cell function has not been studied in detail. In the present study, we have investigated this functional interaction. RESEARCH DESIGN AND METHODS: The presence of leptin receptors (ObR) was demonstrated by RT-PCR analysis, Western blot, and immunocytochemistry. Electrical activity was analyzed by patch-clamp and Ca(2+) signals by confocal microscopy. Exocytosis and glucagon secretion were assessed using fluorescence methods and radioimmunoassay, respectively. RESULTS: The expression of several ObR isoforms (a-e) was detected in glucagon-secreting alphaTC1-9 cells. ObRb, the main isoform involved in leptin signaling, was identified at the protein level in alphaTC1-9 cells as well as in mouse and human alpha-cells. The application of leptin (6.25 nmol/l) hyperpolarized the alpha-cell membrane potential, suppressing the electrical activity induced by 0.5 mmol/l glucose. Additionally, leptin inhibited Ca(2+) signaling in alphaTC1-9 cells and in mouse and human alpha-cells within intact islets. A similar result occurred with 0.625 nmol/l leptin. These effects were accompanied by a decrease in glucagon secretion from mouse islets and were counteracted by the phosphatidylinositol 3-kinase inhibitor, wortmannin, suggesting the involvement of this pathway in leptin action. CONCLUSIONS: These results demonstrate that leptin inhibits alpha-cell function, and, thus, these cells are involved in the adipoinsular communication.

Radiobinding assay for detecting autoantibodies to single epitopes.

Autoantibodies, a hallmark of autoimmune disease, are directed against diverse antigens and epitopes. This diversity aids in characterising disease progression and identifying disease-associated autoantibodies. Here we aimed to develop a sensitive assay to detect and quantify epitope-specific autoantibodies. We generated constructs to integrate known peptide epitopes into the TrxA protein, and used these to in vitro synthesize radio-labelled proteins for a radiobinding assay (RBA). This assay was first validated with an animal model using mice immunized with the MOG(40-55) peptide. Using type 1 diabetes-associated protein tyrosine phosphatase-like autoantigen IA-2 as a human model, we expressed IA-2(609-621), IA-2(619-631), or IA-2(609-631) peptide in the TrxA construct and used the RBA to test sera from 113 patients with type 1 diabetes and 87 controls. Antibodies were detected to the IA-2(609-631) epitope in 37 of 113 patient sera and one control; 17 sera were reactive to the IA-2(609-621) (JM1) epitope, and 16 to the IA-2(619-631) (JM2) epitope. Interestingly, a novel third epitope (JM3) was identified in 7 sera that reacted to IA-2(609-631) but not the JM1 and JM2 sub-specificities. An ELISA using IA-2(609-631) was less sensitive than the RBA, as it detected only some of the JM1 and JM3 antibody positive sera and none of the JM2. Mutagenesis of single IA-2(609-631) amino acids in combination with the RBA showed that antibodies to JM2 and JM3 were highly diverse in their specific residue requirements. Our strategy using in vitro synthesized antigens and the RBA was thus a highly sensitive and versatile method to identify epitopes and quantify epitope-specific antibodies.

Salicylates increase insulin secretion in healthy obese subjects.

CONTEXT: Conflicting results on the effects of salicylates on glucose tolerance in subjects with normal glucose tolerance or type 2 diabetes have been reported. OBJECTIVE: The objective of the study was to investigate the effects of a salicylate derivative (triflusal) on insulin sensitivity and insulin secretion. DESIGN, SETTING, AND PARTICIPANTS: This was a double-blind, randomized, crossover study with three treatment periods corresponding to two dose levels of triflusal and placebo in healthy obese subjects. MAIN OUTCOME MEASURES: Insulin sensitivity and insulin secretion, evaluated through frequently sampled iv glucose tolerance test that was performed after each treatment period, were measured. Insulin secretion was also evaluated in vitro in mice and human islets of Langerhans. RESULTS: The administration of triflusal led to decreased fasting serum glucose concentration in the study subjects. Insulin sensitivity did not significantly change after each treatment period. Insulin secretion, however, significantly increased in a dose-dependent fashion after each triflusal treatment period. The administration of 800 mum of the main triflusal metabolite to whole mice islets of Langerhans led to a sustained increase in intracellular calcium concentration level. This was followed by a significantly increase in insulin secretion. In human islets, 200 mum of 2-hydroxy-4-trifluoromethylbenzoic acid was sufficient to increase insulin release. CONCLUSIONS: The administration of a salicylate compound led to lowering of serum glucose concentration. We suggest that this effect was mediated through increased insulin secretion induced by salicylate directly on the beta-cell.

Phosphorylation events implicating p38 and PI3K mediate tungstate-effects in MIN6 beta cells.

Oral administration of sodium tungstate is an effective treatment for diabetes in animal models. Several lines of evidence indicate the pancreatic beta cell as one of the targets of tungstate action. Here, we examined the molecular mechanism by which this compound exerts its effects on the beta cell line MIN6. Tungstate treatment induced phosphorylation and subsequent activation of p38 and PI3K which in turn are implicated in tungstate PDX-1 nuclear localization and activation. Although no effect was observed in glucose-induced insulin secretion we found that tungstate activates basal insulin release, a process driven, at least in part, by activation of p38. These results show a direct involvement of p38 and PI3K phosphorylation in the mechanism of action of tungstate in the beta cell.

Monoclonal antibody 76F distinguishes IA-2 from IA-2beta and overlaps an autoantibody epitope.

IA-2 and IA-2beta are highly related proteins that are autoantigens in type 1 diabetes, and provide a model for developing reagents and assays that distinguish similar proteins with unique autoantibody epitopes. Monoclonal antibodies (mAb) to IA-2 and IA-2beta were prepared and tested for their ability to bind to the related proteins and their ability to compete for specific autoantibody epitope binding by sera from patients with type 1 diabetes. Monoclonal antibodies that specifically bound IA-2 (76F) or bound both IA-2 and IA-2beta (A9) were isolated and characterized. 76F mAb recognized IA-2 of human, rat and mouse origin in native and denatured forms and had an epitope specificity for residues 626-630 (FEYQD) which are found in the juxtamembrane (JM) region of human and mouse IA-2, but not IA-2beta. This region overlaps with the autoantibody epitope JM2. Binding to the 76F monoclonal antibody was specifically inhibited by sera with antibodies to the JM2 epitope but not with antibodies to the adjacent JM1 epitope, indicating that unique epitopes can be distinguished by this approach. 76F mAb has the unique property to distinguish between the two closely related autoantigens IA-2 and IA-2beta by targeting an IA-2 specific epitope of the juxtamembrane region. The findings define an approach to develop assays for specific antibody epitope measurements which may be relevant for disease prognosis and monitoring intervention therapies.

Humoral autoimmune responses to glutamic acid decarboxylase have similar target epitopes and subclass that show titer-dependent disease association.

Glutamic acid decarboxylase (GAD) is an autoantigen in stiff man syndrome (SMS) and type 1 diabetes (T1DM). Different GAD autoantibody characteristics in these disorders have suggested distinct underlying mechanisms of autoimmunity. Here, it is shown that increased prevalence of autoantibodies to GAD65 amino terminal and GAD67 epitopes and autoantibodies of IgG2, IgG3, or IgG4 subclass in patients with SMS (P < 0.001 vs. T1DM) are secondary to the markedly higher autoantibody titers in SMS patients (P < 0.0001) and that autoantibody epitopes and subclasses were similar when patients were matched for autoantibody titer. Exposure to autoantigen in the disorders is likely to involve similar humoral antigenic determinants, but different B cell regulation.

Diabetes and abnormal glucose tolerance in women with previous gestational diabetes.

OBJECTIVE: In Spanish women with gestational diabetes mellitus (GDM), we aimed to study the progression to diabetes and abnormal glucose tolerance (AGT) and identify predictive factors. RESEARCH DESIGN AND METHODS: In 696 women with GDM and 70 control women, glucose tolerance was evaluated postpartum and at 5-year intervals. RESULTS: In the GDM group, the cumulative risk for diabetes and AGT was 13.8 and 42.4% after 11 years compared with 0 and 2.8% in control women, respectively (P < 0.05). Independent predictive factors for diabetes were previous hyperglycemia, four abnormal glucose values on the diagnostic oral glucose tolerance test (OGTT) or overt diabetes during pregnancy, 2-h blood glucose on the diagnostic OGTT >/=11.7 mmol/l, gestational age at diagnosis <24 weeks, and prepregnancy BMI >/=26.4 kg/m(2). All of these factors (some with different cutoff points) in addition to fasting glycemia were predictors of AGT also. The risk was nonlinear. Four abnormal glucose values on the diagnostic OGTT or overt diabetes during pregnancy was the strongest predictive factor for diabetes (relative risk 3.92), and prepregnancy BMI was the predictive factor with the highest attributable fraction in the whole group (13.3%). When first postpartum OGTT data were included in the analysis, predictors changed, but the overall prediction was similar. CONCLUSIONS: Spanish women with GDM have an increased risk of diabetes and AGT. Predictive factors display a nonlinear relationship. The strongest predictive factor for diabetes was four abnormal glucose values on the diagnostic OGTT or overt diabetes during pregnancy; the factor with the highest attributable fraction in the whole group was prepregnancy BMI.

Different regulated expression of the tyrosine phosphatase-like proteins IA-2 and phogrin by glucose and insulin in pancreatic islets: relationship to development of insulin secretory responses in early life.

IA-2 and phogrin are tyrosine phosphatase-like proteins that may mediate interactions between secretory granules and cytoskeleton in islets and neuroendocrine tissues. We investigated factors that regulate IA-2 and phogrin expression and their relationship to maturation of insulin secretory responses that occur after birth. Islet content of IA-2, but not phogrin, increased during the first 10 days of life in rats, when insulin secretion in response to glucose increased to adult levels. In cultured 5-day-old rat islets, IA-2 protein and mRNA was increased by glucose and agents that potentiate insulin secretion by the cAMP pathway. Addition of insulin increased IA-2 protein levels and insulin biosynthesis without affecting IA-2 mRNA. Blocking insulin secretion with diazoxide or insulin action with insulin receptor antibodies inhibited glucose-induced increases in IA-2 protein, but not those of mRNA. Phogrin expression was unchanged by all agents. Thus, IA-2 is regulated at the mRNA level by glucose and elevated cAMP, whereas locally secreted insulin modulates IA-2 protein levels by stimulating biosynthesis. In contrast, phogrin expression is insensitive to factors that modify beta-cell function. These results demonstrate differential regulation of two closely related secretory granule components and identify IA-2 as a granule membrane protein subject to autocrine regulation by insulin.

Two distinctly HLA-associated contiguous linear epitopes uniquely expressed within the islet antigen 2 molecule are major autoantibody epitopes of the diabetes-specific tyrosine phosphatase-like protein autoantigens.

The related tyrosine phosphatase-like proteins islet Ag (IA)-2 and IA-2beta are autoantigens of type 1 diabetes in humans. Autoantibodies are predominantly against IA-2, and IA-2-specific epitopes are major autoantibody targets. We used the close homology of IA-2 and IA-2beta to design chimeras and mutants to identify humoral IA-2-specific epitopes. Two major IA-2 epitopes that are absent from the related autoantigens IA-2beta and IA-2Delta 13 splice variant ICA512.bdc were found contiguous to each other within IA-2 juxtamembrane amino acids 611-620 (epitope JM1) and 621-630 (epitope JM2). JM1 and JM2 are recognized by sera from 67% of patients with IA-2 Abs, and relatives of patients with type 1 diabetes having Abs to either JM epitope had a >50% risk for developing type 1 diabetes within 6 years, even in the absence of diabetes-associated HLA genotypes. Remarkably, the presence of Abs to one of these two epitopes was mutually exclusive of the other; JM2 Abs and not JM1 Abs were found in relatives with HLA DR3/4, DR4/13, or DR1/4 genotypes; and the binding of autoantibodies to the JM2 epitope, but not the JM1 epitope, markedly affected proteolysis of IA-2. This is a unique demonstration of HLA-associated B cell responses to epitopes within a single autoantigen in humans and is consistent with modification of Ag processing by specific Ab-influencing peptide presentation by HLA molecules.

Características de los anticuerpos antiislotes del adulto, diabético tipo I de reciente diagnóstico, familiares de primer grado de diabéticos tipo 1 y diabetes gestacional / ICA characteristics in latent autoimmune diabetes of adults, new onset type diabetics, first degree relatives of type l diabetics and gestational diabetes

Se estudiaron las características de los sueros con anticuerpos antiislotes pancreáticos (ICA+) títulos, ICA sobre páncreas de ratón (ICA-NR), reactividad a extractos glucolipídicos pancreáticos (REGP) y asociación a anticuerpos anti-GAD65) en diferentes grupos de sujetos: diabetes autoinmune del adulto (LADA, n = 20), diabéticos tipo 1 de reciente diagnóstico (DMIDrd, n = 43), familiares de primer grado de diabéticos tipo 1 (FPG, n = 31) y mujeres con diabetes gestacional (DG, n = 10). Se detectaron ICA e ICA-NR por la técnica de inmunofluorescencia indirecta y los anticuerpos anti-GAD65, por un método RIA de inmunoprecipitación. Se utilizó la fase superior de extractos pancreáticos humanos que contienen glucolípidos para medir REGP de los ICA. Se determinaron las características de los ICA en los diferentes grupos: LADA: alta frecuencia en sus títulos (ü 80 unidades JDF) (80 porciento), anticuerpos anti-GAD65 (100 porciento) y baja frecuencia de ICA-NR (15 porciento) y REGP (15 porciento); DMIDrd: alta frecuencia de anticuerpos anti-GAD65 (72 porciento), ICA-NR (81 porciento) y REGP (86 porciento); FPG: alta frecuencia de ICA-NR (93 porciento) y REGP (87 porciento); DG: Bajos títulos de ICA (< 20 unidades JDF) (60 porciento) y alta frecuencia de REGP (80 porciento), aunque la REGP fue generalmente parcial. Se comprobó que en los grupos estudiados, el proceo de pérdida de la tolerancia inmunológica es disímil porque los ICA reconocen a determinantes antigénicos diferentes. Estos resultados son también importantes para seleccionar el marcador inmunológico correcto para la predicción del proceso autoinmune en cada entidad(AU)