Pancreatic Beta-cell Survival Research Articles

Sleeve gastrectomy, but not duodenojejunostomy, preserves total beta-cell mass in Goto-Kakizaki rats evaluated by three-dimensional optical projection tomography.

In type 2 diabetes mellitus, there is a progressive loss of beta-cell mass. Bariatric surgery has in recent investigations showed promising results in terms of diabetes remission, but little is established regarding the effect of surgery on the survival or regeneration of pancreatic beta-cells. In this study, we aim to explore how bariatric surgery with its subsequent hormonal alterations affects the islets of Langerhans. Twenty-four Goto-Kakizaki rats were operated with duodenojejunostomy (DJ), sleeve gastrectomy (SG) or sham operation. From the 38th week after surgery, body weight, fasting blood glucose, glycosylated hemoglobin, mixed meal tolerance with repeated measures of insulin, glucagon-like peptide 1, gastrin and total ghrelin were evaluated. Forty-six weeks after surgery, the animals were euthanized and the total beta-cell mass in all animals was examined by three-dimensional volume quantification by optical projection tomography based on the signal from insulin-specific antibody staining. Body weight did not differ between groups (P(g) = 0.37). SG showed lower fasting blood glucose compared to DJ and sham (P(g) = 0.037); HbA1c levels in SG were lower compared to DJ only (p < 0.05). GLP-1 levels were elevated for DJ compared to SG and sham (P(g) = 0.001), whereas gastrin levels were higher in SG compared to the two other groups (P(g) = 0.002). Beta-cell mass was significantly greater in animals operated with SG compared to both DJ and sham (p = 0.036). Sleeve gastrectomy is superior to duodenojejunostomy and sham operation when comparing the preservation of beta-cell mass 46 weeks after surgery in Goto-Kakizaki rats. This could be related to both the increased gastrin levels and the long-term improvement in glycemic parameters observed after this procedure.

The microRNA-200 family regulates pancreatic beta cell survival in type 2 diabetes.

Pancreatic beta cell death is a hallmark of type 1 (T1D) and type 2 (T2D) diabetes, but the molecular mechanisms underlying this aspect of diabetic pathology are poorly understood. Here we report that expression of the microRNA (miR)-200 family is strongly induced in islets of diabetic mice and that beta cell-specific overexpression of miR-200 in mice is sufficient to induce beta cell apoptosis and lethal T2D. Conversely, mir-200 ablation in mice reduces beta cell apoptosis and ameliorates T2D. We show that miR-200 negatively regulates a conserved anti-apoptotic and stress-resistance network that includes the essential beta cell chaperone Dnajc3 (also known as p58IPK) and the caspase inhibitor Xiap. We also observed that mir-200 dosage positively controls activation of the tumor suppressor Trp53 and thereby creates a pro-apoptotic gene-expression signature found in islets of diabetic mice. Consequently, miR-200-induced T2D is suppressed by interfering with the signaling of Trp53 and Bax, a proapoptotic member of the B cell lymphoma 2 protein family. Our results reveal a crucial role for the miR-200 family in beta cell survival and the pathophysiology of diabetes.

Pretreatment with obestatin reduces the severity of ischemia/reperfusion-induced acute pancreatitis in rats

Obestatin, as ghrelin, has been originally extracted from the stomach, which remains its major source. Previous studies have shown that administration of obestatin exhibits protective and healing-promoting effects in several organs, including the stomach and kidney. In pancreas, pretreatment with obestatin inhibits the development of cerulein-induced acute pancreatitis and promotes survival of pancreatic beta cells and human islets. The aim of the present study was to check the universality of protective effect of obestatin in the pancreas. For this reason we investigated the influence of obestatin administration on the development of ischemia/reperfusion-induced pancreatitis. Acute pancreatitis was induced by pancreatic ischemia followed by reperfusion of the gland. Obestatin (4, 8 or 16nmol/kg/dose) was administered intraperitoneally twice: 0.5h before exposure to ischemia, and 3h after the first injection. The effect of obestatin on the course of necrotizing pancreatitis was assessed after 6-h reperfusion, and included histological, functional, and biochemical analyses. Treatment with obestatin reduced morphological signs of pancreatic damage including edema, vacuolization of acinar cells, hemorrhages, acinar necrosis, and leukocyte infiltration of the gland. These effects were accompanied by an improvement of pancreatic DNA synthesis and superoxide dismutase activity, and a decrease in serum level of lipase and pro-inflammatory interleukin-1β. Moreover pretreatment with obestatin reduced myeloperoxidase activity and malondialdehyde concentration in pancreatic tissue of rats with acute pancreatitis. Conclusions: Administration of obestatin inhibits the development of ischemia/reperfusion-induced acute pancreatitis. This observation, taken together with previous findings that obestatin protects the pancreas against cerulein-induced pancreatitis, indicates that protective effect of obestatin in the pancreas is universal and independent of the primary cause of acute pancreatitis.

Cocoa-rich diet attenuates beta cell mass loss and function in young Zucker diabetic fatty rats by preventing oxidative stress and beta cell apoptosis.

We have recently shown that cocoa flavanols may have anti-diabetic potential by promoting survival and function of pancreatic beta-cells in vitro. In this work, we investigated if a cocoa-rich diet is able to preserve beta-cell mass and function in an animal model of type 2 diabetes and the mechanisms involved. Our results showed that cocoa feeding during the prediabetic state attenuates hyperglycaemia, reduces insulin resistant, and increases beta cell mass and function in obese Zucker diabetic rats. At the molecular level, cocoa-rich diet prevented beta-cell apoptosis by increasing the levels of Bcl-xL and decreasing Bax levels and caspase-3 activity. Cocoa diet enhanced the activity of endogenous antioxidant defenses, mainly glutathione peroxidase, preventing thus oxidative injury induced by the pre-diabetic condition and leading to apoptosis prevention. These findings provide the first in vivo evidence that a cocoa-rich diet may delay the loss of functional beta-cell mass and delay the progression of diabetes by preventing oxidative stress and beta-cell apoptosis.

Pancreatic Beta Cell Survival and Signaling Pathways: Effects of Type 1 Diabetes-Associated Genetic Variants.

Type 1 diabetes (T1D) is a complex autoimmune disease in which pancreatic beta cells are specifically destroyed by the immune system. The disease has an important genetic component and more than 50 loci across the genome have been associated with risk of developing T1D. The molecular mechanisms by which these putative T1D candidate genes modulate disease risk, however, remain poorly characterized and little is known about their effects in pancreatic beta cells. Functional studies in in vitro models of pancreatic beta cells, based on techniques to inhibit or overexpress T1D candidate genes, allow the functional characterization of several T1D candidate genes. This requires a multistage procedure comprising two major steps, namely accurate selection of genes of potential interest and then in vitro and/or in vivo mechanistic approaches to characterize their role in pancreatic beta cell dysfunction and death in T1D. This chapter details the methods and settings used by our groups to characterize the role of T1D candidate genes on pancreatic beta cell survival and signaling pathways, with particular focus on potentially relevant pathways in the pathogenesis of T1D, i.e., inflammation and innate immune responses, apoptosis, beta cell metabolism and function.

Nephrin, a transmembrane protein, is involved in pancreatic beta-cell survival signaling

Nephrin, a cell surface signaling receptor, regulates podocyte function in health and disease. We study the role of nephrin in β-cell survival signaling. We report that in mouse islet β-cells and the mouse pancreatic beta-cell line (βTC-6 cells) nephrin is associated and partly co-localized with PI3-kinase. Incubation of cells with functional anti-nephrin antibodies induced nephrin clustering at the plasma membrane, nephrin phosphorylation and recruitment of PI3-kinase to nephrin thus resulting in increased PI3K-dependent Akt phosphorylation and augmented phosphorylation/inhibition of pro-apoptotic Bad and FoxO. Nephrin silencing abolished Akt activation and increased susceptibility of cells to apoptosis. High glucose impaired nephrin signaling, increased nephrin internalization and up-regulated PKCα expression. Interestingly, a marked decrease in nephrin expression and phosphorylated Akt was observed in pancreatic islets of db/db lepr−/− diabetic mice. Our findings revealed that nephrin is involved in β-cell survival and suggest that glucose-induced changes in nephrin signaling may contribute to gradual pancreatic β-cell loss in type 2 diabetes.

The Role of PTHrP in Pancreatic Beta-Cells and Implications for Diabetes Pathophysiology and Treatment

Diabetes is one of the fastest growing diseases worldwide, with an immense economic and health burden attached. It is now well accepted that a deficiency of functional insulin-producing pancreatic beta-cells is the main cause for all forms of diabetes. Several approaches are being taken to increase functional beta-cell mass. These include differentiation of new beta-cells from stem cells or progenitor cells, transdifferentiation of beta-cells from other mature cell types, as well as finding ways to enhance the function, proliferation, survival, and regeneration of preexisting beta-cells. This article enumerates on the role of parathyroid hormone-related protein (PTHrP) and its mode of action on pancreatic beta-cell function, proliferation, and survival in rodents as well as in human beta-cells. A further understanding of the mechanism of action of PTHrP and its role in the normal physiology and pathophysiology of the beta-cell will be important for its potential use in future as a therapeutic treatment for diabetes.

Targeting type 2 diabetes: lessons from a knockout model of insulin receptor substrate 2.

Insulin receptor substrate 2 (IRS2) is a widely expressed protein that regulates crucial biological processes including glucose metabolism, protein synthesis, and cell survival. IRS2 is part of the insulin - insulin-like growth factor (IGF) signaling pathway and mediates the activation of the phosphotidylinositol 3-kinase (PI3K)-Akt and the Ras-mitogen-activated protein kinase (MAPK) cascades in insulin target tissues and in the pancreas. The best evidence of this is that systemic elimination of the Irs2 in mice (Irs2(-/-)) recapitulates the pathogenesis of type 2 diabetes (T2D), in that diabetes arises as a consequence of combined insulin resistance and beta-cell failure. Indeed, work using this knockout mouse has confirmed the importance of IRS2 in the control of glucose homeostasis and especially in the survival and function of pancreatic beta-cells. These studies have shown that IRS2 is critically required for beta-cell compensation in conditions of increased insulin demand. Importantly, islets isolated from T2D patients exhibit reduced IRS2 expression, which supports the likely contribution of altered IRS2-dependent signaling to beta-cell failure in human T2D. For all these reasons, the Irs2(-/-) mouse has been and will be essential for elucidating the inter-relationship between beta-cell function and insulin resistance, as well as to delineate therapeutic strategies to protect beta-cells during T2D progression.

Proteasome dysfunction mediates high glucose-induced apoptosis in rodent beta cells and human islets.

The ubiquitin/proteasome system (UPS), a major cellular protein degradation machinery, plays key roles in the regulation of many cell functions. Glucotoxicity mediated by chronic hyperglycaemia is detrimental to the function and survival of pancreatic beta cells. The aim of our study was to determine whether proteasome dysfunction could be involved in beta cell apoptosis in glucotoxic conditions, and to evaluate whether such a dysfunction might be pharmacologically corrected. Therefore, UPS activity was measured in GK rats islets, INS-1E beta cells or human islets after high glucose and/or UPS inhibitor exposure. Immunoblotting was used to quantify polyubiquitinated proteins, endoplasmic reticulum (ER) stress through CHOP expression, and apoptosis through the cleavage of PARP and caspase-3, whereas total cell death was detected through histone-associated DNA fragments measurement. In vitro, we found that chronic exposure of INS-1E cells to high glucose concentrations significantly decreases the three proteasome activities by 20% and leads to caspase-3-dependent apoptosis. We showed that pharmacological blockade of UPS activity by 20% leads to apoptosis in a same way. Indeed, ER stress was involved in both conditions. These results were confirmed in human islets, and proteasome activities were also decreased in hyperglycemic GK rats islets. Moreover, we observed that a high glucose treatment hypersensitized beta cells to the apoptotic effect of proteasome inhibitors. Noteworthily, the decreased proteasome activity can be corrected with Exendin-4, which also protected against glucotoxicity-induced apoptosis. Taken together, our findings reveal an important role of proteasome activity in high glucose-induced beta cell apoptosis, potentially linking ER stress and glucotoxicity. These proteasome dysfunctions can be reversed by a GLP-1 analog. Thus, UPS may be a potent target to treat deleterious metabolic conditions leading to type 2 diabetes.

14-3-3zeta Regulates Adipogenesis, Glucose Tolerance and Insulin Sensitivity in Mice

Adipogenesis is a complex process governed by a distinct network of adipogenic transcription factors. As obesity is a growing health concern, a complete understanding of the mechanisms that govern adipocyte differentiation is critical for the development of novel therapies. Molecular adaptors, such as the 14-3-3 protein family, integrate and coordinate signalling events, but whether they facilitate adipocyte differentiation is not known. Therefore, we hypothesized that they are critical for adipogenesis. We utilized knockout mice lacking 14-3-3zeta, an isoform, which we previously reported to regulate pancreatic beta-cell survival. Throughout their lifespan, knockout mice were significantly smaller than wild-type controls. This was attributed to decreased adiposity and smaller adipocytes, as assessed by DEXA and histological studies. Deletion of 14-3-3zeta was also associated with immature adipocytes as serum levels of leptin were significantly decreased, in addition to the expression of mature adipocyte genes (Hsl, Atgl, and Fasn). Knockout mice also exhibited glucose intolerance and insulin resistance, as measured by intraperitoneal glucose (2 g/kg) and insulin (1.5 U/kg) tolerance tests. To further examine the roles of 14-3-3zeta in adipogenesis, 3T3-L1 pre-adipocytes were transfected with 14-3-3zeta siRNA, which blocked differentiation, as observed by Oil Red-O staining and quantitative PCR measurements of Ppparg, Slc2a4, Hsl and Fasn. Furthermore, depletion of 14-3-3zeta promoted the rapid degradation of the critical adipogenic factor, C/EBP-delta, resulting in attenuated nuclear translocation during differentiation. Collectively, this work identifies 14-3-3zeta as a novel regulator of adipogenesis, glucose homeostasis and insulin action, and highlights a potential therapeutic target for the treatment of obesity.

The orphan nuclear receptor small heterodimer partner negatively regulates pancreatic beta cell survival and hyperglycemia in multiple low-dose streptozotocin-induced type 1 diabetic mice

The small heterodimer partner (SHP; NR0B2) regulates the transcription of a variety of target genes and controls a variety of physiological functions in various tissues. However, the role of SHP in beta cell has not been fully determined yet. We used SHP knockout (SHP KO) mice to investigate the role of SHP in multiple low-dose streptozotocin (MLDS)-induced diabetes. Blood glucose and insulin levels were measured until 20 days, and intraperitoneal glucose tolerance and glucose-stimulated insulin secretion tests were performed. The expression of apoptotic genes and beta cell markers were detected by quantitative realtime-polymerase chain reaction, immunostaining and western blot analysis. SHP KO mice showed significantly lower blood glucose, higher insulin levels, and enhanced glucose tolerance compared with wild type (WT) mice after MLDS treatment. Moreover, beta cell mass and pancreatic insulin content were remarkably increased in SHP KO mice. In the response to glucose stimulation, islets of SHP KO showed increased insulin secretion via up-regulation of beta cell enriched transcription factors compared to WT mice after streptozotocin (STZ) treatment. In quantification for beta cell apoptosis at day 1 post STZ treatment, the SHP KO mice showed significantly increased anti-apoptotic gene expression and decreased release of apoptotic markers cytochrome c, smac/diablo, and only a few apoptotic beta cells were found in SHP KO pancreas through inactivation of caspase-3, compared to those of WT. These data demonstrate that SHP deficiency ameliorates hyperglycemia and preserves islet function by inhibiting apoptosis of pancreatic beta cells and up-regulating of their enriched transcriptional factors.

Autophagy induced by glibenclamide serves as a defense against apoptosis in INS-1 rat insulinoma cells

Glibenclamide, a blocker of ATP-sensitive potassium channels, has been found to induce apoptosis in some cell types, including pancreatic beta-cells. Since autophagy plays doubleedged roles in pancreatic beta-cell survival, frequently through cross-talking with apoptosis, we investigated if glibenclamide induced autophagy in INS-1 rat insulinoma cells and the influence of autophagy on apoptosis. Mammalian target of rapamycin (mTOR) is a negative regulator of autophagy. As one of the substrates of mTOR, p70 S6 kinase (p70 S6K) is phosphorylated upon mTOR activation. Our results showed that glibenclamide induced an elevated protein level of the autophagy marker LC3-II, while decreasing phosphorylated p70 S6K, indicative of inhibition on mTOR signaling in INS-1 cells. Furthermore, inhibiting glibenclamide-induced autophagy via knocking down the autophagy essential gene Atg7 decreased cell viability and increased apoptosis in INS-1 cells. Our results indicate that glibenclamide induces autophagy in INS-1 cells, and that autophagy activation is exerting a protective activity against apoptosis.

Critical role of c-Kit in beta cell function: increased insulin secretion and protection against diabetes in a mouse model

The receptor tyrosine kinase, c-Kit, and its ligand, stem cell factor, control a variety of cellular processes, including pancreatic beta cell survival and differentiation as revealed in c-Kit ( Wv ) mice, which have a point mutation in the c-Kit allele leading to loss of kinase activity and develop diabetes. The present study further investigated the intrinsic role of c-Kit in beta cells, especially the underlying mechanisms that influence beta cell function. We generated a novel transgenic mouse model with c-KIT overexpression specifically in beta cells (c-KitβTg) to further examine the physiological and functional roles of c-Kit in beta cells. Isolated islets from these mice were used to investigate the underlying molecular pathway of c-Kit in beta cells. We also characterised the ability of c-Kit to protect animals from high-fat-diet-induced diabetes, as well as to rescue c-Kit ( Wv ) mice from early onset of diabetes. c-KitβTg mice exhibited improved beta cell function, with significantly improved insulin secretion, and increased beta cell mass and proliferation in response to high-fat-diet-induced diabetes. c-KitβTg islets exhibited upregulation of: (1) insulin receptor and IRSs; (2) Akt and glycogen synthase kinase 3β phosphorylation; and (3) transcription factors important for islet function. c-KIT overexpression in beta cells also rescued diabetes observed in c-Kit ( Wv ) mice. These findings demonstrate that c-Kit plays a direct protective role in beta cells, by regulating glucose metabolism and beta cell function. c-Kit may therefore represent a novel target for treating diabetes.

In vitro effects of mycophenolic acid on survival, function, and gene expression of pancreatic beta-cells

Post-transplant diabetes mellitus represents an important complication of prolonged immunosuppressive treatment after solid organ transplantation. The immunosuppressive toxicity, responsible for a persistent impairment of glucose metabolism in pancreatic islet-transplanted patients, is mainly attributed to calcineurin inhibitors and steroids, while other immunosuppressive molecules (azathioprine and mycophenolic acid, MPA) are considered not to have a toxic effect. In the present study, in vitro effects of MPA have been investigated in mouse beta-cell lines (βTC-1 and βTC-6) and in purified human pancreatic islets. βTC-1, βTC-6, and human pancreatic islets were exposed to various concentrations of MPA for different times. Consequently, we evaluated the viability, the induction of apoptosis, the glucose-stimulated insulin secretion, and the expression of β-cell function genes (Isl1, Pax6, Glut-2, glucokinase) and apoptosis-related genes (Bax and Bcl2). βTC-1, βTC-6, and human islets treated, respectively, for 48 and 72 h with 15-30 nM MPA showed altered islet architecture, as compared with control cells. We observed for βTC-1 and βTC-6 almost 70% reduction in cell viability; three to sixfold induction of TUNEL/apoptotic-positive cells quantified by FACS analysis. A twofold increase in apoptotic cells was observed in human islets after MPA exposure associated with strong inhibition of glucose-stimulated insulin secretion. Furthermore, we showed significant down-regulation of gene expression of molecules involved in β-cell function and increase rate between Bax/Bcl2. Our data demonstrate that MPA has an in vitro diabetogenic effect interfering at multiple levels with survival and function of murine and human pancreatic β-cells.

Ubiquitin C-terminal hydrolase L1 is required for pancreatic beta cell survival and function in lipotoxic conditions

Ubiquitin C-terminal hydrolase L1 (UCHL1) is associated with neurodegenerative diseases and has been suggested to have roles in pancreatic beta cells. Our proteomic analysis revealed that UCHL1 was the most increased protein in MIN6 cells exposed to palmitate. The present study used a genetic loss-of-function model to test the hypothesis that UCHL1 is required for normal beta cell function and fate under lipotoxic conditions. Human islets, mouse islets and MIN6 cells were used to analyse UCHL1 protein levels and regulation of UCHL1 by palmitate. The levels of free mono-ubiquitin and poly-ubiquitinated proteins were assessed. Gracile axonal dystrophy (GAD) mutant mice lacking UCHL1 were fed a normal or lipotoxic high-fat diet. Glucose tolerance, insulin tolerance and insulin secretion were assessed in vivo. Beta cell death and proliferation were assessed by TUNEL and proliferating cell nuclear antigen (PCNA) staining. Insulin secretion, calcium signalling, endoplasmic reticulum (ER) stress, apoptosis and SNARE protein levels were assessed in vitro. UCHL1 protein, which was highly specific to beta cells, was increased by palmitate at basal glucose, but not in the context of hyperglycaemia associated with frank diabetes. Although islet development and function were initially normal in Uchl1 (-/-) mice, a 4-week high-fat diet caused glucose intolerance and impaired insulin secretion. Uchl1 (-/-) mice had increased ER stress and beta cell apoptosis. The levels of SNARE proteins were dysregulated in Uchl1 (-/-) islets. Palmitate-stimulated vesicle-associated membrane protein 2 (VAMP2) ubiquitination was modulated by a chemical UCHL1 inhibitor. Together, these data suggest that UCHL1 has essential functional and anti-apoptotic roles in beta cells under stress conditions associated with lipotoxicity.

Survival of pancreatic beta cells is partly controlled by a TCF7L2-p53-p53INP1-dependent pathway

The transcription factor T-cell factor 7-like 2 (TCF7L2) confers type 2 diabetes risk mainly through impaired insulin secretion, perturbed incretin effect and reduced beta-cell survival. The aim of this study was to identify the molecular mechanism through which TCF7L2 influences beta-cell survival. TCF7L2 target genes in INS-1 cells were identified using Chromatin Immunoprecipitation. Validation of targets was obtained by: siRNA silencing, real-time quantitative polymerase chain reaction, electrophoretic mobility shift assay, luciferase reporter assays and western blot. Apoptosis rate was measured by DNA degradation and caspase-3 content. Islet viability was estimated by measuring metabolic rate. TCF7L2 binds to 3646 gene promoters in INS-1 cells in high or low glucose, including Tp53, Pten, Uggt1, Adamts9 and Fto. SiRNA-mediated reduction in TCF7L2 activity resulted in increased apoptosis and increased expression of Tp53, which resulted in elevated p53 protein activity and an increased expression of the p53 target gene Tp53inp1 (encoding p53-induced-nuclear-protein 1). Reversing the increase in p53INP1 protein expression, seen after Tcf7l2 silencing, protected INS-1 cells from Tcf7l2 depletion-induced apoptosis. This result was replicated in primary rat islets. The risk T-allele of rs7903146 is associated with increased TCF7L2 mRNA expression and transcriptional activity. On the other hand, in vitro silencing of TCF7L2 lead to increased apoptosis. One possibility is that the risk T-allele increases expression of an inhibitory TCF7L2 isoform with lower transcriptional activity. These results identify the p53-p53INP1 pathway as a molecular mechanism through which TCF7L2 may affect beta-cell survival and established a molecular link between Tcf7l2 and two type 2 diabetes-associated genes, Tp53inp1 and Adamts9.

Flavonol kaempferol improves chronic hyperglycemia-impaired pancreatic beta-cell viability and insulin secretory function

Considerable evidence shows that chronic hyperglycemia can cause pancreatic beta-cell dysfunction, which contributes to progressive deterioration of glucose homeostasis and overt diabetes. In the present study, we found that kaempferol, a flavonol compound present in various Chinese medicinal herbs, has cytoprotective effects on cultured clonal beta-cells and pancreatic human islets. Kaempferol treatment dose-dependently promoted viability, inhibited cellular apoptosis, and reduced caspase-3 activity in beta-cells and human islets exposed to chronic high glucose, with 10 μM kaempferol exerting the maximum effect. In addition, kaempferol treatment improved the expression of anti-apoptotic proteins Akt and Bcl-2 that was significantly reduced in beta-cells and human islets chronically exposed to hyperglycemia. Furthermore, exposure of beta-cells and human islets to kaempferol restored high glucose-attenuated intracellular cAMP and ATP production. Inhibition of protein kinase A or Akt activation ablated the anti-apoptotic effect of kaempferol. These cytoprotective effects of kaempferol were associated with improved insulin secretory function and synthesis in beta-cells and human islets. These findings provide evidence that kaempferol may be a naturally occurring anti-diabetic compound by protecting pancreatic beta-cell survival and function in a hostile environment that would otherwise lead to type 2 diabetes.

Adipocyte expression of the glucose-dependent insulinotropic polypeptide receptor involves gene regulation by PPARγ and histone acetylation

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone that exerts insulinotropic and growth and survival effects on pancreatic β-cells. Additionally, there is increasing evidence supporting an important role for GIP in the regulation of adipocyte metabolism. In the current study we examined the molecular mechanisms involved in the regulation of GIP receptor (GIPR) expression in 3T3-L1 cells. GIP acted synergistically with insulin to increase neutral lipid accumulation during progression of 3T3-L1 preadipocytes to the adipocyte phenotype. Both GIPR protein and mRNA expression increased during 3T3-L1 cell differentiation, and this increase was associated with upregulation of nuclear levels of sterol response element binding protein 1c (SREBP-1c) and peroxisome proliferator-activated receptor γ (PPARγ), as well as acetylation of histones H3/H4. The PPARγ receptor agonists LY171883 and rosiglitazone increased GIPR expression in differentiated 3T3-L1 adipocytes, whereas the antagonist GW9662 ablated expression. Additionally, both PPARγ and acetylated histones H3/H4 were shown to bind to a region of the GIPR promoter containing the peroxisome proliferator response element (PPRE). Knockdown of PPARγ in differentiated 3T3-L1 adipocytes, using RNA interference, reduced GIPR expression, supporting a functional regulatory role. Taken together, these studies show that GIP and insulin act in a synergistic manner on 3T3-L1 cell development and that adipocyte GIPR expression is upregulated through a mechanism involving interactions between PPARγ and a GIPR promoter region containing an acetylated histone region.

Adiponectin Enhances Insulin Sensitivity by Increasing Hepatic IRS-2 Expression via a Macrophage-Derived IL-6-Dependent Pathway

Insulin resistance is often associated with impeded insulin signaling due either to decreased concentrations or functional modifications of crucial signaling molecules including insulin receptor substrates (IRS) in the liver. Many actions of adiponectin, a well-recognized antidiabetic adipokine, are currently attributed to the activation of two critical molecules downstream of AdipoR1 and R2: AMP-activated kinase (AMPK) and peroxisome proliferator-activated receptor α (PPARα). However, the direct effects of adiponectin on insulin signaling molecules remain poorly understood. We show here that adiponectin upregulates IRS-2 through activation of signal transducer and activator of transcription-3 (STAT3). Surprisingly, this activation is associated with IL-6 production from macrophages induced by adiponectin through NFκB activation independent of its authentic receptors, AdipoR1 and AdipoR2. These data have unraveled an insulin-sensitizing action initiated by adiponectin leading to upregulation of hepatic IRS-2 via an IL-6 dependent pathway through a still unidentified adiponectin receptor.

Translationally controlled tumour protein (TCTP) is a novel glucose-regulated protein that is important for survival of pancreatic beta cells.

This study used proteomics and biochemical approaches to identify novel glucose-regulated proteins and to unveil their role in pancreatic beta cell function. Translationally controlled tumour protein (TCTP) was identified to be one such protein, and further investigations into its function and regulation were carried out. Global protein profiling of beta cell homogenates following glucose stimulation was performed using two-dimensional gel electrophoresis. Proteins were identified by mass spectroscopy analysis. Immunoblotting was used to investigate alterations in TCTP protein levels in response to glucose stimulation or cell stress induced by palmitate. To investigate the biological function of TCTP, immunolocalisation, gene knockdown and overexpression of Tctp (also known as Tpt1) were performed. Apoptosis was measured in Tctp knockdown or Tctp-overexpressing cells. Glucose-stimulated insulin secretion was carried out in Tctp knockdown cells. TCTP was identified as a novel glucose-regulated protein, the level of which is increased at stimulatory glucose concentration. Glucose also induced TCTP dephosphorylation and its partial translocation to the mitochondria and the nucleus. TCTP protein levels were downregulated in response to cell stress induced by palmitate or thapsigargin treatments. Gene knockdown by small interfering RNA led to increased apoptosis, whereas overproduction of TCTP prevented palmitate-induced cell death. Regulation of TCTP protein levels by glucose is likely to be an important cyto-protective mechanism for pancreatic beta cells against damage caused by hyperglycaemia. In contrast, high concentration of palmitate causes cell stress, reduction in TCTP levels and consequently reduced cell viability. Our results imply that TCTP levels influence the sensitivity of beta cells to apoptosis.