Genetic regulation of metabolic pathways in beta-cells disrupted by hyperglycemia

The emerging role of oxylipins in thrombosis and diabetes

The prevalence of cardiovascular disease (CVD), the leading cause of death in the US, is predicted to increase due to the shift in age of the general population and increase in CVD risk factors such as obesity and diabetes. New therapies are required to decrease the prevalence of CVD risk factors (obesity and diabetes) as well as reduce atherothrombosis, the major cause of CVD related mortality. Oxylipins, bioactive metabolites derived from the oxygenation of polyunsaturated fatty acids, play a role in the progression of CVD risk factors and thrombosis. Aspirin, a cyclooxygenase-1 inhibitor, decreases atherothrombotic associated mortality by 25%. These potent effects of aspirin have shown the utility of modulating oxylipin signaling pathways to decrease CVD mortality. The role of many oxylipins in the progression of CVD, however, is still uncertain or controversial. An increased understanding of the role oxylipins play in CVD risk factors and thrombosis could lead to new therapies to decrease the prevalence of CVD and its associated mortality.

A Preexistent Hypoxic Gene Signature Predicts Impaired Islet Graft Function and Glucose Homeostasis

We examined whether hypoxic exposure prior to the event of transplantation would have a positive or negative effect upon later islet graft function. Mouse islets exposed to hypoxic culture were transplanted into syngeneic recipients. Islet graft function, β-cell physiology, as well as molecular changes were examined. Expression of hypoxia-response genes in human islets pre- and posttransplant was examined by microarray. Hypoxia-preexposed murine islet grafts provided poor glycemic control in their syngeneic recipients, marked by persistent hyperglycemia and pronounced glucose intolerance with failed first- and second-phase glucose-stimulated insulin secretion in vivo. Mechanistically, hypoxic preexposure stabilized HIF-1α with a concomitant increase in hypoxic-response genes including LDHA, and a molecular gene set, which would favor glycolysis and lactate production and impair glucose sensing. Indeed, static incubation studies showed that hypoxia-exposed islets exhibited dysregulated glucose responsiveness with elevated basal insulin secretion. Isolated human islets, prior to transplantation, express a characteristic hypoxia-response gene expression signature, including high levels of LDHA, which is maintained posttransplant. Hypoxic preexposure of an islet graft drives a HIF-dependent switch to glycolysis with subsequent poor glycemic control and loss of glucose-stimulated insulin secretion (GSIS). Early intervention to reverse or prevent these hypoxia-induced metabolic gene changes may improve clinical islet transplantation.

Interaction between cytokines and inflammatory cells in islet dysfunction, insulin resistance and vascular disease

Inflammation is an established pathogenic player in insulin resistance, islet demise and atherosclerosis. The complex interactions between cytokines, immune cells and affected tissues result in sustained inflammation in diabetes and atherosclerosis. 12- and 15-lipoxygenase (LO), such as 12/15-LO, produces a variety of metabolites through peroxidation of fatty acids and potentially contributes to the complex molecular crosstalk at the site of inflammation. 12- and 15-LO pathways are frequently activated in tissues affected by diabetes and atherosclerosis including adipose tissue (AT), islets and the vasculature. Moreover, mice with whole body and tissue-specific knockout of 12/15-LO are protected against insulin resistance, hyperglycaemia and atherosclerosis supporting functional contribution of 12- and 15-LO pathways in diabetes and atherosclerosis. Recently, it has emerged that there is a temporal regulation of the particular isoforms of 12- and 15-LO in human AT and islets during the development of type 1 and type 2 diabetes and obesity. Analyses of tissues affected by diabetes and atherosclerosis also implied the roles of interleukin (IL)-12 and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-1 (NOX-1) in islets and IL-17A in atherosclerosis. Future studies should aim to test the efficacy of inhibitions of these mediators for treatment of diabetes and atherosclerosis.

β-cell dedifferentiation in diabetes is important, but what is it?

This commentary discusses the concept of β-cell dedifferentiation in diabetes, which is important but not well defined. A broad interpretation is that a state of differentiation has been lost, which means changes in gene expression as well as in structural and functional elements. Thus, a fully mature healthy β cell will have its unique differentiation characteristics, but maturing cells and old β cells will have different patterns of gene expression and might therefore be considered as dedifferentiated. The meaning of dedifferentiation is now being debated because β cells in the diabetic state lose components of their differentiated state, which results in severe dysfunction of insulin secretion. The major cause of this change is thought to be glucose toxicity (glucotoxicity) and that lowering glucose levels with treatment results in some restoration of function. An issue to be discussed is whether dedifferentiated β cells return to a multipotent precursor cell phenotype or whether they follow a different pathway of dedifferentiation.

Metabolic signaling in fuel-induced insulin secretion

The pancreatic islet β cell senses circulating levels of calorigenic nutrients to secrete insulin according to the needs of the organism. Altered insulin secretion is linked to various disorders such as diabetes, hypoglycemic states, and cardiometabolic diseases. Fuel stimuli, including glucose, free fatty acids, and amino acids, promote insulin granule exocytosis primarily via their metabolism in β cells and the production of key signaling metabolites. This paper reviews our current knowledge of the pathways involved in both positive and negative metabolic signaling for insulin secretion and assesses the role of established and candidate metabolic coupling factors, keeping recent developments in focus.

A novel mechanism regulating insulin secretion involving Herpud1 in mice

AIMS/HYPOTHESIS

Type 2 diabetes results from beta cell dysfunction after prolonged physiological stress, which causes oversecretion of insulin. We recently found that insulin hypersecretion is mediated by at least two genes. Among mouse models of type 2 diabetes, the DBA/2 mouse strain is more susceptible to diabetes than is the C57BL/6J (B6J) strain. One distinctive feature of the DBA/2 mouse is that it hypersecretes insulin, independent of changes in insulin sensitivity; we identified Nnt as a gene responsible for this trait.

METHODS

To identify the other gene(s) affecting insulin hypersecretion, we tested a panel of recombinant inbred BXD strains, which have different combinations of B6 and DBA/2 alleles.

RESULTS

We found that 25% of the BXD strains hypersecreted insulin in response to glucose. Microarray profiling of islets from high- and low-secretor strains showed that at least four genes were differentially expressed. One gene was consistently underexpressed in islets from both DBA/2 and the high-secretor BXD strains. This gene (Herpud1 or Herp) encodes the 54 kDa endoplasmic reticulum stress-inducible protein (HERP) that resides in the integral endoplasmic reticulum membrane. To test directly whether Herpud1 can interact with Nnt, Herpud1 was either knocked down or overexpressed in MIN6 cells. These results showed that when Herpud1 was suppressed, Nnt expression was reduced, while overexpression of Herpud1 led to increased Nnt expression. Furthermore, Herpud1 suppression resulted in significantly decreased glucose-stimulated insulin secretion in the DBA/2 islets but not B6J islets.

CONCLUSIONS/INTERPRETATION

We conclude that Herpud1 regulates insulin secretion via control of Nnt expression.

Cellular responses of novel human pancreatic β-cell line, 1.1B4 to hyperglycemia

The novel human-derived pancreatic β-cell line, 1.1B4 exhibits insulin secretion and β-cell enriched gene expression. Recent investigations of the cellular responses of this novel cell line to lipotoxicity and cytokine toxicity revealed similarities to primary human β cells. The current study has investigated the responses of 1.1B4 cells to chronic 48 and 72 h exposure to hyperglycemia to probe mechanisms of human β-cell dysfunction and cell death. Exposure to 25 mM glucose significantly reduced insulin content (p<0.05) and glucokinase activity (p<0.01) after 72 h. Basal insulin release was unaffected but acute secretory response to 16.7 mM glucose was impaired (p<0.05). Insulin release stimulated by alanine, GLP-1, KCl, elevated Ca (2+) and forskolin was also markedly reduced after exposure to hyperglycemia (p<0.001). In addition, PDX1 protein expression was reduced by 58% by high glucose (p<0.05). Effects of hyperglycemia on secretory function were accompanied by decreased mRNA expression of INS, GCK, PCSK1, PCSK2, PPP3CB, GJA1, ABCC8, and KCNJ11. In contrast, exposure to hyperglycemia upregulated the transcription of GPX1, an antioxidant enzyme involved in detoxification of hydrogen peroxide and HSPA4, a molecular chaperone involved in ER stress response. Hyperglycemia-induced DNA damage was demonstrated by increased % tail DNA and olive tail moment, assessed by comet assay. Hyperglycemia-induced apoptosis was evident from increased activity of caspase 3/7 and decreased BCL2 protein. These observations reveal significant changes in cellular responses and gene expression in novel human pancreatic 1.1B4 β cells exposed to hyperglycemia, illustrating the usefulness of this novel human-derived cell line for studying human β-cell biology and diabetes.

Islet β cell mass in diabetes and how it relates to function, birth, and death

In type 1 diabetes (T1D) β cell mass is markedly reduced by autoimmunity. Type 2 diabetes (T2D) results from inadequate β cell mass and function that can no longer compensate for insulin resistance. The reduction of β cell mass in T2D may result from increased cell death and/or inadequate birth through replication and neogenesis. Reduction in mass allows glucose levels to rise, which places β cells in an unfamiliar hyperglycemic environment, leading to marked changes in their phenotype and a dramatic loss of glucose-stimulated insulin secretion (GSIS), which worsens as glucose levels climb. Toxic effects of glucose on β cells (glucotoxicity) appear to be the culprit. This dysfunctional insulin secretion can be reversed when glucose levels are lowered by treatment, a finding with therapeutic significance. Restoration of β cell mass in both types of diabetes could be accomplished by either β cell regeneration or transplantation. Learning more about the relationships between β cell mass, turnover, and function and finding ways to restore β cell mass are among the most urgent priorities for diabetes research.

The molecular mechanisms of pancreatic β-cell glucotoxicity: recent findings and future research directions

It is well established that regular physiological stimulation by glucose plays a crucial role in the maintenance of the β-cell differentiated phenotype. In contrast, prolonged or repeated exposure to elevated glucose concentrations both in vitro and in vivo exerts deleterious or toxic effects on the β-cell phenotype, a concept termed as glucotoxicity. Evidence indicates that the latter may greatly contribute to the pathogenesis of type 2 diabetes. Through the activation of several mechanisms and signaling pathways, high glucose levels exert deleterious effects on β-cell function and survival and thereby, lead to the worsening of the disease over time. While the role of high glucose-induced β-cell overstimulation, oxidative stress, excessive Unfolded Protein Response (UPR) activation, and loss of differentiation in the alteration of the β-cell phenotype is well ascertained, at least in vitro and in animal models of type 2 diabetes, the role of other mechanisms such as inflammation, O-GlcNacylation, PKC activation, and amyloidogenesis requires further confirmation. On the other hand, protein glycation is an emerging mechanism that may play an important role in the glucotoxic deterioration of the β-cell phenotype. Finally, our recent evidence suggests that hypoxia may also be a new mechanism of β-cell glucotoxicity. Deciphering these molecular mechanisms of β-cell glucotoxicity is a mandatory first step toward the development of therapeutic strategies to protect β-cells and improve the functional β-cell mass in type 2 diabetes.

The zinc finger protein ZBTB20 regulates transcription of fructose-1,6-bisphosphatase 1 and β cell function in mice

BACKGROUND & AIMS

Fructose-1,6-bisphosphatase (FBP)-1 is a gluconeogenic enzyme that regulates glucose metabolism and insulin secretion in β cells, but little is known about how its transcription is controlled. The zinc finger protein ZBTB20 regulates glucose homeostasis, so we investigated its effects on expression of FBP-1.

METHODS

We analyzed gene expression using real-time reverse-transcription polymerase chain reaction, immunoblotting, and immunohistochemistry. We generated mice with β cell-specific disruption of Zbtb20 using Cre/LoxP technology. Expression of Zbtb20 in β cells was reduced using small interfering RNAs, and promoter occupancy and transcriptional regulation were analyzed by chromatin immunoprecipitation and reporter assays.

RESULTS

ZBTB20 was expressed at high levels by β cells and other endocrine cells in islets of normal mice; expression levels were reduced in islets from diabetic db/db mice. Mice with β cell-specific knockout of Zbtb20 had normal development of β cells but had hyperglycemia, hypoinsulinemia, glucose intolerance, and impaired glucose-stimulated insulin secretion. Islets isolated from these mice had impaired glucose metabolism, adenosine triphosphate production, and insulin secretion after glucose stimulation in vitro, although insulin secretion returned to normal levels in the presence of KCl. ZBTB20 knockdown with small interfering RNAs impaired glucose-stimulated insulin secretion in the β cell line MIN6. Expression of Fbp1 was up-regulated in β cells with ZBTB20 knockout or knockdown; impairments to glucose-stimulated insulin secretion were restored by inhibition of FBPase activity. ZBTB20 was recruited to the Fbp1 promoter and repressed its transcription in β cells.

CONCLUSIONS

The transcription factor ZBTB20 regulates β cell function and glucose homeostasis in mice. It might be a therapeutic target for type 2 diabetes mellitus.

Endoplasmic reticulum stress and insulin biosynthesis: a review

Insulin resistance and pancreatic beta cell dysfunction are major contributors to the pathogenesis of diabetes. Various conditions play a role in the pathogenesis of pancreatic beta cell dysfunction and are correlated with endoplasmic reticulum (ER) stress. Pancreatic beta cells are susceptible to ER stress. Many studies have shown that increased ER stress induces pancreatic beta cell dysfunction and diabetes mellitus using genetic models of ER stress and by various stimuli. There are many reports indicating that ER stress plays an important role in the impairment of insulin biosynthesis, suggesting that reduction of ER stress could be a therapeutic target for diabetes. In this paper, we reviewed the relationship between ER stress and diabetes and how ER stress controls insulin biosynthesis.

PPARs: fatty acid sensors controlling metabolism

The peroxisome proliferator activated receptors (PPARs) are nuclear receptors that play key roles in the regulation of lipid metabolism, inflammation, cellular growth, and differentiation. The receptors bind and are activated by a broad range of fatty acids and fatty acid derivatives and they thereby serve as major transcriptional sensors of fatty acids. Here we review the function, regulation, and mechanism of the different PPAR subtypes with special emphasis on their role in the regulation of lipid metabolism.

Glucose-induced O₂ consumption activates hypoxia inducible factors 1 and 2 in rat insulin-secreting pancreatic beta-cells

Background

Glucose increases the expression of glycolytic enzymes and other hypoxia-response genes in pancreatic beta-cells. Here, we tested whether this effect results from the activation of Hypoxia-Inducible-factors (HIF) 1 and 2 in a hypoxia-dependent manner.

Methodology/Principal Findings

Isolated rat islets and insulin-secreting INS-1E cells were stimulated with nutrients at various pO₂ values or treated with the HIF activator CoCl₂. HIF-target gene mRNA levels and HIF subunit protein levels were measured by real-time RT-PCR, Western Blot and immunohistochemistry. The formation of pimonidazole-protein adducts was used as an indicator of hypoxia. In INS-1E and islet beta-cells, glucose concentration-dependently stimulated formation of pimonidazole-protein adducts, HIF1 and HIF2 nuclear expression and HIF-target gene mRNA levels to a lesser extent than CoCl₂ or a four-fold reduction in pO₂. Islets also showed signs of HIF activation in diabetic Lepr^db/db but not non-diabetic Lepr^db/+ mice. In vitro, these glucose effects were reproduced by nutrient secretagogues that bypass glycolysis, and were inhibited by a three-fold increase in pO₂ or by inhibitors of Ca²⁺ influx and insulin secretion. In INS-1E cells, small interfering RNA-mediated knockdown of Hif1α and Hif2α, alone or in combination, indicated that the stimulation of glycolytic enzyme mRNA levels depended on both HIF isoforms while the vasodilating peptide adrenomedullin was a HIF2-specific target gene.

Conclusions/Significance

Glucose-induced O₂ consumption creates an intracellular hypoxia that activates HIF1 and HIF2 in rat beta-cells, and this glucose effect contributes, together with the activation of other transcription factors, to the glucose stimulation of expression of some glycolytic enzymes and other hypoxia response genes.

Upregulation of β-cell genes and improved function in rodent islets following chronic glucokinase activation

Glucokinase (GK) plays a critical role in controlling blood glucose; GK activators have been shown to stimulate insulin secretion acutely both in vitro and in vivo. Sustained stimulation of insulin secretion could potentially lead to β-cell exhaustion; this study examines the effect of chronic GK activation on β-cells. Gene expression and insulin secretion were measured in rodent islets treated in vitro with GKA71 for 72 h. Key β-cell gene expression was measured in rat, mouse and global GK heterozygous knockout mouse islets (gk(del/wt)). Insulin secretion, after chronic exposure to GKA71, was measured in perifused rat islets. GKA71 acutely increased insulin secretion in rat islets in a glucose-dependent manner. Chronic culture of mouse islets with GKA71 in 5 mmol/l glucose significantly increased the expression of insulin, IAPP, GLUT2, PDX1 and PC1 and decreased the expression of C/EBPβ compared with 5 mmol/l glucose alone. Similar increases were shown for insulin, GLUT2, IAPP and PC1 in chronically treated rat islets. Insulin mRNA was also increased in GKA71-treated gk(del/wt) islets. No changes in GK mRNA were observed. Glucose-stimulated insulin secretion was improved in perifused rat islets following chronic treatment with GKA71. This was associated with a greater insulin content and GK protein level. Chronic treatment of rodent islets with GKA71 showed an upregulation of key β-cell genes including insulin and an increase in insulin content and GK protein compared with glucose alone.

Perinatal exposure to bisphenol A at reference dose predisposes offspring to metabolic syndrome in adult rats on a high-fat diet

Bisphenol A (BPA), a widely used environmental endocrine disruptor, has been reported to disrupt glucose homeostasis. BPA exposure may be a risk factor for type 2 diabetes. In this study, we investigated the effects of early-life BPA exposure on metabolic syndrome in rat offspring fed a normal diet and a high-fat diet. Pregnant Wistar rats were exposed to BPA (50, 250, or 1250 μg/kg · d) or corn oil throughout gestation and lactation by oral gavage. Offspring were fed a normal diet or a high-fat diet after weaning. Body weight, parameters of glucose and lipid metabolism, morphology, and function of β-cells were measured in offspring. On a normal diet, perinatal exposure to 50 μg/kg · d BPA resulted in increased body weight, elevated serum insulin, and impaired glucose tolerance in adult offspring. On a high-fat diet, such detrimental effects were accelerated and exacerbated. Furthermore, severe metabolic syndrome, including obesity, dyslipidemia, hyperleptindemia, hyperglycemia, hyperinsulinemia, and glucose intolerance, was observed in high-fat-fed offspring perinatally exposed to 50 μg/kg · d BPA. No adverse effect of perinatal BPA exposure at 250 and 1250 μg/kg · d was observed no matter on a normal diet or a high-fat diet. These results suggest that perinatal exposure to BPA at reference dose, but not at high dose, impairs glucose tolerance in adult rat offspring on a normal diet and predisposes offspring to metabolic syndrome at adult on a high-fat diet. High-fat diet intake is a trigger that initiates adverse metabolic effects of BPA.

Eicosanoids, β-cell function, and diabetes

Arachidonic acid (AA) is metabolized by cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) enzymes into eicosanoids, which are involved in diverse diseases, including type 1 and type 2 diabetes. During the last 30 years, evidence has been accumulated that suggests important functions for eicosanoids in the control of pancreatic β-cell function and destruction. AA metabolites of the COX pathway, especially prostaglandin E(2) (PGE(2)), appear to be significant factors to β-cell dysfunction and destruction, participating in the pathogenesis of diabetes and its complications. Several elegant studies have contributed to the sorting out of the importance of 12-LOX eicosanoids in cytokine-mediated inflammation in pancreatic β cells. The role of CYP eicosanoids in diabetes is yet to be explored. A recent publication has demonstrated that stabilizing the levels of epoxyeicosatrienoic acids (EETs), CYP eicosanoids, by inhibiting or deleting soluble epoxide hydrolase (sEH) improves β-cell function and reduces β-cell apoptosis in diabetes. In this review we summarize recent findings implicating these eicosanoid pathways in diabetes and its complications. We also discuss the development of animal models with targeted gene deletion and specific enzymatic inhibitors in each pathway to identify potential targets for the treatment of diabetes and its complications.

Gastrin treatment stimulates β-cell regeneration and improves glucose tolerance in 95% pancreatectomized rats

β-Cell mass reduction is a central aspect in the development of type 1 and type 2 diabetes, and substitution or regeneration of the lost β-cells is a potentially curative treatment of diabetes. To study the effects of gastrin on β-cell mass in rats with 95% pancreatectomy (95%-Px), a model of pancreatic regeneration, rats underwent 95% Px or sham Px and were treated with [15 leu] gastrin-17 (Px+G and S+G) or vehicle (Px+V and S+V) for 15 d. In 95% Px rats, gastrin treatment reduced hyperglycemia (280 ± 52 mg vs. 436 ± 51 mg/dl, P < 0.05), and increased β-cell mass (1.15 ± 0.15 mg)) compared with vehicle-treated rats (0.67 ± 0.15 mg, P < 0.05). Gastrin treatment induced β-cell regeneration by enhancing β-cell neogenesis (increased number of extraislet β-cells in Px+G: 0.42 ± 0.05 cells/mm(2) vs. Px+V: 0.27 ± 0.07 cells/mm(2), P < 0.05, and pancreatic and duodenal homeobox 1 expression in ductal cells of Px+G: 1.21 ± 0.38% vs. Px+V: 0.23 ± 0.10%, P < 0.05) and replication (Px+G: 1.65 ± 0.26% vs. S+V: 0.64 ± 0.14%; P < 0.05). In addition, reduced β-cell apoptosis contributed to the increased β-cell mass in gastrin-treated rats (Px+G: 0.07 ± 0.02%, Px+V: 0.23 ± 0.05%; P < 0.05). Gastrin action on β-cell regeneration and survival increased β-cell mass and improved glucose tolerance in 95% Px rats, supporting a potential role of gastrin in the treatment of diabetes.

Glucose-dependent changes in SNARE protein levels in pancreatic β-cells

Prolonged exposure to high glucose concentration alters the expression of a set of proteins in pancreatic β-cells and impairs their capacity to secrete insulin. The cellular and molecular mechanisms that lie behind this effect are poorly understood. In this study, three either in vitro or in vivo models (cultured rat pancreatic islets incubated in high glucose media, partially pancreatectomized rats, and islets transplanted to streptozotozin-induced diabetic mice) were used to evaluate the dependence of the biological model and the treatment, together with the cell location (insulin granule or plasma membrane) of the affected proteins and the possible effect of sustained insulin secretion, on the glucose-induced changes in protein expression. In all three models, islets exposed to high glucose concentrations showed a reduced expression of secretory granule-associated vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins synaptobrevin/vesicle-associated membrane protein 2 and cellubrevin but minor or no significant changes in the expression of the membrane-associated target-SNARE proteins syntaxin1 and synaptosomal-associated protein-25 and a marked increase in the expression of synaptosomal-associated protein-23 protein. The inhibition of insulin secretion by the L-type voltage-dependent calcium channel nifedipine or the potassium channel activator diazoxide prevented the glucose-induced reduction in islet insulin content but not in vesicle-SNARE proteins, indicating that the granule depletion due to sustained exocytosis was not involved in the changes of protein expression induced by high glucose concentration. Altogether, the results suggest that high glucose has a direct toxic effect on the secretory pathway by decreasing the expression of insulin granule SNARE-associated proteins.

High pancreatic n-3 fatty acids prevent STZ-induced diabetes in fat-1 mice: inflammatory pathway inhibition

OBJECTIVE

Because of confounding factors, the effects of dietary n-3 polyunsaturated fatty acids (PUFA) on type 1 diabetes remain to be clarified. We therefore evaluated whether fat-1 transgenic mice, a well-controlled experimental model endogenously synthesizing n-3 PUFA, were protected against streptozotocin (STZ)-induced diabetes. We then aimed to elucidate the in vivo response at the pancreatic level.

RESEARCH DESIGN AND METHODS

β-Cell destruction was produced by multiple low-doses STZ (MLD-STZ). Blood glucose level, plasma insulin level, and plasma lipid analysis were then performed. Pancreatic mRNA expression of cytokines, the monocyte chemoattractant protein, and GLUT2 were evaluated as well as pancreas nuclear factor (NF)-κB p65 and inhibitor of κB (IκB) protein expression. Insulin and cleaved caspase-3 immunostaining and lipidomic analysis were performed in the pancreas.

RESULTS

STZ-induced fat-1 mice did not develop hyperglycemia compared with wild-type mice, and β-cell destruction was prevented as evidenced by lack of histological pancreatic damage or reduced insulin level. The prevention of β-cell destruction was associated with no proinflammatory cytokine induction (tumor necrosis factor-α, interleukin-1β, inducible nitric oxide synthase) in the pancreas, a decreased NF-κB, and increased IκB pancreatic protein expression. In the fat-1-treated mice, proinflammatory arachidonic-derived mediators as prostaglandin E₂ and 12-hydroxyeicosatetraenoic acid were decreased and the anti-inflammatory lipoxin A₄ was detected. Moreover, the 18-hydroxyeicosapentaenoic acid, precursor of the anti-inflammatory resolvin E1, was highly increased.

CONCLUSIONS

Collectively, these findings indicate that fat-1 mice were protected against MLD-STZ-induced diabetes and pointed out for the first time in vivo the beneficial effects of n-3 PUFA at the pancreatic level, on each step of the development of the pathology-inflammation, β-cell damage-through cytokine response and lipid mediator production.

Obesity and nutrient sensing TOR pathway in flies and vertebrates: Functional conservation of genetic mechanisms

The global prevalence of obesity has grown to epidemic proportions, and 400 million people are now considered to be obese. Excessive accumulation of dietary lipids (obesity) is a known risk factor for the development of deleterious metabolic conditions and has been strongly linked to the progression of heart disease and type 2 diabetes. Investigating the origin and effects of high-fat diet (HFD)-induced obesity and its genetic mediators is an important step in understanding the mechanisms that contribute to obesity. However, the mechanisms that underlie HFD pathophysiology have yet to be elucidated fully. Here we describe recent work in a Drosophila model to investigate the origin and genetic mechanisms that could underlie HFD-induced obesity, type 2 diabetes and cardiac dysfunction.

Evidence for activation of inflammatory lipoxygenase pathways in visceral adipose tissue of obese Zucker rats

Central obesity is associated with low-grade inflammation that promotes type 2 diabetes and cardiovascular disease in obese individuals. The 12- and 5-lipoxygenase (12-LO and 5-LO) enzymes have been linked to inflammatory changes, leading to the development of atherosclerosis. 12-LO has also been linked recently to inflammation and insulin resistance in adipocytes. We analyzed the expression of LO and proinflammatory cytokines in adipose tissue and adipocytes in obese Zucker rats, a widely studied genetic model of obesity, insulin resistance, and the metabolic syndrome. mRNA expression of 12-LO, 5-LO, and 5-LO-activating protein (FLAP) was upregulated in adipocytes and adipose tissue from obese Zucker rats compared with those from lean rats. Concomitant with increased LO gene expression, the 12-LO product 12-HETE and the 5-LO products 5-HETE and leukotriene B4 (LTB4) were also increased in adipocytes. Furthermore, upregulation of key proinflammatory markers interleukin (IL)-6, TNFα, and monocyte chemoattractant protein-1 were observed in adipocytes isolated from obese Zucker rats. Immunohistochemistry indicated that the positive 12-LO staining in adipose tissue represents cells in addition to adipocytes. This was confirmed by Western blotting in stromal vascular fractions. These changes were in part reversed by the novel anti-inflammatory drug lisofylline (LSF). LSF also reduced p-STAT4 in visceral adipose tissue from obese Zucker rats and improved the metabolic profile, reducing fasting plasma glucose and increasing insulin sensitivity in obese Zucker rats. In 3T3-L1 adipocytes, LSF abrogated the inflammatory response induced by LO products. Thus, therapeutic agents reducing LO or STAT4 activation may provide novel tools to reduce obesity-induced inflammation.

Pathological aspects of lipid peroxidation

Lipid peroxidation (LPO) product accumulation in human tissues is a major cause of tissular and cellular dysfunction that plays a major role in ageing and most age-related and oxidative stress-related diseases. The current evidence for the implication of LPO in pathological processes is discussed in this review. New data and literature review are provided evaluating the role of LPO in the pathophysiology of ageing and classically oxidative stress-linked diseases, such as neurodegenerative diseases, diabetes and atherosclerosis (the main cause of cardiovascular complications). Striking evidences implicating LPO in foetal vascular dysfunction occurring in pre-eclampsia, in renal and liver diseases, as well as their role as cause and consequence to cancer development are addressed.

Functional and pathological roles of the 12- and 15-lipoxygenases

The 12/15-lipoxygenase enzymes react with fatty acids producing active lipid metabolites that are involved in a number of significant disease states. The latter include type 1 and type 2 diabetes (and associated complications), cardiovascular disease, hypertension, renal disease, and the neurological conditions Alzheimer's disease and Parkinson's disease. A number of elegant studies over the last thirty years have contributed to unraveling the role that lipoxygenases play in chronic inflammation. The development of animal models with targeted gene deletions has led to a better understanding of the role that lipoxygenases play in various conditions. Selective inhibitors of the different lipoxygenase isoforms are an active area of investigation, and will be both an important research tool and a promising therapeutic target for treating a wide spectrum of human diseases.

Fructose-1,6-bisphosphatase regulates glucose-stimulated insulin secretion of mouse pancreatic beta-cells

Pancreatic β-cells can precisely sense glucose stimulation and accordingly adjust their insulin secretion. Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme, but its physiological significance in β-cells is not established. Here we determined its physiological role in regulating glucose sensing and insulin secretion of β-cells. Considerable FBPase mRNA was detected in normal mouse islets and β-cell lines, although their protein levels appeared to be quite low. Down-regulation of FBP1 in MIN6 cells by small interfering RNA could enhance the glucose-stimulated insulin secretion (GSIS), whereas FBP1-overexpressing MIN6 cells exhibited decreased GSIS. Inhibition of FBPase activity in islet β-cells by its specific inhibitor MB05032 led to significant increase of their glucose utilization and cellular ATP to ADP ratios and consequently enhanced GSIS in vitro. Pretreatment of mice with the MB05032 prodrug MB06322 could potentiate GSIS in vivo and improve their glucose tolerance. Therefore, FBPase plays an important role in regulating glucose sensing and insulin secretion of β-cells and serves a promising target for diabetes treatment.

Activation of pancreatic-duct-derived progenitor cells during pancreas regeneration in adult rats

The adult pancreas has considerable capacity to regenerate in response to injury. We hypothesized that after partial pancreatectomy (Px) in adult rats, pancreatic-duct cells serve as a source of regeneration by undergoing a reproducible dedifferentiation and redifferentiation. We support this hypothesis by the detection of an early loss of the ductal differentiation marker Hnf6 in the mature ducts, followed by the transient appearance of areas composed of proliferating ductules, called foci of regeneration, which subsequently form new pancreatic lobes. In young foci, ductules express markers of the embryonic pancreatic epithelium - Pdx1, Tcf2 and Sox9 - suggesting that these cells act as progenitors of the regenerating pancreas. The endocrine-lineage-specific transcription factor Neurogenin3, which is found in the developing embryonic pancreas, was transiently detected in the foci. Islets in foci initially resemble embryonic islets in their lack of MafA expression and lower percentage of beta-cells, but with increasing maturation have increasing numbers of MafA(+) insulin(+) cells. Taken together, we provide a mechanism by which adult pancreatic duct cells recapitulate aspects of embryonic pancreas differentiation in response to injury, and contribute to regeneration of the pancreas. This mechanism of regeneration relies mainly on the plasticity of the differentiated cells within the pancreas.

Gene expression profiles of Beta-cell enriched tissue obtained by laser capture microdissection from subjects with type 2 diabetes

Background

Changes in gene expression in pancreatic beta-cells from type 2 diabetes (T2D) should provide insights into their abnormal insulin secretion and turnover.

Methodology/Principal Findings

Frozen sections were obtained from cadaver pancreases of 10 control and 10 T2D human subjects. Beta-cell enriched samples were obtained by laser capture microdissection (LCM). RNA was extracted, amplified and subjected to microarray analysis. Further analysis was performed with DNA-Chip Analyzer (dChip) and Gene Set Enrichment Analysis (GSEA) software. There were changes in expression of genes linked to glucotoxicity. Evidence of oxidative stress was provided by upregulation of several metallothionein genes. There were few changes in the major genes associated with cell cycle, apoptosis or endoplasmic reticulum stress. There was differential expression of genes associated with pancreatic regeneration, most notably upregulation of members of the regenerating islet gene (REG) family and metalloproteinase 7 (MMP7). Some of the genes found in GWAS studies to be related to T2D were also found to be differentially expressed. IGF2BP2, TSPAN8, and HNF1B (TCF2) were upregulated while JAZF1 and SLC30A8 were downregulated.

Conclusions/Significance

This study made possible by LCM has identified many novel changes in gene expression that enhance understanding of the pathogenesis of T2D.

Protective unfolded protein response in human pancreatic beta cells transplanted into mice

BACKGROUND

There is great interest about the possible contribution of ER stress to the apoptosis of pancreatic beta cells in the diabetic state and with islet transplantation.

METHODS AND FINDINGS

Expression of genes involved in ER stress were examined in beta cell enriched tissue obtained with laser capture microdissection (LCM) from frozen sections of pancreases obtained from non-diabetic subjects at surgery and from human islets transplanted into ICR-SCID mice for 4 wk. Because mice have higher glucose levels than humans, the transplanted beta cells were exposed to mild hyperglycemia and the abnormal environment of the transplant site. RNA was extracted from the LCM specimens, amplified and then subjected to microarray analysis. The transplanted beta cells showed an unfolded protein response (UPR). There was activation of many genes of the IRE-1 pathway that provide protection against the deleterious effects of ER stress, increased expression of ER chaperones and ERAD (ER-associated protein degradation) proteins. The other two arms of ER stress, PERK and ATF-6, had many down regulated genes. Downregulation of EIF2A could protect by inhibiting protein synthesis. Two genes known to contribute to apoptosis, CHOP and JNK, were downregulated.

CONCLUSIONS

Human beta cells in a transplant site had UPR changes in gene expression that protect against the proapoptotic effects of unfolded proteins.

Targeting orphan nuclear receptor SHP in the treatment of metabolic diseases

IMPORTANCE OF THE FIELD

The orphan nuclear receptor small heterodimer partner (SHP; NR0B2) is an atypical nuclear receptor that contains a ligand-binding domain, but lacks the conserved DNA-binding domain. Since its discovery, SHP has been identified as a key transcriptional regulatory factor of genes involved in diverse metabolic pathways.

AREAS COVERED IN THIS REVIEW

We performed a Medline/Pubmed search to find published studies on the role of SHP in the regulation of metabolism in the liver, pancreatic islets, blood vessel, and kidney and on the feasibility of using SHP as a therapeutic target in metabolic disease.

WHAT THE READER WILL GAIN

In this review, we discuss the function of SHP as regulator of cholesterol, lipid and glucose metabolism, and the role of SHP in metabolic and fibrotic diseases.

TAKE HOME MESSAGE

The reviewed studies suggested that SHP could be a major target for therapeutic intervention in metabolic and fibrotic diseases, including metabolic syndrome and its complications. However, for SHP-targeted therapy, there are some outstanding issues, including the small number of known inducers of SHP and the lack of sufficient data in humans.

Peroxisome proliferator-activated receptor alpha (PPARalpha) protects against oleate-induced INS-1E beta cell dysfunction by preserving carbohydrate metabolism

AIMS/HYPOTHESIS

Pancreatic beta cells chronically exposed to fatty acids may lose specific functions and even undergo apoptosis. Generally, lipotoxicity is triggered by saturated fatty acids, whereas unsaturated fatty acids induce lipodysfunction, the latter being characterised by elevated basal insulin release and impaired glucose responses. The peroxisome proliferator-activated receptor alpha (PPARalpha) has been proposed to play a protective role in this process, although the cellular mechanisms involved are unclear.

METHODS

We modulated PPARalpha production in INS-1E beta cells and investigated key metabolic pathways and genes responsible for metabolism-secretion coupling during a culture period of 3 days in the presence of 0.4 mmol/l oleate.

RESULTS

In INS-1E cells, the secretory dysfunction primarily induced by oleate was aggravated by silencing of PPARalpha. Conversely, PPARalpha upregulation preserved glucose-stimulated insulin secretion, essentially by increasing the response at a stimulatory concentration of glucose (15 mmol/l), a protection we also observed in human islets. The protective effect was associated with restored glucose oxidation rate and upregulation of the anaplerotic enzyme pyruvate carboxylase. PPARalpha overproduction increased both beta-oxidation and fatty acid storage in the form of neutral triacylglycerol, revealing overall induction of lipid metabolism. These observations were substantiated by expression levels of associated genes.

CONCLUSIONS/INTERPRETATION

PPARalpha protected INS-1E beta cells from oleate-induced dysfunction, promoting both preservation of glucose metabolic pathways and fatty acid turnover.

PPARdelta is a fatty acid sensor that enhances mitochondrial oxidation in insulin-secreting cells and protects against fatty acid-induced dysfunction

The peroxisome proliferator-activated receptor delta (PPARdelta) is implicated in regulation of mitochondrial processes in a number of tissues, and PPARdelta activation is associated with decreased susceptibility to ectopic lipid deposition and metabolic disease. Here, we show that PPARdelta is the PPAR subtype expressed at the highest level in insulinoma cells and rat pancreatic islets. Furthermore, PPARdelta displays high transcriptional activity and acts in pronounced synergy with retinoid-X-receptor (RXR). Interestingly, unsaturated fatty acids mimic the effects of synthetic PPARdelta agonists. Using short hairpin RNA-mediated knockdown, we demonstrate that the ability of unsaturated fatty acids to stimulate fatty acid metabolism is dependent on PPARdelta. Activation of PPARdelta increases the fatty acid oxidation capacity in INS-1E beta-cells, enhances glucose-stimulated insulin secretion (GSIS) from islets, and protects GSIS against adverse effects of prolonged fatty acid exposure. The presented results indicate that the nuclear receptor PPARdelta is a fatty acid sensor that adapts beta-cell mitochondrial function to long-term changes in unsaturated fatty acid levels. As maintenance of mitochondrial metabolism is essential to preserve beta-cell function, these data indicate that dietary or pharmacological activation of PPARdelta and RXR may be beneficial in the prevention of beta-cell dysfunction.

Towards better understanding of the contributions of overwork and glucotoxicity to the beta-cell inadequacy of type 2 diabetes

Type 2 diabetes (T2D) is characterized by reduction of beta-cell mass and dysfunctional insulin secretion. Understanding beta-cell phenotype changes as T2D progresses should help explain these abnormalities. The normal phenotype should differ from the state of overwork when beta-cells compensate for insulin resistance to keep glucose levels normal. When only mild hyperglycaemia develops, beta-cells are subjected to glucotoxicity. As hyperglycaemia becomes more severe, so does glucotoxicity. beta-Cells in all four of these situations should have separate phenotypes. When assessing phenotype with gene expression, isolated islets have artefacts resulting from the trauma of isolation and hypoxia of islet cores. An advantage comes from laser capture microdissection (LCM), which obtains beta-cell-rich tissue from pancreatic frozen sections. Valuable data can be obtained from animal models, but the real goal is human beta-cells. Our experience with LCM and gene arrays on frozen pancreatic sections from cadaver donors with T2D and controls is described. Although valuable data was obtained, we predict that the approach of taking fresh samples at the time of surgery is an even greater opportunity to markedly advance our understanding of how beta-cell phenotype evolves as T2D develops and progresses.

Glucose regulation of islet stress responses and beta-cell failure in type 2 diabetes

Pancreatic beta-cells exposed to high glucose concentrations display altered gene expression, function, survival and growth that may contribute to the slow deterioration of the functional beta-cell mass in type 2 diabetes. These glucotoxic alterations may result from various types of stress imposed by the hyperglycaemic environment, including oxidative stress, endoplasmic reticulum stress, cytokine-induced apoptosis and hypoxia. The glucose regulation of oxidative stress-response and integrated stress-response genes in cultured rat islets follows an asymmetric V-shaped profile parallel to that of beta-cell apoptosis, with a large increase at low glucose and a moderate increase at high vs. intermediate glucose concentrations. These observations suggest that both types of stress could play a role in the alteration of the functional beta-cell mass under states of prolonged hypoglycaemia and hyperglycaemia. In addition, beta-cell demise under glucotoxic conditions may also result from beta-cell hypoxia and, in vivo, from their exposure to inflammatory cytokines released locally by non-endocrine islet cells. A better understanding of the relative contribution of each type of stress to beta-cell glucotoxicity and of their pathophysiological cause in vivo may lead to new therapeutic strategies to prevent the slow deterioration of the functional beta-cell mass in glucose intolerant and type 2 diabetic patients.

The glucose transporter 2 undergoes plasma membrane endocytosis and lysosomal degradation in a secretagogue-dependent manner

In beta-cells of the pancreas, the glucose transporter (GLUT)-2 facilitative glucose transporter protein is localized to the plasma membrane and functions as part of the glucose sensing mechanism for the stimulation of insulin secretion. We observed that expressed GLUT2 protein in the cultured Min6B1 cell line undergoes enhanced endocytosis at high extracellular glucose concentrations that stimulate insulin secretion. Moreover, the internalized GLUT2 protein undergoes rapid degradation induced by chronic high-glucose or arginine stimulation but does not undergo plasma membrane recycling or accumulation in any microscopically apparent intracellular membrane compartment. The rapid degradation of GLUT2 was prevented by lysosomal inhibition (chloroquine) concomitant with the accumulation of GLUT2 in endomembrane structures. In contrast, neither endocytosis nor the lack of internal membrane localized GLUT2 remained completely unaffected by proteosomal inhibition (lactacystin) or an heat shock protein-90 inhibitor (geldanamycin). Moreover, the endocytosis and degradation of GLUT2 was specific for beta-cells because expression of GLUT2 in 3T3L1 adipocytes remained cell surface localized and did not display a rapid rate of degradation. Together, these data demonstrate that hyperglycemia directly affects beta-cell function and activates a trafficking pathway that results in the rapid endocytosis and degradation of the cell surface GLUT2 glucose transporter.

Glucose-6-phosphatase catalytic subunit gene family

Glucose-6-phosphatase catalyzes the hydrolysis of glucose 6-phosphate (G6P) to glucose and inorganic phosphate. It is a multicomponent system located in the endoplasmic reticulum that comprises several integral membrane proteins, namely a catalytic subunit (G6PC) and transporters for G6P, inorganic phosphate, and glucose. The G6PC gene family contains three members, designated G6PC, G6PC2, and G6PC3. The tissue-specific expression patterns of these genes differ, and mutations in all three genes have been linked to distinct diseases in humans. This minireview discusses the disease association and transcriptional regulation of the G6PC genes as well as the biological functions of the encoded proteins.

Islet beta cell failure in the 60% pancreatectomised obese hyperlipidaemic Zucker fatty rat: severe dysfunction with altered glycerolipid metabolism without steatosis or a falling beta cell mass

AIMS/HYPOTHESIS

The Zucker fatty (ZF) rat subjected to 60% pancreatectomy (Px) develops moderate diabetes by 3 weeks. We determined whether a progressive fall in beta cell mass and/or beta cell dysfunction contribute to beta cell failure in this type 2 diabetes model.

METHODS

Partial (60%) or sham Px was performed in ZF and Zucker lean (ZL) rats. At 3 weeks post-surgery, beta cell mass and proliferation, proinsulin biosynthesis, pancreatic insulin content, insulin secretion, and islet glucose and lipid metabolism were measured.

RESULTS

ZL-Px rats maintained normal glycaemia and glucose-stimulated insulin secretion (GSIS) despite incomplete recovery of beta cell mass possibly due to compensatory enhanced islet glucose metabolism and lipolysis. ZF-Px rats developed moderate hyperglycaemia (14 mmol/l), hypertriacylglycerolaemia and relative hypoinsulinaemia. Despite beta cell mass recovery and normal arginine-induced insulin secretion, GSIS and pancreatic insulin content were profoundly lowered in ZF-Px rats. Proinsulin biosynthesis was not reduced. Compensatory increases in islet glucose metabolism above those observed in ZF-Sham rats were not seen in ZF-Px rats. Triacylglycerol content was not increased in ZF-Px islets, possibly due to lipodetoxification by enhanced lipolysis and fatty acid oxidation. Fatty acid accumulation into monoacylglycerol and diacylglycerol was increased in ZF-Px islets together with a 4.5-fold elevation in stearoyl-CoA desaturase mRNA expression.

CONCLUSIONS/INTERPRETATION

Falling beta cell mass, reduced proinsulin biosynthesis and islet steatosis are not implicated in early beta cell failure and glucolipotoxicity in ZF-Px rats. Rather, severe beta cell dysfunction with a specific reduction in GSIS and marked depletion of beta cell insulin stores with altered lipid partitioning underlie beta cell failure in this animal model of type 2 diabetes.

p38 MAPK is a major regulator of MafA protein stability under oxidative stress

Mammalian MafA/RIPE3b1 is an important glucose-responsive transcription factor that regulates function, maturation, and survival of beta-cells. Increased expression of MafA results in improved glucose-stimulated insulin secretion and beta-cell function. Because MafA is a highly phosphorylated protein, we examined whether regulating activity of protein kinases can increase MafA expression by enhancing its stability. We demonstrate that MafA protein stability in MIN6 cells and isolated mouse islets is regulated by both p38 MAPK and glycogen synthase kinase 3. Inhibiting p38 MAPK enhanced MafA stability in cells grown under both low and high concentrations of glucose. We also show that the N-terminal domain of MafA plays a major role in p38 MAPK-mediated degradation; simultaneous mutation of both threonines 57 and 134 into alanines in MafA was sufficient to prevent this degradation. Under oxidative stress, a condition detrimental to beta-cell function, a decrease in MafA stability was associated with a concomitant increase in active p38 MAPK. Interestingly, inhibiting p38 MAPK but not glycogen synthase kinase 3 prevented oxidative stress-dependent degradation of MafA. These results suggest that the p38 MAPK pathway may represent a common mechanism for regulating MafA levels under oxidative stress and basal and stimulatory glucose concentrations. Therefore, preventing p38 MAPK-mediated degradation of MafA represents a novel approach to improve beta-cell function.

Molecular analysis of db gene-related pancreatic beta cell dysfunction; evidence for a compensatory mechanism inhibiting development of diabetes in the db gene heterozygote

The db gene homozygous, but not heterozygous, mice develop diabetes with severe beta-cell damage. We investigated the molecular mechanism underlying db gene-associated pancreatic beta-cell dysfunction. Islet morphology, beta-cell function, and gene expression profiles specific for pancreatic islet cells were compared among db gene homozygous(db/db), heterozygous (db/m) and unrelated m/m mice. The beta-cell ratio decreased in db/db mice with age, but not in non-diabetic db/m and m/m mice. The islet insulin content was lower, but the triglyceride content was higher in db/db than other mice. The islet cell specific gene expression profiles analyzed by laser capture microdissection method and morphological findings suggested an augmentation of beta-cell apoptosis, oxidative stress and ER stress in db/db mice. Interestingly, insulin I and II, anti-apoptotic bcl-2, and proliferation promoting ERK-1 gene expressions were significantly upregulated in db/m mice. An impaired glucose tolerance was shown in m/m mice fed a high fat diet, but not in db/m mice, in which a higher insulin response and increased beta-cell mass were observed. Expressions of insulin I and II, bcl-2, and ERK-1 gene were increased in db/m mice, but not in m/m fed a high fat diet. The present results strongly suggest that the db gene heterozygote, but not homozygote, acquires a compensatory mechanism suppressing beta-cell apoptosis and augmenting the capacity of beta-cell function.

MIN6 is not a pure beta cell line but a mixed cell line with other pancreatic endocrine hormones

MIN6 cells retains glucose-stimulated insulin secretion (GSIS) as isolated islets. We comprehensively evaluated the gene expression and production of other islet hormones in MIN6 cells. Islet hormones were demonstrated by immunohistochemical staining and measured by ELISA. The gene expression profiles of MIN6 cells were compared with those in the mouse islets obtained by the laser capture micro-dissection (LCM). MIN6 cells excreted insulin, glucagon, somatostatin and ghrelin. They expressed mRNAs of insulin I and II, proglucagon, somatostatin, pancreatic polypeptide (PP) and ghrelin which were shown in the mouse pancreatic islet core and periphery obtained by LCM. A variety of genes closely related to the islet hormone producing cells were expressed in MIN6. Confocal laser scanning microscopy revealed that MIN6 cells included not only insulin positive cells but also insulin and glucagon or somatostin double positive cells. Glucagon, somatostatin and ghrelin were detectable in the culture medium. The present study clearly demonstrated that MIN6 produce pancreatic endocrine cells. It would be possible to use this cell line as a model to research the development, cell differentiation and function of pancreatic islets.

Laser capture microdissection of human pancreatic beta-cells and RNA preparation for gene expression profiling

Human beta-cell gene profiling is a powerful tool for understanding beta-cell biology in normal and pathological conditions. The assessment is complicated when isolated islets are studied because of contamination by non-beta-cells and the exposure to the trauma of isolation that causes changes in gene expression. These limitations can be overcome by dissecting the beta-cells from the pancreatic tissue directly using the laser capture microdissection (LCM) technique. LCM allows the sampling of specific cell types from tissue sections. The technique requires morphological criteria or specific stains for targeted cells, and the protocols must preserve the condition of the sought-after macromolecules. We have developed a protocol of rapid tissue dehydration followed by identification of human beta-cells by their intrinsic autofluorescence, which allows laser microdissection for gene-profiling studies.

Role of nuclear receptors in the modulation of insulin secretion in lipid-induced insulin resistance

In healthy individuals, a hyperbolic relationship exists between whole-body insulin-sensitivity and insulin secretion. Thus, for any difference in insulin-sensitivity, a reciprocal proportionate change occurs in insulin secretion. Such a feedback loop is evident in healthy individuals ingesting diets high in saturated fat and in late pregnancy where, despite lipid-induced insulin resistance, glucose tolerance is maintained through augmented GSIS (glucose-stimulated insulin secretion). NRs (nuclear receptors) are members of a superfamily of ligand-regulated and orphan transcription factors. On activation by a cognate ligand, many ligand-activated NRs recruit the RXR (retinoid X receptor) for heterodimer formation. Such NRs include the PPARs (peroxisome-proliferator-activated receptors), which are involved in lipid sensing and liporegulation. PPARs exert important lipid-lowering effects in vivo, thereby opposing the development of lipid-induced insulin resistance by relieving the inhibition of insulin-stimulated glucose disposal by muscle and lowering the necessity for augmented GSIS to counter lipid-induced insulin resistance. Long-chain fatty acids are proposed as natural PPAR ligands and some specific endogenous pathways of lipid metabolism are believed to generate PPAR agonists. Other NRs, e.g. the LXR (liver X receptor), which senses expansion of the metabolically active pool of cholesterol, and the FXR (farnesoid X receptor; NR1H4), which, like the LXR, is involved in sterol metabolism, also modulate systemic lipid levels and insulin-sensitivity. In this review, we discuss how these NRs impact insulin secretion via effects on the insulin-sensitivity-insulin secretion feedback loop and, in some cases, via direct effects on the islet itself. In addition, we discuss interactions between these nutrient/metabolite-responsive NRs and NRs that are central to the action of metabolically important hormones, including (i) the glucocorticoid receptor, critical for maintaining glucose homoeostasis in stress, inflammation and during fasting, and (ii) the thyroid hormone receptors, vital for maintenance of oxidative functions. We present data indicating that the RXR occupies a key role in directly modulating islet function and that its heterodimerization with at least two of its partners modulates GSIS.

12-Lipoxygenase-knockout mice are resistant to inflammatory effects of obesity induced by Western diet

Inflammation is a key pathological process in the progression of atherosclerosis and type 2 diabetes. 12/15-lipoxygenase (12-LO), an enzyme involved in fatty acid metabolism, may contribute to inflammatory damage triggered by stressors such as obesity and insulin resistance. We hypothesized that mice lacking 12-LO are protected against inflammatory-mediated damage associated with a "western" diet. To test this hypothesis, age-matched male 12-LO knockout (12-LOKO) and wild-type C57BL/6 (B6) mice were fed either a standard chow or western diet and assessed for several inflammatory markers. Western-fed B6 mice showed expected reductions in glucose and insulin tolerance compared with chow-fed mice. In contrast, western-fed 12-LOKO mice maintained glucose and insulin tolerance similar to chow-fed mice. Circulating proinflammatory cytokines, tumor necrosis factor-alpha and interleukin-6, were increased in western B6 mice but not 12-LOKO mice, whereas the reported protective adipokine, adiponectin, was decreased only in western B6 mice. 12-LO activity was significantly elevated by western diet in islets from B6 mice. Islets from 12-LOKO mice did not show western-diet-induced islet hyperplasia or increases in caspase-3 apoptotic staining observed in western-fed B6 mice. Islets from 12-LOKO mice were also protected from reduced glucose-stimulated insulin secretion observed in islets from western-fed B6 mice. In visceral fat, macrophage numbers and monocyte chemoattractant protein-1 expression were elevated in western B6 mice but not 12-LOKO mice. These data suggest that 12-LO activation plays a role in western-diet-induced damage in visceral fat and islets. Inhibiting 12-LO may provide a new therapeutic approach to prevent inflammation-mediated metabolic consequences of excess fat intake.

Restoring a functional beta-cell mass in diabetes

Type 1 and type 2 diabetes have often been presented as disease forms that profoundly differ in the presence and pathogenic significance of a reduced beta-cell mass. We review evidence indicating that the beta-cell mass in type 1 diabetes is usually not decreased by at least 90% at clinical onset, and remains often detectable for years after diagnosis at age above 15 years. Clinical and experimental evidence also exists for a reduced beta-cell mass in type 2 diabetes where it can be the cause for and/or the consequence of dysregulated beta-cell functions. With beta-cell mass defined as number of beta-cells, these views face the limitation of insufficient data and methods for human organs. Because beta-cells can occur under different phenotypes that vary with age and with environmental conditions, we propose to use the term functional beta-cell mass as an assessment of a beta-cell population by the number of beta-cells and their phenotype or functional state. Assays exist to measure functional beta-cell mass in isolated preparations. We selected a glucose-clamp test to evaluate functional beta-cell mass in type 1 patients at clinical onset and in type 1 recipients following intraportal islet cell transplantation. Comparison of the data with those in non-diabetic controls helps targeting and monitoring of therapeutic interventions.

Fructose-1,6-bisphosphatase overexpression in pancreatic beta-cells results in reduced insulin secretion: a new mechanism for fat-induced impairment of beta-cell function

OBJECTIVE

Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme that is upregulated in islets or pancreatic beta-cell lines exposed to high fat. However, whether specific beta-cell upregulation of FBPase can impair insulin secretory function is not known. The objective of this study therefore is to determine whether a specific increase in islet beta-cell FBPase can result in reduced glucose-mediated insulin secretion.

RESEARCH DESIGN AND METHODS

To test this hypothesis, we have generated three transgenic mouse lines overexpressing the human FBPase (huFBPase) gene specifically in pancreatic islet beta-cells. In addition, to investigate the biochemical mechanism by which elevated FBPase affects insulin secretion, we made two pancreatic beta-cell lines (MIN6) stably overexpressing huFBPase.

RESULTS

FBPase transgenic mice showed reduced insulin secretion in response to an intravenous glucose bolus. Compared with the untransfected parental MIN6, FBPase-overexpressing cells showed a decreased cell proliferation rate and significantly depressed glucose-induced insulin secretion. These defects were associated with a decrease in the rate of glucose utilization, resulting in reduced cellular ATP levels.

CONCLUSIONS

Taken together, these results suggest that upregulation of FBPase in pancreatic islet beta-cells, as occurs in states of lipid oversupply and type 2 diabetes, contributes to insulin secretory dysfunction.

Effects of c-MYC activation on glucose stimulus-secretion coupling events in mouse pancreatic islets

Alteration of pancreatic beta-cell survival and Preproinsulin gene expression by prolonged hyperglycemia may result from increased c-MYC expression. However, it is unclear whether c-MYC effects on beta-cell function are compatible with its proposed role in glucotoxicity. We therefore tested the effects of short-term c-MYC activation on key beta-cell stimulus-secretion coupling events in islets isolated from mice expressing a tamoxifen-switchable form of c-MYC in beta-cells (MycER) and their wild-type littermates. Tamoxifen treatment of wild-type islets did not affect their cell survival, Preproinsulin gene expression, and glucose stimulus-secretion coupling. In contrast, tamoxifen-mediated c-MYC activation for 2-3 days triggered cell apoptosis and decreased Preproinsulin gene expression in MycER islets. These effects were accompanied by mitochondrial membrane hyperpolarization at all glucose concentrations, a higher resting intracellular calcium concentration ([Ca(2+)](i)), and lower glucose-induced [Ca(2+)](i) rise and islet insulin content, leading to a strong reduction of glucose-induced insulin secretion. Compared with these effects, 1-wk culture in 30 mmol/l glucose increased the islet sensitivity to glucose stimulation without reducing the maximal glucose effectiveness or the insulin content. In contrast, overnight exposure to a low H(2)O(2) concentration increased the islet resting [Ca(2+)](i) and reduced the amplitude of the maximal glucose response as in tamoxifen-treated MycER islets. In conclusion, c-MYC activation rapidly stimulates apoptosis, reduces Preproinsulin gene expression and insulin content, and triggers functional alterations of beta-cells that are better mimicked by overnight exposure to a low H(2)O(2) concentration than by prolonged culture in high glucose.

On the role of uncoupling protein-2 in pancreatic beta cells

Pancreatic beta cells secrete insulin when blood glucose levels are high. Dysfunction of this glucose-stimulated insulin secretion (GSIS) is partly responsible for the manifestation of type 2 diabetes, a metabolic disorder that is rapidly becoming a global pandemic. Mitochondria play a central role in GSIS by coupling glucose oxidation to production of ATP, a signal that triggers a series of events that ultimately leads to insulin release. Beta cells express a mitochondrial uncoupling protein, UCP2, which is rather surprising as activity of such a protein is anticipated to lower the efficiency of oxidative phosphorylation, and hence to impair GSIS. The mounting evidence demonstrating that insulin secretion is indeed blunted by UCP2 agrees with this prediction, and has provoked the idea that UCP2 activity contributes to beta cell pathogenesis and development of type 2 diabetes. Although this notion may be correct, the evolved function of UCP2 remains unclear. With this paper we aim to provide a brief account of the present state of affairs in this field, suggest a physiological role for UCP2, and highlight some of our own recent results.

Gene expression of purified beta-cell tissue obtained from human pancreas with laser capture microdissection

CONTEXT

Human beta-cell gene profiling is a powerful tool for understanding beta-cell biology in normal and pathological conditions. Assessment is complicated when isolated islets are studied because of contamination by non-beta-cells and the trauma of the isolation procedure.

OBJECTIVE

The objective was to use laser capture microdissection (LCM) of human beta-cells from pancreases of cadaver donors and compare their gene expression with that of handpicked isolated islets.

DESIGN

Endogenous autofluorescence of beta-cells facilitated procurement of purified beta-cell tissue from frozen pancreatic sections with LCM. Gene expression profiles of three microdissected beta-cell samples and three isolated islet preparations were obtained. The array data were normalized using DNA-Chip Analyzer software (Harvard School of Public Health, Boston, MA), and the lower confidence bound evaluated differentially expressed genes. Real-time PCR was performed on selected acinar genes and on the duct cell markers, carbonic anhydrase II and keratin 19.

RESULTS

Endogenous autofluorescence facilitates the microdissection of beta-cell rich tissue in human pancreas. When compared with array profiles of purified beta-cell tissue, with lower confidence bound set at 1.2, there were 4560 genes up-regulated and 1226 genes down-regulated in the isolated islets. Among the genes up-regulated in isolated islets were pancreatic acinar and duct genes, chemokine genes, and genes associated with hypoxia, apoptosis, and stress. Quantitative RT-PCR confirmed the differential expression of acinar gene transcripts and the duct marker carbonic anhydrase II in isolated islets.

CONCLUSION

LCM makes it possible to obtain beta-cell enriched tissue from human pancreas sections without the trauma and ischemia of islet isolation.

UCP-2 and UCP-3 proteins are differentially regulated in pancreatic beta-cells

Background

Increased uncoupling protein-2 (UCP-2) expression has been associated with impaired insulin secretion, whereas UCP-3 protein levels are decreased in the skeleton muscle of type-2 diabetic subjects. In the present studies we hypothesize an opposing effect of glucose on the regulation of UCP-2 and UCP-3 in pancreatic islets.

Methodology

Dominant negative UCP-2 and wild type UCP-3 adenoviruses were generated, and insulin release by transduced human islets was measured. UCP-2 and UCP-3 mRNA levels were determined using quantitative PCR. UCP-2 and UCP-3 protein expression was investigated in human islets cultured in the presence of different glucose concentrations. Human pancreatic sections were analyzed for subcellular localization of UCP-3 using immunohistochemistry.

Principal Findings

Dominant negative UCP-2 expression in human islets increased insulin secretion compared to control islets (p<0.05). UCP-3 mRNA is expressed in human islets, but the relative abundance of UCP-2 mRNA was 8.1-fold higher (p<0.05). Immunohistochemical analysis confirmed co-localization of UCP-3 protein with mitochondria in human beta-cells. UCP-2 protein expression in human islets was increased ∼2-fold after high glucose exposure, whereas UCP-3 protein expression was decreased by ∼40% (p<0.05). UCP-3 overexpression improved glucose-stimulated insulin secretion.

Conclusions

UCP-2 and UCP-3 may have distinct roles in regulating beta-cell function. Increased expression of UCP-2 and decreased expression of UCP-3 in humans with chronic hyperglycemia may contribute to impaired glucose-stimulated insulin secretion. These data imply that mechanisms that suppress UCP-2 or mechanisms that increase UCP-3 expression and/or function are potential therapeutic targets to offset defects of insulin secretion in humans with type-2 diabetes.

Reversal of islet GIP receptor down-regulation and resistance to GIP by reducing hyperglycemia in the Zucker rat

In type 2 diabetes (T2DM) beta-cell responsiveness to glucose-dependent insulinotropic polypeptide (GIP) is reduced. In a model of T2DM, the VDF Zucker rat, GIP receptor mRNA and protein levels were shown to be down-regulated. Possible restoration of responsiveness to GIP in Zucker rats by reducing hyperglycemia has been examined. ZDF rats with extreme hyperglycemia demonstrated greater islet GIP receptor mRNA down-regulation (94.3+/-3.8%) than ZF rats (48.8+/-22.8%). GIP receptor mRNA levels in ZDF rats returned to 83.0+/-17.9% of lean following normalization of hyperglycemia by phlorizin treatment and pancreas perfusions demonstrated markedly improved GIP responsiveness. Treatment of VDF rats with a DP IV inhibitor (P32/98) resulted in improved glucose tolerance and restored sensitivity to GIP in isolated pancreata. These findings support the proposal that GIP receptor down-regulation in rodent T2DM is secondary to chronic hyperglycemia and that normalization of glycemia can restore GIP sensitivity.