Chronic exposure of HIT cells to high glucose concentrations paradoxically decreases insulin gene transcription and alters binding of insulin gene regulatory protein

Calcium/calmodulin-dependent protein kinase IV involvement in the pathophysiology of glucotoxicity in rat pancreatic β-cells

Glucotoxicity is a critical component of the pathophysiology of type 2 diabetes mellitus; however, the molecular mechanisms of glucotoxicity are still not fully understood. We have attempted to determine the protein kinases involved in glucotoxicity in pancreatic β-cells by the use of a new technique. Using Multi-PK antibodies, which are capable of detecting a wide variety of protein kinases, we analyzed the protein kinase that correlated with insulin synthesis in INS-1 cells under glucotoxic conditions. When expression patterns of protein kinases in INS-1 cells were analyzed by Western blotting with Multi-PK antibodies, a kinase of 63 kd was significantly reduced concomitant with the decrease of insulin secretion under glucotoxic conditions. To identify the 63-kd kinase, we used a unique 2-dimensional gel electrophoretic technique and MicroRotofor (Bio-Rad Laboratories, Tokyo, Japan) electrophoresis. From the molecular size of a native kinase/cyanogen bromide fragment and pI value, the 63-kd protein kinase was deduced to be CaMKIV. This was confirmed by Western blotting analysis using anti-CaMKIV antibodies. The decreased CaMKIV levels under glucotoxic conditions recovered to original levels after changing the medium to a normal glucose concentration. Recombinant CaMKIV was degraded in a Ca²+-dependent manner by incubation with cell lysates from INS-1 cells under glucotoxic conditions, and degradation was protected by calpain inhibitor. Furthermore, CaMKIV was reduced in the pancreatic islets of diabetic Otsuka Long-Evans Tokushima fatty rats, whereas that of nondiabetic Long-Evans Tokushima Otsuka rats was not. This study suggests that the abnormal regulation of CaMKIV is a component of β-cell dysfunction caused by high glucose.

The activation of the rat insulin gene II by BETA2 and PDX-1 in rat insulinoma cells is repressed by Pax6

The transcriptional transactivator Pax6 binds the pancreatic islet cell-specific enhancer sequence (PISCES) of the rat insulin I gene. However the human, mouse, and rat insulin gene II promoters do not contain a PISCES element. To analyze the role of Pax6 in those PISCES-less promoters, we investigated its influence on rat insulin gene II expression and included in our studies the main activators: pancreatic and duodenal homeobox protein-1 (PDX-1) and BETA2/E47. Luciferase assays, Northern blots, and RIA were used to study effects of Pax6 overexpression, gel shift and chromatin precipitation assays to study its binding to the DNA, and yeast two-hybrid assays and glutathione S transferase capture assays to investigate its interactions with PDX-1 and BETA2. Finally, glucose-dependent intracellular transport of Pax6 was demonstrated by fluorescence microscopy. Overexpression of Pax6 prevents activation of the rat insulin II gene by BETA2 and PDX-1 and hence suppresses insulin synthesis and secretion. In vitro, Pax6 binds to the A-boxes, thereby blocking binding of PDX-1, and at the same time, its paired domain interacts with BETA2. Fluorescence microscopy demonstrated that the nuclear-cytoplasmic localization of Pax6 and PDX-1 are oppositely regulated by glucose. From the results, it is suggested that at low concentrations of glucose, Pax6 is localized in the nucleus and prevents the activation of the insulin gene by occupying the PDX-1 binding site and by interacting with BETA2.

Regulation of MafA expression in pancreatic beta-cells in db/db mice with diabetes

OBJECTIVE

Islet beta-cells loose their ability to synthesize insulin under diabetic conditions, which is at least partially due to the decreased activity of insulin transcription factors such as MafA. Although an in vitro study showed that reactive oxygen species (ROS) decrease MafA expression, the underlying mechanism still remains unclear. In this study, we examined the effects of c-Jun, which is known to be upregulated by ROS, on the expression of MafA under diabetic conditions.

RESEARCH DESIGN AND METHODS

To examine the protein levels of MafA and c-Jun, we performed histological analysis and Western blotting using diabetic db/db mice. In addition, to evaluate the possible effects of c-Jun on MafA expression, we performed adenoviral overexpression of c-Jun in the MIN6 beta-cell line and freshly isolated islets. RESULTS MafA expression was markedly decreased in the islets of db/db mice, while in contrast c-Jun expression was increased. Costaining of these factors in the islets of db/db mice clearly showed that MafA and insulin levels are decreased in c-Jun-positive cells. Consistent with these results, overexpression of c-Jun significantly decreased MafA expression, accompanied by suppression of insulin expression. Importantly, MafA overexpression restored the insulin promoter activity and protein levels that were suppressed by c-Jun. These results indicate that the decreased insulin biosynthesis induced by c-Jun is principally mediated by the suppression of MafA activity.

CONCLUSIONS

It is likely that the augmented expression of c-Jun in diabetic islets decreases MafA expression and thereby reduces insulin biosynthesis, which is often observed in type 2 diabetes.

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.

JNK1 and IKKbeta: molecular links between obesity and metabolic dysfunction

Inflammation is thought to underlie the pathogenesis of many chronic diseases. It is now established that obesity results in a state of chronic low-grade inflammation thought to contribute to several metabolic disorders, including insulin resistance and pancreatic islet dysfunction. The protein kinases JNK1 and IKKbeta have been found to serve as critical molecular links between obesity, metabolic inflammation, and disorders of glucose homeostasis. The precise mechanisms of these linkages are still being investigated. However, as we discuss here, JNK1 and IKKbeta are activated by almost all forms of metabolic stress that have been implicated in insulin resistance or islet dysfunction. Furthermore, both JNK1 and IKKbeta are critically involved in the promotion of diet-induced obesity, metabolic inflammation, insulin resistance, and beta-cell dysfunction. Understanding the molecular mechanisms by which JNK1 and IKKbeta mediate obesity-induced metabolic stress is likely to be of importance for the development of new treatments for a variety of obesity-associated diseases.

Safe and simple emergency department discharge therapy for patients with type 2 diabetes mellitus and severe hyperglycemia

OBJECTIVE

To investigate the safety and effectiveness of 2 simple discharge regimens for use in patients with type 2 diabetes mellitus (DM2) and severe hyperglycemia, who present to the emergency department (ED) and do not need to be admitted.

METHODS

We conducted an 8-week, open-label, randomized controlled trial in 77 adult patients with DM2 and blood glucose levels of 300 to 700 mg/dL seen in a public hospital ED. Patients were randomly assigned to receive glipizide XL, 10 mg orally daily (G group), versus glipizide XL, 10 mg orally daily, plus insulin glargine, 10 U daily (G+G group). The primary outcome was to maintain safe fasting glucose and random glucose levels of <350 and <500 mg/dL up to 4 weeks and <300 and <400 mg/dL, respectively, thereafter and to have no return ED visits (responders).

RESULTS

Baseline characteristics were similar between the 2 treatment groups. The primary outcome was achieved in 87% of patients in both treatment groups. The enrollment mean blood glucose values of 440 and 467 mg/dL in the G and G+G groups, respectively, declined by the end of week 1 to 298 and 289 mg/dL and by week 8 to 140 and 135 mg/dL, respectively. Homeostasis model assessment of beta-cell function and early insulin response improved 7-fold and 4-fold, respectively, in responders at the end of the 8-week study.

CONCLUSION

Sulfonylurea with and without use of a small dose of insulin glargine rapidly improved blood glucose levels and beta-cell function in patients with DM2. Use of sulfonylurea alone once daily can be considered a safe discharge regimen for such patients and an effective bridge between ED intervention and subsequent follow-up.

Activation of autophagy through modulation of 5'-AMP-activated protein kinase protects pancreatic beta-cells from high glucose

Chronic hyperglycaemia is detrimental to pancreatic beta-cells by causing impaired insulin secretion and diminished beta-cell function through glucotoxicity. Understanding the mechanisms underlying beta-cell survival is crucial for the prevention of beta-cell failure associated with glucotoxicity. Autophagy is a dynamic lysosomal degradation process that protects organisms against metabolic stress. To date, little is known about the physiological function of autophagy in the pathogenesis of diabetes. In the present study, we explored the roles of autophagy in the survival of pancreatic beta-cells exposed to high glucose using pharmacological and genetic manipulation of autophagy. We demonstrated that chronic high glucose increases autophagy in rat INS-1 (832/13) cells and pancreatic islets, and that this increase is enhanced by inhibition of 5'-AMP-activated protein kinase. Our results also indicate that stimulation of autophagy rescues pancreatic beta-cells from high-glucose-induced cell death and inhibition of autophagy augments caspase-3 activation, suggesting that autophagy plays a protective role in the survival of pancreatic beta-cells. Greater knowledge of the molecular mechanisms linking autophagy and beta-cell survival may unveil novel therapeutic targets needed to preserve beta-cell function.

Multiple chromatin-bound protein kinases assemble factors that regulate insulin gene transcription

During the onset of diabetes, pancreatic beta cells become unable to produce sufficient insulin to maintain blood glucose within the normal range. Proinflammatory cytokines have been implicated in impaired beta cell function. To understand more about the molecular events that reduce insulin gene transcription, we examined the effects of hyperglycemia alone and together with the proinflammatory cytokine interleukin-1beta (IL-1beta) on signal transduction pathways that regulate insulin gene transcription. Exposure to IL-1beta in fasting glucose activated multiple protein kinases that associate with the insulin gene promoter and transiently increased insulin gene transcription in beta cells. In contrast, cells exposed to hyperglycemic conditions were sensitized to the inhibitory actions of IL-1beta. Under these conditions, IL-1beta caused the association of the same protein kinases, but a different combination of transcription factors with the insulin gene promoter and began to reduce transcription within 2 h; stimulatory factors were lost, RNA polymerase II was lost, and inhibitory factors were bound to the promoter in a kinase-dependent manner.

Adaptation and failure of pancreatic beta cells in murine models with different degrees of metabolic syndrome

The events that contribute to the expansion of beta-cell mass and enhanced beta-cell function in insulin-resistant states have not been elucidated fully. Recently, we showed that beta-cell adaptation failed dramatically in adult, insulin-resistant POKO mice, which contrasts with the appropriate expansion of beta cells in their ob/ob littermates. Thus, we hypothesised that characterisation of the islets in these mouse models at an early age should provide a unique opportunity to: (1) identify mechanisms involved in sensing insulin resistance at the level of the beta cells, (2) identify molecular effectors that contribute to increasing beta-cell mass and function, and (3) distinguish primary events from secondary events that are more likely to be present at more advanced stages of diabetes. Our results define the POKO mouse as a model of early lipotoxicity. At 4 weeks of age, it manifests with inappropriate beta-cell function and defects in proliferation markers. Other well-recognised pathogenic effectors that were observed previously in 16-week-old mice, such as increased reactive oxygen species (ROS), macrophage infiltration and endoplasmic reticulum (ER) stress, are also present in both young POKO and young ob/ob mice, indicating the lack of predictive power with regards to the severity of beta-cell failure. Of interest, the relatively preserved lipidomic profile in islets from young POKO mice contrasted with the large changes in lipid composition and the differences in the chain length of triacylglycerols in the serum, liver, muscle and adipose tissue in adult POKO mice. Later lipotoxic insults in adult beta cells contribute to the failure of the POKO beta cell. Our results indicate that the rapid development of insulin resistance and beta-cell failure in POKO mice makes this model a useful tool to study early molecular events leading to insulin resistance and beta-cell failure. Furthermore, comparisons with ob/ob mice might reveal important adaptive mechanisms in beta cells with either therapeutic or diagnostic potential.

Beta-cell deterioration during diabetes: what's in the gun?

Clinical management of patients with type 2 diabetes (T2D) successfully prevents extreme hyperglycemia but does not precisely control glucose levels throughout the day. The pathogenesis of T2D is akin to a double-barrel shotgun. The first trigger causes an explosion that sets genetic expression of the disease in motion; the second trigger discharges a host of environmental factors that worsen its clinical course. Candidate shells include glucolipotoxicity, cytokines, oxidative and endoplasmic reticulum stress and insulin resistance. This review considers how each candidate adversely impacts beta-cell function to create the downward spiral of glycemic control. Their roles in pathogenesis raise possibilities for new drug therapies designed to protect against adverse effects of residual hyperglycemia in patients treated with conventional drugs.

ERp46 is reduced by high glucose and regulates insulin content in pancreatic beta-cells

Our studies focus on ERp46, an endoplasmic reticulum (ER) component, and analyze its involvement in glucose toxicity and in insulin production. Differences in pancreatic beta-TC-6 cell proteome under conditions of low vs. high glucose were examined by proteomic approaches, including two-dimensional gel electrophoresis, image analysis, and mass spectrometry. Among differentially expressed proteins, ERp46, a novel endoplasmic reticulum component, was examined further. The expression of ERp46 in pancreatic sections was analyzed by immunocytochemistry, and high glucose-induced alterations of expression were evaluated in cultured beta-cells, in isolated pancreatic islets, and in the pancreas of db/db diabetic animals. Inhibition of ERp46 expression by siRNA was performed to study its role in insulin production, in secretion, and in ER stress. Proteomic analysis led to identification of 46 differentially expressed spots corresponding to 23 proteins. Since ERp46 is a novel protein with a possible crucial role in secretory cells, we further analyzed its role in beta-cell function. ERp46 expression is reduced in high glucose concentration in beta-TC-6 cells and in isolated murine islets. Further analysis revealed high expression of ERp46 in pancreatic islets compared with exocrine tissue. Interestingly, a marked decrease in ERp46 expression was found in the pancreatic islets of db/db mice. Most importantly, siRNA-mediated knockdown of ERp46 in cultured beta-cells led to a significant decrease in the insulin content; however, no alterations in insulin mRNA levels were observed under these conditions. In addition, reduced expression of ERp46 by siRNA increased the expression of CHOP and peIF2a, indicating development of ER stress. We conclude that ERp46 may be an important component in the phenomenon of "glucose toxicity" involved in insulin production at the posttranslational level.

High extracellular glucose inhibits exocytosis through disruption of syntaxin 1A-containing lipid rafts

Diabetes is characterized by high blood glucose which eventually impairs the secretion of insulin. Glucose directly affects cholesterol biosynthesis and may in turn affect cellular structures that depend on the sterol, including lipid rafts that help organize the secretory apparatus. Here, we investigated the long-term effects of glucose upon lipid rafts and secretory granule dynamics in pancreatic beta-cells. Raft fractions, identified by the presence of GM1 and flotillin, contained characteristically high levels of cholesterol and syntaxin 1A, the t-SNARE which tethers granules to the plasma membrane. Seventy-two hours exposure to 28mM glucose resulted in approximately 30% reduction in membrane cholesterol, with consequent redistribution of raft markers and syntaxin 1A throughout the plasma membrane. Live cell imaging indicated loss of syntaxin 1A from granule docking sites, and fewer docked granules. In conclusion, glucose-mediated inhibition of cholesterol biosynthesis perturbs lipid raft stability, resulting in a loss of syntaxin 1A from granule docking sites and inhibition of insulin secretion.

Role of MafA in pancreatic beta-cells

Pancreatic beta-cell-specific insulin gene expression is regulated by a variety of pancreatic transcription factors and the conserved A3, C1 and E1 elements in the insulin gene enhancer region are very important for activation of insulin gene. Indeed, PDX-1 binding to the A3 element and NeuroD binding to the E1 element are crucial for insulin gene transcription. Recently, C1 element-binding transcription factor was identified as MafA, which is a basic-leucine zipper transcription factor and functions as a potent transactivator for the insulin gene. Under diabetic conditions, chronic hyperglycemia gradually deteriorates pancreatic beta-cell function, which is accompanied by decreased expression and/or DNA binding activities of MafA and PDX-1. Furthermore, MafA overexpression, together with PDX-1 and NeuroD, markedly induces insulin biosynthesis in various non-beta-cells and thereby is a useful tool to efficiently induce insulin-producing surrogate beta-cells. These results suggest that MafA plays a crucial role in pancreatic beta-cells and could be a novel therapeutic target for diabetes.

Beta-cell failure as a complication of diabetes

Type 2 diabetes mellitus is a complex disease characterized by beta-cell failure in the setting of insulin resistance. In early stages of the disease, pancreatic beta-cells adapt to insulin resistance by increasing mass and function. As nutrient excess persists, hyperglycemia and elevated free fatty acids negatively impact beta-cell function. This happens by numerous mechanisms, including the generation of reactive oxygen species, alterations in metabolic pathways, increases in intracellular calcium and the activation of endoplasmic reticulum stress. These processes adversely affect beta-cells by impairing insulin secretion, decreasing insulin gene expression and ultimately causing apoptosis. In this review, we will first discuss the regulation of beta-cell mass during normal conditions. Then, we will discuss the mechanisms of beta-cell failure, including glucotoxicity, lipotoxicity and endoplasmic reticulum stress. Further research into mechanisms will reveal the key modulators of beta-cell failure and thus identify possible novel therapeutic targets. Type 2 diabetes mellitus is a multifactorial disease that has greatly risen in prevalence in part due to the obesity and inactivity that characterize the modern Western lifestyle. Pancreatic beta-cells possess the potential to greatly expand their function and mass in both physiologic and pathologic states of nutrient excess and increased insulin demand. beta-cell response to nutrient excess occurs by several mechanisms, including hypertrophy and proliferation of existing beta-cells, increased insulin production and secretion, and formation of new beta-cells from progenitor cells [1, 2]. Failure of pancreatic beta-cells to adequately expand in settings of increased insulin demand results in hyperglycemia and diabetes. In this review, we will first discuss the factors involved in beta-cell growth and then discuss the mechanisms by which beta-cell expansion fails and leads to beta-cell failure and diabetes (Fig. 1).

The role of CHOP messenger RNA expression in the link between oxidative stress and apoptosis

Low expression of antioxidant enzymes makes pancreatic beta-cells susceptible to cell damage by oxidative stress. Pancreatic beta-cell loss caused by endoplasmic reticulum stress is associated with the onset of diabetes mellitus. The present studies were undertaken to investigate a possible involvement of proapoptotic gene CHOP in pancreatic beta-cells damage by oxidative stress. The induction of CHOP messenger RNA and apoptosis were investigated in betaHC-9 cells after the oxidative stress by hydrogen peroxide and ribose. Latter was examined after the suppression of CHOP by small interfering RNA. For in vivo study, the pancreatic beta-cells were examined in CHOP-knockout (KO) mice after multiple low-dose streptozotocin (MLDS) administration. In betaHC-9 cells, both hydrogen peroxide and ribose obviously increased apoptotic cells, accompanied with enhanced CHOP messenger RNA expression. However, the number of apoptotic cells by those stimulations was significantly reduced by the addition of small interfering RNA against CHOP. In vivo study also showed that CHOP-KO mice were less susceptible to diabetes after MLDS administration. Although the oxidative stress marker level was similar to that of MLDS-treated wild type, the pancreatic beta-cell area was maintained in CHOP-KO mice. The present studies showed that CHOP should be important in pancreatic beta-cell injury by oxidative stress and indicate that CHOP may play a role in the development of pancreatic beta-cell damage on the onset of diabetes mellitus.

Abnormalities in insulin secretion in type 2 diabetes mellitus

Type 2 diabetes mellitus is a multifactorial disease, due to decreased glucose peripheral uptake, and increased hepatic glucose production, due to reduced both insulin secretion and insulin sensitivity. Multiple insulin secretory defects are present, including absence of pulsatility, loss of early phase of insulin secretion after glucose, decreased basal and stimulated plasma insulin concentrations, excess in prohormone secretion, and progressive decrease in insulin secretory capacity with time. beta-cell dysfunction is genetically determined and appears early in the course of the disease. The interplay between insulin secretory defect and insulin resistance is now better understood. In subjects with normal beta-cell function, increase in insulin is compensated by an increase in insulin secretion and plasma glucose levels remain normal. In subjects genetically predisposed to type 2 diabetes, failure of beta-cell to compensate leads to a progressive elevation in plasma glucose levels, then to overt diabetes. When permanent hyperglycaemia is present, progressive severe insulin secretory failure with time ensues, due to glucotoxicity and lipotoxicity, and oxidative stress. A marked reduction in beta-cell mass at post-mortem examination of pancreas of patients with type 2 diabetes has been reported, with an increase in beta-cell apoptosis non-compensated by neogenesis.

The relationship between fasting hyperglycemia and insulin secretion in subjects with normal or impaired glucose tolerance

To assess the relationship between the fasting plasma glucose (FPG) concentration and insulin secretion in normal glucose tolerance (NGT) and impaired glucose tolerance (IGT) subjects, 531 nondiabetic subjects with NGT (n = 293) and IGT (n = 238; 310 Japanese and 232 Mexican Americans) received an oral glucose tolerance test (OGTT) with measurement of plasma glucose, insulin, and C-peptide every 30 min. The insulin secretion rate was determined by plasma C-peptide deconvolution. Insulin sensitivity (Matsuda index) was measured from plasma insulin and glucose concentrations. The insulin secretion/insulin resistance (IS/IR) or disposition index was calculated as DeltaISR/DeltaG / IR. As FPG increased in NGT subjects, the IS/IR index declined exponentially over the range of FPG from 70 to 125 mg/dl. The relationship between the IS/IR index and FPG was best fit with the equation: 28.8 exp(-0.036 FPG). For every 28 mg/dl increase in FPG, the IS/IR index declined by 63%. A similar relationship between IS/IR index and FPG was observed in IGT. However, the decay constant was lower than in NGT. The IS/IR index for early-phase insulin secretion (0-30 min) was correlated with the increase in FPG in both NGT and IGT (r = -0.43, P < 0.0001 and r = -0.20, P = 0.001, respectively). However, the correlation between late-phase insulin secretion (60-120 min) and FPG was not significant. In conclusion, small increments in FPG, within the "normal" range, are associated with a marked decline in glucose-stimulated insulin secretion and the decrease in insulin secretion with increasing FPG is greater in subjects with NGT than IGT and primarily is due to a decline in early-phase insulin secretion.

PDX-1 and MafA play a crucial role in pancreatic beta-cell differentiation and maintenance of mature beta-cell function

Pancreatic and duodenal homeobox factor-1 (PDX-1) plays a crucial role in pancreas development, beta-cell differentiation, and maintenance of mature beta-cell function. PDX-1 expression is maintained in pancreatic precursor cells during pancreas development but becomes restricted to beta-cells in mature pancreas. In mature beta-cells, PDX-1 transactivates the insulin and other genes involved in glucose sensing and metabolism such as GLUT2 and glucokinase. MafA is a recently isolated beta-cell-specific transcription factor which functions as a potent activator of insulin gene transcription. Furthermore, these transcription factors play an important role in induction of insulin-producing cells in various non-beta-cells and thus could be therapeutic targets for diabetes. On the other hand, under diabetic conditions, expression and/or activities of PDX-1 and MafA in beta-cells are reduced, which leads to suppression of insulin biosynthesis and secretion. It is likely that alteration of such transcription factors explains, at least in part, the molecular mechanism for beta-cell glucose toxicity found in diabetes.

Glucolipotoxicity: fuel excess and beta-cell dysfunction

Glucotoxicity, lipotoxicity, and glucolipotoxicity are secondary phenomena that are proposed to play a role in all forms of type 2 diabetes. The underlying concept is that once the primary pathogenesis of diabetes is established, probably involving both genetic and environmental forces, hyperglycemia and very commonly hyperlipidemia ensue and thereafter exert additional damaging or toxic effects on the beta-cell. In addition to their contribution to the deterioration of beta-cell function after the onset of the disease, elevations of plasma fatty acid levels that often accompany insulin resistance may, as glucose levels begin to rise outside of the normal range, also play a pathogenic role in the early stages of the disease. Because hyperglycemia is a prerequisite for lipotoxicity to occur, the term glucolipotoxicity, rather than lipotoxicity, is more appropriate to describe deleterious effects of lipids on beta-cell function. In vitro and in vivo evidence supporting the concept of glucotoxicity is presented first, as well as a description of the underlying mechanisms with an emphasis on the role of oxidative stress. Second, we discuss the functional manifestations of glucolipotoxicity on insulin secretion, insulin gene expression, and beta-cell death, and the role of glucose in the mechanisms of glucolipotoxicity. Finally, we attempt to define the role of these phenomena in the natural history of beta-cell compensation, decompensation, and failure during the course of type 2 diabetes.

The role of IRE1alpha in the degradation of insulin mRNA in pancreatic beta-cells

Background

The endoplasmic reticulum (ER) is a cellular compartment for the biosynthesis and folding of newly synthesized secretory proteins such as insulin. Perturbations to ER homeostasis cause ER stress and subsequently activate cell signaling pathways, collectively known as the Unfolded Protein Response (UPR). IRE1α is a central component of the UPR. In pancreatic β-cells, IRE1α also functions in the regulation of insulin biosynthesis.

Principal Findings

Here we report that hyperactivation of IRE1α caused by chronic high glucose treatment or IRE1α overexpression leads to insulin mRNA degradation in pancreatic β-cells. Inhibition of IRE1α signaling using its dominant negative form prevents insulin mRNA degradation. Islets from mice heterozygous for IRE1α retain expression of more insulin mRNA after chronic high glucose treatment than do their wild-type littermates.

Conclusions/Significance

These results reveal a role of IRE1α in insulin mRNA expression under ER stress conditions caused by chronic high glucose. The rapid degradation of insulin mRNA could provide immediate relief for the ER and free up the translocation machinery. Thus, this mechanism would preserve ER homeostasis and help ensure that the insulin already inside the ER can be properly folded and secreted. This adaptation may be crucial for the maintenance of β-cell homeostasis and may explain why the β-cells of type 2 diabetic patients with chronic hyperglycemia stop producing insulin in the absence of apoptosis. This mechanism may also be involved in suppression of the autoimmune type 1 diabetes by reducing the amount of misfolded insulin, which could be a source of “neo-autoantigens.”

Pleiotropic Roles of PDX-1 in the Pancreas

It is well known that pancreatic and duodenal homeobox factor-1 (PDX-1) plays a pleiotropic role in the pancreas. In the developing pancreas, PDX-1 is involved in both pancreas formation and beta-cell differentiation. In mature beta-cells, PDX-1 transactivates insulin and other beta-cell-related genes such as GLUT2 and glucokinase. Furthermore, PDX-1 plays an important role in the induction of insulin-producing cells in various non-beta-cells and is thereby a possible therapeutic target for diabetes. On the other hand, under diabetic conditions, expression and/or activity of PDX-1 in beta-cells is reduced, which leads to suppression of insulin biosynthesis and secretion. It is likely that PDX-1 inactivation explains, at least in part, the molecular mechanism for beta-cell glucose toxicity found in diabetes.

Regulation of insulin gene expression and insulin production in Nile tilapia (Oreochromis niloticus)

Compared to mammals, little is known about insulin gene expression in fish. Using transient transfection experiments and mammalian insulinoma cell lines we demonstrate that transcription of the Nile tilapia (Oreochromis niloticus) insulin gene is (a) regulated in a beta-cell-specific manner; and (b) not sensitive to the glucose stimulations. Deletion analysis of the 1575 bp 5' insulin gene flanking sequence revealed that cooperative interactions between regulatory elements within the proximal (-1 to -396 bp) and the distal (-396 bp to -1575 bp) promoter regions were necessary for induction of the beta-cell-specific transcription. Effects of glucose and arginine on endogenous insulin secretion, translation, and transcription in isolated tilapia Brockmann bodies were determined using Northern hybridization, Western analysis, and quantitative RT-PCR. Similar to the regulation of mammalian insulin, we found that increases of glucose (1-70 mM) and arginine (0.4-25 mM) induced insulin secretion. However, transcription of the insulin gene was activated only by extremely high concentrations of glucose and arginine added simultaneously. When stimulated for 24 h with low concentrations of both inducers or with either of them added separately, tilapia beta-cells were able to replenish secreted insulin and to maintain insulin stores at a constant level without elevations of the insulin mRNA levels. Since the basal level of insulin mRNA was approximately 3.7-fold higher in tilapia beta-cells than it is in mammalian beta-cells, insulin production in tilapia cells probably relies on an enlarged intracellular insulin mRNA pool and does not require the transcriptional activation of the insulin gene.

Chronic oxidative stress as a mechanism for glucose toxicity of the beta cell in type 2 diabetes

Type 2 diabetes is characterized by a relentless decline in pancreatic islet beta cell function and worsening hyperglycemia despite optimal medical treatment. Our central hypothesis is that residual hyperglycemia, especially after meals, generates reactive oxygen species (ROS), which in turn causes chronic oxidative stress on the beta cell. This hypothesis is supported by several observations. Exposure of isolated islets to high glucose concentrations induces increases in intracellular peroxide levels. The beta cell has very low intrinsic levels of antioxidant proteins and activities and thus is very vulnerable to ROS. Treatment with antioxidants protects animal models of type 2 diabetes against complete development of phenotypic hyperglycemia. The molecular mechanisms responsible for the glucose toxic effect on beta cell function involves disappearance of two important regulators of insulin promoter activity, PDX-1 and MafA. Antioxidant treatment in vitro prevents disappearance of these two transcription factors and normalizes insulin gene expression. These observations suggest that the ancillary treatment with antioxidants may improve outcomes of standard therapy of type 2 diabetes in humans.

The expanding role of oxidative stress, renin angiotensin system, and beta-cell dysfunction in the cardiometabolic syndrome and Type 2 diabetes mellitus

The incidence of obesity, cardiometabolic syndrome (CMS), and type 2 diabetes mellitus (DM2), as well as their devastating cardiovascular consequences, keep rising with increasing human and economical costs. For a long time, insulin resistance has been the main player in the pathogenesis and treatment of DM2, but every day more knowledge is gained about the central role of beta-cell failure, not only in the appearance of hyperglycemia but also in the failure of the pharmacological therapy. beta-Cell failure implies impairment of glucosestimulated insulin secretion and loss of beta-cell mass. Hyperglycemia, elevated circulating fatty acids, inadequate local activation of renin angiotensin system, and chronic low grade inflammation are conditions that coexist in the CMS and DM2 that turn out to be deleterious for the beta-cell functioning and existance. Excessive oxidative stress secondary to increased production of reactive oxygen species and decreased availability of antioxidants is a possible common converging point of the multiple noxious stimuli. Activation of the NADPH oxidase complex secondary to angiotensin II stimulation is of interest, as its pharmacological blockade has beneficial effects. New knowledge about the intimate mechanisms of oxidative-stress induced beta-cell failure will provide new therapeutic targets against CMS and DM2.

Metabolic syndrome: psychosocial, neuroendocrine, and classical risk factors in type 2 diabetes

This article summarizes some aspects of stress in the metabolic syndrome at the psychosocial, tissue, and cellular levels. The metabolic syndrome is a valuable research concept for studying population health and social-biological translation. The cluster of cardiovascular risk factors labeled the metabolic syndrome is linked with low socioeconomic status. Systematic differences in diet and physical activity contribute to social patterning of the syndrome. In addition, psychosocial factors including chronic work stress are linked with its development. Psychosocial factors could lead to metabolic perturbations and increase cardiovascular risk via activation of neuroendocrine responses, for example, in the autonomic nervous system and in several hormonal pathways. High glucocorticoid levels will promote lipid storage in visceral rather than subcutaneous adipose tissue. Adipocytes secrete several proinflammatory cytokines, which considered major contributors to increase in oxidants and cell injury. Upregulation of heme oxygenase 1 (HO-1) and peroxidase in the early development of diabetes produces a decrease in oxidative-mediated injury. Increased HO activity is associated with a significant decrease in superoxide, endothelial cell shedding and blood pressure. Finally, it is proposed that overexpression of glutathione peroxidase in beta cells may protect beta cell deterioration from oxidative stress during development of diabetes and hyperglycemia and this may result in attenuation of beta cell failure. If this proves to be the case, then the scene will be set to develop glutathione peroxidase mimetics for use in preclinical and clinical trials.

Pancreatic islet beta-cell and oxidative stress: the importance of glutathione peroxidase

Pancreatic beta-cell function continuously deteriorates in type 2 diabetes despite optimal treatment regimens, which has been attributed to hyperglycemia itself via formation of excess levels of reactive oxygen species (ROS). Glutathione peroxidase GPx), by virtue of its ability to catabolize both H(2)O(2) and lipid peroxides, is uniquely positioned to protect tissues from ROS. The level of this antioxidant in beta cells is extremely low and overexpression of GPx in islets provides enhanced protection against oxidative stress. This suggests that GPx mimetics may represent a valuable ancillary treatment that could add a novel layer of protection for the beta-cell.

Impact of mitochondrial ROS production in the pathogenesis of diabetes mellitus and its complications

In this review, the impacts of mitochondrial reactive oxygen species (ROS) on diabetes and its complications are described. In endothelial cells, high-glucose treatment increases mitochondrial ROS and normalization of the ROS production by inhibitors of mitochondrial metabolism, or by overexpression of UCP-1 or MnSOD, prevents glucose-induced activation of PKC, formation of AGE, and accumulation of sorbitol, all of which are believed to be the main molecular mechanisms of diabetic complications. Glomerular hyperfiltration, one of the characteristics of early diabetic nephropathy, may be caused by mitochondrial ROS through activation of COX-2 gene transcription, followed by PGE2 overproduction. In pancreatic beta cells, hyperglycemia also increases mitochondrial ROS, which suppresses the first phase of glucose-induced insulin secretion, at least in part, through the suppression of GAPDH activity. In liver cells, similar to that in hyperglycemia, TNF-alpha increases mitochondrial ROS, which in turn activates apoptosis signal-regulating kinase 1 (ASK1) and c-jun NH2-terminal kinases (JNK), increases serine phosphorylation of IRS-1, and decreases insulin-stimulated tyrosine phosphorylation of IRS-1, leading to insulin resistance. These results suggest the importance of mitochondrial ROS in the pathogenesis of diabetes mellitus and its complications through modification of various cellular events in many tissues, including vessels, kidney, pancreatic beta cells, and liver.

Glucotoxicity in the INS-1 rat insulinoma cell line is mediated by the orphan nuclear receptor small heterodimer partner

Prolonged elevations of glucose concentration have deleterious effects on beta-cell function. One of the hallmarks of such glucotoxicity is a reduction in insulin gene expression, resulting from decreased insulin promoter activity. Small heterodimer partner (SHP; NR0B2) is an atypical orphan nuclear receptor that inhibits nuclear receptor signaling in diverse metabolic pathways. In this study, we found that sustained culture of INS-1 cells at high glucose concentrations leads to an increase in SHP mRNA expression, followed by a decrease in insulin gene expression. Inhibition of endogenous SHP gene expression by small interfering RNA partially restored high-glucose-induced suppression of the insulin gene. Adenovirus-mediated overexpression of SHP in INS-1 cells impaired glucose-stimulated insulin secretion as well as insulin gene expression. SHP downregulates insulin gene expression via two mechanisms: by downregulating PDX-1 and MafA gene expression and by inhibiting p300-mediated pancreatic duodenal homeobox factor 1-and BETA2-dependent transcriptional activity from the insulin promoter. Finally, the pancreatic islets of diabetic OLETF rats express SHP mRNA at higher levels than the islets from LETO rats. These results collectively suggest that SHP plays an important role in the development of beta-cell dysfunction induced by glucotoxicity.

Glucocorticoid receptor mediated repression of human insulin gene expression is regulated by PGC-1α

Transcriptional regulation of the insulin gene plays a critical role in maintenance of pancreatic beta cell function in response to various stimuli. Here, we used INS-1 cells to test the hypothesis that PGC-1alpha regulates human insulin gene transcription by modulating glucocorticoid (GR) binding to the insulin gene promoter. Analysis of the human insulin promoter region revealed that the suppressive region regulated by GR and PGC-1alpha is localized from -362 to -257 bp. To locate the GR binding site in the human insulin promoter region, EMSAs were performed with candidate GR binding sequences and confirmed that a palindromic region (Palin, -284 to -274 bp) specifically interacts with GR. We also found that the Palin-binding activity of GR is increased in the presence of PGC-1alpha. These findings suggest that PGC-1alpha elevates the binding of GR to Palin and thereby enhances the GR-mediated inhibition of human insulin transcription.

Glucose induces MafA expression in pancreatic beta cell lines via the hexosamine biosynthetic pathway

MafA is a basic leucine zipper transcription factor that regulates gene expression in both the neuroretina and pancreas. Within the pancreas, MafA is exclusively expressed in the beta cells and is involved in insulin gene transcription, insulin secretion, and beta cell survival. The expression of the mafA gene within beta cells is known to increase in response to high glucose levels by an unknown mechanism. In this study, we demonstrate that pyruvate, which is produced by glycolysis from glucose, is not sufficient to induce mafA gene expression compared with high glucose. This suggests that the signal for MafA induction is independent of ATP levels and that a metabolic event occurring upstream of pyruvate production leads to the induction of MafA. Furthermore, insulin secretion mediated by high glucose is not important for MafA expression. However, the addition of glucosamine to beta cell lines stimulates MafA expression in the absence of high glucose, and inhibition of the hexosamine biosynthetic pathway in the presence of high glucose abolishes MafA induction. Moreover, we demonstrate that the expression of UDP-N-acetylglucosaminyl transferase, the enzyme mediating O-linked glycosylation of cytosolic and nuclear proteins, is essential for glucose-dependent MafA expression. Consistent with this observation, inhibition of N-acetylglucosaminidase, the enzyme involved in the removal of the O-GlcNAc modification from proteins, with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate stimulates MafA expression under low glucose conditions. The presented data suggest that MafA expression mediated by high glucose requires flux through the hexosamine biosynthetic pathway and the O-linked glycosylation of an unknown protein(s) by UDP-N-acetylglucosaminyl transferase.

PDX-1 and MafA in β-cell differentiation and dysfunction

Pancreatic and duodenal homeobox factor-1 (PDX-1) plays crucial roles in pancreas development and β-cell differentiation, and in maintaining mature β-cell function. MafA is a recently isolated β-cell-specific transcription factor that functions as a potent activator of insulin gene transcription. Also, these pancreatic transcription factors play a crucial role in inducing surrogate β-cells from non-β-cells and, thus, could be therapeutic targets for diabetes. Conversely, expression and/or activities of PDX-1 and MafA in β-cells are reduced under diabetic conditions, which leads to suppression of insulin biosynthesis and secretion. It is likely that alteration of such transcription factors explains, at least in part, the molecular mechanism for β-cell glucose toxicity.

Diabetes, glucose toxicity, and oxidative stress: A case of double jeopardy for the pancreatic islet beta cell

Diabetes is commonly referred to in terms of type 1 and type 2. Both forms involve pancreatic islet beta-cell abnormalities, characterized by death in type 1 and accelerated apoptosis in type 2. The resultant chronic hyperglycemia leads to chronic oxidative stress for all tissues because glucose in abnormally high concentrations forms reactive oxygen species. It has been repeatedly emphasized that this can lead to oxidative damage in the classical secondary targets of diabetes, such as eyes, kidneys, nerves, and blood vessels. However, it has been much less appreciated that the beta cell itself is also a prime target, a case of double jeopardy. This situation is all the more pernicious because islets contain among the lowest levels of antioxidant enzyme activities compared to other tissues. This adverse effect of high glucose concentrations is referred to as glucose toxicity. A major manifestation of glucose toxicity in the beta cell is defective insulin gene expression, diminished insulin content, and defective insulin secretion. The molecular mechanisms involve the development of decreased levels of two very important insulin promoter transcription factors, PDX-1 and MafA. Studies with animal models of type 2 diabetes have established that pharmacologic protection against oxidative stress ameliorates the severity of diabetes progression. Translational research with humans is now under way to ascertain whether this protection can be provided to patients experiencing inadequate glycemic control.

Contributions of beta-cell dysfunction and insulin resistance to the pathogenesis of impaired glucose tolerance and impaired fasting glucose

Impaired glucose tolerance (IGT) and impaired fasting glucose (IFG) are intermediate states in glucose metabolism that exist between normal glucose tolerance and overt diabetes. Epidemiological studies demonstrate that the two categories describe distinct populations with only partial overlap, suggesting that different metabolic abnormalities characterize IGT and IFG. Insulin resistance and impaired beta-cell function, the primary defects observed in type 2 diabetes, both can be detected in subjects with IGT and IFG. However, clinical studies suggest that the site of insulin resistance varies between the two disorders. While subjects with IGT have marked muscle insulin resistance with only mild hepatic insulin resistance, subjects with IFG have severe hepatic insulin resistance with normal or near-normal muscle insulin sensitivity. Both IFG and IGT are characterized by a reduction in early-phase insulin secretion, while subjects with IGT also have impaired late-phase insulin secretion. The distinct metabolic features present in subjects with IFG and IGT may require different therapeutic interventions to prevent their progression to type 2 diabetes.

MafA expression and insulin promoter activity are induced by nicotinamide and related compounds in INS-1 pancreatic beta-cells

Nicotinamide has been reported to induce differentiation of precursor/stem cells toward a beta-cell phenotype, increase islet regeneration, and enhance insulin biosynthesis. Exposure of INS-1 beta-cells to elevated glucose leads to reduced insulin gene transcription, and this is associated with diminished binding of pancreatic duodenal homeobox factor 1 (PDX-1) and mammalian homologue of avian MafA/l-Maf (MafA). Nicotinamide and other low-potency poly(ADP-ribose) polymerase (PARP) inhibitors were thus tested for their ability to restore insulin promoter activity. The low-potency PARP inhibitors nicotinamide, 3-aminobenzamide, or PD128763 increased expression of a human insulin reporter gene suppressed by elevated glucose. In contrast, the potent PARP-1 inhibitors PJ34 or INO-1001 had no effect on promoter activity. Antioxidants, including N-acetylcysteine, lipoic acid, or quercetin, only minimally induced the insulin promoter. Site-directed mutations of the human insulin promoter mapped the low-potency PARP inhibitor response to the C1 element, which serves as a MafA binding site. INS-1 cells exposed to elevated glucose had markedly reduced MafA protein and mRNA levels. Low-potency PARP inhibitors restored MafA mRNA and protein levels, but they had no affect on PDX-1 protein levels or binding activity. Increased MafA expression by low-potency PARP inhibitors was independent of increased MafA protein or mRNA stability. These data suggest that low-potency PARP inhibitors increase insulin biosynthesis, in part, through a mechanism involving increased MafA gene transcription.

Role of the renin-angiotensin system in the endocrine pancreas: implications for the development of diabetes

Activation of the renin-angiotensin system has a pivotal role in the pathogenesis of diabetic complications. However, recent evidence suggests that it may also contribute to the development of diabetes itself. In the endocrine pancreas, all the components of an active renin-angiotensin system are present, which modulate a range of activities including local blood flow, hormone release and prostaglandin synthesis. In both types 1 and 2 diabetes, there is an up-regulation of its expression and activity in the endocrine pancreas. Whether these changes have a direct pathogenetic role or reflect a response to local stress or tissue injury remains to be established. Angiotensin-mediated increases in oxidative stress, inflammation and free fatty acids levels potentially contribute to beta-cell dysfunction in diabetes. In addition, activation of the renin-angiotensin system appears to potentiate the action of other pathogenic pathways including glucotoxicity, lipotoxicity and advanced glycation. In experimental models of type 2 diabetes, blockade of the renin-angiotensin system with angiotensin converting enzyme inhibitors or angiotensin receptor antagonists results in the improvement of islet structure and function. Moreover, the incidence of de novo diabetes appears to be significantly reduced by blockade of the renin-angiotensin system in clinical studies. At least two large controlled trials are currently underway to study the role of renin-angiotensin system in the development of diabetes. It is hoped that these studies will demonstrate the true potential of the blockade of the renin-angiotensin system for the prevention of diabetes.

ERK1/2-dependent Activation of Transcription Factors Required for Acute and Chronic Effects of Glucose on the Insulin Gene Promoter

The insulin promoter is both positively and negatively regulated in response to conditions to which pancreatic beta-cells are exposed. Exposure of intact rat islets and INS-1 pancreatic beta-cells to 11 mm glucose for minutes to hours results in an enhancement in the rate of insulin gene transcription assessed with a reporter linked to the insulin gene promoter. In contrast, chronic exposure of rat islets or beta-cells to 11 mm glucose results in loss of the glucose responsiveness of the insulin gene promoter. By 48 h, glucose inhibits insulin gene promoter activity. Here we show that not only the acute effect of elevated glucose to stimulate the insulin gene promoter but also the chronic effect of elevated glucose to inhibit the insulin gene promoter depend on ERK1/2 mitogen-activated protein kinase activity. In examining the underlying mechanism, we found that acute exposure to 11 mm glucose resulted in the binding of the transcription factors NFAT and Maf to the glucose-responsive A2C1 element of the insulin gene promoter. An NFAT and C/EBP-beta complex was observed in cells chronically exposed to 11 mm glucose. Formation of NFAT-Maf and NFAT-C/EBP-beta complexes was sensitive to inhibitors of ERK1/2 and calcineurin, consistent with our previous finding that activation of ERK1/2 by glucose required calcineurin activity and the well documented regulation of NFAT by calcineurin. These results indicate that the ERK1/2 pathway modulates partners of NFAT, which may either stimulate or repress insulin gene transcription during stimulatory and chronic exposure to elevated glucose.

Elevated cAMP level attenuates 2-deoxy-d-ribose-induced oxidative damage in pancreatic beta-cells

Glucose toxicity to pancreatic beta-cells is defined as irreversible beta-cell damage, including apoptosis, caused by chronic exposure to high glucose levels in type 2 diabetes. Oxidative stress is an important mechanism for glucose toxicity to pancreatic beta-cells. Reducing sugars produce reactive oxygen species through autoxidation and protein glycosylation. 2-Deoxy-d-ribose (dRib) is a reducing sugar with high reactivity. We investigated whether cAMP-stimulating agents could protect beta-cells from dRib-induced oxidative damage. HIT-T15 cells were cultured with various concentrations of dRib for 24 h. We measured cell survival, intracellular cAMP and H2O2 levels, and apoptosis. dRib decreased cell survival in a dose- and time-dependent manner and markedly increased intracellular H2O2 levels and apoptosis. N-Acetyl-l-cysteine decreased dRib-induced rises in intracellular H2O2 and apoptosis to control levels. Forskolin, IBMX, and dbcAMP markedly elevated intracellular cAMP levels and significantly attenuated dRib-induced cytotoxicity and apoptosis, but had no influence on the dRib-induced rise in intracellular H2O2 levels. These results demonstrate that dRib produced oxidative stress and apoptosis in pancreatic beta-cells and that elevated intracellular cAMP levels reduced dRib-induced damage, independent of reactive oxygen species metabolism.

Free fatty acids increase PGC-1alpha expression in isolated rat islets

PGC-1alpha mRNA and protein are elevated in islets from multiple animal models of diabetes. Overexpression of PGC-1alpha impairs glucose-stimulated insulin secretion (GSIS). However, it is not well known which metabolic events lead to upregulation of PGC-1alpha in the beta-cells under pathophysiological condition. In present study, we have investigated effects of chronic hyperlipidemia and hyperglycemia on PGC-1alpha mRNA expression in isolated rat islets. Isolated rat islets are chronically incubated with 0, 0.2 and 0.4 mM oleic acid/palmitic acid (free fatty acids, FFA) or 5.5 and 25 mM glucose for 72 h. FFA dose-dependently increases PGC-1alpha mRNA expression level in isolated islets. FFA also increases PGC-1alpha expression in mouse beta-cell-derived beta TC3 cell line. In contrast, 25 mM glucose decreases expression level of PGC-1alpha. Inhibition of PGC-1alpha by siRNA improves FFA-induced impairment of GSIS in islets. These data suggest that hyperlipidemia and hyperglycemia regulate PGC-1alpha expression in islets differently, and elevated PGC-1alpha by FFA plays an important role in chronic hyperlipidemia-induced beta-cell dysfunction.

PDX-1/VP16 fusion protein, together with NeuroD or Ngn3, markedly induces insulin gene transcription and ameliorates glucose tolerance

Diabetes is the most prevalent and serious metabolic disease, and the number of diabetic patients worldwide is increasing. The reduction of insulin biosynthesis in pancreatic beta-cells is closely associated with the onset and progression of diabetes, and thus it is important to search for ways to induce insulin-producing cells in non-beta-cells. In this study, we showed that a modified form of the pancreatic and duodenal homeobox factor 1 (PDX-1) carrying the VP16 transcriptional activation domain (PDX-1/VP16) markedly increases insulin biosynthesis and induces various pancreas-related factors in the liver, especially in the presence of NeuroD or neurogenin 3 (Ngn3). Furthermore, in streptozotocin-induced diabetic mice, PDX-1/VP16 overexpression, together with NeuroD or Ngn3, drastically ameliorated glucose tolerance. Thus PDX-1/VP16 expression, together with NeuroD or Ngn3, markedly induces insulin gene transcription and ameliorates glucose tolerance. This approach warrants further investigation and may have utility in the treatment of diabetes.

Oxidative stress-mediated, post-translational loss of MafA protein as a contributing mechanism to loss of insulin gene expression in glucotoxic beta cells

Glucose toxicity in pancreatic islet beta cells causes loss of insulin gene expression, content, and secretion due to loss of binding of transcription factors, most notably PDX-1 and RIPE-3b1 activator, to the promoter region of the insulin gene. Recently, RIPE-3b1 activator was cloned and identified as the mammalian homologue of avian MafA/Maf-L (MafA). This enabled us to carry out more extensive studies of the role of MafA in glucotoxicity than were hitherto possible. Northern analysis of glucotoxic HIT-T15 cells revealed normal amounts of MafA mRNA, but Western analysis demonstrated a 97 +/- 1% reduction in MafA protein (p < 0.0001). The proteasome is a likely site for MafA degradation as lactacystin, an irreversible proteasome inhibitor, caused an accumulation of MafA protein. Antioxidants have previously been shown to prevent the adverse effects of glucose toxicity on beta cell function both in vivo and in vitro. In the current study, chronic culturing of HIT-T15 cells with the antioxidant N-acetylcysteine (NAC) prevented loss of MafA protein (late passage = 18.9 +/- 10.4% of early passage, p < 0.001; late passage with NAC = 68.7 +/- 19.7% of early passage, p = not significant) and loss of DNA binding (late passage = 63.7 +/- 9% of early passage, p < 0.02; late passage with NAC = 116 +/- 10% of early passage, p = not significant). Additionally, transient transfection of PDX-1 or MafA cDNA into glucotoxic cells increased PDX-1 and MafA protein levels and individually increased insulin promoter activity (untreated = 34%, PDX-1 = 70%, MafA = 78%; percentage of activity of early passage cells), whereas the combined transfection of MafA and PDX-1 completely restored insulin promoter activity. This recovery of promoter activity following transient transfection had no effect on endogenous insulin mRNA. However, adenoviral infection of MafA and PDX-1 significantly increased endogenous insulin mRNA levels by 93% (121 +/- 9 versus 233 +/- 18 density light units; n = 5, p < 0.001). We conclude that the absence of MafA protein from beta cells via chronic oxidative stress contributes importantly to the loss of endogenous insulin gene expression as glucose toxicity develops.

Cell lines derived from pancreatic islets

The islets of Langerhans play a major role in control of metabolic fuel homeostasis. The rapid increase in incidence of diabetes worldwide has spurred renewed interest in islet cell biology. However, gaining a detailed understanding of islet function at a molecular and biochemical level has been complicated by the difficulty and high cost associated with isolation of pancreatic islets. Until recently, islet-derived cell lines have represented sub-optimal surrogates for primary cells for functional studies due to their undifferentiated or unstable phenotypic features. New approaches have resulted in isolation and characterization of rodent insulinoma cell lines that retain many key functional attributes of normal islets and have become useful tools in the study of islet cell biology.

d-Glyceraldehyde Causes Production of Intracellular Peroxide in Pancreatic Islets, Oxidative Stress, and Defective Beta Cell Function via Non-mitochondrial Pathways

D-Glyceraldehyde (D-GLYC) is usually considered to be a stimulator of insulin secretion but theoretically can also form reactive oxygen species (ROS), which can inhibit beta cell function. We examined the time- and concentration-dependent effects of D-GLYC on insulin secretion, insulin content, and formation of ROS. We observed that a 2-h exposure to 0.05-2 mM D-GLYC potentiated glucose-stimulated insulin secretion (GSIS) in isolated Wistar rat islets but that higher concentrations inhibited GSIS. A 24-h exposure to 2 mm D-GLYC inhibited GSIS, decreased insulin content, and increased intracellular peroxide levels (2.14 +/- 0.31-fold increase, n = 4, p < 0.05). N-Acetylcysteine (10 mM) prevented the increase in intracellular peroxides and the adverse effects of d-GLYC on GSIS. In the presence of 11.1 but not 3.0 mm glucose, koningic acid (10 microM), a specific glyceraldehyde-3-phosphate dehydrogenase inhibitor, increased intracellular peroxide levels (1.88 +/- 0.30-fold increase, n = 9, p < 0.01) and inhibited GSIS (control GSIS = p < 0.001; koningic acid GSIS, not significant). To determine whether oxidative phosphorylation was the source of ROS formation, we cultured rat islets with mitochondrial inhibitors. Neither rotenone or myxothiazol prevented D-GLYC-induced increases in islet ROS. Adenoviral overexpression of manganese superoxide dismutase also failed to prevent the effect of D-GLYC to increase ROS levels. These observations indicate that exposure to excess D-GLYC increases reactive oxygen species in the islet via non-mitochondrial pathways and suggest the hypothesis that the oxidative stress associated with elevated D-GLYC levels could be a mechanism for glucose toxicity in beta cells exposed chronically to high glucose concentrations.

[Pathogenesis of type 2 diabetes mellitus]

"Common" type 2 diabetes mellitus is a multifactorial disease. Hyperglycemia is related to a decrease in glucose peripheral uptake, and to an increase in hepatic glucose production, due to reduced insulin secretion and insulin sensitivity. Multiple insulin secretory defects are present, including loss of basal pulsatility, lack of early phase of insulin secretion after intravenous glucose administration, decreased basal and stimulated plasma insulin concentrations, excess in prohormone secretion, and progressive decrease in insulin secretory capacity with time. These genetically determined abnormalities appear early in the course of the disease. Insulin resistance affects muscle, liver, and adipose tissue. For the same plasma insulin levels, peripheral glucose uptake and hepatic glucose production suppressibility are lower in diabetic patients than in controls. It results from aging of the population and from "western" lifestyle, with progressive increase in mean body weight, due to excess in energy intake, decreased energy expenses and low physical activity level.

NEW ASPECTS

The role of beta-cell dysfunction, as well as the interplay between insulin secretory defect and insulin resistance are now better understood. In subjects with normal beta-cell function, increase in insulin needs secondary to insulin resistance is compensated by an increase in insulin secretion adjusted to maintain plasma glucose levels to normal. In subjects genetically predisposed to type 2 diabetes, failure of beta-cell to compensate for increased needs is responsible for a progressive elevation in plasma glucose levels, then for overt type 2 diabetes. This adaptative phenomenon is called beta-cell compensation of insulin resistance. The lack of compensation is responsible for type 2 diabetes. When permanent hyperglycemia is present, progressive insulin secretory failure with time ensues, due to glucotoxicity and to lipotoxicity.

PERSPECTIVES

Simple changes in lifestyle, such regular moderate physical activity, and control of body weight, should permit to avoid the explosion in prevalence of type 2 diabetes. This has been evidenced by the results of prospective studies aiming at preventing conversion from impaired glucose tolerance to diabetes. In patients with permanent hyperglycemia not controlled by lifestyle changes, metabolic defects are the targets of specific therapy intervention with antidiabetic oral agents, such as insulin secretagogues, insulin sensitizers, and inhibitors of hepatic glucose production.

Reduced beta-cell mass and altered glucose sensing impair insulin-secretory function in betaIRKO mice

Pancreatic beta-cell-restricted knockout of the insulin receptor results in hyperglycemia due to impaired insulin secretion, suggesting that this cell is an important target of insulin action. The present studies were undertaken in beta-cell insulin receptor knockout (betaIRKO) mice to define the mechanisms underlying the defect in insulin secretion. On the basis of responses to intraperitoneal glucose, approximately 7-mo-old betaIRKO mice were either diabetic (25%) or normally glucose tolerant (75%). Total insulin content was profoundly reduced in pancreata of mutant mice compared with controls. Both groups also exhibited reduced beta-cell mass and islet number. However, insulin mRNA and protein were similar in islets of diabetic and normoglycemic betaIRKO mice compared with controls. Insulin secretion in response to insulin secretagogues from the isolated perfused pancreas was markedly reduced in the diabetic betaIRKOs and to a lesser degree in the nondiabetic betaIRKO group. Pancreatic islets of nondiabetic betaIRKO animals also exhibited defects in glyceraldehyde- and KCl-stimulated insulin release that were milder than in the diabetic animals. Gene expression analysis of islets revealed a modest reduction of GLUT2 and glucokinase gene expression in both the nondiabetic and diabetic mutants. Taken together, these data indicate that loss of functional receptors for insulin in beta-cells leads primarily to profound defects in postnatal beta-cell growth. In addition, altered glucose sensing may also contribute to defective insulin secretion in mutant animals that develop diabetes.