Concordant glucose induction of glucokinase, glucose usage, and glucose-stimulated insulin release in pancreatic islets maintained in organ culture

(Re-)Viewing Role of Intracellular Glucose Beyond Extracellular Regulation of Glucose-Stimulated Insulin Secretion by Pancreatic Cells

For glucose-stimulated insulin secretion (GSIS) by pancreatic β-cells in animals, it is believed that ATP generated from glucose metabolism is primarily responsible. However, this ignores two well-established aspects in literature: (a) intracellular ATP generation from other sources resulting in an overall pool of ATP, regardless of the original source, and (b) that intracellular glucose transport is 10- to 100-fold higher than intracellular glucose phosphorylation in β-cells. The latter especially provides an earlier unaddressed, but highly appealing, observation pertaining to (at least transient) the presence of intracellular glucose molecules. Could these intracellular glucose molecules be responsible for the specificity of GSIS to glucose (instead of the widely believed ATP production from its metabolism)? In this work, we provide a comprehensive compilation of literature on glucose and GSIS using various cellular systems - all studies focus only on the extracellular role of glucose in GSIS. Further, we carried out a comprehensive analysis of differential gene expression in Mouse Insulinoma 6 (MIN6) cells, exposed to low and high extracellular glucose concentrations (EGC), from the existing whole transcriptome data. The expression of other genes involved in glycolysis, Krebs cycle, and electron transport chain was found to be unaffected by EGC, except Gapdh, Atp6v0a4, and Cox20. Remarkably, 3 upregulated genes (Atp6v0a4, Cacnb4, Kif11) in high EGC were identified to have an association with cellular secretion. Using glucose as a possible ligand for the 3 proteins, computational investigations were carried out (that will require future 'wet validation', both in vitro and in vivo, e.g., using primary islets and animal models). The glucose-affinity/binding scores (in kcal/mol) obtained were also compared with glucose binding scores for positive controls (GCK and GLUT2), along with negative controls (RPA1, KU70-80, POLA1, ACAA1A, POLR1A). The binding affinity scores of glucose molecules for the 3 proteins were found to be closer to positive controls. Therefore, we report the glucose binding ability of 3 secretion-related proteins and a possible direct role of intracellular glucose molecules in GSIS.

The physiological role of β-cell heterogeneity in pancreatic islet function

Endocrine cells within the pancreatic islets of Langerhans are heterogeneous in terms of transcriptional profile, protein expression and the regulation of hormone release. Even though this heterogeneity has long been appreciated, only within the past 5 years have detailed molecular analyses led to an improved understanding of its basis. Although we are beginning to recognize why some subpopulations of endocrine cells are phenotypically different to others, arguably the most important consideration is how this heterogeneity affects the regulation of hormone release to control the homeostasis of glucose and other energy-rich nutrients. The focus of this Review is the description of how endocrine cell heterogeneity (and principally that of insulin-secreting β-cells) affects the regulation of hormone secretion within the islets of Langerhans. This discussion includes an overview of the functional characteristics of the different islet cell subpopulations and describes how they can communicate to influence islet function under basal and glucose-stimulated conditions. We further discuss how changes to the specific islet cell subpopulations or their numbers might underlie islet dysfunction in type 2 diabetes mellitus. We conclude with a discussion of several key open questions regarding the physiological role of islet cell heterogeneity.

Glucose transporters in pancreatic islets

The fine-tuning of glucose uptake mechanisms is rendered by various glucose transporters with distinct transport characteristics. In the pancreatic islet, facilitative diffusion glucose transporters (GLUTs), and sodium-glucose cotransporters (SGLTs) contribute to glucose uptake and represent important components in the glucose-stimulated hormone release from endocrine cells, therefore playing a crucial role in blood glucose homeostasis. This review summarizes the current knowledge about cell type-specific expression profiles as well as proven and putative functions of distinct GLUT and SGLT family members in the human and rodent pancreatic islet and further discusses their possible involvement in onset and progression of diabetes mellitus. In context of GLUTs, we focus on GLUT2, characterizing the main glucose transporter in insulin-secreting β-cells in rodents. In addition, we discuss recent data proposing that other GLUT family members, namely GLUT1 and GLUT3, render this task in humans. Finally, we summarize latest information about SGLT1 and SGLT2 as representatives of the SGLT family that have been reported to be expressed predominantly in the α-cell population with a suggested functional role in the regulation of glucagon release.

Reducing Glucokinase Activity to Enhance Insulin Secretion: A Counterintuitive Theory to Preserve Cellular Function and Glucose Homeostasis

Pancreatic beta-cells are the only cells in the body that can synthesize and secrete insulin. Through the process of glucose-stimulated insulin secretion, beta-cells release insulin into circulation, stimulating GLUT4-dependent glucose uptake into peripheral tissue. Insulin is normally secreted in pulses that promote signaling at the liver. Long before type 2 diabetes is diagnosed, beta-cells become oversensitive to glucose, causing impaired pulsatility and overstimulation in fasting levels of glucose. The resulting hypersecretion of insulin can cause poor insulin signaling and clearance at the liver, leading to hyperinsulinemia and insulin resistance. Continued overactivity can eventually lead to beta-cell exhaustion and failure at which point type 2 diabetes begins. To prevent or reverse the negative effects of overstimulation, beta-cell activity can be reduced. Clinical studies have revealed the potential of beta-cell rest to reverse new cases of diabetes, but treatments lack durable benefits. In this perspective, we propose an intervention that reduces overactive glucokinase activity in the beta-cell. Glucokinase is known as the glucose sensor of the beta-cell due to its high control over insulin secretion. Therefore, glycolytic overactivity may be responsible for hyperinsulinemia early in the disease and can be reduced to restore normal stimulus-secretion coupling. We have previously reported that reducing glucokinase activity in prediabetic mouse islets can restore pulsatility and enhance insulin secretion. Building on this counterintuitive finding, we review the importance of pulsatile insulin secretion and highlight how normalizing glucose sensing in the beta cell during prediabetic hyperinsulinemia may restore pulsatility and improve glucose homeostasis.

The Central Role of Glucokinase in Glucose Homeostasis: A Perspective 50 Years After Demonstrating the Presence of the Enzyme in Islets of Langerhans

It is hypothesized that glucokinase (GCK) is the glucose sensor not only for regulation of insulin release by pancreatic β-cells, but also for the rest of the cells that contribute to glucose homeostasis in mammals. This includes other cells in endocrine pancreas (α- and δ-cells), adrenal gland, glucose sensitive neurons, entero-endocrine cells, and cells in the anterior pituitary. Glucose transport is by facilitated diffusion and is not rate limiting. Once inside, glucose is phosphorylated to glucose-6-phosphate by GCK in a reaction that is dependent on glucose throughout the physiological range of concentrations, is irreversible, and not product inhibited. High glycerol phosphate shuttle, pyruvate dehydrogenase, and pyruvate carboxylase activities, combined with low pentose-P shunt, lactate dehydrogenase, plasma membrane monocarboxylate transport, and glycogen synthase activities constrain glucose-6-phosphate to being metabolized through glycolysis. Under these conditions, glycolysis produces mostly pyruvate and little lactate. Pyruvate either enters the citric acid cycle through pyruvate dehydrogenase or is carboxylated by pyruvate carboxylase. Reducing equivalents from glycolysis enter oxidative phosphorylation through both the glycerol phosphate shuttle and citric acid cycle. Raising glucose concentration increases intramitochondrial [NADH]/[NAD⁺] and thereby the energy state ([ATP]/[ADP][Pi]), decreasing [Mg²⁺ADP] and [AMP]. [Mg²⁺ADP] acts through control of K_ATP channel conductance, whereas [AMP] acts through regulation of AMP-dependent protein kinase. Specific roles of different cell types are determined by the diverse molecular mechanisms used to couple energy state to cell specific responses. Having a common glucose sensor couples complementary regulatory mechanisms into a tightly regulated and stable glucose homeostatic network.

Y1 receptor deficiency in β-cells leads to increased adiposity and impaired glucose metabolism

Insulin secretion from pancreatic β-cells is critical for maintaining glucose homeostasis and deregulation of circulating insulin levels is associated with the development of metabolic diseases. While many factors have been implicated in the stimulation of insulin secretion, the mechanisms that subsequently reduce insulin secretion remain largely unexplored. Here we demonstrate that mice with β-cell specific ablation of the Y1 receptor exhibit significantly upregulated serum insulin levels associated with increased body weight and adiposity. Interestingly, when challenged with a high fat diet these β-cell specific Y1-deficient mice also develop hyperglycaemia and impaired glucose tolerance. This is most likely due to enhanced hepatic lipid synthesis, resulting in an increase of lipid accumulation in the liver. Together, our study demonstrates that Y1 receptor signaling negatively regulates insulin release, and pharmacological inhibition of Y1 receptor signalling for the treatment of non-insulin dependent diabetes should be taken into careful consideration.

Nitric Oxide Activates β-Cell Glucokinase by Promoting Formation of the "Glucose-Activated" State

The release of insulin from the pancreas is tightly controlled by glucokinase (GCK) activity that couples β-cell metabolism to changes in blood sugar. Despite having only a single glucose-binding site, GCK displays positive glucose cooperativity. Ex vivo structural studies have identified several potential protein conformations with varying levels of enzymatic activity, yet it is unclear how living cells regulate GCK cooperativity. To better understand the cellular regulation of GCK activation, we developed a homotransfer Förster resonance energy transfer (FRET) GCK biosensor and used polarization microscopy to eliminate fluorescence crosstalk from FRET quantification and improve the signal-to-noise ratio. This approach enhanced sensor contrast compared to that seen with the heterotransfer FRET GCK reporter and allowed observation of individual GCK states using an automated method to analyze FRET data at the pixel level. Mutations known to activate and inhibit GCK activity produced distinct anisotropy distributions, suggesting that at least two conformational states exist in living cells. A high glucose level activated the biosensor in a manner consistent with GCK's enzymology. Interestingly, glucose-free conditions did not affect GCK biosensor FRET, indicating that there is a single low-activity state, which is counter to proposed structural models of GCK cooperativity. Under low-glucose conditions, application of chemical NO donors efficiently shifted GCK to the more active conformation. Notably, GCK activation by mutation, a high glucose level, a pharmacological GCK activator, or S-nitrosylation all shared the same FRET distribution. These data suggest a simplified model for GCK activation in living cells, where post-translational modification of GCK by S-nitrosylation facilitates a single conformational transition that enhances GCK enzymatic activity.

Adaptive response of rat pancreatic β-cells to insulin resistance induced by monocrotophos: Biochemical evidence

Our previous findings clearly suggested the role of duration of exposure to monocrotophos (MCP) in the development of insulin resistance. Rats exposed chronically to MCP developed insulin resistance with hyperinsulinemia without overt diabetes. In continuation of this vital observation, we sought to delineate the biochemical mechanisms that mediate heightened pancreatic β-cell response in the wake of MCP-induced insulin resistance in rats. Adult rats were orally administered (0.9 and 1.8mg/kgb.w/d) MCP for 180days. Terminally, MCP-treated rats exhibited glucose intolerance, hyperinsulinemia, and potentiation of glucose-induced insulin secretion along with elevated levels of circulating IGF1, free fatty acids, corticosterone, and paraoxonase activity. Biochemical analysis of islet extracts revealed increased levels of insulin, malate, pyruvate and ATP with a concomitant increase in activities of cytosolic and mitochondrial enzymes that are known to facilitate insulin secretion and enhanced shuttle activities. Interestingly, islets from MCP-treated rats exhibited increased insulin secretory potential ex vivo compared to those isolated from control rats. Further, MCP-induced islet hypertrophy was associated with increased insulin-positive cells. Our study demonstrates the impact of the biological interaction between MCP and components of metabolic homeostasis on pancreatic beta cell function/s. We speculate that the heightened pancreatic beta cell function evidenced may be mediated by increased IGF1 and paraoxonase activity, which effectively counters insulin resistance induced by chronic exposure to MCP. Our findings emphasize the need for focused research to understand the confounding environmental risk factors which may modulate heightened beta cell functions in the case of organophosphorus insecticide-induced insulin resistance. Such an approach may help us to explain the sharp increase in the prevalence of type II diabetes worldwide.

Real-time imaging of intracellular hydrogen peroxide in pancreatic islets

A real-time method to measure intracellular hydrogen peroxide (H₂O₂) would be very impactful in characterizing rapid changes that occur in physiologic and pathophysiologic states. Current methods do not provide the sensitivity, specificity and spatiotemporal resolution needed for such experiments on intact cells. We developed the use of HyPer, a genetic indicator for H₂O₂ that can be expressed in the cytosol (cyto-HyPer) or the mitochondria (mito-HyPer) of live cells. INS-1 cells or islets were permeabilized and the cytosolic HyPer signal was a linear function of extracellular H₂O₂, allowing fluorescent cyto-HyPer signals to be converted into H₂O₂ concentrations. Glucose increased cytosolic H₂O₂, an effect that was suppressed by overexpression of catalase. Large perturbations in pH can influence the HyPer signal, but inclusion of HEPES [4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid] in the perfusate prevented pH changes, but did not affect glucose-induced cyto-HyPer signals, suggesting that this effect is largely pH-independent. Using the assay, two fundamental questions were addressed. Knockdown of superoxide dismutase 2 (SOD2), the mitochondrial form of SOD, completely suppressed glucose-induced H₂O₂ Furthermore, glucose also induced mitochondrial superoxide and H₂O₂ production, which preceded the appearance of cytosolic H₂O₂ Therefore, glucose-induced H₂O₂ largely originated from mitochondria. Finally, the glucose-induced HyPer signal was less than 1/20th of that induced by toxic levels of H₂O₂ Overall, the use of HyPer for real-time imaging allowed resolution of acute changes in intracellular levels of H₂O₂ and will have great utility for islet studies involving mechanisms of H₂O₂-mediated signaling and oxidative stress.

When is it MODY? Challenges in the Interpretation of Sequence Variants in MODY Genes

The genomics revolution has raised more questions than it has provided answers. Big data from large population-scale resequencing studies are increasingly deconstructing classic notions of Mendelian disease genetics, which support a simplistic correlation between mutational severity and phenotypic outcome. The boundaries are being blurred as the body of evidence showing monogenic disease-causing alleles in healthy genomes, and in the genomes of individu-als with increased common complex disease risk, continues to grow. In this review, we focus on the newly emerging challenges which pertain to the interpretation of sequence variants in genes implicated in the pathogenesis of maturity-onset diabetes of the young (MODY), a presumed mono-genic form of diabetes characterized by Mendelian inheritance. These challenges highlight the complexities surrounding the assignments of pathogenicity, in particular to rare protein-alerting variants, and bring to the forefront some profound clinical diagnostic implications. As MODY is both genetically and clinically heterogeneous, an accurate molecular diagnosis and cautious extrapolation of sequence data are critical to effective disease management and treatment. The biological and translational value of sequence information can only be attained by adopting a multitude of confirmatory analyses, which interrogate variant implication in disease from every possible angle. Indeed, studies which have effectively detected rare damaging variants in known MODY genes in normoglycemic individuals question the existence of a sin-gle gene mutation scenario: does monogenic diabetes exist when the genetic culprits of MODY have been systematical-ly identified in individuals without MODY?

Transgenerational effects of obesity and malnourishment on diabetes risk in F2 generation

Transgenerational inheritance of various diseases and phenotypes has been demonstrated in diverse species and involves various epigenetic markers. Obesity and malnourishment are nutritional stresses that have effects on offspring through increasing their risk of diabetes and/or obesity. Obesity and malnourishment both affect glucose metabolism and alter oxidative stress parameters in key organs. We induced obesity and malnutrition in F0 female rats by the use of obesogenic diet and protein-deficient diet, respectively. F0 obese and malnourished females were mated with control males and their offspring (F1 generation) were maintained on control diets. The male and female F1 offspring were mated with controls and the resultant offspring (F2 generation) were maintained on control diet. Glucose-sensing markers, glucose metabolism, indicators of insulin resistance and oxidative stress parameters were assessed during fetal development and till the adulthood of the offspring. Glucose-sensing genes were significantly over-expressed in distinct fetal tissues of F2 offspring of malnourished F1 females (F2-MF1F), specifically in fetal pancreas, liver, and adipose tissue. Nuclear and mitochondrial 8-oxo-dG DNA content was significantly elevated in F2-MF1F fetal pancreas. Maternal FBG was significantly elevated in F2-MF1F and F2 offspring of obese F1 females (F2-OF1F) during pregnancy. Males and females offspring of F2-OF1 exhibited significantly elevated FBG and impaired OGTT. Offspring of F2-MF1F showed similar results, while that of F2-MF1M did not significantly deviate from controls. F2-OF1F and F2-MF1F offspring exhibited significant deviation in insulin levels and HOMA-IR levels from controls. Malnourishment has a stronger transgenerational effect through maternal line compared to obesity and malnourishment through paternal line in increasing risk of diabetes in F2 generation.

Glucokinase activation is beneficial or toxic to cultured rat pancreatic islets depending on the prevailing glucose concentration

In rat pancreatic islets, β-cell gene expression, survival, and subsequent acute glucose stimulation of insulin secretion (GSIS) are optimally preserved by prolonged culture at 10 mM glucose (G10) and markedly altered by culture at G5 or G30. Here, we tested whether pharmacological glucokinase (GK) activation prevents these alterations during culture or improves GSIS after culture. Rat pancreatic islets were cultured 1-7 days at G5, G10, or G30 with or without 3 μM of the GK activator Ro 28-0450 (Ro). After culture, β-cell apoptosis and islet gene mRNA levels were measured, and the acute glucose-induced increase in NAD(P)H autofluorescence, intracellular calcium concentration, and insulin secretion were tested in the absence or presence of Ro. Prolonged culture of rat islets at G5 or G30 instead of G10 triggered β-cell apoptosis and reduced their glucose responsiveness. Addition of Ro during culture differently affected β-cell survival and glucose responsiveness depending on the glucose concentration during culture: it was beneficial to β-cell survival and function at G5, detrimental at G10, and ineffective at G30. In contrast, acute GK activation with Ro increased the glucose sensitivity of islets cultured at G10 but failed at restoring β-cell glucose responsiveness after culture at G5 or G30. We conclude that pharmacological GK activation prevents the alteration of β-cell survival and function by long-term culture at G5 but mimics glucotoxicity when added to G10. The complex effects of glucose on the β-cell phenotype result from changes in glucose metabolism and not from an effect of glucose per se.

Src regulates insulin secretion and glucose metabolism by influencing subcellular localization of glucokinase in pancreatic β-cells

Aims/Introduction

Src, a non‐receptor tyrosine kinase, regulates a wide range of cellular functions, and hyperactivity of Src is involved in impaired glucose metabolism in pancreatic β‐cells. However, the physiological role of Src in glucose metabolism in normal, unstressed β‐cells remains unclear. In the present study, we investigated the role of Src in insulin secretion and glucose metabolism.

Materials and Methods

Src was downregulated using small interfering ribonucleic acid in INS‐1 cells, and glucose‐induced insulin secretion, adenosine triphosphate content, intracellular calcium concentration, glucose utilization and glucokinase activity were measured. Expression levels of messenger ribonucleic acid and protein of glucokinase were examined by semiquantitative real‐time polymerase chain reaction and immunoblotting, respectively. Cells were fractionated by digitonin treatment, and subcellular localization of glucokinase was examined by immunoblotting. Interaction between glucokinase and neuronal nitric oxide synthase was estimated by immunoprecipitation.

Results

In Src downregulated INS‐1 cells, glucose‐induced insulin secretion was impaired, whereas insulin secretion induced by high K⁺ was not affected. Intracellular adenosine triphosphate content and elevation of intracellular calcium concentration by glucose stimulation were suppressed by Src downregulation. Src downregulation reduced glucose utilization in the presence of high glucose, which was accompanied by a reduction in glucokinase activity without affecting its expression. However, Src downregulation reduced glucokinase in soluble, cytoplasmic fraction, and increased it in pellet containing intaracellular organelles. In addition, interaction between glucokinase and neuronal nitric oxide synthase was facilitated by Src downregulation.

Conclusions

Src plays an important role in glucose‐induced insulin secretion in pancreatic β‐cells through maintaining subcellular localization and activity of glucokinase.

Pancreatic PYY Is Critical in the Control of Insulin Secretion and Glucose Homeostasis in Female Mice

Insulin secretion is tightly controlled through coordinated actions of a number of systemic and local factors. Peptide YY (PYY) is expressed in α-cells of the islet, but its role in control of islet function such as insulin release is not clear. In this study, we generated a transgenic mouse model (Pyy(tg/+)/Rip-Cre) overexpressing the Pyy gene under the control of the rat insulin 2 gene promoter and assessed the impact of islet-released PYY on β-cell function, insulin release, and glucose homeostasis in mice. Our results show that up-regulation of PYY in islet β-cells leads to an increase in serum insulin levels as well as improved glucose tolerance. Interestingly, PYY-overproducing mice show increased lean mass and reduced fat mass with no significant changes in food intake or body weight. Energy expenditure is also increased accompanied by increased respiratory exchange ratio. Mechanistically, the enhanced insulin levels and improved glucose tolerance are primarily due to increased β-cell mass and secretion. This is associated with alterations in the expression of genes important for β-cell proliferation and function as well as the maintenance of the β-cell phenotype. Taken together, these data demonstrate that pancreatic islet-derived PYY plays an important role in controlling glucose homeostasis through the modulation of β-cell mass and function.

Role of Islet Glucokinase, Glucose Metabolism, and Insulin Pathway in the Enhancing Effect of Islet Neogenesis-Associated Protein on Glucose-Induced Insulin Secretion

OBJECTIVE

To demonstrate the role of islet glucokinase, glucose metabolism, and intracellular insulin mediators in the enhancing effect of islet neogenesis-associated protein pentadecapeptide (INGAP-PP) on glucose-induced insulin secretion.

METHODS

Islets from normal rats were cultured for 4 days in the absence or presence of 10 μg/mL INGAP-PP, with/without Wortmannin or LY294002. Islets were incubated with different glucose concentrations to measure insulin secretion and content, hexokinase and glucokinase activity, glucose oxidation and utilization, glucokinase, insulin receptor, insulin receptor substrate (IRS)-1/2, and PI3K concentration and phosphorylation.

RESULTS

The INGAP-PP significantly increased insulin release at high but not at low glucose concentration, glucokinase activity, glucose metabolism, glucokinase, insulin receptor, IRS-2 and PI3K protein concentration, insulin receptor and IRS-1/2 tyrosine phosphorylation, and the association of p85 with IRS-1. Wortmannin and LY294002 blocked INGAP-PP effect on insulin secretion and glucokinase protein levels in a dose-dependent manner.

CONCLUSIONS

The enhancing effect of INGAP-PP on glucose-induced insulin release could be partly ascribed to its effect on glucokinase activity and glucose metabolism and is mainly mediated by the PI3K/AKT pathway. These results, together with the low hypoglycemia risk associated with the use of INGAP-PP, offer a new alternative for diabetes prevention and treatment.

Recognition and Management of Individuals With Hyperglycemia Because of a Heterozygous Glucokinase Mutation

Glucokinase-maturity-onset diabetes of the young (GCK-MODY), also known as MODY2, is caused by heterozygous inactivating mutations in the GCK gene. GCK gene mutations are present in ∼1 in 1,000 of the population, but most are not diagnosed. They are common causes of MODY (10-60%): persistent incidental childhood hyperglycemia (10-60%) and gestational diabetes mellitus (1-2%). GCK-MODY has a unique pathophysiology and clinical characteristics, so it is best considered as a discrete genetic subgroup. People with GCK-MODY have a defect in glucose sensing; hence, glucose homeostasis is maintained at a higher set point resulting in mild, asymptomatic fasting hyperglycemia (5.4-8.3 mmol/L, HbA1c range 5.8-7.6% [40-60 mmol/mol]), which is present from birth and shows slight deterioration with age. Even after 50 years of mild hyperglycemia, people with GCK-MODY do not develop significant microvascular complications, and the prevalence of macrovascular complications is probably similar to that in the general population. Treatment is not recommended outside pregnancy because glucose-lowering therapy is ineffective in people with GCK-MODY and there is a lack of long-term complications. In pregnancy, fetal growth is primarily determined by whether the fetus inherits the GCK gene mutation from their mother. Insulin treatment of the mother is only appropriate when increased fetal abdominal growth on scanning suggests the fetus is unaffected. The impact on outcome of maternal insulin treatment is limited owing to the difficulty in altering maternal glycemia in these patients. Making the diagnosis of GCK-MODY through genetic testing is essential to avoid unnecessary treatment and investigations, especially when patients are misdiagnosed with type 1 or type 2 diabetes.

Reversal of diabetes following transplantation of an insulin-secreting human liver cell line: Melligen cells

As an alternative to the transplantation of islets, a human liver cell line has been genetically engineered to reverse type 1 diabetes (TID). The initial liver cell line (Huh7ins) commenced secretion of insulin in response to a glucose concentration of 2.5 mmol/l. After transfection of the Huh7ins cells with human islet glucokinase, the resultant Melligen cells secreted insulin in response to glucose within the physiological range; commencing at 4.25 mmol/l. Melligen cells exhibited increased glucokinase enzymatic activity in response to physiological glucose concentrations, as compared with Huh7ins cells. When transplanted into diabetic immunoincompetent mice, Melligen cells restored normoglycemia. Quantitative real-time polymerase chain reaction (qRT-PCR) revealed that both cell lines expressed a range of β-cell transcription factors and pancreatic hormones. Exposure of Melligen and Huh7ins cells to proinflammatory cytokines (TNF-α, IL-1β, and IFN-γ) affected neither their viability nor their ability to secrete insulin to glucose. Gene expression (microarray and qRT-PCR) analyses indicated the survival of Melligen cells in the presence of known β-cell cytotoxins was associated with the expression of NF-κB and antiapoptotic genes (such as BIRC3). This study describes the successful generation of an artificial β-cell line, which, if encapsulated to avoid allograft rejection, may offer a clinically applicable cure for T1D.

Phenotypic severity of homozygous GCK mutations causing neonatal or childhood-onset diabetes is primarily mediated through effects on protein stability

Mutations in glucokinase (GCK) cause a spectrum of glycemic disorders. Heterozygous loss-of-function mutations cause mild fasting hyperglycemia irrespective of mutation severity due to compensation from the unaffected allele. Conversely, homozygous loss-of-function mutations cause permanent neonatal diabetes requiring lifelong insulin treatment. This study aimed to determine the relationship between in vitro mutation severity and clinical phenotype in a large international case series of patients with homozygous GCK mutations. Clinical characteristics for 30 patients with diabetes due to homozygous GCK mutations (19 unique mutations, including 16 missense) were compiled and assigned a clinical severity grade (CSG) based on birth weight and age at diagnosis. The majority (28 of 30) of subjects were diagnosed before 9 months, with the remaining two at 9 and 15 years. These are the first two cases of a homozygous GCK mutation diagnosed outside infancy. Recombinant mutant GCK proteins were analyzed for kinetic and thermostability characteristics and assigned a relative activity index (RAI) or relative stability index (RSI) value. Six of 16 missense mutations exhibited severe kinetic defects (RAI ≤ 0.01). There was no correlation between CSG and RAI (r(2) = 0.05, P = 0.39), indicating that kinetics alone did not explain the phenotype. Eighty percent of the remaining mutations showed reduced thermostability, the exceptions being the two later-onset mutations which exhibited increased thermostability. Comparison of CSG with RSI detected a highly significant correlation (r(2) = 0.74, P = 0.002). We report the largest case series of homozygous GCK mutations to date and demonstrate that they can cause childhood-onset diabetes, with protein instability being the major determinant of mutation severity.

Nutritional regulation of glucokinase: a cross-species story

The glucokinase (GK) enzyme (EC 2.7.1.1.) is essential for the use of dietary glucose because it is the first enzyme to phosphorylate glucose in excess in different key tissues such as the pancreas and liver. The objective of the present review is not to fully describe the biochemical characteristics and the genetics of this enzyme but to detail its nutritional regulation in different vertebrates from fish to human. Indeed, the present review will describe the existence of the GK enzyme in different animal species that have naturally different levels of carbohydrate in their diets. Thus, some studies have been performed to analyse the nutritional regulation of the GK enzyme in humans and rodents (having high levels of dietary carbohydrates in their diets), in the chicken (moderate level of carbohydrates in its diet) and rainbow trout (no carbohydrate intake in its diet). All these data illustrate the nutritional importance of the GK enzyme irrespective of feeding habits, even in animals known to poorly use dietary carbohydrates (carnivorous species).

Control of insulin secretion by cytochrome C and calcium signaling in islets with impaired metabolism

The aim of the study was to assess the relative control of insulin secretion rate (ISR) by calcium influx and signaling from cytochrome c in islets where, as in diabetes, the metabolic pathways are impaired. This was achieved either by culturing isolated islets at low (3 mm) glucose or by fasting rats prior to the isolation of the islets. Culture in low glucose greatly reduced the glucose response of cytochrome c reduction and translocation and ISR, but did not affect the response to the mitochondrial fuel α-ketoisocaproate. Unexpectedly, glucose-stimulated calcium influx was only slightly reduced in low glucose-cultured islets and was not responsible for the impairment in glucose-stimulated ISR. A glucokinase activator acutely restored cytochrome c reduction and translocation and ISR, independent of effects on calcium influx. Islets from fasted rats had reduced ISR and cytochrome c reduction in response to both glucose and α-ketoisocaproate despite normal responses of calcium. Our data are consistent with the scenario where cytochrome c reduction and translocation are essential signals in the stimulation of ISR, the loss of which can result in impaired ISR even when calcium response is normal.

Identification of the ubiquitin-like domain of midnolin as a new glucokinase interaction partner

Glucokinase acts as a glucose sensor in pancreatic beta cells. Its posttranslational regulation is important but not yet fully understood. Therefore, a pancreatic islet yeast two-hybrid library was produced and searched for glucokinase-binding proteins. A protein sequence containing a full-length ubiquitin-like domain was identified to interact with glucokinase. Mammalian two-hybrid and fluorescence resonance energy transfer analyses confirmed the interaction between glucokinase and the ubiquitin-like domain in insulin-secreting MIN6 cells and revealed the highest binding affinity at low glucose. Overexpression of parkin, an ubiquitin E3 ligase exhibiting an ubiquitin-like domain with high homology to the identified, diminished insulin secretion in MIN6 cells but had only some effect on glucokinase activity. Overexpression of the elucidated ubiquitin-like domain or midnolin, containing exactly this ubiquitin-like domain, significantly reduced both intrinsic glucokinase activity and glucose-induced insulin secretion. Midnolin has been to date classified as a nucleolar protein regulating mouse development. However, we could not confirm localization of midnolin in nucleoli. Fluorescence microscopy analyses revealed localization of midnolin in nucleus and cytoplasm and co-localization with glucokinase in pancreatic beta cells. In addition we could show that midnolin gene expression in pancreatic islets is up-regulated at low glucose and that the midnolin protein is highly expressed in pancreatic beta cells and also in liver, muscle, and brain of the adult mouse and cell lines of human and rat origin. Thus, the results of our study suggest that midnolin plays a role in cellular signaling of adult tissues and regulates glucokinase enzyme activity in pancreatic beta cells.

Insights into the pathogenicity of rare missense GCK variants from the identification and functional characterization of compound heterozygous and double mutations inherited in cis

OBJECTIVE

To demonstrate the importance of using a combined genetic and functional approach to correctly interpret a genetic test for monogenic diabetes.

RESEARCH DESIGN AND METHODS

We identified three probands with a phenotype consistent with maturity-onset diabetes of the young (MODY) subtype GCK-MODY, in whom two potential pathogenic mutations were identified: [R43H/G68D], [E248 K/I225M], or [G261R/D217N]. Allele-specific PCR and cosegregation were used to determine phase. Single and double mutations were kinetically characterized.

RESULTS

The mutations occurred in cis (double mutants) in two probands and in trans in one proband. Functional studies of all double mutants revealed inactivating kinetics. The previously reported GCK-MODY mutations R43H and G68D were inherited from an affected father and unaffected mother, respectively. Both our functional and genetic studies support R43H as the cause of GCK-MODY and G68D as a neutral rare variant.

CONCLUSIONS

These data highlight the need for family/functional studies, even for previously reported pathogenic mutations.

Discovery of a novel site regulating glucokinase activity following characterization of a new mutation causing hyperinsulinemic hypoglycemia in humans

Type 2 diabetes is a global problem, and current ineffective therapeutic strategies pave the way for novel treatments like small molecular activators targeting glucokinase (GCK). GCK activity is fundamental to beta cell and hepatocyte glucose metabolism, and heterozygous activating and inactivating GCK mutations cause hyperinsulinemic hypoglycemia (HH) and maturity onset diabetes of the young (MODY) respectively. Over 600 naturally occurring inactivating mutations have been reported, whereas only 13 activating mutations are documented to date. We report two novel GCK HH mutations (V389L and T103S) at residues where MODY mutations also occur (V389D and T103I). Using recombinant proteins with in vitro assays, we demonstrated that both HH mutants had a greater relative activity index than wild type (6.0 for V389L, 8.4 for T103S, and 1.0 for wild type). This was driven by an increased affinity for glucose (S(0.5), 3.3 ± 0.1 and 3.5 ± 0.1 mm, respectively) versus wild type (7.5 ± 0.1 mm). Correspondingly, the V389D and T103I MODY mutants had markedly reduced relative activity indexes (<0.1). T103I had an altered affinity for glucose (S(0.5), 24.9 ± 0.6 mm), whereas V389D also exhibited a reduced affinity for ATP and decreased catalysis rate (S(0.5), 78.6 ± 4.5 mm; ATP(K(m)), 1.5 ± 0.1 mm; K(cat), 10.3 ± 1.1s(-1)) compared with wild type (ATP(K(m)), 0.4 ± <0.1; K(cat), 62.9 ± 1.2). Both Thr-103 mutants showed reduced inhibition by the endogenous hepatic inhibitor glucokinase regulatory protein. Molecular modeling demonstrated that Thr-103 maps to the allosteric activator site, whereas Val-389 is located remotely to this position and all other previously reported activating mutations, highlighting α-helix 11 as a novel region regulating GCK activity. Our data suggest that pharmacological manipulation of GCK activity at locations distal from the allosteric activator site is possible.

Research and development of glucokinase activators for diabetes therapy: theoretical and practical aspects

Glucokinase Glucokinase (GK GK ; EC 2.7.1.1.) phosphorylates and regulates glucose metabolism in insulin-producing pancreatic beta-cells, hepatocytes, and certain cells of the endocrine and nervous systems allowing it to play a central role in glucose homeostasis glucose homeostasis . Most importantly, it serves as glucose sensor glucose sensor in pancreatic beta-cells mediating glucose-stimulated insulin biosynthesis and release and it governs the capacity of the liver to convert glucose to glycogen. Activating and inactivating mutations of the glucokinase gene cause autosomal dominant hyperinsulinemic hypoglycemia and hypoinsulinemic hyperglycemia in humans, respectively, illustrating the preeminent role of glucokinase in the regulation of blood glucose and also identifying the enzyme as a potential target for developing antidiabetic drugs antidiabetic drugs . Small molecules called glucokinase activators (GKAs) glucokinase activators (GKAs) which bind to an allosteric activator allosteric activator site of the enzyme have indeed been discovered and hold great promise as new antidiabetic agents. GKAs increase the enzyme's affinity for glucose and also its maximal catalytic rate. Consequently, they stimulate insulin biosynthesis and secretion, enhance hepatic glucose uptake, and augment glucose metabolism and related processes in other glucokinase-expressing cells. Manifestations of these effects, most prominently a lowering of blood glucose, are observed in normal laboratory animals and man but also in animal models of diabetes and patients with type 2 diabetes mellitus (T2DM T2DM ) type 2 diabetes mellitus (T2DM) . These compelling concepts and results sustain a strong R&D effort by many pharmaceutical companies to generate GKAs with characteristics allowing for a novel drug treatment of T2DM.

PREB regulates transcription of pancreatic glucokinase in response to glucose and cAMP

Prolactin regulatory element binding (PREB) is a transcription factor that regulates prolactin promoter activity in rat anterior pituitary. The PREB protein is not only expressed in the anterior pituitary but also in the pancreas. We have recently reported that in pancreatic beta-cells, PREB regulates the transcription of the insulin gene in response to glucose stimulation. In the current study, we have examined the role of PREB in regulating glucokinase (GK) in pancreatic beta-cells. To analyse the effects of PREB on GK gene transcription, we employed a reporter gene assay. In the cells expressing or with knocked down PREB, GK expression was determined. GK expression was regulated by glucose and cAMP, and both glucose and cAMP stimulated the expression of PREB in a dose-dependent manner. Conversely, overexpression of PREB using a PREB-expressing adenovirus increased the expression of the GK protein. GK enzymatic activity was also significantly increased in the cells that stably expressed PREB. In addition, PREB induced GK promoter activity. Chromatin immunoprecipitation (ChIP) analyses showed that PREB mediated its transcriptional effect by binding to the PREB-responsive cis-element of the GK promoter. Finally, we used siRNA to inhibit PREB expression in cells and demonstrated that the knockdown of PREB attenuated the effects of glucose and cAMP on GK expression. Our data show that in pancreatic -cells, PREB regulates the transcription of the GK gene in response to glucose and cAMP.

Proteomics and islet research

The complementary disciplines of genomics and proteomics offer better insights into the molecular mechanisms of diseases. While genomics hunts for defining our static genetic substrate, proteomics explores the structure and function of proteins expressed by a cell or tissue type under specified conditions. In the past decade, proteomics has been revolutionized by the application of techniques such as two-dimensional gel electrophoresis (2DGE), mass spectrometry (MS), and protein arrays. These techniques have tremendous potential for biomarker development, target validation, diagnosis, prognosis, and optimization of treatment in medical care, especially in the field of islet and diabetes research. This chapter will highlight the contributions of proteomic technologies toward the dissection of complex network of signaling molecules regulating islet function, the identification of potential biomarkers, and the understanding of mechanisms involved in the pathogenesis of diabetes.

Update on mutations in glucokinase (GCK), which cause maturity-onset diabetes of the young, permanent neonatal diabetes, and hyperinsulinemic hypoglycemia

Glucokinase is a key regulatory enzyme in the pancreatic beta-cell. It plays a crucial role in the regulation of insulin secretion and has been termed the glucose sensor in pancreatic beta-cells. Given its central role in the regulation of insulin release it is understandable that mutations in the gene encoding glucokinase (GCK) can cause both hyper- and hypoglycemia. Heterozygous inactivating mutations in GCK cause maturity-onset diabetes of the young (MODY) subtype glucokinase (GCK), characterized by mild fasting hyperglycemia, which is present at birth but often only detected later in life during screening for other purposes. Homozygous inactivating GCK mutations result in a more severe phenotype presenting at birth as permanent neonatal diabetes mellitus (PNDM). A growing number of heterozygous activating GCK mutations that cause hypoglycemia have also been reported. A total of 620 mutations in the GCK gene have been described in a total of 1,441 families. There are no common mutations, and the mutations are distributed throughout the gene. The majority of activating mutations cluster in a discrete region of the protein termed the allosteric activator site. The identification of a GCK mutation in patients with both hyper- and hypoglycemia has implications for the clinical course and clinical management of their disorder.

Transcription factor AP-2beta inhibits glucose-induced insulin secretion in cultured insulin-secreting cell-line

AIM

We previously identified the transcription factor activating enhancer-binding protein-2beta (AP-2beta) gene as a new candidate for conferring susceptibility to type 2 diabetes. To ascertain the possible involvement of AP-2beta in the pathogenesis of type 2 diabetes we examined the effects of AP-2beta on glucose-induced insulin secretion.

METHODS

We measured the insulin secretion stimulated by glucose, tolbutamide, or KCl in the HIT-T15 cells infected with adenovirus vectors encoding AP-2beta or LacZ (control).

RESULTS

We identified clear expression of AP-2beta in isolated rat pancreatic islets and in HIT-T15 cells. Glucose-induced increase in insulin secretion was significantly inhibited in AP-2beta-overexpressing cells (LacZ, 5.0+/-0.8 ng h(-1)mg(-1) protein; AP-2beta, 1.7+/-0.2 ng h(-1)mg(-1) protein; P=0.0015), whereas insulin expression was the same in both types of cells. Tolbutamide-induced insulin secretion was also suppressed in the AP-2beta-overexpressing cells, but KCl-induced insulin secretion was not affected by AP-2beta overexpression. In addition, Kir6.2 and glucokinase expression was significantly decreased in the AP-2beta-overexpressing cells.

CONCLUSION

We identified for the first time that AP-2beta expressed and functioned in insulin-secreting cell-line HIT-T15. These results suggest that AP-2beta contributes to susceptibility to type 2 diabetes by inhibiting glucose-induced insulin secretion in pancreatic beta cells.

Identification of a novel beta-cell glucokinase (GCK) promoter mutation (-71G>C) that modulates GCK gene expression through loss of allele-specific Sp1 binding causing mild fasting hyperglycemia in humans

OBJECTIVE

Inactivating mutations in glucokinase (GCK) cause mild fasting hyperglycemia. Identification of a GCK mutation has implications for treatment and prognosis; therefore, it is important to identify these individuals. A significant number of patients have a phenotype suggesting a defect in glucokinase but no abnormality of GCK. We hypothesized that the GCK beta-cell promoter region, which currently is not routinely screened, could contain pathogenic mutations; therefore, we sequenced this region in 60 such probands.

RESEARCH DESIGN AND METHODS

The beta-cell GCK promoter was sequenced in patient DNA. The effect of the identified novel mutation on GCK promoter activity was assessed using a luciferase reporter gene expression system. Electrophoretic mobility shift assays (EMSAs) were used to determine the impact of the mutation on Sp1 binding.

RESULTS

A novel -71G>C mutation was identified in a nonconserved region of the human promoter sequence in six apparently unrelated probands. Family testing established cosegregation with fasting hyperglycemia (> or = 5.5 mmol/l) in 39 affected individuals. Haplotype analysis in the U.K. family and four of the Slovakian families demonstrated that the mutation had arisen independently. The mutation maps to a potential transcriptional activator binding site for Sp1. Reporter assays demonstrated that the mutation reduces promoter activity by up to fourfold. EMSAs demonstrated a dramatic reduction in Sp1 binding to the promoter sequence corresponding to the mutant allele.

CONCLUSIONS

A novel beta-cell GCK promoter mutation was identified that significantly reduces gene expression in vitro through loss of regulation by Sp1. To ensure correct diagnosis of potential GCK-MODY (maturity-onset diabetes of the young) cases, analysis of the beta-cell GCK promoter should be included.

Assessing the potential of glucokinase activators in diabetes therapy

Glucokinase, a unique isoform of the hexokinase enzymes, which are known to phosphorylate D-glucose and other hexoses, was identified during the past three to four decades as a new, promising drug target for type 2 diabetes. Glucokinase serves as a glucose sensor of the insulin-producing pancreatic islet beta-cells, controls the conversion of glucose to glycogen in the liver and regulates hepatic glucose production. Guided by this fundamental knowledge, several glucokinase activators are now being developed, and have so far been shown to lower blood glucose in several animal models of type 2 diabetes and in initial trials in humans with the disease. Here, the scientific basis and current status of this new approach to diabetes therapy are discussed.

Glucokinase activators in diabetes management

Type 2 diabetes is a chronic metabolic disease that adversely affects both the quality and longevity of life. The disease is characterised by elevated blood glucose (hyperglycaemia) that is associated with microvascular complications and increased macrovascular risk. Existing oral agents, either alone or in combination, do not exhibit adequate or sustained glucose lowering efficacy in Type 2 diabetics. Consequently, there is an unmet medical need for improved antidiabetic agents which are both more effective at lowering glucose and which display sustained efficacy over a number of years. Such agents would allow present glycaemic treatment targets to be achieved with greater success. Glucokinase activators (GKAs) represent a novel class of glucose-lowering agents. Preclinical data supports the notion that these agents act to lower blood glucose through effects in both the liver and pancreas. It is predicted that this dual compartment mechanism of action of GKAs will translate to robust glucose lowering in diabetic patients. The potential benefits and risks associated with the pharmacological activation of glucokinase are evaluated. The status of GKAs in preclinical and clinical development is assessed are the future prospects of these agents.

SREBP1 is required for the induction by glucose of pancreatic beta-cell genes involved in glucose sensing

Previous studies have reported both positive and negative effects of culture of islets at high glucose concentrations on regulated insulin secretion. Here, we have reexamined this question in mouse islets and determined the role of changes in lipid synthesis in the effects of glucose. Glucose-stimulated insulin secretion (GSIS) and gene expression were examined in islets from C57BL/6 mice or littermates deleted for sterol-regulatory element binding protein-1 (SREBP1) after 4 days of culture at high glucose concentrations. Culture of control islets at 30 versus 8 mmol/l glucose led to enhanced secretion at both basal (3 mmol/l) and stimulatory (17 mmol/l) glucose concentrations and to enhanced triacylglycerol accumulation. These changes were associated with increases in the expression of genes involved in glucose sensing (glucose transporter 2, glucokinase, sulfonylurea receptor 1, inwardly rectifying K(+) channel 6.2), differentiation (pancreatic duodenal homeobox 1), and lipogenesis (Srebp1, fatty acid synthase, acetyl-coenzyme A carboxylase 1, stearoyl-coenzyme A desaturase 1). When cultured at either 8 or 30 mmol/l glucose, SREBP1-deficient (SREBP1(-/-)) islets displayed reduced GSIS and triacylglycerol content compared with normal islets. Correspondingly, glucose induction of the above genes in control islets was no longer observed in SREBP1(-/-) mouse islets. We conclude that enhanced lipid synthesis mediated by SREBP1c-dependent genes is required for the adaptive changes in islet gene expression and insulin secretion at high glucose concentrations.

Lipid-induced beta-cell dysfunction in vivo in models of progressive beta-cell failure

We determined the effect of 48-h elevation of plasma free fatty acids (FFA) on insulin secretion during hyperglycemic clamps in control female Wistar rats (group a) and in the following female rat models of progressive beta-cell dysfunction: lean Zucker diabetic fatty (ZDF) rats, both wild-type (group b) and heterozygous for the fa mutation in the leptin receptor gene (group c); obese (fa/fa) Zucker rats (nonprediabetic; group d); obese prediabetic (fa/fa) ZDF rats (group e); and obese (fa/fa) diabetic ZDF rats (group f). FFA induced insulin resistance in all groups but increased C-peptide levels (index of absolute insulin secretion) only in obese prediabetic ZDF rats. Insulin secretion corrected for insulin sensitivity using a hyperbolic or power relationship (disposition index or compensation index, respectively, both indexes of beta-cell function) was decreased by FFA. The decrease was greater in normoglycemic heterozygous lean ZDF rats than in Wistar controls. In obese "prediabetic" ZDF rats with mild hyperglycemia, the FFA-induced decrease in beta-cell function was no greater than that in obese Zucker rats. However, in overtly diabetic obese ZDF rats, FFA further impaired beta-cell function. In conclusion, 1) the FFA-induced impairment in beta-cell function is accentuated in the presence of a single copy of a mutated leptin receptor gene, independent of hyperglycemia. 2) In prediabetic ZDF rats with mild hyperglycemia, lipotoxicity is not accentuated, as the beta-cell mounts a partial compensatory response for FFA-induced insulin resistance. 3) This compensation is lost in diabetic rats with more marked hyperglycemia and loss of glucose sensing.

Time-dependent mechanisms in beta-cell glucose sensing

The relation between plasma glucose and insulin release from pancreatic beta-cells is not stationary in the sense that a given glucose concentration leads to a specific rate of insulin secretion. A number of time-dependent mechanisms appear to exist that modify insulin release both on a short and a longer time scale. Typically, two phases are described. The first phase, lasting up to 10 min, is a pulse of insulin release in response to fast changes in glucose concentration. The second phase is a more steady increase of insulin release over minutes to hours, if the elevated glucose concentration is sustained. The paper describes the glucose sensing mechanism via the complex dynamics of the key enzyme glucokinase, which controls the first step in glucose metabolism: phosphorylation of glucose to glucose-6-phosphate. Three time-dependent phenomena (mechanisms) are described. The fastest, corresponding to the first phase, is a delayed negative feedback regulating the glucokinase activity. Due to the delay, a rapid glucose increase will cause a burst of activity in the glucose sensing system, before the glucokinase is down-regulated. The second mechanism corresponds to the translocation of glucokinase from an inactive to an active form. As the translocation is controlled by the product(s) of the glucokinase reaction rather than by the substrate glucose, this mechanism gives a positive, but saturable, feedback. Finally, the release of the insulin granules is assumed to be enhanced by previous glucose exposure, giving a so-called glucose memory to the beta-cells. The effect depends on the insulin release of the cells, and this mechanism constitutes a second positive, saturable feedback system. Taken together, the three phenomena describe most of the glucose sensing behaviour of the beta-cells. The results indicate that the insulin release is not a precise function of the plasma glucose concentration. It rather looks as if the beta-cells just increase the insulin production, until the plasma glucose has returned to normal. This type of integral control has the advantage that the precise glucose sensitivity of the beta-cells is not important for normal glucose homeostasis.

A novel glucokinase activator modulates pancreatic islet and hepatocyte function

The glucose-sensing enzyme glucokinase (GK) plays a key role in glucose metabolism. We report here the effects of a novel glucokinase activator, LY2121260. The activator enhanced GK activity via binding to the allosteric site located in the hinge region of the enzyme. LY2121260 stimulated insulin secretion in a glucose-dependent manner in pancreatic beta-cells and increased glucose use in rat hepatocytes. In addition, incubation of beta-cells with the GK activator resulted in increased GK protein levels, suggesting that enhanced insulin secretion on chronic treatment with a GK activator may be due to not only changed enzyme kinetics but also elevated enzyme levels. Animals treated with LY2121260 showed an improved glucose tolerance after oral glucose challenge. These results support the concept that GK activators represent a new class of compounds that increase both insulin secretion and hepatic glucose use and in doing so may prove to be effective agents for the control of blood glucose levels in patients with type 2 diabetes.

Glucose-induced changes of multiple mouse islet proteins analysed by two-dimensional gel electrophoresis and mass spectrometry

AIMS/HYPOTHESIS

The aim of this study was to investigate molecular mechanisms of glucose-induced changes in islets of Langerhans by analysing global changes in protein patterns of islets exposed to elevated glucose concentrations.

METHODS

Islets were isolated from C57BL/6J mice and used either directly or after exposure to 11 mmol/l glucose for 24 h. Islet protein profiles were obtained by two-dimensional gel electrophoresis, and protein spots were identified by peptide mass fingerprinting using mass spectrometry.

RESULTS

Two-dimensional gels of freshly isolated islets and islets exposed to 11 mmol/l glucose contained 1,074 and 1,254 spots, respectively. The number of differentially expressed spots was 379, with 20 spots appearing as new proteins in islets exposed to 11 mmol/l glucose. We identified 124 spots corresponding to 77 protein entries and generated a reference map from freshly isolated islets. Actin, alpha enolase, cytokeratin 8, endoplasmin, glucose-regulated proteins, heat shock proteins, peroxiredoxins, prohormone convertase 2, protein disulphide isomerase, superoxide dismutase, tubulin, and V-type H+ -ATPase (V1 subunit A) were upregulated in islets exposed to 11 mmol/l glucose. In contrast, exocrine proteins and secretagogin were downregulated in these islets compared with in freshly isolated islets.

CONCLUSIONS/INTERPRETATION

The islet proteome approach revealed simultaneous changes in protein patterns of islets exposed to elevated glucose concentrations, indicating enhanced insulin synthesis, granular mobilisation and maturation, and increased stress response. The changes may be of relevance for the understanding of altered islet function in the hyperglycaemic state. It is expected that the islet reference map will become an important tool for dissecting multifactorial islet processes.

Increased glucose sensitivity of both triggering and amplifying pathways of insulin secretion in rat islets cultured for 1 wk in high glucose

Chronic hyperglycemia has been shown to induce either a lack of response or an increased sensitivity to glucose in pancreatic beta-cells. We reinvestigated this controversial issue in a single experimental model by culturing rat islets for 1 wk in 10 or 30 mmol/l glucose (G10, Controls; or G30, High-glucose islets) before testing the effect of stepwise glucose stimulation from G0.5 to G20 on key beta-cell stimulus-secretion coupling events. Compared with Controls, the glucose sensitivity of High-glucose islets was markedly increased, leading to maximal stimulation of oxidative metabolism and both triggering and amplifying pathways of insulin secretion in G6 rather than G20, hence to loss of glucose effect above G6. This enhanced glucose sensitivity occurred despite an approximately twofold increase in islet uncoupling protein 2 mRNA expression. Besides this increased glucose sensitivity, the maximal glucose stimulation of insulin secretion in High-glucose islets was reduced by approximately 50%, proportionally to the reduction of insulin content. In High-glucose islets, changes in (45)Ca(2+) influx induced by glucose and diazoxide were qualitatively similar but quantitatively smaller than in Control islets and, paradoxically, did not lead to detectable changes in the intracellular Ca(2+) concentration measured by microspectrofluorimetry (fura PE 3). In conclusion, after 1 wk of culture in G30, the loss of glucose stimulation of insulin secretion in the physiological range of glucose concentrations (G5-G10) results from the combination of an increased sensitivity to glucose of both triggering and amplifying pathways of insulin secretion and an approximately 50% reduction in the maximal glucose stimulation of insulin secretion.

Glucokinase (GCK) mutations in hyper- and hypoglycemia: maturity-onset diabetes of the young, permanent neonatal diabetes, and hyperinsulinemia of infancy

Glucokinase is a key regulatory enzyme in the pancreatic beta-cell. It plays a crucial role in the regulation of insulin secretion and has been termed the pancreatic beta-cell sensor. Given its central role in the regulation of insulin release, it is understandable that mutations in the gene encoding glucokinase (GCK) can cause both hyperglycemia and hypoglycemia. Heterozygous inactivating mutations in GCK cause maturity-onset diabetes of the young (MODY), characterized by mild hyperglycemia, which is present at birth, but is often only detected later in life during screening for other purposes. Homozygous inactivating GCK mutations result in a more severe phenotype, presenting at birth as permanent neonatal diabetes mellitus (PNDM). Several heterozygous activating GCK mutations that cause hypoglycemia have also been reported. A total of 195 mutations in the GCK gene have been described, in a total of 285 families. There are no common mutations and the mutations are distributed throughout the gene. Mutations that cause hypoglycemia are located in various exons in a discrete region of the protein termed the heterotropic allosteric activator site. The identification of a GCK mutation in hyper- and hypoglycemia has implications for the clinical course and clinical management of the disorder.

Oxidative stress is a mediator of glucose toxicity in insulin-secreting pancreatic islet cell lines

Pancreatic beta cells secrete insulin in response to changes in the extracellular glucose. However, prolonged exposure to elevated glucose exerts toxic effects on beta cells and results in beta cell dysfunction and ultimately beta cell death (glucose toxicity). To investigate the mechanism of how increased extracellular glucose is toxic to beta cells, we used two model systems where glucose metabolism was increased in beta cell lines by enhancing glucokinase (GK) activity and exposing cells to physiologically relevant increases in extracellular glucose (3.3-20 mm). Exposure of cells with enhanced GK activity to 20 mm glucose accelerated glycolysis, but reduced cellular NAD(P)H and ATP, caused accumulation of intracellular reactive oxygen species (ROS) and oxidative damage to mitochondria and DNA, and promoted apoptotic cell death. These changes required both enhanced GK activity and exposure to elevated extracellular glucose. A ROS scavenger partially prevented the toxic effects of increased glucose metabolism. These results indicate that increased glucose metabolism in beta cells generates oxidative stress and impairs cell function and survival; this may be a mechanism of glucose toxicity in beta cells. The level of beta cell GK may also be critical in this process.

A rat model for the energetic regulation of gonadotropin secretion: role of the glucose-sensing mechanism in the brain

Energy availability has been considered to regulate gonadal activity by modulating the release of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) at various reproductive phases, such as lactation and puberty in domestic as well as wild animals. Experimental models with rats and sheep have demonstrated that fasting or glucoprivation suppresses pulsatile LH release. From those experiments, the information on energy deficiency is considered to be detected by specific central sensors and conveyed to the hypothalamus to regulate LH release as well as food intake. Noradrenergic neurons, originating in the medulla oblongata and projecting to the hypothalamic paraventricular nucleus (PVN), is reported to be one of the pathways mediating the response of LH release to energy deficiency. The other component is considered to be an energy-sensing mechanism in the brain. Glucose or other oxidizable fuels may function as a metabolic signal to regulate LH release. Previous studies suggest the presence of a glucose-sensing mechanism in the rat hindbrain. From our previous results in the rat, the ependymocytes lining the wall of the cerebroventricle could possibly serve as a glucose sensor to regulate GnRH/LH release. Greater understanding of the nature of the energy-sensing mechanism in the brain will contribute to the nutritional manipulation of reproductive performance in domestic animals in various conditions.

A functional link between glucokinase binding to insulin granules and conformational alterations in response to glucose and insulin

Glucokinase (GK) activity is essential for the physiological regulation of insulin secretion by glucose. Because the enzyme exerts nearly total control over glucose metabolism in the beta-cell, even small changes in GK activity exert effects on glucose-stimulated insulin secretion and, consequently, the blood glucose concentration. Using quantitative imaging of multicolor fluorescent proteins fused to GK, we found that the association of GK with insulin granules is regulated by glucose in the beta-cell. Glucose stimulation increased the rate of fluorescence recovery after photobleaching of GK to insulin granules, indicating that GK is released into the cytoplasm after glucose stimulation. Changes in fluorescence resonance energy transfer between two different fluorescent protein variants inserted on opposing ends of GK were observed after glucose stimulation and correlated with increased enzyme activity. Furthermore, glucose-stimulated changes in GK regulation were blocked by two inhibitors of insulin secretion. Insulin treatment restored GK regulation in inhibited cells and stimulated GK translocation and activation by itself. Together, these data support a model for post-translational regulation of GK whereby insulin regulates both the association of GK with secretory granules and the activity of the enzyme within the pancreatic beta-cell.

Acute hyperglycemia alters the ability of the normal beta-cell to sense and respond to glucose

Impaired glucose tolerance (IGT) and non-insulin-dependent diabetes mellitus (NIDDM) are associated with an impaired ability of the beta-cell to sense and respond to small changes in plasma glucose. The aim of this study was to establish whether acute hyperglycemia per se plays a role in inducing this defect in beta-cell response. Seven healthy volunteers with no family history of NIDDM were studied on two occasions during a 12-h oscillatory glucose infusion with a periodicity of 144 min. Once, low-dose glucose was infused at a mean rate of 6 mg x kg(-1) x min(-1) and amplitude 33% above and below the mean rate, and, once, high-dose glucose was infused at 12 mg x kg(-1) x min(-1) and amplitude 16% above and below the mean rate. Mean glucose levels were significantly higher during the high-dose compared with the low-dose glucose infusion [9.5 +/- 0.8 vs. 6.8 +/- 0.2 mM (P < 0.01)], resulting in increased mean insulin secretion rates [ISRs; 469.1 +/- 43.8 vs. 268.4 +/- 29 pmol/min (P < 0.001)] and mean insulin levels [213.6 +/- 46 vs. 67.9 +/- 10.9 pmol/l (P < 0.008)]. Spectral analysis evaluates the regularity of oscillations in glucose, insulin secretion, and insulin at a predetermined frequency. Spectral power for glucose, ISR, and insulin was reduced during the high-dose glucose infusion [11.8 +/- 1.4 to 7.0 +/- 1.6 (P < 0.02), 7.6 +/- 1.5 to 3.2 +/- 0.5 (P < 0.04), and 10.5 +/- 1.6 to 4.6 +/- 0.7 (P < 0.01), respectively]. In conclusion, short-term infusion of high-dose glucose to obtain glucose levels similar to those previously seen in IGT subjects results in reduced spectral power for glucose, ISR, and insulin. The reduction in spectral power previously observed for ISR in IGT or NIDDM subjects may be due partly to hyperglycemia.

Cyclic adenosine 3',5'-monophosphate increases pancreatic glucokinase activity and gene expression

Comparison of the pancreatic and hepatic glucokinase gene transcripts reveals tissue-specific control of expression and the existence of two distinct promoters in a single glucokinase gene. The existence of alternate promoters suggests that separate factors regulate glucokinase transcription in the two tissues. Hepatic glucokinase expression has been shown to be repressed by cAMP; however, in the pancreatic beta-cell it is unlikely that cAMP represses glucokinase activity, as cAMP is known to positively affect glucose-induced insulin secretion, a process that in mature islets requires pancreatic glucokinase activity. In this work we demonstrate that cAMP indeed has a stimulatory effect on pancreatic glucokinase. The cyclic nucleotide stimulates pancreatic glucokinase activity after 3-h incubation, and maximal effects are observed after 6 and 12 h of treatment. Using the bDNA assay, a sensitive signal amplification technique, we detected relative increases in glucokinase messenger RNA levels of 40.5 +/- 7.5% after 3-h incubation with cAMP. This stimulatory effect was increased to 106.3 +/- 22% after 6-h incubation and sustained up to 12 h of incubation. Inhibition of gene transcription by actinomycin D abolishes cAMP-induced glucokinase activity. In transfected fetal islets, cAMP increased the activity of the -1000 bp rat glucokinase promoter by 60 +/- 6%. These data demonstrate that cAMP has a stimulatory effect on pancreatic glucokinase gene expression and that the nucleotide has opposite effects on pancreatic and hepatic glucokinase, supporting the concept that glucokinase transcription in the liver and that in the beta-cell differ.

Interactions between effects of adrenalectomy and diet on insulin secretion in fa/fa Zucker rats

Our objective was to determine if a cafeteria-type diet with increased fat content would block the decrease in insulin secretion induced by adrenalectomy in obese rats. Five week old Zucker (fa/fa) rats were adrenalectomized. One week later, half of the adrenalectomized groups, and age-matched, sham-operated animals were given a diet of 16% fat and 44% carbohydrate. Control animals were maintained on standard rat chow (4.6% fat and 49% carbohydrate). After 4 weeks on the diets, in vivo measurements included caloric intake, weight gain, plasma corticosterone, triglyceride, free fatty acids, and oral glucose tolerance tests. In vitro measurements included glucose-stimulated insulin secretion, glucose phosphorylating activity, islet triglyceride content, and fatty acid oxidizing activity of cultured islets. Generally, the cafeteria diet did not block the effects of adrenalectomy on in vitro insulin secretion parameters, even though in sham-operated animals weight gain and insulin resistance was induced by the diet in vivo. Adrenalectomy and the diet exerted independent effects on glucose phosphorylation and fatty acid oxidation in islets. In conclusion, adrenalectomy decreased the elevated insulin secretion in fa/fa rats. The failure of a cafeteria diet enriched in fat to block the adrenalectomy-mediated changes in B-cell function indicates the importance of glucocorticoids and centrally-mediated effects on insulin secretion and other metabolic parameters.Key words: obesity, insulin secretion, islets of Langerhans, adrenalectomy, high fat diet.

Characterization of glucokinase regulatory protein-deficient mice

The glucokinase regulatory protein (GKRP) inhibits glucokinase competitively with respect to glucose by forming a protein-protein complex with this enzyme. The physiological role of GKRP in controlling hepatic glucokinase activity was addressed using gene targeting to disrupt GKRP gene expression. Heterozygote and homozygote knockout mice have a substantial decrease in hepatic glucokinase expression and enzymatic activity as measured at saturating glucose concentrations when compared with wild-type mice, with no change in basal blood glucose levels. Interestingly, when assayed under conditions to promote the association between glucokinase and GKRP, liver glucokinase activity in wild-type and null mice displayed comparable glucose phosphorylation capacities at physiological glucose concentrations (5 mM). Thus, despite reduced hepatic glucokinase expression levels in the null mice, glucokinase activity in the liver homogenates was maintained at nearly normal levels due to the absence of the inhibitory effects of GKRP. However, following a glucose tolerance test, the homozygote knockout mice show impaired glucose clearance, indicating that they cannot recruit sufficient glucokinase due to the absence of a nuclear reserve. These data suggest both a regulatory and a stabilizing role for GKRP in maintaining adequate glucokinase in the liver. Furthermore, this study provides evidence for the important role GKRP plays in acutely regulating of hepatic glucose metabolism.

The role of glucose and its metabolism in the regulation of glucokinase expression in isolated human pancreatic islets

Previous reports concerning the regulation of glucokinase expression in beta cells have been done using cell models from rodent origin. Evidence is lacking so far to implicate the same regulatory mechanisms in human cells. In this study, we investigate the effects of glucose on the expression of glucokinase using isolated human pancreatic islets. High glucose (16.7 mM), in a time-dependent manner, increases the amount of immunoreactive glucokinase (+150% after 7 days culture, P < 0.01) without apparent changes in glucokinase gene expression, suggesting that glucose exerts its effect at a posttranscriptional level. Mannose, but not the nonmetabolized hexoses, 3-O-methylglucose or 2-deoxyglucose, increases glucokinase protein content. Even though these findings are compatible with an involvement of signals derived from glucose metabolism, additional data argue against this hypothesis: (i) a glucokinase inhibitor (mannoheptulose) does not block glucose-induced increase in glucokinase content and (ii) other metabolic fuels (amino acids) are ineffective. We suggest that the glucose molecule, by mechanisms yet to be defined, but probably not involving its metabolism, regulates human glucokinase expression.