Enzymatic, metabolic and secretory patterns in human islets of type 2 (non-insulin-dependent) diabetic patients

Beta Cell Dysfunction in Youth- and Adult-Onset Type 2 Diabetes: An Extensive Narrative Review with a Special Focus on the Role of Nutrients

Traditionally a disease of adults, type 2 diabetes (T2D) has been increasingly diagnosed in youth, particularly among adolescents and young adults of minority ethnic groups. Especially, during the recent COVID-19 pandemic, obesity and prediabetes have surged not only in minority ethnic groups but also in the general population, further raising T2D risk. Regarding its pathogenesis, a gradually increasing insulin resistance due to central adiposity combined with a progressively defective β-cell function are the main culprits. Especially in youth-onset T2D, a rapid β-cell activity decline has been observed, leading to higher treatment failure rates, and early complications. In addition, it is well established that both the quantity and quality of food ingested by individuals play a key role in T2D pathogenesis. A chronic imbalance between caloric intake and expenditure together with impaired micronutrient intake can lead to obesity and insulin resistance on one hand, and β-cell failure and defective insulin production on the other. This review summarizes our evolving understanding of the pathophysiological mechanisms involved in defective insulin secretion by the pancreatic islets in youth- and adult-onset T2D and, further, of the role various micronutrients play in these pathomechanisms. This knowledge is essential if we are to curtail the serious long-term complications of T2D both in pediatric and adult populations.

Altered glycolysis triggers impaired mitochondrial metabolism and mTORC1 activation in diabetic β-cells

Chronic hyperglycaemia causes a dramatic decrease in mitochondrial metabolism and insulin content in pancreatic β-cells. This underlies the progressive decline in β-cell function in diabetes. However, the molecular mechanisms by which hyperglycaemia produces these effects remain unresolved. Using isolated islets and INS-1 cells, we show here that one or more glycolytic metabolites downstream of phosphofructokinase and upstream of GAPDH mediates the effects of chronic hyperglycemia. This metabolite stimulates marked upregulation of mTORC1 and concomitant downregulation of AMPK. Increased mTORC1 activity causes inhibition of pyruvate dehydrogenase which reduces pyruvate entry into the tricarboxylic acid cycle and partially accounts for the hyperglycaemia-induced reduction in oxidative phosphorylation and insulin secretion. In addition, hyperglycaemia (or diabetes) dramatically inhibits GAPDH activity, thereby impairing glucose metabolism. Our data also reveal that restricting glucose metabolism during hyperglycaemia prevents these changes and thus may be of therapeutic benefit. In summary, we have identified a pathway by which chronic hyperglycaemia reduces β-cell function.

Role of Reactive Oxygen Species in Glucose Metabolism Disorder in Diabetic Pancreatic β-Cells

The dysfunction of pancreatic β-cells plays a central role in the onset and progression of type 2 diabetes mellitus (T2DM). Insulin secretory defects in β-cells are characterized by a selective impairment of glucose stimulation, and a reduction in glucose-induced ATP production, which is essential for insulin secretion. High glucose metabolism for insulin secretion generates reactive oxygen species (ROS) in mitochondria. In addition, the expression of antioxidant enzymes is very low in β-cells. Therefore, β-cells are easily exposed to oxidative stress. In islet studies using a nonobese T2DM animal model that exhibits selective impairment of glucose-induced insulin secretion (GSIS), quenching ROS generated by glucose stimulation and accumulated under glucose toxicity can improve impaired GSIS. Acute ROS generation and toxicity cause glucose metabolism disorders through different molecular mechanisms. Nuclear factor erythroid 2-related factor 2 (Nrf2), a transcription factor, is a master regulator of antioxidant defense and a potential therapeutic target in oxidative stress-related diseases, suggesting the possible involvement of Nrf2 in β-cell dysfunction caused by ROS. In this review, we describe the mechanisms of insulin secretory defects induced by oxidative stress in diabetic β-cells.

Elucidating the metabolic characteristics of pancreatic β-cells from patients with type 2 diabetes (T2D) using a genome-scale metabolic modeling

Diabetes is a global health problem caused primarily by the inability of pancreatic β-cells to secrete adequate insulin. Despite extensive research, the identity of factors contributing to the dysregulated metabolism-secretion coupling in the β-cells remains elusive. The present study attempts to capture some of these factors responsible for the impaired β-cell metabolism-secretion coupling that contributes to diabetes pathogenesis. The metabolic-flux profiles of pancreatic β-cells were predicted using genome-scale metabolic modeling for ten diabetic patients and ten control subjects. Analysis of these flux states shows reduction in the mitochondrial fatty acid oxidation and mitochondrial oxidative phosphorylation pathways, that leads to decreased insulin secretion in diabetes. We also observed elevated reactive oxygen species (ROS) generation through peroxisomal fatty acid β-oxidation. In addition, cellular antioxidant defense systems were found to be attenuated in diabetes. Our analysis also uncovered the possible changes in the plasma metabolites in diabetes due to the β-cells failure. These efforts subsequently led to the identification of seven metabolites associated with cardiovascular disease (CVD) pathogenesis, thus establishing its link as a secondary complication of diabetes.

MicroRNA Sequences Modulated by Beta Cell Lipid Metabolism: Implications for Type 2 Diabetes Mellitus

Alterations in lipid metabolism within beta cells and islets contributes to dysfunction and apoptosis of beta cells, leading to loss of insulin secretion and the onset of type 2 diabetes. Over the last decade, there has been an explosion of interest in understanding the landscape of gene expression which influences beta cell function, including the importance of small non-coding microRNA sequences in this context. This review sought to identify the microRNA sequences regulated by metabolic challenges in beta cells and islets, their targets, highlight their function and assess their possible relevance as biomarkers of disease progression in diabetic individuals. Predictive analysis was used to explore networks of genes targeted by these microRNA sequences, which may offer new therapeutic strategies to protect beta cell function and delay the onset of type 2 diabetes.

A direct look at the dysfunction and pathology of the β cells in human type 2 diabetes

β cells uniquely produce and secrete insulin under the control of several, integrated signals, to maintain blood glucose concentrations within a narrow physiological interval. β cell failure is key to the onset and progression of type 2 diabetes, due to impaired function and reduced mass. In this review we focus on several features of human β cell dysfunction and pathology in type 2 diabetes, as revealed by direct assessment of isolated islet traits and examination of pancreatic tissue from organ donors, surgical samples or autoptic specimens. Insulin secretion defects and pathology findings are discussed in relation to some of the major underlying mechanisms, to also provide clues for conceiving better prevention and treatment of type 2 diabetes by targeting the pancreatic β cells.

Loss of Mitochondrial DNA by Gemcitabine Triggers Mitophagy and Cell Death

Gemcitabine (2,2-difluorodeoxycytidine nucleic acid), an anticancer drug exhibiting a potent ability to kill cancer cells, is a frontline chemotherapy drug. Although some chemotherapeutic medicines are known to induce nuclear DNA damage, no investigation into mitochondrial DNA (mtDNA) damage currently exists. When we treated insulinoma pancreatic β-cells (line INS-1) with high mitochondrial activity with gemcitabine for 24 h, the mtDNA contents were decreased. Gemcitabine induced a decrease in the number of mitochondria and the average potential of mitochondrial membrane in the cell but increased the superoxide anion radical levels. We observed that treatment with gemcitabine to induce cell death accompanied by autophagy-related protein markers, Atg5 and Atg7; these were significantly prevented by the autophagy inhibitors. The localization of Atg5 co-occurred with the location of mitochondria with membranes having high potential and mitophagy in cells treated with gemcitabine. The occurrence of mitophagy was inhibited by the inhibitors of the phosphatidylinositol 3-kinase/Akt pathway. Our results led us to the conclusion that gemcitabine induced cell death through mitophagy with the loss of mtDNA. These findings may provide a rationale for the combination of mtDNA damage with mitophagy in future clinical applications for cancer cells.

Mitochondrial Bioenergetics and Dynamics in Secretion Processes

Secretion is an energy consuming process that plays a relevant role in cell communication and adaptation to the environment. Among others, endocrine cells producing hormones, immune cells producing cytokines or antibodies, neurons releasing neurotransmitters at synapsis, and more recently acknowledged, senescent cells synthesizing and secreting multiple cytokines, growth factors and proteases, require energy to successfully accomplish the different stages of the secretion process. Calcium ions (Ca²⁺) act as second messengers regulating secretion in many of these cases. In this setting, mitochondria appear as key players providing ATP by oxidative phosphorylation, buffering Ca²⁺ concentrations and acting as structural platforms. These tasks also require the concerted actions of the mitochondrial dynamics machinery. These proteins mediate mitochondrial fusion and fission, and are also required for transport and tethering of mitochondria to cellular organelles where the different steps of the secretion process take place. Herein we present a brief overview of mitochondrial energy metabolism, mitochondrial dynamics, and the different steps of the secretion processes, along with evidence of the interaction between these pathways. We also analyze the role of mitochondria in secretion by different cell types in physiological and pathological settings.

The pathogenetic role of β-cell mitochondria in type 2 diabetes

Mitochondrial metabolism is a major determinant of insulin secretion from pancreatic β-cells. Type 2 diabetes evolves when β-cells fail to release appropriate amounts of insulin in response to glucose. This results in hyperglycemia and metabolic dysregulation. Evidence has recently been mounting that mitochondrial dysfunction plays an important role in these processes. Monogenic dysfunction of mitochondria is a rare condition but causes a type 2 diabetes-like syndrome owing to β-cell failure. Here, we describe novel advances in research on mitochondrial dysfunction in the β-cell in type 2 diabetes, with a focus on human studies. Relevant studies in animal and cell models of the disease are described. Transcriptional and translational regulation in mitochondria are particularly emphasized. The role of metabolic enzymes and pathways and their impact on β-cell function in type 2 diabetes pathophysiology are discussed. The role of genetic variation in mitochondrial function leading to type 2 diabetes is highlighted. We argue that alterations in mitochondria may be a culprit in the pathogenetic processes culminating in type 2 diabetes.

Transcribing β-cell mitochondria in health and disease

Background

The recent genome-wide association studies (GWAS) of Type 2 Diabetes (T2D) have identified the pancreatic β-cell as the culprit in the pathogenesis of the disease. Mitochondrial metabolism plays a crucial role in the processes controlling release of insulin and β-cell mass. This notion implies that mechanisms controlling mitochondrial function have the potential to play a decisive pathogenetic role in T2D.

Scope of the review

This article reviews studies demonstrating that there is indeed mitochondrial dysfunction in islets in T2D, and that GWAS have identified a variant in the gene encoding transcription factor B1 mitochondrial (TFB1M), predisposing to T2D due to mitochondrial dysfunction and impaired insulin secretion. Mechanistic studies of the nature of this pathogenetic link, as well as of other mitochondrial transcription factors, are described.

Major conclusions

Based on this, it is argued that transcription and translation in mitochondria are critical processes determining mitochondrial function in β-cells in health and disease.

Pancreatic Beta Cell Identity in Humans and the Role of Type 2 Diabetes

Pancreatic beta cells uniquely synthetize, store, and release insulin. Specific molecular, functional as well as ultrastructural traits characterize their insulin secretion properties and survival phentoype. In this review we focus on human islet/beta cells, and describe the changes that occur in type 2 diabetes and could play roles in the disease as well as represent possible targets for therapeutical interventions. These include transcription factors, molecules involved in glucose metabolism and insulin granule handling. Quantitative and qualitative insulin release patterns and their changes in type 2 diabetes are also associated with ultrastructural features involving the insulin granules, the mitochondria, and the endoplasmic reticulum.

Isocitrate-to-SENP1 signaling amplifies insulin secretion and rescues dysfunctional β cells

Insulin secretion from β cells of the pancreatic islets of Langerhans controls metabolic homeostasis and is impaired in individuals with type 2 diabetes (T2D). Increases in blood glucose trigger insulin release by closing ATP-sensitive K+ channels, depolarizing β cells, and opening voltage-dependent Ca2+ channels to elicit insulin exocytosis. However, one or more additional pathway(s) amplify the secretory response, likely at the distal exocytotic site. The mitochondrial export of isocitrate and engagement with cytosolic isocitrate dehydrogenase (ICDc) may be one key pathway, but the mechanism linking this to insulin secretion and its role in T2D have not been defined. Here, we show that the ICDc-dependent generation of NADPH and subsequent glutathione (GSH) reduction contribute to the amplification of insulin exocytosis via sentrin/SUMO-specific protease-1 (SENP1). In human T2D and an in vitro model of human islet dysfunction, the glucose-dependent amplification of exocytosis was impaired and could be rescued by introduction of signaling intermediates from this pathway. Moreover, islet-specific Senp1 deletion in mice caused impaired glucose tolerance by reducing the amplification of insulin exocytosis. Together, our results identify a pathway that links glucose metabolism to the amplification of insulin secretion and demonstrate that restoration of this axis rescues β cell function in T2D.

Glucose plus metformin compared with glucose alone on β-cell function in mouse pancreatic islets

Metformin is currently the first drug of choice for treatment of type II diabetes. The primary function of metformin is to decrease hepatic glucose production mainly by inhibiting gluconeogenesis. The aim of the present study was to investigate the effects of glucose alone (control groups) and glucose and metformin (treatment groups) on pancreatic islets functions. Pancreatic islets were isolated by collagenase digestion and incubated for 24 or 48 h in RPMI-1640 containing 5 mmol/l glucose (control groups 1 and 2, respectively) or 24 h with 25 mmol/l glucose (control group 3) and 15 µmol/l metformin (treatment groups 1, 2 and 3, corresponding to the control groups, respectively). Subsequently, the rate of insulin output from islets, pancreatic and duodenal homeobox 1 (Pdx-1) and insulin genes expression and islet viability were assayed. The rate of insulin secretion in a 5 mmol/l glucose concentration in the 48 h treatment group increased significantly compared with that of the 24 h treatment group (P<0.05). An increase of the glucose concentration (25 mmol/l) caused insulin secretion to increase compared with that of 5 mmol/l glucose. Pdx-1 gene expression in treatment group 2 significantly decreased compared with the control group 2 (P<0.05). The the Pdx-1 gene expression in treatment group 2 decreased compared with that of the treatment group 1. The expression of the insulin gene in treatment group 1 increased compared with control group 1, and in treatment group 2, there was a 2-fold increase in insulin gene expression compared with control group 2. The insulin gene expression in treatment group 2 increased compared with treatment group 1. The percentage of islet cell viability was increased in treatment group 3 by ~40% compared with the islet cells of treatment groups 1 and 2 (P<0/05). These data indicate that glucose and metformin have direct effects on β-cell function.

Supplementation of Coenzyme Q10 among Patients with Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a major cause of morbidity and mortality with ever increasing prevalence in the United States and worldwide. There is growing body of evidence suggesting that mitochondrial dysfunction secondary to oxidative stress plays a critical role in the pathogenesis of T2DM. Coenzyme Q10 is an important micronutrient acting on the electron transport chain of the mitochondria with two major functions: (1) synthesis of adenosine triphosphate (ATP); and (2) a potent antioxidant. Deficiency in coenzyme Q10 is often seen in patients with T2DM. Whether restoration of coenzyme Q10 will help alleviate oxidative stress, preserve mitochondrial function, and thus improve glycemic control in T2DM is unclear. This article reviews the relationships among oxidative stress, mitochondrial dysfunction, and T2DM and examines the evidence for potential use of coenzyme Q10 as a supplement for the treatment of T2DM.

CNX-011-67, a novel GPR40 agonist, enhances glucose responsiveness, insulin secretion and islet insulin content in n-STZ rats and in islets from type 2 diabetic patients

BACKGROUND

GPR40 is a G-protein coupled receptor regulating free fatty acid induced and also glucose induced insulin secretion. We generated neonatally-streptozotocin-treated female rats (n-STZ) and treated them with CNX-011-67, a GPR40 agonist to examine the role of GPR40 in modulation of glucose metabolism, insulin secretion and content.

METHODS

Female n-STZ animals were orally administered with CNX-011-67 (15 mg/kg body weight, twice daily) or with vehicle for 8 weeks (n = 8 per group). Glucose tolerance in treated animals and insulin secretion, islet insulin content and gene expression in isolated islets were determined. Islets from type 2 diabetic mellitus (T2DM) patients were treated with different concentrations of glucose in presence or absence of CNX-011-67 and insulin secretion was measured.

RESULTS

Treatment of n-STZ rats with GPR40 agonist CNX-011-67 enhanced insulin secretion in response to oral glucose load on day 0 and this response persisted during the treatment period. The treatment also produced a 'memory effect' during which insulin secretion in response to oral glucose load remained enhanced, for a week, even in absence of the agonist. Activation of GPR40 enhanced responsiveness of islets to glucose and increased glucose induced insulin secretion and islet insulin content. An increase in islet mRNA expression of GCK, PDX1, insulin and PC was also observed. Acute treatment of islets from n-STZ rats with GPR40 agonist enhanced cellular ATP content. Activation of GPR40 enhanced mitochondrial calcium level in NIT-1 insulinoma cells. CNX-011-67 increased insulin secretion in islets from T2DM patients which were non-responsive to increased glucose concentration

CONCLUSIONS

Our data provide evidence that activation of GPR40 with CNX-011-67 stimulates glucose metabolism, enhances glucose responsiveness, increases insulin secretion and content and that pharmacological activation of GPR40 will prove beneficial for treatment of T2DM.

The pancreatic beta cells in human type 2 diabetes

Bell-cell (beta-cell) impairment is central to the development and progression of human diabetes, as a result of the combined effects of genetic and acquired factors. Reduced islet number and/or reduced beta cells amount in the pancreas of individuals with Type 2 diabetes have been consistently reported. This is mainly due to increased beta cell death, not adequately compensated for by regeneration. In addition, several quantitative and/or qualitative defects of insulin secretion have been observed in Type 2 diabetes, both in vivo and ex vivo with isolated islets. All this is associated with modifications of islet cell gene and protein expression. With the identification of several susceptible Type 2 diabetes loci, the role of genotype in affecting beta-cell function and survival has been addressed in a few studies and the relationships between genotype and beta-cell phenotype investigated. Among acquired factors, the importance of metabolic insults (in particular glucotoxicity and lipotoxicity) in the natural history of beta-cell damage has been widely underlined. Continuous improvements in our knowledge of the beta cells in human Type 2 diabetes will lead to more targeted and effective strategies for the prevention and treatment of the disease.

The function and the role of the mitochondrial glycerol-3-phosphate dehydrogenase in mammalian tissues

Mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is not included in the traditional textbook schemes of the respiratory chain, reflecting the fact that it is a non-standard, tissue-specific component of mammalian mitochondria. But despite its very simple structure, mGPDH is a very important enzyme of intermediary metabolism and as a component of glycerophosphate shuttle it functions at the crossroads of glycolysis, oxidative phosphorylation and fatty acid metabolism. In this review we summarize the present knowledge on the structure and regulation of mGPDH and discuss its metabolic functions, reactive oxygen species production and tissue and organ specific roles in mammalian mitochondria at physiological and pathological conditions.

Streptozotocin-nicotinamide-induced diabetes in the rat. Characteristics of the experimental model

Administration of both streptozotocin (STZ) and nicotinamide (NA) has been proposed to induce experimental diabetes in the rat. STZ is well known to cause pancreatic B-cell damage, whereas NA is administered to rats to partially protect insulin-secreting cells against STZ. STZ is transported into B-cells via the glucose transporter GLUT2 and causes DNA damage leading to increased activity of poly(ADP-ribose) polymerase (PARP-1) to repair DNA. However, exaggerated activity of this enzyme results in depletion of intracellular NAD(+) and ATP, and the insulin-secreting cells undergo necrosis. The protective action of NA is due to the inhibition of PARP-1 activity. NA inhibits this enzyme, preventing depletion of NAD(+) and ATP in cells exposed to STZ. Moreover, NA serves as a precursor of NAD(+) and thereby additionally increases intracellular NAD(+) levels. The severity of diabetes in experimental rats strongly depends on the doses of STZ and NA given to these animals. Therefore, in diabetic rats, blood glucose may be changed in a broad range--from slight hyperglycemia to substantial hyperglycemia compared with control animals. Similarly, blood insulin may be only slightly decreased or substantial hypoinsulinemia may be induced. In vitro studies demonstrated that the insulin-secretory response to glucose is attenuated in STZ-NA-induced diabetic rats compared with control animals. This is due to reduced B-cell mass as well as metabolic defects in the insulin-secreting cells. Results of numerous experiments have demonstrated that this model of diabetes is useful in studies of different aspects of diabetes.

The role of peroxidation of mitochondrial membrane phospholipids in pancreatic β -cell failure

Type 2 diabetes (T2D) is characterized by peripheral insulin resistance and pancreatic islet β-cell failure. Accumulating evidence indicates that mitochondrial dysfunction is a central contributor to β-cell failure in the pathogenesis of T2D. This review focuses on mechanisms whereby reactive oxygen species (ROS) produced by β-cell in response to metabolic stress affect mitochondrial structure and function and lead to β-cell failure. Specifically, ROS oxidize mitochondrial membrane phospholipids such as cardiolipin, which impairs membrane integrity and leads to cytochrome c release and apoptosis. In addition, ROS activate UCP2 via peroxidation of the mitochondrial membrane phospholipids, which results in proton leak leading to reduced ATP synthesis and content in β-cells - critical parameters in the regulation of glucose-stimulated insulin secretion. Group VIA Phospholipase A2 (iPLA2β) appears to be a component of a mechanism for repairing mitochondrial phospholipids that contain oxidized fatty acid substituents, and genetic or acquired iPLA2β-deficiency increases β-cell mitochondrial susceptibility to injury from ROS and predisposes to development of T2D. Interventions that attenuate the adverse effects of ROS on β-cell mitochondrial phospholipids may prevent or retard the development of T2D.

Mitochondrial dysfunction and β-cell failure in type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is the most common human endocrine disease and is characterized by peripheral insulin resistance and pancreatic islet β-cell failure. Accumulating evidence indicates that mitochondrial dysfunction is a central contributor to β-cell failure in the evolution of T2DM. As reviewed elsewhere, reactive oxygen species (ROS) produced by β-cell mitochondria as a result of metabolic stress activate several stress-response pathways. This paper focuses on mechanisms whereby ROS affect mitochondrial structure and function and lead to β-cell failure. ROS activate UCP2, which results in proton leak across the mitochondrial inner membrane, and this leads to reduced β-cell ATP synthesis and content, which is a critical parameter in regulating glucose-stimulated insulin secretion. In addition, ROS oxidize polyunsaturated fatty acids in mitochondrial cardiolipin and other phospholipids, and this impairs membrane integrity and leads to cytochrome c release into cytosol and apoptosis. Group VIA phospholipase A₂ (iPLA₂β) appears to be a component of a mechanism for repairing mitochondrial phospholipids that contain oxidized fatty acid substituents, and genetic or acquired iPLA₂β-deficiency increases β-cell mitochondrial susceptibility to injury from ROS and predisposes to developing T2DM. Interventions that attenuate ROS effects on β-cell mitochondrial phospholipids might prevent or retard development of T2DM.

Role of endogenous ROS production in impaired metabolism-secretion coupling of diabetic pancreatic β cells

One of the characteristics of type 2 diabetes is that the insulin secretory response of β cells is selectively impaired to glucose. In the Goto-Kakizaki (GK) rat, a genetic model of type 2 diabetes mellitus, glucose-induced insulin secretion is selectively impaired due to deficient ATP production derived from impaired glucose metabolism. In addition, islets in GK rat and human type 2 diabetes are oxidatively stressed. In this issue, role of endogenous reactive oxygen species (ROS) production in impaired metabolism-secretion coupling of diabetic pancreatic β cells is reviewed. In β cells, ROS is endogenously produced by activation of Src, a non-receptor tyrosine kinase. Src inhibitors restore the impaired insulin release and impaired ATP elevation by reduction in ROS production in diabetic islets. Src is endogenously activated in diabetic islets, since the level of Src pY416 in GK islets is higher than that in control islets. In addition, exendin-4, a glucagon-like peptide-1 (GLP-1) receptor agonist, decreases Src pY416 and glucose-induced ROS production and ameliorates impaired ATP production dependently on Epac in GK islets. These results indicate that GLP-1 signaling regulates endogenous ROS production due to Src activation and that incretin has unique therapeutic effects on impaired glucose metabolism in diabetic β cells.

Metabolomic analyses reveal profound differences in glycolytic and tricarboxylic acid cycle metabolism in glucose-responsive and -unresponsive clonal β-cell lines

Insulin secretion from pancreatic β-cells is controlled by complex metabolic and energetic changes provoked by exposure to metabolic fuels. Perturbations in these processes lead to impaired insulin secretion, the ultimate cause of T2D (Type 2 diabetes). To increase our understanding of stimulus-secretion coupling and metabolic processes potentially involved in the pathogenesis of T2D, a comprehensive investigation of the metabolic response in the glucose-responsive INS-1 832/13 and glucose-unresponsive INS-1 832/2 β-cell lines was performed. For this metabolomics analysis, we used GC/MS (gas chromatography/mass spectrometry) combined with multivariate statistics. We found that perturbed secretion in the 832/2 line was characterized by disturbed coupling of glycolytic and TCA (tricarboxylic acid)-cycle metabolism. The importance of this metabolic coupling was reinforced by our observation that insulin secretion partially could be reinstated by stimulation of the cells with mitochondrial fuels which bypass glycolytic metabolism. Furthermore, metabolic and functional profiling of additional β-cell lines (INS-1, INS-1 832/1) confirmed the important role of coupled glycolytic and TCA-cycle metabolism in stimulus-secretion coupling. Dependence of the unresponsive clones on glycolytic metabolism was paralleled by increased stabilization of HIF-1α (hypoxia-inducible factor 1α). The relevance of a similar perturbation for human T2D was suggested by increased expression of HIF-1α target genes in islets from T2D patients.

Differences between human and rodent pancreatic islets: low pyruvate carboxylase, atp citrate lyase, and pyruvate carboxylation and high glucose-stimulated acetoacetate in human pancreatic islets

Anaplerosis, the net synthesis in mitochondria of citric acid cycle intermediates, and cataplerosis, their export to the cytosol, have been shown to be important for insulin secretion in rodent beta cells. However, human islets may be different. We observed that the enzyme activity, protein level, and relative mRNA level of the key anaplerotic enzyme pyruvate carboxylase (PC) were 80-90% lower in human pancreatic islets compared with islets of rats and mice and the rat insulinoma cell line INS-1 832/13. Activity and protein of ATP citrate lyase, which uses anaplerotic products in the cytosol, were 60-75% lower in human islets than in rodent islets or the cell line. In line with the lower PC, the percentage of glucose-derived pyruvate that entered mitochondrial metabolism via carboxylation in human islets was only 20-30% that in rat islets. This suggests human islets depend less on pyruvate carboxylation than rodent models that were used to establish the role of PC in insulin secretion. Human islets possessed high levels of succinyl-CoA:3-ketoacid-CoA transferase, an enzyme that forms acetoacetate in the mitochondria, and acetoacetyl-CoA synthetase, which uses acetoacetate to form acyl-CoAs in the cytosol. Glucose-stimulated human islets released insulin similarly to rat islets but formed much more acetoacetate. β-Hydroxybutyrate augmented insulin secretion in human islets. This information supports previous data that indicate beta cells can use a pathway involving succinyl-CoA:3-ketoacid-CoA transferase and acetoacetyl-CoA synthetase to synthesize and use acetoacetate and suggests human islets may use this pathway more than PC and citrate to form cytosolic acyl-CoAs.

Regulation of insulin secretion: role of mitochondrial signalling

Pancreatic beta cells are specialised endocrine cells that continuously sense the levels of blood sugar and other fuels and, in response, secrete insulin to maintain normal fuel homeostasis. During postprandial periods an elevated level of plasma glucose rapidly stimulates insulin secretion to decrease hepatic glucose output and promote glucose uptake into other tissues, principally muscle and adipose tissues. Beta cell mitochondria play a key role in this process, not only by providing energy in the form of ATP to support insulin secretion, but also by synthesising metabolites (anaplerosis) that can act, both intra- and extramitochondrially, as factors that couple glucose sensing to insulin granule exocytosis. ATP on its own, and possibly modulated by these coupling factors, triggers closure of the ATP-sensitive potassium channel, resulting in membrane depolarisation that increases intracellular calcium to cause insulin secretion. The metabolic imbalance caused by chronic hyperglycaemia and hyperlipidaemia severely affects mitochondrial metabolism, leading to the development of impaired glucose-induced insulin secretion in type 2 diabetes. It appears that the anaplerotic enzyme pyruvate carboxylase participates directly or indirectly in several metabolic pathways which are important for glucose-induced insulin secretion, including: the pyruvate/malate cycle, the pyruvate/citrate cycle, the pyruvate/isocitrate cycle and glutamate-dehydrogenase-catalysed alpha-ketoglutarate production. These four pathways enable 'shuttling' or 'recycling' of these intermediate(s) into and out of mitochondrion, allowing continuous production of intracellular messenger(s). The purpose of this review is to present an account of recent progress in this area of central importance in the realm of diabetes and obesity research.

Glucose sensing in the pancreatic beta cell: a computational systems analysis

Background

Pancreatic beta-cells respond to rising blood glucose by increasing oxidative metabolism, leading to an increased ATP/ADP ratio in the cytoplasm. This leads to a closure of K_ATP channels, depolarization of the plasma membrane, influx of calcium and the eventual secretion of insulin. Such mechanism suggests that beta-cell metabolism should have a functional regulation specific to secretion, as opposed to coupling to contraction. The goal of this work is to uncover contributions of the cytoplasmic and mitochondrial processes in this secretory coupling mechanism using mathematical modeling in a systems biology approach.

Methods

We describe a mathematical model of beta-cell sensitivity to glucose. The cytoplasmic part of the model includes equations describing glucokinase, glycolysis, pyruvate reduction, NADH and ATP production and consumption. The mitochondrial part begins with production of NADH, which is regulated by pyruvate dehydrogenase. NADH is used in the electron transport chain to establish a proton motive force, driving the F₁F₀ ATPase. Redox shuttles and mitochondrial Ca²⁺ handling were also modeled.

Results

The model correctly predicts changes in the ATP/ADP ratio, Ca²⁺ and other metabolic parameters in response to changes in substrate delivery at steady-state and during cytoplasmic Ca²⁺ oscillations. Our analysis of the model simulations suggests that the mitochondrial membrane potential should be relatively lower in beta cells compared with other cell types to permit precise mitochondrial regulation of the cytoplasmic ATP/ADP ratio. This key difference may follow from a relative reduction in respiratory activity. The model demonstrates how activity of lactate dehydrogenase, uncoupling proteins and the redox shuttles can regulate beta-cell function in concert; that independent oscillations of cytoplasmic Ca²⁺ can lead to slow coupled metabolic oscillations; and that the relatively low production rate of reactive oxygen species in beta-cells under physiological conditions is a consequence of the relatively decreased mitochondrial membrane potential.

Conclusion

This comprehensive model predicts a special role for mitochondrial control mechanisms in insulin secretion and ROS generation in the beta cell. The model can be used for testing and generating control hypotheses and will help to provide a more complete understanding of beta-cell glucose-sensing central to the physiology and pathology of pancreatic β-cells.

Lower succinyl-CoA:3-ketoacid-CoA transferase (SCOT) and ATP citrate lyase in pancreatic islets of a rat model of type 2 diabetes: knockdown of SCOT inhibits insulin release in rat insulinoma cells

Succinyl-CoA:3-ketoacid-CoA transferase (SCOT) is a mitochondrial enzyme that catalyzes the reversible transfer of coenzyme-A from acetoacetyl-CoA to succinate to form acetoacetate and succinyl-CoA. mRNAs of SCOT and ATP citrate lyase were decreased 55% and 58% and enzyme activities were decreased >70% in pancreatic islets of the GK rat, a model of type 2 diabetes. INS-1 832/13 cells were transfected with shRNAs targeting SCOT mRNA to generate cell lines with reduced SCOT activity. Two cell lines with >70% knockdown of SCOT activity showed >70% reduction in glucose- or methyl succinate-plus-beta-hydroxybutyrate-stimulated insulin release. Less inhibition of insulin release was observed with two cell lines with less knockdown of SCOT. Previous studies showed knockdown of ATP citrate lyase in INS-1 832/13 cells does not lower insulin release. The results further support work that suggests mitochondrial pathways involving SCOT which supply acetoacetate for export to the cytosol are important for insulin secretion.

Decreased levels of metabolic enzymes in pancreatic islets of patients with type 2 diabetes

AIMS/HYPOTHESIS

Glucose-stimulated insulin secretion is defective in patients with type 2 diabetes. We sought to acquire new information about enzymes of glucose metabolism, with an emphasis on mitochondrial enzymes, by comparing pancreatic islets of type 2 diabetes patients with those of non-diabetic controls.

METHODS

Expression of genes encoding 13 metabolic enzymes was estimated with microarrays and activities of up to nine metabolic enzymes were measured.

RESULTS

The activities of the mitochondrial enzymes, glycerol phosphate dehydrogenase, pyruvate carboxylase (PC) and succinyl-CoA:3-ketoacid-CoA transferase (SCOT) were decreased by 73%, 65% and 92%, respectively, in the diabetic compared with the non-diabetic islets. ATP citrate lyase, a cytosolic enzyme of the mitochondrial citrate pyruvate shuttle, was decreased 57%. Activities of propionyl-CoA carboxylase, NADP-isocitrate dehydrogenase, cytosolic malic enzyme, aspartate aminotransferase and malate dehydrogenase were not significantly different from those of the control. The low activities of PC and SCOT were confirmed with western blots, which showed that their protein levels were low. The correlation of relative mRNA signals with enzyme activities was good in four instances, moderate in four instances and poor in one instance. In diabetic islets, the mRNA signal of the islet cell-enriched transcription factor musculoaponeurotic fibrosarcoma oncogene homologue A, which regulates expression of islet genes, including the PC gene, was decreased to 54% of the control level. PC activity and protein levels in the non-diabetic islets were significantly lower than in islets from non-diabetic rodents.

CONCLUSIONS/INTERPRETATION

Low levels of certain islet metabolic enzymes, especially mitochondrial enzymes, are associated with human type 2 diabetes.

Pancreatic beta-cell mass in European subjects with type 2 diabetes

Decreases in both beta-cell function and number can contribute to insulin deficiency in type 2 diabetes. Here, we quantified the beta-cell mass in pancreas obtained at autopsy of 57 type 2 diabetic (T2D) and 52 non-diabetic subjects of European origin. Sections from the body and tail were immunostained for insulin. The beta-cell mass was calculated from the volume density of beta-cells (measured by point-counting methods) and the weight of the pancreas. The pancreatic insulin concentration was measured in some of the subjects. beta-cell mass increased only slightly with body mass index (BMI). After matching for BMI, the beta-cell mass was 41% (BMI < 25) and 38% (BMI 26-40) lower in T2D compared with non-diabetic subjects, and neither gender nor type of treatment influenced these differences. beta-cell mass did not correlate with age at diagnosis but decreased with duration of clinical diabetes (24 and 54% lower than controls in subjects with <5 and >15 years of overt diabetes respectively). Pancreatic insulin concentration was 30% lower in patients. In conclusion, the average beta-cell mass is about 39% lower in T2D subjects compared with matched controls. Its decrease with duration of the disease could be a consequence of diabetes that, with further impairment of insulin secretion, contributes to the progressive deterioration of glucose homeostasis. We do not believe that the small difference in beta-cell mass observed within 5 years of onset could cause diabetes in the absence of beta-cell dysfunction.

An overview of pancreatic beta-cell defects in human type 2 diabetes: Implications for treatment

Type 2 diabetes is the most common form of diabetes in humans. It results from a combination of factors that impair beta-cell function and tissue insulin sensitivity. However, growing evidence is showing that the beta-cell is central to the development and progression of this form of diabetes. Reduced islet and/or insulin-containing cell mass or volume in Type 2 diabetes has been reported by several authors. Furthermore, studies with isolated Type 2 diabetic islets have consistently shown both quantitative and qualitative defects of glucose-stimulated insulin secretion. The impact of genotype in affecting beta-cell function and survival is a very fast growing field or research, and several gene polymorphisms have been associated with this form of diabetes. Among acquired factors, glucotoxicity, lipotoxicity and altered IAPP processing are likely to play an important role. Interestingly, however, pharmacological intervention can improve several defects of Type 2 diabetes islet cells in vitro, suggesting that progression of the disease might not be relentless.

The endoplasmic reticulum in pancreatic beta cells of type 2 diabetes patients

AIMS/HYPOTHESIS

Pancreatic beta cells have highly developed endoplasmic reticulum (ER) due to their role in insulin secretion. Since ER stress has been associated with beta cell dysfunction, we studied several features of beta cell ER in human type 2 diabetes.

METHODS

Pancreatic samples and/or isolated islets from non-diabetic controls (ND) and type 2 diabetes patients were evaluated for insulin secretion, apoptosis (electron microscopy and ELISA), morphometric ER assessment (electron microscopy), and expression of ER stress markers in beta cell prepared by laser capture microdissection and in isolated islets.

RESULTS

Insulin release was lower and beta cell apoptosis higher in type 2 diabetes than ND islets. ER density volume was significantly increased in type 2 diabetes beta cells. Expression of alpha-mannosidase (also known as mannosidase, alpha, class 1A, member 1) and UDP-glucose glycoprotein glucosyl transferase like 2 (UGCGL2), assessed by microarray and/or real-time reverse transcriptase polymerase chain reaction (RT-PCR), differed between ND and type 2 diabetes beta cells. Expression of immunoglobulin heavy chain binding protein (BiP, also known as heat shock 70 kDa protein 5 [glucose-regulated protein, 78 kDa] [HSPA5]), X-box binding protein 1 (XBP-1, also known as XBP1) and C/EBP homologous protein (CHOP, also known as damage-inducible transcript 3 [DDIT3]) was not higher in type 2 diabetes beta cell or isolated islets cultured at 5.5 mmol/l glucose (microarray and real-time RT-PCR) than in ND samples. When islets were cultured for 24 h at 11.1 mmol/l glucose, there was induction of BiP and XBP-1 in type 2 diabetes islets but not in ND islets.

CONCLUSIONS/INTERPRETATION

Beta cell in type 2 diabetes showed modest signs of ER stress when studied in pancreatic samples or isolated islets maintained at physiological glucose concentration. However, exposure to increased glucose levels induced ER stress markers in type 2 diabetes islet cells, which therefore may be more susceptible to ER stress induced by metabolic perturbations.

Islet gene expression and function in type 2 diabetes; studies in the Goto-Kakizaki rat and humans

Defective beta-cell function with resulting impairment of glucose-stimulated insulin release is a critical factor in the pathogenesis of type 2 diabetes. Accumulated studies in pancreatic islets of the spontaneously diabetic Goto-Kakizaki (GK) rat suggest that this is a useful animal model of type 2 diabetes. The GK rat is non-obese, and abnormal glucose regulation develops early in life in association with impaired insulin secretion. There are some differences in islet morphology and function reported between different GK rat colonies. In addition to reduction of beta-cell mass, a number of beta-cell defects have been described with possible relevance for the reduced insulin secretion. Interestingly, some of these defects have also been shown in isolated islets from type 2 diabetic humans. The polygenic nature of diabetes heredity in the GK rat may well resemble the genetic basis in the majority of patients with type 2 diabetes. Here, we review studies concerning beta-cell function and islet gene expression in the GK rat and compare it with the limited number of investigations on similar topics in isolated islets from patients with type 2 diabetes.

Impaired metabolism-secretion coupling in pancreatic beta-cells: role of determinants of mitochondrial ATP production

Glucose-induced insulin secretion from beta-cells is often impaired in diabetic condition and by exposure to diabetogenic pharmacological agents. In pancreatic beta-cells, intracellular glucose metabolism regulates exocytosis of insulin granules, according to metabolism-secretion coupling in which glucose-induced mitochondrial ATP production plays an essential role. Impaired glucose-induced insulin secretion often results from impaired glucose-induced ATP elevation in beta-cells. Mitochondrial ATP production is driven by the proton-motive force including mitochondrial membrane potential (DeltaPsi(m)) generated by the electron transport chain. These electrons are derived from reducing equivalents, generated in the Krebs cycle and transferred from cytosol by the shuttles. Here, roles of the determinants of mitochondrial ATP production in impaired glucose-induced insulin secretion are discussed. Cytosolic alkalization, H(+) leak in the inner membrane by uncoupler (e.g. free fatty acid exposure), decrease in the supply of electron donors including NADH and FADH(2) to the respiratory chain, and endogenous mitochondrial ROS (e.g. Na(+)/K(+)-ATPase inhibition) all reduce hyperpolarlization of DeltaPsi(m) and ATP production, causing decresed glucose-induced insulin release. The decrease in the supply of NADH and FADH(2) to the respiratory chain derives from impairments in glucose metabolism including glycolysis (e.g. MODY2 and exposure to NO) and the shuttles (e.g. diabetic state and exposure to ketone body).

Nicotinamide nucleotide transhydrogenase: a link between insulin secretion, glucose metabolism and oxidative stress

This paper reviews recent studies on the role of Nnt (nicotinamide nucleotide transhydrogenase) in insulin secretion and detoxification of ROS (reactive oxygen species). Glucose-stimulated insulin release from pancreatic beta-cells is mediated by increased metabolism. This elevates intracellular [ATP], thereby closing KATP channels (ATP-sensitive potassium channels) and producing membrane depolarization, activation of voltage-gated Ca2+ channels, Ca2+ influx and, consequently, insulin secretion. The C57BL/6J mouse displays glucose intolerance and reduced insulin secretion, which results from a naturally occurring deletion in the Nnt gene. Transgenic expression of the wild-type Nnt gene in C57BL/6J mice rescues the phenotype. Knockdown of Nnt in the insulin-secreting cell line MIN6 with small interfering RNA dramatically reduced Ca2+ influx and insulin secretion. Similarly, mice carrying ENU (N-ethyl-N-nitrosourea)-induced loss-of-function mutations in Nnt were glucose intolerant and secreted less insulin during a glucose tolerance test. Islets isolated from these mice showed impaired insulin secretion in response to glucose, but not to the KATP channel blocker tolbutamide. This is explained by the fact that glucose failed to elevate ATP in Nnt mutant islets. Nnt is a nuclear-encoded mitochondrial protein involved in detoxification of ROS. beta-Cells isolated from Nnt mutant mice showed increased ROS production on glucose stimulation. We hypothesize that Nnt mutations enhance glucose-dependent ROS production and thereby impair beta-cell mitochondrial metabolism, possibly via activation of uncoupling proteins. This reduces ATP production and lowers KATP channel activity. Consequently, glucose-dependent electrical activity and insulin secretion are impaired.

Nutrient control of insulin secretion in isolated normal human islets

Pancreatic islets were isolated from 16 nondiabetic organ donors and, after culture for approximately 2 days in 5 mmol/l glucose, were perifused to characterize nutrient-induced insulin secretion in human islets. Stepwise increases from 0 to 30 mmol/l glucose (eight 30-min steps) evoked concentration-dependent insulin secretion with a threshold at 3-4 mmol/l glucose, K(m) at 6.5 mmol/l glucose, and V(max) at 15 mmol/l glucose. An increase from 1 to 15 mmol/l glucose induced biphasic insulin secretion with a prominent first phase (peak increase of approximately 18-fold) and a sustained, flat second phase ( approximately 10-fold increase), which were both potentiated by forskolin. The central role of ATP-sensitive K(+) channels in the response to glucose was established by abrogation of insulin secretion by diazoxide and reversible restoration by tolbutamide. Depolarization with tolbutamide or KCl (plus diazoxide) triggered rapid insulin secretion in 1 mmol/l glucose. Subsequent application of 15 mmol/l glucose further increased insulin secretion, showing that the amplifying pathway is operative. In control medium, glutamine alone was ineffective, but its combination with leucine or nonmetabolized 2-amino-bicyclo [2,2,1]-heptane-2-carboxylic acid (BCH) evoked rapid insulin secretion. The effect of BCH was larger in low glucose than in high glucose. In contrast, the insulin secretion response to arginine or a mixture of four amino acids was potentiated by glucose or tolbutamide. Palmitate slightly augmented insulin secretion only at the supraphysiological palmitate-to-albumin ratio of 5. Inosine and membrane-permeant analogs of pyruvate, glutamate, or succinate increased insulin secretion in 3 and 10 mmol/l glucose, whereas lactate and pyruvate had no effect. In conclusion, nutrient-induced insulin secretion in normal human islets is larger than often reported. Its characteristics are globally similar to those of insulin secretion by rodent islets, with both triggering and amplifying pathways. The pattern of the biphasic response to glucose is superimposable on that in mouse islets, but the concentration-response curve is shifted to the left, and various nutrients, in particular amino acids, influence insulin secretion within the physiological range of glucose concentrations.

The pancreatic beta-cell in human Type 2 diabetes

AIM

There is growing evidence that the beta-cell is central to the development of Type 2 diabetes. In this brief review we discuss the factors predisposing to beta-cell dysfunction and some characteristics of islet cells in Type 2 diabetes.

DATA SYNTHESIS

Several genes have been associated with islet cell dysfunction in Type 2 diabetes, including those encoding for transcription factors, glucose metabolism proteins, molecules of the insulin signaling pathways, and several others. On the other hand, many environmental factors can directly or indirectly affect pancreatic islet cells, and possibly contribute to the development and/or progression of Type 2 diabetes. In this regard, the role of prolonged exposure to high glucose (glucotoxicity) and high fatty acid (lipotoxicity) concentrations seems to be of particular relevance. More recently, it has been possible to directly evaluate some properties of pancreatic islets prepared from Type 2 diabetic donors. Consistently, a marked decrease in insulin secretion during glucose stimulation has been found, although the secretory response to amino acids or sulphonylurea is usually less severely affected. In addition, increased beta-cell apoptosis in Type 2 diabetes islets has been reported. Interestingly, experimental data show that in vitro manipulation of human Type 2 diabetes islets by agents that are able to reduce oxidative stress can improve beta-cell function and survival.

CONCLUSION

Available data are consistent with the concept that the defect of the beta-cell is of primary importance in Type 2 diabetes; the evidence that some alterations in Type 2 diabetes beta-cells can be reverted, at least in vitro, may open new perspectives in the treatment of this disease.

Proteomic Profiling of Hepatic Endoplasmic Reticulum-associated Proteins in an Animal Model of Insulin Resistance and Metabolic Dyslipidemia

Hepatic insulin resistance and lipoprotein overproduction are common features of the metabolic syndrome and insulin-resistant states. A fructose-fed, insulin-resistant hamster model was recently developed to investigate mechanisms linking the development of hepatic insulin resistance and overproduction of atherogenic lipoproteins. Here we report a systematic analysis of protein expression profiles in the endoplasmic reticulum (ER) fractions isolated from livers of fructose-fed hamsters with the intention of identifying new candidate proteins involved in hepatic complications of insulin resistance and lipoprotein dysregulation. We have profiled hepatic ER-associated proteins from chow-fed (control) and fructose-fed (insulin-resistant) hamsters using two-dimensional gel electrophoresis and mass spectrometry. A total of 26 large scale two-dimensional gels of hepatic ER were used to identify 34 differentially expressed hepatic ER protein spots observed to be at least 2-fold differentially expressed with fructose feeding and the onset of insulin resistance. Differentially expressed proteins were identified by matrix-assisted laser desorption ionization-quadrupole time of flight (MALDI-Q-TOF), MALDI-TOF-postsource decay, and database mining using ProteinProspector MS-fit and MS-tag or the PROWL ProFound search engine using a focused rodent or mammalian search. Hepatic ER proteins ER60, ERp46, ERp29, glutamate dehydrogenase, and TAP1 were shown to be more than 2-fold down-regulated, whereas alpha-glucosidase, P-glycoprotein, fibrinogen, protein disulfide isomerase, GRP94, and apolipoprotein E were all found to be up-regulated in the hepatic ER of the fructose-fed hamster. Seven isoforms of ER60 in the hepatic ER were all shown to be down-regulated at least 2-fold in hepatocytes from fructosefed/insulin-resistant hamsters. Implications of the differential expression of positively identified protein factors in the development of hepatic insulin resistance and lipoprotein abnormalities are discussed.

Functional and molecular defects of pancreatic islets in human type 2 diabetes

To shed further light on the primary alterations of insulin secretion in type 2 diabetes and the possible mechanisms involved, we studied several functional and molecular properties of islets isolated from the pancreata of 13 type 2 diabetic and 13 matched nondiabetic cadaveric organ donors. Glucose-stimulated insulin secretion from type 2 diabetic islets was significantly lower than from control islets, whereas arginine- and glibenclamide-stimulated insulin release was less markedly affected. The defects were accompanied by reduced mRNA expression of GLUT1 and -2 and glucokinase and by diminished glucose oxidation. In addition, AMP-activated protein kinase activation was reduced. Furthermore, the expression of insulin was decreased, and that of pancreatic duodenal homeobox-1 (PDX-1) and forkhead box O1 (Foxo-1) was increased. Nitrotyrosine and 8-hydroxy-2'-deoxyguanosine concentrations, markers of oxidative stress, were significantly higher in type 2 diabetic than control islets, and they were correlated with the degree of glucose-stimulated insulin release impairment. Accordingly, 24-h exposure to glutathione significantly improved glucose-stimulated insulin release and decreased nitrotyrosine concentration, with partial recovery of insulin mRNA expression. These results provide direct evidence that the defects of insulin secretion in type 2 diabetic islets are associated with multiple islet cell alterations. Most importantly, the current study shows that the functional impairment of type 2 diabetic islets can be, at least in part, reversible. In this regard, it is suggested that reducing islet cell oxidative stress is a potential target of human type 2 diabetes therapy.

Functional and morphological alterations of mitochondria in pancreatic beta cells from type 2 diabetic patients

AIMS/HYPOTHESIS

Little information is available on the insulin release properties of pancreatic islets isolated from type 2 diabetic subjects. Since mitochondria represent the site where important metabolites that regulate insulin secretion are generated, we studied insulin release as well as mitochondrial function and morphology directly in pancreatic islets isolated from type 2 diabetic patients.

METHODS

Islets were prepared by collagenase digestion and density gradient purification, and insulin secretion in response to glucose and arginine was assessed by the batch incubation method. Adenine nucleotides, mitochondrial membrane potential, the expression of UCP-2, complex I and complex V of the respiratory chain, and nitrotyrosine levels were evaluated and correlated with insulin secretion.

RESULTS

Compared to control islets, diabetic islets showed reduced insulin secretion in response to glucose, and this defect was associated with lower ATP levels, a lower ATP/ADP ratio and impaired hyperpolarization of the mitochondrial membrane. Increased protein expression of UCP-2, complex I and complex V of the respiratory chain, and a higher level of nitrotyrosine were also found in type 2 diabetic islets. Morphology studies showed that control and diabetic beta cells had a similar number of mitochondria; however, mitochondrial density volume was significantly higher in type 2 diabetic beta cells.

CONCLUSIONS/INTERPRETATION

In pancreatic beta cells from type 2 diabetic subjects, the impaired secretory response to glucose is associated with a marked alteration of mitochondrial function and morphology. In particular, UCP-2 expression is increased (probably due to a condition of fuel overload), which leads to lower ATP, decreased ATP/ADP ratio, with consequent reduction of insulin release.

Pathways in beta-cell stimulus-secretion coupling as targets for therapeutic insulin secretagogues

Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.

A high carbohydrate diet does not induce hyperglycaemia in a mitochondrial glycerol-3-phosphate dehydrogenase-deficient mouse

AIMS/HYPOTHESIS

The electrons of the glycolysis-derived reduced form of NADH are transferred to mitochondria through the NADH shuttle system. There are two NADH shuttles: the glycerol phosphate and malate-aspartate shuttle. Mice with a targeted disruption of mitochondrial glycerol-3-phosphate dehydrogenase, a rate-limiting enzyme of the glycerol phosphate shuttle, are not diabetic and have normal islet glucose-induced secretion. In this study, we analyzed if environmental factors, such as a high carbohydrate diet could contribute to the development of Type 2 diabetes mellitus in mice with a specific defective genetic background.

METHODS

The mice were fed with a high carbohydrate diet for 1 and 6 months, and several biochemical parameters were analysed. The mitochondrial respiratory activity was assayed by polarography; and the islet function was studied by islet perifusion and pancreas perfusion.

RESULTS

The high carbohydrate diet induced hyperglycaemia, hyperinsulinaemia, and islet hyperplasia in the wild-type and heterozygote mice. Activity of the respiratory chain complex I also increased in these mice. In contrast, these effects were not observed in the null mice fed with the diet; in addition, these null mice had an increased insulin sensitivity compared to wild-type mice.

CONCLUSION/INTERPRETATION

The phenotype of the mice with an impairment of NADH shuttles does not worsen when fed a high carbohydrate diet; moreover, the diet does not compromise islet function.

Role of NADH shuttles in glucose-induced insulin secretion from fetal beta-cells

The NADH shuttle system, which transports the substrate for oxidative metabolism directly from the cytosol to the mitochondrial electron transport chain, has been shown to be essential for glucose-induced activation of mitochondrial metabolism and insulin secretion in adult beta-cells. We examined the role of these shuttles in the fetal beta-cell, which is immature in being unable to secrete insulin in response to glucose. The activity and concentration of the two key enzymes of the NADH shuttles, mitochondrial glycerol phosphate dehydrogenase (mGPDH) and mitochondrial malate dehydrogenase (mMDH), were eight- and threefold lower, respectively, in fetal compared with adult rat islets. Likewise, mGPDH and mMDH activity was fivefold lower in islet-like cell clusters (ICCs) and sevenfold lower in purified beta-cells compared with adult islets in the pig. The low level of enzyme activity was a result of low gene expression of the mitochondrial enzymes in the fetal beta-cells. Increasing NADH shuttle activity by transduction of fetal rat islets with mGPDH cDNA enabled the fetal islets to secrete insulin when stimulated with glucose. We concluded that the immaturity of the NADH shuttles contributes to the inability of fetal beta-cells to secrete insulin in response to glucose.

Normal thyroid thermogenesis but reduced viability and adiposity in mice lacking the mitochondrial glycerol phosphate dehydrogenase

The mitochondrial glycerol phosphate dehydrogenase (mGPD) is important for metabolism of glycerol phosphate for gluconeogenesis or energy production and has been implicated in thermogenesis induced by cold and thyroid hormone treatment. mGPD in combination with the cytosolic glycerol phosphate dehydrogenase (cGPD) is proposed to form the glycerol phosphate shuttle, catalyzing the interconversion of dihydroxyacetone phosphate and glycerol phosphate with net oxidation of cytosolic NADH. We made a targeted deletion in Gdm1 and produced mice lacking mGPD. On a C57BL/6J background these mice showed a 50% reduction in viability compared with wild-type littermates. Uncoupling protein-1 mRNA levels in brown adipose tissue did not differ between mGPD knockout and control pups, suggesting normal thermogenesis. Pups lacking mGPD had decreased liver ATP and slightly increased liver glycerol phosphate. In contrast, liver and muscle metabolites were normal in adult animals. Adult mGPD knockout animals had a normal cold tolerance, normal circadian rhythm in body temperature, and demonstrated a normal temperature increase in response to thyroid hormone. However, they were found to have a lower body mass index, a 40% reduction in the weight of white adipose tissue, and a slightly lower fasting blood glucose than controls. The phenotype may be secondary to consequences of the obligatory production of cytosolic NADH from glycerol metabolism in the mGPD knockout animal. We conclude that, although mGPD is not essential for thyroid thermogenesis, variations in its function affect viability and adiposity in mice.

Sp/Krüppel-like transcription factors are essential for the expression of mitochondrial glycerol phosphate dehydrogenase promoter B

The human mitochondrial glycerol phosphate dehydrogenase (hmGPD) is abundant in the normal pancreatic insulin cell and its level is lowered to 50% by high glucose and diabetes. Using Drosophila and INS-1 cells, we have analysed the hmGPD gene promoter B to characterize cis-elements and trans-acting factors that affect its regulation. We identified ten efficient Sp/Krüppel-like transcription-factor-binding sites in the promoter sequence. These sites include four GC-boxes (CCCGCCC at -227, -68, -46 and GGGCGAG at -382), three GT-boxes (CCCCACCC at -350, CCCACACCC at -257, and CACCCGCCC at -48), and three CT/GA-boxes (TCCCTCCC at -262, GGGAGGGAG at -129, and GGGAGGAGGA at -107). Transfection of Drosophila SL2 cells, which lack Sp/Krüppel-like factors, with constructs encoding either Sp1, Sp3, Sp4 or erythroid Krüppel-like factor (EKLF) specifically activates the hmGPD promoter B up to 50-fold. Promoter activation requires the Sp1 activation and the DNA binding domains. Co-transfected EKLF was synergistic with either Sp1 or Sp3. On the other hand, the basic Krüppel-like factor (BKLF) inhibited Sp1-and EKLF-mediated promoter activation. Similarly, constructs encoding either GA-binding protein (GABP) or PU.1 inhibited Sp1-mediated promoter activation. Oligonucleotide 'decoys' with potential transcription factor binding sites decreased promoter activity in both INS-1 and Drosophila cells. Significant loss of Sp- and EKLF-mediated activation was observed with some internal as well as sequential 5' deletions of the promoter DNA. The level of Sp1 protein was reduced by 50% in INS-1 cells chronically exposed to a high concentration of glucose. The results demonstrate that Sp/Krüppel-like factors are essential for mGPD gene expression.

Interaction of nateglinide with K(ATP) channel in beta-cells underlies its unique insulinotropic action

Nateglinide is a novel insulinotropic agent for the treatment of type 2 diabetes. It is a D-phenylalanine derivative, chemically distinct from repaglinide and sulphonylureas (glyburide or glimepiride). Although each agent is known to stimulate insulin release via the signaling cascade initiated by closure of ATP-dependent K+ (K(ATP)) channels in pancreatic beta-cells, the pharmacological effect of nateglinide is reportedly fast-acting, short-lasting, sensitive to ambient glucose and more resistant to metabolic inhibition. The aim of the present study was to elucidate the molecular mechanism(s) underlying the distinct properties of the insulinotropic action of nateglinide. By using the patch-clamp methods, we comparatively characterized the potency and kinetics of the effect of these agents on K(ATP) channels in rat beta-cells at normal vs. elevated glucose and under physiological condition vs. experimentally induced metabolic inhibition. Our results demonstrated that the mode of the action of nateglinide on K(ATP) current was unique in (a) glucose dependency; (b) increased potency and efficacy under ATP depletion and uncoupling of mitochondrial oxidative phosphorylation than physiological condition; (c) substantially more rapid onset and offset kinetics. The data provide mechanistic rationale for the unique in vivo and ex vivo activity profile of nateglinide and may contribute to reduced hypoglycemic potential associated with excessive insulin secretion.

Site-directed mutations in the FAD-binding domain of glycerophosphate dehydrogenase: catalytic defects with preserved mitochondrial anchoring of the enzyme in transfected COS-7 cells

Single-strand conformational polymorphism analysis of mitochondrial FAD-linked glycerophosphate dehydrogenase (mGDH) gene has revealed mutations in both the calcium- and FAD-binding domains of this enzyme in some diabetic patients. It was now investigated whether site-directed mutations in the FAD-binding domain of the mGDH gene may affect the mitochondrial anchoring and catalytic activity of the enzyme. COS-7 cells were transfected with plasmid cDNA coding for either wild-type or mutated human mGDH (G --> A substitutions at positions 352, 355, and 364 and A --> C substitution at position 390) fused, when required, at the N-terminus of green fluorescent protein. The activity of mGDH was measured by both radioisotopic ((3)HOH production from l-[2-(3)H]glycerol 3-phosphate) and colorimetric (iodoformazan formation) procedures. In cells transfected with the mGDHwt-EGFP or mGDHmut-EGFP constructs, the fused protein was found by confocal microscopy exclusively in the mitochondria, colocalized with a mitochondrial marker. In homogenates of COS-7 cells transfected with mGDHmut, however, the catalytic activity of the enzyme was decreased, this coinciding with low ratios between both the activities measured in the absence/presence of exogenous FAD and the results obtained by the colorimetric/radioisotopic procedure. Thus, although the present site-directed mutations of the mGDH gene failed to impair the mitochondrial anchoring of the enzyme, they led to catalytic defects that were, in some respect, comparable to those previously encountered in the lymphocytes or islets of type 2 diabetic patients.

Redox regulation of cytosolic glycerol-3-phosphate dehydrogenase: Cys(102) is the target of the redox control and essential for the catalytic activity

Cytosolic glycerol-3-phosphate dehydrogenase (cG3PDH) occupies the branch point between the glycolytic pathway and triglyceride biosynthesis. However, the regulatory mechanism of the cG3PDH activity has remained obscure. Here we report that cG3PDH is efficiently inhibited by modification of the thiol group through a redox mechanism. In this study, we found that sodium selenite and nitric oxide (NO) donors such as S-nitroso-N-acetylpenicillamine and 3-morpholinosydnonimine inhibited cG3PDH activity, and that similar effects could be achieved with selenium metabolites such as selenocysteine and selenomethionine. Furthermore, we found that reducing agents, such as dithiothreitol and beta-mercaptoethanol, restored the cG3PDH activity suppressed by selenite and NO both in vitro and in cultured cells. Buthionine sulfoximine depleted levels of both reduced glutathione and the oxidized form but had no effect on the suppression of cG3PDH activity by selenite in cultured cells. Moreover, thiol-reactive agents, such as N-ethylmaleimide and o-iodosobenzoic acid, blocked the enzyme activity of cG3PDH through the modification of redox-sensitive cysteine residues in cG3PDH. The inhibitor of NO synthase, L-N(G)-nitro-arginine, restored the cG3PDH activity inhibited by NO in cultured cells, whereas the inhibitor of guanylyl cyclase, 1H-[1,2,4] oxadiazole[4,3-alpha] quinoxalin-1-one (ODQ), has no effect. NO directly inhibits cG3PDH activity not via a cGMP-dependent mechanism. Finally, using site-directed mutagenesis, we found that Cys(102) of cG3PDH was sensitive to both selenite and NO. From the results, we suggest that cG3PDH is a target of cellular redox regulation.

Expression of glucose transporter-2, glucokinase and mitochondrial glycerolphosphate dehydrogenase in pancreatic islets during rat ontogenesis

To gain better insight into the insulin secretory activity of fetal beta cells in response to glucose, the expression of glucose transporter 2 (GLUT-2), glucokinase and mitochondrial glycerol phosphate dehydrogenase (mGDH) were studied. Expression of GLUT-2 mRNA and protein in pancreatic islets and liver was significantly lower in fetal and suckling rats than in adult rats. The glucokinase content of fetal islets was significantly higher than of suckling and adult rats, and in liver the enzyme appeared for the first time on about day 20 of extrauterine life. The highest content of hexokinase I was found in fetal islets, after which it decreased progressively to the adult values. Glucokinase mRNA was abundantly expressed in the islets of all the experimental groups, whereas in liver it was only present in adults and 20-day-old suckling rats. In fetal islets, GLUT-2 and glucokinase protein and their mRNA increased as a function of increasing glucose concentration, whereas reduced mitochondrial citrate synthase, succinate dehydrogenase and cytochrome c oxidase activities and mGDH expression were observed. These findings, together with those reported by others, may help to explain the decreased insulin secretory activity of fetal beta cells in response to glucose.