What couples glycolysis to mitochondrial signal generation in glucose-stimulated insulin secretion?

Pancreatic islet beta-cells are poised to generate metabolic messengers in the mitochondria that link glucose metabolism to insulin exocytosis. This is accomplished through the tight coupling of glycolysis to mitochondrial activation. The messenger molecules ATP and glutamate are produced after the metabolism of glycolysis-derived pyruvate in the mitochondria. The entry of monocarboxylates such as pyruvate into the beta cell is limited, explaining why overexpression of monocarboxylate transporters unravels pyruvate-stimulated insulin secretion. NADH generated by glycolysis is efficiently reoxidized by highly active mitochondrial shuttles rather than by lactate dehydrogenase. Overexpression of this enzyme does not alter glucose-stimulated insulin secretion, suggesting that NADH availability restricts the conversion of pyruvate to lactate in the beta cell. These metabolic features permit the fuel function of glucose to be extended to the generation of signaling molecules, which increases cytosolic Ca2+ and promotes insulin exocytosis.     

The mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) and glucose homeostasis: Has it been overlooked?

Background

Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis.

Scope of review

A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways.

Major conclusions

PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis.

General significance

The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.     

Impact of mitochondrial calcium on the coupling of metabolism to insulin secretion in the pancreatic beta-cell

Mitochondria play an essential role in metabolism-secretion coupling in the pancreatic beta-cell. Dysfunction of the organelle leads to impaired glucose-stimulated insulin secretion, as exemplified by the rare disease mitochondrial diabetes, which is caused by mutations in the mitochondrial DNA. In the excitable beta-cell, mitochondria generate ATP and possibly other coupling factors that promote plasma membrane depolarization and calcium influx triggering insulin exocytosis. Cytosolic calcium signals are relayed into the mitochondria, where the ion potentiates oxidative metabolism. Hormones such as glucagon-like peptide 1 (GLP-1) or neurotransmitter secretagogues stimulate the beta-cell by activating different signal transduction pathways eventually also raising mitochondrial calcium. Likewise, pharmacological inhibition of the Na(+)/Ca(2+) exchanger of the inner mitochondrial membrane augments intra-organellar calcium and insulin secretion. Islets obtained after autopsy from type 2 diabetic patients have altered mitochondrial morphology impaired glucose oxidation and reduced ATP generation, explaining defective insulin secretion. We hypothesize that the improvement of glucose-stimulated insulin secretion by sulfonylurea compounds in type 2 diabetic patients is in part due to their capacity to raise mitochondrial calcium, which is beneficial for the generation of metabolic coupling factors.     

Mitochondria-derived glutamate at the interplay between branched-chain amino acid and glucose-induced insulin secretion

In pancreatic beta-cells, glutamate has been proposed to mediate insulin secretion as a glucose-derived factor, although it is also considered for its sole catabolic function. Hence, changes in cellular glutamate levels are a matter of debate. Here, we investigated the effects of glucose and the glutamate precursor glutamine on kinetics of glutamate levels together with insulin secretion in INS-1E beta-cells. Preincubation at low (1 mM) glucose resulted in reduced cellular glutamate levels, which were doubled by exposure to glutamine. In glutamine-deprived cells, 5 mM glucose restored glutamate concentrations. Incubation at 15 mM glucose increased cellular glutamate, along with stimulation of insulin secretion, following both glutamine-free and glutamine-rich preincubations. Nuclear magnetic resonance (NMR) spectroscopy of INS-1E cells exposed to 15 mM D-[1-(13)C]glucose revealed glutamate as the major glucose metabolic product. Branched-chain amino acids, such as leucine, reduced cellular glutamate levels at low and intermediate glucose. This study demonstrates that glucose stimulates glutamate generation, whereas branched-chain amino acids promote competitive glutamate expenditure.     

Citrate oscillates in liver and pancreatic beta cell mitochondria and in INS-1 insulinoma cells

Oscillations in citric acid cycle intermediates have never been previously reported in any type of cell. Here we show that adding pyruvate to isolated mitochondria from liver, pancreatic islets, and INS-1 insulinoma cells or adding glucose to intact INS-1 cells causes sustained oscillations in citrate levels. Other citric acid cycle intermediates measured either did not oscillate or possibly oscillated with a low amplitude. In INS-1 mitochondria citrate oscillations are in phase with NAD(P) oscillations, and in intact INS-1 cells citrate oscillations parallel oscillations in ATP, suggesting that these processes are co-regulated. Oscillations have been extensively studied in the pancreatic beta cell where oscillations in glycolysis, NAD(P)/NAD(P)H and ATP/ADP ratios, plasma membrane electrical activity, calcium levels, and insulin secretion have been well documented. Because the mitochondrion is the major site of ATP synthesis and NADH oxidation and the only site of citrate synthesis, mitochondria need to be synchronized for these factors to oscillate. In suspensions of mitochondria from various organs, most of the citrate is exported from the mitochondria. In addition, citrate inhibits its own synthesis. We propose that this enables citrate itself to act as one of the cellular messengers that synchronizes mitochondria. Furthermore, because citrate is a potent inhibitor of the glycolytic enzyme phosphofructokinase, the pacemaker of glycolytic oscillations, citrate may act as a metabolic link between mitochondria and glycolysis. Citrate oscillations may coordinate oscillations in mitochondrial energy production and anaplerosis with glycolytic oscillations, which in the beta cell are known to parallel oscillations in insulin secretion.     

Icilin induces G1 arrest through activating JNK and p38 kinase in a TRPM8-independent manner

Mechanistic models of glucose stimulated insulin secretion (GSIS) established in minimal media in vitro, may not accurately describe the complexity of coupling metabolism with insulin secretion that occurs in vivo. As a first approximation, we have evaluated metabolic pathways in a typical growth media, DMEM as a surrogate in vivo medium, for comparison to metabolic fluxes observed under the typical experimental conditions using the simple salt-buffer of KRB. Changes in metabolism in response to glucose and amino acids and coupling to insulin secretion were measured in INS-1 832/13 cells. Media effects on mitochondrial function and the coupling efficiency of oxidative phosphorylation were determined by fluorometrically measured oxygen consumption rates (OCRs) combined with ³¹P NMR measured rates of ATP synthesis. Substrate preferences and pathways into the TCA cycle, and the synthesis of mitochondrial 2nd messengers by anaplerosis were determined by ¹³C NMR isotopomer analysis of the fate of [U-¹³C] glucose metabolism.Despite similar incremental increases in insulin secretion, the changes of OCR in response to increasing glucose from 2.5 to 15 mM were blunted in DMEM relative to KRB. Basal and stimulated rates of insulin secretion rates were consistently higher in DMEM, while ATP synthesis rates were identical in both DMEM and KRB, suggesting greater mitochondrial uncoupling in DMEM. The relative rates of anaplerosis, and hence synthesis and export of 2nd messengers from the mitochondria were found to be similar in DMEM to those in KRB. And, the correlation of total PC flux with insulin secretion rates in DMEM was found to be congruous with the correlation in KRB. Together, these results suggest that signaling mechanisms associated with both TCA cycle flux and with anaplerotic flux, but not ATP production, may be responsible for the enhanced rates of insulin secretion in more complex, and physiologically-relevant media.     

Impaired anaplerosis and insulin secretion in insulinoma cells caused by small interfering RNA-mediated suppression of pyruvate carboxylase

Anaplerosis, the synthesis of citric acid cycle intermediates, by pancreatic beta cell mitochondria has been proposed to be as important for insulin secretion as mitochondrial energy production. However, studies designed to lower the rate of anaplerosis in the beta cell have been inconclusive. To test the hypothesis that anaplerosis is important for insulin secretion, we lowered the activity of pyruvate carboxylase (PC), the major enzyme of anaplerosis in the beta cell. Stable transfection of short hairpin RNA was used to generate a number of INS-1 832/13-derived cell lines with various levels of PC enzyme activity that retained normal levels of control enzymes, insulin content, and glucose oxidation. Glucose-induced insulin release was decreased in proportion to the decrease in PC activity. Insulin release in response to pyruvate alone, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) plus glutamine, or methyl succinate plus beta-hydroxybutyrate was also decreased in the PC knockdown cells. Consistent with a block at PC, the most PC-deficient cells showed a metabolic crossover point at PC with increased basal and/or glucose-stimulated pyruvate plus lactate and decreased malate and citrate. In addition, in BCH plus glutamine-stimulated PC knockdown cells, pyruvate plus lactate was increased, whereas citrate was severely decreased, and malate and aspartate were slightly decreased. The incorporation of 14C into lipid from [U-14C]glucose was decreased in the PC knockdown cells. The results confirm the central importance of PC and anaplerosis to generate metabolites from glucose that support insulin secretion and even suggest PC is important for insulin secretion stimulated by noncarbohydrate insulin secretagogues.     

Biotin enhances ATP synthesis in pancreatic islets of the rat, resulting in reinforcement of glucose-induced insulin secretion

Previous studies showed that biotin enhanced glucose-induced insulin secretion. Changes in the cytosolic ATP/ADP ratio in the pancreatic islets participate in the regulation of insulin secretion by glucose. In the present study we investigated whether biotin regulates the cytosolic ATP/ADP ratio in glucose-stimulated islets. When islets were stimulated with glucose plus biotin, the ATP/ADP ratio increased to approximately 160% of the ATP/ADP ratio in islets stimulated with glucose alone. The rate of glucose oxidation, assessed by CO(2) production, was also about 2-fold higher in islets treated with biotin. These increasing effects of biotin were proportional to the effects seen in insulin secretion. There are no previous reports of vitamins, such as biotin, directly affecting ATP synthesis. Our data indicate that biotin enhances ATP synthesis in islets following the increased rate of substrate oxidation in mitochondria and that, as a consequence of these events, glucose-induced insulin release is reinforced by biotin.     

Diabetes mellitus: Metabolic effects and oxidative stress

Diabetes mellitus is a complex polygenic pathology, which is characterized by numerous metabolic disorders. Progressive hyperglycemia developing during this disease causes clinically significant tissue damage and is considered as a main risk factor of micro- and macrovascular complications leading to retinopathy, nephropathy, and neuropathy. Hyperglycemia-depended oxidative stress and impairments in nitric oxide bioavailability play an essential role in the pathogenesis of diabetes and its complications. Homeostasis of glucose maintained by metabolic effects of insulin includes an increase of glucose uptake by skeletal muscles and suppression of glucose production by the liver. M. Brownlee (2005) put forward a hypothesis assuming that oxidative stress is the main mechanism of diabetic tissue damages. According to this hypothesis, mitochondrial dysfunction and superoxide anion radical hyperproduction by mitochondria is the principal mechanism of activation of four pathways of hyperglycemia-induced impairments under diabetes. Two cell signaling cascades regulate the glucose homeostasis: insulin-mediated glucose uptake (IMGU) in skeletal muscles, liver, and heart and glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells. In addition to nonspecific irreversible oxidative damage of DNA, protein and lipid molecules reactive oxygen and nitrogen species induce cell and tissue damage, activating a number of cell stress-sensitive signaling cascades. Stress-dependent serine phosphorylation of insulin receptor substrate (IRS) proteins decreases its capacity for tyrosine phosphorylation and may accelerate degradation of IRS. This process underlies the molecular mechanism of oxidative stress-induced insulin resistance.     

Mitochondrial metabolism sets the maximal limit of fuel-stimulated insulin secretion in a model pancreatic beta cell: a survey of four fuel secretagogues

The precise metabolic steps that couple glucose catabolism to insulin secretion in the pancreatic beta cell are incompletely understood. ATP generated from glycolytic metabolism in the cytosol, from mitochondrial metabolism, and/or from the hydrogen shuttles operating between cytosolic and mitochondrial compartments has been implicated as an important coupling factor. To identify the importance of each of these metabolic pathways, we have compared the fates of four fuel secretagogues (glucose, pyruvate, dihydroxyacetone, and glycerol) in the INS1-E beta cell line. Two of these fuels, dihydroxyacetone and glycerol, are normally ineffective as secretagogues but are enabled by adenovirus-mediated expression of glycerol kinase. Comparison of these two particular fuels allows the effect of redox state on insulin secretion to be evaluated since the phosphorylated products dihydroxyacetone phosphate and glycerol phosphate lie on opposite sides of the NADH-consuming glycerophosphate dehydrogenase reaction. Based upon measurements of glycolytic metabolites, mitochondrial oxidation, mitochondrial matrix calcium, and mitochondrial membrane potential, we find that insulin secretion most tightly correlates with mitochondrial metabolism for each of the four fuels. In the case of glucose stimulation, the high control strength of glucose phosphorylation sets the pace of glucose metabolism and thus the rate of insulin secretion. However, bypassing this reaction with pyruvate, dihydroxyacetone, or glycerol uncovers constraints imposed by mitochondrial metabolism, each of which attains a similar maximal limit of insulin secretion. More specifically, we found that the hyperpolarization of the mitochondrial membrane, related to the proton export from the mitochondrial matrix, correlates well with insulin secretion. Based on these findings, we propose that fuel-stimulated secretion is in fact limited by the inherent thermodynamic constraints of proton gradient formation.     

Mitochondrial dysfunction and increased reactive oxygen species impair insulin secretion in sphingomyelin synthase 1-null mice

Sphingomyelin synthase 1 (SMS1) catalyzes the conversion of ceramide to sphingomyelin. Here, we generated and analyzed SMS1-null mice. SMS1-null mice exhibited moderate neonatal lethality, reduced body weight, and loss of fat tissues mass, suggesting that they might have metabolic abnormality. Indeed, analysis on glucose metabolism revealed that they showed severe deficiencies in insulin secretion. Isolated mutant islets exhibited severely impaired ability to release insulin, dependent on glucose stimuli. Further analysis indicated that mitochondria in mutant islet cells cannot up-regulate ATP production in response to glucose. We also observed additional mitochondrial abnormalities, such as hyperpolarized membrane potential and increased levels of reactive oxygen species (ROS) in mutant islets. Finally, when SMS1-null mice were treated with the anti-oxidant N-acetyl cysteine, we observed partial recovery of insulin secretion, indicating that ROS overproduction underlies pancreatic β-cell dysfunction in SMS1-null mice. Altogether, our data suggest that SMS1 is important for controlling ROS generation, and that SMS1 is required for normal mitochondrial function and insulin secretion in pancreatic β-cells.     

Computational Modeling of Glucose Transport in Pancreatic β-Cells Identifies Metabolic Thresholds and Therapeutic Targets in Diabetes

Pancreatic β-cell dysfunction is a diagnostic criterion of Type 2 diabetes and includes defects in glucose transport and insulin secretion. In healthy individuals, β-cells maintain plasma glucose concentrations within a narrow range in concert with insulin action among multiple tissues. Postprandial elevations in blood glucose facilitate glucose uptake into β-cells by diffusion through glucose transporters residing at the plasma membrane. Glucose transport is essential for glycolysis and glucose-stimulated insulin secretion. In human Type 2 diabetes and in the mouse model of obesity-associated diabetes, a marked deficiency of β-cell glucose transporters and glucose uptake occurs with the loss of glucose-stimulated insulin secretion. Recent studies have shown that the preservation of glucose transport in β-cells maintains normal insulin secretion and blocks the development of obesity-associated diabetes. To further elucidate the underlying mechanisms, we have constructed a computational model of human β-cell glucose transport in health and in Type 2 diabetes, and present a systems analysis based on experimental results from human and animal studies. Our findings identify a metabolic threshold or “tipping point” whereby diminished glucose transport across the plasma membrane of β-cells limits intracellular glucose-6-phosphate production by glucokinase. This metabolic threshold is crossed in Type 2 diabetes and results in β-cell dysfunction including the loss of glucose stimulated insulin secretion. Our model further discriminates among molecular control points in this pathway wherein maximal therapeutic intervention is achieved.     

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.     

Metabolomics Analysis Reveals that AICAR Affects Glycerolipid,Ceramide and Nucleotide Synthesis Pathways in INS-1 Cells

AMPK regulates many metabolic pathways including fatty acid and glucose metabolism, both of which are closely associated with insulin secretion in pancreatic β-cells. Insulin secretion is regulated by metabolic coupling factors such as ATP/ADP ratio and other metabolites generated by the metabolism of nutrients such as glucose, fatty acid and amino acids. However, the connection between AMPK activation and insulin secretion in β-cells has not yet been fully elucidated at a metabolic level. To study the effect of AMPK activation on glucose stimulated insulin secretion, we applied the pharmacological activator 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) to an INS-1 (832/13) β-cell line. We measured the change in 66 metabolites in the presence or absence of AICAR using different stable isotopic labeled nutrients to probe selected pathways. AMPK activation by AICAR increased basal insulin secretion and reduced the glucose stimulation index. Although ATP/ADP ratios were not strongly affected by AICAR, several other metabolites and pathways important for insulin secretion were affected by AICAR treatment including long-chain CoAs, malonyl-CoA, 3-hydroxy-3 methylglutaryl CoA, diacylglycerol, and farnesyl pyrophosphate. Tracer studies using ¹³C-glucose revealed lower glucose flux in the purine and pyrimidine pathway and in the glycerolipid synthesis pathway. Untargeted metabolomics revealed reduction in ceramides caused by AICAR that may explain the beneficial role of AMPK in protecting β-cells from lipotoxicity. Taken together, the results provide an overall picture of the metabolic changes associated with AICAR treatment and how it modulates insulin secretion and β-cell survival.     

Mitochondria and endoplasmic reticulum: mitochondria-endoplasmic reticulum interplay in type 2 diabetes pathophysiology

Mitochondria and endoplasmic reticulum (ER) are two important metabolic organelles for the maintenance of cellular homeostasis and their functional defects are suspected to participate to the aetiology of type 2 diabetes (T2D). Particularly, excessive lipid intake and/or ectopic lipid accumulation in tissues (referred as lipotoxicity) are involved in alterations of both organelles and are closely linked to peripheral insulin resistance and defective insulin secretion. Since, mitochondria and ER are physically and functionally interconnected, their respective alterations during T2D could be interrelated. However, the mechanisms that coordinate the interplay between mitochondrial dysfunction and ER stress, and its relevance in the control of glucose homeostasis are unknown. Among these mechanisms, we will discuss on the potential role of altered mitochondria/ER crosstalk in organelle dysfunctions and in T2D pathophysiology.     

Effects of adenosine A(1) receptor antagonism on insulin secretion from rat pancreatic islets

Adenosine is known to influence different kinds of cells, including beta-cells of the pancreas. However, the role of this nucleoside in the regulation of insulin secretion is not fully elucidated. In the present study, the effects of adenosine A(1) receptor antagonism on insulin secretion from isolated rat pancreatic islets were tested using DPCPX, a selective adenosine A(1) receptor antagonist. It was demonstrated that pancreatic islets stimulated with 6.7 and 16.7 mM glucose and exposed to DPCPX released significantly more insulin compared with islets incubated with glucose alone. The insulin-secretory response to glucose and low forskolin appeared to be substantially potentiated by DPCPX, but DPCPX was ineffective in the presence of glucose and high forskolin. Moreover, DPCPX failed to change insulin secretion stimulated by the combination of glucose and dibutyryl-cAMP, a non-hydrolysable cAMP analogue. Studies on pancreatic islets also revealed that the potentiating effect of DPCPX on glucose-induced insulin secretion was attenuated by H-89, a selective inhibitor of protein kinase A. It was also demonstrated that formazan formation, reflecting metabolic activity of cells, was enhanced in islets exposed to DPCPX. Moreover, DPCPX was found to increase islet cAMP content, whereas ATP was not significantly changed. These results indicate that adenosine A(1) receptor blockade in rat pancreatic islets potentiates insulin secretion induced by both physiological and supraphysiological glucose concentrations. This effect is proposed to be due to increased metabolic activity of cells and increased cAMP content.     

Glucose metabolism, second messengers and insulin secretion

Insulin secretion from beta cells of the islets of Langerhans in the endocrine pancreas is regulated by glucose, glucose metabolites, metabolic intermediates such as ATP, acetyl CoA and reduced pyridine nucleotides, and classical second messengers. Receptor responses transduced by guanine nucleotide binding proteins modulate metabolic activity, the generation of second messengers, and cell depolarization during stimulus-response coupling in the beta cell. This review will consider insulin secretion as regulated by glucose metabolic pathways and second messengers.     

Chronic high glucose induced INS‐1β cell mitochondrial dysfunction: A comparative mitochondrial proteome with SILAC

As glucose‐stimulated insulin secretion of pancreatic β cell is triggered and promoted by the metabolic messengers derived from mitochondria, mitochondria take a central stage in the normal function of β cells. β cells in diabetics were chronically exposed to hyperglycemia stimulation, which have been reported to exert deleterious effects on β‐cell mitochondria. However, the mechanism of the toxic effects of hyperglycemia on β‐cell mitochondria was not clear. In this study, we characterized the biological functional changes of rat INS‐1β cells and their mitochondria with chronic exposure to hyperglycemia and created a research model of chronic hyperglycemia‐induced dysfunctional β cells with damaged mitochondria. Then, SILAC‐based quantitative proteomic approach was used to compare the mitochondrial protein expression from high glucose treated INS‐1β cells and control cells. The expression of some mitochondrial proteins was found with significant changes. Functional classification revealed most of these proteins were related with oxidative phosphorylation, mitochondrial protein biosynthesis, substances metabolism, transport, and cell death. These results presented some useful information about the effect of glucotoxicity on the β‐cell mitochondria.     

Adiponectin--its role in metabolism and beyond.

Adiponectin is a recently identified adipose tissue-derived protein (adipocytokine) with important metabolic effects. It is exclusively expressed in adipose tissue and released into the circulation. Adiponectin expression and/or secretion is increased by insulin like growth factor-1 and ionomycin, and decreased by tumor necrosis factor-alpha, glucocorticoids, beta-adrenergic agonists and cAMP. Data for insulin are somewhat inconclusive. Moreover, adiponectin expression and secretion are increased by activators of peroxisome proliferator-activated receptor (PPAR)-gamma. Besides inhibiting inflammatory pathways, recombinant adiponectin increases insulin sensitivity and improves glucose tolerance in various animal models. This insulin-sensitizing effect appears to be mostly attributable to enhanced suppression of glucose production, but beneficial effects on muscle cannot be excluded. In humans, plasma adiponectin concentrations exceed those of any other hormone by a thousand times; they decrease with obesity and are positively associated with whole-body insulin sensitivity. Therefore, low adiponectin may contribute to the decrease in whole-body insulin sensitivity that accompanies obesity. Furthermore, there is increasing evidence that genetic variants in the adiponectin gene itself and/or in genes encoding adiponectin-regulatory proteins--such as PPAR-gamma--may be associated with hypoadiponectinemia, insulin resistance and type 2 diabetes. This suggests that adiponectin may reflect PPAR-gamma activity in vivo. Finally, reversal or alleviation of hypoadiponectinemia may represent a target for development of drugs improving insulin sensitivity and glucose tolerance.     

Dual effect of insulin on in vitro leptin secretion by adipose tissue

Although it is widely accepted that insulin stimulates leptin secretion, a dual action was observed using a validated in vitro system, i.e., an early (less than 48 h) inhibitory action, followed later (48-96 h) by a clear-cut stimulation. While the inhibitory phase was observed at every glucose concentration tested (from 1 to 25 mM), the stimulatory phase required the presence of physiological or supraphysiological glucose concentrations. In fact, leptin secretion was virtually eliminated in the presence of glucose uptake inhibitors. This dual effect of insulin was not due to modifications of the ob mRNA levels, suggesting that it depends entirely on posttranslational mechanisms. In conclusion, insulin appears to induce an early inhibition of leptin secretion by the adipose cell, followed later by a stimulatory effect secondary to the metabolic changes triggered by the insulin-induced increase in glucose uptake.