A pyruvate cycling pathway involving cytosolic NADP-dependent isocitrate dehydrogenase regulates glucose-stimulated insulin secretion

Pharmacological targeting of the mitochondrial phosphatase PTPMT1

The dual-specificity protein tyrosine phosphatases (PTPs) play integral roles in the regulation of cell signaling. There is a need for new tools to study these phosphatases, and the identification of inhibitors potentially affords not only new means for their study, but also possible therapeutics for the treatment of diseases caused by their dysregulation. However, the identification of selective inhibitors of the protein phosphatases has proven somewhat difficult. PTP localized to mitochondrion 1 (PTPMT1) is a recently discovered dual-specificity phosphatase that has been implicated in the regulation of insulin secretion. Screening of a commercially available small-molecule library yielded alexidine dihydrochloride, a dibiguanide compound, as an effective and selective inhibitor of PTPMT1 with an in vitro concentration that inhibits response by 50% of 1.08 microM. A related dibiguanide analog, chlorhexidine dihydrochloride, also significantly inhibited PTPMT1, albeit with lower potency, while a monobiguanide analog showed very weak inhibition. Treatment of isolated rat pancreatic islets with alexidine dihydrochloride resulted in a dose-dependent increase in insulin secretion, whereas treatment of a pancreatic beta-cell line with the drug affected the phosphorylation of mitochondrial proteins in a manner similar to genetic inhibition of PTPMT1. Furthermore, knockdown of PTPMT1 in rat islets rendered them insensitive to alexidine dihydrochloride treatment, providing evidence for mechanism-based activity of the inhibitor. Taken together, these studies establish alexidine dihydrochloride as an effective inhibitor of PTPMT1, both in vitro and in cells, and support the notion that PTPMT1 could serve as a pharmacological target in the treatment of type II diabetes.

Glucose induces expression of rat pyruvate carboxylase through a carbohydrate response element in the distal gene promoter

Pyruvate carboxylase is an enzyme of the so-called pyruvate cycling pathways, which have been proposed to contribute to glucose-stimulated insulin secretion in pancreatic beta-cells. In the rat insulinoma cell line 832/13, transcripts from both the distal and proximal gene promoter for pyruvate carboxylase are up-regulated by glucose, with pyruvate carboxylase being expressed mainly from the distal gene promoter. At position -408 to -392 relative to the transcription start site, the distal gene promoter was found to contain a ChoRE (carbohydrate response element). Its deletion abolishes glucose responsiveness of the promoter, and the sequence can mediate glucose responsiveness to a heterologous gene promoter. ChREBP (carbohydrate response element-binding protein) and its dimerization partner Mlx (Max-like protein X) bind to the ChoRE in vitro. ChREBP further binds to the distal promoter region at a high glucose concentration in situ. The E-box-binding transcription factors USF1/2 (upstream stimulatory factor 1/2) and E2A variant 2 [also known as E47 and TCF3 (transcription factor 3)] can also bind to the ChoRE. Overexpression of E2A diminishes the magnitude of the glucose response from the pyruvate carboxylase ChoRE. This illustrates that competition between ChREBP-Mlx and other factors binding to the ChoRE affects glucose responsiveness. We conclude that a ChoRE in the distal gene promoter contributes to the glucose-mediated expression of pyruvate carboxylase.

Chronic reduction of the cytosolic or mitochondrial NAD(P)-malic enzyme does not affect insulin secretion in a rat insulinoma cell line

The cytosolic malic enzyme (ME1) has been suggested to augment insulin secretion via the malate-pyruvate and/or citrate-pyruvate shuttles, through the production of NADPH or other metabolites. We used selectable vectors expressing short hairpin RNA (shRNA) to stably decrease Me1 mRNA levels by 80-86% and ME1 enzyme activity by 78-86% with either of two shRNAs in the INS-1 832/13 insulinoma cell line. Contrary to published short term ME1 knockdown experiments, our long term targeted cells showed normal insulin secretion in response to glucose or to glutamine plus 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid. We found no increase in the mRNAs and enzyme activities of the cytosolic isocitrate dehydrogenase or glucose-6-phosphate dehydrogenase, which also produce cytosolic NADPH. There was no compensatory induction of the mRNAs for the mitochondrial malic enzymes Me2 or Me3. Interferon pathway genes induced in preliminary small interfering RNA experiments were not induced in the long term shRNA experiments. We repeated our study with an improved vector containing Tol2 transposition sequences to produce a higher rate of stable transferents and shortened time to testing, but this did not alter the results. We similarly used stably expressed shRNA to reduce mitochondrial NAD(P)-malic enzyme (Me2) mRNA by up to 95%, with severely decreased ME2 protein and a 90% decrease in enzyme activity. Insulin release to glucose or glutamine plus 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid remained normal. The maintenance of robust insulin secretion after lowering expression of either one of these malic enzymes is consistent with the redundancy of pathways of pyruvate cycling and/or cytosolic NADPH production in insulinoma cells.

Mitochondria mediated cell death in diabetes

Mitochondrial dysfunction plays a role in the pathogenesis of a wide range of diseases that involve disordered cellular fuel metabolism and survival/death pathways, including neurodegenerative diseases, cancer and diabetes. Cytokine, virus recognition and cellular stress pathways converging on mitochondria cause apoptotic and/or necrotic cell death of beta-cells in type-1 diabetes. Moreover, since mitochondria generate crucial metabolic signals for glucose stimulated insulin secretion (GSIS), mitochondrial dysfunction underlies both the functional derangement of GSIS and (over-nutrition) stress-induced apoptotic/necrotic beta-cell death, hallmarks of type-2 diabetes. The apparently distinct mechanisms governing beta-cell life/death decisions during the development of diabetes provide a remarkable example where remote metabolic, immune and stress signalling meet with mitochondria mediated apoptotic/necrotic death pathways to determine the fate of the beta-cell. We summarize the main findings supporting such a pivotal role of mitochondria in beta-cell death in the context of current trends in diabetes research.

High glucose inhibits glucose-6-phosphate dehydrogenase, leading to increased oxidative stress and beta-cell apoptosis

Patients with type 2 diabetes lose beta cells, but the underlying mechanisms are incompletely understood. Glucose-6-phosphate dehydrogenase (G6PD) is the principal source of the major intracellular reductant, NADPH, which is required by many enzymes, including enzymes of the antioxidant pathway. Previous work from our laboratory has shown that high glucose impairs G6PD activity in endothelial and kidney cells, which leads to decreased cell survival. Pancreatic beta cells are highly sensitive to increased ROS. This study aimed to determine whether G6PD and NADPH play central roles in beta-cell survival. Human and mouse islets, MIN6 cell line, and G6PD deficient mice were studied. High glucose inhibited G6PD expression and activity. Inhibition of G6PD with siRNA led to increased ROS and apoptosis, decreased proliferation, and impaired insulin secretion. High glucose decreased insulin secretion, which was improved by overexpressing G6PD. G6PD-deficient mice had smaller islets and impaired glucose tolerance compared with control mice, which suggests that G6PD deficiency per se leads to beta-cell dysfunction and death. G6PD plays an important role in beta-cell function and survival. High-glucose-mediated decrease in G6PD activity may provide a mechanistic explanation for the gradual loss of beta cells in patients with diabetes.

Insulin release, peripheral insulin resistance and muscle function in protein malnutrition: a role of tricarboxylic acid cycle anaplerosis

Pancreatic β-cells and skeletal muscle act in a synergic way in the control of systemic glucose homeostasis. Several pyruvate-dependent and -independent shuttles enhance tricarboxylic acid cycle intermediate (TACI) anaplerosis and increase β-cell ATP:ADP ratio, triggering insulin exocytotic mechanisms. In addition, mitochondrial TACI cataplerosis gives rise to the so-called metabolic coupling factors, which are also related to insulin release. Peripheral insulin resistance seems to be related to skeletal muscle fatty acid (FA) accumulation and oxidation imbalance. In this sense, exercise has been shown to enhance skeletal muscle TACI anaplerosis, increasing FA oxidation and by this manner restores insulin sensitivity. Protein malnutrition reduces β-cell insulin synthesis, release and peripheral sensitivity. Despite little available data concerning mitochondrial metabolism under protein malnutrition, evidence points towards reduced β-cell and skeletal muscle mitochondrial capacity. The observed decrease in insulin synthesis and release may reflect reduced anaplerotic and cataplerotic capacity. Furthermore, insulin release is tightly coupled to ATP:ADP rise which in turn is related to TACI anaplerosis. The effect of protein malnutrition upon peripheral insulin resistance is time-dependent and directly related to FA oxidation capacity. In contrast to β-cells, TACI anaplerosis and cataplerosis pathways in skeletal muscle seem to control FA oxidation and regulate insulin resistance.

Phosphoenolpyruvate cycling via mitochondrial phosphoenolpyruvate carboxykinase links anaplerosis and mitochondrial GTP with insulin secretion

Pancreatic beta-cells couple the oxidation of glucose to the secretion of insulin. Apart from the canonical K(ATP)-dependent glucose-stimulated insulin secretion (GSIS), there are important K(ATP)-independent mechanisms involving both anaplerosis and mitochondrial GTP (mtGTP). How mtGTP that is trapped within the mitochondrial matrix regulates the cytosolic calcium increases that drive GSIS remains a mystery. Here we have investigated whether the mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) is the GTPase linking hydrolysis of mtGTP made by succinyl-CoA synthetase (SCS-GTP) to an anaplerotic pathway producing phosphoenolpyruvate (PEP). Although cytosolic PEPCK (PEPCK-C) is absent, PEPCK-M message and protein were detected in INS-1 832/13 cells, rat islets, and mouse islets. PEPCK enzymatic activity is half that of primary hepatocytes and is localized exclusively to the mitochondria. Novel (13)C-labeling strategies in INS-1 832/13 cells and islets measured substantial contribution of PEPCK-M to the synthesis of PEP. As high as 30% of PEP in INS-1 832/13 cells and 41% of PEP in rat islets came from PEPCK-M. The contribution of PEPCK-M to overall PEP synthesis more than tripled with glucose stimulation. Silencing the PEPCK-M gene completely inhibited GSIS underscoring its central role in mitochondrial metabolism-mediated insulin secretion. Given that mtGTP synthesized by SCS-GTP is an indicator of TCA flux that is crucial for GSIS, PEPCK-M is a strong candidate to link mtGTP synthesis with insulin release through anaplerotic PEP cycling.

A conserved role for the mitochondrial citrate transporter Sea/SLC25A1 in the maintenance of chromosome integrity

Histone acetylation plays essential roles in cell cycle progression, DNA repair, gene expression and silencing. Although the knowledge regarding the roles of acetylation of histone lysine residues is rapidly growing, very little is known about the biochemical pathways providing the nucleus with metabolites necessary for physiological chromatin acetylation. Here, we show that mutations in the scheggia (sea)-encoded Sea protein, the Drosophila ortholog of the human mitochondrial citrate carrier Solute carrier 25 A1 (SLC25A1), impair citrate transport from mitochondria to the cytosol. Interestingly, inhibition of sea expression results in extensive chromosome breakage in mitotic cells and induces an ATR-dependent cell cycle arrest associated with a dramatic reduction of global histone acetylation. Notably, loss of SLC25A1 in short interfering RNA (siRNA)-treated human primary fibroblasts also leads to chromosome breaks and histone acetylation defects, suggesting an evolutionary conserved role for Sea/SLC25A1 in the regulation of chromosome integrity. This study therefore provides an intriguing and unexpected link between intermediary metabolism and epigenetic control of genome stability.

Role for malic enzyme, pyruvate carboxylation, and mitochondrial malate import in glucose-stimulated insulin secretion

Pyruvate cycling has been implicated in glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells. The operation of some pyruvate cycling pathways is proposed to necessitate malate export from the mitochondria and NADP(+)-dependent decarboxylation of malate to pyruvate by cytosolic malic enzyme (ME1). Evidence in favor of and against a role of ME1 in GSIS has been presented by others using small interfering RNA-mediated suppression of ME1. ME1 was also proposed to account for methyl succinate-stimulated insulin secretion (MSSIS), which has been hypothesized to occur via succinate entry into the mitochondria in exchange for malate and subsequent malate conversion to pyruvate. In contrast to rat, mouse beta-cells lack ME1 activity, which was suggested to explain their lack of MSSIS. However, this hypothesis was not tested. In this report, we demonstrate that although adenoviral-mediated overexpression of ME1 greatly augments GSIS in rat insulinoma INS-1 832/13 cells, it does not restore MSSIS, nor does it significantly affect GSIS in mouse islets. The increase in GSIS following ME1 overexpression in INS-1 832/13 cells did not alter the ATP-to-ADP ratio but was accompanied by increases in malate and citrate levels. Increased malate and citrate levels were also observed after INS-1 832/13 cells were treated with the malate-permeable analog dimethyl malate. These data suggest that although ME1 overexpression augments anaplerosis and GSIS in INS-1 832/13 cells, it is not likely involved in MSSIS and GSIS in pancreatic islets.

Quantifying interactions within the NADP(H) enzyme network in Drosophila melanogaster

In this report, we use synthetic, activity-variant alleles in Drosophila melanogaster to quantify interactions across the enzyme network that reduces nicotinamide adenine dinucleotide phosphate (NADP) to NADPH. We examine the effects of large-scale variation in isocitrate dehydrogenase (IDH) or glucose-6-phosphate dehydrogenase (G6PD) activity in a single genetic background and of smaller-scale variation in IDH, G6PD, and malic enzyme across 10 different genetic backgrounds. We find significant interactions among all three enzymes in adults; changes in the activity of any one source of a reduced cofactor generally result in changes in the other two, although the magnitude and directionality of change differs depending on the gene and the genetic background. Observed interactions are presumably through cellular mechanisms that maintain a homeostatic balance of NADPH/NADP, and the magnitude of change in response to modification of one source of reduced cofactor likely reflects the relative contribution of that enzyme to the cofactor pool. Our results suggest that malic enzyme makes the largest single contribution to the NADPH pool, consistent with the results from earlier experiments in larval D. melanogaster using naturally occurring alleles. The interactions between all three enzymes indicate functional interdependence and underscore the importance of examining enzymes as components of a network.

The molecular determinants of de novo nucleotide biosynthesis in cancer cells

Tumor cells increase the use of anabolic pathways to satisfy the metabolic requirements associated with a high growth rate. Transformed cells take up and metabolize nutrients such as glucose and glutamine at high levels that support anabolic growth. Oncogenic signaling through the PI3K/Akt and Myc pathways directly control glucose and glutamine uptake, respectively. In order to achieve elevated rates of nucleotide biosynthesis, neoplastic cells must divert carbon from PI3K/Akt-induced glycolytic flux into the nonoxidative branch of the pentose phosphate pathway to generate ribose-5-phosphate. This redirection of glucose catabolism appears to be regulated by cytoplasmic tyrosine kinases. Myc-induced glutamine metabolism also increases the abundance and activity of different rate-limiting enzymes that produce the molecular precursors required for de novo nucleotide synthesis. In this review, we will focus on recent progress in understanding how glucose and glutamine metabolism is redirected by oncogenes in order to support de novo nucleotide biosynthesis during proliferation and how metabolic reprogramming can be potentially exploited in the development of new cancer therapies.

Matrix alkalinization: a novel mitochondrial signal for sustained pancreatic beta-cell activation

Nutrient secretagogues activate mitochondria of the pancreatic beta-cell through the provision of substrate, hyperpolarisation of the inner mitochondrial membrane and mitochondrial calcium rises. We report that mitochondrial matrix pH, a parameter not previously studied in the beta-cell, also exerts an important control function in mitochondrial metabolism. During nutrient stimulation matrix pH alkalinises, monitored by the mitochondrial targeted fluorescent pH-sensitive protein mtAlpHi or (31)P-NMR inorganic phosphate chemical shifts following saturation transfer. Compared with other cell types, the resting mitochondrial pH was surprisingly low, rising from pH 7.25 to 7.7 during nutrient stimulation of rat beta-cells. As cytosolic alkalinisation to the nutrient was of much smaller amplitude, the matrix alkalinisation was accompanied by a pronounced increase of the DeltapH across the inner mitochondrial membrane. Furthermore, matrix alkalinisation closely correlates with the cytosolic ATP net increase, which is also associated with elevated ATP synthesis rates in mitochondria. Preventing DeltapH increases in permeabilised cells abrogated substrate-driven ATP synthesis. We propose that the mitochondrial pH and DeltapH are key determinants of mitochondrial energy metabolism and metabolite transport important for cell activation.

Role of cytosolic NADP+-dependent isocitrate dehydrogenase in ischemia-reperfusion injury in mouse kidney

Cytosolic NADP+-dependent isocitrate dehydrogenase (IDPc) synthesizes reduced NADP (NADPH), which is an essential cofactor for the generation of reduced glutathione (GSH), the most abundant and important antioxidant in mammalian cells. We investigated the role of IDPc in kidney ischemia-reperfusion (I/R) in mice. The activity and expression of IDPc were highest in the cortex, modest in the outer medulla, and lowest in the inner medulla. NADPH levels were greatest in the cortex. IDPc expression in the S1 and S2 segments of proximal tubules was higher than in the S3 segment, which is much more susceptible to I/R. IDPc protein was also highly expressed in the mitochondrion-rich intercalated cells of the collecting duct. IDPc activity was 10- to 30-fold higher than the activity of glucose-6-phosphate dehydrogenase, another producer of cytosolic NADPH, in various kidney regions. This study identifies that IDPc may be the primary source of NADPH in the kidney. I/R significantly reduced IDPc expression and activity and NADPH production and increased the ratio of oxidized glutathione to total glutathione [GSSG/(GSH+GSSG)], resulting in kidney dysfunction, tubular cell damage, and lipid peroxidation. In LLC-PK(1) cells, upregulation of IDPc by IDPc gene transfer protected the cells against hydrogen peroxide, enhancing NADPH production, inhibiting the increase of GSSG/(GSH+GSSG), and reducing lipid peroxidation. IDPc downregulation by small interference RNA treatment presented results contrasting with the upregulation. In conclusion, these results demonstrate that IDPc is expressed differentially along tubules in patterns that may contribute to differences in susceptibility to injury, is a major enzyme in cytosolic NADPH generation in kidney, and is downregulated with I/R.

Malic enzyme is present in mouse islets and modulates insulin secretion

AIMS/HYPOTHESIS

The pyruvate-malate shuttle is a metabolic cycle in pancreatic beta cells and is important for beta cell function. Cytosolic malic enzyme (ME) carries out an essential step in the shuttle by converting malate to pyruvate and generating NADPH. In rat islets the pyruvate-malate shuttle may regulate insulin secretion and it has been shown to play a critical role in adaptation to obesity and insulin resistance. However, ME has not been demonstrated in mouse islets and three reports indicate that mouse islets contain no ME activity. If mouse islets lack ME, rat and mouse islets must regulate insulin secretion by different mechanisms.

METHODS

We measured ME activity by a fluorometric enzymatic assay and Me mRNA by real-time PCR. ME activity was also measured in streptozotocin-treated mouse islets. FACS-purified beta cells were obtained from MIP-GFP mouse islets, agouti-L obese mouse islets and mouse beta cell line MIN-6. Insulin secretion and NADPH/NADP(+) ratios were measured in Me siRNA-treated beta cells.

RESULTS

ME activity and Me mRNA were present in C57BL/6 mouse islets. ME activity was reduced in streptozotocin-treated mouse islets. ME activity was also measurable in FACS-purified mouse beta cells. In addition, ME activity was significantly increased in obese agouti-L mouse islets and the mouse MIN-6 cell line. Me siRNA inhibited ME activity and reduced glucose-stimulated insulin secretion and also inhibited NADPH products.

CONCLUSIONS/INTERPRETATION

Mouse islets contain ME, which plays a significant role in regulating insulin secretion.

Metabolic cycling in control of glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) is central to normal control of metabolic fuel homeostasis, and its impairment is a key element of beta-cell failure in type 2 diabetes. Glucose exerts its effects on insulin secretion via its metabolism in beta-cells to generate stimulus/secretion coupling factors, including a rise in the ATP/ADP ratio, which serves to suppress ATP-sensitive K(+) (K(ATP)) channels and activate voltage-gated Ca(2+) channels, leading to stimulation of insulin granule exocytosis. Whereas this K(ATP) channel-dependent mechanism of GSIS has been broadly accepted for more than 30 years, it has become increasingly apparent that it does not fully describe the effects of glucose on insulin secretion. More recent studies have demonstrated an important role for cyclic pathways of pyruvate metabolism in control of insulin secretion. Three cycles occur in islet beta-cells: the pyruvate/malate, pyruvate/citrate, and pyruvate/isocitrate cycles. This review discusses recent work on the role of each of these pathways in control of insulin secretion and builds a case for the particular relevance of byproducts of the pyruvate/isocitrate cycle, NADPH and alpha-ketoglutarate, in control of GSIS.

Requirement for aralar and its Ca2+-binding sites in Ca2+ signal transduction in mitochondria from INS-1 clonal beta-cells

Aralar, the mitochondrial aspartate-glutamate carrier present in beta-cells, is a component of the malate-aspartate NADH shuttle (MAS). MAS is activated by Ca2+ in mitochondria from tissues with aralar as the only AGC isoform with an S0.5 of approximately 300 nm. We have studied the role of aralar and its Ca2+-binding EF-hand motifs in glucose-induced mitochondrial NAD(P)H generation by two-photon microscopy imaging in INS-1 beta-cells lacking aralar or expressing aralar mutants blocked for Ca2+ binding. Aralar knock-down in INS-1 beta-cell lines resulted in undetectable levels of aralar protein and complete loss of MAS activity in isolated mitochondria and in a 25% decrease in glucose-stimulated insulin secretion. MAS activity in mitochondria from INS-1 cells was activated 2-3-fold by extramitochondrial Ca2+, whereas aralar mutants were Ca2+ insensitive. In Ca2+-free medium, glucose-induced increases in mitochondrial NAD(P)H were small (1.3-fold) and unchanged regardless of the lack of aralar. In the presence of 1.5 mm Ca2+, glucose induced robust increases in mitochondrial NAD(P)H (approximately 2-fold) in cell lines with wild-type or mutant aralar. There was a approximately 20% reduction in NAD(P)H response in cells lacking aralar, illustrating the importance of MAS in glucose action. When small Ca2+ signals that resulted in extremely small mitochondrial Ca2+ transients were induced in the presence of glucose, the rise in mitochondrial NAD(P)H was maintained in cells with wild-type aralar but was reduced by approximately 50% in cells lacking or expressing mutant aralar. These results indicate that 1) glucose-induced activation of MAS requires Ca2+ potentiation; 2) Ca2+ activation of MAS represents a larger fraction of glucose-induced mitochondrial NAD(P)H production under conditions where suboptimal Ca2+ signals are associated with a glucose challenge (50 versus 20%, respectively); 3) inactivation of EF-hand motifs in aralar prevents activation of MAS by small Ca2+ signals. The results suggest a possible role for aralar and MAS in priming the beta-cell by Ca2+-mobilizing neurotransmitter or hormones.

The role of pyruvate carboxylase in insulin secretion and proliferation in rat pancreatic beta cells

AIMS/HYPOTHESIS

Pyruvate carboxylase (PC) or pyruvate dehydrogenase (PDH) is required to transfer carbons from pyruvate into the Krebs cycle. PC activity is preserved in the islets of obese animals, but it is reduced in the islets of animal models of type 2 diabetes, suggesting that PC is important in beta cell adaptation to insulin resistance and that PC reduction may lead to beta cell failure.

METHODS

To confirm the significance of PC, we first lowered activity using Pc (now known as Pcx) small interfering RNA (siRNA) in INS-1 cells and in dispersed rat islet cells. Second, we overexpressed PC in INS-1 cells, and third, we inhibited PDH by overexpressing the gene encoding pyruvate dehydrogenase kinase 4 (Pdk4) in INS-1 cells.

RESULTS

Treatment of INS-1 cells or dispersed rat islet cells with Pc siRNA resulted in a significant reduction in insulin secretion in both cell types and reduced proliferation in INS-1 cells. This treatment also reduced the content of oxaloacetate, malate and ATP, as well as the NADPH:NADP(+) ratio and activity of the pyruvate-malate shuttle. Overexpression of PC in INS-1 cells led to an elevation of insulin secretion and cell proliferation, whereas inhibition of PDH activity by overexpressing Pdk4 in INS-1 cells did not reduce insulin secretion.

CONCLUSIONS/INTERPRETATION

Our findings indicate that the PC pathway in beta cells might play a key role in pyruvate metabolism, insulin secretion and cell proliferation.

The islet beta-cell: fuel responsive and vulnerable

The pancreatic beta-cell senses blood nutrient levels and is modulated by neurohormonal signals so that it secretes insulin according to the need of the organism. Nutrient sensing involves marked metabolic activation, resulting in the production of coupling signals that promote insulin biosynthesis and secretion. The beta-cell's high capacity for nutrient sensing, however, necessitates reduced protection to nutrient toxicity. This potentially explains why in susceptible individuals, chronic fuel surfeit results in beta-cell failure and type 2 diabetes. Here we discuss recent insights into first, the biochemical basis of beta-cell signaling in response to glucose, amino acids and fatty acids, and second, beta-cell nutrient detoxification. We emphasize the emerging role of glycerolipid/fatty acid cycling in these processes.

Silencing of cytosolic or mitochondrial isoforms of malic enzyme has no effect on glucose-stimulated insulin secretion from rodent islets

We have previously demonstrated a role for pyruvate cycling in glucose-stimulated insulin secretion (GSIS). Some of the possible pyruvate cycling pathways are completed by conversion of malate to pyruvate by malic enzyme. Using INS-1-derived 832/13 cells, it has recently been shown by other laboratories that NADP-dependent cytosolic malic enzyme (MEc), but not NAD-dependent mitochondrial malic enzyme (MEm), regulates GSIS. In the current study, we show that small interfering RNA-mediated suppression of either MEm or MEc results in decreased GSIS in both 832/13 cells and a new and more glucose- and incretin-responsive INS-1-derived cell line, 832/3. The effect of MEm to suppress GSIS in these cell lines was linked to a substantial decrease in cell growth, whereas MEc suppression resulted in decreased NADPH, shown previously to be correlated with GSIS. However, adenovirus-mediated delivery of small interfering RNAs specific to MEc and MEm to isolated rat islets, while leading to effective suppression of the targets transcripts, had no effect on GSIS. Furthermore, islets isolated from MEc-null MOD1(-/-) mice exhibit normal glucose- and potassium-stimulated insulin secretion. These results indicate that pyruvate-malate cycling does not control GSIS in primary rodent islets.

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.

Capillary LC-MS for high sensitivity metabolomic analysis of single islets of Langerhans

Reversed-phase, packed capillary liquid chromatography interfaced by electrospray ionization to mass spectrometry was explored as an analytical method for determination of metabolites in microscale tissue samples using single islets of Langerhans as a model system. With the use of a 75 microm inner diameter column coupled to a quadrupole ion trap mass spectrometer in full scan mode, detection limits of 0.1-33 fmol were achieved for glycoloytic and tricarboxylic acid cycle metabolites. Reproducible processing of islets for analysis with little loss of metabolites was performed by rapid freezing followed by methanol-water extraction. The method yielded 20 microL of extract of which just 15 nL was injected suggesting the potential for performing multiple assays on the same islet. Approximately 200 presumed metabolites could be detected, of which 22 were identified by matching retention times and MS/MS spectra to standards. Relative standard deviations for peak detection was from 7 to 18% and was unaffected by storage for up to 11 days. The method was used to detect changes in metabolism associated with increasing extracellular islet glucose concentration from 3 to 20 mM yielding results largely consistent with known metabolism of islets. Because most previous studies of islet metabolism have only observed a few compounds at once and require far more tissue, this measurement method represents a significant advance for studies of metabolism of islets and other microscale samples.

Chronic suppression of acetyl-CoA carboxylase 1 in beta-cells impairs insulin secretion via inhibition of glucose rather than lipid metabolism

Acetyl-CoA carboxylase 1 (ACC1) currently is being investigated as a target for treatment of obesity-associated dyslipidemia and insulin resistance. To investigate the effects of ACC1 inhibition on insulin secretion, three small interfering RNA (siRNA) duplexes targeting ACC1 (siACC1) were transfected into the INS-1-derived cell line, 832/13; the most efficacious duplex was also cloned into an adenovirus and used to transduce isolated rat islets. Delivery of the siACC1 duplexes decreased ACC1 mRNA by 60-80% in 832/13 cells and islets and enzyme activity by 46% compared with cells treated with a non-targeted siRNA. Delivery of siACC1 decreased glucose-stimulated insulin secretion (GSIS) by 70% in 832/13 cells and by 33% in islets. Surprisingly, siACC1 treatment decreased glucose oxidation by 49%, and the ATP:ADP ratio by 52%, accompanied by clear decreases in pyruvate cycling activity and tricarboxylic acid cycle intermediates. Exposure of siACC1-treated cells to the pyruvate cycling substrate dimethylmalate restored GSIS to normal without recovery of the depressed ATP:ADP ratio. In siACC1-treated cells, glucokinase protein levels were decreased by 25%, which correlated with a 36% decrease in glycogen synthesis and a 33% decrease in glycolytic flux. Furthermore, acute addition of the ACC1 inhibitor 5-(tetradecyloxy)-2-furoic acid (TOFA) to beta-cells suppressed [(14)C]glucose incorporation into lipids but had no effect on GSIS, whereas chronic TOFA administration suppressed GSIS and glucose metabolism. In sum, chronic, but not acute, suppression of ACC1 activity impairs GSIS via inhibition of glucose rather than lipid metabolism. These findings raise concerns about the use of ACC inhibitors for diabetes therapy.

Genetic networks of liver metabolism revealed by integration of metabolic and transcriptional profiling

Although numerous quantitative trait loci (QTL) influencing disease-related phenotypes have been detected through gene mapping and positional cloning, identification of the individual gene(s) and molecular pathways leading to those phenotypes is often elusive. One way to improve understanding of genetic architecture is to classify phenotypes in greater depth by including transcriptional and metabolic profiling. In the current study, we have generated and analyzed mRNA expression and metabolic profiles in liver samples obtained in an F2 intercross between the diabetes-resistant C57BL/6 leptin^ob/ob and the diabetes-susceptible BTBR leptin^ob/ob mouse strains. This cross, which segregates for genotype and physiological traits, was previously used to identify several diabetes-related QTL. Our current investigation includes microarray analysis of over 40,000 probe sets, plus quantitative mass spectrometry-based measurements of sixty-seven intermediary metabolites in three different classes (amino acids, organic acids, and acyl-carnitines). We show that liver metabolites map to distinct genetic regions, thereby indicating that tissue metabolites are heritable. We also demonstrate that genomic analysis can be integrated with liver mRNA expression and metabolite profiling data to construct causal networks for control of specific metabolic processes in liver. As a proof of principle of the practical significance of this integrative approach, we illustrate the construction of a specific causal network that links gene expression and metabolic changes in the context of glutamate metabolism, and demonstrate its validity by showing that genes in the network respond to changes in glutamine and glutamate availability. Thus, the methods described here have the potential to reveal regulatory networks that contribute to chronic, complex, and highly prevalent diseases and conditions such as obesity and diabetes.

Although numerous quantitative trait loci (QTL) influencing disease-related phenotypes have been detected through gene mapping and positional cloning, identifying individual genes and their potential roles in molecular pathways leading to disease remains a challenge. In this study, we include transcriptional and metabolic profiling in genomic analyses to address this limitation. We investigated an F2 intercross between the diabetes-resistant C57BL/6 leptin^ob/ob and the diabetes-susceptible BTBR leptin^ob/ob mouse strains that segregates for genotype and diabetes-related physiological traits; blood glucose, plasma insulin and body weight. Our study shows that liver metabolites (comprised of amino acids, organic acids, and acyl-carnitines) map to distinct genetic regions, thereby indicating that tissue metabolites are heritable. We also demonstrate that genomic analysis can be integrated with liver mRNA expression and metabolite profiling data to construct causal, testable networks for control of specific metabolic processes in liver. We apply an in vitro study to confirm the validity of this integrative method, and thus provide a novel approach to reveal regulatory networks that contribute to chronic, complex, and highly prevalent diseases and conditions such as obesity and diabetes.

Dual compartmental localization and function of mammalian NADP+-specific isocitrate dehydrogenase in yeast

Isozymes of NADP+-specific isocitrate dehydrogenase (IDP) provide NADPH in cytosolic, mitochondrial, and peroxisomal compartments of eukaryotic cells. Analyses of purified IDP isozymes from yeast and from mouse suggest a general correspondence of pH optima for catalysis and pI values with pH values reported for resident cellular compartments. However, mouse IDP2, which partitions between cytosolic and peroxisomal compartments in mammalian cells, exhibits a broad pH optimum and an intermediate pI value. Mouse IDP2 was found to similarly colocalize in both cellular compartments when expressed in yeast at levels equivalent to those of endogenous yeast isozymes. The mouse enzyme can compensate for loss of yeast cytosolic IDP2 and of peroxisomal IDP3. Removal of the peroxisomal targeting signal of the mouse enzyme precludes both localization in peroxisomes and compensation for loss of yeast IDP3.

NAD+/NADH and NADP+/NADPH in cellular functions and cell death: regulation and biological consequences

Accumulating evidence has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could belong to the fundamental common mediators of various biological processes, including energy metabolism, mitochondrial functions, calcium homeostasis, antioxidation/generation of oxidative stress, gene expression, immunological functions, aging, and cell death: First, it is established that NAD mediates energy metabolism and mitochondrial functions; second, NADPH is a key component in cellular antioxidation systems; and NADH-dependent reactive oxygen species (ROS) generation from mitochondria and NADPH oxidase-dependent ROS generation are two critical mechanisms of ROS generation; third, cyclic ADP-ribose and several other molecules that are generated from NAD and NADP could mediate calcium homeostasis; fourth, NAD and NADP modulate multiple key factors in cell death, such as mitochondrial permeability transition, energy state, poly(ADP-ribose) polymerase-1, and apoptosis-inducing factor; and fifth, NAD and NADP profoundly affect aging-influencing factors such as oxidative stress and mitochondrial activities, and NAD-dependent sirtuins also mediate the aging process. Moreover, many recent studies have suggested novel paradigms of NAD and NADP metabolism. Future investigation into the metabolism and biological functions of NAD and NADP may expose fundamental properties of life, and suggest new strategies for treating diseases and slowing the aging process.

Fuel-induced amplification of insulin secretion in mouse pancreatic islets exposed to a high sulfonylurea concentration: role of the NADPH/NADP+ ratio

AIMS/HYPOTHESIS

The aim of this study was to examine whether the cytosolic NADPH/NADP+ ratio of beta cells serves as an amplifying signal in fuel-induced insulin secretion and whether such a function is mediated by cytosolic alpha-ketoglutarate.

METHODS

Pancreatic islets and islet cells were isolated from albino mice by collagenase digestion. Insulin secretion of incubated or perifused islets was measured by ELISA. The NADPH and NADP+ content of incubated islets was determined by enzymatic cycling. The cytosolic Ca2+ concentration ([Ca2+]c) in islets was measured by microfluorimetry and the activity of ATP-sensitive K+ channels in islet cells by patch-clamping.

RESULTS

Both 30 mmol/l glucose and 10 mmol/l alpha-ketoisocaproate stimulated insulin secretion and elevated the NADPH/NADP+ ratio of islets preincubated in the absence of fuel. The increase in the NADPH/NADP+ ratio was abolished in the presence of 2.7 micromol/l glipizide (closing all ATP-sensitive K+ channels). However, alpha-ketoisocaproate, but not glucose, still stimulated insulin secretion. That glipizide did not inhibit alpha-ketoisocaproate-induced insulin secretion was not the result of elevated [Ca2+]c, as glucose caused a more marked [Ca2+]c increase. Insulin release triggered by glipizide alone was moderately amplified by dimethyl alpha-ketoglutarate (which is cleaved to produce cytosolic alpha-ketoglutarate), but there was no indication of a signal function of cytosolic alpha-ketoglutarate.

CONCLUSIONS/INTERPRETATION

The results strongly suggest that the NADPH/NADP+ ratio in the beta cell cytosol does not serve as an amplifying signal in fuel-induced insulin release. The study supports the view that amplification results from the intramitochondrial production of citrate by citrate synthase and from the associated export of citrate into the cytosol.

A role for ATP-citrate lyase, malic enzyme, and pyruvate/citrate cycling in glucose-induced insulin secretion

In pancreatic beta-cells, metabolic coupling factors generated during glucose metabolism and pyruvate cycling through anaplerosis/cataplerosis processes contribute to the regulation of insulin secretion. Pyruvate/citrate cycling across the mitochondrial membrane leads to the production of malonyl-CoA and NADPH, two candidate coupling factors. To examine the implication of pyruvate/citrate cycling in glucose-induced insulin secretion (GIIS), different steps of the cycle were inhibited in INS 832/13 cells by pharmacological inhibitors and/or RNA interference (RNAi) technology: mitochondrial citrate export, ATP-citrate lyase (ACL), and cytosolic malic enzyme (ME1). The inhibitors of the di- and tri-carboxylate carriers, n-butylmalonate and 1,2,3-benzenetricarboxylate, respectively, reduced GIIS, indicating the importance of transmitochondrial transport of tri- and dicarboxylates in the action of glucose. To directly test the role of ACL and ME1 in GIIS, small hairpin RNA (shRNA) were used to selectively decrease ACL or ME1 expression in transfected INS 832/13 cells. shRNA-ACL reduced ACL protein levels by 67%, and this was accompanied by a reduction in GIIS. The amplification/K(ATP)-independent pathway of GIIS was affected by RNAi knockdown of ACL. The ACL inhibitor radicicol also curtailed GIIS. shRNA-ME1 reduced ME1 activity by 62% and decreased GIIS. RNAi suppression of either ACL or ME1 did not affect glucose oxidation. However, because ACL is required for malonyl-CoA formation, inhibition of ACL expression by shRNA-ACL decreased glucose incorporation into palmitate and increased fatty acid oxidation in INS 832/13 cells. Taken together, the results underscore the importance of pyruvate/citrate cycling in pancreatic beta-cell metabolic signaling and the regulation of GIIS.

Normal flux through ATP-citrate lyase or fatty acid synthase is not required for glucose-stimulated insulin secretion

It has been proposed that de novo synthesis of long-chain acyl-CoA (LC-CoA) is a signal for glucose-stimulated insulin secretion (GSIS). Key enzymes involved in synthesis of fatty acids from glucose include ATP-citrate lyase (CL) and fatty acid synthase (FAS). An inhibitor of CL, hydroxycitrate (HC), has been reported to inhibit insulin secretion in some laboratories but not in others. Here we show that high concentrations of NaCl created during preparation of HC by standard methods explain the inhibition of GSIS, and that removal of the excess NaCl prevents the effect. To further investigate the role of CL, two small interfering RNA adenoviruses (Ad-siCL2 and Ad-siCL3) were generated. Ad-siCL3 reduced CL mRNA levels by 92 +/- 6% and CL protein levels by 75 +/- 4% but did not affect GSIS in 832/13 cells compared with cells treated with a control adenovirus (Ad-siControl). Similar results were obtained with Ad-siCL2. Ad-siCL3-treated cells also exhibited a 52 +/- 7% reduction in cytosolic oxaloacetate, an 83 +/- 4% reduction in malonyl-CoA levels, and inhibition of [U-(14)C]glucose incorporation into lipid by 43 +/- 4%, all expected metabolic out-comes of CL suppression. Similarly, treatment of 832/13 cells with a recombinant adenovirus specific to FAS (Ad-siFAS) reduced FAS mRNA levels by 81 +/- 2% in 832/13 cells, resulting in a 59 +/- 4% decrease in [U-(14)C]glucose incorporation into lipid, without affecting GSIS. Finally, treatment of primary rat islets with Ad-siCL3 or Ad-siFAS reduced CL and FAS mRNA levels by 65 +/- 4% and 52 +/- 3%, respectively, but had no effect on GSIS relative to Ad-siControl-treated islets. These findings demonstrate that a normal rate of flux of glucose carbons through CL and FAS is not required for GSIS in insulinoma cell lines or rat islets.

Feasibility of pathways for transfer of acyl groups from mitochondria to the cytosol to form short chain acyl-CoAs in the pancreatic beta cell

The mitochondria of pancreatic beta cells are believed to convert insulin secretagogues into products that are translocated to the cytosol where they participate in insulin secretion. We studied the hypothesis that short chain acyl-CoA (SC-CoAs) might be some of these products by discerning the pathways of SC-CoA formation in beta cells. Insulin secretagogues acutely stimulated 1.5-5-fold increases in acetoacetyl-CoA, succinyl-CoA, malonyl-CoA, hydroxymethylglutaryl-CoA (HMG-CoA), and acetyl-CoA in INS-1 832/13 cells as judged from liquid chromatography-tandem mass spectrometry measurements. Studies of 12 relevant enzymes in rat and human pancreatic islets and INS-1 832/13 cells showed the feasibility of at least two redundant pathways, one involving acetoacetate and the other citrate, for the synthesis SC-CoAs from secretagogue carbon in mitochondria and the transfer of their acyl groups to the cytosol where the acyl groups are converted to SC-CoAs. Knockdown of two key cytosolic enzymes in INS-1 832/13 cells with short hairpin RNA supported the proposed scheme. Lowering ATP citrate lyase 88% did not inhibit glucose-induced insulin release indicating citrate is not the only carrier of acyl groups to the cytosol. However, lowering acetoacetyl-CoA synthetase 80% partially inhibited glucose-induced insulin release indicating formation of SC-CoAs from acetoacetate in the cytosol is important for insulin secretion. The results indicate beta cells possess enzyme pathways that can incorporate carbon from glucose into acetyl-CoA, acetoacetyl-CoA, and succinyl-CoA and carbon from leucine into these three SC-CoAs plus HMG-CoA in their mitochondria and enzymes that can form acetyl-CoA, acetoacetyl-CoA, malonyl-CoA, and HMG-CoA in their cytosol.

A kinetic core model of the glucose-stimulated insulin secretion network of pancreatic beta cells

The construction and characterization of a core kinetic model of the glucose-stimulated insulin secretion system (GSIS) in pancreatic beta cells is described. The model consists of 44 enzymatic reactions, 59 metabolic state variables, and 272 parameters. It integrates five subsystems: glycolysis, the TCA cycle, the respiratory chain, NADH shuttles, and the pyruvate cycle. It also takes into account compartmentalization of the reactions in the cytoplasm and mitochondrial matrix. The model shows expected behavior in its outputs, including the response of ATP production to starting glucose concentration and the induction of oscillations of metabolite concentrations in the glycolytic pathway and in ATP and ADP concentrations. Identification of choke points and parameter sensitivity analysis indicate that the glycolytic pathway, and to a lesser extent the TCA cycle, are critical to the proper behavior of the system, while parameters in other components such as the respiratory chain are less critical. Notably, however, sensitivity analysis identifies the first reactions of nonglycolytic pathways as being important for the behavior of the system. The model is robust to deletion of malic enzyme activity, which is absent in mouse pancreatic beta cells. The model represents a step toward the construction of a model with species-specific parameters that can be used to understand mouse models of diabetes and the relationship of these mouse models to the human disease state.

The mitochondrial citrate/isocitrate carrier plays a regulatory role in glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) is mediated in part by glucose metabolism-driven increases in ATP/ADP ratio, but by-products of mitochondrial glucose metabolism also play an important role. Here we investigate the role of the mitochondrial citrate/isocitrate carrier (CIC) in regulation of GSIS. Inhibition of CIC activity in INS-1-derived 832/13 cells or primary rat islets by the substrate analogue 1,2,3-benzenetricarboxylate (BTC) resulted in potent inhibition of GSIS, involving both first and second phase secretion. A recombinant adenovirus containing a CIC-specific siRNA (Ad-siCIC) dose-dependently reduced CIC expression in 832/13 cells and caused parallel inhibitory effects on citrate accumulation in the cytosol. Ad-siCIC treatment did not affect glucose utilization, glucose oxidation, or ATP/ADP ratio but did inhibit glucose incorporation into fatty acids and glucose-induced increases in NADPH/NADP+ ratio relative to cells treated with a control siRNA virus (Ad-siControl). Ad-siCIC also inhibited GSIS in 832/13 cells, whereas overexpression of CIC enhanced GSIS and raised cytosolic citrate levels. In normal rat islets, Ad-siCIC treatment also suppressed CIC mRNA levels and inhibited GSIS. We conclude that export of citrate and/or isocitrate from the mitochondria to the cytosol is an important step in control of GSIS.