Mitochondrial leak metabolism induces the Spemann-Mangold Organizer via Hif-1α in Xenopus.

An instructive role for metabolism in embryonic patterning is emerging, although a role for mitochondria is poorly defined. We demonstrate that mitochondrial oxidative metabolism establishes the embryonic patterning center, the Spemann-Mangold Organizer, via hypoxia-inducible factor 1α (Hif-1α) in Xenopus. Hypoxia or decoupling ATP production from oxygen consumption expands the Organizer by activating Hif-1α. In addition, oxygen consumption is 20% higher in the Organizer than in the ventral mesoderm, indicating an elevation in mitochondrial respiration. To reconcile increased mitochondrial respiration with activation of Hif-1α, we discovered that the "free" c-subunit ring of the F1Fo ATP synthase creates an inner mitochondrial membrane leak, which decouples ATP production from respiration at the Organizer, driving Hif-1α activation there. Overexpression of either the c-subunit or Hif-1α is sufficient to induce Organizer cell fates even when ß-catenin is inhibited. We propose that mitochondrial leak metabolism could be a general mechanism for activating Hif-1α and Wnt signaling.

Review and Hypothesis: A Potential Common Link Between Glial Cells, Calcium Changes, Modulation of Synaptic Transmission, Spreading Depression, Migraine, and Epilepsy-H.

There is significant evidence to support the notion that glial cells can modulate the strength of synaptic connections between nerve cells, and it has further been suggested that alterations in intracellular calcium are likely to play a key role in this process. However, the molecular mechanism(s) by which glial cells modulate neuronal signaling remains contentiously debated. Recent experiments have suggested that alterations in extracellular H+ efflux initiated by extracellular ATP may play a key role in the modulation of synaptic strength by radial glial cells in the retina and astrocytes throughout the brain. ATP-elicited alterations in H+ flux from radial glial cells were first detected from Müller cells enzymatically dissociated from the retina of tiger salamander using self-referencing H+-selective microelectrodes. The ATP-elicited alteration in H+ efflux was further found to be highly evolutionarily conserved, extending to Müller cells isolated from species as diverse as lamprey, skate, rat, mouse, monkey and human. More recently, self-referencing H+-selective electrodes have been used to detect ATP-elicited alterations in H+ efflux around individual mammalian astrocytes from the cortex and hippocampus. Tied to increases in intracellular calcium, these ATP-induced extracellular acidifications are well-positioned to be key mediators of synaptic modulation. In this article, we examine the evidence supporting H+ as a key modulator of neurotransmission, review data showing that extracellular ATP elicits an increase in H+ efflux from glial cells, and describe the potential signal transduction pathways involved in glial cell-mediated H+ efflux. We then examine the potential role that extracellular H+ released by glia might play in regulating synaptic transmission within the vertebrate retina, and then expand the focus to discuss potential roles in spreading depression, migraine, epilepsy, and alterations in brain rhythms, and suggest that alterations in extracellular H+ may be a unifying feature linking these disparate phenomena.

Parkinson's disease protein DJ-1 regulates ATP synthase protein components to increase neuronal process outgrowth.

Familial Parkinson's disease (PD) protein DJ-1 mutations are linked to early onset PD. We have found that DJ-1 binds directly to the F1FO ATP synthase ß subunit. DJ-1's interaction with the ß subunit decreased mitochondrial uncoupling and enhanced ATP production efficiency while in contrast mutations in DJ-1 or DJ-1 knockout increased mitochondrial uncoupling, and depolarized neuronal mitochondria. In mesencephalic DJ-1 KO cultures, there was a progressive loss of neuronal process extension. This was ameliorated by a pharmacological reagent, dexpramipexole, that binds to ATP synthase, closing a mitochondrial inner membrane leak and enhancing ATP synthase efficiency. ATP synthase c-subunit can form an uncoupling channel; we measured, therefore, ATP synthase F1 (ß subunit) and c-subunit protein levels. We found that ATP synthase ß subunit protein level in the DJ-1 KO neurons was approximately half that found in their wild-type counterparts, comprising a severe defect in ATP synthase stoichiometry and unmasking c-subunit. We suggest that DJ-1 enhances dopaminergic cell metabolism and growth by its regulation of ATP synthase protein components.

The mitochondrial complex V-associated large-conductance inner membrane current is regulated by cyclosporine and dexpramipexole.

Inefficiency of oxidative phosphorylation can result from futile leak conductance through the inner mitochondrial membrane. Stress or injury may exacerbate this leak conductance, putting cells, and particularly neurons, at risk of dysfunction and even death when energy demand exceeds cellular energy production. Using a novel method, we have recently described an ion conductance consistent with mitochondrial permeability transition pore (mPTP) within the c-subunit of the ATP synthase. Excitotoxicity, reactive oxygen species-producing stimuli, or elevated mitochondrial matrix calcium opens the channel, which is inhibited by cyclosporine A and ATP/ADP. Here we show that ATP and the neuroprotective drug dexpramipexole (DEX) inhibited an ion conductance consistent with this c-subunit channel (mPTP) in brain-derived submitochondrial vesicles (SMVs) enriched for F1FO ATP synthase (complex V). Treatment of SMVs with urea denatured extramembrane components of complex V, eliminated DEX- but not ATP-mediated current inhibition, and reduced binding of [(14)C]DEX. Direct effects of DEX on the synthesis and hydrolysis of ATP by complex V suggest that interaction of the compound with its target results in functional conformational changes in the enzyme complex. [(14)C]DEX bound specifically to purified recombinant b and oligomycin sensitivity-conferring protein subunits of the mitochondrial F1FO ATP synthase. Previous data indicate that DEX increased the efficiency of energy production in cells, including neurons. Taken together, these studies suggest that modulation of a complex V-associated inner mitochondrial membrane current is metabolically important and may represent an avenue for the development of new therapeutics for neurodegenerative disorders.

Paper-based colorimetric enzyme linked immunosorbent assay fabricated by laser induced forward transfer.

We report the Laser Induced Forward Transfer (LIFT) of antibodies from a liquid donor film onto paper receivers for application as point-of-care diagnostic sensors. To minimise the loss of functionality of the active biomolecules during transfer, a dynamic release layer was employed to shield the biomaterial from direct exposure to the pulsed laser source. Cellulose paper was chosen as the ideal receiver because of its inherent bio-compatibility, liquid transport properties, wide availability and low cost, all of which make it an efficient and suitable platform for point-of-care diagnostic sensors. Both enzyme-tagged and untagged IgG antibodies were LIFT-printed and their functionality was confirmed via a colorimetric enzyme-linked immunosorbent assay. Localisation of the printed antibodies was exhibited, which can allow the creation of complex 2-d patterns such as QR codes or letters for use in a final working device. Finally, a calibration curve was determined that related the intensity of the colour obtained to the concentration of active antibodies to enable quantitative assessment of the device performance. The motivation for this work was to implement a laser-based procedure for manufacturing low-cost, point-of-care diagnostic devices on paper.

Expression profiling of primary and metastatic ovarian tumors reveals differences indicative of aggressive disease.

The behavior and genetics of serous epithelial ovarian cancer (EOC) metastasis, the form of the disease lethal to patients, is poorly understood. The unique properties of metastases are critical to understand to improve treatments of the disease that remains in patients after debulking surgery. We sought to identify the genetic and phenotypic landscape of metastatic progression of EOC to understand how metastases compare to primary tumors. DNA copy number and mRNA expression differences between matched primary human tumors and omental metastases, collected at the same time during debulking surgery before chemotherapy, were measured using microarrays. qPCR and immunohistochemistry validated findings. Pathway analysis of mRNA expression revealed metastatic cancer cells are more proliferative and less apoptotic than primary tumors, perhaps explaining the aggressive nature of these lesions. Most cases had copy number aberrations (CNAs) that differed between primary and metastatic tumors, but we did not detect CNAs that are recurrent across cases. A six gene expression signature distinguishes primary from metastatic tumors and predicts overall survival in independent datasets. The genetic differences between primary and metastatic tumors, yet common expression changes, suggest that the major clone in metastases is not the same as in primary tumors, but the cancer cells adapt to the omentum similarly. Together, these data highlight how ovarian tumors develop into a distinct, more aggressive metastatic state that should be considered for therapy development.

Quantitative exploration of the contribution of settlement, growth, dispersal and grazing to the accumulation of natural marine biofilms on antifouling and fouling-release coatings.

The accumulation of microbial biofilms on ships' hulls negatively affects ship performance and efficiency while also playing a role in the establishment of even more detrimental hard-fouling communities. However, there is little quantitative information on how the accumulation rate of microbial biofilms is impacted by the balance of the rates of cell settlement, in situ production (ie growth), dispersal to surrounding waters and mortality induced by grazers. These rates were quantified on test panels coated with copper-based antifouling (AF) or polymer-based fouling-release (FR) coatings by using phospholipids as molecular proxies for microbial biomass. The results confirmed the accepted modes of efficacy of these two types of coatings. In a more extensive set of experiments with only the FR coatings, it was found that seasonally averaged cellular production rates were 1.5 ± 0.5 times greater than settlement and the dispersal rates were 2.7 ± 0.8 greater than grazing. The results of this study quantitatively describe the dynamic balance of processes leading to the accumulation of microbial biofilm on coatings designed for ships' hulls.

Identification of ovarian cancer metastatic miRNAs.

Serous epithelial ovarian cancer (EOC) patients often succumb to aggressive metastatic disease, yet little is known about the behavior and genetics of ovarian cancer metastasis. Here, we aim to understand how omental metastases differ from primary tumors and how these differences may influence chemotherapy. We analyzed the miRNA expression profiles of primary EOC tumors and their respective omental metastases from 9 patients using miRNA Taqman qPCR arrays. We find 17 miRNAs with differential expression in omental lesions compared to primary tumors. miR-21, miR-150, and miR-146a have low expression in most primary tumors with significantly increased expression in omental lesions, with concomitant decreased expression of predicted mRNA targets based on mRNA expression. We find that miR-150 and miR-146a mediate spheroid size. Both miR-146a and miR-150 increase the number of residual surviving cells by 2-4 fold when challenged with lethal cisplatin concentrations. These observations suggest that at least two of the miRNAs, miR-146a and miR-150, up-regulated in omental lesions, stimulate survival and increase drug tolerance. Our observations suggest that cancer cells in omental tumors express key miRNAs differently than primary tumors, and that at least some of these microRNAs may be critical regulators of the emergence of drug resistant disease.

Effects of dexpramipexole on brain mitochondrial conductances and cellular bioenergetic efficiency.

Cellular stress or injury can result in mitochondrial dysfunction, which has been linked to many chronic neurological disorders including amyotrophic lateral sclerosis (ALS) and Parkinson's disease (PD). Stressed and dysfunctional mitochondria exhibit an increase in large conductance mitochondrial membrane currents and a decrease in bioenergetic efficiency. Inefficient energy production puts cells, and particularly neurons, at risk of death when energy demands exceed cellular energy production. Here we show that the candidate ALS drug dexpramipexole (DEX; KNS-760704; ((6R)-4,5,6,7-tetrahydro-N6-propyl-2,6-benzothiazole-diamine) and cyclosporine A (CSA) inhibited increases in ion conductance in whole rat brain-derived mitochondria induced by calcium or treatment with a proteasome inhibitor, although only CSA inhibited calcium-induced permeability transition in liver-derived mitochondria. In several cell lines, including cortical neurons in culture, DEX significantly decreased oxygen consumption while maintaining or increasing production of adenosine triphosphate (ATP). DEX also normalized the metabolic profile of injured cells and was protective against the cytotoxic effects of proteasome inhibition. These data indicate that DEX increases the efficiency of oxidative phosphorylation, possibly by inhibition of a CSA-sensitive mitochondrial conductance.

The level of menadione redox-cycling in pancreatic ß-cells is proportional to the glucose concentration: role of NADH and consequences for insulin secretion.

Pancreatic ß-cells release insulin in response to elevation of glucose from basal (4-7mM) to stimulatory (8-16mM) levels. Metabolism of glucose by the ß-cell results in the production of low levels of reactive oxygen intermediates (ROI), such as hydrogen peroxide (H(2)O(2)), a newly recognized coupling factor linking glucose metabolism to insulin secretion. However, high and toxic levels of H(2)O(2) inhibit insulin secretion. Menadione, which produces H(2)O(2) via redox cycling mechanism in a dose-dependent manner, was investigated for its effect on ß-cell metabolism and insulin secretion in INS-1 832/13, a rat ß-cell insulinoma cell line, and primary rodent islets. Menadione-dependent redox cycling and resulting H(2)O(2) production under stimulatory glucose exceeded several-fold those reached at basal glucose. This was paralleled by a differential effect of menadione (0.1-10µM) on insulin secretion, which was enhanced at basal, but inhibited at stimulatory glucose. Redox cycling of menadione and H(2)O(2) formation was dependent on glycolytically-derived NADH, as inhibition of glycolysis and application of non-glycogenic insulin secretagogues did not support redox cycling. In addition, activity of plasma membrane electron transport, a system dependent in part on glycolytically-derived NADH, was also inhibited by menadione. Menadione-dependent redox cycling was sensitive to the NQO1 inhibitor dicoumarol and the flavoprotein inhibitor diphenylene iodonium, suggesting a role for NQO1 and other oxidoreductases in this process. These data may explain the apparent dichotomy between the stimulatory and inhibitory effects of H(2)O(2) and menadione on insulin secretion.

Bcl-xL regulates metabolic efficiency of neurons through interaction with the mitochondrial F1FO ATP synthase.

Anti-apoptotic Bcl2 family proteins such as Bcl-x(L) protect cells from death by sequestering apoptotic molecules, but also contribute to normal neuronal function. We find in hippocampal neurons that Bcl-x(L) enhances the efficiency of energy metabolism. Our evidence indicates that Bcl-x(L)interacts directly with the ß-subunit of the F(1)F(O) ATP synthase, decreasing an ion leak within the F(1)F(O) ATPase complex and thereby increasing net transport of H(+) by F(1)F(O) during F(1)F(O) ATPase activity. By patch clamping submitochondrial vesicles enriched in F(1)F(O) ATP synthase complexes, we find that, in the presence of ATP, pharmacological or genetic inhibition of Bcl-x(L) activity increases the membrane leak conductance. In addition, recombinant Bcl-x(L) protein directly increases the level of ATPase activity of purified synthase complexes, and inhibition of endogenous Bcl-x(L) decreases the level of F(1)F(O) enzymatic activity. Our findings indicate that increased mitochondrial efficiency contributes to the enhanced synaptic efficacy found in Bcl-x(L)-expressing neurons.

Catechol metabolites of endogenous estrogens induce redox cycling and generate reactive oxygen species in breast epithelial cells.

Estrogens are major risk factors for the development of breast cancer; they can be metabolized to catechols, which are further oxidized to DNA-reactive quinones and semiquinones (SQs). These metabolites are mutagenic and may contribute to the carcinogenic activity of estrogens. Redox cycling of the SQs and subsequent generation of reactive oxygen species (ROS) is also an important mechanism leading to DNA damage. The SQs of exogenous estrogens have been shown to redox cycle, however, redox cycling and the generation of ROS by endogenous estrogens has never been characterized. In the present studies, we determined whether the catechol metabolites of endogenous estrogens, including 2-hydroxyestradiol, 4-hydroxyestradiol, 4-hydroxyestrone and 2-hydroxyestriol, can redox cycle in breast epithelial cells. These catechol estrogens, but not estradiol, estrone, estriol or 2-methoxyestradiol, were found to redox cycle and generate hydrogen peroxide (H(2)O(2)) and hydroxyl radicals in lysates of three different breast epithelial cell lines: MCF-7, MDA-MB-231 and MCF-10A. The generation of ROS required reduced nicotinamide adenine dinucleotide phosphate as a reducing equivalent and was inhibited by diphenyleneiodonium, a flavoenzyme inhibitor, indicating that redox cycling is mediated by flavin-containing oxidoreductases. Using extracellular microsensors, catechol estrogen metabolites stimulated the release of H(2)O(2) by adherent cells, indicating that redox cycling occurs in viable intact cells. Taken together, these data demonstrate that catechol metabolites of endogenous estrogens undergo redox cycling in breast epithelial cells, resulting in ROS production. Depending on the localized concentrations of catechol estrogens and enzymes that mediate redox cycling, this may be an important mechanism contributing to the development of breast cancer.

Plasma membrane electron transport in pancreatic ß-cells is mediated in part by NQO1.

Plasma membrane electron transport (PMET), a cytosolic/plasma membrane analog of mitochondrial electron transport, is a ubiquitous system of cytosolic and plasma membrane oxidoreductases that oxidizes cytosolic NADH and NADPH and passes electrons to extracellular targets. While PMET has been shown to play an important role in a variety of cell types, no studies exist to evaluate its function in insulin-secreting cells. Here we demonstrate the presence of robust PMET activity in primary islets and clonal ß-cells, as assessed by the reduction of the plasma membrane-impermeable dyes WST-1 and ferricyanide. Because the degree of metabolic function of ß-cells (reflected by the level of insulin output) increases in a glucose-dependent manner between 4 and 10 mM glucose, PMET was evaluated under these conditions. PMET activity was present at 4 mM glucose and was further stimulated at 10 mM glucose. PMET activity at 10 mM glucose was inhibited by the application of the flavoprotein inhibitor diphenylene iodonium and various antioxidants. Overexpression of cytosolic NAD(P)H-quinone oxidoreductase (NQO1) increased PMET activity in the presence of 10 mM glucose while inhibition of NQO1 by its inhibitor dicoumarol abolished this activity. Mitochondrial inhibitors rotenone, antimycin A, and potassium cyanide elevated PMET activity. Regardless of glucose levels, PMET activity was greatly enhanced by the application of aminooxyacetate, an inhibitor of the malate-aspartate shuttle. We propose a model for the role of PMET as a regulator of glycolytic flux and an important component of the metabolic machinery in ß-cells.

Redox cycling and increased oxygen utilization contribute to diquat-induced oxidative stress and cytotoxicity in Chinese hamster ovary cells overexpressing NADPH-cytochrome P450 reductase.

Diquat and paraquat are nonspecific defoliants that induce toxicity in many organs including the lung, liver, kidney, and brain. This toxicity is thought to be due to the generation of reactive oxygen species (ROS). An important pathway leading to ROS production by these compounds is redox cycling. In this study, diquat and paraquat redox cycling was characterized using human recombinant NADPH-cytochrome P450 reductase, rat liver microsomes, and Chinese hamster ovary (CHO) cells constructed to overexpress cytochrome P450 reductase (CHO-OR) and wild-type control cells (CHO-WT). In redox cycling assays with recombinant cytochrome P450 reductase and microsomes, diquat was 10-40 times more effective at generating ROS compared to paraquat (K(M)=1.0 and 44.2µM, respectively, for H(2)O(2) generation by diquat and paraquat using recombinant enzyme, and 15.1 and 178.5µM, respectively for microsomes). In contrast, at saturating concentrations, these compounds showed similar redox cycling activity (V(max)≈6.0nmol H(2)O(2)/min/mg protein) for recombinant enzyme and microsomes. Diquat and paraquat also redox cycle in CHO cells. Significantly more activity was evident in CHO-OR cells than in CHO-WT cells. Diquat redox cycling in CHO cells was associated with marked increases in protein carbonyl formation, a marker of protein oxidation, as well as cellular oxygen consumption, measured using oxygen microsensors; greater activity was detected in CHO-OR cells than in CHO-WT cells. These data demonstrate that ROS formation during diquat redox cycling can generate oxidative stress. Enhanced oxygen utilization during redox cycling may reduce intracellular oxygen available for metabolic reactions and contribute to toxicity.

Construction, theory, and practical considerations for using self-referencing of Ca(2+)-selective microelectrodes for monitoring extracellular Ca(2+) gradients.

Ca(2+) signaling in the extra- and intracellular domains is linked to Ca(2+) transport across the plasma membrane. Noninvasive monitoring of these resulting extracellular Ca(2+) gradients with self-referencing of Ca(2+)-selective microelectrodes is used for studying Ca(2+) signaling across Kingdoms. The quantitated Ca(2+) flux enables comparison with changes to intracellular [Ca(2+)] measured with other methods and determination of Ca(2+) transport stoichiometry. Here, we review the construction of Ca(2+)-selective microelectrodes, their physical characteristics, and their use in self-referencing mode to calculate Ca(2+) flux. We also discuss potential complications when using them to measure Ca(2+) gradients near the boundary layers of single cells and tissues.

Physiological and pharmacological characterizations of the larval Anopheles albimanus rectum support a change in protein distribution and/or function in varying salinities.

Ion regulation is a biological process crucial to the survival of mosquito larvae and a major organ responsible for this regulation is the rectum. The recta of anopheline larvae are distinct from other subfamilies of mosquitoes in several ways, yet have not yet been characterized extensively. Here we characterize the two major cell types of the anopheline rectum, DAR and non-DAR cells, using histological, physiological, and pharmacological analyses. Proton flux was measured at the basal membrane of 2%- and 50%-artificial sea water-reared An. albimanus larvae using self-referencing ion-selective microelectrodes, and the two cell types were found to differ in basal membrane proton flux. Additionally, differences in the response of that flux to pharmacological inhibitors in larvae reared in 2% versus 50% ASW indicate changes in protein function between the two rearing conditions. Finally, histological analyses suggest that the non-DAR cells are structurally suited for mediating ion transport. These data support a model of rectal ion regulation in which the non-DAR cells have a resorptive function in freshwater-reared larvae and a secretive function in saline water-reared larvae. In this way, anopheline larvae may adapt to varying salinities.

Windows to cell function and dysfunction: signatures written in the boundary layers.

The medium surrounding cells either in culture or in tissues contains a chemical mix varying with cell state. As solutes move in and out of the cytoplasmic compartment they set up characteristic signatures in the cellular boundary layers. These layers are complex physical and chemical environments the profiles of which reflect cell physiology and provide conduits for intercellular messaging. Here we review some of the most relevant characteristics of the extracellular/intercellular space. Our initial focus is primarily on cultured cells but we extend our consideration to the far more complex environment of tissues, and discuss how chemical signatures in the boundary layer can or may affect cell function. Critical to the entire essay are the methods used, or being developed, to monitor chemical profiles in the boundary layers. We review recent developments in ultramicro electrochemical sensors and tailored optical reporters suitable for the task in hand.

Release and elementary mechanisms of nitric oxide in hair cells.

The enzyme nitric oxide (NO) synthase, that produces the signaling molecule NO, has been identified in several cell types in the inner ear. However, it is unclear whether a measurable quantity of NO is released in the inner ear to confer specific functions. Indeed, the functional significance of NO and the elementary cellular mechanism thereof are most uncertain. Here, we demonstrate that the sensory epithelia of the frog saccule release NO and explore its release mechanisms by using self-referencing NO-selective electrodes. Additionally, we investigated the functional effects of NO on electrical properties of hair cells and determined their underlying cellular mechanism. We show detectable amounts of NO are released by hair cells (>50 nM). Furthermore, a hair-cell efferent modulator acetylcholine produces at least a threefold increase in NO release. NO not only attenuated the baseline membrane oscillations but it also increased the magnitude of current required to generate the characteristic membrane potential oscillations. This resulted in a rightward shift in the frequency-current relationship and altered the excitability of hair cells. Our data suggest that these effects ensue because NO reduces whole cell Ca(2+) current and drastically decreases the open probability of single-channel events of the L-type and non L-type Ca(2+) channels in hair cells, an effect that is mediated through direct nitrosylation of the channel and activation of protein kinase G. Finally, NO increases the magnitude of Ca(2+)-activated K(+) currents via direct NO nitrosylation. We conclude that NO-mediated inhibition serves as a component of efferent nerve modulation of hair cells.

Glucagon-like peptide-1 induced signaling and insulin secretion do not drive fuel and energy metabolism in primary rodent pancreatic beta-cells.

BACKGROUND: Glucagon like peptide-1 (GLP-1) and its analogue exendin-4 (Ex-4) enhance glucose stimulated insulin secretion (GSIS) and activate various signaling pathways in pancreatic beta-cells, in particular cAMP, Ca(2+) and protein kinase-B (PKB/Akt). In many cells these signals activate intermediary metabolism. However, it is not clear whether the acute amplification of GSIS by GLP-1 involves in part metabolic alterations and the production of metabolic coupling factors. METHODOLOGY/PRINICIPAL FINDINGS: GLP-1 or Ex-4 at high glucose caused release (approximately 20%) of the total rat islet insulin content over 1 h. While both GLP-1 and Ex-4 markedly potentiated GSIS in isolated rat and mouse islets, neither had an effect on beta-cell fuel and energy metabolism over a 5 min to 3 h time period. GLP-1 activated PKB without changing glucose usage and oxidation, fatty acid oxidation, lipolysis or esterification into various lipids in rat islets. Ex-4 caused a rise in [Ca(2+)](i) and cAMP but did not enhance energy utilization, as neither oxygen consumption nor mitochondrial ATP levels were altered. CONCLUSIONS/SIGNIFICANCE: The results indicate that GLP-1 barely affects beta-cell intermediary metabolism and that metabolic signaling does not significantly contribute to GLP-1 potentiation of GSIS. The data also indicate that insulin secretion is a minor energy consuming process in the beta-cell, and that the beta-cell is different from most cell types in that its metabolic activation appears to be primarily governed by a "push" (fuel substrate driven) process, rather than a "pull" mechanism secondary to enhanced insulin release as well as to Ca(2+), cAMP and PKB signaling.

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.