The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation

Ca2+-Mediated Signaling Pathways: A Promising Target for the Successful Generation of Mature and Functional Stem Cell-Derived Pancreatic Beta Cells In Vitro

Diabetes mellitus is a chronic disease affecting over 500 million adults globally and is mainly categorized as type 1 diabetes mellitus (T1DM), where pancreatic beta cells are destroyed, and type 2 diabetes mellitus (T2DM), characterized by beta cell dysfunction. This review highlights the importance of the divalent cation calcium (Ca²⁺) and its associated signaling pathways in the proper functioning of beta cells and underlines the effects of Ca²⁺ dysfunction on beta cell function and its implications for the onset of diabetes. Great interest and promise are held by human pluripotent stem cell (hPSC) technology to generate functional pancreatic beta cells from diabetic patient-derived stem cells to replace the dysfunctional cells, thereby compensating for insulin deficiency and reducing the comorbidities of the disease and its associated financial and social burden on the patient and society. Beta-like cells generated by most current differentiation protocols have blunted functionality compared to their adult human counterparts. The Ca²⁺ dynamics in stem cell-derived beta-like cells and adult beta cells are summarized in this review, revealing the importance of proper Ca²⁺ homeostasis in beta-cell function. Consequently, the importance of targeting Ca²⁺ function in differentiation protocols is suggested to improve current strategies to use hPSCs to generate mature and functional beta-like cells with a comparable glucose-stimulated insulin secretion (GSIS) profile to adult beta cells.

Phosphate Dysregulation and Metabolic Syndrome

Phosphorus is one of the most abundant minerals in the human body. It is essential for almost all biochemical activities through ATP formation, intracellular signal transduction, cell membrane formation, bone mineralization, DNA and RNA synthesis, and inflammation modulation through various inflammatory cytokines. Phosphorus levels must be optimally regulated, as any deviations may lead to substantial derangements in glucose homeostasis. Clinical studies have reported that hyperphosphatemia can increase an individual's risk of developing metabolic syndrome. High phosphate burden has been shown to impair glucose metabolism by impairing pancreatic insulin secretion and increasing the risk of cardiometabolic disorders. Phosphate toxicity deserves more attention as metabolic syndrome is being seen more frequently worldwide and should be investigated further to determine the underlying mechanism of how phosphate burden may increase the cardiometabolic risk in the general population.

Mitochondrial Calcium Signaling in Pancreatic β-Cell

Accumulation of calcium in energized mitochondria of pancreatic β-cells is emerging as a crucial process for pancreatic β-cell function. β-cell mitochondria sense and shape calcium signals, linking the metabolism of glucose and other secretagogues to the generation of signals that promote insulin secretion during nutrient stimulation. Here, we describe the role of mitochondrial calcium signaling in pancreatic β-cell function. We report the latest pharmacological and genetic findings, including the first mitochondrial calcium-targeted intervention strategies developed to modulate pancreatic β-cell function and their potential relevance in the context of diabetes.

The Effect of Deflazacort Treatment on the Functioning of Skeletal Muscle Mitochondria in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe hereditary disease caused by a lack of dystrophin, a protein essential for myocyte integrity. Mitochondrial dysfunction is reportedly responsible for DMD. This study examines the effect of glucocorticoid deflazacort on the functioning of the skeletal-muscle mitochondria of dystrophin-deficient mdx mice and WT animals. Deflazacort administration was found to improve mitochondrial respiration of mdx mice due to an increase in the level of ETC complexes (complexes III and IV and ATP synthase), which may contribute to the normalization of ATP levels in the skeletal muscle of mdx animals. Deflazacort treatment improved the rate of Ca2+ uniport in the skeletal muscle mitochondria of mdx mice, presumably by affecting the subunit composition of the calcium uniporter of organelles. At the same time, deflazacort was found to reduce the resistance of skeletal mitochondria to MPT pore opening, which may be associated with a change in the level of ANT2 and CypD. In this case, deflazacort also affected the mitochondria of WT mice. The paper discusses the mechanisms underlying the effect of deflazacort on the functioning of mitochondria and contributing to the improvement of the muscular function of mdx mice.

Diabetes Mellitus, Mitochondrial Dysfunction and Ca2+-Dependent Permeability Transition Pore

Diabetes mellitus is one of the most common metabolic diseases in the developed world, and is associated either with the impaired secretion of insulin or with the resistance of cells to the actions of this hormone (type I and type II diabetes, respectively). In both cases, a common pathological change is an increase in blood glucose—hyperglycemia, which eventually can lead to serious damage to the organs and tissues of the organism. Mitochondria are one of the main targets of diabetes at the intracellular level. This review is dedicated to the analysis of recent data regarding the role of mitochondrial dysfunction in the development of diabetes mellitus. Specific areas of focus include the involvement of mitochondrial calcium transport systems and a pathophysiological phenomenon called the permeability transition pore in the pathogenesis of diabetes mellitus. The important contribution of these systems and their potential relevance as therapeutic targets in the pathology are discussed.

Mitochondrial ion channels in pancreatic β-cells: Novel pharmacological targets for the treatment of Type 2 diabetes

Pancreatic beta‐cells are central regulators of glucose homeostasis. By tightly coupling nutrient sensing and granule exocytosis, beta‐cells adjust the secretion of insulin to the circulating blood glucose levels. Failure of beta‐cells to augment insulin secretion in insulin‐resistant individuals leads progressively to impaired glucose tolerance, Type 2 diabetes, and diabetes‐related diseases. Mitochondria play a crucial role in β‐cells during nutrient stimulation, linking the metabolism of glucose and other secretagogues to the generation of signals that promote insulin secretion. Mitochondria are double‐membrane organelles containing numerous channels allowing the transport of ions across both membranes. These channels regulate mitochondrial energy production, signalling, and cell death. The mitochondria of β‐cells express ion channels whose physio/pathological role is underappreciated. Here, we describe the mitochondrial ion channels identified in pancreatic β‐cells, we further discuss the possibility of targeting specific β‐cell mitochondrial channels for the treatment of Type 2 diabetes, and we finally highlight the evidence from clinical studies.

LINKED ARTICLES

This article is part of a themed issue on Cellular metabolism and diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.10/issuetoc

Duchenne muscular dystrophy is associated with the inhibition of calcium uniport in mitochondria and an increased sensitivity of the organelles to the calcium-induced permeability transition

Duchenne muscular dystrophy (DMD) is characterized by a pronounced and progressive degradation of the structure of skeletal muscles, which decreases their strength and lowers endurance of the organism. At muscular dystrophy, mitochondria are known to undergo significant functional changes, which is manifested in a decreased efficiency of oxidative phosphorylation and impaired energy metabolism of the cell. It is believed that the DMD-induced functional changes of mitochondria are mainly associated with the dysregulation of Ca²⁺ homeostasis. This work examines the kinetic parameters of Ca²⁺ transport and the opening of the Ca²⁺-dependent MPT pore in the skeletal-muscle mitochondria of the dystrophin-deficient C57BL/10ScSn-mdx mice. As compared to the organelles of wild-type animals, skeletal-muscle mitochondria of mdx mice have been found to be much less efficient in respect to Ca²⁺ uniport, with the kinetics of Na⁺-dependent Ca²⁺ efflux not changing. The data obtained indicate that the decreased rate of Ca²⁺ uniport in the mitochondria of mdx mice may be associated with the increased level of the dominant negative subunit of Ca²⁺ uniporter (MCUb). The experiments have also shown that in mdx mice, skeletal-muscle mitochondria have low resistance to the induction of MPT, which may be related to a significantly increased expression of adenylate translocator (ANT2), a possible structural element of the MPT pore. The paper discusses how changes in the expression of calcium uniporter and putative components of the MPT pore caused by the development of DMD can affect Ca²⁺ homeostasis of skeletal-muscle mitochondria.

Mitochondrial oxidative stress mediates high-phosphate-induced secretory defects and apoptosis in insulin-secreting cells

Inorganic phosphate (Pi) plays an important role in cell signaling and energy metabolism. In insulin-releasing cells, Pi transport into mitochondria is essential for the generation of ATP, a signaling factor in metabolism-secretion coupling. Elevated Pi concentrations, however, can have toxic effects in various cell types. The underlying molecular mechanisms are poorly understood. Here, we have investigated the effect of Pi on secretory function and apoptosis in INS-1E clonal β-cells and rat pancreatic islets. Elevated extracellular Pi (1~5 mM) increased the mitochondrial membrane potential (ΔΨm), superoxide generation, caspase activation, and cell death. Depolarization of the ΔΨm abolished Pi-induced superoxide generation. Butylmalonate, a nonselective blocker of mitochondrial phosphate transporters, prevented ΔΨm hyperpolarization, superoxide generation, and cytotoxicity caused by Pi. High Pi also promoted the opening of the mitochondrial permeability transition (PT) pore, leading to apoptosis, which was also prevented by butylmalonate. The mitochondrial antioxidants mitoTEMPO or MnTBAP prevented Pi-triggered PT pore opening and cytotoxicity. Elevated extracellular Pi diminished ATP synthesis, cytosolic Ca(2+) oscillations, and insulin content and secretion in INS-1E cells as well as in dispersed islet cells. These parameters were restored following preincubation with mitochondrial antioxidants. This treatment also prevented high-Pi-induced phosphorylation of ER stress proteins. We propose that elevated extracellular Pi causes mitochondrial oxidative stress linked to mitochondrial hyperpolarization. Such stress results in reduced insulin content and defective insulin secretion and cytotoxicity. Our data explain the decreased insulin content and secretion observed under hyperphosphatemic states.

Respective effects of oxygen and energy substrate deprivation on beta cell viability

Deficit in oxygen and energetic substrates delivery is a key factor in islet loss during islet transplantation. Permeability transition pore (PTP) is a mitochondrial channel involved in cell death. We have studied the respective effects of oxygen and energy substrate deprivation on beta cell viability as well as the involvement of oxidative stress and PTP opening. Energy substrate deprivation for 1h followed by incubation in normal conditions led to a cyclosporin A (CsA)-sensitive-PTP-opening in INS-1 cells and human islets. Such a procedure dramatically decreased INS-1 cells viability except when transient removal of energy substrates was performed in anoxia, in the presence of antioxidant N-acetylcysteine (NAC) or when CsA or metformin inhibited PTP opening. Superoxide production increased during removal of energy substrates and increased again when normal energy substrates were restored. NAC, anoxia or metformin prevented the two phases of oxidative stress while CsA prevented the second one only. Hypoxia or anoxia alone did not induce oxidative stress, PTP opening or cell death. In conclusion, energy substrate deprivation leads to an oxidative stress followed by PTP opening, triggering beta cell death. Pharmacological prevention of PTP opening during islet transplantation may be a suitable option to improve islet survival and graft success.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Uncoupling and reactive oxygen species (ROS)--a double-edged sword for β-cell function? "Moderation in all things"

The ability of the mitochondrion to (a) manage fuel import to oxidize for adenosine tri-phosphate (ATP) generation while (b) protecting itself and the cellular environment from electron leak, which can generate highly reactive oxygen species (ROS) is a delicate balancing act. ATP is the currency of the cell and as such serves a signaling function as a substrate partner to many kinases and ion channels. While various ROS species have been viewed as a dangerous and toxic group of molecules, it also has a role as a signal derived from mitochondria, as well as other enzymatic sources: a double-edged sword. Current efforts to understand the biochemical mechanisms affected by ROS as a signal--usually noted to be hydrogen peroxide (H(2)O(2))--are exciting, but this duality of ROS effects also pose challenges in managing its levels to protect cells. The mitochondrial uncoupling protein-2 (UCP2), UCP3, and the permeability transition pore have been integral to efforts to try to understand what role mitochondrial-derived ROS have in cells. In this piece we reflect on mitochondrial ROS and uncoupling proteins as signaling regulators. It seems that when it comes to ROS and uncoupling the proverb "Moderation in all things" is apt.

Regulation of pancreatic β-cell survival by nitric oxide: clinical relevance

The reduction of pancreatic β-cell mass is an important factor in the development of type 1 and type 2 diabetes. Understanding the mechanisms that regulate the maintenance of pancreatic β-cell mass as well as β-cell death is necessary for the establishment of therapeutic strategies. In this context, nitric oxide (NO) is a diatomic, gaseous, highly reactive molecule with biological activity that participates in the regulation of pancreatic β-cell mass. Two types of cellular responses can be distinguished depending on the level of NO production. First, pancreatic β-cells exposed to inflammatory cytokines, lipid stress or hyperglycaemia produce high concentrations of NO, mainly due to the activation of inducible NO synthase (iNOS), thus promoting cell death. Meanwhile, under homeostatic conditions, low concentrations of NO, constitutively produced by endothelial NO synthase (eNOS), promote cell survival. Here, we will discuss the current knowledge of the NO-dependent mechanisms activated during cellular responses, emphasizing those related to the regulation of cell survival.

Protection of pancreatic INS-1 β-cells from glucose- and fructose-induced cell death by inhibiting mitochondrial permeability transition with cyclosporin A or metformin

Hyperglycemia is detrimental to β-cell viability, playing a major role in the progression of β-cell loss in diabetes mellitus. The permeability transition pore (PTP) is a mitochondrial channel involved in cell death. Recent evidence suggests that PTP inhibitors prevent hyperglycemia-induced cell death in human endothelial cells. In this work, we have examined the involvement of PTP opening in INS-1 cell death induced by high levels of glucose or fructose. PTP regulation was studied by measuring the calcium retention capacity in permeabilized INS-1 cells and by confocal microscopy in intact INS-1 cells. Cell death was analyzed by flow cytometry. We first reported that metformin and cyclosporin A (CsA) prevented Ca²⁺-induced PTP opening in permeabilized and intact INS-1 cells. We then showed that incubation of INS-1 cells in the presence of 30 mM glucose or 2.5 mM fructose induced PTP opening and led to cell death. As both metformin and CsA prevented glucose- and fructose- induced PTP opening, and hampered glucose- and fructose- induced cell death, we conclude that PTP opening is involved in high glucose- and high fructose- induced INS-1 cell death. We therefore suggest that preventing PTP opening might be a new approach to preserve β-cell viability.

The dicarboxylate carrier plays a role in mitochondrial malate transport and in the regulation of glucose-stimulated insulin secretion from rat pancreatic beta cells

AIMS/HYPOTHESIS

We have previously described a strong correlation between pyruvate cycling and insulin secretion. We have also demonstrated a particularly important role for a pyruvate-isocitrate cycling pathway involving the mitochondrial citrate/isocitrate carrier (CIC) and cytosolic NADP-dependent isocitrate dehydrogenase. CIC requires cytosolic malate as a counter-substrate during citrate and isocitrate export. Thus, considering that the mitochondrial dicarboxylate carrier (DIC) provides an important source of cytosolic malate, we investigated the potential role of DIC in control of glucose-stimulated insulin secretion (GSIS).

METHODS

We used pharmacological and small interfering RNA (siRNA) tools to assess the role of DIC in insulin release in clonal insulin-secreting 832/13 cells and isolated rat islets.

RESULTS

Butylmalonate, an inhibitor of malate transport, reduced cytosolic malate and citrate levels, and inhibited GSIS in a dose-dependent manner in 832/13 cells. Suppression of DIC expression resulted in inhibition of GSIS by 5% to 69%, the extent of inhibition of insulin secretion being proportional to the level of Dic (also known as Slc25a10) gene knockdown. The most effective siRNA duplex against Dic did not affect glucose utilisation, glucose oxidation or ATP/ADP ratio, but did suppress glucose-induced increments of the NADPH/NADP(+) ratio. Confirmation of our results in primary cultures of isolated rat islets showed that butylmalonate and an adenovirus expressing an siRNA against Dic-inhibited GSIS.

CONCLUSIONS/INTERPRETATION

Malate transport by DIC may play an important role in GSIS, possibly by providing cytosolic malate as a counter-substrate for citrate and/or isocitrate export by CIC. These studies also suggest that malate transport by DIC is (1) a critical component of NADPH production mediated by pyruvate-cycling and (2) regulates GSIS.

Mitochondrial dysfunction induced by pancreatic and crotalic (Crotalus durissus terrificus) phospholipases A2 on rabbit proximal tubules suspensions

In the present study we show that phospholipases A2 isolated from porcine pancreas (PP-PLA2) and Crotalus durissus terrificus snake venom (SV-PLA2) induced dose-dependent increases of LDH release from rabbit proximal tubules in suspension. Both porcine and crotalic PLA(2)s induced increases in non-esterified fatty acid (NEFA) levels (microg of NEFA/mg of tubule protein). It was observed that the NEFA levels in the pellets were higher than in the supernatant for both PLA2, and were dose-dependent for the crotalic PLA2 group. Furthermore, snake venom PLA2 induced a decrease in mitochondrial membrane potential (DeltaPsi(m)) assessed by both JC-1 uptake and safranin O uptake. Porcine PLA2 produced no effects on JC-1 uptake with the highest concentrations and an unexpected increase in the group treated with the lowest concentration. In contrast, the safranin O method revealed decreases of energization with both phospholipases, so it had higher sensitivity to the presence of the increased NEFA levels. Addition of delipidated bovine serum albumin (dBSA) completely reversed the effects induced by phospholipases on DeltaPsi(m) measured with safranin O. Incubation with pancreatic and crotalic phospholipases A2 produced no changes on cell ATP levels. We conclude that the treatment of proximal tubule suspensions with porcine or crotalic phospholipases disturbed membrane integrity as well as mitochondrial function. Specific early NEFA-mediated mitochondrial effects of the phospholipases used in the present study are indicated by the benefit provided by dBSA.

Dominance of cytokine- over FasL-induced impairment of the mitochondrial transmembrane potential (Deltapsim) in the pancreatic beta-cell line NIT-1

Mitochondria of pancreatic beta-cells are potential targets of intrinsic and extrinsic apoptotic pathways in the autoimmune pathogenesis of type 1 diabetes. We aimed to investigate whether cytokine- and FasLigand (FasL)-induced apoptosis is associated with impaired mitochondrial transmembrane potential (Deltapsim) in the pancreatic beta-cell line NIT-1. NIT-1 cells were exposed to the interleukin-1beta/interferon-gamma (IL-1beta/IFN-gamma) cytokine combination to induce apoptosis in vitro. Low concentrations of cytokines resulted in Deltapsim impairment, and increasing concentrations had only a minor additional effect. Treatment with the inducible nitric oxide synthase (iNOS) inhibitor Nw-nitro-L-arginine methyl ester hydrochloride (L-NAME) prevented cytokine-mediated Deltapsim impairment, implying that cytokines affect Deltapsim via nitric oxide. The broad-spectrum caspase inhibitor Z-VAD(Ome)-FMK (ZVAD) revealed dichotomic actions. In the presence of ZVAD, cytokine-induced nitrite generation was increased but cell death and Deltapsim impairment were reduced. Deltapsim impairment was also reduced by inhibitors of caspases 1, 6 and 8. Induction of Fas by IL-1beta/IFN-gamma coupled with activation by Super-FasL augmented cytokine-induced cell death. We observed a clear dominance of cytokine- over FasL-induced effects on Deltapsim. Our findings show that IL-1beta/IFN-gamma cytokines have a strong effect to impair Deltaym and prime beta-cells for apoptosis via the intrinsic pathway mediated by iNOS and caspases. Furthermore, at least in NIT-1 cells, the extrinsic FasL/Fas pathway has only a minor additive effect on cytokine-induced Deltapsim impairment.

Free fatty acid-induced reduction in glucose-stimulated insulin secretion: evidence for a role of oxidative stress in vitro and in vivo

OBJECTIVE

An important mechanism in the pathogenesis of type 2 diabetes in obese individuals is elevation of plasma free fatty acids (FFAs), which induce insulin resistance and chronically decrease beta-cell function and mass. Our objective was to investigate the role of oxidative stress in FFA-induced decrease in beta-cell function.

RESEARCH DESIGN AND METHODS

We used an in vivo model of 48-h intravenous oleate infusion in Wistar rats followed by hyperglycemic clamps or islet secretion studies ex vivo and in vitro models of 48-h exposure to oleate in islets and MIN6 cells.

RESULTS

Forty-eight-hour infusion of oleate decreased the insulin and C-peptide responses to a hyperglycemic clamp (P < 0.01), an effect prevented by coinfusion of the antioxidants N-acetylcysteine (NAC) and taurine. Similar to the findings in vivo, 48-h infusion of oleate decreased glucose-stimulated insulin secretion ex vivo (P < 0.01) and induced oxidative stress (P < 0.001) in isolated islets, effects prevented by coinfusion of the antioxidants NAC, taurine, or tempol (4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl). Forty-eight-hour infusion of olive oil induced oxidative stress (P < 0.001) and decreased the insulin response of isolated islets similar to oleate (P < 0.01). Islets exposed to oleate or palmitate and MIN6 cells exposed to oleate showed a decreased insulin response to high glucose and increased levels of oxidative stress (both P < 0.001), effects prevented by taurine. Real-time RT-PCR showed increased mRNA levels of antioxidant genes in MIN6 cells after oleate exposure, an effect partially prevented by taurine.

CONCLUSIONS

Our data are the first demonstration that oxidative stress plays a role in the decrease in beta-cell secretory function induced by prolonged exposure to FFAs in vitro and in vivo.

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.

alpha-lipoic acid regulates AMP-activated protein kinase and inhibits insulin secretion from beta cells

AIMS/HYPOTHESIS

The antioxidant compound alpha-lipoic acid (alpha-LA) possesses antidiabetic and anti-obesity properties. In the hypothalamus, alpha-LA suppresses appetite and prevents obesity by inhibiting AMP-activated protein kinase (AMPK). Given the therapeutic potential of alpha-LA for the treatment of type 2 diabetes and obesity, and the importance of AMPK in beta cells, we examined the effect of alpha-LA on pancreatic beta cell function.

MATERIALS AND METHODS

Isolated rat islets and MIN6 beta cells were treated acutely (15-90 min) or chronically (18-24 h) with alpha-LA or the known AMPK-activating compounds 5'-amino-imidazole-4-carboxamide ribonucleoside (AICAR) and metformin. Insulin secretion, the AMPK-signalling pathway, mitochondrial function and cell growth were assessed.

RESULTS

Acute or chronic treatment of islets and MIN6 cells with alpha-LA led to dose-dependent rises in phosphorylation of the AMPK alpha-subunit and acetyl CoA carboxylase. Chronic exposure to alpha-LA, AICAR or metformin caused a reduction in insulin secretion. alpha-LA inhibited the p70 s6 kinase translational control pathway, and inhibited MIN6 growth in a manner similar to rapamycin. Unlike AICAR and metformin, alpha-LA also acutely inhibited insulin secretion. Examination of the effect of alpha-LA on mitochondrial function showed that acute treatment with this compound elevated reactive oxygen species (ROS) production and enhanced mitochondrial depolarisation induced by Ca(2+).

CONCLUSIONS/INTERPRETATION

This study is the first to demonstrate that alpha-LA directly affects beta cell function. The chronic effects of alpha-LA include AMPK activation and reductions in insulin secretion and content, and cell growth. Acutely, alpha-LA also inhibits insulin secretion, an effect probably involving the ROS-induced impairment of mitochondrial function.

Assessment of the integrity of poly(caprolactone)-b-poly(ethylene oxide) micelles under biological conditions: a fluorogenic-based approach

The integrity of block copolymer micelles is important for their effectiveness and successful delivery of the incorporated drugs. Here we evaluate the integrity of poly(caprolactone)-b-poly(ethylene oxide) micelles in media of varying chemical complexity and in cells by using fluorogenic micelles. Fluorogenic dye fluorescein-5-carbonyl azide diacetate was covalently attached to the micelle-core-forming part of the block copolymer, poly(caprolactone). The fluorescence was not detectable unless the poly(caprolactone)21-b-poly(ethylene oxide)45 micelles were destroyed and the fluorogenic dye was activated by deesterification. The fluorescence of the activated dye from destroyed micelles was easily detectable in various media and in cells. Micelles were stable in simple media such as phosphate-buffered saline but disassembled to varying extents with increasing chemical complexity of the media and addition of serum. The integrity of the internalized micelles within the cells showed a time-dependent decrease but remained largely preserved (80%) after 20 h of incubation with cells. A proof of principle was also demonstrated in vivo in mice. The fluorogenic approach to micelle integrity assessment presented herein should lend itself to other block copolymer micelles and assessments of their integrity in complex biological systems in vitro and in vivo.

Acute glutathione depletion restricts mitochondrial ATP export in cerebellar granule neurons

Decreases in GSH pools detected during ischemia sensitize neurons to excitotoxic damage. Thermodynamic analysis predicts that partial GSH depletion will cause an oxidative shift in the thiol redox potential. To investigate the acute bioenergetic consequences, neurons were exposed to monochlorobimane (mBCl), which depletes GSH by forming a fluorescent conjugate. Neurons transfected with redox-sensitive green fluorescent protein showed a positive shift in thiol redox potential synchronous with the formation of the conjugate. Mitochondria within neurons treated with mBCl for 1 h failed to hyperpolarize upon addition of oligomycin to inhibit their ATP synthesis. A decreased ATP turnover was confirmed by monitoring neuronal oxygen consumption in parallel with mitochondrial membrane potential (Deltapsi(m)) and GSH-mBCl formation. mBCl progressively decreased cell respiration, with no effect on mitochondrial proton leak or maximal respiratory capacity, suggesting adequate glycolysis and a functional electron transport chain. This approach to "state 4" could be mimicked by the adenine nucleotide translocator inhibitor bongkrekic acid, which did not further decrease respiration when administered after mBCl. The cellular ATP/ADP ratio was decreased by mBCl, and consistent with mitochondrial ATP export failure, respiration could not respond to an increased cytoplasmic ATP demand by plasma membrane Na(+) cycling; instead, mitochondria depolarized. More prolonged mBCl exposure induced mitochondrial failure, with Deltapsi(m) collapse followed by cytoplasmic Ca(2+) deregulation. The initial bioenergetic consequence of neuronal GSH depletion in this model is thus an inhibition of ATP export, which precedes other forms of mitochondrial dysfunction.

Glutathione Redox State Regulates Mitochondrial Reactive OxygenProduction

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) is poorly understood. Following one dose of TCDD (5 microg/kg body weight), mitochondrial succinate-dependent production of superoxide and H2O2 in mouse liver doubled at 7-28 days, then subsided by day 56; concomitantly, levels of GSH and GSSG increased in both cytosol and mitochondria. Cytosol displayed a typical oxidative stress response, consisting of diminished GSH relative to GSSG, decreased potential to reduce protein-SSG mixed disulfide bonds (type 1 thiol redox switch) or protein-SS-protein disulfide bonds (type 2 thiol redox switch), and a +10 mV change in GSSG/2GSH reduction potential. In contrast, mitochondria showed a rise in reduction state, consisting of increased GSH relative to GSSG, increases in type 1 and type 2 thiol redox switches, and a -25 mV change in GSSG/2GSH reduction potential. Comparing Ahr(-/-) knock-out and wild-type mice, we found that TCDD-induced thiol changes in both cytosol and mitochondria were dependent on the aromatic hydrocarbon receptor (AHR). GSH was rapidly taken up by mitochondria and stimulated succinate-dependent H2O2 production. A linear dependence of H2O2 production on the reduction potential for GSSG/2GSH exists between -150 and -300 mV. The TCDD-stimulated increase in succinate-dependent and thiol-stimulated production of reactive oxygen paralleled a four-fold increase in formamidopyrimidine DNA N-glycosylase (FPG)-sensitive cleavage sites in mitochondrial DNA, compared with a two-fold increase in nuclear DNA. These results suggest that TCDD produces an AHR-dependent oxidative stress in mitochondria, with concomitant mitochondrial DNA damage mediated, at least in part, by an increase in the mitochondrial thiol reduction state.