Control of oxygen free radical formation from mitochondrial complex I: roles for protein kinase A and pyruvate dehydrogenase kinase

Mitochondrial c-Jun NH2-terminal kinase prevents the accumulation of reactive oxygen species and reduces necrotic damage in neural tumor cells that lack trophic support

In response to different stress signals, the c-Jun NH(2)-terminal kinase (JNK) can trigger cell death. However, JNK also facilitates the survival and cell cycle progression of tumor cells by mechanisms that are poorly defined. Here, we show that schwannoma RN22 cells can survive and proliferate under serum-free conditions although serum withdrawal rapidly induces mitochondrial fission and swelling. Although the morphologic changes observed in the mitochondria did not trigger cytochrome c release, they were accompanied by an increase in the mitochondrial membrane potential (DeltaPsi(M)) and of immunoreactivity for active JNK in these organelles. Pharmacologic inhibition of JNK provoked a further increase of the DeltaPsi(M), an increase in reactive oxygen species (ROS) production, and a sustained decrease in cell viability due to necrosis. This increase in necrosis was prevented by the presence of ROS scavengers. Immunoreactivity for active JNK was also observed in the mitochondria of neuroblastoma 1E-115 and neuroblastoma 2a neuroblastoma cell lines on serum withdrawal, whereas active JNK was barely detected in serum-deprived fibroblasts. Accordingly, the reduction in neural tumor cell viability induced by JNK inhibition was largely attenuated in serum-deprived fibroblasts. These data indicate that local activation of JNK in the mitochondria can protect against necrotic cell death associated with ROS production, facilitating the growth of neural tumor cells subjected to serum deprivation.

Synergistic effects of caloric restriction with maintained protein intake on skeletal muscle performance in 21-month-old rats: a mitochondria-mediated pathway

Caloric restriction (CR) delays the onset of age-related mitochondrial abnormalities but does not prevent the decline in ATP production needed to sustain muscle protein fractional synthesis rate (FSR) and contractile activity. We hypothesized that improving mitochondrial activity and FSR using a CR diet with maintained protein intakes could enhance myofibrillar protein FSR and consequently improve muscle strength in aging rats. Wistar rats (21 months old) were fed either an ad libitum (AL), 40% protein-energy restricted (PER) or 40% AL-isonitrogenous energy restricted (ER) diet for 5 months. ATP production, electron transport chain activity, reactive oxygen species (ROS) generation, protein carbonyl content and FSR were determined in both tibialis anterior (TA) and soleus muscle mitochondria. Myosin and actin FSR and grip force were also investigated. The ER diet led to improved mitochondrial activity and ATP production in the TA and soleus muscles in comparison with PER. Furthermore, mitochondrial FSR in the TA was enhanced under the ER diet but diminished under the PER. Mitochondrial protein carbonyl content was decreased by both the ER and PER diets. The ER diet was able to improve myosin and actin FSR and grip force. Therefore, the synergistic effects of CR with maintained protein intake may help to limit the progression of sarcopenia by optimizing the turnover rates and functions of major proteins in skeletal muscle.

Phosphorylation of B14.5a subunit from bovine heart complex I identified by titanium dioxide selective enrichment and shotgun proteomics

Shotgun proteomics was used to study the steady phosphorylation state of NADH:ubiquinone oxidoreductase (complex I) subunits from bovine heart mitochondria. A total tryptic digestion of enzymatically active complex I was performed, and the resulting peptide mixture was subjected to phosphopeptide enrichment by the use of titanium dioxide (TiO2). The phosphopeptide-enriched fraction was separated and analyzed with nanoscale reverse-phase HPLC-ESI-MS/MS in single information-dependent acquisition. Hence two phosphorylated complex I subunits were detected: 42 kDa and B14.5a. Phosphorylation of 42-kDa subunit at Ser-59 has already been determined with fluorescent phosphoprotein-specific gel staining and mass spectrometry (Schilling, B., Aggeler, R., Schulenberg, B., Murray, J., Row, R. H., Capaldi, R. A., and Gibson, B. W. (2005) Mass spectrometric identification of novel phosphorylation site in subunit NDUFA10 of bovine mitochondrial complex I. FEBS Lett. 579, 2485-2490). In our work, this finding was confirmed using a non-gel-based approach. In addition, we report novel phosphorylation on B14.5a nuclear encoded subunit. We demonstrated evidence of the phosphorylation site at Ser-95 residue by collision-induced dissociation experiments on three different molecular ions of two tryptic phosphopeptides of B14.5a.

Systemic imbalance of essential metals and cardiac gene expression in rats following acute pulmonary zinc exposure

It was recently demonstrated that particulate matter (PM) containing water-soluble zinc produces cardiac injury following pulmonary exposure. To investigate whether pulmonary zinc exposure produces systemic metal imbalance and direct cardiac effects, male Wistar Kyoto (WKY) rats (12-14 wk age) were intratracheally (IT) instilled with saline or 2 micromol/kg zinc sulfate. Temporal analysis was performed for systemic levels of essential metals (zinc, copper, and selenium), and induction of zinc transporter-2 (ZT-2) and metallothionein-1 (MT-1) mRNA in the lung, heart, and liver. Additionally, cardiac gene expression profile was evaluated using Affymetrix GeneChips (rat 230A) arrays to identify zinc-specific effects. Pulmonary zinc instillation produced an increase in plasma zinc to approximately 20% at 1 and 4 h postexposure with concomitant decline in the lung levels. At 24 and 48 h postexposure, zinc levels rose significantly (approximately 35%) in the liver. At these time points, plasma and liver levels of copper and selenium also increased significantly, suggesting systemic disturbance in essential metals. Zinc exposure was associated with marked induction of MT-1 and ZT-2 mRNA in lung, heart, and liver, suggesting systemic metal sequestration response. Given the functional role of zinc in hundreds of proteins, the gene expression profiles demonstrated changes that are expected based on its physiological role. Zinc exposure produced an increase in expression of kinases and inhibition of expression of phosphatases; up- or downregulation of genes involved in mitochondrial function; changes in calcium regulatory proteins suggestive of elevated intracellular free calcium and increases in sulfotransferases; upregulation of potassium channel genes; and changes in free radical-sensitive proteins. Some of these expression changes are reflective of a direct effect of zinc on myocardium following pulmonary exposure, which may result in impaired mitochondrial respiration, stimulated cell signaling, altered Ca2+ homeostasis, and increased transcription of sulfotransferases. Cardiotoxicity may be an outcome of acute zinc toxicosis and occupational exposures to metal fumes containing soluble zinc. Imbalance of systemic metal homeostasis as a result of pulmonary zinc exposure may underlie the cause of extrapulmonary effects.

Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance

Brief, intense exercise training may induce metabolic and performance adaptations comparable to traditional endurance training. However, no study has directly compared these diverse training strategies in a standardized manner. We therefore examined changes in exercise capacity and molecular and cellular adaptations in skeletal muscle after low volume sprint-interval training (SIT) and high volume endurance training (ET). Sixteen active men (21 +/- 1 years, ) were assigned to a SIT or ET group (n = 8 each) and performed six training sessions over 14 days. Each session consisted of either four to six repeats of 30 s 'all out' cycling at approximately 250% with 4 min recovery (SIT) or 90-120 min continuous cycling at approximately 65% (ET). Training time commitment over 2 weeks was approximately 2.5 h for SIT and approximately 10.5 h for ET, and total training volume was approximately 90% lower for SIT versus ET ( approximately 630 versus approximately 6500 kJ). Training decreased the time required to complete 50 and 750 kJ cycling time trials, with no difference between groups (main effects, P Collapse

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MESH Headings

• Adaptation, Physiological
• Adult
• Electron Transport Complex IV/metabolism
• Energy Metabolism
• Exercise/physiology
• Exercise Tolerance
• Glycogen/metabolism
• Humans
• Male
• Muscle, Skeletal/metabolism
• Muscle, Skeletal/physiology
• Physical Education and Training/methods
• Physical Endurance/physiology
• Time Factors

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Affiliation(s)

Martin J Gibala Department of Kinesiology IWC AB122, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4K1, Canada.
Jonathan P Little
Martin van Essen
Geoffrey P Wilkin
Kirsten A Burgomaster
Adeel Safdar
Sandeep Raha
Mark A Tarnopolsky

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Poly(ADP-ribose) polymerase-1 hyperactivation and impairment of mitochondrial respiratory chain complex I function in reperfused mouse hearts

Poly(ADP-ribose) polymerase-1 (PARP-1), the most abundant member of the PARP family, is a nuclear enzyme that catalyzes ADP-ribose transfer from NAD+ to specific acceptor proteins in response to DNA damage. Excessive PARP-1 activation is an important cause of infarction and contractile dysfunction in heart tissue during interruptions of blood flow. The mechanisms by which PARP-1 inhibition and disruption dramatically improve metabolic recovery and reduce oxidative stress during cardiac reperfusion have not been fully explored. We developed a mouse heart experimental protocol to test the hypothesis that mitochondrial respiratory complex I is a downstream mediator of beneficial effects of PARP-1 inhibition or disruption. Pharmacological inhibition of PARP-1 activity produced no deterioration of hemodynamic function in C57BL/6 mouse hearts. Hearts from PARP-1 knockout mice also exhibited normal baseline contractility. Prolonged ischemia-reperfusion produced a selective defect in complex I function distal to the NADH dehydrogenase component. PARP-1 inhibition and PARP-1 gene disruption conferred equivalent protection against mitochondrial complex I injury and were strongly associated with improvement in myocardial energetics, contractility, and tissue viability. Interestingly, ischemic preconditioning abolished cardioprotection stimulated by PARP-1 gene disruption. Treatment with the antioxidant N-(2-mercaptopropionyl)-glycine or xanthine oxidase inhibitor allopurinol restored the function of preconditioned PARP-1 knockout hearts. This investigation establishes a strong association between PARP-1 hyperactivity and mitochondrial complex I dysfunction in cardiac myocytes. Our findings advance understanding of metabolic regulation in myocardium and identify potential therapeutic targets for prevention and treatment of ischemic heart disease.

Energy converting NADH:quinone oxidoreductase (complex I)

NADH:quinone oxidoreductase (complex I) pumps protons across the inner membrane of mitochondria or the plasma membrane of many bacteria. Human complex I is involved in numerous pathological conditions and degenerative processes. With 14 central and up to 32 accessory subunits, complex I is among the largest membrane-bound protein assemblies. The peripheral arm of the L-shaped molecule contains flavine mononucleotide and eight or nine iron-sulfur clusters as redox prosthetic groups. Seven of the iron-sulfur clusters form a linear electron transfer chain between flavine and quinone. In most organisms, the seven most hydrophobic subunits forming the core of the membrane arm are encoded by the mitochondrial genome. Most central subunits have evolved from subunits of different hydrogenases and bacterial Na+/H+ antiporters. This evolutionary origin is reflected in three functional modules of complex I. The coupling mechanism of complex I most likely involves semiquinone intermediates that drive proton pumping through redox-linked conformational changes.

Mitochondrial Complex I: structural and functional aspects

This review examines two aspects of the structure and function of mitochondrial Complex I (NADH Coenzyme Q oxidoreductase) that have become matter of recent debate. The supramolecular organization of Complex I and its structural relation with the remainder of the respiratory chain are uncertain. Although the random diffusion model [C.R. Hackenbrock, B. Chazotte, S.S. Gupte, The random collision model and a critical assessment of diffusion and collision in mitochondrial electron transport, J. Bioenerg. Biomembranes 18 (1986) 331-368] has been widely accepted, recent evidence suggests the presence of supramolecular aggregates. In particular, evidence for a Complex I-Complex III supercomplex stems from both structural and kinetic studies. Electron transfer in the supercomplex may occur by electron channelling through bound Coenzyme Q in equilibrium with the pool in the membrane lipids. The amount and nature of the lipids modify the aggregation state and there is evidence that lipid peroxidation induces supercomplex disaggregation. Another important aspect in Complex I is its capacity to reduce oxygen with formation of superoxide anion. The site of escape of the single electron is debated and either FMN, iron-sulphur clusters, and ubisemiquinone have been suggested. The finding in our laboratory that two classes of hydrophobic inhibitors have opposite effects on superoxide production favours an iron-sulphur cluster (presumably N2) is the direct oxygen reductant. The implications in human pathology of better knowledge on these aspects of Complex I structure and function are briefly discussed.

Regulation by the cAMP cascade of oxygen free radical balance in mammalian cells

A study is presented of the effect of the cAMP cascade on oxygen metabolism in mammalian cell cultures. Serum-starvation of the cell cultures resulted in depression of the forward NADH-ubiquinone oxidoreductase activity of complex I, decreased content of glutathione, and enhancement of the cellular level of H2O2. Depressed transcription of cytosolic Cu/Zn-SOD 1, mitochondrial glutathione peroxidase and catalase was also observed. Activation of the cAMP cascade reversed the depression of the activity of complex I and the accumulation of H2O2. The effect of cAMP involved the cAMP-dependent protein kinase.

Is the mitochondrial free radical theory of aging intact?

The present state of the mitochondrial free radical theory of aging is reviewed. Available studies do not support the hypothesis that antioxidants control the rate of aging because: (a) they correlate inversely with maximum longevity in vertebrates, and (b) increasing their concentration by different methods does not increase maximum lifespan. On the other hand, comparative studies consistently show that long-lived mammals and birds have low rates of mitochondrial reactive oxygen species (ROS) production and low levels of oxidative damage in their mitochondrial DNA. Furthermore, caloric restriction, which extends longevity, also decreases mitochondrial ROS production at complex I and lowers mtDNA oxidative damage. Recent data show that these changes can also be obtained with protein restriction without strong caloric restriction. Another trait of long-lived mammals and birds is the possession of low degrees of unsaturation in their cellular membranes. This is mainly due to minimizing the presence of highly unsaturated fatty acids such as 22:6n-3 and emphasizing the presence of less unsaturated fatty acids such as 18:2n-6 in long-lived animals, without changing the total amount of polyunsaturated fatty acids. This leads to lower levels of lipid peroxidation and lipoxidation-derived protein modification in long-lived species. Taken together, available information is consistent with the predictions of the mitochondrial free radical theory of aging, although definitive proof and many mechanistic details are still lacking.

TFAM-dependent and independent dynamics of mtDNA levels in C2C12 myoblasts caused by redox stress

TFAM is an essential protein factor for the initiation of transcription of the mtDNA. It also functions as a packaging factor, which stabilizes the mtDNA pool. To investigate the regulatory role of TFAM for regeneration and proliferation of the mtDNA pool, we exposed the muscle cell line C2C12 to a severe redox stress (H2O2) or to a moderate redox stress (GSH depletion), determined the dynamics of the mtDNA levels and correlated this with the TFAM protein levels. H2O2 caused a concentration-dependent loss of mtDNA molecules. The mtDNA levels recovered slowly within 3 days after H2O2 stress. The TFAM protein was less degraded than the mtDNA indicating an accumulation of TFAM protein per mtDNA after H2O2 stress. Overexpression of TFAM did not protect against the mtDNA loss after H2O2 stress but shortened the recovery time. GSH depletion led to a proliferation of the mtDNA pool. Although the mtDNA levels increased the TFAM protein levels were unaffected by the GSH depletion. We conclude that the accumulation of the TFAM protein after H2O2 stress contributes to the regeneration of the mtDNA pool but that other mechanisms, independent from the TFAM protein amount have to be postulated to explain the proliferation of the mtDNA pool after GSH depletion.

Proteomics of bovine mitochondrial RNA-binding proteins: HES1/KNP-I is a new mitochondrial resident protein

Proteomic analysis of bovine mitochondrial proteins with affinity to polyAdenylate or polyUridylate was performed in an effort to identify novel RNA-binding mitochondrial proteins. We have used 2D gel electrophoresis and MALDI-QqTOF mass spectrometry to identify a total of 64 proteins, of which 51 have defined mitochondrial function including 6 known RNA-binding proteins. HES1/KNP-I from the polyA-binding fraction of mitochondrial Triton extract showed exclusive mitochondrial localization when expressed in GFP-tagged form. The HES1/KNP-I gene is on human chromosome 21q22.3 and may be involved in several disorders mapped to that region. Thus, HES1/KNP-I is a proven mitochondrial resident protein with apparent tight membrane association and tentative RNA-binding properties.

Differences in the neurotoxicity profile induced by ATP and ATPgammaS in cultured cerebellar granule neurons

Extracellular ATP and P2 receptors may play a crucial role in the neurodegeneration of the CNS. Here, we investigated in neuronal cerebellar granule cultures the biological effect of the quite stable P2 receptor agonist ATPgammaS and compare it to the cytotoxic action of ATP. Time-course experiments showed that 500 microM ATPgammaS causes 50-100% cell death in 15-24 h. As proved by pharmacological means, ATPgammaS toxicity apparently involves neither indirect activation of NMDA receptors, nor ectonucleotidase activities, nor nucleoside transport and intracellular purine metabolism. Moreover, ATPgammaS induces detrimental effects without modifying the expression of several P2X and P2Y receptor proteins. Cell death instead occurs after extracellular release of the cytosolic enzyme lactic dehydrogenase and inhibition of the overall activity of the intracellular dehydrogenases. Moreover, ATPgammaS causes transient outflow of cytochrome c from mitochondria (maximal 2.5-fold stimulation in 4 h), it raises the intracellular reactive oxygen species (about four-fold in 1 h) and cAMP levels (about 40% in 15 min-4 h). Among several P2 receptor antagonists, only pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium promotes 80-100% neuroprotection.

Downregulation of NDUFA1 and other oxidative phosphorylation-related genes is a consistent feature of basal cell carcinoma

Basal cell carcinoma (BCC) is the most common cutaneous malignancy that, like other tumours, possesses a heterogeneous genetic composition. In order to select genes with consistent changes in expression among these tumours, we analysed BCC microarray expression data by using a novel approach, termed correlative analysis of microarrays (CAM). CAM is a nested, non-parametric method designed to qualitatively select candidates based on their individual, similar effects upon an array-wide closeness measure. We applied the CAM method to expression data generated by two-channel cDNA microarray experiments, where 21 BCC and patient-matched normal skin specimens were examined. Fifteen candidate genes were selected, with six overexpressed and nine underexpressed in BCC vs. normal skin. Five of the nine consistently downregulated genes in the tumour samples are involved in mitochondrial function and the oxidative phosphorylation (OXPHOS) pathway. One of these genes was the 7.5-kDa subunit, NADH dehydrogenase (ubiquinone) alpha subcomplex-1 (NDUFA1), an accessory component of OXPHOS complex-I that is essential for respiratory activity. These findings support the hypothesis that irregularities in mitochondrial function are involved in neoplasia. Suppression of NDUFA1 expression could represent a key pathogenic mechanism in the development of BCC.

Mitochondrial Dok-4 Recruits Src Kinase and Regulates NF-κB Activation in Endothelial Cells

The downstream of kinase (Dok) family of adapter proteins consists of at least five members structurally characterized by an NH2-terminal tandem of conserved pleckstrin homology and phosphotyrosine binding domains linked to a unique COOH-terminal region. To determine the role of the novel adapter protein Dok-4 in endothelial cells, we first investigated the cell localization of Dok-4. Most surprisingly, immunofluorescence microscopy, cell fractionation studies, and studies with enhanced green fluorescent protein chimeras showed that wild type Dok-4 (Dok-4-WT) specifically localized in mitochondria. An NH2-terminal deletion mutant of Dok-4 (Dok-4-(deltaN11-29)), which lacks the mitochondrial targeting sequence, could not accumulate in mitochondria. Co-immunoprecipitation revealed an interaction of c-Src with Dok-4-WT in endothelial cells. Most interestingly, overexpression of Dok-4-WT, but not Dok-4-(deltaN1-99), increased mitochondrial c-Src expression, whereas knock-down of endogenous Dok-4 with a small interfering RNA vector greatly inhibited mitochondrial localization of c-Src, suggesting a unique function for Dok-4 as an anchoring protein for c-Src in mitochondria. Dok-4-WT significantly decreased 39-kDa subunit complex I expression. PP2, a specific Src kinase inhibitor, prevented the Dok-4-mediated complex I decrease, suggesting the involvement of Src kinase in regulation of complex I expression. Dok-4-WT enhanced tumor necrosis factor-alpha (TNF-alpha)-mediated reactive oxygen species (ROS) production, supporting the functional relevance of a Dok-4-Src-complex I/ROS signaling pathway in mitochondria. Finally, Dok-4 enhanced TNF-alpha-mediated NF-kappaB activation, whereas this was inhibited by transfection with Dok-4 small interfering RNA. In addition, Dok-4-induced NF-kappaB activation was also inhibited by transfection of a dominant negative form of c-Src. These data suggest a role for mitochondrial Dok-4 as an anchoring molecule for the tyrosine kinase c-Src, and in turn as a regulator of TNF-alpha-mediated ROS production and NF-kappaB activation.

Mass spectrometric identification of a novel phosphorylation site in subunit NDUFA10 of bovine mitochondrial complex I

Mitochondrial Complex I (NADH:ubiquinone oxidoreductase) consists of at least 46 subunits. Phosphorylation of the 42-kDa subunit NDUFA10 was recently reported using a novel phosphoprotein stain [Schulenberg et al. (2003) Analysis of steady-state protein phosphorylation in mitochondria using a novel fluorescent phosphosensor dye. J. Biol. Chem. 278, 27251]. Two smaller Complex I phosphoproteins, ESSS and MWFE, and their sites of modification, have since been determined [Chen et al. (2004) The phosphorylation of subunits of complex I from bovine heart mitochondria. J. Biol. Chem. 279, 26036]. Here we identify the site of phosphorylation in NDUFA10 from bovine heart mitochondria by tandem mass spectrometry. A single phosphopeptide spanning residues 47-60 was identified and confirmed by synthesis to be (47)LITVDGNICSGKpSK(60), establishing serine-59 as the site of phosphorylation.

Inducing coproporphyria in rat hepatocyte cultures using cyclic AMP and cyclic AMP-releasing agents

Cyclic AMP (c-AMP), added on its own to rat hepatocyte cultures, caused a marked accumulation of coproporphyrin III. The results obtained by comparing the effect of c-AMP to that of exogenous 5-aminolevulinate (ALA), and from adding c-AMP and ALA together, indicated that the coproporphyrinogen III metabolism was blocked, even though no inhibition of the relevant enzyme, coproporphyrinogen oxidase, could be demonstrated. Preferential accumulation of coproporphyrin could also be produced in cultures of rat hepatocytes by agents that raise the cellular levels of cyclic AMP, such as glucagon. The effect of supplementing the culture medium with triiodothyronine (T3) on the response of rat hepatocytes to c-AMP was also investigated. T3, which is known to stimulate mitochondrial respiration, uncoupling O2 consumption from ATP synthesis, produced a c-AMP-like effect when given on its own and potentiated the effect of c-AMP, with an apparent increase in the severity of the metabolic block. It is suggested that an oxidative mechanism may be activated in c-AMP and T3-induced coproporphyria, preferentially involving the mitochondrial compartment, leading to oxidation of porphyrinogen intermediates of haem biosynthesis, especially coproporphyrinogen. Coproporphyin, the fully oxidized aromatic derivative produced, cannot be metabolized and will therefore accumulate.

The human mitochondrial proteome: oxidative stress, protein modifications and oxidative phosphorylation

Mitochondria are one of the most complex of subcellular organelles and play key roles in many cellular functions including energy production, fatty acid metabolism, pyrimidine biosynthesis, calcium homeostasis, and cell signaling. In recent years, we and other groups have attempted to identify the complete set of proteins that are localized to human mitochondria as a way to better understand its cellular functions and how it communicates with other cell compartment in complex signaling pathways such as oxidative stress and apoptosis. Indeed, there is an increasing interest in understanding the molecular details of oxidative stress and the mitochondrial role in this process, as well as assessing how mitochondrial proteins become damaged or posttranslationally modified as a consequence of a major change in a cell's redox status. In this review, we report on the current status of the human mitochondrial proteome with an emphasis towards understanding how mitochondrial proteins, especially the proteins that make up the respiratory chain or oxidative phosphorylation (OXPHOS) enzymes, are modified in various models of age-related diseases such as cancer and Parkinson's disease (PD).

Superoxide production by NADH:ubiquinone oxidoreductase (complex I) depends on the pH gradient across the mitochondrial inner membrane

The relationship between protonmotive force and superoxide production by mitochondria is poorly understood. To address this issue, the rate of superoxide production from complex I of rat skeletal muscle mitochondria incubated under a variety of conditions was assessed. By far, the largest rate of superoxide production was from mitochondria respiring on succinate; this rate was almost abolished by rotenone or piericidin, indicating that superoxide production from complex I is large under conditions of reverse electron transport. The high rate of superoxide production by complex I could also be abolished by uncoupler, confirming that superoxide production is sensitive to protonmotive force. It was inhibited by nigericin, suggesting that it is more dependent on the pH gradient across the mitochondrial inner membrane than on the membrane potential. These effects were examined in detail, leading to the conclusions that the effect of protonmotive force was mostly direct, and not indirect through changes in the redox state of the ubiquinone pool, and that the production of superoxide by complex I during reverse electron transport was at least 3-fold more sensitive to the pH gradient than to the membrane potential.

Molecular genetics of complex I-deficient Chinese hamster cell lines

The work from our laboratory on complex I-deficient Chinese hamster cell mutants is reviewed. Several complementation groups with a complete defect have been identified. Three of these are due to X-linked mutations, and the mutated genes for two have been identified. We describe null mutants in the genes for the subunits MWFE (gene: NDUFA1) and ESSS. They represent small integral membrane proteins localized in the Ialpha (Igamma) and Ibeta subcomplexes, respectively [J. Hirst, J. Carroll, I.M. Fearnley, R.J. Shannon, J.E. Walker. The nuclear encoded subunits of complex I from bovine heart mitochondria. Biochim. Biophys. Acta 1604 (7-10-2003) 135-150.]. Both are absolutely essential for assembly and activity of complex I. Epitope-tagged versions of these proteins can be expressed from a poly-cistronic vector to complement the mutants, or to be co-expressed with the endogenous proteins in other hamster cell lines (mutant or wild type), or human cells. Structure-function analyses can be performed with proteins altered by site-directed mutagenesis. A cell line has been constructed in which the MWFE subunit is conditionally expressed, opening a window on the kinetics of assembly of complex I. Its targeting, import into mitochondria, and orientation in the inner membrane have also been investigated. The two proteins have recently been shown to be the targets for a cAMP-dependent kinase [R. Chen, I.M. Fearnley, S.Y. Peak_Chew, J.E. Walker. The phosphorylation of subunits of complex I from bovine heart mitochondria. J. Biol. Chem. xx (2004) xx-xx.]. The epitope-tagged proteins can be cross-linked with other complex I subunits.

Regulation of NADH/CoQ oxidoreductase: do phosphorylation events affect activity?

We had previously suggested that phosphorylation of proteins by mitochondrial kinases regulate the activity of NADH/CoQ oxidoreductase. Initial data showed that pyruvate dehydrogenase kinase (PDK) and cAMP-dependent protein kinase A (PKA) phosphorylate mitochondrial membrane proteins. Upon phosphorylation with crude PDK, mitochondria appeared to be deficient in NADH/cytochrome c reductase activity associated with increased superoxide production. Conversely, phosphorylation by PKA resulted in increased NADH/cytochrome c reductase activity and decreased superoxide formation. Current data confirms PKA involvement in regulating Complex I activity through phosphorylation of an 18 kDa subunit. Beef heart NADH/ cytochrome c reductase activity increases to 150% of control upon incubation with PKA and ATP-gamma-S. We have cloned the four human isoforms of PDK and purified beef heart Complex I. Incubation of mitochondria with PDK isoforms and ATP did not alter Complex I activity or superoxide production. Radiolabeling of mitochondria and purified Complex I with PDK failed to reveal phosphorylated proteins.

In search of the function of the peripheral-type benzodiazepine receptor

The peripheral-type benzodiazepine receptor (PBR) is a mitochondrial protein, involved in the regulation of cholesterol transport from the outer to the inner mitochondrial membrane, the rate-determining step in steroid hormone biosynthesis. Molecular modeling of PBR suggested that it might function as a channel for cholesterol. Indeed, cholesterol uptake and transport by bacteria cells was induced upon PBR expression. Amino acid deletion, site-directed mutagenesis, and structural studies identified a cholesterol recognition/interaction amino acid consensus sequence in the cytoplasmic carboxy-terminus of the receptor. In vitro reconstitution experiments demonstrated that the 18 kDa PBR protein binds with high affinity both drug ligands and cholesterol. In situ and in vitro studies indicated that in steroidogenic cells the StAR-induced cholesterol import into mitochondria was mediated by the outer mitochondrial membrane PBR. In search of the tissue specificity of PBR expression it was shown that the high levels of PBR expression in steroidogenic cells are due, at least in part, to the expression of Sp1/Sp3 transcription factors. Moreover, PBR's function in cholesterol transport was found to be conserved across kingdoms because a PBR-homologous Arabidopsis sequence when expressed in bacteria protoplasts caused a ligand-induced uptake of cholesterol suggesting that the Arabidopsis PBR homologue is involved in steroid import in plant mitochondria. In conclusion, these studies suggest that PBR's ability to bind and transport cholesterol is a well-conserved function of this ubiquitous protein. Expression of specific transcription factors results in the overexpression of PBR and increased cholesterol transport into mitochondria associated with a specialized function (steroidogenesis). In other tissues, PBR expression might be part of the mitochondrial membrane biogenesis process involved in increased cell proliferation (cancer, gliosis) and tissue repair (nerve damage and ischemia-reperfusion injury).

Calcium, ATP, and ROS: a mitochondrial love-hate triangle

The mitochondrion is at the core of cellular energy metabolism, being the site of most ATP generation. Calcium is a key regulator of mitochondrial function and acts at several levels within the organelle to stimulate ATP synthesis. However, the dysregulation of mitochondrial Ca2+homeostasis is now recognized to play a key role in several pathologies. For example, mitochondrial matrix Ca2+overload can lead to enhanced generation of reactive oxygen species, triggering of the permeability transition pore, and cytochrome c release, leading to apoptosis. Despite progress regarding the independent roles of both Ca2+and mitochondrial dysfunction in disease, the molecular mechanisms by which Ca2+can elicit mitochondrial dysfunction remain elusive. This review highlights the delicate balance between the positive and negative effects of Ca2+and the signaling events that perturb this balance. Overall, a “two-hit” hypothesis is developed, in which Ca2+plus another pathological stimulus can bring about mitochondrial dysfunction.

Chronic treatment of mitochondrial disease patients with dichloroacetate

Clinical features are reported for 37 patients with various mitochondrial disorders, treated with sodium dichloroacetate (DCA) for 3 weeks to 7 years (mean 3.25 years) at 11-50 mg/kg/day (34.6+/-13.1) in an open-label format. DCA pharmacokinetics showed half-times approximately 86 min for the first intravenous dose of 50 mg/kg, 3.2 h for a subsequent intravenous dose 4-6 h later, and 11 h after continued oral dosing of 12.5-25 mg/kg twice daily. Basal blood and CSF lactate (mean values at entry 29.6 and 46.8 mg/dL, respectively) decreased at 3 months (to 18.1 and 34.2, respectively) and 12 months (to 17.7 and 33.1, respectively). There was some attenuation of the blood lactate response to oral fructose but not glucose, although the baseline lactate was lower with DCA. A standardized neurologic inventory showed stabilization or improvement over one year. The subjective impression of overall disease course was worsening in 21.6%, improvement in 48.6%, and no discernable effect in 29.7%. Among 8 patients who had 17 stroke-like events in 0.25-5 years prior to study entry, there were a total of 2 events over 3-6 years of treatment. In two cases institution of DCA resulted in dramatic relief of severe headaches which had been refractory to narcotics. Given variability of symptoms and limited understanding of natural history of mitochondrial disease, it is difficult to determine the efficacy of DCA in this open-label study, but there did appear to be some cases in which there were at least temporary benefits.

NADH oxidase activity of rat cardiac sarcoplasmic reticulum regulates calcium-induced calcium release

NADH and Ca2+ have important regulatory functions in cardiomyocytes related to excitation-contraction coupling and ATP production. To elucidate elements of these functions, we examined the effect of NADH on sarcoplasmic reticulum (SR) Ca2+ release and the mechanisms of this regulation. Physiological concentrations of cytosolic NADH inhibited ryanodine receptor type 2 (RyR2)-mediated Ca2+-induced Ca2+ release (CICR) from SR membranes (IC50=120 micromol/L) and significantly lowered single channel open probability. In permeabilized single ventricular cardiomyocytes, NADH significantly inhibited the amplitude and frequency of spontaneous Ca2+ release. Blockers of electron transport prevented the inhibitory effect of NADH on CICR in isolated membranes and permeabilized cells, as well as on the activity of RyR2 channels reconstituted in lipid bilayer. An endogenous NADH oxidase activity from rat heart copurified with SR enriched with RyR2. A significant contribution by mitochondria was excluded as NADH oxidation by SR exhibited >9-fold higher catalytic activity (8.8 micromol/mg protein per minute) in the absence of exogenous mitochondrial complex I (ubiquinone) or complex III (cytochrome c) electron acceptors, but was inhibited by rotenone and pyridaben (IC50=2 to 3 nmol/L), antimycin A (IC50=13 nmol/L), and diphenyleneiodonium (IC50=28 micromol/L). Cardiac junctional SR treated with [3H](trifluoromethyl)diazirinyl-pyridaben specifically labeled a single 23-kDa PSST-like protein. These data indicate that NADH oxidation is tightly linked to, and essential for, negative regulation of the RyR2 complex and is a likely component of an important physiological negative-feedback mechanism coupling SR Ca2+ fluxes and mitochondrial energy production.

Oxidized low-density lipoprotein and 15-deoxy-delta 12,14-PGJ2 increase mitochondrial complex I activity in endothelial cells

Oxidized lipids are capable of initiating diverse cellular responses through both receptor-mediated mechanisms and direct posttranslational modification of proteins. Typically, exposure of cells to low concentrations of oxidized lipids induces cytoprotective pathways, whereas high concentrations result in apoptosis. Interestingly, mitochondria can contribute to processes that result in either cytoprotection or cell death. The role of antioxidant defenses such as glutathione in adaptation to stress has been established, but the potential interaction with mitochondrial function is unknown and is examined in this article. Human umbilical vein endothelial cells (HUVEC) were exposed to oxidized LDL (oxLDL) or the electrophilic cyclopentenone 15-deoxy-Delta 12,14-PGJ2 (15d-PGJ2). We demonstrate that complex I activity, but not citrate synthase or cytochrome-c oxidase, is significantly induced by oxLDL and 15d-PGJ2. The mechanism is not clear at present but is independent of the induction of GSH, peroxisome proliferator-activated receptor (PPAR)-gamma, and PPAR-alpha. This response is dependent on the induction of oxidative stress in the cells because it can be prevented by nitric oxide, probucol, and the SOD mimetic manganese(III) tetrakis(4-benzoic acid) porphyrin chloride. This increased complex I activity appears to contribute to protection against apoptosis induced by 4-hydroxynonenal.

A promoter genotype and oxidative stress potentially link resistin to human insulin resistance

Insulin resistance is a component of type 2 diabetes and often precedes pancreatic beta-cell failure. Contributing factors include obesity and a central pattern of fat accumulation with a strong genetic component. The adipocyte secreted hormone resistin has been proposed as a link between the adipocyte and insulin resistance by inhibition of insulin-stimulated glucose uptake and/or blocking adipocyte differentiation. Here we report that the G/G genotype of a single nucleotide polymorphism (SNP) in the promoter of the human resistin gene, -180C>G, had significantly increased basal promoter activity in adipocytes. These data were recapitulated in vivo, where G/G homozygotes had significantly higher resistin mRNA levels in human abdominal subcutaneous fat. A significant interaction was also found between the -180C>G SNP, a marker of oxidative stress (NAD[P]H quinone oxidoreductase mRNA) and homeostasis model assessment of insulin resistance. In addition, resistin mRNA was positively and independently correlated with insulin resistance and hepatic fat as measured by liver X-ray attenuation. These data implicate resistin in the pathophysiology of the human insulin resistance syndrome, an effect mediated by the -180C>G promoter SNP and potentially cellular oxidative stress.

Tumor cytotoxicity by endothelial cells. Impairment of the mitochondrial system for glutathione uptake in mouse B16 melanoma cells that survive after in vitro interaction with the hepatic sinusoidal endothelium

High GSH content associates with high metastatic activity in B16-F10 melanoma cells cultured to low density (LD B16M). GSH homeostasis was investigated in LD B16M cells that survive after adhesion to the hepatic sinusoidal endothelium (HSE). Invasive B16M (iB16M) cells were isolated using anti-Met-72 monoclonal antibodies and flow cytometry-coupled cell sorting. HSE-derived NO and H(2)O(2) caused GSH depletion and a decrease in gamma-glutamylcysteine synthetase activity in iB16M cells. Overexpression of gamma-glutamylcysteine synthetase heavy and light subunits led to a rapid recovery of cytosolic GSH, whereas mitochondrial GSH (mtGSH) further decreased during the first 18 h of culture. NO and H(2)O(2) damaged the mitochondrial system for GSH uptake (rates in iB16M were approximately 75% lower than in LD B16M cells). iB16M cells also showed a decreased activity of mitochondrial complexes II, III, and IV, less O(2) consumption, lower ATP levels, higher O(2) and H(2)O(2) production, and lower mitochondrial membrane potential. In vitro growing iB16M cells maintained high viability (>98%) and repaired HSE-induced mitochondrial damages within 48 h. However, iB16M cells with low mtGSH levels were highly susceptible to TNF-alpha-induced oxidative stress and death. Therefore depletion of mtGSH levels may represent a critical target to challenge survival of invasive cancer cells.