Mitochondrial transcription factor A (TFAM): Roles in maintenance of mtDNA and cellular functions

Effects of overexpression of mitochondrial transcription factor A on lifespan and oxidative stress response in Drosophila melanogaster

Mitochondrial transcription factor A (TFAM) plays a role in the maintenance of mitochondrial DNA (mtDNA) by packaging mtDNA, forming the mitochondrial nucleoid. There have been many reports about a function of TFAM at the cellular level, but only a few studies have been done in individual organisms. Here we examined the effects of TFAM on the Drosophila lifespan and oxidative stress response, by overexpressing TFAM using the GAL4/UAS system. Under standard conditions, the lifespan of TFAM-overexpressing flies was shorter than that of the control flies. However, the lifespan of TFAM-overexpressing flies was longer when they were treated with 1% H(2)O(2). These results suggest that even though excess TFAM has a negative influence on lifespan, it has a defensive function under strong oxidative stress. In the TFAM-overexpressing flies, no significant changes in mtDNA copy number or mtDNA transcription were observed. However, the results of a total antioxidant activity assay suggest the possibility that TFAM is involved in the elimination of oxidative stress. The present results clearly show the effects of TFAM overexpression on the lifespan of Drosophila under both standard conditions and oxidative stress conditions, and our findings contribute to the understanding of the physiological mechanisms involving TFAM in mitochondria.

Mitochondrial DNA variation is associated with elite athletic status in the Polish population

There is mounting evidence that genetic factors located in mitochondrial and nuclear genomes influence sport performance. Certain mitochondrial haplogroups and polymorphisms were associated with the status of elite athlete, especially in endurance performance. The aim of our study was to assess whether selected mitochondrial DNA (mtDNA) and nuclear DNA variants are associated with elite athlete performance in a group of 395 elite Polish athletes (213 endurance athletes and 182 power athletes) and 413 sedentary controls. Our major finding was that the mtDNA haplogroup H and HV cluster influence endurance performance at the Olympic/World Class level of performance (P = 0.018 and P = 0.0185, respectively). We showed that two polymorphisms located in the mtDNA control region were associated with achieving the elite performance level either in the total athlete's group as compared with controls (m.16362C, 3.8% vs 9.2%, respectively, P = 0.0025, odds ratio = 0.39, 95% confidence interval: 0.21-0.72), or in the endurance athletes as compared with controls (m.16080G, 2.35% vs 0%, respectively, P = 0.004). Our results indicate that mtDNA variability affects the endurance capacity rather than the power one. We also propose that mtDNA haplogroups and subhaplogroups, as well as individual mtDNA polymorphisms favoring endurance performance, could be population-specific, reflecting complex cross-talk between nuclear and mitochondrial genomes.

A role for sequestosome 1/p62 in mitochondrial dynamics, import and genome integrity

As a signaling scaffold, p62/sequestosome (p62/SQSTM1) plays important roles in cell signaling and degradation of misfolded proteins. While localization of p62 to mitochondria has been reported, a description of its function once there, remains unclear. Here, we report that p62 is localized to mitochondria in non-stressed situations and demonstrate that deficiency in p62 exacerbates defects in mitochondrial membrane potential and energetics leading to mitochondrial dysfunction. We report on the relationship between mitochondrial protein import and p62. In a p62 null background, mitochondrial import of the mitochondrial transcription factor TFAM is disrupted. When p62 is returned, mitochondrial function is restored to more normal levels. We identify for the first time that p62 localization plays a role in regulating mitochondrial morphology, genome integrity and mitochondrial import of a key transcription factor. We present evidence that these responses to the presence of p62 extend beyond the protein's immediate influence on membrane potential.

An analogue of resveratrol HS-1793 exhibits anticancer activity against MCF-7 cells via inhibition of mitochondrial biogenesis gene expression

Resveratrol is a phytoalexin and polyphenol derived from grapes, berries, and peanuts. It has been shown to mediate death of a wide variety of cancer cells. Although resveratrol is considered an important potential chemotherapeutic agent, it is required at high doses to achieve a biologically or physiologically significant effect, which may be impractical for treating cancer. Thus, a more stable and potent derivative of resveratrol, with more effective tumoricidal activity, must be developed. A novel resveratrol analog, HS-1793, has recently been synthesized and was determined to exhibit a greater decrease in cancer cell viability than resveratrol. However, the underlying mechanism of HS-1793-induced cancer cell death remains unknown. We thus investigated the mechanism by which HS-1793 induces cell death and assessed whether this occurs through a mitochondrial-mediated mechanism. Using the MCF-7 breast cancer cell line, we determined that HS-1793 treatment significantly increased cell death at a relatively low dose compared with resveratrol. HS-1793 treatment more significantly decreased mitochondrial membrane potential, cellular ATP concentration, and cellular oxygen consumption rate than resveratrol treatment. At the molecular level, HS-1793 treatment down-regulated the expression of major mitochondrial biogenesis-regulating proteins, including mitochondrial transcriptional factor A (TFAM), Tu translation elongation factor (TUFM), and single-stranded DNA-binding protein. We conclude that HS- 1793 acts by regulating the expression of TFAM and TUFM, leading to a block in normal mitochondrial function, which sensitizes cancer cells to cell death. We therefore propose that HS-1793 can be a useful chemosensitization agent, which together with other such agents can efficiently target cancer cells.

Modeling RNA polymerase interaction in mitochondria of chordates

Background

In previous work, we introduced a concept, a mathematical model and its computer realization that describe the interaction between bacterial and phage type RNA polymerases, protein factors, DNA and RNA secondary structures during transcription, including transcription initiation and termination. The model accurately reproduces changes of gene transcription level observed in polymerase sigma-subunit knockout and heat shock experiments in plant plastids. The corresponding computer program and a user guide are available at http://lab6.iitp.ru/en/rivals. Here we apply the model to the analysis of transcription and (partially) translation processes in the mitochondria of frog, rat and human. Notably, mitochondria possess only phage-type polymerases. We consider the entire mitochondrial genome so that our model allows RNA polymerases to complete more than one circle on the DNA strand.

Results

Our model of RNA polymerase interaction during transcription initiation and elongation accurately reproduces experimental data obtained for plastids. Moreover, it also reproduces evidence on bulk RNA concentrations and RNA half-lives in the mitochondria of frog, human with or without the MELAS mutation, and rat with normal (euthyroid) or hyposecretion of thyroid hormone (hypothyroid). The transcription characteristics predicted by the model include: (i) the fraction of polymerases terminating at a protein-dependent terminator in both directions (the terminator polarization), (ii) the binding intensities of the regulatory protein factor (mTERF) with the termination site and, (iii) the transcription initiation intensities (initiation frequencies) of all promoters in all five conditions (frog, healthy human, human with MELAS syndrome, healthy rat, and hypothyroid rat with aberrant mtDNA methylation). Using the model, absolute levels of all gene transcription can be inferred from an arbitrary array of the three transcription characteristics, whereas, for selected genes only relative RNA concentrations have been experimentally determined. Conversely, these characteristics and absolute transcription levels can be obtained using relative RNA concentrations and RNA half-lives known from various experimental studies. In this case, the “inverse problem” is solved with multi-objective optimization.

Conclusions

In this study, we demonstrate that our model accurately reproduces all relevant experimental data available for plant plastids, as well as the mitochondria of chordates. Using experimental data, the model is applied to estimate binding intensities of phage-type RNA polymerases to their promoters as well as predicting terminator characteristics, including polarization. In addition, one can predict characteristics of phage-type RNA polymerases and the transcription process that are difficult to measure directly, e.g., the association between the promoter’s nucleotide composition and the intensity of polymerase binding. To illustrate the application of our model in functional predictions, we propose a possible mechanism for MELAS syndrome development in human involving a decrease of Phe-tRNA, Val-tRNA and rRNA concentrations in the cell. In addition, we describe how changes in methylation patterns of the mTERF binding site and three promoters in hypothyroid rat correlate with changes in intensities of the mTERF binding and transcription initiations. Finally, we introduce an auxiliary model to describe the interaction between polysomal mRNA and ribonucleases.

Maintenance of mitochondrial genome distribution by mitochondrial AAA+ protein ClpX

The segregation of mitochondrial DNA (mtDNA) is important for the maintenance and transmission of the genome between generations. Recently, we clarified that human mitochondrial transcription factor A (TFAM) is required for equal distribution and symmetric segregation of mtDNA in cultured cells; however, the molecular mechanism involved is largely unknown. ClpX is an ATPase associated with various cellular activities (AAA+) proteins that localize to the mitochondrial matrix and is suggested to associate with mtDNA. In this study, we found that RNAi-mediated knockdown of ClpX in HeLa cells resulted in enlarged mtDNA nucleoids, which is very similar to that observed in TFAM-knockdown cells in several properties. The expression of TFAM protein was not significantly reduced in ClpX-knockdown cells. However, the enlarged mtDNA nucleoids caused by ClpX-knockdown were suppressed by overexpression of recombinant TFAM and the phenotype was not observed in knockdown with ClpP, a protease subunit of ClpXP. Endogenous ClpX and TFAM exist in close vicinity, and ClpX enhanced DNA-binding activity of TFAM in vitro. These results suggest that human ClpX, a novel mtDNA regulator, maintains mtDNA nucleoid distribution through TFAM function as a chaperone rather than as a protease and its involvement in mtDNA segregation.

Mitochondrial DNA maintenance is regulated in human hepatoma cells by glycogen synthase kinase 3β and p53 in response to tumor necrosis factor α

During chronic liver inflammation, up-regulated Tumor Necrosis Factor alpha (TNF-α) targets hepatocytes and induces abnormal reactive oxygen species (ROS) production responsible for mitochondrial DNA (mtDNA) alterations. The serine/threonine Glycogen Synthase Kinase 3 beta (GSK3β) plays a pivotal role during inflammation but its involvement in the maintenance of mtDNA remains unknown. The aim of this study was to investigate its involvement in TNF-α induced mtDNA depletion and its interrelationship with p53 a protein known to maintain mtDNA copy numbers. Using quantitative polymerase chain reaction (qPCR) we found that at 30 min in human hepatoma HepG2 cells TNF-α induced 0.55±0.10 mtDNA lesions per 10 Kb and a 52.4±2.8% decrease in mtDNA content dependent on TNF-R1 receptor and ROS production. Both lesions and depletion returned to baseline from 1 to 6 h after TNF-α exposure. Luminol-amplified chemiluminescence (LAC) was used to measure the rapid (10 min) and transient TNF-α induced increase in ROS production (168±15%). A transient 8-oxo-dG level of 1.4±0.3 ng/mg DNA and repair of abasic sites were also measured by ELISA assays. Translocation of p53 to mitochondria was observed by Western Blot and co-immunoprecipitations showed that TNF-α induced p53 binding to GSK3β and mitochondrial transcription factor A (TFAM). In addition, mitochondrial D-loop immunoprecipitation (mtDIP) revealed that TNF-α induced p53 binding to the regulatory D-loop region of mtDNA. The knockdown of p53 by siRNAs, inhibition by the phosphoSer(15)p53 antibody or transfection of human mutant active GSK3βS9A pcDNA3 plasmid inhibited recovery of mtDNA content while blockade of GSK3β activity by SB216763 inhibitor or knockdown by siRNAs suppressed mtDNA depletion. This study is the first to report the involvement of GSK3β in TNF-α induced mtDNA depletion. We suggest that p53 binding to GSK3β, TFAM and D-loop could induce recovery of mtDNA content through mtDNA repair.

Targeting mitochondrial biogenesis for promoting health

Mitochondrial biogenesis is a key physiological process that is required for normal growth and development and for maintenance of ongoing cellular energy requirements during aging. Of equivalent and or greater importance is the regulated enhancement of mitochondrial biogenesis upon physiological demand coupled to multiple cellular insults. Basically, cellular survival mechanisms following a variety of disease-related pathophysiological insults are entrained by convergent mechanisms designed to regain homeostatic control of mitochondrial biogenesis. Recent molecular studies represent a clearly defined approach to maximize normative cellular expression of mitochondrial biogenesis for maintenance of cellular energy requirements and as an anti-aging strategy in healthy human populations. This report focuses on mitochondrial transcription factor A, peroxisome proliferator-activated receptor gamma coactivator 1-alpha, PINK1 and Parkin. Designing agents to target mitochondrial function represents a compelling therapeutic strategy for enhancement of cellular expression of mitochondrial biogenesis in diverse human populations afflicted with metabolic, degenerative, neurodegenerative, and metastatic diseases.<br />

Mitochondria and cell bioenergetics: increasingly recognized components and a possible etiologic cause of Alzheimer's disease

SIGNIFICANCE

Mitochondria and brain bioenergetics are increasingly thought to play an important role in Alzheimer's disease (AD).

RECENT ADVANCES

Data that support this view are discussed from the perspective of the amyloid cascade hypothesis, which assumes beta-amyloid perturbs mitochondrial function, and from an opposite perspective that assumes mitochondrial dysfunction promotes brain amyloidosis. A detailed review of cytoplasmic hybrid (cybrid) studies, which argue mitochondrial DNA (mtDNA) contributes to sporadic AD, is provided. Recent AD endophenotype data that further suggest an mtDNA contribution are also summarized.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Biochemical, molecular, cybrid, biomarker, and clinical data pertinent to the mitochondria-bioenergetics-AD nexus are synthesized and the mitochondrial cascade hypothesis, which represents a mitochondria-centric attempt to conceptualize sporadic AD, is discussed.

The manner in which DNA is packaged with TFAM has an impact on transcription activation and inhibition

For successful mitochondrial transgene expression, an optimal packaging exogenous DNA is an important issue. We report herein on the effects of DNA packaged with mitochondrial transcription factor A (TFAM), which packages mitochondrial DNA (mtDNA), on the transcription process. Our initial findings indicated that the transcription of the TFAM/DNA complex was activated, when the complex was formed at an optimal ratio. We also found that TFAM has a significant advantage over protamine, a nuclear DNA packaging protein, from the viewpoint of transcription efficiency. This result indicates that TFAM can be useful packaging protein for exogenous DNA to achieve mitochondrial transgene expression.

Mitochondrial transcription factor A serves as a danger signal by augmenting plasmacytoid dendritic cell responses to DNA

Plasmacytoid dendritic cells (pDC) are potent APCs known to regulate immune responses to self-Ags, particularly DNA. The mitochondrial fraction of necrotic cells was found to most potently promote human pDC activation, as reflected by type I IFN release, which was dependent upon the presence of mitochondrial DNA and involved TLR9 and receptors for advanced glycation end products. Mitochondrial transcription factor A (TFAM), a highly abundant mitochondrial protein that is functionally and structurally homologous to high mobility group box protein 1, was observed to synergize with CpG-containing oligonucleotide, type A, DNA to promote human pDC activation. pDC type I IFN responses to TFAM and CpG-containing oligonucleotide, type A, DNA indicated their engagement with receptors for advanced glycation end products and TLR9, respectively, and were dependent upon endosomal processing and PI3K, ERK, and NF-κB signaling. Taken together, these results indicate that pDC contribute to sterile immune responses by recognizing the mitochondrial component of necrotic cells and further incriminate TFAM and mitochondrial DNA as likely mediators of pDC activation under these circumstances.

Damage to mtDNA in liver injury of patients with extrahepatic cholestasis: the protective effects of mitochondrial transcription factor A

Oxidative stress and mitochondrial dysfunction are involved in the pathogenesis of chronic liver cholestasis. Mitochondrial DNA (mtDNA) is highly susceptible to oxidative stress and mtDNA damage leads to mitochondrial dysfunction. This study aimed to investigate the mtDNA alterations that occurred during liver injury in patients with extrahepatic cholestasis. Along with an increase in malondialdehyde (MDA) levels and a decrease in ATP levels, extrahepatic cholestatic patients presented a significant increase in mitochondrial 8-hydroxydeoxyguanosine (8-OHdG) levels and decreases in mtDNA copy number, mtDNA transcript levels, and mtDNA nucleoid structure. In L02 cells, glycochenodeoxycholic acid (GCDCA) induced similar damage to the mtDNA and mitochondria. In line with the mtDNA alterations, the mRNA and protein levels of mitochondrial transcription factor A (TFAM) were significantly decreased both in cholestatic patients and in GCDCA-treated L02 cells. Moreover, overexpression of TFAM could efficiently attenuate the mtDNA damage induced by GCDCA in L02 cells. However, without its C-tail, ΔC-TFAM appeared less effective against the hepatotoxicity of GCDCA than the wild-type TFAM. Overall, our study demonstrates that mtDNA damage is involved in liver damage in extrahepatic cholestatic patients. The mtDNA damage is attributable to the loss of TFAM. TFAM has mtDNA-protective effects against the hepatotoxicity of bile acid during cholestasis.

Mitochondrial gene replacement in human pluripotent stem cell-derived neural progenitors

Human pluripotent stem cell-derived neural progenitor (hNP) cells are an excellent resource for understanding early neural development and neurodegenerative disorders. Given that many neurodegenerative disorders can be correlated with defects in the mitochondrial genome, optimal utilization of hNP cells requires an ability to manipulate and monitor changes in the mitochondria. Here, we describe a novel approach that uses recombinant human mitochondrial transcription factor A (rhTFAM) protein to transfect and express a pathogenic mitochondrial genome (mtDNA) carrying the G11778A mutation associated with Leber's hereditary optic neuropathy (LHON) disease, into dideoxycytidine (ddC)-treated hNPs. Treatment with ddC reduced endogenous mtDNA and gene expression, without loss of hNP phenotypic markers. Entry of G11778A mtDNA complexed with the rhTFAM was observed in mitochondria of ddC-hNPs. Expression of the pathogenic RNA was confirmed by restriction enzyme analysis of the SfaN1-digested cDNA. On the basis of the expression of neuron-specific class III beta-tubulin, neuronal differentiation occurred. Our results show for the first time that pathogenic mtDNA can be introduced and expressed into hNPs without loss of phenotype or neuronal differentiation potential. This mitochondrial gene replacement technology allows for creation of in vitro stem cell-based models useful for understanding neuronal development and treatment of neurodegenerative disorders.

Cardiac-specific deletion of SOCS-3 prevents development of left ventricular remodeling after acute myocardial infarction

OBJECTIVES

The study investigated the role of myocardial suppressor of cytokine signaling-3 (SOCS3), an intrinsic negative feedback regulator of the janus kinase and signal transducer and activator of transcription (JAK-STAT) signaling pathway, in the development of left ventricular (LV) remodeling after acute myocardial infarction (AMI).

BACKGROUND

LV remodeling after AMI results in poor cardiac performance leading to heart failure. Although it has been shown that JAK-STAT-activating cytokines prevent LV remodeling after AMI in animals, little is known about the role of SOCS3 in this process.

METHODS

Cardiac-specific SOCS3 knockout mice (SOCS3-CKO) were generated and subjected to AMI induced by permanent ligation of the left anterior descending coronary artery.

RESULTS

Although the initial infarct size after coronary occlusion measured by triphenyltetrazolium chloride staining was comparable between SOCS3-CKO and control mice, the infarct size 14 days after AMI was remarkably inhibited in SOCS3-CKO, indicating that progression of LV remodeling after AMI was prevented in SOCS3-CKO hearts. Prompt and marked up-regulations of multiple JAK-STAT-activating cytokines including leukemia inhibitory factor and granulocyte colony-stimulating factor (G-CSF) were observed within the heart following AMI. Cardiac-specific SOCS3 deletion enhanced multiple cardioprotective signaling pathways including STAT3, AKT, and extracellular signal-regulated kinase (ERK)-1/2, while inhibiting myocardial apoptosis and fibrosis as well as augmenting antioxidant expression.

CONCLUSIONS

Enhanced activation of cardioprotective signaling pathways by inhibiting myocardial SOCS3 expression prevented LV remodeling after AMI. Our data suggest that myocardial SOCS3 may be a key molecule in the development of LV remodeling after AMI.

Crosstalk between mitochondrial (dys)function and mitochondrial abundance

A controlled regulation of mitochondrial mass through either the production (biogenesis) or the degradation (mitochondrial quality control) of the organelle represents a crucial step for proper mitochondrial and cell function. Key steps of mitochondrial biogenesis and quality control are overviewed, with an emphasis on the role of mitochondrial chaperones and proteases that keep mitochondria fully functional, provided the mitochondrial activity impairment is not excessive. In this case, the whole organelle is degraded by mitochondrial autophagy or "mitophagy." Beside the maintenance of adequate mitochondrial abundance and functions for cell homeostasis, mitochondrial biogenesis might be enhanced, through discussed signaling pathways, in response to various physiological stimuli, like contractile activity, exposure to low temperatures, caloric restriction, and stem cells differentiation. In addition, mitochondrial dysfunction might also initiate a retrograde response, enabling cell adaptation through increased mitochondrial biogenesis.

Human apurinic/apyrimidinic endonuclease 1 translocalizes to mitochondria after photodynamic therapy and protects cells from apoptosis

Photodynamic therapy (PDT) is an effective therapeutic regime for lung cancer. Mitochondrial functional failure is considered to be one of the most important factors causing cell death after PDT. However, the detailed mechanisms that are involved are still unclear. We previously reported that apurinic/apyrimidinic endonuclease (APE1) plays a critical role in regulating sensitivity to PDT in the lung cancer A549 cell line. An important mitochondrial regulatory role for APE1 has recently been reported, so therefore we explored the role of APE1 in cell survival after PDT-induced oxidative stress through regulation of mitochondrial function. We first observed that photoirradiation induced the mitochondrial translocation of APE1. The ability of APE1 to regulate mitochondrial membrane potential and reactive oxygen species (ROS) production after photoirradiation was tested in APE1 knockdown A549 cells. APE1-deficient A549 cells were characterized as having a lower mitochondrial membrane potential and higher ROS production, which led to increased apoptosis through the mitochondrial pathway after PDT. Additionally, unexpected activity of APE1 was observed in mitochondria: the control of mitochondrial transcriptional activity by redox regulation of mitochondrial transcription factor A (TFAM). Furthermore, two dominant-negative mutants of APE1 were overexpressed to enhance their individual activities in mitochondria. The results suggest that both these APE1 activities play a role in the regulation of mitochondrial function but through different mechanisms. The present study not only provides possible mechanisms for APE1 in regulating survival after photoirradiation but also uncovers a new activity of APE1 in mitochondria.

Differential responses of white adipose tissue and brown adipose tissue to caloric restriction in rats

Caloric restriction (CR) slows the aging process and extends longevity, but the exact underlying mechanisms remain debatable. It has recently been suggested that the beneficial action of CR may be mediated in part by adipose tissue remodeling. Mammals have two types of adipose tissue: white adipose tissue (WAT) and brown adipose tissue (BAT). In this study, proteome analysis using two-dimensional gel electrophoresis combined with MALDI-TOF MS, and subsequent analyses were performed on both WAT and BAT from 9-month-old male rats fed ad libitum or subjected to CR for 6 months. Our findings suggest that CR activates mitochondrial energy metabolism and fatty acid biosynthesis in WAT. It is likely that in CR animals WAT functions as an energy transducer from glucose to energy-dense lipid. In contrast, in BAT CR either had no effect on, or down-regulated, the mitochondrial electron transport chain, but enhanced fatty acid biosynthesis. This suggests that in CR animals BAT may change its function from an energy consuming system to an energy reservoir system. Based on our findings, we conclude that WAT and BAT cooperate to use energy effectively via a differential response of mitochondrial function to CR.

Effect of mitochondrial transcription factor a overexpression on motor neurons in amyotrophic lateral sclerosis model mice

Increasing evidence indicates that oxidative stress is an important mechanism underlying motor neuron (MN) degeneration in amyotrophic lateral sclerosis (ALS). Mitochondrial DNA (mtDNA) is highly susceptible to oxidative damage and has little potential for repair, although mitochondrial transcription factor A (TFAM) plays essential roles in maintaining mitochondrial DNA by reducing oxidative stress, promoting mtDNA transcription, and regulating mtDNA copy number. To analyze a possible therapeutic effect of TFAM on ALS pathology, double transgenic mice overexpressing G93A mutant SOD1 (G93ASOD1) and human TFAM (hTFAM) were newly generated in the present study. Rotarod scores were better in G93ASOD1/hTFAM double-Tg mice than G93ASOD1 single-Tg mice at an early symptomatic stage, 15 and 16 weeks of age, with a 10% extension of the onset age in double-Tg mice. The number of surviving MNs was 30% greater in double-Tg mice with end-stage disease, at 19 weeks, with remarkable reductions in the amount of the oxidative stress marker 8-OHdG and the apoptotic marker cleaved caspase 3 and with preserved COX1 expression. Double-immunofluorescence study showed that hTFAM was expressed specifically in MNs and microglia in the spinal cords of double-Tg mice. The present study suggests that overexpression of TFAM has a potential to reduce oxidative stress in MN and delay onset of the disease in ALS model mice. © 2012 Wiley Priodicals, Inc.

Lipopolysaccharide-induced mitochondrial DNA depletion

Hepatic energy depletion has been described in severe sepsis, and lipopolysaccharide (LPS) has been shown to cause mitochondrial DNA (mtDNA) damage. To clarify the mechanisms of LPS-induced mtDNA damage and mitochondrial alterations, we treated wild-type (WT) or transgenic manganese superoxide dismutase-overerexpressing (MnSOD(+++)) mice with a single dose of LPS (5 mg/kg). In WT mice, LPS increased mitochondrial reactive oxygen species formation, hepatic inducible nitric oxide synthase (NOS) mRNA and protein, tumor necrosis factor-alpha, interleukin-1 beta, and high-mobility group protein B1 concentrations. Six to 48 h after LPS administration (5 mg/kg), liver mtDNA levels, respiratory complex I activity, and adenosine triphosphate (ATP) contents were decreased. In addition, LPS increased interferon-β concentration and decreased mitochondrial transcription factor A (Tfam) mRNA, Tfam protein, and mtDNA-encoded mRNAs. Morphological studies showed mild hepatic inflammation. The LPS (5 mg/kg)-induced mtDNA depletion, complex I inactivation, ATP depletion, and alanine aminotransferase increase were prevented in MnSOD(+++) mice or in WT mice cotreated with 1400W (a NOS inhibitor), (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride, monohydrate (a superoxide scavenger) or uric acid (a peroxynitrite scavenger). The MnSOD overexpression delayed death in mice challenged by a higher, lethal dose of LPS (25 mg/kg). In conclusion, LPS administration damages mtDNA and alters mitochondrial function. The protective effects of MnSOD, NOS inhibitors, and superoxide or peroxynitrite scavengers point out a role of the superoxide anion reacting with NO to form mtDNA- and protein-damaging peroxynitrite. In addition to the acute damage caused by reactive species, decreased levels of mitochondrial transcripts contribute to mitochondrial dysfunction.

Nickel exposure induces oxidative damage to mitochondrial DNA in Neuro2a cells: the neuroprotective roles of melatonin

Recent studies suggest that oxidative stress and mitochondrial dysfunction play important roles in the neurotoxicity of nickel. Because mitochondrial DNA (mtDNA) is highly vulnerable to oxidative stress and melatonin can efficiently protect mtDNA against oxidative damage in various pathological conditions, the aims of this study were to determine whether mtDNA oxidative damage was involved in the neurotoxicity of nickel and to assay the neuroprotective effects of melatonin in mtDNA. In this study, we exposed mouse neuroblastoma cell lines (Neuro2a) to different concentrations of nickel chloride (NiCl(2), 0.125, 0.25, and 0.5 mm) for 24 hr. We found that nickel significantly increased reactive oxygen species (ROS) production and mitochondrial superoxide levels. In addition, nickel exposure increased mitochondrial 8-hydroxyguanine (8-OHdG) content and reduced mtDNA content and mtDNA transcript levels. Consistent with this finding, nickel was found to destroy mtDNA nucleoid structure and decrease protein levels of Tfam, a key protein component for nucleoid organization. However, all the oxidative damage to mtDNA induced by nickel was efficiently attenuated by melatonin pretreatment. Our results suggest that oxidative damage to mtDNA may account for the neurotoxicity of nickel. Melatonin has great pharmacological potential in protecting mtDNA against the adverse effects of nickel in the nervous system.

The mitochondrial transcription and packaging factor Tfam imposes a U-turn on mitochondrial DNA

Tfam, a DNA binding protein with tandem HMG (high mobility group)-box domains, plays a central role in expression, maintenance, and organization of the mitochondrial genome. It activates transcription from mitochondrial promoters and organizes the mitochondrial genome into nucleoids. Using X-ray crystallography, we show that human Tfam forces promoter DNA to undergo a U-turn, reversing the direction of the DNA helix. Each HMG-box domain wedges into the DNA minor groove to generate two kinks on one face of the DNA. On the opposite face, a positively charged α-helix serves as a platform to facilitate DNA bending. The structural principles underlying DNA bending converge with those of the unrelated HU family proteins, which play analogous architectural roles in organizing bacterial nucleoids. The functional importance of this extreme DNA bending is promoter-specific and appears related to the orientation of Tfam on the promoters.

The role of microglial mtDNA damage in age-dependent prolonged LPS-induced sickness behavior

Microglia are the main cellular source of oxidation products and inflammatory molecules in the brain during aging. The accumulation of mitochondrial DNA (mtDNA) oxidative damage in microglia during aging results in the increased production of reactive oxygen species (ROS). The increased intracellular ROS, in turn, activates a redox-sensitive nuclear factor-κB (NF-κB) to provoke excessive neuroinflammation, resulting in memory deficits and the prolonged behavioral consequence of infection. Besides its role in regulating the gene copy number, mitochondrial transcription factor A (TFAM) is closely associated with the stabilization of mtDNA structures. Lipopolysaccharide (LPS) induces the generation of ROS from the actively respirating mitochondria as well as NADPH oxidase, and leads to the subsequent activation of the NF-κB-dependent inflammatory pathway in aging microglia. The overexpression of human TFAM improves the age-dependent prolonged LPS-induced sickness behaviors by ameliorating the mtDNA damage and reducing the resultant redox-regulated inflammatory responses. Therefore, 'microglia-aging' plays important roles in the age-dependent enhanced behavioral consequences of infection.

Human telomerase acts as a hTR-independent reverse transcriptase in mitochondria

Human telomerase reverse transcriptase (hTERT) is localized to mitochondria, as well as the nucleus, but details about its biology and function in the organelle remain largely unknown. Here we show, using multiple approaches, that mammalian TERT is mitochondrial, co-purifying with mitochondrial nucleoids and tRNAs. We demonstrate the canonical nuclear RNA [human telomerase RNA (hTR)] is not present in human mitochondria and not required for the mitochondrial effects of telomerase, which nevertheless rely on reverse transcriptase (RT) activity. Using RNA immunoprecipitations from whole cell and in organello, we show that hTERT binds various mitochondrial RNAs, suggesting that RT activity in the organelle is reconstituted with mitochondrial RNAs. In support of this conclusion, TERT drives first strand cDNA synthesis in vitro in the absence of hTR. Finally, we demonstrate that absence of hTERT specifically in mitochondria with maintenance of its nuclear function negatively impacts the organelle. Our data indicate that mitochondrial hTERT works as a hTR-independent reverse transcriptase, and highlight that nuclear and mitochondrial telomerases have different cellular functions. The implications of these findings to both the mitochondrial and telomerase fields are discussed.

Impaired mitochondrial biogenesis in hippocampi of rats with chronic seizures

Mitochondrial dysfunction has been suggested to be a contributing factor of epilepsy, but the underlying mechanisms are not completely explored. Mitochondrial biogenesis is involved in regulation of mitochondrial content, morphology, and function. In the current study, we show mitochondrial biogenesis severely impaired in hippocampi of rats with chronic seizures induced by pilocarpine, as evidenced by decreased mitochondrial DNA (mtDNA) content and decreased mtDNA-encoded protein level. Furthermore, we show mtDNA transcription and replication reduced in rats with chronic seizures. These defects were independent of downregulation of mitochondrial biogenesis-related factors, such as peroxisome proliferator-activated receptor gamma coactivator-1α, nuclear respiratory factor-1, and mitochondrial transcription factor A (Tfam), but depended on reduced Tfam-DNA binding activity. The present study suggests novel mechanisms for mitochondrial dysfunction during chronic seizures.

Expression of key regulators of mitochondrial biogenesis in growth hormone receptor knockout (GHRKO) mice is enhanced but is not further improved by other potential life-extending interventions

Mitochondrial biogenesis is essential for cell viability. Growth hormone receptor knockout (GHRKO), calorie restriction, and surgical visceral fat removal constitute experimental interventions to delay aging and increase life span. We examined the expression of known regulators of mitochondriogenesis: peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α), adenosine monophosphate (AMP)-activated protein kinase (AMPK), sirtuin-1 (SIRT-1) and sirtuin-3 (SIRT-3), endothelial nitric oxide synthase (eNOS), nuclear respiratory factor-1, mitochondrial transcription factor A (TFAM), and mitofusin-2 (MFN-2) in the skeletal muscles and hearts of control and calorie-restricted female GHRKO mice and in the kidneys of male GHRKOs after visceral fat removal or sham surgery. Expression of PGC-1α in skeletal muscles, AMPK, SIRT-1, SIRT-3, eNOS, and MFN-2 in the heart and PGC-1α, AMPK, SIRT-3, eNOS, and MFN-2 in kidneys was increased in GHRKO mice but was not affected by calorie restriction or visceral fat removal. GHRKO mice have increased expression of key regulators of mitochondriogenesis, which is not improved further by calorie restriction or visceral fat removal.

Early long-term L-T3 replacement rescues mitochondria and prevents ischemic cardiac remodelling in rats

3,5,3′-Levo-triiodothyronine (L-T3) is essential for DNA transcription, mitochondrial biogenesis and respiration, but its circulating levels rapidly decrease after myocardial infarction (MI). The main aim of our study was to test whether an early and sustained normalization of L-T3 serum levels after MI exerts myocardial protective effects through a mitochondrial preservation. Seventy-two hours after MI induced by anterior interventricular artery ligation, rats were infused with synthetic L-T3 (1.2 μg/kg/day) or saline over 4 weeks. Compared to saline, L-T3 infusion restored FT3 serum levels at euthyroid state (3.0 ± 0.2 versus 4.2 ± 0.3 pg/ml), improved left ventricular (LV) ejection fraction (39.5 ± 2.5 versus 65.5 ± 6.9%), preserved LV end-systolic wall thickening in the peri-infarct zone (6.34 ± 3.1 versus 33.7 ± 6.21%) and reduced LV infarct-scar size by approximately 50% (all P < 0.05). Moreover, L-T3 significantly increased angiogenesis and cell survival and enhanced the expression of nuclear-encoded transcription factors involved in these processes. Finally, L-T3 significantly increased the expression of factors involved in mitochondrial DNA transcription and biogenesis, such as hypoxic inducible factor-1α, mitochondrial transcription factor A and peroxisome proliferator activated receptor γ coactivator-1α, in the LV peri-infarct zone. To further explore mechanisms of L-T3 protective effects, we exposed isolated neonatal cardiomyocytes to H₂O₂ and found that L-T3 rescued mitochondrial biogenesis and function and protected against cell death via a mitoKATP dependent pathway. Early and sustained physiological restoration of circulating L-T3 levels after MI halves infarct scar size and prevents the progression towards heart failure. This beneficial effect is likely due to enhanced capillary formation and mitochondrial protection.

Effects of a 300 mT static magnetic field on human umbilical vein endothelial cells

This study describes the effects of a static magnetic field (SMF) on cell growth and DNA integrity of human umbilical vein endothelial cells (HUVECs). Fast halo assay was used to investigate nuclear damage; quantitative polymerase chain reaction (QPCR), standard PCR, and real-time PCR were used to evaluate mitochondrial DNA integrity, content, and gene expression. HUVECs were continually exposed to a 300 mT SMF for 4, 24, 48, and 72 h. Compared to control samples (unexposed cultures) the SMF-exposed cells did not show a statistically significant change in their viability. Conversely, the static field was shown to be significant after 4 h of exposure, inducing damage on both the nuclear and mitochondrial levels, reducing mitochondrial content and increasing reactive oxygen species. Twenty-four hours of exposure increased mitochondrial DNA content as well as expression of one of the main genes related to mitochondrial biogenesis. No significant differences between exposed and sham cultures were found after 48 and 72 h of exposure. The results suggest that a 300 mT SMF does not cause permanent DNA damage in HUVECs and stimulates a transient mitochondrial biogenesis.

Mitochondrial transcription factor A overexpression and base excision repair deficiency in the inner ear of rats with D-galactose-induced aging

Oxidative damage to mtDNA is associated with excessive reactive oxygen species production. The mitochondrial common deletion (mtDNA 4977-bp and 4834-bp deletion in humans and rats, respectively) is the most typical and frequent form of mtDNA damage associated with aging and degenerative diseases. The accumulation of the mitochondrial common deletion has been proposed to play a crucial role in age-related hearing loss (presbycusis). However, the mechanisms underlying the formation and accumulation of mtDNA deletions are still obscure. In the present study, a rat mimetic aging model induced by D-Gal was used to explore the origin of deletion mutations and how mtDNA repair systems modulate this process in the inner ear during aging. We found that the mitochondrial common deletion was greatly increased and mitochondrial base excision repair capacity was significantly reduced in the inner ear in D-Gal-treated rats as compared with controls. The overexpression of mitochondrial transcription factor A induced by D-Gal significantly stimulated mtDNA replication, resulting in an increase in mtDNA copy number. In addition, an age-related loss of auditory sensory cells in the inner ear was observed in D-Gal-treated rats. Taken together, our data suggest that mitochondrial base excision repair capacity deficiency and an increase in mtDNA replication resulting from mitochondrial transcription factor A overexpression may contribute to the accumulation of mtDNA deletions in the inner ear during aging. This study also provides new insights into the development of presbycusis.

MnSOD overexpression prevents liver mitochondrial DNA depletion after an alcohol binge but worsens this effect after prolonged alcohol consumption in mice

Both acute and chronic alcohol consumption increase reactive oxygen species (ROS) formation and lipid peroxidation, whose products damage hepatic mitochondrial DNA (mtDNA). To test whether manganese superoxide dismutase (MnSOD) overexpression modulates acute and chronic alcohol-induced mtDNA lesions, transgenic MnSOD-overexpressing (TgMnSOD(+++)) mice and wild-type (WT) mice were treated by alcohol, either chronically (7 weeks in drinking water) or acutely (single intragastric dose of 5 g/kg). Acute alcohol administration increased mitochondrial ROS formation, decreased mitochondrial glutathione, depleted and damaged mtDNA, durably increased inducible nitric oxide synthase (NOS) expression, plasma nitrites/nitrates and the nitration of tyrosine residues in complex V proteins and decreased complex V activity in WT mice. These effects were prevented in TgMnSOD(+++) mice. In acutely alcoholized WT mice, mtDNA depletion was prevented by tempol, a superoxide scavenger, L-NAME and 1400W, two NOS inhibitors, or uric acid, a peroxynitrite scavenger. In contrast, chronic alcohol consumption decreased cytosolic glutathione and increased hepatic iron, lipid peroxidation products and respiratory complex I protein carbonyls only in ethanol-treated TgMnSOD(+++) mice but not in WT mice. In chronic ethanol-fed TgMnSOD(+++) mice, but not WT mice, mtDNA was damaged and depleted, and the iron chelator, deferoxamine (DFO), prevented this effect. In conclusion, MnSOD overexpression prevents mtDNA depletion after an acute alcohol binge but aggravates this effect after prolonged alcohol consumption, which selectively triggers iron accumulation in TgMnSOD(+++) mice but not in WT mice. In the model of acute alcohol binge, the protective effects of MnSOD, tempol, NOS inhibitors and uric acid suggested a role of the superoxide anion reacting with NO to form mtDNA-damaging peroxynitrite. In the model of prolonged ethanol consumption, the protective effects of DFO suggested the role of iron reacting with hydrogen peroxide to form mtDNA-damaging hydroxyl radical.

Frequent truncating mutation of TFAM induces mitochondrial DNA depletion and apoptotic resistance in microsatellite-unstable colorectal cancer

The mitochondrial transcription factor A (TFAM) is required for mitochondrial DNA (mtDNA) replication and transcription. Disruption of TFAM results in heart failure and premature aging in mice. But very little is known about the role of TFAM in cancer development. Here, we report the identification of frequent frameshift mutations in the coding mononucleotide repeat of TFAM in sporadic colorectal cancer (CRC) cell lines and in primary tumors with microsatellite instability (MSI), but not in microsatellite stable (MSS) CRC cell lines and tumors. The presence of the TFAM truncating mutation, in CRC cells with MSI, reduced the TFAM protein level in vivo and in vitro and correlated with mtDNA depletion. Furthermore, forced overexpression of wild-type TFAM in RKO cells carrying a TFAM truncating mutation suppressed cell proliferation and inhibited RKO cell-induced xenograft tumor growth. Moreover, these cells showed more susceptibility to cisplatin-induced apoptosis due to an increase of cytochrome b (Cyt b) expression and its release from mitochondria. An interaction assay between TFAM and the heavy-strand promoter (HSP) of mitochondria revealed that mutant TFAM exhibited reduced binding to HSP, leading to reduction in Cyt b transcription. Collectively, these data provide evidence that a high incidence of TFAM truncating mutations leads to mitochondrial copy number reduction and mitochondrial instability, distinguishing most CRC with MSI from MSS CRC. These mutations may play an important role in tumorigenesis and cisplatin-induced apoptotic resistance of most microsatellite-unstable CRCs.

Biochemical properties of Caenorhabditis elegans HMG-5, a regulator of mitochondrial DNA

Caenorhabditis elegans HMG-5, which is encoded by F45E4.9, contains two high mobility group (HMG) box domains and shows sequence similarity with mammalian mitochondrial transcription factor A (TFAM). In this study, using soaking RNA interference, we found that knockdown of HMG-5 reduced the amount of mtDNA in P0 hermaphrodites, suggesting it as functional orthologue of mammalian TFAM. We also examined the biochemical property of HMG-5 in mammalian cells and in vitro. We found that HMG-5 localized to the mitochondria in human cultured cells and was included in the NP-40-insoluble fraction in which mtDNA and TFAM were enriched. By immunoprecipitation analysis, HMG-5 was found to associate with human mitochondrial DNA (mtDNA) in the cells. In vitro binding experiment also showed that HMG-5 binds to C. elegans mtDNA and plasmid DNA, indicating its feature as a non-specific DNA-binding protein. Furthermore, it was found that HMG-5 can interact with itself. These results demonstrate that HMG5 shares similar biochemical properties with mammalian TFAM as a nucleoid factor. HMG-5 could be a good candidate for investigating mtDNA metabolism in multicellular organisms.

Role and treatment of mitochondrial DNA-related mitochondrial dysfunction in sporadic neurodegenerative diseases

Several sporadic neurodegenerative diseases display phenomena that directly or indirectly relate to mitochondrial function. Data suggesting altered mitochondrial function in these diseases could arise from mitochondrial DNA (mtDNA) are reviewed. Approaches for manipulating mitochondrial function and minimizing the downstream consequences of mitochondrial dysfunction are discussed.

Different effects of oleate vs. palmitate on mitochondrial function, apoptosis, and insulin signaling in L6 skeletal muscle cells: role of oxidative stress

The type of free fatty acids (FFAs), saturated or unsaturated, is critical in the development of insulin resistance (IR), since the degree of saturation correlates with IR. We compared the effects of the saturated FFA palmitate, the unsaturated FFA oleate, and a mixture of each on the production of mitochondrial reactive oxygen species (mtROS), mitochondrial DNA (mtDNA) damage, mitochondrial function, apoptosis, and insulin-signaling pathway in skeletal muscle cells. Only palmitate caused a significant increase of mtROS production, which correlated with concomitant mtDNA damage, mitochondrial dysfunction, induction of JNK, apoptosis, and inhibition of insulin signaling. Blocking de novo synthesis of ceramide abolished the effects of palmitate on mtROS production, viability, and insulin signaling. Oleate alone did not cause mtROS generation and mtDNA damage, and its addition to palmitate prevented palmitate-induced mtDNA damage, increased total ATP levels and cell viability, and prevented palmitate-induced apoptosis and inhibition of insulin-stimulated Akt (Ser(473)) phosphorylation. The peroxisome proliferator activator receptor-γ coactivator 1α (PGC-1α) protein level and promoter activity were decreased at concentrations of palmitate ≥0.5 mM, whereas addition of oleate increased both PGC-1α level and promoter activity. Expression of the mitochondrial transcription factor (TFAM) was significantly diminished after palmitate but not oleate treatment. Addition of the ROS scavenger, N-acetylcystein (NAC), to palmitate restored both the expression and promoter activity of PGC-1α as well as TFAM expression. We propose that 1) mtROS generation is the initial event in the induction of mitochondrial dysfunction and consequent apoptosis and the inhibition of insulin signaling and that 2) oleate ameliorates palmitate-induced mitochondrial dysfunction and thus may contribute to the prevention of palmitate-induced IR.

The levels of male gametic mitochondrial DNA are highly regulated in angiosperms with regard to mitochondrial inheritance

The mechanisms that regulate mitochondrial inheritance are not yet clear, even though it is 100 years since the first description of non-Mendelian genetics. Here, we quantified the copy numbers of mitochondrial DNA (mtDNA) in the gametic cells of angiosperm species. We demonstrate that each egg cell from Arabidopsis thaliana, Antirrhinum majus, and Nicotiana tabacum possesses 59.0, 42.7, and 73.0 copies of mtDNA on average, respectively. These values are equivalent to those in Arabidopsis mesophyll cells, at 61.7 copies per cell. On the other hand, sperm or generative cells from Arabidopsis, A. majus, and N. tabacum possess minor amounts of mtDNA, at 0.083, 0.47, and 1 copy on average, respectively. We further reveal a 50-fold degradation of mtDNA during pollen development in A. majus. In contrast, markedly high levels of mtDNA are found in the male gametic cells of Cucumis melo and Pelargonium zonale (1296.3 and 256.7 copies, respectively). Our results provide direct evidence for mitochondrial genomic insufficiency in the eggs and somatic cells and indicate that a male gamete of an angiosperm may possess mtDNA at concentrations as high as 21-fold (C. melo) or as low as 0.1% (Arabidopsis) of the levels in somatic cells. These observations reveal the existence of a strong regulatory system for the male gametic mtDNA levels in angiosperms with regard to mitochondrial inheritance.

Bilobalide protects mitochondrial function in ovariectomized rats by up-regulation of mRNA and protein expression of cytochrome c oxidase subunit I

Bilobalide (BB), a sesquiterpene trilactone from Ginkgo biloba has been proposed to have protective effects on mitochondrial function. Using ovariectomized rats to mimic the post-menopausal pathophysiological changes in women, this study demonstrated that BB treatment could prevent estrogen withdrawal-induced decrease in mitochondrial adenosine triphosphate content, cytochrome c oxidase subunit I (COXI) mRNA and protein levels and COX activity in hippocampal tissues as effectively as estradiol benzoate. But neither ovariectomy nor BB treatment affected citrate synthase activity. These results suggested that BB was able to regulate COX activity via up-regulation of the gene and protein expression of its mitochondrial DNA-coded subunits, and modulation of COX activity by BB might contribute to its protective effects on mitochondrial function. Given that ovariectomy induces decrease in estrogen levels similar to that of menopause, BB may be useful in developing therapy for neurodegenerative diseases such as Alzheimer's disease in post-menopausal females.

Waves of gene regulation suppress and then restore oxidative phosphorylation in cancer cells

We posit the following hypothesis: Independently of whether malignant tumors are initiated by a fundamental reprogramming of gene expression or seeded by stem cells, "waves" of gene expression that promote metabolic changes occur during carcinogenesis, beginning with oncogene-mediated changes, followed by hypoxia-induced factor (HIF)-mediated gene expression, both resulting in the highly glycolytic "Warburg" phenotype and suppression of mitochondrial biogenesis. Because high proliferation rates in malignancies cause aglycemia and nutrient shortage, the third (second oncogene) "wave" of adaptation stimulates glutaminolysis, which in certain cases partially re-establishes oxidative phosphorylation; this involves the LKB1-AMPK-p53, PI3K-Akt-mTOR axes and MYC dysregulation. Oxidative glutaminolysis serves as an alternative pathway compensating for cellular ATP. Together with anoxic glutaminolysis it provides pyruvate, lactate, and the NADPH pool (alternatively to pentose phosphate pathway). Retrograde signaling from revitalized mitochondria might constitute the fourth "wave" of gene reprogramming. In turn, upon reversal of the two Krebs cycle enzymes, glutaminolysis may partially (transiently) function even during anoxia, thereby further promoting malignancy. The history of the carcinogenic process within each malignant tumor determines the final metabolic phenotype of the selected surviving cells, resulting in distinct cancer bioenergetic phenotypes ranging from the highly glycolytic "classic Warburg" to partial or enhanced oxidative phosphorylation. We discuss the bioenergetically relevant functions of oncogenes, the involvement of mitochondrial biogenesis/degradation in carcinogenesis, the yet unexplained Crabtree effect of instant glucose blockade of respiration, and metabolic signaling stemming from the accumulation of succinate, fumarate, pyruvate, lactate, and oxoglutarate by interfering with prolyl hydroxylase domain enzyme-mediated hydroxylation of HIFα prolines.

Peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1alpha) and sirtuin 1 (SIRT1) reside in mitochondria: possible direct function in mitochondrial biogenesis

The transcriptional co-activator PGC-1alpha and the NAD(+)-dependent deacetylase SIRT1 are considered important inducers of mitochondrial biogenesis because in the nucleus they regulate transcription of nucleus-encoded mitochondrial genes. We demonstrate that PGC-1alpha and SIRT1 are also present inside mitochondria and are in close proximity to mtDNA. They interact with mitochondrial transcription factor A (TFAM) as assessed by confocal microscopy analysis and by blue native-PAGE. Nucleoid purification allowed us to identify SIRT1 and PGC-1alpha as proteins associated with native and cross-linked nucleoids, respectively. After mtDNA immunoprecipitation analysis, carried out on mitochondrial extracts, we found that PGC-1alpha is present on the same D-loop region recognized by TFAM. Finally, by oligonucleotide pulldown assay, we found PGC-1alpha and SIRT1 associated with the TFAM consensus sequence (human mitochondrial transcription factor-binding site H). The results obtained suggest that in mitochondria PGC-1alpha and SIRT1 may function as their nuclear counterparts and represent the genuine factors mediating the cross-talk between nuclear and mitochondrial genome. Finally, this work adds new knowledge on the function of SIRT1 and PGC-1alpha and highlights the direct involvement of such proteins in regulation of mitochondrial biogenesis.

The causes of cancer revisited: "mitochondrial malignancy" and ROS-induced oncogenic transformation - why mitochondria are targets for cancer therapy

The role of oncoproteins and tumor suppressor proteins in promoting the malignant transformation of mammalian cells by affecting properties such as proliferative signalling, cell cycle regulation and altered adhesion is well established. Chemicals, viruses and radiation are also generally accepted as agents that commonly induce mutations in the genes encoding these cancer-causing proteins, thereby giving rise to cancer. However, more recent evidence indicates the importance of two additional key factors imposed on proliferating cells that are involved in transformation to malignancy and these are hypoxia and/or stressful conditions of nutrient deprivation (e.g. lack of glucose). These two additional triggers can initiate and promote the process of malignant transformation when a low percentage of cells overcome and escape cellular senescence. It is becoming apparent that hypoxia causes the progressive elevation in mitochondrial ROS production (chronic ROS) which over time leads to stabilization of cells via increased HIF-2alpha expression, enabling cells to survive with sustained levels of elevated ROS. In cells under hypoxia and/or low glucose, DNA mismatch repair processes are repressed by HIF-2alpha and they continually accumulate mitochondrial ROS-induced oxidative DNA damage and increasing numbers of mutations driving the malignant transformation process. Recent evidence also indicates that the resulting mutated cancer-causing proteins feedback to amplify the process by directly affecting mitochondrial function in combinatorial ways that intersect to play a major role in promoting a vicious spiral of malignant cell transformation. Consequently, many malignant processes involve periods of increased mitochondrial ROS production when a few cells survive the more common process of oxidative damage induced cell senescence and death. The few cells escaping elimination emerge with oncogenic mutations and survive to become immortalized tumors. This review focuses on evidence highlighting the role of mitochondria as drivers of elevated ROS production during malignant transformation and hence, their potential as targets for cancer therapy. The review is organized into five main sections concerning different aspects of "mitochondrial malignancy". The first concerns the functions of mitochondrial ROS and its importance as a pacesetter for cell growth versus senescence and death. The second considers the available evidence that cellular stress in the form of hypoxic and/or hypoglycaemic conditions represent two of the major triggering events for cancer and how oncoproteins reinforce this process by altering gene expression to bring about a common set of changes in mitochondrial function and activity in cancer cells. The third section presents evidence that oncoproteins and tumor suppressor proteins physically localize to the mitochondria in cancer cells where they directly regulate malignant mitochondrial programs, including apoptosis. The fourth section covers common mutational changes in the mitochondrial genome as they relate to malignancy and the relationship to the other three areas. The last section concerns the relevance of these findings, their importance and significance for novel targeted approaches to anti-cancer therapy and selective triggering in cancer cells of the mitochondrial apoptotic pathway.

AMPK-mediated regulation of transcription in skeletal muscle

Skeletal muscle phenotype plays a critical role in human performance and health, and skeletal muscle oxidative capacity is a key determinant of exercise tolerance. More recently, defective muscle oxidative metabolism has been implicated in a number of conditions associated with the metabolic syndrome, cardiovascular disease and muscle-wasting disorders. AMPK (AMP-activated protein kinase) is a critical regulator of cellular and organismal energy balance. AMPK has also emerged as a key regulator of skeletal muscle oxidative function, including metabolic enzyme expression, mitochondrial biogenesis and angiogenesis. AMPK mediates these processes primarily through alterations in gene expression. The present review examines the role of AMPK in skeletal muscle transcription and provides an overview of the known transcriptional substrates mediating the effects of AMPK on skeletal muscle phenotype.

Mitochondrial DNA content and expression of genes involved in mtDNA transcription, regulation and maintenance during human fetal development

The mitochondrial biogenesis and adequate energy production are important for fetal growth and early postnatal adaptation. The aim of the study was to characterize mitochondrial DNA (mtDNA) content and expression patterns of POLG, TFAM, NRF1,NRF2 and PGC1 family of regulated coactivators (PGC1A, PGC1B and PRC) involved in the mtDNA transcription, regulation and maintenance in human fetal tissues during second trimester of gestation. Further the mRNA expression profiles of selected cytochrome c oxidase (COX) subunits were analysed. Moreover enzyme activities of COX and CS and protein levels of COX subunits were analysed. DNA, RNA and proteins were isolated from 26 pairs of fetal liver and muscle samples obtained at autopsy after termination of pregnancy for genetic indications unrelated to OXPHOS deficiency between 13th and 28th week of gestation. This work offers a broad view on the mtDNA content changes in two different tissues during the second trimester of gestation and in the corresponding tissues after birth. The important differences in expression of POLG, TFAM, NRF2 genes and family PGC1 coactivators were found between the fetal tissues. The significant tissue-specific changes in expression of selected COX subunits on mRNA level (COX4 and MTCO2) were observed. Further the considerable differences in enzyme activities of COX and CS are demonstrated between fetal and postnatal phase. In conclusion our study indicates that the fetal developing tissues might differ in the control of mitochondrial biogenesis depending on their energy demand and the age of gestation. Moreover the gene expression is changed mainly on transcriptional level through fetal period.

Hepatic mitochondrial DNA depletion after an alcohol binge in mice: probable role of peroxynitrite and modulation by manganese superoxide dismutase

Alcohol consumption increases reactive oxygen species (ROS) formation, which can damage mitochondrial DNA (mtDNA) and alter mitochondrial function. To test whether manganese superoxide dismutase (MnSOD) modulates acute alcohol-induced mitochondrial alterations, transgenic MnSOD-overexpressing (MnSOD(+++)) mice, heterozygous knockout (MnSOD(+/-)) mice, and wild-type (WT) littermates were sacrificed 2 or 24 h after intragastric ethanol administration (5 g/kg). Alcohol administration further increased MnSOD activity in MnSOD(+++) mice, but further decreased it in MnSOD(+/-) mice. In WT mice, alcohol administration transiently increased mitochondrial ROS formation, decreased mitochondrial glutathione, depleted and damaged mtDNA, and decreased complex I and V activities; alcohol durably increased inducible nitric-oxide synthase (NOS) expression, plasma nitrites/nitrates, and the nitration of tyrosine residues in complex V proteins. These effects were prevented in MnSOD(+++) mice and prolonged in MnSOD(+/-) mice. In alcoholized WT or MnSOD(+/-) mice, mtDNA depletion and the nitration of tyrosine residues in complex I and V proteins were prevented or attenuated by cotreatment with tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl), a superoxide scavenger; N(omega)-nitro-l-arginine methyl ester and N-[3-(aminomethyl)benzyl]acetamidine (1,400W), two NOS inhibitors; or uric acid, a peroxynitrite scavenger. In conclusion, MnSOD overexpression prevents, and MnSOD deficiency prolongs, mtDNA depletion after an acute alcohol binge in mice. The protective effects of MnSOD, tempol, NOS inhibitors, and uric acid point out a role of the superoxide anion reacting with NO to form mtDNA-damaging peroxynitrite.

Parkinson's disease: Exit toxins, enter genetics

Parkinson's disease was long considered a non-hereditary disorder. Despite extensive research trying to find environmental risk factors for the disease, genetic variants now stand out as the major causative factor. Since a number of genes have been implicated in the pathogenesis it seems likely that several molecular pathways and downstream effectors can affect the trophic support and/or the survival of dopamine neurons, subsequently leading to Parkinson's disease. The present review describes how toxin-based animal models have been valuable tools in trying to find the underlying mechanisms of disease, and how identification of disease-linked genes in humans has led to the development of new transgenic rodent models. The review also describes the current status of the most common genetic susceptibility factors for Parkinson's disease identified up to today.