Uncouplers of oxidative phosphorylation

The bioenergetic landscape of cancer

BACKGROUND

Bioenergetic remodeling of core energy metabolism is essential to the initiation, survival, and progression of cancer cells through exergonic supply of adenosine triphosphate (ATP) and metabolic intermediates, as well as control of redox homeostasis. Mitochondria are evolutionarily conserved organelles that mediate cell survival by conferring energetic plasticity and adaptive potential. Mitochondrial ATP synthesis is coupled to the oxidation of a variety of substrates generated through diverse metabolic pathways. As such, inhibition of the mitochondrial bioenergetic system by restricting metabolite availability, direct inhibition of the respiratory Complexes, altering organelle structure, or coupling efficiency may restrict carcinogenic potential and cancer progression.

SCOPE OF REVIEW

Here, we review the role of bioenergetics as the principal conductor of energetic functions and carcinogenesis while highlighting the therapeutic potential of targeting mitochondrial functions.

MAJOR CONCLUSIONS

Mitochondrial bioenergetics significantly contribute to cancer initiation and survival. As a result, therapies designed to limit oxidative efficiency may reduce tumor burden and enhance the efficacy of currently available antineoplastic agents.

Deoxynivalenol triggers the expression of IL-8-related signaling cascades and decreases protein biosynthesis in primary monocyte-derived cells

Humans and their immune system are confronted with mold-contaminated food and/or mold-contaminated air in daily life and indoor activities. This results in metabolic stress and unspecific disease symptoms. Other studies provided evidence that exposure to mold is associated with the etiology of allergies. Deoxynivalenol (DON) is of great concern due to its frequent occurrence in toxically relevant concentrations. The exposure to this toxin is a permanent health risk for both humans and farm animals because DON cannot be significantly removed during standard milling and processing procedures. However, the direct effect on immunity or hematology is poorly defined because most investigations could not separate the effect of DON-contaminated feed intake. Due to the widespread distribution of DON after rapid absorption, it is not surprising that DON is known to affect the immune system. The immune system of the organism has one important function, to defend against the invasion of unknown substances/organisms. This study shows for the first time a synergistic effect of both-low physiological DON-doses in combination with low LPS-doses with the focus on the IL-8 expression on protein and RNA level. Both doses were found in vivo. IL-8 together with other anorectic cytokines like IL-1β can affect the food intake and anorexia. We could also show that a calcium-response is not involved in the increased IL-8 production after acute DON stimulation with high or low concentrations.

Pyrazinoic acid, the active form of the anti-tuberculosis drug pyrazinamide, and aromatic carboxylic acid analogs are protonophores

Pyrazinoic acid is the active form of pyrazinamide, a first-line antibiotic used to treat Mycobacterium tuberculosis infections. However, the mechanism of action of pyrazinoic acid remains a subject of debate, and alternatives to pyrazinamide in cases of resistance are not available. The work presented here demonstrates that pyrazinoic acid and known protonophores including salicylic acid, benzoic acid, and carbonyl cyanide m-chlorophenyl hydrazone all exhibit pH-dependent inhibition of mycobacterial growth activity over a physiologically relevant range of pH values. Other anti-tubercular drugs, including rifampin, isoniazid, bedaquiline, and p-aminosalicylic acid, do not exhibit similar pH-dependent growth-inhibitory activities. The growth inhibition curves of pyrazinoic, salicylic, benzoic, and picolinic acids, as well as carbonyl cyanide m-chlorophenyl hydrazone, all fit a quantitative structure-activity relationship (QSAR) derived from acid-base equilibria with R2 values > 0.95. The QSAR model indicates that growth inhibition relies solely on the concentration of the protonated forms of these weak acids (rather than the deprotonated forms). Moreover, pyrazinoic acid, salicylic acid, and carbonyl cyanide m-chlorophenyl hydrazone all caused acidification of the mycobacterial cytoplasm at concentrations that inhibit bacterial growth. Thus, it is concluded that pyrazinoic acid acts as an uncoupler of oxidative phosphorylation and that disruption of proton motive force is the primary mechanism of action of pyrazinoic acid rather than the inhibition of a classic enzyme activity.

Long-term super-resolution inner mitochondrial membrane imaging with a lipid probe

The inner mitochondrial membrane (IMM) generates power to drive cell function, and its dynamics control mitochondrial health and cellular homeostasis. Here, we describe the cell-permeant, lipid-like small molecule MAO-N3 and use it to assemble high-density environmentally sensitive (HIDE) probes that selectively label and image the IMM in live cells and multiple cell states. MAO-N3 pairs with strain-promoted azide-alkyne click chemistry-reactive fluorophores to support HIDE imaging using confocal, structured illumination, single-molecule localization and stimulated emission depletion microscopy, all with significantly improved resistance to photobleaching. These probes generate images with excellent spatial and temporal resolution, require no genetic manipulations, are non-toxic in model cell lines and primary cardiomyocytes (even under conditions that amplify the effects of mitochondrial toxins) and can visualize mitochondrial dynamics for 12.5 h. This probe will enable comprehensive studies of IMM dynamics with high temporal and spatial resolution.

Solid-Phase Compatible Silane-Based Cleavable Linker Enables Custom Isobaric Quantitative Chemoproteomics

Mass spectrometry-based chemoproteomics has emerged as an enabling technology for functional biology and drug discovery. To address limitations of established chemoproteomics workflows, including cumbersome reagent synthesis and low throughput sample preparation, here, we established the silane-based cleavable isotopically labeled proteomics (sCIP) method. The sCIP method is enabled by a high yielding and scalable route to dialkoxydiphenylsilane fluorenylmethyloxycarbonyl (DADPS-Fmoc)-protected amino acid building blocks, which enable the facile synthesis of customizable, isotopically labeled, and chemically cleavable biotin capture reagents. sCIP is compatible with both MS1- and MS2-based quantitation, and the sCIP-MS2 method is distinguished by its click-assembled isobaric tags in which the reporter group is encoded in the sCIP capture reagent and balancer in the pan cysteine-reactive probe. The sCIP-MS2 workflow streamlines sample preparation with early stage isobaric labeling and sample pooling, allowing for high coverage and increased sample throughput via customized low cost six-plex sample multiplexing. When paired with a custom FragPipe data analysis workflow and applied to cysteine-reactive fragment screens, sCIP proteomics revealed established and unprecedented cysteine-ligand pairs, including the discovery that mitochondrial uncoupling agent FCCP acts as a covalent-reversible cysteine-reactive electrophile.

Carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone-induced toxicities in rats: comparative study with other mitochondrial uncouplers (2,4-dinitrophenol, OPC-163493 and tolcapone)

FCCP (carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone) is known to inhibit oxidative phosphorylation as a protonophore, dissipating the proton gradient across the inner mitochondrial membrane. To understand the toxicity of FCCP, 3-day, 2- and 4-week repeated oral dose studies were performed in male rats. In the 3-day and 2-week repeated dose toxicity studies, observations included salivation, increased body temperature, and dead and moribund animals. Increased liver weight was observed in conjunction with hydropic degeneration and centrilobular necrosis of hepatocytes. In addition, pathological changes were observed in the pancreas, testis, epididymal duct, stomach and parotid gland. Electron microscopic examination revealed mitochondrial pleomorphism in the hepatocytes. Swelling of mitochondria was observed in the alpha cells and beta cells of the pancreas. Dilatation of rough endoplasmic reticulum, Golgi bodies and loss of secretory granules were also noted in the beta cells of the pancreas. FCCP was also compared with three other mUncouplers (DNP, OPC-163493 and tolcapone) with regard to in vitro mitochondrial uncoupling (mUncoupling) activities. FCCP produced the peak ΔOCR (oxygen consumption rate) at the lowest concentration (0.4 μM), followed by OPC-163493, tolcapone, and DNP, based on peak values in ascending order of concentration (2.5, 10, and 50 μM, respectively). Considering the relationship between the mUncoupling activity and toxicity profile of the four mUncouplers, there is no parallel relationship between the in vitro mUncoupling activity and the degree of in vivo toxicity. These findings may contribute to the efficient development of new mitochondrial uncoupler candidates.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-023-00189-x.

Deletion of GPR30 Drives the Activation of Mitochondrial Uncoupling Respiration to Induce Adipose Thermogenesis in Female Mice

Thermogenic adipocytes possess a promising approach to combat obesity with its capability promoting energy metabolism. We previously discovered that deletion of GPR30 (GPRKO), a presumably membrane-associated estrogen receptor, protected female mice from developing obesity, glucose intolerance, and insulin resistance when challenged with a high-fat diet (HFD). In vivo, the metabolic phenotype of wild type (WT) and GPRKO female mice were measured weekly. Acute cold tolerance test was performed. Ex vivo, mitochondrial respiration of brown adipose tissue (BAT) was analyzed from diet-induced obese female mice of both genotypes. In vitro, stromal vascular fractions (SVF) were isolated for beige adipocyte differentiation to investigate the role of GPR30 in thermogenic adipocyte. Deletion of GPR30 protects female mice from hypothermia and the mitochondria in BAT are highly energetic in GPRKO animals while the WT mitochondria remain in a relatively quiescent stage. Consistently, GPR30 deficiency enhances beige adipocyte differentiation in white adipose tissue (WAT) and activates the thermogenic browning of subcutaneous WAT due to up-regulation of UCP-1, which thereby protects female mice from HFD-induced obesity. GPR30 is a negative regulator of thermogenesis, which at least partially contributes to the reduced adiposity in the GPRKO female mice. Our findings provide insight into the mechanism by which GPR30 regulates fat metabolism and adiposity in female mice exposed to excess calories, which may be instrumental in the development of new therapeutic strategies for obesity.

Callyspongiolide kills cells by inducing mitochondrial dysfunction via cellular iron depletion

The highly cytotoxic marine natural product callyspongiolide holds great promise as a warhead of antibody-drug conjugate in cancer therapeutics; however, the mechanism underlying its cytotoxicity remains unclear. To elucidate how callyspongiolide kills cells, we employed label-free target identification with thermal stability-shift-based fluorescence difference in two-dimensional (2-D) gel electrophoresis (TS-FITGE), which allowed observation of a unique phenomenon of protein-spot separation on 2-D gels upon treatment with callyspongiolide at increasing temperatures. During our exploration of what proteins were associated with this phenomenon as well as why it happens, we found that callyspongiolide induces mitochondrial/lysosomal dysfunction and autophagy inhibition. Moreover, molecular biology studies revealed that callyspongiolide causes lysosomal dysfunction, which induces cellular iron depletion and leads to mitochondrial dysfunction and subsequent cytotoxicity. Notably, these effects were rescued through iron supplementation. Although our approach was unable to reveal the direct protein targets of callyspongiolide, unique phenomena observed only by TS-FITGE provided critical insight into the mechanism of action of callyspongiolide and specifically its cytotoxic activity via induction of mitochondrial dysfunction through cellular iron depletion caused by lysosomal deacidification, which occurred independent of known programmed cell death pathways.

In order to elucidate how callyspongiolide, a potent cytotoxic marine natural product, kills human lung cancer cells, Ha and Park employed TS-FITGE technique, a label-free target identification method with thermal stability-shift-based fluorescence difference in 2-D gel electrophoresis, allowing them to observe protein-spot separation upon treatment in increasing temperatures. They found that callyspongiolide induces lysosomal dysfunction followed by mitochondrial dysfunction as well as iron depletion, which sheds light on the mechanism of action of callyspongiolide.

Reduction of the New Tetrazolium Dye, Alamar Blue™, in Cultured Rat Hepatocytes and Liver Fractions

Alamar Blue™ (AB) is a new tetrazolium dye substrate that has been introduced as an alternative cell viability indicator. AB is reduced by intracellular reductases to a product which is exported from cells and can be quantified by fluorescent or spectrophotometric methods. We investigated the processes by which AB was reduced in liver cytosolic, microsomal or mitochondrial fractions and in cultured rat hepatocytes. AB reduction was catalysed by all liver fractions in an NADPH-dependent and NADH-dependent manner; the cytosolic fraction catalysed the highest rate of AB reduction. All of these activities were inhibited by dicoumarol (10μM), except AB reduction catalysed by NADH in mitochondrial fractions, which was resistant to the effects of dicoumarol and the metabolic inhibitors, but sensitive to inhibition by mercury (II) chloride. In hepatocyte cultures, AB reduction was stimulated by dicoumarol (10μM), menadione (10μM), rotenone (10μM), lactate (1–10mM) and fluoride (3-10mM). Potassium cyanide, ethanol and malonate had little effect. The results from this study suggest that AB is reduced in an NADPH-quinone oxidoreductase-dependent fashion, but that superoxide may also be involved in the reduction of AB. The modulation of AB reduction by lactate means that AB reduction may be modified by alterations in intermediary metabolism which are not a reflection of cell lethality. Therefore, great care should be exercised when using AB reduction as a viability indicator.

Effects of the Photosystem II Inhibitors CCCP and DCMU on Hydrogen Production by the Unicellular Halotolerant Cyanobacterium Aphanothece halophytica

The unicellular halotolerant cyanobacterium Aphanothece halophytica is a potential dark fermentative producer of molecular hydrogen (H₂) that produces very little H₂ under illumination. One factor limiting the H₂ photoproduction of this cyanobacterium is an inhibition of bidirectional hydrogenase activity by oxygen (O₂) obtained from splitting water molecules via photosystem II activity. The present study aimed to investigate the effects of the photosystem II inhibitors carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on H₂ production of A. halophytica under light and dark conditions and on photosynthetic and respiratory activities. The results showed that A. halophytica treated with CCCP and DCMU produced H₂ at three to five times the rate of untreated cells, when exposed to light. The highest H₂ photoproduction rates, 2.26 ± 0.24 and 3.63 ± 0.26  μmol H₂g⁻¹ dry weight h⁻¹, were found in cells treated with 0.5 μM CCCP and 50 μM DCMU, respectively. Without inhibitor treatment, A. halophytica incubated in the dark showed a significant increase in H₂ production compared with cells that were incubated in the light. Only CCCP treatment increased H₂ production of A. halophytica during dark incubation, because CCCP functions as an uncoupling agent of oxidative phosphorylation. The highest dark fermentative H₂ production rate of 39.50 ± 2.13  μmol H₂g⁻¹ dry weight h⁻¹ was found in cells treated with 0.5 μM CCCP after 2 h of dark incubation. Under illumination, CCCP and DCMU inhibited chlorophyll fluorescence, resulting in a low level of O₂, which promoted bidirectional hydrogenase activity in A. halophytica cells. In addition, only CCCP enhanced the respiration rate, further reducing the O₂ level. In contrast, DCMU reduced the respiration rate in A. halophytica.

Transient heat release during induced mitochondrial proton uncoupling

Non-shivering thermogenesis through mitochondrial proton uncoupling is one of the dominant thermoregulatory mechanisms crucial for normal cellular functions. The metabolic pathway for intracellular temperature rise has widely been considered as steady-state substrate oxidation. Here, we show that a transient proton motive force (pmf) dissipation is more dominant than steady-state substrate oxidation in stimulated thermogenesis. Using transient intracellular thermometry during stimulated proton uncoupling in neurons of Aplysia californica, we observe temperature spikes of ~7.5 K that decay over two time scales: a rapid decay of ~4.8 K over ~1 s followed by a slower decay over ~17 s. The rapid decay correlates well in time with transient electrical heating from proton transport across the mitochondrial inner membrane. Beyond ~33 s, we do not observe any heating from intracellular sources, including substrate oxidation and pmf dissipation. Our measurements demonstrate the utility of transient thermometry in better understanding the thermochemistry of mitochondrial metabolism.

Expression of the Alternative Oxidase Influences Jun N-Terminal Kinase Signaling and Cell Migration

Downregulation of Jun N-terminal kinase (JNK) signaling inhibits cell migration in diverse model systems. In Drosophila pupal development, attenuated JNK signaling in the thoracic dorsal epithelium leads to defective midline closure, resulting in cleft thorax. Here we report that concomitant expression of the Ciona intestinalis alternative oxidase (AOX) was able to compensate for JNK pathway downregulation, substantially correcting the cleft thorax phenotype. AOX expression also promoted wound-healing behavior and single-cell migration in immortalized mouse embryonic fibroblasts (iMEFs), counteracting the effect of JNK pathway inhibition. However, AOX was not able to rescue developmental phenotypes resulting from knockdown of the AP-1 transcription factor, the canonical target of JNK, nor its targets and had no effect on AP-1-dependent transcription. The migration of AOX-expressing iMEFs in the wound-healing assay was differentially stimulated by antimycin A, which redirects respiratory electron flow through AOX, altering the balance between mitochondrial ATP and heat production. Since other treatments affecting mitochondrial ATP did not stimulate wound healing, we propose increased mitochondrial heat production as the most likely primary mechanism of action of AOX in promoting cell migration in these various contexts.

Mitochondrial Transfer from Wharton's Jelly Mesenchymal Stem Cell to MERRF Cybrid Reduces Oxidative Stress and Improves Mitochondrial Bioenergetics

Myoclonus epilepsy associated with ragged-red fibers (MERRF) is a maternally inherited mitochondrial disease affecting neuromuscular functions. Mt.8344A>G mutation in mitochondrial DNA (mtDNA) is the most common cause of MERRF syndrome and has been linked to an increase in reactive oxygen species (ROS) level and oxidative stress, as well as impaired mitochondrial bioenergetics. Here, we tested whether WJMSC has therapeutic potential for the treatment of MERRF syndrome through the transfer of mitochondria. The MERRF cybrid cells exhibited a high mt.8344A>G mutation ratio, enhanced ROS level and oxidative damage, impaired mitochondrial bioenergetics, defected mitochondria-dependent viability, exhibited an imbalance of mitochondrial dynamics, and are susceptible to apoptotic stress. Coculture experiments revealed that mitochondria were intercellularly conducted from the WJMSC to the MERRF cybrid. Furthermore, WJMSC transferred mitochondria exclusively to cells with defective mitochondria but not to cells with normal mitochondria. MERRF cybrid following WJMSC coculture (MF+WJ) demonstrated improvement of mt.8344A>G mutation ratio, ROS level, oxidative damage, mitochondrial bioenergetics, mitochondria-dependent viability, balance of mitochondrial dynamics, and resistance against apoptotic stress. WJMSC-derived mitochondrial transfer and its therapeutic effect were noted to be blocked by F-actin depolymerizing agent cytochalasin B. Collectively, the WJMSC ability to rescue cells with defective mitochondrial function through donating healthy mitochondria may lead to new insights into the development of more efficient strategies to treat diseases related to mitochondrial dysfunction.

Optical visualisation of thermogenesis in stimulated single-cell brown adipocytes

The identification of brown adipose deposits in adults has led to significant interest in targeting this metabolically active tissue for treatment of obesity and diabetes. Improved methods for the direct measurement of heat production as the signature function of brown adipocytes (BAs), particularly at the single cell level, would be of substantial benefit to these ongoing efforts. Here, we report the first application of a small molecule-type thermosensitive fluorescent dye, ERthermAC, to monitor thermogenesis in BAs derived from murine brown fat precursors and in human brown fat cells differentiated from human neck brown preadipocytes. ERthermAC accumulated in the endoplasmic reticulum of BAs and displayed a marked change in fluorescence intensity in response to adrenergic stimulation of cells, which corresponded to temperature change. ERthermAC fluorescence intensity profiles were congruent with mitochondrial depolarisation events visualised by the JC-1 probe. Moreover, the averaged fluorescence intensity changes across a population of cells correlated well with dynamic changes such as thermal power, oxygen consumption, and extracellular acidification rates. These findings suggest ERthermAC as a promising new tool for studying thermogenic function in brown adipocytes of both murine and human origins.

Impaired oxidative phosphorylation regulates necroptosis in human lung epithelial cells

Cellular metabolism can impact cell life or death outcomes. While metabolic dysfunction has been linked to cell death, the mechanisms by which metabolic dysfunction regulates the cell death mode called necroptosis remain unclear. Our study demonstrates that mitochondrial oxidative phosphorylation (OXPHOS) activates programmed necrotic cell death (necroptosis) in human lung epithelial cells. Inhibition of mitochondrial respiration and ATP synthesis induced the phosphorylation of mixed lineage kinase domain-like protein (MLKL) and necroptotic cell death. Furthermore, we demonstrate that the activation of AMP-activated protein kinase (AMPK), resulting from impaired mitochondrial OXPHOS, regulates necroptotic cell death. These results suggest that impaired mitochondrial OXPHOS contributes to necroptosis in human lung epithelial cells.

Hypoxia Induces a Metabolic Shift and Enhances the Stemness and Expansion of Cochlear Spiral Ganglion Stem/Progenitor Cells

Previously, we demonstrated that hypoxia (1% O₂) enhances stemness markers and expands the cell numbers of cochlear stem/progenitor cells (SPCs). In this study, we further investigated the long-term effect of hypoxia on stemness and the bioenergetic status of cochlear spiral ganglion SPCs cultured at low oxygen tensions. Spiral ganglion SPCs were obtained from postnatal day 1 CBA/CaJ mouse pups. The measurement of oxygen consumption rate, extracellular acidification rate (ECAR), and intracellular adenosine triphosphate levels corresponding to 20% and 5% oxygen concentrations was determined using a Seahorse XF extracellular flux analyzer. After low oxygen tension cultivation for 21 days, the mean size of the hypoxia-expanded neurospheres was significantly increased at 5% O₂; this correlated with high-level expression of hypoxia-inducible factor-1 alpha (Hif-1α), proliferating cell nuclear antigen (PCNA), cyclin D1, Abcg2, nestin, and Nanog proteins but downregulated expression of p27 compared to that in a normoxic condition. Low oxygen tension cultivation tended to increase the side population fraction, with a significant difference found at 5% O₂ compared to that at 20% O₂. In addition, hypoxia induced a metabolic energy shift of SPCs toward higher basal ECARs and higher maximum mitochondrial respiratory capacity but lower proton leak than under normoxia, where the SPC metabolism was switched toward glycolysis in long-term hypoxic cultivation.

Weight loss by Ppc-1, a novel small molecule mitochondrial uncoupler derived from slime mold

Mitochondria play a key role in diverse processes including ATP synthesis and apoptosis. Mitochondrial function can be studied using inhibitors of respiration, and new agents are valuable for discovering novel mechanisms involved in mitochondrial regulation. Here, we screened small molecules derived from slime molds and other microorganisms for their effects on mitochondrial oxygen consumption. We identified Ppc-1 as a novel molecule which stimulates oxygen consumption without adverse effects on ATP production. The kinetic behavior of Ppc-1 suggests its function as a mitochondrial uncoupler. Serial administration of Ppc-1 into mice suppressed weight gain with no abnormal effects on liver or kidney tissues, and no evidence of tumor formation. Serum fatty acid levels were significantly elevated in mice treated with Ppc-1, while body fat content remained low. After a single administration, Ppc-1 distributes into various tissues of individual animals at low levels. Ppc-1 stimulates adipocytes in culture to release fatty acids, which might explain the elevated serum fatty acids in Ppc-1-treated mice. The results suggest that Ppc-1 is a unique mitochondrial regulator which will be a valuable tool for mitochondrial research as well as the development of new drugs to treat obesity.

Succinate dehydrogenase inhibition leads to epithelial-mesenchymal transition and reprogrammed carbon metabolism

BACKGROUND

Succinate dehydrogenase (SDH) is a mitochondrial metabolic enzyme complex involved in both the electron transport chain and the citric acid cycle. SDH mutations resulting in enzymatic dysfunction have been found to be a predisposing factor in various hereditary cancers. Therefore, SDH has been implicated as a tumor suppressor.

RESULTS

We identified that dysregulation of SDH components also occurs in serous ovarian cancer, particularly the SDH subunit SDHB. Targeted knockdown of Sdhb in mouse ovarian cancer cells resulted in enhanced proliferation and an epithelial-to-mesenchymal transition (EMT). Bioinformatics analysis revealed that decreased SDHB expression leads to a transcriptional upregulation of genes involved in metabolic networks affecting histone methylation. We confirmed that Sdhb knockdown leads to a hypermethylated epigenome that is sufficient to promote EMT. Metabolically, the loss of Sdhb resulted in reprogrammed carbon source utilization and mitochondrial dysfunction. This altered metabolic state of Sdhb knockdown cells rendered them hypersensitive to energy stress.

CONCLUSIONS

These data illustrate how SDH dysfunction alters the epigenetic and metabolic landscape in ovarian cancer. By analyzing the involvement of this enzyme in transcriptional and metabolic networks, we find a metabolic Achilles' heel that can be exploited therapeutically. Analyses of this type provide an understanding how specific perturbations in cancer metabolism may lead to novel anticancer strategies.

Mitochondrial dysfunction in some triple-negative breast cancer cell lines: role of mTOR pathway and therapeutic potential

INTRODUCTION

Triple-negative breast cancer (TNBC) is a subtype of highly malignant breast cancer with poor prognosis. TNBC is not amenable to endocrine therapy and often exhibit resistance to current chemotherapeutic agents, therefore, further understanding of the biological properties of these cancer cells and development of effective therapeutic approaches are urgently needed.

METHODS

We first investigated the metabolic alterations in TNBC cells in comparison with other subtypes of breast cancer cells using molecular and metabolic analyses. We further demonstrated that targeting these alterations using specific inhibitors and siRNA approach could render TNBC cells more sensitive to cell death compared to other breast cancer subtypes.

RESULTS

We found that TNBC cells compared to estrogen receptor (ER) positive cells possess special metabolic characteristics manifested by high glucose uptake, increased lactate production, and low mitochondrial respiration which is correlated with attenuation of mTOR pathway and decreased expression of p70S6K. Re-expression of p70S6K in TNBC cells reverses their glycolytic phenotype to an active oxidative phosphorylation (OXPHOS) state, while knockdown of p70S6K in ER positive cells leads to suppression of mitochondrial OXPHOS. Furthermore, lower OXPHOS activity in TNBC cells renders them highly dependent on glycolysis and the inhibition of glycolysis is highly effective in targeting TNBC cells despite their resistance to other anticancer agents.

CONCLUSIONS

Our study shows that TNBC cells have profound metabolic alterations characterized by decreased mitochondrial respiration and increased glycolysis. Due to their impaired mitochondrial function, TNBC cells are highly sensitive to glycolytic inhibition, suggesting that such metabolic intervention may be an effective therapeutic strategy for this subtype of breast cancer cells.

S-nitrosation of mitochondrial connexin 43 regulates mitochondrial function

S-nitrosation (SNO) of connexin 43 (Cx43)-formed channels modifies dye uptake in astrocytes and gap junctional communication in endothelial cells. Apart from forming channels at the plasma membrane of several cell types, Cx43 is also located at the inner membrane of myocardial subsarcolemmal mitochondria (SSM), but not in interfibrillar mitochondria (IFM). The absence or pharmacological blockade of mitochondrial Cx43 (mtCx43) reduces dye and potassium uptake. Lack of mtCx43 is associated with loss of endogenous cardioprotection by ischemic preconditioning (IPC), which is mediated by formation of reactive oxygen species (ROS). Whether or not mitochondrial Lucifer Yellow (LY), ion uptake, or ROS generation are affected by SNO of mtCx43 and whether or not cardioprotective interventions affect SNO of mtCx43 remains unknown. In SSM from rat hearts, application of NO donors (48 nmol to 1 mmol) increased LY uptake (0.5 mmol SNAP 38.4 ± 7.1 %, p < 0.05; 1 mmol GSNO 28.1 ± 7.4 %, p < 0.05) and the refilling rate of potassium (SNAP 227.9 ± 30.1 %, p < 0.05; GSNO 122.6 ± 28.1 %, p < 0.05). These effects were absent following blockade of Cx43 hemichannels by carbenoxolone as well as in IFM lacking Cx43. Unlike potassium, the sodium permeability was not affected by application of NO. Furthermore, mitochondrial ROS formation was increased following NO application compared to control SSM (0.5 mmol SNAP 22.9 ± 1.8 %, p < 0.05; 1 mmol GSNO 40.6 ± 7.1 %, p < 0.05), but decreased in NO treated IFM compared to control (0.5 mmol SNAP 14.4 ± 4 %, p < 0.05; 1 mmol GSNO 13.8 ± 4 %, p < 0.05). NO donor administration to isolated SSM increased SNO of mtCx43 by 109.2 ± 15.8 %. Nitrite application (48 nmol) to mice was also associated with elevated SNO of mtCx43 by 59.3 ± 18.2 % (p < 0.05). IPC by four cycles of 5 min of ischemia and 5 min of reperfusion increased SNO of mtCx43 by 41.6 ± 1.7 % (p < 0.05) when compared to control perfused rat hearts. These data suggest that SNO of mtCx43 increases mitochondrial permeability, especially for potassium and leads to increased ROS formation. The increased amount of SNO mtCx43 by IPC or nitrite administration may link NO and Cx43 in the signal transduction cascade of cardioprotective interventions.

Organic anion transporting polypeptides and organic cation transporter 1 contribute to the cellular uptake of the flavonoid quercetin

Flavonoids such as quercetin and kaempferol mediate several health protective effects, e.g., anticancer effects. They are inhibitors of organic anion transporting polypeptides (OATP) and organic cation transporters (e.g., OCT2). However, little is known whether such transporters contribute to the cellular uptake of flavonoids. Therefore, we investigated the cellular uptake of kaempferol and quercetin using HEK293 cell lines stably expressing different human OATPs or OCT1. Kaempferol was not a substrate of any of the investigated transporters (OATP1A2, OATP1B1, OATP1B3, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP5A1, and OCT1). Quercetin showed a significantly higher uptake into the HEK293-OATP1A2, HEK293-OATP2A1, HEK293-OATP2B1, and HEK293-OCT1 cells compared to control cells. The OATP1A2-, OATP2B1-, and OCT1-mediated quercetin uptake was inhibited by known inhibitors such as naringin, cyclosporin A, and quinidine, respectively. The cellular accumulation of quercetin into HEK293-OATP2A1 cells was not inhibited by prostaglandin E2 and diclofenac. The ionophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) reduced the net uptake of quercetin by increasing the uptake in the HEK293-control cells and causing no significant change in the HEK293-OATP2B1 cells indicating that quercetin follows the FCCP-driven proton flux through the plasma membrane. In addition to passive diffusion, the SLC transporters OATP1A2, OATP2B1, and OCT1 contribute to cellular accumulation of quercetin.

High-affinity glucose transport in Aspergillus nidulans is mediated by the products of two related but differentially expressed genes

Independent systems of high and low affinity effect glucose uptake in the filamentous fungus Aspergillus nidulans. Low-affinity uptake is known to be mediated by the product of the mstE gene. In the current work two genes, mstA and mstC, have been identified that encode high-affinity glucose transporter proteins. These proteins' primary structures share over 90% similarity, indicating that the corresponding genes share a common origin. Whilst the function of the paralogous proteins is little changed, they differ notably in their patterns of expression. The mstC gene is expressed during the early phases of germination and is subject to CreA-mediated carbon catabolite repression whereas mstA is expressed as a culture tends toward carbon starvation. In addition, various pieces of genetic evidence strongly support allelism of mstC and the previously described locus sorA. Overall, our data define MstC/SorA as a high-affinity glucose transporter expressed in germinating conidia, and MstA as a high-affinity glucose transporter that operates in vegetative hyphae under conditions of carbon limitation.

The bacterial secondary metabolite 2,4-diacetylphloroglucinol impairs mitochondrial function and affects calcium homeostasis in Neurospora crassa

The bacterial secondary metabolite 2,4-diacetylphloroglucinol (DAPG) is of interest as an active ingredient of biological control strains of Pseudomonas fluorescens and as a potential lead pharmaceutical molecule because of its capacity to inhibit growth of diverse microbial and non-microbial cells. The mechanism by which this occurs is unknown and in this study the filamentous fungus Neurospora crassa was used as a model to investigate the effects of DAPG on a eukaryotic cell. Colony growth, conidial germination and cell fusion assays confirmed the inhibitory nature of DAPG towards N. crassa. A number of different fluorescent dyes and fluorescent protein reporters were used to assess the effects of DAPG treatment on mitochondrial and other cellular functions. DAPG treatment led to changes in mitochondrial morphology, and rapid loss of mitochondrial membrane potential. These effects are likely to be responsible for the toxicity of DAPG. It was also found that DAPG treatment caused extracellular calcium to be taken up by conidial germlings leading to a transient increase in cytosolic free Ca(2+) with a distinct concentration dependent Ca(2+) signature.

Imaging of mitochondrial and non-mitochondrial responses in cultured rat hippocampal neurons exposed to micromolar concentrations of TMRM

Tetramethylrhodamine methyl ester (TMRM) is a fluorescent dye used to study mitochondrial function in living cells. Previously, we reported that TMRM effectively labeled mitochondria of neurons deep within mouse brain slices. Use of micromolar concentration of dye, which was required to get sufficient staining for two-photon imaging, resulted in typical fluctuations of TMRM. With prolonged exposure, we recorded additional responses in some neurons that included slow oscillations and propagating waves of fluorescence. (Note: We use the terms “fluctuation” to refer to a change in the fluorescent state of an individual mitochondrion, “oscillation” to refer to a localized change in fluorescence in the cytosol, and “wave” to refer to a change in cytosolic fluorescence that propagated within a cell. Use of these terms does not imply any underlying periodicity.) In this report we describe similar results using cultured rat hippocampal neurons. Prolonged exposure of cultures to 2.5 µM TMRM produced a spontaneous increase in fluorescence in some neurons, but not glial cells, after 45–60 minutes that was followed by slow oscillations, waves, and eventually apoptosis. Spontaneous increases in fluorescence were insensitive to high concentrations of FCCP (100 µM) and thapsigargin (10 µM) indicating that they originated, at least in part, from regions outside of mitochondria. The oscillations did not correlate with changes in intracellular Ca²⁺, but did correlate with differences in fluorescence lifetime of the dye. Fluorescence lifetime and one-photon ratiometric imaging of TMRM suggested that the spontaneous increase and subsequent oscillations were due to movement of dye between quenched (hydrophobic) and unquenched (hydrophilic) compartments. We propose that these movements may be correlates of intracellular events involved in early stages of apoptosis.

Sulindac sulfide inhibits sarcoendoplasmic reticulum Ca2+ ATPase, induces endoplasmic reticulum stress response, and exerts toxicity in glioma cells: relevant similarities to and important differences from celecoxib

Malignant gliomas have low survival expectations regardless of current treatments. Nonsteroidal anti-inflammatory drugs (NSAIDs) prevent cell transformation and slow cancer cell growth by mechanisms independent of cyclooxygenase (COX) inhibition. Certain NSAIDs trigger the endoplasmic reticulum stress response (ERSR), as revealed by upregulation of molecular chaperones such as GRP78 and C/EBP homologous protein (CHOP). Although celecoxib (CELE) inhibits the sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA), an effect known to induce ERSR, sulindac sulfide (SS) has not been reported to affect SERCA. Here, we investigated these two drugs for their effects on Ca(2+) homeostasis, ERSR, and glioma cell survival. Our findings indicate that SS is a reversible inhibitor of SERCA and that both SS and CELE bind SERCA at its cyclopiazonic acid binding site. Furthermore, CELE releases additional Ca(2+) from the mitochondria. In glioma cells, both NSAIDS upregulate GRP78 and activate ER-associated caspase-4 and caspase-3. Although only CELE upregulates the expression of CHOP, it appears that CHOP induction could be associated with mitochondrial poisoning. In addition, CHOP induction appears to be uncorrelated with the gliotoxicity of these NSAIDS in our experiments. Our data suggest that activation of ERSR is primarily responsible for the gliotoxic effect of these NSAIDS. Because SS has good brain bioavailability, has lower COX-2 inhibition, and has no mitochondrial effects, it represents a more appealing molecular candidate than CELE to achieve gliotoxicity via activation of ERSR.

N-terminally glutamate-substituted analogue of gramicidin A as protonophore and selective mitochondrial uncoupler

Limited uncoupling of oxidative phosphorylation could be beneficial for cells by preventing excessive generation of reactive oxygen species. Typical uncouplers are weak organic acids capable of permeating across membranes with a narrow gap between efficacy and toxicity. Aimed at designing a nontoxic uncoupler, the protonatable amino acid residue Glu was substituted for Val at the N-terminus of the pentadecapeptide gramicidin A (gA). The modified peptide [Glu1]gA exhibited high uncoupling activity in isolated mitochondria, in particular, abolishing membrane potential at the inner mitochondrial membrane with the same or even larger efficacy as gA. With mitochondria in cell culture, the depolarizing activity of [Glu1]gA was observed at concentrations by an order of magnitude lower than those of gA. On the contrary, [Glu1]gA was much less potent in forming proton channels in planar lipid bilayers than gA. Remarkably, at uncoupling concentrations, [Glu1]gA did not alter cell morphology and was nontoxic in MTT test, in contrast to gA showing high toxicity. The difference in the behavior of [Glu1]gA and gA in natural and artificial membranes could be ascribed to increased capability of [Glu1]gA to permeate through membranes and/or redistribute between different membranes. Based on the protective role of mild uncoupling, [Glu1]gA and some other proton-conducting gA analogues may be considered as prototypes of prospective therapeutic agents.

Mitochondrial oxidative stress and respiratory chain dysfunction account for liver toxicity during amiodarone but not dronedarone administration

The role played by oxidative stress in amiodarone-induced mitochondrial toxicity is debated. Dronedarone shows pharmacological properties similar to those of amiodarone but several differences in terms of toxicity. In this study, we analyzed the effects of the two drugs on liver mitochondrial function by administering an equivalent human dose to a rat model. Amiodarone increased mitochondrial H(2)O(2) synthesis, which in turn induced cardiolipin peroxidation. Moreover, amiodarone inhibited Complex I activity and uncoupled oxidative phosphorylation, leading to a reduction in the hepatic ATP content. We also observed a modification of membrane phospholipid composition after amiodarone administration. N-acetylcysteine completely prevented such effects. Although dronedarone shares with amiodarone the capacity to induce uncoupling of oxidative phosphorylation, it did not show any of the oxidative effects and did not impair mitochondrial bioenergetics. Our data provide important insights into the mechanism of mitochondrial toxicity induced by amiodarone. These results may greatly influence the clinical application and toxicity management of these two antiarrhythmic drugs.

UCP2 regulates energy metabolism and differentiation potential of human pluripotent stem cells

It has been assumed, based largely on morphologic evidence, that human pluripotent stem cells (hPSCs) contain underdeveloped, bioenergetically inactive mitochondria. In contrast, differentiated cells harbour a branched mitochondrial network with oxidative phosphorylation as the main energy source. A role for mitochondria in hPSC bioenergetics and in cell differentiation therefore remains uncertain. Here, we show that hPSCs have functional respiratory complexes that are able to consume O(2) at maximal capacity. Despite this, ATP generation in hPSCs is mainly by glycolysis and ATP is consumed by the F(1)F(0) ATP synthase to partially maintain hPSC mitochondrial membrane potential and cell viability. Uncoupling protein 2 (UCP2) plays a regulating role in hPSC energy metabolism by preventing mitochondrial glucose oxidation and facilitating glycolysis via a substrate shunting mechanism. With early differentiation, hPSC proliferation slows, energy metabolism decreases, and UCP2 is repressed, resulting in decreased glycolysis and maintained or increased mitochondrial glucose oxidation. Ectopic UCP2 expression perturbs this metabolic transition and impairs hPSC differentiation. Overall, hPSCs contain active mitochondria and require UCP2 repression for full differentiation potential.

Measuring mitochondrial function in intact cardiac myocytes

Mitochondria are involved in cellular functions that go beyond the traditional role of these organelles as the power plants of the cell. Mitochondria have been implicated in several human diseases, including cardiac dysfunction, and play a role in the aging process. Many aspects of our knowledge of mitochondria stem from studies performed on the isolated organelle. Their relative inaccessibility imposes experimental difficulties to study mitochondria in their natural environment-the cytosol of intact cells-and has hampered a comprehensive understanding of the plethora of mitochondrial functions. Here we review currently available methods to study mitochondrial function in intact cardiomyocytes. These methods primarily use different flavors of fluorescent dyes and genetically encoded fluorescent proteins in conjunction with high-resolution imaging techniques. We review methods to study mitochondrial morphology, mitochondrial membrane potential, Ca(2+) and Na(+) signaling, mitochondrial pH regulation, redox state and ROS production, NO signaling, oxygen consumption, ATP generation and the activity of the mitochondrial permeability transition pore. Where appropriate we complement this review on intact myocytes with seminal studies that were performed on isolated mitochondria, permeabilized cells, and in whole hearts.

A computational model of cytosolic and mitochondrial [ca] in paced rat ventricular myocytes

We carried out a series of experiment demonstrating the role of mitochondria in the cytosolic and mitochondrial Ca(2+) transients and compared the results with those from computer simulation. In rat ventricular myocytes, increasing the rate of stimulation (1~3 Hz) made both the diastolic and systolic [Ca(2+)] bigger in mitochondria as well as in cytosol. As L-type Ca(2+) channel has key influence on the amplitude of Ca(2+)-induced Ca(2+) release, the relation between stimulus frequency and the amplitude of Ca(2+) transients was examined under the low density (1/10 of control) of L-type Ca(2+) channel in model simulation, where the relation was reversed. In experiment, block of Ca(2+) uniporter on mitochondrial inner membrane significantly reduced the amplitude of mitochondrial Ca(2+) transients, while it failed to affect the cytosolic Ca(2+) transients. In computer simulation, the amplitude of cytosolic Ca(2+) transients was not affected by removal of Ca(2+) uniporter. The application of carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP) known as a protonophore on mitochondrial membrane to rat ventricular myocytes gradually increased the diastolic [Ca(2+)] in cytosol and eventually abolished the Ca(2+) transients, which was similarly reproduced in computer simulation. The model study suggests that the relative contribution of L-type Ca(2+) channel to total transsarcolemmal Ca(2+) flux could determine whether the cytosolic Ca(2+) transients become bigger or smaller with higher stimulus frequency. The present study also suggests that cytosolic Ca(2+) affects mitochondrial Ca(2+) in a beat-to-beat manner, however, removal of Ca(2+) influx mechanism into mitochondria does not affect the amplitude of cytosolic Ca(2+) transients.

Hydrogen Sulfide Oxidation Is Coupled to Oxidative Phosphorylation in Mitochondria of Solemya reidi

Solemya reidi, a gutless clam found in sulfide-rich habitats, contains within its gills bacterial symbionts thought to oxidize sulfur compounds and provide a reduced carbon food source to the clam. However, the initial step or steps in sulfide oxidation occur in the animal tissue, and mitochondria isolated from both gill and symbiont-free foot tissue of the clam coupled the oxidation of sulfide to oxidative phosphorylation [adenosine triphosphate (ATP) synthesis]. The ability of Solmya reidi to exploit directly the energy in sulfide for ATP synthesis is unprecedented, and suggests that sulfide-habitat animals that lack bacterial symbionts may also use sulfide as an inorganic energy source.

Localization and Characterization of the Carbon Tetrachloride Transformation Activity of Pseudomonas sp. Strain KC

Previous research has established that Pseudomonas sp. strain KC rapidly transforms carbon tetrachloride (CT) to carbon dioxide (45 to 55%), a nonvolatile fraction (45 to 55%), and a cell-associated fraction ((equiv)5%) under denitrifying, iron-limited conditions. The present study provides additional characterization of the nonvolatile fraction, demonstrates that electron transfer plays a role in the transformation, and establishes the importance of both extracellular and intracellular factors. Experiments with (sup14)C-labeled CT indicate that more than one nonvolatile product is produced during CT transformation by strain KC. One of these products, accounting for about 20% of the [(sup14)C]CT transformed, was identified as formate on the basis of its elution time from an ion-exchange column, its boiling point, and its conversion to (sup14)CO(inf2) when incubated with formate dehydrogenase. Production of formate requires transfer of two electrons to the CT molecule. The role of electron transfer was also supported by experiments demonstrating that stationary-phase cells that do not transform CT can be stimulated to transform CT when supplemented with acetate (electron donor), nitrate (electron acceptor), or a protonophore (carbonyl cyanide m-chlorophenylhydrazone). The location of transformation activity was also evaluated. By themselves, washed cells did not transform CT to a significant degree. Occasionally, CT transformation was observed by cell-free culture supernatant, but this activity was not reliable. Rapid and reliable CT transformation was only obtained when washed whole cells were reconstituted with culture supernatant, indicating that both extracellular and intracellular factors are normally required for CT transformation. Fractionation of culture supernatant by ultrafiltration established that the extracellular factor or factors are small, with an apparent molecular mass of less than 500 Da. The extracellular factor or factors were stable after lyophilization to powder and were extractable with acetone. Addition of micromolar levels of iron inhibited CT transformation in whole cultures, but the level of iron needed to inhibit CT transformation was over 100-fold higher for washed cells reconstituted with a 10,000-Da supernatant filtrate. Thus, the inhibitory effects of iron are exacerbated by a supernatant factor or factors with a molecular mass greater than 10,000 Da.

Slow non-specific accumulation of 2'-deoxy and 2'-O-methyl oligonucleotide probes at mitochondria in live cells

Molecular beacons (MBs) have the potential to provide a powerful tool for rapid RNA detection in living cells, as well as monitoring the dynamics of RNA expression in response to external stimuli. To exploit this potential, it is necessary to distinguish true signal from background signal due to non-specific interactions. Here, we show that, when cyanine-dye labeled 2'-deoxy and 2'-O-methyl oligonucleotide probes are inside living cells for >5 h, most of their signals co-localize with mitochondrial staining. These probes include random-sequence MB, dye-labeled single-strand linear oligonucleotide and dye-labeled double-stranded oligonucleotide. Using carbonyl cyanide m-chlorophenyl hydrazone treatment, we found that the non-specific accumulation of oligonucleotide probes at mitochondria was driven by mitochondrial membrane potential. We further demonstrated that the dye-labeled oligonucleotide probes were likely on/near the surface of mitochondria but not inside mitochondrial inner membrane. Interestingly, oligonucleotides probes labeled respectively with Alexa Fluor 488 and Alexa Fluor 546 did not accumulate at mitochondria, suggesting that the non-specific interaction between dye-labeled ODN probes and mitochondria is dye-specific. These results may help design and optimize fluorescence imaging probes for long-time RNA detection and monitoring in living cells.

A silybin-phospholipid complex prevents mitochondrial dysfunction in a rodent model of nonalcoholic steatohepatitis

Mitochondrial dysfunction and oxidative stress are determinant events in the pathogenesis of nonalcoholic steatohepatitis. Silybin has shown antioxidant, anti-inflammatory, and antifibrotic effects in chronic liver disease. We aimed to study the effect of the silybin-phospholipid complex (SILIPHOS) on liver redox balance and mitochondrial function in a dietary model of nonalcoholic steatohepatitis. To accomplish this, glutathione oxidation, mitochondrial oxygen uptake, proton leak, ATP homeostasis, and H(2)O(2) production rate were evaluated in isolated liver mitochondria from rats fed a methionine- and choline-deficient (MCD) diet and the MCD diet plus SILIPHOS for 7 and 14 weeks. Oxidative proteins, hydroxynonenal (HNE)- and malondialdehyde (MDA)-protein adducts, and mitochondrial membrane lipid composition were also measured. Treatment with SILIPHOS limited glutathione depletion and mitochondrial H(2)O(2) production. Moreover, SILIPHOS preserved mitochondrial bioenergetics and prevented mitochondrial proton leak and ATP reduction. Finally, SILIPHOS limited the formation of HNE- and MDA-protein adducts. In conclusion, SILIPHOS is effective in preventing severe oxidative stress and preserving hepatic mitochondrial bioenergetics in nonalcoholic steatohepatitis induced by the MCD diet. The modifications of mitochondrial membrane fatty acid composition induced by the MCD diet are partially prevented by SILIPHOS, conferring anti-inflammatory and antifibrotic effects. The increased vulnerability of lipid membranes to oxidative damage is limited by SILIPHOS through preserved mitochondrial function.

Ultrastructural changes to the tegumental system and the gastrodermal cells in adult Fasciola hepatica following in vivo treatment with the experimental fasciolicide, compound alpha

Sheep infected with the triclabendazole-susceptible, Cullompton isolate of Fasciola hepatica were dosed with 15 mg/kg of compound alpha at 12 weeks post-infection. Adult flukes were recovered from the bile ducts at 24, 48 and 72 h post-treatment (p.t.). Ultrastructural changes to the flukes were assessed using transmission electron microscopy (TEM), with a view to gathering information on the mechanism(s) of action for compound alpha and on the possible route of its entry into F. hepatica. The tegumental syncytium was more severely affected than the gut at all time-points p.t. with compound alpha, suggesting a predominantly trans-tegumental route of uptake. Disruption to the tegumental system became increasingly severe over time. A stress response was observed at 24 h p.t. and took the form of blebbing and increases in the production and transport of secretory bodies. By 72 h p.t., extensive tegumental loss and degeneration of the tegumental cell bodies had occurred. Degeneration of subtegumental tissues and internal flooding were also observed. Changes in the gastrodermal cells were slow to develop: reduced secretory activity was evident at 72 h p.t.. There was progressive disruption to the somatic muscle layers, with disorganization of the muscle blocks and loss of muscle fibres.

Alterations of hepatic ATP homeostasis and respiratory chain during development of non-alcoholic steatohepatitis in a rodent model

BACKGROUND

Mitochondrial dysfunction is considered a key player in non-alcoholic steatohepatitis (NASH) but no data are available on the mitochondrial function and ATP homeostasis in the liver during NASH progression. In the present paper we evaluated the hepatic mitochondrial respiratory chain activity and ATP synthesis in a rodent model of NASH development.

MATERIALS AND METHODS

Male Wistar rats fed a High Fat/Methionine-Choline Deficient (MCD) diet to induce NASH or a control diet (SHAM), and sacrificed after 3, 7 and 11 weeks. The oxidative phosphorylation, the F(0)F(1)ATPase (ATP synthase) and the ATP content were assessed in liver mitochondria.

RESULTS

NASH mitochondria exhibited an increased rate of substrate oxidation at 3 weeks, which returned to below the normal level at 7 and 11 weeks, concomitantly with the coupling between the phosphorylation activity and the mitochondrial respiration (ADP/O). Uncoupling of NASH liver mitochondria did not allow the recovery of the maximal respiration rate at 7 and 11 weeks. The ATPase (ATP synthase) activity was similar in NASH and SHAM rats, but the mitochondrial ATP content was significantly lower in NASH livers.

CONCLUSIONS

The loss of hepatic ATP stores is not dependent on the F(0)F(1)-ATPase but resides in the respiratory chain. Dysfunction of both Complex I and II of the mitochondrial respiratory chain during NASH development implies a mitochondrial adaptive mechanism occurring in the early stages of NASH.

In vitro-matured rat oocytes have low mitochondrial deoxyribonucleic acid and adenosine triphosphate contents and have abnormal mitochondrial redistribution

OBJECTIVE

To study the development and function of mitochondria in in vitro-matured rat oocytes derived from follicles of different sizes.

DESIGN

Experimental animal study.

SETTING

Department of Anatomy at the University of Hong Kong.

ANIMAL(S)

Immature female Sprague-Dawley rats that were 25 days of age.

INTERVENTION(S)

Immature oocytes were collected from rat ovarian follicles of different sizes and were induced to mature in vitro.

MAIN OUTCOME MEASURE(S)

The number of copies of mitochondrial DNA, mitochondrial activity, adenosine triphosphate content of matured oocytes, and rates of fertilization and blastulation were determined.

RESULT(S)

The mitochondrial DNA copy number of oocytes increased linearly with the diameter of antral follicles. The mitochondrial DNA copy number, adenosine triphosphate content, and proportion of oocytes with peripheral distribution of mitochondria in in vitro-matured oocytes from small antral follicles were significantly lower than those from preovulatory follicles and in vivo-matured oocytes. Compared with in vitro-matured oocytes from small antral follicles, those from preovulatory follicles and in vivo-matured oocytes also had significantly better fertilization potential and higher blastulation rate.

CONCLUSION(S)

The inferior developmental potential of in vitro-matured oocytes may be attributed partly to a reduced number of mitochondria, resulting in insufficient production of adenosine triphosphate for required developmental events.

Hymenolepis diminuta: mitochondrial transhydrogenase as an additional site for anaerobic phosphorylation

Employing adult Hymenolepis diminuta SMP and exogenous pyridine nucleotide-generating systems, reduced pyridine nucleotide-dependent net 32P incorporation into ATP was examined. NADH supported rotenone-sensitive 32P incorporation and this rate increased markedly with fumarate addition, in keeping with an active fumarate reductase. Interestingly, corresponding evaluations with NADPH did not result in detectable phosphorylation in the absence or presence of fumarate. However, with NAD addition, but without NAD generation, active NADPH-dependent phosphorylation occurred, thereby demonstrating mitochondrial transhydrogenase involvement, and 32P incorporation increased significantly with fumarate addition. More importantly, in the presence of rotenone and both NADPH and NAD generation, significant net 32P incorporation was noted, but was undetectable in the presence of DCCD or protonophores (e.g., niclosamide). Without NAD generation, minimal phosphorylation occurred. These data demonstrate that with ongoing NADPH and NAD generation, the H. diminuta, proton-translocating, mitochondrial transhydrogenase can serve as an additional anaerobic phosphorylation site. A model is presented.

Low mitochondrial DNA and ATP contents contribute to the absence of birefringent spindle imaged with PolScope in in vitro matured human oocytes

BACKGROUND

Birefrigent meiotic spindle in live human oocytes can be visualized by the PolScope. This study investigated the relationship between birefrigent meiotic spindle and cytoplasmic mitochondrial DNA (mtDNA) and ATP contents in in vitro matured human oocytes.

METHODS

Oocytes at germinal vesicle stage were collected and cultured for 24-48 h with or without the metabolic inhibitor, carbonyl cyanide p-(tri-fluromethoxy) phenyl-hydrazone (FCCP). All in vitro matured oocytes were examined by PolScope for the presence of meiotic spindle, then the oocytes were used for either intracytoplasmic sperm injection or the measurement of mitochondrial quantity and ATP content.

RESULTS

Meiotic spindles were observed in 51.3% (60/117) of the in vitro matured oocytes. Oocytes with detectable meiotic spindle contained significantly higher mtDNA copies (637 250 +/- 237 606 versus 491 454 +/- 153 406, P = 0.027) and ATP content (1.97 +/- 0.38 versus 1.65 +/- 0.32 pmol, P = 0.028) when compared with those without detectable meiotic spindle. However, in vitro matured oocytes showed a significantly reduced rate of positive meiotic spindle and a lower ATP content when cultured with FCCP. A lower incidence of normal fertilization and good quality embryos were observed if meiotic spindles were not detected.

CONCLUSIONS

Low mtDNA and ATP content might contribute to the absence of birefringent spindle imaged with the PolScope in human in vitro matured oocytes.

Catalysis of NADH-->NADP+ transhydrogenation by adult Hymenolepis diminuta mitochondria

Hymenolepis diminuta mitochondria catalyze nonenergy-linked and energy-linked NADH-->NADP(+) transhydrogenations, with the latter driven by electron-transport dependent NADH oxidation (electron transport-driven, ETD) or ATP hydrolysis (ATP-driven, ATPD). Using submitochondrial particles, NADH-->NADP(+) transhydrogenations were characterized further. ETD and ATPD reactions were enhanced by bovine serum albumin (BSA) and were inhibited by N,N'-dicyclohexylcarbodiimide (DCCD), carbonyl cyanide 3-chlorophenylhydrazone (CCCP), carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP), and niclosamide. The nonenergy-linked reaction was unaffected by these additives. Except for DCCD inhibition of the ATPD reaction, BSA mitigated inhibitor effects on energy-linked activities. BSA enhanced NADH oxidase (but not ATPase) activity. Although DCCD inhibited NADH oxidase and ATPase, BSA only lessened oxidase inhibition. With protonophores, an increase in NADH oxidase (but not ATPase) activity was suggested. Oxidase inhibition by rotenone was unaffected by BSA. The ATP-hydrolyzed/NADPH-formed for the ATPD reaction was almost unity. A model for H. diminuta energy-linked transhydrogenation is presented.

Role of mitochondrial Na+ concentration, measured by CoroNa red, in the protection of metabolically inhibited MDCK cells

In ischemic or hypoxic tissues, elevated cytosolic calcium levels can induce lethal processes. Mitochondria, besides the endoplasmic reticulum, play a key role in clearing excessive cytosolic Ca2+. In a previous study, it was suggested that the clearance of cytosolic Ca2+, after approximately 18 min of metabolic inhibition (MI) in renal epithelial cells, occurs via the reverse action of the mitochondrial Na+/Ca2+ exchanger (NCX). For further investigating the underlying mechanism, changes in the mitochondrial Na+ concentration ([Na+](m)) were monitored in metabolically inhibited MDCK cells. CoroNa Red, a sodium-sensitive fluorescence probe, was used to monitor [Na+]m. In the first 15 min of MI, a twofold increase of [Na+]m was observed reaching 113 +/- 7 mM, whereas the cytosolic Na+ concentration ([Na+]c) elevated threefold, to a level of 65 +/- 6 mM. In the next 45 min of MI, [Na+]m dropped to 91 +/- 7 mM, whereas [Na+]c further increased to 91 +/- 4 mM. The striking rise in [Na+]m is likely sufficient to sustain the driving force for mitochondrial Ca2+ uptake via the NCX. Furthermore, when CGP-37157, a specific inhibitor of the mitochondrial NCX, was applied during MI, the second-phase drop of [Na+]m was completely abolished. The obtained results support the hypothesis that the mitochondrial NCX reverses after approximately 15 min of MI. Moreover, because the cellular homeostasis can recover after MI, the mitochondria likely protect MDCK cells from injury during MI by the reversal of the mitochondrial NCX. This study is the first to report [Na+]m measurements in nonpermeabilized living cells.

Amiloride uptake and toxicity in fission yeast are caused by the pyridoxine transporter encoded by bsu1+ (car1+)

Amiloride, a diuretic drug that acts by inhibition of various sodium transporters, is toxic to the fission yeast Schizosaccharomyces pombe. Previous work has established that amiloride sensitivity is caused by expression of car1+, which encodes a protein with similarity to plasma membrane drug/proton antiporters from the multidrug resistance family. Here we isolated car1+ by complementation of Saccharomyces cerevisiae mutants that are deficient in pyridoxine biosynthesis and uptake. Our data show that Car1p represents a new high-affinity, plasma membrane-localized import carrier for pyridoxine, pyridoxal, and pyridoxamine. We therefore propose the gene name bsu1+ (for vitamin B6 uptake) to replace car1+. Bsu1p displays an acidic pH optimum and is inhibited by various protonophores, demonstrating that the protein works as a proton symporter. The expression of bsu1+ is associated with amiloride sensitivity and pyridoxine uptake in both S. cerevisiae and S. pombe cells. Moreover, amiloride acts as a competitor of pyridoxine uptake, demonstrating that both compounds are substrates of Bsu1p. Taken together, our data show that S. pombe and S. cerevisiae possess unrelated plasma membrane pyridoxine transporters. The S. pombe protein may be structurally related to the unknown human pyridoxine transporter, which is also inhibited by amiloride.

Impairment of mitochondrial oxidative phosphorylation in rat fatty liver exposed to preservation-reperfusion injury

BACKGROUND/AIMS

As the impairment of the cellular energy metabolism contributes to the failure of fatty liver grafts after transplantation, we aimed to determine whether steatosis affects the oxidative phosphorylation activity during preservation.

METHODS

Rat normal and fatty livers were preserved for 18 h and then reperfused with warm oxygenated solution. The oxidative phosphorylation, the F(0)F(1)-ATPase and the Complex I activities were assessed in isolated mitochondria before and after preservation, and during reperfusion. The ALT release and portal pressure were monitored during reperfusion.

RESULTS

The baseline phosphorylation activity was similar in normal and steatotic mitochondria. After cold preservation, the respiratory control index and state 3 respiration decreased significantly only in steatotic livers. Reperfusion induced a further deterioration in either group. Contrary to normal liver, uncoupling of fatty liver mitochondria allowed the recovery of the maximal respiration rate only using succinate (Complex II-dependent substrate), but not glutamate-malate (Complex I-dependent). Complex I dysfunction was confirmed spectrophotometrically. The ATPase activity was also significantly lower in fatty livers. Finally, ALT release and portal pressure were greater in steatotic livers.

CONCLUSIONS

The alteration of the oxidative phosphorylation activity during preservation is greatly exacerbated by fatty infiltration likely resulting from damage of the respiratory chain Complex I and of the F(0)F(1)-ATP synthase.

Yessotoxin, a shellfish biotoxin, is a potent inducer of the permeability transition in isolated mitochondria and intact cells

The diarrhetic poisoning by bivalve molluscs, diarrhetic shellfish poisoning, is due to consumption of mussels containing biotoxins produced by some Dinoflagellate species. Toxic effects of yessotoxin (YTX) include morphological alterations of mitochondria from heart and liver but the biochemical basis for these alterations is completely unknown. This paper demonstrates that YTX is a very powerful compound that opens the permeability transition pore (PTP) of the inner mitochondrial membrane of rat liver mitochondria at nanomolar concentrations. The effect requires the presence of a permissive level of calcium, by itself incapable of opening the pore. The direct effect of YTX on PTP is further confirmed by the inhibition exerted by cyclosporin A (CsA) that is known as a powerful inhibitor of PTP opening. Moreover, YTX induces membrane depolarization as shown by the quenching of tetramethylrhodamine methyl ester (TMRM), also prevented by the addition of CsA. YTX caused PTP opening in Morris Hepatoma 1C1 cells, as shown by the occurrence of CsA-sensitive depolarization within minutes of the addition of submicromolar concentrations of the toxin. These results provide a biochemical basis for the mitochondrial alterations observed in the course of intoxication with YTX, offering the first clue into the pathogenesis of diseases caused by YTX, and providing a novel tool to study the PTP in situ.