Alterations in bioenergetic function induced by Parkinson's disease mimetic compounds: lack of correlation with superoxide generation

Complex I superoxide anion production is necessary and sufficient for complex I inhibitor-induced dopaminergic neurodegeneration in Caenorhabditis elegans

Parkinson's Disease (PD) is the 2nd most prevalent neurodegenerative disease, but there is currently no cure and limited understanding of the pathogenesis resulting in dopaminergic neurodegeneration. Inhibitors of electron transport chain Complex I (CI) have long been associated with and are now used to model PD, but CI inhibition results in multiple effects including ATP depletion and reactive oxygen species (ROS) generation. The lack of tools to isolate effects of CI inhibition have rendered it difficult to determine which mechanistic step is critical for CI inhibitor-induced dopaminergic neurodegeneration. Here we report that CI-derived superoxide anion, not ATP depletion, is the critical driver of CI inhibitor-induced dopaminergic neurodegeneration in the model organism Caenorhabditis elegans. We first use SuperNova, a light-activated ROS-generating protein, fused to CI to demonstrate that in absence of enzymatic inhibition CI-localized ROS production is sufficient to drive morphological damage and loss of function of the dopaminergic neurons. Second, we prevented superoxide anion production during exposure to the CI inhibitors rotenone and pyridaben and report a full rescue of CI inhibitor-induced degeneration and functional loss, without rescue of inhibitor-induced ATP depletion. We highlight the importance of mitochondrial superoxide anion generation in the pathogenesis of PD and build a foundation for further definition of the pathways activated by mitochondrial ROS that led to neuronal dysfunction and death. Identification of these underlying mechanisms allows for future prevention of toxicant exposure-induced PD based on mechanistic knowledge.

An iridium(iii)-based photosensitizer disrupting the mitochondrial respiratory chain induces ferritinophagy-mediated immunogenic cell death

Cancer cells have a strategically optimized metabolism and tumor microenvironment for rapid proliferation and growth. Increasing research efforts have been focused on developing therapeutic agents that specifically target the metabolism of cancer cells. In this work, we prepared 1-methyl-4-phenylpyridinium-functionalized Ir(iii) complexes that selectively localize in the mitochondria and generate singlet oxygen and superoxide anion radicals upon two-photon irradiation. The generation of this oxidative stress leads to the disruption of the mitochondrial respiratory chain and therefore the disturbance of mitochondrial oxidative phosphorylation and glycolysis metabolisms, triggering cell death by combining immunogenic cell death and ferritinophagy. To the best of our knowledge, this latter is reported for the first time in the context of photodynamic therapy (PDT). To provide cancer selectivity, the best compound of this work was encapsulated within exosomes to form tumor-targeted nanoparticles. Treatment of the primary tumor of mice with two-photon irradiation (720 nm) 24 h after injection of the nanoparticles in the tail vein stops the primary tumor progression and almost completely inhibits the growth of distant tumors that were not irradiated. Our compound is a promising photosensitizer that efficiently disrupts the mitochondrial respiratory chain and induces ferritinophagy-mediated long-term immunotherapy.

Metabolic dysfunctions in the intranigral rotenone model of Parkinson's disease

Parkinson disease (PD) is a chronic neurodegenerative disorder characterized by a progressive loss of dopamine neurons in the substantia nigra pars compacta (SNpc). In the last years, a growing interest to study the relationship between metabolic dysfunction and neurodegenerative disease like PD has emerged. This study aimed to evaluate the occurrence of possible changes in metabolic homeostasis due to intranigral rotenone administration, a neurotoxin that damages dopaminergic neurons leading to motor impairments mimicking those that happen in PD. Male Wistar rats were distributed into two groups: sham (n = 10) or rotenone (n = 10). Sham group received, bilaterally, within the SNpc, 1 µL of vehicle dimethyl-sulfoxide (DMSO) and the experimental group was bilaterally injected with 1 µL of rotenone (12 µg/µL). Twenty-four hours after the stereotaxic surgeries, the animals underwent the open field test followed by subsequent peripheral blood and cerebrospinal fluid (CSF) samples collection for biochemical testing. The results showed that rotenone was able to replicate the typical motor behavior impairment seen in the disease, i.e., decrease in locomotion (P = 0.05) and increase in immobility (P = 0.01) with a strong correlation (r = - 0.85; P < 0.0001) between them. In addition, it was demonstrated that this model is able to decrease plasmatic total-cholesterol (P = 0.04) and HDL-cholesterol (P = 0.007) potentially impacting peripheral metabolism. Hence, it was revealed a potential ability to reproduce relevant metabolic dysfunctions like hyperglycemia which could be explained by acute and systemic mitochondrial rotenone toxicity and SNpc nigral toxicity. Such mechanisms may still be responsible for the potential occurrence of CSF-hyperglycemia (d = 0.7). Since intranigral rotenone is an early phase model of PD, the present results open a new road for studies aiming to investigate metabolic changes in PD.

Nanostars Carrying Multifunctional Neurotrophic Dendrimers Protect Neurons in Preclinical In Vitro Models of Neurodegenerative Disorders

A challenge in neurology is the lack of efficient brain-penetrable neuroprotectants targeting multiple disease mechanisms. Plasmonic gold nanostars are promising candidates to deliver standard-of-care drugs inside the brain but have not been trialed as carriers for neuroprotectants. Here, we conjugated custom-made peptide dendrimers (termed H3/H6), encompassing motifs of the neurotrophic S100A4-protein, onto star-shaped and spherical gold nanostructures (H3/H6-AuNS/AuNP) and evaluated their potential as neuroprotectants and interaction with neurons. The H3/H6 nanostructures crossed a model blood-brain barrier, bound to plasma membranes, and induced neuritogenesis with the AuNS, showing higher potency/efficacy than the AuNP. The H3-AuNS/NP protected neurons against oxidative stress, the H3-AuNS being more potent, and against Parkinson's or Alzheimer's disease (PD/AD)-related cytotoxicity. Unconjugated S100A4 motifs also decreased amyloid beta-induced neurodegeneration, introducing S100A4 as a player in AD. Using custom-made dendrimers coupled to star-shaped nanoparticles is a promising route to activate multiple neuroprotective pathways and increase drug potency to treat neurodegenerative disorders.

Plant Polyphenols as Neuroprotective Agents in Parkinson's Disease Targeting Oxidative Stress

Parkinson's disease (PD) is the second most prevalent progressive neurodegenerative disorder characterized by the degeneration of dopaminergic neurons in the human midbrain. Various ongoing research studies are competing to understand the pathology of PD and elucidate the mechanisms underlying neurodegeneration. Current pharmacological treatments primarily focused on improving dopamine metabolism in PD patients, despite the side effects of long-term usage. In recent years, it is recognized that oxidative stress-mediated pathways lead to neurodegeneration in the brain, which is associated with the pathophysiology of PD. The importance of oxidative stress is often less emphasized when developing potential therapeutic approaches. Natural plant antioxidants have been shown to mediate the oxidative stress-induced effects in PD, which has gained considerable attention in both in vitro and in vivo studies. Yet, clinical trials on natural polyphenol compounds are limited, restricting the potential use of these compounds as an alternative treatment for PD. Therefore, this review provides an understanding of the oxidative stress-induced effects in PD by elucidating the underlying events contributing to oxidative stress and explore the potential use of polyphenols in improving the oxidative status in PD. Preclinical findings have supported the potential of polyphenols in providing neuroprotection against oxidative stress-induced toxicity in PD. However, limiting factors, such as safety and bioavailability of polyphenols, warrant further investigations so as to make them the potential target for clinical applications in the treatment and management of PD.

Amauroderma rugosum Protects PC12 Cells against 6-OHDA-Induced Neurotoxicity through Antioxidant and Antiapoptotic Effects

Amauroderma rugosum (AR) is a dietary mushroom in the Ganodermataceae family whose pharmacological activity and medicinal value have rarely been reported. In this study, the antioxidant capacity and neuroprotective effects of AR were investigated. The aqueous extract of AR was confirmed to contain phenolic compounds, polysaccharides, and triterpenes. The results of 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) and total antioxidant capacity assays revealed that AR extract scavenged reactive oxygen species. Moreover, AR extract decreased the cytotoxicity, oxidative stress, mitochondrial dysfunction, and apoptosis of PC12 cells induced by 6-hydroxydopamine (6-OHDA). In addition, 6-OHDA upregulated the expressions of proapoptotic proteins and downregulated the Akt (protein kinase B)/mTOR- (mammalian target of rapamycin-) and MEK (mitogen-activated protein kinase kinase)/ERK- (extracellular signal-regulated kinases-) dependent signaling pathways. These effects of 6-OHDA were abolished or partially reversed by AR extract. Furthermore, the neuroprotective effects of AR in 6-OHDA-treated PC12 cells were significantly abolished by Akt and MEK inhibitor. Thus, AR extract possesses neuroprotective effects, probably through its antioxidant and antiapoptotic effects. These findings suggest the potential application of AR in the prevention or treatment of oxidative stress-related neurodegenerative diseases such as Parkinson's disease.

Multiple metabolic changes mediate the response of Caenorhabditis elegans to the complex I inhibitor rotenone

Rotenone, a mitochondrial complex I inhibitor, has been widely used to study the effects of mitochondrial dysfunction on dopaminergic neurons in the context of Parkinson's disease. Although the deleterious effects of rotenone are well documented, we found that young adult Caenorhabditis elegans showed resistance to 24 and 48 h rotenone exposures. To better understand the response to rotenone in C. elegans, we evaluated mitochondrial bioenergetic parameters after 24 and 48 h exposures to 1 μM or 5 μM rotenone. Results suggested upregulation of mitochondrial complexes II and V following rotenone exposure, without major changes in oxygen consumption or steady-state ATP levels after rotenone treatment at the tested concentrations. We found evidence that the glyoxylate pathway (an alternate pathway not present in higher metazoans) was induced by rotenone exposure; gene expression measurements showed increases in mRNA levels for two complex II subunits and for isocitrate lyase, the key glyoxylate pathway enzyme. Targeted metabolomics analyses showed alterations in the levels of organic acids, amino acids, and acylcarnitines, consistent with the metabolic restructuring of cellular bioenergetic pathways including activation of complex II, the glyoxylate pathway, glycolysis, and fatty acid oxidation. This expanded understanding of how C. elegans responds metabolically to complex I inhibition via multiple bioenergetic adaptations, including the glyoxylate pathway, will be useful in interrogating the effects of mitochondrial and bioenergetic stressors and toxicants.

Role of oxidative metabolism in osseointegration during spinal fusion

Spinal fusion is a commonly performed orthopedic surgery. Autologous bone graft obtained from the iliac crest is frequently employed to perform spinal fusion. Osteogenic bone marrow stromal (a.k.a. mesenchymal stem) cells (BMSCs) are believed to be responsible for new bone formation and development of the bridging bone during spinal fusion, as these cells are located in both the graft and at the site of fusion. Our previous work revealed the importance of mitochondrial oxidative metabolism in osteogenic differentiation of BMSCs. Our objective here was to determine the impact of BMSC oxidative metabolism on osseointegration of the graft during spinal fusion. The first part of the study was focused on correlating oxidative metabolism in bone graft BMSCs to radiographic outcomes of spinal fusion in human patients. The second part of the study was focused on mechanistically proving the role of BMSC oxidative metabolism in osseointegration during spinal fusion using a genetic mouse model. Patients’ iliac crest-derived graft BMSCs were identified by surface markers. Mitochondrial oxidative function was detected in BMSCs with the potentiometric probe, CMXRos. Spinal fusion radiographic outcomes, determined by the Lenke grade, were correlated to CMXRos signal in BMSCs. A genetic model of high oxidative metabolism, cyclophilin D knockout (CypD KO), was used to perform spinal fusion in mice. Graft osseointegration in mice was assessed with micro-computed tomography. Our study revealed that higher CMXRos signal in patients’ BMSCs correlated with a higher Lenke grade. Mice with higher oxidative metabolism (CypD KO) had greater mineralization of the spinal fusion bridge, as compared to the control mice. We therefore conclude that higher oxidative metabolism in BMSCs correlates with better spinal fusion outcomes in both human patients and in a mouse model. Altogether, our study suggests that promoting oxidative metabolism in osteogenic cells could improve spinal fusion outcomes for patients.

From cell lines to pluripotent stem cells for modelling Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder characterized by loss of dopaminergic (DAergic) neurons in the substantia nigra (SN) that contributes to the main motor symptoms of the disease. At present, even if several advancements have been done in the last decades, the molecular and cellular mechanisms involved in the pathogenesis are far to be fully understood. Accordingly, the establishment of reliable in vitro experimental models to investigate the early events of the pathogenesis represents a key issue in the field. However, to mimic and reproduce in vitro the complex neuronal circuitry involved in PD-associated degeneration of DAergic neurons still remains a highly challenging issue. Here we will review the in vitro PD models used in the last 25 years of research, ranging from cell lines, primary rat or mice neuronal cultures to the more recent use of human induced pluripotent stem cells (hiPSCs) and, finally, the development of 3D midbrain organoids.

Tetramethylpyrazine Analogue T-006 Exerts Neuroprotective Effects against 6-Hydroxydopamine-Induced Parkinson's Disease In Vitro and In Vivo

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), and there is no cure for it at present. We have previously reported that the tetramethylpyrazine (TMP) derivative T-006 exhibited beneficial effects in Alzheimer's disease (AD) models. However, its effect on PD remains unclear. In the present study, we investigated the neuroprotective effects and underlying mechanisms of T-006 against 6-hydroxydopamine- (6-OHDA-) induced lesions in in vivo and in vitro PD models. Our results demonstrated that T-006 alleviated mitochondrial membrane potential loss and restored the energy metabolism and mitochondrial biogenesis that were induced by 6-OHDA in PC12 cells. In addition, animal experiments showed that administration of T-006 significantly attenuated the 6-OHDA-induced loss of tyrosine hydroxylase- (TH-) positive neurons in the SNpc, as well as dopaminergic nerve fibers in the striatum, and also increased the concentration of dopamine and its metabolites (DOPAC, HVA) in the striatum. Functional deficits were restored following T-006 treatment in 6-OHDA-lesioned mice, as demonstrated by improved motor coordination and rotational behavior. In addition, we found that the neuroprotective effects of T-006 were mediated, at least in part, by the activation of both the PKA/Akt/GSK-3β and CREB/PGC-1α/NRF-1/TFAM pathways. In summary, our findings demonstrate that T-006 could be developed as a novel neuroprotective agent for PD, and the two pathways might be promising therapeutic targets for PD.

Intrinsic Effects of Gold Nanoparticles on Oxygen-Glucose Deprivation/Reperfusion Injury in Rat Cortical Neurons

This study aimed to investigate the potential effects of gold nanoparticles (Au-NPs) on rat cortical neurons exposed to oxygen-glucose deprivation/reperfusion (OGD/R) and to elucidate the corresponding mechanisms. Primary rat cortical neurons were exposed to OGD/R, which is commonly used in vitro to mimic ischemic injury, and then treated with 5- or 20-nm Au-NPs. We then evaluated cell viability, apoptosis, oxidative stress, and mitochondrial respiration in these neurons. We found that 20-nm Au-NPs increased cell viability, alleviated neuronal apoptosis and oxidative stress, and improved mitochondrial respiration after OGD/R injury, while opposite effects were observed for 5-nm Au-NPs. In terms of the underlying mechanisms, we found that Au-NPs could regulate Akt signaling. Taken together, these results show that 20-nm Au-NPs can protect primary cortical neurons against OGD/R injury, possibly by decreasing apoptosis and oxidative stress, while activating Akt signaling and mitochondrial pathways. Our results suggest that Au-NPs may be potential therapeutic agents for ischemic stroke.

Glycolysis-Derived Compounds From Astrocytes That Modulate Synaptic Communication

Based on the concept of the tripartite synapse, we have reviewed the role of glucose-derived compounds in glycolytic pathways in astroglial cells. Glucose provides energy and substrate replenishment for brain activity, such as glutamate and lipid synthesis. In addition, glucose metabolism in the astroglial cytoplasm results in products such as lactate, methylglyoxal, and glutathione, which modulate receptors and channels in neurons. Glucose has four potential destinations in neural cells, and it is possible to propose a crossroads in “X” that can be used to describe these four destinations. Glucose-6P can be used either for glycogen synthesis or the pentose phosphate pathway on the left and right arms of the X, respectively. Fructose-6P continues through the glycolysis pathway until pyruvate is formed but can also act as the initial compound in the hexosamine pathway, representing the left and right legs of the X, respectively. We describe each glucose destination and its regulation, indicating the products of these pathways and how they can affect synaptic communication. Extracellular L-lactate, either generated from glucose or from glycogen, binds to HCAR1, a specific receptor that is abundantly localized in perivascular and post-synaptic membranes and regulates synaptic plasticity. Methylglyoxal, a product of a deviation of glycolysis, and its derivative D-lactate are also released by astrocytes and bind to GABA_A receptors and HCAR1, respectively. Glutathione, in addition to its antioxidant role, also binds to ionotropic glutamate receptors in the synaptic cleft. Finally, we examined the hexosamine pathway and evaluated the effect of GlcNAc-modification on key proteins that regulate the other glucose destinations.

Methods for assessing mitochondrial quality control mechanisms and cellular consequences in cell culture

Mitochondrial quality is under surveillance by autophagy, the cell recycling process which degrades and removes damaged mitochondria. Inadequate autophagy results in deterioration in mitochondrial quality, bioenergetic dysfunction, and metabolic stress. Here we describe in an integrated work-flow to assess parameters of mitochondrial morphology, function, mtDNA and protein damage, metabolism and autophagy regulation to provide the framework for a practical assessment of mitochondrial quality. This protocol has been tested with cell cultures, is highly reproducible, and is adaptable to studies when cell numbers are limited, and thus will be of interest to researchers studying diverse physiological and pathological phenomena in which decreased mitochondrial quality is a contributory factor.

Rapamycin increases oxidative metabolism and enhances metabolic flexibility in human cardiac fibroblasts

Inhibition of mTOR signaling using rapamycin has been shown to increase lifespan and healthspan in multiple model organisms; however, the precise mechanisms for the beneficial effects of rapamycin remain uncertain. We have previously reported that rapamycin delays senescence in human cells and that enhanced mitochondrial biogenesis and protection from mitochondrial stress is one component of the benefit provided by rapamycin treatment. Here, using two models of senescence, replicative senescence and senescence induced by the presence of the Hutchinson-Gilford progeria lamin A mutation, we report that senescence is accompanied by elevated glycolysis and increased oxidative phosphorylation, which are both reduced by rapamycin. Measurements of mitochondrial function indicate that direct mitochondria targets of rapamycin are succinate dehydrogenase and matrix alanine aminotransferase. Elevated activity of these enzymes could be part of complex mechanisms that enable mitochondria to resume their optimal oxidative phosphorylation and resist senescence. This interpretation is supported by the fact that rapamycin-treated cultures do not undergo a premature senescence in response to the replacement of glucose with galactose in the culture medium, which forces a greater reliance on oxidative phosphorylation. Additionally, long-term treatment with rapamycin increases expression of the mitochondrial carrier protein UCP2, which facilitates the movement of metabolic intermediates across the mitochondrial membrane. The results suggest that rapamycin impacts mitochondrial function both through direct interaction with the mitochondria and through altered gene expression of mitochondrial carrier proteins.

Mitochondrial Complex I Reversible S-Nitrosation Improves Bioenergetics and Is Protective in Parkinson's Disease

AIMS

This study was designed to explore the neuroprotective potential of inorganic nitrite as a new therapeutic avenue in Parkinson's disease (PD).

RESULTS

Administration of inorganic nitrite ameliorates neuropathology in phylogenetically distinct animal models of PD. Beneficial effects are not confined to prophylactic treatment and also occur if nitrite is administered when the pathogenic cascade is already active. Mechanistically, the effect is mediated by both complex I S-nitrosation, which under nitrite administration is favored over formation of other forms of oxidation, and down-stream activation of the antioxidant Nrf2 pathway. Nitrite also rescues respiratory reserve capacity and increases proton leakage in LRRK2 PD patients' dermal fibroblasts.

INNOVATION

The study proposes an unprecedented approach based on the administration of the nitrosonium donor nitrite to contrast complex I and redox anomalies in PD. Dysfunctional mitochondrial complex I propagates oxidative stress in PD, and treatments mitigating this defect may, therefore, limit disease progression. Therapeutic complex I targeting has been successfully achieved in ischemia/reperfusion by using nitrosonium donors such as nitrite to reversibly modify its subunits and protect from oxidative damage after reperfusion. This evidence led to the innovative hypothesis that nitrite could exert protective effects also in pathological conditions where complex I dysfunction occurs in normoxia, such as in PD.

CONCLUSIONS

Overall, these results demonstrate that administration of inorganic nitrite improves mitochondrial function in PD, and it, therefore, represents an amenable intervention to hamper disease progression. Antioxid. Redox Signal. 28, 44-61.

Teaching the basics of reactive oxygen species and their relevance to cancer biology: Mitochondrial reactive oxygen species detection, redox signaling, and targeted therapies

Reactive oxygen species (ROS) have been implicated in tumorigenesis (tumor initiation, tumor progression, and metastasis). Of the many cellular sources of ROS generation, the mitochondria and the NADPH oxidase family of enzymes are possibly the most prevalent intracellular sources. In this article, we discuss the methodologies to detect mitochondria-derived superoxide and hydrogen peroxide using conventional probes as well as newly developed assays and probes, and the necessity of characterizing the diagnostic marker products with HPLC and LC-MS in order to rigorously identify the oxidizing species. The redox signaling roles of mitochondrial ROS, mitochondrial thiol peroxidases, and transcription factors in response to mitochondria-targeted drugs are highlighted. ROS generation and ROS detoxification in drug-resistant cancer cells and the relationship to metabolic reprogramming are discussed. Understanding the subtle role of ROS in redox signaling and in tumor proliferation, progression, and metastasis as well as the molecular and cellular mechanisms (e.g., autophagy) could help in the development of combination therapies. The paradoxical aspects of antioxidants in cancer treatment are highlighted in relation to the ROS mechanisms in normal and cancer cells. Finally, the potential uses of newly synthesized exomarker probes for in vivo superoxide and hydrogen peroxide detection and the low-temperature electron paramagnetic resonance technique for monitoring oxidant production in tumor tissues are discussed.

Mito-Apocynin Prevents Mitochondrial Dysfunction, Microglial Activation, Oxidative Damage, and Progressive Neurodegeneration in MitoPark Transgenic Mice

AIMS

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive motor deficits and degeneration of dopaminergic neurons. Caused by a number of genetic and environmental factors, mitochondrial dysfunction and oxidative stress play a role in neurodegeneration in PD. By selectively knocking out mitochondrial transcription factor A (TFAM) in dopaminergic neurons, the transgenic MitoPark mice recapitulate many signature features of the disease, including progressive motor deficits, neuronal loss, and protein inclusions. In the present study, we evaluated the neuroprotective efficacy of a novel mitochondrially targeted antioxidant, Mito-apocynin, in MitoPark mice and cell culture models of neuroinflammation and mitochondrial dysfunction.

RESULTS

Oral administration of Mito-apocynin (10 mg/kg, thrice a week) showed excellent central nervous system bioavailability and significantly improved locomotor activity and coordination in MitoPark mice. Importantly, Mito-apocynin also partially attenuated severe nigrostriatal degeneration in MitoPark mice. Mechanistic studies revealed that Mito-apo improves mitochondrial function and inhibits NOX2 activation, oxidative damage, and neuroinflammation.

INNOVATION

The properties of Mito-apocynin identified in the MitoPark transgenic mouse model strongly support potential clinical applications for Mito-apocynin as a viable neuroprotective and anti-neuroinflammatory drug for treating PD when compared to conventional therapeutic approaches.

CONCLUSION

Collectively, our data demonstrate, for the first time, that a novel orally active apocynin derivative improves behavioral, inflammatory, and neurodegenerative processes in a severe progressive dopaminergic neurodegenerative model of PD. Antioxid. Redox Signal. 27, 1048-1066.

A review of the basics of mitochondrial bioenergetics, metabolism, and related signaling pathways in cancer cells: Therapeutic targeting of tumor mitochondria with lipophilic cationic compounds

The present review is a sequel to the previous review on cancer metabolism published in this journal. This review focuses on the selective antiproliferative and cytotoxic effects of mitochondria-targeted therapeutics (MTTs) in cancer cells. Emerging research reveals a key role of mitochondrial respiration on tumor proliferation. Previously, a mitochondria-targeted nitroxide was shown to selectively inhibit colon cancer cell proliferation at submicromolar levels. This review is centered on the therapeutic use of MTTs and their bioenergetic profiling in cancer cells. Triphenylphosphonium cation conjugated to a parent molecule (e.g., vitamin-E or chromanol, ubiquinone, and metformin) via a linker alkyl chain is considered an MTT. MTTs selectively and potently inhibit proliferation of cancer cells and, in some cases, induce cytotoxicity. MTTs inhibit mitochondrial complex I activity and induce mitochondrial stress in cancer cells through generation of reactive oxygen species. MTTs in combination with glycolytic inhibitors synergistically inhibit tumor cell proliferation. This review discusses how signaling molecules traditionally linked to tumor cell proliferation affect tumor metabolism and bioenergetics (glycolysis, TCA cycle, and glutaminolysis).

Dieldrin-induced neurotoxicity involves impaired mitochondrial bioenergetics and an endoplasmic reticulum stress response in rat dopaminergic cells

Mitochondria are sensitive targets of environmental chemicals. Dieldrin (DLD) is an organochlorine pesticide that remains a human health concern due to high lipid bioaccumulation, and it has been epidemiologically associated to an increased risk for Parkinson's disease (PD). As mitochondrial dysfunction is involved in the etiology of PD, this study aimed to determine whether DLD impaired mitochondrial bioenergetics in dopaminergic cells. Rat immortalized dopaminergic N27 cells were treated for 24 or 48h with one dose of either a solvent control, 2.5, 25, or 250μM DLD. Dopaminergic cells treated with 250μM DLD showed increased Casp3/7 activity at 24 and 48h. DLD also caused a dose dependent reduction in cell viability of ∼25-30% over 24h. No significant effects on cell viability, apoptosis, nor cytotoxicity were detected at 24 or 48h with 2.5μM DLD. Following a 24h exposure to 2.5 and 25μM DLD, viable cells were subjected to a mitochondrial stress test using the Seahorse XFe24 Extracellular Flux Analyzer. Following three independent experiments conducted for rigor, dopaminergic cells that were treated with 2.5 and 25μM DLD consistently showed a reduction in maximum respiration and spare capacity compared to the control group. Molecular responses were measured to determine mechanisms of DLD-induced mitochondrial dysfunction. There were no changes in transcripts associated with mitochondrial membrane potential and permeability (e.g. Ant, Hk1, Tspo, Vdac), nor PI3 K/Akt/mTor signaling or mitochondrial-associated apoptotic factors (Bax, Bcl2, Casp3). However, transcript levels for Chop/Gadd153 (DNA Damage Inducible Transcript 3), an apoptotic gene activated following endoplasmic reticulum (ER) stress, were 3-fold higher in N27 cells treated with DLD, suggesting that DLD-induced mitochondrial dysfunction is related to ER stress. Dopamine cells were also assessed for changes in tyrosine hydroxylase (TH) protein, which did not differ among treatments. This study demonstrates that DLD impairs oxidative respiration in dopamine cells, and ER stress is hypothesized to be associated with the DLD-induced mitochondrial dysfunction. This is important as ER stress is also linked to PD. This study presents mechanistic insight into pesticide-induced mitochondrial dysfunction using a chemical that is reported to be associated to a higher risk for neurodegenerative disease.

Metabolic Dysfunction in Parkinson's Disease: Bioenergetics, Redox Homeostasis and Central Carbon Metabolism

The loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the accumulation of protein inclusions (Lewy bodies) are the pathological hallmarks of Parkinson's disease (PD). PD is triggered by genetic alterations, environmental/occupational exposures and aging. However, the exact molecular mechanisms linking these PD risk factors to neuronal dysfunction are still unclear. Alterations in redox homeostasis and bioenergetics (energy failure) are thought to be central components of neurodegeneration that contribute to the impairment of important homeostatic processes in dopaminergic cells such as protein quality control mechanisms, neurotransmitter release/metabolism, axonal transport of vesicles and cell survival. Importantly, both bioenergetics and redox homeostasis are coupled to neuro-glial central carbon metabolism. We and others have recently established a link between the alterations in central carbon metabolism induced by PD risk factors, redox homeostasis and bioenergetics and their contribution to the survival/death of dopaminergic cells. In this review, we focus on the link between metabolic dysfunction, energy failure and redox imbalance in PD, making an emphasis in the contribution of central carbon (glucose) metabolism. The evidence summarized here strongly supports the consideration of PD as a disorder of cell metabolism.

Redox-based therapeutics in neurodegenerative disease

This review describes recent developments in the search for effective therapeutic agents that target redox homeostasis in neurodegenerative disease. The disruption to thiol redox homeostasis in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis is discussed, together with the experimental strategies that are aimed at preventing, or at least minimizing, oxidative damage in these diseases. Particular attention is given to the potential of increasing antioxidant capacity by targeting the Nrf2 pathway, the development of inhibitors of NADPH oxidases that are likely candidates for clinical use, together with strategies to reduce nitrosative stress and mitochondrial dysfunction. We describe the shortcomings of compounds that hinder their progression to the clinic and evaluate likely avenues for future research.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Mitochondria-targeted metformins: anti-tumour and redox signalling mechanisms

Reports suggest that metformin exerts anti-cancer effects in diabetic individuals with pancreatic cancer. Thus, metformin is currently being repurposed as a potential drug in cancer treatment. Studies indicate that potent metformin analogues are required in cancer treatment because of the low bioavailability of metformin in humans at conventional antidiabetic doses. We proposed that improved mitochondrial targeting of metformin by attaching a positively charged lipophilic triphenylphosphonium group will result in a new class of mitochondria-targeted metformin analogues with significantly enhanced anti-tumour potential. Using this approach, we synthesized various mitochondria-targeted metformin analogues with different alkyl chain lengths. Results indicate that the antiproliferative effects increased with increasing alkyl chain lengths (100-fold to 1000-fold). The lead compound, mito-metformin₁₀, potently inhibited mitochondrial respiration through inhibition of complex I, stimulation of superoxide and hydrogen peroxide formation and activation of AMPK. When used in combination with ionizing radiation, mito-metformin₁₀ acted as a radiosensitizer of pancreatic cancer cells. Because of the 1000-fold-higher potency of mitochondria-targeted metformin₁₀, therapeutically effective plasma concentrations likely can be achieved in cancer patients.

Recent developments in detection of superoxide radical anion and hydrogen peroxide: Opportunities, challenges, and implications in redox signaling

In this review, some of the recent developments in probes and assay techniques specific for superoxide (O2-) and hydrogen peroxide (H2O2) are discussed. Over the last decade, significant progress has been made in O2- and H2O2 detection due to syntheses of new redox probes, better understanding of their chemistry, and development of specific and sensitive assays. For superoxide detection, hydroethidine (HE) is the most suitable probe, as the product, 2-hydroxyethidium, is specific for O2-. In addition, HE-derived dimeric products are specific for one-electron oxidants. As red-fluorescent ethidium is always formed from HE intracellularly, chromatographic techniques are required for detecting 2-hydroxyethidium. HE analogs, Mito-SOX and hydropropidine, exhibit the same reaction chemistry with O2- and one-electron oxidants. Thus, mitochondrial superoxide can be unequivocally detected using HPLC-based methods and not by fluorescence microscopy. Aromatic boronate-based probes react quantitatively with H2O2, forming a phenolic product. However, peroxynitrite and hypochlorite react more rapidly with boronates, forming the same product. Using ROS-specific probes and HPLC assays, it is possible to screen chemical libraries to discover specific inhibitors of NADPH oxidases. We hope that rigorous detection of O2- and H2O2 in different cellular compartments will improve our understanding of their role in redox signaling.

Mitochondria-regulated formation of endothelium-derived extracellular vesicles shifts the mediator of flow-induced vasodilation

To examine the effect of endothelium-derived extracellular vesicles (eEVs) on the mediator of flow-induced dilation (FID), composition, formation, and functional effects on the mediator of FID were examined from two different eEV subtypes, one produced from ceramide, while the other was produced from plasminogen-activator inhibitor 1 (PAI-1). Using video microscopy, we measured internal-diameter changes in response to increases in flow in human adipose resistance arteries acutely exposed (30 min) to eEVs derived from cultured endothelial cells exposed to ceramide or PAI-1. FID was significantly impaired following exposure to 500K/ml (K = 1,000) of ceramide-induced eEVs (Cer-eEVs) but unaffected by 250K/ml. FID was reduced in the presence of PEG-catalase following administration of 250K/ml of Cer-eEVs and PAI-1 eEVs, whereas N^ω-nitro-l-arginine methyl ester (l-NAME) had no effect. Pathway analysis following protein composition examination using liquid chromatography tandem mass spectrometry (LC-MS/MS) demonstrated that both subtypes were strongly linked to similar biological functions, primarily, mitochondrial dysfunction. Flow cytometry was used to quantify eEVs in the presence or absence of l-phenylalanine-4'-boronic acid (PBA) and mitochondria-targeted [93-boronophenyl)methyl]triphenyl-phosphonium (mito-PBA), cytosolic and mitochondrial-targeted antioxidants, respectively. eEV formation was significantly and dramatically reduced with mito-PBA treatment. In conclusion, eEVs have a biphasic effect, with higher doses impairing and lower doses shifting the mediator of FID from nitric oxide (NO) to hydrogen peroxide (H₂O₂). Despite differences in protein content, eEVs may alter vascular function in similar directions, regardless of the stimulus used for their formation. Furthermore, mitochondrial ROS production is required for the generation of these vesicles.NEW & NOTEWORTHY The vascular effect of endothelium-derived extracellular vesicles (eEVs) is biphasic, with higher doses decreasing the magnitude of flow-induced dilation (FID) compared with lower doses that shift the mediator of FID from nitric oxide to H₂O₂ eEVs may cause vascular dysfunction via similar pathways despite being formed from different stimuli, although both require mitochondrial reactive oxygen species for their formation.

Mitochondrial transfer of mesenchymal stem cells effectively protects corneal epithelial cells from mitochondrial damage

Recent studies have demonstrated that mesenchymal stem cells (MSCs) can donate mitochondria to airway epithelial cells and rescue mitochondrial damage in lung injury. We sought to determine whether MSCs could donate mitochondria and protect against oxidative stress-induced mitochondrial dysfunction in the cornea. Co-culturing of MSCs and corneal epithelial cells (CECs) indicated that the efficiency of mitochondrial transfer from MSCs to CECs was enhanced by Rotenone (Rot)-induced oxidative stress. The efficient mitochondrial transfer was associated with increased formation of tunneling nanotubes (TNTs) between MSCs and CECs, tubular connections that allowed direct intercellular communication. Separation of MSCs and CECs by a transwell culture system revealed no mitochiondrial transfer from MSCs to CECs and mitochondrial function was impaired when CECs were exposed to Rot challenge. CECs with or without mitochondrial transfer from MSCs displayed a distinct survival capacity and mitochondrial oxygen consumption rate. Mechanistically, increased filopodia outgrowth in CECs for TNT formation was associated with oxidative inflammation-activated NFκB/TNFαip2 signaling pathways that could be attenuated by reactive oxygen species scavenger N-acetylcysteine (NAC) treatment. Furthermore, MSCs grown on a decellularized porcine corneal scaffold were transplanted onto an alkali-injured eye in a rabbit model. Enhanced corneal wound healing was evident following healthy MSC scaffold transplantation. And transferred mitochondria was detected in corneal epithelium. In conclusion, mitochondrial transfer from MSCs provides novel protection for the cornea against oxidative stress-induced mitochondrial damage. This therapeutic strategy may prove relevant for a broad range of mitochondrial diseases.

Mitochondria-Targeted Analogues of Metformin Exhibit Enhanced Antiproliferative and Radiosensitizing Effects in Pancreatic Cancer Cells

Metformin (Met) is an approved antidiabetic drug currently being explored for repurposing in cancer treatment based on recent evidence of its apparent chemopreventive properties. Met is weakly cationic and targets the mitochondria to induce cytotoxic effects in tumor cells, albeit not very effectively. We hypothesized that increasing its mitochondria-targeting potential by attaching a positively charged lipophilic substituent would enhance the antitumor activity of Met. In pursuit of this question, we synthesized a set of mitochondria-targeted Met analogues (Mito-Mets) with varying alkyl chain lengths containing a triphenylphosphonium cation (TPP(+)). In particular, the analogue Mito-Met10, synthesized by attaching TPP(+) to Met via a 10-carbon aliphatic side chain, was nearly 1,000 times more efficacious than Met at inhibiting cell proliferation in pancreatic ductal adenocarcinoma (PDAC). Notably, in PDAC cells, Mito-Met10 potently inhibited mitochondrial complex I, stimulating superoxide and AMPK activation, but had no effect in nontransformed control cells. Moreover, Mito-Met10 potently triggered G1 cell-cycle phase arrest in PDAC cells, enhanced their radiosensitivity, and more potently abrogated PDAC growth in preclinical mouse models, compared with Met. Collectively, our findings show how improving the mitochondrial targeting of Met enhances its anticancer activities, including aggressive cancers like PDAC in great need of more effective therapeutic options. Cancer Res; 76(13); 3904-15. ©2016 AACR.

Ablation of the mitochondrial complex IV assembly protein Surf1 leads to increased expression of the UPR(MT) and increased resistance to oxidative stress in primary cultures of fibroblasts

Mice deficient in the electron transport chain (ETC) complex IV assembly protein SURF1 have reduced assembly and activity of cytochrome c oxidase that is associated with an upregulation of components of the mitochondrial unfolded protein response (UPR^MT) and increased mitochondrial number. We hypothesized that the upregulation of proteins associated with the UPR^MT in response to reduced cytochrome c oxidase activity in Surf1^−/− mice might contribute to increased stress resistance. To test this hypothesis we asked whether primary cultures of fibroblasts from Surf1^−/− mice exhibit enhanced resistance to stressors compared to wild-type fibroblasts. Here we show that primary dermal fibroblasts isolated from Surf1^−/− mice have increased expression of UPR^MT components ClpP and Hsp60, and increased expression of Lon protease. Fibroblasts from Surf1^−/− mice are significantly more resistant to cell death caused by oxidative stress induced by paraquat or tert-Butyl hydroperoxide compared to cells from wild-type mice. In contrast, Surf1^−/− fibroblasts show no difference in sensitivity to hydrogen peroxide stress. The enhanced cell survival in response to paraquat or tert-Butyl hydroperoxide in Surf1^−/− fibroblasts compared to wild-type fibroblasts is associated with induced expression of Lon, ClpP, and Hsp60, increased maximal respiration, and increased reserve capacity as measured using the Seahorse Extracellular Flux Analyzer. Overall these data support a protective role for the activation of the UPR^MT in cell survival.

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Surf1^−/− mice fibroblasts exhibit upregulation of proteins involved in the UPR^MT.

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Mitochondrial specific oxidative stressors induce UPR^MT in mammalian fibroblasts.

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Surf1^−/− fibroblasts exhibit enhanced mitochondrial specific stress resistance.

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Surf1^−/− fibroblasts have increased maximal respiration and respiratory reserve.

Parkinsonian toxin-induced oxidative stress inhibits basal autophagy in astrocytes via NQO2/quinone oxidoreductase 2: Implications for neuroprotection

Oxidative stress (OS) stimulates autophagy in different cellular systems, but it remains controversial if this rule can be generalized. We have analyzed the effect of chronic OS induced by the parkinsonian toxin paraquat (PQ) on autophagy in astrocytoma cells and primary astrocytes, which represent the first cellular target of neurotoxins in the brain. PQ decreased the basal levels of LC3-II and LC3-positive vesicles, and its colocalization with lysosomal markers, both in the absence and presence of chloroquine. This was paralleled by increased number and size of SQSTM1/p62 aggregates. Downregulation of autophagy was also observed in cells chronically exposed to hydrogen peroxide or nonlethal concentrations of PQ, and it was associated with a reduced astrocyte capability to protect dopaminergic cells from OS in co-cultures. Surprisingly, PQ treatment led to inhibition of MTOR, activation of MAPK8/JNK1 and MAPK1/ERK2-MAPK3/ERK1 and upregulation of BECN1/Beclin 1 expression, all signals typically correlating with induction of autophagy. Reduction of OS by NMDPEF, a specific NQO2 inhibitor, but not by N-acetylcysteine, abrogated the inhibitory effect of PQ and restored autophagic flux. Activation of NQO2 by PQ or menadione and genetic manipulation of its expression confirmed the role of this enzyme in the inhibitory action of PQ on autophagy. PQ did not induce NFE2L2/NRF2, but when it was co-administered with NMDPEF NFE2L2 activity was enhanced in a SQSTM1-independent fashion. Thus, a prolonged OS in astrocytes inhibits LC3 lipidation and impairs autophagosome formation and autophagic flux, in spite of concomitant activation of several pro-autophagic signals. These findings outline an unanticipated neuroprotective role of astrocyte autophagy and identify in NQO2 a novel pharmacological target for its positive modulation.

NADPH oxidase promotes Parkinsonian phenotypes by impairing autophagic flux in an mTORC1-independent fashion in a cellular model of Parkinson's disease

Oxidative stress and aberrant accumulation of misfolded proteins in the cytosol are key pathological features associated with Parkinson's disease (PD). NADPH oxidase (Nox2) is upregulated in the pathogenesis of PD; however, the underlying mechanism(s) of Nox2-mediated oxidative stress in PD pathogenesis are still unknown. Using a rotenone-inducible cellular model of PD, we observed that a short exposure to rotenone (0.5 μM) resulted in impaired autophagic flux through activation of a Nox2 dependent Src/PI3K/Akt axis, with a consequent disruption of a Beclin1-VPS34 interaction that was independent of mTORC1 activity. Sustained exposure to rotenone at a higher dose (10 μM) decreased mTORC1 activity; however, autophagic flux was still impaired due to dysregulation of lysosomal activity with subsequent induction of the apoptotic machinery. Cumulatively, our results highlight a complex pathogenic mechanism for PD where short- and long-term oxidative stress alters different signaling pathways, ultimately resulting in anomalous autophagic activity and disease phenotype. Inhibition of Nox2-dependent oxidative stress attenuated the impaired autophagy and cell death, highlighting the importance and therapeutic potential of these pathways for treating patients with PD.

Alterations in mitochondrial dynamics induced by tebufenpyrad and pyridaben in a dopaminergic neuronal cell culture model

Tebufenpyrad and pyridaben are two agro-chemically important acaricides that function like the known mitochondrial toxicant rotenone. Although these two compounds have been commonly used to kill populations of mites and ticks in commercial greenhouses, their neurotoxic profiles remain largely unknown. Therefore, we investigated the effects of these two pesticides on mitochondrial structure and function in an in vitro cell culture model using the Seahorse bioanalyzer and confocal fluorescence imaging. The effects were compared with rotenone. Exposing rat dopaminergic neuronal cells (N27 cells) to tebufenpyrad and pyridaben for 3h induced dose-dependent cell death with an EC50 of 3.98μM and 3.77μM, respectively. Also, tebufenpyrad and pyridaben (3μM) exposure induced reactive oxygen species (ROS) generation and m-aconitase damage, suggesting that the pesticide toxicity is associated with oxidative damage. Morphometric image analysis with the MitoTracker red fluorescent probe indicated that tebufenpyrad and pyridaben, as well as rotenone, caused abnormalities in mitochondrial morphology, including reduced mitochondrial length and circularity. Functional bioenergetic experiments using the Seahorse XF96 analyzer revealed that tebufenpyrad and pyridaben very rapidly suppressed the basal mitochondrial oxygen consumption rate similar to that of rotenone. Further analysis of bioenergetic curves also revealed dose-dependent decreases in ATP-linked respiration and respiratory capacity. The luminescence-based ATP measurement further confirmed that pesticide-induced mitochondrial inhibition of respiration is accompanied by the loss of cellular ATP. Collectively, our results suggest that exposure to the pesticides tebufenpyrad and pyridaben induces neurotoxicity by rapidly initiating mitochondrial dysfunction and oxidative damage in dopaminergic neuronal cells. Our findings also reveal that monitoring the kinetics of mitochondrial respiration with Seahorse could be used as an early neurotoxicological high-throughput index for assessing the risk that pesticides pose to the dopaminergic neuronal system.

Antiproliferative effects of mitochondria-targeted cationic antioxidants and analogs: Role of mitochondrial bioenergetics and energy-sensing mechanism

One of the proposed mechanisms for tumor proliferation involves redox signaling mediated by reactive oxygen species such as superoxide and hydrogen peroxide generated at moderate levels. Thus, the antiproliferative and anti-tumor effects of certain antioxidants were attributed to their ability to mitigate intracellular reactive oxygen species (ROS). Recent reports support a role for mitochondrial ROS in stimulating tumor cell proliferation. In this study, we compared the antiproliferative effects and the effects on mitochondrial bioenergetic functions of a mitochondria-targeted cationic carboxyproxyl nitroxide (Mito-CP), exhibiting superoxide dismutase (SOD)-like activity and a synthetic cationic acetamide analog (Mito-CP-Ac) lacking the nitroxide moiety responsible for the SOD activity. Results indicate that both Mito-CP and Mito-CP-Ac potently inhibited tumor cell proliferation. Both compounds altered mitochondrial and glycolytic functions, and intracellular citrate levels. Both Mito-CP and Mito-CP-Ac synergized with 2-deoxy-glucose (2-DG) to deplete intracellular ATP, inhibit cell proliferation and induce apoptosis in pancreatic cancer cells. We conclude that mitochondria-targeted cationic agents inhibit tumor proliferation via modification of mitochondrial bioenergetics pathways rather than by dismutating and detoxifying mitochondrial superoxide.

Alterations in energy/redox metabolism induced by mitochondrial and environmental toxins: a specific role for glucose-6-phosphate-dehydrogenase and the pentose phosphate pathway in paraquat toxicity

Parkinson’s disease (PD) is a multifactorial disorder with a complex etiology including genetic risk factors, environmental exposures, and aging. While energy failure and oxidative stress have largely been associated with the loss of dopaminergic cells in PD and the toxicity induced by mitochondrial/environmental toxins, very little is known regarding the alterations in energy metabolism associated with mitochondrial dysfunction and their causative role in cell death progression. In this study, we investigated the alterations in the energy/redox-metabolome in dopaminergic cells exposed to environmental/mitochondrial toxins (paraquat, rotenone, 1-methyl-4-phenylpyridinium [MPP⁺], and 6-hydroxydopamine [6-OHDA]) in order to identify common and/or different mechanisms of toxicity. A combined metabolomics approach using nuclear magnetic resonance (NMR) and direct-infusion electrospray ionization mass spectrometry (DI-ESI-MS) was used to identify unique metabolic profile changes in response to these neurotoxins. Paraquat exposure induced the most profound alterations in the pentose phosphate pathway (PPP) metabolome. ¹³C-glucose flux analysis corroborated that PPP metabolites such as glucose-6-phosphate, fructose-6-phosphate, glucono-1,5-lactone, and erythrose-4-phosphate were increased by paraquat treatment, which was paralleled by inhibition of glycolysis and the TCA cycle. Proteomic analysis also found an increase in the expression of glucose-6-phosphate dehydrogenase (G6PD), which supplies reducing equivalents by regenerating nicotinamide adenine dinucleotide phosphate (NADPH) levels. Overexpression of G6PD selectively increased paraquat toxicity, while its inhibition with 6-aminonicotinamide inhibited paraquat-induced oxidative stress and cell death. These results suggest that paraquat “hijacks” the PPP to increase NADPH reducing equivalents and stimulate paraquat redox cycling, oxidative stress, and cell death. Our study clearly demonstrates that alterations in energy metabolism, which are specific for distinct mitochondiral/environmental toxins, are not bystanders to energy failure but also contribute significant to cell death progression.

Profiling and targeting of cellular bioenergetics: inhibition of pancreatic cancer cell proliferation

BACKGROUND

Targeting both mitochondrial bioenergetics and glycolysis pathway is an effective way to inhibit proliferation of tumour cells, including those that are resistant to conventional chemotherapeutics.

METHODS

In this study, using the Seahorse 96-well Extracellular Flux Analyzer, we mapped the two intrinsic cellular bioenergetic parameters, oxygen consumption rate and proton production rate in six different pancreatic cancer cell lines and determined their differential sensitivity to mitochondrial and glycolytic inhibitors.

RESULTS

There exists a very close relationship among intracellular bioenergetic parameters, depletion of ATP and anti-proliferative effects (inhibition of colony-forming ability) in pancreatic cancer cells derived from different genetic backgrounds treated with the glycolytic inhibitor, 2-deoxyglucose (2-DG). The most glycolytic pancreatic cancer cell line was exquisitely sensitive to 2-DG, whereas the least glycolytic pancreatic cancer cell was resistant to 2-DG. However, when combined with metformin, inhibitor of mitochondrial respiration and activator of AMP-activated protein kinase, 2-DG synergistically enhanced ATP depletion and inhibited cell proliferation even in poorly glycolytic, 2-DG-resistant pancreatic cancer cell line. Furthermore, treatment with conventional chemotherapeutic drugs (e.g., gemcitabine and doxorubicin) or COX-2 inhibitor, celecoxib, sensitised the cells to 2-DG treatment.

CONCLUSIONS

Detailed profiling of cellular bioenergetics can provide new insight into the design of therapeutic strategies for inhibiting pancreatic cancer cell metabolism and proliferation.

Oxidative stress, redox signaling, and autophagy: cell death versus survival

SIGNIFICANCE

The molecular machinery regulating autophagy has started becoming elucidated, and a number of studies have undertaken the task to determine the role of autophagy in cell fate determination within the context of human disease progression. Oxidative stress and redox signaling are also largely involved in the etiology of human diseases, where both survival and cell death signaling cascades have been reported to be modulated by reactive oxygen species (ROS) and reactive nitrogen species (RNS).

RECENT ADVANCES

To date, there is a good understanding of the signaling events regulating autophagy, as well as the signaling processes by which alterations in redox homeostasis are transduced to the activation/regulation of signaling cascades. However, very little is known about the molecular events linking them to the regulation of autophagy. This lack of information has hampered the understanding of the role of oxidative stress and autophagy in human disease progression.

CRITICAL ISSUES

In this review, we will focus on (i) the molecular mechanism by which ROS/RNS generation, redox signaling, and/or oxidative stress/damage alter autophagic flux rates; (ii) the role of autophagy as a cell death process or survival mechanism in response to oxidative stress; and (iii) alternative mechanisms by which autophagy-related signaling regulate mitochondrial function and antioxidant response.

FUTURE DIRECTIONS

Our research efforts should now focus on understanding the molecular basis of events by which autophagy is fine tuned by oxidation/reduction events. This knowledge will enable us to understand the mechanisms by which oxidative stress and autophagy regulate human diseases such as cancer and neurodegenerative disorders.

Protein kinase D1 (PKD1) phosphorylation promotes dopaminergic neuronal survival during 6-OHDA-induced oxidative stress

Oxidative stress is a major pathophysiological mediator of degenerative processes in many neurodegenerative diseases including Parkinson’s disease (PD). Aberrant cell signaling governed by protein phosphorylation has been linked to oxidative damage of dopaminergic neurons in PD. Although several studies have associated activation of certain protein kinases with apoptotic cell death in PD, very little is known about protein kinase regulation of cell survival and protection against oxidative damage and degeneration in dopaminergic neurons. Here, we characterized the PKD1-mediated protective pathway against oxidative damage in cell culture models of PD. Dopaminergic neurotoxicant 6-hydroxy dopamine (6-OHDA) was used to induce oxidative stress in the N27 dopaminergic cell model and in primary mesencephalic neurons. Our results indicated that 6-OHDA induced the PKD1 activation loop (PKD1S744/S748) phosphorylation during early stages of oxidative stress and that PKD1 activation preceded cell death. We also found that 6-OHDA rapidly increased phosphorylation of the C-terminal S916 in PKD1, which is required for PKD1 activation loop (PKD1S744/748) phosphorylation. Interestingly, negative modulation of PKD1 activation by RNAi knockdown or by the pharmacological inhibition of PKD1 by kbNB-14270 augmented 6-OHDA-induced apoptosis, while positive modulation of PKD1 by the overexpression of full length PKD1 (PKD1^WT) or constitutively active PKD1 (PKD1^S744E/S748E) attenuated 6-OHDA-induced apoptosis, suggesting an anti-apoptotic role for PKD1 during oxidative neuronal injury. Collectively, our results demonstrate that PKD1 signaling plays a cell survival role during early stages of oxidative stress in dopaminergic neurons and therefore, positive modulation of the PKD1-mediated signal transduction pathway can provide a novel neuroprotective strategy against PD.

High-throughput assays for superoxide and hydrogen peroxide: design of a screening workflow to identify inhibitors of NADPH oxidases

Recent progress characterizing the reaction mechanism(s) of fluorescent probes with reactive oxygen species has made it possible to rigorously analyze these reactive species in biological systems. We have developed rapid high throughput-compatible assays for monitoring cellular production of superoxide radical anion and hydrogen peroxide using hydropropidine and coumarin boronic acid probes, respectively. Coupling plate reader-based fluorescence measurements with HPLC-based simultaneous monitoring of superoxide radical anion and hydrogen peroxide provides the basis for the screening protocol for NADPH oxidase (Nox) inhibitors. Using this newly developed approach along with the medium-throughput plate reader-based oximetry and EPR spin trapping as confirmatory assays, it is now eminently feasible to rapidly and reliably identify Nox enzyme inhibitors with a markedly lower rate of false positives. These methodological advances provide an opportunity to discover selective inhibitors of Nox isozymes, through enhanced conceptual understanding of their basic mechanisms of action.

Loss of AMP-activated protein kinase in X-linked adrenoleukodystrophy patient-derived fibroblasts and lymphocytes

X-Adrenoleukodystrophy (X-ALD) is a peroxisomal disorder characterized by accumulation of very-long-chain (VLC) fatty acids, which induces inflammatory disease and alterations in cellular redox, both of which are reported to play a role in the pathogenesis of the severe form of the disease (childhood cerebral ALD). While the mutation defect in ABCD1 gene is common to all forms of X-ALD it fails to account for the spectrum of phenotypic variability seen in X-ALD patients, strongly suggesting a role for as yet unidentified modifier gene(s). Here we report, for the first time, loss of AMP-activated protein kinase alpha1 (AMPKα1) in patient-derived fibroblasts and lymphocytes of the severe cerebral form of X-ALD (ALD), and not in the milder adrenomyeloneuropathy (AMN) form. Decrease in AMPK was observed at both protein and mRNA levels. AMPK loss in ALD patient-derived fibroblasts was associated with increased ubiquitination. Using the Seahorse Bioscience XF(e)96 Flux Analyzer for measuring the mitochondrial oxygen consumption and extracellular acidification rate we show that ALD patient-derived fibroblasts have a significantly lower "metabolic state" than AMN fibroblasts. Unstimulated ALD patient-derived lymphocytes had significantly higher proinflammatory gene expression. Selective AMPK loss represents a novel physiopathogenic factor in X-ALD disease mechanism. Strategies aimed at upregulating/recovering AMPK levels might have beneficial therapeutic effects in X-ALD.

Autophagy as an essential cellular antioxidant pathway in neurodegenerative disease

Oxidative stress including DNA damage, increased lipid and protein oxidation, are important features of aging and neurodegeneration suggesting that endogenous antioxidant protective pathways are inadequate or overwhelmed. Importantly, oxidative protein damage contributes to age-dependent accumulation of dysfunctional mitochondria or protein aggregates. In addition, environmental toxins such as rotenone and paraquat, which are risk factors for the pathogenesis of neurodegenerative diseases, also promote protein oxidation. The obvious approach of supplementing the primary antioxidant systems designed to suppress the initiation of oxidative stress has been tested in animal models and positive results were obtained. However, these findings have not been effectively translated to treating human patients, and clinical trials for antioxidant therapies using radical scavenging molecules such as α-tocopherol, ascorbate and coenzyme Q have met with limited success, highlighting several limitations to this approach. These could include: (1) radical scavenging antioxidants cannot reverse established damage to proteins and organelles; (2) radical scavenging antioxidants are oxidant specific, and can only be effective if the specific mechanism for neurodegeneration involves the reactive species to which they are targeted and (3) since reactive species play an important role in physiological signaling, suppression of endogenous oxidants maybe deleterious. Therefore, alternative approaches that can circumvent these limitations are needed. While not previously considered an antioxidant system we propose that the autophagy-lysosomal activities, may serve this essential function in neurodegenerative diseases by removing damaged or dysfunctional proteins and organelles.

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Significant oxidative damage occurs in neurodegenerative disease brains.

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Effective in animal models with single toxins, antioxidants are ineffective in clinical trials.

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The failure of antioxidant therapy maybe due to propagation of cellular damage.

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Autophagic clearance of diverse damaged molecules may provide antioxidant mechanisms.

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Further mechanistic and translational studies on autophagy therapy are needed.

Aging risk factors and Parkinson's disease: contrasting roles of common dietary constituents

Aging is a Parkinson's disease (PD) risk factor. It is suggested here that certain dietary components may either contribute to or ameliorate PD risk. There is evidence, which indicates that excessive carbohydrate (glucose or fructose) catabolism is a cause of mitochondrial dysfunction in PD, one consequence is increased production of methylglyoxal (MG). However, other dietary components (carnosine and certain plant extracts) not only scavenge MG but can also influence some of the biochemical events (signal transduction, stress protein synthesis, glycation, and toxin generation) associated with PD pathology. As double blind, placebo-controlled carnosine supplementation studies have revealed beneficial outcomes in humans, it is suggested that MG scavengers such as carnosine be further explored for their therapeutic potential toward PD.

How Parkinsonian toxins dysregulate the autophagy machinery

Since their discovery, Parkinsonian toxins (6-hydroxydopamine, MPP+, paraquat, and rotenone) have been widely employed as in vivo and in vitro chemical models of Parkinson's disease (PD). Alterations in mitochondrial homeostasis, protein quality control pathways, and more recently, autophagy/mitophagy have been implicated in neurotoxin models of PD. Here, we highlight the molecular mechanisms by which different PD toxins dysregulate autophagy/mitophagy and how alterations of these pathways play beneficial or detrimental roles in dopamine neurons. The convergent and divergent effects of PD toxins on mitochondrial function and autophagy/mitophagy are also discussed in this review. Furthermore, we propose new diagnostic tools and discuss how pharmacological modulators of autophagy/mitophagy can be developed as disease-modifying treatments for PD. Finally, we discuss the critical need to identify endogenous and synthetic forms of PD toxins and develop efficient health preventive programs to mitigate the risk of developing PD.

Compartmentalized oxidative stress in dopaminergic cell death induced by pesticides and complex I inhibitors: distinct roles of superoxide anion and superoxide dismutases

The loss of dopaminergic neurons induced by the parkinsonian toxins paraquat, rotenone, and 1-methyl-4-phenylpyridinium (MPP(+)) is associated with oxidative stress. However, controversial reports exist regarding the source/compartmentalization of reactive oxygen species (ROS) generation and its exact role in cell death. We aimed to determine in detail the role of superoxide anion (O2(•-)), oxidative stress, and their subcellular compartmentalization in dopaminergic cell death induced by parkinsonian toxins. Oxidative stress and ROS formation were determined in the cytosol, intermembrane (IMS), and mitochondrial matrix compartments, using dihydroethidine derivatives and the redox sensor roGFP, as well as electron paramagnetic resonance spectroscopy. Paraquat induced an increase in ROS and oxidative stress in both the cytosol and the mitochondrial matrix prior to cell death. MPP(+) and rotenone primarily induced an increase in ROS and oxidative stress in the mitochondrial matrix. No oxidative stress was detected at the level of the IMS. In contrast to previous studies, overexpression of manganese superoxide dismutase (MnSOD) or copper/zinc SOD (CuZnSOD) had no effect on alterations in ROS steady-state levels, lipid peroxidation, loss of mitochondrial membrane potential (ΔΨm), and dopaminergic cell death induced by MPP(+) or rotenone. In contrast, paraquat-induced oxidative stress and cell death were selectively reduced by MnSOD overexpression, but not by CuZnSOD or manganese-porphyrins. However, MnSOD also failed to prevent ΔΨm loss. Finally, paraquat, but not MPP(+) or rotenone, induced the transcriptional activation of the redox-sensitive antioxidant response elements (ARE) and nuclear factor kappa-B (NF-κB). These results demonstrate a selective role of mitochondrial O2(•-) in dopaminergic cell death induced by paraquat, and show that toxicity induced by the complex I inhibitors rotenone and MPP(+) does not depend directly on mitochondrial O2(•-) formation.

HPLC-based monitoring of products formed from hydroethidine-based fluorogenic probes--the ultimate approach for intra- and extracellular superoxide detection

BACKGROUND

Nearly ten years ago, we demonstrated that superoxide radical anion (O2⋅¯) reacts with the hydroethidine dye (HE, also known as dihydroethidium, DHE) to form a diagnostic marker product, 2-hydroxyethidium (2-OH-E(+)). This particular product is not derived from reacting HE with other biologically relevant oxidants (hydrogen peroxide, hydroxyl radical, or peroxynitrite). This discovery negated the longstanding view that O2⋅¯ reacts with HE to form the other oxidation product, ethidium (E(+)). It became clear that due to the overlapping fluorescence spectra of E(+) and 2-OH-E(+), fluorescence-based techniques using the "red fluorescence" are not suitable for detecting and measuring O2⋅¯ in cells using HE or other structurally analogous fluorogenic probes (MitoSOX(TM) Red or hydropropidine). However, using HPLC-based assays, 2-OH-E(+) and analogous hydroxylated products can be easily detected and quickly separated from other oxidation products.

SCOPE OF REVIEW

The principles discussed in this chapter are generally applicable in free radical biology and medicine, redox biology, and clinical and translational research. The assays developed here could be used to discover new and targeted inhibitors for various superoxide-producing enzymes, including NADPH oxidase (NOX) isoforms.

MAJOR CONCLUSIONS

HPLC-based approaches using site-specific HE-based fluorogenic probes are eminently suitable for monitoring O2⋅¯ in intra- and extracellular compartments and in mitochondria. The use of fluorescence-microscopic methods should be avoided because of spectral overlapping characteristics of O2⋅¯-derived marker product and other, non-specific oxidized fluorescent products formed from these probes.

GENERAL SIGNIFICANCE

Methodologies and site-specific fluorescent probes described in this review can be suitably employed to delineate oxy radical dependent mechanisms in cells under physiological and pathological conditions. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Mitochondrial quality, dynamics and functional capacity in Parkinson's disease cybrid cell lines selected for Lewy body expression

Background

Lewy bodies (LB) are a neuropathological hallmark of Parkinson’s disease (PD) and other synucleinopathies. The role their formation plays in disease pathogenesis is not well understood, in part because studies of LB have been limited to examination of post-mortem tissue. LB formation may be detrimental to neuronal survival or merely an adaptive response to other ongoing pathological processes. In a human cytoplasmic hybrid (cybrid) neural cell model that expresses mitochondrial DNA from PD patients, we observed spontaneous formation of intracellular protein aggregates (“cybrid LB” or CLB) that replicate morphological and biochemical properties of native, cortical LB. We studied mitochondrial morphology, bioenergetics and biogenesis signaling by creating stable sub-clones of three PD cybrid cell lines derived from cells expressing CLB.

Results

Cloning based on CLB expression had a differential effect on mitochondrial morphology, movement and oxygen utilization in each of three sub-cloned lines, but no long-term change in CLB expression. In one line (PD63_CLB), mitochondrial function declined compared to the original PD cybrid line (PD63_Orig) due to low levels of mtDNA in nucleoids. In another cell line (PD61_Orig), the reverse was true, and cellular and mitochondrial function improved after sub-cloning for CLB expression (PD61_CLB). In the third cell line (PD67_Orig), there was no change in function after selection for CLB expression (PD67_CLB).

Conclusions

Expression of mitochondrial DNA derived from PD patients in cybrid cell lines induced the spontaneous formation of CLB. The creation of three sub-cloned cybrid lines from cells expressing CLB resulted in differential phenotypic changes in mitochondrial and cellular function. These changes were driven by the expression of patient derived mitochondrial DNA in nucleoids, rather than by the presence of CLB. Our studies suggest that mitochondrial DNA plays an important role in cellular and mitochondrial dysfunction in PD. Additional studies will be needed to assess the direct effect of CLB expression on cellular and mitochondrial function.

Hydropropidine: a novel, cell-impermeant fluorogenic probe for detecting extracellular superoxide

Here we report the synthesis and characterization of a membrane-impermeant fluorogenic probe, hydropropidine (HPr(+)), the reduction product of propidium iodide, for detecting extracellular superoxide (O(2)(•-)). HPr(+) is a positively charged water-soluble analog of hydroethidine (HE), a fluorogenic probe commonly used for monitoring intracellular O(2)(•-). We hypothesized that the presence of a highly localized positive charge on the nitrogen atom would impede cellular uptake of HPr(+) and allow for exclusive detection of extracellular O(2)(•-). Our results indicate that O(2)(•-) reacts with HPr(+) (k=1.2×10(4) M(-1) s(-1)) to form exclusively 2-hydroxypropidium (2-OH-Pr(2+)) in cell-free and cell-based systems. This reaction is analogous to the reaction between HE and O(2)(•-) (Zhao et al., Free Radic. Biol. Med.34:1359-1368; 2003). During the course of this investigation, we also reassessed the rate constants for the reactions of O(2)(•-) with HE and its mitochondria targeted analog (Mito-HE or MitoSOX Red) and addressed the discrepancies between the present values and those reported previously by us. Our results indicate that the rate constant between O(2)(•-) and HPr(+) is slightly higher than that of HE and O(2)(•-) and is closer to that of Mito-HE and O(2)(•-). Similar to HE, HPr(+) undergoes oxidation in the presence of various oxidants (peroxynitrite-derived radicals, Fenton's reagent, and ferricytochrome c) forming the corresponding propidium dication (Pr(2+)) and the dimeric products (e.g., Pr(2+)-Pr(2+)). In contrast to HE, there was very little intracellular uptake of HPr(+). We conclude that HPr(+) is a useful probe for detecting O(2)(•-) and other one-electron oxidizing species in an extracellular milieu.

Distinct effects of rotenone, 1-methyl-4-phenylpyridinium and 6-hydroxydopamine on cellular bioenergetics and cell death

Parkinson's disease is characterized by dopaminergic neurodegeneration and is associated with mitochondrial dysfunction. The bioenergetic susceptibility of dopaminergic neurons to toxins which induce Parkinson's like syndromes in animal models is then of particular interest. For example, rotenone, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its active metabolite 1-methyl-4-phenylpyridinium (MPP(+)), and 6-hydroxydopamine (6-OHDA), have been shown to induce dopaminergic cell death in vivo and in vitro. Exposure of animals to these compounds induce a range of responses characteristics of Parkinson's disease, including dopaminergic cell death, and Reactive Oxygen Species (ROS) production. Here we test the hypothesis that cellular bioenergetic dysfunction caused by these compounds correlates with induction of cell death in differentiated dopaminergic neuroblastoma SH-SY5Y cells. At increasing doses, rotenone induced significant cell death accompanied with caspase 3 activation. At these concentrations, rotenone had an immediate inhibition of mitochondrial basal oxygen consumption rate (OCR) concomitant with a decrease of ATP-linked OCR and reserve capacity, as well as a stimulation of glycolysis. MPP(+) exhibited a different behavior with less pronounced cell death at doses that nearly eliminated basal and ATP-linked OCR. Interestingly, MPP(+), unlike rotenone, stimulated bioenergetic reserve capacity. The effects of 6-OHDA on bioenergetic function was markedly less than the effects of rotenone or MPP(+) at cytotoxic doses, suggesting a mechanism largely independent of bioenergetic dysfunction. These studies suggest that these dopaminergic neurotoxins induce cell death through distinct mechanisms and differential effects on cellular bioenergetics.