Hydroxynonenal and uncoupling proteins: a model for protection against oxidative damage

UCP2, a Member of the Mitochondrial Uncoupling Proteins: An Overview from Physiological to Pathological Roles

UCP2 is an uncoupling protein homolog to UCP1. Unlike UCP1, which participates in non-shivering thermogenesis by uncoupling oxidative phosphorylation (OXPHOS), UCP2 does not perform a canonical H+ leak, consuming the protonmotive force (Δp) through the inner mitochondrial membrane. The UCP2 biological role is elusive. It can counteract oxidative stress, acting with a "mild uncoupling" process to reduce ROS production, and, in fact, UCP2 activities are related to inflammatory processes, triggering pathological conditions. However, the Δp dissipation by UCP2 activity reduces the mitochondrial ATP production and rewires the bioenergetic metabolism of the cells. In all likelihood, UCP2 works as a carrier of metabolites with four carbon atoms (C4), reversing the anaerobic glycolysis-dependent catabolism to OXPHOS. Indeed, UCP2 can perform catalysis in dual mode: mild uncoupling of OXPHOS and metabolite C4 exchange of mitochondria. In vivo, the UCP2 features in the biology of mitochondria promote healthy ageing, increased lifespan, and can assure cerebro- and cardiovascular protection. However, the pathological conditions responsible for insulin secretion suppression are dependent on UCP2 activity. On balance, the uncertain biochemical mechanisms dependent on UCP2 do not allow us to depict the protective role in mitochondrial bioenergetics.

Reactive Carbonyl Species and Protein Lipoxidation in Atherogenesis

Atherosclerosis is a multifactorial disease of medium and large arteries, characterized by the presence of lipid-rich plaques lining the intima over time. It is the main cause of cardiovascular diseases and death worldwide. Redox imbalance and lipid peroxidation could play key roles in atherosclerosis by promoting a bundle of responses, including endothelial activation, inflammation, and foam cell formation. The oxidation of polyunsaturated fatty acids generates various lipid oxidation products such as reactive carbonyl species (RCS), including 4-hydroxy alkenals, malondialdehyde, and acrolein. RCS covalently bind to nucleophilic groups of nucleic acids, phospholipids, and proteins, modifying their structure and activity and leading to their progressive dysfunction. Protein lipoxidation is the non-enzymatic post-translational modification of proteins by RCS. Low-density lipoprotein (LDL) oxidation and apolipoprotein B (apoB) modification by RCS play a major role in foam cell formation. Moreover, oxidized LDLs are a source of RCS, which form adducts on a huge number of proteins, depending on oxidative stress intensity, the nature of targets, and the availability of detoxifying systems. Many systems are affected by lipoxidation, including extracellular matrix components, membranes, cytoplasmic and cytoskeletal proteins, transcription factors, and other components. The mechanisms involved in lipoxidation-induced vascular dysfunction are not fully elucidated. In this review, we focus on protein lipoxidation during atherogenesis.

Adenine nucleotide translocase: Current knowledge in post-translational modifications, regulations and pathological implications for human diseases

Adenine nucleotide translocases (ANTs) are central to mitochondrial integrity and bioenergetic metabolism. This review aims to integrate the progresses and knowledge on ANTs over the last few years, contributing to a potential implication of ANTs for various diseases. Structures, functions, modifications, regulators and pathological implications of ANTs for human diseases are intensively demonstrated here. ANTs have four isoforms (ANT1-4), responsible for exchanging ATP/ADP, possibly composing of pro-apoptotic mPTP as a major component, and mediating FA-dependent uncoupling of proton efflux. ANT can be modified by methylation, nitrosylation and nitroalkylation, acetylation, glutathionylation, phosphorylation, carbonylation and hydroxynonenal-induced modifications. Compounds, including bongkrekic acid, atractyloside calcium, carbon monoxide, minocycline, 4-(N-(S-penicillaminylacetyl)amino) phenylarsonous acid, cardiolipin, free long-chain fatty acids, agaric acid, long chain acyl-coenzyme A esters, all have an ability to regulate ANT activities. ANT impairment leads to bioenergetic failure and mitochondrial dysfunction, contributing to pathogenesis of diseases, such as diabetes (deficiency), heart disease (deficiency), Parkinson's disease (reduction), Sengers Syndrome (decrease), cancer (isoform shifting), Alzheimer's Disease (coaggregation with Tau), Progressive External Opthalmoplegia (mutation), and Fascioscapulohumeral muscular dystrophy (overexpression). This review improves the understanding of the mechanism of ANT in pathogenesis of human diseases, and opens a window for novel therapeutic strategies targeted on ANT in diseases.

Deconstructing the molecular genetics behind the PINK1/Parkin axis in Parkinson’s disease using Drosophila melanogaster as a model organism

Background

Parkinson’s disease (PD) is a multifactorial neurodegenerative disorder marked by the death of nigrostriatal dopaminergic neurons in response to the compounding effects of oxidative stress, mitochondrial dysfunction and protein aggregation. Transgenic Drosophila models have been used extensively to decipher the underlying genetic interactions that exacerbate neural health in PD. Autosomal recessive forms of the disease have been linked to mutations in the serine/threonine kinase PINK1(PTEN-Induced Putative Kinase 1) and E3 ligase Parkin, which function in an axis that is conserved in flies. This review aims to probe the current understanding of PD pathogenesis via the PINK1/Parkin axis while underscoring the importance of several molecular and pharmacologic rescues brought to light through studies in Drosophila.

Main body

Mutations in PINK1 and Parkin have been shown to affect the axonal transport of mitochondria within dopaminergic neurons and perturb the balance between mitochondrial fusion/fission resulting in abnormal mitochondrial morphology. As per studies in flies, ectopic expression of Fwd kinase and Atg-1 to promote fission and mitophagy while suppressing fusion via MUL1 E3 ligase may aid to halt mitochondrial aggregation and prolong the survival of dopaminergic neurons. Furthermore, upregulation of Hsp70/Hsp90 chaperone systems (Trap1, CHIP) to target misfolded mitochondrial respiratory complexes may help to preserve their bioenergetic capacity. Accumulation of reactive oxygen species as a consequence of respiratory complex dysfunction or antioxidant enzyme deficiency further escalates neural death by inducing apoptosis, lipid peroxidation and DNA damage. Fly studies have reported the induction of canonical Wnt signalling to enhance the activity of transcriptional co-activators (PGC1α, FOXO) which induce the expression of antioxidant enzymes. Enhancing the clearance of free radicals via uncoupling proteins (UCP4) has also been reported to ameliorate oxidative stress-induced cell death in PINK1/Parkin mutants.

Conclusion

While these novel mechanisms require validation through mammalian studies, they offer several explanations for the factors propagating dopaminergic death as well as promising insights into the therapeutic importance of transgenic fly models in PD.

Mitochondrial Uncoupling Proteins (UCPs) as Key Modulators of ROS Homeostasis: A Crosstalk between Diabesity and Male Infertility?

Uncoupling proteins (UCPs) are transmembrane proteins members of the mitochondrial anion transporter family present in the mitochondrial inner membrane. Currently, six homologs have been identified (UCP1-6) in mammals, with ubiquitous tissue distribution and multiple physiological functions. UCPs are regulators of key events for cellular bioenergetic metabolism, such as membrane potential, metabolic efficiency, and energy dissipation also functioning as pivotal modulators of ROS production and general cellular redox state. UCPs can act as proton channels, leading to proton re-entry the mitochondrial matrix from the intermembrane space and thus collapsing the proton gradient and decreasing the membrane potential. Each homolog exhibits its specific functions, from thermogenesis to regulation of ROS production. The expression and function of UCPs are intimately linked to diabesity, with their dysregulation/dysfunction not only associated to diabesity onset, but also by exacerbating oxidative stress-related damage. Male infertility is one of the most overlooked diabesity-related comorbidities, where high oxidative stress takes a major role. In this review, we discuss in detail the expression and function of the different UCP homologs. In addition, the role of UCPs as key regulators of ROS production and redox homeostasis, as well as their influence on the pathophysiology of diabesity and potential role on diabesity-induced male infertility is debated.

Mitochondrial proton leaks and uncoupling proteins

Non-shivering thermogenesis in brown adipose tissue is mediated by uncoupling protein 1 (UCP1), which provides a carefully regulated proton re-entry pathway across the mitochondrial inner membrane operating in parallel to the ATP synthase and allowing respiration, and hence thermogenesis, to be released from the constraints of respiratory control. In the 40 years since UCP1 was first described, an extensive, and frequently contradictory, literature has accumulated, focused on the acute physiological regulation of the protein by fatty acids, purine nucleotides and possible additional factors. The purpose of this review is to examine, in detail, the experimental evidence underlying these proposed mechanisms. Emphasis will be placed on the methodologies employed and their relation to the physiological constraints under which the protein functions in the intact cell. The nature of the endogenous, UCP1-independent, proton leak will also be discussed. Finally, the troubled history of the putative novel uncoupling proteins, UCP2 and UCP3, will be evaluated.

The Role of Mitochondria in the Mechanisms of Cardiac Ischemia-Reperfusion Injury

Mitochondria play a critical role in maintaining cellular function by ATP production. They are also a source of reactive oxygen species (ROS) and proapoptotic factors. The role of mitochondria has been established in many aspects of cell physiology/pathophysiology, including cell signaling. Mitochondria may deteriorate under various pathological conditions, including ischemia-reperfusion (IR) injury. Mitochondrial injury can be one of the main causes for cardiac and other tissue injuries by energy stress and overproduction of toxic reactive oxygen species, leading to oxidative stress, elevated calcium and apoptotic and necrotic cell death. However, the interplay among these processes in normal and pathological conditions is still poorly understood. Mitochondria play a critical role in cardiac IR injury, where they are directly involved in several pathophysiological mechanisms. We also discuss the role of mitochondria in the context of mitochondrial dynamics, specializations and heterogeneity. Also, we wanted to stress the existence of morphologically and functionally different mitochondrial subpopulations in the heart that may have different sensitivities to diseases and IR injury. Therefore, various cardioprotective interventions that modulate mitochondrial stability, dynamics and turnover, including various pharmacologic agents, specific mitochondrial antioxidants and uncouplers, and ischemic preconditioning can be considered as the main strategies to protect mitochondrial and cardiovascular function and thus enhance longevity.

Absence of Uncoupling Protein-3 at Thermoneutrality Impacts Lipid Handling and Energy Homeostasis in Mice

The role of uncoupling protein-3 (UCP3) in energy and lipid metabolism was investigated. Male wild-type (WT) and UCP3-null (KO) mice that were housed at thermoneutrality (30 °C) were used as the animal model. In KO mice, the ability of skeletal muscle mitochondria to oxidize fatty acids (but not pyruvate or succinate) was reduced. At whole animal level, adult KO mice presented blunted resting metabolic rates, energy expenditure, food intake, and the use of lipids as metabolic substrates. When WT and KO mice were fed with a standard/low-fat diet for 80 days, since weaning, they showed similar weight gain and body composition. Interestingly, KO mice showed lower fat accumulation in visceral adipose tissue and higher ectopic fat accumulation in liver and skeletal muscle. When fed with a high-fat diet for 80 days, since weaning, KO mice showed enhanced energy efficiency and an increased lipid gain (thus leading to a change in body composition between the two genotypes). We conclude that UCP3 plays a role in energy and lipid homeostasis and in preserving lean tissues by lipotoxicity, in mice that were housed at thermoneutrality.

Uncoupling proteins: Martin Klingenberg's contributions for 40 years

The uncoupling protein (UCP1) is a proton (H⁺) transporter in the mitochondrial inner membrane. By dissipating the electrochemical H⁺ gradient, UCP1 uncouples respiration from ATP synthesis, which drives an increase in substrate oxidation via the TCA cycle flux that generates more heat. The mitochondrial uncoupling-mediated non-shivering thermogenesis in brown adipose tissue is vital primarily to mammals, such as rodents and new-born humans, but more recently additional functions in adult humans have been described. UCP1 is regulated by β-adrenergic receptors through the sympathetic nervous system and at the molecular activity level by nucleotides and fatty acid to meet thermogenesis needs. The discovery of novel UCP homologs has greatly contributed to the understanding of human diseases, such as obesity and diabetes. In this article, we review the progress made towards the molecular mechanism and function of the UCPs, in particular focusing on the influential contributions from Martin Klingenberg's laboratory. Because all members of the UCP family are potentially promising drug targets, we also present and discuss possible approaches and methods for UCP-related drug discovery.

Proatherogenic effects of 4-hydroxynonenal

4-hydroxy-2-nonenal (HNE) is a α,β-unsaturated hydroxyalkenal generated by peroxidation of n-6 polyunsaturated fatty acid. This reactive carbonyl compound exhibits a huge number of biological properties that result mainly from the formation of HNE-adducts on free amino groups and thiol groups in proteins. In the vascular system, HNE adduct accumulation progressively leads to cellular dysfunction and tissue damages that are involved in the progression of atherosclerosis and related diseases. HNE contributes to the atherogenicity of oxidized LDL, by forming HNE-apoB adducts that deviate the LDL metabolism to the scavenger receptor pathway of macrophagic cells, and lead to the formation of foam cells. HNE activates transcription factors (Nrf2, NF-kappaB) that (dys)regulate various cellular responses ranging from hormetic and survival signaling at very low concentrations, to inflammatory and apoptotic effects at higher concentrations. Among a variety of cellular targets, HNE can modify signaling proteins involved in atherosclerotic plaque remodeling, particularly growth factor receptors (PDGFR, EGFR), cell cycle proteins, mitochondrial and endoplasmic reticulum components or extracellular matrix proteins, which progressively alters smooth muscle cell proliferation, angiogenesis and induces apoptosis. HNE adducts accumulate in the lipidic necrotic core of advanced atherosclerotic lesions, and may locally contribute to macrophage and smooth muscle cell apoptosis, which may induce plaque destabilization and rupture, thereby increasing the risk of athero-thrombotic events.

The conserved regulation of mitochondrial uncoupling proteins: From unicellular eukaryotes to mammals

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier protein family and mediate regulated proton leak across the inner mitochondrial membrane. Free fatty acids, aldehydes such as hydroxynonenal, and retinoids activate UCPs. However, there are some controversies about the effective action of retinoids and aldehydes alone; thus, only free fatty acids are commonly accepted positive effectors of UCPs. Purine nucleotides such as GTP inhibit UCP-mediated mitochondrial proton leak. In turn, membranous coenzyme Q may play a role as a redox state-dependent metabolic sensor that modulates the complete activation/inhibition of UCPs. Such regulation has been observed for UCPs in microorganisms, plant and animal UCP1 homologues, and UCP1 in mammalian brown adipose tissue. The origin of UCPs is still under debate, but UCP homologues have been identified in all systematic groups of eukaryotes. Despite the differing levels of amino acid/DNA sequence similarities, functional studies in unicellular and multicellular organisms, from amoebae to mammals, suggest that the mechanistic regulation of UCP activity is evolutionarily well conserved. This review focuses on the regulatory feedback loops of UCPs involving free fatty acids, aldehydes, retinoids, purine nucleotides, and coenzyme Q (particularly its reduction level), which may derive from the early stages of evolution as UCP first emerged.

Effects of aging on mitochondrial function in skeletal muscle of American American Quarter Horses

Skeletal muscle function, aerobic capacity, and mitochondrial (Mt) function have been found to decline with age in humans and rodents. However, not much is known about age-related changes in Mt function in equine skeletal muscle. Here, we compared fiber-type composition and Mt function in gluteus medius and triceps brachii muscle between young (age 1.8 ± 0.1 yr, n = 24) and aged (age 17-25 yr, n = 10) American Quarter Horses. The percentage of myosin heavy chain (MHC) IIX was lower in aged compared with young muscles (gluteus, P = 0.092; triceps, P = 0.012), while the percentages of MHC I (gluteus; P < 0.001) and MHC IIA (triceps; P = 0.023) were increased. Mass-specific Mt density, indicated by citrate synthase activity, was unaffected by age in gluteus, but decreased in aged triceps (P = 0.023). Cytochrome-c oxidase (COX) activity per milligram tissue and per Mt unit decreased with age in gluteus (P < 0.001 for both) and triceps (P < 0.001 and P = 0.003, respectively). Activity of 3-hydroxyacyl-CoA dehydrogenase per milligram tissue was unaffected by age, but increased per Mt unit in aged gluteus and triceps (P = 0.023 and P < 0.001, respectively). Mt respiration of permeabilized muscle fibers per milligram tissue was unaffected by age in both muscles. Main effects of age appeared when respiration was normalized to Mt content, with increases in LEAK, oxidative phosphorylation capacity, and electron transport system capacity (P = 0.038, P = 0.045, and P = 0.007, respectively), independent of muscle. In conclusion, equine skeletal muscle aging was accompanied by a shift in fiber-type composition, decrease in Mt density and COX activity, but preserved Mt respiratory function.

Hormetic and regulatory effects of lipid peroxidation mediators in pancreatic beta cells

Nutrient sensing mechanisms of carbohydrates, amino acids and lipids operate distinct pathways that are essential for the adaptation to varying metabolic conditions. The role of nutrient-induced biosynthesis of hormones is paramount for attaining metabolic homeostasis in the organism. Nutrient overload attenuate key metabolic cellular functions and interfere with hormonal-regulated inter- and intra-organ communication, which may ultimately lead to metabolic derangements. Hyperglycemia and high levels of saturated free fatty acids induce excessive production of oxygen free radicals in tissues and cells. This phenomenon, which is accentuated in both type-1 and type-2 diabetic patients, has been associated with the development of impaired glucose tolerance and the etiology of peripheral complications. However, low levels of the same free radicals also induce hormetic responses that protect cells against deleterious effects of the same radicals. Of interest is the role of hydroxyl radicals in initiating peroxidation of polyunsaturated fatty acids (PUFA) and generation of α,β-unsaturated reactive 4-hydroxyalkenals that avidly form covalent adducts with nucleophilic moieties in proteins, phospholipids and nucleic acids. Numerous studies have linked the lipid peroxidation product 4-hydroxy-2E-nonenal (4-HNE) to different pathological and cytotoxic processes. Similarly, two other members of the family, 4-hydroxyl-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), have also been identified as potential cytotoxic agents. It has been suggested that 4-HNE-induced modifications in macromolecules in cells may alter their cellular functions and modify signaling properties. Yet, it has also been acknowledged that these bioactive aldehydes also function as signaling molecules that directly modify cell functions in a hormetic fashion to enable cells adapt to various stressful stimuli. Recent studies have shown that 4-HNE and 4-HDDE, which activate peroxisome proliferator-activated receptor δ (PPARδ) in vascular endothelial cells and insulin secreting beta cells, promote such adaptive responses to ameliorate detrimental effects of high glucose and diabetes-like conditions. In addition, due to the electrophilic nature of these reactive aldehydes they form covalent adducts with electronegative moieties in proteins, phosphatidylethanolamine and nucleotides. Normally these non-enzymatic modifications are maintained below the cytotoxic range due to efficient cellular neutralization processes of 4-hydroxyalkenals. The major neutralizing enzymes include fatty aldehyde dehydrogenase (FALDH), aldose reductase (AR) and alcohol dehydrogenase (ADH), which transform the aldehyde to the corresponding carboxylic acid or alcohols, respectively, or by biding to the thiol group in glutathione (GSH) by the action of glutathione-S-transferase (GST). This review describes the hormetic and cytotoxic roles of oxygen free radicals and 4-hydroxyalkenals in beta cells exposed to nutritional challenges and the cellular mechanisms they employ to maintain their level at functional range below the cytotoxic threshold.

Hepatoprotective effect of grape seed proanthocyanidins on Cadmium-induced hepatic injury in rats: Possible involvement of mitochondrial dysfunction, inflammation and apoptosis

The present study was undertaken to evaluate the possible ameliorative role of grape seed proanthocyanidins (GSP) against Cadmium (Cd) induced hepatic inflammation, apoptosis and hepatic mitochondrial toxicity in rats. Male Wistar rats were distributed in four experimental groups: control, GSP, Cd and Cd + GSP. Exposure to a hepatotoxic dose of Cd (5 mg/kg BW) caused liver damage, coupled with enhanced reactive oxygen species (ROS) generation, increased inflammation and apoptosis in liver with increased DNA damage in hepatocytes of rats. Mitochondria were isolated from the hepatic tissues of rats from each group. Our results showed significant decrease in the tri-carboxylic acid cycle enzymes, increased mitochondrial swelling, inhibition of cytochrome c oxidase activity and complex I-III, II-III and IV mediated electron transfer, decreased mitochondrial ATPases, a reduction in calcium content and mitochondrial oxygen consumption in Cd treated rats. All these molecular changes caused by Cd were alleviated by the pre-supplementation with GSP (100 mg/kg BW). The ultra structural changes in the liver also support our findings. From our results, it is clearly indicated that the free radical scavenging, metal chelating and antioxidant potentials of GSP might be the possible reason, responsible for the rescue action against Cd induced mitochondrial damage in the liver of rats.

4-Hydroxynonenal induces Nrf2-mediated UCP3 upregulation in mouse cardiomyocytes

4-Hydroxy-2-nonenal (HNE) is a highly cytotoxic product of lipid peroxidation. Nevertheless, at low concentrations, it is able to mediate cell signaling and to activate protective pathways, including that of the transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2). In addition, HNE activates uncoupling proteins (UCPs), mitochondrial inner membrane proteins that mediate uncoupling of oxidative phosphorylation and have been proposed to protect against oxidative stress. It is not known, however, whether HNE might induce UCP expression via Nrf2 to cause mitochondrial uncoupling. We investigated the effects of HNE on UCP3 expression in mouse cardiomyocytes and the involvement of Nrf2. HNE induced the nuclear accumulation of Nrf2 and enhanced UCP3 expression, effects prevented by the antioxidant N-acetylcysteine. ChIP assays indicated that Nrf2 bound to the Ucp3 promoter after HNE treatment, increasing its expression. Cardiomyocytes treated with Nrf2- or UCP3-specific siRNA were less tolerant to HNE as reflected by increased cell death, and Nrf2 siRNA prevented HNE-induced UCP3 upregulation. The treatment with HNE greatly altered cardiomyocyte bioenergetics, increasing the proton leak across the inner mitochondrial membrane and severely decreasing the maximal respiratory capacity and the respiratory reserve capacity. These findings confirm that low HNE doses activate Nrf2 in cardiomyocytes and provide the first evidence of Nrf2 binding to the Ucp3 promoter in response to HNE, leading to increased protein expression. These results suggest that the upregulation of UCP3 mediated by Nrf2 in response to HNE might be important in the protection of the heart under conditions of oxidative stress such as ischemia-reperfusion.

4-Hydroxy-nonenal-A Bioactive Lipid Peroxidation Product

This review on recent research advances of the lipid peroxidation product 4-hydroxy-nonenal (HNE) has four major topics: I. the formation of HNE in various organs and tissues, II. the diverse biochemical reactions with Michael adduct formation as the most prominent one, III. the endogenous targets of HNE, primarily peptides and proteins (here the mechanisms of covalent adduct formation are described and the (patho-) physiological consequences discussed), and IV. the metabolism of HNE leading to a great number of degradation products, some of which are excreted in urine and may serve as non-invasive biomarkers of oxidative stress.

UCP2, a mitochondrial protein regulated at multiple levels

An ever-increasing number of studies highlight the role of uncoupling protein 2 (UCP2) in a broad range of physiological and pathological processes. The knowledge of the molecular mechanisms of UCP2 regulation is becoming fundamental in both the comprehension of UCP2-related physiological events and the identification of novel therapeutic strategies based on UCP2 modulation. The study of UCP2 regulation is a fast-moving field. Recently, several research groups have made a great effort to thoroughly understand the various molecular mechanisms at the basis of UCP2 regulation. In this review, we describe novel findings concerning events that can occur in a concerted manner at various levels: Ucp2 gene mutation (single nucleotide polymorphisms), UCP2 mRNA and protein expression (transcriptional, translational, and protein turn-over regulation), UCP2 proton conductance (ligands and post-transcriptional modifications), and nutritional and pharmacological regulation of UCP2.

Fatty acids are key in 4-hydroxy-2-nonenal-mediated activation of uncoupling proteins 1 and 2

The production of reactive oxygen species (ROS) in mitochondria is very sensitive to the proton motive force and may be decreased by mild uncoupling, mediated e.g. by mitochondrial uncoupling proteins (UCPs). UCPs were conversely hypothesized to be activated by ROS. Conclusions from experiments studying the reactive product of lipid peroxidation 4-hydroxy-2-nonenal (HNE) in isolated mitochondria and UCP knock-out mice are highly controversial. Here we investigated the molecular mechanism of HNE action by evaluating the separate contributions of lipid and protein phases of the membrane and by comparing UCP1 and UCP2, which were reconstituted in planar lipid bilayers. We demonstrated that aldehyde does not directly activate either UCP1 or UCP2. However, HNE strongly potentiated the membrane conductance increase (G_m) mediated by different long-chain fatty acids in UCP-containing and in UCP-free membranes and this suggest the involvement of both lipid-mediated and protein-mediated mechanisms with FA playing the central role. G_m increase was concentration-dependent and exhibited a typical saturation kinetic with the binding constant 0.3 mM. By using Electron Paramagnetic Resonance, membrane fluidity change could be excluded as a cause for the HNE-mediated increase in the presence of FA. The impact of the HNE binding to definite positively charged UCP amino acid residues is discussed as a possible protein-mediated mechanism of the UCP activation.

The transcription factor Nrf2 promotes survival by enhancing the expression of uncoupling protein 3 under conditions of oxidative stress

Uncoupling protein 3 (UCP3) is a member of the mitochondrial inner membrane carrier superfamily that modulates energy efficiency by catalyzing proton conductance and thus decreasing the production of superoxide anion. However, its role during oxidative stress and the underlying regulatory and molecular mechanisms remain poorly understood. We sought to investigate how UCP3 expression is regulated by oxidative stress and to evaluate the putative antioxidant role of this protein. H2O2 treatment increased UCP3 expression and the nuclear accumulation of the transcription factor Nrf2 in C2C12 and HL-1 cells. Nrf2 siRNA prevented H2O2-induced UCP3 expression, increasing oxidative stress and cell death. ChIP assays identified an antioxidant-response element (ARE) within the UCP3 promoter that bound Nrf2 after exposure to H2O2. Luciferase reporter experiments confirmed increased ARE activity in H2O2-treated HL-1 cells. Importantly, H2O2 increased the UCP3-mediated proton leak, suggesting a role for this protein in attenuating ROS-induced damage. Nrf2 nuclear accumulation and increased UCP3 protein were also detected in intact mouse heart subjected to a condition known to increase ROS generation. This is the first study to demonstrate that H2O2 augments UCP3 expression and it provides the first evidence of Nrf2 binding to the UCP3 promoter in response to oxidative challenge. These findings suggest that UCP3 functions as a member of the cellular antioxidant defense system that protects against oxidative stress in vivo. In conclusion, we have identified a novel regulatory process induced by an oxidative insult whereby the expression of the mitochondrial protein UCP3 is driven by the Nrf2 transcription factor, which decreases ROS production and prevents cell death.

Mitochondrial pathways, permeability transition pore, and redox signaling in cardioprotection: therapeutic implications

Reperfusion therapy is the indispensable treatment of acute myocardial infarction (AMI) and must be applied as soon as possible to attenuate the ischemic insult. However, reperfusion is responsible for additional myocardial damage likely involving opening of the mitochondrial permeability transition pore (mPTP). A great part of reperfusion injury occurs during the first minute of reperfusion. The prolonged opening of mPTP is considered one of the endpoints of the cascade to myocardial damage, causing loss of cardiomyocyte function and viability. Opening of mPTP and the consequent oxidative stress due to reactive oxygen and nitrogen species (ROS/RNS) are considered among the major mechanisms of mitochondrial and myocardial dysfunction. Kinases and mitochondrial components constitute an intricate network of signaling molecules and mitochondrial proteins, which interact in response to stressors. Cardioprotective pathways are activated by stimuli such as preconditioning and postconditioning (PostC), obtained with brief intermittent ischemia or with pharmacological agents, which drastically reduce the lethal ischemia/reperfusion injury. The protective pathways converging on mitochondria may preserve their function. Protection involves kinases, adenosine triphosphate-dependent potassium channels, ROS signaling, and the mPTP modulation. Some clinical studies using ischemic PostC during angioplasty support its protective effects, and an interesting alternative is pharmacological PostC. In fact, the mPTP desensitizer, cyclosporine A, has been shown to induce appreciable protections in AMI patients. Several factors and comorbidities that might interfere with cardioprotective signaling are considered. Hence, treatments adapted to the characteristics of the patient (i.e., phenotype oriented) might be feasible in the future.

Mitochondria as a source and target of lipid peroxidation products in healthy and diseased heart

The heart is a highly oxidative organ in which cardiomyocyte turnover is virtually absent, making it particularly vulnerable to accumulation of lipid peroxidation products (LPP) formed as a result of oxidative damage. Reactive oxygen and nitrogen species are the most common electrophiles formed during lipid peroxidation and lead to the formation of both stable and unstable LPP. Of the LPP formed, highly reactive aldehydes are a well-recognized causative factor in ageing and age-associated diseases, including cardiovascular disease and diabetes. Recent studies have identified that the mitochondria are both a primary source and target of LPP, with specific emphasis on aldehydes in cardiomyocytes and how these affect the electron transport system and Ca(2+) balance. Numerous studies have found that there are functional consequences in the heart following exposure to specific aldehydes (acrolein, trans-2-hexanal, 4-hydroxynonenal and acetaldehyde). Because these LPP are known to form in heart failure, cardiac ischaemia-reperfusion injury and diabetes, they may have an underappreciated role in the pathophysiology of these disease processes. Lipid peroxidation products are involved in the transcriptional regulation of endogenous anti-oxidant systems. Recent evidence demonstrates that transient increases in LPP may be beneficial in cardioprotection by contributing to mitohormesis (i.e. induction of anti-oxidant systems) in cardiomyocytes. Thus, exploitation of the cardioprotective actions of the LPP may represent a novel therapeutic strategy for future treatment of heart disease.

Mitochondrial neuronal uncoupling proteins: a target for potential disease-modification in Parkinson's disease

This review gives a brief insight into the role of mitochondrial dysfunction and oxidative stress in the converging pathogenic processes involved in Parkinson's disease (PD). Mitochondria provide cellular energy in the form of ATP via oxidative phosphorylation, but as an integral part of this process, superoxides and other reactive oxygen species are also produced. Excessive free radical production contributes to oxidative stress. Cells have evolved to handle such stress via various endogenous anti-oxidant proteins. One such family of proteins is the mitochondrial uncoupling proteins (UCPs), which are anion carriers located in the mitochondrial inner membrane. There are five known homologues (UCP1 to 5), of which UCP4 and 5 are predominantly expressed in neural cells. In a series of previous publications, we have shown how these neuronal UCPs respond to 1-methyl-4-phenylpyridinium (MPP+; toxic metabolite of MPTP) and dopamine-induced toxicity to alleviate neuronal cell death by preserving ATP levels and mitochondrial membrane potential, and reducing oxidative stress. We also showed how their expression can be influenced by nuclear factor kappa-B (NF-κB) signaling pathway specifically in UCP4. Furthermore, we previously reported an interesting link between PD and metabolic processes through the protective effects of leptin (hormone produced by adipocytes) acting via UCP2 against MPP+-induced toxicity. There is increasing evidence that these endogenous neuronal UCPs can play a vital role to protect neurons against various pathogenic stresses including those associated with PD. Their expression, which can be induced, may well be a potential therapeutic target for various drugs to alleviate the harmful effects of pathogenic processes in PD and hence modify the progression of this disease.

Reduced coupling of oxidative phosphorylation in vivo precedes electron transport chain defects due to mild oxidative stress in mice

Oxidative stress and mitochondrial function are at the core of many degenerative conditions. However, the interaction between oxidative stress and in vivo mitochondrial function is unclear. We used both pharmacological (2 week paraquat (PQ) treatment of wild type mice) and transgenic (mice lacking Cu, Zn-superoxide dismutase (SOD1(-/-))) models to test the effect of oxidative stress on in vivo mitochondrial function in skeletal muscle. Magnetic resonance and optical spectroscopy were used to measure mitochondrial ATP and oxygen fluxes and cell energetic state. In both models of oxidative stress, coupling of oxidative phosphorylation was significantly lower (lower P/O) at rest in vivo in skeletal muscle and was dose-dependent in the PQ model. Despite this reduction in efficiency, in vivo mitochondrial phosphorylation capacity (ATPmax) was maintained in both models, and ex vivo mitochondrial respiration in permeabilized muscle fibers was unchanged following PQ treatment. In association with the reduced P/O, PQ treatment led to a dose-dependent reduction in PCr/ATP ratio and increased phosphorylation of AMPK. These results indicate that oxidative stress uncouples oxidative phosphorylation in vivo and results in energetic stress in the absence of defects in the mitochondrial electron transport chain.

Chronic and acute exposure of mouse hearts to fatty acids increases oxygen cost of excitation-contraction coupling

The aim of the present study was to evaluate the underlying processes involved in the oxygen wasting induced by inotropic drugs and acute and chronic elevation of fatty acid (FA) supply, using unloaded perfused mouse hearts from normal and type 2 diabetic ( db/db) mice. We found that an acute elevation of the FA supply in normal hearts, as well as a chronic (in vivo) exposure to elevated FA as in db/db hearts, increased myocardial oxygen consumption (MV̇o2unloaded) due to increased oxygen cost for basal metabolism and for excitation-contraction (EC) coupling. Isoproterenol stimulation, on top of a high FA supply, led to an additive increase in MV̇o2unloaded, because of a further increase in oxygen cost for EC coupling. In db/db hearts, the acute elevation of FA did not further increase MV̇o2. Since the elevation in the FA supply is accompanied by increased rates of myocardial FA oxidation, the present study compared MV̇o2 following increased FA load versus FA oxidation rate by exposing normal hearts to normal and high FA concentration (NF and HF, respectively) and to compounds that either stimulate (GW-610742) or inhibit [dichloroacetate (DCA)] FA oxidation. While HF and NF + GW-610742 increased FA oxidation to the same extent, only HF increased MV̇o2unloaded. Although DCA counteracted the HF-induced increase in FA oxidation, DCA did not reduce MV̇o2unloaded. Thus, in normal hearts, acute FA-induced oxygen waste is 1) due to an increase in the oxygen cost for both basal metabolism and EC coupling and 2) not dependent on the myocardial FA oxidation rate per se, but on processes initiated by the presence of FAs. In diabetic hearts, chronic exposure to elevated circulating FAs leads to adaptations that afford protection against the detrimental effect of an acute FA load, suggesting different underlying mechanisms behind the increased MV̇o2 following acute and chronic FA load.

Acute administration of grape seed proanthocyanidin extract modulates energetic metabolism in skeletal muscle and BAT mitochondria

Proanthocyanidin consumption might reduce the risk of developing several pathologies, such as inflammation, oxidative stress and cardiovascular diseases. The beneficial effects of proanthocyanidins are attributed to their antioxidant properties, although they also can modulate gene expression at the transcriptional level. Little is known about the effect of proanthocyanidins on mitochondrial function and energy metabolism. In this context, the objective of this study was to determine the effect of an acute administration of grape seed proanthocyanidin extract (GSPE) on mitochondrial function and energy metabolism. To examine this effect, male Wistar rats fasted for fourteen hours, and then they were orally administered lard oil containing GSPE or were administered lard oil only. Liver, muscle and brown adipose tissue (BAT) were used to study enzymatic activity and gene expression of proteins related to energetic metabolism. Moreover, the gastrocnemius muscle and BAT mitochondria were used to perform high-resolution respirometry. The results showed that, after 5 h, the GSPE administration significantly lowers plasma triglycerides, free fatty acids, glycerol and urea concentrations. In skeletal muscle, GSPE lowers FATP1 mRNA levels and increases mitochondrial oxygen consumption, using pyruvate as the substrate, suggesting a promotion of glycosidic metabolism. Furthermore, GSPE increased the genetic expression of key genes in energy metabolism such as peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC1α), and modulated the enzyme activity of proteins, which are involved in the citric acid cycle and electron transport chain (ETC) in BAT. In conclusion, GSPE affects mainly the skeletal muscle and BAT mitochondria, increasing their oxidative capacity rapidly after acute supplementation.

Trans-4-oxo-2-nonenal potently alters mitochondrial function

Alzheimer disease elevates lipid peroxidation in the brain and data indicate that the resulting lipid-aldehydes are pathological effectors of lipid peroxidation. The disposition of 4-substituted nonenals derived from arachidonate (20:4, n-6) and linoleate (18:2, n-6) oxidation is modulated by their protein adduction targets, their metabolism, and the nature of the 4-substitutent. Trans-4-oxo-2-nonenal (4-ONE) has a higher toxicity in some systems than the more commonly studied trans-4-hydroxy-2-nonenal (HNE). In this work, we performed a structure-function analysis of 4-hydroxy/oxoalkenal upon mitochondrial endpoints. We tested the hypotheses that 4-ONE, owing to a highly reactive nature, is more toxic than HNE and that HNE toxicity is enantioselective. We chose to study freshly isolated brain mitochondria because of the role of mitochondrial dysfunction in neurodegenerative disorders. Whereas there was little effect related to HNE chirality, our data indicate that in the mitochondrial environment, the order of toxic potency under most conditions was 4-ONE>HNE. 4-ONE uncoupled mitochondrial respiration at a concentration of 5μM and inhibited aldehyde dehydrogenase 2 (ALDH2) activity with an IC(50) of approximately 0.5μM. The efficacy of altering mitochondrial endpoints was ALDH2 inhibition>respiration=mitochondrial swelling=ALDH5A inhibition>GSH depletion. Thiol-based alkenal scavengers, but not amine-based scavengers, were effective in blocking the effects of 4-ONE upon respiration. Quantum mechanical calculations provided insights into the basis for the elevated reactivity of 4-ONE>HNE. Our data demonstrate that 4-ONE is a potent effector of lipid peroxidation in the mitochondrial environment.

Plasticity of oxidative metabolism in variable climates: molecular mechanisms

Converting food to chemical energy (ATP) that is usable by cells is a principal requirement to sustain life. The rate of ATP production has to be sufficient for housekeeping functions, such as protein synthesis and maintaining membrane potentials, as well as for growth and locomotion. Energy metabolism is temperature sensitive, and animals respond to environmental variability at different temporal levels, from within-individual to evolutionary timescales. Here we review principal molecular mechanisms that underlie control of oxidative ATP production in response to climate variability. Nuclear transcription factors and coactivators control expression of mitochondrial proteins and abundance of mitochondria. Fatty acid and phospholipid concentrations of membranes influence the activity of membrane-bound proteins as well as the passive leak of protons across the mitochondrial membrane. Passive proton leak as well as protein-mediated proton leak across the inner mitochondrial membrane determine the efficacy of ATP production but are also instrumental in endothermic heat production and as a defense against reactive oxygen species. Both transcriptional mechanisms and membrane composition interact with environmental temperature and diet, and this interaction between diet and temperature in determining mitochondrial function links the two major environmental variables that are affected by changing climates. The limits to metabolic plasticity could be set by the production of reactive oxygen species leading to cellular damage, limits to substrate availability in mitochondria, and a disproportionally large increase in proton leak over ATP production.

Signaling and cytotoxic functions of 4-hydroxyalkenals

The peroxidation of n-3 and n-6 polyunsaturated fatty acids (PUFAs) and of their hydroperoxy metabolites is a complex process. It is initiated by free oxygen radical-induced abstraction of a hydrogen atom from the lipid molecule followed by a series of nonenzymatic reactions that ultimately generate the reactive aldehyde species 4-hydroxyalkenals. The molecule 4-hydroxy-2E-hexenal (4-HHE) is generated by peroxidation of n-3 PUFAs, such as linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid. The aldehyde product 4-hydroxy-2E-nonenal (4-HNE) is the peroxidation product of n-6 PUFAs, such as arachidonic and linoleic acids and their 15-lipoxygenase metabolites, namely 15-hydroperoxyeicosatetraenoic acid (15-HpETE) and 13-hydroperoxyoctadecadienoic acid (13-HpODE). Another reactive peroxidation product is 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), which is derived from 12-hydroperoxyeicosatetraenoic acid (12-HpETE), the 12-lipoxygenase metabolite of arachidonic acid. Hydroxyalkenals, notably 4-HNE, have been implicated in various pathophysiological interactions due to their chemical reactivity and the formation of covalent adducts with macromolecules. The progressive accumulation of these adducts alters normal cell functions that can lead to cell death. The lipophilicity of these aldehydes positively correlates to their chemical reactivity. Nonetheless, at low and noncytotoxic concentrations, these molecules may function as signaling molecules in cells. This has been shown mostly for 4-HNE and to some extent for 4-HHE. The capacity of 4-HDDE to generate such "mixed signals" in cells has received less attention. This review addresses the origin and cellular functions of 4-hydroxyalkernals.

Role of the transmembrane potential in the membrane proton leak

The molecular mechanism responsible for the regulation of the mitochondrial membrane proton conductance (G) is not clearly understood. This study investigates the role of the transmembrane potential (DeltaPsim) using planar membranes, reconstituted with purified uncoupling proteins (UCP1 and UCP2) and/or unsaturated FA. We show that high DeltaPsim (similar to DeltaPsim in mitochondrial State IV) significantly activates the protonophoric function of UCPs in the presence of FA. The proton conductance increases nonlinearly with DeltaPsim. The application of DeltaPsim up to 220 mV leads to the overriding of the protein inhibition at a constant ATP concentration. Both, the exposure of FA-containing bilayers to high DeltaPsim and the increase of FA membrane concentration bring about the significant exponential Gm increase, implying the contribution of FA in proton leak. Quantitative analysis of the energy barrier for the transport of FA anions in the presence and absence of protein suggests that FA- remain exposed to membrane lipids while crossing the UCP-containing membrane. We believe this study shows that UCPs and FA decrease DeltaPsim more effectively if it is sufficiently high. Thus, the tight regulation of proton conductance and/or FA concentration by DeltaPsim may be key in mitochondrial respiration and metabolism.

Reaction of ferritin with hydrogen peroxide induces lipid peroxidation

Lipid peroxidation is known to be an important factor in the pathologies of many diseases associated with oxidative stress. We assessed the lipid peroxidation induced by the reaction of ferritin with H2O2. When linoleic acid micelles or phosphatidyl choline liposomes were incubated with ferritin and H2O2, lipid peroxidation increased in the presence of ferritin and H2O2 in a concentration-dependent manner. The hydroxyl radical scavengers, azide and thiourea, prevented lipid peroxidation induced by the ferritin/H2O2 system. The iron specific chelator desferoxamine also prevented ferritin/H2O2 systemmediated lipid peroxidation. These results demonstrate the possible role of iron in ferritin/H2O2 system-mediated lipid peroxidation. Carnosine is involved in many cellular defense processes, including free radical detoxification. In this study, carnosine, homocarnosine, and anserine were shown to significantly prevent ferritin/H2O2 system-mediated lipid peroxidation and also inhibited the free radical-generation activity of ferritin. These results indicated that carnosine and related compounds may prevent ferritin/H2O2 system-mediated lipid peroxidation via free radical scavenging.

Mitochondrial uncoupling and lifespan

The quest to understand why we age has given rise to numerous lines of investigation that have gradually converged to include metabolic control by mitochondrial activity as a major player. That is, the ideal balance between nutrient uptake, its transduction into usable energy, and the mitigation of damaging byproducts can be regulated by mitochondrial respiration and output (ATP, reactive oxygen species (ROS), and heat). Mitochondrial inefficiency through proton leak, which uncouples substrate oxidation from ADP phosphorylation, can comprise as much as 30% of the basal metabolic rate. This uncoupling is hypothesized to protect cells from conditions that favor ROS production. Uncoupling can also occur through pharmacological induction of proton leak and activity of the uncoupling proteins. Mitochondrial uncoupling is implicated in lifespan extension through its effects on metabolic rate and ROS production. However, evidence to date does not suggest a consistent role for uncoupling in lifespan. The purpose of this review is to discuss recent work examining how mitochondrial uncoupling impacts lifespan.

GDP and carboxyatractylate inhibit 4-hydroxynonenal-activated proton conductance to differing degrees in mitochondria from skeletal muscle and heart

The lipid peroxidation product 4-hydroxynonenal (HNE) increases the proton conductance of the inner mitochondrial membrane through effects on uncoupling proteins (UCPs) and the adenine nucleotide translocase (ANT); however, the relative contribution of the two carriers to these effects is unclear. To clarify this we isolated mitochondria from skeletal muscle and heart of wild-type and Ucp3 knockout (Ucp3KO) mice. To increase UCP3 expression, some mice were i.p. injected with LPS (12mg/kg body weight). In spite of the increased UCP3 expression levels, basal proton conductance did not change. HNE increased the proton conductance of skeletal muscle and heart mitochondria. In skeletal muscle, this increase was lower in Ucp3KO mice and higher in LPS-treated wild-type mice, and was partially abolished by GDP (UCPs inhibitor) and completely abolished by carboxyatractylate (ANT inhibitor) or addition of both inhibitors. GDP had no effect on HNE-induced conductance in heart mitochondria, but carboxyatractylate or administration of both inhibitors had a partial effect. GDP-mediated inhibition of HNE-activated proton conductance in skeletal muscle mitochondria was not observed in Ucp3KO mice, indicating that GDP is specific for UCP3, at least in muscle. Carboxyatractylate was able to inhibit UCP3, probably through an indirect mechanism. Our results are consistent with the conclusion that, in skeletal muscle, HNE-induced increase in proton conductance is mediated by UCP3 (30%) and ANT, whereas in the heart the increase is mediated by ANT and other carriers, possibly including UCP3.

Hyperglycemia alters the schwann cell mitochondrial proteome and decreases coupled respiration in the absence of superoxide production

Hyperglycemia-induced mitochondrial dysfunction contributes to sensory neuron pathology in diabetic neuropathy. Although Schwann cells (SCs) also undergo substantial degeneration in diabetic neuropathy, the effect of hyperglycemia on the SC mitochondrial proteome and mitochondrial function has not been examined. Stable isotope labeling with amino acids in cell culture (SILAC) was used to quantify the temporal effect of hyperglycemia on the mitochondrial proteome of primary SCs isolated from neonatal rats. Of 317 mitochondrial proteins identified, about 78% were quantified and detected at multiple time points. Pathway analysis indicated that proteins associated with mitochondrial dysfunction, oxidative phosphorylation, the TCA cycle, and detoxification were significantly increased in expression and over-represented. Assessing mitochondrial respiration in intact SCs indicated that hyperglycemia increased the overall rate of oxygen consumption but decreased the efficiency of coupled respiration. Although a glucose-dependent increase in superoxide production occurs in embryonic sensory neurons, hyperglycemia did not induce a substantial change in superoxide levels in SCs. This correlated with a 1.9-fold increase in Mn superoxide dismutase expression, which was confirmed by immunoblot and enzymatic activity assays. These data support that hyperglycemia alters mitochondrial respiration and can cause remodeling of the SC mitochondrial proteome independent of significant contributions from glucose-induced superoxide production.

Mitochondrial redox metabolism: aging, longevity and dietary effects

Mitochondrial redox metabolism has long been considered to play important roles in mammalian aging and the development of age-related pathologies in the major oxidative organs. Both genetic and dietary manipulations of mitochondrial redox metabolism have been associated with the extension of lifespan. Here we provide a broad overview of the circumstantial evidence showing associations between mitochondrial reactive oxygen species (ROS) metabolism, aging and longevity. We address most aspects of mitochondrial ROS metabolism, from superoxide production, to ROS detoxification and the repair/removal of ROS-mediated macromolecular damage. Finally, we discuss the effects of dietary manipulations (e.g. caloric restriction, methionine restriction), dietary deficiencies (e.g. folate) and dietary supplementation (e.g. resveratrol) on mitochondrial ROS metabolism and lifespan.

Rapid turnover of mitochondrial uncoupling protein 3

UCP3 (uncoupling protein 3) and its homologues UCP2 and UCP1 are regulators of mitochondrial function. UCP2 is known to have a short half-life of approx. 1 h, owing to its rapid degradation by the cytosolic 26S proteasome, whereas UCP1 is turned over much more slowly by mitochondrial autophagy. In the present study we investigate whether UCP3 also has a short half-life, and whether the proteasome is involved in UCP3 degradation. UCP3 half-life was examined in the mouse C2C12 myoblast cell line by inhibiting protein synthesis with cycloheximide and monitoring UCP3 protein levels by immunoblot analysis. We show that UCP3 has a short half-life of 0.5-4 h. Rapid degradation was prevented by a cocktail of proteasome inhibitors, supporting a proteasomal mechanism for turnover. In addition, this phenotype is recapitulated in vitro: UCP3 was degraded in mitochondria isolated from rat skeletal muscle or brown adipose tissue with a half-life of 0.5-4 h, but only in the presence of a purified 26S proteasomal fraction. This in vitro proteolysis was also sensitive to proteasome inhibition. This phenotype is in direct contrast with the related proteins UCP1 and the adenine nucleotide translocase, which have long half-lives. Therefore UCP3 is turned over rapidly in multiple cell types in a proteasome-dependent manner.

New extension of the Mitchell Theory for oxidative phosphorylation in mitochondria of living organisms

The Mitchell Theory implies the proton motive force Deltap across the inner mitochondrial membrane as the energy-rich intermediate of oxidative phosphorylation. Deltap is composed mainly of an electrical (DeltaPsi(m)) and a chemical part (DeltapH) and generated by the respiratory chain complexes I, III and IV. It is consumed mostly by the ATP synthase (complex V) to produce ATP. The free energy of electron transport within the proton pumps is sufficient to generate Deltap of about 240 mV. The proton permeability of biological membranes, however, increases exponentially above 130 mV leading to a waste of energy at high values (DeltaPsi(m)>140 mV). In addition, at DeltaPsi(m)>140 mV, the production of the superoxide radical anion O(2)(-) at complexes I, II and III increases exponentially with increasing DeltaPsi(m). O(2)(-) and its neutral product H(2)O(2) (=ROS, reactive oxygen species) induce oxidative stress which participates in aging and in the generation of degenerative diseases. Here we describe a new mechanism which acts independently of the Mitchell Theory and keeps DeltaPsi(m) at low values through feedback inhibition of complex IV (cytochrome c oxidase) at high ATP/ADP ratios, thus preventing the formation of ROS and maintaining high efficiency of oxidative phosphorylation.

UCP-mediated free fatty acid uncoupling of isolated cortical mitochondria from fasted animals: correlations to dietary modulations

Uncoupling proteins (UCP) translocate protons from the mitochondrial intermembrane space to the matrix, thereby "uncoupling" electron transport from the production of ATP. It has been shown that these proteins are highly expressed in animals maintained on the ketogenic diet (KD). Although the exact mechanism remains unclear, it is known that these proteins are activated within a protective antireactive oxygen species (ROS) mechanism by free fatty acids (FFA). In our current studies, mitochondrial samples were probed for the presence of UCP2, which is the most ubiquitously expressed UCP isoform. We found that both traumatic brain injury and fasting upregulated the expression of UCP2, with a synergistic upregulation in fasted injured animals. We then used mitochondria from fasted naive animals to screen a number of FFA for their activation of uncoupling as well as their ability to reduce ROS. We found that arachidonic acid (AA), docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), palmitoleic acid, myristic acid, and butyric acid increased mitochondrial uncoupling when added after oligomycin. These FFA, along with oleic acid, also reduced ROS in mitochondria incubated with oligomycin. In order to correlate our data to KD and fasting, both of which have been shown to be neuroprotective after neurologic insult, we determined the serum levels of FFA in KD and fasted animals using gas chromatography/mass spectroscopy. We also determined brain and cerebrospinal fluid (CSF) FFA levels from fasted animals.

The effect of UCP3 overexpression on mitochondrial ROS production in skeletal muscle of young versus aged mice

Uncoupling protein 3 (UCP3) is suggested to protect mitochondria against aging and lipid-induced damage, possibly via modulation of reactive oxygen species (ROS) production. Here we show that mice overexpressing UCP3 (UCP3Tg) have a blunted age-induced increase in ROS production, assessed by electron spin resonance spectroscopy, but only after addition of 4-hydroxynonenal (4-HNE). Mitochondrial function, assessed by respirometry, on glycolytic substrate was lower in UCP3Tg mice compared to wild types, whereas this tended to be higher on fatty acids. State 4o respiration was higher in UCP3Tg animals. To conclude, UCP3 overexpression leads to increased state 4o respiration and, in presence of 4-HNE, blunts the age-induced increase in ROS production.

Atheroprotective and plaque-stabilizing effects of enzymatically modified isoquercitrin in atherogenic apoE-deficient mice

OBJECTIVE

Enzymatically modified isoquercitrin (EMIQ), isoquercitrin with malto-oligosaccharides, has been recognized as "generally recognized as safe" by the Flavor and Extracts Manufacturers Association in the United States since 2003. The long-term antiatherogenic effect of EMIQ was examined using apolipoprotein E (apoE)-deficient atherogenic mice.

METHODS

Male apoE-deficient mice (6 wk old) were fed with a high-fat diet alone or a diet containing EMIQ for 14 wk. At 20 wk old, atherosclerotic lesions in the aorta and aortic sinus were measured by morphometry and histomorphometry.

RESULTS

In apoE-deficient mice, EMIQ did not significantly affect body weight, plasma total cholesterol, triacylglycerol, and high-density lipoprotein cholesterol throughout the experiment. EMIQ significantly suppressed the aortic atherosclerotic lesion area (control 8.8 +/- 3.5% versus EMIQ 4.4 +/- 1.5%, mean +/- SD, P = 0.022). Similarly, atherosclerotic plaque lesions in the aortic sinus were significantly reduced by EMIQ (control 37.7 +/- 3.6% versus EMIQ 30.2 +/- 2.0%, P = 0.010). Of note, the immunostained area for macrophage or 4-hydroxy-2-nonenal, a well-recognized marker of oxidative stress, at the plaque in the aortic sinus was markedly suppressed, whereas the area for collagen or smooth muscle cell were increased by EMIQ, suggesting a plaque-stabilizing effect of EMIQ.

CONCLUSION

EMIQ has atheroprotective and plaque-stabilizing effects.

Stimulation of mitochondrial proton conductance by hydroxynonenal requires a high membrane potential

Mild uncoupling of oxidative phosphorylation, caused by a leak of protons back into the matrix, limits mitochondrial production of ROS (reactive oxygen species). This proton leak can be induced by the lipid peroxidation products of ROS, such as HNE (4-hydroxynonenal). HNE activates uncoupling proteins (UCP1, UCP2 and UCP3) and ANT (adenine nucleotide translocase), thereby providing a negative feedback loop. The mechanism of activation and the conditions necessary to induce uncoupling by HNE are unclear. We have found that activation of proton leak by HNE in rat and mouse skeletal muscle mitochondria is dependent on incubation with respiratory substrate. In the presence of HNE, mitochondria energized with succinate became progressively more leaky to protons over time compared with mitochondria in the absence of either HNE or succinate. Energized mitochondria must attain a high membrane potential to allow HNE to activate uncoupling: a drop of 10–20 mV from the resting value is sufficient to blunt induction of proton leak by HNE. Uncoupling occurs through UCP3 (11%), ANT (64%) and other pathways (25%). Our findings have shown that exogenous HNE only activates uncoupling at high membrane potential. These results suggest that both endogenous HNE production and high membrane potential are required before mild uncoupling will be triggered to attenuate mitochondrial ROS production.

Energization-dependent endogenous activation of proton conductance in skeletal muscle mitochondria

Leak of protons into the mitochondrial matrix during substrate oxidation partially uncouples electron transport from phosphorylation of ADP, but the functions and source of basal and inducible proton leak in vivo remain controversial. In the present study we describe an endogenous activation of proton conductance in mitochondria isolated from rat and mouse skeletal muscle following addition of respiratory substrate. This endogenous activation increased with time, required a high membrane potential and was diminished by high concentrations of serum albumin. Inhibition of this endogenous activation by GDP [classically considered specific for UCPs (uncoupling proteins)], carboxyatractylate and bongkrekate (considered specific for the adenine nucleotide translocase) was examined in skeletal muscle mitochondria from wild-type and Ucp3-knockout mice. Proton conductance through endogenously activated UCP3 was calculated as the difference in leak between mitochondria from wild-type and Ucp3-knockout mice, and was found to be inhibited by carboxyatractylate and bongkrekate, but not GDP. Proton conductance in mitochondria from Ucp3-knockout mice was strongly inhibited by carboxyatractylate, bongkrekate and partially by GDP. We conclude the following: (i) at high protonmotive force, an endogenously generated activator stimulates proton conductance catalysed partly by UCP3 and partly by the adenine nucleotide translocase; (ii) GDP is not a specific inhibitor of UCP3, but also inhibits proton translocation by the adenine nucleotide translocase; and (iii) the inhibition of UCP3 by carboxyatractylate and bongkrekate is likely to be indirect, acting through the adenine nucleotide translocase.

Mitochondria as a target for the cardioprotective effects of nitric oxide in ischemia-reperfusion injury

During cardiac ischemia-reperfusion (IR) injury, excessive generation of reactive oxygen species (ROS) and overload of Ca(2+) at the mitochondrial level both lead to opening of the mitochondrial permeability transition (PT) pore on reperfusion. This can result in the depletion of ATP, irreversible oxidation of proteins, lipids, and DNA within the cardiomyocyte, and can trigger cell-death pathways. In contrast, mitochondria are also implicated in the cardioprotective signaling processes of ischemic preconditioning (IPC), to prevent IR-related pathology. Nitric oxide (NO*) has emerged as a potent effector molecule for a variety of cardioprotective strategies, including IPC. Whereas NO* is most noted for its activation of the "classic" soluble guanylate cyclase (sGC) signaling pathway, emerging evidence indicates that NO can directly act on mitochondria, independent of the sGC pathway, affording acute cardioprotection against IR injury. These direct effects of NO* on mitochondria are the focus of this review.

Effects of 4-hydroxynonenal on mitochondrial 3-hydroxy-3-methylglutaryl (HMG-CoA) synthase

Chronic ethanol consumption causes increased production of reactive oxygen species in hepatic mitochondria accompanied by elevations in products of lipid peroxidation such as 4-hydroxynonenal (4-HNE). In the current study we investigated the effects of chronic ethanol consumption on a prominent protein-4-HNE adduct in liver mitochondria. Male Sprague-Dawley rats were fed a liquid diet for 31 days in which ethanol constituted 36% of total calories. Immunoblot analyses of liver mitochondria from ethanol-fed and control animals, using an antibody to a 4-HNE-protein adduct, demonstrated elevated 4-HNE binding (+50%) to a mitochondrial protein of approximately 55 kDa due to chronic ethanol consumption. Analysis of this protein using AspN digestion and tandem mass spectrometry identified it as the mitochondrial form of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase. Activity of the activated form of this enzyme was unchanged in livers from ethanol-fed animals, but the protein level was elevated by 36%, which suggests a compensatory mechanism to maintain constant levels of synthase activity in the mitochondrion in the face of continuous inactivation by 4-HNE. Treatment of isolated mitochondria with 4-HNE demonstrated that the enzyme activity decreased as a function of 4-HNE concentration and with time of exposure. This study demonstrates that ethanol consumption increases the formation of a 4-HNE adduct with mitochondrial HMG-CoA synthase, which has the potential to inactivate the enzyme in situ.

Induction of Endogenous Uncoupling Protein 3 Suppresses Mitochondrial Oxidant Emission during Fatty Acid-supported Respiration

Uncoupling protein 3 (UCP3) expression increases dramatically in skeletal muscle under metabolic states associated with elevated lipid metabolism, yet the function of UCP3 in a physiological context remains controversial. Here, in situ mitochondrial H(2)O(2) emission and respiration were measured in permeabilized fiber bundles prepared from both rat and mouse (wild-type) gastrocnemius muscle after a single bout of exercise plus 18 h of recovery (Ex/R) that induced a approximately 2-4-fold increase in UCP3 protein. Elevated uncoupling activity (i.e. GDP inhibitable) was evident in Ex/R fibers only upon the addition of palmitate (known activator of UCP3) or under substrate conditions eliciting substantial rates of H(2)O(2) production (i.e. respiration supported by succinate or palmitoyl-L-carnitine/malate but not pyruvate/malate), indicative of UCP3 activation by endogenous reactive oxygen species. In mice completely lacking UCP3 (ucp3(-/-)), Ex/R failed to induce uncoupling activity. Surprisingly, when UCP3 activity was inhibited by GDP (rats) or in the absence of UCP3 (ucp3(-/-)), H(2)O(2) emission was significantly (p < 0.05) higher in Ex/R versus non-exercised control fibers. Collectively, these findings demonstrate that the oxidant emitting potential of mitochondria is increased in skeletal muscle during recovery from exercise, possibly as a consequence of prolonged reliance on lipid metabolism and/or altered mitochondrial biochemistry/morphology and that induction of UCP3 in vivo mediates an increase in uncoupling activity that restores mitochondrial H(2)O(2) emission to non-exercised, control levels.

Aging process modulates nonlinear dynamics in liver cell metabolism

Regulatory dynamics of energy metabolism in living cells entails a coordinated response of multiple enzyme networks that operate under non-equilibrium conditions. Here we show that mitochondrial dysfunctions associated with the aging process significantly modify nonlinear dynamical signatures in free radical generation/removal, thereby altering energy metabolism in liver cells. We support our data with a plausible biochemical mechanism for modified bioenergetics that involves uncoupling protein-2 that is up-regulated in aged cells as an adaptive response to mitigate increased oxidative stress. Combining high spatial and temporal resolution imaging and bio-energetic measurements, our work provides experimental support to the hypothesis that mitochondria manifest nonlinear dynamical behavior for efficiently regulating energy metabolism in intact cells, and any partial or complete reduction in this behavior would contribute to organ dysfunctions including the aging process and other disease processes.

Mitochondrial Dysfunction

Innovative noninvasive methods open a new window on the cell in vivo. This window reveals that the tempo of mitochondrial dysfunction with age varies among muscles and in proportion to Type II muscle fiber content. Exercise training can reverse age-related dysfunction, thereby providing an intervention to slow the pace of aging and disability in the elderly.

Mitochondrial function, fibre types and ageing: new insights from human muscle in vivo

Mitochondrial changes are at the centre of a wide range of maladies, including diabetes, neurodegeneration and ageing-related dysfunctions. Here we describe innovative optical and magnetic resonance spectroscopic methods that non-invasively measure key mitochondrial fluxes, ATP synthesis and O(2) uptake, to permit the determination of mitochondrial coupling efficiency in vivo (P/O: half the ratio of ATP flux to O(2) uptake). Three new insights result. First, mitochondrial coupling can be measured in vivo with the rigor of a biochemical determination and provides a gold standard to define well-coupled mitochondria (P/O approximately 2.5). Second, mitochondrial coupling differs substantially among muscles in healthy adults, from values reflective of well-coupled oxidative phosphorylation in a hand muscle (P/O = 2.7) to mild uncoupling in a leg muscle (P/O = 2.0). Third, these coupling differences have an important impact on cell ageing. We found substantial uncoupling and loss of cellular [ATP] in a hand muscle indicative of mitochondrial dysfunction with age. In contrast, stable mitochondrial function was found in a leg muscle, which supports the notion that mild uncoupling is protective against mitochondrial damage with age. Thus, greater mitochondrial dysfunction is evident in muscles with higher type II muscle fibre content, which may be at the root of the preferential loss of type II fibres found in the elderly. Our results demonstrate that mitochondrial function and the tempo of ageing varies among human muscles in the same individual. These technical advances, in combination with the range of mitochondrial properties available in human muscles, provide an ideal system for studying mitochondrial function in normal tissue and the link between mitochondrial defects and cell pathology in disease.