Synergy of fatty acid and reactive alkenal activation of proton conductance through uncoupling protein 1 in mitochondria

Mitochondrial reactive oxygen species and adipose tissue thermogenesis: Bridging physiology and mechanisms

Brown and beige adipose tissues can catabolize stored energy to generate heat, relying on the principal effector of thermogenesis: uncoupling protein 1 (UCP1). This unique capability could be leveraged as a therapy for metabolic disease. Numerous animal and cellular models have now demonstrated that mitochondrial reactive oxygen species (ROS) signal to support adipocyte thermogenic identity and function. Herein, we contextualize these findings within the established principles of redox signaling and mechanistic studies of UCP1 function. We provide a framework for understanding the role of mitochondrial ROS signaling in thermogenesis together with testable hypotheses for understanding mechanisms and developing therapies.

Being right on Q: shaping eukaryotic evolution

Reactive oxygen species (ROS) formation by mitochondria is an incompletely understood eukaryotic process. I proposed a kinetic model [BioEssays (2011) 33, 88–94] in which the ratio between electrons entering the respiratory chain via FADH₂ or NADH (the F/N ratio) is a crucial determinant of ROS formation. During glucose breakdown, the ratio is low, while during fatty acid breakdown, the ratio is high (the longer the fatty acid, the higher is the ratio), leading to higher ROS levels. Thus, breakdown of (very-long-chain) fatty acids should occur without generating extra FADH₂ in mitochondria. This explains peroxisome evolution. A potential ROS increase could also explain the absence of fatty acid oxidation in long-lived cells (neurons) as well as other eukaryotic adaptations, such as dynamic supercomplex formation. Effective combinations of metabolic pathways from the host and the endosymbiont (mitochondrion) allowed larger varieties of substrates (with different F/N ratios) to be oxidized, but high F/N ratios increase ROS formation. This might have led to carnitine shuttles, uncoupling proteins, and multiple antioxidant mechanisms, especially linked to fatty acid oxidation [BioEssays (2014) 36, 634–643]. Recent data regarding peroxisome evolution and their relationships with mitochondria, ROS formation by Complex I during ischaemia/reperfusion injury, and supercomplex formation adjustment to F/N ratios strongly support the model. I will further discuss the model in the light of experimental findings regarding mitochondrial ROS formation.

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.

Mitochondrial oxidative metabolism and uncoupling proteins in the failing heart

Despite significant progress in cardiovascular medicine, myocardial ischemia and infarction, progressing eventually to the final end point heart failure (HF), remain the leading cause of morbidity and mortality in the USA. HF is a complex syndrome that results from any structural or functional impairment in ventricular filling or blood ejection. Ultimately, the heart's inability to supply the body's tissues with enough blood may lead to death. Mechanistically, the hallmarks of the failing heart include abnormal energy metabolism, increased production of reactive oxygen species (ROS) and defects in excitation-contraction coupling. HF is a highly dynamic pathological process, and observed alterations in cardiac metabolism and function depend on the disease progression. In the early stages, cardiac remodeling characterized by normal or slightly increased fatty acid (FA) oxidation plays a compensatory, cardioprotective role. However, upon progression of HF, FA oxidation and mitochondrial oxidative activity are decreased, resulting in a significant drop in cardiac ATP levels. In HF, as a compensatory response to decreased oxidative metabolism, glucose uptake and glycolysis are upregulated, but this upregulation is not sufficient to compensate for a drop in ATP production. Elevated mitochondrial ROS generation and ROS-mediated damage, when they overwhelm the cellular antioxidant defense system, induce heart injury and contribute to the progression of HF. Mitochondrial uncoupling proteins (UCPs), which promote proton leak across the inner mitochondrial membrane, have emerged as essential regulators of mitochondrial membrane potential, respiratory activity and ROS generation. Although the physiological role of UCP2 and UCP3, expressed in the heart, has not been clearly established, increasing evidence suggests that these proteins by promoting mild uncoupling could reduce mitochondrial ROS generation and cardiomyocyte apoptosis and ameliorate thereby myocardial function. Further investigation on the alterations in cardiac UCP activity and regulation will advance our understanding of their physiological roles in the healthy and diseased heart and also may facilitate the development of novel and more efficient therapies.

Skeletal muscle mitochondria exhibit decreased pyruvate oxidation capacity and increased ROS emission during surgery-induced acute insulin resistance

Development of acute insulin resistance represents a negative factor after surgery, but the underlying mechanisms are not fully understood. We investigated the postoperative changes in insulin sensitivity, mitochondrial function, enzyme activities, and release of reactive oxygen species (ROS) in skeletal muscle and liver in pigs on the 2nd postoperative day after major abdominal surgery. Peripheral and hepatic insulin sensitivity were assessed by D-[6,6-²H₂]glucose infusion and hyperinsulinemic euglycemic step clamping. Surgical trauma elicited a decline in peripheral insulin sensitivity (∼34%, P<0.01), whereas hepatic insulin sensitivity remained unchanged. Intramyofibrillar (IFM) and subsarcolemma mitochondria (SSM) isolated from skeletal muscle showed a postoperative decline in ADP-stimulated respiration (V(ADP)) for pyruvate (∼61%, P<0.05, and ∼40%, P<0.001, respectively), whereas V(ADP) for glutamate and palmitoyl-L-carnitine (PC) was unchanged. Mitochondrial leak respiration with PC was increased in SSM (1.9-fold, P<0.05) and IFM (2.5-fold, P<0.05), indicating FFA-induced uncoupling. The activity of the pyruvate dehydrogenase complex (PDC) was reduced (∼32%, P<0.01) and positively correlated to the decline in peripheral insulin sensitivity (r=0.748, P<0.05). All other mitochondrial enzyme activities were unchanged. No changes in mitochondrial function in liver were observed. Mitochondrial H₂O₂ and O₂·⁻ emission was measured spectrofluorometrically, and H₂O₂ was increased in SSM, IFM, and liver mitochondria (∼2.3-, ∼2.5-, and ∼2.3-fold, respectively, all P<0.05). We conclude that an impairment in skeletal muscle mitochondrial PDC activity and pyruvate oxidation capacity arises in the postoperative phase along with increased ROS emission, suggesting a link between mitochondrial function and development of acute postoperative insulin resistance.

How to deal with oxygen radicals stemming from mitochondrial fatty acid oxidation

Oxygen radical formation in mitochondria is an incompletely understood attribute of eukaryotic cells. Recently, a kinetic model was proposed, in which the ratio between electrons entering the respiratory chain via FADH2 or NADH determines radical formation. During glucose breakdown, the ratio is low; during fatty acid breakdown, the ratio is high (the ratio increasing--asymptotically--with fatty acid length to 0.5, when compared with 0.2 for glucose). Thus, fatty acid oxidation would generate higher levels of radical formation. As a result, breakdown of fatty acids, performed without generation of extra FADH2 in mitochondria, could be beneficial for the cell, especially in the case of long and very long chained ones. This possibly has been a major factor in the evolution of peroxisomes. Increased radical formation, as proposed by the model, can also shed light on the lack of neuronal fatty acid oxidation and tells us about hurdles during early eukaryotic evolution. We specifically focus on extending and discussing the model in light of recent publications and findings.

How the mitochondrion was shaped by radical differences in substrates: what carnitine shuttles and uncoupling tell us about mitochondrial evolution in response to ROS

As free-living organisms, alpha-proteobacteria produce reactive oxygen species (ROS) that diffuse into the surroundings; once constrained inside the archaeal ancestor of eukaryotes, however, ROS production presented evolutionary pressures - especially because the alpha-proteobacterial symbiont made more ROS, from a variety of substrates. I previously proposed that ratios of electrons coming from FADH2 and NADH (F/N ratios) correlate with ROS production levels during respiration, glucose breakdown having a much lower F/N ratio than longer fatty acid (FA) breakdown. Evidently, higher endogenous ROS formation did not hinder eukaryotic evolution, so how were its disadvantages mitigated? I propose that the resulting selection pressures favoured the evolution of a variety of eukaryotic 'innovations': peroxisomes for FA breakdown, carnitine shuttles, the linkage of beta-oxidation to antioxidant properties, uncoupling proteins (UCPs) and using mitochondrial uncoupling during beta-oxidation to reduce ROS. Recently observed relationships between peroxisomes and mitochondria further support the model.

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.

Coenzyme Q10 depletion in medical and neuropsychiatric disorders: potential repercussions and therapeutic implications

Coenzyme Q10 (CoQ10) is an antioxidant, a membrane stabilizer, and a vital cofactor in the mitochondrial electron transport chain, enabling the generation of adenosine triphosphate. It additionally regulates gene expression and apoptosis; is an essential cofactor of uncoupling proteins; and has anti-inflammatory, redox modulatory, and neuroprotective effects. This paper reviews the known physiological role of CoQ10 in cellular metabolism, cell death, differentiation and gene regulation, and examines the potential repercussions of CoQ10 depletion including its role in illnesses such as Parkinson's disease, depression, myalgic encephalomyelitis/chronic fatigue syndrome, and fibromyalgia. CoQ10 depletion may play a role in the pathophysiology of these disorders by modulating cellular processes including hydrogen peroxide formation, gene regulation, cytoprotection, bioenegetic performance, and regulation of cellular metabolism. CoQ10 treatment improves quality of life in patients with Parkinson's disease and may play a role in delaying the progression of that disorder. Administration of CoQ10 has antidepressive effects. CoQ10 treatment significantly reduces fatigue and improves ergonomic performance during exercise and thus may have potential in alleviating the exercise intolerance and exhaustion displayed by people with myalgic encepholamyletis/chronic fatigue syndrome. Administration of CoQ10 improves hyperalgesia and quality of life in patients with fibromyalgia. The evidence base for the effectiveness of treatment with CoQ10 may be explained via its ability to ameliorate oxidative stress and protect mitochondria.

A lipidomics analysis of the relationship between dietary fatty acid composition and insulin sensitivity in young adults

Relative to diets enriched in palmitic acid (PA), diets rich in oleic acid (OA) are associated with reduced risk of type 2 diabetes. To gain insight into mechanisms underlying these observations, we applied comprehensive lipidomic profiling to specimens collected from healthy adults enrolled in a randomized, crossover trial comparing a high-PA diet to a low-PA/high-OA (HOA) diet. Effects on insulin sensitivity (SI) and disposition index (DI) were assessed by intravenous glucose tolerance testing. In women, but not men, SI and DI were higher during HOA. The effect of HOA on SI correlated positively with physical fitness upon enrollment. Principal components analysis of either fasted or fed-state metabolites identified one factor affected by diet and heavily weighted by the PA/OA ratio of serum and muscle lipids. In women, this factor correlated inversely with SI in the fasted and fed states. Medium-chain acylcarnitines emerged as strong negative correlates of SI, and the HOA diet was accompanied by lower serum and muscle ceramide concentrations and reductions in molecular biomarkers of inflammatory and oxidative stress. This study provides evidence that the dietary PA/OA ratio impacts diabetes risk in women.

Flies, worms and the Free Radical Theory of ageing

Drosophila and Caenorhabditis elegans have provided the largest body of evidence addressing the Free Radical Theory of ageing, however the evidence has not been unequivocally supportive. Oxidative damage to DNA is probably not a major contributor, damage to lipids is assuming greater importance and damage to proteins probably the source of pathology. On balance the evidence does not support a primary role of oxidative damage in ageing in C. elegans, perhaps because of its particular energy metabolic and stress resistance profile. Evidence is more numerous, varied and consistent and hence more compelling for Drosophila, although not conclusive. However there is good evidence for a role of oxidative damage in later life pathology. Future work should: 1/ make more use of protein oxidative damage measurements; 2/ use inducible transgenic systems or pharmacotherapy to ensure genetic equivalence of controls and avoid confounding effects during development; 3/ to try to delay ageing, target interventions which reduce and/or repair protein oxidative damage.

Fatty acids change the conformation of uncoupling protein 1 (UCP1)

UCP1 catalyzes proton leak across the mitochondrial inner membrane to disengage substrate oxidation from ATP production. It is well established that UCP1 is activated by fatty acids and inhibited by purine nucleotides, but precisely how this regulation occurs remains unsettled. Although fatty acids can competitively overcome nucleotide inhibition in functional assays, fatty acids have little effect on purine nucleotide binding. Here, we present the first demonstration that fatty acids induce a conformational change in UCP1. Palmitate dramatically changed the binding kinetics of 2'/3'-O-(N-methylanthraniloyl)-GDP, a fluorescently labeled nucleotide analog, for UCP1. Furthermore, palmitate accelerated the rate of enzymatic proteolysis of UCP1. The altered kinetics of both processes indicate that fatty acids change the conformation of UCP1, reconciling the apparent discrepancy between existing functional and ligand binding data. Our results provide a framework for how fatty acids and nucleotides compete to regulate the activity of UCP1.

Hydroxynonenal, a lipid peroxidation end product, stimulates uncoupling protein activity in Acanthamoeba castellanii mitochondria; the sensitivity of the inducible activity to purine nucleotides depends on the membranous ubiquinone redox state

We studied the influence of exogenously generated superoxide and exogenous 4-hydroxy-2-nonenal (HNE), a lipid peroxidation end product, on the activity of the Acanthamoeba castellanii uncoupling protein (AcUCP). The superoxide-generating xanthine/xanthine oxidase system was insufficient to induce mitochondrial uncoupling. In contrast, exogenously added HNE induced GTP-sensitive AcUCP-mediated mitochondrial uncoupling. In non-phosphorylating mitochondria, AcUCP activation by HNE was demonstrated by increased oxygen consumption accompanied by a decreased membrane potential and ubiquinone (Q) reduction level. The HNE-induced GTP-sensitive proton conductance was similar to that observed with linoleic acid. In phosphorylating mitochondria, the HNE-induced AcUCP-mediated uncoupling decreased the yield of oxidative phosphorylation. We demonstrated that the efficiency of GTP to inhibit HNE-induced AcUCP-mediated uncoupling was regulated by the endogenous Q redox state. A high Q reduction level activated AcUCP by relieving the inhibition caused by GTP while a low Q reduction level favoured the inhibition. We propose that the regulation of UCP activity involves a rapid response through the endogenous Q redox state that modulates the inhibition of UCP by purine nucleotides, followed by a late response through lipid peroxidation products resulting from an increase in the formation of reactive oxygen species that modulate the UCP activation.

Functional characterization of UCP1 in mammalian HEK293 cells excludes mitochondrial uncoupling artefacts and reveals no contribution to basal proton leak

Mechanistic studies on uncoupling proteins (UCPs) not only are important to identify their cellular function but also are pivotal to identify potential drug targets to manipulate mitochondrial energy transduction. So far, functional and comparative studies of uncoupling proteins in their native environment are hampered by different mitochondrial, cellular and genetic backgrounds. Artificial systems such as yeast ectopically expressing UCPs or liposomes with reconstituted UCPs were employed to address crucial mechanistic questions but these systems also produced inconsistencies with results from native mitochondria. We here introduce a novel mammalian cell culture system (Human Embryonic Kidney 293 - HEK293) to study UCP1 function. Stably transfected HEK293 cell lines were derived that contain mouse UCP1 at concentrations comparable to tissue mitochondria. In this cell-based test system UCP1 displays native functional behaviour as it can be activated with fatty acids (palmitate) and inhibited with purine nucleotides guanosine-diphosphate (GDP). The catalytic centre activity of the UCP1 homodimer in HEK293 is comparable to activities in brown adipose tissue supporting functionality of UCP1. Importantly, at higher protein levels than in yeast mitochondria, UCP1 in HEK293 cell mitochondria is fully inhibitable and does not contribute to basal proton conductance, thereby emphasizing the requirement of UCP1 activation for therapeutic purposes. These findings and resulting analysis on UCP1 characteristics demonstrate that the mammalian HEK293 cell system is suitable for mechanistic and comparative functional studies on UCPs and provides a non-confounding mitochondrial, cellular and genetic background.

Expression of uncoupling proteins in a mammalian cell culture system (HEK293) and assessment of their protein function

Immortalised cultured cells are powerful tools to assess the function of ectopically expressed proteins. However, it must be ensured that the protein of interest is functional in the host system and display native behaviour. In particular, mitochondrial uncoupling proteins (UCPs) displayed (non-native) artefactual uncoupling when expressed in yeast or can possess functions upon reconstitution in proteoliposomes that cannot be reproduced in isolated mitochondria. In the light of newly discovered UCP1 orthologues and paralogues (UCP2, UCP3, plant UCP), comparative functional studies require a system with identical mitochondrial, cellular, and genetic backgrounds. In this chapter, the protocols for the ectopic expression of mouse UCP1 in the human embryonic kidney (HEK293) cell line are introduced. In isolated cell culture mitochondria, the proton leak can be measured and modulators of UCP1 activity can be tested. As mouse UCP1 in this system shows native behaviour, this may be a suitable system to directly compare the functional relationships between different UCPs.

Relationship of electrophilic stress to aging

This review begins with the premise that an organism's life span is determined by the balance between two countervailing forces: (i) the sum of destabilizing effects and (ii) the sum of protective longevity-assurance processes. Against this backdrop, the role of electrophiles is discussed, both as destabilizing factors and as signals that induce protective responses. Because most biological macromolecules contain nucleophilic centers, electrophiles are particularly reactive and toxic in a biological context. The majority of cellular electrophiles are generated from polyunsaturated fatty acids by a peroxidation chain reaction that is readily triggered by oxygen-centered radicals, but propagates without further input of reactive oxygen species (ROS). Thus, the formation of lipid-derived electrophiles such as 4-hydroxynon-2-enal (4-HNE) is proposed to be relatively insensitive to the level of initiating ROS, but to depend mainly on the availability of peroxidation-susceptible fatty acids. This is consistent with numerous observations that life span is inversely correlated to membrane peroxidizability, and with the hypothesis that 4-HNE may constitute the mechanistic link between high susceptibility of membrane lipids to peroxidation and shortened life span. Experimental interventions that directly alter membrane composition (and thus their peroxidizability) or modulate 4-HNE levels have the expected effects on life span, establishing that the connection is not only correlative but causal. Specific molecular mechanisms are considered, by which 4-HNE could (i) destabilize biological systems via nontargeted reactions with cellular macromolecules and (ii) modulate signaling pathways that control longevity-assurance mechanisms.

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.

Test systems to study the structure and function of uncoupling protein 1: a critical overview

The discovery of active brown adipose tissue (BAT) in healthy adult humans has renewed interest in the biology of this organ. BAT is capable of distributing nutrient energy in the form of heat allowing small mammals to efficiently defend their body temperature when acutely exposed to the cold. On the other hand BAT might be a target for the treatment of obesity and related diseases, as its pharmacological activation could allow release of excess energy stored in white adipose tissue depots. Energy dissipation in BAT depends on the activity of uncoupling protein 1 (UCP1), therefore a BAT-based obesity therapy requires a detailed understanding of structure and function of UCP1. Although UCP1 has been in the focus of research since its discovery, central questions concerning its mechanistic function and regulation are not yet resolved. They have been addressed in native mitochondria but also in several test systems, which are generally used to lower inter-experimental variability and to simplify analysis conditions. Different test systems have contributed to our current knowledge about UCP1 but of course all of them have certain limitations. We here provide an overview about research on UCP1 structure and function in test systems. So far, these have nearly exclusively been employed to study rodent and not human UCP1. Considering that the amino acid sequence of mouse and human UCP1 is only 79% identical, it will be essential to test whether the human version has a similarly high catalytic activity, allowing a relevant amount of energy dissipation in human BAT. Besides the issue of comparable mechanistic function a sufficiently high expression level of human UCP1 is a further prerequisite for anti-obesity therapeutic potential. Treatments which induce BAT hyperplasia and UCP1 expression in humans might therefore be equally important to discover as mere activators of the thermogenic process.

Molecular cloning of amphioxus uncoupling protein and assessment of its uncoupling activity using a yeast heterologous expression system

The present study describes the molecular cloning of a novel cDNA fragment from amphioxus (Branchiostoma belcheri) encoding a 343-amino acid protein that is highly homologous to human uncoupling proteins (UCP), this protein is therefore named amphioxus UCP. This amphioxus UCP shares more homology with and is phylogenetically more related to mammalian UCP2 as compared with UCP1. To further assess the functional similarity of amphioxus UCP to mammalian UCP1 and -2, the amphioxus UCP, rat UCP1, and human UCP2 were separately expressed in Saccharomyces cerevisiae, and the recombinant yeast mitochondria were isolated and assayed for the state 4 respiration rate and proton leak, using pYES2 empty vector as the control. UCP1 increased the state 4 respiration rate by 2.8-fold, and the uncoupling activity was strongly inhibited by GDP, while UCP2 and amphioxus UCP only increased the state 4 respiration rate by 1.5-fold and 1.7-fold in a GDP-insensitive manner, moreover, the proton leak kinetics of amphioxus UCP was very similar to UCP2, but much different from UCP1. In conclusion, the amphioxus UCP has a mild, unregulated uncoupling activity in the yeast system, which resembles mammalian UCP2, but not UCP1.

The regulation and turnover of mitochondrial uncoupling proteins

Uncoupling proteins (UCP1, UCP2 and UCP3) are important in regulating cellular fuel metabolism and as attenuators of reactive oxygen species production through strong or mild uncoupling. The generic function and broad tissue distribution of the uncoupling protein family means that they are increasingly implicated in a range of pathophysiological processes including obesity, insulin resistance and diabetes mellitus, neurodegeneration, cardiovascular disease, immunity and cancer. The significant recent progress describing the turnover of novel uncoupling proteins, as well as current views on the physiological roles and regulation of UCPs, is outlined.

Not all mitochondrial carrier proteins support permeability transition pore formation: no involvement of uncoupling protein 1

The mPTP (mitochondrial permeability transition pore) is a non-specific channel that is formed in the mitochondrial inner membrane in response to several stimuli, including elevated levels of matrix calcium. The pore is proposed to be composed of the ANT (adenine nucleotide translocase), voltage-dependent anion channel and cyclophilin D. Knockout studies, however, have demonstrated that ANT is not essential for permeability transition, which has led to the proposal that other members of the mitochondrial carrier protein family may be able to play a similar function to ANT in pore formation. To investigate this possibility, we have studied the permeability transition properties of BAT (brown adipose tissue) mitochondria in which levels of the mitochondrial carrier protein, UCP1 (uncoupling protein 1), can exceed those of ANT. Using an improved spectroscopic assay, we have quantified mPTP formation in de-energized mitochondria from wild-type and Ucp1KO (Ucp1-knockout) mice and assessed the dependence of pore formation on UCP1. When correctly normalized for differences in mitochondrial morphology, we find that calcium-induced mPTP activity is the same in both types of mitochondria, with similar sensitivity to GDP (approximately 50% inhibited), although the portion sensitive to cyclosporin A is higher in mitochondria lacking UCP1 (approximately 80% inhibited, compared with approximately 60% in mitochondria containing UCP1). We conclude that UCP1 is not a component of the cyclosporin A-sensitive mPTP in BAT and that playing a role in mPTP formation is not a general characteristic of the mitochondrial carrier protein family but is, more likely, restricted to specific members including ANT.

Uncoupling protein 1 inhibition by purine nucleotides is under the control of the endogenous ubiquinone redox state

We studied non-esterified fatty acid-induced uncoupling of heterologously expressed rat UCP1 (uncoupling protein 1) in yeast mitochondria, as well as UCP1 in rat BAT (brown adipose tissue) mitochondria. The proton-conductance curves and the relationship between the ubiquinone reduction level and membrane potential were determined in non-phosphorylating BAT and yeast mitochondria. The ADP/O method was applied to determine the ADP phosphorylation rate and the relationship between the ubiquinone reduction level and respiration rate in yeast mitochondria. Our studies of the membranous ubiquinone reduction level in mitochondria demonstrate that activation of UCP1 leads to a purine nucleotide-sensitive decrease in the ubiquinone redox state. Results obtained for non-phosphorylating and phosphorylating mitochondria, as the endogenous ubiquinone redox state was gradually varied by a lowering rate of the ubiquinone-reducing or ubiquinol-oxidizing pathways, indicate that the endogenous ubiquinone redox state has no effect on non-esterified fatty acid-induced UCP1 activity in the absence of GTP, and can only regulate this activity through sensitivity to inhibition by the purine nucleotide. At a given oleic acid concentration, inhibition by GTP diminishes when ubiquinone is reduced sufficiently. The ubiquinone redox state-dependent alleviation of UCP1 inhibition by the purine nucleotide was observed at a high ubiquinone reduction level, when it exceeded 85-88%.

Uncoupling protein-1 (UCP1) contributes to the basal proton conductance of brown adipose tissue mitochondria

Proton leak pathways uncouple substrate oxidation from ATP synthesis in mitochondria. These pathways are classified as basal (not regulated) or inducible (activated and inhibited). Previously it was found that over half of the basal proton conductance of muscle mitochondria was catalyzed by the adenine nucleotide translocase (ANT), an abundant mitochondrial anion carrier protein. To determine whether ANT is the unique protein catalyst, or one of many proteins that catalyze basal proton conductance, we measured proton leak kinetics in mitochondria isolated from brown adipose tissue (BAT). BAT can express another mitochondrial anion carrier, UCP1, at concentrations similar to ANT. Basal proton conductance was measured under conditions where UCP1 and ANT were catalytically inactive and was found to be lower in mitochondria from UCP1 knockout mice compared to wild-type. Ablation of another abundant inner membrane protein, nicotinamide nucleotide transhydrogenase, had no effect on proton leak kinetics in mitochondria from liver, kidney or muscle, showing that basal proton conductance is not catalyzed by all membrane proteins. We identify UCP1 as a second protein propagating basal proton leak, lending support to the hypothesis that basal leak pathways are perpetrated by members of the mitochondrial anion carrier family but not by other mitochondrial inner membrane proteins.

HDMCP uncouples yeast mitochondrial respiration and alleviates steatosis in L02 and hepG2 cells by decreasing ATP and H2O2 levels: a novel mechanism for NAFLD

BACKGROUND/AIMS

To explore the uncoupling activity of hepatocelluar downregulated mitochondrial carrier protein (HDMCP) in a yeast expression system and its function in non-alcoholic fatty liver disease (NAFLD).

METHODS

Molecular cloning and RT-PCR were used for yeast protein expression and uncoupling activity was assessed. Western blot analysis was used to determine HDMCP level in rat NAFLD and steatotic L02 and hepG2 cell models where their presence was confirmed by pathologic (Nile red and H-E staining) and biochemical changes. RNA interference was used to knock down HDMCP level and mitochondrial ATP and hydroperoxide levels were measured for potential mechanism exploration.

RESULTS

We found a significant GDP insensitive uncoupling activity of HDMCP in yeast mitochondria and its increased expression in animal and cell models. HDMCP was significantly increased with culture time and steatosis was aggravated when HDMCP level was knocked down. Furthermore, we found that HDMCP might function through promoting ATP depletion and decreasing H(2)O(2) production.

CONCLUSION

This study adds supportive data to the hypothesis that HDMCP might be a long postulated liver-specific uncoupling protein and broadens our understanding of the pathogenesis of NAFLD. More importantly, HDMCP might become a novel drug target for its ability in alleviating hepatic steatosis.

Effect of aging, caloric restriction, and uncoupling protein 3 (UCP3) on mitochondrial proton leak in mice

Mitochondrial proton leak may modulate reactive oxygen species (ROS) production and play a role in aging. The purpose of this study was to determine proton leak across the life span in skeletal mitochondria from calorie-restricted and UCP2/3 overexpressing mice. Proton leak in isolated mitochondria and markers of oxidative stress in whole tissue were measured in female C57BL/6J mice fed ad-libitum (WT-Control) or a 30% calorie-restricted (WT-CR) diet, and in mice overexpressing UCP2 and UCP3 (Positive-TG), their non-overexpressing littermates (Negative-TG) and UCP3 knockout mice (UCP3KO). Proton leak in WT-CR mice was lower than that of control mice at 8 and 26 months of age. The Positive-TG mice had greater proton leak than the Negative-TG and UCP3KO mice at 8 months of age, but this difference disappeared by 19 and 26 months. Lipid peroxidation was generally lower in WT-CR vs. WT-Control mice and UCP3KO mice had greater concentrations of T-BARS (thiobarbituric acid reactive substances, a measure of lipid peroxidation) than did Positive-TG and Negative-TG. The results of this study indicate that sustained increases in muscle mitochondrial proton leak are not responsible for alterations in life span with calorie restriction or UCP3 overexpression in mice. However, UCP3 may contribute to the actions of CR through mechanisms distinct from increasing basal proton leak.

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.

Cold-induced alterations of phospholipid fatty acyl composition in brown adipose tissue mitochondria are independent of uncoupling protein-1

The recruitment process induced by acclimation of mammals to cold includes a marked alteration in the acyl composition of the phospholipids of mitochondria from brown adipose tissue: increases in 18:0, 18:2(n-6), and 20:4(n-6) and decreases in 16:0, 16:1, 18:1, and 22:6(n-3). A basic question is whether these alterations are caused by changes in the concentration of uncoupling protein-1 (UCP1) or the thermogenesis it mediates-implying that they are secondary effects-or whether they are an integrated, independent part of the recruitment process. This question was addressed here using wild-type and UCP1-ablated C57BL/6 mice acclimated to 24 degrees C or 4 degrees C. In wild-type mice, the phospholipid fatty acyl composition of mitochondria from brown adipose tissue showed the changes in response to cold that were expected from observations in other species and strains. The changes were specific, as different changes occurred in skeletal muscle mitochondria. In UCP1-ablated mice, cold acclimation induced acyl alterations in brown adipose tissue that were qualitatively identical and quantitatively similar to those in wild-type mice. Therefore, neither the increased content of UCP1 nor mitochondrial uncoupling altered the effect of cold on acyl composition. Cold acclimation in wild-type mice had little effect on phospholipid acyl composition in muscle mitochondria, but cold-acclimation in UCP1-ablated mice caused significant alterations, probably due to sustained shivering. Thus, the alterations in brown adipose tissue phospholipid acyl composition are revealed to be an independent part of the recruitment process, and their functional significance for thermogenesis should be elucidated.

Functional characterisation of UCP1 in the common carp: uncoupling activity in liver mitochondria and cold-induced expression in the brain

Mammalian uncoupling protein 1 (UCP1) mediates nonshivering thermogenesis in brown adipose tissue. We previously reported on the presence of a UCP1 orthologue in ectothermic fish and observed downregulation of UCP1 gene expression in the liver of the common carp. Neither the function of UCP1, nor the mode of UCP1 activation is known in carp liver mitochondria. Here, we compared the proton conductance at 25 degrees C of liver mitochondria isolated from carp either maintained at 20 degrees C (warm-acclimated, WA) or exposed to 8 degrees C (cold-acclimated, CA) water temperature for 7-10 days. Liver mitochondria from WA carp had higher state four rates of oxygen consumption and greater proton conductance at high membrane potential. Liver mitochondria from WA, but not from CA, carp showed a strong increase in proton conductance when palmitate (or 4-hydroxy-trans-2-nonenal, HNE) was added, and this inducible proton conductance was prevented by addition of GDP. This fatty acid sensitive proton leak is likely due to the expression of UCP1 in the liver of WA carp. The observed biochemical properties of proton leak strongly suggest that carp UCP1 is a functional uncoupling protein with broadly the same activatory and inhibitory characteristics as mammalian UCP1. Significant UCP1 expression was also detected in our previous study in whole brain of the carp. We here observed a twofold increase of UCP1 mRNA in carp brain following cold exposure, suggesting a role of UCP1 in the thermal adaptation of brain metabolism. In situ hybridization located the UCP1 gene expression to the optic tectum responsible for visual system control, the descending trigeminal tract and the solitary tract. Taken together, this study characterises uncoupling protein activity in an ectotherm for the first time.

Fatty acid transport protein 1 is required for nonshivering thermogenesis in brown adipose tissue

Nonshivering thermogenesis in brown adipose tissue (BAT) generates heat through the uncoupling of mitochondrial beta-oxidation from ATP production. The principal energy source for this process is fatty acids that are either synthesized de novo in BAT or are imported from circulation. How uptake of fatty acids is mediated and regulated has remained unclear. Here, we show that fatty acid transport protein (FATP)1 is expressed on the plasma membrane of BAT and is upregulated in response to cold stimuli, concomitant with an increase in the rate of fatty acid uptake. In FATP1-null animals, basal fatty acid uptake is reduced and remains unchanged following cold exposure. As a consequence, FATP1 knockout (KO) animals display smaller lipid droplets in BAT and fail to defend their core body temperature at 4 degrees C, despite elevated serum free fatty acid levels. Similarly, FATP1 is expressed by the BAT-derived cell line HIB-1B upon differentiation, and both fatty acid uptake and FATP1 protein levels are rapidly elevated following isoproterenol stimulation. Stimulation of fatty uptake by isoproterenol required both protein kinase A and mitogen-activated kinase signaling and is completely dependent on FATP1 expression, as small-hairpin RNA-mediated knock down of FATP1 abrogated the effect.