Uncoupling proteins 2 and 3 are highly active H(+) transporters and highly nucleotide sensitive when activated by coenzyme Q (ubiquinone)

The mitochondrial uncoupling-protein homologues

Uncoupling protein(UCP)1 is an integral membrane protein that is located in the mitochondrial inner membrane of brown adipocytes. Its physiological role is to mediate a regulated, thermogenic proton leak. UCP2 and UCP3 are recently identified UCP1 homologues. They also mediate regulated proton leak, and might function to control the production of superoxide and other downstream reactive oxygen species. However, their role in normal physiology remains unknown. Recent studies have shown that UCP2 has an important part in the pathogenesis of type-2 diabetes. The obscure roles of the UCP homologues in normal physiology, together with their emerging role in pathophysiology, provide exciting potential for further investigation.

Targeting cellular energy production in neurological disorders

The concepts of energy dysregulation and oxidative stress and their complicated interdependence have rapidly evolved to assume primary importance in understanding the pathophysiology of numerous neurological disorders. Therefore, neuroprotective strategies addressing specific bioenergetic defects hold particular promise in the treatment of these conditions (i.e., amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease, Friedreich's ataxia, mitochondrial cytopathies and other neuromuscular diseases), all of which, to some extent, share 'the final common pathway' leading to cell death through either necrosis or apoptosis. Compounds such as creatine monohydrate and coenzyme Q(10) offer substantial neuroprotection against ischaemia, trauma, oxidative damage and neurotoxins. Miscellaneous agents, including alpha-lipoic acid, beta-OH-beta-methylbutyrate, riboflavin and nicotinamide, have also been shown to improve various metabolic parameters in brain and/or muscle. This review will highlight the biological function of each of the above mentioned compounds followed by a discussion of their utility in animal models and human neurological disease. The balance of this work will be comprised of discussions on the therapeutic applications of creatine and coenzyme Q(10).

Thyroid-hormone effects on putative biochemical pathways involved in UCP3 activation in rat skeletal muscle mitochondria

In vitro, uncoupling protein 3 (UCP3)-mediated uncoupling requires cofactors [e.g., superoxides, coenzyme Q (CoQ) and fatty acids (FA)] or their derivatives, but it is not yet clear whether or how such activators interact with each other under given physiological or pathophysiological conditions. Since triiodothyronine (T3) stimulates lipid metabolism, UCP3 expression and mitochondrial uncoupling, we examined its effects on some biochemical pathways that may underlie UCP3-mediated uncoupling. T3-treated rats (Hyper) showed increased mitochondrial lipid-oxidation rates, increased expression and activity of enzymes involved in lipid handling and increased mitochondrial superoxide production and CoQ levels. Despite the higher mitochondrial superoxide production in Hyper, euthyroid and hyperthyroid mitochondria showed no differences in proton-conductance when FA were chelated by bovine serum albumin. However, mitochondria from Hyper showed a palmitoyl-carnitine-induced and GDP-inhibited increased proton-conductance in the presence of carboxyatractylate. We suggest that T3 stimulates the UCP3 activity in vivo by affecting the complex network of biochemical pathways underlying the UCP3 activation.

Biological validation of coenzyme Q redox state by HPLC-EC measurement: relationship between coenzyme Q redox state and coenzyme Q content in rat tissues

The properties of coenzymes Q (CoQ9 and CoQ10) are closely linked to their redox state (CoQox/total CoQ) x 100. In this work, CoQ redox state was biologically validated by high performance liquid chromatography-electrochemical measurement after modulation of mitochondrial electron flow of cultured cells by molecules increasing (rotenone, carbonyl cyanide chlorophenylhydrazone) or decreasing (antimycin) CoQ oxidation. The tissue specificity of CoQ redox state and content were investigated in control and hypoxic rats. In control rats, there was a strong negative linear regression between tissular CoQ redox state and CoQ content. Hypoxia increased CoQ9 redox state and decreased CoQ9 content in a negative linear relationship in the different tissues, except the heart and lung. This result demonstrates that, under conditions of mitochondrial impairment, CoQ redox control is tissue-specific.

Coenzyme Q10 affects expression of genes involved in cell signalling, metabolism and transport in human CaCo-2 cells

Coenzyme Q10 is an essential cofactor in the electron transport chain and serves as an important antioxidant in both mitochondria and lipid membranes. CoQ10 is also an obligatory cofactor for the function of uncoupling proteins. Furthermore, dietary supplementation affecting CoQ10 levels has been shown in a number of organisms to cause multiple phenotypic effects. However, the molecular mechanisms to explain pleiotrophic effects of CoQ10 are not clear yet and it is likely that CoQ10 targets the expression of multiple genes. We therefore utilized gene expression profiling based on human oligonucleotide sequences to examine the expression in the human intestinal cell line CaCo-2 in relation to CoQ10 treatment. CoQ10 caused an increased expression of 694 genes at threshold-factor of 2.0 or more. Only one gene was down-regulated 1.5-2-fold. Real-time RT-PCR confirmed the differential expression for seven selected target genes. The identified genes encode proteins involved in cell signalling (n = 79), intermediary metabolism (n = 58), transport (n = 47), transcription control (n = 32), disease mutation (n = 24), phosphorylation (n = 19), embryonal development (n = 13) and binding (n = 9). In conclusion, these findings indicate a prominent role of CoQ10 as a potent gene regulator. The presently identified comprehensive list of genes regulated by CoQ10 may be used for further studies to identify the molecular mechanism of CoQ10 on gene expression.

Specificity of coenzyme Q10 for a balanced function of respiratory chain and endogenous ubiquinone biosynthesis in human cells

Coenzyme Q (Q) is an obligatory component of both respiratory chain and uncoupling proteins. Also, Q acts as an antioxidant in cellular membranes. Several neurodegenerative diseases are associated with modifications of Q10 levels. For these reasons, therapies based on Q supplementation in the diet are currently studied in order to mitigate the symptoms of these diseases. However, the incorporation of exogenous Q also affects aging process in nematodes probably affecting reactive oxygen species (ROS) production. The aim of the present work is to clarify if supplementation with both Q10 and Q6 isoforms affects mitochondrial Q10 content, respiratory chain activity and ROS levels in human cells. Cells incorporated exogenously added Q10 and Q6 isoforms into mitochondria that produced changes in mitochondrial activity depending on the side chain length. Supplementation with Q10, but not with Q6, increased mitochondrial Q-dependent activities. However, Q6 affected the mitochondrial membrane potential, ROS production, and increased the protein levels of both catalase and Mn-superoxide dismutase (Mn-SOD). Also, Q6 induced a transient decrease in endogenous mitochondrial Q10 levels by increasing its catabolism. These results show that human cells supplemented with Q6 undergo a mitochondrial impairment, which is not observed with Q10 supplementation.

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

In this mini review we summarize recent studies from our laboratory that show the involvement of superoxide and the lipid peroxidation product 4-hydroxynonenal in the regulation of mitochondrial uncoupling. Superoxide produced during mitochondrial respiration is a major cause of the cellular oxidative damage that may underlie degenerative diseases and ageing. Superoxide production is very sensitive to the magnitude of the mitochondrial protonmotive force, so can be strongly decreased by mild uncoupling. Superoxide is able to give rise to other reactive oxygen species, which elicit deleterious effects primarily by oxidizing intracellular components, including lipids, DNA and proteins. Superoxide-induced lipid peroxidation leads to the production of reactive aldehydes, including 4-hydroxynonenal. These aldehydic lipid peroxidation products are in turn able to modify proteins such as mitochondrial uncoupling proteins and the adenine nucleotide translocase, converting them into active proton transporters. This activation induces mild uncoupling and so diminishes mitochondrial superoxide production, hence protecting against disease and oxidative damage at the expense of energy production.

Role of energy charge and AMP-activated protein kinase in adipocytes in the control of body fat stores

As indicated by in vitro studies, both lipogenesis and lipolysis in adipocytes depend on the cellular ATP levels. Ectopic expression of mitochondrial uncoupling protein 1 (UCP1) in the white adipose tissue of the aP2-Ucp1 transgenic mice reduced obesity induced by genetic or dietary manipulations. Furthermore, respiratory uncoupling lowered the cellular energy charge in adipocytes, while the synthesis of fatty acids (FA) was inhibited and their oxidation increased. Importantly, the complex metabolic changes triggered by ectopic UCP1 were associated with the activation of AMP-activated protein kinase (AMPK), a metabolic master switch, in adipocytes. Effects of several typical treatments that reduce adiposity, such as administration of leptin, beta-adrenoceptor agonists, bezafibrate, dietary n-3 polyunsaturated FA or fasting, can be compared with a phenotype of the aP2-Ucp1 mice. These situations generally lead to the upregulation of mitochondrial UCPs and suppression of the cellular energy charge and FA synthesis in adipocytes. On the other hand, FA oxidation is increased. Moreover, it has been shown that AMPK in adipocytes can be activated by adipocyte-derived hormones leptin and adiponectin, and also by insulin-sensitizes thiazolidinediones. Thus, it is evident that metabolism of adipose tissue itself is important for the control of body fat content and that the cellular energy charge and AMPK are involved in the control of lipid metabolism in adipocytes. The reciprocal link between synthesis and oxidation of FA in adipocytes represents a prospective target for the new treatment strategies aimed at reducing obesity.

Uncoupling protein-3 sensitizes cells to mitochondrial-dependent stimulus of apoptosis

The mitochondrial uncoupling protein-3 is a member of the mitochondrial carrier protein family. As a homologue of the thermogenic brown fat uncoupling protein-1, it possesses a mitochondrial uncoupling activity and thus can influence cell energy metabolism but its exact biological function remains unclear. In the present study, uncoupling protein-3 was expressed in 293 cells using the tetracycline-inducible system and its impact on cell bioenergetics and responsiveness to the apoptotic stimulus was determined. The induction of uncoupling protein-3 expression in mitochondria did not lead to uncontrolled respiratory uncoupling in intact cells. However, it caused a GDP-inhibition of state 4 respiration and a GDP-induced re-polarization of the inner mitochondrial membrane in the presence of fatty acids, in agreement with its expected physiological behavior as an uncoupling protein (UCP). Uncoupling protein-3 expression did not cause apoptosis per se but increased the responsiveness of the cells to a mitochondrial apoptotic stimulus (i.e., addition of staurosporine in the culture medium). It enhanced caspase 3 and caspase 9 activation and favored cytochrome c release. Moreover, cells in which uncoupling protein-3 expression had been induced showed a higher mitochondrial Bax/Bcl-2 ratio essentially due to enhanced translocation of Bax from cytosol to mitochondria. Finally, the induction of uncoupling protein-3 also increased the sensitivity of mitochondria to open the permeability transition pore in response to calcium. It is concluded that the presence of uncoupling protein-3 in mitochondria sensitizes cells to apoptotic stimuli involving mitochondrial pathways.

Role of UCP3 in state 4 respiration during contractile activity-induced mitochondrial biogenesis

In an effort to better characterize uncoupling protein-3 (UCP3) function in skeletal muscle, we assessed basal UCP3 protein content in rat intermyofibrillar (IMF) and subsarcolemmal (SS) mitochondrial subfractions in conjunction with measurements of state 4 respiration. UCP3 content was 1.3-fold ( P < 0.05) greater in IMF compared with SS mitochondria. State 4 respiration was 2.6-fold greater ( P < 0.05) in the IMF subfraction than in SS mitochondria. GDP attenuated state 4 respiration by ∼40% ( P < 0.05) in both subfractions. The UCP3 activator oleic acid (OA) significantly increased state 4 respiration in IMF mitochondria only. We used chronic electrical stimulation (3 h/day for 7 days) to investigate the relationship between changes in UCP3 protein expression and alterations in state 4 respiration during contractile activity-induced mitochondrial biogenesis. UCP3 content was increased by 1.9- and 2.3-fold in IMF and SS mitochondria, respectively, which exceeded the concurrent 40% ( P < 0.05) increase in cytochrome- c oxidase activity. Chronic contractile activity increased state 4 respiration by 1.4-fold ( P < 0.05) in IMF mitochondria, but no effect was observed in the SS subfraction. The uncoupling function of UCP3 accounted for 50–57% of the OA-induced increase in state 4 respiration in IMF mitochondria, which was independent of the induced twofold difference in UCP3 content due to chronic contractile activity. Thus modifications in UCP3 function are more important than changes in UCP3 expression in modifying state 4 respiration. This effect is evident in IMF but not SS mitochondria. We conclude that UCP3 at physiological concentrations accounts for a significant portion of state 4 respiration in both IMF and SS mitochondria, with the contribution being greater in the IMF subfraction. In addition, the contradiction between human and rat training studies with respect to UCP3 protein expression may partly be explained by the greater than twofold difference in mitochondrial UCP3 content between rat and human skeletal muscle.

Starvation-sensitive UCP 3 protein expression in thymus and spleen mitochondria

To date, UCP 3 has only been associated with skeletal muscle and brown adipose tissue (BAT). Using RT-PCR/PCR methodology, we show that human spleen and human thymus contain UCP 3. In addition, using peptide antibodies, previously demonstrated to be selective for UCP 3, we show that UCP 3 protein is present in mitochondria isolated from rat thymus and mitochondria isolated from reticulocytes, monocytes and lymphocytes of rat spleen. UCP 3 protein expression is also starvation-sensitive. UCP 3 abundance is augmented in mitochondria isolated from thymus and mitochondria isolated from lymphocytes of the spleen from fasted rats when compared to fed controls. The results are consistent with a role for UCP 3 in developing lymphocytes, thymus atrophy and fatty acid utilisation in spleen and thymus.

Ubiquinone is not required for proton conductance by uncoupling protein 1 in yeast mitochondria

Q (coenzyme Q or ubiquinone) is reported to be a cofactor obligatory for proton transport by UCPs (uncoupling proteins) in liposomes [Echtay, Winkler and Klingenberg (2000) Nature (London) 408, 609-613] and for increasing the binding of the activator retinoic acid to UCP1 [Tomás, Ledesma and Rial (2002) FEBS Lett. 526, 63-65]. In the present study, yeast ( Saccharomyces cerevisiae ) mutant strains lacking Q and expressing UCP1 were used to determine whether Q was required for UCP function in mitochondria. Wild-type yeast strain and two mutant strains (CENDeltaCOQ3 and CENDeltaCOQ2), both not capable of synthesizing Q, were transformed with the mouse UCP1 gene. UCP1 activity was measured as fatty acid-dependent, GDP-sensitive proton conductance in mitochondria isolated from the cells. The activity of UCP1 was similar in both Q-containing and -deficient yeast mitochondria. We conclude that Q is neither an obligatory cofactor nor an activator of proton transport by UCP1 when it is expressed in yeast mitochondria.

Mitochondrial uncoupling protein 1 expressed in the heart of transgenic mice protects against ischemic-reperfusion damage

BACKGROUND

Mitochondrial respiration is the main source of energy in aerobic animal cells and is adapted to the energy demand by respiratory coupling. Uncoupling proteins (UCPs) perturb respiratory coupling by inducing a proton leak through the mitochondrial inner membrane. Although this could lead to deleterious energy waste, it may prevent the production of oxygen radicals when the rate of phosphorylation of ADP into ATP is low, whereas oxygen and substrate availability to mitochondria is high. The latter conditions are encountered during cardiac reperfusion after ischemia and are highly relevant to heart infarction.

METHODS AND RESULTS

Heart function of 6 transgenic mice expressing high amounts of UCP1 and of 6 littermate controls was compared in isolated perfused hearts in normoxia, after 40-minute global ischemia, and on reperfusion. In normoxia, oxygen consumption, contractility (quantified as the rate-pressure product), and their relationship (energetic yield) were similar in controls and transgenic mice. Although UCP1 expression did not alter the sensitivity to ischemia, it significantly improved functional recovery on reperfusion. After 60 minutes of reperfusion, contractility was 2-fold higher in transgenic mice than in controls. Oxygen consumption remained significantly depressed in controls (53+/-27% of control), whereas it recovered strikingly to preischemic values in transgenic mice, showing uncoupling of respiration by UCP1 activity. Glutathione and aconitase, markers of oxidative damage, indicated lower oxidative stress in transgenic mice.

CONCLUSIONS

UCP1 activity is low under normoxia but is induced during ischemia-reperfusion. The presence of UCP1 mitigates reperfusion-induced damage, probably because it lowers mitochondrial hyperpolarization at reperfusion.

Absence of UCP3 in brown adipose tissue does not impair nonshivering thermogenesis

We report on a novel Djungarian hamster mutant lineage that exhibits a loss of uncoupling protein (UCP) 3 mRNA and protein in brown adipose tissue (BAT), whereas UCP3 expression in skeletal muscle is only mildly diminished. In response to 2 d of cold exposure, UCP3 mRNA was 4.5-fold elevated in BAT of wild-type hamsters but remained undetectable in mutant hamsters. Notably, in BAT of warm- and cold-exposed mutant hamsters, UCP1 and UCP2 mRNA levels were increased. The tissue specificity of UCP3 deficiency suggests that the underlying unknown mutation impairs a factor controlling UCP3 gene expression selectively in brown adipocytes. In wild-type but not mutant primary brown adipocytes, UCP3 gene expression was stimulated by treatment with peroxisome proliferator activated receptor (PPAR) ligands. This implies that the underlying mutation causing UCP3 deficiency is expressed within brown adipocytes and disrupts PPAR-dependent transactivation of the UCP3 gene. On the functional level, we found no direct phenotypic consequences of altered UCP expression in BAT. The absence of UCP3 in BAT of cold-acclimated mutant hamsters affected neither maximal nonshivering thermogenesis elicited by noradrenaline nor the uncoupled respiration of isolated mitochondria in the presence of oligomycin and in response to palmitate.

Uncoupling protein 2 and 3 in marsupials: identification, phylogeny, and gene expression in response to cold and fasting in Antechinus flavipes

We searched for the presence of uncoupling protein genes so far unknown in marsupials and monotremes and identified uncoupling protein 2 (UCP2) and UCP3 full-length cDNAs in libraries constructed from the marsupials Antechinus flavipes and Sminthopsis macroura. Marsupial UCP2 is 89–90% identical to rodent UCP2, whereas UCP3 exhibits 80% identity to mouse UCP3. A phylogenetic tree including all known UCPs positions the novel marsupial UCP2 and UCP3 at the base of the mammalian orthologs. In the 5′-untranslated region of UCP2 a second open reading frame encoding for a 36-amino acid peptide was identified which is highly conserved in all vertebrate UCP2 transcripts. Analysis of tissue specificity in A. flavipes with homologous cDNA probes revealed ubiquitous presence of UCP2 mRNA and striated muscle specificity of UCP3 mRNA resembling the known expression pattern in rodents. Neither UCP2 nor UCP3 gene expression was stimulated in adipose tissue and skeletal muscle of cold exposed A. flavipes. However, UCP3 mRNA expression was upregulated 6-fold in heart and 2.5-fold in skeletal muscle as reported for rodents in response to fasting. Furthermore, UCP3 mRNA seems to be coregulated with PDK4 mRNA, indicating a relation to enhanced lipid metabolism. In contrast, UCP2 gene expression was not regulated in response to fasting in adipose tissue and skeletal muscle but was diminished in the lung and increased in adipose tissue. Taken together, the sequence analysis, tissue specificity and physiological regulation suggest a conserved function of UCP2 and UCP3 during 130 million years of mammalian evolution.

Superoxide-mediated activation of uncoupling protein 2 causes pancreatic beta cell dysfunction

Failure to secrete adequate amounts of insulin in response to increasing concentrations of glucose is an important feature of type 2 diabetes. The mechanism for loss of glucose responsiveness is unknown. Uncoupling protein 2 (UCP2), by virtue of its mitochondrial proton leak activity and consequent negative effect on ATP production, impairs glucose-stimulated insulin secretion. Of interest, it has recently been shown that superoxide, when added to isolated mitochondria, activates UCP2-mediated proton leak. Since obesity and chronic hyperglycemia increase mitochondrial superoxide production, as well as UCP2 expression in pancreatic beta cells, a superoxide-UCP2 pathway could contribute importantly to obesity- and hyperglycemia-induced beta cell dysfunction. This study demonstrates that endogenously produced mitochondrial superoxide activates UCP2-mediated proton leak, thus lowering ATP levels and impairing glucose-stimulated insulin secretion. Furthermore, hyperglycemia- and obesity-induced loss of glucose responsiveness is prevented by reduction of mitochondrial superoxide production or gene knockout of UCP2. Importantly, reduction of superoxide has no beneficial effect in the absence of UCP2, and superoxide levels are increased further in the absence of UCP2, demonstrating that the adverse effects of superoxide on beta cell glucose sensing are caused by activation of UCP2. Therefore, superoxide-mediated activation of UCP2 could play an important role in the pathogenesis of beta cell dysfunction and type 2 diabetes.

Metabolism and function of coenzyme Q

Coenzyme Q (CoQ) is present in all cells and membranes and in addition to be a member of the mitochondrial respiratory chain it has also several other functions of great importance for the cellular metabolism. This review summarizes the findings available to day concerning CoQ distribution, biosynthesis, regulatory modifications and its participation in cellular metabolism. There are a number of indications that this lipid is not always functioning by its direct presence at the site of action but also using e.g. receptor expression modifications, signal transduction mechanisms and action through its metabolites. The biosynthesis of CoQ is studied in great detail in bacteria and yeast but only to a limited extent in animal tissues and therefore the informations available is restricted. However, it is known that the CoQ is compartmentalized in the cell with multiple sites of biosynthesis, breakdown and regulation which is the basis of functional specialization. Some regulatory mechanisms concerning amount and biosynthesis are established and nuclear transcription factors are partly identified in this process. Using appropriate ligands of nuclear receptors the biosynthetic rate can be increased in experimental system which raises the possibility of drug-induced upregulation of the lipid in deficiency. During aging and pathophysiological conditions the tissue concentration of CoQ is modified which influences cellular functions. In this case the extent of disturbances is dependent on the localization and the modified distribution of the lipid at cellular and membrane levels.

Uncoupling protein 2, but not uncoupling protein 1, is expressed in the female mouse reproductive tract

Uncoupling proteins (UCPs) are transporters of the inner mitochondrial membrane. Whereas UCP1 is uniquely present in brown adipose tissue where it uncouples respiration from ATP synthesis and activates respiration and heat production, UCP2 is present in numerous tissues, and its exact function remains to be clarified. Two sets of data provided the rationale for this study: (i) the intriguing report that UCP1 is present in uterus of mice (Nibbelink, M., Moulin, K., Arnaud, E., Duval, C., Penicaud, L., and Casteilla, L. (2001) J. Biol. Chem. 276, 47291-47295); and (ii) an observation that Ucp2(-/-) female mice (homozygous matings) have smaller litters compared with Ucp2(+/+) animals (S. Rousset and A.-M. Cassard-Doulcier, unpublished observations). These data prompted us to examine the expression of UCP1 and UCP2 in the reproductive tract of female mice. Using wild type, Ucp1(-/-) mice, and Ucp2(-/-) mice, we were unable to detect UCP1 in uterus of mice with appropriate antibodies, and we conclude that the signal assigned to UCP1 by others was neither UCP1 nor UCP2. Using a polyclonal antibody against UCP2 and tissues from Ucp2(-/-) mice as controls, UCP2 was detected in ovary, oviduct, and uterus. Expression of Ucp2 mRNA was also observed in ovary and uterus using in situ hybridization analysis. Bone marrow transplantation experiments revealed that the UCP2 signal of the ovary was restricted to ovarian cells. UCP2 level in ovary decreased during follicular growth and increased during the pre-ovulatory period, during which aspects of an inflammatory process are known to exist. Because UCP2 down-regulates reactive oxygen species, a role in the regulation of inflammatory events linked to the preparation of ovulation is suggested.

Uncoupling protein 2 influences dopamine secretion in PC12h cells

Uncoupling protein 2 (UCP2) belongs to the UCP family, and is distributed in many organs including the brain. Although UCP2 is known to be related to many functions such as the regulation of insulin secretion or the scavenging of the radicals, the role of UCP2 in the central nervous system remains unclear. In this report, rat UCP2 (rUCP2) and its mutants were overexpressed in the PC12h cells to determine the physiological roles played by UCP2 in neural cells and to elucidate the mechanisms that regulate these functions. It was found that rUCP2 was activated by the stimulation of the cAMP-protein kinase A (PKA) cascade. Moreover, the activation of rUCP2 suppressed intracellular ATP levels and inhibited the cAMP-dependent increase of dopamine secretion. Thus, UCP2 appears to be regulated by the excitatory stimulus via the cAMP-PKA cascade and serves to negatively control the synaptic output by reducing intracellular ATP levels.

Uncoupling protein 2 prevents neuronal death including that occurring during seizures: a mechanism for preconditioning

The mitochondrial uncoupling protein (UCP2) is expressed in selected regions of the brain. Here we demonstrate that up-regulation of UCP2 is part of a neuroprotective set of responses to various cellular stresses in vitro and in vivo. PC12 cells, when transfected with UCP2, were protected against free radical-induced cell death. Seizure activity was associated with elevated UCP2 levels and mitochondrial uncoupling activity. In transgenic mice that expressed UCP2 constitutively in the hippocampus before seizure induction, a robust reduction in cell death was seen. Because UCP2 increased mitochondrial number and ATP levels with a parallel decrease in free radical-induced damage, it is reasonable to suggest that mitochondrial UCPs precondition neurons by dissociating cellular energy production from that of free radicals to withstand the harmful effects of cellular stress occurring in a variety of neurodegenerative disorders, including epilepsy.

Bone marrow transplantation reveals the in vivo expression of the mitochondrial uncoupling protein 2 in immune and nonimmune cells during inflammation

The mitochondrial uncoupling protein 2 (UCP2) is expressed in spleen, lung, intestine, white adipose tissue, and immune cells. Bone marrow transplantation in mice was used to assess the contribution of immune cells to the expression of UCP2 in basal condition and during inflammation. Immune cells accounted for the total amount of UCP2 expression in the spleen, one-third of its expression in the lung, and did not participate in its expression in the intestine. LPS injection stimulated UCP2 expression in lung, spleen, and intestine in both immune and non-immune cells. Successive injections of LPS and dexamethasone or N-acetyl-cysteine prevented the induction of UCP2 in all three tissues, suggesting that oxygen free radical generation plays a role in UCP2 regulation. Finally, both previous studies and our data show that there is down-regulation of UCP2 in immune cells during their activation in the early stages of the LPS response followed by an up-regulation in UCP2 during the later stages to protect all cells against oxidative stress.

Sensitivity of Caenorhabditis elegans clk-1 mutants to ubiquinone side-chain length reveals multiple ubiquinone-dependent processes

Ubiquinone (coenzyme Q, or Q) is a membrane constituent, whose head group is capable of accepting and donating electrons and whose lipidic side chain is composed of a variable number of isoprene subunits. A possible role for Q as a dietary antioxidant for treating conditions that involve altered cellular redox states is being intensely studied. Mutations in the clk-1 gene of the nematode Caenorhabditis elegans affect numerous physiological rates including behavioral rates, developmental rates, reproduction, and life span. clk-1 encodes a protein associated with the inner mitochondrial membrane that is necessary for Q biosynthesis in C. elegans. clk-1 mutants do not synthesize Q but accumulate demethoxyubiquinone, a Q synthesis intermediate that is able to partially sustain mitochondrial respiration in worms as well as in mammals. Recently, we and others have found that exogenous Q is necessary for the fertility and development of clk-1 mutants. Here, we take advantage of the clk-1 genetic model to identify structural features of Q that are functionally important in vivo. We show that clk-1 mutants are exquisitely sensitive to the length of the side chain of the Q they consume. We also identified differential sensitivity to Q side-chain length between null alleles of clk-1 (qm30 and qm51) and the weaker allele e2519. This allows us to propose a model where we distinguish several types of Q-dependent processes in vivo: processes that are very sensitive to Q side-chain length and processes that are permissive to Q with shorter chains.

A function for novel uncoupling proteins: antioxidant defense of mitochondrial matrix by translocating fatty acid peroxides from the inner to the outer membrane leaflet

It is hypothesized that mitochondrial uncoupling proteins operate as carriers of fatty acid peroxide anions. This is assumed to result in electrophoretic extrusion of such anions from the inner to the outer leaflet of the inner mitochondrial membrane, being driven by membrane potential (mitochondrial interior negative). In this way, the inner leaflet is ridded of fatty acid peroxides that otherwise can form very aggressive oxidants damaging mitochondrial DNA, aconitase, and other mitochondrial matrix-localized components of vital importance. The steady-state concentration the fatty acid peroxides is known to be low. This explains why UCP2, 3, 4, and 5 are present in small amounts usually insufficient to make a large contribution to the H+ conductance of the mitochondrial membrane.

A signalling role for 4-hydroxy-2-nonenal in regulation of mitochondrial uncoupling

Oxidative stress and mitochondrial dysfunction are associated with disease and aging. Oxidative stress results from overproduction of reactive oxygen species (ROS), often leading to peroxidation of membrane phospholipids and production of reactive aldehydes, particularly 4-hydroxy-2-nonenal. Mild uncoupling of oxidative phosphorylation protects by decreasing mitochondrial ROS production. We find that hydroxynonenal and structurally related compounds (such as trans-retinoic acid, trans-retinal and other 2-alkenals) specifically induce uncoupling of mitochondria through the uncoupling proteins UCP1, UCP2 and UCP3 and the adenine nucleotide translocase (ANT). Hydroxynonenal-induced uncoupling was inhibited by potent inhibitors of ANT (carboxyatractylate and bongkrekate) and UCP (GDP). The GDP-sensitive proton conductance induced by hydroxynonenal correlated with tissue expression of UCPs, appeared in yeast mitochondria expressing UCP1 and was absent in skeletal muscle mitochondria from UCP3 knockout mice. The carboxyatractylate-sensitive hydroxynonenal stimulation correlated with ANT content in mitochondria from Drosophila melanogaster expressing different amounts of ANT. Our findings indicate that hydroxynonenal is not merely toxic, but may be a biological signal to induce uncoupling through UCPs and ANT and thus decrease mitochondrial ROS production.

Quantitation of blood lymphocyte mitochondrial DNA for the monitoring of antiretroviral drug-induced mitochondrial DNA depletion

OBJECTIVE

To investigate the impact of antiretroviral treatment on the mitochondrial DNA (mtDNA) content of peripheral blood mononuclear cells (PBMCs) from HIV-1-infected patients.

DESIGN

As absolute mtDNA copy numbers widely differ between individuals, we performed a longitudinal analysis where the patient's first historical specimen was obtained as a baseline reference for relative comparison with subsequent samples from that patient.

METHODS

mtDNA and nuclear DNA quantitation per cell (beta-globin gene copies) were both measured by real-time polymerase chain reaction analysis of whole DNA extracts of 361 serial live-cryopreserved PBMCs collected in former trials and clinical follow-ups from 60 individuals with established or recently acquired HIV-1 infections before and during administration of various antiviral combination therapies.

RESULTS

mtDNA amounts were stable or increasing over years of natural HIV-1 infection in untreated patients (n = 7), consistent with our finding of a lack of differences in mtDNA copy numbers in patients with either a long established or recent lentivirus infection. Our quantitation system revealed significant changes in mtDNA copy number depending on the designated triple, quadruple, or quintuple anti-HIV drug combinations. Zidovudine + zalcitabine + ritonavir and zidovudine + lamivudine + didanosine regularly lead to mtDNA depletion in each of the treated patients, whereas none of 7 patients (and 35 cell specimens) receiving a stavudine + lamivudine + indinavir combination had any significant mtDNA content variations. In 7 patients, mtDNA copy numbers returned to pretreatment levels and/or higher levels without any interruption of the previously mtDNA-depleting antiretroviral drug combination.

CONCLUSION

Our assay system allowed the detection of significant changes in the mtDNA content of PBMCs from HIV-1-infected patients taking antiretroviral drugs, as has been reported in the literature with other detection systems. Yet, mtDNA copy numbers regularly diminished during administration of some but not all nucleoside analog-containing combinations. This, plus the occasional finding that depleted mtDNA contents spontaneously increased to baseline levels and/or higher levels during uninterrupted treatment, should raise a note of caution about resorting to the PBMC mtDNA marker for monitoring of antiretroviral drug-related mitochondrial toxicities.

Reconstitution of recombinant uncoupling proteins: UCP1, -2, and -3 have similar affinities for ATP and are unaffected by coenzyme Q10

The successful development of recombinant expression and reconstitution protocols has enabled a detailed study of the transport properties and regulation of the uncoupling proteins (UCP). We optimized conditions of isolation and refolding of bacterially expressed uncoupling proteins and reexamined the transport properties and regulation of bacterially expressed UCP1, -2, and -3 reconstituted in liposomes. We show for the first time that ATP inhibits UCP1, -2, and -3 with similar affinities. The Ki values for ATP inhibition were 50 microm (UCP1), 70 microm (UCP2), and 120 microm (UCP3) at pH 7.2. These affinities for ATP are similar to those obtained with native UCP1 isolated from brown adipose tissue mitochondria (Ki = 65 microm at pH 7.2). The Vmax values for proton transport were also similar among the UCPs, ranging from 8 to 20 micromol.min(-1).mg(-1), depending on experimental conditions. We also examined the effect of coenzyme Q on fatty acid-catalyzed proton flux in liposomes containing recombinant UCP1, -2, and -3. We found that coenzyme Q had no effect on the fatty acid-dependent proton transport catalyzed by any of the UCPs nor did it affect nucleotide regulation of the UCPs. We conclude that coenzyme Q is not a cofactor of UCP-mediated proton transport.

Selective detection of UCP 3 expression in skeletal muscle: effect of thyroid status and temperature acclimation

A novel peptide antibody to UCP 3 is characterized which is sensitive and discriminatory for UCP 3 over UCP 2, UCP 1 and other mitochondrial transporters. The peptide antibody detects UCP 3 expression in E. coli, COS cells and yeast expression systems. The peptide antibody detects a single approximately 33 kDa protein band in mitochondria from isolated rat skeletal muscle, mouse and rat brown adipose tissue, and in whole muscle groups (soleus and extensor digitorum longus) from mice. No 33 kDa band is detectable in isolated mitochondria from liver, heart, brain, kidney and lungs of rats, or gastrocnemius mitochondria from UCP 3 knock-out mice. From our data, we conclude that the peptide antibody is detecting UCP 3 in skeletal muscle, skeletal muscle mitochondria and brown adipose tissue mitochondria. It is also noteworthy that the peptide antibody can detect human, mouse and rat forms of UCP 3. Using the UCP 3 peptide antibody, we confirm and quantify the increased (2.8-fold) UCP 3 expression observed in skeletal muscle mitochondria isolated from 48-h-starved rats. We show that UCP 3 expression is increased (1.6-fold) in skeletal muscle of rats acclimated over 8 weeks to 8 degrees C and that UCP 3 expression is decreased (1.4-fold) in rats acclimated to 30 degrees C. Furthermore, UCP 3 expression is increased (2.3-fold) in skeletal muscle from hyperthyroid rats compared to euthyroid controls. In addition, we show that UCP 3 expression is only coincident with the mitochondrial fraction of skeletal muscle homogenates and not peroxisomal, nuclear or cytosolic and microsomal fractions.

Coenzyme Q induces nigral mitochondrial uncoupling and prevents dopamine cell loss in a primate model of Parkinson's disease

Parkinson's disease is characterized by dopamine cell loss of the substantia nigra. Parkinson's disease and the neurotoxin 1-methyl-4-phenyl-1,2,5,6 tetrahydropyridine may destroy dopamine neurons through oxidative stress. Coenzyme Q is a cofactor of mitochondrial uncoupling proteins that enhances state-4 respiration and eliminate superoxides. Here we report that short-term oral administration of coenzyme Q induces nigral mitochondrial uncoupling and prevents dopamine cell loss after 1-methyl-4-phenyl-1,2,5,6 tetrahydropyridine administration in monkeys.

Mitochondrial uncoupling protein 2 in the central nervous system: neuromodulator and neuroprotector

Uncoupling proteins (UCPs) are localized in the inner membrane of the mitochondria in diverse tissues and decrease mitochondrial membrane potential. The first of these proteins, UCP1, was discovered in brown adipose tissue, where it has a well-described role in thermogenesis. The functional significance of other UCPs, including UCP2, is less well understood. Here we summarize the recent advancements on the role of UCP2 in the brain and portray this uncoupler as an important player in normal neuronal function as well as a key cell death-suppressing device. These previously unknown functions of UCPs offer new avenues not only for the better understanding of these proteins but also for the furthering of our knowledge on the central nervous system in healthy and disease states.

Activating omega-6 polyunsaturated fatty acids and inhibitory purine nucleotides are high affinity ligands for novel mitochondrial uncoupling proteins UCP2 and UCP3

UCP2 (the lowest Km values: 20 and 29 microm, respectively) for omega-6 polyunsaturated FAs (PUFAs), all-cis-8,11,14-eicosatrienoic and all-cis-6,9,12-octadecatrienoic acids, which are also the most potent agonists of the nuclear PPARbeta receptor in the activation of UCP2 transcription. omega-3 PUFA, cis-5,8,11,14,17-eicosapentaenoic acid had lower affinity (Km, 50 microm), although as an omega-6 PUFA, arachidonic acid exhibited the same low affinity as lauric acid (Km, approximately 200 microm). These findings suggest a possible dual role of some PUFAs in activating both UCPn expression and uncoupling activity. UCP2 (UCP3)-dependent H+ translocation activated by all tested FAs was inhibited by purine nucleotides with apparent affinity to UCP2 (reciprocal Ki) decreasing in order: ADP > ATP approximately GTP > GDP >> AMP. Also [3H]GTP ([3H]ATP) binding to isolated Escherichia coli (Kd, approximately 5 microm) or yeast-expressed UCP2 (Kd, approximately 1.5 microm) or UCP3 exhibited high affinity, similar to UCP1. The estimated number of [3H]GTP high affinity (Kd, <0.4 microm) binding sites was (in pmol/mg of protein) 182 in lung mitochondria, 74 in kidney, 28 in skeletal muscle, and approximately 20 in liver mitochondria. We conclude that purine nucleotides must be the physiological inhibitors of UCPn-mediated uncoupling in vivo.

Intrinsic and extrinsic uncoupling of oxidative phosphorylation

This article reviews parameters of extrinsic uncoupling of oxidative phosphorylation (OxPhos) in mitochondria, based on induction of a proton leak across the inner membrane. The effects of classical uncouplers, fatty acids, uncoupling proteins (UCP1-UCP5) and thyroid hormones on the efficiency of OxPhos are described. Furthermore, the present knowledge on intrinsic uncoupling of cytochrome c oxidase (decrease of H(+)/e(-) stoichiometry=slip) is reviewed. Among the three proton pumps of the respiratory chain of mitochondria and bacteria, only cytochrome c oxidase is known to exhibit a slip of proton pumping. Intrinsic uncoupling was shown after chemical modification, by site-directed mutagenesis of the bacterial enzyme, at high membrane potential DeltaPsi, and in a tissue-specific manner to increase thermogenesis in heart and skeletal muscle by high ATP/ADP ratios, and in non-skeletal muscle tissues by palmitate. In addition, two mechanisms of respiratory control are described. The first occurs through the membrane potential DeltaPsi and maintains high DeltaPsi values (150-200 mV). The second occurs only in mitochondria, is suggested to keep DeltaPsi at low levels (100-150 mV) through the potential dependence of the ATP synthase and the allosteric ATP inhibition of cytochrome c oxidase at high ATP/ADP ratios, and is reversibly switched on by cAMP-dependent phosphorylation. Finally, the regulation of DeltaPsi and the production of reactive oxygen species (ROS) in mitochondria at high DeltaPsi values (150-200 mV) are discussed.

Fasting, lipid metabolism, and triiodothyronine in rat gastrocnemius muscle: interrelated roles of uncoupling protein 3, mitochondrial thioesterase, and coenzyme Q

We investigated the role of uncoupling protein 3 (UCP3) during fasting and examined the effect of triiodothyronine (T3) administration in such a condition. The possible involvement of mitochondrial thioesterase (MTE I) and the role of putative cofactors, such as coenzyme Q (CoQ), was also examined. Here, we report that fasting induced a more than twofold elevation in the expression and activity of MTE I, and an increase in UCP3 expression, without any associated uncoupling activity. Administration of T3 to fasting rats further up-regulated UCP3 as well as MTE I expression, markedly enhanced MTE I enzyme activity and prevented the impairment of the uncoupling activity of UCP3 normally seen during fasting. Indeed, T3-treatment induced an UCP3-dependent decrease in mitochondrial membrane potential, which was abolished by the addition of either GDP or superoxide dismutase (SOD). T3 administration also prevented the marked decrease of CoQ levels observed in fasting rats and this provides evidence that also, in vivo, CoQ represents an essential cofactor for the UCP3-mediated uncoupling. The data also show that MTE I and UCP3 are likely involved in the same biochemical mechanism and that UCP3 postulated functions, such as lipid handling and uncoupling, are not mutually exclusive but may coexist in vivo.

Thyroid hormone and uncoupling proteins

Thyroid hormone (TH/T3) exerts many of its effects on energy metabolism by affecting gene transcription. However, although this is an important target for T3, only a limited number of T3-responsive genes have been identified and studied. Among these, the genes for uncoupling proteins (UCPs) have attracted the interest of scientists. Although the role of UCP1 seems quite well established, uncertainty surrounds the physiological function of the recently discovered UCP1 analogs, UCP2 and UCP3. The literature suggests that T3 affects both the expression and the activity of each of these UCPs but further studies are needed to establish whether the mechanisms activated by the hormone are the same. Recently, because of their larger range of expression, much attention has been devoted to UCP2 and UCP3. Most detailed studies on the involvement of these proteins as mediators of the effects of T3 on metabolism have focused on UCP3 because of its expression in skeletal muscle. T3 seems to be unique in having the ability to stimulate the expression and activity of UCP3 and this may be related to the capacity of T3 to activate the integrated biochemical processes linked to UCP activity, such as those related to fatty acids, coenzyme Q and free radicals.

Bioenergetic approaches for neuroprotection in Parkinson's disease

There is considerable evidence suggesting that mitochondrial dysfunction and oxidative damage may play a role in the pathogenesis of Parkinson's disease (PD). This possibility has been strengthened by recent studies in animal models, which have shown that a selective inhibitor of complex I of the electron transport gene can produce an animal model that closely mimics both the biochemical and histopathological findings of PD. Several agents are available that can modulate cellular energy metabolism and that may exert antioxidative effects. There is substantial evidence that mitochondria are a major source of free radicals within the cell. These appear to be produced at both the iron-sulfur clusters of complex I as well as the ubiquinone site. Agents that have shown to be beneficial in animal models of PD include creatine, coenzyme Q(10), Ginkgo biloba, nicotinamide, and acetyl-L-carnitine. Creatine has been shown to be effective in several animal models of neurodegenerative diseases and currently is being evaluated in early stage trials in PD. Similarly, coenzyme Q(10) is also effective in animal models and has shown promising effects both in clinical trials of PD as well as in clinical trials in Huntington's disease and Friedreich's ataxia. Many other agents show good human tolerability. These agents therefore are promising candidates for further study as neuroprotective agents in PD.

Effect of thyroid state on rate and sites of H2O2 production in rat skeletal muscle mitochondria

The purpose of this study was to investigate the effects of thyroid state on rates and sites of H(2)O(2) production in rat muscle mitochondria. With Complex I- and Complex II-linked substrates, hypothyroidism decreased and hyperthyroidism increased the rates of O(2) consumption during State 4 and State 3 respiration and the rates of H(2)O(2) release during State 4 respiration. During State 3, the rates of H(2)O(2) release were not affected by thyroid state. However, the mitochondrial capacity to remove H(2)O(2) increased in the transition from hypothyroid to hyperthyroid state, thus suggesting that an increase in H(2)O(2) production rate also occurred in such a transition during State 3 respiration. The observation that mitochondrial coenzyme Q levels and cytochrome oxidase activities are higher in the hyperthyroid and lower in the hypothyroid groups suggests that the modifications of H(2)O(2) production are due to a modulation by thyroid hormone of the mitochondrial content of autoxidizable electron carriers. This idea is supported by measurements of H(2)O(2) release in the presence of respiratory inhibitors. In fact, such measurements indicate that the thyroid state-linked changes in H(2)O(2) production occur at both generator sites of the respiratory chain.

Distribution and breakdown of labeled coenzyme Q10 in rat

Radioactive coenzyme Q(10) ([(3)H]CoQ) was synthesized in a way that the metabolites produced retained the radioactivity. Administration of the lipid to rats intraperitoneally resulted in an efficient uptake into the circulation, with high concentrations found in spleen, liver, and white blood cells; lower concentrations in adrenals, ovaries, thymus, and heart; and practically no uptake in kidney, muscle, and brain. In liver homogenate most [(3)H]CoQ appeared in the organelles, but it was also present in the cytosol and transport vesicles. Mitochondria, purified on a metrizamide gradient, had a very low concentration of [(3)H]CoQ, which was mainly present in the lysosomes. All organs that took up the labeled lipid also contained water-soluble metabolites. The majority of metabolites excreted through the kidney and appeared in the urine. Some metabolites were also present in the feces, which further contained nonmetabolized [(3)H]CoQ, excreted through the bile. The major metabolites were purified from the urine, and the mass spectrometric fragmentation showed that these compounds, containing the ring with a short side chain, are phosphorylated. Thus, the results demonstrate that CoQ is metabolized in all tissues, the metabolites are phosphorylated in the cells, transported in the blood to the kidney, and excreted into the urine.

Involvement of retinoid X receptor alpha in coenzyme Q metabolism

The nuclear retinoid X receptor alpha (RXRalpha) is the heterodimer partner in several nuclear receptors, some of them regulating lipid biosynthesis. Since coenzyme Q (CoQ) levels are greatly modified in aging and a number of diseases, we have investigated the involvement of RXRalpha in the biosynthetic regulation of this lipid by using a hepatocyte-specific RXRalpha-deficient mouse strain (RXRalpha-def). In the receptor-deficient liver, the amount of CoQ decreased to half of the control, and it was demonstrated that this decrease was caused by a significantly lowered rate of biosynthesis. On the other hand, induction of CoQ was extensive in both control and RXRalpha-def liver using the peroxisomal inducer di(2-ethylhexyl)phthalate (DEHP). Since the RXRalpha deficiency was specific to liver, no change in CoQ content or biosynthesis was observed in kidney. The other mevalonate pathway lipids, cholesterol and dolichol, were unchanged in the RXRalpha-def liver. Upon treatment with DEHP, cholesterol decreased in the control but remained unchanged in the receptor-deficient mice. In control mice, cold exposure elevated CoQ levels by 60%, but this induction did not occur in the liver of RXRalpha-def mice. In contrast, PPARalpha-null mice, which lack induction upon treatment with peroxisomal inducers, respond to cold exposure and CoQ content is increased. The amount of cholesterol decreased in both control and RXRalpha-def liver upon cold treatment. The results demonstrate that RXRalpha is required for CoQ biosynthesis and for its induction upon cold treatment, but does not appear to be involved in the basic synthesis of cholesterol and dolichol. The receptor is not involved in the elevated CoQ biosynthesis during peroxisomal induction.

Primary coenzyme Q10 deficiency and the brain

Our findings in 19 new patients with cerebellar ataxia establish the existence of an ataxic syndrome due to primary CoQ10 deficiency and responsive to CoQ10 therapy. As all patients presented cerebellar ataxia and cerebellar atrophy, this suggests a selective vulnerability of the cerebellum to CoQ10 deficiency. We investigated the regional distribution of coenzyme Q10 in the brain of adult rats and in the brain of one human subject. We also evaluated the levels of coenzyme Q9 (CoQ9) and CoQ10 in different brain regions and in visceral tissues of rats before and after oral administration of CoQ10. Our results show that in rats, amongst the seven brain regions studied, cerebellum contains the lowest level of CoQ. However, the relative proportion of CoQ10 was the same (about 30% of total CoQ) in all regions studied. The level of CoQ10 is much higher in brain than in blood or visceral tissue, such as liver, heart, or kidney. Daily oral administration of CoQ10 led to substantial increases of CoQ10 concentrations only in blood and liver. Of the four regions of one human brain studied, cerebellum again had the lowest CoQ10y concentration.

Superoxide activates mitochondrial uncoupling protein 2 from the matrix side. Studies using targeted antioxidants

Superoxide activates nucleotide-sensitive mitochondrial proton transport through the uncoupling proteins UCP1, UCP2, and UCP3 (Echtay, K. S., et al. (2002) Nature 415, 1482-1486). Two possible mechanisms were proposed: direct activation of the UCP proton transport mechanism by superoxide or its products and a cycle of hydroperoxyl radical entry coupled to UCP-catalyzed superoxide anion export. Here we provide evidence for the first mechanism and show that superoxide activates UCP2 in rat kidney mitochondria from the matrix side of the mitochondrial inner membrane: (i) Exogenous superoxide inhibited matrix aconitase, showing that external superoxide entered the matrix. (ii) Superoxide-induced uncoupling was abolished by low concentrations of the mitochondrially targeted antioxidants 10-(6'-ubiquinonyl)decyltriphenylphosphonium (mitoQ) or 2-[2-(triphenylphosphonio)ethyl]-3,4-dihydro-2,5,7,8-tetramethyl-2H-1-benzopyran-6-ol bromide (mitoVit E), which are ubiquinone (Q) or tocopherol derivatives targeted to the matrix by covalent attachment to triphenylphosphonium cation. However, superoxide-induced uncoupling was not affected by similar concentrations of the nontargeted antioxidants Q(o), Q(1), decylubiquinone, vitamin E, or 6-hydroxy-2,5,7,8-tetramethylchroman 2-carboxylic acid (TROLOX) or of the mitochondrially targeted but redox-inactive analogs decyltriphenylphosphonium or 4-chlorobutyltriphenylphosphonium. Thus matrix superoxide appears to be necessary for activation of UCP2 by exogenous superoxide. (iii) When the reduced to oxidized ratio of mitoQ accumulated by mitochondria was increased by inhibiting cytochrome oxidase, it induced nucleotide-sensitive uncoupling that was not inhibited by external superoxide dismutase. Under these conditions quinols are known to produce superoxide, and because mitoQ is localized within the mitochondrial matrix this suggests that production of superoxide in the matrix was sufficient to activate UCP2. Furthermore, the superoxide did not need to be exported or to cycle across the inner membrane to cause uncoupling. We conclude that superoxide (or its products) exerts its uncoupling effect by activating the proton transport mechanism of uncoupling proteins at the matrix side of the mitochondrial inner membrane.

Skeletal muscle mitochondrial free-fatty-acid content and membrane potential sensitivity in different thyroid states: involvement of uncoupling protein-3 and adenine nucleotide translocase

The effect of triiodothyronine (T3) on mitochondrial efficiency could be related to an increase in the concentrations of some proteins, such as uncoupling proteins (UCPs). Free fatty acids (FFA) seem to be a cofactor essential for the uncoupling activity of UCP3. In this paper, we report that the hypothyroidism-hyperthyroidism transition is accompanied by increases: (i) in the endogenous levels of mitochondrial FFA and (ii) in the sensitivity to FFA shown by the mitochondrial respiration rate and membrane potential, which correlated with the level of UCP3 protein. The level of the mRNA for adenine-nucleotide translocase-1 (ANT) was not affected by the thyroid state, while the ANT contribution to FFA-induced changes in mitochondrial uncoupling was low in the hypothyroid and euthyroid states but became more relevant in the hyperthyroid state at the highest concentration of FFA.

Development and fertility in Caenorhabditis elegans clk-1 mutants depend upon transport of dietary coenzyme Q8 to mitochondria

The Caenorhabditis elegans clk-1 mutants lack coenzyme Q(9) and instead accumulate the biosynthetic intermediate demethoxy-Q(9) (DMQ(9)). clk-1 animals grow to reproductive adults, albeit slowly, if supplied with Q(8)-containing Escherichia coli. However, if Q is withdrawn from the diet, clk-1 animals either arrest development as young larvae or become sterile adults depending upon the stage at the time of the withdrawal. To understand this stage-dependent response to a Q-less diet, the quinone content was determined during development of wild-type animals. The quinone content varies in the different developmental stages in wild-type fed Q(8)-replete E. coli. The amounts peak at the second larval stage, which coincides with the stage of arrest of clk-1 larvae fed a Q-less diet from hatching. Levels of the endogenously synthesized DMQ(9) are high in the clk-1(qm30)-arrested larvae and sterile adults fed Q-less food. Comparison of quinones from animals fed a Q-replete or a Q-less diet establishes that the Q(8) present is assimilated from the E. coli. Furthermore, this E. coli-specific Q(8) is present in mitochondria isolated from fertile clk-1(qm30) adults fed a Q-replete diet. These results suggest that the uptake and transport of dietary Q(8) to mitochondria prevent the arrest and sterility phenotypes of clk-1 mutants and that DMQ is not functionally equivalent to Q.

High level of uncoupling protein 1 expression in muscle of transgenic mice selectively affects muscles at rest and decreases their IIb fiber content

The mitochondrial uncoupling protein of brown adipose tissue (UCP1) was expressed in skeletal muscle and heart of transgenic mice at levels comparable with the amount found in brown adipose tissue mitochondria. These transgenic mice have a lower body weight, and when related to body weight, food intake and energy expenditure are increased. A specific reduction of muscle mass was observed but varied according to the contractile activity of muscles. Heart and soleus muscle are unaffected, indicating that muscles undergoing regular contractions, and therefore with a continuous mitochondrial ATP production, are protected. In contrast, the gastrocnemius and plantaris muscles showed a severely reduced mass and a fast to slow shift in fiber types promoting mainly IIa and IIx fibers at the expense of fastest and glycolytic type IIb fibers. These observations are interpreted as a consequence of the strong potential dependence of the UCP1 protonophoric activity, which ensures a negligible proton leak at the membrane potential observed when mitochondrial ATP production is intense. Therefore UCP1 is not deleterious for an intense mitochondrial ATP production and this explains the tolerance of the heart to a high expression level of UCP1. In muscles at rest, where ATP production is low, the rise in membrane potential enhances UCP1 activity. The proton return through UCP1 mimics the effect of a sustained ATP production, permanently lowering mitochondrial membrane potential. This very likely constitutes the origin of the signal leading to the transition in fiber types at rest.

Uncoupling protein 2 knockout mice have enhanced insulin secretory capacity after a high-fat diet

Uncoupling protein 2 (UCP2) may act as an important regulator of insulin secretion. In this study, beta-cell function in UCP2-deficient mice was examined after a 45% high-fat diet (HFD) to assess its role during the development of diet-induced type 2 diabetes. HFD-fed UCP2 (-/-) mice have lower fasting blood glucose and elevated insulin levels when compared with wild-type (WT) mice. UCP2 (-/-) mice also have enhanced beta-cell glucose sensitivity compared with WT mice after HFD, a result that is due in part to the deterioration of glucose responsiveness in WT mice. HFD-fed UCP2 (-/-) mice have increased insulin secretory capacity as a result of increased pancreatic beta-cell mass and insulin content per islet. Islets from WT mice exposed to 0.5 mmol/l palmitate for 48 h have significantly reduced mitochondrial membrane potential, ATP concentrations, and glucose responsiveness compared with UCP2 (-/-) islets, suggesting that elevated UCP2 in WT mice increases proton leak and decreases mitochondrial ATP production. Highly increased carnitine palmitoyl transferase-1 gene expression in UCP2 (-/-) mice is suggestive of enhanced fatty acid oxidizing capacity, particularly after HFD stress. These results further establish UCP2 as a component in glucose sensing and suggest a possible new aspect of UCP2 function during the progression of type 2 diabetes.

Old and new determinants in the regulation of energy expenditure

Bw gain is controlled by energy intake on one hand and expenditure on the other. The components of energy expenditure are basal metabolism, exercise induced thermogenesis and adaptive thermogenesis. In this short review we shall discuss the main determinants of adaptive thermogenesis.

Cold-induced mitochondrial uncoupling and expression of chicken UCP and ANT mRNA in chicken skeletal muscle

Although bird species studied thus far have no distinct brown adipose tissue (BAT) or a related thermogenic tissue, there is now strong evidence that non-shivering mechanisms in birds may play an important role during cold exposure. Recently, increased expression of the duckling homolog of the avian uncoupling protein (avUCP) was demonstrated in cold-acclimated ducklings [Raimbault et al., Biochem. J. 353 (2001) 441-444]. Among the mitochondrial anion carriers, roles for the ATP/ADP antiporter (ANT) as well as UCP variants in thermogenesis are proposed. The present experiments were conducted (i) to examine the effects of cold acclimation on the fatty acid-induced uncoupling of oxidative phosphorylation in skeletal muscle mitochondria and (ii) to clone the cDNA of UCP and ANT homologs from chicken skeletal muscle and study differences compared to controls in expression levels of their mRNAs in the skeletal muscle of cold-acclimated chickens. The results obtained here show that suppression of palmitate-induced uncoupling by carboxyatractylate was greater in the subsarcolemmal skeletal muscle mitochondria from cold-acclimated chickens than that for control birds. An increase in mRNA levels of avANT and, to lesser degree, of avUCP in the skeletal muscle of cold-acclimated chickens was also found. Taken together, the present studies on cold-acclimated chickens suggest that the simultaneous increments in levels of avANT and avUCP mRNA expression may be involved in the regulation of thermogenesis in skeletal muscle.