Alkylsulfonates activate the uncoupling protein UCP1: implications for the transport mechanism

Proton leak through the UCPs and ANT carriers and beyond: A breath for the electron transport chain

Mitochondria produce heat as a result of an ineffective H+ cycling of mitochondria respiration across the inner mitochondrial membrane (IMM). This event present in all mitochondria, known as proton leak, can decrease protonmotive force (Δp) and restore mitochondrial respiration by partially uncoupling the substrate oxidation from the ADP phosphorylation. During impaired conditions of ATP generation with F1FO-ATPase, the Δp increases and IMM is hyperpolarized. In this bioenergetic state, the respiratory complexes support H+ transport until the membrane potential stops the H+ pump activity. Consequently, the electron transfer is stalled and the reduced form of electron carriers of the respiratory chain can generate O2∙¯ triggering the cascade of ROS formation and oxidative stress. The physiological function to attenuate the production of O2∙¯ by Δp dissipation can be attributed to the proton leak supported by the translocases of IMM.

Mitochondrial proton leaks and uncoupling proteins

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

Mitochondrial electron transport chain, ROS generation and uncoupling (Review)

The mammalian mitochondrial electron transport chain (ETC) includes complexes I-IV, as well as the electron transporters ubiquinone and cytochrome c. There are two electron transport pathways in the ETC: Complex I/III/IV, with NADH as the substrate and complex II/III/IV, with succinic acid as the substrate. The electron flow is coupled with the generation of a proton gradient across the inner membrane and the energy accumulated in the proton gradient is used by complex V (ATP synthase) to produce ATP. The first part of this review briefly introduces the structure and function of complexes I-IV and ATP synthase, including the specific electron transfer process in each complex. Some electrons are directly transferred to O₂ to generate reactive oxygen species (ROS) in the ETC. The second part of this review discusses the sites of ROS generation in each ETC complex, including sites I_F and I_Q in complex I, site II_F in complex II and site III_Qo in complex III, and the physiological and pathological regulation of ROS. As signaling molecules, ROS play an important role in cell proliferation, hypoxia adaptation and cell fate determination, but excessive ROS can cause irreversible cell damage and even cell death. The occurrence and development of a number of diseases are closely related to ROS overproduction. Finally, proton leak and uncoupling proteins (UCP_S) are discussed. Proton leak consists of basal proton leak and induced proton leak. Induced proton leak is precisely regulated and induced by UCPs. A total of five UCPs (UCP1-5) have been identified in mammalian cells. UCP1 mainly plays a role in the maintenance of body temperature in a cold environment through non-shivering thermogenesis. The core role of UCP2-5 is to reduce oxidative stress under certain conditions, therefore exerting cytoprotective effects. All diseases involving oxidative stress are associated with UCPs.

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.

The distinct bioenergetic properties of the human UCP1

The uncoupling protein UCP1 from brown adipose tissue is a mitochondrial carrier which allows dissipation of metabolic energy as heat. We have characterized the human UCP1 (HsUCP1) recombinantly expressed in Saccharomyces cerevisiae and we demonstrate that HsUCP1 is activated by fatty acids and retinoids in a nucleotide sensitive manner just as its rodent orthologs. However, in the absence of regulators, rodent UCP1 presents a high ohmic proton conductance that cannot be detected in HsUCP1. Since the human protein can be activated in a nucleotide sensitive manner, we conclude that it must have lost selectively the basal proton conductance.

UCPs, at the interface between bioenergetics and metabolism

The first member of the uncoupling protein (UCP) family, brown adipose tissue uncoupling protein 1 (UCP1), was identified in 1976. Twenty years later, two closely related proteins, UCP2 and UCP3, were described in mammals. Homologs of these proteins exist in other organisms, including plants. Uncoupling refers to a deterioration of energy conservation between substrate oxidation and ADP phosphorylation. Complete energy conservation loss would be fatal but fine-tuning can be beneficial for processes such as thermogenesis, redox control, and prevention of mitochondrial ROS release. The coupled/uncoupled state of mitochondria is related to the permeability of the inner membrane and the proton transport mediated by activated UCPs underlies the uncoupling activity of these proteins. Proton transport by UCP1 is activated by fatty acids and this ensures thermogenesis. In vivo in absence of this activation UCP1 remains inhibited with no transport activity. A similar situation now seems unlikely for UCP2 and UCP3 and while activation of their proton transport has been described its physiological relevance remains uncertain and their influence can be envisaged as a result of another transport pathway that takes place in the absence of activation. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

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.

Metabolically inert perfluorinated fatty acids directly activate uncoupling protein 1 in brown-fat mitochondria

The metabolically inert perfluorinated fatty acids perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) can display fatty acid-like activity in biological systems. The uncoupling protein 1 (UCP1) in brown adipose tissue is physiologically (re)activated by fatty acids, including octanoate. This leads to bioenergetically uncoupled energy dissipation (heat production, thermogenesis). We have examined here the possibility that PFOA/PFOS can directly (re)activate UCP1 in isolated mouse brown-fat mitochondria. In wild-type brown-fat mitochondria, PFOS and PFOA overcame GDP-inhibited thermogenesis, leading to increased oxygen consumption and dissipated membrane potential. The absence of this effect in brown-fat mitochondria from UCP1-ablated mice indicated that it occurred through activation of UCP1. A competitive type of inhibition by increased GDP concentrations indicated interaction with the same mechanistic site as that utilized by fatty acids. No effect was observed in heart mitochondria, i.e., in mitochondria without UCP1. The stimulatory effect of PFOA/PFOS was not secondary to non-specific mitochondrial membrane permeabilization or to ROS production. Thus, metabolic effects of perfluorinated fatty acids could include direct brown adipose tissue (UCP1) activation. The possibility that this may lead to unwarranted extra heat production and thus extra utilization of food resources, leading to decreased fitness in mammalian wildlife, is discussed, as well as possible negative effects in humans. However, a possibility to utilize PFOA-/PFOS-like substances for activating UCP1 therapeutically in obesity-prone humans may also be envisaged.

Dangerous liaisons between detergents and membrane proteins. The case of mitochondrial uncoupling protein 2

The extraction of membrane proteins from their native environment by detergents is central to their biophysical characterization. Recent studies have emphasized that detergents may perturb the structure locally and modify the dynamics of membrane proteins. However, it remains challenging to determine whether these perturbations are negligible or could be responsible for misfolded conformations, altering the protein's function. In this work, we propose an original strategy combining functional studies and molecular simulations to address the physiological relevance of membrane protein structures obtained in the presence of detergents. We apply our strategy to a structure of isoform 2 of an uncoupling protein (UCP2) binding an inhibitor recently obtained in dodecylphosphocholine detergent micelles. Although this structure shares common traits with the ADP/ATP carrier, a member of the same protein family, its functional and biological significance remains to be addressed. In the present investigation, we demonstrate how dodecylphosphocholine severely alters the structure as well as the function of UCPs. The proposed original strategy opens new vistas for probing the physiological relevance of three-dimensional structures of membrane proteins obtained in non-native environments.

Mechanism of fatty-acid-dependent UCP1 uncoupling in brown fat mitochondria

Mitochondrial uncoupling protein 1 (UCP1) is responsible for nonshivering thermogenesis in brown adipose tissue (BAT). Upon activation by long-chain fatty acids (LCFAs), UCP1 increases the conductance of the inner mitochondrial membrane (IMM) to make BAT mitochondria generate heat rather than ATP. Despite being a member of the family of mitochondrial anion carriers (SLC25), UCP1 is believed to transport H(+) by an unusual mechanism that has long remained unresolved. Here, we achieved direct patch-clamp measurements of UCP1 currents from the IMM of BAT mitochondria. We show that UCP1 is an LCFA anion/H(+) symporter. However, the LCFA anions cannot dissociate from UCP1 due to hydrophobic interactions established by their hydrophobic tails, and UCP1 effectively operates as an H(+) carrier activated by LCFA. A similar LCFA-dependent mechanism of transmembrane H(+) transport may be employed by other SLC25 members and be responsible for mitochondrial uncoupling and regulation of metabolic efficiency in various tissues.

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.

Mitochondrial proton and electron leaks

Mitochondrial proton and electron leak have a major impact on mitochondrial coupling efficiency and production of reactive oxygen species. In the first part of this chapter, we address the molecular nature of the basal and inducible proton leak pathways, and their physiological importance. The basal leak is unregulated, and a major proportion can be attributed to mitochondrial anion carriers, whereas the proton leak through the lipid bilayer appears to be minor. The basal proton leak is cell-type specific and correlates with metabolic rate. The inducible leak through the ANT (adenine nucleotide translocase) and UCPs (uncoupling proteins) can be activated by fatty acids, superoxide or lipid peroxidation products. The physiological role of inducible leak through UCP1 in mammalian brown adipose tissue is heat production, whereas the roles of non-mammalian UCP1 and its paralogous proteins, in particular UCP2 and UCP3, are not yet resolved. The second part of the chapter focuses on the electron leak that occurs in the mitochondrial electron transport chain. Exit of electrons prior to the reduction of oxygen to water at cytochrome c oxidase causes superoxide production. As the mechanisms of electron leak are crucial to understanding their physiological relevance, we summarize the mechanisms and topology of electron leak from complexes I and III in studies using isolated mitochondria. We also highlight recent progress and challenges of assessing electron leak in the living cell. Finally, we emphasize the importance of proton and electron leak as therapeutic targets in body mass regulation and insulin secretion.

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.

Identification of the mitochondrial carrier that provides Yarrowia lipolytica with a fatty acid-induced and nucleotide-sensitive uncoupling protein-like activity

Uncoupling proteins (UCPs) are mitochondrial carriers distributed throughout the eukaryotic kingdoms. While genes coding for UCPs have been identified in plants and animals, evidences for the presence of UCPs in fungi and protozoa are only functional. Here, it is reported that in the yeast Yarrowia lipolytica there is a fatty acid-promoted and GDP-sensitive uncoupling activity indicating the presence of a UCP. The uncoupling activity is higher in the stationary phase than in the mid-log growth phase. The in silico search on the Y. lipolytica genome led to the selection of two genes with the highest homology to the UCP family, XM_503525 and XM_500457. By phylogenetic analysis, XP_503525 was predicted to be an oxaloacetate carrier while XP_500457 would be a dicarboxylate carrier. Each of these two genes was cloned and heterologously expressed in Saccharomyces cerevisiae and the resulting phenotype was analyzed. The transport activity of the two gene products confirmed the phylogenetic predictions. In addition, only mitochondria isolated from yeasts expressing XP_503525 showed bioenergetic properties characteristic of a UCP: the proton conductance was increased by linoleic acid and inhibited by GDP. It is concluded that the XM_503525 gene from Y. lipolytica encodes for an oxaloacetate carrier although, remarkably, it also displays an uncoupling activity stimulated by fatty acids and inhibited by nucleotides.

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.

Mitochondrial uncouplers with an extraordinary dynamic range

We have discovered that some weak uncouplers (typified by butylated hydroxytoluene) have a dynamic range of more than 10(6) in vitro: the concentration giving measurable uncoupling is less than one millionth of the concentration causing full uncoupling. They achieve this through a high-affinity interaction with the mitochondrial adenine nucleotide translocase that causes significant but limited uncoupling at extremely low uncoupler concentrations, together with more conventional uncoupling at much higher concentrations. Uncoupling at the translocase is not by a conventional weak acid/anion cycling mechanism since it is also caused by substituted triphenylphosphonium molecules, which are not anionic and cannot protonate. Covalent attachment of the uncoupler to a mitochondrially targeted hydrophobic cation sensitizes it to membrane potential, giving a small additional effect. The wide dynamic range of these uncouplers in isolated mitochondria and intact cells reveals a novel allosteric activation of proton transport through the adenine nucleotide translocase and provides a promising starting point for designing safer uncouplers for obesity therapy.

Uncoupling effect of lauryl sulfate on mitochondria can be mediated by release of bound endogenous fatty acids

The mechanism of uncoupling by lauryl sulfate (LS) has been studied. The very fact that uncoupling by low concentration of LS (a strong acid) resembles very much that by fatty acids (weak acids) was used as an argument against the fatty acid cycling scheme of uncoupling where protonated fatty acids operate as a protonophore. We have found that rat liver and heart muscle mitochondria can be uncoupled by low (70 microM) LS concentration in a fashion completely arrested by the ATP/ADP antiporter inhibitor carboxyatractylate (CAtr). On the other hand, uncoupling by two-fold higher LS concentration is not sensitive to CAtr. Addition of oleate desensitizes mitochondria to low LS so that addition of bovine serum albumin becomes necessary to recouple mitochondria. The data are accounted for assuming that low LS releases endogenous fatty acids from some mitochondrial depots, and these fatty acids are responsible for uncoupling. As to high LS, it causes a nonspecific (CAtr-insensitive) damage to the mitochondrial membrane.

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.

Polyunsaturated fatty acids activate human uncoupling proteins 1 and 2 in planar lipid bilayers

Uncoupling proteins 1 (UCP1) and 2 (UCP2) belong to the family of mitochondrial anion transporters and share 59% sequence identity with each other. Whereas UCP1 was shown to be responsible for the rapid production of heat in brown adipose tissue, the primary function and transport properties of ubiquitously expressed UCP2 are controversially discussed. Here, for the first time, the activation pattern of the recombinant human UCP2 in comparison to the recombinant human UCP1 are studied using a well-defined system of planar lipid bilayers. It is shown that despite apparently different physiological functions, hUCP2 exhibited its protonophoric function similar to hUCP1--exclusively in the presence of long-chain fatty acids (FA). The calculated hUCP2 transport rate of 4.5 s(-1) is the same order of magnitude, as shown previously for UCP1. It leads to the conclusion that the differences in the activity of both proteins in living mitochondria are based exclusively on their different expression level. Both proteins are activated much more effectively by polyunsaturated than by saturated FA. The proton and total membrane conductances increased in the range palmitic < oleic < eicosatrienoic < linoleic < retinoic < arachidonic acids. The higher uncoupling protein (UCP)-dependent conductance in the presence of polyunsaturated FA is explained on the basis of the FA cycling hypothesis.

UCP2, UCP3, avUCP, what do they do when proton transport is not stimulated? Possible relevance to pyruvate and glutamine metabolism

Uncoupling proteins (UCPs) are specialized members of the mitochondrial transporter family. They allow passive proton transport through the mitochondrial inner membrane. This activity leads to uncoupling of mitochondrial respiration and to energy waste, which is well documented with UCP1 in brown adipose tissue. The uncoupling activity of the new UCPs (discovered after 1997), such as UCP2 and UCP3 in mammals or avUCP in birds, is more difficult to characterize. However, extensive data support the idea that the new UCPs are involved in the control of reactive oxygen species (ROS) generation. This fits with the hypothesis that mild uncoupling caused by the UCPs prevents ROS production. Activators and inhibitors regulate the proton transport activity of the UCPs. In the absence of activators of proton transport, the UCP allows the permeation of other ions. We suggest that this activity has physiological significance and, for example, UCP3 expressed in glycolytic muscle fibres may be a passive pyruvate transporter ensuring equilibrium between glycolysis and oxidative phosphorylation. Induction of UCP2 expression by glutamine strengthens the proposal that new UCPs could act to determine the choice of mitochondrial substrate. This would obviously have an impact on mitochondrial bioenergetics and ROS production.

Fatty acid activation of the uncoupling proteins requires the presence of the central matrix loop from UCP1

Noradrenaline signals the initiation of brown fat thermogenesis and the fatty acids liberated by the hormone-stimulated lipolysis act as second messengers to activate the uncoupling protein UCP1. UCP1 is a mitochondrial transporter that catalyses the re-entry of protons to the mitochondrial matrix thus allowing a regulated discharge of the proton gradient. The high affinity of UCP1 for fatty acids is a distinct feature of this uncoupling protein. The uncoupling proteins belong to a protein superfamily formed by the mitochondrial metabolite carriers. Members of this family present a tripartite structure where a domain containing two transmembrane helices, linked by a long hydrophilic loop, is repeated three times. Using protein chimeras, where the repeats had been swapped between UCP1 and UCP3, it has been shown that the central third of UCP1 is necessary and sufficient for the response of the protein to fatty acids. We have extended those studies and in the present report we have generated protein chimeras where different regions of the second repeat of UCP1 have been sequentially replaced with their UCP2 counterparts. The resulting chimeras present a progressive degradation of the characteristic bioenergetic properties of UCP1. We demonstrate that the presence of the second matrix loop is necessary for the high affinity activation of UCP1 by fatty acids.

Uncoupling proteins: a role in protection against reactive oxygen species--or not?

A physiological function of the original uncoupling protein, UCP1, is well established: UCP1 is the molecular background for nonshivering thermogenesis. The functions of the "novel" UCPs, UCP2 and UCP3, are still not established. Recent discussions imply that all UCPs may play a role in protection against reactive oxygen species (ROS). Here we examine critically the evidence that UCP1, UCP2 and UCP3 are stimulated by ROS (superoxide) or ROS products (4-hydroxy-2-nonenal), and that the UCPs actually diminish oxidative damage. We conclude that, concerning UCP1, it is unlikely that it has such a role; concerning UCP2/UCP3, most evidence for physiologically significant roles in this respect is still circumstantial.

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

The kinetics of proton transport through mammalian UCP1 (uncoupling protein 1) expressed in yeast mitochondria were measured. There was little or no UCP1 activity in the absence of added palmitate, but significant activity in its presence. The activator 4-HNE (4-hydroxy-2-nonenal) had little effect when added alone, but significantly enhanced proton conductance in the presence of added palmitate. Activation of the proton conductance of UCP1 was synergistic: proton conductance in the presence of both palmitate and 4-HNE was significantly greater than the sum of the individual effects. Mitochondria from control yeast transformed with empty vector showed no such synergy, showing that synergy is a property of UCP1. Activation by the 4-HNE analogue trans-cinnamate showed essentially the same characteristics as activation by 4-HNE. Mitochondria from brown adipose tissue also showed synergistic activation of GDP-sensitive proton conductance by palmitate and 4-HNE. These results show that reactive alkenals activate the proton conductance of UCP1 more strongly when fatty acids are also added, with implications for both mechanistic and physiological models of UCP1 activation.

The efficiency and plasticity of mitochondrial energy transduction

Since it was first realized that biological energy transduction involves oxygen and ATP, opinions about the amount of ATP made per oxygen consumed have continually evolved. The coupling efficiency is crucial because it constrains mechanistic models of the electron-transport chain and ATP synthase, and underpins the physiology and ecology of how organisms prosper in a thermodynamically hostile environment. Mechanistically, we have a good model of proton pumping by complex III of the electron-transport chain and a reasonable understanding of complex IV and the ATP synthase, but remain ignorant about complex I. Energy transduction is plastic: coupling efficiency can vary. Whether this occurs physiologically by molecular slipping in the proton pumps remains controversial. However, the membrane clearly leaks protons, decreasing the energy funnelled into ATP synthesis. Up to 20% of the basal metabolic rate may be used to drive this basal leak. In addition, UCP1 (uncoupling protein 1) is used in specialized tissues to uncouple oxidative phosphorylation, causing adaptive thermogenesis. Other UCPs can also uncouple, but are tightly regulated; they may function to decrease coupling efficiency and so attenuate mitochondrial radical production. UCPs may also integrate inputs from different fuels in pancreatic beta-cells and modulate insulin secretion. They are exciting potential targets for treatment of obesity, cachexia, aging and diabetes.

A new look at UCP 1

There has been a resurgence of interest in mitochondrial uncoupling protein 1 due to a desire to understand the regulation of the prominent role it plays in control of metabolic flux in brown adipose tissue and non-shivering thermogenesis, combined with the fact that UCP 1 acts as a paradigm for other novel less abundant uncoupling proteins. In this manuscript, we review the recent evidence for detection, purification, identification and function of UCP 1 in thymus mitochondria. In addition, we review the two proposed mechanisms for fatty acid dependent UCP 1 activity, namely (a) the flippase (flip-flop) model and (b) the cofactor/activation model, and the implication for these models of recent data showing that glucose-O-omega-palmitate cannot facilitate UCP 1 activity.

Evolutionarily distinct residues in the uncoupling protein UCP1 are essential for its characteristic basal proton conductance

The uncoupling proteins (UCPs) are mitochondrial transporters that modulate the efficiency of oxidative phosphorylation. Members of this family have been described in many phyla within the animal and plant kingdoms, as well as in fungi. The mammalian uncoupling protein UCP1 is activated by fatty acids and inhibited by nucleotides. In the absence of both regulators, UCP1 presents a high ohmic proton conductance that is a unique property of this carrier. The increasing number of protein sequences available has enabled us to apply a sequence analysis approach to investigate transporter function. We reconstructed a robust phylogeny of UCPs and used comparative sequence analysis to search for phylogenetically shared derived sequence features that may confer distinct properties on UCP1. We assessed the functional relevance of shared derived UCP1 residues by substituting them with their counterparts in UCP2, and expressing the protein chimeras in yeast. We found that substitution of both Glu134 and Met140 abolishes the basal proton permeability of UCP1 while preserving fatty acid activation and its nucleotide inhibition.

Undecanesulfonate does not allosterically activate H+ uniport mediated by uncoupling protein-1 in brown adipose tissue mitochondria

Undecanesulfonate is transported by uncoupling protein-1. Its inability to induce H+ uniport with reconstituted uncoupling protein-1 supports fatty acid cycling hypothesis. Rial et al. [Rial, E., Aguirregoitia, E., Jimenez-Jimenez, J., & Ledesma, A. (2004). Alkylsulfonates activate the uncoupling protein UCP1: Implications for the transport mechanism. Biochimica et Biophysica Acta, 1608, 122-130], have challenged the fatty acid cycling by observing uncoupling of brown adipose tissue mitochondria due to undecanesulfonate, interpreted as allosteric activation of uncoupling protein-1. We have estimated undecanesulfonate effects after elimination of endogenous fatty acids by carnitine cycle in the presence or absence of bovine serum albumin. We show that the undecanesulfonate effect is partly due to fatty acid release from albumin when undecanesulfonate releases bound fatty acid and partly represents a non-specific uncoupling protein-independent acceleration of respiration, since it proceeds also in rat heart mitochondria lacking uncoupling protein-1 and membrane potential is not decreased upon addition of undecanesulfonate without albumin. When the net fatty acid-induced uncoupling was assayed, the addition of undecanesulfonate even slightly inhibited the uncoupled respiration. We conclude that undecanesulfonate does not allosterically activate uncoupling protein-1 and that fatty acid cycling cannot be excluded on a basis of its non-specific effects.

Physiological functions of the mitochondrial uncoupling proteins UCP2 and UCP3

Evidence for the physiological functions of UCP2 and UCP3 is critically reviewed. They do not mediate adaptive thermogenesis, but they may be significantly thermogenic under specific pharmacological conditions. There is strong evidence that the mild regulated uncoupling they cause attenuates mitochondrial ROS production, protects against cellular damage, and diminishes insulin secretion. Evidence that they export fatty acids physiologically is weak. UCP2 and UCP3 are important potential targets for treatment of aging, degenerative diseases, diabetes, and perhaps obesity.

The reactions catalysed by the mitochondrial uncoupling proteins UCP2 and UCP3

The mitochondrial uncoupling proteins UCP2 and UCP3 may be important in attenuating mitochondrial production of reactive oxygen species, in insulin signalling (UCP2), and perhaps in thermogenesis and other processes. To understand their physiological roles, it is necessary to know what reactions they are able to catalyse. We critically examine the evidence for proton transport and anion transport by UCP2 and UCP3. There is good evidence that they increase mitochondrial proton conductance when activated by superoxide, reactive oxygen species derivatives such as hydroxynonenal, and other alkenals or their analogues. However, they do not catalyse proton leak in the absence of such acute activation. They can also catalyse export of fatty acid and other anions, although the relationship of anion transport to proton transport remains controversial.

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.

Activation by retinoids of the uncoupling protein UCP1

The uncoupling protein from brown adipose tissue (UCP1) is a transporter that catalyzes a regulated discharged of the mitochondrial proton gradient. The proton conductance in UCP1 is inhibited by nucleotides and activated by fatty acids. We have recently shown that all-trans-retinoic acid (ATRA) is a high-affinity activator of UCP1. In the present report, we have set to analyze the structural requirements for the ligands that activate UCP1 and particularly the specificity for different retinoids. For this purpose, we have developed a new protocol to determine the activity of UCP1 in respiring yeast mitochondria that can be adapted for high-throughput screenings. Our results evidence differences between the structural requirements for the activation by fatty acids and retinoids. Thus, although all active retinoids must possess a carboxylate, the introduction of additional polar groups renders them inactive. The linear and rigid structure of these molecules suggests the existence of a long hydrophobic binding pocket. We postulate that the access to the retinoid binding site must occur from the lipid bilayer and this could be at the interface between two transmembrane alpha-helices.

Native UCP1 displays simple competitive kinetics between the regulators purine nucleotides and fatty acids

Elucidation of the regulation of uncoupling protein 1 (UCP1) activity in its native environment, i.e. the inner membrane of brown-fat mitochondria, has been hampered by the presence of UCP1-independent, quantitatively unresolved effects of investigated regulators on the brown-fat mitochondria themselves. Here we have utilized the availability of UCP1-ablated mice to dissect UCP1-dependent and UCP1-independent effects of regulators. Using a complex-I-linked substrate (pyruvate), we found that UCP1 can mediate a 4-fold increase in thermogenesis when stimulated with the classical positive regulator fatty acids (oleate). After demonstrating that the fatty acids act in their free form, we found that UCP1 increased fatty acid sensitivity approximately 30-fold (as compared with the 1.5-fold increase reported earlier based on nominal fatty acid values). By identifying the UCP1-mediated fraction of the response, we could conclude that the interaction between purine nucleotides (GDP) and fatty acids (oleate) unexpectedly displayed simple competitive kinetics. In GDP-inhibited mitochondria, oleate apparently acted as an activator. However, only a model in which UCP1 is inherently active (i.e."activating" fatty acids cannot be included in the model), where GDP functions as an inhibitor with a K(m) of 0.05 mm, and where oleate functions as a competitive antagonist for the GDP effect (with a K(i) of 5 nm) can fit all of the experimental data. We conclude that, when examined in its native environment, UCP1 functions as a proton (equivalent) carrier in the absence of exogenous or endogenous fatty acids.