The mechanism of proton transport mediated by mitochondrial uncoupling proteins

Mitochondrial lipidomes are tissue specific - low cholesterol contents relate to UCP1 activity

Lipid composition is conserved within sub-cellular compartments to maintain cell function. Lipidomic analyses of liver, muscle, white and brown adipose tissue (BAT) mitochondria revealed substantial differences in their glycerophospholipid (GPL) and free cholesterol (FC) contents. The GPL to FC ratio was 50-fold higher in brown than white adipose tissue mitochondria. Their purity was verified by comparison of proteomes with ER and mitochondria-associated membranes. A lipid signature containing PC and FC, calculated from the lipidomic profiles, allowed differentiation of mitochondria from BAT of mice housed at different temperatures. Elevating FC in BAT mitochondria prevented uncoupling protein (UCP) 1 function, whereas increasing GPL boosted it. Similarly, STARD3 overexpression facilitating mitochondrial FC import inhibited UCP1 function in primary brown adipocytes, whereas a knockdown promoted it. We conclude that the mitochondrial GPL/FC ratio is key for BAT function and propose that targeting it might be a promising strategy to promote UCP1 activity.

Structural mechanisms of mitochondrial uncoupling protein 1 regulation in thermogenesis

In mitochondria, the oxidation of nutrients is coupled to ATP synthesis by the generation of a protonmotive force across the mitochondrial inner membrane. In mammalian brown adipose tissue (BAT), uncoupling protein 1 (UCP1, SLC25A7), a member of the SLC25 mitochondrial carrier family, dissipates the protonmotive force by facilitating the return of protons to the mitochondrial matrix. This process short-circuits the mitochondrion, generating heat for non-shivering thermogenesis. Recent cryo-electron microscopy (cryo-EM) structures of human UCP1 have provided new molecular insights into the inhibition and activation of thermogenesis. Here, we discuss these structures, describing how purine nucleotides lock UCP1 in a proton-impermeable conformation and rationalizing potential conformational changes of this carrier in response to fatty acid activators that enable proton leak for thermogenesis.

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.

Structural basis of purine nucleotide inhibition of human uncoupling protein 1

Mitochondrial uncoupling protein 1 (UCP1) gives brown adipose tissue of mammals its specialized ability to burn calories as heat for thermoregulation. When activated by fatty acids, UCP1 catalyzes the leak of protons across the mitochondrial inner membrane, short-circuiting the mitochondrion to generate heat, bypassing ATP synthesis. In contrast, purine nucleotides bind and inhibit UCP1, regulating proton leak by a molecular mechanism that is unclear. We present the cryo-electron microscopy structure of the GTP-inhibited state of UCP1, which is consistent with its nonconducting state. The purine nucleotide cross-links the transmembrane helices of UCP1 with an extensive interaction network. Our results provide a structural basis for understanding the specificity and pH dependency of the regulatory mechanism. UCP1 has retained all of the key functional and structural features required for a mitochondrial carrier-like transport mechanism. The analysis shows that inhibitor binding prevents the conformational changes that UCP1 uses to facilitate proton leak.

Dietary choline increases brown adipose tissue activation markers and improves cholesterol metabolism in female APOE*3-Leiden.CETP mice

OBJECTIVES

Studies in mice have recently linked increased dietary choline consumption to increased incidence of obesity-related metabolic diseases, while several clinical trials have reported an anti-obesity effect of high dietary choline intake. Since the underlying mechanisms by which choline affects obesity are incompletely understood, the aim of the present study was to investigate the role of dietary choline supplementation in adiposity.

METHODS

Female APOE*3-Leiden.CETP mice, a well-established model for human-like lipoprotein metabolism and cardiometabolic diseases, were fed a Western-type diet supplemented with or without choline (1.2%, w/w) for up to 16 weeks.

RESULTS

Dietary choline reduced body fat mass gain, prevented adipocyte enlargement, and attenuated adipose tissue inflammation. Besides, choline ameliorated liver steatosis and damage, associated with an upregulation of hepatic genes involved in fatty acid oxidation. Moreover, choline reduced plasma cholesterol, as explained by a reduction of plasma non-HDL cholesterol. Mechanistically, choline reduced hepatic VLDL-cholesterol secretion and enhanced the selective uptake of fatty acids from triglyceride-rich lipoprotein (TRL)-like particles by brown adipose tissue (BAT), consequently accelerating the clearance of the cholesterol-enriched TRL remnants by the liver.

CONCLUSIONS

In APOE*3-Leiden.CETP mice, dietary choline reduces body fat by enhancing TRL-derived fatty acids by BAT, resulting in accelerated TRL turnover to improve hypercholesterolemia. These data provide a mechanistic basis for the observation in human intervention trials that high choline intake is linked with reduced body weight.

Activating ligands of Uncoupling protein 1 identified by rapid membrane protein thermostability shift analysis

OBJECTIVE

Uncoupling protein 1 (UCP1) catalyses mitochondrial proton leak in brown adipose tissue to facilitate nutrient oxidation for heat production, and may combat metabolic disease if activated in humans. During the adrenergic stimulation of brown adipocytes, free fatty acids generated from lipolysis activate UCP1 via an unclear interaction. Here, we set out to characterise activator binding to purified UCP1 to clarify the activation process, discern novel activators and the potential to target UCP1.

METHODS

We assessed ligand binding to purified UCP1 by protein thermostability shift analysis, which unlike many conventional approaches can inform on the binding of hydrophobic ligands to membrane proteins. A detailed activator interaction analysis and screening approach was carried out, supported by investigations of UCP1 activity in liposomes, isolated brown fat mitochondria and UCP1 expression-controlled cell lines.

RESULTS

We reveal that fatty acids and other activators influence UCP1 through a specific destabilising interaction, behaving as transport substrates that shift the protein to a less stable conformation of a transport cycle. Through the detection of specific stability shifts in screens, we identify novel activators, including the over-the-counter drug ibuprofen, where ligand analysis indicates that UCP1 has a relatively wide structural specificity for interacting molecules. Ibuprofen successfully induced UCP1 activity in liposomes, isolated brown fat mitochondria and UCP1-expressing HEK293 cells but not in cultured brown adipocytes, suggesting drug delivery differs in each cell type.

CONCLUSIONS

These findings clarify the nature of the activator-UCP1 interaction and demonstrate that the targeting of UCP1 in cells by approved drugs is in principle achievable as a therapeutic avenue, but requires variants with more effective delivery in brown adipocytes.

Mitochondrial contributions to vascular endothelial dysfunction, arterial stiffness, and cardiovascular diseases

Cardiovascular disease (CVD) affects one in three adults and remains the leading cause of death in America. Advancing age is a major risk factor for CVD. Recent plateaus in CVD-related mortality rates in high-income countries after decades of decline highlight a critical need to identify novel therapeutic targets and strategies to mitigate and manage the risk of CVD development and progression. Vascular dysfunction, characterized by endothelial dysfunction and large elastic artery stiffening, is independently associated with an increased CVD risk and incidence and is therefore an attractive target for CVD prevention and management. Vascular mitochondria have emerged as an important player in maintaining vascular homeostasis. As such, age- and disease-related impairments in mitochondrial function contribute to vascular dysfunction and consequent increases in CVD risk. This review outlines the role of mitochondria in vascular function and discusses the ramifications of mitochondrial dysfunction on vascular health in the setting of age and disease. The adverse vascular consequences of increased mitochondrial-derived reactive oxygen species, impaired mitochondrial quality control, and defective mitochondrial calcium cycling are emphasized, in particular. Current evidence for both lifestyle and pharmaceutical mitochondrial-targeted strategies to improve vascular function is also presented.

Anti-Obesity Effects of Artemisia annua Extract in Zucker Fatty Rats and High-Fat Diet Sprague Dawley Rats through Upregulation of Uncoupling Protein 1

Background

Obesity is a widespread disease and is caused mainly by excessive adipocyte differentiation and fat accumulation. Peroxisome proliferation-activated receptor γ (PPARγ) and CCAAT/enhancer-binding proteins (C/EBP) are major components for regulating adipocyte differentiation. Uncoupling protein 1 (UCP1) is a transmembrane protein that can convert white fat to brown adipose tissue. Artemisia annua L. has long been used in East Asia as an herbal drug for anti-oxidant, anti-bacterial, and anti-obesity purposes.

Methods

We investigated the effects of water extracts of A. annua (WEAA) in C3H10T1/2, a mesenchymal stem cell line, by measuring the level of intracellular fat accumulation and the expression of genes associated with adipocyte differentiation. We also evaluated anti-obesity effects of WEAA in Zucker rats, a genetic model for the study of obesity, and in Sprague Dawley rats with high-fat diet (HFD)-induced obesity.

Results

In this study, WEAA reduced the expression levels of PPARγ and C/EBPα in C3H10T1/2 cells, as well as the expression of enzymes that regulate fatty acid metabolism. In the Zucker fatty rat model and the HFD-induced obesity rat model, WEAA significantly decreased adipogenic differentiation and white fat accumulation between the scapulae, in contrast to the brown fat that remained unchanged between the groups. A. annua suppressed the expression of the adipocyte differentiation-promoting genes, while increasing the expression of UCP1.

Conclusion

These results indicated that WEAA could reduce adipocyte differentiation and fat accumulation in in vitro and in vivo model systems, resulting in suppression of obesity and the occurrence of fatty liver due to a HFD.

Molecular Dynamics Simulations of Mitochondrial Uncoupling Protein 2

Molecular dynamics (MD) simulations of uncoupling proteins (UCP), a class of transmembrane proteins relevant for proton transport across inner mitochondrial membranes, represent a complicated task due to the lack of available structural data. In this work, we use a combination of homology modelling and subsequent microsecond molecular dynamics simulations of UCP2 in the DOPC phospholipid bilayer, starting from the structure of the mitochondrial ATP/ADP carrier (ANT) as a template. We show that this protocol leads to a structure that is impermeable to water, in contrast to MD simulations of UCP2 structures based on the experimental NMR structure. We also show that ATP binding in the UCP2 cavity is tight in the homology modelled structure of UCP2 in agreement with experimental observations. Finally, we corroborate our results with conductance measurements in model membranes, which further suggest that the UCP2 structure modeled from ANT protein possesses additional key functional elements, such as a fatty acid-binding site at the R60 region of the protein, directly related to the proton transport mechanism across inner mitochondrial membranes.

Ulmus parvifolia Jacq. Exhibits Antiobesity Properties and Potentially Induces Browning of White Adipose Tissue

The bark of Ulmus parvifolia Jacq. (UP) was traditionally used as a diuretic and to treat intestinal inflammation. With modern evidence of the correlation of diuretics, gut inflammation, and obesity, our study has shown the antiobesity effects of the bark of UP. UP treatment reduced lipid production and adipogenic genes in vitro. In vivo studies revealed that UP 100 mg/kg and UP 300 mg/kg treatment significantly reduced mouse weight without reducing food intake, indicating increased energy expenditure. UP significantly reduced the weight of epididymal and subcutaneous adipose tissue and decreased liver weight. Histological analysis revealed improvement in the progression of nonalcoholic fatty liver disease and epididymal white adipose tissue hypertrophy induced by a HFD. Real-Time PCR of epididymal adipose tissue revealed significant increases of uncoupling protein-1 (UCP-1) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) expression after UP 300 mg/kg treatments. Phosphorylation of AMP-activated protein α (AMPKα) was increased, while phosphorylation of Acetyl-CoA Carboxylase (ACC) was reduced. Our findings reveal the ability of UP to reduce the occurrence of obesity through increased browning of white adipose tissue via increased AMPKα, PPARγ, PGC-1α, and UCP-1 expression.

The contribution of uncoupling protein 2 to mitochondrial Ca2+ homeostasis in health and disease - A short revisit

Considering the versatile functions attributed to uncoupling protein 2 (UCP2) in health and disease, a profound understanding of the protein's molecular actions under physiological and pathophysiological conditions is indispensable. This review aims to revisit and shed light on the fundamental molecular functions of UCP2 in mitochondria, with particular emphasis on its intricate role in regulating mitochondrial calcium (Ca²⁺) uptake. UCP2's modulating effect on various vital processes in mitochondria makes it a crucial regulator of mitochondrial homeostasis in health and disease.

Phosphocholine accumulation and PHOSPHO1 depletion promote adipose tissue thermogenesis

Phosphocholine phosphatase-1 (PHOSPHO1) is a phosphocholine phosphatase that catalyzes the hydrolysis of phosphocholine (PC) to choline. Here we demonstrate that the PHOSPHO1 transcript is highly enriched in mature brown adipose tissue (BAT) and is further induced by cold and isoproterenol treatments of BAT and primary brown adipocytes. In defining the functional relevance of PHOPSPHO1 in BAT thermogenesis and energy metabolism, we show that PHOSPHO1 knockout mice are cold-tolerant, with higher expression of thermogenic genes in BAT, and are protected from high-fat diet-induced obesity and development of insulin resistance. Treatment of mice with the PHOSPHO1 substrate phosphocholine is sufficient to induce cold tolerance, thermogenic gene expression, and allied metabolic benefits. Our results reveal a role of PHOSPHO1 as a negative regulator of BAT thermogenesis, and inhibition of PHOSPHO1 or enhancement of phosphocholine represent innovative approaches to manage the metabolic syndrome.

Alternative splicing of UCP1 by non-cell-autonomous action of PEMT

OBJECTIVE

Phosphatidylethanolamine methyltransferase (PEMT) generates phosphatidylcholine (PC), the most abundant phospholipid in the mitochondria and an important acyl chain donor for cardiolipin (CL) biosynthesis. Mice lacking PEMT (PEMTKO) are cold-intolerant when fed a high-fat diet (HFD) due to unclear mechanisms. The purpose of this study was to determine whether PEMT-derived phospholipids are important for the function of uncoupling protein 1 (UCP1) and thus for maintenance of core temperature.

METHODS

To test whether PEMT-derived phospholipids are important for UCP1 function, we examined cold-tolerance and brown adipose (BAT) mitochondria from PEMTKO mice with or without HFD feeding. We complemented these studies with experiments on mice lacking functional CL due to tafazzin knockdown (TAZKD). We generated several conditional mouse models to study the tissue-specific roles of PEMT, including mice with BAT-specific knockout of PEMT (PEMT-BKO).

RESULTS

Chow- and HFD-fed PEMTKO mice completely lacked UCP1 protein in BAT, despite a lack of difference in mRNA levels, and the mice were accordingly cold-intolerant. While HFD-fed PEMTKO mice exhibited reduced mitochondrial CL content, this was not observed in chow-fed PEMTKO mice or TAZKD mice, indicating that the lack of UCP1 was not attributable to CL deficiency. Surprisingly, the PEMT-BKO mice exhibited normal UCP1 protein levels. Knockout of PEMT in the adipose tissue (PEMT-AKO), liver (PEMT-LKO), or skeletal muscle (PEMT-MKO) also did not affect UCP1 protein levels, suggesting that lack of PEMT in other non-UCP1-expressing cells communicates to BAT to suppress UCP1. Instead, we identified an untranslated UCP1 splice variant that was triggered during the perinatal period in the PEMTKO mice.

CONCLUSIONS

PEMT is required for UCP1 splicing that yields functional protein. This effect is derived by PEMT in nonadipocytes that communicates to BAT during embryonic development. Future research will focus on identifying the non-cell-autonomous PEMT-dependent mechanism of UCP1 splicing.

The Mechanism FA-Dependent H+ Transport by UCP1

Uncoupling protein 1 (UCP1) is an integral protein of the inner mitochondrial membrane (IMM) that is expressed specifically in brown and beige fat depots. UCP1 is responsible for the production of heat to control core body temperature, the regulation of fat metabolism, and the energy balance. As an uncoupling protein, UCP1 transports H⁺ across the IMM in presence of long-chain fatty acids (FA), which makes brown fat mitochondria produce heat at the expense of ATP. However, the exact mechanism of UCP1 action has remained difficult to elucidate, because direct methods for studying currents generated by UCP1 were unavailable. Recently, the patch-clamp technique was successfully applied to brown and beige fat mitochondria to directly study H⁺ currents across the IMM and characterize UCP1 function. A new model of the UCP1 mechanism was proposed based on the patch-clamp analysis. In this model, both FA anions (FA^-) and H⁺ are transport substrates of UCP1, and UCP1 operates as a non-canonical FA^-/H⁺ symporter. Here, we summarize recent findings obtained with the patch-clamp technique that describe how UCP1 can transport not only H⁺ but also FA^-.

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.

Cluh plays a pivotal role during adipogenesis by regulating the activity of mitochondria

Cluh is a cytosolic protein that is known to specifically bind the mRNAs of nuclear-encoded mitochondrial proteins and play critical roles in mitochondrial biogenesis. Here, we report the role of Cluh in adipogenesis. Our study shows that mRNA expression of Cluh is stimulated during adipogenesis, and that cAMP/Creb signalling increases its transcription. Cluh depletion impaired proper adipocyte differentiation, with reductions seen in lipid droplets and adipogenic marker gene expression. Interestingly, the inductions of the brown adipocyte-specific genes, Ucp1, Cidea and Cox7a1, are severely blocked by Cluh depletion during brown adipogenesis. Mitochondrial respiration and the stability of mRNAs encoding mitochondrial proteins are reduced by Cluh depletion during brown adipogenesis. These results suggest that Cluh, which is induced during adipogenesis, promotes the post-transcriptional regulation of mitochondrial proteins and supports differentiation.

Important Trends in UCP3 Investigation

Membrane uncoupling protein 3 (UCP3), a member of the mitochondrial uncoupling protein family, was discovered in 1997. UCP3's properties, such as its high homology to other mitochondrial carriers, especially to UCP2, its short lifetime and low specificity of UCP3 antibodies, have hindered progress in understanding its biological function and transport mechanism over decades. The abundance of UCP3 is highest in murine brown adipose tissue (BAT, 15.0 pmol/mg protein), compared to heart (2.7 pmol/mg protein) and the gastrocnemius muscle (1.7 pmol/mg protein), but it is still 400-fold lower than the abundance of UCP1, a biomarker for BAT. Investigation of UCP3 reconstituted in planar bilayer membranes revealed that it transports protons only when activated by fatty acids (FA). Although purine nucleotides (PN) inhibit UCP3-mediated transport, the molecular mechanism differs from that of UCP1. It remains a conundrum that two homologous proton-transporting proteins exist within the same tissue. Recently, we proposed that UCP3 abundance directly correlates with the degree of FA β-oxidation in cell metabolism. Further development in this field implies that UCP3 may have dual function in transporting substrates, which have yet to be identified, alongside protons. Evaluation of the literature with respect to UCP3 is a complex task because (i) UCP3 features are often extrapolated from its "twin" UCP2 without additional proof, and (ii) the specificity of antibodies against UCP3 used in studies is rarely evaluated. In this review, we primarily focus on recent findings obtained for UCP3 in biological and biomimetic systems.

Mitochondrial Uncoupling Proteins: Subtle Regulators of Cellular Redox Signaling

SIGNIFICANCE

Mitochondria are the energetic, metabolic, redox, and information signaling centers of the cell. Substrate pressure, mitochondrial network dynamics, and cristae morphology state are integrated by the protonmotive force Δp or its potential component, ΔΨ, which are attenuated by proton backflux into the matrix, termed uncoupling. The mitochondrial uncoupling proteins (UCP1-5) play an eminent role in the regulation of each of the mentioned aspects, being involved in numerous physiological events including redox signaling. Recent Advances: UCP2 structure, including purine nucleotide and fatty acid (FA) binding sites, strongly support the FA cycling mechanism: UCP2 expels FA anions, whereas uncoupling is achieved by the membrane backflux of protonated FA. Nascent FAs, cleaved by phospholipases, are preferential. The resulting Δp dissipation decreases superoxide formation dependent on Δp. UCP-mediated antioxidant protection and its impairment are expected to play a major role in cell physiology and pathology. Moreover, UCP2-mediated aspartate, oxaloacetate, and malate antiport with phosphate is expected to alter metabolism of cancer cells.

CRITICAL ISSUES

A wide range of UCP antioxidant effects and participations in redox signaling have been reported; however, mechanisms of UCP activation are still debated. Switching off/on the UCP2 protonophoretic function might serve as redox signaling either by employing/releasing the extra capacity of cell antioxidant systems or by directly increasing/decreasing mitochondrial superoxide sources. Rapid UCP2 degradation, FA levels, elevation of purine nucleotides, decreased Mg²⁺, or increased pyruvate accumulation may initiate UCP-mediated redox signaling.

FUTURE DIRECTIONS

Issues such as UCP2 participation in glucose sensing, neuronal (synaptic) function, and immune cell activation should be elucidated. Antioxid. Redox Signal. 29, 667-714.

Brown Adipose Tissue Energy Metabolism in Humans

The demonstration of metabolically active brown adipose tissue (BAT) in humans primarily using positron emission tomography coupled to computed tomography (PET/CT) with the glucose tracer 18-fluorodeoxyglucose (18FDG) has renewed the interest of the scientific and medical community in the possible role of BAT as a target for the prevention and treatment of obesity and type 2 diabetes (T2D). Here, we offer a comprehensive review of BAT energy metabolism in humans. Considerable advances in methods to measure BAT energy metabolism, including nonesterified fatty acids (NEFA), chylomicron-triglycerides (TG), oxygen, Krebs cycle rate, and intracellular TG have led to very good quantification of energy substrate metabolism per volume of active BAT in vivo. These studies have also shown that intracellular TG are likely the primary energy source of BAT upon activation by cold. Current estimates of BAT's contribution to energy expenditure range at the lower end of what would be potentially clinically relevant if chronically sustained. Yet, 18FDG PET/CT remains the gold-standard defining method to quantify total BAT volume of activity, used to calculate BAT's total energy expenditure. Unfortunately, BAT glucose metabolism better reflects BAT's insulin sensitivity and blood flow. It is now clear that most glucose taken up by BAT does not fuel mitochondrial oxidative metabolism and that BAT glucose uptake can therefore be disconnected from thermogenesis. Furthermore, BAT thermogenesis is efficiently recruited upon repeated cold exposure, doubling to tripling its total oxidative capacity, with reciprocal reduction of muscle thermogenesis. Recent data suggest that total BAT volume may be much larger than the typically observed 50-150 ml with 18FDG PET/CT. Therefore, the current estimates of total BAT thermogenesis, largely relying on total BAT volume using 18FDG PET/CT, may underestimate the true contribution of BAT to total energy expenditure. Quantification of the contribution of BAT to energy expenditure begs for the development of more integrated whole body in vivo methods.

Cell-free production and characterisation of human uncoupling protein 1-3

The uncoupling proteins (UCPs) leak protons across the inner mitochondrial membrane, thus uncoupling the proton gradient from ATP synthesis. The main known physiological role for this is heat generation by UCP1 in brown adipose tissue. However, UCPs are also believed to be important for protection against reactive oxygen species, fine-tuning of metabolism and have been suggested to be involved in disease states such as obesity, diabetes and cancer.

Structural studies of UCPs have long been hampered by difficulties in sample preparation with neither expression in yeast nor refolding from inclusion bodies in E. coli yielding sufficient amounts of pure and stable protein. In this study, we have developed a protocol for cell-free expression of human UCP1, 2 and 3, resulting in 1 mg pure protein per 20 mL of expression media. Lauric acid, a natural UCP ligand, significantly improved protein thermal stability and was therefore added during purification. Secondary structure characterisation using circular dichroism spectroscopy revealed the proteins to consist of mostly α-helices, as expected. All three UCPs were able to bind GDP, a well-known physiological inhibitor, as shown by the Fluorescence Resonance Energy Transfer (FRET) technique, suggesting that the proteins are in a natively folded state.

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A protocol for cell-free expression of human uncoupling protein 1–3 is described.

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Addition of native membrane components increased expression levels.

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Addition of lauric acid increased protein stability in solution.

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CD spectroscopy confirms alpha-helical secondary structure as expected.

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All proteins binds GDP as demonstrated by Fluorescence Resonance Energy Transfer.

Small-Molecule Uncoupling Protein Mimics: Synthetic Anion Receptors as Fatty Acid-Activated Proton Transporters

Uncoupling proteins (UCPs) regulate energy expenditure in living cells by inducing proton leakage across the mitochondrial inner membrane, thereby uncoupling adenosine diphosphate phosphorylation from nutrient oxidation. The proton transport activity of UCP1 and UCP2 requires activation by fatty acids. We report here the first examples of synthetic neutral anion receptors performing this biologically important fatty acid-activated function in phospholipid bilayers. We have shown that a tripodal thiourea possesses poor H⁺/OH^- transport activity without fatty acids, but in the presence of long-chain fatty acids is "switched on" as a proton transporter with an activity close to that of a commonly used protonophore. The fatty acid-enhanced proton transport was also observed for other hydrogen and halogen bond-based synthetic anion transporters. We propose that these compounds induce proton permeability by catalyzing transbilayer movement ("flip-flop") of anionic forms of fatty acids, so allowing the fatty acids to complete a proton transport cycle. Several lines of evidence have been provided to support such a fatty acid cycling mechanism. Our findings open up new applications of anion receptor chemistry and provide important clues for understanding biological activities of synthetic anion transporters and potentially the uncoupling mechanism of naturally occurring membrane proteins.

UCP1: A transporter for H+ and fatty acid anions

Adaptive thermogenesis regulates core body temperature, controls fat deposition, and contributes strongly to the overall energy balance. This process occurs in brown fat and requires uncoupling protein 1 (UCP1), an integral protein of the inner mitochondrial membrane. Classic biochemical studies revealed the general principle of adaptive thermogenesis: in the presence of long-chain fatty acids (FA), UCP1 increases the permeability of the inner mitochondrial membrane for H⁺, which makes brown fat mitochondria produce heat rather than ATP. However, the exact mechanism by which UCP1 increases the membrane H⁺ conductance in a FA-dependent manner has remained a fundamental unresolved question. Recently, the patch-clamp technique was successfully applied to the inner mitochondrial membrane of brown fat to directly characterize the H⁺ currents carried by UCP1. Based on the patch-clamp data, a new model of UCP1 operation was proposed. In brief, FA anions are transport substrates of UCP1, and UCP1 operates as an unusual FA anion/H⁺ symporter. Interestingly, in contrast to short-chain FA anions, long-chain FA anions cannot easily dissociate from UCP1 due to strong hydrophobic interactions established by their carbon tails, and a single long-chain FA participates in many H⁺ transport cycles. Therefore, in the presence of long-chain FA, endogenous activators of brown fat thermogenesis, UCP1 effectively operates as an H⁺ uniport. In addition to their transport function, long-chain FA competitively remove tonic inhibition of UCP1 by cytosolic purine nucleotides, thus enabling activation of the thermogenic H⁺ leak through UCP1 under physiological conditions.

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.

Uncoupling protein 1 binds one nucleotide per monomer and is stabilized by tightly bound cardiolipin

Uncoupling protein 1 (UCP1) catalyzes fatty acid-activated, purine nucleotide-sensitive proton leak across the mitochondrial inner membrane of brown adipose tissue to produce heat, and could help combat obesity and metabolic disease in humans. Studies over the last 30 years conclude that the protein is a dimer, binding one nucleotide molecule per two proteins, and unlike the related mitochondrial ADP/ATP carrier, does not bind cardiolipin. Here, we have developed novel methods to purify milligram amounts of UCP1 from native sources by using covalent chromatography that, unlike past methods, allows the protein to be prepared in defined conditions, free of excess detergent and lipid. Assessment of purified preparations by TLC reveal that UCP1 retains tightly bound cardiolipin, with a lipid phosphorus content equating to three molecules per protein, like the ADP/ATP carrier. Cardiolipin stabilizes UCP1, as demonstrated by reconstitution experiments and thermostability assays, indicating that the lipid has an integral role in the functioning of the protein, similar to other mitochondrial carriers. Furthermore, we find that UCP1 is not dimeric but monomeric, as indicated by size exclusion analysis, and has a ligand titration profile in isothermal calorimetric measurements that clearly shows that one nucleotide binds per monomer. These findings reveal the fundamental composition of UCP1, which is essential for understanding the mechanism of the protein. Our assessment of the properties of UCP1 indicate that it is not unique among mitochondrial carriers and so is likely to use a common exchange mechanism in its primary function in brown adipose tissue mitochondria.

Trends in thermostability provide information on the nature of substrate, inhibitor, and lipid interactions with mitochondrial carriers

Mitochondrial carriers, including uncoupling proteins, are unstable in detergents, which hampers structural and mechanistic studies. To investigate carrier stability, we have purified ligand-free carriers and assessed their stability with a fluorescence-based thermostability assay that monitors protein unfolding with a thiol-reactive dye. We find that mitochondrial carriers from both mesophilic and thermophilic organisms exhibit poor stability in mild detergents, indicating that instability is inherent to the protein family. Trends in the thermostability of yeast ADP/ATP carrier AAC2 and ovine uncoupling protein UCP1 allow optimal conditions for stability in detergents to be established but also provide mechanistic insights into the interactions of lipids, substrates, and inhibitors with these proteins. Both proteins exhibit similar stability profiles across various detergents, where stability increases with the size of the associated detergent micelle. Detailed analysis shows that lipids stabilize carriers indirectly by increasing the associated detergent micelle size, but cardiolipin stabilizes by direct interactions as well. Cardiolipin reverses destabilizing effects of ADP and bongkrekic acid on AAC2 and enhances large stabilizing effects of carboxyatractyloside, revealing that this lipid interacts in the m-state and possibly other states of the transport cycle, despite being in a dynamic interface. Fatty acid activators destabilize UCP1 in a similar way, which can also be prevented by cardiolipin, indicating that they interact like transport substrates. Our controls show that carriers can be soluble but unfolded in some commonly used detergents, such as the zwitterionic Fos-choline-12, which emphasizes the need for simple validation assays like the one used here.

Antioxidant and Regulatory Role of Mitochondrial Uncoupling Protein UCP2 in Pancreatic β-cells

Research on brown adipose tissue and its hallmark protein, mitochondrial uncoupling protein UCP1, has been conducted for half a century and has been traditionally studied in the Institute of Physiology (AS CR, Prague), likewise UCP2 residing in multiple tissues for the last two decades. Our group has significantly contributed to the elucidation of UCP uncoupling mechanism, fully dependent on free fatty acids (FFAs) within the inner mitochondrial membrane. Now we review UCP2 physiological roles emphasizing its roles in pancreatic β-cells, such as antioxidant role, possible tuning of redox homeostasis (consequently UCP2 participation in redox regulations), and fine regulation of glucose-stimulated insulin secretion (GSIS). For example, NADPH has been firmly established as being a modulator of GSIS and since UCP2 may influence redox homeostasis, it likely affects NADPH levels. We also point out the role of phospholipase iPLA2 isoform  in providing FFAs for the UCP2 antioxidant function. Such initiation of mild uncoupling hypothetically precedes lipotoxicity in pancreatic β-cells until it reaches the pathological threshold, after which the antioxidant role of UCP2 can be no more cell-protective, for example due to oxidative stress-accumulated mutations in mtDNA. These mechanisms, together with impaired autocrine insulin function belong to important causes of Type 2 diabetes etiology.

Brown adipose tissue as an anti-obesity tissue in humans

During the 11th Stock Conference held in Montreal, Quebec, Canada, world-leading experts came together to present and discuss recent developments made in the field of brown adipose tissue biology. Owing to the vast capacity of brown adipose tissue for burning food energy in the process of thermogenesis, and due to demonstrations of its presence in adult humans, there is tremendous interest in targeting brown adipose tissue as an anti-obesity tissue in humans. However, the future of such therapeutic approaches relies on our understanding of the origin, development, recruitment, activation and regulation of brown adipose tissue in humans. As reviewed here, the 11th Stock Conference was organized around these themes to discuss the recent progress made in each aspect, to identify gaps in our current understanding and to further provide a common groundwork that could support collaborative efforts aimed at a future therapy for obesity, based on brown adipose tissue thermogenesis.

Involvement of heparan sulfate 6-O-sulfation in the regulation of energy metabolism and the alteration of thyroid hormone levels in male mice

Here, we report that male heparan sulfate 6-O-sulfotransferase-2 (Hs6st2) knockout mice showed increased body weight in an age-dependent manner even when fed with a normal diet and showed a phenotype of impaired glucose metabolism and insulin resistance. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that the expression of mitochondrial uncoupling proteins Ucp1 and Ucp3 was reduced in the interscapular brown adipose tissue (BAT) of male Hs6st2 knockout mice, suggesting reduced energy metabolism. The serum level of thyroid-stimulating hormone was significantly higher and that of thyroxine was lower in the knockout mice. When cultures of brown adipocytes from wild-type and Hs6st2 knockout mice isolated and differentiated in vitro were treated with FGF19 (fibroblast growth factor 19) or FGF21 in the presence or the absence of heparitinase I, phosphorylation of p42/p44 mitogen-activated protein (MAP) kinase was reduced. Heparan sulfate (HS) 6-O-sulfation was reduced not only in BAT but also in the thyroid tissue of the knockout mice. Thus, 6-O-sulfation in HS seems to play an important role in mediating energy metabolism by controlling thyroid hormone levels and signals from the FGF19 subfamily proteins, and the alteration of the HS composition may result in metabolic syndrome phenotypes such as altered glucose and insulin tolerance.

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.

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

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

Uncoupling protein 1 of brown adipocytes, the only uncoupler: a historical perspective

Uncoupling protein 1 (UCP1), is a unique mitochondrial membranous protein devoted to adaptive thermogenesis, a specialized function performed by brown adipocytes. Whereas the family of mitochondrial metabolite carriers comprises ∼40 members, UCP1 is the only memberable to translocate protons through the inner membrane of brown adipocyte mitochondria. By this process, UCP1 uncouples respiration from ATP synthesis and therefore provokes energy dissipation in the form of heat while, also stimulating high levels of fatty acid oxidation. UCP1 homologs were identified but they are biochemically and physiologically different from UCP1. Thirty five years after its identification, UCP1 still appears as a fascinating component. The recent renewal of the interest in human brown adipose tissue makes UCP1 as a potential target for strategies of treatment of metabolic disorders.

The regulation and physiology of mitochondrial proton leak

Mitochondria couple respiration to ATP synthesis through an electrochemical proton gradient. Proton leak across the inner membrane allows adjustment of the coupling efficiency. The aim of this review is threefold: 1) introduce the unfamiliar reader to proton leak and its physiological significance, 2) review the role and regulation of uncoupling proteins, and 3) outline the prospects of proton leak as an avenue to treat obesity, diabetes, and age-related disease.

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

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

Acute stimulation of white adipocyte respiration by PKA-induced lipolysis

OBJECTIVE

We examined the effect of β-adrenergic receptor (βAR) activation and cAMP-elevating agents on respiration and mitochondrial uncoupling in human adipocytes and probed the underlying molecular mechanisms.

RESEARCH DESIGN AND METHODS

Oxygen consumption rate (OCR, aerobic respiration) and extracellular acidification rate (ECAR, anaerobic respiration) were examined in response to isoproterenol (ISO), forskolin (FSK), and dibutyryl-cAMP (DB), coupled with measurements of mitochondrial depolarization, lipolysis, kinase activities, and gene targeting or knock-down approaches.

RESULTS

ISO, FSK, or DB rapidly increased oxidative and glycolytic respiration together with mitochondrial depolarization in human and mouse white adipocytes. The increase in OCR was oligomycin-insensitive and contingent on cAMP-dependent protein kinase A (PKA)-induced lipolysis. This increased respiration and the uncoupling were blocked by inhibiting the mitochondrial permeability transition pore (PTP) and its regulator, BAX. Interestingly, compared with lean individuals, adipocytes from obese subjects exhibited reduced OCR and uncoupling capacity in response to ISO.

CONCLUSIONS

Lipolysis stimulated by βAR activation or other maneuvers that increase cAMP levels in white adipocytes acutely induces mitochondrial uncoupling and cellular energetics, which are amplified in the absence of scavenging BSA. The increase in OCR is dependent on PKA-induced lipolysis and is mediated by the PTP and BAX. Because this effect is reduced with obesity, further exploration of this uncoupling mechanism will be needed to determine its cause and consequences.

A haplotype at the UCP1 gene locus contributes to genetic risk for type 2 diabetes in Asian Indians (CURES-72)

BACKGROUND

The gene encoding for uncoupling protein-1 (UCP1) is considered to be a candidate gene for type 2 diabetes because of its role in thermogenesis and energy expenditure. The objective of the study was to examine whether genetic variations in the UCP1 gene are associated with type 2 diabetes and its related traits in Asian Indians.

METHODS

The study subjects, 810 type 2 diabetic subjects and 990 normal glucose tolerant (NGT) subjects, were chosen from the Chennai Urban Rural Epidemiological Study (CURES), an ongoing population-based study in southern India. The polymorphisms were genotyped using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Linkage disequilibrium (LD) was estimated from the estimates of haplotypic frequencies.

RESULTS

The three polymorphisms, namely -3826A-->G, an A-->C transition in the 5'-untranslated region (UTR) and Met229Leu, were not associated with type 2 diabetes. However, the frequency of the A-C-Met (-3826A-->G-5'UTR A-->C-Met229Leu) haplotype was significantly higher among the type 2 diabetic subjects (2.67%) compared with the NGT subjects (1.45%, P < 0.01). The odds ratio for type 2 diabetes for the individuals carrying the haplotype A-C-Met was 1.82 (95% confidence interval, 1.29-2.78, P = 0.009).

CONCLUSIONS

The haplotype, A-C-Met, in the UCP1 gene is significantly associated with the increased genetic risk for developing type 2 diabetes in Asian Indians.

Myocardial fatty acid metabolism in health and disease

There is a constant high demand for energy to sustain the continuous contractile activity of the heart, which is met primarily by the beta-oxidation of long-chain fatty acids. The control of fatty acid beta-oxidation is complex and is aimed at ensuring that the supply and oxidation of the fatty acids is sufficient to meet the energy demands of the heart. The metabolism of fatty acids via beta-oxidation is not regulated in isolation; rather, it occurs in response to alterations in contractile work, the presence of competing substrates (i.e., glucose, lactate, ketones, amino acids), changes in hormonal milieu, and limitations in oxygen supply. Alterations in fatty acid metabolism can contribute to cardiac pathology. For instance, the excessive uptake and beta-oxidation of fatty acids in obesity and diabetes can compromise cardiac function. Furthermore, alterations in fatty acid beta-oxidation both during and after ischemia and in the failing heart can also contribute to cardiac pathology. This paper reviews the regulation of myocardial fatty acid beta-oxidation and how alterations in fatty acid beta-oxidation can contribute to heart disease. The implications of inhibiting fatty acid beta-oxidation as a potential novel therapeutic approach for the treatment of various forms of heart disease are also discussed.

Type 1 diabetic akita mouse hearts are insulin sensitive but manifest structurally abnormal mitochondria that remain coupled despite increased uncoupling protein 3

OBJECTIVE

Fatty acid-induced mitochondrial uncoupling and oxidative stress have been proposed to reduce cardiac efficiency and contribute to cardiac dysfunction in type 2 diabetes. We hypothesized that mitochondrial uncoupling may also contribute to reduced cardiac efficiency and contractile dysfunction in the type 1 diabetic Akita mouse model (Akita).

RESEARCH DESIGN AND METHODS

Cardiac function and substrate utilization were determined in isolated working hearts and in vivo function by echocardiography. Mitochondrial function and coupling were determined in saponin-permeabilized fibers, and proton leak kinetics was determined in isolated mitochondria. Hydrogen peroxide production and aconitase activity were measured in isolated mitochondria, and total reactive oxygen species (ROS) were measured in heart homogenates.

RESULTS

Resting cardiac function was normal in Akita mice, and myocardial insulin sensitivity was preserved. Although Akita hearts oxidized more fatty acids, myocardial O(2) consumption was not increased, and cardiac efficiency was not reduced. ADP-stimulated mitochondrial oxygen consumption and ATP synthesis were decreased, and mitochondria showed grossly abnormal morphology in Akita. There was no evidence of oxidative stress, and despite a twofold increase in uncoupling protein 3 (UCP3) content, ATP-to-O ratios and proton leak kinetics were unchanged, even after perfusion of Akita hearts with 1 mmol/l palmitate.

CONCLUSIONS

Insulin-deficient Akita hearts do not exhibit fatty acid-induced mitochondrial uncoupling, indicating important differences in the basis for mitochondrial dysfunction between insulin-responsive type 1 versus insulin-resistant type 2 diabetic hearts. Increased UCP3 levels do not automatically increase mitochondrial uncoupling in the heart, which supports the hypothesis that fatty acid-induced mitochondrial uncoupling as exists in type 2 diabetic hearts requires a concomitant increase in ROS generation.

Within brown-fat cells, UCP1-mediated fatty acid-induced uncoupling is independent of fatty acid metabolism

In the present investigation, we have utilized the availability of UCP1(-/-) mice to examine a wide range of previously proposed lipid activators of Uncoupling Protein 1 (UCP1) in its native environment, i.e. in the brown-fat cells. A non-metabolizable fatty acid analogue, beta,beta cent-methyl-substituted hexadecane alpha,omega-dicarboxylic acid (Medica-16) is a potent UCP1 (re)activator in brown-fat cells, despite its bipolar structure. All-trans-retinoic acid activates UCP1 within cells, whereas beta-carotene only does so after metabolism. The UCP1-dependent effects of fatty acids are positively correlated with their chain length. Medium-chain fatty acids are potent UCP1 activators in cells, despite their lack of protonophoric properties in mitochondrial membranes. Thus, neither the ability to be metabolized nor an innate uncoupling/protonophoric ability is a necessary property of UCP1 activators within brown-fat cells.

A CD study of uncoupling protein-1 and its transmembrane and matrix-loop domains

Conformations of the prototypic UCP-1 (uncoupling protein-1) and its TM (transmembrane) and ML (matrix-loop) domains were studied by CD spectroscopy. Recombinant, untagged mouse UCP-1 and a hexahistidine-tagged version of the protein were obtained in high purity following their overexpression in Escherichia coli. The TM and ML domains of hamster UCP-1 were chemically synthesized. Conformations of both recombinant UCP-1 proteins were dominantly helical (40-50%) in digitonin micelles. Binding of the purine nucleotides GDP and GTP to UCP-1, detected in the near-UV CD region, supported the existence of the functional form of the protein in digitonin micelles. All individual TM and ML peptides, except the third ML domain, adopted helical structures in aqueous trifluoroethanol, which implies that, in addition to six TM segments, at least two of the ML domains of the UCP-1 can form helical structures in membrane interface regions. TM and ML domains interacted with vesicles composed of the main phospholipids of the inner membrane of mitochondria, phosphatidylcholine, phosphatidylethanolamine and cardiolipin, to adopt dominantly beta- and/or unordered conformations. Mixtures of UCP-1 peptide domains spontaneously associated in aqueous, phospholipid vesicles and digitonin micelle environments to form ordered conformations, which exhibited common features with the conformations of the full-length proteins. Thermal denaturations of UCP-1 and its nine-peptide-domain assembly in digitonin were co-operative but not reversible. Assembly of six TM domains in lipid bilayers formed ion-conducting units with possible helical bundle conformations. Consequently, covalent connection between peptide domains, tight domain interactions and TM potential are essential for the formation of the functional conformation of UCP-1.

Mitochondrial Ca2+, the secret behind the function of uncoupling proteins 2 and 3?

The underlying molecular action of the novel uncoupling proteins 2 and 3 (UCP2 and UCP3) is still under debate. The proteins have been implicated in many cell functions, including the regulation of insulin secretion and regulation of reactive oxygen species (ROS) generation. These effects have mainly been explained by suggesting that the proteins establish a proton leak through the inner mitochondrial membrane (IMM). However, accumulating data question this mechanism and suggest that UCP2 and UCP3 may play other roles, including carrying free fatty acids from the matrix towards the intermembrane space, or contributing to the mitochondrial Ca(2+) uniport. Accordingly, in this review we reflect on these actions of UCP2/UCP3 and discuss alternative explanations for the molecular mechanisms by which UCP2/UCP3 might contribute to aspects of cell function. Based on the potential role of UCP2/UCP3 in regulating mitochondrial Ca(2+) uptake, we propose a scheme whereby these proteins integrate Ca(2+)-dependent signal transduction and energy metabolism in order to meet the energy demand of the cell for its continuous response, adaptation, and stimulation to environmental input.

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.

High-fat diet, muscular lipotoxicity and insulin resistance

A high dietary fat intake and low physical activity characterize the current Western lifestyle. Dietary fatty acids do not stimulate their own oxidation and a surplus of fat is stored in white adipose tissue, liver, heart and muscle. In these organs intracellular lipids serve as a rapidly-available energy source during, for example, physical activity. However, under conditions of elevated plasma fatty acid levels and high dietary fat intake, conditions implicated in the development of modern diseases such as obesity and type 2 diabetes mellitus, fat accumulation in liver and muscle (intramyocellular lipids; IMCL) is associated with the development of insulin resistance. Recent data suggest that IMCL are specifically harmful when combined with reduced mitochondrial function, both conditions that characterize type 2 diabetes. In the (pre)diabetic state reduced expression of the transcription factor PPARgamma co-activator-1alpha (PGC-1alpha), which is involved in mitochondrial biogenesis, has been suggested to underlie the reduced mitochondrial function. Importantly, the reduction in PGC-1alpha may be a result of low physical activity, consumption of high-fat diets and high plasma fatty acid levels. Mitochondrial function can also be impaired as a result of enhanced mitochondrial damage by reactive oxygen species. Fatty acids in the vicinity of mitochondria are particularly prone to lipid peroxidation. In turn, lipid peroxides can induce oxidative damage to mitochondrial RNA, DNA and proteins. The mitochondrial protein uncoupling protein 3, which is induced under high-fat conditions, may serve to protect mitochondria against lipid-induced oxidative damage, but is reduced in the prediabetic state. Thus, muscular lipotoxicity may impair mitochondrial function and may be central to insulin resistance and type 2 diabetes mellitus.

Beta-adrenergic regulation of uncoupling protein expression in swine

This study examined the beta-adrenergic regulation of uncoupling protein (UCP) 2 and UCP3 gene expression in porcine tissues. In vitro experiments examined changes in UCP2 and UCP3 gene expression in middle (MSQ) and outer (OSQ) subcutaneous adipose tissues from crossbred neutered male pigs. Incubation of tissue slices (24 h) with 0 to 1000 nM isoproterenol increased UCP2 and UCP3 mRNA abundance in MSQ and OSQ, relative to 18S rRNA (P<0.05). For the in vivo experiment, nine randomly selected pigs (80 kg) were presented with a diet supplemented with 10.0 ppm ractopamine for 2 weeks. Another eight pigs were maintained on a control diet. Dietary ractopamine did not affect adipose UCP2 or UCP3 gene expression (P>0.05). However, UCP2 mRNA abundance was depressed in semitendinosus white (STW, P<0.05) and semitendinosus red (STR, P<0.001) by ractopamine feeding. Also, ractopamine decreased UCP3 mRNA abundance by 28% in STW (P<0.05). The in vitro data suggest that beta-adrenergic agonists directly affect adipose tissue UCP expression, although these adipose effects can be masked by the in vivo physiology. The in vivo data indicate that beta-adrenergic agonists may function in regulating UCP2 and UCP3 expression in selected muscles.