Expression of uncoupling protein-3 and mitochondrial activity in the transition from hypothyroid to hyperthyroid state in rat skeletal muscle

Increased uncoupling protein 3 content does not affect mitochondrial function in human skeletal muscle in vivo

Phosphocreatine (PCr) resynthesis rate following intense anoxic contraction can be used as a sensitive index of in vivo mitochondrial function. We examined the effect of a diet-induced increase in uncoupling protein 3 (UCP3) expression on postexercise PCr resynthesis in skeletal muscle. Nine healthy male volunteers undertook 20 one-legged maximal voluntary contractions with limb blood flow occluded to deplete muscle PCr stores. Exercise was performed following 7 days consumption of low-fat (LF) or high-fat (HF) diets. Immediately following exercise, blood flow was reinstated, and muscle was sampled after 20, 60, and 120 seconds of recovery. Mitochondrial coupling was assessed by determining the rate of PCr resynthesis during recovery. The HF diet increased UCP3 protein content by approximately 44% compared with the LF diet. However, this HF diet-induced increase in UCP3 expression was not associated with any changes in the rate of muscle PCr resynthesis during conditions of maximal flux through oxidative phosphorylation. Muscle acetylcarnitine, free-creatine, and lactate concentrations during recovery were unaffected by the HF diet. Taken together, our findings demonstrate that increasing muscle UCP3 expression does not diminish the rate of PCr resynthesis, allowing us to conclude that the primary role of UCP3 in humans is not uncoupling.

Cold-induced enhancement of avian uncoupling protein expression, heat production, and triiodothyronine concentrations in broiler chicks

The relationships among avian uncoupling protein (avUCP) mRNA expression, heat production, and thyroid hormone metabolism were investigated in 7-14-day-old broiler chicks (Gallus gallus) exposed to a low temperature (cold-exposed chicks, CE) or a thermoneutral temperature (TN). After 7 days of exposure, CE chicks exhibited higher heat production (+83%, P<0.01), avUCP mRNA expression (+20%, P<0.01), and circulating triiodothyronine (T(3)) levels (+104%, P=0.07) for non-statistically different body weights and feed intake between 3 and 7 days of exposure as compared to TN chicks. Plasma thyroxine (T(4)) concentration was clearly decreased in CE chicks (-33%, P=0.06). The lower hepatic inner-ring deiodination activity (-47%) and the higher renal outer-ring deiodination activity (+75%) measured in CE compared to TN chicks could partly account for their higher plasma T(3) concentrations. This study describes for the first time the induction of avUCP mRNA expression by low temperature in chickens, as it has been previously shown in ducklings, and supports the possible involvement of avUCP in avian thermogenesis.

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

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

Molecular mechanisms linking calorie restriction and longevity

Calorie-restricted feeding retards the rate of ageing in mammalian and invertebrate species. The molecular mechanisms underlying this effect include a lower rate of accrual of tissue oxidative damage that is associated with a significantly lower rate of mitochondrial free radical generation in rodent species. To identify the important sites of control and regulation for mitochondrial free radical generation during ageing and calorie-restricted feeding, metabolic control analysis is being applied to the study of mitochondrial bioenergetics. With ageing an increase in the mitochondrial proton leak is observed in mouse hepatocytes and in rat skeletal muscle. Limited data suggest that calorie-restricted feeding lowers the inner mitochondrial membrane potential and this may explain the reduced rate of free radical generation. A lowered unsaturation/saturation index is observed for mitochondrial membrane lipids in calorie-restricted rodents resulting in an altered membrane structure and function. Plasma concentrations of insulin and triiodothyronine are significantly lower under calorie-restricted feeding conditions and these hormones exert transcriptional control over desaturase enzymes that are important in the control of membrane lipid unsaturation. A loss of double bonds should make the mitochondrial membranes more resistant to peroxidation damage and would also reduce the proton conductance of the membrane, raising the membrane potential at a given respiration rate. This effect however, appears to be offset by other membrane changes that may include increased activity of uncoupling proteins. These unidentified adaptations increase the proton leak in calorie-restricted animals resulting in a lowering of the membrane potential and ROS generation.

Possible physiological roles of mitochondrial uncoupling proteins--UCPn

Five mitochondrial uncoupling proteins exist in the human gemone: UCP2, expressed ubiquitously; UCP1, exclusively in brown adipose tissue (BAT); UCP3, predominantly in muscle; UCP4 and BMCP (UCP5), in brain. UCP4 is the ancestral prototype from which the other UCPn diverged. Findings on the level of organism and reconstituted recombinant proteins demonstrated that UCPn exhibit a protonophoric function, documented by overexpression in mice, L6 myotubes, INS1 cells, muscle, and yeast. In a few cases (yeast), this protonophoric function was correlated with elevated fatty acid (FA) levels. Reconstituted UCPn exhibited nucleotide-sensitive FA induced H(+) uniport. Two mechanisms, local buffering or FA cycling were suggested as an explanation. A basic UCPn role with mild uncoupling is to accelerate metabolism and reduce reactive oxygen species. UCP2 (UCP3) roles were inferred from transcriptional up-regulation mediated by FAs via peroxisome proliferator-activated receptors, cytokines, leptin signalling via hypothalamic pathway, and by thyroide and beta2 adrenergic stimulation. The latter indicated a role in catecholamine-induced thermogenesis in skeletal muscle. UCP2 (UCP3) may contribute to body weight regulation, although obesity was not induced in knockout (KO) mice. An obesity reduction in middle-aged humans was associated with the less common allele of -866 G/A polymorphism in the ucp2 gene promoter enhancing the exon 8 insertion: deletion transcript ratio. Up-regulated UCP2 transcription by pyrogenic cytokines (tumour necrosis factor alpha (TNFalpha)) suggested a role in fever. UCP2 could induce type 2 diabetes as developed from obesity due to up-regulated UCP2 transcription by FAs in pancreatic beta-cells. UCPn might be pro-apoptotic as well as anti-apoptotic, depending on transcriptional and biochemical regulation.

De novo expression of uncoupling protein 3 is associated to enhanced mitochondrial thioesterase-1 expression and fatty acid metabolism in liver of fenofibrate-treated rats

Uncoupling protein 3 (UCP3) is a member of the mitochondrial carrier superfamily, preferentially expressed in skeletal muscle. Its function is not fully understood and it is debated whether it uncouples oxidative phosphorylation as does UCP1 in brown adipose tissue. Recent evidences suggest a role for UCP3 in the flux of fatty acids in and out mitochondria and their utilization in concert with mitochondrial thioesterase-1 (MTE-1). In fact, mice overexpressing muscle UCP3 also show high levels of MTE-1. Fenofibrate is a hypolipidemic drug that prevents body weight gain in diet-induced obese rats and enhances lipid metabolism by activating peroxisome proliferator-activated receptors (PPARs). Because fatty acids and fenofibrate stimulate PPARs and in turn UCP3, we investigated whether UCP3 expression might be induced 'de novo' in situations of increased hepatic mitochondrial fatty acid utilization caused by a combined effect of a high-fat diet and fenofibrate treatment. We also investigated whether Mte-1 expression and beta-oxidation were affected. We show here that Ucp3 is induced in liver of fenofibrate-treated rats at the mRNA and protein level. Expression was restricted to hepatocytes and was unevenly distributed in the liver. No increase in cell proliferation, inflammatory or fibrotic responses was found. Mte-1 expression and mitochondrial beta-oxidation were upregulated. Thus, Ucp3 can be transactivated in tissues where it is normally silent and fenofibrate can attain this effect in liver. The data demonstrate that UCP3 is involved in fatty acid utilization and support the notion that UCP3 and MTE-1 are linked within the same metabolic pathway.

The role of uncoupling proteins in pathophysiological states

Until very recently, the uncoupling protein-1 (UCP1), present only in brown adipose tissue (BAT), was considered to be the only mitochondrial carrier protein that stimulated heat production by dissipating the proton gradient generated during respiration across the inner mitochondrial membrane and therefore uncoupling respiration from ATP synthesis. Recently, new uncoupling proteins, UCP2, UCP3, and UCP4, and brain mitochondrial carrier protein-1 (BMCP-1) have been described in mammalian tissues. The present review deals with the possible role of these proteins in different pathological conditions involving alterations in energy balance such as obesity or cachexia. In conclusion, the emergence of the UCP family has altered the approaches to bioenergetics and stressed the importance of uncoupling respiration in different pathophysiological conditions. An extensive qualitative and quantitative characterization of the new members of the UCP family in mammalian tissues will allow a better understanding of the molecular and regulatory mechanisms of thermogenesis and energy metabolism. At this point, we hope that the knowledge presented in the present review will not only stimulate a debate about the role of the UCP family in disease but also lead to applications beneficial for human health.

In vivo regulation of human skeletal muscle gene expression by thyroid hormone

Thyroid hormones are key regulators of metabolism that modulate transcription via nuclear receptors. Hyperthyroidism is associated with increased metabolic rate, protein breakdown, and weight loss. Although the molecular actions of thyroid hormones have been studied thoroughly, their pleiotropic effects are mediated by complex changes in expression of an unknown number of target genes. Here, we measured patterns of skeletal muscle gene expression in five healthy men treated for 14 days with 75 microg of triiodothyronine, using 24,000 cDNA element microarrays. To analyze the data, we used a new statistical method that identifies significant changes in expression and estimates the false discovery rate. The 381 up-regulated genes were involved in a wide range of cellular functions including transcriptional control, mRNA maturation, protein turnover, signal transduction, cellular trafficking, and energy metabolism. Only two genes were down-regulated. Most of the genes are novel targets of thyroid hormone. Cluster analysis of triiodothyronine-regulated gene expression among 19 different human tissues or cell lines revealed sets of coregulated genes that serve similar biologic functions. These results define molecular signatures that help to understand the physiology and pathophysiology of thyroid hormone action.

Are the effects of T3 on resting metabolic rate in euthyroid rats entirely caused by T3 itself?

Because we previously reported that T3 and 3,5-diiodo-L-thyronine (3,5-T2) both increase resting metabolic rate (RMR), 3,5-T2 could be another thyroidal regulator of energy metabolism. This effect of 3,5-T2 is evident in rats made hypothyroid by propylthiouracil and iopanoic acid, not in normal euthyroid (N) rats. Possibly, under euthyroid conditions, active 3,5-T2 may need to be formed intracellularly from a precursor such as T3. We tested this hypothesis by giving a single injection of T3 to N rats and comparing the time course of the variations in RMR with those of the changes in the serum and hepatic levels of 3,5-T2. Acute injection had an evident effect on RMR, 25 h earlier, in N rats than in rats made hypothyroid by propylthiouracil and iopanoic acid, maximal values (+40%) being reached in the former at 24-26 h. In N rats, the simultaneous injection of actinomycin D with the T3 inhibited the late part of the effect (after 24 h) more strongly than the early part (14-24 h). In serum and liver, 3,5-T2 levels were increased significantly at 12-24 h after T3 injection into N rats, a time at which RMR was rising rapidly to peak. These results seem to indicate that when T3 is injected into N animals, not all the effects on RMR are attributable to T3 itself, the early effect presumably being largely because of its in vivo deiodination to 3,5-T2. Because the effects of T3 and 3,5-T2 are additive, in N rats, the two iodothyronines probably cooperate in vivo to determine the total metabolic rate.

The basal proton conductance of skeletal muscle mitochondria from transgenic mice overexpressing or lacking uncoupling protein-3

The ability of native uncoupling protein-3 (UCP3) to uncouple mitochondrial oxidative phosphorylation is controversial. We measured the expression level of UCP3 and the proton conductance of skeletal muscle mitochondria isolated from transgenic mice overexpressing human UCP3 (UCP3-tg) and from UCP3 knockout (UCP3-KO) mice. The concentration of UCP3 in UCP3-tg mitochondria was approximately 3 microg/mg protein, approximately 20-fold higher than the wild type value. UCP3-tg mitochondria had increased nonphosphorylating respiration rates, decreased respiratory control, and approximately 4-fold increased proton conductance compared with the wild type. However, this increased uncoupling in UCP3-tg mitochondria was not caused by native function of UCP3 because it was not proportional to the increase in UCP3 concentration and was neither activated by superoxide nor inhibited by GDP. UCP3 was undetectable in mitochondria from UCP3-KO mice. Nevertheless, UCP3-KO mitochondria had unchanged respiration rates, respiratory control ratios, and proton conductance compared with the wild type under a variety of assay conditions. We conclude that uncoupling in UCP3-tg mice is an artifact of transgenic expression, and that UCP3 does not catalyze the basal proton conductance of skeletal muscle mitochondria in the absence of activators such as superoxide.

Flow cytometry of isolated mitochondria during development and under some pathological conditions

Mitochondria play an essential role in the generation of the energy needed for eukaryotic cell life and in the release of molecules involved in initiation of cell death. Here we review the changes in isolated mitochondrial fluorescent populations as distinguished by flow cytometry during postnatal development and their regulation by thyroid hormones and catecholamines. The use of flow cytometry in the study of mitochondrial changes occurring under hypothyroidism, alcohol abuse and aging is also reviewed.

Control of energy metabolism by iodothyronines

One of the most widely recognized effects of thyroid hormones (TH) in adult mammals is their influence over energy metabolism. In the past, this has received much attention but, possibly because of the complex mode of action of thyroid hormones, no universally accepted mechanism to explain this effect has been put forward so far. Significant advances in our understanding of the biochemical processes involved in the actions of TH have been made in the last three decades and now it seems clear that TH can act through both nuclear-mediated and extranuclear-mediated pathways. TH increase energy expenditure, partly by reducing metabolic efficiency, with control of specific genes at the transcriptional level, being is thought to be the major molecular mechanism. However, both the number and the identity of the thyroid-hormone-controlled genes remain unknown, as do their relative contributions. The recent discovery of uncoupling proteins (UCPs) (in addition to UCP1 in brown adipose tissue) in almost all tissues in animals, including humans, has opened new perspectives on the understanding of the mechanisms involved in the regulation of energy metabolism by thyroid hormones. Other approaches have included the various attempts made to attribute changes in respiratory activity to a direct influence of thyroid hormones over the mitochondrial energy-transduction apparatus. In addition, an increasing number of studies has revealed that TH active in the regulation of energy metabolism include not only T3, but also other iodothyronines present in the biological fluids, such as 3,5-diiodothyronine (3,5-T2). This, in turn, may make it possible to explain some of the effects exerted by TH on energy metabolism that cannot easily be attributed to T3.

Mitochondrial uncoupling as a target for drug development for the treatment of obesity

Mitochondrial proton cycling is responsible for a significant proportion of basal or standard metabolic rate, so further uncoupling of mitochondria may be a good way to increase energy expenditure and represents a good pharmacological target for the treatment of obesity. Uncoupling by 2,4-dinitrophenol has been used in this way in the past with notable success, and some of the effects of thyroid hormone treatment to induce weight loss may also be due to uncoupling. Diet can alter the pattern of phospholipid fatty acyl groups in the mitochondrial membrane, and this may be a route to uncoupling in vivo. Energy expenditure can be increased by stimulating the activity of uncoupling protein 1 (UCP1) in brown adipocytes either directly or through beta 3-adrenoceptor agonists. UCP2 in a number of tissues, UCP3 in skeletal muscle and the adenine nucleotide translocase have also been proposed as possible drug targets. Specific uncoupling of muscle or brown adipocyte mitochondria remains an attractive target for the development of antiobesity drugs.

Effect of triiodothyronine on mitochondrial energy coupling in human skeletal muscle

The mechanism underlying the regulation of basal metabolic rate by thyroid hormone remains unclear. Although it has been suggested that thyroid hormone might uncouple substrate oxidation from ATP synthesis, there are no data from studies on humans to support this hypothesis. To examine this possibility, we used a novel combined (13)C/(31)P nuclear magnetic resonance (NMR) approach to assess mitochondrial energy coupling in skeletal muscle of seven healthy adults before and after three days of triiodothyronine (T(3)) treatment. Rates of ATP synthesis and tricarboxylic acid (TCA) cycle fluxes were measured by (31)P and (13)C NMR spectroscopy, respectively, and mitochondrial energy coupling was assessed as the ratio. Muscle TCA cycle flux increased by approximately 70% following T(3) treatment. In contrast, the rate of ATP synthesis remained unchanged. Given the disproportionate increase in TCA cycle flux compared with ATP synthesis, these data suggest that T(3) promotes increased thermogenesis in part by promoting mitochondrial energy uncoupling in skeletal muscle.

A mitochondrial uncoupling artifact can be caused by expression of uncoupling protein 1 in yeast

Uncoupling protein 1 (UCP1) from mouse was expressed in yeast and the specific (GDP-inhibitable) and artifactual (GDP-insensitive) effects on mitochondrial uncoupling were assessed. UCP1 provides a GDP-inhibitable model system to help interpret the uncoupling effects of high expression in yeast of other members of the mitochondrial carrier protein family, such as the UCP1 homologues UCP2 and UCP3. Yeast expressing UCP1 at modest levels (approx. 1 microg/mg of mitochondrial protein) showed no growth defect, normal rates of chemically uncoupled respiration and an increased non-phosphorylating proton conductance that was completely GDP-sensitive. The catalytic-centre activity of UCP1 in these yeast mitochondria was similar to that in mammalian brown-adipose-tissue mitochondria. However, yeast expressing UCP1 at higher levels (approx. 11 microg/mg of mitochondrial protein) showed a growth defect. Their mitochondria had depressed chemically uncoupled respiration rates and an increased proton conductance that was partly GDP-insensitive. Thus, although UCP1 shows native behaviour at modest levels of expression in yeast, higher levels (or rates) of expression can lead to an uncoupling that is not a physiological property of the native protein and is therefore artifactual. This observation might be important in the interpretation of results from experiments in which the functions of UCP1 homologues are verified by their ability to uncouple yeast mitochondria.

Cold-induced changes in the energy coupling and the UCP3 level in rodent skeletal muscles

The mechanism of thermoregulatory uncoupling of respiration and phosphorylation in skeletal muscles has been studied. It is found that 24 h cold exposure results in (i) a 3-fold increase in the amount of UCP3 protein in rat skeletal muscle mitochondria, and (ii) pronounced lowering of the membrane potential in isolated rat or mouse skeletal muscle mitochondria. The decrease in membrane potential is reversed by adding bovine serum albumin. Cold exposure is also found to sensitize the membrane potential to the uncoupling action of added fatty acid (laurate). After laurate addition, the recoupling effects of GDP and carboxyatractylate decrease whereas that of albumin increases in mitochondria from cold-treated rats or mice. Changes similar to those induced by cold can be initiated by the in vivo addition of thyroxine. Cold exposure does not affect energy coupling in liver mitochondria. The possible involvement of UCP3 isoforms in nucleotide-sensitive and -insensitive uncoupling is discussed.

T(3) increases mitochondrial ATP production in oxidative muscle despite increased expression of UCP2 and -3

Triiodothyronine (T(3)) increases O(2) and nutrient flux through mitochondria (Mito) of many tissues, but it is unclear whether ATP synthesis is increased, particularly in different types of skeletal muscle, because variable changes in uncoupling proteins (UCP) and enzymes have been reported. Thus Mito ATP production was measured in oxidative and glycolytic muscles, as well as in liver and heart, in rats administered T(3) for 14 days. Relative to saline-treated controls, T(3) rats had 80, 168, and 62% higher ATP production in soleus muscle, liver, and heart, respectively, as well as higher activities of citrate synthase (CS; 63, 90, 25%) and cytochrome c oxidase (COX; 119, 225, 52%) in the same tissues (all P < 0.01). In plantaris muscle of T(3) rats, CS was only slightly higher (17%, P < 0.05) than in controls, and ATP production and COX were unaffected. mRNA levels of COX I and III were 33 and 47% higher in soleus of T(3) rats (P < 0.01), but there were no differences in plantaris. In contrast, UCP2 and -3 mRNAs were 2.5- to 14-fold higher, and protein levels were 3- to 10-fold higher in both plantaris and soleus of the T(3) group. We conclude that T(3) increases oxidative enzymes and Mito ATP production and Mito-encoded transcripts in oxidative but not glycolytic rodent tissues. Despite large increases in UCP expression, ATP production was enhanced in oxidative tissues and maintained in glycolytic muscle of hyperthyroid rats.

Mitochondrial proton leak: a role for uncoupling proteins 2 and 3?

In mitochondria ATP synthesis is not perfectly coupled to oxygen consumption due to proton leak across the mitochondrial inner membrane. Quantitative studies have shown that proton leak contributes to approximately 25% of the resting oxygen consumption of mammals. Proton leak plays a role in accounting for differences in basal metabolic rate. Thyroid studies, body mass studies, phylogenic studies and obesity studies have all shown that increased mass-specific metabolic rate is linked to increased mitochondrial proton leak. The mechanism of the proton leak is unclear. Evidence suggests that proton leak occurs by a non-specific diffusion process across the mitochondrial inner membrane. However, the high degree of sequence homology of the recently cloned uncoupling proteins UCP 2 and UCP 3 to brown adipose tissue UCP 1, and their extensive tissue distribution, suggest that these novel uncoupling proteins play a role in proton leak. Early indications from reconstitution experiments and several in vitro expression studies suggest that the novel uncoupling proteins uncouple mitochondria. Furthermore, mice overexpressing UCP 3 certainly show a phenotype consistent with increased metabolism. The evidence for a role for these novel UCPs in mitochondrial proton leak is reviewed.

Increased uncoupling proteins and decreased efficiency in palmitate-perfused hyperthyroid rat heart

The physiological role of mitochondrial uncoupling proteins (UCPs) in heart and skeletal muscle is unknown, as is whether mitochondrial uncoupling of oxidative phosphorylation by fatty acids occurs in vivo. In this study, we found that UCP2 and UCP3 protein content, determined using Western blotting, was increased by 32 and 48%, respectively, in hyperthyroid rat heart mitochondria. Oligomycin-insensitive respiration rate, a measure of mitochondrial uncoupling, was increased in all mitochondria in the presence of palmitate: 36% in controls and 71 and 100% with 0.8 and 0.9 mM palmitate, respectively, in hyperthyroid rat heart mitochondria. In the isolated working heart, 0.4 mM palmitate significantly lowered cardiac output by 36% and cardiac efficiency by 38% in the hyperthyroid rat heart. Thus increased mitochondrial UCPs in the hyperthyroid rat heart were associated with increased uncoupling and decreased myocardial efficiency in the presence of palmitate. In conclusion, a physiological effect of UCPs on fatty acid oxidation has been found in heart at the mitochondrial and whole organ level.

Mitochondrial proton leak and the uncoupling protein 1 homologues

Mitochondrial proton leak is the largest single contributor to the standard metabolic rate (SMR) of a rat, accounting for about 20% of SMR. Yet the mechanisms by which proton leak occurs are incompletely understood. The available evidence suggests that both phospholipids and proteins in the mitochondrial inner membrane are important determinants of proton conductance. The uncoupling protein 1 homologues (e.g. UCP2, UCP3) may play a role in mediating proton leak, but it is unlikely they account for all of the observed proton conductance. Experimental data regarding the functions of these proteins include important ambiguities and contradictions which must be addressed before their function can be confirmed. The physiological role of the proton leak, and of the uncoupling protein 1 homologues, remains similarly unclear.

Triiodothyronine-mediated up-regulation of UCP2 and UCP3 mRNA expression in human skeletal muscle without coordinated induction of mitochondrial respiratory chain genes

Triiodothyronine (T3) increases mitochondrial respiration and promotes the uncoupling between oxygen consumption and ATP synthesis. T3 effect is mediated partly through transcriptional control of genes encoding mitochondrial proteins. We determined the effect of T3 on mRNA levels of uncoupling proteins (UCP) and proteins involved in the biogenesis of the respiratory chain in human skeletal muscle and on UCP2 mRNA expression in adipose tissue. Ten young, healthy males received 75 to 100 5g of T3 per day for 14 days. The increase in plasma-free T3 levels was associated with an increase of resting metabolic rate and a decrease of respiratory quotient. In skeletal muscle, treatment with T3 induced a twofold increase of both UCP2 and UCP3 mRNA levels (p c oxidase subunits 2 and 4, nuclear respiratory factor 1, mitochondrial transcription factor A, and the co-activator PGC1 did not change during the treatment. In adipose tissue, UCP2 mRNA levels increased threefold. The direct effect of T3 on skeletal muscle an d adipose tissue UCP2 and UCP3 mRNA expression was demonstrated in vitro in human primary cultures. Our data show that T3 induces UCP2 and UCP3 mRNA expression in humans. In skeletal muscle, UCP regulation by T3 is not associated with the transcriptional regulation of respiratory chain proteins.

Restriction of energy intake, energy expenditure, and aging

Energy restriction (ER), without malnutrition, increases maximum life span and retards the development of a broad array of pathophysiological changes in laboratory rodents. The mechanism responsible for the retardation of aging by ER is, however, unknown. One proposed explanation is a reduction in energy expenditure (EE). Reduced EE may increase life span by decreasing the number of oxygen molecules interacting with mitochondria, thereby lowering reactive oxygen species (ROS) production. As a step toward testing this hypothesis, it is important to determine the effect of ER on EE. Several whole-body, organ, and cellular studies have measured the influence of ER on EE. In general, whole-body studies have reported an acute decrease in mass-adjusted EE that disappears with long-term ER. Organ-specific studies have shown that decreases in EE of liver and gastrointestinal tract are primarily responsible for initial reductions in EE with ER. These data, however, do not determine whether cellular EE is altered with ER. Three major processes contributing to resting EE at the cellular level are mitochondrial proton leak, Na(+)-K(+)-ATPase activity, and protein turnover. Studies suggest that proton leak and Na(+)-K(+)-ATPase activity are decreased with ER, whereas protein turnover is either unchanged or slightly increased with ER. Thus, two of the three major processes contributing to resting EE at the cellular level may be decreased with ER. Although additional cellular measurements are needed, the current results suggest that a lowering of EE could be a mechanism for the action of ER.

Impact of endotoxin on UCP homolog mRNA abundance, thermoregulation, and mitochondrial proton leak kinetics

Linking tissue uncoupling protein (UCP) homolog abundance with functional metabolic outcomes and with expression of putative genetic regulators promises to better clarify UCP homolog physiological function. A murine endotoxemia model characterized by marked alterations in thermoregulation was employed to examine the association between heat production, UCP homolog expression, and mitochondrial proton leak ("uncoupling"). After intraperitoneal lipopolysaccharide (LPS, approximately 6 mg/kg) injection, colonic temperature (T(c)) in adult female C57BL6/J mice dropped to a nadir of approximately 30 degrees C by 8 h, preceded by a four- to fivefold drop in liver UCP2 and UCP5/brain mitochondrial carrier protein 1 mRNA levels, with no change in their hindlimb skeletal muscle (SKM) expression. SKM UCP3 mRNA rose fivefold during development of hypothermia and was correlated with an LPS-induced increase in plasma free fatty acid concentration. UCP2 and UCP5 transcripts recovered about three- to sixfold in both tissues starting at 6-8 h, preceding a recovery of T(c) between 16 and 24 h. SKM UCP3 followed an opposite pattern. Such results are not consistent with an important influence of UCP3 in driving heat production but do not preclude a role for UCP2 or UCP5 in this process. The transcription coactivator PGC-1 displayed a transient LPS-evoked rise (threefold) or drop (two- to fivefold) in SKM and liver expression, respectively. No differences between control and LPS-treated mouse liver or SKM in vitro mitochondrial proton leak were evident at time points corresponding to large differences in UCP homolog expression.

Lack of obesity and normal response to fasting and thyroid hormone in mice lacking uncoupling protein-3

Uncoupling protein-3 (UCP3) is a mitochondrial protein that can diminish the mitochondrial membrane potential. Levels of muscle Ucp3 mRNA are increased by thyroid hormone and fasting. Ucp3 has been proposed to influence metabolic efficiency and is a candidate obesity gene. We have produced a Ucp3 knockout mouse to test these hypotheses. The Ucp3 (-/-) mice had no detectable immunoreactive UCP3 by Western blotting. In mitochondria from the knockout mice, proton leak was greatly reduced in muscle, minimally reduced in brown fat, and not reduced at all in liver. These data suggest that UCP3 accounts for much of the proton leak in skeletal muscle. Despite the lack of UCP3, no consistent phenotypic abnormality was observed. The knockout mice were not obese and had normal serum insulin, triglyceride, and leptin levels, with a tendency toward reduced free fatty acids and glucose. Knockout mice showed a normal circadian rhythm in body temperature and motor activity and had normal body temperature responses to fasting, stress, thyroid hormone, and cold exposure. The base-line metabolic rate and respiratory exchange ratio were the same in knockout and control mice, as were the effects of fasting, a beta3-adrenergic agonist (CL316243), and thyroid hormone on these parameters. The phenotype of Ucp1/Ucp3 double knockout mice was indistinguishable from Ucp1 single knockout mice. These data suggest that Ucp3 is not a major determinant of metabolic rate but, rather, has other functions.

Uncoupling protein-2 and -3 messenger ribonucleic acids in adipose tissue and skeletal muscle of healthy males: variability, factors affecting expression, and relation to measures of metabolic rate

Mitochondrial uncoupling protein-2 and -3 (UCP2 and UCP3) may be involved in the modulation of resting metabolic rate and energy balance. To investigate their variability, the influence of this on the variability of energy expenditure, and potential regulatory factors of the expression of the corresponding genes, we measured their messenger ribonucleic acids (mRNAs) in muscle and white adipose tissue of lean, healthy men and correlated the abundance of these mRNAs (attomoles per microg total RNA) with measures of resting metabolic rate, hormone levels (thyroid hormones, insulin, glucagon, leptin, and catecholamines), and fuels potentially involved in energy balance regulation. We also investigated whether the thiazolidinedione, troglitazone, stimulates UCP2 and UCP3 mRNA levels to follow up on the observation that this antidiabetic drug increases the levels of expression in cultured cells. We found UCP2 and UCP3 mRNA levels to be highly variable and poorly correlated with measures of energy expenditure and with most factors affecting energy balance. Only nocturnal urinary norepinephrine excretion could explain a significant fraction of the variability in both UCP2 and UCP3 expression in muscle, but not adipose tissue. Thyroid hormone and norepinephrine excretion were found to contribute to the variability of resting metabolic rate, but this could not be explained by an effect on UCP mRNAs. Troglitazone affected neither the expression of UCPs nor the hormones or the measures of metabolic rate investigated. In conclusion, our results show that the expression of UCP2 and UCP3 genes is quite variable in healthy males and that this variability does not explain that in resting energy expenditure, and suggest that sympathetic activity is an important potential regulator of the expression of these proteins in skeletal muscle. However, the data do not support the concept that regulation of the expression of these genes is the most important level of control of UCP3 and UCP2 functions, and other levels of control have to be invoked.

First evidence of uncoupling protein-2 (UCP-2) and -3 (UCP-3) gene expression in piglet skeletal muscle and adipose tissue

Uncoupling proteins (UCPs) facilitate proton transport inside the mitochondria and decrease the proton gradient, leading to heat production. Until now, the presence of UCP1 or other UCP homologs had not been detected in tissues of pig, a species where evidence for the presence of brown adipose tissue has only been provided in 2-3 month old animals. In the light of the improving knowledge on the UCPs family, we decided to examine both UCP2 and UCP3 mRNA expression in piglet skeletal muscle and adipose tissue. Using RT-PCR we have successfully cloned a partial UCP2 sequence and a complete UCP3 cDNA. UCP3's open reading frame (936bp) shares 90, 89 and 85% similarity with bovine, human and rat UCP3 nucleotide sequences, respectively. In 3-5 day old piglets, these genes are expressed in adipose tissue and in both longissimus thoracis (LT) and rhomboïdeus (RH) muscles, without any effect of muscle metabolic type. This is in good agreement with the measurement of the same membrane potential in mitochondria isolated from both types of muscles. In triiodothyronine-treated piglets, UCP3 mRNA is more expressed in LT than in RH muscle. These genes may be involved in the control of the energy metabolism of the piglet.

Uncoupling protein homologs: emerging views of physiological function

The widespread occurrence of excess weight and related diseases demands that efforts be made to understand energy expenditure from the gene to the whole animal. For some time, it has been understood that mitochondrial oxidation of fuels generates an electrochemical gradient via outward pumping of protons by the electron transport chain. ATP production via F(1)F(0) ATP synthase is then facilitated by the inward flux of protons down the gradient. There is a growing appreciation that a significant portion of the metabolic rate of endotherms is attributable to counteracting "proton leak" (uncoupling), wherein a flux of protons down the electrochemical gradient generates heat independently of ATP production. Proton leak is especially apparent in thermogenic brown adipose tissue, which expresses a tissue-specific uncoupling protein (UCP1). The recent discovery of widely expressed putative UCP1 homologs [UCP2, UCP3, UCP4, UCP5/brain mitochondrial carrier protein-1 (BMCP1)] raised the possibility that innate proton leak and metabolic rate are regulated by UCP1-like proteins. On the basis of current published data, one may not exclude the possibility that UCP homologs influence metabolic rate.

UCP2 and UCP3 rise in starved rat skeletal muscle but mitochondrial proton conductance is unchanged

The relationship between UCP2 and UCP3 expression and mitochondrial proton conductance of rat skeletal muscle was examined. Rats were starved for 24 h and the levels of UCP2 and UCP3 mRNA and UCP3 protein were determined by Northern and Western blots. Proton conductance was measured by titrating mitochondrial respiration rate and membrane potential with malonate. Starvation increased UCP2 and UCP3 mRNA levels more than 5-fold and 4-fold, respectively, and UCP3 protein levels by 2-fold. However, proton conductance remained unchanged. These results suggest either that Northern and Western blots do not reflect the levels of active protein or that these UCPs do not catalyse the basal proton conductance in skeletal muscle mitochondria.

T(3) stimulates resting metabolism and UCP-2 and UCP-3 mRNA but not nonphosphorylating mitochondrial respiration in mice

The molecular basis for variations in resting metabolic rate (RMR) within a species is unknown. One possibility is that variations in RMR occur because of variations in uncoupling protein 2 (UCP-2) and uncoupling protein 3 (UCP-3) expression, resulting in mitochondrial proton leak differences. We tested the hypothesis that UCP-2 and -3 mRNAs positively correlate with RMR and proton leak. We treated thyroidectomized and sham-operated mice with triiodothyronine (T(3)) or vehicle and measured RMR, liver, and skeletal muscle mitochondrial nonphosphorylating respiration and UCP-2 and -3 mRNAs. T(3) stimulated RMR and liver UCP-2 and gastrocnemius UCP-2 and -3 expression. Mitochondrial respiration was not affected by T(3) and did not correlate with UCP-2 and -3 mRNAs. Gastrocnemius UCP-2 and -3 expression did correlate with RMR. We conclude 1) T(3) did not influence intrinsic mitochondrial properties such as membrane structure and composition, and 2) variations in UCP-2 and -3 expression may partly explain variations in RMR. One possible explanation for these data is that T(3) stimulates the leak in vivo but not in vitro because a posttranslational regulator of UCP-2 and -3 is not retained in the mitochondrial fraction.

Action of thyroid hormones at the cellular level: the mitochondrial target

Thyroid hormones exert profound effects on the energy metabolism. An inspection of the early and more recent literature shows that several targets at the cellular level have been identified. Since their effects on the nuclear signalling pathway have already been well-defined and extensively reviewed, this article focuses on the regulation of mitochondrial activity by thyroid hormones. Mitochondria, by virtue of their biochemical functions, are a natural candidate as a direct target for the calorigenic effects of thyroid hormones. To judge from results coming from various laboratories, it is quite conceivable that mitochondrial activities are regulated both directly and indirectly. Not only triiodo-L-thyronine, but also diiodothyronines are active in regulating the energy metabolism. They influence the resting metabolism in rats with 3,5-diiodo-L-thyronine seeming to show a clearer effect.