Up-regulation of uncoupling proteins by beta-adrenergic stimulation in L6 myotubes

Investigating Carvedilol's Repurposing for the Treatment of Non-Small Cell Lung Cancer via Aldehyde Dehydrogenase Activity Modulation in the Presence of β-Adrenergic Agonists

Repurposing existing drugs appears to be a potential solution for addressing the challenges in the treatment of non-small cell lung cancer (NSCLC). β-adrenoceptor antagonist drugs (β-blockers) have tumor-inhibiting effects, making them promising candidates for potential NSCLC treatment. This study investigates the anticancer potential of a subset of β-blockers in NSCLC cell lines; A549 and H1299. Additionally, it investigates the underlying mechanism behind β-blockers' anticancer effect by influencing a potential novel target named aldehyde dehydrogenase (ALDH). The MTT assay assessed β-blockers' cytotoxicity on both cell lines, while Western blot and NADH fluorescence assays evaluated their influence on ALDH protein expression and activity. Carvedilol (CAR) was the most effective blocker in reducing cell survival of A549 and H1299 with IC50 of 18 µM and 13.7 µM, respectively. Significantly, CAR led to a 50% reduction in ALDH expression and 80% decrease in ALDH activity in A549 cells, especially when combined with β-agonists, in comparison to the control. This effect might be attributed to β-agonist blockade or an alternative pathway. This novel finding adds to our understanding of CAR's multifaceted anticancer properties, implying that combining CAR with β-agonists could be a useful strategy for lung cancer treatment.

Effects of Thermal Conditioning on Changes in Hepatic and Muscular Tissue Associated With Reduced Heat Production and Body Temperature in Young Chickens

Effects of increased summer temperatures on poultry production are becoming more pronounced due to global warming, so it is important to consider approaches that might reduce heat stress in chickens. Thermal conditioning in chickens in the neonatal period can improve thermotolerance and reduce body temperature increases when birds are exposed to high ambient temperature later in life. The objective of this study was to investigate physiological and molecular changes associated with heat production and hence body temperature regulation under high ambient temperatures in thermally conditioned chicks. Three-day-old broiler chicks (Chunky) were thermally conditioned by exposure to a high ambient temperature (40°C) for 12 h while control chicks were kept at 30°C. Four days after the treatment, both groups were exposed to 40°C for 15 or 90 min. The increase in rectal temperature during 90 min of exposure to a high ambient temperature was less in thermally conditioned than control chicks. At 15-min of re-exposure treatment, gene expression for uncoupling protein and carnitine palmitoyletransferase 1, key molecules in thermogenesis and fatty acid oxidation, were significantly higher in pectoral muscle of control chicks but not conditioned chicks. Hepatic argininosuccinate synthase (ASS) decreased and hepatic argininosuccinate lyase (ASL) increased after reexposure to a high temperature. The concentrations of hepatic arginosuccinic acid, and ASS and ASL expression, were upregulated in conditioned chicks compared with the control chicks, indicating activity of the urea cycle could be enhanced to trap more energy to reduce heat production in conditioned chicks. These results suggest thermal conditioning can reduce the increase in heat production in muscles of chickens that occurs in high ambient temperatures to promote sensible heat loss. Conditioning may also promote energy trapping process in the liver by altering the heat production system, resulting in an alleviation of the excessive rise of body temperature.

Expression of genes involved in carbohydrate-lipid metabolism in muscle and fat tissues in the initial stage of adult-age obesity in fed and fasted mice

C57Bl mice exhibit impaired glucose metabolism by the late adult age under standard living conditions. The aim of this study was to evaluate white adipose tissue (WAT), brown adipose tissue (BAT), and skeletal muscle expression of genes involved in carbohydrate‐lipid metabolism at postpubertal stages preceding the late adult age in C57Bl mice. Muscle mRNA levels of uncoupling protein 3 (Ucp3) and carnitine palmitoyltransferase 1 (Cpt1) (indicators of FFA oxidation), WAT mRNA levels of hormone‐sensitive lipase (Lipe) and lipoprotein lipase (Lpl) (indicators of lipolysis and lipogenesis), muscle and WAT mRNA levels of the type 4 glucose transporter Slc2a4 (indicators of insulin‐dependent glucose uptake), and BAT mRNA levels of uncoupling protein 1 (Ucp1) (indicator of thermogenesis) were measured in fed and 16 h‐fasted mice in three age groups: 10‐week‐old (young), 15‐week‐old (early adult), and 30‐week‐old (late adult). Weight gain from young to early adult age was not accompanied by changes in WAT and BAT indexes and biochemical blood parameters. Weight gain from early to late adult age was accompanied by increased WAT and BAT indexes and decreased glucose tolerance. Muscle Ucp3 and Cpt1 mRNA levels and WAT Lipe and Slc2a4 mRNA levels increased from young to early adult age and then sharply decreased by the late adult age. Moreover, BAT Ucp1 mRNA level decreased in the late adult age. Fasting failed to increase muscle Cpt1 mRNA levels in late adult mice. These transcriptional changes could contribute to impaired glucose metabolism and the onset of obesity in late adult mice during normal development.

Whole-Body Vibration Partially Reverses Aging-Induced Increases in Visceral Adiposity and Hepatic Lipid Storage in Mice

At old age, humans generally have declining muscle mass and increased fat deposition, which can increase the risk of developing cardiometabolic diseases. While regular physical activity postpones these age-related derangements, this is not always possible in the elderly because of disabilities or risk of injury. Whole-body vibration (WBV) training may be considered as an alternative to physical activity particularly in the frail population. To explore this possibility, we characterized whole-body and organ-specific metabolic processes in 6-month and 25-month old mice, over a period of 14 weeks of WBV versus sham training. WBV training tended to increase blood glucose turnover rates and stimulated hepatic glycogen utilization during fasting irrespective of age. WBV was effective in reducing white fat mass and hepatic triglyceride content only in old but not in young mice and these reductions were related to upregulation of hepatic mitochondrial uncoupling of metabolism (assessed by high-resolution respirometry) and increased expression of uncoupling protein 2. Because these changes occurred independent of changes in food intake and whole-body metabolic rate (assessed by indirect calorimetry), the liver-specific effects of WBV may be a primary mechanism to improve metabolic health during aging, rather than that it is a consequence of alterations in energy balance.

Human Uncoupling Protein-3 and Obesity: An Update

The cloning of the uncoupling protein (UCP)1 homologs UCP2 and UCP3 has raised considerable interest in the mechanism. The expression of UCP3 mainly in skeletal muscle mitochondria and the potency of the skeletal muscle as a thermogenic organ made UCP3 an attractive target for studies toward manipulation of energy expenditure to fight disorders such as obesity and type 2 diabetes. Overexpressing UCP3 in mice resulted in lean, hyperphagic mice. However, the lack of an apparent phenotype in mice lacking UCP3 triggered the search for alternative functions of UCP3. The observation that fatty acid levels significantly affect UCP3 expression has given UCP3 a position in fatty acid handling and/or oxidation. Emerging data indicate that the primary physiological role of UCP3 may be the mitochondrial handling of fatty acids rather than the regulation of energy expenditure through thermogenesis. It has been proposed that UCP3 functions to export fatty acid anions away from the mitochondrial matrix. In doing so, fatty acids are exchanged with protons, explaining the uncoupling activity of UCP3. The exported fatty acid anions may originate from hydrolysis of fatty acid esters by a mitochondrial thioesterase, or they may have entered the mitochondria as nonesterified fatty acids by incorporating into and flip-flopping across the mitochondrial inner membrane. Regardless of the origin of the fatty acid anions, this putative function of UCP3 might be of great importance in protecting mitochondria against fatty acid accumulation and may help to maintain muscular fat oxidative capacity.

Transforming growth factor β₁ oppositely regulates the hypertrophic and contractile response to β-adrenergic stimulation in the heart

BACKGROUND

Neuroendocrine activation and local mediators such as transforming growth factor-β₁ (TGF-β₁) contribute to the pathobiology of cardiac hypertrophy and failure, but the underlying mechanisms are incompletely understood. We aimed to characterize the functional network involving TGF-β₁, the renin-angiotensin system, and the β-adrenergic system in the heart.

METHODS

Transgenic mice overexpressing TGF-β₁ (TGF-β₁-Tg) were treated with a β-blocker, an AT₁-receptor antagonist, or a TGF-β-antagonist (sTGFβR-Fc), were morphologically characterized. Contractile function was assessed by dobutamine stress echocardiography in vivo and isolated myocytes in vitro. Functional alterations were related to regulators of cardiac energy metabolism.

RESULTS

Compared to wild-type controls, TGF-β₁-Tg mice displayed an increased heart-to-body-weight ratio involving both fibrosis and myocyte hypertrophy. TGF-β₁ overexpression increased the hypertrophic responsiveness to β-adrenergic stimulation. In contrast, the inotropic response to β-adrenergic stimulation was diminished in TGF-β₁-Tg mice, albeit unchanged basal contractility. Treatment with sTGF-βR-Fc completely prevented the cardiac phenotype in transgenic mice. Chronic β-blocker treatment also prevented hypertrophy and ANF induction by isoprenaline, and restored the inotropic response to β-adrenergic stimulation without affecting TGF-β₁ levels, whereas AT₁-receptor blockade had no effect. The impaired contractile reserve in TGF-β₁-Tg mice was accompanied by an upregulation of mitochondrial uncoupling proteins (UCPs) which was reversed by β-adrenoceptor blockade. UCP-inhibition restored the contractile response to β-adrenoceptor stimulation in vitro and in vivo. Finally, cardiac TGF-β₁ and UCP expression were elevated in heart failure in humans, and UCP--but not TGF-β₁--was downregulated by β-blocker treatment.

CONCLUSIONS

Our data support the concept that TGF-β₁ acts downstream of angiotensin II in cardiomyocytes, and furthermore, highlight the critical role of the β-adrenergic system in TGF-β₁-induced cardiac phenotype. Our data indicate for the first time, that TGF-β₁ directly influences mitochondrial energy metabolism by regulating UCP3 expression. β-blockers may act beneficially by normalizing regulatory mechanisms of cellular hypertrophy and energy metabolism.

β-Adrenergic inhibition of contractility in L6 skeletal muscle cells

The β-adrenoceptors (β-ARs) control many cellular processes. Here, we show that β-ARs inhibit calcium depletion-induced cell contractility and subsequent cell detachment of L6 skeletal muscle cells. The mechanism underlying the cell detachment inhibition was studied by using a quantitative cell detachment assay. We demonstrate that cell detachment induced by depletion of extracellular calcium is due to myosin- and ROCK-dependent contractility. The β-AR inhibition of L6 skeletal muscle cell detachment was shown to be mediated by the β₂-AR and increased cAMP but was surprisingly not dependent on the classical downstream effectors PKA or Epac, nor was it dependent on PKG, PI3K or PKC. However, inhibition of potassium channels blocks the β₂-AR mediated effects. Furthermore, activation of potassium channels fully mimicked the results of β₂-AR activation. In conclusion, we present a novel finding that β₂-AR signaling inhibits contractility and thus cell detachment in L6 skeletal muscle cells by a cAMP and potassium channel dependent mechanism.

Regulation of the expression of the avian uncoupling protein 3 by isoproterenol and fatty acids in chick myoblasts: possible involvement of AMPK and PPARalpha?

The avian uncoupling protein 3 (UCP3), mainly expressed in muscle tissue, could be involved in fatty acid (FA) metabolism, limitation of reactive oxygen species production, and/or nonshivering thermogenesis. We recently demonstrated that UCP3 mRNA expression was increased by isoproterenol (Iso), a β-agonist, in chicken Pectoralis major. This upregulation was associated with changes in FA metabolism and variations in the activation of AMP-activated protein kinase (AMPK) and in the expression of the transcription factors peroxisome proliferator-activated receptor (PPAR)α, PPARβ/δ, and PPARγ coactivator-1α (PGC-1α). The aim of the present study was to elucidate the mechanisms involving AMPK and PPARα in UCP3 regulation in primary cultures of chick myoblasts. Avian UCP3 mRNA expression, associated with p38 mitogen-activated protein kinase (p38 MAPK) activation, was increased by Iso and/or FAs. The PKA pathway mediated the effects of Iso on UCP3 expression. FA stimulation also led to AMPK activation. Furthermore, the direct involvement of AMPK on UCP3 regulation was shown by using 5-aminoimidazole-4-carboxyamide ribonucleoside and Compound C. The use of the p38 MAPK inhibitor SB202190, which was associated with AMPK activation, also dramatically enhanced UCP3 mRNA expression. Finally the PPARα agonist WY-14643 strongly increased UCP3 mRNA expression. This study highlights the control of UCP3 expression by the β-adrenergic system and FA in chick myoblasts and demonstrates that its expression is directly regulated by AMPK and by PPARα. Overexpression of avian UCP3 might modulate energy utilization or limit oxidative stress when mitochondrial metabolism of FA is triggered by catecholamines.

Interaction between the UCP2 -866 G>A polymorphism, diabetes, and beta-blocker use among patients with acute coronary syndromes

OBJECTIVE

UCP2 -866G>A (rs659366) has been implicated in cardiometabolic disease and represents a novel candidate gene for beta-blocker response, particularly among patients with diabetes. We assessed the function of -866G>A and its role as a modifier of beta-blocker treatment outcomes by diabetes status in an acute coronary syndrome (ACS) cohort.

METHODS

ACS patients with genetic samples and 12 months of follow-up for cardiac rehospitalizations or death (n=468) were assessed. The influence of -866G>A on beta-blocker treatment outcomes was evaluated in those with diabetes and without. To assess functional correlates of -866G>A, we compared uncoupling protein 2 (UCP2) expression in the skeletal muscle of obese participants by genotype and compared the activity of UCP2 luciferase promoters with -866G and -866A alleles.

RESULTS

An interaction between -866G>A and beta-blocker treatment was found in individuals with diabetes (P=0.002) but not those without (P=0.79). Among G/G individuals with diabetes, discharge beta-blocker use was associated with an 80% reduction in cardiac rehospitalization (adjusted hazard ratio: 0.20; 95% confidence interval: 0.04-1.02). In contrast, among A-carrier patients with diabetes, there was an 11-fold increase in cardiac rehospitalizations with discharge beta-blocker therapy (adjusted hazard ratio: 11.75; 95% confidence interval: 1.28-108.2). Promoter activity assays showed that -866G had greater cyclic AMP response element binding protein-responsiveness compared with -866A, and compared with -866A carriers G/G individuals exhibited increased UCP2 expression in the skeletal muscle.

CONCLUSION

We identified a significant interaction between -866G>A and beta-blocker response among ACS patients with diabetes. Furthermore, -866G conferred greater gene transcriptional activity than -866A in cell lines and in obese patients. These findings may help us gain insight into the mechanisms underlying the beneficial and detrimental effects of beta-blockers in those with diabetes.

The beta-adrenergic system is involved in the regulation of the expression of avian uncoupling protein in the chicken

Avian uncoupling protein (avUCP) is orthologous to UCP3, which is suggested to be involved in fatty acid metabolism and to limit the mitochondrial production of reactive oxygen species in mammals. In the chicken, the role and regulation of avUCP remain to be clarified. The aim of this study was to explore the control of avUCP expression by the beta-adrenergic system, known to be involved in avian thermoregulation and lipid utilization, and in UCP expression in mammals. Therefore, we measured the expression of avUCP mRNA and protein in the Pectoralis major muscle of chickens injected with the beta(2) agonist isoproterenol, and we investigated the potential pathways involved in the regulation of avUCP mRNA expression. Avian UCP mRNA expression was increased 7-fold 4h after isoproterenol injection, leading to a tendency to a 40% increase in avUCP protein 24h post-injection. This increase was preceded, 30 min after isoproterenol injection, by changes in the chicken thyroid status and in the muscular expression of PPARalpha, PPARbeta/delta, and PPARgamma coactivator-1alpha (PGC-1alpha). Moreover, the analysis of the avUCP promoter sequence suggested potential binding sites for PPARs and for thyroid hormone receptors. We also detected the activation of AMP-activated protein kinase, which has recently been reported to be involved in UCP3 regulation in mammals. This study presents for the first time evidence of beta-adrenergic control on avUCP messenger expression in chicken muscle and suggests the potential involvement of AMPK and several transcription factors in this regulation.

Effects of ingesting a commercial thermogenic product on hemodynamic function and energy expenditure at rest in males and females

The purpose of this study was to examine the effects of a commercially available thermogenic product (TP) on resting energy expenditure (REE) and hemodynamic variables in a randomized, double-blind, placebo (PL)-controlled study. Eight male (age: 23.0 +/- 3.70 years, weight: 95.77 +/- 16.44 kg, height: 182.4 +/- 7.87 cm) and 10 female (age: 23.6 +/- 4.81 years, weight: 67.25 +/- 5.74 kg, height: 172.42 +/- 10.31 cm) physically active individuals participated in this study. Participants reported to the laboratory on a 10-h fast and performed baseline testing on REE, heart rate, and blood pressure. Participants were then randomly assigned to ingest 3 capsules of either an experimental TP or a vitamin E PL. Criterion variables were then measured at 1-, 2-, and 3-h post ingestion. Data were analyzed by 2-factor analysis of variance (ANOVA) using SPSS, version 16.0 (SPSS Inc., Chicago, Ill.). Supplementation of the TP resulted in a significant main effect for time (p = 0.040) and for interaction (p < 0.01) in REE when compared with PL. Post hoc analysis revealed that there was no significant difference (p > 0.05) between groups at baseline, but the TP group was significantly higher (p < 0.01) than the PL group at 1-, 2-, and 3-h post, with peak values being achieved at 2-h post time point. The TP group also experienced an overall increase in REE by 17.3%, 19.6%, and 15.3% at the 1-, 2-, and 3-h time points, respectively, over baseline values. Conversely, the PL group experienced a reduction in REE by 2.5%, 1.8%, and 0.3% at the same time points compared with baseline values. There was no significant change in heart rate, systolic blood pressure, or diastolic blood pressure in either group. Taken on a daily basis, a TP may increase overall energy expenditure. Caloric expenditure significantly increased at all 3 time points in the TP group, whereas the PL group experienced no change in energy expenditure.

Expression profiling of skeletal muscle following acute and chronic beta2-adrenergic stimulation: implications for hypertrophy, metabolism and circadian rhythm

Background

Systemic administration of β-adrenoceptor (β-AR) agonists has been found to induce skeletal muscle hypertrophy and significant metabolic changes. In the context of energy homeostasis, the importance of β-AR signaling has been highlighted by the inability of β_1-3-AR-deficient mice to regulate energy expenditure and susceptibility to diet induced obesity. However, the molecular pathways and gene expression changes that initiate and maintain these phenotypic modulations are poorly understood. Therefore, the aim of this study was to identify differential changes in gene expression in murine skeletal muscle associated with systemic (acute and chronic) administration of the β₂-AR agonist formoterol.

Results

Skeletal muscle gene expression (from murine tibialis anterior) was profiled at both 1 and 4 hours following systemic administration of the β₂-AR agonist formoterol, using Illumina 46K mouse BeadArrays. Illumina expression profiling revealed significant expression changes in genes associated with skeletal muscle hypertrophy, myoblast differentiation, metabolism, circadian rhythm, transcription, histones, and oxidative stress. Differentially expressed genes relevant to the regulation of muscle mass and metabolism (in the context of the hypertrophic phenotype) were further validated by quantitative RT-PCR to examine gene expression in response to both acute (1-24 h) and chronic administration (1-28 days) of formoterol at multiple timepoints. In terms of skeletal muscle hypertrophy, attenuation of myostatin signaling (including differential expression of myostatin, activin receptor IIB, phospho-Smad3 etc) was observed following acute and chronic administration of formoterol. Acute (but not chronic) administration of formoterol also significantly induced the expression of genes involved in oxidative metabolism, including hexokinase 2, sorbin and SH3 domain containing 1, and uncoupling protein 3. Interestingly, formoterol administration also appeared to influence some genes associated with the peripheral regulation of circadian rhythm (including nuclear factor interleukin 3 regulated, D site albumin promoter binding protein, and cryptochrome 2).

Conclusion

This is the first study to utilize gene expression profiling to examine global gene expression in response to acute β₂-AR agonist treatment of skeletal muscle. In summary, systemic administration of a β₂-AR agonist had a profound effect on global gene expression in skeletal muscle. In terms of hypertrophy, β₂-AR agonist treatment altered the expression of several genes associated with myostatin signaling, a previously unreported effect of β-AR signaling in skeletal muscle. This study also demonstrates a β₂-AR agonist regulation of circadian rhythm genes, indicating crosstalk between β-AR signaling and circadian cycling in skeletal muscle. Gene expression alterations discovered in this study provides insight into many of the underlying changes in gene expression that mediate β-AR induced skeletal muscle hypertrophy and altered metabolism.

Body fat mass reduction and up-regulation of uncoupling protein by novel lipolysis-promoting plant extract

We have found natural products exhibiting lipolysis-promoting activity in subcutaneous adipocytes, which are less sensitive to hormones than visceral adipocytes. The activities and a action mechanisms of a novel plant extract of Cirsium oligophyllum (CE) were investigated in isolated adipocytes from rat subcutaneous fat, and its fat-reducing effects by peroral administration and topical application were evaluated in vivo. CE-induced lipolysis was synergistically enhanced by caffeine, a phosphodiesterase inhibitor, and was reduced by propranolol, a β adrenergic antagonist. The peroral administration of 10% CE solution to Wistar rats for 32 days reduced body weight gain, subcutaneous, and visceral fat weights by 6.6, 26.2, and 3.0%, respectively, as compared to the control group. By the topical application of 2% of this extract to rats for 7 days, weight of subcutaneous fat in the treated skin was reduced by 23.2%. This fat mass reduction was accompanied by the up-regulation of uncoupling protein 1 (UCP), a principal thermogenic mitochondrial molecule related to energy dissipating, in subcutaneous fat and UCP3 in skin except for the fat layer. These results indicate that CE promotes lipolysis via a mechanism involving the β adrenergic receptor, and affects the body fat mass. This fat reduction may be partially due to UCP up-regulation in the skin including subcutaneous fat. This is the first report showing that repeated lipolysis promotion through CE administration may be beneficial for the systematic suppression of body fat accumulation or the control of fat distribution in obesity.

The acute effects of the thermogenic supplement Meltdown on energy expenditure, fat oxidation, and hemodynamic responses in young, healthy males

The purpose of this study was to evaluate the effects of a thermogenic supplement, Meltdown, on energy expenditure, fat oxidation, and hemodynamics before and after maximal treadmill exercise. In a double-blind, randomized, placebo-controlled, cross-over design, 12 male participants underwent two testing sessions after consuming either the Meltdown or placebo supplement. While in a fasted state, participants rested for one hour, orally ingested either Meltdown or placebo and rested for another hour, performed a maximal treadmill exercise test, and then rested for another hour. Throughout the testing protocol, resting energy expenditure (REE) and respiratory exchange ratio (RER) were assessed. In addition, heart rate (HR) and blood pressure (BP) were assessed before and after exercise. Meltdown increased REE significantly more than placebo at 45 min (1.44 ± 0.25 vs. 1.28 ± 0.23 kcal/min; p = 0.003), 60 min (1.49 ± 0.28 vs. 1.30 ± 0.22 kcal/min; p = 0.025), and 120 min (1.51 ± 0.26 vs. 1.33 ± 0.27 kcals/min; p = 0.014) post-ingestion. Meltdown significantly decreased RER at 30 min (0.84 ± 0.03 vs. 0.91 ± 0.04; p = 0.022) and 45 min post-ingestion (0.82 ± 0.04 vs. 0.89 ± 0.05; p = 0.042), and immediately post-exercise (0.83 ± 0.05 vs. 0.90 ± 0.07; p = 0.009). Furthermore, over the course of the evaluation period, area under the curve assessment demonstrated that REE was significantly increased with Meltdown compared to placebo (992.5 ± 133.1 vs. 895.1 ± 296.1 kcals; p = 0.043), while RER was significantly less than placebo (5.55 ± 0.61 vs. 5.89 ± 0.44; p = 0.002) following ingestion. HR and BP were not significantly affected prior to exercise with either supplement (p > 0.05) and the exercise-induced increases for HR and BP decreased into recovery and were not different between supplements (p > 0.05). These data suggest that Meltdown enhances REE and fat oxidation more than placebo for several hours after ingestion in fully rested and post-exercise states without any adverse hemodynamic responses associated with maximal exercise.

Expression of uncoupling proteins in human skin and skin-derived cells

Uncoupling protein (UCP) is a mitochondrial membrane protein that uncouples oxidative phosphorylation. The physiological function of major isoforms of UCPs is related to the control of body temperature and reactive oxygen species production. Although skin is an important organ for heat radiation and protection against stress, the expression and function of UCPs in the skin have remained unclear. The expression of UCPs in human skin and its derived cells was researched at the mRNA and protein levels. The effects of norepinephrine (NE) and 9-cis retinoic acid (RA) on UCP expression were also investigated. The expression of UCP1 mRNA was found in the human epidermis and was upregulated in differentiated keratinocytes. UCP1 expression in keratinocytes was synergistically upregulated by NE and RA treatment. Significant expression of UCP2 and UCP3 was observed also in cultured keratinocytes and fibroblasts. By immunohistochemistry, localization of UCP1 was found in the granular layer of the epidermis, sweat glands, hair follicles, and sebaceous glands of various sites in the human body. UCP3 was widely found in the dermis. This showed that UCPs exist in human skin, with their expression being under hormonal control. These findings are in stark contrast with the well-accepted view of UCP1 expression being exclusive to brown adipose tissue.

Mitochondrial uncoupling protein 3 and its role in cardiac- and skeletal muscle metabolism

Uncoupling protein 3 (UCP3), is primarily expressed in skeletal muscle mitochondria and has been suggested to be involved in mediating energy expenditure via uncoupling, hereby dissipating the mitochondrial proton gradient necessary for adenosine triphosphate (ATP) synthesis. Although some studies support a role for UCP3 in energy metabolism, other studies pointed towards a function in fatty acid metabolism. Thus, the protein is up regulated or high when fatty acid supply to the mitochondria exceeds the capacity to oxidize fatty acids and down regulated or low when oxidative capacity is high or improved. Irrespective of the exact operating mechanism, UCP3 seems to protect mitochondria against lipid-induced oxidative stress, which makes this protein a potential player in the development of type 2 diabetes mellitus. Next to skeletal muscle, UCP3 is also expressed in cardiac muscle where its role is relatively unexplored. Interestingly, energy deficiency in cardiac muscle is associated to heart failure and UCP3 might contribute to this energy deficiency. It has been suggested that UCP3 decreases energy status via uncoupling of mitochondrial respiration, but the available data does not provide a unified answer. In fact, the results obtained regarding cardiac UCP3 are very similar as in skeletal muscle, implying that its physiological function can be extrapolated. Therefore, cardiac UCP3 can just as well serve to protect the heart against lipid-induced oxidative stress, similar to the function described for skeletal muscle UCP3. The present review will deal with the available literature on both skeletal muscle- and cardiac UCP3 to elucidate its physiological function in these tissues.

Influence of ß2-adrenoceptor gene polymorphisms on diet-induced thermogenesis

The sympathetic nervous system is involved in the control of energy metabolism and expenditure. Diet-induced thermogenesis is mediated partly by the ß-adrenergic component of this system. The aim of the present study was to investigate the role of genetic variation in the ß2-adrenoceptor in diet-induced thermogenesis. Data from twenty-four subjects (fourteen men and ten women; BMI 26·7(sem 0·8) kg/m2; age 45·2(sem1·4) years) with different polymorphisms of the ß2-adrenoceptor at codon 16 (Gly16Gly, Gly16Arg or Arg16Arg) were recruited for this study. Subjects were given a high-carbohydrate liquid meal, and the energy expenditure, respiratory exchange ratio, and plasma concentrations of NEFA, glycerol, glucose, insulin and catecholamines were measured before and over 4 h after the meal. The AUC of energy expenditure (diet-induced thermogenesis) was not significantly different between polymorphism groups, nor was the response of any of the other measured variables to the meal. In a multiple regression model, the only variable that explained a significant proportion (32 %) of the variation in diet-induced thermogenesis was the increase in plasma adrenaline in response to the meal (P<0·05). The ß2-adrenoceptor codon16 polymorphisms did not contribute significantly. In conclusion, an independent contribution of the codon 16 polymorphism of the ß2-adrenoceptor gene to the variation in thermogenic response to a high-carbohydrate meal could not be demonstrated. The interindividual variation in thermogenic response to the meal was correlated with variations in the plasma adrenaline response to the meal.

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.

The orphan nuclear receptor, NOR-1, is a target of beta-adrenergic signaling in skeletal muscle

beta-Adrenergic receptor (beta-AR) agonists induce Nur77 mRNA expression in the C2C12 skeletal muscle cell culture model and elicit skeletal muscle hypertrophy. We previously demonstrated that Nur77 (NR4A1) is involved in lipolysis and gene expression associated with the regulation of lipid homeostasis. Subsequently it was demonstrated by another group that beta-AR agonists and cold exposure-induced Nur77 expression in brown adipocytes and brown adipose tissue, respectively. Moreover, NOR-1 (NR4A3) was hyperinduced by cold exposure in the nur77(-/-) animal model. These studies underscored the importance of understanding the role of NOR-1 in skeletal muscle. In this context we observed 30-480 min of beta-AR agonist treatment significantly and transiently increased expression of the orphan nuclear receptor NOR-1 in both mouse skeletal muscle tissue (plantaris) and C2C12 skeletal muscle cells. Specific beta(2)- and beta(3)-AR agonists had similar effects as the pan-agonist and were blocked by the beta-AR antagonist propranolol. Moreover, in agreement with these observations, isoprenaline also significantly increased the activity of the NOR-1 promoter. Stable exogenous expression of a NOR-1 small interfering RNA (but not the negative control small interfering RNA) in skeletal muscle cells significantly repressed endogenous NOR-1 mRNA expression and led to changes in the expression of genes involved in the control of lipid use and muscle mass underscored by a dramatic increase in myostatin mRNA expression. Concordantly the myostatin promoter was repressed by NOR-1 expression. In conclusion, NOR-1 is highly responsive to beta-adrenergic signaling and regulates the expression of genes controlling fatty acid use and muscle mass.

Multiple signalling pathways involved in beta2-adrenoceptor-mediated glucose uptake in rat skeletal muscle cells

1. Beta-adrenoceptor (AR) agonists increase 2-deoxy-[3H]-D-glucose uptake (GU) via beta2-AR in rat L6 cells. The beta-AR agonists, zinterol (beta2-AR) and (-)-isoprenaline, increased cAMP accumulation in a concentration-dependent manner (pEC50=9.1+/-0.02 and 7.8+/-0.02). Cholera toxin (% max increase 141.8+/-2.5) and the cAMP analogues, 8-bromo-cAMP (8Br-cAMP) and dibutyryl cAMP (dbcAMP), also increased GU (196.8+/-13.5 and 196.4+/-17.3%). 2. The adenylate cyclase inhibitor, 2',5'-dideoxyadenosine (50 microM), significantly reduced cAMP accumulation to zinterol (100 nM) (109.7+35.0 to 21.6+4.5 pmol well(-1)), or forskolin (10 microM) (230.1+/-58.0 to 107.2+/-26.3 pmol well(-1)), and partially inhibited zinterol-stimulated GU (217+/-26.3 to 176.1+/-20.4%). The protein kinase A (PKA) inhibitor, 4-cyano-3-methylisoquinoline (100 nM), did not inhibit zinterol-stimulated GU. The PDE4 inhibitor, rolipram (10 microM), increased cAMP accumulation to zinterol or forskolin, and sensitised the GU response to zinterol, indicating a stimulatory role of cAMP in GU. 3. cAMP accumulation studies indicated that the beta2-AR was desensitised by prolonged stimulation with zinterol, but not forskolin, whereas GU responses to zinterol increased with time, suggesting that receptor desensitisation may be involved in GU. Receptor desensitisation was not reversed by inhibition of PKA or Gi. 4. PTX pretreatment (100 ng ml(-1)) inhibited insulin or zinterol-stimulated but not 8Br-cAMP or dbcAMP-stimulated GU. The PI3K inhibitor, LY294002 (1 microM), inhibited insulin- (174.9+/-5.9 to 142.7+/-2.7%) and zinterol- (166.9+/-7.6 to 141.1+/-8.1%) but not 8 Br-cAMP-stimulated GU. In contrast to insulin, zinterol did not cause phosphorylation of Akt. 5. The results suggest that GU in L6 cells involves three mechanisms: (1) an insulin-dependent pathway involving PI3K, (2) a beta2-AR-mediated pathway involving both cAMP and PI3K, and (3) a receptor-independent pathway suggested by cAMP analogues that increase GU independently of PI3K. PKA appears to negatively regulate beta2-AR-mediated GU.

The role of mitochondrial uncoupling in 3,4-methylenedioxymethamphetamine-mediated skeletal muscle hyperthermia and rhabdomyolysis

Use of the popular club drug ecstasy (3,4-methylenedioxymethamphetamine, MDMA) can result in life-threatening hyperthermia and rhabdomyolysis. Recent studies show a link between skeletal muscle uncoupling proteins in MDMA-mediated hyperthermia. The mechanisms by which MDMA interacts with skeletal muscle mitochondria are largely unknown. The present study was designed to comprehensively evaluate the effects of MDMA on bioenergetics and toxicity of skeletal muscle. Using (31)P nuclear magnetic resonance (NMR) and serum creatine kinase levels, we demonstrate evidence for uncoupling of oxidative phosphorylation in the skeletal muscle of MDMA (40 mg/kg)-treated rats. In vivo, rats treated with MDMA had significantly elevated serum creatine kinase levels, a marker of rhabdomyolysis, 4 h post-MDMA treatment (955 +/- 132 IU/l) compared with saline-treated controls (373.2 +/- 59 IU/l). beta-ATP signal areas after MDMA treatment showed significant reductions (15%) from the baseline values with corresponding increases in inorganic phosphate (88% increases) and decreases in intracellular pH. Clark electrode experiments on isolated skeletal muscle mitochondria in vitro (1-5 mM MDMA) and ex vivo in MDMA-treated animals demonstrated no evidence of uncoupling of oxidative phosphorylation. In vitro experiments using L6 myotubules cocultured with primary hepatocytes demonstrated the presence of uncoupling protein-3 in the L6 myotubules, but no evidence of a direct effect of MDMA or its potential metabolites on cellular creatine kinase concentrations. These findings suggest that MDMA uncouples skeletal muscle mitochondria in vivo but that this uncoupling is the result of indirect mechanisms.

Effect of zinc-alpha2-glycoprotein (ZAG) on expression of uncoupling proteins in skeletal muscle and adipose tissue

The plasma protein zinc-alpha2-glycoprotein (ZAG) has been shown to be identical with a lipid mobilizing factor capable of inducing loss of adipose tissue in cancer cachexia through an increased lipid mobilization and utilization. The ability of ZAG to induce uncoupling protein (UCP) expression has been determined using in vitro models of adipose tissue and skeletal muscle. ZAG induced a concentration-dependent increase in the expression of UCP-1 in primary cultures of brown, but not white, adipose tissue, and this effect was attenuated by the beta3-adrenergic receptor (beta3-AR) antagonist SR59230A. A 6.5-fold increase in UCP-1 expression was found in brown adipose tissue after incubation with 0.58 microM ZAG. ZAG also increased UCP-2 expression 3.5-fold in C2C12 murine myotubes, and this effect was also attenuated by SR59230A and potentiated by isobutylmethylxanthine, suggesting a cyclic AMP-mediated process through interaction with a beta3-AR. ZAG also produced a dose-dependent increase in UCP-3 in murine myotubes with a 2.5-fold increase at 0.58 microM ZAG. This effect was not mediated through the beta3-AR, but instead appeared to require mitogen activated protein kinase. These results confirm the ability of ZAG to directly influence UCP expression, which may play an important role in lipid utilization during cancer cachexia.

Chronic low-frequency stimulation upregulates uncoupling protein-3 in transforming rat fast-twitch skeletal muscle

The purpose of this investigation was to examine the temporal changes in uncoupling protein (UCP)-3 expression, as well as related adaptive changes in mitochondrial density and fast-to-slow fiber type transitions during chronically enhanced contractile activity. We examined the effects of 1-42 days of chronic low-frequency electrical stimulation (CLFS), applied to rat tibialis anterior (TA) for 10 h/day, on the expression of UCP-3 and concomitant changes in myosin heavy chain (MHC) protein expression and increases in oxidative capacity. UCP-3 protein content increased from 1 to 12 days, reaching 1.5-fold over control (P < 0.0005); it remained elevated for up to 42 days. In contrast, UCP-3 mRNA decreased in response to CLFS, reaching a level that was threefold lower than control (P < 0.0007). The activities of the mitochondrial reference enzymes citrate synthase (EC 4.1.3.7) and 3-hydroxyacyl-CoA-dehydrogenase (EC 1.1.1.35), which are known to increase in proportion to mitochondrial density, progressively increased up to an average of 2.3-fold (P < 0.00001). These changes were accompanied by fast-to-slow fiber type transitions, characterized by a shift in the pattern of MHC expression (P <0.0002): MHCI and MHCIIa expression increased by 1.7- and 4-fold, whereas MHCIIb displayed a 2.4-fold reduction. We conclude that absolute increases in UCP-3 protein content in the early adaptive phase were associated with the genesis of mitochondria containing a normal complement of UCP-3. However, during exposure to long-term CLFS, mitochondria were generated with a lower complement of UCP-3 and coincided with the emergence of a growing population of oxidative type IIA fibers.

Role of lipid-mobilising factor (LMF) in protecting tumour cells from oxidative damage

Lipid-mobilising factor (LMF) is produced by cachexia-inducing tumours and is involved in the degradation of adipose tissue, with increased oxidation of the released fatty acids through an induction of uncoupling protein (UCP) expression. Since UCP-2 is thought to be involved in the detoxification of free radicals if LMF induced UCP-2 expression in tumour cells, it might attenuate free radical toxicity. As a model system we have used MAC13 tumour cells, which do not produce LMF. Addition of LMF caused a concentration-dependent increase in UCP-2 expression, as determined by immunoblotting. This effect was attenuated by the β3 antagonist SR59230A, suggesting that it was mediated through a β3 adrenoreceptor. Co-incubation of LMF with MAC13 cells reduced the growth-inhibitory effects of bleomycin, paraquat and hydrogen peroxide, known to be free radical generators, but not chlorambucil, an alkylating agent. There was no effect of LMF alone on cellular proliferation. These results indicate that LMF antagonises the antiproliferative effect of agents working through a free radical mechanism, and may partly explain the unresponsiveness to the chemotherapy of cachexia-inducing tumours.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Effect of thyroid hormone on uncoupling protein-3 mRNA expression in rat heart and skeletal muscle

Thyroid hormones (triiodothyronine [T3] and thyroxine [T4]) stimulate UCP-3 expression in skeletal muscle. We examined whether thyroid hormone-induced changes in uncoupling protein (UCP)-3 mRNA expression are related to directs effects of T3 or reflect secondary effects of the hormone through stimulation of renin-angiotensin or beta-adrenergic systems. Hyperthyroidism was produced by three injections of 100 microg T3/100 g body weight on alternate days with or without concomitant treatment with either captopril (an angiotensin-converting enzyme [ACE] inhibitor), propranolol (a beta-blocker) or clenbuterol (a beta2-agonist). The relative abundance of UCP-3 mRNA was measured in ventricular myocardium and skeletal muscle (gastrocnemius and soleus). T3 resulted in a significant increase in the relative abundance of UCP-3 in heart and skeletal muscle (p < 0.05), and the effect was not altered by captopril or propanolol; the inhibitors alone had no effect of UCP-3 mRNA content. There was no synergistic or additive effect of T3 and clenbuterol on UCP-3 mRNA expression in skeletal muscle. Increased UCP-3 mRNA levels were associated with increased UCP-3 protein expression in skeletal muscle. We conclude that the effect of T3 on UCP-3 expression in cardiac and skeletal muscle is not dependent on either angiotensin II or the beta-adrenergic system and probably reflects a direct action of the hormone on UCP-3 gene expression.

Reduction of diet-induced obesity in transgenic mice overexpressing uncoupling protein 3 in skeletal muscle

AIMS/HYPOTHESIS

It has been suggested that uncoupling protein 3 (UCP3) can increase energy expenditure, thereby regulating body weight. Although studies on UCP3 knock-out mice suggest that lack of UCP3 function does not cause obesity or Type 2 diabetes, it is possible that up-regulation of UCP3 function improves these disorders or their clinical sequelae. A 10- to 20-fold increase of UCP3 gene expression is achievable through physiological or pharmacological stimuli. We examined the phenotype of transgenic mice with approximately 18-fold overexpression of mouse UCP3 mRNA in skeletal muscle.

METHODS

We generated transgenic mice with approximately 18-fold overexpression of mouse UCP3 mRNA in skeletal muscle under control of the skeletal muscle-specific muscle creatine kinase gene promoter. The phenotype of these mice was analysed either on a standard diet or on a 4-week high-fat diet.

RESULTS

In mice on standard chow, there was no difference in body weight, oxygen consumption and mitochondrial protonmotive force between transgenic mice and non-transgenic littermates. However, transgenic mice tended to have lower body weight, increased oxygen consumption and decreased mitochondrial protonmotive force than the control mice. Transgenic mice on a 4-week high-fat diet consumed much more oxygen and had noticeably less weight gain and less epididymal fat, as well as better glucose tolerance than non-transgenic littermates.

CONCLUSIONS/INTERPRETATION

Our study shows that 18-fold overexpression of UCP3 mRNA in the skeletal muscle reduced diet-induced obesity. An 18-fold increase of UCP3 mRNA can be attained by physiological or pharmacological stimuli, suggesting that UCP3 has therapeutic potential in the treatment of obesity.

Effect of beta1- and beta2-adrenergic stimulation on energy expenditure, substrate oxidation, and UCP3 expression in humans

In humans, beta-adrenergic stimulation increases energy and fat metabolism. In the case of beta1-adrenergic stimulation, it is fueled by an increased lipolysis. We examined the effect of beta2-adrenergic stimulation, with and without a blocker of lipolysis, on thermogenesis and substrate oxidation. Furthermore, the effect of beta1-and beta2-adrenergic stimulation on uncoupling protein 3 (UCP3) mRNA expression was studied. Nine lean males received a 3-h infusion of dobutamine (DOB, beta1) or salbutamol (SAL, beta2). Also, we combined SAL with acipimox to block lipolysis (SAL+ACI). Energy and substrate metabolism were measured continuously, blood was sampled every 30 min, and muscle biopsies were taken before and after infusion. Energy expenditure significantly increased approximately 13% in all conditions. Fat oxidation increased 47 +/- 7% in the DOB group and 19 +/- 7% in the SAL group but remained unchanged in the SAL+ACI condition. Glucose oxidation decreased 40 +/- 9% upon DOB, remained unchanged during SAL, and increased 27 +/- 11% upon SAL+ACI. Plasma free fatty acid (FFA) levels were increased by SAL (57 +/- 11%) and DOB (47 +/- 16%), whereas SAL+ACI caused about fourfold lower FFA levels compared with basal levels. No change in UCP3 was found after DOB or SAL, whereas SAL+ACI downregulated skeletal muscle UCP3 mRNA levels 38 +/- 13%. In conclusion, beta2-adrenergic stimulation directly increased energy expenditure independently of plasma FFA levels. Furthermore, this is the first study to demonstrate a downregulation of skeletal muscle UCP3 mRNA expression after the lowering of plasma FFA concentrations in humans, despite an increase in energy expenditure upon beta2-adrenergic stimulation.

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

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

Regulation of intracellular cyclic AMP in skeletal muscle cells involves the efflux of cyclic nucleotide to the extracellular compartment

(1) This report analyses the intracellular and extracellular accumulation of cyclic AMP in primary rat skeletal muscle cultures, after direct and receptor-dependent stimulation of adenylyl cyclase (AC). (2) Isoprenaline, calcitonin gene-related peptide (CGRP) and forskolin induced a transient increase in the intracellular cyclic AMP that peaked 5 min after onset stimulation. (3) Under stimulation with isoprenaline or CGRP, the intracellular cyclic AMP initial rise was followed by an exponential decline, reaching 46 and 52% of peak levels in 10 min, respectively. (4) Conversely, the forskolin-dependent accumulation of intracellular cyclic AMP decreased slowly and linearly, reaching 49% of the peak level in 30 min. (5) The loss of intracellular cyclic AMP from peak levels, induced by direct or receptor-induced activation of AC, was followed by an increase in the extracellular cyclic AMP. (6) This effect was independent on PDEs, since it was obtained in the presence of 3-isobutyl-1-methylxanthine (IBMX). (7) Besides, in isoprenaline treated cells, the beta-adrenoceptor antagonist propranolol reduced both intra- and extracellular accumulation of cyclic AMP, whereas the organic anion transporter inhibitor probenecid reduced exclusively the extracellular accumulation. (8) Together our data show that direct or receptor-dependent activation of skeletal muscle AC results in a transient increase in the intracellular cyclic AMP, despite the continuous presence of the stimulus. The temporal declining of intracellular cyclic AMP was not dependent on the cyclic AMP breakdown but associated to the efflux of cyclic nucleotide to the extracellular compartment, by an active transport since it was prevented by probenecid.

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.

Characterization of the beta-adrenoceptor subtype involved in mediation of glucose transport in L6 cells

1. The receptor that mediates the increase in glucose transport (GT) in response to beta-adrenoceptor (beta-AR) agonists was characterized in the rat skeletal muscle cell line L6, using the 2-deoxy-[(3)H]-D-glucose assay. 2. The beta(3)-AR agonist BRL37344 (pEC(50) = 6.89 +/- 0.21), the beta-AR agonist isoprenaline (pEC(50) = 8.99 +/ -0.24) and the beta(2)-AR agonist zinterol (pEC(50) = 9.74 +/- 0.15) increased GT as did insulin (pEC(50) = 6.93 +/- 0.15). The highly selective beta(3)-AR agonist CL316243 only weakly stimulated GT. 3. The pK(B) values calculated from the shift of the pEC(50) values of the agonists in the presence of the beta(1)-AR selective antagonist CGP 20712A or the beta(3)-AR selective antagonist SR 59230A were not indicative of activation of beta(1)- or beta(3)-ARs. Only (-)-propranolol and the beta(2)-AR selective antagonist ICI 118551 caused marked rightward shifts of CR curves to isoprenaline (pK(B) = 10.2 +/- 0.2 and 9.6 +/- 0.3), zinterol (pK(B) = 9.0 +/- 0.1 and 9.4 +/- 0.3) and BRL 37344 (pK(B) = 9.4 +/- 0.3 and 8.4 +/- .2), indicating participation of beta(2)-ARs. 4. The pharmacological analysis was supported by reverse transcription and polymerase chain reaction analysis of L6 mRNA, which showed high levels of expression of beta(2)-AR but not beta(1)- or beta(3)-AR in these cells. 5. Forskolin and dibutyryl cyclic AMP produced negligible increases in GT while the phosphatidylinositol-3 kinase inhibitor, wortmannin, significantly decreased both insulin- and zinterol-stimulated GT, suggesting a possible interaction between the insulin and beta(2)-AR pathways. 6. This study demonstrates that beta(2)-ARs mediate the increase in GT in L6 cells to beta-AR agonists, including the beta(3)-AR selective agonist BRL 37344. This effect does not appear to be directly related to increases in cyclic AMP but requires P13K.

[Mitochondrial uncoupling protein as a target of pharmacotherapy for obesity]

Uncoupling protein (UCP) is a transporter family present in the mitochondrial inner membrane, and as its name suggests, it uncouples respiration from ATP synthesis by dissipating the transmembrane proton gradient as heat. UCP is now recognized as a key molecule in metabolic thermogenesis such as cold- and diet-induced heat production, which is a significant component of energy expenditure, and its dysfunction contributes to the development of obesity. Among the UCP family, UCP-1 is expressed exclusively in brown adipose tissue (BAT), while UCP-2 is present in many organs and UCP-3 is in skeletal muscle. BAT thermogenesis by UCP-1, which has been studied most extensively, is controlled directly by sympathetic nerves principally through the beta-adrenergic action of norepinephrine. Since the beta 3-adrenoceptor is present primarily in adipose tissues, its selective agonists stimulate BAT thermogenesis and also lipid mobilization in white adipose tissue without any noticeable effect on beta 1- and beta 2-adrenoceptos. Therefore, beta 3-adrenoceptor agonists would be promising for the pharmacotherapy of obesity. UCP gene expression is up regulated by ligands for nuclear receptors such as thyroid hormone receptor, peroxisome proliferator-activated receptors (PPAR) and retinoid-X receptor. Long chain fatty acids and some of their metabolites are known to activate PPAR and thereby lead to abundant expression of UCP, which may also contribute to increase in energy expenditure and prevention of obesity. The activity of UCP is suppressed by purine nucleotides but activated by fatty acids. Thus, fatty acids increase UCP-mediated thermogenesis by direct activation of UCP and also by increased gene expression, implying some specific fatty acids or their derivatives as an effective anti-obesity tool.