Regulation of UCP1 and UCP3 in arctic ground squirrels and relation with mitochondrial proton leak.

Uncoupling protein (UCP) 1 (UCP1) catalyzes a proton leak in brown adipose tissue (BAT) mitochondria that results in nonshivering thermogenesis (NST), but the extent to which UCP homologs mediate NST in other tissues is controversial. To clarify the role of UCP3 in mediating NST in a hibernating species, we measured Ucp3 expression in skeletal muscle of arctic ground squirrels in one of three activity states (not hibernating, not hibernating and fasted for 48 h, or hibernating) and housed at 5 degrees C or -10 degrees C. We then compared Ucp3 mRNA levels in skeletal muscle with Ucp1 mRNA and UCP1 protein levels in BAT in the same animals. Ucp1 mRNA and UCP1 protein levels were increased on cold exposure and decreased with fasting, with the highest UCP1 levels in thermogenic hibernators. In contrast, Ucp3 mRNA levels were not affected by temperature but were increased 10-fold during fasting and >3-fold during hibernation. UCP3 protein levels were increased nearly fivefold in skeletal muscle mitochondria isolated from fasted squirrels compared with nonhibernators, but proton leak kinetics in the presence of BSA were unchanged. Proton leak in BAT mitochondria also did not differ between fed and fasted animals but did show classical inhibition by the purine nucleotide GDP. Levels of nonesterified fatty acids were highest during hibernation, and tissue temperatures during hibernation were related to Ucp1, but not Ucp3, expression. Taken together, these results do not support a role for UCP3 as a physiologically relevant mediator of NST in muscle.     

Shifts in thermoregulatory strategy during ontogeny in harp seals (Pagophilus groenlandicus)

Heat balance can be difficult for young and/or small animals in polar regions because environmental conditions in combination with small body size or physiological immaturity can increase heat loss. We investigated how thermoregulatory patterns change with ontogeny in 5 age classes of harp seal (Pagophilus groenlandicus) from birth to post-molt to further understand the timing of thermoregulatory development in relation to their potential vulnerability to ongoing fluctuations in the extent and stability of Arctic pack ice. We measured changes in the amount, conductivity, and resistance of the seal pups׳ insulative layers (blubber and fur), the potential for endogenous heat-generation by shivering (muscle enzyme activity), and nonshivering thermogenesis (NST; brown adipose tissue (BAT) uncoupling protein 1 (UCP1) expression and mitochondrial density). There was no significant difference in blubber conductivity among age classes, though the amount of blubber insulation significantly increased from birth to weaning. Pelage conductivity was low (0.12±0.01 W m⁻¹ °C⁻¹) except in 9-day old pups (0.40±0.08 W m⁻¹ °C⁻¹); the significantly higher conductivity may signal the beginning of the molt, and this age group may be the most vulnerable to early water entry. Citrate synthase activity significantly increased (49.68±3.26 to 75.08±3.52 μmol min⁻¹ g wet weight⁻¹) in the muscle; however it is unlikely that increasing a single enzyme greatly impacts heat generation. BAT of younger pups contained UCP1, though expression and mitochondrial density quickly declined, and the ability of pups to produce heat via NST was lost by weaning. While total thermal resistance did not differ, neonatal and early nursing animals gained the majority of their thermal resistance from lanugo (82.5±0.03%); however, lanugo is not insulative when wet, and NST may be important to maintain euthermia and dry the coat if early immersion in water occurs. By late nursing, blubber seems sufficient as insulation (75.87±0.01% of resistance after 4 weeks), but high conductivity of fur may be responsible for retention of UCP1 expression. Weaned animals rely on blubber insulation, and no longer need NST, as wetted fur is no longer a threat to euthermia.     

The molecular and biochemical basis of nonshivering thermogenesis in an African endemic mammal, Elephantulus myurus

Uncoupling protein 1 (UCP1) mediated nonshivering thermogenesis (NST) in brown adipose tissue (BAT) is an important avenue of thermoregulatory heat production in many mammalian species. Until recently, UCP1 was thought to occur exclusively in eutherians. In the light of the recent finding that UCP1 is already present in fish, it is of interest to investigate when UCP1 gained a thermogenic function in the vertebrate lineage. We elucidated the basis of NST in the rock elephant shrew, Elephantulus myurus (Afrotheria: Macroscelidea). We sequenced Ucp1 and detected Ucp1 mRNA and protein restricted to brown fat deposits. We found that cytochrome c oxidase activity was highest in these deposits when compared with liver and skeletal muscle. Consistent with a thermogenic function of UCP1 isolated BAT mitochondria showed increased state 4 respiration in the cold, as well as palmitate-induced, GDP-sensitive proton conductance, which was absent in liver mitochondria. On the whole animal level, evidence of thermogenic function was further corroborated by an increased metabolic response to norepinephrine (NE) injection. Cold acclimation (18 degrees C) led to an increased basal metabolic rate relative to warm acclimation (28 degrees C) in E. myurus, but there was no evidence of additional recruitment of NE-induced NST capacity in response to cold acclimation. In summary, we showed that BAT and functional UCP1 are already present in a member of the Afrotheria, but the seasonal regulation and adaptive value of NST in Afrotherians remain to be elucidated.     

Uncoupling Protein 1 Decreases Superoxide Production in Brown Adipose Tissue Mitochondria

In thermogenic brown adipose tissue, uncoupling protein 1 (UCP1) catalyzes the dissipation of mitochondrial proton motive force as heat. In a cellular environment of high oxidative capacity such as brown adipose tissue (BAT), mitochondrial uncoupling could also reduce deleterious reactive oxygen species, but the specific involvement of UCP1 in this process is disputed. By comparing brown adipose tissue mitochondria of wild type mice and UCP1-ablated litter mates, we show that UCP1 potently reduces mitochondrial superoxide production after cold acclimation and during fatty acid oxidation. We address the sites of superoxide production and suggest diminished probability of “reverse electron transport” facilitated by uncoupled respiration as the underlying mechanism of reactive oxygen species suppression in BAT. Furthermore, ablation of UCP1 represses the cold-stimulated increase of substrate oxidation normally seen in active BAT, resulting in lower superoxide production, presumably avoiding deleterious oxidative damage. We conclude that UCP1 allows high oxidative capacity without promoting oxidative damage by simultaneously lowering superoxide production.     

Antioxidant properties of UCP1 are evolutionarily conserved in mammals and buffer mitochondrial reactive oxygen species

Mitochondrial uncoupling reduces reactive oxygen species (ROS) production and appears to be important for cellular signaling/protection, making it a focus for the treatment of metabolic and age-related diseases. Whereas the physiological role of uncoupling protein 1 (UCP1) of brown adipose tissue is established for thermogenesis, the function of UCP1 in the reduction of ROS in cold-exposed animals is currently under debate. Here, we investigated the role of UCP1 in mitochondrial ROS handling in the Lesser hedgehog tenrec (Echinops telfairi), a unique protoendothermic Malagasy mammal with recently identified brown adipose tissue (BAT). We show that the reduction of ROS by UCP1 activity also occurs in BAT mitochondria of the tenrec, suggesting that the antioxidative role of UCP1 is an ancient mammalian trait. Our analysis shows that the quantity of UCP1 displays strong control over mitochondrial hydrogen peroxide release, whereas other factors, such as mild cold, nonshivering thermogenesis, oxidative capacity, and mitochondrial respiration, do not correlate. Furthermore, hydrogen peroxide release from recoupled BAT mitochondria was positively associated with mitochondrial membrane potential. These findings led to a model of UCP1 controlling mitochondrial ROS release and, presumably, being controlled by high membrane potential, as proposed in the canonical model of “mild uncoupling”. Our study further promotes a conserved role for UCP1 in the prevention of oxidative stress, which was presumably established during evolution before UCP1 was physiologically integrated into nonshivering thermogenesis.     

Sources of heat during nonshivering thermogenesis in Djungarian hamsters: a dominant role of brown adipose tissue during cold adaptation

Summary To assess the thermogenic importance of BAT in Djungarian hamsters we removed about 40% of their BAT and compared their thermogenic abilities before and after the operation. BAT was weighed and assayed for its respiratory properties (Cox, mitochondria). Following removal of BAT we observed considerable reductions of NST. The comparison of NST with BAT weight and with respiratory properties of BAT following partial removal of BAT revealed that at least three different pathways for heat production were involved in NST. In cold-adapted hamsters (values for warm-adapted hamsters in parentheses) we estimated that 66.2% (37.0%) of all NST was produced by mitochondrial respiration in BAT; 16.3% (38.4%) was produced in other organ sites but required the presence of BAT, i.e. there was a mediatory action of BAT on thermogenesis in other organ sites. A further 11.5% (23%) of NST occurred outside of and independent of BAT. Mitochondrial respiration in BAT was the only compartment of NST which increased its contribution during cold adaptation (238 mW to 1,062 mW), whereas the other sources of heat remained largely unchanged.Abbreviations BAT brown adipose tissue - BATex partial removal of brown adipose tissue - BMR basal metabolic rate at thermoneutrality - Cox cytochrome c oxidase - NA noradrenaline - NST nonshivering thermogenesis     

Seasonal changes in thermogenesis and body mass in wild Mongolian gerbils (Meriones unguiculatus)

Seasonal adjustments in body mass (BM), nonshivering thermogenesis (NST) and several physiological, hormonal, and biochemical markers were measured in wild-trapped Mongolian gerbils (Meriones unguiculatus) from Inner Mongolia, China. Sexual differences were detected in BM, NST, brown adipose tissue (BAT) mass, and mitochondrial protein content. BM and NST in males were higher in winter (January) and spring (May) than in summer (August), and BM of females was also the highest in winter, but NST remained relatively constant throughout the year. Cytochrome c oxidase activity and mitochondrial uncoupling protein 1 (UCP1) content in BAT were enhanced in winter in males or females, respectively. Serum leptin concentration was the lowest in winter and positively correlated with BM and body fat mass but was negatively correlated with BAT UCP1 content. These data suggest that wild Mongolian gerbils do not depend on a decrease in BM, but instead increase their thermogenic capacity to cope with cold stress. Leptin may be involved in the seasonal regulation in energy balance and thermogenesis in field Mongolian gerbils.     

Effects of photoperiod on energy budgets and thermogenesis in Mongolian gerbils (Meriones unguiculatus)

(1)

To investigate the role of photoperiod on the regulation of energy budgets and thermogenesis in Mongolian gerbils, body mass (BM), body fat mass (BFM), basal metabolic rate (BMR), nonshivering thermogenesis (NST), gross energy intake (GEI), mitochondrial cytochrome c oxidase (COX) activity and uncoupling protein1 (UCP1) content of brown adipose tissue (BAT), and serum tri-iodothyronine (T₃), thyroxine (T₄) and leptin levels were measured.     

Browning deficiency and low mobilization of fatty acids in gonadal white adipose tissue leads to decreased cold-tolerance of transglutaminase 2 knock-out mice

During cold-exposure ‘beige’ adipocytes with increased mitochondrial content are activated in white adipose tissue (WAT). These cells, similarly to brown adipose tissue (BAT), dissipate stored chemical energy in the form of heat with the help of uncoupling protein 1 (UCP1). We investigated the effect of tissue transglutaminase (TG2) ablation on the function of ATs in mice. Although TG2^+/+ and TG2^−/− mice had the same amount of WAT and BAT, we found that TG2^+/+ animals could tolerate acute cold exposure for 4 h, whereas TG2^−/− mice only for 3 h. Both TG2^−/− and TG2^+/+ animals used up half of the triacylglycerol content of subcutaneous WAT (SCAT) after 3 h treatment; however, TG2^−/− mice still possessed markedly whiter and higher amount of gonadal WAT (GONAT) as reflected in the larger size of adipocytes and lower free fatty acid levels in serum. Furthermore, lower expression of ‘beige’ marker genes such as UCP1, TBX1 and TNFRFS9 was observed after cold exposure in GONAT of TG2^−/− mice, paralleled with a lower level of UCP1 protein and a decreased mitochondrial content. The detected changes in gene expression of Resistin and Adiponectin did not provoke glucose intolerance in the investigated TG2^−/− mice, and TG2 deletion did not influence adrenaline, noradrenaline, glucagon and insulin production. Our data suggest that TG2 has a tissue-specific role in GONAT function and browning, which becomes apparent under acute cold exposure.     

Uncoupling mechanism and redox regulation of mitochondrial uncoupling protein 1 (UCP1)

Brown adipose tissue (BAT) and brown in white (brite) adipose tissue, termed also beige adipose tissue, are major sites of mammalian nonshivering thermogenesis. Mitochondrial uncoupling protein 1 (UCP1), specific for these tissues, is the key factor for heat production. Recent molecular aspects of UCP1 structure provide support for the fatty acid cycling model of coupling, i.e. when UCP1 expels fatty acid anions in a uniport mode from the matrix, while uncoupling. Protonophoretic function is ensured by return of the protonated fatty acid to the matrix independent of UCP1. This mechanism is advantageous for mitochondrial uncoupling and compatible with heat production in a pro-thermogenic environment, such as BAT. It must still be verified whether posttranslational modification of UCP1, such as sulfenylation of Cys253, linked to redox activity, promotes UCP1 activity. BAT biogenesis and UCP1 expression, has also been linked to the pro-oxidant state of mitochondria, further endorsing a redox signalling link promoting an establishment of pro-thermogenic state. We discuss circumstances under which promotion of superoxide formation exceeds its attenuation by uncoupling in mitochondria and throughout point out areas of future research into UCP1 function.     

Maximal thermogenic capacity and non-shivering thermogenesis in the South American subterranean rodent Ctenomys talarum

Subterranean rodents inhabit closed tunnel systems that are hypoxic and hypercapnic and buffer aboveground ambient temperature. In contrast to other strictly subterranean rodents, Ctenomys talarum exhibits activity on the surface during foraging and dispersion and hence, is exposed also to the aboveground environment. In this context, this species is a valuable model to explore how the interplay between underground and aboveground use affects the relationship among basal metabolic rate (BMR), cold-induced maximum metabolic rate (MMR), shivering (ST), and non-shivering thermogenesis (NST). In this work, we provide the first evidence of the presence of NST, including the expression of uncoupling proteins in brown adipose tissue (BAT), and shivering thermogenesis in Ctenomys talarum, a species belonging to the most numerous subterranean genus, endemic to South America. Our results show no differences in BMR, cold-induced MMR, and NST between cold- (15?°C) and warm- (25?°C) acclimated individuals. Furthermore, thermal acclimation had no effect on the expression of mitochondrial uncoupling protein 1 (UCP1) in BAT. Only cytochrome c oxidase (COX) content and activity increased during cold acclimation. When interscapular BAT was removed, NST decreased more than 30?%, whereas cold-induced MMR remained unchanged. All together, these data suggest that cold-induced MMR reaches a maximum in warm-acclimated individuals and so a probable ceiling in NST and UCP1 expression in BAT. Possible thermogenic mechanisms explaining the increase in the oxidative capacity, mediated by COX in BAT of cold-acclimated individuals and the role of ST in subterranean life habits are proposed.     

Brown adipose tissue mitochondria: modulation by GDP and fatty acids depends on the respiratory substrates

The UCP1 [first UCP (uncoupling protein)] that is found in the mitochondria of brown adipocytes [BAT (brown adipose tissue)] regulates the heat production, a process linked to non-shivering thermogenesis. The activity of UCP1 is modulated by GDP and fatty acids. In this report, we demonstrate that respiration and heat released by BAT mitochondria vary depending on the respiratory substrate utilized and the coupling state of the mitochondria. It has already been established that, in the presence of pyruvate/malate, BAT mitochondria are coupled by faf-BSA (fatty-acid-free BSA) and GDP, leading to an increase in ATP synthesis and mitochondrial membrane potential along with simultaneous decreases in both the rates of respiration and heat production. Oleate restores the uncoupled state, inhibiting ATP synthesis and increasing the rates of both respiration and heat production. We now show that in the presence of succinate: (i) the rates of uncoupled mitochondria respiration and heat production are five times slower than in the presence of pyruvate/malate; (ii) faf-BSA and GDP accelerate heat and respiration as a result and, in coupled mitochondria, these two rates are accelerated compared with pyruvate/malate; (iii) in spite of the differences in respiration and heat production noted with the two substrates, the membrane potential and the ATP synthesized were the same; and (iv) oleate promoted a decrease in heat production and respiration in coupled mitochondria, an effect different from that observed using pyruvate/malate. These effects are not related to the production of ROS (reactive oxygen species). We suggest that succinate could stimulate a new route to heat production in BAT mitochondria.     

Suppression of thermogenic capacity during reproduction in primiparous brandt's voles (Microtus brandtii)

• (1)To investigate the changes of brown adipose tissue (BAT) thermogenic capacity in primiparous Brandt's voles during different phases of reproduction, BAT weight, mitochondrial protein concentration, cytochrome c oxidase (COX) activity, and uncoupling protein (UCP1) contents were measured.
• (2)Both cytochrome c oxidase activity and UCP1 contents decreased significantly during lactation, suggesting that thermogenic capacity was suppressed.
• (3)The decrease of thermogenic capacity during reproduction, especially during lactation, is compensation to the large demand of energy for reproduction. This is advantageous for energy conservation and lactation in Brandt's voles. UCP1 is the base of molecular thermogenesis of BAT in Brandt's voles.

     

PEX13 is required for thermogenesis of white adipose tissue in cold-exposed mice

Non-shivering thermogenesis (NST) is a heat generating process controlled by the mitochondria of brown adipose tissue (BAT). In the recent decade, ‘functionally’ acting brown adipocytes in white adipose tissue (WAT) has been identified as well: the so-called process of the ‘browning’ of WAT. While the importance of uncoupling protein 1 (UCP1)-oriented mitochondrial activation has been intensely studied, the role of peroxisomes during the browning of white adipocytes is poorly understood. Here, we assess the change in peroxisomal membrane proteins, or peroxins (PEXs), during cold stimulation and importantly, the role of PEX13 in the cold-induced remodeling of white adipocytes. PEX13, a protein that originally functions as a docking factor and is involved in protein import into peroxisome matrix, was highly increased during cold-induced recruitment of beige adipocytes within the inguinal WAT of C57BL/6 mice. Moreover, beige-induced 3 T3-L1 adipocytes and stromal vascular fraction (SVF) cells by exposure to the peroxisome proliferator-activated receptor gamma (PPARγ) agonist rosiglitazone showed a significant increase in mitochondrial thermogenic factors along with peroxisomal proteins including PEX13, and these were confirmed in SVF cells with the beta 3 adrenergic receptor (β3AR)-selective agonist CL316,243. To verify the relevance of PEX13, we used the RNA silencing method targeting the Pex13 gene and evaluated the subsequent beige development in SVF cells. Interestingly, siPex13 treatment suppressed expression of thermogenic proteins such as UCP1 and PPARγ coactivator 1 alpha (PGC1α). Overall, our data provide evidence supporting the role of peroxisomal proteins, in particular PEX13, during beige remodeling of white adipocytes.     

Immortal brown adipocytes from p53-knockout mice: differentiation and expression of uncoupling proteins

Brown adipose tissue (BAT) is the specific site for metabolic heat production in mammals. To establish a novel immortal brown adipocyte cell line, the stromal-vascular fraction containing preadipocytes was obtained from interscapular BAT of mice deficient of a tumor-suppressor gene p53. The p53-deficient cells, tentatively named as HB2 cells, could be cultured in vitro after repeated passages and differentiated into adipocytes in the presence of insulin, T3 and/or troglitazone, expressing some adipocyte-specific genes and accumulating intracellular lipid droplets. The mRNA level of uncoupling protein 1 (UCP1), a mitochondrial protein specifically present in brown adipocytes, was undetectable in HB2 preadipocytes, but increased after adipose differentiation. In HB2 adipocytes, UCP1 mRNA expression was markedly activated after stimulation of the beta-adrenergic receptor pathway. The mRNA of UCP2 and UCP3, recently cloned isoforms of UCP1, were also detected in HB2 adipocytes, but their levels were not influenced by adrenergic stimulation. Thus HB2 cells seem useful for in vitro studies of BAT and UCP functions.     

Tissue-specific expression and cold-induced mRNA levels of uncoupling proteins in the Djungarian hamster

The uncoupling protein 1 (UCP1), a mitochondrial transmembrane protein, is responsible for adaptive thermogenesis in brown adipose tissue (BAT). Two UCP1 homologues, UCP2 and UCP3, were recently discovered, but it is controversial whether they also play a role in energy homeostasis. Djungarian hamster UCPs were found to exhibit high similarity with homologues known in other species. UCP1 mRNA was restricted to BAT, UCP2 mRNA was expressed in multiple tissues, and UCP3 mRNA was detected mainly in BAT and skeletal muscles. We examined the cold-induced regulation of hamster UCP mRNA levels and tested their correlation with serum free fatty acid (FFA) concentrations. In BAT UCP1, UCP2, and UCP3 expression was upregulated in the cold, but the increase and time course of increase differed. In skeletal muscle, UCP2 and UCP3 mRNA levels were not altered. Cold-induced changes of serum FFA levels correlated with the stimulation of UCP1 mRNA in BAT but not with UCP2 and UCP3.     

Nonshivering thermogenesis in a marsupial (the tasmanian bettong Bettongia gaimardi) is not attributable to brown adipose tissue

The Tasmanian bettong (Bettongia gaimardi, a marsupial) is a rat-kangaroo that increases nonshivering thermogenesis (NST) in response to norepinephrine (NE). This study attempted to assess whether brown adipose tissue (BAT), a specialized thermogenic effector, is involved in NST in the bettong. Regulatory NST, indicated by resting oxygen consumption (Vo2) of the whole body, was measured under conscious conditions at 20 degrees C with various stimuli: cold (4 degrees -5 degrees C) or warm (25 degrees C) acclimation, NE injection, and the beta3-adrenoceptor agonist (BRL) 37344. In line with the functional studies in vivo, the presence of BAT was evaluated by examining the expression of the uncoupling protein 1 (UCP1) with both rat cDNA and oligonucleotide probes. Both NE and BRL 37344 significantly stimulated NST in the bettong. After cold acclimation of the animals (at 4 degrees -5 degrees C for 2 wk), the resting Vo2 was increased by 15% and the thermogenic effect of NE was enhanced; warm-acclimated animals showed a slightly depressed response. However, no expression of UCP1 was detected in bettongs either before or after cold exposure (2 wk). These data suggest that the observed NST in the marsupial bettong is not attributable to BAT.     

Tissue-specific functional interaction between apolipoproteins A1 and E in cold-induced adipose organ mitochondrial energy metabolism

White (WAT) and brown (BAT) adipose tissue, the two main types of adipose organ, are responsible for lipid storage and non-shivering thermogenesis, respectively. Thermogenesis is a process mediated by mitochondrial uncoupling protein 1 (UCP1) which uncouples oxidative phosphorylation from ATP production, leading to the conversion of free fatty acids to heat. This process can be triggered by exposure to low ambient temperatures, caloric excess, and the immune system. Recently mitochondrial thermogenesis has also been associated with plasma lipoprotein transport system. Specifically, apolipoprotein (APO) E3 is shown to have a bimodal effect on WAT thermogenesis that is highly dependent on its site of expression. Similarly, APOE2 and APOE4 differentially affect BAT and WAT mitochondrial metabolic activity in processes highly modulated by APOA1. Furthermore, the absence of classical APOA1 containing HDL (APOA1-HDL), is associated with no measurable non-shivering thermogenesis in WAT of mice fed high fat diet. Based on these previous observations which indicate important regulatory roles for both APOA1 and APOE in adipose tissue mitochondrial metabolic activity, here we sought to investigate the potential roles of these apolipoproteins in BAT and WAT metabolic activation in mice, following stimulation by cold exposure (7 °C). Our data indicate that APOA1-HDL promotes metabolic activation of BAT only in the presence of very low levels (virtually undetectable) of APOE3-containing HDL (APOE3-HDL), which acts as an inhibitor in this process. In contrast, induction of WAT thermogenesis is subjected to a more complicated regulation which requires the combined presence of both APOA1-HDL and APOE3-HDL.     

Fusion of the Endoplasmic Reticulum and Mitochondrial Outer Membrane in Rats Brown Adipose Tissue: Activation of Thermogenesis by Ca2+

Brown adipose tissue (BAT) mitochondria thermogenesis is regulated by uncoupling protein 1 (UCP 1), GDP and fatty acids. In this report, we observed fusion of the endoplasmic reticulum (ER) membrane with the mitochondrial outer membrane of rats BAT. Ca²⁺-ATPase (SERCA 1) was identified by immunoelectron microscopy in both ER and mitochondria. This finding led us to test the Ca²⁺ effect in BAT mitochondria thermogenesis. We found that Ca²⁺ increased the rate of respiration and heat production measured with a microcalorimeter both in coupled and uncoupled mitochondria, but had no effect on the rate of ATP synthesis. The Ca²⁺ concentration needed for half-maximal activation varied between 0.08 and 0.11 µM. The activation of respiration was less pronounced than that of heat production. Heat production and ATP synthesis were inhibited by rotenone and KCN.Liver mitochondria have no UCP1 and during respiration synthesize a large amount of ATP, produce little heat, GDP had no effect on mitochondria coupling, Ca²⁺ strongly inhibited ATP synthesis and had little or no effect on the small amount of heat released. These finding indicate that Ca²⁺ activation of thermogenesis may be a specific feature of BAT mitochondria not found in other mitochondria such as liver.     

Brown adipose tissue: Updates in cellular and molecular biology

Brown adipose tissue (BAT) is mainly composed of adipocytes, it is highly vascularized and innervated, and can be activated in adult humans. Brown adipocytes are responsible for performing non-shivering thermogenesis, which is exclusively mediated by uncoupling protein (UCP) -1 (a protein found in the inner mitochondrial membrane), the hallmark of BAT, responsible for the uncoupling of the proton leakage from the ATP production, therefore, generating heat (i.e. thermogenesis). Besides UCP1, other compounds are essential not only to thermogenesis, but also to the proliferation and differentiation of BAT, including peroxisome proliferator-activated receptor (PPAR) family, PPARgamma coactivator 1 (PGC1)-alpha, and PRD1-BF-1-RIZ1 homologous domain protein containing protein (PRDM) -16. The sympathetic nervous system centrally regulates thermogenesis through norepinephrine, which acts on the adrenergic receptors of BAT. This bound leads to the initialization of the many pathways that may activate thermogenesis in acute and/or chronic ways. In summary, this mini-review aims to demonstrate the latest advances in the knowledge of BAT.