Human brown adipose tissue: Underestimated target in metabolic disease?

Active brown adipose tissue (BAT) has, since it rediscovery in adult humans in 2009, received much attention for its ability to increase energy expenditure when activated. By means of mitochondrial uncoupling activity BAT's main function is to produce heat instead of storing energy such as in white adipose tissue (WAT). Therefore, BAT is considered a new potential target to treat obesity and the metabolic syndrome. However, the contribution of this thermogenic tissue is still a matter of debate among researchers.The aim of this review is to give an overview of the differences between classical brown adipocytes and inducible beige adipocytes in humans, and the potential activators of BAT in humans. Furthermore newly described genetic markers for identification of these two types of brown adipocytes are examined. Finally, the potential of the current measurement techniques, and the contribution of BAT activity to whole body energy expenditure are discussed.     

Glycerokinase activity in brown adipose tissue: a sympathetic regulation?

The effect of brown adipose tissue (BAT) sympathetic hemidenervation on the activity of glycerokinase (GyK) was investigated in different physiological conditions. In rats fed a balanced diet, the activity of the enzyme was approximately 50% lower in BAT-denervated pads than in intact, innervated pads. In rats adapted to a high-protein, carbohydrate-free diet, norepinephrine turnover rates and BAT GyK activity were already reduced, and BAT denervation resulted in a further decrease in the activity of the enzyme. Cold acclimation of normally fed rats at 4 degrees C for 10 days markedly increased the activity of the enzyme. Cold exposure (4 degrees C) for 6 h was insufficient to stimulate BAT GyK, but the activity of the enzyme was already increased after 12 h of cold exposure. The cold-induced BAT GyK stimulation was completely blocked in BAT-denervated pads. The data indicate that an adequate sympathetic flow to BAT is required for the maintenance of normal levels of GyK activity and for the enzyme response to situations, such as cold exposure, which markedly increase BAT sympathetic flow.     

Iodothyronine 5′-deiodinase activity in rat brown adipose tissue during development

Iosothyronine 5′-deiodinase activity in rat brown adipose tissue has a characteristic pattern of developmental changes that is completely different from that of the liver. Fetal brown fat exhibits an extremely high iodothyronine 5′-deiodinase activity that is approx. 10-fold that in adult rats. Even though brown fat iodothyronine 5′-deiodinase activity falls suddenly at birth, there is a new peak in the activity around days 5–7 of life, whereas it remaines very low afterwards. Just after birth, brown adipose tissue iodothyronine 5′-deiodinase activity is already capable of stimulation by noradrenaline. The postnatal peak in brown fat iodothyronine 5′-deiodinase correlates with the known increase in the thermogenic activity of the tissue in the neonatal rat, thus reinforcing the suggestion that local 3′,3,5-triiodothyronine generation could be an important event related to thermogeneis in brown adispose tissue. However, the high fetal activity was only slightly related to the thermogenic activity of brown fat. Moreover, the increased iodothyronine 5′-deiodinase activity of brown adipose tissue during fetal and neonatal life suggests a substantial contribution by brown fat in the overall extrahydroidal 3′,3,5-triiodothyronine production in these physiological periods.     

Metabolic effects of tumour necrosis factor-α on rat brown adipose tissue

Intravenous administration of a single dose (100 g/kg bw) of recombinant tumour necrosis factor- (TNF, cachectin) to rats increased the rate ofin vitro fatty acid synthesis in interscapular brown adipose tissue (IBAT) from both glucose and alanine, without changes in the oxidation of these substrates to¹⁴CO₂. Lactate production and glycerol release were also unaffected by treatment with the cytokine. Additionally, the presence of TNF in the incubation media did not affect fatty acid synthesis, suggesting an indirect effect of the cytokine. The activities of different enzymes of glucose and alanine metabolism such as hexokinase, phosphofructokinase, pyruvate kinase, glucose-6-phosphate dehydrogenase and alanine transaminase, did not suffer changes as a consequence of TNF administration. The same applied to the enzymatic activities involved in fatty acid synthesis such as fatty acid synthase, acetyl-CoA carboxylase and ATP-citrate lyase. Conversely, citrate levels in IBAT were increased in animals treated with TNF, suggesting that it could be the cause for the increased fatty acid synthesis in this tissue.     

Increased α1 receptor density in brown adipose tissue of cafeteria-fed rats

A possible general corollary between ₁-receptor density in brown adipose tissue and the degree of activation of the tissue was investigated. For this purpose, the effect of cafeteria feeding on ₁-adrenergic receptors in brown adipose tissue of seven-week-old female rats was studied by the use of the ₁-antagonist (³H)prazosin. In cafeteria-fed rats, the K_D of the ₁-receptor for (³H)prazosin was unchanged (about 0.35 nM), but the receptor density was doubled (up to 40 fmol per mg of membrane protein). This was also observed when the results were expressed per unit of a plasma-membrane marker (5-nucleotidase). It was concluded that an increased ₁-receptor density is seen not only in cold-acclimated rats, but also in other conditions where brown fat is activated, and a possible general physiological significance of ₁-adrenergic pathways in brown adipose tissue is discussed.     

Is the "mammalian" brown fat-specific mitochondrial uncoupling protein present in adipose tissues of birds?

1. Mitochondria were isolated from the furcular, subcutaneous, abdominal, nape and lateral adipose tissue depots of five species of bird (pheasant, Japanese quail, pigeon, house sparrow and great tit) acclimatized to the Northern winter. 2. Mitochondrial proteins were separated by SDS-polyacrylamide gel electrophoresis, blotted onto nitrocellulose membranes, and probed for the presence of the 32,000-33,000 Mr uncoupling protein characteristic of "mammalian" brown adipose tissue, using an anti-(ground squirrel uncoupling protein)serum. 3. Immunoreactivity consistent with uncoupling protein was not detected in mitochondria from any of the avian adipose tissues. Immunoreactivity was, however, evident in mitochondria from perirenal or interscapular adipose tissue from a range of mammals--rats, mice, golden hamsters, Orkney voles, wood mice, pipistrelle bats, wood lemmings, and newborn lambs, cattle, reindeer and red deer. 4. These results provide biochemical evidence that "mammalian-like" thermogenic brown adipose tissue is absent from avian species; adipose tissues in birds appear to be functionally "white".     

Do molluscs possess indoleamine 2,3-dioxygenase?

Indoleamine 2,3-dioxygenase (IDO)-like myoglobin (Mb) was discovered in 1989 in the buccal mass of the abalone Sulculus diversicolor, and it has since been isolated from several archaegastropods. The amino acid sequences and genomic structures of IDO-like Mbs show significant homology with those of mammalian IDOs, suggesting that they have evolved from a common ancestral gene. However, details of the evolutionary relationships between them remain unknown. Here, we isolated a novel multicopy gene from Sulculus named molluscan IDO-like protein (MIP). The amino acid sequences of MIPs show the highest homology (about 60% identity) with Sulculus IDO-like Mb, and their exon/intron structures are also highly homologous. However, MIPs are mainly expressed in the gut whereas IDO-like Mb was found only in the buccal mass, suggesting that MIPs are not simply isoforms of IDO-like Mb. A bacterial expression study showed that MIP is a heme-binding protein, and that His335 is the proximal ligand of heme. Although we could not detect IDO activity using a recombinant glutathione S-transferase (GST)-MIP fusion protein in the present study, MIP should have some function other than that of an oxygen carrier like myoglobin, and it might in fact be molluscan IDO.     

α- andβ-adrenergic control of thermogenin mRNA expression in brown adipose tissue

By the use of an earlier characterised cDNA clone, CIN-1, corresponding to a sequence of the mRNA coding for the brown-fat specific uncoupling protein, thermogenin, the amount of thermogenin mRNA found in the brown adipose tissue of mice was quantitatively investigated under different physiological and pharmacological conditions.It was found that a 4 hr cold stress led to a 7-fold increase in the amount of thermogenin mRNA; injection of norepinephrine had a significant but smaller effect. Most notably, isoprenaline (-agonist) and phenylephrine (-agonist) had in themselves no effect, but when injected together were able to increase the mRNA level synergistically. In 4 hr cold-stressed mice, norepinephrine, isoprenaline and cholera toxin could all further potentiate the effect of the cold stress itself on the mRNA level. Insulin and the glucocorticoid dexamethasone both had weak stimulatory effects on the mRNA level.It is concluded that an increase in intracellular cAMP levels is a necessary and perhaps sufficient stimulus for the increase in thermogenin gene expression. However, at least underin vivo conditions, this increase requires stimulation of both - and-adrenergic pathways.     

Do cereal mixed-linked β-glucans possess immune-modulating activities?

β-glucans are known for their immune-modulating properties. However, the heterogeneity of these glucose polymers makes a distinction between the different sources and structures necessary-a fact that has been little allowed for in the literature. We have focused on β-glucans from cereals as they are already used as functional food ingredients due to their established cholesterol lowering effect. Cereal β-glucans have shown in vitro activity on cytokine secretion, phagocytic activity and cytotoxicity of isolated immune cells, and activation of the complement system. Animal studies suggest a possible protective effect against an intestinal parasite, against bacterial infection, and a synergistic effect in antibody-dependent cellular cytotoxicity. Animal studies have shown activity of orally applied cereal β-glucans indicating uptake or interaction with cells of the gastrointestinal tract. However, uptake is still debated, interaction with intestinal epithelial cells has been suggested but not clarified, and mechanisms of action remain largely unknown. So far, cereal β-glucans have not shown immune modulation in the few conducted human studies and further studies are needed to clarify their effect.     

Effects of ovariectomy and 17-β estradiol replacement on rat brown adipose tissue mitochondrial function

Taking into account the sexual dimorphism previously reported regarding mitochondrial function and biogenesis in brown adipose tissue, the aim of the present study was to go further into these differences by investigating the effect of ovariectomy and 17-β estradiol (E2) replacement on brown adipose tissue mitochondrial function. In this study, fourteen-week-old control female and ovariectomized female Wistar rats were used. Rats were ovariectomized at 5 weeks of age and were treated every 2 days with placebo (OVX group) or E2 (10 μg/kg) (OVX+E2 group) for 4 weeks before sacrifice. We studied the levels of oxidative capacity, antioxidant defence and oxidative damage markers in brown adipose tissue. Moreover, the levels of key elements of mitochondrial biogenesis as well as UCP1 protein levels, as an index of mitochondrial thermogenic capacity, were also determined. In response to ovariectomy, mitochondrial proliferation increased, resulting in less functional mitochondria, since oxidative capacity and antioxidant defences decreased. Although E2 supplementation was able to restore the serum levels of E2 shown by control rats, the treatment reverted the effects of the ovariectomy only in part, and oxidative and antioxidant capacities in OVX+E2 rats did not reach the levels shown by control females. Taking these results into account, we suggest that ovarian hormones are responsible, at least in part, for the sexual dimorphism in BAT mitochondrial function. However, other signals produced by ovary, rather than E2, would play an important role in the control of mitochondrial function in BAT.     

Do tropical wetland plants possess convective gas flow mechanisms?

? Internal pressurization and convective gas flow, which can aerate wetland plants more efficiently than diffusion, are common in temperate species. Here, we present the first survey of convective flow in a range of tropical plants. ? The occurrence of pressurization and convective flow was determined in 20 common wetland plants from the Mekong Delta in Vietnam. The diel variation in pressurization in culms and the convective flow and gas composition from stubbles were examined for Eleocharis dulcis, Phragmites vallatoria and Hymenachne acutigluma, and related to light, humidity and air temperature. ? Nine of the 20 species studied were able to build up a static pressure of > 50 Pa, and eight species had convective flow rates higher than 1 ml min(-1). There was a clear diel variation, with higher pressures and flows during the day than during the night, when pressures and flows were close to zero. ? It is concluded that convective flow through shoots and rhizomes is a common mechanism for below-ground aeration of tropical wetland plants and that plants with convective flow might have a competitive advantage for growth in deep water.     

Are hypothalamic neurons transsynaptically connected to porcine adipose tissue?

Specific anatomical sites and pathways responsible for mediating metabolic and neuroendocrine effects of leptin are still poorly understood. Therefore, we examined distribution of leptin receptor-containing neurons transsynaptically connected with the porcine fat tissue by means of combined viral transneuronal tracing and immunohistochemical staining method. Pseudorabies virus (PRV) was injected into the perirenal fat tissue in pigs, and after survival periods of 3, 5, 7, 9, and 11 days, hypothalami were processed immunohistochemically with primary antisera against PRV and leptin receptor (OBR). PRV labeled neurons were found in paraventricular nucleus (PVN), ventromedial nucleus (VMN), anterior hypothalamic area (AHA), preoptic area (PA), arcuate nucleus (ARC), and supraoptic nucleus (SON) by nine days after injection of the virus. Double-labeling immunofluorescence demonstrated that OBR were co-localized in nearly all virus-infected neurons. The present results provide the first morphological data demonstrating a multisynaptic circuit of neurons of CNS origin which innervates porcine fat tissue.     

Do wild brown lemurs reconcile? Not always

Post-conflict behaviour has been widely investigated in anthropoid primates but not extensively in prosimians. Here, we report the reconciliation pattern of a 14-individual group of wild brown lemurs (Eulemur fulvus) in the Berenty gallery forest (South Madagascar). We found that reconciliation occurs only in the non-feeding context and that it works in reducing the risk of renewed aggression. Thus, reconciliation would provide an immediate positive feedback that is probably independent of the quality of the relationship between opponents. Reconciliation may also be a hic-et-nunc mechanism, needed to avoid conflict spreading across group members, possibly leading to social disruption.     

Do test tube-grown pathogenic mycobacteria possess a protective capsule?

Abstract Intracellularly-growing pathogenic mycobacteria are surrounded inside host-phagosomes by a protective capsule, also called the 'electron-transparent zone' (ETZ). Whether this capsule is formed during active intracellular growth of these bacteria or is present even in test tube-growing bacilli is not yet elucidated. The present electron microscopic investigation used immunoelectron cytochemistry and also a new method using gelatin-Lowicryl embedding. This morphological study showed for the first time that test tube-grown pathogenic species ( Mycobacterium avium and M. tuberculosis ) are effectively surrounded by a capsule-like structure resembling intramacrophagic ETZ which was absent in the nonpathogenic species M. smegmatis and M. aurum .     

Can non-shivering thermogenesis in brown adipose tissue following NA injection be quantified by changes in overlying surface temperatures using infrared thermography?

We aimed to investigate whether infra red thermography (IRT) can be used to measure and quantify non-shivering thermogenesis (NST) in the short-tailed field vole Microtus agrestis, by directly comparing it with a standard method, i.e. metabolic response following Noradrenaline injection (NA). Mean skin surface temperature overlying Brown adipose tissue (BAT) depot was 0.82 degrees C higher than mean surface temperature that did not overly BAT. The difference in temperature increased by 1.26 degrees C after NA was administered. Mean skin surface temperature overlying BAT increased by 0.32 degrees C after NA was administered; however, surface temperature decreased by 1.32 degrees C after saline was administered. Mean skin surface temperature overlying BAT did not change significantly between warm and cold acclimated voles; in contrast metabolic peak following NA injection significantly increased in cold acclimated voles. There was no significant correlation between change in surface temperature after NA injection and metabolic peak following NA injection. The results of this study suggest that IRT is not a sensitive enough method to measure changes in NST capacity in BAT following NA injection, or to detect changes in NST capacity induced by cold acclimation. However, IRT can distinguish between skin surfaces overlying BAT and skin surfaces that do not.