Adaptive thermogenesis and thermal conductance in wild-type and UCP1-KO mice

Brite/beige fat and UCP1 - is it thermogenesis?

The presence of two distinct types of adipose tissue, which have opposing functions, has been known for decades. White adipose tissue (WAT) is the main tissue of energy storage, while brown adipose tissue (BAT) dissipates energy as heat and is required for non-shivering thermoregulation. In the last few years, a third type of adipocyte was identified, termed the brite ("brown and white") or beige adipocyte. Their physiological control and role, however, are not fully clarified. Brite/beige adipocytes have a positive impact on systemic metabolism that is generally explained by the thermogenesis of brite/beige adipocytes; although thermogenesis has not been directly measured but is mostly inferred by gene expression data of typical thermogenic genes such as uncoupling protein 1 (UCP1). Here we critically review functional evidence for the thermogenic potential of brite/beige adipocytes, leading to the conclusion that direct measurements of brite/beige adipocyte bioenergetics, beyond gene regulation, are pivotal to quantify their thermogenic potential. In particular, we exemplified that the massive induction of UCP1 mRNA during the browning of isolated subcutaneous adipocytes in vitro is not reflected in significant alterations of cellular bioenergetics. Herein, we demonstrate that increases in mitochondrial respiration in response to beta-adrenergic stimulus can be independent of UCP1. Using HEK293 cells expressing UCP1, we show how to directly assess UCP1 function by adequate activation in intact cells. Finally, we provide a guide on the interpretation of UCP1 activity and the pitfalls by solely using respiration measurements. The functional analysis of beige adipocyte bioenergetics will assist to delineate the impact of browning on thermogenesis, possibly elucidating additional physiological roles and its contribution to systemic metabolism, highlighting possible avenues for future research. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

Dietary fucoxanthin increases metabolic rate and upregulated mRNA expressions of the PGC-1alpha network, mitochondrial biogenesis and fusion genes in white adipose tissues of mice

The mechanism for how fucoxanthin (FX) suppressed adipose accumulation is unclear. We aim to investigate the effects of FX on metabolic rate and expressions of genes related to thermogenesis, mitochondria biogenesis and homeostasis. Using a 2 × 2 factorial design, four groups of mice were respectively fed a high sucrose (50% sucrose) or a high-fat diet (23% butter + 7% soybean oil) supplemented with or without 0.2% FX. FX significantly increased oxygen consumption and carbon dioxide production and reduced white adipose tissue (WAT) mass. The mRNA expressions of peroxisome proliferator-activated receptor (PPAR) γ coactivator-1α (PGC-1α), cell death-inducing DFFA-like effecter a (CIDEA), PPARα, PPARγ, estrogen-related receptor α (ERRα), β3-adrenergic receptor (β3-AR) and deiodinase 2 (Dio2) were significantly upregulated in inguinal WAT (iWAT) and epididymal WAT (eWAT) by FX. Mitochondrial biogenic genes, nuclear respiratory factor 1 (NRF1) and NRF2, were increased in eWAT by FX. Noticeably, FX upregulated genes of mitochondrial fusion, mitofusin 1 (Mfn1), Mfn2 and optic atrophy 1 (OPA1), but not mitochondrial fission, Fission 1, in both iWAT and eWAT. In conclusion, dietary FX enhanced the metabolic rate and lowered adipose mass irrespective of the diet. These were associated with upregulated genes of the PGC-1α network and mitochondrial fusion in eWAT and iWAT.

RIP140 represses the "brown-in-white" adipocyte program including a futile cycle of triacylglycerol breakdown and synthesis

Receptor-interacting protein 140 (RIP140) is a corepressor of nuclear receptors that is highly expressed in adipose tissues. We investigated the role of RIP140 in conditionally immortal preadipocyte cell lines prepared from white or brown fat depots. In white adipocytes, a large set of brown fat-associated genes was up-regulated in the absence of RIP140. In contrast, a relatively minor role can be ascribed to RIP140 in the control of basal gene expression in differentiated brown adipocytes because significant changes were observed only in Ptgds and Fabp3. The minor role of RIP140 in brown adipocytes correlates with the similar histology and uncoupling protein 1 and CIDEA staining in knockout compared with wild-type brown adipose tissue (BAT). In contrast, RIP140 knockout sc white adipose tissue (WAT) shows increased numbers of multilocular adipocytes with elevated staining for uncoupling protein 1 and CIDEA. Furthermore in a white adipocyte cell line, the markers of BRITE adipocytes, Tbx1, CD137, Tmem26, Cited1, and Epsti1 were repressed in the presence of RIP140 as was Prdm16. Microarray analysis of wild-type and RIP140-knockout white fat revealed elevated expression of genes associated with cold-induced expression or high expression in BAT. A set of genes associated with a futile cycle of triacylglycerol breakdown and resynthesis and functional assays revealed that glycerol kinase and glycerol-3-phosphate dehydrogenase activity as well as [³H]glycerol incorporation were elevated in the absence of RIP140. Thus, RIP140 blocks the BRITE program in WAT, preventing the expression of brown fat genes and inhibiting a triacylglycerol futile cycle, with important implications for energy homeostasis.

Intact brown adipose tissue thermogenesis is required for restorative sleep responses after sleep loss

Metabolic signals related to feeding and body temperature regulation have profound effects on vigilance. Brown adipose tissue (BAT) is a key effector organ in the regulation of metabolism in several species, including rats and mice. Significant amounts of active BAT are also present throughout adulthood in humans. The metabolic activity of BAT is due to the tissue-specific presence of the uncoupling protein-1 (UCP-1). To test the involvement of BAT thermogenesis in sleep regulation, we investigated the effects of two sleep-promoting stimuli in UCP-1-deficient mice. Sleep deprivation by gentle handling increased UCP-1 mRNA expression in BAT and elicited rebound increases in non-rapid-eye-movement sleep and rapid-eye-movement sleep accompanied by elevated slow-wave activity of the electroencephalogram. The rebound sleep increases were significantly attenuated, by ~ 35-45%, in UCP-1-knockout (KO) mice. Wild-type (WT) mice with capsaicin-induced sensory denervation of the interscapular BAT pads showed similar impairments in restorative sleep responses after sleep deprivation, suggesting a role of neuronal sleep-promoting signaling from the BAT. Exposure of WT mice to 35 °C ambient temperature for 5 days led to increased sleep and body temperature and suppressed feeding and energy expenditure. Sleep increases in the warm environment were significantly suppressed, by ~ 50%, in UCP-1-KO animals while their food intake and energy expenditure did not differ from those of the WTs. These results suggest that the metabolic activity of the BAT plays a role in generating a metabolic environment that is permissive for optimal sleep. Impaired BAT function may be a common underlying cause of sleep insufficiency and metabolic disorders.

Hypothermia activates adipose tissue to promote malignant lung cancer progression

Microenvironment has been increasingly recognized as a critical regulator of cancer progression. In this study, we identified early changes in the microenvironment that contribute to malignant progression. Exposure of human bronchial epithelial cells (BEAS-2B) to methylnitrosourea (MNU) caused a reduction in cell toxicity and an increase in clonogenic capacity when the temperature was lowered from 37°C to 28°C. Hypothermia-incubated adipocyte media promoted proliferation in A549 cells. Although a hypothermic environment could increase urethane-induced tumor counts and Lewis lung cancer (LLC) metastasis in lungs of three breeds of mice, an increase in tumor size could be discerned only in obese mice housed in hypothermia. Similarly, coinjections using differentiated adipocytes and A549 cells promoted tumor development in athymic nude mice when adipocytes were cultured at 28°C. Conversely, fat removal suppressed tumor growth in obese C57BL/6 mice inoculated with LLC cells. Further studies show hypothermia promotes a MNU-induced epithelial-mesenchymal transition (EMT) and protects the tumor cell against immune control by TGF-β1 upregulation. We also found that activated adipocytes trigger tumor cell proliferation by increasing either TNF-α or VEGF levels. These results suggest that hypothermia activates adipocytes to stimulate tumor boost and play critical determinant roles in malignant progression.

Assessment of brown adipose tissue function

In this review we discuss practical considerations for the assessment of brown adipose tissue in rodent models, focusing on mice. The central aim of the review is to provide a critical appraisal of the utility of specialized techniques for assessing brown adipose tissue function in vivo. We cover several of the most common specialized methods for analysing brown adipose tissue function in vivo, including assessment of maximal thermogenic capacity by indirect calorimetry and the measurement of sympathetic tone to brown adipose tissue. While these techniques are powerful, they are not readily available to all laboratories; therefore we also cover several simple measurements that, particularly in combination, can be used to determine if a mouse model is likely to have alterations in brown adipose tissue function. Such techniques include: pair feeding, analysis of brown adipose tissue lipid content and mRNA and protein markers of brown adipose tissue activation.

Anti-Obesity Effect of n-3 Polyunsaturated Fatty Acids in Mice Fed High-Fat Diet Is Independent of Cold-Induced Thermogenesis

Long-chain n-3 polyunsaturated fatty acids (LC n-3 PUFA) exert beneficial effects on health and they could help to prevent development of obesity and associated metabolic disorders. In our previous studies in mice fed high-fat (cHF; ~60 % calories as fat) diet and maintained at 20 °C, dietary LC n-3 PUFA could counteract accretion of body fat, without inducing mitochondrial uncoupling protein 1 (UCP1) in adipose tissue, suggesting that the anti-obesity effect was not linked to adaptive (UCP1-mediated) thermogenesis. To exclude a possible dependence of the anti-obesity effect on any mechanism inducible by cold, experiments were repeated in mice maintained at thermoneutrality (30 °C). Male C57BL/6J mice were fed either cHF diet, or cHF diet supplemented with LC n-3 PUFA, or standard diet for 7 months. Similarly as at 20 °C, the LC n-3 PUFA supplementation reduced accumulation of body fat, preserved lipid and glucose homeostasis, and induced fatty acid re-esterification in epididymal white adipose tissue. Food consumption was not affected by LC n-3 PUFA intake. Our results demonstrated anti-obesity metabolic effect of LC n-3 PUFA, independent of cold-induced thermogenesis and they suggested that induction of fatty acid re-esterification creating a substrate cycle in white fat, which results in energy expenditure, could contribute to the anti-obesity effect.

Detection of thermogenesis in rodents in response to anti-obesity drugs and genetic modification

Many compounds and genetic manipulations are claimed to confer resistance to obesity in rodents by raising energy expenditure. Examples taken from recent and older literature, demonstrate that such claims are often based on measurements of energy expenditure after body composition has changed, and depend on comparisons of energy expenditure divided by body weight. This is misleading because white adipose tissue has less influence than lean tissue on energy expenditure. Application of this approach to human data would suggest that human obesity is usually due to a low metabolic rate, which is not an accepted view. Increased energy expenditure per animal is a surer way of demonstrating thermogenesis, but even then it is important to know whether this is due to altered body composition (repartitioning), or increased locomotor activity rather than thermogenesis per se. Regression analysis offers other approaches. The thermogenic response to some compounds has a rapid onset and so cannot be due to altered body composition. These compounds usually mimic or activate the sympathetic nervous system. Thermogenesis occurs in, but may not be confined to, brown adipose tissue. It should not be assumed that weight loss in response to these treatments is due to thermogenesis unless there is a sustained increase in 24-h energy expenditure. Thyroid hormones and fibroblast growth factor 21 also raise energy expenditure before they affect body composition. Some treatments and genetic modifications alter the diurnal rhythm of energy expenditure. It is important to establish whether this is due to altered locomotor activity or efficiency of locomotion. There are no good examples of compounds that do not affect short-term energy expenditure but have a delayed effect. How and under what conditions a genetic modification or compound increases energy expenditure influences the decision on whether to seek drugs for the target or take a candidate drug into clinical studies.

Dynamic changes in lipid droplet-associated proteins in the "browning" of white adipose tissues

The morphological and functional differences between lipid droplets (LDs) in brown (BAT) and white (WAT) adipose tissues will largely be determined by their associated proteins. Analysing mRNA expression in mice fat depots we have found that most LD protein genes are expressed at higher levels in BAT, with the greatest differences observed for Cidea and Plin5. Prolonged cold exposure, which induces the appearance of brown-like adipocytes in mice WAT depots, was accompanied with the potentiation of the lipolytic machinery, with changes in ATGL, CGI-58 and G0S2 gene expression. However the major change detected in WAT was the enhancement of Cidea mRNA. Together with the increase in Cidec, it indicates that LD enlargement through LD-LD transference of fat is an important process during WAT browning. To study the dynamics of this phenotypic change, we have applied 4D confocal microscopy in differentiated 3T3-L1 cells under sustained β-adrenergic stimulation. Under these conditions the cells experienced a LD remodelling cycle, with progressive reduction on the LD size by lipolysis, followed by the formation of new LDs, which were subjected to an enlargement process, likely to be CIDE-triggered, until the cell returned to the basal state. This transformation would be triggered by the activation of a thermogenic futile cycle of lipolysis/lipogenesis and could facilitate the molecular mechanism for the unilocular to multilocular transformation during WAT browning. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.

UCP1 mRNA does not produce heat

Because of the possible role of brown adipose tissue and UCP1 in metabolic regulation, even in adult humans, there is presently considerable interest in quantifying, from in-vitro data, the thermogenic capacities of brown and brite/beige adipose tissues. An important issue is therefore to establish which parameters are the most adequate for this. A particularly important issue is the relevance of UCP1 mRNA levels as estimates of the degree of recruitment and of the thermogenic capacity resulting from differences in physiological conditions and from experimental manipulations. By solely following UCP1 mRNA levels in brown adipose tissue, the conclusion would be made that the tissue's highest activation occurs after only 6h in the cold and then successively decreases to being only some 50% elevated after 1month in the cold. However, measurement of total UCP1 protein levels per depot ("mouse") reveals that the maximal thermogenic capacity estimated in this way is reached first after 1month but represents an approx. 10-fold increase in thermogenic capacity. Since this in-vitro measure correlates quantitatively and temporally with the acquisition of nonshivering thermogenesis, this must be considered the most physiologically relevant parameter. Similarly, observations that cold acclimation barely increases UCP1 mRNA levels in classical brown adipose tissue but leads to a 200-fold increase in UCP1 mRNA levels in brite/beige adipose tissue depots may overemphasise the physiological significance of these depots, as the high fold-increases are due to very low initial levels, and the UCP1 mRNA levels reached are at least an order of magnitude lower than in brown adipose tissue; furthermore, based on total UCP1 protein amounts, the brite/beige depots attain only about 10% of the thermogenic capacity of the classical brown adipose tissue depots. Consequently, inadequate conclusions may be reached if UCP1 mRNA levels are used as a proxy for the metabolic significance of recruited versus non-recruited brown adipose tissue and for estimating the metabolic significance of brown versus brite/beige adipose tissues. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.

Searching for ways to switch on brown fat: are we getting warmer?

Obesity rates are increasing alongside those of its co-morbidities, placing a huge strain on health systems across the globe. Evidence points to inappropriate levels of ectopic lipid accumulation outside of adipose tissue being a major factor in the progression of many of these diseases. Brown adipose tissue (BAT) has a huge capacity to remove lipids from the circulatory system to fuel thermogenesis. Multiple studies have now confirmed the existence of active BAT in adult humans, making strategies aimed at activating it a potential therapeutic option in obese subjects. In recent years, researchers working in murine models have found a wide range of endogenous molecules with specific roles regulating BAT. These findings place BAT firmly within the wider network of physiological regulation covering global metabolism. They also highlight the possibility of targeting thermogenesis in a safe and specific manner to remove potentially harmful lipids released from stressed or failing white adipose tissue in obese states.

A VASP-Rac-soluble guanylyl cyclase pathway controls cGMP production in adipocytes

The ubiquitous second messenger cyclic guanosine monophosphate (cGMP) plays an important role in metabolism and promotes brown adipocyte differentiation. We showed that ablation of the gene encoding vasodilator-stimulated phosphoprotein (VASP), a major downstream component of the cGMP signaling cascade, increased cellular cGMP content in brown and white adipocytes and mouse embryonic fibroblasts. VASP-deficient cells showed increased activation of Rac1, which in turn increased the abundance of the cGMP-producing enzyme soluble guanylyl cyclase (sGC), the main receptor for nitric oxide. Consequently, loss of VASP caused increased cGMP concentrations and enhanced brown adipocyte differentiation. Consistent with the in vitro data, we found increased energy expenditure in VASP-deficient mice and exposure to cold triggered enhanced lipolysis and cellular respiration in VASP-deficient brown fat cells. In addition, VASP-deficient mice exhibited increased development of brown-like adipocytes in white fat. Our data revealed that a VASP to Rac to sGC negative feedback loop limited cGMP production, thereby regulating adipogenesis and energy homeostasis.

Functional characterization of UCP1 in mammalian HEK293 cells excludes mitochondrial uncoupling artefacts and reveals no contribution to basal proton leak

Mechanistic studies on uncoupling proteins (UCPs) not only are important to identify their cellular function but also are pivotal to identify potential drug targets to manipulate mitochondrial energy transduction. So far, functional and comparative studies of uncoupling proteins in their native environment are hampered by different mitochondrial, cellular and genetic backgrounds. Artificial systems such as yeast ectopically expressing UCPs or liposomes with reconstituted UCPs were employed to address crucial mechanistic questions but these systems also produced inconsistencies with results from native mitochondria. We here introduce a novel mammalian cell culture system (Human Embryonic Kidney 293 - HEK293) to study UCP1 function. Stably transfected HEK293 cell lines were derived that contain mouse UCP1 at concentrations comparable to tissue mitochondria. In this cell-based test system UCP1 displays native functional behaviour as it can be activated with fatty acids (palmitate) and inhibited with purine nucleotides guanosine-diphosphate (GDP). The catalytic centre activity of the UCP1 homodimer in HEK293 is comparable to activities in brown adipose tissue supporting functionality of UCP1. Importantly, at higher protein levels than in yeast mitochondria, UCP1 in HEK293 cell mitochondria is fully inhibitable and does not contribute to basal proton conductance, thereby emphasizing the requirement of UCP1 activation for therapeutic purposes. These findings and resulting analysis on UCP1 characteristics demonstrate that the mammalian HEK293 cell system is suitable for mechanistic and comparative functional studies on UCPs and provides a non-confounding mitochondrial, cellular and genetic background.

Chronic cold acclimation increases thermogenic capacity, non-shivering thermogenesis and muscle citrate synthase activity in both wild-type and brown adipose tissue deficient mice

The purpose of this study was to determine whether chronic cold exposure would increase the aerobic capacity of skeletal muscle in UCP-dta mice, a transgenic line lacking brown adipose tissue (BAT). Wild type and UCP-dta mice were acclimated to either warm (23 °C), or cold (4 °C) conditions. Cold increased muscle oxidative capacity nearly equivalently in wild-type and UCP-dta mice, but did not affect the respiratory function of isolated mitochondria. Summit metabolism ( ̇V O2summit) and norepinephrine-induced thermogenesis ( ̇V O2NST) were significantly lower in UCP-dta mice relative to wild-type mice regardless of temperature treatment, but both were significantly higher in cold relative to warm acclimated mice. BAT mass was significantly higher in the cold relative to warm acclimated wild-type mice, but not in cold acclimated UCP-dta mice. BAT citrate synthase activity was lower in transgenic animals regardless of acclimation temperature and BAT citrate synthase activity per depot was significantly higher only in the cold acclimated wild-type mice. Muscle citrate synthase activity was increased in both genotypes. As defects in muscle oxidative function have been observed with obesity and type 2 diabetes, these results suggest that chronic cold exposure is a useful intervention to drive skeletal muscle oxidative capacity in mouse models of obesity.

The regulation and physiology of mitochondrial proton leak

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

Using brown adipose tissue to treat obesity - the central issue

Current therapeutic strategies are proving inadequate to deal with growing obesity rates because of the inherent resistance of the human body to weight loss. The activation of human brown adipose tissue (BAT) represents an opportunity to increase energy expenditure and weight loss alongside improved lipid and glucose homeostasis. Research into the regulation of BAT has made increasing the thermogenic capacity of an individual to treat metabolic disease a plausible strategy, despite thermogenesis being under tight central nervous system control. Previous therapies targeted at the sympathetic nervous system have had deleterious effects because of a lack of organ specificity, but advances in our understanding of central BAT regulatory systems might open up better strategies to specifically stimulate BAT in obese individuals to aid weight reduction.

Human brown fat and obesity: methodological aspects

Much is known about brown adipose tissue (BAT) in rodents. Its function is to generate heat in response to low environmental temperatures and to diet or overfeeding. The knowledge about BAT in humans is still rather limited despite the recent rediscovery of its functionality in adults. This review highlights the information available on the contribution of BAT in increasing human energy expenditure in relation to obesity. Besides that methodological aspects will be discussed that need special attention in order to unravel the heat producing capacity of human BAT, the recruitment of the tissue, and its functionality.