Defining the lineage of thermogenic perivascular adipose tissue

Brown adipose tissue can expend large amounts of energy, and therefore increasing its size or activity is a promising therapeutic approach to combat metabolic disease. In humans, major deposits of brown fat cells are found intimately associated with large blood vessels, corresponding to perivascular adipose tissue (PVAT). However, the cellular origins of PVAT are poorly understood. Here, we determine the identity of perivascular adipocyte progenitors in mice and humans. In mice, thoracic PVAT develops from a fibroblastic lineage, consisting of progenitor cells (Pdgfra+, Ly6a+ and Pparg-) and preadipocytes (Pdgfra+, Ly6a+ and Pparg+), which share transcriptional similarity with analogous cell types in white adipose tissue. Interestingly, the aortic adventitia of adult animals contains a population of adipogenic smooth muscle cells (Myh11+, Pdgfra- and Pparg+) that contribute to perivascular adipocyte formation. Similarly, human PVAT contains presumptive fibroblastic and smooth muscle-like adipocyte progenitor cells, as revealed by single-nucleus RNA sequencing. Together, these studies define distinct populations of progenitor cells for thermogenic PVAT, providing a foundation for developing strategies to augment brown fat activity.

Adipose tissue growth and development: the modulating role of ambient temperature

Adipose tissue is usually laid down in small amounts in the foetus and is characterised as possessing small amounts of the brown adipose tissue-specific mitochondrial uncoupling protein (UCP)1. In adults, a primary factor determining the abundance and function of UCP1 is ambient temperature. Cold exposure causes activation and the rapid generation of heat through the free flow of protons across the mitochondria with no requirement to convert ADP to ATP. In rodents, housing at an ambient temperature below thermoneutrality promotes the appearance of beige like adipocytes. These arise as discrete regions of UCP1 containing cells in white fat depots. There is increasing evidence to show that to gain credible translational results on brown and beige fat function in rodent models that they should be housed at thermoneutrality. This not only reflects the type of environment in which humans spend a majority of their time, but is in accord with the rise of global temperature caused by industrialisation and the uncontrolled burning of fossil fuels. There is now good evidence in adult humans, that stimulating brown fat can improve glucose homeostasis which can be achieved either by nutritional or pharmacological interventions. The challenge, therefore, is to establish credible developmental models in animals maintained at thermoneutrality which will elucidate the true impact of nutrition. The primary focus should fall specifically on the components of breast milk and how these modulate long term effects on brown or beige fat development and function.

Both proliferation and lipogenesis of brown adipocytes contribute to postnatal brown adipose tissue growth in mice

Brown adipose tissue (BAT) is the primary non-shivering thermogenesis organ in mammals, which plays essential roles in maintaining the body temperature of infants. Although the development of BAT during embryogenesis has been well addressed in rodents, how BAT grows after birth remains unknown. Using mouse interscapular BAT (iBAT) as an example, we studied the cellular and molecular mechanisms that regulate postnatal BAT growth. By analyzing the developmental dynamics of brown adipocytes (BAs), we found that BAs size enlargement partially accounts for iBAT growth. By investigating the BAs cell cycle activities, we confirmed the presence of proliferative BAs in the neonatal mice. Two weeks after birth, most of the BAs exit cell cycle, and the further expansion of the BAT was mainly due to lipogenesis-mediated BAs volume increase. Microscopy and fluorescence-activated cell sorting analyses suggest that most BAs are mononuclear and diploid. Based on the developmental dynamics of brown adipocytes, we propose that the murine iBAT has two different growth phases between birth and weaning: increase of BAs size and number in the first two weeks, and BAs size enlargement thereafter. In summary, our data demonstrate that both lipogenesis and proliferation of BAs contribute to postnatal iBAT growth in mice.

Dose threshold for radiation induced fetal programming in a mouse model at 4 months of age: Hepatic expression of genes and proteins involved in glucose metabolism and glucose uptake in brown adipose tissue

Exposure to ionizing radiation contributing to negative health outcomes is a widespread concern. However, the impact of low dose and sub-lethal dose radiation (SLDR) exposures remain contentious, particularly in pregnant women who represent a vulnerable group. The fetal programming hypothesis states that an adverse in utero environment or stress during development of an embryo or fetus can result in permanent physiologic changes often resulting in progressive metabolic dysfunction with age. To assess changes in gene expression profiles of glucose/insulin signaling and lipid metabolism caused by radiation exposure in utero, pregnant C57Bl/6J mice were irradiated using a dose response ranging from low dose to SLDR and compared to a Sham-irradiated group. mRNA expression analysis in 16 week old offspring (n = 84) revealed that genes involved in metabolic function including glucose metabolism, insulin signaling and lipid metabolism were unaffected by prenatal radiation exposures up to 300 mGy. However, female offspring of dams exposed to 1000 mGy had upregulated expression of genes contributing to insulin resistance and gluconeogenesis. In a second cohort of mice, the effects of SLDR on fetal programming of hepatic SOCS3 and PEPCK protein expression were assessed. 4 month old female offspring of dams irradiated at 1000 mGy had: 1) increased liver weights, 2) increased hepatic expression of proteins involved in glucose metabolism and 3) increased ¹⁸F-fluorodeoxyglucose (FDG) uptake in interscapular brown adipose tissue (IBAT) measured by positron emission tomography (PET) (n = 25). The results of this study indicate that prenatal radiation exposure does not affect metabolic function up to 300 mGy and 1000 mGy may be a threshold dose for sex-specific alterations in glucose uptake and hepatic gene and protein expression of SOCS3, PEPCK, PPARGC1A and PPARGC1B. These findings suggest that SLDR doses alter glucose uptake in IBAT and hepatic gene and protein expression of offspring and these changes may progress with age.

Enoyl coenzyme A hydratase 1 combats obesity and related metabolic disorders by promoting adipose tissue browning

Browning of white adipose tissue (WAT) has been recognized as an important strategy for the treatment of obesity, insulin resistance, and diabetes. Enoyl coenzyme A hydratase 1 (ECH1) is a widely known enzyme involved in lipid metabolism. However, whether and how ECH1 is implicated in browning of WAT remain obscure. Adeno-associated, virus-mediated genetic engineering of ECH1 in adipose tissue was used in investigations in mouse models of obesity induced by a high-fat diet (HFD) or browning induced by cold exposure. Metabolic parameters showed that ECH1 overexpression decreased weight gain and improved insulin sensitivity and lipid profile after 8 wk of an HFD. Further work revealed that these changes were associated with enhanced energy expenditure and increased appearance of brown-like adipocytes in inguinal WAT, as verified by a remarkable increase in uncoupling protein 1 and thermogenic gene expression. In vitro, ECH1 induced brown fat-related gene expression in adipocytes differentiated from primary stromal vascular fractions, whereas knockdown of ECH1 reversed this effect. Mechanistically, ECH1 regulated the thermogenic program by inhibiting mammalian target of rapamycin signaling, which may partially explain the potential mechanism for ECH1 regulating adipose browning. In summary, ECH1 may participate in the pathology of obesity by regulating browning of WAT, which probably provides us with a new therapeutic strategy for combating obesity.

Electroacupuncture Regulates Inguinal White Adipose Tissue Browning by Promoting Sirtuin-1-Dependent PPARγ Deacetylation and Mitochondrial Biogenesis

BACKGROUND

Previous studies had suggested that electroacupuncture (EA) can promote white adipose tissue (WAT) browning to counter obesity. But the mechanism was still not very clear.

AIM

In this study, we aim to study the effect of EA on promoting inguinal WAT (iWAT) browning and its possible mechanism.

METHOD

Three-week-old rats were randomly divided into a normal diet (ND) group and a high-fat diet (HFD) group. After 10 weeks, the HFD rats were grouped into HFD + EA group and HFD control group. Rats in the EA group were electro-acupunctured for 4 weeks on Tianshu (ST25) acupoint under gas anesthesia with isoflurane, while the rats in HFD group were under gas anesthesia only. Body weight and cumulative food intake were monitored, and H&E staining was performed to assess adipocyte area. The effect of EA on WAT was assessed by qPCR, immunoblotting, immunoprecipitation and Co-immunoprecipitation. Mitochondria were isolated from IWAT to observe the expression of mitochondrial transcription factor A (TFAM).

RESULTS

The body weight, WAT/body weight ratio and cumulative food consumption obviously decreased (P < 0.05) in the EA group. The expressions of brown adipose tissue (BAT) markers were increased in the iWAT of EA rats. Nevertheless, the mRNA expressions of WAT genes were suppressed by 4-week EA treatment. Moreover, EA increased the protein expressions of SIRT-1, PPARγ, PGC-1α, UCP1 and PRDM16 which trigger the molecular conversion of iWAT browning. The decrease of PPARγ acetylation was also found in EA group, indicating EA could advance WAT-browning through SIRT-1 dependent PPARγ deacetylation pathway. Besides, we found that EA could activate AMPK to further regulate PGC-1α-TFAM-UCP1 pathway to induce mitochondrial biogenesis.

CONCLUSION

In conclusion, EA can remodel WAT to BAT through inducing SIRT-1 dependent PPARγ deacetylation, and regulating PGC-1α-TFAM-UCP1 pathway to induce mitochondrial biogenesis. This may be one of the mechanisms by which EA affects weight loss.

Transcriptomic evidence that cortisol alters perinatal epicardial adipose tissue maturation

Cortisol administration during late gestation in ewes, modeling maternal stress, resulted in transcriptomic changes suggesting altered maturation and metabolic changes to the offspring heart. This study investigates the effects of cortisol on epicardial adipose tissue (EAT), a visceral fat pad associated with adverse cardiovascular conditions in adults. Pregnant ewes were treated with either 1 mg·kg^-1·day^-1 cortisol from 115 days gestation to term and EAT collected from term fetuses (control: n = 8, maternal cortisol 1 mg·kg^-1·day^-1: n = 6). To compare the effects of cortisol to the normal maturation in EAT, we also modeled the normal changes in gene expression in EAT at the transition from in utero to postnatal life using the EAT from control fetuses and from two-week-old lambs (control: n = 7). Transcriptomic modeling was used to identify pathways altered by maternal cortisol overexposure. Transcriptomic modeling confirmed the brown fat phenotype of EAT at term and a transition toward white fat at 2 wk of age in EAT of control fetuses/lambs and highlighted a role of immune responses, including complement coagulation, and serotonin in this transition. Maternal cortisol (1 mg·kg^-1·day^-1) increased the lipid peroxidation product 4-hydroxynonenal in EAT of term fetuses but did not affect the number of activated macrophages or size of the lipid droplets in the depot; transcriptomics suggested an earlier metabolic maturation of EAT via, in part, increased immune responses.

Effect of ambient temperature on the proliferation of brown adipocyte progenitors and endothelial cells during postnatal BAT development in Syrian hamsters

In Syrian hamsters, brown adipose tissue (BAT) develops postnatally through the proliferation and differentiation of brown adipocyte progenitors. In the study reported here, we investigated how ambient temperature influenced BAT formation in neonatal hamsters. In both hamsters raised at 23 or 30 °C, the interscapular fat changed from white to brown coloration in an age-dependent manner and acquired the typical morphological features of BAT by day 16. However, the expression of uncoupling protein 1, a brown adipocyte marker, and of vascular endothelial growth factor α were lower in the group raised at 30 °C than in that raised at 23 °C. Immunofluorescent staining revealed that the proportion of Ki67-expressing progenitors and endothelial cells was lower in the 30 °C group than in the 23 °C group. These results indicate that warm ambient temperature suppresses the proliferation of brown adipocyte progenitors and endothelial cells and negatively affects the postnatal development of BAT in Syrian hamsters.

Brown adipose tissue development and function and its impact on reproduction

Although brown adipose tissue (BAT) is one of the smallest organs in the body, it has the potential to have a substantial impact on both heat production as well as fat and carbohydrate metabolism. This is most apparent at birth, which is characterised with the rapid appearance and activation of the BAT specific mitochondrial uncoupling protein (UCP)1 in many large mammals. The amount of brown fat then gradually declines with age, an adaptation that can be modulated by the thermal environment. Given the increased incidence of maternal obesity and its potential transmission to the mother's offspring, increasing BAT activity in the mother could be one mechanism to prevent this cycle. To date, however, all rodent studies investigating maternal obesity have been conducted at standard laboratory temperature (21°C), which represents an appreciable cold challenge. This could also explain why offspring weight is rarely increased, suggesting that future studies would benefit from being conducted at thermoneutrality (~28°C). It is also becoming apparent that each fat depot has a unique transcriptome and show different developmental pattern, which is not readily apparent macroscopically. These differences could contribute to the retention of UCP1 within the supraclavicular fat depot, the most active depot in adult humans, increasing heat production following a meal. Despite the rapid increase in publications on BAT over the past decade, the extent to which modifications in diet and/or environment can be utilised to promote its activity in the mother and/or her offspring remains to be established.

Prenatal PPARα activation by clofibrate increases subcutaneous fat browning in male C57BL/6J mice fed a high-fat diet during adulthood

We tested the hypothesis that prenatal administration of PPARα agonist clofibrate may permanently increase browning capacity of developing white adipose tissue (WAT). Pregnant C57BL/6J mice were fed a basal diet, without (C) or with 0.5% clofibrate (CF, a PPARα agonist) throughout pregnancy. After parturition, only male offspring were used; all suckled their mothers (who were eating the C diet) and after weaning, they ate a standard chow diet for 4 wk, followed by a high-fat diet (HFD) for 5 wk. Administration of CF up-regulated serum concentrations and hepatic expression of FGF21 in fetuses, with a return to basal levels after CF withdrawal. At postnatal day 84 (P84), CF-offspring had significantly higher expression of thermogenic genes (Ucp1, Cidea, Ppara Ppargc1a, Cpt1b) and UCP1 protein levels in response to HFD in inguinal fat, but not in retroperitoneal (combined with perirenal) or epididymal fat. Based on UCP1 levels in inguinal fat on P7, P14, and P21, appearance of the transient brown-adipocyte phenotype seemed to be hastened by CF exposure. We concluded that giving CF to pregnant mice programmed greater HFD-induced WAT browning in subcutaneous, but not in visceral fat, in their male offspring at adulthood.

Comparative Transcriptomic and Epigenomic Analyses Reveal New Regulators of Murine Brown Adipogenesis

Increasing energy expenditure through brown adipocyte recruitment is a promising approach to combat obesity. We report here the comprehensive profiling of the epigenome and transcriptome throughout the lineage commitment and differentiation of C3H10T1/2 mesenchymal stem cell line into brown adipocytes. Through direct comparison to datasets from differentiating white adipocytes, we systematically identify stage- and lineage-specific coding genes, lncRNAs and microRNAs. Utilizing chromatin state maps, we also define stage- and lineage-specific enhancers, including super-enhancers, and their associated transcription factor binding motifs and genes. Through these analyses, we found that in brown adipocytes, brown lineage-specific genes are pre-marked by both H3K4me1 and H3K27me3, and the removal of H3K27me3 at the late stage is necessary but not sufficient to promote brown gene expression, while the pre-deposition of H3K4me1 plays an essential role in poising the brown genes for expression in mature brown cells. Moreover, we identify SOX13 as part of a p38 MAPK dependent transcriptional response mediating early brown cell lineage commitment. We also identify and subsequently validate PIM1, SIX1 and RREB1 as novel regulators promoting brown adipogenesis. Finally, we show that SIX1 binds to adipogenic and brown marker genes and interacts with C/EBPα, C/EBPβ and EBF2, suggesting their functional cooperation during adipogenesis.

Cdkn1c Boosts the Development of Brown Adipose Tissue in a Murine Model of Silver Russell Syndrome

The accurate diagnosis and clinical management of the growth restriction disorder Silver Russell Syndrome (SRS) has confounded researchers and clinicians for many years due to the myriad of genetic and epigenetic alterations reported in these patients and the lack of suitable animal models to test the contribution of specific gene alterations. Some genetic alterations suggest a role for increased dosage of the imprinted CYCLIN DEPENDENT KINASE INHIBITOR 1C (CDKN1C) gene, often mutated in IMAGe Syndrome and Beckwith-Wiedemann Syndrome (BWS). Cdkn1c encodes a potent negative regulator of fetal growth that also regulates placental development, consistent with a proposed role for CDKN1C in these complex childhood growth disorders. Here, we report that a mouse modelling the rare microduplications present in some SRS patients exhibited phenotypes including low birth weight with relative head sparing, neonatal hypoglycemia, absence of catch-up growth and significantly reduced adiposity as adults, all defining features of SRS. Further investigation revealed the presence of substantially more brown adipose tissue in very young mice, of both the classical or canonical type exemplified by interscapular-type brown fat depot in mice (iBAT) and a second type of non-classic BAT that develops postnatally within white adipose tissue (WAT), genetically attributable to a double dose of Cdkn1c in vivo and ex-vivo. Conversely, loss-of-function of Cdkn1c resulted in the complete developmental failure of the brown adipocyte lineage with a loss of markers of both brown adipose fate and function. We further show that Cdkn1c is required for post-transcriptional accumulation of the brown fat determinant PR domain containing 16 (PRDM16) and that CDKN1C and PRDM16 co-localise to the nucleus of rare label-retaining cell within iBAT. This study reveals a key requirement for Cdkn1c in the early development of the brown adipose lineages. Importantly, active BAT consumes high amounts of energy to generate body heat, providing a valid explanation for the persistence of thinness in our model and supporting a major role for elevated CDKN1C in SRS.

Silver Russell syndrome is a severe developmental disorder characterised by low birth weight, sparing of the head and neonatal hypoglycemia. SRS adults are small and can be extremely thin, lacking body fat. Numerous genetic and epigenetic mutations have been linked to SRS primarily involving imprinted genes, but progress has been hampered by the lack of a suitable animal model. Here we describe a mouse model of the rare micro duplications reported in some SRS patients, which recapitulated many of the defining features of SRS, including extreme thinness. We showed that these mice possessed substantially more of the energy consuming brown adipose tissue (BAT), driven by a double dose of the imprinted Cdkn1c gene. We further show that Cdkn1c is required for the postranscriptional accumulation of the BAT determinant PRDM16 and that these proteins co-localise to the nucleus of in a rare label-retaining cell within BAT. These data suggest that Cdkn1c contributes to the development of BAT by modulating PRDM16 and supports a major role for this gene in SRS.

The influence of sex steroids on adipose tissue growth and function

Obesity remains a major global health concern. Understanding the metabolic influences of the obesity epidemic in the human population on maintenance of a healthy weight and metabolic profile is still of great significance. The importance and role of white adipose tissue has been long established, particularly with excess adiposity. Brown adipose tissue (BAT), however, has only recently been shown to contribute significantly to the metabolic signature of mammals outside the previously recognised role in small mammals and neonates. BAT's detection in adults has led to a renewed interest and is now considered to be a potential therapeutic target to prevent excess white fat accumulation in obesity, a theory further promoted by the recent discovery of beige fat. Adipose tissue distribution varies significantly between genders. Pre-menopausal females often show enhanced lower and peripheral fat deposition in adiposity deposition compared to the male profile of central and visceral fat accumulation with obesity. This sex disparity is partly attributed to the different effects of sex hormone profiles and interactions on the adipose tissue system. In this review, we explore this intricate relationship and show how modifications in the effects of sex hormones impact on both brown and white adipose tissues. We also discuss the impact of sex hormones on activation of the hypothalamic-pituitary-adrenal (HPA) axis and how the three pathways between adiposity, HPA and sex steroids can have a major contribution to the prevention or maintenance of obesity and therefore on overall health.

Disruption of insulin signaling in Myf5-expressing progenitors leads to marked paucity of brown fat but normal muscle development

Insulin exerts pleiotropic effects on cell growth, survival, and metabolism, and its role in multiple tissues has been dissected using conditional knockout mice; however, its role in development has not been studied. Lineage tracing experiments have demonstrated that interscapular brown adipose tissue (BAT) arises from a Myf5-positive lineage shared with skeletal muscle and distinct from the majority of white adipose tissue (WAT) precursors. In this study, we sought to investigate the effects of impaired insulin signaling in the Myf5-expressing precursor cells by deleting the insulin receptor gene. Mice lacking insulin receptor in the Myf5 lineage (Myf5IRKO) have a decrease of interscapular BAT mass; however, muscle development appeared normal. Histologically, the residual BAT had decreased cell size but appeared mature and potentially functional. Expression of adipogenic inhibitors preadipocyte factor-1, Necdin, and wingless-type MMTV integration site member 10a in the residual BAT tissue was nonetheless increased compared with controls, and there was an enrichment of progenitor cells with impaired adipogenic differentiation capacity, suggesting a suppression of adipogenesis in BAT. Surprisingly, when cold challenged, Myf5IRKO mice did not show impaired thermogenesis. This resistance to cold could be attributed to an increased presence of uncoupling protein 1-positive brown adipocytes in sc WAT as well as increased expression of lipolytic activity in BAT. These data suggest a critical role of insulin signaling in the development of interscapular BAT from Myf5-positive progenitor cells, but it appears to be dispensable for muscle development. They also underscore the importance of compensatory browning of sc WAT in the absence of BAT for thermoregulation.

Role of developmental transcription factors in white, brown and beige adipose tissues

In this review we discuss the role of developmental transcription factors in adipose tissue biology with a focus on how these developmental genes may contribute to regional variation in adipose tissue distribution and function. Regional, depot-specific, differences in lipid handling and signalling (lipolysis, lipid storage and adipokine/lipokine signalling) are important determinants of metabolic health. At a cellular level, preadipocytes removed from their original depot and cultured in vitro retain depot-specific functional properties, implying that these are intrinsic to the cells and not a function of their environment in situ. High throughput screening has identified a number of developmental transcription factors involved in embryological development, including members of the Homeobox and T-Box gene families, that are strongly differentially expressed between regional white adipose tissue depots and also between brown and white adipose tissue. However, the significance of depot-specific developmental signatures remains unclear. Developmental transcription factors determine body patterning during embryogenesis. The divergent developmental origins of regional adipose tissue depots may explain their differing functional characteristics. There is evidence from human genetics that developmental genes determine adipose tissue distribution: in GWAS studies a number of developmental genes have been identified as being correlated with anthropometric measures of adiposity and fat distribution. Additionally, compelling functional studies have recently implicated developmental genes in both white adipogenesis and the so-called 'browning' of white adipose tissue. Understanding the genetic and developmental pathways in adipose tissue may help uncover novel ways to intervene with the function of adipose tissue in order to promote health.

[Energetics of child's organisim: qualitative and quantitative specifics]

The review summarizes data on the age transformations of energy metabolism observed in the human ontogeny from birth to maturity. Shown that elevated basal energy in child is not associated with the activity of the growth processes. As maturing body structures qualitative changes of metabolic processes appear, which generally lead to economizing and expand functional range. At the same time, even in the adult organism there are some specialized structures (a.e. brown adipose tissue), providing energy dissipation and thereby participate in maintaining energy and substrate homeostasis. Suggested that the increased metabolic rate in childhood forms the body's resistance thanks to the "gyroscopic effect" and thus allows safely overcome the most intense and important stages of ontogenesis.

To each its own: Thermoregulatory strategy varies among neonatal polar phocids

Cold environmental conditions and small body size promote heat loss and may create thermoregulatory challenges for marine mammals born in polar regions. However, among polar-born phocid seal species there are variations in physical attributes and environmental conditions at birth, allowing for an interesting contrast in thermoregulatory strategy. We compared thermoregulatory strategies through morphometrics, sculp attributes (conductivity and resistance), nonshivering thermogenesis (NST via uncoupling protein 1; UCP1), and muscle thermogenesis (via enzyme activity) in neonatal harp (Pagophilus groenlandicus), hooded (Cystophora cristata), and Weddell seals (Leptonychotes weddellii). Harp seals are the smallest at birth (9.8±0.7 kg), rely on lanugo (82.49±3.70% of thermal resistance), and are capable of NST through expression of UCP1 in brown adipose tissue (BAT). In contrast, hooded seal neonates (26.8±1.3 kg) have 2.06±0.23 cm of blubber, accounting for 38.19±6.07% of their thermal resistance. They are not capable of NST, as UCP1 is not expressed. The large Weddell seal neonates (31.5±4.9 kg) rely on lanugo (89.85±1.25% of thermal resistance) like harp seals, but no evidence of BAT was found. Muscle enzyme activity was highest in Weddell seal neonates, suggesting that they rely primarily on muscle thermogenesis. Similar total thermal resistance, combined with marked differences in thermogenic capacity of NST and ST among species, strongly supports that thermoregulatory strategy in neonatal phocids is more closely tied to pups' surface area to volume ratio (SA:V) and potential for early water immersion rather than mass and ambient environmental conditions.

Fatty acid binding protein 4 and 5 play a crucial role in thermogenesis under the conditions of fasting and cold stress

Hypothermia is rapidly induced during cold exposure when thermoregulatory mechanisms, including fatty acid (FA) utilization, are disturbed. FA binding protein 4 (FABP4) and FABP5, which are abundantly expressed in adipose tissues and macrophages, have been identified as key molecules in the pathogenesis of overnutrition-related diseases, such as insulin resistance and atherosclerosis. We have recently shown that FABP4/5 are prominently expressed in capillary endothelial cells in the heart and skeletal muscle and play a crucial role in FA utilization in these tissues. However, the role of FABP4/5 in thermogenesis remains to be determined. In this study, we showed that thermogenesis is severely impaired in mice lacking both FABP4 and FABP5 (DKO mice), as manifested shortly after cold exposure during fasting. In DKO mice, the storage of both triacylglycerol in brown adipose tissue (BAT) and glycogen in skeletal muscle (SkM) was nearly depleted after fasting, and a biodistribution analysis using 125I-BMIPP revealed that non-esterified FAs (NEFAs) are not efficiently taken up by BAT despite the robustly elevated levels of serum NEFAs. In addition to the severe hypoglycemia observed in DKO mice during fasting, cold exposure did not induce the uptake of glucose analogue 18F-FDG by BAT. These findings strongly suggest that DKO mice exhibit pronounced hypothermia after fasting due to the depletion of energy storage in BAT and SkM and the reduced supply of energy substrates to these tissues. In conclusion, FABP4/5 play an indispensable role in thermogenesis in BAT and SkM. Our study underscores the importance of FABP4/5 for overcoming life-threatening environments, such as cold and starvation.

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±0.01Wm(-1)°C(-1)) except in 9-day old pups (0.40±0.08Wm(-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±3.52μmolmin(-1)gwetweight(-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.

Expression and subcellular localization of estrogen receptors α and β in human fetal brown adipose tissue

CONTEXT

Brown adipose tissue (BAT) has the unique ability of generating heat due to the expression of mitochondrial uncoupling protein 1 (UCP1). A recent discovery regarding functional BAT in adult humans has increased interest in the molecular pathways of BAT development and functionality. An important role for estrogen in white adipose tissue was shown, but the possible role of estrogen in human fetal BAT (fBAT) is unclear.

OBJECTIVE

The objective of this study was to determine whether human fBAT expresses estrogen receptor α (ERα) and ERβ. In addition, we examined their localization as well as their correlation with crucial proteins involved in BAT differentiation, proliferation, mitochondriogenesis and thermogenesis including peroxisome proliferator-activated receptor γ (PPARγ), proliferating cell nuclear antigen (PCNA), PPARγ-coactivator-1α (PGC-1α), and UCP1.

DESIGN

The fBAT was obtained from 4 human male fetuses aged 15, 17, 20, and 23 weeks gestation. ERα and ERβ expression was assessed using Western blotting, immunohistochemistry, and immunocytochemistry. Possible correlations with PPARγ, PCNA, PGC-1α, and UCP1 were examined by double immunofluorescence.

RESULTS

Both ERα and ERβ were expressed in human fBAT, with ERα being dominant. Unlike ERβ, which was present only in mature brown adipocytes, we detected ERα in mature adipocytes, preadipocytes, mesenchymal and endothelial cells. In addition, double immunofluorescence supported the notion that differentiation in fBAT probably involves ERα. Immunocytochemical analysis revealed mitochondrial localization of both receptors.

CONCLUSION

The expression of both ERα and ERβ in human fBAT suggests a role for estrogen in its development, primarily via ERα. In addition, our results indicate that fBAT mitochondria could be targeted by estrogens and pointed out the possible role of both ERs in mitochondriogenesis.

Developmental androgen excess programs sympathetic tone and adipose tissue dysfunction and predisposes to a cardiometabolic syndrome in female mice

Among women, the polycystic ovarian syndrome (PCOS) is considered a form of metabolic syndrome with reproductive abnormalities. Women with PCOS show increased sympathetic tone, visceral adiposity with enlarged adipocytes, hypoadiponectinemia, insulin resistance, glucose intolerance, increased inactive osteocalcin, and hypertension. Excess fetal exposure to androgens has been hypothesized to play a role in the pathogenesis of PCOS. Previously, we showed that neonatal exposure to the androgen testosterone (NT) programs leptin resistance in adult female mice. Here, we studied the impact of NT on lean and adipose tissues, sympathetic tone in cardiometabolic tissues, and the development of metabolic dysfunction in mice. Neonatally androgenized adult female mice (NTF) displayed masculinization of lean tissues with increased cardiac and skeletal muscle as well as kidney masses. NTF mice showed increased and dysfunctional white adipose tissue with increased sympathetic tone in both visceral and subcutaneous fat as well as increased number of enlarged and insulin-resistant adipocytes that displayed altered expression of developmental genes and hypoadiponectinemia. NTF exhibited dysfunctional brown adipose tissue with increased mass and decreased energy expenditure. They also displayed decreased undercarboxylated and active osteocalcin and were predisposed to obesity during chronic androgen excess. NTF showed increased renal sympathetic tone associated with increased blood pressure, and they developed glucose intolerance and insulin resistance. Thus, developmental exposure to testosterone in female mice programs features of cardiometabolic dysfunction, as can be observed in women with PCOS, including increased sympathetic tone, visceral adiposity, insulin resistance, prediabetes, and hypertension.

The aging myostatin null phenotype: reduced adiposity, cardiac hypertrophy, enhanced cardiac stress response, and sexual dimorphism

The natural aging process results in the physiological decline of multiple tissues and organ systems. Changes commonly occur with middle age and include decreased skeletal muscle mass, bone mineral density, cardiac output, and insulin sensitivity, and increased adiposity, all of which can contribute to the onset of sarcopenia, osteoporosis, heart failure, or type 2 diabetes. Recent studies suggest that myostatin may influence many of these systems. We therefore sought to determine whether they are affected by aging, especially in 'middle-aged' Mstn-/- mice (12-20 months old (m.o.)). Although body weights were similar in wild-type (WT) and Mstn-/- mice, lean fat-free mass and skeletal muscles composed of predominantly type I, II, and mixed fibers were significantly heavier in Mstn-/- mice. These differences were accompanied by lower total adiposity, especially in female mice, white and brown fat pad weights, and adipocyte size. Hearts were heavier in Mstn-/- mice across a large age range (3-24 m.o.) and exhibited signs of dilated cardiomyopathy at rest, which include lower strain measurements compared with WT myocardium. However, Mstn-/- mice responded better to isoproterenol stress tests with greater increases in fractional shortening and ejection fraction-differences that were again more apparent in females and which are consistent with physiological cardiac hypertrophy. Spleens and kidneys were also smaller, although histologically normal, in Mstn-/- mice. These data together suggest that attenuating myostatin could potentially prevent or possibly treat pathological conditions that develop with age. Additional studies are nevertheless needed to definitively assess potential risks to cardiac function.

Effect of premature aging on murine adipose tissue

To evaluate the effect of aging on adipose tissue development, subcutaneous (SC) and gonadal (GON) white and peri-aortic brown adipose tissues were analyzed of 10 and 30 week old mice deficient in the clock gene Bmal1 (brain and muscle arnt like protein 1) (Bmal1(-/-)) and wild-type littermates (Bmal1(+/+)) kept on a standard fat diet. At both ages, daily food intake was significantly decreased for Bmal1(-/-) mice, associated with reduced hypothalamic expression of PPARα. Between 10 and 30 weeks of age, the total body weight of Bmal1(+/+) mice increased significantly, but that of Bmal1(-/-) mice did not change. Whereas for Bmal1(+/+) mice, both SC and GON fat mass increased with age, these decreased for Bmal1(-/-) mice. This was associated with increased adipocyte size with age for Bmal1(+/+) but not for Bmal1(-/-) mice. Adipose tissue related angiogenesis was not affected by genotype or aging. Peri-aortic brown adipose tissue mass in 30 week old Bmal1(-/-) mice was significantly reduced as compared to age-matched Bmal1(+/+) mice. Comparison of gene expression profiles in SC and GON adipose tissues of both genotypes revealed very marked effects of Bmal1 gene deletion in itself on PAI-1 (4- to 13-fold downregulation), whereas the associated effect of premature aging was striking for leptin (90- to 130-fold downregulation). Thus, premature aging in Bmal1(-/-) mice kept on normal chow was associated with reduced adiposity.

Age-related decrease in cold-activated brown adipose tissue and accumulation of body fat in healthy humans

Brown adipose tissue (BAT) can be identified by (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) combined with X-ray computed tomography (CT) in adult humans. The objective of this study was to clarify the relationship between BAT and adiposity in healthy adult humans, particularly to test the idea that decreased BAT activity may be associated with body fat accumulation with age. One hundred and sixty-two healthy volunteers aged 20-73 years (103 males and 59 females) underwent FDG-PET/CT after 2-h cold exposure at 19 °C with light clothing. Cold-activated BAT was detected in 41% of the subjects (BAT-positive). Compared with the BAT-negative group, the BAT-positive group was younger (P < 0.01) and showed a lower BMI (P < 0.01), body fat content (P < 0.01), and abdominal fat (P < 0.01). The incidence of cold-activated BAT decreased with age (P < 0.01), being more than 50% in the twenties, but less than 10% in the fifties and sixties. The adiposity-related parameters showed some sex differences, but increased with age in the BAT-negative group (P < 0.01), while they remained unchanged from the twenties to forties in the BAT-positive group, in both sexes. These results suggest that decreased BAT activity may be associated with accumulation of body fat with age.

White adipose tissue re-growth after partial lipectomy in high fat diet induced obese wistar rats

The effects of partial removal of epididymal (EPI) and retroperitoneal (RET) adipose tissues (partial lipectomy) on the triacylglycerol deposition of high fat diet induced obese rats were analyzed, aiming to challenge the hypothesized body fat regulatory system. Male 28-day-old wistar rats received a diet enriched with peanuts, milk chocolate and sweet biscuits during the experimental period. At the 90th day of life, rats were submitted to either lipectomy (L) or sham surgery. After 7 or 30 days, RET, EPI, liver, brown adipose tissue (BAT), blood and carcass were obtained and analyzed. Seven days following surgery, liver lipogenesis rate and EPI relative weight were increased in L. After 30 days, L, RET and EPI presented increased lipogenesis, lipolysis and percentage of small area adipocytes. L rats also presented increased liver malic enzyme activity, BAT lipogenesis, and triacylglycerol and corticosterone serum levels. The partial removal of visceral fat pads affected the metabolism of high fat diet obese rats, which leads to excised tissue re-growth and possibly compensatory growth of non-excised depots at a later time.

Absence of thyroid hormone activation during development underlies a permanent defect in adaptive thermogenesis

Type 2 deiodinase (D2), which is highly expressed in brown adipose tissue (BAT), is an enzyme that amplifies thyroid hormone signaling in individual cells. Mice with inactivation of the D2 pathway (D2KO) exhibit dramatically impaired thermogenesis in BAT, leading to hypothermia during cold exposure and a greater susceptibility to diet-induced obesity. This was interpreted as a result of defective acute activation of BAT D2. Here we report that the adult D2KO BAT has a permanent thermogenic defect that stems from impaired embryonic BAT development. D2KO embryos have normal serum T3 but due to lack of D2-generated T3 in BAT, this tissue exhibits decreased expression of genes defining BAT identity [i.e. UCP1, PGC-1alpha and Dio2 (nonfunctional)], which results in impaired differentiation and oxidative capacity. Coinciding with a reduction of these T3-responsive genes, there is oxidative stress that in a cell model of brown adipogenesis can be linked to decreased insulin signaling and decreased adipogenesis. This discovery highlights the importance of deiodinase-controlled thyroid hormone signaling in BAT development, where it has important metabolic repercussions for energy homeostasis in adulthood.

Brown adipose tissue--a new role in humans?

New targets for pharmacological interventions are of great importance to combat the epidemic of obesity. Brown adipose tissue could potentially represent one such target. Unlike white adipose tissue, brown adipose tissue has the ability to dissipate energy by producing heat rather than storing it as triglycerides. In small mammals, the presence of active brown adipose tissue is pivotal for the maintenance of body temperature and possibly to protect against the detrimental effects of surplus energy intake. Animal studies have shown that expansion and/or activation of brown adipose tissue counteracts diet-induced weight gain and related disorders such as type 2 diabetes mellitus. Several independent studies have now confirmed the presence of functional brown adipose tissue in adult humans, for whom this tissue is probably metabolically beneficial given its association with both low BMI and low total adipose tissue content. Over the past few years, knowledge of the transcriptional control and development of brown adipose tissue has increased substantially. Thus, several possible targets that may be useful for the expansion and/or activation of this tissue by pharmacological means have been identified. Whether or not brown adipose tissue will be useful in the battle against obesity remains to be seen. However, this possibility is certainly well worth exploring.

Acute brown adipose tissue temperature response to cold in monosodium glutamate-treated Siberian hamsters

Neonatal monosodium glutamate (MSG) administration increases adiposity, decreases energy expenditure and is associated with arcuate nucleus (Arc) destruction. Disrupted brown adipose tissue (BAT) thermogenesis underlies some of these effects, although, interscapular BAT temperature (T(IBAT)) has not been measured. Therefore, we tested the effects of neonatal MSG or vehicle administration in Siberian hamsters and, when they were adults, measured T(IBAT) during acute cold exposure. The Arc and its projection to the hypothalamic paraventricular nucleus (PVH) are both components of the CNS outflow circuits to IBAT, with the latter implicated in BAT thermogenesis that could be compromised by MSG treatment. Using a viral transneuronal tract tracer, pseudorabies virus (PRV), we also tested whether the components of these circuits were intact. As adults, MSG-treated hamsters had significantly increased body mass and some white fat pad masses, markedly reduced Arc Nissl and neuropeptide staining, and PVH neuropeptide fiber staining. Cold-exposed (18 h at 5 degrees C) MSG- and vehicle-treated hamsters initially maintained T(IBAT), but the ability of the former waned after 2 h being significantly decreased by 18 h. PRV immunoreactive fibers/cells were not altered by neonatal MSG treatment despite substantial Arc and PVH destruction. MSG- and vehicle-treated hamsters given an exogenous norepinephrine challenge showed identical increases in the duration and peak of T(IBAT). Thus, the inability of MSG-treated animals to sustain T(IBAT) in the cold is not due to any obvious MSG-induced deletions of central sympathetic outflow circuits to IBAT, but appears to be extrinsic to the tissue nevertheless.

Developmental origin of fat: tracking obesity to its source

The development of obesity not only depends on the balance between food intake and caloric utilization but also on the balance between white adipose tissue, which is the primary site of energy storage, and brown adipose tissue, which is specialized for energy expenditure. In addition, some sites of white fat storage in the body are more closely linked than others to the metabolic complications of obesity, such as diabetes. In this Review, we consider how the developmental origins of fat contribute to its physiological, cellular, and molecular heterogeneity and explore how these factors may play a role in the growing epidemic of obesity.

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.

Ontogenic loss of brown adipose tissue sensitivity to beta-adrenergic stimulation in the ovine

In ruminants and other large animals, expression of uncoupling protein-1 (UCP1) in brown adipose tissue (BAT) is confined to the perinatal period when it plays a key role in nonshivering thermogenesis. This study determined whether loss of expression of the BAT phenotype was due to reduced response to a beta-agonist, isoprenaline, and expression of the peroxisome proliferator-activated receptor (PPAR) family [PPARalpha, PPARgamma, PPAR coactivator 1alpha (PGC-1alpha)], which regulates UCP1 gene expression. Perirenal adipose tissue (PAT) was sampled from ovine fetuses, newborn lambs, and lambs on d 1, 5, 7, and 21 of life. UCP1 mRNA and protein in PAT increased from d 123 of fetal life to reach a maximum at birth followed by a rapid decrease over the first 5 d of life. Expression of the coactivator, PGC-1alpha and PPAR alpha, peaked between fetal day 123 and birth, and then declined to undetectable levels in the first days of life. In vivo administration of isoprenaline was able to induce expression of UCP1, PGC-1alpha, and PPARalpha in BAT up to 5 d of age but thereafter was ineffective. In vitro addition of beta-receptor, PPARalpha, and PPARgamma agonists were unable to overcome the suppression of UCP1, PPARalpha, and PPARgamma expression observed in differentiated adipocytes prepared from 30-d-old compared with 1-d-old lambs. These data are consistent with a model in which postnatal loss of UCP1 expression and beta-adrenergic induction of the brown adipocyte phenotype is due to loss of expression of PGC-1alpha and PPARalpha.

Critical roles of the p160 transcriptional coactivators p/CIP and SRC-1 in energy balance

Several transcriptional coactivators have been implicated in modulating the transcriptional activities of nuclear hormone receptors in vitro. Potential roles of these cofactors in important physiological processes such as energy homeostasis remain unknown. We report here that a developmental arrest in interscapular brown fat and defective adaptive thermogenesis occur in mice lacking both the p160 family transcriptional coactivators SRC-1 and p/CIP due to a failure in induction of selective PPARgamma target genes involved in adipogenesis and mitochondrial uncoupling. In the absence of p/CIP and SRC-1, mice eat more food on both regular chow and a high-fat diet because of decreased blood leptin levels. However, the p/CIP(-/-)/SRC-1(-/-) mice are lean and resistant to high-fat-diet-induced obesity. They exhibit increased basal metabolic rates and heightened levels of physical activity. Therefore, p/CIP and SRC-1 play critical roles in energy balance by controlling both energy intake and energy expenditure.

Prediction of preadipocyte differentiation by gene expression reveals role of insulin receptor substrates and necdin

The insulin/IGF-1 (insulin-like growth factor 1) signalling pathway promotes adipocyte differentiation via complex signalling networks. Here, using microarray analysis of brown preadipocytes that are derived from wild-type and insulin receptor substrate (Irs) knockout animals that exhibit progressively impaired differentiation, we define 374 genes/expressed-sequence tags whose expression in preadipocytes correlates with the ultimate ability of the cells to differentiate. Many of these genes, including preadipocyte factor-1 (Pref-1) and multiple members of the Wnt signalling pathway, are related to early adipogenic events. Necdin is also markedly increased in Irs knockout cells that cannot differentiate, and knockdown of necdin restores brown adipogenesis with downregulation of Pref-1 and Wnt10a expression. Insulin receptor substrate proteins regulate a necdin-E2F4 interaction that represses peroxisome-proliferator-activated receptor gamma (PPARgamma) transcription via a cyclic AMP response element binding protein (CREB)-dependent pathway. Together these define a key signalling network that is involved in brown preadipocyte determination.

Hormonal and nutritional regulation of adipose tissue mitochondrial development and function in the newborn

Growth, development, and maturation of adipose tissue in the fetus can determine both survival at birth as well as having longer term consequences for adult disease. The mitochondrial proteins uncoupling protein (UCP) 1, voltage dependent anion channel (VDAC), and cytochrome c have an important role in cellular energy regulation. Activity of these proteins is particularly important during the transition from fetal to neonatal life when cellular energy requirements are at near maximal rates. The regulation of these proteins by endocrine factors is highly complex and may be dependent on both fetal number and maternal nutrition. The cytokine hormones leptin and prolactin have well established functions in energy regulation and lactation respectively. However, recent data proposes a role in regulation of mitochondrial proteins, particularly UCP1, and thermogenesis. Cortisol is an adrenal hormone with a critical role in fetal tissue maturation, especially the lung. It has now been shown to influence the abundance of UCP1 in the fetus, a role that may in part be regulated by the metabolically active thyroid hormone triiodothyronine. A greater understanding of the regulation of mitochondrial proteins within adipose tissue by endocrine and nutritional factors is likely to be important in preventing neonatal morbidity and mortality. It could also add substantially to our understanding of pathological conditions such as obesity and non-insulin dependent diabetes.

Brown adipose tissue development and metabolism in ruminants1

We conducted several experiments to better understand the relationship between brown adipose tissue (BAT) metabolism and thermogenesis. In Exp. 1, we examined perirenal (brown) and sternum s.c. adipose tissue in 14 Wagyu x Angus neonates infused with norepinephrine (NE). Perirenal adipocytes contained numerous large mitochondria with well-differentiated cristae; sternum s.c. adipocytes contained a few, small mitochondria, with poorly developed cristae. Lipogenesis from acetate was high in BAT but barely detectable in sternum s.c. adipose tissue. In Exp. 2, we compared perirenal and tailhead adipose tissues between NE-infused Angus (n = 6) and Brahman (n = 7) newborn calves. Brahman BAT contained two-to-three times as many total beta-receptors as Angus BAT. The mitochondrial UCP1:28S rRNA ratio was greater in Brahman BAT than in BAT from Angus calves. Lipogenesis from acetate and glucose again was high, but lipogenesis from palmitate was barely detectable. Tail-head s.c. adipose tissue from both breed types contained adipocytes with distinct brown adipocyte morphology. In Exp. 3, three fetuses of each breed type were taken at 96, 48, 24, 14, and 6 d before expected parturition, and at parturition. Lipogenesis from acetate and glucose in vitro decreased 97% during the last 96 d of gestation in both breed types, whereas the UCP1 gene expression tripled during gestation in both breed types. At birth, palmitate esterification was twice as high in Angus than in Brahman BAT and was at least 100-fold higher than in BAT from NE-infused calves from Exp. 2. Uncoupling protein-1 mRNA was readily detectable in tailhead s.c. adipose tissue in all fetal samples. In Exp. 4, male Brahman and Angus calves (n = 5 to 7 per group) were assigned to 1) newborn treatment (15 h of age), 2) 48 h of warm exposure (22 degrees C) starting at 15 h of age, or 3) 48 h of cold exposure (4 degrees C) starting at 15 h of age. Brahman BAT adipocytes shrank with cold exposure, whereas Angus BAT adipocytes did not. Similarly, BAT from neonatal lambs (Exp. 5; n = 6 per group) was depleted of lipid in response to cold exposure, although UCP1 gene expression persisted. In Exp. 4, NE stimulated lipogenesis from palmitate in BAT incubated in vitro. Lipogenesis from palmitate was higher in Angus than in Brahman BAT, and increased with both warm and cold exposure. These studies suggest that BAT from Brahman calves may be exhausted of lipid shortly after birth during times of cold exposure.

Thermoregulation in rats during early postnatal maturation: importance of nitric oxide

Experiments were carried out to determine the role of nitric oxide in mediating autonomic and behavioral thermoregulatory control in rat pups on postnatal days 1-2, 5-6, and 10-11. For an experiment, each pup received a subcutaneous injection of vehicle, NG-nitro-d-arginine methyl ester (d-NAME; 100 mg/kg), or NG-nitro-l-arginine methyl ester (l-NAME; 100 mg/kg) before being placed in a metabolic chamber or in a thermocline with a linear temperature gradient of 23 to 43°C. In the metabolic chamber, oxygen consumption and core temperature were measured as ambient temperature was decreased from 40 to 15°C over a 60-min period. Decreasing ambient temperature elicited an increase in oxygen consumption in all age groups that received vehicle or d-NAME. The lower critical temperature and peak oxygen consumption upon exposure to cold after vehicle were 41 ± 10 ml · kg-1 · min-1 at 30°C, 43 ± 12 ml · kg-1 · min-1 at 28°C, and 55 ± 11 ml · kg-1 · min-1 at 25°C in the 1- to 2-, 5- to 6-, and 10- to 11-day-old pups, respectively. Administration of l-NAME abolished the oxygen consumption response to cold in the 1- to 2- and 5- to 6-day-old pups and significantly attenuated the oxygen consumption response to cold in the 10- to 11-day-old pups. Selected ambient temperature in the thermocline was not significantly affected by prior administration of d-NAME or l-NAME compared with vehicle. Thus our data provide evidence that the nitric oxide system plays a role in mediating autonomic but not behavioral thermoregulatory control in rat pups during early postnatal maturation.

Differential effects of age and sex on the postnatal responsiveness of brown adipose tissue to prolactin administration in rats

Previous studies have shown that prolactin administration to pregnant rats results in offspring with enhanced abundance of the brown adipose tissue-specific uncoupling protein (UCP) 1. The present study therefore aimed to determine whether a similar effect was observed after birth and if the sex of the animal further influenced the responsiveness of brown adipose tissue mitochondria to prolactin administration. Daily prolactin injections were therefore commenced at 15, 35 or 60 days of age and continued for 4 days. The abundance of UCP1 was unchanged with age in females but decreased between 15 and 35 days in males and was lower in males than females by 60 days of age. Cytochrome c abundance remained unchanged with postnatal age in both males and females and was consistently higher in males at each sampling age. Prolactin decreased the abundance of UCP1 and cytochrome c when administered to female rats at 35 and 60 days of age, but had no effect at 15 days. In contrast, prolactin had no effect on UCP1 in male rats at any age, but did stimulate the abundance of cytochrome c at 15 days of age. In conclusion, the administration of prolactin to postnatal rats over the period in which maturation of the hypothalamic-pituitary axis and brown adipose tissue function is occurring did not enhance UCP1 abundance. In females, prolactin actually caused a reduction in UCP1 suggesting that in rats it is only prior to birth that prolactin has a stimulatory role on brown adipose tissue development.

Unexpected deposition of brown fat in mammary gland during postnatal development

Mammary fat tissue is crucial for mammary ductal morphogenesis in both fetal and adult mice. There are two kinds of adipocytes, the energy-storing white and the energy-dissipating brown adipocyte. The precise identity of the types of adipocyte in the mammary gland has never been investigated but was always assumed to be only white fat. In this study, we show that both white and brown adipocytes are present in the postnatal mammary gland. The amount of brown adipose tissue (BAT) examined by histology and electron microscopy correlates with the transcript levels of uncoupling protein 1, which is a mitochondrial carrier expressed exclusively in BAT. Uncoupling protein 1 mRNAs are the highest during prepuberty, decrease upon puberty, and are finally undetectable in the adult mammary gland. The analysis of a BAT-depleted mouse model showed that depletion of mammary BAT in early postnatal development induces epithelial differentiation. Alveolar structures were formed along all ducts and were functional since they produced beta-casein. However, mammary transplantation experiments indicated that a systemic effect was responsible for epithelium differentiation. Our data suggest that BAT negatively regulates the differentiation of mammary epithelial cells in a systemic manner during prepubertal ductal outgrowth.

Effects of rearing temperature on sympathoadrenal activity in young adult rats

Animals reared at 18 degrees C exhibit enhanced innervation of brown adipose tissue (BAT) and greater cold tolerance as adults, yet gain more weight when fed an enriched diet compared with rats reared at 30 degrees C. To explore this paradox, sympathoadrenal activity was examined using techniques of [(3)H]norepinephrine ([(3)H]NE) turnover and urinary catecholamine excretion in male and female rats reared until 2 mo of age at 18 or 30 degrees C. Gene expression in BAT was also analyzed for several sympathetically related proteins. Although [(3)H]NE turnover in heart did not differ between groups, [(3)H]NE turnover in BAT was consistently elevated in the 18 degrees C-reared animals, even 2 mo after removal from the cool environment. Gene expression for uncoupling proteins 1 and 3, GLUT-4, leptin, and the alpha(1A)-adrenergic receptor was more abundant in BAT and the increase in epinephrine excretion with fasting suppressed in 18 degrees C-reared animals. These studies demonstrate that obesity consequent to exposure to 18 degrees C in early life occurs despite tonic elevation of sympathetic input to BAT. Diminished adrenal epinephrine responsiveness to fasting may play a contributory role.

Differential effects of leptin on thermoregulation and uncoupling protein abundance in the neonatal lamb

As the role of leptin in energy balance in neonate is unknown, we investigated the effect of acute (2 h) and chronic (7 days) administration of leptin (100 microg/day) on thermoregulation and mitochondrial protein abundance in adipose tissue. The concentration of uncoupling protein (UCP)1 and voltage-dependent anion channel (VDAC) located on the inner and outer mitochondrial membranes, respectively, were measured. Administration of leptin prevented the normal decline in colonic temperature over the first few hours and days after birth. It subsequently accelerated the loss of both mRNA and protein for UCP1 but had no effect on VDAC abundance. At seven days of age, colonic temperature was correlated strongly with both mRNA abundance and thermogenic potential of UCP1 in leptin-treated but not control lambs, indicating more effective use of UCP1 for heat production following leptin administration. Leptin had no effect on weight gain or adipose tissue deposition; at one day of age only, leptin mRNA was correlated positively with adipose tissue weight. In conclusion, leptin administration to neonatal lambs improves thermoregulation and promotes the loss of UCP1 in brown adipose tissue.

Long-term suppression of weight gain, adiposity, and serum insulin by central leptin gene therapy in prepubertal rats: effects on serum ghrelin and appetite-regulating genes

Intracerebroventricular administration of recombinant adeno-associated virus (rAAV) encoding the rat leptin gene (rAAV-lep) to 24-d-old female and male rats suppressed postpubertal weight gain for extended periods by decreasing food consumption and adiposity, as reflected by lowered serum leptin, insulin, and FFA. Serum ghrelin levels were increased in young but not older rats. Central rAAV-lep therapy also increased energy expenditure through nonshivering thermogenesis in younger rats as shown by expression of uncoupling protein mRNA in brown adipose tissue. The sustained decrease in appetite seemingly resulted from attenuation of appetite-stimulating neuropeptide Y and enhancement of appetite-inhibiting melanocortin signalings in the hypothalamus. Neither the onset of pubertal sexual maturation nor reproductive cyclicity in adult female rats was affected by the sustained reduction in energy consumption and weight gain. These findings demonstrate that central leptin gene therapy in prepubertal rats is a novel therapy to control postpubertal weight gain, adiposity, and hyperinsulinemia for extended periods.

Norepinephrine specifically stimulates ribonucleotide reductase subunit R2 gene expression in proliferating brown adipocytes: mediation via a cAMP/PKA pathway involving Src and Erk1/2 kinases

We have examined whether a qualitative switch occurs in the response of the ribonucleotide reductase (RNR) genes to the effect of the physiological cAMP-elevating agent norepinephrine (NE) during the development of brown adipocytes. Basal expression of the genes for both RNR subunits, R1 and R2, was high in proliferating cells, but was markedly down-regulated in parallel with adipocyte differentiation. NE stimulation, which promotes DNA synthesis and proliferation of brown preadipocytes, resulted in an increased expression of the R2 gene in proliferating cells (1.6-fold), but was without effect on R1 expression. In contrast, NE stimulation of confluent differentiating brown adipocytes reduced both R1 and R2 expression. The NE stimulation of R2 expression in preadipocytes was mimicked by forskolin and abolished by H89, demonstrating mediation via cAMP and protein kinase A (PKA). Also, inhibitors of Src and of Erk1/2 kinases markedly reduced NE-stimulated R2 expression. We conclude that adrenergic stimulation of brown adipocytes by NE specifically elevates expression of the RNR subunit R2 gene in the proliferative stage of brown adipocyte development, the mediating pathway being a cAMP/PKA cascade further involving Src and the MAP kinase Erk1/2. These results suggest that adrenergic stimulation of brown adipocyte proliferation may act at the level of gene expression of the limiting subunit for RNR activity, R2, and demonstrate a qualitative switch in the response of the R2 gene to cAMP-elevating agents as a consequence of the switch from proliferating to differentiating cell status.

Angiotensinogen-deficient mice exhibit impairment of diet-induced weight gain with alteration in adipose tissue development and increased locomotor activity

White adipose tissue is known to contain the components of the renin-angiotensin system, which gives rise to angiotensin II from angiotensinogen (AGT). Recent evidence obtained in vitro and ex vivo is in favor of angiotensin II acting as a trophic factor of adipose tissue development. To determine whether AGT plays a role in vivo in this process, comparative studies were performed in AGT-deficient (agt(-/-)) mice and control wild-type mice. The results showed that agt(-/-) mice gain less weight than wild-type mice in response to a chow or high fat diet. Adipose tissue mass from weaning to adulthood appeared altered rather specifically, as both the size and the weight of other organs were almost unchanged. Food intake was similar for both genotypes, suggesting a decreased metabolic efficiency in agt(-/-) mice. Consistent with this hypothesis, cellularity measurement indicated hypotrophy of adipocytes in agt(-/-) mice with a parallel decrease in the fatty acid synthase activity. Moreover, AGT-deficient mice exhibited a significantly increased locomotor activity, whereas metabolic rate and mRNA levels of uncoupling proteins remained similar in both genotypes. Thus, AGT appears to be involved in the regulation of fat mass through a combination of decreased lipogenesis and increased locomotor activity that may be centrally mediated.

Cloning of BUG demonstrates the existence of a brown preadipocyte distinct from a white one

BACKGROUND

Several indirect arguments agree with the existence of a brown preadipocyte distinct from a white one. Nevertheless, to date, no molecular marker has been available to directly in vivo demonstrate this hypothesis.

OBJECTIVE

The aim of this study was to find a gene expressed in brown preadipocyte but not in white and to use it as a molecular marker to analyse brown preadipocyte recruitment in different physiological and physiopathological situations.

METHOD

Differential display was performed on stromal-vascular and adipocyte fractions of white and brown adipose tissues in rat.

RESULTS

We identified a new gene, BUG, preferentially expressed in the stromal-vascular fraction of brown fat vs other adipose tissues fractions in adult rat. This RNA is also highly expressed in heart and, to a lesser extent, in other tissues such as kidney and brain. The BUG transcript is detected by in situ hybridization in putative preadipocytes within brown adipose tissue. Its level is transiently and specifically up-regulated during early stages of brown preadipocyte differentiation in a primary culture system, before the acquisition of late brown adipocyte phenotype. During development, BUG can be detected before the emergence of UCP-1 expression. In adult rats, BUG expression is inversely associated to brown adipose tissue (BAT) activation during cold exposure as well as in obese animals.

CONCLUSIONS

The pattern of BUG expression agrees with an early divergence between brown and white adipocyte lineages. It also reveals the existence of a pool of committed brown preadipocytes within BAT that are recruited during cold exposure. BUG expression is increased in obese animals, suggesting that an early defect in brown preadipocyte differentiation could account for impaired BAT function in genetically obese rats.

Emergence during development of the white-adipocyte cell phenotype is independent of the brown-adipocyte cell phenotype

In mammals, two types of adipose tissue are present, brown and white. They develop sequentially, as brown fat occurs during late gestation whereas white fat grows mainly after birth. However, both tissues have been shown to have great plasticity. Thus an apparent transformation of brown fat into white fat takes place during post-natal development. This observation raises questions about a possible conversion of brown into white adipocytes during development, although indirect data argue against this hypothesis. To investigate such questions in vivo, we generated two types of transgenic line. The first carried a transgene expressing Cre recombinase specifically in brown adipocytes under the control of the rat UCP1 promoter. The second corresponded to an inactive lacZ gene under the control of the human cytomegalovirus promoter. This dormant gene is inducible by Cre because it contains a Stop sequence between two loxP sequences, separating the promoter from the coding sequence. Adipose tissues of progeny derived by crossing independent lines established from both constructs were investigated. LacZ mRNA corresponding to the activated reporter gene was easily detected in brown fat and not typically in white fat, even by reverse transcriptase PCR experiments. These data represent the first direct experimental proof that, during normal development, most white adipocytes do not derive from brown adipocytes.

Brown adipocyte precursor cells: a morphological study

The origin of brown adipocyte precursor cells is to date unknown. Some authors believe they arise from vascular cells, others from interstitial cells. The purpose of the present ultrastructural study was to find markers in rat fetal and perinatal adipose tissue that can be used to identify brown adipose precursor cells. The study was carried out on the interscapular brown adipose tissue of fetal (fetuses of 19 and 21 days) and perinatal rats (pups of 4 and 12 hours and of 1, 3, 5, 7, 9, 11, 13, and 15 days). The analysis focused on stem cells and showed the characteristic presence of typical mitochondria which make their identification as brown adipocyte precursor cells inequivocal. These cells were frequently observed in a pericytic position. Also some endothelial cells were characterised by typical mitochondria and abundant glycogen. These data seem to support the hypothesis that brown adipocytes originate from vascular cells.

Transgenic mice overexpressing leptin accumulate adipose mass at an older, but not younger, age

Sensitivity to leptin is associated with a normal regulation of the adipose mass, whereas decreased leptin sensitivity results in elevated adipose tissue stores. To address whether the effects of chronic hyperleptinemia are sustained with age, we generated transgenic mice that overexpress leptin under the control of the fat specific aP2 promoter/enhancer. At 6-9 weeks of age, transgenic mice overexpressed 5-fold more human leptin than endogenous mouse levels and had consistently low body weights, with reduced brown and white fat depots characterized by adipocytes either devoid of or containing minute lipid droplets. However, at 33-37 weeks, despite continuous secretion of human leptin, the transgenic mice showed a rebound effect characterized by an increase in body weight, accumulation of adipose mass, and lipid-filled adipocytes. Thus, this mouse model exhibits a two-stage phenotype, with respect to fat accumulation. In addition, plasma glucose, triglycerides, and cholesterol levels were markedly depressed in young, but not older, transgenic mice. A detrimental consequence of early hyperleptinemia was a failure of the transgenic mice to acclimatize to the cold, as a result of depleted fat stores within their brown adipocytes. Cold exposure was tolerated after a 2-week high-fat diet or at an older age when fat depots had naturally accumulated. Treatment of the older transgenic mice with large doses of leptin stimulated weight loss, demonstrating that the leptin pathway still responds to pharmacological levels of leptin. Overall, these studies show that moderate hyperleptinemia in normal mice results in a sensitivity of the adipose mass to leptin at a younger (but not older) age. The mechanisms that lead to the accumulation of fat at an older age remain largely unknown, and this hyperleptinemic mouse model will allow the uncovering of at least some of these mechanisms.