Brown Adipose Tissue Composition Research Articles

Repeated short excursions from thermoneutrality suffice to restructure brown adipose tissue

Given the presence of brown adipose tissue in adult humans, an important issue is whether human brown adipose tissue is recruitable. Cold exposure is the canonical recruitment treatment; however, in experimental animals (mice), recruitment of brown adipose tissue is normally induced by placing the mice in constant cold, a procedure not feasible in humans. For possible translational applications, we have therefore investigated whether shorter daily excursions from thermoneutrality would suffice to qualitatively and quantitatively induce recruitment in mice. Mice, housed at thermoneutrality (30 °C) to mimic human conditions, were transferred every day for 4 weeks to cool conditions (18 °C), for 0, 15, 30, 120 and 420 min (or placed constantly in 18 °C). On the examination day, the mice were not exposed to cold. Very short daily exposures (≤30 min) were sufficient to induce structural changes in the form of higher protein density in brown adipose tissue, changes that may affect the identification of the tissue in e.g. computer tomography and other scan studies. To estimate thermogenic capacity, UCP1 protein levels were followed. No UCP1 protein was detectable in inguinal white adipose tissue. In the interscapular brown adipose tissue, a remarkable two-phase reaction was seen. Very short daily exposures (≤30 min) were sufficient to induce a significant increase in total UCP1 levels. For attainment of full cold acclimation, the mice had, however, to remain exposed to the cold. The studies indicate that marked alterations in brown adipose tissue composition can be induced in mammals through relatively modest stimulation events.

Association between supraclavicular brown adipose tissue composition at birth and adiposity gain from birth to 6 months of age.

BackgroundBrown Adipose Tissue (BAT) is associated with higher energy expenditure and lower adiposity in adults. However, the relationship between BAT composition and adiposity in early life is unknown. The objective of this study was to test the hypothesis that brown fat composition at birth is prospectively associated with adiposity gain during the first six months of postnatal life.MethodsN=35 healthy infants were followed prospectively from intrauterine life and birth through 6 months age. Dixon MRI scans were conducted during the neonatal period to characterize supraclavicular BAT composition. Dual-energy X-ray absorptiometry to assess total body composition was performed within the first month of life and at 6 months age.ResultsAfter adjusting for potential confounding factors, a more brown-like composition (smaller fat fraction) of the supraclavicular BAT depot was associated with a smaller increase in percent body fat over the first six months of postnatal life.ConclusionsA more brown-like BAT composition at birth appears to be protective against excess adiposity gain in early life. Newborn BAT tissue may constitute a target for prevention strategies against the subsequent development of obesity.

Brown and white adipose tissue lipid composition in three successive progenies of rats: Effects of ethanol exposure

The effect of ethanol exposure on the fatty acid composition of brown and white adipose tissue in three successive rat progenies at the end of an experimental period (24 weeks) was studied. Ethanol‐treated rats received a standard rat chow diet and 5, 10 and 15% ethanol in the ad libitum drinking fluid over 3 successive weeks. Then a concentration of 20% ethanol was maintained for 5 additional weeks up to the end of the experimental period. The males and females in the ethanol treated group were mated to obtain the 1st generation of offspring. Then female and male rats from the 1st generation were mated to obtain the 2nd generation. Finally, males and females from the 2nd generation were mated to obtain the 3rd generation of ethanol treated rats. Another group served as control and received only water and a standard rat chow diet. The control group was handled in the same way as the other experimental groups. In the 1st and 2nd generations the percentage of stearic acid (18:0) decreased and palmitoleic (16:ln7) and oleic acid (18:ln9) increased in both adipose tissues of ethanol‐treated rats with respect to control. Additionally, n‐3 and n‐6 series were reduced both in brown and white adipose tissues. In the 3rd generation the fatty acid composition of the white adipose tissue was similar to that of control rats. Thus, no significant difference in essential fatty acids and oleic acid (18:ln9) were found. However, the fatty acid composition of the brown adipose tissue, in the 3rd generation, was similar to that observed in the 1st and 2nd generation. Thus, a decrease in essential fatty acids and an increase in oleic acid (18:ln9) was found. This suggests adaptation to ethanol consumption during successive progenies in white adipose tissue. However, in brown adipose tissue the values indicate a triglyceride storing during the thermogenesis, which is more important to newborns.

Fatty acid composition of brown adipose tissue in genetically heat-tolerant FOK rats.

The phospholipid fatty acid composition of brown adipose tissue (BAT) was examined in inbred heat-tolerant FOK rats and compared with that in conventional Wistar rats not previously exposed to heat. The FOK rats showed higher unsaturation states, as indicated by higher levels of polyunsaturated fatty acids and a higher unsaturation index and polyunsaturated fatty acids/saturated fatty acids ratio. This higher level of unsaturation was characterized by the higher amount of polyunsaturated fatty acids such as linoleic acid, arachidonic acid and docosahexaenoic acid. It may be concluded that the increased docosahexaenoic acid level in BAT phospholipids brings about the hyperplasia of BAT, causing an enhancement of its in vivo thermogenic activity as well as the systemic non-shivering thermogenesis observed in heat-tolerant FOK rats.

In vitro thermogenesis and phospholipid fatty acid composition of brown adipose tissue in fasted and refed rats.

Membrane phospholipids are known for their role in the regulation of membrane structures and functions. Membrane phospholipid fatty acid docosahexaenoic acid (DHA) has been recently indicated to be important for the regulation of cellular activities, including metabolic regulation. Our previous studies have indicated the involvement of DHA in the regulation of brown adipose tissue (BAT) thermogenesis. The objective of the present study is to examine the changes in BAT phospholipid fatty acid composition including DHA and thermogenic activity in fasted and refed rats. Phospholipid content per microgram DNA was decreased in rats fasted for 72 h and it was not restored to the control level by refeeding for 72 h. Phospholipid fatty acid composition of BAT, as expressed by mol%, was modified in the fasted rats. Most notably, DHA, which constituted about 89% of the n-3 polyunsaturated fatty acids, was decreased concomitant with the increase in the n-6 polyunsaturated fatty acid arachidonic acid. The monounsaturated to saturated fatty acid ratio, which is an index of Delta(9)-desaturase activity and membrane fluidity, was decreased. Thermogenesis, as measured by the in vitro oxygen consumption of BAT, was suppressed in the fasted rats. All of the above changes were restored to normal levels after refeeding the fasted rats for 72 h. In vitro oxygen consumption correlated with the level of DHA and monounsaturated to saturated fatty acid ratio. These results indicate that the modification of phospholipid fatty acid composition, especially the modification of n-3 polyunsaturated fatty acid DHA, and membrane fluidity are related to BAT thermoregulation in fasted and refed rats.

Thermogenesis and fatty acid composition of brown adipose tissue in rats rendered hyperthyroid and hypothyroid-with special reference to docosahexaenoic acid.

The effects of hyperthyroidism and hypothyroidism on brown adipose tissue (BAT) thermogenesis and phospholipid fatty acid composition were investigated in rats. Chronic triiodothyronine (T3) treatment (hyperthyroidism) increased the interscapular BAT pad weight, its triacylglycerol content, and its DNA content. It did not affect basal and noradrenaline-stimulated in vitro oxygen consumption of BAT expressed per microg DNA, although it significantly increased the oxygen consumption of the whole BAT pad. T3 treatment had little effect on phospholipid content and phospholipid fatty acid composition. In contrast, chronic methimazole treatment (hypothyroidism) decreased the BAT pad weight and the triacylglycerol content, but did not significantly change the DNA content in comparison with the control. It significantly decreased the noradrenaline-stimulated BAT oxygen consumption expressed per microg DNA and per BAT pad, but did not change the basal oxygen consumption. Methimazole treatment significantly affected phospholipid content and phospholipid fatty acid composition. Among the major fatty acids of BAT, it decreased docosahexaenoic acid (DHA), arachidonic acid, palmitic acid, palmitoleic acid, and oleic acid, and it increased linoleic acid, stearic acid, and eicosapentaenoic acid. A regression analysis revealed a positive relationship between in vitro respiration and DHA levels in phospholipids (r = 0.404, p<0.05). These results suggest that thyroid hormones have trophic action on BAT and are necessary for BAT thermogenic activity. This study also suggests that DHA is involved in the regulation of BAT thermogenic activity, as we previously indicated.

Fatty acid composition of brown adipose tissue in dietary obese rats.

The effects of both dietary obesity and a food deprivation period of 24 hours on fatty acid composition of brown adipose tissue have been investigated. Long time exposure to a hypercaloric high-fat diet such as the cafeteria diet induced an important tissue fatty acid accumulation, mainly for the major saturated and monounsaturated fatty acids. Notable metabolic differences have been observed in the behaviour of control and obese rats facing a food deprivation period: a preferential utilization of the most abundant saturated fatty acids in control rats and a minor response in obese rats, with a greater fat accumulation in the interscapular brown adipose tissue.

Effects of nitric oxide synthase inhibition on phospholipid fatty acid composition of brown adipose tissue.

We investigated the effects of nitric oxide synthase inhibition on the phospholipid fatty acid composition of brown adipose tissue by administering chronic N omega-nitro-L-arginine methyl ester (L-NAME) (0.01%) to rats via drinking water. L-NAME caused a significant decrease in C14:0, C16:0, C16:1 and C18:1, with a concomitant increase in C18:0 and C18:2, L-NAME treatment elicited an increase in the unsaturation index. There were no significant changes in other fatty acids including the polyunsaturated fatty acids docosahexaenoate (DHA, C22:6) and arachidonate (C20:4). Polyunsaturated fatty acid linoleate (C18:2) was increased and the arachidonate index decreased in the L-NAME-treated group, suggesting a diminished conversion of C18:2 to C20:4.

Changes in brown adipose tissue composition during fasting and refeeding of diet-induced obese mice.

The objective of this work was to evaluate how obesity would influence the changes in brown fat (BAT) thermogenic capacity during fasting-refeeding. Mice fed either chow or chow + high-fat supplement for 6 wk had body weights of 34 +/- 1 and 43 +/- 1 g, respectively. They were fasted for 48 h followed by ad libitum refeeding for up to 5 days. Loss of carcass fat was similar between food-deprived mice previously fed chow or chow + high-fat supplement. However, even after a 48-h fast, obese mice still had a carcass fat content much greater than that of chow-fed mice. Brown fat atrophy caused by food deprivation was characterized by reductions in tissue weight, fat, mitochondrial proteins and uncoupling protein (UCP), without change in tissue DNA. Obesity did not alter the rate or extent of brown fat atrophy. Upon refeeding 48-h-fasted lean and obese mice, there was recovery of BAT thermogenic capacity that was similar between the two groups. In chow-fed mice, an intact neural input was essential for recovery of BAT thermogenic capacity during refeeding. These results indicate that food deprivation triggers an immediate adaptive response in mice previously fed chow or chow + a high-fat supplement and that reduction in brown fat thermogenic capacity during fasting and its recovery during refeeding appear little affected by the size of the animal energy reserves.

Postnatal changes in fatty acids composition of brown adipose tissue.

It has been demonstrated that thermogenic activity of brown adipose tissue (BAT) is higher during the early postnatal period, decreasing towards a low adult level. The present study examined postnatal changes in the lipid composition of BAT. BAT from pre-weaning rats at 4 and 14 days old showed the following differences in lipid composition compared to that from adults of 12 weeks old. (i) Relative weight of interscapular BAT to body weight was markedly greater. (ii) BAT-triglyceride (TG) level was lower, while BAT-phospholipid (PL)level was higher. (iii) In TG fatty acids (FA) polyunsaturated fatty acids (PU; mol%), arachidonate index (AI), unsaturation index (UI) and PU/saturated FA (SA) were higher; rare FA such as eicosadienoate, bishomo-gamma-linolenic acid and lignoceric acid in mol % were also higher. (iv) In PL-FA monounsaturated FA (MU) in mol % was lower; PU mol %, AI and UI were higher. These features in BAT of pre-weaning rats resembled those in the cold-acclimated adults, suggesting a close relationship of the PL-FA profile to high activity of BAT.

Lipid composition of brown adipose tissue mitochondria and microsomes in hyperthyroid rats

1. 1. The effects of triiodothyronine on the lipid composition of rat brown adipose tissue (BAT) mitochondria and microsomes was investigated by high performance liquid chromatography (HPLC). 2. 2. An increase of about 20% was noted in mitochondrial cholesterol and phospholipids, while a decrease of about 20% for both total cholesterol and phospholipids was observed in microsomes from hyperthyroid rats. 3. 3. The BAT phospholipid composition was altered significantly in mitochondria from T 3-treated rats with an increase (41%) of cardiolipin and a decrease (18%) in phosphatidylcholine. 4. 4. In microsomes, a decrease by 25% in phosphatidylinositol was accompanied by a similar additional percentage increase in phosphatidylethanolamine. 5. 5. Important alterations in the fatty acid pattern were found in mitochondrial neutral lipids.

Rare fatty acids in brown fat are substrates for thermogenesis during arousal from hibernation.

Because brown adipose tissue lipids are the preferred substrate for thermogenesis during arousal from hibernation, the fatty acid composition of brown fat lipids was followed during cold acclimation and during a hibernation bout. In control golden hamsters (living at 22 degrees C), the fatty acid composition of the white adipose tissue closely resembled that of the food, but brown adipose tissue contained more animal-derived fatty acids. As an effect of acclimation to cold, the fatty acid composition of brown adipose tissue changed to resemble that of the food, and no marked differences between white and brown adipose tissue were then evident. During a hibernation bout, a major part of the fatty acids accumulated in brown fat during entry into hibernation consisted of "rare" acids, such as homo-gamma-linoleic acid. Homo-gamma-linoleic, together with eicosadienoic acid and lignoceric acid, was preferentially utilized during the early phase of arousal. During this phase, "bulk" fatty acids, such as linoleic acid, were spared, whereas in late arousal, linoleic acid was the preferred substrate. It was concluded that rare fatty acids are of quantitative significance in brown adipose tissue during hibernation and arousal.

Effects of repeated cycles of fasting-refeeding on brown adipose tissue composition in mice.

Mice fasted for 24 h showed reductions in carcass fat and gonadal fat depots and atrophy of brown adipose tissue (BAT) that was characterized by loss of protein and succinate dehydrogenase. These changes were reversed on 24 h of refeeding. Cycling mice experienced 14 cycles of 1 day of fast followed by 2 days of refeeding, whereas control mice were fed ad libitum. Weight loss during each fast remained constant, and the animals lost and regained in excess of twice their initial weights within 6 wk. However, final weight and carcass and gonadal fat weights were similar to those of animals fed ad libitum. Total food intake was similar between cycling mice and those fed ad libitum suggesting an increase in feeding efficiency. There was no development of resistance to food deprivation since the preceding fasting experience of the animal had no effect on weight and carcass fat loss during a 24- or 48-h fast. Norepinephrine-stimulated oxygen consumption that was reduced in cycling mice was probably the result of a reduction of BAT thermogenic capacity. BAT succinate dehydrogenase content and the concentration of uncoupling protein in isolated mitochondria were significantly reduced. These changes in BAT composition were not observed when the refeeding period of each cycle was increased to 6 days. These results suggest that reduced energy expenditure in BAT may play a role in the conservation of energy during intermittent and frequent bouts of food deprivation.

Thyroid hormone effects on neonatal thermogenesis.

It is clear that thyroid hormones modulate non-shivering heat production of the newborn mammal. While some obvious thyroid hormone effects on thermogenesis can be demonstrated in adult, cold-acclimated or hibernating animals, these findings cannot be directly applied to the newborn, in which changes in amount and composition of brown adipose tissue, as well as sympathetic and thyroid system maturational events, are occurring. Thyroid hormones do influence the prenatal development and subsequent responses of brown adipose tissue in neonates through primary actions on brown adipose tissue iodothyronine 5'monodeiodinase and mitochondrial uncoupling protein. Secondary effects involving brown adipocyte growth, lipid composition, oxidation proteins, and sympathoadrenal activity are of lesser importance in thyroid hormone modulation of newborn thermogenesis. The acute surge in thyroid hormones that occurs at birth seems of limited significance with regard to neonatal thermogenesis. The stimulation of sympathoadrenal activity at the time of birth, as well as continued in situ conversion of T4 to T3 in the brown adipocyte, are of critical importance in stimulating and modulating heat production in the newborn.

Effects of maternal hypocaloric diet feeding on neonatal rat brown adipose tissue.

Hypocaloric diet (control diet diluted 1:1 with cellulose) feeding during pregnancy caused a reduction in the body weight of rat pups at birth whereas the main parameters of brown adipose tissue composition and thermogenic activity were unaffected. When prenatally underfed rat pups were nursed by untreated dams eating control diet during lactation, early (day 4 of life) and late (day 13 of life) neonatal body weight was rehabilitated and brown adipose tissue remained essentially unchanged. When prenatally underfed rat pups were nursed by dams that were fed with the hypocaloric diet during lactation, neonatal body weight continued lower at the two mentioned days of life and the overall thermogenic capacity of brown fat (GDP binding/g body weight) was substantially depressed. The reduction of the brown fat thermogenic capacity in these pups is mainly due to a hypotrophy of the mitochondrial component of the tissue as indicated by the lowered cytochrome oxidase activity. Results indicate that mild maternal underfeeding during lactation may depress brown adipose tissue thermogenic function in neonates whereas similar hypocaloric intakes during pregnancy did not alter brown fat thermogenic capacity.

Effects of fasting and food restriction on brown adipose tissue composition in normal and dystrophic hamsters.

Fasting for 36-48 h or food restriction (30% reduction of daily food intake for 6 weeks) caused brown adipose tissue (BAT) atrophy in hamsters. Fasting-induced atrophy was characterized by reductions in tissue mass, DNA, protein, and thermogenin. By contrast, food restriction had no effect on tissue cellularity (DNA) but markedly reduced the tissue protein and thermogenin contents. The concentration of thermogenin in isolated mitochondria was unchanged by fasting or food restriction. Dystrophic hamsters had a reduced BAT mass when compared with weight-matched control hamsters. This resulted from a reduction in tissue cellularity since BAT DNA, protein and thermogenin contents were all reduced. The extent of binding of [3H]guanosine diphosphate to isolated mitochondria and their content of thermogenin were similar in normal and dystrophic hamsters. In response to cold exposure, as in normal hamsters, BAT of dystrophic hamsters grew and the tissue thermogenin increased, but the mitochondrial concentration of thermogenin did not change. In response to fasting, in contrast with normal hamsters, there was no significant reduction in BAT DNA in dystrophic animals and the loss of tissue protein was reduced. However, the relative changes in BAT composition during chronic food restriction were similar in normal and dystrophic animals. Thus, reduction in hamster BAT thermogenic capacity during food deprivation may occur by loss of cells and (or)reduction in the tissue protein and thermogenin contents. The extent of protein and (or) DNA loss may be dependent upon the original tissue mass and the severity of food deprivation.

Sequential changes in brown adipose tissue composition, cytochrome oxidase activity and GDP binding throughout pregnancy and lactation in the rat

The sequential appearance of changes in interscapular brown adipose tissue composition, cytochrome oxidase activity and GDP binding was studied throughout pregnancy and lactation in the rat. Brown adipose tissue was hypertrophied during pregnancy because of progressive lipid accumulation, whereas its mitochondrial component and GDP binding to brown fat mitochondria were unchanged. In early lactation (day 5) there was a decrease in the overall GDP binding to brown fat only because of the lower mitochondrial protein content. In late stages of lactation (days 10 and 15), the amount of tissue and its mitochondrial protein content were minimal and the GDP binding per mitochondrial protein decreased substantially. Scatchard analysis in day-15-lactating rats indicated a large decrease in GDP binding sites without any changes in affinity. It is concluded that the diminished thermogenic activity of brown fat in lactation is attained through changes at different structural levels of the tissue occurring in a characteristic sequential trend; first a reduction in its mitochondrial component, and only later, at nid-lactation, a decrease in the specific mitochondrial proton conductance pathway activity.

Effect of temperature on lipoprotein lipase and lipogenic enzyme activities in brown adipose tissue of hypophysectomized rats.

It has been shown that the same modifications on the composition of brown adipose tissue (BAT) which are normally induced following cold stimulation are also observed in hypophysectomized rats acclimated either at 28 degrees C or 15 degrees C. To test the possibility of BAT stimulation in hypophysectomized rats, we have determined some enzymatic activities known to modulate the energy supply to that organ. Seven week old Long-Evans rats were hypophysectomized. Three weeks later, they were exposed to either 28 degrees C or 15 degrees C ambient temperature for five or six weeks. Hypophysectomized rats were compared to age matched or weight matched controls. Total lipoprotein lipase activity (LPL) (triglyceride uptake) was enhanced in BAT of 28 degrees C hypophysectomized rats compared to controls. Cold acclimation led to a large increased activity. Total LPL activity was comparable in BAT of hypophysectomized and control rats. Total malic enzyme and glucose-6-phosphate dehydrogenase activities (in situ lipogenesis) were doubled in BAT of 28 degrees C hypophysectomized compared to controls. A large enhancement was observed in BAT of either 15 degrees C control or 15 degrees C hypophysectomized rats. Among the studied organs (liver, white adipose tissue, heart, BAT) hypophysectomy promotes the three enzyme activities only in BAT. These variations were discussed with relation to the effect of hypophysectomy on brown adipose tissue at 15 degrees C and 28 degrees C.

Changes in metabolic rate and brown adipose tissue composition during nutritional rehabilitation of postnatally undernourished rats.

Rats previously undernourished (PU) during the first 3 postnatal weeks gained less weight than controls during the subsequent rehabilitation period, in spite of similar levels of food intake per body weight 0.75. Resting oxygen consumption (ml/min/kg0.75) was significantly elevated (15-22%) in the PU group after 36 days of age but this difference was almost completely abolished by beta-adrenergic blockade with propranolol. PU rats also showed a greater thermogenic activity after noradrenaline (25 micrograms/100 g b.wt.) than controls. The mass of brown adipose tissue (BAT, mg/100 g b.wt.) was reduced by undernutrition but during rehabilitation BAT increased in PU rats to a value greater than that of controls. These results indicate that the low weight gain of PU rats may result partly from a high metabolic rate due to sympathetic activation of BAT.

Functional disconnection of brown adipose tissue in hypothalamic obesity in rats.

The metabolic responses to electrical nerve stimulation, norepinephrine or octanoate additions were studied using continuous monitoring of NAD(P)H/NADP redox state by reflexion spectrophotometry of interscapular brown adipose tissues from control and ventromedial hypothalamic (VMH) lesioned rats. The responses to these stimuli were all greatly decreased already 3 days after VMH lesions, indicating a reduced cell capacity to oxidize free fatty acids. Measurements of interscapular brown adipose tissue composition 4-5 weeks after VMH lesions showed a decrease of both DNA concentration and total content, indicating some tissue involution. It is concluded that the involvement of the ventromedial hypothalamus in the activation of brown adipose tissue provided a possible anatomical clue concerning pathways connecting thermal and weight regulations.