Cold-induced Nonshivering Thermogenesis Research Articles

Cold-induced thermogenesis requires neutral-lipase-mediated intracellular lipolysis in brown adipocytes.

Long-chain fatty acids (FAs) are the major substrates fueling brown adipose tissue (BAT) thermogenesis. Investigation of mouse models has previously called into question the contribution of brown adipocyte intracellular lipolysis to cold-induced non-shivering thermogenesis. Here, we determined the role of the lipolytic enzymes, adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL), in BAT thermogenesis. Brown fat from mice with inducible brown-adipocyte-specific deletion of ATGL and HSL (BAHKO) is hypertrophied with increased lipid droplet size and preserved mitochondria area and density. Maintenance of body temperature during cold exposure is compromised in BAHKO mice in the fasted but not in the fed state. This altered response to cold is observed in various thermal and nutritional conditions. Positron emission tomography-computed tomography using [11C]-acetate and [11C]-palmitate shows abolished cold-induced BAT oxidative activity and impaired FA metabolism in BAHKO mice. Our findings show that brown adipocyte intracellular lipolysis is required for BAT thermogenesis.

Glucose regulation of adipose tissue browning by CBP/p300- and HDAC3-mediated reversible acetylation of CREBZF

Glucose is required for generating heat during cold-induced nonshivering thermogenesis in adipose tissue, but the regulatory mechanism is largely unknown. CREBZF has emerged as a critical mechanism for metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as nonalcoholic fatty liver disease (NAFLD). We investigated the roles of CREBZF in the control of thermogenesis and energy metabolism. Glucose induces CREBZF in human white adipose tissue (WAT) and inguinal WAT (iWAT) in mice. Lys208 acetylation modulated by transacetylase CREB-binding protein/p300 and deacetylase HDAC3 is required for glucose-induced reduction of proteasomal degradation and augmentation of protein stability of CREBZF. Glucose induces rectal temperature and thermogenesis in white adipose of control mice, which is further potentiated in adipose-specific CREBZF knockout (CREBZF FKO) mice. During cold exposure, CREBZF FKO mice display enhanced thermogenic gene expression, browning of iWAT, and adaptive thermogenesis. CREBZF associates with PGC-1α to repress thermogenic gene expression. Expression levels of CREBZF are negatively correlated with UCP1 in human adipose tissues and increased in WAT of obese ob/ob mice, which may underscore the potential role of CREBZF in the development of compromised thermogenic capability under hyperglycemic conditions. Our results reveal an important mechanism of glucose sensing and thermogenic inactivation through reversible acetylation.

Cold-inducible lncRNA266 promotes browning and the thermogenic program in white adipose tissue.

Cold-induced nonshivering thermogenesis has contributed to the improvement of several metabolic syndromes caused by obesity. Several long noncoding RNAs (lncRNAs) have been shown to play a role in brown fat biogenesis and thermogenesis. Here we show that the lncRNA lnc266 is induced by cold exposure in inguinal white adipose tissue (iWAT). In vitro functional studies reveal that lnc266 promotes brown adipocyte differentiation and thermogenic gene expression. At room temperature, lnc266 has no effects on white fat browning and systemic energy consumption. However, in a cold environment, lnc266 promotes white fat browning and thermogenic gene expression in obese mice. Moreover, lnc266 increases core body temperature and reduces body weight gain. Mechanistically, lnc266 does not directly regulate Ucp1 expression. Instead, lnc266 sponges miR-16-1-3p and thus abolishes the repression of miR-16-1-3p on Ucp1 expression. As a result, lnc266 promotes preadipocyte differentiation toward brown-like adipocytes and stimulates thermogenic gene expression. Overall, lnc266 is a cold-inducible lncRNA in iWAT, with a key role in white fat browning and the thermogenic program.

78-OR: Adipose Lysosomal Acid Lipase Is Indispensable for Starvation- and Cold-Induced Lipolysis

Lipolysis is a metabolic process by which triglycerides (TGs) are hydrolyzed into free fatty acids (FFAs) and glycerol to serve as fuels during fasting or cold-induced nonshivering thermogenesis. Cytoplasmic lipases are thought to be the predominant mechanism of liberating FFA from lipid stores. However, lysosomes also contain lipolytic activity via lysosomal acid lipase (Lipa). In adipocytes, the primary lipid-storing cell in the body with significant involvement in metabolism, the roles of Lipa and lysosomal lipolysis are unclear. We found that Lipa expression increases in the adipose tissue of mice in response to fasting, cold exposure, and CL316,243 (CL) treatment, a mimic of cold-induced lipolysis. Similar results were obtained in cultured adipocytes with nutrient-depletion or isoproterenol treatment, mimicking fasting or cold, respectively. Treatment of C57BL/6J mice with Lalistat (Lali), a specific inhibitor of Lipa, impaired thermogenesis, as evidenced by suppression of cold- and fasting-enhanced increases in plasma FFAs levels during cold exposure or CL treatment. Lipolysis was inhibited by Lali treatment in cultured adipocytes and fresh adipose tissue of C57BL/6J mice under fasting- or cold-mimic conditions. To conduct more detailed in vivo studies, we developed adipose-specific Lipa knockout (A-Lipa) mice, which exhibited lower plasma FFAs when challenged with fasting, cold exposure, or CL treatment. Furthermore, A-Lipa mice were significantly thermogenesis-impaired with exposure to cold or treatment with CL. To determine whether autophagy is involved in these phenomena, adipose-specific Atg5 knockout (A-Atg5) mice were challenged with fasting, cold exposure, or CL treatment. As was found in A-Lipa mice, A-Atg5 mice had lower levels of plasma FFAs and obliteration of stress-induced thermogenesis. Overall, our data suggest a previously unrecognized contribution of lysosomal lipolysis in adipocytes, possibly involving autophagy-related lipophagy. Disclosure Y.Yeh: None. B.Razani: None. Funding American Heart Association (897628); National Institutes of Health (DK131188)

Taiwanese green propolis ameliorates metabolic syndrome via remodeling of white adipose tissue and modulation of gut microbiota in diet-induced obese mice

Excessive energy intake leads to dysbiosis of intestinal microbiota and puts surrounding tissues under oxidative stress and inflammation, contributing to the development of metabolic syndrome. Taiwanese green propolis (TGP) exhibits a broad spectrum of biological activities, including anti-bacterial, anti-inflammatory, and antioxidant properties. However, the benefits of TGP on metabolic syndrome have not been explained in detail. In this study, we examined the preventive effects of TGP on high-fat diet (HFD)-induced obesity. The results showed that TGP supplementation at 1000 ppm improved condition such as hyperlipidemia, fat accumulation, liver steatosis, and whitening of brown adipose tissue (BAT) in mice. In addition, we observed more cold-induced non-shivering thermogenesis by BAT in TGP treatment with 1000 ppm group. At lower dose of 500 ppm, TGP improved glucose intolerance and insulin insensitivity in HFD mice and restructured the composition of gut microbiota to reduce dysbiosis, which involved an increase in the abundance of metabolism-related bacteria such as Lachnospiraceae NK4A136 group and the decrease in Desulfovibrio. The change of dominant microbiota was associated with the homeostasis of blood glucose and lipid. Transcriptome and micro-western array analysis revealed that TGP supplementation at 500 ppm promoted the browning and adipogenesis in white adipose tissue (WAT), blocked inflammation signaling and attenuated reactive oxygen species, contributing to healthy WAT remodeling and offsetting negative metabolic effects of obesity. We concluded that TGP modulated the function of BAT, WAT, and gut microbiota, bringing a balance to the glucose and lipid homeostasis in the body.

Candidate biomarkers in brown adipose tissue for post-mortem diagnosis of fatal hypothermia.

Post-mortem diagnosis of fatal hypothermia (FHT) is challenging in forensic practice because traditional morphological and biochemical methods lack specificity. Recent studies have reported that brown adipose tissue (BAT) is activated during cold-induced non-shivering thermogenesis in mammals, but BAT has not been used to diagnose FHT. The aim of this study was to identify novel biomarkers in BAT for FHT based on morphological changes and differential protein expression. Two FHT animal models were created by exposing mice to 4 or -20 °C at 50% humidity. Morphologically, the unilocular lipid droplet content was significantly increased in BAT of FHT model mice compared with that of control mice. Proteomics analysis revealed a total of 283 and 266 differentially expressed proteins (DEPs) between the 4 or -20 °C FHT subgroups and control group, respectively. In addition, 140 proteins were shared between the FHT subgroups. GO and KEGG analyses revealed that the shared DEPs were mainly enriched in pathways associated with metabolism, oxidative phosphorylation, and thermogenesis. Further screening (|log2FC| > 1.6, q-value (FDR) < 0.05) identified GMFB, KDM1A, DDX6, RAB1B, SHMT-1, CLPTM1, and LMF1 as candidate biomarkers of FHT. Subsequent validation experiments were performed in FHT model mice using classic immunohistochemistry and western blotting. RAB1B and GMFB expression was further verified in BAT specimens from human cases of FHT. The results demonstrate that BAT can be used as a target organ for FHT diagnosis employing RAB1B and GMFB as biological markers, thus providing a new strategy for the post-mortem diagnosis of FHT in forensic practice.

Intraindividual difference between supraclavicular and subcutaneous proton density fat fraction is associated with cold-induced thermogenesis

Brown adipose tissue (BAT) activity is triggered by cold exposure resulting in an increased resting energy expenditure, called cold-induced non-shivering thermogenesis (CIT). Magnetic resonance (MR)-based proton density fat fraction (PDFF) of the supraclavicular fossa has been proposed as a surrogate marker of human BAT. The present study investigates supraclavicular PDFF in relation to CIT. For this prospective cross-sectional study 39 adults were recruited, from a cross-sectional study, exploring energy expenditure after cold exposure compared to thermoneutral conditions. Participants underwent additional MR examination of neck, pelvis, and abdomen. Supraclavicular and subcutaneous gluteal adipose tissue depots were segmented semi-automatically. Mean PDFF was assessed for each compartment and the delta PDFF was calculated as the difference of both. Correlation analysis of supraclavicular PDFF to CIT was performed for the whole cohort and subgroups, sorted by body mass index (BMI) and body fat percentage. Median age of participants (61.5% female) was 27 years. BMI ranged from 19.0 to 38.5 kg/m2, with body fat percentages from 4.6% to 45.3%. Median supraclavicular PDFF of 82.5% and median gluteal PDFF of 91.1%, were significantly different (P<0.0001). Median delta PDFF was 8.8% (3.9-21.9%). Mean CIT was 4.7%±9.0%. No statistically significant correlation of supraclavicular PDFF and CIT was found in the whole cohort and in most of the observed subgroups. Just the subgroup with normal body fat percentage revealed significant correlations between supraclavicular PDFF and CIT (rho =-0.59; P=0.003). Delta PDFF was significantly associated with CIT (rho =0.36; P=0.026). PDFF is influenced by adiposity. Therefore, if supraclavicular PDFF is used as approach to indirectly assess BAT presence, body composition should be considered. Delta PDFF, as the difference between gluteal and supraclavicular PDFF, may be a marker of CIT.

Cold Exposure Distinctively Modulates Parathyroid and Thyroid Hormones in Cold-Acclimatized and Non-Acclimatized Humans.

Cold-induced activation of thermogenesis modulates energy metabolism, but the role of humoral mediators is not completely understood. We aimed to investigate the role of parathyroid and thyroid hormones in acute and adaptive response to cold in humans. Examinations were performed before/after 15 minutes of ice-water swimming (n = 15) or 120 to 150 minutes of cold-induced nonshivering thermogenesis (NST) applied to cold-acclimatized (n = 6) or non-acclimatized (n = 11) individuals. Deep-neck brown adipose tissue (BAT) was collected from non-acclimatized patients undergoing elective neck surgery (n = 36). Seasonal variations in metabolic/hormonal parameters of ice-water swimmers were evaluated. We found that in ice-water swimmers, PTH and TSH increased and free T3, T4 decreased after a 15-minute winter swim, whereas NST-inducing cold exposure failed to regulate PTH and free T4 and lowered TSH and free T3. Ice-water swimming-induced increase in PTH correlated negatively with systemic calcium and positively with phosphorus. In non-acclimatized men, NST-inducing cold decreased PTH and TSH. Positive correlation between systemic levels of PTH and whole-body metabolic preference for lipids as well as BAT volume was found across the 2 populations. Moreover, NST-cooling protocol-induced changes in metabolic preference for lipids correlated positively with changes in PTH. Finally, variability in circulating PTH correlated positively with UCP1/UCP1, PPARGC1A, and DIO2 in BAT from neck surgery patients. Our data suggest that regulation of PTH and thyroid hormones during cold exposure in humans varies by cold acclimatization level and/or cold stimulus intensity. Possible role of PTH in NST is indicated by its positive relationships with whole-body metabolic preference for lipids, BAT volume, and UCP1 content.

311-OR: CREBZF as a Novel Target of Glucose Sensing Signaling in the Control of Adaptive Thermogenesis in the Adipose Tissue

Although glucose is generally considered to be required for generating heat during cold-induced nonshivering thermogenesis in the adipose tissue, whether it engages nutrient sensing pathways in the adipose tissue to negatively regulate thermogenesis and impairs metabolic homeostasis remains largely unknown. Here we report CREB/ATF bZIP transcription factor CREBZF as a key target of adipocyte glucose sensing signaling that exerts a negative effect on the adaptive thermogenesis in the adipose tissue. We found that CREBZF was significantly induced in inguinal white adipose tissue (iWAT) in response to cold exposure or β3AR agonist treatment, which was associated with an induced expression of glucose transporter genes such as GLUT1 and GLUT4. To elucidate the function of CREBZF, we generated adipose-specific CREBZF knockout (CREBZF FKO) mice. Compared with WT mice, CREBZF FKO mice exhibited enhanced browning of iWAT and thermogenic capacity in response to acute and chronic cold exposure. Moreover, CREBZF FKO mice were less cold sensitive. In the adipocyte, CREBZF was induced by glucose treatment, which was associated with increased acetylation of CREBZF. Interestingly, the protein stability of CREBZF was dynamically regulated by reversible acetylation mediated by transacetylase CREB-binding protein (CBP) and deacetylase HDAC. Deletion of CBP ameliorated glucose-induced CREBZF. Proteomic analysis was performed to identify potential acetylation sites of CREBZF. Furthermore, CREBZF associated with PGC-1α and inhibited its transcription activity. Together, we propose that in addition to its role as a fuel source to promote thermogenesis, glucose may act as a chemical signal to stimulate CREBZF activity and cause impaired browning and thermogenesis in the adipose tissue. This study uncovers an unknown mechanism of glucose sensing in the adipocyte that contributes to homeostasis of energy metabolism. Disclosure A. Cui: None. Z. Liu: None. F. Ma: None. J. Bai: None. Y. Xue: None. Y. Liu: None. Z. Hu: None. Y. Han: None. F.F. Zhang: None. J. Gao: None. X. Gao: None. Y. Li: None. Funding National Key Research and Development Program of China (2017YFC0909601); National Natural Science Foundation of China (31471129, 31671224); Chinese Academy of Sciences (KJZD-EW-G20-02); K.C. Wong Education Foundation (to Y.L.)

Cold-Induced Thermogenesis Depends on ATGL-Mediated Lipolysis in Cardiac Muscle, but Not Brown Adipose Tissue

SummaryFatty acids (FAs) activate and fuel UCP1-mediated non-shivering thermogenesis (NST) in brown adipose tissue (BAT). Release of FAs from intracellular fat stores by adipose triglyceride lipase (ATGL) is considered a key step in NST. Accordingly, the severe cold intolerance of global ATGL knockout (AKO) mice has been attributed to defective BAT lipolysis. Here we show that this conclusion is incorrect. We demonstrate that although the BAT-specific loss of ATGL impairs BAT lipolysis and alters BAT morphology, it does not compromise the β3-adrenergic thermogenic response or cold-induced NST. Instead, NST depends on nutrient supply or lipolysis in white adipose tissue during fasting, suggesting that circulating energy substrates are sufficient to fuel NST. Cold intolerance in AKO mice is not caused by BAT dysfunction as previously suspected but by severe cardiomyopathy. We conclude that functional NST requires adequate substrate supply and cardiac function, but does not depend on ATGL-mediated lipolysis in BAT.

Lipolysis in Brown Adipocytes Is Not Essential for Cold-Induced Thermogenesis in Mice

Lipid droplet (LD) lipolysis in brown adipose tissue (BAT) is generally considered to be required for cold-induced nonshivering thermogenesis. Here, we show that mice lacking BAT Comparative Gene Identification-58 (CGI-58), a lipolytic activator essential for the stimulated LD lipolysis, have normal thermogenic capacity and are not cold sensitive. Relative to littermate controls, these animals had higher bodytemperatures when they were provided food during cold exposure. The increase in body temperature in the fed, cold-exposed knockout mice was associated with increased energy expenditure and with increased sympathetic innervation and browning of white adipose tissue (WAT). Mice lacking CGI-58 in both BAT and WAT were cold sensitive, but only in the fasted state. Thus, LD lipolysis in BAT is not essential for cold-induced nonshivering thermogenesis invivo. Rather, CGI-58-dependent LD lipolysis in BAT regulates WAT thermogenesis, and our data uncover an essential role of WAT lipolysis in fueling thermogenesis during fasting.

Loss of UCP2 impairs cold-induced non-shivering thermogenesis by promoting a shift toward glucose utilization in brown adipose tissue

Uncoupling protein 2 (UCP2) was discovered in 1997 and classified as an uncoupling protein largely based on its homology of sequence with UCP1. Since its discovery, the uncoupling function of UCP2 has been questioned and there is yet no consensus on the true function of this protein. UCP2 was first proposed to be a reactive oxygen species (ROS) regulator and an insulin secretion modulator. More recently, it was demonstrated as a regulator of the mitochondrial fatty acid oxidation, which prompted us to investigate its role in the metabolic and thermogenic functions of brown adipose tissue. We first investigated the role of UCP2 in affecting the glycolysis capacity by evaluating the extracellular flux in cells lacking UCP2. We thereafter investigated the role of UCP2 in BAT thermogenesis with positron emission tomography using the metabolic tracers [11C]-acetate (metabolic activity), 2-deoxy-2-[18F]-fluoro-d-glucose (18FDG, glucose uptake) and 14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid [18FTHA, non-esterified fatty acid (NEFA) uptake]. The effect of the β3-adrenoreceptor (ADRB3) selective agonist, CL316,243 (CL), on BAT 18FDG and 18FTHA uptakes, as well as 11C-acetate activity was assessed in UCP2KO and UCP2WT mice exposed at room temperature or adapted to cold. Our results suggest that despite the fact that UCP2 does not have the uncoupling potential of UCP1, its contribution to BAT thermogenesis and to the adaptation to cold exposure appears crucial. Notably, we found that the absence of UCP2 promoted a shift toward glucose utilization and increased glycolytic capacity in BAT, which conferred a better oxidative/thermogenic activity/capacity following an acute adrenergic stimulation. However, following cold exposure, a context of high-energy demand, BAT of UCP2KO mice failed to adapt and thermogenesis was impaired. We conclude that UCP2 regulates BAT thermogenesis by favouring the utilization of NEFA, a process required for the adaptation to cold.

NT-PGC-1α activation attenuates high-fat diet-induced obesity by enhancing brown fat thermogenesis and adipose tissue oxidative metabolism.

The transcriptional coactivator peroxisome proliferator–activated receptor γ coactivator (PGC)-1α and its splice variant N terminal (NT)-PGC-1α regulate adaptive thermogenesis by transcriptional induction of thermogenic and mitochondrial genes involved in energy metabolism. We previously reported that full-length PGC-1α (FL-PGC-1α) is dispensable for cold-induced nonshivering thermogenesis in FL-PGC-1α−/− mice, since a slightly shorter but functionally equivalent form of NT-PGC-1α (NT-PGC-1α254) fully compensates for the loss of FL-PGC-1α in brown and white adipose tissue. In the current study, we challenged FL-PGC-1α−/− mice with a high-fat diet (HFD) to investigate the effects of diet-induced thermogenesis on HFD-induced obesity. Despite a large decrease in locomotor activity, FL-PGC-1α−/− mice exhibited the surprising ability to attenuate HFD-induced obesity. Reduced fat mass in FL-PGC-1α−/− mice was closely associated with an increase in body temperature, energy expenditure, and whole-body fatty acid oxidation (FAO). Mechanistically, FL-PGC-1α−/− brown adipose tissue had an increased capacity to oxidize fatty acids and dissipate energy as heat, in accordance with upregulation of thermogenic genes UCP1 and DIO2. Furthermore, augmented expression of FAO and lipolytic genes in FL-PGC-1α−/− white adipose tissue was highly correlated with decreased fat storage in adipose tissue. Collectively, our data highlight a protective effect of NT-PGC-1α on diet-induced obesity by enhancing diet-induced thermogenesis and FAO.

Adaptive thermogenesis with weight loss in humans

Adaptive thermogenesis (AT) with weight loss refers to underfeeding-associated fall in resting and non-resting energy expenditure (REE, non-REE); this is independent of body weight and body composition. In humans, the existence of AT was inconsistently shown and its clinical significance has been questioned. Discrepant findings are mainly due to different definitions of AT, the use of various and nonstandardized study protocols, and the limits of accuracy of methods to assess energy expenditure. With controlled underfeeding, AT takes more than 2 wk to develop. AT accounts to an average of 0.5 MJ (or 120 kcal) with a considerable between subject variance. Low-sympathetic nervous system activity, 3,5,3'-tri-iodothyronine (T3) and leptin are likely to add to AT; however, the kinetic changes of their plasma levels with underfeeding differ from the time course of AT and controlled intervention studies substituting and titrating these hormones are rare in humans. AT in response to underfeeding is independent of thermogenesis in response to either diet or cold. Although fat-free mass (FFM) and, thus, liver, and skeletal muscle are considered as major sites of AT, cold-induced nonshivering thermogenesis relates to the metabolism of brown adipose tissue (BAT). In humans, diet-induced thermogenesis is related to postprandial substrate metabolism of FFM with a questionable role of BAT. Obviously, the REE component of AT differs from and its non-REE component with respect to organ contribution as well as mechanisms. Thus, AT cannot be considered as unique. AT should be characterized based on individual components of daily energy expenditure, detailed body composition analyses, and mathematical modeling. The biological basis of AT as well as the influences of age, sex, obesity, stress, and inflammation remain to be established in humans.

Brown adipose tissue oxidative metabolism contributes to energy expenditure during acute cold exposure in humans.

Brown adipose tissue (BAT) is vital for proper thermogenesis during cold exposure in rodents, but until recently its presence in adult humans and its contribution to human metabolism were thought to be minimal or insignificant. Recent studies using PET with 18F-fluorodeoxyglucose (18FDG) have shown the presence of BAT in adult humans. However, whether BAT contributes to cold-induced nonshivering thermogenesis in humans has not been proven. Using PET with 11C-acetate, 18FDG, and 18F-fluoro-thiaheptadecanoic acid (18FTHA), a fatty acid tracer, we have quantified BAT oxidative metabolism and glucose and nonesterified fatty acid (NEFA) turnover in 6 healthy men under controlled cold exposure conditions. All subjects displayed substantial NEFA and glucose uptake upon cold exposure. Furthermore, we demonstrated cold-induced activation of oxidative metabolism in BAT, but not in adjoining skeletal muscles and subcutaneous adipose tissue. This activation was associated with an increase in total energy expenditure. We found an inverse relationship between BAT activity and shivering. We also observed an increase in BAT radio density upon cold exposure, indicating reduced BAT triglyceride content. In sum, our study provides evidence that BAT acts as a nonshivering thermogenesis effector in humans.

Recruitment of Brown Adipose Tissue as a Therapy for Obesity-Associated Diseases

Brown adipose tissue (BAT) has been recognized for more than 20 years to play a key role in cold-induced non-shivering thermogenesis (CIT, NST), and body weight homeostasis in animals. BAT is a flexible tissue that can be recruited by stimuli (including small molecules in animals), and atrophies in the absence of a stimulus. In fact, the contribution of BAT (and UCP1) to resting metabolic rate and healthy body weight homeostasis in animals (rodents) is now well established. Many investigations have shown that resistance to obesity and associated disorders in various rodent models is due to increased BAT mass and the number of brown adipocytes or UCP1 expression in various depots. The recent discovery of active BAT in adult humans has rekindled the notion that BAT is a therapeutic target for combating obesity-related metabolic disorders. In this review, we highlight investigations performed in rodents that support the contention that activation of BAT formation and/or function in obese individuals is therapeutically powerful. We also propose that enhancement of brown adipocyte functions in white adipose tissue (WAT) will also regulate energy balance as well as reduce insulin resistance in obesity-associated inflammation in WAT.

Three years with adult human brown adipose tissue

The presence of active brown adipose tissue in adult humans has been recognized in general physiology only since 2007. The intervening three years established that the depots originally observed by (18)F-fluoro-deoxy-glucose positron emission tomography (FDG PET) scanning techniques really are brown adipose tissue depots because they are enriched for uncoupling protein 1 (UCP1). Reports of low apparent prevalence of brown adipose tissue based on retrospective studies of hospital records of FDG PET scans markedly underestimate true prevalence because such studies only reflect acute activity state; consequently, such retrospective studies cannot be conclusively analysed for factors influencing activity and amount of brown adipose tissue. Dedicated studies show that the true prevalence is 30-100%, depending on cohort. Warm temperature during the investigation-as well as adrenergic antagonists-inhibit tissue activity. There is probably no sexual dimorphism in the prevalence of brown adipose tissue. Outdoor temperature may affect the amount of brown adipose tissue, and the amount is negatively correlated with age and obesity. The presence of brown adipose tissue is associated with cold-induced nonshivering thermogenesis, and the tissue may be a major organ for glucose disposal. The decline in brown adipose tissue amount with increasing age may account for or aggravate middle-age obesity. Maintained activation of brown adipose tissue throughout life may thus protect against obesity and diabetes.

Mitochondrial respiration in muscle and liver from cold-acclimated hypothyroid rats.

The effects of long-term cold exposure on muscle and liver mitochondrial oxygen consumption in hypothyroid and normal rats were examined. Thyroid ablation was performed after 8-wk acclimation to 4 degrees C. Hypothyroid and normal controls remained in the cold for an additional 8 wk. At the end of 16-wk cold exposure, all hypothyroid rats were alive and normothermic and had normal body weight. At ambient temperature (24 degrees C), thyroid ablation induced a 65% fall in muscle mitochondrial oxygen consumption, which was reversed by thyroxine but not by norepinephrine administration. After cold acclimation was reached, suppression of thyroid function reduced muscle mitochondrial respiration by 30%, but the hypothyroid values remained about threefold higher than those in hypothyroid muscle in the warm. Blockade of beta- and alpha1-adrenergic receptors in both hypothyroid and normal rats produced hypothermia in vivo and a fall in muscle, liver, and brown adipose tissue mitochondria respiration in vitro. In normal rats, cold acclimation enhanced muscle respiration by 35%, in liver 18%, and in brown adipose tissue 450% over values in the warm. The results demonstrate that thyroid hormones, in the presence of norepinephrine, are major determinants of thermogenic activity in muscle and liver of cold-acclimated rats. After thyroid ablation, cold-induced nonshivering thermogenesis replaced 3,5,3'-triiodothyronine-induced thermogenesis, and normal body temperature was maintained.

Cellular proliferation and UCP content in brown adipose tissue of cold-exposed aging Fischer 344 rats.

Previous investigations have demonstrated that older vs. younger rats respond to cold exposure with blunted cold-induced nonshivering thermogenesis of brown adipose tissue (BAT). This reduction in nonshivering thermogenesis is associated with reduced mass and blunted nonshivering thermogenic capacity of BAT. The purpose of this study was to test the hypothesis that brown fat in 26-mo-old Fischer 344 (F344) male rats has an impaired capacity to respond to the trophic stimulus of chronic cold exposure with increases in cell number, mass, and uncoupling protein (UCP) content. To test this hypothesis, the response of BAT to chronic cold exposure was evaluated in young and old rats. We exposed 6-, 12-, and 26-mo-old F344 male rats to 10 degrees C for 5 days and measured interscapular BAT (IBAT) mass, cell size and proliferation, and mitochondrial UCP1 content. Plasma concentrations of insulin-like growth factor I (IGF-I) and norepinephrine IBAT mass, cell proliferation, or UCP1 content in response to chronic cold, whereas the 6-mo-old rats had a nearly 2-fold cold-induced increase in IBAT mass, a 26-fold increase in cell proliferation, and a 4-fold increase in UCP1 content. Cold exposure also produced an increase of 29, 19, and 20% in mature brown adipocyte cell size of the 6-, 12-, and 26-mo-old animals, respectively. Plasma levels of IGF-I were unaffected by cold at all ages, whereas NE levels were increased by the cold exposure and by increasing age. These data support the hypothesis that brown fat in old F344 rats does not respond to the trophic stimulus of chronic cold exposure to the same degree as younger animals. Moreover, these data indicate that the observed cold- or age-induced changes in levels of growth factors evaluated in this study were not associated with the lack of cold-induced preadipocyte proliferation or increased UCP1 in brown fat of the 26-mo-old rats.

Effects of diet and hormonal status on brown fat lipoprotein lipase activity.

Brown adipose tissue (BAT) is considered to be important for diet and cold-induced non-shivering thermogenesis in rodents through its ability to generate heat by fatty acid (FA) oxidation [1,2]. The source of FA varies within nutritional and physiological status: FA may be provided from endogenous triacylglycerol, de now fatty acid synthesis or from circulating triacylglycerol-rich lipogfoteins via lipoprotein l i r e , (LPL). There are con icting and variable reports o nutritional and hormonal influences on LPL activities in BAT e.g. [3-61. In the present study, we investigated the response of interscapular BAT (IBAT) LPL activity to dietary manipulations (starvation; chronic [ 10 day] fat feeding; chow re-feeding after starvation) and changes in hormonal status (late pregnancy; hyperthyroidism) associated with altered energy intake and/or expenditure. Female albino Wistar rats (180-220 g) were maintained on a 12-h light/l2-h dark cycle (light from 10:00 am.) at 20 ? 2'C. Ad libitum-fed rats were allowed free access to standard (52% carbohydrate/3% lipid) chow or synthetic diets, one with a macronutrient composition comparable to standard chow (termed low-fat diet) and one relatively high in (saturated) fat (22% lipid by wt., termed high-fat diet). Pregnant rats were sampled at day 19 of gestation (term is 23 days). LPL activities were measured in acetone-dried preparations of freeze-clamped tissue as described in ref. [7]. IBAT LPL activities measured in control (chow-fed) rats in the fed state (Table 1) were lower than those of intersca ular white adipose tissue [8]. Within the adlibitum-kd groups the highest IBAT LPL activities were observed in rats fed the high-fat diet (Table 1). These were 1.8 fold higher than those fed chow or low-fat diet. In another study, a high-fat diet did not increase IBAT LPL at 28oC but increases in IBAT LPL activities were seen at 40C [9]. IBAT LPL activities of rats fed standard chow ad libitum were not influenced by late (19 day) pregnancy (Table 1); under the same nutritional conditions, rates of de now FA synthesis in IBAT were significantly suppressed (Holness, M.J. & Sugden, M.C., unpublished results). Experimental hyperthyroidism [induced by S.C. injection of 100 pg tri-iodothyronine/lOO g body wt./day for 3 days) increased IBAT LPL activities to 2.91 f 0.31 (6) nmol of FA released/min per mg of acetone-dried tissue ( P <0.05 versus control): like pregnancy, hyperthyroidism is associated with increased metabolic expenditure but there is no requirement for nutrient conservation. Starvation (72 h) is associated with decreased IBAT LPL activities in Wistar rats [lo]. In the present study, employing a less protracted (18-24 h) starvation period, IBAT LPL activities were enhanced in rats previously-fed either chow or low-fat synthetic diet (Table 1). Notably, the already-high LPL activities observed in rats adapted to the high-fat synthetic diet were not further increased by starvation (Table 1). Furthermore, starvation of latepregnant rats for 24 h did not significantly increase IBAT LPL activities (Table 1). Chow re-feeding after 24 h-starvation failed to reverse the starvation-induced increases in IBAT LPL activities observed in control rats: instead, further increases in IBAT LPL activity were observed such that at 2 h after re-feedin LPL activities 14.34 f 0.41 (12) nmol of FA releasedrmin per mg of acetone-dried tissue] were 2.2 fold higher (P <0.05) than those measured in the fed state. Table 1. IBAT LPL activities during nutritional and hormonal manipulations