Brown Adipose Tissue Dysfunction Research Articles

Functional brown adipose tissue limits cardiomyocyte injury and adverse remodeling in catecholamine-induced cardiomyopathy

Brown adipose tissue (BAT) has well recognized thermogenic properties mediated by uncoupling protein 1 (UCP1); more recently, BAT has been demonstrated to modulate cardiovascular risk factors. To investigate whether BAT also affects myocardial injury and remodeling, UCP1-deficient (UCP1−/−) mice, which have dysfunctional BAT, were subjected to catecholamine-induced cardiomyopathy. At baseline, there were no differences in echocardiographic parameters, plasma cardiac troponin I (cTnI) or myocardial fibrosis between wild-type (WT) and UCP1−/− mice. Isoproterenol infusion increased cTnI and myocardial fibrosis and induced left ventricular (LV) hypertrophy in both WT and UCP1−/− mice. UCP1−/− mice also demonstrated exaggerated myocardial injury, fibrosis, and adverse remodeling, as well as decreased survival. Transplantation of WT BAT to UCP1−/− mice prevented the isoproterenol-induced cTnI increase and improved survival, whereas UCP1−/− BAT transplanted to either UCP1−/− or WT mice had no effect on cTnI release. After 3days of isoproterenol treatment, phosphorylated AKT and ERK were lower in the LV's of UCP1−/− mice than in those of WT mice. Activation of BAT was also noted in a model of chronic ischemic cardiomyopathy, and was correlated to LV dysfunction. Deficiency in UCP1, and accompanying BAT dysfunction, increases cardiomyocyte injury and adverse LV remodeling, and decreases survival in a mouse model of catecholamine-induced cardiomyopathy. Myocardial injury and decreased survival are rescued by transplantation of functional BAT to UCP1−/− mice, suggesting a systemic cardioprotective role of functional BAT. BAT is also activated in chronic ischemic cardiomyopathy.

The Whitening of Brown Fat and Its Implications for Weight Management in Obesity.

Systemic inflammation resulting from dysfunction of white adipose tissue (WAT) accelerates the pathologies of diabetes and cardiovascular diseases. In contrast to WAT, brown adipose tissue (BAT) is abundant in mitochondria that produce heat by uncoupling respiratory chain process of ATP synthesis. Besides BAT's role in thermogenesis, accumulating evidence has shown that it is involved in regulating systemic metabolism. Studies have analyzed the "browning" processes of WAT as a means to combat obesity, whereas few studies have focused on the impact and molecular mechanisms that contribute to obesity-linked BAT dysfunction--a process that is associated with the "whitening" of this tissue. Compared to WAT, a dense vascular network is required to support the high energy consumption of BAT. Recently, vascular rarefaction was shown to be a significant causal factor in the whitening of BAT in mouse models. Vascular insufficiency leads to mitochondrial dysfunction and loss in BAT and contributes to systemic insulin resistance. These data suggest that BAT "whitening," resulting from vascular dysfunction, can impact obesity and obesity-linked diseases. Conversely, agents that promote BAT function could have utility in the treatment of these conditions.

Adipose Tissue ‐ Vasculature Interactions

Obesity and metabolic dysfunction diminish lifespan, predominantly by increasing the frequency and severity of vascular diseases. It is becoming increasingly recognized that obesity contributes to cardio-metabolic disease because it promotes the development of a chronic and systemic inflammatory state. Key to the development of this inflammatory state is the status of multiple bioactive substances, referred to as adipokines, that are secreted by adipose tissues. In lean, healthy organisms, adipose tissue produces anti-inflammatory adipokines that serve to protect the vasculature from stresses. Under conditions of obesity, adipose tissue becomes dysfunctional and produces a pro-inflammatory repertoire of adipokines that contribute to vascular diseases. In addition to the influence that adipokines have on vascular function, it is also appreciated that adipose tissue vascularity influences adipose tissue function. Recent studies have shown that brown adipose tissue is particularly sensitive to perturbations in its vascular supply. Compared to white adipose tissue that functions to store fat (and is most often associated with obesity), brown adipose tissue possesses abundant mitochondria that consumes fat to produce heat through uncoupled respiration. Recent work has shown that overnutrition leads to the dysfunction of brown adipose tissue, giving it a “whitened” appearance, via a collapse of VEGF-dependent angiogenesis. These findings highlight the important interplay between the adipose tissue and the vasculature that is mediated by adipokines and angiogenic growth factors.

Brown Adipocyte Dysfunction in UCP1 Knockout Mice leads to Age‐Dependent Development of Type 2 Diabetes

Since the discovery of functional brown adipose tissue (BAT) in adult humans, an inverse correlation between aging, type 2 diabetes and the amount and activity of the BAT‐depots have been reported. Upon activation, BAT converts the energy of free fatty acids and glucose oxidation into heat through the mitochondrial carrier protein,uncoupling protein 1 (UCP1). BAT has recently been demonstrated to increase triglyceride uptake and insulin sensitivity. Although the thermogenic responses of UCP1 KO mice have been well studied, little is known about the short and long term effects of UCP1 deficiency and how it affects BAT function and metabolism in general.We used RNAseq to analyze and compare the BAT transcriptome of WT and UCP1 KO mice, under thermoneutral conditions at baseline and after catecholamine induced activation. The effects of aging on glucose and insulin tolerance and molecular markers of type 2 diabetes were investigated in WT and UCP1 KO mice in thermoneutrality for up to 1 year.Whole transcriptome analysis, followed by qPCR and immunoblotting, revealed marked dysfunction of key metabolic pathways in activated BAT from UCP1KO mice compared to WT mice, while at baseline thermoneutral conditions the BAT transcriptome from UCP1KO and WT mice were similar. Further, UCP1KO mice age‐dependently develop hallmarks of type 2 diabetes, such as impaired glucose tolerance and increased insulin resistance, without developing an obesity phenotype.We conclude that UCP1 KO mice are a model of severe BAT dysfunction. Long‐term BAT dysfunction is associated with age‐dependent development of hallmarks of type 2 diabetes.

Abstract 20251: A Novel Role for Brown Adipose Tissue in Cardioprotection

Introduction: Brown adipose tissue (BAT), upon activation by natriuretic peptides and catecholamines, converts the energy of free fatty acids and glucose oxidation into heat (thermogenesis) through uncoupling protein 1 (UCP1). BAT activation has also been reported to increase triglyceride uptake and improve insulin resistance in obesity. Hypothesis: We hypothesized that activated BAT limits myocardial injury and remodeling in a catecholamine-induced cardiomyopathy model. Methods: Isoproterenol (ISO, 60 mg/kg/day) or saline were infused in wild type (WT, n=12) and UCP1-deficient (UCP1-/-, n=11) mice for 14 days using an osmotic minipump. Plasma cardiac troponin I levels (cTnI) were measured after 3 days of treatment. Echocardiograms were performed before and after treatment for 14 days. BAT and LV were harvested for RNA and histology. The experiments were repeated in mice that had undergone prior BAT transplantation (WT to WT n=8; WT to UCP1-/- n=7; UCP1-/- to UCP1-/- n=6). Results: At baseline LV size, mass and ejection fraction (LVEF) were similar in WT and UCP1-/- mice, cTnI was undetectable. Gene expression in BAT of UCP1-/- mice revealed decreased lipid metabolism and oxidative phosphorylation. After 3 days of ISO treatment plasma cTnI was markedly increased in UCP1-/- mice compared to WT (Table). After 14 days of ISO infusion UCP1-/- mice developed greater LV hypertrophy and fibrosis than WT mice (Table). Transplantation of UCP1-/- BAT to UCP1-/- or WT mice did not decrease isoproterenol-induced cTnI release, however, transplantation of WT BAT to UCP1-/- mice reversed the increased cTnI and adverse remodeling. Conclusions: UCP1-deficiency causes BAT dysfunction, and increases cardiomyocyte injury and adverse LV remodeling in a mouse model of catecholamine-induced cardiomyopathy. Cardiomyocyte injury and adverse LV remodeling are rescued by transplantation of functional BAT in to UCP1-/- mice, suggesting that functional BAT is cardioprotective.

Prdm16 Is Required for the Maintenance of Brown Adipocyte Identity and Function in Adult Mice

Prdm16 is a transcription factor that regulates the thermogenic gene program in brown and beige adipocytes. However, whether Prdm16 is required for the development or physiological function of brown adipose tissue (BAT) in vivo has been unclear. By analyzing mice that selectively lacked Prdm16 in the brown adipose lineage, we found that Prdm16 was dispensable for embryonic BAT development. However, Prdm16 was required in young mice to suppress the expression of white-fat-selective genes in BAT through recruitment of the histone methyltransferase Ehmt1. Additionally, Prdm16 deficiency caused a severe adult-onset decline in the thermogenic character of interscapular BAT. This resulted in BAT dysfunction and cold sensitivity but did not predispose the animals to obesity. Interestingly, the loss of brown fat identity due to ablation of Prdm16 was accelerated by concurrent deletion of the closely related Prdm3 gene. Together, these results show that Prdm16 and Prdm3 control postnatal BAT identity and function.

Molecular imaging of brown adipose tissue in health and disease

Brown adipose tissue (BAT) has transformed from an interfering tissue in oncological (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to an independent imaging research field. This review takes the perspective from the imaging methodology on which human BAT research has come to rely on heavily. This review analyses relevant PubMed-indexed publications that discuss molecular imaging methods of BAT. In addition, reported links between BAT and human diseases such as obesity are discussed, and the possibilities for imaging in these fields are highlighted. Radiopharmaceuticals aiming at several different biological mechanisms of BAT are discussed and evaluated. Prospective, dedicated studies allow visualization of BAT function in a high percentage of human subjects. BAT dysfunction has been implicated in obesity, linked with diabetes and associated with cachexia and atherosclerosis. Presently, (18)F-FDG PET/CT is the most useful tool for evaluating therapies aiming at BAT activity. In addition to (18)F-FDG, other radiopharmaceuticals such as (99m)Tc-sestamibi, (123)I-metaiodobenzylguanidine (MIBG), (18)F-fluorodopa and (18)F-14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid (FTHA) may have a potential for visualizing other aspects of BAT activity. MRI methods are under continuous development and provide the prospect of functional imaging without ionizing radiation. Molecular imaging of BAT can be used to quantitatively assess different aspects of BAT metabolic activity.

Effects of granulocyte-colony stimulating factor on obesity and energy metabolism in male Otsuka-Long-Evans-Tokushima fatty rats

Purpose: The structure and signaling pathways of G-CSF are similar to those of leptin. G-CSF also has immune-modulator properties, reduces reactive oxygen species in intracellular spaces and protects mitochondrial functions. Given that obesity and diabetes are associated with adipocyte inflammation and mitochondrial dysfunction, we hypothesized that G-CSF was a plausible candidate for reducing body weight. Methods: We treated twelve 38-week-old male Otsuka-Long-Evans-Tokushima fatty rats (OLETF, diabetic phenotype) and twelve age-matched male Long-Evans-Tokushima rats (LETO, healthy control) with 200 μg/day G-CSF or saline for 5 consecutive days and followed them up for 8 weeks. Energy expenditure (EEx) was measured weekly after the treatments by an indirect calorimeter. Body composition was measured by Dual Energy X-ray Absorptiometry (DEXA) 1 week before and 8 weeks after the treatments. Expression of appetite mediators including Proopiomelanocortin (POMC), Neuropeptide Y (NPY), Cocaine-Amphetamine-Related Transcript (CART) and Agouti-Related Peptide (AgRP) in hypothalamus was measured by quantitative PCR. Expression of Uncoupling Protein-1 (UCP-1) in Brown Adipose Tissue (BAT) was measured by both quantitative PCR and western blotting. Results: In OLETF, body weight reduction was larger in the G-CSF-treated group than the saline-treated group 8 weeks after the treatments (6.9% vs. 4.3%, p < 0.05). There was no effect on body weight in LETO, and food intake did not change in any group. The reduction of body fat mass in the G-CSF-treated group was larger than that in the saline-treated group in OLETF (48.14g vs. 36.46g, p < 0.05), but unaffected in LETO. The EEx was higher from 5 weeks after G-CSF treatment in OLETF, but unaffected in LETO. Cholesterol, triglyceride and interleukin-6 decreased after G-CSF treatment only in OLETF. The mRNA expression of POMC and NPY increased in hypothalamus after G-CSF treatment in both OLETF and LETO, while the mRNA expression of CART and AgRP did not differ between the G-CSF-treated groups and the saline-treated groups in both OLETF and LETO. Both mRNA and protein expressions of UCP-1 in BAT of the G-CSF-treated group were higher in OLETF, while not different in LETO. G-CSF receptor was also identified in BAT by RT-PCR. Conclusions: G-CSF reduces body weight in a diabetic model, and this may be associated with the restoration of BAT dysfunction in the diabetic model. Therefore, G-CSF is a possible candidate for treating obesity in diabetic subjects.

Altered thermogenesis and impaired bone remodeling in Misty mice

Fat mass may be modulated by the number of brown-like adipocytes in white adipose tissue (WAT) in humans and rodents. Bone remodeling is dependent on systemic energy metabolism and, with age, bone remodeling becomes uncoupled and brown adipose tissue (BAT) function declines. To test the interaction between BAT and bone, we employed Misty (m/m) mice, which were reported be deficient in BAT. We found that Misty mice have accelerated age-related trabecular bone loss and impaired brown fat function (including reduced temperature, lower expression of Pgc1a, and less sympathetic innervation compared to wild-type (+/ +)). Despite reduced BAT function, Misty mice had normal core body temperature, suggesting heat is produced from other sources. Indeed, upon acute cold exposure (4°C for 6 hours), inguinal WAT from Misty mice compensated for BAT dysfunction by increasing expression of Acadl, Pgc1a, Dio2, and other thermogenic genes. Interestingly, acute cold exposure also decreased Runx2 and increased Rankl expression in Misty bone, but only Runx2 was decreased in wild-type. Browning of WAT is under the control of the sympathetic nervous system (SNS) and, if present at room temperature, could impact bone metabolism. To test whether SNS activity could be responsible for accelerated trabecular bone loss, we treated wild-type and Misty mice with the β-blocker, propranolol. As predicted, propranolol slowed trabecular bone volume/total volume (BV/TV) loss in the distal femur of Misty mice without affecting wild-type. Finally, the Misty mutation (a truncation of DOCK7) also has a significant cell-autonomous role. We found DOCK7 expression in whole bone and osteoblasts. Primary osteoblast differentiation from Misty calvaria was impaired, demonstrating a novel role for DOCK7 in bone remodeling. Despite the multifaceted effects of the Misty mutation, we have shown that impaired brown fat function leads to altered SNS activity and bone loss, and for the first time that cold exposure negatively affects bone remodeling.

Epidemiological and Experimental Links between Air Pollution and Type 2 Diabetes

There is increasing evidence suggesting links between exposure to environmental toxins and susceptibility to type 2 diabetes mellitus (DM). In this review, we summarize the experimental evidence to support this association that has been noted in many epidemiologic studies. Inflammation in response to particulate matter (PM(2.5)) exposure in air pollution represents a common mechanism that may interact with other pro-inflammatory influences in diet and life style to modulate susceptibility to cardiometabolic diseases. The role of innate immune cytokines released from macrophages in the lung is well known. In addition, chemokine triggers in response to air-pollution exposure may mediate a cellular response from the bone marrow/spleen through toll-like receptors (TLRs) and Nucleotide Oligomerization Domain receptors (NLRs) pathways to mediate inflammatory response in organs. Emerging data also seem to support a role for PM(2.5) exposure in endoplasmic reticulum stress-induced apoptosis and in brown adipose tissue dysfunction. Decreased expression of UCP1 in brown adipose tissue may account for reduced thermogenesis providing another link between PM(2.5) and insulin resistance. The implications of an experimental link between air-pollution exposure and type 2 DM are profound as air pollution is a pervasive risk factor throughout the world and even modest alleviation in exposure may provide substantial public health benefits.

Chronic dietary supplementation of proanthocyanidins corrects the mitochondrial dysfunction of brown adipose tissue caused by diet-induced obesity in Wistar rats

The present study aims to determine the effects of grape seed proanthocyanidin extract (GSPE) on brown adipose tissue (BAT) mitochondrial function in a state of obesity induced by diet. Wistar male rats were fed with a cafeteria diet (Cd) for 4 months; during the last 21 d, two groups were treated with doses of 25 and 50mg GSPE/kg body weight. In the BAT, enzymatic activities of citrate synthase, cytochrome c oxidase (COX) and ATPase were determined and gene expression was analysed by real-time PCR. The mitochondrial function of BAT was determined in fresh mitochondria by high-resolution respirometry using both pyruvate and carnitine-palmitoyl-CoA as substrates. The results show that the Cd causes an important decrease in the gene expression of sirtuin 1, nuclear respiratory factor 1, isocitrate dehydrogenase 3γ and COX5α and, what is more telling, decreases the levels of mitochondrial respiration both with pyruvate and canitine-palmitoyl-CoA. Most of these parameters, which are indicative of mitochondrial dysfunction due to diet-induced obesity, are improved by chronic supplementation of GSPE. The beneficial effects caused by the administration of GSPE are exhibited as a protection against weight gain, in spite of the Cd the rats were fed. These data indicate that chronic consumption of a moderate dose of GSPE can correct an energy imbalance in a situation of diet-induced obesity, thereby improving the mitochondrial function and thermogenic capacity of the BAT.

Brown Adipose Tissue Can Be Activated or Inhibited within an Hour before 18F‐FDG Injection: A Preliminary Study with MicroPET

Brown adipose tissue (BAT) is emerging as a potential target for treating human obesity. It has been indicated that BAT is rich in innervations of sympathetic nerve control. Using 18F-FDG microPET imaging, this study aims at evaluating how factors related to sympathetic activation/inhibition changed BAT metabolism of mice. BAT 18F-FDG uptake were semiquantitatively evaluated in different groups of mice under temperature (cold or warm stimulus) or pharmacological interventions (norepinephrine, epinephrine, isoprenaline, or propranolol) and were compared with the corresponding controls. It was found that BAT activation can be stimulated by cold exposure (P = 1.96 × 10−4), norepinephrine (P = .002), or both (P = 2.19 × 10−6) within an hour before 18F-FDG injection and can also be alleviated by warming up (P = .001) or propranolol lavage (P = .027). This preliminary study indicated that BAT function could be evaluated by 18F-FDG PET imaging through short-term interventions, which paved the way for further investigation of the relationship between human obesity and BAT dysfunction.

Chronic leptin treatment does not prevent the development of obesity in transgenic mice with brown fat deficiency.

With the exception of ob/ob mice, circulating plasma leptin is elevated in all other obese rodents as well as in obese humans, suggesting that leptin resistance rather than leptin deficiency is a characteristic feature of obesity. The exact molecular mechanisms leading to leptin resistance and the applicability of exogenous leptin to overcome resistance to the anorectic effect of the hormone, are insufficiently characterized. The aim of this study was to investigate whether chronic leptin administration could prevent the development of obesity and its associated disorders in transgenic mice with toxigene mediated ablation of brown adipose tissue (BAT). Daily injections of leptin were started at the age of 6 weeks, when body weight, food intake and plasma leptin levels of transgenics were not different from control mice. Over the next 6 weeks, leptin treated transgenics showed the same excessive body weight gain as transgenic mice injected with saline. Leptin treatment was furthermore not able to prevent the development of hyperphagia, hyperglycaemia, hyperinsulinaemia and hyperlipidaemia in transgenic mice. In contrast, control mice injected with leptin had significantly lower body weight, food intake and plasma triglycerides than those treated with saline. In summary, leptin treatment was not able to prevent the development of obesity and its associated abnormalities in transgenic mice with BAT deficiency. This data suggests that intact BAT function is of critical importance for leptin's effect on food intake and energy expenditure, and that primary dysfunction of BAT is associated with leptin resistance, even when hyperleptinaemia is not yet present.

Activation of the Nuclear Receptor Peroxisome Proliferator-activated Receptor γ Promotes Brown Adipocyte Differentiation

Brown adipose tissue (BAT) functions in non-shivering and diet-induced thermogenesis via its capacity for uncoupled mitochondrial respiration. BAT dysfunction in rodents is associated with severe defects in energy homeostasis, resulting in obesity and hyperglycemia. Here, we report that the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma), a prostaglandin-activated transcription factor recently implicated as a central regulator of white adipose tissue differentiation, also regulates brown adipocyte function. PPARgamma is abundantly expressed in both embryonic and adult BAT. Treatment of CD-1 rats with the PPARgamma-selective ligand BRL49653, an anti-diabetic drug of the thiazolidinedione class, results in marked increases in the mass of interscapular BAT. In vitro, BRL49653 induces the terminal differentiation of the brown preadipocyte cell line HIB-1B as judged by both changes in cell morphology and expression of uncoupling protein and other adipocyte-specific mRNAs. These data demonstrate that PPARgamma is a key regulatory factor in brown adipocytes and suggest that PPARgamma functions not only in the storage of excess energy in white adipose tissue but also in its dissipation in BAT.