Direct assessment of brown adipose tissue as a site of systemic tri-iodothyronine production in the rat

Distinct effects of physical and functional ablation of brown adipose tissue on T3-dependent pathological cardiac remodeling

Heart failure tends to deteriorate in colder climates, heightening the risk of major adverse cardiovascular events. Brown adipose tissue (BAT) serves as both a thermogenic organ and an atypical site for triiodothyronine (T3) synthesis in response to cold. This study investigates the potential role of BAT in contributing to abdominal aortic constriction (AAC)-induced pathological cardiac remodeling during cold exposure. In this study, we developed a mouse model of pathological cardiac remodeling using AAC. Physical excision of interscapular BAT (iBATx) was performed during cold exposure, and T3 synthesis levels were measured. Additionally, the impact of uncoupling protein 1 (UCP1) knockout on thermogenic function and pathological cardiac remodeling was investigated. In vitro studies were conducted to assess the effect of T3 on cardiomyocyte hypertrophy induced by phenylephrine (PE). Physical removal of interscapular BAT during cold exposure decreased T3 synthesis and mitigated pathological cardiac remodeling. Conversely, UCP1 knockout eliminated thermogenic function during cold exposure, while preserving BAT integrity increased T3 synthesis and exacerbated pathological cardiac remodeling. In vitro, T3 further aggravated cardiomyocyte hypertrophy caused by PE. These findings underscore the distinct effects of physical and functional BAT ablation on pathological cardiac remodeling, primarily through altering T3 levels rather than thermogenesis in cold environments. This research provides new insights into the differential roles of BAT in cardiac health, particularly under cold exposure conditions.

Thermogenic Capacity of Human Supraclavicular Brown Fat and Cold-Stimulated Brain Glucose Metabolism

Human brain metabolism is susceptible to temperature changes. It has been suggested that the supraclavicular brown adipose tissue (BAT) protects the brain from these fluctuations by regulating heat production through the presence of uncoupling protein 1 (UCP-1). It remains unsolved whether inter-individual variation in the expression of UCP-1, which represents the thermogenic capacity of the supraclavicular BAT, is linked with brain metabolism during cold stress. Ten healthy human participants underwent 18F-FDG PET scanning of the brain under cold stimulus to determine brain glucose uptake (BGU). On a separate day, an excision biopsy of the supraclavicular fat—the fat proximal to the carotid arteries supplying the brain with warm blood—was performed to determine the mRNA expression of the thermogenic protein UCP-1. Expression of UCP-1 in supraclavicular BAT was directly related to the whole brain glucose uptake rate determined under cold stimulation (rho = 0.71, p = 0.03). In sub-compartmental brain analysis, UCP-1 expression in supraclavicular BAT was directly related to cold-stimulated glucose uptake rates in the hypothalamus, medulla, midbrain, limbic system, frontal lobe, occipital lobe, and parietal lobe (all rho ≥ 0.67, p < 0.05). These relationships were independent of body mass index and age. When analysing gene expressions of BAT secretome, we found a positive correlation between cold-stimulated BGU and DIO2. These findings provide evidence of functional links between brain metabolism under cold stimulation and UCP-1 and DIO2 expressions in BAT in humans. More research is needed to evaluate the importance of these findings in clinical outcomes, for instance, in examining the supporting role of BAT in cognitive functions under cold stress.

Blood Sampling for Arteriovenous Difference Measurements Across Interscapular Brown Adipose Tissue in Rat

A classic physiological approach to assess the specific uptake or release of circulating factors in organs and tissues is to measure concentration differences between venous and arterial blood. For interscapular brown adipose tissue (iBAT), the anatomic distribution of its vascularization, which drains most of the blood into Sulzer's vein, allows for local measurement of arteriovenous differences. The use of this procedure to monitor oxygen concentration changes was fundamental for the recognition of BAT as the main site of adaptive non-shivering thermogenesis. More recently, this technique has regained importance as a means to identify BAT-secreted regulatory molecules, such as fibroblast growth factor-21 and the chemokine CXCL14. In this chapter, we provide a detailed description of an optimized and feasible protocol to determine arteriovenous differences across iBAT. We include tips and practical advice for using this powerful tool to study BAT metabolism and secretory activity in rats as an experimental model.

Brown/Beige adipose tissues and the emerging role of their secretory factors in improving metabolic health: The batokines

Brown and beige adipose tissues are the primary sites for adaptive non-shivering thermogenesis. Although they have been known principally for their thermogenic effects, in recent years, it has emerged that, just like white adipose tissue (WAT), brown and beige adipose tissues also play an important role in the regulation of metabolic health through secretion of various brown adipokines (batokines) in response to various physiological cues. These secreted batokines target distant organs and tissues such as the liver, heart, skeletal muscles, brain, WAT, and perform various local and systemic functions in an autocrine, paracrine, or endocrine manner. Brown and beige adipose tissues are therefore now receiving increasing levels of attention with respect to their effects on various other organs and tissues. Identification of novel secreted factors by these tissues may help in the discovery of drug candidates for the treatment of various metabolic disorders such as obesity, type-2 diabetes, skeletal deformities, cardiovascular diseases, dyslipidemia. In this review, we comprehensively describe the emerging secretory role of brown/beige adipose tissues and the metabolic effects of various brown/beige adipose tissues secreted factors on other organs and tissues in endocrine/paracrine manners, and as well as on brown/beige adipose tissue itself in an autocrine manner. This will provide insights into understanding the potential secretory role of brown/beige adipose tissues in improving metabolic health.

Mechanisms linking adipose tissue inflammation to cardiac hypertrophy and fibrosis

Adipose tissue is classically recognized as the primary site of lipid storage, but in recent years has garnered appreciation for its broad role as an endocrine organ comprising multiple cell types whose collective secretome, termed as adipokines, is highly interdependent on metabolic homeostasis and inflammatory state. Anatomical location (e.g. visceral, subcutaneous, epicardial etc) and cellular composition of adipose tissue (e.g. white, beige, and brown adipocytes, macrophages etc.) also plays a critical role in determining its response to metabolic state, the resulting secretome, and its potential impact on remote tissues. Compared with other tissues, the heart has an extremely high and constant demand for energy generation, of which most is derived from oxidation of fatty acids. Availability of this fatty acid fuel source is dependent on adipose tissue, but evidence is mounting that adipose tissue plays a much broader role in cardiovascular physiology. In this review, we discuss the impact of the brown, subcutaneous, and visceral white, perivascular (PVAT), and epicardial adipose tissue (EAT) secretome on the development and progression of cardiovascular disease (CVD), with a particular focus on cardiac hypertrophy and fibrosis.

Thermogenesis in Adipose Tissue Activated by Thyroid Hormone

Thermogenesis is the production of heat that occurs in all warm-blooded animals. During cold exposure, there is obligatory thermogenesis derived from body metabolism as well as adaptive thermogenesis through shivering and non-shivering mechanisms. The latter mainly occurs in brown adipose tissue (BAT) and muscle; however, white adipose tissue (WAT) also can undergo browning via adrenergic stimulation to acquire thermogenic potential. Thyroid hormone (TH) also exerts profound effects on thermoregulation, as decreased body temperature and increased body temperature occur during hypothyroidism and hyperthyroidism, respectively. We have termed the TH-mediated thermogenesis under thermoneutral conditions “activated” thermogenesis. TH acts on the brown and/or white adipose tissues to induce uncoupled respiration through the induction of the uncoupling protein (Ucp1) to generate heat. TH acts centrally to activate the BAT and browning through the sympathetic nervous system. However, recent studies also show that TH acts peripherally on the BAT to directly stimulate Ucp1 expression and thermogenesis through an autophagy-dependent mechanism. Additionally, THs can exert Ucp1-independent effects on thermogenesis, most likely through activation of exothermic metabolic pathways. This review summarizes thermogenic effects of THs on adipose tissues.

Angiotensin-(1-7), Adipokines and Inflammation

Nowadays the adipose tissue is recognized as one of the most critical endocrine organs releasing many adipokines that regulate metabolism, inflammation and body homeostasis. There are several described adipokines, including the renin-angiotensin system (RAS) components that are especially activated in some diseases with increased production of angiotensin II and several pro-inflammatory hormones. On the other hand, RAS also expresses angiotensin-(1-7), which is now recognized as the main peptide on counteracting Ang II effects. New studies have shown that increased activation of ACE2/Ang-(1-7)/MasR arm can revert and prevent local and systemic dysfunctions improving lipid profile and insulin resistance by modulating insulin actions, and reducing inflammation. In this context, the present review shows the interaction and relevance of Ang-(1-7) effects on regulating adipokines, and as one adipokine itself, modulating body homeostasis, with emphasis on its anti-inflammatory properties, especially in the context of metabolic disorders with focus on obesity and type 2 diabetes mellitus pandemic.

Brown Adipokines

Brown adipokines are regulatory factors secreted by brown and beige adipocytes that exhibit endocrine, paracrine, and autocrine actions. Peptidic and non-peptidic molecules, including miRNAs and lipids, are constituents of brown adipokines. Brown adipose tissue remodeling to meet thermogenic needs is dependent on the secretory properties of brown/beige adipocytes. The association between brown fat activity and a healthy metabolic profile, in relation to energy balance and glucose and lipid homeostasis, is influenced by the endocrine actions of brown adipokines. A comprehensive knowledge of the brown adipocyte secretome is still lacking. Advancements in the identification and characterization of brown adipokines will facilitate therapeutic interventions for metabolic diseases, as these molecules are obvious candidates to therapeutic agents. Moreover, identification of brown adipokines as circulating biomarkers of brown adipose tissue activity may be particularly useful for noninvasive assessment of brown adipose tissue alterations in human pathologies.

The Lives and Times of Brown Adipokines

Brown adipose tissue (BAT) is responsible for adaptive non-shivering thermogenesis. Moreover, brown fat secretes regulatory factors, so-called brown adipokines, that have autocrine, paracrine, and endocrine actions. Brown adipokines are either polypeptides or nonpeptidic molecules including lipid molecules and microRNAs. The secretory properties of brown fat are essential for tissue remodeling adaptations to thermogenic necessities. The endocrine properties of brown adipokines are thought to contribute to the association between BAT activity and a healthy metabolic profile in relation to glucose and lipid homeostasis. The identification and characterization of brown adipokines may allow the discovery of circulating biomarkers of BAT activity in humans, and will lead to the development of candidate tools for therapeutic interventions in metabolic diseases.

MicroRNA Regulation of Brown Adipogenesis and Thermogenic Energy Expenditure

Obesity, diabetes, and associated metabolic diseases have become global epidemics. Obesity results from excess accumulation of white fat, while brown and its related beige fat function to dissipate energy as heat, thus counteracting obesity and its related metabolic disorders. Understanding the regulatory mechanisms for both white and brown adipogenesis provides new insights for prevention and treatment of these metabolic diseases. In addition to traditional gene transcription and translation, microRNA (miRNA) represents a new layer of regulatory mechanism in many biological processes and has attracted a great deal of research interests in exploring their roles in physiological and pathophysiological conditions. This review focuses on the recent advances of regulating brown adipogenesis and energy metabolism by miRNAs, aiming to delineate the regulatory principles of miRNAs on this unique aspect of energy homeostasis.

Brown adipose tissue as a secretory organ

Brown adipose tissue (BAT) is the main site of adaptive thermogenesis and experimental studies have associated BAT activity with protection against obesity and metabolic diseases, such as type 2 diabetes mellitus and dyslipidaemia. Active BAT is present in adult humans and its activity is impaired in patients with obesity. The ability of BAT to protect against chronic metabolic disease has traditionally been attributed to its capacity to utilize glucose and lipids for thermogenesis. However, BAT might also have a secretory role, which could contribute to the systemic consequences of BAT activity. Several BAT-derived molecules that act in a paracrine or autocrine manner have been identified. Most of these factors promote hypertrophy and hyperplasia of BAT, vascularization, innervation and blood flow, processes that are all associated with BAT recruitment when thermogenic activity is enhanced. Additionally, BAT can release regulatory molecules that act on other tissues and organs. This secretory capacity of BAT is thought to be involved in the beneficial effects of BAT transplantation in rodents. Fibroblast growth factor 21, IL-6 and neuregulin 4 are among the first BAT-derived endocrine factors to be identified. In this Review, we discuss the current understanding of the regulatory molecules (the so-called brown adipokines or batokines) that are released by BAT that influence systemic metabolism and convey the beneficial metabolic effects of BAT activation. The identification of such adipokines might also direct drug discovery approaches for managing obesity and its associated chronic metabolic diseases.

Adipokines and the Endocrine Role of Adipose Tissues

The last two decades have witnessed a shift in the consideration of white adipose tissue as a mere repository of fat to be used when food becomes scarce to a true endocrine tissue releasing regulatory signals, the so-called adipokines, to the whole body. The control of eating behavior, the peripheral insulin sensitivity, and even the development of the female reproductive system are among the physiological events controlled by adipokines. Recently, the role of brown adipose tissue in human physiology has been recognized. The metabolic role of brown adipose tissue is opposite to white fat; instead of storing fat, brown adipose tissue is a site of energy expenditure via adaptive thermogenesis. There is growing evidence that brown adipose tissue may have its own pattern of secreted hormonal factors, the so-called brown adipokines, having distinctive biological actions on the overall physiological adaptations to enhance energy expenditure.

Brown adipose tissue: The heat is on the heart

The study of brown adipose tissue (BAT) has gained significant scientific interest since the discovery of functional BAT in adult humans. The thermogenic properties of BAT are well recognized; however, data generated in the last decade in both rodents and humans reveal therapeutic potential for BAT against metabolic disorders and obesity. Here we review the current literature in light of a potential role for BAT in beneficially mediating cardiovascular health. We focus mainly on BAT's actions in obesity, vascular tone, and glucose and lipid metabolism. Furthermore, we discuss the recently discovered endocrine factors that have a potential beneficial role in cardiovascular health. These BAT-secreted factors may have a favorable effect against cardiovascular risk either through their metabolic role or by directly affecting the heart.

Brown adipose tissue: a potential target in the fight against obesity and the metabolic syndrome

BAT (brown adipose tissue) is the main site of thermogenesis in mammals. It is essential to ensure thermoregulation in newborns. It is also found in (some) adult humans. Its capacity to oxidize fatty acids and glucose without ATP production contributes to energy expenditure and glucose homoeostasis. Brown fat activation has thus emerged as an attractive therapeutic target for the treatment of obesity and the metabolic syndrome. In the present review, we integrate the recent advances on the metabolic role of BAT and its relation with other tissues as well as its potential contribution to fighting obesity and the metabolic syndrome.

An endocrine role for brown adipose tissue?

White adipose tissue is recognized as both a site of energy storage and an endocrine organ that produces a myriad of endocrine factors called adipokines. Brown adipose tissue (BAT) is the main site of nonshivering thermogenesis in mammals. The amount and activity of brown adipocytes are associated with protection against obesity and associated metabolic alterations. These effects of BAT are traditionally attributed to its capacity for the oxidation of fatty acids and glucose to sustain thermogenesis. However, recent data suggest that the beneficial effects of BAT could involve a previously unrecognized endocrine role through the release of endocrine factors. Several signaling molecules with endocrine properties have been found to be released by brown fat, especially under conditions of thermogenic activation. Moreover, experimental BAT transplantation has been shown to improve glucose tolerance and insulin sensitivity mainly by influencing hepatic and cardiac function. It has been proposed that these effects are due to the release of endocrine factors by brown fat, such as insulin-like growth factor I, interleukin-6, or fibroblast growth factor-21. Further research is needed to determine whether brown fat plays an endocrine role and, if so, to comprehensively identify which endocrine factors are released by BAT. Such research may reveal novel clues for the observed association between brown adipocyte activity and a healthy metabolic profile, and it could also enlarge a current view of potential therapeutic tools for obesity and associated metabolic diseases.

Brown fat and obesity: the next big thing?

Brown adipose tissue (BAT) is well recognised to have an important role in the maintenance of body temperature in animals and human neonates, its thermogenic action affected by a tissue-specific uncoupling protein; fatty acid oxidation within the numerous brown adipocyte mitochondria is rendered inefficient leading to heat, rather than adenosine triphosphate (ATP), production. BAT was believed to show rapid involution in early childhood, leaving only vestigial amounts in adults. However, recent evidence suggests that its expression in adults is far more common than previously appreciated, with a higher likelihood of detection in women and leaner individuals. It is conceivable that BAT activity might reduce the risk of developing obesity since fat stores are used for thermogenesis, and a directed enhancement of adipocyte metabolism might have value in weight reduction. However, it is as yet unclear how such manipulation of BAT might be achieved; even in animal models, the control of thermogenic activity is incompletely understood. Even so, there is still much to interest the endocrinologist in BAT, with a range of hormones affecting adipocyte activity. This may either contribute to normal physiological function, or the phenotypical presentation of states of pathological hormone excess or deficiency. Thus, the gender differences in BAT distribution may be attributable to the differential effects of male and female sex hormones, whilst BAT expansion may drive the weight loss associated with catecholamine-producing phaeochromocytomas. These observations support an important influence of the endocrine system on BAT activity and offer new potential targets in the treatment of obesity.

Thermogenic activation induces FGF21 expression and release in brown adipose tissue

FGF21 is a novel metabolic regulator involved in the control of glucose homeostasis, insulin sensitivity, and ketogenesis. The liver has been considered the main site of production and release of FGF21 into the blood. Here, we show that, after thermogenic activation, brown adipose tissue becomes a source of systemic FGF21. This is due to a powerful cAMP-mediated pathway of regulation of FGF21 gene transcription. Norepinephrine, acting via β-adrenergic, cAMP-mediated, mechanisms and subsequent activation of protein kinase A and p38 MAPK, induces FGF21 gene transcription and also FGF21 release in brown adipocytes. ATF2 binding to the FGF21 gene promoter mediates cAMP-dependent induction of FGF21 gene transcription. FGF21 release by brown fat in vivo was assessed directly by analyzing arteriovenous differences in FGF21 concentration across interscapular brown fat, in combination with blood flow to brown adipose tissue and assessment of FGF21 half-life. This analysis demonstrates that exposure of rats to cold induced a marked release of FGF21 by brown fat in vivo, in association with a reduction in systemic FGF21 half-life. The present findings lead to the recognition of a novel pathway of regulation the FGF21 gene and an endocrine role of brown fat, as a source of FGF21 that may be especially relevant in conditions of activation of thermogenic activity.

Oleoyl-oestrone inhibits lipogenic, but maintains thermogenic, gene expression of brown adipose tissue in overweight rats

In the present study we intended to determine how BAT (brown adipose tissue) maintained thermogenesis under treatment with OE (oleoyl-oestrone), a powerful slimming hormone that sheds off body lipid but maintains the metabolic rate. Overweight male rats were subjected to daily gavages of 10 nmol/g of OE or vehicle (control) for 10 days. A PF (pair-fed) vehicle-receiving group was used to discount the effects attributable to energy availability limitation. Interscapular BAT mass, lipid, DNA, mRNA and the RT-PCR (real-time PCR) expression of lipid and energy metabolism genes for enzymes and regulatory proteins were measured. BAT mass and lipid were decreased in OE and PF, with the latter showing a marked reduction in tissue mRNA. Maintenance of perilipin gene expression in PF and OE rats despite the loss of lipid suggests the preservation of the vacuolar interactive surface, a critical factor for thermogenic responsiveness. OE and, to a lesser extent, PF maintained the expression of genes controlling lipolysis and fatty acid oxidation, but markedly decreased the expression of those genes involved in lipogenic and acyl-glycerol synthesis. OE did not affect UCP1 (uncoupling protein 1) (decreased in PF), β3 adrenergic receptors or hormone-sensitive lipase gene mRNAs, which may translate in maintaining a full thermogenic system potential. OE rats were able to maintain a less energetically stressed BAT (probably through glucose utilization) than PF rats. These changes were not paralleled in PF rats, in which lower thermogenesis and glucose preservation resulted in a heavier toll on internal fat stores. Thus the mechanism of action of OE is more complex and tissue-specific than previously assumed.

Unique regulation of thyroid hormone metabolism during fasting in the house musk shrew (Suncus murinus, Insectivora: Soricidae)

The active hormone, 3,3',5-triiodothyronine (T3) is derived from thyroxine (T4) by the action of iodothyronine 5'-deiodinases (5'-D). By now two types of 5'-D have been identified; Type 1 (D1) and type 2 (D2). A relative contribution of these isotypes to the circulating T3 levels in the human remains to be determined whereas a number of reports indicate that, under physiological conditions, D1 plays a major role in maintaining circulating T3 levels in rodents. In both human and rodents, sickness and starvation reduce serum T3 concentration mainly through decrease in D1 activity. Recently, we found that the house musk shrew (Suncus murinus, Insectivora: Soricidae) has a different tissue distribution of D1 activity. Because compared to rodents D1 activity in the shrew was found only in liver at a much reduced level, D2 rather than D1 may play a role in the maintenance of serum T3. Therefore, we questioned how D1 and D2 activities change in fasted shrews and how these changes affect circulating thyroid hormone levels. We thus starved shrews for 24, 48 or 72 h and measured changes in serum concentration of T3, T4, and 3,3',5'-triiodothyronine (reverse T3, rT3) and D1 activities as well as its mRNA expression in liver. D2 activities were also measured in brown adipose tissue (BAT) and cerebral cortex of shrews. Unlike in human and rodents, T3 levels in shrews remained constant during fasting while T4 levels tended to decrease, resulting in an increase in its T3/T4 ratio. On the other hand, changes in rT3 levels were similar to those in human and rodents, being elevated with fasting. D1 mRNA and its activity were significantly reduced in the liver whereas D2 activities in BAT and cerebral cortex were increased by fasting. These results indicated that fasting in shrews also reduced hepatic D1 activity but it did not affect circulating T3 levels. The increased T3/T4 ratio together with increased D2 activity in BAT and cerebral cortex with fasting suggest that D2 rather than D1 is responsible for the maintenance of T3 levels in the house musk shrew.

Thyroid hormones as molecular determinants of thermogenesis

Thyroid hormones (TH) are major modulators of energy metabolism and thermogenesis. It is generally believed that 3,5,3'-triiodo-l-thyronine (T3) is the only active form of TH, and that most of its effects are mediated by nuclear T3 receptors, which chiefly affect the transcription of target genes. Some of these genes encode for the proteins involved in energy metabolism. However, a growing volume of evidence now indicates that other iodothyronines may be biologically active. Several mechanisms have been proposed to explain the calorigenic effect of TH, but none has received universal acceptance. Cold acclimation/exposure and altered nutritional status are physiological conditions in which a modulation of energy expenditure is particularly important. TH seem to be deeply involved in this modulation, and this article will review some aspects of their possible influence in these conditions.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Uncoupling protein-3 gene expression in skeletal muscle during development is regulated by nutritional factors that alter circulating non-esterified fatty acids

Uncoupling protein-3 gene expression in skeletal muscle is up-regulated during postnatal development of mice. A high-carbohydrate diet at weaning induces a decrease in uncoupling protein-3 mRNA levels that does not occur when mice were weaned onto a high-fat diet. Uncoupling protein-3 mRNA levels do not increase in response to fasting in young pups. Only after day 15 of life, when fasting increases serum non-esterified fatty acids, uncoupling protein-3 mRNA is up-regulated by starvation. Over-nutrition or under-nutrition during lactation increases or decreases, respectively, uncoupling protein-3 mRNA expression in skeletal muscle. Regulation of uncoupling protein-3 gene expression in skeletal muscle during development is mediated by ontogenic and nutritional factors determining changes in circulating non-esterified fatty acids.

Brown fat is essential for cold-induced thermogenesis but not for obesity resistance in aP2-Ucp mice

The role of brown adipose tissue in total energy balance and cold-induced thermogenesis was studied. Mice expressing mitochondrial uncoupling protein 1 (UCP-1) from the fat-specific aP2 gene promoter (heterozygous and homozygous aP2-Ucp transgenic mice) and their nontransgenic C57BL6/J littermates were used. The transgenic animals are resistant to obesity induced by a high-fat diet, presumably due to ectopic synthesis of UCP-1 in white fat. These animals exhibited atrophy of brown adipose tissue, as indicated by smaller size of brown fat and reduction of its total UCP-1 and DNA contents. Norepinephrine-induced respiration (measured in pentobarbital sodium-anesthetized animals) was decreased proportionally to the dosage of the transgene, and the homozygous (but not heterozygous) transgenic mice exhibited a reduction in their capacity to maintain body temperature in the cold. Our results indicate that the role of brown fat in cold-induced thermogenesis cannot be substituted by increased energy expenditure in other tissues.

Tissue metabolism and plasma levels of thyroid hormones in critically ill very premature infants

Thyroid status was characterized in very preterm infants (gestational age < or =32 wk; n = 61) from birth through d 14, and in infants who died within 16 d after delivery (n = 10), where it was also correlated with metabolism of iodothyronines in peripheral tissues (brain, liver, kidney, skeletal muscle, and adipose tissue). At 3 d of life, mean plasma levels of thyroxine, triiodothyronine, and TSH started to decrease, being lower in the critically ill compared with healthy premature neonates. Activities of the three iodothyronine deiodinases enzymes (type I, II, and III, respectively) were detected in all postmortem tissue samples, except for absence of the type II activity in kidney. All activities were the highest in liver and differed in other tissues. Lack of correlation between the type I activity in liver (and kidney), and plasma levels of thyroid hormones suggested that the thyroid was the primary source of circulating triiodothyronine. On the other hand, namely in brain, correlations between activity of the deiodinases and plasma hormone levels were found which suggested a complex control by thyroid hormones of their own metabolism. High activity of type III in liver, adipose tissue, and skeletal muscle demonstrated a role of these tissues in thyroid hormones degradation. Results support the view that peripheral tissues of very preterm infants are engaged in local generation of triiodothyronine, and inactivation of thyroid hormones, but do not represent a major source of circulating triiodothyronine.

beta-Adrenergic stimulation of interleukin-1alpha and interleukin-6 expression in mouse brown adipocytes

Mouse brown adipocytes in primary culture were shown to contain high levels of mRNA for interleukin-1alpha (IL-1alpha) which could be further stimulated up to 9-fold by norepinephrine (NE). Even higher stimulation by NE, up to 40-fold, was found in case of interleukin-6 (IL-6). Time-course of activation of both genes was biphasic, but the response of IL-6 gene was slower than of IL-1alpha gene. IL-1alpha mRNA level reached the maximum after 1 h and the second, lower increase, occurred after 8 h. IL-6 mRNA level showed first maximum after 2 h, but the highest level was found after 8 h. Similarly to NE, the expression of IL-1alpha and IL-6 genes was stimulated by selective beta-adrenergic agonist isoproterenol, beta3-selective agonist CGP-12117, forskoline and db-cAMP. The activation of both genes by CGP-12177 was dose-dependent with the optimum at 100 nM concentration. Stimulation of alpha-adrenergic receptors by cirazoline and oxymetazoline was without any effect. When the expression of IL-6 was studied at the protein level, the stimulation of IL-6 gene via beta3-receptors resulted in secretion of IL-6 up to the concentration 10 ng/ml culture media in 24 h. The results indicate a new type of regulation of expression of IL-1alpha and IL-6 genes in brown adipocytes by catecholamines acting via beta3-adrenergic receptors. The resulting increase in IL-6 production by brown adipocytes could significantly contribute to systemic levels of IL-6.

Replacement therapy for hypothyroidism with thyroxine alone does not ensure euthyroidism in all tissues, as studied in thyroidectomized rats

We have studied whether, or not, tissue-specific regulatory mechanisms provide normal 3,5,3'-triiodothyronine (T3) concentrations simultaneously in all tissues of a hypothyroid animal receiving thyroxine (T4), an assumption implicit in the replacement therapy of hypothyroid patients with T4 alone. Thyroidectomized rats were infused with placebo or 1 of 10 T4 doses (0.2-8.0 micrograms per 100 grams of body weight per day). Placebo-infused intact rats served as controls. Plasma and 10 tissues were obtained after 12-13 d of infusion. Plasma thyrotropin and plasma and tissue T4 and T3 were determined by RIA. Iodothyronine-deiodinase activities were assayed using cerebral cortex, liver, and lung. No single dose of T4 was able to restore normal plasma thyrotropin, T4 and T3, as well as T4 and T3 in all tissues, or at least to restore T3 simultaneously in plasma and all tissues. Moreover, in most tissues, the dose of T4 needed to ensure normal T3 levels resulted in supraphysiological T4 concentrations. Notable exceptions were the cortex, brown adipose tissue, and cerebellum, which maintained T3 homeostasis over a wide range of plasma T4 and T3 levels. Deiodinase activities explained some, but not all, of the tissue-specific and dose related changes in tissue T3 concentrations. In conclusion, euthyroidism is not restored in plasma and all tissues of thyroidectomized rats on T4 alone. These results may well be pertinent to patients on T4 replacement therapy.

Induction of type II iodothyronine 5'-deiodinase and mitochondrial uncoupling protein in brown adipocytes differentiated in cell culture

Brown adipocytes differentiated in primary cell culture were found to contain a type II iodothyronine 5'-deiodinase (5'D). Incubation of confluent cells with norepinephrine or dibutyryl-cAMP caused up to 17-fold increase in 5'D activity with a maximum after 8 h. Activation of 5'D required mRNA and protein synthesis and was accompanied by parallel, up to 5.8-fold increase in the amount of mitochondrial uncoupling protein with a maximum after 24 h. Analysis of adrenergic stimulation of 5'D suggested predominant involvement of the beta-receptors and increased intracellular cAMP levels, while the contribution of alpha 1-receptors was small.

Ontogeny and perinatal modulation of gene expression in rat brown adipose tissue. Unaltered iodothyronine 5'-deiodinase activity is necessary for the response to environmental temperature at birth

We have performed a sequential study on the abundance of the mRNA for uncoupling protein (UCP), subunit II of cytochrome-c oxidase (COII) and lipoprotein lipase in brown adipose tissue during the fetal and postnatal periods. Moreover, we have determined whether these parameters can be modulated by ambient temperature in the early hours after birth, and at which point in development this sensitivity first appears. UCP gene expression in the fetal and neonatal period has particular features when compared with overall mitochondriogenesis (COII mRNA expression) or with the expression of lipoprotein lipase mRNA. There is a specific induction of UCP gene expression between days 18 and 19 of pregnancy followed by a specific increase of UCP gene expression in utero and a further increase after birth. The acquisition of the physiological apparatus capable of the response to UCP and lipoprotein lipase gene expression to the environmental temperature is not achieved until the last day of fetal development. This result suggests that mechanisms of beta-adrenergic modulation of gene expression in brown fat are already established at birth. From an experiment on iopanoic acid treatment of pregnant mothers, it was concluded that iodothyronine 5'-deiodinase activity is not necessary for the expression of the mRNAs for UCP, COII and lipoprotein lipase in the fetus whereas it is necessary for the acquisition of temperature sensitivity to these parameters at birth.

Thyroid hormones and 5'-deiodinase in rat brown adipose tissue during fetal life

Brown adipose tissue (BAT) iodothyronine 5'-deiodinase (5'D) activities are very high during fetal life but decrease 10-fold a few hours before birth. Accordingly, BAT 3,5,3'-triiodothyronine (T3) concentrations are also very high. The temporal patterns of changes in BAT 5'-D and fetal plasma insulin are similar (and differ from the pattern for catecholamines) but are not superimposable. A causal role for insulin in the activation of fetal BAT 5'-D is therefore not supported by the data. Maternal thyroidectomy leads to a decrease in the total and relative weight of fetal BAT and to a 30-50% increase in BAT 5'-D activities; BAT thyroid hormone concentrations are essentially unchanged. Fetal hypothyroidism was induced by giving methimazole and resulted in a marked decrease of BAT thyroxine (T4) and T3 concentrations. This treatment increased BAT 5'-D activity only on day 21 of gestation, but no effect was observed on day 20. The fetal 5'-D response to thyroid hormones infused into the methimazole-treated dams was studied at 21 days of gestation. The increase in BAT 5'-D induced by methimazole treatment was prevented by T4 infused into control dams but not by T3. In fetuses from thyroidectomized dams, the pattern of 5'-D regulation by thyroid hormones was impaired. It is suggested that the high concentrations of thyroid hormones present in fetal BAT might participate in the general maturation and development of fetal BAT.

Iodothyronine 5'-deiodinase activity as an early event of prenatal brown-fat differentiation in bovine development

Iodothyronine 5'-deiodinase activity appears to be a type I enzyme in bovine brown adipose tissue, on the basis of its high Km for 3,3',5'-tri-iodothyronine ('reverse T3') (in the micromolar range) and sensitivity to propylthiouracil inhibition. This enzyme activity is already detectable in perirenal adipose tissue of bovine fetuses in the second month of gestation, reaches peak values around the seventh month of fetal life, declines before birth, becomes lower after parturition and finally undetectable in the adult cow. Iodothyronine 5'-deiodinase activity is present in the pericardic, peritoneal and intermuscular adipose depots of the neonatal calf, but it is always undetectable in the subcutaneous adipose tissue. It is concluded that iodothyronine 5'-deiodinase is a specific feature of brown fat in the bovine species that is not shared by white adipose tissue. white adipose tissue. Peak values of 5'-deiodinating activity appear as an early event in the prenatal differentiation programme of bovine brown-fat cells as they occur when uncoupling-protein-gene expression first starts.

Iodothyronine 5'-deiodinase activity and thyroid hormone content in brown adipose tissue during the breeding cycle of the rat

Brown adipose tissue iodothyronine 5'-deiodinase activity is significantly lower in 17-day pregnant rats compared with virgin controls and remains low during late pregnancy and lactation. It fully recovers with abrupt weaning, but only partially with spontaneous weaning. Even though this profile of changes is remarkably in step with the known pattern of modifications in brown fat thermogenesis during the breeding cycle, the lowered iodothyronine 5'-deiodinase activity appearing between days 15 and 17 of pregnancy occurs earlier than the reduction in brown adipose tissue thermogenesis. Brown fat 3,3',5-tri-iodothyronine content is also reduced in late pregnant, early and mid-lactating rats, most probably as a consequence of the lowered 5'-deiodination of thyroxine in situ. Acute insulin treatment increases brown fat iodothyronine 5'-deiodinase activity in virgin animals as well as in late-pregnant and lactating rats, despite the lowered basal enzyme activity levels in the latter groups. Thus an impaired response to insulin in brown fat does not appear to be a factor leading to the lowered iodothyronine 5'-deiodinase activity during late pregnancy and lactation.

Evidence for a differential physiological modulation of brown fat iodothyronine 5'-deiodinase activity in the perinatal period

Brown adipose tissue iodothyronine 5'-deiodinase increases progressively in fetuses from the day 17 of pregnancy on, it reaches peak values on the 20th day of gestation and declines in the last days of fetal life as well as during the first day of life. Birth of premature fetuses causes a sudden drop in the enzyme activity. Postmaturity is associated to a decrease in brown fat 5'-deiodinase similar to that found after birth in fetuses born at term. In the first hours of life brown fat iodothyronine 5'-deiodinase is essentially insensitive to the cold-stimulus. Present data indicates that, differently from adult rats, brown fat iodothyronine 5'-deiodinase activity during the perinatal period is dissociated from the thermogenic activity of the tissue. It is suggested that factors different from the action of the sympathetic nervous system may play a main role in brown fat iodothyronine 5'-deiodinase activity modulation in the fetal and neonatal life.

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.

Amino acid and glucose uptake by rat brown adipose tissue. Effect of cold-exposure and acclimation

The net uptake/release of glucose, lactate and amino acids from the bloodstream by the interscapular brown adipose tissue of control, cold-exposed and cold-acclimated rats was estimated by measurement of arteriovenous differences in their concentrations. In the control animals amino acids contributed little to the overall energetic needs of the tissue; glucose uptake was more than compensated by lactate efflux. Cold-exposure resulted in an enhancement of amino acid utilization and of glucose uptake, with high lactate efflux. There was a net glycine and proline efflux that partly compensated the positive nitrogen balance of the tissue; amino acids accounted for about one-third of the energy supplied by glucose to the tissue. Cold-acclimation resulted in a very high increase in glucose uptake, with a parallel decrease in lactate efflux and amino acid consumption. Branched-chain amino acids, however, were more actively utilized. This was related with a much higher alanine efflux, in addition to that of glycine and proline. It is suggested that most of the glucose used during cold-exposure is returned to the bloodstream as lactate under conditions of active lipid utilization, amino acids contributing their skeletons largely in anaplerotic pathways. On the other hand, cold-acclimation resulted in an important enhancement of glucose utilization, with lowered amino acid oxidation. Amino acids are thus used as metabolic substrates by the brown adipose tissue of rats under conditions of relatively scarce substrate availability, but mainly as anaplerotic substrates, in parallel to glucose. Cold-acclimation results in a shift of the main substrates used in thermogenesis from lipid to glucose, with a much lower need for amino acids.

Phorbol esters, protein kinase C, and thyroxine 5'-deiodinase in brown adipocytes

Protein kinase C activity has been identified in the rat brown adipocyte. About 60% of this activity is found in the cytosolic fraction under basal conditions, and 12-O-tetradecanoylphorbol 13-acetate (TPA) causes a rapid shift from the cytosol to the particulate fraction. Norepinephrine and phenylephrine cause a similar redistribution that can be blocked by prazosin but not by alprenolol. alpha 1-Adrenergic agonists cause three- to fivefold stimulation of type 2 iodothyronine 5'-deiodinase activity in brown adipocytes. TPA has no effect on basal deiodinase activity and reduces the response of the enzyme to alpha 1-adrenergic agonists. These results suggest that the translocation of protein kinase C from cytosol to particulate fraction is not sufficient to increase deiodinase activity but can modulate the alpha 1-adrenergic agonist-mediated responses in these cells.

Factors influencing the altered thermogenic response of rat brown adipose tissue in streptozotocin-diabetes

1. Adipocytes were isolated from the interscapular brown fat of male rats maintained at 21 degrees C. These animals were controls, streptozotocin-diabetics or 2-day insulin-treated diabetics. 2. With adipocytes from diabetic animals, maximum rates of noradrenaline-stimulated O2 uptake were decreased by 58%, and the Bmax. of [3H]GDP binding to mitochondria was decreased by 55%. Insulin administration reversed both of these changes. 3. Streptozotocin-diabetes increased basal lipolysis in adipocytes incubated with adenosine deaminase (1 unit/ml), decreased the EC50 (concn. giving 50% of maximum effect) for noradrenaline, but did not change the maximum rate of noradrenaline-stimulated lipolysis. Except for some small differences at very low concentrations (10-100 pM), diabetes or insulin treatment did not alter the sensitivity of noradrenaline-stimulated lipolysis or O2 uptake to the inhibitory effect of N6-phenylisopropyladenosine. It is therefore concluded that the lesion(s) in thermogenesis in diabetes are not attributable to any changes in lipolysis. 4. Blood flow through interscapular brown fat, measured by accumulation of [14C]DDT [14C-labelled 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane] was increased by 2.3-fold 70 min after a single administration of insulin to diabetic rats. This treatment decreased blood flow through epididymal white fat by 58%. 5. Propranolol treatment of diabetic rats muted the ability of insulin treatment to increase the maximum rate of noradrenaline-stimulated O2 uptake, suggesting that this action of insulin may be a secondary one rather than a direct effect of the hormone on the adipocytes.