BMP4-mediated brown fat-like changes in white adipose tissue alter glucose and energy homeostasis

Expression of bone morphogenetic protein 4 (BMP4) in adipocytes of white adipose tissue (WAT) produces “white adipocytes” with characteristics of brown fat and leads to a reduction of adiposity and its metabolic complications. Although BMP4 is known to induce commitment of pluripotent stem cells to the adipocyte lineage by producing cells that possess the characteristics of preadipocytes, its effects on the mature white adipocyte phenotype and function were unknown. Forced expression of a BMP4 transgene in white adipocytes of mice gives rise to reduced WAT mass and white adipocyte size along with an increased number of a white adipocyte cell types with brown adipocyte characteristics comparable to those of beige or brite adipocytes. These changes correlate closely with increased energy expenditure, improved insulin sensitivity, and protection against diet-induced obesity and diabetes. Conversely, BMP4-deficient mice exhibit enlarged white adipocyte morphology and impaired insulin sensitivity. We identify peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC1α) as the target of BMP signaling required for these brown fat-like changes in WAT. This effect of BMP4 on WAT appears to extend to human adipose tissue, because the level of expression of BMP4 in WAT correlates inversely with body mass index. These findings provide a genetic and metabolic basis for BMP4’s role in altering insulin sensitivity by affecting WAT development.     

From the Cover: Prolonged daily light exposure increases body fat mass through attenuation of brown adipose tissue activity

Disruption of circadian rhythmicity is associated with obesity and related disorders, including type 2 diabetes and cardiovascular disease. Specifically, prolonged artificial light exposure associates with obesity in humans, although the underlying mechanism is unclear. Here, we report that increasing the daily hours of light exposure increases body adiposity through attenuation of brown adipose tissue (BAT) activity, a major contributor of energy expenditure. Mice exposed to a prolonged day length of 16- and 24-h light, compared with regular 12-h light, showed increased adiposity without affecting food intake or locomotor activity. Mechanistically, we demonstrated that prolonged day length decreases sympathetic input into BAT and reduces β3-adrenergic intracellular signaling. Concomitantly, prolonging day length decreased the uptake of fatty acids from triglyceride-rich lipoproteins, as well as of glucose from plasma selectively by BAT. We conclude that impaired BAT activity is an important mediator in the association between disturbed circadian rhythm and adiposity, and anticipate that activation of BAT may overcome the adverse metabolic consequences of disturbed circadian rhythmicity.Modern world society is subjected to disturbances of circadian rhythms by shift work, sleep deprivation, and environmental light pollution. Importantly, the increasing prevalence of obesity is associated with a disrupted sleep-wake pattern in humans (1) and coincides with the availability of artificial light (2, 3). Additionally, a recent study revealed a relationship between exposure to light at night and obesity in a cross-sectional analysis of over 100,000 women (4). Light input is the most important cue for generation of circadian (∼24 h) rhythms by the master clock. Both in rodents and humans the master clock is situated in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is responsible for synchronization of peripheral clocks throughout the body, which is mediated by endocrine and neuronal signals (5). A causal role for a disturbed circadian rhythm in the development of obesity has been demonstrated by animal studies. Mice with genetically dysfunctional clock genes develop obesity and insulin resistance (6–9). Moreover, specific ablation of the SCN induces acute weight gain (10). These results indicate a crucial role for the SCN in the regulation of adiposity.Interestingly, we previously showed that prolonged light exposure only is sufficient to enhance weight gain in mice. Constant light disrupts the central circadian clock, evidenced by an immediate reduction in the circadian amplitude of SCN electrical activity. Moreover, constant light induces body weight gain and insulin resistance, even faster than high-fat diet, which was not caused by increased food intake or reduced locomotor activity (11). Therefore, disruption of the central biological clock likely induces weight gain by decreasing energy expenditure.Recently, it has been recognized that brown adipose tissue (BAT) importantly contributes to energy expenditure. BAT combusts high amounts of triglycerides (TG) into heat, a process called thermogenesis that is mediated by uncoupling protein 1 (UCP1). Interestingly, SCN neurons project onto BAT and injection of glutamate into the SCN increases BAT thermogenesis in rats (12, 13). This finding indicates that BAT may mediate the association between circadian rhythmicity and energy expenditure. Therefore, the aim of this study was to shed light on the association between prolonged light exposure and obesity in humans by investigating the effect of day length on BAT activity in mice in relation to body fat gain, independent of ambient temperature. We demonstrate that daily light exposure negatively associates with the uptake of TG-derived fatty acids and glucose from plasma by BAT, pointing to decreased activity of the tissue. Furthermore, we show that increasing daily light exposure decreases BAT activity through reduced sympathetic stimulation.     

Remodelling the inguinal adipose sensory system to switch on the furnace: Electroacupuncture stimulation induces brown adipose thermogenesis

Brown and white adipose tissue mediate thermogenesis through the thermogenetic centre of the brain, but safe methods for activating thermogensis and knowledge of the associated molecular mechanisms are lacking. We investigated body surface electroacupuncture stimulation (ES) at ST25 (targeted at the abdomen) induction of brown adipose thermogenesis and the neural mechanism of this process. Inguinal white adipose tissue (iWAT) and interscapular brown adipose tissue (iBAT) were collected and the thermogenic protein expression levels were measured to evaluate iBAT thermogenesis capacity. The thermogenic centre activating region and sympathetic outflow were evaluated based on neural electrical activity and c-fos expression levels. iWAT sensory axon plasticity was analysed with whole-mount adipose tissue imaging. ES activated the sympathetic nerves in iBAT and the c-fos-positive cells induced sympathetic outflow activation to the iBAT from the medial preoptic area (MPA), the dorsomedial hypothalamus (DM) and the raphe pallidus nucleus (RPA). iWAT denervation mice exhibited decreased c-fos-positive cells in the DM and RPA, and lower recombinant uncoupling orotein 1 peroxisome proliferator-activated receptor, β3-adrenergic receptor, and tyrosine hydroxylase expression. Remodelling the iWAT sensory axons recovered the signal from the MPA to the RPA and induced iBAT thermogenesis. The sympathetic denervation attenuated sensory nerve density. ES induced sympathetic outflow from the thermogenetic centres to iBAT, which mediated thermogenesis. iWAT sensory axon remodelling induced the MPA–DM–RPA–iBAT thermogenesis pathway.     

Significance and application of melatonin in the regulation of brown adipose tissue metabolism: relation to human obesity

A worldwide increase in the incidence of obesity indicates the unsuccessful battle against this disorder. Obesity and the associated health problems urgently require effective strategies of treatment. The new discovery that a substantial amount of functional brown adipose tissue (BAT) is retained in adult humans provides a potential target for treatment of human obesity. BAT is active metabolically and disposes of extra energy via generation of heat through uncoupling oxidative phosphorylation in mitochondria. The physiology of BAT is readily regulated by melatonin, which not only increases recruitment of brown adipocytes but also elevates their metabolic activity in mammals. It is speculated that the hypertrophic effect and functional activation of BAT induced by melatonin may likely apply to the human. Thus, melatonin, a naturally occurring substance with no reported toxicity, may serve as a novel approach for treatment of obesity. Conversely, because of the availability of artificial light sources, excessive light exposure after darkness onset in modern societies should be considered a potential contributory factor to human obesity as light at night dramatically reduces endogenous melatonin production. In the current article, the potential associations of melatonin, BAT, obesity and the medical implications are discussed.     

Impaired metabolic response to nerve stimulation in brown adipose tissue of hypothyroid rats

In brown adipose tissue of the rat, chemically or surgically induced hypothyroidism caused the following effects. A large decrease of the magnitude of the metabolic response to electrical nerve stimulation. The deactivation half-time of the response was reduced to 70% of the control value, with no change in catechol O-methyltransferase activity. Pre-incubation of tissues with norepinephrine, 10(-5) M, increased the response to subsequent nerve stimulation almost to that of the controls. The catecholamine analogue dose-response curves were shifted to the right. The shift was very pronounced for isoproterenol (K50 426 nM versus 2 nM), somewhat less marked for norepinephrine (7373 nM versus 194) and very slight for phenylephrine (2803 nM versus 1649); there was almost no change in Emax values. An increase of octanoate oxidative capacity. A decrease of the capacity of the stereoselective binding of (-)-[3H]dihydroalprenolol of the high-affinity (Kd 2.0 nM) sites to a fourth of the control value and an increase by a factor of 2.9 of the Kd of the low-affinity binding sites. This decrease of binding to the beta-receptors was not sufficient quantitatively to explain the decrease in the metabolic response, suggesting the existence of an additional defective reaction which could occur between the binding to the beta-receptors and the activation of the triglyceride lipase. These results show that the sharp decrease of the metabolic response of brown adipose tissue to nerve stimulation has multiple causes. The findings are discussed in the context of the drastic decrease of cold resistance in hypothyroid rats.     

Non-shivering thermogenesis in brown adipose tissue and aging

The aging-induced decrease in non-shivering thermogenic capacity was prevented in the genetically CCK-A receptor deficient, diabetic obese OLETF rats, provably by an increase in the responsiveness of BAT to glucagon. The potential metabolic capacities can be sustained until aged period.     

Molecular pathways regulating the formation of brown‐like adipocytes in white adipose tissue

Adipose tissue is functionally composed of brown adipose tissue and white adipose tissue. The unique thermogenic capacity of brown adipose tissue results from expression of uncoupling protein 1 in the mitochondrial inner membrane. On the basis of recent findings that adult humans have functionally active brown adipose tissue, it is now recognized as playing a much more important role in human metabolism than was previously thought. More importantly, brown‐like adipocytes can be recruited in white adipose tissue upon environmental stimulation and pharmacologic treatment, and this change is associated with increased energy expenditure, contributing to a lean and healthy phenotype. Thus, the promotion of brown‐like adipocyte development in white adipose tissue offers novel possibilities for the development of therapeutic strategies to combat obesity and related metabolic diseases. In this review, we summarize recent advances in understanding the molecular mechanisms involved in the recruitment of brown‐like adipocyte in white adipose tissue. Copyright © 2014 John Wiley & Sons, Ltd.     

Factors involved in white‐to‐brown adipose tissue conversion and in thermogenesis: a review

Obesity is the result of energy intake chronically exceeding energy expenditure. Classical treatments against obesity do not provide a satisfactory long‐term outcome for the majority of patients. After the demonstration of functional brown adipose tissue in human adults, great effort is being devoted to develop therapies based on the adipose tissue itself, through the conversion of fat‐accumulating white adipose tissue into energy‐dissipating brown adipose tissue. Anti‐obesity treatments that exploit endogenous, pharmacological and nutritional factors to drive such conversion are especially in demand. In the present review, we summarize the current knowledge about the various molecules that can be applied in promoting white‐to‐brown adipose tissue conversion and energy expenditure and the cellular mechanisms involved.     

Effects of Short Photoperiod and Melatonin Treatment on Thermogenesis in the Syrian Hamster

This study investigated whether short photoperiod or melatonin-treatment could alter the thermogenic capacity of Syrian hamsters. Exposure of hamsters to short photoperiod and to exogenous melatonin treatment induced gonadal regression and hypertrophy of brown adipose tissue (BAT). Short photoperiod and melatonin-induced BAT hypertrophy was not accompanied by any change in noradrenaline (NA) turnover in this tissue. The concentration of NA was significantly decreased in hypertrophied BAT, indicating that sympathetic innervation in BAT did not effect its hypertrophy. No improvement in nonshivering thermogenic capacity was noticed in hamsters with increased BAT mass. However, capability for shivering thermogenesis seemed to be enhanced in melatonin-treated hamsters. These observations suggest that melatonin, in addition to mediating short photoperiod-induced gonadal regression in the Syrian hamster, also brings about thermoregulatory adjustments necessary for hibernation.     

From the Cover: Protein carbamylation is a hallmark of aging

Aging is a progressive process determined by genetic and acquired factors. Among the latter are the chemical reactions referred to as nonenzymatic posttranslational modifications (NEPTMs), such as glycoxidation, which are responsible for protein molecular aging. Carbamylation is a more recently described NEPTM that is caused by the nonenzymatic binding of isocyanate derived from urea dissociation or myeloperoxidase-mediated catabolism of thiocyanate to free amino groups of proteins. This modification is considered an adverse reaction, because it induces alterations of protein and cell properties. It has been shown that carbamylated proteins increase in plasma and tissues during chronic kidney disease and are associated with deleterious clinical outcomes, but nothing is known to date about tissue protein carbamylation during aging. To address this issue, we evaluated homocitrulline rate, the most characteristic carbamylation-derived product (CDP), over time in skin of mammalian species with different life expectancies. Our results show that carbamylation occurs throughout the whole lifespan and leads to tissue accumulation of carbamylated proteins. Because of their remarkably long half-life, matrix proteins, like type I collagen and elastin, are preferential targets. Interestingly, the accumulation rate of CDPs is inversely correlated with longevity, suggesting the occurrence of still unidentified protective mechanisms. In addition, homocitrulline accumulates more intensely than carboxymethyl-lysine, one of the major advanced glycation end products, suggesting the prominent role of carbamylation over glycoxidation reactions in age-related tissue alterations. Thus, protein carbamylation may be considered a hallmark of aging in mammalian species that may significantly contribute in the structural and functional tissue damages encountered during aging.Aging is a complex process resulting from the combination of a large number of genetic and acquired factors leading to a decline of organism functions. Cellular senescence, telomere shortening, decreased proliferative capacity, mitochondrial DNA single mutations, and inflammation influence the aging process (1–3). Protein aging is also actively involved in tissue aging. During their biological life, proteins are exposed to various alterations caused by nonenzymatic posttranslational modifications (NEPTMs), like glycation, oxidation, carbonylation, or carbamylation, that contribute to functional and structural alterations of their properties (4).Among them, glycation has generally been recognized to significantly contribute to aging processes. Glycation refers to the binding of sugar carbonyl groups to protein amino groups, resulting in the formation of a Schiff base, which rapidly undergoes a molecular rearrangement to form an Amadori product. These products can be further exposed to irreversible oxidative processes, which lead to the generation of a variety of complex compounds called advanced glycation end products (AGEs). Because glycation and oxidative reactions are closely linked, it is more suitable to name the whole pathway “glycoxidation.” It has been recognized that AGEs accumulated in organisms in an age-dependent manner. N^ε-carboxymethyl-lysine (CML) and pentosidine, two major AGEs, were found in several tissues, like kidney, bone, eye, skeletal muscle, cartilage, arterial wall, or brain, and shown to be correlated to the risk of adverse aging-related outcomes (5–10). Because AGE formation is cumulative and irreversible, glycoxidation particularly affects extracellular matrix (ECM) proteins because of their long biological life. Indeed, glycation promotes collagen cross-linking involved in stiffness and decreased elasticity of skin (11) but also, modifies matrix proteins of other tissues, contributing, for example, to the development of vessel rigidity. This phenomenon is associated with the higher prevalence of cardiovascular diseases and could predict adverse cardiovascular events in both healthy subjects and high-risk patients (12). Moreover, it has been recently shown that exogenous AGEs brought by diet contributed to endothelial dysfunction, arterial stiffness, and aging (13).The justified interest expressed for glycoxidation may, however, have distracted attention from other important pathophysiological mechanisms of aging. Indeed, it has been shown that another NEPTM, carbamylation, participated in protein molecular aging. This reaction corresponds to the binding of isocyanic acid to free amino groups and preferentially occurs on the ε-NH₂ of lysine residues generating homocitrulline (HCit) (Fig. 1), the most characteristic carbamylation-derived product (CDP) (14, 15) that can be specifically quantified (16). Isocyanic acid is mainly formed by the spontaneous dissociation of urea into cyanate and ammonia (15) but may also derive from thiocyanate through myeloperoxidase action (17, 18). Isocyanic acid generated from biomass burning, biofuel use, or tobacco use has been described as a minor environmental source (19).Open in a separate windowFig. 1.Carbamylation reaction.The occurrence of in vivo carbamylation has been known since 1960, when Stark et al. (20) reported that cyanate was able to react with amino acids and proteins. The first deleterious effects of carbamylation in vivo were evidenced in the 1970s. At that time, patients with sickle cell disease were treated with urea or cyanate to promote the carbamylation of HbS, increasing its affinity for oxygen and decreasing its capacity of aggregation. However, these patients developed cataract, which was attributed to the carbamylation of lens proteins (21). Apart from this context, other pathological implications of carbamylation have been discussed only recently (22–24). The intensity of this reaction is particularly amplified during chronic kidney disease (CKD) because of hyperuremia (25, 26). High concentrations of carbamylated plasma proteins constitute a significant risk factor for cardiovascular events and mortality in hemodialysis patients (26, 27). Recently, our laboratory has evidenced in a mouse model that tissue proteins, including skin collagen, were more intensely carbamylated during CKD and accumulated in the organism (28). In addition, carbamylation rate is increased in atherosclerotic plaques because of myeloperoxidase release from inflammatory cells, participating in the development of atherosclerosis in patients with CKD or type 2 diabetes, even in the absence of renal impairment (23, 29, 30).However, to date, no data are available on the involvement of carbamylation in aging. Such a finding would be of potential relevance, because previous studies have evidenced that carbamylation induced deleterious effects on biological functions of proteins (23, 31–33). In this paper, we have examined the age-related evolution of carbamylation of skin proteins and the two major matrix proteins, type I collagen and elastin, in three mammalian species (human, murine, and bovine). Our results show a progressive increase of carbamylated proteins in skin and suggest that carbamylation may be considered a general feature of tissue aging at least as important as glycation.