Dietary fatty acid metabolism of brown adipose tissue in cold-acclimated men

No effect of acute normobaric hypoxia on plasma triglyceride levels in fasting healthy men

Circulating fatty acids are a major systemic energy source in the fasting state as well as a determinant of hepatic triglycerides (TG)-rich very-low-density lipoprotein production. Upon acute hypoxia, sympathetic arousal induces adipose tissue lipolysis, resulting in an increase in circulating nonesterified fatty acids (NEFA). Animal studies suggest that TG clearance may also be strongly reduced under hypoxia, though this effect has been shown to be dependent on temperature. Whether the hypoxia-induced rise in blood fatty acid concentrations affects fasting TG levels in humans under thermoneutral conditions remains unknown. TG, NEFA, and glycerol levels were measured in fasted healthy young men (n = 10) exposed for 6 h to either normoxia (ambient air) or acute hypoxia (fraction of inspired oxygen = 0.12) in a randomized, crossover design. Participants were casually clothed and rested in front of a fan in an environmental chamber maintained at 28 °C during each trial. Under hypoxia, a significantly greater increase in NEFA occurred (condition × time interaction, p = 0.049) and glycerol levels tended to be higher (condition × time, p = 0.104), suggesting an increase in adipose tissue lipolysis. However, plasma TG levels did not change over time and did not differ between the normoxia and hypoxia conditions. In conclusion, acute exposure to normobaric hypoxia under thermoneutral condition in healthy men during fasting state increased lipolysis without affecting circulating TG.

Targeting thermogenesis in brown fat and muscle to treat obesity and metabolic disease

Brown fat is emerging as an interesting and promising target for therapeutic intervention in obesity and metabolic disease. Activation of brown fat in humans is associated with marked improvement in metabolic parameters such as levels of free fatty acids and insulin sensitivity. Skeletal muscle is another important organ for thermogenesis, with the capacity to induce energy-consuming futile cycles. In this Review, we focus on how these two major thermogenic organs - brown fat and muscle - act and cooperate to maintain normal body temperature. Moreover, in the light of disease-relevant mechanisms, we explore the molecular pathways that regulate thermogenesis in brown fat and muscle. Brown adipocytes possess a unique cellular mechanism to convert chemical energy into heat: uncoupling protein 1 (UCP1), which can short-circuit the mitochondrial proton gradient. However, recent research demonstrates the existence of several other energy-expending 'futile' cycles in both adipocytes and muscle, such as creatine and calcium cycling. These mechanisms can complement or even substitute for UCP1-mediated thermogenesis. Moreover, they expand our view of cold-induced thermogenesis from a special feature of brown adipocytes to a more general physiological principle. Finally, we discuss how thermogenic mechanisms can be exploited to expend energy and hence offer new therapeutic opportunities.

Abnormal Myocardial Dietary Fatty Acid Metabolism and Diabetic Cardiomyopathy

Patients with diabetes are at very high risk of hospitalization and death from heart failure. Increased prevalence of coronary heart disease, hypertension, autonomic neuropathy, and kidney failure all play a role in this increased risk. However, cardiac metabolic abnormalities are now recognized to play a role in this increased risk. Increased reliance on fatty acids to produce energy might predispose the diabetic heart to oxidative stress and ischemic damage. Intramyocellular accumulation of toxic lipid metabolites leads to a number of cellular abnormalities that might also contribute to cardiac remodelling and cardiac dysfunction. However, fatty acid availability from circulation and from intracellular lipid droplets to fuel the heart is critical to maintain its function. Fatty acids delivery to the heart is very complex and includes plasma nonesterified fatty acid flux as well as triglyceride-rich lipoprotein-mediated transport. Although many studies have shown a cross-sectional association between enhanced fatty acid delivery to the heart and reduction in left ventricular function in subjects with prediabetes and diabetes, these mechanisms change very rapidly during type 2 diabetes treatment. The present review focuses on the role of fatty acids in cardiac function, with particular emphasis on the possible role of early abnormalities of dietary fatty acid metabolism in the development of diabetic cardiomyopathy.

Shivering and nonshivering thermogenesis in skeletal muscles

Humans have inherited complex neural circuits which drive behavioral, somatic, and autonomic thermoregulatory responses to defend their body temperature. While they are well adapted to dissipate heat in warm climates, they have a reduced capacity to preserve it in cold environments. Consequently, heat production is critical to defending their core temperature. As in other large mammals, skeletal muscles are the primary source of heat production recruited in cold-exposed humans. This is achieved voluntarily in the form of contractions from exercising muscles or involuntarily in the form of contractions from shivering muscles and the recruitment of nonshivering mechanisms. This review describes our current understanding of shivering and nonshivering thermogenesis in skeletal muscles, from the neural circuitry driving their recruitment to the metabolic substrates that fuel them. The presence of these heat-producing mechanisms can be measured in vivo by combining indirect respiratory calorimetry with electromyography or biomedical imaging modalities. Indeed, much of what is known regarding shivering in humans and other animal models stems from studies performed using these methods combined with in situ and in vivo neurologic techniques. More recent investigations have focused on understanding the metabolic processes that produce the heat from both contracting and noncontracting mechanisms. With the growing interest in the potential therapeutic benefits of shivering and nonshivering skeletal muscle to counter the effects of neuromuscular, cardiovascular, and metabolic diseases, we expect this field to continue its growth in the coming years.

Cold-Induced Brown Adipose Tissue Activity Alters Plasma Fatty Acids and Improves Glucose Metabolism in Men

CONTEXT

Mounting evidence suggests beneficial effects of brown adipose tissue (BAT) activation on glucose and lipid metabolism in humans. It is unclear whether cold-induced BAT activation affects not only insulin sensitivity but also insulin secretion. Likewise, the role in clearing circulating fatty acids (FAs) has not been fully explored.

OBJECTIVE

Exploring the effects of cold-induced BAT activation on insulin sensitivity and secretion, as well as on plasma FA profiles.

DESIGN

Fifteen healthy men participated in a cross-balanced repeated within-subject study with two experimental conditions. Subjects were exposed to thermoneutrality (22°C) and to moderate cold (18.06°C, shivering excluded) by use of a water-perfused whole body suit. Cold-induced BAT activation was quantified by [18F]-fluorodeoxyglucose positron emission tomography-computed tomography in a subset of volunteers. A Botnia clamp procedure was applied to determine pancreatic first phase insulin response (FPIR) and insulin sensitivity. Hormones and metabolites, including 26 specific plasma FAs, were sampled throughout the experiment.

RESULTS

Cold exposure induced BAT activity. Plasma noradrenaline and dopamine concentrations increased in response to cold. Peripheral glucose uptake and insulin sensitivity significantly improved by ∼20%, whereas FPIR remained stable. Lignoceric acid (C24:0) concentrations increased, whereas levels of eicosanoic acid (C20:1n9), nervonic acid (C24:1n9), and behenic acid (C22:0) decreased.

CONCLUSIONS

Cold-exposure induces sympathetic nervous system activity and BAT metabolism in humans, resulting in improved glucose metabolism without affecting pancreatic insulin secretion. In addition, BAT activation is associated with altered circulating concentrations of distinct FAs. These data support the concept that human BAT metabolism significantly contributes to whole body glucose and lipid utilization in a coordinated manner.

Distinct cellular and molecular mechanisms for β3 adrenergic receptor-induced beige adipocyte formation

Beige/brite adipocytes are induced within white adipose tissues (WAT) and, when activated, consume glucose and fatty acids to produce heat. Classically, two stimuli have been used to trigger a beiging response: cold temperatures and β3-adrenergic receptor (Adrb3) agonists. These two beiging triggers have been used interchangeably but whether these two stimuli may induce beiging differently at cellular and molecular levels remains unclear. Here, we found that cold-induced beige adipocyte formation requires Adrb1, not Adrb3, activation. Adrb1 activation stimulates WAT resident perivascular (Acta2+) cells to form cold-induced beige adipocytes. In contrast, Adrb3 activation stimulates mature white adipocytes to convert into beige adipocytes. Necessity tests, using mature adipocyte-specific Prdm16 deletion strategies, demonstrated that adipocytes are required and are predominant source to generate Adrb3-induced, but not cold-induced, beige adipocytes. Collectively, we identify that cold temperatures and Adrb3 agonists activate distinct cellular populations that express different β-adrenergic receptors to induce beige adipogenesis.

Excess accumulation of a type of fat called white fat is associated with obesity and metabolic problems. White fat cells store energy. White fat tissue also contains some beige fat cells, which burn fats and sugars to produce heat. Cold temperatures trigger the production and activity of beige fat cells, which allows the body to stay warm. People with obesity tend to have less beige fat and more white fat. This has led scientists to test whether treatments that increase the number of beige fat cells a person has could reduce fat mass and improve metabolism.

To develop treatments that increase beige fat, scientists must first understand where it comes from and how cold and other factors stimulate its growth. Recent studies have shown that smooth muscle cells, which surround blood vessel walls, make cold-induced beige fat cells. A widely used drug that turns on the β3 adrenergic receptor, which is found in the cell membrane, also boosts the creation of beige fat cells. Yet, it was not clear exactly how cold or this drug triggers the production of beige fat.

Now, Jiang et al. show that drugs that target β3 adrenergic receptors cause white fat cells in mice to change into beige fat cells. The experiments also showed that cold turns on a different receptor called the β1 adrenergic receptor on smooth muscle cells causing them to make beige fat cells. This shows that there is more than one source for beige fat cells in the body and that different strategies for increasing beige fat cell numbers do not work the same way.

More studies are needed to learn whether beige fat cells produced after exposure to cold or drugs behave in the same way and have similar affects on metabolism. This could help scientists determine if one of these strategies could make a better treatment for obesity or other metabolic disorders.

The color of fat and its central role in the development and progression of metabolic diseases

Excess caloric intake does not always translate to an expansion of the subcutaneous adipose tissue (SAT) and increase in fat mass. It is now recognized that adipocyte type (white, WAT, or brown, BAT), size (large vs. small) and metabolism are important factors for the development of cardiometabolic diseases. When the subcutaneous adipose tissue is not able to expand in response to increased energy intake the excess substrate is stored as visceral adipose tissue or as ectopic fat in tissues as muscle, liver and pancreas. Moreover, adipocytes become dysfunctional (adiposopathy, or sick fat), adipokines secretion is increased, fat accumulates in ectopic sites like muscle and liver and alters insulin signaling, increasing the demand for insulin secretion. Thus, there are some subjects that despite having normal weight have the metabolic characteristics of the obese (NWMO), while some obese expand their SAT and remain metabolically healthy (MHO). In this paper we have reviewed the recent findings that relate the metabolism of adipose tissue and its composition to metabolic diseases. In particular, we have discussed the possible role of dysfunctional adipocytes and adipose tissue resistance to the antilipolytic effect of insulin on the development of impaired glucose metabolism. Finally we have reviewed the possible role of BAT vs. WAT in the alteration of lipid and glucose metabolism and the recent studies that have tried to stimulate browning in human adipose tissue.

Metabolic regulation and the anti-obesity perspectives of human brown fat

Activation of brown adipose tissue (BAT) in adult humans increase glucose and fatty acid clearance as well as resting metabolic rate, whereas a prolonged elevation of BAT activity improves insulin sensitivity. However, substantial reductions in body weight following BAT activation has not yet been shown in humans. This observation raise the possibility for feedback mechanisms in adult humans in terms of a brown fat-brain crosstalk, possibly mediated by batokines, factors produced by and secreted from brown fat. Batokines also seems to be involved in BAT recruitment by stimulating proliferation and differentiation of brown fat progenitors. Increasing human BAT capacity could thus include inducing brown fat biogenesis as well as identifying novel batokines. Another attractive approach would be to induce a brown fat phenotype, the so-called brite or beige fat, within the white fat depots. In adult humans, white fat tissue transformation into beige has been observed in patients with pheochromocytoma, a norepinephrine-producing tumor. Interestingly, human beige fat is predominantly induced in regions that were BAT during early childhood, possibly reflecting that a presence of human beige progenitors is depot specific and originating from BAT. In conclusion, to utilize the anti-obesity potential of human BAT focus should be directed towards identifying novel regulators of brown and beige fat progenitor cells, as well as feedback mechanisms of BAT activation. This would allow for identification of novel anti-obesity targets.

Shivering thermogenesis in humans: Origin, contribution and metabolic requirement

As endotherms, humans exposed to a compensable cold environment rely on an increase in thermogenic rate to counteract heat lost to the environment, thereby maintaining a stable core temperature. This review focuses primarily on the most important contributor of heat production in cold-exposed adult humans, shivering skeletal muscles. Specifically, it presents current understanding on (1) the origins of shivering, (2) the contribution of shivering to total heat production and (3) the metabolic requirements of shivering. Although shivering had commonly been measured as a metabolic outcome measure, considerable research is still needed to clearly identify the neuroanatomical structures and circuits that initiate and modulate shivering and drives the shivering patterns (continuous and burst shivering). One thing is clear, the thermogenic rate in humans can be maintained despite significant inter-individual differences in the thermogenic contribution of shivering, the muscles recruited in shivering, the burst shivering rate and the metabolic substrates used to support shivering. It has also become evident that the variability in burst shivering rate between individuals, despite not influencing heat production, does play a key role in orchestrating metabolic fuel selection in the cold. In addition, advances in our understanding of the thermogenic role of brown adipose tissue have been able to explain, at least in part, the large inter-individual differences in the contribution of shivering to total heat production. Whether these differences in the thermogenic role of shivering have any bearing on cold endurance and survival remains to be established. Despite the available research describing the relative thermogenic importance of shivering skeletal muscles in humans, the advancement in our understanding of how shivering is initiated and modulated is needed. Such research is critical to consider strategies to either reduce its role to improve occupational performance or exploit its metabolic potential for clinical purposes.

Molecular imaging of postprandial metabolism

Disordered postprandial metabolism of energy substrates is one of the main defining features of prediabetes and contributes to the development of several chronic diseases associated with obesity, such as type 2 diabetes and cardiovascular diseases. Postprandial energy metabolism has been studied using classical isotopic tracer approaches that are limited by poor access to splanchnic metabolism and highly dynamic and complex exchanges of energy substrates involving multiple organs and systems. Advances in noninvasive molecular imaging modalities, such as PET and MRI/magnetic resonance spectroscopy (MRS), have recently allowed important advances in our understanding of postprandial energy metabolism in humans. The present review describes some of these recent advances, with particular focus on glucose and fatty acid metabolism in the postprandial state, and discusses current gaps in knowledge and new perspectives of application of PET and MRI/MRS for the investigation and treatment of human metabolic diseases.

Beyond obesity – thermogenic adipocytes and cardiometabolic health

The global prevalence of obesity and related cardiometabolic disease continues to increase through the 21st century. Whilst multi-factorial, obesity is ultimately caused by chronic caloric excess. However, despite numerous interventions focussing on reducing caloric intake these either fail or only elicit short-term changes in body mass. There is now a focus on increasing energy expenditure instead which has stemmed from the recent ‘re-discovery’ of cold-activated brown adipose tissue (BAT) in adult humans and inducible ‘beige’ adipocytes. Through the unique mitochondrial uncoupling protein 1 (UCP1), these thermogenic adipocytes are capable of combusting large amounts of chemical energy as heat and in animal models can prevent obesity and cardiometabolic disease. At present, human data does not point to a role for thermogenic adipocytes in regulating body weight or fat mass but points to a pivotal role in regulating metabolic health by improving insulin resistance as well as glucose and lipid homeostasis. This review will therefore focus on the metabolic benefits of BAT activation and the mechanisms and signalling pathways by which these could occur including improvements in insulin signalling in peripheral tissues, systemic lipid and cholesterol metabolism and cardiac and vascular function.