Caloric Restriction and Diet-Induced Weight Loss Do Not Induce Browning of Human Subcutaneous White Adipose Tissue in Women and Men with Obesity

Calorie restriction prevents diet-induced insulin resistance independently of PGC-1-driven mitochondrial biogenesis in white adipose tissue

Calorie restriction (CR) exerts remarkable, beneficial effects on glucose homeostasis by mechanisms that are not fully understood. Given the relevance of white adipose tissue (WAT) in glucose homeostasis, we aimed at identifying the main cellular processes regulated in WAT in response to CR in a pathologic context of obesity. For this, a gene-expression profiling study was first conducted in mice fed ad libitum or subjected to 40% CR. We found that the gene network related to mitochondria was the most highly upregulated in WAT by CR. To study the role that increased mitochondrial biogenesis plays on glucose homeostasis following CR, we generated a mouse model devoid of the coactivators peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1)α and PGC-1β specifically in adipocytes. Our results show that mice lacking PGC-1s in adipocytes are unable to increase mitochondrial biogenesis in WAT upon CR. Despite a blunted induction of mitochondrial biogenesis in response to calorie deprivation, mice lacking adipose PGC-1s still respond to CR by improving their glucose homeostasis. Our study demonstrates that PGC-1 coactivators are major regulators of CR-induced mitochondrial biogenesis in WAT and that increased mitochondrial biogenesis and oxidative function in adipose tissue are not required for the improvement of glucose homeostasis mediated by CR.-Pardo, R., Vilà, M., Cervela, L., de Marco, M., Gama-Pérez, P., González-Franquesa, A., Statuto, L., Vilallonga, R., Simó, R., Garcia-Roves, P. M., Villena, J. A. Calorie restriction prevents diet-induced insulin resistance independently of PGC-1-driven mitochondrial biogenesis in white adipose tissue.

Common traits between the beige fat-inducing stimuli

Adipose tissues play an essential role in regulating the metabolic homeostasis and can be found in almost all parts of the body. Excessive adiposity leads to obesity and can contribute to metabolic and other disorders. Adipocytes show remarkable plasticity in their function, which can be pushed toward energy storage, or energy expenditure-a `browning' of fat. Browning is controlled by the cellular milieu of the adipose tissue, with sympathetic innervation and by immune responses as key integrators of the signals that promote browning. Here, we describe the latest contributions to our understanding of how different metabolic stimuli can shape the adipocyte function. We especially focus on the role of the gut microbiota and the negative energy balance in regulating the browning.

Role of Human Brown Fat in Obesity, Metabolism and Cardiovascular Disease: Strategies to Turn Up the Heat

Human brown adipose tissue (BAT) was re-discovered in 2009 by several independent groups, who showed that it is present and active in adults, as judged from the profound uptake of the glucose analogue radiotracer ¹⁸F-fluorodeoxyglucose in positron-emission tomography and computed tomography scan analysis after cold exposure. A potential clinical implication of activating BAT relates to its high metabolic activity and its potential role in stimulating energy expenditure (i.e. resting energy expenditure, meal-induced thermogenesis, and cold-induced thermogenesis), which makes it an attractive target to reduce adiposity. Moreover, due to its ability to oxidise glucose and lipids, BAT activation may also potentially exert beneficial metabolic and cardiovascular effects through reducing glucose and lipid levels, respectively. This review describes the potential role of human BAT in the prevention and treatment of obesity, metabolism, and cardiovascular disease focusing on its impact on energy expenditure and management of body fat accumulation as well as on glucose and lipid metabolism. This article also summarises the strategies that are currently being studied to activate human BAT.

Weight Loss and Adipose Tissue Browning in Humans: The Chicken or the Egg?

Recruitment of thermogenic adipocytes within white fat depots represents a promising strategy to increase energy expenditure. Negative energy balance has been reported to promote adipose tissue browning in rodents. In a recent issue of Cell Reports, Barquissau et al. show that caloric restriction-associated weight loss does not induce browning of subcutaneous abdominal white fat in obese humans.

Toward an Understanding of How Immune Cells Control Brown and Beige Adipobiology

Immune cells were recently found to have an unexpected involvement in controlling the thermogenic activity of brown and beige adipose tissue. Here, we review how macrophages, eosinophils, type 2 innate lymphoid cells, and T lymphocytes are linked to this process. In particular, the recruitment of alternatively activated macrophages and eosinophils is associated with brown fat activation and white fat browning. Conversely, pro-inflammatory immune cell recruitment represses the thermogenic activity of brown and beige adipose tissues via cytokines that inhibit noradrenergic signaling. Macrophages also influence the noradrenergic tone by degrading norepinephrine locally and by inhibiting sympathetic innervation over time.

Sex differences in the response to dietary restriction in rodents

Dietary restriction (DR) remains the most reproducible and consistent laboratory intervention to extend lifespan and improve health in mammals. DR has been primarily characterized in males due to issues of cost, perceived heightened variability amongst females, and the misconception that the reproductive system is the only important difference between sexes in mammals. In reality, existing data point to clear sex differences in mammalian responses to DR. Here we discuss recent advances in our understanding of sex differences in the responses to DR in rodent models.