A Third Variable in Obesity: The Effects of Brown Adipose Tissue on Thermogenesis

Approaches to weight management which consider only energy intake and/or expenditure do not consistently lead to favorable outcomes. A third variable, thermogenesis, must also be considered in a comprehensive understanding of obesity· Three types of thermogenesis have been outlined-shivering thermogenesis, nonshivering thermogenesis (NST), and diet-induced thermogenesis (DIT). The latter two types of thermogenesis, NST and DIT, may share a common biochemical mechanism which leads to heat production in brown adipose tissue (BAT) which is unchecked by energy needs. Four categories of studies are reviewed which implicate BAT as an important factor in DIT and point to commonalities in NST and DIT. More research is necessary to fully understand the role of BAT in human obesity

Wireless optogenetics protects against obesity via stimulation of non-canonical fat thermogenesis.

Cold stimuli and the subsequent activation of β-adrenergic receptor (β-AR) potently stimulate adipose tissue thermogenesis and increase whole-body energy expenditure. However, systemic activation of the β3-AR pathway inevitably increases blood pressure, a significant risk factor for cardiovascular disease, and, thus, limits its application for the treatment of obesity. To activate fat thermogenesis under tight spatiotemporal control without external stimuli, here, we report an implantable wireless optogenetic device that bypasses the β-AR pathway and triggers Ca2+ cycling selectively in adipocytes. The wireless optogenetics stimulation in the subcutaneous adipose tissue potently activates Ca2+ cycling fat thermogenesis and increases whole-body energy expenditure without cold stimuli. Significantly, the light-induced fat thermogenesis was sufficient to protect mice from diet-induced body-weight gain. The present study provides the first proof-of-concept that fat-specific cold mimetics via activating non-canonical thermogenesis protect against obesity

Single cell analysis reveals immune cell-adipocyte crosstalk regulating the transcription of thermogenic adipocytes.

Immune cells are vital constituents of the adipose microenvironment that influence both local and systemic lipid metabolism. Mice lacking IL10 have enhanced thermogenesis, but the roles of specific cell types in the metabolic response to IL10 remain to be defined. We demonstrate here that selective loss of IL10 receptor α in adipocytes recapitulates the beneficial effects of global IL10 deletion, and that local crosstalk between IL10-producing immune cells and adipocytes is a determinant of thermogenesis and systemic energy balance. Single Nuclei Adipocyte RNA-sequencing (SNAP-seq) of subcutaneous adipose tissue defined a metabolically-active mature adipocyte subtype characterized by robust expression of genes involved in thermogenesis whose transcriptome was selectively responsive to IL10Rα deletion. Furthermore, single-cell transcriptomic analysis of adipose stromal populations identified lymphocytes as a key source of IL10 production in response to thermogenic stimuli. These findings implicate adaptive immune cell-adipocyte communication in the maintenance of adipose subtype identity and function

Perilipin regulates the thermogenic actions of norepinephrine in brown adipose tissue

In response to cold, norepinephrine (NE)-induced triacylglycerol hydrolysis (lipolysis) in adipocytes of brown adipose tissue (BAT) provides fatty acid substrates to mitochondria for heat generation (adaptive thermogenesis). NE-induced lipolysis is mediated by protein kinase A (PKA)-dependent phosphorylation of perilipin, a lipid droplet-associated protein that is the major regulator of lipolysis. We investigated the role of perilipin PKA phosphorylation in BAT NE-stimulated thermogenesis using a novel mouse model in which a mutant form of perilipin, lacking all six PKA phosphorylation sites, is expressed in adipocytes of perilipin knockout (Peri KO) mice. Here, we show that despite a normal mitochondrial respiratory capacity, NE-induced lipolysis is abrogated in the interscapular brown adipose tissue (IBAT) of these mice. This lipolytic constraint is accompanied by a dramatic blunting (∼70%) of the in vivo thermal response to NE. Thus, in the presence of perilipin, PKA-mediated perilipin phosphorylation is essential for NE-dependent lipolysis and full adaptive thermogenesis in BAT. In IBAT of Peri KO mice, increased basal lipolysis attributable to the absence of perilipin is sufficient to support a rapid NE-stimulated temperature increase (∼3.0°C) comparable to that in wild-type mice. This observation suggests that one or more NE-dependent mechanism downstream of perilipin phosphorylation is required to initiate and/or sustain the IBAT thermal response

Mitochondrial lipoylation integrates age-associated decline in brown fat thermogenesis.

Thermogenesis in brown adipose tissue (BAT) declines with age; however, what regulates this process remains poorly understood. Here, we identify mitochondria lipoylation as a previously unappreciated molecular hallmark of aged BAT in mice. Using mitochondrial proteomics, we show that mitochondrial lipoylation is disproportionally reduced in aged BAT through a post-transcriptional decrease in the iron-sulfur (Fe-S) cluster formation pathway. A defect in the Fe-S cluster formation by the fat-specific deletion of Bola3 significantly reduces mitochondrial lipoylation and fuel oxidation in BAT, leading to glucose intolerance and obesity. In turn, enhanced mitochondrial lipoylation by α-lipoic acid supplementation effectively restores BAT function in old mice, thereby preventing age-associated obesity and glucose intolerance. The effect of α-lipoic acids requires mitochondrial lipoylation via the Bola3 pathway and does not depend on the anti-oxidant activity of α-lipoic acid. These results open up the possibility to alleviate the age-associated decline in energy expenditure by enhancing the mitochondrial lipoylation pathway

The Role of the Sympathetic Nervous System in the Hypothermic Effect of d-Fenfluramine

Experiments in this dissertation were conducted to characterize the effects of d-fenfluramine on body temperature and the mechanisms by which d-fenfluramine alter body temperature. The experiments were conducted in conscious male Sprague-Dawley rats. Body temperature was measured in all animals using telemetry. The results of the experiments indicated that d-fenfluramine altered body temperature in animals kept 28, 22, 16 and 4 degrees Centigrade. D-fenfluramine produced hyperthermia in animals kept at 28 degrees Centigrade and varying degrees hypothermia at normal and cooler ambient temperatures. Further experiments were conducted to explore the effects of d-fenfluramine on brown adipose tissue (BAT) thermogenesis, cutaneous vascular tone and whole body oxygen consumption. In animals kept at 22 and 4 degrees Centigrade, we found that d-fenfluramine activated BAT, as indicated by a decrease in BAT norepinephrine content, to the same magnitude. Thus, the hypothermia seen at normal and cooler ambient temperature was not due to lack of BAT activation. Also, activation of BAT by d-fenfluramine was mediated through the sympathetic nervous system and through release of central serotonin, since ganglionic blocker pentolinium and serotonin reuptake inhibitor fluoxetine blocked d-fenfluramine-mediated BAT activation. In animals kept at 16 degrees Centigrade, d-fenfluramine increased tail-skin temperature (Tsk), an index of cutaneous vascular tone, indicating that d-fenfluramine produced cutaneous vasodilation. d-fenfluramine-induced increase in Tsk was mediated through withdrawal of the sympathetic vasoconstrictor tone to the tail, since pentolinium blocks this effect. In animals kept at 28 degrees Centigrade, d-fenfluramine produced a decrease in Tsk, indicating vasoconstriction. The effects of d-fenfluramine on the Tsk were mediated through release of serotonin, since fluoxetine blocked these effects. D-fenfluramine increased whole body oxygen consumption, an index of metabolic activity and the increase was due to BAT activation, since pentolinium prevented the increase. Thus, although d-fenfluramine increased metabolic activity through BAT activation, the increase was insufficient to make up for the heat loss produced by cutaneous vasodilation and thus produces hypothermia. The hyperthermia seen at 28oC is due to activation of BAT and the subsequent inability of the animal to lose the excess heat due to cutaneous vasoconstriction produced by d-fenfluramine at 28 degrees Centigrade

Mechanisms of Energy Balance in Obesity

The proper understanding of obesity requires a multifaceted approach. Behavioral considerations of eating and activity patterns do not account for the large between- and within-subjects variance associated with the energy-balance equation. Sources of adaptive and dispositional variance in metabolic rates are reviewed and suggested to be a likely source of importance for the proper conceptualization and intervention of obesity. Five proposed mechanisms of metabolic variation are reviewed with consideration of the supporting evidence for each mechanism. The generalizability of some of the proposed mechanisms is limited because of the scope of past research. However, the roles of lipoprotein lipase in fat storage and brown adipose tissue in thermogenesis are intriguing possibilities for future research with humans