Brown adipose tissue: physiological function and evolutionary significance

Abstract

In modern eutherian (placental) mammals, brown adipose tissue (BAT) evolved as a specialized thermogenic organ that is responsible for adaptive non-shivering thermogenesis (NST). For NST, energy metabolism of BAT mitochondria is increased by activation of uncoupling protein 1 (UCP1), which dissipates the proton motive force as heat. Despite the presence of UCP1 orthologues prior to the divergence of teleost fish and mammalian lineages, UCP1's significance for thermogenic adipose tissue emerged at later evolutionary stages. Recent studies on the presence of BAT in metatherians (marsupials) and eutherians of the afrotherian clade provide novel insights into the evolution of adaptive NST in mammals. In particular studies on the 'protoendothermic' lesser hedgehog tenrec (Afrotheria) suggest an evolutionary scenario linking BAT to the onset of eutherian endothermy. Here, we review the physiological function and distribution of BAT in an evolutionary context by focusing on the latest research on phylogenetically distinct species.

Effects of thyroid hormones on thermogenesis and energy partitioning

“…in lambs and rabbits it disappears several days 265 to month after birth (reviewed in (Oelkrug et al 2015). In humans, BAT was regarded to be 266 restricted to a short period after birth, but recently has been detected in adults using This fits to data in humans where no change in FGF21 was observed in hyperthyroidism or 381…”

“…Non-shivering thermogenesis is mainly initiated by a cutaneous cold-sensation which 67 via cold sensitive thermoreceptors activates the sympathetic nervous system (SNS) 68 (Nakamura and Morrison 2008). Subsequently, the SNS stimulates the release of 69 noradrenaline from postganglionic sympathetic fibres and induces β3-adrenergic receptor 70 (ARβ3)-mediated pathways that activate lipolysis and the release of free fatty acids (FFA, 71 reviewed in (Oelkrug, et al 2015)). FFA stimulate mitochondrial energy dissipation by 72 activation of the uncoupling protein 1 (UCP1) in the inner mitochondrial membrane.…”

A Review of Natural Stimulant and Non‐stimulant Thermogenic Agents

“…β‐3 Adrenergic stimulation activates cellular adenylyl cyclase, which leads to the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP), activation of protein kinase A, and phosphorylation and activation of triacylglycerol lipase. The phosphorylated lipase converts triacylglycerols (white storage fat) into free fatty acids, which act to release uncoupling protein 1 (UCP1) by overriding the inhibition caused by purine nucleotides (adenosine diphosphate (ADP), guanosine diphosphate (GDP), and adenosine triphosphate (ATP)) (Oelkrug et al , 2015). The UCP1‐mediated uncoupling of the respiratory chain in mitochondria ultimately produces the metabolic heat or thermogenesis (Fedorenko et al , 2012).…”

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

“…Recent studies have shown that the adipose tissue is not simply a deposit of lipids for energy storage but also interacts with other organs through secretion of adipokines and free fatty acids (FFA) [3]. Moreover, different types of adipocytes have been identified: white (WAT), brown (BAT) and the recently discovered beige (BeAT) [4], [5], [6]. The main characteristic of the brown and beige adipocytes is the presence of mitochondria and the possibility of fat burning with the production of heat (i.e.…”