UCP-1 gene is activated by norepinephrine stimulation, which also protects brown adipocytes by inhibiting apoptosis.
In the past, it was thought that uncoupling proteins were expressed only in brown adipose tissue. Recently, several similar uncoupling proteins have been discovered in other tissues. UCP-2 is linked to hyperinsulimenia and obesity and may be involved in regulation of body weight. UPC-3 is expressed in skeletal muscles and may account for the thermogenic effects of thyroid hormone. UPC-4 is a brain-specific molecule.
Metabolism of lipid in brown adipose tissue generates heat in process known as thermogenesis.
Hibernating animals have large amounts of brown adipose tissue. The tissue serves as a ready source of lipid.
When oxidized, it produces heat to warm the blood flowing through the brown fat on arousal from hibernation and in the maintenance of body temperature in the cold. This type of heat production is known as nonshivering thermogenesis.
Brown adipose tissue is also present in nonhibernating animals and humans and again serves as a source of heat. As in the mobilization of lipid in white adipose tissue, lipid is mobilized, and heat is generated by brown adipocytes when they are stimulated by the sympathetic nervous system. Therefore, normally present brown adipose tissue can most likely be induced and function in the context of human adaptive thermogenesis. Future research is being directed toward finding mechanisms for increased brown fat differentiation, which may potentially be an attractive treatment in both diet-induced and genetically acquired obesity.