Adipose: Tissue Function

Brown Adipose Tissue (BAT)

Non-shivering thermogenesis. Brown adipocytes utilize oxygen and lipids as substrates to produce heat. The functional thermogenic unit consists of a brown adipocyte maintained within a structural network of connective tissue with access to a rich blood supply and innervation. The vascular network serves the BAT by both delivering substrate and signaling molecules to the organ as well as carrying away the heat product to the body. Therefore, access to an ample vascular network is necessary in order to achieve maximal generation and distribution of the BAT-generated heat. Heat generation is achieved by a mitochondrial protein known as uncoupling protein-1 (UCP1) or thermogenin. It allows for combustion of fatty acids in the respiratory chain without the production of ATP; instead, heat is the form of energy that is released.

Signal transmission. The ventromedial (VML) hypothalamic nucleus of the brain coordinates information regarding body energy reserves and body temperature. When a thermogenic demand is sensed by the VML, the information is relayed via the sympathetic nervous system. The neurotransmitter norepinephrine (NE) is released and binds β-3 adrenergic receptors in the BAT to trigger an intracellular cascade that eventually leads to the generation of heat and an increased body temperature.

Thermogenic demand. Pre-adipocytes represent a rapidly accessible stem cell population that can replicate and differentiate into mature BAT under situations of increased thermogenic demand.

White Adipose Tissue (WAT)

Steroid hormone metabolism. Adipose tissue serves a crucial role in processing steroid hormones produced in the adrenal glands and gonads. This processing is referred to as “tissue-specific pre-receptor steroid hormone metabolism” and is necessary for full activation or inactivation of [Page 19]the circulating steroid hormones. The enzymes required to perform this process are extensive and include cytochrome P450-dependent aromatase, 3β-hydroxysteroid dehydrogenase (HSD), 3αHSD, 11βHSD1, 17βHSD, 17α-hydroxylase, 5α-reductase, and UDP-glucuronosyltransferase 2B15.

Aromatase is an especially important adipose enzyme in that it converts androgens into estrogens. In postmenopausal women, gonadal synthesis of estrogens becomes diminished and adipose tissue accounts for all of the circulating estrogen.

Reservoir for energy storage. Despite the large capacity of adipose tissue to secrete proteins and metabolize steroid hormones, the major secretory product of WAT is fatty acids. Adipocytes store triglycerides internally as a lipid droplet through an enzymatic process. First, triacylglycerides absorbed from the diet or synthesized in the liver reach their storage site (adipose tissue) and are converted into fatty acids via the enzyme lipoprotein lipase (LPL). They are then combined with the metabolic glucose product glycerol phosphate to reform triacylglyceride inside the adipocyte. When the cell receives signals that the body requires use of the free fatty acids for energy, they are then broken down via an enzyme called hormone sensitive lipase (HSL) that breaks apart the stored triglycerides to release free fatty acids. HSL responds to hormones such as catecholamines and glucagon to increase the free fatty acid concentration in the plasma so that it can be utilized for energy. Therefore, the sympathetic nervous system is a primary modulator of triacylglycerol breakdown.

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