Brown adipose tissue and metabolism

There are two main types of adipose tissue in the body: white and brown. While white adipose tissue is located primarily beneath the skin throughout the body, brown adipose tissue is only found in a few specific locations. In rodents, like most mammals, brown fat is located in the subscapular region and in the areas surrounding the aorta; it is ubiquitous in both newborns and adults. In humans, brown fat was originally thought to be prevalent only in newborns, but newer studies indicate that brown fat exists in the neck and upper chest of adult humans. In fact, it has also been discovered that the amount of brown adipose tissue in adult humans varies with an individual’s body mass index (BMI).

To understand the function of brown fat, scientists took adult rats under anesthesia and inserted thermal probes in the midbrain, colon, and interscapular fat. The scientists then proceeded to measure the rats’ body temperature at these various locations while the rats were exposed to external temperatures of 31°C and 20°C. The scientists also recorded the amount of oxygen the adult rats consumed during the experiment; all of the results are detailed below in Figure 1.

Figure 1: Change in the inner body temperature of the rat colon, brain, and interscapular fat as the rat is exposed to an 11°C drop in external temperature. At the bottom of the graph, the amount of consumed oxygen as a function of time is recorded.

Another team of scientists discovered that the brown color of brown adipose tissue was due to the fact that the brown adipose cells were filled with mitochondria. And upon further investigation, the brown adipose mitochondria were discovered to contain a unique protein called UCP, which stands for UnCoupling Protein. The scientists found that UCP is usually present in a membrane, and UCP is a special transporter protein that uses a fatty acid inside of the transporter protein to facilitate the transport of hydrogen ions across the membrane. The rate of diffusion of H+ ions across UCP depends only on the relative concentration of H+ ions. Figure 2 is shows the location of UCP relative to the ATPase and the electron transport chain, all of which are located in a phospholipid bilayer

Figure 2: Brown adipose mitochondria differ from regular mitochondria in the addition of UCP. This drawing depicts the relative location of UCP, ATPase, and the electron transport chain inside of brown adipose tissue. All proteins are embedded in a membrane indicated by the grey line.

Sources: • Adapted from a paper by Donhoffer SZ, Sárdy F., Szegvári GY, (1964) Brown Adipose Tissue and Thermoregulatory Heat Production in the Rat. Nature 203, 765 - 766 and from a paper by Chavin KD, Yang S, Lin HZ, Chatham J, Chacko VP, et al. (1999) Obesity induces expression of uncoupling protein-2 in hepatocytes and promotes liver ATP depletion. J Biol Chem 274: 5692–5700 and • Figure 1 from Donhoffer S., Sardy F., Szegvari G. (1964) "Brown Adipose Tissue and Thermoregulatory Heat Production in the Rat." Nature 203: 765-6. • Figure 2 Adapted from Ledesma A, García de Lacoba M, and Rial E. (2002) The mitochondrial uncoupling proteins. Genome Biology Vol 3: reviews3015 - reviews3015.9