Thyroid hormone and uncoupling proteins.

Thyroid hormone (TH/T3) exerts many of its effects on energy metabolism by affecting gene transcription. However, although this is an important target for T3, only a limited number of T3-responsive genes have been identified and studied. Among these, the genes for uncoupling proteins (UCPs) have attracted the interest of scientists. Although the role of UCP1 seems quite well established, uncertainty surrounds the physiological function of the recently discovered UCP1 analogs, UCP2 and UCP3. The literature suggests that T3 affects both the expression and the activity of each of these UCPs but further studies are needed to establish whether the mechanisms activated by the hormone are the same. Recently, because of their larger range of expression, much attention has been devoted to UCP2 and UCP3. Most detailed studies on the involvement of these proteins as mediators of the effects of T3 on metabolism have focused on UCP3 because of its expression in skeletal muscle. T3 seems to be unique in having the ability to stimulate the expression and activity of UCP3 and this may be related to the capacity of T3 to activate the integrated biochemical processes linked to UCP activity, such as those related to fatty acids, coenzyme Q and free radicals.