The Human Uncoupling Protein-3 Gene

Abstract

Uncoupling protein-3 (UCP3) is a recently identified candidate mediator of adaptive thermogenesis in humans. Unlike UCP1 and UCP2, UCP3 is expressed preferentially and at high levels in human skeletal muscle and exists as short and long form transcripts, UCP3 S and UCP3 L . UCP3 S is predicted to encode a protein which lacks the last 37 C-terminal residues of UCP3 L . In the present study, we have defined the intron-exon structure for the human UCP3 gene and determined that UCP3 S is generated when a cleavage and polyadenylation signal (AATAAA) located in the last intron prematurely terminates message elongation. In addition we have mapped UCP3 to the distal segment of human chromosome 11q13 (between framework markers D11S916 and D11S911), adjacent to UCP2. Of note, UCP2 and UCP3 in both mice and humans colocalize in P1 and BAC genomic clones indicating that these two UCPs are located within 75-150 kilobases of each other and most likely resulted from a gene duplication event. Previous studies have noted that mouse UCP2 maps to a region of chromosome 7 which is coincident with three independently mapped quantitative trait loci for obesity. Our study shows that UCP3 is also coincident with these quantitative trait loci raising the possibility that abnormalities in UCP3 are responsible for obesity in these models.The control of body weight involves a regulated balance between energy intake and expenditure. Energy expenditure can be divided into three components (1): resting metabolic rate, physical activity, and adaptive thermogenesis, the latter being defined as the component of energy expenditure that changes in response to environmental stimuli such as cold exposure or chronic dietary excess. In rodents, an important site of adaptive thermogenesis is brown adipose tissue (reviewed in Ref.2) where uncoupling protein-1 (UCP1), 1 expressed exclusively in brown adipocytes (3, 4), promotes proton transport across the mitochondrial inner membrane. UCP1 decreases the proton electrochemical potential gradient, uncoupling fuel oxidation from ADP availability (reviewed in Refs. 5 and 6). Activation of UCP1, therefore, causes increased consumption of calories and generation of heat. UCP1-mediated effects on energy expenditure are regulated by changes in the level of sympathetic nervous system activity in brown fat. Cold exposure and overfeeding cause increased sympathetic stimulation of brown fat, simulating UCP1-mediated uncoupling and energy expenditure. The importance of this is demonstrated by the fact that mice lacking UCP1 are cold-intolerant (7). UCP1 is also regulated by purine di-and trinucleotides (ATP, ADP, GTP, and GDP) and free fatty acids, which inhibit and stimulate uncoupling activity, respectively (reviewed in Refs. 5 and 6).UCP1 may be of lesser importance in humans in whom the mass of brown adipose tissue is limited. Instead, skeletal muscle is thought to be a major site of adpative thermogenesis (8 -12). UCP2 (13-15) is a recently described UCP1 homologue which, unlike UCP1, is expressed in most tissue...

The uncoupling protein homologues: UCP1, UCP2, UCP3, StUCP and AtUCP

“…The structure of the UCP2 and UCP3 genes is similar to that of the UCP1 gene, the coding sequence being distributed over six exons in each of the three genes. However, the UCP2 and UCP3 genes differ from the UCP1 gene by the presence of two or one untranslated exons located on the 5h side of the UCP2 and UCP3 gene respectively [146][147][148][149][150][151]. The structure of the genes of plant UCPs is somewhat different, since nine exons covering the coding sequence were detected in the gene encoding AtUCP (M. Laloi, unpublished work).…”

“…Following the initial cloning of UCP2 and UCP3 cDNAs, it was observed that these two genes are within less than 100 kb of each other [57,146,155]. In fact, it was demonstrated that the two genes are adjacent, the UCP2 gene being only 7 or 8 kb downstream of the UCP3 gene in mouse and human respectively [149,151,156].…”

“…The immediate vicinity of the two genes strongly suggested a duplication event. The UCP3\UCP2 locus is localized on mouse chromosome 7, human chromosome 11 [30,57,146], and rat chromosome 1 [155].…”

“…The gross and detailed structure of the human and mouse UCP2 [147][148][149][150]163,164] and UCP3 [146,151,[165][166][167][168] genes have been determined ( Figure 6). A particular feature of the mouse and human UCP2 genes is the presence of several ATGs in-frame with an open reading frame for an unknown peptide of 36 amino acids in exon 2, whereas the UCP2 coding sequence begins in exon 3.…”

“…cDNA cloning of UCP3 revealed that the human gene is expressed as two splice variants generated by alternative splicing of the last intron ; the predicted amino acid sequences correspond to a putative 312-amino-acid protein (UCP3 L ) and a 275-amino-acid protein (UCP3 S ) lacking the last potential transmembrane domain [55,56]. The two UCP3 mRNAs are expressed in similar amounts in human skeletal muscle [146,169]. The existence of two forms of UCP3 mRNA has not been reported in rodents.…”