Changes in uncoupling protein-2 and-3 expression in aging rat skeletal muscle, liver, and heart

Uncoupling protein (UCP)-2 and -3 mediate mitochondrial (mt) proton leak in vitro and are potential regulators of energy expenditure and ATP productio n. Aging is associated with alteration of tissue functions, suggesting impa ired mtATP production. To determine whether age-related changes in UCP expr ession occur, we measured the transcript levels of UCP-2 and -3 in skeletal muscle, liver, and heart in 6- and 27-mo-old rats. UCP-2 transcripts were higher in old animals in the white (+100%) and red (+70%, both P< 0.04) gas trocnemius muscle and in the liver (+300%, P< 0.03), whereas they were comp arable in the heart in both age groups. UCP-2 transcript levels correlated positively with mitochondrial-encoded cytochrome c oxidase transcripts norm alized for mtDNA (P< 0.01) and negatively with mtDNA copy number (P< 0.001) . UCP-3 transcripts were lower in the less oxidative white (-50%, P< 0.04) and unchanged in the more oxidative red (-15%, P = 0.41) gastrocnemius musc le in old animals. Similar changes at protein level were confirmed by UCP-2 protein in aging liver (+300%, P< 0.01) and UCP-2 (+85%, P< 0.05) and UCP- 3 (-30%, P = 0.4) protein in aging mixed gastrocnemius muscle. Aging is thu s associated with tissue-specific changes of UCP-2 and -3 gene expression. Increased UCP-2 expression may limit ATP production and is related to mitoc hondrial gene expression in aging muscles and liver. Different age-related changes may reflect differential regulation of UCP-2 and -3 in skeletal mus cle. The current data suggest a potential role of uncoupling proteins to al ter energy production in aging tissues.