Possible role of avian uncoupling protein in down-regulating mitochondrial superoxide production in skeletal muscle of fasted chickens

Little is known about the precise physiological roles of uncoupling protein 1 (UCP1) homologs (UCP2, UCP3, avian UCP) whose levels are up-regulated during fasting. UCPs in skeletal muscle are thought to play a role in the regulation of lipids as fuel substrates, and/or in controlling the production of reactive oxygen species (ROS). The aim of this investigation, using skeletal muscle from fasted chickens, was to examine alterations in the expression of genes encoding for avian UCP and key enzymes relevant to lipid flux across the mitochondrial β-oxidation pathway. We also clarified whether an increase in avUCP content could be associated with altered ROS production by mitochondria. Transcription levels of avUCP and CPT-I genes were increased 7.7- and 9.5-fold after a 24 h fast and slightly diminished but remained about 5.0- and 7.7-fold higher than baseline levels, respectively, after 48 h of fasting. In contrast, members of the β-oxidation pathway, LCAD and 3HADH, were gradually up-regulated from 12 to 48 h of fasting. This suggests that processes involved in the transfer and oxidation of fatty acids are up-regulated differently during the initial stage of fasting. Analysis of ROS production by lucigenin-derived chemiluminescence showed that the FFA-sensitive portion of carboxyatractyloside-upregulated ROS production was greater in skeletal muscle mitochondria from 24 h-fasted chickens compared with control, which leads us to postulate that ROS production is potentially down-regulated by UCP. The possible involvement of a backlog of fatty acid for oxidation, observed in chickens after a 24 h fast, in a transmembrane gradient of free non-oxidized fatty acids is also discussed.