Are uncoupling protein-1 homologs contributors to mitochondrial proton leak and resting metabolism?

摘要

Variations in resting metabolic rate (RMR) may be an important factor in an animal's susceptibility to obesity because it constitutes a substantial component of 24-hour energy expenditure, As reviewed in Chapter 1, thyroid hormone affects RMR through changes in the rates of ATP-dependent and -independent cellular reactions. Mitochondrial proton leak is an ATP-independent and thyroid hormone-dependent process of unknown origin which may be an important contributor to RMR. Recent identification of uncoupling protein-1 homologs (e.g., UCP-2 and UCP-3) has led to the hypothesis that they are important contributors to RMR and adaptive thermogenesis because they facilitate proton leak. As reviewed in Chapter 2, studies to date have not provided sufficient evidence to support this hypothesis. The experiments detailed in Chapter 4 addressed the possibility that variations in RMR with thyroid status can partly be accounted for by variations in UCP-2 and UCP-3 expression and proton leak. I tested the hypothesis that UCP-2 & 3 messenger RNA (mRNA) levels positively correlate with RMR and proton leak. T₃ stimulated RMR and liver UCP-2 and muscle UCP-2 & 3 expression, but non-phosphorylating mitochondrial respiration was not affected by T₃ and did not correlate with UCP-2 & 3 expression. Although the data indicated that variations in UCP-2 & 3 mRNAs may partly explain variations in RMR, the discordance between UCP-2 & 3 mRNAs and proton leak suggests that the leak in vitro may not reflect that in vivo because a post-translational regulator of UCP-2 & 3 is not retained in the isolated mitochondria. The experiments in Chapter 5 were designed to further test the possibility that UCP-2 and UCP-3 facilitate proton leak. Since the uncoupling activity of UCP-1 is inhibited by purine nucleotides, I hypothesized that liver and muscle proton leak is inhibited by purine nucleotides. Data obtained from the experiments did not support this hypothesis; however, the data indicated that high concentrations of cytidine 5′-monophosphate (CMP) stimulated liver and muscle proton leak. Proteins similar in size to UCP-2 & 3 were partially purified by CMP affinity chromatography, suggesting that the effect of CMP on proton leak may be mediated via UCP-2 & 3.