Uncoupling protein-3 is a molecular determinant for the regulation of resting metabolic rate by thyroid hormone

Thyroid hormones increase energy expenditure, partly by reducing metabolic efficiency. The control of specific genes at the transcriptional level is t hought to be the major molecular mechanism. However, both the number and th e identity of the thyroid hormone-controlled genes remain unknown, as do th eir relative contributions. Uncoupling protein-3, a recently identified mem ber of the mitochondrial transporter superfamily and one that is predominan tly expressed in skeletal muscle, has the potential to be a molecular deter minant for thyroid thermogenesis. However, changes in mitochondrial proton conductance and resting metabolic rate after physiologically mediated chang es in uncoupling protein-3 levels have not been described. Here, in a study on hypothyroid rats given a single injection of T-3, we describe a strict correlation in terms of time course between the induced increase in uncoupl ing protein-3 expression (at mRNA and protein levels) and decrease in mitoc hondrial respiratory efficiency, on the one hand, and the increase in resti ng metabolic rate, on the other. First, we describe our finding that uncoup ling protein-3 is present and regulated by T3 only in metabolically relevan t tissues (such as skeletal muscle and heart). Second, we follow the time c ourse (at 0, 6, 12, 24, 48, 65, 96, and 144 h) of both uncoupling protein-3 mRNA levels and mitochondrial uncoupling protein-3 density in gastrocnemiu s muscle and heart. In both tissues, the maximal (12-fold) increase in unco upling protein-3 density was reached at 65 h. The resting metabolic rate [I O2(kg(0.75))(-1) h(-1)] showed the same time course, and at 65 h the increa se vs. time zero was 45% (1.316 +/- 0.026 vs. 0.940 +/- 0.007; P < 0.001). At the same time point, gastrocnemius muscle mitochondria showed a signific antly higher nonphosphorylating respiration rate (nanoatoms of oxygen per m in/mg protein; increase vs. time zero, 40%; 118 +/- 4 vs. 85 +/- 9; P < 0.0 5), whereas the membrane potential decreased by 8% (168 +/- 2 vs. 182 +/- 4 ; P < 0.05). These data are diagnostic of mitochondrial uncoupling. The res ults reported here provide the first direct in vivo evidence that uncouplin g protein-3 has the potential to act as a molecular determinant in the regu lation of resting metabolic rate by T-3.