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Numerous studies have established that thyroid hormones stimulate protein synthesis,'-8 but the mechanism of the stimulation remains obscure and controversial. The first observations of the stimulation in cell-free systems also provided evidence of mitochondrial participation in the mechanism of the effect." 2 Mfixing experiments with subcellular fractions from euthyroid and hyperthyroid rat liver implicated a change in the mitochondrial fraction as the cause of most of the increase in microsomal protein synthesis observed in hyperthyroid homogenates. Thyroxine added directly in vitro to the identical cell-free system which exhibited the effects of in vivo hormone administration also stimulated amino acid incorporation into microsomal protein.", 2 The stimulation in vitro was localized to the transfer of transfer-RNA-bound amino acid to microsomal9 or ribosomal'0 protein but was not dependent on any effect on RNA synthesis." Indeed, it was unaffected by inhibitors of RNA synthesis and was exerted equally well on synthetic polyribonucleotide-directed polypeptide synthesis." Recent studies with a reticulocyte lysate system have demonstrated that thyroxine can stimulate the synthesis of the a and f3 chains of hemoglobin and that the stimulation is a translational effect exerted primarily at a late stage in the assembly or elongation of the nascent polypeptide chain.'2 In agreement with the evidence of mitochondrial involvement in the mechanism of the effect following its in vivo administration," 2 the stimulation of microsomal protein synthesis by thyroxine in vitro exhibited an absolute requirement for mitochondria in the reaction mixture.2 9 " Kinetic studies and studies of the partial reactions of the effect have demonstrated that it is not thyroxine itself but a consequence of a prior thyroxine-mitochondrial interaction which is responsible for the increased ribosomal protein synthetic activity.7' 1' The studies carried out in this laboratory, both in vivo and in vitro, have provided several independent lines of evidence all consistently pointing to a stimulation of microsomal protein synthesis by thyroid hormones which is cytoplasmic in origin, localized at the level of translation and not of transcription, and secondary to a reaction between the hormone and the mitochondrial components of the cell. AIlore recently, other workers observed that the depressed rate of protein synthesis in cell-free preparations from thyroid-deficient rats could be raised toward normal by prior replacement therapy in vivo and that the effect was manifest in reaction mixtures devoid of mitochondria. 3 4 The stimulation of protein synthesis did not become apparent, however, until at least 30 hours following a dose of hormone in vivo.4 In contrast, nuclear RNA turnover, 4 RNA polymerase activity,14 and microsomal RNA content4 were found to be increased after latent

Mechanisms of stimulation of protein synthesis by thyroid hormones in vivo.