The uncoupling protein thermogenin during acclimation: indications for pretranslational control

To analyze the regulation of the content of the uncoupling protein thermogenin in brown adipose tissue, we have selected a physiological transition phase during which to investigate the relationship between the level of mRNA and the level of the ensuing protein product. Mice preacclimated to 28 degrees C were transferred to 4 degrees C. Cold acclimation led to the expected increases in brown fat total protein and RNA content. Two recruited proteins were analyzed: the cytosolic glycerol-3-phosphate dehydrogenase and the mitochondrial uncoupling protein thermogenin. The activity of the dehydrogenase acutely followed the level of the corresponding mRNA, indicating pretranslational control. However, for thermogenin there was a marked time delay between the establishment of the fully recruited level of thermogenin mRNA (after only approximately 4 h of cold exposure) and that of thermogenin itself (after > 3 wk). By reiterative computer simulation, it was investigated whether a model only involving pretranslational regulation could be invoked for either system. For glycerol-phosphate dehydrogenase, a plausible model could be constructed, provided the protein half-life was shorter than approximately 24 h. Despite the long time delay between full thermogenin mRNA recruitment and full thermogenin protein recruitment, a plausible pretranslational control model could also be constructed, provided that the protein half-life was approximately 5 days. This computed value was in good agreement with the half-life obtained from independent thermogenin half-life studies. It is implied that pretranslational control may suffice to explain the regulation of thermogenin content in brown adipose tissue during a warm-to-cold transition period.