High basal expression and 3,5,3'-triiodothyronine regulation of messenger ribonucleic acid S14 in lipogenic tissues.

We recently reported that T3 and the high carbohydrate lipogenic diet elicit a brisk and marked increase in the rat liver mRNA coding for the cytosolic protein Spot 14 (17,010 mol wt; 4.9 pI). By means of a hybridization assay, we have shown that the response of hepatic mRNAS14 to T3 and dietary manipulation is analogous to the response of many hepatic lipogenic enzymes. We extend our previous studies by examining mRNAS14 expression and regulation in other lipogenic and nonlipogenic tissues. The relative levels of mRNAS14 expression in three fat depots (epididymal, retroperitoneal, and brown fat) and lactating mammary gland are 10- and 4-fold higher, respectively, than that in euthyroid male liver. Expression of mRNAS14 in liver, epididymal fat, and mammary gland is augmented by T3, whereas feeding a lipogenic diet augments mRNAS14 in liver and fat only. In contrast, the relative levels of mRNAS14 in various nonlipogenic tissues (brain, heart, kidney, lung, spleen, testes, and pituitary) are 7% that in liver or less. Neither diet nor thyroid status influenced mRNAS14 levels in the nonlipogenic tissues. We also found that hepatic mRNAS14 in the 15-day-old rat is expressed at 0.3% of the adult (2-month) hepatic level and increases 186-fold from 15 to 30 days of age. We speculate that low levels of mRNAS14 in neonatal rat liver may be due to high fat in the milk diet. The presence of high basal levels of mRNAS14 in lipogenic tissues and its regulation by T3 and diet suggest that the Spot 14 protein may play an important role in some aspect of synthesis, metabolism, transport, or storage of lipid used in energy production. Further, our results emphasize that the presence of T3 receptors in a tissue is not sufficient to confer T3 regulation of mRNAS14 expression. Additional tissue-specific factors are required for the regulation of the Spot 14 gene.