Rapid effects of triiodothyronine on hepatic gene expression. Hybridization analysis of tissue-specific triiodothyronine regulation of mRNAS14.

We recently reported that 3,5,3'-triiodothyronine (T3) causes a rapid increase in the translational activity of a specific mRNA (mRNAS14) coding for a protein designated "Spot 14" (Mr = 18,000; 4.9 pI) using two-dimensional gel electrophoresis of in vitro translated proteins. The current studies suggest that Spot 14 is a cytosolic protein. Using a plasmid-derived cDNAS14 and dot hybridization, we have directly measured changes in hepatic mRNAS14 levels in response to T3 and other physiologic stimuli. Following injection of hypothyroid rats with a receptor-saturating dose of T3, a 20-min lag preceded the induction of hepatic mRNAS14 to levels 16-fold above basal values within 4 h. This is the most rapid effect of T3 on hepatic gene expression documented to date. Both during time course studies in hypothyroid animals injected with T3 as well as in the intact euthyroid rat, hepatic mRNAS14 levels varied up to 3-fold over a 24-h period in a pattern consistent with circadian variation. A lipogenic diet also significantly elevated hepatic mRNAS14. Thus, T3, dietary, and the unknown factors responsible for circadian variation influence hepatic mRNAS14 expression. Blot and dot hybridization analysis indicated that mRNAS14 expression in nonhepatic tissues was significantly below euthyroid hepatic values. Responsivity of mRNAS14 in nonhepatic tissue to in vivo T3 administration was also either substantially diminished or totally absent. Thus, expression and regulation of mRNAS14 is tissue specific.