Thyroid hormone metabolism in primary cultures of fetal rat brain cells.

The metabolism of thyroxine 3,5,3',5'-tetraiodothyronine, (T4) and 3,5,3'-triiodothyronine (T3) was studied in primary cultures of dispersed fetal rat brain cells. Cultured brain cells actively metabolized both T4 and T3 by enzyme catalyzed deiodination reactions which increase (type II 5'-deiodinase) or decrease (type I 5'-deiodinase and type III 5-deiodinase) the bioactivity of thyroid hormone. Homogenates of cultured brain cells showed both type I and type II 5'-deiodinating activities and these two enzymes tended to differ in their time course of appearance. Cultures exposed to 10 microM cytosine arabinoside for 16 h showed up to a 70% reduction in type I activity without decreasing the type II enzyme suggesting that the type II enzyme is associated with non-dividing neuronal cells. The predominant pathway for T4 and T3 metabolism in situ was tyrosyl-ring or type III 5'-deiodination which followed first order kinetics with a t1/2 of 70 min. T4 to T3 conversion by the type II enzyme was consistently observed after correcting for the degradation of newly formed T3 by the type III enzyme. In situ, both type II and type II enzymes were thiol-dependent and both activities were inhibited by iopanoic acid. Type III 5-deiodination of T4 produced 34 fmol 3,3,5'-triiodothyronine (rT3)/h per 10(6) cells at 10 mM dithiothreitol (DTT) and 97 fmol of rT3/h per 10(6) cells at 50 mM DTT. T3 production by the type II enzyme was 1.2 and 4.4 fmol of T3/h per 10(6) cells at 10 and 50 mM DTT, respectively. Thyroid hormone deficient culture conditions increased type II enzyme activity by 4-5-fold within 48 h and this was prevented in a dose-dependent fashion by supplementing the media with increasing amounts of T3. These data indicate that primary cultures of dispersed brain cells mimic the intact cerebral cortex with respect to the metabolism of thyroid hormone and the regulatory mechanisms which defend cerebrocortical T3 levels. The vigorous metabolism of both T4 and T3 by these cultures may explain some of the difficulties in demonstrating thyroid hormone-dependent biochemical changes at physiologically relevant levels of thyroid hormone.