The iodothyronine deiodinases are a family of selenoproteins that metabolize thyroxine and other thyroid hormones to active and inactive metabolites in a number of tissues including brain. Using primary cultures of rat astroglial cells as a model system, we demonstrate that the mRNA for the type II iodothyronine deiodinase (DII) selenoenzyme is rapidly and markedly induced by forskolin and 8-bromo-cAMP. The induction of DII activity, however, was significantly impaired by culturing cells in selenium-deficient medium for 7 days. Under such conditions, the addition of selenium resulted in a rapid increase in cAMP-induced DII activity that was dose-dependent, with maximal effects noted within 2 h. Cycloheximide blocked this effect of selenium on restoring cAMP-induced DII activity, whereas actinomycin D did not. These data demonstrate that the DII selenoenzyme is expressed in cultured astrocytes and that the induction of DII activity by cAMP analogues appears to be mediated, at least in part, by pretranslational mechanisms. Furthermore, selenium deprivation impairs the expression of DII activity at the level of translation.Recent molecular cloning studies have identified cDNAs that code for a family of structurally related iodothyronine deiodinases (1-4). These oxidoreductases catalyze the removal of iodide from the phenolic (5Ј-position) or the tyrosyl (5-position) ring of thyroxine (T 4 ) 1 to form the active compound 3,5,3Ј-triiodothyronine (T 3 ) or the inactive metabolite 3,3Ј,5Ј-triiodothyronine, respectively (5). Based on both their unique functional properties and the structural information derived from cDNA sequences, three isoforms designated type I, II, and III iodothyronine deiodinases (DI, DII, and DIII, respectively) have been identified.The cDNAs for all three deiodinase isoforms contain within their coding regions an in-frame TGA triplet that has been demonstrated to code for the uncommon amino acid selenocysteine (1, 2, 4, 6, 7). This residue plays an essential role in the function of these enzymes; mutagenesis of the TGA triplet to a cysteine codon markedly reduces catalytic activity (1-3). It thus appears that the more potent nucleophilic capability of selenium, as compared with sulfur, is required for efficient deiodination. This thesis has been underscored by the identification of cDNAs coding for homologues of these enzymes from several fish, amphibian, and mammalian species (3, 8 -10). All such cDNAs isolated to date have demonstrated strict conservation of the active-site selenocysteine codon.Complementary DNAs that code for DII from Rana catesbeiana (3), rat (designated rBAT1-1), and human have been identified most recently (4). The proteins coded by these cDNAs are highly conserved; rBAT1-1 DII demonstrates 73 and 87% amino acid identity to the R. catesbeiana and human homologues, respectively. In addition, expression studies have demonstrated that these enzymes manifest all of the unique characteristics of DII (3, 4, 7).Evidence that the rBAT1-1 DII cDNA and its human and amphibian homo...
