RESUMO
Several liganded nuclear receptors have alternative ligands acting in a tissue-specific fashion and playing important biological roles. We present evidence that 3,5-diiodothyronine (T(2)), a naturally occurring iodothyronine that results from T(3) outer-ring deiodination, is an alternative ligand for thyroid hormone receptor ß1 (TRß1). In tilapia, 2 TRß isoforms differing by 9 amino acids in the ligand-binding domain were cloned. Binding and transactivation studies showed that T(2) activates the human and the long tilapia TRß1 isoform, but not the short one. A chimeric human TRß1 (hTRß1) that contained the 9-amino-acid insert showed no response to T(2), suggesting that the conformation of the hTRß1 naturally allows T(2) binding and that other regions of the receptor are implicated in TR activation by T(2). Indeed, further analysis showed that the N terminus is essential for T(2)-mediated transactivation but not for that by T(3) in the long and hTRß1, suggesting a functional interaction between the N-terminal domain and the insertion in the ligand-binding domain. To establish the functional relevance of T(2)-mediated TRß1 binding and activation, mRNA expression and its regulation by T(2) and T(3) was evaluated for both isoforms. Our data show that long TRß1expression is 10(6)-fold higher than that of the short isoform, and T(3) and T(2) differentially regulate the expression of these 2 TRß1 isoforms in vivo. Taken together, our results prompted a reevaluation of the role and mechanism of action of thyroid hormone metabolites previously believed to be inactive. More generally, we propose that classical liganded receptors are only partially locked to very specific ligands and that alternative ligands may play a role in the tissue-specific action of receptors.