Further characterization of thyroid hormone response elements in the human type 1 iodothyronine deiodinase gene

The increased type 1 iodothyronine deiodinase expression in hyperthyroid patients increases the fraction of plasma T3 generated from T4 by the propylthiouracil-sensitive pathway. In this study, we extend our analysis of the thyroid hormone response elements (TREs) in the 5' flanking region of the human dio1 gene. The 5' TRE (TRE2), a direct repeat separated by 4 bp (DR+4) at -660 bp, arises from an A to G substitution in an Alu sequence, the first example of this phenomenon. An SP1 binding site immediately 5' to TRE2 increases basal expression of a 430-bp dio1 promoter-chloramphenicol acetyltransferase construct in the presence of unliganded thyroid hormone receptor, thus decreasing T3 responsiveness, but does not do so when this complex is placed in its more 5' wild-type location. The two octameric binding sites of TRE1, a retinoid X-receptor independent DR+10 structure at -90, can be exchanged or inverted without loss of T3 response potency, despite significant changes in thyroid hormone receptor binding, as assessed by gel shift assays. However, the retinoic acid response of the 716-bp dio1 5' flanking region is unaffected by elimination of TRE2 but is lost with mutations in TRE1. These findings indicate the importance of functional analyses of potential ligand-responsive transcription factors, as well as the influence of position, on TRE function and interaction with basal transcription factors. The unusual features of these TREs emphasize the need to consider alternatives to canonical half-site arrangements of receptor binding sites and contexts in the evaluation of T3- and retinoic acid-responsive genes.