The human, but not rat, dio2 gene is stimulated by thyroid transcription factor-1 (TTF-1)

Types 1 and 2 iodothyronine deiodinases (D1 and D2) catalyze the production of T-3 from T-4, D2 mRNA is abundant in the human thyroid but very low in adult rat thyroid, whereas D1 activity is high in both. To understand the m olecular regulation of these genes in thyroid cells, the effect of thyroid transcription factor 1 (TTF-1) and the paired domain-containing protein 8 ( Pax-8) on the transcriptional activity of the deiodinase promoters were stu died. Both the approximately 6.5-kb hdio2 sequence and its most 3' 633 bp w ere activated 10-fold by transiently expressed TTF-1 in COS-7 cells, but th e hdio1 was unaffected. Surprisingly, the response of the rdio2 gene to TTF -1 was only 3-fold despite the 73% identity with the proximal 633-bp region of hdio2 including complete conservation of a functional cAMP response ele ment at -90, Neither human nor rat dio2 nor human diol was induced by Pax-8 , The binding affinity of four putative TTF-1 binding sites in hdio2 were c ompared by a semiquantitative gel retardation assay using in vitro expresse d TTF-1 homeodomain protein. Only two sites, D and C1 (both of which are ab sent in rdio2), had significant affinity. Functional analyses showed that b oth sites are required for the full response to TTF-1. These results can ex plain the differential expression of dio2 in thyroid and potentially other tissues in humans and rats.