Characterization of the 5 '-flanking and 5 '-untranslated regions of the cyclic adenosine 3 ',5 '-monophosphate-responsive human type 2 iodothyroninedeiodinase gene

The type 2 iodothyronine deiodinase (D2) catalyzes T-4 activation. In human s, unlike rodents, it is widely expressed, and its action probably contribu tes to both intracellular and plasma T-3 pools. We have isolated the 6.5-kb 5'-flanking region (FR) and the previously uncloned 553 nucleotides (nt) o f the 5'-untranslated region (UTR) of hdio2. The 5'-UTR is complex, with th ree transcription start sites (TSS) (708, 31, and similar to 24 nt 5' to th e ATG), an alternatively spliced approximately 300-nt intron in the 5'-UTR, and three short open reading frames 5' to the initiator ATG. The previousl y reported approximately 7.5-kb D2 messenger RNA (mRNA) is actually an ap-p roximately 7-kb doublet that is present in thyroid, pituitary, cardiac and skeletal muscle, and possibly brain, but with only the longer transcript in placenta. A canonical cAMP response element-binding protein-binding site i s present at about 90 bp 5' to the most 5'-TSS. It accounts for the robust response of the 6.8-kb hdio2 5'-FR to protein kinase A. Forskolin increases D2 mRNA in human thyroid cells, which may explain the high D2 mRNA in Grav es' thyroid and thyroid adenomas. The hdio2 gene structure and Northern blo t results suggest that D2 expression is tightly controlled and tissue speci fic.