Transcriptional regulation of iodothyronine deiodinases during embryonic development

A single dose of chicken growth hormone (cGH) or dexamethasone acutely incr eases circulating T-3 levels in 18-day-old chicken embryos through a reduct ion of hepatic type III iodothyronine deiodinase (D3). The data in the pres ent study suggest that this decrease in D3 is induced by a direct downregul ation of hepatic D3 gene transcription, The lack of effect of cGH or dexame thasone on brain and kidney D3 activity, furthermore suggests that both hor mones affect peripheral thyroid hormone metabolism in a tissue specific man ner, Dexamethasone administration also results in an increase in brain type II iodothyronine deiodinase (D2) activity and mRNA levels that is also reg ulated at a transcriptional level. In contrast, however, cGH has no effect on brain D2 activity, thereby suggesting that either GH cannot pass through the blood-brain barrier in chicken or that cGH and dexamethasone regulate thyroid hormone deiodination by different mechanisms. In addition, the very short half-life of D2 and D3 (t(1 2) < 1 h) in comparison with the longer half life of type I iodothyronine deiodinase (D1, t(1 2) > 8 h), allows for D2 and D3 to play a more prominent role in the acute regulation of periphe ral thyroid hormone metabolism than D1. (C) 2001 Elsevier Science Ireland L td. All rights reserved.