부산대학교 도서관

Mammalian thyroid hormone receptors (TRs) have multiple isoforms, including the bona fide receptors that bind [T.sub.3] (TR[alpha]1, TR[beta]1 and TR[beta]2) and a non-hormone-binding variant, TR[alpha]2. Intriguingly, TR[alpha]2 is strongly expressed in the brain, where its mRNA levels exceed those of functional TRs. Ablation of TR[alpha]2 in mice results in over-expression of TR[alpha]1, and a complex phenotype with low levels of free [T.sub.3] and [T.sub.4], without elevated TSH levels, suggesting an alteration in the negative feedback at the hypothalamic-pituitary level. As the hypothesis of a potential TRH response defect has never been tested, we explored the functional role of TR[alpha]2 in negative feedback on transcription of hypothalamic thyrotropin, Trh. The in vivo transcriptional effects of TR[alpha]2 on hypothalamic Trh were analysed using an in vivo reporter gene approach. Effects on Trh-luc expression were examined to that of two, [T.sub.3] positively regulated genes used as controls. Applying in vivo gene transfer showed that TR[alpha]2 over-expression in the mouse hypothalamus abrogates [T.sub.3]-dependent repression of Trh and [T.sub.3] activation of positively regulated promoters, blocking their physiological regulation. Surprisingly, loss of function studies carried out by introducing a shTR[alpha]2 construct in the hypothalamus also blocked physiological [T.sub.3] dependent regulation. Thus, modulating hypothalamic TR[alpha]2 expression by either gain or loss of function abrogated [T.sub.3] dependent regulation of Trh transcription, producing constant transcriptional levels insensitive to feedback. This loss of physiological regulation was reflected at the level of the endogenous Trh gene, were gain or loss of function held mRNA levels constant. These results reveal the as yet undescribed dominant negative role of TR[alpha]2 over TR[alpha]1 effect on hypothalamic Trh transcription.
Introduction Thyroid hormone (TH) production is controlled by the hypothalamic peptide Thyrotropin Releasing Hormone (TRH). [T.sub.3] exerts negative feedback on Trh transcription mainly through the beta forms of the thyroid [...]