Abstract
Thyroid hormone is essential for metabolism, growth, development and cognitive function. Thyroid hormone is secreted in two forms, the biologically active 3,5,3’-triiodothyronine (T3) and the inactive thyroxine (T4) that must be converted to T3 in order to gain biological activity. This reaction is catalyzed by type II deiodinase (D2), a thioredoxin-fold containing selenoenzyme; its activity plays an essential role in thyroid hormone activation. The thyroid gland is regulated by the hypothalamic-pituitary-thyroid (HPT) axis. It responds to food availability via leptin induction of thyrotropin releasing hormone (TRH) in the hypothalamus and thyroid stimulating hormone (TSH) in the pituitary. These actions stimulate the production and secretion of T3 and T4 from the thyroid. This current work describes the exciting new finding that food availability also activates thyroid hormone action via the induction of D2 as measured, in part, through the conversion of T4 to T3. Herein, we have characterized the mechanism whereby D2 is activated via insulin/IGF-1 signaling through the PI3K-mTORC2-Akt pathway. To understand how nutritional signals affect D2 expression and activity, we studied mouse skeletal muscle and a mesothelioma cell line (MSTO-211H) and show that D2 is transcriptionally upregulated during the transition from fasting to refeeding or upon switching from 0.1 to 10% FBS, respectively. The underlying mechanism is the transcriptional de-repression of DIO2 through the activation of the mTORC2 signaling pathway, since lentiviral knockdown of rictor abrogates this effect. Forkhead box O1 (FOXO1), a downstream target of mTORC2, transcriptionally represses DIO2; this was determined using its specific inhibitor AS1842856 and through adenoviral infection of constitutively active FOXO1. These results were confirmed by ChIP studies in MSTO-211H cells indicating that within 4h of exposure to 10% FBS-containing media, FOXO1 binding to the DIO2 promoter markedly decreases and the DIO2 promoter is activated. Studies using the insulin-receptor FOXO1 KO mouse indicate that insulin is a key signaling molecule in this process. Similar results were found using IGF-1 as a stimulant. This study concludes that FOXO1 represses DIO2 during fasting and DIO2 de-repression is stimulated by nutritional activation of the PI3K-mTORC2-Akt-FOXO1 pathway. Through these studies, we have been able to characterize a novel mechanism of D2 regulation that is both locally and temporally regulated. It mechanistically describes how the production of T3 fluctuates in response to nutritional stimuli independently of the HPT axis. This mechanism linking thyroid hormone activation via D2 induction to food availability describes how the body adjusts T3 production to an appropriate level in the conditions of fasting and refeeding.