Inhibition of the type 2 iodothyronine deiodinase underlies the elevated plasma TSH associated with amiodarone treatment

Understanding selenoprotein function and regulation through the use of rodent models

Selenium (Se) is an essential micronutrient. Its biological functions are associated with selenoproteins, which contain this trace element in the form of the 21st amino acid, selenocysteine. Genetic defects in selenocysteine insertion into proteins are associated with severe health issues. The consequences of selenoprotein deficiency are more variable, with several selenoproteins being essential, and several showing no clear phenotypes. Much of these functional studies benefited from the use of rodent models and diets employing variable levels of Se. This review summarizes the data obtained with these models, focusing on mouse models with targeted expression of individual selenoproteins and removal of individual, subsets or all selenoproteins in a systemic or organ-specific manner. This article is part of a Special Issue entitled: Cell Biology of Metals.

Prolonged high iodine intake is associated with inhibition of type 2 deiodinase activity in pituitary and elevation of serum thyrotropin levels

Our previous epidemiological study indicated that excessive intake of iodine could potentially lead to hypothyroidism. In the present study, we aimed to investigate the time and dose effect of iodine intake on serum thyrotropin (thyroid-stimulating hormone, TSH) levels and to explore the non-autoimmune regulation of serum TSH by pituitary type 2 deiodinase (D2). A total of 360 Wistar rats were randomly divided into five groups depending on administered iodine dosages (folds of physiological dose): normal iodine (NI), 3-fold iodine (3HI), 6-fold iodine (6HI), 10-fold iodine (10HI) and 50-fold iodine (50HI). At 4, 8, 12 and 24 weeks after administration of sodium iodide, blood was collected for serum TSH measurement by chemiluminescent immunoassay. Pituitaries were also excised for measurement of TSHβ subunit expression, D2 expression and activity, monocarboxylate transporter 8 (MCT8) and thyroid hormone receptor β2 isoform (TRβ2) levels. The results showed that iodine intake of 10HI and 50HI significantly increased pituitary and serum TSH levels from 8 to 24 weeks (P < 0·05 v. NI). Excess iodine had no effect on D2 mRNA or protein expression; however, 10HI and 50HI administration significantly inhibited pituitary D2 activities from 8 to 24 weeks (P < 0·05 v. NI). Iodine had no effect on MCT8 or TRβ2 protein levels. We conclude that prolonged high iodine intake inhibits pituitary D2 activity and induces elevation of serum TSH levels. These findings may provide a potential mechanism of iodine excess-induced overt and subclinical hypothyroidism.

Type 2 deiodinase at the crossroads of thyroid hormone action

Thyroid hormone action can be customized on a cell-specific fashion through the controlled action of the deiodinase group of enzymes, which are homodimeric thioredoxin fold containing selenoproteins. Whereas the type II deiodinase (D2) initiates thyroid hormone signaling by activating the pro-hormone thyroxine (T4) to the biologically active T3 molecule, the type III deiodinase (D3) terminates thyroid hormone action by catalyzing the inactivation of both T4 and T3 molecules. Deiodinases play a role in thyroid hormone homeostasis, development, growth and metabolic control by affecting the intracellular levels of T3 and thus gene expression on a cell-specific basis. Whereas both Dio2 and Dio3 are transcriptionally regulated, ubiquitination of D2 is a switch mechanism that controls D2 activity and intracellular T3 production. The hedgehog-inducible WSB-1 and the yeast Doa10 mammalian ortholog TEB4 are two E3 ligases that inactivate D2 via ubiquitination. Inactivation involves disruption of the D2:D2 dimer and can be reversed via two ubiquitin-specific proteases, USP20 and USP33, rescuing catalytic activity and T3 production. The ubiquitin-based switch mechanism that controls D2 activity illustrates how different cell types fine-tune thyroid hormone signaling, making D2 a suitable target for pharmacological intervention. This article reviews the cellular and molecular aspects of D2 regulation and the current models of D2-mediated thyroid hormone signaling.

Physiological role and regulation of iodothyronine deiodinases: a 2011 update

T4 is a prohormone secreted by the thyroid. T4 has a long half life in circulation and it is tightly regulated to remain constant in a variety of circumstances. However, the availability of iodothyronine selenodeiodinases allow both the initiation or the cessation of thyroid hormone action and can result in surprisingly acute changes in the intracellular concentration of the active hormone T3, in a tissue- specific and chronologically-determined fashion, in spite of the constant circulating levels of the prohormone. This fine-tuning of thyroid hormone signaling is becoming widely appreciated in the context of situations where the rapid modifications in intracellular T3 concentrations are necessary for developmental changes or tissue repair. Given the increasing availability of genetic models of deiodinase deficiency, new insights into the role of these important enzymes are being recognized. In this review, we have incorporated new information regarding the special role played by these enzymes into our current knowledge of thyroid physiology, emphasizing the clinical significance of these new insights.