Atypical Expression of Type 2 Iodothyronine Deiodinase in Thyrotrophs Explains the Thyroxine-Mediated Pituitary Thyrotropin Feedback Mechanism

Abstract

T(4), the main product of thyroid secretion, is a critical signal in plasma that mediates the TSH-negative feedback mechanism. As a prohormone, T(4) must be converted to T(3) to acquire biological activity; thus, type 2 iodothyronine deiodinase (D2) is expected to play a critical role in this feedback mechanism. However, the mechanistic details of this pathway are still missing because, counterintuitively, D2 activity is rapidly lost in the presence of T(4) by a ubiquitin-proteasomal mechanism. In the present study, we demonstrate that D2 and TSH are coexpressed in rat pituitary thyrotrophs and that hypothyroidism increases D2 expression in these cells. Studies using two murine-derived thyrotroph cells, TtT-97 and TalphaT1, demonstrate high expression of D2 in thyrotrophs and confirm its sensitivity to negative regulation by T(4)-induced proteasomal degradation of this enzyme. Despite this, expression of the Dio2 gene in TalphaT1 cells is higher than their T(4)-induced D2 ubiquitinating capacity. As a result, D2 activity and net T(3) production in these cells are sustained, even at free T(4) concentrations that are severalfold above the physiological range. In this system, free T(4) concentrations and net D2-mediated T(3) production correlated negatively with TSHbeta gene expression. These results resolve the apparent paradox between the homeostatic regulation of D2 and its role in mediating the critical mechanism by which T(4) triggers the TSH-negative feedback.

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…Using depleted serum has been shown to induce cellular hypothyroidism, as assessed by the expression level of T 3 -responsive genes, in as little as 24 hours (259). In theory, the time needed to achieve hypothyroidism could be shortened by frequently changing the media (e.g., every few hours instead of daily); this would accelerate the depletion of intracellular T 3 stores.…”

“…In the GH3 somatotroph cell line, exposure to T 3 increases growth hormone expression in a TRb2-mediated manner (532), while in GH-secreting GC cells, T 3 decreases D2 mRNA levels (143). The thyrotroph-derived cell line TaT1 also responds to physiological levels of T 3 by decreasing TSHb and D2 expression (259,533).…”

Physiological role and regulation of iodothyronine deiodinases: A 2011 update

“…Studies using two murine-derived thyrotroph cells, TtT-97 and TalphaT1, demonstrate high expression of D2 in thyrotrophs (11). D2 produces T3 from circulating T4 allowing the pituitary and hypothalamus to monitor both serum T3 and T4 independently (7).…”

Negative Regulation of the Thyrotropin β Gene by Thyroid Hormone

“…While GSU is common to follicle stimulating hormone (FSH), luteinizing hormone (LH) and chorionic gonadotropin (CG), TSH is specific to TSH alone. Although the concentration of serum T4 is much higher than that of T3, T4 is converted to T3 by deiodinase (Dio) in the TSH-producing cells (thyrotrophs) of the pituitary (Christoffolete et al, 2006), and T3 exhibits biological activity as a thyroid hormone (Gereben et al, 2008). T3 inhibits expression of both TSH and GSU at the transcriptional level (Shupnik et al, 1989).…”