Thyroid Hormone Signaling in a Triiodothyronine-Only Hormonal Environment - PROJECT SUMMARY/ ABSTRACT Thyroid hormones (THs), T4 and T3, are synthesized in the thyroid gland under the influence of TSH, and play a critical role in regulating thermogenesis, energy expenditure (EE), and even thyroid function. T4 is primarily formed at the N-terminus of thyroglobulin (Tg), while de novo synthesis of T3 (the most active TH) occurs mostly at the C-terminal ChEL domain of Tg. In the body, all T4 is produced in the thyroid, while T3 is generated largely through deiodination of T4 in various tissues. Hypothyroidism, affecting 5-10% of the population, often presents with goiter, reduced EE, and weight gain. Although T4 treatment normalizes TSH in hypothyroid patients, it often fails to restore EE, and weight gain persists. T3 treatment aids weight loss but results in wide fluctuations in blood T3 levels. To study the consequences of a “T3-only” hormonal environment on body metabolism, we created a novel genetic knock-in (KI) mouse that expresses only the Tg-ChEL domain (lacking the remainder of the Tg molecule) under TSH regulation, named ChEL-KI: this animal produces normal serum T3 levels with negligible T4. Interestingly, ChEL-KI mice have hypothyroidism by TSH criteria, but sustain normal body temperature even after cold exposure and expend more energy than euthyroid mice under sub-thermoneutral (<30°C) conditions, when adaptive thermogenesis contributes significantly to heat production. Brown adipose tissue and skeletal muscle are key sites of adaptive thermogenesis, and under certain situations, when one is deficient, the other compensates, with skeletal muscle consuming more energy. Additionally, ChEL-KI mice develop a larger goiter with increased markers of T3 action and thyroid cell proliferation compared to “conventionally hypothyroid” mice with similarly high serum TSH. T3 increases thyrocyte proliferation in culture, and exuberant thyroidal T3 may contribute to such overgrowth. In addition, while serum IGF-1 (a stimulator of thyrocyte proliferation) is reduced in hypothyroid mice, it remains normal in ChEL-KI mice, likely supported by normal serum T3. Here, we seek to dissect the metabolic and proliferative roles of T3 when T4 is deficient: 1) We hypothesize that physiological levels of T3 (in animals with negligible T4) support energetically costly thermoregulation, contributing to increased EE. We will characterize the effects of T3 on the metabolic profiles (including EE) in mice lacking T4 at thermoneutrality and under mild cold exposure; the activation of adaptive thermogenesis mechanisms in these animals; and their vulnerability to developing obesity. 2) We hypothesize that T3, when T4 is lacking, supports proliferation of TSH-stimulated thyrocytes and thyroid enlargement through direct T3 action in the thyroid gland and mechanisms involving IGF-1. We will quantitatively assess the role of T3 in regulating thyroid cell proliferation and differentiation, and thyroid gland mass when serum TSH is high. We will also probe the importance of the IGF-1 and TSH pathways to the dose-dependent effects of T3 on the thyroidal expression of genes involved in cell cycle regulation and cell differentiation. These studies of thyroid hormone action will pave the way for advancements in the treatment of hypothyroidism, and for targeting pathways that help to promote weight loss.
