Tri-iodothyronine De Novo Synthesis within Thyroglobulin

CINTIA E. CITTERIO; PETER ARVAN

Ann Arbor

Simposio; Department of Internal Medicine 24th Annual Research Symposium; 2016

TUniversity of Michigan, Department of Internal Medicine

Thyroid Hormones regulate crucial physiological functions and their synthesis is initiated within the precursor protein, thyroglobulin (TG). TG is a large (2746 residue) glycoprotein synthesized in thyrocytes and post-translationally iodinated on multiple monoiodotyrosine (MIT) and diiodotyrosine (DIT) residues. Upon iodination, chemical coupling of DIT-DIT, or MIT-DIT, within the TG protein, initiates formation of thyroxine (T4) and tri-iodothyronine (T3), respectively. In two common thyroid diseases ? hypothyroidism caused by iodine deficiency, and autoimmune hyperthyroidism (Graves? disease) ? a central mechanism includes massive hyperstimulation of thyrocytes by Thyroid Stimulating Hormone (TSH). Interestingly, both conditions result in increased T3 relative to T4 production in the thyroid gland. In this study, we have investigated the hypothesis that in thyrocytes hyperstimulated by TSH, the structure of the TG may be altered to favor chemical coupling of residues involved in T3 formation. We treated PCCl3 thyrocytes with lower and hyperstimulating concentrations of TSH and collected secreted TG. To examine the ability of secreted TG to form de novo T3 within its structure, we performed iodination in vitro followed by Western blotting with monoclonal anti-T3 antibodies. Alternatively, we added a TSH receptor-stimulating antibody (KSAb1) that mimics Graves? disease. To validate our method, we analyzed new T3 formation within TG from mice hyperstimulated by TSH and mice with genetic deletion of TSH receptors. Altogether, we found that TG synthesized in thyrocytes stimulated by TSH is intrinsically more capable of de novo T3 formation. Moreover, we found that T3 formation occurs at special sites within the carboxyl-terminal region of TG.