Defining the In Vivo Role of mTORC1 in Thyrocytes by Studying the TSC2 Conditional Knockout Mouse Model

Camila Ludke Rossetti; Bruna Lourençoni Alves; Flavia Leticia Martins Peçanha; Aime T Franco; Vania Nosé; Everardo Magalhaes Carneiro; John Lew; Ernesto Bernal-Mizrachi; Joao Pedro Werneck-de-Castro

doi:10.1089/thy.2024.0053

Defining the In Vivo Role of mTORC1 in Thyrocytes by Studying the TSC2 Conditional Knockout Mouse Model

Thyroid. 2024 May 8. doi: 10.1089/thy.2024.0053. Online ahead of print.

Affiliations

¹ Division of Endocrinology, Diabetes and Metabolism, University of Miami, Miami, Florida, USA.
² Obesity and Comorbidities Research Center (OCRC), Department of Structural and Functional Biology, Institute of Biology, University of Campinas, Campinas, Brazil.
³ Division of Endocrinology and Diabetes, Children's Hospital of Philadelphia, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
⁴ Massachusetts General Hospital, Harvard University, Boston, Massachusetts, USA.
⁵ Department of Surgery, University of Miami, Miami, Florida, USA.
⁶ Miami VA Health Care System, Miami, Florida, USA.

PMID: 38661550
DOI: 10.1089/thy.2024.0053

Abstract

Background: The thyroid gland is susceptible to abnormal epithelial cell growth, often resulting in thyroid dysfunction. The serine-threonine protein kinase mechanistic target of rapamycin (mTOR) regulates cellular metabolism, proliferation, and growth through two different protein complexes, mTORC1 and mTORC2. The PI3K-Akt-mTORC1 pathway's overactivity is well associated with heightened aggressiveness in thyroid cancer, but recent studies indicate the involvement of mTORC2 as well. Methods: To elucidate mTORC1's role in thyrocytes, we developed a novel mouse model with mTORC1 gain of function in thyrocytes by deleting tuberous sclerosis complex 2 (TSC2), an intracellular inhibitor of mTORC1. Results: The resulting TPO-TSC2^KO mice exhibited a 70-80% reduction in TSC2 levels, leading to a sixfold increase in mTORC1 activity. Thyroid glands of both male and female TPO-TSC2^KO mice displayed rapid enlargement and continued growth throughout life, with larger follicles and increased colloid and epithelium areas. We observed elevated thyrocyte proliferation as indicated by Ki67 staining and elevated cyclin D3 expression in the TPO-TSC2^KO mice. mTORC1 activation resulted in a progressive downregulation of key genes involved in thyroid hormone biosynthesis, including thyroglobulin (Tg), thyroid peroxidase (Tpo), and sodium-iodide symporter (Nis), while Tff1, Pax8, and Mct8 mRNA levels remained unaffected. NIS protein expression was also diminished in TPO-TSC2^KO mice. Treatment with the mTORC1 inhibitor rapamycin prevented thyroid mass expansion and restored the gene expression alterations in TPO-TSC2^KO mice. Although total thyroxine (T4), total triiodothyronine (T3), and TSH plasma levels were normal at 2 months of age, a slight decrease in T4 and an increase in TSH levels were observed at 6 and 12 months of age while T3 remained similar in TPO-TSC2^KO compared with littermate control mice. Conclusions: Our thyrocyte-specific mouse model reveals that mTORC1 activation inhibits thyroid hormone (TH) biosynthesis, suppresses thyrocyte gene expression, and promotes growth and proliferation.

Keywords: hypothyroidism; thyrocyte proliferation; thyroid function; thyroid hormone synthesis.