[Apoptosis and the thyroid: the Fas pathway]

Area of Science:

• Immunology
• Cell Biology
• Endocrinology

Background:

• The Fas/Fas-L pathway is a key mediator of apoptosis, involving Fas receptor on target cells and Fas ligand on cytotoxic cells.
• Thyroid gland volume is regulated by thyroid-stimulating hormone (TSH) and thyrocyte apoptosis, with normal thyrocytes expressing Fas but not Fas-L.
• Thyrocytes possess resistance mechanisms to Fas-mediated apoptosis, involving inhibitors of apoptotic signal transduction.

Purpose of the Study:

• To investigate the role of the Fas/Fas-L apoptotic pathway in normal thyroid physiology and various thyroid pathologies.
• To elucidate the mechanisms of thyrocyte apoptosis regulation in Hashimoto's thyroiditis, Graves' disease, and thyroid cancer.

Main Methods:

• Analysis of Fas and Fas-L expression and function in thyroid cells and infiltrating lymphocytes in different thyroid conditions.
• Assessment of apoptotic pathways and signaling in normal thyrocytes, and in thyrocytes from patients with thyroiditis and cancer.
• Correlation of soluble Fas levels with the clinical course of Graves' disease.

Main Results:

• In Hashimoto's thyroiditis, thyrocytes aberrantly express Fas-L, leading to fratricide apoptosis, while infiltrating lymphocytes are apoptosis-resistant (Bcl2 overexpression).
• In Graves' disease, reduced thyrocyte apoptosis and goiter formation are linked to repressed Fas expression and increased soluble Fas, correlating with disease activity.
• Thyroid cancer cells exhibit resistance to apoptosis and induce apoptosis in antitumoral lymphocytes via Fas counter-attack.

Conclusions:

• The Fas/Fas-L pathway plays a critical, albeit complex, role in thyroid homeostasis and pathogenesis.
• Aberrant Fas/Fas-L signaling contributes to thyrocyte loss in Hashimoto's thyroiditis and influences disease progression in Graves' disease.
• Fas-mediated apoptosis evasion is a hallmark of thyroid cancer, enabling tumor growth and immune evasion.