EXPRESSION OF NITRIC-OXIDE SYNTHASE-III IN HUMAN THYROID FOLLICULAR CELLS - EVIDENCE FOR INCREASED EXPRESSION IN HYPERTHYROIDISM

Nitric oxide mediates a wide array of cellular functions in many tissu es. It is generated by three known isoforms of nitric oxide synthases (NOS). Recently the endothelial isoform, NOSIII, was shown to be abund antly expressed in the rat thyroid gland and its expression increased in goitrous glands. In this study, we analyzed whether NOSIII is expre ssed in human thyroid tissue and whether levels of expression vary in different states of thyroid gland function. Semiquantitative RT-PCR wa s used to assess variations in NOSIII gene expression in seven patient s with Graces' disease, one with a TSH-receptor germline mutation and six hypothyroid patients (Hashimoto's thyroiditis). Protein expression and subcellular localization were determined by immunohistochemistry (two normal thyroids, five multinodular goiters, ten hyperthyroid pati ents and two hypothyroid patients). NOSIII mRNA was detected in all sa mples: the levels were significantly higher in tissues from hyperthyro id patients compared with euthyroid and hypothyroid patients. NOSIII i mmunoreactivity was detected in vascular endothelial cells, but was al so found in thyroid follicular cells. In patients with Graves' disease , the immunostaining was diffusely enhanced in all follicular cells. A more intense signal was observed in toxic adenomas and in samples obt ained from a patient with severe hyperthyroidism due to an activating mutation in the TSH receptor. In multinodular goiters, large follicles displayed a weak signal whereas small proliferative follicles showed intense immunoreactivity near the apical plasma membrane. In hypothyro id patients, NOSIII immunoreactivity was barely detectable. In summary , NOSIII is expressed both in endothelial. cells and thyroid follicula r cells. The endothelial localization of NOSIII is consistent with a r ole for nitric oxide in the vascular control of the thyroid. NOSIII ex pression in thyroid follicular cells and the variations in its immunor eactivity suggest a possible role for nitric oxide in thyrocyte functi on and/or growth.