Iodide transport: implications for health and disease

Disorders of the thyroid gland are among the most common conditions diagnosed and managed by pediatric endocrinologists. Thyroid hormone synthesis depends on normal iodide transport and knowledge of its regulation is fundamental to understand the etiology and management of congenital and acquired thyroid conditions such as hypothyroidism and hyperthyroidism. The ability of the thyroid to concentrate iodine is also widely used as a tool for the diagnosis of thyroid diseases and in the management and follow up of the most common type of endocrine cancers: papillary and follicular thyroid cancer. More recently, the regulation of iodide transport has also been the center of attention to improve the management of poorly differentiated thyroid cancer. Iodine deficiency disorders (goiter, impaired mental development) due to insufficient nutritional intake remain a universal public health problem. Thyroid function can also be influenced by medications that contain iodide or interfere with iodide metabolism such as iodinated contrast agents, povidone, lithium and amiodarone. In addition, some environmental pollutants such as perchlorate, thiocyanate and nitrates may affect iodide transport. Furthermore, nuclear accidents increase the risk of developing thyroid cancer and the therapy used to prevent exposure to these isotopes relies on the ability of the thyroid to concentrate iodine. The array of disorders involving iodide transport affect individuals during the whole life span and, if undiagnosed or improperly managed, they can have a profound impact on growth, metabolism, cognitive development and quality of life.

Switching from MAPK-dependent to MAPK-independent repression of the sodium-iodide symporter in 2D and 3D cultured normal thyroid cells

Loss of sodium-iodide symporter (NIS) expression in thyroid tumour cells primarily caused by constitutive MAPK pathway activation is often refractory to small molecule MAPK inhibitors. Suggested mechanisms are rebound MAPK signalling and activation of alternative signalling pathways. Here we provide evidence that failure to recover down-regulated NIS by MEK inhibition is not specific to tumour cells. NIS mRNA levels remained repressed in TSH-stimulated primary thyroid cells co-treated with epidermal growth factor (EGF) and pan-MEK inhibitor U0126 in the presence of 5% fetal bovine serum or, independently of serum, in 3D cultured thyroid follicles. This led to inhibited iodide transport and iodination. In contrast, U0126 restituted thyroglobulin synthesis in EGF-treated follicular cells. Serum potentiated TSH-stimulated NIS expression in 2D culture. U0126 blocked down-regulation of NIS only in serum-starved cells with a diminished TSH response. Together, this suggests that morphogenetic signals modify the expression of NIS and recovery response to MEK inhibition.