Characterization of the iodide transport system of the thyroid gland and its hormonal regulation

Abstract

The thyroid is an endocrine gland that synthesizes the hormones tri-iodothyronine (T{dollar}\sb3{dollar}) and thyroxine (T{dollar}\sb4{dollar}). Production and release of these hormones is stimulated by thyroid stimulating hormone (TSH) from the adenohypophysis. T{dollar}\sb3{dollar} and T{dollar}\sb4{dollar} are of major significance for intermediary metabolism in virtually all tissues, and play a crucial role in the maturation of the lungs and nervous system in the fetus and the newborn. Iodine is an essential component of T{dollar}\sb3{dollar} and T{dollar}\sb4{dollar}, and the transport of I{dollar}\sp-{dollar} into the thyroid is the first step in the biosynthetic pathway of these hormones. Insufficient dietary supply of iodine, a major health issue worldwide, leads to decreased thyroid hormone biosynthesis which can cause goiter and cretinism. The active transport of iodide (I{dollar}\sp-{dollar}) into the thyroid gland is mediated by the Na{dollar}\sp+{dollar}/I{dollar}\sp-{dollar} symporter, an intrinsic plasma membrane protein located in the thyroid follicular cells.;To further characterize this transport process, a protocol was devised to prepare functional membrane vesicles (MV) from FRTL-5 cells, a line of highly differentiated rat thyroid cells in culture. Kinetic parameters of Na{dollar}\sp+{dollar}/I{dollar}\sp-{dollar} symport activity in these vesicles, Na{dollar}\sp+{dollar} dependence, vesicle sidedness, intravesicular volume, stoichiometry and electrogenicity were assessed. New modes of regulation of Na{dollar}\sp+{dollar}/I{dollar}\sp-{dollar} symport activity by TSH were elucidated in FRTL-5 MV whereby TSH is now thought to stimulate Na{dollar}\sp+{dollar}/I{dollar}\sp-{dollar} symport activity by activating symporter molecules that reside in the plasma membrane in an inactive state.;In an effort to identify the Na{dollar}\sp+{dollar}/I{dollar}\sp-{dollar} symporter, an {dollar}\sp{lcub}125{rcub}{dollar}photolabeling method was developed to label thyroid polypeptides with high affinity for I{dollar}\sp-{dollar}. Only polypeptides from I{dollar}\sp-{dollar}-transporting cells were {dollar}\sp{lcub}125{rcub}{dollar}I-labeled, suggesting a strong link between polypeptide labeling and the ability of cells to transport I{dollar}\sp-{dollar}. NH{dollar}\sb2{dollar}-terminal sequencing information was obtained for several polypeptides one of which was a 90 kDa {dollar}\sp{lcub}125{rcub}{dollar}I-labeled polypeptide, identified to be the chaperone calnexin. In FRTL-5 cells, immunoblot analysis with anti-calnexin antibody revealed that TSH increases thyroid calnexin content. Moreover, regulation of calnexin by TSH was confirmed in vivo.