Kuliawat R, Arvan P
Division of Endocrinology, Beth Israel Hospital, Harvard Medical School, Boston, Massachusetts 02215.
J Biol Chem. 1994 Feb 18;269(7):4922-7.
Thyroid follicles perform several functions that depend upon epithelial polarity: secretion of thyroglobulin (Tg) to the apical lumen, uptake of iodide for Tg iodination, and the manufacture of thyroid hormone for delivery to the bloodstream. In this report we examine Tg processing by thyroid epithelial monolayers cultured on porous filters. Basolateral 125I uptake resulted in thyrotropin-dependent radiolabeling of Tg in cells and apical medium. Polarized thyrocytes iodinated exogenous gamma globulins (IgG), demonstrating labeling in the apical extracellular space. Apical catalase addition inhibited the appearance of apical [125I]IgG and [125I]Tg, but had no effect on cell-associated [125I]Tg, indicating additional iodination of Tg in an intracellular compartment. A similar conclusion was drawn from radioiodination experiments at 20 degrees C. Intracellular iodination was selective for Tg forms receiving prior Golgi carbohydrate modifications. During a 2-h chase, [125I]Tg was exported from cells to apical medium, while modest amounts of thyroxine were secreted with a majority to the basolateral medium. Neither radioiodination at 20 degrees C nor apical catalase addition blocked formation or secretion of [125I]thyroxine during the chase. Thus in filter-grown thyroid epithelial cells, prior to apical extracellular iodination, intracellular iodination of Tg begins the process leading to formation of thyroxine.
将甲状腺球蛋白（Tg）分泌到顶端腔隙，摄取碘用于Tg碘化，以及制造甲状腺激素以输送到血液中。在本报告中，我们研究了在多孔滤器上培养的甲状腺上皮单层对Tg的加工过程。基底外侧摄取125I导致细胞和顶端培养基中Tg的促甲状腺激素依赖性放射性标记。极化的甲状腺细胞对外源γ球蛋白（IgG）进行碘化，显示在顶端细胞外空间有标记。向顶端添加过氧化氢酶可抑制顶端[125I]IgG和[125I]Tg的出现，但对细胞相关的[125I]Tg没有影响，表明在细胞内区室中Tg有额外的碘化。从20℃的放射性碘化实验得出了类似的结论。细胞内碘化对先前接受高尔基体碳水化合物修饰的Tg形式具有选择性。在2小时的追踪过程中，[125I]Tg从细胞分泌到顶端培养基中，而少量甲状腺素分泌，大部分分泌到基底外侧培养基中。在追踪过程中，20℃的放射性碘化和顶端添加过氧化氢酶均未阻断[125I]甲状腺素的形成或分泌。因此，在滤器培养的甲状腺上皮细胞中，在顶端细胞外碘化之前，Tg的细胞内碘化开始了导致甲状腺素形成的过程。
