Marinò M, McCluskey R T
Pathology Research Laboratory, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts 02129, USA.
Am J Physiol Cell Physiol. 2000 Nov;279(5):C1295-306. doi: 10.1152/ajpcell.2000.279.5.C1295.
Thyroglobulin (Tg), the thyroid hormone precursor, is synthesized by thyrocytes and secreted into the colloid. Hormone release requires uptake of Tg by thyrocytes and degradation in lysosomes. This process must be precisely regulated. Tg uptake occurs mainly by micropinocytosis, which can result from both fluid-phase pinocytosis and receptor-mediated endocytosis. Because Tg is highly concentrated in the colloid, fluid-phase pinocytosis or low-affinity receptors should provide sufficient Tg uptake for hormone release; high-affinity receptors may serve to target Tg away from lysosomes, through recycling into the colloid or by transcytosis into the bloodstream. Several apical receptors have been suggested to play roles in Tg uptake and intracellular trafficking. A thyroid asialoglycoprotein receptor may internalize and recycle immature forms of Tg back to the colloid, a function also attributed to an as yet unidentified N-acetylglucosamine receptor. Megalin mediates Tg uptake by thyrocytes, especially under intense thyroid-stimulating hormone stimulation, resulting in transcytosis of Tg from the colloid to the bloodstream, a function that prevents excessive hormone release.
甲状腺球蛋白（Tg）是甲状腺激素的前体，由甲状腺细胞合成并分泌到滤泡胶体中。激素释放需要甲状腺细胞摄取Tg并在溶酶体中降解。这个过程必须受到精确调控。Tg的摄取主要通过微胞饮作用，这可能由液相胞饮作用和受体介导的内吞作用引起。由于Tg在滤泡胶体中高度浓缩，液相胞饮作用或低亲和力受体应为激素释放提供足够的Tg摄取；高亲和力受体可能通过循环回到滤泡胶体或通过转胞吞作用进入血液，使Tg远离溶酶体。几种顶端受体被认为在Tg摄取和细胞内运输中起作用。一种甲状腺去唾液酸糖蛋白受体可能将未成熟形式的Tg内化并循环回到滤泡胶体，这一功能也归因于一种尚未确定的N-乙酰葡糖胺受体。巨膜蛋白介导甲状腺细胞摄取Tg，尤其是在强烈的促甲状腺激素刺激下，导致Tg从滤泡胶体转胞吞到血液中，这一功能可防止激素过度释放。
