Circulating thyroglobulin transcytosed by thyroid cells is complexed with secretory components of its endocytic receptor megalin

After its endocytosis from the colloid, some thyroglobulin (Tg) is transcyt osed intact across thyrocytes, accounting in part for its presence in the c irculation. We previously showed that megalin (gp330), an endocytic Tg rece ptor, mediates apical to basolateral Tg transcytosis. Here we investigated whether a portion of megalin remains combined with Tg after its transcytosi s, using studies with cultured thyroid cells and in, vivo observations. FRTL-5 cells, a rat thyroid cell line, cultured on filters in dual chambers form tight junctions and exhibit features of polarity, with expression of megalin exclusively on the upper (apical) surface. After the addition of un labeled Tg to the upper chamber and incubation at 37 C, some Tg was transcy tosed intact across FRTL-5 cells into the lower chamber. Two antimegalin ec todomain antibodies precipitated transcytosed Tg in fluids collected from t he lower chamber. After the addition of Tg to surface-biotinylated FRTL-5 c ells, an anti-Tg antibody and the two antimegalin ectodomain antibodies pre cipitated high molecular mass biotinylated material in fluids collected fro m the lower chamber, corresponding to much of the megalin ectodomain, as we ll as smaller amounts of lower molecular mass material. The results indicat e that Tg transcytosed across FRTL-5 cells remains complexed with megalin e ctodomain components, which we refer to as megalin secretory components. In aminotriazole-treated rats, which develop increased megalin-mediated Tg transcytosis, antimegalin antibodies precipitated some of the Tg in the ser um. Tg was also precipitated by antimegalin antibodies in sera from patient s with Graves' disease, in which we found increased megalin expression on t he apical surface of thyrocytes. In contrast, in thyroidectomized patients with metastatic papillary thyroid carcinoma, in whom Tg is directly secrete d by neoplastic thyroid cells into the circulation rather than transcytosed , serum Tg was not precipitated by antimegalin antibodies. The detection of Tg-megalin complexes may help identify the source of serum Tg in patients with thyroid diseases.