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Circulating Thyroglobulin Transcytosed by Thyroid Cells Is Complexed with Secretory Components of Its Endocytic Receptor Megalin¹

Pathology Research Laboratory (M.M., N.M., D.A., R.T.M.) and Department of Pathology (A.B.C.), Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts 02129; Department of Endocrinology, University of Pisa (M.M., L.C., F.L., A.P.), 56100 Pisa, Italy; and Department of Pathology, University of Athens (N.M., S.T.B.), 10554 Athens, Greece

Address all correspondence and requests for reprints to: Dr. Michele Marinò, Department of Endocrinology, University of Pisa, Via Paradisa 2, 56100 Pisa, Italy. E-mail: m.marino{at}endoc.med.unipi.it

After its endocytosis from the colloid, some thyroglobulin (Tg) is transcytosed intact across thyrocytes, accounting in part for its presence in the circulation. We previously showed that megalin (gp330), an endocytic Tg receptor, mediates apical to basolateral Tg transcytosis. Here we investigated whether a portion of megalin remains combined with Tg after its transcytosis, 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 unlabeled Tg to the upper chamber and incubation at 37 C, some Tg was transcytosed intact across FRTL-5 cells into the lower chamber. Two antimegalin ectodomain antibodies precipitated transcytosed Tg in fluids collected from the lower chamber. After the addition of Tg to surface-biotinylated FRTL-5 cells, an anti-Tg antibody and the two antimegalin ectodomain antibodies precipitated high molecular mass biotinylated material in fluids collected from the lower chamber, corresponding to much of the megalin ectodomain, as well as smaller amounts of lower molecular mass material. The results indicate that Tg transcytosed across FRTL-5 cells remains complexed with megalin ectodomain 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 serum. Tg was also precipitated by antimegalin antibodies in sera from patients with Graves’ disease, in which we found increased megalin expression on the apical surface of thyrocytes. In contrast, in thyroidectomized patients with metastatic papillary thyroid carcinoma, in whom Tg is directly secreted 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.

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