ENDOCYTOSIS OF ALBUMIN AND THYROGLOBULIN AT THE BASOLATERAL MEMBRANE OF THYROCYTES ORGANIZED IN FOLLICLES

Serum proteins such as albumin are present inside thyroid follicles in both normal and pathological situations. To analyze the mechanism of entry of these proteins, we investigated the ability of polarized thyr ocytes to internalize soluble molecules at their basolateral pole. Exp eriments were conducted on in vitro reconstituted thyroid follicles us ing BSA and pig thyroglobulin (Tg) coupled to gold particles for elect ron microscopy, conjugated to fluorescein for conventional and confoca l fluorescence microscopy, or radioiodinated for biochemical measureme nts. Incubations were carried out at 37 C. BSA and Tg coupled to gold particles were rapidly internalized from the culture medium and sequen tially found in small vesicles and early endosomes and in late endosom es and lysosomes. Fluorescence microscope analyses revealed that the m ajority of cells forming reconstituted thyroid follicles are capable o f internalizing BSA and Tg, but that Tg was more efficiently endocytos ed than BSA. Using radioiodinated ligands, it was observed that the en docytosis of Tg was 10 times higher than that of BSA. The internalizat ion of [I-125]Tg was inhibited by increasing concentrations of unlabel ed Tg. In contrast, endocytosis of I-125-labeled BSA was independent o f the unlabeled BSA concentration. Experiments performed at 4 C indica ted the presence of a basolateral membrane binding activity for [I-125 ]Tg; the Tg concentration that reduced the binding of labeled Tg by 50 % ranged from 4-6 mu M. These data are evidence of a process of intern alization of soluble molecules at the basolateral pole of thyrocytes, with BSA being internalized by fluid phase endocytosis and Tg by selec tive endocytosis. Our findings explain how serum albumin can enter thy roid follicles and disclose a new cellular handling and transport path way of Tg. We propose that selective uptake of Tg operating on molecul es secreted at the basolateral surface of thyrocytes could control the amount of Tg released in the circulation.