THYROGLOBULIN MOLECULES INTERNALIZED BY THYROCYTES ARE SORTED IN EARLY ENDOSOMES AND PARTIALLY RECYCLED BACK TO THE FOLLICULAR LUMEN

Thyroglobulin (Tg) molecules stored in thyroid follicle lumens are het erogenous in terms of iodine and hormone contents. It has been suggest ed that thyroid hormone is preferentially produced from the most highl y iodinated Tg molecules and that thyrocytes are capable of selecting these molecules. The cellular localization as well as the molecular ba sis of such a selection process are not known. The present work was un dertaken to determine whether there is selectivity at the step of endo cytosis and, if not, to discover other possible mechanisms. Studies we re conducted on reconstituted thyroid follicles (RTF) in culture. We c ompared the ability of thyrocytes to internalize Tg and an exogenous p rotein, BSA, which is neither iodinated nor glycosylated. To identify the protein, Tg and BSA were coupled to gold particles of different si ze and microinjected in a fixed ratio into the lumen of RTF. Neither o f the two protein gold probes detected by transmission electron micros cope bound at the cell surface, and both entered the cells at a simila r rate and were concentrated in early endosomes. After 20 min, both Tg -G and BSA-G were segregated into distinct vacuolar structures. At 60 min, the intracellular content of BSA-G (mainly in prelysosomes and ly sosomes) was 2- to 3-fold higher than that of Tg-G. At the same time, there was a marked reduction in the BSA-G/Tg-G ratio in the lumen. The differences between the Tg-G and BSA-G distribution patterns that wer e amplified in TSH-treated RTF are in keeping with a back-transfer of internalized Tg toward the lumen. The existence of a cell to lumen tra nsport of previously endocytosed Tg was further documented using intra lumenal [I-125]Tg as a marker. RTF pulse labeled with tracer amounts o f [I-125]iodide were shortly incubated with TSH to induce [I-125]Tg en docytosis, and the fate of internalized [I-125]Tg was studied in a cha se incubation period of up to 4 h. At 20 C, where the degradation of i nternalized Tg is blocked, we observed a time-dependent decrease in in tracellular [I-125]Tg and a corrresponding increase in the lumenal [I- 125]Tg content. This cell to lumen [I-125]Tg transfer was inhibited by primaquine. In conclusion, our data show that 1) the thyroid apical e ndocytic process does not exhibit selectivity for Tg; 2) the thyrocyte possesses a sorting machinery for endocytosed ligands; and 3) interna lized Tg molecules can be recycled back to the follicular lumen. These findings led us to propose a mechanism for the preferential utilizati on of well iodinated Tg in thyroid hormone production; high and low io dinated Tg molecules internalized at random would be sorted inside the cells, the former being conveyed to lysosomal compartments and the la tter being recycled to the lumen for completion of hormone synthesis. This postulated pathway requires direct experimental groundings.