Role of megalin (gp330) in transcytosis of thyroglobulin by thyroid cells - A novel function in the control of thyroid hormone release

When thyroglobulin (Tg) is endocytosed by thyrocytes and transported to lys osomes, thyroid hormones (T4 and T3) are released. However, some internaliz ed Tg is transcytosed intact into the bloodstream, thereby avoiding proteol ytic cleavage. Here we show that megalin (gp330), a Tg receptor on thyroid cells, plays a role in Tg transcytosis. Following incubation with exogenous rat Tg at 37 degrees C, Fisher rat thyroid (FRTL-5) cells, a differentiate d thyroid cell line, released T3 into the medium. However, when cells were incubated with Tg plus either of two megalin competitors, T3 release was in creased, suggesting that Tg internalized by megalin bypassed the lysosomal pathway, possibly with release of undegraded Tg from cells. To assess this possibility, we performed experiments in which FRTL-5 cells were incubated with either unlabeled or I-125-labeled Tg at 37 degrees C to allow internal ization, treated with heparin to remove cell surface-bound Tg, and further incubated at 37 degrees C to allow Tg release. Intact 330-kDa Tg was releas ed into the medium, and the amount released was markedly reduced by megalin competitors. To investigate whether Tg release resulted from transcytosis, we studied FRTL-5 cells cultured as polarized layers with tight junctions on permeable filters in the upper chamber of dual chambered devices, Follow ing the addition of Tg to the upper chamber and incubation at 37 degrees C, intact 330-kDa Tg was found in fluids collected from the lower chamber. Th e amount recovered was markedly reduced by megalin competitors, indicating that megalin mediates Tg transcytosis. We also studied Tg transcytosis in v ivo, using a rat model of goiter induced by aminotriazole, in which increas ed release of thyrotropin induces massive colloid endocytosis, This was ass ociated with increased megalin expression on thyrocytes and increased serum Tg levels, with reduced serum T3 levels, supporting the conclusion that me galin mediates Tg transcytosis. Tg transcytosis is a novel function of mega lin, which usually transports ligands to lysosomes, Megalin-mediated transc ytosis may regulate the extent of thyroid hormone release.