Preferential megalin-mediated transcytosis of low-hormonogenic thyroglobulin: A control mechanism for thyroid hormone release

Abstract

Hormone secretion by thyrocytes occurs by fluid phase uptake and lysosomal degradation of the prohormone thyroglobulin (Tg). However, some Tg internalized by megalin bypasses lysosomes and is transcytosed across cells and released into the bloodstream. Because the hormone content of Tg is variable, we investigated whether this affects transcytosis. We found that rat Tg with a low hormone content [low-hormonogenic rat Tg (low-horm-rTg)] is transcytosed by megalin across thyroid FRTL-5 cells to a greater extent than rat Tg with a high hormone content [hormonogenic rat Tg (horm-rTg)]. In immunoprecipitation experiments, the Tg sequence Arg-2489-Lys-2503 (required for binding to megalin and heparan sulfate proteoglycans) was found to be more exposed in low-horm-rTg, which accounted for its preferential transcytosis. Thus, removal of surface heparan sulfate proteoglycans from FRTL-5 cells or blocking of 2489 -2503 reduced transcytosis of low-horm-rTg to a greater extent than that of horm-rTg. Preferential transcytosis of low-horm-rTg affected hormone release. Thus, the increase in hormone release from horm-rTg in FRTL-5 cells determined by megalin blocking (due to reduced transcytosis and enhanced Tg degradation) was rescued by low-horm-rTg, suggesting that megalin is required for effective hormone release. This finding was confirmed in a small number of megalin-deficient mice, which had serological features resembling mild hypothyroidism. Reduced hormone formation within Tg in vivo, due to treatment of rats with aminotriazole or of patients with Graves' disease with methimazole, resulted in increased Tg transcytosis via megalin, in confirmation of results with FRTL-5 cells. Our study points to a major role of megalin in thyroid homeostasis with possible implications in thyroid diseases.

The Vps10p-domain receptor family

“…Hormone release is precisely regulated and requires uptake of thyroglobulin by thyrocytes, degradation in lysosomes, transcytosis, and recycling of the precursor. It is well established that Megalin, a member of the LDL receptor family, mediates thyroglobulin uptake and transcytosis [57]. A recent study demonstrates that Sortilin is expressed in thyrocytes in a thyroid-stimulating hormone (TSH)-dependent manner and that it interacts with thyroglobulin in postendocytic endosomes and partakes in thyroglobulin recycling [55].…”

Thyrocyte-specific Gq/G11 deficiency impairs thyroid function and prevents goiter development

“…The recently described endocytosis of thyroglobulin via megalin is followed by the apical to basolateral transcytosis of thyroglobulin with low hormone content (35,36). The transcytotic removal of hormone-poor thyroglobulin is believed to increase lysosomal degradation of hormone-rich thyroglobulin and hence hormone secretion.…”

Intracellular retention of thyroglobulin in the absence of the low-density lipoprotein receptor-associated protein (RAP) is likely due to premature binding to megalin in the biosynthetic pathway

“…Megalin is expressed on the apical surface of TEC, directly facing the colloid [9], where it serves as an endocytic receptor for the thyroid hormone precursor thyroglobulin (Tg) [7,10,11]. Once synthesized by thyrocytes, Tg is secreted into the follicle lumen where it undergoes hormone formation by iodine coupling of its tyrosyl residues [11]. Hormone-rich Tg molecules proceed further into the Abstract Objective The low-density lipoprotein receptor associated protein (RAP) is expressed by thyroid epithelial cells (TEC) in a TSH-dependent manner.…”

“…COS-7 cells transiently transfected with hRAP, mTg or both, expressed the two proteins accordingly. RAP was found almost exclusively in cell extracts, whereas Tg was found follicle, arranging into aggregates that form the colloid, or, alternatively, they undergo proteolytic cleavage, resulting in hormone release [11]. Tg enzymatic digestion occurs especially within lysosomes, which follows fluid phase micropinocytosis of Tg by thyrocytes [11].…”