Megalin in thyroid physiology and pathology.

Megalin, a member of the low density lipoprotein endocytic receptor family, is expressed on the apical surface of thyroid epithelial cells, directly facing the follicle lumen, where colloid is stored in high concentrations. Studies in vivo and with cultured thyroid cells have provided evidence that megalin expression on thyroid cells is TSH-dependent. Thyroglobulin (Tg), the major protein component of the colloid and the precursor of thyroid hormones, binds to megalin with high affinity and megalin mediates in part its uptake by thyrocytes. Tg internalized by megalin avoids the lysosomal pathway and is delivered by transepithelial transport (transcytosis) to the basolateral membrane of thyrocytes, from which it is released into the bloodstream. This process competes with pathways leading to thyroid hormone release from Tg molecules, which occurs following internalization of Tg molecules from the colloid by other means of uptake (fluid phase endocytosis or endocytosis mediated by low affinity receptors) that result in proteolytic cleavage in the lyosomes. During transcytosis of Tg, a portion of megalin (secretory component) remains complexed with Tg and enters the circulation, where its detection may serve as a tool to identify the origin of serum Tg in patients with thyroid diseases. Tg endocytosis via megalin is facilitated by the interaction of Tg with cell surface heparan sulfate proteoglycans, which occurs via a carboxyl terminal heparin binding site of Tg functionally related with a major megalin binding site. Although autoantibodies against megalin can be found in the serum of approximately 50% of patients with autoimmune thyroiditis, a role of megalin in this and other thyroid diseases remains to be established.