In vitro studies of the thyroglobulin degradation pathway: endocytosis and delivery of thyroglobulin to lysosomes, release of thyroglobulin cleavage products--iodotyrosines and iodothyronines
Abstract
Iodinated thyroglobulin stored in the thyroid follicular lumen is subjected to an internalization process and thought to be transferred into the lysosomal compartment for proteolytic cleavage and thyroid hormone release. In the present study, we have designed in vitro models to study: 1) the transfer of endocytosed thyroglobulin into lysosomes, and 2) the intracellular fate of free thyroid hormones and iodinated precursors generated by intralysosomal proteolysis of thyroglobulin. Open follicles prepared from pig thyroid tissue by collagenase treatment were used to probe the delivery of exogenous thyroglobulin to lysosomes via the differentiated apical cell membrane. Open follicles were incubated with pure [125I]thyroglobulin with or without unlabeled thyroglobulin in the presence or in the absence of chloroquine. Subcellular fractionation on a Percoll gradient showed that [125I]thyroglobulin was internalized and present in low (for the major part) and high density thyroid vesicles. In chloroquine-treated open follicles, we observed the appearance of a definite fraction of [125I]thyroglobulin in a lysosome subpopulation having the expected properties of phagolysosomes or secondary lysosomes. In contrast, in control open follicle...Continue Reading
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