heparitin-sulfate and Hypothyroidism

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.

Topics: Adult; Amitrole; Animals; Dose-Response Relationship, Drug; Endocytosis; Enzyme-Linked Immunosorbent Assay; Female; Graves Disease; Heparitin Sulfate; Homozygote; Hormones; Humans; Hypothyroidism; Low Density Lipoprotein Receptor-Related Protein-2; Male; Methimazole; Mice; Mice, Transgenic; Middle Aged; Models, Biological; Precipitin Tests; Rats; Rats, Inbred Lew; Thyroglobulin; Thyroid Gland; Thyroid Hormones

Proteoglycans and hyaluronic acid synthesized by human skin fibroblasts in culture were characterized, and the effect of thyroid hormone deficiency was examined. The fibroblasts in culture synthesize hyaluronic acid, dermatan sulfate (DS) proteoglycans and heparan sulfate (HS) proteoglycans. Hyaluronic acid is almost exclusively secreted into the medium. Among the proteoglycans synthesized during 24 h label, about 70% were secreted into the medium and the remaining 30% were associated with the cell layer. About 70% of proteoglycans secreted into the medium contained DS and the remaining 30% contained HS. For cell-associated proteoglycans, 60% contained HS and the remainder contained DS. The size distributions of the glycosaminoglycans from both DS and HS proteoglycans were similar, with an average Mr of approximately 30,000. Incubation of fibroblasts in thyroid hormone deficient medium increased net synthesis of hyaluronic acid (approximately 50%) and all species of proteoglycans (approximately 85%). 3H/35S ratios in the chondroitin 4-sulfate disaccharide isolated with HPLC were not altered in thyroid hormone deficient cultures, indicating that the specific activity of 3H in UDP-N-acetylhexosamine precursors did not change. The increased incorporation of 3H into hyaluronic acid and of 3H and 35S into DS and HS proteoglycans thus indicates increased net synthesis. Degradation of cell-associated proteoglycans was not influenced by thyroid hormone deficiency.

Topics: Adolescent; Animals; Borohydrides; Cattle; Cells, Cultured; Chondroitin Lyases; Chromatography; Chromatography, High Pressure Liquid; Culture Media; Dermatan Sulfate; Female; Fibroblasts; Glucosamine; Heparitin Sulfate; Humans; Hyaluronic Acid; Hypothyroidism; Macromolecular Substances; Papain; Proteoglycans; Sulfates

Urinary excretion of acid mucopolysaccharides and glycopeptides in the urine of an untreated patient with cretinism were measured before and after thyroid hormone replacement. Urinary uronic acid and hexose excretion in the CPC-precipitated fraction increased four to ten times after thyroid hormone administration. The maximum excretion was observed after 2 months of thyroid replacement. The excreted acid mucopolysaccharides consisted of chondroitin sulphate A/C and minor quantities of heparan sulphate. Urinary excretion of glycopeptides, particularly small molecular glycopeptides rose also four to five-fold with thyroid hormone administration. These data suggest that thyroid hormone markedly influences the metabolism of acid mucopolysaccharides and glycoproteins. Possibly, the lack of thyroid hormone caused a decreased activity of various lysosomal glycosidases and sulphatases.

Topics: Child; Chondroitin Sulfates; Dermatan Sulfate; Glycopeptides; Glycosaminoglycans; Heparitin Sulfate; Hexoses; Humans; Hypothyroidism; Male; Sialic Acids; Triiodothyronine; Uronic Acids