Abstract
The key proteins responsible for hormone synthesis in the thyroid are glycosylated. Oligosaccharides strongly affect the function of glycosylated proteins. Both thyroid-stimulating hormone (TSH) secreted by the pituitary gland and TSH receptors on the surface of thyrocytes contain N-glycans, which are crucial to their proper activity. Thyroglobulin (Tg), the protein backbone for synthesis of thyroid hormones, is a heavily N-glycosylated protein, containing 20 putative N-glycosylated sites. N-oligosaccharides play a role in Tg transport into the follicular lumen, where thyroid hormones are produced, and into thyrocytes, where hyposialylated Tg is degraded. N-glycans of the cell membrane transporters sodium/iodide symporter and pendrin are necessary for iodide transport. Some changes in glycosylation result in abnormal activity of the thyroid and alteration of the metabolic clearance rate of hormones. Alteration of glycan structures is a pathological process related to the progression of chronic diseases such as thyroid cancers and autoimmunity. Thyroid carcinogenesis is accompanied by changes in sialylation and fucosylation, β1,6-branching of glycans, the content and structure of poly-LacNAc chains, as well as O-GlcNAcylation, while in thyroid autoimmunity the main processes affected are sialylation and fucosylation. The glycobiology of the thyroid gland is an intensively studied field of research, providing new data helpful in understanding the role of the sugar component in thyroid protein biology and disorders.
Highlights
The thyroid gland is crucial to the regulation of metabolism, development and growth, acting via the thyroid hormones triiodothyronine (T3) and thyroxine (T4) [1]
Synthesis of thyroid hormones is regulated by pituitary thyroid-stimulating hormone (TSH), which binds to its thyroid-stimulating hormone receptor (TSHR) on the surface of thyroid follicular cells [6]
Lectin histochemical staining of sialic acid (SA) in tissue specimens of four human thyroid carcinomas showed that cancer transformation of thyroid follicular epithelial cells to Papillary thyroid carcinoma (PTC) and follicular thyroid cancer (FTC) is associated with an increase of sialylation [105,106]
Summary
The thyroid gland is crucial to the regulation of metabolism, development and growth, acting via the thyroid hormones triiodothyronine (T3) and thyroxine (T4) [1]. Synthesis of glycans requires transfer of monosaccharides from activated nucleoside triphosphate donors to a sugar acceptor [11] This process is catalyzed by glycosyltransferases (GTs), which play a key regulatory role in glycosylation; they are responsible for the attachment of sugar moieties to the nascent oligosaccharide structure. The impact of Tg oligosaccharides on recognition by specific antibodies was shown in the porcine model [29] These findings were fundamental to further studies aimed at decoding the role of glycans in thyroid proteins that are important in thyrocyte physiology and pathology. Differences in glycosylation have been shown between TSH subunits; α subunit glycans are mainly sialylated and monosulfated, whereas the β subunit contains more disulfated and core-fucosylated structures [40]
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