Insights into the posttranslational structural heterogeneity of thyroglobulin and its role in the development, diagnosis, and management of benign and malignant thyroid diseases

Abstract

Thyroglobulin (Tg) is the major glycoprotein produced by the thyroid gland, where it serves as a template for thyroid hormone synthesis and as an intraglandular store of iodine. Measurement of Tg levels in serum is of great practical importance in the follow-up of differentiated thyroid carcinoma (DTC), a setting in which elevated levels after total thyroidectomy are indicative of residual or recurrent disease. The most recent methods for serum Tg measurement are monoclonal antibody-based and are highly sensitive. However, major challenges remain regarding the interpretation of the results obtained with these immunometric methods, particularly in patients with endogenous antithyroglobulin antibodies or in the presence of heterophile antibodies, which may produce falsely low or high Tg values, respectively. The increased prevalence of antithyroglobulin antibodies in patients with DTC, as compared with the general population, raises the very pertinent possibility that tumor Tg may be more immunogenic. This inference makes sense, as the tumor microenvironment (tumor cells plus normal host cells) is characterized by several changes that could induce posttranslational modification of many proteins, including Tg. Attempts to understand the structure of Tg have been made for several decades, but findings have generally been incomplete due to technical hindrances to analysis of such a large protein (660 kDa). This review article will explore the complex structure of Tg and the potential role of its marked heterogeneity in our understanding of normal thyroid biology and neoplastic processes. Arch Endocrinol Metab. 2016;60(1):66-75

Glycosylation in the Thyroid Gland: Vital Aspects of Glycoprotein Function in Thyrocyte Physiology and Thyroid Disorders

“…The Tg molecule is O - and N -glycosylated [ 8 ], and around 10% of its molecular mass is related to oligosaccharides [ 71 , 72 ]. The polypeptide chain of human Tg (hTg) contains 20 putative N -glycosylation sites, of which 16 Asn are glycosylated [ 34 ].…”

“…Porcine Tg was also reported to contain sulfated N -linked carbohydrate chains [ 74 ]. The structure of the O -glycans identified on Tg is still largely unknown [ 72 ].…”

“…Glycans are necessary for intracellular and extracellular transport of Tg, protein folding iodination and hormone synthesis, and the proper functioning and immunoreactivity of Tg [ 72 , 75 ]. N -glycans located in the N-terminal domain of Tg play a role in iodination of the tyrosine residue and in iodotyrosine coupling.…”

Endoplasmic reticulum stress inhibits expression of genes involved in thyroid hormone synthesis and their key transcriptional regulators in FRTL-5 thyrocytes

“…Besides glycosylation, TG undergoes posttranslational modification by phosphorylation of carbohydrate chains (50%) and serine (30%) and tyrosine residues (20%) [ 53 ]. Phosphorylation has been suggested to take place in the Golgi complex and is thought to be important for directing TG through the Golgi complex to the endosomes and, finally, to the lysosomes [ 54 ]. To study the effect of ER stress on phosphorylation of TG in FRTL-5 cells, proteins were isolated also from TM-treated FRTL-5 cells using a buffer containing phosphatase-inhibitors.…”

De novo triiodothyronine formation from thyrocytes activated by thyroid-stimulating hormone

“…During its complex trafficking through the intracellular transport pathway of thyrocytes, TG undergoes considerable post-translational processing prior to its secretion and iodination (20). Many of these post-translational modifications are regulated indirectly by TSH-induced changes in the gene expression and activity of TG processing enzymes (21).…”