Abstract
Thyroid peroxidase (TPO) is an enzyme and autoantigen expressed in thyroid and breast tissues. Thyroid TPO undergoes a complex maturation process however, nothing is known about post-translational modifications of breast-expressed TPO. In this study, we have investigated the biochemical properties of TPO expressed in normal and cancerous human breast tissues, and the maturation process and antigenicity of TPO present in a panel of human breast tissue-derived cell lines. We found that the molecular weight of breast TPO was slightly lower than that of thyroid TPO due to decreased glycosylation and as suggest results of Western blot also shorter amino acid chain. Breast TPO exhibit enzymatic activity and isoelectric point comparable to that of thyroid TPO. The biochemical properties of TPO expressed in mammary cell lines and normal thyrocytes are similar regarding glycan content, molecular weight and isoelectric point. However, no peroxidase activity and dimer formation was detected in any of these cell lines since the majority of TPO protein was localized in the cytoplasmic compartment, and the TPO expression at the cell surface was too low to detect its enzymatic activity. Lactoperoxidase, a protein highly homologous to TPO expressed also in breast tissues, does not influence the obtained data. TPO expressed in the cell lines was recognized by a broad panel of TPO-specific antibodies. Although some differences in biochemical properties between thyroid and breast TPO were observed, they do not seem to be critical for the overall three-dimensional structure. This conclusion is supported by the fact that TPO expressed in breast tissues and cell lines reacts well with conformation-sensitive antibodies. Taking into account a close resemblance between both proteins, especially high antigenicity, future studies should investigate the potential immunotherapies directed against breast-expressed TPO and its specific epitopes.
Highlights
Human thyroid peroxidase (TPO), the crucial enzyme responsible for biosynthesis of hormones by the thyroid gland, catalyzes iodination and coupling of tyrosine residues in thyroglobulin, which leads to the synthesis of triiodothyronine and thyroxine [1, 2]
Biochemical properties of TPO expressed in human breast tissues major autoantigen in autoimmune thyroid disease (AITD)
The amount of carbohydrate residues attached to the breast TPO protein core is lower than in the thyroid TPO molecule
Summary
Human thyroid peroxidase (TPO), the crucial enzyme responsible for biosynthesis of hormones by the thyroid gland, catalyzes iodination and coupling of tyrosine residues in thyroglobulin, which leads to the synthesis of triiodothyronine and thyroxine [1, 2]. A majority of polyclonal TPO-specific antibodies (TPOAbs) present in sera of AITD patients react with epitopes located on two discontinuous, three-dimensional integrity-dependent immunodominant regions (IDR) on the surface of the TPO molecule, termed A and B (IDR-A and–B) [3,4,5]. These regions have been detected both in antigenic competition experiments with a panel of murine monoclonal antibodies (mAbs) [6] and using recombinant human Fab fragments [7, 8]. The TPO protein maturation and trafficking require the assistance of thyrocyte endoplasmic reticulum chaperones: calreticulin, calnexin and BiP [13, 14]
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