The microsomal/peroxidase antigen: modulation of its expression in thyroid cells

Preferential megalin-mediated transcytosis of low-hormonogenic thyroglobulin: a control mechanism for thyroid hormone release

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.

Thyroid stimulating hormone upregulates secretion of cathepsin B from thyroid epithelial cells

Constant levels of thyroid hormones in the blood are principal requirements for normal vertebrate development. Their release depends on the regulated proteolysis of thyroglobulin which is extracellularly stored in the follicle lumen under resting conditions. Thyroglobulin is proteolytically degraded to a major part in lysosomes, but in part also extracellularly leading to the release of thyroxine. Extracellularly occurring lysosomal enzymes are most probably involved in the proteolytic release of thyroxine. In this study we have analyzed the secretion of cathepsin B by thyroid follicle cells (primary cells as well as FRTL-5 cells) and its regulation by thyroid stimulating hormone, which stimulated the secretory release of the proenzyme as well as of mature cathepsin B. Within one to two hours of stimulation with thyroid stimulating hormone, the cathepsin B activity associated with the plasma membrane increased significantly. This increase correlated closely with the localization of lysosomes in close proximity to the plasma membrane of cultured thyrocytes as well as with the thyroxine liberating activity of thyrocyte secretion media. These observations indicate that thyroid stimulating hormone induces the secretion of cathepsin B, which contributes to the extracellular release of thyroxine by thyrocytes.

Cysteine proteinases mediate extracellular prohormone processing in the thyroid

Thyroglobulin, the precursor of thyroid hormones, is extracellularly stored in a highly condensed and covalently cross-linked form. Solublization of thyroglobulin is facilitated by cysteine proteinases like cathepsins B and K which are proteolytically active at the surface of thyroid epithelial cells. The cysteine proteinases mediate the processing of thyroglobulin by limited extracellular proteolysis at the apical plasma membrane, thereby rapidly liberating thyroxine. The trafficking of cysteine proteinases in thyroid epithelial cells includes their targeting to lysosomes where they become maturated before being transported to the apical plasma membrane and, thus, into the extracellular follicle lumen. We propose that thyroid stimulating hormone regulates extracellular proteolysis of thyroglobulin in that it enhances the rate of exocytosis of lysosomal proteins at the apical plasma membrane. Later, thyroid stimulating hormone upregulates thyroglobulin synthesis and its secretion into the follicle lumen for subsequent compaction by covalent cross-linking. Hence, cycles of thyroglobulin proteolysis and thyroglobulin deposition might result in the regulation of the size of the luminal content of thyroid follicles. We conclude that the biological significance of extracellularly acting cysteine proteinases of the thyroid is the rapid utilization of thyroglobulin for the maintenance of constant thyroid hormone levels in vertebrate organisms.

Thyrotropin chronically regulates the pool of thyroperoxidase and its intracellular distribution: a quantitative confocal microscopic study

The regulation of thyroperoxidase (TPO) expression and of its intracellular distribution was studied in porcine thyroid cells cultured on porous bottom filters. Cells were cultured for 18 days in the absence or in the presence of thyrotropin (TSH) and with or without iodide. Microsomes were purified and analyzed by electrophoresis. TPO was detected by immunoblotting with polyclonal anti-porcine TPO antibodies and quantified by scanning the bands. The amount of TPO was increased 2-fold by TSH. High concentrations of iodide (1-50 microM, added daily) decreased the level of TPO. Confocal microscopy served to determine the intracellular localization of TPO and its quantitative distribution. Intracellular and surface-located TPO was detected by fluorescein-labeled antibodies on saponin-treated cells. Quantitative confocal microscopy showed that TSH increased the total amount of TPO 2-fold as for immunoblotting. The highest amount of TPO was found in the perinuclear area and between the nucleus and the Golgi apparatus. Only 4% of TPO was present on the apical surface and about 1% on the basolateral membrane; the remainder (about 95%) was inside the cells. TSH did not change these relative contents. TSH modified the intracellular distribution of the enzyme, increasing the TPO pool from the perinuclear area to apical membrane. This domain could be a site of storage of TPO. Adding a physiological concentration of iodide (0.5 microM, daily) did not influence the intracellular distribution of TPO. We concluded that chronic TSH stimulation 1) increased 2-fold the pool of TPO but did not change the relative proportion of TPO inside the cells and on the apical surface, and 2) modified the intracellular distribution of vesicular TPO, the major part of which was accumulated in the perinuclear and cytoplasmic area under the subapical domain of the polarized cells.

Effect of anti-thyroid peroxidase (TPO) antibodies on TPO activity measured by chemiluminescence assay

A chemiluminescence method was developed to measure thyroid peroxidase (TPO) activity and the inhibitory effect of anti-TPO antibodies in purified porcine TPO. The TPO preparation was characterized kinetically and controlled by Western-blotting technique. The chemiluminescence method proved to be reproducible and much more sensitive than the widely used guaiacol method, being able to detect TPO concentrations of 2.21 × 10−5 g/L vs 6.63 × 10−2g/L with the latter. Otherwise, the determinations with the two methods correlated well (r = 0.76). Investigating the effect of IgGs from 23 hypothyroid patients on measured TPO activity, we detected inhibition in 19 cases with the chemiluminescence technique (15 with the guaiacol method). Anti-TPO antibodies showed competitive inhibition of TPO activity with respect to the substrate guaiacol. In both systems, the inhibition is present in the IgG F(ab′)2 fragment. We conclude that the high sensitivity of chemiluminescence detection allows routine determination of the inhibition of TPO activity by anti-TPO antibodies.

Adherence of RFD-1 positive dendritic cells to the basal surface of thyroid follicular cells in Graves' disease

HLA-DR positive cells infiltrating Graves' thyroid tissue were examined for their expression of cell-specific immunological markers using light and electron microscopic immunostaining of frozen sections and isolated open thyroid follicles. Graves' glands (n = 21) were enriched of CD68 and Leu-M5/CD11c positive monocytes/macrophages as well as CD4 and CD8 lymphocytes. These cell types were distributed juxta-follicular as well as in other tissue areas. Only the RFD-1 antibody, considered to label antigen-presenting cells including dendritic cells, identified cells invariably located close to the interstitial follicular surface. After follicle isolation, RFD-1 cells were enriched compared to Leu-M5 cells and exclusively adherent to the follicular epithelium. The plasma membrane of RFD-1 positive cells were in intimate contact with the basolateral membrane of the thyrocytes, sometimes extending deeply into the intercellular space of the epithelium. Parallel labelling experiments suggested that the follicle-adhering RFD-1 cells also expressed HLA-DR. Our findings show that in human thyroid glands with Graves' disease RFD-1 positive cells with a dendritic morphology establish direct contact with the follicular epithelium. In view of the fact that both HLA-DR and RFD-1 are associated with antigen-presenting functions it is suggested that a direct interaction of dendritic cells with thyrocytes might be an important component of the autoimmune reaction in Graves' disease.

Interference of thyroperoxidase on immuno-cytochemical determination of steroid receptors in thyroid tissue

The presence of sexual steroid receptor proteins in thyroid tissue has been previously demonstrated by biochemical means. The aim of this study was to determine the estrogen (ER) and progesterone (PR) receptors in malignant (12 papillary and 1 follicular carcinoma) and nonmalignant (19 multinodular goiters, 1 Graves' disease, 1 Hashimoto's thyroiditis) thyroid diseases using immunocytochemical assay employing monoclonal anti-ER and anti-PR antibodies and the peroxidase-antiperoxidase technique. Positive results were obtained in 24/34 (70%) for ER (ER-ICA+) and 22/34 (64%) for PR (PR-ICA+). To evaluate the possible interference of thyroperoxidase in the immunostaining, in consecutive sections of a positive specimen, primary antibody or primary antibody plus bridging antibody or PAP complex was omitted. Using these modified procedures, staining distribution was similar to that obtained by the standard procedure: in contrast, no staining was found in the positive control, i.e. a breast cancer specimen. The inhibition of the endogenous peroxidase caused a loss of staining in both the standard and modified procedures on thyroid specimens; no staining modification was obtained in the positive control. These results suggest that the staining observed in thyroid tissue is not specific and related to the activity of thyroperoxidase on chromogen solution. The complete loss of staining after peroxidase inhibition appears to be in contrast with the results obtained by biochemical method, and different antigenicity of thyroid receptors in comparison with breast receptors may explain this discrepancy.

Thyroid autoimmunity

Antigenic structure remains a major focus in thyroid immunology. The genes for three major thyroid antigens--thyroglobulin, thyroid peroxidase and the thyrotropin receptor--were sequenced in the late 1980's, and epitopes for antibody and T cells have been reported within the last year. In addition, new evidence for selective use of T-cell receptor V gene segments in human thyroid infiltrates may point the way to specific immunotherapy.