Immunocytochemical study of localization and traffic of thyroid peroxidase/microsomal antigen

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Structural and functional aspects of thyroid peroxidase

Thyroperoxidase (TPO) is the enzyme involved in thyroid hormone synthesis. Although many studies have been carried out on TPO since it was first identified as being the thyroid microsomal antigen involved in autoimmune thyroid disease, previous authors have focused more on the immunological than on the biochemical aspects of TPO during the last few years. Here, we review the latest contributions in the field of TPO research and provide a large reference list of original publications. Given this promising background, scientists and clinicians will certainly continue in the future to investigate the mechanisms whereby TPO contributes to hormone synthesis and constitutes an important autoantigen involved in autoimmune thyroid disease, and the circumstances under which the normal physiological function of this enzyme takes on a pathological role.

Intracellular Trafficking of Thyroid Peroxidase to the Cell Surface

For thyroid hormone synthesis, thyroid peroxidase (TPO) molecules must be transported from the endoplasmic reticulum via the Golgi complex to be delivered at the cell surface to catalyze iodination of secreted thyroglobulin. Like other glycoproteins, TPO molecules in transit to the cell surface have the potential to acquire endoglycosidase H resistance as a consequence of Golgi-based modification of their N-linked carbohydrates, and measurement of the intracellular distribution of TPO has often relied on this assumption. To examine TPO surface distribution in thyrocyte cell lines, we prepared new antibodies against rat TPO. Antibody reactivity was first established upon expression of recombinant rat (r) TPO in 293 cells, which were heterogeneous for surface expression as determined by flow cytometry. By cell fractionation, surface rTPO fractionated distinctly from internal pools of TPO (that co-fractionate with calnexin), yet surface TPO molecules remained endoglycosidase H (endo H)-sensitive. Although the FRTL5 (and PC Cl3) rat thyrocyte cell line also exhibits almost no endo H-resistant TPO, much of the endogenous rTPO is localized to the cell surface by immunofluorescence. Similar results were obtained by fractionation of FRTL5 cell membranes on sucrose gradients. We conclude that in FRTL5 cells, a large fraction of rTPO is delivered to the plasma membrane yet does not acquire Golgi-type processing of its N-glycans. Rat and mouse thyroid tissue TPO also shows little or no endo H resistance, although cell fractionation still needs to be optimized for these tissues.

Polarized trafficking of thyrocyte proteins in MDCK cells

Recent studies suggest striking similarities between polarized protein sorting in thyrocytes and MDCK epithelial cells, including apical trafficking of thyroglobulin (Tg), thyroid peroxidase, and aminopeptidase N; as well as basolateral targeting of heparan sulfate proteoglycans, thrombospondin 1 (TSP1), type 1 5'-deiodinase, sodium-potassium ATPase, and the thyrotropin receptor. In this report, we have firstly expressed in stably transfected MDCK II cells a range of truncation mutants lacking up to 78% of the C-terminus of TSP1; these studies indicate that the N-terminal region containing the heparin binding domain is sufficient for basolateral targeting of TSP1. Secondly, we have stably transfected MDCK II cells with both Tg and sodium-iodide symporter (NIS) cDNAs, obtaining clones that simultaneously express both thyroid-specific proteins at the apical and basolateral cell surfaces, respectively. These studies represent promising early steps towards designing artificial thyrocytes by thyroid gene transfer into MDCK cells.

Cell type-dependent differences in thyroid peroxidase cell surface expression

Recently, it has been suggested that only approximately 2% of human thyroid peroxidase (hTPO(933)) reaches the surface of stably transfected (Chinese hamster ovary) cells, most being degraded intracellularly, and this might be representative of thyroid peroxidase (TPO) behavior in thyrocytes (Fayadat, L., Siffroi-Fernandez, S., Lanet, J., and Franc, J.-L. (2000) J. Biol. Chem. 275, 15948-15954). In agreement, in stably transfected Madin-Darby canine kidney clones, nonpermeabilized cells exhibit wild-type hTPO(933) immunofluorescence (apically) on <10% of that found in permeabilized cells, where an endoplasmic reticulum pattern is observed. Further, a C-terminally truncated, membrane-anchorless hTPO(848) is also retained in the endoplasmic reticulum of stably transfected Madin-Darby canine kidney cells. However, by contrast, in Chinese hamster ovary cells after transient transfection, hTPO(933) immunofluorescence is detected equally well in nonpermeabilized and permeabilized cells, indicating that a large portion of hTPO(933) is present at the cell surface; furthermore, hTPO(848) is efficiently secreted. Further, using an antiserum not cross-reacting with rat TPO, we find by immunofluorescence that in stable clones of PC Cl3 (rat) thyrocytes, considerably more ( approximately 50%) of the cells exhibit hTPO(933) at the cell surface. However, cell surface biotinylation and endoglycosidase H digestion assays appear to under-represent the extent of hTPO(933) transport, presumably because protein folding limits both Golgi carbohydrate modification and accessibility of lysines in the extracellular domain. We conclude that cell type-specific factors may facilitate stable expression of TPO at the cell surface of thyrocytes.

Evaluation of the monoclonal antibody antithyroperoxidase MoAb47 in the diagnostic decision of cold thyroid nodules by fine-needle aspiration

Fine needle aspiration (FNA) of cold thyroid nodules has become the first line diagnostic decision for electing which patients need surgery. In order to improve FNA accuracy, the monoclonal antithyroperoxidase (TPO) antibody (MoAb47) was tested. A total of 554 patients were included in this study and among them, 208 were referred to surgery. The results of FNA compared to the final histological diagnosis revealed a sensitivity and a specificity of 94 and 55% respectively, while the sensitivity and specificity of TPO immunodetection on the same cases reached 98 and 83% respectively. By combining the two methods, the 3 false-negative of FNA and 60% of suspicious cytology corresponding to histological benign lesions were correctly identified by immunocytochemistry. With better results than FNA alone, TPO immunodetection with MoAb47 represents a useful adjunct to conventional cytology for selecting patients for surgery.

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.

Polarized distribution and delivery of plasma membrane proteins in thyroid follicular epithelial cells

Thyroid follicular cells coordinate several oppositely located surface enzyme activities. Recent studies have raised questions about the basic mechanisms used to achieve thyroid surface polarity. We investigated these mechanisms in primary thyroid epithelial monolayers cultured on porous filters. In the steady state, most Na+/K(+)-ATpase and aminopeptidase N were available for surface biotinylation, and these proteins exhibited physiological distributions (basolateral and apical, respectively). Glycosylphosphatidylinositol-anchored proteins were also apically distributed. By pulse-chase, newly synthesized transmembrane proteins exhibited polarized surface delivery that was oriented similarly to that observed at steady state. Little time elapsed between acquisition of Golgi-specific processing and cell surface arrival. Interestingly, when either newly synthesized or steady state-labeled thyroid peroxidase was similarly analyzed, only approximately 30% of the enzyme was ever detected at the cell surface. Of this, the majority was localized apically. The data suggest that most thyroid peroxidase remains intracellular in these monolayers, consistent with the possibility of intracellular iodination activity in addition to apical extracellular iodination. Nevertheless, in filter-polarized thyrocytes, most newly synthesized plasma membrane proteins appear to be sorted in the Golgi complex for direct delivery to apical and basolateral domains.

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.

Immunopurification and characterization of thyroid autoantibodies with dual specificity for thyroglobulin and thyroperoxidase

The presence of autoantibodies (aAbs) to thyroglobulin (TG) and thyroperoxidase (TPO) in most of the patients with autoimmune thyroid disease is now well documented. Studies of these aAbs suggested that some, termed TGPO aAbs, could interact with both TG and TPO. This hypothesis was investigated using IgG fraction from a pool of 25 patients' sera with high TG and TPO aAb titres. Immunopurification of TG, TPO and TGPO aAbs was carried out by sequential affinity chromatography using a large quantity of highly purified human TG and TPO. TGPO aAbs, obtained absorption-elution of affinity purified TG aAbs onto a TPO column, were found to represent about 20% of the TG reactive aAbs and 0.23% of the total amount of IgG. Purified TGPO aAbs were characterized and compared to specific TG and TPO aAbs. In contrast to TG and TPO aAbs which recognized only their target antigen, TGPO aAbs showed high affinity interactions with both TG and TPO. As compared to TG aAbs, TGPO aAbs displayed similar affinity for native TG and higher affinity for denatured TG. Compared to TPO aAbs, TGPO aAbs showed lower affinity for both native and denatured TPO. TGPO aAbs also differed from specific TG and TPO aAbs with regard to IgG subclass distribution and antigen fine specificities as determined by monoclonal antibody assisted mapping of TG and TPO surface epitopes. Taken together, these data indicate that TGPO aAbs are effectively present in the serum of patients with autoimmune thyroid disease. TGPO aAbs may be considered as a subpopulation of TG aAbs with the unique property to cross-react with TPO. The existence of aAbs cross-reacting with these functionally and antigenically related thyroid molecules could lead to a re-examination of the emergence of thyroid autoimmunity.

Immunohistochemical study of thyroid peroxidase in normal, hyperplastic, and neoplastic human thyroid tissues

An immunohistochemical study using two monoclonal antibodies (MoAb 30 and MoAb 47) against thyroid peroxidase (TPO) was performed on surgical specimens of human thyroid carcinoma (n = 65), adenoma (n = 70) and Graves' disease (n = 10). Normal adjacent thyroid tissue was used as positive control. Monoclonal antibody 30 reacted significantly with all adenoma and most carcinoma, whereas MoAb 47 reacted with 66 adenoma but only two carcinoma. Of the four adenomas that did not react with MoAb 47, three were of the fetal type. Both carcinoma reacting with MoAb 47 were of the well-differentiated follicular type. These findings further confirm the hypothesis that thyroid carcinoma is associated with changes in the quantity and antigenic properties of TPO. Although the alterations in antigenic behavior revealed by MoAb 47 are not 100% specific, they may allow more accurate diagnosis of malignancy in thyroid tumors.

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

Evidence has accumulated in the last few years that the expression of the microsomal/peroxidase antigen (M/TPO-Ag) in thyroid cells is induced by TSH, through pathways which involve intracellular cAMP accumulation and protein synthesis. These data have been found true in any thyroid system studied so far, both in terms of immunologic and enzymatic activity of TPO. TSH and cAMP also increase the levels of the specific mRNA for TPO in thyroid cells from different species. Whether this phenomenon is due to a direct transcriptional regulation of the TPO gene, as shown in dog thyroid cells, or to posttranscriptional effects, as it would appear in FRTL-5 cells, remains to be clarified by future experiments. Thyroid stimulating antibody (TSAb) of Graves' disease also stimulates the expression of M/TPO-Ag. This finding gives further support to the relevance of TSAb in the pathogenesis of hyperthyroidism and explains the well known observation that the "microsomal" antigen is particularly abundant in glands of Graves' patients. The modulation of M/TPO-Ag surface expression by TSH can explain the decrease of circulating anti-MAb observed during L-thyroxine therapy in hypothyroid patients with Hashimoto's thyroiditis. Other agents, such as methimazole and sodium iodide, which influence thyroid cell function, do not directly interfere with the expression of M/TPO-Ag. Cytokines, such as gamma-interferon, interleukin-1, and interleukin-6 have been shown to inhibit the TSH-induced increase of TPO mRNA, but further investigations are required to elucidate the exact role of cytokines in the regulation of M/TPO-Ag expression.