Acid protease activity in thyroid gland from patients with Graves' disease

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

Identification of New Rare Variants Associated With Familial Autoimmune Thyroid Diseases by Deep Sequencing of Linked Loci

CONTEXT

Genetic risk factors play a major role in the pathoetiology of autoimmune thyroid diseases (AITD). So far, only common risk variants have been identified in AITD susceptibility genes. Recently, rare genetic variants have emerged as important contributors to complex diseases, and we hypothesized that rare variants play a key role in the genetic susceptibility to AITD.

OBJECTIVE

We aimed to identify new rare variants that are associated with familial AITD.

METHODS

We performed deep sequencing of 3 previously mapped AITD-linked loci (10q, 12q, and 14q) in a dataset of 34 families in which AITD clustered (familial AITD).

RESULTS

We identified 13 rare variants, located in the inositol polyphosphate multikinase (IPMK) gene, that were associated with AITD (ie, both Graves' disease [GD] and Hashimoto's thyroiditis [HT]); 2 rare variants, within the dihydrolipoamide S-succinyltransferase (DLST) and zinc-finger FYVE domain-containing protein (ZFYVE1) genes, that were associated with GD only; and 3 rare variants, within the phosphoglycerate mutase 1 pseudogene 5 (PGAM1P5), LOC105369879, and methionine aminopeptidase 2 (METAP2) genes, that were associated with HT only.

CONCLUSION

Our study demonstrates that, in addition to common variants, rare variants also contribute to the genetic susceptibility to AITD. We identified new rare variants in 6 AITD susceptibility genes that predispose to familial AITD. Of these, 3 genes, IPMK, ZFYVE1, and METAP2, are mechanistically involved in immune pathways and have been previously shown to be associated with autoimmunity. These genes predispose to thyroid autoimmunity and may serve as potential therapeutic targets in the future.

Interferon-α Triggers Autoimmune Thyroid Diseases via Lysosomal-Dependent Degradation of Thyroglobulin

CONTEXT

Autoimmune thyroid diseases (AITDs) arise from complex interactions among genetic, epigenetic, and environmental factors. Thyroglobulin (TG) is a major susceptibility gene for both Graves disease and Hashimoto thyroiditis. Interferon-α (IFNα), a cytokine secreted during viral infections, has emerged as a key trigger of AITD. We have shown that IFNα upregulates TG transcription; however, how the upregulation of TG transcription by IFNα triggers AITD is still unknown.

OBJECTIVE

To evaluate how IFNα triggers AITD by testing its effects on TG processing.

DESIGN

We exposed human thyroid cells to IFNα and evaluated its effects on TG expression and processing.

RESULTS

Human thyroid cells exposed to INFα had increased levels of TG mRNA but reduced TG protein levels, indicating TG protein degradation. IFNα induced endoplasmic reticulum stress, but surprisingly, neither the use of chemical chaperones nor proteasome inhibitor prevented IFNα-induced TG degradation. IFNα also increased LysoTracker staining and autophagy flux measured by net light chain 3 (LC3)-II and p62 fluxes. In addition, expression of autophagy markers LC3 and autophagy-related gene 5 was higher in thyroid tissues from patients with AITD. Finally, blocking lysosomal degradation prevented IFNα-induced degradation of TG.

CONCLUSION

We have shown in this study IFNα-induced lysosomal-dependent degradation of TG in human thyroid cells. Our findings suggest that during viral infections, local thyroidal IFNα production can lead to lysosomal TG degradation, releasing pathogenic TG peptides that can trigger AITD.

Cathepsin D in normal and neoplastic thyroid tissues

BACKGROUND

Cathepsin D is a widely distributed lysosomal acidic endopeptidase. It is an estrogen-regulated protein that is a prognostic factor in breast cancer. The aim of this study was to measure cathepsin D concentrations in thyroid tissues and to correlate these concentrations with clinical and pathologic parameters.

METHODS

Cathepsin D and thyroglobulin concentrations were measured in the cytosol of normal thyroid tissues (n = 14), benign nodules (n = 6), and thyroid carcinomas (n = 32) with an immunoradiometric assay. Statistical analysis was based on the Kruskal-Wallis and Wilcoxon tests and on the Spearman rank correlation coefficient.

RESULTS

The mean level of cathepsin D, expressed as picomoles per milligram protein minus thyroglobulin, was higher in the 32 carcinomas, 29.1 +/- 15.5, than in the 14 normal thyroid tissues, 8.4 +/- 2.5 (p < 0.001) or in the 6 benign nodules, 11.2 +/- 7.3 (p = 0.003). Cathepsin D concentrations correlated with tumor size; Spearman rank correlation coefficient was rs = 0.44 (p = 0.012). No significant difference was found regarding histologic type. Cathepsin D concentrations were inversely correlated with the thyroglobulin level in the tumor; Spearman rank correlation coefficient was rs = -0.60 (p < 0.001).

CONCLUSIONS

Cathepsin D concentration is higher in thyroid carcinoma than in normal thyroid tissue. Increased cathepsin D concentrations correlate with thyroid tumor size but not with histologic type. Further studies should be done to confirm the potential prognostic value of cathepsin D in patients with thyroid carcinomas.

Estrogen receptors and cathepsin D in human thyroid tissue

BACKGROUND

To investigate the significance of estrogen receptors (ER) in the pathogenesis of thyroid dysplasia, the authors analyzed, by analogy with breast cancers, ER and three estrogen-regulated proteins: progesterone receptor (PR), cathepsin D, and pS2 protein, in cytosols of 42 human thyroid tissues.

METHODS

ER and PR were measured by an immunoenzymatic assay and cathepsin D and pS2 by an immunoradiometric assay. Tissue specimens included 7 normal tissues, 6 benign nodules, 8 toxic adenomas, 7 from patients with Graves disease, and 14 carcinomas.

RESULTS

ER was present at very low concentrations, with no statistical difference between neoplastic and nonneoplastic tissues. The mean levels of cathepsin D, expressed as pmol/mg protein minus thyroglobulin, were higher in the 14 carcinomas (P = 0.0003), the 7 specimens from patients with Graves disease (P = 0.006), and the 8 toxic adenomas (P = 0.04) than in the 7 normal thyroid tissues. A significant difference also was observed between the carcinomas (P = 0.003) and six benign nodules. Compared to TNM parameters, cathepsin D concentrations correlated with tumor size: higher cathepsin D levels were found in pT4 than in pT2 and pT3 carcinomas. All the tissues tested were negative for PR and pS2 protein.

CONCLUSIONS

The results clearly indicate a significant difference between neoplastic and normal thyroid tissue in terms of the amount of cathepsin D, but not that of ER. This suggests that cathepsin D probably is not regulated by estrogen but simply is a marker of protease activity during invasion by thyroid carcinomas.

Enhanced acid protease activity of lysosomes from papillary thyroid carcinoma

In vitro lysosomal acid protease activity was studied in human papillary thyroid carcinoma (n = 13). As a control, morphologically normal thyroid tissue from the same patient was used in each individual case of carcinoma. Although a marked variation may be observed between individual cases, each examined papillary thyroid carcinoma showed significantly greater activity of acid proteases, both per unit weight of wet thyroid tissue and per unit of lysosomal proteins, in comparison to the corresponding control (range, 24%-248%). In conclusion, it is suggested that enhanced proteolytic activity of lysosomal acid proteases in papillary carcinoma is probably a result of disturbance in catabolic degradation of the thyroglobulin molecule in malignantly transformed thyroid tissue.

Acid thyroglobulin protease activities in human diseased thyroid glands

The lysosomal acid protease is thought to be a main enzyme in hydrolysis of thyroglobulin. The protease activity of lysosome-rich fractions of various diseased thyroid glands were assayed by the sensitive and reproducible method developed in our laboratory. They included 78 diseased thyroid glands; 37 thyrotoxic goitres, five irradiated thyroids, five Hashimoto's thyroiditis, one granulomatous thyroiditis, six multinodular goitres, 14 'cold' adenomas, four 'hot' adenomas with three paranodular tissues, and six carcinomas. Fifteen paranodular tissues of cold and solitary thyroid nodules were served as controls. The protease activities were significantly increased in thyroid tissues of thyrotoxic goitres, Hashimoto's thyroiditis, adenomas and adenocarcinomas. In thyrotoxic goitres, the protease activity correlated well with the mean area size of colloid follicles. The 'hot' adenomas had higher protease activity than 'cold' ones. The protease activity of multinodular goitres was nearly normal. Only the irradiated thyroid glands had low protease activities. Therefore the high protease activities in hyperfunctioning goitres seem to reflect hormone secretory activities. On the other hand, cold neoplastic goitres seem to have lysosomal protease activities which are not associated with thyroidal secretory process. The assay of acid protease activity will be useful to understand the pathophysiology of lysosomal system in diseased thyroid glands.