Autocrine growth factors produced in the thyroid

Thyroid cancer and the immune system: a model for effective immune surveillance

Differentiated thyroid cancers, including papillary and follicular variants, are a useful model with which to examine interactions between cancer and the immune system. Differentiated thyroid cancers are detected in only 20,000 individuals annually in the USA, but thyroid microcarcinomas (< 1 cm in diameter) are far more common. This suggests that the immune system might restrain the growth of these microcarcinomas. On the clinical level, patients with lymphocytes that infiltrate into papillary thyroid cancer have improved survival, supporting the notion that immune system activation might improve this. Together, these observations suggest that the growth and distant spread of thyroid carcinoma are suppressed by mechanisms of immune surveillance, possibly involving lymphocytes, macrophages and their secreted products. In this review, we examine the general hypothesis of immune surveillance and the data pertaining to the roles of lymphocytes, dendritic cells and cytokines in the immune response against thyroid cancers.

Expression of receptor-type tyrosine kinase, Axl, and its ligand, Gas6, in pediatric thyroid carcinomas around chernobyl

Protein-tyrosine kinases (PTKs) play a crucial role in regulating thyroid cell growth and differentiation. It is known that PTKs are activated by ionizing radiation, but the critical pathways through ionizing radiation-induced malignant transformation are not well defined. The Axl protein is a member of a new family of receptor tyrosine kinases, which the ligand, Gas6, a protein S-related molecule, has recently been identified. We previously clarified the expression of Axl and mitogenic action of Gas6 in thyroid carcinoma cells. To investigate the involvement of Gas6-Axl signaling in pediatric thyroid carcinoma further, we examined tissues obtained from 17 patients with thyroid papillary carcinomas from Gomel, Belarus, by immunohistochemistry and in situ hybridization. Axl and Gas6 were overexpressed in 76.5% and 70.6 % of cases, respectively. Solid component and invasive front tended to overexppress Gas6 and Axl in papillary carcinomas. Eighty-five percent of Axl positive cases coexpressed Gas6. In situ hybridization also confirmed the presence of axl mRNA in thyroid carcinoma tissues. These findings suggest that Axl and Gas6 expression might be involved in childhood thyroid tumorigenesis around Chernobyl.

Natriuretic peptides increase cAMP production in human thyrocytes via the natriuretic peptide clearance receptor (NPR-C)

The relationship between natriuretic peptides and adenylyl cyclase/cAMP signal transduction has generally been shown to be an inhibitory one, mediated via the NPR-C receptor coupled to adenylyl cyclase by inhibitory G proteins (Gi). In the present studies, we have investigated the modulation of cAMP by natriuretic peptides in a long-term culture of human thyroid cells. Competition of [125I] rat ANF binding to human thyrocytes (HTU-5) by rat ANF (99-126) and by the NPR-C-specific analog C-ANF (4-23) indicated that greater than 97% of the ANF binding sites on HTU-5 cells are of the NPR-C type. However, rather than inhibiting intracellular cAMP in these cells, ANF increased maximal cAMP to 200-300% of control value. The ANF-induced increase in cAMP was duplicated by C-ANF (4-23). Basal cAMP content was reduced, and the response to ANF was abolished when the cells were grown in low (0.5%) serum without the addition of pituitary and hypothalamic extracts. CNP-22 also increased cAMP above control in HTU-5 cells identically to ANF. Neither ANF nor C-ANF (4-23) had any effect on cAMP in a culture of rat aortic smooth muscle cells. These results provide the first evidence for a positive effect of natriuretic peptides on cAMP mediated through the NPR-C, suggesting the possibility of an alternative mode of signaling by this receptor subtype.

Expression of the Axl receptor tyrosine kinase in human thyroid carcinoma

Protein tyrosine kinases (PTKs) play a crucial role in regulating thyroid cell growth and differentiation. The Axl protein is a member of a new family of receptor tyrosine kinases, of which the ligand, Gas6, a protein S-related molecule, recently was proved to be a mitogenic factor for human thyroid cells. To further investigate the involvement of Axl in human thyroid carcinoma, we examined tissues obtained from 81 patients with thyroid carcinomas, 18 with adenomas, and 13 with adenomatous goiters by immunohistochemistry and in situ hybridization. In addition, among the thyroid carcinomas, we compared Axl expression levels with the grade of differentiation and lymph node metastasis of the carcinoma. Axl was expressed faintly in adenomatous goiter and adenomas, but not in normal thyroid tissues. Among the 81 cases of thyroid carcinoma, 70 (86.4%) showed a positive staining for the Axl protein. Immunopositive (+ +) was detected in papillary carcinomas and anaplastic carcinomas. The level of Axl expression, however, had no correlation with the presence of lymph node metastasis in thyroid carcinomas. In situ hybridization also confirmed the presence of axl mRNA in thyroid carcinoma tissues. These findings suggest that Axl expression may be closely involved in human thyroid tumorigenesis.

Regulation of transforming growth factor beta 1 messenger ribonucleic acid expression in porcine thyroid follicles in vitro by growth factors, iodine, or delta-iodolactone

Transforming growth factor beta 1 (TGF beta 1) is an autocrine growth factor for thyrocytes and is supposed to be the mediator of iodine-induced growth inhibition of thyroid epithelial cells, but this is still controversial. We further investigated this hypothesis using intact porcine thyroid follicles ex vivo in a three-dimensional culture system. In this culture system it has been shown previously that both iodide as well as delta-iodolactone, the putative iodocompound mediating thyroid cell proliferation, inhibit growth of these follicles. We measured the amount of TGF beta 1 mRNA expression in these follicles after treatment either with thyrotropin (TSH), epidermal growth factor (EGF), or transforming growth factor alpha (TGF alpha) for growth stimulation or with inorganic iodine or delta-iodolactone in concentrations known to inhibit growth. TGF beta 1-mRNA was detected by Northern blot analysis. The known major transcript of 2.5 kb was detected in a steady state level up to 48 hours in untreated thyroid follicles. EGF and TGF alpha (5 ng/mL each) enhanced TGF beta 1 mRNA about threefold within 4 and 8 hours. This increase of TGF beta 1 mRNA was slightly decreased by simultaneous incubation with delta-iodolactone (1 microM) or iodide (40 microM KI). In contrast, both TSH (1 mU/mL) and forskolin (16 microM) decreased TGF beta 1 mRNA expression to about 70%, and this effect was abolished when follicles were pretreated with iodide (40 microM KI) in a concentration known to inhibit TSH action on cyclic adenosine monophosphate (cAMP) formation and proliferation. Iodide or delta-iodolactone alone had no significant effect on basal TGF beta 1 mRNA expression. We conclude that the growth inhibitory effect of iodide as well as of delta-iodolactone is not mediated through TGF beta 1 in intact porcine thyroid follicles ex vivo. The stimulatory effect of EGF and TGF alpha on TGF beta 1 expression might be related to extracellular matrix modulation during proliferation.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.