Effects of forskolin on the morphology and function of the rat thyroid cell strain, FRTL-5: comparison with the effects of thyrotrophin

Regulation of thyroid cell proliferation by TSH and other factors: a critical evaluation of in vitro models

TSH via cAMP, and various growth factors, in cooperation with insulin or IGF-I stimulate cell cycle progression and proliferation in various thyrocyte culture systems, including rat thyroid cell lines (FRTL-5, WRT, PC Cl3) and primary cultures of rat, dog, sheep and human thyroid. The available data on cell signaling cascades, cell cycle kinetics, and cell cycle-regulatory proteins are thoroughly and critically reviewed in these experimental systems. In most FRTL-5 cells, TSH (cAMP) merely acts as a priming/competence factor amplifying PI3K and MAPK pathway activation and DNA synthesis elicited by insulin/IGF-I. In WRT cells, TSH and insulin/IGF-I can independently activate Ras and PI3K pathways and DNA synthesis. In dog thyroid primary cultures, TSH (cAMP) does not activate Ras and PI3K, and cAMP must be continuously elevated by TSH to directly control the progression through G(1) phase. This effect is exerted, at least in part, via the cAMP-dependent activation of the required cyclin D3, itself synthesized in response to insulin/IGF-I. This and other discrepancies show that the mechanistic logics of cell cycle stimulation by cAMP profoundly diverge in these different in vitro models of the same cell. Therefore, although these different thyrocyte systems constitute interesting models of the wide diversity of possible mechanisms of cAMP-dependent proliferation in various cell types, extrapolation of in vitro mechanistic data to TSH-dependent goitrogenesis in man can only be accepted in the cases where independent validation is provided.

Thyrocyte integration, and thyroid folliculogenesis and tissue regeneration: perspective for thyroid tissue engineering

The thyroid gland is composed of many ball-like structures called thyroid follicles, which are supported by the interfollicular extracellular matrix (ECM) and a capillary network. The component thyrocytes are highly integrated in their specific structural and functional polarization. In conventional monolayer and floating culture systems, thyrocytes cannot organize themselves into follicles with normal polarity. In contrast, in 3-D collagen gel culture, thyrocytes easily form stable follicles with physiological polarity. Integration of thyrocyte growth and differentiation results ultimately in thyroid folliculogenesis. This culture method and subacute thyroiditis are two promising models for addressing mechanisms of folliculogenesis, because thyroid-follicle formation actively occurs both in the culture system and at the regenerative phase of the disorder. The understanding of the mechanistic basis of folliculogenesis is prerequisite for generation of artificial thyroid tissue, which would enable a more physiological strategy to the treatment of hypothyroidism caused by various diseases and surgical processes than conventional hormone replacement therapy. We review here thyrocyte integration, and thyroid folliculogenesis and tissue regeneration. We also briefly discuss a perspective for thyroid tissue regeneration and engineering.

Protein tyrosine phosphatase-eta expression is upregulated by the PKA-dependent and is downregulated by the PKC-dependent pathways in thyroid cells

We have recently reported the isolation of a rat cDNA encoding a receptor-type tyrosine phosphatase, which appears to be a marker of thyroid differentiation. To elucidate the molecular mechanisms underlying r-PTPeta expression in normal thyroid cells both in vitro and in vivo, we investigated the regulation of r-PTPeta expression in cultured thyrocytes (the rat cell line PC Cl 3) and in an animal model of TSH-dependent thyroid goitrogenesis. In vitro studies showed that mRNA expression of r-PTPeta in thyroid cells is induced in a time- and dose-dependent manner by the activation of growth- and differentiation-linked PKA pathways (TSH and forskolin), whereas it is down-regulated by the activation of the proliferative dedifferentiating PKC-dependent transduction pathway (TPA). However, the regulation of r-PTPeta expression by TSH and TPA, respectively, is observed only in normal thyroid cells, but is lost in transformed thyroid cells. In vivo studies with thiouracil-fed rats demonstrated that increased serum levels of TSH up-regulated r-PTPeta mRNA expression in parallel with the stimulation of thyroid growth and function. The reduction of blood TSH levels due to iodide refeeding to goitrous rats determined a marked down-regulation of r-PTPeta expression, in parallel with involution of thyroid hyperplasia. Taken together these results demonstrate that the phosphatase r-PTPeta is regulated by the two main thyroid regulatory pathways and suggest that it may play an important role in the growth and differentiation of thyroid cells.

Phagocytosis of fluorescent beads by rat thyroid follicular cells (FRTL-5): comparison with iodide trapping as an index of functional activity of thyrocytes in vitro

The ability of FRTL-5 rat thyroid follicular cells to engulf latex beads by phagocytosis was evaluated using flow cytometry and compared to iodide trapping in response to selected growth factors, second messengers, and chemicals. Cell suspensions were analyzed to determine the percentage of fluorescence-positive cells as well as the fluorescence intensity of positive cells. Phagocytosis was stimulated by forskolin, cholera toxin, 8-Br-cAMP, calcitriol, and transforming growth factor-beta. In contrast, phagocytosis was inhibited by insulin, calcium, and aminotriazole, but not by sodium iodide. The results of this study showed that phagocytosis of latex beads was regulated in a manner similar to iodide trapping and could be altered by the addition of numerous compounds. Phagocytic activity was stimulated by both cAMP-dependent and cAMP-independent pathways. Flow cytometric evaluation of phagocytosis of fluorescent latex beads represents a simple, rapid, nonradioactive index of thyroid function in vitro.

Impairment of the TSH signal transduction system in human thyroid carcinoma cells

In order to further evaluate the role of TSH in the proliferation and the differentiation of human thyroid carcinoma cells, we have analyzed the function of the TSH receptor in the established thyroid carcinoma cell lines NPA and WRO. The TSH signal transduction system in the carcinoma cells was also compared with that in normal thyroid cells. Although unresponsiveness to bovine and human TSH was demonstrated by measurement of cAMP production and [3H]thymidine incorporation after treatment of TSH, cAMP production was induced after stimulation of these cells by forskolin, cholera toxin, and isoproterenol. Specific binding to 125I-TSH was demonstrated in both NPA and WRO cells in addition to the existence of a TSH receptor mRNA and thyroglobulin mRNA species, although thyroid-specific gene expression in these cells was not regulated by TSH. These findings suggest that the unresponsiveness to TSH in these cells may be due to an abnormality of TSH receptor-G protein coupling rather than to a decreased level of TSH-receptor expression or a Gs protein abnormality.

The regulation and integration of thyroid follicular differentiation and function

As in a number of other endocrine tissues in which expression of differentiated function is dependent upon a stable cellular architecture, the control of differentiation of the thyroid follicle requires a coordination between the ultrastructural and morphological responses to a number of endocrine growth factors, which may ultimately involve autocrine or paracrine-mediated effects within the immediate follicular microenvironment. Through analogy with other cell types, expression of appropriate differentiation characteristics within the thyroid follicle may involve the activation of specific c-oncogenes within each of the component cells. With the recent development and application of oncogene transfection technology, application of such procedures to the thyroid follicular cell should prove to be a particularly fruitful area for future research within the thyroid gland, leading to elucidation of the mechanisms whereby the cellular responses to growth and tissue-differentiating factors are mediated. Clearly however, further consideration must also be made of the roles played in maintaining follicular stability by both physical and chemical interactions between the component cells and the immediate extracellular environment. The contributory roles played by basement membrane and cell-surface components in cellular recognition, together with the physical effects imposed upon the apical surfaces of the follicle by luminal thyroglobulin have already been identified as fundamental factors in this respect. It is also readily apparent that morphological differentiation of the follicle bears critically upon the chemical characteristics of the microenvironment through the ability of the latter to promote expression of specific apical/basal recognition characteristics of the component cells, and thus maintain the unidirectional polarity upon which the functional capacity of the thyroid follicle is so critically-dependent.