In accordance with the available data most authors conclude that epidermal growth factor (EGF) has very little or no effect on FRTL-5 cells. This has been viewed as a serious handicap of this cell line. In the present study we cultivated three strains of FRTL-5 cells from different sources and assessed their response to EGF with regard to proliferation, function and differentiation. Cell proliferation was assessed by counting in a Coulter cell counter after culturing cells at suboptimal conditions in well plates. Cell function was studied by measuring iodide uptake. Cell differentiation was examined immunocytochemically by staining monolayer cultures for thyroglobulin (Tg) and EGF receptor (EGFr) as well as morphologically by microscopical evaluation of monolayer cultures. All three FRTL-5 cell lines investigated express EGFr. In two wild type FRTL-5 cell lines EGF stimulates growth, an effect that is enhanced by the presence of TSH, and partially inhibits iodide uptake. A third mutated strain of FRTL-5 cells does not respond to EGF. Tg expression can be demonstrated immunocytochemically in EGF-treated cells as well as in controls. Morphologically, in monolayer culture EGF-treated cells cannot be distinguished from controls. Contrary to previous reports, these studies demonstrate EGF effects on FRTL-5 cells that are consistent with EGF effects established in other thyroid follicular cells.

While the multifunctional proteins of the transforming growth factor-beta (TGF-beta) family have a potent antiproliferative effect on thyroid follicular cell growth, increased expression of TGF-beta in proliferating thyroid cells and in thyroid tumours has recently been described, suggesting a secondary counter-regulatory role of these proteins. We have studied further this apparent paradox in vitro using FRTL-5 cells, 5 continuous cell strains from feline multinodular goitres (MNG) and 29 primary cultures prepared from human MNG. While dose dependent inhibition of FRTL-5 cell growth was confirmed, a variable fraction of these cells was naturally resistant towards TGF-beta 1, thus explaining the large interassay variability of growth inhibition (36 to 98% within 2 days, n = 19). After 40 days of continuous exposure, FRTL-5 cells became fully refractory towards TGF-beta 1 inhibition, due to the selective growth of naturally resistant subclones, as demonstrated for example by microscopic observation of three-dimensionally growing collagen-embedded cell clusters. The refractoriness could still be demonstrated even after several cell passages. In addition, 2 out of 5 feline thyroid cell strains obtained from feline MNG and 18 out of 29 primary cultures from human MNG showed a high degree of refractoriness towards TGF-beta. We conclude that constitutively TGF-beta resistant cells may occur in thyroid glands and that persistent TGF-beta refractoriness may secondarily be acquired. Resistant cells may escape regular growth control mechanisms and hence may contribute to the notorious heterogeneity of thyroid growth and to nodular transformation.

Extracellular matrix (ECM) and basement membrane (BM) components were studied by immunohistological methods in native rat thyroid tissue, and in rat thyroid tissue and FRTL-5 cells cultured in a three-dimensional alginate bead system. In all three situations, the presence of collagen IV, laminin, perlecan, and fibronectin was demonstrated. There were marked differences between rat thyroid tissue and FRTL-5 cells in culture. Rat thyroid tissue maintained a follicular structure, whereas FRTL-5 cells did not form follicles. Rat thyroid cells multiplied more slowly than FRTL-5 cells and thyroglobulin (Tg) was visible in the follicular lumen, while in FRTL-5 cells Tg was only seen intracellularly. Tg iodination was much lower in FRTL-5 cells than in rat cells. In rat thyroid cells, positive staining for collagen IV, laminin, and perlecan was seen in thin membranes around individual follicles, and for fibronectin around groups of follicles. In FRTL-5 cells, these ECM/BM components could be identified, but were not organized into equally regular networks around groups of cells. These results demonstrate that of the two types of cells examined, primary cultures of rat thyroid cells in alginate beads maintain structural and functional similarities to native thyroid tissue and would therefore be suitable for future in vitro studies of thyroidal ECM/BM and their interrelationship with growth and function of this organ. FRTL-5 cells cultured in alginate beads show some functional, but not structural similarities to native thyroid tissue and so would be less valuable for use in such studies.

The extracellular matrix (ECM) and basement membranes (BM, a specialized form of ECM) greatly influence proliferation, differentiation, and function of cells and the structure of tissues. While a considerable amount of information is available on thyroid cellular proliferation, differentiation and function, much less is known about thyroid ECM and BM. In this study the presence of the ECM/BM components fibronectin, collagen IV, alpha1, beta1, gamma1 laminin, several laminin variants, osteonectin, and perlecan was demonstrated in cryosections of nonadenomatous and toxic adenoma human thyroid tissue. Also, positive immunohistochemical staining for collagen IV, laminin, perlecan, and fibronectin was obtained in sections of human thyroid tissue cultured in a three-dimensional (alginate) culture system. The present study provides methods and data that will facilitate the investigation of the interaction between cells and ECM in thyroid tissue.

One of the earliest responses of the thyroid cells to TSH is macropinocytosis with formation of intracellular colloid droplets. We demonstrate here that increasing stimulation with TSH not only elicits a highly individual macropinocytotic response among different follicular cells but that the fraction of TSH-responsive cells is also a function of the TSH dose. After pretreatment with T4, mice and rats were injected ip with bovine TSH and killed 2 h later. The macropinocytotic response to TSH was evaluated on periodic acid-Schiff-stained 3-microns sections of the thyroids in terms of droplet number per 25 follicles and, in addition, by assessing recruitment, i.e. percentage of droplet-containing cells. Both variables increased with increasing TSH stimulation until they reached a plateau at about 9 mU TSH in mice and at about 300 mU TSH in rats: the percentage of droplet-containing cells gradually increased in mice from 2% (no TSH) to 67% (9 mU TSH) and in rats from 11% (no TSH) to 54% (300 mU TSH). Overall pinocytotic response as well as thyrocyte recruitment could be modified by extra- and intrathyroidal factors: for example, pretreatment of the mice with an iodine-deficient diet increased the maximal percentage of droplet containing cells to nearly 90%. Obviously, two separate components of the macropinocytotic response of the thyroid gland to TSH can be distinguished: the first is the gradually increasing fraction of droplet-containing cells, the second is the well known increase of the number of colloid droplets in each TSH-responsive cell with progressive TSH stimulation. Recruitment of thyrocytes with a gradually increasing natural threshold to a hormonal stimulus appears to be a fundamental mechanism in the thyroid gland and possibly in other organs.