Epidermal growth factor inhibits thyrotropin-mediated synthesis of tissue-specific proteins in cultured ovine thyroid cells

Nuclear localization of epidermal growth factor and epidermal growth factor receptors in human thyroid tissues

Epidermal growth factor (EGF) has widespread growth effects, and in some tissues proliferation is associated with the nuclear localization of EGF and epidermal growth factor receptor (EGFR). In the thyroid, EGF promotes growth but differs from thyrotropin (TSH) in inhibiting rather than stimulating functional parameters. We have therefore studied the occurrence and cellular distribution of EGF and EGFR in normal thyroid, in Graves' disease, where growth is mediated through the thyrotropin receptor (TSHR), and in a variety of human thyroid tumors. In the normal gland the staining was variable, but largely cytoplasmic, for both EGF and EGFR. In Graves' disease there was strong cytoplasmic staining for both EGF and EGFR, with frequent positive nuclei. Nuclear positivity for EGF and particularly for EGFR was also a feature of both follicular adenomas and follicular carcinomas. Interestingly, nuclear staining was almost absent in papillary carcinomas. These findings document for the first time the presence of nuclear EGF and EGFR in thyroid. Their predominant occurrence in tissues with increased growth (Graves' disease, follicular adenoma, and carcinoma) may indicate that nuclear EGF and EGFR play a role in growth regulation in these conditions. The absence of nuclear EGF and EGFR in papillary carcinomas would suggest that the role played by EGF in growth control differs between papillary carcinoma and follicular adenomas/carcinomas of the thyroid.

Regulation of myo-inositol transport during the growth and differentiation of thyrocytes: a link with thyroid-stimulating hormone-induced phospholipase A2 activity

The Vmax of myo-inositol transport increased 3-fold during epidermal growth factor (EGF)-induced growth and thyroid-stimulating hormone. (TSH)-induced differentiation in primary cultures of sheep and human thyrocytes. The Km remained unaltered. This up-regulation required the presence of insulin. The TSH-induced rise in myo-inositol transport commenced 8 to 16 h after the initial stimulus and achieved a plateau at 24 h. In human thyrocytes the change in Vmax was accompanied by an increase in the steady-state levels of mRNA for the myo-inositol transporter following treatment with either ligand. Examination of the metabolites of myo-inositol showed few significant changes after treatment of sheep thyrocytes with EGF for 24 h. This is consistent with maintenance of the intracellular concentration of myo-inositol as the cells enlarge in preparation for cell division. In TSH-treated cells, however, up-regulation of myo-inositol transport was linked with increased myo-inositol cycling across the cell membrane, increased phospholipase A2-mediated turnover of phosphatidylinositol and a concomitant increase in arachidonic acid turnover. Increased levels of myo-inositol phosphates were also noted 24 h after TSH treatment. These results indicate the initiation of secondary signalling events many hours after the primary stimulus.

Thyrotropin modulation of epidermal growth factor (EGF) binding to receptors on cultured thyroid cells

Previous studies had shown that epidermal growth factor (EGF) will stimulate growth of cultured thyroid cells in vitro, and TSH will stimulate total assayable EGF receptor in cultured porcine thyroid cells. In this study, we report the effect of TSH on EGF binding to human thyroid cells. Addition of bTSH (1 mU/mL) in binding buffer during receptor assay stimulated specific EGF binding to cells, with an increase of 44% observed over the control after 1 h incubation at 37 degrees C. Affinity crosslinking of the [125I]EGF-receptor complex showed a single labeled band with molecular size of 170 kD. No additional band was detected in the presence of TSH. Preincubation of cells with chloroquine, which inhibits lysosomal degradative enzyme activity, caused a continuous accumulation of bound EGF over a 4 h study period at 37 degrees C, and TSH stimulated an increase in internalized EGF. In the presence of chloroquine, total specific bound EGF was linearly correlated to incubation time up to 4 h and can be expressed as Bound = slope*time+intercept (time0) Addition of TSH during the binding assay significantly increased the value of the slope when compared to control (p < 0.002). The rate at which prebound [125I]EGF was released into medium was not affected by the presence of TSH, indicating that TSH-enhanced binding may not be attributed to a reduction in EGF degradation. Coincubation of thyroid cells with EGF at 0 and 1 ng/mL and increasing concentrations of TSH (0-10 mU/mL) indicated that EGF stimulated thymidine incorporation, although TSH failed to synergistically enhance EGF-stimulated cell growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of protein synthesis by thyrotropin and epidermal growth factor in human thyrocytes: role of morphological changes

The effect of thyrotropin (TSH) and epidermal growth factor (EGF) on the synthesis of proteins has been studied using two-dimensional gel electrophoresis in primary cultures of thyroid cells developing as a monolayer or that remained associated as dense aggregates. (1) A 4-day treatment of monolayer cells by TSH or dibutyryl cAMP enhanced the synthesis of 26 proteins and decreased that of 19 others. (2) The synthesis of 29 proteins was similarly modified by TSH and dibutyryl cAMP in both types of culture organizations. Both agents stimulated the synthesis of thyroperoxidase and of proliferating cell nuclear antigen (PCNA)/cyclin and decreased that of actin and of a high Mr isoform of tropomyosin. (3) TSH induced the retraction of monolayer cells. Its effect on the synthesis of many proteins was mimicked by culturing unstimulated cells as dense aggregates instead of monolayers which similarly affected cell morphology. (4) EGF alone had no effect on protein synthesis in monolayer cells but it inhibited both the morphological changes induced by TSH and dibutyryl cAMP and the effect of these agents on the synthesis of 23 proteins including thyroperoxidase.

IN CONCLUSION

(1) TSH and cAMP induce both proliferation and the expression of differentiation in thyroid cells while EGF has a small mitogenic effect but a marked inhibitory action on differentiation expression; (2) many TSH (cAMP) and EGF effects on the pattern of protein synthesis might be related to morphological changes; (3) the expression of the differentiation marker thyroperoxidase and of the mitogenic marker PCNA/cyclin appears independent of cell configuration and morphology.

Differentiation expression during proliferative activity induced through different pathways: in situ hybridization study of thyroglobulin gene expression in thyroid epithelial cells

In canine thyrocytes in primary culture, our previous studies have identified three mitogenic agents and pathways: thyrotropin (TSH) acting through cyclic AMP (cAMP), EGF and its receptor tyrosine protein kinase, and the phorbol esters that stimulate protein kinase C. TSH enhances, while EGF and phorbol esters inhibit, the expression of differentiation. Given that growth and differentiation expression are often considered as mutually exclusive activities of the cells, it was conceivable that the differentiating action of TSH was restricted to noncycling (Go) cells, while the inhibition of the differentiation expression by EGF and phorbol esters only concerned proliferating cells. Therefore, the capacity to express the thyroglobulin (Tg) gene, the most prominent marker of differentiation in thyrocytes, was studied in proliferative cells (with insulin) and in quiescent cells (without insulin). Using cRNA in situ hybridization, we observed that TSH (and, to a lesser extent, insulin and insulin-like growth factor I) restored or maintained the expression of the Tg gene. Without these hormones, the Tg mRNA content became undetectable in most of the cells. EGF and 12-0-tetradecanoyl phorbol-13-acetate (TPA) inhibited the Tg mRNA accumulation induced by TSH (and/or insulin). Most of the cells (up to 90%) responded to both TSH and EGF. Nevertheless, the range of individual response was quite variable. The effects of TSH and EGF on differentiation expression were not dependent on insulin and can therefore be dissociated from their mitogenic effects. Cell cycling did not affect the induction of Tg gene. Indeed, the same cell distribution of Tg mRNA content was observed in quiescent cells stimulated by TSH alone, or in cells approximately 50% of which had performed one mitotic cycle in response to TSH + insulin. Moreover, after proliferation in "dedifferentiating" conditions (EGF + serum + insulin), thyrocytes had acquired a fusiform fibroblast-like morphology, and responded to TSH by regaining a characteristic epithelial shape and high Tg mRNA content. 32 h after the replacement of EGF by TSH, cells in mitosis presented the same distribution of the Tg mRNA content as the rest of the cell population. This implies that cell cycling (at least 27 h, as previously shown) did not affect the induction of the Tg gene which is clearly detectable after a time lag of at least 24 h. The data unequivocally show that the reexpression of differentiation and proliferative activity are separate but fully compatible processes when induced by cAMP in thyrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of thyroperoxidase, thyroglobulin and iodide levels in sheep thyroid cells by TSH, tumor promoters and epidermal growth factor

Using sheep thyroid cells in culture, we have studied the effects of thyroid stimulating hormone (TSH), epidermal growth factor (EGF) and the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) on the activity and expression of both thyroglobulin (Tg) and thyroid peroxidase (TPO) and on the ability of cells to trap and organify iodide. Using Western blotting techniques, we found that TSH increased the absolute cellular levels of Tg. The optimum TSH concentration for Tg mRNA production was between 0.1-1.0 mU/ml. Thyroglobulin mRNA levels were stimulated by TSH but detectable levels were also present in cultures grown in its absence containing cortisol, insulin, transferrin, somatostatin and glycyl-lysyl-histidyl acetate. Unlike Tg, TPO protein levels were found to be completely dependent upon TSH. A time course of TSH stimulation of TPO mRNA showed increases after 8 h of TSH stimulation, whereas induction of Tg mRNA by TSH was seen at 24 h. Iodide trapping and organification were also TSH-dependent processes, showing maximum activities at 300-500 muU/ml of TSH. The addition of 10 nM TPA caused a biphasic decrease in radiolabeled pertechnetate uptake, with complete inhibition being seen at 14 h. Inhibition of iodide organification occurred more rapidly. TPA and EGF (1 nM) reduced the amount of newly synthesized Tg in TSH-stimulated cells by 50% but the absolute amount of Tg within the cells was not markedly inhibited at these early times.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroglobulin gene expression as a differentiation marker in primary cultures of calf thyroid cells

A system of calf thyroid follicular cells in primary cultures has been developed to investigate the control of thyroglobulin gene expression in normal cells in vitro. In low (0.1%) serum conditions, the cells remained quiescent and formed dense aggregates surrounded by slowly spreading cells. High expression of thyroid-specific differentiation markers such as thyroglobulin (Tg) mRNA accumulation and iodide transport required the continuous exposure of cells to thyrotropin (TSH) or other adenylate cyclase activators (cholera toxin and forskolin). In the absence of TSH, Tg mRNA decreased to low but still detectable levels. Addition of TSH, forskolin or cholera toxin restored high Tg gene expression. Hydrocortisone moderately stimulated basal Tg mRNA accumulation and strongly potentiated the effect of TSH. Growth promoters including serum (1-10%), epidermal growth factor (EGF), fibroblast growth factor (FGF) and 12-O-tetradecanoylphorbol 13-acetate (TPA) induced calf thyroid cells to develop as a monolayer and inhibited both basal and TSH-stimulated expression of specialized functions. Moreover, only a partial restoration of this expression was achieved after addition of TSH or forskolin to well spread-out cells that had proliferated in response to EGF or serum. The results show that in calf thyroid cells, iodide transport and Tg gene expression are regulated by TSH through cyclic AMP; hydrocortisone potentiates this effect on Tg gene expression, while all growth promoting factors inhibit the expression of these differentiated functions.

Factors influencing the growth of normal and neoplastic thyroid tissue

TSH activates the adenylate cyclase and the phosphoinositide turnover-protein kinase C-calcium systems in thyroid cells and appears to have an important but variable role in controlling the growth of both normal and neoplastic thyroid tissues. Species differences, experimental conditions, and tissue or tumor cell heterogeneity may account for this variability. Although TSH seems to be an important physiologic growth factor, it is neither the exclusive growth factor for the thyroid gland nor absolutely necessary for the effect of other thyroid growth factors. TSH may work in concert with other growth factors such as EGF. Some growth factors influence thyroid growth through TSH, whereas others do not.

Thyrotropin-induced formation of functional follicles in primary cultures of ovine thyroid cells

In primary cultures of ovine thyroid cells, TSH induced the expression of several differentiated functions including the formation of follicles, and synthesis and storage of iodinated thyroglobulin in the follicular lumen. In the present report, these follicles were shown by transmission (TEM) and scanning electron microscopy (SEM) to be intact, comprised of two or more cells and to possess numerous microvilli on the inner cell membranes facing the follicular lumen. The TSH-induced formation of follicles was reversible and dynamic, with the kinetics of formation preceding that of iodination. The follicles were further demonstrated to be functional in terms of thyroglobulin storage and iodination.