Differential expression of thyroglobulin gene in normal and transformed thyroid cells

Abstract

We have mesured the synthesis of thyroglobulin in two differentiated cell lines, FRTL-5 and FRTL-424, and two transformed thyroid cell lines, FRA and 1-5G. The untransformed cells actively synthesized and exported thyroglobulin in the medium: however, the FRTL-5 cell line synthesized seven times less thyroglobulin than the FRTL-424 cell line, even though both cell types contained equal amounts of functional thyroglobulin mRNA. In contrast the transformed cells expressed extremely low levels of thyroglobulin mRNA, even though there was no detectable change in gene structure or copy number as determined by Southern blot analysis. On the basis of these data we conclude that (a) the different levels of thyroglobulin synthesis in the two untransformed cell lines are due to stable post-transcriptional alterations in the biosynthesis of thyroglobulin and (b) the transformation of thyroid cells results in a substantial reduction in thyroglobulin gene expression.The biosynthesis of thyroglobulin is a distinctive feature of differentiated thyroid cells. This protein represents about 70% of the newly synthesized proteins of the tyroid gland in vivo [l]. Recently information has been obtained on the primary structure of the protein [la] and on the structural organization of the rat [2] and human [3] genes. The regulation of the expression of thyroglobulin gene, however, is still a difficult problem to approach. An ideal system for the investigation of the molecular steps of thyroglobulin biosynthesis would be a collection of cell strains defective in one or more steps of the thyroglobulin expression pathway. The availability of rat thyroid cell lines, normal and transformed, together with DNA probes, makes this aim realistic.In this report, as the first approach to the study of the mechanisms involved in the control of the thyroglobulin biosynthesis, we have quantified the expression of the thyroglobulin gene at the mRNA and protein levels in untransformed and transformed thyroid cell lines [4, 51. We suggest that the lower rate of expression of thyroglobulin in the FRTL-5 cell line is due to a stable post-transcriptional alteration in the biosynthesis of thyroglobulin. Morphological analysis of the two untransformed cell lines correlates well with the biochemical results. In addition we have measured the amount of thyroglobulin and thyroglobulin mRNA in two transformed cell lines derived from different thyroid tumours: thyroglobulin was not detectable in either cell line and only the FRA cells produced detectable levels of thyroglobulin mRNA. ~~

Concomitant activation of MEK-1 and Rac-1 increases the proliferative potential of thyroid epithelial cells, without affecting their differentiation

“…Similar results were obtained by simultaneous cotransfection of MEKDN3/ S218E/S222D and Val-12 Rac in FRTL-5 cells studies have shown that oncogenic Ras causes potent tumorigenic transformation of these cells, resulting in growth factor-independent proliferation, morphological transformation, anchorage-independent growth and tumor formation when cells are injected into nude mice (Fusco et al, 1987). Furthermore, in cells transformed by Ras as well as by a variety of other oncogenes, thyroid di erentiation is suppressed (Avvedimento et al, 1985;Francis-Lang et al, 1992). Loss of the di erentiated phenotype is a common feature of malignant transformation.…”

An Adult Mouse Thyroid Side Population Cell Line that Exhibits Enriched Epithelial–Mesenchymal Transition

“…Most mitochondrial matrixes were not enriched, and the cytoplasm, especially in small cells, was slightly granular and less contrasted, suggesting that SPTL cells were not fully differentiated. These characteristics were in contrast to those found in rat thyroid-derived FRTL-424 and FRTL-5 cells, and rat follicular cells, where many enriched mitochondria and welldeveloped ER were found (25,26). In a previous study, half of the thyroid SP cells were positive for SCA1, a candidate stem-cell marker for various tissues (27)(28)(29).…”

“…The newly established SPTL cell line expresses SCA1 at 100%, while SCA1 expression decreases once the SPTL cells start differentiating, thus suggesting that SCA1 could be used as a marker for thyroid stem/progenitor cells. Electron microscopy revealed that SPTL cells are not fully differentiated compared with rat thyroid-derived FRTL-424 and FRTL-5 cells and rat follicular cells (25,26). It is believed that this cell line is not the result of contaminated fibroblasts that overgrew during passages because (i) an increased expression of Nkx2-1 was not observed when NIH3T3 cells as representative mouse fibroblast cells were cultured in low serum; (ii) no fibers were detected in the electron microscope images, which quite often can be seen if the cells are fibroblast; and (iii) the expression of the thyroid-specific transcription factor NKX2-1 was increased when cells were cultured in low serum (see below).…”

Enhanced sensitivity to apoptosis in Ras-transformed thyroid cells

“…Similarly, Ras transformation confers hormone-independent proliferation to established rat thyroid cells (Fusco et al, 1987;Berlingieri et al, 1990;Miller et al, 1998). In these cells, constitutive expression of activated Ras abolishes di erentiated gene expression (Avvedimento et al, 1985(Avvedimento et al, , 1991Fusco et al, 1987;Berlingieri et al, 1990;Francis-Lang et al, 1992;Kupperman et al, 1996). Conversely, de-di erentiation was a rare event following Ras transformation of primary Wistar rat thyrocytes (Burns et al, 1992) as in human thyrocytes (Lemoine et al, 1990;Gire et al, 1999).…”