Dissociation between transformed and differentiated phenotype in rat thyroid epithelial cells after transformation with a temperature-sensitive mutant of the Kirsten murine sarcoma virus

FAK phosphorylation by ERK primes ras-induced tyrosine dephosphorylation of FAK mediated by PIN1 and PTP-PEST

Activated Ras has been found in many types of cancer. However, the mechanism underlying Ras-promoted tumor metastasis remains unclear. We demonstrate here that activated Ras induces tyrosine dephosphorylation and inhibition of FAK mediated by the Ras downstream Fgd1-Cdc42-PAK1-MEK-ERK signaling cascade. ERK phosphorylates FAK S910 and recruits PIN1 and PTP-PEST, which colocalize with FAK at the lamellipodia of migrating cells. PIN1 binding and prolyl isomerization of FAK cause PTP-PEST to interact with and dephosphorylate FAK Y397. Inhibition of FAK mediated by this signal relay promotes Ras-induced cell migration, invasion, and metastasis. These findings uncover the importance of sequential modification of FAK-by serine phosphorylation, isomerization, and tyrosine dephosphorylation--in the regulation of FAK activity and, thereby, in Ras-related tumor metastasis.

v-K-ras leads to preferential farnesylation of p21(ras) in FRTL-5 cells: multiple interference with the isoprenoid pathway

The isoprenoid pathway in FRTL-5 thyroid cells was found to be deeply altered on transformation with v-K-ras. A dramatic overall reduction of protein prenylation was found in v-K-ras-transformed cells in comparison with the parent FRTL-5 cells, as shown by labeling cells with [3H]mevalonic acid. This phenomenon was accompanied by a relative increase of p21(ras) farnesylation and by a decrease of the ratio between the amounts of geranylgeraniol and farnesol bound to prenylated proteins. Analysis of protein prenylation in FRTL-5 cells transformed by a temperature-sensitive mutant of the v-K-ras oncogene indicated that these variations represent an early and specific marker of active K-ras. Conversely, FRTL-5 cells transformed with Harvey-ras showed a pattern of [3H]-mevalonate (MVA)-labeled proteins similar to that of nontransformed cells. The K-ras oncogene activation also resulted in an overall decrease of [3H]-MVA incorporation into isopentenyl-tRNA together with an increase of unprocessed [3H]-MVA and no alteration in [3H]-MVA uptake. The effects of v-K-ras on protein prenylation could be mimicked in FRTL-5 cells by lowering the concentration of exogenous [3H]-MVA whereas increasing the [3H]-MVA concentration did not revert the alterations observed in transformed cells. Accordingly, v-K-ras expression was found to: (i) down-regulate mevalonate kinase; (ii) induce farnesyl-pyrophosphate synthase expression; and (iii) augment protein farnesyltransferase but not protein geranylgeranyl-transferase-I activity. Among these events, mevalonate kinase down-regulation appeared to be related strictly to differential protein prenylation. This study represents an example of how expression of the v-K-ras oncogene, through multiple interferences with the isoprenoid metabolic pathway, may result in the preferential farnesylation of the ras oncogene product p21(ras).

Multiple mechanisms of interference between transformation and differentiation in thyroid cells

Transformation of the thyroid cell line FRTL-5 results in loss or reduction of differentiation as measured by the expression of thyroglobulin and thyroperoxidase, two proteins whose genes are exclusively expressed in thyroid follicular cells. The biochemical mechanisms leading to this phenomenon were investigated in three cell lines obtained by transformation of FRTL-5 cells with Ki-ras, Ha-ras, and polyomavirus middle-T oncogenes. With the ras oncogenes, transformation leads to undetectable expression of the thyroglobulin and thyroperoxidase genes. However, the mechanisms responsible for the extinction of the differentiated phenotype seem to be different for the two ras oncogenes. In Ki-ras-transformed cells, the mRNA encoding TTF-1, a transcription factor controlling thyroglobulin and thyroperoxidase gene expression, is severely reduced. On the contrary, nearly wild-type levels of TTF-1 mRNA are detected in Ha-ras-transformed cells. Furthermore, overexpression of TTF-1 can activate transcription of the thyroglobulin promoter in Ki-ras-transformed cells, whereas it has no effect on thyroglobulin transcription in the Ha-ras-transformed line. Expression of polyoma middle-T antigen in thyroid cells leads to only a reduction of differentiation and does not severely affect either the activity or the amount of TTF-1. Another thyroid cell-specific transcription factor, TTF-2, is more sensitive to transformation, since it disappears in all three transformed lines, and probably contributes to the reduced expression of the differentiated phenotype.

Multiple mechanisms of interference between transformation and differentiation in thyroid cells

Transformation of the thyroid cell line FRTL-5 results in loss or reduction of differentiation as measured by the expression of thyroglobulin and thyroperoxidase, two proteins whose genes are exclusively expressed in thyroid follicular cells. The biochemical mechanisms leading to this phenomenon were investigated in three cell lines obtained by transformation of FRTL-5 cells with Ki-ras, Ha-ras, and polyomavirus middle-T oncogenes. With the ras oncogenes, transformation leads to undetectable expression of the thyroglobulin and thyroperoxidase genes. However, the mechanisms responsible for the extinction of the differentiated phenotype seem to be different for the two ras oncogenes. In Ki-ras-transformed cells, the mRNA encoding TTF-1, a transcription factor controlling thyroglobulin and thyroperoxidase gene expression, is severely reduced. On the contrary, nearly wild-type levels of TTF-1 mRNA are detected in Ha-ras-transformed cells. Furthermore, overexpression of TTF-1 can activate transcription of the thyroglobulin promoter in Ki-ras-transformed cells, whereas it has no effect on thyroglobulin transcription in the Ha-ras-transformed line. Expression of polyoma middle-T antigen in thyroid cells leads to only a reduction of differentiation and does not severely affect either the activity or the amount of TTF-1. Another thyroid cell-specific transcription factor, TTF-2, is more sensitive to transformation, since it disappears in all three transformed lines, and probably contributes to the reduced expression of the differentiated phenotype.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

Neoplastic transformation inactivates specific trans-acting factor(s) required for the expression of the thyroglobulin gene

The expression of rat thyroglobulin gene is repressed following the transformation of rat thyroid cells with Kirsten murine sarcoma virus. The expression of a reporter gene fused to the thyroglobulin promoter is down-regulated in transformed thyroid cells in transient or in stable transfection assays. DNase and exonuclease III cleavage-protection analysis reveals that a promoter binding activity located at -60 base pairs from the transcription start site is substantially reduced in transformed thyroid cells. The repression in the transformed cells of the reporter gene joined to the thyroglobulin promoter can be reversed by fusion with normal differentiated thyroid cells. Fusion of transformed thyroid cells to liver cells does not reactivate the reporter under control of the thyroglobulin promoter.

One- and two-step transformations of rat thyroid epithelial cells by retroviral oncogenes

A system of epithelial cells is described in which it is possible to study the number and the nature of genes capable of conferring the malignant phenotype. Two fully differentiated, hormone-responsive cell lines from rat thyroid glands are presented which are susceptible to one-step or two-step transformation upon infection with several murine acute retroviruses. After infection, both cell lines became independent from their thyrotropic hormone requirement for growth. However, complete transformation was achieved with one of the cell lines (FRTL-5 Cl 2), whereas the other cell line (PC Cl 3) failed to grow in agar and to give rise to tumors in vivo. The latter cell line was susceptible to complete transformation upon cooperation of the v-ras-Ha and the human c-myc oncogenes.

One- and two-step transformations of rat thyroid epithelial cells by retroviral oncogenes

A system of epithelial cells is described in which it is possible to study the number and the nature of genes capable of conferring the malignant phenotype. Two fully differentiated, hormone-responsive cell lines from rat thyroid glands are presented which are susceptible to one-step or two-step transformation upon infection with several murine acute retroviruses. After infection, both cell lines became independent from their thyrotropic hormone requirement for growth. However, complete transformation was achieved with one of the cell lines (FRTL-5 Cl 2), whereas the other cell line (PC Cl 3) failed to grow in agar and to give rise to tumors in vivo. The latter cell line was susceptible to complete transformation upon cooperation of the v-ras-Ha and the human c-myc oncogenes.

Increased level of thyroglobulin mRNA in a human familial goiter

Thyroglobulin (Tg) and its specific mRNA were analyzed in the thyroid gland of two cases of familial goiter. A three-fold increase in the level of stable protein and functional Tg mRNA was found in the goiter of one of the two patients in whom a desiodase defect was demonstrated. Normal concentrations of both Tg and its mRNA were found in the other goiter in which no enzymatic defect could be shown. Our results suggest that the increase of Tg mRNA levels found in the goiter with a desiodase defect was due to a chronic stimulation of the gland by TSH.

A mos oncogene-containing retrovirus, myeloproliferative sarcoma virus, transforms rat thyroid epithelial cells and irreversibly blocks their differentiation pattern

Differentiated, cloned rat thyroid epithelial cells (424 cells) were infected with a wild-type and a temperature-sensitive strain of the myeloproliferative variant of the Moloney murine sarcoma virus. The thyroid cells were productively infected and transformed by both virus strains and displayed some of the typical properties of malignant cells, such as morphological changes, growth in soft agar, and in vivo tumorigenicity. The acquisition of the transformed phenotype by the virus-infected cells was accompanied by a loss of the typical differentiated features of the thyroid epithelium, such as thyroglobulin (TG) secretion, iodide uptake, and dependence for growth on six factors including thyrotropin, the physiological thyroid stimulator. TG mRNA could not be demonstrated in cells transformed by both viral strains, suggesting a block at the level of the TG gene transcription. While the transformed state of the cell clones infected with the temperature-sensitive strain could be reverted by shifting the cultures to the temperature nonpermissive for transformation (39 degrees C), no reversion of the differentiated functions took place after such a shift, showing that the v-mos oncogene irreversibly shuts off the differentiation of thyroid epithelial cells in vitro. These results demonstrate, for the first time, an oncogenic potential of the v-mos oncogene family towards differentiated epithelial cells in vitro.