The block of thyroglobulin synthesis, which occurs upon transformation of rat thyroid epithelial cells, is at the transcriptional level and it is associated with methylation of the 5' flanking region of the gene

Recent advances in molecular biology of thyroid cancer and their clinical implications

Thyroid cancer is the most common endocrine malignancy. With a rapidly rising incidence in recent years, novel efficient management strategies are increasingly needed for this cancer. Remarkable advances have occurred in understanding several major biologic areas of thyroid cancer, including the molecular alterations for the loss of radioiodine avidity of thyroid cancer, the pathogenic role of the MAP kinase and PI3K/Akt pathways and their related genetic alterations, and the aberrant methylation of functionally important genes in thyroid tumorigenesis and pathogenesis. These exciting advances provide unprecedented opportunities for the development of molecular-based novel diagnostic, prognostic, and therapeutic strategies for thyroid cancer.

Gene methylation in thyroid tumorigenesis

Aberrant gene methylation plays an important role in human tumorigenesis, including thyroid tumorigenesis. Many tumor suppressor genes are aberrantly methylated in thyroid cancer, and some even in benign thyroid tumors, suggesting a role of this epigenetic event in early thyroid tumorigenesis. Methylation of some of these genes tends to occur in certain types of thyroid cancer and is related to specific signaling pathways. For example, methylation of PTEN and RASSF1A genes occurs mostly in follicular thyroid cancer, and its tumorigenic role may be related to the phosphatidylinositol 3-kinase/Akt signaling pathway, whereas methylation of genes for tissue inhibitor of metalloproteinase-3, SLC5A8, and death-associated protein kinase occurs in papillary thyroid cancer and is related to the BRAF/MAPK kinase/MAPK pathway. Methylation of thyroid-specific genes, such as those for sodium/iodide symporter and thyroid-stimulating hormone receptor, is also common in thyroid cancer. Although its tumorigenic role is not clear, methylation, and hence silencing, of these thyroid-specific genes is a cause for the failure of clinical radioiodine treatment of thyroid cancer. Unlike gene methylation, histone modifications have been relatively poorly investigated in thyroid tumors. Future studies need to emphasize the mechanistic aspects of these two types of epigenetic alterations to uncover new molecular mechanisms in thyroid tumorigenesis and to provide novel therapeutic targets for thyroid cancer.

Disruption of cell-type-specific methylation at the Maspin gene promoter is frequently involved in undifferentiated thyroid cancers

Cancer-associated DNA hypomethylation is as prevalent as cancer-linked hypermethylation, but the biological significance of DNA hypomethylation in carcinogenesis is less understood. The expression of Maspin (mammary serpin) in differentiated normal cells is regulated by epigenetic modifications in a cell-type-specific manner. Paradoxical Maspin expression due to epigenetic modification has been addressed in several cancer cell types. To elucidate the role of the Maspin gene in thyroid cancer, we studied methylation status in the promoter region and its expression in six human undifferentiated thyroid cancer cell lines and in specimens from 92 primary thyroid tumors, consisting of six follicular adenomas, 56 well-differentiated thyroid cancers (WDTCs), 17 poorly differentiated thyroid cancers (PDTCs) and 13 undifferentiated thyroid cancers (UDTCs). Three of the six cell lines overexpressed Maspin mRNA and its protein product, but the remaining three did not. The methylation status at the promoter region was inversely correlated with Maspin expression. In Maspin-negative cell lines, Maspin expression was induced by treatment with 5-aza-2'-deoxycytidine, a DNA demethylating agent. Immunoreactivity for Maspin protein was frequently detected in UDTCs (8/13, 62%) and PDTCs (7/17, 41%). Immunoreactivity for Maspin was diffusely positive in UDTCs, and was restricted to dedifferentiated components of the tumor in PDTCs. Positive immunoreactivity was infrequent in WDTCs (1/56, 2%), and all follicular adenomas and normal thyroid glands were completely negative. Their methylation status evaluated by the methylation-specific PCR method showed a good inverse correlation with their immunoreactivity in surgically resected specimens. Our data suggest that overexpression of Maspin by DNA hypomethylation is closely associated with morphological dedifferentiation in thyroid cancers.

Regulation of the rat thyrotropin receptor gene by the methylation-sensitive transcription factor GA-binding protein

The GA-binding protein (GABP), a transcription factor with a widespread tissue distribution, consists of two subunits, a and beta1, and acts as a potent positive regulator of various genes. The effect of GABP on transcription of the TSH receptor (TSHR) gene in rat FRTL-5 thyroid cells has now been investigated. Both deoxyribonuclease I footprint analysis and gel mobility-shift assays indicated that bacterially expressed glutathione S-transferase fusion proteins of GABP subunits bind to a region spanning nucleotides (nt) -116 to -80 of the TSHR gene. In gel mobility-shift assays, nuclear extracts of FRTL-5 cells and FRT cells yielded several specific bands with a probe comprising nt -116 to -80. Supershift assays with antibodies to GABPalpha and to GABPbeta1 showed that GABP was a component of the probe complexes formed by the nuclear extracts. Immunoblot analysis confirmed the presence of both GABP subunits in the nuclear extracts. A reporter gene construct containing the TSHR gene promoter was activated, in a dose-dependent manner, in FRTL-5 cells by cotransfection with constructs encoding both GABPalpha and GABPbeta1. Both GABP binding to and activation of the TSHR gene promoter were prevented by methylation of CpG sites at nt -93 and -85. These CpG sites were highly methylated (>82%) in FRT cells and completely demethylated in FRTL-5 cells, consistent with expression of the TSHR gene in the latter, but not the former. These results suggest that GABP regulates transcription of the TSHR gene in a methylation-dependent manner and that methylation of specific CpG sites and the methylation sensitivity of GABP contribute to the failure of FRT cells to express the endogenous TSHR gene.

Inhibition of HMGI-C protein synthesis suppresses retrovirally induced neoplastic transformation of rat thyroid cells

Elevated expression of the three high-mobility group I (HMGI) proteins (HMGI, HMGY, and HMGI-C) has previously been correlated with the presence of a highly malignant phenotype in epithelial and fibroblastic rat thyroid cells and in experimental thyroid, lung, mammary, and skin carcinomas. Northern (RNA) blot and run-on analyses demonstrated that the induction of HMGI genes in transformed thyroid cells occurs at the transcriptional level. An antisense methodology to block HMGI-C protein synthesis was then used to analyze the role of this protein in the process of thyroid cell transformation. Transfection of an antisense construct for the HMGI-C cDNA into normal thyroid cells, followed by infection with transforming myeloproliferative sarcoma virus or Kirsten murine sarcoma virus, generated cell lines that expressed significant levels of the retroviral transforming oncogenes v-mos or v-ras-Ki and removed the dependency on thyroid-stimulating hormones. However, in contrast with untransfected cells or cells transfected with the sense construct, those containing the antisense construct did not demonstrate the appearance of any malignant phenotypic markers (growth in soft agar and tumorigenicity in athymic mice). A great reduction of the HMGI-C protein levels and the absence of the HMGI(Y) proteins was observed in the HMGI-C antisense-transfected, virally infected cells. Therefore, the HMGI-C protein seems to play a key role in the transformation of these thyroid cells.

Demonstration of a heterogeneous transcription pattern of thyroglobulin mRNA in human thyroid tissues

Previous reports on human thyroglobulin (hTg) modifications in thyroid carcinomas prompted us to study hTg mRNA in thyroid adenomas and carcinomas. The quantification of hTg mRNA showed a decrease in its levels of expression in both pathological conditions which differed by a factor of 2 between adenomas and carcinomas. Furthermore, PCR was used to analyse the characteristics of hTg mRNA by amplifying 4 regions of the hTg mRNA. When applied to 2 normal, 17 benign and 13 malignant pathological tissue specimens, PCR showed no modification in the size of Tg mRNA. However, abnormal sized cDNAs appeared in all tissues with no distinction between the pathologies; the Restriction Fragment Length Polymorphism study of these cDNAs suggests the existence of alternate splicing patterns in thyroglobulin mRNAs.

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.