Expression of the RET/PTC1 oncogene impairs the activity of TTF-1 and Pax-8 thyroid transcription factors.
The most frequent genetic alterations described thus far in human papillary thyroid carcinomas are somatic rearrangements of the RET proto-oncogene, which generate the chimeric RET/PTC oncogenes. We recently found that the expression of the RET/PTC1 oncogene blocked the expression of the thyroid-differentiated phenotype in rat thyroid epithelial cell line PC CI 3 (PC). Here, we show that this block occurs at a transcriptional level; indeed, the thyroid-specific thyroglobulin and thyroperoxidase gene promoters were inactive in PC-PTC cells. Specific transcription factors, namely, TTF-1 and Pax-8, regulate the expression of differentiated functions in thyroid cells. Here, we show that Pax-8 is expressed at reduced levels in PC-PTC cells and that its adoptive overexpression is unable to restore the activity of target promoters. In contrast, TTF-1 expression is unaltered in PC-PTC cells; however, by using a synthetic promoter that contains its specific target sequence, we demonstrate that TTF-1 is inactive in PC-PTC cells. We conclude that the RET/PTC1 oncogene alters the expression of the thyroid-differentiated phenotype by at least two different mechanisms, ie., down-regulation of Pax-8 protein and mRNA expression and impaired function of TTF-1 and Pax-8, which occurs at a posttranslational level.
De Vita
G
G
Dipartimento de Biologia e Patologia Cellulare e Molecolare, Facoltà di Medicina e Chirurgia, Università di Napoli Federico II, Naples, Italy.
Zannini
M
M
Cirafici
A M
AM
Melillo
R M
RM
Di Lauro
R
R
Fusco
A
A
Santoro
M
M
D000818:Animals; D002291:Carcinoma, Papillary / Q000235:genetics; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000378:metabolism*; D015536:Down-Regulation; D029721:Drosophila Proteins; D006801:Humans; D009687:Nuclear Proteins / Q000378:metabolism*; D009857:Oncogenes; D000072636:PAX8 Transcription Factor; D051761:Paired Box Transcription Factors; D011401:Promoter Regions, Genetic; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D051096:Proto-Oncogene Proteins c-ret; D051381:Rats; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics*; D013964:Thyroid Neoplasms / Q000235:genetics; D000074482:Thyroid Nuclear Factor 1; D015534:Trans-Activators / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*
D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; C000715470:MAS1 protein, human; C000614412:NKX2-1 protein, human; C000614416:Nkx2-1 protein, rat; D009687:Nuclear Proteins; D000072636:PAX8 Transcription Factor; C083325:PAX8 protein, human; D051761:Paired Box Transcription Factors; C499497:Pax8 protein, rat; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins; D000074482:Thyroid Nuclear Factor 1; D015534:Trans-Activators; D014157:Transcription Factors; D051096:Proto-Oncogene Proteins c-ret; D020794:Receptor Protein-Tyrosine Kinases; C416649:Ret protein, Drosophila; C495001:Ret protein, rat