EXPRESSION OF THE RET PTC1 ONCOGENE IMPAIRS THE ACTIVITY OF TTF-1 ANDPAX-8 THYROID TRANSCRIPTION FACTORS/

The most frequent genetic alterations described thus far in human papi llary thyroid carcinomas are somatic rearrangements of the RET proto-o ncogene, which generate the chimeric RET/PTC oncogenes. We recently fo und that the expression of the RET/PTC1 oncogene blocked the expressio n of the thyroid-differentiated phenotype in rat thyroid epithelial ce ll line PC CI 3 (PC), Here, we show that this block occurs at a transc riptional level; indeed, the thyroid-specific thyroglobulin and thyrop eroxidase gene promoters were inactive in PC-PTC cells, Specific trans cription factors, namely, TTF-l 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 ove rexpression is unable to restore the activity of target promoters, In contrast, TTF-l expression is unaltered in PC-PTC cells; however, by u sing a synthetic promoter that contains its specific target sequence, we demonstrate that TTF-l is inactive in PC-PTC cells, We conclude tha t the RET/PTC1 oncogene alters the expression of the thyroid-different iated phenotype by at least two different mechanisms, Le., down-regula tion of Pax-8 protein and mRNA expression and impaired function of TTF -l and Pax-8, which occurs at a posttranslational level.