Differential expression of thyroglobulin gene in normal and transformed thyroid cells

We have measured 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.