In vitro synthesis of a thyroglobulin precursor by porcine thyroid membrane-bound ribosomes in a heterologous cell-free system

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.

Synthesis of high molecular weight thyroglobulin peptides by thyroid polysome in vitro

Bovine thyroid polysomes were isolated under conditions which had yielded large polysomes in other systems. Between 25 and 40% of the protein synthesized by these polysomes could be precipitated by thyroglobulin antibody. When these immunoprecipitates were separated by dodecyl sulfate-polyacrylamide gel electrophoresis (4% running gel), over 50% of the radioactivity was located in the regions of polypeptides greater than 100,000 daltons. Between 11 and 13% of the total radioactivity was found as a single peak co-migrating with the main band of bovine thyroglobulin (Mr = 330,000). Peaks of radioactivity were also found in regions of molecular weights between 130,000 and 200,000. When the immunoprecipitates were separated in a 10% running gel, about 50% of the radioactivity was located in the top 8 mm of the gel. Most of the remaining radioactivity was distributed in regions corresponding to molecular weights greater than 68,000. No peak of radioactivity was seen corresponding to peptides of 15,000 daltons.