Lack of thyroglobulin synthesis as an indicator of early random dedifferentiation of the Fischer rat thyroid cell line FRTL-5.

Fischer rat thyroid cells (FRTL-5) are widely used for the study of thyroid cell function. With age or transformation the characteristics of FRTL-5 cells change, leading to poor differentiation and accelerated growth. Thyroglobulin production from FRTL-5 cells indicates differentiation to a higher extent than does the cyclic adenosine-3',5'-monophosphate (cAMP) response to thyrotropin (TSH) and growth. This study investigated the release of the differentiation marker thyroglobulin from FRTL-5 cells into the medium during various early subcultures compared with cAMP response and cell growth. Growth was measured by 3H-thymidine incorporation into DNA and by the amount of DNA in each well. Cell differentiation was measured by release of cAMP and thyroglobulin from the cells. TSH (1 U/l) stimulated 3H-thymidine incorporation (p<0.001), release of cAMP (p<0.1x10(-6)) and thyroglobulin (p<0.1x10(-6)). However, five passages showed an unexpected lack of thyroglobulin production (less than 15 ng/microg DNA, i.e. below the limit of detection). These results were reproduced from two different cell stocks. The passages which had lost their ability to produce thyroglobulin demonstrated a significant increase in DNA content (p<0.001), and a highly significant decrease in cAMP response to TSH (p<0.00001) compared with the passages that continued to produce thyroglobulin. This study shows that FRTL-5 cells randomly change their biological properties, which is consistent with dedifferentiation of cells in early passages. Lack of thyroglobulin production is a marker of this dedifferentiation.