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 ce lls change, leading to poor differentiation and accelerated growth. Thyrogl obulin production from FRTL-5 cells indicates differentiation to a higher e xtent than does the cyclic adenosine-3',5'-monophosphate (cAMP) response to thyrotropin (TSH) and growth. This study investigated the release of the d ifferentiation marker thyroglobulin from FRTL-5 cells into the medium durin g various early subcultures compared with cAMP response and cell growth. Gr owth was measured by H-3-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 H-3-thymidine inco rporation (p<0.001), release of cAMP (p < 0.1 x 10(-6)) and thyrogIobulin ( p < 0.1 x 10(-6)). However, five passages showed an unexpected lack of thyr oglobulin production (less than 15 ng/mu g DNA, i.e. below the limit of det ection). 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 significan t decrease in cAMP response to TSH (p < 0.00001) compared with the passages that continued to produce thyroglobulin. This study shows that FRTL-5 cell s randomly change their biological properties, which is consistent with ded ifferentiation of cells in early passages. Lack of thyroglobulin production is a marker of this dedifferentiation.