THE FISCHER RAT-THYROID CELL-LINE FRTL-5 EXHIBITS A NONDIPLOID KARYOTYPE

The FRTL-5 cell line is a stable thyroid cell line derived from the th yroid gland of the Fischer rat under defined culture conditions, which has been widely adopted as a model system for the study of thyroid ce ll function and for bioassay. While characterizing by flow cytometry F RTL-5 cells that were supplied to this laboratory by ATCC (American Ty pe Cell Collection), we discovered that the cells (ATCC CRL8305) were not diploid, having approximately twofold the DNA content relative to a diploid control. The increase in DNA content also applied to cells o riginally supplied by the ATCC (described as passage 14) that when cou nted in metaphase had a modal chromosomal count of 84, indicating tetr aploid status, double the expected 42 of a diploid rat cell. In a priv ate communication, the ATCC confirmed these findings which nevertheles s are contrary to previous literature reports where they were reported to be diploid. Tetraploid cells are less sensitive to thyrotropin (TS H) as measured by cyclic adenosine monophosphate (cAMP) production, co mpared with diploid cells (p = < 0.001). Despite similar H-3-thymidine uptake in 0.2% fetal calf serum, tetraploid cells show increased 3H-t hymidine uptake in 5% fetal calf serum in the absence of TSH (p = 0.00 1). The origin of these chromosomal changes is unclear, but these find ings must raise doubts regarding the suitability of the tetraploid FRT L-5 cell line as a model for studies of human or animal thyroid physio logy.