RESUMO
BACKGROUND/AIM: Fisher rat thyroid cells (FRTL-5) display increased proliferation, reduced follicularization and decreased thyroxin release with repeated sub-culturing. These changes occur earlier and more rapidly following exposure to ionizing radiation. We hypothesized that altered transforming growth factor-ß1 (TGF-ß1) signaling contributes to these differences. MATERIALS AND METHODS: Assessments included FRTL-5 cell growth rate and quantification of TGF-ß1 ligand and receptors. The levels and activity of Smads2, 3 and 4 were measured by western blotting and the ability of TGF-ß1 to regulate cyclin A and plasminogen activator inhibitor type 1 (PAI-1) activity was assessed using transfection assays. RESULTS: TGF-ß1 production increased after radiation but returned to control levels after repeated sub-culturing. There was no difference in TGF-ß1 levels between un-irradiated cells at low versus high-passage number. TGF-ß1 receptors and basal levels of Smads2, 3 and 4 remained unchanged. However, there were significant changes in cell proliferation, TGF-ß1-mediated Smads2 and 3 activation and in TGF-ß1's ability to regulate cyclin A and PAI-1 transcription in irradiated and repeatedly sub-cultured cells (p<0.05). CONCLUSION: Collectively, these results support the conclusion that alterations in the TGF-ß1 pathway contribute to phenotypic changes in FRTL-5 cells as a function of passage number and radiation.
