Effect of thyrotropin and cAMP on FRTL5 cell growth in a serum free medium

TSH-induced proliferation of FRTL5 cells was studied in a chemically serum-free, defined medium. FRTL5 cells incubated for several days in a medium lacking of serum and hormones were fully able to respond to TSH in terms of cAMP production and iodine uptake. In the same medium, TSH stimulated FRTL5 cell growth as assessed by thymidine incorporation, DNA content and cell count. In these experimental conditions the cellular doubling time was of 7.5 days as compared to 40 h when calf serum was present together with TSH. Cholera toxin, forskolin and (bu)2cAMP, substances able to selectively increase intracellular cAMP levels, were not as efficient as TSH in inducing FRTL5 cell growth. However, both TSH and (bu)2cAMP, in a similar way, induced c-myc gene expression and cellular progression through the prereplicative phase of cell division. These data demonstrate that cAMP-dependent mechanisms are only partially responsible for TSH-induced FRTL5 cell growth, thus suggesting a role of cAMP-independent mechanisms.