Insulin and insulin-like growth factor-I (IGF-I) regulate differentiation as well as growth in FRTL-5 cells

Thyroglobulin (Tg) is the major protein synthesized by the thyroid cell and is the macromolecular precursor of thyroid hormones. Thyrotropin (TSH) has been shown to increase Tg synthesis approximately 2-fold both in vivo (Van Heuverswyn et al. 1984, 1985) and in rat FRTL-5 thyroid cells (Ambesi-Impiombato et al. 1982; Avvedimento et al. 1984) a continuous functioning line whose growth is TSH dependent but requires medium containing insulin and insulin-like growth factor-I (IGF-I) (Ambesi-Impiombato et al. 1982). In evaluating the role of insulin and IGF-I, we found that 1) they can induce the synthesis of Tg in the absence of TSH and cell growth, 2) their action was at the level of Tg mRNA transcription, and 3) their action on Tg synthesis was quantitatively greater than and additive to the effect of TSH.