Thyrotropin Potentiation of Insulin-Like Growth Factor-I Dependent Deoxribonucleic Acid Synthesis in FRTL-5 Cells: Mediation by an Autocrine Amplification Factor(s)*

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Abstract
Studies were undertaken to determine the mechanism(s) by which TSH and insulin-like growth factors (IGFs) act synergistically to stimulate DNA synthesis in FRTL-5 cells. As observed in previous studies, the response of these cells to a combination of TSH plus IGFs (or micromolar concentrations of insulin) greatly surpass the sum of the effects of the individual hormones when acting alone. Part of this synergism was eliminated when media containing TSH and IGF-I were replaced every 4 h with fresh media. This suggested that part of the synergism between TSH and IGF-I on cell proliferation is mediated by an amplification factor(s) (AF) released from FRTL-5 cells during incubation. The AF was not specific for thyroid cells, however, since conditioned medium from TSH treated FRTL-5 cells was also found to potentiate the mitogenic effect of IGF-I in the human fibroblast cell line GM3652. It is unlikely that the AF activity secreted by these cells in response to TSH is either IGF or an IGF-binding protein, since the anti-IGF monoclonal antibody sm 1.2 did not attenuate the synergism between TSH and high concentrations of insulin on thymidine incorporation. Analysis of thymidine incorporation into DNA at different times after different patterns of exposure to TSH, IGF-I, or TSH plus IGF-I suggested that at least part of the synergism between the two hormones resulted from increasing the number of quiescent cells recruited into the cell cycle. These results suggested that the TSH-dependent AF might be acting as a competence factor. In a preliminary screen of candidate growth factors, only fibroblast growth factor (FGF) simulated the effect of AF, and its effect was smaller than that obtained with TSH-treated FRTL-5 cells. After preincubation with TSH, FRTL-5 cells exhibited greatly increased responsivity to the mitogenic effects of IGF-I that was manifested by both increased sensitivity to IGF-I, as judged by a decreased EC50, and an increase in their maximum response. TSH pretreatment, likewise, amplified subsequent DNA synthesis in response to serum and tetradecanoyl phorbol acetate. Thus, the mitogenic effect of TSH in FRTL-5 cells is due not only its stimulation of IGF production, but also to its stimulation of one or more AF that greatly enhance the responsivity of these cells to mitogenic stimuli.

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