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Thyroid cell proliferation is regulated by the concerted action of TSH/cAMP and serum growth factors. The specific contributions of cAMP-dependent vs. -independent signals to cell cycle progression are not well understood. We examined the molecular basis for the synergistic effects of TSH and serum on G1/S phase cell cycle progression in rat thyroid cells. Although strictly required for thyroid cell proliferation, TSH failed to stimulate G1 phase cell cycle progression. Together with serum, TSH increased the number of cycling cells. TSH enhanced the effects of serum on retinoblastoma protein hyperphosphorylation, cyclin-dependent kinase 2 activity, and cyclin A expression. Most notably, TSH and serum elicited strikingly different effects on p27 localization. TSH stimulated the nuclear accumulation of p27, whereas serum induced its nuclear export. Unexpectedly, TSH enhanced the depletion of nuclear p27 in serum-treated cells. Furthermore, only combined treatment with TSH and serum led to rapamycin-sensitive p27 turnover. Together, TSH and serum stimulated p70S6K activity that remained high through S phase. These data suggest that TSH regulates cell cycle progression, in part, by increasing the number of cycling cells through p70S6K-mediated effects on the localization of p27.

Thyrotropin and Serum Regulate Thyroid Cell Proliferation through Differential Effects on p27 Expression and Localization