Proliferative response to cyclic AMP elevation of thyroid epithelium in suspension culture

The role of cyclic AMP (cAMP) as an intracellular growth signal in thyroid epithelial (follicular) cells has been studied using primary suspension cultures of rat thyroid follicles closely resembling the in vivo state. TSH at 0.1 mU/ml in the presence of insulin (0.08 microgram/ml) induced a 9.6-fold increase in [3H]thymidine incorporation with a peak after 48-72 h. The response to the cAMP elevating agent cholera toxin (10 ng/ml) was identical both in timing, magnitude and dependence on insulin. A lower amplitude response occurred with forskolin. The data further support the conclusion that elevation of intracellular cAMP concentration is a major, if not the only, signal required for the proliferative response of the follicular cell to its physiological mitogen, TSH.