Growth factor control of rat thyroid follicular cell proliferation

We have investigated the proliferative responses of rat thyroid follicular cells in serum-free culture to a range of growth factors including TSH, epidermal growth factor, and insulin, added singly or in combination. Follicles released from normal thyroids by collagenase/dispase digestion were cultured in suspension in agarose-coated microtiter plates to prevent monolayer formation. Growth responses were measured by [3H] thymidine incorporation and by autoradiography over successive 24- or 36-h periods. Insulin, even in the absence of other growth factors, stimulated [3H]thymidine incorporation in a concentration-dependent manner, rising from basal levels of 486 +/- (SE) 18 cpm to 4222 +/- 367 cpm/5 X 10(4) cells at 8 micrograms/ml. In contrast, TSH alone had no effect. In the presence of threshold levels (0.08 micrograms/ml) of insulin, however, there was a highly significant (P less than 0.001) response to TSH, [3H]thymidine incorporation rising from 1089 +/- 163 cpm in the absence of TSH to a maximum of 7548 +/- 585 with 1 mU/ml TSH. There was a synergistic interaction between insulin and TSH over the concentrations tested. Epidermal growth factor either alone or in combination with insulin failed to produce a significant response. Parallel autoradiographic studies were concordant with the [3H]thymidine incorporation data. We conclude that whereas in the absence of other growth factors TSH is unable to stimulate DNA synthesis in isolated rat thyroid follicles, the inclusion of just a single growth factor, insulin, permits a marked response. These observations emphasize the need for inclusion of appropriate permissive growth factor(s) when assessing the in vitro effect of a suspected tissue-specific mitogen.