Using a clonal line of TSH-dependent functional rat thyroid cells, the effects of TSH on cAMP production and cell growth, as measured by cell number and [14C]thymidine incorporation, were compared. The withdrawal of TSH from the medium stopped the growth of cells, as measured by cell number. After 24 h in TSH-free medium, an acute challenge with 1 × 10-11 M pure TSH caused a 40-fold increase in the cAMP level; this response increased to greater than 80-fold as TSH withdrawal from the culture system was prolonged over 10 days. In contrast to these acute effects on cAMP levels, an increase in [14C]thymidine incorporation required 20-fold higher TSH concentrations and waned to an unresponsive level after 10 days of TSH withdrawal. After withdrawal of TSH from the medium for 10 days, readdition of TSH could return cell growth to normal rates only after a 2-day lag period. In terms of the lag, the TSH effect on cell number was paralleled by its effect on thymidine incorporation, not cAMP levels; (Bu)2-cAMP at concentrations as high as 1 × 10-3 M could not duplicate the effect of TSH on either cell number or thymidine incorporation. The effect of TSH on thymidine incorporation was blocked by anti-TSH, but not by an antibody to contaminant proteins in crude TSH preparations. Crude TSH could increase thymidine incorporation, but was nearly 1/100th less effective based on its bioactivity measurements, i.e. contained an inhibitor in addition to an activator of thymidine incorporation. The inhibitor activity affected the cAMP response in a parallel manner. These observations are consistent with separate bioeffects of TSH mediated through different domains of the TSH receptor or different transmission pathways. The data suggest that discriminant separation of these bioeffects in primary cultures of human thyroid cells, wherein TSH challenge augments cAMP production but fails to elevate [14C]thymidine incorporation, does not reflect the absence of a TSH mitogenic activity but, rather, demonstrates a condition wherein the TSH receptor is uncoupled from the message transmission pathway important for cell growth. The data also suggest that the TSH effect on cell growth requires an action beyond that of activating the adenylate cyclase system
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