Effect of monensin on DNA synthesis in cultured human thyroid cells.
Monensin is a carboxylic ionophore which perturbs the structure and function of the Golgi apparatus and lysosomes. In the present study, we investigated the functional significance of these organelle in the growth factor-mediated cell proliferation in cultured human thyroid cells from normal and Graves' disease. DNA synthesis was estimated by [3H]-thymidine uptake and flow cytometric analysis. Monensin inhibited both [3H]-thymidine uptake in a dose-dependent manner and the transition of G1 to S phase determined by flow cytometric analysis. Monensin partially blocked the effect of bovine TSH in normal thyroid cells. [3H]-thymidine uptake was suppressed to 56.7 +/- 37.3% of the control value with bTSH and monensin, but it was still higher than those with monensin alone (21.9 +/- 15.0% of the control). The percentage of cells in the S phase was also increased from 7.64 +/- 1.91% with monensin alone to 11.54 +/- 2.82% with bTSH at t = 24h. Forskolin or 12-O-tetradecanoylphorbol 13-acetate (TPA) could not mimic the action of TSH. On the other hand, insulin and EGF most effectively counteracted monensin-induced inhibition of DNA synthesis in Graves' thyroid cells. [3H]-thymidine uptake was not completely inhibited, being 73.5 +/- 24.0% with EGF, 105.0 +/- 25.4% with insulin, and 49.2 +/- 6.6% with monensin alone, respectively. The percentage of cells in the S phase also increased from 8.31 +/- 2.61% with monensin alone to 11.25 +/- 4.27% with EGF and 12.86 +/- 3.12% with insulin. In conclusion, the functional maintenance of the Golgi apparatus and lysosomes is necessary for DNA synthesis in both normal and Graves' thyroid cells, in which bTSH, insulin, and EGF might be differently involved in the regulation of DNA synthesis.