EFFECT OF MONENSIN ON DNA-SYNTHESIS IN CULTURED HUMAN THYROID-CELLS

Monensin is a carboxylic ionophore which perturbs the structure and fu nction of the Golgi apparatus and lysosomes. In the present study, we investigated the functional significance of these organella in the gro wth factor-mediated cell proliferation in cultured human thyroid cells from normal and Graves' disease. DNA synthesis was estimated by [H-3] -thymidine uptake and flow cytometric analysis. Monensin inhibited bot h [H-3]-thymidine uptake in a dose-dependent manner and the transition of G1 to S phase determined by flow cytometric analysis. Monensin par tially blocked the effect of bovine TSH in normal thyroid cells. [H-3] -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 monens in 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 1 1.54+/-2.82% with bTSH at t=24h. Forskolin or 12-O-tetradecanoylphorbo l 13-acetate (TPA) could not mimic the action of TSH. On the other han d, insulin and EGF most effectively counteracted monensin-induced inhi bition of DNA synthesis in Graves' thyroid cells. [H-3]-thymidine upta ke 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+/-2.6 1% with monensin alone to 11.25+/-4.27% with EGF and 12.86+/-3.12% wit h insulin. In conclusion, the functional maintenance of the Golgi appa ratus and lysosomes is necessary for DNA synthesis in both normal and Graves' thyroid cells, in which b-FSH, insulin, and EGF might be diffe rently involved in the regulation of DNA synthesis.