Effects of Long-Term Exposure to Iodine Excess on the Apoptosis of Thyrocytes in Wistar Rats

Objective: To explore the effects of long-term exposure to more than adequate and excessive iodine on the apoptosis of thyrocytes in Wistar rats. Methods: Wistar rats, divided into four groups: Group 0 (control, treated with 4 mu g/day of iodine), Group 1 (1.5x control dose, i.e. 6 mu g/day of iodine), Group 2 (3x control dose, i.e. 12 mu g/day of iodine), and Group 3 (6x control dose, i.e. 24 mu g/day of iodine) had oral iodine intake for 1, 2, 4, and 8 months before being sacrificed for study. After the initial treatment course of 8 months, some rats in each group continued to have an iodine intake at 4 mu g/day for extra 3 months. Urinary iodine concentration was measured by arsenic/cerium catalyzing spectrophotography. In addition, the early apoptosis was assessed by flow cytometry with Annexin V-FITC staining, while the late apoptosis was evaluated by TUNEL microscopy. Furthermore, the rate of cell death and cell cycle kinetics were determined by flow cytometry with propidium iodide (PI) staining. Moreover, the level of intracellular reactive oxygen species (ROS) quantified by the fluorescence intensity was measured via flow cytometry with a DCFH-DA probe. Immunohistochemistry technique was also used for the detection of the expression of Bcl-2 and Bax. Results: The apoptotic rate and ROS production in thyrocytes were significantly elevated in Groups 2 and 3 after 4 and 8 months of treatment compared to the control group (P<0.05). Groups 2 and 3 were also found to have greatly higher percentage of cells in S phase after 4 and 8 months of exposure (5%-6% vs. 3%, P<0.05), with a significant reduction in cells in G0G1-phase (64%-67% vs. 80%, P<0.05). These differences disappeared after normal iodine intake for extra 3 months. The expressions of Bcl-2 and Bax among different groups remained unchanged. Positive correlations were observed between the apoptotic rate, ROS level and urinary iodine concentration after 4 and 8 months of excessive iodine exposure (r is in the range of 0.736 similar to 0.855, all of Ps<0.01). Conclusion: The experimental data indicated that more apoptosis of thyrocytes after exposure to excessive iodine was attributed to the increased free radical production, other than regulated Bcl-2 and Bax expression. The effects of iodine excess are reversible.