The mechanisms by which thyroid stimulating hormone (TSH) regulates the expression and activity of sodium iodide symporter (NIS) through cAMP-PKA have been partially elucidated by many studies. However, the effects of the TSH-mediated PLC-IP3 signaling pathway on the expression of NIS and how intracellular iodinated compounds interfere with these signaling pathways are poorly understood. In this study, we investigated the effects of the TSH-mediated cAMP-PKA and PLC-IP3 pathways on the expression of NIS in the presence of various intracellular iodinated compounds. The intracellular iodinated compounds were formed by treating cells with different concentrations of iodine with or without methimazole (MMI), an inhibitor of iodine organification, in a pig monolayer thyrocyte in vitro. A high concentration of iodine increased NIS expression at the mRNA and protein levels; however, this phenomenon was not observed in the presence of MMI. Both the cAMP-PKA and PLC-IP3 signaling pathways inhibited the expression of NIS at low iodine concentrations; however, in thyrocytes treated with high concentrations of iodine, the effect of cAMP-PKA on the expression of NIS changed from inhibition to promotion, while the PLC-IP3 pathway continued to inhibit NIS expression. These findings indicate that intracellular iodinated compounds affect NIS expression through the TSH-mediated cAMP-PKA and PLC-IP3 pathways.

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