Regulation of Sodium Iodide Symporter Gene Expression in FRTL-5 Rat Thyroid Cells

The sodium iodide symporter (NIS), first identified in FRTL-5 cells, plays a critical role in iodide transport in the thyroid gland and in the production of the iodine-containing thyroid hormones. The aim of our study was to examine the regulation of NIS RNA steady-state levels and protein expression as well as functional activity in FRTL-5 cells. FRTL-5 cells cycling in media containing thyrotropin (TSH) were incubated for 48 hours with dexamethasone (10^-8-10^-5 M), triiodothyronine (T₃; 10^-9-10^-6 M), methimazole (100 μM), propylthiouracil (PTU; 100 μM), perchlorate (10 μM) and potassium iodide (40 μM). In other experiments, cells were treated for 48 hours with various cytokines including interleukin-6 (IL-6) (100 U/mL), interferon-γ (IFN-γ) (100 U/mL), tumor necrosis factor-α (TNF-α) (10 ng/ml), IL-1α (100 U/mL), and IL-1β (100 U/mL). Northern blot analysis using a ³²P-labeled rat NlS-specific cDNA probe (nucleotides 1397-1937) revealed NIS mRNA as a single species of approximately 3 kb. When normalized for β-actin mRNA signal intensities, NIS RNA steady-state levels in viable FRTL-5 cells were suppressed by approximately 80% after incubation with dexamethasone and T₃ in a concentration-dependent manner. Iodide accumulation was decreased by up to 40% after incubation with dexamethasone and T₃, respectively, in a concentration-dependent manner. Using a rabbit polyclonal rNIS-specific antibody, Western blot analysis of FRTL-5 cell membranes revealed a 60% and 70% suppression of NIS protein expression after treatment with T₃ (0.1 μM) and dexamethasone (1 μM), respectively. In additon, NIS RNA steady-state levels were decreased by approximately 50% after treatment of monolayers with methimazole, PTU, and potassium iodide, respectively. Incubation with methimazole and PTU resulted in a 20% and 25% decrease of iodide accumulation, respectively, whereas potassium iodide suppressed iodide accumulation by approximately 50%. Treatment of FRTL-5 cells with IL-6 and IL-1β resulted in a 30% decrease of NIS RNA steady-state levels. IL-6 did not alter NIS functional activity, but IL-1β suppressed iodide accumulation by approximately 25%. IFN-γ and perchlorate failed to alter NIS RNA steady-state levels. In contrast to IFN-γ that had no effect on iodide accumulation, perchlorate almost completely suppressed iodide accumulation. TNF-α and IL-lα failed to alter NIS RNA steady-state levels in higher passage numbers of FRTL-5 cells, whereas treatment with TNF-α and IL-1α of early passages of FRTL-5 cells (<20 cell passages) resulted in a 70% and 40% decrease of NIS RNA steady-state levels, respectively, and in a 20% suppression of NIS functional activity. In conclusion, our data suggest that various agents known to affect iodide transport are capable of differentially altering NIS gene expression and function in cultured thyroid cells. Suppression of NIS gene expression and function by certain cytokines may be responsible, at least in part, for the impaired radioiodine uptake by thyroid tissue in certain forms of thyroiditis.