The sodium/iodide symporter (NIS) mediates the active transport of iodide from the bloodstream into thyrocytes. NIS function is strategic for the diagnosis and treatment of various thyroid diseases. In addition, a promising anti-cancer strategy based on targeted NIS gene transfer in non-thyroidal cells is currently developed. However, only little information is available concerning the molecular mechanism of NIS-mediated iodide translocation. Ten small molecules have recently been identified using a high-throughput screening method for their inhibitory effect on iodide uptake of NIS-expressing mammalian cells. In the present study, we analyzed these compounds for their rapid and reversible effects on the iodide-induced current in NIS-expressing Xenopus oocytes. Four molecules almost completely inhibited the iodide-induced current; for three of them the effect was irreversible, for one compound the initial current could be fully re-established after washout. Three molecules showed a rapid inhibitory effect of about 75%, half of which was reversible. Another three compounds inhibited the iodide-induced current from 10 to 50%. Some molecules altered the membrane conductance by themselves, i.e. in the absence of iodide. For one of these molecules the observed effect was also found in water-injected oocytes whereas for some others the iodide-independent effect was associated with NIS expression. The tested molecules show a surprisingly high variability in their possible mode of action, and thus are promising tools for further functional characterization of NIS on a molecular level, and they could be useful for medical applications.

The active transport of iodide from the bloodstream into thyroid follicular cells is mediated by the Na⁺/I⁻ symporter (NIS). We studied mouse NIS (mNIS) and found that it catalyzes iodide transport into transfected cells more efficiently than human NIS (hNIS). To further characterize this difference, we compared ¹²⁵I uptake in the transiently transfected human embryonic kidney (HEK) 293 cells. We found that the V _max for mNIS was four times higher than that for hNIS, and that the iodide transport constant (K _m) was 2.5-fold lower for hNIS than mNIS. We also performed immunocytolocalization studies and observed that the subcellular distribution of the two orthologs differed. While the mouse protein was predominantly found at the plasma membrane, its human ortholog was intracellular in ∼40% of the expressing cells. Using cell surface protein-labeling assays, we found that the plasma membrane localization frequency of the mouse protein was only 2.5-fold higher than that of the human protein, and therefore cannot alone account for the difference in the obtained V _max values. We reasoned that the observed difference could also be caused by a higher turnover number for iodide transport in the mouse protein. We then expressed and analyzed chimeric proteins. The data obtained with these constructs suggest that the iodide recognition site could be located in the region extending from the N-terminus to transmembrane domain 8, and that the region between transmembrane domain 5 and the C-terminus could play a role in the subcellular localization of the protein.