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This study analyzes the uptake and antiproliferative effect of two different chemical forms of iodine, iodide (I−) and molecular iodine (I2), in MCF-7 cells, which are inducible for the Na+/I− symporter (NIS) and positive for pendrin (PDS). The mouse fibroblast cell line NIH3T3 was used as control. Our results show that in MCF-7 cells, I− uptake is sustained and dependent on NIS, whereas I2 uptake is transient with a maximal peak at 10 min and a final retention of 10% of total uptake. In contrast, no I− was taken up by NIH3T3 cells, and although I2 was captured with the same time pattern as in MCF-7 cells, its uptake was significantly lower, and it was not retained within the cell. The uptake of I2 is independent of NIS, PDS, Na+, and energy, but it is saturable and dependent on protein synthesis, suggesting a facilitated diffusion system. Radioiodine was incorporated into protein and lipid fractions only with I2 treatment. The administration of non-radiolabeled I2 and 6-iodo-5-hydroxy-8,11,14-eicosatrienoic acid (6-iodolactone, an iodinated arachidonic acid), but not KI, significantly inhibited proliferation of MCF-7 cells. Proliferation of NIH3T3 cells was not inhibited by 20 μM I2. In conclusion, these results demonstrate that I2 uptake does not depend on NIS or PDS; they suggest that in mammary cancer cells, I2 is taken up by a facilitated diffusion system and then covalently bound to lipids or proteins that, in turn, inhibit proliferation.
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Previous reports have documented the antiproliferative properties of I2 and the arachidonic acid (AA) derivative 6-iodolactone (6-IL) in both thyroid and mammary glands. In this study, we characterized the cellular pathways activated by these molecules and their effects on cell cycle arrest and apoptosis in normal (MCF-12F) and cancerous (MCF-7) breast cells. Low-to-moderate concentrations of I2 (10–20 μM) cause G1 and G2/M phase arrest in MCF-12F and caspase-dependent apoptosis in MCF-7 cells. In normal cells, only high doses of I2 (40 μM) induced apoptosis, and this effect was mediated by poly (ADP-ribose) polymerase-1 (PARP1) and the apoptosis-induced factor, suggesting an oxidative influence of iodine at high concentrations. Our data indicate that both I2 and 6-IL trigger the same intracellular pathways and suggest that the antineoplasic effect of I2 in mammary cancer involves the intracellular formation of 6-IL. Mammary cancer cells are known to contain high concentrations of AA, which might explain why I2 exerts apoptotic effects at lower concentrations only in tumoral cells.