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Background:High iodine inhibits papillary thyroid carcinoma cell proliferation associated with TGFβ pathway activation,although thyroid cancers are frequently refractory to TGFβsignaling.The TGFβpathway is proved to be regulated by miR-146b-5p.MicroRNAs are small noncoding RNAs that inhibit target mRNA translation and have emerged as potent modulators of tumorigenesis.Although the BRAFV600E mutation is the most prevalent alteration in thyroid cancer,the impact of iodine intake on BRAF-mediated oncogenesis remains unclear.In this study,we aimed to determine the influence of high iodine on miR-146b-5p expression and relevant functions in thyroid cells.Methods: BCPAP cells,a papillary thyroid cell lineage harboring BRAF1799T >A mutation,were treated with10-3M NaI.Cell growth was analyzed by cell counting and the MTT assay.The expression of smad4 and thyroid-specific(natrium-iodide symporter [Nis] and thyroid-stimulating hormone receptor [Tshr])proteins were analyzed by Western blotting.Thyroidspecific gene and miR-146b-5p expression was further analyzed by quantitative reverse transcription(RT)–polymerase chain reaction.Luciferase assays were used to quantify Smad4 3-UTR/miR-146b-5p.Results: A significant inhibition of proliferation was observed,along with no significant variation in cell death rate,in the iodine-treated cells.High iodine reduced the expression of miR-146b-5p.Overexpression of miR-146b-5p blocked high iodine-induced inhibition of cell proliferation.Furthermore,miR-146b-5p attenuated the loss of Nis and increase of smad4 gene and protein expression induced by high iodine induction.Smad4 was validated as a miR-146b-5p target by luciferase assays,which revealed reduced luminescence associated with miR-146b-5p interaction in Smad4 3-UTR.Conclusion: Our results indicate an antioncogenic role for excess iodine during BRAFV600E mutation in thyroid cancer.High iodine abrogates activation of miR-146b-5p,a identified Smad4 regulator,restores responsiveness to TGFβ signaling.Our results indicate that iodine exerts protective effects in thyroid cells,attenuating acute BRAF oncogene-mediated microRNA deregulation.