High iodine concentration attenuates RET/PTC3 oncogene activation in thyroid follicular cells.
BACKGROUND
Papillary thyroid carcinoma (PTC) is frequently associated with a RET gene rearrangement that generates a RET/PTC oncogene. RET/PTC is a fusion of the tyrosine kinase domain of RET to the 5' portion of a different gene. This fusion results in a constitutively active MAPK pathway, which plays a key role in PTC development. The RET/PTC3 fusion is primarily associated with radiation-related PTC. Epidemiological studies show a lower incidence of PTC in radiation-exposed regions that are associated with an iodine-rich diet. Since the influence of excess iodine on the development of thyroid cancer is still unclear, the aim of this study is to evaluate the effect of high iodine concentrations on RET/PTC3-activated thyroid cells.


METHODS
PTC3-5 cells, a rat thyroid cell lineage harboring doxycycline-inducible RET/PTC3, were treated with 10(-3) M NaI. Cell growth was analyzed by cell counting and the MTT assay. The expression and phosphorylation state of MAPK pathway-related (Braf, Erk, pErk, and pRet) and thyroid-specific (natrium-iodide symporter [Nis] and thyroid-stimulating hormone receptor [Tshr]) proteins were analyzed by Western blotting. Thyroid-specific gene expression was further analyzed by quantitative reverse transcription (RT)-polymerase chain reaction.


RESULTS
A significant inhibition of proliferation was observed, along with no significant variation in cell death rate, in the iodine-treated cells. Further, iodine treatment attenuated the loss of Nis and Tshr gene and protein expression induced by RET/PTC3 oncogene induction. Finally, iodine treatment reduced Ret and Erk phosphorylation, without altering Braf and Erk expression.


CONCLUSIONS
Our results indicate an antioncogenic role for excess iodine during thyroid oncogenic activation. These findings contribute to a better understanding of the effect of iodine on thyroid follicular cells, particularly how it may play a protective role during RET/PTC3 oncogene activation.
Fiore
Ana Paula Zen Petisco
AP
Department of Cell and Developmental Biology, Institute of Biomedical Sciences, University of São Paulo, São Paulo 05508-000, Brazil.
Fuziwara
Cesar Seigi
CS
Kimura
Edna Teruko
ET
D000704:Analysis of Variance; D000818:Animals; D015153:Blotting, Western; D002452:Cell Count; D016923:Cell Death / Q000187:drug effects / Q000235:genetics; D002460:Cell Line; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D007455:Iodine / Q000008:administration & dosage*; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000235:genetics; D056918:Nuclear Receptor Coactivators / Q000235:genetics* / Q000378:metabolism; D050596:Oncogene Fusion / Q000187:drug effects* / Q000235:genetics; D009857:Oncogenes / Q000187:drug effects* / Q000235:genetics; D010766:Phosphorylation / Q000187:drug effects / Q000235:genetics; D020033:Protein Isoforms / Q000235:genetics; D051096:Proto-Oncogene Proteins c-ret / Q000235:genetics* / Q000378:metabolism; D051381:Rats; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D013961:Thyroid Gland / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D013997:Time Factors
D056918:Nuclear Receptor Coactivators; D020033:Protein Isoforms; D007455:Iodine; D051096:Proto-Oncogene Proteins c-ret