Suppression of BRAF/MEK/MAP kinase pathway restores expression of iodide-metabolizing genes in thyroid cells expressing the V600E BRAF mutant.
OBJECTIVE
The V600E BRAF mutant plays an important role in the pathogenesis of papillary thyroid cancer (PTC) and is associated with loss of expression of thyroid iodide-metabolizing genes. This study was done to investigate the restorability of expression of these genes by suppressing the BRAF/extracellular signal-regulated kinase kinase (MEK)/mitogen-activated protein (MAP) kinase pathway in V600E BRAF-harboring thyroid cells and to explore the mechanisms involved.


METHODS
We used inducible expression of V600E BRAF, small interfering RNA transfection, and MEK-specific inhibitor to alter the MAP kinase pathway activities and subsequently examined the changes in expression, promoter activities, and methylation status of thyroid genes.


RESULTS
MEK inhibitor U0126 or cessation of V600E BRAF expression in PCCL3 cells restored expression of thyroid genes silenced by induced expression of V600E BRAF. U0126 also restored the expression of these genes in V600E BRAF-harboring PTC-derived NPA cells. Knockdown of BRAF by specific small interfering RNA restored expression of some of these genes in NPA cells. Luciferase reporter assay using thyroid-stimulating hormone receptor gene as a model showed that the promoter activity was modulated by the MAP kinase pathway. Promoter methylation in association with DNA methyltransferase expression played a role in gene silencing by MAP kinase pathway in NPA cells.


CONCLUSIONS
We showed the restorability of expression of thyroid iodide-metabolizing genes silenced by V600E BRAF, and linked this process to gene methylation in PTC cells. The results provide clinical implications that therapeutic targeting at the BRAF/MEK/MAP kinase pathway may be a good approach in restoring thyroid gene expression for effective radioiodine therapy for BRAF mutation-harboring PTC.
Liu
Dingxie
D
Division of Endocrinology and Metabolism, Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, USA.
Hu
Shuiying
S
Hou
Peng
P
Jiang
David
D
Condouris
Stephen
S
Xing
Mingzhao
M
D000818:Animals; D002070:Butadienes / Q000494:pharmacology; D002291:Carcinoma, Papillary / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D007454:Iodides / Q000737:chemistry / Q000378:metabolism*; D020930:MAP Kinase Kinase Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D020935:MAP Kinase Signaling System / Q000187:drug effects; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D009570:Nitriles / Q000494:pharmacology; D011401:Promoter Regions, Genetic; D048493:Proto-Oncogene Proteins B-raf / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics; D051381:Rats; D011989:Receptors, Thyrotropin / Q000235:genetics; D013964:Thyroid Neoplasms / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism*; D014162:Transfection
D002070:Butadienes; D007454:Iodides; D009570:Nitriles; D034741:RNA, Small Interfering; D011989:Receptors, Thyrotropin; C113580:U 0126; D048493:Proto-Oncogene Proteins B-raf; D020928:Mitogen-Activated Protein Kinases; D020930:MAP Kinase Kinase Kinases