HISTONE DEACETYLASE INHIBITION AFFECTS SODIUM IODIDE SYMPORTER (NIS) EXPRESSION AND INDUCES ¹³¹I CYTOTOXICITY IN ANAPLASTIC THYROID CANCER CELLS.

Background: Anaplastic thyroid cancers (ATC) represent only 1-2% of all thyroid tumors, but they account for up to 50% of the mortality. Treatment of differentiated thyroid carcinomas is well standardized and the use of radioiodine represents an essential step; in contrast, there is no standardized therapeutic approach for anaplastic tumors and their prognosis is poor. The resistance of anaplastic thyroid cancer to radioiodine treatment is principally due to the absence of expression of the sodium iodide symporter (NIS), mainly due to epigenetic silencing. The acetylation status of histones is involved in the epigenetic control of gene expression and is usually disrupted in advanced thyroid cancer. Histone deacetylase inhibitors have been demonstrated as potent anticancer drugs with several different effects on cell viability and differentiation. Methods: Stabilized anaplastic thyroid cancer cell lines (BHT-101 and CAL-62) and primary cultures from patients who underwent thyroidectomy for anaplastic thyroid cancer were treated with the histone deacetylase inhibitor LBH589. After treatment, we evaluated the expression and function of NIS. Gene expression was evaluated by real-time PCR (RT-PCR); NIS promoter activity was determined with a luciferase reporter assay; and protein expression was assessed through immunofluorescence. We tested the protein function by 125I uptake and efflux experiments; finally the cytotoxic effect of 131I was determined with a clonogenic assay. Results: Our results demonstrate that treatment with LBH589 leads to NIS RNA expression as shown by RT-PCR and luciferase assay, and to protein expression as determined by immunofluorescence in vitro and by immunohistochemistry in xenograft tumors. Moreover, 125I uptake and efflux experiments show the correct protein function and iodine retention, that translate into cytotoxicity effects, as demonstrated by a clonogenic assay with 131I . Conclusions: This study supplies a new potential strategy for the treatment of ATC by modifying gene expression with the aim of inducing responsiveness towards radioiodine therapy.