S. Jang1, X. Yu1, S. K. Odorico1, M. Clark1, C. Schienebeck2, W. Tang2, H. Chen1 1University Of Wisconsin,Department Of Surgery,Madison, WI, USA 2University Of Wisconsin,Department Of Chemistry,Madison, WI, USA
Introduction: There are limited effective therapies for aggressive thyroid malignancies including anaplastic and poorly differentiated cancers. Compounds targeting histone deacetylases (HDAC) have shown promising antitumor activities in others cancers. In order to develop novel therapies for these aggressive thyroid cancers, we synthesized a new group of analogues targeting HDACs named AB1 to AB13 which has different linkers between a metal chelating group and a hydrophobic cap. Therefore, the purpose of this study was to screen out the most effective compounds and evaluate the therapeutic efficacy in aggressive thyroid cancers.
Methods: Anaplastic (HTh7 and 8505C) and metastatic follicular (FTC236) thyroid cancer cells were treated with thirteen AB analogues using various concentrations, and the IC50 was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The most effective compounds were selected based on the IC50 to further study the molecular mechanisms of growth regulation. Both cell cycle regulatory proteins and apoptosis markers were analyzed by the Western blot. In addition, the expression levels of the thyrocyte-specific genes were quantified by real-time PCR to assess the drug potency of inducing re-differentiation in aggressive thyroid cancer cells.
Results: Among all the analogues, AB1, 4, 5, 6, 13 showed very limited cytotoxicity effect while AB7, 8, 9, 11, and 12 presented with moderate efficacy to inhibit cell growth. AB2, AB3, and AB10 demonstrated the lowest IC50 values of the thirteen screened drug analogues (Table 1). The AB analogues showed less cytotoxicity against human fibroblasts WI38. AB2, AB3, and AB10 treatment resulted in an increase of apoptosis markers including cleaved poly adenosine diphosphate ribose polymerase (PARP) and cleaved caspase 3 in a dose dependent manner in all three cancer cell lines. Additionally, the expression of cell cycle regulatory proteins p21/WAF1 and p27 Kip increased with the treatment of ABs while cyclin D1 decreased dose-dependently. Furthermore, AB2, AB3, and AB10 were able to induce various of thyrocyte specific genes in all the cell lines indicated by increased level of sodium iodide symporter (NIS), paired box gene 8 (PAX8), thyroid transcription factor 1 (TTF1), TTF2, and thyroid stimulating hormone receptors (TSHR).
Conclusion: Novel synthetic HDAC inhibitors AB2, AB3 and AB10 suppress thyroid cancer cell growth via cell cycle arrest and apoptosis. They also induce cell re-differentiation which could make aggressive thyroid cancer cells more susceptible to radioactive iodine therapy. Therefore, these compounds could be new options for patients with aggressive thyroid cancers.
