MULTIPLE PLURIPOTENT STEM CELL MARKERS IN HUMAN ANAPLASTIC THYROID CANCER: THE PUTATIVE UPSTREAM ROLE OF SOX-2

Background: Anaplastic Thyroid Carcinoma (ATC) is a rare and aggressive endocrine tumor, with highly undifferentiated morphology. It has been suggested that cancer stem cells (CSCs) might play a central role in ATC. The objectives of this study were the following: 1) to characterize CSCs from ex vivo ATC specimens by investigating the expression of several pluripotent stem cell markers; 2) to evaluate in vitro drug resistance modifications after specific CSC transcription factor switch off. Methods: Ex vivo: eight formalin-fixed, paraffin-embedded ATC specimens were analyzed by RT and qRT-PCR and immunohistochemistry. In vitro: in ATC SW1736 cells the expression levels of OCT-4, NANOG and ABCG2 and the sensitivity to either cisplatin or doxorubicin were evaluated after silencing. Results: OCT-4, KLF4 and SOX2 transcription factors and C-KIT and THY-1 stem surface antigens showed variable up-regulation in all ATC cases. The SW1736 cell line was characterized by a high percentage of stem population (10.4 ± 2.1 % of cells were aldehyde dehydrogenase positive) and a high expression of several CSC markers (SOX2, OCT4, NANOG, C-MYC, SSEA4). SOX2 silencing down-regulated OCT-4, NANOG and ABCG2. SOX2 silencing sensitized SW1736 cells, causing a significant cell death increase (1.8 fold) in comparison to control cells with 10 µM cisplatin (93.9±3.4% vs. 52.6±9.4%, p<0.01) and 2.7 fold with 0.5µM doxorubicin (45.8±9.9% vs. 17.1±3.4% p<0.01). ABCG2 silencing caused increased cell death with both cisplatin (74.9±1.4%) and doxorubicin treatment (74.1±0.1%) vs. no-target-treated cells (respectively, 45.8±1.0% and 48.6±1.0%, p<0.001). Conclusions: The characterization of CSCs in ATC through the analysis of multiple pluripotent stem cell markers might be useful in identifying cells with a stem-like phenotype capable of resisting conventional chemotherapy. In addition, our data demonstrate that SOX2 switch-off through ABCG2 transporter down-regulation has a major role in overcoming CSC chemotherapy resistance.