Background: Anaplastic thyroid cancer (ATC) is highly aggressive and has very limited treatment options. Recent studies suggest that cancer stem cell (CSC) activity in ATC could underlie this recurrence and resistance to treatment. The recent approval by the U.S. Food and Drug Administration of the combined treatment of BRAF and MEK inhibitors for ATC patients has shown some efficacy in patients harboring the BRAFV600E mutation. However, it was unknown whether the combined treatment could affect the CSC activity. This study explores the effects of the BRAF and MEK inhibitors on CSC activity in human ATC cells. Methods: Using three human ATC cells, THJ-11T, THJ-16T, and 8505C cells, we evaluated the effects of dabrafenib (a BRAF kinase inhibitor), trametinib (an MEK inhibitor), or a combined treatment of the two drugs on the CSC activity by tumorsphere formation, Aldefluor assays, expression profiles of key CSC markers, immunohistochemistry, and in vivo xenograft mouse models. Furthermore, we also used confocal imaging to directly visualize the effects on drugs on CSCs by the SORE6-mCherry reporter in cultured cells and xenograft tumor cells. Results: The BRAF inhibitor, dabrafenib, had weak efficacy, while the MEK inhibitor, trametinib, showed strong efficacy in attenuating the CSC activity, as evidenced by suppression of CSC marker expression, tumorsphere formation, and Aldefluor assays. Using ATC cells expressing a fluorescent CSC SORE6 reporter, we showed reduction of CSC activity in the rank order of combined > trametinib > dabrafenib through in vitro and in vivo xenograft models. Molecular analyses showed that suppression of CSC activity by these drugs was, in part, mediated by attenuation of the transcription by dampening the RNA polymerase II activity. Conclusions: Our analyses demonstrated the presence of CSCs in ATC cells. The inhibition of CSC activity by the MEK signaling could partially account for the efficacy of the combined treatment shown in ATC patients. However, our studies also showed that not all CSC activity was totally abolished, which may account for the recurrence observed in ATC patients. Our findings have provided new insights into the molecular basis of efficacy and limitations of these drugs in ATC patients.
Keywords: BRAF inhibitor; MEK inhibitor; anaplastic thyroid cancer; cancer stem cells; xenograft models.