Valproic Acid Sensitizes TRAIL-Resistant Anaplastic Thyroid Carcinoma Cells to Apoptotic Cell Death

Background. Anaplastic thyroid carcinoma (ATC) is an aggressive human tumor associated with a median survival of 2-6 months. TRAIL, as a ligand of death receptors, is known to induce apoptotic cell death in several cancer cells. However, TRAIL treatment alone is not effective against TRAIL-resistant cancer cells. This study was designed to investigate whether valproic acid (VPA) enhances apoptotic cell death of TRAIL-resistant ATC cells and to identify the mechanism of cell death of ATC cells by combination treatment with VPA and TRAIL. Methods. To evaluate the cytotoxic effect of TRAIL and/or VPA on ATC cells, we used the MTT assay. The effects of VPA and TRAIL on apoptosis were assessed using FACS analysis (Annexin-V/PI stain) and Western blotting. Results. The combination of VPA with TRAIL significantly induced apoptotic cell death compared with 8505C and ARO cells treated with TRAIL alone. The protein levels of cleaved caspase-8, -3, and PARP were increased in VPA and TRAIL co-treated ARO cells. The combination induced the activation of JNK and the phosphorylation of FADD and c-Jun but not p38. However, pretreatment with caspase inhibitors reduced the expression of cleaved caspase-8, -3, and PARP in co-treated ARO cells. SP600125 remarkably reduced the expression of cleaved caspase-8, -3, and PARP and the phosphorylation of FADD and c-Jun, as well as apoptotic cell death. Conclusions. VPA sensitized TRAIL-resistant ATC cells to apoptotic cell death through involvement of the JNK pathway. Thus, the combination of VPA and TRAIL may be a promising therapy for ATC.