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Background-HSP90 is a chaperone protein regulating several client proteins involved in thyroid cancer development. The purpose of this study is to mechanisticially evaluate a novel natural-product HSP90 inhibitor in thyroid cancer cell lines for future translational applications.

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e17003 Background: Epithelial cancers, particularly lung cancer and head and neck squamous cell carcinoma (HNSCC), continue to pose formidable challenges in clinical practice. Novel chemotherapeutic agents have been developed, but even those have limited long-term benefits in the treatment of these tumors, illustrating the need to continue to improve systemic therapy for affected patients. The objective of the present study was to investigate the effect of withaferin A (WA), a plant-derived small molecule, on cancer cell growth, heat shock protein expression and induction of apoptosis in human HNSCCs. Methods: MDA1986 and JMAR HNSCC cells were used in all experiments. The effect of WA on cell viability was determined by MTS assay. Apoptosis and mitochondrial membrane potential changes were assessed by annexin V/propidium iodide and JC-1 staining respectively using standard flow cytometry methods. Effect of WA on modulation of heat shock proteins was determined by Western blot analysis. Results: Withaferin A reduces cell viability in both MDA1986 and JMAR cells with IC50 levels of 265 ± 5 nM by MTS assay, which is 5 fold higher than its reported activity in breast cancer cells. WA completely down-regulates HSP90beta, GRP94, and TRAP-1 expression at 250 nM concentration in HNSCC cells at 24 hours treatment. In addition, WA markedly increased HSP70 levels and mildly increased HSP27 levels in a dose-dependent manner at 24 hours treatment. Flow cytometry with Annexin V/PI staining shows that 5 μM WA treatment for 24 hours induced apoptosis in 63% of MDA1986 and 60% of JMAR cells. WA at 5 μM also reduced mitochondrial membrane potential by JC-1 staining with flow cytometry to less than 10% of controls in both JMAR and MDA1986 cells at 24 hours treatment. Conclusions: Withaferin A is a potent novel inhibitor of HSP90 in human HNSCCs. In addition to HSP modulation, its anticancer mechanistic effects involve apoptotic cell death through the mitochondrial pathway. This molecule shows promise for further in vivo studies to establish preclinical proof of concept as a novel anticancer therapy in this disease. No significant financial relationships to disclose.

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