Analysis of Type of Cell Death Induced by Topoisomerase Inhibitor SN-38 in Human Oral Squamous Cell Carcinoma Cell Lines

Despite frequent use of topoisomerase inhibitors (TIs) as antitumor agents, their application to oral squamous cell carcinoma (OSCC) has not been reported. We investigated three inhibitors of topoisomerase 1 [camptothecin, irinotecan, SN-38 (active metabolite of irinotecan)] and two inhibitors of topoisomerase II (etoposide, teniposide) for their cytotoxicity towards a total of 15 human tumor cell lines and normal cultured cells. All TIs exhibited higher cytotoxicity towards tumor cell lines (OSCC, glioblastoma, myelogenous leukemia) as compared with normal mesenchymal (gingival fibroblast, pulp cell, periodontal ligament fibroblast) and epithelial cells (skin keratinocytes). Among TIs, SN-38 had the highest cytotoxicity towards OSCC cell lines, with a tumor specificity index of 1321 compared to mesenchymal cells and 22 compared with epithelial cells. SN-38 induced different types of cell death in two OSCC cell lines: apoptosis (caspase-3 activation and internucleosomal DNA fragmentation) in HSC-2 cells and autophagy (formation of autophagosome and secondary lysosome) in HSC-4 cells. The cell death of HSC-2 and HSC-4 cells was significantly inhibited by pre-treatment with caspase inhibitor (Z-VAD-FMK) and autophagy inhibitors (3-methyladenine, bafilomycin A1), respectively. The present study demonstrated that SN-38 is highly cytotoxic to OSCC cell lines, regardless of the type of induced cell death, suggesting its future application for chemotherapy of OSCC.