Synergistic cytotoxicity of Rana catesbeiana ribonuclease and IFN-γ, on hepatoma cells

RC-RNase purified from Rana catesbeiana (bullfrog) oocytes is a pyrimidine-guanine sequence-specific ribonuclease. RC-RNase is derived from the RNase superfamily genes exerting distinct ribonucleolytic activity and possesses cytotoxicity to tumor cells, but rarely to primary cells. In this study, we utilized RC-RNase to function with antiproliferative cytokines. The combination with TNF-α or TNF-β would not aggravate cell death. However, the combination with IFN-γ could induce synergistic cytotoxicity verified by XTT assays toward three hepatoma cell lines bearing different differentiation stages. The distinct cytotoxicity from RC-RNase or RC-RNase/IFN-γ on different hepatoma cells was correlated with the differentiation extent but not the proliferation rate of the cells. Despite the synergistic cytotoxicity and severe mitochondrial disruptions in the RC-RNase/IFN-γ-treated cells, we scarcely detected any significant feature of apoptosis or necrosis by FACS analysis on annexin-VI propidium iodide staining. The mechanisms of cell death triggered by RC-RNase or RC-RNase/IFN-γ require further investigation.