Glutathione as a Mediator of the In Vitro Cytotoxicity of a Green Tea Polyphenol Extract

Abstract

ABSTRACTThe 24-hr cytotoxicities of a green tea polyphenol (GTP) extract to cell lines derived from the human oral cavity were assessed using the neutral red (NR) assay. The sequence of sensitivity was carcinoma HSC-2 cells > immortalized gingival GT1 fibroblasts > normal gingival HGF-2 fibroblasts. The GTP extract generated hydrogen peroxide (H2O2) in cell culture medium and in phosphate buffer, albeit to a lesser extent. A 3-hr exposure to the GTP extract lowered the intracellular glutathione (GSH) content of the HSC-2 cells, but stimulated that of the GT1 and HGF-2 fibroblasts. The cytotoxicity of a 4-hr exposure of the GTP extract to the HSC-2 and GT1, but not to the HGF-2, cells was lessened in the presence of 2.5 mM GSH. Conversely, a 0.5 hr preexposure to the glutathione depleter, 1-chloro-2-dinotrobenzene (CDNB) at 25 μM, potentiated the 24-hr cytotoxicity of the GTP extract to the HSC-2 and GT1, but not to the HGF-2, cells. Using a cell-free system, it was shown that the GTP extract quickly depleted GSH, with depletion greatly enhanced at an alkaline pH, thus, correlating with the enhanced generation of H2O2 by the GTP extract observed at alkaline pH. Apparently, a mode of cytotoxicity of the GTP extract, in particular to the carcinoma HSC-2 cells, was to induce oxidative stress, as noted by the generation of H2O2, the depletion of intracellular GSH, the protection afforded by extracellular GSH, and cell hypersensitivity after pretreatment with CDNB.