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

ABSTRACT

The 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.

KEYWORDS :

• Glutathione
• Green Tea Polyphenols
• In Vitro Cytotoxicity
• Oxidative Stress