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(-)Epigallocatechin-3-gallate (EGCG), the principal polyphenol in green tea, has been shown to inhibit the growth of many cancer cell lines and to suppress the phosphorylation of epidermal growth factor receptor (EGFR). We observed similar effects of EGCG in esophageal squamous cell carcinoma KYSE 150 cells. Pretreatment of KYSE 150 cells with EGCG for 0.5 to 24 h in HAM’s F12 and RPMI 1640 mixed medium at 37°C, before the addition of epidermal growth factor (EGF), resulted in a decreased level of phosphorylated EGFR (by 85% to 32%). Prolonged treatment with EGCG (8 or 24 h) also decreased EGFR protein level (by 80%). Pretreatment with EGCG also caused inhibitory effects on platelet derived growth factor receptor (PDGFR) phosphorylation level. These effects of EGCG were greatly diminished by the addition of superoxide dismutase (SOD, 5 U/ml) or SOD plus catalase (30 U/ml) to the cell culture medium. Under these cell culture conditions, EGCG was unstable, with a half-life of approximately 30 min; EGCG dimers and other oxidative products were formed. The presence of SOD in the culture medium stabilized EGCG and increased its half-life to longer than 24 h, and some EGCG epimerized to (+)-gallocatechin-3-gallate (GCG). A mechanism of superoxide radical-dependent dimerization of EGCG and H2O2 formation is proposed. The stabilization of EGCG by SOD in the culture medium potentiated the activity of EGCG in inhibiting KYSE 150 cell growth. The results suggest that in cell culture conditions, the auto-oxidation of EGCG leads to EGFR inactivation, but the inhibition of cell growth is due to EGCG directly. The presently observed auto-oxidation of EGCG may or may not occur in vivo. In future studies of EGCG and other polyphenolic compounds in cell culture, SOD may be added to stabilize EGCG and to avoid possible artifacts. (Supported by NIH grant, CA88961 and CA56673)

[Proc Amer Assoc Cancer Res, Volume 46, 2005]