Pro-oxidative activities of (-)-epigallocatechin-3-gallate: Auto-oxidation and induction of oxidative stress.

摘要

(-)-Epigallocatechin-3-gallate (EGCG), the principal polyphenol in green tea, has been suggested as a chemopreventive agent against cancer in both cell line and animal studies. The pro-oxidative activities of EGCG involved in the anti-cancer effects need to be studied. The aims of this research were: (1) to characterize the auto-oxidation of EGCG and its products under cell culture conditions; (2) to study the dependency of growth factor receptor inactivation and cell growth inhibition on EGCG auto-oxidation; and (3) to determine possible EGCG oxidation in cancer cells and organs and tumors of EGCG-treated mice.; Under culture conditions EGCG was unstable, with a half-life of 30 min; EGCG dimers and other oxidative products were formed. EGCG was stabilized by the presence of superoxide dismutase (SOD) in the cell culture medium, with the half-life elongated to 24 h. A mechanism of EGCG auto-oxidation is proposed. Pretreatment of cancer cell lines with EGCG before the addition of epidermal growth factor (EGF) led to decreased levels of EGF receptor (EGFR) and phosphorylated EGFR. These effects of EGCG were prevented or delayed by the addition of SOD (5 U/ml), or SOD plus catalase (30 U/ml), to the cell culture medium. Similar phenomena were observed with HER-2/neu and PDGF-Rbeta in other cancer cell lines. EGCG inhibited cancer cell growth in the presence of SOD/catalase and/or serum, probably through auto-oxidation-independent actions. This effect was less dramatic compared to that of auto-oxidized EGCG.; Even in the presence of SOD/catalase, EGCG treatment still resulted in ROS production in both H1299 and KYSE 150 cells, causing DNA damage reflected by the phosphorylation of one histone 2A isomer (MAX). Daily i.p. injection of 40 mg/kg EGCG for 4 weeks increased the levels of pH2A.X and metallothionein in the liver of NCR nu/nu mice. These markers were little affected in kidney, lung, and small intestine or by treatments through oral administration of EGCG. EGCG treatments through i.p. injection (40 mg/kg) or diet (0.2%) increased the pH2A.X level in H1299 xenograft tumors that the animals carry. The relationship between the EGCG-induced oxidative stress and the inhibitory effects remains to be determined.