Activation of antioxidant-response element (ARE), mitogen-activated protein kinases (MAPKs) and caspases by major green tea polyphenol components during cell survival and death

Green tea polyphenols (GTP) have been demonstrated to suppress tumorigenesi s in several chemical-induced animal carcinogenesis models, and predicted a s promising chemopreventive agents in human. Recent studies of CTP extracts showed the involvement of mitogen-activated protein kinases (MAPKs) in the regulation of Phase II enzymes gene expression and induction of apoptosis. In the current work we compared the biological actions of five green tea c atechins: Iii induction of ARE reporter gene, (2) activation of MAP kinases , (3) cytotoxicity in human hepatoma HepG2-C8 cells, and 14) caspase activa tion in human cervical squamous carcinoma HeLa cells. For the induction of phase II gene assay, (-)-epigallocatechin-3-gallate (EGCG) and (-)-epicatec hin-3-gallate (ECG) potently induced antioxidant response element (ARE)-med iated luciferase activity, with induction observed at 25 muM with EGCG. The induction of ARE reporter gene appears to be structurally related to the 3 -gallate group. Comparing the activation of MAPK by the five polyphenols, o nly EGCG showed potent activation of all three MAPKs (ERK, JNK and p38) in a dose- and time-dependent manner, whereas EGC activated ERK and p38. In th e concentration range of 25 muM to 1 mM, EGCG and ECC strongly suppressed H epG2-ARE-C8 cell-growth. To elucidate the mechanisms of green tea polypheno l-induced apoptosis, we measured the activation of an important cell death protein, caspase-3 induced by EGCG, and found that caspase-3 was activated in a dose- and time-dependent manner. Interestingly, the activation of casp ase-3 was a relatively late event (peaked at 16 h), whereas activation of M APKs was much earlier (peaked at 2 h). It is possible, that at low concentr ations of EGCG, activation of MAPK leads to ARE-mediated gene expression in cluding phase II detoxifying enzymes. Whereas at higher concentrations of E CCG, sustained activation of MAPKs such as JNK leads to apoptosis. These me chanisms are currently under investigation in our laboratory. As the most a bundant catechin in CTP extract, we found that EGCG potently induced ARE-me diated gene expression, activated MAP kinase pathway, stimulated caspase-3 activity, and induced apoptosis. These mechanisms together with others, may contribute to the overall chemopreventive function of EGCG itself as well as the GTP.