Genoprotection and genotoxicity of green tea (Camellia sinensis): Are they two sides of the same redox coin?

Objectives: Regular intake of green tea associates with lower DNA damage and increased resistance of DNA to oxidant challenge. However, in vitro pro-oxidant effects of green tea have been reported. Both effects could be mediated by hydrogen peroxide (H2O2) which is generated by autoxidation of tea catechins. In large amounts, H2O2 is genotoxic, but low concentrations could activate the redox-sensitive antioxidant response element (ARE) via the Keap-1/Nrf2 redox switch, inducing genoprotective adaptations. Our objective was to test this hypothesis. Methods: Peripheral lymphocytes from healthy volunteers were incubated for 30 minutes at 37 degrees C in freshly prepared tea solutions (0.005, 0.01, 0.05% w/v (7, 14, 71 mu mol/l total catechins) in phosphate buffered saline (PBS), with PBS as control) in the presence and absence of catalase (CAT). H2O2 in tea was measured colorimetrically. Oxidation-induced DNA lesions were measured by the Fpg-assisted comet assay. Results: H2O2 concentrations in 0.005, 0.01, and 0.05% green tea after 30 minutes at 37 degrees C were, respectively, similar to 3, similar to 7, and similar to 52 mu mol/l. Cells incubated in 0.005 and 0.01% tea showed less (P < 0.001) DNA damage compared to control cells. Cells treated with 0.05% green tea showed similar to 50% (P < 0.001) more DNA damage. The presence of CAT prevented this damage, but did not remove the genoprotective effects of low-dose tea. No significant changes in expression of ARE-associated genes (HMOX1, NRF2, KEAP1, BACH1, and hOGG1) were seen in cells treated with tea or tea + CAT. Conclusion: Genoprotection by low-dose green tea could be due to direct antioxidant protection by green tea polyphenols, or to H2O2-independent signalling pathways.