Possible mechanisms of green tea effects on health via modulation of redox balance and DNA damage & repair in vitro and in vivo

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Possible mechanisms of green tea effects on health via modulation of redox balance and DNA damage & repair in vitro and in vivo


Author: Ho, Kin-chun Cyrus
Title: Possible mechanisms of green tea effects on health via modulation of redox balance and DNA damage & repair in vitro and in vivo
Degree: M.Phil.
Year: 2013
Subject: Green tea -- Analysis.
DNA repair.
Active oxygen in the body.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Health Technology and Informatics
Pages: xxi, 239 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2652667
URI: http://theses.lib.polyu.edu.hk/handle/200/7212
Abstract: It has been previously reported that regular intake of green tea is associated with lower basal oxidation-induced damage to DNA and increased resistance of lymphocytic DNA to oxidant challenge. However, in vitro pro-oxidant, genotoxic and cytotoxic effects of green tea polyphenols at high concentrations have been reported in the literature. This differential effect observed in the in vitro and in vivo studies may be mediated by H₂O₂, which in large amounts is genotoxic, but in small amounts could activate the redox-sensitive Antioxidant Response Element (ARE) via the Keap-1/Nrf2 redox switch, inducing cytoprotection. This study, consisting of both in vitro and in vivo parts, aimed to test this hypothesis. For the in vitro part of the study, pooled lymphocytes from venous blood of 5 healthy volunteers were incubated for 30 min at 37°C in 1) freshly prepared green tea (Pre-rain Loong-cheng tea) solutions (0.005%, 0.01%, 0.05%w/v in PBS, with PBS as control), 2) tea solutions with addition of catalase (CAT) and, 3) tea solutions with addition of catalase and superoxide dismutase (CAT+SOD). The H₂O₂ concentrations of 0.005%, 0.01% and 0.05% tea solutions after 30 min at 37°C were, respectively, 3, 7 and 52μM. Genoprotective effect of green tea on lymphocytic DNA (assessed using the Fpg-assisted comet assay) at lower doses, i.e. 0.005% and 0.01%, was observed but a damaging effect was seen at higher dose of 0.05% tea. Addition of enzymes to remove H₂O₂ lowered the DNA damage induced by the 0.05% tea solution, but did not result in loss of genoprotection seen with the low doses. Gene expression of HMOX1, NRF2, KEAP1, BACH1, and hOGG1 were not found to be altered in any treatments compared to corresponding PBS control.
For the in vivo part, a controlled human intervention trial was conducted to investigate the acute (up to 2 hours post-ingestion) effects of a single dose of 1.5% pre-rain Loong-cheng green tea and effects on fasting samples of 7 days of twice daily 1% tea intake. Subjects (n=16) were randomised to take either tea (n=8) or water (n=8) first, and after a 4-week washout period they were crossed over to the other treatment. Samples were collected pre- and post both treatments. Results showed that single dose and regular intake of green tea were associated with relative decreases of ~30% (p<0.05) in lymphocytic DNA damage in samples collected at 60min and 120min after the single dose, and in fasting samples after 7 days of regular intake of green tea. Furthermore, after 7 days' supplementation with green tea, lymphocytic hOGG1 activity (measured using a variation of the comet assay; p<0.001), and HO-1 protein expression (p<0.05) were increased. However, no significant changes in total antioxidant capacity (as the FRAP value) in plasma or in gene expression of ARE-related factors (HMOX1, NRF2, KEAP1, BACH1, NQO1, GSTα, and XRCC5) were observed. A significant, inverse correlation was seen between plasma catechin concentrations and oxidation-induced DNA lesions, and a positive correlation between catechins and hOGG1 activity was seen. This is the first study to investigate the effect of green tea on redox-controlled adaptive cytoprotection in a controlled human intervention trial. Genoprotection by green tea was confirmed in both the in vitro and in vivo parts of the study, and the H₂O₂-induced genotoxic effects of high dose tea were also confirmed in the in vitro part. However, results did not support the initiation of the Nrf2/ARE signalling pathway by green tea as we saw no changes in gene expression. The in vitro genoprotection seen with low dose tea could be due to direct antioxidant protection by green tea polyphenols, or to H₂O₂-independent ARE-induction. The supplementation-related effects could also be due to direct antioxidant effects of the absorbed catechins, but could be due to post-translational effects on hOGG1, through which DNA repair activity is enhanced, and HO-1 and its cytoprotective effects.

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