Green tea is one of the beverages consumed in the highest quantity in the world. Epidemiologic research has revealed that individuals who drink a lot of green are less likely to develop cancer (28
). Very recently, a relationship between the consumption of green tea and a reduced risk of type 2 diabetes was reported (23
). Green tea contains many ingredients considered to promote health such as polyphenolic flavonoids, of which epigallocatechin gallate (EGCG) is the major constituent. Evidence is mounting that EGCG has anticarcinogenic activity in vitro (3
), which may support the results of the epidemiologic research on the correlation between drinking green tea and the risk of morbidity from cancer.
Many studies have been done on the biological activity of green tea extract (GTE) and individual catechins in vitro. EGCG is widely accepted as an antioxidant. For example, EGCG scavenges superoxide anion radicals (O2·−
), hydrogen peroxide (H2
), hydroxy radicals (HO·
), peroxyl radicals, singlet oxygen, and peroxynitrite (5
). The one-electron reduction potential of EGCG under standard conditions is 550 mV, a value lower than that of glutathione (920 mV) and comparable to that of α-tocopherol (480 mV) (13
). Besides directly scavenging reactive oxygen/nitrogen species, EGCG chelates redox-active metal ions, such as iron and copper, leading to a reduction in the production of reactive oxygen species. Accordingly, many food supplements or beverages containing a high concentration of EGCG (>1 mM) have been developed, and therefore, the physiological function of EGCG in vivo with a high-dose ingestion remains to be elucidated.
In contrast to its antioxidative activity, recent experiments in vitro indicate that EGCG produces reactive oxygen species. For example, EGCG promotes apoptosis and has bactericidal activity, which is attributed to its ability to reduce O2
to yield H2
). We have previously found that EGCG has an adverse effect on the protection of Escherichia coli
cells against oxidative damage in the presence of H2
and copper ions (21
). In addition, we have recently revealed that mutant strains of the budding yeast Saccharomyces cerevisiae
that are defective in Yap1 and/or Skn7 show increased sensitivity to GTE (52
). Both Yap1 and Skn7 are transcription factors critical for the response to oxidative stress. We have also reported that the expression of TRX2
, which codes for the antioxidant enzyme thioredoxin and is one of the target genes for both Yap1 and Skn7 (32
), is induced following treatment with GTE (52
). Consequently, mutants defective in Yap1 and/or Skn7 showed increased susceptibility to GTE (52
). These results imply that GTE induces a response to oxidative stress in yeast.
In spite of numerous reports about the effects of polyphenolic flavonoids on the mammalian system, intriguingly, little attention has been paid to the use of yeast to analyze the biological activity of tea polyphenols, though this organism has provided an excellent model for the study of many biological events. In the present study, to assess whether GTE actually causes oxidative stress-related responses in yeast, we determined the activity levels of oxidative-stress-responsive transcription factors, i.e., Yap1 and Msn2 in S. cerevisiae
and Pap1 in the fission yeast Schizosaccharomyces pombe
. The activities of these transcription factors are regulated through their nucleocytoplasmic localization (14
). We show that these transcription factors are concentrated in the nuclei of cells treated with GTE or EGCG. Additionally, we demonstrate that the nuclear localization of Yap1 and Msn2 is repressed by catalase but not by superoxide dismutase. In regard to Yap1, the formation of intramolecular disulfide bonds has been proposed as crucial for its nuclear localization (10
). We verify the correlation between the GTE- and EGCG-induced nuclear localization of Yap1 and the transcriptional activation of TRX2
using various Cys-replaced Yap1 mutants. Finally, we show that both GTE and EGCG produce H2
under weak alkaline conditions, which induce a response to oxidative stress in yeast cells.