Autooxidation of green tea polyphenols generates H2O2 and quinones which have been implicated in cell death induced by these polyphenols. While autooxidation of epigallocatechin-3-gallate (EGCG), the major green tea polyphenol, has been shown in vitro where oxygen tension is high, the role of reactive oxygen species in the action of EGCG in vivo is currently gaining attention. When DU145 prostate cancer cells were treated in vitro with EGCG, PKC isoenzymes were inactivated. This inactivation was strongly correlated with EGCG-induced cell death. Cell-permeable catalase partially blocked EGCG-induced inactivation of PKC and cell death suggesting the involvement of H2O2 in these processes. EGCG-induced cell death was enhanced by 3-amino-1,2,4-triazole, a catalase inhibitor, suggesting an inhibitory role for intracellular catalase in this process. Furthermore, the inactivated PKC isoenzymes were regenerated to their active form by thiol agents in the test tube or by the thioredoxin reductase (TR) system in cells. This suggests sulfhydryl oxidation of PKC is induced by EGCG-mediated ROS generation. Previously, others have demonstrated inactivation of TR by a quinone generated by autooxidation of EGCG. We found that EGCG treatment of intact DU145 cells inactivated TR, presumably by generation of quinone. Such inactivation of TR enhanced the net increase in inactivation of PKC due to a decrease in reversal of PKC oxidative modification. Thus, the combined actions of H2O2 and quinones induce more inactivation of PKC and cell death than either agent alone. Cotreatment of cells with auranofin, a specific inhibitor of TR, enhanced EGCG-induced inactivation of PKC and cell death. Conversely, a three-fold increase in the induction of TR, by pretreatment of DU145 cells with selenite for 48 h, completely abolished both EGCG-induced inactivation of PKC and induction of cell death. This suggests a possible antagonistic interaction between selenium and EGCG. Dicumarol, an inhibitor of phase II enzyme quinone reductase, enhanced inactivation of TR, presumably by accumulation of quinones. Conversely, an induction of quinone reductase by tertiary butylhydroquinone, decreased EGCG-induced inactivation of TR, presumably due to less accumulation of EGCG-derived quinones. This decreased inactivation of TR subsequently led to a net decrease in inactivation of PKC isoenzymes and cell death. This suggests that QR is a negative regulator of EGCG-induced inactivation of PKC and cell death. Thus, EGCG oxidation products H2O2 and quinone appear to mediate EGCG-induced cell death via inactivation of PKC isoenzymes and TR respectively. Furthermore, the data suggests that both EGCG-induced PKC inactivation and cell death are antagonized by an increase in TR and QR. This study was supported by the National Cancer Institute grant CA099216.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1609. doi:1538-7445.AM2012-1609
