Inhibition of β-catenin/Tcf activity by white tea, green tea, and epigallocatechin-3-gallate (EGCG): minor contribution of H 2O 2 at physiologically relevant EGCG concentrations

Abstract

Epigallocatechin-3-gallate (EGCG) is the major polyphenol present in white tea and green tea. Recently, it was reported that the addition of EGCG and other tea polyphenols to cell culture media, minus cells, generated significant levels of H 2O 2, with the corollary that this might represent an `artifact' in cell culture studies which seek to examine the chemopreventive mechanisms of tea. We show here that in cell growth media with and without serum, and in growth media containing human embryonic kidney 293 (HEK293) cells plus serum, physiologically relevant concentrations of EGCG (⩽25 μM) generated H 2O 2 with a peak concentration of the order of 10–12 μM. However, addition of 20 μM H 2O 2 directly to HEK293 cells transiently transfected with wild-type or mutant β-catenin constructs and TCF-4 had no significant effect on β-catenin/TCF-4 reporter activity or β-catenin expression levels. In contrast, 2–25 μM EGCG inhibited β-catenin/TCF-4 reporter activity in a concentration-dependent fashion and there was a concomitant reduction in β-catenin protein levels in the cell lysates without changes in TCF-4 expression. The inhibition of reporter activity was recapitulated by white tea and green tea, each tested at a 25 μM EGCG equivalent concentration in the assay, and this was unaffected by the addition of exogenous catalase. The results indicate that physiologically relevant concentrations of tea and EGCG inhibit β-catenin/TCF-4 reporter activity in HEK293 cells due to reduced expression of β-catenin and that this is unlikely to be an artifact of H 2O 2 generation under the assay conditions used here. These data are consistent with the findings from in vivo studies, showing the suppression of intestinal polyps by tea, via an apparent down-regulation of β-catenin and Wnt target genes.