Role of Catechin Quinones in the Induction of EpRE-Mediated Gene Expression

Abstract

In the present study, the ability of green tea catechins to induce electrophile-responsive element (EpRE)mediated gene expression and the role of their quinones in the mechanism of this induction were investigated. To this end, Hepa1c1c7 mouse hepatoma cells were used, stably transfected with a luciferase reporter gene under the expression regulation of an EpRE from the human NAD(P)H:quinone oxidoreductase 1 (NQO1) gene. The results obtained show that several, but not all, catechins tested are able to induce EpRE-mediated gene transcription, with epigallocatechin gallate (EGCG) and gallocatechin gallate (GCG), both containing a pyrogallol and a galloyl moiety, being the most powerful inducers. Moreover, it was demonstrated that the EpRE-mediated response to catechins was increased in cells with reduced cellular glutathione (GSH) levels and decreased in cells with increased levels of GSH, corroborating a role for catechin quinones. The intrinsic capacity of catechins to form quinone type metabolites upon their oxidation was demonstrated using incubations of epigallocatechin (EGC) and EGCG with tyrosinase and the GSH-trapping method. Glutathione conjugates formed in these incubations were identified as 2′-glutathionyl-EGC, 2′,6′-diglutathionyl-EGC, 2′-glutathionyl-EGCG, and 2′,6′diglutathionyl-EGCG, supporting the formation of quinone type metabolites involving especially the pyrogallol moiety of these catechins. Formation of the EGCG-quinone-glutathionyl adducts was also observed in the EpRE-LUX cellular system. This further supports the importance of the pyrogallol moiety for the quinone chemistry of the catechins. Finally, the presence of the pyrogallol moiety in the catechins also results in a relatively lower half-wave oxidation potential (E 1/2 ) and calculated heat of formation (DHF) for conversion of the catechins to their corresponding quinones, pointing at an increased ability to become oxidized. Altogether, our studies reveal that catechins, especially those containing a pyrogallol moiety, induce EpRE-mediated detoxifying gene expression and that this induction is likely to be the result of their quinone chemistry.

The influence of stereochemistry on the antioxidant properties of catechin epimers

“…The antioxidant action of catechin epimers was observed not only in in vitro studies but also in ex vivo and in vivo studies [12,[26][27][28]. Since pH range of different human body fluids and tissues varies widely, from pH [12]; SC 50 concentration of catechins required to give a 50 % decrease in the signal intensity of DPPH Á radical; c [24], IC 50 concentration of catechins required to give a 50 % decrease in the lipid peroxidation assay; d [14]; e [25] 1.0-2.0 in the stomach, pH 5.7-6.5 in the duodenum, pH 5.5-7.0 in the colon, pH 5.9-6.8 in jejunum, pH 6.8 in esophagus, through pH 7.3-7.7 in ileum, pH 7.4 in blood and other body fluids to the pH 7.6-8.6 in pancreas [15,16], the effect of pH on the antioxidant properties of catechin epimers may be of biological relevance.…”

Antioxidant and Cytoprotective Properties of Three Egyptian Cyperus Species Using Cell-free and Cell-based Assays

“…Previous reports showed the presence of polar classes of secondary metabolites in these two species such as polyphenols, alkaloids and essential oils which are known to possess antioxidant activities [18][19][20][21][22][23].…”

Chemical and Biological Mechanisms of Phytochemical Activation of NRF2 and Importance in Disease Prevention

“…Importantly, the oxidized species is much more reactive toward thiolates, and its conjugation products have been detected with glutathione (GSH), N -acetylcysteine (NAC), cysteine [34, 35], and protein cysteine residues [36]. Similarly, EGCG contains several aromatic 1,2-dihydroxy units, and thus can form quinones that react with isolated and protein thiolates, as demonstrated in both biochemical experiments and cells [37, 38]. EGCG has also been found in mouse urine as the S -cysteinyl–EGCG conjugate after a high oral dose [39].…”

“…EGCG has also been found in mouse urine as the S -cysteinyl–EGCG conjugate after a high oral dose [39]. Furthermore, the oxidative conversion of phenolic compounds to Michael acceptors has been shown to correlate strongly with cytoprotective enzyme induction in studies evaluating tert- butylhydroquinone (tBHQ) [40] and EGCG analogs [38], as well as a broad series of phenols [41]. As further evidence that the oxidation of phenols is a prerequisite to ARE induction, Cu 2+ or other oxidized transition metal cations in the media strongly stimulated the ARE induction potential of para- and ortho- hydroquinones [42].…”