| The cytochrome (CYP) P450 monooxygenase system metabolizes drugs, toxins and chemical carcinogens. CYP1A1 and CYP1A2 enzymes are central in the bioactivation of numerous carcinogens. CYP1A enzymes are substrate-inducible and transcriptional activation is mediated by the aryl hydrocarbon receptor (AhR). Inhibition of chemically induced cancers by green tea and its components has been demonstrated in vitro and in vivo in many organ sites, and one mechanism proposed for its chemopreventive properties is the ability to modulate CYP enzymes. Therefore, we examined the effect of the green tea extract Polyphenon 100 (P100) on AhR-mediated human CYP1A expression, and investigated the contribution of individual flavonoids. In HepG2 cells, P100 inhibited the transcription of a human CYP1A1 promoter-driven reporter gene induced by the AhR ligand 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) in a concentration-dependent manner and inhibited the induced accumulation of CYP1A1 mRNA and protein; P100 alone induced CYP1A1 expression. P100 blocked TCDD-induced binding of the AhR to DNA in HepG2 cells and in vitro in isolated hepatic cytosol and caused nuclear translocation of a transiently transfected AhR-YFP fusion protein in COS-1 cells, identifying P100 as an AhR agonist/antagonist. To identify active component(s) we similarly examined the four major flavonoids in P100. We found that (−)-epigallocatechin gallate (EGCG) blocked the TCDD response, but not as markedly as P100; furthermore, EGCG functioned as a strict AhR antagonist (i.e. EGCG alone did not induce CYP1A1 expression). To identify possible synergistic activity among the flavonoids, a “reconstitution” of P100 (P100R)—a mixture of the four components mirroring the amount of each found in P100—was examined. We determined that the mixture of flavonoids was required for the full AhR agonist/antagonist effects of P100. Effects of P100, P100R and EGCG on CYP1A1 and CYP1A2 expression were confirmed using primary human hepatocytes. These studies demonstrate that green tea, and EGCG to a lesser extent, can inhibit TCDD-induced human CYP1A expression. The effect of P100 can not be attributed to a single component, but is due to the effects of a complex mixture. Chemoprevention strategies using green tea as a whole, rather than a single flavonoid, may be more effective. |
