Differential effects of flavonoid compounds on tumor promoter-induced activation of the human CYP1A2 enhancer

Flavonoids, a family of naturally occurring polyphenolic compounds found in many fruits, nuts, vegetables, and beverages, appear to inhibit tumor prom otion as part of their chemopreventive properties, To investigate at the mo lecular level the ability of flavonoids to inhibit tumor-promoting activity , we developed a cell line designed to screen for flavonoids that block the tumor promoter-mediated induction of activator protein-1 (AP-1) transcript ional activity. This cell line, T2Luc, is a HepG2-derived cell line stably integrated with a region of the human CYP1A2 5'-Banking gene containing two AP-1 binding sites linked to the thymidine kinase promoter-driven grey luc iferase reporter gene. Treatment of T2Luc with a commercial extract of gree n tea alone had no effect on luciferase activity, but did block the inducti on of luciferase when cells were further challenged with the tumor promoter phorbol 12-O-tetradecanoate 13-acetate (TPA), In contrast, treatment of ce lls with the flavonoid quercetin alone activated luciferase activity in a c oncentration-dependent manner and enhanced the TPA-induced transcription of luciferase, Gel mobility shift assays using nuclear extracts from cells tr eated with green tea extracts or TPA alone revealed induced binding of AP-1 proteins to the CYP1A2 3'AP-1 site, Pretreatment with green tea extracts d id not inhibit the TPA-induced formation of AP-I complexes. Quercetin treat ment alone slightly enhanced binding of AP-1 complexes to this site, Our re sults suggest that these dietary chemopreventive agents may work through di fferent pathways to modulate gene expression. (C) Academic Press. Press.