Abstract

Recent studies in our laboratory in the human hepatic and intestinal cell lines Hep G2 and Caco-2 have demonstrated induction of UGT1A1 by the flavonoid chrysin (5,7-dihydroxyflavone) using catalytic activity assays and Western and Northern blotting. In the present study, we examined which features of the flavonoid structures were associated with induction of UGT1A1 and whether common drug-metabolizing enzyme inducers also produce this induction. We also determined whether flavonoid treatment affected sulfate conjugation and CYP1A1 activity. We used intact Hep G2 cells for these studies, with chrysin as the model substrate. Both glucuronidation and sulfation were measured. Hep G2 cells were pretreated for 3 days with 25 μM concentrations of 22 flavonoids (n = 4–12). Only four flavonoids demonstrated induction of glucuronidation similar to that of chrysin (i.e., 3–5-fold in the intact cells). These were acacetin, apigenin, luteolin, and diosmetin, all of which, like chrysin, are 5,7-dihydroxyflavones with varying substituents in the B-ring. 5-Hydroxy-7-methoxyflavone and 5-methyl-7-hydroxyflavone produced a modest 1.5 to 2-fold induction, whereas all other flavonoids examined were without effect. None of the flavonoids caused more than a modest change in sulfation activity (60–140% of control). In contrast, all tested 5,7-dihydroxyflavones and -flavonols induced CYP1A1 activity (ethoxyresorufin deethylation). Of seven common drug-metabolizing enzyme inducers only 3-methylcholanthrene and oltipraz showed modest induction of chrysin glucuronidation but not 2,3,7,8-tetrachlorodibenzo-p-dioxin or phenobarbital. Together, these results strongly suggest that the flavonoid induction of UGT1A1 is through a novel nonaryl hydrocarbon receptor-mediated mechanism.