Inhibition of depurinating estrogen-DNA adduct formation by natural compounds

Specific metabolites of estrogens, catechol estrogen-3,4-quinones, if produced in relatively large amounts, can become chemical carcinogens by reacting with DNA to form predominandy depurinating DNA adducts. Estradiol (E₂)-3,4-quinone (Q) reacts with DNA to form predominantly the depurinating DNA adducts, 4-hydroxyestradiol (OHE₂)-1-N3Ade and 4-OHE₂-1-N7Gua. The depurinating adducts induce mutations by error-prone repair. We have conducted a study in which selected natural chemopreventing agents, N-acetylcysteine (NAcCys), melatonin, reduced lipoic acid, and resveratrol, have been tested for their ability to prevent the reaction of E₂-3,4-Q with DNA. When DNA was incubated with E ₂-3,4-Q or lactoperoxidase-activated 4-OHE₂ in the presence of an antioxidant, the formation of the N3Ade and N7Gua adducts was reduced. E₂-3,4-Q or lactoperoxidase-oxidized 4-OHE₂ (87 μM final concentration) was incubated with calf-thymus DNA and one of the antioxidants at different ratios (1:0, 1:0.3, 1:1, and 1:3 with respect to E₂-3,4-Q or 4-OHE₂) at 37°C. After 10 h, the DNA was precipitated, and the supernatant was analyzed by using ultraperformance liquid chromatography/tandem mass spectrometry (LC/MS/MS). As anticipated, resveratrol and melatonin did not affect the formation of the depurinating adducts when E₂-3,4-Q was reacted with DNA in their presence. On the other hand, NAcCys and lipoic acid (reduced form) showed a significant inhibition of the formation of the depurinating adducts by E₂-3,4-Q. With reaction of lactoperoxidase-activated 4-OHE₂ with DNA, resveratrol achieved the highest level of inhibition, NAcCys and reduced lipoic acid produced moderate inhibition, and melatonin had the least inhibition. These results demonstrate that all four selected compounds can inhibit the formation of depurinating estrogen-DNA adducts and set the stage for studies of their ability to inhibit adduct formation and malignant transformation in mammary epithelial cells. This approach is highly useful for identifying agents to prevent the initiation of human cancers, especially breast and prostate cancer.