Formation of the depurinating N3adenine and N7guanine adducts by reaction of DNA with hexestrol-3′,4′-quinone or enzyme-activated 3′-hydroxyhexestrol: Implications for a unifying mechanism of tumor initiation by natural and synthetic estrogens

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The nonsteroidal synthetic estrogen hexestrol (HES), which is diethylstilbestrol hydrogenated at the C-3–C-4 double bond, is carcinogenic. Its major metabolite is the catechol, 3′-OH-HES, which can be metabolically converted to the catechol quinone, HES-3′,4′-Q. Study of HES was undertaken with the scope to substantiate evidence that natural catechol estrogen-3,4-quinones are endogenous carcinogenic metabolites. HES-3′,4′-Q was previously shown to react with deoxyguanosine to form the depurinating adduct 3′-OH-HES-6′-N7Gua by 1,4-Michael addition [Jan S-T, Devanesan PD, Stack DE, Ramanathan R, Byun J, Gross ML, et al. Metabolic activation and formation of DNAadducts of hexestrol,a synthetic nonsteroidal carcinogenic estrogen. Chem Res Toxicol 1998;11:412–9.]. We report here formation of the depurinating adduct 3′-OH-HES-6′-N3Ade by reaction of HES-3′,4′-Q with Ade by 1,4-Michael addition. The structure of the N3Ade adduct was established by NMR and MS. We also report here formation of the depurinating 3′-OH-HES-6′-N7Gua and 3′-OH-HES-6′-N3Ade adducts by reaction of HES-3′,4′-Q with DNA or by activation of 3′-OH-HES by tyrosinase, lactoperoxidase, prostaglandin H synthase or 3-methylcholanthrene-induced rat liver microsomes in the presence of DNA. The N3Ade adduct was released instantaneously from DNA, whereas the N7Gua adduct was released with a half-life of approximately 3 h. Much lower (<1%) levels of unidentified stable adducts were detected in the DNA from these reactions. These results are similar to those obtained by reaction of endogenous catechol estrogen-3,4-quinones with DNA. The similarities extend to the instantaneously-depurinating N3Ade adducts and relatively slowly-depurinating N7Gua adducts. The endogenous estrogens, estrone and estradiol, their 4-catechol estrogens and HES are carcinogenic in the kidney of Syrian golden hamsters. These results suggest that estrone (estradiol)-3,4-quinones and HES-3′,4′-Q are the ultimate carcinogenic metabolites of the natural and synthetic estrogens, respectively. Reaction of the electrophilic quinones by 1,4-Michael addition with DNA at the nucleophilic N-3 of Ade and N-7 of Gua is suggested to be the major critical step in tumor initiation by these compounds.