Spectral characterization of fluorescently labeled catechol estrogen 3,4-quinone-derived N7 guanine adducts and their identification in rat mammary gland tissue.

The oxidation of carcinogenic 4-hydroxycatechol estrogens (CE) of estrone (E1) and estradiol (E2) to catechol estrogen 3,4-quinones (CE-3,4-Q) results in electrophilic intermediates that covalently bind to DNA to form depurinating adducts [Cavalieri et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 10937]. These DNA adducts, 4-OHE1-1-N7Gua and 4-OHE2-1-N7Gua, are nonfluorescent. To utilize laser-excited fluorescence methods, the catechol estrogen-derived metabolites and adducts were labeled with a fluorescent marker. The 4-OHEi-1-N7Gua adduct standards (i = 1, 2) and 4-OHEi metabolites have been derivatized with 1-pyrenesulfonyl chloride and investigated by low-temperature spectroscopy under non-line-narrowing and line-narrowing conditions. Molecular modeling studies assisted in interpretation of the fluorescence spectra; energetically favored structures of the 4-OHE2-1-N7Gua-dipyrene adduct and 4-OHE2-dipyrene metabolite reveal unique conformations which, in agreement with fluorescence data, show a significant pi-pi interaction of pyrene labels with guanine and/or the aromatic ring of catechol estrogen. The conformation obtained for the 4-OHE2-1-N7Gua-dipyrene adduct appears to be conducive to mixing of its pipi state with pyrene-guanine charge-transfer states, consistent with the experimentally observed strong electron-phonon coupling. Non-line-narrowed and line-narrowed spectra obtained at 77 and 4.2 K, respectively, are shown to distinguish 4-OHE2-1-N7Gua-dipyrene adducts from 4-OHE2-dipyrene metabolites. These standards have subsequently been used for the spectroscopic identification of depurinating DNA adducts formed in a tissue culture experiment where rat mammary gland tissue was treated with the estrogen quinone E2-3,4-Q. The depurinating adduct formed is 4-OHE2-1-N7Gua.

Synthesis and structure elucidation of estrogen quinones conjugated with cysteine, N-acetylcysteine, and glutathione.

Catechol estrogen quinones (CE-Q) have been implicated as ultimate carcinogenic metabolites in estrogen-induced carcinogenesis. CE-Q may covalently bind to DNA to initiate cancer. These quinones can also be conjugated with glutathione, a reaction that prevents damage to DNA by CE-Q. The glutathione conjugates are then catabolized through mercapturic acid biosynthesis to cysteine and N-acetylcysteine conjugates. This may be the most important detoxification pathway of CE-Q. The chemical synthesis and characterization of these conjugates are the first essential steps to better understand their function in biological systems. Eighteen conjugates were synthesized by reaction of estrone-3,4-quinone (E1-3, 4-Q), estradiol-3,4-quinone (E2-3,4-Q), estrone-2,3-quinone (E1-2, 3-Q), or estradiol-2,3-quinone (E2-2,3-Q) with various sulfur nucleophiles, RSH, in which R is the cysteine, N-acetylcysteine, or glutathione moiety. Reactions of E1-3,4-Q and E2-3,4-Q produce regiospecifically 4-OHE1-2-SR and 4-OHE2-2-SR, respectively, in almost quantitative yield. E1-2,3-Q and E2-2,3-Q react regioselectively and quantitatively to form 2-OHE1(E2)-1-SR and 2-OHE1(E 2)-4-SR, in which the 1-isomers are always the major products. The ratio between 1 and 4 isomers is 3.5 for cysteine, 2.7 for N-acetylcysteine, and 2.5 for glutathione. The synthesized conjugates will be used as standards in the identification of these compounds formed in biological systems.

Metabolic activation and formation of DNA adducts of hexestrol, a synthetic nonsteroidal carcinogenic estrogen.

Hexestrol (HES), a synthetic nonsteroidal estrogen, is carcinogenic in Syrian golden hamsters. The major metabolite of HES is its catechol, 3'-OH-HES, which can be metabolically converted to the electrophilic catechol quinone, HES-3',4'-Q, by peroxidases and cytochrome P450. Standard adducts were synthesized by reacting HES-3',4'-Q with dG and dA to produce the adducts 3'-OH-HES-6'(alpha, beta)-N7Gua and HES-3',4'-Q-6'-N6dA, respectively. When HES-3',4'-Q was reacted with calf thymus DNA, 3'-OH-HES-6'(alpha,beta)-N7Gua was identified by HPLC and tandem mass spectrometry as the depurinating adduct, with minor amounts of stable adducts. 3'-OH-HES was bound to DNA after activation by horseradish peroxidase, lactoperoxidase, or rat liver microsomes. The depurinating adduct 3'-OH-HES-6'(alpha, beta)-N7Gua was identified in these systems at levels of 65, 41, and 11 micromol/mol of DNA-P, respectively. Unidentified stable adducts were observed in much lower amounts and were quantified by the 32P-postlabeling method. Similarly to 3'-OH-HES, the catechol metabolites of the natural steroidal estrogens estrone (E1) and estradiol (E2), namely, 2-OHE1, 4-OHE1, 2-OHE2, and 4-OHE2, can be oxidized to their corresponding quinones by peroxidases and cytochrome P450. The quinones of the carcinogenic 4-OHE1 and 4-OHE2 have chemical and biochemical properties similar to those of HES-3',4'-Q. The results suggest that formation of HES-3',4'-Q may be a critical event in tumor initiation by HES and that HES is an excellent model compound to corroborate the hypothesis that estrogen-3,4-quinones are ultimate carcinogenic metabolites of the natural steroidal estrogens E1 and E2.

Molecular origin of cancer: catechol estrogen-3,4-quinones as endogenous tumor initiators.

Cancer is a disease that begins with mutation of critical genes: oncogenes and tumor suppressor genes. Our research on carcinogenic aromatic hydrocarbons indicates that depurinating hydrocarbon-DNA adducts generate oncogenic mutations found in mouse skin papillomas (Proc. Natl. Acad. Sci. USA 92:10422, 1995). These mutations arise by mis-replication of unrepaired apurinic sites derived from the loss of depurinating adducts. This relationship led us to postulate that oxidation of the carcinogenic 4-hydroxy catechol estrogens (CE) of estrone (E1) and estradiol (E2) to catechol estrogen-3,4-quinones (CE-3, 4-Q) results in electrophilic intermediates that covalently bind to DNA to form depurinating adducts. The resultant apurinic sites in critical genes can generate mutations that may initiate various human cancers. The noncarcinogenic 2-hydroxy CE are oxidized to CE-2,3-Q and form only stable DNA adducts. As reported here, the CE-3,4-Q were bound to DNA in vitro to form the depurinating adduct 4-OHE1(E2)-1(alpha,beta)-N7Gua at 59-213 micromol/mol DNA-phosphate whereas the level of stable adducts was 0.1 micromol/mol DNA-phosphate. In female Sprague-Dawley rats treated by intramammillary injection of E2-3,4-Q (200 nmol) at four mammary glands, the mammary tissue contained 2.3 micromol 4-OHE2-1(alpha, beta)-N7Gua/molDNA-phosphate. When 4-OHE1(E2) were activated by horseradish peroxidase, lactoperoxidase, or cytochrome P450, 87-440 micromol of 4-OHE1(E2)-1(alpha, beta)-N7Gua was formed. After treatment with 4-OHE2, rat mammary tissue contained 1.4 micromol of adduct/mol DNA-phosphate. In each case, the level of stable adducts was negligible. These results, complemented by other data, strongly support the hypothesis that CE-3,4-Q are endogenous tumor initiators.

Molecular characteristics of catechol estrogen quinones in reactions with deoxyribonucleosides.

Estrogens can have two roles in the induction of cancer: stimulating proliferation of cells by receptor-mediated processes, and generating electrophilic species that can covalently bind to DNA. The latter role is thought to proceed through catechol estrogen metabolites, which can be oxidized to o-quinones that bind to DNA. Four estrogen-deoxyribonucleoside adducts were synthesized by reaction of estrone 3,4-quinone (E1-3,4-Q), 17 beta-estradiol 3,4-quinone (E2-3,4-Q), or estrone 2,3-quinone (E1-2,3-Q) with deoxyguanosine (dG) or deoxyadenosine (dA) in CH3CO2H/H2O (1:1). Reaction of E1-3,4-Q or E2-3,4-Q with dG produced specifically 7-[4-hydroxyestron-1(alpha, beta)-yl]guanine (4-OHE1-1(alpha, beta)-N7Gua) or 7-[4-hydroxyestradiol-1(alpha, beta)-yl]-guanine (4-OHE2-1(alpha, beta)-N7Gua), respectively, in 40% yield, with loss of deoxyribose. These two quinones did not react with dA, deoxycytidine, or thymidine. When E1-2,3-Q was reacted with dG or dA, N2-(2-hydroxyestron-6-yl)deoxyguanosine (2-OHE1-6-N2dG, 10% yield) and N6-(2-hydroxyestron-6-yl)deoxyadenosine (2-OHE1-6-N6dA, 80% yield), respectively, were formed. These adducts provide insight into the type of DNA damage that can be caused by o-quinones of the catechol estrogens. The estrogen 3,4-quinones are expected to produce depurinating guanine adducts that are lost from DNA, generating apurinic sites, whereas the 2,3-quinones would form stable adducts that remain in DNA, unless repaired. The adducts reported here will be used as references in studies to elucidate the structure of estrogen adducts in biological systems.

Radical cations of benzo[a]pyrene and 6-substituted derivatives: reaction with nucleophiles and DNA.

1. Oxidation of benzo[a]pyrene (BP) by I2 in the presence of AgClO4 in benzene generates the BP.+ClO4-.AgI complex. This same method was used to produce radical cations from 6-FBP, 6-ClBP, 6-BrBP and 6-CH3BP. 2. Reaction of the BP, 6-FBP, 6-ClBP and 6-BrBP radical cation perchlorates with H2O produced BP 1,6-, 3,6- and 6,12- dione, whereas 6-CH3BP.+ClO4-.AgI yielded 6-CH2OHBP. 3. When BP.+ClO4-.AgI and 6-FBP.+ClO4-.AgI were reacted with NaOAc in H2O/CH3CN (9:1), 6-OAcBP was formed, in addition to the quinones. In the case of 6-ClBP.+ClO4-.AgI, a small amount of 1-OAc-6-ClBP and 3-OAc-6-ClBP was formed in addition to the diones, whereas for 6-BrBP and 6-CH3BP the reaction products were BP diones and 6-CH2OHBP respectively. 4. These results confirm the localization of charge in the BP.+ at C-6, followed by C-1 and C-3. 5. The reaction of BP with NOBF4 in CH2Cl2 produced BP.+BF4-, radical cation free of complexation with inorganic salts. 6. Reaction of BP.+BF4- with DNA produced the depurinating adducts BP-6-C8Gua, BP-6-C8dGua and BP-6-N7Gua.