Tumor-initiating activity of 4-fluoro-7,12-dimethylbenz[a]anthracene and 1,2,3,4-tetrahydro-7,12-dimethylbenz[a]anthracene in female SENCAR mice

The molecular etiology and prevention of estrogen-initiated cancers: Ockham's Razor: Pluralitas non est ponenda sine necessitate. Plurality should not be posited without necessity

Elucidation of estrogen carcinogenesis required a few fundamental discoveries made by studying the mechanism of carcinogenesis of polycyclic aromatic hydrocarbons (PAH). The two major mechanisms of metabolic activation of PAH involve formation of radical cations and diol epoxides as ultimate carcinogenic metabolites. These intermediates react with DNA to yield two types of adducts: stable adducts that remain in DNA unless removed by repair and depurinating adducts that are lost from DNA by cleavage of the glycosyl bond between the purine base and deoxyribose. The potent carcinogenic PAH benzo[a]pyrene, dibenzo[a,l]pyrene, 7,12-dimethylbenz[a]anthracene and 3-methylcholanthrene predominantly form depurinating DNA adducts, leaving apurinic sites in the DNA that generate cancer-initiating mutations. This was discovered by correlation between the depurinating adducts formed in mouse skin by treatment with benzo[a]pyrene, dibenzo[a,l]pyrene or 7,12-dimethylbenz[a]anthracene and the site of mutations in the Harvey-ras oncogene in mouse skin papillomas initiated by one of these PAH. By applying some of these fundamental discoveries in PAH studies to estrogen carcinogenesis, the natural estrogens estrone (E1) and estradiol (E2) were found to be mutagenic and carcinogenic through formation of the depurinating estrogen-DNA adducts 4-OHE1(E2)-1-N3Ade and 4-OHE1(E2)-1-N7Gua. These adducts are generated by reaction of catechol estrogen quinones with DNA, analogously to the DNA adducts obtained from the catechol quinones of benzene, naphthalene, and the synthetic estrogens diethylstilbestrol and hexestrol. This is a weak mechanism of cancer initiation. Normally, estrogen metabolism is balanced and few estrogen-DNA adducts are formed. When estrogen metabolism becomes unbalanced, more catechol estrogen quinones are generated, resulting in higher levels of estrogen-DNA adducts, which can be used as biomarkers of unbalanced estrogen metabolism and, thus, cancer risk. The ratio of estrogen-DNA adducts to estrogen metabolites and conjugates has repeatedly been found to be significantly higher in women at high risk for breast cancer, compared to women at normal risk. These results indicate that formation of estrogen-DNA adducts is a critical factor in the etiology of breast cancer. Significantly higher adduct ratios have been observed in women with breast, thyroid or ovarian cancer. In the women with ovarian cancer, single nucleotide polymorphisms in the genes for two enzymes involved in estrogen metabolism indicate risk for ovarian cancer. When polymorphisms produce high activity cytochrome P450 1B1, an activating enzyme, and low activity catechol-O-methyltransferase, a protective enzyme, in the same woman, she is almost six times more likely to have ovarian cancer. These results indicate that formation of estrogen-DNA adducts is a critical factor in the etiology of ovarian cancer. Significantly higher ratios of estrogen-DNA adducts to estrogen metabolites and conjugates have also been observed in men with prostate cancer or non-Hodgkin lymphoma, compared to healthy men without cancer. These results also support a critical role of estrogen-DNA adducts in the initiation of cancer. Starting from the perspective that unbalanced estrogen metabolism can lead to increased formation of catechol estrogen quinones, their reaction with DNA to form adducts, and generation of cancer-initiating mutations, inhibition of estrogen-DNA adduct formation would be an effective approach to preventing a variety of human cancers. The dietary supplements resveratrol and N-acetylcysteine can act as preventing cancer agents by keeping estrogen metabolism balanced. These two compounds can reduce the formation of catechol estrogen quinones and/or their reaction with DNA. Therefore, resveratrol and N-acetylcysteine provide a widely applicable, inexpensive approach to preventing many of the prevalent types of human cancer.

Carbocations from oxidized metabolites of benzo[a]anthracene: a computational study of their methylated and fluorinated derivatives and guanine adducts

Structure-reactivity relationships and substituent effects on carbocation stability in benzo[a]anthracene (BA) derivatives have been studied computationally at the B3LYP/6-31G and MP2/6-31G levels. Bay-region carbocations are formed by O-protonation of the 1,2-epoxides in barrierless processes. This process is energetically more favored as compared to carbocation generation via zwitterion formation/O-protonation, via single electron oxidation to generate a radical cation, or via benzylic hydroxylation. Relative carbocation stabilities were determined in the gas phase and in water as solvent (PCM method). Charge delocalization mode in the BA carbocation framework was deduced from NPA-derived changes in charges, and substitution by methyl or fluorine was studied at different positions selected on basis of the carbocation charge density. A bay-region methyl group produces structural distortion with consequent deviation from planarity of the aromatic system, which destabilizes the epoxide, favoring ring opening. Whereas fluorine substitution at sites bearing significant positive charge leads to carbocation stabilization by fluorine p-pi back-bonding, a fluorine atom at a ring position which presented negative charge density leads to inductive destabilization. Methylated derivatives are less sensitive to substituent effects as compared to the fluorinated analogues. Although the solvent decreases the exothermicity of the epoxide ring-opening reactions due to greater stabilization of the reactants, it provokes no changes in relative reactivities. Relative energies in the resulting bay-region carbocations are examined taking into account the available biological activity data on these compounds. In selected cases, quenching of bay-region carbocations was investigated by analyzing relative energies (in the gas phase and in water) and geometries of their guanine adducts formed via covalent bond formation with the exocyclic amino group and with the N-7.

beta-Carotene-mediated inhibition of a DNA adduct induced by 7,12-dimethylbenz(a)anthracene and 7-hydroxymethyl-12-methylbenz(a)anthracene in mouse mammary gland in vitro

The influence of beta-carotene on the formation of DNA-adducts induced by 7,12-dimethylbenz(a)anthracene (DMBA) and 7-hydroxymethyl-12-methylbenz(a)anthracene (7-OHM-12-MBA) during transformation of mouse mammary cells in organ culture was analysed. Treatment with beta-carotene (10(-8)-10(-5) mol/l) caused inhibition (48.8-94.4%) of an adduct (VI), which was detectable in DNA samples from DMBA-treated mammary glands. Out of six adducts, derived from further analysis of DNA samples from 7-OHM-12-MBA-treated glands, adduct f eluted in the same fraction as adduct (VI), indicating these adducts were analogous. Likewise, adduct f was also inhibited by beta-carotene. Boronate chromatographic analysis revealed this particular adduct was a syn-dihydrodiol epoxide product. Adduct inhibition was detectable both at the start and after DMBA treatment. alpha-Tocopherol and canthaxanthin were ineffective in inhibiting adducts. It is reasonable to conclude that beta-carotene-mediated modification of adducts is associated with the inhibition of a syn-adduct, which is derived from further metabolism of a 7-OHM-12-MBA intermediate.

The approach to understanding aromatic hydrocarbon carcinogenesis. The central role of radical cations in metabolic activation

Polycyclic aromatic hydrocarbons (PAH) are carcinogens requiring metabolic activation to react with cellular macromolecules, the initial event in carcinogenesis. Cytochrome P450 mediates binding of PAH to DNA by two pathways of activation. One-electron oxidation to form radical cations is the major pathway of activation for the most potent carcinogenic PAH, whereas monooxygenation to form bay-region diol epoxides is generally a minor pathway. For benzo[a]pyrene and 7,12-dimethylbenz[a]-anthracene, 80% and 99%, respectively, of the DNA adducts formed by rat liver microsomes or in mouse skin arise via the radical cation. Therefore, studies of PAH activation should begin by considering one-electron oxidation as the primary mechanism.

Radical cations in aromatic hydrocarbon carcinogenesis

Most carcinogens, including polycyclic aromatic hydrocarbons (PAH), require metabolic activation to produce the ultimate electrophilic species that bind covalently with cellular macromolecules to trigger the cancer process. Metabolic activation of PAH can be understood in terms of two main pathways: one-electron oxidation to yield reactive intermediate radical cations and monooxygenation to produce bay-region diol epoxides. The reason we have postulated that one-electron oxidation plays an important role in the activation of PAH derives from certain common characteristics of the radical cation chemistry of the most potent carcinogenic PAH. Two main features common to these PAH are: 1) a relatively low ionization potential, which allows easy metabolic removal of one electron, and 2) charge localization in the PAH radical cation that renders this intermediate specifically and efficiently reactive toward nucleophiles. Equally important, cytochrome P-450 and mammalian peroxidases catalyze one-electron oxidation. This mechanism plays a role in the binding of PAH to DNA. Chemical, biochemical and biological evidence will be presented supporting the important role of one-electron oxidation in the activation of PAH leading to initiation of cancer.

Cytotoxicity, anchorage-independent growth, and DNA adduct formation in human neonatal fibroblasts by 1,2,3,4-tetrahydro-7,12-dimethylbenz(a)-anthracene (TH-DMBA), its six aryl fluoro regioisomers, and an exo methylene tautomer

1,2,3,4-Tetrahydro-7,12-dimethylbenz(a)anthracene (TH-DMBA), its six possible fluorosubstituted regioisomers, and the C-7 exo methylene tautomer of the 11F derivative have been investigated for their cytotoxicity and for their ability to induce anchorage-independent growth and to form adducts in human neonatal foreskin fibroblasts. All compounds tested exhibited a low level of cytotoxicity, determined as percent cloning efficiency, up to a final concentration of 30 micrograms/ml. Except for 5F-TH-DMBA and the C-7 exo methylene tautomer, all compounds induced anchorage-independent growth of neonatal foreskin fibroblasts in soft agar at all concentrations tested (1, 3, 10 and 30 micrograms/ml). The C-7 exo methylene tautomer induced anchorage-independent growth only at a concentration of 10 micrograms/ml. Among the compounds tested the 6F derivative was the most effective compound at 1 microgram/ml. The D-ring fluoro isomers induced anchorage-independent growth at a frequency comparable to TH-DMBA itself, with the 11F derivative being the least effective of the four D-ring regioisomers. All compounds except 5F-TH-DMBA formed detectable adducts with cellular DNA as determined by 32P postlabeling procedures, when the cells were treated at 1 microgram/ml. Two adducts were detected in cells treated with TH-DMBA and four adducts were detected in DNA obtained from cells treated with 6F-TH-DMBA. The level of bonding for the D-ring fluoro isomers was quantitatively less and sometimes qualitatively different than that for TH-DMBA. For the D-ring compounds, the ability to induce anchorage-independent growth frequency correlated with the total quantity of adduct formed. The C-7 exo methylene tautomer formed a single adduct and the level of bonding was less than one adduct per 10(9) nucleotides. Analysis of these results led to the proposal that the planar anthracene ring structure (rings B, C, and D) of TH-DMBA and possibly oxidative metabolism at benzylic carbon 4 of the A-ring are important to DNA bonding and initiation of induction of anchorage-independent growth.

Tumor-initiating activity in mouse skin and carcinogenicity in rat mammary gland of fluorinated derivatives of benzo[a]pyrene and 3-methylcholanthrene

Comparative studies of tumor-initiating activity in mouse skin and carcinogenicity in rat mammary gland were conducted with benzo[a]pyrene (BP) and 3-methylcholanthrene (MC) derivatives. SENCAR mice were initiated with BP, 6-fluorobenzo[a]pyrene (6-FBP), 6-methylBP, 7-FBP, 8-FBP, 9-FBP, 10-FBP, or 10-azaBP and promoted with tetradecanoyl phorbol acetate. The same compounds plus BP 7,8-dihydrodiol were tested by intramammillary injection in female Sprague-Dawley rats. Tumor-initiating activity in mice and/or carcinogenicity in rats were observed for BP, 6-methylBP, 6-, 7-, 8-, and 10-FBP, whereas 9-FBP was inactive in both experiments and 10-azaBP was only marginally active in the mammary gland. BP 7,8-dihydrodiol was carcinogenic in rat mammary gland, although it was less potent than BP. MC, 8-FMC, 10-FMC, and 3-methylcholanthrylene were also tested in Sprague-Dawley rats by intramammillary injection. All compounds were carcinogenic, with MC displaying the most potent activity. The less potent carcinogenic activity of BP 7,8-dihydrodiol in the mammary gland, compared with BP, and the moderate-to-weak tumor-initiating and/or carcinogenic activity of 7-, 8-, and 10-FBP suggest that the bay-region diol-epoxide pathway does not play a significant role in the activation of BP in these two target tissues. Similarly, the carcinogenic activity of 8-FMC and 10-FMC, in which the bay-region diol-epoxide pathway is blocked, suggests that this mechanism of activation is not important in the carcinogenicity of MC in rat mammary gland.

Ab initio study on the molecular structure of trans-1,2-dihydroxy-1,2-dihydro-8-fluoronaphthalene

The molecular geometries of two conformations (diequatorial and diaxial) of trans-1,2-dihydroxy-1,2-dihydro-8-fluoronaphthalene have been refined the ab initio gradient method at the 4-21G level to determine the effect of fluoro substitution on the conformational and structural properties of naphthalene dihydrodiols. As with trans-1,2-dihydroxy-1,2-dihydronaphthalene, the conformation with diequatorial hydroxyl groups is the most stable. The structural differences for the fluorinated and unfluorinated naphthalene dihydrodiols are discussed and the possible consequences of the structural and conformational trends on the metabolism of dihydrodiols to dihydrodiol epoxides are considered.

Tumor-initiating activity of the 9,10-dihydrodiol- and 9,10-dihydrodiol-7,8-epoxide of 3-methylcholanthrene in SENCAR mice

The skin tumor-initiating activities of several bay-region metabolites of 3-methylcholanthrene (3-MCA) were determined in SENCAR mice. 3-MCA-anti-9,10-diol-7,8-epoxide possessed weak tumor-initiating activity when tested at 100 and 200 nmol/mouse doses (0.27 and 0.67 papillomas per mouse after 18 weeks of promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA)). 3-MCA-trans-9,10-diol at initiating doses of 50 and 100 nmol/mouse was as active as 3-MCA. 3-MCA-trans-9,10-diol was also tested for mutagenic activity toward V79 cells in cell-mediated assays and found to be approximately 2-times more potent than 3-MCA. The data suggest that 3-MCA-trans-9,10-diol is a proximate carcinogen for mouse skin.

Mutagenic activity of methyl- and fluoro-substituted derivatives of polycyclic aromatic hydrocarbons in a human hepatoma (HepG2) cell-mediated assay

Several series of methyl- and fluoro-substituted polycyclic aromatic hydrocarbon (PAH) derivatives were tested for mutagenic activity in cell-mediated assays with cells of the human hepatoma cell line, HepG2, as PAH activators. The mutagenic activity of dibenz[a,h]anthracene [DB(a,h)A] increased progressively with the substitution of a methyl group at one or both non-benzo bay-region sites. At a concentration of 0.25 micrograms/ml, the mutation frequencies induced by DB(a,h)A, and 7,14-diMeDB(a,H)A were 1.3, 13.1 and 59.0 6-thioguanine-resistant colonies/10(5) viable V79 cells, respectively. Methyl groups at non-benzo bay-region sites in 3-methylcholanthrene and benzo[e]pyrene had little effect on mutagenic activity at the highest concentration which could be tested (2 micrograms/ml). The presence of a fluorine atom on the bay-region A-ring of 7,12-dimethylbenz[a]anthracene (DMBA) drastically drastically reduced mutagenic activity. At a concentration of 1.0 micrograms/ml, the mutation frequencies induced by DMBA, 1-fluoro-DMBA and 4-fluoro-DMBA were 50.5, 6.8 and 1.6, respectively. On the other hand, the mutation frequency was increased 6-fold when the fluoro substituent was in the 10-position of the D-ring of DMBA. Thus, for the most part, the relative mutagenic activities of these compounds in the HepG2 cell-mediated assay paralleled their skin tumor-initiating activity in SENCAR mice reported earlier (DiGiovanni et al., 1982, 1983a, b). These studies demonstrate the value of the HepG2 cell line as an exogenous, intact human cell activation system in short-term assays designed to evaluate the genotoxic effects of PAHs.

Stereoselective metabolism of 8- and 9-fluorobenzo[a]pyrene by rat liver microsomes: absolute configurations of trans-dihydrodiol metabolites

Rat liver microsomal metabolism of 8-fluorobenzo[a]pyrene (8-fluoro-BaP) generated 3-hydroxy-8-fluoro-BaP, 8-fluoro-BaP trans-4,5-dihydrodiol, and 8-fluoro-BaP, 3,6-quinone as major products. A minor metabolite of 8-fluoro-BaP was tentatively assigned as 8-fluoro-BaP 9,10-dihydrodiol. Metabolism of 9-fluorobenzo[a]pyrene (9-fluoro-BaP) gave 3-hydroxy-9-fluoro-BaP, 9-fluoro-BaP trans-4,5-dihydrodiol, 9-fluoro-BaP trans-7,8-dihydrodiol, and 9-fluoro-BaP 3,6-quinone. All three dihydrodiol metabolites were optically active. Comparison of the circular dichroism spectra of BaP 4R,5R-dihydrodiol, 6-bromo-BaP 7R,8R-dihydrodiol, and 6-fluoro-BaP 7R,8R-dihydrodiol with those of the respective dihydrodiol metabolites allowed assignments of an R,R absolute configuration to the major enantiomers of the three dihydrodiol metabolites.

Comparative metabolism of 7,12-dimethylbenz[a]-anthracene and its non-carcinogenic 2-fluoro analogue by Syrian hamster embryo cells

The metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) and its non-carcinogenic 2-fluoro analogue (2F-DMBA) by Syrian hamster embyro (SHE) cells has been studied using high pressure liquid chromatography (HPLC) and fluorescence spectroscopy. Metabolites produced by SHE cells were compared chromatographically to those produced on a larger scale by liver microsomal preparations and previously identified by gas chromatography and mass spectrometry. At least 2 (possibly 3) phenol metabolites, none of which appear to be in the A-ring, were formed from [3H] 2F-DMBA and totalled only 3% of the organic extractable activity present in the media at 24 h. On the other hand, 3 A-ring phenols (DMBA-2-ol, DMBA-3-ol and DMBA-4-ol) comprising almost 12% of the total organic extractable radioactivity at 24 h were identified as metabolites in SHE cell culture media. For both hydrocarbons the major organic extractable metabolite present at 24 h was the respective 8,9-dihydro-dihydroxydiol (DMBA 45%, 2F-DMBA 39%). Thus, substitution of fluorine for hydrogen at the 2-carbon of DMBA appears to block or greatly reduce the A-ring metabolism of this compound but has relatively little effect on D-ring oxidation. Therefore loss of the carcinogenic/mutagenic activity of DMBA correlates with the extent of A-ring metabolism including, possibly, the bay region diol epoxide.

The effect of steric strain in the bay-region of polycyclic aromatic hydrocarbons: tumorigenicity of alkyl-substituted benz[a]anthracenes

3,6-Dimethylcholanthrene (3,6-DMC) and 7,11,12-trimethylbenz[a]anthrene (7,11,12-TMBA) were tested for tumor-initiating activity on mouse skin as an approach to evaluate the potential role of steric strain in the bay-region on tumorigenic potency. Methyl-substitution at the 6-position of 3-methylcholanthrene (3-MC) increases steric strain in the bay-region of the hydrocarbon as it does at the 12-position of benz[a]anthracene (BA) causing both hydrocarbons to become non-planar. 3,6-DMC had at least 2- to 3-fold higher tumor-initiating activity than did 3-MC. Introduction of an 11-methyl group in 7,12-dimethylbenz[a]anthracene (7,12-DMBA) results in the formation of a more highly hindered (buttressing effect) hydrocarbon. 7,11,12-TMBA had 5% or less of the tumor-initiating activity of 7,12-DMBA, although the hydrocarbon still had relatively high tumorigenic activity on mouse skin. The results obtained with 3,6-DMC and studies reported previously with other methyl-substituted hydrocarbons, show that hydrocarbons possessing steric strain in the bay-region of the molecule can have enhanced tumorigenic activity. The basis of this steric effect remains unclear, however, as a result of the decreased tumorigenic activity of the 11-methyl-substituted derivative of 7,12-DMBA. The weak tumor-initiating activity of BA was enhanced at least 4- to 8-fold as a result of methyl-substitution at the 6- and 8-positions (6,8-dimethylbenz[a]anthracene). The higher tumorigenic activity of 6,8-dimethylbenz[a]anthracene compared to BA is consistent with a presumed decrease in metabolic detoxification of the dimethyl-substituted derivative at the 5,6- and 8,9-double bonds.