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

Regiospecifically Fluorinated Polycyclic Aromatic Hydrocarbons via Julia-Kocienski Olefination and Oxidative Photocyclization. Effect of Fluorine Atom Substitution on Molecular Shape

A modular synthesis of regiospecifically fluorinated polycyclic aromatic hydrocarbons (PAHs) is described. 1,2-Diarylfluoroalkenes, synthesized via Julia-Kocienski olefination (70-99% yields), were converted to isomeric 5- and 6-fluorobenzo[c]phenanthrene, 5-and 6-fluorochrysene, and 9- and 10-benzo[g]chrysene (66-83% yields) by oxidative photocyclization. Photocyclization to 6-fluorochrysene proceeded more slowly than conversion of 1-styrylnaphthalene to chrysene. Higher fluoroalkene dilution led to a more rapid cyclization. Therefore, photocyclizations were performed at higher dilutions. To evaluate the effect of fluorine atom on molecular shapes, X-ray data for 5- and 6-fluorobenzo[c]phenanthrene, 6-fluorochrysene, 9- and 10-fluorobenzo[g]chrysene, and unfluorinated chrysene as well as benzo[g]chrysene were obtained and compared. The fluorine atom caused a small deviation from planarity in the chrysene series and decreased nonplanarity in the benzo[c]phenanthrene derivatives, but its influence was most pronounced in the benzo[g]chrysene series. A remarkable flattening of the molecule was observed in 9-fluorobenzo[g]chrysene, where the short 2.055 Å interatomic distance between bay-region F-9 and H-8, downfield shift of H-8, and a 26.1 Hz coupling between F-9 and C-8 indicate a possible F-9···H-8 hydrogen bond. In addition, in 9-fluorobenzo[g]chrysene, the stacking distance is short at 3.365 Å and there is an additional interaction between the C-11-H and C-10a of a nearby molecule that is almost perpendicular.

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.

Synthesis and structure determination of 6-methylbenzo[a]pyrene-deoxyribonucleoside adducts and their identification and quantitation in vitro and in mouse skin

Activation of the moderate carcinogen 6-methylbenzo[a]pyrene (6-CH(3)BP) by one-electron oxidation to form DNA adducts was studied. Iodine oxidation of 6-CH(3)BP in the presence of dGuo produces BP-6-CH(2)-N(2)dGuo, BP-6-CH(2)-N7Gua and a mixture of 6-CH(3)BP-(1&3)-N7Gua, whereas in the presence of Ade the adducts BP-6-CH(2)-N1Ade, BP-6-CH(2)-N3Ade, BP-6-CH(2)-N7Ade and 6-CH(3)BP-(1&3)-N1Ade are obtained. Furthermore, for the first time an aromatic hydrocarbon radical cation afforded an adduct with dThd, the stable adduct BP-6-CH(2)-N3dThd. Formation of these adducts indicates that the 6-CH(3)BP radical cation has charge localized at the 6, 1 and 3 position. When 6-CH(3)BP was activated by horseradish peroxidase in the presence of DNA, two depurinating adducts were identified, BP-6-CH(2)-N7Gua (48%) and 6-CH(3)BP-(1&3)-N7Gua (23%), with 29% unidentified stable adducts. In the binding of 6-CH(3)BP catalyzed by rat liver microsomes, the same two depurinating adducts, BP-6-CH(2)-N7Gua (22%) and 6-CH(3)BP-(1&3)-N7Gua (10%), were identified, with 68% unidentified stable adducts. In 6-CH(3)BP-treated mouse skin, the two depurinating adducts, BP-6-CH(2)-N7Gua and 6-CH(3)BP-(1&3)-N7Gua, were identified. Although quantitation of these two adducts was not possible due to coelution of metabolites on HPLC, they appeared to be the major adducts found in mouse skin. These results show that 6-CH(3)BP forms depurinating adducts only with the guanine base of DNA, both in vitro and in mouse skin. The weaker reactivity of 6-CH(3)BP radical cation vs. BP radical cation could account for the weaker tumor-initiating activity of 6-CH(3)BP in comparison to that of BP.

Potent carcinogenicity of 2,7-dinitrofluorene, an environmental pollutant, for the mammary gland of female Sprague-Dawley rats

Nitrofluorene compounds are environmental pollutants chiefly from incomplete combustion. This study examined carcinogenicities after one intramammary injection of 2-nitrofluorene (2-NF), 2, 7-dinitrofluorene (2,7-diNF) or dimethyl sulfoxide (DMSO) (solvent control) to 30-day-old and of 2-NF, 9-OH-2-NF, 9-oxo-2-NF, 2,7-diNF, 9-oxo-2,7-diNF, 2,5-dinitrofluorene, 9-oxo-2,4,7-trinitrofluorene, N-OH-2-acetylaminofluorene (N-OH-2-AAF) (carcinogen control) or DMSO to 50-day-old female Sprague-Dawley rats. In 30- and 50-day-old rats 6 and 8 glands/rat, respectively, were injected with 2.04 micromol of compound in 50 microliter/gland of DMSO. Whereas all compounds including DMSO yielded combined malignant and benign mammary tumor incidences of 33-87% by week 82 after injection, 2,7-diNF produced 100 and 93% incidences significantly (P < 0.001) sooner than did DMSO, i.e. by weeks 23-49 and 18-48 after treatment of 30- and 50-day-old rats, respectively. Rats treated with 2,7-diNF and 9-oxo-2,7-diNF had significantly (P < 0.0001) and marginally (P = 0. 0536) more mammary tumors, respectively, than DMSO-treated rats. In 2,7-diNF-treated rats, the ratio of malignant to benign mammary tumors was 5.4, whereas in all other groups it was <0.5. N-OH-2-AAF, a potent tumorigen when applied to the mammary gland as a solid or in suspension, did not yield the expected tumorigenicity here. The contrasting tumorigenic potencies of 2,7-diNF and N-OH-2-AAF may have been prompted by differences in their solubilities in DMSO. Thus, the poorly soluble 2,7-diNF was slowly absorbed from the injection sites since residues (up to 0.9% of the dose injected) were recovered even after 45 weeks. The data indicate prolonged exposure of the mammary gland to 2,7-diNF and suggest that contamination of the environment with 2,7-diNF, even at low levels, poses substantial carcinogenic risk.

Relating aromatic hydrocarbon-induced DNA adducts and c-H-ras mutations in mouse skin papillomas: the role of apurinic sites

Mouse skin tumors contain activated c-H-ras oncogenes, often caused by point mutations at codons 12 and 13 in exon 1 and codons 59 and 61 in exon 2. Mutagenesis by the noncoding apurinic sites can produce G-->T and A-->T transversions by DNA misreplication with more frequent insertion of deoxyadenosine opposite the apurinic site. Papillomas were induced in mouse skin by several aromatic hydrocarbons, and mutations in the c-H-ras gene were determined to elucidate the relationship among DNA adducts, apurinic sites, and ras oncogene mutations. Dibenzo[a,l]pyrene (DB[a,l]P), DB[a,l]P-11,12-dihydrodiol, anti-DB[a,l]P-11,12-diol-13,14-epoxide, DB[a,l]P-8,9-dihydrodiol, 7,12-dimethylbenz[a]anthracene (DMBA), and 1,2,3,4-tetrahydro-DMBA consistently induced a CAA-->CTA mutation in codon 61 of the c-H-ras oncogene. Benzo[a]pyrene induced a GGC-->GTC mutation in codon 13 in 54% of tumors and a CAA-->CTA mutation in codon 61 in 15%. The pattern of mutations induced by each hydrocarbon correlated with its profile of DNA adducts. For example, both DB[a,l]P and DMBA primarily form DNA adducts at the N-3 and/or N-7 of deoxyadenosine that are lost from the DNA by depurination, generating apurinic sites. Thus, these results support the hypothesis that misreplication of unrepaired apurinic sites generated by loss of hydrocarbon-DNA adducts is responsible for transforming mutations leading to papillomas in mouse skin.

Evaluation of the tumor-initiating activity of 4-, 5-, 6-, and 7-fluorobenzo[b]fluoranthene in mouse skin

Benzo[b]fluoranthene and 4-, 5-, 6-, and 7-fluorobenzo[b]fluoranthene were evaluated for tumor-initiating activity in mouse skin. These fluorinated benzo[b]fluoranthene derivatives were assayed at doses of 400, 120, 40, and 10 nmol per mouse. Similar tumorigenic activity was observed for benzo[b]fluoranthene and 5-fluorobenzo[b]fluoranthene. While 4-fluorobenzo[b]fluoranthene did produce a significant tumorigenic response at each dose assayed, substantially fewer tumors per mouse were observed compared to benzo[b]fluoranthene at initiator doses at or above 120 nmol. Only moderate tumorigenic activity was observed for 6- and 7-fluorobenzo[b]fluoranthene. Both of these fluorinated derivatives were significantly less tumorigenic (P < 0.05) than 4-fluorobenzo[b]fluoranthene when administered at initiator doses at or below 120 nmol. These results were unanticipated in view of data which indicate that metabolism of trans-9,10-dihydro-9,10-dihydroxybenzo[b]-fluoranthene to trans-9,10-dihydro-5,9,10-trihydroxybenzo[b]fluoranthene represents a principal activation mechanism of benzo[b]fluoranthene in mouse skin. The potential of fluorine substitution not only to inhibit metabolism, but also to alter the genotoxic activity of those metabolites which do form could explain the tumorigenic activity observed with these fluorinated derivatives of benzo[b]fluoranthene. These data suggest caution in the interpretation of results based exclusively upon the assumption that the only influence of fluorine substitution is inhibition of the formation of specific metabolites.

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.

Tumorigenicity of 6-halogenated derivatives of benzo[a]pyrene in mouse skin and rat mammary gland

Studies of the tumorigenicity of 6-halogenated derivatives of benzo[a]pyrene (BP) can provide evidence about the role of the 6 position in the carcinogenic activation of BP. Female Swiss and A-strain mice were treated on the skin with BP, 6-fluorobenzo[a]pyrene (6-FBP), 6-chlorobenzo[a]pyrene (6-C1BP), 6-bromobenzo[a]pyrene (6-BrBP) and 6-iodobenzo[a]pyrene (6-IBP) by repeated application, and in some cases by initiation-promotion. While BP was more potent than 6-FBP, only these two compounds exhibits tumor-initiating and carcinogenic activity in mouse skin. Female Sprague-Dawley rats were treated with BP, 6-FBP, 6-ClBP, and 6-BrBP by intramammillary injection. BP and 6-FBP induced high levels of mammary epithelial tumors and fibrosarcomas. 6-ClBP elicited only a high percentage of fibrosarcomas, whereas 6-BrBP induced a few adenocarcinomas. These results indicate that chloro or bromo substitution at C-6 in BP reduces or eliminates carcinogenic activity. Conversely, 6-FBP, from which the fluoro substituent has been chemically and metabolically removed by one-electron oxidation, displays a moderate carcinogenic activity which is consistent with activation by either one-electron oxidation or monooxygenation.

Carcinogenicity of aromatic hydrocarbons directly applied to rat mammary gland

To obtain some initial evidence on the mechanism(s) of activation of PAH in rat mammary gland, we studied the carcinogenicity of a series of PAH directly applied to this tissue. A series of PAH which are or are not expected to be activated by one-electron oxidation because of their low or high ionization potential (IP), respectively, were tested. The compounds were dispersed as fine powders on an exposed mammary gland of female Sprague-Dawley rats. 5-Methylchrysene, dibenz[a,h]anthracene and benz[a]anthracene, which have relatively high IP, were inactive. In contrast, three PAH with relatively low IP, 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene (BP), and 3-methylcholanthrene (MC), were potent carcinogens, 6-MethylBP, with low IP, and 7-methyl-benz[a]anthracene, with borderline IP, elicited only mesenchymal tumors, whereas BP 7,8-dihydrodiol and cyclopenta[cd]pyrene were inactive. A series of MC derivatives substituted at C-1 or C-2 was tested. Substituents at C-1, the position of activation in the one-electron oxidation pathway, generally suppressed carcinogenic activity. Substitution at C-2 did not eliminate carcinogenic activity, with the exception of MC2-one. These results provide initial information suggesting that one-electron oxidation may be a mechanism of activation for PAH in the mammary gland.