In vivo formation of benzo(alpha)pyrene diol epoxide-deoxyadenosine adducts in the skin of mice susceptible to benzo(alpha)pyrene-induced carcinogenesis

Fluorescence measurements of DNA-bound metabolites of benzo(a)pyrene derivatives with different carcinogenic effects

(+/-)-trans-dihydroxy-7,8-dihydrobenzo(a)pyrene (BP-7,8-diol) and 9-hydroxybenzo(a)pyrene (9-OH-BP) were metabolized by rat liver microsomes in the presence of calf thymus DNA, resulting in preferential DNA binding of fluorescent (+)-anti-BP-7,8-diol-9,10-epoxide (BPDE) and 9-OH-BP-4,5-epoxide, respectively. When the DNA is denatured the fluorescence intensities of the bound metabolites change in a characteristic manner. Fluorescence decay measurements show that the intensity changes are due to changes in lifetimes of the excited states. Model substances for the bound metabolites were studied in solvents of different polarity. We found that the fluorescence changes observed after denaturation of the DNA may be explained as solvent polarity effects, so that denaturation forces the bound metabolites from a more hydrophobic environment to a hydrophilic one. Fluorescence depolarization studies as a function of temperature in combination with previous linear dichroism studies show that both BPDE and 9-OH-BP-4,5-epoxide form rigidly associated complexes with native DNA.

The effect of fluoro substituents on reactivity of 7-methylbenz[a]anthracene diol epoxides

The present study has examined potential mechanisms for the influence of F-substituents on the biologic activity of methylbenz[a]anthracenes. DNA adducts derived from reaction of the racemic bay-region anti-diol epoxides of 7-methylbenz[a]anthracene, and its 9- and 10- fluoro derivatives, with calf thymus DNA in vitro were partially characterized. All three hydrocarbon diol epoxides produced similar DNA adduct profiles upon reaction with calf thymus DNA in vitro that were composed of two deoxyganosine and two deoxyadenosine adducts (tentatively identified as trans addition products). The extent of covalent binding to calf thymus DNA, as estimated by 32P-postlabeling, was similar for all three diol epoxides. The reactivity of the unsubstituted and 10-F-substituted diol epoxide was further assessed by measuring overall pseudo-first-order rate constants for hydrolysis in water or 0.1 M Tris-HCl buffer, pH 7.0, and in the presence or absence of native or denatured DNA. The rate constant for hydrolysis of 7-methylbenz[a]anthracene diol epoxide in the absence of DNA was similar to that of 10-F-7-methylbenz[a]anthracene diol epoxide (t1/2 = 138 min vs 115 min in water, respectively, and 93 vs 83 min in 0.1 M Tris-HCl buffer, respectively). In addition, the presence of DNA accelerated hydrolysis rates to similar extents for both diol expoxides. The skin tumor-initiating activities of the 9- and 10-F-substituted 3,4-diols of 7-methyl-, 12-methyl-, and 7,12-dimethylbenz[a]anthracene were determined in SENCAR mice. The presence of F-substituents in the 9- or 10- position did not enhance or in some cases reduced the tumor-initiating activity of the 3,4-diols of these hydrocarbons. Collectively, these results, as well as previous results from our laboratory, suggest that the influence of a F-substituent at position 10 of the benz[a]anthracene nucleus is not due to increased or altered reactivity of the bay-region diol epoxide but rather likely on the initial formation of the 3,4-diol.

Methods used for analyses of "environmentally" damaged nucleic acids

In this review, we present various techniques, currently applied in many laboratories, which are useful in the detection of "environmentally"-induced damage to DNA. These techniques include: (a) chromatographic methods, which allow determination of chemical changes within DNA, be they formation of adducts with or oxidation of bases in DNA; (b) electrophoretic methods, which facilitate finding the site(s) in DNA where that chemical modification occurred; and (c) immunological assays, which help to detect DNA damage using externally produced antibodies that recognize the specific chemical changes in DNA or its fragments, as well as by detection of autoantibodies that develop in response to environmental exposures of animals and humans.

Promotion as a factor in carcinogenesis

Promotion is any factor which results in the increased cellular replication of initiated or transformed cells. We argue that cytotoxicity is not a necessary component of promotion and that, therefore, the existence of a threshold for promotion is unlikely.

Multistage carcinogenesis in mouse skin

The mouse skin model of multistage carcinogenesis has for many years provided a conceptual framework for studying carcinogenesis mechanisms and potential means for inhibiting specific stages of carcinogenesis. The process of skin carcinogenesis involves the stepwise accumulation of genetic change ultimately leading to malignancy. Initiation, the first step in multistage skin carcinogenesis involves carcinogen-induced genetic changes. A target gene identified for some skin tumor initiators is c-Ha-ras. The second step, the promotion stage, involves processes whereby initiated cells undergo selective clonal expansion to form visible premalignant lesions termed papillomas. The process of tumor promotion involves the production and maintenance of a specific and chronic hyperplasia characterized by a sustained cellular proliferation of epidermal cells. These changes are believed to result from epigenetic mechanisms such as activation of the cellular receptor, protein kinase C, by some classes of tumor promoters. The progression stage involves the conversion of papillomas to malignant tumors, squamous cell carcinomas. The accumulation of additional genetic changes in cells comprising papillomas has been correlated with tumor progression, including trisomies of chromosomes 6 and 7 and loss of heterozygosity. The current review focuses on the mechanisms involved in multistage skin carcinogenesis, a summary of known inhibitors of specific stages and their proposed mechanisms of action, and the relevance of this model system to human cancer.

Mechanism of action of food-associated polycyclic aromatic hydrocarbon carcinogens

The polycyclic aromatic hydrocarbon carcinogens are formed in the inefficient combustion of organic matter and contaminate foods through direct deposition from the atmosphere or during cooking or smoking of foods. These potent carcinogens and mutagens require metabolism to dihydrodiol epoxide metabolites in order to express their biological activities. In vitro studies show that these reactive metabolites can react with the bases in DNA with different specificities depending upon the hydrocarbon from which they are derived. Thus, the more potent carcinogens react more extensively with adenine residues in DNA than do the less potent carcinogens, with the result that mutation at A . T base pairs is enhanced for the more potent carcinogens. In the past few years, considerable clarification of the mechanism of metabolic activation have been achieved and the focus for the immediate future is expected to be on how the reactive metabolites actually bring about biological responses.

Benzo(a)pyrene metabolism by lymphocytes from normal individuals and individuals carrying the Mediterranean variant of glucose-6-phosphate dehydrogenase

In vitro growing human lymphocytes (HL) and fibroblasts, isolated from glucose-6-phosphate dehydrogenase (G6PD)-deficient subjects (Mediterranean variant), show a sharp decrease in this enzymatic activity and in NADPH:NADP+ ratio. These cells are less able than controls to hydroxylate benzo(a)pyrene (BaP) when tested in the absence of an exogenous NADPH-generating system. They exhibit great resistance to the toxic effect of BaP. G6PD-deficient fibroblasts are less prone than controls to in vitro transformation by BaP. To investigate whether this depends on a decreased production of active BaP metabolites and BaP:DNA adducts by G6PD-deficient cells, BaP metabolism was studied in G6PD-deficient HL cultured in vitro in the presence of mitogens and treated with BaP for 24 hr. HPLC profiles of organo- and water-soluble metabolites revealed that both types of benzo(a)anthracene (BaA)-induced HL produced: 4,5-, 7,8-, 9,10-diols, 1,3-, 3,6-quinones, 3-, 9-hydroxy and 2 peaks of more polar metabolites. There was a 25-76% decrease of organo- and water-soluble metabolites in the G6PD-deficient cells. When HL were incubated with 7,8-diol, the formation of metabolites mutagenic for Salmonella typhimurium (His-) was very low in G6PD-deficient cells. BaP:deoxyadenosine (dAde) and BaP:deoxyguanosine (dGua) adducts were identified after incubation of both types of HL with BaP. There was a 31-79% fall in adduct formation by G6PD-deficient cells. Our results indicate that G6PD-deficient human lymphocytes are less able to metabolize BaP than normal lymphocytes. We suggest that the NADPH pool is inadequate, in deficient cells, for active BaP metabolism.

Formation of benzo[a]pyrene-DNA adducts by microsomal enzymes: comparison of maternal and fetal liver, fetal hematopoietic cells and placenta

The formation of benzo[a]pyrene (BP)-DNA adducts was studied in vitro in the presence of microsomes prepared from the isolated labyrinth zone of the rat placenta, the hematopoietic erythroblast cells of the fetal liver, the fetal liver, as well as the maternal liver. Pregnant rats received beta-naphthoflavone (beta NF; 15 mg/kg, i.p.) on day 17 gestation. One day later, placentae, fetal and maternal livers were obtained and hematopoietic erythroblast cells were separated from hepatocytes in the fetal livers. The respective microsomal fractions were incubated in the presence of calf thymus DNA, NADPH-regenerating system and [3H]BP (300 microCi) at 37 degrees C for 30 min. Following beta NF pretreatment, the levels of covalent binding (pmol/mg DNA/mg microsomal protein) for maternal liver, fetal liver, placenta and erythroblast cells were: 28.4, 2.4, 0.31 and 3.9, respectively, with the hematopoietic erythroblast cells being the most active among fetal tissue preparations. The extent of transplacental induction compared to control was greatest in the hematopoietic cells (18-fold) followed by fetal liver (16-fold) and labyrinth zone (5-fold). Further experiments characterized the BP-DNA adducts formed by induced microsomes. DNA was isolated, purified and digested sequentially with DNase I, snake venom phosphodiesterase type II and alkaline phosphatase type III. The deoxynucleoside-BP adducts were purified on a Sephadex LH-20 column and then separated on HPLC and the adducts were quantitated radiometrically. Seven distinct adducts were separated on HPLC and named A-G in order of elution. Adduct B was prominent in all preparations (22-55% total radioactivity). The adduct profile and retention time for peak B is similar to that reported for the adduct formed by microsomal activation of 9-hydroxy BP. Peak D constituted a major fraction (19%) in maternal liver profiles in comparison with the three fetal tissue preparations (8%). In subsequent experiments, peak D was shown to be derived from reaction of (+/-)7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) with DNA. Peak C was unique to erythroblast cell and labyrinth profiles, while peak G was specific for maternal liver and fetal liver profiles. These results demonstrate that fetal liver and its hematopoietic cells are significant sites of BP bioactivation which may contribute to the fetal toxicity of polyaromatic hydrocarbons.

Pathways involved in the metabolism and activation of polycyclic hydrocarbons

The metabolism and activation of the polycyclic aromatic hydrocarbons has been reviewed and the original contributions made to this area by Professor E. Boyland have been placed in context. The reactions involved in the formation, via epoxides, of hydroxylated derivatives have been outlined and conjugations with glucuronic and sulphuric acids and with glutathione have been discussed. Examples of secondary hydroxylation reactions have been given and the possible role that phenolic hydroxyl groups may play in activating epoxides considered. Mechanism by which polycyclic hydrocarbons are activated by metabolism to epoxides of various types have been included, mainly by reference to benzo[a]pyrene, benz[a]anthracene and chrysene. The tissue and species specific effects of polycyclic hydrocarbons have been referred to and the tissues that may act as targets in man for the initiation of malignancy by polycyclic hydrocarbons mentioned.

Comparative studies of the metabolic activation of chrysene in rodent and human skin

Metabolism and activation of chrysene was examined in mouse, rat and human skin using a short-term organ culture technique. Mouse skin released larger quantities of free dihydrodiols into the culture medium than either rat or human skin and greater quantities of chrysene metabolites became covalently bound to the DNA of mouse skin. The stereochemistry of the chrysene-1,2-diol that was formed by each skin type was examined using high-performance liquid chromatography (HPLC) with a chiral stationary phase to resolve the enantiomers. It was found that in each case the (-)-enantiomer predominated. When hydrolysates of DNA extracted from rodent or human skin that had been treated with 3H-labelled chrysene were chromatographed on Sephadex LH-20 columns, the elution profiles of the hydrocarbon-DNA adducts were found to vary between the species studied. Further examination using HPLC showed that some of the adducts formed in skin had the chromatographic characteristics of adducts formed when the anti-isomer of the 'bay-region' diol-epoxide of chrysene (r-1,t-2-dihydroxy-t-3,4-oxy-1,2,3,4-tetrahydrochrysene) reacted with DNA and that others had the characteristics of triol-epoxide adducts.

Covalent binding of benzo[a]pyrene diol epoxide to DNA of mouse skin: in vivo persistence of adducts formation

In the first 9 d after topical application of a single dose of benzo[a]pyrene to the dorsal skin of C3H mice, the half-lives of benzo[a]pyrene diol epoxide-DNA adducts and of DNA were determined to be approximately 5 d. These data indicate that, in proliferating mouse skin, benzo[a]pyrene diol epoxide-DNA lesions are not repaired, but are diluted from the genome at a rate equivalent to DNA turnover (i.e., replication versus degradation). Subsequent to this initial period, benzo[a]pyrene diol epoxide-DNA adduct removal continues, but at a much reduced rate. At 30 d posttreatment with benzo[a]pyrene, approximately 15% of the adducts are still detectable; however, their half-lives had increased to 30 d. Similar experiments with a hairless mouse showed that, although the amount of adduct formation was lower initially, the kinetics of adduct disappearance and persistence were essentially the same as found with the C3H mouse. The data obtained in this work are consistent with the hypothesis that benzo[a]pyrene diol epoxide adducts persist in a subpopulation of skin cells long after their disappearance by DNA turnover would predict.

Distribution and macromolecular binding of benzo[a]pyrene in SENCAR and BALB/c mice following topical and oral administration

When benzo[a]pyrene (BaP) is used as the initiator in initiation-promotion assays, the topical route of administration has been shown to produce a greater epidermal tumor incidence than do other routes of administration, particularly the oral route. In addition, different strains of mice exhibit varying degrees of susceptibility to two-stage epidermal tumorigenesis using BaP. The SENCAR strain is known to be far more sensitive to epidermal tumor formation following BaP initiation than are other strains, such as the BALB/c strain. To investigate the possible contribution of distribution and binding to DNA in such route and strain differences, the distribution and macromolecular binding of [3H]BaP was examined in the skin, liver, lung, and stomach of SENCAR and BALB/c mice following topical or oral administration of BaP at time periods ranging from 0.5 to 48 h. Levels of labeled material in skin were higher, and the binding of BaP to epidermal DNA was greater following topical administration than following oral administration for mice of both strains. The much greater binding of BaP to epidermal DNA following topical administration may account for the much greater epidermal tumor incidence in mice following topical administration of BaP. Following topical administration, BALB/c mice had generally higher levels of labeled material in whole skin than did SENCAR mice, and the binding of BaP to epidermal DNA at 48 h was greater in BALB/c mice than in SENCAR mice following either route of administration. Thus, the known differences between these two strains in susceptibility to epidermal tumor formation when BaP is used as an initiator cannot be explained on the basis of differences in tissue distribution or the amount of binding to epidermal DNA at the time periods examined.

An in vitro approach to studying cutaneous metabolism and disposition of topically applied xenobiotics

The extent to which cutaneous metabolism may be involved in the penetration and fate of topically applied xenobiotics was examined by metabolically viable and structurally intact mouse skin in organ culture. Evidence that skin penetration of certain chemicals is coupled to cutaneous metabolism was based upon observations utilizing [14C]benzo[a]pyrene (BP). As judged by the recovery of radioactivity in the culture medium 24 hr after in vitro topical application of [14C]BP to the skin from both control and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced C3H mice, skin penetration of BP was higher in the induced tissue. All classes of metabolites of BP were found in the culture medium; water-soluble metabolites predominated and negligible amounts of unmetabolized BP were found. As shown by enzymatic hydrolysis of the medium, TCDD induction resulted in shifting the cutaneous metabolism of BP toward the synthesis of more water-soluble conjugates. Differences in the degree of covalent binding of BP, via diol epoxide intermediates to epidermal DNA, from control and induced tissues were observed. These differences may reflect a change in the pathways of metabolism as a consequence of TCDD induction. These results indicated that topically applied BP is metabolized by the skin during its passage through the skin; and the degree of percutaneous penetration and disposition of BP was dependent upon the metabolic status of the tissue. This suggests that cutaneous metabolism may play an important role in the translocation and subsequent physiological disposition of topically applied BP.

Structure elucidation of a 6-methylbenzo[a]pyrene-DNA adduct formed by horseradish peroxidase in vitro and mouse skin in vivo

Activation of polycyclic aromatic hydrocarbons (PAH) by horseradish peroxidase (HRP) with H2O2 has been studied as a model system for one-electron oxidation. This peroxidase has been used to catalyze binding of 6-[14C]methylbenzo[a]pyrene (BP-6-CH3) to DNA, which was purified, hydrolyzed to deoxyribonucleosides and analyzed by high pressure liquid chromatography (HPLC). The predominant hydrocarbon-DNA adduct observed was identified as BP-6-CH3 bound at the 6-methyl group to the 2-amino group of dG, confirming that activation by HRP occurs by one-electron oxidation. When DNA from mouse skin treated in vivo with [14C]BP-6-CH3 was purified, hydrolyzed and analyzed by HPLC, a profile was observed which was qualitatively similar to that from the peroxidase system. In particular, the identified adduct with the hydrocarbon bound at the 6-methyl group to the 2-amino group of dG was obtained. These results demonstrate that one-electron oxidation is the mechanism of activation by HRP for aromatic hydrocarbons and indicate that the same mechanism may occur in mouse skin, a target tissue for hydrocarbon carcinogenesis.

Metabolic activation of benzo[a]pyrene in human skin maintained in short-term organ culture

The metabolic activation of benzo[a]pyrene (BP) was examined in six samples of human skin after topical application of the hydrocarbon to the skin in short-term organ culture. The results show that all of the samples were capable of metabolizing BP to water-soluble products and to ether-soluble products that included the 4,5-, 7,8- and 9,10-dihydrodiols and a product which had chromatographic properties identical with those of authentic trans-11,12-dihydro-11,12-dihydroxybenzo[a]pyrene (BP-11,12-diol). The major BP-deoxyribonucleoside adduct detected in each skin sample appeared to be formed from the reaction of r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BP-7,8-diol 9,10-oxide) with deoxyguanosine residues in DNA.

Metabolism and activation of benzo[a]pyrene by mouse and rat skin in short-term organ culture and in vivo

The metabolic activation of BP was examined in mouse and rat skin in vivo and in short-term organ culture. In mouse skin, larger quantities of ether- and water-soluble metabolites were formed and more BP became bound covalently to DNA and protein than in rat skin. Qualitative differences in the formation of dihydrodiol metabolites and of BP-deoxyribonucleoside adducts between mouse and rat skin were also observed. Organ culture techniques may not provide a true model of metabolic activation in vivo because it was found that the covalent binding of BP to DNA and protein was reduced in skin maintained in culture despite an accumulation of dihydrodiol and other ether-soluble metabolites. In addition, the proportions of the syn- and anti-isomers of BP-7,8-diol 9,10-oxide involved in the formation of adducts with deoxyguanosine differed between skin treated in organ culture and in vivo.

Persistence of benzo[alpha]pyrenediolepoxide DNA adduct in mouse skin

The covalently bound products of [3H]benzo[alpha]pyrene (BP) were determined in the DNA isolated from the skin of mice 3 weeks after application of the carcinogen. In the relatively rapidly proliferating skin the persistence of BP-DNA adducts was observed, 50% of which were unmodified nucleosides. Small amounts of (7R) BPDE I-dG adduct (5% from the initial formation) were found after 3 weeks. The minor adducts observed at 18 h after treatment were excised.

Induction and repair of DNA strand breaks in cultured human fibroblasts exposed to various phenols and dihydrodiols of benzo[a]pyrene

Cultured human fibroblasts from healthy donors were incubated for 30 min with nine different benzo[a]pyrene (BP) derivatives in the presence or absence of liver microsomes from 3-methylcholanthrene treated rats. The induction and repair of DNA strand breaks were analysed by alkaline unwinding and separation of double and single stranded DNA (SS-DNA) by hydroxylapatite chromatography immediately after the incubation or at various times after the treatment. In the absence of microsomes DNA stand breaks were detected in fibroblasts exposed to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP) and the three BP dihydrodiols (BP-4,5-, BP-7,8- or BP-9,10-dihydrodiol). After removal of the BP derivatives from the medium the DNA strand breaks disappeared within 24 h. alpha-Naphthoflavone (alpha-NF) caused a decrease in the induction of strand breaks by 1-, 3- and 9-OH-BP but did not affect the induction of strand breaks in cells exposed to BP-7,8-dihydrodiol. In the presence of microsomes DNA strand breaks were found after exposure to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP), as well as BP-7,8- and 9,10-dihydrodiol. In contrast BP-4,5-dihydrodiol did not induce strand breaks under these conditions. The induction of strand breaks by BP-7,8-dihydrodiol was enhanced in the presence of cytosine-1-beta-D-arabinofuranoside (AraC). In all cases the DNA strand breaks had disappeared 24 h after removal of the BP derivatives and microsomes except after treatment with BP-7,8-dihydrodiol.

Formation of covalent deoxyribonucleic acid benzo[a]pyrene 4,5-epoxide adduct in mouse and rat skin

The covalently bound products of [3H]benzo[a]pyrene (BP) were determined in the DNA from the skin of mice and rats. The quality and the distribution of bound products was similar. The formation of products corresponding to 4,5-dihydro-4,5-epoxy benzo[a]pyrene (BPE) and to a further metabolite of 9-hydroxybenzo[a]pyrene bound to DNA were found in the 2 species at the 24 h end point.