The persistence of benzo[a]pyrene diol-epoxide deoxyguanosine adduct in mouse skin and its disappearance in rat skin

Dietary curcumin post-treatment enhances the disappearance of B(a)P-derived DNA adducts in mouse liver and lungs

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Effects of dietary curcumin on disappearance of DNA adducts were studied.

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Curcumin post-treatment enhanced the disappearance of BPDE-DNA adducts in liver/lungs.

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Curcumin post-treatment augmented B(a)P-induced apoptosis during 24–120 h.

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Curcumin post-treatment increased PCNA in B(a)P-treated tissues during 7–28 days.

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Curcumin-mediated increase in apoptosis and cell proliferation decreased DNA adducts.

To study the post-treatment effects of dietary curcumin on the levels of benzo(a)pyrene [B(a)P]-induced DNA adducts, mice were administered oil or B(a)P and randomized into 7 subgroups after 24 h. One of the subgroups from both the oil and B(a)P groups was killed at 24 h while the remaining 6 subgroups were shifted to powdered control or 0.05% curcumin diet and killed after 24, 72 and 120 h (experiment 1), and 7, 14, and 28 days (experiment 2). Quantitative comparisons of BPDE-DNA nuclear adducts (area and intensity) in immunohistochemically stained lungs and liver sections was carried out by IHC profiler. A time-dependent decrease in the levels of adducts in B(a)P-treated animals was further enhanced by curcumin exposure compared to the levels in time-matched controls. To assess the contribution of apoptosis and cell proliferation in observed curcumin-mediated enhanced decrease of BPDE-DNA adducts, comparative evaluation of apoptosis and cell proliferation markers was undertaken. Results suggested enhancement of B(a)P-induced apoptosis in liver and lungs by curcumin during 24–120 h while no such enhancement was observed at 7–28 days. Results suggest curcumin-mediated enhancement in apoptosis (experiment 1) and adduct dilution (experiment 2) to be the reason for the observed higher decrease of BPDE-DNA adducts.

Comparison of oxidant-generation and BP-diol activation by bone marrow cells from C57Bl/6 and DBA/2 mice: implications for risk of bone marrow toxicity induced by polycyclic hydrocarbons

Neutrophil-derived oxidants have been implicated in both the damage to biomolecules and the metabolic activation of xenobiotics. Bone marrow, a relatively neutrophil-rich tissue with low cytochrome P450 activity, is subject to toxicity from orally administered benzo[a]pyrene (BP) in mice with noninducible P450 monooxygenase systems. Thus, we have compared the oxidant generation and chemical activation by neutrophilic cells isolated from femurs of male DBA/2 and C57Bl/6 mice, strains that are susceptible and nonsusceptible, respectively, to bone marrow toxicity from BP. Oxidant generation of neutrophilic preparations was assayed by superoxide anion generation and oxidant-dependent chemiluminescence (CL) from luminol or lucigenin. In all assays, cells from DBA/2 mice demonstrated increased oxidant generation. CL from BP-7,8-dihydrodiol (BP-diol) has previously been shown to correlate with its ability to elicit genotoxic effects. A twofold enhancement of oxidant-dependent CL from BP-diol was observed with TPA-stimulated neutrophilic cells from DBA/2 mice as compared to cells from C57Bl/6 mice. DBA/2-derived bone marrow cells also activated more BP-diol to a DNA covalent binding species than did bone marrow cells derived from the less BP-sensitive C57Bl/6 mouse. Tetraol analysis of BP-diol metabolism by activated bone marrow cells confirmed this greater bioactivation of the diol by DBA/2-derived cells. These results suggest that the increased risk of DBA/2 mice for BP-induced bone marrow toxicity may be related to their greater ability to bioactivate xenobiotics through oxidant-dependent mechanisms.

Neutrophil-derived oxidants as mediators of chemical activation in bone marrow

Neutrophil-derived oxidants have been implicated in both damage to biomolecules and the metabolic activation of xenobiotics. Since the bone marrow is a relatively neutrophil-rich tissue which is subject to xenobiotic toxicity, we have characterized the oxidant generating capability of neutrophilic cells isolated from femurs of male C57BL/6J mice. Addition of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to neutrophil preparations (70 +/- 5% ring neutrophils and metamyelocytes) elicited superoxide anion generation, as indicated by superoxide dismutase (SOD)-inhibitable acetylated cytochrome c reduction, and oxidant-dependent chemiluminescence (CL) from luminol or lucigenin. The interaction of benzo[a]pyrene-7,8-dihydrodiol (BP-diol), a proximate carcinogenic metabolite of benzo[a]pyrene (BP), with TPA-stimulated bone marrow neutrophils resulted in azide-inhibitable CL (90%) indicative of its myeloperoxidase-dependent oxidation to an excited-state intermediate. Covalent binding of [3H]BP-diol to exogenous DNA was similarly increased 3-fold in the presence of TPA-stimulated bone marrow neutrophils. Recently, our laboratory has shown that in addition to CL, TPA-stimulated human polymorphonuclear leukocytes can activate BP-diol to an intermediate which covalently binds to DNA and elicits mutagenicity in Salmonella typhimurium TA100. These observations combined with our current results suggest a possible role for neutrophil-derived oxidants in the mechanisms of chemically-induced bone marrow toxicity.

Stereoselective aspects of the metabolic activation of benzo[a]pyrene by human skin in vitro

Benzo[a]pyrene (BP) is activated within tissues in both a regio- and a stereoselective manner and, since human skin is sensitive to tumour induction by polycyclic aromatic hydrocarbons (PAH), the steroselective metabolism of BP in this tissue has been investigated. Samples of skin from eleven individuals were treated with [3H]BP in short-term organ culture. Two samples were also treated with mixtures of [14C](+)- and (-)-trans-7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (BP-7,8-dihydrodiol) in varying proportions. Following application of [3H] BP, more 7,8-dihydrodiol was recovered from the skin itself than from the culture fluid in ten cases; no 7.8-dihydrodiol was detected in extracts from the eleventh. The 7,8-dihydrodiol metabolite was extracted predominantly (range 74-greater than 99%) as the (-)-enantiomer in nine of these ten patients, although proportionately more (+)-enantiomer was recovered from the culture fluid than from the skin in each case. The relative proportions of [3H]BP tetrols derived from syn- and anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydroxybenzo[a]pyrene (BPDE) detected in these extracts was more variable. When skin samples were treated with [14C]BP-7,8-dihydrodiol, more anti- than syn-BPDE-derived tetrols were extracted, irrespective of the optical purity of the dihydrodiol applied. These findings provide evidence for interindividual variations in the stereoselective metabolism of BP, which may be of some importance in determining individual susceptibility to PAH-induced skin carcinogenesis.

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.

Comparison of benzo[a]pyrene-DNA adduct levels in mouse and rat epidermis and dermis

Male Swiss mice and Wistar rats were treated topically with 250 nmol/mouse and 750 nmol/rat of [3H]benzo[a]pyrene ([3H]BaP). The initial level of total BaP--DNA and the individual modified deoxyribonucleoside adducts were similar in the skin epidermis of the two species. The concentration of these adducts was approximately 3 times less in both mouse and rat dermis. The decreased amount of BaP bound to DNA of mouse dermis may be related to the resistance of this tissue to the carcinogenic action of BaP. The ability of both the mouse (susceptible) and rat (resistant) skin to form BaP-bound products similar in nature and ratio in the epidermal and dermal DNA, suggests that other mechanisms are involved in the difference in the biological response of epidermis versus dermis to the carcinogenic effect of BaP.

Benzo[a]pyrene metabolism and DNA adduct formation mediated by English sole liver enzymes

Tritiated benzo[a]pyrene (BaP) and (+/-)-7,8-dihydroBaP (BaP 7,8-dihydrodiol) were incubated with English sole liver microsomes in the presence of salmon testes DNA. The modified deoxynucleosides were isolated by Sephadex LH-20 column chromatography and analyzed by reverse-phase high-pressure liquid chromatography (HPLC). A single, major adduct (60-68% of the total modified deoxynucleosides) was formed when either BaP or BaP 7,8-dihydrodiol was incubated with sole liver microsomes and DNA. Although other minor BaP-DNA adducts were formed, none represented greater than 3% of the total adducts. The major adduct had a retention time on HPLC identical to that of the N2-[10 beta (7 beta, 8 alpha, 9 alpha-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene)yl]-deoxy-guanosine (7R-anti-BPDE/trans-dG) adduct formed when anti-BPDE, the ultimate carcinogen of BaP in mammals, was incubated with DNA. Analysis of the Bay region tetrols showed that only the 7 alpha, 8 beta, 9 beta, 10 alpha-tetrahydroxy-7,8,9,10-tetrahydroBaP, a hydrolysis product of the anti-BPDE, was formed when BaP was incubated with sole liver microsomes. When BaP 7,8-dihydrodiol was used as the substrate, the 7 alpha, 8 beta, 9 beta, 10 alpha-, 7 alpha, 8 beta, 9 alpha, 10 beta-, and 7 alpha, 8 beta, 9 alpha, 10 alpha-tetrahydroxy-7,8,9,10-tetrahydroBaP's were formed, indicating the formation of both anti- and syn-BPDE. The ratio of tetrols of anti-BPDE/syn-BPDE was 2; however, the ratio of adducts of anti-BPDE/syn-BPDE was 20. Thus, the findings show that hepatic microsomes of English sole, a fish species having a high incidence of liver neoplasia in chemically contaminated estuaries, metabolized BaP and BaP 7,8-dihydrodiol stereoselectively to form predominantly the 7R-anti-BPDE/trans-dG adduct.

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.