Characterization of covalent adriamycin-DNA adducts

Adriamycin is a popular antineoplastic agent whose ability to form covalent adducts with DNA has been correlated to cellular apoptosis (programmed cell death) in tumor models. We have isolated and purified this adduct formed under oxido-reductive (Fenton) conditions in Tris buffer. We show by homo- and heteronuclear NMR spectroscopy that the covalent Adriamycin-DNA adduct is structurally equivalent to that resulting from direct reaction with formaldehyde. Covalent linkage of the drug to one of the DNA strands confers remarkable stability to the duplex, indicated by a 162-fold reduction in the rate of strand displacement compared with the complex with noncovalently bound drug. Glyceraldehyde also engenders covalent Adriamycin-DNA complexes, providing a possible relevant biological context for in vivo adduct formation.

Toxicokinetics of sulfur mustard and its DNA-adducts in the hairless guinea pig

In order to provide a quantitative basis for pretreatment and therapy of intoxications with sulfur mustard (SM) the toxicokinetics of this agent as well as its major DNA-adduct were studied in male hairless guinea pigs for the intravenous, respiratory and percutaneous routes. The study comprised measurement of the concentration-time course of SM in blood and measurement of the concentrations of intact SM and its adduct to guanine in various tissues at several time points after administration of, or exposure to SM. SM was analyzed in blood and tissues by gas chromatography with automated thermodesorption injection and mass-spectrometric detection. DNA-adducts were measured via an immuno-slot-blot method. In contrast with nerve agents of the phosphofluoridate type, SM partitions strongly to various organs, especially the lung, spleen, liver and bone marrow. The respiratory toxicity of SM appears to be local, rather than systemic. Surprisingly, the maximum concentration of SM in blood upon percutaneous exposure to 1 LCt50 (10,000 mg.min.m-3, estimated) is approximately 6-fold higher than that for nose--only exposure to 3 LCt50 (2,400 mg.min.m-3). Pretreatment of hairless guinea pigs with the potential scavengers N-acetyl cysteine or cysteine isopropyl ester did not significantly increase the LCt50-value for nose--only exposure to SM vapor.

Mechanism(s) of turmeric-mediated protective effects against benzo(a)pyrene-derived DNA adducts

The effects of turmeric feeding before and after benzo(a)pyrene [B(a)P] exposure on the levels of B(a)P-derived DNA adducts were studied in tissues of Swiss mice employing (32)P-postlabelling analysis. A reduction in the levels of B(a)P-derived DNA adducts in liver, lung, and forestomach was observed in animals pre-treated with 0.2 or 1% turmeric diet and exposed to B(a)P by oral intubation when compared to animals receiving standard laboratory diet and B(a)P. The observed decrease was not due to dilution caused by nascent DNA synthesis. Comparative evaluation of levels of B(a)P-derived DNA adducts in tissues of animals shifted to 0.2 or 1% turmeric diet after 24 h of oral intubation of B(a)P with those continued on standard laboratory diet did not suggest enhanced disappearance/repair of B(a)P-derived DNA adducts due to exposure to turmeric. Further, pre-treatment of mice with 1% turmeric diet significantly reduced the B(a)P-induced increase in activity of cytochrome P450 (CYP450) isozymes CYP 1A1 and 1A2 in liver, lung, and forestomach of mice. In addition, hepatic glutathione S-transferase (GST) was found to be elevated in turmeric pre-treated mice. Thus turmeric-mediated decrease in induction of phase-I enzymes in liver, lung, and forestomach of mice and enhancement of hepatic GST appear to play an important role in reducing the B(a)P-induced DNA damage in target and non-target tissues.

Myeloperoxidase--463A variant reduces benzo[a]pyrene diol epoxide DNA adducts in skin of coal tar treated patients

The skin of atopic dermatitis patients provides an excellent model to study the role of inflammation in benzo[a]pyrene (BaP) activation, since these individuals are often topically treated with ointments containing high concentrations of BaP. In this study we have determined, by HPLC with fluorescence detection, the BaP diol epoxide (BPDE)-DNA adduct levels in human skin after topical treatment with coal tar and their modulation by the -463G-->A myeloperoxidase (MPO) polymorphism, which reduces MPO mRNA expression. BPDE-DNA adduct levels were 2.2 and 14.2 adducts/10(8) nt for MPO-463AA/AG and -463GG, respectively. The predominant BaP tetrol observed was tetrol I-1, which is derived after hydrolysis of the anti-BPDE-DNA adduct. The tetrol I-1/II-2 ratio, corresponding to the anti/syn ratio, was 6.7. The (32)P-post-labeling assay was also performed and thin layer chromatograms showed a major spot with a chromatographic location corresponding to BPDE-DNA. The mean values of the BPDE-DNA adduct spots were 3.8 +/- 2.4 per 10(8) nt for MPO-463AA/AG (n = 3) and 18.4 +/- 11.0 per 10(8) nt for MPO-463GG (n = 7), respectively (P = 0.03). One individual with the homozygous mutant genotype (-463AA) even had a 13-fold lower adduct level (1.4 per 10(8) nt) as compared to MPO-463GG subjects. In conclusion, these data show for the first time: (i) the in vivo formation of BPDE-DNA adducts in human skin treated with coal tar; (ii) that the MPO-463AA/AG genotype reduced BPDE-DNA adduct levels in human skin.

DNA adduct formation in northern pike (Esox lucius) exposed to a mixture of benzo[a]pyrene, benzo[k]fluoranthene and 7H-dibenzo[c, g]carbazole: time-course and dose-response studies

The time-course and dose dependent formation of DNA adducts in juvenile northern pike (Esox lucius) following a single exposure to a mixture of benzo[a]pyrene (BaP), benzo[k]fluoranthene (BkF) and 7H-dibenzo[c,g]carbazole (DBC) were investigated by use of the (32)P-postlabelling assay. A complex adduct pattern was detected in liver and intestine of exposed fish. For the time-course studies fish were exposed either by oral administration or by intraperitoneal (i.p.) injection. Following a single i.p. injection of the mixture (40micromole/kg body weight of each substance) significantly elevated DNA adduct levels were detected in the liver after 1 day. Adduct levels were higher in liver than in intestine, in which significant elevation were detected from day 3 to 12. Following exposure via food (80micromole/kg body weight of each substance), adduct levels were detected in both liver and intestine 1 day after exposure, and continued to increase until day 3 in liver and day 6 in intestine. Calculation of a binding index, which compensates for differences in dosage, resulted in much higher adduct formation (five times in liver and 22 times in intestine) following oral exposure. Pikes receiving single oral doses of 12.5, 50, 100 or 200micromole/kg body weight of each substance exhibited significantly higher adduct levels in both liver and intestine compared to controls. Hepatic adduct levels were also higher in fish given 100 and 200micromole/kg compared to 12.5micromole/kg. Results from this study show that DNA adducts are rapidly formed in juvenile northern pike following both i.p. injection and feeding of a mixture of BaP, BkF and DBC. A maximum level was reached within a few days, which then persisted at approximately the same level for at least 9-12 days. The results also shows that higher levels of adducts were obtained following oral administration compared to i.p. injection, particularly in the intestine.

Metabolism in vitro and in vivo of the DNA base adduct, M1G

Oxidative damage is considered a major contributing factor to genetic diseases including cancer. Our laboratory is evaluating endogenously formed DNA adducts as genomic biomarkers of oxidative injury. Recent efforts have focused on investigating the metabolic stability of adducts in vitro and in vivo. Here, we demonstrate that the base adduct, M1G, undergoes oxidative metabolism in vitro in rat liver cytosol (RLC, Km = 105 microM and vmax/Km = 0.005 min-1 mg-1) and in vivo when administered intravenously to male Sprague Dawley rats. LC-MS analysis revealed two metabolites containing successive additions of 16 amu. One- and two-dimensional NMR experiments showed that oxidation occurred first at the 6-position of the pyrimido ring, forming 6-oxo-M1G, and then at the 2-position of the imidazole ring, yielding 2,6-dioxo-M1G. Authentic 6-oxo-M1G was chemically synthesized and observed to undergo metabolism to 2,6-dioxo-M1G in RLC (Km = 210 microM and vmax/Km = 0.005 min-1 mg-1). Allopurinol partially inhibited M1G metabolism (75%) and completely inhibited 6-oxo-M1G metabolism in RLC. These inhibition studies suggest that xanthine oxidase is the principal enzyme acting on M1G in RLC and the only enzyme that converts 6-oxo-M1G to 2,6-dioxo-M1G. Both M1G and 6-oxo-M1G are better substrates (5-fold) for oxidative metabolism in RLC than the deoxynucleoside, M1dG. Alternative repair pathways or biological processing of M1dG makes the fate of M1G of interest as a potential marker of oxidative damage in vivo.

Genotoxicity of complex PAH mixtures recovered from contaminated lake sediments as assessed by three different methods

Although human exposure generally occurs to mixtures of chemicals, limited toxicological information is available to characterize the potential interactions of the components of environmental mixtures. This study was conducted to compare the genotoxicity of chemically characterized polycyclic aromatic hydrocarbon (PAH) mixtures using in vitro and in vivo techniques. A total of three extracts (E1-E3) were selected from sediment samples collected from a lake adjacent to an abandoned coal gasification site. Sediments were collected on a grid moving downstream and away from the most likely source of PAH contamination, with E1 collected closest to the shore, E2 at an intermediate distance, and E3 furthest from the shore. The sediment samples were extracted in methylene chloride and methanol, dried, and redissolved in an appropriate solvent for evaluation in a battery of genotoxicity assays. Samples were evaluated for their ability to produce point mutations in bacteria and DNA adducts in vitro without metabolic activation or in vivo. Samples were also analyzed using GC/MS. Sample E1 had both the highest concentration of benzo(a)pyrene (BP) (46.5 ppm) and carcinogenic PAHs and, using 32P-postlabeling, induced the highest adduct levels overall in vitro and in vivo. Sample E2, which had a BP concentration of 14 ppm, induced the greatest number of revertants in the bacterial mutagenicity assay. Sample E3, which had the lowest level of carcinogenic PAHs and BP, induced the lowest adduct levels. However, E3 was capable of inducing a positive genotoxic response in bacteria (with S9), although the slope of the response at lower doses was less than that of E2. The in vivo data showed that the major adduct formed by E1 and E2 was a BP adduct. This information could not have been obtained with the Salmonella or in vitro postlabeling tests. Among internal organs, the extracts of all three samples induced the greatest adduct levels in the lung, similarly to previous complex PAH mixtures studied. These data demonstrate the limitations of predicting genotoxic or carcinogenic potential based on chemical analysis or a single biological test. The results suggest that mixture interactions, cytotoxicity and metabolism are likely to have an influence on the potential of a complex mixture of chemicals to produce a carcinogenic effect. In addition, the concentration of genotoxic PAHs and both in vitro and in vivo DNA adduct formations were decreased with increasing distance from the shoreline.

Regulation of the formation of the major diethylstilbestrol-DNA adduct and some evidence of its structure

Diethylstilbestrol (DES) induces kidney tumors in hamsters. In previous studies, DES has been shown by 32P-post-labeling analysis to bind covalently to DNA in vivo and in vitro and DES-DNA adduct formation has been suggested to play a key role in DES-induced carcinogenicity. In this study, we have examined the influence of the dose of DES, age of animals and organ specificity on adduct formation in hamsters. In addition, we examined the characteristics of DES-DNA adduct formation in vitro and the structure of the major adduct. DES-DNA adducts were detected in liver and kidney of hamsters treated with at least 20 mg/kg DES. Adduct concentrations were higher at higher doses or in older compared to younger animals. The covalent binding of DES to DNA catalyzed by hamster liver microsomes required cumene hydroperoxide as cofactor, whereas with NADPH, adducts were barely detectable, presumably because the reactive metabolic intermediate DES quinone was reduced to DES. The major DES-DNA adduct formed in vitro was purified by semipreparative and analytical high pressure liquid chromatography. It is concluded that DES-DNA adducts are formed from DES quinone at very low rates in vitro and occur at low levels in vivo, even when hamsters receive very large doses of DES. The dependence of DES-DNA adduct concentrations in vitro on organic hydroperoxide cofactors required for cytochrome P450-mediated DES quinone formation indicates that stilbene-DNA adduction may occur only under conditions of oxidative stress.

Alteration of benzo(a)pyrene biotransformation by resveratrol in ApcMin/+ mouse model of colon carcinogenesis

Epidemiological surveys have revealed that environmental and dietary factors contribute to most of the human cancers. Our earlier studies have shown that resveratrol (RVT), a phytochemical reduced the tumor number, size and incidence of dysplasias induced by benzo(a)pyrene (BaP), an environmental toxicant in the Apc^Min/+ mouse model of colon cancer. In this study we investigated to ascertain whether the preventive effects of RVT on BaP-induced colon carcinogenesis is a result of altered BaP biotransformation by RVT. For the first group of mice, 100 μg BaP/kg bw was administered in peanut oil via oral gavage over a 60 day period. For the second group, 45 μg RVT/kg bw was co-administered with BaP. For the third group, RVT was administered for 1 week prior to BaP exposure. Blood, colon and liver were collected from control and BaP/RVT-treated mice at 60 days post-BaP & RVT exposure. We have assayed activities and expression (protein & mRNA) of drug metabolizing enzymes such as cytochrome P4501A1 (CYP1A1), CYP1B1, and glutathione-S-transferase (GST) in colon and liver samples from the treatment groups mentioned above. An increased expression of CYP1A1 in liver and colon and of CYP1B1 in liver of BaP-treated mice was seen, while RVT inhibited the extent of biotransformation mediated by these enzymes in the respective tissue samples. In the case of GST, an increased expression in colon of BaP alone-treated mice was noted when RVT was administered prior to BaP or simultaneously with BaP. However, there is no change in liver GST expression between BaP and RVT treatment groups. The concentrations of BaP aqueous (phase II) metabolites were found to be greater than the organic (phase I) metabolites, suggesting that RVT slows down the phase I metabolism (metabolic activation) of BaP, while enhancing phase II metabolism (detoxification). Additionally, the BaP-DNA adduct concentrations measured in colon and liver of BaP + RVT-treated mice were low relative to their BaP counterparts. Taken together, our findings strongly suggest that RVT alleviates BaP-induced colon carcinogenesis by impairing biotransformation pathways and DNA adduct formation, and therefore holds promise as a chemopreventive agent.

The binding of benzo(a)pyrene to DNA components of differing sequence complexity

An examination has been made of the binding, both in vitro and in vivo, of the benzo(a)pyrene (BP) adduct to DNA components of differing sequence complexity. Annealing was performed at low renaturation temperatures in the presence of high concentrations of formamide to minimize hydrocarbon-induced depurination. BHK-DNA was modified in vitro using a tritiated derivative of the ultimate carcinogenic metabolite of BP, 7alpha,8beta-di-hydroxy-9beta,10beta-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BPDE). Co-renaturation of this modfied DNA with [14C]-thymidine-labelled BHK-DNA demonstrated that the hydrocarbon adduct did not interfere with strand annealing and showed that the BP adduct was distributed randomly throughout all DNA sequence classes. However, when the DNA of cells in culture was modified by [3H]BP, following metabolic activation, and mixed with [14C]-thymidine-labelled DNA, a small but reproducible difference in the renaturation of the two labels was found. This difference in renaturation profiles was not due to base-compositional effects since a similar result was found when the alternate 14C-label was present in guanine bases, the principal site of BP modification. The small difference in the renaturation of the two radioactive labels indicated an enrichment of the hydrocarbon on the most rapidly-renaturing sequence components (the palindromic and highly repetitive sequences) where it amounted to between 19 and 64% increased modification.

Dose-response relationship, kinetics of formation, and persistence of S-[2-(N7-guanyl)-ethyl]glutathione-DNA adduct in livers of channel catfish (Ictalurus punctatus) exposed in vivo to ethylene dichloride

Formation of DNA adducts by reactive chemicals or their metabolites are often a precursor of mutagenesis and other adverse effects. Studies in juvenile channel catfish (Ictalurus punctatus) were conducted to determine the dose-response, kinetics of formation, and persistence of S-[2-(N7-guanyl)ethyl]glutathione hepatic-DNA adducts following a 4-h in vivo aqueous exposure to ethylene dichloride (EDC) at several dose levels. S-[2-(N7-guanyl)ethyl] glutathione adducts were detectable in liver tissue after 2 h of exposure and were still detectable three weeks after a single pulse exposure (detection limit=approximately 10 fmol, approximately 1 DNA adduct in 10(7) bases). Pretreatment of catfish with the glutathione-depleting agent diethylmaleate significantly reduced the level of tissue glutathione levels and, as a result, DNA adducts were not detected in pretreated fish. Catfish may serve as a useful sentinel species for detecting DNA-reactive chemicals in aquatic systems.

[DNA adducts as biological markers for human exposure to polycyclic aromatic compounds]

Polycyclic Aromatic Hydrocarbons (PAH) in human carcinogenesis is undisputed. Measurements of PAH-DNA adduct levels in easily accessible white blood cells therefore represent useful early endpoints in exposure intervention or preventive study. The successful applicability of DNA adducts as early endpoints depends on several criteria: (1) Adduct levels in easily accessible surrogate tissues should reflect adduct levels in target tissues (2) Toxic-kinetics and the temporal relevance should be properly defined (3) Sources of inter and intra individual variability must be know and controllable (4) Adduct analysis must have advantages as compared to other markers of PAH exposure. Higher proportions of subjects with detectable DNA adduct levels were found in exposed individuals as compared with non-exposed subjects, but saturation may occur at high exposure. Furthermore, DNA adducts levels varied according to changes in exposure. Intra-individual variation during continuous exposure was low over a short period of time (weeks), but varied significantly when longer time periods (months) were investigated. Intra-individual variation is currently only partly explained by genetic polymorphisms in genes involved in PAH metabolism. DNA adducts measurements have three advantages over traditional exposure assessment: (1) they can smooth the extreme variability in exposure which is typical for environmental toxicants and may integrate exposure over longer periods of time (2) Biological monitoring of DNA adducts accounts for all exposure routes (3) DNA adduct may account for inter-individual differences in uptake, elimination, distribution, metabolism and repair amongst exposed individuals. There is a sufficiently large scientific basis to justify the application of DNA adduct measurements as biomarkers in exposure assessment. However, their use in risk assessment requires further investigation.