New syntheses of N-(guanosin-8-yl)-4-aminobiphenyl and its 5'-monophosphate

Genetic and epigenetic changes in rat preneoplastic liver tissue induced by 2-acetylaminofluorene

Genotoxic carcinogens, including 2-acetylaminofluorene (2-AAF), in addition to exerting their genotoxic effects, often cause a variety of non-genotoxic alterations in cells. It is believed that these non-genotoxic effects may be indispensable events in tumorigenesis; however, there is insufficient knowledge to clarify the role of carcinogens in both the genetic and epigenetic changes in premalignant tissues and a lack of conclusive information on the link between epigenetic alterations and carcinogenic exposure. In the current study, we investigated whether or not the mechanism of 2-AAF-induced hepatocarcinogenesis consists of both genotoxic (genetic) and non-genotoxic (epigenetic) alterations. Male and female Sprague-Dawley rats were fed NIH-31 diet containing 0.02% of 2-AAF for 6, 12, 18 or 24 weeks. The levels of DNA adducts obtained from 2-AAF in liver and kidney tissues were assessed by high-performance liquid chromatography combined with electrospray tandem mass spectrometry (HPLC-ES-MS/MS). N-(Deoxyguanosine-8-yl)-2-aminofluorene was the major adduct detected at all time points in both tissues. Global DNA methylation in the livers and kidneys, as determined by an HpaII-based cytosine extension assay and by HPLC-ES-MS/MS, did not change over the 24-week period. In the livers of male rats, there was a progressive decrease of global and long interspersed nucleotide element-1-associated histone H4 lysine 20 trimethylation, as well as hypermethylation of the p16(INK4A) gene. These epigenetic changes were not observed in the livers of female rats or the kidneys of both sexes. Importantly, morphological evidence of formation and progression of neoplastic process was observed in the liver of male rats only. In conclusion, we have demonstrated that exposure of rats to genotoxic hepatocarcinogen 2-AAF, in addition to formation of 2-AAF-specific DNA lesions, resulted in substantial alterations in cellular epigenetic status.

Sequence effects of aminofluorene-modified DNA duplexes: thermodynamic and circular dichroism properties

Circular dichroism (CD) and UV-melting experiments were conducted with 16 oligodeoxynucleotides modified by the carcinogen 2-aminofluorene, whose sequence around the lesion was varied systematically [d(CTTCTNG[AF]NCCTC), N = G, A, C, T], to gain insight into the factors that determine the equilibrium between base-displaced stacked (S) and external B-type (B) duplex conformers. Differing stabilities among the duplexes can be attributed to different populations of S and B conformers. The AF modification always resulted in sequence-dependent thermal (T(m)) and thermodynamic (-DeltaG degrees ) destabilization. The population of B-type conformers derived from eight selected duplexes (i.e. -AG*N- and -CG*N-) was inversely proportional to the -DeltaG degrees and T(m) values, which highlights the importance of carcinogen/base stacking in duplex stabilization even in the face of disrupted Watson-Crick base pairing in S-conformation. CD studies showed that the extent of the adduct-induced negative ellipticities in the 290-350 nm range is correlated linearly with -DeltaG degrees and T(m), but inversely with the population of B-type conformations. Taken together, these results revealed a unique interplay between the extent of carcinogenic interaction with neighboring base pairs and the thermodynamic properties of the AF-modified duplexes. The sequence-dependent S/B heterogeneities have important implications in understanding how arylamine-DNA adducts are recognized in nucleotide excision repair.

A Modified Host Cell Reactivation Assay to Measure DNA Repair Capacity for Removing 4-Aminobiphenyl Adducts: A Pilot Study of Bladder Cancer

As DNA repair plays an important role in genetic susceptibility to bladder cancer, assessment of the DNA repair phenotype is critical for the molecular epidemiology of bladder cancer. In this study, we developed and applied an assay using the luciferase (luc) reporter gene in a host-cell reactivation assay to measure DNA repair capacity for DNA damage induced by 4-aminobiphenyl (4-ABP), a well-studied aromatic amine and a known bladder carcinogen. We observed a dose-response relationship for 4-ABP dosage and DNA repair capacity (luc activity). We then applied this assay to measure DNA repair capacity in a pilot study of 89 pairs of bladder cancer patients and healthy controls matched by age, gender, and ethnicity, and we found that DNA repair capacity was significantly lower in cases than in controls (13.0% versus 14.4%; P = 0.006). Poor DNA repair capacity was associated with 3.42-fold increased bladder cancer risk. Further analysis revealed that intermediate and low levels of DNA repair capacity increased bladder cancer risk to 3.43-fold and 4.97-fold, respectively, compared with individuals with the most efficient DNA repair capacity. Moreover, ever smokers with suboptimal DNA repair capacity exhibited a 6.06-fold increased risk compared with never smokers with normal DNA repair capacity. In conclusion, our results support the hypothesis that deficient DNA repair capacity for 4-ABP induced DNA damage and increases bladder cancer risk. Our assay provides a new tool to specifically quantify DNA repair capacity in bladder cancer studies and, therefore, contributes to our goal of further elucidating bladder carcinogenesis.

Synthesis and characterization of nucleoside derivatives, n-(benzoyl)-n-(deoxyguanosin-8-yl)4-aminobiphenyl and n-(2'-deoxyguanosin-8-yl)4-aminobiphenyl via alpha-phenyl-n-(4-biphenyl)nitrone

Lead tetraacetate (LTA) oxidation of alpha-Phenyl-N-(4-bipheny])nitrone (8) to give a new ultimate carcinogen, N-acetoxy-N-benzoyl-4-aminobiphenyl (9) which was reacted with deoxyguanosine (dG) at pH 6.9 to give nucleoside derivative, N-(benzoyl)-N-(deoxyguanosin-8-yl)-4-aminobiphenyl (10). Following debenzoylation with sodium carbonate-methanol leads to N-(2'-deoxyguanosin-8-yl)-4-aminobiphenyl (11).

Mechanism of oxidative DNA damage induced by carcinogenic 4-aminobiphenyl

DNA adduct formation is thought to be a major cause of DNA damage by carcinogenic aromatic amines. We investigated the ability of an aromatic amine, 4-aminobiphenyl (4-ABP) and its N-hydroxy metabolite (4-ABP(NHOH)) to cause oxidative DNA damage, using (32)P-labeled human DNA fragments from the p53 tumor suppressor gene and the c-Ha-ras-1 protooncogene. 4-ABP(NHOH) was found to cause Cu(II)-mediated DNA damage, especially at thymine residues. Addition of the endogenous reductant NADH led to dramatic enhancement of this process. Catalase and bathocuproine, a Cu(I)-specific chelator, reduced the amount of DNA damage, suggesting the involvement of H(2)O(2) and Cu(I). 4-ABP(NHOH) dose-dependently induced 8-hydroxy-2'-deoxyguanosine (8-OHdG) formation in the presence of Cu(ll) and NADH. 4-ABP(NHOH) conversion to nitrosobiphenyl, as measured by UV-visible spectroscopy, occurred rapidly in the presence of Cu(II), suggesting Cu(II)-mediated autoxidation. Increased amounts of 8-OHdG were found in HL-60 cells compared to the H(2)O(2)-resistant clone HP100 following 4-ABP(NHOH) treatment, further supporting the involvement of H(2)O(2). The present study demonstrates that an N-hydroxy derivative of 4-ABP induces oxidative DNA damage through H(2)O(2) in both a cell-free system and in cultured human cells. We conclude that, in addition to DNA adduct formation, oxidative DNA damage may play an important role in the carcinogenic process of 4-ABP.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Mechanism of C8 alkylation of guanine residues by activated arylamines: evidence for initial adduct formation at the N7 position

Aromatic amines are bioactivated to electrophilic compounds that react with DNA, predominantly at the C8 position of guanine bases. This site is weakly nucleophilic and it has been proposed that the C8 adduct is the final product after initial N7-adduct formation. To consider this possibility, we reacted several C8-substituted guanine derivatives with N-acetoxy-2-aminofluorene, prepared in situ from 2-acetylsalicylic acid and N-hydroxy-2-aminofluorene. With C8,N9-dimethylguanine, an adduct was isolated in good yield that was consistent, by NMR and mass spectral characterization, with a structure involving carcinogen substitution at the N7 position of guanine and linked through the 2-aminofluorenyl nitrogen--N-(C8,N9- dimethylguanin-N7-yl)-2-aminofluorene. This adduct could be easily reduced with NaBH4, consistent with the proposed N7-adduct structure. The same reaction was also carried out with C8-methylguanosine and C8-methyldeoxyguanosine and similar adducts were isolated. In contrast, C8-bromoguanosine reacted with N-acetoxy-2-aminofluorene to yield the C8-substituted arylamine adduct N-(guanosin-C8-yl)-2-aminofluorene directly. These products are uniquely consistent with a scheme in which C8-adduct formation is preceded by an initial electrophilic substitution on the N7 atom, which is postulated to be a general reaction for activated arylamines and heterocyclic amines.

Development of fast atom bombardment mass spectral methods for the identification of carcinogen-nucleoside adducts

An analytical strategy using fast atom bombardment (FAB) ionization and tandem mass spectrometry has been developed to determine the molecular weight and major fragment ions, and to provide limited structural characterization of low picomole levels of carcinogen-nucleoside adducts. This strategy consists of three main components: (1) the sensitivity for analysis by FAB combined with mass spectrometry is increased via chemical derivatization; (2) the nucleoside adducts are selectively detected by using constant neutral loss scans; and (3) structurally characteristic fragments are obtained by using daughter ion scans. Trimethylsilyl derivatized arylamine-nucleoside adducts have been detected at levels as low as a few picomoles by using this approach. After experimental determination of the mass of the BH 2 (+) fragment ion, daughter ion spectra have been used to probe the structure specificity associated with collision-activated decomposition of this fragment. With model C-8 substituted arylamine adducts [N-(deoxyguanosin-8-yl)-4-aminobiphenyl, N-(deoxyadenosin--yl)-4-aminobiphenyl, and N-(deoxyguanosin-8-yl)-2-aminofluorene], nucleoside-specific and carcinogen-specific fragmentation have been observed in daughter ion spectra.

Development of an avidin-biotin amplified enzyme-linked immunoassay for detection of DNA adducts of the human bladder carcinogen 4-aminobiphenyl

4-Aminobiphenyl (ABP) is a known human urinary bladder carcinogen which is present in tobacco smoke and may be ubiquitous in the environment. As a biological monitor of carcinogen exposure, we have developed an immunological method for measuring the predominant carcinogen-DNA adduct of ABP, N-(deoxyguanosin-8-yl)-ABP (dG-C8-ABP). Rabbits were immunized with keyhole limpet hemocyanin (KLH) conjugate prepared by a periodate oxidation and coupling of N-(guanosin-8-yl)-ABP (rG-C8-ABP) to the protein. The resulting polyclonal antisera was systematically characterized using dual inhibitor methodology augmented by specialized computer and software support; and a competitive avidin-biotin enzyme-linked immunoassay (A-B ELISA) assay employing polyclonal rabbit anti-KLH-(rG-C8-ABP) was developed. Under the assay conditions described, the detection limit for dG-C8-ABP was 18 fmol/well. The relative lack of reactivity toward ABP, N-acetyl-4-aminobiphenyl, N-(deoxyadenosin-8-yl)-ABP, N-(deoxyguanosin-8-yl)-2-aminofluorene and deoxyguanosine as inhibitors indicated that primary specificity involves epitopes found on the purine and biphenyl rings. Results emphasize the need to define polyclonal anti-adduct sera operationally in the context of the antigen/assay system used to evaluate it. Assay sensitivity was achieved by decreasing the amount of antibody and solid-phase antigen in the competitive portion of the assay and the use of avidin-biotin as well as enzymatic amplification. This methodology is a useful alternative to other ultrasensitive techniques and should be directly applicable to the detection of ABP-DNA adducts in exposed human populations.

Prostaglandin H synthase-mediated reaction of carcinogenic arylamines with tRNA and homopolyribonucleotides

Prostaglandin H synthase mediates the reaction of an extensive series of carcinogenic arylamines with tRNA. Structure-activity relationships suggest that benzidine is especially reactive due to extended conjugation between the 4,4'-diamino groups. In trapping experiments with homopolyribonucleotides, benzidine reacts with polyguanylic acid but 4-aminobiphenyl reacts with polycytidylic acid. The nitrenium ion of 4-aminobiphenyl (formed by N,O-acyltransferase activation of N-hydroxy-4-acetylaminobiphenyl) reacts primarily with polyguanylic acid and to a lesser extent with polyadenylic acid. The results suggest that arylamine activation by prostaglandin H synthase does not involve nitrenium ion formation.

Arylhydroxylamine-induced ribonucleic acid chain cleavage and chromatographic analysis of arylamine-ribonucleic acid adducts

Reaction of N-hydroxy-2-aminofluorene (N-OH-AF) with rRNA at pH 5.0 decreased the molecular weight of the polynucleotide. Toluene-soluble aryl derivatives were released on hydrolysis of fluorenylamine- and biphenylamine-substituted RNA by treatment with venom phosphodiesterase and alkaline phosphatase. These data suggested that arylhydroxylamines, activated by incubation at pH 5.0 or by enzymatic O-acetylation, might react with the phosphate group of RNA to give unstable phosphate triesters. Spontaneous hydrolysis of these triesters would result in cleavage of the polynucleotide chain. Further enzymatic hydrolysis of the phosphate esters would yield nonpolar arylamine derivatives. Enzymatically degraded 4-aminobiphenyl(ABP)-RNA adducts were examined by high performance liquid chromatography (HPLC) for the presence of a putative phosphorylated adduct. Synthetic standards of the C-8-guanosine monophosphate-ABP adduct (ABP-GMP) and o-aminobiphenyl-O-phosphate were used as markers in the analysis of the digested RNA. A phosphate adduct of ABP was undetectable by these methods. The data also indicated that the ABP-GMP formed in the acyltransferase-mediated binding of N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) to RNA is readily degraded during the enzymatic digestion of the RNA adduct.