Genetic control of predominantly error-free replication through an acrolein-derived minor-groove DNA adduct

Acrolein, an α,β-unsaturated aldehyde, is generated in vivo as the end product of lipid peroxidation and from metabolic oxidation of polyamines, and it is a ubiquitous environmental pollutant. The reaction of acrolein with the N2 of guanine in DNA leads to the formation of γ-hydroxy-1-N²-propano-2' deoxyguanosine (γ-HOPdG), which can exist in DNA in a ring-closed or a ring-opened form. Here, we identified the translesion synthesis (TLS) DNA polymerases (Pols) that conduct replication through the permanently ring-opened reduced form of γ-HOPdG ((r) γ-HOPdG) and show that replication through this adduct is mediated via Rev1/Polη-, Polι/Polκ-, and Polθ-dependent pathways, respectively. Based on biochemical and structural studies, we propose a role for Rev1 and Polι in inserting a nucleotide (nt) opposite the adduct and for Pols η and κ in extending synthesis from the inserted nt in the respective TLS pathway. Based on genetic analyses and biochemical studies with Polθ, we infer a role for Polθ at both the nt insertion and extension steps of TLS. Whereas purified Rev1 and Polθ primarily incorporate a C opposite (r) γ-HOPdG, Polι incorporates a C or a T opposite the adduct; nevertheless, TLS mediated by the Polι-dependent pathway as well as by other pathways occurs in a predominantly error-free manner in human cells. We discuss the implications of these observations for the mechanisms that could affect the efficiency and fidelity of TLS Pols.

Potential Metabolic Activation of Representative Alkylated Polycyclic Aromatic Hydrocarbons 1-Methylphenanthrene and 9-Ethylphenanthrene Associated with the Deepwater Horizon Oil Spill in Human Hepatoma (HepG2) Cells

Exposure to petrogenic polycyclic aromatic hydrocarbons (PPAHs) is the major human health hazard associated with the Deepwater Horizon oil spill. Alkylated phenanthrenes are the most abundant PPAHs present in the crude oil and could contaminate the food chain. We describe the metabolism of a C₁-phenanthrene regioisomer 1-methylphenanthrene (1-MP) and a C₂-phenanthrene regioisomer 9-ethylphenanthrene (9-EP) in human HepG2 cells. The structures of the metabolites were identified by HPLC-UV-fluorescence detection and LC-MS/MS. Side chain hydroxylation of 1-MP and 9-EP was observed as the major metabolic pathway. The formation of 1-(hydroxymethyl)-phenanthrene was confirmed by reference to an authentic synthetic standard. However, formation of the bioactivated sulfate was not detected. Tetraols were also identified as signature metabolites of 1-MP and 9-EP, indicating that metabolic activation occurred via the diol-epoxide pathway. O-Monosulfonated-catechols were discovered as signature metabolites of the o-quinone pathway of metabolic activation of 1-MP and 9-EP, respectively. The identification of O-monosulfonated-catechols supports the metabolic activation of 1-MP and 9-EP by P450 and AKR isozymes followed by metabolic detoxification of the o-quinone through interception of redox cycling by phase II isozymes. The signature metabolites identified could be used as biomarkers of human exposure to 1-MP and 9-EP resulting from oil spills.

Potential Metabolic Activation of a Representative C4-Alkylated Polycyclic Aromatic Hydrocarbon Retene (1-Methyl-7-isopropyl-phenanthrene) Associated with the Deepwater Horizon Oil Spill in Human Hepatoma (HepG2) Cells

Exposure to petrogenic polycyclic aromatic hydrocarbons (PPAHs) in the food chain is the major human health hazard associated with the Deepwater Horizon oil spill. C4-Phenanthrenes are representative PPAHs present in the crude oil and could contaminate the seafood. We describe the metabolism of a C4-phenanthrene regioisomer retene (1-methyl-7-isopropyl-phenanthrene) in human HepG2 cells as a model for metabolism in human hepatocytes. Retene because of its sites of alkylation cannot be metabolized to a diol-epoxide. The structures of the metabolites were identified by HPLC-UV-fluorescence detection and LC-MS/MS. O-Monosulfonated-retene-catechols were discovered as signature metabolites of the ortho-quinone pathway of PAH activation catalyzed by aldo-keto reductases. We also found evidence for the formation of bis-ortho-quinones where the two dicarbonyl groups were present on different rings of retene. The identification of O-monosulfonated-retene-catechol and O-bismethyl-O-monoglucuronosyl-retene-bis-catechol supports metabolic activation of retene by P450 and aldo-keto reductase isozymes followed by metabolic detoxification of the ortho-quinone through interception of redox cycling by catechol-O-methyltransferase, uridine 5'-diphospho-glucuronosyltransferase, and sulfotransferase isozymes. We propose that catechol conjugates could be used as biomarkers of human exposure to retene resulting from oil spills.

Metabolism of an Alkylated Polycyclic Aromatic Hydrocarbon 5-Methylchrysene in Human Hepatoma (HepG2) Cells

Exposure to polycyclic aromatic hydrocarbons (PAHs) in the food chain is the major human health hazard associated with the Deepwater Horizon oil spill. C1-chrysenes are representative PAHs present in the crude oil and have been detected in contaminated sea food in amounts that exceed their permissible safety thresholds. We describe the metabolism of the most carcinogenic C1-chrysene regioisomer, 5-methylchrysene (5-MC), in human HepG2 cells. The structures of the metabolites were identified by HPLC-UV-fluorescence detection and LC-MS/MS. 5-MC-tetraol, a signature metabolite of the diol-epoxide pathway, was identified as reported previously. Novel O-monosulfonated-5-MC-catechol isomers and O-monomethyl-O-monosulfonated-5-MC-catechol were discovered, and evidence for their precursor ortho-quinones was obtained. The identities of O-monosulfonated-5-MC-1,2-catechol, O-monomethyl-O-monosulfonated-5-MC-1,2-catechol, and 5-MC-1,2-dione were validated by comparison to authentic synthesized standards. Dual metabolic activation of 5-MC involving the formation of bis-electrophiles, i.e., a mono-diol-epoxide and a mono-ortho-quinone within the same structure, bis-diol-epoxides, and bis-ortho-quinones is reported for the first time. Evidence was also obtained for minor metabolic conversion of 5-MC to form monohydroxylated-quinones and bis-phenols. The identification of 5-MC-tetraol, O-monosulfonated-5-MC-1,2-catechol, O-monomethyl-O-monosulfonated-5-MC-1,2-catechol, and 5-MC-1,2-dione supports metabolic activation of 5-MC by P450 and AKR isozymes followed by metabolic detoxification of the ortho-quinone through interception of redox cycling by COMT and SULT isozymes. The major metabolites, O-monosulfonated-catechols and tetraols, could be used as biomarkers of human exposure to 5-MC resulting from oil spills.

A daidzein-daunomycin conjugate improves the therapeutic response in an animal model of ovarian carcinoma

The use of daunomycin against neoplasms is limited due to its severe cardiotoxicity. The cytotoxicity of daunomycin can be minimized by linking it to an affinity tag. Since ovarian cancer cells are sensitive to isoflavone action, we synthesized a daidzein daunomycin conjugate. In MLS human ovarian cancer cells, the conjugate was shown to have a larger cytotoxic effect than daunomycin per se at a low concentration. The conjugate was then tested in vivo in mice carrying MLS xenografts. Tumour growth in the groups of conjugate and daunomycin was inhibited by >50% as compared to vehicle treated mice. In contrast to daunomycin treated mice, no weight reduction or death was seen in mice treated with the conjugate. In vivo imaging of the fluorescence signal generated by daunomycin indicated uptake of both conjugate and daunomycin by the tumour. Tumour fluorescence was, however, higher in the conjugate treated mice than in the daunomycin treated mice, thus suggesting specific delivery of the drug to the tumour. Histological examination of myocardial tissue indicated that only the daunomycin, but not conjugate treated mice showed cardiac damage. These results indicate that targeting of daunomycin via carboxymethyldaidzein retains daunomycin's cytotoxic effects while averting its toxicity in an ovarian xenograft.

Synthesis and mutagenesis of the butadiene-derived N3 2'-deoxyuridine adducts

1,3-Butadiene is a known carcinogen and mutagen that acts through a variety of metabolic intermediates that react with DNA, forming stable and unstable lesions on dG, dA, dC, and dT. The N3 2'-deoxyuridine adducts are a highly stable, stereoisomeric mixture of adducts derived from the reaction of cytosine with the monoepoxide metabolite of butadiene, followed by spontaneous deamination. In this study, the phosphoramidites and subsequent oligodeoxynucleotides containing the N3 2'-deoxyuridine adducts have been constructed and characterized. Using a single-stranded shuttle vector DNA, the mutagenic potential of these adducts has been tested following replication in mammalian cells. Replication past the N3 2'-deoxyuridine adducts was found to be highly mutagenic with an overall mutation yield of approximately 97%. The major mutations that were observed were C to T transitions and C to A transversions. In vitro, these adducts posed a complete block to both the Klenow fragment of Escherichia coli polymerase I and polymerase epsilon, while these lesions significantly blocked polymerase delta. These data suggested a possible involvement of bypass polymerases in the in vivo replication of these lesions. Overall, these findings indicate that the N3 2'-deoxyuridine adducts are highly mutagenic lesions that may contribute to butadiene-mediated carcinogenesis.

Mutagenicity of stereochemical configurations of 1,2-epoxybutene and 1,2:3,4-diepoxybutane in human lymphblastoid cells

The carcinogenicity of 1,3-butadiene (BD) is related to its bioactivation to several DNA-reactive metabolites; accumulating evidence suggests that the stereochemistry of these BD intermediates may play a significant role in the mutagenic and carcinogenic actions of the parent compound. The objective of this study was to evaluate the cytotoxicity and mutagenicity of stereochemical forms of 1,2-epoxybutene (EB) and 1,2:3,4-diepoxybutane (DEB), two genotoxic BD metabolites, in a human lymphoblastoid cell line, TK6. Cytotoxicity was measured by comparing cloning efficiencies in chemical-exposed cells versus those in control cells. The hypoxanthine-guanine phosphoribosyltransferase (HPRT) and thymidine kinase (TK) mutant frequencies (MFs) were measured using a cell cloning assay. HPRT mutants collected from cells exposed to the three forms of DEB were analyzed by PCR to characterize large genetic alterations. All the three stereoisomers of DEB caused increased HPRT and TK MFs compared to the concurrent control samples. There were no significant differences in cytotoxicity or mutagenicity among the three isomers of DEB in TK6 cells. Molecular analysis of HPRT mutants revealed similar distributions of types of mutations among the three isomers of DEB. There were also no statistically significant differences in mutagenic efficiencies between the two isomers of EB in TK6 cells. These results were consistent with the in vivo findings that there was little difference in the mutagenic efficiencies of racemic-DEB versus meso-DEB in rodents. Thus, in terms of mutagenic efficiency, stereochemical configurations of EB and DEB are not likely to play a significant role in the mutagenicity and carcinogenicity of BD.

Separation of synthetic oligonucleotide dithioates from monothiophosphate impurities by anion-exchange chromatography on a mono-q column

A method using a strong anion-exchange liquid-chromatography column, Mono-Q, has been developed for high-resolution analysis and purification of oligonucleotide dithioates, which were synthesized by an automated, solid-phase, phosphorothioamidite chemistry. High-resolution separation of oligonucleotide phosphorodithioates from monothiophosphate impurities was obtained. High-resolution separation was also demonstrated at pH 8. The separation of oligonucleotide dithioates was found to be linearly dependent on the number of sulfurs for the same sequence length. Thiocyanate, SCN-, as eluting anion, can be used to purify oligonucleotides containing a high percentage of phosphorodithioate linkages in lower salt concentrations and provides better separation than chloride as eluting anion.

Utilization of tetrabutylammonium triphenyldifluorosilicate (TBAT) in the synthesis of 6-fluoropurine nucleosides

Tetrabutylammonium triphenydifluorosilicate (TBAT) has been found to be a useful reagent for the conversion of 6-chloropurine nucleosides to 6-fluoropurine derivatives. The 6-chloropurine nucleosides were reacted with trimethylamine to form quaternary trimethylammonium salts which were treated in situ with TBAT in DMF to effect conversion to the 6-fluoro derivatives in yields of 59-72%.

Heterocyclic N-dithiocarboxylates as cadmium antagonists: 4-hydroxypiperidine- and 4-carboxamidopiperidine-N-dithiocarboxylate

Sodium 4-hydroxypiperidine-N-dithiocarboxylate(HP-N-DTC) and sodium 4-carboxamidopiperidine-N-dithiocarboxylate (CAP-N-DTC) were synthesized by the reaction of each piperidine analog with CS2 in the presence of NaOH. Following isolation and characterization, each dithiocarboxylate (DTC) when added to an aqueous solution of CdCl2 formed a water-insoluble complex with Cd which was shown by elemental analyses to consist of DTC:Cd in a 2:1 molar ratio. HP-N-DTC was a highly effective antagonist of acute Cd toxicity in mice, while CAP-N-DTC was less effective. When seven equimolar doses of each DTC were administered to mice which had received a non-lethal dose of CdCl2 along with 1.0 microCi of 109CdCl2 50 days earlier, both effected marked reductions of renal Cd levels. CAP-N-DTC was much more effective than HP-N-DTC in reducing the whole body Cd burden, apparently due to its more rapid depletion of hepatic Cd accompanied by an enhanced rate of Cd excretion. The data illustrate that subtle structural modifications of this class of heavy metal binding agents can yield compounds with impressive differences in pharmacologic activity.