Mutagenicity of oxidized microsomal metabolites of 1-nitropyrene in Chinese hamster ovary cells

The role of epoxide hydrolases in health and disease

Epoxide hydrolases (EH) are ubiquitously expressed in all living organisms and in almost all organs and tissues. They are mainly subdivided into microsomal and soluble EH and catalyze the hydration of epoxides, three-membered-cyclic ethers, to their corresponding dihydrodiols. Owning to the high chemical reactivity of xenobiotic epoxides, microsomal EH is considered protective enzyme against mutagenic and carcinogenic initiation. Nevertheless, several endogenously produced epoxides of fatty acids function as important regulatory mediators. By mediating the formation of cytotoxic dihydrodiol fatty acids on the expense of cytoprotective epoxides of fatty acids, soluble EH is considered to have cytotoxic activity. Indeed, the attenuation of microsomal EH, achieved by chemical inhibitors or preexists due to specific genetic polymorphisms, is linked to the aggravation of the toxicity of xenobiotics, as well as the risk of cancer and inflammatory diseases, whereas soluble EH inhibition has been emerged as a promising intervention against several diseases, most importantly cardiovascular, lung and metabolic diseases. However, there is reportedly a significant overlap in substrate selectivity between microsomal and soluble EH. In addition, microsomal and soluble EH were found to have the same catalytic triad and identical molecular mechanism. Consequently, the physiological functions of microsomal and soluble EH are also overlapped. Thus, studying the biological effects of microsomal or soluble EH alterations needs to include the effects on both the metabolism of reactive metabolites, as well as epoxides of fatty acids. This review focuses on the multifaceted role of EH in the metabolism of xenobiotic and endogenous epoxides and the impact of EH modulations.

Mutation spectra induced by 1-nitropyrene 4,5-oxide and 1-nitropyrene 9,10-oxide in the supF gene of human XP-A fibroblasts

1-Nitropyrene 4,5-oxide and 1-nitropyrene 9,10-oxide are oxidative metabolites that are responsible for the mutagenicity of 1-nitropyrene. In this study, the mutation spectra induced by oxidative metabolites in human cells were determined using a shuttle vector assay. The mutation frequencies induced by 1-nitropyrene 9,10-oxide were 2-3 times higher than those induced by 1-nitropyrene 4,5-oxide. The base substitutions induced by 1-nitropyrene 4,5-oxide were G --> A transitions, G --> C transversions, and G --> T transversions. In the case of 1-nitropyrene 9,10-oxide, G --> A transitions, G --> T transversions, A --> G transitions and G --> C transversions were observed. Most base substitution mutations induced by oxidative metabolites occurred at the guanine sites in the supF gene. These sequence-specific hot spots were commonly identified as 5'-GA sequences for both metabolites. On the other hand, the sequence-specific hot spots at the adenine sites were identified as 5'-CAC sequences for 1-nitropyrene 9,10-oxide. These results suggest that the oxidative metabolites of 1-nitropyrene induce sequence-specific DNA mutations at the guanine and adenine sites at high frequency.

1-Nitropyrene efficiently induces mitotic recombination in Saccharomyces cerevisiae

Nitropyrene, a mutagenic and carcinogenic component of diesel exhaust, has been shown to be a potent bacterial and mammalian mutagen. There is, however, some controversy regarding the genotoxic effects of 1-nitropyrene towards yeast. To obtain insights into the mechanisms of 1-nitropyrene-induced mutations in Saccharomyces cerevisiae, we have attempted to characterize the genetic alterations that inactivate the endogenous CAN1 gene either in haploid cells or in heterozygous diploid cells. 1-Nitropyrene, without any activation treatment, showed a substantial toxic effect until 500 microM. The mutation frequency in haploid cells treated with 500 microM of 1-nitropyrene was 1.59 x 10(-5), which is 15-fold higher than the control value. Sequencing of mutants indicated that both frameshifts and base substitutions were increased. In diploid cells treated with 500 microM of 1-nitropyrene, the frequency with which can1Delta::LEU2/can1Delta::LEU2 was converted from CAN1/can1Delta::LEU2, a phenotypic change from a canavanine-sensitive to canavanine-resistant form, was 8.59 x 10(-4), which is 9.15-fold higher than the spontaneous level. More than 99% of the 1-nitropyrene-induced mutations in canavanine-resistant diploid cells constituted a gene conversion or crossover. Chromosome loss was not increased after treatment with 1-nitropyrene. These results suggest that 1-nitropyrene is an agent that efficiently induces point mutations, gene conversion, and crossover, but not chromosome loss, in S. cerevisiae.

Epoxide hydrolases: biochemistry and molecular biology

Epoxides are organic three-membered oxygen compounds that arise from oxidative metabolism of endogenous, as well as xenobiotic compounds via chemical and enzymatic oxidation processes, including the cytochrome P450 monooxygenase system. The resultant epoxides are typically unstable in aqueous environments and chemically reactive. In the case of xenobiotics and certain endogenous substances, epoxide intermediates have been implicated as ultimate mutagenic and carcinogenic initiators Adams et al. (Chem. Biol. Interact. 95 (1995) 57-77) Guengrich (Properties and Metabolic roles 4 (1982) 5-30) Sayer et al. (J. Biol. Chem. 260 (1985) 1630-1640). Therefore, it is of vital importance for the biological organism to regulate levels of these reactive species. The epoxide hydrolases (E.C. 3.3.2. 3) belong to a sub-category of a broad group of hydrolytic enzymes that include esterases, proteases, dehalogenases, and lipases Beetham et al. (DNA Cell Biol. 14 (1995) 61-71). In particular, the epoxide hydrolases are a class of proteins that catalyze the hydration of chemically reactive epoxides to their corresponding dihydrodiol products. Simple epoxides are hydrated to their corresponding vicinal dihydrodiols, and arene oxides to trans-dihydrodiols. In general, this hydration leads to more stable and less reactive intermediates, however exceptions do exist. In mammalian species, there are at least five epoxide hydrolase forms, microsomal cholesterol 5,6-oxide hydrolase, hepoxilin A(3) hydrolase, leukotriene A(4) hydrolase, soluble, and microsomal epoxide hydrolase. Each of these enzymes is distinct chemically and immunologically. Table 1 illustrates some general properties for each of these classes of hydrolases. Fig. 1 provides an overview of selected model substrates for each class of epoxide hydrolase.

Molecular characterization of mutation and comparison of mutation profiles in the hprt gene of Chinese hamster ovary cells treated with benzo[a]pyrene trans-7,8-diol-anti-9,10-epoxide, 1-nitrobenzo[a]pyrene trans-7,8-diol-anti-9,10-epoxide, and 3-nitrobenzo[a]pyrene trans-7,8- diol-anti-9,10-epoxide

Both 1- and 3-nitrobenzo[a]pyrene (nitro-BaP) are environmental contaminants, potent mutagens in Salmonella, and moderate mutagens in Chinese hamster ovary (CHO) cells. The mutagenicity of their oxidized metabolites,trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-epoxy -7,8,9,10-tetrahydro-1-nitrobenzo[a]pyrene (1-nitro-BaP-DE) and trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydro-3-nitrobenzo[a]- pyrene (3-nitro-BaPDE), together with trans-7,8-dihydroxy-anti-9, 10-ep- oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BaP-DE), was determined in CHO-K1 cells, and the resulting mutations at the hprt locus were characterized by polymerase chain reaction (PCR) amplification of reverse-transcribed hprt mRNA, followed by DNA sequence analysis. The mutant frequencies, in mutants/10(6) clonable cells, at 30 and 100 ng/ml, were BaP-DE, 248 and 456; 1-nitro-BaP-DE, 68 and 260; 3-nitro-BaP-DE, 81 and 232, respectively. In general, the three diolepoxides exhibited similar mutational spectra: 1) 64% (23/36 sequenced mutants) of BaP-DE, 53% (19/36) of 1-nitro-BaP-DE, and 64% (23/36) of 3-nitro-BaP-DE mutants resulted from simple base pair substitution, with the predominant mutation being G-->T transversion; 2) 90%, 100%, and 100% of mutations at G:C had the mutated dG on the nontranscribed DNA strand; and 3) about one quarter of the mutants produced by each mutagen had one or more PCR products with partial or complete exon deletions. The mutagens induced few frameshifts or complex mutations. Among the differences in mutational specificity for the three diolepoxides, the proportion of substituted dGs with 3' purines was significant (P < 0.05) for BaP-DE (16/19, 84%) and 3-nitro-BaP-DE (17/20, 85%), but not significant for 1-nitro-BaP-DE-induced mutants (11/17, 65%, P > 0.05). Also, high proportions of BaP-DE and 3-nitro-BaP-DE base pair substitutions at G:C occurred in DNA sequence contexts of 5'-GG-3', 5'-GGA-3', and 5'-TGGA-3', while the proportions of 1-nitro-BaP-DE mutants in these contexts were often lower. The results indicate that nitro substitution at C1 or C3 of BaP-DE reduces mutational potency in CHO cells and appears to have only subtle effects upon the mutational pattern in the hprt gene.

Metabolism of 1-nitropyrene in mice: transport across the placenta and mammary tissues

The distribution and metabolism of the environmental pollutant 1-nitropyrene was studied in C57B1/6N mice following oral or intraperitoneal dosing. When administered by gavage, 1-nitropyrene and its metabolites demonstrated biphasic elimination kinetics from the blood, with half-lives of 0.3 and 1.8 days and a distribution volume of 74 ml. Intraperitoneal administration resulted in similar biphasic elimination, with half-lives of 0.5 and 3 days and a distribution volume of 98 ml. Treating pregnant C57B1/6N (C3H sire) mice by gavage resulted in similar absorption and elimination kinetics of 1-nitropyrene and metabolites, except that the distribution volume increased to 123 ml. 1-Nitropyrene and/or its metabolites (0.7% of the administered dose) crossed the placenta and accumulated in the fetuses and amniotic fluid, with both C-oxidized and nitroreduced metabolites being detected. Suckling neonates accumulated 1-nitropyrene and its metabolites when their dams were administered 1-nitropyrene by gavage. Each neonate received approximately 0.1% of the administered dose and demonstrated the presence of both C-oxidized and nitroreduced metabolites. These results demonstrate that this environmental pollutant is capable of crossing the placenta or mammary tissues to expose the offspring to a potentially genotoxic compound.

Metabolic activation of 1-nitropyrene to a mammalian cell mutagen and a carcinogen

1. The mutagenicity of 1-nitropyrene metabolites in Chinese hamster ovary (CHO) cells, in the absence of rat liver S9, decreased in the order 6-hydroxy-1-nitropyrene > 1-nitropyrene 9,10-oxide > 1-nitropyrene 4,5-oxide approximately 3-hydroxy-1-nitropyrene approximately 8-hydroxy-1-nitropyrene > 1-nitropyrene. The order of mutagenicity with rat liver S9 was 1-nitropyrene 4,5-oxide approximately 6-hydroxy-1-nitropyrene approximately 1-nitropyrene 9,10-oxide > 3-hydroxy-1-nitropyrene approximately 1-nitropyrene > 8-hydroxy-1-nitropyrene. 2. 1-Nitropyrene 4,5-oxide reacted with calf thymus DNA to give one or several closely related adducts. The same adducts were detected in CHO cells incubated with 1-nitropyrene 4,5-oxide. Inclusion of a nitroreductase, xanthine oxidase, in the incubations with calf thymus DNA resulted in the formation of an additional adduct identified as N-(deoxyguanosin-8-yl)-1-aminopyrene (dG-C8-AP). 3. 1-Nitropyrene 9,10-oxide reacted with calf thymus DNA to give an adduct pattern similar to that observed with 1-nitropyrene 4,5-oxide. Incubation of 1-nitropyrene 9,10-oxide with CHO cells resulted in the formation of the same adducts along with dG-C8-AP. 4. dG-C8-AP and N-(deoxyguanosin-8-yl)-1-amino-x-nitropyrene (x = 3, 6 or 8; dG-C8-ANP) were detected in injection site DNA from Sprague-Dawley rats treated with 1-nitropyrene. In mammary gland DNA, dG-C8-AP and an unidentified adduct were found. dG-C8-ANP was the only DNA adduct detected in the livers of newborn CD-1 mice and the lungs of A/J mice dosed with 1-nitropyrene.

Genotoxicity of pyrene oxide and 1-nitropyrene oxides in hepatocyte primary culture/DNA repair test

The genotoxicity of a pyrene oxide, 1-nitropyrene (NP) oxides and other related compounds was examined in the hepatocyte primary culture (HPC)/DNA repair test. Pyrene 4,5-oxide and both 1-NP-4,5-oxide and 1-NP-9,10-oxide elicited clearly positive responses of DNA repair. In this assay, 1-NP itself was weakly positive. However, other related chemicals such as pyrene, 1-nitro-3-hydroxypyrene, 1-nitro-6-hydroxypyrene, and 1-nitro-8-hydroxypyrene did not generate positive responses.