Formation of DNA adducts in mouse tissues after intratracheal instillation of 1-nitropyrene

Mechanistic Basis for the Bypass of a Bulky DNA Adduct Catalyzed by a Y-Family DNA Polymerase

1-Nitropyrene (1-NP), an environmental pollutant, induces DNA damage in vivo and is considered to be carcinogenic. The DNA adducts formed by the 1-NP metabolites stall replicative DNA polymerases but are presumably bypassed by error-prone Y-family DNA polymerases at the expense of replication fidelity and efficiency in vivo. Our running start assays confirmed that a site-specifically placed 8-(deoxyguanosin-N(2)-yl)-1-aminopyrene (dG(1,8)), one of the DNA adducts derived from 1-NP, can be bypassed by Sulfolobus solfataricus DNA polymerase IV (Dpo4), although this representative Y-family enzyme was paused strongly by the lesion. Pre-steady-state kinetic assays were employed to determine the low nucleotide incorporation fidelity and establish a minimal kinetic mechanism for the dG(1,8) bypass by Dpo4. To reveal a structural basis for dCTP incorporation opposite dG(1,8), we solved the crystal structures of the complexes of Dpo4 and DNA containing a templating dG(1,8) lesion in the absence or presence of dCTP. The Dpo4·DNA-dG(1,8) binary structure shows that the aminopyrene moiety of the lesion stacks against the primer/template junction pair, while its dG moiety projected into the cleft between the Finger and Little Finger domains of Dpo4. In the Dpo4·DNA-dG(1,8)·dCTP ternary structure, the aminopyrene moiety of the dG(1,8) lesion, is sandwiched between the nascent and junction base pairs, while its base is present in the major groove. Moreover, dCTP forms a Watson-Crick base pair with dG, two nucleotides upstream from the dG(1,8) site, creating a complex for "-2" frameshift mutation. Mechanistically, these crystal structures provide additional insight into the aforementioned minimal kinetic mechanism.

DNA adduct formation by the environmental contaminant 3-nitrobenzanthrone after intratracheal instillation in rats

3-Nitrobenzanthrone (3-NBA) is an environmental pollutant and suspected human carcinogen found in emissions from diesel and gasoline engines and on the surface of ambient air particulate matter; human exposure to 3-NBA is likely to occur primarily via the respiratory tract. In our study female Sprague Dawley rats were treated by intratracheal instillation with a single dose of 0.2 or 2 mg/kg body weight of 3-NBA. Using the butanol enrichment version of the (32)P-postlabeling method, DNA adduct formation by 3-NBA 48 hr after intratracheal administration in different organs (lung, pancreas, kidney, urinary bladder, heart, small intestine and liver) and in blood was investigated. The same adduct pattern consisting of up to 5 DNA adduct spots was detected by thin layer chromatography in all tissues and blood and at both doses. Highest total adduct levels were found in lung and pancreas (350 +/- 139 and 620 +/- 370 adducts per 10(8) nucleotides for the high dose and 39 +/- 18 and 55 +/- 34 adducts per 10(8) nucleotides for the low dose, respectively) followed by kidney, urinary bladder, heart, small intestine and liver. Adduct levels were dose-dependent in all organs (approximately 10-fold difference between doses). It was demonstrated by high performance liquid chromatography (HPLC) that all 5 3-NBA-derived DNA adducts formed in rats after intratracheal instillation are identical to those formed by other routes of application and are, as previously shown, formed from reductive metabolites bound to purine bases. Although total adduct levels in the blood were much lower (41 +/- 27 and 9.5 +/- 1.9 adducts per 10(8) nucleotides for the high and low dose, respectively) than those found in the lung, they were related to dose and to the levels found in lung. These results show that uptake of 3-NBA by the lung induces high levels of specific DNA adducts in several organs of the rat and an identical adduct pattern in DNA from blood. Therefore, 3-NBA-DNA adducts present in the blood are useful biomarkers for exposure to 3-NBA and may help to assess the effective biological dose in humans exposed to it.

Neoplastic potential of rat tracheal epithelial cell lines induced by 1-nitropyrene and dibenzo(a,i)pyrene

Our previous study showed that both 1-nitropyrene (1-NP) and dibenzo(a,i)pyrene (DBP) induced enhanced growth variants (EGVs) in primary cultures of rat tracheal epithelial (RTE) cells exposed in vivo. Cell lines were established from some of the EGVs. Further studies, using anchorage-independent growth in soft agar and tumorigenicity in athymic nude mice, were performed to determine the neoplastic potential of EGVs induced by 1-NP and DBP. Results show that three of five from DBP- and five of five from 1-NP-induced cell lines displayed anchorage-independent growth. The colony forming efficiency (CFE) from DBP-induced cell lines was 0.067 per thousand and CFE from 1-NP-induced cell lines was 0.151 per thousand. There is a significant difference between the two CFEs (mu = 12.08, P<0. 01). Two of five DBP- and five of five 1-NP-induced cell lines produced squamous cell carcinomas (SCC) in nude mice. The rate of tumorigenicity counted by injected sites was 20% (6/30) for DBP-induced cell lines and 57% (17/30) for 1-NP-induced cell lines. There is a significant difference between the results of tumorigenicity from the cell lines induced by the two different compounds (chi(2)=8.53, P<0.01). Neither of the two cell lines from spontaneously developed foci grew in soft agar or produced SCC in nude mice. It seems that the neoplastic potential of transformed RTE cells induced by 1-NP was higher than that of DBP.

In vitro and in vivo transformation in rat tracheal epithelial cells exposed to diesel emission particles and related compounds

The rat tracheal epithelial (RTE) cell transformation assay was performed to determine the transforming activity of diesel emission particles (DEPs) and two related compounds, 1-nitropyrene (1-NP) and dibenzo(a,i)pyrene (DBP). RTE cells were treated with these agents in vitro and in vivo. Transformed cells from foci induced by these agents were passaged over 20 times to establish immortal cell lines. Results show that (1) DEPs- and 1-NP-induced cell transformation only with the in vivo exposure (30-75 mg/kg bw DEPs and 15-60 mg/kg bw 1-NP); (2) positive dose-related responses to DBP were found with both in vitro (0.05-0.50 microg/ml) and in vivo (7.5-30 mg/kg bw) exposures; (3) the fraction of transformed foci becoming cell lines was in the order of 1-NP(25/48) > DBP(8/28) > DEPs(0/30). These results indicate that (1) DEPs, 1-NP and DBP are capable of transforming rat tracheal epithelial cells, however, the transforming activity of DEPs and -NP may be dependent on metabolic activation, and (2) transformed cells induced by DEPs have a very low probability, if any, of becoming cell lines.

Role of O-acetyltransferase in activation of oxidised metabolites of the genotoxic environmental pollutant 1-nitropyrene

The genotoxic environmental contaminant 1-nitropyrene is metabolised in mammalian systems by pathways more complex than the straightforward nitroreduction which accounts for most of its biological activity in bacteria. In order to evaluate the role of O-acetyltransferase (OAT) activity in generation of genotoxic intermediates from 1-nitropyrene, the mutagenicity of the major primary oxidised metabolites of 1-nitropyrene was characterised in the Ames Salmonella typhimurium plate incorporation assay with strain TA98, and with variants of TA98 deficient (TA98/1,8-DNP6) or enhanced (YG1024) in O-acetyltransferase. 1-Nitropyren-3-ol was more mutagenic in the absence than in the presence of S9, while 1-nitropyren-4-ol, 1-nitropyren-6-ol and 1-nitropyren-8-ol required S9 for maximum expression of mutagenicity. 1-Nitropyren-4-ol (176 rev/nmol without S9, 467 rev/nmol with S9 in TA98) and 1-nitropyren-6-ol (13 rev/nmol without S9, 266 rev/nmol with S9 in TA98) were overall the most potent nitropyrenol isomers assayed. 1-Acetamidopyren-8-ol and 1-acetamidopyrene 4,5-quinone were only minimally active. 1-Acetamidopyren-3-ol exhibited direct-acting mutagenicity. 1-Acetamidopyren-6-ol, previously shown to be a major contributor to mutagenicity in the urines of rats dosed with 1-nitropyrene (Ball et al., 1984b), was confirmed as a potent (359 rev/nmol) S9-dependent mutagen. Both the direct-acting and the S9-dependent mutagenicity of all the compounds studied was enhanced in the OAT-overproducing strain and much diminished (though not always entirely lost) in the OAT-deficient strain, showing that OAT amplifies expression of the genotoxicity of these compounds. 1-Acetamidopyren-6-ol required both S9 and OAT activity in order to exhibit any mutagenicity; this finding strongly implicates N-hydroxylation followed by O-esterification, as opposed to further S9-catalyzed ring oxidation, as a major route of activation for urinary metabolites of 1-nitropyrene.

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.