Formation of reactive 1-nitropyrene metabolites by lung microsomes and isolated lung cells

Tissue-specific metabolic activation and mutagenicity of 3-nitrobenzanthrone in MutaMouse

3-Nitrobenzanthrone (3-NBA) is a mutagen and suspected human carcinogen detected in diesel exhaust, airborne particulate matter, and urban soil. We investigated the tissue specific mutagenicity of 3-NBA at the lacZ locus of transgenic MutaMouse following acute single dose or 28-day repeated-dose oral administration. In the acute high dose (50 mg/kg) exposure, increased lacZ mutant frequency was observed in bone marrow and colonic epithelium, but not in liver and bladder. In the repeated-dose study, a dose-dependent increase in lacZ mutant frequency was observed in bone marrow and liver (2- and 4-fold increase above control), but not in lung or intestinal epithelium. In addition, a concentration-dependent increase in mutant frequency (8.5-fold above control) was observed for MutaMouse FE1 lung epithelial cells exposed in vitro. 1-Nitropyrene reductase, 3-NBA reductase, and acetyltransferase activities were measured in a variety of MutaMouse specimens in an effort to link metabolic activation and mutagenicity. High 3-NBA nitroreductase activities were observed in lung, liver, colon and bladder, and detectable N-acetyltransferase activities were found in all tissues except bone marrow. The relatively high 3-NBA nitroreductase activity in MutaMouse tissues, as compared with those in Salmonella TA98 and TA100, suggests that 3-NBA is readily reduced and activated in vivo. High 3-NBA nitroreductase levels in liver and colon are consistent with the elevated lacZ mutant frequency values, and previously noted inductions of hepatic DNA adducts. Despite an absence of induced lacZ mutations, the highest 3-NBA reductase activity was detected in lung. Further studies are warranted, especially following inhalation or intratracheal exposures.

In vivo metabolism and genotoxic effects of nitrated polycyclic aromatic hydrocarbons

During incomplete combustion of organic matter, nitro-polycyclic aromatic hydrocarbons (nitro-PAHs), are formed in a reaction that is catalyzed by a low pH. 2-Nitrofluorene (NF), a marker for nitro-PAHs, is metabolized in vivo by two different routes. After inhalation, potent mutagenic metabolites, hydroxylated nitrofluorenes (OH-NFs), are formed. The metabolites are distributed by systemic circulation. After oral administration, NF is reduced to the corresponding amine, a reaction mediated by the intestinal microflora. This metabolite is acetylated to 2-acetylaminofluorene (AAF), a potent carcinogen. Further ring-hydroxylation of AAF leads to detoxification and excretion. Induction of cytochrome P450s affects the metabolism, and more OH-NFs are formed. As a consequence, more mutagenic metabolites are found in the circulation. OH-NFs are excreted in the bile as, in terms of mutagenicity, totally harmless glucuronide conjugates. When these conjugates are excreted via the bile, intestinal beta-glucuronidase can liberate direct-acting mutagens in the intestine. Thus, inhalation of NF can lead to formation of potent mutagens in the intestine. NF is a direct-acting mutagen in bacterial assays and an initiator and promoter of the carcinogenic process, and gives rise to DNA adduct formation in laboratory animals.

Chemically induced DNA damage in isolated rabbit lung cells

By use of an isolation procedure including centrifugal elutriation and density gradient centrifugation, relatively pure fractions of Clara cells and type II cells were obtained from rabbit lungs. These cells and alveolar macrophages isolated by lavage were exposed to methyl methanesulfonate (MMS), 1,2-dibromo-3-chloropropane (DBCP), 1-nitropyrene (1-NP), 2-nitrofluorene (2-NF), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), N'-nitrosonornicotine (NNN), N-nitrosoheptamethyleneimine (NHMI) or phorbol 12-myristate 13-acetate (TPA). DNA damage measured as alkali-labile sites and/or single-strand breaks was then determined in the different lung cells by an automated alkaline elution system. The direct-acting compound MMS showed similar DNA-damaging effect in Clara cells, type II cells and alveolar macrophages. The nematocide DBCP, activated by both P450- and glutathione S-transferase(s)-dependent pathways, caused considerably less DNA damage in macrophages than in Clara or type II cells. Similar differences between the lung cells in induction of DNA damage as observed with DBCP were demonstrated after exposure to the activation-dependent nitrosamines NNK and NHMI and the tumor promoter TPA. The other test substances (1-NP, 2-NF, NNN) did not cause any marked DNA damage measured by the alkaline elution technique. These findings are in agreement with the known metabolic capacity of these cell types, indicating that Clara and type II cells are possible primary targets for lung toxic/carcinogenic compounds.

Formation of genotoxic products from N-nitrosoheptamethyleneimine (NHMI), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) by isolated rabbit lung cells

The genotoxic potentials of N-nitrosoheptamethyleneimine (NHMI), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) were studied in fresh preparations of Clara cells and type II cells isolated by centrifugal elutriation and density gradient centrifugation, and macrophages from rabbit lung. The activation of the compounds to bacterial mutagens was assayed in the Salmonella mutagenicity test using strains of TA 100 and TA 1530 preincubated with test chemicals and cells placed in chambers with nucleopore membranes to separate cells and bacteria. Unscheduled DNA synthesis was measured by incorporation of [3H]-thymidine in the cells after exposure to the compounds. NHMI, NNK and NNN were not activated to bacterial mutagens by Clara cells, type II cells or macrophages, presumably because the reactive metabolites generated were not released into the incubation medium. However, NHMI and NNK increased unscheduled DNA synthesis in Clara cells, and the highest repair activity was found after incubation with NNK. The effect of NNN was only marginal. This indicates that NHMI and NNK are genotoxic in the rabbit lung and that the Clara cells are involved in the metabolic activation of these compounds.

1,6-Dinitropyrene causes spindle disturbances and chromosomal damage in V79 Chinese hamster cells

We have investigated the cytogenetic effect of 1,6-dinitropyrene (1,6-DNP) in Chinese hamster V79 cells. The chemical caused a dose-dependent increase in the incidence of initial and full C-mitoses, polyploid mitoses, ana-telophases with lagging chromosomes, non-disjunction and multipolar configurations, in a range of 0.05-5 microM. These findings indicate that 1,6-DNP interferes with the functioning of the spindle apparatus in V79 cells. Early signs of spindle disturbances were seen at 1,6-DNP concentrations which only moderately reduced cell growth and division. Analysis of structural chromosomal aberrations revealed the appearance of chromatid-type aberrations with open breaks and exchanges accompanied by gaps. The results indicate that 1,6-DNP is both a spindle-disturbing and a clastogenic agent in V79 cells.