Part 3. Assessment of genotoxicity and oxidative stress after exposure to diesel exhaust from U.S. 2007-compliant diesel engines: report on 1- and 3-month exposures in the ACES bioassay

Human health hazards due to diesel exhaust (DE*) exposure have been associated with both solvent and combustion components. In the past, diesel engine exhaust components have been linked to increased mutagenicity in cultures of Salmonella typhimurium and mammalian cells (Tokiwa and Ohnishi 1986). In addition, DE has been shown to increase both the incidence of tumors and the induction of 8-hydroxy-deoxyguanosine adducts (8-OHdG) in ICR mice (Ichinose et al. 1997). Furthermore, DE is composed of a complex mixture of polycyclic aromatic hydrocarbons (PAHs) and particulates. One such PAH, 3-nitrobenzanthrone (3-NBA), has been identified in DE and found in urban air. 3-NBA has been observed to induce micronucleus formation in DNA of human hepatoma cells (Lamy et al. 2004). The purpose of the current research, which is part of the Advanced Collaborative Emissions Study (ACES), a multidisciplinary program being carried out by the Health Effects Institute and the Coordinating Research Council, is to determine whether improvements in the engineering of heavy-duty diesel engines reduce the oxidative stress and genotoxic risk associated with exposure to DE components. To this end, the genotoxicity and oxidative stress of DE from an improved diesel engine was evaluated in bioassays of tissues from Wistar Han rats and C57BL/6 mice exposed to DE. Genotoxicity was measured as strand breaks using an alkaline-modified comet assay. To correlate possible DNA damage found by the comet assay, measurement of DNA-adduct formation was evaluated by a competitive enzyme-linked immunosorbent assay (ELISA) to determine the levels of free 8-OHdG found in the serum of the animals exposed to DE. 8-OHdG is a specific modified base indicating an oxidative type of DNA damage to DNA nucleotides. In addition, a thiobarbituric acid reactive substances (TBARS) assay was used to assess oxidative stress and damage in the form of lipid peroxidation in the hippocampus region of the brains of DE-exposed animals. Results from the comet assay showed no significant differences in rats between the control and exposed groups (P = 0.53, low exposure; P = 0.92, medium exposure; P = 0.77, high exposure) after 1 month of DE exposure. There were no differences between sexes in the responses of rats to these exposures. Likewise, there were no significant differences found after 3 months of exposure. Similarly, no significant differences were found between the mice exposed for 1 and 3 months to DE, nor were any differences found between sexes. Measurements of 8-OHdG in both mice and rats showed no significant difference among DE exposure groups (P = 0.46, mice; P = 0.86, rats). In mice, measured 8-OHdG was lower in the 3-month group than the 1-month group. In rats, the inverse was true. In mice, no significant differences in the levels of lipid peroxidation, as measured by TBARS, were found between the controls and DE exposure groups (P = 0.92), nor were there any differences between sexes. In rats, comparisons between the control and low-exposure groups approached significance, but no significant differences were found between the other DE exposure groups. Additionally, in rats, there were no significant differences between the 1- and 3-month DE exposure groups.

3,4-Epoxy-1-butene, a reactive metabolite of 1,3-butadiene, induces somatic mutations in Xpc-null mice

Xpc-null (Xpc-/-) mice, deficient in the global genome repair subpathway of nucleotide excision repair (NER-GGR), were exposed by intraperitoneal (i.p.) injection to a 300 mg/kg mutagenic dose of 3,4-epoxy-1-butene (EB), to investigate NER's potential role in repairing butadiene (BD) epoxide DNA lesions. Mutagenic sensitivity was assessed using the Hprt assay. Xpc-/- mice were significantly more sensitive to EB exposure, exhibiting an average 2.8-fold increase in Hprt mutant frequency (MF) relative to those of exposed Xpc+/+ (wild-type) mice. As a positive control for NER-GGR, additional mice were exposed by i.p. injection to a 150 mg/kg mutagenic dose of benzo[a]pyrene (B[a]P). The Xpc-/- mice had MFs 2.9-fold higher than those of exposed Xpc+/+ mice. These results suggest that NER-GGR plays a role in recognizing and repairing some of the DNA adducts formed following in vivo exposure to EB. Additional research is needed to examine the response of Xpc-/- mice, as well as other NER-deficient strains, to inhaled BD. Furthermore, it is likely that alternative DNA repair pathways also are involved in restoring genomic integrity compromised by BD-epoxide DNA damage. Collaborative studies are currently underway to address these critical issues.

Gene expression, cell localization, and evolution of rodent submandibular gland androgen-binding protein

A small dimeric androgen-binding protein (Abp) secreted by mouse (Mus) submandibular salivary glands has been hypothesized to function in mate-selection. The alpha-subunit (Abpa) evolves rapidly under natural selection. However, cellular site(s) of synthesis, mode of function, and patterns of evolution of this biologically important protein are otherwise unknown. We used a radiolabeled riboprobe and in situ hybridization to mouse (Mus) Abpa mRNA to localize Abpa synthesis to submandibular gland acinar cells. We next used a quantitative fluorescent thermal cycler (real-time PCR) to determine relative expression levels (normalized with a constitutively expressed ribosomal gene [S15/rig; rat insulinoma gene]) in male, female, and sexually immature European striped field mice (Apodemus agrarius). We hypothesized that gender or age-related differences might occur in production of a salivary protein related to sexual selection, but found no significant differences within our sample. Finally, we also used reverse transcription-PCR of mRNAs isolated from submandibular salivary glands to determine the Abpa allele in the striped wood mouse and compare it to published information on the homologue in Mus musculus domesticus.

Subchronic exposure of BALB/c and C57BL/6 strains of Mus musculus to the radioactive environment of the Chornobyl, Ukraine exclusion zone

Environmental contamination resulting from the Chornobyl, Ukraine, disaster offers a unique opportunity to examine the in vivo biological effects of chronic, low-dose exposure to radiation. Laboratory studies of acute exposure to ionizing radiation have been used to estimate risk and potential human health effects by the extrapolation of laboratory data to situations of low-dose environmental radiation exposure. Few studies, however, have explored the biological consequences of low-dose exposure via in situ environmental radiation in a sentinel species. In the present study, laboratory strains of Mus musculus (BALB/c and 57BL/6) were placed in environmental enclosures in the Red Forest region of the Chornobyl exclusion zone. Blood samples were obtained every 10 d, and the micronucleus (MN) test was employed to assess the potential for cytogenetic damage from exposure to Chornobyl radiation. Radionuclide uptake was monitored throughout the study, and dose was estimated for each individual as well as for their offspring. Total dose for the mice experimentally exposed to this environment averaged 1,162 mGy for BALB/c (30 d) and 1,629 mGy for C57BL/6 (40 d). A higher MN frequency for both strains was observed at day 10, although this change was only statistically significant in the C57BL/6 mice (chi2/3 = 13.41, p = 0.003). Subsequent samples from C57BL/6 resulted in values at or less than the initial frequencies. In BALB/c mice, an increase in MN was also evident at day 30 (chi2/3 = 10.38, p = 0.006). The experimental design employed here allows for the incorporation of traditional laboratory strains, as well as transgenic strains of Mus, as sentinels of environmental radiation contamination.

Cytochrome b sequences in black-crowned night-herons (Nycticorax nycticorax) from heronries exposed to genotoxic contaminants

DNA sequence analysis of a 215 base-pair region of the mitochondrial cytochrome b gene was used to examine genetic variation and search for evidence of an increased mutation rate in black-crowned night-herons. We examined five populations exposed to environmental contamination (primarily PAHs and PCBs) and one reference population from the eastern U.S. There was no evidence of a high mutation rate even within populations previously shown to exhibit increased variation in DNA content among somatic cells as a result of petroleum exposure. Three haplotypes were observed among 99 individuals. The low level of variability could be evidence for a genetic bottleneck, or that cytochrome b is too conservative for use in population genetic studies of this species. With the exception of one population from Louisiana, pair-wise Phist estimates were very low, indicative of little population structure and potentially high rates of effective migration among populations.

Experimental exposure of naive bank voles (Clethrionomys glareolus) to the Chornobyl, Ukraine, environment: a test of radioresistance

Previous studies have demonstrated no difference in micronucleus (MN) frequencies between wild rodents chronically exposed to the environmental radiation contamination of the Chornobyl (Ukraine) exclusion zone and those inhabiting reference populations. The aim of the present study was to test the hypothesis that a population of bank voles (Clethrionomys glareolus) has developed radioresistance as a result of 14 years of chronic, low-dose radiation exposure. Naive voles were placed in environmental enclosures in the Red Forest region of the exclusion zone for 30 d. Blood samples were obtained at regular intervals, and the MN assay was used to assess chromosomal damage. Additionally, radionuclide uptake was monitored throughout the study, and dose was documented for each individual as well as for their offspring. Total dose for the voles experimentally exposed in this environment averaged 1.09 Gy (36.20 mGy d(-1)) for the 30-d study period. Our results indicate that exposure to radiation levels well above regulatory statutes did not result in an increased MN frequency. Furthermore, our results do not support the hypothesis that voles chronically exposed to these radiation levels have developed a genetic basis for radioresistance that is unique from that present in naive populations. The use of C. glareolus as a sentinel species for environmental studies of radiation contamination and the question of whether the MN assay is an appropriate endpoint for studies of low-dose, chronic radiation exposure are also discussed.

Accumulation of 137Cesium and 90Strontium from abiotic and biotic sources in rodents at Chornobyl, Ukraine

Bank voles (Clethrionomys glareolus) and laboratory strains of house mice (Mus musculus BALB and C57BL) were relocated into enclosures in a highly contaminated area of the Red Forest near the Chornobyl (Ukraine) Reactor 4 to evaluate the uptake rates of 137Cs and 90Sr from abiotic sources. Mice were provided with uncontaminated food supplies, ensuring that uptake of radionuclides was through soil ingestion, inhalation, or water. Mice were sampled before introduction and were reanalyzed every 10 d for 137Cs uptake. Levels of 90Sr were assessed in subsamples from the native populations and in experimental animals at the termination of the study. Uptake rates in house mice were greater than those in voles for both 137Cs and 90Sr. Daily uptake rates in house mice were estimated at 2.72 x 10(12) unstable atoms per gram (whole body) for 137Cs and 4.04 x 10(10) unstable atoms per gram for 90Sr. Comparable rates in voles were 2.26 x 10(11) unstable atoms per gram for 137Cs and 1.94 x 10(10) unstable atoms per gram for 90Sr. By comparing values from voles in the enclosures to those from wild voles caught within 50 m of the enclosures, it was estimated that only 8.5% of 137Cs was incorporated from abiotic sources, leaving 91.5% being incorporated by uptake from biotic materials. The fraction of 90Sr uptake from abiotic sources was at least 66.7% (and was probably much higher). Accumulated whole-body doses during the enclosure periods were estimated as 174 mGy from intramuscular 137Cs and 68 mGy by skeletal 90Sr in house mice over 40 d and 98 mGy from 137Cs and 19 mGy from 90Sr in voles over 30 d. Thus, uptake of radionuclides from abiotic materials in the Red Forest at Chornobyl is an important source of internal contamination.

Consequences of polluted environments on population structure: the bank vole (Clethrionomys glareolus) at Chornobyl

The accident at the Chornobyl Nuclear Power Plant in April 1986, released 100-200 million Curies of radioactive material into the surrounding environment. To investigate the possible genetic and population effects resulting from chronic exposure to this environmental radiation, we have examined mitochondrial DNA (control region) sequences from bank voles, Clethrionomys glareolus, inhabiting contaminated sites. Our analysis indicates genetic diversity is elevated in the contaminated sites when compared to relatively uncontaminated reference sites. This may be attributed to either an increased mutation rate in the mtDNA control region or immigration of individuals from surrounding areas into the contaminated environment. Although our observations do indicate that the contaminated areas represent sink populations, we cannot statistically discriminate between these two alternatives at this time. In addition, we have been unable to attribute any significant detrimental effects to bank vole populations inhabiting the contaminated Chornobyl environment based on these data. This is particularly paradoxical considering bank voles in the contaminated areas harbor the highest radiocesium (137Cs) body burdens and external dose rates of any mammal ever measured. Our long-term research on the bank vole indicates that several factors, including contaminants, may affect haplotype dynamics both spatially and temporally. These multifarious influences subsequently affect population genetic estimates typically used to address the effects of environmental pollution on animal populations. Finally, we provide a general framework for designing experiments investigating the role contaminants play in altering the genetic characteristics of exposed populations.

Mixed-function oxygenases, oxidative stress, and chromosomal damage measured in lesser scaup wintering on the Indiana Harbor Canal

During the winter of 1993-1994, male lesser scaup (Aythya affinis) were collected on the heavily polluted Indiana Harbor Canal, East Chicago, Indiana, and examined for several bioindicators of chemical exposure. Livers were analyzed for activities of three cytochrome P450-associated monooxygenases and four measures of oxidative stress. Blood and spleen were analyzed by flow cytometry for chromosomal damage. In a concurrent study, scaup tissues were analyzed for organic and inorganic contaminants. Ethoxyresorufin-O-dealkylase (EROD) activity in livers of scaup collected in January 1994 was significantly higher than in livers of scaup collected in March 1994 or in livers of reference birds. Three hepatic monooxygenase activities were each significantly correlated with polycyclic aromatic hydrocarbon (PAH) concentrations in scaup carcasses. Thiobarbituric acid (TBA) activity in scaup livers was positively correlated with iron, boron, and lead concentrations in livers and polychlorinated biphenyl concentrations in carcasses. TBA activity was negatively correlated with protein-bound thiol activity and mercury concentrations in livers. The coefficient of variation of DNA content in scaup blood cells was correlated with PAH concentrations in scaup carcasses. This is the first field study with birds to demonstrate a correlation between liver monooxygenase activity and carcass PAH concentrations and to show a direct correlation between PAH concentrations in tissues and somatic chromosomal damage in blood.

Multiparametric assessment of bursal lymphocyte apoptosis

When bursal lymphocytes are placed in cell culture, they undergo an apoptotic form of cell death that can be inhibited by phorbol esters and protein synthesis inhibitors. The goal of the current study was to evaluate the time course of this process and the inhibition of this process using several different assays to detect apoptosis: (1) terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) of lymphocyte DNA strand breaks with dUTP-FITC; (2) propidium iodide (PI) staining of lymphocyte chromatin; (3) chloromethyl-x-rosamine (CMX-Ros) binding to lymphocyte mitochondria; (4) merocyanine-540 (MC-540) binding to the lymphocyte plasma membrane; (5) flow cytometric analysis of light scatter from lymphocytes; (6) analysis of genomic DNA from lymphocytes by agarose gel electrophoresis; and (7) cellular caspase-3 activity of lymphocytes. When bursal lymphocyte apoptosis was analyzed as a function of time, or inhibited by phorbol esters or cycloheximide, all of these assays corroborated the apoptotic process. However, treatment of lymphocytes with a cytotoxic level of the proteinase inhibitor, n-ethylmaleimide (NEM) resulted in a putative, necrotic form of cell death that revealed discrepancies among the various assays in the detection of apoptotic cells. Specifically, the CMX-Ros and MC-540 assays erroneously detected the necrotic cells as being apoptotic cells following NEM treatment. These findings indicate the need for additional assays and appropriate treatment controls to verify the apoptotic process when using the CMX-Ros and MC-540 assays.