DNA shuffling of a family of genes from diverse species accelerates directed evolution

DNA shuffling is a powerful process for directed evolution, which generates diversity by recombination, combining useful mutations from individual genes. Libraries of chimaeric genes can be generated by random fragmentation of a pool of related genes, followed by reassembly of the fragments in a self-priming polymerase reaction. Template switching causes crossovers in areas of sequence homology. Our previous studies used single genes and random point mutations as the source of diversity. An alternative source of diversity is naturally occurring homologous genes, which provide 'functional diversity'. To evaluate whether natural diversity could accelerate the evolution process, we compared the efficiency of obtaining moxalactamase activity from four cephalosporinase genes evolved separately with that from a mixed pool of the four genes. A single cycle of shuffling yielded eightfold improvements from the four separately evolved genes, versus a 270- to 540-fold improvement from the four genes shuffled together, a 50-fold increase per cycle of shuffling. The best clone contained eight segments from three of the four genes as well as 33 amino-acid point mutations. Molecular breeding by shuffling can efficiently mix sequences from different species, unlike traditional breeding techniques. The power of family shuffling may arise from sparse sampling of a larger portion of sequence space.

Comparison of pleural responses of rats and hamsters to subchronic inhalation of refractory ceramic fibers

In the present subchronic study, we compared pleural inflammation, visceral pleural collagen deposition, and visceral and parietal pleural mesothelial cell proliferation in rats and hamsters identically exposed to a kaolin-based refractory ceramic fiber, (RCF)-1 by nose-only inhalation exposure, and correlated the results to translocation of fibers to the pleural cavity. Fischer 344 rats and Syrian golden hamsters were exposed to 650 fibers/cc of RCF-1, for 4 hr/day, 5 days/week for 12 weeks. Following 4 and 12 weeks of exposure, and after a 12-week recovery period, pleural lavage fluid was analyzed for cytologic and biochemical evidence of inflammation. Visceral and parietal pleural mesothelial cell proliferation was assessed by immunocytochemical detection of bromodeoxyuridine incorporation. Pleural collagen was quantitated using morphometric analysis of lung sections stained with Sirius Red. Fiber-exposed rats and hamsters had qualitatively similar pleural inflammation at each time point. Mesothelial cell proliferation was more pronounced in hamsters than in rats at each time point and at each site. In both species, the mesothelial cell labeling index was highest in the parietal pleural mesothelial cells lining the surface of the diaphragm at each time point. Hamsters but not rats had significantly elevated collagen in the visceral pleura at the 12-week postexposure time point. Fibers were found in the pleural cavities of both species at each time point. These fibers were generally short and thin. These results suggest that mesothelial cell proliferation and fibroproliferative changes in the pleura of rodents following short-term inhalation exposure are associated with fiber translocation to the pleura and may be predictive of chronic pleural disease outcomes following long-term exposure.

Pulmonary and pleural responses in Fischer 344 rats following short-term inhalation of a synthetic vitreous fiber. I. Quantitation of lung and pleural fiber burdens

The pleura is an important target tissue of fiber-induced disease, although it is not known whether fibers must be in direct contact with pleural cells to exert pathologic effects. In the present study, we determined the kinetics of fiber movement into pleural tissues of rats following inhalation of RCF-1, a ceramic fiber previously shown to induce neoplasms in the lung and pleura of rats. Male Fischer 344 rats were exposed by nose-only inhalation to RCF-1 at 89 mg/m3 (2645 WHO fibers/cc), 6 hr/day for 5 consecutive days. On Days 5 and 32, thoracic tissues were analyzed to determine pulmonary and pleural fiber burdens. Mean fiber counts were 22 x 10(6)/lung (25 x 10(3)/pleura) at Day 5 and 18 x 10(6)/lung (16 x 10(3)/pleura) at Day 32. Similar geometric mean lengths (GML) and diameters (GMD) of pulmonary fiber burdens were observed at both time points. Values were 5 microns for GML (geometric standard deviation GSD approximately 2.3) and 0.3 micron for GMD (GSD approximately 1.9), with correlations between length and diameter (tau) of 0.2-0.3. Size distributions of pleural fiber burdens at both time points were approximately 1.5 microns GML (GSD approximately 2.0) and 0.09 micron GMD (GSD approximately 1.5; tau approximately 0.2-0.5). Few fibers longer than 5 microns were observed at either time point. These findings demonstrate that fibers can rapidly translocate to pleural tissues. However, only short, thin (< 5 microns in length) fibers could be detected over the 32-day time course of the experiment.

Pulmonary and pleural responses in Fischer 344 rats following short-term inhalation of a synthetic vitreous fiber. II. Pathobiologic responses

The pleura is a target site for toxic effects induced by a variety of fibrous particulates, including both natural mineral and man-made vitreous fibers. We examined selected cytological and biochemical indicators of inflammation in both the pleural compartment and pulmonary parenchyma in F344 rats following inhalation of RCF-1, a kaolin-based ceramic fiber. Male F344 rats were exposed by inhalation to 89 mg/m3 (2645 WHO fibers/cc) RCF-1 6 hr/day for 5 consecutive days. In lung parenchyma, cytological and biochemical inflammatory responses occurred rapidly following exposure. In contrast, pleural responses were delayed in onset and of a much smaller magnitude than those observed in lung. At both Day 1 and Day 28 postexposure, increased quantities of lactate dehydrogenase, N-acetyl glucosaminidase, alkaline phosphatase, albumin, and neutrophils were present in bronchoalveolar lavage fluid. These responses were attenuated at the latter time point. No significant responses were detected in pleural lavage fluid until 28 days following exposure, at which time elevated numbers of macrophages and eosinophils, but not neutrophils, were observed. Increased albumin and fibronectin were also observed in PLF at this latter time point. These findings demonstrate that the onset of pleural and pulmonary responses following inhalation of RCF-1 are temporally separated and that pleural injury may increase in severity with time following exposure. The increase in severity of pleural inflammation found in the postexposure period cannot be readily explained by fiber translocation.

Expression of insulin-like growth factor II in spontaneously immortalized rat mesothelial and spontaneous mesothelioma cells: a potential autocrine role of insulin-like growth factor II

Insulin-like growth factors (IGFs) are polypeptides that play an important role in cellular proliferation and differentiation. The present study examines the role of IGFs in the growth of mesothelial cells. Cell lines derived from normal rat mesothelium as well as lines derived from spontaneous rat mesotheliomas were found to express RNA transcripts for IGF-II. In contrast, cell lines derived from asbestos-induced rat mesotheliomas did not express this growth factor. All cell lines expressed receptors for IGF-I and IGF-II, as well as insulin receptors. Coexpression of IGF-II and its cognate receptor suggested that IGF-II was acting as an autocrine growth factor in the spontaneously immortalized cells and the cells derived from the spontaneous tumors. The biological activity of IGF-II secreted by the cell lines into conditioned medium could be neutralized using an IGF-II-specific antibody. Growth was inhibited in a dose-dependent manner; at the highest antibody concentration used (100 micrograms anti-IGF-II/ml), cell growth was decreased to 47% of control values. This inhibition was partially reversible by treatment of the cultures with IGF-II (91% of the control). These data suggest that IGF-II expression may be involved in the spontaneous alteration of rat mesothelial cells and may function as an autocrine or paracrine growth factor to modulate the growth of these cells in vitro and in vivo. Ubiquitous expression of IGF-II by cells that have not been exposed to asbestos and the lack of IGF-II expression by asbestos-transformed cells suggest that the mechanisms of changes in growth factor expression differ in mesothelial cells transformed by different pathways.

Immunohistochemical localization of p53, PCNA, and TGF-alpha proteins in formaldehyde-induced rat nasal squamous cell carcinomas

Mutation of the p53 tumor suppressor gene is a common event in many human cancers and has been specifically associated with invasive squamous cell carcinoma of the human skin and respiratory tract. Alterations in the p53 gene have also been identified in certain rodent tumors, including formaldehyde-induced nasal squamous cell carcinomas. Overexpression of transforming growth factor-alpha (TGF-alpha) is associated with carcinomas of the head and neck and respiratory tract in human patients and formaldehyde-induced rat nasal squamous cell carcinomas. Sections of rat noses containing tumors and other formaldehyde-induced lesions from rats exposed to 15 ppm formaldehyde vapor were examined using immunohistochemical techniques to detect and identify potential relationships between the presence and distribution of p53, proliferating cell nuclear antigen (PCNA), and TGF-alpha proteins. The five tumors that had p53 mutations were for mutant p53 protein by immunohistochemistry and three of six tumors with no detected p53 mutations were also immunoreactive for p53 protein. The presence, pattern, and distribution of p53 staining in tissue sections depended on the morphology of the lesion. PCNA immunoreactivity was strikingly similar in pattern and distribution to p53 immunoreactivity. The pattern and distribution of immunoreactivity for TGF-alpha did not directly correlate with the other markers. Mutation of the p53 tumor suppressor gene may be an important step in the progression of formaldehyde-induced nasal carcinogenesis in the rat. This study demonstrated that immunohistochemistry is a useful tool for the identification of sites within tumors that might have p53 mutations.

Autocrine growth stimulation by transforming growth factor alpha in asbestos-transformed rat mesothelial cells

Although the association between asbestos exposure and mesothelioma development has been established for decades, very little is known regarding the molecular mechanism(s) by which asbestos fibers induce this disease. In this series of experiments, the potential for transforming growth factor alpha (TGF-alpha) to act as an autocrine growth factor in transformed mesothelial cells was examined in rats, a model system frequently used to assess the tumorigenic potential of fibrous particulates. Both asbestos-transformed cells and spontaneously transformed cells expressed functional EGF receptors, although only the asbestos-transformed cells expressed TGF-alpha. Expression of TGF-alpha transcripts was correlated with secretion of picogram amounts of growth factor into conditioned medium by the asbestos-transformed cells. In addition, whereas TGF-alpha inhibited the growth of spontaneously transformed mesothelial cells, it stimulated the growth of asbestos-transformed cells. Neutralizing antibody that recognized TGF-alpha secreted by the asbestos-transformed cells was able to inhibit the growth of these cells. Taken together, these data indicate that TGF-alpha acts as an autocrine growth factor for asbestos-transformed rat mesothelial cells. Therefore, in asbestos-transformed mesothelial cells, altered production and responsiveness to TGF-alpha distinguish these cells from spontaneously transformed mesothelial cells. These data suggest that differences in mesothelioma etiology may be reflected in differences in the molecular alterations present in these tumors.

Mesothelial cell proliferation induced by intrapleural instillation of man-made fibers in rats and hamsters

Long-term inhalation exposure to a biopersistent man-made ceramic fiber (RCF 1) results in a high incidence of pleural mesotheliomas in Syrian golden hamsters but not in identically exposed rats. To understand better the mechanisms involved in the intraspecies pathobiology of fiber-exposed mesothelium, the ability of the two different man-made fibers to induce cell proliferation in hamster and rat pleural mesothelial cells was investigated. Three dose levels of either glass fibers (MMVF 10) or ceramic fibers (RCF 1) were instilled intrapleurally into male Fischer 344 rats and male Syrian Golden hamsters. Rats and hamsters were exposed to approximately equal numbers of long thin fibers per kilogram of body weight using a single intrapleural instillation. Bromodeoxyuridine (BrdU) was administered via an implanted osmotic pump, and mesothelial cell proliferation was assessed at 7 and 28 days postinstillation (PI) using immunocytochemical visualization of labeled S-phase cells. Both rats and hamsters exhibited dose-dependent increases in proliferation of pleural mesothelial cells following exposure to both fiber types. Interspecies differences in mesothelial cell proliferation were noted for fiber type and pleural site. At 28 days PI, RCF-induced mesothelial cell proliferation was found to be more pronounced in hamsters than in rats in the caudal visceral pleural. Comparing both fibers either by equal mass or by equal fiber numbers, mesothelial cell proliferation in RCF 1-treated animals was higher than in animals exposed to MMVF 10, especially in hamsters, and may be a factor in the difference in mesothelioma induced by the two fibers. The higher sustained (28 day) mesothelial cell proliferation in the visceral pleural of hamsters exposed to RCF may contribute to the species-specific differences in mesothelioma incidence found in long-term rodent inhalation studies.

Pleural lesions in Syrian golden hamsters and Fischer-344 rats following intrapleural instillation of man-made ceramic or glass fibers

The mesothelium is a target of the toxic and carcinogenic effects of certain natural mineral and man-made fibers. Long-term inhalation of a ceramic fiber (RCF-1) results in a high incidence of pleural mesotheliomas in Syrian golden hamsters but not in identically exposed Fischer-344 rats. The present study compared the histopathology of the early pleural response in rats and hamsters instilled with artificial fibers. Groups of Syrian golden hamsters and Fischer-344 rats were instilled with ceramic (RCF-1) or glass (MMVF-10) fibers directly into the pleural space. Each species received approximately equal numbers of long, thin fibers per g body weight. Fiber-induced lesions were compared 7 and 28 days postinstillation. Both hamsters and rats developed qualitatively similar dose-dependent inflammatory lesions that were not fiber-type specific. Both species developed fibrosis in conjunction with inflammation in the visceral pleura, but a striking interspecies difference was noted in the pattern of mesothelial cell response. Hamsters developed greater surface mesothelial cell proliferation and had focal aggregates of mesothelial cells embedded deep within regions of visceral pleural fibrosis. It is hypothesized from the present study that the marked fiber-induced proliferative mesothelial cell response of the hamster visceral pleura may explain the high number of pleural mesotheliomas found in long-term fiber studies in this species.

Characterization of cell lines derived from formaldehyde-induced nasal tumors in rats

Cell lines derived from formaldehyde-induced nasal tumors in Fischer 344 rats were established. All of the lines were found to be epithelial and aneuploid and to express keratin, transforming growth factor-alpha, and epidermal growth factor receptor transcripts. Two of four lines were tumorigenic upon injection into nude mice, and these lines also contained point mutations in the p53 suppressor gene. The data indicate that these lines possess characteristics that make them a valuable tool for the study of chemically induced respiratory tract carcinogenesis.

p53 mutations in formaldehyde-induced nasal squamous cell carcinomas in rats

Formaldehyde induces squamous cell carcinomas in the nasal passages of rats following chronic inhalation exposure at concentrations of > or = 10 ppm. We have examined the complementary DNA of the tumor suppressor gene p53 from 11 primary formaldehyde-induced tumors for mutation using DNA sequence analysis. A polymerase chain reaction-amplified fragment of the rat p53 complementary DNA containing the evolutionarily conserved regions II-V was directly sequenced from each tumor. Point mutations in the p53 complementary DNA sequence were found in 5 of 11 of the tumors analyzed. These data demonstrate p53 point mutations in formaldehyde-induced squamous cell carcinomas and indicate a common alteration in certain rat and human squamous cell carcinomas of the respiratory tract.

Identification of heptanal and nonanal in bronchoalveolar lavage from rats exposed to low levels of ozone

Heptanal and nonanal are identified from in vitro studies as potential biomarkers of exposure to ozone, the former resulting from ozonation of palmitoleic acid and the latter from oleic acid. An analytical method is developed based on derivatization using O-pentafluorobenzylhydroxylamine HCl and gas chromatography. These molecules also are present in the lung lavage of Sprague-Dawley rats exposed to 1.3 ppm ozone for 10 hr. These results suggest aldehydes may be useful dosimeters for ozone and indicate that unsaturated fatty acids in the lung lining fluid layer undergo ozonation in vivo.

Characterization of platelet-derived growth factor and platelet-derived growth factor receptor expression in asbestos-induced rat mesothelioma

Although altered expression of platelet-derived growth factor (PDGF) is a hallmark of human mesothelioma, expression of PDGF receptors has not been characterized in this cell type. In addition, the expression of this growth factor and its cognate receptor in rodent mesothelioma has not been investigated. In this study, examination of transformed mesothelial cells derived from asbestos-induced rat mesotheliomas revealed that these cells expressed high affinity PDGF receptors (Kd = 0.5 nM) and receptor number was 1.6 x 10(5)/cell. Western analysis using antibodies specific for either the alpha-type or beta-type PDGF receptor and Northern analysis using probes specific for alpha- and beta-type receptor RNA transcripts indicated that these cells expressed beta-type PDGF receptors but that alpha-type receptors could not be detected. However, when the mesothelioma-derived cells were examined for the expression of PDGF, no expression of this growth factor could be detected. The transformed cells expressed no detectable A- or B-chain PDGF RNA transcripts; and using a competitive enzyme immunoassay specific for isoforms containing the B chain of PDGF and a sandwich enzyme-linked immunosorbent assay specific for A-chain-containing isoforms, neither AA, nor AB, nor BB isoforms of this growth factor could be detected in medium conditioned by these cells. The absence of alterations in PDGF expression in rat mesothelioma, in contrast to the data for the human disease, suggests that the production of this growth factor by transformed mesothelial cells may be species specific.

Trisomy of rat chromosome 1 associated with mesothelial cell transformation

Identification of specific chromosomal aberrations in transformed mesothelial cells is an important step in elucidating the mechanism of transformation of these cells which are targets for occupational and environmental carcinogens, such as asbestos fibers. Cytogenetic analysis of normal rat mesothelial cell lines revealed that at late passage (p20-p34), trisomy of chromosome 1 was present in greater than 80% of the cells in four spontaneously immortalized lines examined, whereas at early passage (p8-p10), only 15-44% of the cells had trisomy 1. Trisomy of chromosome 1 had increased in the population as a function of passage, suggesting that cells with trisomy 1 had a selective growth advantage under in vitro culture conditions and that this alteration was associated with transformation. A commercially available rat mesothelial cell line (4/4 RM4, ATCC), was also found to have a duplication of a portion of the long arm of chromosome 1. To determine if chromosome 1 alterations have relevance to the transformed phenotype in vivo, a neoplastic cell line was established from a spontaneous rat mesothelioma. At passage 15, trisomy of chromosome 1 was observed in 26% of the metaphases in this line. However, when these cells were injected into nude mice, 99% of the cells from the resulting tumor contained an additional copy of chromosome 1. Therefore, trisomy 1 also conferred a selective growth advantage in vivo and/or was associated with the malignant subpopulation in the tumor derived cell line. These studies suggest that chromosome 1 contains a gene(s) involved in transformation of rat mesothelial cells.

Examination of potential mechanisms of carcinogenicity of 1,4-dioxane in rat nasal epithelial cells and hepatocytes

Several long-term studies with 1,4-dioxane (dioxane) have shown it to induce liver tumors in mice, and nasal and liver tumors in rats when administered in amounts from 0.5 to 1.8% in the drinking water (Argus et al. 1965; Kociba et al. 1974; National Cancer Institute, 1978). In order to examine potential mechanisms of action, chemically-induced DNA repair (as an indicator of DNA reactivity) and cell proliferation (as an indicator of promotional activity) were examined in nasal turbinate epithelial cells and hepatocytes of male Fischer-344 rats treated with dioxane. Neither dioxane nor 1,4-dioxane-2-one, one of the proposed metabolites, exhibited activity in the in vitro primary rat hepatocyte DNA repair assay, even from cells that had been isolated from animals given either 1 or 2% dioxane in the drinking water for 1 week to induce enzymes that might be responsible for producing genotoxic metabolites. No activity was seen in the in vivo hepatocyte DNA repair assay in animals given a single dose of up to 1000 mg/kg dioxane or up to 2% dioxane in the drinking water for 1 week. Treatment of rats with 1.0% dioxane in the drinking water for 5 days yielded no increase in liver/body weight nor induction of palmitoyl CoA oxidase, indicating that dioxane does not fit into the class of peroxisomal proliferating carcinogens. The percentage of cells in DNA synthesis phase (S-phase) was determined by administration of 3H-thymidine and subsequent quantitative histoautoradiography. The hepatic labeling index (LI) did not increase at either 24 or 48 h following a single dose of 1000 mg/kg dioxane.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of growth factor and growth factor receptor RNA in rat pleural mesothelial cells in culture

Mineral fiber-induced pleural mesothelioma in the rat is a suitable model for asbestos-induced mesothelioma in humans. A proposed mechanism for the genesis of mesotheliomas is the initiation of an autocrine pathway leading to unregulated growth of the mesothelium. To understand if changes in the expression of mRNA of critical growth factors and receptors occur in target mesothelial cells, it is first necessary to characterize the pattern of expression of these genes in normal mesothelial cells. Rat mesothelial cells were isolated from the parietal pleura and strains of these cells were propagated in vitro. The cells were diploid, had epithelial gross morphology and ultrastructure, and coexpressed keratins and vimentin. Northern blot analysis demonstrated that the cells expressed transforming growth factor beta 1 and fibroblast growth factor. Transcripts for transforming growth factor alpha, platelet-derived growth factor A-chain, and platelet-derived growth factor B-chain were not detected. Receptors for platelet-derived growth factor, epidermal growth factor, and insulin were detected. Although normal mesothelial cells express receptors for these growth factors, no production of their corresponding ligands by these cells could be detected, suggesting that autocrine stimulation of growth via the production of such factors may be specific to transformed mesothelial cells.

The induction of DNA-strand breaks and unscheduled DNA synthesis in F-344 rat hepatocytes following in vivo administration of caprolactam or benzoin

Benzoin (ZOIN) and caprolactam (CAP) were administered by gavage to Fischer 344 rats at a dose of 750 mg/kg and the hepatocytes isolated 12, 24 or 48 h after treatment. The isolated hepatocytes were subsequently examined for the induction of DNA-strand breaks (SB) and unscheduled DNA synthesis (UDS). Neither ZOIN nor CAP induced SB or UDS in hepatocytes, however ZOIN did induce an increase in the fraction of cells in S-phase 24 h after treatment. These results correlate well with the observed lack of carcinogenicity of these compounds.

A protocol and guide for the in vivo rat hepatocyte DNA-repair assay

The in vivo rat hepatocyte DNA-repair assay is a valuable tool in assessing the genotoxic activity of chemical agents. An advantage of the system is that it reflects the complex patterns of uptake, distribution, metabolism, detoxification and excretion that actually occur in the whole animal. This article provides a typical procedure and guidelines for conducting the rat in vivo hepatocyte DNA-repair assay.

A protocol and guide for the in vitro rat hepatocyte DNA-repair assay

The in vitro rat-hepatocyte DNA-repair assay is a valuable tool in assessing the genotoxic activity of chemical agents. An advantage of the assay is that the target cells themselves are metabolically competent, so that the patterns of metabolic activation and detoxification closely reflect those in the whole animal. This article provides a typical procedure and guidelines for conducting the rat in vitro hepatocyte DNA-repair assay.

Formaldehyde mutagenesis and formation of DNA-protein crosslinks in human lymphoblasts in vitro

Human lymphoblasts were exposed in vitro to various concentrations of formaldehyde (HCHO) in single and multiple treatment regimens to determine relative mutagenic efficiency. Single treatments of HCHO (0-150 microM X 2 h) resulted in a nonlinear increase in induced mutant fraction at the thymidine kinase locus with increasing slope at concentrations above 125 microM. Only HCHO exposures of 125 microM X 2 h or greater produced significant effects on the growth rate of the lymphoblasts. Cultures were also exposed to either three treatments of 50 microM X 2 h, five treatments of 30 microM X 2 h, or ten treatments of 15 microM X 2 h; multiple treatments were administered on different days. These multiple treatments resulted in increases in mutant fraction, although their combined effect was less than a single treatment of equivalent concentration X time (150 microM X 2 h). Exposure of lymphoblasts to four treatments of 150 microM X 2 h HCHO failed to induce mutations at the ouabain resistance locus. Cultures of lymphoblasts receiving a single treatment of HCHO (0-600 microM X 2 h) were analyzed by the alkaline elution technique to detect the presence of DNA-protein crosslinks. HCHO treatment resulted in a significant nonlinear increase in DNA-protein crosslinks at concentrations greater than 50 microM X 2 h, which correlated with the onset of significant toxicity in this cell line. Holding the culture for 24 h resulted in complete removal of the crosslinks. These data indicate that both the induction of mutations and the formation of DNA-protein crosslinks by HCHO are nonlinear functions in human lymphoblasts and occur at overlapping concentration ranges.

Assessment of the genotoxic effects of methyl chloride in human lymphoblasts

The activity of methyl chloride was measured in 4 genotoxicity assays. In an established human lymphoblast line, a 3-h treatment with 0-5% methyl chloride resulted in a dose-related increase in mutant fraction at the thymidine kinase locus and induction of sister-chromatid exchange. No increase in DNA damage, as measured by alkaline elution, was detected in the lymphoblasts at concentrations of methyl chloride shown to be mutagenic. Also, a concentration-related increase in 8-azaguanine-resistant fraction in Salmonella typhimurium was observed following a 3-h treatment with atmospheres containing 0-20% methyl chloride. Thus, methyl chloride is a weak, direct-acting mutagen for bacteria and human cells in culture.

The assessment of DNA damage and repair in rat nasal epithelial cells

A methodology for the isolation and culture of rat nasal turbinates was developed with the aim of studying chemically-induced genotoxic effects in the nasal epithelium. DNA damage, as assessed by alkaline elution, and unscheduled DNA synthesis, as measured by quantitative autoradiography, were observed in the cells of the respiratory and olfactory epithelium as a result of in vitro treatment with methyl methanesulfonate. The capacity of nasal epithelium to metabolize the promutagen dimethylnitrosamine (DMN) was demonstrated by the induction of both DNA strand breakage and the induction of unscheduled DNA synthesis. Although a response was observed in all of the epithelia examined, differences in response to DMN were observed that were not attributable to the differences in the cell types present (e.g., respiratory versus olfactory epithelium). The respiratory epithelial cells of the maxilloturbinate were found to be resistant to the induction of DNA damage at concentrations of 0.5 and 1.0 mM DMN, whereas the respiratory epithelial cells of the nasoturbinates were susceptible. In contrast, the response of the olfactory epithelium was similar to the response observed in the cells of the nasoturbinates. These techniques should prove valuable in assessing chemically-induced DNA damage and repair in this target tissue.

Lack of genotoxic activity of di(2-ethylhexyl)phthalate (DEHP) in rat and human hepatocytes

Di(2-ethylhexyl)phthalate (DEHP) is a widely used plasticizer which has been reported to induce a statistically significant increase in the incidence of hepatocellular carcinomas in female Fischer-344 rats (8/50) when administered in the diet at 12 000 p.p.m. for two years. Numerous studies with cells in culture have failed to show any genotoxic activity associated with DEHP. Because DEHP induces multiple changes in the liver, such as peroxisomal proliferation, it was possible that these alterations could result in genotoxic effects in the treated whole animal that would not be seen in cells in culture. Accordingly, the ability of DEHP to induce DNA damage or repair was examined in rat hepatocytes in vivo and in vitro and in human hepatocytes in vitro. Unscheduled DNA synthesis was measured by incorporation of [3H]thymidine into primary hepatocyte cultures immediately isolated from treated animals or hepatocyte cultures incubated directly with DEHP. DNA damage was measured by alkaline elution of cellular DNA from the same cultures. In vivo-in vitro treatment regimens were: (i) female rats, 12 000 p.p.m. DEHP in the diet for 30 days; (ii) female rats, 12 000 p.p.m. in the diet for 30 days, followed by 500 mg/kg DEHP by gavage 2 h before sacrifice; (iii) male rats, 500 mg/kg DEHP by gavage 2, 12, 24, or 48 h before sacrifice; and (iv) male rats, 150 mg/kg/day by gavage for 14 days. In vitro conditions were 0.1, 1.0 and 10.0 mM DEHP in the cultures for 18 h. Primary cultures of human hepatocytes were prepared from freshly discarded surgical material and exposed to the same concentration of DEHP. Concentrations up to 0.5 mM mono(2-ethylhexyl)phthalate, a principal metabolite of DEHP, were also examined in the human hepatocyte assay. No chemically induced DNA damage or repair was observed in vivo or in vitro in rat or human hepatocytes under any of the conditions employed. However, an increase in the percentage of cells in S-phase in the animals given DEHP was observed. These data indicate that DEHP does not exhibit direct genotoxic activity in the animals even with a treatment regimen which eventually produced tumors in a long term bioassay, and that both rat and human hepatocytes are similar in their lack of a genotoxic response to DEHP exposure in culture.

Measurement of genotoxic activity in multiple tissues following inhalation exposure to dimethylnitrosamine

Chemically-induced DNA repair was measured as unscheduled DNA synthesis (UDS) in selected tissues isolated from rats following in vivo exposure to inhaled dimethylnitrosamine (DMN). UDS was evaluated in epithelial cells from rat nasal turbinates and trachea, in hepatocytes and in pachytene spermatocytes from the same treated animal. At nominal concentrations of 500 and 1000 ppm of DMN in air, chemically-induced DNA repair was observed in the epithelial cells of the upper respiratory system. DMN also entered the circulation, as evidenced by a strong DNA-repair response in hepatocytes. No DNA repair was observed in pachytene spermatocytes indicating either that DMN or its active metabolites did not reach the testes in sufficient concentration to induce DNA repair or that the testes lacked the capability to metabolically activate the compound. These results illustrate the potential of this approach to assess the organ-specific genotoxicity of environmental chemicals.