Alterations in the K-ras and p53 genes in rat lung tumors

Epigenetic Changes in Alveolar Type II Lung Cells of A/J Mice Following Intranasal Treatment with Lipopolysaccharide

Lipopolysaccharide (LPS) is a bacterial endotoxin present in cigarette smoke. LPS is known to induce inflammation and to increase the size and the multiplicity of lung tumors induced by tobacco-specific nitrosamines. However, the means by which LPS contributes to pulmonary carcinogenesis are not known. One possible mechanism includes LPS-mediated epigenetic deregulation, which leads to aberrant expression of genes involved in DNA repair, tumor suppression, cell cycle progression, and cell growth. In the present work, epigenetic effects of LPS were examined in alveolar type II lung cells of A/J mice. Type II cells were selected because they serve as progenitors of lung adenocarcinomas in smoking induced lung cancer. A/J mice were intranasally treated with LPS, followed by isolation of alveolar type II cells from the lung using cell panning. Global levels of DNA methylation and histone acetylation were quantified by mass spectrometry, while genome-wide transcriptomic changes were characterized by RNA-Seq. LPS treatment was associated with epigenetic changes including decreased cytosine formylation and reduced histone H3K14 and H3K23 acetylation, as well as altered expression levels of genes involved in cell adhesion, inflammation, immune response, and epigenetic regulation. These results suggest that exposure to inflammatory agents in cigarette smoke leads to early epigenetic changes in the lung, which may collaborate with genetic changes to drive the development of lung cancer.

Molecular fingerprints of environmental carcinogens in human cancer

Identification of specific molecular changes (fingerprints) is important to identify cancer etiology. Exploitable biomarkers are related to DNA, epigenetics, and proteins. DNA adducts are the turning point between environmental exposures and biological damage. DNA mutational fingerprints are induced by carcinogens in tumor suppressor and oncogenes. In an epigenetic domain, methylation changes occurs in specific genes for arsenic, benzene, chromium, and cigarette smoke. Alteration of specific microRNA has been reported for environmental carcinogens. Benzo(a)pyrene, cadmium, coal, and wood dust hits specific heat-shock proteins and metalloproteases. The multiple analysis of these biomarkers provides information on the carcinogenic mechanisms activated by exposure to environmental carcinogens.

Multi-step lung carcinogenesis model induced by oral administration of N-nitrosobis(2-hydroxypropyl)amine in rats

N-Nitrosobis(2-hydroxypropyl)amine (BHP) was first synthesized by Krüger et al. (1974), and has been shown to primarily induce pancreatic duct adenocarcinomas by a subcutaneous injection in Syrian hamsters. By contrast, the carcinogenic effect of BHP has been indicated at the different target organs in rats, namely the lung. When rats are received by an oral administration of BHP in drinking water for 25 weeks, a high incidence of lung carcinomas are induced, which include adenocarcinomas, squamous cell carcinomas and combined squamous cell and adenocarcinomas. So many similarities are observed in terms of not only histological appearances but also gene alterations between human and BHP-induced rat lung cancers. Moreover, the step by step development of lung lesions, from preneoplastic lesions to cancers in rat lung carcinogenesis by BHP offers a good model to investigate the mechanisms underlying the pathogenesis of lung cancers. Because data for genetic and epigenetic alterations have indeed been accumulated during the BHP-induced rat lung carcinogenesis, we will introduce them in this review and hence demonstrate that this lung carcinogenesis model provides a useful opportunity for the research on the pathogenesis of lung cancers of both humans and rats.

The cellular and molecular carcinogenic effects of radon exposure: a review

Radon-222 is a naturally occurring radioactive gas that is responsible for approximately half of the human annual background radiation exposure globally. Chronic exposure to radon and its decay products is estimated to be the second leading cause of lung cancer behind smoking, and links to other forms of neoplasms have been postulated. Ionizing radiation emitted during the radioactive decay of radon and its progeny can induce a variety of cytogenetic effects that can be biologically damaging and result in an increased risk of carcinogenesis. Suggested effects produced as a result of alpha particle exposure from radon include mutations, chromosome aberrations, generation of reactive oxygen species, modification of the cell cycle, up or down regulation of cytokines and the increased production of proteins associated with cell-cycle regulation and carcinogenesis. A number of potential biomarkers of exposure, including translocations at codon 249 of TP53 in addition to HPRT mutations, have been suggested although, in conclusion, the evidence for such hotspots is insufficient. There is also substantial evidence of bystander effects, which may provide complications when calculating risk estimates as a result of exposure, particularly at low doses where cellular responses often appear to deviate from the linear, no-threshold hypothesis. At low doses, effects may also be dependent on cellular conditions as opposed to dose. The cellular and molecular carcinogenic effects of radon exposure have been observed to be both numerous and complex and the elevated chronic exposure of man may therefore pose a significant public health risk that may extend beyond the association with lung carcinogenesis.

Beryllium metal II. a review of the available toxicity data

Beryllium metal was classified in Europe collectively with beryllium compounds, e.g. soluble salts. Toxicological equivalence was assumed despite greatly differing physicochemical properties. Following introduction of the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) regulation, beryllium metal was classified as individual substance and more investigational efforts to appropriately characterize beryllium metal as a specific substance apart from soluble beryllium compounds was required. A literature search on toxicity of beryllium metal was conducted, and the resulting literature compiled together with the results of a recently performed study package into a comprehensive data set. Testing performed under Organisation for Economic Co-Operation and Development guidelines and Good Laboratory Practice concluded that beryllium metal was neither a skin irritant, an eye irritant, a skin sensitizer nor evoked any clinical signs of acute oral toxicity; discrepancies between the current legal classification of beryllium metal in the European Union (EU) and the experimental results were identified. Furthermore, genotoxicity and carcinogenicity were discussed in the context of the literature data and the new experimental data. It was concluded that beryllium metal is unlikely to be a classical nonthreshold mutagen. Effects on DNA repair and morphological cell transformation were observed but need further investigation to evaluate their relevance in vivo. Animal carcinogenicity studies deliver evidence of carcinogenicity in the rat; however, lung overload may be a species-specific confounding factor in the existing studies, and studies in other species do not give convincing evidence of carcinogenicity. Epidemiology has been intensively discussed over the last years and has the problem that the studies base on the same US beryllium production population and do not distinguish between metal and soluble compounds. It is noted that the correlation between beryllium exposure and carcinogenicity, even including the soluble compounds, remains under discussion in the scientific community and active research is continuing.

Comparative levels of O6-methylguanine, pyridyloxobutyl-, and pyridylhydroxybutyl-DNA adducts in lung and liver of rats treated chronically with the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a lung carcinogen in rats and may be a cause of lung cancer in smokers. NNK is metabolized by cytochromes P450 to intermediates that react with DNA forming methyl, pyridyloxobutyl (POB), and pyridylhydroxybutyl (PHB) adducts, which are critical in carcinogenesis. The methyl adduct O(6)-methylguanine (O(6)-methyl-G) has miscoding properties, but there are no reports on levels of this adduct in rats treated chronically with NNK in the drinking water, nor has its levels been compared with those of POB- and PHB-DNA adducts. We used liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring to quantify O(6)-methyl-G in lung and liver DNA of rats treated with a carcinogenic dose of 10 ppm of NNK in the drinking water and sacrificed after 1, 2, 5, 10, 16, and 20 weeks. The maximal level of O(6)-methyl-G in lung DNA, 2550 +/- 263 fmol/mg DNA, was reached at 5 weeks and was significantly greater (P < 0.05) at that point than all other adducts (measured previously) except O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine. Overall levels of O(6)-methyl-G in lung were intermediate between those of total POB- and PHB-DNA adducts. In liver, the wave of O(6)-methyl-G peaked at 2 weeks while that of total POB-DNA adducts peaked at 10 weeks, and levels of total PHB-DNA adducts were low throughout. The results of this study demonstrate that substantial amounts of O(6)-methyl-G are formed at various time points in lung and liver DNA of rats treated chronically with NNK, supporting its role in carcinogenesis.

Tyrosine-dependent oxidative DNA damage induced by carcinogenic tetranitromethane

Tetranitromethane (TNM) is used as an oxidizer in rocket propellants and explosives and as an additive to increase the cetane number of diesel fuel. TNM was reported to induce pulmonary adenocarcinomas and squamous cell carcinomas in mice and rats. However, the mechanisms underlying carcinogenesis induced by TNM has not yet been clarified. We previously revealed that nitroTyr and nitroTyr-containing peptides caused Cu(II)-dependent DNA damage in the presence of P450 reductase, which is considered to yield nitroreduction. Since TNM is a reagent for nitration of Tyr in proteins and peptides, we have hypothesized that TNM-treated Tyr and Tyr-containing peptides induce DNA damage by the modification of Tyr. We examined DNA damage induced by TNM-treated amino acids or peptides using (32)P-5'-end-labeled DNA fragments obtained from the human p53 tumor suppressor gene and the c-Ha-ras-1 protooncogene. TNM-treated Tyr and Lys-Tyr-Lys induced DNA damage including the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine in the presence of Cu(II) and NADH. DNA damage was inhibited by catalase and bathocuproine, indicating the involvement of H(2)O(2) and Cu(I). The cytosine residue of the ACG sequence complementary to codon 273, well-known hotspots of the p53 gene, was cleaved with piperidine and Fpg treatments. On the other hand, nitroTyr and Lys-nitroTyr-Lys did not induce DNA damage in the presence of Cu(II) and NADH. Time-of-flight mass spectrometry confirmed that reactions between Lys-Tyr-Lys and TNM yielded not only Lys-nitroTyr-Lys but also Lys-nitrosoTyr-Lys. Therefore, it is speculated that the nitrosotyrosine residue can induce oxidative DNA damage in the presence of Cu(II) and NADH. It is concluded that Tyr-dependent DNA damage may play an important role in the carcinogenicity of TNM. TNM is a new type of carcinogen that induces DNA damage not by itself but via Tyr modification.

Air pollution and cancer: biomarker studies in human populations

Large cohort studies in the U.S. and in Europe suggest that air pollution may increase lung cancer risk. Biomarkers can be useful to understand the mechanisms and to characterize high-risk groups. Here we describe biomarkers of exposure, in particular DNA adducts as well as markers of early damage, including mutagenicity, other endpoints of genotoxicity and molecular biomarkers of cancer. Several studies found an association between external measures of exposure to air pollution and increased levels of DNA adducts, with an apparent levelling-off of the dose-response relationship. Also, numerous experimental studies in vitro and in vivo have provided unambiguous evidence for genotoxicity of air pollution. In addition, due to the organic extracts of particulate matter [especially various polycyclic aromatic hydrocarbon (PAH) compounds], particulate air pollution induces oxidative damage to DNA. The experimental work, combined with the data on frequent oxidative DNA damage in lymphocytes in people exposed to urban air pollution, suggests 8-oxo-dG as one of the important promutagenic lesions. Lung cancer develops through a series of progressive pathological changes occurring in the respiratory epithelium. Molecular alterations such as loss of heterozygosity, gene mutations and aberrant gene promoter methylation have emerged as potentially promising molecular biomarkers of lung carcinogenesis. Data from such studies relevant for emissions rich in PAHs are also summarized, although the exposure circumstances are not directly relevant to outdoor air pollution, in order to shed light on potential mechanisms of air pollution-related carcinogenesis.

Carbon black should not be classified as a human carcinogen based on rodent bioassay data

Numerous epidemiology studies have failed to adequately demonstrate an increased risk of lung cancer due to occupational exposure to carbon black (CB). CB is not carcinogenic to mice (oral, skin or inhalation), hamsters (inhalation or intratracheal), guinea pigs (inhalation), rabbits (skin or inhalation), primates (skin or inhalation) or rats (oral). Only studies conducted by inhalation and intratracheal administration in rats have shown significant increases in benign and malignant lung tumors and lesions described as benign cystic keratinizing squamous-cell (KSC) tumors. CB-induced lung tumor formation, including KSC lesions, occurs only in rats. An expert panel reviewing KSC lesions (induced in rats by TiO2 or p-aramid) concluded that KSC lesions are not seen in humans. Lung tumors in humans are primarily located in the bronchial airways, whereas in the rat they occur in the parenchyma and are alveolar in origin. This species-specific response (tumor formation and KSC lesions) by the rat to CB, not seen in any other laboratory species and which has not been reported in humans, strongly suggests that the results of the rat inhalation bioassay should not be considered directly relevant when assessing human risk. Therefore, CB should not be classified as carcinogenic to humans based on the rodent bioassay data.

Comparative study on Tp53 gene mutations in lung tumors from rats exposed to 239Pu, 237Np and 222Rn

The tumor suppressor gene Tp53 was analyzed by polymerase chain reaction-amplification of genomic DNA extracted from paraffin-embedded tissue sections of rat lung tumors to compare mutations that occurred after inhalation exposures to plutonium dioxide, neptunium dioxide, or radon and radon progenies. Exons 5 to 8 of the gene were amplified in 16 plutonium-, 23 neptunium- and 15 radon-induced lung tumors, and their polymerase chain reaction products were examined for mutations by single strand conformational polymorphism analysis and direct sequencing method. Two point mutations were detected in the plutonium-induced tumors, i.e., a guanine to adenine transition at codon 219 of exon 6 and a cytosine to thymine transition at codon 266 of exon 8. Although only one point mutation was found at codon 175 of exon 5 (cytosine to thymine transition) from neptunium-induced tumors, no mutations were detectable from radon-induced tumors. These results indicate that the abnormalities of the Tp53 gene might not be so critical for the pulmonary carcinogenesis after the inhalation of different alpha emitters, even though the presence and frequencies of the Tp53 gene mutations were different.

Carcinogenic potential and genomic instability of beryllium sulphate in BALB/c-3T3 cells

Occupational exposure to beryllium (Be) and Be compounds occurs in a wide range of industrial processes. A large number of workers are potentially exposed to this metal during manufacturing and processing, so there is a concern regarding the potential carcinogenic hazard of Be. Studies were performed to determine the carcinogenic potential of beryllium sulfate (BeSO4) in cultured mammalian cells. BALB/c-3T3 cells were treated with varying concentrations of BeSO4 for 72 h and the transformation frequency was determined after 4 weeks of culturing. Concentrations from 50-200 microg BeSO4/ml, caused a concentration-dependent increase (9-41 fold) in transformation frequency. Non-transformed BALB/c-3T3 cells and cells from transformed foci induced by BeSO4 were injected into both axillary regions of nude mice. All ten Be-induced transformed cell lines injected into nude mice produced fibrosarcomas within 50 days after cell injection. No tumors were found in nude mice receiving non-transformed BALB/c-3T3 cells 90 days post-injection. Gene amplification was investigated in K-ras, c-myc, c-fos, c-jun, c-sis, erb-B2 and p53 using differential PCR while random amplified polymorphic DNA fingerprinting was employed to detect genomic instability. Gene amplification was found in K-ras and c-jun, however no change in gene expression or protein level was observed in any of the genes by Western blotting. Five of the 10 transformed cell lines showed genetic instability using different random primers. In conclusion, these results indicate that BeSO4 is capable of inducing morphological cell transformation in mammalian cells and that transformed cells induced by BeSO4 are potentially tumorigenic. Also, cell transformation induced by BeSO4 may be attributed, in part, to the gene amplification of K-ras and c-jun and some BeSO4-induced transformed cells possess neoplastic potential resulting from genomic instability.

Expression and NNK reducing activities of carbonyl reductase and 11beta-hydroxysteroid dehydrogenase type 1 in human lung

The tobacco specific nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK), which is found in high amounts in tobacco products, is believed to play an important role in lung cancer induction in smokers. NNK requires metabolic activation by cytochrome P450 mediated alpha-hydroxylation to exhibit its carcinogenic properties. On the other hand, NNK is inactivated by carbonyl reduction to its alcohol-equivalent 4-methylnitrosamino-1-(3-pyridyl)-1-butanol (NNAL) followed by glucuronidation and final excretion into urine or bile. Carbonyl reduction and alpha-hydroxylation are the predominant pathways in man, and it has been postulated that the extent of these competing pathways determines the individual susceptibility to lung cancer. Moreover, only a minor part of all habitual smokers develop lung cancer, suggesting the existence of susceptibility genes. Microsomal 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD 1) (EC 1.1.1.146) and cytosolic carbonyl reductase (CR) (EC 1.1.1.184) have been shown to be mainly responsible for NNAL formation in liver and lung. In the present study, we performed comparative investigations of human lung tissue samples from several patients with respect to the expression and activity of 11beta-HSD 1 and carbonyl reductase. We observed varying levels in 11beta-HSD 1 and carbonyl reductase expression in these patients, as revealed by RT-PCR and ELISA. Also, the tissue samples showed a different activity and inhibitor profile for both enzymes. According to our results, variations in the expression and activity of NNK carbonyl reducing enzymes may constitute a major determinant in the overall NNK detoxification capacity and thus may be linked to the great differences observed in the individual susceptibility of tobacco-smoke related lung cancer.

Purification and characterization of oxidoreductases-catalyzing carbonyl reduction of the tobacco-specific nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) in human liver cytosol

1. Four enzymes were purified to homogeneity from human liver cytosol and were demonstrated to be responsible for carbonyl reduction of the tobacco-specific nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK). 2. Carbonyl reductase (EC 1.1.1.184), a member of the short-chain dehydrogenase/reductase (SDR) superfamily, was compared with three isoenzymes of the aldo-keto reductase (AKR) superfamily in terms of enzyme kinetics, co-substrate dependence and inhibition pattern. 3. AKR1C1, 1C2 and 1C4, previously designated as dihydrodiol dehydrogenases (DD1, DD2 and DD4), showed lower K(m) (0.2, 0.3 and 0.8 mM respectively) than did carbonyl reductase (7 mM), whereas carbonyl reductase exhibited the highest enzyme efficiency (Vmax/K(m)) for NNK. Multiplication of enzyme efficiencies with the relative quantities of individual enzymes in cytosol resulted in a rough estimate of their contributions to total alcohol metabolite formation. These were approximately 60% for carbonyl reductase, 20% each for AKR1C1 and 1C2, and 1% for AKR1C4. 4. Except for AKR1C4, the enzymes had a strong preference for NADPH over NADH, and the highest activities were measured with an NADPH-regenerating system. Carbonyl reductase activity was extensively inhibited by menadione, rutin and quercitrin, whereas medroxyprogesterone acetate, phenolphthalein and flufenamic acid were potent inhibitors of AKR1C1, 1C2 and 1C4. 5. In conclusion, cytosolic members of the SDR and AKR superfamilies contribute to reductive NNK detoxification in human liver, the enzymes responsible being carbonyl reductase and aldoketo reductases of the AKRIC subfamily.

No consistent pattern of mutations in p53 and ras genes in liver tumors of rat treated with the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX)

The frequency of point mutations in p53 (exons 4-7) and in Ki-ras, Ha-ras, and N-ras (exons 1 and 2) and the expression of p53 protein were evaluated in the liver tumors of Wistar rats of a 104-week carcinogenicity study on 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a chlorine disinfection by-product in drinking water. Mutations were analyzed in 16 hepatocellular adenomas, 7 hepatocellular carcinomas, 23 cholangiomas, and 2 cholangiocarcinomas of the MX-treated animals and one hepatocellular carcinoma and cholangiocarcinoma in control animals using PCR-SSCP (polymerase chain reaction-single-strand conformation polymorphism) or PCR-TGGE (temperature gradient gel electrophoresis) and direct sequencing. The expression of the p53 protein (wild-type and mutated protein) was detected by immunohistochemistry (CM5 antibody). The expression of p53 and that of the proliferating cell nuclear antigen (PCNA, 19 A2) were also evaluated in livers of female animals exposed to MX for 1 week, 3 weeks, or 18 weeks. Altogether, four mutations were found in p53 in three tumors, in two hepatocellular adenomas, and one cholangiocarcinoma, all in females receiving the highest MX dose (6. 6 mg/kg/day) of the study. Three of the mutations were G:C --> A:T transitions and one was an A:T --> T:A transversion. The mutations were scattered at different codons and positions of the codon. One hepatocellular adenoma contained two p53 mutations. All cholangiomas and cholangiocarcinomas, but no hepatocellular adenomas and carcinomas, overexpressed the p53 protein. MX treatment did not induce p53 expression at any age in the liver or alter the expression of the PCNA in the liver of younger animals. The p53 protein was overexpressed in hyperplastic bile ducts in aged rats but not in bile ducts of younger rats (up to 24 weeks). No mutations were observed in either Ki-ras, Ha-ras, or N-ras of the liver tumors. These data suggest that point mutations in p53, Ki-ras, Ha-ras, and N-ras are not involved in the MX-induced liver carcinogenesis in rats.

Tobacco smoke carcinogens and lung cancer

The complexity of tobacco smoke leads to some confusion about the mechanisms by which it causes lung cancer. Among the multiple components of tobacco smoke, 20 carcinogens convincingly cause lung tumors in laboratory animals or humans and are, therefore, likely to be involved in lung cancer induction. Of these, polycyclic aromatic hydrocarbons and the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone are likely to play major roles. This review focuses on carcinogens in tobacco smoke as a means of simplifying and clarifying the relevant information that provides a mechanistic framework linking nicotine addiction with lung cancer through exposure to such compounds. Included is a discussion of the mechanisms by which tobacco smoke carcinogens interact with DNA and cause genetic changes--mechanisms that are reasonably well understood--and the less well defined relationship between exposure to specific tobacco smoke carcinogens and mutations in oncogenes and tumor suppressor genes. Molecular epidemiologic studies of gene-carcinogen interactions and lung cancer--an approach that has not yet reached its full potential--are also discussed, as are inhalation studies of tobacco smoke in laboratory animals and the potential role of free radicals and oxidative damage in tobacco-associated carcinogenesis. By focusing in this review on several important carcinogens in tobacco smoke, the complexities in understanding tobacco-induced cancer can be reduced, and new approaches for lung cancer prevention can be envisioned.

Association of p53, K-ras and proliferating cell nuclear antigen with rat lung lesions following exposure to simulated nuclear fuel particles

Expression of p53, K-ras, and proliferating cell nuclear antigen (PCNA) and mutations of p53 and K-ras genes in lung lesions of Han/Wistar rats were investigated by immunohistochemistry and direct DNA sequencing following a long-term exposure of animals to neutron-activated UO2 particles. The p53 protein was overexpressed in all five malignant tumors, in 62% of benign tumors, and in 42% of hyperplastic lesions examined. K-ras protein and PCNA levels were only slightly elevated in all types of lung lesions. In three malignant tumors a C-->T transition was detected in codon 288 (human 290) of the p53 gene, but this mutation was not present in seven other tumors analyzed. No mutations were detected in codons 12/13 and 61 of the K-ras gene in any of the five tumors analyzed. Our findings suggest that K-ras overexpression is a rare alteration, whereas p53 protein overexpression (sometimes associated with mutated p53 gene), as assessed with the CM5 antibody, is a relatively common phenomenon in hot particle-induced preneoplastic and neoplastic rat lung lesions.

K-ras mutations in 239PuO2 canine lung neoplasms

Single-strand conformational polymorphism (SSCP) analysis and direct sequencing methods were used to examine lung tumors derived from a cohort of beagle dogs with inhalational exposures to 239PuO2. These exposures were done at Pacific Northwest Laboratories where 18-month-old beagle dogs were given 239PuO2 by single-dose inhalation and allowed to live out their life-spans. Formalin-fixed paraffin-embedded blocks of tissues from 25 dogs exposed to 239PuO2 by aerosol inhalation which later developed lung tumors were available for this study. Two of 25 tumors had mutations within exon 1 of K-ras detected by SSCP analysis. Both mutations were GGT to GAT transitions at codon 12 confirmed by direct sequencing experiments. One was an adenocarcinoma from the medium-high exposure group and the other was a broncheolo-alveolar carcinoma from the medium-low exposure group. The rate of K-ras mutations in plutonium-induced lung tumors described herein (8%) was greater than previously described in canine plutonium-induced lung tumors (0%), but was less than that which we have described in spontaneous canine lung cancer (16%), less than that reported for human spontaneous non-small cell lung cancer (13-36%) and less than that described in rats with spontaneous lung cancer (40%) or lung tumors following 239Pu inhalation exposure (46%).

Role of the cytosine DNA-methyltransferase and p16INK4a genes in the development of mouse lung tumors

CpG island methylation is an epigenetic modification of DNA associated with the silencing of gene transcription. The p16INK4a (p16) tumor suppressor gene is inactivated in human non-small cell lung cancers (NSCLCs) by either homozygous deletion or aberrant methylation. Inactivation of tumor suppressor genes by methylation has been linked in part to altered activity of the cytosine DNA-methyltransferase (DNA-MTase), the enzyme that catalyzes DNA methylation at CpG sites. The purpose of these studies was to define the role of DNA-MTase and p16 in the development of murine lung cancer. DNA-MTase activity was determined in alveolar type II and Clara cells from A/J and C3H mice that exhibit high and low susceptibility, respectively, for lung tumor formation. Increased DNA-MTase activity leading to an increase in overall DNA methylation was found only in alveolar type II cells, the target for murine adenocarcinomas. Both DNA-MTase and DNA methylation changes were detected 7 days after carcinogen exposure and, thus, were early events in neoplastic evolution. In addition, enzyme activity increased incrementally during lung cancer progression. Expression of p16 was detected in all primary lung tumors from A/J mice; however, levels of expression differed by up to 15-fold between tumors. The apparent low levels of expression seen in approximately half of the tumors was not attributed to methylation of the p16 gene. In contrast to the detection of p16 expression in primary tumors, this gene was deleted in four tumor-derived cell lines induced in the A/J mouse by NNK. The results from these studies indicate that the modulation of DNA-MTase activity was cell specific, segregated with susceptibility, and occurred early in neoplastic evolution. Thus, the marked increase in enzyme activity detected in alveolar type II cells after carcinogen treatment could be a major factor contributing to the high susceptibility for chemical-induced neoplasia associated with the A/J mouse strain. The inactivation of the p16 gene in murine cancers induced by NNK most likely arises as a late event via homozygous deletion.