Mutations of the Ki-ras protooncogene in 3-methylcholanthrene and urethan-induced and butylated hydroxytoluene-promoted lung tumors of strain A/J and SWR mice

Pulmonary Inflammation and KRAS Mutation in Lung Cancer

Chronic lung infection and lung cancer are two of the most important pulmonary diseases. Respiratory infection and its associated inflammation have been increasingly investigated for their role in increasing the risk of respiratory diseases including chronic obstructive pulmonary disease (COPD) and lung cancer. Kirsten rat sarcoma viral oncogene (KRAS) is one of the most important regulators of cell proliferation, differentiation, and survival. KRAS mutations are among the most common drivers of cancer. Lung cancer harboring KRAS mutations accounted for ~25% of the incidence but the relationship between KRAS mutation and inflammation remains unclear. In this chapter, we will describe the roles of KRAS mutation in lung cancer and how elevated inflammatory responses may increase KRAS mutation rate and create a vicious cycle of chronic inflammation and KRAS mutation that likely results in persistent potentiation for KRAS-associated lung tumorigenesis. We will discuss in this chapter regarding the studies of KRAS gene mutations in specimens from lung cancer patients and in animal models for investigating the role of inflammation in increasing the risk of lung tumorigenesis driven primarily by oncogenic KRAS.

Activation of liver X receptor inhibits the development of pulmonary carcinomas induced by 3-methylcholanthrene and butylated hydroxytoluene in BALB/c mice

We previously reported that LXR ligand, T0901317, inhibited the growth of inoculated Lewis lung carcinoma in C57BL/6 mice by activating IFN-γ production. However, the effects of T0901317 on carcinogen-induced pulmonary carcinomas remain unknown. In this study, we initially conducted a statistical analysis on the data of human lung cancer samples extracted from the TCGA database, and determined that survival rate/time of lung cancer patients and grade of lung adenocarcinoma were positively and negatively related to lung IFN-γ levels, respectively. We then determined the inhibitory effects of T0901317 on mouse pulmonary carcinomas induced by 3-methylcholanthrene (MCA) and butylated hydroxytoluene (BHT) or urethane. We found that T0901317 reduced morbidity and mortality in MCA/BHT-injected BALB/c mice by inhibiting lung adenocarcinoma. T0901317 also protected C57BL/6 mice, but not IFN-γ deficient (IFN-γ^−/−, C57BL/6 background) mice, against MCA/BHT-induced lung hyperplasia/inflammation. In addition, we determined that T0901317 inhibited urethane-induced lung tumors in BABL/c mice. Furthermore, we determined that T0901317 prevented metastasis of 4T1 breast cancer cells in BALB/c mice. Administration of T0901317 substantially increased serum IFN-γ levels and lung IFN-γ expression in BABL/c and C57BL/6 mice. Taken together, our study demonstrates that LXR inhibits MCA/BHT-induced pulmonary carcinomas in BABL/c mice and the inhibition is associated with induction of IFN-γ production.

Differential roles of STAT3 in the initiation and growth of lung cancer

Signal transducer and activator of transcription 3 (STAT3) is linked to multiple cancers, including pulmonary adenocarcinoma. However, the role of STAT3 in lung cancer pathogenesis has not been determined. Using lung epithelial-specific inducible knockout strategies, we demonstrate that STAT3 has contrasting roles in the initiation and growth of both chemically and genetically induced lung cancers. Selective deletion of lung epithelial STAT3 in mice before cancer induction by the smoke carcinogen, urethane, resulted in increased lung tissue damage and inflammation, K-Ras oncogenic mutations and tumorigenesis. Deletion of lung epithelial STAT3 after establishment of lung cancer inhibited cancer cell proliferation. Simultaneous deletion of STAT3 and expression of oncogenic K-Ras in mouse lung elevated pulmonary injury, inflammation and tumorigenesis, but reduced tumor growth. These studies indicate that STAT3 prevents lung cancer initiation by maintaining pulmonary homeostasis under oncogenic stress, whereas it facilitates lung cancer progression by promoting cancer cell growth. These studies also provide a mechanistic basis for targeting STAT3 to lung cancer therapy.

Use of the γH2AX assay for assessing the genotoxicity of polycyclic aromatic hydrocarbons in human cell lines

The development of in vitro genotoxic assays as an alternative method to animal experimentation is of growing interest in the context of the implementation of new regulations on chemicals. However, extrapolation of toxicity data from in vitro systems to in vivo models is hampered by the fact that in vitro systems vary in their capability to metabolize chemicals, and that biotransformation can greatly influence the experimental results. Therefore, much attention has to be paid to the cellular models used and experimental conditions. Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic ubiquitous pollutants. Human exposure to PAHs is mainly from food origin. In this study, a detailed analysis of the biotransformation capabilities of three human cell lines commonly used for in vitro testing (HepG2, ACHN and Caco-2) was undertaken using 3 model PAHs (benzo(a)pyrene [B(a)P], fluoranthene [FLA] and 3-methylcholanthrene [3-MC]). Concomitantly the genotoxicity of these PAHs was investigated in different cell lines, using a new genotoxic assay (H2AX) in 96-well plates. The metabolic rates of B(a)P, FLA and 3-MC were similar in HepG2 and Caco-2 cell lines, respectively, though with the production of different metabolites. The ACHN cell line was shown to express very limited metabolic capabilities. We demonstrated that the PAHs having a high metabolic rate (B(a)P and 3-MC) were genotoxic from 10(-7) molar in both HepG2 and Caco-2 cells. The present study shows that H2AX measurement in human cell lines competent for the metabolism, is an efficient and sensitive genotoxic assay requiring less cells and time than other currently available tests.

Mutagenicity of 3-methylcholanthrene, pcb3, and 4-oh-pcb3 in the lung of transgenic bigblue rats

Recent findings of high levels of predominantly lower chlorinated biphenyls in indoor and outdoor air open the question of possible health consequences. Lower chlorinated biphenyls are more readily metabolized to reactive and potentially harmful intermediates, acting as mutagens and cancer initiators. The goal of this study was to assess the mutagenicity of PCB3 in the lungs of rats. Male BigBlue® 334 Fisher transgenic rats, which carry the bacterial lacI gene as a target of mutagenicity, were given intraperitoneal injections of corn oil, 3-methylcholanthrene (3-MC, positive control), 4-monochlorobiphenyl (PCB3) or its metabolite 4-hydroxy-PCB3 (4-OH-PCB3) weekly for 4 weeks. Lungs tissue was harvested to determine mutant frequencies, mutation spectra, and pathological changes. 3-MC caused a 15-fold increase in mutant frequency and an increase in transversion type mutations; a very early occurrence of this type of mutation in lung tissue was previously identified in Ki-ras oncogenes of lung tumors from 3-MC exposed mice. The 2-fold increase in the mutant frequency after treatment with PCB3 and 4-OH-PCB3 was not statistically significant, but a shift in the mutation spectra, especially with PCB3, and an increase in mutations outside of the hotspot region for spontaneous mutations (bp 1-400), suggest that PCB3 and possibly 4-OH-PCB3 are mutagenic in the rat lung.

Genetic alterations in cancer knowledge system: analysis of gene mutations in mouse and human liver and lung tumors

Mutational incidence and spectra for genes examined in both human and mouse lung and liver tumors were analyzed using the National Institute of Environmental Health Sciences (NIEHS) Genetic Alterations in Cancer (GAC) knowledge system. GAC is a publicly available, web-based system for evaluating data obtained from peer-reviewed studies of genetic changes in tumors associated with exposure to chemical, physical, or biological agents, as well as spontaneous tumors. In mice, mutations in Kras2 and Hras-1 were the most common events reported for lung and liver tumors, respectively, whether chemically induced or spontaneous. There was a significant difference in Kras2 mutation incidence for spontaneous versus induced mouse lung tumors and in Hras-1 mutation incidence and spectrum for spontaneous versus induced mouse liver tumors. The major gene changes reported for human lung and liver tumors were in KRAS2 (lung only) and TP53. The KRAS2 mutation incidence was similar for spontaneous and asbestos-induced human lung tumors, while the TP53 mutation incidence differed significantly. Aflatoxin B1, hepatitis B virus, hepatitis C virus, and vinyl chloride all caused TP53 mutations in human liver tumors, but the mutation spectrum for each agent differed. The incidence of KRAS2 mutations in human compared to mouse lung tumors differed significantly, as did the incidence of Hras and p53 gene mutations in human compared to mouse liver tumors. Differences observed in the mutation spectra for agent-induced compared to spontaneous tumors and similarities in spectra for structurally similar agents support the concept that mutation spectra can serve as a "fingerprint" of exposure based on chemical structure.

Final report on the safety assessment of BHT(1)

BHT is the recognized name in the cosmetics industry for butylated hydroxytoluene. BHT is used in a wide range of cosmetic formulations as an antioxidant at concentrations from 0.0002% to 0.5%. BHT does penetrate the skin, but the relatively low amount absorbed remains primarily in the skin. Oral studies demonstrate that BHT is metabolized. The major metabolites appear as the carboxylic acid of BHT and its glucuronide in urine. At acute doses of 0.5 to 1.0 g/kg, some renal and hepatic damage was seen in male rats. Short-term repeated exposure to comparable doses produced hepatic toxic effects in male and female rats. Subchronic feeding and intraperitoneal studies in rats with BHT at lower doses produced increased liver weight, and decreased activity of several hepatic enzymes. In addition to liver and kidney effects, BHT applied to the skin was associated with toxic effects in lung tissue. BHT was not a reproductive or developmental toxin in animals. BHT has been found to enhance and to inhibit the humoral immune response in animals. BHT itself was not generally considered genotoxic, although it did modify the genotoxicity of other agents. BHT has been associated with hepatocellular and pulmonary adenomas in animals, but was not considered carcinogenic and actually was associated with a decreased incidence of neoplasms. BHT has been shown to have tumor promotion effects, to be anticarcinogenic, and to have no effect on other carcinogenic agents, depending on the target organ, exposure parameters, the carcinogen, and the animal tested. Various mechanism studies suggested that BHT toxicity is related to an electrophillic metabolite. In a predictive clinical test, 100% BHT was a mild irritant and a moderate sensitizer. In provocative skin tests, BHT (in the 1% to 2% concentration range) produced positive reactions in a small number of patients. Clinical testing did not find any depigmentation associated with dermal exposure to BHT, although a few case reports of depigmentation were found. The Cosmetic Ingredient Review Expert Panel recognized that oral exposure to BHT was associated with toxic effects in some studies and was negative in others. BHT applied to the skin, however, appears to remain in the skin or pass through only slowly and does not produce systemic exposures to BHT or its metabolites seen with oral exposures. Although there were only limited studies that evaluated the effect of BHT on the skin, the available studies, along with the case literature, demonstrate no significant irritation, sensitization, or photosensitization. Recognizing the low concentration at which this ingredient is currently used in cosmetic formulations, it was concluded that BHT is safe as used in cosmetic formulations.

Inhibitory effects of cinnamaldehyde on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced lung carcinogenesis in rasH2 mice

Previously we reported a lack of modification by cinnamaldehyde (CNMA) of development of lung proliferative lesions induced by urethane in CB6F1-TgHras2 (rasH2) mice. In the present study, we re-evaluated CNMA effects using the same rasH2 strain and non-transgenic littermates initiated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Sixteen mice/strain/sex received intraperitoneal NNK injections at a dose of 3 mg/mouse once a week for 2 weeks followed by free feeding of commercial diet containing 5000 ppm CNMA for 26 weeks. Additional groups were maintained without NNK injection and/or CNMA feeding for 28 weeks. Lung tumors were induced by NNK in both rasH2 and non-Tg males and females at incidence ranging from 63 to 100%. CNMA treatment significantly reduced the combined incidence of adenomas and carcinomas from 86 to 31% in rasH2 males (P<0.05), but no significant influence was evident in females. The multiplicity of NNK-induced lung tumors was also significantly reduced in rasH2 males given CNMA (P<0.01). Similar effects were also observed in non-Tg females given CNMA after NNK initiation. The results of our study strongly indicate that CNMA is capable of inhibiting development of NNK-initiated pulmonary tumorigenesis in rasH2 and non-Tg mice.

Etheno-adduct-forming chemicals: from mutagenicity testing to tumor mutation spectra

During the past 25 years, ethenobases have emerged as a new class of DNA lesions with promutagenic potential. Ethenobases were first investigated as DNA reaction products of vinyl chloride, an occupational carcinogen causing angiosarcoma of the liver (ASL). They were subsequently shown to be formed by several carcinogenic agents, including urethane (ethyl carbamate), and more recently, to occur in various tissues of unexposed humans and rodents. The endogenous source of ethenobases in DNA is thought to be a lipid peroxidation (LPO) product. Initial studies on metabolic activation, mutagenicity and carcinogenicity moved to the analyses of the formation of ethenobases in vivo and to the determination of their promutagenic properties. Quantification of etheno adducts in vivo became possible with the development of ultrasensitive techniques of analysis. To study the miscoding properties of ethenobases, the initial assays on the fidelity of replication or of transcription were replaced by site-directed mutagenesis assays in vivo. Ethenobases generate mainly base pair substitution mutations. With the advent of new techniques of molecular biology, mutations were investigated in the ras and p53 genes of tumors induced by vinyl chloride and urethane. In liver tumors induced by vinyl chloride, specific mutational patterns were found in the Ki-ras gene in human ASL, in the Ha-ras gene in hepatocellular carcinoma (HCC) in rats, and in the p53 gene in human and rat ASL. In tumors induced by urethane in mice, codon 61 of the Ha-ras gene (liver, skin) and of the Ki-ras gene (lung) seems to be a characteristic target. These tumor mutation spectra are compatible with the promutagenic properties of etheno adducts and with their formation in target tissues, suggesting that ethenobases can be initiating lesions in carcinogenesis. Another recent focus has been given to the repair of etheno adducts, and DNA glycosylases able to excise these adducts in vitro have been identified. The last two decades have brought ethenobases to light as potentially important DNA lesions in carcinogenesis. More research is needed to better understand the environmental and genetic factors that affect the formation and persistence of ethenobases in vivo.

Genotoxicity of 3-methylcholanthrene in liver of transgenic big Blue mice

Transgenic mice provide a unique tool for studying the tissue specificity and mutagenic potential of chemicals. Because 3-methylcholanthrene (3MC) was found mutagenic in bacteria, clastogenic in bone marrow, and induces DNA adducts in animals, we were interested to determinine whether this xenobiotic provokes (1) cell proliferation, (2) transcriptional activity changes, (3) DNA adducts, and (4) hepatic mutations in transgenic Big Blue mice carrying the lambdaLIZ phage shuttle vector. Big Blue C57/Bl male mice were treated with a single intraperitoneal dose of 80 mg/kg 3MC for 1, 3, 6, 14, or 30 days. Cell proliferation was checked by 5-bromo-2-deoxyuridine labeling and immunohistochemical detection. The maximal increase of the mitotic index was evidenced after 3 days (2.9 times the control value; P < 0.01). The relative nucleus area, reflecting the transcriptional activity, was also the highest in the treated group after 3 days: 1.86 times the control value, on average (P < 0.01). Four major DNA adducts, determined according to the [(32)P]-postlabeling method, were evidenced in liver DNA of treated mice, 6 days after the treatment: the spot intensities increased in a time-dependent manner. The mutant frequency of liver DNA was the highest after 14 days: 20.3 +/- 2.9 x 10(-5) in the treated vs. 7.6 +/- 2.7 x 10(-5) in the control mice (P < 0.01). Sequencing of the lambda lacI mutant plaques showed mainly G:C --> T:A and C:G --> A:T transversions. In conclusion, 3MC at first induced nuclear enlargement and a slight increase of cell proliferation in liver, followed by parallel formation of DNA adducts and mutations. This study shows how transgenic models allow in vivo evaluation of mechanistically simultaneous endpoints.

Strain-dependent lung tumor formation in mice transplacentally exposed to 3-methylcholanthrene and post-natally exposed to butylated hydroxytoluene

The carcinogenic effects of in utero exposure to 3-methylcholanthrene (MC) have been demonstrated in the tumor-resistant C57BL/6 (B6) and DBA (D2) strains of mice. In this study, we determined the effects of in utero exposure to MC in BALB/c mice, a strain which demonstrates greater susceptibility to lung tumor induction, and compared our findings with those previously found in [D2xB6D2F(1)]F(2) mice. In addition, we assessed the molecular pathogenesis of the chemically induced tumors and examined the effects of the putative lung tumor promoter butylated hydroxytoluene (BHT) in BALB/c mice. BALB/c mice were treated on day 17 of gestation with 5, 15 or 45 mg/kg MC and 6 weeks after birth with BHT for 6 consecutive weeks. Mice were killed at 6 months of age. Ki-ras, p16Ink4a and p19ARF gene loci were amplified from paraffin-embedded lung tumor tissue and screened for the presence of point mutations via allele-specific oligonucleotide hybridization and single strand conformation polymorphism (SSCP) analyses. Ki-ras point mutations were found in 56% (20/36) of BALB/c lung tumors, with 33% (2/6) of the hyperplasias, 58% (10/19) of the adenomas and 73% (8/11) of the carcinomas exhibiting point mutations at this gene locus. Similar incidences of Ki-ras mutations were previously found following transplacental exposure of [D2xB6D2F(1)]F(2) mice to MC and treatment of adult A/J mice with urethane. Interestingly, a strain-dependent difference was observed in the mutational spectrum. Sixty-two and 38% of the lung lesions in BALB/c mice exhibited G-->C and G-->T transversions, respectively, in contrast to the 13 and 84% incidences previously observed in [D2xB6D2F(1)]F(2) mice. SSCP analysis of the tumor suppressor gene p16Ink4a showed a 6% incidence of point mutations, consistent with that found in [D2xB6D2F(1)]F(2) mice. No mutations were found in exon 1beta of the p19ARF gene of either strain. BHT, a lung tumor promoter in adult mice, had no statistically significant effects on either tumor incidence, tumor multiplicity or the mutational spectrum produced in the Ki-ras gene by in utero MC treatment. However, though not significant, there was an observable trend in increased tumor multiplicity in mice co-treated with BHT. These data demonstrate the transplacental carcinogenic effect of MC in BALB/c mice and show that mutagenic damage to Ki-ras is a critical early event mediating murine lung tumorigenesis in both the tumor-sensitive and tumor-resistant strains. Unlike what occurs when adult BALB/c mice are treated with MC, BHT does not appear to significantly promote the formation of lung tumors following transplacental exposure to MC, possibly due to the rapid growth and cell proliferation in the developing organism. Strain-dependent differences in the Ki-ras mutational spectrum may be associated with their differential susceptibility to lung tumor initiation.

Susceptibility to urethane carcinogenesis of transgenic mice carrying a human prototype c-Ha-ras gene (rasH2 mice) and its modification by butylhydroxytoluene

The susceptibility of rasH2 mice to urethane lung carcinogenesis and the modifying effects of butylhydroxytoluene (BHT) on development of pulmonary lesions were examined. Single i.p. injections of urethane at 250 mg/kg in males or 500 mg/kg in females induced alveolar/bronchiolar adenomas within 6 weeks. At 4 weeks after the injection with a dose of 1000 mg/kg, adenomas occurred in both sexes. BHT administration increased the multiplicity of hyperplasias observed 3 weeks after the urethane injection and additionally caused adenomas which did not occur in the urethane alone-treated animals. The overall data suggest the possibility of rapid assays for lung carcinogens using rasH2 mice.

Polycyclic aromatic hydrocarbons: correlations between DNA adducts and ras oncogene mutations

This review describes a series of studies on the tumorigenic activities of polycyclic aromatic hydrocarbons (PAHs) in various experimental animal model systems, their abilities to form PAH-DNA adducts in target tissues, and their abilities to mutate ras oncogenes in PAH-induced tumors. The review is limited to those PAHs that do not contain nitrogen, for which ras mutations have been detected in induced tumors, and for which some information is available about the structures of the DNA adducts induced in the target tissue. In general, PAHs that form DNA adducts at deoxyadenosine induce mutations at codon 61, whereas those PAHs that form DNA adducts at deoxyguanosine primarily induce mutations at codons 12 or 13. Those PAHs that induce adducts at both bases induce both types of mutations. These correlations provide evidence for the involvement of adduct-directed mutations in ras in the etiology of these tumors. The induced mutation spectra in ras may in fact point back to the identity of the type of adduct formed.

The relationship between 1,2-dimethylhydrazine dose and the induction of colon tumours: tumour development in female SWR mice does not require a K-ras mutational event

In this study we have investigated the relationship between the dose of 1,2-dimethylhydrazine (DMH) and the yield (and location) of tumours in a mouse strain susceptible to colon tumour induction. Female SWR mice were injected with 6.8 mg/kg DMH i.p. once a week for 1, 5, 10 and 20 weeks and the animals were followed for almost 2 years. Administration of increasing doses of DMH resulted in a dose-dependent decrease in survival time. Colon tumours developed in 26, 76 and 87% of mice given a total dose of 34, 68 and 136 mg/kg DMH, respectively: no tumours were detected in animals treated with a total dose of 6.8 mg/kg. Most colon tumours (79%) were located in the distal colon with the remainder being found in the mid colon and none were detected in either the proximal colon or small intestine. As mutations in the K-ras gene are thought to be key events in the pathogenesis of human and rodent colon tumours, we determined the frequency of codon 12 and 13 K-ras mutations in these tumours by restriction site mutation analysis and/or DNA sequencing. A total of 50 colon tumour samples were analysed for codon 12 mutations and of these 29 were also screened for codon 13 mutations. No mutations were detected in either of these codons. The mutational activation of the K-ras gene is not an essential step in the development of DMH-induced colon tumours in female SWR mice and if similar considerations apply to humans, then the aetiological role of alkylating agents may be underestimated from the prevalence of K-ras GC-->AT transitions in human tumours.

Analysis of the c-myc, K-ras and p53 genes in methylcholanthrene-induced mouse sarcomas

We have examined 63 methylcholanthrene (MCA)-induced mouse sarcomas for possible correlations of mutations involving the c-myc, ras and p53 genes. The c-myc gene was found to be amplified in 18 of these sarcomas (29%). Polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis and subsequent direct sequencing identified 18 cases carrying K-ras mutation at codons 12, 13 and 61 (29%). No mutation was detected in the H-ras and N-ras genes. Mutations of the p53 gene in exons 5 to 8 were found in 45 cases (71%). Comparison of these mutations revealed that out of 18 cases with c-myc gene amplifications, 10 carried K-ras mutations (56%) and 14 carried p53 mutations (78%). In contrast, among 45 cases of sarcomas without c-myc gene amplification, 8 were found to have K-ras mutations (18%). The same 45 cases were found to have 31 p53 mutations (69%). The present study suggests a strong correlation between c-myc gene amplification and K-ras gene mutation (P < 0.01). p53 gene mutation was frequently found among MCA-induced mouse sarcomas, indicating the importance of this mutation in the etiology of these tumors. However, p53 mutations were present in sarcomas regardless of the state of c-myc amplification and K-ras mutation. Therefore, a defect in the p53 gene is independent of amplification of the c-myc gene or point mutation of the K-ras gene.

Molecular pathogenesis of transplacentally induced mouse lung tumors

Previous studies from this and other laboratories have shown that treatment of pregnant mice with 3-methylcholanthrene (MC) caused lung tumors in the offspring, the incidence of which correlated with fetal inducibility of Cyp1a1. Analysis of paraffin-embedded lung tissue for Ki-ras-2 mutations indicated that 79% of the lesions examined contained point mutations in codons 12 and 13 of the Ki-ras-2 gene locus, the majority of which (84%) were G-->T transversions. The mutational spectrum was dependent on the tumor stage, as both the incidence of mutation and type of mutation produced correlated with malignant progression of the tumor. Mutations occurred in 60% of the hyperplasias, 80% of the adenomas, and 100% of the adenocarcinomas. In the tumors with mutations, GLY12-->CYS12 transversions occurred in 100% of the hyperplasias, 42% of the adenomas, and 14% of the adenocarcinomas. GLY12-->VAL12 transversions were not observed in hyperplasias and occurred in 42% of the adenomas and 57% of the adenocarcinomas. The remaining ASP12 and ARG13 mutations occurred only in adenomas (17%) and adenocarcinomas (29%). The tumors were also analyzed for alterations in the structure or function of the tumor suppressor genes Rb, p53, and Cdkn2a. No mutations were observed in exons 5-8 of the p53 gene. SSCP analysis demonstrated that 2 of 15 lung tumors contained shifted bands at the Cdkn2a gene locus. Sequence analysis had identified these as mutations in exon 2, with a CAC-->TAC transition at base 301 (HIS74-->TYR74) in tumor 23-1 and GGG-->GAG transition at base 350 (GLY90-->GLU90) in tumor 36-1. Northern blot analysis of the larger tumors revealed that 14 of 14 of these large lung tumors exhibited markedly decreased expression of Rb gene transcripts. These results were confirmed by immunohistochemistry. The larger tumors, which exhibited features of adenocarcinomas, showed a marked reduction or almost complete absence of nuclear pRb staining compared with smaller adenomas and normal lung tissue. The results suggest that Ki-ras-2 mutations are an early and frequent event in lung tumorigenesis, and that the type of mutation produced by environmental chemicals can influence the carcinogenic potential of the tumor. The results obtained with the Cdkn2a and Rb genes suggest that alterations in the Rb regulatory axis may play a key role in the pathogenesis of the pulmonary tumors and appear to occur later in the neoplastic process. It appears from these experiments that the combination of mutated Ki-ras-2 and alterations in the Rb regulatory gene locus, which are frequent alterations in human lung tumors, may be the preferred pathway for lung tumor pathogenesis in mice exposed transplacentally to environmental carcinogens.

Ki-ras and p53 mutations are early and late events, respectively, in urethane-induced pulmonary carcinogenesis in A/J mice

In the A/J strain of mice, urethane (ethyl carbamate) induces lung hyperplasia, adenoma, and adenocarcinoma in a time-dependent manner. These distinct morphological stages may correlate with sequential molecular genetic changes in this mouse model. To test this hypothesis, we investigated the presence of mutations involving Ki-ras and p53 in urethane-induced lung lesions in A/J mice at early and late stages of tumorigenesis. We precisely microdissected 40 lung lesions from paraffin-embedded sections. Ki-ras mutations around codon 61 and p53 mutations in exons 5-8 were identified by polymerase chain reaction-single-strand conformation polymorphism and DNA sequencing techniques. In 29 early-stage lung lesions classified as hyperplasias (seven) or adenomas (22), we observed 19 Ki-ras mutations (66%), including three silent mutations and one double mutation at different codons, and one silent p53 mutation (3.5%). In 11 late-stage adenomas, we identified nine activating Ki-ras mutations (82%) and four missense p53 mutations (36%). These results indicate that Ki-ras mutations arise early, whereas p53 mutations occur relatively late during the benign stages of urethane-induced lung carcinogenesis in A/J mice.

The activation of K-ras gene at an early stage of lung tumorigenesis in mice

To clarify the exact timing of K-ras gene mutational activation in lung tumorigenesis of mice, we applied a sensitive mutant allele specific amplification (MASA) method to pulmonary DNA from urethane-treated mice. The activation of K-ras gene with 61st codon AT mutation was detected in the lungs of mice at day 14 but not day 7 after urethane treatment by MASA. The mutation of MASA products was also checked by XbaI restriction fragment length polymorphism analysis and DNA sequencing. These data suggest that the mutation of K-ras gene in the lungs of mice treated with urethane occurred at the early stage of lung tumorigenesis.