The formation of 3-methylcholanthrene-initiated lung tumors correlates with induction of cytochrome P450IA1 by the carcinogen in fetal but not adult mice

Dibenzo[def,p]chrysene transplacental carcinogenesis in wild-type, Cyp1b1 knockout, and CYP1B1 humanized mice

The cytochrome P450 (CYP) 1 family is active toward numerous environmental pollutants, including polycyclic aromatic hydrocarbons (PAHs). Utilizing a mouse model, null for Cyp1b1 and expressing human CYP1B1, we tested the hypothesis that hCYP1B1 is important for dibenzo[def,p]chrysene (DBC) transplacental carcinogenesis. Wild-type mCyp1b1, transgenic hCYP1B1 (mCyp1b1 null background), and mCyp1b1 null mice were assessed. Each litter had an equal number of siblings with Ahr^b-1/d and Ahr^d/d alleles. Pregnant mice were dosed (gavage) on gestation day 17 with 6.5 or 12 mg/kg of DBC or corn oil. At 10 months of age, mortality, general health, lymphoid disease and lung tumor incidence, and multiplicity were assessed. hCYP1B1 genotype did not impact lung tumor multiplicity, but tended to enhance incidence compared to Cyp1b1 wild-type mice (P = 0.07). As with Cyp1b1 in wild-type mice, constitutive hCYP1B1 protein is non-detectable in liver but was induced with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Wild-type mice were 59% more likely to succumb to T-cell Acute Lymphoblastic Leukemia (T-ALL). Unlike an earlier examination of the Ahr genotype in this model (Yu et al., Cancer Res, 2006;66:755-762), but in agreement with a more recent study (Shorey et al., Toxicol Appl Pharmacol, 2013;270:60-69), this genotype was not associated with lung tumor incidence, multiplicity, or mortality. Sex was not significant with respect to lung tumor incidence or mortality but males exhibited significantly greater multiplicity. Lung tumor incidence was greater in mCyp1b1 nulls compared to wild-type mice. To our knowledge, this is the first application of a humanized mouse model in transplacental carcinogenesis. © 2016 Wiley Periodicals, Inc.

Transplacental carcinogenesis with dibenzo[def,p]chrysene (DBC): timing of maternal exposures determines target tissue response in offspring

Dibenzo[def,p]chrysene (DBC) is a transplacental carcinogen in mice (15mg/kg; gestation day (GD) 17). To mimic residual exposure throughout pregnancy, dams received four smaller doses of DBC (3.75mg/kg) on GD 5, 9, 13 and 17. This regimen alleviated the previously established carcinogenic responses in the thymus, lung, and liver. However, there was a marked increase in ovarian tumors (females) and hyperplastic testes (males). [(14)C]-DBC (GD 17) dosing revealed transplacental distribution to fetal tissues at 10-fold lower concentrations than in paired maternal tissue and residual [(14)C] 3weeks post-dose. This study highlights the importance of developmental stage in susceptibility to environmental carcinogens.

Transplacental benzene exposure increases tumor incidence in mouse offspring: possible role of fetal benzene metabolism

Childhood cancer is the leading cause of disease-related death in children aged 1-14 years in Canada and the USA and it has been hypothesized that transplacental exposure to environmental carcinogens such as benzene may contribute to the etiology of these cancers. Our objectives were to determine if transplacental benzene exposure increased tumor incidence in mouse offspring and assess fetal benzene metabolism capability. Pregnant CD-1 and C57Bl/6N mice were given intraperitoneal injections of corn oil, 200 mg/kg, or 400 mg/kg benzene on gestational days 8, 10, 12 and 14. A significant increase in tumor incidence was observed in CD-1, but not C57BL/6N, 1-year-old offspring exposed transplacentally to 200 mg/kg benzene. Hepatic and hematopoietic tumors were predominantly observed in male and female CD-1 offspring, respectively. Female CD-1 offspring exposed transplacentally to 200 mg/kg benzene had significantly suppressed bone marrow CD11b(+) cells 1 year after birth, correlating with reduced colony-forming unit granulocyte/macrophage numbers in 2-day-old pups. CD-1 and C57Bl/6N maternal blood benzene levels and fetal liver benzene, t, t-muconic acid, hydroquinone and catechol levels were analyzed by gas chromatography/mass spectrometry. Significant strain-, gender- and dose-related differences were observed. Male CD-1 fetuses had high hydroquinone levels, whereas females had high catechol levels after maternal exposure to 200 mg/kg benzene. This is the first demonstration that transplacental benzene exposure can induce hepatic and hematopoietic tumors in mice, which may be dependent on fetal benzene metabolism capability.

Strain-specific induction of murine lung tumors following in utero exposure to 3-methylcholanthrene

Fetal mice are more sensitive to chemical carcinogens than are adults. We previously demonstrated that resistant offspring of a DBA/2 x (C57BL/6 x DBA2) backcross exhibited a high incidence of lung tumors 12-13 mo after transplacental exposure to 3-methylcholanthrene (MC). We compared the effects of in utero treatment with MC on lung tumor incidence in the offspring of intermediately susceptible BALB/c (C), resistant C57BL/6 (B6), and reciprocal crosses between these strains. Pregnant mice were treated with 45 mg/kg of MC on day 17 of gestation and tumor incidence, multiplicity, and the Ki-ras mutational spectrum determined in the offspring 12-18 mo after birth. Tumor incidences in C mice and reciprocal crosses were 86% and 100%, respectively, while B6 mice demonstrated resistance to tumorigenesis, with a tumor incidence of 11%. Tumor multiplicities in C, B6C, CB6, and B6 mice were 3.3 +/- 3.2, 5.8 +/- 3.2, 5.0 +/- 2.7, and <0.1, respectively. Ki-ras mutations, which occurred chiefly in the K(s) allele (96%), were found in 79-81% of reciprocally crossed F1 mice, 64% of C mice, and 50% of B6 mice, with the Val(12), Asp(12), and Arg(13) mutations associated with more aggressive tumors. A subset of these mice was used to demonstrate the utility of computer tomography (CT) for the visualization and measurement of lung tumors in the submillimeter range in vivo. Based on known genetic differences in murine strains for lung cancer, our results suggest the presence of a previously unidentified genetic factor(s) which appears to specifically influence lung tumorigenesis following exposure to carcinogens during fetal development.

Species difference in the induction of hepatic CYP1A subfamily enzymes, especially CYP1A2, by 2-methoxy-4-nitroaniline among rats, mice, and guinea pigs

Species difference in the induction of hepatic cytochrome P450 CYP1A subfamily enzymes by 2-methoxy-4-nitroaniline (2-MeO-4-NA) was investigated among male F344 rats, C57BL/6 Cr mice, and Hartley guinea pigs. All species of animals were treated with a single ip injection of 2-MeO-4-NA (0.44 mmol/kg body weight), and changes in levels of the mRNA and protein of hepatic cytochrome P4501A (CYP1A) subfamily enzymes were examined by the methods of RT-PCR and Western blot, respectively. In addition, hepatic microsomal enzyme activities were measured using methoxyresorufin and ethoxyresorufin as substrates of CYP1A2 and CYP1A1, respectively. The overall results of the RT-PCR, Western blot, and measurement of the enzyme activity indicated that 2-MeO-4-NA-mediated induction of hepatic CYP1A subfamily enzymes, especially CYP1A2, occurred only in rats but not any other species of animals examined and that the species difference in the CYP1A induction was not necessarily correlated with that in pharmacokinetics of 2-MeO-4-NA. Furthermore, a luciferase reporter gene assay for screening of the ligands of arylhydrocarbon receptor (AhR) using a rat hepatic cell line suggested that 2-MeO-4-NA is not an AhR ligand. The present findings demonstrate for the first time the species difference in the 2-MeO-4-NA-mediated induction of hepatic CYP1A subfamily enzymes between rats and other rodents, mice and guinea pigs, and further propose an AhR-independent pathway for 2-MeO-4-NA-mediated induction in rats.

Expression of glutathione S-transferases in fetal lung and liver tissue from parental strains and F1 crosses between C57BL/6 and BALB/c F1 mice following in utero exposure to 3-methylcholanthrene

GST isoforms have been extensively studied in adult tissues but little is known about the composition and levels of these enzymes in fetal tissues. As part of our ongoing studies to determine the potential role of metabolic enzymes in mediating the differential susceptibility of different strains of mice to lung tumorigenesis following in utero exposure to 3-methylcholanthrene (MC), we screened for GST enzyme activity and for expression of the individual GSTalpha, pi, mu, and theta isoforms in murine fetal lung and liver tissues isolated from the parental strains and F1 crosses between C57BL/6 (B6) and BALB/c (C) mice. Using 1-chloro-2,4-dinitrobenzene (CDNB) as a substrate, we found that treatment with MC had no effect on the levels of GST enzyme activity in either the fetal lung or liver in either of the two parental strains or their F1 crosses. Low levels of expression of each of the four enzymes were detected by Western blotting in both fetal lung and liver tissues in all four strains. A statistically significant 3.5-fold induction was observed only for GSTmu in the fetal lung of the parental strain of BALB/c mice 48 h after exposure to MC. None of the other enzymes showed any significant differences in the levels of expression following exposure to MC. Although strain-specific differences in the expression of the GSTs that were independent of MC treatment were observed, they could not account for the differences previously observed in either the Ki-ras mutational spectrum or lung tumor incidence in the different strains of mice. Similar results were obtained when the maternal metabolism of MC was assayed in liver microsomal preparations. The results are consistent with previous studies showing low levels and poor inducibility of phase II enzymes during gestation, and demonstrate for the first time that all four of the major GST enzymes are expressed in fetal tissues. While the high inducibility of activating enzymes, such as Cyp1a1, and low, uninducible levels of phase II conjugating enzymes probably account for the high susceptibility of the fetus to transplacentally induced tumor formation, the results also suggest that factors other than metabolism may account for the strain-specific differences in susceptibility to carcinogen-mediated lung tumor induction following in utero exposure to chemical carcinogens.

In utero Exposure of Mice to Dibenzo[a,l]Pyrene Produces Lymphoma in the Offspring: Role of the Aryl Hydrocarbon Receptor

Lymphoma and leukemia are the most common cancers in children and young adults; in utero carcinogen exposure may contribute to the etiology of these cancers. A polycyclic aromatic hydrocarbon (PAH), dibenzo[a,l]pyrene (DBP), was given to pregnant mice (15 mg/kg body weight, gavage) on gestation day 17. Significant mortalities in offspring, beginning at 12 weeks of age, were observed due to an aggressive T-cell lymphoblastic lymphoma. Lymphocytes invaded numerous tissues. All mice surviving 10 months, exposed in utero to DBP, exhibited lung tumors; some mice also had liver tumors. To assess the role of the aryl hydrocarbon receptor (AHR) in DBP transplacental cancer, B6129SF1/J (AHR(b-1/d), responsive) mice were crossed with strain 129S1/SvIm (AHR(d/d), nonresponsive) to determine the effect of maternal and fetal AHR status on carcinogenesis. Offspring born to nonresponsive mothers had greater susceptibility to lymphoma, irrespective of offspring phenotype. However, when the mother was responsive, an AHR-responsive phenotype in offspring increased mortality by 2-fold. In DBP-induced lymphomas, no evidence was found for TP53, beta-catenin, or Ki-ras mutations but lung adenomas of mice surviving to 10 months of age had mutations in Ki-ras codons 12 and 13. Lung adenomas exhibited a 50% decrease and a 35-fold increase in expression of Rb and p19/ARF mRNA, respectively. This is the first demonstration that transplacental exposure to an environmental PAH can induce a highly aggressive lymphoma in mice and raises the possibility that PAH exposures to pregnant women could contribute to similar cancers in children and young adults.

Induction of Cyp1a1 and Cyp1b1 and formation of DNA adducts in C57BL/6, Balb/c, and F1 mice following in utero exposure to 3-methylcholanthrene

Fetal mice are more sensitive to chemical carcinogens than are adults. Previous studies from our laboratory demonstrated differences in the mutational spectrum induced in the Ki-ras gene from lung tumors isolated from [D2 x B6D2F1]F2 mice and Balb/c mice treated in utero with 3-methylcholanthrene (MC). We thus determined if differences in metabolism, adduct formation, or adduct repair influence strain-specific responses to transplacental MC exposure in C57BL/6 (B6), Balb/c (BC), and reciprocal F1 crosses between these two strains of mice. The induction of Cyp1a1 and Cyp1b1 in fetal lung and liver tissue was determined by quantitative fluorescent real-time PCR. MC treatment caused maximal induction of Cyp1a1 and Cyp1b1 RNA 2-8 h after injection in both organs. RNA levels for both genes then declined in both fetal organs, but a small biphasic, secondary increase in Cyp1a1 was observed specifically in the fetal lung 24-48 h after MC exposure in all four strains. Cyp1a1 induction by MC at 4 h was 2-5 times greater in fetal liver (7000- to 16,000-fold) than fetal lung (2000- to 6000-fold). Cyp1b1 induction in both fetal lung and liver was similar and much lower than that observed for Cyp1a1, with induction ratios of 8- to 18-fold in fetal lung and 10- to 20-fold in fetal liver. The overall kinetics and patterns of induction were thus very similar across the four strains of mice. The only significant strain-specific effect appeared to be the relatively poor induction of Cyp1b1 in the parental strain of B6 mice, especially in fetal lung tissue. We also measured the levels of MC adducts and their disappearance from lung tissue by the P(32) post-labeling assay on gestation days 18 and 19 and postnatal days 1, 4, 11, and 18. Few differences were seen between the different strains of mice; the parental strain of B6 mice had nominally higher levels of DNA adducts 2 (gestation day 19) and 4 (postnatal day 1) days after injection, although this was not statistically significant. These results indicate that differences in Phase I metabolism of MC and formation of MC-DNA adducts are unlikely to account for the marked differences observed in the Ki-ras mutational spectrum seen in previous studies. Further, the results suggest that other genetic factors may interact with chemical carcinogens in determining individual susceptibility to these agents during development.

Predictive values of traditional animal bioassay studies for human perinatal carcinogenesis risk determination

The many physiological, biochemical, and structure differences between rodents and humans, especially with regard to gestation and fetal development, invite questions as to the utility of rodent models for the prediction of risk of perinatal carcinogenesis in humans and for extrapolation of mechanistic studies. Here, the relevance of basic generalities, derived from rodent perinatal studies, to human contexts is considered. The cross-species usefulness of these generalities was upheld by the example of carcinogen activation and detoxification as determining factors. These have been established in rodent studies and recently indicted in humans by investigations of genetic polymorphisms in cytochromes P450, N-acetyltransferase, myeloperoxidase, quinone reductase, and glutathione S-transferase. Also, published data have been analyzed comparatively for diethylstilbestrol and irradiation, the two known human transplacental carcinogenic agents. At similar doses to those experienced by humans, both diethylstilbestrol and X- and gamma-irradiation in rodents and dogs yielded increased tumors at rates similar to those for humans. In rodents, there was a clearly negative relationship between total diethylstilbestrol dose and tumors per dose unit, and a similar pattern was suggested for radiation. Diethylstilbestrol had transgenerational effects that did not diminish over three generations. Overall, this analysis of the published literature indicates that there are basic qualitative and quantitative similarities in the responsiveness of human and rodent fetuses to carcinogens, and that dose effects may be complex and in need of further investigation.

Transplacental lung carcinogenesis: molecular mechanisms and pathogenesis

A wide variety of studies in both animal models and human populations have demonstrated age-related differences in the susceptibility of the developing organism to environmentally prevalent toxicants. While this differential susceptibility has been clearly established, the mechanistic basis for these age-related differences is still poorly understood. The developing fetus utilizes many of the same metabolic and signaling pathways as adult organisms in responding to environmental agents. However, it is becoming increasingly evident that the fetus is not a "little adult" and exhibits unique biochemical responses and gene expression profiles to chemical and physical agents. Because of the rapid growth and developmental changes that occur during gestation, the fetus represents a particularly challenging research subject as a result of the dynamic alterations that occur in gene expression pathways as gene systems are activated or repressed during specific stages of development. Thus, an understanding of the mechanism(s) that render the developing organism more or less susceptible to specific carcinogenic agents is crucial for both regulatory decisions regarding the determination of safe levels of toxic chemicals released into the environment and also for determining the effects of therapeutic compounds in younger age groups and pregnant women. Concentrating on studies from the author's laboratory, this review will highlight recent research on the molecular pathogenesis of transplacentally induced tumors. While focusing on the lung, other animal models and recent human epidemiological studies will also be discussed to contrast similarities and differences in the developing and adult organisms in terms of responses to toxic chemicals, including metabolism of environmentally prevalent toxicants and alterations in gene systems at the molecular level.

A framework for assessing risks to children from exposure to environmental agents

In recent years there has been an increasing focus in environmental risk assessment on children as a potentially susceptible population. There also has been growing recognition of the need for a systematic approach for organizing, evaluating, and incorporating the available data on children's susceptibilities in risk assessments. In this article we present a conceptual framework for assessing risks to children from environmental exposures. The proposed framework builds on the problem formulation-->analysis-->risk characterization paradigm, identifying at each phase the questions and issues of particular importance for characterizing risks to the developing organism (from conception through organ maturation). The framework is presented and discussed from the complementary perspectives of toxicokinetics and toxicodynamics.

Induction of a high incidence of lung tumors in C57BL/6 mice with multiple ethyl carbamate injections

Murine pulmonary adenomas progress to malignancy with many similarities to human pulmonary adenocarcinoma, the most common form of lung cancer. Inbred mice vary in their susceptibility to lung tumor development, and induced genetic modifications are a powerful tool for understanding this susceptibility. Many transgenic and null mutations relevant to lung cancer pathogenesis were derived on the highly resistant C57BL/B6 (B6) background. Since the inability to reliably induce lung tumors in B6 mice limits these studies, we systematically examined several carcinogenesis protocols in B6 mice. Ten weekly ethyl carbamate (EC) doses caused a nearly 100% lung tumor incidence with a tumor multiplicity >2; multiple EC dosing is thus an alternative to the time-consuming transfer of transgenes and null mutations to susceptible backgrounds.

Alterations at the Ink4a locus in transplacentally induced murine lung tumors

The malignant phenotype results from multiple genetic alterations, including the activation of oncogenes and inactivation of tumor suppressor genes. Activation of the Ki-ras oncogene has been implicated as an early event in the pathogenesis of lung adenocarcinomas in humans and experimental animal models. Previous studies from this laboratory have shown that, following treatment of pregnant [D2 x B6D2F(1)]F(2) or Balb/c mice with the polycyclic aromatic hydrocarbon, 3-methylcholanthrene (MC), lung tumors from the transplacentally exposed offspring exhibited a high incidence of mutations in the Ki-ras gene. The role of genetic alterations at other oncogenic or tumor suppressor loci that can mediate lung tumor initiation and/or progression have not been well characterized in either human or murine models. Using the transplacental carcinogenesis model, which results in the induction of both lung and liver tumors following in utero exposure to MC, the results of this and our previous studies show that alterations in the Ink4a locus occur in only 15 and 27% of the lung and liver tumors, respectively. Preliminary data also suggests that the type of mutation induced in the Ki-ras gene following the initial exposure to MC may influence lung tumor progression. These results imply that damage to the Ink4a gene is not a frequent pathway to malignant progression in mouse lung and liver tumors following in utero exposure to environmental carcinogens.

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.

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.

Role of tumor suppressor genes in transplacental lung carcinogenesis

Most human cancers involve multiple genetic changes, including activation of oncogenes such as Ki-ras-2 (Kras2) and inactivation of any one of a number of tumor suppressor genes such as p53 and members of the retinoblastoma (Rb) regulatory axis. As part of an ongoing project to determine how in utero exposure to chemical carcinogens affects the molecular pathogenesis of murine lung tumors, the p53 and p16Cdkn2a genes were analyzed by using paraffin-embedded lung tissues from mice treated transplacentally with 3-methylcholanthrene. Single-strand conformation polymorphism analysis of exons 5-8 of the p53 gene, as well as their flanking introns, demonstrated an absence of mutations at this gene locus. However, a genetic polymorphism was identified at nt 708 in intron 4 of the DBA/2 strain of mice 5 bp downstream of a 3' branching-point splice signal. Analysis of exons 1 and 2 of the Cdkn2a gene by single-strand conformation polymorphism and sequence analyses revealed mutations in exon 2 in 7% of the tumors examined. Tumor 23-1 exhibited a CAC-->TAC transition at nt 301 (His74-->Tyr74), and tumor 36-1 exhibited a GGG-->GAG transition at nucleotide 350 (Gly90-->Glu90). Northern blot analysis of 14 of the larger tumors showed a marked decrease in the levels of Rb RNA expression. Immunohistochemical analysis revealed a spectrum of pRb expression, with the smaller adenomas showing moderate numbers of nuclei with heterogeneous staining for pRb in contrast with a highly reduced or near-complete absence of expression in the nuclei of larger tumors with features of adenocarcinomas. The low incidence of mutations at tumor suppressor loci suggested that inactivation of tumor suppressor genes was a late event in murine lung tumor pathogenesis. The identification of both mutations at the Cdkn2a gene locus and reduced levels of Rb expression combined with previous studies demonstrating a high incidence of mutated Kras2 alleles in these tumors implies that alterations of the Rb regulatory axis, in combination with mutation of Kras2, may be the preferred pathway for the pathogenesis of pulmonary tumors in transplacentally exposed mice.

Expression of the cytochrome P4502E and 2B gene families in the lungs and livers of nonpregnant, pregnant, and fetal hamsters

Members of the cytochrome P4502E and 2B gene families have been implicated in the activation of nitrosamines to reactive species capable of binding to cellular macromolecules and initiating tumor formation in various rodent species. This study was initiated to determine the relative prevalence of these isozymes and their response to ethanol during pregnancy and late gestation. Nonpregnant and pregnant hamsters were given a 10% ethanol solution in their drinking water for 10 days (gestation days 5-15) prior to being killed. RNA blot analysis of liver and lung tissue from nonpregnant, pregnant, and fetal hamsters demonstrated tissue-specific expression of CYP2E and 2B in adult and fetal animals. The levels of RNA expression of both P450s in fetal hamsters were less than 30% of nonpregnant adult values. In pregnant hamsters, the hepatic levels of CYP2E and 2B RNAs were decreased compared to nonpregnant animals. In contrast, the pulmonary levels of CYP2B RNA were increased in pregnant versus non-pregnant hamsters, while no effect of pregnancy on the levels of CYP2E RNA was seen. Although rats contain a single CYP2E1 gene transcript. Northern analysis demonstrated the presence of 1.8 and 2.8 kb bands in both liver and lung tissue of the hamster. Pretreatment with ethanol had little effect on the levels of either P450 RNA species in the lungs or livers of nonpregnant, pregnant, and fetal hamsters. These results demonstrate differences in the levels of expression of members of the CYP2E and 2B gene families during pregnancy and late gestation compared to nonpregnant adult hamsters. Fetal animals, like the adults, apparently respond to ethanol treatment by altering the levels of these P450 isozymes at the post-transcriptional level.

Induction of cytochrome P-450IA1 in fetal rat liver by a single dose of 3-methylcholanthrene

Pregnant Sprague-Dawley rats were treated with a single ip dose of either olive oil or 40 mg/kg of 3-methylcholanthrene on gestation day 20 and sacrificed at various times after injection. Determination of aryl hydrocarbon hydroxylase activity 24 hr after injection revealed that treatment with 3-methylcholanthrene resulted in a 10.5-fold stimulation of enzymatic activity in liver 800 x g supernatants. Western blot analysis with monoclonal antibody 1-7-1 confirmed these results and demonstrated the presence of a 3-methylcholanthrene-inducible P-450 isozyme. Using Northern and slot blot techniques, the induction of steady-state levels of CYPIA1 RNA was shown to occur as early as 4 hr following 3-methylcholanthrene injection. CYPIA1 RNA levels were induced 31.6-fold over values obtained from oil-treated tissues at this time. This appears to be the optimal time to study changes in the levels of CYPIA1 RNA gene expression in the fetus following transplacental exposure to polycyclic aromatic hydrocarbons. By 12 to 24 hr postinjection, the induction of CYPIA1 RNA levels declined to 3.5- to 8.5-fold above control values. These results demonstrate that the kinetics of induction of the CYPIA1 gene during the fetal period differed from that seen in adults.