Differential induction of fetal mouse liver and lung cytochromes P-450 by beta-naphthoflavone and 3-methylcholanthrene

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.

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.

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.

Prenatal expression of N-acetyltransferases in C57Bl/6 mice

Exposure to carcinogens such as 4-aminobiphenyl (4ABP), found in tobacco smoke and other combustion products, results in the formation of detectable levels of 4ABP-hemoglobin adducts in cord blood and 4ABP-DNA adducts in conceptal tissue. The presence of these adducts requires that the parent compound undergo biotransformation. When exposure occurs in utero, the maternal, placental and conceptal tissues are all possible sites for the formation of DNA-reactive products. One step in the activation of 4ABP is catalyzed by N-acetyltransferases (NAT). The expression of NAT was evaluated in gestational day (GD) 10-18 conceptal tissues from C57Bl/6 mice. There was a quantitative increase in NAT1 and NAT2 mRNAs with increasing gestational age that was also reflected in age-related changes in functional protein measured as 4ABP-NAT activity. The ability to acetylate 4ABP increased from GD10 to 18 and was lower in conceptal tissue than in adult liver. The potential toxicologic significance of prenatal NAT expression was assessed by formation of 4ABP-DNA adducts. At GD 15 and 18, 4ABP-DNA adducts were detected by immunohistochemistry 24 h following a single oral dose of 120 mg 4ABP/kg. Based on nuclear fluorescence, conceptual 4ABP-DNA adducts were present at similar levels at GD15 and 18. Levels of 4ABP-DNA adducts were significantly higher in maternal liver compared with the conceptus. Results from this study show that both NAT genes were expressed prenatally and that functional enzymes were present. These data support the possible in situ generation of reactive products by the conceptus. The relative contributions of maternal activation of 4ABP and that by the conceptus remain to be determined.

Pulmonary Cyp1A1 and CYP1A2 levels and activities in adult male and female offspring of rats exposed during gestation and lactation to 2,3,7, 8-tetrachlorodibenzo-p-dioxin

The levels and activities of pulmonary microsomal CYP1A1 and CYP1A2 in 40-day-old male and female, and 120-day-old male offspring of pregnant rats treated with five weekly 0.1 microg/kg doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during gestation and lactation were compared with those in age-matched offspring of untreated dams. The CYP1A1-preferential activity, ethoxyresorufin O-deethylase (EROD), was comparably induced 5.3- and 6.4-fold in 40-day-old male and female offspring, respectively, but was not induced in 120-day-old male offspring, of TCDD-treated dams. Similarly, CYP1A1 protein was induced in 40-day-old female or male offspring of untreated dams but was undetectable in 120-day-old offspring of untreated or treated dams. CYP1A2 activity, as measured by the bioactivation of 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) to mutagens in the Ames assay, was elevated 11.1- and 5.5-fold in 40-day-old female and male offspring, respectively, of TCDD-treated dams, but was unaffected by TCDD exposure in 120-day-old offspring. CYP1A2 protein was undetectable in 40-day-old male or female offspring of untreated dams or in 120-day-old male offspring of treated or untreated dams; it was detected in 40-day-old offspring of treated dams, at a level that was higher in females than in males. The results show that gestational and lactational exposure to TCDD causes long-lasting and gender-preferential induction of CYP1A1 as well as CYPIA2 in the lungs of rat offspring.

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.

Transfer of the food mutagen PhIP to foetuses and newborn mice following maternal exposure

The food mutagen PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) is a liver carcinogen in neonatal mice. In this study the transfer of PhIP to foetuses and neonates following a single ip dose (4.7-5.2 mg/kg body weight) to pregnant or lactating C57Bl/6 mice was studied by tissue extraction and HPLC analysis. A transplacental transfer of unchanged [3H]PhIP to foetuses was demonstrated; the highest foetal levels were observed at late gestation. Autoradiography of mice injected iv with [2-14C]PhIP (1.4 mg/kg) during late gestation showed a high and selective localization of radioactivity in the pigmented parts of the foetal eye and a moderate level of radioactivity in the foetal liver, gastro-intestinal contents, urine and in the uterine fluid. HPLC analysis of stomach contents and tissues of newborn mice exposed for 4 hr to lactating dams dosed ip with [3H]PhIP (5.2 mg/kg) showed the presence of unchanged PhIP suggesting excretion of PhIP in the milk of the dams. The results in this study raise concern that exposure to PhIP during pregnancy and nursing may result in a transfer of this food mutagen to foetuses and infants.

Developmental expression of cytochrome P450s within intrasplenically transplanted fetal hepatocytes from spontaneously hypertensive rats

Fetal hepatocytes were harvested at 20 days of gestation from spontaneously hypertensive rats (SHR) and then transplanted into recipient adult SHR spleens. Morphological examination of the recipient spleens revealed that after 4 wk, large masses of hepatocytes were present in the red pulp with apparent cord-like structures. Of major significance was the fact that hepatocyte transplanted spleens were able to express several families of cytochrome P450 (cyto P450) proteins 2-6 wk after transplantation. Thus, the transplanted hepatocyte tissue appeared to grow and develop a cyto P450 metabolic system. Immunochemical determinations revealed that cytos P450 IA1, P450 IIB1, P450 p, P450 HLp, and P450 LA omega were over detected without any prior induction. None of the cyto P450s could be detected in control nontransplanted spleens, however, all were detected in fetal hepatocyte microsomes before transplantation. All were intensely expressed 6 wk after transplantation, however P450 IA1 and P450 IIB1 did not appear to be expressed by 2 wk after transplantation. These results demonstrate that fetal hepatocytes can be successfully transplanted to the spleens of recipient hosts, and that the fetal hepatocytes appear to grow and develop functional cyto P450 metabolizing systems. Therefore, transplantation of fetal hepatocytes to splenic tissue represents a unique in vivo system of extra hepatic metabolizing tissue.

Cytochrome P4502A-mediated coumarin 7-hydroxylation and testosterone hydroxylation in mouse and rat lung

Pulmonary coumarin 7-hydroxylase, testosterone hydroxylase and other P450-mediated activities were compared in the mouse and rat. Coumarin 7-hydroxylase activity was 20 pmol/mg/min. in mouse and 4 pmol/mg/min. in rat lung microsomes. Liver values were 180 (mouse) and 1 (rat) pmol/mg/min. Km values of rat and mouse lung coumarin 7-hydroxylase were about 1 microM whereas the rat liver Km value was > 100 microM. Phenobarbital and 3-methylcholanthrene did not affect rat lung (or liver) coumarin 7-hydroxylase activity. Anti-Cyp2a-5 antibody effectively inhibited mouse and rat lung coumarin 7-hydroxylase and testosterone 15 alpha-hydroxylations but failed to block these activities in the rat liver. In immunoblot analysis anti-Cyp2a-5 antibody recognized the 50-kDa Cyp2a-4/5 protein in mouse lung microsomes. A P450 protein co-migrating with Cyp2a-5 was also detected in rat lung microsomes. Cyp2a-5 cDNA probe hybridized with a 1.8-kb mRNA species in rat lung RNA fraction. The hybridization signal was not increased by 3-methylcholanthrene or phenobarbital. These data suggest that the mouse lung expresses Cyp2a-5 which differs from the liver enzyme only in its regulation and that the rat lung contains a P450 isoform(s) belonging to the 2A subfamily which may be orthologous with the mouse Cyp2a-4/5 catalyzing coumarin 7-hydroxylase and testosterone 15 alpha-hydroxylase activities. The recently reported rat lung CYP2A3 (Kimura et al.) gene product is a candidate for the observed coumarin 7-hydroxylase activity in the rat lung.

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.

Organogenesis-stage cytochrome P450 isoforms: utilization of PCR for detection of CYP1A1 mRNA in rat conceptal tissues

Utilizing the reverse transcriptase-linked polymerase chain reaction, we analyzed the capacity of three groups of rat conceptal tissues to express cytochrome P4501A1 (CYP1A1) mRNA during the dysmorphogenesis-sensitive stage of organogenesis. The visceral yolk sac, ectoplacental cone and embryo proper each were investigated on day 12 of gestation with and without prior exposure in utero to 3-methylcholanthrene as inducing agent. With two sets of discriminating oligonucleotide primers, definitive, reproducible signals were detectable only in tissues from 3-methylcholanthrene preexposed conceptuses. Signals of highest intensity were observed with visceral yolk sac tissues and signals of lowest intensity were observed with tissues of the embryo per se. Specificities of the amplified cDNAs were verified using Southern blotting with hybridization to an internal oligonucleotide probe. The results indicate that organogenesis-stage conceptual tissues of the rat will express CYP1A1 mRNA in response to environmental transregulating agents.

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.

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

The administration of 3-methylcholanthrene (MC) to pregnant mice results in the formation of lung tumors in the offspring. Previous work has shown that fetuses demonstrating inducibility of aryl hydrocarbon metabolism develop two to five times more lung tumors than induction-nonresponsive littermates. In this study, the effects of fetal versus adult MC exposure were compared with regard to both induction of aryl hydrocarbon hydroxylase activity (AHH) in lung and dependence of lung tumorigenesis on the Ah genotype. In inducible (C57BL/6 X DBA/2)F1 fetal lung supernatants, a single ip injection of 100 mg/kg of MC to the mothers resulted in a maximal 50-fold induction of AHH activity by 8 hr, which persisted for 48 hr. The enzyme data agreed well with RNA blot analysis, as MC caused maximal induction of P450IA1 RNA by 4 hr. For comparison, adult (F1 X DBA/2) mice were given three weekly injections of 100 mg/kg MC and tumor incidences were determined after 16 weeks. No differences were observed between responsive and nonresponsive mice of either sex in the number of mice bearing lung tumors, nor did the tumor multiplicity differ between responsive and nonresponsive males. However, noninducible female mice had a significantly higher tumor multiplicity than their inducible counterparts (p less than 0.025). Single ip injections of MC to adult F1 mice revealed that lung AHH activity was increased only 4- to 7-fold in the adult animal compared to the large fetal induction ratio. The difference in the magnitude of induction was due to the higher constitutive levels of AHH activity seen in adult tissue (4- to 14-fold greater than maximal basal fetal levels), as fetal and adult supernatants showed similar levels of induced activity following MC treatment. These results suggest that the correlation between susceptibility to MC-initiated lung tumors and induction of cytochrome P450IA1 is a unique property of the fetus and may be due, in part, to the low basal levels of fetal activating enzymes and their high induction ratio during the fetal period.