Urethan Carcinogenesis in Newborn, Suckling, and Adult Mice of C57BL, C3H, BC3FJ, C3Hf, and SWR Strains

Influence of Thymectomy, Splenectomy, and Cortisone on Carcinogenesis

Groups of outbred albino CTM mice were either thymectomized or splenectomized at 4 weeks of age and administered urethan 0.4 percent in the drinking water for 10 days at 5 weeks. In comparison with intact, urethan-treated animals, the thymectomized, urethan-treated mice had a slightly lower incidence of malignant lymphomas (19 against 24 %), a marked decrease of mammary tumors (31 against 62%), and increased incidence of lung adenomas (84 against 48%) and of skin papillomas (10 against 1%). Thymectomized, but otherwise untreated mice developed less lymphomas and mammary tumors than intact, untreated controls. Splenectomy did not modify significantly the tumor incidence. In a second experiment, groups of CTM mice were administered either 1 mg of cortisone s. c. daily for 10 days or urethan in the drinking water for 5 days followed by 5 days of cortisone or viceversa, or urethan alone. No major differences in the tumor incidence in the various groups were observed, but for a decreased incidence of thymic lymphosarcomas in the group with cortisone after urethan. In a third experiment, CTM mice were given 2 mg urethan within the first day after birth and thymectomized or sham operated during the anesthesia caused by the urethan. Both groups had a very high incidence of hepatomas and lung adenomas, and developed only few lymphomas and mammary tumors. Finally, a group of C3H mice thymectomized at birth under Nembutal anesthesia, developed a considerably lower incidence of hepatomas and mammary tumors than sham operated animals.

Hepatocarcinogenesis: A Polygenic Model of Inherited Predisposition to Cancer

The murine inbred strain C3H provides an experimental model of inherited predisposition to hepatocellular cancer. Hepatocellular neoplastic lesions induced by chemical carcinogens reach a volume 10-100-fold greater in C3H mice than in genetically resistant strains. However, the huge strain differences in tumor size are explained by relatively small differences (10%-30%) in tumor cell kinetics. Genetic linkage experiments in different crosses demonstrated that six unlinked hepatocarcinogen sensitivity ( Hcs) and two hepatocarcinogen resistance ( Hcr) loci determined quantitative variations in susceptibility to hepatocarcinogenesis. Such results provide the genetic basis for the strain variations in susceptibility to hepatocarcinogenesis and demonstrate a new model of polygenic inheritance of predisposition to cancer.

Role of the Immune System in the Growth of an Isotransplanted Urethan-Induced Lymphosarcoma

Two groups of (SWR×C57BL)F1 mice were injected subcutaneously with two quantitatively different cell suspensions from a uretan-induced thymic lymphoma of the same hybrid combination. Each of these groups was divided before the neoplastic isograft into the following sub-groups: controls, cyclophosphamide injected, X-irradiated and BCG injected mice. Percentage of takes, tumor diameters and mortality provided the parameters for the evaluation of the tumor growth. With the higher inoculum of neoplastic cells (5×105) only cyclophosphamide was effective in enhancing the take percentage 10 days after the isograft. With the smaller inoculum (5 × 103) all treatments were effective on tumor growth although the inhibitory action of BCG was not as strong as the enhancing action of the immune depressive treatments. The results are interpreted as evidence of a specific antigenicity of urethan-induced lymphoma.

Primary Immune Response in Swiss Mice Injected at Birth with a Single Dose of 7,12-Dymethylbenz [α] anthracene

The primary immune response of 7,12-dimethylbenz[α] anthracene (DMBA) newborn treated outbred Swiss mice and untreated control animals of the same strain was determined at 10, 20, 30, 50, 75, 100, 150 and 200 days of age, according to the agar plaque technique of Jerne et al. The number of spleen antibody plaque forming cells (APFC) as well as the number of nucleated spleen cells was measured at time of sacrifice. The tumors observed in the treated groups were subcutaneous sarcomas and fibrosarcomas at the site of injection, and malignant lymphomas usually of the poor differentiated-cell type, either limited to the thymus or with extrathymic diffusion; in some instances animals with both subcutaneous tumors and malignant lymphomas were found. The number of APFC per spleen was constantly and markedly lower in the DMBA treated animals than in the control animals. Among the tumor bearing animals minimum numbers of APFC were recorded for the malignant lymphomas. According to the data of the present study and of previously reported observations, the possible relationship between immunodepression after DMBA treatment at birth and tumor development following the carcinogen injection is discussed.

Electron Microscopic Study of Viral Particles in the Pancreas of Normal and Leukemic Mice

An electron microscopic study of viral particles was conducted on pancreas, thymus, and bone marrow of normal and leukemic (C57BL x C3Hf) F1 (BC3) and SWR mice. The leukemic mice had developed thymic lymphosarcoma after neonatal injection of N-nitrosomethylurea or urethan. An intense proliferation of type C particles morphologically identical to the murine leukemia virus was found in the acinar cells of the exocrine pancreas of BC3 mice. The particles were predominantly located in intracytoplasmic vacuoles but they were also numerous in dilated cisternae of the rough endoplasmic reticulum and in extracellular spaces. A substantial quantitative difference of type C particles between the pancreases of leukemic and healthy mice was not observed. No viral particles were found in the pancreas of SWR mice. In the thymus and bone marrow of BC3 and SWR mice a small number of type C particles and intracisternal type A particles was found in all the experimental and control groups. The observation of a high number of type C particles in the exocrine pancreas of BC3 mice is discussed in relation to a viral etiology of chemically-induced lymphomas.

Spontaneous and Urethan-Induced Tumor Incidence in B6C3F1 versus B6Cf1 Mice

The incidences of spontaneous tumors of the murine hybrids (C57BL/6J X C3Hf)F1 (B6C3F1) and (C57BL/6J X BALB/c)F1 (B6CF1) were compared in untreated mice kept until 110 weeks of age. Male B6C3F1 and B6CF1 mice had respectively 16% and 20% incidence of lymphomas, 26% and 4% of liver tumors and 12% and 22% of lung tumors. Among B6C3F1 and B6CF1 females, a 36% and 12% incidence of lymphomas, a 6% and zero incidence of liver tumors, and a 4% and 16% of lung tumors were observed. A few other tumors were seen in both hybrids. Groups of male and female mice of the 2 hybrids received 5 i.p. injections of 1000 mg/kg urethan once every other day starting at 10 days of age, and were kept under observation until 65-80 weeks of age. Treated B6C3F1 mice had an earlier mortality than B6CF1 mice due to tumor development. The statistical analysis, allowing for survival, showed a significantly higher lymphoma incidence in male and female B6C3F1 than B6CF1 mice, which had instead a higher incidence of lung tumors. Hepatocellular tumors were seen in both sexes of the 2 hybrids, with a higher frequency in B6C3F1 mice. Male mice of both hybrids had a higher incidence of liver tumors than females.

Isonicotinic Acid Hydrazide and Urethan Carcinogenesis in suckling SWR Mice

Five i.p. injections of 1 mg urethan/g body weight on alternate days administered to 10-day old SWR mice resulted in the development of thymic lymphosarcomas in 60 % and of lung adenomas in nearly 100 % of the animals, irrespective of sex. Concurrent treatment with isonicotinic acid hydrazide (INH), 0.1 mg/g body weight, administered five times on alternate days starting at 9 days of age, did not modify the carcinogenic effect of urethan. The treatment with INH alone did not induce a higher tumor incidence than in untreated controls.

Electron Microscopic Study of Viruses in Mouse Thymuses during Leukemogenesis by Urethan

A systematic search for viral particles was carried out in thymuses of C3Hf inbred mice treated with leukemogenic doses of urethan. Thymuses collected during the tumor latent period and thymic lymphosarcomas were examined. A group of 38 mice was treated, starting at 10 days of age, with 5 i.p. doses of urethan (lmg/g body weight) once every two days. This treatment induces approximately 30 per cent thymic lymphosarcomas in C3Hf mice, after an average period of 20 weeks, whereas no spontaneous thymic lymphosarcomas develop in untreated controls. Six animals with thymic lymphosarcoma were killed between 16–32 weeks of age; the others were sacrificed at 3,5,10 and 14 weeks of age when they did not have any recognizable thymic tumor either grossly or microscopically. As control, thymuses of a group of 28 untreated mice were examined. Mature and immature type C particles morphologically indistinguishable from the murine leukemogenic viruses and intracisternal A particles were observed. The immature C particles and intracisternal A particles were found in all the groups of treated and untreated mice. Intracisternal A particles were more numerous in the untreated than in the treated animals. The number of A particles in the neoplastic thymuses was far inferior than that observed in the treated animals killed before development of lymphoma. The amount of immature C particles was always much less than that of A particles. No significant difference in the number of immature C particles was observed in all experimental groups. Mature C particles were only seen in the thymus of a few treated or untreated mice, never in thymic lymphosarcomas. Within the limits of this investigation, the results presented are not consistent with the view that urethan may act by stimulating viral multiplication.

Susceptibilities of p53 Knockout and rasH2 Transgenic Mice to Urethane-Induced Lung Carcinogenesis are Inherited from their Original Strains

In the present study, susceptibility of CB6F1 mice carrying the human prototype c-Ha- ras gene (rasH2 mice) and p53 gene knockout mice ( p53 (+/−) mice) to urethane-induced lung carcinogenesis was compared under the same experimental conditions. Both strains were administered 500 ppm urethane in their drinking water for 3 weeks. At week 26, lung adenocarcinomas and adenomas were observed in 53% and 100% of rasH2 mice, respectively, and lung adenomas were observed in 67% of rasH2 littermate (non-Tg) mice. However, lung tumors were not observed in either p53 (+/−) or p53 (+/+) mice. Peliosis hepatis and hepatic hemangiomas were observed in 27% and 67% of p53 (+/−) mice, but only in 6.7% and 6.7% of the rasH2 animals, respectively. Under the same experimental conditions, BALB/c mice, the strain of origin of the rasH2 mice, developed lung adenomas at an incidence of 93%, whereas none of the C57BL/6 original strain for p53 (+/−) mice developed lung tumors. Peliosis hepatis was observed in 40% of the C57BL/6 mice, but not in BALB/c mice; hepatic and splenic hemangiomas were not observed in these animals. These results indicate that organ susceptibility of rasH2 and p53 (+/−) mice is inherited from their strains of origin, the rasH2 and BALB/c lines being much more sensitive to the induction of pulmonary carcinogenesis.

Genetic control of resistance to hepatocarcinogenesis by the mouse Hpcr3 locus

The genome of the BALB/c mouse strain provides alleles that dominantly inhibit hepatocellular tumor development in F1 crosses with the highly hepatocarcinogenesis-susceptible C3H/He strain. Genome-wide linkage analysis using a 1536-single-nucleotide polymorphism array in a (C3H/He x BALB/c)F2 intercross population treated with urethane to induce hepatocellular tumor development revealed a locus with a major role in the resistance to hepatocarcinogenesis. This locus, designated hepatocarcinogen resistance 3 (Hpcr3) and mapping to central chromosome 15, showed a linkage at LOD score = 16.52 and accounted for 40% of the phenotypical variance. The BALB/c-derived allele at Hpcr3 reduced tumor-occupied area of the liver up to 25-fold, in a semidominant way. Additional minor loci were mapped to chromosomes 1, 10, and 18. A gene expression profile of normal adult mouse liver showed a significant association with susceptibility of BALB/c, C3H/He, and F1 mice to hepatocarcinogenesis and identified the genes expressed in the Hpcr3 locus region; moreover, this analysis implicated the E2F1 pathway in the modulation of the phenotype susceptibility to hepatocarcinogenesis.

CONCLUSION

These findings, indicating the complex genetics of dominant resistance to hepatocarcinogenesis, represent a step toward the identification of the genes underlying this phenotype.

Effect of ethanol on the tumorigenicity of urethane (ethyl carbamate) in B6C3F1 mice

Urethane is a carcinogen to which there is widespread exposure through the consumption of fermented foods and alcoholic beverages. In this study, we have assessed the carcinogenicity of urethane in combination with ethanol. Male and female B6C3F(1) mice (48 mice per sex per group) were exposed to 0, 10, 30, or 90 ppm urethane in the presence of 0%, 2.5%, or 5% ethanol in drinking water ad libitum for two years, at which time the extent of tumorigenesis was assessed. Additional mice (four per sex per group) received the same doses for four weeks to assess serum levels of urethane and ethanol, DNA adduct formation, and the induction of microsomal cytochromes P450, cell proliferation, and apoptosis. Urethane decreased cell replication in the livers of female, but not male, mice, decreased cell replication in the lungs of both sexes, and induced cytochrome P450 2E1 in the livers of female mice. Hepatic levels of the DNA adduct 1,N(6)-ethenodeoxyadenosine were increased by exposure to urethane and decreased by treatment with ethanol. Animal weights and survival were not affected by ethanol; in contrast, urethane administration decreased body weights and survival. Urethane caused dose-dependent increases in liver, lung, and harderian gland adenoma or carcinoma and hemangiosarcoma of the liver and heart in both sexes, mammary gland and ovarian tumors in females, and squamous cell papilloma or carcinoma of the skin and forestomach in males. The increase in hepatocellular tumors occurred in a relatively linear manner and was attributed to the formation of 1,N(6)-ethenodeoxyadenosine in hepatic DNA coupled with an increase in cell replication. Hemangiosarcomas were observed only at the 90 ppm urethane dose and were probably a result of high-dose urethane-induced toxicity. Lung alveolar/bronchiolar and harderian gland adenoma or carcinoma increased in a relatively linear manner, suggestive of a genotoxic mechanism for tumor induction. Ethanol induced a dose-dependent trend in hepatocellular adenoma or carcinoma in male mice, with the incidence being marginally increased at the highest dose. In female mice administered 10 ppm and 90 ppm urethane, ethanol caused dose-related increases in alveolar/bronchiolar adenoma or carcinoma and hemangiosarcoma of the heart, respectively. This may be due to ethanol decreasing the first-pass clearance of urethane, thus, increasing systemic distribution. In male mice a different relationship was observed: ethanol caused a dose-related decrease in alveolar/bronchiolar and harderian gland adenoma or carcinoma in mice administered 30 ppm urethane.

Pas1 haplotype-dependent genetic predisposition to lung tumorigenesis in rodents: a meta-analysis

Rodent species and strains show wide variations in susceptibility to lung tumorigenesis. In mice, hierarchical clustering of 29 inbred laboratory strains by pulmonary adenoma susceptibility 1 (Pas1) locus polymorphisms separated the strains into either an A/J- or a C57BL/6J-type Pas1 haplotype. A pooled analysis (including >8500 mice) of studies on spontaneous and chemically induced lung tumorigenesis in these strains revealed a significantly higher risk of spontaneous lung tumors [odds ratio (OR) 12.17; 95% confidence interval (CI) 9.00-16.45] as well as of chemically induced lung tumors (OR 15.14; 95% CI 12.51-18.31) in the A/J-type haplotype. Strain differences were observed with six different carcinogens, suggesting that Pas1 locus activity is carcinogen-independent. Thus, the present meta-analysis indicates a link between the genetic control of spontaneous and chemically induced lung tumor susceptibility in mice. The Pas1 susceptibility allele is frequent in the population of inbred mouse strains, whereas a counterpart appears to be absent or rare in rat and hamster strains. These findings might help in the interpretation of results of rodent carcinogenicity bioassays and assessing the risk of lung carcinogenesis from chemicals.

Metabolic activation capacity of neonatal mice in relation to the neonatal mouse tumorigenicity bioassay

The neonatal mouse tumorigenicity bioassay is a well-developed animal model that has recently been recommended as an alternative tumorigenicity bioassay by the International Conference on Harmonization (ICH) for Technical Requirements for the Registration of Pharmaceuticals for Human Use. There are sufficient data to conclude that this animal model is highly sensitive to genotoxic chemical carcinogens that exert their tumorigenicity through mechanisms involving the formation of covalently bound exogenous DNA adducts that lead to mutation. On the other hand, it is not sensitive to chemical carcinogens that exert tumorigenicity through a secondary mechanism. The metabolizing enzymes present in the neonatal mouse, particularly the cytochromes P450, are critical factors in determining the tumorigenic potency of a chemical tested in this bioassay. However, compared to the metabolizing enzymes of the adult mouse and rat, the study of the metabolizing enzymes in neonatal mouse tissues has been relatively limited.

Genetics of liver tumor susceptibility in mice

A good experimental model of genetic predisposition to hepatocellular tumors is the murine strain C3H. These tumors share morphologic similarities with human hepatocellular tumors. After a treatment with a single small dose of chemical carcinogen, the C3H mice show a high susceptibility to the growth of hepatocellular neoplastic lesions, that reach a volume > 100-fold as compared to the corresponding lesions of genetically resistant strains. Genetic linkage analysis experiments were conducted in 2 different crosses, with the C3H as one of the parental strains, and the other parental strains being represented by mice genetically resistant to hepatocarcinogenesis (A/J, M. spretus). Six different regions, on chromosomes 2, 5, 7, 8, 12, and 19 showed a significant linkage with hepatocellular tumor development. These results provide the genetic basis for the strain variations seen in susceptibility to hepatocarcinogenesis, indicating polygenic inheritance of this trait.

Multiple molecular alterations in mouse lung tumors

Twenty-five mouse lung tumors induced by a single urethan treatment in female A/J, BALB/c, and (A/J x C3H/He)F1 (AC3) mice were analyzed for the presence of mutations at codon 61 of the Ki-ras gene and for the expression of the surfactant protein A (SP-A), retinoblastoma (Rb), growth arrest-specific-3 (gas-3), p53, c-myc, and thymidylate synthase (TS) genes. Ki-ras codon 61 mutations were detected in 22 of 25 tumor samples without differences among strains. In comparison with normal lungs, all the tumors showed increased SP-A mRNA levels, indicating their derivation from alveolar type II pneumocytes or Clara cells. Rb and gas-3 transcripts were instead found in all tumors at about tenfold and about 20-fold reduced levels, respectively. No apparent structural alterations or loss of heterozygosity at the Rb locus was detected in any tumors. The p53 mRNA was observed without variation in quantity or size in lung tumors and normal tissue. A threefold to fivefold c-myc overexpression was observed, without amplification of the gene. TS expression was only slightly increased, indicating no great differences in cell proliferation between lung tumors and normal tissue. Our data suggest that the pathogenesis of urethan-induced lung tumors in mice involves specific and recurrent molecular alterations (Ki-ras mutations, decrease of Rb and gas-3 expression, and increase of c-myc expression) that could represent different steps in lung carcinogenesis.

Long-term assays for carcinogenicity

During the past two decades, the rodent bioassay for detection of chemical carcinogens has reached a high standard of performance with both an increased number of animals and dose levels and a more detailed assessment of findings. However, the basic principles of testing and evaluation of results have remained essentially unchanged. Problems such as the length of the testing, use of maximum tolerated dose (MTD), selection of strains, variability of spontaneous tumors, discordant results between mouse and rat, and the classification of chemical carcinogens according to their mechanism of action have all remained unsolved. By contrast, the results of short-term tests and of other biological analyses do not always show a direct correlation with those of the long-term bioassays; this can be interpreted as an indication of different mechanisms of carcinogenicity. Currently available medium-term tests may detect carcinogenic activity of chemicals at particular organs in a period of time (weeks to months) relatively shorter than that of the 2-year carcinogenesis bioassay, and they may also provide additional information on mechanisms of carcinogenicity.

Two-stage liver carcinogenesis in the mouse

A two-stage protocol for studying liver carcinogenesis was applied to the mouse. The protocol includes the treatment of 7-day-old mice with a single low dose of an initiating agent (diethylnitrosamine, NDEA), promotion starting after weaning and lasting about 20 weeks, and histologic analysis, at 30 weeks of age, of hepatocellular nodules on H&E stained sections. A stereologic analysis of results allows the evaluation of nodule frequency and size. Using this protocol in B6C3 and B6C mice, we have identified the promoting activity of the phenobarbital-like enzyme inducer, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), which was found to be a strong hyperplaseogenic agent for mouse liver. These studies also indicated that the different susceptibility to hepatocarcinogenesis in B6C3 and B6C mice may be related to a higher susceptibility of B6C3 than B6C initiated liver cells to growth stimulation. A long-term study showed that B6C mice have a low incidence of spontaneous liver tumors but are susceptible to chemical hepatocarcinogenesis and, therefore, they may be an alternative model to B6C3 mice in carcinogenesis bioassays.

Carcinogenicity study in mice on pildralazine, a hydralazinelike antihypertensive compound

Pildralazine, a hydralazinelike antihypertensive vasodilator containing a free hydrazine group, was administered in the drinking water to male and female B6C3F1 and to female BALB/c mice at 100, 200, and 400 ppm dose levels for 80 weeks. The animals were kept under observation until 130-133 weeks of age, when the experiment was terminated. A transplacental-infantile bioassay was also carried out with pildralazine administered in the drinking water at 200 and 400 ppm dose levels to female C57BL/6J mice for 1 week before mating with C3Hf males and during mating and pregnancy. The progeny received the same doses for 10 weeks after birth and were kept under observation until 80-85 weeks of age, when the experiment was terminated. In both long-term and transplacental-infantile assays, control and treated groups developed the pattern of tumors usually observed in the strains used and no tumor type at any site appeared to be related to treatment.

Comparison of urethane-induced sister-chromatid exchanges in various murine strains, and the effect of enzyme inducers

The induction of sister-chromatid exchanges (SCEs) by urethane, 150 and 300 mg/kg administered i.p., was examined in bone-marrow cells of AKR, BALB/c, C3Hf, C57BL/6J and DBA/2 male mice. In all strains, the base-line level of SCE/cell was similar, ranging from 4.3 to 8.7, and the response increased with the dose of urethane. DBA/2 mice were the most susceptible to urethane at both dose levels, with 30.6 SCE/cell after treatment with 300 mg/kg, whereas the response of the other strains was from 17.4 to 21.5 SCE/cell at the same urethane dose. Pretreatment of C57BL/6J and DBA/2 mice with phenobarbital decreased the SCE frequencies induced by urethane, 300 mg/kg, to 70%, whereas a prior administration of beta-naphthoflavone reduced SCE levels in C57BL/6J but not in DBA/2 mice.