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STRAIN-SPECIFIC SENSITIVITY TO INDUCTION OF MURINE LUNG TUMORS FOLLOWING IN UTERO EXPOSURE TO 3-METHYLCHOLANTHRENE
Citation:
JENNINGS-GEE, J. E., M. XU, J. E. MOORE, G. B. NELSON, S. T. DANCE, S. LEONE-KABLER, N. D. KOCK, T. P. MCCOY, J. A. ROSS, A. J. TOWNSEND, AND M. S. MILLER. STRAIN-SPECIFIC SENSITIVITY TO INDUCTION OF MURINE LUNG TUMORS FOLLOWING IN UTERO EXPOSURE TO 3-METHYLCHOLANTHRENE. Presented at Genetics and Environmental Mutagenesis Society, Chapel Hill, NC, October 26, 2005.
Description:
We previously demonstrated that different strains of fetal mice were more sensitive to lung tumor induction by 3-methylcholanthrene (MC) than were adults. Offspring from either a D2 x B6D2F1 backcross or from parental Balb/c mice exhibited a similar high incidence of lung tumors 6 to 12 months after transplacental exposure to MC. We thus compared the effects of in utero treatment with MC on lung tumor induction in the offspring of intermediately susceptible Balb/c (Bc), resistant C57BL/6 (B6), and reciprocal crosses between these two strains. Pregnant mice were treated with 45 mg/kg of MC on day 17 of gestation. Tumor incidence, multiplicity, and Ki-ras mutational spectrum were determined in the offspring 12-18 months after birth. Tumor incidences in Bc mice and reciprocal crosses between the two strains were 83% and 100%, respectively, while B6 mice demonstrated a remarkable resistance to tumorigenesis, with a tumor incidence of 11%. Tumor multiplicities in Bc, B6Bc, BcB6, and B6 mice were 3.1 ± 3.2, 5.5 ± 3.4, 4.8 ± 2.9, and <0.1, respectively, counting only lesions that were discrete, individual nodules. Ki-ras mutations in the lung tumors, which occurred chiefly in the KS allele (96%), were found in 79 to 81 % of B6Bc and BcB6 mice, 64% of Bc mice, and 50% of B6 mice, with Val12, Asp 12, and Arg13 mutations associated with more aggressive tumors. Expression of Cyp1a1 and Cyp1b1, which are involved in the metabolic activation of MC, were examined by real time RT-PCR and similar kinetics of induction and decline of both RNAs were observed in all four strains. Maximal induction of both RNAs occurred 2 to 8 hrs after injection with MC in both organs in all 4 strains. Cyp1a1 induction was 2 to 5 times greater in the fetal liver (7000- to 16,000-fold) than fetal lung (2000- to 6000-fold), and significantly higher than Cyp1b1 induction, which ranged from 8- to 20-fold for both organs. The only significant strain-specific effect seen was a particularly poor induction of Cyp1b1 in B6 mice, especially in the fetal lung. We also determined whether strain-specific differences in Phase II metabolism could account for the differences in fetal susceptibility. Using CDNB as a substrate, we found that treatment with MC had no effect on the levels of GST enzyme activity in either organ in any of the 4 strains of mice. Western blot analysis demonstrated very low levels of expression of GST isoforms α, θ, π, and µ in both lung and liver supernatants from all 4 strains. With the exception of GSTµ, which showed a 3.5-fold induction in the lung of Bc mice 48 hrs after exposure, there were no significant effects seen among the other isozymes. DNA adduct formation and repair were also assessed at several time points between 1 to l8 days (2 weeks post-natal) post-injection. Few differences were noted between the four strains. Collectively, while high Cyp1a1 induction and low, uninducible levels of phase II conjugating enzymes probably accounts for the increased overall susceptibility of the fetus to transplacentally-induced tumor formation, metabolic differences among the 4 strains and differences in adduct formation could not account for differences in lung tumor susceptibility observed in B6 mice relative to the other strains. These results suggest that another dominantly acting genetic locus may influence lung tumorigenesis specifically following in utero exposure to carcinogens during fetal development. This abstract does not necessarily reflect EPA policy. (Supported by EPA STAR grant R829428-01-0)