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) str ains 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 t o lung tumor induction, and compared our findings with those previously fou nd in [D2 x B6D2F(1)]F-2 mice. In addition, we assessed the molecular patho genesis of the chemically induced tumors and examined the effects of the pu tative lung tumor promoter butylated hydroxytoluene (BHT) in BALB/c mice. B ALB/c mice were treated on day 17 of gestation with 5, 15 or 45 mg/kg MC an d 6 weeks after birth with BHT for 6 consecutive weeks. Mice were killed at 6 months of age. Ki-ras, p16(Ink4a) and p19(ARF) gene loci were amplified from paraffin-embedded lung tumor tissue and screened for the presence of p oint mutations via allele-specific oligonucleotide hybridization and single strand conformation polymorphism (SSCP) analyses. Ki-ras point mutations w ere found in 56% (20/36) of BALB/c lung tumors, with 33% (2/6) of the hyper plasias, 58% (10/19) of the adenomas and 73% (8/11) of the carcinomas exhib iting point mutations at this gene locus, Similar incidences of Ki-ras muta tions were previously found following transplacental exposure of [D2 X B6D2 F(1)]F-2 mice to MC and treatment of adult A/J mice with urethane, Interest ingly, a strain-dependent difference was observed in the mutational spectru m, 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% incidence s previously observed in [D2 X B6D2F(1)]F-2 mice. SSCP analysis of the tumo r suppressor gene p16(Ink4a) showed a 6% incidence of point mutations, cons istent with that found in [D2 x B6D2F(1)]F-2 mice. No mutations were found in exon 1 beta of the p19(ARF) gene of either strain. BHT, a lung tumor pro moter in adult mice, had no statistically significant effects on either tum or incidence, tumor multiplicity or the mutational spectrum produced in the Ki-ms gene by in utero MC treatment. However, though not significant, ther e was an observable trend in increased tumor multiplicity in mice co-treate d with BHT, These data demonstrate the transplacental carcinogenic effect o f 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 formatio n of lung tumors following transplacental exposure to MC, possibly due to t he rapid growth and cell proliferation in the developing organism. Strain-d ependent differences in the Ki-ms mutational spectrum may be associated wit h their differential susceptibility to lung tumor initiation.