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Human ovarian cancer is predominantly of epithelial cell origin (>90% of malignant tumors) and most often presents at an advanced stage with poor prognosis. Most animal models of ovarian carcinoma yield thecal/granulosa cell tumors, rather than adenocarcinomas. Induction of adenocarcinoma in 10-45% of rats following an ovarian implantation of 7,12-dimethylbenz[a]anthracene (DMBA) coated silk suture has been reported. Here, DMBA of 99% purity was melted at 124 degrees C to impregnate a 1 cm length of sterile suture for direct ovarian implantation in Wistar Furth rats at 7 weeks of age. DMBA-treated ovaries showed a nearly complete loss of primary follicles and degeneration of granulosa cells at 16 weeks, consistent with the known toxic response of the ovary to direct DMBA application. No tumors were present. Untreated right ovaries and sham dimethyl sulfoxide-treated ovaries were normal. Ovarian tumors in DMBA-treated rats were first noted at 26 weeks post implantation reaching a cumulative tumor incidence of 77% (23/30) at 52 weeks. Controls showed no evidence of tumor at 52 weeks (0/31). Tumor histology was distributed as well differentiated adenocarcinoma (1/23), poorly differentiated adenocarcinoma (8/23), thecal/granulosa cell tumor (8/23), undifferentiated sarcoma (5/23) and one undifferentiated carcinoma with no adeno character. Tumors occasionally seeded to peritoneal mesentery, spleen and abdominal wall. Adenocarcinomas appeared to originate from the ovarian surface epithelium, with focal papillary extension into cystic space. Epithelial derived tumor cells positively react with antibodies to cytokeratin (8/8), epithelial cell adhesion molecule (Ep-CAM 5/5) and prostaglandin synthetase-1 (COX-1 4/4). Vimentin positive epithelial cells when present in adenocarcinomas (4/7), showed perinuclear staining, quite distinct from the uniformly stained stromal cells in thecal/granulosa cell tumors (8/8). The thecal/granulosa cell tumors were Ep-CAM negative (0/5) and weakly COX-1 positive (4/4). Thus, the DMBA suture model in rats yields epithelial derived tumors histologically similar to humans and should prove suitable for the testing of preventive or therapeutic agents.

p53 transgenic mice carrying a dominant negative mutation were crossed with Ink4A/Arf heterozygous-deficient mice to investigate whether there is a synergy between these two germ-line mutations in promoting carcinogen-induced lung tumor progression in mice. Mice with a p53 dominant negative mutation and Ink4A/Arf heterozygous deficiency exhibited >20-fold increase in tumor volume compared with approximately 4-fold increase in Ink4A/Arf heterozygous-deficient mice and a 9-fold increase in mice with only the p53 dominant negative mutation. The effect of Ink4A/Arf heterozygous deficiency on lung tumor progression occurred late in the carcinogenesis process (>30 weeks after carcinogen treatment). In addition, most of the lung tumors (approximately 80%) from mice with a p53 mutation and deletion of Ink4A/Arf were lung adenocarcinomas. In contrast, lung adenocarcinomas were seen in <10% of the lung tumors from the wild-type mice and approximately 50% of the lung tumors from Ink4a/Arf heterozygous-deficient or p53 mutant mice. These results indicate a significant synergistic interaction between the presence of a mutant p53 transgene and the Ink4A/Arf deletion during lung tumor progression (P < 0.01). The usefulness of this new mouse model in lung cancer chemoprevention was examined. The chemopreventive efficacy of budesonide was examined in wild-type mice, mice with Ink4A/Arf heterozygous deficiency, mice with a mutation in the p53 gene, or mice with both a mutation in the p53 gene and deletion in the Ink4A/Arf locus. Mice treated with budesonide displayed an average of 90% inhibition of lung tumor progression in a standard 18-week chemoprevention assay, regardless of p53 and/or Ink4A/Arf status. However, the efficacy of budesonide against lung tumor progression decreased from 94 to 77% (P = 0.07) in mice with alterations in both p53 and Ink4A/Arf in a 40-week chemoprevention assay. Similarly, when mice bearing established lung adenomas were treated with budesonide, genotype-dependent differential effects of budesonide in wild-type and mutant mice were clearly revealed with a 82, 64, 45, and 33% decrease in tumor volume in wild-type mice, p53(+/+)Ink4a/Arf(+/-) mice, p53(+/-)Ink4a/Arf(+/+) mice, and p53(+/-)Ink4a/Arf(+/-), respectively. Thus, mutant mice with alterations in p53 and/or Ink4A/Arf exhibited a significant resistance to chemoprevention by budesonide. Because p53 and Ink4a/Arf mutations are the most prevalent mutations in human lung cancers, the effectiveness of chemopreventive agents on the mutant A/J mice containing alterations with p53 and Ink4a/Arf is the best preclinical estimate of their efficacy in humans. Thus, the mutant A/J mouse model should prove useful for chemoprevention studies.

Previous observation has shown that the wild-type Kras2 allele is a suppressor of lung cancer in mice. Here we report that loss of heterozygosity (LOH) of chromosome 12p was detected in approximately 50% of human lung adenocarcinomas and large cell carcinomas, and Kras2 mutations were detected at codon 12 in approximately 40% of adenocarcinomas and large cell carcinomas. Interestingly, all of the lung adenocarcinomas and large cell carcinomas containing a Kras2 mutation exhibited allelic loss of the wild-type Kras2 allele when a correlation between LOH of the region on chromosome 12p and Kras2 mutation was made. These results from human lung cancer tissues provide a strong evidence in support of our previous observation in mouse models that the wild-type Kras2 is a tumor suppressor of lung cancer.