High susceptibility of human c-Ha-ras proto-oncogene transgenic rats to carcinogenesis: a cancer-prone animal model

Abstract

Transgenic animals carrying human c-Ha-ras proto-oncogene, v-Ha-ras transgenic mice, pim-1 transgenic mice and several knockout mice deficient of tumor suppressor genes, such as p53, have been shown to exhibit increased carcinogen susceptibility. As a result, studies into practical application and medium-term screening of environmental carcinogens are under way. Given the advantages of rat models characterized by larger organ size, abundant information regarding preneoplasias and virus-free constitution, we have concentrated on the generation of transgenic rats bearing copies of the human c-Ha-ras proto-oncogene and shown the Hras128 strain to be extremely sensitive to the induction of mammary carcinomas, and to a lesser extent, lesions in the urinary bladder, esophagus and skin. In most, if not all, the mammary cancers mutations of the transgene but not the endogenous H-ras gene are present, appearing to occur early in the process of tumorigenesis, which involves proliferation of cells in TEB and intraductal hyperplasia before carcinomas arise. Preliminary findings suggest that this is independent of endogenous ovarian hormones, although inhibited by soy isoflavones and promoted by atrazine and nonylphenols. Although further studies of the mechanisms are clearly necessary, the model appears to have great potential for screening purposes, not only for modifiers active in the breast, but also other organs where tumors characterized by ras gene mutations develop.

Figure 1

Rapid development of mammary carcinomas with three different carcinogens, MNU, DMBA and PhIP. Hras128 and its wild‐type counterparts were given a single dose of MNU (50 mg/kg, i.v.) or DMBA (40 mg/kg, i.g.) on day 50 after birth, then killed on week 8 or 12, respectively. PhIP (100 mg/kg) was given i.g. eight times over a 4 week‐period followed by death on week 12. Multiple mammary carcinomas developed in almost all the transgenic rats, whereas the incidence and multiplicities were far lower in the wild‐type rats.

Figure 2

Data for mutations that occurred in the transgene but not the endogenous Hras gene in all treatment groups. T, transgene; E, endogenous Hras gene; ^†numbers indicate the number of tumors examined.

Figure 3

Results of periodical observation of preneoplastic and carcinoma lesion development in abdominal‐inguinal mammary glands after injection of N‐methyl‐N‐nitrosourea (MNU) in Hras128.

Figure 4

Schematic presentation of carcinogenesis. Terminal endbuds, targets of carcinogens, have proliferative potential as a result of expression of the mutated transgene. Hyperplastic change occurs rapidly after application of carcinogens. The TEB shown at the lower left indicates normal mammary duct development in wild‐type rats.

Figure 5

Summary of data for the organ specificity for susceptibility to tumor induction, transgene mutations and endogenous ras gene mutations.

Figure 6

Schematic presentation of relevant signal pathways in Hras128 rats. The mutated c‐Ha‐ras oncogene plays a dominant role in activation of the MAPK pathway by binding to Raf.