Our current focus is on the use of transgenic mouse models to understand the molecular mechanisms of ras mediated tumorigenesis. The transgenic mouse model we are currently employing for this work is Tg.AC. Tg.AC is an important basic research model because 30% of human tumors have been estimated to involve Ras signaling. At the programmatic level, the Tg.AC mouse model is being evaluated by the National Toxicology Program (NTP) as an adjunct to the conventional two-year bioassay. The short-term 26-week bioassay relies on the empirical observation that Tg.AC transgenic mice produce skin papillomas when topically treated with chemical carcinogens. Created on the FVB/N mouse, only one of five founder lines displayed the unique characteristics now associated with the Tg.AC mouse. Tg.AC transgenic mice carry a transgene consisting of an oncogenic v-Ha-ras coding region flanked 5' by a mouse zeta-globin promoter and 3' by an SV-40 polyadenylation sequence. Located proximal to the centromere on chromosome 11, the transgene is transcriptionally silent until activated by full thickness wounding, UV light, or chemical carcinogens. This discriminating tumorigenic response does not occur when Tg.AC mice are treated with structurally similar non-carcinogenic chemicals. Expression of the transgene is a required early event that drives cellular proliferation resulting in clonal expansion and tumor formation. Genetic and epigenetic mechanism may contribute to this induced response. A very unique and specific assemblage of cis regions and transcription factors are required to activate the transgene following treatments with carcinogens. The goal of this project is to understand the induced tumorigenic response of Tg.AC mice and the dependence on the v-Ha-ras transgene. Line-1 elements are also known to flank the transgene integration site and may play a role in its response; therefore, we investigated Line 1 retrotransposition in tumor cells. In a collaboration with Dr. James Mason (LMG) we have demonstrated that Line-1 retrotransposition in human cancer cells induces P53 dependent apoptosis. TPA is a strong tumorigen in Tg.AC mice and has been associated with oxidative stress induced DNA damage. To investigate the requirments for ROS lesions in skin tumorigenesis we used a compromised Pol Beta DNA repair mouse model crossed to Tg.AC and measured changes in tumorigenesis. KSR knockout mice repress ras mediated tumorigenesis by blocking signaling through EGFr. To investigate the role of the EGF/MAPK pathway in Tg.AC mice we crossed Tg.AC to KSR knockout mice. We can demonstrate a reduction of tumorigenesis in vivo and a hair phenotype similar to EGFr KO mice. In our molecular studies of the transgene integration site we discovered a new ABC oxysterol transport gene. This gene appears to transport cholesterol and may be responsible for the transcriptional activation of the ras gene in Tg.AC mice. We have also investigated the transcription factors that bind to the basal promoter of Tg.AC mice by using foot print and gel shift analysis. To determine the effect of the transgene integration site in addition to the ras transgene expression we have used gene array to measure gene expression changes in Tg.AC skin prior to and after transgene expression. These expression profiles will provide insight into the early gene expression events that ocurr in skin tumorigenesis.
