The role of v -Ha -ras transgene in tumor development in the Tg.AC transgenic mouse.

Abstract

Protecting the public against potential cancer causing agents and discovering therapeutic interventions for existing cancers has been a major focus of scientific research. Advances in the development of transgenic animal models offer scientists improved methods to screen chemicals for carcinogenic and anticarcinogenic activity. The Tg.AC transgenic mouse, harboring an activated viral Harvey ras (v-Ha-ras) oncogene is being proposed as an alternative screening method to the conventional, labor-intensive and costly 2-year rodent cancer bioassay. Uncertainties surrounding the mechanism of the model and a lack of a clear understanding regarding the role of the transgene in tumorigenesis have, however, raised questions regarding it's reliability to detect a wide range of chemical carcinogens that can act via a variety of mechanisms and routes of exposure. The aim of the current investigations was to examine the role of the v-Ha-ras transgene in tumor development and it's impact on the utility of the Tg.AC mouse model as a carcinogen-screening tool. We ascertain that the role of the transgene in the Tg.AC mouse is unique to the skin and that two distinct mechanisms exist for regulation of the transgene. The first is constitutive regulation occurring in tissues expressing nuclear transcription factors (e.g. GATA-3) required for zeta-globin driven expression of the transgene. We hypothesize that constitutive expression of the transgene does not confer oncogenic potential. The second is an inducible mechanism of transgene expression unique to the skin and this is a function of tissue specific nuclear transcription factors expressed in response to chemical carcinogen treatment. We hypothesize that these transcription factors interact with a novel TPA response element (TRE)-like binding site contained within the palindromic region of the zeta-globin promoter and this inducible transgene activation confers oncogenic potential. Although the role of the transgene in tumor development appears to be unique to the skin, the utility of the model as a carcinogenic screen and as a tool for mechanistic investigations of chemical carcinogens and anticarcinogens has been established through these investigations. A high correlation between known chemical activity (carcinogenic, anticarcinogenic, noncarcinogenic) and that predicted by the Tg.AC model was apparent as was the model's ability to provide some information regarding the mode of action for each chemical used in the evaluation. An unexpected difference in phenotypic tumor response was observed for genotoxic (benzo[a]pyrene) versus non-genotoxic (TPA-tetradecanoyl phorbol acetate) chemicals indicating further utility of this transgenic model to discriminate these two classes of carcinogens. The current investigations extend the characterization of this transgenic animal in the areas of transgene regulation and the utility of the model as a carcingen-screening tool. Overall this work sheds light on current mechanistic uncertainties surrounding the model and further support its utility as a short-term carcinogenicity screen and research tool.