The long-range goal of this project is to elucidate molecular and cellular mechanisms important in breast cancer causation, utilizing the BALB/c mouse model and chemical carcinogens. The approach has been to examine the expression of endogenous viral (MMTV) genes and cellular oncogenes in different stages of mammary tumorigenesis. Our data indicate that multiple pathways are operative in the genesis of BALB/c mammary tumors. This renewal application combines in vivo and in vitro approaches in order to define general types of molecular and cellular changes that occur at defined stages in breast cancer development. We speculate that one possible initiating event in the process (which can be induced by multiple agents) may be the altered expression of a trans-regulatory gene, coupled with the activation of one or more cellular oncogenes by chemical carcinogen. The MMTV LTR gene product may sometimes be involved in the early states of tumorigenesis in BALB/c mice. The following specific aims are presented. The first is an extension of in vivo studies just completed. The other two aims are new directions that will be long-term efforts. 1) Multiple transformation assays. Normal and preneoplastic mammary cells will be exposed to selected oncogenes that represent different sites of oncogene action in growth-control pathways (fos, erb-B, H-ras,neu). Combinations of oncogenes will be tested to identify functional activities necessary to mediate complete transformation of mammary epithelial cells. A powerful feature of the mouse system is that the cell phenotypes (normal, preneoplastic, malignant) can be assessed by transplantation back into cleared mammary fat pads of syngeneic mice, providing in vivo correlates of in vitro assays. These studies will also better define the nature of preneoplastic mammary cells. 2) The structure and function of the MMTV LTR gene product will be determined. Protein will be synthesized using a baculovirus expression vector system and specific antisera prepared. Assays using the CAT reporter gene will test if the LTR protein has trans-acting ability. Transgenic mice will be made to allow detailed characterization of the LTR protein. 3) Both in vivo (transgenic mice) and in vitro gene or growth-modulating genes display increased susceptibility to transformation by the chemical carcinogen DMBA. This program will provide new understanding of molecular and cellular changes leading to breast cancer.