WAP-T1 transgenic mice express SV40-TAg under control of the whey acidic protein (WAP) promoter, which directs activity of this strong viral oncogene to luminal cells of the mammary gland. the formation of hyperplastic lesions. and advanced tumor stages WAP-T1 (2C6). Malignant tumors develop late at low frequency in glands of WAP-T1 mice. They reveal a gene expression profile that recapitulates the phenotype of aggressive human cancers (18). The data suggest that carcinogenesis in resting glands of WAP-T1 is largely delayed or halted at the stage of hyperplastic lesions. The cellular composition of hyperplasia and the SB 202190 status of TAg expressing epithelial cells in these lesions compared to lactating glands and advanced tumor stages are not defined. It was speculated that TAg expression in WAP-T1 selects for certain epithelial cell types. A gene expression analysis showed that WAP-T1 tumor samples are enriched in transcription factors relevant for embryonic stem cell maintenance. It led one to assume that TAg expression may favor survival and proliferation of cells displaying features of epithelial stem or progenitor cells (19). But not only stem or progenitor cells but also cells at advanced stages of differentiation have been proposed to generate hyperplastic lesions in transgenic mouse models (20C24). Epithelia of the mouse mammary gland reveal a complex composition, SB 202190 marked by stem and progenitor cells, terminally differentiated cells, and regulatory units, such as hormone sensing cells (25). They rapidly change composition and functional status of the layer in dependence of developmental stages and environmental ATF1 signals. This raises the question whether oncogenic activity of TAg in WAP-T1 mice at the early stage of hyperplasia randomly targets epithelial cells or promotes selection of a distinct cell type. Gene expression analysis of advanced WAP-T1 tumors identified at least two different tumor entities, which completely differ in marker expression: (i) low grade tumors, exhibiting a basal-like and morphologically differentiated phenotype with loss of chromosomes 2 and 19 and (ii) high grade tumors marked by strong expression of the gene and by co-expression of keratin 8/18, SB 202190 keratin 6, and the mesenchymal marker vimentin (26). But, a heterogeneous cell composition of advanced tumors does not necessarily contradict the idea that TAg selects for a distinct epithelial cell type. Data obtained with a tumor cell line derived from WAP-T1 glands showed that tumor cells are equipped with phenotypic plasticity, which for instance allows these cells to acquire a mesenchymal or an epithelial phenotype depending on the tumor environment (27). Our data show that hyperplasia in resting glands of WAP-T1 mice are uniformly composed of cells differentiating along the alveolar lineage. The results suggest that expression of the viral oncogene in luminal epithelial cells pre-disposed to alveologenesis induces unscheduled proliferation of differentiating cells and thereby causes formation of hyperplasia. Materials and Methods Mice Inbred BALB/c and the transgenic WAP-SV40 early region mouse line SB 202190 T1 (6) were housed under SPF conditions in accordance with official regulations for care and use of laboratory animals (UKCCCR Guidelines for the Welfare of Animals in Experimental Neoplasia) and approved by Hamburgs Authority for Health (Nr. 24/96). Preparation of mouse mammary glands and isolation of luminal cell subpopulations Mammary glands were collected at indicated time points from virgin mice, lactating mice, and uniparous mice of the BALB/c or WAP-T1 strains, respectively. Lymph nodes and tumors sometimes present at late stages in WAP-T1 mice were removed. Mammary glands to be used for RNA extraction were snap-frozen in liquid nitrogen. Glands intended for immunofluorescence were embedded in Shandon Cryomatrix (Thermo Scientific) and frozen at ?80C. To extract cells for subsequent FACS-sorting all mammary glands from one mouse were pooled in L15 Medium (Sigma-Aldrich), transferred to a sterile Petri dish and minced with scalpels. The organoid suspension was digested in serum-free L15 Medium with 3?mg/ml Collagenase Type I (Life Technologies) and 1.5?mg/ml trypsin (Sigma-Aldrich) for 1?h at 37C. Cells were collected by centrifugation at 300??for 5?min and washed once in L15?+?10% fetal calf serum (FCS). Red.