Stromal PTEN inhibits the expansion of mammary epithelial stem cells through Jagged-1. Mus musculus

Fibroblasts within the mammary tumor microenvironment are active participants in carcinogenesis mediating both tumor initiation and progression. Our group has previously demonstrated that genetic loss of PTEN in mammary fibroblasts induces an oncogenic secretome that remodels the extracellular milieu accelerating ErbB2-driven mammary tumor progression. While these prior studies highlighted a tumor suppressive role for stromal PTEN, how the adjacent normal epithelium transforms in response to PTEN loss was not previously addressed. To identify these early events, we have evaluated both phenotypic and genetic changes within the pre-neoplastic mammary epithelium of mice with and without stromal PTEN expression. We report that fibroblast-specific PTEN deletion greatly restricts mammary ductal elongation and induces aberrant alveolar side-branching. These mice concomitantly exhibit an expansion of the mammary epithelial stem cell (MaSC) enriched basal/myoepithelial population and further genome wide expression analysis followed by gene set enrichment analysis (GSEA) revealed that NOTCH signaling is diminished in these cells. NOTCH3 was confirmed to be downregulated in the MaSC-enriched pool by confirmatory qRT-PCR and immunofluorescence. Mechanistically, JAGGED-1, a transmembrane ligand for the NOTCH receptor, is downregulated in the PTEN-null fibroblasts leading to a loss in the paracrine activation of NOTCH signaling from the surrounding stroma. Overall design: Each biological replicate of FACS sorted Lin-CD24+CD29Hi MaSC-enriched epithelial subpopulations was collected by pooling pre-neoplastic mammary tissue from 6-8 virgin ErbB2;Ptenfl/fl or ErbB2;Fsp-cre;Ptenfl/fl (8-9 weeks of age) mice. Microarray on three MaSC-enriched samples from both genetic groups was performed with GeneChip® Mouse Transcriptome Assay 1.0 (Affymetrix, Santa Clara, California, USA) at the OSUCCC Genomics Facility.