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

ABSTRACT: 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.

ORGANISM(S): Mus musculus

PROVIDER: GSE84190 | GEO | 2016/09/09

SECONDARY ACCESSION(S): PRJNA328249

REPOSITORIES: GEO

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Project description:Identification and transcriptome analysis of mammary stem cells (MaSC) are important steps toward understanding the molecular and cellular bases of mammary epithelial growth, homeostasis and tissue repair. We identified MaSC based upon their ability to retain bromodeoxyuridine (BrdU) for an extended period. Using laser microdissection, we isolated four categories of cells from the mammary epithelium of female bovine calves. These categories included label retaining epithelial cells (LREC) from the basal layer (LRECb), basal control cells (ECb), LREC from embedded layers (LRECe) and embedded control cells (ECe). Microarray analysis revealed 592 genes that were differentially expressed between LRECb and ECb, and 110 genes between LRECe and ECe. Our analysis supported classification of LRECb as MaSC and LRECe as progenitor cells. Enriched expression of genes in LRECb were associated with stem cell attributes and identified enrichment of Wnt, Notch and MAPK pathways of self renewal and proliferation. Genes expressed in LRECe revealed retention of some stem-like properties along with up-regulation of differentiation factors and the Notch pathway involved in lineage commitment. The basal epithelium provided for the stem cell niche, characterized as a microenvironment containing numerous survival and proliferation factors, growth suppressors and chromatin modifiers in balance state. We conclude that BrdU label retention identifies stem and progenitor cells, wherein MaSC are located in the basal epithelial layer and committed progenitor cells are localized in more apical layers. Results provide novel candidate markers for bovine MaSC with potential applicability to the study of MaSC in other species. Five Holstein heifers were injected with bromodeoxyuridine (BrdU) for five consecutive days and sacrificed 45 days later, at which time mammary samples were obtained and processed. Cryosections were prepared and immunostained to detect epithelial cells that retained BrdU label (LREC). Four classes of cells were excised by laser microdissection, which included LREC and control cells in the basal and in the suprabasal layers of the mammary epithelium. For each cell type, cells were lysed, RNA was reverse transcribed and cDNA amplified for microarray analysis. Twenty microarrays were utilized for sample hybridization and another array for hybridization of amplified cDNA from a water blank.

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

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