title: New insights into lineage restriction of mammary gland epithelium using parity-identified mammary epithelial cells.
abstract: BACKGROUND
  Parity-identified mammary epithelial cells (PI-MECs) are an interesting cellular subset because they survive involution and are a presumptive target for transformation by human epidermal growth factor receptor 2 (HER2)/neu in mammary tumors. Depending on the type of assay, PI-MECs have been designated lobule-restricted progenitors or multipotent stem/progenitor cells. PI-MECs were reported to be part of the basal population of mammary epithelium based on flow cytometry. We investigated the cellular identity and lineage potential of PI-MECs in intact mammary glands.
  
  
  METHODS
  We performed a quantitative and qualitative analysis of the contribution of PI-MECs to mammary epithelial cell lineages in pregnant and involuted mammary glands by immunohistochemistry, fluorescence-activated cells sorting (FACS), and quantitative polymerase chain reaction. PI-MECs were labeled by the activation of Whey Acidic Protein (WAP)-Cre during pregnancy that results in permanent expression of yellow fluorescent protein.
  
  
  RESULTS
  After involution, PI-MECs are present exclusively in the luminal layer of mammary ducts. During pregnancy, PI-MECs contribute to the luminal layer but not the basal layer of alveolar lobules. Strikingly, whereas all luminal estrogen receptor (ER)-negative cells in an alveolus can be derived from PI-MECs, the alveolar ER-positive cells are unlabeled and reminiscent of Notch2-traced L cells. Notably, we observed a significant population of unlabeled alveolar progenitors that resemble PI-MECs based on transcriptional and histological analysis.
  
  
  CONCLUSIONS
  Our demonstration that PI-MECs are luminal cells underscores that not only basal cells display multi-lineage potential in transplantation assays. However, the lineage potential of PI-MECs in unperturbed mammary glands is remarkably restricted to luminal ER-negative cells of the secretory alveolar lineage. The identification of an unlabeled but functionally similar population of luminal alveolar progenitor cells raises the question of whether PI-MECs are a unique population or the result of stochastic labeling. Interestingly, even when all luminal ER-negative cells of an alveolus are PI-MEC-derived, the basal cells and hormone-sensing cells are derived from a different source, indicating that cooperative outgrowth of cells from different lineages is common in alveologenesis.
authors.forename: Ted H-T
authors.initials: TH
authors.lastname: Chang
authors.forename: Kamini
authors.initials: K
authors.lastname: Kunasegaran
authors.forename: Gerard A
authors.initials: GA
authors.lastname: Tarulli
authors.forename: Duvini
authors.initials: D
authors.lastname: De Silva
authors.forename: P Mathijs
authors.initials: PM
authors.lastname: Voorhoeve
authors.forename: Alexandra M
authors.initials: AM
authors.lastname: Pietersen
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics*; D051927:CD24 Antigen / Q000378:metabolism; D002364:Caseins / Q000378:metabolism; D002454:Cell Differentiation; D019070:Cell Lineage; D004268:DNA-Binding Proteins / Q000378:metabolism; D004847:Epithelial Cells / Q000166:cytology*; D005260:Female; D005434:Flow Cytometry; D039503:Integrin alpha6 / Q000378:metabolism; D008164:Luminescent Proteins / Q000235:genetics*; D008321:Mammary Glands, Animal / Q000166:cytology*; D051379:Mice; D008822:Mice, Transgenic; D008894:Milk Proteins / Q000494:pharmacology; D039902:Multipotent Stem Cells / Q000166:cytology*; D010298:Parity; D011247:Pregnancy; D011960:Receptors, Estrogen / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D051927:CD24 Antigen; D002364:Caseins; C497890:Cd24a protein, mouse; D004268:DNA-Binding Proteins; C500759:Elf5 protein, mouse; D039503:Integrin alpha6; D008164:Luminescent Proteins; D008894:Milk Proteins; D011960:Receptors, Estrogen; D014157:Transcription Factors; C024471:whey acidic proteins; C054562:yellow fluorescent protein, Bacteria