title: Estrogen receptor-alpha and progesterone receptor are expressed in label-retaining mammary epithelial cells that divide asymmetrically and retain their template DNA strands.
abstract: BACKGROUND
  Stem cells of somatic tissues are hypothesized to protect themselves from mutation and cancer risk through a process of selective segregation of their template DNA strands during asymmetric division. Mouse mammary epithelium contains label-retaining epithelial cells that divide asymmetrically and retain their template DNA.
  
  
  METHODS
  Immunohistochemistry was used in murine mammary glands that had been labeled with [3H]thymidine during allometric growth to investigate the co-expression of DNA label retention and estrogen receptor (ER)-alpha or progesterone receptor (PR). Using the same methods, we investigated the co-localization of [3H]thymidine and ER-alpha or PR in mammary tissue from mice that had received treatment with estrogen, progesterone, and prolactin subsequent to a long chase period to identify label-retaining cells.
  
  
  RESULTS
  Label-retaining epithelial cells (LRECs) comprised approximately 2.0% of the entire mammary epithelium. ER-alpha-positive and PR-positive cells represented about 30-40% of the LREC subpopulation. Administration of estrogen, progesterone, and prolactin altered the percentage of LRECs expressing ER-alpha.
  
  
  CONCLUSIONS
  The results presented here support the premise that there is a subpopulation of LRECs in the murine mammary gland that is positive for ER-alpha and/or PR. This suggests that certain mammary LRECs (potentially stem cells) remain stably positive for these receptors, raising the possibility that LRECs comprise a hierarchy of asymmetrically cycling mammary stem/progenitor cells that are distinguished by the presence or absence of nuclear steroid receptor expression.
authors.affiliation: Mammary Biology and Tumorigenesis Laboratory, National Cancer Institute, National Institute of Health, Bethesda, Maryland, USA. boothbr@mail.nih.gov
authors.forename: Brian W
authors.initials: BW
authors.lastname: Booth
authors.forename: Gilbert H
authors.initials: GH
authors.lastname: Smith
mesh_terms: D000818:Animals; D002455:Cell Division / Q000502:physiology*; D004247:DNA; D004847:Epithelial Cells / Q000502:physiology*; D047628:Estrogen Receptor alpha / Q000096:biosynthesis*; D005260:Female; D008321:Mammary Glands, Animal / Q000166:cytology / Q000503:physiopathology*; D051379:Mice; D011980:Receptors, Progesterone / Q000096:biosynthesis*; D013698:Templates, Genetic
chemical_list: D047628:Estrogen Receptor alpha; D011980:Receptors, Progesterone; D004247:DNA