title: Epithelial cell-directed efferocytosis in the post-partum mammary gland is necessary for tissue homeostasis and future lactation.
abstract: BACKGROUND
  Mammary glands harbor a profound burden of apoptotic cells (ACs) during post-lactational involution, but little is known regarding mechanisms by which ACs are cleared from the mammary gland, or consequences if this process is interrupted. We investigated AC clearance, also termed efferocytosis, during post-lactational remodeling, using mice deficient for MerTK, Axl, and Tyro3, three related receptor tyrosine kinases (RTKs) regulating macrophage-mediated efferocytosis in monocytes. MerTK expression, apoptosis and the accumulation of apoptotic debris were examined in histological sections of MerTK-deficient, Axl/Tyro3-deficient, and wild-type mammary glands harvested at specific time points during lactation and synchronized involution. The ability of primary mammary epithelial cells (MECs) to engulf ACs was assessed in culture. Transplant of MerTK-deficient mammary epithelium into cleared WT mammary fat pads was used to assess the contribution of WT mammary macrophages to post-lactational efferocytosis.
  
  
  RESULTS
  ACs induced MerTK expression in MECs, resulting in elevated MerTK levels at the earliest stages of involution. Loss of MerTK resulted in AC accumulation in post-lactational MerTK-deficient mammary glands, but not in Axl and Tyro3-deficient mammary glands. Increased vascularization, fibrosis, and epithelial hyperproliferation were observed in MerTK-deficient mammary glands through at least 60 days post-weaning, due to failed efferocytosis after lactation, but did not manifest in nulliparous mice. WT host-derived macrophages failed to rescue efferocytosis in transplanted MerTK-deficient mammary epithelium.
  
  
  CONCLUSIONS
  Efferocytosis by MECs through MerTK is crucial for mammary gland homeostasis and function during the post-lactational period. Efferocytosis by MECs thus limits pathologic consequences associated with the apoptotic load following lactation.
authors.affiliation: UNC-Lineberger Comprehensive Cancer Center, 450 West Ave, Chapel Hill, North Carolina 27599, USA.
authors.forename: Melissa
authors.initials: M
authors.lastname: Sandahl
authors.forename: Debra M
authors.initials: DM
authors.lastname: Hunter
authors.forename: Karen E
authors.initials: KE
authors.lastname: Strunk
authors.forename: H Shelton
authors.initials: HS
authors.lastname: Earp
authors.forename: Rebecca S
authors.initials: RS
authors.lastname: Cook
mesh_terms: D000818:Animals; D017209:Apoptosis; D004847:Epithelial Cells / Q000378:metabolism*; D005260:Female; D006706:Homeostasis; D007774:Lactation; D008264:Macrophages; D008321:Mammary Glands, Animal / Q000166:cytology* / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D010587:Phagocytosis; D016133:Polymerase Chain Reaction; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D000076202:c-Mer Tyrosine Kinase; D000094963:Axl Receptor Tyrosine Kinase
chemical_list: D011518:Proto-Oncogene Proteins; C097072:MERTK protein, human; D020794:Receptor Protein-Tyrosine Kinases; C084944:TYRO3 protein, human; D000076202:c-Mer Tyrosine Kinase; D000094963:Axl Receptor Tyrosine Kinase; C000722290:AXL receptor tyrosine kinase, mouse