Impaired involution of mammary glands in the absence of milk fat globule EGF factor 8

Abstract

During the involution of mammary glands, epithelial cells undergo apoptosis and are cleared for the next cycle of lactation. The clearance of apoptotic epithelial cells is mediated by neighboring epithelial cells and by macrophages that migrate into the mammary glands. Here, we report that milk fat globule EGF factor 8 (MFG-E8), a secreted glycoprotein that binds to apoptotic cells by recognizing phosphatidylserine, was expressed by epithelial cells and macrophages in mammary glands and was involved in engulfment of apoptotic cells. A deficiency of MFG-E8 caused the accumulation of a large number of milk fat globules (MFGs) in the mammary ducts during involution, indicating that the excess MFGs were cleared by an MFG-E8-dependent mechanism. The MFG-E8 ؊/؊ mice developed mammary duct ectasia with periductal mastitis, and the redevelopment of the mammary gland for their second litter was impaired. These results demonstrate that MFG-E8-mediated phagocytosis of apoptotic epithelial cells and MFGs is important for efficient involution of mammary glands.exosomes ͉ mastitis ͉ phosphatidylserine M ammary glands are unique mammalian organs that are developed to nurse offspring. For each pregnancy, the glands undergo a cycle of development, lactation, and involution. In each cycle, mammalian epithelial cells proliferate, differentiate, and die. The involution of mammary glands is triggered when the suckling stimulus is lost, usually upon weaning, and milk accumulates in the glands. The involution takes Ϸ10 days in mice and can be divided into two phases (1-3). In the first phase, which is reversible and lasts 48 h after weaning, mammary epithelial cells lose their differentiated function. In the second phase, the basement membranes and extracellular matrix in the mammary glands are degraded by proteases, leading to destruction of the lobular-alveolar architecture of the mammary glands. As involution progresses, mammary epithelial cells are removed and adipocytes concomitantly reappear, and the lobular-alveolar structure is reorganized to approach that of virgin glands (4).During the involution process, epithelial cells undergo apoptosis and are shed into the lumen (5). At the initial stage of involution, these apoptotic cells are thought to be engulfed mainly by neighboring epithelial cells; in the later stage, they seem to be cleared by macrophages that migrate into the gland (3, 6). Milk fat globules (MFGs) are minute globules carrying fat that are secreted from the epithelial cells into the lumen during lactation (7). Upon weaning, many MFGs remain in the lumen, and they seem to be engulfed by epithelial cells lining the lobules in the mammary gland (8). How these epithelial cells and macrophages recognize and engulf apoptotic cells and MFGs has not been well elucidated.Many molecules expressed on the surface of apoptotic cells or phagocytes have been proposed as ligands and receptors for the engulfment of apoptotic cells (9 -11). Mouse MFG EGF factor 8 (MFG-E8), called lactadherin in humans, is a 72-kDa glycoprotein s...

Serum-Derived Plasminogen Is Activated by Apoptotic Cells and Promotes Their Phagocytic Clearance

“…Of note, plasminogen-deficient mice display delayed involution of the mammary glands (24,25). This process is closely linked to apoptotic cell removal and comparably impaired in mice lacking the engulfment factor MFG-E8 (26). Further studies will help to elucidate whether plasminogen deficiency is associated with defects in corpse clearance and the onset of chronic inflammation and autoimmunity.…”

MFG-E8/lactadherin regulates cyclins D1/D3 expression and enhances the tumorigenic potential of mammary epithelial cells

“…In line with this, functionally blocking antibodies against MFG-E8 potentiated the effects of chemotherapeutic agents on experimental melanomas, colon carcinomas and lymphomas (Jinushi et al, 2009). In the mammary gland, MFG-E8 expression increases during pregnancy, lactation and subsequent mammary gland involution (Oshima et al, 1999;Hanayama and Nagata, 2005). It was demonstrated that MFG-E8 is required for mammary gland branching morphogenesis (Ensslin and Shur, 2007).…”

“…The binding of MFG-E8 to integrins on macrophages and to phosphatidylserine and phosphatidylethanolamine residues on apoptotic cells establishes intercellular interactions that promote the subsequent engulfment of apoptotic cells (Hanayama et al, 2002(Hanayama et al, , 2004. Similarly, MFG-E8 is required for the removal of apoptotic cells by mammary epithelial cells during mammary gland involution (Atabai et al, 2005;Hanayama and Nagata, 2005). MFG-E8 binding to integrins regulates a variety of signaling pathways.…”

p63(TP63) elicits strong trans-activation of the MFG-E8/lactadherin/BA46 gene through interactions between the TA and ΔN isoforms

“…In contrast, this study dealt with the functional p63 proteins localized to undifferentiated keratinocytes in the basal layer of stratified epithelia and in carcinomas derived from the stage. MFG-E8-L, the mouse-specific long isoform, having a Pro/Thr-enriched domain, facilitates phagocytosis of apoptotic cells by macrophages as demonstrated by recent studies with MFGE8-deficient mice (Hanayama et al, 2002;Hanayama and Nagata, 2005). However, the other isoform, MFG-E8-S, lacking the phagocytosisfacilitating activity is more ubiquitously, abundantly expressed (Watanabe et al, 2005) and expected to facilitate various types of cell-cell interactions.…”