Cell–matrix interactions during development and apoptosis of the mouse mammary gland in vivo

Abstract

Epithelial cell survival is dependent on extracellular signals provided by both soluble factors and by adhesion. In the mammary gland, extensive apoptosis of epithelial cells occurs rapidly when lactation ceases, but the mechanism of apoptosis induction is not known. In tissue culture, mammary epithelial cells require laminin as a survival ligand and specific ␤1 integrins are necessary to suppress apoptosis. To explore the possibility that dynamic changes in cell-matrix interactions contribute to the onset of apoptosis during mammary involution in vivo, a detailed immunohistochemical analysis of the expression of integrin subunits and their extracellular matrix ligands during mouse mammary gland development has been performed. The kinetics of apoptosis were determined by using tissue samples obtained from virgin, pregnant, lactating, and involuting gland. The maximal elevation of apoptosis occurred within 24 hr of weaning as determined by histologic analysis and caspase-3 staining. A wide variety of laminin subunits, together with nidogen-1 and -2, and perlecan were identified within the basement membrane region of epithelial ducts, lobules, and alveoli in both human and mouse mammary gland. However, no change in the distribution of any of the basement membrane proteins or their cognate integrin receptors was observed during the transition from lactation to apoptosis. Instead, we discovered that altered ligand-binding conformation of the ␤1 integrin to a nonbinding state coincided with the immediate onset of mammary apoptosis. This finding may provide a novel dynamic mechanism for inhibiting the transduction of extracellular matrix survival signals, thereby contributing to the onset of apoptosis in a developmental context in vivo.

Regulation of clusterin expression in mammary epithelial cells

“…This implies that the function of β1 integrin is required for the correct initiation of involution. On the other hand, it has been reported that the level of ligand-bound β1 integrin declined when apoptosis of mammary epithelial cells began during involution (38). From all these observations, we speculate that the fall of ligand-bound β1 integrin levels leads to the drop in clusterin expression at day 3 of involution.…”

RETRACTED ARTICLE: Regulation and Functional Relevance of Milk Fat Globules and Their Components in the Mammary Gland

“…Degradation of nuclear DNA into fragments that form ladders on agarose gels, apoptotic morphology of epithelial cells exhibiting chromatin condensation, and activation of effector caspases, all characteristic markers of apoptosis, are observed within 2 d of withdrawing pups. 48,[50][51][52][53] Apoptotic mammary epithelial cells must be cleared immediately by phagocytes in involuting mammary glands to prevent inflammation and autoimmune response against intracellular antigens released by the dying cells. Phagocytosis of apoptotic mammary epithelial cells by both macrophages and residual living epithelial cells has been demonstrated with convincing data, and three potential fates of apoptotic mammary epithelial cells have been suggested: release into the lumen, and phagocytosis by neighboring alveolar epithelial cells or macrophages.…”

Role of DDC-4/sFRP-4, a secreted Frizzled-related protein, at the onset of apoptosis in mammary involution

“…27 Recent work, however, tends to show that the earliest events of apoptosis occur before detectable changes in extracellular matrix or cognate integrin receptors can be detected. 28 Other survival signals of importance are the insulin-like growth factors (IGFs), whose function can be blocked by IGF-binding proteins (IGFBP). For example, IGFBP-5 is produced very early at the onset of involution; hence, it could well be a relevant mediator of apoptosis in mammary involution.…”