Cytokine modulation of mammary gland development and involution

In vivo and in vitro studies were used to examine the role of cytokines in mammary gland development and involution. We describe the establishment and characterization of a caprine mammary epithelial cell line (CMEC) and a caprine mammary myoepithelial cell line (CMMyoEC). CMECs are epithelial based on morphology and expression of cytoskeletal proteins. They respond to endocrine and cytokine growth factors and show functional and morphologic differentiation in culture. CMMyoECs are myoepithelial based on morphology and expression of α-smooth muscle actin and vimentin. The cells undergo functional differentiation reflective of a contractile response to exogenous oxytocine. Myoepithelial cells show an augmented proliferation response when cultured with epithelial cell culture supernatant. Co-culture experiments with the two cell lines demonstrate the interactions required for organ-like development in vitro. We also observe the production of an epithelial-derived myoepithelial chemotattractant. In in vivo studies on the role of apoptosis in mammary gland involution, morphological examination and TUNEL assay demonstrate the induction of cell death processes shortly after the secession of milk removal. Apoptosis is observed to occur at a consistent rate, with no more than 5% of the total epithelial cells between 7 to 21 days of involution showing signs of cell death. Immunohistochemical studies suggest that TNFα expressed by epithelial cells acts as an autocrine mediator of apoptosis. TNF-R1 is expressed on the apical surface of lactating mammary epithelial cells and shed as cytoplasmic fragments as part of the normal apocrine lactation process. During involution, the receptor is found to be associated with the cell membrane suggesting it\u27s role in the induction of signal transduction for apoptosis. In vitro the caprine mammary epithelial cell line also shows sensitivity to TNFα-induced and involuting mammary gland secretion-induced cell death. These in vitro and in vivo studies suggest that post-lactation mammary gland involution is cytokine mediated.