Context-Dependent Function of Myoepithelial Cells in Breast Morphogenesis and Neoplasia

Myoepithelial cells (MEPs) are specialized cells derived from epithelial progenitor cells, yet they also express the contractile machinery of smooth muscle cells. MEPs are prominent in glandular tissues where their function is to help expel secretions generated by the glandular epithelial cells. In the breast, MEPs are part of the bi-layered breast epithelium that line ducts and alveoli positioned perpendicular to the luminal epithelial cells (LEPs), separated from the surrounding stroma by the basement membrane. Researchers have recognized MEPs as important regulators of structural and functional behavior of LEPs, namely having role in polarization of LEPs, and regulating milk production. Furthermore, they have also been proposed to act as tumor suppressors as their presence inhibits invasion of cancer cells into the surrounding stroma. There is, however, accumulating evidence that MEPs in normal breast, carcinoma in situ and in invasive breast cancer differ significantly in terms of marker expression and this may truly interfere with their ability to behave as tumor suppressors. The term myoepithelial cell is often used synonymously with basal cell. While all MEPs, due to their position, can be referred to as basal cells, some basal cells do not fulfill the criteria of being MEPs. Synonymous use of these terms may hold true under normal conditions but careful interpretation of these terms should be used in breast cancer. In recent years, partial myoepithelial differentiation and epithelial to mesenchymal transition (EMT) have been shown to be associated with, and in some cases, necessary for cancer invasion and metastasis. In this review, we will discuss the context-dependent role of MEPs in breast morphogenesis, tumor suppression, and also the appearance of basal or partial myoepithelial differentiation in aggressive forms of breast cancer.