Cell adhesion to the apical pole of epithelium: a function of cell polarity

Human uterine epithelium displays a distinct polarized organization with apical, lateral, and basal plasma membrane domains. Although non-adhesive throughout most of the menstrual cycle, epithelial cells allow attachment of trophoblast cells to their apical pole during embryo implantation. A recent hypothesis postulates that epithelial cells turn off genes for apical-basal polarity and turn on genes for a more mesenchyme-like phenotype allowing cell-cell interaction with trophoblast. Using an in vitro assay human uterine cell lines (RL95-2, HEC-1-A, AN3-CA) were selected on the basis of adhesiveness for trophoblast-type cells (JAR). Subsequently, uterine cells were examined for epithelium-specific ultrastructure using transmission electron microscopy, and for the expression of E-cadherin, alpha 6-, beta 1-, beta 4-integrin subunits and cytokeratin using immunocytochemistry, confocal laser scanning microscopy, and surface replication technique. HEC-1-A monolayers are non-adhesive for JAR cells and appear highly polarized expressing E-cadherin, alpha 6-, beta 1-, beta 4-integrin subunits, and cytokeratin. Both, integrins and E-cadherin, are present at the lateral membrane. RL95-2 monolayers which are adhesive for JAR cells appear non-polarized. Like HEC-1-A cells, RL95-2 cells express E-cadherin, alpha 6-, beta 1-, and beta 4-integrin subunits, and cytokeratin. In contrast to HEC-1-A cells, integrins and E-cadherin are distributed at the entire cell surface. AN3-CA monolayers are non-adhesive for JAR cells and appear non-polarized. Cells lack epithelial-specific markers such as keratin and E-cadherin. They show only low expression of alpha 6-, beta 1-integrin subunits and lack beta 4-integrin subunit. Conversely, they express vimentin. Thus, modulation of the epithelial phenotype of uterine cells, i.e. loss of apical-basal polarity, might prepare the apical cell pole for cell-cell interaction with trophoblast. However, loss of cell polarity would not lead to enhancement of adhesiveness for trophoblast if accompanied by a loss of epithelium-specific adhesion molecules.