Phosphatidylinositol 3-Kinase is required for adherens junction-dependent mammary epithelial cell spheroid formation

Adherens junctions facilitate and maintain epithelial cell-cell adhesion. T his is true of mammary epithelial cells, both in two dimensional monolayers and in three-dimensional basement membrane cultures. Using the immortalize d, functional mouse mammary epithelial scp2 cell line, we found that pharma cological inhibition of phosphatidylinositol 3-kinase (PI3-kinase) disrupte d adherens junctions. In monolayers, this disruption was associated with de creased E-cadherin and beta -catenin at sites of cell-cell contact and decr eased association of both proteins with the cytoskeleton. Changes in the di stribution of f-actin after PI3-kinase inhibition suggest that this disrupt ion of adherens junctions may be mediated by alterations to the cytoskeleto n. In basement membrane cultures, PI3-kinase inhibition reversibly prevente d adherens junction-dependent spheroid formation and differentiative milk p rotein gene expression, both in scp2 cells and in a second mouse mammary ep ithelial cell line, EpH4. Decreasing the calcium concentration in the cultu re medium produced similar, although less dramatic, phenotypic effects. The se data indicate that adherens junctions contribute, at least in part, to t he efficient induction of basement membrane-dependent differentiation of ma mmary epithelial cells.