Physical connections between feeder cells and recipient normal mammary epithelial cells

Cells of the LA7 rat mammary tumor line stimulate proliferation of normal mouse mammary epithelial cells in culture when in direct physical contact with them. We examined junctional connections between these LA7 feeders and the recipient mouse mammary epithelial cells in order to study the role these junctions may play in growth signaling. Tight junctions and desmosomes between LA7 feeders and mouse mammary cells were detected by immunocytochemistry. These junctions connected every cell of either type with each of its neighbors. Adherens junctions, although evident between mouse mammary cells, could not be detected between LA7 and mouse mammary cells or between LA7 cells themselves. However, E-cadherin, the transmembrane protein of adherens junctions, was present in LA7 cell lysates. beta-catenin, which normally binds cadherins, was detected at the borders of LA7 cells. Presence of gap junctions between LA7 and mouse mammary cells was determined by traverse of lucifer yellow from an injected LA7 cell to surrounding mouse mammary epithelial cells. The experiments thus indicate that three types of intercellular junctions occur between cells active in direct cell-cell-stimulated proliferation signaling.