Extracellular matrix selectively modulates the response of mammary epithelial cells to different soluble signaling ligands.

In adherent cells, cell-substratum interactions are essential for the propagation of some growth factor signaling events. However, it has not been resolved to what extent different types of extracellular matrix regulate the signals elicited by different soluble ligands. Our previous work has shown that prolactin signaling in mammary epithelium requires a specific cell interaction with the basement membrane and does not occur in cells plated on collagen I. We have now investigated whether the proximal signaling pathways triggered by insulin, epidermal growth factor (EGF), and interferon-gamma are differentially regulated in primary mammary epithelial cell cultures established on basement membrane and collagen I. Two distinct signaling pathways triggered by insulin exhibited a differential requirement for cell-matrix interactions. Activation of insulin receptor substrate (IRS) and phosphatidylinositol 3-kinase was restricted to cells contacting basement membrane, whereas the phosphorylation of Erk occurred equally in cells on both substrata. The amplitude and duration of insulin-triggered IRS-1 phosphorylation and its association with phosphatidylinositol 3-kinase were strongly enhanced by cell-basement membrane interactions. The mechanism for inhibition of IRS-1 phosphorylation in cells cultured on collagen I may in part be mediated by protein-tyrosine phosphatase activity since vanadate treatment somewhat alleviated this effect. In contrast to the results with insulin, cell adhesion to collagen I conferred greater response to EGF, leading to higher levels of tyrosine phosphorylation of the EGF receptor and Erk. The mechanism for increased EGF signaling in cells adhering to collagen I was partly through an increase in EGF receptor expression. The interferon-gamma-activated tyrosine phosphorylation of Jak2 and Stat3 was independent of the extracellular matrix. It is well recognized that the cellular environment determines cell phenotype. We now suggest that this may occur through a selective modulation of growth factor signal transduction resulting from different cell-matrix interactions.