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 propa gation of some growth factor signaling events. However, it has not been res olved to what extent different types of extracellular matrix regulate the s ignals elicited by different soluble ligands. Our previous work has shown t hat prolactin signaling in mammary epithelium requires a specific cell inte raction with the basement membrane and does not occur in cells plated on co llagen I. We have now investigated whether the proximal signaling pathways triggered by insulin, epidermal growth factor (EGF), and interferon-gamma a re differentially regulated in primary mammary epithelial cell cultures est ablished on basement membrane and collagen I. Two distinct signaling pathwa ys triggered by insulin exhibited a differential requirement for cell-matri x interactions. Activation of insulin receptor substrate (IRS) and phosphat idylinositol 3-kinase was restricted to cells contacting basement membrane, whereas the phosphorylation of Erk occurred equally in cells on both subst rata. The amplitude and duration of insulin-triggered IRS-1 phosphorylation and its association with phosphatidylinositol 3-kinase were strongly enhan ced by cell-basement membrane interactions. The mechanism for inhibition of IRS-1 phosphorylation in cells cultured on collagen I may in part be media ted by protein-tyrosine phosphatase activity since vanadate treatment somew hat alleviated this effect. In contrast to the results with insulin, cell a dhesion to collagen I conferred greater response to EGF, leading to higher levels of tyrosine phosphorylation of the EGF receptor and Erk. The mechani sm for increased EGF signaling in cells adhering to collagen I was partly t hrough an increase in EGF receptor expression, The interferon-gamma-activat ed tyrosine phosphorylation of Jak2 and Stat3 was independent of the extrac ellular matrix. It is well recognized that the cellular environment determi nes cell phenotype. We now suggest that this may occur through a selective modulation of growth factor signal transduction resulting from different ce ll-matrix interactions.