Matrix/Integrin Interaction Activates the Mitogen-activated Protein Kinase, p44erk-1 and p42erk-2

Cell adhesion to extracellular matrix proteins is a dynamic process leading to dramatic changes in the cell phenotype. Integrins are one of the major receptor families that mediate cell-matrix contact. Evidence that integrins can act as signal transducing molecules has accumulated over the past few years. We report here that p44erk-1 and p42erk-2 mitogen-activated protein (MAP) kinases are rapidly phosphorylated on tyrosine residues upon adhesion of human skin fibroblasts to fibronectin or upon cross-linking of beta 1 integrins with antibody. The tyrosine phosphorylation of both kinases is associated with increased enzymatic activity. Pretreatment of the cells with cytochalasin D, which selectively disrupts the network of the actin filaments, completely inhibits this adhesion-mediated MAP kinase activation. Thus, our findings indicate that ligation of beta 1 integrins induces an increase in both tyrosine phosphorylation and enzymatic activity of p44erk-1 and p42erk-2 MAP kinases, and that the integrity of the actin cytoskeleton is essential in this process. Since MAP kinase behaves as a convergence point for diverse receptor-initiated signaling events at the plasma membrane, this serine/threonine kinase plays a key role and helps to account for the diversity of integrin-dependent cell functions.