Integrin-mediated Activation of MEK and Mitogen-activated Protein Kinase Is Independent of Ras

Abstract

The integrins are a family of cell surface receptors that mediate adhesive interactions with the extracellular matrix and also generate signals that influence cell growth and differentiation. Ligation and clustering of integrins causes activation and autophosphorylation of focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase, and results in the transient activation of p42 and p44 mitogen-activated protein (MAP) kinases. Initial evidence has suggested that the integrin signaling pathway may share common elements with the canonical Ras signal transduction cascade activated by peptide mitogens such as epidermal growth factor (EGF). In this report we demonstrate that Raf-1 and MAP or extracellular signal-related kinase kinase (MEK), key cytoplasmic kinases of the Ras cascade, are activated subsequent to integrin-mediated adhesion of mouse NIH 3T3 fibroblasts. We also show that MAP kinase is downstream of MEK in the integrin signaling pathway. However, in contrast to the receptor tyrosine kinase signaling cascade, integrin-mediated signal transduction seems to be largely independent of Ras. Dominant negative inhibitors of Ras-dependent signaling failed to block integrin-mediated activation of MEK. In addition, while treatment with the peptide mitogen EGF clearly increased GTP-loading of Ras, little effect was observed in response to integrin-dependent cell adhesion. Thus, integrin-mediated activation of MEK and MAP kinase in 3T3 cells occurs primarily by a mechanism that is distinct from the Ras signal transduction cascade.