Integrin-mediated RON growth factor receptor phosphorylation requires tyrosine kinase activity of both the receptor and c-Src

Cooperation between integrins and growth factor receptors plays an importan t role in the regulation of cell growth, differentiation, and survival. The function of growth factor receptor tyrosine kinases (RTKs) can be regulate d by cell adhesion to extracellular matrix (ECM) even in the absence of lig and, We investigated the pathway involved in integrin-mediated RTK activati on, using RON, the receptor for macrophage-stimulating protein. Adhesion of RON-expressing epithelial cells to ECM caused phosphorylation of RON, whic h depended on the kinase activity of both RON itself and c-Src. This conclu sion is based on these observations: 1) ECM-induced RON phosphorylation was inhibited in cells expressing kinase-inactive c-Src; 2) active c-Src could phosphorylate immunoprecipitated RON from ECM-stimulated cells but not fro m unstimulated cells; and 3) ECM did not cause RON phosphorylation in cells expressing kinase-dead RON, nor could active c-Src phosphorylate RON immun oprecipitated from these cells. The data fit a pathway in which ECM-induced integrin aggregation causes both c-Src activation and RON oligomerization followed by RON kinase-dependent autophosphorylation; this results in RON b ecoming a target for activated c-Src, which phosphorylates additional tyros ines on RON. Integrin-induced epidermal growth factor receptor (EGFR) phosp horylation also depended on both EGFR and c-Src kinase activities. This seq uence appears to be a general pathway for integrin-dependent growth factor RTK activation.