Stimulation of beta1 integrins on fibroblasts induces PDGF independent tyrosine phosphorylation of PDGF beta-receptors.

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Abstract
We report that integrin-mediated signaling induces a rapid and transient tyrosine phosphorylation of platelet-derived growth factor (PDGF) beta-receptors in human diploid foreskin AG 1518 fibroblasts. A transient tyrosine phosphorylation of PDGF beta-receptors was evident one and two hours after cells had been plated on collagen type I and fibronectin, as well as on immobilized anti-integrin subunit IgG, but not on poly-L-lysine. In contrast EGF or PDGF alpha-receptors were not phosphorylated on tyrosine residues under these conditions. Tyrosine phosphorylation of PDGF beta-receptors induced by plating on collagen type I was inhibited by cytochalasin D and herbimycin A, unaffected by cycloheximide and enhanced by orthovanadate. Furthermore, a transient phosphorylation of PDGF beta-receptors occurred when AG 518 fibroblasts were cultured in three-dimensional collagen lattices or exposed to external strain exerted through centrifugation. The latter effect was evident already after two minutes. Clustering of cell surface beta1 integrins led to PDGF beta-receptor phosphorylation both in suspended and firmly attached AG 1518 fibroblasts. Plating of cells on collagen type I, fibronectin, and anti-beta1-integrin IgG resulted in the formation of PDGF beta-receptor aggregates as detected by immunofluorescence. Suramin or anti-PDGF-BB IgG had no effect on the plating-induced tyrosine phosphorylation of PDGF beta-receptors. PDGF-B chain mRNA, or protein, were not detected in AG 1518 fibroblasts. Our data suggest that a ligand-independent PDGF beta-receptor activation during cell adhesion and early phases of cell spreading is involved in integrin-mediated signaling in fibroblasts, and constitutes parts of a mechanism for cells to respond during the dynamic phases of externally applied tension as well as fibroblast-mediated tension during cell adhesion and collagen gel contraction.