PLATELET-DERIVED GROWTH-FACTOR STIMULATES GROWTH-FACTOR RECEPTOR-BINDING PROTEIN-2 ASSOCIATION WITH SHC IN VASCULAR SMOOTH-MUSCLE CELLS

Platelet-derived growth factor (PDGF) stimulates smooth muscle cell proliferation and migration in vascular disorders such as atherosclerosis and restenosis. Growth factor receptor binding protein-2 (GRB2) and Shc have been shown to link growth factor receptor activation with guanine nucleotide exchange on p21-ras. We have examined this pathway in cultures of rat A10 vascular smooth muscle cells. Our data demonstrate that PDGF stimulates tyrosine phosphorylation on Shc in a concentration- and time-dependent manner that parallels PDGF beta-receptor activation. Immunoprecipitates of Shc from cells exposed to PDGF revealed Shc.GRB2 complexes. She immune complexes also contained PDGF beta-receptors. Complex formation was maximal with 30 ng/ml PDGF and peaked within 10 min of exposure. Although PDGF beta-receptors contain a putative GRB2 binding site, activated receptors failed to bind GRB2 directly. Evaluation of Shc from membrane and cytosolic fractions of A10 cells showed little redistribution of Shc following PDGF exposure. Cytosolic Shc bound only GRB2, whereas, membrane-associated Shc complexed with GRB2, the PDGF beta-receptor, Src, and additional tyrosine phosphorylated proteins. We conclude that Shc serves as a primary docking protein for GRB2 in smooth muscle cells and is critical for proliferation in response to PDGF.