PLATELET-DERIVED GROWTH-FACTOR STIMULATES PHOSPHORYLATION OF GROWTH-FACTOR RECEPTOR-BINDING PROTEIN-2 IN VASCULAR SMOOTH-MUSCLE CELLS

Growth factor receptor binding protein-2 (GRB2) couples growth factor receptor activation to the p21(ras) nucleotide exchange factor son-of- sevenless (SOS). Son-of-sevenless can serve as a substrate for mitogen -activated protein kinases and may be subject to feed back regulation in mitogen-stimulated cells. Herein, we demonstrate phosphorylation on GRB2 in rat A10 Vascular smooth muscle cells exposed to platelet-deri ved growth factor (PDGF). Lysates from smooth cells stimulated with PD GF revealed a shift in the electrophoretic mobility of GRB2. Further i nvestigation confirmed that phosphorylation on GRB2 accompanied this m obility shift. Phosphorylation on GRB2 was time dependent and correlat ed with PDGF receptor activation. The time-course for phosphorylation of GRB2 and subsequent decay corresponded with other events characteri stic of platelet-derived growth factor signaling. GRB2 was not phospho rylated in cells treated with phorbol 12-myristate 13-acetate, and dow n-regulation of protein kinase C failed to attenuate phosphorylation o n GRB2 in response to growth factor. Analysis of GRB2 immune complexes revealed a kinase activity capable of phosphorylating GRB2 in vitro a nd demonstrated that the kinase activated in response to PDGF may phys ically associate with GRB2 signaling complexes.