Stimulation of tyrosine-specific phosphorylation by platelet-derived growth factor

Abstract

Recent studies have shown that tyrosine-specific protein kinases may be involved in both virus transformation and growth stimulation with polypeptide growth factors. Thus, several onc gene products possess such kinase activity^1–9 and, further, binding of epidermal growth factor (EGF)¹⁰ to A-431 cells stimulates phosphorylation of tyrosine residues of both membrane and cytoplasmic proteins^11–16. Platelet-derived growth factor (PDGF), a 30,000 molecular weight (M_r) polypeptide, is the major growth-promoting factor in serum for connective tissue-derived cells and glial cells in culture^17–22. High-affinity binding of ¹²⁵I-PDGF to a specific receptor on such cells has recently been demonstrated; thus, human foreskin fibroblasts were found to have some 300,000 PDGF receptors per cell, with an affinity constant of 1 nM (ref. 23). The addition of PDGF to human glial cells and fibroblasts produces various phenotypic changes, similar to those induced by EGF²⁴ and tumour viruses²⁵, which include rapid induction of membrane ruffling, reduction in cell adhesion, change in structure of the actin cytoskeleton and increased mitotic activity (B.W. et al., unpublished data). These considerations suggested that PDGF binding to cellular receptors might similarly stimulate kinase activity. The present report demonstrates such an effect: when incubated with plasma membranes from human fibroblasts or glial cells, PDGF induces the phosphorylation of tyrosine residues of membrane proteins with apparent molecular weights of 175,000 and 130,000.