Increased phosphotyrosine content and inhibition of proliferation in EGF-treated A431 cells.

Abstract

Epidermal growth factor (EGF), which binds to specific high-affinity cell-surface receptors, stimulates replication of a number of cell types1,2. In vitro EGF stimulates a membrane-associated protein kinase which catalyses phosphorylation of the EGF receptor at tyrosine residues3,4. The transforming proteins of several RNA tumour viruses are protein kinases which also specifically catalyse phosphorylation at tyrosine residues5–10. An elevated level of phosphotyrosine is found in cells transformed by Rous sarcoma, Fujinami sarcoma, PRCII, Y73, Snyder–Theilin and Gardner–Armstrong feline sarcoma and Abelson murine leukaemia viruses5,10–12. At least four of these viruses, which encode distinct protein kinases, catalyse phosphorylation of tyrosine residues in the same cellular substrate proteins13. In vitro EGF-stimulated protein kinase catalyses the phosphorylation of anti-p60src heavy chains, suggesting that this enzyme recognizes similar substrate determinants to p60src (refs 14,15). Here we demonstrate that EGF treatment of A431 human epidermoid carcinoma cells increases phosphotyrosine content, indicating that EGF stimulates tyro sine-specific protein kinase activity in vivo as well as in vitro. In contrast to Rous sarcoma virus (RSV) transformation, EGF inhibits replication of A431 cells. This inhibition by EGF is influenced by both cell density and tissue culture substratum.