Growth factor-stimulated protein phosphorylation in G0/G1-arrested fibroblasts. Two distinct classes of growth factors with potentiating effects.

Protein phosphorylation of G0/G1-arrested Chinese hamster lung fibroblasts (CC139 line) has been analyzed following stimulation by fetal calf serum (FCS) or by a variety of growth factors. FCS stimulated the phosphorylation of three major polypeptides separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis: a nuclear protein with a Mr of 62,000 daltons, the ribosomal protein S6, and a cytosoluble peptide of 27,000 daltons. These phosphorylations occurred rapidly after serum stimulation (1 min for the 27,000-dalton peptide, 5 min for S6 and the 62,000-dalton proteins) and were maximal after 30 min. In nonstimulated cells the 27,000-dalton phosphopeptide exists in two forms with isoelectric points of 5.7 and 6.0; serum increased the amount of the most acidic form. At low concentrations, the "commitment" growth factors, alpha-thrombin, eye-derived growth factor (EDGF), platelet-derived growth factor (PDGF), stimulated phosphorylation of the 27,000-dalton peptide. At higher concentrations, these factors alone reinitiated DNA synthesis and, like FCS, stimulated phosphorylation of the three major peptides. In contrast, and suggesting a different mechanism of action, "progression" factors such as insulin (1-10 micrograms/ml) and multiplication-stimulating activity (MSA) are unable to stimulate phosphorylation of the 27,000-dalton peptide. However, insulin or MSA which are known to potentiate the mitogenic action of alpha-thrombin, PDGF, EDGF, ... were also found to potentiate phosphorylation of the ribosomal protein S6. These results support the existence of two classes of growth factors and suggest that protein phosphorylation is an early event involved in the control of the cellular G0 leads to G1 transition.