Phospholipase C-γ, a substrate for PDGF receptor kinase, is not phosphorylated on tyrosine during the mitogenic response to CSF-1

Abstract

Quiescent mouse NIH3T3 cells expressing a transduced human c-fms gene encoding the receptor for colony stimulating factor-1 (CSF-1) were stimulated with mitogenic concentrations of platelet-derived growth factor (PDGF) or CSF-1. Immunoprecipitated phospholipase C-Γ (PLC-γ) was phosphorylated on tyrosine and calcium was mobilized following treatment of intact cells with PDGF. In contrast, only trace amounts of phosphotyrosine were incorporated into PLC-γ and no intracellular calcium signal was detected after CSF-1 stimulation. Similarly, CSF-1 treatment did not stimulate phosphorylation of PLC-γ on tyrosine in a CSF-1-dependent, SV40-immortalized mouse macrophage cell line that expresses high levels of the CSF-1 receptor. In fibroblasts, antiserum to PLC-γ co-precipitated a fraction of the tyrosine phosphorylated form of the PDGF receptor (PDGF-R) after ligand stimulation, implying that phosphorylated PDGF-R and PLC-γ were associated in a stable complex. Pre-treatment of cells with orthovanadate also led to tyrosine phosphorylation of PLC-γ which was significantly enhanced by PDGF, but not by CSF-1. Thus, although the PDGF and CSF-1 receptors are structurally related and appear to be derived from a single ancestor gene, only PDGF-induced mitogenesis in fibroblasts correlated with tyrosine phosphorylation of PLC-γ.