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Platelet-derived growth factor (PDGF) stimulates the proliferation of quiescent fibroblasts through a series of events initiated by activation of tyrosine kinase activity of the PDGF receptor at the cell surface. Physiologically significant substrates for this or other growth factor receptor or oncogene tyrosine kinases have been difficult to identify. Phospholipase C (PLC), a key enzyme of the phosphoinositide pathway, is believed to be an important site for hormonal regulation of the hydrolysis of phosphatidylinositol 4,5-bisphosphate, which produces the intracellular second-messenger molecules inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. Treatment of BALB/c 3T3 cells with PDGF led to a rapid (within 1 min) and significant (greater than 50-fold) increase in PLC activity, as detected in eluates of proteins from a phosphotyrosine immunoaffinity matrix. This PDGF-stimulated increase in phosphotyrosine-immunopurified PLC activity occurred for up to 12 h after addition of growth factor to quiescent cells. Interestingly, the PDGF stimulation occurred at 3 as well as 37 degrees C and in the absence or presence of extracellular Ca2+. Immunoprecipitation of cellular proteins with monoclonal antibodies specific for three distinct cytosolic PLC isozymes demonstrated the presence of a 145-kilodalton isozyme, PLC-gamma (formerly PLC-II), in BALB/c 3T3 cells. Furthermore, these immunoprecipitation studies showed that PLC-gamma is rapidly phosphorylated on tyrosine residues after PDGF stimulation. The results suggest that mitogenic signaling by PDGF is coincident with tyrosine phosphorylation of PLC-gamma.
Wahl et al. (Sat,) studied this question.