PLATELET-DERIVED GROWTH-FACTOR INCREASES THE INVIVO ACTIVITY OF PHOSPHOLIPASE-C-GAMMA-1 AND PHOSPHOLIPASE-C-GAMMA-2

Upon binding to its cell surface receptor, platelet-derived growth factor (PDGF) causes the tyrosine phosphorylation of phospholipase C-gamma-1 (PLC-gamma-1) and stimulates the production of diacylglycerol and inositol 1,4,5-triphosphate. We showed that following stimulation by PDGF, rat-2 cells overexpressing PLC-gamma-1 display an increase in the levels of both tyrosine-phosphorylated PLC-gamma-1 and inositol phosphates compared with the parental rat-2 cells. This increased responsiveness to PDGF is a direct effect of PLC-gamma-1 overexpression, as a cell line expressing similar levels of an enzymatically inactive point mutant of PLC-gamma-1, PLC(gamma-1)335Q, did not show elevated inositol phosphate production in response to PDGF. Hematopoietic cells express PLC-gamma-2, a PLC isoform that is closely related to PLC-gamma-1. When rat-2 cells overexpressing PLC-gamma-2 were treated with PDGF, an increase in both the tyrosine phosphorylation and the in vivo activity of PLC-gamma-2 was observed. Aluminum fluoride (AIF4-), a universal activator of PLC linked to G-proteins, did not produce an increase in the levels of inositol phosphates in either of the overexpressing cell lines compared with parental rat-2 cells, demonstrating that PLC-gamma isoforms respond specifically to a receptor with tyrosine kinase activity.