ACTIVATION OF PHOSPHOLIPASE-D INDUCED BY PLATELET-DERIVED GROWTH-FACTOR IS DEPENDENT UPON THE LEVEL OF PHOSPHOLIPASE C-GAMMA-1

Abstract

The mechanism of phospholipase D (PLD) activation by platelet derived growth factor (PDGF) was examined using a NIH 3T3 fibroblast cell line (3T3-gamma 1) that stably overexpresses PLC-gamma 1 isozyme. Immunoblot analysis revealed that 3T3-gamma 1 cells contained about 10-fold more PLC-gamma 1 than a control cell line (3T3-C) transfected with expression vector lacking PLC-gamma 1 cDNA. PDGF-stimulated PLD activation was 10-fold greater in 3T3-gamma 1 cells than in 3T3-C cells, indicating that PLD activation is dependent upon the level of PLC-gamma 1. Phorbol 12-myristate 13-acetate (PMA) treatment increased PLD activity to a similar extent in both 3T3-gamma 1 cells and control cells. Pretreatment with tyrosine kinase inhibitors including staurosporine and genistein decreased PLD activity by 82.6% and 87.2%, respectively, and completely blocked tyrosine phosphorylation of PDGF receptor and PLC-gamma 1 in 3T3-gamma 1 cells stimulated with PDGF. Moreover, downregulation of protein kinase C by pretreatment of PMA caused complete inhibition of PDGF- and PMA-stimulated PLD activation. Therefore, these results suggest that PDGF-induced PLD activation may be a consequence of primary stimulation of PLC-gamma 1 and that PLD may play a role downstream hom PLC-gamma 1 in PDGF-triggered mitogenesis.