Platelet-derived growth factor stimulates a biphasic mobilization of arachidonic acid in Swiss 3T3 cells. The role of phospholipase A2.

Stimulation of quiescent Swiss 3T3 cells with platelet-derived growth factor (PDGF) increased the initial rate of cytosolic phospholipase A2 activity by 95 +/- 6% over extracts from control cells. Cytosolic phospholipase A2 activity increased rapidly following PDGF treatment (near maximum stimulation by 2.5 min) and was dose-dependent (EC50 = 2 ng/ml). Epidermal growth factor, vasopressin, and phorbol 12,13-dibutyrate also increased cytosolic phospholipase A2 activity but did not produce a sustained mobilization of arachidonic acid in these cells. Detailed kinetic analysis of PDGF-induced arachidonic acid mobilization revealed a biphasic release of 3H radioactivity into the extracellular medium. A first, rapid phase, occurred within 15 min which, like the activation of cytosolic phospholipase A2 activity, was independent of de novo RNA and protein synthesis. After 20 min of stimulation, a second phase became evident which accounts for the majority of arachidonic acid mobilized by PDGF. This second phase was abolished in the presence of either cycloheximide or actinomycin D. Both inhibitors blocked the release of arachidonic acid rather than inhibiting cyclooxygenase activity and consequently prostaglandin E2 production. These findings demonstrate a biphasic mobilization of arachidonic acid in Swiss 3T3 cells by PDGF. Cytosolic phospholipase A2 activity could contribute to the rapid first phase but not the second major phase, which is dependent upon de novo protein synthesis.