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 pho spholipase A2 activity increased rapidly following PDGF treatment (nea r maximum stimulation by 2.5 min) and was dose-dependent (EC50 = 2 ng/ ml). Epidermal growth factor, vasopressin, and phorbol 12,13-dibutyrat e also increased cytosolic phospholipase A2 activity but did not produ ce a sustained mobilization of arachidonic acid in these cells. Detail ed kinetic analysis of PDGF-induced arachidonic acid mobilization reve aled a biphasic release of H-3 radioactivity into the extracellular me dium. A first, rapid phase, occurred within 15 min which, like the act ivation 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 ac id 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 acti vity and consequently prostaglandin E2 production. These findings demo nstrate 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 u pon de novo protein synthesis.