Serum stimulation of phospholipase A₂ and prostaglandin release in 3T3 cells is associated with platelet-derived growth-promoting activity

Sera from mouse, rat, and calf sources stimulate cellular phospholipase A₂ acitivity (PLase; phosphatide 2-acylhydrolase, EC 3.1.1.4) and prostaglandin synthesis in 3T3 Swiss mouse fibroblasts, releasing up to 33% of biosynthetically incorporated [³H]arachidonic acid as hydrolysis products in 1 hr. The PLase stimulated by mouse serum exhibits specificity for arachidonic acid residues on phospholipids. It is stimulated 2.5-fold by 1.8 mM Ca²⁺ in the presence of 5 μM divalent cation ionophore A23187, consistent with a Ca²⁺-dependent enzyme possessing a cytoplasmic Ca²⁺ binding site. The percentage maximal PLase- and growth-stimulating activities of the three sera exhibit similar concentration dependencies, with the homologous (mouse) serum exhibiting the highest specific activities. Confluent 3T3 cells deplete PLase-stimulating activity from medium containing calf serum at a rate similar to the depletion of cell growth-stimulating activity. The PLase-stimulating activity in rat and mouse sera is derived from the leukocyte fraction of blood, presumably from platelets. In rat leukocyte lysates the PLase-stimulating activity exhibits properties similar to those reported for platelet-derived cell growth factors from rat and human sources - i.e., stability to exposure to 100°C for 2 min or to pH 2 or pH 11, and cationic properties. Purified preparations of human platelet-derived growth factor also exhibit 3T3 PLase-stimulating activity.