The regulation of phospholipase-A2 (PLA-2) by cytokines expressing hematopoietic growth-stimulating properties.

Various growth factors released by macrophages and other cell types modulate normal hematopoiesis. The physiological mechanisms whereby these molecules interact with specific target cells are ill defined. Eicosanoids, the products of fatty acid metabolism, are known to regulate cell proliferation and differentiation. The release of membrane-bound phospholipid by phospholipase-A2 (PLA-2) is the first critical step in the initiation of membrane remodeling and eventually eicosanoid synthesis. We report here data that demonstrates how various cytokines exhibit a marked hydrolytic activity mediated through PLA-2 against both [1-14C] oleic acid- and [1-14C] arachidonic acid-labeled Escherichia coli (micelle) substrates. PLA-2 extracts were prepared from neutrophils elicited by injecting rats ip with 8% glycogen. The rate of hydrolysis of free fatty acids from the phospholipid substrate was found to be linear, rapid, and pH dependent and was calculated to be 30 nmoles of phospholipid/hr/mg protein lysate. Cytokines (i.e., interleukin-1 [IL-1, human and murine recombinant, alpha], mouse lung cell-derived colony-stimulating factor [L-CSF], granulocyte-macrophage colony-stimulating factor [murine recombinant GM-CSF], tumor necrosis factor [murine recombinant TNF-alpha], and granulocyte colony-stimulating factor [human recombinant, G-CSF] all induced PLA-2 activity with the release of free fatty acids above basal levels. In contrast, lipopolysaccharide (LPS), interleukin-2, (IL-2, human recombinant), and macrophage colony-stimulating factor (M-CSF) did not significantly activate PLA-2 hydrolysis. The activation of this membrane-bound enzyme-substrate complex by these growth factors may serve as a mechanism whereby the appropriate target cells expressing receptors respond through either direct or secondary signals leading to the formation of free fatty acids with the eventual synthesis of prostanoid or lipoxygenase products, resulting in cellular proliferation and differentiation.