Phospholipase A2-activating protein induces the synthesis of IL-1 and TNF in human monocytes

Phospholipase A2-activating protein (PLAP) is an important mediator of eicosanoid generation. PLAP can also be found in high concentrations in synovial fluid from patients with rheumatoid arthritis, and injection of PLAP into animal joints results in an inflammatory, rheumatoid-like lesion. We have demonstrated previously that TNF-alpha and IL-1 beta stimulate formation of PLAP before phospholipase A2 (PLA2) enzyme activation and production of eicosanoids. To further explore the mechanisms found in the inflammatory response, we examined the ability of PLAP to stimulate release of TNF and IL-1 from human peripheral blood monocytes. TNF and IL-1 protein levels were measured by ELISA, and IL-1 and TNF mRNA were determined by Northern blotting. PLAP, PLAP peptide, and melittin, a bee venom PLA2 activator with homology with PLAP, all increased IL-1 and TNF production in a time- and dose-dependent manner. Heat-denatured PLAP and actin (an irrelevant protein) failed to exert this effect. PLAP stimulation of TNF and IL-1 could be enhanced with co-treatment of cells with free fatty acids, such as arachidonic or linoleic acid, but it was not blocked completely by PLA2 inhibitors. These results demonstrate not only that synthesis of PLAP can be stimulated by cytokines, but also that PLAP may regulate cytokine synthesis and thus perpetuate an immune or inflammatory response.