A catalytically independent physiological function for human acute phase protein group IIA phospholipase A2: cellular...

Human group IIA phospholipase A2 (IIA PLA2) is an acute phase protein first identified at high concentrations in synovial fluid from patients with rheumatoid arthritis. Its physiological role has since been debated; the enzyme has a very high affinity for anionic phospholipid interfaces but expresses almost zero activity with zwitterionic phospholipid substrates, because of a lack of interfacial binding. We have prepared the cysteine-containing mutant (S74C) to allow the covalent attachment of fluorescent reporter groups. We show that fluorescently labeled IIA was taken up by phorbol 12-myristate 13-acetate-activated THP-1 cells in an energy-dependent process involving cell surface heparan sulfate proteoglycans. Uptake concurrently involved significant cell swelling, characteristic of macropinocytosis and the fluorescent enzyme localized to the nucleus. The endocytic process did not necessitate enzyme catalysis, ruling out membrane phospholipid hydrolysis as an essential requirement. The enzyme produced supramolecular aggregates with anionic phospholipid vesicles as a result of bridging between particles, a property that is unique to this globally cationic IIA PLA2. Uptake of such aggregates labeled with fluorescent anionic phospholipid was dramatically enhanced by the IIA protein, and uptake involved binding to heparan sulfate proteoglycans on activated THP-1 cells. A physiological role for this protein is proposed that involves the removal of anionic extracellular cell debris, including anionic microparticles generated as a result of trauma, infection, and the inflammatory response, and under such conditions serum levels of IIA PLA2 can increase approximately 1000-fold. A similar pathway may be significant in the uptake into cells of anionic vector DNA involving cationic lipid transfection protocols.