Effect of platelet-activating factor acetylhydrolase on the formation and action of minimally oxidized low density lipoprotein

Mildly oxidized low density lipoprotein (MM-LDL) pro-duced by oxidative enzymes or cocultures of human artery wall cells induces endothelial cells to produce monocyte che-motactic protein-1 and to bind monocytes. HDL prevents the formation ofMM-LDL by cocultures of artery wall cells. Using albumin treatment and HPLC we have isolated and partially characterized bioactive oxidized phospholipids in MM-LDL. Platelet activating factor-acetylhydrolase (PAF-AH), a serine esterase, hydrolyzes short chain acyl groups esterified to the sn-2 position of phospholipids such as PAF and particular oxidatively fragmented phospholipids. Treat-ment of MM-LDL with PAF-AH (2-4 x 10-2 U/ml) elimi-nated the ability of MM-LDL to induce endothelial cells to bind monocytes. When HDL protected against the forma-tion of MM-LDL by cocultures, lysophosphatidylcholine was detected in HDL; whereas when HDL was pretreated with diisopropyl fluorophosphate, HDL was no longer pro-tective and lysophosphatidylcholine was undetectable. HPLC analysis also revealed that the active oxidized phos-pholipid species in MM-LDL had been destroyed after PAF-AH treatment. In addition, treatment of MM-LDL with al-bumin removed polar phospholipids that, when reisolated, induced monocyte binding to endothelial cells. These polar phospholipids, when treated with PAF-AH, lost biological activity and were no longer detected by HPLC. These results suggest that PAF-AH in HDL protects against the produc-tion and activity of MM-LDL by facilitating hydrolysis of active oxidized phospholipids to lysolipids, thereby destroy-ing the biologically active lipids in MM-LDL. (J. Clin. Invest