Localization of Nonpancreatic Secretory Phospholipase A 2 in Normal and Atherosclerotic Arteries

Abstract

Secretory nonpancreatic type II phospholipase A 2 (snpPLA 2 ) hydrolyzes fatty acids at the sn-2 position in phospholipids releasing free fatty acids (FFAs) and lysophospholipids. These products may act as intracellular second messengers or can be further metabolized into proinflammatory lipid mediators. The presence of snpPLA 2 in extracellular fluids and serum during inflammation has suggested a role of the enzyme in this process. However, the presence of snpPLA 2 in a variety of normal tissues suggests that snpPLA 2 may also have physiological functions. Atherosclerosis appears to have an inflammatory component. Here we report on the snpPLA 2 localization in normal and atherosclerotic lesions and on the properties of the isolated enzyme. A strong snpPLA 2 immunoreactivity was observed in the arterial media that was colocalized with α-actin–positive vascular smooth muscle cells (SMCs) in both normal and atherosclerotic vessels. In aortic atherosclerotic lesions, snpPLA 2 was observed colocalized with CD68-positive macrophages and HHF-35–positive SMCs and extracellularly in the lipid core. snpPLA 2 was isolated from human normal arteries and from aorta with lesions. The enzyme was isolated by acid extraction of normal arterial tissues followed by immunoaffinity chromatography. The purified snpPLA 2 had an expected molecular weight of 14 kD by polyacrylamide gel electrophoresis and appeared as a single band in immunoblotting. The enzymatic activity was followed by measuring release of fatty acids from phospholipid liposomes or LDL as substrates. The enzymatic activity was inhibited with two specific inhibitors for human snpPLA 2 : (1) monoclonal antibody 187 and (2) LY311727, a synthetic selective inhibitor. The mRNA for snpPLA 2 was detected with reverse transcriptase polymerase chain reaction. These results indicate that snpPLA 2 is present in human arteries and that it is able to hydrolyze phospholipids in LDL. The results support the hypothesis that snpPLA 2 can release proinflammatory lipids at places of LDL deposition in the arterial wall.