Lipolysis of LDL by Human Secretory Phospholipase A ₂ Induces Particle Fusion and Enhances the Retention of LDL to Human Aortic Proteoglycans

Abstract

Abstract-The first morphological sign of atherogenesis is the accumulation of extracellular lipid droplets in the proteoglycan-rich subendothelial layer of the arterial intima. Secretory nonpancreatic phospholipase A 2 (snpPLA 2 ), an enzyme capable of lipolyzing LDL particles, is found in the arterial extracellular matrix and in contact with the extracellular lipid droplets. We have recently shown that in the presence of heparin, lipolysis of LDL with bee venom PLA 2 induces aggregation and fusion of the particles. Here, we studied the effect of human snpPLA 2 on the integrity of LDL particles and on their interaction with human aortic proteoglycans. In addition, the capacity of the proteoglycans to retain PLA 2 -lipolyzed LDL particles was tested in a microtiter well assay. We found that lipolysis of LDL induced fusion of proteoglycan-bound LDL particles, which increased their binding strength to the proteoglycans. Moreover, lipolysis of LDL with snpPLA 2 under physiological salt and albumin concentrations induced a 3-fold increase in the amount of LDL bound to proteoglycans. The results imply a role for PLA 2 in the retention and accumulation of LDL to the proteoglycan matrix in atherosclerosis. Key Words: phospholipases Ⅲ LDL Ⅲ fusion Ⅲ retention Ⅲ proteoglycans I nitiation of atherosclerosis in both humans 1 and experimental animals 2-4 is characterized by the appearance of extracellular lipid droplets in the proteoglycan (PG)-rich subendothelial layer. Experimental models have shown that similar droplets can be formed directly from LDL particles both in situ 2,5 and in vivo. 2,6 Moreover, both chemical analyses 7,8 and measurements of the size 9,10 of the extracellular lipid droplets in human arterial lesions suggest that the majority of the droplets originate from LDL particles. Because native LDL particles do not fuse into such lipid droplets, the LDL particles must undergo modification in the arterial intima. Indeed, the lipid droplets isolated from the arterial intima have features suggesting that they may have been derived from plasma LDL by extensive modification (reviewed by Öörni et al 11 ). Modification of LDL in vitro by proteolytic enzymes, 12-14 by oxidative compounds, 15 or by lipolytic enzymes such as sphingomyelinase, 13,15-17 phospholipase C, 18,19 or phospholipase A 2 (PLA 2 ) 17,20 has been shown to induce aggregation, fusion, or both aggregation and fusion of the particles. See p 884Phospholipase A 2 s are enzymes that catalyze the hydrolysis of the sn-2 fatty acyl ester bond in phospholipids, yielding a free fatty acid and a lysophospholipid. Recently, type II secretory nonpancreatic phospholipase A 2 (snpPLA 2 ), which is capable of lipolyzing LDL, 21 has been shown to be located in both atherosclerotic and nonatherosclerotic arterial intima [22][23][24][25][26] and to be associated with extracellular matrix structures and lipid droplets. 24 Interestingly, lipolysis of LDL by bee venom PLA 2 reduces the size of the LDL particles in the absence of glycosaminoglycans (GAGs) 17,20,27 but ...

Group V Secretory Phospholipase A2-modified Low Density Lipoprotein Promotes Foam Cell Formation by a SR-A- and CD36-independent Process That Involves Cellular Proteoglycans

“…Previous studies have shown that lipolysis of LDL with sPLA 2 results in an increased affinity for proteoglycans secreted by human arterial smooth muscle cells (47) and proteoglycans isolated from human aorta (20). In a recent study, Flood et al (24) identified the molecular basis for this altered interaction.…”

“…Accumulating evidence indicates that sPLA 2 hydrolysis of LDL-PL results in structural alterations of the particles that promote lipid accumulation in the vessel wall and enhances macrophage uptake. Hydrolysis of LDL by sPLA 2 in vitro results in an increased affinity for proteoglycans, which would be expected to increase retention in the subendothelium (20,21). Lipolysis of LDL by sPLA 2 leads to conformational changes in apoB-100 and reorganization of lipids that induce particle aggregation (22)(23)(24).…”

Cysteine Protease Cathepsin F Is Expressed in Human Atherosclerotic Lesions, Is Secreted by Cultured Macrophages, and Modifies Low Density Lipoprotein Particles in Vitro

“…The degree of aggregation and/or fusion of proteolyzed [ 3 H]cholesteryl linoleate-labeled LDL was determined by rate zonal ultracentrifugation (37), as described previously (38). Briefly, a linear NaBr gradient (d ϭ 1.006 -1.10 g/ml) was layered on top of 50-l samples of modified [ 3 H]cholesteryl linoleate-LDL in 250 l of 40% NaBr (w/v) and centrifuged at 33,000 rpm in a SW 40 Ti rotor (Beckman) for 1 h at 20°C.…”

Relation of Secretory Phospholipase A2 and High-Sensitivity C-Reactive Protein to Chlamydia Pneumoniae Infection in Acute Coronary Syndromes

“…Secretory PLA2, is stored in the secretory granules of immune cells such as leukocytes and mast cells and participates in arachidonic acid metabolism during the progression of inflammation. 18,19 High concentrations of sPLA2 are found in inflammatory exudate. 20 Presently, expression of sPLA2 is thought to be promoted by IL-1, as well as by inflammatory cytokines such as interferon and IL-6, lipopolysaccharides, and substances that elevate intracellular concentrations of cyclic adenosine monophos- phate, such as forskolin.…”

“…16 Recent interest has focused on secretory group IIA phospholipase A2 (sPLA2) because of its potential role in the accumulation of oxidized LDL in vascular walls as well as in the progression of atherosclerosis and the development of CAD. [17][18][19][20] The value of sPLA2 as a potential marker for the risk of cardiovascular events has received considerable attention.…”