Phospholipase A2 type II binds to extracellular matrix biglycan: modulation of its activity on LDL by colocalization in glycosaminoglycan matrixes

We recently reported the presence of secretory, nonpancreatic phospholipase A2 type II (snpPLA2; EC 3.1.1.4) in human atherosclerotic arteries (Hurt-Camejo et al, Arterioscler Thromb Vasc Biol. 1997;17:300-309). SnpPLA2 may generate the proinflammatory products lysophospholipids and free fatty acids, thus contributing to atherogenesis when acting on low density lipoproteins (LDLs) retained in the arterial wall. Immunohistochemical studies showed that smooth muscle cells (SMCs) in human arterial tissue are the main sources of snpPLA2. In cultures of human arterial SMCs, snpPLA2 interacts with versican and smaller heparan/chondroitin sulfate proteoglycans (PGs) secreted as soluble components into the medium. In the present study, we investigated the binding of snpPLA2 to extracellular matrix (ECM) PGs produced by SMCs. The results show that snpPLA2 can bind to the ECM at physiological salt concentrations. ECM-bound snpPLA2 was active, hydrolyzing phosphatidylcholine-containing micelles. Soluble chondroitin-6-sulfate at concentrations >1 micromol/L, but not heparin or heparan sulfate, was able to release ECM-bound snpPLA2. The PG mainly involved in the binding of snpPLA2 was identified as biglycan. Perlecan was also present in the ECM synthesized by SMCs, but it contributed less to the binding of snpPLA2. Experiments with immobilized glycosaminoglycans indicated that snpPLA2 hydrolyzed 7-fold more LDL phospholipids when the lipoprotein and the enzyme were colocalized in a matrix with chondroitin-6-sulfate compared with one with heparin. These data suggest that retention of snpPLA2 in ECMs of different composition may modulate the enzymatic activity of snpPLA2 toward LDL. The results presented in this work support the hypothesis of the potential contribution of snpPLA2 to atherosclerosis.