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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1999;19:1276-1283.)
© 1999 American Heart Association, Inc.

Atherosclerosis and Lipoproteins

Lipolytic Modification of LDL by Phospholipase A₂ Induces Particle Aggregation in the Absence and Fusion in the Presence of Heparin

Jukka K. Hakala; Katariina Öörni; Mika Ala-Korpela; Petri T. Kovanen

From the Wihuri Research Institute, Kalliolinnantie 4, FIN-00140 Helsinki.

Correspondence to Petri T. Kovanen, Wihuri Research Institute, Kalliolinnantie 4, FIN-00140 Helsinki, Finland. E-mail Petri.Kovanen{at}wri.fi

Abstract—One of the first events in atherogenesis is modification of low density lipoprotein (LDL) particles in the arterial wall with ensuing formation of aggregated and fused lipid droplets. The accumulating particles are relatively depleted in phosphatidylcholine (PC). Recently, secretory phospholipase A₂ (PLA₂), an enzyme capable of hydrolyzing LDL PC into fatty acid and lysoPC molecules, has been found in atherosclerotic arteries. There is also evidence that both LDL and PLA₂ bind to the glycosaminoglycan (GAG) chains of extracellular proteoglycans in the arterial wall. Here we studied the effect of heparin GAG on the lipolytic modification of LDL by PLA₂. Untreated LDL, heparin-treated LDL, and heparin-bound LDL were lipolyzed with bee venom PLA₂. In the presence of albumin, lipolysis resulted in aggregation in all 3 preparations of the LDL particles. Lipolysis of untreated LDL did not result in aggregation if albumin was absent from the reaction medium, and the lipolytic products accumulated in the particles rendering them negatively charged. However, heparin-treated and heparin-bound lipolyzed LDL particles aggregated even in the absence of albumin. Importantly, in the presence of albumin, some of the heparin-treated and heparin-bound lipolyzed LDL particles fused, the proportion of fused particles being substantially greater when LDL was bound to heparin during lipolysis. In summary, lipolysis of LDL PC by PLA₂ under physiological conditions, which allow transfer of the lipolytic degradation products to albumin, leads to fusion of LDL particles in the presence, but not in the absence, of heparin. Thus, it is possible that within the GAG meshwork of the arterial intima, PLA₂-induced modification of LDL is one source of the lipid droplets during atherogenesis.

Key Words: aggregation • fusion • heparin • LDL • phospholipase A₂

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