MINIMAL OXIDATION AND STORAGE OF LOW-DENSITY LIPOPROTEINS RESULT IN AN INCREASED SUSCEPTIBILITY TO PHOSPHOLIPID HYDROLYSIS BY PHOSPHOLIPASEA(2)

In vitro-studies have shown that phospholipid hydrolysis of low densit y lipoproteins (LDL) by bee venom or porcine pancreatic phospholipase A(2) (PLA(2)) leads to an increased uptake of these lipoproteins by ma crophages transforming them into foam cells. Recently, a secretory pho spholipase A(2), group II, was detected in human atherosclerotic plaqu es. In order to investigate the role of this enzyme in the pathogenesi s of atherosclerosis, a structurally identical human secretory PLA, wa s purified from the medium of HepG2 cells stimulated with interleukin- 6 and tumor necrosis factor-alpha. The activity of the purified enzyme towards the phospholipids of native and modified low density lipoprot eins was compared with the activity towards Escherichia coli-membranes and other phospholipid substrates. Compared to E. coli-membranes, nat ive LDL proved to be a poor substrate for group II PLA(2). After mild oxidation induced by copper ions or by 2,2-azobis(2-amidinopropane) (A APH), the susceptibility of LDL to phospholipid hydrolysis was found t o be increased by 25 and 23%, respectively, whereas extensive copper-m ediated oxidation caused a decreased hydrolysis. Aging of LDL at 6 deg rees C for weeks or at 37 degrees C for hours resulted in an increase in PLA(2)-catalyzed phospholipid hydrolysis of up to 26-fold. LDL prot ected from oxidation by probucol during aging showed a lesser increase in susceptibility to phospholipid hydrolysis. Our results suggest tha t PLA(2), group II, can increase the atherogenicity of LDL by its abil ity to hydrolyze the phospholipids of these lipoproteins, especially a fter modifications that are likely to occur in vivo. (C) 1997 Elsevier Science Ireland Ltd.