Normal high density lipoprotein inhibits three steps in the formation of mildly oxidized low density lipoprotein: steps 2 and 3

Treatment of human artery wall cells with apolipoprotein A-I (apoA-I), but not apoA-II, with an apoA-I peptide mimetic, or with high density lipoprote in (HDL), or paraoxonase, rendered the cells unable to oxidize low density lipoprotein (LDL), Human aortic wall cells were found to contain 12-lipoxyg enase (12-LO) protein. Transfection of the cells with antisense to 12-LO (b ut not sense) eliminated the 12-LO protein and prevented LDL-induced monocy te chemotactic activity. Addition of 13(S)-hydroperoxyoctadecadienoic acid [13(S)-HPODE] and 15(S)-hydroperoxyeicosatetraenoic acid [15(S)-HPETE] dram atically enhanced the nonenzymatic oxidation of both 1-palmitoyl-2-arachido noyl-sn-glycero-3-phosphocholine (PAPC) and cholesteryl linoleate, On a mol ar basis 13(S)-HPODE and 15(S)-HPETE were approximately two orders of magni tude greater in potency than hydrogen peroxide in causing the formation of biologically active oxidized phospholipids (m/z 594, 610, and 828) from PAP C. Purified paraoxonase inhibited the biologic activity of these oxidized p hospholipids. HDL from 10 of 10 normolipidemic patients with coronary arter y disease, who were neither diabetic nor receiving hypolipidemic medication s, failed to inhibit LDL oxidation by artery wall cells and failed to inhib it the biologic activity of oxidized PAPC, whereas HDL from 10 of 10 age- a nd sex-matched control subjects did. We conclude that a) mildly oxidized LD L is formed in three steps, one of which involves 12-LO and each of which c an be inhibited by normal HDL, and b) HDL from at least some coronary arter y disease patients with normal blood lipid levels is defective both in its ability to prevent LDL oxidation by artery wall cells and in its ability to inhibit the biologic activity of oxidized PAPC.