Lipoprotein-associated phospholipase A2, platelet-activating factor acetylhydrolase, generates two bioactive products during the oxidation of low-density lipoprotein: use of a novel inhibitor

A novel and potent azetidinone inhibitor of the lipoprotein-associated phospholipase A₂ (Lp-PLA₂), i.e. platelet-activating factor acetylhydrolase, is described for the first time. This inhibitor, SB-222657 (K_i = 40±3 nM, k_obs/[I] = 6.6×10⁵ M^-1·s^-1), is inactive against paraoxonase, is a poor inhibitor of lecithin:cholesterol acyltransferase and has been used to investigate the role of Lp-PLA₂ in the oxidative modification of lipoproteins. Although pretreatment with SB-222657 did not affect the kinetics of low-density lipoprotein (LDL) oxidation by Cu²⁺ or an azo free-radical generator as determined by assay of lipid hydroperoxides (LOOHs), conjugated dienes and thiobarbituric acid-reacting substances, in both cases it inhibited the elevation in lysophosphatidylcholine content. Moreover, the significantly increased monocyte chemoattractant activity found in a non-esterified fatty acid fraction from LDL oxidized by Cu²⁺ was also prevented by pretreatment with SB-222657, with an IC₅₀ value of 5.0±0.4 nM. The less potent diastereoisomer of SB-222657, SB-223777 (K_i = 6.3±0.5 µM, k_obs/[I] = 1.6×10⁴ M^-1·s^-1), was found to be significantly less active in both assays. Thus, in addition to generating lysophosphatidylcholine, a known biologically active lipid, these results demonstrate that Lp-PLA₂ is capable of generating oxidized non-esterified fatty acid moieties that are also bioactive. These findings are consistent with our proposal that Lp-PLA₂ has a predominantly pro-inflammatory role in atherogenesis. Finally, similar studies have demonstrated that a different situation exists during the oxidation of high-density lipoprotein, with enzyme(s) other than Lp-PLA₂ apparently being responsible for generating lysophosphatidylcholine.