Particle size and platelet activating factor-acetylhydrolase influence LDL oxidation by an artery wall coculture system

Small dense LDL has enhanced susceptibility to in vitro copper mediated oxidation (LDL-OX). However, LDL-OX in the absence of serum and artery wall cells may not reliably reflect LDL modification in vivo. Additionally, it is unknown if there exist any differences in the atherogenic effects resulting from oxidation of buoyant and dense LDL and whether this process is influenced by the LDL associated enzyme platelet activating factor-acetylhydrolase(PAF-AH). To more closely test the effect that particle size and PAF-AH may have on in vivo modification of LDL we prepared buoyant (B-LDL: 1.022-1.034 g/ml) and dense LDL (D-LDL:1.040-1.054 g/ml) and evaluated their LDL-OX (by measuring conjugated diene formation) and their ability to stimulate monocyte chemotaxis and adherence to endothelial cells after oxidation in a smooth muscle/endothelial cell coculture system containing 7.5% lipoprotein-deficient serum (LPDS). D-LDL particles were significantly smaller (22.5 ± 1.6 vs. 27.5 ± 5.4 nm, p<0.05) and had greater LDL-OX compared to B-LDL. LDL-OX lag time correlated with LDL particle size (r=0.82, p<0.05). LDL conditioned by cocultures stimulated monocyte chemotaxis and adherence to endothelial cells in the order D-LDL>B-LDL=unconditioned LDL. Since PAF-AH reportedly reduces the atherogenic properties of oxidized LDL, but is progressively lost during LDL oxidation we evaluated the effect of PAF-AH inactivation by phenylmethylsulfonylfluoride treatment. LDL-OX was increased in both B-LDL and D-LDL after PAF-AH inactivation, yet the change in chemotaxis differed (B-LDL 1.3±.8 vs. D-LDL 8.6 ±4.3 monocytes per field, p <0.05). In conclusion, D-LDL particles are more susceptible to oxidation even in a serum-containing in vitro model of the artery wall. Additionally, PAF-AH protects both LDL subfractions from oxidation when present, however its inactivation in D-LDL markedly enhances D-LDL's bioactivity. Thus, D-LDL may be particulary atherogenic because its enhanced susceptibility to oxidation may also accelerate the inactivation of PAF-AH, further stimulating monocyte chemotaxis and adherence.