Cytotoxicity of myeloperoxidase/nitrite-oxidized low-density lipoprotein toward endothelial cells is due to a high 7β-hydroxycholesterol to 7-ketocholesterol ratio

Oxygenated cholesterols (oxysterols) formed during oxidation of low-density lipoprotein (LDL) are associated with endothelial dysfunction and atherogenesis. We compared the profile of oxysterols in modified human LDL obtained on reaction with myeloperoxidase/H2O2 plus nitrite (MPO/H2O2/nitrite-oxLDL) with that on Cu2+-catalyzed oxidation. The 7β-hydroxycholesterol/7-ketocholesterol ratio was markedly higher in MPO/H2O2/nitrite-oxLDL than in Cu2+-oxidized LDL (7.9 ± 3.0 versus 0.94 ± 0.10). Like MPO/H2O2/nitrite-oxLDL, 7β-hydroxycholesterol was cytotoxic toward endothelial cells through eliciting oxidative stress. Cytotoxicity was accompanied by DNA fragmentation and was prevented by the NADPH oxidase inhibitor apocynin, suggesting stimulation of NADPH oxidase-mediated O2− formation. 7-Ketocholesterol was only cytotoxic when added alone, whereas a 1:1-mixture with 7β-hydroxycholesterol surprisingly was noncytotoxic. We conclude from our data that (i) 7β-hydroxycholesterol is a pivotal cytotoxic component of oxidized LDL, (ii) 7-ketocholesterol protects against 7β-hydroxycholesterol in oxysterol mixtures or oxLDL, (iii) the 7β-hydroxycholesterol/7-ketocholesterol ratio is a crucial determinant for cytotoxicity of oxidized LDL species and oxysterol mixtures, and (iv) the low share of 7-ketocholesterol explains the higher cytotoxicity of MPO/H2O2/nitrite-oxLDL than other forms of oxidized LDL. The dietary polyphenol (−)-epicatechin inhibited not only formation but also cytotoxic actions of both oxLDL and oxysterols.