Relevance and mechanism of oxysterol stereospecifity in coronary artery disease

Cholesterol oxidation products (oxysterols) are markers for in vitro LDL oxidation. They are potent inducers of programmed cell death and are also found in high concentrations inside atherosclerotic lesions. Among physiologically occurring oxysterols, 7β-OH-cholesterol suggests an increase of lipid peroxidation in vivo. In the underlying study, we quantified free plasma oxysterols by means of gas chromatography in patients with stable coronary artery disease (CAD). Total free plasma oxysterols were elevated more than 2-fold in patients with stable CAD (233 ± 49 vs 108 ± 19 ng/ml, n = 22, P < 0.05) compared to a control group (n = 20) with similar atherogenic risk profile and angiographically normal coronary arteries. We found that 7-ketocholesterol, as well as the β-isomers of epoxide (25.7 ± 10.0 vs 7.3 ± 1.4 ng/ml, P = 0.07) and 7β-OH-cholesterol (65.1 ± 15.7 vs 19.4 ± 8.9 ng/ml, P < 0.01), was mainly responsible for this increase. To elucidate a potential relevance of oxysterol stereospecificity in regard to endothelial damage, we further conducted in vitro experiments using human arterial endothelial cells (HAECs). Surprisingly, β-isomers exerted an up to 10-fold higher amount of cell death in equivalent doses when compared to α-isomers. The greater cytotoxic potential of β-isomers was due to increased apoptosis, preceded by mitochondrial release of cytochrome c with subsequent caspase-3 activation. Stereospecific release of cytochrome c depended on the presence of an intact cytoplasmic membrane, hinting at the existence of a putative oxysterol receptor or a direct stereospecific effect on membrane biology. Finally, both isoforms of oxysterols directly released cytochrome c only in conjunction with protein containing cytosol and endoplasmatic reticulum. Free plasma oxysterol levels, particularly 7-ketocholesterol, β-epoxide and 7β-OH-cholesterol, are elevated in patients with stable CAD, independent of their LDL cholesterol levels. Due to the highly increased cytotoxicity of oxysterol β-isomers in vitro, they may represent important atherogenic risk factors.