In vitro oxidised HDL exerts a cytotoxic effect on macrophages

The cytotoxic effect of native high density lipoprotein (n-HDL) and oxidised high density lipoprotein (ox-HDL) on macrophages was studied and compared with that of low density lipoprotein (LDL). Copper-mediated oxidation of HDL and LDL was conducted in vitro and assessed by the analysis of conjugated dienes (CD). The kinetics of CD production during lipoprotein oxidation showed that HDL, relative to LDL, exhibited a shorter lag phase (47.7 +/- 17.8 vs. 82.9 +/- 24.5 min), higher diene production (242.2 +/- 23.0 vs 210.4 +/- 14.9 nmol/mg lipid) and reached maximal diene concentration in less time (100.0 +/- 35.4 vs 136.4 +/- 27.9 min). The maximal rate of CD production was 5.38 +/- 1.30 nmol/mg lipid/min for HDL and 4.42 +/- 0.60 nmol/mg lipid/min for LDL. Vitamin E concentration was higher in HDL than in LDL (2.76 +/- 0.41 vs. 2.19 +/- 0.33 micrograms alpha-tocopherol equivalent/mg lipid). Ox-HDL and oxidised LDL (ox-LDL), under the same experimental conditions, were cytotoxic to macrophages in a dose-dependent manner. At the same protein, or total mass concentration, ox-HDL was less cytotoxic than ox-LDL. However, when both lipoproteins were compared at the same lipid or cholesterol concentrations, ox-HDL was equally or more cytotoxic than ox-LDL. In conclusion, HDL is more susceptible to in vitro oxidation than LDL and the resultant modification of HDL converts this lipoprotein into a cytotoxic particle.