Cell-mediated oxidation of LDL: Comparison of different cell types of the atherosclerotic lesion

The three major cell types of the human atherosclerotic lesion - macrophages (Mø), smooth muscle cells (SMC) and endothelial cells (EC) - were compared for their ability to oxidise low density lipoprotein (LDL) in vitro under identical conditions. Near-confluent cultures were incubated for up to 48 h with 50 μg protein/ml LDL in Ham's F10 medium supplemented with 7 μM Fe2+. All three cell types oxidised LDL readily using our culture conditions. After 24 and 48 h, the degree of LDL oxidation was in the order: Mø > SMC > EC when based on cell growth area and EC > SMC > Mø when based on cellular DNA content. However, LDL oxidation in vitro progressed more slowly between 24 and 48 h, probably due to increasing toxicity to the cells and/or depletion of polyunsaturated fatty acids. We therefore compared the time of onset of LDL oxidation. The earliest increase in LDL oxidation was always apparent with SMC. Gas chromatography revealed that LDL oxidation by all three cell types followed a similar pattern. The polyunsaturated fatty acids linoleic acid (18:2) and arachidonic acid (20:4) were depleted (to 10.3-18.1% and 4.5-24.7% respectively, compared to native LDL), whereas the content of stearic acid (18:0) and oleic acid (18:1) remained unchanged. Cholesterol was depleted (to 54.1-75.6% of native LDL) with a concomitant rise in 7β-hydroxycholesterol (to 60.6-128.1 μg/mg LDL). This corresponds to a conversion of 4.9, 9.5 and 10.4% of LDL cholesterol in EC-, SMC- and Mø-modified LDL respectively. All three cell types showed significant toxicity in the oxidising culture after 24 h. The possible relevance to LDL oxidation in atherosclerosis is discussed.