Chylomicron-remnant-induced foam cell formation and cytotoxicity: a possible mechanism of cell death in atherosclerosis

The effects of chylomicron remnants on cytoplasmic lipid loading and cell viability were assessed in cultures of human monocyte-derived macrophages and rabbit arterial smooth muscle cells. At a cholesterol concentration of 150 μg/ml, chylomicron remnants induced substantial cytoplasmic lipid loading of macrophages, but not of smooth muscle cells, within 6 h of exposure. Chylomicron remnants were found to be cytotoxic to macrophages and smooth muscle cells, although the latter were generally more resistant. Chylomicron remnants contained no detectable oxysterols (> 1 ng) and contained less non-esterified (‘free’) fatty acids than non-lipolysed nascent chylomicrons. Chylomicron-remnant-induced cytotoxicity appeared to be time- and dose-dependent. Macrophage and smooth muscle cell viability were inversely related to the production of superoxide free radicals and were significantly improved in the combined presence of superoxide dismutase and catalase. Collectively, our data suggest that, in macrophages, cell viability is compromised as a consequence of superoxide free radical production following uptake of chylomicron remnants. We would suggest that, in arterial smooth muscle cells, chylomicron-remnant-induced cell death also occurs as a consequence of superoxide free radical production. Our observations in the present study suggest that macrophage foam cells in atherosclerotic plaques might be derived from the cellular uptake of chylomicron remnants. Furthermore, arterial accumulation of chylomicron remnants might contribute to plaque destabilization as a consequence of cell death following superoxide free radical production by macrophages and smooth muscle cells.