Phagocytosis of aggregated lipoprotein by macrophages: low density lipoprotein receptor-dependent foam-cell formation

Macrophages and oxidized low density lipoproteins in the pathogenesis of atherosclerosis

Oxidized low density lipoprotein (LDL) may play an important role in the pathogenesis of atherosclerosis. Recent evidence strongly suggests that oxidized LDL is present in atherosclerotic lesions in vivo: 1) LDL isolated from human and rabbit lesions (but not from normal intima) resembles oxidized LDL in its physical, chemical and immunological properties; 2) Oxidized LDL and/or oxidation specific lipid-protein adducts can be demonstrated in human and rabbit lesions by immunocytochemical techniques; 3) Human and rabbit serum contains autoantibodies against oxidized LDL and oxidation specific lipid-protein adducts; 4) atherosclerotic lesions contain IgG that recognizes oxidized LDL and 5) antioxidant therapy slows the development of atherosclerotic lesions in rabbits. Atherosclerosis in human and rabbit arteries may be linked to macrophage-induced oxidative modification of LDL mediated by 15-lipoxygenase which leads to an enhanced uptake of LDL in macrophages by way of the scavenger receptor(s). The identification of LDL oxidation as one of the key events in the early pathogenesis of atherosclerosis offers an interesting possibility to reduce atherosclerosis by antioxidants, enzyme inhibitors and other compounds that protect LDL against oxidative damage and/or reduce the subsequent harmful effects of oxidized LDL on various cellular functions.

Arterial metabolism of lipoproteins in relation to atherogenesis

In this short review we have concentrated on the ways in which modification of LDL structure may account for foam cell formation. We have presented in vivo evidence as well as in vitro evidence supporting the proposition that modification of native LDL is a prerequisite for foam cell formation and atherogenesis. Actually, oxidized LDL can contribute to atherogenesis in other ways as well. Oxidized LDL is chemotactic for circulating monocytes, yet inhibits the motility of the tissue macrophage as shown by Quinn et al. Also, oxidized LDL is cytotoxic as discussed above and this could play a crucial role in the transition from the fatty streak lesion to the clinically more consequential fibrous plaque and complicated lesion. If further research supports the importance of LDL modification in atherogenesis, a whole new array of possibilities opens itself to us for intervention. Anything that interferes with the relevant modifications of the LDL structure would presumably be additive to interventions lowering the plasma concentration of LDL. At the moment, the only such intervention that appears to be feasible is prevention of LDL oxidation. Possibly we may find ways to interfere with immune mechanisms that are involved in some patients; conceivably we might be able to interfere with the aggregation of LDL with itself or with other complexes in the artery wall that appear also to favor initiation of the atherogenic process.