Scavenger receptors and modified lipoproteins: fatal attractions?

Lipoproteins modified by oxidation, glycation, alkylation, and nitration are generated by oxidative stress during inflammation, diabetes, and inadequate supply of dietary antioxidants. A family of genes, the scavenger receptors, recognizes and internalizes modified lipoproteins, making them susceptible to degradation. Clearance of modified lipoproteins by scavenger receptors occurs mainly in macrophages, dendritic cells, and Kupffer cells of the liver. However, scavenger receptor expression also occurs in other cells, such as endothelial cells, aortic smooth muscle cells, neuronal cells, and keratinocytes. Thus, the local clearance of oxidized low-density lipoprotein and the resolution of inflammatory processes may rely in part on the expression of scavenger receptors in "nonprofessional" phagocytes. Uptake of oxidized low-density lipoprotein, without an efficient machinery to degrade them and uncontrolled expression of scavenger receptors, may lead to cellular deregulation, apoptosis, and formation of foam cells. Diseases accompanied by oxidation of lipoproteins, such as atherosclerosis, Alzheimer disease, glomerulosclerosis, ataxia with vitamin E deficiency, and possibly age-dependent lipofuscin deposition, may share a common pathogenetic feature. This review will focus on foam cell formation, mainly within the atherosclerotic lesion, and the possible involvement of aberrant regulation of the scavenger receptor genes. To date, the regulatory mechanisms at the basis of scavenger receptor gene expression and their roles in atherosclerosis and other diseases are not well established. Knowledge on this subject could lead to a better understanding of the pathogenesis, prevention, and therapy of these diseases.