Lipid-modifying enzymes and macrophage scavenger receptors that may affect foam cell formation and atherogenesis. From left to right: putative lipoprotein modifiers, macrophage scavenger receptors, and fat-laden foam cells. Arrows from the lipoprotein-modifying enzymes indicate the components of lipoproteins they can act on. Red arrows within the macrophage indicate whether a given scavenger receptor mediates cholesterol uptake or efflux. Circles in macrophages indicate intracellularly accumulated cholesteryl esters. CD68 is included in this figure because it has been shown to bind oxidized LDL, but its role in modified LDL uptake is not yet clear. SR-A, scavenger receptor class A type I/II (9–11); SR-BI, scavenger receptor class B type I (16, 24). MPO, myeloperoxidase; 12/15-LO, lipoxygenase (25); sPLA₂, secretory phospholipase A₂ (26); SMase, sphingomyelinase; CE, cholesteryl ester; PL, phospholipids.