Sphingomyelinase converts lipoproteins from apolipoprotein E knockout mice into potent inducers of macrophage foam cell formation

The apoE knockout (EO) mouse is one of the most widely used animal models of atherosclerosis, and there may be similarities to chylomicron remnant-induced atherosclerosis in humans. Although the lesions of these mice contain large numbers of cholesteryl ester (CE)-laden macrophages (foam cells), EO plasma lipoproteins are relatively weak inducers of cholesterol esterification in macrophages. Previous in vivo work has suggested that arterial wall sphingomyelinase (SMase) may promote atherogenesis in the EO mouse, perhaps by inducing subendothelial lipoprotein aggregation and subsequent foam cell formation. The goal of the present study was to test the hypothesis that the modification of EO lipoproteins by SMase converts these lipoproteins into potent inducers of macrophage foam cell formation. When d<1.063 EO lipoproteins were pretreated with SMase and then incubated with EO macrophages, cellular CE mass and stimulation of the cholesterol esterification pathway were increased 5-fold compared with untreated lipoproteins. SMase-treated EO lipoproteins were more potent stimulators of cholesterol esterification than either EO lipoproteins in the presence of lipoprotein lipases or oxidized EO lipoproteins. The uptake and degradation of SMase-treated EO lipoproteins by macrophages were saturable and specific and substantially inhibited by partial proteolysis of cell-surface proteins. Uptake and degradation were diminished by an anti-apoB antibody and by competition with human S-f 100-400 hypertriglyceridemic VLDL, raising the possibility that a receptor that recognizes apoB-48 might be involved. In conclusion, SMase-modification of EO lipoproteins, a process previously shown to occur in lesions, may be an important mechanism for foam cell formation in this widely studied model of atherosclerosis. Moreover, the findings in this report may provide important clues regarding the atherogenicity of chylomicron remnants in humans.