Sphingomyelinase induces aggregation and fusion of small very low-density lipoprotein and intermediate-density lipoprotein particles and increases their retention to human arterial proteoglycans

Objectives - Infiltration of low- density lipoprotein ( LDL) into subendothelial space is an early step in atherosclerosis. In addition to LDL particles, small very low - density lipoprotein ( sVLDL) and intermediate- density lipoprotein ( IDL) particles are also able to enter the arterial intima and be retained within the subendothelial extracellular matrix. Here we compared how proteolysis with alpha- chymotrypsin and phospholipid hydrolysis with phospholipase A(2) or sphingomyelinase ( SMase) of sVLDL, IDL, and LDL particles can influence their aggregation, fusion, and binding to human arterial proteoglycans in vitro. Methods and Results - In each of the 3 lipoprotein classes, the particles became only slightly aggregated with alpha- chymotrypsin or phospholipase A(2). However, the particles strongly aggregated when treated with SMase. The aggregated/ fused particles were found to bind to proteoglycans in proteoglycan affinity chromatography more tightly than the native- sized counterparts. In addition, in a microtiter well assay, the binding of SMase- treated lipoproteins was enhanced: the amounts of proteoglycan- bound SMase- treated LDL, IDL, and sVLDL were 4-, 5-, and 20- fold higher, respectively, than the amounts of proteoglycan- bound native lipoproteins. Conclusion - These results imply a specific role for SMase as an sVLDL- and IDL- modifying enzyme and also suggest a novel mechanism of lipid accumulation in atherogenesis, namely enhanced retention of atherogenic triglyceride- rich lipoprotein particles in intimal areas expressing extracellular SMase activity.