Mechanistic roles of lipoprotein lipase and sphingomyelinase in low density lipoprotein aggregation

The initiation of atherosclerosis involves retention of colloidal atherogenic lipoproteins, primarily low density lipoprotein (LDL), in the arterial intima. This retention occurs when LDL binds to smooth muscle cell extracellular matrix (SMC ECM), and is enhanced by lipoprotein lipase (LpL) and sphingomyelinase (Smase). Here we use a fluorescence assay and dynamic light scattering to study the individual and combined effects of these two enzymes on LDL aggregation. Our results show: (1) LpL is self-sufficient to induce LDL aggregation with aggregate sizes up to ∼400 nm; (2) Smase induces LDL aggregation due to generation of ceramide and subsequent hydrophobic interactions; (3) Smase hydrolysis of LpL-induced LDL aggregates does not cause further aggregation and results in a ∼3-fold diminished production of ceramide, while LpL treatment of Smase-induced aggregates does enhance aggregation; (4) The simultaneous addition of LpL and Smase causes increased variability in aggregation with final sizes ranging from 50 to 110 nm. Our data suggest a new proatherogenic function for LpL, namely, bridging between LDL particles causing their aggregation and consequently enhanced retention by SMC ECM. The mechanism of LpL-and-Smase-mediated LDL aggregation and binding to SMC ECM provides specific points of intervention to design novel effective antiatherogenic therapeutics.

Aggregation of colloidal LDL in human arteries leads to increased buildup of arterial plaque. We study the roles of sphingomyelinase and lipoprotein lipase, which are implicated in atherosclerosis, on LDL aggregation mechanisms.Figure optionsDownload high-quality image (156 K)Download as PowerPoint slideHighlights
► Atherogenic modification of colloidal LDL particles is investigated here.
► We show that: lipoprotein lipase (LpL) is self sufficient to induce LDL aggregation (up to ∼400 nm).
► Sphingomyelinase (Smase) treatment of these aggregates produces ∼3-fold less ceramide.
► LpL-and-Smase-mediated LDL aggregation shows increased variability of aggregate sizes.
► These mechanisms provide points of intervention for new antiatherogenic therapies.