Size-Selective Uptake of Sphingomyelinase-Modified Low Density Lipoprotein Aggregates by Macrophages

The accumulation of foam cells in the subendothelial region of arteries is a key feature of atherosclerotic lesional tissue. We have previously shown that the aggregation of low density lipoprotein (LDL) particles, which is known to enhance the extent of foam cell formation, depends directly on the concentration of ceramide that is generated in the LDL phospholipid monolayer. Here, we focus on the effect of LDL aggregate particle sizes on their subsequent uptake by macrophages. Our data shows the first measurement of uptake as a function of aggregate size and also the first direct comparison of uptake resulting from neutral, Mg²⁺-dependent Sphingomyelinase (Smase)-catalyzed and vortex mixing-mediated LDL aggregation. Size-selectivity was observed in both types of samples. Particle sizes, ranging from radii of 12-212 nm and 12-78 nm, for Smase-treated and vortexed samples, respectively, resulted in maximal uptake (~58 and ~118 µg sterol/mg protein) at intermediate aggregate sizes (132 nm and 53 nm), which was consistent with micrographs. Maximal uptake at intermediate sizes indicates competition between different controlling factors. This study shows that Smase-catalyzed LDL aggregation is a significant factor in the size-dependant uptake of LDL by macrophages and therefore in the progression of atherosclerotic lesions and the pathophysiology of atherosclerosis.